Silyl Ligands Research Articles

Silylene [PSiP] Pincer Cobalt(II) Chloride–Catalyzed Alkene Hydrosilylation

ABSTRACTSilylene metal complexes exhibit excellent catalytic activity and selectivity in homogeneous catalysis. However, there are relatively few examples of silylene metal complexes as catalysts for the hydrosilylation of alkenes. In order to study the effect of a silylene ligand on the catalytic performance of [PSiP] pincer Co(II) chloride for alkene hydrosilylation, the PMe3 ligand in (MeSi(C6H4‐PPh2)2)Co (PMe3)Cl (1) was replaced by a silylene ligand, and a silylene [PSiP] pincer Co(II) chloride, (MeSi(C6H4‐PPh2)2)Co (PhC (NtBu)2Si)Cl (2), was used as catalyst to study its catalytic activity in alkene hydrosilylation. It was confirmed that silylene Complex 2 has excellent catalytic activity for alkene hydrosilylation with Ph2SiH2 as a hydrogen source. For alkene hydrosilylation, compared to phosphine Complex 1, silylene Complex 2 does not need NaBHEt3 as an additive. In addition, for aromatic alkenes, silylene Complex 2 has anti‐Markovnikov selectivity, while phosphine Complex 1 provides Markovnikov products in the presence of NaBHEt3.

Identity of the Silyl Ligand in an Iron Silyl Complex Influences Olefin Hydrogenation: An Experimental and Computational Study.

In this study, we explore the selective synthesis of iron silyl complexes using the reaction of an iron mesityl complex (MesCCC)FeMes(Py) with various hydrosilanes. These resulting iron silyl complexes, (MesCCC)Fe(SiH2Ph)(Py)(N2), (MesCCC)Fe(SiMe2Ph)(Py)(N2), and (MesCCC)Fe[SiMe(OSiMe3)2](Py)(N2), serve as effective precatalysts for olefin hydrogenation. The key to their efficiency in catalysis lies in the specific nature of the silyl ligand attached to the iron center. Experimental observations, supported by density functional theory (DFT) simulations, reveal that the catalytic performance correlates with the relative stability of dihydrogen and hydride species associated with each iron silyl complex. The stability of these intermediates is crucial for efficient hydrogen transfer during the catalytic cycle. The DFT simulations help to quantify these stability factors, showing a direct relationship between the silyl ligand's electronic and steric properties and the overall catalytic activity. Complexes with certain silyl ligands exhibit better performance due to the optimal balance between the stability and reactivity of the key active catalyst. This work highlights the importance of ligand design in the development of iron-based hydrogenation catalysts.

Recent progress in transition metal complexes featuring silylene as ligands.

Silylenes, divalent silicon(II) compounds, once considered highly reactive and transient species, are now widely employed as stable synthons in main-group and coordination chemistry for myriad applications. The synthesis of stable silylenes represents a major breakthrough, which led to extensive exploration of silylenes in stabilizing low-valent main-group elements and as versatile ligands in coordination chemistry and catalysis. In recent years, the exploration of transition metal complexes stabilized with silylene ligands has captivated significant research attention. This is due to their robust σ-donor characteristics and capacity to stabilize transition metals in low valent states. It has also been demonstrated that the transition metal complexes of silylenes are effective catalysts for hydroboration, hydrosilylation, hydrogenation, hydrogen isotope exchange reactions, and small molecule activation chemistry. This review article focuses on the recent progress in the synthesis and catalytic application of transition metal complexes of silylenes.

Recent Advances on the Chemistry of Transition Metal Complexes with Monoanionic Bidentate Silyl Ligands.

The chemistry of transition-metal (TM) complexes with monoanionic bidentate (κ2-L,Si) silyl ligands has considerably grown in recent years. This work summarizes the advances in the chemistry of TM-(κ2-L,Si) complexes (L=N-heterocycle, phosphine, N-heterocyclic carbene, thioether, ester, silylether or tetrylene). The most common synthetic method has been the oxidative addition of the Si-H bond to the metal center assisted by the coordination of L. The metal silicon bond distances in TM-(κ2-L,Si) complexes are in the range of metal-silyl bond distances. TM-(κ2-L,Si) complexes have proven to be effective catalysts for hydrosilylation and/or hydrogenation of unsaturated molecules among other processes.

