Silver dichloroacetate:: A compound with weak Ag-Cl bonding interactions and an extraordinary range of 35Cl NQR frequencies

Silver complexes of halocarbons and silver salts of halogenated organic acids (for example, silver chloroacetate) often show secondary Ag center dot center dot center dot X bonding interactions and unusually low Cl-35 NQR frequencies, due to secondary bonding of chlorines to silver atoms. The crystal structure of silver dichloroacetate has been determined at 100 K and shows six crystallographically-inequivalent chlorines. The structure is built from Ag-2(OOCCHCl2)(2) dimers, similar to those found in silver chloroacetate; in both compounds the dimers are linked by additional Ag-O and Ag-Cl bonds. In the structure of silver dichloroacetate, two distinct conformations of the dichloromethyl groups are present. Two chlorines have no silver neighbors closer than 3.50 angstrom; two bridge to one Ag atom each, at Ag center dot center dot center dot Cl secondary bond distances of 2.8203(4) and 3.0196(4) angstrom, and two are apical, coordinating to at least two Ag neighbors each, at longer bond distances of 3.1401 (3)-3.3704(4) angstrom. Such very long distances are nevertheless shorter than the sum of the van der Waals radii of silver and chlorine, ca. 3.45 angstrom. The Cl-35 NQR spectrum of silver dichloroacetate at 77 K shows six signals scattered over the broad range from 35.600 to 38.498 MHz. Their EFG asymmetry parameters eta were measured by the Fourier analysis of the slow beats in the spin echo envelope of the NQR signal of polycrystalline samples. The two highest-frequency chlorines have relatively low eta values, 0.075 and 0.106, as befits Cl atoms not coordinated to Ag, and are placed by their conformations far from the carboxylate plane. The two middle-frequency chlorines have higher g values, 0.167 and 0.168, as expected for bridging Cl atoms. The two low-frequency chlorines have lower eta values of 0.114 and 0.129, as expected for apical Cl atoms. For purposes of comparison, eta values for Ag-2(OOCCH2Cl)(2), Na(OOCCH2Cl), and Ca(OOCCH2Cl)(2) center dot H2O were also recorded. So far, we have not observed any significant effect on the 35 Cl NQR parameters of halogenated organic anions coordinated to hard-acid metal ions (K+, Rb+, Ca2+). The effects of the different conformations of the Cl2CH groups on the broad NQR frequency range are also discussed. (C) 2008 Elsevier B.V. All rights reserved.