13 research outputs found
MetalâLigand Bifunctional Catalysis: The âAcceptedâ Mechanism, the Issue of Concertedness, and the Function of the Ligand in Catalytic Cycles Involving Hydrogen Atoms
For
years, following the ideas of Shvo and Noyori, the core assumption
of metalâligand bifunctional molecular catalysis has relied
on the direct involvement of the chelating ligand in the catalytic
reaction via a reversible proton (H<sup>+</sup>) transfer through
cleavage/formation of one of its XâH bonds (X = O, N, C). A
recently revised mechanism of the Noyori asymmetric hydrogenation
reaction (Dub, P. A. et al. <i>J. Am. Chem. Soc.</i> <b>2014</b>, <i>136</i>, 3505) suggests that the ligand
is rather involved in the catalytic reaction via the stabilization
of determining transition states through NâH···O
hydrogen-bonding interactions (HBIs) and not via a reversible H<sup>+</sup> transfer, behaving in a chemically intact manner within the
productive cycle or <i>predominantly</i> in a chemically
intact manner within productive cycles. By reexamining selected examples
of computational mechanistic studies involving bifunctional catalysts
from the literature in the years between 2012â2017, the purpose
of this work is to point out common misconceptions in modeling concerted
reactions and show that the actual stepwise nature of key transition
states unveils a more complicated <i>catalytic reaction pool</i> (all conceivable catalytic pathways and their crossovers). Such
a realization can not only potentially result in a reconsideration
of the âacceptedâ mechanism but also lead us to a new
conceptual understanding of the role that the ligand plays in the
reaction. The ultimate goal of this paper is, therefore, to encourage
the reader to reconsider the function of the ligand in catalytic cycles
of hydrogenation/dehydrogenation with bifunctional catalysts, which
until recently has relied almost exclusively on a chemically noninnocent
ligand
Air-Stable NNS (ENENES) Ligands and Their Well-Defined Ruthenium and Iridium Complexes for Molecular Catalysis
We
introduce ENENES, a new family of air-stable and low-cost NNS
ligands bearing NH functionalities of the general formula EÂ(CH<sub>2</sub>)<sub><i>m</i></sub>NHÂ(CH<sub>2</sub>)<sub><i>n</i></sub>SR, where E is selected from âNC<sub>4</sub>H<sub>8</sub>O, âNC<sub>4</sub>H<sub>8</sub>, or âNÂ(CH<sub>3</sub>)<sub>2</sub>, <i>m</i> and <i>n</i> =
2 and/or 3, and R = Ph, Bn, Me, or SR (part of a thiophenyl fragment).
The preparation and characterization of more than 15 examples of well-defined
Ru and Ir complexes supported by these ligands that are relevant to
bifunctional metalâligand M/NH molecular catalysis are reported.
Reactions of NNS ligands with suitable Ru or Ir precursors afford
rich and diverse solid-state and solution chemistries, producing monometallic
molecules as well as bimetallics in which the ligand coordinates to
the metal via either bidentate (Îș<sup>2</sup>[<i>N,N</i>âČ] or Îș<sup>2</sup>[<i>N</i>âČ<i>,S</i>]) or tridentate (Îș<sup>3</sup>[<i>N,N</i>âČ<i>,S</i>]) binding modes, depending on the basicity
of the sulfur atom, CH<sub>2</sub> chain length (<i>m</i> or <i>n</i> parameter), or identity of the transition
metal. In the case of Ir, ligands bearing benzyl substituents lead
to unprecedented Îș<sup>4</sup>[<i>N,N</i>âČ<i>,S,C</i>]-tetradentate core-structure complexes of the type
[Ir<sup>III</sup>HClÂ{Îș<sup>4</sup>(<i>N,N</i>âČ<i>,S,C</i>)âligand}], resulting from <i>ortho</i>-metalation via CâH oxidative addition. Fourteen of these
Ru and Ir complexes have been crystallographically characterized.
