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, Tp^RR′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 π^* = 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-BuCu]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-BuCu]₂ bound to NO at 23 °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 Tp^t-BuCu- (NO₃). The geometry of 1 was shown by X-ray crystallography to be approximately C_3v-distorted tetrahedral, with bond distances Cu-N = 1.759(6) Å and N-O = 1.108(7) Å and bond angle Cu-N-O = 163.4(6)° (crystal data: monoclinic, space group P2₁/n (No. 14) at −101 °C, a = 10.28(1) Å,b- 17.40(2) Å,c= 16.12(1) Å,β = 90.0(1)°, V = 2882(8) ų, Z = 4, R - 0.052 and R_w = 0.065 for 2866 reflections with I > 3σ(I) and 293 variable parameters after correction for twinning). Distinctive spectroscopic features of the nitrosyls 1 and 2 include (i) ν(¹⁴NO,¹⁵NO) = 1712, 1679 cm⁻¹ (1) and ν(¹⁴NO,¹⁵NO) = 1720, 1687 cm⁻¹ (2); (ii) MLCT bands at 494 nm (1) and 478 nm (2) with ϵ ~ 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 λ_max values; (v) EPR spectra with g_e ~ g_⊥ > g_║ = 1.83 and large nitrogen (A _⊥¹⁴NO = 27 × 10⁻⁴ cm⁻¹) and copper hyperfine splitting. These spectroscopic features were shown to contrast with those of the series Tp^t-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 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 π*(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.