Advances in Accessing Rare Oxidation States of Nickel for Catalytic Innovation

Zubaydi S.A., Waske S., Akyildiz V., Starbuck H.F., Majumder M., Moore C.E., Kalyani D., Sevov C.S.

The selective cross-coupling of two alkyl electrophiles to construct complex molecules remains a challenge in organic synthesis1,2. Known reactions are optimized for specific electrophiles and are not amenable to interchangeably varying electrophilic substrates that are sourced from common alkyl building blocks, such as amines, carboxylic acids and halides3–5. These limitations restrict the types of alkyl substrate that can be modified and, ultimately, the chemical space that can be explored6. Here we report a general solution to these limitations that enables a combinatorial approach to alkyl–alkyl cross-coupling reactions. This methodology relies on the discovery of unusually persistent Ni(alkyl) complexes that can be formed directly by oxidative addition of alkyl halides, redox-active esters or pyridinium salts. The resulting alkyl complexes can be isolated or directly telescoped to couple with a second alkyl electrophile, which represent cross-selective reactions that were previously unknown. The utility of this synthetic capability is showcased in the rapid diversification of amino acids, natural products, pharmaceuticals and drug-like building blocks by various combinations of dehalogenative, decarboxylative or deaminative coupling. In addition to a robust scope, this work provides insights into the organometallic chemistry of synthetically relevant Ni(alkyl) complexes through crystallographic analysis, stereochemical probes and spectroscopic studies. Alkyl–alkyl cross-coupling reactions are achieved by forming nickel–alkyl complexes directly by oxidative addition of alkyl halides, redox-active esters or pyridinium salts, and then combining them with a second alkyl electrophile.

Saeb R., Nöthling N., Cornella J.

Saeb R., Boulenger B., Cornella J.

Ghosh S., Rooj A., Chakrabortty R., Ganesh V.

Chakrabortty R., Ghosh S., Ganesh V.

Rubel C.Z., He W., Wisniewski S.R., Engle K.M.

Pahar S., Sharma V., Raj K.V., Sangole M.P., George C.P., Singh K., Vanka K., Gonnade R.G., Sen S.S.

AbstractThe reaction of a nickel(II) chloride complex containing a tridentate β‐diketiminato ligand with a picolyl group [2,6‐iPr2‐C6H3NC(Me)CHC(Me)NH(CH2py)]Ni(II)Cl (1)] with KSi(SiMe3)3 conveniently afforded a nickel(I) radical with a T‐shaped geometry (2). The compound‘s metalloradical nature was confirmed through electron paramagnetic resonance (EPR) studies and its reaction with TEMPO, resulting in the formation of a highly unusual three‐membered nickeloxaziridine complex (3). When reacted with disulfide and diselenide, the S−S and Se−Se bonds were cleaved, and a coupled product was formed through carbon atom of the pyridine‐imine group. The nickel(I) radical activates dihydrogen at room temperature and atmospheric pressure to give the monomeric nickel hydride.

Jevtović M., Pevec A., Turel I., Radanović D., Milčić M., Gruden M., Zlatar M., Mitić D., Anđelković K., Čobeljić B.

In this study, the properties of nickel(II) thiosemicarbazone (complex 1) and nickel(III) (complex 2) hydrazone complexes were investigated using single crystal X-ray diffraction analysis, electron paramagnetic resonance, infrared spectroscopy, UV-Vis spectroscopy, molar conductivity and Density Functional Theory (DFT) calculations. The large difference in magnetic moments led us to suspect that we had obtained nickel complexes of different oxidation states (3.5 μB and 2.1 μB). This was verified after recording the Electron Paramagnetic Resonance (EPR) spectra for complex 2. The g value of 2.018 is consistent with a low spin (S = 1/2) Ni(III) species. DFT calculations are in agreement with the EPR data and show a doublet spin state of complex 2 with the unpaired electron practically completely located in the first coordination sphere (76.5%). All the findings show that the studied complex have different structural and electronic properties as a function of the oxidized state of nickel. In general, nickel(III) complexes are more difficult to obtain than nickel(II) complexes due to the stability of the nickel(II) oxidation state. Considering that this is the first Ni(III) hydrazone complex to be synthesised without an oxidising agent, its importance lies in providing insight into the fundamental properties and structure of Ni(III) hydrazone complexes. This could help to improve their synthesis and further investigate their applications.

