From manganese to group VI metals: development and investigation of base metal catalysts for homogeneous hydrogenation reactions / Author Thomas Vielhaber

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JOHANNES KEPLER UNIVERSITY LINZ Altenberger Straße 69 4040 Linz, Austria jku.at Author Thomas Vielhaber Submitted at Institute of Catalysis Thesis Supervisor DI Dr. Christoph Topf Co-Supervisor

Univ.-Prof. Dr. Marko Hapke

December 2019

FROM MANGANESE TO GROUP

VI METALS: DEVELOPMENT

AND INVESTIGATION OF BASE

METAL CATALYSTS FOR

HOMOGENEOUS

HYDROGENATION REACTIONS

Master’s Thesis

to confer the academic degree of

Diplom-Ingenieur

in the Master’s Program

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SWORN DECLARATION

I hereby declare under oath that the submitted Master’s Thesis has been written solely by me without any third-party assistance, information other than provided sources or aids have not been used and those used have been fully documented. Sources for literal, paraphrased and cited quotes have been accurately credited.

The submitted document here present is identical to the electronically submitted text document.

Linz,

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ACKNOWLEDGEMENTS

First of all, I would like to express my very great appreciation to Univ.-Prof. Dr. Marko Hapke and the entire Institute of Catalysis for providing the opportunity to perform the research necessary for finishing my Master’s program. I would also like to thank the members of the Institute for the friendly atmosphere and their willingness to help.

I gratefully acknowledge my colleague Kirill Faust MSc for solving the molecular structures of the metal complexes presented in this thesis via X-Ray diffraction analyses. Furthermore, I wish to thank assoc. Univ.-Prof. Dr. Markus Himmelsbach from the Institute of Analytical Chemistry at the JKU for recording high resolution mass spectra.

My special thanks are extended to my parents and Mag.a_{Marlene Eder for their support and} encouragement throughout my whole study.

Finally, I would like to express the deepest appreciation to my supervisor DI Dr. Christoph Topf for his professional guidance and valuable assistance during my research work. Without his constructive recommendations and expertise this Master’s Thesis would not have been possible.

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ABSTRACT

I herein report on the development of an in situ formed manganese(I) hydrogenation catalyst suitable for the reduction of various ketones, aldehydes, and activated C-C double bonds. Special emphasis was placed on the optimization of the reaction conditions as well as the elaboration of the scope and limitations of the catalytic system.

Furthermore, the research work included the syntheses and characterization of group VI metal complexes applicable for homogeneous hydrogenation reactions. For this purpose, a pertinent anionic bromocarbonyl precursor complex was prepared on which a variety of ligand substitution reactions involving multidentate ligands incorporating the N-H motif was explored. In addition, this two-step synthesis was compared to known methods from the literature in order to check its efficiency. Consequently, a series of different neutral coordination compounds was prepared and subsequently tested for the ability to catalyze the homogeneous hydrogenation of selected carbonyl substrates. In this context, the influence of different additives and solvents on the catalyst performance was examined. Regrettably, most of the investigated group VI metal complexes were either inactive or showed only low catalytic activity. The best catalyst performance was observed with the PN molybdenum and chromium complexes in the homogeneous hydrogenation of acetophenone to yield the corresponding secondary alcohol.

Moreover, research efforts also focused on ligand substitution reactions on the tungsten(II) complex [CpW(CO)3Cl] conducted with certain bi- and tridentate ligands. The straightforward synthesis of these compounds allowed to generate an unprecedented class of cationic tungsten(II) complexes. These complexes were tested as catalysts for the homogeneous hydrogenation of carbonyl moieties in the presence of different additives. Whereas the tungsten(II) complex incorporating the PNP ligand displayed some activity for the hydrogenation of acetophenone, the overall reactivity of these coordination compounds was found to be very low.

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5.9. Ligand substitution reactions on [CpW(CO)3Cl] ... 68 5.10. Ligand substitution reactions with SN ... 70 5.11. Ligand substitution reactions with SNS ... 71 5.12. Hydrogenation reactions conducted with complexes incorporating group VI metals ... 72 6. Used Abbreviations ... 73 7. References ... 74

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1. Introduction

1.1. Catalytic hydrogenation reactions

The addition of two hydrogen atoms across multiple bond(s) incorporated in a substrate, such as C-C, C-N, or C-O motifs, is called hydrogenation. This type of reaction is indispensable for the large-scale processes of the chemical industry but is also an important catalytic method in academia. The title transformation plays a key role in the synthesis of fine chemicals, pharmaceuticals, agrochemicals and many more.[1]

The most striking feature of catalytic hydrogenation is the intrinsic excellent atom economy. The actual value of this pivotal parameter is close to 100% provided that hydrogen gas is used as the reducing agent and that the catalyst works selectively. Furthermore, the solvent as well as excess hydrogen must be recycled to achieve the given value. The given catalytic process does not generate high amounts of waste and is therefore essential for sustainable synthesis. Stoichiometric reducing agents such as lithium aluminium hydride, which are commonly applied in traditional synthesis for reductive transformations produce large amounts of undesired waste and are mostly pyrophoric. Hence, modern contemporary catalysis aims at the substitution of these unpleasant reductants by gaseous hydrogen in combination with a suitable catalyst.[1,2] In general, catalytic hydrogenation reactions are facilitated by either homogeneous or heterogeneous catalysts. The latter is mainly applied in industrial processes, since the catalyst is easily separated from the reaction solution on completion of the reaction. For homogeneous hydrogenations, laborious catalyst recovery and product isolation steps are inevitable. These complex separation methods entail enormous costs as well as complicated engineering setups. With respect to heterogeneous systems, precious metals such as platinum, rhodium, ruthenium or palladium take the spotlight as they exhibit superior stability, selectivity and activity. These active metals are often immobilized on a support in order the enhance both stability and activity of the catalyst. In addition, a promoter is often added for fine tuning of the catalytic properties of the solid material. The nature of the support is of great importance because it dictates both metal distribution and dispersion that significantly influence the outcome of the reaction. Moreover, the diffusion of the substrate to the catalytically active centers is effectively controlled by the carrier material. The promoter itself is inactive but improves the catalytic performance regarding selectivity, activity and/or stability because it modifies the electronic as well as the geometric structure of the catalyst. [1-3]

Owing to the limited long-term availability of precious metals and their high price, modern catalysis strives for replacing noble-metal-based catalysts by base metals. The most extensively used non-noble metals for hydrogenation reactions are cobalt, copper, and nickel. These metals found widespread applications in many industrial processes, but the associated reactions proceed under relatively harsh reaction conditions and higher catalyst loadings when compared to their precious congeners. Apart from that, the employed base metals often outperform their noble counterparts

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with respect to chemoselectivity. The hardening of fatty acids and edible oils may serve as an instructive example for a highly efficient hydrogenation process catalyzed by an heterogenous base-metal catalyst. This reaction, which is nowadays the largest single application of hydrogenation, was already discovered in the end of the 19th _{century by} _{the French chemist} Sabatier and has rapidly evolved to a global industry. The employed Ni-catalyst combines high selectivity, low costs and is crucial for the control of the isomerization reaction. In contrast, noble metals strongly enable both isomerization and migration of the double bond, which lead to many side products.[1]

