Structural Variation in Transition-Metal Bispidine Compounds

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Abstract
The experimentally determined molecular structures of 40 transition metal complexes with the tetradentate bispyridine‐substituted bispidone ligand, 2,4‐bis(2‐pyridine)‐3,7‐diazabicyclo[3.3.1]nonane‐9‐one [M(bisp)XYZ]n+; M=CrIII, MnII, FeII, CoII, CuII, CuI, ZnII; X, Y, Z=mono‐ or bidentate co‐ligands; penta‐, hexa‐ or heptacoordinate complexes) are characterized in detail, supported by force‐field and DFT calculations. While the bispidine ligand is very rigid (N3⋅⋅⋅N7 distance=2.933±0.025 Å), it tolerates a large range of metal–donor bond lengths (2.07 Å<Σ(MN)/4<2.35 Å). Of particular interest is the ratio of the bond lengths between the metal center and the two tertiary amine donors (0.84 Å<MN3/MN7<1.05 Å) and the fact that, in terms of this ratio there seem to be two clusters with MN3<MN7 and MN3≥MN7. Calculations indicate that the two structural types are close to degenerate, and the structural form therefore depends on the metal ion, the number and type of co‐ligands, as well as structural variations of the bispidine ligand backbone. Tuning of the structures is of importance since the structurally differing complexes have very different stabilities and reactivities.