ALKYLIDYNE(CARBABORANE) COMPLEXES OF THE GROUP-6 METALS .10. FACILE PROTON-INDUCED DEGRADATION OF 13-VERTEX TUNGSTA(CARBORANE) POLYHEDRA

Protonation (HBF4.Et2O) of the 13-vertex alkylidyne(carborane)tungsten salts [NEt4]-[W(=CR)(CO)2(eta6-7,9-R'2-7,9-C2B10H10)] (6, R = C6H4Me-4, Me; R' = H, Me) with 1 mol equiv of acid in the presence of donor molecules L (CO, PhC=CPh, PPh3), gives neutral 13- or 12-vertex tungstacarborane species, depending on the nature of L. The 13-verteX WC2B10 products have the general formula [W(CO)2(L)n{eta6-7,9-R'2-7,9-C2B10H9-11-(CH2R)}] (8a, L = CO, R = C6H4Me-4, R' = Me, n = 2; 8b, L = PhC=CPh, R = C6H4Me-4, R' = Me, n = 1; 8c, L = PhC=CPh, R = Me, R' = Me, n = 1) and result from migration and insertion into a B-H bond of an initially formed alkylidene {C(H)R} group. These compounds are thermally unstable, slowly decomposing in CH2Cl2 Solution to give the 12-vertex WC2B9 species [W(CO)2(L)n(eta5-7,9-R'2-7,9-C2B9H9)] (9a, L = CO, R'= Me, n = 2; 9b, L = PhC=CPh, R'= Me, n = 1), a process involving formal loss of both a BH fragment and a CHR moiety. For the particular case where L = PPh3, transformation of the WC2B10 framework is so rapid that the only species isolated by protonating the alkylidyne(carborane) salts are the compounds [W(CO)2(PPh3)2(eta5-7,9-R'2-7,9-C2B9H9)] (9c, R' = Me; 9d, R' = H) having the 2,1,7-WC2B9 cage framework. In the absence of substrate molecules, protonation of the species 6 with ca. 0.5 mol equiv of HBF4.Et2O gives the ditunpten compounds [NEt4][W2{mu-eta2-C(R)CO}(CO)3(eta6-7,9-R'2-7,9-C2B10H10){eta6-7,9-R'2-7,9-C2B10H9-11-(CH2R)}] (10). The structure of the complex with R = Me, R' = Me, and [Au-(PPh3)2]+ as the cation has been established by X-ray diffraction. Crystals are triclinic, space group P1BAR (No. 2) with a = 9.398(1) angstrom, b = 18.181(5) angstrom, c = 20.491(3) angstrom, alpha = 69.07(2)-degrees, beta = 88.75(1)-degrees, gamma = 85.91(1)-degrees, and Z = 2. The anion consists of two 13-vertex WC2B10 metallacarboranes connected by a W-W bond [2.778(1) angstrom] bridged by a mu-eta2-C(Me)CO ketenyl ligand [mu-C-W 2.22(1) and 2.27(1) angstrom, W-CO 2.44(2) angstrom], and a three-center two-electron B-H half arrow pointing right W linkage. The metallacarborane cage which does not form the exopolyhedral B-H half arrow pointing right W bond carries a CH2Me substituent at the boron atom that is in a beta site with respect to the carbon atoms of the metal-ligating CBCBBB ring. In addition to the X-ray crystal structure determination, the IR and NMR data for the new complexes are reported and discussed.