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ChemRxiv208.pdf (6.58 MB)
Scandium Alkyl and Hydride Complexes Supported by a Pentadentate Diborate Ligand: Reactions with CO2 and N2O
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submitted on 10.08.2018 and posted on 13.08.2018by Warren Piers, Daniel W. Beh, Iker del Rosal, Maron Laurent, Benjamin
S. Gelfand, Chris Gendy, Jian-Bin Li
and hydrido scandium complexes of the dianionic pentatdentate ligand B2Pz4Py
are reported. The key starting material (B2Pz4Py)ScCl is
readily prepared and alkylated with organolithium reagents RLi (R = CH3,
CH2SiMe3, CH2SiMe2Ph, CH2CH2CH3
and CH2CHMe2) to form alkyl derivatives in 61-93% yields.
These compounds are very thermally stable and do not undergo sigma bond
metathesis reactions with dihydrogen. The hydrido complex was prepared from (B2Pz4Py)ScCl
and NaHBEt3 in 80% yield and was found to be more stable by 28 kcal
mol-1 as a dimer, rather than a monomeric hydrido complex. However,
the monomer is accessible through dissociation of the dimer at 80˚C. All of the
compounds (B2Pz4Py)ScR react with water to form the
bridging oxo dimer (B2Pz4Py)ScOSc(B2Pz4Py).
The reactivity of the hydrido and methyl complexes towards carbon dioxide was
explored; heating to 80˚C results in the formation of k2 formato and acetate complexes, respectively.
The mechanisms were studied via density function theory and distinct transition
states for insertion of CO2 into the Sc-R (R = H, CH3)
were found, with the insertion into the Sc-CH3 being more
enthalpically difficult (by 18 kcal mol-1) than insertion into Sc-H.
The slow rate of reaction between [(B2Pz4Py)ScH]2
and CO2 is attributed to the barrier associated with dimer
dissociation. In both insertion reactions, the kinetic products are k1
formato or acetate complexes that are only slightly less stable than the
derivatives. The k1
compounds can therefore be trapped by treating the k2 isomers with tris-pentafluorophenyl borane.