RESUMEN
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.
RESUMEN
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.
RESUMEN
Recent advances in heterobimetallic chemistry have revealed the potential for mixed-metal systems to facilitate reactions that are unattainable with their single-metal components. This perspective explores the pairing of nickel(0) complexes with organo-alkali-metal reagents, which yield highly reactive alkali-metal nickelates. These previously underexplored systems have re-emerged as a promising area of research, with recent studies uncovering their unique bonding and structural motifs. Furthermore, the discovery of nickelates as potential intermediates in cross-coupling reactions has provided the foundation for the development and mechanistic understanding of stoichiometric and catalytic transformations.
RESUMEN
The Ni-catalyzed cross-coupling of aryl ethers is a powerful synthetic tool to transform widely available phenol derivatives into functionalized aromatic molecules. Recent theoretical and experimental mechanistic studies have identified the involvement of heterobimetallic nickelates as key intermediates that facilitate the challenging transformation under mild conditions and often without the need for external ligands or additives. In this work, based on calculations performed at the density functional theory (DFT) level and by comparison with spectroscopic and kinetic data, we investigate the mechanism of the Ni(COD)2-catalyzed cross-coupling of 2-methoxynaphthalene with PhLi and assess the speciation of lithium nickelate intermediates. The crucial role of solvent on the reaction is explained, and the multiple roles played by lithium are unveiled. Experimental studies have identified key lithium nickelate species which support and help to evolve the calculated reaction mechanism and ultimately complete the catalytic cycle. Based on this new mechanistic knowledge, a well-known experimental challenge of these transformations, the so-called "naphthalene problem" which restricts the use of electrophilic coupling partners to π-extended systems, can be addressed to enable the cross-coupling of unbiased aryl ethers under mild conditions.
RESUMEN
By combining the Lewis acid Zn(C6F5)2 with nucleophilic diarylzinc (ZnAr2) reagents, we report the atom-efficient arylation of N-tosylimines under mild conditions. Mechanistic studies through the isolation of key intermediates reveal how the two zinc species act cooperatively to activate the imine substrate and regenerate the ZnAr2 reagent, enabling a limiting 50 mol% to be employed.
Asunto(s)
Ácidos de Lewis , Zinc , Indicadores y ReactivosRESUMEN
By exploiting the high aggregation of aliphatic lithium acetylides, here we report the synthesis and structural analysis of polynuclear lithium nickelate clusters in which up to 10 equivalents of organolithium can co-complex per Ni(0) centre. Exposure of the Ni(0)-ate clusters to dry air provides an alternative route to homoleptic Ni(II)-ates.
RESUMEN
Whilst low-valent nickelates have recently been proposed as intermediates in Ni-catalysed reactions involving polar organometallics, their isolation and characterisation is often challenging due to their high sensitivity and reactivity. Advancing the synthetic, spectroscopic and structural insights of these heterobimetallic systems, here we report a new family of alkyne supported alkali-metal nickelates of the formula Li4(solv)n(Ar)4Ni2{µ2:η2,η2-Ph-CîC-Ph} (where solv = Et2O, THF; Ar = Ph, o-Tol, naphthyl, 4-tBu-C6H4) which can be accessed through the combination of Ni(COD)2, Ph-CîC-Ph and the relevant lithium aryl in a 2 : 1 : 4 ratio. Demonstrating the versatility of this approach, the sodium and potassium nickelates can also be accessed when using PhNa or via alkali-metal exchange with AMOtBu (AM = Na, K). When employing bulky or structurally constrained aryl-lithiums, mononickel complexes of the formula Li2(solv)n(Ar)2Ni{η2-Ph-CîC-Ph} are instead obtained, highlighting the structural diversity of alkali-metal nickelates bearing alkyne ligands. Expanding the catalytic potential of these systems, their ability to promote the catalytic cyclotrimerisation of diphenylacetylene to hexaphenylbenzene was explored, with mononickel compounds bearing electron rich aryl-substituents displaying the best performance.
