A Stable Homoleptic Organometallic Iron(IV) Complex

Using Iron L-Edge and Nitrogen K-Edge X-ray Absorption Spectroscopy to Improve the Understanding of the Electronic Structure of Iron Carbene Complexes

Iron-centered N-heterocyclic carbene compounds have attracted much attention in recent years due to their long-lived excited states with charge transfer (CT) character. Understanding the orbital interactions between the metal and ligand orbitals is of great importance for the rational tuning of the transition metal compound properties, e.g., for future photovoltaic and photocatalytic applications. Here, we investigate a series of iron-centered N-heterocyclic carbene complexes with +2, + 3, and +4 oxidation states of the central iron ion using iron L-edge and nitrogen K-edge X-ray absorption spectroscopy (XAS). The experimental Fe L-edge XAS data were simulated and interpreted through restricted-active space (RAS) and multiplet calculations. The experimental N K-edge XAS is simulated and compared with time-dependent density functional theory (TDDFT) calculations. Through the combination of the complementary Fe L-edge and N K-edge XAS, direct probing of the complex interplay of the metal and ligand character orbitals was possible. The σ-donating and π-accepting capabilities of different ligands are compared, evaluated, and discussed. The results show how X-ray spectroscopy, together with advanced modeling, can be a powerful tool for understanding the complex interplay of metal and ligand.

Unusual S=1 Four-Coordinate Fe(IV) Complexes Supported by Bisamide Ligands: Syntheses, Characterization, and Electronic Structures

The catalytic relevance of Fe(IV) species in non-heme iron catalysis has motivated synthetic advances in well-defined five- and six-coordinate Fe(IV) complexes for a better understanding of their fundamental electronic structures and reactivities. Herein, we report the syntheses of FeDipp2 and FeMes2, a pair of unusual four-coordinate non-heme formally Fe(IV) complexes with S=1 ground states supported by strongly donating bisamide ligands. By combining spectroscopic characterization and computational modeling, we found that small variations in ligand aryl substituents resulted in substantial changes in both structures and bonding. This work highlights the strong donor capabilities and modularity of the bisamide ligand set. More broadly, it is a critical contribution to the utilization of ligand design to modulate molecular geometries and electronic structures of low-coordinate, high-valent iron complexes.

Tailoring the Photophysical Properties of a Homoleptic Iron(II) Tetra N-Heterocyclic Carbene Complex by Attaching an Imidazolium Group to the (C∧N∧C) Pincer Ligand─A Comparative Study

We here report the synthesis of the homoleptic iron(II) N-heterocyclic carbene (NHC) complex [Fe(miHpbmi)2](PF6)4 (miHpbmi = 4-((3-methyl-1H-imidazolium-1-yl)pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) and its electrochemical and photophysical properties. The introduction of the π-electron-withdrawing 3-methyl-1H-imidazol-3-ium-1-yl group into the NHC ligand framework resulted in stabilization of the metal-to-ligand charge transfer (MLCT) state and destabilization of the metal-centered (MC) states. This resulted in an improved excited-state lifetime of 16 ps compared to the 9 ps for the unsubstituted parent compound [Fe(pbmi)2](PF6)2 (pbmi = (pyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)) as well as a stronger MLCT absorption band extending more toward the red spectral region. However, compared to the carboxylic acid derivative [Fe(cpbmi)2](PF6)2 (cpbmi = 1,1'-(4-carboxypyridine-2,6-diyl)bis(3-methylimidazol-2-ylidene)), the excited-state lifetime of [Fe(miHpbmi)2](PF6)4 is the same, but both the extinction and the red shift are more pronounced for the former. Hence, this makes [Fe(miHpbmi)2](PF6)4 a promising pH-insensitive analogue of [Fe(cpbmi)2](PF6)2. Finally, the excited-state dynamics of the title compound [Fe(miHpbmi)2](PF6)4 was investigated in solvents with different viscosities, however, showing very little dependency of the depopulation of the excited states on the properties of the solvent used.

