Syntheses, structures and properties of palladium(II) complexes with photochromic 4-methoxyazobenzene

Two palladium(II) complexes with the photochromic ligand 4-methoxyazobenzene (MeOazbH), [Pd(MeOazb) (PhCN) (H2O)] (ClO4) (1) and [Pd(MeOazbH)(2)Cl-2] (2) were prepared and their molecular structures determined by X-ray crystallography. 1: triclinic, <P(1)over bar>, a=9.684(3), b=11.833(2), c=9.731(3) Angstrom, alpha=102.41(1), beta=94.54(3), gamma=94.82(2)degrees, V=1079.7(4) Angstrom(3), Z=2. 2: triclinic, P (1) over bar, a=9.497(3), b=9.719(4), c=8.226(3) Angstrom, alpha=96.43(4), beta=109.24(3), gamma=62.13(2)degrees, V=632.9(4) Angstrom(3), Z=1. The molecular structure of complex 1 shows a significantly distorted square planar coordination around the Pd atom and the MeOazb group forms a five-membered chelate ring with the metal ion by means of a sigma-bond at an ortho-carbon atom and the lone pair of the further nitrogen. In complex 2 the palladium atom lies on an inversion center, surrounded in an essentially square planar fashion by the symmetry related two MeOazbH ligands and two chloride ions. In contrast with the ortho-metalation reaction in 1, MeOazbH in 2 acts as a monodentate ligand to bind the metal center via a Pd-N sigma-bond. Electronic spectroscopic studies of the two complexes together with the free ligand were carried out in solution at room temperature and upon irradiation of light; while rigidity imparted to the MeOazb linkage by palladation in 1 inhibits its photochromic reaction, the monodentate-coordinated MeOazbH in 2 shows a trans-cis photoisomerization.