Cobalt Complexes

Cobalt(II) complex 54 (Co^tBu-MeCN) was first synthesized and crystallized by SAMANTA.^[419] In this work the synthetic and crystallization procedures were optimized and reactivity studies as well as catalysis experiments were conducted.

tBu CH2

py pz

75 3.4 Complex Synthesis and Reactivity 3.4.2.1 Synthesis

Co^tBu-MeCN was synthesized according to the reaction depicted in Scheme 3.18. Cobalt(II) tetrafluoroborate was combined with a suspension of HL^tBu and KO^tBu in MeCN. Pure material was obtained by layering a MeCN solutions with Et2O as red‐brown plates in up to 52 % yield.

Scheme 3.18: Synthesis of [(Co(MeCN)2)2L^tBu](BF4)3 54 from HL^tBu and KO^tBu.

Co^tBu-MeCN crystallized in the monoclinic space group P12/n1 with two molecules in the unit cell. Similar to the zinc complexes the cobalt ions are coordinated in a distorted square pyramidal geometry (τ5 = 0.10), having one acetonitrile ligand in the axial and one in the equatorial position of each cobalt ion (Figure 3.21). The bonds to the axial ligands are longer (2.083 A vs. 1.893 A ) and the metal atoms are slightly above the plane of the four donor atoms in the base plane of the pyramidal arrangement (dCo‐plane = 0.242 A ). The metal separation in the symmetric complex Co^tBu-MeCN is about 0.1 A longer than in the zinc complex (dCo‐Co = 4.437 A ). The separation of the phosphorus atoms and the backbone plane including the pyrazole, the pyridines and methylene groups is larger than that of the metal ions.

Figure 3.21: Thermal displacement ellipsoids (shown at 30 % probability) of the molecular structure of [(Co(MeCN)2)2L^tBu](BF4)3 54 in two different orientations. Counterions, hydrogen atoms and solvent molecules were omitted for clarity.

Further characterization of the cobalt(II) complex proved to be difficult in the way that NMR spectroscopy and mass spectrometry were not suitable methods for this complex.

Paramagnetic NMR measurements showed some rather broad lines, but less than expected and not assignable (Figure 3.25). Other methods such as UV‐vis or IR spectroscopy provided no structural information. This limitation became especially a problem when the reactivity of this complex was investigated (see chapter 3.4.2.2). To enhance the crystallization properties, the

54

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3 Two-in-one Pincer Ligands and Their Metal Complexes for Catalysis

large non‐coordinating anion tetraphenylborate was introduced, but no crystals of the product of the salt metathesis could be obtained. Also the structural characterization of cobalt complexes with chloride or bromide or with ligand HL^iPr failed, although many reaction conditions and crystallization attempts were tested.

The magnetic properties of Co^tBu-MeCN were investigated by EPR spectroscopy (frozen MeCN solution) and by temperature dependent measurements of the magnetic susceptibility with a SQUID magnetometer (solid material, Figure 3.22, left). The magnetic data revealed that the cobalt(II) ions have a low‐spin configuration with one unpaired electron each (S = ½). Via superexchange through the pyrazole unit the two spins can interact with each other leading to possible spin states St = 0 and St = 1. The energetic separation of these two states is represented by the coupling parameter J for the isotropic interactions (ESt = 1 – ESt = 0 = ‐2J). The magnetic susceptibility was recorded in the temperature range from 295 K to 2 K and the data was fitted using the HEISENBERG‐DIRAC‐VAN‐VLECK Hamiltonian 𝐻̂ = −2𝐽𝑆̂₁𝑆̂₂+ 𝑔𝜇_𝐵(𝑆⃗₁+ 𝑆⃗₂)𝐵⃗⃗. At ambient temperature (295 K) a 𝛸_𝑀𝑇 value of 1.24 cm³Kmol^‐1 was observed, which is larger than the calculated spin only value for two uncoupled S = ½ spins (0.75 cm³Kmol^‐1). This indicated also an impact of the orbital angular momentum and with that a larger LANDE factor than two.

Upon lowering the temperature, the Χ_𝑀𝑇 value decreased to 0.58 cm³Kmol^‐1. The data was best fitted with a small antiferromagnetic coupling constant J = –1.25 cm^‐1 and temperature independent paramagnetism (TIP, 453 · 10^‐6 cm³mol^‐1).

