Phosphasilenes or silylidenephosphanes are a class of compounds with silicon-phosphorus double bonds. [1] Since the electronegativity of phosphorus (2.1) is higher than that of silicon (1.9), the "Si=P" moiety of phosphasilene is polarized. [2] [3] The degree of polarization can be tuned by altering the coordination numbers of the Si and P centers, or by modifying the electronic properties of the substituents. [2] The phosphasilene Si=P double bond is highly reactive, yet with the choice of proper substituents, it can be stabilized via donor-acceptor interaction or by steric congestion. [3]
The landmark discovery of the first phosphasilene by NMR spectroscopy was made in 1984 by Bickelhaupt et al. [4] The first phosphasilene came with bulky aryl substituents at the phosphorus and silicon atoms. [4] Almost a decade after this spectroscopic observation, the first structural characterization of phosphasilene was achieved in 1993 by Niecke et al. [5] The successful isolation of phosphasilenes with silicon-phosphorus double bonds represents one of the discoveries that challenged and disproved the "double-bond rule".
An important synthetic pathway towards phosphasilene is the 1,2-elimination reactions of silylphosphane derivatives. The first metastable arylphosphasilenes were accessed by Bickelhaupt et al. via the deprotonation of in situ formed (chlorosilyl)phosphanes ArP(H)–(Cl)SiAr2 using organolithium bases. [2] [4] [6] [7] As shown in the scheme on the left, it is also possible to use pre-formed lithium phosphanides Ar'P(H)Li as both a phosphorus source and a base. However, the latter synthetic pathway involves formation of primary phosphines ArPH2, which can be difficult of be separated from phosphasilenes. [2] Despite such a drawback, this strategy has been successfully applied by Niecke et al. to obtain a series of 1,3-diphospha-2-silaallyl anions, which serve as precursors for 2-phosphanylphosphasilenes. [2] [8]
Applying an analogous strategy, Driess and coworkers developed an effective approach for synthesizing P-silyl phosphasilenes via the thermal elimination of LiF from corresponding lithium (flouorosilyl)phosphanides. [9] [10] [11]
Phosphasilenes with 4-coordinate silicon, which can also be viewed as silylene-stabilized phosphinidene, can be synthesized based on the reactivities of stable silyene complexes. [2] For example, Inoue and coworkers demonstrated that benzamidinate-stabilized phosphanylsilylene can give rise to corresponding Si- and P-trimethylsilyl-substituted phosphasilene via thermal rearrangement, while the reaction can also yield 4-disila-1,3-diphosphacyclobuta-diene with the addition of a mild chlorinating agent Ph3PCl2. [12] [2] In these phosphasilenes with four-coordinate silicon, even though formally five bonds are drawn around silicon, the Si–P π bonds are calculated to be strongly polarized towards the P atoms. The contribution from phosphorus and silicon to the π orbitals are calculated to be 87.53% and 12.47%, respectively.
The unstable parent phosphasilene H2Si=PH has been generated in the gas phase by the reacting atomic silicon with phosphine PH3, and identified via matrix isolation spectroscopy methods. [14] Density functional theory (DFT) calculations suggests that in the ground state, H2Si=PH exists in a singlet spin state, with Cs-symmetric planar geometry. [14] At the B3LYP/6-311+G** level of theory, the Si=P bond length and the Si-P-H bond angle are calculated to be 2.084 Å and 90.7o. [14] The Si=P bond dissociation energy is 75.0 kcal mol−1 at the B97-D/6-31G(d) level of theory; while the π-bond energy, Dπ(Si=P) is 36.6/35.9 kcal mol−1. [13] The frontier orbitals of the parent phosphasilene consists of the π bonding and π anti-bonding orbitals: π(Si=P) and π*(Si=P) correspond to the HOMO and LUMO, respectively. HOMO-1 was calculated to be the lone pair on phosphorus n(P). [15]
Driess and coworkers prepared thermally stable "half"-parent phosphasilene R2Si=PH (R2Si = (tBu3Si)(iPr3C6H2)Si), which is the first example of phosphasilene with a terminal PH group. [16] This species was obtained as a mixture of E/Z isomers, thus its 31P NMR spectrum featured two doublets with 29Si satellites (δ=123.1, 1J (P, H)=123 Hz, 1J (P, Si)=157 Hz and δ=134.2 ppm, 1J (P, H)=131 Hz, 1J (P, Si)=130 Hz). [16] These 1J (P, H) coupling constants are much smaller compared to those of secondary phosphanes (R2PH) and phosphaalkenes with a PH group, which indicates that the phosphasilene phosphorus atom possesses more 3p character. [16] X-ray crystallography of this "half"-parent phosphasilene species shows that the silicon atom occupies a trigonal-planar coordination environment. [16] The Si=P bond distance was reported to be 2.094(5) Å, which is about 7% shorter than a typical silicon-phosphorus single bond, but only slightly longer than that of P-silyl-substituted phosphasilenes, [16] which suggests that the potential of Si=P bond on a potential energy surface is relatively shallow. [16] [17]
