Syntheses and Structures of Molybdenum(IV) Complexes with 3,5-Dimethyl- and 3,5-Di-tert-butylpyrazolato Ligands. Variable Coordination Modes and C−H Bond Activation

Abstract

Treatment of tetrakis(dimethylamido)molybdenum(IV) (Mo(NMe 2 ) 4 ) with 4 equiv of 3,5-dimethylpyrazole (Me 2 pzH) in pentane yields Mo(Me 2 pzH)(Me 2 pz) 3 (η 2 -CH 2 NHCH 3 ) (1), whereby the reaction with 3,5-di-tert-butylpyrazole (t-Bu 2 pzH) leads to Mo(t-Bu 2 pz) 3 (NMe 2 ) (2), independent of the molar ratio. The homoleptic Mo(t-Bu 2 pz) 4 (3) was synthesized from tetrachlorobis(tetrahydrofurano)molybdenum(IV) (MoCl 4 (thf) 2 ) with 4 equiv of 3,5-di-tert-butylpyrazolatopotassium (t-Bu 2 pzK). Magnetic measurements show the three complexes to be paramagnetic with two unpaired electrons (1, 3.0; 2, 2.8; and 3, 2.8 μ B , respectively). The molecular structures of all compounds determined by X-ray diffraction analysis are reported. Complex 1 reveals a η 2 -coordinate CH 2 NHCH 3 ligand arranged in an azametal-lacyclic fashion, verified by a small Mo-C-N angle and a typical C-N single bond length. The bonding mode of the pyrazolato ligands in all three compounds 1, 2, and 3 is influenced by the steric bulk around the metal center leading to η 1 -, slipped η 2 -, and η 2 -coordinate ligands.