Synthesis and dehydrative condensation of square-planar mono- and dinuclear hydroxopalladium complexes with the hydrotris(3,5-diisopropylpyrazolyl)borato ligand (TpiPr2), TpiPr2(Py)Pd-OH, and (mu-OH)2[PdTpiPr2(H2O)]2

Mechanistic Studies for Pd(II)(O2) Reduction Generating Pd(0) and H2O: Formation of Pd(OH)2 as a Key Intermediate

Molecular oxygen (O₂) remains to be an ideal yet underutilized feedstock for the oxidative transformation of organic substrates and renewable energy systems such as fuel cells. Palladium (Pd) has shown particular promise in enabling these applications. The present study describes a Pd-mediated O₂ reduction to water via C-H activation of 9,10-dihydroanthracene (DHA) by a Pd(II) η²-peroxo complex 1O₂. The reaction yields stoichiometric anthracene and Pd(0) product 1 and is notable in two respects. First, plots of concentrations of the reaction participants over time have distinctly sigmoidal shapes, indicating that conversion accelerates over time and implying autocatalysis. Second, the reaction proceeds via a genuine monometallic Pd(II) dihydroxide 1(OH)₂ directly observed to grow and decay as an intermediate. Confirming its role as an intermediate, the dihydroxide 1(OH)₂ was found to mediate C-H oxidation of DHA on par in activity with the peroxo compound 1O₂. Mechanistic studies with density functional theory (DFT) calculations suggested that both 1O₂ and 1(OH)₂ react with DHA by hydrogen atom transfer (HAT) and that autocatalysis in the 1O₂ reaction results from oxidative addition of the initial Pd(II) complex 1O₂ to the Pd(0) product 1. This reaction forms a transient bis(μ-oxo) Pd(II) dimer 1O₂1 that is more active in the HAT oxidation of DHA than the initial 1O₂.

A pseudotetrahedral nickel(II) complex with a tridentate oxazoline-based scorpionate ligand: chlorido[tris(4,4-dimethyloxazolin-2-yl)phenylborato]nickel(II)

Poly(pyrazol-1-yl)borates have been utilized extensively in coordination compounds due to their high affinity toward cationic metal ions on the basis of electrostatic interactions derived from the mononegatively charged boron centre. The original poly(pyrazol-1-yl)borates, christened `scorpionates', were pioneered by the late Professor Swiatoslaw Trofimenko and have expanded to include various borate ligands with N-, P-, O-, S-, Se- and C-donors. Scorpionate ligands with boron-carbon bonds, rather than the normal boron-nitrogen bonds, have been developed and in these new types of scorpionate ligands, amines and azoles, such as pyridines, imidazoles and oxazolines, have been employed as N-donors instead of pyrazoles. Furthermore, a variety of bis- and tris(oxazolinyl)borate ligands, including chiral ones, have been developed. Tris(oxazolin-2-yl)borates work as facially capping tridentate chelating ligands in the same way as tris(pyrazol-1-yl)borates. In the title compound, [Ni(C₂₁H₂₉BN₃O₃)Cl], the Ni^II ion is coordinated by three N atoms from the facially capping tridentate chelating tris(4,4-dimethyloxazolin-2-yl)phenylborate ligand and a chloride ligand in a highly distorted tetrahedral geometry. The Ni-Cl bond length [2.1851 (5) Å] is comparable to those found in a previously reported tris(3,5-dimethylpyrazol-1-yl)hydroborate derivative [2.1955 (18) and 2.150 (2) Å]. The molecular structure deviates from C_3v symmetry due to the structural flexibility of the tris(4,4-dimethyloxazolin-2-yl)phenylborate ligand.

Classical and non-classical redox reactions of Pd(II) complexes containing redox-active ligands

Reactivity studies of a Pd(II)-verdazyl complex reveal novel ligand-centred reduction processes which trigger pseudo-reductive elimination at Pd. Reaction of the complex with water induces a ligand-centred redox disproportionation. The reduced verdazyl ligands can also be reversibly protonated.

