Reactions of Benzyne−Nickel(0) Complexes with Thioalkynes: Preparation and Coordination of Polydentate Thioethers Containing a 2,3-Naphthylene Backbone

Self-complementary nickel halides enable multifaceted comparisons of intermolecular halogen bonds: fluoride ligands vs. other halides

Studies of X–Ni–C₆F₄I···X–Ni–C₆F₄I halogen-bonded networks reveal pronounced differences between fluoride (X = F) and other halides: the ¹⁹F-MAS NMR spectrum is a sensitive probe of the halogen bond.

The syntheses of three series of complexes designed with self-complementary motifs for formation of halogen bonds between an iodotetrafluorophenyl ligand and a halide ligand at square-planar nickel are reported, allowing structural comparisons of halogen bonding between all four halides C₆F₄I···X–Ni (X = F, Cl, Br, I). In the series trans-[NiX(2,3,5,6-C₆F₄I)(PEt₃)₂] 1pX and trans-[NiX(2,3,4,5-C₆F₄I)(PEt₃)₂] (X = F, Cl, Br, I) 1oX, the iodine substituent on the benzene ring was positioned para and ortho to the metal, respectively. The phosphine substituents were varied in the series, trans-[NiX(2,3,5,6-C₆F₄I)(PEt₂Ph)₂] (X = F, I) 2pX. Crystal structures were obtained for the complete series 1pX, and for 1oF, 1oCl, 1oI and 2pI. All these complexes exhibited halogen bonds in the solid state, of which 1pF exhibited unique characteristics with a linear chain, the shortest halogen bond d(C₆F₄I···F–Ni) = 2.655(5) Å and the greatest reduction in halogen bond distance (I···F) compared to the sum of the Bondi van der Waals radii, 23%. The remaining complexes form zig-zag chains of halogen bonds with distances also reduced with respect to the sum of the van der Waals radii. The magnitude of the reductions follow the pattern F > Cl ∼ Br > I, 1pX > 1oX, consistent with the halogen bond strength following the same order. The variation in the I···X–Ni angles is consistent with the anisotropic charge distribution of the halide ligand. The temperature dependence of the X-ray structure of 1pF revealed a reduction in halogen bond distance of 0.055(7) Å on cooling from 240 to 111 K. Comparison of three polymorphs of 1oI shows that the halogen bond geometry may be altered significantly by the crystalline environment. The effect of the halogen bond on the ¹⁹F NMR chemical shift in the solid state is demonstrated by comparison of the magic-angle spinning NMR spectra of 1pF and 1oF with that of a complex incapable of halogen bond formation, trans-[NiF(C₆F₅)(PEt₃)₂] 3F. Halogen bonding causes deshielding of δ_iso in the component of the tensor perpendicular to the nickel coordination plane. The results demonstrate the potential of fluoride ligands for formation of halogen bonds in supramolecular structures.

A study on an unusual SN2 mechanism in the methylation of benzyne through nickel-complexation

In this study, three reaction mechanisms of a benzyne-nickel (Ni) complex ([Ni(C₆H₄)(dcpe)]) with iodomethane during the methylation process were investigated, namely (a) S_N2 reaction of the benzyne-Ni complex with iodomethane, (b) concerted σ-bond metathesis during the bond breaking/forming processes, and (c) oxidative addition of iodomethane to the Ni-center and the subsequent reductive elimination process. DFT calculations revealed that the reaction barrier of the S_N2 reaction is slightly lower than those of the other mechanisms. The results of orbital analyses suggest that [Ni(C₆H₄)(dcpe)] forms a metallacycle structure between benzyne and the Ni^II (3d⁸) center instead of the η²-structure with the Ni⁰ (3d¹⁰) center. The metallacycle structures became inappropriate as the intermediates of oxidative addition in the formation of the Ni^II-Me bond, avoiding further oxidation to the high-valent Ni^IV. The high free energy along σ-bond metathesis was generated from the steric hindrance, thus invoking methylation and Ni-I bond formation concertedly.

Preparation of Aryne-Nickel Complexes from ortho-Borylaryl Triflates

A facile method for preparing diverse aryne-nickel complexes from readily synthesized ortho-borylaryl triflates is described. Exploratory synthetic applications, including the synthesis of 1,2-difunctionalized arenes, based on the nucleophilic character of the aryne-nickel complexes are also demonstrated.