However, depending on the costs, metal recyclability, activity, and selectivity one can choose between precious and non-noble metals, but the selectivity of a catalyst is the most important criterion. High selectivity is crucial for an efficient use of the feedstock since this will ensure high yield of the products while at the same time waste production is minimized. Moreover, the purification costs in the subsequent work-up will be reduced. [1,2]

As mentioned above, heterogeneous hydrogenation protocols dominate the field owing to their outstanding catalytic performance and simple separation of the catalysts compared to their homogeneous counterparts. However, the very first homogeneous hydrogenation catalysts were developed in the 1960s by Vaska and Wilkinson (Figure 1). These complexes are so-called precatalysts and may be activated by oxidative addition of hydrogen to form the hydride species that brings about the desired catalytic transformation.[4-6]

Figure 1: First hydrogenation catalysts reported by Wilkinson (1965) and Vaska (1962).[5,6]

If special requirements on enantioselectivity have to be met, asymmetric hydrogenation protocols that rely on homogeneous catalysts are indispensable. The development of the corresponding soluble transition metal complexes is rapidly evolving since the last decades. The first asymmetric hydrogenation process performed on large-scale was the synthesis of L-Dopa. This therapeutic agent was developed by Knowles in the 1970s and is mainly applied for the clinical treatment of Parkinson’s disease. The asymmetric transformation is performed by a cationic rhodium complex in the presence of gaseous hydrogen as the reductant.[2,7]

Another example of a homogeneous hydrogenation process which is already implemented on industrial scale is the reduction of esters catalyzed by the popular Ru-MACHO complex. The (pre)catalyst developed by Takasago Int. Corp. is shown in Figure 2. This ruthenium(II) complex features a PNP-ligand, exhibits high selectivity and is mainly applied in the synthesis of the

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antibacterial drug levofloxacin. It should also be mentioned that this catalyst can stop the catalytic transformation at the stage of the intermediate hemiacetal.[7-9]

Figure 2: Ru-Macho catalyst employed for the hydrogenation of esters (Kuriyama et al.).[8,9]

Despite that, hydrogenations are also applied in the chemical industry for the selective reduction of ketones using ruthenium(II) based catalysts. In analogy to these catalysts, several research groups have recently developed new complexes incorporating base-metals such as manganese in conjunction with multidentate ligands bearing the N-H motif. To date, the noble catalysts still dominate the field because they overwhelm their first-row analogues in activity, stability and shelf life.[7]

1.2. Activation of H

2

in homogeneous hydrogenation reactions

In order to add two hydrogen atoms onto the unsaturated substrate, the hydrogen molecule has to be activated by a catalyst. One way for H2-activation is oxidative addition, which is shown in Figure 3. In oxidative addition, the H-H bond is initially polarized by a transition metal (at least adopting a d2_{-configuration) leading to weakening of the respective bond. Subsequent} backbonding from the transition metal into the antibonding orbital of the H-H bond gives rise to a dihydride species that enables the hydrogenation process [10,11]

Figure 3: H2-acitvation by oxidative addition (top), activation step of Wilkinson’s (pre)catalysts (bottom).[10,11]

Another activation pathway for hydrogen gas is homolytic cleavage of the H-H bondby a suitable catalyst. As exemplified in Figure 4, the paramagnetic cobalt(II) complex [Co(CN)5]3- activates the hydrogen molecule homolytically leading to the 18-electron complex ion [HCo(CN)5]3- with cobalt

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in the oxidation state +III. Subsequently, a hydrogen atom is transferred onto the substrate molecule leading to an organic radical species. In the final step, the benzylic radical, which is resonance-stabilized, abstracts another hydrogen atom from the second cobalt hydride. As a result, the unsaturated substrate and the active 17-electron species [Co(CN)5]3- are formed.[10,11]

Figure 4: Homolytic activation of H2 demonstrated by Iguchi’s complex [Co(CN)5]3-.[11]

The hydrogen molecule can also be activated by heterolytic cleavage of the H-H bond, which is illustrated in Figure 5. It can occur in either an inter- or an intramolecular fashion but in both activation modes an appropriate proton acceptor (B) is necessary. This type of activation can be performed by several transition metal complexes in which a hydride is delivered to the metal center, whereas the proton is transferred to the proton acceptor (B). In contrast to oxidative addition, this reaction type produces a monohydride species.[10,12]

Figure 5: Inter- and intramolecular heterolytic activation of H2.[10,12]

The coordination compound [RuHCl(PPh3)3] is well known for his ability to cleave H2 through σ-bond metathesis. In general, there exist two possible mechanism for this activation type. In the concerted mechanism (Figure 6), bond breaking as well as bond formation occur simultaneously in one step, whereas in the σ-complex assisted metathesis (σ-CAM) the reaction proceeds via the intermediate formation of discrete σ-complexes. The reason for this activation pathway is the fact that the pertinent ruthenium(II) complex cannot form a sufficiently stable ruthenium (IV) species, thus rendering H2-acitvation by oxidative addition an energetically disfavored process. Furthermore, complexes with a d0_{-konfiguration such as [{LuH(η}5_-C

5Me5)2}2] or [(η5-C5H5)MR2] (M = Ti, Zr, Hf) are also suitable for H2-acitavition by σ-bond metathesis because their metal is already in the highest oxidation state.[11,13,14]

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Figure 6: Concerted mechanism of σ-bond metathesis.[10]

A very important mode by which polar organic substrates are hydrogenated is the outer sphere mechanism (Figure 7).During the course of the respective catalytic transformation the substrate is solely interacting with the ligand of the transition metal complex and a hydride is transferred from the metal to the carbon atom of the C=O double bond. Simultaneously, an adjacent amine fragment of the ligand transfers a proton to the oxygen atom of the carbonyl derivative. The resulting 16-electron species then cleaves a hydrogen molecule in a heterolytic manner and reforms the catalytically active complex. Compared to other redox-transformations, the oxidations state of the metal atom remains unchanged during the conversion of the substrate.[11]

Figure 7: Asymmetric Hydrogenation by Noyori’s catalyst.[11]

This catalytic mechanism is of particular interest in a variety of asymmetric hydrogenation reactions. Noyori’s hydrogenation catalyst shows outstanding catalytic performance in the enantioselective formation of alcohols from functionalized ketones with H2 as the principal reductant. These catalysts typically incorporate a ruthenium(II) metal center held by chiral diphosphines and diamines (Figure 8) and operate at ambient temperature with high turnover numbers as well as excellent enantioselectivity and chemoselectivity.[7]

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1.3. Transfer hydrogenation

Another method that has to be mentioned in the context of adding two hydrogen atoms across an unsaturated bond is transfer hydrogenation. In this catalytic transformation gaseous hydrogen is replaced by a liquid (usually isopropanol or formic acid), which acts both as solvent and reducing agent. Transfer hydrogenation is especially beneficial for industrial applications owing to the ease of handling isopropanol as well as the avoidance of explosive gaseous hydrogen. A typical transfer hydrogenation employing benzophenone as substrate is shown in Figure 9.[11]

Figure 9: Example of a transfer hydrogenation with isopropanol as H2-transfer reagent.[11]

A typical catalyst for transfer hydrogenation is the ruthenium(II) complex [RuCl2(PPh3)3] established by Bäckvall et al. The group reported that this catalyst is suitable for the reduction of aromatic and aliphatic ketones in the presence of sodium hydroxide as promoter. The reactions were performed in isopropanol at 82 °C but they did not observe any catalytic activity in the absence of base. Hence, it has to be inferred that an isopropoxide anion is generated after addition of the base, which coordinates to the metal center. Subsequently, acetone and the hydride species are formed upon β-hydride elimination.

