Unique Complexation of 1,4-Diaza-1,3-butadiene Ligand on Half-Metallocene Fragments of Niobium and Tantalum

We report the synthesis of half-metallocene 1,4-diaza-1,3-butadiene (dad) complexes of niobium and tantalum and the flexible coordination modes of the dad ligand. Treatment of MCl4(η5-C5R5) (1a, M = Ta, R = CH3; 1b, M = Ta, R = H; 2a, M = Nb, R = CH3; 2b, M = Nb, R = H) with 1 equiv of a dilithium salt of 1,4-bis(p-methoxyphenyl)-1,4-diaza-1,3-butadiene (p-MeOC6H4-dad) in THF yielded the corresponding half-sandwich 1,4-diaza-1,3-butadiene complexes MCl2(η5-C5R5)(η4-supine-p-MeOC6H4-dad) (3a, M = Ta, R = CH3; 3b, M = Ta, R = H; 4a, M = Nb, R = CH3; 4b, M = Nb, R = H). Similar reaction of 1a with the dianions of 1,4-bis(p-tolyl)-1,4-diaza-1,3-butadiene (p-Tol-dad), 1,4-bis(o-tolyl)-1,4-diaza-1,3-butadiene (o-Tol-dad), and 1,4-dicyclohexyl-1,4-diaza-1,3-butadiene (Cy-dad) afforded tantalum complexes TaCl2Cp*(η4-supine-p-Tol-dad) (5) (Cp* = pentamethylcyclopentadienyl), TaCl2Cp*(η4-supine-o-Tol-dad) (6), and TaCl2Cp*(η4-supine-Cy-dad) (7), respectively. When a dinuclear Ta(III) complex [TaCl2Cp*]2 (8) was treated with 1,4-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene (( i Pr)2C6H3-dad), a η2-C,N-imine complex TaCl2Cp*(η2-C,N-( i Pr)2C6H3-dad) (9) was formed. Dialkylation of the dichloro complexes caused the change of the orientation of the dad ligand from supine fashion to a prone one of dimethyl complexes TaMe2Cp*(η4-prone-dad) (10, dad = p-MeOC6H4-dad; 11, dad = o-Tol-dad) and bis(benzyl) complexes M(CH2Ph)2Cp*(η4-prone-dad) (12, M = Ta, dad = p-MeOC6H4-dad; 13, M = Nb, dad = p-MeOC6H4-dad; 14, M = Ta, dad = Cy-dad), while a mono(benzyl) complex Ta(CH2Ph)ClCp*(η4-supine-o-Tol-dad) (16) kept the supine orientation. The bis(benzyl) complexes 12−14 thus isolated were thermally stable in solid state, but in solution these complexes gradually decomposed to give the corresponding benzylidene complexes M(CHPh)Cp*(η4-prone-dad) (17, M = Ta, dad = p-MeOC6H4-dad; 18, M = Nb, dad = p-MeOC6H4-dad; 19, M = Ta, dad = Cy-dad) with the release of toluene. Treatments of 3a,b, 4a,b, 5, and 7 with a slight excess of 1,3-butadiene−magnesium adduct afforded mixed-ligand 16-electron complexes M(η5-C5R5)(η2-N,N‘-p-MeOC6H4-dad)(η4-s-cis-1,3-butadiene) (20a, M = Ta, R = CH3; 20b, M = Ta, R = H; 21a, M = Nb, R = CH3; 21b, M = Nb, R = H) and TaCp*(η2-N,N‘-dad)(η4-s-cis-1,3-butadiene) (22, dad = p-Tol-dad; 23, dad = Cy-dad), respectively. These complexes showed characteristic intense red or purple color due to LMCT bands where the filled N(pπ) orbital is donated to the empty M(dπ