Efficient Reduction of Carbon Dioxide to Methanol Equivalents Catalyzed by Two-Coordinate Amido–Germanium(II) and −Tin(II) Hydride Complexes

The bulky amido–germanium­(II) and −tin­(II) hydride complexes, L^†EH [E = Ge or Sn; L^† = -N­(Ar^†) (SiPr^<i>i</i>₃); Ar^† = C₆H₂Pr^<i>i</i>{C­(H)­Ph₂}₂-4,2,6], which are two-coordinate in solution, are shown to be efficient and highly selective “single-site” catalysts for the reduction of CO₂ to methanol equivalents (MeOBR₂), using HBpin or HBcat as the hydrogen source. L^†SnH is the most active non-transition metal catalyst yet reported for such reductions, yielding turnover frequencies of up to 1188 h^–1 at room temperature. Computational studies have identified two thermodynamically and kinetically viable catalytic pathways by which these reductions may operate. Spectroscopic investigations have identified several reaction intermediates, which leads to the conclusion that one of these reaction pathways predominates in the experimental situation. Stoichiometric reactivity studies have shed further light on the reaction mechanisms in operation and indicate that the involvement of the second reaction pathway cannot be ruled out. This study highlights the potential of relatively cheap, main group complexes as viable alternatives to transition metal-based systems in the catalytic transformation of small molecules

Qualitative Estimation of the Single-Electron Transfer Step Energetics Mediated by Samarium(II) Complexes: A “SOMO–LUMO Gap” Approach

Lanthanide II organometallic complexes usually initiate reactions via a single-electron transfer (SET) from the metal to a bonded substrate. Extensive mechanistic studies were carried out for lanthanide III complexes in which no change of oxidation state is involved. Some case-dependent strategies were reported by our group in order to account for a SET event in organometallic computed studies. In the present study, we show that analysis of DFT orbital spectra allows differentiating between exothermic and endothermic electron transfer. This methodology appears to be general; it allows differentiating between lanthanide centers and substituent effects on metallocenes. For that purpose, we considered mainly various samarocene adducts as well as a SmI₂ complex explicitly solvated by THF. Comparison between DFT methods and <i>ab initio</i> (CAS-SCF and HF) computational level revealed that the SOMO–LUMO gap computed at the DFT B3PW91 level, in combination with small-core RECPs and standard basis sets, offers a qualitative estimation of the energetics of the SET that is in line with both CAS-SCF calculations and experimental results when available. This orbital-based approach, based on DFT calculation, affords a fast and efficient methodology for pioneer exploration of the reactivity of lanthanide­(II) mediated by SET

Changing the Charge: Electrostatic Effects in Pd-Catalyzed Cross-Coupling

A stable dianionic 14-electron Pd(0) complex supported by monoanionic carboranyl phosphines is reported. This complex rapidly undergoes the oxidative addition of Cl-C₆H₅ at room temperature and is a competent catalyst for Kumada cross-coupling. The isosteric PdL₂ complex, supported by neutral <i>o</i>-carboranyl phosphines, does not display the same reactivity. The high reactivity of the dianionic Pd(0) complex toward chloroarenes can be explained by electrostatic effects that promote both formation of monophosphine-ligated LPd⁰ and stabilization of the transition state during oxidative addition. This mode of stabilization is distinct from the well-known π-arene interactions of biaryl phosphines, in that it occurs both on and off cycle

Hydroaminomethylation of Styrene Catalyzed by Rhodium Complexes Containing Chiral Diphosphine Ligands and Mechanistic Studies: Why Is There a Lack of Asymmetric Induction?

