C-H bond activation via concerted metalation-deprotonation at a palladium(iii) center.

Herein we report the direct observation of C-H bond activation at an isolated mononuclear Pd(iii) center. The oxidation of the Pd(ii) complex (MeN4)PdII(neophyl)Cl (neophyl = -CH2C(CH3)2Ph; MeN4 = N,N'-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) using the mild oxidant ferrocenium hexafluorophosphate (FcPF6) yields the stable Pd(iii) complex [(MeN4)PdIII(neophyl)Cl]PF6. Upon the addition of an acetate source, [(MeN4)PdIII(neophyl)Cl]PF6 undergoes Csp2-H bond activation to yield the cyclometalated product [(MeN4)PdIII(cycloneophyl)]PF6. This metalacycle can be independently prepared, allowing for a complete characterization of both the starting and final Pd(iii) complexes. The C-H activation step can be monitored directly by EPR and UV-Vis spectroscopies, and kinetic isotope effect (KIE) studies suggest that either a pre-association step such as an agostic interaction may be rate limiting, or that the C-H activation is partially rate-limiting in conjunction with ligand rearrangement. Density functional theory calculations support that the reaction proceeds through a κ3 ligand coordination and that the flexible ligand structure is important for this transformation. Overall, this study represents the first example of discrete C-H bond activation occurring at a Pd(iii) center through a concerted metalation-deprotonation mechanism, akin to that observed for Pd(ii) and Pd(iv) centers.

Reduction of highly bulky triphenolamine molybdenum nitrido and chloride complexes.

Transition metal nitrides are key intermediates in the catalytic reduction of dinitrogen to ammonia. To date, transition metal nitride complexes with the triphenolamine (TPA) ligand have not been reported and the system with the ligand has been much less studied for ammonia formation compared with other systems. Herein, we report a series of molybdenum complexes supported by a sterically demanding TPA ligand, including a nitrido complex NMo(TPA). We achieved the stoichiometric conversion of the nitride moiety into ammonia under ambient conditions by adding proton and electron sources to NMo(TPA). However, the catalytic turnover for N2 reduction to ammonia was not observed in the triphenolamine ligand system unlike the Schrock system-triamidoamine ligand. Density functional theory calculation revealed that the molybdenum center favors binding NH3 over N2 by 16.9 kcal mol-1 and the structural lability of the trigonal bipyramidal (TBP) molybdenum complex seems to prevent catalytic turnover. Our systematic study showed that the electronegativity and bond length of ancillary ligands determine the preference between N2 and NH3, suggesting a systematic design strategy for improvement.

Fixation of Dinitrogen at an Asymmetric Binuclear Titanium Complex.

A new type of dititanium dinitrogen complex supported by a triphenolamine (TPA) ligand is reported. Analysis by single-crystal X-ray diffraction and Raman and NMR spectroscopy reveals different coordination geometries for the two titanium centers. Hence, coordination of TPA and a nitrogen ligand results in trigonal-bipyramidal geometry, while an octahedral titanium center is obtained upon additional coordination of an ethoxide generated upon C-O bond cleavage in a diethyl ether solvent molecule. The titanium complex successfully generates ammonia in the presence of an excess amount of PCy3HI and KC8 in 154% yield (per titanium atom). A titanium complex with a bulkier TPA does not form a dinitrogen complex, and mononuclear titanium dinitrogen complexes were not accessible, presumably because of the high tendency of early transition metals to form binuclear dinitrogen complexes.

Concurrent Formation of N-H Imines and Carbonyl Compounds by Ruthenium-Catalyzed C-C Bond Cleavage of ß-Hydroxy Azides.

A commercial cyclopentadienylrutenium dicarbonyl dimer ([CpRu(CO)2]2) efficiently catalyzes the formation of N-H imines and carbonyl compounds simultaneously from ß-hydroxy azides via C-C bond cleavage under visible light. Density functional theory calculations for the cleavage reaction support the mechanism involving chelation of alkoxy azide species and liberation of nitrogen as the driving force. The synthetic utility of the reaction was demonstrated by a new amine synthesis promoted by chemoselective allylation of imine and synthesis of isoquinoline.

First-row early transition metal complexes with a highly sterically demanding triisopropylphenyl amino triphenolate ligand: synthesis and applications.

Amino triphenolate ligands have been widely used for the synthesis of various transition metal complexes aiming at various applications such as ring-opening polymerization, olefin polymerization, and sulfoxidation. However, the introduction of highly sterically demanding aromatic substituents, such as triisopropylphenyl (TRIP), to the amino triphenolate ligand has not been previously reported probably due to the synthetic difficulty. In six-step reactions using commercial materials, a highly sterically demanding amino triphenolate ligand was successfully synthesized, and early transition metal complexes (Ti, V, Cr, Mn) supported by the ligand were also obtained and fully characterized. In addition, titanium and chromium complexes were further used for catalytic sulfoxidation, and polymerization of ethylene, respectively.

N-heterocyclic carbene nitric oxide radicals.

N-Heterocyclic carbene-stabilized nitric oxide radicals were prepared by direct addition of nitric oxide to two N-heterocyclic carbenes in solution phase. The compounds were fully characterized by X-ray crystallography and EPR. The nitric oxide moiety in the solid compounds obtained can be thermally transferred to another N-heterocyclic carbene, suggesting potential applications to NO delivery.