Sakya S. Sen

High yield access to silylene RSiCl (R = PhC(NtBu)2) and its reactivity toward alkyne: synthesis of stable disilacyclobutene.

Two new approaches for synthesizing RSiCl, (R = PhC(NtBu)(2)) are reported by the reaction of RSiHCl(2) with bis-trimethyl silyl lithium amide and N-heterocyclic carbene respectively. In the former method silylene is produced in 90% yield. The silylene was treated with biphenyl alkyne to afford the disilacyclobutene system. This is a rare example of two five-coordinate silicon centers arranged adjacent to each other in a four-membered ring. Furthermore, we fluorinated the four-membered ring by trimethyltin fluoride to obtain the fluoro substituted disilacyclobutene.

Chemistry of functionalized silylenes

Recent years have witnessed important developments in organosilicon chemistry, courtesy to the high yield access of stable monomeric chlorosilylene (LSiCl, L = PhC(NtBu)2) and N-heterocyclic carbene (NHC) stabilized dichlorosilylene (L1SiCl2, L1 = 1,3-bis(2,6-iPr2C6H3)imidazol-2-ylidene) by dehydrochlorination technique using strong bases such as N-heterocyclic carbenes or LiN(SiMe3)2 as HCl scavenger. The chemistry of functionalized silylenes is markedly different from the previously reported dicoordinate silylenes. Their utility as synthons is well documented which paves the way for several striking compounds, e.g. monosilaoxiranes, CSi2P derivative, CSi3P cation, a dimer of silaisonitrile (Si2N2) with dicoordinate Si atoms etc. This study also unravels that these functionalized silylenes are proficient at activating phosphorus, and the functional group attached to the α-position of the silicon(II) centre plays a key role to determine the final product. The reaction of P4 with LSiCl affords a zwitterionic Si2P2 ring, while with LSiN(SiMe3)2 it gives an acyclic Si2P4 derivative. Another recent achievement in this field is the successful isolation of a valence isomer of disilyne known as the inter-connected bis-silylene. This perspective portrays the chemistry of functionalized silylenes which will attract great attention, heading for the further development of organosilicon chemistry.

A remarkable base-stabilized bis(silylene) with a silicon(I)-silicon(I) bond.

Double and triple covalent bonds are ubiquitous in carbon chemistry and have been studied for more than two centuries but were unusual with its congener in the periodic table, silicon. Initial attempts to synthesize such compounds were unsuccessful, resulting in the formation of polymeric substances. This situation changed when West and co-workers in 1981 synthesized a compound containing a Si Si double bond (R2Si=SiR2, R = Me3C6H2), in which each Si atom has a formal oxidation state of + II. Key to the discovery of stable compounds containing Si Si double bonds was the protection of the double bonds by bulky substituents, which provide kinetic stability. Apeloig et al. showed that silylene has a singlet ground state, and the B1 triplet state lies significantly higher in energy. Moreover, recent calculations indicate that the energy difference between the singlet and triplet states of silylene is around 18–21 kcalmol . This singlet–triplet energy difference of the silylene fragments is the main reason for the weakness of the Si=Si double bond. This now generally accepted model originated from Carter, Goddard, Malrieu, and Trinquier (CGMT), who described the doublebond topology as being a function of the energy difference between the singlet and the triplet state of the carbene-like fragments formally constituting the double bond. Compounds with double-bonded silicon are established and have been studied in detail during the last decade. In 2004, Sekiguchi and co-workers as well as Wiberg et al. were successful in isolating compounds containing Si Si triple bonds (RSi SiR; R = Si(iPr){CH(SiMe3)2}2 and R = SiMe(SitBu3)2) in which the formal oxidation state of Si is + I. [7] Subsequently, Robinson and co-workers synthesized two compounds, one with a Si Si single bond having formal oxidation state + I and another with a Si Si double bond in which the formal oxidation state of Si is 0 (RClSi SiClR and RSi=SiR, R = 1,3-bis-(2,6-diisopropylphenyl)imidazol-2-ylidene). In addition to disilenes and disilynes, a number of other unusual stable compounds with low-coordinate Si atoms have been described. To our knowledge, only one compound consisting of a Si Si single bond and a lone pair of electrons on each Si atom was reported to date. The compound was stabilized by an N-heterocyclic carbene, and one chlorine atom was attached to each silicon center making the formal oxidation state of the silicon atoms + I. This situation is unique, because each Si center which features a lone pair of electrons is simultaneously involved in bonding. These two attributes are usually associated with extreme instability. In view of this chemistry, we became interested in synthesizing a compound with a Si Si single bond stabilized by a monoanionic ligand, thus avoiding the lone pair of electrons taking part in any bonding. We were recently successful in using an amidinate ligand with tBu substituents on the nitrogen atoms to stabilize heteroleptic silylenes and a Ge dimer. It seems that such a ligand might also stabilize a Si compound with a Si Si single bond. Our preliminary results in this direction are reported herein. The reaction of tert-butylcarbodiimide with one equivalent of PhLi in diethyl ether and subsequent treatment with SiCl4 afforded [PhC(NtBu)2]SiCl3 (1, Scheme 1). Compound 1

