NHC-stabilized copper(I) aryl complexes and their transmetalation reaction with aryl halides

Direct Observation of Alkyl Group Transmetalation from Boron to Copper: Impact of Structure Modification and the Critical Role of Copper-Oxygen Preassociation in Stereospecificity

Mechanistic investigations of prospective intermediates in copper-catalyzed cross-coupling reactions with alkyllithium-activated alkylboronic esters are described. Complexes between (NHC)copper and the borate salts were characterized by X-ray crystallography and NMR, and the rate of intramolecular transmetalation of an alkyl group from boron to copper was measured. Derived activation parameters were correlated with those determined by DFT analysis and give insight into the mechanism of stereospecific transmetalation.

Cuprophilic Interactions in Polymeric [Cu10O2(Mes)6]n

The properties of cuprophilic compounds and the underlying fundamental principles responsible for the Cu(I)···Cu(I) interactions have been the subject of intense research as their diverse structural and physical attributes are being explored. In this light, we performed a new study of the compound [Cu10O2(Mes)6] reported by Haakansson et al. using state of the art experimental and theoretical analysis techniques. Doing this, we found the compound to be a polymer in the solid state, best written as [Cu10O2(Mes)6]n, with unsupported Cu(I)···Cu(I) contacts linking the monomers (2.776 Å). The monomeric unit also exhibits various cuprophilic contacts bridged by mesityl and/or oxo ligands. The compound was analyzed in its solid state, revealing luminescent properties resulting from two distinct fluorescent emissions, as well as in solution, in which its polymeric structure reversibly decomposes. A quantum theory of atoms in molecules (QTAIM) analysis based on density functional theory (DFT) calculations allows to characterize the various Cu(I)···Cu(I) contacts, in which only a few, and not necessarily the shortest, are associated with a bond critical point. Additionally, an energy decomposition analysis of the bonding between monomers indicates that it is dominated by dispersion forces in which the ligands play a dominant role, resulting in bonding energies significantly larger than found in previous DFT investigations based on less bulky models.

π-Bonding of Group 11 Metals to a Tantalum Alkylidyne Alkyl Complex Promotes Unusual Tautomerism to Bis-alkylidene and CO2 to Ketenyl Transformation

A salt metathesis synthetic strategy is used to access rare tantalum/coinage metal (Cu, Ag, Au) heterobimetallic complexes. Specifically, complex [Li(THF)2][Ta(CtBu)(CH2tBu)3], 1, reacts with (IPr)MCl (M = Cu, Ag, Au, IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) to afford the alkylidyne-bridged species [Ta(CH2tBu)3(μ-CtBu)M(IPr)] 2-M. Interestingly, π-bonding of group 11 metals to the Ta─C moiety promotes a rare alkylidyne alkyl to bis-alkylidene tautomerism, in which compounds 2-M are in equilibrium with [Ta(CHtBu)(CH2tBu)2(μ-CHtBu)M(IPr)] 3-M. This equilibrium was studied in detail using NMR spectroscopy and computational studies. This reveals that the equilibrium position is strongly dependent on the nature of the coinage metal going down the group 11 triad, thus offering a new valuable avenue for controlling this phenomenon. Furthermore, we show that these uncommon bimetallic couples could open attractive opportunities for synergistic reactivity. We notably report an uncommon deoxygenative carbyne transfer to CO2 resulting in rare examples of coinage metal ketenyl species, (tBuCCO)M(IPr), 4-M (M = Cu, Ag, Au). In the case of the Ta/Li analogue 1, the bis(alkylidene) tautomer is not detected, and the reaction with CO2 does not cleanly yield ketenyl species, which highlights the pivotal role played by the coinage metal partner in controlling these unconventional reactions.

Fluoride Abstraction Induced by Tris(pentafluoroethyl)difluorophosphorane: A Convenient Way to Synthesize Cationic N-Heterocyclic Carbene- and Cyclic (Alkyl)(amino)carbene-Ligated Copper Alkyne and Arene Complexes