Poly(imidazolyliden-yl)borato Complexes of Tungsten: Mapping Steric vs. Electronic Features of Facially Coordinating Ligands

A convenient synthesis of [HB(HImMe)3](PF6)2 (ImMe = N-methylimidazolyl) is decribed. This salt serves in situ as a precursor to the tris(imidazolylidenyl)borate Li[HB(ImMe)3] pro-ligand upon deprotonation with nBuLi. Reaction with [W(≡CC6H4Me-4)(CO)2(pic)2(Br)] (pic = 4-picoline) affords the carbyne complex [W(≡CC6H4Me-4)(CO)2{HB(ImMe)3}]. Interrogation of experimental and computational data for this compound allow a ranking of familiar tripodal and facially coordinating ligands according to steric (percentage buried volume) and electronic (νCO) properties. The reaction of [W(≡CC6H4Me-4)(CO)2{HB(ImMe)3}] with [AuCl(SMe2)] affords the heterobimetallic semi-bridging carbyne complex [WAu(μ-CC6H4Me-4)(CO)2(Cl){HB(ImMe)3}].

High-Efficiency Photoinduced Charge Separation in Fe(III)carbene Thin Films

Symmetry-breaking charge separation in molecular materials has attracted increasing attention for optoelectronics based on single-material active layers. To this end, Fe(III) complexes with particularly electron-donating N-heterocyclic carbene ligands offer interesting properties with a 2LMCT excited state capable of oxidizing or reducing the complex in its ground state. In this Communication, we show that the corresponding symmetry-breaking charge separation occurs in amorphous films of pristine [Fe(III)L2]PF6 (L = [phenyl(tris(3-methylimidazol-2-ylidene))borate]-). Excitation of the solid material with visible light leads to ultrafast electron transfer quenching of the 2LMCT excited state, generating Fe(II) and Fe(IV) products with high efficiency. Sub-picosecond charge separation followed by recombination in about 1 ns could be monitored by transient absorption spectroscopy. Photoconductivity measurements of films deposited on microelectrode arrays demonstrated that photogenerated charge carriers can be collected at external contacts.

Homoleptic Fe(III) and Fe(IV) Complexes of a Dianionic C3-Symmetric Scorpionate

High-valent iron species have been implicated as key intermediates in catalytic oxidation reactions, both in biological and synthetic systems. Many heteroleptic Fe(IV) complexes have now been prepared and characterized, especially using strongly π-donating oxo, imido, or nitrido ligands. On the other hand, homoleptic examples are scarce. Herein, we investigate the redox chemistry of iron complexes of the dianonic tris-skatylmethylphosphonium (TSMP^2-) scorpionate ligand. One-electron oxidation of the tetrahedral, bis-ligated [(TSMP)₂Fe^II]^2- leads to the octahedral [(TSMP)₂Fe^III]^-. The latter undergoes thermal spin-cross-over both in the solid state and solution, which we characterize using superconducting quantum inference device (SQUID), Evans method, and paramagnetic nuclear magnetic resonance spectroscopy. Furthermore, [(TSMP)₂Fe^III]^- can be reversibly oxidized to the stable high-valent [(TSMP)₂Fe^IV]⁰ complex. We use a variety of electrochemical, spectroscopic, and computational techniques as well as SQUID magnetometry to establish a triplet (S = 1) ground state with a metal-centered oxidation and little spin delocalization on the ligand. The complex also has a fairly isotropic g-tensor (g_iso = 1.97) combined with a positive zero-field splitting (ZFS) parameter D (+19.1 cm^-1) and very low rhombicity, in agreement with quantum chemical calculations. This thorough spectroscopic characterization contributes to a general understanding of octahedral Fe(IV) complexes.

Crystallographic Evidence of η1-Coordination of Bulky Aminopyridine in Halide-Containing Iron (II) Complexes

Reaction of N-(2,6-diisopropylphenyl)-[6-(2,4,6-triisopropylphenyl)-pyridine-2-yl]-amine (ApH) in equimolar ratio with anhydrous FeBr2 and FeI2 in tetrahydrofuran (THF) afforded, after workup in toluene, the first examples of mono(aminopyridine) Fe(II) complexes, [ApHFeBr(µ-Br)]2 (1) and [ApHFeI2(thf)] (2), respectively. X-ray analysis shows 1 to be dimeric, whereas compound 2 is monomeric. In both cases, aminopyridine ligands show rare η1-coordination to Fe through pyridine nitrogen atom. Compound 1 exhibits intramolecular N–H⋯Br hydrogen bonds [3.363 Å] with an N–H⋯Br angle of 158.84°. Hirshfeld surface and fingerprint plots identify the significant intermolecular interactions in the crystal network. Both compounds crystallized in the monoclinic space group. For compound 1, C2/c, the cell parameters are: a  =  25.5750(5) Å, b  =  10.5150(5) Å, c = 18.9610(8) Å, β  =  97.892(5)°, V  =  5050.7(3) A3, Z  =  4. For compound 2, P21/c, the cell parameters are: a  =  10.3180(7) Å, b  =  16.1080(10) Å, c  =  18.6580(11) Å, β  =  102.038(5)°, V  = 3032.8(3) A3, Z  =  4.

