Arene Displacement Research Articles

Arene displacement, C-H activation and acetonitrile insertion reactions enabled by coordination of a functionalized iminophosphorane to a RuII-p-cymene scaffold.

Reaction of a sterically demanding iminophosphorano-phosphine, Ph2PCH2Ph2PNAr* (here onwards referred to as PCPNAr*; Ar* = 2,6-dibenzhydryl-4-methylphenyl), (1) with [Ru(η6-p-cymene)Cl2]2 yielded three different types of complex, [RuCl2{(η6-p-cymene)(PCPNAr*)-κ1-P}] (2), [RuCl{(P(O)CPNAr*)(κ2-N,C)(C-η6-arene)}] (3) and [RuCl{(POCPNAr*)(κ2-N,C-o)(C-η6-arene)}] (4), depending on the reaction conditions via CH activation, tethered η6-arene coordination, ortho-metallation or PN bond cleavage/rearrangement reactions. Interestingly, a similar reaction in CH3CN in the presence of AgBF4 resulted in the insertion of CH3CN into the PN bond to form a novel metallacycle [Ru(NCMe)3{(PC2PN(CH3)CNAr*)-κ3-N,N,P}][BF4]2 (5) containing 4- and 5-membered rings via an aza-Wittig type reaction. Complex 4 showed very good catalytic activity in the transfer hydrogenation of carbonyl compounds.

Facile Arene Ligand Exchange in p-Cymene Ruthenium(II) Complexes of Tertiary P-Chiral Ferrocenyl Phosphines.

Half-sandwich arene–metal complexes are commonly used for specific applications. Herein, we report facile arene ligand exchange reactions of two ruthenium(II) complexes of tertiary P-stereogenic ferrocenyl phosphines. By mild photochemical activation, the displacement of p-cymene and subsequent tethering by η6-coordination of the terminal phenyl ring of a biphenylyl-substituted ferrocenyl phosphine were enabled. Furthermore, the spontaneous p-cymene displacement in a 2-methoxyphenyl-containing ferrocenyl phosphine and ensuing coordination of the ligand as a P,O chelate were examined. For both reactions, theoretical calculations of the general course of the reaction confirmed the experimental findings. The ease of the controlled arene displacement reported here can offer new pathways for the synthesis and design of novel tailor-made catalysts.

Photochemically Induced Reductive Elimination as a Route to a Zirconocene Complex with a Strongly Activated N2 Ligand

The zirconocene dinitrogen complex [{(η(5)-C5Me4H)2Zr}2(μ2,η(2),η(2)-N2)] was synthesized by photochemical reductive elimination from the corresponding zirconium bis(aryl) or aryl hydride complexes, providing a high-yielding, alkali metal-free route to strongly activated early-metal N2 complexes. Mechanistic studies support the intermediacy of zirconocene arene complexes that in the absence of sufficient dinitrogen promote C-H activation or undergo comproportion to formally Zr(III) complexes. When N2 is in excess arene displacement gives rise to strong dinitrogen activation.

Microwave‐Assisted Organometallic Syntheses: Formation of Dinuclear [(Arene)Ru(μ‐Cl)3RuCl(L–L′)] Complexes (L–L′: Chelate Ligands with P‐, N‐, or S‐Donor Atoms) by Displacement of Arene π Ligands (Eur. J. Inorg. Chem. 8/2009)

AbstractThe cover picture shows the molecular structure of a dinuclear ruthenium complex, which was obtained by an arene displacement reaction in a microwave oven. Although microwave heating has been used extensively in organic synthesis, there are relatively few reports about its application in preparative organometallic chemistry. On p. 1003 ff, K. Severin et al. describe that [(arene)Ru(μ‐Cl)3RuCl(L?L′)] complexes with a diverse set of chelate ligands L?L′ are easily accessible by microwave heating. It should be mentioned, though, that the instrument used for their experiments was slightly more sophisticated than the one shown on the cover.

