Kinetics and mechanism of oxidative addition of MeI to binuclear cycloplatinated complexes containing biphosphine bridges: Effects of ligands

High-Valent Pyrazolate-Bridged Platinum Complexes: A Joint Experimental and Theoretical Study

Complexes [{Pt(C^C*)(μ-pz)}₂] (HC^C*_A = 1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1H-imidazol-2-ylidene 1a, HC^C*_B = 1-phenyl-3-methyl-1H-imidazol-2-ylidene 1b) react with methyl iodide (MeI) at room temperature in the dark to give compounds [{Pt^IV(C^C*)Me(μ-pz)}₂(μ-I)]I (C^C*_A2a, C^C*_B2b). The reaction of 1a with benzyl bromide (BnBr) in the same conditions afforded [Br(C^C*_A)Pt^III(μ-pz)₂Pt^III(C^C*_A)Bn] (5a), which by heating in BnBr(l) became [{Pt^IV(C^C*_A)Bn(μ-pz)}₂(μ-Br)]Br (6a). Experimental investigations and density functional theory (DFT) calculations on the mechanisms of these reactions from 1a revealed that they follow a S_N2 pathway in the two steps of the double oxidative addition (OA). Based on the DFT investigations, species such as [(C^C*_A)Pt^III(μ-pz)₂Pt^III(C^C*_A)R]X (RX = MeI Int-Me, BnBr Int-Bn) and [(C^C*_A)Pt^II(μ-pz)₂Pt^IV(C^C*_A)(R)X] (RX = MeI Int′-Me, BnBr Int′-Bn) were proposed as intermediates for the first and the second OA reactions, respectively. In order to put the mechanisms on firmer grounds, Int-Me was prepared as [(C^C*_A)Pt^III(μ-pz)₂Pt^III(C^C*_A)Me]BF₄ (3a′) and used to get [I(C^C*_A)Pt^III(μ-pz)₂Pt^III(C^C*_A)Me](4a), [(C^C*_A)Pt^II(μ-pz)₂Pt^IV(C^C*_A)(Me)I](Int′-Me), and [{Pt^IV(C^C*)Me(μ-pz)}₂(μ-I)]BF₄ (2a′). The single-crystal X-ray structures of 2a, 2b, 3a′, and 5a along with the mono- and bi-dimensional ¹H and ¹⁹⁵Pt{¹H} NMR spectra of all the named species allowed us to compare structural and spectroscopic data for high-valent complexes with the same core [{Pt(C^C*)(μ-pz)}₂] but different oxidation states.

Experimental and theoretical investigations on the mechanisms of OA reactions of MeI and BnBr to [{Pt^II(C^C*)(μ-pz)}₂] allowed us to get high-valent pyrazolate-bridged platinum compounds: Pt₂(III,III), Pt₂(II,IV), and Pt₂(IV,IV). Their stability and structural and spectroscopic features have been compared.

Tetranuclear rollover cyclometalated organoplatinum-rhenium compounds; C-I oxidative addition and C-C reductive elimination using a rollover cycloplatinated dimer

The novel tetranuclear Pt(IV)-Re(VII) complex [Pt₂Me₄(OReO₃)₂(PMePh₂)₂(µ-bpy-2H)], 4, is synthesized through the reaction of silver perrhenate with a new rollover cycloplatinated(IV) complex [Pt₂Me₄I₂(PMePh₂)₂(µ-bpy-2H)], 3. In complex 4, while 2,2'-bipyridine (bpy) acts as a linker between two Pt metal centers, oxygen acts as a mono-bridging atom between Pt and Re centers through an unsupported Pt(IV)-O-Re(VII) bridge. The precursor rollover cycloplatinated(IV) complex 3 is prepared by the MeI oxidative addition reaction of the rollover cycloplatinated(II) complex [Pt₂Me₂(PMePh₂)₂(µ-bpy-2H)], 2. Complex 2 shows a metal-to-ligand charge-transfer band in the visible region, which was used to investigate the kinetics and mechanism of its double MeI oxidative addition reaction. Based on the experimental findings, the classical S_N2 mechanism was suggested for both steps and supported by computational studies. All complexes are fully characterized using multinuclear NMR spectroscopy and elemental analysis. Attempts to grow crystals of the rollover cycloplatinated(IV) dimer 3 yielded a new dimer rollover cyclometalated complex [Pt₂I₂(PMePh₂)₂(µ-bpy-2H)], 5, presumably from the C-C reductive elimination of ethane. The identity of complex 5 was confirmed by single crystal X-ray diffraction analysis.

Metal-Metal Cooperation in the Oxidation of a Flapping Platinum Butterfly by Haloforms: Experimental and Theoretical Evidence

