EVIDENCE OF AN ALTERNATIVE MECHANISM FOR THE REDUCTIVE COUPLING OF ISONITRILES BY ELECTROPHILIC 1-SILA-3-ZIRCONACYCLOBUTANE COMPLEXES - STRUCTURAL CHARACTERIZATION OF THE BICYCLIC ENEDIAMIDO COMPLEXES CP2ZR(N(CME3)C(CH2SIME2CH2) = CN(R)), WHERE R = TERT-BUTYL AND 2,6-XYLYL

The reductive coupling of 2 equiv of CN-t-Bu by Cp2 activated Zr(CH2SiMe2CH2) proceeds sequentially with the formation of the eta-2-iminoacyl complex Cp2 activated Zr(N(CMe3)CCH2SiMe2CH2) (1) and the eta-2-iminoacyl imine complex Cp2 activated Zr(N(CMe3)C-C(=NCMe3)CH2SiMe2CH2) (2), which upon heating to 120-degrees-C rearranges to the bicyclic enediamido complex Cp2 activated Zr(N(CMe3)C(CH2SiMe2CH2)=CN(CMe3)) (3). Labeling experiments using a C-13-enriched sample of 1 indicate that the insertion of the second equivalent of CN-t-Bu proceeds with nucleophilic displacement of the original eta-2-iminoacyl group and does not involve the breaking of the C(eta-2-iminoacyl)-C (methylene) bond of 1. Treatment of 1 with 2,6-xylyl isocyanide proceeds directly at 25-degrees-C to yield the bicyclic enediamidate complex Cp2 activated Zr(N(CMe3)C(CH2SiMe2CH2)=CN(xy)) (5). From kinetic measurements within the temperature range 80-120-degrees-C, the activation parameters for the first-order conversion of 2 to 3 are DELTA-H* = 25.6 (5) kcal/mol and DELTA-S double-ended-dagger = 5.0 (9) eu. The corresponding reaction of 2 with 2,6-xylyl isocyanide affords only 3 upon heating, thus ruling out the possibility of isocyanide deinsertion being important during this intramolecular rearrangement. Complexes 3 and 5 were characterized by infrared and electronic spectroscopy, by variable-temperature NMR measurements, and by elemental analyses. Their molecular structures were determined by X-ray crystallographic methods. Crystal data for 3 at 22-degrees-C: monoclinic space group P2(1) with a = 9.218 (2) angstrom, b = 9.186 (2) angstrom, c = 14.923 (2) angstrom, beta = 101.95 (2)-degrees, and Z = 2. For 5 at 22-degrees-C: monoclinic P2(1)/c with a = 15.775 (3) angstrom, b = 8.169 (2) angstrom, c = 20.924 (4) angstrom, beta = 97.36 (1)-degrees, and Z = 4. The collective outcome of these experiments is consistent with an alternative mechanism being operative for the reductive coupling of isocyanides by a 1-sila-3-zirconacyclobutane complex.