Graphical Abstract: Eur. J. Inorg. Chem. 8/2009

AbstractThe cover picture shows the molecular structure of a dinuclear ruthenium complex, which was obtained by an arene displacement reaction in a microwave oven. Although microwave heating has been used extensively in organic synthesis, there are relatively few reports about its application in preparative organometallic chemistry. On p. 1003 ff, K. Severin et al. describe that [(arene)Ru(μ‐Cl)3RuCl(L?L′)] complexes with a diverse set of chelate ligands L?L′ are easily accessible by microwave heating. It should be mentioned, though, that the instrument used for their experiments was slightly more sophisticated than the one shown on the cover.

Microwave‐Assisted Organometallic Syntheses: Formation of Dinuclear [(Arene)Ru(μ‐Cl)3RuCl(L–L′)] Complexes (L–L′: Chelate Ligands with P‐, N‐, or S‐Donor Atoms) by Displacement of Arene π Ligands

AbstractMicrowave heating was employed to promote arene displacement in reactions of [{(p‐cymene)RuCl2}2] or [{(1,3,5‐C6H3iPr3)RuCl2}2] with neutral chelate ligands L–L′ [L–L′: 1,1′‐bis(diphenylphosphanyl)methane, 1,1′‐bis(diphenylphosphanyl)ferrocene, (S)‐BINAP, (S,S)‐DIOP, N,N′‐bis(2,4,6‐trimethylphenyl)‐1,2‐ethanediylidenediamine], (R)‐Ph‐PHOX, and 3‐(phenylsulfanylpropyl)diphenylphosphane. The reactions gave complexes of the general formula [(arene)Ru(μ‐Cl)3RuCl(L–L′)] in good yield. The synthesis of [(p‐cymene)Ru(μ‐Cl)3RuCl{PPh2(CH2)3NH2}] (22) was accomplished in two steps via the intermediate [{(p‐cymene)RuCl2}2{μ‐PPh2(CH2)3NH2}] (21). The structures of [(1,3,5‐C6H3iPr3)Ru(μ‐Cl)3RuCl(dppf)] (16), [(1,3,5‐C6H3iPr3)Ru(μ‐Cl)3RuCl{(S)‐BINAP}] (17), and [(p‐cymene)Ru(μ‐Cl)3RuCl(MesNCHCHNMes)] (18) were determined by single‐crystal X‐ray diffraction.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Synthesis and Structure of Novel (η1:η6-Aminoalkylarene)RuII Complexes

Abstract A series of mononuclear tethered complexes, RuCl2(η1:η6-aminoalkylarene), in which the η6-arene group and the ligated protic amine group are connected by suitable aliphatic carbon chains, were synthesized from RuCl2(η6-PhCO2C2H5)(aminoalkylarene-κ-N) complexes via an intramolecular arene displacement reaction.

A laser flash photolysis, matrix isolation, and DFT investigation of (η 6-C 6H 5Y)Cr(CO) 3 (Y = NH 2, OCH 3, H, CHO, or CO 2CH 3)

The quantum yield for arene displacement from (η 6-C 6H 5Y)Cr(CO) 3 was measured in 1,1,2-trifluorotrichloroethane (Y = NH 2, OCH 3, H, CHO, or CO 2CH 3). Values of 0.24, 0.27, 0.15, 0.17, and 0.32 were obtained respectively ( λ exc. = 355 nm). These values are significantly higher than those measured for photoinduced arene loss in hydrocarbon solvents using the same excitation wavelength. Laser flash photolysis of (η 6-C 6H 5Y)Cr(CO) 3 in 1,1,2-trifluorotrichloroethane ( λ exc. = 355 nm) resulted in the rapid formation (<10 ns) of Cr(CO) 6. Matrix isolation experiments on (η 6-C 6H 5Y)Cr(CO) 3 (Y = H or CHO) at 12 K in CH 4 or CO-doped CH 4 matrixes using monochromatic irradiation confirmed the presence of two discrete excited states, one leading to CO-loss and the other to arene-loss. The results correlate with the calculated electron drift in the excited state derived from density functional theory and time dependent density functional theory calculations.