Properties and reactions of μ-(dihydroxo)dipalladium(II) bisporphyrin complexes

μ-(dihydroxo)dipalladium(II) bisporphyrin was obtained by the reaction of dicationic bis(aqua)Pd(II) complexes of N21, N22-bridged porphyrin with NaOH or triethylamine. The reaction gives a mixture of two isomers and caused deprotonation from the active methylene compounds to give organopalladium(II) porphyrin complexes under neutral conditions.

Mechanistic studies of Pd-catalyzed regioselective aryl C-H bond functionalization with strained alkenes: origin of regioselectivity

Mechanistic studies of a palladium-catalyzed regioselective aryl C-H functionalization of 2-pyrrole phenyl iodide with norbornene are presented. Kinetic and spectroscopic analyses together with crystallographic data provide evidence for intermediates in a proposed stepwise mechanism. On the basis of the mechanistic studies, the origin of the regioselectivity is due to a ligand exchange between I(-) and HO(-) on the norbornyl palladium complex. These mechanistic studies also implicate that either alkoxide or water is responsible for the formation of the palladacycle, but a reversible ring-opening-ring-closing process of the palladacycle with HX can retard the rate of reaction of a key intermediate. The significant aspects of the proposed mechanism are discussed in detail.

Resting state and elementary steps of the coupling of aryl halides with thiols catalyzed by alkylbisphosphine complexes of palladium

Detailed mechanistic studies on the coupling of aryl halides with thiols catalyzed by palladium complexes of the alkylbisphosphine ligand CyPF-(t)Bu (1-dicyclohexylphosphino-2-di-tert-butylphosphinoethylferrocene) are reported. The elementary steps that constitute the catalytic cycle, i.e. oxidative addition, transmetalation and reductive elimination, have been studied, and their relative rates are reported. Each of the steps of the catalytic process occurs at temperatures that are much lower than those required for the reactions catalyzed by a combination of palladium precursors and CyPF-(t)Bu. To explain these differences in rates between the catalytic and stoichiometric reactions, studies were conducted to identify the resting state of the catalyst of the reactions catalyzed by a combination of Pd(OAc)(2) and CyPF-(t)Bu, a combination of Pd(dba)(2) and CyPF-(t)Bu, or the likely intermediate Pd(CyPF-(t)Bu)(Ar)(Br). These data show that the major palladium complex in each case lies off of the catalytic cycle. The resting state of the reactions catalyzed by Pd(OAc)(2) and CyPF-(t)Bu was the palladium bis-thiolate complex [Pd(CyPF-(t)Bu)(SR)(2)] (R = alkyl or aryl). The resting state in reactions catalyzed by Pd(2)(dba)(3) and CyPF-(t)Bu was the binuclear complex [Pd(CyPF-(t)Bu)](2)(mu(2),eta(2)-dba) (9). The resting states of reactions of both aromatic and aliphatic thiols catalyzed by [Pd(CyPF-(t)Bu)(p-tolyl)(Br)] (3a) were the hydridopalladium thiolate complexes [Pd(CyPF-(t)Bu)(H)(SR)] (R= alkyl and aryl). All these palladium species have been prepared independently, and the mechanisms by which they enter the catalytic cycle have been examined in detail. These features of the reaction catalyzed by palladium and CyPF-(t)Bu have been compared with those of reactions catalyzed by the alkylbisphosphine DiPPF and Pd(OAc)(2) or Pd(dba)(2). Our data indicate that the resting states of these reactions are similar to each other and that our mechanistic conclusions about reactions catalyzed by palladium and CyPF-(t)Bu can be extrapolated to reactions catalyzed by complexes of other electron-rich bisphosphines.

Synthesis, characterization and spectroscopic studies of the dihydrobis(1,2,3-benzotriazolyl)borate anion and its complexes with MCl2·py2

The preparation of sodium dihydrobis(1,2,3-benzotriazolyl)borate was realised by refluxing one mole of sodium borohydride with two moles of 1,2,3-benzotriazole in toluene over a period of 12 h. Its complexes with MCl2?py2 [where M= Mn(II), Fe(II), Co(II), Ni(II), Cu(II) and py = Pyridine] were characterized by elemental analysis as well as magnetic, spectroscopic and conductivity measurements. On the basis of these studies, it is proposed that the geometry of all the complexes is octahedral. The ligand field parameters 10 Dq, B and ? show extensive overlap between the M-L orbital. The molar conductance of 10-3Msolutions of the complexes in DMSO suggest them to be non-ionic in nature.

