Pd-N Distances Research Articles

Palladium complexes bearing calixpyrrole ligands with pendant hydrogen bond donors: Synthesis, structural characterization, electrochemistry and dihydrogen evolution

A series of three calixpyrrole ligands (1a-c) with pendant nitrogen-based hydrogen bond donors, R, were synthesized and coordinated to palladium to produce metal complexes (2a-c), where R = NH2 (a); NHC(O)CH3 (b); or NHC(O)OC(CH3)3 (c). The calixpyrrole compounds were generated using a Schiff-base reaction starting with 5,5′-diformyl-2,2′-diphenyldipyrromethane and an aniline precursor. The deprotonated calixpyrrole species were subsequently bound to palladium to generate distorted square planar complexes. The pendant groups were not coordinated to the metal with Pd-N distances of 3.36 to 4.79 Å. The electrochemical properties of the palladium complexes were also explored, and 2a-c displayed two irreversible oxidations above 0.0 V vs ferrocene/ferrocenium (Fc/Fc+), as well as two irreversible reductions below −0.70 V vs Fc/Fc+. Interestingly, the free ligands showed similar electrochemical features, suggesting redox non-innocence. Preliminary reactivity studies indicated the palladium complexes did not activate small molecules, but they did catalyze H2 evolution in the presence of acid. The onset of catalysis for 2a-c was approximately 0.4–0.5 V more positive than a glassy carbon electrode, and the active species could undergo 500 scans without significant changes in activity. The catalysts were found to be heterogeneous in nature and adsorbed onto the working electrode.

Synthesis, structure and in vitro anticancer activity of Pd(II) complexes of mono- and bis-pyrazolyl-s-triazine ligands

The square planar complexes [Pd(MPT)Cl2] (1) and [Pd(BPT)Cl]ClO4 (2) were synthesized by the reaction of the 4,4′-(6-(3,5-dimethyl-1H-pyrazol-1-yl)-1,3,5-triazine-2,4-diyl)dimorpholine (MPT) and N-methyl-N-phenyl-4,6-di(1H-pyrazol-1-yl)-1,3,5-triazin-2-amine (BPT) ligands with PdCl2 (1:1) in acetone under thermal conditions, respectively. In complex 1, the Pd(II) ion is coordinated with the MPT ligand as a bidentate NN-chelate, augmented with two chloride ligands in cis positions. In complex 2, the Pd(II) ion is coordinated with the BPT ligand as a tridentate N-chelate in a pincer fashion, together with one chloride ligand. Hirshfeld analysis indicated that complex 1 is packed with a significant quantity of Cl…H (20.2%), O…H (6.8–8.1%) and N…H (10.8–11,7%) hydrogen bonds, as well as some CH…π (8.1–9.0%) interactions and C…O contacts (1.1–2.0%). On the other hand, O…H (13.8%), CH…π (16.5%) and anion-π stacking (C…O: 1.2%) are the most important interactions in 2. The atoms in molecules topological parameters correlate well with the Pd-N distances. In vitro anticancer experiments showed that both complexes have higher activity than their free ligands against MDA-MB-231 and MCF-7 cell lines. Complex 2 showed the highest cytotoxic activity with IC50 = 13.5 and 18.6 µg/mL against MDA-MB-231 and MCF-7, respectively, compared to 30.5 µg/mL for complex 1.

Synthesis and characterization of nano-peanuts of lead(II) coordination polymer [Pb(qcnh)(NO3)2]n with ultrasonic assistance: A new precursor for the preparation of pure-phase nano-sized PbO

A sonochemical method was used to synthesize nano-peanuts of a new lead(II) coordination 1D polymer, [Pb(qcnh)(NO3)2]n (1), where qcnh=2-quinolincarbaldehyde nicotinohydrazide. The compound was characterized by scanning electron microscopy (SEM), elemental analysis, IR spectroscopy, thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), X-ray powder diffraction (XRD), and single crystal X-ray analysis. The X-ray structure revealed that the Pb(II) atom is coordinated by one oxygen and three nitrogen atoms from two qcnh ligands and five oxygen atoms from three nitrate ligands in an 8+1 fashion with a PbN3O6 donor set. One of the PdN distances in the vicinity of the central atom is a bit longer (Pb1N1=2.939(4) Å), which shows the effect of the 6s2 lone electron pair localized within the valence shell of the lead(II) atom. PbO nanoparticles were obtained by thermolysis of 1 at 180°C with oleic acid as a surfactant. The average diameter of the nanoparticles was estimated by XRD to be 28nm. The morphology and size of the prepared PbO nanoparticles were further studied using SEM.

