Main parts of the results presented in this subchapter were published:
2.2.4 Dynamic of a macrocyclic dicopper(I) NHC complex in solution
2.2.4 Dynamic of a macrocyclic dicopper(I) NHC complex in solution
7 was obtained by reaction of calix[4]-2H-imidazolium[2]pyridine with Mes4Cu4 in MeCN.
After work-up of the reaction mixture (see experimental section) a pale yellow solid was obtained. Single crystals of 7 suitable for X-ray diffraction are collected, after diffusion of Et2O into a complex solution in MeCN.
Scheme 2.5: Reaction of calix[4]-2H-imidazolium[2]pyridine with Mes4Cu4 results in the formation of complex 7.
7 crystallizes in the space group P1̅. The molecular structure in solid state shows in agreement with the analogous silver(I) complex[62] a dinuclear complex, in which the metal ions are coordinated by two carbene-C atoms in an almost linear fashion (C13–Cu1–C2 175.9°) (Figure 2.20). In the corresponding silver complex a more acute angle of 169.9° is observed.
7 shows a Cu(1)/Cu(1’) distance of 4.00 Å. This distance is quite large in comparison to the disilver complex (2.87 Å), which shows argentophilic interactions.[62] In the molecular structure in solid state, the ligand scaffold is twisted around the N(1)–Cu(1)–Cu(1’)–N(1’) axis (Figure 2.20, right). Cu(1)–C(2) and Cu(1)–C(13) distances are 1.90 Å, in agreement with in the literature known copper(I) carbene complexes.[36,63] The Cu(1)∙∙∙∙N(1) distance of 2.26 Å indicates a weak interaction between these two atoms. In general, metal-pyridinyl bond lengths range between 1.9 Å and 2.1 Å.[26,64,65] Relevant bond lengths are summarized in Table 2.1. The ESI-MS shows two peaks at m/z = 314 (100) for [L5Cu2]2+ and m/z = 773 (22) for [L5Cu2(PF6)]+ (Figure 2.21).
2.2.4 Dynamic of a macrocyclic dicopper(I) NHC complex in solution
Figure 2.20: Molecular structure (50% probability thermal ellipsoids) of the cationic part of 7. H atoms are omitted for clarity.
Table 2.1: Selected bond lengths [Å] and angles [°] of complex 7.
Atoms Bond lengths
2.2.4 Dynamic of a macrocyclic dicopper(I) NHC complex in solution
1H NMR spectrum of 7 in MeCN-d3 shows one set of signals, with a diastereotopic splitting of the proton resonances H5 of the two CH2 groups. In view of the molecular structure in solid state one doublet of the AX spin system in the NMR spectrum is assigned to the two protons which are pointing towards the outside of the ring and the second doublet to the resonances of the two inner protons, orientated to the inside ring. In the latter, the protons are more shielded because of proximity to the two copper(I) ions resulting in a high-field shift of 426 Hz (Figure 2.22).
Figure 2.22: Top: 1H NMR spectrum of 7 in MeCN-d3 at 298 K and 500 MHz shows a diastereotopic splitting of the proton resonances H5 of the CH2 groups and a shift of the CH2 groups of 426 Hz. (*) shows DCM as an impurity. Bottom: In 13C{1H} NMR spectrum a carbene-C peak at 182.3 ppm and one set of signals are observed.
VT 1H NMR spectra of complex 7 in MeCN-d3 show a coalescence temperature of the proton resonances H5 at Tc = 353 K (Figure 2.23) which allows to determine the rate constants k of the ring flip (Scheme 2.6) in the C2h symmetric complex at different temperatures by line
2.2.4 Dynamic of a macrocyclic dicopper(I) NHC complex in solution
shape analysis of the CH2 peaks H5 in the 1H NMR spectra (see experimental section 10.2.2).[66,67] The rate constants at different temperatures are listed in Table 2.2. Based on the temperature dependence of the rate constant k and an Eyring plot (Figure 2.24, left), an activation enthalpy of ΔH‡ = 45.9 kJ∙mol–1±0.92 kJ∙mol–1 and an activation entropy of ΔS‡ = –65.7 J∙mol-1∙K–1±1.3 J∙mol–1∙K–1 are determined, and by an Arrhenius plot the activation energy Ea = 48.5 kJ∙mol–1±0.97 kJ∙mol–1 is obtained (Figure 2.24, right). A comparison of the kinetic data with similar copper, silver or gold complexes is not possible, because comparable data are unavailable in the literature.
