Synthesis of Zwitterionic Triphosphenium Transition Metal Complexes: A Boron Atom Makes The Difference

A collection of zwitterionic
phosphanide metal carbonyl coordination complexes has been synthesized
and fully characterized, representing the first isolated series of
metal complexes for the triphosphenium family of compounds. The dicoordinate
phosphorus atom of the zwitterion is formally in the +1 oxidation
state and can coordinate to one metal, 2M (M = Cr, Mo,
W) and 2Fe, or two metals, a Co2(CO)6 fragment 4, depending on the starting reagents. All
complexes have been isolated in greater than 80% yield, and structures
were confirmed crystallographically. Metrical parameters are consistent
with 1 being a weak donor and results in long metal–phosphorus
bonds being observed in all cases. Unique bimetallic structures, 3M (M = Cr, Mo, W), consisting of a M­(CO)5 fragment
on phosphorus and a piano-stool M­(CO)3 fragment on a boron
phenyl group have been identified in the 31P­{1H} NMR spectra and confirmed using X-ray diffraction studies. Use
of the borate backbone in 1, which renders the molecule
zwitterionic, proves to be a determining factor in whether these metal
complexes will form; the halide salt of a cationic triphosphenium
ion, 6[Br], shows no evidence for formation of the analogous
metal complexes by 31P­{1H} NMR spectroscopy,
and tetraphenylborate salts, 6[BPh4] and 7[BPh4], produce complexes that are unstable.