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

Abstract

Recently, the synthesis of hexadentate ligands based on N,N′‐bis‐(2,2′‐bipyridine‐6‐ylmethyl)‐2,2′‐biphenylenediamine (1a–d) and the corresponding iron(II) complexes (3a–d) was reported by our group. In this contribution we present the synthesis of the analogous cobalt(II) complexes 4a–d. Together with the iron(II) complex the electrochemical behavior of the complexes 3 and 4 was investigated by cyclic voltammetry (CV). The aminomethyl substructure was identified as the main source of ligand degradation caused by chemical oxidation with air. Upon exposure to air the amine group in complexes 3 and 4 is oxidized to imine and even amide groups. Some examples (8, 9) of the oxidation products were characterized by X‐ray structure analysis. In order to increase the robustness of the FeII and CoII complexes towards oxidation, the ligand scaffold of 1a was modified by N‐methylation of the amino group yielding the tertiary amine 2. The corresponding iron(II) and cobalt(II) complexes employing 2 as ligand were synthesized {[Fe(2)][PF6]2 (5), [Co(2)][PF6]2 (6)} and fully characterized, their redox behavior and spin‐state was investigated by CV and Evans' method. It was found, that the introduced N‐methyl group leads to a substantial anodic shift of the MII–MIII redox potential and the stabilization of FeII and CoII high‐spin state. By means of X‐ray structure analysis these effects could be explained by repulsive steric effects of the methyl group.

1H NMR spectroscopic elucidation in solution of the kinetics and thermodynamics of spin crossover for an exceptionally robust Fe2+ complex

“…53 A substantial effect of N-methylation on the metrics of both, SCO and redox, became evident in previous work on a structurally related complex, employing a 2,2′-diamino-1,1′biphenyl bridge. 9 For instance, in this latter case, the methylation of the secondary amine resulted in a dramatic shift of the spin-state energies. Thermal SCO behavior applied to the non-N-methylated complex (T1 2 = 403 K), whereas purely HS character arises for the N-methylated complex.…”

“…At the same time, the oxidation potential for the Fe 2+/3+ redox transition shifted by 300 mV to more anodic potentials. 9 Both effects were ascribed to steric reasons; that is, to the repulsion between the methyl groups and the biphenyl core that hinders the approach of the ligand to the metal center and therefore favors the HS state or the Fe 2+/3+ -ion with the larger ion diameter.…”

“…In this regard, we have recently introduced Fe 2+ SCO complexes employing a hexadentate ligand that can be obtained by condensation of 2,2′-diaminobiphenyles and [2,2′-bipyridine]-6-carbaldehyde or 6′-substituted derivatives. [7][8][9] The complexes are stable against hydrolysis in water, but strongly coordinating solvents like dmso replace the hexadentate ligands. We also found that the presence of aminomethylene units directly bound to the aromatic ring increases the sensitivity towards oxidation 9,10 leading to imines as well as amides.…”

“…[7][8][9] The complexes are stable against hydrolysis in water, but strongly coordinating solvents like dmso replace the hexadentate ligands. We also found that the presence of aminomethylene units directly bound to the aromatic ring increases the sensitivity towards oxidation 9,10 leading to imines as well as amides. N-Methylation shuts down this sensitivity but, at the same time, shifts the HS/LS equilibrium completely to the HS state side.…”

“…N-Methylation shuts down this sensitivity but, at the same time, shifts the HS/LS equilibrium completely to the HS state side. 9 To circumvent this problem, the 2,2′-diaminobiphenyl bridge can be replaced by aliphatic diamines, e.g., 1,2-diaminocyclohexanes (4). Complexes of this type were reported before by Constable, 11 Kol 12 and Blackman.…”