The rates of reaction of N2CHSiMe3 with HMo(CO) 3Cp (Cp = η5-C5H5) in heptane obey the rate law -d[HMo(CO)3Cp]/dt = k[HMo(CO)3Cp][N 2CHSiMe3] (k = 0.035 ± 0.01 M-1 s -1 at 0 °C; Δ H‡= 11.7 ± 2.0 kcal/mol and ΔS‡ = -22.0 ± 3.0 cal/(mol K)). Isotopic scrambling between DMo(CO)3Cp and N2CHSiMe 3 occurs at a rate faster than the overall reaction. Reversible 1,2-addition to form the tightly bound intermediate [Me3SiCH 2Nβ=Nαδ+]][ δ-M(CO)3Cp] is proposed as the first step of the reaction. Spectroscopic and computational data support this formulation. The contact ion pairs can undergo heterolytic cleavage to ions or homolytic cleavage to radicals, and the solvent influence on kobs (THF > toluene > heptane) is interpreted in terms of this model. The enthalpy of this reaction has been measured by solution calorimetry at 272 K in THF: ΔH = -11.6 ± 1.2 kcal/mol. These data, together with computed organic reaction energies allow estimation of the bond strength between the three-electron donors · N2CHSiMe3 and · Mo(CO) 2Cp to be 25 ± 5 kcal/mol stronger than the two-electron Mo-CO bond. Coordination of N2CHSiMe3 to the complexes M(PR3)2(CO)3 (M = Mo, W; R = Cy, iPr; Cy = cyclohexyl; iPr = isopropyl) alters the course of reaction with HMo(CO)3Cp. The stoichiometric reaction of Me3SiCH= N=NMo(PiPr3)2(CO)3 with 2 equiv of HMo(CO)3Cp produces SiMe4, Mo(N2)(P iPr3)2(CO)3, and [Mo(CO) 3Cp]2. In the presence of excess N2CHSiMe 3 this reaction is catalytic and has been used to experimentally measure the heat of hydrogenation of N2CHSiMe3 to N 2 and SiMe4 by 2 equiv of HMo(CO)3Cp. The derived enthalpy of formation of N2CHSiMe3 (5.8 ± 3.0 kcal/mol) is in reasonable agreement with high-level theoretical calculations. X-ray crystal structure data are reported for W(CO) 2(N2CH2SiMe3)Cp: triclinic, space group P1̄, a = 6.3928(7) Å, b = 10.6551(12) Å, c = 10.8766(12) Å, α = 100.632(2)°, β= 96.254(2)°, V = 721.32 Å3, Z = 2.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry