N,N′‐Bis(2,2′‐bipyridine‐6‐ylmethyl)‐2,2′‐biphenylenediamines: A Tuneable Ligand Scaffold for Room Temperature Fe²⁺ SCO Complexes

Abstract

Condensation and subsequent reduction of 2,2′‐diaminobiphenyls 5 with 6′‐ and 5′‐substituted 6‐carbaldehyde‐2,2′‐bipyridines 4 yielded N,N′‐bis(2,2′‐bipyridine‐6‐ylmethyl)‐2,2′‐biphenyl‐enediamines 7, which were employed as hexadendate ligands with N6 donor sets in the synthesis of dicationic [Fe2+(7‐κ6N)] complexes 8. Dependent on the substitution pattern the respective complexes are found in the HS state (8b and 8c) or show SCO behaviour. By means of temperature‐dependent susceptibility measurements, usingEvans' method, the thermodynamic parameters ΔH, ΔS and T1/2 for the SCO have been determined. T1/2 as well as ΔH are remarkably susceptible to substitution next to the central C–C bond of the biphenyl bridge.

Modification of a Hexadentate Amine based Ligand System by N‐Methylation and Effects on Spin State and Redox ­Behavior of the Corresponding Transition Metal Complexes

“…CH 2 Cl 2 / H 2 O) and following counterion exchange with NaBF 4 or NH 4 PF 6 . Due to the d 7 configuration of Co II , complexes 4 are in every case paramagnetic, leading to spreading of signals in the respective 1 H NMR spectra from 130 ppm to about –20 ppm, as also observed for the analogous paramagnetic Fe II complexes 3b and 3c ,38 which is exemplarily shown in Figure 1 for 4a . The number of signals observed for complexes 4 points towards a C 2 symmetric structure, similar to analogous Fe II complexes 3 .…”

“…In contrast to the Fe II ‐SCO complex 3a the N ‐methylated complex 5 is a high‐spin complex at all temperatures, which reveals the drastic influence of the N ‐methyl group on the spin‐state of Fe II . This can be attributed to the steric influence of the additional methyl group at the amino function ( vide infra ), similarly to the steric influence of the substituents in the ortho position of the terminal pyridine found in high‐spin Fe II complexes 3b and 3c 38. In light of the found high‐spin state of complex 5 it is not surprising that the analogous cobalt(II) complex ( 6 ) is also found in its pure high‐spin state as cobalt(II) low‐spin complexes require a stronger ligand field than low‐spin iron(II) complexes (Figure 2).…”

“…Conversion of the secondary amine 1a to the tertiary amine 2 was achieved by treatment with formaldehyde and sodium borohydride 43. The synthesis of complexes 3 – 7 was accomplished in an analogous manner as previously published 38. An overview of the accomplished synthesis strategy is given in Scheme .…”

“…The cobalt complexes 4 are yielded similarly to iron complexes 3 38 by reaction of the free hexadentate base 1 with Co(NO 3 ) 2 · 6H 2 O in ethanol or a mixture of solvents (e.g. CH 2 Cl 2 / H 2 O) and following counterion exchange with NaBF 4 or NH 4 PF 6 .…”

“…Recently, we described the synthesis of a series of hexadentate nitrogen based ligands 1a – d based on a biphenylene‐2,2′‐diamino backbone with additional bipyridine moieties,38 similar to other ligand systems reported by Constable et al39–41 and Malachowski 42. Depending on the substitution pattern, the respective iron(II) complexes either exhibited Spin Crossover (SCO) behavior with transition temperatures T ½ well above room temperature, or were present as high‐spin complexes.…”