Stoichiometric and Catalytic Reactivity of Ni(6-Mes)(PPh3)2

The coordination of alkali-metal nickelates to organic π-systems: synthetic, structural and spectroscopic insights

Low-valent nickelates have recently been shown to be key intermediates in challenging cross-coupling reactions using aryl ethers as electrophiles. Key for the success of these transformations is the activation of the substrate through π-coordination to the nickelate intermediate, however there is still limited knowledge about the fundamental structure and coordination chemistry of these heterobimetallic complexes. Herein, we report the synthesis, structures, and spectroscopic analysis of a diverse family of alkali-metal nickelates derived from phenyl-alkali-metal reagents and Ni(ttt-CDT), where ttt-CDT = trans,trans,trans-1,5,9-cyclododecatriene. The co-complexation of PhLi with Ni(ttt-CDT) was found to yield 1 : 1, 2 : 1 or 4 : 2 lithium nickelates depending on the stoichiometry and reaction conditions employed. The high lability of the ttt-CDT ligand enables facile ligand exchange with an assorted series of organic π-acceptors, ranging from polyaromatic hydrocarbons to ketones, imines and nitriles. For anthracene and phenanthrene, a homologous series of Li, Na and K nickelates could be obtained, which lead to different structural motifs or degrees of aggregation in the solid-state spanning solvated monomers to complex polymeric arrangements. For π-extended systems such as perylene or coronene, competing single-electron-transfer to give the corresponding radical anions was observed, illustrating the highly reducing nature of the alkali-metal nickelates. X-ray crystallographic analysis and NMR spectroscopy of the phenyl-alkali-metal nickelates reveal extreme back-bonding from Ni(0) to the organic π-acceptors due to strong σ-donation from the carbanionic ligands.

Stoichiometric and Catalytic Lithium Nickelate-Mediated C-F Bond Alkynylation of Fluoroarenes

Low-valent nickelates have recently been shown to be key intermediates that facilitate challenging cross-coupling reactions under mild conditions. Expanding the synthetic potential of these heterobimetallic complexes, herein we report the success of trilithium nickelate Li3(TMEDA)3Ni(C≡C-Ph)3 in promoting stoichiometric C-F activation of assorted aryl fluorides furnishing novel mixed Li/Ni(0) or Li/Ni(II) species depending on the substrate and conditions employed. These stoichiometric successes can be upgraded to catalytic regimes to enable the atom-efficient alkynylation of aryl fluorides and polyfluoroarenes with lithium acetylides and precatalyst Ni(COD)2, which operates without the intervention of external ligands, Cu cocatalysts, or additives.

A mesoionic carbene stabilized nickel(II) hydroxide complex: a facile precursor for C-H activation chemistry

We report the synthesis of a new nickel(II) hydroxide complex 2 supported by a rigid, tridentate triazolylidene-carbazolid ligand. The complex can be accessed in high yields following a simple and stepwise extraction protocol using dichloromethane and aqueous ammonium chloride followed by aqeous sodium hydroxide solution. We found that complex 2 is highly basic, undergoing various deprotonation/desilylation reactions with E-H and C-H acidic and silylated compounds. In this context we synthesized a variety of novel, functionalized nickel(II) complexes with trimethylsilylolate (3), trityl sulfide (4), tosyl amide (5), azido (6), pyridine (7), acetylide (8, 9), fluoroarene (10 & 11) and enolate (12) ligands. We furthermore found that 2 reacts with malonic acid dimethyl ester in a knoevennagel-type condensation reaction, giving access to a new enolate ligand in complex 13, consisting of two malonic acid units. Furthermore, complex 2 reacts with acetonitrile to form the cyanido complex 14. The formation of complexes 13 and 14 is particularly interesting, as they underline the potential of complex 2 in both C-C bond formation and cleavage reactions.

