Synthesis and fluxional behaviour of allylpalladium complexes with poly (pyrazol-1-yl)methane ligands

Operando Spectroscopic and Kinetic Characterization of Aerobic Allylic C-H Acetoxylation Catalyzed by Pd(OAc)2/4,5-Diazafluoren-9-one

Allylic C-H acetoxylations are among the most widely studied palladium(II)-catalyzed C-H oxidation reactions. While the principal reaction steps are well established, key features of the catalytic mechanisms are poorly characterized, including the identity of the turnover-limiting step and the catalyst resting state. Here, we report a mechanistic study of aerobic allylic acetoxylation of allylbenzene with a catalyst system composed of Pd(OAc)₂ and 4,5-diazafluoren-9-one (DAF). The DAF ligand is unique in its ability to support aerobic catalytic turnover, even in the absence of benzoquinone or other co-catalysts. Herein, we describe operando spectroscopic analysis of the catalytic reaction using X-ray absorption and NMR spectroscopic methods that allow direct observation of the formation and decay of a palladium(I) species during the reaction. Kinetic studies reveal the presence of two distinct kinetic phases: (1) a burst phase, involving rapid formation of the allylic acetoxylation product and formation of the dimeric Pd^I complex [Pd^I(DAF)(OAc)]₂, followed by (2) a post-burst phase that coincides with evolution of the catalyst resting state from the Pd^I dimer into a π-allyl-Pd^II species. The data provide unprecedented insights into the role of ancillary ligands in supporting catalytic turnover with O₂ as the stoichiometric oxidant and establish an important foundation for the development of improved catalysts for allylic oxidation reactions.

Quantum mechanical study on the different fluxional mechanisms in a [Pd(η2-olefin) complex

In this work, using quantum mechanics, different fluxional mechanisms in a Pd(η2-olefin) complex including sulfur inversion, olefin pseudo rotation, Pd–olefin bond dissociation, propeller-like olefin rotation and a bimolecular process have been investigated theoretically in a solvent environment using the polarisable continuum model (PCM). The activation energy, activation enthalpy and activation Gibbs free energy of the different pathways have been calculated and compared with each other and with experimental results. These activation energies for olefin pseudo rotation, Pd–olefin bond dissociation and propeller-like olefin rotation are greater in comparison with those for the sulfur inversion and bimolecular process, which demonstrates that these latter mechanisms are the dominating paths and make the main contributions in the reaction. The computed activation enthalpies of the sulfur inversion and bimolecular process pathways are in good agreement with the experimental values. All of the calculations have been performed using a hybrid density functional method (B3LYP) in the solution phase.

Coordination, organometallic and related chemistry of tris(pyrazolyl)methane ligands

Tris(pyrazolyl)methanes are the neutral analogues of the widely exploited and highly useful tris(pyrazolyl)hydroborates, yet by comparison with their boron based counterparts their chemistry is underdeveloped. Recent breakthroughs in the synthesis of ring-substituted tris(pyrazolyl)methanes offer the opportunity for the development of this useful and promising class of ligand. This review summarises the current state of the coordination and organometallic chemistry of tris(pyrazolyl)methanes and highlights areas in which development is likely to occur.

Pd(II) complexes with polydentate nitrogen ligands. Molecular recognition and dynamic behavior involving Pd-N bond rupture. X-ray molecular structures of [[Pd(C6HF4)2](bpzpm)] and [[Pd(eta 3-C4H7)]2(bpzpm)] (CF3SO3)2 [bpzpm = 4,6-bis(pyrazol-1-yl)pyrimidine]

