Towards Noble‐Metal‐Free Dyads: Ground and Excited State Tuning by a Cobalt Dimethylglyoxime Motif Connected to an Iron N‐Heterocyclic Carbene Photosensitizer

Abstract

Heteroleptic iron based complexes bearing the 2,6bis[3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine motif and a polypridine ligand have been synthesized and characterized in their ground and excited state. This series of complexes includes a first example of a hetero-bimetallic complex connecting an iron N-heterocyclic carbene photosensitizer with a cobalt dimethylglyoxime fragment. Focus is set on the influence of the linker and cobalt center as second ligand at the iron center on the photophysics. While electronic absorption spec- [a] linked second metal can be employed to optimize the photophysics of such systems. ResultsBased on our previously published heteroleptic complex bearing a tridentate bis-NHC ligand and a terpy (terpy = 2,2′:6′,2′′terpyridine) ligand, [19] the related complex [FeL1(pyterpy)][PF 6 ] 2 1 was synthesized (pyterpy = L2 = 4′-(4′′′-pyridyl)-2,2′:6′,2′′-terpyridine; L1 = 2,6-bis[3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine) in order to utilize the fact that the electronic nature of the substituents attached to the 4′ position of terpy allows significant control over the photophysical properties. [21] By employing 4-pyridyl, pendant-functionalization by protonation, [18] methylation [22,23] or coordination of metal fragments [24] allow multiple applications. Based on 1, the synthesis and structure of a new bimetallic complex 3 shown in Scheme 1, which connects an iron(II) center and a chlorobis(dimethylglyoximato)cobalt(III) motif, is presented (dimethylglyoximato = dmgH). For comparison, the N-methylated complex 2 was also prepared. Such systems can be described as metal-containing topographical analogues of N,N-Dialkyl-4,4-bipyridinium salts (see orange colored fragments in Scheme 1). [23] These salts, also known as viologens, are electron transfer reagents in many photochemical applications due to their reversible reduction properties. [25] To the best of our knowledge, 3 is the first example of a bimetallic complex connecting a NHC-ligand based iron(II) complex with a cobalt(III) dmgH motif. The former are promising candidates for iron-based photosensitizers, [9] the latter are known to Scheme 1. Route of synthesis for complexes 1-3. The viologen motif is marked in orange (DIPP = 2,6-diisopropylphenyl).

Excited‐State Kinetics of an Air‐Stable Cyclometalated Iron(II) Complex

“…In the electronic absorption spectra, 2 shows a strong panchromatic absorption up into the NIR region (Figure ) with broadened peaks due to the lower symmetry and concomitant larger t 2g splitting in comparison to 3 . The MLCT maximum of 2 at λ max =618 nm ( ϵ max =0.92 10 4 m −1 cm −1 ) is redshifted compared to that of 3 ( λ max =552 nm, ϵ max =1.12 10 4 m −1 cm −1 ) due to the destabilized t 2g orbitals (Figure ).…”

Design and Synthesis of Photoactive Iron N-Heterocyclic Carbene Complexes

“…This intermediate has the advantage that it can either be reacted with tpy-type ligands as in the case of [Fe(pbdppi)(pytpy)] 2+ and its derivatives or with another carbene-containing ligand which led to [Fe(bpymi)(pdbppi)] 2+ (Figure 20). The employed Fe(II) oxidation state in this intermediate also leads to a significant increase in yields of the heteroleptic complexes ranging from 52% to 60% [24,36]. The pytpy ligand offers the possibility of being modified when coordinated to the iron center thus allowing to influence the properties of the complex by simple means.…”

“…Heteroleptic FeNHC-pytpy complexes modified by methylation or introduction of a CoCl(dmg) 2 unit leading to hetero-bimetallic compound Fe-Co[36].…”

Photophysics and Photochemistry of Iron Carbene Complexes for Solar Energy Conversion and Photocatalysis

“…Figure 8illustrates how selected ligand motifs, including the low π* with the protonated pytpy-ligand heteroleptic complex 7, can be used to rationally influence the photo(electro)chemical properties of the FeNHC complexes. More recently, Zimmer et al came to a similar conclusion with complexes 9 and the corresponding methylated complex 10[56].Catalysts 2020, 10, x FOR PEER REVIEW 13 of 32…”

“…This result highlights the importance of steric effects of the bulky side-groups. In contrast, introduction of iso-propyl and hexanyl sidegroups (complexes 3 and 4) did not influence the photophysics as much (Table 1) [31,[56][57][58].…”