Tamao et al. reported a series of π-conjugated phosphasilenes stabilized by Eind groups (Eind=1,1,3,3,5,5,7,7-octaethyl-s-hydrindacen-4-yl). [18] These systems feature Si=P units that are highly coplanar with the aromatic ring, allowing strong π → π* absorptions. The coplanarity is made possible by the rigidity of the two Eind groups that are oriented trans and perpendicular with respect to the Si=P bond. [18] The Si=P bond length observed by X-ray crystallography are ca. 2.09-2.10 Å, which are typical for phosphasilenes. [18]
The bonding of π-conjugated phosphasilenes has been probed by DFT calculations at the B3LYP/6-31G** level. The HOMO was calculated to represent mostly the 3pπ(Si–P), while the LUMO featured significant contribution from the 3pπ*(Si–P)–2pπ*(phenyl) conjugation. The HOMO-1 orbital involves the 3n–2pπ conjugation, which originate from the presence of lone pair on the phosphorus atom and the π-orbital on the Eind benzene ring. [18]
By installing electron-donating substituents on silicon and electron-withdrawing substituents on phosphorus, the Si=P bond polarization can be decreased and even reversed through the "push-pull" interaction of the substituents with opposing electronic effects. [19] Applying this design strategy, Escudié et al. prepared stable "push-pull" phosphasilene (tBu2MeSi)2Si=PMes* (Mes* = 2,4,6-tri-tertbutylphenyl) with electron-donating silyl groups on Si and an electron-withdrawing aryl group on P. [19] Computations on the model compound (Me3Si)2Si=PMes (Mes = 2,4,6-trimethylphenyl) demonstrate that n(P) and π*(Si=P) correspond to the HOMO and LUMO, respectively. [19] The relatively small energy gap between the interacting occupied (n(P)) and vacant (π*(Si=P)) molecular orbitals gives rise to a large paramagnetic contribution, [20] which explains the extreme deshielding of the doubly bonded Si and P atoms, as well as the red shift in the UV spectrum that are observed in (tBu2MeSi)2Si=PMes*. [19] In the "push-pull" phosphasilene prepared by Escudié and co-workers, the Si=P bond length was reported to be 2.1114(7) Å, which is longer than what was observed in most other reported phosphasilenes (2.062(1)–2.094(5) Å). [19]
Driess et al. demonstrated that stable metallophosphasilenes of the type R2Si=PM can be prepared from metalation reaction of "half"-parent phosphasilenes R2Si=PH (R2Si = (tBu3Si)(iPr3C6H2)Si). [16] NMR spectroscopic studies demonstrated that metalation led to the deshielding of the 31P nucleus, while the low coordinate 29Si atom in the metallophosphasilene became more shielded. Driess and coworkers explained this observation by proposing that the stabilization of the non-bonding orbital at phosphorus through n(P) → σ*(Si-Si) hyperconjugation is more effective after metalation. [16] This is due to the higher negative partial charge at the phosphorus atom in the metallophosphasilene. As shown in the scheme below, this shielding effects is analogous to what has been observed for related alkali-metal-substituted disilenylides of the type [M(R3Si)Si=Si(SiR3)2]. [16] [21] [22] The Si=P distance in the metallophosphasilene that Driess et al. synthesized was reported to be 2.064(1) Å, which is significantly shorter than that of the "half"-parent compound (R2Si=PH) from which R2Si=PM was derived. [16] This contraction of the Si=P bond, together with a slight elongation of the Si–Si bond and a shrinkage of the P-Si-Si angle, has been rationalized by the increased hyperconjugative interactions in the R2Si=PM system. [16]
Due to the n(N)–π*(Si=P) orbital interaction, there exists strong delocalization of electron density from nitrogen to phosphorus in phosphasilene with amino-substituents at silicon. [2] Among the resonance structures shown in the figure on the left, the structure in the middle with zwitterionic structure was calculated to have a significant contribution. [2] Therefore, compared to the parent phosphasilene, the double-bond character of the Si–P is significantly reduced in phosphasilene with amino-substituents at Si, giving rise to longer Si–P bond distance.