Eta6-mesityl,eta1-imidazolinylidene-carbene-ruthenium(II) complexes: catalytic activity of their allenylidene derivatives in alkene metathesis and cycloisomerisation reactions.

The reaction of electron-rich carbene-precursor olefins containing two imidazolinylidene moieties [(2,4,6-Me(3)C(6)H(2)CH(2))NCH(2)CH(2)N(R)Cdbond;](2) (2a: R=CH(2)CH(2)OMe, 2 b R=CH(2)Mes), bearing at least one 2,4,6-trimethylbenzyl (R=CH(2)Mes) group on the nitrogen atom, with [RuCl(2)(arene)](2) (arene=p-cymene, hexamethylbenzene) selectively leads to two types of complexes. The cleavage of the chloride bridges occurs first to yield the expected (carbene) (arene)ruthenium(II) complex 3. Then a further arene displacement reaction takes place to give the chelated eta(6)-mesityl,eta(1)-carbene-ruthenium complexes 4 and 5. An analogous eta(6)-arene,eta(1)-carbene complex with a benzimidazole frame 6 was isolated from an in situ reaction between [RuCl(2)(p-cymene)](2), the corresponding benzimidazolium salt and cesium carbonate. On heating, the RuCl(2)(imidazolinylidene) (p-cymene) complex 8, with p-methoxybenzyl pendent groups attached to the N atoms, leads to intramolecular p-cymene displacement and to the chelated eta(6)-arene,eta(1)-carbene complex 9. On reaction with AgOTf and the propargylic alcohol HCtbond;CCPh(2)OH, compounds 4-6 were transformed into the corresponding ruthenium allenylidene intermediates (4-->10, 5-->11, 6-->12). The in situ generated intermediates 10-12 were found to be active and selective catalysts for ring-closing metathesis (RCM) or cycloisomerisation reactions depending on the nature of the 1,6-dienes. Two complexes [RuCl(2)[eta(1)-CN(CH(2)C(6)H(2)Me(3)-2,4,6)CH(2)CH(2)N- (CH(2)CH(2)OMe)](C(6)Me(6))] 3 with a monodentate carbene ligand and [RuCl(2)[eta(1)-CN[CH(2)(eta(6)-C(6)H(2)Me(3)-2,4,6)]CH(2)CH(2)N-(CH(2)C(6)H(2)Me(3)-2,4,6)]] 5 with a chelating carbene-arene ligand were characterised by X-ray crystallography.

New Dihydride− and Alkene−η6-Arene Complexes of Iridium

The synthesis of iridium derivatives of general formula [(η6-arene)IrH2(PR3)]BF4 {arene = benzene: R = iPr (1), Cy (2); R = iPr: arene = toluene (4), 1,3,5-trimethylbenzene (5), 1,2,4-trimethylbenzene (6), hexamethylbenzene (7), 1-methylstyrene (8), phenol (9), aniline (10)} and [(η6-arene)Ir(η2-C2H4)(PiPr3)]BF4 {arene = benzene (23), toluene (24), 1,3,5-trimethylbenzene (25), hexamethylbenzene (26)} is described. 1 and 2 have been obtained by treatment of [Ir(μ-OMe)(cod)]2 with [HPR3]BF4, followed by reaction with H2, in acetone/benzene mixtures. A similar synthetic methodology has been used to prepare the naphthalene complex [(η6-C10H8)IrH2(PiPr3)]BF4 (16) and the thiophene derivative [(η5-SC4H4)IrH2(PiPr3)]BF4 (17). Compounds 4−10 have been prepared from 1 by arene substitution in acetone solutions. The substitution reactions involved a tris-acetone intermediate, which has been characterized in acetone-d6, [IrH2(OC(CD3)2)3(PiPr3)]+ (3). The η6-aniline ligand of 10 can be displaced by three N-bonded anilines, to give the complex [IrH2(κ-N, NH2Ph)3(PiPr3)]BF4 (11). Other N-containing arenes such as quinoline and isoquinoline gave only products with N-coordinating ligands, 12−15. The treatment of 1 with NaBPh4 led to the zwitterionic compound [Ph3B(η6-Ph)IrH2(PiPr3)] (18). This complex has been used as an arene ligand to obtain the compounds [Ph2B{(η6-Ph)IrH2(PiPr3)}2]BF4 (19), [PhB{(η6-Ph)IrH2(PiPr3)}3](BF4)2 (20), and [B{(η6-Ph)IrH2(PiPr3)}4](BF4)3 (21), in which the BPh4- anion is coordinated to two, three, or four iridium centers, respectively. Treatment of 1 with styrene afforded the Ir(I) compound [(η6-C6H5Et)Ir(η2-CH2CHPh)(PiPr3)]BF4 (22). Under similar conditions, the reaction of 1, 4, and 5 with ethylene produced ethane and the ethylene complexes 23−25. Compound 26 and the zwitterionic complex [Ph3B(η6-Ph)Ir(η2-C2H4)(PiPr3)] (27) have been prepared by arene displacement from 23. 1 has been found to catalyze, under mild conditions, the hydrogenation of several unsaturated substrates, including imines. The molecular structure of 23 has been determined by X-ray crystallography.

