STRUCTURES AND REACTIVITY OF (C5H4ME)2ZR(CH2CH2R)(CH3CN)N+ COMPLEXES - COMPETITION BETWEEN INSERTION AND BETA-H ELIMINATION

A series of cationic alkyl complexes (C5H4Me)2Zr(CH2CH2R)(CH3CN)n+ (3b,c-7b,c; R = H, CH3, CH2CH3, Ph, CMe3) is generated by reaction of the corresponding THF complexes (C5H4Me)2Zr(CH2CH2R)(THF)+ (3a-7a) with excess CH3CN. In CD2Cl2 these complexes exist as equilibrium mixtures of rapidly exchanging mono(CH3CN) species (3b-7b) and bis(CH3CN) species (3c-7c). K(eq) for CH3CN dissociation from the ethyl complex 3c is estimated to be 0.5(3) M (20-degrees-C) from the variation of H-1 NMR ethyl chemical shifts vs [CD3CN]. NMR data establish that the mono(nitrile) ethyl complexes 3b and (C5H4Me)2Zr(CH2CH3)(tBuCN)+ (12) adopt beta-agostic structures analogous to those of (C5H4Me)2Zr(CH2CH2R)(PMe3)+ species. Key data include high-field H-1 and C-13 ZrCH2CH3 resonances, large J(Calpha-H) values, and reduced J(Calpha-Cbeta) values (27 Hz). The J values are similar to values for cyclobutanes and thus reflect the reduced Zr-C-C and C-C-H(br) angles and concomitant hybridization changes associated with the beta-agostic structure, rather than extensive distortion toward an olefin hydride structure. By analogy to (C5H4Me)2Zr(CH2CH2R)(PMe3)+ systems, the higher alkyls 4b-7b also likely adopt beta-agostic structures. In CD2Cl2 solution containing excess CH3CN as a trapping reagent, 4b,c-7b,c undergo clean beta-H elimination and subsequent rapid CH3CN insertion at 23-degrees-C to yield (C5H4Me)2Zr-{N=C(H)(Me)}(CH3CN)+(11)and olefin. Under these conditions, ethyl system 3b,c undergoes competitive CH3CN insertion leading to (C5H4Me)2Zr{N=C(Et)(Me)}(CH3CN)+ (10, 84%) and beta-H elimination leading to 11 (16%). Kinetic studies support a mechanism in which mono-(CH3CN) complex 3b undergoes competitive insertion and beta-H elimination followed by rapid trapping; k(insert) = 4.38(9) x 10(-4) s-1 and k(beta-elim) = 8.20(13) X 10(-5) s-1 at 20.0(4)-degrees-C. Kinetic studies of the reaction of ZrCH2CH2tBu system 7b,c support an analogous mechanism in which mono-(CH3CN) complex 7b undergoes rate-limiting beta-H elimination (k(beta-elim) = 9.4(1) X 10(-4) s-1) and k(insert) much less than k(beta-elim). Alkyl/aryl substituents on the beta-carbon of (C5H4Me)2Zr(CH2CH2R)(CH3CN)+ influence both insertion and beta-H elimination rates.