Syntheses and properties of molecular nickel(II) hydride, methyl, and nickel(I) complexes supported by trimethylphosphane and (2-diphenylphosphanyl)thiophenolato and -naphtholato ligands

Mechanistic Study of an Improved Ni Precatalyst for Suzuki-Miyaura Reactions of Aryl Sulfamates: Understanding the Role of Ni(I) Species

Nickel precatalysts are potentially a more sustainable alternative to traditional palladium precatalysts for the Suzuki-Miyaura coupling reaction. Currently, there is significant interest in Suzuki-Miyaura coupling reactions involving readily accessible phenolic derivatives such as aryl sulfamates, as the sulfamate moiety can act as a directing group for the prefunctionalization of the aromatic backbone of the electrophile prior to cross-coupling. By evaluating complexes in the Ni(0), (I), and (II) oxidation states we report a precatalyst, (dppf)Ni(o-tolyl)(Cl) (dppf = 1,1'-bis(diphenylphosphino)ferrocene), for Suzuki-Miyaura coupling reactions involving aryl sulfamates and boronic acids, which operates at a significantly lower catalyst loading and at milder reaction conditions than other reported systems. In some cases it can even function at room temperature. Mechanistic studies on precatalyst activation and the speciation of nickel during catalysis reveal that Ni(I) species are formed in the catalytic reaction via two different pathways: (i) the precatalyst (dppf)Ni(o-tolyl)(Cl) undergoes comproportionation with the active Ni(0) species; and (ii) the catalytic intermediate (dppf)Ni(Ar)(sulfamate) (Ar = aryl) undergoes comproportionation with the active Ni(0) species. In both cases the formation of Ni(I) is detrimental to catalysis, which is proposed to proceed via a Ni(0)/Ni(II) cycle. DFT calculations are used to support experimental observations and provide insight about the elementary steps involved in reactions directly on the catalytic cycle, as well as off-cycle processes. Our mechanistic investigation provides guidelines for designing even more active nickel catalysts.

Nickel-catalyzed release of H2 from formic acid and a new method for the synthesis of zerovalent Ni(PMe3)4

Ni(PMe₃)₄, which can be obtained by reaction of either Ni(py)₄(O₂CH)₂ or Ni(O₂CH)₂·2H₂O with PMe₃, serves as a catalyst for the release of H₂ from formic acid.

Brønsted acid-promoted C-F bond activation in [P,S]-ligated neutral and anionic perfluoronickelacyclopentanes

Treatment of L2Ni(CF2)44a-c (L = PPh3, PPh2Me, pyridine) with an external Lewis acid (trimethylsilyl triflate) gives new L-functionalized fluoronickelacyclopentanes 5a-c. Complexes L2Ni(CF2)44a,b react with the thiol form of the bidentate ligand 1,2,4-(HS)(Ph2P)Me(C6H3) [P,SH] through a unique Brønsted acid-promoted Cα-F bond activation mechanism, affording phosphine-functionalized nickelacycles bearing a phosphinothiolate ligand 6a-b. Furthermore, substituting monodentate ligands in L2Ni(CF2)44a-c with the deprotonated form of the bidentate ligand [P,S(-)] leads to the first anionic perfluoronickelacycle 7. The anionic metallacyle reacts with phosphonium salts [PHPh3](Br) and [PHPh2Me](Br) to yield HF and phosphine-functionalized nickelacycles 6a,b that still contain the terminal thiolate moiety.

Heteropolytopic phosphanylarylthiolato ligands: formation of cis isomers of nickel(II), palladium(III) and platinum(III) complexes with 1-P(Biph)-2-SHC6H4 (Biph = 1,1'-biphenyl-2,2'-diyl)

Reaction of the ditopic phosphanylarylthiol 1-P(Biph)-2-SHC(6)H(4) (BiphPSH, Biph = 1,1'-biphenyl-2,2'-diyl), prepared by lithiation-electrophilic substitution, with NiCl(2)·6H(2)O, Na(2)[PdCl(4)] and [PtI(2)(cod)] (cod = 1,5-cyclooctadiene) in a 2:1 ratio and in the presence of NEt(3) led to formation of exclusively cis isomers of the square-planar complexes cis-[M{(1-P(Biph)-2-S-C(6)H(4))-κ(2)S,P}(2)] ([M{(BiphPS)-κ(2)S,P}(2)]; M = Ni (1), Pd (2), Pt (3)). Density functional calculations support the assumption that this is probably due to intramolecular π-π interaction of the biphenyl groups, which results in enhanced stability of the cis isomers. Compound 1 is the first example of a structurally characterised mononuclear cis-bis(phosphanylthiolato)nickel(III) complex. Small amounts of the trinuclear complex [{PtI(1-P(Biph)-μ-2-S-C(6)H(4)-κ(2)S,P)}(3)] (4) are also formed besides the mononuclear platinum bis-chelate complex 3.