Synthesis and Computational Study of a Pyridylcarbene Fe(II) Complex: Unexpected Effects of fac/mer Isomerism in Metal-to-Ligand Triplet Potential Energy Surfaces

Abstract

The synthesis and the steady-state absorption spectrum of a new pyridine-imidazolylidene Fe(II) complex (Fe-NHC) are presented. A detailed mechanism of the triplet metal-to-ligand charge-transfer states decay is provided on the basis of minimum energy path (MEP) calculations used to connect the lowest-lying singlet, triplet, and quintet state minima. The competition between the different decay pathways involved in the photoresponse is assessed by analyzing the shapes of the obtained potential energy surfaces. A qualitative difference between facial ( fac) and meridional ( mer) isomers' potential energy surface (PES) topologies is evidenced for the first time in iron-based complexes. Indeed, the mer complex shows a steeper triplet path toward the corresponding MC minimum, which lies at a lower energy as compared to the fac isomer, thus pointing to a faster triplet decay of the former. Furthermore, while a major role of the metal-centered quintet state population from the tripletMC region is excluded, we identify the enlargement of iron-nitrogen bonds as the main normal modes driving the excited-state decay.

Impact of the fac/mer Isomerism on the Excited-State Dynamics of Pyridyl-carbene Fe(II) Complexes

“…Figure S16. Coherently with what observed for fac-C1 and mer-C1, 23 the main coordinate driving the process is the enlargement of the Fe-N1 bond, which has a bond distance of 2.82 Å at the T 1 ( 3 MC) min geometry ( Fig. 4a).…”

“…Recently, and to influence the angular strain, we have reported the first example of iron complex with bidentate pyridylcarbene ligands C1. 23 The asymmetric nature of the employed ligand led to different geometrical isomers. In fact, C1 was obtained as a mixture of mostly the meridional (mer) isomer with a concomitant fraction of the facial (fac) isomer in a 14:1 ratio.…”

“…In particular, the low-lying triplet PES of the mer isomer, leading to the MC population and deactivation, was found to be much steeper than the one of the corresponding fac isomer. 23 This aspect is intimately linked to the larger stabilization of the 3 MC state in the mer arrangement as compared to the fac one. Altogether, our molecular modelling results indicate a much faster deactivation for the mer isomer.…”

Is Iron the New Ruthenium?

“…[14,31] Recent computational work by Monaria nd co-workers demonstrated that there could be subtle but important differences in the 3 MLCT deactivation pathways for fac and mer isomers of the same Fe II -carbene complex, and they concluded that FeÀN bond elongation was the key normalc oordinate leading to triplet relaxation. [32] Their work underscores the importance of complete calculation of the potential-energy surfaces to adequately describe excited-state relaxation.…”

Design and Synthesis of Photoactive Iron N-Heterocyclic Carbene Complexes

“…The use of a bidentate pyridine-imidazole-derived ligand led to [Fe(mpi) 3 ] 2+ (mpi = 3-methyl-1-(pyridin-2-yl)-imidazol-2-ylidene) which exists in two different isomers with either meridional or facial coordination of the carbene/pyridine moieties (Figure 15) [54]. When synthesized the complex is obtained in a 14:1 mer/fac mixture that is inseparable.…”