SYNTHESIS AND STRUCTURAL AND SPECTROSCOPIC CHARACTERIZATION OF MONONUCLEAR COPPER NITROSYL COMPLEXES - MODELS FOR NITRIC-OXIDE ADDUCTS OF COPPER PROTEINS AND COPPER-EXCHANGED ZEOLITES

We report the synthesis and characterization of the first examples of well-characterized mononuclear copper nitrosyl complexes, TpRR'CuNO (T p(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 e lectrons) compounds model a possible intermediate in nitrite reduction by the copper nitrite reductase from Achromobacter cycloclastes and p rovide chemical precedent for NO coordination to isolated copper sites in proteins and in zeolites. Compounds 1 and 2 were synthesized by tr eating the copper(I) complexes [Tp(t-Bu)Cu]2 or Tp(RR')CuL (R = tert-b utyl, R' = H, L = CH3CN; R = R' = Ph, L = CH3CN; R = R' = Ph, L = 3,5- diphenylpyrazole) with NO (1 atm) in organic solvent. Manometry measur ements showed that NO binding is weak, reversible, and temperature dep endent, 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 yie ld the respective Cu(I) adducts. Reaction of 1 with O2 afforded the kn own complex Tpt-BuCu-(No3). The geometry of 1 was shown by X-ray cryst allography to be approximately C3v-distorted tetrahedral, with bond di stances 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 twinni ng). Distinctive spectroscopic features of the nitrosyls 1 and 2 inclu de (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 f eatures or electronic absorptions above 600 nm; (iv) strong features i n 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 wer e 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 simila r to that of 1, but which can be described unambiguously as Cu(II) com plexes. Significant spectroscopic features of 3-6 include d --> d tran sitions observable at low energies (>900 nm) in their electronic absor ption 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 investi gations suggests that the nitrosyl complexes are best described by a m olecular orbital picture in which the orbitals with mostly copper d ch aracter are fully populated and the unpaired electron resides in a pri marily pi(NO) orbital. This qualitative bonding description was suppo rted by all-electron ab initio Hartree-Fock and post-Hartree-Fock calc ulations carried out for a simple analog, the Cu(NH3)3NO radical catio n, with the heavy atoms constrained to the geometry found from the cry stal structure of 1. Spectroscopic features the synthetic compounds sh are with purported Cu-NO adducts in both copper proteins and copper-ex changed 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.