Double Oxidative Addition Research Articles

Non-Palindromic C∧C∧P Platinum and Palladium Pincer Complexes Showing Intense Phosphorescence via Direct Spin-Forbidden S0 → T1 Excitation.

The synthesis of C∧C∧P pre-ligands based on a dicyclohexylphosphine-substituted biphenyl framework is reported. The pre-ligands form the respective non-palindromic pincer complexes of PtII and PdII via double oxidative addition and subsequent comproportionation or C-H activation. The complexes of PtII as well as PdII emit similar green phosphorescence efficiently in the solid state, the former also in solution albeit with less intensity. The most fascinating photophysical feature, however, is a direct singlet-triplet (S0 → T1) excitation of this phosphorescence in the spectral window between the emission and the major singlet-singlet UV absorption. The S0 → T1 excitation spectra show a rich vibronic pattern, which is especially pronounced for the solid samples at cryogenic temperatures. The molar extinction of the lowest-energy singlet-triplet absorption band of the homologous Pt and Pd complexes as well as that of the Pt complex with a different (NHC) ancillary ligand were determined in tetrahydrofuran solutions. Quantum efficiencies of triplet formation (by intersystem crossing) via the "standard" excitation pathway S0 → Sn → T1 were determined for the Pt complexes and found to be different in dependence of the ancillary ligand.

Mechanistic studies: Comparative investigation of nickel(0)-catalyzed selective decarbonylative cross-coupling of cyclic anhydrides with alkyl bromides or aryl triflates

The mechanisms of the nickel-catalyzed decarbonylative cross-coupling of cyclic anhydrides with alkyl bromides, and the nickel-catalyzed non-decarbonylative cross-coupling of cyclic anhydrides with aryl triflates are investigated using density functional theory (DFT) calculations. The results show that the decarbonylative reaction has a radical-involved mechanism with cyclic anhydrides being activated first by the nickel catalyst. The Ni center undergoes a valence change of Ni(0)-Ni(II)-Ni(III)-Ni(I)-Ni(II)-Ni(0). While the non-decarbonylative reaction has a double oxidative addition mechanism with aryl triflates being activated first by the nickel catalyst. The Ni center undergoes a valence change of Ni(0)-Ni(II)-Ni(I)-Ni(III)-Ni(I)-Ni(0). The reasons that affect the selective decarbonylation of the coupling products of cyclic anhydrides are explained from the perspective of substrate activation priority and key intermediate geometric structure. These theoretical insights may shed light on the understanding and development of transition-metal-catalyzed selective decarbonylative CO bond activation reactions.

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 [{PtIV(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)PtIII(μ-pz)2PtIII(C^C*A)Bn] (5a), which by heating in BnBr(l) became [{PtIV(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 SN2 pathway in the two steps of the double oxidative addition (OA). Based on the DFT investigations, species such as [(C^C*A)PtIII(μ-pz)2PtIII(C^C*A)R]X (RX = MeI Int-Me, BnBr Int-Bn) and [(C^C*A)PtII(μ-pz)2PtIV(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)PtIII(μ-pz)2PtIII(C^C*A)Me]BF4 (3a′) and used to get [I(C^C*A)PtIII(μ-pz)2PtIII(C^C*A)Me](4a), [(C^C*A)PtII(μ-pz)2PtIV(C^C*A)(Me)I](Int′-Me), and [{PtIV(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.

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 SN2 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.

Boranediyl- and Diborane(4)-1,2-diyl-Bridged Platinum A-Frame Complexes.

Diplatinum A‐frame complexes with a bridging (di)boron unit in the apex position were synthesized in a single step by the double oxidative addition of dihalo(di)borane precursors at a bis(diphosphine)‐bridged Pt0 2 complex. While structurally analogous to well‐known μ‐borylene complexes, in which delocalized dative three‐center‐two‐electron M‐B‐M bonding prevails, theoretical investigations into the nature of Pt−B bonding in these A‐frame complexes show them to be rare dimetalla(di)boranes displaying two electron‐sharing Pt−B σ‐bonds. This is experimentally reflected in the low kinetic stability of these compounds, which are prone to loss of the (di)boron bridgehead unit.

Synthetic access to a phosphorescent non-palindromic pincer complex of palladium by a double oxidative addition - comproportionation sequence.

A highly luminescent non-palindromic [(C^C^N)Pd] pincer complex forms upon reacting pyridine-substituted 2,2'-diiodo-biphenyl with [Pd(PPh3)4]. This case study establishes for the first time that the title compound is formed via a double oxidative addition - comproportionation sequence. DFT and TDDFT calculations complement mechanistic and photophysical characterizations.