Isolation and Reactivity of Silepins with a Sterically Demanding Silyl Ligand

Silacycloheptatriene (silepin) species are novel silicon compounds reported in recent years. The interplay between the “closed” silepin and the “open” silylene form enables an enhanced stability of the low valent species while maintaining a high reactivity towards small molecules. In this work, two new silepins of similar structures to literature known compounds bearing modified silyl ligands are reported. A unique intramolecular activation of an aromatic hydrogen is found, and the respective formed hydrosilanes are characterized. We further synthesized iron(0) carbonyl complexes of known and new silepins in order to investigate their electronic properties relative to each other to gain more insight into substitution effect in such compounds.

Catalytic Reduction of Cyanide to Ammonia and Methane at a Mononuclear Fe Site.

Nitrogenase enzymes catalyze nitrogen reduction (N2R) to ammonia and also the reduction of non-native substrates, including the 7H+/6e- reduction of cyanide to CH4 and NH3. CN- and N2 are isoelectronic, and it is hence fascinating to compare the mechanisms of synthetic Fe catalysts capable of both CN- and N2 reduction. Here, we describe the catalytic reduction of CN- to NH3 and CH4 by a highly selective (P3Si)Fe(CN) catalyst (P3Si represents a tris(phosphine)silyl ligand). Catalysis is driven in the presence of excess acid ([Ph2NH2]OTf) and reductant ((C6H6)2Cr), with turnover as high as 73 demonstrated. This catalyst system is also modestly competent for N2R and structurally related to other tris(phosphine)Fe-based N2R catalysts. The choice of catalyst and reductant is important to observe high yields. Mechanistic studies elucidate several intermediates of CN- reduction, including iron isocyanides (P3SiFeCNH+/0) and terminal iron aminocarbynes (P3SiFeCNH2+/0). Aminocarbynes are isoelectronic to iron hydrazidos (Fe═N-NH2+/0), which have been invoked as selectivity-determining intermediates of N2R (NH3 versus N2H4 products). For the present CN- reduction catalysis, reduction of aminocarbyne P3SiFeCNH2+ is proposed to be rate but not selectivity contributing. Instead, by comparison with the reactivity of a methylated aminocarbyne analogue (P3SiFeCNMe2), and associated computational studies, formation of a Fischer carbene (P3SiFeC(H)(NH2)+) intermediate that is on path for either CH4 and NH3 (6 e-) or CH3NH2 (4 e-) products is proposed. From this carbene intermediate, pathways to the observed CH4 and NH3 products (distinct from CH3NH2 formation) are considered to compare and contrast the (likely) mechanism/s of CN- and N2 reduction.

X-type silyl ligands for transition-metal catalysis.

Given the critical importance of novel ligand development for transition-metal (TM) catalysis, as well as the resurgence of the field of organosilicon chemistry and silyl ligands, to summarize the topic of X-type silyl ligands for TM catalysis is highly attractive and timely. This review particularly emphasizes the unique σ-donating characteristics and trans-effects of silyl ligands, highlighting their crucial roles in enhancing the reactivity and selectivity of various catalytic reactions, including small molecule activation, Kumada cross-coupling, hydrofunctionalization, C-H functionalization, and dehydrogenative Si-O coupling reactions. Additionally, future developments in this field are also provided, which would inspire new insights and applications in catalytic synthetic chemistry.

Dinitrogen silylation catalyzed by silylene cobalt(I) and silylene iron(I) chlorides.

