Reaction of molecular oxygen with a Pd(II)-hydride to produce a Pd(II)-hydroperoxide: experimental evidence for an HX-reductive-elimination pathway

The reaction of molecular oxygen with a Pd(II)-hydride species to form a Pd(II)-hydroperoxide represents one of the proposed catalyst reoxidation pathways in Pd-catalyzed aerobic oxidation reactions, but well-defined examples of this reaction were discovered only recently. Here, we present a mechanistic study of the reaction of O2 with trans-(IMes) 2Pd(H)(OBz), 1 (IMes = 1,3-dimesitylimidazol-2-ylidene), which yields trans-(IMes) 2Pd(OOH)(OBz), 2. The reaction was monitored by (1)H NMR spectroscopy in benzene-d6, and kinetic studies reveal a two-term rate law, rate = k1[1] + k2[1][BzOH], and a small deuterium kinetic isotope effect, k(Pd-H)/k(Pd-D) = 1.3(1). The rate is independent of the oxygen pressure. The data support a stepwise mechanism for the conversion of 1 into 2 consisting of rate-limiting reductive elimination of BzOH from 1 followed by rapid reaction of molecular oxygen with (IMes) 2Pd(0) and protonolysis of a Pd-O bond of the eta(2)-peroxo complex (IMes) 2Pd(O2). Benzoic acid and other protic additives (H2O, ArOH) catalyze the oxygenation reaction, probably by stabilizing the transition state for reductive elimination of BzOH from 1. This study provides the first experimental validation of the mechanism traditionally proposed for aerobic oxidation of Pd-hydride species.     

Kinetics and mechanism of the reactions of Pd(II) complexes with azoles and diazines. Crystal structure of [Pd(bpma)(H2O)](ClO4)2.2H2O

Substitution reactions of the complexes [Pd(bpma)(H2O)]2+, [Pd(bpma)Cl]+, [Pd(dien)(H2O)]2+ and [Pd(dien)Cl]+, where bpma = bis(2-pyridylmethyl)amine and dien = diethylentriamine or 1,5-diamino-3-azapentane, with some nitrogen-donor ligands such as triazole, pyrazole, pyrimidine, pyrazine and pyridazine, were studied in an aqueous 0.10 M NaClO4 at pH 2.8 using variable-temperature and -pressure stopped-flow spectrophotometry. The second-order rate constants indicate that the Pd(II) complexes of bpma, viz. [Pd(bpma)(H2O)]2+ and [Pd(bpma)Cl]+, are more reactive than the complexes of dien, viz. [Pd(dien)(H2O)]2+ and [Pd(dien)Cl]+. Also, the aqua complexes, [Pd(bpma)(H2O)]2+ and [Pd(dien)(H2O)]2+, are much more reactive than the corresponding chloro complexes. The most reactive nucleophile of the five-membered rings is triazole and for the six-membered rings the most reactive one is pyridazine. Activation parameters were determined for all reactions and the negative entropies and volumes of activation (Delta S++, Delta V++) support an associative ligand substitution mechanism. The crystal structure of [Pd(bpma)(H2O)](ClO4)2.2H2O was determined by X-ray diffraction. Crystals are monoclinic with the space group P2(1)/c. The coordination geometry of [Pd(bpma)(H2O)]2+ is distorted square-planar. The Pd-N (central) bond distance, 1.958(5) A, is shorter than the other two Pd-N distances, 2.007(5) and 2.009(5) A. The Pd-O distance is 2.043(5) A.     

X-ray structure characterization of palladium(II) ternary complexes of pyridinedicarboxylic and phthalic acid with phenanthroline and bipyridine

The crystal structures of the series of four ternary complexes, [Pd(phen)(2,6-PDCA)].4H(2)O (1) (phen=1,10-phenanthroline; 2,6-PDCA=2,6-pyridinedicarboxylic acid), [Pd(bpy)(2,3-PDCA)].3H(2)O (2) (bpy=2,2'-bipyridineand; 2,3-PDCA=2,3-pyridinedicarboxylic acid) and [Pd(phen)(PHT)].2.5H(2)O (3) (PHT=o-phthalic acid ) and [Pd(bpy)(PHT)].1.5H(2)O (4), are determined and the coordination modes of palladium(II) ternary complexes are characterized. All complexes take the mononuclear Pd(II) complexes, in which central Pd(II) atom of each complex has a similar distorted square-planar four coordination geometry. In all complexes, the aromatic heterocyclic compounds, phen and bpy, behave as a bidentate N, N' ligand. In the complex 1 and 2, 2,6-PDCA and 2,3-PDCA behave as a bidentate N, O ligand, and in complex 3 and 4, PHT behaves as a bidentate O, O' ligand.     

