Free Arene Research Articles

Ruthenium/η5-Phenoxo-Catalyzed Amination of Phenols with Amines.

Ruthenium(II) complexes are known to form η6-arene complexes with benzene-containing compounds through π-coordination, a property extensively utilized to initiate reactions not typically observed with free arenes. A prime example is nucleophilic aromatic substitution, where ruthenium-complexed aryl halides undergo nucleophilic attack, allowing the direct synthesis of diverse aromatic compounds by displacing halides with nucleophiles. However, this activation relies on the electron-withdrawing effect of the Ru(II) species, as well as is hindered by the resistance of η6-arenes to arene exchange. In the previous pursuit of catalysis, the emphasis of ligand design has centered on promoting arene exchange. In this study, we extended the ruthenium activation strategy to umpolung substitution reactions of phenols. The amination proceeds through a direct condensation between phenols and amines, with a key intermediate identified as [bis(η5-phenoxo)Ru], which is in situ generated from a commercially available ruthenium catalyst. In comparison with the well-studied cyclopentadienyl (Cp) type ligands, we demonstrated that an η5-phenoxo motif, as a superior alternative to Cp, contributes to the amination of phenols in two crucial ways: its less electron-donating nature enhances the withdrawing effect of the ruthenium unit, facilitating substitution on the phenol complex; its distinctive behavior in arene exchange allows for conducting the amination with a catalytic amount of metal. Additionally, hydrogen bonding, wherein the phenoxo serves as the acceptor, was found to be important for the substitution. The versatility of this ruthenium-catalyzed amination was validated by performing reactions with a diverse array of phenols exhibiting various electronic properties, in combination with a wide range of primary amines. This work exemplifies the expansion of the scope of π-coordination activation in catalysis through innovative ligand development.

Synthesis and Application of Silylene-Stabilized Low-Coordinate Ag(I)–Arene Cationic Complexes

We report the first examples of N-heterocyclic silylene-stabilized monocoordinate Ag(I) cationic complexes weakly bound to the free arene rings (C6H6, C6Me6, and C7H8). Further, the application of these electrophilic Ag(I) complexes as catalysts has been investigated toward A3-coupling reactions, which afforded a series of propargylamines in good to excellent yields with low catalyst loading under a solvent-free condition (19 examples).

Catalyst‐Free Polyhydroboration of Dodecaborate Yields Highly Photoluminescent Ionic Polyarylated Clusters

AbstractThe incorporation of polyhedral boranes into novel photoluminescent materials is an area with increasing interest. While the neutral carboranes have been widely investigated for this purpose, the dodecaborate ion has received much less attention. Herein we report a significant expansion to the scope of substitution reactions for the dodecaborate ion, whereby this cluster was observed to react directly with a wide range of arenes in a step‐wise and controlled manner. In the products of these reactions, the dodecaborate ion serves as a core upon which up to nine mono‐ or polycyclic aromatic hydrocarbon ligands are exohedrally bonded. Spectroscopic evidence suggests the presence of conjugation between the π systems of the aryl ligands and the dodecaborate core, resulting in materials which exhibit high solution‐phase photoluminescence, as well as molar absorptivities and Stokes shifts that are significantly larger than those of the free arenes from which they were derived. We propose that this broad reactivity is a valuable synthetic tool for the incorporation of polyhedral boron into novel organic structures.

Catalyst-Free Polyhydroboration of Dodecaborate Yields Highly Photoluminescent Ionic Polyarylated Clusters.

The incorporation of polyhedral boranes into novel photoluminescent materials is an area with increasing interest. While the neutral carboranes have been widely investigated for this purpose, the dodecaborate ion has received much less attention. Herein we report a significant expansion to the scope of substitution reactions for the dodecaborate ion, whereby this cluster was observed to react directly with a wide range of arenes in a step-wise and controlled manner. In the products of these reactions, the dodecaborate ion serves as a core upon which up to nine mono- or polycyclic aromatic hydrocarbon ligands are exohedrally bonded. Spectroscopic evidence suggests the presence of conjugation between the π systems of the aryl ligands and the dodecaborate core, resulting in materials which exhibit high solution-phase photoluminescence, as well as molar absorptivities and Stokes shifts that are significantly larger than those of the free arenes from which they were derived. We propose that this broad reactivity is a valuable synthetic tool for the incorporation of polyhedral boron into novel organic structures.