Air- and moisture-stable complexes of the type <i>trans</i>-[Ru<sup>II</sup>Cl<sub>2</sub>{Îș<sup>3</sup>[<i>N,N</i>âČ<i>,S</i>]âligand}Â(L)] (L = PPh<sub>3</sub>, PCy<sub>3</sub>, DMSO), and others, effect the selective hydrogenation
of methyl trifluoroacetate into the important synthon trifluoroacetaldehyde
methyl hemiacetal in basic methanol under relatively mild conditions
(35â40 °C, 25 bar H<sub>2</sub>) with reasonable turnover
numbers (i.e., > 1000), whereas the air-stable Ir monohydride complexes
[Ir<sup>III</sup>HClÂ{Îș<sup>4</sup>(<i>N,N</i>âČ<i>,S,C</i>)âligand}] exhibit excellent catalytic activities
and high chemoselectivity for the same reaction, reaching turnover
numbers of >10âŻ000
Why Does Alkylation of the NâH Functionality within M/NH Bifunctional Noyori-Type Catalysts Lead to Turnover?
Molecular metal/NH
bifunctional Noyori-type catalysts are remarkable
in that they are among the most efficient artificial catalysts developed
to date for the hydrogenation of carbonyl functionalities (loadings
up to âŒ10<sup>â5</sup> mol %). In addition, these catalysts
typically exhibit high Cî»O/Cî»C chemo- and enantioselectivities.
This unique set of properties is traditionally associated with the
operation of an unconventional mechanism for homogeneous catalysts
in which the chelating ligand plays a key role in facilitating the
catalytic reaction and enabling the aforementioned selectivities by
delivering/accepting a proton (H<sup>+</sup>) via its NâH bond
cleavage/formation. A recently revised mechanism of the Noyori hydrogenation
reaction (Dub, P. A. et al. <i>J. Am. Chem. Soc</i>. <b>2014</b>, <i>136</i>, 3505) suggests that the NâH
bond is not cleaved but serves to stabilize the turnover-determining
transition states (TDTSs) via strong NâH···O
hydrogen-bonding interactions (HBIs). The present paper shows that
this is consistent with the largely ignored experimental fact that
alkylation of the NâH functionality within M/NH bifunctional
Noyori-type catalysts leads to detrimental catalytic activity. The
purpose of this work is to demonstrate that decreasing the strength
of this HBI, ultimately to the limit of its complete absence, are
conditions under which the same alkylation may lead to beneficial
catalytic activity
Why Does Alkylation of the NâH Functionality within M/NH Bifunctional Noyori-Type Catalysts Lead to Turnover?
Molecular metal/NH
bifunctional Noyori-type catalysts are remarkable
in that they are among the most efficient artificial catalysts developed
to date for the hydrogenation of carbonyl functionalities (loadings
up to âŒ10<sup>â5</sup> mol %). In addition, these catalysts
typically exhibit high Cî»O/Cî»C chemo- and enantioselectivities.
This unique set of properties is traditionally associated with the
operation of an unconventional mechanism for homogeneous catalysts
in which the chelating ligand plays a key role in facilitating the
catalytic reaction and enabling the aforementioned selectivities by
delivering/accepting a proton (H<sup>+</sup>) via its NâH bond
cleavage/formation. A recently revised mechanism of the Noyori hydrogenation
reaction (Dub, P. A. et al. <i>J. Am. Chem. Soc</i>. <b>2014</b>, <i>136</i>, 3505) suggests that the NâH
bond is not cleaved but serves to stabilize the turnover-determining
transition states (TDTSs) via strong NâH···O
hydrogen-bonding interactions (HBIs). The present paper shows that
this is consistent with the largely ignored experimental fact that
alkylation of the NâH functionality within M/NH bifunctional
Noyori-type catalysts leads to detrimental catalytic activity. The
purpose of this work is to demonstrate that decreasing the strength
of this HBI, ultimately to the limit of its complete absence, are
conditions under which the same alkylation may lead to beneficial
catalytic activity
Air-Stable NNS (ENENES) Ligands and Their Well-Defined Ruthenium and Iridium Complexes for Molecular Catalysis
We
introduce ENENES, a new family of air-stable and low-cost NNS
ligands bearing NH functionalities of the general formula EÂ(CH<sub>2</sub>)<sub><i>m</i></sub>NHÂ(CH<sub>2</sub>)<sub><i>n</i></sub>SR, where E is selected from âNC<sub>4</sub>H<sub>8</sub>O, âNC<sub>4</sub>H<sub>8</sub>, or âNÂ(CH<sub>3</sub>)<sub>2</sub>, <i>m</i> and <i>n</i> =
2 and/or 3, and R = Ph, Bn, Me, or SR (part of a thiophenyl fragment).
