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

Treatment of tetrakis(dimethylamido)molybdenum(IV) (Mo(NMe2)4) with 4 equiv of 3,5-dimethylpyrazole (Me2pzH) in pentane yields Mo(Me2pzH)(Me2pz)3(η2-CH2NHCH3) (1), whereby the reaction with 3,5-di-tert-butylpyrazole (t-Bu2pzH) leads to Mo(t-Bu2pz)3(NMe2) (2), independent of the molar ratio. The homoleptic Mo(t-Bu2pz)4 (3) was synthesized from tetrachlorobis(tetrahydrofurano)molybdenum(IV) (MoCl4(thf)2) with 4 equiv of 3,5-di-tert-butylpyrazolatopotassium (t-Bu2pzK). Magnetic measurements show the three complexes to be paramagnetic with two unpaired electrons (1, 3.0; 2, 2.8; and 3, 2.8 μB, respectively). The molecular structures of all compounds determined by X-ray diffraction analysis are reported. Complex 1 reveals a η2-coordinate CH2NHCH3 ligand arranged in an azametallacyclic 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 η1-, “slipped” η2-, and η2-coordinate ligands.