In this contribution, Co(PMe3)3Cl (1), bis(silylene) cobalt chlorides Co(LSi:)2(PMe3)2Cl (LSi: = {PhC(NtBu)2}SiCl (2); {p-CH3C6H4C(NtBu)2}SiCl (3); and {p-tBuC6H4C(NtBu)2}SiCl (4)) and bis(silylene) iron chlorides Fe(LSi:)2(PMe3)2Cl (LSi: = {PhC(NtBu)2}SiCl (5); {p-CH3C6H4C(NtBu)2}SiCl (6); {p-tBuC6H4C(NtBu)2}SiCl (7) and Fe(PMe3)2Cl2 (8)) were synthesized to study the effects of different metals and silylene ligands on the catalytic activity of complexes 1-8 in dinitrogen silylation reaction. The experimental results indicate that there is no substantial difference in catalytic activity between the phosphine cobalt complex 1 and the silylene cobalt chlorides 2-4 although the cobalt silylene complex 2 has slightly better catalytic activity than complexes 1, 3 and 4 in the dinitrogen silylation. Silylene iron complexes 5-7 are more active than the phosphine iron complex 8. Among the three silylene iron(I) chlorides 5-7, complex 5 is the most effective catalyst for dinitrogen silylation and 402 equiv. of N(SiMe3)3 could be obtained per Fe atom. In the dinitrogen silylation reaction catalyzed by iron complexes, the introduction of the silylene ligand made the silylene iron complexes 5-7 more active than the phosphine iron complex 8. In addition, iron chlorides 5-8 are more effective catalysts than cobalt(I) chlorides 1-4 for the dinitrogen silylation reaction. Complexes 3, 4, 6 and 7 were new complexes, and their molecular structures were determined by single crystal X-ray diffraction analysis.

Effects of silylene ligands on the catalytic activity of [PSiP] pincer cobalt(ii) chloride for N2 silylation

Three silyl [PSiP] pincer cobalt(ii) chlorides [(2-Ph2PC6H4)2MeSiCo(Cl)(PMe3)] (1), [(2-iPr2PC6H4)2MeSiCo(Cl)(PMe3)] (2) and [(2-Ph2PC6H4)2MeSiCo(Cl)(LSi:)] (LSi: = {PhC(NtBu)2}SiCl) (3) were used as catalysts for dinitrogen silylation was studied.

Pd–Si complexes of the type ClPd(μ2-pyO)4SiR (R = Me, Ph, Bn, Allyl, κO-(pyO)PdCl(η3-allyl); pyO = pyridine-2-olate): The influence of substituent R on the Pd–Si bond

The reactions of organosiliconpyridine-2-olates (pyridyl-2-oxysilanes) RSi(pyO)3 (pyO = pyridine-2-olate, R = Me (1a), Ph (1b), Bn (1c) and Allyl (1d)) and [PdCl2(NCMe)2] in chloroform afforded the hexacoordinate silicon complexes RSi(μ2-pyO)4PdCl (R = Me (2a), Ph (2b), Bn (2c) and Allyl (2d), respectively), which feature a Pd–Si bond, in which the Pd atom is the formal lone pair donor toward Si. The new compounds 2b, 2c, 2d were characterized with multi-nuclear NMR spectroscopy and elemental analysis. The effect of the Si-bound substituent R on the trans-disposed Pd–Si bond was studied by single-crystal X-ray diffraction and computational analyses (e.g., Natural Localized Molecular Orbitals, NLMO; topological analyses of the electron density at the bond critical point with Quantum Theory of Atoms-In-Molecules, QTAIM). A structurally related byproduct, (η3-allyl)ClPd(pyO)Si(μ2-pyO)4PdCl 2d’, which formed along with target product 2d and features an Si–O bond trans to Pd–Si, was included in this systematic study. Another byproduct from the synthesis of 2d, the pentanuclear complex ClPd(μ2-pyO)2Si(μ2-pyO)2Pd(μ2-pyO)2Si(μ2-pyO)2PdCl (compound 3) was characterized crystallographically. This compound features pentacoordinate Si atoms within trigonal–bipyramidal Si(O4Pd) coordination spheres with equatorial Pd–Si bonds to the terminal Pd atoms. The Pd–Si bond situation in this compound was elucidated with the aid of computational analyses. QTAIM analyses of 3 in conjunction with a model compound PdSi4, which features two silyl groups and two silylene ligands, indicate topological properties of the electron density at the Pd–Si bond critical point which are similar to Pd–Si bonds of silyl and silylene compounds. The latter exhibit greater similarity, which indicates features of a Pd←Si bond. In contrast, NLMO analyses of 3 identify a polar covalent Pd–Si bond with predominant Pd contribution (formal Pd→Si donation).

