Jesús Campos

Efficient selective and atom economic catalytic conversion of glycerol to lactic acid

The availability of glycerol is rapidly increasing due to the expanding biodiesel industry, which produces this polyol as the main waste material. Several value-added chemicals have been synthesized using glycerol as a feedstock; however, the conversion of glycerol to lactic acid has been investigated to a lesser extent despite the numerous and novel uses of lactic acid. We report a family of iridium complexes as the first homogeneous catalysts for the conversion of glycerol to lactic acid. These have higher activity and selectivity than the previously reported heterogeneous systems. In addition, hydrogen gas is generated as a useful byproduct. Unlike prior systems, the reactions can be performed in air, under mild conditions and without solvent. Our method has even been applied to samples of crude glycerol waste derived from the biodiesel industry without prior purification, albeit with somewhat lower activity while maintaining the same high selectivity.

Electrochemical Activation of Cp* Iridium Complexes for Electrode-Driven Water-Oxidation Catalysis

Organometallic iridium complexes bearing oxidatively stable chelate ligands are precursors for efficient homogeneous water-oxidation catalysts (WOCs), but their activity in oxygen evolution has so far been studied almost exclusively with sacrificial chemical oxidants. In this report, we study the electrochemical activation of Cp*Ir complexes and demonstrate true electrode-driven water oxidation catalyzed by a homogeneous iridium species in solution. Whereas the Cp* precursors exhibit no measurable O2-evolution activity, the molecular species formed after their oxidative activation are highly active homogeneous WOCs, capable of electrode-driven O2 evolution with high Faradaic efficiency. We have ruled out the formation of heterogeneous iridium oxides, either as colloids in solution or as deposits on the surface of the electrode, and found indication that the conversion of the precursor to the active molecular species occurs by a similar process whether carried out by chemical or electrochemical methods. This work makes these WOCs more practical for application in photoelectrochemical dyads for light-driven water splitting.

Synthesis and Reactivity of a Cationic Platinum(II) Alkylidene Complex

The last decades have witnessed an explosive growth of interest in the chemistry of transition-metal carbenes, 2] which has mainly been due to their important catalytic applications. A great deal of effort has converged in recent years on platinumand gold-catalyzed reactions that occur with participation of carbene species as active reaction intermediates. For these systems F rstner and co-workers have established an analogy between metal carbenes and metal-stabilized carbocationic structures and proposed the term carbenoid to illustrate this ambiguity. 4] For the Group 10 elements, carbene complexes with carbocyclic-, N-heterocyclic-, and heteroatom-stabilizedcarbene ligands are known. However, related complexes that contain {M=C(R)(R’)} units, where R and R are hydrogen or hydrocarbyl groups are very rare. The first such nickel complex was reported by Mindiola and Hillhouse in 2002 and contains a Ni center bound to a C(Ph)2 carbene fragment. Interestingly, only a year later, a cationic Pd derivative of a non-heteroatom-stabilized carbene ligand C(p-tolyl)2 was prepared and structurally characterized by X-ray crystallography. The latter complex features a rather long Pd Ccarbene bond (1.976 ), which is indicative of only a minor stabilization of the carbene s donor by back-bonding from filled palladium dp orbitals. In view of the importance of cationic platinum(II) alkylidene species, [Pt=C(R)(R’)], in catalysis 5] we planned to generate and characterize some compounds of this kind. Over the years, we have succeeded in ascertaining the structural properties of related cationic Ir alkylidenes. Studies of the energetics of {Pt=CH2} + by ab initio calculations revealed a high bond energy that is largely due to relativistic effects. On the assumption that the bonding for a cationic {Pt=C(R)(R’)} unit follows the dative model, and taking into account that similar to Pd little backdonation from the cationic platinum(II) center to the alkylidene can be expected, it is reasonable to think that failure to isolate such species is most probably due to their high reactivity rather than to intrinsic weakness of the platinum–alkylidene bond. Indeed, Menj n, Forni s, and coworkers have demonstrated recently that, contrary to isolable [M]=CF2 derivatives of Group 8 and 9 metals, the analogous [Pt]=CF2 units are highly reactive and need extra stabilization by formation of a chelating pyridinium ylide structure. We envisaged that sterically protected platinum bis(metallacycles) containing five-membered rings as a result of metalation of aryl phosphine ligands (structures I and II in Scheme 1) could be suitable candidates for this enterprise, as cationic benzylidene structures could then be readily generated by a-hydride abstraction. Related platinum metallacycles have already been reported. 20] With the express intention of hindering C C coupling between the benzylidene and benzyl termini of the desired cationic alkylidene formulations, a trans geometry like II in Scheme 1 was chosen. Reaction of [PtCl2(cod)] (cod = 1,5-cyclooctadiene) with the bulky xylyl phosphine 21] PiPr2Xyl (Xyl = 2,6-Me2C6H3) under the conditions of Scheme 2 yielded the desired trans bis(metallacycle) 1 as a colorless microcrystalline solid, in yields of isolated product of close to 80%. Complex 1 was fully characterized by microanalysis and NMR spectroscopy (see the Supporting Information).

