Organoscandium chemistry

Organoscandium chemistry is the chemistry of organometallic compounds containing a carbon to scandium chemical bond.[1][2] The interest in organoscandium compounds is mostly academic but several compound classes find practical application in catalysis, especially in polymerization. A common precursor is scandium chloride.

As with the other elements in group 3 – e.g. yttrium, forming organoyttrium compounds – and the lanthanides, the dominant oxidation state for scandium in organometallic compounds is +3 (electron configuration [Ar] 3d¹4s²). The members of this group also have large ionic radii with vacant s,p and d orbitals (88 pm for Sc³⁺ compared to 67 pm for Al³⁺) and as a result they behave as hard Lewis acids and tend to have high coordination numbers of 9 to 12. The metal to ligand chemical bond is largely ionic.

Ligands

Most organoscandium compounds have at least one cyclopentadienyl (Cp) ligand. The dominant species are CpScX₂, Cp₂ScX and Cp₃Sc. The mononuclear compounds are of limited utility because of poor steric shielding which makes them prone to nucleophilic addition and solvent attack.

Cp₂ScCl can be synthesised from sodium cyclopentadienide:

ScCl₃ + 2NaCp → ScCp₂Cl + 2 NaCl

Chlorine can be replaced by a host of other ligands for example by an allyl group in reaction with allylmagnesium bromide:

ScCp₂Cl + C₃H₅MgBr → ScCp₂(η³-C₃H₅) + MgClBr

Cp₂ScX compounds are dimers with X forming a bridging ligand. Dimerization is avoided in presence of coordinating solvent such as THF or MeCN or when the Cp group is replaced by a bulkier ligand such as the Cp* group. The compound Cp^*₂ScCl is a stable monomer.

The trinuclear compound ScCp₃ can also be synthesized from NaCp:

ScCl₃ + 3 NaCp → ScCp₃ + 3 NaCl

This compound is sensitive to air and moisture and forms an infinite chain with one bridging Cp group and two pentahapto Cp units. Once again the chain can be broken up with excess coordinating solvent.

The synthesis of tris(allyl)scandium has been reported by reaction of ScCl₃ with allylpotassium in THF.[3] In Sc(C₃H₅)₃(THF)₂ two allyl ligands are η³ coordinated and one allyl ligand is η¹ coordinated.

Lower oxidation states (+2, +1, 0) have also been observed in organoscandium compounds.[4][5][6][7].

References

1. Synthesis of Organometallic Compounds: A Practical Guide Sanshiro Komiya Ed. 1997
2. C. Elschenbroich, A. Salzer Organometallics : A Concise Introduction (2nd Ed) (1992) from Wiley-VCH: Weinheim. ISBN 3-527-28165-7
3. Allyl complexes of scandium: synthesis and structure of neutral, cationic and anionic derivatives Sabine Standfuss, Elise Abinet, Thomas P. Spaniol and Jun Okuda Chem. Commun., 2011, Advance Article doi:10.1039/C1CC14180E
4. Polly L. Arnold; F. Geoffrey; N. Cloke; Peter B. Hitchcock & John F. Nixon (1996). "The First Example of a Formal Scandium(I) Complex: Synthesis and Molecular Structure of a 22-Electron Scandium Triple Decker Incorporating the Novel 1,3,5-Triphosphabenzene Ring". J. Am. Chem. Soc. 118 (32): 7630–7631. doi:10.1021/ja961253o.
5. F. Geoffrey N. Cloke; Karl Khan & Robin N. Perutz (1991). "η-Arene complexes of scandium(0) and scandium(II)". J. Chem. Soc., Chem. Commun. (19): 1372–1373. doi:10.1039/C39910001372.
6. Ana Mirela Neculai; Dante Neculai; Herbert W. Roesky; Jörg Magull; Marc Baldus; et al. (2002). "Stabilization of a Diamagnetic Sc^IBr Molecule in a Sandwich-Like Structure". Organometallics. 21 (13): 2590–2592. doi:10.1021/om020090b.
7. Polly L. Arnold; F. Geoffrey; N. Cloke & John F. Nixon (1998). "The first stable scandocene: synthesis and characterisation of bis(η-2,4,5-tri-tert-butyl-1,3-diphosphacyclopentadienyl)scandium(II)". Chem. Commun. (7): 797–798. doi:10.1039/A800089A.