Results and Discussion

1.2 Results and Discussion

The σ-mesityl copper(I) NHC complex 1 is synthesized according to the method established for I (Scheme 1.2).^[8] In situ deprotonation of 1,3-bis(mesityl) imidazolium chloride with n-BuLi and addition of Mes4Cu4 results in the formation of 1 as a colorless powder.

Scheme 1.2: Synthesis route of the formation of 1.

Colorless single crystals are obtained by cooling a saturated complex solution of 1 in toluene at –24 °C for 4 days. 1 crystallizes in the space group P21/c (Figure 1.1), like complex I^[8]. The copper-carbene bond length Cu–C(1) in 1 is 1.903 Å (Figure 1.1) and in agreement with that of I (Table 1.1)^[8] as well as in the range of in the literature known Cu–C NHC bonds and Cu–C NHC bond lengths described later in this thesis (chapter 2).^[11,12] The distance between the copper ion and the mesityl ligand Cu–C(22) is 1.924 Å and the angle C(1)–Cu–C(22) is 173.43°. An almost linear coordination of the copper(I) ion by the two ligands is observed.^[11]

Compared to complex I, the Cu^I–Cipso bond is slightly longer and the angle C(1)–Cu–C(22) is smaller (Table 1.1). One reason for these differences is the slightly different capability of both NHC ligands for undergoing π–back-donation from the metal. The π–donation from the p orbital of the two N atoms into the free pπ orbital of the carbene C atom results into a significant electronic charge in the formally empty pπ orbital of the carbene.^[13] The π-delocalization in imidazol-2-ylidenes is higher than in imidazolin-2-ylidenes.^[14,15] The imidazolin-2-ylidene ligand is a better π-acceptor than the imidazole-2-ylidene ligand, due to less electron delocalization in the carbene pπ orbitals from the N donors. Less π–backdonation from the copper(I) (d¹⁰) to the imidazol-2-ylidene ligand results into an elongation of the copper-mesityl bond in comparison to the complex I.

1.2 Results and Discussion

Figure 1.1: Molecular structure (50% probability thermal ellipsoids) of 1. Hydrogen atoms are omitted for clarity.

Table 1.1: Comparison of bond lengths and angles of 1 and I^[8].

Elemental analysis confirmed the expected composition of the complex and underlines the purity of the compound (see experimental section). In the context of copper deposition reactions this issue is of enormous importance, to avoid side reactions or accumulation of impurities on a surface. ESI-MS of 1 in THF shows two dominant peaks at m/z = 671.40 (100) [(NHC)₂Cu]⁺ and m/z = 853.37 (40) [(NHC)₂Cu₂Mes]⁺ which demonstrates the instability of the complex under ESI-MS conditions (Figure 1.2). These conditions degrade the complex and a more stable, linear coordinated [(NHC)₂Cu]⁺ complex cation is detected.

This result is not surprising, since it emphasizes the intrinsic properties of NHCs, including the strong σ-donor character of the ligand^[16], and it underlines the utility of this complex for undergoing substitution reactions at the mesityl position. [(NHC)2Cu2Mes]⁺ is a result of an aggregation process under ESI-MS conditions. It is proposed that the mesityl group takes the function of a linker between two NHC coordinated copper(I) cores.

Meyer and Stollenz et al.^[17,18] reported several copper(I) mesityl complexes featuring a three-center two-electron (3c2e) coordination motif. The behavior of the complex is in agreement

Atoms 1 I

Cu–C(1) 1.90(3) 1.90(3)

Cu(1)–C(22) 1.92(4) 1.91(4)

C(1)–Cu–C(22) 173.4 175.5

1.2 Results and Discussion

Figure 1.2: ESI-MS of 1 in THF. Inset shows the simulated (top) and experimental (bottom) isotopic distribution pattern of the peak at m/z = 671.40 (100) assigned to [(NHC)₂Cu]⁺.

Thermal decomposition of 1 was determined via TG/DSC analysis (Figure 1.3). The decomposition of 1 occurs in two stages. The first decomposition of the complex occurs between approximately 100°C to 200 °C (Δm = –26.21%). A nearly thermostable behavior of the resulting product is observed in the temperature range from 200 °C to 330 °C. The second decomposition occurs from 330 °C to 450 °C (Δm = –40.15%) and at 800 °C is a residue of temperature separation of almost 130 °C while the decomposition of I can be described as an almost continuous decomposition process. A comparison of mass differences between 1 and I suggests that in the first initiation phase probably C₉H₁₂ (Δm = –24.62%) is released.

220 330 440 550 660 770 880 990 1100 1210 1320 1430 0

1.2 Results and Discussion

Figure 1.3: Left TG/DSC analysis of 1 with a heating rate of 5 K/min and right TG/DSC analysis combined with MS of I with the same heating rate.^[8] © 2016 ELSEVIER.

The TG/DSC analysis of 1 motivates to synthesize the oxalate-bridged dinuclear copper(I) NHC complex 2, which should also give a clean deposition to elemental copper, in agreement with the dicopper(I) µ-1,2,3,4-oxolato NHC complex II.

2 is synthesized according to the reaction in Scheme 1.3 and analogous to the synthesis route for complex II.^[8] Addition of one equivalent of oxalic acid at –78 °C to 1 and reaction of the components at r.t. overnight results in the formation of complex 2. After further work-up (see experimental section) a colorless powder is obtained. Colorless single crystals are isolated after layering a saturated THF complex solution with cooled Et2O at 4 °C (see experimental section).

Scheme 1.3: Synthesis of the unsaturated dinuclear oxalato-bridged copper(I) NHC complex 2.

