We report the synthesis and characterization of the first examples of well-characterized mononuclear copper nitrosyl complexes, TpRR'CuNO (Tp(RR') = tris(3-R,5-R'-pyrazolyl)hydroborate: 1, R = t-Bu, R' = H; 2, R = R' = Ph). These novel {CuNO}11 (10 metal d + 1 NO pi* = 11 total electrons) compounds model a possible intermediate in nitrite reduction by the copper nitrite reductase from Achromobacter cycloclastes and provide chemical precedent for NO coordination to isolated copper sites in proteins and in zeolites. Compounds 1 and 2 were synthesized by treating the copper(I) complexes [Tp(t-Bu)Cu]2 or Tp(RR')CuL (R = tert-butyl, R' = H, L = CH3CN; R = R' = Ph, L = CH3CN; R = R' = Ph, L = 3,5-diphenylpyrazole) with NO (1 atm) in organic solvent. Manometry measurements showed that NO binding is weak, reversible, and temperature dependent, with only 38(3)% of the available copper in solutions of [Tp(t-Bu)Cu]2 bound to NO at 23-degrees-C. Irreversible displacement of the nitrosyl ligand was effected by addition of excess CH3CN or CO to yield the respective Cu(I) adducts. Reaction of 1 with O2 afforded the known complex Tpt-BuCu-(No3). The geometry of 1 was shown by X-ray crystallography to be approximately C3v-distorted tetrahedral, with bond distances Cu-N = 1.759(6) angstrom and N-O = 1.108(7) angstrom and bond angle Cu-N-O = 163.4(6)degrees (crystal data: monoclinic, space group P2(1)/n (No. 14) at -101-degrees-C, a = 10.28(1) angstrom, b = 17.40(2) angstrom, c = 16.12(1) angstrom, beta = 90.0(1)degrees, V = 2882(8) angstrom3, Z = 4, R = 0.052 and R(w) = 0.065 for 2866 reflections with I > 3sigma(I) and 293 variable parameters after correction for twinning). Distinctive spectroscopic features of the nitrosyls 1 and 2 include (i) nu((NO)-N-14, (NO)-N-15 = 1712,1679 cm-1 (1) and nu((NO)-N-14, (NO)-N-15 = 1720,1687 cm-1 (2); (ii) MLCT bands at 494 nm (1) and 478 nm (2) with epsilon approximately 1400 M-1 cm-1; (iii) a lack of MCD features or electronic absorptions above 600 nm; (iv) strong features in their MCD spectra corresponding to the MLCT lambda(max) values; (v) EPR spectra with g(e) is similar to g perpendicular-to > g parallel-to = 1.83 and large nitrogen (A perpendicular-to (NO)-N-14 = 27 X 10(-4) cm1) and copper hyperfine splitting. These spectroscopic features were shown to contrast with those of the series Tpt-BuCuX (3, X = Cl-; 4, X = Br-; 5, X = CF3SO3-; 6, X = N3-) that have geometries very similar to that of 1, but which can be described unambiguously as Cu(II) complexes. Significant spectroscopic features of 3-6 include d --> d transitions observable at low energies (>900 nm) in their electronic absorption and MCD spectra, LMCT bands between 300-600 nm, and rhombic EPR signals with all g values >2.0 and large copper hyperfine in the high-field component consistent with ground states for the complexes having substantial d(z)2 character. The combined evidence from these investigations suggests that the nitrosyl complexes are best described by a molecular orbital picture in which the orbitals with mostly copper d character are fully populated and the unpaired electron resides in a primarily pi*(NO) orbital. This qualitative bonding description was supported by all-electron ab initio Hartree-Fock and post-Hartree-Fock calculations carried out for a simple analog, the Cu(NH3)3NO radical cation, with the heavy atoms constrained to the geometry found from the crystal structure of 1. Spectroscopic features the synthetic compounds share with purported Cu-NO adducts in both copper proteins and copper-exchanged zeolites are discussed, as is the possible relevance of these compounds to proposed intermediates in the reduction of nitrite ion by copper-containing nitrite reductase.