Providing support in favor of the existence of a Pd(II)/Pd(IV) catalytic cycle in a Heck-type reaction

The complex [Pd(O,N,C-L)(OAc)], in which L is a monoanionic pincer ligand derived from 2,6-diacetylpyridine, reacts with 2-iodobenzoic acid at room temperature to afford the very stable pair of Pd(IV) complexes (OC-6-54)- and (OC-6-26)-[Pd(O,N,C-L)(O,C-C(6)H(4)CO(2)-2)I] (1.5:1 molar ratio, at -55?°C). These complexes and the Pd(II) species [Pd(O,N,C-L)(OX)] and [Pd(O,N,C-L')(NCMe)]ClO(4), (X = MeC(O) or ClO(3), L' = another monoanionic pincer ligand derived from 2,6-diacetylpyridine), are precatalysts for the arylation of CH(2)=CHR (R = CO(2)Me, CO(2)Et, Ph) using IC(6)H(4)CO(2)H-2 and AgClO(4). These catalytic reactions have been studied and a tentative mechanism is proposed. The presence of two Pd(IV) complexes was detected by ESI(+)-MS during the catalytic process. All the data obtained strongly support a Pd(II)/Pd(IV) catalytic cycle.     

Kinetic and mechanistic study on the reactions of [Pt(bpma)(H2O)]2+ and [Pd(bpma)(H2O)]2+ with some nucleophiles. Crystal structure of [Pd(bpma)(py)](ClO4)2

Substitution reactions of the complexes [Pd(bpma)(H2O)]2+ and [Pt(bpma)(H2O)]2+, where bpma = bis(2-pyridylmethyl)amine, with TU, DMTU and TMTU for both complexes and Cl-, Br-, I- and SCN- for the platinum complex, were studied in aqueous 0.10 M NaClO4 at pH 2.5 using a variable-temperature stopped-flow spectrophotometer. The pKa value for the coordinated water molecule in [Pd(bpma)(H2O)]2+ (6.67) is a unit higher than that of [Pt(bpma)(H2O)]2+. The observed pseudo-first-order rate constants k(obs) (s(-1)) obeyed the equation k(obs) = k2[Nu] (Nu = nucleophile). The second-order rate constants indicate that the Pd(II) complex is a factor of 10(3) more reactive than Pt(II) complex. The nucleophile reactivity attributed to the steric hindrance in case of TMTU and the inductive effect for DMTU was found to be DMTU > TU > TMTU for [Pt(bpma)(H2O)]2+ and DMTU approximately TU > TMTU for [Pd(bpma)(H2O)]2+. The trend for ionic nucleophile was I- > SCN- > Br- > Cl-, an order linked to their polarizability and the softness or hardness of the metal. Activation parameters were determined for all reactions and the negative entropies of activation (Delta S++) support an associative ligand substitution mechanism. The X-ray crystal structure of [Pd(bpma)(py)](ClO4)2 was determined; it belongs to the triclinic space group P1 and has one formula unit in the unit cell. The unit cell dimensions are a = 8.522(2), b = 8.627(2), c = 16.730(4) A; alpha = 89.20(2), beta = 81.03(2), gamma = 60.61(2) degrees ; V = 1055.7(5) A3. The structure was solved using direct methods in WinGX's implementation of SHELXS-97 and refined to R = 0.054. The coordination geometry of [Pd(bpma)(py)]2+ is distorted square-planar. The Pd-N(central) bond distance, 1.996(3) A, is shorter than the other two Pd-N distances, 2.017(3) and 2.019(3) A. The Pd-N(pyridine) distance is 2.037(3) A.     

