Ir<sup>II</sup>(ethene):  Metal or Carbon Radical?

One-electron oxidation of [(Me<i><sub>n</sub></i>tpa)Ir<sup>I</sup>(ethene)]<sup>+</sup> complexes (Me<sub>3</sub>tpa = <i>N,N,N</i>-tri(6-methyl-2-pyridylmethyl)amine; Me<sub>2</sub>tpa = <i>N</i>-(2-pyridylmethyl)-<i>N,N,</i>-di[(6-methyl-2-pyridyl)methyl]-amine) results in relatively stable, five-coordinate Ir<sup>II</sup>−olefin species [(Me<i><sub>n</sub></i>tpa)Ir<sup>II</sup>(ethene)]<sup>2+</sup> (<b>1</b><sup>2+</sup>:  <i>n </i>= 3; <b>2</b><sup>2+</sup>:  <i>n </i>= 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, <b>1</b><sup>2+</sup> and <b>2</b><sup>2+</sup> behave as moderately reactive metallo-radicals. Radical coupling of <b>1</b><sup>2+</sup> with NO in acetone occurs <i>at the metal</i>, resulting in dissociation of ethene and formation of the stable nitrosyl complex [(Me<sub>3</sub>tpa)Ir(NO)]<sup>2+</sup> (<b>6</b><sup>2+</sup>). In the coordinating solvent MeCN, <b>1</b><sup>2+</sup> generates more reactive radicals; [(Me<sub>3</sub>tpa)Ir(MeCN)(ethene)]<sup>2+</sup> (<b>9</b><sup>2+</sup>) by MeCN coordination, and [(Me<sub>3</sub>tpa)Ir<sup>II</sup>(MeCN)]<sup>2+</sup> (<b>10</b><sup>2+</sup>) by substitution of MeCN for ethene. Complex <b>10</b><sup>2+</sup> is a metallo-radical, like <b>1</b><sup>2+</sup> but more reactive. DFT calculations indicate that <b>9</b><sup>2+</sup> is intermediate between the slipped-olefin Ir<sup>II</sup>(CH<sub>2</sub>CH<sub>2</sub>) and ethyl radical Ir<sup>III</sup>−CH<sub>2</sub>−CH<sub>2</sub>· resonance structures, of which the latter prevails. The ethyl radical character of <b>9</b><sup>2+</sup> allows radical type addition reactions <i>at the ethene ligand</i>. Complex <b>2</b><sup>2+</sup> behaves similarly in MeCN. In the absence of further reagents, <b>1</b><sup>2+</sup> and <b>2</b><sup>2+</sup> convert to the ethylene bridged species [(Me<i><sub>n</sub></i>tpa)(MeCN)Ir<sup>III</sup>(μ<sub>2</sub>-C<sub>2</sub>H<sub>4</sub>)Ir<sup>III</sup>(MeCN)(Me<sub>3</sub>tpa)]<sup>4+</sup> (<i>n </i>= 3:  <b>3</b><sup>4+</sup>; <i>n</i> = 2:  <b>4</b><sup>4+</sup>) in MeCN. In the presence of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxo), formation of <b>3</b><sup>4+</sup> from <b>1</b><sup>2+</sup> in MeCN is completely suppressed and only [(Me<sub>3</sub>tpa)Ir<sup>III</sup>(TEMPO<sup>-</sup>)(MeCN)]<sup>2+</sup> (<b>7</b><sup>2+</sup>) is formed. This is thought to proceed via radical coupling of TEMPO <i>at the metal </i><i>center</i> of <b>10</b><sup>2+</sup>. In the presence of water, hydrolysis of the coordinated acetonitrile fragment of <b>7</b><sup>2+</sup> results in the acetamido complex [(Me<sub>3</sub>tpa)Ir<sup>III</sup>(NHC(O)CH<sub>3</sub>))(TEMPO<i>H</i>)]<sup>2+</sup> (<b>8</b><sup>2+</sup>).