Mechanochemical and solution synthesis, and crystal structures and IR and solid-state (CPMAS) NMR spectroscopy of some bis(triphenylphosphine)silver(i) mono- and di-hydrogencitrate systems

Abstract

The complex [(Ph₃P)₂Ag(H₂cit)]·EtOH (1; H₂cit⁻ = dihydrogencitrate = C₆H₇O₇⁻) contains [(Ph₃P)₂Ag(H₂cit)] molecules in which the silver atom is coordinated to two PPh₃ molecules and the two oxygen atoms of one of the ‘terminal’/1-carboxylate groups of the dihydrogencitrate group. The molecules form centrosymmetric hydrogen-bonded dimers in the solid. In [{(Ph₃P)₂Ag}₂(Hcit)], (2), unsymmetrical deprotonation of the citrate grouping is found, from the 1- and 3- (i.e. terminal and central) carboxylates: [(Ph₃P)₂Ag(O₂CCH₂C(OH) (CH₂COOH)CO₂)Ag(PPh₃)₂]. The above complexes, as well as [(Ph₃P)₃Ag(H₂cit)] (3) were prepared via conventional solution methods, involving the reaction of trisilver(I) citrate, citric acid and triphenylphosphine, and by a mechanochemical method involving the reaction of silver(I) oxide, citric acid and triphenylphosphine. IR studies of 1–3 show the presence of coordinated carboxylate and free carboxylic acid groups in the mono- and di-hydrogencitrate ligands, and the formation of 2 from 1 shows that dihydrogencitrate deprotonation can occur upon dissolution of 1 in protic solvents. High-field (9.40 T) ³¹P CPMAS NMR spectra were recorded and analysed, yielding heteronuclear ¹J(^107/109Ag,³¹P) and homonuclear ²J(³¹P,³¹P) spin–spin coupling constants.