Review of Small Molecule Activation Transition Metal Catalysis

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Eur J Inorg Chem. 2020 Sep seven; 2020(33): 3131–3142.

Modest Molecule Activation by Two‐Coordinate Acyclic Silylenes

Dr. Shiori Fujimori

^one Section of Chemistry, WACKER‐Found of Silicon Chemistry and Catalysis Enquiry Eye, Technische Universität München, Lichtenbergstraße 4, 85748 Garching bei München Germany,

Prof. Shigeyoshi Inoue

^one Department of Chemical science, WACKER‐Institute of Silicon Chemistry and Catalysis Research Center, Technische Universität München, Lichtenbergstraße iv, 85748 Garching bei München Germany,

Abstract

In recent decades, the chemistry of stable silylenes (R₂Si:) has evolved significantly. The start major development in this chemistry was the isolation of a silicocene which is stabilized past the Cp* (Cp* = η⁵‐C₅Me₅) ligand in 1986 and after the isolation of a showtime N‐heterocyclic silylene (NHSi:) in 1994. Since the groundbreaking discoveries, a big number of isolable circadian silylenes and college coordinated silylenes, i.e. Si(Two) compounds with coordination number greater than ii, have been prepared and the backdrop investigated. Withal, the first isolable two‐coordinate acyclic silylene was finally reported in 2012. The achievements in the synthesis of acyclic silylenes take immune for the utilization of silylenes in small-scale molecule activation including inert H₂ activation, a procedure previously exclusive to transition metals. This minireview highlights the developments in silylene chemistry, specifically two‐coordinate acyclic silylenes, including experimental and computational studies which investigate the extremely high reactivity of the acyclic silylenes.

Keywords: Silylenes, Acyclic compounds, Reaction mechanisms, Small-scale molecule activation, Silicon

Abstract

Silylenes (:SiR_ii), the heavier analogues of carbenes (:CR₂), have recently shown fascinating modes of reactivity, which are conventionally the domain of transition‐metal complexes. This minireview focuses on the developments in silylene chemistry, specifically two‐coordinate acyclic silylenes, including experimental and computational studies attempting to investigate the remarkable high reactivity of the acyclic silylenes.

i. Introduction

The cleavage of rigid σ‐bonds, such as H–H bond, is a key step in a lot of important catalytic processes, conventionally the domain of transition metals.1 The power of transition metals to bind reversibly with diverse functional groups enables transition‐metal complexes to perform as effective catalysts. In contempo decades, the field of master‐group compounds has grown significantly and a variety of low‐valent main‐group compounds which show interesting reactivity have been reported.2 Silicon, the second most arable element in the Globe'south crust, is especially of interest due to its high natural abundance and depression‐toxicity. Bail activation reactions using low‐valent silicon species in place of transition‐metallic complexes are of importance, as the onetime are more environmentally friendly and toll‐effective than the latter. Over the by four decades, many low‐valent silicon compounds have been prepared using sterically demanding ligands (kinetic stabilization) and/or electronically stabilizing ligands based on heteroatom substituents (thermodynamic stabilization).iii Silylenes (:SiR₂), the silicon analogues of carbenes (:CR₂), have gained much attending due to their propensity to selectively actuate small molecules.[2e] While the ground electronic country of carbenes (singlet or triplet) depends on the nature of the pendent substituents, silylenes by and large exhibit a singlet basis state.four The frontier molecular orbitals of singlet‐state silylenes consist of a high energy lone pair (HOMO) and an bachelor vacant p‐orbital (LUMO). This dual donor/acceptor character (ambiphilicity) mimics the frontier d‐orbitals found in transition metals.[2a] As such, the activation of inert molecules (such as H_ii) using silylenes has been shown to be possible, a process previously exclusive to transition metals.

In full general, silylenes are of loftier reactivity, have short lifetimes and tend to undergo facile dimerization, oligomerization or polymerization. For example, silylenes bearing sterically beefy substituents such as Mes (Mes = ii,4,6‐Me₃C₆H_ii), dimerize to form the corresponding disilenes (R₂Si=SiR₂).v Therefore, kinetic and/or thermodynamic stabilization is required to isolate such silylenes equally stable compounds. One of important developments in primary‐grouping chemistry was the isolation of a disilene Mes₂Si=SiMes₂ which was formed through the dimerization of the transient divalent silylene :SiMes_two at 77 Thou.6 Since the groundbreaking discovery, a variety of isolable cyclic silylenes and functionalized silylenes with a college coordinated silicon(2) atom have been reported to appointment (Figure ​ 1). One of the meaning developments in the chemical science of stable silylenes is the isolation of the dodecamethylsilicocene :SiCp*₂ (ane) (Cp* = η⁵‐C₅Me₅) as a Si(2) compound with higher coordination number by Jutzi and co‐workers in 1986.7 Silicocene 1 is stabilized using the thermodynamic stabilization effect of the Cp* ligands. In 1994, Denk and co‐workers reported the first stable 2‐coordinate Northward‐heterocyclic silylene (NHSi) 2,8 which is the silicon analogue of the stable Due north‐heterocyclic carbene (NHC) isolated by Arduengo and co‐workers in 1991.9 Subsequent to this, the groups of Lappert and Gehrhus succeeded in isolating the benzo‐fused silylenes iii.10 In 2006, the six‐membered NHSi iv was described by Driess and co‐workers.11 Roesky and co‐workers reported the first base‐free bis‐silylene 5 in 2011.12 A large number of examples of NHSi's, which are stabilized past the effect of cyclic systems forth with the interaction from the lone pairs on the directly bonded nitrogen atoms to the vacant 3p orbital on the silicon atom, were reported.13 In 1999, Kira and co‐workers succeeded in the synthesis of the beginning isolable cyclic dialkylsilylene 6 using the kinetic stabilization effect past the sterically bulky dialkyl based helmet‐type ligand.xiv Furthermore, Driess and co‐workers reported the synthesis of carbocyclic silylenes 7 bearing ii phosphonium ylides which exhibit comparable aromatic graphic symbol.15

Selected examples of isolable silylenes stabilized by thermodynamic and/or kinetic stabilization effects.