Due to the higher electronegativity of phosphorus and the polarized nature of the Si=P moiety, phosphasilene tend to react with Lewis acids and bases at the phosphorus atom and silicon atom, respectively. The reaction of phosphasilene with Lewis acids, which usually occurs at the Lewis-basic two-coordinate phosphorus atom, is also dependent upon the nature of the reactants. [2]
Lewis bases have been shown to react with both three-coordinate and four-coordinate silicon atoms. The coordination of Lewis bases to the three-coordinate silicon atom of phosphasilene can lead to effective stabilization of the Si=P moiety. [2] For example, it has been demonstrated that unstable phosphasilene can react with DMAP and small N-heterocyclic carbene (NHC) to afford the corresponding stable complexes. [23] In phosphasilenes stabilized by NHC, the Si=P bonds are elongated and the negative charge gets localized on the phosphorus atom. Among the resonance structures shown in the figure below, Natural Resonance Theory (NRT) indicates that the third structure with a Si–P single bond is predominant, carrying a resonance weight of 76.3%. [2] [24] Therefore, these stabilized phosphasilene can also be interpreted as a silyene-phosphinidene adduct. [2]
For donor-stabilized phosphasilene with four coordinate silicon, the reaction with Lewis bases may lead to ligand exchange at the silicon atom. [25]
Metalation of phosphasilenes gives rise to either complexes featuring the coordination of the phosphorus lone pair to a metal center [26] [3] [24] or P-metalated phosphasilenes. [27] [16] [28] In the former case, the binding of the phosphasilenes to transition metals via the phosphorus lone pair reduces the double-bond character of the Si–P bond. [2] Some examples of this type of phosphasilene transition metal complexes are shown below.
Driess and coworkers first observed the formation of P-metalated phosphasilenes of the latter case: P-ferrio-substituted phosphasilene R2Si=P[Fe(CO)2(η5-C5H5)] (R = 2,4,6-iPr3C6H2). [27] They further demonstrated that P-metalated phosphasilenes R2Si=PM can be obtained by metalating "half"-parent phosphasilenes, which substitutes the R2Si=PH hydrogen atom with transition metal-containing fragments. [3] [16]
Phosphasilenes with significant zwitterionic characters undergoes facile hemolytic cleavage of the fragile Si=P bond. This can be utilized for the liberation and transfer of phosphinidene (:PH) to unsaturated organic molecules. [29] Driess et al. demonstrated that a fragile "half"-parent phosphasilene LSi=PH (L = CH[(C=CH2)CMe(NAr)2]; Ar = 2,6-iPr2C6H3) with highly shielded PH moiety is capable of transferring :PH to NHC. [29]
Theoretical investigation by DFT (B3LYP/6-31G(d) level) revealed that this phosphasilene bears two highly localized lone pairs on the phosphorus atom due to the LSi=PH ↔ LSi–P+H− resonance. Based on natural bond orbital (NBO) analysis, the σ bond of Si=P involves even contributions from Si and P, while the π bond (HOMO-1) is strongly polarized to the phosphorus atom. This indicates that the π bond between silicon and phosphorus is not predominant, supporting the significance of the zwitterionic resonance structure in the description of Si–P bonding. [29]
The Si=P moiety of phosphasilene has been reported to demonstrate small molecule activation reactivities analogous to those observed in Si=Si, P=C, and other heavier alkene analogues. For example, phosphasilene with silyl substituents has been shown to activate white phosphorus (P4) under relatively mild reaction conditions to form 1,2,3-triphospha-4-silabicyclo[1.1.0]butanes, [9] which is similar to disilenes' reactivity. [31] [2] Analogous to the behavior of phosphaalkenes, [32] phosphasilene can also activate chalcogens such as S8 and Te to form unstable three-membered ring compounds. [7]
Using a fragile zwitterionic "half-parent" phosphasilene L'Si=PH (L' = CH[(C=CH2)CMe(NAr)2], Ar = 2,6-iPr2C6H3), Driess and coworkers demonstrated that an unusual N-H activation reactivity can be achieved by the Si=P moiety in ammonia, affording L'Si(NH2)PH2 species. [23] This represent a rare example of catalyst-free 1,2-hydroamination reaction that's been reported in heavier alkene analogues. [2] [33] [34]
Tetrahedrane is a hypothetical platonic hydrocarbon with chemical formula C4H4 and a tetrahedral structure. The molecule would be subject to considerable angle strain and has not been synthesized as of 2021. However, a number of derivatives have been prepared. In a more general sense, the term tetrahedranes is used to describe a class of molecules and ions with related structure, e.g. white phosphorus.
