Synthesis, Structure, and Electrochemical Properties of Copper(I) Complexes with S/N Homoscorpionate and Heteroscorpionate Ligands

Dinuclear Cu(I) complexes with bifunctionalized homoscorpionate ligands, hydrotris(thioxotriazolyl)borato [Li(TrMe,o-Py) (1) and Li(TrMes,Me) (2)], and the heteroscorpionate ligand hydro[bis(thioxotriazolyl)-3-(2-pyridyl)pyrazolyl]borato [K(BrMespzo-Py)] (3) were synthesized and crystallographically characterized. The complexes [Cu(TrMes,Me)]2 (4) and [Cu(TrMe,o-Py)]2 (5) exhibit a similar coordination geometry where every metal is surrounded by three thioxo groups in a trigonal arrangement. The presence of a [B-H···Cu] three-center-two-electron interaction in both compounds causes the overall coordination to become tetrahedrally distorted (S3H coordination for each metal). The complex [Cu(BrMespzo-Py)]2 (6) presents a trigonal geometry in which the metals interact with two thioxo groups and a bridging pyrazolyl nitrogen atom. A weak contact with a pyridine nitrogen atom completes the coordination of the metals (S2N,Nʹ coordination for each metal). [Cu(TrMes,Me)]2, [Cu(TrMe,o-Py)]2, and [Cu(BrMespzo-Py)]2 exhibit fluxional behavior in solution as evidenced by variable-temperature NMR spectroscopy, and for 5 and 6 two species in equilibrium [in the ratio 2/1 for 5 (CDCl3) and 3/2 for 6 (CD2Cl2)] are distinguishable in the 1H NMR spectra at 270 K. 2D-NOESY spectra recorded at 270 K assisted in the attribution of solution molecular geometries for each isomer of 5 and 6. The free energy of activation ((delta)G(double dagger)Tc) was determined for both equilibria from the evaluation of the coalescence temperature. DFT calculations were performed to describe plausible molecular geometry for the minor isomer of 5 and 6 and to propose a possible mechanism of interconversion between major and minor isomers. Cyclic voltammograms were recorded in CH2Cl2 (3 and 6) or CH2Cl2/CH3CN (1/1, v/v) (2, 4, and 5) solutions using 0.1 M TBAHFP or TBAOTf as supporting electrolytes. [Cu(TrMes,Me)]2, [Cu(TrMe,o-Py)]2, and [Cu(BrMespzo-Py)]2 exhibit a quasi-reversible Cu(I)/Cu(II) redox behavior with Epa = +719 mV and Epc = +538 mV for 4, Epa = +636 mV and Epc = -316 mV for 5, and Epa = +418 mV and Epc = -319 mV for 6.