Apart from these cyclic silylenes, a number of Lewis base stabilized silylenes have been prepared.16 In this decade, many studies on three coordinate Si(Two) compounds have been demonstrated and their interesting electronic features and fascinating reactivity have been revealed. A significant discovery in this field was the synthesis of three coordinate silylenes begetting halogens [:SiX₂(L)] (10 = element of group vii) which are widely used every bit precursors to synthesize novel silicon compounds. In 2006, the first example of isolable monomeric chlorosilylene (:SiCl[PhC(N ^t Bu)₂]) 8 stabilized by an amidinate ligand was reported by Roesky and co‐workers.17 NHC‐stabilized dihalosilylenes [:SiX₂(NHC) (X = element of group vii)] 9 are also indispensable edifice blocks in synthetic chemical science.[16c], eighteen Another remarkable recent achievement in such three coordinate systems are the grooming of hydrosilylenes [:Si(H)R] which are attractive compounds for applications in catalytic transformations such as the hydrosilylation of alkenes, alkynes and carbonyl compounds. There are only a few examples of isolated hydrosilylenes without using Lewis acid stabilization.xix Kato and Baceiredo reported the isolation of a three coordinate Si(II) hydride 10a which is stabilized by an intramolecular phosphine coordination.[19a] Similarly, the phenyl‐substituted silylene 10b stabilized by a similar phosphine based ligand was isolated.20 In addition, iv coordinate silicon(Two) compounds, e.g. tetraphosphorus coordinated Si(II) compound (11), accept been reported.21 While many isolable cyclic silylenes13, 22 and Lewis base of operations stabilized silylenes16 have been described to date, only a few examples of simple dicoordinate acyclic silylenes are known, because the isolation of such silylenes equally stable compounds is synthetically challenging due to their highly reactive nature.

In a previous theoretical study, Wang and Ma investigated the pocket-size molecule activation, specifically H_ii, to the diverseness of cyclic and acyclic silylenes.23 Some of the central factors which influence the reactivity of silylenes towards H_two activation are the Man–LUMO and singlet–triplet gaps. In the instance of NHSi'southward which exhibit big Human being–LUMO and singlet–triplet energy gaps, high activation energies are required to reach the corresponding product. Another important factor in the reaction beliefs of silylenes is the geometry around the silicon middle, peculiarly the bending at the silicon atom. When the band strain is big, a higher barrier is required in breaking a H₂ molecule. For instance, the activation energy for the three‐membered silylene ring, silacyclopropenylidene (53.15 kcal/mol), is much higher compared with that of the acyclic dimethylsilylene (:SiMe₂) (13.31 kcal/mol) due to the band strain along with the 2π‐electrons‐delocalization on the C–C–Si ring (Figure ​ 2). Similarly, in the case of nitrogen‐substituted systems, the college activation barrier (63.46 kcal/mol) for the N‐heterocyclic silylene is required to reach the product than that (45.59 kcal/mol) of the diaminosilylene [:Si(NH_two)₂].23 Information technology is too establish that the splitting of H_ii with acyclic silylenes is more exothermic (–50.89 kcal/mol for dimethylsilylene, –23.34 kcal/mol for diaminosilylene) than that of cyclic silylenes (–18.65 kcal/mol for silacyclopropenylidene, –6.65 kcal/mol for Due north‐heterocyclic silylene). In a recent computational written report, Kuriakose and Vanka investigated the single site small molecule activation past acyclic silylenes and the undesired side reaction.24 In these systems during H₂ splitting the undesired side reaction, which leads to decomposition of the silylenes forming the products :Si(H)R' and HR (decomposition reaction), would exist competitive to the desired reaction in which both hydrogen atoms demark to the same atom to grade the tetravalent RSi(R')H₂ product (single site reaction) (R/R' = thiolato/thiolato, boryl/amido, or silyl/amido). The written report indicated that the angle at the silicon center also affects the preference of silylenes for the single site or the decomposition pathway. When the angle becomes fifty-fifty smaller, the dissociation pathway is favored over the unmarried site pathway significantly.

The activation energy and the exothermic free energy for the H_two insertion reaction with the acyclic silylene (black line) and the cyclic silylene (light dark-green line), calculated at the B3LYP/half-dozen‐311+Grand** level of theory.23 Transition state (TS), addition production (Pr).

In the last three decades, the chemistry of stable silylenes has grown significantly and has been subject field to many recent reviews regarding isolable cyclic silylenes,xiii, 22 functionalized silylenes with higher coordinate silicon(II) centers,16 and their application towards small molecule activation and catalytic reactions.[2c], 25 Despite recent progress, the activation of inert molecules such every bit H_ii using silylenes remains deficient. The computational studies unsaid that acyclic silylenes with a highly obtuse angle at the silicon center would have a depression‐lying triplet excited state and allow for the activation of inert molecules. Therefore, the study of such silylenes may open up new doors to the reactivity of principal‐group compounds as transition metal mimics. The focus of this minireview is the properties and key reactivity highlights from simple dicoordinate acyclic silylenes.

