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

The N,N'-di(toluenesulfonyl)-2,11-diaza[3,3](2,6)pyridinophane (TsN4) precursor was sought after as a starting point for the preparation of various symmetric and asymmetric pyridinophane-derived ligands. Various procedures to synthesize TsN4 had been published, but the crucial problem had been the purification of TsN4 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 TsN4·HCl. This optimized synthesis of TsN4 also led to the development of symmetric RN4 derivatives as well as the asymmetric derivative N-(tosyl)-2,11-diaza[3,3](2,6)pyridinophane (TsHN4). Using this TsHN4 precursor, different N-substituents can be added to create a library of asymmetric RR'N4 macrocyclic ligands. These asymmetric RR'N4 derivatives expand the utility of the RN4 framework in coordination chemistry and the ability to study the electronic, steric, and denticity effects of these pyridinophane ligands on the metal center.

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. These Ni(III) species undergo transmetalation and/or reductive elimination reactions to form new C-C or C-heteroatom bonds and are also competent catalysts for Kumada and Negishi cross-coupling reactions. Overall, these results provide strong evidence for the direct involvement of organometallic Ni(III) species in cross-coupling reactions and oxidatively induced C-heteroatom bond formation reactions.

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.

Photoreductive chlorine elimination from a Ni(iii)Cl2 complex supported by a tetradentate pyridinophane ligand.

Herein we report the isolation, characterization, and photoreactivity of a stable NiIII dichloride complex supported by a tetradentate pyridinophane N-donor ligand. Upon irradiation, this complex undergoes an efficient photoreductive chlorine elimination reaction, both in solution and the solid-state. Subsequently, the NiIIICl2 species can be regenerated via a reaction with PhICl2.

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

Several new PdII and PdIII complexes supported by electronically and sterically tuned tetradentate pyridinophane ligands MeN4OMe, MeN4, and tBuN4 were isolated and fully characterized (MeN4OMe: N,N'-dimethyl-2,11-diaza[3,3](2,6)-para-methoxypyridinophane; MeN4: N,N'-dimethyl-2,11-diaza[3,3](2,6)pyridinophane; tBuN4: 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 RN4 ligands have a pronounced effect on the electronic properties of the corresponding PdIII 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 PdIII complexes are mainly dictated by their frontier molecular orbitals that have major atomic contributions from the Pd center (mainly the Pd dz2 atomic orbital) and the axial N atom donors.

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.

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. By contrast, the Pd(IV) complexes have a pseudo-octahedral geometry and undergo selective non-radical C-C or C-Cl bond formation that is controlled by the ability of the complex to access a five-coordinate intermediate.