The ligands 4,6-bis(pyrazol-1-yl)pyrimidine (bpzpm) and 4,6-bis(4-methylpyrazol-1-yl)pyrimidine (Me-bpzpm) were synthesized and their reactions with some palladium derivatives explored. Mononuclear or dinuclear neutral or cationic complexes were obtained by reaction of the ligands with 1 or 2 equiv of Pd(C6XF4)2(cod) (cod = 1,5-cyclooctadiene; X = F, H) or the palladium fragment [Pd(eta 3-2-Me-C3H4)(S)2]+ (S = acetone). The reaction of the dinuclear derivatives with 1 equiv of the respective free ligand immediately led to the regeneration of the mononuclear complexes. Except in the case of the synthesis of [[Pd(C6HF4)2][Pd(C6F5)2](bpzpm)], where two similar metallic groups are present, all attempts to obtain dinuclear asymmetric complexes with two different palladium fragments failed. Instead, the dinuclear symmetric complexes were formed. This result could be considered as an example of molecular recognition with the ligand acting as a ditopic receptor. This behavior is comparable to chemical symbiosis but in this case applied to the ligand rather than to the metal center as occurs normally. The polyfluorophenyl rings are situated on average in a perpendicular orientation with respect to the coordination plane. Their restricted rotation results in several atropoisomers for the complexes with m-C6HF4. Different cross-reaction experiments were carried out, and these showed the mobility of the metallic fragments, with the more difficult process being that involving the more strongly bonded polyfluorophenyl palladium groups. By means of 1H NMR variable temperature studies, the interconversion of the two isomers of [[Pd(eta 3-C4H7)]2-(bpzpm)]Tf2 (Tf = CF3SO3) was analyzed. In the case of [[Pd(eta 3-C4H7)](bpzpm)]Tf the existence of two processes, an intramolecular apparent allyl rotation and an intermolecular exchange of the allylpalladium fragments, has been demonstrated. Different delta Gc++ values at the coalescence temperatures have also been determined. An X-ray single-crystal analysis was carried out on [[Pd(eta 3-C4H7)]2(bpzpm)]Tf2, which crystallizes in the monoclinic system, space group I2/m, with a = 9.368(2), b = 16.191(3), c = 20.228(6) A, beta = 101.26(3), and Z = 4. Compound [[Pd(C6HF4)2](bpzpm)] crystallized in the triclinic system, space group P1, with a = 8.845(6), b = 12.6609(9), c = 12.826(3) A, alpha = 88.45(2), beta = 74.36(3), gamma = 89.32(2), and Z = 2.

Synthesis and Characterization of Palladium(II) Complexes with New Polydentate Nitrogen Ligands. Dynamic Behavior Involving Pd-N Bond Rupture. X-ray Molecular Structure of [{Pd(eta(3)-C(4)H(7))}(2)(Me-BPzTO)](4-MeC(6)H(4)SO(3)) [Me-BPzTO = 4,6-Bis(4-methylpyrazol-1-yl)-1,3,5-triazin-2-olate]

The ligands 2,4,6-tris(4-methylpyrazol-1-yl)-1,3,5-triazine (Me-TPzT), 2,4,6-tris(4-bromopyrazol-1-yl)-1,3,5-triazine (Br-TPzT), and 2-methoxy-4,6-bis(4-methylpyrazol-1-yl)-1,3,5-triazine (Me-BPzTOMe) have been synthesized and their reactions with some palladium derivatives explored. The palladium fragment [Pd(eta(3)-2-Me-C(3)H(4))(S)(2)](+), S = acetone, reacts in acetone with Me-TPzT or Br-TPzT in a 3:1 molar ratio to generate new complexes in which two allylpalladium fragments are present and the TPzT ligands have been partially hydrolyzed: [{Pd(eta(3)-C(4)H(7))}(2)(X-BPzTO)]A, X-BPzTO = 4,6-bis[4-methyl(or bromo)pyrazol-1-yl]-1,3,5-triazin-2-olate (X = Me, A = BF(4), 1; A = PF(6), 2; A = CF(3)SO(3), 3; A = p-MeC(6)H(4)SO(3), 4; X = Br, A = CF(3)SO(3), 5). When the ligand Me-BPzTOMe is made to react with only 1 equiv of the palladium solvate, compound 6, [Pd(eta(3)-2-Me-C(3)H(4))(Me-BPzTOMe)]CF(3)SO(3), is isolated. Reaction of 6 with another 1 equiv of the palladium derivative leads to 3. The intermediate 7, [{Pd(eta(3)-2-Me-C(3)H(4))}(2)(Me-BPzTOMe)]CF(3)SO(3), has been isolated as an almost pure compound. The reaction of Me-BPzTOMe with 1 equiv of [Pd(C(6)F(5))(2)(cod)] (cod = 1,5-cyclooctadiene) leads to the complex [Pd(C(6)F(5))(2)(Me-BPzTOMe)], 8. Attention has been focused on the dynamic behavior, related with metallotropic phenomena, of the new complexes. (1)H NMR variable-temperature studies of complexes 1, 6, and 8 have been carried out. For 8, only one static species is observed, while, for 1 and 6, two isomers are detected at low temperature. Different DeltaG(c)() activation energies at the coalescence temperature have been determined and are ascribed to processes implying Pd-N bond ruptures. For 6, two different barriers are detected, corresponding to Pd-N(triazine) or Pd-N(pyrazole) bond ruptures. From the DeltaG(c)() data, it is concluded that the main driving force of the hydrolysis process is the formation of a better coordinating ligand. The molecular structure of 4 has been determined by X-ray diffraction. The meso isomer, in which the two C-Me axes of the allylic groups are oriented in the same direction, is found in the solid state.