C-H bond activation 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 (N4)Pd(neophyl)Cl (neophyl = -CHC(CH)Ph; N4 = ,'-dimethyl-2,11-diaza[3.3](2,6)pyridinophane) using the mild oxidant ferrocenium hexafluorophosphate (FcPF) yields the stable Pd(iii) complex [(N4)Pd(neophyl)Cl]PF.

Electronic versus steric effects of pyridinophane ligands on Pd(iii) complexes.

Several new Pd and Pd complexes supported by electronically and sterically tuned tetradentate pyridinophane ligands N4, N4, and N4 were isolated and fully characterized (N4: N,N'-dimethyl-2,11-diaza[3,3](2,6)-para-methoxypyridinophane; N4: N,N'-dimethyl-2,11-diaza[3,3](2,6)pyridinophane; N4: N,N'-di-tert-butyl-2,11-diaza[3,3](2,6)pyridinophane). Cyclic voltammetry studies, UV-vis and EPR spectroscopy, and X-ray crystallography were employed to reveal that the steric properties of the N-substituents of the N4 ligands have a pronounced effect on the electronic properties of the corresponding Pd complexes, while the electronic tuning of the ligand pyridyl groups has a surprisingly minimal effect. An explanation for these observations was provided by DFT and TD-DFT calculations which suggest that the electronic properties of the Pd complexes are mainly dictated by their frontier molecular orbitals that have major atomic contributions from the Pd center (mainly the Pd d atomic orbital) and the axial N atom donors.

Improved synthesis of symmetrically & asymmetrically N-substituted pyridinophane derivatives.

The N,N'-di(toluenesulfonyl)-2,11-diaza[3,3](2,6)pyridinophane (N4) precursor was sought after as a starting point for the preparation of various symmetric and asymmetric pyridinophane-derived ligands. Various procedures to synthesize N4 had been published, but the crucial problem had been the purification of N4 from the larger 18- and 24-membered azamacrocycles. Most commonly, column chromatography or other laborious methods have been utilized for this separation, yet we have found an alternate selective dissolution method upon protonation which allows for multi-gram scale output of N4·HCl.

Stable bis(trifluoromethyl)nickel(III) complexes.

Organometallic Ni(III) intermediates have been proposed in several Nickel-catalyzed cross-coupling reactions, yet no isolated bis(hydrocarbyl)Ni(III) complexes have been reported to date. Herein we report the synthesis and detailed characterization of stable organometallic Ni(III) complexes that contain two trifluoromethyl ligands and are supported by tetradentate N-donor ligands (R)N4 (R = Me or tBu). Interestingly, the corresponding Ni(II) precursors undergo facile oxidation, including aerobic oxidation, to generate uncommonly stable organometallic Ni(III) complexes that exhibit limited reactivity.

Organometallic nickel(III) complexes relevant to cross-coupling and carbon-heteroatom bond formation reactions.

Nickel complexes have been widely employed as catalysts in C-C and C-heteroatom bond formation reactions. In addition to Ni(0) and Ni(II) intermediates, several Ni-catalyzed reactions are proposed to also involve odd-electron Ni(I) and Ni(III) oxidation states. We report herein the isolation, structural and spectroscopic characterization, and organometallic reactivity of Ni(III) complexes containing aryl and alkyl ligands.

Structural and reactivity comparison of analogous organometallic Pd(III) and Pd(IV) complexes.

The tetradentate ligands (R)N4 ((R)N4 = N,N'-di-alkyl-2,11-diaza[3,3](2,6)pyridinophane, R = Me or iPr) were found to stabilize cationic ((R)N4)PdMe(2) and ((R)N4)PdMeCl complexes in both Pd(III) and Pd(IV) oxidation states. This allows for the first time a direct structural and reactivity comparison of the two Pd oxidation states in an identical ligand environment. The Pd(III) complexes exhibit a distorted octahedral geometry, as expected for a d(7) metal center, and display unselective C-C and C-Cl bond formation reactivity.