Metal–Ligand Bifunctional Catalysis: The “Accepted” 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 metal–ligand bifunctional molecular catalysis has relied on the direct involvement of the chelating ligand in the catalytic reaction via a reversible proton (H⁺) transfer through cleavage/formation of one of its X–H 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 N–H···O hydrogen-bonding interactions (HBIs) and not via a reversible H⁺ 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 2012–2017, 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 “accepted” 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 E­(CH₂)_<i>m</i>NH­(CH₂)_<i>n</i>SR, where E is selected from −NC₄H₈O, −NC₄H₈, or −N­(CH₃)₂, <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 metal–ligand 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 (κ²[<i>N,N</i>′] or κ²[<i>N</i>′<i>,S</i>]) or tridentate (κ³[<i>N,N</i>′<i>,S</i>]) binding modes, depending on the basicity of the sulfur atom, CH₂ 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 κ⁴[<i>N,N</i>′<i>,S,C</i>]-tetradentate core-structure complexes of the type [Ir^IIIHCl­{κ⁴(<i>N,N</i>′<i>,S,C</i>)–ligand}], resulting from <i>ortho</i>-metalation via C–H 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^IICl₂{κ³[<i>N,N</i>′<i>,S</i>]–ligand}­(L)] (L = PPh₃, PCy₃, DMSO), and others, effect the selective hydrogenation of methyl trifluoroacetate into the important synthon trifluoroacetaldehyde methyl hemiacetal in basic methanol under relatively mild conditions (35–40 °C, 25 bar H₂) with reasonable turnover numbers (i.e., > 1000), whereas the air-stable Ir monohydride complexes [Ir^IIIHCl­{κ⁴(<i>N,N</i>′<i>,S,C</i>)–ligand}] exhibit excellent catalytic activities and high chemoselectivity for the same reaction, reaching turnover numbers of >10 000

Why Does Alkylation of the N–H 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 ∼10^–5 mol %). In addition, these catalysts typically exhibit high CO/C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⁺) via its N–H 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 N–H bond is not cleaved but serves to stabilize the turnover-determining transition states (TDTSs) via strong N–H···O hydrogen-bonding interactions (HBIs). The present paper shows that this is consistent with the largely ignored experimental fact that alkylation of the N–H 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 N–H 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 ∼10^–5 mol %). In addition, these catalysts typically exhibit high CO/C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⁺) via its N–H 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 N–H bond is not cleaved but serves to stabilize the turnover-determining transition states (TDTSs) via strong N–H···O hydrogen-bonding interactions (HBIs). The present paper shows that this is consistent with the largely ignored experimental fact that alkylation of the N–H 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 E­(CH₂)_<i>m</i>NH­(CH₂)_<i>n</i>SR, where E is selected from −NC₄H₈O, −NC₄H₈, or −N­(CH₃)₂, <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 metal–ligand 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 (κ²[<i>N,N</i>′] or κ²[<i>N</i>′<i>,S</i>]) or tridentate (κ³[<i>N,N</i>′<i>,S</i>]) binding modes, depending on the basicity of the sulfur atom, CH₂ 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 κ⁴[<i>N,N</i>′<i>,S,C</i>]-tetradentate core-structure complexes of the type [Ir^IIIHCl­{κ⁴(<i>N,N</i>′<i>,S,C</i>)–ligand}], resulting from <i>ortho</i>-metalation via C–H 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^IICl₂{κ³[<i>N,N</i>′<i>,S</i>]–ligand}­(L)] (L = PPh₃, PCy₃, DMSO), and others, effect the selective hydrogenation of methyl trifluoroacetate into the important synthon trifluoroacetaldehyde methyl hemiacetal in basic methanol under relatively mild conditions (35–40 °C, 25 bar H₂) with reasonable turnover numbers (i.e., > 1000), whereas the air-stable Ir monohydride complexes [Ir^IIIHCl­{κ⁴(<i>N,N</i>′<i>,S,C</i>)–ligand}] exhibit excellent catalytic activities and high chemoselectivity for the same reaction, reaching turnover numbers of >10 000

Syntheses and Reactivity Studies of Square-Planar Diamido–Pyridine Complexes Based on Earth-Abundant First-Row Transition Elements

The new square-planar complexes M­[NNN]­(pyridine) (M = Fe (<b>1</b>), Co­(<b>2</b>); NNN = 2,6-bis­(2,6-diisopropylphenylamidomethyl)­pyridine) 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 Co­[NNN.H]₂ (<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-imino–pyridine complex Ni­[PDI]Cl (<b>4</b>) (PDI = 2,6-bis­(2,6-diisopropylphenyliminomethyl)­pyridine), 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 imino–amino pyridine ligand Co­[NNN′]­Cl₂ (<b>5</b>) (NNN′ = 2-(2,6-(diisopropylphenyliminomethyl)-6-(diisopropylphenylamidomethyl)-pyridine) in good yield

