Stereoisomerism as an Origin of Different Reactivities of Ir(III) PC(sp3)P Pincer Catalysts.

Two stereoisomers of pentacoordinate iridium(III) hydridochloride with triptycene-based PC(sp3)P pincer ligand (1,8-bis(diisopropylphosphino)triptycene), 1 and 2, differ by the orientation of hydride ligand relative to the bridgehead ring of triptycene. According to DFT/B3PW91/def2-TZVP calculations performed, an equatorial Cl ligand can relatively easily change its position in 1, whereas that is not the case in 2. Both complexes 1 and 2 readily bind the sixth ligand to protect the empty coordination site. Variable temperature spectroscopic (NMR, IR, and UV-visible) studies show the existence of two isomers of hexacoordinate complexes 1·MeCN, 2·MeCN, and 2·Py with acetonitrile or pyridine coordinated trans to hydride or trans to metalated C(sp3), whereas only the equatorial isomer is found for 1·Py. These complexes are stabilized by various intramolecular noncovalent C-H···Cl interactions that are affected by the rotation of isopropyls or pyridine. The substitution of MeCN by pyridine is slow yielding axial Py complexes as kinetic products and the equatorial Py complexes as thermodynamic products with faster reactions of 1·L. Ultimately, that explains the higher activity of 1 in the catalytic alkenes' isomerization observed for allylbenzene, 1-octene, and pent-4-enenitrile, which proceeds as an insertion/elimination sequence rather than through the allylic mechanism.

Dinitrogen Splitting Coupled to Protonation.

The coupling of electron- and proton-transfer steps provides a general concept to control the driving force of redox reactions. N2 splitting of a molybdenum dinitrogen complex into nitrides coupled to a reaction with Brønsted acid is reported. Remarkably, our spectroscopic, kinetic, and computational mechanistic analysis attributes N-N bond cleavage to protonation in the periphery of an amide pincer ligands rather than the {Mo-N2 -Mo} core. The strong effect on electronic structure and ultimately the thermochemistry and kinetic barrier of N-N bond cleavage is an unusual case of a proton-coupled metal-to-ligand charge transfer process, highlighting the use of proton-responsive ligands for nitrogen fixation.

Hydrogen bonding and proton transfer to ruthenium hydride complex CpRuH(dppe): metal and hydride dichotomy.

The combination of variable temperature (190-297 K) IR and NMR spectroscopy studies with quantum-chemical calculations at the DFT/B3PW91 and AIM level had the aim to determine the mechanism of proton transfer to CpRuH(dppe) (1, dppe = Ph(2)P(CH(2))(2)PPh(2)) and the structures of intermediates. Dihydrogen bond (DHB) formation was established in the case of interaction with weak proton donors like CF(3)CH(2)OH. Low-temperature protonation (at about 200 K) by stronger proton donors leads via DHB complex to the cationic nonclassical complex [CpRu(η(2)-H(2))(dppe)](+) (2). Thermodynamic parameters of DHB formation (for CF(3)CH(2)OH: ΔH°(HB) = -4.9 ± 0.2 kcal·mol(-1), ΔS°(HB) = -17.8 ± 0.7 cal·mol(-1)·K(-1)) and proton transfer (for (CF(3))(2)CHOH: ΔH°(PT) = -5.2 ± 0.3 kcal·mol(-1), ΔS°(PT) = -23 ± 1 cal·mol(-1)·K(-1)) were determined. Above 240 K 2 transforms into trans-[CpRu(H)(2)(dppe)](+) (3) yielding a mixture of 2 and 3 in 1:2 ratio. Kinetic analysis and activation parameters for the "[Ru(η(2)-H(2))](+) → trans-[Ru(H)(2)](+)" transformation indicate reversibility of this process in contrast to irreversible intramolecular isomerization of the Cp* analogue. Calculations show that the driving force of this process is greater stability (by 1.5 kcal·mol(-1) in ΔE scale) of the dihydride cation in comparison with the dihydrogen complex. The calculations of the potential energy profile indicate the low barrier for deprotonation of 2 suggesting that the formation of trans-[CpRu(H)(2)(dppe)](+) proceeds via deprotonation of [Ru(η(2)-H(2))](+) to DHB complex, formation of hydrogen bond with Ru atom and subsequent proton transfer to the metal site.