Metallabenzenes.

Many similarities exist between metallabenzenes and conventional arenes. Among these similarities are structural features such as ring planarity and the absence of bond length alternation, spectroscopic features such as downfield chemical shifts for ring protons, and chemical reactions such as electrophilic aromatic substitution and arene displacement from (arene)Mo(CO)3. All of these features, taken together, strongly support the thesis that metallabenzenes represent a new class of aromatic compounds, one in which metal d orbitals participate fully with carbon p orbitals in the formation of ring pi-bonds. However, it is also apparent that metallabenzenes are much more prone to isomerization reactions than are conventional arenes. This appears to be particularly true of first-row and second-row metallabenzenes, where the metal-carbon bond strengths are weaker. In these systems, carbene migratory insertion often leads to cyclopentadienyl-metal products. pi-Coordination of metallabenzenes to other metal centers generally stabilizes the metallabenzene moieties while maintaining their aromatic character. Among metallabenzenes coordinated in this way, there are representatives from all three transition-metal rows (Fe, Ni, Mo, Ru, and Ir). The metal atom in pi-coordinated metallabenzenes is displaced out of the ring and away from the complexing metal center. The reason for this displacement in most cases appears to be steric repulsion between ligands on the two metal centers. However, other subtle effects may contribute. For example, metal displacement leads to more favorable internal angles at the alpha-carbons and a better orientation of C alpha p orbitals toward the complexing metal center. While no dominant synthetic strategy for constructing metallabenzenes has emerged, cyclization reactions involving metal-thiocarbonyl, metal-alkylidyne, and metal-alkylidene precursors have proved useful. In addition, approaches involving pentadienyl reagents as the source of ring carbons have yielded notable successes. Vinylcyclopropene reagents have recently led to isolation of the first example of a metallabenzene valence isomer--a metallabenzvalene--and its subsequent conversion to a planar metallabenzene. Finally, interligand attacks of butadienyls on carbonyls have produced a variety of transient oxy- or alkoxy-substituted metallabenzene species. The development of new synthetic approaches, particularly systematic approaches that can be used with a variety of transition metals, is the key issue facing metallabenzene chemists. One hundred and thirty-five years after Kekulé's celebrated dream, aromatic chemistry continues to be a fascinating and provocative research topic. Metallabenzenes represent one of the "new frontiers" that promise to keep aromatic chemistry vibrant well into the 21st century.

