Am anganese alkylc omplex containing ac hiral bis(oxazolinyl-methylidene)isoindoline pincer ligand is ap recatalyst for ac atalytic system of unprecedented activity and selectivity in the enantioselective hydroboration of ketones, thus producing preparatively useful chiral alcohols in excellent yields with up to greater than 99 %ee. It is applicable for both aryl alkyla nd dialkylk etone reduction under mild reaction conditions (TOF > 450 h À1 at À40 8 8C). The earth-abundant base-metal catalyst operates at very low catalyst loadings (as low as 0.1 mol %) and with ah igh level of functional-group tolerance.T here is evidence for the existence of two distinct mechanistic pathways for manganese-catalyzed hydride transfer and their role for enantiocontrol in the selectivity-determining step is presented.

The carbazole-based pincer ligand R(CbzPNP)H (R = i Pr, t Bu) has been used for the synthesis and characterization of various low- and high-spin cobalt complexes. Upon treatment of the high-spin complexes R(CbzPNP)­CoCl (2 R -Co II Cl) with NaHBEt3, the selective formation of cobalt­(II) hydride 3 i Pr -Co II H and T-shaped cobalt­(I) complex 4 t Bu -Co I was observed, depending on the substituents at the phosphorus atoms. For an unambiguous characterization of the reaction products, a density functional theory (DFT) supported paramagnetic NMR analysis was carried out, which established the electron configuration and the oxidation states of the metal atoms, thus demonstrating the significant impact of ligand substitution on the outcome of the reaction. A distinct one-electron reactivity was found for 4 t Bu -Co I in the dehalogenation of t BuCl and cleavage of PhSSPh. On the other hand, the CoI species displayed two-electron redox behavior in the oxidative addition of dihydrogen. The resulting dihydride complex 6 t Bu -Co III (H) 2 was found to display sluggish reactivity toward alkenes, whereas the cobalt­(II) hydride 3 i Pr -Co II H was successfully employed in the catalytic hydrogenation of unhindered alkenes. The stoichiometric hydrogenolysis of 8 i Pr -Co II Bn at elevated pressure (10 bar) led to a rapid cleavage of the Co–C bond to yield hydride complex 3 i Pr -Co II H. On the other hand, treatment of 2 i Pr -Co II Cl with phenethylmagnesium chloride directly resulted in the formation of 3 i Pr -Co II H, indicating facile β-H elimination of the alkene insertion product (reversibly) generated in the catalytic hydrogenation. On the basis of these observations, a mechanistic pathway involving a key σ-bond metathesis step of the CoII-alkyl species is proposed.

This study focused on the synthesis and characterization of a range of low-valent, high-spin iron(II) complexes supported by a carbazole-based PNP pincer-type ligand. The addition of the lithiated ligand (PNP)Li to FeCl(THF) yielded the chlorido complex (PNP)FeCl (1), which could be readily converted to the four-coordinate iron(II) alkyl complexes (PNP)FeR [R = CHSiMe (3a), Me (3b), CHPh (3c)]. These iron(II) complexes were fully characterized by X-ray analysis and a comprehensive, density-functional-theory-assisted study with complete assignment of their paramagnetic H andC NMR spectra. Treatment of 1 with KHBEt or the addition of molecular hydrogen to (PNP)FeR afforded a high-spin iron(II) PNP hydrido complex, which was identified as the dimer [(PNP)Fe(μ-H)] (4) with two bridging hydrido ligands between the iron centers. Exposing complexes 1 and 4 to carbon monoxide led to the corresponding six-coordinate, diamagnetic complexes (PNP)Fe(CO)Cl (2) and (PNP)Fe(CO)H (5), of which 2 was present as cis/trans isomers. Furthermore, 4 was found to be an active catalyst for the hydrogenation of alkenes.

The association constants and enthalpies for the binding of hydrogen bond donors to group 10 transition metal complexes featuring a single fluoride ligand (trans-[Ni(F)(2-C5NF4)(PR3)2], R = Et 1a, Cy 1b, trans-[Pd(F)(4-C5NF4)(PCy3)2] 2, trans-[Pt(F){2-C5NF2H(CF3)}(PCy3)2] 3 and of group 4 difluorides (Cp2MF2, M = Ti 4a, Zr 5a, Hf 6a; Cp*2MF2, M = Ti 4b, Zr 5b, Hf 6b) are reported. These measurements allow placement of these fluoride ligands on the scales of organic H-bond acceptor strength. The H-bond acceptor capability β (Hunter scale) for the group 10 metal fluorides is far greater (1a 12.1, 1b 9.7, 2 11.6, 3 11.0) than that for group 4 metal fluorides (4a 5.8, 5a 4.7, 6a 4.7, 4b 6.9, 5b 5.6, 6b 5.4), demonstrating that the group 10 fluorides are comparable to the strongest organic H-bond acceptors, such as Me3NO, whereas group 4 fluorides fall in the same range as N-bases aniline through pyridine. Additionally, the measurement of the binding enthalpy of 4-fluorophenol to 1a in carbon tetrachloride (-23.5 ± 0.3 kJ mol(-1)) interlocks our study with Laurence's scale of H-bond basicity of organic molecules. The much greater polarity of group 10 metal fluorides than that of the group 4 metal fluorides is consistent with the importance of pπ-dπ bonding in the latter. The polarity of the group 10 metal fluorides indicates their potential as building blocks for hydrogen-bonded assemblies. The synthesis of trans-[Ni(F){2-C5NF3(NH2)}(PEt3)2], which exhibits an extended chain structure assembled by hydrogen bonds between the amine and metal-fluoride groups, confirms this hypothesis.

A comprehensive experimental and computational mechanistic study of the highly enantioselective hydroboration of ketones catalyzed by a manganese(II) alkyl boxmi pincer complex is reported. The catalyst operates at low catalyst loadings (down to 0.01 mol %) under very mild conditions (typically -40 °C) and facilitates the reduction of both aryl alkyl and dialkyl ketones with excellent selectivity (up to >95%ee). Catalyst activation pathways were investigated, demonstrating that a manganese(II) hydride and a manganese(II) alkoxide species are part of the catalytic cycle and can be generated via σ-bond metathesis of the alkyl precursor with the borane or by alcoholysis. Extensive kinetic experiments based on a "one-pot sequential kinetic profiling" approach under various conditions in combination with kinetic simulations reveal that two catalytic cycles are effective with this earth-abundant base metal catalyst: (i) a minor MnH/borane-mediated insertion cycle, in which the subsequent, product-releasing metathesis step is rate determining ( k = 0.076 s), giving a background reaction, which is zeroth order in substrate concentrations, and (ii) a major MnOR/borane-based alkoxide exchange process, leveraging the high-barrier metathesis via the affiliation to an insertion step. The latter features non-integer reaction orders in both reagents due to a combination of an adduct formation step ( k = 2.12 M s, k = 0.49 s) and a substrate insertion step of comparable rates ( k = 3.74 M s). The kinetic findings are underpinned by high-level density functional theory calculations of the mechanism, control experiments, and kinetic isotope effect/Hammett/Eyring analysis in different concentration regimes. The study highlights the role of a rigorous mechanistic understanding of homogeneous catalytic processes in 3d metals for rational catalyst discovery and optimization.