Ni-Catalyzed Reductive Coupling of Electron-Rich Aryl Iodides with Tertiary Alkyl Halides

This work illustrates the reductive coupling of electron-rich aryl halides with tertiary alkyl halides under Ni-catalyzed cross-electrophile coupling conditions, which offers an efficient protocol for the construction of all carbon quaternary stereogenic centers. The mild and easy-to-operate reaction tolerates a wide range of functional groups. The utility of this method is manifested by the preparation of cyclotryptamine derivatives, wherein successful incorporation of 7-indolyl moieties is of particular interest as numerous naturally occurring products are composed of these key scaffolds. DFT calculations have been carried out to investigate the proposed radical chain and double oxidative addition pathways, which provide useful mechanistic insights into the part of the reaction that takes place in solution.

Theoretical studies of methyl iodide oxidative addition to adjacent metal centers in diplatinum(II) complexes

The density functional theory calculations were used to study the electronic and steric effects between two adjacent platinum centers on the oxidative addition of methyl iodide to dimeric complex [Me2Pt(μ-bipym)PtMe2], in which bipym=2,2′-bipyrimidine. A double MeI oxidative addition was considered and the classical SN2 mechanism was suggested for both steps, and the involved transition states and intermediates were proposed. Consistent with the suggested mechanism, large negative ΔS‡ values were found in each step. The calculated energy barrier was larger in the second step as compared to the first step because of the electronic effects transmitted through the 2,2′-bipyrimidine ligand and the steric effects. In a comparative study, the monomeric complex [PtMe2(bipym)] was used in the MeI oxidative reaction as a “calibration reaction” to evaluate the effect of bipym ligand on the rate of the reaction of MeI with dimeric complex, [Me2Pt(μ-bipym)PtMe2].

Thermal and Photochemical Ring-Bromination in Naphthyl-, Naphthdiyl-, and Dicarboximideperyl-Platinum Complexes

Brominated polycyclic aromatic compounds are important synthons, but their synthesis can be difficult. Herein, we report that Pt(IV) centers σ-bonded to naphthalene and a dicarboximideperylene activate the ring systems to selective thermal and photochemical bromination. Thus, trans-Pt(PEt3)2(Br)3(4-bromo-1-naphthyl) and Br2 give trans-Pt(PEt3)2(Br)3(7,4-dibromo-1-naphthyl). Introduction of a second Pt(IV) center is achieved by double oxidative addition of 1,4-dibromonaphthalene to 2Pt(PEt3)4. Bromination of [trans-Pt(PEt3)2Br]2(1,4-naphthdiyl) yields [trans-Pt(PEt3)2(Br)3]2(1,4-naphthdiyl), which further brominates on the ring to give [trans-Pt(PEt3)2(Br)3]2(6,7-dibromo-1,4-naphthdiyl). Photoreduction of the Pt(IV) centers with 1-hexene gives first mixed-valent [trans-Pt(PEt3)2(Br)3][trans-Pt(PEt3)2(Br)](6,7-dibromo-1,4-naphthdiyl) and then [trans-Pt(PEt3)2Br]2(6,7-dibromo-1,4-naphthdiyl). Photoreduction of trans-Pt(PEt3)2(Br)3(PMI) (PMI = N-(2,5-di-tert-butylphenyl)perylen-3-yl-9,10-dicarboximide) withou...

Palladium-catalyzed intramolecular reductive cross-coupling of Csp2-Csp3 bond formation.

A Pd-catalyzed efficient reductive cross-coupling reaction without metallic reductant to construct a Csp(2) -Csp(3) bond has been reported. A Pd(IV) complex was proposed to be a key intermediate, which subsequently went through double oxidative addition and double reductive elimination to produce the cross-coupling products by involving Pd(0/II/IV) in one transformation. The oxidative addition from Pd(II) to Pd(IV) was partially demonstrated to be a radical process by self-oxidation of substrate without additional oxidants. Furthermore, the solvent was proved to be the reductant for this transformation through XPS analysis.