Catalytic B-N Dehydrogenation Using Frustrated Lewis Pairs: Evidence for a Chain-Growth Coupling Mechanism.

The catalytic dehydrogenation of ammonia- and amine-boranes by a dimethylxanthene-derived frustrated Lewis pair is described. Turnover is facilitated on a thermodynamic basis by the ready release of H2 from the weakly basic PPh2-containing system. In situ NMR studies and the isolation of intermediates from stoichiometric reactions support a mechanism initiated by B-H activation, followed by end-growth BN coupling involving the terminal NH bond of the bound BN fragment and a BH bond of the incoming borane monomer.

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., ΔS(⧧)) 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).

A stable heavier group 14 analogue of vinylidene

Vinylidene (H2C=C) is a member of the family of compounds of composition CH (and isomeric with ethyne, HC≡CH), but it has been observed only transiently-with a lifetime in the region of 0.1 ns. Indeed, no simple (non-base-stabilized) compounds of the type R2E=E have been characterized structurally for any of the group 14 elements. Here we show that by employing the bulky and strongly electron-donating boryl ligand (HCDippN)2B (Dipp, 2,6-iPr2C6H3), a simple monomeric digermavinylidene compound, (boryl)2GeGe, can be synthesized and is stable at room temperature. Both its formation via the two-electron chemical oxidation of the symmetrical Ge0 compound K2[(boryl)GeGe(boryl)] and its subsequent reaction chemistry (for example, with H2), are consistent with a high substituent lability and the accessibility of both 1,1- and 1,2-substitution patterns. Structural and computational studies of [(HCDippN)2B]2GeGe reveal a weak Ge-Ge double bond-the π component of which contributes to the highest occupied molecular orbital (HOMO)-with a Ge-centred lone pair as the HOMO-1.

Stable Iridium(IV) Complexes of an Oxidation-Resistant Pyridine-Alkoxide Ligand: Highly Divergent Redox Properties Depending on the Isomeric Form Adopted

The preparation of the facial and meridional isomers of [Ir(pyalk)3] (pyalk = 2-(2-pyridyl)isopropanoate), as model complexes for a powerful water oxidation catalyst, is reported. The strongly donating N3O3 ligand set is very oxidation-resistant, yet promotes facile metal-centered oxidation to form stable Ir(IV) compounds. The Ir(III/IV) reduction potentials of the two isomers differ by 340 mV despite the identical ligand set. A ligand field rationalization is advanced and supported by DFT calculations.

A Systematic Study of Structure and E-H Bond Activation Chemistry by Sterically Encumbered Germylene Complexes.