Interconnected Bis-Silylenes: A New Dimension in Organosilicon Chemistry

The past two decades have brought remarkable advances in organosilicon chemistry with the isolation of stable silylenes, persila-allene, and disilynes. The extension of this list gives an impression that it will continue to flourish. The judicous employment of sterically appropriate ligands has enabled the synthesis and isolation of compounds with low-valent silicon. Recently, for example, interconnected bis-silylenes were isolated where the two Si atoms are connected by a σ-bond and each Si atom is possessing a lone pair of electrons. The formal oxidation state of each Si atom in the interconnected bis-silylene is +1, so bis-silylenes can be considered as the valence isomers of disilynes. In this Account, we describe the synthesis of interconnected bis-silylenes and assess their potential as a new building block in organosilicon chemistry. In 2009, we reported the isolation of a bis-silylene ((PhC(NtBu)(2))(2)Si(2)) stabilized by a sterically bulky benz-amidinato ligand with tBu substituents on the nitrogen atoms. Prior to our work, Robinson and co-workers described the synthesis of a N-heterocyclic carbene stabilized bis-silylene. In following years, just two more interconnected bis-silylenes have been reported. Density functional theory calculations to establish the geometric and electronic structures of the reported bis-silylenes have shown that the Wiberg bond index (WBI) for all the reported bis-silylenes is ~1. The synthesis of stable (PhC(NtBu)(2))(2)Si(2) prompted explorations of its reactivity. An important facet of silylene chemistry involves oxidative addition at the Si(II) center with unsaturated substrates, a reaction also available for bis-silylenes. Due to the three reaction sites (two lone pairs of electrons and a labile Si(I)-Si(I) single bond) in the interconnected bis-silylenes, we expect novel product formation. A labile Si-Si bond facilitates the reactions of (PhC(NtBu)(2))(2)Si(2) with diphenyl alkyne or adamantyl phosphaalkyne which afforded 1,4- disilabenzene and 1,3-disilacarbaphosphide (CSi(2)P) derivatives, respectively. The former is a noteworthy addition to the silicon analogues of benzene, and the latter serves as a heavy cyclobutadiene. With white phosphorus, a cyclic Si(2)P(2) derivative, an analogue of cyclobutadiene was obtained. The most predominant structural feature of these heavy cyclobutadienes is the presence of two-coordinate P atoms.

Striking Stability of a Substituted Silicon(II) Bis(trimethylsilyl)amide and the Facile Si–Me Bond Cleavage without a Transition Metal Catalyst

Silicon(II) bis(trimethylsilyl)amide (LSiN(SiMe(3))(2), L= PhC(NtBu)(2)) (2) has been synthesized by the reaction of LSiHCl(2) with KN(SiMe(3))(2) in 1:2 molar ratio in high yield where 1 equiv of the latter functions as a dehydrochlorinating agent. 2 exhibits a high stability up to 154 °C and can be handled in open air for a short period of time without any appreciable decomposition. An amazing five-membered cyclic silene (3) results from the cleavage of one Si-Me bond of 2 with an adamantyl phosphaalkyne. 3 is the first example of a heavy cyclopentene derivative which consists of four different elements, C, N, Si, and P. Both compounds are characterized by multinuclear NMR spectroscopy, EI-mass spectrometry, and single crystal X-ray diffraction studies.