Chiral PSiSi-Ligand Enabled Iridium-Catalyzed Atroposelective Intermolecular C-H Silylation.

Catalytic enantioselective intermolecular C-H silylation offers an efficient approach for the rapid construction of chiral organosilicon compounds, but remains a significant challenge. Herein, a new type of chiral silyl ligand is developed, which enables the first iridium-catalyzed atroposelective intermolecular C-H silylation reaction of 2-arylisoquinolines. This protocol features mild reaction conditions, high atom economy, and remarkable yield with excellent stereoselectivity (up to 99% yield, 99% ee), delivering enantioenriched axially chiral silane platform molecules with facile convertibility. Key to the success of this unprecedented transformation relies on a novel chiral PSiSi-ligand, which facilitates the intermolecular C-H silylation process with perfect chem-, regio- and stereo-control via a multi-coordinated silyl iridium complex.

Chiral PSiSi‐Ligand Enabled Iridium‐Catalyzed Atroposelective Intermolecular C−H Silylation

AbstractCatalytic enantioselective intermolecular C−H silylation offers an efficient approach for the rapid construction of chiral organosilicon compounds, but remains a significant challenge. Herein, a new type of chiral silyl ligand is developed, which enables the first iridium‐catalyzed atroposelective intermolecular C−H silylation reaction of 2‐arylisoquinolines. This protocol features mild reaction conditions, high atom economy, and remarkable yield with excellent stereoselectivity (up to 99 % yield, 99 % ee), delivering enantioenriched axially chiral silane platform molecules with facile convertibility. Key to the success of this unprecedented transformation relies on a novel chiral PSiSi‐ligand, which facilitates the intermolecular C−H silylation process with perfect chem‐, regio‐ and stereo‐control via a multi‐coordinated silyl iridium complex.

Crystal structure and DFT calculations of Cp2NbH(SiIMe2)(SiFMe2): an asymmetric bis(silyl) niobocene hydride complex.

An asymmetric bis(silyl) niobocene hydride complex, namely, bis(η5-cyclopentadienyl)(fluorodimethylsilyl)hydrido(iododimethylsilyl)niobium, [Nb(C5H5)2(C2H6FSi)(C2H6ISi)H] or Cp2NbH(SiIMe2)(SiFMe2), has been studied to determine the effect of the silyl ligand on the position of the hydride attached to the Nb atom. It has been shown that when a Group 17 atom is substituted onto one of the silyl ligands, there is a greater interaction between the hydride and this ligand, as demonstrated by a shorter Si...H distance. In the present work, we have investigated the effect when the silyl ligands are substituted by different Group 17 atoms. We present here the structure and DFT calculations of Cp2NbH(SiIMe2)(SiFMe2), showing that the position of the hydride is located between the two silyl ligands. The results from our investigation show that the hydride is closer to the silyl ligand that is substituted by fluorine.

Effects of silylene ligands on the performance of carbonyl hydrosilylation catalyzed by cobalt phosphine complexes.