Synthesis, redox chemistry and EPR spectroscopy of the mixed-sandwich complexes (η-arene)(η-cycloheptatrienyl)metal(z+) (M = Cr or Mo; z = 1 or 2): crystal structures of the redox pair [Cr(η-C6H5Me)(η-C7H6C6H4Me-4)][PF6]n (n = 1 or 2)

The mixed sandwich complexes [M(η-arene)(η-C7H6R′)]+ (M = Cr, R′ = H, arene = C6H4Me2-1,4, 1a; or C6H3Me3-1,3,5, 2a; R′ = C6H4Me-4, arene = C6H5Me, 3a; M = Mo, R′ = H, arene = C6H3Me3-1,3,5, 4a) were prepared by reflux of [M(CO)3(η-C7H6R′)]+ in the appropriate arene solvent. Reflux of [Mo(η-C6H5Me)(η-C7H7)]+ with an excess of HCCBut in acetone affords [Mo(η-C6H3But3-1,3,5)(η-C7H7)]+, 5a. Cyclic voltammetric studies in NCMe reveal that each of 1a, 2a, 3a and 5a undergoes reversible one-electron oxidation processes to give the corresponding, isolable 17-electron radical dications, 1b, 2b, 3b and 5b which have been characterised by EPR spectroscopy. NMR data for the 18-electron monocations suggest an enhanced electron density at the arene ring in the chromium derivatives by comparison with molybdenum analogues and this is reflected in the stability of complexes 1a, 2a and 3a towards arene displacement reactions. The crystal structure of 3a reveals only a small asymmetry in the average chromium-to-ring carbon bond lengths for the arene and cycloheptatrienyl rings. One-electron oxidation of 3a to give 3b results in a small increase in metal-to-ring distances (ca. 0.02 Å) consistent with a HOMO which is essentially non-bonding with respect to the metal–ring interaction.

Bonding of halogenated arenes in photogenerated (arene)M(CO) 5 complexes (M = Cr, Mo, W)

Detailed kinetics studies of arene displacement by the trapping nucleophile, 1-hexene, from (haloarene)Cr(CO) 5 complexes (haloarene = X-arene; X = F, Cl, Br) and selected Mo and W analogues generated by pulsed laser flash photolysis have been conducted. All reactions obey a ‘reversible dissociation-competition for the [M(CO) 5] intermediate’ mechanism. The systematic variations in rate constants and activation parameters for unimolecular dissociation of haloarenes in the presence of the trap strongly support coordinate covalent bonding of Cl and Br to Cr, where Cr-Br Bonds are stronger than are Cr- Cl bonds. In contrast, evidence indicates that fluorobenzene coordinates more weakly, bound through an arene ring edge. Carbonyl stretching frequencies for the complexes are diagnostic for the type of bonding, and enthalpies of activation decrease in the order W > Mo > Cr. The failure to observe more than one reaction pathway where more than one complex is generated upon photolysis may suggest very rapid linkage isomerization to afford the thermodynamically most stable (X-arene)Cr(CO) 5 complex.

Asymmetric Induction in η2-Arene Complexes of Pentaammineosmium(II)

A series of η2-arene complexes (2a−i) of the form [Os(NH3)5(ArOCHR1CH3)]2+ was prepared in which the alkoxy substituent R1 bears a potential hydrogen bond acceptor. Using an ester or amide group as the hydrogen bond acceptor, a two-point interaction was achieved between the pentaammineosmium system and the organic ligand. For these complexes, a single coordination diastereomer was observed. For compound 2a, evidence for hydrogen bonding between the carbonyl oxygen and the pentaammineosmium fragment was obtained through 1H NMR studies and by a comparison of the rate of arene displacement for 2a compared to that of the parent complex [Os(NH3)5(anisole)]2+. Complex 2a undergoes stereospecific protonation to form a 4H-anisolium product 3a. Complex 3a undergoes nucleophilic addition of a silylketene acetal at C3 to generate an alkoxydiene complex (4) as a single diastereomer. Hydrolysis of 4 removes the chiral auxiliary, and subsequent oxidative decomplexation yields a cyclohexenone with a new stereogenic center.

