Rollover-assisted C(sp2)-C(sp3) bond formation

Regioselective C(sp2)-C(sp3) Coupling Mediated by Classical and Rollover Cyclometalation

By taking advantage of a sequence of oxidative addition/reductive elimination reactions, Pt(II) cyclometalated derivatives are able to promote a rare C(sp2)-C(sp3) bond coupling, resulting in the production of novel methyl-substituted pyridines and bipyridines. Starting from 6-phenyl-2,2'-bipyridine, the step-by-step full sequence of reactions has been followed, leading to the unprecedented 3-methyl-6-phenyl-2,2'-bipyridine, which was isolated and fully characterized. The synthesis involves the following steps: (1) rollover cyclometalation to give the starting complex [Pt(N^C)(DMSO)Me]; (2) the synthesis of a more electron-rich complex [Pt(N^C)(PPh3)Me] by the substitution of DMSO with triphenylphosphine; (3) oxidative addition with methyl iodide to give the Pt(IV) complex [Pt(N^C)(PPh3)(Me)2(I)]; (4) iodide abstraction with silver tetrafluoborate to give an unstable pentacoordinate intermediate, which rapidly evolves through a carbon-carbon reductive coupling, forming a new C(sp3)-C(sp2) bond; (5) finally, the extrusion and characterization of the newly formed 3-methyl-6-phenyl-2,2'-bipyridine. The reaction has been therefore extended to a well-known classical cyclometalating ligand, 2-phenylpyridine, demonstrating that the method is not restricted to rollover derivatives. Following the same step-by-step procedure, 2-phenylpyridine was converted to 2-o-tolyl-pyridine, displaying the potential application of the method to the larger family of classical cyclometalated complexes. The application of this protocol may be useful to convert an array of heterocyclic compounds to their methyl- or alkyl-substituted analogs.

Classical vs. Non-Classical Cyclometalated Pt(II) Complexes

Rollover cyclometalated complexes constitute a family of derivatives which differ from classical cyclometalated species in certain aspects. Various potential application fields have been developed for both classes of compounds, which have both similarities and differences. In order to uncover the relationships and distinctions between these two families of compounds, four Pt(II) cyclometalated complexes derived from 2-phenylpyridine (ppy) and 2,2'-bipyridine (bpy), assumed as prototypical ligands, were compared. For this study, an electron rich isostructural and isoelectronic pair of compounds, [Pt(N^C)Me(PPh3)], and an electron-poorer compound, [Pt(N^C)Cl(PPh3)] were chosen (N^C = ppy or bpy). DFT calculations, cyclic voltammetry, and UV-Vis spectra also helped to shed light into these species. Due to the presence of the more electronegative nitrogen in place of a C-H group, the rollover bpy-H ligand becomes a slightly weaker donor than the classical ppy-H ligand, and hence, generates (slightly) more stable cyclometalated complexes, lower energy frontier molecular orbitals, and electron-poorer platinum centers. On the whole, it was revealed that classical and rollover complexes have overall structural similarity, which contrasts to their somewhat different chemical behavior.

C-Cl Oxidative Addition and C-C Reductive Elimination Reactions in the Context of the Rhodium-Promoted Direct Arylation

A cycle of stoichiometric elemental reactions defining the direct arylation promoted by a redox-pair Rh(I)-Rh(III) is reported. Starting from the rhodium(I)-aryl complex RhPh{κ3-P,O,P-[xant(PiPr2)2]} (xant(PiPr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene), the reactions include C-Cl oxidative addition of organic chlorides, halide abstraction from the resulting six-coordinate rhodium(III) derivatives, C-C reductive coupling between the initial aryl ligand and the added organic group, oxidative addition of a C-H bond of a new arene, and deprotonation of the generated hydride-rhodium(III)-aryl species to form a new rhodium(I)-aryl derivative. In this context, the kinetics of the oxidative additions of 2-chloropyridine, chlorobenzene, benzyl chloride, and dichloromethane to RhPh{κ3-P,O,P-[xant(PiPr2)2]} and the C-C reductive eliminations of biphenyl and benzylbenzene from [RhPh2{κ3-P,O,P-[xant(PiPr2)2]}]BF4 and [RhPh(CH2Ph){κ3-P,O,P-[xant(PiPr2)2]}]BF4, respectively, have been studied. The oxidative additions generally involve the cis addition of the C-Cl bond of the organic chloride to the rhodium(I) complex, being kinetically controlled by the C-Cl bond dissociation energy; the weakest C-Cl bond is faster added. The C-C reductive elimination is kinetically governed by the dissociation energy of the formed bond. The C(sp3)-C(sp2) coupling to give benzylbenzene is faster than the C(sp2)-C(sp2) bond formation to afford biphenyl. In spite of that a most demanding orientation requirement is needed for the C(sp3)-C(sp2) coupling than for the C(sp2)-C(sp2) bond formation, the energetic effort for the pregeneration of the C(sp3)-C(sp2) bond is lower. As a result, the weakest C-C bond is formed faster.

