Rémi Tirfoin

Cooperative Bond Activation and Catalytic Reduction of Carbon Dioxide at a Group 13 Metal Center

A single-component ambiphilic system capable of the cooperative activation of protic, hydridic and apolar HX bonds across a Group 13 metal/activated β-diketiminato (Nacnac) ligand framework is reported. The hydride complex derived from the activation of H2 is shown to be a competent catalyst for the highly selective reduction of CO2 to a methanol derivative. To our knowledge, this process represents the first example of a reduction process of this type catalyzed by a molecular gallium complex.

Stable GaX2, InX2 and TlX2 radicals

The chemistry of the Group 13 metals is dominated by the +1 and +3 oxidation states, and simple monomeric M(II) species are typically short-lived, highly reactive species. Here we report the first thermally robust monomeric MX2 radicals of gallium, indium and thallium. By making use of sterically demanding boryl substituents, compounds of the type M(II)(boryl)2 (M = Ga, In, Tl) can be synthesized. These decompose above 130 °C and are amenable to structural characterization in the solid state by X-ray crystallography. Electron paramagnetic resonance and computational studies reveal a dominant metal-centred character for all three radicals (>70% spin density at the metal). M(II) species have been invoked as key short-lived intermediates in well-known electron-transfer processes; consistently, the chemical behaviour of these novel isolated species reveals facile one-electron shuttling processes at the metal centre.

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.

Exploiting Electrostatics To Generate Unsaturation: Oxidative GeE Bond Formation Using a Non π‐Donor Stabilized [R(L)Ge:]+ Cation

The two-coordinate germanium cation [(IDipp){(Me3Si)2CH}Ge:](+) has been synthesized, which lacks π-donor stabilization of the metal center and consequently has a very small HOMO-LUMO gap (187 kJ mol(-1)). It undergoes a variety of facile oxidative bond-forming reactions, most notably allowing access to the first examples of Group 14 metal cations containing M=E multiple bonds (E = C, N). The use of an electrostatic (rather than purely steric) strategy to discourage aggregation means that less bulky systems (for example, containing a primary alkylidene fragment, =CHR) are accessible.

A molecular 'traffic light': highly selective cyanide sensing in aqueous media by a CpFe(indenyl)-functionalized borane.

The CpFe(indenyl)-functionalized borane {η(5)-(4-dimesitylboryl)indenyl}(η(5)-cyclopentadienyl)iron is available in two simple steps from organic precursors (in ca. 50% yield) and binds cyanide in protic media (1:1 thf-H2O) with an accompanying green to red/pink colour change. Extremely high selectivity over fluoride and hydroxide and a detection limit of ca. 10 ppm represent a highly desirable combination among borane derived cyanide receptors.

Cobalt Boryl Complexes: Enabling and Exploiting Migratory Insertion in Base‐Metal‐Mediated Borylation

Cobalt boryl complexes, which have only been sporadically reported, can be accessed systematically with remarkable (but controllable) variation in the nature of the M-B bond. Complexes incorporating a very strong trans σ-donor display unparalleled inertness, reflected in retention of the M-B bond even in the presence of extremely strong acid. By contrast, the use of the strong π-acceptor CO in the trans position, results in significant Co-B elongation and to labilization of the boryl ligand via unprecedented CO migratory insertion. Such chemistry provides a pathway for the generation of coordinative unsaturation, thereby enabling ligand substitution and/or substrate assimilation. Alkene functionalization by boryl transfer, a well-known reaction for noble metals such as Rh or Pt, can thus be effected by an 18-electron base-metal complex.

Frustrated Lewis Pairs as Molecular Receptors: Colorimetric and Electrochemical Detection of Nitrous Oxide

While the idea of sterically enforced Lewis acid/base frustration dates back as far as 1942, exploitation of the latent reactivity of such systems constitutes a significant recent development in small-molecule activation. Synergistic reactivity involving both donor and acceptor components has led to the trapping of molecules such as CO2 and N2O, [3,4] and to the cleavage of relatively inert bonds, such as that in H2 for catalytic processes. 6] Given that the Lewis acidic component of many “frustrated Lewis pairs” (FLPs) is a halogenated triarylborane, and that related ferrocene-containing systems are accessible, we hypothesized that electrochemical or colorimetric detection, even of relatively unreactive molecules, might be possible by employing an FLP approach. The conversion of a three-coordinate ferrocenyl borane to a fourcoordinate borate is typically accompanied by a change in both the Fe redox potential and the electronic absorption maxima of the ferrocenyl chromophore, and such systems have previously been shown to be effective in detecting anionic analytes. In particular, given the trapping of nitrous oxide by the tBu3P/B(C6F5)3 FLP reported by Stephan and co-workers, [4] and the reactivity of N2O toward IMes described by Severin and co-workers (see Scheme 1), we hypothesized that the combination of a bulky Lewis base (e.g. a tertiary phosphine or N-heterocyclic carbene) and an electron-deficient ferrocenyl borane (for example, of the type FcB(C6(Hal)5)2; Fc = ferrocenyl = (h-C5H5)Fe(h -C5H4)) might constitute a viable colorimetric/electrochemical protocol for the detection of the potent environmental pollutant N2O. [10] The selective detection of N2O in general is problematic because 1) it is an intrinsically poor ligand for transition metal centers; and 2) its reactivity as an oxygen atom transfer reagent is common to a number of other species. While the synergistic reactivity of the Lewis acid and base components in an FLP provides a solution to the former problem, careful choice of FLP components is required to develop a system which not only signals the presence of N2O, but also gives a null response to exposure to other O-atom sources (including O2). Initial studies focused on the interaction of the known adduct IMesN2O (Scheme 1) with FcB(C6F5)2 (Scheme 2a). 9] In the event, the formation of the simple 1:1 complex IMes·N2O·BFc(C6F5)2 (1) in solution was confirmed