2. Synthesis

Although a variety of isolable circadian and Lewis base stabilized silylenes take been studied, only a scattering of isolable dicoordinate acyclic silylenes have been reported. While diamino substituted acyclic silylenes have been observed by NMR and UV/Vis spectroscopy, these compounds are unstable at ambient temperature.26 The beginning examples of isolable dicoordinate acyclic silylenes 14 and 17a were reported in 2012.27, 28 (Scheme ​ 1 and Scheme ​ 2). The sterically demanding boryl‐substituted silylene :Si{B(NDippCH)_ii}{N(SiMe₃)‐Dipp} (fourteen) (Dipp = 2,6‐ ⁱ Pr₂C_sixH_three), was prepared by the reaction of a tribromo(amino)silane 12 with two equivalents of lithiumboryl reagent 13.27 Alternatively, 12 reacts readily with two equivalents of hypersilylpotassium [Chiliad(THF)₂][Si(SiMe₃)₃] to give the respective silyl‐substituted silylene :Si{Si(SiMe₃)₃}{North(SiMe₃)‐Dipp} (15) (Scheme ​ ane).29 In the solid country, both silylenes fourteen and xv exhibit remarkable thermal robustness (T _d = ca. 140 °C for 14 and fifteen). The synthesis of an extensive series of arylthio substituted silylenes 17a–c was reported (Scheme ​ 2).28, 30 Bis(arylthiolato)silylene :Si(SAr^Me6)_ii (17a) [Ar^Me6 = ii,six‐(two,4,half-dozen‐Me₃C_half-dozenH₂)₂C₆H₃] was synthesized by the reduction of Br₂Si(SAr^Me6)₂ 17a with Jones' Mg(I)–Mg(I) circuitous [(Nacnac)Mg]₂ {Nacnac = HC‐[(Me)CN(Mes)]_ii}.28 Silylene 17a was found to be stable up to 146 °C. However, attempts to synthesize the silylenes :Si(SAr ^iPr4)₂ (17b) [Ar ^iPr4 = ii,6‐(2,6‐ ⁱ Pr_iiC₆H_iii)₂C₆H₃] and :Si(SAr ^iPr6)_ii (17c) [Ar ^iPr6 = 2,six‐(ii,4,half-dozen‐ ⁱ Pr₃C_sixH_two)_iiC₆H₃] were unsuccessful due to the bulkier substituents.xxx Alternatively, the reduction of the dibromobisthiolato Si(IV), Br_iiSi(SAr ^iPr4)_two 16b and Br_twoSi(SAr ^iPr6)_ii 16c, with Rieke'southward magnesium31 and a catalytic corporeality of anthracene afforded the silylenes :Si(SAr ^iPr4)₂ (17b) and :Si(SAr ^iPr6)_two (17c). The endeavor to synthesize the silylene :Si(SAr^Me6)₂ (17a) by the reduction using only Rieke's magnesium was unsuccessful. Merely amide substituted silylene :Si(TBoN)_ii (19) stabilized by two extremely beefy boryl‐amido ligands, [Due north(SiMe₃){B(DAB)}]^– [TBoN; DAB = (DippNCH)₂], was synthesized by the groups of Jones and Aldridge (Scheme ​ 3).32 The reaction of Li[TBoN] eighteen with the dichlorosilylene :SiCl_ii(IPr) (ix) [IPr = :C(HCNDipp)₂] gave a mixture of the silylene :Si(TBoN)_two 19 and IPr in a ratio of 1:1. Silylene nineteen is thermally stable both in the solid state (one thousand.p. 152–160 °C) and in hydrocarbon solutions at room temperature. Recently, our group succeeded in isolating the first example of acyclic neutral silanone which comprise a planar iii‐coordinate Si cantlet.33 Interestingly, while silanone 20 is stable in the solid state at ambient temperature, a solution (C_half-dozenD₆ or [D_viii]THF) of xx is fully converted into the imino(siloxy)silylene :Si(OSi ^t Bu₃)(IPrN) (21) at room temperature within 48h (Scheme ​ 4). The Aldridge group reported the bis(boryloxy) silylene :Si[(HCDippN)₂BO]₂ (24) (Scheme ​ five). The reaction of 22 with half an equivalent of SiI₄ furnished the intermediate [(HCDippN)_twoBO]₂SiI₂ 23. Subsequent reduction of 23 with Jones' reagent at 80 °C resulted in the formation of the bis(boryloxy) silylene 24 which is thermally stable at eighty °C over several days.34 The Rivard grouping utilized a bulky vinylic ligand [(^MeIPr)CH]^–, (^MeIPr = (MeCNDipp)₂C) to generate the vinyl(silyl)silylene 27 stabilized past a carbon‐based donor. The handling of a toluene solution of (^MeIPrCH)SiBr_iii (26) with two equivalents of [K(THF)₂][Si(SiMe_three)₃] afforded the silylene 27 (Scheme ​ vi).35 Compound 27 is remarkably stable both in the solid state (m.p. 110–112 °C) and in benzene solution at ambience temperature for a couple of months.

Synthesis of amido(boryl)silylene fourteen and amido(silyl)silylene xv.

Synthesis of bis(arylthiolato)silylenes 17a–c.

Synthesis of bis(amido)silylene 19.

Synthesis of imino(siloxy)silylene 21.

Synthesis of bis(boryloxy) silylene 24.

Synthesis of vinyl(silyl)silylene 27.

