Crystal Structure of the Quasitetrahedral Iridium(i) Complex, Ir(coch₂cme₃)[P(p-tolyl)₃]₂[C₂(CO₂Me)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)

Complexes trans-RIr(CO)L₂(R = Me, CH₂CMe₃; L = PPh₃or P(p-tolyl)₃) are cyclotrimerization catalysts for activated acetylenes. Stoichiometric reaction between C₂(C0₂Me)₂and trans-Me₃CCH₂Ir(CO)[P-tolyl)₃]₂results in the formation of Ir(COCH₂CMe₃)[P(p-tolyl)₃]₂[C₂(C0₂Me)₂], an unusual tetrahedral, 16-electron, iridium(I) complex. An X-ray diffraction study confirms the geometry. the complex Ir(COCH₂CMe₃)[P(p-tolyl)₃]₂[C₂(C0₂Me)₂] crystallizes in the centrosymmetric monoclinic space group P2₁/c with a = 11.125 (2) Å, b = 21.725 (6) Å,c = 20.889 (5) Å, β= 93.465 (16)°, V = 5039 (2) ų, and Z = 4. Single-crystal X-ray diffraction data (Mo Ka, 26 = 5-45°) were collected and the structure solved and refined to R_F- 6.2% for those 5196 data with |F°3σ|F°| > 3<r(|F°|) [R_F= 5.0% for 4433 data with| F°| > 6(r(|F°|)]. I1 f we regard the T/²-C₂(C0₂Me)₂ligand 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) Å, the Ir-C(acyl) bond length is 1.951 (13) Å, and Ir…cent (center of acetylenic linkage) = 1.941 Å. Interligand angles are P(1)—Ir—P(2) = 105.20 (10)°, P(l)-Ir-C(acyl) = 94.20 (36)°, P(l)-Ir-cent = 122.80°, P(2)-Ir-C(acyl) = 91.86 (36)°, P(2)-Ir-cent = 123.28°, and C(acyl)-Ir-cent = 111.15°. 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.