Self-assembly of tetranuclear iron(II) compounds by a combination of rod- and clamp-shaped bridging units

Dicyanomethanides, [RC(CN)(2)](-) (1(-)), are bridging ligands with an angu lar clamp-type shape. In the tripod/iron(II) system, [tripod = CH3C(CH2PPh2 )(3)], they form binuclear species with three of the clamp-type ligands bri dging two iron centers. An appropriate stoichiometric mixture of the tripod ligand, iron(II) salts and the dicyanomethanide ligand leads to the exclus ive formation of [tripodFe{mu -NC-C(R)-CN)(3)-Fetripod](+) (3(+)), Altogeth er, seven constituent groups arrange themselves in this type of aggregate. The process of self-aggregation is distinctly modified by the presence of c yanide ions that may act as linear rod-type bridging ligands. Equimolar mix tures of tripod, iron(II) salts, dicyanomethanide ligands and cyanide form tetranuclear aggregates [(tripodFe)(3){mu -NC-C(R)-CN}(3){mu -CN}(3)FeX](+) (5(+)). In these species, the iron centers form a trigonal-pyramidal arran gement with three octahedrally coordinated low-spin tripodiron(II) entities in the basal plane and a tetrahedrally coordinated high-spin iron(II) at t he apex. The iron centers in the basal plane are connected by mu (2)-bridgi ng dicyanomethanide ligands. The apical iron center is coordinated by the N -termini of three cyano ligands, which complete the octahedral coordination of the basal low-spin iron(II) centers. The fourth, external ligand X at t he tetrahedrally coordinated high-spin iron(H) apex can be varied, but in m ost cases it is found to be a terminally coordinated dicyanomethanide entit y, Without counting this variable ligand X, thirteen constituent groups of four different types selectively aggregate to form the cage compounds 5(+.) While the dinuclear compounds 3 are diamagnetic, the tetranuclear species 5(+) have magnetic moments close to the spin-only value of mu (S.O.) = 4.9 mu (B), for tetrahedral high-spin iron(II). The Mossbauer spectra are in ag reement with the description of 5(+) above, which contain three low-spin tr ipodiron(II) entities and one high-spin iron(II) center. The synthesis, spe ctroscopic data, cyclic voltammetric data, Mossbauer data and X-ray analyti cal data for a series of compounds of types 3(+) and 5(+) are presented in this paper.