Trisoxazolinylborate

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Trisoxazolinylborate ligand Trisoxazolinylborate ligand.jpg
Trisoxazolinylborate ligand

Tris(oxazolinyl)borate compounds are a class of tridentate ligands; often abbreviated ToR, where R is the substituent on the oxazoline ring. Most commonly the substituent is either a methyl, propyl, tert-butyl or hydrogen. The formation of anionic boron backbone with addition of a phenyl group on boron allows the ligand to strongly bind to the metal center. It results in a more robust complex. [1]

Contents

Tris(oxazolinyl)borates can be characterised as scorpionate ligands and may be compared to tris(pyrazolyl)borate and trisoxazoline ligands. In bulky pyrazolylborate (Tp) derivatives, isomerization may occur via 1,2-shifts; additionally B–N bond cleavage is a common decomposition pathways for the Tp ligands. The oxazoline-based ligands with B-C linkages avoid these decomposition problems. [1]

Synthesis

Tris(4,4-dimethyl-2-oxazolinyl)phenyl borate (To ) ToM ligand.jpg
Tris(4,4-dimethyl-2-oxazolinyl)phenyl borate (To )
Tris(4S-isopropyl-2-oxazolinyl)phenylborate (To ) ToP ligand.jpg
Tris(4S-isopropyl-2-oxazolinyl)phenylborate (To )

The first example of a trisoxazolinylborate ligand was tris(4,4-dimethyl-2-oxazolinyl)phenyl borate (ToM). This was prepared by a reaction of dichlorophenylborane with 3 equivalents of 2-lithio-4,4-dimethyl-2-oxazolide. [2] Later variants, such as tris(4S-isopropyl-2-oxazolinyl)phenylborate (ToP) have been prepared in an analogous manner. [3]

PhBCl2 + 3 Li(Oxaz-Me2) → PhB(Oxaz-Me2)3 + 3 LiCl

Complexes of ToR

The first coordination complexes made using ToM ligands were based around zirconium (IV), as the sterically bulky ligands were able to stabilise the highly reactive metal centers. ToMZr(IV) complexes were prepared by salt metathesis using LiToM and TlToM and ZrCl4. The formed complex ToMZrCl3 was found to be quite robust and showed C3V symmetry in both solution and solid state. [4]

Preparation of (To )ZrCl3 ligand Preparation of ToMZrCl3.png
Preparation of (To )ZrCl3 ligand

Lithium tris(4,4-dimethyl-2-oxazolin-2-yl) phenyl borate (LiToM) is used as a transfer agent. However TlToM frequently is as a more effective transfer agent than LiToM because of the higher solubility of the Tl salt and the insolubility of thallium chloride by-products. In contrast, lithium halide byproducts from preparations employing LiToM can cause purification problems. [1]

Another example for the coordinating chemistry of ToM is the formation of ToMMgMe by the reaction of equimolar amounts of HToM and MgMe2(O2C4H8)2. In addition, the reaction of two equivalents of HToM with MgMe2(O2C4H8)2 gives the homoleptic ToM2Mg compound. This compound can also be obtained by the reaction between one equivalent of HToM and ToMMgMe revealing that Mg in ToMMgMe is an active center for the chemical reactions. According to 1H NMR spectroscopic data, ToM2Mg shows Cs symmetry. In these reactions HToM is used as the transfer agent. [2] Coordination chemistry of iridium(I) centers with ToP has been shown by the preparation of [Ir(ToP)(COD)] (COD =1,5-C8H12) by the reaction of LiToP and 0.5 equivalent of [Ir(μ-Cl)(COD)]2. [3]

Preparation of (To )2Mg ligand Preparation of ToM2Mg.png
Preparation of (To )2Mg ligand

Catalysis

ToMMgMe is an effective precatalyst for the cross-dehydrocoupling of Si-H bonds in organosilanes and N-H bonds in amines to give Si-N bonds and H2. [2] Furthermore, tris(oxazolinyl)borate yttrium alkyl and amide compounds (ToMYR2) can be used as precatalysts for the cyclization of aminoalkenes. [5]

See also

Related Research Articles

Scorpionate ligand

The term scorpionate ligand refers to a tridentate ligand which would bind to a metal in a fac manner. The most popular class of scorpionates are the hydrotris(pyrazolyl)borates or Tp ligands. These were also the first to become popular. These ligands first appeared in journals in 1966 from the then little-known DuPont chemist of Ukrainian descent, Swiatoslaw Trofimenko. Trofimenko called this discovery "a new and fertile field of remarkable scope".