Figure 3.22: Temperature dependent measurement of the magnetic susceptibility (left)^[419] and EPR spectrum in MeCN at 161 K (right) of [(Co(MeCN)2)2L^tBu](BF4)3 54.

Electron paramagnetic resonance spectroscopy (EPR) showed less than expected and extremely broad signals at 161K which could not be fitted in an adequate way (Figure 3.22, right). The g‐values of the rhombic spectrum were estimated to gx = 1.97, gy = 2.28 and gz = 2.72. These values are in sharp contrast to the fused pincer dinuclear cobalt complex of FIEDLER (Figure 3.2), which exhibited diamagnetic behavior.^[266] To increase the resolution of the signal the temperature has to be lowered, but this data was not available to date.

77 3.4 Complex Synthesis and Reactivity The redox properties of Co^tBu-MeCN were investigated by electrochemistry measurements. The cyclic voltammogram and the square wave voltammogram showed four irreversible reduction events (Figure 3.23, left). The first two are well separated while the third and fourth step are close together with a separation of only 80 mV. The first reduction is much easier to access with a potential of almost 1 V less negative compared to the second reduction. A shoulder of the first reduction wave could not be explained so far. It was assumed that the first two reductions are metal centered producing a dinuclear cobalt(I) compound. The third and fourth event was assigned to be ligand‐based, since the zinc complexes exhibited events at similar potentials (Figure 3.16).

Figure 3.23: Cyclic voltammogram (measured in 0.1 M nBu4NPF6 in MeCN) of Co^tBu-MeCNat different scan rates (left) and square wave voltammogram (at 100 mV/s) with the assigned reduction potentials (right).

An elemental analyses performed with carefully dried single crystalline material showed that loss of almost one acetonitrile per molecule could not be avoided. However, the best values are in good agreement with the formula [Co2L^tBu(MeCN)3](BF4)3 (see Chapter 5.5.2.4).

3.4.2.2 Reactivity

To study the reactivity of the cobalt(II) complex Co^tBu-MeCN, different reagents such as bases, reductants and hydride or alkyl transfer agents were applied. With regards to the concept of the activation of small molecules, especially the reduced cobalt(I) and the deprotonated species (Scheme 3.19, pathways A and B) are of interest.

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3 Two-in-one Pincer Ligands and Their Metal Complexes for Catalysis

Scheme 3.19: Potential reactivity pathways for [(Co(MeCN)2)2L^tBu](BF4)3 54: deprotonation of the side arm(s) (A);

reduction to cobalt(I) or even further (B); alkylation of the cobalt centers (C) and formation of a cobalt hydride complex for the activation of small molecules (D).

Hence, Co^tBu-MeCN was treated with different strong bases (KO^tBu, LiO^tBu and KH) in MeCN or CH2Cl2 and the reaction was followed by UV‐vis spectroscopy. When the reaction was conducted in MeCN, a slight color change from a red‐brown solution to a rather brown solution was observed. The maximum of the absorption band, however, did not change. Somehow the intensity of the π‐π* ligand band at 258 nm increased upon each addition of KO^tBu, without ending in a saturation (Figure 3.24, left). Changing the base to LiO^tBu, similar absorption changes were observed. The reactions were slower at lower temperature, but showed the same behavior. This indicated that a deprotonation reaction of Co^tBu-MeCN did not take place. The reason for that might be that strong bases may deprotonate bound acetonitrile and with that totally different reactions may occur.^[420] Unfortunately, the poor solubility of Co^tBu-MeCN in other solvents than MeCN and CH2Cl2 rendered the use of different solvents impossible. It was thought that the dearomatization as a consequence of the deprotonation would drastically change the electronic structure of the complex and therefore a much distinct absorption maximum was expected.

54

79 3.4 Complex Synthesis and Reactivity

Figure 3.24: UV-vis spectra of reactions of [(Co(MeCN)2)2L^tBu](BF4)3 54 with LiO^tBu in MeCN (left) and in DCM (right).

A reaction in dichloromethane (with LiO^tBu for solubility reasons) supported this hypothesis, since in contrast to the reaction in MeCN, in CH2Cl2 no increase of the absorption bands was observed (Figure 3.24, right). In this experiment a slight shift of the absorption maximum by 7 nm was observed. The reason for that might be a change in the coordination sphere of the cobalt centers. The use of potassium hydride as base yielded intense dark blue reaction mixtures but no products could be identified by different spectroscopic techniques.

