Heteroditopic Chelating NHC Ligand-Supported Co^III Complexes: Catalysts for the Reductive Functionalization of Carbon Dioxide under Ambient Conditions

Synthesis and characterization of heteroditopic chelating NHC ligand-supported air stable CoIII–NHC complexes (1a–d), featuring variable triazole N-substituents and thus, being structurally tunable, are reported. These complexes were observed to be very effective catalysts for the reductive functionalization of CO2 with aromatic amines using hydrosilane under ambient conditions (1 bar of CO2 pressure and room temperature) to yield diverse N-formylated amines, and importantly, the catalytic activity of the complexes was found to be reasonably tuned by the triazole N-substituents, which is probably due to some electronic modulations, supported by electrochemical analysis, rather than any considerable steric alterations as indicated by the percent buried volume calculation. Notably, the corresponding CoIII–NHC complexes generated in situ were also found to be equally effective. It is worth mentioning that this is the first report on the effective N-formylation of less nucleophilic aromatic primary amines by employing a homogeneous Co complex, to the best of our knowledge. In addition, control experiments suggest that this protocol proceeds via Co hydride and formoxysilane intermediate formation

Heteroditopic Chelating NHC Ligand-Supported Co^III Complexes: Catalysts for the Reductive Functionalization of Carbon Dioxide under Ambient Conditions

Synthesis and characterization of heteroditopic chelating NHC ligand-supported air stable CoIII–NHC complexes (1a–d), featuring variable triazole N-substituents and thus, being structurally tunable, are reported. These complexes were observed to be very effective catalysts for the reductive functionalization of CO2 with aromatic amines using hydrosilane under ambient conditions (1 bar of CO2 pressure and room temperature) to yield diverse N-formylated amines, and importantly, the catalytic activity of the complexes was found to be reasonably tuned by the triazole N-substituents, which is probably due to some electronic modulations, supported by electrochemical analysis, rather than any considerable steric alterations as indicated by the percent buried volume calculation. Notably, the corresponding CoIII–NHC complexes generated in situ were also found to be equally effective. It is worth mentioning that this is the first report on the effective N-formylation of less nucleophilic aromatic primary amines by employing a homogeneous Co complex, to the best of our knowledge. In addition, control experiments suggest that this protocol proceeds via Co hydride and formoxysilane intermediate formation

Mixed Alkyl Hydrido Complexes of Zinc: Synthesis, Structure, and Reactivity

The (NNNN)-type macrocycle 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane (Me₃TACD, 1,4,7-Me₃[12]­aneN₄) reacted with 1 equiv of ZnEt₂ under ethane elimination to give the mononuclear ethyl complex [(Me₃TACD)­ZnEt] (<b>1</b>). Upon treatment of (Me₃TACD)H with 2 equiv of ZnEt₂, the dinuclear complex [(Me₃TACD)­(ZnEt)­(ZnEt₂)] (<b>2</b>) was formed, which was converted with an additional 1 equiv of (Me₃TACD)H to <b>1</b>. Reaction of <b>1</b> with PhSiH₃ led to the formation of a tetranuclear ethyl hydrido complex [{(Me₃TACD)­ZnEt}₂(ZnEtH)₂] (<b>3</b>). Single-crystal X-ray diffraction study revealed <b>3</b> to be a centrosymmetric dimer featuring two [(Me₃TACD)­ZnEt] units coordinated to a [Zn­(μ-H)₂Zn] core via amido nitrogen atoms of the Me₃TACD ligands. Substitution of the two [(Me₃TACD)­ZnEt] units in <b>3</b> by N-heterocyclic carbene IMes [1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene] gave [(IMes)­ZnEtH]₂ (<b>4b</b>). The mixed alkyl hydrido complexes [(IMes)­ZnRH]₂ (R = Me, <b>4a</b>; Et, <b>4b</b>) were alternatively synthesized in quantitative yield by reacting [(IMes)­ZnR₂] (R = Me, Et) with [(IMes)­ZnH₂]₂ in 2:1 ratio. Methyl complex <b>4a</b> reacted with CO₂ (<i>p</i>(CO₂) = 0.5 bar) under facile insertion of CO₂ into Zn–H bonds to give dinuclear formate complex [(IMes)­ZnMe­(O₂CH)]₂ (<b>5a</b>). Treatment of <b>4b</b> with CO₂ (<i>p</i>(CO₂) = 0.5 bar) afforded a mixture of di- and trinuclear formate complexes [(IMes)­ZnEt­(O₂CH)]₂ (<b>5b</b>) and [(IMes)₂Zn₃Et₃(O₂CH)₃] (<b>6</b>) under elimination of one IMes as CO₂ adduct <b>IMes</b>·<b>CO</b>_<b>2</b>

