Organoscandium chemistry is the chemistry of organometallic compounds containing a carbon to scandium chemical bond. 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.
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. In Sc(C3H5)3(THF)2 two allyl ligands are η3 coordinated and one allyl ligand is η1 coordinated.
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