Apart from that, transfer hydrogenations are also applicable for the reduction of imines and are often employed in asymmetric catalysis. [11,16]

1.4. Homogeneous hydrogenation with manganese complexes

The first manganese(I) complex suitable for homogeneous hydrogenation reactions was reported in 2016 by the Beller group.[17,18]_{More specifically, they described the hydrogenation of aldehydes,} ketones, nitriles and even esters in the presence of different manganese(I) PNP pincer complexes. From that time on, the research field of manganese(I)-based catalysts evolved expeditiously resulting in the improvement of the catalysts as well as in the enlargement of the substrate scope. Especially for carbonyl compounds, the manganese complexes showed remarkable catalytic performance. A collection of the developed manganese catalysts suitable for the hydrogenation of carbonyl derivates is outlined in Figure 10. All this manganese precatalysts have in common that they incorporate a pincer-type ligand with a donor nitrogen atom in the center. The activation of the presented precatalysts eventually leads to the active monohydride species. The figure also illustrates the structural diversity of pincer-type ligands employed in this catalytic systems. [19-22]

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Figure 10: Various manganese-based precatalysts featuring pincer-type ligands, the active catalyst, and the ligand

platforms for these catalytic assemblies.[19]

So far, (pre)catalysts incorporating pincer-type ligands were discussed but there are also manganese(I) complexes featuring bidentate ligands, which exhibit considerable catalytic activity. In 2017, Sortais and coworkers reported a catalyst that effectively mediates the transfer hydrogenation of ketones to alcohols.[23]_{They used isopropanol as hydrogen donor and a} manganese(I) complex bearing the inexpensive and phosphine-free ligand 2-picolylamine. Strikingly, the pertinent catalyst operates at ambient temperature using a relatively low loading of 0.1 mol%. Moreover, the same group also developed a carbonyl manganese(I) catalyst featuring a bidentate pyridinyl-phosphine ligand, which was employed in the hydrogenation of carbonyl derivates.[24]

In the same year, Khusnutdinova et al. reported on an efficient catalytic protocol for the hydrogenation of carbon dioxide to formats by employing a manganese(I) (pre)catalyst incorporating 6,6′-dihydroxy-2,2′-bipyridine as ligand in the presence of diazabicycloundecene.[25] Furthermore, the report disclosed that carbon dioxide is also converted to formamide using a secondary amine as an additive.

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Another report from Pidko et al. in 2017, demonstrated the hydrogenation of esters to alcohols effected by a manganese-PN complex. The catalytic transformation of carboxylic acid esters is performed at 100 °C with low catalyst loading (0.2 mol%) whereas KOtBu was employed as an additive in sub-stoichiometric amounts (75 mol%).[26]

Later in 2018, Kirchner et al. communicated an air-stable bisphosphine manganese(I) complex, which is suitable for the hydrogenation of nitriles and ketones. For the reduction of nitriles, a rather high amount of base (20 mol%) is required. Moreover, this reaction proceeds at higher temperatures (100 °C) compared to carbonyl compounds (50 °C).[27]

All these examples addressed the hydrogenation of several polar functional groups and the catalysts discussed so far did not display any activity towards the conversion of C-C double and triple bonds.

In fall 2019, Kirchner et al. reported the very first additive-free manganese(I) based catalytic protocol suitable for the hydrogenation of alkenes to the corresponding alkane under mild reaction conditions.[28]_{. Hydrogenation reactions catalyzed by manganese(I) based catalysts usually} proceed by an outer sphere mechanism. In this report, the precatalyst sustains migratory insertion of the alkyl motif leading to the formation of an acyl complex, which subsequently undergoes hydrogenolysis. Consequently, an aldehyde as well as the active manganese(I) monohydride species are formed.

The reduction of an aldimine to the corresponding secondary amine by transfer hydrogenation was reported by the Sortais group in 2019.[29]_{For this hydrogenation reaction they used their} previously reported bromotricarbonyl(2-picolylamine)manganese(I) catalyst and employed small amounts of KOtBu as the additive.

All of the presented pre(catalysts) featuring bidentate ligands are summarized in Figure 11.

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Figure 12 provides an overview of organic substrates that are amenable to homogeneous hydrogenation catalyzed by manganese-based complexes.

Figure 12: Suitable organic substrates for homogeneous hydrogenation reactions catalyzed by manganese-based

complexes. [17-28]

As mentioned above, ruthenium complexes are already well-established for asymmetric hydrogenations that are conducted in the chemical industry but owing to the low abundance and high price of ruthenium its replacement by more inexpensive metals, e.g. manganese, is desirable and highly sought-after. The price per mole of these two metals as well as their common precursor are juxtaposed in Table 1.[22]

Table 1: Comparison of prices for ruthenium and manganese.[22]

Metal Metal price / € mol-1 _{Metal precursor} _{Precursor price / € mol}-1

ruthenium 138.312 [(PPh3)3RuH(CO)Cl] 79049.2

manganese 0.097 [Mn(CO)5Br] 10995.6

Looking at the table, one will recognize that the price of ruthenium is significantly higher than that of manganese. Thus, one might be tempted to assume that manganese is an attractive substitute to the noble metal catalysts, but the main drawback of these base-metal catalysts is their lack in catalytic performance, which is fairly lower compared to their precious metal congeners. In addition, the synthesis of manganese(I) catalysts commences with the quite expensive precursor [Mn(CO)5Br]. Hence, ongoing research is necessary to provide competitive alternatives to ruthenium-based catalysts.[22]

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1.5. Homogeneous hydrogenation with group VI metal complexes

Hitherto, reports on homogeneous hydrogenation reactions catalysed by group VI metal complexes are still scarce. To name one example, Berke et al. reported highly active, molybdenum and tungsten amide catalysts for the hydrogenation of nitriles, secondary imines and carbon dioxide.[30-32]_{The activation of the precatalysts is outlined in Figure 13. In their study of the} activation step, they observed heterolytic cleavage of hydrogen to afford a monohydride species and an amine. Furthermore, the hydrogen molecule can either approach from the CO or from the side of the NO ligand. Consequently, an isomeric mixture composed of the cis- and trans-product is formed. In the case of tungsten, the formation of the cis- and trans-product is irreversible and hence the addition of molecular hydrogen gives rise to two different hydride species that are not interconvertible.

Figure 13: Hydrogen activation by Berke’s molybdenum and tungsten complexes.[30]

Another molybdenum PNP pincer complex from Bernskoetter et al. was also employed for the reduction of carbon dioxide to obtain formate.[33]_{In order to improve the catalytic performance of} this catalyst the reaction was performed under basic conditions in the presence of the Lewis-acidic salt lithium triflate.

In a recent report from the Beller group, a molybdenum PNP-pincer complex was deployed for the hydrogenation of ketones and styrene derivates (Figure 14).[34]_{They performed these} hydrogenation reactions in the presence of NaHBEt3 (0.5 M in THF) and used equimolar amounts of this additive relative to the catalyst. Furthermore, harsher conditions are necessary for the reduction of styrenes compared to the acetophenone derivates. The report also reveals that this catalyst is effective in the reduction of C-C and C-N triple bonds, though insufficient selectivity was observed.

