Synthetic, structural and electrochemical studies of cationic chromium(VI) and molybdenum(VI) bis(imido) complexes supported by 1,4,7-triazacyclononane macrocycles

Treatment of [Cr(N^tBu)₂Cl₂] or [Mo(N^tBu)₂Cl₂(dme)] with 1,4,7-triazacyclononane (tacn) in THF, followed by metathesis using NaClO₄ in aqueous medium, readily afforded [M(tacn)(N^tBu)₂Cl]ClO₄ (M = Cr 1 or Mo 2); [M(Me₃tacn)(N^tBu)₂Cl]Cl (M = Cr 3 or Mo 4) were similarly prepared using the metal precursors and 1,4,7-trimethyl-1,4,7-triazacyclononane (Me₃tacn). Complexes 1-4 are resistant to hydrolysis, but do not mediate imido group transfer reactions. Distinctive UV-vis absorption bands are tentatively assigned to imido-to-metal LMCT transitions. The crystal structures of 1 and 2·CH₃CN show that the 3 nitrogen atoms of tacn are not equidistant from the respective metal centres. A severely distorted imido moiety (imido angle 151.8(4)°) has been observed in the structure of 1 but not 2·CH₃CN, and is attributed to steric and crystal packing factors rather than electronic effects. Metathesis of both chloride groups in 3 using AgOTf(OTf = SO₃CF₃) yielded [Cr(Me₃tacn)(N^tBu)₂(OTf)]OTf 5. The cyclic voltammograms of 1-5 in acetonitrile consist of a quasi-reversible couple (+0.86 to +1.20 V vs. Cp₂Fe^0/+) and an irreversible wave (-1.04 to -2.08 V vs. Cp₂Fe^0/+); the former couple is assigned to an imido-centred oxidation. © The Royal Society of Chemistry 1999.