We herein report the convenient synthesis of different N-heterocyclic carbene (NHC)- and cyclic (alkyl)(amino)carbene (cAAC)-ligated copper cations using the weakly coordinating tris(pentafluoroethyl)trifluorophosphate counterion (FAP anion, [(C2F5)3PF3]-). The reaction of the fluorido complexes [(carbene)CuF] (carbene = NHC, cAACMe) 2a-2f and the tris(pentafluoroethyl)difluorophosphorane (C2F5)3PF2 in the presence of alkynes or arenes led to fluoride transfer from Cu to the phosphorane with formation of the cationic transition metal complexes [(carbene)Cu(L)]+ and the weakly coordinating counteranion [(C2F5)3PF3]- (FAP). Using this method, the complexes [(IDipp)Cu(L)]+FAP- (IDipp = 1,3-bis(2,6-di-iso-propylphenyl)-imidazolin-2-ylidene; L = PhC≡CPh, 4d; PhC≡CMe, 5d), [(cAACMe)Cu(L)]+FAP- (cAACMe = 1-(2,6-di-iso-propylphenyl)-3,3,5,5-tetramethyl-pyrrolidin-2-ylidene; L = PhC≡CPh, 4f; PhC≡CMe, 5f), [(SIDipp)Cu(C6Me6)]+FAP- (6e), (SIDipp = 1,3-bis(2,6-di-iso-propylphenyl)-imidazolidine-2-ylidene), and [(cAACMe)Cu(C6Me6)]+FAP- (6f) have been synthesized and characterized. The complexes [(IDipp)Cu(C6Me6)]+FAP- (6d) and [(cAACMe)Cu(C6Me6)]+FAP- (6f) have been used as catalysts for the copper(I)-catalyzed cycloaddition of benzyl azide to terminal alkynes.

Applications of Transition Metal-Catalyzed ortho-Fluorine-Directed C-H Functionalization of (Poly)fluoroarenes in Organic Synthesis

The synthesis of organic compounds efficiently via fewer steps but in higher yields is desirable as this reduces energy and reagent use, waste production, and thus environmental impact as well as cost. The reactivity of C-H bonds ortho to fluorine substituents in (poly)fluoroarenes with metal centers is enhanced relative to meta and para positions. Thus, direct C-H functionalization of (poly)fluoroarenes without prefunctionalization is becoming a significant area of research in organic chemistry. Novel and selective methodologies to functionalize (poly)fluorinated arenes by taking advantage of the reactivity of C-H bonds ortho to C-F bonds are continuously being developed. This review summarizes the reasons for the enhanced reactivity and the consequent developments in the synthesis of valuable (poly)fluoroarene-containing organic compounds.

Heteroleptic Silver(I) and Gold(I) N-Heterocyclic Carbene Complexes: Structural Characterization, Computational Analysis, Tyrosinase Inhibitory, and Biological Effects

Derivatization of (NHC)M-Cl (M = Ag, Au) with selected sulfur donors from the family of dialkyldithiophosphates and bis(2-mercapto-1-methylimidazolyl)borate ligands gave a series of heteroleptic mononuclear complexes. In single-crystal X-ray diffraction analysis, Ag(I) complexes adopted a trigonal planar geometry, while Au(I) complexes are near-linear. TD-DFT and hole-electron analyses of the selected complexes gave insight into the electronic features of the metal complexes. In vitro cellular tests were conducted on the human cancerous breast cell line MCF-7 using 2 and 8. The antibacterial activities of complexes 1, 2, 3, 7, 8, and IPr-Ag-Cl were also screened against Gram-positive (Staphylococcus aureus PTCC 1112) and Gram-negative (Escherichia coli PTCC 1330) bacteria. Antityrosinase and hemolytic effects of the selected compounds were also determined.

N-Heterocyclic carbene and cyclic (alkyl)(amino)carbene ligated half-sandwich complexes of chromium(II) and chromium(I)

The synthesis and characterization of a series of Cr(II) N-Heterocyclic Carbene (NHC) complexes of the type [{Cr(NHC)Cl(μ-Cl)}2] and [(Cyp)Cr(NHC)X] (Cyp = η5-C5H5, cyclopentadienyl; η5-C5Me5, pentamethylcyclopentadienyl; X = Cl, η3-C3H5; NHC = IMeMe, IiPrMe, IMes, IDipp) as well as the cyclic (alkyl)(amino)carbene cAACMe ligated complexes [(η5-C5H5)Cr(cAACMe)X] (X = Cl, NPh2), [(η5-C9H7)Cr(cAACMe)Cl] (C9H7 = Ind, indenyl) and [(η5-C13H9)Cr(cAACMe)Cl] (C13H9 = Fl, fluorenyl) are reported. The reduction of [(η5-C5Me5)Cr(IMeMe)Cl] with KC8 in the presence of CO afforded the NHC ligated Cr(I) metallo-radical [(η5-C5Me5)Cr(IMeMe)(CO)2]. Quantum chemical calculations performed on [(η5-C5Me5)Cr(IMeMe)(CO)2] confirm for this complex a predominantly chromium centered radical.