Aryne Complexes of Zerovalent Metals of the Nickel Triad

The chemistry of dihapto-aryne complexes of the zerovalent Group 10 metals of general formula [M(η2-aryne)L2] (M = Ni, Pd, Pt; L = various tertiary phosphines) is reviewed, with emphasis on the highly reactive nickel(0) compounds (aryne = benzyne, C6H4; 4,5-difluorobenzyne, 4,5-C6H2F2; 2,3-naphthalyne, 2,3-C10H6; L2 = 2 PEt3, 2 PiPr3, 2 PCy3, dcpe). These can be generated by alkali metal reduction of the appropriate (2-halogenoaryl)nickel(ii) halide precursors, such as [NiX(2-XC6H4)L2], which in turn are accessible by oxidative addition of the 1,2-dihaloarene to nickel(0) precursors such as [Ni(1,5-COD)2]. The X-ray structure of [Ni(η2-C6H4)(dcpe)] shows that this compound is a typical 16-electron Ni(0) (3d10) species in which benzyne acts as a 2π-electron donor. Several unusual organonickel compounds derived from [Ni(η2-4,5-C6H2F2)(PEt3)2] have been isolated recently, including [Ni2(μ-η2:η2-4,5-C6H2F2)(PEt3)4], in which a 4π-electron donor 4,5-difluorobenzyne is located at right-angles to a pair of nickel atoms. Free benzyne can be intercepted by both [Ni(η2-C2H4)(dcpe)] and [Pt(η2-C2H4)(PPh3)2], but the resulting benzyne complexes rapidly insert benzyne to give the appropriate η1:η1-2,2′-biphenylyl complexes. [Pt(η2-C6H4)(PPh3)2] also undergoes rapid ortho-metallation to give [PtPh(2-C6H4PPh2)(PPh3)]. However, a trapping reaction has been used to make the first 1,4-benzdiyne complex, [{Ni(dcpe)2}2(μ-η2:η2-1,4-C6H2)] by treatment of the 4-fluorobenzyne complex [Ni(η2-4-FC6H3)(dcpe)] with LiTMP. The use of alkali metals in the preparation of the η2-benzyne complexes is avoided in a more recently developed procedure, which starts from (2-bromophenyl)boronic acid, and is based on Suzuki–Miyaura coupling. This procedure has made accessible for the first time an aryne complex of palladium(0), [Pd(η2-C6H4)(PCy3)2], and the labile nickel(0) complex [Ni(η2-C6H4)(PPh3)2]. The aryne-nickel(0) complexes Ni(η2-aryne)L2 (L2 = 2 PEt3, dcpe) undergo sequential insertions into the aryne-metal bond with unsaturated molecules, such as CO, C2F4, substituted alkynes, substituted diynes, alkynylphosphines, and alkynyl thioethers, often with considerable regioselectivity. After the reductive elimination of two nickel-carbon σ-bonds, a variety of interesting polycyclic compounds can be obtained.

Nickel-Mediated Regioselective [2 + 2 + 2] Cycloaddition of Carboryne with Alkynes

Transition metal-benzyne complexes have found many applications in organic synthesis, mechanistic studies, and the synthesis of functional materials. In sharp contrast, the chemistry of transition metal-carboryne complexes, especially late transition metal complexes, is virtually unknown. This communication reports a novel nickel-mediated regioselective [2 + 2 + 2] cycloaddition reaction of carboryne with alkynes via the Ni-carboryne intermediate (eta2-C2B10H10)Ni(PPh3)2. Because of the bulkiness of the carborane moiety, a high regioselectivity was achieved in the reactions involving unsymmetrical alkynes. This work furnishes a novel method for the preparation of highly substituted benzocarboranes which are difficult to obtain by other methods.

Preparation of benzyne complexes of group 10 metals by intramolecular suzuki coupling of ortho-metalated phenylboronic esters: molecular structure of the first benzyne-palladium(0) complex

A series of nickel(II) and palladium(II) aryl complexes substituted in the ortho position of the aromatic ring by a (pinacolato)boronic ester group, [MBr[o-C(6)H(4)B(pin)]L(2)] (M = Ni, L(2) = 2PPh(3) (2a), 2PCy(3) (2b), 2PEt(3) (2c), dcpe (2d), dppe (2e), and dppb (2f); M = Pd, L(2) = 2PPh(3) (3a), 2PCy(3) (3b), and dcpe (3d)), has been prepared. Many of these complexes react readily with KO(t)Bu to form the corresponding benzyne complexes [M(eta(2)-C(6)H(4))L(2)] (M = Ni, L(2) = 2PPh(3) (4a), 2PCy(3) (4b), 2PEt(3) (4c), dcpe (4d); M = Pd, L(2) = 2PCy(3) (5b)). This reaction can be regarded as an intramolecular version of a Suzuki cross-coupling reaction, the driving force for which may be the steric interaction between the boronic ester group and the phosphine ligands present in the precursors 2 and 3. Complex 3d also reacts with KO(t)Bu, but in this case disproportionation of the initially formed eta(2)-C(6)H(4) complex (5d) leads to a 1:1 mixture of a novel dinuclear palladium(I) complex, [(dcpe)Pd(mu(2)-C(6)H(4))Pd(dcpe)] (6), and a 2,2'-biphenyldiyl complex, [Pd(2,2'-C(6)H(4)C(6)H(4))(dcpe)] (7d). Complexes 2a, 3b, 3d, 4b, 5b, 6, and 7d have been structurally characterized by X-ray diffraction; complex 5b is the first example of an isolated benzyne-palladium(0) species.