In order to study the effects of silylene ligands on the catalytic activity of carbonyl hydrosilylation catalyzed by cobalt phosphine complexes, readily available model catalysts are required. In this contribution, a comparative study of the hydrosilylation of aldehydes and ketones catalyzed by tris(trimethylphosphine) cobalt chloride, CoCl(PMe3)3 (1), and bis(silylene) cobalt chloride, Co(LSi:)2(PMe3)2Cl (2, LSi: = {PhC(NtBu)2}SiCl), is presented. It was found that both complexes 1 and 2 are good catalysts for the hydrosilylation of aldehydes and ketones under mild conditions. This catalytic system has a broad substrate scope and selectivity for multi-functional substrates. Silylene complex 2 shows higher activity than complex 1, bearing phosphine ligands, for aldehydes, but conversely, for ketones, the activity of complex 1 is higher than that of complex 2. It is worth noting that in the process of mechanistic studies the intermediates (PMe3)3Co(H)(Cl)(PhH2Si) (3) and (LSi:)2(PMe3)Co(H)(Cl)(PhH2Si) (4) were isolated from the stoichiometric reactions of 1 and 2 with phenylsilane, respectively. Further experiments confirmed that complex 3 is a real intermediate. A possible catalytic mechanism for the hydrosilylation of carbonyl compounds catalyzed by 1 was proposed based on the experimental investigation and literature reports, and this mechanism was further supported by DFT studies. The bis(silylene) complex 4 showed complicated behavior in solution. A series of experiments were designed to study the catalytic mechanism for the hydrosilylation of carbonyl compounds catalyzed by complex 2. According to the experimental results, the hydrosilylation of aldehydes catalyzed by 1 proceeds via a different mechanism than that of the analogous reaction with complex 2 as the catalyst. In the case of ketones, complex 4 is a real intermediate, indicating that both 1 and 2 catalyze the reaction by the same mechanism. The molecular structures of 3 and 4 were determined by single crystal X-ray diffraction analysis.

Di- and Trinuclear Complexes of Pd(0) and Pt(0) with Bridging Silylene Ligands: Structures with a Coordinatively Unsaturated Metal Center and Their Reactions with Alkynes

Di- and Trinuclear Complexes of Pd(0) and Pt(0) with Bridging Silylene Ligands: Structures with a Coordinatively Unsaturated Metal Center and Their Reactions with Alkynes

Early Lanthanide(III) Ate Complexes Featuring Ln-Si Bonds (Ln = La, Ce): Synthesis, Structural Characterization, and Bonding Analysis.

While research on lanthanide (Ln) complexes with silyl ligands is receiving growing attention, significantly unbalanced efforts have been devoted to different Ln elements. In comparison with the intense investigations on Ln elements such as Sm and Yb, the chemistry of silyl lanthanum and cerium complexes is much slower to develop, and no solid-state structure of a silyl lanthanum complex has been reported so far. In this research, four types of ate complexes, including [(DME)3Li][Cp3LnSi(H)Mes2], [(18-crown-6)K][Cp3LnSi(CH3)Ph2], [(DME)3Li][Cp3LnSiPh3], and [(12-crown-4)2Na] [Cp3LnSi(Ph)2Si(H)Ph2] (Ln = La, Ce), were synthesized by reacting [(DME)3Na][Cp3La(μ-Cl)LaCp3] or Cp3Ce(THF) with alkali metal silanides. All of the synthesized silyl Ln ate complexes were structurally characterized. La-Si bond lengths are in a range of 3.1733(4)-3.1897(10) Å, and the calculated formal shortness ratios of the La-Si bonds (1.071.08) are comparable to those in the reported silyl complexes having other Ln metal centers. The Ce-Si bond lengths (3.1415(6)-3.1705(9) Å) are within the typical range of reported silyl cerium ate complexes. 29Si solid-state NMR measurements on the diamagnetic silyl lanthanum complexes were conducted, and large one-bond hyperfine splitting constants arising from = 7/2) were resolved. Computational studies on these silyl lanthanum and cerium complexes suggested the polarized covalent feature of the Ln-Si bonds, which is in line with the measured large 1J139La-Si splitting constants.

N,N′-Di-tert-butyl-P,P-diphenylphosphinimidic Amidato-κN,κN′]chlorosilicon-κSi-tetracarbonyliron

The title complex {[Ph2P(tBuN)2](Cl)Si:->Fe(CO)4} (2) was synthesized via the reaction of chlorosilylene [Ph2P(tBuN)2]SiCl (1), supported by an iminophosphonamide ligand with Fe(CO)5 in THF. The molecular structure of 2 was fully characterized by NMR (1H, 13C, 29Si, and 31P) and IR spectroscopies, as well as single-crystal X-ray diffraction (SCXRD) analysis. In the SCXRD analysis of 2, the silylene ligand was located in the axial positions of the coordination sphere of the central iron atom and other sites were occupied by carbonyl ligands.