Reactions of carbon monoxide with free palladium oxide clusters: strongly size dependent competition between adsorption and combustion

The gas phase reactions of carbon monoxide with small mass-selected clusters of palladium, Pd(x)(+) (x = 2-7), and their oxides, Pd(x)O(+) (x = 2-7) and Pd(x)O(2)(+) (x = 4-6), have been investigated in a radio frequency ion trap operated under multi-collision conditions. The bare palladium clusters were found to readily adsorb CO yielding a highly size dependent product pattern. Most interestingly, the reactions of the pre-oxidized palladium clusters with CO lead to very similar product distributions of Pd(x)(CO)(z)(+) complexes as in the case of the corresponding pure Pd(x)(+) clusters. Consequently, it has been concluded that the investigated palladium oxide clusters efficiently oxidize CO under formation of the bare clusters, which further adsorb CO molecules yielding the previously observed Pd(x)(CO)(z)(+) product complex distributions. This CO combustion reaction has been observed even at temperatures as low as 100 K. However, for Pd(2)O(+), Pd(6)O(+), Pd(6)O(2)(+), and Pd(7)O(+) a competing reaction channel yielding palladium oxide carbonyls Pd(x)O(CO)(z)(+) could be detected. The latter adsorption reaction may even hamper the CO combustion under certain reaction conditions and indicates enhanced activation barriers involved in the CO oxidation and/or the CO(2) elimination process on these clusters.     

Reactivity of ammonia ligands of the antitumor agent cisplatin: a unique dodecanuclear Pt4,Pd4,Ag4 platform for four cytosine model nucleobases

The reaction of a potential mono(nucleobase) model adduct of cisplatin, cis-[Pt(NH(3))(2)(1-MeC-N3)(H(2)O)](2+) (6; 1-MeC: 1-methylcytosine), with the electrophile [Pd(en)(H(2)O)(2)](2+) (en: ethylenediamine) at pH approximately 6 yields a kinetic product X which is likely to be a dinuclear Pt,Pd complex containing 1-MeC(-)-N3,N4 and OH bridges, namely cis-[Pt(NH(3))(2)(1-MeC(-)-N3,N4)(OH)Pd(en)](2+). Upon addition of excess Ag(+) ions, conversion takes place to form a thermodynamic product, which, according to (1)H NMR spectroscopy and X-ray crystallography, is dominated by a mu-NH(2) bridge between the Pt(II) and Pd(II) centers. X-ray crystallography reveals that the compound crystallizes out of solution as a dodecanuclear complex containing four Pt(II), four Pd(II), and four Ag(+) entities: [{Pt(2)(1-MeC(-)-N3,N4)(2)(NH(3))(2)(NH(2))(2)(OH)Pd(2)(en)(2)Ag}(2){Ag(H(2)O)}(2)](NO(3))(10) 6 H(2)O (10) is composed of a roughly planar array of the 12 metal ions, in which the metal ions are interconnected by mu-NH(2) groups (between Pt and Pd centers), mu-OH groups (between pairs of Pt atoms), and metal-metal donor bonds (Pt-->Ag, Pd-->Ag). The four 1-methylcytosinato ligands, which are stacked pairwise, as well as the four NH(3) ligands and parts of the en rings, are approximately perpendicular to the metal plane. Two of the four Ag ions (Ag2, Ag2') of 10 are labile in solution and show the expected behavior of Ag(+) ions in water, that is, they are readily precipitated as AgCl by Cl(-) ions. The resulting pentanuclear complex [Pt(2)Pd(2)Ag(1-MeC(-))(2)(NH(2))(2)(OH)(NH(3))(2)(en)(2)](NO(3))(4)7 H(2)O (11) largely maintains the structural features of one half of 10. The other two Ag(+) ions (Ag1, Ag1') of 10 are remarkably unreactive toward excess NaCl. In fact, the pentanuclear complex [Pt(2)Pd(2)AgCl(1-MeC(-))(2)(NH(2))(2)(OH)(NH(3))(2)(en)(2)](NO(3))(3)4.5 H(2)O (12), obtained from 10 with excess NaCl, displays a Cl(-) anion bound to the Ag center (2.459(3) A) and is thus a rare case of a crystallized "AgCl molecule".     

Isomerization reaction of head-to-head alpha-pyridonato-bridged ethylenediaminepalladium(II) binuclear complex, [Pd2(en)2(C5H4NO)2]2+, in aqueous solution

