Remote Effects of Axial Ligand Substitution in Heterometallic CrCr···M Chains

The heterometallic complexes CrCrM(dpa)4Cl2 (dpa = 2,2′-dipyridylamide) featuring linear Cl–CrCr···M–Cl chains can regiospecifically be modified via axial ligand substitution to yield OTf–CrCr···M–Cl chains (OTf = triflate) with M being Fe, Mn, or Co. The effect of OTf substitution on the Cr side of the molecule has an unusual and profound structural impact on the square-pyramidal transition metal M. Specifically, elongation of the four equatorial M–Npy bonds and the axial M–Cl bonds by 0.03 and 0.09 Å for Fe and 0.07 and 0.11 Å for Mn is observed. The longer M–Cl and M–Npy bonds result from subtle interactions between the equatorial dpa ligand and the three metal ions. The equatorial dpa ligand responds to the introduction of the more labile OTf ligand at Cr by binding more strongly to this Cr ion which in turn weakens bonding to M. The ligand field experienced by M can be tuned by changing the Cr axial ligand, and this effect is observed in electrochemical measurements of the iron compounds.