Syntheses and Reactivity Studies of Square-Planar Diamido–Pyridine Complexes Based on Earth-Abundant First-Row Transition Elements

The new square-planar complexes M­[NNN]­(pyridine) (M = Fe (<b>1</b>), Co­(<b>2</b>); NNN = 2,6-bis­(2,6-diisopropylphenylamidomethyl)­pyridine) 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 Co­[NNN.H]₂ (<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-imino–pyridine complex Ni­[PDI]Cl (<b>4</b>) (PDI = 2,6-bis­(2,6-diisopropylphenyliminomethyl)­pyridine), 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 imino–amino pyridine ligand Co­[NNN′]­Cl₂ (<b>5</b>) (NNN′ = 2-(2,6-(diisopropylphenyliminomethyl)-6-(diisopropylphenylamidomethyl)-pyridine) in good yield

Reactivity of (Triphos)FeBr₂(CO) towards sodium borohydrides

<p>The addition of CO to (Triphos)FeBr₂ (Triphos = PhP(CH₂CH₂PPh₂)₂) resulted in formation of six-coordinate (Triphos)FeBr₂(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 NaEt₃BH and NaBH₄ was investigated. Adding 1 eq. of NaEt₃BH to (Triphos)FeBr₂(CO) resulted in formation of (Triphos)FeH(Br)(CO), while the addition of 2.2 eq. afforded previously described (Triphos)Fe(CO)₂. In contrast, adding 2.2 eq. of NaBH₄ to (Triphos)FeBr₂(CO) resulted in carbonyl dissociation and formation of diamagnetic (Triphos)FeH(<i>η</i>²-BH₄), which has been structurally characterized. Notably, efforts to prepare (Triphos)FeH(<i>η</i>²-BH₄) following 2.2 eq. NaBH₄ addition to (Triphos)FeBr₂ 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 tris­(amidate) lanthanide complexes is described. Addition of K₃[N­(<i>o</i>-PhNC­(O)<i>^t</i>Bu)₃] to LnX₃ (LnX₃ = LaBr₃, CeI₃, and NdCl₃) in <i>N</i>,<i>N</i>-dimethylformamide (DMF) results in the generation of dinuclear complexes, [Ln­(N­(<i>o</i>-PhNC­(O)^<i>t</i>Bu)₃)­(DMF)]₂­(μ-DMF) (Ln = La (<b>1</b>), Ce (<b>2</b>), Nd­(<b>3</b>)), in good yields. Syntheses of tetranuclear complexes, [Ln­(N­(<i>o</i>-PhNC­(O)^<i>t</i>Bu)₃)]₄ (Ln = Ce (<b>4</b>), Nd­(<b>5</b>)), resulted from protonolysis of Ln­[N­(SiMe₃)₂]₃ (Ln = Ce, Nd) with N­(<i>o</i>-PhNCH­(O)^<i>t</i>Bu)₃. In the solid-state, complexes <b>1</b>–<b>5</b> exhibit coordination modes of the tripodal tris­(amidate) ligand that are unique to the 4f elements and have not been previously observed in transition metal systems

Iron Complex-Catalyzed Ammonia–Borane Dehydrogenation. A Potential Route toward B–N-Containing Polymer Motifs Using Earth-Abundant Metal Catalysts

Ammonia–borane (NH₃BH₃, AB) has garnered interest as a hydrogen storage material due to its high weight percent hydrogen content and ease of H₂ release relative to metal hydrides. As a consequence of dehydrogenation, B–N-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 Fe­(PCy₃)­[N­(SiMe₃)₂]₂ (<b>1</b>) and four-coordinate Fe­(DEPE)­[N­(SiMe₃)₂]₂ (<b>2</b>) yield a mixture of (NH₂BH₂)_<i>n</i> and (NHBH)_<i>n</i> products with up to 1.7 equiv of H₂ released per AB but cannot be recycled (DEPE = 1,2-bis­(diethylphosphino)­ethane). In contrast, Fe supported by a bidentate P–N 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> (Fe­(N–N)­(P–P), <b>3</b>), only generates (NH₂BH₂)_<i>n</i> and does so in minutes at room temperature. This marked difference in reactivity may be the result of the chemistry of Fe­(II) vs Fe(0)