Synthesis of the Elusive (R3P)2MF2(M = Pd, Pt) Complexes

The synthesis and characterization of previously unknown palladium(II) and platinum(II) difluoro phosphine complexes are described. These complexes can be obtained either via a halide metathesis reaction with AgF in dichloromethane or by reacting the corresponding dimethyl complexes with XeF2. While the Pt(II) complexes can be prepared with both aryl- and alkyl-phosphine ligands, the stability of the Pd(II) complexes is limited to those having cis-oriented trialkyl phosphine ligands.

Insertion reactions into Pd[bond]O and Pd[bond]N bonds: preparation of alkoxycarbonyl, carbonato, carbamato, thiocarbamate, and thioureide complexes of palladium(II)

Mononuclear palladium hydroxo complexes of the type [Pd(N[bond]N)(C(6)F(5))(OH)] [(N[bond]N = 2,2'-bipyridine (bipy), 4,4'-dimethyl-2,2'-bipyridine (Me(2)bipy), 1,10-phenanthroline (phen), or N,N,N',N'-tetramethylethylenediamine (tmeda)] have been prepared by reaction of [Pd(N[bond]N)(C(6)F(5))(acetone)]ClO(4) with KOH in methanol. These hydroxo complexes react, in methanol, with CO (1 atm, room temperature) to yield the corresponding methoxycarbonyl complexes [Pd(N[bond]N)(C(6)F(5))(CO(2)Me)]. Similar alkoxycarbonyl complexes [Pd(N[bond]N)(C(6)F(5))(CO(2)R)] (N[bond]N = bis(3,5-dimethylpyrazol-1-yl)methane); R = Me, Et, or (i)Pr) are obtained when [Pd(N[bond]N)(C(6)F(5))Cl] is treated with KOH in the corresponding alcohol ROH and CO is bubbled through the solution. The reactions of [Pd(N[bond]N)(C(6)F(5))(OH)] (N[bond]N = bipy or Me(2)bipy) with CO(2), in tetrahydrofuran, lead to the formation of the binuclear carbonate complexes [(N[bond]N)(C(6)F(5))Pd(mu-eta(2)-CO(3))Pd(C(6)F(5))(N[bond]N)]. Complexes [Pd(N[bond]N)(C(6)F(5))(OH)] react in alcohol with PhNCS to yield the corresponding N-phenyl-O-alkylthiocarbamate complexes [Pd(N[bond]N)(C(6)F(5))[SC(OR)NPh]]. Similarly, the reaction of [Pd(bipy)(C(6)F(5))(OH)] with PhNCO in methanol gives the N-phenyl-O-methylcarbamate complex [Pd(bipy)(C(6)F(5))[NPhC(O)OR]]. The reactions of [(N[bond]N)Pd(C(6)F(5))(OH)] with PhNCS in the presence of Et(2)NH yield the corresponding thioureidometal complexes [Pd(N[bond]N)(C(6)F(5))[NPhCSNR(2)]]. The crystal structures of [Pd(tmeda)(C(6)F(5))(CO(2)Me)], [Pd(2)(Me(2)bipy)(2)(C(6)F(5))(2)(mu-eta(2)-CO(3))].2CH(2)Cl(2), and [Pd(tmeda)(C(6)F(5))[SC(OMe)NPh]] have been determined.

Synthesis and characterization of a series of (hydroperoxo)-, (alkylperoxo)-, and (mu-peroxo)palladium complexes containing the hydrotris(3,5-diisopropylpyrazolyl)borato ligand (Tp(iPr2)): (Tp(iPr2))(py)Pd-OO-X [X = H, t-Bu, Pd(Tp(iPr2))(py)] and (Tp(iPr2))(py)Pd-(mu-kappa(1):kappa(2)-OO)-PdTp(iPr2)

