Cyclometalation Process Research Articles

Enantiopure [6]-Azairidahelicene by Dynamic Kinetic Resolution of a Configurationally Labile [4]-Helicene.

A pair of enantiopure [6]-azairidahelicenes incorporating chirality at the metal center and on the helicenic ligand were synthesized by dynamic kinetic resolution (dkr) of a configurationally labile [4]-helicenic ligand (4-(2-pyridyl)-benzo[g]phenanthrene, L1H) using bis-cyclometalated chiral-at-metal only iridium(III) precursors as chiral inductors. The origin of the observed dkr is attributed to the different conformation and stability of diastereomeric reaction intermediates formed during the cyclometalation process. The isolated enantiomers exhibited circularly polarized phosphorescence (CPP), with |gphos| values of 1.8×10-3.

Enantiopure [6]‐Azairidahelicene by Dynamic Kinetic Resolution of a Configurationally Labile [4]‐Helicene

AbstractA pair of enantiopure [6]‐azairidahelicenes incorporating chirality at the metal center and on the helicenic ligand were synthesized by dynamic kinetic resolution (dkr) of a configurationally labile [4]‐helicenic ligand (4‐(2‐pyridyl)‐benzo[g]phenanthrene, L1H) using bis‐cyclometalated chiral‐at‐metal only iridium(III) precursors as chiral inductors. The origin of the observed dkr is attributed to the different conformation and stability of diastereomeric reaction intermediates formed during the cyclometalation process. The isolated enantiomers exhibited circularly polarized phosphorescence (CPP), with |gphos| values of 1.8×10−3.

P-Stereogenic threonine derived Ir–P,N complexes. Reversible cyclometallation and asymmetric hydrogenation reactions

A threonine-derived phosphine-oxazoline iridium catalyst is reported, showcasing excellent selectivity in the hydrogenation of N-Boc-2,3-diarylallylamines. The reversible cyclometallation process between the square planar Ir(I) complex and the corresponding octahedral Ir(III) species is thoroughly investigated, while considering the influence of substitution at the phenyl group of the oxazoline on this equilibrium. X-Ray analysis reveals that the cyclometallated tridentate ligand adopts a fac coordination mode. Furthermore, the catalytic performance of these complexes in the asymmetric hydrogenation is evaluated and the utilization of Ir(III) pre-catalysts or Ir(I)/Ir(III) mixtures in hydrogenation reactions is proposed based on the observed equilibrium dynamics.

Cyclometallated ruthenium complexes with P-stereogenic monophosphines containing a polycyclic aromatic substituent

Reactions of optically pure P-stereogenic ortho-tolyl substituted phosphines with [RuCl2(p-cymene)]2 afforded the corresponding κP-coordinated ruthenium(II) dichlorides (C1′, C2’) even in the presence of sodium acetate. In contrast, the ruthenium cyclometallated (κ2-C,P) complexes (C3–C9) were obtained with phosphines containing a polycyclic aromatic substituent (L3-L9), namely 1-naphthyl, 9-phenanthryl or 1-pyrenyl. Some diastereoselectivity in the cyclometallation process has been observed for the most bulky ligands. The new compounds have been used as catalytic precursors in the reduction of acetophenone to 1-phenylethanol by transfer hydrogenation.

Synthetic and Computational Studies on the Thermal and Photochemical Reactions of [NPN]TaMe3 (NPN = PhP(CH2SiMe2NPh)2) and [MesNPN]TaMe3 (MesNPN = PhP(CH2SiMe2N(2,4,6-Me3C6H2))2)

The thermolysis of [PhP(CH2SiMe2NPh)2]TaMe3 leads to the elimination of methane and the formation of cyclometalated derivative [PhP(CH2SiMe2NPh)(CH2SiMe2N-o-C6H4)]TaMe2, which was characterized by NMR spectroscopy and single crystal X-ray analysis. Computational studies confirm the expected four-membered transition state involving an ortho-N-phenyl-C–H bond and a Ta-methyl unit. The photolysis of [PhP(CH2SiMe2NPh)2]TaMe3 takes a different course; loss of methane also occurs but results in the formation the methylidene complex, [PhP(CH2SiMe2NPh)2]Ta═CH2(Me), which was characterized by NMR spectroscopy. Attempts to block the cyclometalation process by replacement of the N-phenyl substituent with N-Mesityl (Mesityl = 2,4,6-Me3C6H2) is also reported. With this bulkier ancillary ligand, the reactions are more complicated with multiple products being observed in an overall slow process. The reactions of the trimethyl, the cyclometalated product and the methylidene with H2 were also investigated and found to exhibit different rates of hydrogenolysis. This has implications for some of the steps in the reaction of [PhP(CH2SiMe2NPh)2]TaMe3 with H2 to generate dinuclear tetrahydride ([PhP(CH2SiMe2NPh)2]Ta)2(μ-H)4.

