This contribution presents a detailed comparison of solid-state and solution structural properties for bis[(hydrotris(1-pyrazolyl)borato)copper(I)] ([HBpz3Cu]2) and bis[(hydrotris(3,5-dimethyl-1-pyrazolyl)borato)copper(I)] ([HB(3,5-Me2pz)3Cu]2). The former compound crystallizes in the triclinic space group P1, with one dimeric molecule in a unit cell of dimensions a = 9.264 (1) Å, b = 7.638 (1) Å, and c = 8.181 (1) Å, α = 92.96 (1)°, β = 101.71 (1)°, and γ = 89.40 (1)°. Full-matrix least-squares refinement gave a final value of the conventional R factor (on F) of 0.037 for 2111 reflections having F2 > 3σ(F2). The structure consists of discrete centrosymmetric dimeric molecules, with each HBpz3 ligand presenting a terminally bonded pyrazole ligand to each of the two copper atoms in the dimer (Cu-N(av) = 1.947 (2) Å), and a symmetrically bridging pyrazole to both copper atoms (Cu-N(av) = 2.239 (2) Å). The copper-copper distance is 2.660 (1) Å. The complex [HB(3,5-Me2pz)3Cu]2 crystallizes in the triclinic space group P1 with one molecule in a cell of dimensions a = 10.245 (4) Å, b = 10.694 (3) Å, c = 10.034 (3) Å, α = 115.94 (1)°, β = 112.13 (1)°, and γ = 94.34 (1)°. Full-matrix least-squares refinement gave a final conventional R factor (on F) of 0.047 for 2786 unique reflections with F2 > 3σ(F2). The structure consists of centrosymmetric dimeric molecules with each HB(3,5-Me2pz)3 ligand presenting two terminally bonded pyrazole units to one copper atom, and one to the other copper atom of the dimer (Cu-N(av) = 1.986 (3) Å). Any pyrazole bridging interaction is minimal. The copper-copper distance is 2.506 (1) Å. In both [HBpz3Cu]2 and in [HB(3,5-Me2pz)3Cu]2, the major distortion of the ligand from C3v local symmetry involves twisting about the B-N bonds. In solution, [HBpz3Cu]2 is dimeric. All pyrazole rings are equivalent in the 1HNMR spectrum down to -130°. The methylated analogue is extensively dissociated in solution. These results imply that pairs of Cu(I) ions in protein polyimidazole binding sites may exhibit high conformational mobility and considerable variation in coordination geometry. Short copper-copper contacts and bridging by imidazole ligands are also conceivable.
|Number of pages||8|
|Journal||Journal of the American Chemical Society|
|Publication status||Published - 1976|
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