Organoberyllium chemistry

Organoberyllium chemistry involves the synthesis and properties of

The beryllium

Coordination in beryllium can range from a coordination number of two to four.

(NacNac).

General characteristics

Organoberyllium chemistry is limited to academic research due to the cost and toxicity of beryllium.

Organoberyllium compounds consist of a beryllium atom with an

dipole moment

.

Lighter organoberyllium compounds are often considered

Compounds such as these hydrides can coordinate with carbenes such as

N-heterocyclic carbene

to form crystals. The propensity for co-crystallization suggests applications in organocatalysis.

Compounds

Beryllium can form a variety of organoberyllium compounds, including ring structures,

.

Dimethylberyllium has the same crystal structure as dimethylmagnesium^[13] and can be used to synthesize beryllium azide and beryllium hydride.

Ring structure

Organoberyllium structures can consist of an aryl,

Halides

Beryllium halides are formed by a combination of halogen with a beryllium atom. Beryllium halides are mostly covalent in nature except for the fluoride which is more ionic. They can be used as Lewis acid catalysts. Preparation for these compounds varies by the halogen. Beryllium halides are among the most common starting points to form complexes with other types of ligand.[20]^[2] Halides can donate 2 electrons into the beryllium center with a charge of −1.

Phosphines

Organoberyllium phosphines are another class of compounds that is used in synthesis.

with filled metal orbitals. Beryllium can coordinate with a phosphine due to its good π-acceptor ability, which is used extensively in beryllium chemistry literature. An organoberyllium phosphine can be prepared through coordination with a beryllium halide to form a four-coordinate tetrahedral compound.

Carbenes

An organoberyllium carbene consists of a

(CAAC).

N-Hetereocyclic carbenes

Beryllium can coordinate with an N-hetereocyclic carbene (NHC).[22]^[23][24] NHCs are defined as heterocyclic species containing a carbene carbon and at least one nitrogen atom within the ring structure.^[21] NHCs have found numerous applications in some of the most important catalytic transformations in chemical industry, but their reactivity in coordinating with main group elements especially with beryllium’s potential as a reactive organocatalyst has opened new areas of research.^[25]

Cyclic alkyl amino carbenes (CAAC)

Beryllium can coordinate with cyclic alkyl amino carbene (CAAC) ligands and can form beryllium radicals which can be present with beryllium complexes (BeR₂). A CAAC ligand coordinates a 2 electron -1 charge into the beryllium center.^[26] CAAC has an "amino" substituent and an "alkyl" sp³ carbon atom. CAACs are very good σ donors (higher HOMO) and π acceptors (lower LUMO) compared to NHCs. In addition, the lower heteroatom stability of the carbene center in CAAC compared to NHC results in a lower ΔE.

β-Diketiminates (NacNac)

β-Diketiminates (BDI, also known as NacNac), are a commonly used class of supporting ligands that have been successfully adopted to stabilize an extensive range of metal ions from the s, p, d, and f-blocks in multiple oxidation states.^[27] The popularity of these monoanionic N-donor ligands can be explained by their convenient access and high stereoelectronic coordination. This enables the separation of highly reactive coordinatively unsaturated complexes. Moreover, studies have demonstrated the utility of this class of ligands for designing active catalysts for various transformations. So, because of that, beryllium can properly coordinate with β-diketiminate compounds due to the high reactivity and stereo electronic coordination with the beryllium thus a Be NacNac compound is also common in organoberyllium chemistry.

Synthesis

Synthesis of organoberyllium compounds is limited but literature have shown that beryllium can react with halides, alkyls, alloxides and other organic compounds. Alkylation of beryllium halide is one of the most widely-used method in beryllium chemistry.^[28]

Transmetallation

A transmetallation involves a ligand transfer to one another such as this:

MR₂ + Be → BeR₂ + M

M is not limited to any

.

In this case organoberyllium can form reactions such as:

Ph

₂, which forms a crystal structure from this reaction.

Alkylation

Cp₂Be. The solid-state structure suggests that the two rings are bound to the beryllium differently such that one is designated η⁵

and the other η¹

Alkylation of beryllium halide is another common method to react to make an organoberyllium compound such as this:

2 MR¹ + BeR²₂ → BeR¹₂ + 2 MR²₂

M is not limited to any main group and/or transition metal. R¹ is not limited to phenyl, methyl, methyl oxide, carbene etc. R² can be any halide such as fluoride, bromide, iodide, or chloride.

An example of such reaction is the synthesis of bis(cyclopentadienyl)beryllium (

cyclopentadienide

:

2 K[Cp] + BeCl₂ → [Cp]₂Be + 2 KCl

Low oxidation beryllium chemistry

While Be(II) is one of the more common oxidation states, there is also further research on a Be(I) and Be(0) complex. Low-valent main group compounds have recently become desirable synthetic targets due to their interesting reactivity comparable to transition metal complexes. In one work, stabilized cyclic (alkyl)(amino)carbene ligands were used to isolate and characterize the first neutral compounds containing beryllium, with the Be(0) compound stabilized by a strongly σ-donating and π-accepting cyclic CAAC ligand.[29]

Me

and (CH₂)₅ and Dipp is otherwise known as 2,6-diisopropylphenyl.

Be(I) is another example of a rare phenomenon and few publications were reported, but one example of a Be(I) was a CAAC ligand already coordinated with Be. Gilliard and his group created a more stable beryllium radical cation.^[6] Because of well-established challenges concerning the reduction of Be(II) to Be(I), they pursued the radical via an oxidation strategy using TEMPO ((2,2,6,6-Tetramethylpiperidin-1-yl) oxyl). This reaction resulted in a Be(I) compound just by stabilizing the Be radical.

See also

• Group 2 organometallic chemistry
• Beryllium

References