Organometallics 1993,12, 544-553
544
Structures and Reactivity of ( C S H ~ M ~ ) ~ Z ~ ( C H ~(CHaCN),+ C H ~ R ) Complexes. Competition between Insertion and /3-H Elimination Yun W. Alelyunas, Zhaoyu Guo, Robert E. LaPointe, and Richard F. Jordan' Department of Chemistry, University of Iowa, Iowa City, Iowa 52242 Received June 15,1992
A series of cationic alkyl complexes (C~H~M~)~Z~(CHZCHZR)(CH~CN),+ (3b,c-7b,c; R = H, CH3,CHzCH3, Ph, CMe3) is generated by reaction of the corresponding T H F complexes (C5H4Me)zZr(CH2CHzR)(THF)+(38-78) with excess CH3CN. In CDzClz these complexes exist as equilibrium mixtures of rapidly exchanging mono(CH&N) species (3b-7b) and bis(CH3CN) species (3c-7c). Kegfor CH3CN dissociation from the ethyl complex 3c is estimated to be 0.5(3) M (20 "C) from the variation of lH NMR ethyl chemical shifts vs [CDSCNI. NMR data establish that the mono(nitri1e) ethyl complexes 3b and (C~H~M~)ZZ~(CHZCH~)(~B~CN)+ (12) adopt species. Key data @-agosticstructures analogous to those of (C5H4Me)zZr(CH~CH2R)(PMe3)+ include high-field lH and 13C ZrCHzCH3 resonances, large Jc,-Hvalues, and reduced J c " ~ values (27 Hz). The Jvalues are similar to values for cyclobutanes and thus reflect the reduced Z d - C and C-C-HI,, angles and concomitant hybridization changes associated with the @-agostic structure, rather than extensive distortion toward an olefin hydride structure. By analogy to (C5H4Me)zZr(CH2CHzR)(PMe3)+ systems, the higher alkyls 4&7b also likely adopt 8-agostic structures. In CD2C12 solution containing excess CH3CN as a trapping reagent, 4b,c-7b,c undergo clean j3-H elimination and subsequent rapid CH3CN insertion a t 23 OC to yield (CSH4Me)zZr{N=C(H)(Me)J(CH&N)+ (11) and olefin. Under these conditions, ethyl system 3b,c undergoes competitive CH3CN insertion leading to (C5H4Me)ZZr~N--L(Et)(Me))(CH,CN)+ (10,84%) and @-Helimination leading to 11 (16%). Kinetic studies support a mechanism in which mono(CH3CN) complex 3b undergoes competitive insertion and @-Helimination followed by rapid s - and ~ ka-elim = 8.20(13) X t3-l a t 20.0(4) "C. Kinetic studies trapping; kinsed = 4.38(9) X of the reaction of ZrCHzCH+Bu system 7b,c support an analogous mechanism in which mono(CH3CN) complex 7b undergoes rate-limiting @-Helimination (kp-elm = 9.4(1) X 104 s-l) and hinsed