Head-to-head bis(alpha-pyridonato)-bridged bis(ethylenediamine)dipalladium(ii), HH-[Pd(2)(en)(2)(alpha-pyridonato)(2)](ClO(4))(2), was synthesized and structurally characterized by X-ray crystallography. The (1)H NMR spectra show that the head-to-head (HH) dimer produces the head-to-tail (HT) dimer and monomers ([Pd(en)(alpha-pyridone)(2)](2+), [Pd(en)(H(2)O)(alpha-pyridone)](2+), [Pd(en)(H(2)O)(2)](2+), etc.) in aqueous solution, and the relative amount of dimers to monomers is dependent on the total concentration of the HH dimer dissolved as well as the acidity of the solution. It was found that the formation of the HH and HT dimers from the monomers is fast, and the HT dimer is produced from the HH dimer only via coexisting monomers, i.e., there is no direct isomerization path between the HH and HT dimers. The kinetic analyses for the HH <==>HT isomerization reaction with time-resolved (1)H NMR measurements revealed that the reaction proceeds via first-order kinetics, which was explained based on a relaxation process. The rate determining step for HH <==>HT isomerization is the reaction step between the mono-alpha-pyridone complex and the bis-alpha-pyridone complex, [Pd(en)(H(2)O)(alpha-pyridone)](2+)+alpha-pyridone <==> [Pd(en)(alpha-pyridone)(2)](2+).     

Density functional theory analysis of the reaction pathway for methane oxidation to acetic acid catalyzed by Pd2+ in sulfuric acid

Density functional theory has been used to investigate the thermodynamics and activation barriers associated with the direct oxidation of methane to acetic acid catalyzed by Pd2+ cation in concentrated sulfuric acid. Pd2+ cations in such solutions are ligated by two bisulfate anions and by one or two molecules of sulfuric acid. Methane oxidation is initiated by the addition of CH4 across one of the Pd-O bonds of a bisulfate ligand to form Pd(HSO4)(CH3)(H2SO4)2. The latter species will react with CO to produce Pd(HSO4)(CH3CO)(H2SO4)2. The most likely path to the final products is found to be via oxidation of Pd(HSO4)(CH3)(H2SO4)2 and Pd(HSO4)(CH3CO)(H2SO4)2 to form Pd(eta2-HSO4)(HSO4)2(CH3)(H2SO4) and Pd(eta2-HSO4)(HSO4)2(CH3CO)(H2SO4), respectively. CH3HSO4 or CH3COHSO4 is then produced by reductive elimination from the latter two species, and CH(3)COOH is then formed by hydrolysis of CH3COHSO4. The loss of Pd2+ from solution to form Pd(0) or Pd-black is predicted to occur via reduction with CO. This process is offset, though, by reoxidation of palladium by either H2SO4 or O2.     

Pentafluorophenyl imidato palladium(II) complexes: catalysts for Suzuki cross-coupling reactions

Novel N-bonded imidato complexes of general formula [Pd(N-N)(C6F5)(imidate)](imidate = maleimidate, succinimidate or phthalimidate; N-N = 2,2'-bipyridine (bipy), 4,4'-dimethyl-2,2'-bipyridine (Me2bipy) or N,N,N',N'-tetramethylethylenediamine (tmeda)), [NBu4][Pd(C6F5)(H2O)(succinimidate)2] and [NBu4][Pd(C6F5)(L)(succinimidate)2](L = PPh3 or t-BuNC) have been synthesised. These complexes are air-, light- and moisture-stable. The crystal structures of [Pd(tmeda)(C6F5)(maleimidate)].H2O.0.5CHCl3, [NBu4][Pd(C6F5)(H2O)(succinimidate)2].H2O and [NBu4][Pd(C6F5)(t-BuNC)(succinimidate)2].2H2O have been determined by X-ray diffraction. Many of these new complexes are shown to be active phosphine-free palladium catalysts/precatalysts for the Suzuki cross-coupling reactions of aryl bromides and aryl chlorides with phenylboronic acid.     

Controlled synthesis of hydroxyapatite-supported palladium complexes as highly efficient heterogeneous catalysts

Achieving precise control of active species on solid surfaces is one of the most important goals in the development of highly functionalized heterogeneous catalysts. The treatment of hydroxyapatites with PdCl(2)(PhCN)(2) gives two new types of hydroxyapatite-bound Pd complexes. Using the stoichiometric hydroxyapatite, Ca(10)(PO(4))(6)(OH)(2), we found that monomeric PdCl(2) species can be grafted on its surface, which are easily transformed into Pd(0) particles with narrow size distribution in the presence of alcohols. Such metallic Pd species can effectively promote alcohol oxidation using molecular oxygen and are shown to give a remarkably high TON of up to 236 000. Another monomeric Pd(II) phosphate complex can be generated at a Ca-deficient site of the nonstoichiometric hydroxyapatite, Ca(9)(HPO(4))(PO(4))(5)(OH), affording a catalyst with Pd(II) structure and high activity for the Heck and Suzuki reactions. To the best of our knowledge, the PdHAP are one of the most active heterogeneous catalysts for both alcohol oxidation under an atmospheric O(2)() pressure and the Heck reaction reported to date. These Pd catalysts are recyclable in the above organic reactions. Our approach to catalyst preparation based on the control of Ca/P ratios of hydroxyapatites represents a particularly attractive method for the nanoscale design of catalysts.     