Scheme 2.6: Ring flip in the C2h symmetric complex 7.
353 K 343 K 333 K 323 K 313 K 303 K 293 K 283 K 273 K 263 K
2.2.4 Dynamic of a macrocyclic dicopper(I) NHC complex in solution
Figure 2.24: Eyring plot (left) and Arrhenius plot (right) based on the line broadening analysis of the diastereotopic splitting of the CH2 groups H5 in a temperature regime between 343 K and 273 K. An activation enthalpy of ΔH‡ = 45.9±0.92 kJ∙mol–1 and an activation entropy of ΔS‡ = –65.7±1.3 J∙mol–1∙K–1 were determined (R2 = 0.98) as well as an activation energy of Ea = 48.5±0.97 kJ∙mol–1 (R2 = 0.98).
Table 2.2: Rate constants at different temperatures derived by line shape analysis of the 1H NMR peaks of the CH2 group H5.
2.3 Conclusion and Outlook
2.3 Conclusion and Outlook
The presented results show the versatility of mesitylcopper(I) as a precursor for copper(I) carbene complex synthesis. Beside the previously presented synthesis of the complexes 3-6, also the synthesis of 7 was successful, broadening the scope of new CuI NHC complexes by the mesitylcopper route. All complexes showed diverse structural properties. It was previously shown, that by variation of the central bridge (methylene, propylene or pyridinyl) different coordination motives and different nuclearities of the resulting copper complexes can be obtained. In this work it was shown that 3 and 4 strongly rearrange in MeCN from oligonuclear complexes to dinuclear copper(I) complexes.
The pyridinyl-bridged dinuclear copper(I) complex 5 shows a dynamic behavior in solution, monitored by VT 1H NMR spectroscopy. At room temperature and elevated temperatures a three center two electron interaction of the internal pyridinyl groups with the copper(I) ions is observed. In contrast to this, at lower temperatures the copper(I) cores interact with the peripheral pyridinyl groups. This rearrangement affects the proton resonances in such a way, that chemical shift changes of up to 1 ppm are observed.
In 6, a high flexibility of the ligand scaffold is monitored by 1H NMR spectroscopy. No energy barrier over a broad temperature range is obtained, due to weak interaction of the pyridinyl groups with the copper(I) ion and a fast ring flip, which is reflected by an apparent D2h symmetry of the complex in the 1H NMR spectrum. Herein presented CV of complex 6, as well as CV measurements of the copper(II) complex, confirmed a EC mechanism for the oxidation of the copper(I) complex or the reduction of the copper(II) complex. This observation is based on rearrangement of the ring by coordination or de-coordination of the pyridinyl groups to the copper center. In the future, this configurational change can be further analyzed by CV simulations at different complex concentrations, to obtain an estimated E1/2
potential and data of the ring flip.
In contrast to 6, 7 shows a coalescence temperature at approximately 353 K in MeCN and the activation parameters (ΔH‡, ΔS‡, Ea) for the ring flip of the complex are determined.
These five examples show the importance of detailed studies of copper(I) NHC in both solid state and in solution to understand the coordination environment of the metal ions and composition of the metal complexes in different states of matter. The 1H DOSY NMR spectroscopy has several constraints and can only give an average picture of the movement of
2.3 Conclusion and Outlook
complex is not investigated in this thesis. This interaction can have consequences on the dynamics of the molecule in solution and can change the 1H DOSY NMR spectra. Additional experiments are necessary to prove the theory of a certain ion pairing.