INSERTION OF ISOCYANIDES INTO THE ZR-C BOND OF RUTHENIUM ZIRCONIUM DIMETALLOALKENES - SPECTROSCOPIC AND STRUCTURAL COMPARISONS OF THE ETA-2-IMINOACYL COMPLEXES CP(PME3)2RUCH=CRC(NTERTBU)ZRCLCP2 (R = H VS R = ME) AND KINETICS OF ISOMERIZATION OF A THERMODYNAMICALLY UNSTABLE ETA-2-IMINOACYL COMPLEX

The reaction of Cp(PMe3)2RuCH = CHZrClCp2 with (t)BuNC produces an eta(2)-iminoacyl complex, Cp-(PMe3)2RuCH = CHC(N(t)Bu)ZrClCp2. The initially formed isomer of this compound is the ''N-outside'' isomer (2Hk), where the N-donor portion of the eta(2)-iminoacyl ligand is on the outside position of the three ligation sites of the Cp2Zr moiety. This kinetic isomer cleanly rearranges to the thermodynamic ''N-inside'' isomer (2Ht). Activation parameters determined for this rearrangement are DELTA-H double-ended-dagger = 19.9 +/- 0.6 kcal mol-1, DELTA-S double-ended-dagger = -3.4 +/- 2.2 cal K-1 mol-1, and DELTA-G double-ended-dagger (298 K) = 20.9 kcal mol-1. Spectroscopic and crystallographic data for 2Ht indicate a significant contribution from a zwitterionic resonance form that is facilitated by the disparate electronic properties of the electron-rich ruthenium and the electron-deficient zirconium moieties. The reaction of (t)BuNC with the analogous methylated dimetalloalkene Cp(PMe3)2RuCH = C-(CH3)ZrClCp2 gives the eta(2)-iminoacyl complex Cp(PMe3)2RuCH = C(CH3)C(N(t)Bu)ZrClCp2 (2Me). Spectroscopic and crystallographic data for 2Me indicate that the presence of a single methyl group has a profound influence on the structural and electronic properties of this eta(2)-iminoacyl complex, compared to 2Ht. In contrast to 2Ht, the metals and the organic bridging group in 2Me are twisted out of planarity in order to avoid an unfavorable interaction of the methyl group and the (t)Bu group. Spectroscopic and structural data suggest negligible contribution from the zwitterionic resonance form in this compound, due to steric inhibition of resonance. The reaction of the less sterically demanding isocyanide MeNC with Cp(PMe3)2RuCH = C(R)ZrClCp2 (R = H, CH3) gives Cp(PMe3)2RuCH = CHC(NMe)ZrClCp2 (3H) and Cp(PMe3)2RuCH = C(CH3)C(NMe)ZrClCp2 (3Me). Spectroscopic data for 3H and 3Me and structural data for 3Me indicate a contribution from a zwitterionic resonance form for both of these eta(2)-iminoacyl complexes. Crystallographic data for 2Ht: orthorhombic space group Pnma, a = 16.640 (3) angstrom, b = 13.936 (3) angstrom, c = 13.290 (3), V = 3082 (2) angstrom 3, Z = 4. Crystallographic data for 2Me: monoclinic space group P2(1)/c, a = 13.386 (3) angstrom, b = 22.514 (6) angstrom, c = 10.639 (3), beta = 91.89 (2)-degrees, V = 3205 (3) angstrom 3, Z = 4. Crystallographic data for 3Me: monoclinic space group P2(1)/n, a = 13.031 (3) angstrom, b = 14.880 (4) angstrom, c = 15.594 (3), beta = 108.08 (2)-degrees, V = 2874 (2) angstrom 3, Z = 4.