103 research outputs found
Methane CâH Activation via 3d Metal Methoxide Complexes with Potentially Redox-Noninnocent Pincer Ligands: A Density Functional Theory Study
This paper reports
a density functional theory study of 3d transition-metal
methoxide complexes with potentially redox-noninnocent pincer supporting
ligands for methane CâH bond activation to form methanol (L<sub><i>n</i></sub>M-OMe + CH<sub>4</sub> â L<sub><i>n</i></sub>MâMe + CH<sub>3</sub>OH). The three types
of tridentate pincer ligands [terpyridine (NNN), bisÂ(2-pyridyl)Âphenyl-<i>C</i>,<i>N</i>,<i>N</i>âČ (NCN), and
2,6-bisÂ(2-phenyl)Âpyridine-<i>N</i>,<i>C</i>,<i>C</i>âČ (CNC)] and different first-row transition metals
(M = Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are used to elucidate the
reaction mechanism as well as the effect of the metal identity on
the thermodynamics and kinetics of a methane activation reaction.
Spin-density analysis indicates that some of these systems, the NNN
and NCN ligands, have redox-noninnocent character. A four-centered,
kite-shaped transition state, Ï-bond metathesis, or oxidative
hydrogen migration has been found for methane activation for the complexes
studied. Calculations suggest that the d electron count is a more
significant factor than the metal formal charge in controlling the
thermodynamics and kinetics of CâH activation and late 3d metal
methoxides, with high d counts preferred. Notably, early-to-middle
metals tend toward oxidative hydrogen migration and late metals undergo
a pathway that is more akin to Ï-bond metathesis, suggesting
that metal methoxide complexes that favor Ï-bond metathesis
pathways for methane activation will yield lower barriers for CâH
activation
Effect of Ancillary Ligands on Oxidative Addition of CH<sub>4</sub> to Ta(III) Complexes Ta(OC<sub>2</sub>H<sub>4</sub>)<sub>3</sub>A (A = B, Al, CH, SiH, N, P): A Density Functional Theory Study
A DFT study of oxidative
addition of methane to TaÂ(OC<sub>2</sub>H<sub>4</sub>)<sub>3</sub>A (where A may act as ancillary ligand)
was conducted to understand how A may affect the propensity of the
complex to undergo oxidative addition. Among the A groups studied,
they can be a Lewis acid (B or Al), a saturated, electron-precise
moiety (CH or SiH), a Ï-donor (N), or a Ï-donor/Ï-acid
(P). By varying A, we seek to understand how changing the electronic
properties of A can affect the kinetics and thermodynamics of methane
CâH activation by these complexes. For every reaction two transition
states (H or CH<sub>3</sub> trans to A) leading to two corresponding
products were identified. For all A, the TS with H trans to A is favored
kinetically; except for SiH and CH, the kinetically favored product
is not thermodynamically favored. For the kinetic products, the Î<i>G</i><sup>⧧</sup> values for A = B, Al are highest among
the 2p and 3p elements, respectively. Upon moving from electron-deficient
to electron-rich moieties (P and N) the computed CâH activation
barrier for the kinetic product decreases significantly. Thus, changing
A greatly influences the barrier for methane CâH oxidative
addition by these complexes
Effect of Ancillary Ligands (A) on Oxidative Addition of CH<sub>4</sub> to Rhenium(III) Complexes: A = B, Al, CH, SiH, N, and P Using MP2, CCSD(T), and MCSCF Methods
A computational
study
of oxidative addition (OA) of methane to
ReÂ(OC<sub>2</sub>H<sub>4</sub>)<sub>3</sub>A (A = ancillary ligand,
which thus may interact with the metal) was carried out. The choice
of ancillary ligands has been made based on their electronic properties:
A = B or Al (Lewis acid), CH or SiH (electron precise), N (Ï-donor),
and P (Ï-donor/Ï-acid). The main objective of this study
was to understand how variation in A affects the structural and electronic
properties of the reactant d<sup>4</sup>-ReÂ(III) complex, which can
ultimately tune the kinetics and thermodynamics of OA. Results obtained
from MP2 calculations revealed that, for OA of CH<sub>4</sub> to ReÂ(OC<sub>2</sub>H<sub>4</sub>)<sub>3</sub>A, the order of Î<i>G</i><sup>âĄ</sup> for a choice of ancillary ligand is B > Al
>
SiH > CH > N > P. Single point calculations for Î<i>G</i><sup>âĄ</sup> obtained with CCSDÂ(T) showed excellent
agreement
with those computed with MP2 methods. MCSCF calculations indicated
that oxidative addition transition states are well described by a
single electronic configuration, giving further confidence in the
MP2 approach used for geometry optimization and Î<i>G</i><sup>âĄ</sup> determination, and that the transition states
are more electronically similar to the d<sup>4</sup>-ReÂ(III) reactant
than the d<sup>2</sup>-ReÂ(V) product
Control of CâH Bond Activation by Mo-Oxo Complexes: p<i>K</i><sub>a</sub> or Bond Dissociation Free Energy (BDFE)?