The preparation and characterization of more than 15 examples of well-defined
Ru and Ir complexes supported by these ligands that are relevant to
bifunctional metalâligand M/NH molecular catalysis are reported.
Reactions of NNS ligands with suitable Ru or Ir precursors afford
rich and diverse solid-state and solution chemistries, producing monometallic
molecules as well as bimetallics in which the ligand coordinates to
the metal via either bidentate (Îș<sup>2</sup>[<i>N,N</i>âČ] or Îș<sup>2</sup>[<i>N</i>âČ<i>,S</i>]) or tridentate (Îș<sup>3</sup>[<i>N,N</i>âČ<i>,S</i>]) binding modes, depending on the basicity
of the sulfur atom, CH<sub>2</sub> chain length (<i>m</i> or <i>n</i> parameter), or identity of the transition
metal. In the case of Ir, ligands bearing benzyl substituents lead
to unprecedented Îș<sup>4</sup>[<i>N,N</i>âČ<i>,S,C</i>]-tetradentate core-structure complexes of the type
[Ir<sup>III</sup>HClÂ{Îș<sup>4</sup>(<i>N,N</i>âČ<i>,S,C</i>)âligand}], resulting from <i>ortho</i>-metalation via CâH oxidative addition. Fourteen of these
Ru and Ir complexes have been crystallographically characterized.
Air- and moisture-stable complexes of the type <i>trans</i>-[Ru<sup>II</sup>Cl<sub>2</sub>{Îș<sup>3</sup>[<i>N,N</i>âČ<i>,S</i>]âligand}Â(L)] (L = PPh<sub>3</sub>, PCy<sub>3</sub>, DMSO), and others, effect the selective hydrogenation
of methyl trifluoroacetate into the important synthon trifluoroacetaldehyde
methyl hemiacetal in basic methanol under relatively mild conditions
(35â40 °C, 25 bar H<sub>2</sub>) with reasonable turnover
numbers (i.e., > 1000), whereas the air-stable Ir monohydride complexes
[Ir<sup>III</sup>HClÂ{Îș<sup>4</sup>(<i>N,N</i>âČ<i>,S,C</i>)âligand}] exhibit excellent catalytic activities
and high chemoselectivity for the same reaction, reaching turnover
numbers of >10âŻ000
Syntheses and Reactivity Studies of Square-Planar DiamidoâPyridine Complexes Based on Earth-Abundant First-Row Transition Elements
The new square-planar complexes MÂ[NNN]Â(pyridine)
(M = Fe (<b>1</b>), CoÂ(<b>2</b>); NNN = 2,6-bisÂ(2,6-diisopropylphenylamidomethyl)Âpyridine)
were synthesized and fully characterized to investigate small molecule
activation on this platform and also associated ligand innocence.
