Synthesis and Reactivities of New NCN-Type Pincer Complexes of Nickel

Reactions of cyclonickelated complexes with hydroxylamines and TEMPO˙: isolation of new TEMPOH adducts of Ni(II) and their reactivities with nucleophiles and oxidants

This contribution describes a study on the reactivities of the complexes [{κP,κC-(i-Pr)2PO-Ar}Ni(μ-Br)]2, 1a-d (Ar: C6H4, a; 3-Cl-C6H3, b; 3-OMe-C6H3, c; 4-OMe-napthalenyl, d), with hydroxylamines in the presence of TEMPO˙ (TEMPO˙ = (2,2,6,6-tetramethylpiperidinyl-1-yl)oxyl). The results of this study showed that treating 1a-d with a mixture of Et2NOH and TEMPO˙ did not afford the desired oxidation-induced functionalization of the Ni-aryl moiety in 1a-d, giving instead the corresponding κO-TEMPOH adducts [{κP,κC-(i-Pr)2PO-Ar}Ni(Br)(κO-TEMPOH)], 3a-d (TEMPOH = N-hydroxy-2,2,6,6-tetramethylpiperidine). The TEMPOH moiety in these zwitterionic compounds 3 can be displaced by a large excess of acetonitrile (MeCN), 10 equiv. of morpholine, or 1-2 equivalents of imidazole. Although these reactions have given the authenticated products [{κP,κC-(i-Pr)2PO-C6H4}Ni(Br)(NCMe)], 4a, [{κP,κC-(i-Pr)2PO-C6H4}Ni(Br)(morpholine)], 5a, and [{κP,κC-(i-Pr)2PO-C6H4}Ni(imidazole)2]Br, 6a, a few other products were also detected by NMR, indicating that the observed reactivities are far more complex than simple substitution of the TEMPOH moiety. Similarly, treating 3a with AgOC(O)CF3 results in the isolation of [{κP,κC-(i-Pr)2PO-C6H4}Ni{OC(O)CF3}(κO-TEMPOH)], 7a, arising from the substitution of the bromo ligand, whereas spectroscopic evidence suggests further reactivity, possibly including displacement of TEMPOH and oxidative decomposition.

Iron-catalyzed one-pot cyclization and amination of 2-alkynylthioanisoles using nitrosobenzenes as the amine source

A direct strategy for the synthesis of 3-phenylaminobenzothiophene via iron-catalyzed cyclization of 2-alkynylthioanisoles and nitrosoarenes is presented in this work.

Aerobic oxidation-functionalization of the aryl moiety in van Koten's pincer complex (NCN)Ni(ii)Br: relevance to carbon–heteroatom coupling reactions promoted by high-valent nickel species

Treating the pincer complex (NCN)NiBr with protic substrates HX (X = OH, OR, or NR2) under aerobic conditions leads to C–X functionalization of the pincer ligand. The crucial importance of aerobic conditions for the success of this coupling reaction implies the formation of high-valent intermediates during the course of the reaction.

Which Type of Pincer Complex Is Thermodynamically More Stable? Understanding the Structures and Relative Bond Strengths of Group 10 Metal Complexes Supported by Benzene-Based PYCYP Pincer Ligands

