Structural and Infrared Spectroscopic Studies of Some Novel Mechanochemically Accessed Adducts of Silver(I) Oxoanion Salts with Thiourea

Abstract

An investigation of the silver(I) nitrate/thiourea ("tu") system, exploiting the uniquely powerful access provided by the recently developed combination of mechanochemical synthesis coupled with vibrational spectroscopy, has resulted in the identification of solid adducts AgNO3/tu (x:y) for x:y = 1:1, 1:1.5, 15:23 (= 1:1.5333), and 1:3. This contrasts with previously published claims for the existence of 1:2 and 1:4 complexes and was confirmed in the case of the 1:3 and 15:23 complexes by X-ray crystal structure determinations on samples prepared by crystallization from aqueous solution. The 1:3 sulfate and perchlorate complexes were also prepared for comparison with the corresponding nitrate compound. In the 1:3 nitrate complex, the one independent silver atom major component (0.78(1)) is closely trigonal planar AgS3 (Ag-S 2.518(2)-2.592(1) A, Sigma(S-Ag-S) 359.7(0) degrees); there are minor nearby components (0.11(1)) which may be regarded as four-coordinate, forming a putative one-dimensional polymer. The sulfate, obtained as its tetrahydrate, has a cation of the familiar binuclear form [{(tu)2Ag(mu-S-tu)}2](SO4) x 4 H2O, while the perchlorate, previously also characterized with the binuclear cation in the anhydrate, has now been isolated as the hemihydrate, having a single-stranded polymeric cation, [...Ag(tu)2(mu-S-tu)...](infinity|infinity)(ClO4) x 0.5 H2O, similar to that observed in CuX/tu (1:3) (X = Cl, Br, I). The remarkable 15:23 nitrate complex (shown to be distinct from the 1:1.5 complex) may be regarded as essentially ionic [Ag15(tu)23(ONO2)](infinity|infinity)(14+)(NO3(-)-)14 (the cation incorporating one O-nitrate ion loosely associated) and comprising one-dimensional polymer strands made up of four-, six-, and eight-membered Ag(2n)S(2n) rings, cross-linked by the sulfur of one of the tu ligands. The nu(CS) and nu(alpha)(CN) bands in the infrared spectra of the complete series of adducts show a monotonic wavenumber decrease and increase, respectively, with decreasing tu content of the adduct, indicating a decrease in the C-S bond strength and a concomitant increase in the C-N bond strength with increasing Ag-S bond strength. The increase in Ag-S bond strength with decreasing tu content is also indicated by an increase in the wavenumbers of the bands assigned to nu(AgS) in the 140-200 cm(-1) region in the far-IR. These and the structural data for the 1:3 and 15:23 complexes provide the basis for establishing a correlation between nu(AgS) and the Ag-S bond length d(AgS) for silver complexes involving the tu ligand.