Low-Coordinated Multiply-Bonded Dirhenium Complexes and Low-Valent Diniobium Complexes: Syntheses and Reactivity Study

Abstract

Reaction of [nBu4N]2[Re2Cl8] with Li[HC (N-2,4,6-Me3C6H2)] produced two intermediate Re26+ complexes, [nBu4N]{(μ-Cl) Cl4Re2[μ-η2-HC(N-2,4,6-Me3C6H2)2]2} (3) and [nBu4N]2{(μ-Cl)2 Cl4Re2[μ-η2-HC(N-2,4,6-Me3C6H2)2]2} (4), which have two bridging amidinate ligands. Complex 3 has a chloride atom bridging to the rhenium centers, while 4 has two chloride atoms bridging to the rhenium. The dirhenium quadruple bond length of 3 and 4 are 2.2492(4) A and 2.262(3) A, respectively. Treatment of 3 and 4 with three equiv of zinc powder in the mixture solvents of THF/Toluene engendered the formation of a quadruply-bonded dirhenium complex Re2Cl2(H)2 [μ-η2-HC(N-2,4,6-Me2C6H2)2]2 (2). In addition, Re2Cl3(H) [μ-η2-HC(N-2,4,6-Me2C6H2)2]2 (5) was also isolated when the solvent is toluene. The dirhenium quadruple bond length of 2 and 5 are 2.1853(6) A and 2.1890(7) A, respectively. Subsequent reduction of 3 and 4 with three equiv of zinc powder gave the first example of the quintuply-bonded dirhenium complex Re2Cl2[μ-η2-HC (N-2,4,6-Me2C6H2)2]2(6) (vide supra) and the quadruply-bonded complex (μ-η2-H){ReCl[μ-η2-HC (N-2,4,6-Me2C6H2)2]ReCl[μ-η2-HC(N-2,4,6-Me2C6H2)(N-2-Me-4-Me-6- (η1-CH2)C6H2)} (7) (vide supra), which has C−H bond activation of methyl group. Complexes 6 and 7 are in equilibrium in a ratio of 1:1 crystal. The dirhenium quintuple bond length of 6 is 2.1852(13) A. The dirhenium quadruple bond length of 7 is 2.1848(6) A. The results display that the dirhenium bond length are all similar. Moreover, the dirhenium(II) complex ClRe[μ-η2-H2C(PPh2)2]ReCl [μ-η2-H2C(PPh(η1-C6H4)2] (12) was prepared form the reaction of [nBu4N]2[Re2Cl8] with three equiv of zinc powder and two equiv of bis(diphenylphosphino)methane in THF. The formation of 12 is possibly via double C−H bond activation. Surprisingly, a novel tetrarhenium complex {ClRe[μ-η2-H2C(PPh2)2]ReCl[μ-η2-H2C(PPh2)2(PPh(η1-C6H4)]} (μ-Cl)2(μ-O){ClRe[μ-η2-H2C(PPh2)2]ReCl[μ-η2-H2C(PPh2)2(PPh (η1-C6H4)]} (15) is generated under the same condition. The possible formation of 15 is assumed from the reaction of ClRe(H)[μ-η2- H2C(PPh2)2]ReCl[μ-η2-H2C(PPh2)2(PPh(η1-C6H4))] (10) with water. Besides, the reaction between diphenylacetylene, and complex 10 yielded C2H(Ph)2ClRe[μ-η2-H2C(PPh2)2]ReCl[μ-η2-H2C(PPh2)2(PPh(η1-C6H4)] (18), which was formed via 1,2-insertion of diphenylacetylene. Treatment of NbCl3(DME) with Li[HC(N-2,6-iPr2C6H3)2] in the different solvents of toluene and THF engendered the formation of a diniobium complex (μ-Cl)3[Li(THF)2(μ-Cl)2][Nb(κ2-HC(N-2,6- iPr2C6H3)]2 (19) and a mononiobium complex NbCl2(THF) [η2-HC(N-2,6-iPr2C6H3)2] (20), respectively. To further investigate the reactivity of the diniobium complex 19 with organic azides. The diniobium cmplexes (N3H)Nb{[(μ-Cl)2[μ-η2-HC(N-2,6-iPrC6H3)]2} Nb(N3)2 (21) and (H2O)Nb{[(μ-Cl)2[μ-η2-HC(N-2,6- iPrC6H3)]2}Nb(N3)2 (22) are afforded from reaction of 19 with six equiv sodium azide. The results are all similar and show that the sodium azide were reduced and released one molecular of nitrogen. Reaction of 19 with p-tolyl-azide or 1-adamantyl azide gave rise to the formation of two mononiobium complexes (η1-p-tolyN)NbCl2(THF)[η2-HC (N-2,6-iPr2C6H3)2] (25) and (η1-AdN)NbCl2(THF)[η2-HC (N-2,6-iPr2C6H3)2] (26), respectively. The results are all similar and display that p-tolyl-azide or 1-adamantyl azide were reduced and released one molecular of nitrogen.