N-Heterocyclic carbene boryl anion

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A Generic NHC Boryl Anion Larger Generic NHC Boryl Anion.png
A Generic NHC Boryl Anion

An N-heterocyclic carbene boryl anion is an isoelectronic structure of an N-heterocyclic carbene (NHC), where the carbene carbon is replaced with a boron atom that has a -1 charge. [1] NHC boryl anions have a planar geometry, and the boron atom is considered to be sp2-hybridized. They serve as extremely strong bases, as they are very nucleophilic. [1] They also have a very strong trans influence, due to the σ-donation coming from the boron atom. [2] NHC boryl anions have stronger electron-releasing character when compared to normal NHCs. [3] These characteristics make NHC boryl anions key ligands in many applications, such as polycyclic aromatic hydrocarbons, and more commonly low oxidation state main group element bonding.

Contents

Synthesis

Ever since the first crystalline carbene structure was isolated by Arduengo ins 1990, tuning different properties of NHCs has been a popular area of study in main group chemistry. [4] The first NHC boryl anion was synthesized by Segawa in 2006. [1] The precursor to the complex was first synthesized by a diimine reduction by magnesium followed by a reaction with BBr3. The final complex was synthesized through cleavage on a boron-bromide bond in a bromo-diazaborole complex by lithium naphthalenide. [1] This reaction made a boryllithium complex, where the boron atom shows strong structural similarity to a free boryl anion. These similarities show that boron has the anionic -1 charge and is recognized as an isoelectronic compound to a singlet carbene. [1] The key to this synthesis was bulky R substituents on the nitrogen which prevented dimerization, something that is common in boron chemistry. [5] These bulky substituents and low temperatures provided successful isolation of the species. [6]

Different Boryllithium Backbones That Were Synthesized Differing Backbones.jpg
Different Boryllithium Backbones That Were Synthesized
Synthesis of the First NHC Boryl Anion NHC Boryl Anion Synthesis.png
Synthesis of the First NHC Boryl Anion
A "Naked" Boryl Anion Naked Boryl Anion.jpg
A "Naked" Boryl Anion

Differing Boryllithium Backbones

After the first synthesis of the NHC boryl anion, Segawa continued to synthesize other NHC boryl anions by switching the backbones of the complexes. In 2008, it was found that by using the same reducing conditions as the first boryl anions, many other NHC boryl anions could be synthesized. [7]

The "Naked" Boryl Anion

A "naked" boryl anion, in which there is no cation near the -1 boron, can be synthesized through an amide metathesis reaction. What is formed is a borylpotassium dimer, in which the K+ ions interact weakly with both the carbons on the substituents on the nitrogens and also the boron centers. [8] The K-B bond distances are >3.1 Å, which is much greater than the sum of the covalent radii. Additionally, the N-B-N bond angle is very close to the calculated gas-phase anion, leading to the conclusion that the boryl anion is as "free" as possible. [8]

Reactivity

NHC boryl ligands tend to be strong σ donors but π acceptors. [9]

Bonding with Group 1 and 2 Elements

When the NHC boryl anion is in the form of a boryllithium salt, it has displayed reactivity with CO, one of the most important building blocks in the industrial field. [10] The complex goes through an insertion reaction, where the CO is inserted into the B-Li bond to make a short-lived intermediate species. This reaction shows promising applications in carbonylative coupling reactions, where CO insertion is necessary. [10]

Mechanism of CO Bond Insertion SVG CO Bond Insertion.svg
Mechanism of CO Bond Insertion
B-Mg single bond formed using NHC Boryl Anion Mg Synthesis of NHB.svg
B-Mg single bond formed using NHC Boryl Anion

In 2007, the first B-Mg single bond was synthesized using an NHC boryl anion as the ligand. The B-Mg bonds are slightly longer than the sum of the covalent radii, but this has been attributed to weakened Coulombic interaction due to coordination of the solvent, which was THF in this experiment. [11] This solvent interaction also affects the geometry of the molecule, as the crystal structure shows that the Mg atom has a distorted sp3-hybridized center. However, the results show that the Mg-B bond has ionic character and can be considered a single bond. [11] Another Mg-B bond was synthesized by reacting the NHC boryl anion with a Mg compound in a 2:1 ratio. This Mg atom also had a distorted tetrahedral coordination, which was also attributed to the coordination of the solvent (THF). [12]