Frustrated Lewis pair-like splitting of aromatic C–H bonds and abstraction of halogen atoms by a cationic [(FPNP)Pt]+species

This manuscript explores new chemistry that can be related to the unobserved 14-electron [(FPNP)Pt]+ transient (FPNP = (4-F-2-(iPr2P)C6H3)2N). Its reactivity can be accessed via abstraction of triflate from (FPNP)PtOTf (1) with K[B(C6F5)4] in various solvents serving as substrates. With benzene, toluene and fluorobenzene, net heterolytic splitting of an aromatic C–H bond across the N–Pt bond is observed, without any detectable intermediates, leading to the [(FPN(H)P)Pt–Ar]+ products (Ar = Ph, 3a; Ar = C6H4Me, 3b (ortho), 3c (meta), 3d (para); Ar = o-C6H4F, 3e). The latter can be alternatively prepared by protonation of the neutral (FPNP)Pt–Ar compounds. Compounds 3a–3e do not release free arene under thermolysis at 80 °C, and compounds 3b/c/d do not interconvert under ambient temperature. With chlorobenzene and bromobenzene, the kinetic product is the κ1-Cl or κ1-Br adduct [(FPNP)Pt–Cl–Ph]+ (4) or [(FPNP)Pt–Br–Ph]+ (10). Compound 4 rearranges into a C–H splitting product [(FPN(H)P)Pt–C6H4Cl]+ (3f), while 10 slowly reacts by formal transfer of Br atom to Pt. An analogous Cl atom transfer to Pt is observed upon the reaction of 1 with K[B(C6F5)4] in dichloromethane, producing [(FPNP)PtCl][B(C6F5)4] (9a) which features an oxidized FPNP ligand framework. X-Ray diffractometry established structures of [(FPN(H)P)Pt–C6H4F-o][B(C6F5)4] (3e, disordered rotamers), [(FPN(H)P)Pt–C6H4Me][B(C6F5)4] (disordered meta- and para-isomers 3c/d), and [(FPNP)PtCl][HCB11Cl11] (9b). DFT calculations at the PBE0 and M06-L levels on the free [(FPNP)Pt]+ cation predict a relatively small (10–12 kcal mol−1) separation between the singlet and the triplet states. The relatively low triplet energy is probably related to the viability of the unexpected halogen atom abstraction reactions.

Synthesis and Reactivity of (η5-Hydroxyalkylcyclohexadienyl)Mn(CO)3 Complexes

(η5-1-Hydroxyalkylcyclohexadienyl)tricarbonylmanganese complexes 3a and 3b have been prepared, and their rearomatization delivered compounds 5 and 6, the first benzylic alcohol derivatives coordinated to the cationic Mn(CO)3 tripod. The structure of 5 has been determined by X-ray crystallography. 1-Hydroxyalkyl 3a, 7, 8, 2-hydroxyalkyl 19, 20, and 3-hydroxyalkyl 25, 26 (η5-cyclohexadienyl)tricarbonylmanganese complexes have been synthesized and studied toward trifluoroacetylation of the hydroxy group when submitted to the action of NEt3 and (CF3CO)2O. Only the 2-hydroxyalkyl-substituted complexes eclipsed by a Mn−CO bond of the Mn(CO)3 tripod gave the expected trifluoroacetate derivatives 21 and 22. For the 1-hydroxyalkyl-substituted complexes, elimination of the trifluoroacetoxy group took place followed by decoordination of the Mn(CO)3 moiety and trifluoroacetylation, giving rise to trifluoromethyltrienones 10, 11, and 12, whose stuctures were established by X-ray analysis. For 3-hydroxyalkyl-substituted complexes, elimination of the trifluoroacetoxy group also occurred, yielding free arenes 27 and 28 by decoordination of the Mn(CO)3 entity.

Dual Triggering of DNA Binding and Fluorescence via Photoactivation of a Dinuclear Ruthenium(II) Arene Complex

