SYNTHESIS OF AN ETA(2)-N-2-TITANIUM DIAZOALKANE COMPLEX WITH BOTH IMIDO-LIKE AND METAL CARBENE-LIKE REACTIVITY PATTERNS

The novel eta(2)-N-2-diazoalkane complex CpTi-2(N2CHSiMe3) (2) has be en prepared by addition of (trimethylsilyl)diazomethane to CpTi-2(C2H 4) (1). The structure of 2 reveals nearly symmetric Ti-N distances and an N-N distance similar to 0.1 Angstrom longer than that of the free diazoalkane. Compound 2 loses dinitrogen in solution under mild condit ions to give the fulvene complex CpFvTiCH(2)SiMe(3) (4). Thermolysis of 2 in the presence of 1-alkenes yields the trans-alpha,beta-disubsti tuted titanacyclobutane complexes CpTi-2(CH(SiMe3)CH(R)CH2) (R = H (6 ), R = Ph (7), R = Me (8), R = Et (9)). The regio- and stereochemistry of the titanacyclobutane complexes was determined by a combination of one- and two-dimensional NMR techniques. The NMR assignment was suppo rted in the case of 8 by an X-ray diffraction study. In addition to co nfirming the regio- and stereochemistry of the metallacycle, the X-ray structure of 8 shows that unlike most titanacyclobutanes, the metalla cycle ring is puckered. A kinetic study of the formation of 7 from 2 a nd styrene revealed that the reaction is first order in 2 and zero ord er in styrene. The rate constant for this reaction is identical to tha t measured for the conversion of 2 to 4. The kinetic study supports a mechanism involving initial rate-determining loss of dinitrogen to for m a carbene complex intermediate which undergoes hydrogen transfer fro m a Cp methyl to give 4, or in the presence of styrene is trapped to form 7. In addition to its metal carbene-like reactions involving N-2 loss, complex 2 undergoes a variety of transformations in which N-2 is retained in the final product. These include cycloaddition reactions with alkynes and allene, as well as reversible reactions with Lewis ba ses to form adducts. These transformations, which are similar to react ions of group IV imido complexes, demonstrate the imide-like character of the diazoalkane ligand.