Palladium(II) complexes with highly basic imidazolin-2-imines and their reactivity toward small bio-molecules.

A series of novel Pd(ii) complexes with chelating mono(imidazolin-2-imine) and bis(imidazolin-2-imine) ligands were synthesized. The crystal structures of [Pd(DMEAIm(iPr))Cl2] and [Pd(DPENIm(iPr))Cl2] were determined by X-ray diffraction analysis. The reactivity of the six Pd(ii) complexes, namely, [Pd(en)Cl2], [Pd(EAIm(iPr))Cl2], [Pd(DMEAIm(iPr))Cl2], [Pd(DPENIm(iPr))Cl2], [Pd(BL(iPr))Cl2] and [Pd(DACH(Im(iPr))2)Cl2], were investigated. Spectrophotometric acid-base titrations were performed to determine the pKa values of the coordinated water molecules in [Pd(en)(H2O)2](2+), [Pd(EAIm(iPr))(H2O)2](2+), [Pd(DMEAIm(iPr))(H2O)2](2+), [Pd(DPENIm(iPr))(H2O)2](2+), [Pd(BL(iPr))(H2O)2](2+) and [Pd(DACH(Im(iPr))2)(H2O)2](2+). The substitution of the chloride ligands in these complexes by TU, l-Met, l-His and Gly was studied under pseudo-first-order conditions as a function of the nucleophile concentration and temperature using stopped-flow techniques; the sulfur-donor nucleophiles have shown better reactivity than nitrogen-donor nucleophiles. The obtained results indicate that there is a clear correlation between the nature of the imidazolin-2-imine ligands and the acid-base characteristics and reactivity of the resulting Pd(ii) complexes; the order of reactivity of the investigated Pd(ii) complexes is: [Pd(en)Cl2] > [Pd(EAIm(iPr))Cl2] > [Pd(DMEAIm(iPr))Cl2] > [Pd(DPENIm(iPr))Cl2] > [Pd(BL(iPr))Cl2] > [Pd(DACH(Im(iPr))2)Cl2]. The solubility measurements revealed good solubility of the studied imidazolin-2-imine complexes in water, despite the fact that these Pd(ii) complexes are neutral complexes. Based on the performed studies, three unusual features of the novel imidazolin-2-imine Pd(ii) complexes are observed, that is, good solubility in water, very low reactivity and high pKa values. The coordination geometries around the palladium atoms are distorted square-planar; the [Pd(DMEAIm(iPr))Cl2] complex displays Pd-N distances of 2.013(2) and 2.076(2) Å, while the [Pd(DPENIm(iPr))Cl2] complex displays similar Pd-N distances of 2.034(4) and 2.038(3) Å. The studied systems are of interest because little is known about the substitution behavior of imidazolin-2-imine Pd(ii) complexes with bio-molecules under physiological conditions.

Unexpected formation and crystal structure of tetra-kis-(1H-pyrazole-κN (2))-palladium(II) dichloride.

The title salt, [Pd(C3H4N2)4]Cl2, was obtained unexpectedly by the reaction of palladium(II) dichloride with equimolar amounts of 1-chloro-1-nitro-2,2,2-tris-(pyrazol-yl)ethane in methanol solution. The Pd(2+) cation is located on an inversion centre and has a square-planar coordination sphere defined by four N atoms of four neutral pyrazole ligands. The average Pd-N distance is 2.000 (2) Å. The two chloride anions are not coordinating to Pd(2+). They are connected to the complex cations through N-H⋯Cl hydrogen bonds. In addition, C-H⋯Cl hydrogen bonds are observed, leading to a three-dimensional linkage of cations and anions.

Syntheses, solid state and solution structures of the palladium(ii) complexes of malonamide-derived open-chain and macrocyclic ligands