NHC Boryl Anion Reactivity with Be Be Synthesis Using NHC Boryl Anion.svg
NHC Boryl Anion Reactivity with Be

The first Be-B bond was reported in 2014, however this bond showed more covalent character, rather than the ionic bond that was reported in the Mg analogue of this complex. This complex was formed by reacting two equivalents of the NHC boryl anion with BeCl2 using benzene as the solvent. [13] In 2020, however, a very interesting reaction between the NHC boryl anion and Be was reported. [12] In this case, the boryl anion was reacted with a Be complex, and rather than forming a bond to, and receiving σ-donation from the boron atom, it reacted with one of the carbons in the backbone of the anion. [12] Although the mechanism of this reaction is unclear, it is believed that one of the backbone protons becomes deprotonated, allowing the Be to bind to the positively charged carbons. This compound is extremely stable even at room temperature, and more studies are being completed to further understand the mechanism of this reaction. [12]

Bonding with Main Group Elements

The NHC boryl anion has also been used to achieve B=B double bonds, but in a tetraborane species rather than the diborane molecule. For this synthesis, an extra boron atom was added to the NHC boryl anion, and then was reduced, forcing dimerization between the molecules and allowing for a H-bridged tetraborane species to occur. [14] Although the complex is H-bridged, the inner B-B bond distance lies between reported double and triple bond lengths. Additionally, the NPA charges on the central B-B moiety are negative, showing that the boryl anions donate electron density, leading to the conclusion that a B=B double bond is occurring. [14]

With specific reaction conditions, a disilane single or double bond can be achieved using the NHC boryl anion. [15] To make a Si-Si single bond, a NHC boryl silane compound is reduced by KC8 in DME solvent. To make a Si-Si double bond, a slightly different NHC boryl silane compound is reduced in KC8 in THF solvent. [15]

Disilane Complex Disilane Complex with NHC Boryl Anion.svg
Disilane Complex
Disilene Complex Disilene Complex with NHC Boryl Anion.svg
Disilene Complex
Disilyne Complex Si-Si Triple Bond with NHC Boryl Anion.svg
Disilyne Complex

Additionally, a dianionic disilyne (Si-Si triple bond) was reported in the form of a Mg complex. [16] Two equivalents of a NHC boryl silane compound is reduced with Mg in THF, leading to a Mg-Si-Si three-membered ring. The boryl anion groups are arranged in a cis formation, and the Si atoms have planar geometry. [16] Additionally, the Si-Si bond length is calculated to be the sum of the covalent radii for a double bond, and the NPA charges show dianionic character on the Si atoms. [16]

Bonding with Transition Metals

NHC boryl anions have also been investigated for their ability to activate C-H bonds and hydroboration activity, [17] two things that were previously thought to only be completed by transition metal systems. [18] Anionic boryl ligands can covalently bond to transition metals, which is different than how it bonds to main group elements (ionically). [19] These boryl ligands σ-bond but also are able to receive π-back donation into the vacant pz orbital that the boron has. It is said that boryl ligands, like NHC boryl anions, are the most effective ligand in controlling reactivity. [19]

Many transition metal boryl complexes have been synthesized, including silver, gold, copper, and zinc. [20] These complexes give insight into potential intermediates of transition metal catalyst reactions, and provide potential starting materials for both organic and inorganic synthetic chemistry. [20]

Different Transition Metal Complexes Transition Metal with NHC Boryl Anion.svg
Different Transition Metal Complexes

Group 12 metal bonding has almost exclusively had the metals in the +1-oxidation state, but NHC boryl anions have helped synthesize group 12 M-M bonds in the 0-oxidation state. [21] Group 12 metals take part in very weak bonding in the 0-oxidation state due to the filled valence d-orbitals, when the metals have NHC boryl anion ligands, they are able to bond in the 0-oxidation state because of the increase electron density that is donated by the ligand. [21] These molecules are synthesized by first have m-terphenyls as the ligands on the metal, and then an isolobal exchange occurs, placing the boryl ligands onto the metal and allowing for the metal to be in the 0-oxidation state. [21]

