DTIC ADA260299: Correlations Between the Ligand Electrochemical Parameter, EL(L) and the Hammett Substituent Parameter Sigma

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AD-A260 299
OFFICE OF NAVAL RESEARCH
Contract N00014-91-J-1910
R & T Code 4131025
Technical Report #51
Correlations Between the Ligand Electrochemical Parameter, EL(L) and the
Hammett Substituent Parameter, a
By
Hitoshi Masui and A.B.P. Lever*
in
Inorganic Chemistry 08Ir99
York University
Department of Chemistry, 4700 Keele St., North York
Ontario, Canada M3J 1P3
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January 25, 1993 Technical - June 1991 to July 1992
4 TITLE AND SUBTITLE S. FUNDING NUMBERS
Correlations Between the Ligand Electrochemical
Parameter, E L(L) and the Hammett Substituent Parameter a
L C - N00014-91-J-1910
6 AUTHOR(S) * PR - 4131025
Hitoshi Masui and A.B.P. Lever
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REPORT NUMBER
A.B.P. Lever, Department of Chemistry
York University, 4700 Keele St.
North York, Ontario M3J 1P3 Report #51
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Dr. Ronald A. De Marco
Office of Naval Research, Chemistry Division
800 N. Quincy St., Arlington, VA 22217 U.S.A.
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13. ABSTRACT (Maximum 200 words)
Correlations of the electrochemical parameter, i
~~~O~p~a~E~+~a~d~ap arameters,r rsne n iL(L) with Hammett and Taft
parameters, 0pmo+ and o* are presented and discussed. The correlations
provide a means to extend the electrochemical parameter and the Hammett and
Taft databases.
14. SUBJECT TERMS 15. NUMBER OF PAGES
14
Electrochemistry, Hammett Parameter 16. PRICE CODE
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Of REPORT OF THIS PAGE Oi ABSTRACT
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v1.053 hammet.pap 01/11/93 -1-
Contribution from the Dept. of Chemistry,
York University, North York (Toronto),
Ontario, Canada, M3J 1P3.
Correlations Between the Ligand Electrochemical Parameter. ET (D and the Hammett Substituent
Param~eter. a.
By Hitoshi Masui and A. B. P. Lever.
Correlations of the electrochemical parameter, EL(L) with Hammett and Taft
,
parameters, ap, am, a+ and a are presented and discussed. The correlations provide
a means to extend the electrochemical parameter and the Hammett and Taft
databases.
Recently, a ligand electrochemical parameter, EL(L), based upon the Ru II/Ru reduction
potential as an electrochemical standard, was introduced1-3 and developed.4"14 This parameter allows
one to predict the metal-centred redox potential of a variety of complexes based on the additivity of the
EL(L) parameters. The prediction is made using the linear relationship,
E(obs) = SM EL(Li) + 'M (1)
where the slope, SM, and the intercept, IM' are constant for all derivatives of a given metal undergoing a
defined redox process; i.e. having a defined initial and final oxidation state, coordination number,
stereochemistry, and spin state.
Hammett a parameters have been shown to correlate with, inter alia, metal centered
electrochemical potentials in a variety of substituted ligand complexes. 15-31 It follows that, in these
cases, Hammett parameters must also linearly correlate with EL(L) values, as briefly noted for substituted
species.3'32
benzoquinonediimine
vl.053 hammet.pap 01/11/93 -2-
In this exploratory paper we consider possible relationships between Hammett or Taft parameters
with EL(L) parameters. The question of whether it is better to use Op, a m, + or a is discussed briefly.
From a pragmatic point of view, correlation with ap or om is preferred since there is a large database of
these parameters for a wide variety of substituents. For example, am and ap values for some 530
substituents are listed in Table I of ref. 33. However, when the substituent is closer to the metal 3id
metal-substituent x interactions are possible, a or a+ may be more appropriate.
It should be possible to use Hammett or Taft parameters to derive EL(L) values for a large variety
of ligands not covered in ref. 1, thereby, greatly expanding the utility of the electrochemical parameter
analysis. In particular the availability of Hammett parameters for a great number of organic functional
groups should permit these EL(L) parameters to be used to derive redox potentials in a variety of
organometallic species. Their availability should spur the synthesis of more exotic complexes by defining
a potential range for their isolation. Further, electrochemically generated EL(L) values may then be used
to obtain a values for hitherto unreported substituents. This paper begins the exploration of such a
correlation but is restricted to a single- rather than multi-parameter correlation.
The relationship between the EL(L) parameter and the a parameter can be deduced from the
mathematical definition of the EL(L) parameter (Eqn. 4b of R-f. I), the Nernst equation34 and the
equations expressing the Hammett free-energy relationship.31,35,36 Thus, the EL(L) parameter of a
substituted ligand, LX, (LH denotes a parent ligand substituted by X which may also generally represent a
collection of substituents as in the case of poly-substituted ligands) is given by:37
EL(LX) = 2.303(RT/nF)P(L)RuII9IZa + EL(LH) (2)
Here, P(L)RuII' the reaction parameter, characterizes the sensitivity of the RuIM redox potential to
substitution at L for the series, and Eo is the sum of the Hammett or Taft parameters of the substituent(s)
attached to L, and EL(LH) is the EL(L) parameter of the unsubstituted ligand. The remaining constants
retain their usual meanings. 1,33-36. The reaction parameter is the fitted parameter.
Based on previous literature,15-31 a correlation can be expected between Zo and EL(L) for
homologous series such as substituted pyridines R-Py, substituted bipyridines,
4-R,4'-R',5-R",5'-R" 'bpy, R, R',R"-phosphines, diketones (RCOCHR'COR"), and
~Avelablit
8DCO"
- '-:IiI~r|S Disat SpeciIaJx
IA .1