Structural and Spectroscopic Characterization of Mononuclear Copper(I) Nitrosyl Complexes: End-on versus Side-on Coordination of NO to Copper(I)

Two crystal structures of the mononuclear copper(I)−nitrosyl complexes [Cu(L3)(NO)] (1) and
[Cu(L3‘)(NO)](ClO4) (2) with the related coligands L3- (hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate)
and L3‘ (tris(3-tert-butyl-5-isopropyl-1-pyrazolyl)methane) are presented. These compounds are then
investigated in detail using a variety of spectroscopic methods. Vibrational spectra show ν(N−O) at 1698
cm-1 and ν(Cu−NO) split at 365/338 cm-1 for 1, which translates to force constants of 12.53 (N−O) and
1.31 mdyn/Å (Cu−NO), respectively. The weak Cu−NO force constant is in agreement with the observed
instability of the Cu−NO bond. Interestingly, complex 2 with the neutral coligand L3‘ shows a stronger
N−O bond, evident from ν(N−O) at 1742 cm-1. This difference is attributed to a true second coordination
sphere effect, where the covalency of the Cu(I)−NO bond is not altered. The EPR spectrum of 1 is in
agreement with the Cu(I)−NO(radical) electronic structure of the complexes, as obtained from density
functional theory (DFT) calculations. In addition, an interesting trend between g∥(gz) and the Cu−N−O
angle is established. Finally, high-quality MCD spectra of 1 are presented and assigned using TD-DFT
calculations. Based on the in-depth spectroscopic characterization of end-on bound NO to copper(I)
presented in this work, it is possible to determine the binding mode of the Cu−NO intermediate of Cu
nitrite reductase studied by Scholes and co-workers (Usov, O. M.; Sun, Y.; Grigoryants, V. M.; Shapleigh,
J. P.; Scholes, C. P., J. Am. Chem. Soc. 2006, 128, 13102−13111) in solution as strongly bent (∼135°)
but likely not side-on.