IrII(ethene):  Metal or Carbon Radical?

One-electron oxidation of [(Me n tpa)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 [(Me n tpa)IrII(ethene)]2+ (1 2+: n = 3; 2 2+: 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, 1 2+ and 2 2+ behave as moderately reactive metallo-radicals. Radical coupling of 1 2+ with NO in acetone occurs at the metal, resulting in dissociation of ethene and formation of the stable nitrosyl complex [(Me3tpa)Ir(NO)]2+ (6 2+). In the coordinating solvent MeCN, 1 2+ generates more reactive radicals; [(Me3tpa)Ir(MeCN)(ethene)]2+ (9 2+) by MeCN coordination, and [(Me3tpa)IrII(MeCN)]2+ (10 2+) by substitution of MeCN for ethene. Complex 10 2+ is a metallo-radical, like 1 2+ but more reactive. DFT calculations indicate that 9 2+ is intermediate between the slipped-olefin IrII(CH2CH2) and ethyl radical IrIII−CH2−CH2· resonance structures, of which the latter prevails. The ethyl radical character of 9 2+ allows radical type addition reactions at the ethene ligand. Complex 2 2+ behaves similarly in MeCN. In the absence of further reagents, 1 2+ and 2 2+ convert to the ethylene bridged species [(Me n tpa)(MeCN)IrIII(μ2-C2H4)IrIII(MeCN)(Me3tpa)]4+ (n = 3: 3 4+; n = 2: 4 4+) in MeCN. In the presence of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxo), formation of 3 4+ from 1 2+ in MeCN is completely suppressed and only [(Me3tpa)IrIII(TEMPO-)(MeCN)]2+ (7 2+) is formed. This is thought to proceed via radical coupling of TEMPO at the metal center of 10 2+. In the presence of water, hydrolysis of the coordinated acetonitrile fragment of 7 2+ results in the acetamido complex [(Me3tpa)IrIII(NHC(O)CH3))(TEMPOH)]2+ (8 2+).