Cationic π-extended heteroaromatics via a catalytic C-H activation annulative alkyne-insertion sequence

Cationic π-conjugated organic molecules have broad applications in materials science as next-generation organic materials. The annulative alkyne-insertion π-extension (AAIPEX) strategy has emerged as a promising synthetic approach for the rapid synthesis of cationic polycyclic heteroaromatic compounds (cPHACs) in a single step. The AAIPEX reaction provides a synthetic shortcut to achieve complex organic molecules from simple (hetero)arene templates and alkynes as π-extending partners, which would otherwise be difficult to achieve using traditional methods. In general, a step-economic AAIPEX protocol proceeds via C-H activation of unfunctionalized heteroarene templates, followed by alkyne insertion-annulation to furnish cPHACs. In this Feature Article, recent progress in the AAIPEX strategy to construct cPHACs is described along with brief illustrations of the resulting cPHACs in luminescence-related applications.

Photophysical Properties and Kinetic Studies of 2-Vinylpyridine-Based Cycloplatinated(II) Complexes Containing Various Phosphine Ligands

A series of cycloplatinated(II) complexes with general formula of [PtMe(Vpy)(PR₃)], Vpy = 2-vinylpyridine and PR₃ = PPh₃ (1a); PPh₂Me (1b); PPhMe₂ (1c), were synthesized and characterized by means of spectroscopic methods. These cycloplatinated(II) complexes were luminescent at room temperature in the yellow–orange region’s structured bands. The PPhMe₂ derivative was the strongest emissive among the complexes, and the complex with PPh₃ was the weakest one. Similar to many luminescent cycloplatinated(II) complexes, the emission was mainly localized on the Vpy cyclometalated ligand as the main chromophoric moiety. The present cycloplatinated(II) complexes were oxidatively reacted with MeI to yield the corresponding cycloplatinated(IV) complexes. The kinetic studies of the reaction point out to an S_N2 mechanism. The complex with PPhMe₂ ligand exhibited the fastest oxidative addition reaction due to the most electron-rich Pt(II) center in its structure, whereas the PPh₃ derivative showed the slowest one. Interestingly, for the PPhMe₂ analog, the trans isomer was stable and could be isolated as both kinetic and thermodynamic product, while the other two underwent trans to cis isomerization.

A Heteropolynuclear Pt-Ag System Having Cycloplatinated Rollover Bipyridyl Units

The synthesis and photophysical properties of the heteropolynuclear Pt-Ag complex having cyclometalated rollover bipyridine ligands (bpy*) and bridging pyrazolato ligands are reported. The Pt₂Ag₂ complex was synthesized by two step reactions from a Pt(II) complex precursor having the rollover bpy* ligand, [Pt(bpy*)(dmso)Cl], with 3,5-dimethylpyrazole (Me₂pzH) and a subsequent replacement of NH protons in the Me₂pzH moieties with the Ag(I) ion. The Pt₂Ag₂ complex exists as a mixture of U- and Z-shaped isomers in solution, whose structures were clearly determined by single-crystal X-ray structural analyses. NMR studies using the single crystals revealed rapid isomerization of the Pt₂Ag₂ complexes in solution, although the Pt₂Ag₂ structures were supported effectively by the multiple metal-metal interactions. Furthermore, the Pt₂Ag₂ framework can capture a Ag(I) ion during the U-Z isomerization to afford a Pt₂Ag₃ core with the formation of Pt → Ag dative bonds. The Pt₂Ag₃ complex showed further aggregation to form a dimer structure in the presence of coordinating solvent via the crystallization process. The formation of Pt → Ag dative bonds significantly changes the emission energy from the Pt₂Ag₂ complex, while the emission spectra of U- and Z-isomers of Pt₂Ag₂ complex almost coincide with each other and their emissive properties are very similar. The density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations revealed the effects of additional Ag(I) ion on the photophysical properties of the heteropolynuclear Pt-Ag complexes bearing the rollover bpy* ligands.

