Alkali Metal-Mediated Schlenk Dimerization of a Phosphorus-Substituted Alkene. Synthesis and Crystal Structures of Li, Na, and K Derivatives of a Phosphorus-Stabilized Butane-1,4-diide Ligand

Treatment of the vinylidene phosphine (Ph2P)2CCH2 (1) with either Li or Na in THF
yields the bis(diphosphinomethanide) complexes {(THF)2Li(Ph2P)2CCH2}2 (2), {(pmdeta)2Li(Ph2P)2CCH2}2·(PhMe)3 (6), and [{(tmeda)1.5Na(Ph2P)2CCH2}2]n (3b), after recrystallization
[tmeda = N,N,N‘,N‘-tetramethylethylenediamine, pmdeta = N,N,N‘,N‘‘,N‘‘-pentamethyldiethylenetriamine]. Treatment of 1 with K under the same conditions results in ligand
degradation to give K(PPh2), which was isolated as the adduct {(pmdeta)K(PPh2)}n (4) after
recrystallization from toluene/pmdeta, and unidentified organopotassium products. The
potassium bis(diphosphinomethanide) [{(Ph2P)2CCH2CH2C(PPh2)2}2K4(pmdeta)(DME)3·(PhMe)2]n (5) may be accessed via a metathesis reaction between 2 and KO-t-Bu in ether,
followed by recrystallization from toluene/DME in the presence of pmdeta [DME = 1,2-dimethoxyethane]. Compounds 3b, 4, 5, and 6 have been characterized by multielement
NMR spectroscopy and X-ray crystallography.