The influence of the pincer platform composition and substitution on the reactivity and physical properties of pincer complexes can be easily explored through different experimental techniques. However, the influence of these factors on the molecular structures and thermodynamic stability of pincer complexes is usually very subtle and cannot always be unambiguously established. To rationalize this subtle influence, a survey of crystallographic data from 130 group 10 metal pincer complexes supported by benzene-based PYCYP pincer ligands, [2,6-(R₂PY)₂C₆H_3-nR'_n]MX (Y = CH₂, NH, O, S; M = Ni, Pd, Pt; R = ^tBu, ⁱPr, Ph, Cy, Me; R' = CO₂Me, ^tBu, CF₃, Ac; n = 0-2; X = F, Cl, Br, I, H, SH, SPh, SBn, Ph, Me, N₃, NCS), was carried out. Theoretical calculations for some selected complexes were performed to evaluate the relative bond strength. It was found that the M-C_ipso bond length decreases following the linker series of CH₂ > NH > O and that the relative M-C_ipso bond strength increases following the linker series of CH₂ < NH < O. In most cases, the M-P bond length decreases following the linker series of NH > CH₂ > O. The relative M-P bond strength increases following the linker series of CH₂ < NH < O. A comparison of the thermochemical balance for the isodesmic displacement of the side-arm interactions with PH₃ as a probe ligand indicated that the Ni-P bond in a PCCCP-type pincer complex is far less difficult to break compared with that in a POCOP-type complex. As a result, with the same donor substituents and the same auxiliary ligand, the POCOP-type pincer complexes are thermodynamically more stable than the PCCCP complexes. The influence of other backbone and donor substitutions as well as the pincer platform composition on the M-C_ipso, M-P, and M-X bond lengths, relative bond strengths, and P-M-P bite angles was also discussed.

Cytotoxicity, anti-tumor effects and structure-activity relationships of nickel and palladium S,C,S pincer complexes against double and triple-positive and triple-negative breast cancer (TNBC) cells

Triple-Negative Breast Cancer (TNBC) is a highly aggressive form of breast cancer. The high rate of metastasis associated with TNBC is attributed to its multidrug resistance, making the treatment of this metastatic condition difficult. The development of metal-based antitumor agents was launched with the discovery of cisplatin, followed by the development of related antitumor drugs such as carboplatin and oxaliplatin. Yet, the severe side effects of this approach represent a limitation for its clinical use. The current search for new metal-based antitumor agents possessing less severe side effects than these platinum-based complexes has focused on various complexes of nickel and palladium, the group 10 congeners of platinum. In this work, we have prepared a series of SCS-type pincer complexes of nickel and palladium featuring a stable meta-phenylene central moiety and two chelating but labile thioamide donor moieties at the peripheries of the ligand. We have demonstrated that the complexes in question, namely L1NiCl, L1NiBr, L1PdCl, L2PdCl, and L3PdCl, are active on the proliferation of estrogen-dependent breast tumor cells (MCF-7 and MC4L2) and triple-negative breast cancer (4 T1). Among the complexes studied, the palladium derivatives were found to be much safer anticancer agents than nickel counterparts; these were thus selected for further investigations for their effects on tumor cell adhesion and migration as well. The results of our studies show that palladium complexes are effective for inhibiting TNBC 4 T1 cells adhesion and migration. Finally, the HOMO and LUMO analysis was used to determine the reactivity and charge transfer within the compounds.

Achiral and chiral NNN-pincer nickel complexes with oxazolinyl backbones: application in transfer hydrogenation of ketones

NNN-based achiral and chiral (oxazolinyl)amido-pincer nickel complexes are developed and employed for the catalytic transfer hydrogenation of ketones.

Reactions of a pincer proligand with copper iodide: bridging instead of C–H metalation

Various precursors of divalent copper have been treated with the meta-disubstituted phenylene-based proligand POC(H)OP (1,3-(i-Pr2PO)2C6H4) with the objective of preparing classical pincer complexes (POCOP)CuX. However, in no case was such species obtained, presumably owing to the difficult C–H metallation step. Analogous reactions of monovalent precursors were also unsuccessful, whereas reaction of POC(H)OP with CuI under different conditions gave the non-metallated adducts {(μ, κP, [Formula: see text]-POC(H)OP)Cu(μ-Ι)}2, 1, {(μ, κP, [Formula: see text]-POC(H)OP)Cu2(μ-Ι)2(DMAP)2}, 2 (DMAP = 4-dimethylaminopyridine), and {(μ, κP, [Formula: see text]-POC(H)OP)Cu2(μ3-Ι)2}2, 3. Treating 1 with DMAP gave the adduct 2, whereas 3 could be obtained by treating 1 with BuLi or by sublimation of 1. The solid state structures of these complexes revealed the tetrahedral geometry that might be anticipated for the d10 Cu(I) centers, in addition to fairly close I–H distances; on the other hand, no C–H interaction (agostic or otherwise) was observed with the Cu centers in any of these structures. The unsuccessful metallation of the C(2)–H moiety is thought to be a result of the strong preference of monovalent copper center to form bridging interactions with iodide and the POC(H)OP ligand; this appears to prevent the approach of the central carbon of the ligand to the Cu centers.