Some disilenes (R₂Si=SiR_ii) are in equilibrium with their monomeric silylene (:SiR₂) form in solution and are therefore synthetically equivalent to silylenes.36, 37 The dissociation energy of the Si=Si double bond in disilenes depends on the singlet–triplet energy gap of the corresponding monomer, therefore the nature of substituents at the silicon cantlet play of import roles. Whilst electropositive and π‐accepting substituents stabilize the triplet country and the Si=Si double bond in the disilenes, electronegative and π‐donating substituents stabilize the singlet and induce to the dissociation of disilenes into the corresponding silylenes.36 Additionally, in some disilenes, the Si=Si bond is weakened past the steric repulsion betwixt the sterically bulky substituents, leading to the dynamic disilene‐silylene equilibria.37 The beginning example of the disilene‐silylene equilibrium was reported in 1993.38 Disilenes Tbt(Mes)Si=Si(Mes)Tbt (28, 29) (Tbt = 2,4,vi‐[CH(SiMe₃)₂]_iiiC_viH₂) bearing sterically bulky substituents was found to undergo thermal dissociation which resulted in the formation of the respective silylene :Si(Mes)Tbt (xxx) (Scheme ​ 7). It was found that the silylene :Si(Mes)Tbt (xxx) tin be trapped with aryl isocyanides ArCN [Ar = Tbt, two,4,6‐(CHMe₂)₃C₆H₂, or 2,4,six‐ ^t Bu₃C₆H₂] to give the corresponding silylene‐isocyanide complexes [Tbt(Mes)Si‐CNAr], which are the showtime isolable silylene‐Lewis base complexes and can human action equally a masked silylene.[38c] In improver, some other silicon compounds behave as masked silylenes.39 The groups of Scheschkewitz and Rzepa reported the isolation of a disilenyl silylene stabilized by an NHC (33) which coexists in equilibrium with the isomeric cyclotrisilene 31 and the free NHC 32 in solution (Scheme ​ 8).[39a], [39b] With regard to this field, our group has demonstrated that DMAP‐stabilized silylenes, :Si(SiR₃)(SiR'₃)(DMAP) [36a: SiR_iii = Si ^t Bu_iii, SiR'₃ = Si(SiMe_iii)_three, 36b: SiR₃ = SiR'₃ = SiMe ^t Bu₂, 36c: SiR₃ = SiR'₃ = Si(SiMe₃)_iii] (DMAP = 4‐N,N‐dimethylaminopyridine), act equally masked silylenes and react with small molecules such as H_ii and ethylene at 65 °C (Scheme ​ 9).[39c] In add-on, the dynamic equilibria between silepins 38a–b and imino(silyl)silylenes :Si(IPrN){Si(SiMe₃)₃} (37a) and :Si(IPrN)( ^t Bu₃) (37b) are revealed by experimental and computational studies (Scheme ​ x).33, xl The intramolecular insertion reaction of the in situ generated silylenes 37a and 37b into the C=C bail of the effluvious ligand framework resulted in the formation of silepins 38a and 38b. Reactivity studies revealed that silepin 38a, serving every bit silylene 37a in situ, is capable of the activation of small molecules (vide infra). Our group also presented an isolable bis(silyl)silylene 40 which is in equilibria with the corresponding tetrasilyldisilene 41. The reaction of dibromosilane {(TMS)_iiiSi}( ^t Bu₃Si)SiBr₂ (39) with KC_eight afforded an equilibrium mixture of forty and 41 (Scheme ​ eleven).41 These compounds 38 and 41 tin can comport as synthetic alternatives to acyclic silylenes.

Dynamic equilibrium between disilenes (28 and 29) and silylene xxx.

Equilibrium of cyclotrisilene 31 and N‐heterocyclic carbene 32 with NHC‐coordinated disilenyl silylene 33.

Synthesis of DMAP‐stabilized silylenes 36a–c.

Dynamic equilibria betwixt silepins 38a–b and imino(silyl)silylenes 37a–b.

Dynamic equilibrium between disilene 41 and bis(silyl)silylene 40.

three. Properties

Several examples of stable 2‐coordinate acyclic silylenes take been structurally characterized using single‐crystal X‐ray diffraction analysis. The information tin can help to explain the compound's reactivity and stability. Selected structural parameters of these acyclic silylenes are shown in Table ​ 1.

Table 1

Selected structural parameters, spectroscopic data, and Human being–LUMO and singlet–triplet energy gaps for the acyclic silylenes

	∠Due east–Si–Eastward'/°	d(Si–E)/Å	29Si chemical shifts (δ)	λ max[nm]	Homo–LUMO gap [eV]	Singlet–triplet gap (kJ/mol)
:Si{B(NDippCH)2}{N(SiMe3)‐Dipp} (fourteen)	B–Si–North	Si–B: 2.066(1)	+439.seven	–	2.04a	103.9a
	109.seven(1)	Si–Northward: 1.731(1)
:Si(SArMe6)2 (17a)	S–Si–S	Si–Due south: two.1607(five)	+285.v	382	4.26b	–
	90.519(19)	Si–South: 2.1560(5)
:Si(SAr iPr4)2 (17b)	S–Si–Due south	Si–S: 2.137(one)	+270.4	405	–	–
	85.08(5)	Si–South: 2.137(1)
:Si(SAr iPr6)2 (17c)	S–Si–S	Si–Due south: 2.089(9)c	+270.9	411	–	–
	84.8(one)
:Si{Si(SiMe3)3}{N(SiMe3)‐Dipp} (15)	Northward–Si–Si	Si–Si: 2.386(ane)	+438.2	–	one.99a	103.7a
	116.91(five)	Si–N: ane.720(i)	+467.five
:Si(TBoN)ii (19)	Due north–Si–N	Si–N: i.7495(10)	+204.6	385	2.55d	158.3d
	110.94(five)	Si–N: 1.7432(x)
:Si(OSi t Buthree)(IPrN) (21)	Northward–Si–O	Si–N: one.661(2)	+58.9	328	4.33e	–
	103.56(8)	Si–O: 1.643(i)
:Si[(HCDippN)2BO]2 (24)	O–Si–O	Si–O: ane.6074(xiv)	+35.five	348	v.45f	–
	100.02(8)	Si–O: 1.6052(14)
:Si(MeIPrCH){Si(SiMe3)3} (27)	C–Si–Si	Si–C: i.798(2)	+432.nine	583	4.79chiliad	–
	101.59(vii)	Si–Si: 2.404(one)
:Si(IPrN){Si(SiMe3)3} (37a)	–	–	+300.0	612	2.96h	103.nineh
:Si(Si t Bu3){Si(SiMe3)3} (40)	–	–	–	–	4.18i	10.5i
:Si[Ga(Br)L]2 (46)	–	–	–	–	2.7j	5.9j

