Spectroscopic and catalytic characterization of Pd–In and Pt–In supported on Al2O3 and SiO2, active catalysts for nitrate hydrogenation (original) (raw)

2008, Applied Catalysis A: General

A mixed-valence complex, [Fe III Fe II L1(μ-OAc) 2 ]BF 4 ·H 2 O, where the ligand H 2 L1 = 2-{[[3-[((bis-( p y r i d i n -2 -y l m e t h y l ) a m i n o ) m e t h y l ) -2 -h y d r o x y -5methylbenzyl](pyridin-2-ylmethyl)amino]methyl]phenol}, has been studied with a range of techniques, and, where possible, its properties have been compared to those of the corresponding enzyme system purple acid phosphatase. The Fe III Fe II and Fe III 2 oxidized species were studied spectroelectrochemically. The temperature-dependent population of the S = 3 / 2 spin states of the heterovalent system, observed using magnetic circular dichroism, confirmed that the dinuclear center is weakly antiferromagnetically coupled (H = −2JS 1 ·S 2 , where J = −5.6 cm −1 ) in a frozen solution. The ligand-to-metal charge-transfer transitions are correlated with density functional theory calculations. The Fe III Fe II complex is electron paramagnetic resonance (EPR)-silent, except at very low temperatures (<2 K), because of the broadening caused by the exchange coupling and zero-field-splitting parameters being of comparable magnitude and rapid spin−lattice relaxation. However, a phosphate-bound Fe III 2 complex showed an EPR spectrum due to population of the S tot = 3 state (J= −3.5 cm −1 ). The phosphatase activity of the Fe III Fe II complex in hydrolysis of bis(2,4dinitrophenyl)phosphate (k cat. = 1.88 × 10 −3 s −1 ; K m = 4.63 × 10 −3 mol L −1 ) is similar to that of other bimetallic heterovalent complexes with the same ligand. Analysis of the kinetic data supports a mechanism where the initiating nucleophile in the phosphatase reaction is a hydroxide, terminally bound to Fe III . It is interesting to note that aqueous solutions of [Fe III Fe II L1(μ-OAc) 2 ] + are also capable of protein cleavage, at mild temperature and pH conditions, thus further expanding the scope of this complex's catalytic promiscuity.