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)(n)PPh2; 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, E-1/2 (ox), of the shorter Pd-P and longer Pd-X bond lengths in the dppm and dppe complexes are high and reflect strong d(xy) (d pi (Pd)) interaction with d(x2-y2) (d sigma(*) (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 d(x2-y2) (d sigma(*)(Pd)) interaction is largely dependent on X. The reduction potential, E1/2(red), which reflects the d(x2-y2) (d sigma(*)(Pd) LUMO energy, and the oxidation potential, E-1/2(red), which reflects the (d(xy) (d pi (Pd) HOMO energy were, determined. The E-1/2(red) and E-1/2(ox) values indicate the cr-bonding properties and pi-interaction in the system, respectively. Good linear relationships were observed between the potentials difference [E-1/2(ox) - E-1/2(red)] and the ligand-field transition energy (d pi(dxy) -> d sigma(*) (d(x2-y2))) in the series of dihalogenopalladium(II) complexes. The linear relationship shows that the energies of the d-d transitions and Delta E(Redox) of the dppm and dppe complexes are less dependent on X- whilst those of the dppp and dppb are largely X- dependent. (C) 2012 Elsevier Ltd. All rights reserved.