Rational design of rhodium complexes featuring κ⁴-N,N,N,N- and κ⁵-N,N,N,P,P-Bis(imino)pyridine ligands

The addition of aminoalkyl-substituted 2,6-bis(imino)pyridine (or pyridine diimine, PDI) ligands to [(COD)RhCl]₂ (COD = 1,5-cyclooctadiene) resulted in the formation of rhodium monochloride complexes with the general formula (^NPDI)RhCl (^NPDI = ^iPr₂ ^NEtPDI or ^Me₂^NPrPDI). The investigation of (^iPr₂^NEtPDI)RhCl and ( ^Me₂^NPrPDI)RhCl by single-crystal X-ray diffraction verified the absence of amine arm coordination and a pseudo square-planar geometry about rhodium. Replacement of the chloride ligand with an outer-sphere anion was achieved by adding AgBF₄ directly to ( ^iPr₂^NEtPDI)RhCl to form [(^iPr ₂^NEtPDI)Rh][BF₄]. Alternatively, this complex was prepared by chelate addition following the salt metathesis reaction between AgBF₄ and [(COD)RhCl]₂. Using the latter method, both [(^NPDI)Rh][BF₄] complexes were isolated and found to exhibit κ⁴-N,N,N,N-PDI coordination regardless of arm length or steric bulk. In contrast, the metallation of ^PPDI chelates featuring alkylphosphane imine substituents (^PPDI = ^Ph ₂^PEtPDI or ^Ph₂^PPrPDI) resulted in the formation of cationic complexes featuring κ⁵-N, N,N,P,P-PDI coordination in all instances, [(^PPDI)Rh][X] (X = Cl, BF₄). Adjusting the metallation stoichiometry allowed the preparation of [(^Ph₂^PPrPDI)Rh][(COD)RhCl₂], as validated by multinuclear NMR spectroscopy and single-crystal X-ray diffraction. The coordinative flexibility of newly prepared PDI ligands has been evaluated following their addition to [(COD)RhCl]₂. Tailoring this chelate class to include donor atoms has allowed the preparation of complexes featuring tetradentate and pentadentate PDI coordination.