The dinuclear RuII arene complexes [{(eta6-arene)RuCl}2(mu-2,3-dpp)](PF6)2, arene=indan (1), benzene (2), p-cymene (3), or hexamethylbenzene (4) and 2,3-dpp=2,3-bis(2-pyridyl)pyrazine, have been synthesized and characterized. Upon irradiation with UVA light, complexes 1 and 2 readily underwent arene loss, while complexes 3 and 4 did not. The photochemistry of 1 was studied in detail. In the X-ray structure of [{(eta6-indan)RuCl}2(mu-2,3-dpp)](PF6)2 (1), 2,3-dpp bridges two RuII centers 6.8529(6) A apart. In water, aquation of 1 in the dark occurs with replacement of chloride with biexponential kinetics and decay constants of 100+/-1 min-1 and 580+/-11 min-1. This aquation was suppressed by 0.1 M NaCl. UV or visible irradiation of 1 in aqueous or methanolic solution led to arene loss. The fluorescence of the unbound arene is approximately 40 times greater than when it is complexed. Irradiation of 1 also had a significant effect on its interactions with DNA. The DNA binding of 1 is increased after irradiation. The non-irradiated form of 1 preferentially formed DNA adducts that only weakly blocked RNA polymerase, while irradiation of 1 transformed the adducts into stronger blocks for RNA polymerase. The efficiency of irradiated 1 to form DNA interstrand cross-links was slightly greater than that of cisplatin in both 10 mM NaClO4 and 0.1 M NaCl. In contrast, the interstrand cross-linking efficiency of non-irradiated 1 in 10 mM NaClO4 was relatively low. An intermediate amount of cross-linking was observed when the sample of DNA already modified by non-irradiated 1 was irradiated. DNA unwinding measurements supported the conclusion that both mono- and bifunctional adducts with DNA can form. These results show that photoactivation of dinuclear RuII arene complexes can simultaneously produce a highly reactive ruthenium species that can bind to DNA and a fluorescent marker (the free arene). Importantly, the mechanism of photoreactivity is also independent of oxygen. These complexes, therefore, have the potential to combine both photoinduced cell death and fluorescence imaging of the location and efficiency of the photoactivation process.

Enhancement of External Spin−Orbit Coupling Effects Caused by Metal−Metal Cooperativity

As part of our efforts to discover simple routes to room-temperature phosphors, we have investigated the interaction of bis(pentafluorophenyl)mercury (1) or trimeric perfluoro-o-phenylene mercury (2) with selected arenes (naphthalene, biphenyl, and fluorene). Solution studies indicate that 2, unlike 1, quenches the fluorescence of naphthalene. When compared to 1, the high quenching efficiency of 2 may be correlated to the higher affinity that 2 displays for arenes as well as to more acute external heavy-atom effects caused by the three mercury atoms. In the crystal, the adducts [1.naphthalene], [1.biphenyl], [1.fluorene], and [2.fluorene] form supramolecular binary stacks in which the arene approaches the mercury centers of 1 or 2 to form Hg-C pi-interactions. Analysis of the electrostatic potential surfaces of the individual components supports the involvement of electrostatic interactions. The luminescence spectra of the adducts show complete quenching of the fluorescence and display heavy-atom-induced emission whose energies and vibronic progressions correspond to the phosphorescence of the respective pure arene. The phosphorescence lifetimes are shortened by 3 or 4 orders of magnitude when compared with those of the free arenes. Taken collectively, the structural, photophysical, and computational results herein suggest that the proximity of the three mercury centers serves to enhance the Lewis acidity of 2, which becomes a better acceptor and a more effective heavy-atom effect inducer than 1.

Lithiation Reactions Catalyzed by Linear and Cross‐Linked Arene‐Based Polymers. Generation of Functionalized Organolithium Compounds.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Lithiation Reactions Catalyzed by Linear and Cross-Linked Arene-Based Polymers. Generation of Functionalized Organolithium Compounds

Lithiation of various substrates, such as chlorinated acetals, α-chloro ether, dichloro derivatives benzo-fused heterocycles, and allyl and benzyl derivatives, with excess lithium powder in the presence of a catalytic amount of soluble linear or insoluble cross-linked arene (naphthalene or biphenyl)-based polymers yields the expected organolithium intermediates. The latter react with electrophiles either in two steps or under Barbier-type reaction conditions to afford the corresponding adducts. The catalyst is easily recuperated by filtration at the end of the process, and the procedure can be regarded as a reasonable alternative to the use of free arenes as electron carrier in lithiation reactions.

Cis–trans Isomerisation of CpRe(CO)2(H)(ArF) (ArF= C6FnH5−n; n = 0–5) is the rate determining step in C–H activation of fluoroarenes: a DFT study

