Compounds of the form [(pap)M(Q2−)] (pap = phenylazopyridine; Q = 3,5-di-tert-
butyl-benzoquinone, M = Pd, 1a and 1b, M = Pt, 2a and 2b; Q = 4-tert-butyl-
benzoquinone, M = Pd, 3a and 3b; M = Pt, 4a and 4b) were synthesized in a one-
pot reaction. The geometrical isomers, which are possible because of the built
in asymmetry of these ligands, have been separated by using different
temperatures and variable solubility. Structural characterization of 1b shows
that the metal centers are in a square planar environment, the pap ligand is
in the unreduced neutral state and the quinones are in the doubly reduced,
Q2−catecholate form. Cyclic voltammetric measurements on the complexes display
two one-electron oxidations and two one-electron reductions. EPR and vis-NIR
spectra of the one-electron oxidized forms of the complexes indicate that the
first oxidation takes place on the Q2− ligands to produce a metal bound
semiquinone (Q˙−) radical. Reduction takes place on the pap ligand, generating
metal bound pap˙− as seen from the 14N (I = 1) coupling in their EPR spectrum.
All the complexes in their [(pap)M(Q2−)] neutral forms show strong absorptions
in the NIR region which are largely LLCT (ligand to ligand charge transfer) in
origin. These NIR bands can be tuned over a wide energy range by varying the
metal center as well as the Q ligand. In addition, the intensity of NIR bands
can be switched on and off by a simple electron transfer at relatively low
potentials. DFT studies were used to corroborate these findings