Stoichiometric reactions of methylparathion with a palladium aryl oxime metallacycle

The reaction of [Pd(3)(OAc)(6)] with (E)-acetophenone oxime and pyridine in CHCl(3) under reflux affords the metallacycle [Pd(OAc)[C,N-(C(6)H(4)C(CH(3))=NOH)-2](py)] (1) as a yellow air-stable complex. The same reaction carried out at room temperature in the absence of pyridine affords the trinuclear oximato complex [Pd(mu-(E)-ON=C(CH(3))Ph)(mu-OAc)](3) (2), which can be converted into 1 upon heating in the presence of pyridine. As indicated by (1)H and (31)P NMR spectroscopy, complex 1 reacts with methylparathion in acetone-d(6)-D(2)O solutions to afford [Pd(SP(=O)(OCH(3))(2))[C,N-(C(6)H(4)C(CH(3))=NOH)-2](py)] (3) and [Pd(mu-SP(=O)(OCH(3))(2))[C,N-(C(6)H(4)C(CH(3))=NOH)-2]](2) (4) as well as free p-nitrophenol. Compounds 1-4 have been characterized by single-crystal X-ray analysis, NMR and EA. Compounds 1 and 3 are mononuclear complexes with the acetate and dimethylthiophosphate ligand, respectively, trans from the phenyl group. Compound 2 is a trinuclear complex whose structure can be derived from that of [Pd(3)(OAc)(6)] by replacing three of the acetate ligands on one side of Pd(3) plane by three N,O-coordinated oximate ligands. Complex 4 is a dinuclear complex in which the two square-planar palladium moieties are linked by the sulfur atoms of the bridging dimethylthiophosphate ligands.     

Chiral bisphosphinite metalloligands derived from a P-chiral secondary phosphine oxide

Interaction of PdCl(2)(MeCN)(2) with 2 equiv of (S(P))-(t)BuPhP(O)H (1H) followed by treatment with Et(3)N gave [Pd((1)(2)H)](2)(micro-Cl)(2) (2). Reaction of 2 with Na[S(2)CNEt(2)] or K[N(PPh(2)S)(2)] afforded Pd[(1)(2)H](S(2)CNEt(2)) (3) or Pd[(1)(2)H)[N(PPh(2)S)(2)] (4), respectively. Treatment of 3 with V(O)(acac)(2) (acac = acetylacetonate) and CuSO(4) in the presence of Et(3)N afforded bimetallic complexes V(O)[Pd(1)(2)(S(2)CNEt(2))](2) (5) or Cu[Pd(1)(2)(S(2)CNEt(2))](2) (6), respectively. X-ray crystallography established the S(P) configuration for the phosphinous acid ligands in 3 and 6, indicating that 1H binds to Pd(II) with retention of configuration at phosphorus. The geometry around Cu in 6 is approximately square planar with the average Cu-O distance of 1.915(3) A. Treatment of 2 with HBF(4) gave the BF(2)-capped compound [Pd((1)(2)BF(2))](2)(micro-Cl)(2) (7). The solid-state structure of 7 containing a PdP(2)O(2)B metallacycle has been determined. Chloride abstraction of 7 with AgBF(4) in acetone/water afforded the aqua compound [Pd((1)(2)BF(2))(H(2)O)(2)][BF(4)] (8) that reacted with [NH(4)](2)[WS(4)] to give [Pd((1)(2)BF(2))(2)](2)[micro-WS(4)] (9). The average Pd-S and W-S distances in 9 are 2.385(3) and 2.189(3) A, respectively. Treatment of [(eta(6)-p-cymene)RuCl(2)](2) with 1H afforded the phosphinous acid adduct (eta(6)-p-cymene)RuCl(2)(1H) (10). Reduction of [CpRuCl(2)](x)() (Cp = eta(5)-C(5)Me(5)) with Zn followed by treatment with 1H resulted in the formation of the Zn(II) phosphinate complex [(CpRu(eta(6)-C(6)H(5)))(t)BuPO(2))](2)(ZnCl(2))(2) (11) that contains a Zn(2)O(4)P(2) eight-membered ring.     