A density
functional theory (DFT) study (BMK/6-31+GÂ(d)) was initiated to investigate
the activation of benzylic carbonâhydrogen bonds by a molybdenum-oxo
complex with a potentially redox noninnocent supporting ligandîža
simple mimic of the active species of the enzyme ethylbenzene dehydrogenase
(EBDH)îžthrough deprotonation (CâH bond heterolysis)
or hydrogen atom abstraction (CâH bond homolysis) routes. Activation
free-energy barriers for neutral and anionic Mo-oxo complexes were
high, but lower for anionic complexes than neutral complexes. Interesting
trends as a function of substituents were observed that indicated
significant H<sup>ÎŽ+</sup> character in the transition states
(TS), which was further supported by the preference for [2 + 2] addition
over HAA for most complexes. Hence, it was hypothesized that CâH
activation by these EBDH mimics is controlled more by the p<i>K</i><sub>a</sub> than by the bond dissociation free energy
of the CâH bond being activated. Therefore, the results suggest
promising pathways for designing more efficient and selective catalysts
for hydrocarbon oxidation based on EBDH active-site mimics
Effect of Ancillary Ligands on Oxidative Addition of CH<sub>4</sub> to Ta(III) Complexes Ta(OC<sub>2</sub>H<sub>4</sub>)<sub>3</sub>A (A = B, Al, CH, SiH, N, P): A Density Functional Theory Study
A DFT study of oxidative
addition of methane to TaÂ(OC<sub>2</sub>H<sub>4</sub>)<sub>3</sub>A (where A may act as ancillary ligand)
was conducted to understand how A may affect the propensity of the
complex to undergo oxidative addition. Among the A groups studied,
they can be a Lewis acid (B or Al), a saturated, electron-precise
moiety (CH or SiH), a Ï-donor (N), or a Ï-donor/Ï-acid
(P). By varying A, we seek to understand how changing the electronic
properties of A can affect the kinetics and thermodynamics of methane
CâH activation by these complexes. For every reaction two transition
states (H or CH<sub>3</sub> trans to A) leading to two corresponding
products were identified. For all A, the TS with H trans to A is favored
kinetically; except for SiH and CH, the kinetically favored product
is not thermodynamically favored. For the kinetic products, the Î<i>G</i><sup>⧧</sup> values for A = B, Al are highest among
the 2p and 3p elements, respectively. Upon moving from electron-deficient
to electron-rich moieties (P and N) the computed CâH activation
barrier for the kinetic product decreases significantly. Thus, changing
A greatly influences the barrier for methane CâH oxidative
addition by these complexes
Methane CâH Activation via 3d Metal Methoxide Complexes with Potentially Redox-Noninnocent Pincer Ligands: A Density Functional Theory Study
This paper reports
a density functional theory study of 3d transition-metal
methoxide complexes with potentially redox-noninnocent pincer supporting
ligands for methane CâH bond activation to form methanol (L<sub><i>n</i></sub>M-OMe + CH<sub>4</sub> â L<sub><i>n</i></sub>MâMe + CH<sub>3</sub>OH). The three types
of tridentate pincer ligands [terpyridine (NNN), bisÂ(2-pyridyl)Âphenyl-<i>C</i>,<i>N</i>,<i>N</i>âČ (NCN), and
2,6-bisÂ(2-phenyl)Âpyridine-<i>N</i>,<i>C</i>,<i>C</i>âČ (CNC)] and different first-row transition metals
(M = Ti, V, Cr, Mn, Fe, Co, Ni, and Cu) are used to elucidate the
reaction mechanism as well as the effect of the metal identity on
the thermodynamics and kinetics of a methane activation reaction.
