Steric and counterion effects on the structure of dipicolylamine nickel complexes

Four nickel complexes each containing an R-2,2′-dipicolylamine ligand species (RDPA; R = benzyl, isopropyl, or tert-butyl) were synthesized and structurally characterized. In the absence of an interfering coordinating counterion, BzDPA and iPrDPA form 1:2 nickel:ligand complexes, with two facial ligands completing an pseudooctahedral nickel(II) coordination environment. In contrast, the sterically hindered tBuDPA ligand instead forms 1:1 metal:ligand complexes, even in the absence of associating counterions. Two novel tBuDPA nickel complexes with different counterions are described: nickel(II) chloride gives rise to an unusual 2Ni–3Cl dimer complex, while nickel(II) nitrate affords a 1:1 nickel:ligand complex which crystallizes with both fac and mer conformations in the same unit cell.

Graphical abstractThe steric spacer in N-substituted dipicolylamine ligands, as well as the counterions, controls 1:2 versus 1:1 metal:ligand complexation; complexes presented include an unusual 2Ni–3Cl dimeric core and an example of a nickel complex with both facial and meridional coordination of the same linear tridentate ligand in different structural isomers present in the same unit cell.Figure optionsDownload full-size imageDownload as PowerPoint slide