IrII(ethene): Metal or Carbon Radical?
2005-02-16T00:00:00Z (GMT) by
One-electron oxidation of [(Mentpa)IrI(ethene)]+ complexes (Me3tpa = N,N,N-tri(6-methyl-2-pyridylmethyl)amine; Me2tpa = N-(2-pyridylmethyl)-N,N,-di[(6-methyl-2-pyridyl)methyl]-amine) results in relatively stable, five-coordinate IrII−olefin species [(Mentpa)IrII(ethene)]2+ (12+: n = 3; 22+: n = 2). These contain a “vacant site” at iridium and a “non-innocent” ethene fragment, allowing radical type addition reactions at both the metal and the ethene ligand. The balance between metal- and ligand-centered radical behavior is influenced by the donor capacity of the solvent. In weakly coordinating solvents, 12+ and 22+ behave as moderately reactive metallo-radicals. Radical coupling of 12+ with NO in acetone occurs at the metal, resulting in dissociation of ethene and formation of the stable nitrosyl complex [(Me3tpa)Ir(NO)]2+ (62+). In the coordinating solvent MeCN, 12+ generates more reactive radicals; [(Me3tpa)Ir(MeCN)(ethene)]2+ (92+) by MeCN coordination, and [(Me3tpa)IrII(MeCN)]2+ (102+) by substitution of MeCN for ethene. Complex 102+ is a metallo-radical, like 12+ but more reactive. DFT calculations indicate that 92+ is intermediate between the slipped-olefin IrII(CH2CH2) and ethyl radical IrIII−CH2−CH2· resonance structures, of which the latter prevails. The ethyl radical character of 92+ allows radical type addition reactions at the ethene ligand. Complex 22+ behaves similarly in MeCN. In the absence of further reagents, 12+ and 22+ convert to the ethylene bridged species [(Mentpa)(MeCN)IrIII(μ2-C2H4)IrIII(MeCN)(Me3tpa)]4+ (n = 3: 34+; n = 2: 44+) in MeCN. In the presence of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxo), formation of 34+ from 12+ in MeCN is completely suppressed and only [(Me3tpa)IrIII(TEMPO-)(MeCN)]2+ (72+) is formed. This is thought to proceed via radical coupling of TEMPO at the metal center of 102+. In the presence of water, hydrolysis of the coordinated acetonitrile fragment of 72+ results in the acetamido complex [(Me3tpa)IrIII(NHC(O)CH3))(TEMPOH)]2+ (82+).