5 research outputs found
A Ruthenium Hydrido Dinitrogen Core Conserved across Multielectron/Multiproton Changes to the Pincer Ligand Backbone
A series
of rutheniumÂ(II) hydrido dinitrogen complexes supported by pincer
ligands in different formal oxidation states have been prepared and
characterized. Treating a ruthenium dichloride complex supported by
the pincer ligand bisÂ(di-<i>tert</i>-butylphosphinoethyl)Âamine
(H-PNP) with reductant or base generates new five-coordinate <i>cis</i>-hydridodinitrogen ruthenium complexes each containing
different forms of the pincer ligand. Further ligand transformations
provide access to the first isostructural set of complexes featuring
all six different forms of the pincer ligand. The conserved <i>cis</i>-hydridodinitrogen structure facilitates characterization
of the π-donor, π-acceptor, and/or σ-donor properties
of the ligands and assessment of the impact of ligand-centered multielectron/multiproton
changes on N<sub>2</sub> activation. Crystallographic studies, infrared
spectroscopy, and <sup>15</sup>N NMR spectroscopy indicate that N<sub>2</sub> remains weakly activated in all cases, providing insight
into the donor properties of the different pincer ligand states. Ramifications
on applications of (pincer)Ru species in catalysis are considered
Ammonia Synthesis from a Pincer Ruthenium Nitride via Metal–Ligand Cooperative Proton-Coupled Electron Transfer
The conversion of
metal nitride complexes to ammonia may be essential
to dinitrogen fixation. We report a new reduction pathway that utilizes
ligating acids and metal–ligand cooperation to effect this
conversion without external reductants. Weak acids such as 4-methoxybenzoic
acid and 2-pyridone react with nitride complex [(<i>H</i>-PNP)ÂRuN]<sup>+</sup> (<i>H-</i>PNP = HNÂ(CH<sub>2</sub>CH<sub>2</sub>P<sup>t</sup>Bu<sub>2</sub>)<sub>2</sub>) to generate
octahedral ammine complexes that are κ<sup>2</sup>-chelated
by the conjugate base. Experimental and computational mechanistic
studies reveal the important role of Lewis basic sites proximal to
the acidic proton in facilitating protonation of the nitride. The
subsequent reduction to ammonia is enabled by intramolecular 2H<sup>+</sup>/2e<sup>–</sup> proton-coupled electron transfer from
the saturated pincer ligand backbone
Correlating Thermodynamic and Kinetic Hydricities of Rhenium Hydrides
The kinetics of hydride transfer from Re(Rbpy)(CO)3H (bpy = 4,4′-R-2,2′-bipyridine;
R = OMe, tBu, Me, H, Br, COOMe, CF3) to CO2 and
seven different cationic N-heterocycles were determined. Additionally,
the thermodynamic hydricities of complexes of the type Re(Rbpy)(CO)3H were established primarily using computational
methods. Linear free-energy relationships (LFERs) derived by correlating
thermodynamic and kinetic hydricities indicate that, in general, the
rate of hydride transfer increases as the thermodynamic driving force
for the reaction increases. Kinetic isotope effects range from inverse
for hydride transfer reactions with a small driving force to normal
for reactions with a large driving force. Hammett analysis indicates
that hydride transfer reactions with greater thermodynamic driving
force are less sensitive to changes in the electronic properties of
the metal hydride, presumably because there is less buildup of charge
in the increasingly early transition state. Bronsted α values
were obtained for a range of hydride transfer reactions and along
with DFT calculations suggest the reactions are concerted, which enables
the use of Marcus theory to analyze hydride transfer reactions involving
transition metal hydrides. It is notable, however, that even slight
perturbations in the steric properties of the Re hydride or the hydride
acceptor result in large deviations in the predicted rate of hydride
transfer based on thermodynamic driving forces. This indicates that
thermodynamic considerations alone cannot be used to predict the rate
of hydride transfer, which has implications for catalyst design
