Nontrigonal pnictogen compounds refer to tricoordinate trivalent pnictogen (phosphorus, arsenic, antimony and bismuth: P, As, Sb and Bi) compounds that are not of typical trigonal pyramidal molecular geometry. By virtue of their geometric constraint, these compounds exhibit distinct electronic structures and reactivities, which bestow on them potential to provide unique nonmetal platforms for bond cleavage reactions.
The first examples of nontrigonal pnictogen compound were synthesized by Arduengo and co-workers in 1984, [1] through condensation of a diketoamine with a phosphorus trihalide in the presence of base. This group reported also on the first systematic investigations into its chemical behavior. [2] Later, on similar routes, the corresponding and isostructural arsenic and antimony species were also synthesized. [3] Other synthetic methods involve deprotonation of OH or NH groups in the presence of ECl3 (E=P, [4] As, Sb [5] and Bi [6] ), salt metathesis [7] or reduction of pentavalent pnictogen compounds. [8]
The molecular structures of nontrigonal pnictogen compounds reveal the steric strain in these molecules, and significantly differing bond angles at the pnictogen atoms indicate a considerable distortion of the coordination spheres. [9]
In particular, the geometry at the central part of these compounds deviate strongly from traditional pnictogen compounds, and indicate molecular strain with an approach to a T-type molecular configuration. With different ligand motifs, the bond angles at pnictogen atoms can vary from 100˚ to almost 180˚. The flattened geometry of these molecules influences the relatively low energetic barriers for inversion of the configuration via planar coordinated pnictogen atoms in the transition state. These low barriers are in accordance with the dynamic behavior and fast equilibration processes observed in ambient temperature NMR. [4]
Results of quantum chemical calculations confirm that in these compounds, the lone pair of electrons at the pnictogen atoms is localized in orbitals with relatively high s-character. From these results, only weak nucleophilicity was derived in accordance with some experimental observations such as the inertness towards benzyl bromide. [4] The LUMO is delocalized but has important contributions from pnictogen empty p orbitals, which should favor a nucleophilic attack of substrates at this position in accordance with experimental findings. [10] The pnictogen atom forms a three-center-four-electron bond with the two flanking nitrogen atoms, which is manifested by the HOMO-2. [5]
For nontrigonal bismuth compounds, a Bi(I) electronic structure could be shown to be most appropriate. Natural bond orbital (NBO) analysis reveals an s-type lone pair and a p-type lone pair at the metal, with the remaining two p orbitals being involved in one two-center-two-electron bond and one three-center-two-electron bond. The p-type lone pair NBO has less than 2 electron occupancy as it is delocalized over the ligand frame. Although considerable Bi(I) character is indicated for the Bi compound, it exhibits reactivity similar to Bi(III) electrophiles, and expresses either a vacant or a filled p orbital at Bi. [6]
From these results, two types of resonance structures can be drawn, one with a filled s-orbital and a vacant p orbital at the pnictogen center, the other one with negative charge on pnictogen, arising from the redox-non-innocent nature of the ligand. This is evident by shorter C-N bond lengths in nontrigonal pnictogen compounds than C-N single bonds in the corresponding ligands. [5] [6] These structures may reflect the specific bonding situation in these strained molecular systems.
These easily available and sterically constrained compounds are potentially suitable for an application in a wide variety of secondary processes such as small molecule activation or the generation of new catalysts based on main-group and transition-metal elements.
Since the LUMOs of nontrigonal pnictogen compounds consist mainly of the vacant p orbitals of the pnictogen nuclei, they could undergo one-electron reduction to afford radical anions if the energy levels of LUMOs are appropriate. For a less sterically hindered compound, the generated radical anion readily dimerizes to form a dianion with a P-P bond. [11] When a sterically encumbered tris-amide ligand is used, stable radical anions bearing T-shaped pnictogen nuclei can be isolated and characterized. [5]
The oxidation of nontrigonal phosphorus compounds and transfer of halogen molecules to the phosphorus atoms to generate phosphoranes with phosphorus atoms in an oxidation state of +5 was achieved by various synthetic procedures. [7] [11] [12] These dihalides are promising starting materials and potentially applicable for the generation of numerous secondary products, but only few reactions have been reported so far in the literature. [11] Nontrigonal phosphorus compounds can also be oxidized by organic azide to yield phosphazenes. [13]
These sterically constrained phosphorus compounds show remarkable reactivity towards protic reagents such as primary amines and alcohols, which results in intermolecular oxidative addition of these O−H and N−H bonds. [4] [14] This reaction tolerates a variety of different substrates, including ammonia and water. [10] [15] Two mechanisms have been suggested for the understanding of the unusual insertion of phosphorus atoms into polar X−H bonds by oxidative addition. [14] [16]
Nontrigonal phosphorus compounds can also react with ammonia–borane to form a formal dihydrogen oxidative addition product. This compound proved to facilitate the catalytic reduction of azobenzene. [17]
The first transition metal complexes of nontrigonal pnictogen compounds have been reported in the 1980s and '90s. [2] [3] Up to now, several complexes have been successfully synthesized, [7] [18] but they have not yet been applied in secondary processes, such as catalytic cycles.