Mechanistic Studies on Ruthenium(II)-Catalyzed Base-Free Transfer Hydrogenation Triggered by Roll-Over Cyclometalation.

The synthesis of 2-substituted pyridine-pyrimidine ligands and their complexation with arene ruthenium(II) chloride moieties is reported. Depending on the electronic and steric influences of the ligand, the catalysts undergo CH activation by roll-over cyclometalation. This process opens up the route to the catalytic transfer hydrogenation of ketones with isopropanol as the hydrogen source under base-free and mild conditions. Barriers related to the roll-over cyclometalation process can be determined experimentally by collision-induced dissociation ESI mass spectrometry. They are supported by DFT calculations and allow the classification of the ligands according to their electronic and steric properties, which is also in accordance with critical bond parameters derived from X-ray structure data. DFT calculations furthermore reveal that the formation of a ruthenium(II) hydrido species is plausible through β-hydride elimination from isopropanol.

Coordinatively Unsaturated T-Shaped Platinum(II) Complexes Stabilized by Small N-Heterocyclic Carbene Ligands. Synthesis and Cyclometalation

The reaction of the N-heterocyclic carbenes (NHCs) IiPr2Me2 and IiPr2 with [PtMe3I]4 does not lead to the clean formation of complexes trans-[PtMeI(NHC)2], as previously found for bulkier NHCs. However, these species can be prepared in high yields by an alternative procedure using the dimethyl species [PtMe2(NHC)2] and iodomethane. The halogenated complexes trans-[PtMeI(NHC)2] have been used as precursors for the formation of the coordinatively unsaturated Pt(II) derivatives [PtMe(NHC)2][SbF6] using AgSbF6 as a halogen abstractor, whereas NaBArF is not reactive enough to completely remove the iodide ligand, leading instead to the dinuclear species [Pt2(μ-I)(Me)2(NHC)4][BArF]. Alternatively, low-coordinate T-shaped complexes [PtMe(NHC)2][BArF] can be prepared by protonation of the dimethyl species [PtMe2(NHC)2] with [H(OEt2)2][BArF]. The complex [PtMe(IiPr2Me2)2][BArF] undergoes a cyclometalation process under mild heating, leading to the derivative [Pt(IiPr2Me2′)(IiPr2Me2)][BArF] (where the prime denotes the cyclometalated ligand). This result is in contrast with the fast cyclometalation observed for the related Pt(II) complex bearing tert-butyl-substituted NHC (ItBu). Different cyclometalation reaction mechanisms have been computationally addressed. DFT calculations indicate that the cyclometalation involving complexes with not very bulky NHCs ligands occurs via cis intermediates. Cyclometalation products bearing tert-butyl-substituted NHCs are kinetically and thermodynamically favored over those involving isopropyl groups.

Luminescent liquid crystalline materials based on palladium(ii) imine derivatives containing the 2-phenylpyridine core

In this work we report our studies concerning the synthesis and characterisation of a series of imine derivatives that incorporate the 2-phenylpyridine (2-ppy) core. These derivatives were used in the cyclometalating reactions of platinum(II) or palladium(II) in order to prepare several complexes with liquid crystalline properties. Depending on the starting materials used as well as the solvents employed, different metal complexes were obtained, some of them showing both liquid crystalline behaviour and luminescence properties at room temperature. It was found that, even if there are two competing coordination sites, the cyclometalation process takes place always at the 2-ppy core with (for Pt) or without (for Pd) the imine bond cleavage. We successfully showed that it is possible to prepare emissive room temperature liquid crystalline materials based on double cyclopalladated heteroleptic complexes by varying the volume fraction of the long flexible alkyl tails on the ancillary benzoylthiourea (BTU) ligands.

