Copper(I) complexes with multidentate NHC ligands and their dynamics in solution
2.2.2 Dynamic of open-chained copper(I) NHC complexes in solution
2.2.2 Dynamic of open-chained copper(I) NHC complexes in solution
Single crystals of complex 3 are obtained by Et2O diffusion into a solution of the complex in MeCN. 3 crystalizes in the space group P21/n and consists of three copper(I) ions. The constitution of the complex in solid state is also confirmed by elemental analysis and fast atom bombardment mass spectrometry (FAB-MS) in agreement with the results published by Chen et al.[36] The 1H NMR spectrum in MeCN-d3 of 3 shows one set of signals and indicates a system with a symmetrical structure (Figure 2.3). These findings can be explained by a rearrangement of the trinuclear copper(I) NHC complex to a dinuclear copper(I) complex under release of one copper ion as [Cu(CD3CN)4]+ in MeCN-d3 (Scheme 2.2). Chen et al.[36]
never mentioned this behavior of the complex and assumed the retention of the trinuclear complex in solution.
Figure 2.3: 1H NMR spectrum of complex 5 in MeCN-d3 at 298 K and 300 MHz. One set of signals is observed.
Scheme 2.2: A rearrangement of the trinuclear oligodentate copper(I) NHC complex in solid state to a dinuclear copper(I) NHC complex and a tetrakisacetonitrile copper(I) complex is proposed by dissolving the complex in MeCN.
2.2.2 Dynamic of open-chained copper(I) NHC complexes in solution
80 °C to –40 °C and confirm the presence of a species with higher symmetry in solution in comparison to the solid state structure (Figure 2.4). The imidazol-2-ylidene proton resonance H4 is not affected by temperature variation, but broadening of all other peaks is observed. No decoalescence of the corresponding proton peaks, down to 233 K is observed. As a consequence of this behavior no activation parameters (ΔH‡ or ΔS‡) can be determined. A necessary condition for the determination of these parameters by NMR spectroscopy is the presence of well resolved spectra below the coalescence temperature to obtain peak widths at half heights of the peaks at a temperature where in ideal case no exchange occurs (see shift differences between free pyridine and coordinated pyridine of 30-150 ppm. Kirchner et al.[53] discussed the chemical shift difference ΔNpy of tri- and tetracoordinated copper(I) complexes bearing bidentate SN ligands. They reported a shift of –30 ppm between the free ligand and the copper(I) complex. Regarding the solid state structure a larger Δδ(Npy) is expected, because the pyridine donor atoms are bound to the peripheral copper ions.
353 K
2.2.2 Dynamic of open-chained copper(I) NHC complexes in solution
Figure 2.5: (left) Comparison of the 15N chemical shift of the pyridine groups in complex 3 (purple) and H2L1(PF6)2 (green) determined by 1H15N HMBC NMR spectroscopy. (right) 1H DOSY NMR of the complex 3 (red) and H2L1(PF6)2 (blue).
A 1H DOSY NMR spectrum of 3 shows only one species in solution (Figure 2.5, right red) with a smaller diffusion coefficient (D298 = 8.97∙10–6 m2s–1) than the free ligand (D298 = 1.07∙10–5 m2s–1). Comparison of the hydrodynamic radii of [H2L1](PF6)2 and 3 considering the Stokes-Einstein equation[54] (1), is in agreement with the composition [(L1)2Cu2]2+. The Stokes-Einstein equation has different restraints (e.g. it assumes spherical molecules) which leads to an inaccuracy of RCu and thus of A for non-spherical molecules.[54]
𝐷 = 𝑘𝑏∙ 𝑇 6 ∙ 𝜋 ∙ 𝜂 ∙ 𝑅𝐻 𝐀 =𝑉𝑉1
2 = (𝑅𝑅Cu
L)3 = (𝐷𝐷L
Cu)3 = (𝐷𝐷Cu
L)−3= (8.97∙101.07∙10−6−5)−3= 1.70 ≈ 2
Furthermore, one has to keep in mind that the 1H DOSY NMR spectrum only can show the average of the dynamics of the molecules in solution based on the NMR instrument time scale and the resolution of the NMR spectra, which strongly depend on the spin-lattice relaxation time T1, which has a strong influence on the signal intensity and the spin-spin relaxation time T2, which has an influence of the line shape of the signal.
