Synthesis of an η2-N2-Titanium Diazoalkane Complex with Both Imido- and Metal Carbene-Like Reactivity Patterns

Abstract

The novel η2-N2-diazoalkane complex Cp*2Ti(N2CHSiMe3) (2) has been prepared by addition of (trimethylsilyl)diazomethane to Cp*2Ti(C2H4) (1). The structure of 2 reveals nearly symmetric Ti−N distances and an N−N distance ∼0.1 Å longer than that of the free diazoalkane. Compound 2 loses dinitrogen in solution under mild conditions to give the fulvene complex Cp*FvTiCH2SiMe3 (4). Thermolysis of 2 in the presence of 1-alkenes yields the trans-α,β-disubstituted titanacyclobutane complexes Cp*2Ti(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 supported in the case of 8 by an X-ray diffraction study. In addition to confirming the regio- and stereochemistry of the metallacycle, the X-ray structure of 8 shows that unlike most titanacyclobutanes, the metallacycle ring is puckered. A kinetic study of the formation of 7 from 2 and styrene revealed that the reaction is first order in 2 and zero order in styrene. The rate constant for this reaction is identical to that measured for the conversion of 2 to 4. The kinetic study supports a mechanism involving initial rate-determining loss of dinitrogen to form a carbene complex intermediate which undergoes hydrogen transfer from 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 N2 loss, complex 2 undergoes a variety of transformations in which N2 is retained in the final product. These include cycloaddition reactions with alkynes and allene, as well as reversible reactions with Lewis bases to form adducts. These transformations, which are similar to reactions of group IV imido complexes, demonstrate the imide-like character of the diazoalkane ligand.