Coordination chemistry of complexes with metalmetal multiple bonds: Reversible coordination of cyanide ion by dimolybdenum and ditungsten hexaalkoxides

Abstract

The dimetal-hexaalkoxides of molybdenum and tungsten (MM) [ 1a : W 2(O t Bu) 6, 1b : W 2(OCH 2 t Bu) 6, 1c : Mo 2(O t Bu) 6, 1d : Mo 2(O t Pr) 6, 1e : Mo 2(OCH 2 t Bu) 6] react reversibly with one equivalent of cyanide ion as its n Bu 4N salt in non-polar media to form mono-adducts of formula [ n Bu 4N] +[M 2(OR) 6(CN)] − ( 2a–e). The spectroscopic data are consistent with the presence of a μ-CN moiety which interacts predominantly as a σ-donor, but do not discount alternative structures which exhibit rapid fluxionality. Addition of a second equivalent (or more) of n Bu 4NCN leads to the reversible formation of 1,2-diadducts of formula [ n Bu 4N] 2 +[M 2(OR) 6(CN) 2] 2− ( 3a, 3b, 3d, 3e). Coordination of a second equivalent of cyanide is significantly less favorable than the first so that the consecutive equilibria may be measured. Determination of K eq for the equilibrium M 2(OR) 6+CN − ⇋ [M 2(OR) 6(CN)] − allows the first quantitative comparison of the Lewis acidity of the reactive dimetal hexaalkoxides. The identity of the metal and steric factors have a significant effect as shown by the following data: for the formation of 2a, Δ H° = − 11.2(1) kcal mol −1, Δ S° = − 18.6(7) eu; 2c, Δ H° = − 8.8(1) kcal mol −1, Δ S° = − 18.8(9) eu; 2d, Δ H° = − 10.5(1) kcal mol −1, Δ S° = − 18.0(8) eu. Complexes 2b and 2e showed no signs of dissociation under similar conditions and thermodynamic parameters associated with their formation may only be estimated (Δ H° = − 16.9 kcal mol −1, Δ S° = − 18.5 eu and Δ H° = − 13.5 kcal mol −1, Δ S° = − 18.5 eu respectively). At high temperatures the equilibrium becomes fast relative to the NMR time scale for molybdenum, and this allows an estimation of the rate of dissociation by dynamic lineshape analysis. For [Mo 2(O t Bu) 6 (CN)] − ( 2c) Δ H‡ = 22.0(5) kcal mol −1, Δ S‡ = 13.5(8) eu while for [Mo 2(OCH 2 t Bu) 6 (CN)] − ( 2e) Δ H‡ = 22.2(5) kcal mol −1, Δ S‡ = 13.9(8) eu. Line broadening was not observed for the tungsten alkoxide complexes 2a–b consistent with the greater thermodynamic strength of the W 2(CN) bond (ΔΔ H° = − 2.4 kcal mol −1). The second equilibrium [M 2(OR) 6(μ-CN)] − + CN − ⇋ [M 2(OR) 6(CN) 2] 2− shows a similar dependence on the nature of the metal and alkoxide: for formation of 3a, Δ H° = − 9.3(5) kcal mol −1, Δ S° = − 29(2) eu; for 3e, Δ H° = − 11.6(5) kcal mol −1, Δ S° = − 29(2) eu. In contrast, 3c was not detected under any conditions (Δ H° = − 6.9 kcal mol −1, Δ S° = − 29.2 eu, estimated) while 3b was undissociated under similar conditions (Δ H° = − 15.0 kcal mol −1, Δ S° = − 29.2 eu, estimated).