Gold(I) Multi-Resonance Thermally Activated Delayed Fluorescent Emitters for Highly Efficient Ultrapure-Green Organic Light-Emitting Diodes

Acceleration of singlet-triplet intersystem crossings (ISC) is instrumental in bolstering triplet exciton harvesting of multi-resonance thermally activated delayed fluorescent (MR-TADF) emitters. This work describes a simple gold(I) coordination strategy to enhance the spin-orbit coupling of green and blue BN(O)-based MR-TADF emitters, which results in a notable increase in rate constants of the spectroscopically observed ISC process to 3×10⁹  s^-1 with nearly unitary ISC quantum yields. Accordingly, the resultant thermally-stable Au^I emitters attained large values of delayed fluorescence radiative rate constant up to 1.3×10⁵ /1.7×10⁵  s^-1 in THF/PMMA film while preserving narrowband emissions (FWHM=30-37 nm) and high emission quantum yields (ca. 0.9). The vapor-deposited ultrapure-green OLEDs fabricated with these Au^I emitters delivered high luminance of up to 2.53×10⁵  cd m^-2 as well as external quantum efficiencies of up to 30.3 % with roll-offs as low as 0.8 % and long device lifetimes (LT₆₀ ) of 1210 h at 1000 cd m^-2 .

Improved synthetic routes to N-heterocyclic carbene-metal-diketonato complexes of gold and copper

In our search for simple synthetic routes to N-heterocyclic carbene (NHC)-metal complexes and their derivatives, we herein report an operationally simple, expedient and scalable method to obtain the widely used NHC-metal-diketonates. The reported complexes are synthesized for the first time under mild, aerobic conditions and in excellent yields in a sustainable manner. The protocol is general with regards to the anionic co-ligand and the ancillary carbene ligands. The spectroscopic and crystallographic characterization of the complexes reveal a bidentate binding mode of the diketonate ligand to copper while the gold-congener is C-bound. Finally, the reported Au complex was shown to be an efficient pre-catalyst for the hydrocarboxylation of alkynes.

N-Heterocyclic carbene and cyclic (alkyl)(amino)carbene complexes of vanadium(III) and vanadium(V)

[VCl₃(THF)₃] offers a convenient entrance point into the chemistry of carbene stabilized V(III) complexes. Herein we report the paramagnetic mono- and biscarbene complexes [VCl₃(cAAC^Me)] 1, [VCl₃(cAAC^Me)(THF)] 1(thf), [VCl₃(IMes)] 2, [{VCl₂(IiPr^Me)(μ-Cl)}₂] 3, [VCl₃(IDipp)] 4, [VCl₃(SIDipp)] 5, [VCl₃(SIDipp)(THF)] 5(thf), [VCl₃(ItBu)] 6, [VCl₃(cAAC^Me)₂] 7 and [VCl₃(IiPr^Me)₂] 8. Reaction of 1 with MesMgCl, MesLi and LiNPh₂ afforded the complexes [VCl₂(Mes)(cAAC^Me)] 9, [cAAC^MeH]⁺[VCl₂Mes₂]^-10 and [VCl₂(NPh₂)(cAAC^Me)] 11. The V(V) complexes [V(O)Cl₃(IDipp)] 12 and [V(O)Cl₃(SIDipp)] 13 were selectively prepared from oxygen oxidation of 4 and 5. [V(O)Cl₃(IDipp)] 12 and [V(O)Cl₃(IMes)] react with isocyanates to yield the NHC-ligated imido complexes [V(N-p-CH₃C₆H₄)Cl₃(IDipp)] 14, [V(N-p-FC₆H₄)Cl₃(IDipp)] 15, [V(N-p-CH₃C₆H₄)Cl₃(SIDipp)] 16, [V(N-p-FC₆H₄)Cl₃(SIDipp)] 17, [V(N-p-CH₃C₆H₄)Cl₃(IMes)] 18 and [V(N-p-FC₆H₄)Cl₃(IMes)] 19.

A detailed theoretical investigation to unravel the molecular mechanism of the ligand-free copper-catalyzed Suzuki cross-coupling reaction

The Suzuki-Miyaura coupling (SMC) represents a very efficacious method for constructing C-C bonds in organic synthesis. The ligand-free variants of SMC have been grabbing attention these days. Despite this momentousness, the mechanistic details of the ligand-free variants are scant in the literature. Herein, we have carried out a detailed mechanistic investigation into the ligand-free Cu-catalyzed SMC of unsaturated organic halides with aryl boronic acid with the aid of density functional theory (DFT) calculations employing the conductor-like polarizable continuum model (CPCM) method. The present study elucidates that in the absence of ancillary ligands on the metal, the substrates, base, and solvent molecules could act as pseudo-ancillary ligands to facilitate the cross-coupling reaction. The investigation further revealed that unsaturated halides like alkynyl halides/vinyl halides could act as good ancillary ligands for copper by forming a Cu-π intermediate and promoting a facile transmetalation process. However, regarding the oxidative addition and reductive elimination steps, a concerted pathway is observed contrary to Pd catalyzed Suzuki coupling, owing to the instability of Cu(III) species and the favourability of Csp²-Csp bond formation. In the whole set of mechanisms explored, oxidative addition/oxidative nucleophilic substitution was the rate-determining step in all the cases. A thermodynamically stable π-coordinated intermediate species where the substrate and base molecule are coordinated to the metal center is identified as the rate-determining species for the ligand-free Suzuki cross-coupling reaction. The presence of the aforesaid intermediate increases the energy span and consequently the activation barrier for the rate-determining step. This study unveiled a theoretical rationale for the high-temperature requirement in the ligand-free Cu-catalyzed SMC reaction.

