Dinitrogen Cleavage by Three-Coordinate Molybdenum(III) Complexes:  Mechanistic and Structural Data

The synthesis and characterization of the complexes Mo[N(R)Ar]_3 (R = C(CD_3)_2CH_3, Ar = 3,5-C_6H_3Me_2), (μ-N_2){Mo[N(R)Ar]_3}_2, (μ-^(15)N_2){Mo[N(R)Ar]_3}_2, NMo[N(R)Ar]_3, ^(15)NMo[N(R)Ar]_3, Mo[N(t-Bu)Ph]_3, (μ-N_2){Mo[N(t-Bu)Ph]_3}_2, and NMo[N(t-Bu)Ph]_3 are described. Temperature-dependent magnetic susceptibility data indicate a quartet ground state for Mo[N(R)Ar]_3. Single-crystal X-ray diffraction studies for Mo[N(R)Ar]_3 and NMo[N(t-Bu)Ph]_3 are described. Extended X-ray absorption fine structure (EXAFS) structural studies for Mo[N(R)Ar]_3, (μ-N_2){Mo[N(R)Ar]_3}_2, and NMo[N(R)Ar]_3 are reported. Temperature-dependent kinetic data are given for the unimolecular fragmentation of (μ-N_2){Mo[N(R)Ar]_3}_2 to 2 equiv of NMo[N(R)Ar]_3 and for the fragmentation of (μ-^(15)N_2){Mo[N(R)Ar]_3}_2 to 2 equiv of ^(15)NMo[N(R)Ar]_3. The temperature dependence of the ^(15)N_2 isotope effect for the latter N_2 cleavage process was fitted to a simple harmonic model, leading to a prediction for the difference in NN stretching frequencies for the two isotopomers. The latter prediction was consistent with the Raman spectroscopic data for (μ-N_2){Mo[N(R)Ar]_3}_2 and (μ-^(15)N_2){Mo[N(R)Ar]_3}_2. The Raman spectroscopic data and EXAFS results are both consistent with an NN bond order of approximately 2 in (μ-N_2){Mo[N(R)Ar]_3}_2. Temperature-dependent magnetic susceptibility data consistent with a triplet ground state are given for (μ-N_2){Mo[N(t-Bu)Ph]_3}_2