Accessing Photoredox Transformations with an Iron(III) Photosensitizer and Green Light

Efficient excited-state electron transfer between an iron(III) photosensitizer and organic electron donors was realized with green light irradiation. This advance was enabled by the use of the previously reported iron photosensitizer, [Fe(phtmeimb)₂]⁺ (phtmeimb = {phenyl[tris(3-methyl-imidazolin-2-ylidene)]borate}, that exhibited long-lived and luminescent ligand-to-metal charge-transfer (LMCT) excited states. A benchmark dehalogenation reaction was investigated with yields that exceed 90% and an enhanced stability relative to the prototypical photosensitizer [Ru(bpy)₃]²⁺. The initial catalytic step is electron transfer from an amine to the photoexcited iron sensitizer, which is shown to occur with a large cage-escape yield. For LMCT excited states, this reductive electron transfer is vectorial and may be a general advantage of Fe(III) photosensitizers. In-depth time-resolved spectroscopic methods, including transient absorption characterization from the ultraviolet to the infrared regions, provided a quantitative description of the catalytic mechanism with associated rate constants and yields.

The Carbene Cannibal: Photoinduced Symmetry-Breaking Charge Separation in an Fe(III) N-Heterocyclic Carbene

Photoinduced symmetry-breaking charge separation (SB-CS) processes offer the possibility of harvesting solar energy by electron transfer between identical molecules. Here, we present the first case of direct observation of bimolecular SB-CS in a transition metal complex, [Fe^IIIL₂](PF₆) (L = [phenyl(tris(3-methylimidazol-1-ylidene))borate]⁻). Photoexcitation of the complex in the visible region results in the formation of a doublet ligand-to-metal charge transfer (²LMCT) excited state (E_0–0 = 2.13 eV), which readily reacts with the doublet ground state to generate charge separated products, [Fe^IIL₂] and [Fe^IVL₂]²⁺, with a measurable cage escape yield. Known spectral signatures allow for unambiguous identification of the products, whose formation and recombination are monitored with transient absorption spectroscopy. The unusual energetic landscape of [Fe^IIIL₂]⁺, as reflected in its ground and excited state reduction potentials, results in SB-CS being intrinsically exergonic (ΔG_CS° ∼ −0.7 eV). This is in contrast to most systems investigated in the literature, where ΔG_CS° is close to zero, and the charge transfer driven primarily by solvation effects. The study is therefore illustrative for the utilization of the rich redox chemistry accessible in transition metal complexes for the realization of SB-CS.

Mechanistic investigation of a visible light mediated dehalogenation/cyclisation reaction using iron(iii), iridium(iii) and ruthenium(ii) photosensitizers

The identification of reaction mechanisms unique to the iron, ruthenium, and iridium PS represents progress towards the long-sought goal of utilizing earth-abundant, first-row transition metals for emerging energy and environmental applications.

A Stable Homoleptic Organometallic Iron(IV) Complex

A homoleptic organometallic Fe^IV complex that is stable in both solution and in the solid state at ambient conditions has been synthesized and isolated as [Fe(phtmeimb)₂ ](PF₆ )₂ (phtmeimb=[phenyl(tris(3-methylimidazolin-2-ylidene))borate]^- ). This Fe^IV N-heterocyclic carbene (NHC) complex was characterized by ¹ H NMR, HR-MS, elemental analysis, scXRD analysis, electrochemistry, Mößbauer spectroscopy, and magnetic susceptibility. The two latter techniques unequivocally demonstrate that [Fe(phtmeimb)₂ ](PF₆ )₂ is a triplet Fe^IV low-spin S=1 complex in the ground state, in agreement with quantum chemical calculations. The electronic absorption spectrum of [Fe(phtmeimb)₂ ](PF₆ )₂ in acetonitrile shows an intense absorption band in the red and near IR, due to LMCT (ligand-to-metal charge transfer) excitation. For the first time the excited state dynamics of a Fe^IV complex was studied and revealed a ≈0.8 ps lifetime of the ³ LMCT excited state of [Fe(phtmeimb)₂ ](PF₆ )₂ in acetonitrile.