Various chiral diphosphine ligands (P–P) have been introduced in the coordination sphere of neutral or cationic rhodium complexes, and the generated species catalyze efficiently the hydroaminomethylation reaction of styrene with piperidine. The diphospholane ligand family is particularly adapted to this tandem reaction leading to the branched amine with good chemo- and regioselectivity. We analyzed in detail the main reasons why the reaction proceeds with no enantioselectivity. Catalytic and HP-NMR experiments reveal the presence of the [Rh­(H)­(CO)₂(P–P)] species as the resting state. DFT calculations allow us to elucidate the mechanism of the hydrogenation of the branched (<i>Z</i>) or (<i>E</i>)-enamine. From the [Rh­(H)­(CO)­(P–P)] active species, the coordination of the two enamine isomers, the hydride transfer, the H₂ activation, and then the final reductive elimination follow similar energetic pathways, explaining the lack of enantioselectivity for the present substrates. Analysis of the energy-demanding steps highlights the formation of the active species as crucial for this rate-limiting hydrogenation reaction

Click-Like Reactions with the Inert HCB₁₁Cl₁₁^– Anion Lead to Carborane-Fused Heterocycles with Unusual Aromatic Character

The chlorinated carba-<i>closo</i>-dodecaborate anion HCB₁₁Cl₁₁^– is an exceptionally stable molecule and has previously been reported to be substitutionally inert at the B–Cl vertices. We present here the discovery of base induced cycloaddition reactions between this carborane anion and organic azides that leads to selective C and B functionalization of the cluster. A single crystal X-ray diffraction study reveals bond lengths in the heterocyclic portion of the ring that are shortened, which suggests electronic delocalization. Molecular orbital analysis of the ensuing heterocycles reveals that two of the bonding orbitals of these systems resemble two of the doubly occupied π-MOs of a simple 5-membered Hückel-aromatic, even though they are entangled in the carborane skeleton. Nucleus independent chemical shift analysis indicates that both the carborane cluster portion of the molecule and the carborane fused heterocyclic region display aromatic character. Computational methods indicate that the reaction likely follows a stepwise addition cyclization pathway

Click-Like Reactions with the Inert HCB₁₁Cl₁₁^– Anion Lead to Carborane-Fused Heterocycles with Unusual Aromatic Character

The chlorinated carba-<i>closo</i>-dodecaborate anion HCB₁₁Cl₁₁^– is an exceptionally stable molecule and has previously been reported to be substitutionally inert at the B–Cl vertices. We present here the discovery of base induced cycloaddition reactions between this carborane anion and organic azides that leads to selective C and B functionalization of the cluster. A single crystal X-ray diffraction study reveals bond lengths in the heterocyclic portion of the ring that are shortened, which suggests electronic delocalization. Molecular orbital analysis of the ensuing heterocycles reveals that two of the bonding orbitals of these systems resemble two of the doubly occupied π-MOs of a simple 5-membered Hückel-aromatic, even though they are entangled in the carborane skeleton. Nucleus independent chemical shift analysis indicates that both the carborane cluster portion of the molecule and the carborane fused heterocyclic region display aromatic character. Computational methods indicate that the reaction likely follows a stepwise addition cyclization pathway

Versatile Reactivity of a Four-Coordinate Scandium Phosphinidene Complex: Reduction, Addition, and CO Activation Reactions

The four-coordinate scandium phosphinidene complex, [LSc­(μ-PAr)]₂ (L = (MeC­(NDIPP)­CHC­(Me)­(NCH₂CH₂N­(^<i>i</i>Pr)₂)), DIPP = 2,6-(^<i>i</i>Pr)₂C₆H₃; Ar = 2,6-Me₂C₆H₃) (<b>1</b>), has been synthesized in good yield, and its reactivity has been investigated. Although <b>1</b> has a bis­(μ-phosphinidene)­discandium structural unit, this coordinatively unsaturated complex shows high and versatile reactivity toward a variety of substrates. First, two-electron reduction occurs when substrates as 2,2′-bipyridine, elemental selenium, elemental tellurium, Me₃PS, or Ph₃PE (E = S, Se) is used, resulting in the oxidative coupling of two phosphinidene ligands 2­[PAr]^2– into a diphosphene ligand [ArP-PAr]^2–. Complex <b>1</b> easily undergoes nucleophilic addition reactions with unsaturated substrates, such as benzylallene, benzonitrile, <i>tert</i>-butyl isocyanide, and CS₂. This complex also shows a peculiar reactivity to CO and Mo­(CO)₆, that includes C–P bond formation, C–C coupling and C–O bond cleavage of CO, to afford novel phosphorus-containing products. In the last two types of reactivity, reaction profiles have been computed (for the insertion of ^<i>t</i>BuNC and the CO activation by <b>1</b>) at the DFT level. The unexpected/surprising sequence of steps in the latter case is also revealed