The angles for the silicon centers in silylenes fourteen, 15, and nineteen [109.7(1)° (14), 116.91(5)° (xv), 110.94(v)° (nineteen)] are wider than those in NHSi'due south [86.02(half-dozen)°–99.31(5)°].8, ten, 11, 12, thirteen Silylenes bearing electronegative siloxy (21) and boryloxy (24) substituents and the vinyl(silyl)silylene (27) showroom narrower bond angles [103.56(eight)° (21); 100.02(8)° (24); 101.59(7)° (27)] than those found in other reported acyclic silylenes. These results suggested the presence of a loftier caste of southward‐character at the alone pair of the silicon center in silylenes. While the angles for the silicon center in the imino(siloxy)silylene 21 and the bis(boryloxy)silylene 24 are similar to each other, the Si–O distances in 24 [i.6074(14) Å and 1.6052(14) Å] are relatively shorter than that in 21 [1.643(1) Å]. This is probable because of the more strongly electron‐donating NHI (N‐heterocyclic imine) ligand leading to the dominant p‐donor contribution in 21 while 24 has two O‐donor ligands. Owing to oxygen's small atomic radius, the birdbrained O–Si–O angle in the bis(boryloxy)silylene 24 [100.02(8)°] is wider than the South–Si–S angles in the bis(arylthiolato)silylenes 17a–c [90.52(two)° (17a), 85.08(5)° (17b), 84.8(1)° (17c)] due to the increased steric repulsion between the beefy boryloxy ligands and charge repulsion between the lone pairs. Additionally, the geometry of silylenes has an issue on the HOMO–LUMO energy, as silylenes which have wider Eastward–Si–E' angles tend to exhibit smaller HOMO–LUMO energy gaps (Table ​ 1). Previous computational studies imply silylenes which exhibit small HOMO–LUMO gaps and coordinative flexibility should exist ideal for selective activation of relatively unreactive small molecules.

Further information regarding the chemical bonding in silylenes is gained by the ²⁹Si NMR spectrum (Tabular array ​ 1). The ²⁹Si NMR chemical shifts for the 2‐coordinate silicon heart of the acyclic bis(amido)silylene nineteen (+204.half-dozen ppm) is downfield shifted relative to those in NHSi'due south (δ = 78–119 ppm).43 In the example of NHSi's, the lone pairs on the nitrogen atoms are parallel to the empty 3p orbital on the silicon atom leading to an efficient π‐overlap and increased shielding of the silylene resonance. Similarly, bis(arylthiolato)silylenes 17a–c and imino(silyl)silylene 37a testify a downfield shift in the ²⁹Si NMR spectrum [+285.five (17a), +270.four (17b), +270.nine (17c), +300.0 (37a) ppm]. These results suggested much less π‐donation from the sulfur or nitrogen atoms to the vacant p orbital on the silicon atom relative to that in NHSi's. Furthermore, amido(boryl)silylene 14, amido(silyl)silylene 15, and vinyl(silyl)silylene 27, which have the wider E–Si–Eastward' angle [101.59(7)°–116.91(5)°] compared with those of the bis(arylthiolato)silylenes 17a–c [84.8(1)°–90.52(2)°] and NHSi's [86.02(half-dozen)°–99.31(5)°], show an enormous downfield betoken [+439.7 (xiv), +438.2, +467.5 (15), +432.9 (27)] in the ²⁹Si NMR spectrum. The results indicate a significantly big electrophilicity of the divalent silicon eye which is reminiscent of that observed in the dialkyl‐substituted cyclic silylene 6 (567.4 ppm).xiv The ²⁹Si NMR spectrum of 21 and 24 feature a significantly highfield resonance [+58.9 (21), +35.5 (24) ppm] compared to other dicoordinate acyclic silylenes, which suggests additional π‐donation by the siloxy or boryloxy ligands.

4. Minor Molecule Activation with Acyclic Silylenes

Equally mentioned in the introduction, depression‐valent main‐group compounds such as silylenes bearing a loftier energy Human being and an energetically accessible LUMO can mimic the reactivity of transition element complexes. The ability of transition metals to facilitate the activation of pocket-size molecules (H₂, CO, alkenes etc.) has enabled the widespread development of homogeneous transition element catalysis. Recently, fundamental catalytic reaction steps (oxidative add-on, insertion reactions, reductive elimination) have been reported for the depression‐valent main‐grouping compounds.two However, the cleavage of rigid σ‐bonds such as H–H past main‐group compounds remain scarce. Silylenes are of extremely high reactivity due to the high‐energy lonely‐pair on silicon (HOMO) and the depression‐lying vacant p orbital (LUMO) that enable to activate these minor molecules. Acyclic silylenes are expected to exhibit high reactivity relative to their cyclic counterparts due to their wide Due east–Si–E' angles and small Human being–LUMO gaps. In this section, the pocket-size molecule activation past two‐coordinate acyclic silylenes is outlined.

