Spectroscopic and Quantum Chemical Investigation of Benzene-1,2-dithiolate-Coordinated Diiron Complexes with Relevance to Dinitrogen Activation.

In this work, a benzene-1,2-dithiolate (bdt) pentamethylcyclopentadienyl di-iron complex [Cp*Fe(µ-η2:η4-bdt)FeCp*] and its [Cp*Fe(bdt)(X)FeCp*] analogues (where X = N2H2, N2H3-, H-, NH2-, NHCH3-, or NO+) were investigated through spectroscopic and computational studies. These complexes are of relevance as model systems for dinitrogen activation in nitrogenase and share with its active site the presence of iron, sulfur ligands, and a very flexible electronic structure. On the basis of a combination of X-ray emission spectroscopy (XES), X-ray crystallography, Mössbauer, NMR, and EPR spectroscopy, the geometric and electronic structure of the series has been experimentally elucidated. All iron atoms were found to be in a local low-spin configuration. When no additional X ligand is bound, the bdt ligand is tilted and features a stabilizing π-interaction with one of the iron atoms. The number of lone-pair orbitals provided by the nitrogen-containing species is crucial to the overall electronic structure. When only one lone-pair is present and the iron atoms are bridged by one atom, a three-center bond occurs, and a direct Fe-Fe bond is absent. If the bridging atom provides two lone-pairs, then an Fe-Fe bond is formed. A recurring theme for all ligands is σ-donation into the unoccupied eg manifolds of both iron atoms and back-donation from the t2g manifolds into the ligand π* orbitals. The latter results in a weakening of the double bond of the bound ligand, and in the case of NO+, it results in a weakening of all bonds that comprise triple bond. The electron-rich thiolates further amplify this effect and can also serve as bases for proton binding. While the above observations have been made for the studied di-iron complexes, they may be of relevance for the active site in nitrogenase, where a similar N2 binding mode may occur allowing for the simultaneous weakening of the N2 σ bond and π bonds.

Raman Spectroscopy as a Method to Investigate Catalytic Intermediates: CO2 Reducing [Re(Cl)(bpy-R)(CO)3] Catalyst.

Complexes of the type [Re(Cl)(bpy-R)(CO)3] (1, bpy = bipyridine, R = (t)Bu, H, CF3) show high catalytic activity for electrochemical CO2 reduction. Application of Raman spectroscopy to these complexes as well as to the doubly reduced species [Re(bpy-R)(CO)3](-) (3), which are the postulated active species, and the monoreduced complex [Re(Cl)(bpy-CF3)(CO)3](-) (2) and comparison with state-of-the-art quantum chemical calculations allows accurate investigation of electronic structures as well as geometries. For doubly reduced complexes, calculations point out a formal closed-shell singlet state only compatible with a formal {Re(I)(bpy-R)(2-)} moiety. In contrast, based on molecular orbital analysis and the change of the actual charge distribution during the overall two-electron reduction, the system is better described as {Re(0)(bpy-R(•))(-)}. Interestingly, the Raman spectra of the monoreduced and doubly reduced complexes with the CF3-substituted bpy ligand are virtually identical, which points to the same overall electronic structure of the bpy species in both complexes. Additional Raman experiments and calculations of [Re(COOH)(bpy)(CO)3] (4) and [Re(bpy)(CO)4]OTf (5), which are proposed to be intermediates of the catalytic cycle for CO2 reduction, confirm the presence of neutral bpy showing that the reducing equivalent stored at the bidentate ligand is involved in the activation of CO2. As such, Raman spectroscopy combined with quantum chemical calculations is an ideal tool to investigate catalysts with redox active ligands, since the spectra give local information about the electronic and geometric structure of the molecule.

Remarkable reactivity of a rhodium(I) boryl complex towards CO2 and CS2: isolation of a carbido complex.

The activation of CO2, CS2 as well as of PhNCO at [Rh(Bpin)(PEt3)3] led to C=X bond cleavage and the formation of {RhXBpin} species (X = O, S, N). Treatment of the boryl complex [Rh(Bpin)(PEt3)3] with 0.5 equivalents of CS2 resulted in the fragmentation of CS2 and the formation of the remarkable µ-carbido complex trans-[Rh2(µ-C)(SBpin)2(PEt3)4].

Catalytic borylation of SCF3-functionalized arenes by rhodium(I) boryl complexes: regioselective C-H activation at the ortho-position.

An unprecedented reaction pathway for the borylation of SCF3-containing arenes using [Rh(Bpin)(PEt3)3] (pin=pinacolato) is reported. Catalytic processes were developed and the functionalizations proceed under mild reaction conditions. The C-H activations occur with a unique regioselectivity for the position ortho to the SCF3 group, which apparently serves as directing group. Borylated SCF3 compounds can serve as versatile building blocks.

Versatile reactivity of a rhodium(I) boryl complex towards ketones and imines.

The rhodium(I) boryl complex [Rh(Bpin)(PEt3)3] (1) reacts with the ketones α,α,α-trifluoroacetophenone and 9-fluorenone by insertion of the C=O bond to give [Rh{η(3)-C(CF3)(OBpin)C6H5}(PEt3)2] (4) and [Rh{η(5)-C13H8(OBpin)}(PEt3)2] (6), whereas the reaction with acetophenone leads to the formation of [Rh(H)(PEt3)3] (2), [Rh(OBpin)(PEt3)3] (3) and (E)-(Ph)CH=CHBpin. Treatment 1 of with ketimines generates [Rh{η(3)-C6H5=C(Ph)N(Ph)(Bpin)}(PEt3)2] (7), [Rh{(η(3)-C12H8)N(Ph)(Bpin)}(PEt3)2] (8) or [Rh{CPh2N(H)(Bpin)}(PEt3)2] (9). The insertion of aldimines into the Rh-B bond gives access to [Rh[η(3)-CH{N(C6H13)Bpin}C6H5](PEt3)2] (11) or [Rh[η(3)-CH{N(Ph)Bpin}C6H5](PEt3)2] (12). The latter is converted into the C-H activation product [Rh{(C6H4)-o-N(Bpin)(CH2Ph)}(PEt3)3] (13). Complex 13 reacts with B2pin2 to yield the boryl complex 1 and the amine PhCH2N(Bpin)(C6H4-o-Bpin).