Polycyclic Aromatic Hydrocarbons

One of the most exciting applications for NHC boryl anions are their place in polycyclic aromatic hydrocarbons, or PAHs. PAHs are normally defined as a molecule that has two or more benzenoid rings and contain no other elements except hydrogen and carbon. They are highly fluorescent, and are naturally found in crude oil and other petrochemical products. [22] It has been shown that replacing the end carbons with a B-N moiety expands the family of PAHs and can serve as functional materials. [23]

PAH with NHC Boryl Anion Units PAH Complex with NHC Boryl Anion.svg
PAH with NHC Boryl Anion Units

Placing boron into PAHs is known to improve and diversity the optoelectronic properties by reducing the LUMO energy level. [24] This lowering of the LUMO energy increases acceptor ability by lowering the energy needed for absorption and emission. The fused NHC boryl anion units add an element of bifunctionality and induce π-conjugation because of the empty pz orbital. [24] The absorption and emission qualities of these molecules are very interesting. PAHs that have a pyrene core all fluoresce blue light under UV light, but their smaller and more planar counterparts had a variety of colors that are emitted. [24] This change in color is attributed to the NHC boryl anion rings contributing more to the smaller PAHs, whereas in the pyrene core there is less effect coming from the boron ligand. These planar NHC boryl anion molecules are very promising in their application to functional materials because they emit light in the near-IR region. [24]

Related Research Articles

In chemistry, the oxidation state, or oxidation number, is the hypothetical charge of an atom if all of its bonds to other atoms were fully ionic. It describes the degree of oxidation of an atom in a chemical compound. Conceptually, the oxidation state may be positive, negative or zero. Beside nearly-pure ionic bonding, many covalent bonds exhibit a strong ionicity, making oxidation state a useful predictor of charge.

<span class="mw-page-title-main">Organoboron chemistry</span> Study of compounds containing a boron-carbon bond

Organoboron chemistry or organoborane chemistry studies organoboron compounds, also called organoboranes. These chemical compounds combine boron and carbon; typically, they are organic derivatives of borane (BH3), as in the trialkyl boranes.

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.

<span class="mw-page-title-main">Persistent carbene</span> Type of carbene demonstrating particular stability

A persistent carbene is an organic molecule whose natural resonance structure has a carbon atom with incomplete octet, but does not exhibit the tremendous instability typically associated with such moieties. The best-known examples and by far largest subgroup are the N-heterocyclic carbenes (NHC), in which nitrogen atoms flank the formal carbene.

The triazol-5-ylidenes are a group of persistent carbenes which includes the 1,2,4-triazol-5-ylidene system and the 1,2,3-triazol-5-ylidene system. As opposed to the now ubiquitous NHC systems based on imidazole rings, these carbenes are structured from triazole rings. 1,2,4-triazol-5-ylidene can be thought of as an analog member of the NHC family, with an extra nitrogen in the ring, while 1,2,3-triazol-5-ylidene is better thought of as a mesoionic carbene (MIC). Both isomers of this group of carbenes benefit from enhanced stability, with certain examples exhibiting greater thermal stability, and others extended shelf life.

<span class="mw-page-title-main">Borirene</span> Chemical compound

Borirenes are a unique class of three-membered heterocyclic compounds characterized by an unsaturated boron atom within their ring structure. First computationally predicted by John Pople and Paul von Rague Schleyer in 1981, the simplest borirene, (CH)2BH, is isoelectronic with the cyclopropenium cation and exhibits Hückel aromaticity. Borirenes undergo ring-opening reactions with polar reagents and form Lewis adducts, due to the significant ring strain in its three-membered structure and the presence of an empty p orbital on the boron atom. The balance of these two properties leads to unique properties as a ligand for transition metals, in addition to observation of photochemical rearrangement and ring expansion. While borirenes were first discovered in the 1980s, new derivatives such as benzoborirenes have led to renewed interest in the field, with their potential applications yet to be fully explored.

<span class="mw-page-title-main">PEPPSI</span> Group of chemical compounds

PEPPSI is an abbreviation for pyridine-enhanced precatalyst preparation stabilization and initiation. It refers to a family of commercially available palladium catalysts developed around 2005 by Prof. Michael G. Organ and co-workers at York University, which can accelerate various carbon-carbon and carbon-heteroatom bond forming cross-coupling reactions. In comparison to many alternative palladium catalysts, Pd-PEPPSI-type complexes are stable to air and moisture and are relatively easy to synthesize and handle.

