Structural comparison of [PdX2(P–P)] complexes (X− = Cl, Br and I; P–P = α,ω-bis(diphenylphosphino)alkanes) and their trends of redox potentials

X-ray crystallographic analyses revealed that the Pd–P and Pd–X bond lengths of [PdX2(P–P)] complexes (X− = Cl, Br and I and P–P = Ph2P(CH2)nPPh2; n = 1: dppm, 2: dppe, 3: dppp, 4: dppb) are: Pd–P (dppm, dppe) < Pd–P (dppp, dppb) and Pd–X (dppm, dppe) > Pd–X (dppp, dppb). The oxidation potentials values, E½(ox), of the shorter Pd–P and longer Pd–X bond lengths in the dppm and dppe complexes are high and reflect strong dxy   (dπ (Pd)) interaction with dx2-y2dx2-y2 (dσ∗ (Pd)) and that this interaction is less dependent on X−. The longer Pd–P and shorter Pd–X bonds for the dppp and dppb complexes reflect that the dx2-y2dx2-y2 (dσ∗ (Pd)) interaction is largely dependent on X−. The reduction potential, E½(red), which reflects the dx2-y2dx2-y2 (dσ∗ (Pd) LUMO energy, and the oxidation potential, E½(ox), which reflects the (dxy (dπ (Pd) HOMO energy were, determined. The E½(red) and E½(ox) values indicate the σ-bonding properties and π-interaction in the system, respectively. Good linear relationships were observed between the potentials difference [E½(ox) − E½(red)] and the ligand-field transition energy (dπ (dxy) → dσ∗ (dx2-y2dx2-y2)) in the series of dihalogenopalladium(II) complexes. The linear relationship shows that the energies of the d–d transitions and ΔE(Redox) of the dppm and dppe complexes are less dependent on X− whilst those of the dppp and dppb are largely X− dependent.

A correlation between crystallogical (X-ray diffraction), electrochemical (redox potentials) and spectroscopic (d–d transition energy) data of the [PdX2(P–P)] complexes is represented. The y axis represents the −E½(red) and −E½(ox) potentials of the complexes against the reference electrode Ag/Ag+ whilst the x axis represents the identical phosphine complexes as a group and their Pd–P bond lengths. The E½(red) values reflect the (dx2-y2dx2-y2 (dσ∗ (Pd)) LUMO energy and E½(ox) values reflect the (dxy (dπ (Pd)) HOMO energy of the [PdX2(P–P)] complexes. The shorter or longer Pd–P bond lengths groupings of the complexes are similar to the groupings along the trends in bond angles and redox potentials such that: dppm, dppe (shorter Pd–P) < dppp, dppb (longer Pd–P); dppm, dppe < 90° and dppp, dppb > 90°; dppm, dppe (short Pd–P) high −E½(ox) potentials; dppp, dppb (long Pd–P) low −E½(ox) potentials.Figure optionsDownload as PowerPoint slideHighlights
► Electrochemical measurements of the complexes were determined.
► The E½(red) values, which reflect the (dx2-y2dx2-y2 (dσ∗ (Pd)) LUMO energy, shift negatively.
► The E½(ox) values, which reflect the (dxy (dπ (Pd)) HOMO energy, shifted positively.
► Good linear relationships between the νd–d, and ΔE(Redox)/V are observed.
► E½(ox) and E½(red) reflect the HOMO and LUMO, respectively of [PdX2(P–P)] complexes.