Direct Base-Assisted C‒H Cyclonickelation of 6-Phenyl-2,2'-bipyridine

The organonickel complexes [Ni(Phbpy)X] (X = Br, OAc, CN) were obtained for the first time in a direct base-assisted arene C(sp²)-H cyclometalation reaction from the rather unreactive precursor materials NiX₂ and HPhbpy (6-phenyl-2,2'-bipyridine) or from the versatile precursor [Ni(HPhbpy)Br₂]₂. Different from previously necessary C‒Br oxidative addition at Ni(0), an extended scan of reaction conditions allowed quantitative access to the title compound from Ni(II) on synthetically useful timescales through base-assisted C‒H activation in nonpolar media at elevated temperature. Optimisation of the reaction conditions (various bases, solvents, methods) identified 1:2 mixtures of acetate and carbonate as unrivalled synergetic base pairs in the optimum protocol that holds promise as a readily usable and easily tuneable access to a wide range of direct nickelation products. While for the base-assisted C‒H metalation of the noble metals Ru, Ir, Rh, or Pd, this acetate/carbonate method has been established for a few years, our study represents the leap into the world of the base metals of the 3d series.

Selective metalation of phenol-type proligands for preparative organometallic chemistry

The selective C-metalation of phenol ester derived proligands is a readily applicable addition to state-of-the-art protocols toward cyclometalated structures, in particular of the base metals.

Catalytically Relevant Intermediates in the Ni-Catalyzed C(sp2)-H and C(sp3)-H Functionalization of Aminoquinoline Substrates

This Article describes the synthesis and characterization of cyclometalated aminoquinoline Ni^II σ-aryl and σ-alkyl complexes that have been proposed as key intermediates in Ni-catalyzed C-H functionalization reactions. These Ni^II complexes serve as competent catalysts for the C-H functionalization of aminoquinoline derivatives with I₂. They also react stoichiometrically with I₂ to form either aryl iodides or β-lactams within minutes at room temperature. Furthermore, they react with Ag^I salts at -30 °C to afford isolable five-coordinate Ni^III species. The Ni^III σ-aryl complexes proved inert toward C(sp²)-I bond-forming reductive elimination under all conditions examined (up to 140 °C in DMF). In contrast, a Ni^III σ-alkyl analogue underwent C(sp³)-N bond-forming reductive elimination at 140 °C in DMF to afford a β-lactam product. However, despite the ability of this latter Ni^III species to participate in stoichiometric product formation, the complex was not a competent catalyst for β-lactam formation. Overall, these results suggest against the intermediacy of Ni^III species in these C-H functionalization reactions.

Impact of Oxidation State on Reactivity and Selectivity Differences between Nickel(III) and Nickel(IV) Alkyl Complexes

Described is a systematic comparison of factors impacting the relative rates and selectivities of C(sp³ )-C and C(sp³ )-O bond-forming reactions at high-valent Ni as a function of oxidation state. Two Ni complexes are compared: a cationic octahedral Ni^IV complex ligated by tris(pyrazolyl)borate and a cationic octahedral Ni^III complex ligated by tris(pyrazolyl)methane. Key features of reactivity/selectivity are revealed: 1) C(sp³ )-C(sp² ) bond-forming reductive elimination occurs from both centers, but the Ni^III complex reacts up to 300-fold faster than the Ni^IV , depending on the reaction conditions. The relative reactivity is proposed to derive from ligand dissociation kinetics, which vary as a function of oxidation state and the presence/absence of visible light. 2) Upon the addition of acetate (AcO^- ), the Ni^IV complex exclusively undergoes C(sp³ )-OAc bond formation, while the Ni^III analogue forms the C(sp³ )-C(sp² ) coupled product selectively. This difference is rationalized based on the electrophilicity of the respective M-C(sp³ ) bonds, and thus their relative reactivity towards outer-sphere S_N 2-type bond-forming reactions.

