Multiple C−H Bond Activation. Threefold-Deprotonated 6-Phenyl-2,2‘-bipyridine as a Bridging Ligand in Dinuclear Platinum(II) Derivatives

Electronic Communication in Binuclear Osmium- and Iridium-Polyhydrides

Reactions of polyhydrides OsH₆(PⁱPr₃)₂ (1) and IrH₅(PⁱPr₃)₂ (2) with rollover cyclometalated hydride complexes have been investigated in order to explore the influence of a metal center on the MH_n unit of the other in mixed valence binuclear polyhydrides. Hexahydride 1 activates an ortho-CH bond of the heterocyclic moiety of the trihydride metal–ligand compounds OsH₃{κ²-C,N-[C₅RH₂N-py]}(PⁱPr₃)₂ (R = H (3), Me (4), Ph (5)). Reactions of 3 and 4 lead to the hexahydrides (PⁱPr₃)₂H₃Os{μ-[κ²-C,N-[C₅RH₂N-C₅H₃N]-N,C-κ²]}OsH₃(PⁱPr₃)₂ (R = H (6), Me (7)), whereas 5 gives the pentahydride (PⁱPr₃)₂H₃Os{μ-[κ²-C,N-[C₅H₃N-C₅(C₆H₄)H₂N]-C,N,C-κ³]}OsH₂(PⁱPr₃)₂ (8). Pentahydride 2 promotes C—H bond activation of 3 and the iridium-dihydride IrH₂{κ²-C,N-[C₅H₃N-py]}(PⁱPr₃)₂ (9) to afford the heterobinuclear pentahydride (PⁱPr₃)₂H₃Os{μ-[κ²-C,N-[C₅H₃N-C₅H₃N]-N,C-κ²]}IrH₂(PⁱPr₃)₂ (10) and the homobinuclear tetrahydride (PⁱPr₃)₂H₂Ir{μ-[κ²-C,N-[C₅H₃N-C₅H₃N]-N,C-κ²]}IrH₂(PⁱPr₃)₂ (11), respectively. Complexes 6–8 and 11 display HOMO delocalization throughout the metal–heterocycle-metal skeleton. Their sequential oxidation generates mono- and diradicals, which exhibit intervalence charge transfer transitions. This notable ability allows the tuning of the strength of the hydrogen–hydrogen and metal–hydrogen interactions within the MH_n units.

The influence of a metal center on the MH_n unit of the other in binuclear osmium- and iridium-polyhydride complexes bearing rollover cyclometalated bipyridines as a bridging ligand has been studied.

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.

Oxidative C-H/C-H Annulation of Imidazopyridines and Indazoles through Rhodium-Catalyzed Vinylene Transfer

Transition-metal-catalyzed C-H activation followed by oxidative annulation with alkynes has been an efficient synthetic tool for the assembly of various polyaromatic scaffolds. Despite the substantial progress in this field, it is still a significant challenge to achieve the synthesis of nonsubstituted vinylene-fused compounds. In this contribution, we report a Rh-catalyzed C-H/C-H vinylene cyclization adopting vinylene carbonate as a "vinylene transfer" agent. This protocol achieves the direct annulative π-extension of imidazole- and pyrazole-fused aromatics.

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.

Simple tuning of the luminescence properties of the double rollover cycloplatinated(ii) structure by halide ligands

The luminescence properties of double rollover cycloplatinated(ii) complexes and the effects of halogen ligands on the luminescence properties are investigated.

Reactivity of a new aryl cycloplatinated(ii) complex containing rollover 2,2′-bipyridine N-oxide toward a series of diphosphine ligands

A new rollover cycloplatinated(ii) complex was prepared. The reactivity of this complex was investigated towards a wide range of diphosphine ligands.

C–H reductive elimination during the reaction of cycloplatinated(ii) complexes with pyridine-2-thione: kinetic follow up

Reactions of the cycloplatinated(ii) complexes [PtR(ppy)(SMe₂)], 1, where ppy = deprotonated 2-phenylpyridine and R = Me or p-MeC₆H₄, with pyridine-2-thione, C₅H₅SN, were studied by UV-vis and ¹H NMR spectroscopies and on the basis of the data a mechanism is proposed.