Synthesis of Stable Silicon Heterocycles by Reaction of Organic Substrates with a Chlorosilylene [PhC(NtBu)2SiCl]

Heteroleptic chlorosilylene (PhC(NtBu)(2)SiCl) (1) reacts with unsaturated organic compounds under oxidative addition. Reactions of 1 with cyclooctatetraene (COT) and a diimine afford [1+4]-cycloaddition products 3 and 6, respectively. In the case of COT, one Si-N bond of the amidinato ligand is cleaved, resulting in tetracoordinate silicon, whereas with a diimine a pentacoordinate silicon is formed. Treatment of 1 with ArN=C=NAr (Ar=2,6-iPr(2)C(6)H(3)) yields silaimine complex 4 with cleavage of one of the C=N bonds. The facile isolation of silaimine complexes is probably due to the kinetic protection afforded by the bulky Ar moiety. When 1 is treated with tert-butyl isocyanate, cleavage of the C=O bond is observed, which leads to formation of the four-membered Si(2)O(2) cycle 5. The same product is formed when 1 is allowed to react with trimethylamine N-oxide. When 1 is treated with diphenyl disulfide, cleavage of the S-S bond occurs to form 7. All products have been characterized by multinuclear NMR spectroscopy, EI mass spectrometry, and elemental analysis. In addition, the molecular structures of 3-6 have been determined by single-crystal X-ray diffraction studies. Collectively, these results suggest that owing to the presence of the lone pair of electrons, the propensity of 1 to undergo oxidative addition is very high.

Stable silaimines with three- and four-coordinate silicon atoms.

The reactions of silylenes with organic azides are quite diverse, depending on the substituents of the silylene center and on the nature of the azide employed. Elusive silaimine with three-coordinate silicon atom L(1)SiN(2,6-Triip(2)-C(6)H(3)) (5) {L(1) = CH[(C═CH(2))(CMe)(2,6-iPr(2)C(6)H(3)N)(2)] and Triip = 2,4,6-triisopropylphenyl} was synthesized by treatment of the silylene L(1)Si (1) with a sterically demanding 2,6-bis(2,4,6-triisopropylphenyl)phenyl azide (2,6-Triip(2)C(6)H(3)N(3)). The reaction of Lewis base-stabilized dichlorosilylene L(2)SiCl(2) (2) {L(2) = 1,3-bis(2,6-iPr(2)C(6)H(3))imidazol-2-ylidene} with Ph(3)SiN(3) afforded four-coordinate silaimine L(2)(Cl(2))SiNSiPh(3) (6). Treatment of 2,6-Triip(2)C(6)H(3)N(3) with L(3)SiCl (3) (L(3) = PhC(NtBu)(2)) yielded silaimine L(3)(Cl)SiN(2,6-Triip(2)-C(6)H(3)) (7) possessing a four-coordinate silicon atom. The reactions of L(3)SiN(SiMe(3))(2) (4) with adamantyl and trimethylsilyl azide furnished silaimine compounds with a four-coordinate silicon atom L(3)(N(Ad)SiMe(3))SiN(SiMe(3)) (8) (Ad = adamantyl) and L(3)(N(SiMe(3))(2))SiN(SiMe(3)) (9). Compound 8 was formed by migration of one of the SiMe(3) groups. Compounds 5-9 are stable under inert atmosphere and were characterized by elemental analysis, NMR spectroscopy, and single-crystal X-ray studies.

B═B and B≡E (E = N and o) multiple bonds in the coordination sphere of late transition metals.

