Polymorphic Lattice and Conformational Effects on the Spin Crossover of [(TpMe,Me)2Fe]

The scorpionate sandwich complex [(TpMe,Me)2Fe], where TpMe,Me is hydrotris(3,5-dimethyl-1-pyrazolyl)borate, has long been of interest because it exhibits spin crossover in the solid state. Previous X-ray crystallographic studies revealed that the complex crystallizes on a triclinic lattice with a nearly ideal D3d geometry. The structure is consistent with high-spin (S = 2) Fe(II) at 298 K, but becomes low-spin (S = 0) at low temperature. We report an alternative trigonal polymorph, in which the complex adopts a distorted conformation of S6 symmetry with a large pyrazolyl ring torsion. This distortion inhibits spin crossover. An X-ray crystal structure performed at 298 K confirms a high-spin state, and this is retained in a second structure at 100 K. Meta-analysis of known [(TpMe,Me)2M] complexes indicates that every example crystallized in either a distorted conformation on an isomorphous trigonal lattice, or an undistorted conformation on a triclinic lattice, depending on the size of the central metal ion. High-spin Fe(II) falls exactly at the dividing line; structures with longer M–N bonds are trigonal, and structures with shorter M–N bonds are triclinic, including low-spin Fe(II). The ligand conformation is coupled to lattice packing, and to spin crossover. Different intermolecular contacts between the lattices are elucidated.