Eva M. Zolnhofer

Coordination complexes of general formula trans ‐[MX ₂ (R ₂ ECH ₂ CH ₂ ER ₂ ) ₂ ] (M ^II =Ti, V, Cr, Mn; E=N or P; R=alkyl or aryl) are a cornerstone of coordination and organometallic chemistry. We investigate the electronic properties of two such complexes, trans ‐[VCl ₂ (tmeda) ₂ ] and trans ‐[VCl ₂ (dmpe) ₂ ], which thus represent trans ‐[MX ₂ (R ₂ ECH ₂ CH ₂ ER ₂ ) ₂ ] where M=V, X=Cl, R=Me and E=N (tmeda) and P (dmpe). These V ^II complexes have S =3/2 ground states, as expected for octahedral d ³ . Their tetragonal distortion leads to zero‐field splitting (zfs) that is modest in magnitude ( D ≈0.3 cm ⁻¹ ) relative to analogous S =1 Ti ^II and Cr ^II complexes. This parameter was determined from conventional EPR spectroscopy, but more effectively from high‐frequency and ‐field EPR (HFEPR) that determined the sign of D as negative for the diamine complex, but positive for the diphosphine, which information had not been known for any trans ‐[VX ₂ (R ₂ ECH ₂ CH ₂ ER ₂ ) ₂ ] systems. The ligand‐field parameters of trans ‐[VCl ₂ (tmeda) ₂ ] and trans ‐[VCl ₂ (dmpe) ₂ ] are obtained using both classical theory and ab initio quantum chemical theory. The results shed light not only on the electronic structure of V ^II in this environment, but also on differences between N and P donor ligands, a key comparison in coordination chemistry.

In large-scale, hydrogen production from water-splitting represents the most promising solution for a clean, recyclable, and low-cost energy source. The realization of viable technological solutions requires suitable efficient electrochemical catalysts with low overpotentials and long-term stability for both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) based on cheap and nontoxic materials. Herein, we present a unique molecular approach to monodispersed, ultra-small, and superiorly active iron phosphide (FeP) electrocatalysts for bifunctional OER, HER, and overall water-splitting. They result from transformation of a molecular iron phosphide precursor, containing a [Fe₂P₃] core with mixed-valence Fe^IIFe^III sites bridged by an asymmetric cyclo-P₍₂₊₁₎ ³⁻ ligand. The as-synthesized FeP nanoparticles act as long-lasting electrocatalysts for OER and HER with low overpotential and high current densities that render them one of the best-performing electrocatalysts hitherto known. The fabricated alkaline electrolyzer delivered low cell voltage with durability over weeks, representing an attractive catalyst for large-scale water-splitting technologies.

The reaction of elemental sulfur with the cis-divacant octahedral complex [(pyrr₂py)Fe(OEt₂)] (1; pyrr₂py^2– = 3,5-^tBu₂-bis(pyrrolyl)pyridine) yields the iron dimer [(pyrr-1-S-pyrrpy)Fe]₂ (2; pyrr-1-S-pyrrpy^2– = 3,5-(^tBu₂-pyrrolyl)(1-S-3,5-^tBu₂-pyrrolyl)pyridine) resulting from a pyrr₂py^2– ligand based S-oxidation of one pyrrole arm. Addition of the phosphorus ylide H₂CPPh₃ to 1 forms the ylide adduct [(pyrr₂py)Fe(CH₂PPh₃)] (3), which upon reaction with elemental sulfur produces a rare example of a sulfurmethylenephosphorane adduct, [(pyrr₂py)Fe(SCH₂PPh₃)] (4). The sulfur-oxidized pyrrole group of the ligand pyrr-1-S-pyrrpy^2– can be reversed, since complex 2 exhibits S atom transferability via the addition of 2 equiv of H₂CPPh₃ to yield a mixture of compounds 3 and 4. For all complexes reported, the ferrous ion remains S = 2. Complexes 2–4 were characterized by single-crystal X-ray diffraction as well as ¹H NMR spectroscopy, solid and solution magnetic studies, and ⁵⁷Fe Mössbauer spectroscopic measurements.