RESUMEN
Advancing the understanding of lithum nickelate complexes, here we report a family of homoleptic organonickelate complexes obtained by reacting Ni(COD)2 and lithium aryl-acetylides in the presence of the bidentate donor TMEDA. These compounds represent rare examples of low-valent transition-metals supported solely by organolithium ligands. Whilst the solid-state structures indicate a hexagonal planar geometry around Ni0 with Ni-Li bonds, bonding analysis via QTAIM, NCI, NBO and ELI methods reveals that the Ni-Li interactions are repulsive in nature, characterising these complexes as tri-coordinated. London dispersion forces between TMEDA and the organic substituents on nickel are found to play a crucial role in the stabilisation and thus isolation of these complexes. Preliminary reactivity studies demonstrate that the homoleptic lithium nickelates undergo stoichiometric cross-coupling with PhI to give dinickel clusters containing both anionic acetylide and neutral alkyne ligands.
RESUMEN
More than four decades ago, a complex identified as the planar homoleptic lithium nickelate "Li3NiPh3(solv)3" was reported by Taube and co-workers. This and subsequent reports involving this complex have lain dormant since; however, the absence of an X-ray diffraction structure leaves questions as to the nature of the Ni-PhLi bonding and the coordination geometry at Ni. By systematically evaluating the reactivity of Ni(COD)2 with PhLi under different conditions, we have found that this classical molecule is instead a unique octanuclear complex, [{Li3(solv)2Ph3Ni}2(µ-η2:η2-C6H4)] (5). This is supported by X-ray crystallography and solution-state NMR studies. A theoretical bonding analysis from NBO, QTAIM, and ELI perspectives reveals extreme back-bonding to the bridging C6H4 ligand resulting in dimetallabicyclobutane character, the lack of a Ni-Ni bond, and pronounced σ-bonding between the phenyl carbanions and nickel, including a weak σC-Li â sNi interaction with the C-Li bond acting as a σ-donor. Employing PhNa led to the isolation of [Na2(solv)3Ph2NiCOD]2 (7) and [Na2(solv)3Ph2(NaC8H11)Ni(COD)]2 (8), which lack the benzyne-derived ligand. These findings provide new insights into the synthesis, structure, bonding and reactivity of heterobimetallic nickelates, whose prevalence in organonickel chemistry and catalysis is likely greater than previously believed.
RESUMEN
Exploiting the cooperative action of Lewis acid Zn(C6F5)2 with diarylzinc reagents, the efficient arylation of N,O-acetals to access diarylmethylamines is reported. Reactions take place under mild reaction conditions without the need for transtion-metal catalysis. Mechanistic investigations have revealed that Zn(C6F5)2 not only acts as a Lewis acid activator, but also enables the regeneration of nucleophilic ZnAr2 species, allowing a limiting 50 mol% to be employed.
RESUMEN
The Ni-catalysed cross-coupling of aryl ethers is a powerful method to forge new C-C and C-heteroatom bonds. However, the inert C(sp2 )-O bond means that a canonical mechanism that relies on the oxidative addition of the aryl ether to a Ni0 centre is thermodynamically and kinetically unfavourable, which suggests that alternative mechanisms may be involved. Here, we provide spectroscopic and structural insights into the anionic pathway, which relies on the formation of electron-rich hetero-bimetallic nickelates by adding organometallic nucleophiles to a Ni0 centre. Assessing the rich co-complexation chemistry between Ni(COD)2 and PhLi has led to the structures and solution-state chemistry of a diverse family of catalytically competent lithium nickelates being unveiled. In addition, we demonstrate dramatic solvent and donor effects, which suggest that the cooperative activation of the aryl ether substrate by Ni0 -ate complexes plays a key role in the catalytic cycle.
RESUMEN
We report the phospha-bora-Wittig reaction for the direct preparation of phosphaalkenes from aldehydes, ketones, esters, or amides. The transient phosphaborene Mes*PâB-NR2 reacts with carbonyl compounds to form 1,2,3-phosphaboraoxetanes, analogues of oxaphosphetane intermediates in the classical Wittig reaction. 1,2,3-Phosphaboraoxetanes undergo thermal or Lewis acid-promoted cycloreversion, yielding phosphaalkenes. Experimental and density functional theory studies reveal far-reaching similarities between classical and phospha-bora-Wittig reactions.
