Mesoionic Complexes of Platinum(II) Derived from “Rollover” Cyclometalation: A Delicate Balance between Pt–C(sp3) and Pt–C(sp2) Bond Cleavage as a Result of Different Reaction Conditions

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.

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.

Phosphine-free ruthenium NCN-ligand complexes and their use in catalytic CO2 hydrogenation

This work investigates the hydrogenation of carbon dioxide to formate catalysed by the phosphine-free Ru complexes Ru(OtBu)(κ³-NCN)(^tBubpy) and RuH(OtBu)(κ²-NCN)(^tBubpy) (OtBu = tert-butoxide, κ²-NCN = 1,3-di(2-methylpyridyl)-4,5-diphenyl-1H-imidazol-2-ylidene, where one pyridyl moiety is not coordinated to Ru, ^tBubpy = 4,4'-di-tert-butyl-2,2'-dipyridyl). A catalytic cycle is proposed for this reaction, supported by computational studies and the characterization of the hydride and the formate intermediates proposed to be involved. Modest catalytic turnovers are demonstrated at relatively low pressures and temperatures. The proposed rate determining step is heterolytic H₂ splitting to regenerate the Ru-H complex, which has an estimated hydricity of approx. 27 kcal mol^-1. The κ²-NCN ligand in the hydride complex undergoes a variety of dynamic processes as detected by EXSY spectroscopy including a pyridyl "roll-over" carbon-hydrogen - ruthenium hydride exchange, possibly occuring via a Perutz-Sabo-Etienne CAM mechanism.

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.

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.

Photophysical study on unsymmetrical binuclear cycloplatinated(ii) complexes

A general synthetic method has been designed to prepare a new series of unsymmetrical binuclear cyclometalated organoplatinum(ii) complexes and the effect of each part on their luminescence properties were investigated.

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.

Reactivity of a half-lantern Pt2(ii,ii) complex with triphenylphosphine: selectivity in a protonation reaction

Reaction of PPh₃ with half-lantern complex [{Pt(ppy)(μ₂-Spy)}₂], 1, gave complex [Pt(ppy)(η¹-S-Spy)(PPh₃)], 2, as a result of platinum–nitrogen bonds dissociation. Complex 2 revealed anionic selectivity upon protonation reaction with acidic species.

Reversible cyclometalation at RhI as a motif for metal–ligand bifunctional bond activation and base-free formic acid dehydrogenation

The first example of base-free catalytic dehydrogenation of formic acid using reversible cyclometalation at Rh(i) is discussed, using a combination of experimental and computational methods.

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.

Acid/base-triggered switching of circularly polarized luminescence and electronic circular dichroism in organic and organometallic helicenes

Electronic circular dichroism and circularly polarized luminescence acid/base switching activity has been demonstrated in helicene-bipyridine proligand 1 a and in its "rollover" cycloplatinated derivative 2 a. Whereas proligand 1 a displays a strong bathochromic shift (>160 nm) of the nonpolarized and circularly polarized luminescence upon protonation, complex 2 a displays slightly stronger emission. This strikingly different behavior between singlet emission in the organic helicene and triplet emission in the organometallic derivative has been rationalized by using quantum-chemical calculations. The very large bathochromic shift of the emission observed upon protonation of azahelicene-bipyridine 1 a has been attributed to the decrease in aromaticity (promoting a charge-transfer-type transition rather than a π-π* transition) as well as an increase in the HOMO-LUMO character of the transition and stabilization of the LUMO level upon protonation.

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

Rollover cyclometalation involves bidentate heterocyclic donors, unusually acting as cyclometalated ligands. The resulting products, possessing a free donor atom, react differently from the classical cyclometalated complexes. Taking advantage of a "rollover"/"retro-rollover" reaction sequence, a succession of oxidative addition and reductive elimination in a series of platinum(II) complexes [Pt(N,C)(Me)(PR3)] resulted in a rare C(sp(2))-C(sp(3)) bond formation to give the bidentate nitrogen ligands 3-methyl-2,2'-bipyridine, 3,6-dimethyl-2,2'-bipyridine, and 3-methyl-2-(2'-pyridyl)-quinoline, which were isolated and characterized. The nature of the phosphane PR3 is essential to the outcome of the reaction. This route constitutes a new method for the activation and functionalization of C-H bond in the C(3) position of bidentate heterocyclic compounds, a position usually difficult to functionalize.

Rollover cyclometalation with 2-(2'-pyridyl)quinoline

Rollover cyclometalation of 2-(2'-pyridyl)quinoline, L, allowed the synthesis of the family of complexes [Pt(L-H)(X)(L')] and [Pt(L*)(X)(L')][BF4] (X = Me, Cl; L' = neutral ligand), the former being the first examples of Pt(II) rollover complexes derived from the ligand L. The ligand L* is a C,N cyclometalated, N-protonated isomer of L, and can also be described as an abnormal-remote pyridylene. The corresponding [Pt(L-H)(Me)(L')]/[Pt(L*)(Me)(L')](+) complexes constitute an uncommon Brønsted-Lowry acid-base conjugated couple. The species obtained were investigated in depth through NMR and UV-vis spectroscopy, cyclic voltammetry, and density functional theory (DFT) methods to correlate different chemico-physical properties with the nature of the cyclometalated ligand (e.g., L vs bipy or L* vs L) and of the neutral ligand (DMSO, CO, PPh3). The crystal structures of [Pt(L-H)(Me)(PPh3)], [Pt(L-H)(Me)(CO)] and [Pt(L*)(Me)(CO)][BF4] were determined by X-ray powder diffraction methods, the latter being the first structure of a Pt(II)-based, protonated, rollover complex to be unraveled. The isomerization of [Pt(L*)(Me)(PPh3)](+) in solution proceeds through a retro-rollover process to give the corresponding adduct [Pt(L)(Me)(PPh3)](+), where L acts as a classical N,N chelating ligand. Notably, the retro-rollover reaction is the first process, among the plethora of Pt-C bond protonolysis reactions reported in the literature, where a Pt-C(heteroaryl) bond is cleaved rather than a Pt-C(alkyl) one.