SYNTHESIS, REACTIVITY, AND CRYSTAL AND MOLECULAR-STRUCTURES OF NB(L)(ETA-6-C6H5-NXN)2B(C6H5-NXN)2, A CLASS OF MONONUCLEAR METAL-COMPOUNDS CONTAINING THE 12-ELECTRON-DONOR TETRAARYLBORATO LIGAND

The one-electron oxidation of Nb(mes)2 (mes = 1,3,5-trimethylbenzene) with ferrocenium tetraarylborate, produces the 16-electron [Nb(mes)2]+ cation, which adds disubstituted symmetrical alkynes to give the ionic compounds [Nb(mes)2)(alkyne)][Y] (alkyne = MeC2Me, Y = BPh4 (1a), Y = B(p-FC6H4)4 (1b), Y = B[3,5-(CF3)2C6H3]4 (1c); alkyne = PhC2Ph, Y = Bph4 (2a), Y = B(p-FC6H4)4 (2b)). 2b crystallizes in space group C2/c (No. 15) with four formula units in cells of dimensions a = 17.640(3) angstrom, b = 16.8386(8) angstrom, c = 19.616(2) angstrom, and beta = 97.230(9)degrees; the [Nb(mes)2(PhC2Ph)]+ cation contains bent eta6-mesitylene ligands, a coordination position being occupied by PhC2Ph as a formal two-electron donor. The alkyne in 1a-c is displaced by CO at room temperature and at atmospheric pressure to give the carbonyl derivatives [Nb-(mes)2CO][Y] (3a-c). Thermal treatment of 1a,b and 2a in toluene gave mesitylene displacement and formation of the neutral covalent compounds containing the uninegative tetraarylborato ligand of formula [Nb(RC2R)(eta6-C6H4X)2B(C6H4X)2] (R = Me, X = H (4a), X = p-F (4b); R = Ph, X = H (5a)). 4a crystallizes in space group P42(1)mBAR (No. 113), with two formula units in cells of dimensions a = 9.734(1) angstrom and c = 11.6333(9) angstrom; 4b crystallizes in space group P2(1)2(1)2, (No. 19) with eight formula units in cells of dimensions a = 11.776(3) angstrom, b = 15.666(3) angstrom, and c = 28.042(5) angstrom. 4a,b both contain the novel bonding arrangement of the tetraarylborato ligand with two interannularly niobium-bridged aryl rings. This new type of bonding is not restricted to the methyl-substituted (4a) or phenyl-substituted (4b) acetylenic derivatives: CO is also compatible with the new bonding arrangement. Alkyne displacement by CO gives the neutral covalent complex [Nb(CO)(eta6-C6H4X)2B(C6H4X)2] (X = H (6a), p-F (6b)). An X-ray diffractometric experiment on 6a has confirmed the novel bonding arrangement of the tetraarylborato ligand for this compound, which crystallizes in space group P2(1)2(1)2, (No. 19) with four formula units in cells of dimensions a = 10.495(2) A, b = 14.586(1) angstrom, and c = 17.174(3) angstrom.