Rollover Cyclometalation as a Valuable Tool for Regioselective C-H Bond Activation and Functionalization

Rollover cyclometalation constitutes a particular case of cyclometallation reaction. This reaction occurs when a chelated heterocyclic ligand loses its bidentate coordination mode and undergoes an internal rotation, after which a remote C-H bond is regioselectively activated, affording an uncommon cyclometalated complex, called "rollover cyclometalated complex". The key of the process is the internal rotation of the ligand, which occurs before the C-H bond activation and releases from coordination a donor atom. The new "rollover" ligand has peculiar properties, being a ligand with multiple personalities, no more a spectator in the reactivity of the complex. The main reason of this peculiarity is the presence of an uncoordinated donor atom (the one initially involved in the chelation), able to promote a series of reactions not available for classic cyclometalated complexes. The rollover reaction is highly regioselective, because the activated C-H bond is usually in a symmetric position with respect to the donor atom which detaches from the metal stating the rollover process. Due to this novel behavior, a series of potential applications have appeared in the literature, in fields such as catalysis, organic synthesis, and advanced materials.

Selectivity in carbon–carbon coupling reactions at palladium(IV) and platinum(IV)

The isomerization and reductive elimination reactions from octahedral organometallic complexes of palladium(IV) and platinum(IV) usually occur through five-coordinate intermediates that cannot be directly detected. This paper reports a computational study of five-coordinate complexes of formulae [PtMe3(bipy)]+, [PtMe2Ph(bipy)]+, and [PtMe(CH2CMe2C6H4)(bipy)]+ (M = Pd or Pt, bipy = 2,2′-bipyridine), particularly with respect to reactivity and selectivity in reductive elimination. All of the complexes are predicted to have square pyramidal structures with the bipy and two R groups in the equatorial positions and one R group in the axial position, and axial–equatorial exchange occurs by a pairwise mechanism, with the transition state having a pinched trigonal bipyramidal (PTBP) stereochemistry, with one nitrogen and two R groups in the trigonal plane. The activation energy for isomerization is lower than that for reductive elimination in all cases. For the complexes [MMe2Ph(bipy)]+, the activation energies for reductive elimination with Me–Me or Me–Ph coupling are similar. For the complexes [MMe(CH2CMe2C6H4)(bipy)]+, the reductive elimination with Me–C6H4 bond formation from the isomer with the methyl group in the axial position is predicted and is attributed to it having the best conformation of the Me and C6H4 groups for C–C bond formation. In all cases, the selectivity for reductive elimination is similar for M = Pd or Pt, but reactivity is higher for M = Pd. The relevance of this work to selectivity in catalysis is discussed.

A double rollover cycloplatinated(ii) skeleton: a versatile platform for tuning emission by chelating and non-chelating ancillary ligand systems

Described here is the synthesis and characterization of heteroleptic binuclear platinum(ii) complexes of the type [Pt2(μ-bpy-2H)(S^S)2] and [Pt2(μ-bpy-2H)(L)2(X)2], containing a 2,2'-bipyridine-based double rollover cycloplatinated core (Pt(μ-bpy-2H)Pt), in combination with the anionic S^S- chelate ligands di(ethyl)dithiocarbamate (dedtc) and O,O'-di(cyclohexyl)dithiophosphate (dcdtp) or non-chelating L/X ancillary ligands (PPh3/Me, t-BuNC/Me, PPh3/SCN and PPh3/N3). The new complexes were characterized using multinuclear (1H, 31P and 195Pt) NMR spectroscopy and some of them additionally using single crystal X-ray diffraction. The absorption and photoluminescence of the complexes show a strong dependence on the ancillary ligands. Upon excitation at 365 nm, in a CH2Cl2 rigid matrix (77 K), the complexes exhibit structured emission bands with λmax between 488 nm and 525 nm and vibrational spacing around 1350 cm-1, indicating the excited states centered on the cyclometalated ligand (3ILCT) with some mixing 3MLCT characteristics. In the case of the PPh3/N3 complex, a dual emission band (orange color) is observed in the solid state at 298 K for which the low energy band arises from an aggregation-induced emission (AIE). Upon lowering the temperature (77 K), thermochromism is observed (orange to yellow) which is accompanied by the intensification of the high energy band (ligand-centered structured band). Finally, in order to rationalize the obtained photophysical data, complete DFT (density functional theory) and TD-DFT (time-dependent DFT) calculations were performed on the selected complexes.

