UNDERSTANDING ELECTRONIC EFFECTS IN ORGANOMETALLIC COMPLEXES - INFLUENCE OF METHYL SUBSTITUTION ON HAFNOCENE DIHALIDES

A series of hafnocene dihalides was synthesized in which the total methyl substitution on the two cyclopentadienide moieties was 0,3,5,6,8, and 10. Each of these methyl-substituted hafnocene derivatives was analyzed by using X-ray photoelectron spectroscopy. The decrease in the Hf(4f7/2) binding energy per methyl group was 0.06 eV. The correlation of binding energies with methyl substitution for a series of hafnocene dichlorides was found to be linear (correlation coefficient of 0.991). Since steric effects are not linearly additive, the additivity of methyl substitution on the hafnocene dihalides established that methyl substitution on the cyclopentadienide rings of the hafnocene dihalides resulted in electron donation through the cyclopentadienide to the complexed transition metal. In order to ensure that the observed changes in binding energy with methyl substitution were not a result of major structural change due to steric interactions, a series of highly crystalline hafnocene dibromide complexes were subjected to single-crystal X-ray analysis. Hafnocene dibromide, 1,2,4-trimethylhafnocene dibromide, 1,2,3,4,5-pentamethylhafnocene dibromide, and 1,1',2,2',3,3',4,4',5,5'-decamethylhafnocene dibromide were found to have Cp-Hf-Cp angles of 130.5, 131.2, 130.3, and 129.8-degrees, respectively. Hafnocene dibromide crystallized in the triclinic space group P1 with a = 7.986 (2) angstrom, b = 12.555 (3) angstrom, and BETA = 79.75 (2)-degrees. The calculated density was 2.746 g/cm3 for four molecules in the unit cell at room temperature. The unweighted R factor was 0.028, and data were collected with the use of a variable-rate omega-20 scan technique and graphite-monochromatized Mo K-alpha radiation. After Lorentz-polarization corrections, 3564 reflections with 20 = 0-54-degrees were observed for [F(o)2 greater-than-or-equal-to 2-sigma-(F(o)2)]. 1,2,4-Trimethylhafnocene dibromide crystallized in the orthorhombic space group Pna2(1) with a = 13.562 (6) angstrom, b = 14.930 (4) angstrom, and c = 7.111 (2) angstrom. The calculated density was 2.355 g/cm3 for four molecules in the unit cell at room temperature. The unweighted R factor was 0.036, and data were collected with the use of a variable-rate omega-20 scan technique and graphitemonochromatized Mo K-alpha radiation. After Lorentz-polarization corrections, 1313 reflections with 20 = 0-55-degrees were observed for [F(o)2 greater-than-or-equal-to 2-sigma(F(o)2)]. 1,2,3,4,5-Pentamethylhafnocene dibromide crystallized in the orthorhombic space group Pnam (a nonstandard setting of Pnma) with a = 9.078 (2) angstrom, b = 12.004 (2) angstrom, and c = 13.547 (7) angstrom. The calculated density was 2.266 g/cm3 for four molecules in the unit cell at room temperature. The unweighted R factor was 0.024, and the data were collected with the use of a variable-rate omega-20 scan technique and graphite-monochromatized Mo K-alpha radiation. After Lorentz-polarization corrections, 1436 reflections with 20 = 0-54-degrees were observed for [F(o)2 greater-than-or-equal-to 2-sigma(F(o)2)]. 1,1',2,2',3,3',4,4',5,5'-Decamethylhafnocene dibromide crystallized in the orthorhombic space group Fmm2 with a = 15.015 (7) angstrom, b = 16.787 (12) angstrom, and c = 8.277 (8) angstrom. The calculated density was 1.938 g/cm3 for four molecules in the unit cell at room temperature. The unweighted R factor was 0.037, and the data were collected with the use of a variable-rate omega-20 scan technique and graphite-monochromatized Mo K-alpha radiation. After Lorentz-polarization corrections, 1105 reflections with 20 = 0-70-degrees were observed for [F(o)2 greater-than-or-equal-to 2-sigma(F(o)2)].