Dehydrative condensation of the hydroxopalladium complex (Tp(iPr2))(py)Pd-OH (1) with hydroperoxides (XOOH: X = H, t-Bu) produces the corresponding (hydroperoxo)-, (Tp(iPr2))(py)Pd-OOH (2a), and (tert-butylperoxo)palladium complexes, (Tp(iPr2))(py)Pd-OOBu(t) (3). Treatment of 2a with PPh(3) results in concomitant ligand displacement giving (Tp(i)(Pr2))(Ph(3)P)Pd-OOH (2b) and oxygenation of PPh(3) giving O=PPh(3). Further condensation between 1 and 2a gives the mu-kappa(1):kappa(1)-peroxo complex (Tp(iPr2))(py)Pd-OO-Pd(Tp(iPr2))(py) (4), while condensation between the bis(mu-hydroxo)dipalladium complex (PdTp(iPr2))(2)(mu-OH)(2) (7) with 2a affords the unsymmetrical mu-kappa(1):kappa(2)-peroxo complex (Tp(iPr2))(py)Pd-OO-PdTp(iPr2) (5). These peroxopalladium complexes 2-5 have been fully characterized by a combination of spectroscopic and crystallographic analyses, which lead to description of the O-O moieties in these complexes as peroxide (O(2)(2-)) with sp(3)-hybridized oxygen atoms. The OOH moiety in 2b is found to interact with the noncoordinated nitrogen atom of the pendant pyrazolyl group through hydrogen bond. The O(2) moieties in 2-5 turn out to be so nucleophilic (basic) as to add across carbon-heteroatom multiple bonds in acetonitrile and acetaldehyde to give the peroxometallacycle Tp(iPr2)Pd[OOC(Me)=N)]Pd(iPr2)(py)(8) (from 2, 4, and 5) and the acetato complex (Tp(iPr2))(py)Pd-OC(=O)CH(3) (9) (from 2-4), respectively. Methyl vinyl ether and styrene, CH(2)=CHY (Y = OMe, Ph), are converted to the corresponding methyl ketones, CH(3)C(=O)Y, upon treatment with 2-4.

Structural characterization and intramolecular aliphatic C-H oxidation ability of M(III)(mu-O)2M(III) complexes of Ni and Co with the hydrotris-(3,5-dialkyl-4-X-pyrazolyl)borate ligands TpMe2,X (X = Me, H, Br) and TpiPr2

Reaction of the dinuclear M(II)-bis(mu-hydroxo) complexes of nickel and cobalt, [(M(II)(TpR)]2(mu-OH)2] (M = Ni; 3Ni M = Co: 3Co), with one equivalent of H2O2 yields the corresponding M(III)-bis(mu-oxo) complexes, [[M(III)(TpR)]2-(mu-O)2] (M=Ni; 2Ni, M=Co: 2Co). The employment of a series of TpMe2,X (TpMe2,X = hydrotris(3,5-dimethyl-4-X-1-pyrazolyl)borate; X = Me, H, Br) as a metal supporting ligand makes it possible to isolate and structurally characterize the thermally unstable M(III)-bis-(mu-oxo) complexes 2Ni and 2Co. Both the starting (3Ni and 3Co) and resulting complexes (2Ni and 2Co) contain five-coordinate metal centers with a slightly distorted square-pyramidal geometry. Characteristic features of the nickel complexes 2Ni, such as the two intense absorptions around 400 and 300 nm in the UV-visible spectra and the apparent diamagnetism, are very similar to those of the previously reported bis(mu-oxo) species of Cu(III) and Ni(III) with ligands other than TpR, whereas the spectroscopic properties of the cobalt complexes 2Co (i.e., paramagnetically shifted NMR signals and a single intense absorption appearing at 350 nm) are clearly distinct from those of the isostructural nickel compounds 2Ni. Thermal decomposition of 2Ni and 2Co results in oxidation of the inner saturated hydrocarbyl substituents of the TpR ligand. Large kH/kD values obtained from the first-order decomposition rates of the TpMe3 and Tp(CD3)2,Me derivatives of 2 evidently indicate that the rate-determining step is an hydrogen abstraction from the primary C-H bond of the methyl substituents. mediated by the M(III)2-(mu-O)2 species. The nickel complex 2Ni shows reactivity about 10(3) times greater than that of the cobalt analogue 2Co. The oxidation ability of the M(III)(mu-O)2M(III) core should be affected by the hindered TpR ligand system, which can stabilize the +2 oxidation state of the metal centers.