Two coordination polymers formulated {Fe(p
md)
2[Ag(CN)
2]
2} (p
md = pyrimidine) have been synthesized andcharacterized. Both polymers, considered to be architectural isomers, display different crystal structures and magneticproperties. Isomer
1 crystallizes in the monoclinic
C2/
c space group with
a = 6.9750(8) Å,
b = 16.1700(9) Å,
c= 14.2020(8) Å,
= 97.954(2)
,
V = 1586.37(14) Å
3, and
Z = 4. The crystal structure of isomer
2 has beenstudied at 250 and 150 K. At both temperatures,
2 displays the orthorhombic
Pccn space group with
a = 15.7700(2) [14.8950(2)] Å,
b = 8.2980(4) [8.1580(4)] Å,
c = 13.4180(6) [13.3480(5)] Å,
V = 1755.87(14) [1621.96(10)] Å
3,and
Z = 4 for 250 [150] K. The iron(II) ions define distorted octahedral [FeN
6] chromophores in both isomers. Theequatorial positions are occupied by four [Ag(CN)
2]
- bridging ligands, which connect the defining layers of twoiron(II) ions. Isomer
1 has two crystallographically distinct [Ag(CN)
2]
- groups; one is essentially linear, while theother is severely distorted [C(5)-Ag(2)-C(5
i)] = 138.8(5)
. This fact facilitates the parallel interpenetration of twolayers, which in addition show short Ag(1)····Ag(2) interactions (distance Ag(1)····Ag(2) = 2.9972(10) Å). Isomer
2 shows only one type of Ag atom, which is slightly bent [C-Ag-C = 161.54(12)
], and as a consequence, thelayers defined are not interpenetrated. In both cases, the axial positions are occupied by the p
md ligands whichinteract with the Ag atoms of adjacent layers defining a 3D coordination polymer. Compound
1 is high spin in thewhole range of temperatures, while
2 undergoes a cooperative high-spin
low-spin effect centered at ca. 184 Kwith a hysteresis loop ca. 5 K wide. The experimental enthalpy and entropy variations were 11.5 ± 0.4 kJ mol
-1and 64 ± 3 J K
-1 mol
-1. Consistency between the experimental thermodynamic data and the magnetic data waschecked in the frame of regular solution theory.