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Figure 14: Hydrogenation of various acetophenone and styrene derivatives catalyzed by a molybdenum PNP pincer

complex in the presence of NaHBEt3 as additive.[34]

All these catalysts incorporate molybdenum or tungsten as well as pincer-type ligands. To the best of our knowledge, chromium complexes featuring multidentate ligands and group VI metal complexes with non-pincer-type ligands for homogeneous hydrogenation reactions have not been reported yet.

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2. Aims and Objectives

The first task of this master’s thesis was the investigation of hydrogenation reactions of carbonyl derivates and C=C double bonds catalyzed by an in situ formed phosphine-free manganese(I) complex.

The detailed objectives of the first part of this thesis included:

• Investigation of the catalytic activity in hydrogenation reactions of ketones, aldehydes as well as C=C double bonds.

• Chemoselectivity studies and determination of the scope and limitations of the catalytic system.

• Optimization of the reaction conditions for suitable substrates.

The second aim of this work was to prepare a variety of novel carbonyl complexes incorporating group VI metals and multidentate ligands that are structural analogs to the manganese(I) hydrogenation catalysts [Mn(CO)3(NN)Br] and [Mn(CO)2(PNP)Br], developed by Sortais et al. and the Beller group, respectively.[17,23]_{These complexes were tested for their ability to catalyze the} homogeneous hydrogenation of various hydrogenation reactions.

The detailed goals of the second part included:

• Synthesis of tetraethylammonium bromidopentacarbonylmetallate NEt4[M(CO)5Br] (M = Cr, Mo, W) as precursor by substitution reaction of [M(CO6)] with tetraethylammonium bromide.

• Investigation of ligand substitution reactions of the bromidopentacarbonylmetallates NEt4[M(CO)5Br] and cyclopentadienylchlorido tricarbonyltungsten(II) [CpW(CO)3Cl] conducted with certain bi- and tridentate ligands bearing the N-H motif.

• Testing of the unprecedented complexes for their ability to catalyze the homogeneous hydrogenation of several substrates.

• Investigation of the influence of the solvent as well as the addition of selected base- or hydride additives on the catalyst performance of the active transition metal complexes.

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3. Results and Discussion

3.1. Homogeneous hydrogenation employing an in situ formed

manganese(I) catalyst

The catalytic performance of the in situ formed manganese(I) precatalyst [Mn(CO)3(NN)(Br)] (NN = 2-picolylamine) as introduced by Sortais et al. for the homogeneous hydrogenation of several polar organic substrates was investigated. This catalyst is already well-established for the transfer hydrogenation of carbonyl derivates but is also readily applied for the hydrogenation of aldimines, which was demonstrated in a recent report.[23,29]_{The standard procedure for this study} is as follows: a 4 mL glass vial was charged with [Mn(CO)5Br] as well as a magnetic stirring bar whereas the solid precursor was hereafter dissolved in 3 mL of THF. Then, 0.5 mmol of the substrate were added to the solution. If solid substrates were tested, the vial was charged with 0.5 mmol of the solid right after the addition of the Mn compound. After adding the ligand 2-picolyamine via a 10 µL Hamilton syringe, KOtBu was poured into the mixture, which caused a color change from yellow to orange. Subsequently, the vials were closed with a septum cap, which was then penetrated with a steel cannula. The reaction vials were placed in a drilled Al-plate (henceforth named inlet) and the 300 mL autoclave was tightly sealed. Depending on the employed substrate, the applied H2-pressure (40-50 bar) as well as the reaction time were adjusted in order to optimize the reaction. In addition, all reactions were performed at 120 °C. On completion of the reaction, the autoclave was cooled to room temperature and depressurized. After removing residual hydrogen upon vigorous stirring of the reaction solution, the composition of the reaction mixture was determined by GC-MS analysis. The general reaction scheme of the catalytic hydrogenation is summarized in Figure 15. The entire substrate scope with conversions and reaction conditions is found in Figure 16, 17 and 18.

Figure 15: General reaction scheme for the hydrogenation of ketones, selected aldehydes and C=C double bonds

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First experiments comparing the well-defined and in situ formed complex disclosed that the catalyst performs best when generated in situ. In contrast, the well-defined complex showed low to zero activity. During the first in situ tests, the addition of the ligand via 10 µL Hamilton syringe proved to be the crucial step in this experiment. In order to ensure the complete addition of the ligand added to the solution it has to be added quickly in one shot. Otherwise, a drop is formed on the cannula of the syringe and the added amount of ligand in the reaction solution is rather erratic. The hydrogenation experiments were carried out in both dry and wet THF whereas it turned out that the water content of the solvent did not influence the outcome of the reaction except for cinnamic acid ester derivates. Therefore, hydrogenation reactions conducted with these esters were carried out in dry THF, pre-dried KOtBu and dry glass vials. Consequently, a significant increase of the conversion (about 10-20%) was observed. For all other reactions, operating under strictly dry conditions was not necessary. Substrates employed in this study included ketones, aldehydes, alkenes and cinnamic acid esters, which incorporate both the ester moiety as well as the C=C double bond.

As can be seen from Figure 16, this catalytic system is suitable for several ketones. Although aldehydes are generally not applicable, complete conversion was found for the aldehyde group of substrate 5. To our surprise, the catalyst favored the aldehyde (96%) over the ketone group. The analysis of the reaction composition showed that the initially formed 4-acetylbenzyl alcohol had already been converted to the corresponding diol to a slight extent (4%).

Since we assume the formation of an intermediate enolate from the respective starting material during the course of the catalytic transformation, different carbonyl substrates equipped with electron withdrawing groups were tested in order to increase the C-H acidity. Consequently, the transformation of substrate 7, 8, and 9 was already completed after 9 h, which is in accordance with the enolate theory. Moreover, the position of the electron withdrawing group in the benzene ring did not influence the outcome of the catalytic process. Surprisingly, compound 10 showed poor conversion, which is inconsistent with the enolate theory since the C-H acidity is comparable to substrates 7-9. Hence, a decrease of the reaction time should have been actually observed. For substrate 11, an increase of the catalyst loading was necessary in order to obtain a decent conversion of the CF3-tagged starting material. Substitution of fluorine by a trifluoromethyl group (12 and 13) had a negative effect on the catalytic performance of this system, thus leading to a reduction in conversion or an increase of reaction time. As expected, the introduction of a second trifluoromethyl group into the benzene ring of acetophenone (15) or propiophenone (14) led to a significant increase of the conversion compared to the monosubstituted ones. It is interesting to note that the disubstituted propiophenone and acetophenone show nearly the same conversion whereas in the case of the monosubstituted substrates the conversion drastically decreases from acetophenone to propiophenone.

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Figure 16: Substrate scope for the hydrogenation of carbonyl derivates catalyzed by the in situ formed manganese(I)

catalyst 1. In addition, the reaction conditions as well as the conversion are given below the substrates.

The given catalytic hydrogenation protocol is not applicable to substrates containing nitro- or hydroxy-groups. In the respective experiments no catalytic conversion of the starting material was observed. One plausible rationale for the nonreaction of hydroxy-substituted acetophenone derivatives is that the added base, which acts as an activator for the precatalyst, is consumed by

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the hydroxide group thus leading to the formation of an alkoxide; hence, no base is left for the activation step.