Pd/Cu bimetallic catalysis to access highly fluorinated biaryls from aryl halides and fluorinated arenes

An efficient Pd/Cu bimetallic cross-coupling catalysis of fluoroaryl halides and fluoroarenes is reported. In situ generation of the Cu nucleophile by rate determining C-H activation of highly fluorinated aryls (≥4 F atoms) leads to high cross-coupling selectivity with little formation of homocoupling products.

Straightforward synthesis of [Cu(NHC)(alkynyl)] and [Cu(NHC)(thiolato)] complexes (NHC = N-heterocyclic carbene)

Synthetic access to monomeric copper-alkynyl and copper-thiolato complexes of the type [(NHC)Cu(R)] (R = alkynyl or thiolato) using a weak base approach is reported. All reported reactions proceed under mild conditions in air and in environmentally acceptable solvents. The novel complexes are fully characterized and single crystal X-ray analyses unambiguously establish the atom connectivity in these mononuclear complexes. The importance of the supporting NHC ligand's steric properties in stabilizing mononuclear complexes is discussed.

Cu-mediated vs. Cu-free selective borylation of aryl alkyl sulfones

A Cu-catalysed borylation of aryl alkyl sulfones was developed for the high yield synthesis of versatile arylboronic esters using a readily prepared NHC-Cu catalyst. In addition, the selective cleavage of either alkyl(C)-sulfonyl or aryl(C)-sulfonyl bonds of a cyclic sulfone via Cu-free or Cu-mediated processes generates the corresponding sulfinate salts, which can be further derivatised to provide sulfonyl-containing boronate esters, such as sulfones and sulfonyl fluorides.

Tris(pentafluoroethyl)difluorophosphorane: A Versatile Fluoride Acceptor for Transition Metal Chemistry

Fluoride abstraction from different types of transition metal fluoride complexes [Ln MF] (M=Ti, Ni, Cu) by the Lewis acid tris(pentafluoroethyl)difluorophosphorane (C2 F5 )3 PF2 to yield cationic transition metal complexes with the tris(pentafluoroethyl)trifluorophosphate counterion (FAP anion, [(C2 F5 )3 PF3 ]- ) is reported. (C2 F5 )3 PF2 reacted with trans-[Ni(iPr2 Im)2 (ArF )F] (iPr2 Im=1,3-diisopropylimidazolin-2-ylidene; ArF =C6 F5 , 1 a; 4-CF3 -C6 F4 , 1 b; 4-C6 F5 -C6 F4 , 1 c) through fluoride transfer to form the complex salts trans-[Ni(iPr2 Im)2 (solv)(ArF )]FAP (2 a-c[solv]; solv=Et2 O, CH2 Cl2 , THF) depending on the reaction medium. In the presence of stronger Lewis bases such as carbenes or PPh3 , solvent coordination was suppressed and the complexes trans-[Ni(iPr2 Im)2 (PPh3 )(C6 F5 )]FAP (trans-2 a[PPh3 ]) and cis-[Ni(iPr2 Im)2 (Dipp2 Im)(C6 F5 )]FAP (cis-2 a[Dipp2 Im]) (Dipp2 Im=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) were isolated. Fluoride abstraction from [(Dipp2 Im)CuF] (3) in CH2 Cl2 or 1,2-difluorobenzene led to the isolation of [{(Dipp2 Im)Cu}2 ]2+ 2 FAP- (4). Subsequent reaction of 4 with PPh3 and different carbenes resulted in the complexes [(Dipp2 Im)Cu(LB)]FAP (5 a-e, LB=Lewis base). In the presence of C6 Me6 , fluoride transfer afforded [(Dipp2 Im)Cu(C6 Me6 )]FAP (5 f), which serves as a source of [(Dipp2 Im)Cu)]+ . Fluoride abstraction of [Cp2 TiF2 ] (7) resulted in the formation of dinuclear [FCp2 Ti(μ-F)TiCp2 F]FAP (8) (Cp=η5 -C5 H5 ) with one terminal fluoride ligand at each titanium atom and an additional bridging fluoride ligand.