A series of new germylene compounds has been synthesized offering systematic variation in the σ- and π-capabilities of the α-substituent and differing levels of reactivity towards E-H bond activation (E=H, B, C, N, Si, Ge). Chloride metathesis utilizing [(terphenyl)GeCl] proves to be an effective synthetic route to complexes of the type [(terphenyl)Ge(ERn )] (1-6: ERn =NHDipp, CH(SiMe3 )2 , P(SiMe3 )2 , Si(SiMe3 )3 or B(NDippCH)2 ; terphenyl=C6 H3 Mes2 -2,6=Ar(Mes) or C6 H3 Dipp2 -2,6=Ar(Dipp) ; Dipp=C6 H3 iPr2 -2,6, Mes=C6 H2 Me3 -2,4,6), while the related complex [{(Me3 Si)2 N}Ge{B(NDippCH)2 }] (8) can be accessed by an amide/boryl exchange route. Metrical parameters have been probed by X-ray crystallography, and are consistent with widening angles at the metal centre as more bulky and/or more electropositive substituents are employed. Thus, the widest germylene units (θ>110°) are found to be associated with strongly σ-donating boryl or silyl ancillary donors. HOMO-LUMO gaps for the new germylene complexes have been appraised by DFT calculations. The aryl(boryl)-germylene system [Ar(Mes) Ge{B(NDippCH)2 }] (6-Mes), which features a wide C-Ge-B angle (110.4(1)°) and (albeit relatively weak) ancillary π-acceptor capabilities, has the smallest HOMO-LUMO gap (119 kJ mol(-1) ). These features result in 6-Mes being remarkably reactive, undergoing facile intramolecular C-H activation involving one of the mesityl ortho-methyl groups. The related aryl(silyl)-germylene system, [Ar(Mes) Ge{Si(SiMe3 )3 }] (5-Mes) has a marginally wider HOMO-LUMO gap (134 kJ mol(-1) ), rendering it less labile towards decomposition, yet reactive enough to oxidatively cleave H2 and NH3 to give the corresponding dihydride and (amido)hydride. Mixed aryl/alkyl, aryl/amido and aryl/phosphido complexes are unreactive, but amido/boryl complex 8 is competent for the activation of E-H bonds (E=H, B, Si) to give hydrido, boryl and silyl products. The results of these reactivity studies imply that the use of the very strongly σ-donating boryl or silyl substituents is an effective strategy for rendering metallylene complexes competent for E-H bond activation.

Reactivity of Cationic Agostic and Carbene Structures Derived from Platinum(II) Metallacycles

This paper describes the formation of new platinacyclic complexes derived from the phosphine ligands PiPr2 Xyl, PMeXyl2 , and PMe2 Ar Xyl 2 (Xyl=2,6-Me2 C6 H3 and Ar Xyl 2=2,6-(2,6-Me2 C6 H3 )2 -C6 H3 ) as well as reactivity studies of the trans-[Pt(C^P)2 ] bis-metallacyclic complex 1 a derived from PiPr2 Xyl. Protonation of compound 1 a with [H(OEt2 )2 ][BArF ] (BArF =B[3,5-(CF3 )2 C6 H3 ]4 ) forms a cationic δ-agostic structure 4 a, whereas α-hydride abstraction employing [Ph3 C][PF6 ] produces a cationic platinum carbene trans-[Pt{PiPr2 (2,6-CH(Me)C6 H3 }{PiPr2 (2,6-CH2 (Me)C6 H3 }][PF6 ] (8). Compounds 4 a and 8 react with H2 to yield the same 1:3 equilibrium mixture of 4 a and trans-[PtH(PiPr2 Xyl)2 ][BArF ] (6), in which one of the phosphine ligands participates in a δ-agostic interaction. DFT calculations reveal that H2 activation by 8 occurs at the highly electrophilic alkylidene terminus with no participation of the metal. The two compounds 4 a and 8 experience C-C coupling reactions of a different nature. Thus, 4 a gives rise to complex trans-[PtH{(E)-1,2-bis(2-(PiPr2 )-3-MeC6 H3 )CHCH}] (7) that contains a tridentate diphosphine-alkene ligand, through agostic CH oxidative cleavage and C-C reductive coupling steps, whereas the C-C coupling reaction in 8 involves classical migratory insertion of its [PtCH] and [PtCH2 ] bonds promoted by platinum coordination of CO or CNXyl. The mechanisms of the CC bond-forming reactions have also been investigated by computational methods.

Mechanism of Hydrogenolysis of an Iridium–Methyl Bond: Evidence for a Methane Complex Intermediate

Evidence for key σ-complex intermediates in the hydrogenolysis of the iridium-methyl bond of (PONOP)Ir(H)(Me)(+) (1) [PONOP = 2,6-bis(di-tert-butylphosphinito)pyridine] has been obtained. The initially formed η(2)-H(2) complex, 2, was directly observed upon treatment of 1 with H(2), and evidence for reversible formation of a σ-methane complex, 5, was obtained through deuterium scrambling from η(2)-D(2) in 2-d(2) into the methyl group of 2 prior to methane loss. This sequence of reactions was modeled by density functional theory calculations. The transition state for formation of 5 from 2 showed significant shortening of the Ir-H bond for the hydrogen being transferred; no true Ir(V) trihydride intermediate could be located. Barriers to methane loss from 2 were compared to those of 1 and the six-coordinate species (PONOP)Ir(H)(Me)(CO)(+) and (PONOP)Ir(H)(Me)(Cl).