C-O and C-N Functionalization of Cationic, NCN-Type Pincer Complexes of Trivalent Nickel: Mechanism, Selectivity, and Kinetic Isotope Effect

This report presents the synthesis of new mono- and dicationic NCN-Ni^III complexes and describes their reactivities with protic substrates. (NCN is the pincer-type ligand κ ^N, κ ^C, κ ^N-2,6-(CH₂NMe₂)₂-C₆H₃.) Treating van Koten's trivalent complex (NCN)Ni^IIIBr₂ with AgSbF₆ in acetonitrile gives the dicationic complex [(NCN)Ni^III(MeCN)₃]²⁺, whereas the latter complex undergoes a ligand-exchange reaction with (NCN)Ni^IIIBr₂ to furnish the related monocationic complex [(NCN)Ni^III(Br)(MeCN)]⁺. These trivalent complexes have been characterized by X-ray diffraction analysis and EPR spectroscopy. Treating these trivalent complexes with methanol and methylamine led, respectively, to C-OCH₃ or C-NH(CH₃) functionalization of the Ni-aryl moiety in these complexes, C-heteroatom bond formation taking place at the ipso-C. These reactions also generate the cationic divalent complex [(NCN)Ni^II(NCMe)]⁺, which was prepared independently and characterized fully. The unanticipated formation of the latter divalent species suggested a comproportionation side reaction between the cationic trivalent precursors and a monovalent species generated at the C-O and C-N bond formation steps; this scenario was supported by direct reaction of the trivalent complexes with the monovalent compound (PPh₃)₃Ni^ICl. Kinetic measurements and density functional theory analysis have been used to investigate the mechanism of these C-O and C-N functionalization reactions and to rationalize the observed inverse kinetic isotope effect in the reaction of [(NCN)Ni^III(Br)(MeCN)]⁺ with CH₃OH/CD₃OD.

Effects of the number of cyclometalated rings and ancillary ligands on the rate of MeI oxidative addition to platinum(ii)–pincer complexes

The reactivity of new organoplatinum(ii)–pincer complexes in their oxidative addition reactions with MeI is related to the ancillary ligand and the number of cyclometalated rings present in the coordination sphere of the Pt centre.

In vitro and in vivo antiproliferative activity of organo-nickel SCS-pincer complexes on estrogen responsive MCF7 and MC4L2 breast cancer cells. Effects of amine fragment substitutions on BSA binding and cytotoxicity

A family of organonickel complexes has been prepared, fully characterized, and tested for their antiproliferative activity against estrogen-responsive human breast cancer cells (MCF7). The three SCS-type pincer ligands HL1, HL2, and HL3 and their corresponding Ni(ii) complexes NiL1, NiL2, and NiL3 have been synthesized and fully characterized, including by single crystal diffraction studies for the complexes. The complexes possess square planar geometry with two symmetrical 5-membered nickellacycles. Fluorescence spectroscopy, circular dichroism measurements, molecular modeling, colorimetric based assay and tumor transplantation studies were used to evaluate the protein binding and antiproliferative activities of these organometallic complexes both in vitro and in vivo. Fluorescence quenching was used to investigate bovine serum albumin (BSA) interaction at different temperatures (293, 303 and 313 K), and the results were analyzed using the classical Stern-Volmer equation, allowing us to propose a dynamic quenching mechanism. Studies in vitro on the antiproliferative activity of the three organonickel complexes against estrogen-responsive human breast cancer cells (MCF7) showed promising antitumor activity for NiL1 containing pyrrolidine fragments. In vivo administration of this compound significantly inhibits tumor growth in estrogen-dependent MC4L2 cancer cells in female BALB/c mice.