Cycloruthenated Complexes from Imine‐Based Heterocycles: Synthesis, Characterization, and Reactivity toward Alkynes

Novel cycloruthenated complexes 2 a-c, 4 a-c, and 6 a, b based on heteroaromatic cores have been synthesized by reaction of a series of heterocycle-based imines with [{RuCl(η(6)-p-cymene)}(2)(μ-Cl)(2)] and Cu(OAc)(2). This approach has proved efficient for the cyclometalation of thiophene, benzothiophene, furan, benzofuran, pyrrole, and indole derivatives. In addition, even a double cyclometalation process over the same heterocyclic ring is possible, yielding unprecedented bimetallic complexes. These ruthenacycles react with 3-hexyne through an unexpected pathway, which involves the coupling of the original imino heterocycle and acetylene followed by dearomatization to afford fused hetero-hydropyridyl ligands bonded to the {Ru(p-cymene)} organometallic moiety (i.e., 7 a-c and 8 a-c). These complexes represent, as far as we know, the first examples of this type of compound within the context of cyclometalation, and an exhaustive analysis of their structure was carried out in solution and solid state. Furthermore, these unique species react with CuCl(2), which promotes the rearomatization and the release of highly valuable aromatic fused bis-heterocycles (i.e., 9 a-c, 10 a-c, 11 a, and 12 a/12 a'), providing a novel and appealing synthetic route to this extraordinary family of molecules.

Ir‐Catalysed Asymmetric Allylic Substitutions with Cyclometalated (Phosphoramidite)Ir Complexes—Resting States, Catalytically Active (π‐Allyl)Ir Complexes and Computational Exploration

Mechanistic aspects of allylic substitutions with iridium catalysts derived from phosphoramidites by cyclometalation were investigated. The determination of resting states by (31)P NMR spectroscopy led to the conclusion that the cyclometalation process is reversible. A novel, one-pot procedure for the preparation of (pi- allyl)Ir complexes was developed, and these complexes were characterised by X-ray crystal structure analyses and spectral data. They are fully active catalysts of the allylic substitution reaction. DFT calculations on the allyl complexes, transition states of the allylic substitution and product olefin complexes gave further mechanistic insight.

Studies on Gas-phase Cyclometalations of [ArNi(PPh 3) n] + ( n = 1 or 2) by Electrospray Ionization Tandem Mass Spectrometry

Gas-phase cyclometalation of [ArNi(PPh(3))(n)](+) (n = 1, 2) complexes have been studied by ESI-MS/MS. The electron-donating substituents of aromatic iodides in the para position were found to inhibit the cyclometalation process of losing ArH, while the electron-withdrawing substituents in the para position were found to enhance it. These results indicate that the cyclometalation process of losing ArH is favored by electron-deficient aromatic groups. In addition, the detailed dissociation pathways of the cationic nickel complexes were studied, and among these pathways, the process of aryl-aryl interchange was also found to proceed in ESI-MS/MS.

Selective cyclometalation of disubstituted acetylenes and ethylene with diethylmagnesium and ethylmagnesium halides in the presence of zirconium complexes

Catalytic cyclometalation of disubstituted acetylenes and ethylene with ethylmagnesium halides EtMgHlg (Hlg = Cl, Br) and diethylmagnesium Et2Mg in the presence of Cp2ZrCl2 gave tetrasubstituted magnesacyclopenta-2,4-dienes and disubstituted magnesacyclopent-2-enes. A probable scheme of formation of cyclic unsaturated organomagnesium compounds was proposed, according to which the reactive intermediates in the cyclometalation process are zirconacyclopentadienes and zirconacyclopentenes generated from Cp2ZrCl2, EtMgHlg, Et2Mg, acetylenes, and ethylene.