Different rotamer states of cytosine nucleobases in heteronuclear PtPd-, PtPd2, and Pt2Pd2Ag complexes derived from [Pt(2,2'-bpy)(1-MeC-N3)2]2+ (1-MeC = 1-methylcytosine): first examples of species with head-head oriented 1-MeC(-) ligands

[Pt(2,2'-bpy)(1-MeC-N3)(2)](NO(3))(2) (1) (2,2'-bpy = 2,2'-bipyridine; 1-MeC = 1-methylcytosine) exists in water in an equilibrium of head-tail and head-head rotamers, with the former exceeding the latter by a factor of ca. 20 at room temperature. Nevertheless, 1 reacts with (en)Pd(II) (en = ethylenediamine) to give preferentially the dinuclear complex [Pt(2,2'-bpy)(1-MeC(-)-N3,N4)(2)Pd(en)](NO(3))(2)·5H(2)O (2) with head-head arranged 1-methylctosinato (1-MeC(-)) ligands and Pd being coordinated to two exocyclic N4H(-) positions. Addition of AgNO(3) to a solution of 2 leads to formation of a pentanuclear chain compound [{Pt(2,2'-bpy)(1-MeC(-))(2)Pd(en)}(2)Ag](NO(3))(5)·14H(2)O (5) in which Ag(+) cross-links two cations of 2 via the four available O2 sites of the 1-MeC(-) ligands. 2 and 5 appear to be the first X-ray structurally characterized examples of di- and multinuclear complexes derived from a Pt(II) species with two cis-positioned cytosinato ligands adopting a head-head arrangement. (tmeda)Pd(II) (tmeda = N,N,N',N'-tetramethylethylenediamine) and (2,2'-bpy)Pd(II) behave differently toward 1 in that in their derivatives the head-tail orientation of the 1-MeC(-) nucleobases is retained. In [Pt(2,2'-bpy)(1-MeC(-))(2){Pd(2,2'-bpy)}(2)](NO(3))(4)·10H(2)O (4), both (2,2'-bpy)Pd(II) entities are pairwise bonded to N4H(-) and O2 sites of the two 1-MeC(-) rings, whereas in [Pt(2,2'-bpy)(1-MeC(-))(2){Pd(tmeda)}(2)(NO(3))](NO(3))(3)·5H(2)O (3) only one of the two (tmeda)Pd(II) units is chelated to N4H(-) and O2. The second (tmeda)Pd(II) is monofunctionally attached to a single N4H(-) site. On the basis of these established binding patterns, ways to the formation of mixed Pt/Pd complexes and possible intermediates are proposed. The methylene protons of the en ligand in 2 are special in that they display two multiplets separated by 0.64 ppm in the (1)H NMR spectrum.     

Sterically hindered electron-withdrawing ligands: the reactions of N-carbazolyl phosphines with rhodium and palladium centres

The series of N-carbazolyl phosphines PPh(3-n)(NC(12)H(8))(n)(n= 1, L1; n= 2, L2; n= 3, L3) has been synthesised using BuLi to generate the N-carbazolyl lithium salt, followed by reaction with the appropriate chlorophosphine. The reactions between [Rh(mu-Cl)(CO)(2)](2) and four equivalents of L1 or L2 gave [RhCl(CO)(L1)(2)] 1 and [RhCl(CO)(L2)(2)] 2, though attempts to synthesise the analogous complex using L3 resulted in the formation of [Rh(mu-Cl)(CO)(L3)](2) 3 instead. The inability of L3 to cleave the chloride bridges can be related to its considerable steric requirements. The electronic properties of L1-3 were assessed by comparison of the nu(CO) values of the [Rh(acac)(CO)(L1-3)] complexes 4-6. The increase in number of N-carbazolyl substituents at the phosphorus atom results in a decrease of the sigma-donor and increase in the pi-acceptor character in the order L1 < L2 < L3. In the reactions of L1-3 with [PdCl(2)(cod)] only L1 was able to displace cod from the metal centre and form [PdCl(2)(L1)(2)] 7. The use of [PdCl(2)(NCMe)(2)] instead of [PdCl(2)(cod)] resulted in the formation of the complexes [PdCl(2)(L1)(2)] 7 from L1, the cyclometallated complex [Pd(mu-Cl)[P(NC(12)H(8))(2)(NC(12)H(7))-kappa(2)P,C]](2) 8 from L3 , and a mixture of [PdCl(2)(L2)(2)] 9 and [Pd(mu-Cl)[PPh(NC(12)H(8))(NC(12)H(7))-kappa(2)P,C]](2) 10 from L2 . The reaction of L3 with [Pd(OAc)(2)] produced the cyclometallated complex [Pd(mu-O(2)CCH(3))[P(NC(12)H(8))(2)(NC(12)H(7))-kappa(2)P,C]](2) 11. The reaction of L3 with [Pd(2)(dba)(3)].CHCl(3) produced the 14-electron complex [Pd(L3)(2)] 12. The X-ray crystal structures of six complexes are reported, all of which show the presence of C-H...Pd hydrogen bonding.     