Because of their unusual structural and bonding motifs, multiply bonded boron compounds are fundamentally important to chemists, leading to enormous research interest. To access these compounds, researchers have introduced sterically demanding ligands that provide kinetic as well as electronic stability. A conceptually different approach to the synthesis of such compounds involves the use of an electron-rich, coordinatively unsaturated transition metal fragment. To isolate the plethora of borane, boryl, and borylene complexes, chemists have also used the coordination sphere of transition metals to stabilize reactive motifs in these molecules. In this Account, we summarize our results showing that increasingly synthetically challenging targets such as iminoboryl (B≡N), oxoboryl (B≡O), and diborene (B═B) fragments can be stabilized in the coordination sphere of late transition metals. This journey began with the isolation of two new iminoboryl ligands trans-[(Cy3P)2(Br)M(B≡N(SiMe3))] (M = Pd, Pt) attached to palladium and platinum fragments. The synthesis involved oxidative addition of the B-Br bond in (Me3Si)2N═BBr2 to [M(PCy3)2] (M = Pt, Pd) and the subsequent elimination of Me3SiBr at room temperature. Variation of the metal, the metal-bound coligands, and the substituent at the nitrogen atom afforded a series of analogous iminoboryl complexes. Following the same synthetic strategy, we also synthesized the first oxoboryl complex trans-[(Cy3P)2BrPt(BO)]. The labile bromide ligand adjacent to platinum makes the complex a viable candidate for further substitution reactions, which led to a number of new oxoboryl complexes. In addition to allowing us to isolate these fundamental compounds, the synthetic strategy is very convenient and minimizes byproducts. We also discuss the reaction chemistry of these types of compounds. In addition to facilitating the isolation of compounds with B≡E (E = N, O) triple bonds, the platinum fragment can also stabilize a diborene (RB═BR) moiety, a bonding motif that thus far had only been accessible when stabilized by N-heterocyclic carbenes (NHCs). In the new π-diborene [(Et3P)2Pt(B2Dur2)] (Dur = 2,3,5,6-Me4-C6H) complex, the diborene ligand receives electron density from Pt, leading to a strong Pt-B bond and a B═B bond. We attribute this result to the very short B═B bond distance (1.51(2) Å) while coordinated to platinum. Overall, an increasing number of chemists are examining the chemistry of multiply bound boron compounds. The isolation of an oxoboryl complex is of special interest not only from a structural standpoint but also because of its orbital similarities to the ubiquitous CO ligand. Detailed computational studies of the π-diborene complex [(Et3P)2Pt(B2Dur2)] show that the bonding properties of this molecule violate the widely accepted Dewar-Chatt-Duncanson (DCD) bonding model.

Reactivity studies of a Ge(I)-Ge(I) compound with and without cleavage of the Ge-Ge bond.

This Communication describes two strikingly different reactivities of a digermylene [{PhC(NtBu)(2)}(2)Ge(2)] (1) featuring a Ge(I)-Ge(I) single bond. In the reaction with azobenzene, 1 affords the oxidative addition product LGeN(Ph)N(Ph)GeL [2; L = PhC(NtBu)(2)], with simultaneous cleavage of the Ge-Ge bond, whereas treatment of 1 with Fe(2)(CO)(9) yields the Lewis acid-base adduct LGe[Fe(CO)(4)]Ge[(Fe(CO)(4)]L (3). Both compounds were characterized by single-crystal X-ray diffraction, NMR spectroscopy, electrospray ionization mass spectrometry, and elemental analysis.

Compounds with Low-Valent p-Block Elements for Small Molecule Activation and Catalysis

The past decade has witnessed staggering progress in the chemistry of compounds with low‐valent main‐group elements. Although these discoveries are mostly fundamental by nature, these compounds show promising reactivity towards small molecule activation. The reactivity of these compounds stems from the modest HOMO–LUMO energy gap; a characteristic known for the transition metals. The journey began in 2005 with the facile activation of dihydrogen by an alkyne analog of germanium [ArGe≡GeAr; Ar=2,6‐Trip2‐C6H3 (Trip=2,4,6‐iPr3‐C6H2)]. Subsequently, tremendous progress has been achieved in understanding and elucidating the potential of these compounds to activate small molecules as well as to use them in a variety of stoichiometric and catalytic transformations. In this review, we focus on developments in the activation of H2, NH3, CO, and CO2 by compounds with multiply bound or open shell main‐group elements. Emphasis will be given to their catalytic activity.