The corresponding results obtained from the catalytic hydrogenation of several conjugated C=C double bonds are of particular interest because the pertinent catalytic reaction is unprecedented with phosphine-free manganese(I) complexes employed as homogeneous catalysts. A recent report from Kirchner et al. describes the reduction of alkenes with molecular hydrogen, but in this case an alkyl bisphosphine manganese(I) complex was applied in the experiments.[28]

Figure 17: Substrate scope for the 1,4-hydrogenation of activated C=C double bonds catalyzed by the in situ formed

manganese(I) catalyst. In addition, the reaction conditions as well as the conversion are given under the structural drawings of the substrates.

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The methyl ester of cinnamic acid gave rise to almost full conversion after a reaction time of 18 h. Substitution of the methyl group by an isopropyl caused a steep decrease of the conversion. In contrast to this, the bulky benzyl ester 23 was fully converted to the unsaturated product after 18 h. Generally, the presence of any substituent in the appendant benzene ring resulted in a drastic decrease of the catalytic activity. On comparing substrate 24 with compound 25, it has to be noted that the difference in reactivity of the methyl and ethyl ester is not very pronounced if one neglects the different positions of the bromine atoms.

For substrate 28, the amount of 2-picolyamine was increased in order to obtain satisfactory results. Surprisingly, the introduction of an additional phenyl group at the α-C atom (29) led to a dramatic increase of the conversion. Hydrogenation of maleic anhydride (31) was performed at 100 °C because this compound tends to deoxygenate at higher temperatures. The substrate was completely converted after a period of 12 h. Again, the catalyst is fully inactive, when nitro compounds are used as substrates (vide supra).

In order to investigate whether this catalyst shows any chemoselectivity towards either the carbonyl group or the C=C double bond, several substrates featuring both functional groups were tested. When trans-chalcone (32) was hydrogenated, decent conversion of the C=C double bond was observed. However, performing the hydrogenation reaction for 72 h and 35 bar resulted in the full reduction of both functional groups.

Figure 18: Tested substrates incorporating both a carbonyl group and a C=C double bond in order to investigate the

chemoselectivity of the given catalytic protocol. The reaction conditions and the conversion are given under the structural drawings of the substrates.

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The catalytic hydrogenation of compounds 35 and 36 solely produced the unsaturated secondary alcohols with the C=C bonds still being intact. Upon comparing the outcome of the hydrogenation of substrate 32 with that of the cyclohexenone derivatives 35 and 36, only the latter two enable viable chemoselectivity. This circumstance may be readily explained by the fact that disubstituted C=C double bonds are more reactive than the sterically more congested trisubstituted analogs. As outlined in Figure 19, the manganese(I) catalyst favors the reduction of the conjugated C=C double bond over the polar carbonyl motif in cinnamic aldehyde 33, thus compound 37 was identified as the major component in the reaction mixture. However, the catalyst still displays some activity towards the aldehyde group. Consequently, two minor products are formed during the reaction, i.e. alkyl alcohol 38 and corresponding cinnamyl alcohol 39.

Figure 19: Composition of the reaction mixture for the hydrogenation of cinnamic aldehyde (33).

In Figure 20, the hydrogenation products of citral (34) are illustrated and again the catalyst did not display any chemoselectivity. Hence, a mixture of three different compounds was obtained. Notably, the alkene group at position 6 is not reduced at all owing to its rather non-polar character compared to the conjugated C=C bond.

Figure 20: Composition of the reaction solution for the hydrogenation of citral (34).

In several experiments described above, a brownish solution was observed on completion of the reaction, which resulted owing to decomposition of the catalytically active manganese complex. Hence, the catalyst solution could not be reused for a second reaction batch.

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3.2. Synthesis of the precursor NEt

4

[M(CO)

5

Br]

The next main goal was the syntheses of group VI metal carbonyl complexes that represent structural analogs to the manganese(I) hydrogenation catalysts [Mn(CO)3(NN)Br] and [Mn(CO)2(PNP)Br].[17,23] For this purpose, the precursors NEt4[M(CO)5Br] (M = Cr, Mo, W) were prepared from the metal hexacarbonyl and tetraethylammonium bromide through ligand substitution.[35]_{The general reaction scheme is outlined in Figure 21. For practical reasons, all} reactions were performed in dry glyme instead of diglyme, which was employed in the literature method. The use of diglyme unnecessarily complicates the work-up procedure of the product owing to its rather high viscosity and boiling point (162 °C).[36]_{In addition, the complex has to be} dried several days in order to remove residual solvent, which is highly undesired since this compound is used as a precursor for further reactions. However, the anionic complex was isolated upon precipitation with diethyl ether, n-pentane or n-hexane, and subsequent filtration through filter paper.

Figure 21: General reaction scheme for the synthesis of the precursors NEt4[M(CO)5Br].

Owing to the fact that the formed precursor slowly decomposes if the reaction time is too long, the temperature as well as the reaction time had to be carefully adjusted for each metal. In addition, the different reactivities of the metal hexacarbonyls also had to be taken into account for the adjustment of the proper reaction conditions. The optimized reaction parameters along with yields and colors of the respective products are summarized in Table 2. The reactions were performed with both pre-dried and wet tetraethylammonium bromide, but no difference in product quality and yield was observed. The complexes are highly hygroscopic, which is indicated by a color change from bright yellow/orange to brown. In addition, the complexes slowly decompose if exposed to air for several weeks. Hence, the coordination compounds have to be stored under exclusion of moisture in an argon atmosphere.

For the preparation of NEt4[Mo(CO)5Br], several attempts were necessary due to the fact that the compound decomposes when the temperature exceeds the boiling point of glyme during the synthesis. Consequently, a green amorphous powder was obtained. To cope with this problem, reactions were carried out slightly beneath 85 °C and the reaction time was increased.

The prepared complexes were characterized by IR spectroscopy and the obtained values are well in accordance with those reported in the literature.[35]

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Table 2: Optimized reaction times and temperatures for the syntheses as well as color and yield of the precursors

[M(CO)5Br][NEt4] (M=Cr, Mo, W).

Complex reaction time / min T / °C Color Yield / %

43a 90 120 orange 84

43b 150 80 yellow 77

43c 80 130 yellow 82

3.3. Synthesis of group VI metal complexes bearing multidentate ligands

As already mentioned in section 3.2, it was attempted to synthesise a variety of group VI metal carbonyl complexes featuring multidentate ligands bearing the N-H motif starting from the anionic precursor complex NEt4[M(CO)5Br]. The substitution of the CO ligands by the multidentate ligand is supposed to permit access to a novel class of group VI metal complexes, which are isoelectronic to the manganese(I) hydrogenation catalysts [Mn(CO)3(NN)Br] and [Mn(CO)2(PNP)Br]. The employed bi- and tridentate ligands are collected in Figure 22.

Figure 22: Structural drawings of the employed multidentate ligands for the syntheses of (new) carbonyl group VI metal

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In general, it was found that the bromido ligand was substituted first. The reaction commences by precipitation of tetraethylammonium bromide (44) from the reaction solution followed by vigorous CO evolution. The reaction was then performed under reduced pressure with the intention to foster the extrusion of the CO ligand from the metal carbonyl and thus substitution of the bromido ligand is thought to be suppressed. However, it was found that in both cases the neutral carbonyl complexes were formed and the anionic coordination compounds were never observed. Rewardingly, most of the generated complexes are not reported yet. A general reaction scheme of the syntheses is depicted in Figure 23.

Figure 23: General reaction scheme for the synthesis of the precursor NEt4[M(CO)5Br] and subsequent ligand substitution reaction with multidentate ligands bearing the N-H motif leading to neutral coordination compounds upon displacement of the bromido ligand. Formation of an anionic coordination unit was not observed.