A silabenzene is a heteroaromatic compound containing one or more silicon atoms instead of carbon atoms in benzene. A single substitution gives silabenzene proper; additional substitutions give a disilabenzene, trisilabenzene, etc.
Silylene is a chemical compound with the formula SiH2. It is the silicon analog of methylene, the simplest carbene. Silylene is a stable molecule as a gas but rapidly reacts in a bimolecular manner when condensed. Unlike carbenes, which can exist in the singlet or triplet state, silylene (and all of its derivatives) are singlets.
Organotin chemistry is the scientific study of the synthesis and properties of organotin compounds or stannanes, which are organometallic compounds containing tin–carbon bonds. The first organotin compound was diethyltin diiodide, discovered by Edward Frankland in 1849. The area grew rapidly in the 1900s, especially after the discovery of the Grignard reagents, which are useful for producing Sn–C bonds. The area remains rich with many applications in industry and continuing activity in the research laboratory.
A transition metal carbene complex is an organometallic compound featuring a divalent organic ligand. The divalent organic ligand coordinated to the metal center is called a carbene. Carbene complexes for almost all transition metals have been reported. Many methods for synthesizing them and reactions utilizing them have been reported. The term carbene ligand is a formalism since many are not derived from carbenes and almost none exhibit the reactivity characteristic of carbenes. Described often as M=CR2, they represent a class of organic ligands intermediate between alkyls (−CR3) and carbynes (≡CR). They feature in some catalytic reactions, especially alkene metathesis, and are of value in the preparation of some fine chemicals.
Diphosphene is a type of organophosphorus compound that has a phosphorus–phosphorus double bond, denoted by R-P=P-R'. These compounds are not common but are of theoretical interest. Normally, compounds with the empirical formula RP exist as rings. However, like other multiple bonds between heavy main-group elements, P=P double bonds can be stabilized by a large steric hindrance from the substitutions. The first isolated diphosphene bis(2,4,6-tri-tert-butylphenyl)diphosphene was exemplified by Masaaki Yoshifuji and his coworkers in 1981, in which diphosphene is stabilized by two bulky phenyl group.
In chemistry, a phosphaalkyne is an organophosphorus compound containing a triple bond between phosphorus and carbon with the general formula R-C≡P. Phosphaalkynes are the heavier congeners of nitriles, though, due to the similar electronegativities of phosphorus and carbon, possess reactivity patterns reminiscent of alkynes. Due to their high reactivity, phosphaalkynes are not found naturally on earth, but the simplest phosphaalkyne, phosphaethyne (H-C≡P) has been observed in the interstellar medium.
In chemistry, the double bond rule states that elements with a principal quantum number (n) greater than 2 for their valence electrons (period 3 elements and higher) tend not to form multiple bonds (e.g. double bonds and triple bonds). The double bonds, when they exist, are often weak due to poor orbital overlap between the n>2 orbitals of the two atoms. Although such compounds are not intrinsically unstable, they instead tend to polymerize. An example is the rapid polymerization that occurs upon condensation of disulfur, the heavy analogue of O2. Numerous violations to the rule exist.
Carbene analogs in chemistry are carbenes with the carbon atom replaced by another chemical element. Just as regular carbenes they appear in chemical reactions as reactive intermediates and with special precautions they can be stabilized and isolated as chemical compounds. Carbenes have some practical utility in organic synthesis but carbene analogs are mostly laboratory curiosities only investigated in academia. Carbene analogs are known for elements of group 13, group 14, group 15 and group 16.
Germylenes are a class of germanium(II) compounds with the general formula :GeR2. They are heavier carbene analogs. However, unlike carbenes, whose ground state can be either singlet or triplet depending on the substituents, germylenes have exclusively a singlet ground state. Unprotected carbene analogs, including germylenes, has a dimerization nature. Free germylenes can be isolated under the stabilization of steric hindrance or electron donation. The synthesis of first stable free dialkyl germylene was reported by Jutzi, et al in 1991.
