Abstract
Variable temperature equilibrium studies were used to derive thermodynamic data for formation of η 1 nitrile complexes with Mo(N[ t Bu]Ar) 3 , 1. (1-AdamantylCN = AdCN: ΔH o = -6 ± 2 kcal mol -1 , ΔS o = -20 ± 7 cal mol -1 K -1 . C 6 H 5 CN = PhCN: ΔH o = -14.5 ± 1.5 kcal mol -1 , ΔS o = -40 ± 5 cal mol -1 K -1 . 2,4,6-(H 3 C) 3 C 6 H 2 CN = MesCN: ΔH o = -15.4 ± 1.5 kcal mol -1 , ΔS o = -52 ± 5 cal mol -1 K -1 .) Solution calorimetric studies show that the enthalpy of formation of 1-[η 2 -NCNMe 2 ] is more exothermic (ΔH o = -22.0 ± 1.0 kcal mol -1 ). Rate and activation parameters for η 1 binding of nitriles were measured by stopped flow kinetic studies (AdCN: ΔH on ‡ = 5 ± 1 kcal mol -1 , ΔS on ‡ = -28 ± 5 cal mol -1 K -1 ; PhCN: ΔH on ‡ = 5.2 ± 0.2 kcal mol -1 , ΔS on ‡ = -24 ± 1 cal mol -1 K -1 ; MesCN: ΔH on ‡ = 5.0 ± 0.3 kcal mol -1 , ΔS on ‡ = -26 ± 1 cal mol -1 K -1 ). Binding of Me 2 NCN was observed to proceed by reversible formation of an intermediate complex 1-[η 1 -NCNMe 2 ] which subsequently forms 1-[η 2 -NCNMe 2 ]: 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-[η 1 -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-[η 2 -NCNMe 2 ] 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-[η 1 - NC(SPh)NMe 2 ] is reported. Trapping of in situ generated 1-[η 1 -NCNMe 2 ] by PhSSPh was successful at low temperatures (-80 to -40°C) as studied by stopped flow methods. If 1-[η 1 -NCNMe 2 ] is not intercepted before isomerization to 1-[η 2 -NCNMe 2 ] 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 η 1 -bound nitriles. A complete reaction profile for reversible ligand binding, η 1 to η 2 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. © 2009 American Chemical Society.