4.1 Activation of H₂

The cleavage of dihydrogen is a key step in numerous homogeneous catalytic processes such every bit the hydrogenation of unsaturated organic compounds and hydroformylation reactions.44 Additionally, the adsorption/regeneration of H₂ is of import processes in potential hydrogen storage materials.45 This desirable reactivity towards H₂ is by and large mediated by transition metals, however it has recently been demonstrated that reduced main‐group centers exhibit this reactivity as well.

Previous theoretical studies on the reactivity of a variety of cyclic and acyclic silylenes towards the H_two activation revealed that the electronic construction features (Human being–LUMO or singlet–triplet energy gaps) in these silylenes have outcome on the accessibility of H₂ activation.23 N‐Heterocyclic silylenes, bis(arylthiolato)silylene :Si(SAr^Me6)_two (17a),28 bis(amido)silylene :Si(TBoN)₂ (19),32 and imino(siloxy)silylene :Si(OSi ^t Bu₃)(IPrN) (21)33 have shown no reaction toward H₂ due to their large Human–LUMO gaps (and its heavily sterically protected silylene center). The first case of the H_ii activation with a silylene was reported in 2012. Amido(boryl)silylene :Si{B(NDippCH)₂}{North(SiMe_iii)‐Dipp} (14) was found to undergo H₂ activation at mild conditions, affording the dihydrosilane H₂Si{B(NDippCH)₂}{Due north(SiMe₃)‐Dipp} (42) (Scheme ​ 12).27 Similarly, amido(silyl)silylene :Si{Si(SiMe₃)₃}{N(SiMe_iii)‐Dipp} (15) has shown reaction towards H₂ at ambient temperatures to yield the respective dihydrosilane H₂Si{Si(SiMe₃)₃}{Northward(SiMe_iii)‐Dipp} (43).29 More recently, our group performed the activation of H₂ with the N‐heterocyclic imino‐ligated silepin 38a, serving as silylene 37a in situ, which resulted in the formation of the corresponding dihydrosilane 44.40 The π‐altruistic substituents (amido or NHI ligands) in silylenes fourteen, xv, and 37a lead to a decreased HOMO–LUMO gap [two.04 eV (xiv), 1.99 eV (15), 2.96 eV (37a)], which allows for the activation of inert molecules. Furthermore, the disilene 41/silylene 40 equilibrium mixture was also constitute to activate H_two under very mild conditions (–40 °C).41 Interestingly, the Homo–LUMO free energy gap in the singlet bis(silyl)silylene 40 (4.18 eV) is comparably larger than those of acyclic silylenes, which are able to activate H₂, and similar to those of acyclic silylenes 17a (4.26 eV) and 21 (iv.33 eV), which have shown no reaction toward H₂. The singlet–triplet energy gap (10.five kJ/mol) for bis(silyl)silylene xl is modest due to the effect of the electropositive bulky silyl substituents. Very recently, the groups of Schulz and Schreiner reported H₂ splitting by a silylene intermediate :Si[Ga(Br)L]₂ (L = HC[C(Me)N(2,six‐ ⁱ Pr₂C_sixH₃)]_ii) (46).46 The treatment of [L(Br)Ga]₂SiBr_two with an equimolar corporeality of LGa at 60 °C under H_ii resulted in the formation of the dihydrosilane H₂Si[Ga(Br)50]₂ (47). Silylene 46 exhibits the lowest HOMO–LUMO gap energy (ii.7 eV) and smallest singlet–triplet gap (5.9 kJ/mol).

Activation of H_two by acyclic silylenes. *Estimated yield by NMR spectroscopy.

four.2 Activation of NH_three

Although many examples of H₂ activation past transition metallic complexes accept been reported, Due north–H bond activation is more than challenging as Werner‐type complexes are readily formed in the reaction with Lewis basic amines.47 The activation of the North–H bonds of ammonia has attracted attention for applications in catalytic transformations such as hydroamination. While very few of examples of N–H bond activation by transition metal complexes are known, it has been revealed that many low‐valent principal‐group compounds undergo such activation processes.[47b]

Bis(amido)silylene 19 reacts with NH₃ to yield triaminosilane 50 together with the secondary amine TBoNH (48) (Scheme ​ 13).32 The plausible machinery involves the formation of diaminosilylene 49 via a σ‐bond metathesis reaction between 19 and NH₃, followed by the oxidation addition to NH₃ to afford the triaminosilane 50.48 This observation is in agreement with a previously computed σ‐bond metathesis H_two activation pathway mediated by silylene.24 Recently, our group reported the reactivity of the imino(siloxy)silylene 21.49 The reaction of silylene 21 with i equivalent of NH₃ affords the hydroamination production 51. It is of annotation that chemical compound 51 even reacts with excessive amounts of NH₃ to yield an unidentified mixture. In the reaction, IPrNH and (H₂N)( ^t Bu₃SiO)Si(H)(NH_two) were formed, like σ‐bail metathesis reaction to that observed for the bis(amido)silylene 19.