The equatorial pyridine solvent moiety could not be removed; a new
bis-ligand species CoÂ[NNN.H]<sub>2</sub> (<b>3</b>) was synthesized
in low yield while attempting to make the base-free derivative. Attempts
to prepare the Ni analogue of <b>1</b> and <b>2</b> instead
yielded crystals of a di-iminoâpyridine complex NiÂ[PDI]Cl (<b>4</b>) (PDI = 2,6-bisÂ(2,6-diisopropylphenyliminomethyl)Âpyridine),
following loss of methylene backbone hydrogen atoms. Structural analysis
indicates that the PDI ligand is a mono-anionic radical. This susceptibility
of the ligand to oxidative dehydrogenation was also shown when the
reaction of <b>2</b> with 2 equiv of trityl chloride yielded
a new complex with an asymmetric iminoâamino pyridine ligand
CoÂ[NNNâČ]ÂCl<sub>2</sub> (<b>5</b>) (NNNâČ = 2-(2,6-(diisopropylphenyliminomethyl)-6-(diisopropylphenylamidomethyl)-pyridine)
in good yield
Syntheses and Reactivity Studies of Square-Planar DiamidoâPyridine Complexes Based on Earth-Abundant First-Row Transition Elements
The new square-planar complexes MÂ[NNN]Â(pyridine)
(M = Fe (<b>1</b>), CoÂ(<b>2</b>); NNN = 2,6-bisÂ(2,6-diisopropylphenylamidomethyl)Âpyridine)
were synthesized and fully characterized to investigate small molecule
activation on this platform and also associated ligand innocence.
The equatorial pyridine solvent moiety could not be removed; a new
bis-ligand species CoÂ[NNN.H]<sub>2</sub> (<b>3</b>) was synthesized
in low yield while attempting to make the base-free derivative. Attempts
to prepare the Ni analogue of <b>1</b> and <b>2</b> instead
yielded crystals of a di-iminoâpyridine complex NiÂ[PDI]Cl (<b>4</b>) (PDI = 2,6-bisÂ(2,6-diisopropylphenyliminomethyl)Âpyridine),
following loss of methylene backbone hydrogen atoms. Structural analysis
indicates that the PDI ligand is a mono-anionic radical. This susceptibility
of the ligand to oxidative dehydrogenation was also shown when the
reaction of <b>2</b> with 2 equiv of trityl chloride yielded
a new complex with an asymmetric iminoâamino pyridine ligand
CoÂ[NNNâČ]ÂCl<sub>2</sub> (<b>5</b>) (NNNâČ = 2-(2,6-(diisopropylphenyliminomethyl)-6-(diisopropylphenylamidomethyl)-pyridine)
in good yield
Reactivity of (Triphos)FeBr<sub>2</sub>(CO) towards sodium borohydrides
<p>The addition of CO to (Triphos)FeBr<sub>2</sub> (Triphos = PhP(CH<sub>2</sub>CH<sub>2</sub>PPh<sub>2</sub>)<sub>2</sub>) resulted in formation of six-coordinate (Triphos)FeBr<sub>2</sub>(CO). This coordination compound was found to have <i>cis</i>-bromide ligands and a <i>mer</i>-Triphos ligand by single crystal X-ray diffraction. Once characterized, the reactivity of this compound toward NaEt<sub>3</sub>BH and NaBH<sub>4</sub> was investigated. Adding 1 eq. of NaEt<sub>3</sub>BH to (Triphos)FeBr<sub>2</sub>(CO) resulted in formation of (Triphos)FeH(Br)(CO), while the addition of 2.2 eq. afforded previously described (Triphos)Fe(CO)<sub>2</sub>. In contrast, adding 2.2 eq. of NaBH<sub>4</sub> to (Triphos)FeBr<sub>2</sub>(CO) resulted in carbonyl dissociation and formation of diamagnetic (Triphos)FeH(<i>η</i><sup>2</sup>-BH<sub>4</sub>), which has been structurally characterized. Notably, efforts to prepare (Triphos)FeH(<i>η</i><sup>2</sup>-BH<sub>4</sub>) following 2.2 eq. NaBH<sub>4</sub> addition to (Triphos)FeBr<sub>2</sub> were unsuccessful. The importance of these observations as they relate to previously reported (Triphos)Fe reactivity and recent developments in Fe catalysis are discussed.</p