The Reactivity of Mercapto Groups against Boron Hydrides in Pincer Ligated Nickel Mercapto Complexes

Several pincer ligated nickel mercapto complexes, [2,6-(R₂ PCH₂ )₂ C₆ H₃ ]NiSH (R=tBu, 1 a; iPr, 1 b), [2,6-(R₂ PO)₂ C₆ H₃ ]NiSH (R=tBu, 2 a; iPr, 2 b) and [4-MeOCO-2,6-(tBu₂ PO)₂ C₆ H₂ ]NiSH (3 a), were synthesized and fully characterized. The reactivity of the mercapto groups against boron hydrides and organic bases was investigated. It was found that the mercapto groups are difficult to be deprotonated by boron hydrides or organic bases. The treatment of complex 2 a or 2 b with an excess amount of catecholborane (HBcat) afforded the corresponding pincer ligated nickel borohydride complexes and the HBcat degradation product. The treatment of complex 1 a, 2 a or 2 b with an excess amount of BH₃ ⋅THF produced the corresponding nickel borohydride species and the S-bridged triborane species THF⋅BH₂ -μ₂ -S(B₂ H₅ ) (5). No reactions between these complexes and organic bases were observed. DFT calculations were carried out to understand this reactivity and get mechanistic insights into the reactions.

A Novel Family of Polyiodo-Bromoantimonate(III) Complexes: Cation-Driven Self-Assembly of Photoconductive Metal-Polyhalide Frameworks

In the presence of different cations, reactions of [SbBr₆ ]^3- and I₂ result in a new family of diverse supramolecular 1D polyiodide-bromoantimonate networks. The coordination number of Sb, as well as geometry of assembling {I_x }^n- polyhalide units, can vary, resulting in unprecedented structural types. The nature of I⋅⋅⋅Br interactions was studied by DFT calculations; estimated energy values are 1.6-6.9 kcal mol^-1 . Some of the compounds showed strong photoconductivity in thin films, suggesting multiple feasible applications in optoelectronics and solar energy conversion.

Synthesis of quinolinyl-based pincer copper(ii) complexes: an efficient catalyst system for Kumada coupling of alkyl chlorides and bromides with alkyl Grignard reagents

Well-defined quinolinamide-based pincer copper complexes have been developed and demonstrated in the Kumada coupling reaction of nonactivated alkyl chlorides and bromides with alkyl magnesium chloride.

Comparison of halogen bonding networks with Ru(ii) complexes and analysis of the influence of the XB interactions on their reactivity

Coordination compounds of formula [Ru(Cl)₂(CNR)₄] are interesting building blocks for the preparation of halogen bonding supramolecular networks, since the chloride ligand is a good XB acceptor. When using I₂ as the XB donor, an unexpected reaction on the ruthenium coordination sphere happens where the chloride ligands are substituted by iodides. The isolation of several intermediates with different substitution degrees and showing XB interactions in a solid state network evidenced the clear influence of the XB species in this unusual reaction process. The extension of the studies to bromine gave the analogous result, i.e. the substitution of the chloride ligands by bromides. Furthermore, changing the organic substituent in the isocyanide ligands from alkyl to aryl does not affect the outcome of the reaction; however the process is faster when the alkyl substituents are present. In the course of the study of these reactions we have isolated a whole range of XB-based networks were interactions such as ClI-I, BrBr-Br, II-I and IBr-Br are present, a systematic comparison of the XB structural features for the different networks isolated and the influence in their reactivity has been performed.

