Reduction of an η²-iminoacyl ligand to η²- iminium enabled by adjacent carbon monoxide ligand replacement with a variable electron donor alkyne ligand in a cationic tungsten(II) bis(acetylacetonate) complex

Cationic iminoacyl-carbonyl tungsten complexes of the type [W(CO) (η²-MeN=CR)(acac)₂]⁺ (acac = acetylacetonate; R = Ph (1a), Me (1b)) easily undergo thermal substitution of CO with two-electron donors to yield [W(L)(η²MeN=CR)(acac) ₂]⁺ (L = tert-butylisonitrile [R = Ph (2a), Me (2b)], 2,6-dimethylphenylisonitrile [R = Me (2c)], triphenylphosphine [R = Ph (3a), Me (3c)], and tricyclohexylphosphine [R = Ph (3b)]). Tricyclohexylphosphine complex 3b exhibits rapid, reversible phosphine ligand exchange at room temperature on the NMR time scale. Photolytic replacement of carbon monoxide with either phenylacetylene or 2-butyne occurs efficiently to form [W(η ²alkyne)(η²-MeN=CR)(acac)₂]⁺ complexes (5a-d) with a variable electron donor η²-alkyne paired with the η²-iminoacyl ligand in the W(II) coordination sphere. PMe₃ adds to 1a or 5b to form [W(L)(η²-MeN=C(PMe ₃)Ph)(acac)₂]⁺ [L = CO (4), MeC≡CMe (6)] via nucleophilic attack at the iminoacyl carbon. Addition of Na[HB(OMe) ₃] to 5b yields W(η²-MeC≡CMe)(η²- MeN=CHPh)(acac)₂, 8, which exhibits alkyne rotation on the NMR time scale. Addition of MeOTf to 8 places a second methyl group on the nitrogen atom to form an unusual cationic η²-iminium complex [W(η²-MeC≡CMe)(η²-Me₂N=CHPh) (acac)₂][OTf] (9[OTf], OTf = SO₃CF₃). X-ray structures of 2,6-dimethylphenylisonitrile complex 2c[BAr′₄] tricyclohexylphosphine complex 3b[BAr′₄], and phenylacetylene complex 5a[BAr′₄] confirm replacement of CO by these ligands in the [W(L)(η²-MeN=CR)(acac)₂]⁺ products. X-ray structures of alkyne-imine complexes 6[BAr′₄] and 8 show products resulting from nucleophilic addition at the iminoacyl carbon, and the X-ray structure of 9[BAr′₄] reflects methylation at the imine nitrogen to form a rare η²-iminium ligand.