The co-thermolysis of As4 and [Cp''2Zr(CO)2] (Cp'' = η5-C5H3tBu2) results in the formation of [Cp''2Zr(η1:1-As4)] (1) in high yields and the arsenic-rich complex [(Cp''2Zr)(Cp''Zr)(µ,η2:2:1-As5)] (2) as a minor product. In contrast to yellow arsenic, 1 is a light-stable, weighable and storable arsenic source for subsequent reactions. This is demonstrated by the transfer reaction of the As42- unit under mild conditions. The reaction of 1 with [Cp'''Fe(µ-Br)]2 (Cp''' = η5-C5H2tBu3) yields the unprecedented bond isomeric complexes [(Cp'''Fe)2(μ,η4:4-As4)] (3a) and [(Cp'''Fe)2(μ,η4:4-cyclo-As4)] (3b). In contrast, the analogous reaction with the CpBn derivative [CpBnFe(µ-Br)]2 (CpBn = η5-C5(CH2(C6H5)5) leads exclusively to the triple decker complex [(CpBnFe)2(μ,η4:4-As4)] (4) possessing the tetraarsabutadiene-type ligand analogous to 3a. To elucidate the stability of the bond-isomers 3a and 3b, DFT calculations were performed. The oxidation of 4 with AgBF4 affords [(CpBnFe)2(μ,η5:5-As5)][BF4] (5), which is a product expanded by an arsenic atom, instead of the expected complex [(CpBnFe)2(μ,η4:4-cyclo-As4)]+.

A comparison of P₄ activations mediated by low‐valent β‐diketiminato (L) cobalt complexes is presented. The formal Co⁰ source [K₂(L³Co)₂(μ₂:η¹,η¹‐N₂)] (1) reacts with P₄ to form a mixture of the monoanionic complexes [K(thf)₆][(L³Co)₂(μ₂:η⁴,η⁴‐P₄)] (2) and [K(thf)₆][(L³Co)₂(μ₂:η³,η³‐P₃)] (3). The analogue Co^I precursor [L³Co(tol)] (4 a), however, selectively yields the corresponding neutral derivative [(L³Co)₂(μ₂:η⁴,η⁴‐P₄)] (5 a). Compound 5 a undergoes thermal P atom loss to form the unprecedented complex [(L³Co)₂(μ₂:η³,η³‐P₃)] (6). The products 2 and 3 can be obtained selectively by an one‐electron reduction of their neutral precursors 5 a and 6, respectively. The electrochemical behaviour of 2, 3, 5 a, and 6 is monitored by cyclic voltammetry and their magnetism is examined by SQUID measurements and the Evans method. The initial Co^I‐mediated P₄ activation is not influenced by applying the structurally different ligands L¹ and L², which is proven by the formation of the isostructural products [(LCo)₂(μ₂:η⁴,η⁴‐P₄)] [L=L³ (5 a), L¹ (5 b), L² (5 c)].

A stable chiral hetero[4]helicene radical cation was synthesized and characterized by UV/Vis absorption and EPR spectroscopy, as well as X‐ray crystallography. For the first time, a combination of chiroptical methods involving ECD, ORD, and VCD, supported by quantum mechanical predictions, enabled the elucidation of the absolute configuration of such open‐shell helical species.

Ball milling TiO₂ anatase together with TiH₂ can create an effective photocatalyst. The process changes the lattice and electronic structure of anatase. Lattice deformation created by mechanical impact combined with hydride incorporation yield electronic gap‐states close to the conduction band of anatase. These provide longer lifetimes of photogenerated charge carriers and lead to an intrinsic cocatalytic activation of anatase for H₂ evolution.

Triarylphosphine oxides (Ar₃P=O) are being intensely studied as electron‐accepting (n‐type) materials. Despite the widespread application of these compounds as electron conductors, experimental data regarding the structural and electronic properties of their negatively charged states remain scarce owing to their propensity for follow‐up chemistry. Herein, a carefully designed triarylphosphine oxide scaffold is disclosed that comprises sterically demanding spirofluorenyl moieties to shield the central phosphoryl (P=O) moiety. This compound undergoes chemical one‐electron reduction to afford an exceptionally stable radical anion, which was isolated and characterized by X‐ray crystallography for the very first time. The experimental data, corroborated by computational studies, shall allow for the construction of phosphine oxide materials with enhanced stability.