Activation of NH₃ by acyclic silylenes.

iv.three C–O Bond Activation

Carbon dioxide is a stiff greenhouse gas in the temper and a versatile feedstock for chemic or material production.50 To date, many studies on carbon capture and storage (CCS) of CO₂ along with its chemical activation and utilization as a C1 source have been demonstrated. While transition metals have been utilized in most CO₂ activation, the development of transition‐metal gratis and eco‐friendly systems have been underexplored. Currently, some low‐valent silicon compounds which undergo CO₂ activation accept been reported.51

Our grouping found that silepin 38a, which behaves as fallow form of imino(silyl)silylene 37a, chop-chop reacts with CO₂ under mild conditions (1 atm, r.t., within 1h) to afford the corresponding silicon carbonate 53 (Scheme ​ xiv).40 Comparable to mechanisms described in literature,[51e], 52 information technology is plausible that the transient silanone (O=Si(IPrN){Si(SiMe₃)₃}) was formed by the oxidative addition of CO₂ and extrusion of CO, followed by the cycloaddition of another molecule of CO_two. While such compounds tend to dimerize,51 our group demonstrated that the isolation of the outset four‐coordinate, monomeric silicon carbonate 53 in loftier yields. The reaction of 14 with CO₂ under mild conditions (1 atm, r.t.) resulted in the formation of the (trimethylsiloxy)iminosilane {(HCDippN)₂B}Si(NDipp)(OSiMe₃) (52).53 It is plausible that the in situ generation of the silanone [O=Si{B(NDippCH)₂}{Due north(SiMe₃)‐Dipp}],54 followed by silyl group migration than the bimolecular reaction with CO_two yields the carbonate.55 Silylene xiv also reacts with CO at ambient temperature to yield 54 which was characterized past standard spectroscopic techniques and X‐ray crystallographic assay (Scheme ​ xv).53 Compound 54 contains two Si(Iv) centers, which demark to ane carbon and two oxygen atoms derived from CO, along with the amide ligand. Although the activation of carbon monoxide and formation of the stable carbonyl complexes under mild condition is well known for transition‐metal complexes, such reaction is nearly unknown for main‐group compounds. Recently, the groups of Schulz and Schreiner reported the isolation of the silylene carbonyl complex [50(Br)Ga]₂Si:–CO (55) (50 = HC[C(Me)North(ii,six‐ ⁱ Pr_iiC₆H_iii)]₂). The reaction of GaL with SiBr₄ nether a CO atmosphere, generates the silylene [L(Br)Ga]₂Si: (46) in situ, subsequently affording the silylene carbonyl circuitous 55.46 Compound 55 is remarkably stable both in the solid country (T _d = 176–177 °C) and in solution, no decomposition was observed in toluene solution upwardly to lxxx °C. Furthermore, silylene carbonyl complex 55 acts as a masked silylene and reacts with H₂ to give the dihydrosilane H_iiSi[Ga(Br)L]₂ (47).

Activation of CO_ii by acyclic silylenes.

Activation of CO by acyclic silylenes.

4.four C=C and C≡C Bonds Activation

The activation of modest organic molecules and the germination of C–C bonds is a fundamentally important procedure for transformation of simple molecules into essential chemical compounds in both academia and industry.56 For this purpose, transition metallic catalysts have been utilized and a big number of useful catalysts have been developed. On the ane mitt, the catalytic bail activation and C–C bail formation are challenging for primary group compounds as their oxidation states vary in a much narrower range. Recently, some main group element compounds accept shown the activation of C–C bonds in neutral organic molecules such as alkenes alkynes as well as its dynamic equilibrium that is a key pace in catalytic processes. Furthermore, it was demonstrated that catalytic activation of alkynes, followed past the formation of C–C bonds by utilizing depression‐valent main‐group compounds is too possible.57

Silylenes are well known to undergo cycloaddition reactions with unsaturated C–C bonds.13, 43 Similarly, silylenes 21, 37a, and 40 reacted with ethylene under mild weather condition (1 atm, r.t.) to course the corresponding cycloaddition products 56, 59, and lx (Scheme ​ xvi).33, 40, 41 In the case of silylene fifteen, the reaction with ethylene at ambience temperatures gave the silirane production Si{CH₂–CH₂}{NDipp(SiMe_iii)}{Si(SiMe₃)₃} (57) in high yields. Furthermore, when compound 57 was heated to threescore °C under an ethylene atmosphere, an exceptional insertion of ethylene into Si–Si bail occurred to yield the modified silirane Si{CH₂–CH₂}{NDipp(SiMe₃)}{CH₂–CH₂–Si(SiMe₃)₃} (58).58 A NMR experiment with deuterated ethylene indicated that the reaction proceeds via migratory insertion of the coordinated ethylene into the Si–Si bond, followed by the formation of the silirane with a C_twoD₄ molecule. The groups of Ability and Tuononen found that silylenes 17a and 17b as well react with ethylene or alkynes to afford the [ane+2] cycloaddition products 61a, 61b, and 62b (Scheme ​ 17).59 Interestingly, the ethylene addition products 61a and 61b were constitute to undergo reversible reactions with ethylene nether ambient atmospheric condition. Notably, while many main‐group compounds can react with ethylene nether mild atmospheric condition, the reversible reaction, which is a key pace in catalytic cycles, remains rare.threescore Products 61a and 61b were characterized using NMR spectroscopy (61a and 61b) and 10‐ray crystallographic analysis (61b). Van't Hoff analysis of the association of ethylene with 17b, as determined past variable‐temperature ¹H NMR spectroscopy, revealed a small value of Gibbs gratis energy (ΔOne thousand _assn = –24.9 kJ/mol at 300 Chiliad), which is comparably more favorable compared with that for the reaction of the phosphine supported Si(II) circuitous reported by the groups of Kato and Baceiredo (–3.0 kJ/mol).[60b] Similarly, the rare reversibility between Si(Ii) and Si(4) compounds was plant in silylenes 37a and 37b which undergo an intermolecular insertion reaction into the C=C bond of the aromatic ligand framework to give silepins 38a and 38b. The equilibrium between 37a and 38a was revealed past experimental and computational studies.

