Trisoxazolinylborate

Tris(oxazolinyl)borate compounds are a class of tridentate ligands; often abbreviated To^R, 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]

Trisoxazolinylborate ligand

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^M)

Tris(4S-isopropyl-2-oxazolinyl)phenylborate (To^P)

The first example of a trisoxazolinylborate ligand was tris(4,4-dimethyl-2-oxazolinyl)phenyl borate (To^M). 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 (To^P) have been prepared in an analogous manner.[3]

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

Complexes of To^R

The first coordination complexes made using To^M ligands were based around zirconium (IV), as the sterically bulky ligands were able to stabilise the highly reactive metal centers. To^MZr(IV) complexes were prepared by salt metathesis using LiTo^M and TlTo^M and ZrCl₄. The formed complex To^MZrCl₃ was found to be quite robust and showed C₃V symmetry in both solution and solid state.[4]

Preparation of (To^M)ZrCl₃ ligand

Lithium tris(4,4-dimethyl-2-oxazolin-2-yl) phenyl borate (LiTo^M) is used as a transfer agent. However TlTo^M frequently is as a more effective transfer agent than LiTo^M 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 LiTo^M can cause purification problems.[1]

Another example for the coordinating chemistry of To^M is the formation of To^MMgMe by the reaction of equimolar amounts of HTo^M and MgMe²(O₂C₄H₈)₂. In addition, the reaction of two equivalents of HTo^M with MgMe₂(O₂C₄H₈)₂ gives the homoleptic To^M₂Mg compound. This compound can also be obtained by the reaction between one equivalent of HTo^M and To^MMgMe revealing that Mg in To^MMgMe is an active center for the chemical reactions. According to 1H NMR spectroscopic data, To^M₂Mg shows C_s symmetry. In these reactions HTo^M is used as the transfer agent.[2] Coordination chemistry of iridium(I) centers with To^P has been shown by the preparation of [Ir(To^P)(COD)] (COD =1,5-C₈H₁₂) by the reaction of LiTo^P and 0.5 equivalent of [Ir(µ-Cl)(COD)]₂.[3]

Preparation of (To^M)₂Mg ligand

Catalysis

To^MMgMe 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 (To^MYR₂) can be used as precatalysts for the cyclization of aminoalkenes.[5]

See also

• Trisoxazolines
• Trispyrazolylborate

References

1. 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. 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. 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.