Mixed Alkyl Hydrido Complexes of Zinc: Synthesis, Structure, and Reactivity

The (NNNN)-type macrocycle 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane (Me₃TACD, 1,4,7-Me₃[12]­aneN₄) reacted with 1 equiv of ZnEt₂ under ethane elimination to give the mononuclear ethyl complex [(Me₃TACD)­ZnEt] (<b>1</b>). Upon treatment of (Me₃TACD)H with 2 equiv of ZnEt₂, the dinuclear complex [(Me₃TACD)­(ZnEt)­(ZnEt₂)] (<b>2</b>) was formed, which was converted with an additional 1 equiv of (Me₃TACD)H to <b>1</b>. Reaction of <b>1</b> with PhSiH₃ led to the formation of a tetranuclear ethyl hydrido complex [{(Me₃TACD)­ZnEt}₂(ZnEtH)₂] (<b>3</b>). Single-crystal X-ray diffraction study revealed <b>3</b> to be a centrosymmetric dimer featuring two [(Me₃TACD)­ZnEt] units coordinated to a [Zn­(μ-H)₂Zn] core via amido nitrogen atoms of the Me₃TACD ligands. Substitution of the two [(Me₃TACD)­ZnEt] units in <b>3</b> by N-heterocyclic carbene IMes [1,3-bis­(2,4,6-trimethylphenyl)­imidazol-2-ylidene] gave [(IMes)­ZnEtH]₂ (<b>4b</b>). The mixed alkyl hydrido complexes [(IMes)­ZnRH]₂ (R = Me, <b>4a</b>; Et, <b>4b</b>) were alternatively synthesized in quantitative yield by reacting [(IMes)­ZnR₂] (R = Me, Et) with [(IMes)­ZnH₂]₂ in 2:1 ratio. Methyl complex <b>4a</b> reacted with CO₂ (<i>p</i>(CO₂) = 0.5 bar) under facile insertion of CO₂ into Zn–H bonds to give dinuclear formate complex [(IMes)­ZnMe­(O₂CH)]₂ (<b>5a</b>). Treatment of <b>4b</b> with CO₂ (<i>p</i>(CO₂) = 0.5 bar) afforded a mixture of di- and trinuclear formate complexes [(IMes)­ZnEt­(O₂CH)]₂ (<b>5b</b>) and [(IMes)₂Zn₃Et₃(O₂CH)₃] (<b>6</b>) under elimination of one IMes as CO₂ adduct <b>IMes</b>·<b>CO</b>_<b>2</b>

Two Different, Metal-Dependent Coordination Modes of a Dicarbene Ligand

The bisimidazolium salt H₂-<b>1</b>(PF₆)₂ featuring a bridging 1,4-phenylene group reacts with 0.5 equiv of [PdCl­(allyl)]₂ in the presence of Cs₂CO₃ to give the dinuclear complex [<b>2</b>]­(PF₆)₂, whereas the reaction of the same bisimidazolium salt with 0.5 equiv of [Ir­(Cl)₂(Cp*)]₂ yields the mononuclear orthometalated complex [<b>3</b>]­PF₆ with one remaining imidazolium unit. The unreacted imidazolium group in complex [<b>3</b>]­PF₆, however, can also be metalated with Rh^III to yield the doubly orthometalated heterobimetallic complex [<b>4</b>]. In complex [<b>4</b>], each M^III center (M = Ir^III and Rh^III) is coordinated by one NHC unit and orthometalates the central aryl ring of the ligand

Facile Reversibility by Design: Tuning Small Molecule Capture and Activation by Single Component Frustrated Lewis Pairs

A series of single component FLPs has been investigated for small molecule capture, with the finding that through tuning of both the thermodynamics of binding/activation and the degree of preorganization (i.e., Δ<i>S</i>^⧧) reversibility can be brought about at (or close to) room temperature. Thus, the dimethylxanthene system {(C₆H₄)₂(O)­CMe₂}­(PMes₂)­(B­(C₆F₅)₂): (i) heterolytically cleaves dihydrogen to give an equilibrium mixture of FLP and H₂ activation product in solution at room temperature and (ii) reversibly captures nitrous oxide (uptake at room temperature, 1 atm; release at 323 K)

Facile Reversibility by Design: Tuning Small Molecule Capture and Activation by Single Component Frustrated Lewis Pairs

A series of single component FLPs has been investigated for small molecule capture, with the finding that through tuning of both the thermodynamics of binding/activation and the degree of preorganization (i.e., Δ<i>S</i>^⧧) reversibility can be brought about at (or close to) room temperature. Thus, the dimethylxanthene system {(C₆H₄)₂(O)­CMe₂}­(PMes₂)­(B­(C₆F₅)₂): (i) heterolytically cleaves dihydrogen to give an equilibrium mixture of FLP and H₂ activation product in solution at room temperature and (ii) reversibly captures nitrous oxide (uptake at room temperature, 1 atm; release at 323 K)