Activation of ethylene past acyclic silylenes. *Estimated yield past NMR spectroscopy.

Equilibrium reactions with ethylene and activation of alkynes by acyclic silylenes 17a–b.

four.5 Activation of P_four

White phosphorus (P₄), which is easily obtained by the reduction of phosphate rock, is widely used as a starting material to synthesize organophosphorus compounds. In industrial processes, phosphorus chloride (PCl_{due north}) and phosphoryl chloride (POCl₃) are precursors to organo‐phosphorus products are prepared past the chlorination or oxychlorination of white phosphorus.61 The eco‐friendly and cantlet efficient method of straight conversion of white phosphorus to phosphine containing products has also been considered.62 Currently, it was demonstrated that direct catalytic transformation of P₄ into organo‐phosphorus compounds using a transition element complex is possible.63 However, the stoichiometric and catalytic reaction of white phosphorus under mild conditions is challenging for both transition metal and main group compounds.

While several cyclic silylenes tin react with P₄, in most cases, the oxidative add-on of a single P–P bond at the silicon center is observed,64 and the controlled reaction of P₄ by chief‐grouping compounds remains scarce.[61a], 65 The handling of the vinyl(silyl)silylene 27 with P₄ resulted in the formation of (^MeIPrCH)Si(P₄){Si(SiMe₃)₃} (63) (Scheme ​ 18).35 It is plausible that compound 63 was formed by the oxidative add-on of a P–P bail of P₄ to silylene 27 with subsequent ane,2‐silyl migration. In this reaction, the cleavage of two P–P bonds of P₄ and the regioselective formation of four new Si–P bonds were observed. In addition, imino(siloxy)silylene 21 was found to occur via the oxidative add-on of i equivalent of P_four to give (IPrN)( ^t Bu_iiiSiO)Si(P₄) (64), which is unlike from that observed for the vinyl(silyl)silylene 27.49

Activation of P_four by acyclic silylenes.

five. Summary and Outlook

Since the discovery of Jutzi's silicocene and Denk'due south NHSi, various circadian silylenes and Lewis base stabilized silylenes accept been reported. However, isolable two‐coordinate acyclic silylenes had been considered to be transient and non‐isolable compounds for a long fourth dimension. The recent synthesis of stable acyclic silylenes has unlocked a new avenue in silicon chemistry, enabling metallomimetic behavior of this Earth‐arable element. In this minireview, we mainly focused on the ii‐coordinate acyclic silylene chemical science containing synthesis, properties, and application for modest molecule activation. Acyclic silylenes begetting wide Eastward–Si–Eastward' angles and pocket-size Homo–LUMO gaps are of extremely high reactivity which lead to the activation of important small molecules such as H₂, NH₃, ethylene, and CO₂. Interestingly, while bis(silyl)silylene 40 exhibits a relatively big Man–LUMO gap (4.18 eV), 40 was found to occur the activation of H₂. The singlet–triplet gap of xl is insufficiently small (10.v kJ/mol). In addition, it was revealed that bis(arylthiolato)silylenes 17a and 17b show equilibrium reactions with ethylene at room temperature which is a key pace in catalytic processes and is rare for silicon due to the unfavorable reduction of Si(Four) to Si(II).

These results for acyclic silylenes bespeak the potential of main group compounds for hereafter applications in the realms of catalytic and materials science. This report is inspiring for the molecular design of new silicon compounds which enable small molecule activation and the unprecedented cleavage of N–N bond in dinitrogen, N₂. Acyclic silylenes are of extremely loftier reactivity due to the high‐energy lone‐pair on silicon (Man) and a low‐lying vacant p‐orbital (LUMO) which may interact with an empty π* orbital and an n‐orbital (a lone pair) on N₂ leading to a weakening of the North–N bond. Furthermore, the hydrogenation and transfer hydrogenation of unsaturated molecules such as alkenes and alkynes mediated by acyclic silylenes is expected to be viable. These studies imply that the steric and electronic tuning of the substituents enable the control of hydrogenation reactions. The accessibility of both Si(Ii) and Si(4) oxidation states may lead to the utilization of silicon compounds in catalysts.

Acknowledgements

This project has received funding from the European union's Horizon 2020 inquiry and innovation programme under the Marie Skłodowska‐Curie grant agreement No 754462 (Fellowship SF), as well as the WACKER Chemie AG and the European Enquiry Quango (SILION 637394). Open admission funding enabled and organized by Projekt DEAL.

Biographies

•

Shiori Fujimori obtained her M.Sc. and Ph.D. degree in chemistry in 2019 at Kyoto University under the supervision of Prof. Norihiro Tokitoh on the synthesis of novel effluvious compounds containing heavier group 14 element. In October 2019, she was awarded a Eurotech‐Marie Curie Fellowship for low‐valent silicon chemistry and joined the group of Prof. Inoue at the TU München.

•

Shigeyoshi Inoue studied at the University of Tsukuba and carried out his doctoral studies under the supervision of Prof. Akira Sekiguchi, obtaining his Ph.D. in 2008. As a Humboldt grantee equally well equally a JSPS grantee, he spent the bookish years 2008–2010 at the TU Berlin in the group of Prof. Matthias Drieß. In 2010, he established an contained enquiry grouping within the framework of the Sofja Kovalevskaja program at the TU Berlin. Since 2015 he has been on the faculty at the TU München. His electric current research interests focus on the synthesis, characterization, and reactivity investigation of compounds containing low‐valent main group elements with unusual structures and unique electronic backdrop, with the goal of finding novel applications in synthesis and catalysis. A particular emphasis is placed on low‐valent silicon and aluminium compounds.

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Source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7507849/

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