The reactions of the precursor complex [Fe2(CO)6(μ‐bdt)] F with PPh3, PPh2Me, PPh2H have been investigated. Treatment of F with the phosphine ligands yielded both mono‐ and disubstituted complexes [Fe2(CO)5(μ‐bdt)(PPh3)] (1), [Fe2(CO)4(μ‐bdt)(PPh3)2] (2), [Fe2(CO)5(μ‐bdt)(PPh2Me)] (3), [Fe2(CO)4(μ‐bdt)(PPh2Me)2] (4), [Fe2(CO)5(μ‐bdt)(PPh2H)] (5) and [Fe2(CO)4(μ‐bdt)(PPh2H)2] (6). Crystal structures have been reported for complexes 1–3. Complexes 1, 3 and 5 participate in electrocatalytic proton reduction in the presence of two distinct acids of varying strengths: HClO4 and CF3CO2H.
The mono-substituted complex [Fe2(CO)5(μ-naphthalene-2-thiolate)2(P(PhOMe-p)3)] was prepared taking after the structural principles from both [NiFe] and [FeFe]-hydrogenase enzymes. Crystal structures are reported for this complex and the all carbonyl analogue. The bridging naphthalene thiolates resemble μ-bridging cysteine amino acids. One of the naphthyl moieties forms π-π stacking interactions with the terminal bulky phosphine ligand in the crystal structure and in calculations. This interaction stabilizes the reduced and protonated forms during electrocatalytic proton reduction in the presence of acetic acid and hinders the rotation of the phosphine ligand. The intramolecular π-π stabilization, the electrochemistry and the mechanism of the hydrogen evolution reaction were investigated using computational approaches.
Reaction of the precursor complexes [Fe2(μ‐SCH2Ph)2(CO)6] 1 and [Fe2(μ‐SEt)2(CO)6] A with the phosphine ligand (L=P(PhOMe‐p)3) yielded the mono‐substituted complexes [Fe2(μ‐SCH2Ph)2(CO)5(P(PhOMe‐p)3)] 2 and [Fe2(μ‐SEt)2(CO)5(P(PhOMe‐p)3)] 3. All the complexes were characterized by various spectroscopic techniques. X‐ray crystal structure has been reported for complex 3. The reduction potentials of the phosphine substituted complexes 2 and 3 appeared at more negative potentials in comparison to the precursor complexes 1 and A. Complexes 1–3 were catalytically active towards proton reduction in the presence of acids.
Hydrogenases
are versatile enzymatic catalysts with an unmet hydrogen
evolution reactivity (HER) from synthetic bio-inspired systems. The
binuclear active site only has one-site reactivity of the distal Fed atom. Here, binuclear complexes [Fe2(CO)5(μ-Mebdt)(P(4-C6H4OCH3)3)] 1 and [Fe2(CO)5(μ-Mebdt)(PPh2Py)] 2 are presented, which show electrocatalytic
activity in the presence of weak acids as a proton source for the
HER. Despite almost identical structural and spectroscopic properties
(bond distances and angles from single-crystal X-ray; IR, UV/vis,
and NMR), introduction of a nitrogen base atom in the phosphine ligand
in 2 markedly changes site reactivity. The bridging benzenedithiolate
ligand Mebdt interacts with the terminal ligand’s phenyl aromatic
rings and stabilizes the reduced states of the catalysts. Although 1 with monodentate phosphine terminal ligands only shows a
distal iron atom HER activity by a sequence of electrochemical and
protonation steps, the lone pair of pyridine nitrogen in 2 acts as the primary site of protonation. This swaps the iron atom
catalytic activity toward the proximal iron for complex 2. Density-functional theory (DFT) calculations reveal the role of
terminal phosphines ligands without/with pendant amines by directing
the proton transfer steps. The reactivity of 1 is a thiol-based
protonation of a dangling bond in 1–
and distal iron hydride mechanism, which may follow either an ECEC
or EECC sequence, depending on the choice of acid. The pendant amine
in 2 enables a terminal ligand protonation and an ECEC
reactivity. The introduction of a terminal nitrogen atom enables the
control of site reactivity in a binuclear system.
A highly efficient homogeneous catalyst system for production of CH3OH from CO2 using single molecular defined ruthenium and rhodium RAPTA-Type catalysts [Ru(ƞ6-p-cymene)X2(PTA)] (X=I(1), Cl(2); PTA=1,3,5-triaza-7-phosphaadamantane and rhodium catalysts [Rh(ƞ5-C5Me5)X2(PTA/PTA-BH3)] (X=Cl(3),...
We present herein a Cp*Co(III)‐half‐sandwich catalyst system for electrocatalytic CO2 reduction in aqueous acetonitrile solution. In addition to an electron‐donating Cp* ligand (Cp*=pentamethylcyclopentadienyl), the catalyst featured a proton‐responsive pyridyl‐benzimidazole‐based N,N‐bidentate ligand. Owing to the presence of a relatively electron‐rich Co center, the reduced Co(I)‐state was made prone to activate the electrophilic carbon center of CO2. At the same time, the proton‐responsive benzimidazole scaffold was susceptible to facilitate proton‐transfer during the subsequent reduction of CO2. The above factors rendered the present catalyst active toward producing CO as the major product over the other potential 2e/2H+ reduced product HCOOH, in contrast to the only known similar half‐sandwich CpCo(III)‐based CO2‐reduction catalysts which produced HCOOH selectively. The system exhibited a Faradaic efficiency (FE) of about 70% while the overpotential for CO production was found to be 0.78 V, as determined by controlled‐potential electrolysis.
Herein we report the first mesoionic carbene (MIC)‐Mn(I) complex Mn‐bim‐MICimz derived from imidazolylidene motif. Structurally the octahedral Mn(I) complex Mn‐bim‐MICimz was assembled with an anionic benzimidazolato‐anchored imidazolylidene MIC‐based bidentate ligand (bim‐MICimz) and four CO ligands, as supported by detailed characterization using NMR and FTIR spectroscopy, mass spectrometry, and single crystal X‐ray diffraction study. We reckoned that the bim‐MICimz ligand would provide a robust and stable bonding with the Mn(I) centre, and also enhance electron density at the Mn(I) centre through its stronger σ‐donating/weaker π‐accepting property. These structural and electronic attributes triggered to exploit Mn‐bim‐MICimz in catalytic hydrogenation of N‐heteroarenes, where efficient hydride (Mn–H) delivery is a key step.
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