The crystal structures of the palladium(II) complexes of the open-chain and macrocyclic ligands PdL(1).3H(2)O, PdL(2).6H(2)O and PdL(3).5H(2)O have been determined (H(2)L(1) = 1,4,8,11-tetraazaundecane-5,7-dione, H(2)L(2) = 1,4,8,11-tetraazacyclotetradecane-5,7-dione, H(2)L(3) = 1,4,8,11-tetraazacyclotridecane-5,7-dione). The coordination polyhedra of the palladium(II) ions in all complexes are formed by two deprotonated amide and two amine donors with Pd-N distances being similar in PdL(1) and PdL(2) and substantially shorter in PdL(3). A detailed analysis of the (1)H NMR spectra of the macrocyclic complexes supports the formation in aqueous solution of only N-meso isomers of both compounds in agreement with the X-ray data. The spectra of the palladium(II) macrocyclic complexes are shifted downfield as a whole as compared to those of the nickel(II) analogues with the shifts being essentially non-uniform. The latter feature can be related to the differences in magnetic anisotropy of the M-N bonds. The maxima of d-d absorption bands of the palladium(II) complexes demonstrate weaker dependence on the macrocycle size as compared to those of the nickel(II) analogues. Both macrocyclic compounds PdL(2).6H(2)O and PdL(3).5H(2)O are characterized by lamellar crystal structures consisting of interleaved layers formed by macrocyclic units and by water molecules with similar metal complex layers and different 2D water sheets. A columnar crystal structure is inherent for PdL(1).3H(2)O with the water molecules present as discrete (H(2)O)(3) clusters.

Synthesis, structural characterization, and catalytic behaviour in Heck coupling of palladium(ii) complexes containing pyrimidine-functionalized N-heterocyclic carbenes

[Pd(L1)2(CH3CN)](PF6)2 (L1 = 1-n-butyl-3-(2-pyrimidyl)imidazolylidene, 3) and [Pd(L2)2](PF6)2 (L2 = 1-(2-picolyl)-3-(2-pyrimidyl)imidazolylidene 4), prepared via carbene transfer reactions of [Ag(L1)2]PF6 (1) and [Ag2(L2)2](PF6)2 (2) with palladium salts, respectively, have been fully characterized by 1H and 13C NMR spectroscopy and elemental analysis. The X-ray crystal structures of complexes 1-4 are reported. Complex 3 is an unusual pentacoordinated palladium complex, in which the palladium is coordinated by two imidazolylidene, two pyridine, and one acetonitrile molecule in a square-pyramidal geometry. The apical position is occupied by a pyrimidine nitrogen atom with a relatively long Pd-N distance (2.762(6) angstroms). Complex is a typical square-planar palladium complex with palladium surrounded by two pairs of cis-arranged pyridine and imidazolylidene ligands. The complexes exhibit good catalytic activities in the Heck coupling reaction of aryl bromides and activated aryl chlorides under mild conditions.

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.

"Swollen" Macrocycles: Palladium(II)-Directed Template Syntheses of Pendant-Arm 14-, 16-, and 18-Membered Macrocycles.

The bis(diamine)palladium(II) cations (diamine = ethane-1,2-diamine, propane-1,3-diamine, or butane-1,4-diamine) all undergo condensation reactions with formaldehyde and nitroethane to produce macromonocycles where each pair of cis-disposed primary amines has been converted to a -NH-CH(2)-C(CH(3))(NO(2))-CH(2)-NH- strap. The 14-membered-ring macrocycle has been previously prepared by condensation around copper(II) and nickel(II), but this does not permit synthesis of the larger ring macrocycles. The macrocyclic complex (6,13-dimethyl-6, 13-dinitro-1,4,8,11-tetraazacyclotetradecane)palladium(II) perchlorate crystallizes in the triclinic space group P&onemacr;, a = 8.105(3) Å, b = 8.370(2) Å, c = 9.437(4) Å, alpha = 69.04(3) degrees, beta = 68.60(3), gamma = 71.53(3) degrees. Complexes of the 16- and 18-membered macrocycles (3,11-dimethyl-3,11-dinitro-1,5,9,13-tetraazacyclohexadecane)palladium(II) perchlorate and (3,12-dimethyl-3,12-dinitro-1,5,10,14-tetraaazacyclooctadecane)palladium(II) perchlorate crystallize in the monoclinic space group P2(1)/c, with a = 8.391(2) Å, b = 12.816(3) Å, c = 23.925(9) Å, and beta = 93.18(2) degrees, and the triclinic space group P&onemacr;, with a = 7.746(5) Å, b = 9.912(5) Å, c = 18.96(2) Å, alpha = 91.76(6) degrees, beta = 101.73(7) degrees, and gamma = 112.83(5) degrees respectively. The larger macrocycles are "swollen" by incorporating longer methylene chains, "swelling" leading to an increase in Pd-N distance and in tetrahedral distortion, with the dominant geometric isomer apparently changing with macrocycle size from anti-disposed nitro pendants (14-membered) to the syn isomer (16-, 18-membered). An irreversible Pd(II/IV) oxidation occurs at ca +1 V (vs Ag/AgCl), varying slightly with ring size, with a multi-electron nitro group reduction observed near -0.8 V in each case. Electronic spectra also vary slightly with ring size.