The cyclic voltammogram of Co^tBu-MeCN (Figure 3.23) exhibited well separated reduction events. The second reduction at a potential of approx. –1.17 V versus Fc/Fc⁺ is perfectly in line with the reduction potential of cobaltocene (–1.33 V versus Fc/Fc⁺ in CH2Cl2).^[421] Assuming the reduction to take place at the cobalt centers, the resulting species should be a dinuclear cobalt(I) d⁸ species, which is diamagnetic in the low spin case. Reduction of the cobalt(II) complex Co^tBu-MeCN with cobaltocene in MeCN resulted in an instant color change of the solution to intense purple. The reaction products were very sensitive to oxygen and moisture.

Upon crystallization yellow needles of the cobaltocenium salt were obtained, but single crystals suitable for X‐ray diffraction analysis of a product resulting from Co^tBu-MeCN could not be isolated. ¹H NMR spectroscopy revealed the presence of paramagnetic compounds, since strongly shifted resonances were observed (Figure 3.25). This might be explained by the formation of a high spin cobalt(I) complex. Like for Co^tBu-MeCN the resonances could not be assigned and therefore the exact number as well as the structures of reaction products remained unknown. In ³¹P NMR spectra neither resonances for the phosphine side arm nor of free ligand were observed.

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3 Two-in-one Pincer Ligands and Their Metal Complexes for Catalysis

Figure 3.25: ¹H NMR spectra of Co^tBu-MeCN (top) and crude products of reactions of Co^tBu-MeCNand [CoCp2] under Ar (middle) and CO atmospheres (bottom)(acetonitrile-d3, 298 K).

To avoid the formation of high spin complexes, the reduction was carried out under an atmosphere of carbon monoxide, which can act as a strong donor ligand supporting low spin configurations. Unfortunately, ¹H NMR spectra of crude products still showed paramagnetic character, although a carbonyl vibration at 1893 cm^‐1 was found in ATR‐IR measurements showing the presence of a CO ligand in the reaction product. The presence of paramagnetic impurities was assumed. If the paramagnetically shifted resonances resulted from a formed carbonyl cobalt(I) complex, it has to have a distorted geometry so that the unusual high spin state is favored. No high spin cobalt(I) complexes bearing a CO ligand were found in literature and also only a very few high spin cobalt(I) complexes with other ligands are known. ^[422,423]

It was also tried to reduce complex Co^tBu-MeCN with a stronger reductant, namely potassium graphite (KC8). The same purple color of the reaction mixture and similar ¹H NMR data were found for the reduction with KC8 in a mixture of cold MeCN and THF. However, the low stability of the product(s) prevented the isolation and identification of the formed species. This fast decomposition (on air within few seconds, under inert conditions at low temperatures within several hours) supports the assumption of the synthesis of a reactive, low valent cobalt species.

Reaction of Co^tBu-MeCN with a Grignard reagent (MeMgBr) or sodium borohydride yielded almost the same purple solutions like the stronger reductants before. Grignard solutions are also used for reduction reactions, therefore the reduction to cobalt(I) might be favored over the transmetallation reaction (Scheme 3.19, pathway C). With hydrazine or azides a rather slight color change of the reaction mixtures was observed indicative for a simple ligand exchange at the cobalt centers, but the products could not be isolated.

The synthesis of cobalt hydride species was attempted (Scheme 3.19 pathway D). The relatively large metal‐metal distance should lead to a dihydride compound rather than a bridged monohydride one. Indeed, a color change of the reaction mixture was observed until approx.

two equivalents of superhydride^® solution (KHBEt3) were added. Further equivalents did not Co^tBu-MeCN

Co^tBu-MeCN + [CoCp2] (Ar)

Co^tBu-MeCN + [CoCp2] (CO)

81 3.4 Complex Synthesis and Reactivity lead to a further color change. Again the identification of the product failed, since single crystals of the compound could not be obtained and other spectroscopic methods provided no structural information. Nevertheless, the formation of a reactive species upon addition of superhydride^® to Co^tBu-MeCN was taken as initiation to investigate of the catalytic activity of this mixture (see Chapter 3.6.2).