Dipyrromethane-diphosphine: the effect of meso substituents on the formation of nickel complexes and on their performance in the transfer hydrogenation of ketones

Three dipyrromethane-diphosphine ligands containing phenyl (L1H2), ethyl (L2H2) and cyclohexyl (L3H2) groups at their meso positions and their nickel complexes were synthesized and structurally characterized. Treatment of Ph2C(C4H3N)2-1,9-(CH2PPh2)2 (L1H2) with [NiCl2(DME)] gave complex [NiCl2(κ2-P,P-L1H2)] 2a. Conversely, the analogous reactions of L2H2 and L3H2 with [NiCl2(DME)] showed a mixture of products containing both a pyrrolide nitrogen coordinated complex of type [Ni(κ4-P,N,N,P-L)] 3 without an exogenous base and a chelated complex of type 2a. In addition, all three ligands react with [NiCl2(DME)] in the presence of a strong base to give a complex of type 3. Furthermore, a novel binuclear Ni(0) complex bearing L1H2 was characterized by X-ray crystallography. Both complexes 2a and 3 (0.5 mol% of loading) catalyze the transfer hydrogenation of a series of aromatic and aliphatic ketones (20 substrates) to their corresponding secondary alcohols using iPrOH as a hydrogen source in the presence of KOH at 100 °C in 6 h. The kinetic trace of the catalytic reaction shows that the meso-phenyl substituted diphosphine coordinated nickel complexes perform better than the other two ligand coordinated nickel complexes.

Triangular Palladium Cluster from Activation of the Si-Si Bond in a Disilane with Phosphine Pendants

A disilane that contains two diphenylphosphino moieties, (Ph₂PCH₂)Ph₂Si-SiPh₂(CH₂PPh), was readily synthesized from the reaction of ClPh₂Si-SiPh₂Cl with (tmeda)Li(CH₂PPh₂). Treatment of the thus-obtained disilane with the palladium(0) precursor [Pd(CN^tBu)₂]₃ led to the exclusive formation of a trinuclear palladium cluster in which three palladium atoms are arranged in a triangular fashion. Single-crystal X-ray diffraction analysis of the obtained triangular cluster revealed that novel silylphosphido chelating ligands were formed via a skeletal rearrangement of the ligand framework.

[Ni(NHC)2 ] as a Scaffold for Structurally Characterized trans [H-Ni-PR2 ] and trans [R2 P-Ni-PR2 ] Complexes

The addition of PPh2 H, PPhMeH, PPhH2 , P(para-Tol)H2 , PMesH2 and PH3 to the two-coordinate Ni0 N-heterocyclic carbene species [Ni(NHC)2 ] (NHC=IiPr2 , IMe4 , IEt2 Me2 ) affords a series of mononuclear, terminal phosphido nickel complexes. Structural characterisation of nine of these compounds shows that they have unusual trans [H-Ni-PR2 ] or novel trans [R2 P-Ni-PR2 ] geometries. The bis-phosphido complexes are more accessible when smaller NHCs (IMe4 >IEt2 Me2 >IiPr2 ) and phosphines are employed. P-P activation of the diphosphines R2 P-PR2 (R2 =Ph2 , PhMe) provides an alternative route to some of the [Ni(NHC)2 (PR2 )2 ] complexes. DFT calculations capture these trends with P-H bond activation proceeding from unconventional phosphine adducts in which the H substituent bridges the Ni-P bond. P-P bond activation from [Ni(NHC)2 (Ph2 P-PPh2 )] adducts proceeds with computed barriers below 10 kcal mol-1 . The ability of the [Ni(NHC)2 ] moiety to afford isolable terminal phosphido products reflects the stability of the Ni-NHC bond that prevents ligand dissociation and onward reaction.

The ambiguous behaviour of diphosphines towards the quasilinear iron(i) complex [Fe(N(SiMe3)2)2]− – between inertness, P–C bond cleavage and C–C double bond isomerisation

A quasilinear iron(i) complex facilitates C–C bond isomerisation or P-aryl bond cleavage of diphosphines but is otherwise inert to simple phosphine coordination.

N-Heterocyclic Carbene Complexes of Copper, Nickel, and Cobalt

The emergence of N-heterocyclic carbenes as ligands across the Periodic Table had an impact on various aspects of the coordination, organometallic, and catalytic chemistry of the 3d metals, including Cu, Ni, and Co, both from the fundamental viewpoint but also in applications, including catalysis, photophysics, bioorganometallic chemistry, materials, etc. In this review, the emergence, development, and state of the art in these three areas are described in detail.

Room Temperature Ni(II) Catalyzed Hydrophosphination and Cyclotrimerization of Alkynes

The catalytic activity of nickel complexes in hydrophosphination involving secondary phosphines is not a commonly studied transformation. Beyond a small number of stand-out examples, many reports in the literature focus on the use of simple nickel salts. β-Diketiminates have been proven to be incredibly effective ligands for catalysis using a range of metal centers. This synthetic study investigates the catalytic ability of a Ni(II) β-diketiminate complex in the hydrophosphination of alkenes and alkynes, with a serendipitous discovery of its ability to effect alkyne cyclotrimerization and phosphine dehydrocoupling.

