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,N‘ 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 (ΔG⧧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.