Valorization of CO₂: Preparation of 2‑Oxazolidinones by Metal–Ligand Cooperative Catalysis with SCS Indenediide Pd Complexes

The capture and utilization of CO₂ to prepare high-value compounds is very attractive chemically and highly desirable socially. Indenediide-based Pd SCS pincer complexes are shown here to promote the carboxylative cyclization of propargylamines leading to 2-oxazolidinones under mild conditions (0.5–1 bar of CO₂, DMSO, 40–80 °C, 1–5 mol % Pd loading). The indenediide Pd complex is competitive with known catalysts. It proved successful for a wide range of propargylamines, including hitherto challenging substrates such as secondary propargylamines bearing tertiary alkyl groups at nitrogen, primary propargylamines, and propargylanilines. Thorough experimental (NMR) and computational (DFT) investigations were undertaken to gain mechanistic insights. Accordingly, (i) the resting state of the catalytic cycle is a Pd DMSO complex; (ii) the indenediide backbone and Pd center act in concert to activate the carbamic acid intermediate and promote its cyclization; (iii) proton shuttling is essential to lower the activation barriers of the initial amine carboxylation as well as of the proton transfers between the ligand backbone and the organic fragments at Pd

Valorization of CO₂: Preparation of 2‑Oxazolidinones by Metal–Ligand Cooperative Catalysis with SCS Indenediide Pd Complexes

The capture and utilization of CO₂ to prepare high-value compounds is very attractive chemically and highly desirable socially. Indenediide-based Pd SCS pincer complexes are shown here to promote the carboxylative cyclization of propargylamines leading to 2-oxazolidinones under mild conditions (0.5–1 bar of CO₂, DMSO, 40–80 °C, 1–5 mol % Pd loading). The indenediide Pd complex is competitive with known catalysts. It proved successful for a wide range of propargylamines, including hitherto challenging substrates such as secondary propargylamines bearing tertiary alkyl groups at nitrogen, primary propargylamines, and propargylanilines. Thorough experimental (NMR) and computational (DFT) investigations were undertaken to gain mechanistic insights. Accordingly, (i) the resting state of the catalytic cycle is a Pd DMSO complex; (ii) the indenediide backbone and Pd center act in concert to activate the carbamic acid intermediate and promote its cyclization; (iii) proton shuttling is essential to lower the activation barriers of the initial amine carboxylation as well as of the proton transfers between the ligand backbone and the organic fragments at Pd

Multimetallic Cooperativity in Uranium-Mediated CO₂ Activation

The metal-mediated redox transformation of CO₂ in mild conditions is an area of great current interest. The role of cooperativity between a reduced metal center and a Lewis acid center in small-molecule activation is increasingly recognized, but has not so far been investigated for f-elements. Here we show that the presence of potassium at a U, K site supported by sterically demanding tris­(<i>tert</i>-butoxy)­siloxide ligands induces a large cooperative effect in the reduction of CO₂. Specifically, the ion pair complex [K­(18c6)]­[U­(OSi­(O^tBu)₃)₄], <b>1</b>, promotes the selective reductive disproportionation of CO₂ to yield CO and the mononuclear uranium­(IV) carbonate complex [U­(OSi­(O^tBu)₃)₄(μ-κ²:κ¹-CO₃)­K₂(18c6)], <b>4</b>. In contrast, the heterobimetallic complex [U­(OSi­(O^tBu)₃)₄K], <b>2</b>, promotes the potassium-assisted two-electron reductive cleavage of CO₂, yielding CO and the U­(V) terminal oxo complex [UO­(OSi­(O^tBu)₃)₄K], <b>3</b>, thus providing a remarkable example of two-electron transfer in U­(III) chemistry. DFT studies support the presence of a cooperative effect of the two metal centers in the transformation of CO₂