Synthesis, Structure, and Electrochemistry of Mononuclear and Face-to-Face Binuclear Orthometalated Complexes of Palladium(II) with N-Monodentate or N(1),N(3)-Bridging 1,3-Di-p-tolyltriazenido Ligands. Dependence on Geometrical Arrangement of the Electronic Communication between Two Equivalent Redox Sites

Abstract

The synthesis, electrochemistry, and structural characterization of the mononuclear complex [Pd{C6H4N(H)NC(CH3)C5H4N}(p-tolN−NNp-tol)] (1) containing the monodentate 1,3-di-p-tolyltriazenido ligand is described. Compound 1 is an example of a stable species containing a Pd−N amido bond cis to a Pd−C aryl bond. Kinetic parameters for the dynamic intramolecular N(1)−N(3) exchange of the monodentate ligand in complex 1 have been calculated. The cis and trans isomers of the orthometalated face-to-face complex [{Pd(C6H4NNC6H5)(μ-p-tolNNNp-tol)}2] (2) have also been prepared, and the crystal structure of the trans isomer is reported. There are noticeable differences in the electrochemical behavior of the mononuclear and binuclear species. From the electrochemical experiments on both isomers of 2 it is possible to recognize different redox sites, to calculate the electronic coupling between them, and to suggest where the reversible electron transfers occur. Each isomers of 2 undergoes two one-electron oxidations and two one-electron reductions. The electronic coupling (ΔE = 0.40 V) at oxidizing potentials is identical for both isomers of complex 2, suggesting that the oxidations occur on the Pd(μ-triazenido)2Pd framework which is common to both isomers. By contrast, the electronic coupling at reducing potentials is greater for cis-2 (ΔE = 0.33 V) than for trans-2 (ΔE = 0.25 V), suggesting that the reduction processes occur on the orthopalladated fragments, which are arranged differently on the two isomers. Thus, electronic communication between two equivalent redox centers in the same molecule depends not only on the nature of the bridging ligand but also on the geometrical arrangement of the redox centers.

Cyclometallation of phenylhydrazones: Synthesis, reactivity, crystal structure analysis and novel trinuclear palladium(II) cyclometallated compounds with [C,N,N′] terdentate ligands

“…As we [18][19][20][21][22] have shown before, when cyclometallated complexes derived from terdentate C,N,N 0 ligands with Pd-N bonds were reacted with tertiary phosphines in the appropriate molar ratio, species with the phosphine coordinated to the metal centre were obtained, with cleavage of, at least, one Pd-N bond; this was prevented in those cases where a vacant coordination was created by initial treatment with a silver(I) salt. However, when the complexes 3a and 1b reported here were reacted directly with triphenylphosphine, without previous removal of the chlorine ligand, no breakage of the Pd-N bond was observed, putting forward the greater strength of the Pd-N pyridine bond in these compounds.…”

“…In the past, we have been interested in palladium(II) and platinum(II) cyclometallated complexes derived from [C,N,X] (X = N, O, S) terdentate ligands [16][17][18][19][20][21][22]. For example, Schiff bases readily react with palladium(II) salts to give mononuclear cyclometallated complexes with the ligand as [C,N,N 0 ] terdentate [18][19][20][21][22].…”

“…For example, Schiff bases readily react with palladium(II) salts to give mononuclear cyclometallated complexes with the ligand as [C,N,N 0 ] terdentate [18][19][20][21][22]. Treatment of the latter with neutral ligands such as tertiary mono-or diphosphines produced cleavage of the metal-N 0 bond prior to ringopening of the five-membered metallacycle.…”

“…Treatment of the latter with neutral ligands such as tertiary mono-or diphosphines produced cleavage of the metal-N 0 bond prior to ringopening of the five-membered metallacycle. When the cyclometallated compound was treated first with a silver(I) salt the chlorine ligand was removed as silver chloride and the vacant coordination site was occupied by the phosphine [18][19][20][21][22]. The diphosphine chelated to the metal only when the complexes were reacted with excess ligand.…”

Nickel(II) complexes of the type [RM(oxam)MR] (oxam: oxalamidinate, R=n-butyl, n-hexyl): the first binuclear n-alkyl nickel complexes

“…For example, García-Herbosa reported about binuclear Pd(II) complexes containing 1,3-di-p -tolyltriazenido bridging ligands and the dependence of their redox properties on cis and trans isomerization of the two equivalent redox sides [28]. In Scheme 2.…”