Trifluoromethylation of a Well-Defined Square-Planar Aryl-NiII Complex involving NiIII /CF3. and NiIV -CF3 Intermediate Species

Ni-mediated trifluoromethylation of an aryl-Br bond in model macrocyclic ligands (L_n -Br) has been thoroughly studied, starting with an oxidative addition at Ni⁰ to obtain well-defined aryl-Ni^II -Br complexes ([L_n -Ni^II ]Br). Abstraction of the halide with AgX (X=OTf^- or ClO₄^- ) thereafter provides [L_n -Ni^II ](OTf). The nitrate analogue has been obtained through a direct C-H activation of an aryl-H bond using Ni^II salts, and this route has been studied by X-ray absorption spectroscopy (XAS). Crystallographic XRD and XAS characterization has shown a tight macrocyclic coordination in the aryl-Ni^II complex, which may hamper direct reaction with nucleophiles. On the contrary, enhanced reactivity is observed with oxidants, and the reaction of [L_n -Ni^II ](OTf) with CF₃⁺ sources afforded L_n -CF₃ products in quantitative yield. A combined experimental and theoretical mechanistic study provides new insights into the operative mechanism for this transformation. Computational analysis indicates the occurrence of an initial single electron transfer (SET) to 5-(trifluoromethyl)dibenzothiophenium triflate (TDTT), producing a transient L₁ -Ni^III /CF₃^. adduct, which rapidly recombines to form a [L₁ -Ni^IV -CF₃ ](X)₂ intermediate species. A final facile reductive elimination affords L₁ -CF₃ . The well-defined square-planar model system studied here permits to gain fundamental knowledge on the rich redox chemistry of nickel, which is sought to facilitate the development of new Ni-based trifluoromethylation methodologies.

Versatile routes for synthesis of diarylamines through acceptorless dehydrogenative aromatization catalysis over supported gold-palladium bimetallic nanoparticles

Diarylamines are an important class of widely utilized chemicals, and development of diverse procedures for their synthesis is of great importance. Herein, we have successfully developed novel versatile catalytic procedures for the synthesis of diarylamines through acceptorless dehydrogenative aromatization. In the presence of a gold-palladium alloy nanoparticle catalyst (Au-Pd/TiO2), various symmetrically substituted diarylamines could be synthesized starting from cyclohexylamines. The observed catalysis of Au-Pd/TiO2 was heterogeneous in nature and Au-Pd/TiO2 could be reused several times without severe loss of catalytic performance. This transformation needs no oxidants and generates molecular hydrogen (three equivalents with respect to cyclohexylamines) and ammonia as the side products. These features highlight the environmentally benign nature of the present transformation. Furthermore, in the presence of Au-Pd/TiO2, various kinds of structurally diverse unsymmetrically substituted diarylamines could successfully be synthesized starting from various combinations of substrates such as (i) anilines and cyclohexanones, (ii) cyclohexylamines and cyclohexanones, and (iii) nitrobenzenes and cyclohexanols. The role of the catalyst and the reaction pathways were investigated in detail for the transformation of cyclohexylamines. The catalytic performance was strongly influenced by the nature of the catalyst. In the presence of a supported gold nanoparticle catalyst (Au/TiO2), the desired diarylamines were hardly produced. Although a supported palladium nanoparticle catalyst (Pd/TiO2) gave the desired diarylamines, the catalytic activity was inferior to that of Au-Pd/TiO2. Moreover, the activity of Au-Pd/TiO2 was superior to that of a physical mixture of Au/TiO2 and Pd/TiO2. The present Au-Pd/TiO2-catalyzed transformation of cyclohexylamines proceeds through complex pathways comprising amine dehydrogenation, imine disproportionation, and condensation reactions. The amine dehydrogenation and imine disproportionation reactions are effectively promoted by palladium (not by gold), and the intrinsic catalytic performance of palladium is significantly improved by alloying with gold. One possible explanation of the alloying effect is the formation of electron-poor palladium species that can effectively promote the β-H elimination step in the rate-limiting amine dehydrogenation.