In 2018, the synthesis and reactivity of a chelating ligand containing a nontrigonal phosphorus center was reported. [19] It is worth noting that, apart from direct metalation of this ligand with RuCl2(PPh3)3, metalation with a ruthenium hydride compound RuHCl(CO)(PPh3)3 yields a complex with net insertion into the Ru−H bond. These ligands, along with recent developments for higher valent states of Sb ligands, [20] may possess rich potential in the field of catalysis and sensing. [21]
A transition metal carbene complex is an organometallic compound featuring a divalent carbon ligand, itself also called a carbene. Carbene complexes have been synthesized from most transition metals and f-block metals, using many different synthetic routes such as nucleophilic addition and alpha-hydrogen abstraction. The term carbene ligand is a formalism since many are not directly derived from carbenes and most are much less reactive than lone carbenes. Described often as =CR2, carbene ligands are intermediate between alkyls (−CR3) and carbynes (≡CR). Many different carbene-based reagents such as Tebbe's reagent are used in synthesis. They also feature in catalytic reactions, especially alkene metathesis, and are of value in both industrial heterogeneous and in homogeneous catalysis for laboratory- and industrial-scale preparation of fine chemicals.
A frustrated Lewis pair (FLP) is a compound or mixture containing a Lewis acid and a Lewis base that, because of steric hindrance, cannot combine to form a classical adduct. Many kinds of FLPs have been devised, and many simple substrates exhibit activation.
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.
Cyclodiphosphazanes are saturated four membered P2N2 ring systems and one of the major classes of cyclic phosphazene compounds. Bis(chloro)cyclodiphosphazanes, (cis-[ClP(μ-NR)]2) are important starting compounds for synthesizing a variety of cyclodiphosphazane derivatives by nucleophilic substitution reactions; are prepared by reaction of phosphorus trichloride (PCl3) with a primary amine (RNH2) or amine hydrochlorides (RNH3Cl).
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.
Cobalt(II)–porphyrin catalysis is a process in which a Co(II) porphyrin complex acts as a catalyst, inducing and accelerating a chemical reaction.
In chemistry, cyclic(alkyl)(amino)carbenes (CAACs) are a family of stable singlet carbene ligands developed by the research group of Guy Bertrand in 2005 at UC Riverside. In marked contrast with the popular N-heterocyclic carbenes (NHCs) which possess two "amino" substituents adjacent to the carbene center, CAACs possess one "amino" substituent and an sp3 carbon atom "alkyl". This specific configuration makes the CAACs very good σ-donors and π-acceptors when compared to NHCs. Moreover the reduced heteroatom stabilization of the carbene center in CAACs versus NHCs also gives rise to a smaller ΔEST.
A borylene is the boron analogue of a carbene. The general structure is R-B: with R an organic moiety and B a boron atom with two unshared electrons. Borylenes are of academic interest in organoboron chemistry. A singlet ground state is predominant with boron having two vacant sp2 orbitals and one doubly occupied one. With just one additional substituent the boron is more electron deficient than the carbon atom in a carbene. For this reason stable borylenes are more uncommon than stable carbenes. Some borylenes such as boron monofluoride (BF) and boron monohydride (BH) the parent compound also known simply as borylene, have been detected in microwave spectroscopy and may exist in stars. Other borylenes exist as reactive intermediates and can only be inferred by chemical trapping.
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.
A phosphetane is a 4-membered organophosphorus heterocycle. The parent phosphetane molecule, which has the formula C3H7P, is one atom larger than phosphiranes, one smaller than phospholes, and is the heavy-atom analogue of azetidines. The first known phosphetane synthesis was reported in 1957 by Kosolapoff and Struck, but the method was both inefficient and hard to reproduce, with yields rarely exceeding 1%. A far more efficient method was reported in 1962 by McBride, whose method allowed for the first studies into the physical and chemical properties of phosphetanes. Phosphetanes are a well understood class of molecules that have found broad applications as chemical building blocks, reagents for organic/inorganic synthesis, and ligands in coordination chemistry.
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.
Gregory H. RobinsonFRSC is an American synthetic inorganic chemist and a Foundation Distinguished Professor of Chemistry at the University of Georgia. Robinson's research focuses on unusual bonding motifs and low oxidation state chemistry of molecules containing main group elements such as boron, gallium, germanium, phosphorus, magnesium, and silicon. He has published over 150 research articles, and was elected to the National Academy of Sciences in 2021.