ESI-MS measurements of single crystals, dissolved in MeCN, confirm the presence of dinuclear species in solution. Two dominant peaks at m/z = 875.08 (100) [(L1)2Cu2(PF6)]+ and m/z = 365.06 (99) [(L1)2Cu2]2+ are observed (Figure 2.6). Overall, the combined evidence from 2D NMR techniques with ESI-MS strongly suggests the presence of the
1 2+
(1) (2)
2.2.2 Dynamic of open-chained copper(I) NHC complexes in solution
Figure 2.6: ESI-MS of 3 in MeCN. The inset shows the experimental and simulated isotopic distribution pattern for the peak at m/z = 875.08 (100) for [(L1)2Cu2 (PF6)]+ .
To further explore the influence of the central bridge on the nuclearity, connectivity and denticity in solid state as well as in solution, the 2H-imidazolium distance of the oligodentate ligand is extended by two CH2 units. Synthesis of the ligand and its copper(I) complex using Mes4Cu4, is performed as described previously.[51,55]
In this work, single crystals of [(L2)5Cu6](PF6)6, suitable for X-ray measurements are obtained by slow diffusion of Et2O into a complex MeCN solution. 4 crystalizes in the space group P1̅. The previously described constitution[51], including bond lengths and bond angles in the complex can now be confirmed by the solid state structure. The exchange of the central unit of the bridge and as consequence thereof higher flexibility of the ligand, results into two trinuclear subunits which are linked to a fifth propylene-bridged ligand (Figure 2.7 and 2.8).
In Figure 2.8 the copper∙∙∙∙copper distances are shown. The obtained crystal structure confirms a hexanuclear copper(I) NHC complex in solid state, where five ligands are surrounding the two trinuclear CuI subunits in a helical fashion (Figure 2.7 and 2.8).
Cuprophilic interactions are observed within the two subunits, and weak interaction between the two internal copper(I) ions Cu(3) and Cu(4) (3.75 Å).
440 660 880 1100 1320 1540 1760 1980 2200 0
873 874 875 876 877 878 879 880 881 882 0
2.2.2 Dynamic of open-chained copper(I) NHC complexes in solution
Figure 2.7: Molecular structure (30% probability thermal ellipsoids) of 4. Counter ions and hydrogen atoms are omitted for clarity. Selected distances [Å] and angles [º]: Cu(1)∙∙∙∙Cu(2) 2.8393(7), Cu(2)∙∙∙∙Cu(3) 2.6501(7), Cu(3)∙∙∙∙Cu(4) 3.7492(6), Cu(2)–N(11) 2.396(4), Cu(1)–C(6) 1.912(4),
C(6)-Cu(1)-C(32) 174.93(2).
Figure 2.8: Schematic drawing of the helical structure of 4. Six copper(I) ions are in the center of a helical
2.2.2 Dynamic of open-chained copper(I) NHC complexes in solution
Figure 2.9: 1H NMR spectrum of complex 5 in MeCN-d3 at 298 K and 500 MHz. One set of signals is observed.
1H NMR spectrum of complex 4 in MeCN-d3 shows, in agreement with complex 3, the presence of a species of higher symmetry than predicted from the molecular structure in solid state (Figure 2.9). VT 1H NMR spectroscopy of complex 4 over a wide temperature range (80 °C to –40 °C) (Figure 2.10) does not show any decoalescence. The signals of the protons H3 and H4 are affected the most by decrease of the temperature from 353 K to 233 K, showing a high-field shift of Δδ = 0.25 ppm and Δδ = 0.27 ppm.
Figure 2.10: VT 1H NMR spectra of complex 4 in the temperature range between 353 K and 233 K at 400 MHz in MeCN-d3. The region between 6.9 ppm and 4.5 ppm was removed to obtain a higher resolution of the peaks.
343 K 333 K
313 K 303 K
283 K 293 K
323 K
263 K 273 K
233 K 243 K
253 K
353 K
2.2.2 Dynamic of open-chained copper(I) NHC complexes in solution
1H DOSY NMR spectroscopy of 4 confirms the presence of one species in solution on the timescale of the experiment, and similar diffusion coefficients of complex 3 and 4 suggests a comparable size of the two complexes (Figure 2.11, D298 = 8.69∙10–6 m2s–1). The ratio between both complexes 4 and 3, based on the Stokes-Einstein equation (2) is 1.09.
Figure 2.11: Stacked 1H DOSY NMR spectra of 3 (red, D298 = 8.97∙10–6 m2s–1) and 4 (green, D298 = 8.69∙10-6 m2s–1) in MeCN-d3 at room temperature.