Noteworthy, all donor atoms of the ligands coordinated to the central atom in the precursor complexes (45a-c) regardless of the respective denticity. Reactions performed with ligands featuring only nitrogen donor atoms (46,49) exhibit high reactivity, thus rendering the one-step synthesis from the metal hexacarbonyl the more appealing alternative route. Moreover, ligands containing the thiophene motif (48, 51) are not suitable for this kind of reaction. In each experiment neither precipitation of tetraethylammonium bromide (44) nor CO evolution were observed. Ligand substitution reactions including ligands with phosphorous donor atoms (47,49) already occurred under relatively mild reaction conditions; however, the syntheses of these complexes were performed at higher temperatures in order to accelerate the rate of the pertinent reactions. The main drawback of this synthetic protocol is that the precursor is not completely transformed into the neutral coordination compound bearing the multidentate ligand. Hence, a mixture composed of the precursor, free ligand and product was obtained. More experiments are necessary in order

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to optimize the reaction and ensure complete conversion. In the next sections the syntheses of the complexes are discussed in detail.

3.3.1. [M(CO)4(NN)]

Both methods starting either from the metal hexacarbonyl compounds (43a-c) or from the prepared precursors NEt4[M(CO)5Br] (45a-c) were tested. The experiments revealed that the picolyl-based ligand 46 exhibits high reactivity. Hence, these complexes were prepared by a one-step synthesis employing the hexacarbonyl as starting material (Figure 24). With the exception of compound 53b, which is only characterized by UV/vis-spectroscopy, these complexes are not reported in the literature yet.[37]

Figure 24: Reaction of the group VI metal hexacarbonyl with ligand 46 in toluene at 130 °C. Reaction time is depending

on the employed hexacarbonyl: Cr: 2 h, Mo: 1 h, W: 6 h.

Reactions were performed through heating a solution of the metal hexacarbonyl and ligand 46 in toluene at reflux for several hours. Depending on the employed metal hexacarbonyl different reaction times were necessary. The reaction including molybdenum was already completed after 1 h and a dark-red solution was obtained. In the case of tungsten, only a slight coloring of the solution was observed after the same reaction time. This leads to the conclusion that the reactivity of the hexacarbonyls is quite different, in which molybdenum exhibits the highest reactivity followed by chromium and tungsten.

Since these novel complexes are highly air-sensitive in solution, the reactions as well as the product isolation steps that include precipitation and subsequent filtration were performed under an argon atmosphere. Notably, the complexes are air-stable for several weeks and exhibit solvatochromic properties. The complexes were obtained in moderate yields of 54 to 67%. The complexes reported herein were all characterized by IR spectroscopy, NMR spectroscopy, high resolution mass spectrometry and X-ray analysis. For X-ray analysis, crystals of 53a, 53b and 53c were obtained by the vapor diffusion technique from n-pentane/toluene at 4 °C. The solved structures are shown in Figure 25, 26, and 27 as ORTEP representations. In Table 3, selected bond lengths, angles and cell parameters are illustrated. One can clearly see that the bond length is increasing from Cr1-N1 to Mo1-N1, which is also true for Cr1-N2 and Mo1-N2. In

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contrast, the bond lengths Mo1-N1 and W1-N1 as well as Mo1-N2 and W1-N2 are in the same range. Since the chromium complex 53a has the shortest bond lengths, the angle N1-M1-N2 is decreasing from chromium to molybdenum and tungsten. Interestingly, the angle between N1-M1-Cn (Cr: n=2, Mo: n=1, W: n=2) remains constant in each complex. Complex 53a also differs in the crystal system and space group compared to 53b and 53c. However, all these complexes exhibit a distorted-octahedral arrangement (Figure 25-27)

Table 3: Selected bond lengths angles and crystal parameters of coordination compounds 53a-c.

Complex M1-N1 bond length / Å M1-N2 bond length / Å N1-M1-N2 angle / ° N1-M1-Cn angle / ° Crystal system Space group Z 53a 2.123 2.143 76.23 172.32 (n=2) monoclinic P21/c 4 53b 2.246 2.283 73.30 172.70 (n=1) orthorhombic P212121 4 53c 2.251 2.281 72.90 172.30 (n=2) orthorhombic P212121 4

Figure 25: ORTEP plot of [Cr(CO)4(NN)] in the crystal P21/c. Thermal ellipsoids are drawn at a probability level of 50% and 23 °C. Hydrogen atoms of the picolyl group are omitted for clarity. The distortion from the ideal octahedral geometry is clearly visible.

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Figure 26: ORTEP representation of [Mo(CO)4(NN)] in the crystal P212121. Thermal ellipsoids are drawn at a probability level of 50% and 23 °C. Hydrogen atoms of the picolyl group are omitted for clarity. The distortion from the ideal octahedral geometry is clearly visible.

Figure 27: ORTEP picture of [W(CO)4(NN)] in the crystal P212121. Thermal ellipsoids are drawn at the 50% probability level and 23 °C. Hydrogen atoms of the picolyl group are omitted for clarity. The distortion from the ideal octahedral geometry is clearly visible.

The results of the IR measurements are summarized in Table 4. All coordination compounds show four CO bands that are all in the same range. Furthermore, two characteristic signals for the amine group were observed.

Table 4: Characteristic CO and NH2 bands of complex 53a-c.

Complex 𝝊̃ (CO) / cm-1 _𝝊_{̃ (NH}

2) / cm-1

53a 2006, 1880, 1846, 1790 3373, 3320

53b 2010, 1887, 1847, 1771 3368, 3314

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3.3.2. [M(CO)4(PN)]

All complexes were synthesized starting from the self-prepared anionic precursor complex and this novel procedure was then compared to the literature method described for similar coordination compounds.[38]

For the sake of convenience and brevity, the synthesized anionic complex 54a was not isolated in the two-step synthesis (Figure 28). The orange precursor solution was filtered and ligand 47 was then added to the warm solution under an argon atmosphere. The precipitation of tetraethylammonium bromide occurred followed by CO evolution whereupon a color change from orange to yellow was observed. However, a slight orange color remained indicating that the precursor was not completely converted. Efforts to shift the equilibrium to the product side by increasing the reaction temperature or performing the reaction under reduced pressure were in vain. Moreover, when performing the reaction with the isolated precursor the same problem was observed. To separate the product from the precursor, recrystallization from an acetone/water mixture (1:1) was performed. Thus, purification step allowed for the isolation of a pristine yellow crystalline product albeit with rather low yield (37%). In the straightforward one-step synthesis as described in the literature, the chromium hexacarbonyl and the tetrafluoroborate salt of ligand 47 were heated at reflux overnight (Figure 28). In order to generate the free ligand in the reaction solution, triethylamine was added prior heating of the mixture. Since the formed product is insoluble in n-octane, the complex precipitated as a greenish solid from the solution. Subsequently, crude 54a was recrystallized from a mixture of acetone/water (1:1). To conclude, both methods are suitable for the preparation of 54a, but the products were obtained in rather low yields.

Figure 28: Synthetic routes for the chromium complex 54a.

