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This contribution describes coordinative/insertive stereoregular homopolymerizations and copolymerizations of styrene and methyl methacrylate (MMA) mediated by a highly active single-site organotitanium catalyst. The catalyst system used to effect these polymerizations of nonpolar and polar olefinic monomers is prepared by in situ Zn reduction of the precursor derived from the reaction (Me(5)Cp)TiMe(3) + Ph(3)C(+)B(C(6)F(5))(4)(-). The resulting catalyst produces polystyrene (>95% syndiotactic, 170 000 g/mol molecular weight; s-PS) by the established coordinative/insertive pathway. The same catalyst mediates polymerization of MMA to poly(methyl methacrylate) (>65% syndiotactic, >70 000 g/mol molecular weight; s-PMMA) by a group transfer protocol-like (GTP-like) pathway (1,4 insertion mechanism). Under optimal conditions, this catalyst also mediates the copolymerization of MMA + styrene (1:19 ratio) at 50 degrees C to yield random approximately 80% coisotactic poly[styrene-co-(methyl methacrylate)] (coiso-PSMMA) which contains approximately 4% MMA. Control experiments argue that a single-site Ti catalyst is the active species for the copolymerization. The catalyst formation process is quite general, and a variety of reducing agents can be substituted for Zn and still effect copolymerization. Control experiments also indicate that known noncoordination copolymerization mechanisms (i.e., ionic or radical) cannot explain this copolymerization. We suggest a new mechanism involving sequential conjugate addition steps to explain these copolymerization results.

The enthalpies of reaction of (Cp*RuCl)4 (Cp* = i/5-C5Me5) with two sterically demanding monodentate tertiary phosphine ligands, leading to the formation of Cp*Ru(PR3)Cl (PR3 = P(CeHn)3 and ' 3) complexes, have been measured by anaerobic solution calorimetry in THE at 30 °C. The enthalpies of reaction associated with the rapid and quantitative reaction of the (Cp*RuCl)4 complex allow for a determination of relative ruthenium-phosphorus bond energy terms and, for the first time, enable a direct solution calorimetric measurement of the relative donating properties of large cone-angle phosphine ligands. Reaction of Cp*Ru-(PR3)C1 with excess phosphine ligand, at 30 °C, has been shown to quantitatively yield the corresponding Cp*Ru(PR'3)2Cl complex and allows for the design of thermochemical cycles, assuring the internal consistency of the thermochemical data. The ruthenium-phosphine bond energy in Cp*Ru(PCy3)Cl was found to be 1.4 ± 0.5 kcal/mol more stable than in the Cp*Ru(P* iPrs)Cl complex.

In this paper, we report a synthetic, molecular structure, thermochemical, and ab initio Hartree−Fock/Moller−Plesset level study of bonding and bonding energetics in the group 6 metallocene oxo series Cp2Mo/(MeCp)2MO, M = Cr, Mo, W. Efficient, high-yield syntheses of the pairs Cp2MH2/(MeCp)2MH2, Cp2MCl2/(MeCp)2MCl2, and Cp2MO/(MeCp)2MO where M = Mo or W are reported. The molecular structure of (MeCp)2WO features a “bent sandwich” geometry with a WO distance of 2.04(1) Å and an average WC(Cp) distance of 2.371(8) Å. Thus, WC(Cp) exhibits a ∼0.07 Å elongation over the corresponding distance in typical Cp2WX2 complexes and a WO distance which appears to be elongated versus what might be expected for a formal triple bond. D(MO) values obtained from (MeCp)2MO silanolytic (Me3SiCl, Me3SiI) batch titration calorimetry are very large:  110(11) kcal/mol (M = Mo) and 132(10) kcal/mol (M = W). The corresponding D(WOTMS) value is determined to be 65(18) kcal/mol. Ab initio relativistic core potential calculations reveal significantly weakened MCp bonding versus that in the corresponding Cp2MCl2 compounds, weakened MO bonding due to population of MO π antibonding levels, and a pronounced accumulation of negative charge on the oxo ligand (consistent with observed nucleophilicity of these complexes). Calculated Cp2MCl2 and Cp2MO molecular geometries and D(MO) values at the MP2 level are in favorable agreement with experiment. The D(MO) and D(MO) data provide significant insight into the chemistry of Cp2MO complexes, especially in regard to constraints on oxo transfer as well as oxametallacycle formation and scission processes.

Single-crystal X-ray diffraction studies were conducted on the following compounds: Cp*Ru(PMe 3 ) 2 Cl (1), Cp*Ru(PPhMe 2 ) 2 Cl (2), Cp*Ru(PMePh 2 ) 2 Cl (3), Cp*Ru(PPh 3 ) 2 Cl (4), Cp*Ru(PEt 3 ) 2 Cl (5), Cp*Ru(AsEt 3 ) 2 Cl (6), Cp*Ru(P n Bu 3 ) 2 Cl (7), and Cp*Ru(dmpm)Cl (8). Structural information obtained from these X-ray studies can be correlated with enthalpies of ligand substitution previously determined from solution calorimetry. The cone angle of the phosphine ligand (monodentate) and the Ru-P bond distance were found to be proportional to the enthalpy of reaction.