Bis(diphenylphosphino)acetylene as bifunctional ligand in cycloplatinated complexes: Synthesis, characterization, crystal structures and mechanism of MeI oxidative addition

Binuclear cycloplatinated(II) complexes with general formula of [Pt2Me2(CˆN)2(μ-dppac)], (1, CˆN = deprotonated 2-phenylpyridine (ppy); 2, CˆN = deprotonated benzo{h}quinoline (bhq)) in which dppac = 1,1′-bis(diphenylphosphino)acetylene, are synthesized by the reaction of [PtMe(SMe2)(CˆN)] with 0.5 equiv of dppac at room temperature. The complexes are fully characterized using multinuclear (1H, 31P and 195Pt) NMR spectroscopy and complex 2 is further identified by single crystal X-ray structure determination. Kinetics of the reaction of complexes 1 and 2 with MeI are investigated in CHCl3 and based on the UV–vis and 31P NMR data, a mechanism involving a double MeI oxidative addition is suggested. The classical SN2 mechanism is proposed for both steps and the involved intermediates are suggested. Although MeI in each step was trans oxidatively added to one of the platinum(II) centers, further trans to cis isomerizations of methyl and iodide ligands were also identified. The rates are almost four times slower in the second step as compared to the first step due to the electronic effects transmitted through the dppac ligand. Reaction rates concerning complex 2, having bhq ligand, are almost 1.3 times slower than those involving the related ppy complex 1. This is attributed to the stronger donor ability of the ppy ligand, as compared to that bhq ligand and is in agreement with the values of 1JPtP observed in the 31P NMR spectra of the complexes 1 and 2. The structure of the Pt(IV)–Pt(IV) dimeric complex [Pt2I2Me4(ppy)2(μ-dppac)], 3, produced by oxidative addition of complex 1 to MeI, is also determined using X-ray diffraction which is the first X-ray structural determination of a diplatinum complex containing two Pt(IV) centers bridged by one dppac ligand.

Synthesis of novel polyfluorene with defined group in the center using aryl dipalladium complex as an initiator

Dinuclear Pd(II) complex DiPd–DPE with a bridging ligand C6H4OC6H4-4,4′ was successfully synthesized by double oxidative addition reaction of Pd(PCy3)4 with 4,4′-dibromodiphenyl ether in good yield. X-ray crystal structure analysis reveals that the Pd atoms have the nearly square-planar coordination geometry with the similar bond lengths of two Pd–Br as well as four Pd–P, implying that two Pd(II) centers have similar reactivity. Using the dinuclear Pd(II) complex DiPd–DPE as an initiator, novel polyfluorene (PF) with a defined group (C6H4OC6H4-4,4′) in the middle of polymer main chain was prepared, demonstrated by MALDI-TOF analysis.

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

The binuclear complex [Pt2Me2(bhq)2(μ-dppf)], 1a, in which bhqH = benzo{h}quinoline and dppf = 1,1′-bis(diphenylphosphino)ferrocene, was synthesized by the reaction of [PtMe(SMe2)(bhq)] with 0.5 equiv of dppf at room temperature. The reaction of Pt(II)–Pt(II) complex 1a with excess MeI gave the Pt(IV)–Pt(IV) complex [Pt2I2Me4(bhq)2(μ-dppf)], 2a. The complexes were fully characterized using multinuclear (1H, 31P and 195Pt) NMR spectroscopy and elemental analysis. The kinetic and mechanism of the reaction of complex 1a with MeI was investigated in CHCl3 and based on the data, obtained from UV–vis and low temperature 31P NMR spectroscopies, a mechanism involving stepwise oxidative addition of MeI to the two Pt(II) centers is suggested. Reaction rates concerning the complex 1a, having bhq ligand, are almost 1.4 times slower than those involving the ppy complex [Pt2Me2(ppy)2(μ-dppf)], ppyH = 2-phenylpyridine, reported previously (Inorg. Chem. 2008, 47, 5441). This is attributed to the stronger donor ability of the ppy ligand as compared to that of the bhq ligand as is further confirmed using density functional theory (DFT) calculations through finding approximate structures for the described complexes. A comparative kinetic study of reaction of the dimeric platinum(II) complex [Pt2Me2(bhq)2(μ-dppm)], 1b, where dppm = bis(diphenylphosphino)methane, with MeI was also performed to investigate the effect of bridging biphosphine ligand on the kinetic and mechanism of the dimeric complexes with MeI. A double MeI oxidative addition was observed for which the classical SN2 mechanism for both steps, as well as the possible intermediates, is suggested.