To "Rollover" or Not? Stereoelectronically Guided C-H Functionalization Pathways from Rhodium-Abnormal NHC Intermediates

Rollover C-H activation with transition-metal complexes has been found to be a difficult but viable pathway to functionalize potentially chelating molecules, which are otherwise reluctant to react further. However, selective rollover or nonrollover C-H activation pathway depends on the stereoelectronic demand of the associated organometallic intermediate(s). The presented work addresses the above issue on abnormal N-heterocyclic carbene (NHC) platform. Catalytic reactions of pyridine-imidazolium substrates with internal alkynes have been selectively guided toward either rollover or nonrollover C-H functionalization route via fulfilling the steric and electronic demands of the relevant rhodium(III)-abnormal NHC metallacyclic intermediates.

An in-depth investigation on the C–I bond activation by rollover cycloplatinated(ii) complexes bearing monodentate phosphane ligands: kinetic and kinetic isotope effect

The oxidative addition reaction of MeI reagent to some cycloplatinated(ii) complexes was performed and kinetically investigated.

C–H activation-annulation on the N-heterocyclic carbene platform

This review highlights the initial development of a new C–H activation–annulation chemistry accessible on the metal–N-heterocyclic carbene platform.

Selective C-C coupling at a Pt(iv) centre: 100% preference for sp2-sp3 over sp3-sp3

The oxidative addition of three different organic halides RX to the non-symmetric platinum(ii) mer coordinated dicyclometallated C^N^C complex 1 yielded short-lived six-coordinate platinum(iv) complexes 2(R) (R = Me, allyl, Bn), with the incoming groups trans across the platinum centre. A spontaneous reductive coupling reaction then occurred with, in each case, a completely chemoselective sp²-sp³ coupling, and exclusively gave R-3, with the newly introduced R group bonded to the previously cyclometallated aryl ring. Following a recyclometallation reaction, the oxidative addition/reductive elimination cycle was repeated and gave the same selectivity. A one-pot route to doubly alkylating the aryl ring was developed. The observed selectivity might have been predicted on the normal basis of a steric barrier associated with non-flat sp³ hybridised groups, but we suggest that it arises from the stereochemistry at the metal, and the orientation of the ligands.

Synthesis, Biological Evaluation, and Molecular Docking Studies on the DNA Binding Interactions of Platinum(II) Rollover Complexes Containing Phosphorus Donor Ligands

Cyclometalated rollover complexes of the type [PtMe(κ² N,C-bipyO-H)(L)] [bipyO-H=cyclometalated 2,2'-bipyridine N-oxide; L=tricyclohexylphosphine (PCy₃ , 2 a), 2-(diphenylphosphino)pyridine (PPh₂ py, 2 b), P(OPh)₃ (2 c)] were synthesized by treating [PtMe(κ² N,C-bipyO-H)(SMe₂ )] (1) with various monodentate phosphine and phosphite ligands. These complexes were characterized by NMR spectroscopy, and the structure of 2 a was confirmed by single-crystal X-ray diffraction. Complex 1 was treated with bis(diphenylphosphino)methane (dppm) at a 1:1 ratio to give the corresponding [PtMe(κ² N,C-bipyO-H)(κ¹ P-dppm)] (3 b) complex, in which the dppm ligand acts as a monodentate pendant ligand. The biological activities of these complexes were evaluated against a panel of four standard cancer cell lines: lung carcinoma (A549), ovarian carcinoma (OV-90 and SKOV3), and breast carcinoma (MCF-7). Complexes 2 c and especially 3 b indicated effective potent cytotoxic activity regarding the cell lines. Electrophoresis mobility shift assays and molecular-modeling investigations were performed to determine the specific binding mode and the binding orientation of these alkylating agents to DNA. Detection of cellular reactive oxygen species was also determined.

Rhodium-catalyzed selective CH functionalization of NNN tridentate chelating compounds via a rollover pathway

Reported herein is the first example of the Rh(NHC)-catalyzed selective bis C-H alkylation of NNN tridentate chelating compounds in reaction with alkenes. The observed excellent site-selectivity can readily be explained by the postulated rollover pathway in the C-H bond activation step. The reaction is highly facile affording bis-alkylated tridentate products in high yields over a broad range of versatile heteroarene substrates and alkene reactants including ethylene gas, thus enabling its applications to be feasible in coordination and synthetic chemistry.

