Dinitrogen Cleavage by Three-Coordinate Molybdenum(III) Complexes: Mechanistic and Structural Data1

Abstract

The synthesis and characterization of the complexes Mo[N(R)Ar] 3 (R = C(CD 3 ) 2 CH 3, Ar = 3,5-C 6 H 3 Me 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 .