Density functional calculations have been used to examine the reaction of {CpRe(CO)2} with fluorobenzenes C6FnH6−n (n = 0–5). Two classes of product have been observed experimentally (using Cp or Cp*): (a) coordination of the arene in an η2 fashion and (b) C–H activation to form a hydrido–aryl complex. Increasing the number of fluorines on the arene ring was shown to favour C–H activation. The thermodynamic and kinetic (reaction path) aspects of these transformations have been examined with DFT (B3PW91) calculations. For a given arene, the rhenium moiety is shown to exhibit the following order of thermodynamic preference for coordination: HCCH site > HCCF site > FCCF site. Binding energies to the different arenes do not follow a clear trend and span ca. 20 kJ mol−1. The Re–C bond energies in CpRe(CO)(H)(C6FnH5−n) span 55 kJ mol−1. Calculated structural parameters agree with the crystal structure of coordination of C6H6 and C6F6. Likewise the binding energy of C6H6 is in good agreement with experimental data. The calculated free energy difference between CpRe(CO)2(η2-C6FnH6−n) and CpRe(CO)2(H)(C6FnH5−n) shows that preference for the hydrido–aryl complex is determined principally by the bond dissociation energy of the C–H bond of the free arene. The binding energy to the η2-arene appears to be only a secondary factor. Three families of complexes are apparent. If there is no F on the carbon ortho to the Re–C bond that is formed, the η2-arene complex is energetically preferred. If there is one F at the ortho position, the energies of the products are similar. In the case of two ortho F substituents, the product of oxidative addition is significantly favoured. In agreement with the calculations, experimental evidence shows that benzene only coordinates to Cp*Re(CO)2, 1,4-C6F2H4 gives a mixture of products and 1,3-C6F2H4 gives only the hydrido–aryl complex. The arene with the stronger C–H bond is the one which gives more oxidative addition product because the Re–C bond energy increases with F substitution (and in particular with ortho F) more than twice as fast as the C–H bond dissociation energy. The reaction path for the overall transformation has been determined. The σ C–H complex is identified as an intermediate on the pathway for the oxidative addition. The initial product of oxidative addition is the cis hydrido–aryl isomer which subsequently isomerizes to the trans isomer. The rate determining step has been found to be the cis–trans isomerisation process and not the oxidation addition step. The cis–trans isomerisation proceeds via an unconventional concerted motion of H and the two COs. The variation of the Re–C bond energy is the dominant factor in determining the changes in the energy barrier between the different fluoroarenes, resulting in strong correlation between the thermodynamics and kinetics of reaction. The activation barriers are therefore also grouped in three families (0 F ortho, 1 F ortho, 2 F ortho).

Preparation of new (η 6-arene)Mn(CO) 3+ complexes involving Pd-catalysed/ exo-hydride abstraction sequence

A novel methodology consisting in a selective palladium cross-coupling reaction starting from (η 5-chlorocyclohexadienyl)Mn(CO) 3 followed by rearomatisation allows the preparation of new cationic (η 6-arene)Mn(CO) 3 complexes, which cannot be obtained directly from the coordination of the corresponding free arenes.

(η5-Halogenocyclohexadienyl)Mn(CO)3 Complexes: Halogen Substitution by Arene Derivatives

Unprecedented coupling reactions on (η5-chlorocyclohexadienyl)tricarbonyl manganese complexes with arenes create a new carbon−carbon bond on the conjugated system, keeping intact the η5-chlorocyclohexadienyl moiety; thus, the removal of the exo hydrogen bond of the sp3 CH2 group yields efficiently new cationic (η6-arene) manganese complexes which cannot be prepared directly from the free arene.

The effect of solubility on the stability of titanium(IV) arene complexes derived from hexasubstituted arenes and TiCl 4

The investigation of new titanium(IV) hexaalkylarene complexes gave new insight into the stability of high-valent metal arene complexes. In contrast to low-valent transition metal arene complexes these complexes are in equilibrium with the free arenes. The stability of the complexes was shown to depend strongly on both the donor ability of the arene and on their solubility. This is unprecedented in transition metal arene chemistry.

Thermodynamics of C−H Activation in Multiple Oxidation States: Comparison of Benzylic C−H Acidities and C−H Bond Dissociation Energies in the Isostructural 16−20-Electron Complexes [Fex(η5-C5R5)(η6-arene)]n,x= 0−IV, R = H or Me,n= −1 to +3

The pKa of the 18-electron complexes [Fe(η5-C5R5)(η6-C6Me6)][PF6] {1a[PF6] (R = H) and 1b[PF6] (R = Me)} and [Fe(η5-C5R5)(η6-C6H5CHPh2)][PF6] {1c[PF6]} have been determined by the direct method in DMSO using bases with known pKa values and found to be 12−14 pKa units lower than the pKa of the free arenes, illustrating the electron-withdrawing character of the CpFe+ and Cp*Fe+ groups. Access to the pKa values for 16-, 17-, 19-, and 20-electron iron-sandwich complexes of this type with various arene structures was available by means of thermodynamic diagrams using the standard redox potentials of the oxidation and reduction of the 18-electron cations 1+ and the deprotonated complexes 2. For instance, the pKa of the 19-electron iron complex 1a (43.5) is about the same as that of free C6Me6 (43−44) whereas that of 1b is even slightly higher (46.4). The pKas of the anionic 20-electron complexes 1a- and 1b- are 7 and 12 units, respectively, higher than that of C6Me6. The pKas of 12+ are around −10, whereas thos...

