CONVERSION OF NITRILES TO NITRENES VIA AZAVINYLIDENES IN LOW-VALENT TUNGSTEN CARBONYL-COMPLEXES

Azavinylidene complexes of the type Tp'(CO)(2)W<--=N=CRNu are available from the reaction of [Tp'(Co)(3)W(N=CR)][BF4] with nucleophiles (Tp' = hydridotris(3,5-dimethylpyrazolyl)borate R = Me, Nu = H (1a); R = Ph, Nu = Et (1d); R = Me, Nu MeO (1e)). Related azavinylidene complexes Tp' (CO)(2)W<--=N=CHR (R = Me (1a); R = Et (1b); R = CH(2)Ph (1c)) form via insertion of nitrile into the W-H bond of Tp'(CO)(3)WH when the metal reagent is photolyzed in the presence of nitriles. Donation of the lone pair of electrons from the azavinylidene nitrogen to the tungsten center, compatible with electron counting guidelines, is reflected in the low IR stretching frequencies of the carbonyl ligands and their downfield carbon resonance in C-13 NMR. NMR spectra and an X-ray structure of Tp'(CO)(2)W<--=N=CHCH(2)Ph (1c) indicate that the azavinylidene ligand is nearly linear ((3)J(WH) = 5.6 Hz, (2)J(WC) = 27 Hz, and alpha(W-N-C) = 166.6(7)degrees). Complex 1c crystallized in the monoclinic space group P2(1)/n with unit cell dimensions of a = 12.163(1), b = 17.207(2), and c 13.312(1) Angstrom and beta = 102.103(7)degrees, with Z = 4, Refinement of 326 variables over 3538 reflections led to R = 4.2% and R(w) = 5.8%. The short W-N distance of 1.871(6) Angstrom indicates a tightly coordinated azavinylidene ligand. As in related dicarbonyl amido complexes, an acute OC-W-CO angle was found in the solid state. Protonation of azavinylidene complexes 1a-d occurs at the ligand carbon atom to effectively oxidize the metal to tungsten(IV) and produce cationic nitrene complexes [Tp(CO)(2)W<--=NCHRR'] [BF4] (R = Me, R' = H(2a); R = Et, R' = H (2b); R = CH(2)Ph, R' = H(2c); R = Ph,R' = Et (2d)). Reaction of the azavinylidene complexes 1a-c with [Ph(3)C][PF6] produces cationic nitrene complexes of the type [Tp'(Co)(2)W<--=NCH(R)CPh(3)[PF6] (R = Me (3a); R = Et 3b); R = CH(2)Ph (3c)) with a chiral substituent on the nitrene, The net result of this sequence of nucleophile, electrophile addition reactions to cationic nitrile complexes is formation of cationic nitrene complexes. These products reflect addition of both the nucleophile and the electrophile to the nitrile alpha-carbon.