A trigonal and hindered tertiary phosphineligand rendered anionic by a niobate anchor: Formation of zwitterionic M(i) (M = Cu, Ag, Au, Rh) complexes

Abstract

Treatment of terminal phosphide anion [PNb(N[Np]Ar)₃]⁻ ([Na(OEt₂)]⁺ salt, Ar = 3,5-Me₂C₆H₃) with two diphenylketene equivalents led to isolation of anion [P(C{CPh₂}O)₂Nb(N[Np]Ar)₃]⁻ in 78% yield as its [Na(THF)]⁺ salt. The latter reacts with diphenylketene (1 equiv) to provide the triple diphenylketene addition product [P(C{CPh₂}O)₃Nb(N[Np]Ar)₃]⁻ in 88% yield as its [Na(THF)]⁺ salt; the same material is obtained alternatively in 93% yield by reaction of diphenylketene (3 equiv) directly with niobium phosphide [Na(OEt₂)][PNb(N[Np]Ar)₃]. The anion [P(C{CPh₂}O)₃Nb(N[Np]Ar)₃]⁻ was also crystallized as its ion-separated [Na(THF)₆]⁺ salt as illuminated by a single-crystal X-ray diffraction study, which also revealed the three-fold symmetric nature of the highly hindered tertiary phosphine comprising the anionic component. Coinage metal monocations of the new, anionic phosphine were prepared via salt elimination; structural studies on the zwitterionic complexes (py)M[P(C{CPh₂}O)₃Nb(N[Np]Ar)₃] (M = Cu, Ag, and Au) showed them to be isostructural in the space groupP2₁/c while illustrating the ensconcement of a two-coordinate coinage metal in a deep binding pocket flanked by three phenyl residues. Rhodium(I) incorporation into the anion's binding pocket illustrated versatility of the latter: in contrast to structural observations for the M(I) complexes (M = Cu, Ag, and Au), in the case of Rh[P(C{CPh₂}O)₃Nb(N[Np]Ar)₃] an X-ray study reveals strong interactions (η⁶ and η², respectively) with two of the three phenyl residues that are proximal to Rh(I) when connected to the phosphine lone pair of the anion.