<span class="mw-page-title-main">Boranylium ions</span>

In chemistry, a boranylium ion is an inorganic cation with the chemical formula BR+
2, where R represents a non-specific substituent. Being electron-deficient, boranylium ions form adducts with Lewis bases. Boranylium ions have historical names that depend on the number of coordinated ligands:

<span class="mw-page-title-main">Cyclic alkyl amino carbenes</span>

Cyclic(alkyl)(amino) carbenes (CAACs) are a class of stable singlet carbene ligands that feature one amino and one sp3 alkyl group adjacent to the carbene carbon atom. CAACs are a subset of N-heterocyclic carbenes (NHCs) in which the replacement of an amino group on the ‘classical’ diaminocarbene with a saturated carbon atom results in a carbene ligand that is both a better σ-donor and π-acceptor than classical NHCs. The lone pair on the nitrogen atoms in classical NHCs allows for π-donation from both nitrogen atoms, while substitution of one nitrogen with a carbon atom results in weaker π-donation from only one nitrogen substituent, thus making CAACs stronger π-acceptors and more electrophilic than classical NHCs. Like NHCs, CAACs have highly tunable steric and electronic properties that make them useful ligands in both transition metal and main group chemistry. CAACs have been shown to be extremely useful in the fields of catalysis and materials science. In chemistry, CAACs have the ability to stabilize highly reactive or unstable molecules and participate in transformations of organic molecules. In materials science, CAACs stabilize species that have promising photophysical properties for organic light emitting diodes (OLEDs) and have been shown to stabilize single molecule magnets (SMMs).

<span class="mw-page-title-main">Borylene</span>

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.

<span class="mw-page-title-main">Stannylene</span> Class of organotin(II) compounds

Stannylenes (R2Sn:) are a class of organotin(II) compounds that are analogues of carbene. Unlike carbene, which usually has a triplet ground state, stannylenes have a singlet ground state since valence orbitals of tin (Sn) have less tendency to form hybrid orbitals and thus the electrons in 5s orbital are still paired up. Free stannylenes are stabilized by steric protection. Adducts with Lewis bases are also known.

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.

<span class="mw-page-title-main">9-Borafluorene</span> Class of chemical compounds

9-borafluorenes are a class of boron-containing heterocycles consisting of a tricyclic system with a central BC4 ring with two fused arene groups. 9-borafluorenes can be thought of as a borole with two fused arene rings, or as a trigonal planar boron atom with an empty p orbital bridging two biphenyl rings. However, 9-borafluorenes are generally less reactive than boroles due to less antiaromatic character and Lewis acidity. Containing highly conjugated π systems, 9-borafluorenes possess interesting photophysical properties. In addition, 9-borafluorenes are good Lewis acids. This combination of properties enables potential uses such as in light-emitting materials, solar cells, and sensors for some molecules.

Coinage metal N-heterocyclic carbene (NHC) complexes refer to transition metal complexes incorporating at least one coinage metal center (M = Cu, Ag, Au) ligated by at least one NHC-type persistent carbene. A variety of such complexes have been synthesized through deprotonation of the appropriate imidazolium precursor and metalation by the appropriate metal source, producing MI, MII, or MIII NHC complexes. While the general form can be represented as (R2N)2C:–M (R = various alkyl or aryl groups), the exact nature of the bond between NHC and M has been investigated extensively through computational modeling and experimental probes. These results indicate that the M-NHC bond consists mostly of electrostatic attractive interactions, with some covalent bond character arising from NHC to M σ donation and minor M to NHC π back-donation. Coinage metal NHC complexes show effective activity as catalysts for various organic transformations functionalizing C-H and C-C bonds, and as antimicrobial and anticancer agents in medicinal chemistry.

