Double isocyanide insertion and C,C-coupling reaction of [(C5Me4)SiMe2(N-t-Bu)]ZrMe2. Structural characterization of the two 1,4-diaza-5-zirconacyclopentene ring conformations for [(C5Me4)SiMe2(N-t-Bu)]Zr[N(R)C(Me)=C(Me)N(R)] complexes

The reactions of 2 equiv of CNR (R = (a) tert-butyl, (b) 2,6-xylyl, (c) Me) with [(C5Me4)SiMe2(N-t-Bu)]ZrMe2, 1, proceed sequentially with isocyanide insertion into both Zr-C(methyl) bands of this 14-electron complex to give the corresponding eta(2)-iminoacyl Zr complexes [(C5Me4)SiMe2(N-t-Bu)]ZrMe[eta(2)-C(Me)NR], 2, and [(C5Me4)SiMe2(N-t-Bu)]Zr[eta(2)-C(Me)NR](2), 3. Subsequent thermolysis of 3 leads to C,C-coupling of the two eta(2)-iminoacyl units and proceeds solely with formation of the enediamidate derivative [(C5Me4)SiMe2(N- t-Bu)Zr[N(R)C(Me)=C(Me)N(R)], 4. Compounds 2a, 3a-c, and 4a-c have been observed and characterized by solution NMR measurements, and the molecular structures of 3a, 3b, 4a, and 4b have been confirmed by crystallographic methods, The nonplanar 1,4-diaza-5-zirconacyclopentene rings of 4a and 4b are folded by ca. 50 degrees in opposite directions along the corresponding N ... N line segment and adopt the prone and supine conformations, respectively. The activation parameters for the first-order intramolecular C,C-coupling reaction leading to the conversion of 3a --> 4a are Delta H double dagger = 24.6(2) kcal/mol and Delta S double dagger = -11.3(7) eu and of 3b --> 4b are Delta H double dagger = 23.9(3) kcal/mol and Delta S double dagger = -10.4(8) eu.