Rollover cyclometalation pathway in rhodium catalysis: dramatic NHC effects in the C-H bond functionalization

Organometallic chelates are readily obtained upon coordination of metal species to multidentate ligands. Because of the robust structural nature, chelation frequently serves as a driving force in the molecular assembly and chemical architecture, and they are used also as an efficient catalyst in numerous reactions. Described herein is the development of a Rh(NHC) catalytic system for the hydroarylation of alkenes and alkynes with 2,2'-bipyridines (bipy) and 2,2'-biquinolines; the most representative chelating molecules. Initially generated (bipy)Rh(NHC) chelates become labile because of the strong trans-effect of N-heterocyclic carbenes, thus weakening a rhodium-pyridyl bond, which is trans to the bound NHC. Subsequent rollover cyclometalation leads to the C-H bond activation, eventually giving rise to double functionalization of chelate molecules. Density functional calculations are in good agreement with our mechanistic proposal based on the experimental data. The present study elucidated for the first time the dramatic NHC effects on the rollover cyclometalation pathway enabling highly efficient and selective bisfunctionalization of 2,2'-bipyridines and 2,2'-biquinolines.

Ion-molecule reactions of "rollover" cyclometalated [Pt(bipy-H)](+) (bipy=2,2'-bipyridine) with dimethyl ether in comparison with dimethyl sulfide: an experimental/computational study

The ion-molecule reactions of dimethyl ether with cyclometalated [Pt(bipy-H)](+) were investigated in gas-phase experiments, complemented by DFT methods, and compared with the previously reported ion-molecule reactions with its sulfur analogue. The initial step corresponds in both cases to a platinum-mediated transfer of a hydrogen atom from the ether to the (bipy-H) ligand, and three-membered oxygen- and sulfur-containing metallacycles serve as key intermediates. Oxidative C--C bond coupling ("dehydrosulfurization"), which dominates the gas-phase ion chemistry of the [Pt(bipy-H)](+) ion with dimethyl sulfide, is practically absent for dimethyl ether. The competition in the formation of C(2)H(4) and CH(2)X (X=O, S) in the reactions of [Pt(bipy-H)](+) with (CH(3))(2)X (X=O, S) as well as the extensive H/D exchange observed in the [Pt(bipy-H)](+)/(CH(3))(2)O system are explained in terms of the corresponding potential-energy surfaces.

Multifaceted coordination of naphthyridine-functionalized N-heterocyclic carbene: a novel "Ir(III)(C--N)(C--C)" compound and its evaluation as transfer hydrogenation catalyst

The 1,8-naphthyridine-functionalized N-heterocyclic carbene 1-benzyl-3-(5,7-dimethyl-1,8-naphthyrid-2-yl)imidazol-2-ylidene (BIN) has been successfully coordinated to Pd(II), W(0), Rh(I), and Ir(III), exhibiting its diverse binding modes. Reaction of BIN x HBr with Ag(2)O, followed by transmetalation with PdCl(2)(COD)(2) provides a cis complex PdCl(2)(kappaC(2)-BIN)(2) (1). Treatment of BIN x HBr with W(CO)(4)(piperidine)(2) in acetonitrile affords a chelate complex W(CO)(4)(kappa(2)C(2),N(1)'-BIN) (2). Reaction of {RhCl(COD)}(2) with KO(t)Bu and subsequent treatment with BIN x HBr in 1:2 and 1:1 ratio results in the mono and dinuclear complexes [Rh(COD)Br(kappaC(2)-BIN)] (3) and [{Rh(COD)Br}(2)(kappaN(8)':kappaC(2)-BIN)] (4), respectively. In complex 3, the "Rh(COD)Br" unit is coordinated to the carbene center, whereas an additional "Rh(COD)Br" unit is attached to naphthyridine nitrogen in complex 4 in an anti arrangement. Under identical reaction condition, a novel Ir(III) complex [Ir(kappa(2)C(2),N(1)'-BIN)(kappa(2)C(3)',C(2)-BIN)(H(2)O)Br]Br (5) has been synthesized. Complex 5 is proved to be catalytically active in hydrogen transfer reaction from (i)PrOH. All complexes have been characterized by spectroscopic methods and X-ray crystallography.