The chemical shift of the pyridine nitrogen atom in 4 is δ(Npy) = –105 ppm, determined by
1H15N HMBC NMR spectroscopy which is almost the same shift like it is observed for complex 3 (Npy= –108 ppm), suggesting a similar weak coordination of the pyridine groups to the copper ions. The dinuclearity of 4 is confirmed by ESI-MS measurement in MeCN (Figure 2.12). Two main peaks at m/z = 933.15 (99) for [(L2)2Cu2(PF6)]+ and at m/z = 395.09 (100) for [L2Cu]+ are observed. The mononuclear species [L2Cu]+ is observed upon fragmentation of [(L2)2Cu2(PF6)]+.
2.2.2 Dynamic of open-chained copper(I) NHC complexes in solution ethylene to hexylene and observed a high structural diversity. The behavior of the copper ions is not comparable with the silver ions using the same ligand scaffold. An expansion of the
440 660 880 1100 1320 1540 1760 1980 2200 0
2.2.2 Dynamic of open-chained copper(I) NHC complexes in solution
alkylene bridge from methylene to propylene results in a higher nuclearity of the complex in solid state, but previous results showed that the two homologous ethylene[51] and propylene- bridged complexes have hexanuclear copper cores in solid state. A comparison between the two homologous copper and silver complexes shows that no prediction of the structure is possible not even upon slight changes of the ligand scaffold or by use of the higher homologous. In conclusion a hexanuclear propylene-bridged copper(I) NHC complex in solid state rearranged to a dinuclear copper(I) NHC complex in solution, bearing two ligands.
A dinuclear copper(I) NHC complex 5 is obtained in the solid state, by exchange of the methylene or propylene bridge by a pyridinyl bridge. Analysis of 5 in solid state[51] and in solution confirms the preservation of the dinuclear copper(I) NHC core. 5 crystalizes in the space group Pnn2.[51] In the crystal structure the two copper(I) ions interact with the central pyridinyl atom in a three-center two-electron (3c2e) configuration, while the external pyridinyl groups do not interact with the metals (Figure 2.14 (a)). 1H NMR spectroscopy of 5 in MeCN-d3 (Figure 2.14 (b) top) shows one set of signals, in analogy to the D2 symmetry of the complex in solid state. A decrease of the temperature results in a rearrangement of the peaks in the 1H NMR spectrum (Figure 2.14 (b), bottom). The reason for this observation is the change of the coordination motif in the complex, which affects the chemical and electronic environment of the protons (Figure 2.14 (a)). Proton resonances were assigned using 2D NMR spectroscopy. Discrimination between the two imidazol-2-ylidene backbone protons H3 and H7 is possible by observation of a NOE from the corresponding imidazol-2-ylidene protons to the meta-pyridinyl protons H4 and H5.
VT 1H NMR spectra in the range 233-353 K give further insides in the highly dynamic behavior of the complex with different coordination modes of the copper cores. At high temperature a three-center two-electron interaction between the copper ions and the internal pyridinyl groups is observed. By decrease of the temperature a stronger interaction with the peripheral pyridinyl groups and the two copper ions is obtained (Figure 2.14). The interaction of the central pyridinyl groups with the two copper centers via a 3c2e bonding situation effects markedly the electronic situation of the protons H1 and H5. Increase of the temperature results into a low-field shift of the para-pyridine peak H1 of almost 1 ppm. At 233 K, NOE between the imidazol-2-ylidene peak H3 and the meta-pyridine H4 is very strong.
2.2.2 Dynamic of open-chained copper(I) NHC complexes in solution
Figure 2.14: (a) Assumed coordination modes of 5 at different temperatures. (b) VT 1H NMR spectra of complex 5 in the temperature range between 353 K and 233 K (400 MHz) in MeCN-d3.
This finding corroborates the coordination of the external pyridinyl groups to the copper centers at low temperatures. Stronger interaction of the external pyridinyl groups with the copper ions leads to an increase of the rotation barrier of latter groups and a stronger coupling of the two protons (H3 and H4) through space. 13C{1H} NMR spectroscopy confirms the different kinds of interactions at low and high temperatures (Figure 2.15). The signals show the same trend like presented for the 1H NMR shifts. An increase of the temperature results in a significant low-field shift of the 13C–1 and 13C–5 signals by a 3c2e interaction (approximately 3 ppm) concomitant a high-field shift of 13C–6 (Figure 2.15).
(a)
(b)
233 K 243 K 253 K 263 K 273 K 283 K 293 K 303 K 313 K 323 K 333 K 343 K 353 K
2.2.2 Dynamic of open-chained copper(I) NHC complexes in solution
Figure 2.15: 13C{1H} NMR spectra of 5 at 353 K (top) and 233 K (bottom) (126 MHz) in MeCN-d3.
2.2.3 Dynamic of a macrocyclic copper(I) NHC complex in solution