59th Annual Report on Research 2014

By carrying oxidation and reductions of complex 1, a series of iron complexes containing bis(amidinato)carbene ligands spanning four oxidation states was obtained (Scheme 2).  An extensive spectroscopic, magnetic, and computational analysis of this series of complexes revealed that the bis(amidinato)-N-heterocyclic carbene complexes are exceptional electron donating ligands, which results in behavior that differs significantly compared to the bis(imino)pyridine complexes.

The magnetic properties of the iron(II) and iron (III) complexes 1 and 5 illustrate the consequence of  the electron donating capabilities of the bis(amidinato)-N-heterocyclic carbene ligands.  Unlike the bis(amino)pyridine complexes, which are high spin complexes at all temperatures greater than 10 K, the bis(amidinato)-N-heterocyclic carbene complexes demonstrate evidence for  an intermediate spin state that pervades at temperatures less than 150 K and undergoes a spin transition to high spin complexes above room temperature (Figure 1). This behavior was further corroborated with variable temperature X-ray crystallography and spin unrestricted DFT calculations, and is consistent with larger ligand field splittings resulting from the more electron donating bis(amidinato)-N-heterocyclic carbene ligands.

One electron reduction of 1 produced the formally iron(I) complex 6 (Scheme 2).  As was the case for bis(amino)pyridine iron complexes, spectroscopic evidence supported by spin unrestricted DFT calculations revealed that bis(amidinato)-N-heterocyclic carbene ligands are redox active in complex 6 (Figure 2). This finding is notable because it is the first example of an isolated complex containing an Arduengo-type N-heterocyclic carbene ligand that is redox active. Interestingly, in contrast to the high oxidation state complexes, which take advantage of the s-donating capabilities of the bis(amidinato)-N-heterocyclic carbene ligands, the low oxidation state complexes utilize the ability for the ligand to act as a p-acceptor.  By having the capacity to interact with the metal center via many bonding modes, several oxidation states and spin states of iron can be stabilized by these ancillary ligands.

With a firm understanding of their electronic structure, we began to examine the catalytic performance of iron complexes containing bis(amidinato)-N-heterocyclic carbene ligands. Considering the superior electron donating capabilities of the ligands, we originally hypothesized that they would be suitable for stabilizing high oxidation states of iron.  In preliminary studies, we revealed that the iron complex 1 catalyzed the formation of diazene products when exposed to azides in the presence of olefins.  This promising result suggested that the bis(amidinato)-N-heterocyclic carbene complexes could support high oxidation states of iron through the formation of an iron amido intermediate.  Ligand modifications to make the complexes more sterically accessible for group transfer reactions made the complexes more reactive for the formation of diazene products, but they unfortunately did not result in any significant group transfer reactions.

In contrast to these findings, in situ reduction of complex 1 in the presence of hydrogen and alkenes led to the rapid hydrogenation of alkenes.  Although the reaction demonstrated limited substrate scope, catalytic hydrogenation was promising considering that the bis(dinitrogen) iron complexes historically used for these reactions were not used.  Future work in this direction will be dedicated to synthesizing the bis(dinitrogen) iron complexes and applying them toward the hydrogenation of alkenes and other reactions such as [2+2] cycloaddition of alkenes and dienes. In a parallel program not supported by the PRF, our lab has discovered that bis(imino)pyridine iron complexes are good catalysts for the ring opening polymerization of cyclic diesters.{JACS 2013, 135, 16553}  Our initial studies revealed that catalytic performance was superior when electron rich catalysts were utilized.  We hypothesized that iron complexes containing the exceptional s-donating bis(amidinato)-N-heterocyclic carbene ligands would be superior catalysts for the polymerization of cyclic diesters.  This hypothesis proved to be correct as the iron alkoxide complexes 8 were found to be very active for the polymerization of cyclic diesters leading to the efficient production of high molecular weight polymer (>300 kg/mol) using small quantities of the iron catalyst (0.02 mol%).{Polyhedron 2014, In Press, DOI:10.1016/j.poly.2014.07.002} These finding demonstrate how the electron donating capabilities of the ligand can be utilized to improve catalytic performance.  Considering our interest in ring opening polymerization, the versatility of the bis(amidinato)-N-heterocyclic carbene ligands will be further pursued in epoxide and lactone polymerization reactions. Finally, the bis(amidinato)-N-heterocyclic carbene and bis(imino)pyridine ligands were evaluated as ancillary ligands in catalytic cross coupling reactions.  There have been many mechanisms proposed that involve multiple oxidation states of iron for iron-catalyzed cross coupling reactions.{ChemSusChem 2009, 2, 396}  We hypothesized that the redox activity of the bis(amidinato)-N-heterocyclic carbene and bis(imino)pyridine ligands would funnel reactions towards one reaction pathway thereby prevent deleterious side reactions resulting from multiple competing reaction pathways.  Additionally, the redox active ligands were expected to provide access to unusual oxidation states of iron that are anticipated to alter catalytic performance.  Moreover, the ligand motif is easily modifiable for assymetric variants, and it is known to discourage irreversible b-hydride elimination reactions{Top. Organomet. Chem. 2009, 26, 107} that have historically been deleterious for cross coupling reactions.  Despite these advantages, there have been no reports that disclose the use of these ligands for catalytic cross coupling reactions.  Recently, we discovered bis(imino)pyridine iron complexes can promote catalytic cross coupling of phenyl Grignard with a variety of alkyl halides (Scheme 3).  Reactions were efficient, and except for benzyl chloride, were selective for cross coupling with little evidence for alkyl halide homo-coupling or b-hydride elimination.  Moving forward, the catalytic cross coupling reaction will be further optimized to limit the amount of Grignard homocoupling product, and the versatility of the bis(imino)pyridine complexes will be extended to other cross coupling reactions, such as Suzuki cross coupling reactions.