Pseudo-Octahedral Iron(II) Complexes with Near-Degenerate Charge Transfer and Ligand Field States at the Franck-Condon Geometry.

Increasing the metal-to-ligand charge transfer (MLCT) excited state lifetime of polypyridine iron(II) complexes can be achieved by lowering the ligand's π* orbital energy and by increasing the ligand field splitting. In the homo- and heteroleptic complexes [Fe(cpmp) ] (1 ) and [Fe(cpmp)(ddpd)] (2 ) with the tridentate ligands 6,2''-carboxypyridyl-2,2'-methylamine-pyridyl-pyridine (cpmp) and N,N'-dimethyl-N,N'-di-pyridin-2-ylpyridine-2,6-diamine (ddpd) two or one dipyridyl ketone moieties provide low energy π* acceptor orbitals. A good metal-ligand orbital overlap to increase the ligand field splitting is achieved by optimizing the octahedricity through CO and NMe units between the coordinating pyridines which enable the formation of six-membered chelate rings.

Fundamental Characterization, Photophysics and Photocatalysis of a Base Metal Iron(II)-Cobalt(III) Dyad.

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.

Electronic Structure of a Diiron Complex: A Multitechnique Experimental Study of [(dppf)Fe(CO) ].

Here we explore the electronic structure of the diiron complex [(dppf)Fe(CO)] [; 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 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.

η -Coordination and Functionalization of the 2-Phosphaethynthiolate Anion at Lanthanum(III)*.

We present the η -coordination of the 2-phosphaethynthiolate anion in the complex (PN) La(SCP) (2) [PN=N-(2-(diisopropylphosphanyl)-4-methylphenyl)-2,4,6-trimethylanilide)]. Structural comparison with dinuclear thiocyanate-bridged (PN) La(μ-1,3-SCN) La(PN) (3) and azide-bridged (PN) La(μ-1,3-N ) La(PN) (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.

When Donors Turn into Acceptors: Ground and Excited State Properties of Fe Complexes with Amine-Substituted Tridentate Bis-imidazole-2-ylidene Pyridine Ligands.

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--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.

Electronic Structure of the Hieber Anion [Fe(CO)(NO)] Revisited by X-ray Emission and Absorption Spectroscopy.

While the Hieber anion [Fe(CO)(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 3d reference Fe(CO) and the formal 3d reference [Fe(CO)] 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.

Hard X-ray spectroscopy: an exhaustive toolbox for mechanistic studies (?).

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.

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

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.

Gold(II) Porphyrins in Photoinduced Electron Transfer Reactions.

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 (P)-Au (P)] are presented (P: tetraaryl porphyrin). Time-resolved spectroscopic studies indicated ultrafast dynamics (k >10  s ) after visible-light excitation, which finally yielded a charge-shifted state [Zn (P )-Au (P)] featuring a gold(II) center. The lifetime of this excited state is quite long due to a comparably slow charge recombination (k ≈3×10  s ).

The Bonding Situation in the Dinuclear Tetra-Hydrido Complex [{CpFe}(μ-H)] Revisited by Hard X-Ray Spectroscopy.

High energy resolution fluorescence detected XANES (HERFD-XANES) and valence-to-core X-ray emission spectroscopy (VtC-XES) are introduced as powerful tools to investigate hydride-iron interaction, the possible iron-iron bond, and iron spin state of the dinuclear tetra-hydrido complex [{CpFe}(μ-H)] (1H, Cp = η-C iPr) by thoroughly accessing the geometric and electronic structure of this complex in comparison to the nonhydride reference [CpCpFe] (1, Cp = CH). The so far observed most intense hydride induced signals in the pre-edge feature of the HERFD-XANES and in the VtC-XES spectra at the iron K-edge allow a precise analysis of the LUMO and HOMO states, respectively, by application of time-dependent density function theory (TD-DFT) and density functional theory (DFT) calculations. The results of these calculations are further employed to understand the oxidation state, spin states, and potential Fe-Fe bonds in this complex.

The Connection between NHC Ligand Count and Photophysical Properties in Fe(II) Photosensitizers: An Experimental Study.

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.

Detection and Characterization of Hydride Ligands in Iron Complexes by High-Resolution Hard X-ray Spectroscopy and Implications for Catalytic Processes.

Two hydride catalysts [Fe(CO)(dppp)H(NO)] (dppp = 1,3-bis(diphenylphosphino)propane) and [Fe(CO)H(NO)(PPh)] in comparison with nonhydride analogues [Fe(dppe)(NO)] (dppe = 1,3-bis(diphenylphosphino)ethane) and [Fe(NO)(PPh)] are investigated with a combination of valence-to-core X-ray emission spectroscopy (VtC-XES) and high-energy resolution fluorescence detected X-ray absorption near-edge structure (HERFD-XANES). To fully understand the experiments and to obtain precise information about molecular levels being involved in the spectral signals, time-dependent density functional theory (TD-DFT) calculations and ground state density functional theory (DFT) calculations are necessary. An excellent agreement between experiment and theory allows the identification of particular spectral signals of the Fe-H group.