Crystal and molecular structure of a second, high-density polymorph of silver malonate

A new orthorhombic polymorph of silver malonate (II) has been synthesized and structurally characterized. Compound II crystallizes in space group Pnma, with a = 12.8180(11), b = 9.2479(8), c = 4.0219(3) Å; V = 476.75(7) Å3; Z = 4; ρcalc = 4.427 g cm−3. Full-matrix least-squares refinement converged to R1 = 0.0099 (I > 2σ(I), 765 data) and wR2 = 0.0264 (F2, 785 data, 51 parameters). The familiar eight-membered Ag2(RCO2)2 ring, characteristic of most silver(I) carboxylate complexes, has a Ag–Ag distance of 2.977(1) Å. Puckered sheets in the crystal ab plane are connected along c to form a three-dimensional coordination polymer. The density of II is >30% higher than that of the monoclinic polymorph (I), first characterized in 1981. A survey of the density ratios of polymorphs in the Cambridge Structural Database indicates that virtually all of the pairs of entries having a density ratio >1.2 involve at least one form with significant void volume, as estimated using PLATON. However, neither I nor II have significant void volume, which suggests that the silver malonate polymorphs represent one of the largest density differences observed to date for a polymorphic pair. To date, it has not been possible to prepare I in our laboratories.