Ir^II(ethene):  Metal or Carbon Radical?

One-electron oxidation of [(Me_ntpa)Ir^I(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 Ir^II−olefin species [(Me_ntpa)Ir^II(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)Ir^II(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 Ir^II(CH₂CH₂) and ethyl radical Ir^III−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 [(Me_ntpa)(MeCN)Ir^III(μ₂-C₂H₄)Ir^III(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)Ir^III(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)Ir^III(NHC(O)CH₃))(TEMPOH)]²⁺ (8²⁺).