Synthesis, structural characterization and reactivity of trisimidazolyl complexes: Copper, zinc, and cadmium species

A new trisimidazolyl ligand, 1,3,5-triethyl-2,4,6-tris(N-methyl-imidazol-2-yl-thiomethyl)benzene, TriMIm, has been prepared and its coordination chemistry with Cu(I), Cd(II), and Zn(II) has been investigated.; TriMIm reacts with [Cu(NCCH₃)₄]BF₄ forming a 1:1 species, [(TriMIm)Cu]BF₄ (1). The Cu ion is in a trigonal planar CuN₃ environment. Variable temperature 1H NMR spectroscopy of 1 supports the existence of enantiomers with a calculated Gibbs free energy of activation for the interconversion process of ΔG‡ = 13 kcal/mol. 1 did not show reactivity towards small molecules, such as CO, O ₂, NO, and PR₃. As a result, the bis-imidazole analogue, [(BiMIm)Cu(NCCH₃)]BF₄ (3) was prepared in search of such reactivity, due to the anticipated labile solvent site. Thus, 3 was shown to reversibly bind CO by a combination of NMR and React FT-IR spectroscopy at low temperature. Nonetheless, 3 decomposed in the presence of dioxygen and was non-reactive toward NO, but, reactive toward NCS−, N₃−, and PMe₃.; The trigonal pyramidal complexes, [(TriMIm)Cd(Sol)](BF₄) ₂ (Sol = OH₂ (4), NCCH₃ (5 )) were independently prepared. The labile aqua ligand was easily replaced in metathetical reaction of 4 with KX (X = N₃, SCN), resulting in formation of discrete monomeric [(TriMIm)Cd(X)]BF₄ complexes. In contrast, treatment of 4 with KSPh resulted in the isolation of a mono-thiophenolato-bridged dimeric complex, [{lcub}(TriMIm)Cd{rcub} ₂(μ—SPh)](BF₄)₃.; Synthesis of two discrete, monomeric zinc-aqua complexes of general formula [(TriMIm)Zn(OH₂)]Y₂ (Y = BF₄ (10), NO₃ (11)) is reported also. Contrary to the initial expectation, X-ray crystallographic data of 11 shows that nitrate anions do not coordinate to the zinc ion. The Zinc ion of 10 and 11 is in a distorted trigonal monopyramidal environment, composed of three coordinated imidazole nitrogens and a water molecule. Both 10 and 11 serve as structural models for the active site of the zinc enzyme carbonic anhydrase. In addition, the dinuclear complex, [{lcub}(TriMIm)Zn{rcub}₂(μ—OH)](BF₄)₃, 12, has been prepared. The cation [{lcub}(TriMIm)Zn{rcub}₂(μ—OH)] 3+ consists of a Zn-(μ-OH)-Zn unit. It is suggested that the Zn-(μ-OH)-Zn unit is present as a structural motif for zinc enzymes like aminopeptidase and phospholipase C.