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Tris(pentafluorophenyl)borane Chemical compound

Tris(pentafluorophenyl)borane, sometimes referred to as "BCF", is the chemical compound (C
6
F
5
)
3
B
. It is a white, volatile solid. The molecule consists of three pentafluorophenyl groups attached in a "paddle-wheel" manner to a central boron atom; the BC
3
core is planar. It has been described as the “ideal Lewis acid” because of its high thermal stability and the relative inertness of the B-C bonds. Related fluoro-substituted boron compounds, such as those containing B−CF
3
groups, decompose with formation of B-F bonds. Tris(pentafluorophenyl)borane is thermally stable at temperatures wide over 200 °C, resistant to oxygen and water-tolerant.

Organoactinide chemistry

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Lithium bis(trimethylsilyl)amide Chemical compound

Lithium bis(trimethylsilyl)amide is a lithiated organosilicon compound with the formula LiN(SiMe3)2. It is commonly abbreviated as LiHMDS (lithium hexamethyldisilazide - a reference to its conjugate acid HMDS) and is primarily used as a strong non-nucleophilic base and as a ligand. Like many lithium reagents, it has a tendency to aggregate and will form a cyclic trimer in the absence of coordinating species.

Sodium tetraphenylborate Chemical compound

Sodium tetraphenylborate is the organic compound with the formula NaB(C6H5)4. It is a salt, wherein the anion consists of four phenyl rings bonded to boron. This white crystalline solid is used to prepare other tetraphenylborate salts, which are often highly soluble in organic solvents. The compound is used in inorganic and organometallic chemistry as a precipitating agent for potassium, ammonium, rubidium, and cesium ions, and some organic nitrogen compounds.

Dimethylphenylphosphine Chemical compound

Dimethylphenylphosphine is an organophosphorus compound with a formula P(C6H5)(CH3)2. The phosphorus is connected to a phenyl group and two methyl groups, making it the simplest aromatic alkylphosphine. It is colorless air sensitive liquid. It is a member of series (CH3)3-n(C6H5)2P that also includes n = 0, n = 2, and n = 3 that are often employed as ligands in metal phosphine complexes.

Organobismuth chemistry

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Oxazoline Chemical compound

Oxazoline is a five-membered heterocyclic chemical compound containing one atom each of oxygen and nitrogen. It was likely first synthesized in 1884 but it was not until 5 years later that Siegmund Gabriel correctly assigned the structure. It was named in-line with the Hantzsch–Widman nomenclature and is part of a family of heterocyclic compounds, where it exists between oxazole and oxazolidine in terms of saturation.

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Dichlorotris(triphenylphosphine)ruthenium(II) Chemical compound

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Metal bis(trimethylsilyl)amides

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Trisoxazolines

Trisoxazolines are a class of tridentate, chiral ligands composed of three oxazoline rings. Despite being neutral they are able to form stable complexes with high oxidation state metals, such as rare earths, due to the chelate effect. The ligands have been investigated for molecular recognition and their complexes are used in asymmetric catalysts and polymerisation.

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References

  1. 1 2 3 Dunne, J.F. (2011). "Stoichiometric and catalytic reactivity of tris(oxazolinyl)borate main group metal compounds". Graduate Theses and Dissertations. doi: 10.31274/etd-180810-27 .
  2. 1 2 3 Dunne, J.F.; Neal, S.R; Engelkemier, J; Arkady, E; Sadow, A.D. (2011). "Tris(oxazolinyl)boratomagnesium-Catalyzed Cross-Dehydrocoupling of Organosilanes with Amines, Hydrazine, and Ammonia". J. Am. Chem. Soc. 133 (42): 16782–5. doi:10.1021/ja207641b. PMID   21958426.
  3. 1 2 Baird, Benjamin; Pawlikowski, A.V; Su, J; Wiencg, J.W; Pruski, M; Sadow, A.D. (2008). "Easily Prepared Chiral Scorpionates: Tris(2-oxazolinyl)boratoiridium(I) Compounds and Their Interactions with MeOTf". Inorg. Chem. 47 (22): 10208–10. doi:10.1021/ic801637s. PMID   18921996.
  4. Dunne, J.F.; Su, J; Ellern, A; Sadow, A.D. (2008). "A New Scorpionate Ligand: Tris(4,4-dimethyl-2-oxazolinyl)borate and Its Zirconium(IV) Complexes". Organometallics. 27 (11): 2399. doi:10.1021/om800252p.
  5. Pawlikowski, A.V.; Ellern, A; Sadow, A.D. (2009). "Ligand Exchange Reactions and Hydroamination with Tris(oxazolinyl)borato Yttrium Compounds". Organometallics. 27 (16): 8020–9. doi:10.1021/ic900689k. PMID   19586044.