Molecular Structures, Electronic Spectra, and ESR Spectra of Bis(4,4’,5,5′-tetramethyl-2,2′-biimidazole)copper(II) Dinitrate and Bis(4,4’,5,5′-tetramethyl-2,2′-biimidazole)zinc(II)_0.90copper(II)_0.10 Dinitrate

The crystal structures of bis(4,4’,5,5′-tetramethyl-2,2′-biimidazole)copper(II) dinitrate (1) and bis(4,4’,5,5′-tetramethyl-2,2′-biimidazole)zinc(II)_0.90Copper(II)_0.10 dinitrate (2) have been determined from single-crystal X-ray data collected by counter methods. Complex 1 (CuC₂₀H₂₈N₁₀O₆) crystallized in space group [formula-omitted] with Z = 2 and a = 7.854 (1) Å, b = 14.303 (2) Å, c = 11.852 (2) Å, α = 78.46 (2)°, β = 94.96 (2)°, and γ = 91.92 (1)°. Complex 2 (Zn_0.9Cu_0.1C₂₀H₂₈N₁₀O₆) also crystallized in [formula-omitted] with Z = 2 and a = 7.812 (1) Å, b = 13.324 (3) Å,c = 13.692 (3) Å,Åα = 72.72 (2)°, β = 107.00 (1)°, and γ = 96.77 (1)°. Structure 1 consists of discrete [Cu(Me₄BIM)₂ONO₂]⁺ cations separated by lattice [formula-omitted] groups. The anisobidentate NO3- ligand exhibits a normal Cu-O bond (2.180 (7) Å) as well as a longer Cu-O semibond (2.569 (8) Å). If the semibond is ignored, the distorted coordination geometry lies closer to the idealized square pyramid than to the idealized trigonal bipyramid. The coordination geometry is completed by three additional equatorial bonds to nitrogen donors (1.987 (5), 2.012 (5), and 2.028 (5) Å) and one apical Cu-N bond (2.210 (6) Å). The structure of 2 consists of discrete [M(Me₄BIM)₂ONO₂]⁺ cations (M = Zn, Cu) separated by [formula-omitted] groups in a lattice not isostructural to that exhibited by 1; no indications of disorder due to the Cu(II) dopant were detected. The Zn(II) coordination mainly differs from that of 1 in that no Zn-ligand bond is obviously apical. Electronic spectral and ESR spectral studies were performed on these complexes as well as those of the parent BIM ligand. Energies of the BIM and Me₄BIM molecular orbitals have been calculated by an INDO/S method. Of particular spectroscopic interest are the ligand orbitals of π₁ - π₁ and π₁ + π₁, character, where π₁ is the HOMO supplied by each imidazole fragment. Both (π¹ - π₁)→ Cu(II) and (π₁ + π₁) Cu(II) ligand to metal charge-transfer (LMCT) absorptions are exhibited by these complexes in the spectral region between the ligand field absorptions and the near-UV absorption edge resulting from ligand π →π^* transitions. These spectroscopic studies point to the unusual result that the Cu(II) chromophore present in 1 persists relatively unchanged in CH₃OH solution and in the lattice of the Zn(II) analogue, which exhibits a somewhat different coordination geometry.