Practical Access to Amines by Platinum-Catalyzed Reduction of Carboxamides with Hydrosilanes: Synergy of Dual Si−H Groups Leads to High Efficiency and Selectivity

The synergetic effect of two Si-H groups leads to efficient reduction of carboxamides to amines by platinum catalysts under mild conditions. The rate of the reaction is dependent on the distance of two Si-H groups; 1,1,3,3-tetramethyldisiloxane (TMDS) and 1,2-bis(dimethylsilyl)benzene are found to be an effective reducing reagent. The reduction of amides having other reducible functional groups such as NO(2), CO(2)R, CN, C horizontal lineC, Cl, and Br moieties proceeds with these groups remaining intact, providing a reliable method for the access to functionalized amine derivatives. The platinum-catalyzed reduction of amides with polymethylhydrosiloxane (PMHS) also proceeds under mild conditions. The reaction is accompanied by automatic removal of both platinum and silicon wastes as insoluble silicone resin, and the product is obtained by simple extraction. A mechanism involving double oxidative addition of TMDS to a platinum center is discussed.

“Synergistic Effects of Two Si−H Groups and a Metal Center” in Transition Metal-Catalyzed Hydrosilylation of Unsaturated Molecules: A Mechanistic Study of the RhCl(PPh3)3-Catalyzed Hydrosilylation of Ketones with 1,2-Bis(dimethylsilyl)benzene

Three rhodadisilacyclopentene complexes are synthesized by the reaction of 1,2-bis(dimethylsilyl)benzene with RhCl(PPh3)3 or Rh(H)(PPh3)4, and their contributions to the catalytic hydrosilylation of acetone are discussed. Treatment of the rhodium precursor RhCl(PPh3)3 or Rh(H)(PPh3)4 with 1,2-bis(dimethylsilyl)benzene affords an unstable Rh(V)-trihydride species having a rhodadisilacyclopentene skeleton, Rh(Me2SiC6H4SiMe2)(H)3(PPh3)2 (1), as a primary product, which is formed by double oxidative addition of 1,2-bis(dimethylsilyl)benzene to the rhodium center. The complex 1 eliminates H2 upon concentration to quantitatively form a Rh(III)-monohydride complex, Rh(SiMe2C6H4SiMe2)(H)(PPh3)2 (2). Further oxidative addition of 1,2-bis(dimethylsilyl)benzene to 2 gives a Rh(III)-trisilyl complex, Rh(Me2SiC6H4SiMe2)(η1-HSiMe2C6H4SiMe2)(PPh3) (3), in which there is an agostic interaction between the Si−H bond and the Rh(III) center. Elimination of H2 from 1 is reversible, and the most effective method for preparing 1 in solution is found to be treatment of 2 with H2. The catalytic behavior of these three new rhodadisilacyclic complexes, RhCl(PPh3)3, and Rh(H)(PPh3)4, in the hydrosilylation of acetone with 1,2-bis(dimethylsilyl)benzene was studied. The results suggest the important contribution of the trihydride 1 in the synergistic effect of two proximate Si−H bonds, leading to an unusual rate enhancement in the hydrosilylation of acetone with 1,2-bis(dimethylsilyl)benzene.

Regioselectivity in alkenyl(aryl)-heteroaryl Suzuki cross-coupling reactions of 2,4-dibromopyridine. A synthetic and mechanistic study

2,4-Dibromopyridine undergoes a regioselective Suzuki cross-coupling reaction at position 2 with several alkenyl(aryl) boronic acids to render 4-bromo-2-carbon substituted pyridines, difficult to be prepared otherwise, in good yields under palladium catalysis, either Pd(PPh 3) 4/TlOH or Pd 2dba 3/PCy 3/K 3PO 4 at 25 °C. This behavior is explained on the basis of the electrophilic character of both C–Br bonds, being their relative reactivity in 2,4-dibromopyridine similar to that in the corresponding monobromopyridines. In addition, the dicoupled compound 6 is not formed through a double oxidative addition of 2,4-dibromopyridine to Pd(PPh 3).