The synthesis of 54b was performed in dry glyme on employing the isolated precursor and the tetrafluoroborate salt of ligand 47. Again, equimolar amounts of triethylamine were added to the reaction solution. After heating the mixture at reflux for a period of 7 h, the solids were filtered off and the complex was precipitated upon slow addition of n-pentane to the clear filtrate. The resulting brown-greenish solid was collected on filter paper and recrystallization from acetone/water furnished a yellow crystalline powder in poor yield.

In analogy to the literature, the synthesis of 54b was performed by simply refluxing a mixture of molybdenum hexacarbonyl, the tetrafluoroborate salt of 47, and triethylamine overnight. Since

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molybdenum hexacarbonyl exhibits the highest reactivity of the group VI metal hexacarbonyls, the reaction already proceeds at a lower reaction temperature. A greenish solid precipitated from the reaction solution, which was subsequently recrystallized from 35 mL of acetone/water (1:1). In contrast to the two-step synthesis, this simple method from the literature gave a crystalline product in fair yields (64%). The reaction scheme of both synthetic routes is illustrated in Figure 29.

Figure 29: Synthetic routes for the preparation of complex 54b.

Since the tungsten hexacarbonyl complex is rather unreactive, a high boiling solvent was employed in the one-step synthesis. The literature recommends the use of mesitylene as solvent but as this solvent was not available, the reaction was conducted in m-xylene, which still has a boiling point high enough to allow the reaction at 170 °C. However, this method did not yield any product (Figure 30). In each experiment, small amounts of a brown residue were obtained, which was analyzed by IR spectroscopy. Applying the novel synthetic procedure that relies on the use of precursor 44 as intermediate yielded the desired product (Figure 30). Again, incomplete conversion of the precursor was observed, which entailed a purification step. The complex was purified by recrystallization from a mixture of acetone/water (1:1) leading to a significant drop in yield.

Figure 30: Reaction scheme of 54c showing the failed synthesis using literature method and the two-step synthesis

with the anionic complex NEt4[M(CO)5Br] (44) as intermediate.

In summary, a novel two-step synthesis for the preparation of group VI metal complexes featuring PN ligands was established. Compared to the literature, this method avoids the use of a high boiling solvent and benefits from a drastically reduced reaction time. However, optimization of this synthesis is still necessary in order to ensure complete conversion. Otherwise an impure product is obtained, which needs to be recrystallized from a mixture of acetone/water (1:1) leading to a significant drop of the yield. The yield of each synthetic route is listed in Table 5.

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In the case of 54b, the one-step synthesis is the method of choice due to the higher reactivity of molybdenum hexacarbonyl compared to chromium and tungsten, thus making the preparation of the active anionic complex superfluous. In contrast, the complex 54c was only successfully synthesized upon employing the anionic precursor as intermediate in order to activate the tungsten species.

Table 5: Comparison of the synthetic yields for the preparation of 54a-c.

Complex Yield / %

one-step synthesis two-step synthesis

54a 28 37

54b 64 16

54c - 39

All complexes were characterized by IR spectroscopy, NMR spectroscopy, high-resolution mass spectrometry, and X-ray diffraction analysis. With the exception of compound 54b, which was mentioned in one report without characterization, none of these complexes have been reported yet.[39]

The results of the 31_{P NMR and IR measurements are summarized in Table 6. Noteworthy, the} phosphorus resonance is shifted to lower frequency from chromium to tungsten. Again, four CO signals and two characteristic bands for the amine group were observed.

Table 6: 31_{P NMR and IR spectral data of complex 54a-c.}

Complex δ(31_{P) / ppm} _𝝊_{̃ (CO) / cm}-1 _𝝊_{̃ (NH}

2) / cm-1

54a 63.3 2006, 1880, 1846, 1790 3373, 3320

54b 42.7 2010, 1887, 1847, 1771 3368, 3314

54c 35.9 2002, 1886, 1840, 1759 3358 3302

In the recorded 31_{P NMR of complex 54c (Figure 31), two satellite peaks were observed owing to} the fact that 31_{P couples to transition metals with a nuclear spin I > 0. The odd multiplicity arises} from the fact that the single 31_{P signal coincides with the center of the multiplet. Hence, the central} peak of the obtained multiplet is extended, which is demonstrated in the blue box in Figure 31.[40]

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Figure 31: 31_{P NMR in CD}₂_Cl₂_{of 54c showing the strong phosphorus singlet (35.8765 ppm) flanked by two satellite} peaks.[40]

Crystals suitable for X-ray analysis were easily obtained by the vapor diffusion technique from THF/n-pentane at 4 °C. A listing of selected bond lengths, angles and crystal parameters are shown in Table 7. ORTEP representations of the structures are depicted in Figure 32, 33, and 34. Bond lengths of M1-N1 and M1-P1 are increasing from chromium to molybdenum but compared to compound 53b and 53c the bond lengths are decreasing from molybdenum to tungsten. Hence, the angle P1-M1-N1 is slightly increasing on going from 54b to 54c. Again, 54a exhibits the greatest angle between P1-M1-N1 owing to the short bond lengths. However, all these complexes feature a distorted-octahedral arrangement (Figure 32-34) and have the same crystal system, space group and formula unit Z. It should also be mentioned that the angle P1-M1-C3 is deviating from ideal linearity and is decreasing from 54a to 54c. Hence, the latter exhibits the maximum deviation with an angle of 168.41°.

Table 7: Selected bond lengths, angles and crystal parameters of coordination compounds 54a-c.

Complex M1-N1 bond length / Å M1-P1 bond length / Å P1-M1-N1 angle / ° P1-M1-C3 angle / ° Crystal system Space group Z 54a 2.030 2.3687 80.32 170.96 monoclinic P21/n 4 54b 2.320 2.5116 77.44 169.05 monoclinic P21/n 4 54c 2.308 2.5005 77.48 168.41 monoclinic P21/n 4

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Figure 32: ORTEP representation of [Cr(CO)4(PN)] in the crystal P21/n. Thermal ellipsoids were drawn at a probability level of 50% and 23 °C. Hydrogen atoms of the phenyl groups are omitted for clarity. The deformation of the octahedral arrangement is visible.

Figure 33: ORTEP plot of [Mo(CO)4(PN)] in the crystal P21/n. Thermal ellipsoids were drawn at a probability level of 50% and 23 °C. Hydrogen atoms of the phenyl groups are omitted for clarity. The deformation of the octahedral arrangement is visible.

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Figure 34: ORTEP picture of [W(CO)4(PN)] in the crystal P21/n. Thermal ellipsoids were drawn at a probability level of 50% and 23 °C. Hydrogen atoms of the phenyl groups are omitted for clarity. The deformation of the octahedral arrangement is visible.

3.3.3. [M(CO)3(NNN)]

The behavior of the NNN ligand (49) is similar to that of the NN ligand (46), thus making the synthesis that starts from the hexacarbonyl the more appealing choice. The complex [Cr(CO)3(NNN)] (55a

)

was prepared in accordance to the literature[41] except that glyme was used as solvent instead of mesitylene (Figure 35). Owing to the high reactivity of ligand 49, a high-boiling solvent was not necessary.

Figure 35: Synthesis of [Cr(CO)3(NNN)] starting from chromium hexacarbonyl 43a.

In order to demonstrate that an ionic precursor complex is also a suitable starting material for the reaction with tridentate ligands incorporating only nitrogen donor atoms, complex 55b was prepared from 43c. As can be seen from the reaction scheme in Figure 36, the ligand substitution reaction is already completed after 15 minutes. However, the main drawback of this synthetic route is that the formed complex 55b as well as tetraethylammonium bromide precipitate from the solution and therefore an impure product is obtained.