Redox-Active Bis(phenolate) N‑Heterocyclic Carbene [OCO] Pincer Ligands Support Cobalt Electron Transfer Series Spanning Four Oxidation States
A new
family of low-coordinate Co complexes supported by three redox-noninnocent
tridentate [OCO] pincer-type bisÂ(phenolate) N-heterocyclic carbene
(NHC) ligands are described. Combined experimental and computational
data suggest that the charge-neutral four-coordinate complexes are
best formulated as CoÂ(II) centers bound to closed-shell [OCO]<sup>2–</sup> dianions, of the general formula [(OCO)ÂCo<sup>II</sup>L] (where L is a solvent-derived MeCN or THF). Cyclic voltammograms
of the [(OCO)ÂCo<sup>II</sup>L] complexes reveal three oxidations accessible
at potentials below 1.2 V vs Fc<sup>+</sup>/Fc, corresponding to generation
of formally CoÂ(V) species, but the true physical/spectroscopic oxidation
states are much lower. Chemical oxidations afford the mono- and dications
of the imidazoline NHC-derived complex, which were examined by computational
and magnetic and spectroscopic methods, including single-crystal X-ray
diffraction. The metal and ligand oxidation states of the monocationic
complex are ambiguous; data are consistent with formulation as either
[(<sup>S</sup>OCO)ÂCo<sup>III</sup>(THF)<sub>2</sub>]<sup>+</sup> containing
a closed-shell [<sup>S</sup>OCO]<sup>2–</sup> diphenolate ligand
bound to a <i>S</i> = 1 CoÂ(III) center, or [(<sup>S</sup>OCO<sup>•</sup>)ÂCo<sup>II</sup>(THF)<sub>2</sub>]<sup>+</sup> with a low-spin CoÂ(II) ion ferromagnetically coupled to monoanionic
[<sup>S</sup>OCO<sup>•</sup>]<sup>−</sup> containing
a single unpaired electron distributed across the [OCO] framework.
The dication is best described as [(<sup>S</sup>OCO<sup>0</sup>)ÂCo<sup>II</sup>(THF)<sub>3</sub>]<sup>2+</sup>, with a single unpaired electron
localized on the d<sup>7</sup> CoÂ(II) center and a doubly oxidized,
charge-neutral, closed-shell <sup>S</sup>OCO<sup>0</sup> ligand.
The combined data provide for the first time unequivocal and structural
evidence for [OCO] ligand redox activity. Notably, varying the degree
of unsaturation in the NHC backbone shifts the ligand-based oxidation
potentials by up to 400 mV. The possible chemical origins of this
unexpected shift, along with the potential utility of the [OCO] pincer
ligands for base-metal-mediated organometallic coupling catalysis,
are discussed
Correlating Thermodynamic and Kinetic Hydricities of Rhenium Hydrides
The kinetics of hydride transfer from Re(Rbpy)(CO)3H (bpy = 4,4′-R-2,2′-bipyridine;
R = OMe, tBu, Me, H, Br, COOMe, CF3) to CO2 and
seven different cationic N-heterocycles were determined. Additionally,
the thermodynamic hydricities of complexes of the type Re(Rbpy)(CO)3H were established primarily using computational
methods. Linear free-energy relationships (LFERs) derived by correlating
thermodynamic and kinetic hydricities indicate that, in general, the
rate of hydride transfer increases as the thermodynamic driving force
for the reaction increases. Kinetic isotope effects range from inverse
for hydride transfer reactions with a small driving force to normal
for reactions with a large driving force. Hammett analysis indicates
that hydride transfer reactions with greater thermodynamic driving
force are less sensitive to changes in the electronic properties of
the metal hydride, presumably because there is less buildup of charge
in the increasingly early transition state. Bronsted α values
were obtained for a range of hydride transfer reactions and along
with DFT calculations suggest the reactions are concerted, which enables
the use of Marcus theory to analyze hydride transfer reactions involving
transition metal hydrides. It is notable, however, that even slight
perturbations in the steric properties of the Re hydride or the hydride
acceptor result in large deviations in the predicted rate of hydride
transfer based on thermodynamic driving forces. This indicates that
thermodynamic considerations alone cannot be used to predict the rate
of hydride transfer, which has implications for catalyst design