Oxidation-State-Dependent Reactivity and Catalytic Properties of a Rh(I) Complex Formed from a Redox-Switchable Hemilabile Ligand

The synthesis and characterization of a phosphinoalkylarene, redox-switchable hemilabile ligand (RHL), (η5-C5H5)Fe(η5-C5H4C6H4OCH2CH2PPh2) (1), are reported. This ligand, which incorporates a redox-active ferrocenyl group, exhibits oxidation-state-dependent bonding properties and, hence, affords electrochemical control over the electronic and steric environments of bound transition metal centers. Two equivalents of 1 complex to Rh(I) to yield a bis(phosphine), η6-arene complex [(η1:η6-(η5-C5H5)Fe(η5-C5H4C6H4OCH2CH2PPh2))(η1-(η5-C5H5)Fe(η5-C5H4C6H4OCH2CH2PPh2))Rh]+BF4- (2). Single-crystal X-ray diffraction studies of 2·1.25CH2Cl2, as well as solution spectroscopic data of 2, are consistent with this formulated piano-stool geometry. Foremost, the properties of 2 as a function of bound RHL state-of-charge are extensively investigated. Interestingly, 2D 1H NMR exchange spectroscopy (EXSY) studies demonstrate significantly faster intramolecular η6-arene, free arene exchange rates only upon oxidation of the lig...

Marked Changes in Relative Nucleophilicity in Comparing SN2Ar Reactions of Free Arenes and Coordinated Arenes. Kinetic Studies of Reactions of (η6-Chlorobenzene)(η5-cyclopentadienyl)iron(II) Tetrafluoroborate with Anionic and Neutral Nucleophiles

Quantitative kinetic studies have been made of reactions of the (η6-chlorobenzene)(η5-cyclopentadienyl)iron(II) cation, (C6H5Cl)Fe(C5H5)+ (1), with methoxide, phenoxide, methanethiolate, benzenethi...

Novel Hemilabile (Phosphinoalkyl)arene Ligands: Mechanistic Investigation of an Unusual Intramolecular, Arene−Arene Exchange Reaction

The novel, hemilabile (phosphinoalkyl)arene ligands ArX(CH2)2PPh2 (1a, Ar = C6H5, X = O; 1b, Ar = C6H5, X = CH2; 1c, Ar = FC6H4, X = CH2) were synthesized and complexed to Rh(I) to form the bis(phosphine), η6-arene piano stool complexes [(η6:η1-ArX(CH2)2PPh2)Rh(η1-ArX(CH2)2PPh2)]BF4 (2a−c). Complexes 2a−c were fully characterized in solution, and complex 2a was characterized by single-crystal X-ray diffraction methods. Two of these complexes, 2a,c, undergo an unusual, degenerate η6-arene, free arene exchange reaction which was studied by 2-D NMR EXSY experiments. A mechanism for the exchange reaction of 2a which involves the formation of a square planar, cis-phosphine, cis-ether Rh(I) complex, [Rh(η2-PhO(CH2)2PPh2)2]BF4 (13), is proposed.

Ultraviolet photoelectron spectra of arene chromium tricarbonyls

He I and He II photoelectron spectra of tricarbonylchromium-complexed benzene derivatives are reported and assigned. Comparisons with the free arenes suggest that in all the cases studied, the spectra can be interpreted in terms of charge donation from the ring to the Cr(CO) 3 group as a whole: electronic effects at Cr are also visible at the C and O atoms as shifts in ionisation energy (IE). IEs for orbitals localised mainly on the substituents also shift, to an extent which depends on the “conjugation” between ring and substituent; conjugation in the complexes appears to be similar to that in the free arenes. Correlations of the observations with those in bis-arene complexes, and in surface-coordinated arenes, are discussed.

Protodeboronation of arylboronic acids and triarylboroxines in Bu 20/THF

The arylboron compounds Ph 3B 3N 3Me 3, Ar 3B 3O 3, or ArB(OH) 2 (Ar  Ph, p-BrC 6H 4, p-MeC 6H 4, m-NH 2C 6H 4 react with [Cr(CO) 6] in refluxing Bu 2O/THF (9:1) to afford the (η 6-arene) tricarbonyl-chromium(O) complexes of the protodeboronated substrates in yields of up to 55%. The proposed mechanism involves protodeboronation of the arylboron compound followed by complexation of the free arene by the chromium carbonyl moiety.