Daniel Gallego

N-heterocyclic silylene complexes in catalysis: new frontiers in an emerging field

The present account is a review of all N-heterocyclic silylene (NHSi) transition metal complexes that have been employed in catalytic transformations, reported up to the present time (2013). NHSi transition metal complexes now enjoy indefatigable attention since their facile isolation was realised by the report of the first isolable NHSis by West and Denk in 1994. Despite considerable research activity since then, in comparison to ubiquitous N-heterocyclic carbene (NHC) complexes, NHSi complexes are still comparatively rare. Accordingly, in comparison to the plethora of reports associated with NHC complexes, implicated in catalytic processes, only scant examples exist for NHSi complexes. Some of these reports include Heck or Suzuki type coupling, alkyne cyclotrimerisation, ketone hydrosilylation, amide reduction or Sonogashira cross-coupling reactions, and are discussed in detail here. These endeavours pave the way for new families of catalysts based on NHSis and highlight the potential future applications of these emerging and rather unexplored complexes in novel catalytic processes.

From a zwitterionic phosphasilene to base stabilized silyliumylidene-phosphide and bis(silylene) complexes.

The reactivity of ylide-like phosphasilene 1 [LSi(TMS)═P(TMS), L = PhC(NtBu)2] with group 10 d(10) transition metals is reported. For the first time, a reaction of a phosphasilene with a transition metal that actually involves the silicon-phosphorus double bond was found. In the reaction of 1 with ethylene bis(triphenylphosphine) platinum(0), a complete silicon-phosphorus bond breakage occurs, yielding the unprecedented dinuclear platinum complex 3 [LSi{Pt(PPh3)}2P(TMS)2]. Spectroscopic, structural, and theoretical analysis of complex 3 revealed the cationic silylene (silyliumylidene) character of the silicon unit in complex 3. Similarly, formation of the analogous dinuclear palladium complex 4 [LSi{Pd(PPh3)}2P(TMS)2] from tetrakis(triphenylphosphine) palladium(0) was observed. On the other hand, in the case of bis(cyclooctadiene) nickel(0) as starting material, a distinctively different product, the bis(silylene) nickel complex 5 [{(LSi)2P(TMS)}Ni(COD)], was obtained. Complex 5 was fully characterized including X-ray diffraction analysis. Density functional theory calculations of the reaction mechanisms showed that the migration of the TMS group in the case of platinum and palladium was induced by the oxidative addition of the transition metal into the silicon-silicon bond. The respective platinum intermediate 2 [LSi{Pt(TMS)(PPh3)}P(TMS)] was also experimentally observed. This is contrasted by the reaction of nickel, in which the equilibrium of phosphasilene 1 and the phosphinosilylene 6 [LSiP(TMS)2] was utilized for a better coordination of the silicon(II) moiety in comparison with phosphorus to the transition metal center.

Facile access to silicon-functionalized bis-silylene titanium(II) complexes.

A series of unprecedented bis-silylene titanium(II) complexes of the type [(η(5)-C(5)H(5))(2)Ti(LSiX)(2)] (L=PhC(NtBu)(2); X=Cl, CH(3), H) has been prepared using a phosphane elimination strategy. Treatment of the [(η(5)-C(5)H(5))(2)Ti(PMe(3))(2)] precursor (1) with two molar equivalents of the N-heterocyclic chlorosilylene LSiCl (2), results in [(η(5)-C(5)H(5))(2)Ti(LSiCl)(2)] (3) with concomitant PMe(3) elimination. The presence of a Si-Cl bond in 3 enabled further functionalization at the silicon(II) center. Accordingly, a salt metathesis reaction of 3 with two equivalents of MeLi results in [(η(5)-C(5)H(5))(2)Ti(LSiMe)(2)] (4). Similarly, the reaction of 3 with two equivalents of LiBHEt(3) results in [(η(5)-C(5)H(5))(2)Ti(LSiH)(2)] (5), which represents the first example of a bis-(hydridosilylene) metal complex. All complexes were fully characterized and the structures of 3 and 4 elucidated by single-crystal X-ray diffraction analysis. DFT calculations of complexes 3-5 were also carried out to assess the nature of the titanium-silicon bonds. Two σ and one π-type molecular orbital, delocalized over the Si-Ti-Si framework, are observed.