Circumventing redox chemistry: synthesis of transition metal boryl complexes from a boryl nucleophile by decarbonylation.

The very strong reducing capabilities of the boryllithium nucleophile (THF)2Li{B(NDippCH)2} (1, Dipp = 2,6-iPr2C6H3) render impractical its use for the direct introduction of the {B(NDippCH)2} ligand via metathesis chemistry into the immediate coordination sphere of transition metals (d(n), with n ≠ 0 or 10). Instead, 1 typically reacts with metal halide, amide and hydrocarbyl electrophiles either via electron transfer or halide abstraction. Evidence for the formation of M-B bonds is obtained only in the case of the d(5) system [{(HCDippN)2B}Mn(THF)(μ-Br)]2. Lower oxidation state metal carbonyl complexes such as Fe(CO)5 and Cr(CO)6 react with 1 via nucleophilic attack at the carbonyl carbon atom to give boryl-functionalized Fischer carbene complexes Fe(CO)4{C(OLi(THF)3)B(NDippCH)2} and Cr(CO)5{C(OLi(THF)2)B(NDippCH)2}. Although C-to-M boryl transfer does not occur for these formally anionic systems, more labile charge neutral bora-acyl derivatives of the type LnM{C(O)B(NDippCH)2} [LnM = Mn(CO)5, Re(CO)5, CpFe(CO)2] can be synthesized, which cleanly lose CO to generate M-B bonds. From a mechanistic standpoint, an archetypal organometallic mode of reactivity, carbonyl extrusion, has thus been shown to be applicable to the boryl ligand class, with (13)C isotopic labeling studies confirming a dissociation/migration pathway. These proof-of-methodology synthetic studies can be extended beyond boryl complexes of the group 7 and 8 metals (for which a number of versatile synthetic routes already exist) to provide access to complexes of cobalt, which have hitherto proven only sporadically accessible.

Formation of sub-valent carbenoid ligands by metal-mediated dehydrogenation chemistry: coordination and activation of H2Ga{(NDippCMe)2CH}

Reactions of the β-diketiminato (‘Nacnac’) stabilized gallium dihydride H2Ga{(NDippCMe)2CH} with a range of mono- and dinuclear metal carbonyl reagents are characterized by loss of dihydrogen and formation of donor/acceptor complexes featuring the Ga(I) carbenoid ligand : Ga{(NDippCMe)2CH}. Thus, far from simply mimicking the chemistry of the corresponding alane H2Al{(NDippCMe)2CH}, which yields κ1 and κ2 Al–H σ-complexes with similar reagents, the weaker nature of Ga–H bonds leads to extensive bond activation chemistry and enables an unprecedented dehydrogenative route to Ga(I) ligand systems. By consideration of the chemistry of dinuclear systems, two alternative pathways are revealed for this chemistry, with either H2 or M–H bonds acting as the ultimate hydrogen sink.

Probing the Limits of Ligand Steric Bulk: Backbone CH Activation in a Saturated N‐Heterocyclic Carbene

The consequences of extremely high steric loading have been probed for late transition metal complexes featuring the expanded ring N-heterocyclic carbene 6-Dipp. The reluctance of this ligand to form 2:1 complexes with d-block metals (rationalised on the basis of its percentage buried volume, % Vbur , of 50.8%) leads to C-H and C-N bond activation processes driven by attack at the backbone β-CH2 unit. In the presence of Ir(I) (or indeed H(+) ) the net result is the formation of an allyl formamidine fragment, while Au(I) brings about an additional ring (re-)closure step via nucleophilic attack at the coordinated alkene. The net transformation of 6-Dipp in the presence of [(6-Dipp)Au](+) represents to our knowledge the first example of backbone C-H activation of a saturated N-heterocyclic carbene, proceeding in this case via a mechanism which involves free carbene in addition to the Au(I) centre.