Lukas Burkhardt

A new base metal iron‐cobalt dyad has been obtained by connection between a heteroleptic tetra‐NHC iron(II) photosensitizer combining a 2,6‐bis[3‐(2,6‐diisopropylphenyl)imidazol‐2‐ylidene]pyridine with 2,6‐bis(3‐methyl‐imidazol‐2‐ylidene)‐4,4′‐bipyridine ligand, and a cobaloxime catalyst. This novel iron(II)‐cobalt(III) assembly has been extensively characterized by ground‐ and excited‐state methods like X‐ray crystallography, X‐ray absorption spectroscopy, (spectro‐)electrochemistry, and steady‐state and time‐resolved optical absorption spectroscopy, with a particular focus on the stability of the molecular assembly in solution and determination of the excited‐state landscape. NMR and UV/Vis spectroscopy reveal dissociation of the dyad in acetonitrile at concentrations below 1 mM and high photostability. Transient absorption spectroscopy after excitation into the metal‐to‐ligand charge transfer absorption band suggests a relaxation cascade originating from hot singlet and triplet MLCT states, leading to the population of the ³MLCT state that exhibits the longest lifetime. Finally, decay into the ground state involves a ³MC state. Attachment of cobaloxime to the iron photosensitizer increases the ³MLCT lifetime at the iron centre. Together with the directing effect of the linker, this potentially makes the dyad more active in photocatalytic proton reduction experiments than the analogous two‐component system, consisting of the iron photosensitizer and Co(dmgH)₂(py)Cl. This work thus sheds new light on the functionality of base metal dyads, which are important for more efficient and sustainable future proton reduction systems.

Here we explore the electronic structure of the diiron complex [(dppf)Fe(CO)₃]^0/+ [1^0/+; dppf = 1,1′-bis(diphenylphosphino)ferrocene] in two oxidation states by an advanced multitechnique experimental approach. A combination of magnetic circular dichroism, X-ray absorption and emission, high-frequency electron paramagnetic resonance (EPR), and Mössbauer spectroscopies is used to establish that oxidation of 1⁰ occurs on the carbonyl iron ion, resulting in a low-spin iron(I) ion. It is shown that an unequivocal result is obtained by combining several methods. Compound 1⁺ displays slow spin dynamics, which is used here to study its geometric structure by means of pulsed EPR methods. Surprisingly, these data show an association of the tetrakis[3,5-bis(trifluoromethylphenyl)]borate counterion with 1⁺.

We present the η³‐coordination of the 2‐phosphaethynthiolate anion in the complex (PN)₂La(SCP) (2) [PN=N‐(2‐(diisopropylphosphanyl)‐4‐methylphenyl)‐2,4,6‐trimethylanilide)]. Structural comparison with dinuclear thiocyanate‐bridged (PN)₂La(μ‐1,3‐SCN)₂La(PN)₂ (3) and azide‐bridged (PN)₂La(μ‐1,3‐N₃)₂La(PN)₂ (4) complexes indicates that the [SCP]⁻ coordination mode is mainly governed by electronic, rather than steric factors. Quantum mechanical investigations reveal large contributions of the antibonding π*‐orbital of the [SCP]⁻ ligand to the LUMO of complex 2, rendering it the ideal precursor for the first functionalization of the [SCP]⁻ anion. Complex 2 was therefore reacted with CAACs which induced a selective rearrangement of the [SCP]⁻ ligand to form the first CAAC stabilized group 15–group 16 fulminate‐type complexes (PN)₂La{SPC(^RCAAC)} (5 a,b, R=Ad, Me). A detailed reaction mechanism for the SCP‐to‐SPC isomerization is proposed based on DFT calculations.

We present the η³‐coordination of the 2‐phosphaethynthiolate anion in the complex (PN)₂La(SCP) (2) [PN=N‐(2‐(diisopropylphosphanyl)‐4‐methylphenyl)‐2,4,6‐trimethylanilide)]. Structural comparison with dinuclear thiocyanate‐bridged (PN)₂La(μ‐1,3‐SCN)₂La(PN)₂ (3) and azide‐bridged (PN)₂La(μ‐1,3‐N₃)₂La(PN)₂ (4) complexes indicates that the [SCP]⁻ coordination mode is mainly governed by electronic, rather than steric factors. Quantum mechanical investigations reveal large contributions of the antibonding π*‐orbital of the [SCP]⁻ ligand to the LUMO of complex 2, rendering it the ideal precursor for the first functionalization of the [SCP]⁻ anion. Complex 2 was therefore reacted with CAACs which induced a selective rearrangement of the [SCP]⁻ ligand to form the first CAAC stabilized group 15–group 16 fulminate‐type complexes (PN)₂La{SPC(^RCAAC)} (5 a,b, R=Ad, Me). A detailed reaction mechanism for the SCP‐to‐SPC isomerization is proposed based on DFT calculations.

In search of new ligand motifs for photoactive iron(II) complexes with long-lived MLCT states, a series of six complexes with tridentate amine-functionalized bis-n-heterocyclic carbene (NHC)-pyridine ligands is presented. In the homoleptic complexes imidazole-, methylimidazole-, or benzimidazole-2-ylidene, NHC donors are employed in combination with pyridine, functionalized in the 4-position by dimethylamine or dibenzylamine. The effects of these different functionalities on the electronic structure of the complexes are examined through detailed ground state characterization by NMR, single crystal X-ray diffraction, as well as electrochemical and spectroscopic methods. The net influence of these different functionalities on orbital–orbital and electrostatic ligand–iron interactions is investigated thoroughly by density functional theory, and changes in the excited state behavior and lifetimes are finally examined by ultrafast optical spectroscopy. Great deviations of the initially expected effects by substitution in 4-position on the photochemical properties are observed, together with a significantly increased π-acceptor interaction strength in the benzimidazole-2-ylidene functionalized complexes.