One-Pot and Step-by-Step N-Assisted CPh−H Activation in 2-(4-Bromophenyl)imidazol[1,2-a]pyridine: Synthesis of a New C,N-Cyclometalated Compound [{Pt(C∧N)(μ-Cl)}2] as Precursor of Luminescent Platinum(II) Compounds

The activation of a CPh−H bond in the phenyl ring of 2-(4-bromophenyl)imidazol[1,2-a]pyridine (HC∧N) by [{Pt(η3-C4H7)(μ-Cl)}2] (η3-C4H7 = η3-2-methylallyl) renders the new cyclometalated complex [{Pt(C∧N)(μ-Cl)}2] (2) with high yield and selectivity. Complex 2 can be achieved directly in a one-pot reaction or step by step through the intermediate [Pt(η3-C4H7)Cl(HC∧N-κN)] (1). Compound 1 could be isolated and fully characterized. The X-ray structure shows the coordination of HC∧N through only the N and the existence of a weak Pt···H−C hydrogen bridging bond (Pt···H1 = 2.78 A, Pt···C1 = 3.365(3) A, Pt−H1−C1 = 120.9°). Hence, the formation of this intermediate could be considered the first step in the cyclometalation process. The mononuclear complexes [PtCl(C∧N)L] (L = tht (3), PPh3 (4), CN-Xyl (5), CN-tBu (6)) were obtained by cleavage of the bridging system in [{Pt(C∧N)(μ-Cl)}2] (2) by the neutral ligands, L. The resulting geometry (trans C, Cl) is that expected from the electronic preferences, taking into...

Palladium-Catalyzed Alkylation of sp2 and sp3 C−H Bonds with Methylboroxine and Alkylboronic Acids: Two Distinct C−H Activation Pathways

Palladium-catalyzed alkylations of sp2 and sp3 C-H bonds with either methylboroxine or alkylboronic acids were developed. Ag2O or AgCO3 is used as a crucial oxidant and promoter for the transmetalation step. Ether, ester, alcohol, and alkene functional groups are tolerated. A new C-H activation pathway differing from the cyclometalation process is elucidated using methylboroxine as the coupling partner.

不活性ｓｐ｀３´炭素‐水素結合の触媒的官能基化

The catalytic transformation of inert C-H bonds to other functional groups is one of the most active research areas in organic and organometallic chemistry because such reactions present possibilities to exploit readily available starting materials, shorten reaction sequences, and synthesize compounds that are otherwise difficult to prepare. In this review, recent advances in the catalytic functionalization of unactivated sp3 C-H bonds are briefly introduced. The sp3 C-H bonds were transformed to C-C and C-O bonds through activation with transition metal catalysts assisted by chelation of directing groups or a cyclometalation process.

Kinetico–mechanistic studies of C–H bond activation on new Pd complexes containing N,N′-chelating ligands

The hybrid imine/amine palladium(II) coordination complexes [PdX2(kappa2-N(imino),N(amino))](X = Cl, AcO; kappa2-N(imino),N(amino)= 4ClC6H4CHNCH2(CH2)nN(CH3)2, n= 1, 2) have been prepared in different isomeric forms which include E/Z arrangement around the C[double bond]N bond of the hybrid ligand and {Pd(kappa(2)-N(imino),N(amino))} ring conformation. The crystal structures of four of them, E-1AcO, Z-1AcO, E-2AcO and E-2Cl, have been determined and the solution behaviour in acetic acid, the common cyclometallating solvent, for all these systems studied. The complexes in acetic acid solution are shown to maintain the structure determined by X-ray crystallography, as they do in deuterated chloroform. Nevertheless, a partial opening equilibrium of the {Pd(kappa2-N(imino),N(amino))} ring is observed by NMR experiments. When the complexes are held in solution for longer periods the corresponding cyclometallated derivatives, 1AcO-CM, 2AcO-CM, 1Cl-CM and 2Cl-CM, containing the {Pd(kappa2-C,N(imino))} palladacycle are obtained, as characterized by 1H NMR spectroscopy. In these compounds the total opening of the N(amino) moiety of the ligand has occurred. The C-H bond activation process has been studied kinetico-mechanistically at different temperatures, pressures and acid concentrations; the results agree with the need of an opening of the chelate ring in [PdX2(kappa2-N(imino),N(amino))] prior to the proper cyclometallation reaction. The values of the enthalpies of activation are higher than those observed for known N-monodentated cyclometallating ligands, as should correspond to the contribution of a ligand dechelation pre-equilibrium. The entropies and volumes of activation are also indicative of this predissociation that include an important amount of contractive ordering. The presence of small amounts of triflic acid in the reaction medium accelerates the reaction to the value observed for N(imino)-monodentate systems, indicating that the full opening of the chelate ring has taken place. For the badly oriented isomeric forms of the ligand in the chelated complex (Z), the cyclometallation process is even more slow and corresponds directly to the reorganization of the ligand to its cyclopalladation-active (E) conformation.