Phosphinidenes are low-valent phosphorus compounds analogous to carbenes and nitrenes, having the general structure RP. The "free" form of these compounds is conventionally described as having a singly-coordinated phosphorus atom containing only 6 electrons in its valence level. Most phosphinidenes are highly reactive and short-lived, thereby complicating empirical studies on their chemical properties. In the last few decades, several strategies have been employed to stabilize phosphinidenes, and researchers have developed a number of reagents and systems that can generate and transfer phosphinidenes as reactive intermediates in the synthesis of various organophosphorus compounds.
tert-Butylphosphaacetylene is an organophosphorus compound. Abbreviated t-BuCP, it was the first example of an isolable phosphaalkyne. Prior to its synthesis, the double bond rule had suggested that elements of Period 3 and higher were unable to form double or triple bonds with lighter main group elements because of weak orbital overlap. The synthesis of t-BuCP discredited much of the double bond rule and opened new studies into the formation of unsaturated phosphorus compounds.
Hydrophosphination is the insertion of a carbon-carbon multiple bond into a phosphorus-hydrogen bond forming a new phosphorus-carbon bond. Like other hydrofunctionalizations, the rate and regiochemistry of the insertion reaction is influenced by the catalyst. Catalysts take many forms, but most prevalent are bases and free-radical initiators. Most hydrophosphinations involve reactions of phosphine (PH3).
Digermynes are a class of compounds that are regarded as the heavier digermanium analogues of alkynes. The parent member of this entire class is HGeGeH, which has only been characterized computationally, but has revealed key features of the whole class. Because of the large interatomic repulsion between two Ge atoms, only kinetically stabilized digermyne molecules can be synthesized and characterized by utilizing bulky protecting groups and appropriate synthetic methods, for example, reductive coupling of germanium(II) halides.
Diphosphagermylenes are a class of compounds containing a divalent germanium atom bound to two phosphorus atoms. While these compounds resemble diamidocarbenes, such as N-heterocyclic carbenes (NHC), diphosphagermylenes display bonding characteristics distinct from those of diamidocarbenes. In contrast to NHC compounds, in which there is effective N-C p(π)-p(π) overlap between the lone pairs of planar nitrogens and an empty p-orbital of a carbene, systems containing P-Ge p(π)-p(π) overlap are rare. Until 2014, the geometry of phosphorus atoms in all previously reported diphosphatetrylenes are pyramidal, with minimal P-Ge p(π)-p(π) interaction. It has been suggested that the lack of p(π)-p(π) in Ge-P bonds is due to the high energetic barrier associated with achieving a planar configuration at phosphorus, which would allow for efficient p(π)-p(π) overlap between the phosphorus lone pair and the empty P orbital of Ge. The resulting lack of π stabilization contributes to the difficulty associated with isolating diphosphagermylene and the Ge-P double bonds. However, utilization of sterically encumbering phosphorus centers has allowed for the isolation of diphosphagermylenes with a planar phosphorus center with a significant P-Ge p(π)-p(π) interaction.
Trisilaallene is a subclass of silene derivatives where a central silicon atom forms double bonds with each of two terminal silicon atoms, with the generic formula R2Si=Si=SiR2. Trisilaallene is a silicon-based analog of an allene, but their chemical properties are markedly different.
An N-Heterocyclic silylene (NHSi) is an uncharged heterocyclic chemical compound consisting of a divalent silicon atom bonded to two nitrogen atoms. The isolation of the first stable NHSi, also the first stable dicoordinate silicon compound, was reported in 1994 by Michael Denk and Robert West three years after Anthony Arduengo first isolated an N-heterocyclic carbene, the lighter congener of NHSis. Since their first isolation, NHSis have been synthesized and studied with both saturated and unsaturated central rings ranging in size from 4 to 6 atoms. The stability of NHSis, especially 6π aromatic unsaturated five-membered examples, make them useful systems to study the structure and reactivity of silylenes and low-valent main group elements in general. Though not used outside of academic settings, complexes containing NHSis are known to be competent catalysts for industrially important reactions. This article focuses on the properties and reactivity of five-membered NHSis.
Plumbylenes (or plumbylidenes) are divalent organolead(II) analogues of carbenes, with the general chemical formula, R2Pb, where R denotes a substituent. Plumbylenes possess 6 electrons in their valence shell, and are considered open shell species.
1-Phosphaallenes is are allenes in which the first carbon atom is replaced by phosphorus, resulting in the structure: -P=C=C<.
Negative hyperconjugation is a theorized phenomenon in organosilicon compounds, in which hyperconjugation stabilizes or destabilizes certain accumulations of positive charge. The phenomenon explains corresponding peculiarities in the stereochemistry and rate of hydrolysis.
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