Scandium alkyl and hydride complexes supported by a pentadentate diborate ligand: reactions with CO₂ and N₂O

Abstract

Alkyl and hydrido scandium complexes of the dianionic pentadentate 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 κ2 formato and acetate complexes, respectively. The mechanisms were studied via density functional theory and distinct transition states for insertion of CO2 into the Sc-R (R = H, CH3) were found, with the insertion into 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 κ1 formato or acetate complexes that are only slightly less stable than the observed κ2 derivatives. The κ1 compounds can therefore be trapped by treating the κ2 isomers with tris-pentafluorophenyl borane.

Diverse Catalytic Systems and Mechanistic Pathways for Hydrosilylative Reduction of CO2

“…Alternatively, M−H can transfer the hydride to CO 2 through an outer‐sphere mechanism, which usually involves CO 2 activation by an external LA . Outer‐sphere hydride transfer that does not involve LA activation has also been discussed, mainly for CO 2 hydrogenation processes . In some main‐group catalyst systems, direct H − transfer from E−H to free or LA‐activated CO 2 has also been proposed …”

Activation of ammonia and hydrazine by electron rich Fe(ii) complexes supported by a dianionic pentadentate ligand platform through a common terminal Fe(iii) amido intermediate

“…8,9,26 The 11 B chemical spectrum also changes drastically from +38.4 ppm for complex 1 Tol to -1.6 ppm for 1 Tol -n NH 3 , which is in the region of diamagnetic metal complexes with this ligand platform. 40 1 Tol -n NH 3 was isolated in 96% yield as a brown solid (Scheme 2) and was characterized by single-crystal X-ray crystallography ( Figure 1). The Fe-N10 bond of 2.072(2) Å confirms the LS configuration by comparison to similar LS Fe-NH 3 adducts.…”

“…We therefore thought of using our tetrapodal dianionic pentadentate B 2 Pz 4 Py ligand platform, which has been successful at accessing a variety of first-row metal complexes. [37][38][39][40][41] The B 2 Pz 4 Py ligand features two borate moieties at the linkage position between the four equatorial pyrazole arms and the axial pyridine, rendering the ligand dianionic and slightly more rigid than the PY5 platform. 42 The charge modification from the PY5 to the B 2 Pz 4 Py platform renders complexes of M(II) ions neutral, and higher oxidation state complexes are generally more accessible with this more electron rich ligand system.…”

Hydrolysis of scandium alkyl derivatives supported by a pentadentate diborate ligand: Interconversion of hydroxo and oxo complexes

“…Hydrolysis of the scandium alkyl complex 1-CH 2 SiMe 3 was examined. Initially, it was assumed that the µ-oxo complex 2 was the thermodynamic product of these reactions, [9] but the system is somewhat more complicated. When solid samples of the alkyl are exposed to ambient atmosphere, reproducible mixtures of 2 and a new dimeric scandium hydroxo complex 3 are produced.…”

“…We recently reported a family of scandium alkyl complexes, 1-R, supported by a dianionic, pentadentate ligand system, B 2 Pz 4 Py (Scheme 2). [9] These compounds have remarkable thermal stability and resistance to sigma bond metathesis reactions [10] common in, for example, decamethylscandocene alkyls. [11] However, they are highly reactive towards protic reagents, including trace amounts of water.…”