Figure 3.3: 1H NMR spectra of HL1 (25 °C) and [CuI2L1]X (X = OTf−, PF6−
for 25 °C, 70 °C, respectively) measured under anhydrous conditions in CD3CN. Heating the [CuI2L1]X sample leads to sharpening of all signals.
1H and 13C NMR analysis of [CuI2L1]X (X = PF6−
or OTf−) in CD3CN at RT revealed one set of resonances which display altered chemical shift values to the corresponding free ligand, HL1, and sodium complex, [NaL1]. In contrast to the latter two systems, the spectra for [CuI2L1]X show significant broadening at RT for all 1H resonances (Figure 3.3), and for the 13C signals corresponding to the pyrazole bridging unit and the methyl groups of the shoulder tertiary nitrogen atoms. In fact, the pyrazole methine carbon atom resonance could only be detected at RT through 1H−13C HSQC experiments. Cooling the sample resulted in an increase in the line widths of all signals. Conversely, heating caused all resonances to sharpen until coupling patterns similar to those for HL1 and [NaL1] were once again resolved. Only one set of 6 peaks corresponding to [CuI2L1]X is apparent at 70 °C in the 13C NMR spectrum (The pyrazole methine carbon could not be observed, even with application of 1H−13C HSQC experiments), and cross-peaks corresponding to 3 nitrogen nuclei are apparent in the 1H−15N HMBC spectrum.
These observations are consistent with the presence of a pyrazole-bridged copper(I) complex which is fluxional in solution, leading to the averaging of the chemical environments of the ligand scaffold at higher temperatures. As [CuI2L1]X possesses only tridentate metal-binding pockets, association of an external co-ligand is plausible.
Coordination of additional donors, such as CD3CN solvent molecules, would induce asymmetry if interacting with a single copper(I) centre, or result in cis-trans isomers if associated to both copper(I) ions in [CuI2L1]X (Scheme 3.6).132 Both of these processes could lead to the observed signal broadening. However, given the well-known tendency of pyrazole-bridging ligands to form trimeric and tetrameric assemblies with coinage metals, such as copper(I), an additional molecule of complex [CuI2L1]X itself is also a viable external co-ligand. Indeed, the observation of multinuclear adducts in the MS experiments described above is consistent with the latter proposal. In order to investigate if any of the aforementioned scenarios apply to the current system, attempts were made to assess the molecular size of [CuI2L1]X in solution.
Scheme 3.6 Coordination of a nitrile solvent molecule to the two copper centres in [CuI2L1]X could result in cis- and trans- isomers, analogously to the situation depicted in Scheme 3.4. The enantiomer of the trans- isomer is not shown above but would also be part of the equilibrium. L = MeCN or EtCN. Charges and counterions are
omitted.
1H NMR DOSY is a technique which is useful for gaining information about the size of a molecule in solution, as it separates signal sets by their diffusion coefficients. A large diffusion coefficient reflects a fast rate of translational motion through solution, and thus implies that the corresponding molecule is relatively small. Conversely, a small diffusion coefficient is indicative of a large molecule. Furthermore, diffusion coefficients allow for an estimate of the molecular radii and volume in solution to be derived (see Experimental Section 8.4 for details). The spectra of [CuI2L1]OTf in CD3CN at RT show the presence of a single species, with a significantly smaller diffusion coefficient (D = 1.07 × 10−9 m2s−1) than that found for equimolar solutions of free ligand HL1 (1.45 × 10−9 m2s−1). The molecular radii calculated from these values reveal that [CuI2L1]OTf (r = 5.5Å) occupies substantially more space in solution than the corresponding ligand HL1 (r = 4.1Å). [CuI2L1]OTf is thus approximately 2.5 times larger in terms of volume in solution.
It is evident from the above described data that [CuI2L1]X is present in solution as a multinuclear aggregate. A tetranuclear species incorporating four copper(I) centres and two ligand molecules was observed in RT ESI-MS experiments, in both positive and negative modes. Furthermore, the difference in diffusion coefficient between [CuI2L1]OTf and HL1 indicate that the molecular volume approximately doubles upon complexation with
copper(I). Although a dimeric tetranuclear species is also plausible,133 pyrazole bridging ligands are well known for forming planar trimeric assemblies with copper(I).127,131 In fact, a multinuclear aggregate containing such a cyclic triangular core was previously isolated and crystallographically characterised upon reaction of CuBr with a binucleating ligand closely related to HL1 (Scheme 3.7, XII).134 Furthermore, solution analysis of this species in MeCN revealed similar MS adducts to those observed for [CuI2L1]OTf, and analogous broadening in the corresponding 1H NMR spectrum. Taken together, the above considerations suggest that [CuI2L1]X may be present in solution as a trimeric hexanuclear species (Scheme 3.7, ([CuI2L1]X)3).
Scheme 3.7: Solid state structure of XII and proposed trimeric assembly of [CuI2L1] to give ([CuI2L1]X)3 in solution. Only one of the possible relative configurations of the ([CuI2L1]X)3 side-arms is depicted. Charges and
counterions are omitted (XII is a neutral complex).
The exact nature of the dynamics displayed by this system are still unclear. It is worth noting that the nuclearity of pyrazole supported copper(I) metallocycles can vary depending on the experimental conditions used for synthesis.127 Furthermore, equilibrium mixtures of trimeric and tetrameric species which interconvert have been observed in solution,131,135 which could account for the observed dynamics. An additional process involving the terminal nitrogen donors might also contribute. While the cyclic pyrazole coordination motif is generally planar in these trimeric assemblies,127 the tertiary amine groups in ([CuI2L1]X)3
likely coordinate from above or below the central plane (Scheme 3.7). Different isomers thus result depending on the relative arrangements of the three distal copper(I) centres, and interconversion between these species is conceivable. Solvent molecules could also additionally coordinate and compete with the tertiary amine donors of the ligand. Such a process is well documented in related copper(I) systems supported by multidentate nitrogen-donor ligands, and is discussed in more detail below (Section 3.3.3). Further studies are needed to fully elucidate the fluxional behaviour observed in this system, and to
corroborate the trimeric assignment of ([CuI2L1]X)3. The speciation and dynamics likely depend on both solvent and temperature. Employing a less-coordinating solvent in place of MeCN may thus offer insights with respect to its possible involvement. The use of alternative solvents could also allow for NMR studies to be conducted at lower temperatures, which may lead to resolution of signal sets for contributing species.