Metathesis between E−C(sp n ) and H−C(sp 3 ) σ‐Bonds (E=Si, Ge; n =2, 3) on an Osmium‐Polyhydride

The silylation of a phosphine of OsH6(PiPr3)2 is performed via net-metathesis between Si−C(spn) and H−C(sp3) σ-bonds (n=2, 3). Complex OsH6(PiPr3)2 activates the Si−H bond of Et3SiH and Ph3SiH to give OsH5(SiR3)(PiPr3)2, which yield OsH4{κ1-P,η2-SiH-[iPr2PCH(Me)CH2SiR2H]}(PiPr3) and R−H (R=Et, Ph), by displacement of a silyl substituent with a methyl group of a phosphine. Such displacement is a first-order process, with activation entropy consistent with a rate determining step occurring via a highly ordered transition state. It displays selectivity, releasing the hydrocarbon resulting from the rupture of the weakest Si-substituent bond, when the silyl ligand bears different substituents. Accordingly, reactions of OsH6(PiPr3)2 with dimethylphenylsilane, and 1,1,1,3,5,5,5-heptamethyltrisiloxane afford OsH5(SiR2R′)(PiPr3)2, which evolve into OsH4{κ1-P,η2-GeH-[iPr2PCH(Me)CH2SiR2H]}(PiPr3) (R=Me, OSiMe3) and R′−H (R′=Ph, Me). Exchange reaction is extended to Et3GeH. The latter reacts with OsH6(PiPr3)2 to give OsH5(GeEt3)(PiPr3)2, which loses ethane to form OsH4{κ1-P,η2-GeH-[iPr2PCH(Me)CH2GeEt2H]}(PiPr3).

Metathesis between E-C(spn ) and H-C(sp3 ) σ-Bonds (E=Si, Ge; n=2, 3) on an Osmium-Polyhydride.

The silylation of a phosphine of OsH6(PiPr3)2 is performed via net‐metathesis between Si−C(sp n ) and H−C(sp3) σ‐bonds (n=2, 3). Complex OsH6(PiPr3)2 activates the Si−H bond of Et3SiH and Ph3SiH to give OsH5(SiR3)(PiPr3)2, which yield OsH4{κ1‐P,η2‐SiH‐[iPr2PCH(Me)CH2SiR2H]}(PiPr3) and R−H (R=Et, Ph), by displacement of a silyl substituent with a methyl group of a phosphine. Such displacement is a first‐order process, with activation entropy consistent with a rate determining step occurring via a highly ordered transition state. It displays selectivity, releasing the hydrocarbon resulting from the rupture of the weakest Si‐substituent bond, when the silyl ligand bears different substituents. Accordingly, reactions of OsH6(PiPr3)2 with dimethylphenylsilane, and 1,1,1,3,5,5,5‐heptamethyltrisiloxane afford OsH5(SiR2R′)(PiPr3)2, which evolve into OsH4{κ1‐P,η2‐GeH‐[iPr2PCH(Me)CH2SiR2H]}(PiPr3) (R=Me, OSiMe3) and R′−H (R′=Ph, Me). Exchange reaction is extended to Et3GeH. The latter reacts with OsH6(PiPr3)2 to give OsH5(GeEt3)(PiPr3)2, which loses ethane to form OsH4{κ1‐P,η2‐GeH‐[iPr2PCH(Me)CH2GeEt2H]}(PiPr3).

An Iridium Complex with a Phosphine‐Pendant Silyl Ligand as an Efficient Catalyst for the (E)‐Selective Semihydrogenation of Alkynes

AbstractTwo iridium disilyl complexes that contain phosphine‐pendant silyl groups have been obtained from the reaction of [(cod)IrCl]2 with a disilane with phosphine pendants, (Ph2PCH2)R2Si−SiR2(CH2PPh) (R=Me or Ph). The iridium complex with methyl‐substituted silyl moieties showed good catalytic performance in the (E)‐selective semihydrogenation of various alkynes at room temperature under 1 atm of H2 without over‐reduction to the corresponding alkanes.