Crystal Structure of the Quasitetrahedral Iridium(I) Complex, Ir(COCH2CMe3)2. An Intermediate in Cyclotrimerization of Activated Alkynes by 16-Electron Alkyl Complexes of Iridium, trans-RIr(CO)L2 (R=Me, CH2CMe3; L=PPh3, P(p-tolyl)3)

Article abstract of DOI:10.1021/ja00251a017

Complexes trans-RIr(CO)L2 (R=Me, CH2CMe3; L=PPh3 or P(p-tolyl)3) are cyclotrimerization catalysts for activated acetylenes.Stoichiometric reaction between C2(CO2Me)2 and trans-Me3CCH2Ir(CO)2 results in the formation of Ir(COCH2CMe3)2,an unusual tetrahedral, 16-electron, iridium(I) complex.An X-ray diffraction study confirms the geometry.The complex Ir(COCH2CMe3)2 crystallizes in the centrosymmetric monoclinic space group P2₁/c with a=11.125 (2) Angstroem, b=21.725 (6) Angstroem, c=20.889 (5) Angstroem, β=93.465 (16)deg, V=5039 (2) Angstroem³, and Z=4.Single-crystal X-ray difraction data (Mo Kα, 2θ=5-45deg) were collected and the structure solved and refined to R_F=6.2percent for those 5196 data with (F₀) >3?((F₀)) F=5.0percent for 4433 data with (F₀ >6?((F₀))>.If we regard the η²-C2(CO2Me)2 ligand as occupying a single stereochemical site (as is usually the case) the molecule is a tetrahedral complex of Ir(I) in which Ir-P=2.334 (3) and 2.314 (3) Angstroem, the Ir-C(acyl) bond length is 1.951 (13) Angstroem, and Ir...cent (center of acetylenic linkage) =1.941 Angstroem.Interligand angles are P(1)-Ir-P(2)=105.20 (10)deg, P(1)-Ir-C(acyl)=94.20 (36)deg, P(1)-Ir-cent =122.80deg, P(2)-Ir-C(acyl)=91.86 (36)deg, P(2)-Ir-cent =123.28deg, and C(acyl)-Ir-cent =111.15deg.The mechanism of the cyclotrimerization reaction involves coordination of an acetylene molecule, an alkyl migration, coordination of a second acetylene molecule, metallacyclopentadiene formation, and cycloaddition of a third acetylene molecule leading to elimination of the derived benzenoid cyclotrimer.