RESEARCH FROM NATIONAL ACADEMY OF SCIENCE PROVIDE NEW INSIGHTS INTO CHEMICAL KINETICS

Science Letter
February 2, 2010

"Absolute rate constants (k(eff)) for the chemical reactions of
Cu(II)(2)(3,5-di-iso-propylsalicylate)(4)(H2O)( 3),
Cu(II)(2)(3,5-di-tert-butylsalicylate)(4),
Cu( II)2(3,5-di-tert-butylsalicylate)(4)(H2O)(4),
Cu(I I)(2)(3,5-dimethylsalicylate)(4)(H2O)(3),
Cu(II)(2 )(3-ethylsalicylate)(4)(H2O),
Cu(II)(2)(3-phenylsa licylate)(4), and
Cu(II)(3,5-di-iso-propylsalicylate)(2)(pyridin e)(2) with
tert-butylperoxyl radical were determined using kinetic electron
paramagnetic resonance measurements in 10% toluene in the hexane
medium at temperatures ranging from -63 degrees C to 2 degrees C.

These antioxidant (AO) chelates were ranked by their reactivity as
follows: 2,6-di-tert-butyl-4-methylphenol >>
Cu(II)2(3,5-di-tert-butylsalicylate)(4) congruent to
Cu(II)(2)(3-phenylsalicylate)(4) >
Cu(II)(2)(3,5-di-iso-propylsalicylate)(4)(H2O )(3) congruent to
Cu(II)(2)(3,5-di-tert-butylsalicylate)(4)(H2O)( 4) congruent to
Cu(II)(2)(3,5-dimethylsalicylate)(4)(H2O)3 >
Cu(II)(2)(3-ethylsalicylate)(4)(H2O) >>
Cu(II)(3,5-di-iso-propylsalicylate)(2)(py ridine)(2) at 20 degrees C.

Differential pulse voltammetry was used to determine redox behavior
of these chelates in CH2Cl2. Two types of salicylic OH groups were
detected in these Cu(II) salicylates, characterized by the presence
or absence of AO reactivity," scientists in Yerevan, Armenia report
(see also Chemical Kinetics).

"One of them was coordinate covalently bonded to Cu(II) via the
oxygen atoms of the salicylic OH groups, displaying oxidation peak
potentials in the range from +650 to 970 mV versus Ag/Ag+. The second
type was intramolecularly hydrogen bonded to carboxylate oxygens,
with an oxidation peak potential in the range from +1100 to 1200 mV
versus Ag/Ag+. It was concluded that non-hydrogen-bonded salicylic
OH groups are responsible for the antiperoxyl radical reactivity of
these chelates, while neither Cu(II) nor salicylate ligands displayed
reactivity with peroxyl radical, It has been established in this
research that axially bonded electron pair donors such as pyridine
and water decrease H-donating reactivity of Cu(II) salicylates by
promoting the formation of intramolecular hydrogen bonding between
the salicylic OH hydrogen atoms and carboxylate oxygen atoms in the
salicylic ligands," wrote L.A. Tavadyan and colleagues, National
Academy of Science.

The researchers concluded: "Dependences of log k(eff) at 20 degrees C
and the anodicoxidation potential (E-pa) for the salicylic OH group on
the difference between symmetric and asymmetric stretching frequencies
of carboxylate groups (in Fourier transform infrared spectra) for
the substituted Cu(II) salicylates were determined."

Tavadyan and colleagues published their study in International Journal
of Chemical Kinetics (Reactivity of Substituted Copper(II) Salicylates
with tert-Butylperoxyl Radical: Structure-Reactivity Relationships.

International Journal of Chemical Kinetics, UNKNOWN DATE;42(1):56-67).

For more information, contact L.A. Tavadyan, National Academy Science,
Institute Chemical Physics, 5-2 Sevak St., Yerevan 0014, Armenia.

Publisher contact information for the International Journal of
Chemical Kinetics is: John Wiley & Sons Inc., 111 River St., Hoboken,
NJ 07030, USA.