Association of dicyanodiphenylacetylenes with silver(I) salts in solution and solid state: Electrospray ionization mass spectrometry samples aggregates at subsaturated concentrations

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 1-3 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 (AgCF ₃SO ₃), silver(I) perchlorate hydrate (AgClO ₄ · xH ₂O, x ~1) or silver(I) hexafluoroantimonate (AgSbF ₆) produces cyclic dimers of composition [Ag(1)(X)] ₂ (X = CF ₃SO ₃ ^- (4) or ClO ₄ ^- (5)) and {[Ag(1)](SbF ₆)} 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 AgCF ₃SO ₃ yields the infinite chain structure [Ag(2)(CF ₃SO ₃)] (7). 2,2'-DCPA coordinates to silver(I) in a transoid conformation resulting in a 'half-bow-tie' motif for the chains. Crystallization of 2,3'-DCPA (3) with AgCF ₃SO ₃ produces the infinite, undulating sheet structure [Ag(3)(CF ₃SO ₃)] (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 AgCF ₃SO ₃, AgClO ₄ · H ₂O, or AgSbF ₆ in acetone or acetonitrile show a distribution of aggregates including [Ag ₂(1) ₂(X)] ⁺ (X = CF ₃SO ₃ ^-, ClO ₄ ^-, or SbF ₆ ^-). The composition of these species corresponds to that of the cyclic dimers of complexes 4-6 minus one counterion, X. With 2,2'-DCPA (2) and AgCF ₃SO ₃ 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 AgCF ₃SO ₃ 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 AgCF ₃SO ₃ in acetone or acetonitrile shows that the aggregate [Ag ₂(L) ₂(CF ₃SO ₃)] ⁺ is most abundant for L = 2,3'-DCPA (1). It is postulated that the relatively high abundance of [Ag ₂(1) ₂(CF ₃SO ₃)] ⁺ 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 ClO ₄ ^- and SbF ₆ ^- as well.