Change in the Spin State of an Fe^III Center upon One N‐to‐O Switch in the Coordination of a 2,6‐Pyridinedicarboxamido Unit: The Effect of Methyl Thioether and Methyl Ether Appendages at the Ligand Periphery

Abstract

Two ligands bearing the 2,6‐pyridinedicarboxamide unit, namely PyPSMeH2 and PyPOMeH2, have been used to prepare complexes of FeIII which have been structurally characterised. The potentially pentadentate thioether ligand PyPSMeH2 affords only the bis complex (Et4N)[Fe(PyPSMe)2] (1), irrespective of the metal to ligand ratio, due to the weak affinity of the thioether groups for the FeIII center. In [Fe(PyPSMe)2]−, the two PyPSMe2− ligands bind to the iron center in a mer fashion and steric crowding among the pendant thioether groups forces one of the four deprotonated carboxamido moieties to bind to the FeIII center through the carbonyl oxygen, giving rise to an FeN5O chromophore. The tridentate ligand PyPOMeH2, in its deprotonated form, also gives rise to a bis complex, (Et4N)[Fe(PyPOMe)2] (2), in which the two PyPOMe2− ligands bind to the iron center in a mer fashion. In this case, however, less steric crowding among the smaller pendant ether groups allows binding of all four carboxamido nitrogens to the FeIII center in [Fe(PyPOMe)2]− giving rise to an FeN6 chromophore. This one N‐to‐O switch in donor atom makes the two complexes 1 and 2 very different. While complex 2 is low spin (g = 2.18, 1.94) much like the other reported FeIII bis complexes with two ligated 2,6‐pyridinedicarboxamido units, complex 1 is high spin (g = 4.3). The spin flip due to a switch in only one N or O donor atom in these two complexes is unprecedented. The two complexes also exhibit different stability towards water. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Modular O- vs. N-coordination of pyridylidene amide ligands to iron determines activity in alcohol oxidation catalysis

“…32 Generally, these doubly anionic ligands coordinate through the nitrogen atoms, but steric congestion indicated that O-coordination is also feasible. 33,34 Bond length analyses in the N-bound para-PYA unit in 6 revealed an average difference of 0.052 Å between Cα-Cβ and Cβ-Cγ, suggesting similar double bond localization as noted in complexes 2 and 3 (Table S6). Remarkably, this bond length alteration is just slightly less pronounced in the O-bound ortho-PYA heterocycle of complex 8 (Δd 0.034 Å; Table S7).…”

Synthesis and characterization of a tris(2-hydroxyphenyl)methane-based cryptand and its triiron(iii) complex

“…20 The N amide -Fe bond distances of 2.064(5) and 2.104(5) Å and the N amide -Fe-N amide bond angle of 145.1(2)°are similar to those reported for the high-spin [Fe-(PyPSMe) 2 ] − complex. 17 Consistent with these structural similarities to the high-spin complex, a room temperature magnetic moment of 10.08 μ B was determined for [(FeCl 2 ) 3 L] 3− by Evan's method using the parallel field formula, 21 which agrees with three S = 3/2 ferric ions. Coordination of the K + cation to O1 likely weakens the donor strength of N1, and results in the longer N amide -Fe bond distance as compared to N3-Fe bond.…”

“…These absorption maxima are comparable to those reported for other ferric complexes of substituted pyridine-2,6-dicarboxamides. 16,17 Attempts to deprotonate the H 6 L with stronger bases, such as methyllithium, were unsuccessful and afforded only a mixture of products. Although related triarylmethanes undergo oxidation via formal hydride loss at the methine carbon atom, 10b the 1 H-NMR spectra of reaction mixtures suggest that this proton can be abstracted by stronger bases, subsequently leading to ligand decomposition.…”

“…The steric demands of H 6 L enforce a 1 : 1 stoichiometry for each pyridine-2,6-dicarboxamide donor and iron(III) cation, which has not been realized for monometallic ferric complexes employing phenyl-substituted pyridine-2,6-dicarboxamide ligands. [16][17][18] In [(FeCl 2 ) 3 L] 3− , each arm coordinates one iron(III) with the metal adopting a slightly distorted square pyramidal coordination geometry (τ = 0.05 with a 0.9% tetragonal distortion 19 ) (Fig. 3).…”

“…The almost perpendicular disposition of the pyridyl ring versus the two neighbouring phenyl rings in [(FeCl 2 ) 3 L] 3− is consistent with that observed for monometallic complexes of substituted N, N′-diphenylpyridine-2,6-dicarboxamide. 16,17 This structural change consequently rotates the equatorial chloride away from the interior cavity, and could be an important feature for substrate access and/or product egress in future efforts. The significant ligand reorganization after metallation is reminiscent of the conformational changes observed during catalytic turnover in many enzyme systems 22 as well as the structural changes reported for molecular devices.…”