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Association of Dicyanodiphenylacetylenes with Silver(I) Salts in Solution and Solid State:  Electrospray Ionization Mass Spectrometry Samples Aggregates at Subsaturated Concentrations

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journal contribution
posted on 29.10.1997, 00:00 by Keith A. Hirsch, Scott R. Wilson, Jeffrey S. Moore
Complexes of 2,3‘-dicyanodiphenylacetylene (2,3‘-DCPA, 1), 2,2‘-dicyanodiphenylacetylene (2,2‘-DCPA, 2), and 2,4‘-dicyanodiphenylacetylene (2,4‘-DCPA, 3) with silver(I) salts have been characterized in the solid state by single-crystal X-ray analysis. In addition, aggregates of compounds 13 and silver(I) ions have been identified in solution by electrospray ionization mass spectrometry (ESI-MS). The topology of the structures in the solid state, namely finite versus infinite, is found to depend on the substitution pattern of nitrile groups on diphenylacetylene. For 2,3‘-DCPA (1), crystallization with silver(I) triflate (AgCF3SO3), silver(I) perchlorate hydrate (AgClO4·xH2O, x ∼ 1), or silver(I) hexafluoroantimonate (AgSbF6) produces cyclic dimers of composition [Ag(1)(X)]2 (X = CF3SO3- (4) or ClO4- (5)) and {[Ag(1)](SbF6)}2 (6). For these structures, 2,3‘-DCPA coordinates to silver(I) ions in a cisoid conformation with respect to the orientation of nitrile groups. Significant deformations of the cyclic dimers are observed as a function of the counterion employed. In contrast to the finite structures involving 2,3‘-DCPA, crystallization of 2,2‘-DCPA (2) with AgCF3SO3 yields the infinite chain structure [Ag(2)(CF3SO3)] (7). 2,2‘-DCPA coordinates to silver(I) in a transoid conformation resulting in a “half-bow-tie” motif for the chains. Crystallization of 2,4‘-DCPA (3) with AgCF3SO3 produces the infinite, undulating sheet structure [Ag(3)(CF3SO3)] (8) in which helical chains of 2,4‘-DCPA coordinated to silver(I) ions are bridged by triflate counterions. Positive ion ESI-MS of solutions of 2,3‘-DCPA (1) and AgCF3SO3, AgClO4·H2O, or AgSbF6 in acetone or acetonitrile show a distribution of aggregates including [Ag2(1)2(X)]+ (X = CF3SO3-, ClO4-, or SbF6-). The composition of these species corresponds to that of the cyclic dimers of complexes 46 minus one counterion, X. With 2,2‘-DCPA (2) and AgCF3SO3 in acetone or acetonitrile, the aggregates [Ag(2)]+ and [Ag(2)2]+ are observed and higher adducts are noted to be present in much lower abundance. It is believed that the predominance of adducts involving one silver(I) ion is due to the formation of chelated species in solution in which 2,2‘-DCPA coordinates to a silver(I) ion in a cisoid conformation. Molecular modeling suggests that such species are viable. ESI-MS of 2,4‘-DCPA (3) and AgCF3SO3 in acetone or acetonitrile shows the existence of [Ag(3)]+ as well as higher aggregates which are less prevalent. For a given ligand, aggregation at concentrations of the ligand and silver(I) salt of ca. 10-3 M is significant in acetone, however, it is largely disrupted in acetonitrile due to the predominance of acetonitrile·Ag(I) adducts. Analysis of the ESI-MS data for all three ligands with AgCF3SO3 in acetone or acetonitrile shows that the aggregate [Ag2(L)2(CF3SO3)]+ is most abundant for L = 2,3‘-DCPA (1). It is postulated that the relatively high abundance of [Ag2(1)2(CF3SO3)]+ is indicative of the formation of a cyclic dimer in solution that resembles complex 4. Similarly, it is believed that cyclic dimers exist in solution with ClO4- and SbF6- as well.

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