A phosphorescent hexa- peri-hexabenzocoronene platinum complex and its time-resolved spectroscopy

Photoluminescence properties of photoactive HBCC 8,2 and HBCPt 2 have been studied. The phenylene-ethynyl-Pt moieties were incorporated onto the bromo functions of HBCBr 2 1 via double oxidative addition of Pt(PPh 3) 4 to 1 followed by CuI-catalyzed dehydrohalogenation reaction of compound 2 and phenylacetylene to give the desired molecule HBC-Pt 2 . Introduction of platinum-phenylacetylene to the hexa- peri-hexabenzocoronenes (HBC) core enhanced the spin–orbit coupling and decreased the phosphorescence lifetime of the triplet state by three orders of magnitude. Photoluminescence measurements revealed unique phosphorescence emission at room temperature and 77 K. On the other hand, symmetric HBC-C 8,2 , bearing no platinum, revealed different delayed emissions depending on the temperature measurement. At room temperature, delayed fluorescence appeared while at 77 K, only phosphorescence could be observed. The triplet energy level has been tuned between 2.3 and 1.7 eV by varying the HBC core substituents. In consequence, combination of both the heavy-atom effect and the increased intersystem crossing in the novel double platinum-substituted HBC lead to the exclusive emission of phosphorescence without any residues of fluorescence in the prompt and delayed emission spectra.

Oxidative Addition of Methyl Iodide to a New Type of Binuclear Platinum(II) Complex: a Kinetic Study

A new organodiplatinum(II) complex cis,cis-[Me2Pt(mu-NN)(mu-dppm)PtMe2] (1), in which NN = phthalazine and dppm = bis(diphenylphosphino)methane, is synthesized by the reaction of cis,cis-[Me2Pt(mu-SMe2)(mu-dppm)PtMe2] with 1 equiv of NN. Complex 1 has a 5d(pi)(Pt) --> pi(imine) metal-to-ligand charge-transfer band in the visible region, which was used to easily follow the kinetics of its reaction with MeI. Meanwhile, the complex contains a robust bridging dppm ligand that holds the binuclear integrity during the reaction. A double MeI oxidative addition was observed, as shown by spectrophotometry and confirmed by a low-temperature 31P NMR study. The classical S(N)2 mechanism was suggested for both steps, and the involved intermediates were suggested. Consistent with the proposed mechanism, the rates of the reactions at different temperatures were slower in benzene than in acetone and large negative deltaS values were found in each step. However, some abnormalities were observed in the related rate constants and deltaS values, which were demonstrated to be due to the associative involvement of the polar acetone molecules in the reactions. The rates are almost 6 times slower in the second step as compared to the first step because of the electronic effects transmitted through the ligands and the steric effects.

C–H and C–C bonds activation by iridium clusters: Synthesis and solid state structure of [Ir 6(CO) 14(μ 3-η 1:η 2:η 1-C 7H 8)

The reaction between norbornene (C 7H 10) and Ir 6(CO) 16 in refluxing toluene yields the substituted cluster [Ir 6(CO) 14(C 7H 8)]. In the solid state, the six iridium atoms define an octahedral metal cage, and one face is capped by the C 7H 8 ligand, coordinated through a 1,2-vinylidene unit. The loss of two hydrogen atoms from the olefin is, presumably, the result of a double oxidative addition of its C–H bonds, followed by elimination of a hydrogen molecule from the cluster. The bonding parameters of the organic fragment, as well as the architecture of the carbonyl ligands, closely match those found in the alkyne-substituted [Ir 6(CO) 14(PhCCPh)] complex. The cluster [Ir 6(CO) 14(C 7H 8)] was characterized through 1H-, 13C- and bidimensional COSY NMR.

Synthesis and structure of 4,6-disubstituted dibenzothiophenes and their use in the preparation of binuclear platinum(ii) complexes

The reaction of two equivalents of [Pt(PEt3)4] with 4,6-dibromodibenzothiophene (DBT-Br2) afforded trans,trans-[(μ-DBT-1κC4:2κC6){PtBr(PEt3)2}2] (5), a unique example of double oxidative addition of a heteroaromatic substrate to Pt(0). The related bimetallic Pt(II) complex, cis,cis-[(μ-DBT-1κC4:2κC6){Pt(η4-cod)Cl}2] (6), was isolated via a double transmetallation of 4,6-bis(trimethylstannyl)dibenzothiophene with [Pt(η4-cod)Cl2], where cod is 1,5-cyclooctadiene. The new complexes were characterised by spectroscopic techniques. The disubstituted dibenzothiophene compounds were prepared by dilithiation of dibenzothiophene, followed by quenching with chlorotrimethylsilane and chlorotrimethylstannane, to give 4,6-(SiMe3)2(SC12H6) (2) and 4,6-(SnMe3)2(SC12H6) (4), respectively. Treatment of 4,6-(SiMe3)2(SC12H6) with bromine produced 4,6-Br2(SC12H6) (3). The X-ray crystal structures of 2 and 3 were determined. An improved method of synthesis of 2,8-dibromodibenzothiophene (1) was also developed.