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

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 AgNO 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 AgS(3) (Ag-S 2.518(2)-2.592(1) angstrom, 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)](SO(4))center dot 4H(2)O, 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)(ClO(4)) center dot 0.5H(2)O, similar to that observed in CuX/tu (1:3) (X = Cl, Br, l). The remarkable 15:23 nitrate complex (shown to be distinct from the 1:1.5 complex) may be regarded as essentially ionic [Ag(15)(tu)(23)(ONO(2))]((infinity vertical bar infinity))(14+)(NO(3)(-))(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(a)(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.