Electronic Coupling in a Highly Preorganized Bimetallic Complex Comprising Pyrazolate‐Bridged CpMn(CO)₂ Moieties

Abstract

By means of a multistep synthetic procedure a dimanganese complex has been prepared, in which a N,N'-bridging pyrazolate ligand spans two CpMn(CO)(2) subunits in a highly preorganized chelate arrangement. The Xray crystallographic analyses of the Mn(I)Mn(I) complex K(+)1(-) and of its non-chelate precursor complex elucidate details of the molecular structure, in particular an unusual pyrazolate binding mode in the solid state and intertwining of the CO ligands in the crowded bimetallic array 1(-). The Mn(I)Mn(I) compound (1(-)), the mixed-valent Mn(I)Mn(II) (1), and the oxidized Mn(II)Mn(II) form (1(+)) have been characterized by various analytical and spectrosopic methods, such as electrochemistry, variable-temperature EPR spectroscopy, IR spectroelectrochemistry, and UV/Vis/NIR spectroelectrochemistry as well as by DFT and TD-DFT calculations. Strong electronic coupling in the mixed-valent complex is observed, but time- (and temperature-) dependent valence detrapping occurs, thus placing 1 in class II according to the Robin and Day assignment, close to the class II/III transition. From variable-temperature EPR spectroscopy a rough estimate of the activation energy and rate for thermal electron transfer can be deduced, with E(th) ( not equal )=13.6 kJ mol(-1) and k(th)=2.6 x 10(10) s(-1) at 298 K. Unexpectedly, no intervalence CT transition for 1 is detected in solution, but one appears in the optical spectrum of solid 1. The conclusions drawn from experiments are fully supported by DFT calculations that were carried out for all three forms of the dimanganese complex. A broken symmetry treatment for mixed-valent 1 reveals almost perfect localization of both spin and charge on one Mn center. According to TD-DFT the first excited states of 1 give rise to the IT processes in the NIR-energy region, as observed in the solid-state spectrum. The HOMOs are located at the Mn ions and are favorably arranged for pi interactions with the bridging pyrazolate.

Mixed‐Valence Ruthenium Complexes Rotating through a Conformational Robin–Day Continuum

“…[40][41][42] Calculated harmonic vibrational frequencies were scaled by an empirical factor of 0.95. [43,44] For + , a two-dimensional relaxed scan of the potentialenergy surface (PES) with a fixed P-Ru-Ru-P dihedral angle Ω (which defines a dihedral angle between the half-sandwich metal complex end groups) and a fixed P-Ru-C 3 -C 4 dihedral angle Θ real (dihedral between bridge and a selected end group) was performed ( Figure 2). Starting from a C i -symmetric structure, both dihedral angles were varied in steps of 10°, from 180° to 0° for Ω and from +50° to -100° for Θ real , to cover a reasonable phase space of the relative conformation of the Ru fragments, and of the phenylene moiety in the bridging ligand.…”

Pyrazole‐Bridged NHC Ligands and Their Dimetallic (Allyl)palladium Complexes

“…[12] A further elaboration is the combination of carbenes with chelating moieties that can act as a bridging unit between two metal ions, giving rise to highly preorganized dimetallic arrangements where the two metal ions are held in close proximity. We and others have recently developed bis(NHC) ligands with a potentially bridging pyridazine moiety, [13,14] and we now report some related systems based on a central pyrazolate core (type II, Scheme 1 with chelating side arms in the 3-and 5-positions of the heterocycle have proven successful for generating manifold dimetallic complexes, [15][16][17][18][19] with valuable applications in bioinorganic modelling and catalysis. [20][21][22] The targeted pyrazolate/NHC hybrid compounds II can formally be described as dinuclear versions of the established type I complexes, where the proximate metal ions in II might cooperate during substrate turnover.…”

Synthesis of a Neutral Mixed‐Valence Diferrocenyl Carborane for Molecular Quantum‐Dot Cellular Automata Applications

“…Implementation of QCA would overcome the power-wall that is pervasive in semiconductor-based computing. [6][7][8][9] Thec arborane 7-Fc + -8-Fc-7,8-nido-[C 2 B 9 H 10 ] À , Fc + FcC 2 B 9 À (3a), was targeted since it combines several attractive features as am olecular switch for QCA applications.S pecifically,t he carborane unit carries ac harge which balances that of the ferrocenium, the delocalized bonding in the carborane bridge potentially allows for electron transfer between the Fe II and Fe III centers,a nd finally,a na ll-neutral null state is possible.T he null state,w here the charge is symmetrically distributed as two ferrocenes and an eutral carborane,isanimportant element for microprocessor clocking in QCA. Desirable traits in am olecular switch include robust thermal and chemical properties,f avorable surface organization, and multiple symmetric ground states.O ne current challenge is that mixed-valence compounds are normally comprised of selec-tively oxidized multi-metal systems,r equiring the need for external counterions to achieve charge balance.U nfortunately,t he external counterion has the possibility of biasing the potential ground states of the molecular switch.…”