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 (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
4−6 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.