2 crystallizes in the space group P1̅. In agreement with the crystal structure of the imidazolin-2-ylidene complex II, the crystal structure of the imidazol-2-ylidene complex shows Ci

symmetry (Figure 1.4). Each copper ion is coordinated in a distorted trigonal-planar fashion

1.2 Results and Discussion

in 2 and II are shorter than in complexes 1 and I, due to increasing electronegativity of the ancillary coligand.^[8]

Figure 1.4: Molecular structure (50% probability thermal ellipsoids) of 2. Hydrogen atoms are omitted for clarity.

An evaluation of the bonding situation in complex II and respective for 2 as well as a comparison of II with oxalato-bridged complexes has already been reported.^[8] The Cu–O bonds in 2 and II lie in the range of the reported copper(I) oxalates with olefin or alkyne ligands (longer; 1.987(1)-2.004(2) Å) and isonitrile complexes (shorter, 2.081(2)-2.122(2) Å).^[6,7] This is again related to the stronger π-acceptor character of the alkynes or alkenes and the σ-donor/π-acceptor character of isonitriles, respectively.^[8]

Table 1.2: Comparison of selected crystallographic data of 2 and II^[8].

The solid state ATR IR spectrum shows a very strong band at 1629 cm^–1 (asymmetric COO stretch) and 1607.6 cm^–1 (s)), reflecting the oxalato-bridge between the two copper ions (Figure 1.5). This stretching frequency is in good agreement with the corresponding values of

Atoms 2 II

Cu–C(1) 1.860(5) 1.860(5)

Cu(1)–O(1) 2.039(4) 2.045(3)

Cu(1)–O(2‘) 2.042(3) 2.048(3) O(1)–Cu(1)–O(2‘) 82.08(1) 81.93(1) C(1)–Cu(1)–O(2‘) 137.9(2) 138.56(2)

1.2 Results and Discussion

in the dinuclear copper oxalato-bridged complexes, mentioned in the literature. Oxalato-complexes with alkene and alkyne co-ligands show COO stretches between 1642-1647 cm^–1 and with isonitrile and phosphine co-ligands show COO stretches between 1617–1622 cm^–1 and 1620-1635 cm^-1)^[6,7] respectively, according to its backbonding capabilities. The backbonding of the copper(I) ions (d¹⁰) to the alkene or alkynes are the strongest and the back donation to the isonitrile is the weakest. In the ATR IR spectrum of the complex in solid state a very strong band at 1629 cm^–1 (asymmetric COO stretch) and 1607.6 cm^–1 (s) is observed, reflecting the oxalato-bridge between the two copper ions (Figure 1.5). This stretching frequency is in good agreement with the corresponding values of in the dinuclear copper oxalato-bridged complexes, mentioned in the literature. Oxalato-complexes with alkene and alkyne co-ligands show COO stretches between 1642-1647 cm^–1 and with isonitrile and phosphine co-ligands show COO stretches between 1617-1622 cm^–1 and 1620-1635 cm^–1[6,7]

respectively, according to its backbonding capabilities. In complex II (1634 cm^–1)^[8] the COO stretch is around 5 cm^–1 higher than in 2. This features the slightly weaker π-acceptor capability of the unsaturated ligand in 2. The reason is the significant electronic charge in the formally empty pπ orbital of the carbene in 2 and a higher π-delocalization in imidazol-2-ylidenes compared to imidazolin-2-imidazol-2-ylidenes.^[19]

Figure 1.5: Solid state ATR IR of the oxalato-bridged dinuclear copper(I) NHC complex 2. At 1629 cm^–1 the asymmetrical COO stretching vibration of the oxalato bridge is observed.

In ¹H and ¹³C{¹H} NMR spectra of 2 in THF-d₈ only one set of singlets is observed. This reflects the high symmetry of the complex in solution (Figure 1.6) within the limitation of the

3500 3000 2500 2000 1500 1000 500

wavenumber [cm^-1] 1629

1.2 Results and Discussion

carbene C⁵is low-field shifted to 185.2 ppm in the ¹³C{¹H} NMR spectrum. At 170.8 ppm appears the ¹³C NMR resonance of the quaternary C⁶ atom of 2 (oxalato bridge of II δ = 170.7 ppm)^[8]. Moreover, the different electronic properties between NHCs, alkynes/alkenes and isonitriles and the effect on the electronic environment of the oxalato carbons are obvious. The ¹³C signals in alkyne and alkene complexes are low-field shifted (171.8-171.4 ppm) and in the isonitrile complex slightly high-field shifted (169.2-168.9 ppm).

In II the chemical shift of the carbene-C is observed at 206.5 ppm and is low-field shifted compared to complex 2. This is a consequence of less electron density at the carbon atom.

Figure 1.6: ¹H and ¹³C{¹H} NMR spectra of 2 in THF-d₈ at 298 K. and (300 MHz and 75 MHz).

ESI-MS of 2 in THF shows one peak at m/z = 671, which is a result of a degradation of the complex under MS conditions. As described before by complexes 1, I and II, the more stable

1.2 Results and Discussion

mononuclear copper(I) dicarbene complex is formed under ionization conditions and no peaks of the dinuclear complex are observed. Elemental analysis confirmed the purity of the compound.

Preliminary thermal stability tests of 2 in a platinum crucible with a heating ramp applied from 25 °C to 800 °C and a heating rate of 5 K∙min^–1 shows a fast decomposition of 2. After the thermostability test, a black decomposition product is obtained inside and outside the crucible. The removal of these deposits is only successful using strong acids. Due to this observation no TG/DSC analysis of 2 was possible. TG/DSC analysis of II suggested the deposition of elemental copper after thermolysis^[8], but so far the exact identity of the resulting material after thermolysis of 2 has not be determined. A comparison of complex 1, 2 with I^[8] and II^[8] indicate that complex 2 seems to be also a precursor for copper deposition, but further experiments are necessary to proof this thesis.