A DFT‐Based Theoretical Investigation of the Mechanism of the PtCl2‐Mediated Cycloisomerization of Allenynes

The mechanism of Pt(II)-catalyzed intramolecular cycloisomerization of allenyne systems has been extensively investigated by DFT calculations. Different mechanistic schemes have been proposed and discussed, including the Alder-ene reaction. The free energy results suggest that the kinetically preferred reaction pathway for precursors that are tri- and tetrasubstituted on the allene moiety should proceed by a five-step mechanism. This would involve formation of a platina(IV)cyclopentene intermediate by selective engagement of the external pi bond of the allene, which would undergo regioselective beta-H elimination from the equatorially disposed methyl group. A metal-induced H migration leads to a second octahedral Pt(IV)-chelate complex, which would yield the expected bicyclic system through an intramolecular migratory insertion step. Therefore, depending on the conformation of the initial eta(4)-reactant complex for trisubstituted patterns, two possible intermediates can be formed that would evolve through different paths. In these cases, the regio- and stereochemical outcomes predicted by the mechanistic scheme proposed agree with experimental data. Substituted precursors on the alkyne moiety follow a distinct, four-step, mechanism also involving an oxidative cyclometalation process to an octahedral Pt(IV) intermediate complex. Theoretical results reveal the kinetic preference for beta-H elimination from the allylic group rather than from the gem-dimethyl group, which should account for the observed regioselectivity.

Double C-H activation in a Rh-NHC complex leading to the isolation of a 14-electron Rh(III) complex.

The solvent-dependent interaction of ItBu with a Rh(I)-olefin complex leads to the isolation of two precursors en route to a unique double cyclometalation process. Abstraction of the chloride ligand from the resulting complex enables the isolation of a "bare" 14-electron Rh(III) complex.

Cyclometalated complexes of Ru(II) with 2-aryl derivatives of quinoline and 1,10-phenanthroline.

Difficulty in cyclometalating 1-(2'-quinolinyl)pyrene and 1,3-di-(2'-quinolinyl)pyrene with Ru(II) led to a more detailed study of the cyclometalation process. A series of 2-aryl-1,10-phenanthrolines, where aryl = phenyl, 2-naphthyl, 1-anthracenyl, and 1-pyrenyl, were treated with [Ru(tpy)Cl(3)] to provide either the N5Cl complex [Ru(tpy)(L)Cl](+) or this same material as a mixture with the N5C cyclometalated species [Ru(tpy)L](+). Steric effects appear to govern the ability of the ligand to attain the near planar conformation required for cyclometalation. The bridged ligand 3,1'-dimethylene-2-(2'-pyrenyl)-1,10-phenanthroline was prepared along with a quinoline analogue. The former species was found to cyclometalate at the C1 of pyrene and afford the N5Cl complex. Both the N5C (P2(1)/n (monoclinic), a = 28.1102(11), b = 8.4638(3), c = 31.2908(12) A, Z = 8) and N5Cl (P-1 (triclinic), a = 11.7235 (10), b = 14.5306(12), c = 14.5725(12) A, Z = 2) complexes were analyzed by X-ray crystallography, and the N5Cl species evidenced a congested environment for pyrene, which is apparently stabilized by pi stacking with tpy. Similar reactions with a series of three 3,2'-bridged derivatives of 2-phenyl-1,10-phenanthroline provide both N5Cl and cyclometalated products in proportions which support the importance of pi stacking. The electronic absorption spectra and redox potentials for these complexes evidence strong sigma donation by the cyclometalated ligand and an apparent insensitivity to the orthogonal 2-aryl group.

New homochiralortho-palladated matrix bearing a bulky substituent at the carbon stereocenter

A new homochiral dimericortho-palladated complex bearing a bulkytert-butyl substituent at the carbon stereocenter was synthesized from optically activeN,N-dimethyl-α-tert-butylbenzylamine. Regioselective activation of only the aromatic C−H bond was shown to occur during the cyclometallation process proceeding under very mild conditions due to steric effects. Spectral characteristics of mononuclear derivatives of the new dimeric complex indicate that the five-membered palladacycle exists predominantly in one of two possible chiral conformations with the axial position of thetert-butyl substituent.