DOUBLE ISOCYANIDE INSERTION AND C,C-COUPLING REACTION OF [(C5ME4)SIME2(N-T-BU)]ZRME2 - STRUCTURAL CHARACTERIZATION OF THE 2 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 i socyanide insertion into both Zr-C(methyl) bands of this 14-electron c omplex 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[e ta(2)-C(Me)NR](2), 3. Subsequent thermolysis of 3 leads to C,C-couplin g 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(M e)N(R)], 4. Compounds 2a, 3a-c, and 4a-c have been observed and charac terized by solution NMR measurements, and the molecular structures of 3a, 3b, 4a, and 4b have been confirmed by crystallographic methods, Th e nonplanar 1,4-diaza-5-zirconacyclopentene rings of 4a and 4b are fol ded by ca. 50 degrees in opposite directions along the corresponding N ... N line segment and adopt the prone and supine conformations, resp ectively. The activation parameters for the first-order intramolecular C,C-coupling reaction leading to the conversion of 3a --> 4a are Delt a 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 a nd Delta S double dagger = -10.4(8) eu.