Trapping five-coordinate platinum(iv) intermediates

The oxidation of three different complexes of the doubly cycloplatinated 2,6-di(4-fluorophenyl)pyridine ligand (namely DMSO, PPh3 and PPr3 derivatives, 1a, 1b and 1c, respectively) with the electrophilic oxidant iodobenzenedichloride was studied. In each case oxidation can yield a simple trans-dichloro platinum(iv) complex (2(t)), which subsequently isomerises to the cis isomer (2(c)). However, by changing the solvent, or performing the reaction in the presence of an additional ligating species, a five-coordinate intermediate can be trapped out and isolated. Thus, cationic species with additional DMSO or pyridine coordinated could be collected for the DMSO and PPh3 derivatives. The PPr3 derivative traps out the reactive five-coordinate species with an agostic interaction that subsequently induces a transcyclometallation reaction to give a complex with a singly cyclometallated pyridine and a cyclometallated phosphine, which was characterised crystallographically, (6c

Methyl Complexes of the Transition Metals

Organometallic chemistry can be considered as a wide area of knowledge that combines concepts of classic organic chemistry, that is, based essentially on carbon, with molecular inorganic chemistry, especially with coordination compounds. Transition-metal methyl complexes probably represent the simplest and most fundamental way to view how these two major areas of chemistry combine and merge into novel species with intriguing features in terms of reactivity, structure, and bonding. Citing more than 500 bibliographic references, this review aims to offer a concise view of recent advances in the field of transition-metal complexes containing M-CH3 fragments. Taking into account the impressive amount of data that are continuously provided by organometallic chemists in this area, this review is mainly focused on results of the last five years. After a panoramic overview on M-CH3 compounds of Groups 3 to 11, which includes the most recent landmark findings in this area, two further sections are dedicated to methyl-bridged complexes and reactivity.

Carbon–sulfur bond reductive coupling from a platinum(ii) thiolate complex

Complex 1 was reacted with different alkyl halides at room temperature. This reaction proceeded via formation of binuclear intermediate complex 3 and through C–S bond reductive coupling to produce alkyl sulfides and corresponding halide complexes 2.

Chiral cyclometalation of 6-(1-phenylbenzyl)-2,2'-bipyridine

A new bipyridine ligand, 6-(1-phenylbenzyl)-2,2'-bipyridine, has been prepared by a multistep synthesis starting from the corresponding substituted pyridine. The coordinating properties of the new ligand have been tested with two d(8) metal ions, Pt(ii) and Pd(ii), to give the cyclometalated complexes [Pt(N,N,C)Cl] and [Pd(N,N,C)Cl], where N,N,C is a terdentate deprotonated bipyridine containing a new stereogenic carbon atom directly generated by C-H bond activation. The single-crystal of the platinum complex has been solved by X-ray diffraction. DFT calculations confirm the presence of a PtH interaction that stabilizes one of the two possible conformers by 14.7 kJ mol(-1) for Pt and 12.9 kJ mol(-1) for Pd. The energy barrier to pass from one conformer to the other is 25.4 and 23.8 kJ mol(-1) respectively. Under different reaction conditions, regioselective activation of a pyridine C-H bond gave the less common cyclometalated rollover complex [Pt(L-H)Me(DMSO)], which was isolated and characterised.

Assembly of symmetrical and unsymmetrical platinum(II) rollover complexes with bidentate phosphine ligands

The reaction of the cyclometalated rollover complex [Pt(bpy-H)(Me)(DMSO)] (bpy-H = cyclometalated 2,2'-bipyridine) with two diphosphines, dppm (1,1-bis(diphenylphosphino)methane) and dppe (1,2-bis(diphenylphosphino)ethane), was investigated. According to the reaction conditions, dppm behaves as a monodentate, bridging or chelated ligand, whereas dppe gave only chelated species. Some aspects of the reactivity of the isolated species were studied, including protonation with [H3O·18-crown-6][BF4] and coordination reactions of mononuclear complexes, obtaining, inter alia, rare examples of unsymmetrical organometallic species with bridging dppm.

Reactivity comparison of five-and six-membered cyclometalated platinum(ii) complexes in oxidative addition reactions

A six-membered cyclometalated complex [PtMe(bzpy)(PPh₃)] (bzpy = 2-benzylpyridinate) undergoes oxidative addition with methyl iodide to give [PtMe₂I(bzpy)(PPh₃)]. The ring size of the chelating unit has a significant impact on the rate of the reaction.

Oxidation of a rollover cycloplatinated(ii) dimer by MeI: a kinetic study

The known rollover cycloplatinated(ii) complex [Pt₂Me₂(PPh₃)₂(μ-bpy-2H)], 1, in which bpy-2H acts as a bridging rollover ligand, reacts with MeI to give a new binuclear rollover cycloplatinated(iv) complex [Pt₂Me₄I₂(PPh₃)₂(μ-bpy-2H)], 2.