NEW NIOBIUM COMPLEXES WITH ALKYNES .2. TETRANUCLEAR COMPOUNDS WITH NB-NB BONDS, AN UNPRECEDENTED TYPE OF TETRACARBON LIGAND, AND OXYGEN IN A RECTANGULAR ENVIRONMENT

Three compounds containing the tetranuclear anion [Nb4OClg{(PhC)4}2]2- have been prepared by (1) reduction of NbCl4(THF)2 with Mg in the presence of PhCCPh, (2) by reduction of NbCl3(PhCCPh)(THF)2 with Na/Hg, and (3) by reduction of NbCl3(PhCCPh)(THF)2 with Na/Hg in the presence of PEt3. The anion consists of a planar, rectangular Nb4 group (Nb-Nb = 2.605 (3) and 3.340 (3) Å) with an oxygen atom at the center (mean Nb-0 = 2.118 (1)Å). The long Nb-Nb edges are doubly bridged by Cl atoms, and there is one terminal Cl atom oneach Nb atom. A C-shaped PhC-C(Ph)C(Ph)-CPh chain clasps each short Nb-Nb edge at the middle and lies in a plane perpendicular to the Nb4O plane. This C4 chain and its mode of bonding to the pair of metal atoms has all C-C distances essentially equal at ca. 1.458 (9) Å.The four Nb-C distances to terminal C atoms have an average value of 2.133 (16) Å, and the four Nb-C distances to inner C atomshave an average value of 2.361 (5) Å. Crystallographic data for the three compounds are as follows: (1) [Mg2Cl3(THF)6]2[Nb4OCl8{(PhC)4}2]6THF, space group Pī, a = 15.200 (3) Å, b = 15.515 (4) Å, c = 16.306 (3) Å, α = 98.93 (2)°, β = 106.90 (2)°, γ = 102.97 (2)°, V= 3484 (3) Å3, Z = 1; (2) Na2[Nb4OCl8{(PhC)4)2]}(THF)6(C6H6)2, space group Pī, a = 11.802 (3) Å, b = 17.311 (2) Å, c = 22.966 (4) Å, α = 98.91 (1)°, β = 91.61 (1)°, 7 = 103.55 (1)°, V = 4496 (3) Å, Z = 2; (3) (HPEt3)2[Nb4OCl8{(PhC)4)2}]2C6H6, space group Pī, a = 12.831 (1) k, b= 13.194 (1) Å, c = 14.219 (1) Å, α = 68.040 (8)°, β = 66.616 (7)°, γ = 66.704 (7)°, V = 1961.0 (4) Å3, Z = 1. © 1990, American Chemical Society. All rights reserved.