Iron(III) complexes of
pyridoxal (
vitamin B6, VB6) or
salicylaldehyde Schiff bases and modified dipicolylamines, namely, [Fe(B)(L)](NO3) (1-5), where B is phenyl-N,N-bis((pyridin-2-yl)methyl)methanamine (phbpa in 1), (anthracen-9-yl)-N,N-bis((pyridin-2-yl)methyl)methanamine (anbpa in 2, 4) and (pyren-1-yl)-N,N-bis((pyridin-2-yl)methyl)methanamine (pybpa in 3, 5) (H2L(1) is 3-hydroxy-5-(hydroxymethyl)-4-(((2-hydroxyphenyl)imino)methyl)-2-methylpyridine (1-3) and H2L(2) is 2-[(2-hydroxyphenyl-imino)methyl]
phenol), were prepared and their uptake in
cancer cells and photocytotoxicity were studied. Complexes 4 and 5, having a non-
pyridoxal Schiff base, were prepared to probe the role of the
pyridoxal group in
tumor targeting and cellular uptake. The PF6
salt (1a) of complex 1 is structurally characterized. The complexes have a distorted six-coordinate FeN4O2 core where the
metal is in the +3 oxidation state with five unpaired electrons. The complexes display a
ligand to
metal charge transfer band near 520 and 420 nm from phenolate to the
iron(III) center. The photophysical properties of the complexes are explained from the time dependent density functional theory calculations. The redox active complexes show a quasi-reversible Fe(III)/Fe(II) response near -0.3 V vs saturated
calomel electrode. Complexes 2 and 3 exhibit remarkable photocytotoxicity in various
cancer cells with IC50 values ranging from 0.4 to 5 μM with 10-fold lower dark toxicity. The cell death proceeded by the apoptotic pathway due to generation of
reactive oxygen species upon light exposure. The nonvitamin complexes 4 and 5 display 3-fold lower photocytotoxicity compared to their VB6 analogues, possibly due to preferential and faster uptake of the
vitamin complexes in the
cancer cells. Complexes 2 and 3 show significant uptake in the endoplasmic reticulum, while complexes 4 and 5 are distributed throughout the cells without any specific localization pattern.