Pd-N Bond Research Articles

Mechanism of catalytic reduction of NO by H 2 or CO on a Pd foil; Role of chemisorbed nitrogen on Pd

The adsorption of NO and its reaction with H 2 over polycrystalline Pd were investigated using flash desorption technique and ultraviolet photoelectron spectroscopy under 10 −5 Pa pressure range of reactants and surface temperatures between 300 and 900 K. NO was adsorbed dissociatively onto the Pd surface above 500 K, and the heat of dissociative adsorption was ca. 126 kJ/mol. Atomic nitrogen was observed to accumulate on the Pd surface during the NO-H 2 reaction, whose desorption rate exhibited second order kinetics and is expressed as follows: V d = 10 −9.8 ± 0.3 exp(−67( kJ/ mol)/ RT) (cm 2/atom·s). Hydrogenation of the adsorbed nitrogen proceeded rapidly at 485 K. It was confirmed from these results that formation of N 2 and NH 3 in the NO-H 2 reaction proceeds through this atomically adsorbed nitrogen. Pd-N bond energy and enthalpies of some intermediate states of the NO-H 2 reaction were estimated.

Arylazo-oximates of ortho-palladated azobenzenes: synthesis, isomerism, and redox behaviour

The synthesis and characterisation of new planar mixed species of the type [PdLA](A =ortho-metallated azobenzene or its substituted derivative) is achieved by splitting the halogeno-bridge in [Pd2Cl2A2] by arylazo-oximes (HL). Triphenylphosphine cleaves the Pd-N(azo) bond in the PdL fragment, giving [PdLA(PPh3)]. Many of the [PdLA] species occur as isomeric mixtures whose compositions have been established using 1H n.m.r. data. The isomerism which originates from different modes of ortho-metallation of A is carried over to [PdLA] from the precursor complex [Pd2Cl2A2]. A simple and accurate 1H n.m.r. method of isomer analysis in [Pd2Cl2A2] is described. Substituent effects on isomer population are consistent with the electrophilic nature of the palladation reaction but steric effects also play dominant roles. The electrochemistry of [PdLA] has been thoroughly examined using cyclic voltammetry. Three couples are observed near –0.8, –1.2, and –1.5 V vs. standard calomel electrode, in acetonitrile. The first two couples correspond to successive one-electron reductions of the azo-functions of L and A respectively. The third couple is due to two-electron metal reduction.

Novel bonding modes of .alpha.-diimines. Synthesis and characterization of [MCl2L(.alpha.-diimine)] and [MCl2(.alpha.-diimine)n] (M = palladium, platinum; L = phosphine, arsine; n = 1, 2) containing .sigma.,.sigma.-N,N', .sigma.-N, or .sigma.-N .tautm. .sigma.-N' bonded .alpha.-diimines

Compounds of the type [PdX2(RN=CHCH=NR),] (n = 1,2; X = C1, Br, I) and [MCl,L(RN=CHCH=NR)] (L = monodentate phosphine or arsine; M = Pd, Pt; R = alkyl) have been synthesized starting from the a-diimine (R-dim) and [PdCI,(PhCN),] or [(MCl,L),], respectively. Three types of metal-a-diimine interactions have been established: (i) up-N,N' chelate bonded R-dim in [PdXz(R-dim)]; (ii) u-N monodentate bonded Rdim in monomeric tr~ns-[PdX~(R-dim)~] ('H and C NMR spectroscopy reveals that the R-dim ligand has the anti conformation with the C-C and Pd-N bonds of the coordinated part of the ligand in the cis configuration. This brings the @-imine proton (S(CH) 9.70) in the proximity of the palladium atom.); (iii) u-N - u-N' bonded R-dim in the monomeric complexes, tr~ns-[M~~Cl,L(R-dim)], which contain monodentate (u-N) bonded R-dim in the slow-exchange limit (-55 C). At 120 OC (fast-exchange limit) the complexes are fluxional by an intramolecular process which involves a pentacoordinate intermediate containing up-N,N' bonded R-dim. These novel bonding modes for the R-dim ligand in these palladium and platinum complexes are a clear indication of the versatile coordination properties of the R-dim ligand compared with ligands such as bpy and phen, which also contain the N=CC=N skeleton.