Fan Y., Kang D.W., Labalme S., Lin W.

Khamrai A., Ganesh V.

We demonstrate the potential of Ni(COD)(DQ), a bench-stable Ni0 complex, as a catalyst for the reductive coupling of aldehydes with alkynes and ynamides, providing silylated allyl alcohols with excellent yields and regioselectivities.

Barbor J.P., Nair V.N., Sharp K.R., Lohrey T.D., Dibrell S.E., Shah T.K., Walsh M.J., Reisman S.E., Stoltz B.M.

Mtshali Z., Conradie J.

A series of 13 tris(polypyridine)nickel(II) complexes were synthesized and subjected to electrochemical (using cyclic voltammetry) and theoretical (using density functional theory, DFT) studies. Experimentally, one oxidation and a couple successive reduction processes are observed for the tris(polypyridine)nickel(II) complexes. The experimental Ni(II/III) oxidation potential follows the same trend as the experimental metal(II/III) oxidation potential of related tris(polypyridine)metal(II) complexes, metal = Co, Os, Fe, Ru and Mn, with the tris(polypyridine)nickel(II) being oxidized at the most positive potential. DFT calculations show that the tris(polypyridine)nickel(II) complexes have an octahedral coordination sphere, while the oxidized tris(polypyridine)nickel(III) complexes exhibit a Jahn-Teller distorted geometry. The change in geometry from octahedral to Jahn-Teller distorted upon oxidation of tris(polypyridine)nickel(II), may contribute to the large peak current potential separations with small peak current ratios experimentally measured for the Ni(II/III) redox process. The DFT calculations further show that the reduced tris(polypyridine)nickel(II) complexes, [Ni(polypyridine)3]n+, contain a Ni(II) central ion ferromagnetically coupled to one (n = 1), two (n = 0) or three (n = -1) polypyridine radicals. The tris(polypyridine)nickel(II) complexes thus exhibit polypyridine ligand-based reduction.

Pan Q., Ping Y., Kong W.

Tran V.T., Kim N., Rubel C.Z., Wu X., Kang T., Jankins T.C., Li Z., Joannou M.V., Ayers S., Gembicky M., Bailey J., Sturgell E.J., Sanchez B.B., Chen J.S., Lin S., et. al.

A flurry of recent research has centered on harnessing the power of nickel catalysis in organic synthesis. These efforts have been bolstered by contemporaneous development of well-defined nickel (pre)catalysts with diverse structure and reactivity. In this report, we present ten different bench-stable, 18-electron, formally zero-valent nickel-olefin complexes that are competent pre-catalysts in various reactions. Our investigation includes preparations of novel, bench stable Ni(COD)(L) complexes (COD = 1,5-cyclooctadiene), in which L = quinone, cyclopentadienone, thiophene-S-oxide, and fulvene. Characterization by NMR, IR, single-crystal X-ray diffraction, cyclic voltammetry, thermogravimetric analysis, and natural bond orbital analysis sheds light on the structure, bonding, and properties of these complexes. Applications in an assortment of nickel-catalyzed reactions underscore the complementary nature of the different pre-catalysts within this toolkit.

Zheng Y., Li C., Liu W., Yu Z.

Present here is a density functional theory (DFT) study of the mechanism and origin of enantioselectivity of Ni-catalyzed desymmetric cyclization of alkyne-tethered malononitriles and aryl boronic acids. The reaction starts from transmetalation and arylnickel addition, followed by trans to cis isomerization to give cis-alkenyl nickel species. The stereodetermining step is the CN insertion, which prefers a transition state with the bystander CN group staying away from the ligand to reduce steric repulsion, and gives the final (R)-product.