Structural Design Parameters for Highly Birefringent Coordination Polymers

A series of coordination polymer
materials incorporating the highly anisotropic 2-(2-pyridyl)-1,10-phenanthroline
(phenpy) building block have been synthesized and structurally characterized.
M­(phenpy)­[Au­(CN)2]2 (M = Cd, Mn) are isostructural
and form a 1-D chain through bridging [Au­(CN)2]− units and extend into a 2-D sheet through aurophilic interactions.
M­(phenpy)­(H2O)­[Au­(CN)2]2·2H2O (M = Cd, Mn, and Zn) are also isostructural but differ from
the first set via the inclusion of a water molecule into the coordination
sphere, resulting in a 1-D topology through aurophilic interactions.
In­(phenpy)­(Cl)2[Au­(CN)2]·0.5H2O forms a dimer through bridging chlorides and contains a free [Au­(CN)2]− unit. In the plane of the primary crystal
growth direction, the birefringence values (Δn) of 0.37(2) (Cd­(phenpy)­[Au­(CN)2]2), 0.50(3)
(In­(phenpy)­(Cl)2[Au­(CN)2]·0.5H2O), 0.56(3) and 0.59(6) (M­(phenpy)­(H2O)­[Au­(CN)2]2·2H2O M = Cd and Zn, respectively) were
determined. β, a structural parameter defined by phenpy units
rotated in the A–C plane
relative to the light propagation (C) direction,
was found to correlate to Δn magnitudes. The
addition of a carbon–carbon double bond to terpy has increased
the molecular polarizability anisotropy of the building block, and
all structures have reduced deviation from planarity in comparison
to terpy and terpy derivative structures, leading to these higher
Δn values, which are among the highest reported
for crystalline solids.