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The yields are in the range of 54% to 59%. Since these coordination compounds are already reported, the complexes were analyzed by IR spectroscopy and the results were compared to the literature (Table 8).[41]

Table 8: Characteristic bands of complex 55a-b.

Complex 𝝊̃ (CO) / cm-1 _𝝊_{̃ (NH) / cm}-1

55a 1886, 1764, 1731 3336

55b 1878, 1755, 1726 3303

3.3.4. [M(CO)3(PNP)]

Group VI metal complexes featuring the PNP ligand (50) were prepared by using the respective anionic precursor. Again, this two-step synthesis was compared to synthetic procedures for similar complexes. [38,42]_{A recent report from the Beller group deals with the synthesis and catalytic} application of the coordination compound [Mo(CO)3(PNP)] in the homogeneous hydrogenation of carbonyl derivatives. Since no catalytic activity was observed in this case, the preparation of the complex was skipped.[34]

As can be seen from Figure 37, the first step of the synthesis includes the preparation of the anionic precursor. As already mentioned in the two-step synthesis of 54a and 54c, the anionic complex 45a was not isolated for convenience. Hence, a 10 wt% solution of 50 in THF was directly added to the orange precursor solution. The resulting mixture was heated for 6 h at reflux and precipitation of 44 as well as CO gas evolution set in. After cooling to room temperature and removing 44 from the reaction mixture by filtration, n-pentane was slowly added to the clear filtrate, which caused the precipitation of orange needles. Since these tiny needles showed a high tendency to accumulate, single crystals suitable for X-ray analysis have not been successfully grown yet. Moreover, complex 56a is air-sensitive, which is indicated by a color change from orange to black upon storing under an ordinary laboratory atmosphere.

Next, coordination compound 56a was prepared in accordance to the literature (Figure 37).[38]_The reaction was carried out in toluene by simply refluxing chromium hexacarbonyl and ligand 50 for a period of 22 h. Since toluene was employed instead of m-xylene, the reaction time was increased in order to ensure complete conversion. Again, orange needles precipitated from the reaction solution upon addition of n-pentane. The crystals were collected on filter paper and 56a was obtained in good yield (83%).

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Figure 37: Synthetic routes for the preparation of complex 56a.

The preparation of complex 56b occurred in a similar manner to the two-step synthesis of coordination compound 56a. After preparing the anionic precursor solution, 1.2 eq. of ligand 50 were added and the mixture was kept at reflux for a period of 6 h. The ammonium salt 44 precipitated from the reaction solution followed by CO evolution. The work-up procedure is analogous to compound 56a. Finally, 56b was obtained as an orange amorphous powder in fair yield (52%).

Application of the literature method for the synthesis of 56a gave very low yields. The reaction was performed in m-xylene instead of mesitylene and the reaction time was increased to 24 h. Both synthetic routes are outlined in Figure 38.

Figure 38: Synthetic routes for the preparation of complex 56b.

The tungsten complexes were analyzed by IR spectroscopy and high resolution mass spectrometry. IR bands of both coordination compounds are in the range of the congeneric molybdenum complex (Table 9).[34]

Table 9: Characteristic bands of complex 56a-b.

Complex 𝝊̃ (CO) / cm-1 _𝝊_{̃ (NH) / cm}-1

56a 1912, 1793, 1758 3272

56b 1917, 1797, 1753 3253

To conclude, the one-step synthesis starting from the hexacarbonyl as well as the method using the anionic coordination unit as intermediate are suitable for the synthesis of 56a and 56b. In contrast to syntheses performed with the related PN ligand, incomplete conversion was not observed. Again, the novel two-step synthesis avoids the use of a high boiling solvent and the

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reaction time was significantly decreased. However, in the case of chromium, the straightforward one-step synthesis from the literature allows to prepare coordination compound 56a in very good yields whereas the two-step synthesis produced a significant lower amount of product. Comparing both synthetic routes for the tungsten complex 56b, one must conclude that the precursor method is privileged, which is well in accordance with the observations from the preparation of 54c. The yields of each synthetic method are summarized in Table 10.

Table 10: Comparison of the one- and two-step synthesis by comparing the yields.

Complex Yield / %

one-step synthesis two-step synthesis

56a 83 47

55b 30 52

3.3.5. [Cr(CO)3(PPP)]

Complex 57 was already reported by Butler et al. in 1988 who synthesized this coordination compound on heating a solution of [(η7_{-cycloheptatriene)tricarbonylchromium(0)] and the PPP} ligand (52) in toluene overnight.[43]_{Since the preparation of [(η}7_-C

7H8)Cr(CO)3] using the method from the literature[44]_{yielded very low amounts of product (5% yield), this synthetic route was not} an option. Therefore, the procedure for the synthesis of the congeneric complexes [Mo(CO)3(PPP)] and [W(CO)3(PPP)] was adopted.[45] A solution of chromium hexacarbonyl (43a) and the ligand 52 was heated to reflux for 16 h upon which a white crystalline powder was obtained after work-up of the reaction solution.

The complex was also prepared starting from the precursor NEt4[Cr(CO)5Br] (45a). The first attempt was carried out in glyme, which led to the same problem that occurred in the reactions beforehand. Hence, a mixture of the precursor, the PPP ligand, and the product was obtained. The recorded 31_{P NMR of the resulting white amorphous powder is outlined in Figure 39 showing} the free ligand (-27.7 ppm) and the formed complex (40.0 ppm).[43]

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Figure 39: 31_{P NMR spectrum recorded in CD}₂_Cl₂_{of the isolated product obtained from the reaction of the precursor} NEt4[Cr(CO)5Br] 45a with the PPP ligand in glyme.

Since the synthetic procedure from the literature relies on the use of DMF as solvent, the reaction was performed in this reaction medium using 45a as starting material. After heating a solution of

45a and 52 to reflux for 6 h, the reaction solution was slowly cooled to room temperature overnight.

Surprisingly, cubic lime green crystals precipitated from the reaction mixture, which were already suitable for X-ray analysis. In addition, the yield was slightly increased from 57% to 67 %. Both synthetic procedures are shown in Figure 40.

Figure 40: Synthesis of 57 using chromium hexacarbonyl 43a or the anionic precursor 45a as starting material.

The appearance of this complex was quite different compared to the product obtained from literature method. In order to guarantee that both methods result in the formation of the same product, IR spectra were recorded. In both cases, two peaks at identical wavenumbers were observed, which is demonstrated in Figure 41.

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2500 2000 1500 1000 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Tr an smitta nce / a .u. Wavenumber  / cm-1 57 from [Cr(CO)₆] 57 from NEt4[Cr(CO)5Br]

Figure 41: Comparison of the complexes obtained from both synthetic routes.

The crystals obtained from the anionic precursor synthetic route were analyzed by X-ray diffraction. The corresponding molecular structure is found in Figure 42. As can be seen from Figure 42, complex 57 adopts an fac coordination geometry, which is in accordance with the literature.[42]_{Selected bond lengths, angles and crystal parameters are listed in Table 11.}

Table 11: Selected bond lengths, angles and crystal parameters of complex 57.

Complex Cr1-P1 bond length / Å Cr1-C1 bond length / Å P1-Cr1-C1 angle / ° Crystal system Space group Z 57 2.3821 1.830 174.91 trigonal R3 3