Lanthanide(III) Di- and Tetra-Nuclear Complexes Supported by a Chelating Tripodal Tris(Amidate) Ligand
Syntheses,
structural, and spectroscopic characterization of multinuclear trisÂ(amidate)
lanthanide complexes is described. Addition of K<sub>3</sub>[NÂ(<i>o</i>-PhNCÂ(O)<i><sup>t</sup></i>Bu)<sub>3</sub>] to LnX<sub>3</sub> (LnX<sub>3</sub> = LaBr<sub>3</sub>, CeI<sub>3</sub>, and NdCl<sub>3</sub>) in <i>N</i>,<i>N</i>-dimethylformamide (DMF) results in the generation of dinuclear complexes,
[LnÂ(NÂ(<i>o</i>-PhNCÂ(O)<sup><i>t</i></sup>Bu)<sub>3</sub>)Â(DMF)]<sub>2</sub>Â(ÎŒ-DMF)
(Ln = La (<b>1</b>), Ce (<b>2</b>), NdÂ(<b>3</b>)),
in good yields. Syntheses of tetranuclear complexes, [LnÂ(NÂ(<i>o</i>-PhNCÂ(O)<sup><i>t</i></sup>Bu)<sub>3</sub>)]<sub>4</sub> (Ln = Ce (<b>4</b>), NdÂ(<b>5</b>)), resulted
from protonolysis of LnÂ[NÂ(SiMe<sub>3</sub>)<sub>2</sub>]<sub>3</sub> (Ln = Ce, Nd) with NÂ(<i>o</i>-PhNCHÂ(O)<sup><i>t</i></sup>Bu)<sub>3</sub>. In the solid-state, complexes <b>1</b>â<b>5</b> exhibit coordination modes of the
tripodal trisÂ(amidate) ligand that are unique to the 4f elements and
have not been previously observed in transition metal systems
Iron Complex-Catalyzed AmmoniaâBorane Dehydrogenation. A Potential Route toward BâN-Containing Polymer Motifs Using Earth-Abundant Metal Catalysts
Ammoniaâborane (NH<sub>3</sub>BH<sub>3</sub>,
AB) has garnered
interest as a hydrogen storage material due to its high weight percent
hydrogen content and ease of H<sub>2</sub> release relative to metal
hydrides. As a consequence of dehydrogenation, BâN-containing
oligomeric/polymeric materials are formed. The ability to control
this process and dictate the identity of the generated polymer opens
up the possibility of the targeted synthesis of new materials. While
precious metals have been used in this regard, the ability to construct
such materials using earth-abundant metals such as Fe presents a more
economical approach. Four Fe complexes containing amido and phosphine
supporting ligands were synthesized, and their reactivity with AB
was examined. Three-coordinate FeÂ(PCy<sub>3</sub>)Â[NÂ(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub> (<b>1</b>) and four-coordinate FeÂ(DEPE)Â[NÂ(SiMe<sub>3</sub>)<sub>2</sub>]<sub>2</sub> (<b>2</b>) yield a mixture
of (NH<sub>2</sub>BH<sub>2</sub>)<sub><i>n</i></sub> and
(NHBH)<sub><i>n</i></sub> products with up to 1.7 equiv
of H<sub>2</sub> released per
AB but cannot be recycled (DEPE = 1,2-bisÂ(diethylphosphino)Âethane).
In contrast, Fe supported by a bidentate PâN ligand (<b>4</b>) can be used in a second cycle to afford a similar product
mixture. Intriguingly, the symmetric analogue of <b>4</b> (FeÂ(NâN)Â(PâP), <b>3</b>), only generates (NH<sub>2</sub>BH<sub>2</sub>)<sub><i>n</i></sub> and does so in minutes at room temperature. This
marked difference in reactivity may be the result of the chemistry
of FeÂ(II) vs Fe(0)