An N-heterocyclic carbene iridium catalyst with metal-centered chirality for enantioselective transfer hydrogenation of imines

A cyclometalating N-heterocyclic carbene iridium complex featuring metal-centered chirality was designed and used for the asymmetric transfer hydrogenation (ATH) of imines. Four strongly σ-donating carbon-based substituents (2 carbenes and 2 phenyl moieties), a chirality transfer directly from the stereogenic metal center to the C[double bond, length as m-dash]N bond of substrates, as well as a restriction of catalyst deactivation by steric demanding substituents, render the new complex one of the most efficient catalysts for ATH of cyclic N-sulfonylimines (down to 0.01 mol% cat., 24 examples, 94-98% ee).

DFT modelling of a diphosphane - N-heterocyclic carbene-Rh(i) pincer complex rearrangement: a computational evaluation of the electronic effects in C-P bond activation

DFT calculations confirmed that the rearrangement of a PCP-Rh-H pincer to a CCP-Rh-phosphane pincer occured by C-P oxidative addition (ΔG^‡ = 29.5 kcal mol^-1, rate-determining step), followed by P-H reductive elimination (ΔG^‡ = 4.8 kcal mol^-1). The oxidative addition proceeded via a 3-centered transition state and is accelerated by electron-withdrawing substituents p- to the reacting C-P bond, resulting in a reaction constant (ρ) of 2.12 for ΔG^‡ and 2.76 for ΔH^‡ in a Hammett-type linear free energy relationship. AIM wavefunction analyses indicated a decrease in the negative charge on the carbon bonded to Rh with a concomitant increase in the positive charge on the latter. The electronic density at the Rh-P bond critical point and the atomic charge on Rh correlate well with the Hammett constants (σ) of the p-substituents. The replacement of the Rh-bound hydride with other anions (CH₃, Ph, t-Bu, OH, F, Cl, and CN) results in a decrease in the OA barrier only for CH₃, which is in accordance with the experimental results. The reductive elimination occurs via a 3-centered (Rh, H, P) transition state, which adopts a conformation wherein the steric clash between the i-Pr groups is minimized, followed by recomplexation of Rh and the newly formed (i-Pr)₂PH by a conformational twist around the Rh-P axis.

[Ru3(6-NHC)(CO)10]: synthesis, characterisation and reactivity of rare 46-electron tri-ruthenium clusters

The room temperature reaction of 6-membered ring N-heterocyclic carbenes with [Ru₃(CO)₁₂] affords [Ru₃(6-NHC)(CO)₁₀], rare examples of coordinatively unsaturated, 46-electron tri-ruthenium clusters. Upon mild heating in the presence of C₅H₅N, H₂ or PPh₃, these compounds lose carbene.

Mono- and dinuclear Ni(i) products formed upon bromide abstraction from the Ni(i) ring-expanded NHC complex [Ni(6-Mes)(PPh3)Br]

New T- and Y-shaped Ni(i) complexes are reported and analysed by DFT and EPR.

NI(ii) phosphine and phosphide complexes supported by a PNP-pyrrole pincer ligand

The reaction between [(PN^pyrP)NiCl] (1, PN^pyrP = 2,5-bis((di-iso-propylphosphino)-methyl)-1H-pyrrolide) and TlPF₆ in the presence of a monodentate phosphine ligand led to cationic nickel phosphine and phosphite complexes, [(PN^pyrP)Ni(PHPh₂)][PF₆] (2), [(PN^pyrP)Ni(PMe₃)][PF₆] (3), and [(PN^pyrP)Ni{P(OMe)₃}][PF₆] (4). Compound 2 can be deprotonated resulting in the generation of a terminal phosphido complex, [(PN^pyrP)Ni(PPh₂)] (5). When 3 is subjected to a base, a methyl proton of PMe₃ is abstracted to afford [(PN^pyrP)Ni(CH₂PMe₂)] (6), containing a methylene bridge between Ni and the external phosphine. Compounds 2-6 were characterized by single crystal X-ray diffraction in addition to multi-nuclear NMR spectroscopy and elemental analysis.