Bonding modes of arenes in photogenerated (arene)Cr(CO) 5 complexes

Detailed kinetics studies of arene displacement by 1-hexene from the ten (arene)Cr(CO) 5 complexes (arene=C 6H 6− n (CH 3) n ; n=2−5) generated by pulsed laser flash photolysis have been conducted. The data indicate that all reactions obey a reversible dissociation-competition for the [Cr(CO) 5] intermediate mechanism. The systematic variations in rate constants for unimolecular dissociation of arenes in the presence of 1-hexene ‘trap’ strongly support ‘edge-on’ bonding of the arene to Cr for the methyl-substituted benzene compounds in which there is a CC ‘ring-edge’ unsubstituted by methyls. Where no such ring-edge is present, i.e., for mesitylene, 1,2,3,5-tetramethylbenzene, 1,2,4,5-tetramethylbenzene and pentamethylbenze, the kinetics data suggest that a rapid equilibrium is e stablished two binding modes, the second possibly through the ring center.

Synthesis, spectroscopic studies and reactivity of η6-arene derivatives of titanium(IV)

Halide abstraction from TiX 4 by AlX 3 in the presence of some aromatic hydrocarbons afforded the η 6-arene complexes of titanium(IV) of formula [TiX 3( η 6-arene)]AlX 4 (arene = 1,3,5-Me 3C 6H 3, X  Cl, Br; arene = 1,2,4,5-Me 4C 6H 2, X  Cl; arene = Me 6C 6, X  Cl, Br, I). The corresponding chloro complexes of 1,4-Me 2C 6H 4, 1,2,3,-Me 3C 6H 3 and 1,2,4-Me 3C 6H 3 were characterized in solution by 13C NMR spectroscopy. Arene displacement from [TiCl 3( η 6-arene)][AlCl 4] occurs with THF or TICp (Cp  C 5H 5 −) to afford TiCl 4(THF) 2 or CpTiCl 3, respectively. The titanium(IV)-arene complexes efficiently promote the hydrogen-deuterium exchange of the ring protons between C 6D 6 and C 6H 6− n Me n .

MONO-η6-ARENE COMPLEXES OF CHROMIUM (II) AND CHROMIUM (0). ARENE EXCHANGE AND ARENE DISPLACEMENT REACTIONS OF η6-FLUOROARENE-DICARBONYLBIS (TRICHLOROSILYL) AND -TRICARBONYLCHROMIUM DERIVATIVES

Abstract In an attempt to obtain chromium-arene derivatives possessing high arene lability, (C6 H5F)Cr(CO)2 (SiCl3)2, (C6H4F2)Cr(CO)2(SiCl3)2, (C6H5F)Cr(CO)3, and (C6H4F2)Cr(CO)3 have been investigated. Kinetics of arene exchange have been studied showing that the Cr(II) derivatives exhibit ten-fold higher rates than the Cr(O) derivatives. Both electronic and steric effects appear to be important, although electronic effects dominate. Arene displacement by phosphine and phosphite ligands has also been studied. Although arene displacement took place, unexpected rearrangement and disproportionation reactions occurred yielding seven-coordinate (CO)4Cr(PR3)(SiCl3)2 species.

Mechanistic aspects of the photochemistry of [CpM(η6-arene)]+ complexes of Fe, Ru and Os

Abstract

Ground-State versus Transition-State Effects in Arene Displacement Reactions of the Complexes (.eta.6-arene)Cr(CO)3: Linear Dependence of Transition-State Energies and Resonance Energies of the Arene Ligands

Ground-State versus Transition-State Effects in Arene Displacement Reactions of the Complexes (.eta.6-arene)Cr(CO)3: Linear Dependence of Transition-State Energies and Resonance Energies of the Arene Ligands

Direct synthesis of (η 6-arene)-tricarbonylchromium complexes by arene displacement from tricarbonyl(η 5-1-methylpyrrole)chromium(0)

Arene displacement from the title complex occurs under unprecedented mild conditions in neutral media to afford (η 6-arene)tricarbonylchromium complexes. The process is most effective with heteroaromatic compounds: the first synthesis of a tricarbonyl(η 6-quinoline)chromium complex and the diastereoselective preparation of a (η 6-indole) complex derived from L-tryptophan are reported as representative examples.