Oligo-nuclear silver thiocyanate complexes with monodentate tertiary phosphine ligands, including novel ‘cubane’ and ‘step’ tetramer forms of AgSCN : PR3 (1 : 1)4

Abstract

Adducts of a number of tertiary pnicogen ligands ER₃ (triphenyl-phosphine and -arsine (PPh₃,AsPh₃), diphenyl,2-pyridylphosphine (PPh₂py), tris(4-fluorophenyl)phosphine (P(C₆H₄-4F)₃), tris(2-tolyl)phosphine (P(o-tol)₃), tris(cyclohexyl)phosphine (PCy₃)), with silver(I) thiocyanate, AgSCN are structurally and spectroscopically characterized. The 1 : 3 AgSCN : ER₃ complexes structurally defined (for PPh₃,AsPh₃ (diversely solvated)) take the form [(R₃E)₃AgX], the thiocyanate X = NCS being N-bound, thus [(Ph₃E)Ag(NCS)]. A 1 : 2 complex with PPh₂py, takes the binuclear form [(pyPh₂P)₂Ag(^SCN_NCS)Ag(PPh₂py)₂] with an eight-membered cyclic core. 1 : 1 complexes are defined with PPh₂py, P(o-tol)₃ and PCy₃; binuclear forms [(R₃P)Ag(^SCN_NCS)Ag(PR₃)] are obtained with P(o-tol)₃ (two polymorphs), while novel isomeric tetranuclear forms, which may be envisaged as dimers of dimers, are obtained with PPh₂py, and, as further polymorphs, with PCy₃; these latter may be considered as extensions of the ‘cubane’ and ‘step’ forms previously described for [(R₃E)AgX]₄ (X = halide) complexes. Solvent-assisted mechanochemical or solvent-assisted solid-state synthesis methods were employed in some cases, where complexes could not be obtained by conventional solution methods, or where such methods yielded a mixture of polymorphs unsuitable for solid-state spectroscopy. The wavenumbers of the ν(CN) bands in the IR spectra are in broad agreement with the empirical rule that distinguishes bridging from terminal bonding, but exceptions occur for compounds that have a double SCN bridged dimeric structure, and replacement of PPh₃ with PPh₂py apparently causes a significant decrease in ν(CN) to well below the range expected for bridging SCN in these structures. ³¹P CP MAS NMR spectra yield additional parameters that allow a correlation between the structures and spectra.