title: Structural and spectroscopic characterization of mononuclear copper(I) nitrosyl complexes: end-on versus side-on coordination of NO to copper(I).
abstract: 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 nu(N-O) at 1698 cm(-1) and nu(Cu-NO) split at 365/338 cm(-1) for 1, which translates to force constants of 12.53 (N-O) and 1.31 mdyn/A (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 nu(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 parallel(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 (approximately 135 degrees) but likely not side-on.
authors.affiliation: Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8571, Japan. kiyoshif@chem.tsukuba.ac.jp
authors.forename: Kiyoshi
authors.initials: K
authors.lastname: Fujisawa
authors.forename: Akira
authors.initials: A
authors.lastname: Tateda
authors.forename: Yoshitaro
authors.initials: Y
authors.lastname: Miyashita
authors.forename: Ken-ichi
authors.initials: K
authors.lastname: Okamoto
authors.forename: Florian
authors.initials: F
authors.lastname: Paulat
authors.forename: V K K
authors.initials: VK
authors.lastname: Praneeth
authors.forename: Anna
authors.initials: A
authors.lastname: Merkle
authors.forename: Nicolai
authors.initials: N
authors.lastname: Lehnert
mesh_terms: D001671:Biochemistry / Q000379:methods*; D002412:Cations; D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D004578:Electron Spin Resonance Spectroscopy; D004581:Electronics; D008024:Ligands; D008956:Models, Chemical; D008968:Molecular Conformation; D015394:Molecular Structure; D009569:Nitric Oxide / Q000737:chemistry*; D009572:Nitrite Reductases / Q000737:chemistry*; D013053:Spectrophotometry / Q000379:methods*; D013056:Spectrophotometry, Ultraviolet / Q000379:methods
chemical_list: D002412:Cations; D008024:Ligands; D009569:Nitric Oxide; D003300:Copper; D009572:Nitrite Reductases