Mechanochemical and solution synthesis, X-ray structure and IR and 31P solid state NMR spectroscopic studies of copper(i) thiocyanate adducts with bulky monodentate tertiary phosphine ligands

Abstract

A number of adducts of copper(I) thiocyanate with bulky tertiary phosphine ligands, and some nitrogen-base solvates, were synthesized and structurally and spectroscopically characterised. CuSCN : PCy₃ (1 : 2), as crystallized from pyridine, is shown by a single crystal X-ray study to be a one-dimensional polymer ...(Cy₃P)₂CuSCN(Cy₃P)₂CuSCN... (1) with the four-coordinate copper atoms linked end-on by S-SCN-N bridging thiocyanate groups. A second form (2), obtained from acetonitrile, was also identified and shown by IR and ³¹P CPMAS NMR spectroscopy to be mononuclear, with the magnitude of the dν_Cu parameter measured from the ³¹P CPMAS and the ν(CN) value from the IR clearly establishing this compound as three-coordinate [(Cy₃P)₂CuNCS]. Two further CuSCN/PCy₃ compounds CuSCN : PCy₃ (1 : 1) (3), and CuSCN : PCy₃ : py (1 : 1 : 1) (4) were also characterized spectroscopically, with the dν_Cu parameters indicating three- and four-coordinate copper sites, respectively. Attempts to obtain a 1 : 2 adduct with tri-t-butylphosphine have yielded, from pyridine, the 1 : 1 adduct as a dimer [(Bu^t₃P)(^SCN_NCS)Cu(PBu^t₃)] (5), while similar attempts with tri-o-tolylphosphine (from acetonitrile and pyridine (= L)) resulted in solvated 1 : 1 : 1 CuSCN : P(o-tol)₃ : L forms as dimeric [{(o-tol)₃P}LCu(^SCN_NCS)CuL{P(o-tol)₃}] (6 and 8). The solvent-free 1 : 1 CuSCN : P(o-tol)₃ adduct (7), obtained by desolvation of 6, was characterized spectroscopically and dν_Cu measurements from the ³¹P CPMAS NMR data are consistent with the decrease in coordination number of the copper atom from four (for 6) (P,N(MeCN)Cu,S,N) to three (for 7) (PCuS,N) upon loss of the acetonitrile of solvation. These results are compared with those previously reported for mononuclear and binuclear PPh₃ adducts which demonstrate a clear tendency for the copper centre to remain four-coordinate. The IR spectroscopic measurements on these compounds show that bands in the far-IR spectra provide a much more definitive criterion for distinguishing between bridging and terminal bonding than does an often-used empirical rule based on ν(CN) in the mid-IR, which leads to the wrong conclusion in some cases.