Color tuning of a nickel complex with a novel N2S2 pyridine-containing macrocyclic ligand

The novel pyridine-containing 14-membered macrocycle 3,11-dithia-7,17-diazabicyclo[11.3.1]heptadeca-1(17),13,15-triene (L), which contains an N2S2 donor set, was synthesized, and its protonation behavior was studied by absorption titration with CH3SO3H. The reaction of L with Pd(II) was studied spectroscopically, and the square-planar complex [Pd(L)](BF4) was isolated and characterized. The reactions between L and NiX2 x 6 H2O (X = BF4, ClO4) in ethanol or acetonitrile afforded the octahedral complexes [Ni(CH3CN)(H2O)(L)](X)2 and [Ni(H2O)2(L)](X)2, respectively. The square-planar complexes [Ni(L)](X)2 were obtained by heating these octahedral complexes. Spectrophotometric titrations of [Ni(L)](BF4)2 were performed with neutral and negatively charged ligands. The color of nitromethane solutions of this square-planar complex turns from red to cyan, purple, blue, yellow-green, and pink following addition of halides, acetonitrile, water, pyridine, and 2,2'-bipyridine, respectively. X-ray structural analyses were carried out on the {[Ni(ClO4)(H2O)(L)][Ni(H2O)2(L)]}(ClO4)3, [Ni(CH3CN)(H2O)(L)](ClO4)2, [{Ni(L)}2(mu-Cl)2](ClO4)2, and [{Ni(L)}2(mu-Br)2]Br2 x 2 CH3NO2 complexes.     

N,N-Dialkyl-2-iodoanilines: a versatile source for the synthesis of Pd(ii) complexes. Synthesis of novel OCP- and CCN-pincer palladium complexes

The reactivity of a series of N,N-dimethyl-2-iodoanilines bearing different chelating "arms" at the 3-position with Pd(2)(dba)(3) has been explored. 3-[(Diphenylphosphino)methyl]-2-iodo-N,N-dimethylaniline reacted with Pd(2)(dba)(3) and PPh(3) under aerobic conditions to give the OCP-pincer complex , which was formed by sequential C(sp(3))-H activation/oxidation at the alpha-position of the aniline N atom. On the other hand, under similar reaction conditions, 3-[2-(dimethylamino)ethyl]-2-iodo-N,N-dimethylaniline afforded the CCN-pincer complex , after a second C-H activation process at the formyl group of the initially formed OCN-pincer complex. In contrast, 2-iodo-3-(1H-1,2,4-triazol-1-ylmethyl)-N,N-dimethylaniline and 2-iodo-3-(pyrazol-1-ylmethyl)-N,N-dimethylaniline reacted with Pd(2)(dba)(3) and PPh(3), respectively, to give the 6-membered azapalladacycles and , in which the aniline nitrogen is merely a spectator substituent. Finally, treatment of iodide complex with Tl(TfO) afforded the CN-bidentate cationic complex. Solid-state structures of palladium complexes, and CH(2)Cl(2).3CH(3)OH.5H(2)O were determined by X-ray analysis.     

Autocatalytic oxidative addition of PhBr to Pd(PtBu3)2 via Pd(PtBu3)2(H)(Br)