Accessing Pincer Bis(carbene) Ni(IV) Complexes from Ni(II) via Halogen and Halogen Surrogates

This communication describes the two-electron oxidation of ((DIPP)CCC)NiX ((DIPP)CCC = bis(diisopropylphenyl-benzimidazol-2-ylidene)phenyl); X = Cl or Br) with halogen and halogen surrogates to form ((DIPP)CCC)NiX3. These complexes represent a rare oxidation state of nickel, as well as an unprecedented reaction pathway to access these species through Br2 and halogen surrogate (benzyltrimethylammonium tribromide). The Ni(IV) complexes have been characterized by a suite of spectroscopic techniques and can readily reduce to the Ni(II) counterpart, allowing for cycling between the Ni(II)/Ni(IV) oxidation states.

Ligand effects on the properties of Ni(iii) complexes: aerobically-induced aromatic cyanation at room temperature

Ligand effect studies on (^RN3C)Ni^IIIL₂ complexes reveal aerobically-induced aromatic cyanation with tBuNC at room temperature for the (^NpN3C)Ni system.

Aromatic Methoxylation and Hydroxylation by Organometallic High-Valent Nickel Complexes

Herein we report the synthesis and reactivity of several organometallic Ni(III) complexes stabilized by a modified tetradentate pyridinophane ligand containing one phenyl group. A room temperature stable dicationic Ni(III)-disolvento complex was also isolated, and the presence of two available cis coordination sites in this complex offers an opportunity to probe the C-heteroatom bond formation reactivity of high-valent Ni centers. Interestingly, the Ni(III)-dihydroxide and Ni(III)-dimethoxide species can be synthesized, and they undergo aryl methoxylation and hydroxylation that is favored by addition of oxidant, which also limits the β-hydride elimination side reaction. Overall, these results provide strong evidence for the involvement of high-valent organometallic Ni species, possibly both Ni(III) and Ni(IV) species, in oxidatively induced C-heteroatom bond formation reactions.

Oxidation of Ni(II) to Ni(IV) with Aryl Electrophiles Enables Ni-Mediated Aryl-CF3 Coupling

This communication describes the synthesis and reactivity of Ni(IV)(aryl)(CF3)2 complexes supported by trispyrazolylborate and 4,4'-di-tert-butylbipyridine ligands. We demonstrate that isolable Ni(IV) complexes can be accessed under mild conditions via the oxidation of Ni(II) precursors with S-(trifluoromethyl)dibenzothiophenium triflate as well as with diaryliodonium and aryl diazonium reagents. The Ni(IV) intermediates undergo high yielding aryl-CF3 bond-forming reductive elimination. These studies support the potential viability of Ni(IV) intermediates in nickel-catalyzed coupling reactions involving diaryliodonium and aryldiazonium electrophiles.

Design, synthesis, and carbon-heteroatom coupling reactions of organometallic nickel(IV) complexes

Homogeneous nickel catalysis is used for the synthesis of pharmaceuticals, natural products, and polymers. These reactions generally proceed via nickel intermediates in the Ni(0), Ni(I), Ni(II), and/or Ni(III) oxidation states. In contrast, Ni(IV) intermediates are rarely accessible. We report herein the design, synthesis, and characterization of a series of organometallic Ni(IV) complexes, accessed by the reaction of Ni(II) precursors with the widely used oxidant S-(trifluoromethyl)dibenzothiophenium triflate. These Ni(IV) complexes undergo highly selective carbon(sp(3))-oxygen, carbon(sp(3))-nitrogen, and carbon(sp(3))-sulfur coupling reactions with exogenous nucleophiles. The observed reactivity has the potential for direct applications in the development of nickel-catalyzed carbon-heteroatom coupling reactions.

Direct metal-free synthesis of diarylamines from 2-nitropropane via the twofold C–H functionalization of arenes

Synthesis of symmetric diarylamines via a twofold intermolecular electrophilic C–H functionalization of electron-rich arenes by umpolung-activated nitroalkane in polyphosphoric acid is demonstrated.