Asymmetric Cationic Phosphines: Synthesis, Coordination Chemistry, and Reactivity.

A single enantiomer of a cationic phosphine, [α-CgPAmHMe]BF4, containing two asymmetric subunits, an amidinium group (AmH) and a phosphacycle (CgP), has been synthesized and isolated. The ligand, which is of an extremely rare class, has been coordinated to Rh(I), Au(I), Ag(I), Cu(I), and Pt(0) to enable an empirical assessment of its donor properties. Analysis of the IR stretching frequency of the carbonyl ligand in trans-[Rh(α-CgPAmHMe)2(CO)Cl](BF4)2 coupled with metric data obtained from crystal-state molecular structures of pertinent complexes confirms the strong π-accepting properties of the ligand. The integrity of the N-P bond is compromised upon addition of base to both [Cu(α-CgPAmHMe)Cl]BF4 and [Ag(α-CgPAmHMe)(OTf)]BF4 where, instead of isolating anticipated chelating and/or bridging forms of the neutral, deprotonated α-CgPAmMe derivative, decomposition products were obtained containing a phosphacycle fragment and/or amidine ligands. The fragility of the N-P bond is also evident in uncoordinated [α-CgPAmHMe]BF4 as treatment with aqueous base releases a neutral amidine fragment and generates the P-P dimer [α,α-CgPP(O)Cg]. These fortuitous observations show [α-CgPAmHMe]BF4 to be a very useful synthon for the potential production of novel asymmetric phosphines.