Spin-density analysis indicates that some of these systems, the NNN
and NCN ligands, have redox-noninnocent character. A four-centered,
kite-shaped transition state, Ï-bond metathesis, or oxidative
hydrogen migration has been found for methane activation for the complexes
studied. Calculations suggest that the d electron count is a more
significant factor than the metal formal charge in controlling the
thermodynamics and kinetics of CâH activation and late 3d metal
methoxides, with high d counts preferred. Notably, early-to-middle
metals tend toward oxidative hydrogen migration and late metals undergo
a pathway that is more akin to Ï-bond metathesis, suggesting
that metal methoxide complexes that favor Ï-bond metathesis
pathways for methane activation will yield lower barriers for CâH
activation
Computational Study of Methane CâH Activation by Earth-Abundant Metal Amide/Aminyl Complexes
Density functional
theory, augmented by multiconfiguration SCF
(MCSCF) simulations, was used to understand the factors that control
methane CâH activation by Earth-abundant, 3d metal (Cr - Ni)
[(Îș<sup>3</sup>-CNC)ÂMÂ(NH<sub>2</sub>)] complexes via hydrogen
atom abstraction (HAA) and [2 + 2] pathways. Calculations suggest
a significant amide/aminyl, i.e., [(Îș<sup>3</sup>-CNC)<sup>2â</sup>M<sup>3+</sup>(NH<sub>2</sub>)<sup>â</sup>] â [(Îș<sup>3</sup>-CNC)<sup>2â</sup>M<sup>2+</sup>(NH<sub>2</sub>)<sup>âą</sup>], admixture in the electronic ground states of these
complexes and thus significant unpaired electron density (radical
character) on the NH<sub>2</sub> ligand. The spin coupling between
the aminyl radical and spin density on the central metal ion is interesting,
particularly for the cobalt aminyl complex, in which both ferromagnetic
and antiferromagnetic triplet states are found to be close in energy
via both DFT and MCSCF methods. Modeled complexes are computed to
have reasonable barriers to methane activation, with Î<i>G</i><sup>⧧</sup> values being in approximately the upper
20s to mid 30s kcal/mol, generally decreasing toward the right in
the 3d series, which loosely tracks with spin density (radical character)
on the aminyl nitrogen, a switch from [2 + 2] to HAA activation pathways,
and more favorable thermodynamics for CâH scission
CâH Bond Activation of Methane by Pt<sup>II</sup>âN-Heterocyclic Carbene Complexes. The Importance of Having the Ligands in the Right Place at the Right Time
A DFT study of methane CâH activation barriers
for neutral
NHCâPt<sup>II</sup>âmethoxy complexes yielded 22.8 and
26.1 kcal/mol for oxidative addition (OA) and oxidative hydrogen migration
(OHM), respectively. Interestingly, this is unlike the case for cationic
NHCâPt<sup>II</sup>âmethoxy complexes, whereby OHM entails
a calculated barrier of 26.9 kcal/mol but the OA barrier is only 14.4
kcal/mol. Comparing transition state (TS) and ground state (GS) geometries
implies an âŒ10 kcal/mol âpenaltyâ to the barriers
arising from positioning the NHC and OMe ligands into a relative orientation
that is preferred in the GS to the orientation that is favored in
the TS. The results thus imply an intrinsic barrier arising from CâH
scission of âŒ15 ± 2 kcal/mol for NHCâPt<sup>II</sup>âmethoxy complexes. Calculations show the importance of designing
CâH activation catalysts where the GS active species is already
structurally âpreparedâ and which either does not need
to undergo any geometric perturbations to access the methane CâH
activation TS or is not energetically prohibited from such perturbations
Mapping the Basicity of Selected 3d and 4d Metal Nitrides: A DFT Study
Nitride complexes have been invoked
as catalysts and
intermediates
in a wide variety of transformations and are noted for their tunable
acid/base properties. A density functional theory study is reported
herein that maps the basicity of 3d and 4d transition metals that
routinely form nitride complexes: V, Cr, Mn, Nb, Mo, Tc, and Ru. Complexes
were gathered from the Cambridge Structural Database, and from the
free energy of protonation, the pKb(N)
of the nitride group was calculated to quantify the impact of metal
identity, oxidation state, coordination number, and supporting ligand
type upon metal-nitride basicity. In general, the basicity of transition
metal nitrides decreases from left to right across the 3d and 4d rows
and increases from 3d metals to their 4d congeners. Metal identity
and oxidation state primarily determine basicity trends; however,
supporting ligand types have a substantial impact on the basicity
range for a given metal. Synergism of these factors in determining
the overall pKb(N) values is discussed,
as are the implications for the catalytic reactivity of metal nitrides
Density Functional Study of Oxygen Insertion into NiobiumâPhosphorus Bonds: Novel Mechanism for Liberating P<sub>3</sub><sup>â</sup> Synthons
We explore the mechanism
of oxygen insertion into niobiumâphosphorus
bonds to liberate synthetically relevant, phosphorus-containing molecules.
Oxygen insertion mechanisms generally proceed through either direct
oxygen insertion from an oxo ligand, Mî»O (oxy-insertion), or
an insertion of an oxygen atom from an external oxidant, OY (BaeyerâVilliger,
BV). Computational methods were employed to elucidate the preferred
mechanism for the liberation of the phosphorus moiety from [(η<sup>2</sup>-P<sub>3</sub>)ÂNbÂ(ODipp)<sub>3</sub>] (Dipp = 2,6-<i><sup>i</sup></i>Pr<sub>2</sub>C<sub>6</sub>H<sub>3</sub>, P<sub>3</sub> = P<sub>3</sub>-SnPh<sub>3</sub>) when treated with pyridine-<i>N</i>-oxide as an external oxidant. Careful analysis of conformational
isomers and energies clearly suggests that the BV mechanism is the
preferred pathway toward phosphorus liberation. Once free, the P<sub>3</sub> moiety can react with 1,3-cyclohexadiene to form the DielsâAlder
product, which is also modeled in the computational study
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