ALKYLATION REACTIONS OF AN ANIONIC ETHYLISOCYANIDE COMPLEX OF W-0 - SYNTHESIS OF ALKYL, ETA-2-IMINOACYL, 1-AZAALLYL AND DIALKYLAMINO CARBYNE COMPLEXES OF TUNGSTEN

Methylation of the ethylisocyanide metallate Na[Cp*W(CO)2(EtNC)] (Cp* = eta-5-C5Me5) (1) with CH3I occurs exclusively at the metal center to yield cis- and trans-Cp*W(CO)2(EtNC)(Me) (2a, 2b) in high yield. The methyl compounds 2a and 2b are converted upon heating to a mixture of the iminoacyl complex Cp*(CO)2W[eta-2-C(NEt)Me] (3) and the 1-azaallyl complex Cp*(CO)2W(eta-3-CH2-CH-NEt) (4). Complex 3 rearranges in refluxing toluene to the thermodynamically favoured isomer 4. By analogy, reaction of 1 with CD3I leads to the isomeric mixture of cis- and trans-Cp*W(CO)2(EtNC)(CD3) (2a', 2b'), which upon heating is transformed to Cp*(CO)2W[eta-2-C(NEt)CD3] (3') and Cp*(CO)2W(eta-3-CD2-CD-NEt) (4'). Complex 3' isomerizes in refluxing toluene to 4'. By contrast to the reaction of 1 with MeI, methylation with Me3OBF4 occurs also at the isocyanide nitrogen and gives the carbyne complex Cp*(CO)2W=CN(Me)Et (5) in low yield. Similarly, EtI alkylates 1 at both the metal center and the isocyanide nitrogen. Reaction at the metal center leads via the isomeric mixture of the ethyl compounds cis- and trans-Cp*W(CO)2(EtNC)(Et) (6a, 6b) to the complexes Cp*(CO)2W[eta-2-C(NEt)Et] (7) and Cp*(CO)2W(eta-3-CH(Me)-CH-NEt) (8), while reaction at the isocyanide nitrogen gives the carbyne complex Cp*(CO)2W=CNEt2 (9). In comparison, alkylation of 1 with Et3OBF4 occurs exclusively at the isocyanide nitrogen to yield 9. By analogy to 3, the iminoacyl complex 7 isomerizes in refluxing toluene to the thermodynamically favoured 1-azaallyl complex 8. A mechanism is suggested for the conversion of 2a/2b to 3 and 4, which involvs a preequilibrium between 2a and 2b followed by methyl migration from the metal center to the isocyanide carbon in 2a to give the 16e iminoacyl intermediate Cp*(CO)2W[eta-1-C(NEt)Me]. This intermediate follows subsequently two competitive pathways. The first one involves simple coordination of the nitrogen to the metal center to give the eta-2-iminoacyl complex 3, the second one an intramolecular H-migration to give the 1-azaallyl complex 4. The same intermediate is also assumed to be involved in the isomerization of 3 to 4. Spectroscopic data show only one isomer for the 1-azaallyl complexes 4 and 8 in solution. The solid state structure of this isomer was established for 4 by a single-crystal X-ray crystallographic study. The molecule adopts the endo conformation with the N-ethyl substituent of the 1-azaallyl ligand in the syn position. For 8 the same conformation is derived from H-1 NMR data. Both N-ethyl- and C-methyl-substituents of the 1-azaallyl ligand occupy syn positions. Variable temperature H-1 NMR spectra of complexes 3 and 7 show these molecules to be fluxional. Two likely mechanisms for this dynamic process are discussed.