Di- and Mononuclear Iron Complexes of N,C,S-Tridentate Ligands Containing an Aminopyridyl Group: Effect of the Pendant Amine Site on Catalytic Properties for Proton Reduction

Abstract

A series of diiron complexes of N,C,S-tridentate ligands containing a 6-, 5-, or 4-amino-2-pyridyl group, [{Fe(μ-L-κ 3 N,C,S)(CO) 2 }Fe(CO) 3 ] (2, L = o-apyBPT; 3, L = mapyBPT; 4, L = p-apyBPT), was synthesized: apyBPT is a doubly deprotonated form of 3′-(amino-2″-pyridyl)-1,1′-biphenyl-2-thiol. Complexes 2−4 were converted to the mononuclear iron(II) complexes trans-[Fe(L-κ 3 N,C,S)(CO)-(PMe 2 Ph) 2 ] (6, L = o-apyBPT; 7, L = m-apyBPT; 8, L = papyBPT). In 2 and 6, the o-amino group is close to Fe bound to the aminopyridyl group. Cyclic voltammograms of 2−4 exhibit two consecutive one-electron reduction events, and catalytic current for proton reduction appears in the presence of acetic acid. The reduction potentials of 2−4 are similar to each other, while the overpotential for proton reduction with o-amino complex 2 is ca. 0.2 V lower than those with 3 and 4. In the mononuclear complexes 6−8, the redox potentials for the Fe III /Fe II couple are dependent on the position of the amino group in the pyridine ring, which is described by electronic and steric effects of the amino group. Such effects on the redox potentials are suppressed in the diiron complexes because the reduction occurs at the diiron core with π-accepting CO ligands, which is supported by DFT calculations. The lower overpotential in 2 compared with 3 and 4 is attributed to the concerted effect of the amino group proximal to the iron center. The amino group probably acts as a proton acceptor and assists the formation of the H−H bond from a hydride on the iron centers and a proton bound to the amino group.

CO Release from N,C,S-Pincer Iron(III) Carbonyl Complexes Induced by Visible-to-NIR Light Irradiation: Mechanistic Insight into Effects of Axial Phosphorus Ligands

“…ORTEP drawings of (a) 2 and (b) 3 with thermal ellipsoids at 50% probability level.Hydrogen atoms, crystal solvents, and ethyl groups of P(OEt) 3 in are omitted for clarity. THF/n-hexane, as reported for 1 48,51. The crystal structures of 2 and 3 are depicted inFigure 2.Selected bond distances and angles of 2 and 3 are listed inTable 1.…”

“…[18][19][20] We recently reported the synthesis and properties of a novel iron complex with an N,C,Stridentate ligand, trans- [Fe(PyBPT) ), where PyBPT is a doubly deprotonated form of 3'-(2''-pyridyl)-1,1'-biphenyl-2-thiol formed via the C-S bond cleavage of the dibenzothiophene derivative. [48][49][50][51] The N,C,S-pincer iron(III) carbonyl complex exhibited unusual stability in solution under ambient conditions in the dark, while the CO ligand readily underwent substitution reaction with the coordinative solvent in response to the short wavelength region of visible light. 52 A theoretical study indicated that SOMO of the iron(III) carbonyl complex mainly consisted of the dπ orbital of the central iron and pπ orbital of the sulfur atom.…”

Iron Carbonyl Complexes Containing N,C,S-Tridentate Ligands with Quinoline, Vinyl, and Benzenethiolate Units

“…The Fe(1)−C(8) bond distances for 1 (1.9456(18) Å) and 2 (1.956(3) Å) are similar to those of diiron complexes derived from PyDBT derivatives or QTs, 33,36,37 and the Fe(1)−C (18) bond trans to C(8) is ca. 0.05 Å longer than the Fe(1)−C (19) bond, indicating the trans influence of the central C donor of the N,C,S-tridentate ligands.…”

“…These findings suggest that complex 1 have a C,Sbridged dinuclear structure with the formula [{Fe(μ-qvbtκ 3 N,C,S)(CO) 2 }Fe(CO) 3 ] (Scheme 2), which is similar to that of diiron carbonyl complexes prepared from pyridyl-substituted dibenzothiophene (PyDBT) or quinolyl-substituted thiophenes (QTs). 33,36,37 In the 1 H NMR spectrum of 2, a significant downfield shift was observed for the quinoline CH adjacent to the vinyl group (7.50 ppm for 1, 9.08 ppm for 2), which is due to the C−H•••O(methoxy) short contact (mentioned later). Complexes 1 and 2 were structurally characterized by X-ray crystallography (Figure 2).…”

“…The Fe(2)−C(7) (2.1512(18) Å for 1, 2.186(3) Å for 2) distances are similar to those of diiron complexes derived from benzothiophene (2.197(4) Å) 38 and QTs (ca. 2.14 Å) 33 rather than from PyDBT derivatives (2.33−2.36 Å), 36,37 indicating the vinyl group is more tightly bound to Fe through π coordination compared with the central phenyl group in PyBPT derivatives.…”