Hexaphosphabenzene is a valence isoelectronic analogue of benzene and is expected to have a similar planar structure due to resonance stabilization and its sp2 nature. Although several other allotropes of phosphorus are stable, no evidence for the existence of P6 has been reported. Preliminary ab initio calculations on the trimerisation of P2 leading to the formation of the cyclic P6 were performed, and it was predicted that hexaphosphabenzene would decompose to free P2 with an energy barrier of 13−15.4 kcal mol−1, and would therefore not be observed in the uncomplexed state under normal experimental conditions. The presence of an added solvent, such as ethanol, might lead to the formation of intermolecular hydrogen bonds which may block the destabilizing interaction between phosphorus lone pairs and consequently stabilize P6. The moderate barrier suggests that hexaphosphabenzene could be synthesized from a [2+2+2] cycloaddition of three P2 molecules. Currently, this is a synthetic endeavour which remains to be conquered.
Arsenic in the solid state can be found as gray, black, or yellow allotropes. These various forms feature diverse structural motifs, with yellow arsenic enabling the widest range of reactivity. In particular, reaction of yellow arsenic with main group and transition metal elements results in compounds with wide-ranging structural motifs, with butterfly, sandwich and realgar-type moieties featuring most prominently.
Aluminium(I) nucleophiles are a group of inorganic and organometallic nucleophilic compounds containing at least one aluminium metal center in the +1 oxidation state with a lone pair of electrons strongly localized on the aluminium(I) center.
Gallium monoiodide is an inorganic gallium compound with the formula GaI or Ga4I4. It is a pale green solid and mixed valent gallium compound, which can contain gallium in the 0, +1, +2, and +3 oxidation states. It is used as a pathway for many gallium-based products. Unlike the gallium(I) halides first crystallographically characterized, gallium monoiodide has a more facile synthesis allowing a synthetic route to many low-valent gallium compounds.
Karsten Meyer is a German inorganic chemist and Chair of Inorganic and General Chemistry at the Friedrich-Alexander University of Erlangen-Nürnberg (FAU). His research involves the coordination chemistry of transition metals as well as uranium coordination chemistry, small molecule activation with these coordination complexes, and the synthesis of new chelating ligands. He is the 2017 recipient of the Elhuyar-Goldschmidt Award of the Spanish Royal Society of Chemistry, the Ludwig-Mond Award of the Royal Society of Chemistry, and the L.A. Chugaev Commemorative Medal of the Russian Academy of Sciences, among other awards. He also serves as an Associate Editor of the journal Organometallics since 2014.
Stable and persistent phosphorus radicals are phosphorus-centred radicals that are isolable and can exist for at least short periods of time. Radicals consisting of main group elements are often very reactive and undergo uncontrollable reactions, notably dimerization and polymerization. The common strategies for stabilising these phosphorus radicals usually include the delocalisation of the unpaired electron over a pi system or nearby electronegative atoms, and kinetic stabilisation with bulky ligands. Stable and persistent phosphorus radicals can be classified into three categories: neutral, cationic, and anionic radicals. Each of these classes involve various sub-classes, with neutral phosphorus radicals being the most extensively studied. Phosphorus exists as one isotope 31P (I = 1/2) with large hyperfine couplings relative to other spin active nuclei, making phosphorus radicals particularly attractive for spin-labelling experiments.
Heteroatomic multiple bonding between group 13 and group 15 elements are of great interest in synthetic chemistry due to their isoelectronicity with C-C multiple bonds. Nevertheless, the difference of electronegativity between group 13 and 15 leads to different character of bondings comparing to C-C multiple bonds. Because of the ineffective overlap between p𝝅 orbitals and the inherent lewis acidity/basicity of group 13/15 elements, the synthesis of compounds containing such multiple bonds is challenging and subject to oligomerization. The most common example of compounds with 13/15 group multiple bonds are those with B=N units. The boron-nitrogen-hydride compounds are candidates for hydrogen storage. In contrast, multiple bonding between aluminium and nitrogen Al=N, Gallium and nitrogen (Ga=N), boron and phosphorus (B=P), or boron and arsenic (B=As) are less common.
Aluminylenes are a sub-class of aluminium(I) compounds that feature singly-coordinated aluminium atoms with a lone pair of electrons. As aluminylenes exhibit two unoccupied orbitals, they are not strictly aluminium analogues of carbenes until stabilized by a Lewis base to form aluminium(I) nucleophiles. The lone pair and two empty orbitals on the aluminium allow for ambiphilic bonding where the aluminylene can act as both an electrophile and a nucleophile. Aluminylenes have also been reported under the names alumylenes and alanediyl.