Bis(μ-trifluoroacetato-O,O')bis{[3-chloro-6-methyl-2-(N-methyl-N-nitrosoamino)phenyl-C,NN=O]palladium(II)}

The title compound, [Pd 2 (C 8 H 8 N 2 ClO) 2 (C 2 F 3 O 2 ) 2 ], contains a palladium(II) centre with pseudo square-planar coordination and a strong interaction with the N-donor atom, which results in a Pd-N distance of 1.938(7)A. The two coordination planes of the dinuclear complex are parallel and form a boat structure.

Electronic structure of transition-metal tetracoordinated complexes. 4. Theoretical ab initio and UV-photoelectron spectroscopy study of nickel(II) and palladium(II) complexes of N,N′-1,3-propaneaminebis(1H-pyrrol-2-ylmethylene) Schiff base

Structural and bonding properties of the Ni and Pd complexes of the title Schiff base have been studied with the aid of both the pseudopotential ab initio method, in partial geometry optimizations, for investigation of ground-state electronic structure and for evaluation of ionization energies, and of He I/He II photoelectron spectroscopy. Theoretical results have provided an accurate description of metal-ligand bonding. It appears dominated by σ interactions involving the upper lying MOs mainly N 2p lone pair in character and, though to a different extent, valence metal subshells. Despite the different functionalities existing in the ligand chromophore, comparable bonding properties, and hence similar Pd-N distances, have been found for the four nitrogen donor atoms. A theoretical explanation of this behaviour is provided. Low ionization energy structures of photoelectron spectra have been assigned in accordance with Δ SCF calculations.

Synthesis, reactivity, and structural studies in trimethylpalladium(IV) chemistry, including PdIMe3(bpy) and [MMe3((pz)3CH)]+ (M = palladium, platinum)

The first synthetic, spectroscopic, and structural studies in (hydrocarbyl)palladium(IV) chemistry are reported. The fac-Pd(IV)Me3 complexes PdIMe3(L2) and [PdMe3(L3)]I are formed on oxidative addition of iodomethane to PdMez2(L2) (L2 = 2,2'-bipyridyl, 1,l0-phenanthroline, other planar bidentates with pyridin-2-yl (py) and 1-methylimidazol-2-y1 (mim) groups) and PdMe2(L3) (L3 = tripodal nitrogen donors containing pyrazol-1-yl (pz), py, and mim groups). The complexes PdIMe3(L2) (L2 = bpy, phen) are stable on storage at ca. -20 C, but in solution reductive elimination of ethane occurs with formation of PdIMe(L2). Palladium(IV) complexes with the other bidentate ligands could be detected in situ by 1H NMR spectroscopy but could not be isolated. 1H NMR studies in CD3CN show that the bidentate ligand complexes form an equilibrium between PdIMe3(L2) and [PdMe3(L2)(CD3CN)]+ and oxidative addition of CD3I or Me1 to PdMe2(bpy) in (CD3)2C0 at low temperature indicates that a cation is formed prior to formation of PdIMe2R(bpy). The complex PdIMe3(bpy) has octahedral geometry, with Pd-I = 2.834 (1) A, Pd-N = 2.188 (5), 2.173 (5) A, and Pd-C = 2.034 (7)-2.046 (7) A. The isolated complexes of the tripod ligands, [PdMe3(L3)]I, are stable at ambient temperature. In solution, the complexes containing pyridine and 1-methylimidazole donor rings as part of the tripod ligand, e.g. [PdMe3((py)(mim)2CH)]I, are more stable than [PdMe3((pz)3CH)]I, although the tetrafluoroborate derivative [PdMe3((pz)3CH)]BF4 appears to be indefinitely stable. The isostructural complexes [MMe3((pz)3CH)]I have essentially identical M-C bond distances, but the Pd-N distances are ca. 0.04 A longer than Pt-N. Crystals of PdIMe3(bpy) are monoclinic, space group P2,/c, with a = 7.917(4) A, b = 9.528(4) A, c = 20.207(8) A, beta = 104.28(4), and Z = 4. The complexes [MMe3((pz)3CH)]I (M = Pd, Pt) are monoclinic, space group C2/c, with Z = 8. Palladium [platinum] complex cells have a = 21.254(8) [21.253(8) A, b = 9.213(5) [9.099(4) A, c = 19.144(8) [19.284(8) A, and beta = 105.65(3) [105.63(3)].