Large effect of a small substitution: competition of dehydration with charge retention and coulomb explosion in gaseous [(bipy(R))Au(mu-O)2Au(bipy(R))]2+ dications

Dinuclear gold(III) clusters with a rhombic Au(2)O(2) core and 2,2'-bipyridyl ligands substituted in the 6-position (bipy(R)) are examined by tandem mass spectrometry. Electrospray ionization of the hexafluorophosphate salts affords the complexes [(bipy(R))Au(mu-O)(2)Au(bipy(R))](2+) as free dications in the gas phase. The fragmentation behavior of the mass-selected dications is probed by means of collision-induced dissociation experiments which reveal an exceptionally pronounced effect of substitution. Thus, for the parent compound with R = H, i.e., [(bipy)Au(mu-O)(2)Au(bipy)](2+), fragmentation at the dicationic stage prevails to result in a loss of neutral H(2)O concomitant with an assumed rollover cyclometalation of the bipyridine ligands. In marked contrast, all complexes with alkyl substituents in the 6-position of the ligands (bipy(R) with R = CH(3), CH(CH(3))(2), CH(2)C(CH(3))(3), and 2,6-C(6)H(3)(CH(3))(2)) as well as the corresponding complex with 6,6'-dimethyl-2,2'-dipyridyl as a ligand exclusively undergo Coulomb explosion to produce two monocationic fragments. It is proposed that the additional steric strain introduced to the central Au(2)O(2) core by the substituents on the bipyridine ligand, in conjunction with the presence of oxidizable C-H bonds in the substituents, crucially affects the subtle balance between dication dissociation under maintenance of the 2-fold charge and Coulomb explosion into two singly charged fragments.

"Roll-over" cyclometalation of 2,2'-bipyridine platinum(II) complexes in the gas phase: a combined experimental and computational study

In a combined experimental/computational investigation, the gas-phase behavior of cationic [Pt(bipy)(CH(3))((CH(3))(2)S)](+) (1) (bipy=2,2'-bipyridine) has been explored. Losses of CH(4) and (CH(3))(2)S from 1 result in the formation of a cyclometalated 2,2'-bipyrid-3-yl species [Pt(bipy-H)](+) (2). As to the mechanisms of ligand evaporation, detailed labeling experiments complemented by DFT-based computations reveal that the reaction follows the mechanistically intriguing "roll-over" cyclometalation path in the course of which a hydrogen atom from the C(3)-position is combined with the Pt-bound methyl group to produce CH(4). Activation of a C-H-bond of the (CH(3))(2)S ligand occurs as well, but is less favored (35 % versus 65 %) as compared to the C(3)-H bond activation of bipy. In addition, the thermal ion/molecule reactions of [Pt(bipy-H)](+) with (CH(3))(2)S have been examined, and for the major pathway, that is, the dehydrogenative coupling of the two methyl groups to form C(2)H(4), a mechanism is suggested that is compatible with the experimental and computational findings. A hallmark of the gas-phase chemistry of [Pt(bipy-H)](+) with the incoming (CH(3))(2)S ligand is the exchange of one (and only one) hydrogen atom of the bipy fragment with the C-H bonds of dimethylsulfide in a reversible "roll-over" cyclometalation reaction. The Pt(II)-mediated conversion of (CH(3))(2)S to C(2)H(4) may serve as a model to obtain mechanistic insight in the dehydrosulfurization of sulfur-containing hydrocarbons.

