SYNTHESIS AND COMPARATIVE CHEMISTRY OF THE EARLY LATE TRANSITION-METAL HETEROBIMETALLACYCLES CPCP'TA(CH2)2IR(CO)(L) AND THEIR MAIN-GROUP ELEMENT LATE TRANSITION-METAL ANALOGS R2P(CH2)2IR(CO)(L)

The synthesis of two tantalum-iridium heterodinuclear complexes formed by the reaction of CpCp'Ta(CH2)(CH3) with IndIr(CO)2 is presented (Cp' = Cp (1a); indenyl (1b)). An X-ray diffraction study of the compound (Ind)CpTa(CH2)2Ir(CO)2 was performed: the Ta(CH2)2Ir ring is planar and the Ta-Ir bond length is 2.858(1) angstrom. These compounds react with phosphines to form substitution and addition products. The crystal structure of the PEt3 adduct Cp2Ta(CH2)2Ir(CO)2(PEt3) (2) is presented; the geometry about the iridium center of 2 is midway between a square-based pyramid and a trigonal bipyramid, and the Ta-Ir bond length is 2.881(1) angstrom. Compound 1a reacts with MeI to form the six-coordinate iridium oxidative addition product CP2Ta(CH2)2Ir(CO)2(CH3)I. The compound CP2Ta(CH2)2Ir(CO)(PPh3) (3) reacts with MeI in an analogous manner (k2nd = 7.61 +/- 0.43 M-1 s-1, THF, -5-degrees-C). In order to assess the effect of the early metal on the chemistry at the late transition metal center, the chelating phosphorus ylide analogues R2P(CH2)2Ir(CO)(PPh3) (R = Ph (6a), Me (6b)) were synthesized by treatment of the ylide anion, R2P(CH2)2Li with Vaska's complex (PPh3)2Ir(CO)Cl. An X-ray diffraction study performed on 6a showed that the Ir(CH2)2P metallacycle, in contrast to that of the Ta-Ir compounds, is non-planar. The ylide P-Ir distance (2.746(1) angstrom) is beyond bonding interaction. Compounds 6a (k2nd = 7.55 +/- 0.30 M-1 s-1, THF, -5-degrees-C) and 6b (k2nd = 17.9 +/-1.4 M-1 s-1, THF, -5-degrees-C) react with CH3I in a fashion analogous to that of the Ta-Ir complexes. Unlike compound 3, both 6a and 6b are highly fluxional in solution due to the dissociation of the PPh3 ligand from the Ir center. In addition, the metallacycle undergoes rapid ring inversion in solution, even at -90-degrees-C. These studies suggest the early metal fragment can act as an electron sink and source for the late transition metal fragment through a direct Ta-Ir bond interaction.