We report that oxidative addition of bromobenzene to Pd(PtBu3)2 occurs by an unusual autocatalytic mechanism. Studies on the effect of various additives showed that the degree of rate acceleration followed the trend: (PtBu3)Pd(Ph)(Br) approximately (HPtBu3)Br < [(PtBu3)Pd(mu-Br)]2 < (PtBu3)2Pd(H)(Br). Studies on the reactions of Pd(PtBu3)2 in the presence of (PtBu3)2Pd(H)(Br) showed that the concentration of (PtBu3)2Pd(H)(Br) decreased only after the Pd(0) complex had been consumed. These data indicated that the catalyst in this process is (PtBu3)2Pd(H)(Br). Thermal decomposition of the three-coordinate oxidative addition product (PtBu3)Pd(Ar)(Br) during the reaction of Pd(PtBu3)2 and bromoarenes ultimately leads to formation of (PtBu3)2Pd(H)(Br). Parallel reactions of bromobenzene with (PtBu3)2Pd(H)(Br) and Pd(PtBu3)2 showed that the bromoarenes reacted considerably faster with the Pd(II) species than with the Pd(0) species. We therefore propose a catalytic cycle for oxidative addition in which PBut3.HBr reacts with the Pd(0) species to form (PtBu3)2Pd(H)(Br), and (PtBu3)2Pd(H)(Br) reacts with the bromoarene, possibly though the anionic species [HPtBu3+][(PtBu3)Pd(Br)-], to form [Pd(PtBu3)(Ar)(Br)].     

A study of the reactions of methionine- and histidine-containing peptides with palladium(II) complexes: The key role of steric crowding on palladium(II) in the selective cleavage of the peptide bond

¹H NMR spectroscopy was applied to study the reactions of palladium(II) complexes, cis-[Pd(dpa)Cl₂] and cis-[Pd(dpa)(H₂O)₂]²⁺ (dpa is 2,2′-dipyridylamine acting as a bidentate ligand) with the dipeptides methionylglycine (Met-Gly) and histidylglycine (His-Gly), and the N-acetylated derivatives of these dipeptides, MeCOMet-Gly and MeCOHis-Gly. All reactions were carried out in the pH range 2.0–2.5 with equimolar amounts of the palladium(II) complex and the peptide at two different temperatures, 25 and 60 °C. In the reactions of cis-[Pd(dpa)Cl₂] and cis-[Pd(dpa)(H₂O)₂]²⁺ with Met-Gly and His-Gly, no hydrolysis of the peptide bond was observed. The final product in these reactions was the [Pd(dpa)₂]²⁺ complex. The square-planar structure of this complex was confirmed by X-ray analysis. The reaction of the cis-[Pd(dpa)(H₂O)₂]²⁺ complex with the MeCOHis-Gly and MeCOMet-Gly peptides under the previously mentioned experimental conditions was remarkably selective in the cleavage of the amide bond involving the carboxylic group of methionine in the side chain. The modes of coordination of cis-[Pd(dpa)Cl₂] and cis-[Pd(dpa)(H₂O)₂]²⁺ in the reactions with the non-acetylated peptides and the total steric inhibition of the hydrolytic reaction between cis-[Pd(dpa)(H₂O)₂]²⁺ and MeCOHis-Gly can be attributed to the steric bulk of the palladium(II) complex. This finding should be taken into consideration in designing new palladium(II) complexes for the regioselective cleavage of peptides and proteins.     

Kinetics and mechanism of the oxidation of hydroquinones by a trans-dioxoruthenium(VI) complex

The kinetics of the oxidation of hydroquinone (H(2)Q) and its derivatives (H(2)Q-X) by trans-[Ru(VI)(tmc)(O)(2)](2+) (tmc = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) have been studied in aqueous acidic solutions and in acetonitrile. In H(2)O, the oxidation of H(2)Q has the following stoichiometry: trans-[Ru(VI)(tmc)(O)(2)](2+) + H(2)Q --> trans-[Ru(IV)(tmc)(O)(OH(2))](2+) + Q. The reaction is first order in both Ru(VI) and H(2)Q, and parallel pathways involving the oxidation of H(2)Q and HQ(-) are involved. The kinetic isotope effects are k(H(2)O)/k(D(2)O) = 4.9 and 1.2 at pH = 1.79 and 4.60, respectively. In CH(3)CN, the reaction occurs in two steps, the reduction of trans-[Ru(VI)(tmc)(O)(2)](2+) by 1 equiv of H(2)Q to trans-[Ru(IV)(tmc)(O)(CH(3)CN)](2+), followed by further reduction by another 1 equiv of H(2)Q to trans-[Ru(II)(tmc)(CH(3)CN)(2)](2+). Linear correlations between log(rate constant) at 298.0 K and the O-H bond dissociation energy of H(2)Q-X were obtained for reactions in both H(2)O and CH(3)CN, consistent with a H-atom transfer (HAT) mechanism. Plots of log(rate constant) against log(equilibrium constant) were also linear for these HAT reactions.