Variable temperature equilibrium studies were used to derive thermodynamic data for formation of η1 nitrile complexes with Mo(N[ tBu]Ar)3, 1. (1-AdamantylCN = AdCN: ΔHo = -6 ± 2 kcal mol-1, ΔSo = -20 ± 7 cal mol-1 K-1. C6H5CN = PhCN: ΔHo = -14.5 ± 1.5 kcal mol-1, ΔS o = -40 ± 5 cal mol-1 K-1. 2,4,6-(H 3C)3C6H2CN = MesCN: ΔH o = -15.4 ± 1.5 kcal mol-1, ΔSo = -52 ± 5 cal mol-1 K-1.) Solution calorimetric studies show that the enthalpy of formation of 1-[η2-NCNMe 2] is more exothermic (ΔHo = -22.0 ± 1.0 kcal mol-1). Rate and activation parameters for η1 binding of nitriles were measured by stopped flow kinetic studies (AdCN: ΔH on‡ = 5 ± 1 kcal mol-1, ΔSon‡ = -28 ± 5 cal mol-1 K-1; PhCN: ΔHon‡ = 5.2 ± 0.2 kcal mol-1, ΔSon‡ = -24 ± 1 cal mol-1 K-1; MesCN: ΔH on‡ = 5.0 ± 0.3 kcal mol-1, ΔSon‡ = -26 ± 1 cal mol-1 K-1). Binding of Me2NCN was observed to proceed by reversible formation of an intermediate complex 1-[η1-NCNMe 2] which subsequently forms 1-[η2-NCNMe2]: ΔH‡k1 = 6.4 ± 0.4 kcal mol -1, ΔS‡k1 = -18 × 2 cal mol-1 K-1, and ΔH‡k2 = 11.1 ± 0.2 kcal mol-1, ΔS‡ k2 = -7.5 ± 0.8 cal mol-1 K-1. The oxidative addition of PhSSPh to 1-[η1-NCPh] is a rapid second-order process with activation parameters: ΔH‡ = 6.7 ± 0.6 kcal mol-1, ΔS‡ = -27 ± 4 cal mol-1 K-1. The oxidative addition of PhSSPh to 1-[η2-NCNMe2] also followed a second-order rate law but was much slower: ΔH‡ = 12.2 η 1.5 kcal mol-1 and ΔS‡ = -25.4 ± 5.0 cal mol-1 K-1. The crystal structure of 1-[η1- NC(SPh)NMe2] is reported. Trapping of in situ generated 1-[η1-NCNMe2] by PhSSPh was successful at low temperatures (-80 to -40°C) as studied by stopped flow methods. If 1-[η1-NCNMe2] is not intercepted before isomerization to 1-[η2-NCNMe2] no oxidative addition occurs at low temperatures. The structures of key intermediates have been studied by density functional theory, confirming partial radical character of the carbon atom in η1-bound nitriles. A complete reaction profile for reversible ligand binding, η1 to η2 isomerization, and oxidative addition of PhSSPh has been assembled and gives a clear picture of ligand reactivity as a function of hapticity in this system.
ASJC Scopus subject areas
- Colloid and Surface Chemistry