Synthesis, Electronic Structure, and Catalytic Activity of Reduced Bis(aldimino)pyridine Iron Compounds: Experimental Evidence for Ligand Participation

Abstract

The two-electron reduction chemistry of the aryl-substituted bis(aldimino)pyridine iron dibromide, ((iPr)PDAI)FeBr(2) ((iPr)PDAI = 2,6-(2,6-(i)Pr(2)-C(6)H(3)-N═CH)(2)C(5)H(3)N), was explored with the goal of generating catalytically active iron compounds and comparing the electronic structure of the resulting compounds to the more well studied ketimine derivatives. Reduction of ((iPr)PDAI)FeBr(2) with excess 0.5% Na(Hg) in toluene solution under an N(2) atmosphere furnished the η(6)-arene complex, ((iPr)PDAI)Fe(η(6)-C(7)H(8)) rather than a dinitrogen derivative. Over time in pentane or diethyl ether solution, ((iPr)PDAI)Fe(η(6)-C(7)H(8)) underwent loss of arene and furnished the dimeric iron compound, [((iPr)PDAI)Fe](2). Crystallographic characterization established a diiron compound bridged through an η(2)-π interaction with an imine arm on an adjacent chelate. Superconducting quantum interference device (SQUID) magnetometry established two high spin ferrous centers each coupled to a triplet dianionic bis(aldimino)pyridine chelate. The data were modeled with two strongly antiferromagnetically coupled, high spin iron(II) centers each with an S = 1 [PDAI](2-) chelate. Two electron reduction of ((iPr)PDAI)FeBr(2) in the presence of 1,3-butadiene furnished ((iPr)PDAI)Fe(η(4)-C(4)H(6)), which serves as a precatalyst for olefin hydrogenation with modest turnover frequencies and catalyst lifetimes. Substitution of the trans-coordinated 1,3-butadiene ligand was accomplished with carbon monoxide and N,N-4-dimethylaminopyridine (DMAP) and furnished ((iPr)PDAI)Fe(CO)(2) and ((iPr)PDAI)Fe(DMAP), respectively. The molecular and electronic structures of these compounds were established by X-ray diffraction, NMR and Mössbauer spectroscopy, and the results compared to the previously studied ketimine variants.