The model 1-DFT for the butterfly complex [{Pt(C^∧C*)(μ-pz)}₂] (1; HC^∧C* = 1-(4-(ethoxycarbonyl)phenyl)-3-methyl-1H-imidazol-2-ylidene) shows two minima in the potential energy surface of the ground state in acetone solution: the butterfly-wing-spreading molecules 1-s, (d_Pt-Pt ≈ 3.20 Å) and the wing-folding molecules 1-f (d_Pt-Pt ≤ 3.00 Å). Both minima are very close in energy (ΔG° = 1.7 kcal/mol) and are connected through a transition state, which lies only 1.9 kcal/mol above 1-s and 0.2 kcal/mol above 1-f. These very low barriers support a fast interconversion process, resembling a butterfly flapping, and the presence of both conformers in acetone solution. However, the 1-f ratio is so low that it is undetectable in the excitation and emission spectra of 1 in 2-MeTHF of diluted solutions (10^-5 M) at 77 K, while it is seen in more concentrated solutions (10^-3 M). In acetone solution, 1 undergoes a [2c, 2e] oxidation by CHX₃ (X = Cl, Br) in the sunlight to render the Pt₂(III,III) compounds [{Pt(C^∧C*)(μ-pz)X)}₂] (X = Cl (2-Cl), Br (2-Br)). In concentrated solutions, 1 can react with CHCl₃ under blue light to give 2-Cl and with CHBr₃ in the dark, the latter rendering the compound [BrPt(C^∧C*)(μ-pz)₂Pt(C^∧C*)CHBr₂] (3-Br) or mixtures of 2-Br and 3-Br if the reaction is performed under an argon atmosphere or in the air, respectively. Mechanistic studies showed that in concentrated solutions the oxidation processes follow a radical mechanism being the MMLCT-based species 1-f, those which trigger the reaction of 1 with CHBr₃ and CHCl₃. In the ground state (S_0f), it promotes the thermal oxidation of 1 by CHBr₃ and in the first singlet excited state (S_1f) the blue-light-driven photooxidation of 1 by CHCl₃. Complexes, 2-Cl, 2-Br, and 3-Br were selectively obtained and fully characterized, showing Pt-Pt distances (ca. 2.6 Å) shorter than that of the starting complex, 1. They are, together with the analogous [{Pt(C^∧C*)(μ-pz)I)}₂] and [IPt(C^∧C*)(μ-pz)₂Pt(C^∧C*)CHI₂], the only dinuclear metal-metal-bonded Pt^III(μ-pz)₂Pt^III compounds reported to date.

Cyclometalated platinum(ii) complexes of 2,2'-bipyridine N-oxide containing a 1,1'-bis(diphenylphosphino)ferrocene ligand: structural, computational and electrochemical studies

The preparation and characterization of new heteronuclear-platinum(ii) complexes containing a 1,1'-bis(diphenylphosphino)ferrocene (dppf) ligand are described. The reaction of the known starting complex [PtMe(κ²N,C-bipyO-H)(SMe₂)], A, in which bipyO-H is a cyclometalated rollover 2,2'-bipyridine N-oxide, with the dppf ligand in a 2 : 1 ratio or an equimolar ratio led to the formation of the corresponding binuclear complex [Pt₂Me₂(κ²N,C-bipyO-H)₂(μ-dppf)], 1, or the mononuclear complex [PtMe(κ¹C-bipyO-H)(dppf)], 2, respectively. According to the reaction conditions, the dppf ligand in 1 and 2 behaves as either a bridging or chelating ligand. All complexes were characterized by NMR spectroscopy. The solid-state structure of 2 was determined by the single-crystal X-ray diffraction method and it was shown that the chelating dppf ligand in this complex was arranged in a "synclinal-staggered" conformation. Also, the occurrence of intermolecular C-H_CpO_bipyO-H interactions in the solid-state gave rise to an extended 1-D network. The electronic absorption spectra and the electrochemical behavior of these complexes are discussed. Density functional theory (DFT) was used for geometry optimization of the singlet states in solution and for electronic structure calculations. The analysis of the molecular orbital (MO) compositions in terms of occupied and unoccupied fragment orbitals in 2 was performed.

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.

Oxidation of Half-Lantern Pt2(II,II) Compounds by Halocarbons. Evidence of Dioxygen Insertion into a Pt(III)-CH3 Bond

The half-lantern compound [{Pt(bzq)(μ-N^S)}2] (1) [bzq = benzo[h]quinoline, HN^S = 2-mercaptopyrimidine (C4H3N2HS)] reacts with CH3I and haloforms CHX3 (X = Cl, Br, I) to give the corresponding oxidized diplatinum(III) derivatives [{Pt(bzq)(μ-N^S)X}2] (X = Cl 2a, Br 2b, I 2c). These compounds exhibit half-lantern structures with short intermetallic distances (∼2.6 Å) due to Pt-Pt bond formation. The halogen abstraction mechanisms from the halocarbon molecules by the Pt2(II,II) compound 1 were investigated. NMR spectroscopic evidence using labeled reagents support that in the case of (13)CH3I the reaction initiates with an oxidative addition through an SN2 mechanism giving rise to the intermediate species [I(bzq)Pt(μ-N^S)2Pt(bzq)((13)CH3)}]. However, with haloforms the reactions proceed through a radical-like mechanism, thermally (CHBr3, CHI3) or photochemically (CHCl3) activated, giving rise to mixtures of species [X(bzq)Pt(μ-N^S)2Pt(bzq)R] (3a-c) and [X(bzq)Pt(μ-N^S)2Pt(bzq)X] (2a-c). In these cases the presence of O2 favors the formation of species 2 over 3. Transformation of 3 into 2 was possible upon irradiation with UV light. In the case of [I(bzq)Pt(μ-N^S)2Pt(bzq)((13)CH3)}] (3d), in the presence of O2 the formation of the unusual methylperoxo derivative [I(bzq)Pt(μ-N^S)2Pt(bzq)(O-O(13)CH3)}] (4d) was detected, which in the presence of (13)CH3I rendered the final product [{Pt(bzq)(μ-N^S)I}2] (2c) and (13)CH3OH.

Photophysical and DFT studies on cycloplatinated complexes: modification in luminescence properties by expanding of π-conjugated systems

The luminescent properties of cycloplatinated complexes containing aryl and SMe₂ ligands, [Pt(p-MeC₆H₄)(ĈN)(SMe₂)], (ĈN = benzo[h]quinolate (bzq), 1, or 2-phenylpyridinate (ppy), 2), were investigated in solution and solid state.

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.