Ir-II(ethene): Metal or carbon radical? .
D.G.H. Hetterscheid, J. Kaiser, E. Reijerse, T.P.J. Peters, S. Thewissen, A.N.J. Blok, J.M.M. Smits, R. de Gelder, B. de Bruin
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY 2005, 127, 6, 1895-1905

Abstract:
One-electron oxidation of [(Me(n)tpa)lr(1)(ethene)](+) complexes (Me(3)tpa = N,N,N-tri(6-methyl-2-pyridylmethyl)amine; Me(2)tpa = N-(2-pyridyl m ethyl) -N,N,-di[(6-methyl-2-pyridyl)methyl]-amine) results in relatively stable, five-coordinate Ir-parallel to-olefin species [(Me(n)tpa)|r(parallel to)(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 [(Me(3)tpa)|r(NO)](2+) (6(2divided by)). In the coordinating solvent MeCN, 1(2+) generates more reactive radicals; [(Me(3)tpa)|r(MeCN)(ethene)}(2+) (9(2divided by)) by MeCN coordination, and [(Me(3)tpa)|r(parallel to)(MeCN)](2+) (10(2+)) by substitution of MeCN for ethene. Complex 10(2divided by) is a metallo-radical, like 1(2+) but more reactive. DFT calculations indicate that 9(2+) is intermediate between the slipped-olefin Ir-parallel to(CH2 = CH2) and ethyl radical Ir-parallel to|-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 22. 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)Ir-lll(mu(2)-C2H4)|r(lll)(MeCN)(Me(3)tpa)](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 [(Me(3)tpa)|r(lll)(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 [(Me(3)tpa)|r(lll)(NHC(O)CH3))-(TEMPOH](2+) (8(2+)).