<span class="mw-page-title-main">Carbones</span> Class of molecules

Carbones are a class of molecules containing a carbon atom in the 1D excited state with a formal oxidation state of zero where all four valence electrons exist as unbonded lone pairs. These carbon-based compounds are of the formula CL2 where L is a strongly σ-donating ligand, typically a phosphine (carbodiphosphoranes) or a N-heterocyclic carbene/NHC (carbodicarbenes), that stabilises the central carbon atom through donor-acceptor bonds. Carbones possess high-energy orbitals with both σ- and π-symmetry, making them strong Lewis bases and strong π-backdonor substituents. Carbones possess high proton affinities and are strong nucleophiles which allows them to function as ligands in a variety of main group and transition metal complexes. Carbone-coordinated elements also exhibit a variety of different reactivities and catalyse various organic and main group reactions.  

<span class="mw-page-title-main">Borepin</span> Aromatic, boron-containing rings

Borepins are a class of boron-containing heterocycles used in main group chemistry. They consist of a seven-membered unsaturated ring with a tricoordinate boron in it. Simple borepins are analogues of cycloheptatriene, which is a seven-membered ring containing three carbon-carbon double bonds, each of which contributes 2π electrons for a total of 6π electrons. Unlike other seven-membered systems such as silepins and phosphepins, boron has a vacant p-orbital that can interact with the π and π* orbitals of the cycloheptatriene. This leads to an isoelectronic state akin to that of the tropylium cation, aromatizing the borepin while also allowing it to act as a Lewis acid. The aromaticity of borepin is relatively weak compared to traditional aromatics such as benzene or even cycloheptatriene, which has led to the synthesis of many fused, π-conjugated borepin systems over the years. Simple and complex borepins have been extensively studied more recently due to their high fluorescence and potential applications in technologies like organic light-emitting diodes (OLEDs) and photovoltaic cells.

<span class="mw-page-title-main">Boraacenes</span> Boron containing acene compounds

Boraacenes are polycyclic aromatic hydrocarbons containing at least one boron atom. Structurally, they are related to acenes, linearly fused benzene rings. However, the boron atom is electron deficient and may act as a Lewis Acid when compared to carbon. This results in slightly less negative charge within the ring, smaller HOMO-LUMO gaps, as well as differences in redox chemistry when compared to their acene analogues. When incorporated into acenes, Boron maintains the planarity and aromaticity of carbon acenes, while adding an empty p-orbital, which can be utilized for the fine tuning of organic semiconductor band gaps. Due to this empty p orbital, however, it is also highly reactive when exposed to nucleophiles like water or normal atmosphere, as it will readily be attacked by oxygen, which must be addressed to maintain its stability.

Gallylenes are a class of gallium species which are electronically neutral and in the +1-oxidation state. This broad definition may include many gallium species, such as oligomeric gallium compounds in which the gallium atoms are coordinated to each other, but these classes of compounds are often referred to as gallanes. In recent literature, the term gallylene has mostly been reserved for low valent gallium species which may have a lone pair, analogous to NHC's or terminal borylenes. They are compounds of academic interest because of their distinctive electronic properties which have been achieved for higher main group elements such as borylenes and carbenes.

<span class="mw-page-title-main">Aluminylene</span>

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.

References

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  22. ↑ Ariese, F.; Goojier, C.; Velthorst, N.H. (18 April 2008). "Application Of Fluorescence Spectroscopic Techniques In The Determination Of Pahs And Pah Metabolites". Techniques and Instrumentation in Analytical Chemistry. 13: 449–480. doi:10.1016/S0167-9244(08)70133-1.
  23. ↑ Weber, Lothar; Eickhoff, Daniel; Chrostowska, Anna; Dargelos, Alain; Darrigan, Clovis; Stammlet, Hans-Georg; Neumann, Beate (25 Oct 2019). "Syntheses and structures of benzo-bis(1,3,2-diazaboroles) and acenaphtho-1,3,2-diazaboroles". Dalton Transactions. 2019 (48): 16911–16921. doi:10.1039/C9DT03818C. PMID   31686076. S2CID   207891622.
  24. 1 2 3 4 Krantz, Kelsie E.; Weisflog, Sarah L.; Frey, Nathan C.; Yang, Wenlong; Dickie, Diane A.; Webster, Charles Edwin; Gilliard Jr., Robert J. (4 May 2020). "Planar, Stair-Stepped, and Twisted: Modulating Structure and Photophysics in Pyrene- and Benzene-Fused N-Heterocyclic Boranes". Chemistry: A European Journal. 26 (44): 10072–10082. doi: 10.1002/chem.202002118 . PMID   32364300. S2CID   218492936.
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