Four homo- and heteroleptic complexes bearing both polypyridyl units and N-heterocyclic carbene (NHC) donor functions are studied as potential noble metal-free photosensitizers. The complexes [Fe(L1)(terpy)][PF], [Fe(L2)][PF], [Fe(L1)(L3)][PF], and [Fe(L3)][PF] (terpy = 2,2':6',2″ terpyridine, L1 = 2,6-bis[3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine, L2 = 2,6-bis[3-isopropylimidazol-2-ylidene]pyridine, L3 = 1-(2,2'-bipyridyl)-3-methylimidazol-2-ylidene) contain tridentate ligands of the C^N^C and N^N^C type, respectively, resulting in a Fe-NHC number between two and four. Thorough ground state characterization by single crystal diffraction, electrochemistry, valence-to-core X-ray emission spectroscopy (VtC-XES), and high energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) in combination with ab initio calculations show a correlation between the geometric and electronic structure of these new compounds and the number of the NHC donor functions. These results serve as a basis for the investigation of the excited states by ultrafast transient absorption spectroscopy, where the lifetime of the MLCT states is found to increase with the NHC donor count. The results demonstrate for the first time the close interplay between the number of NHC functionalities in Fe(II) complexes and their photochemical properties, as revealed in a comparison of the activity as photosensitizers in photocatalytic proton reduction.

For the first time, the potential of noble‐metal‐free iron‐based photosensitizers for photocatalytic water reduction by using homo‐ and heteroleptic carbene complexes was proven. The development of a facile synthesis route for such carbene complexes was delineated. The complexes were investigated by UV/Vis and fluorescence spectroscopy, DFT calculations, cyclic voltammetry, and X‐ray absorption spectroscopy. Their catalytic performance was compared to that of established noble‐metal‐containing photosensitizers such as [Ir(ppy)2(bpy)][PF6] (Hppy = 2‐phenylpyridine, bpy = 2,2′‐bipyridine).

Polynuclear spincrossover (SCO) complexes prepared by the combination of [Fe(DMF)6](2+) and NH2trz (NH2trz = 4-amino-1,2,4-triazole) were studied (2ns(-) = counterion 2-naphthalenesulfonate). It is demonstrated that these [Fe(NH2trz)3](2ns)2 complexes can be dissolved-contrary to common reported experience-in N,N-dimethylformamide (DMF) and, therefore, can be conveniently processed by simple means. The resulting solutions were examined with UV/vis and X-ray absorption spectroscopy (XANES and EXAFS) as well as with small-angle X-ray scattering (SAXS). At a molar NH2trz/Fe(2+) ratio of 3/1, corresponding to the stoichiometric ratio of the ideal coordination compound, [Fe(NH2trz)3](2+) in the low-spin state was found to be in equilibrium with polynuclear species in the high-spin state. The equilibrium can be shifted virtually completely to the side of low-spin Fe(2+) by an excess of the ligand. The polymer therewith formed contains 100 or more Fe(2+) ions and is of a pronounced rigid-rod structure, with Fe-Fe distances around 3.32 Å (in comparison to 3.94 Å of the polynuclear species in the high-spin state). Reversible spin crossover takes place in solution upon a temperature increase to around 60 °C; this process is associated with a shift in equilibrium toward species shorter than the initial polynuclear species.

Bis(pyrazolyl)methane ligands are excellent components of model complexes used to investigate the activity of the enzyme tyrosinase. Combining the N donors 3-tert-butylpyrazole and 1-methylimidazole results in a ligand that is capable of stabilising a (μ-η(2) :η(2) )-dicopper(II) core that resembles the active centre of tyrosinase. UV/Vis spectroscopy shows blueshifted UV bands in comparison to other known peroxo complexes, due to donor competition from different ligand substituents. This effect was investigated with the help of theoretical calculations, including DFT and natural transition orbital analysis. The peroxo complex acts as a catalyst capable of hydroxylating a variety of phenols by using oxygen. Catalytic conversion with the non-biological phenolic substrate 8-hydroxyquinoline resulted in remarkable turnover numbers. In stoichiometric reactions, substrate-binding kinetics was observed and the intrinsic hydroxylation constant, kox , was determined for five phenolates. It was found to be the fastest hydroxylation model system determined so far, reaching almost biological activity. Furthermore, Hammett analysis proved the electrophilic character of the reaction. This sheds light on the subtle role of donor strength and its influence on hydroxylation activity.

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).