While the Hieber anion [Fe(CO)₃(NO)]⁻ has been reincarnated in the last years as an active catalyst in organic synthesis, there is still a debate about the oxidation state of the central Fe atom and the resulting charge of the NO ligand. To shed new light on this question and to understand the Fe–NO interaction in the Hieber anion, it is investigated in comparison to the formal 3d⁸ reference Fe(CO)₅ and the formal 3d¹⁰ reference [Fe(CO)₄]^2– by the combination of valence-to-core X-ray emission spectroscopy (VtC-XES), X-ray absorption near-edge structure spectroscopy (XANES), and high-energy-resolution fluorescence-detected XANES. In order to extract information about the electronic structure, time-dependent density functional theory and ground-state density functional theory calculations are applied. This combination of experimental and computational methods reveals that the electron density at the Fe center of the Hieber resembles that of the isoelectronic [Fe(CO)₄]^2–. These observations challenge recent descriptions of the Hieber anion and reopen the debate about the experimentally and computationally determined Fe oxidation state and charge on the NO ligand.

Established and recent hard X-ray spectroscopic methods in the form of conventional X-ray absorption near edge structure spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS), and the photon-in/photon-out techniques high energy resolution fluorescence detection XANES and valence-to-core X-ray emission spectroscopy (VtC-XES) provide unique opportunities to study mechanisms in metal–organic reactions. The combination of these techniques allows the determination of the local geometric and electronic structures in the form of the numbers of nearest neighbours, their types and distances around an X-ray absorbing atom and the highest occupied and lowest unoccupied molecular levels. Different sample cells for this purpose, which allow high pressure, electrochemical or multi-spectroscopic measurements under inert conditions, are presented and discussed. The potential of HERFD-XANES and VtC-XES to eliminate limitations of conventional EXAFS spectroscopy is established with case studies on the Hieber anion [Fe(CO)₃(NO)]⁻ and the iron hydride complex [Fe(CO)H(NO)(PPh₃)₂]. With VtC-XES the formation of an allyl complex by reaction of [Fe(CO)₃(NO)]⁻ in a catalytic nucleophilic substitution reaction can be followed. Combination of HERFD-XANES and VtC-XES allows the identification and investigation of hydride species, as well as their fate in chemical reactions. On the other hand, in order to investigate the active species formation in iron-catalysed cross coupling reactions, conventional XANES and EXAFS are the method of choice for the moment. For all examples, the advantages and limitations of the hard X-ray toolbox are commented on and the value of the individual methods are compared.

The complex class [Fe(N^N^C)(N^N^N)]⁺ with an Earth‐abundant metal ion has been repeatedly suggested as a chromophore and potential photosensitizer on the basis of quantum chemical calculations. Synthesis and photophysical properties of the parent complex [Fe(pbpy)(tpy)]⁺ (Hpbpy=6‐phenyl‐2,2′‐bipyridine and tpy=2,2′:6′,2′′‐terpyridine) of this new chromophore class are now reported. Ground‐state characterization by X‐ray diffraction, electrochemistry, spectroelectrochemistry, UV/Vis, and X‐ray spectroscopy in combination with DFT calculations proves the high impact of the cyclometalating ligand on the electronic structure. The photophysical properties are significantly improved compared to the prototypical [Fe(tpy)₂]²⁺ complex. In particular, the metal‐to‐ligand absorption extends into the near‐IR and the ³MLCT lifetime increases by 5.5, whereas the metal‐centered excited triplet state is very short‐lived.

In the context of solar‐to‐chemical energy conversion, inspired by natural photosynthesis, the synthesis, electrochemical properties and photoinduced electron‐transfer processes of three novel zinc(II)‐gold(III) bis(porphyrin) dyads [Zn^II(P)–Au^III(P)]⁺ are presented (P: tetraaryl porphyrin). Time‐resolved spectroscopic studies indicated ultrafast dynamics (k >10¹⁰ s⁻¹) after visible‐light excitation, which finally yielded a charge‐shifted state [Zn^II(P^⋅+)–Au^II(P)]⁺ featuring a gold(II) center. The lifetime of this excited state is quite long due to a comparably slow charge recombination (k ≈3×10⁸ s⁻¹). The [Zn^II(P^⋅+)–Au^II(P)]⁺ charge‐shifted state is reductively quenched by amines in bimolecular reactions, yielding the neutral zinc(II)–gold(II) bis(porphyrin) Zn^II(P)–Au^II(P). The electronic nature of this key gold(II) intermediate, prepared by chemical or photochemical reduction, is elucidated by UV/Vis, X‐band EPR, gold L₃‐edge X‐ray absorption near edge structure (XANES) and paramagnetic ¹H NMR spectroscopy as well as by quantum chemical calculations. Finally, the gold(II) site in Zn^II(P)–Au^II(P) is thermodynamically and kinetically competent to reduce an aryl azide to the corresponding aryl amine, paving the way to catalytic applications of gold(III) porphyrins in photoredox catalysis involving the gold(III/II) redox couple.