Synthesis of platinum(II) cyclometallated compounds derived from imines containing pyridyl or pyrimidyl groups

The reaction of compounds [Pt2Me4(µ-SMe2)2] and cis-[PtPh2(SMe2)2] with ligands 4-(2′-C5H4N)C6H4 CHN CH2CH2NMe2 (1a) and 4-(3′,5′-C4H3N2)C6H4CHNCH2CH2NMe2 (1b) carried out in acetone at room temperature produced compounds in which the imines act as bidentate [N,N′] ligands. Analogous compounds could be obtained in refluxing methanol when cis-[PtCl2(DMSO)2] was used as starting material. The corresponding cyclometallation processes leading to platinum(II) compounds with a tridentate [C,N,N′] ligand took place under different reaction conditions, except for metallation of 1a with cis-[PtPh2(SMe2)2], which could not be achieved. All compounds were characterized by usual techniques and [PtMe{4-(3′,5′-C4H3N2)C6H3CHNCH2CH2NMe2}] (3bMe) was also characterized crystallographically.Key words: platinum, imines, pyridyl, pyrimidyl, cyclometallation, crystal structure.

Combination of lacunary polyoxometalates and high-nuclear transition metal clusters under hydrothermal conditions: IX. a series of novel polyoxotungstates sandwiched by octa-copper clusters

The reaction of CuCl(2).2 H2O with trivacant Keggin polyoxoanions K8Na2[A-alpha-GeW9O34].25 H2O or K10[A-alpha-SiW9O34].25 H2O in the presence of 1,2-diaminopropane (dap), ethylenediamine (en) or 2,2'-bipyridine (2,2'-bpy) under hydrothermal conditions afforded five novel hybrid inorganic-organic octa-Cu sandwiched polyoxotungstates (POTs): H4[CuII8(dap)4(H2O)2(B-alpha-GeW9O34)2].13 H2O (1), (H2en)2[CuII8-(en)4(H2O)2(B-alpha-GeW9O34)2].5 H2O(2), (H2en)2[CuII8(en)4(H2O)2(B-alpha-SiW9-O34)2].8 H2O (3), [CuII(H2O)2]H2[CuII8-(en)4(H2O)2(B-alpha-SiW9O34)2] (4), and [CuII2(H2O)2(2,2'-bpy)2]{[CuII(bdyl)]2-[CuII8(2,2'-bpy)4(H2O)2(B-alpha-GeW9-O34)2]}.4 H2O (bdyl=2,2'-bipyridinyl)(5). Additionally, CuCl(2).2 H2O reacts with the mixture of GeO2, Na2WO(4).2 H2O, H2SiW12O(40).2 H2O in the presence of 2,2'-bpy and 4,4'-bpy under hydrothermal conditions leading to another novel mixed-valent octa-Cu sandwiched POT hybrid: [CuI(2,2'-bpy)(4,4'-bpy)]2[{CuI2(2,2'-bpy)2(4,4'-bpy)]2[CuI2CuII6(2,2'-bpy)2(4,4'-bpy)2(B-alpha-GeW9O34)2}].2 H2O (6). 1, 2, and 3 are discrete dimers constructed from two trivacant Keggin [B-alpha-XW9O34]10- (X=GeIV/SiIV) fragments and an octa-Cu cluster whereas 4 displays the 3D (3,6)-connected nets with (4.6(2))(4(2).6(4).8(7).10(2)) topology, which are built by octa-Cu sandwiched polyoxometalate building blocks through copper cation bridges. 5 is a novel 2D layer based on octa-Cu sandwiched POT clusters and [CuII2(bdyl)] units. Interestingly, the rollover metalation of 2,2'-bpy is firstly observed in the system containing the copper complex under hydrothermal conditions. 6 is a discrete mixed-valent octa-Cu sandwiched POT supported by two CuI-complexes [CuI2-(2,2'-bpy)2(4,4'-bpy)]2+ through 4,4'-bpy bridges, which constructs a novel dodeca-copper cluster. Notably, the octa-Cu cluster in 6 is mixed-valent and is different from those in 1-5. To our knowledge, 1-6 represent a rare family of POTs incorporating novel octa-nuclear transition-metal clusters in polyoxometalate chemistry. They were structurally characterized by FT-IR spectra, elemental analysis, thermogravimetric analysis, and single-crystal X-ray diffraction. The magnetic properties of 1, 4, and 5 were quantitatively analyzed by the MAGPACK software package.