RESUMEN
A family of substituted 1,2,4-benzothiadiazine 1-chlorides have been prepared by treatment of N-arylamidines in neat thionyl chloride at reflux. The S(iv) 1-chlorides are readily reduced under mild conditions to persistent 1,2,4-benzothiadiazinyl radicals which have been characterised by EPR spectroscopy and cyclic voltammetry. Crystallographic studies on isolated radicals indicate that the radicals dimerise via pancake bonding in the solid-state, resulting in spin-pairing and net diamagnetism.
RESUMEN
Low molecular weight organic molecules that can accept multiple electrons at high reduction potentials are sought after as electrode materials for high-energy sustainable batteries. To date their synthesis has been difficult, and organic scaffolds for electron donors significantly outnumber electron acceptors. Herein, we report the synthesis and electronic properties of two highly electron-deficient phosphaviologen derivatives from a phosphorus-bridged 4,4'-bipyridine and characterize their electrochemical properties. Phosphaviologen sulfide (PVS) and P-methyl phosphaviologen (PVM) accept two and three electrons at high reduction potentials, respectively. PVM can reversibly accept three electrons between 3-3.6 V vs. Li/Li+ with an equivalent molecular weight of 102 g (mol-1 e-) (262 mA h g-1), making it a promising scaffold for sustainable organic electrode materials having high specific energy densities.
RESUMEN
Phosphane-stabilized phosphenium cations react with silanes to effect either reduction to primary or secondary phosphanes, or formation of P-P bonded species depending upon counteranion. This operates for in situ generated phosphenium cations, allowing catalytic reduction of P(III)-Cl bonds in the absence of strong reducing agents. Anion and substituent dependence studies have allowed insight into the competing mechanisms involved.
RESUMEN
The enhancement of donor strength of ortho-functionalized triarylphosphanes is shown to occur via different mechanisms for O- and N-donor substituents, with internal solvation of the phosphorus center observed for N donors. Nevertheless, the steric congestion about the P center is shown to significantly oppose the increase in donor ability, leading to donation weaker than that expected. A series of mono- and bis-aryl-substituted Ar3PI2 adducts (Ph3-n(o-OMe-C6H4)nPI2, Ph3-n(o-NMe2-C6H4)nPI2, Ph3-n(o-CH2NMe2-C6H4)nPI2 (n = 1, 2)) have been synthesized via the 1:1 reaction of donor-functionalized phosphanes with diiodine. These soft Lewis acid/base adducts exhibit apparent internal solvation of the donor phosphorus by the pendant donor moieties, giving rise to five- or six-coordinate phosphorus atoms acting as both Lewis base and Lewis acid; the first neutral six-coordinate simultaneous P(III) Lewis acid and Lewis base adduct is reported. Single-crystal X-ray diffraction studies reveal unexpectedly weak donor strength for one of the phosphanes, indicating significant steric hindrance as a consequence of internal solvation. Crystallographic interrogation of the corresponding iodophosphonium salts [Ar3PI]X (X = I3, BArF) shows that the cationic complexes experience a still greater influence of the steric bulk of the donor moieties than their neutral precursors. The steric and electronic contributions to bonding have been analyzed through computational studies, determining the factors governing the basicity of these donor-functionalized phosphanes, and show that enhancement of P-centered donor strength occurs by conjugation of lone pairs through the arene rings for oxygen substituents and via internal solvation for the nitrogen donors.
RESUMEN
Paradoxically, N- and O-donor substituted tri-arylphosphanes are shown to be weaker donors than PPh3 when binding the soft Lewis acid moiety [PPh2]+. This arises from internal solvation and rehybridisation at phosphorus, precluding chelation and increasing steric demand, in direct contrast to coordination modes observed for metal complexes.
