Addition of Primary and Secondary Phosphines to Dinuclear Tantalum Hydrides: Synthesis of Phosphides and Phosphinidenes via P−H Bond Activation

The dinuclear complex (RPh[NPN]Ta)2(μ-H)4 (where RPh[NPN] = [RP(CH2SiMe2NPh)2] and
R = Ph, Cy) shows remarkable reactivity with molecular nitrogen, producing the side-on
end-on bound dinitrogen complex (RPh[NPN]Ta)2(μ-η1:η2-N2)(μ-H)2. This tantalum tetrahydride
also promotes other small molecule activation. Addition of secondary phosphines HPR‘2 (R‘
= Ph, Cy) to (RPh[NPN]Ta)2(μ-H)4 accesses trihydride phosphide systems (RPh[NPN]Ta)2(μ-H)3(μ-PR‘2). Addition of primary phosphines H2PR‘ ‘ (R‘ ‘ = Cy, Ad) gives bridging phosphinidene tantalum complexes (RPh[NPN]Ta)2(μ-H)2(μ-PR‘ ‘). Molecular structures as determined
by X-ray crystallography for the complexes (PhPh[NPN]Ta)2(μ-H)3(μ-PCy2) and (CyPh[NPN]Ta)2(μ-H)2(μ-PAd) are presented. Isotopic labeling studies offer mechanistic insights into
the reactivity of this tetrahydride system. Reaction of Cy2PH with (CyPh[NPN]Ta)2(μ-D)4, for
instance, gives solely the monohydride dideuteride complex (CyPh[NPN]Ta)2(μ-H)(μ-D)2(μ-PCy2). This labeling study indicates that D2 loss forms the reactive species, (CyPh[NPN]Ta)2(μ-D)2, which promotes small molecule activation. Addition of CyPH2 to (CyPh[NPN]Ta)2(μ-D)4 gives a series of isotopomers, with an average composition of 0.67 H and 1.33 D,
supporting a 1,2-H2 elimination step to form the tantalum phosphinidenes. These results
are tied into the general reactivity of (RPh[NPN]Ta)2(μ-H)4.