Synthesis and coordination chemistry of the diamido-arsine ligand [NAsN] (NAsN = PhAs(CH2SiMe2NPh)(2)))

The preparation and characterization of the diamido-arsine ligand [NAsN]Li-2(THF) (1) (where NAsN = PhAs(CH2SiMe2NPh)(2)), the protonated ligand precursor [NAsN]H-2 (2), and its coordination chemistry with tantalum is presented. The complex [NAsN]TaMe3 (3) can be synthesized from 1 and TaMe3Cl2. Hydrogenation of 3 did not produce the desired tetrahydride ([NAsN]Ta)(2)(mu-H)(4), instead, activation of Ta-N bonds in the complex produced a modest yield of the free ligand 2. In an attempt to understand the unusual reactivity of 3, a density functional theory investigation of the model complexes 'NAsN'Li-2(OMe2)(2) (4) and 'NAsN'TaMe3 (5) (where 'NAsN' = MeAs(CH2SiH2NMe)(2)) and their phosphine analogs 'NPN'Li-2(OMe2) (7), 'NPN'TaMe3 (8), and a related niobium complex 'NPN'NbMe3 (10) (where 'NPN' = MeP(CH2SiH2NMe)(2)) was undertaken. The difference between the chemistry supported by the As and P ligands originates from the poor binding of As to Ta in these systems and is likely due to a mismatch of the soft As donor and the hard Ta(V) metal centre.