Elucidating the Effect of the Nucleophilicity of the Silyl Group in the Reduction of CO2 to CO Mediated by Silyl‐Copper(I) Complexes

The reduction of CO2 to CO with silyl-copper(I) complexes bearing various silyl groups has been investigated. The silyl-copper(I) complexes [LSi(X)Cu(IPr)] 2-5 (X = OtBu (2), OH (3), H (4), OC6F5 (5); L = CH{C=CH2}(CMe)(NAr)2, IPr = (CHNAr)2C:, Ar = 2,6-iPr2C6H3) bearing OtBu, OH, H, and OC6F5 as functional groups are readily accessible by the activation of the Cu-O and Cu-H bonds in (IPr)CuX with silylene LSi: (1). These complexes are not readily accessible by the commonly used transmetallation reaction, rendering this methodology rather unique and facile in synthesizing silicon-functionalized silyl-metal complexes. The functional groups at the silicon atoms in compounds 2-5 enable the silyl groups to feature different nucleophilicity, which affords different activities toward CO2 reduction to CO. The silyl moieties of complexes 2 and 3, containing electron-donating groups (i.e., OtBu and OH) at the silicon centers, are more nucleophilic than that of compound 4 and 5, bearing a hydride and the electron-withdrawing group OC6F5 at the silicon centers, respectively. Consistent with this observation, compounds 2 and 3 show higher activity in CO2 reduction to CO compared to compounds 4 and 5, and the latter cases are zero-order reactions with respect to 4 and 5 (4: k = 7.8×10(-6) mol L(-1) s(-1); 5: 2.7×10(-8) mol L(-1)  s(-1)). This suggests that the more nucleophilic the silyl moiety in a silyl-copper(I) complex is, the higher is the efficiency in CO2 reduction to CO. In addition, the siloxyl-copper(I) complexes [LSi(X)OCu(IPr)] 6-9 [X = OtBu (6), OH (7), H (8), OC6F5 (9)] were isolated as the products from the corresponding reduction reactions. Complexes 2-4 and 6-8 were characterized by spectroscopic and structural means.

Capturing Elusive Cobaltacycle Intermediates: A Real‐Time Snapshot of the Cp*CoIII‐Catalyzed Oxidative Alkyne Annulation

Despite Cp*CoIII catalysts having emerged as a very attractive alternative to noble transition metals for the construction of heterocyclic scaffolds through C-H activation, the structure of the reactive species remains uncertain. Herein, we report the identification and unambiguous characterization of two long-sought cyclometalated Cp*CoIII complexes that have been proposed as key intermediates in C-H functionalization reactions. The addition of MeCN as a stabilizing ligand plays a crucial role, allowing the access to otherwise highly reactive species. Mechanistic investigations demonstrate the intermediacy of these species in oxidative annulations with alkynes, including the direct observation, under catalytic conditions, of a previously elusive post-migratory insertion seven-membered cobaltacycle.

Chelating N-Heterocyclic Silylenes as Steering Ligands in Catalysis

N-Heterocyclic silylenes (NHSis) have been a key focus in many research groups in recent years and have emerged as a new class of ligands to stabilize transition metal centers. For the most part, the existing examples that have been reported exhibit monodentate NHSi-stabilized complexes that show promise as precursors for catalytic transformations and other small molecule bond-activation reactions. More recently, however, a new class of multidentate NHSi ligands have emerged unraveling new structural motifs and alternative possibilities to stabilize reactive metal centers and use the emerging complexes as precursors in catalytic transformations. Three subclasses of these ligands have emerged: type I [:Si]-X-[Si:], where two NHSi centers are connected via a linker (such as O or some other groups that are noncoordinating to the metal); type II [:Si]-[D:]-[Si:], where the linker [D:] between the two NHSi centers is also capable of acting as a σ-donor or forming a σ-bond to the metal center affording a tridentate or pincer-type ligand system; and finally ligands of type III [Si]-X-[C:], which are mixed NHSi–NHC (N-heterocyclic carbene) ligand systems bearing a nonchelating X linker as with type I . This article will focus on the very recent developments of these multidentate ligand types ( I – III ), with the first reports emerging in 2010, their associated complexes, and the use of these emerging complexes in catalytic transformations.