Volatility and High Thermal Stability in Mid- to Late-First-Row Transition-Metal Diazadienyl Complexes

Treatment of MCl2 (M = Cr, Mn, Fe, Co, Ni) with 2 equiv of lithium metal and 1,4-di-tert-butyl-1,3-diazadiene (tBu2DAD) in tetrahydrofuran at ambient temperature afforded Cr(tBu2DAD)2 (38%), Mn(tBu2DAD)2 (81%), Fe(tBu2DAD)2 (47%), Co(tBu2DAD)2 (36%), and Ni(tBu2DAD)2 (41%). Crystal structure determinations revealed monomeric complexes that adopt tetrahedral coordination environments and were consistent with tBu2DAD radical anion ligands. To evaluate the viability of M(tBu2DAD)2 (M = Cr, Mn, Fe, Co, Ni) as potential film growth precursors, thermogravimetric analyses, preparative sublimations, and solid-state decomposition studies were performed. Mn(tBu2DAD)2 is the most thermally robust among the series, with a solid-state decomposition temperature of 325 °C, a sublimation temperature of 120 °C/0.05 Torr, and a nonvolatile residue of 4.3% in a preparative sublimation. Thermogravimetric traces of all complexes show weight loss regimes from 150 to 225 °C with final percent residues at 500 °C ranging from 1.5 to 3.6%. Thermolysis studies reveal that all complexes except Mn(tBu2DAD)2 decompose into their respective crystalline metal powders under an inert atmosphere. Mn(tBu2DAD)2 may afford amorphous manganese metal upon thermolysis.