4344
N. FILIPESCU, F. GEIGER, C. TRICHILO, AND F. MINN
Electronic and Electron Spin Resonance Spectroscopic Study of Zinc-Reduced Di(4-pyridyl) Ketone Methiodidesla by Nicolae Filipescu,lb Felix E. Geiger, Charles L. Trichilo, and Fredrick L. Minn Department of Chemistry, The George Washington University, Washington,D. C. 80006 and Goddard Space Flight Center, N A S A , Greenbelt, Maryland 80771 (Received July 86, 1970)
The dimethiodide of the title compound with zinc in degassed acetonitrile formed a stable cation radical whose esr spectrum was fully reconstituted. This radical underwent a slow zero-order subsequent reaction with rate constant -2 X loF8mol/(l. hr). The monomethiodide, however, gave a diamagnetic species of the same color which remained stable indefinitely, The mechanism for the reduction is discussed. The N-methyl iodides I and I1 of di(4-pyridyl) ketone
I
I1
are interesting not only because of their structure and functionality but also because of their striking resemblance to model compounds with known physiological activity.2 The free radicals derived from compounds I and I1 are related, for example, to the cationic species obtained from viologens, 3 , 4 the neutral 1-alkyl-4-carbomethoxypyridinyls,6 and the paramagnetic anion of di(4-pyridyl) ketone.e Zinc, magnesium, sodium amalgam, and dithionite reduction has been used by Kosower and coworkers5 to generate free radicals from pyridine derivatives. Here we report the analysis of the electron paramagnetic resonance spectrum of the cation radical obtained on zinc reduction of I1 and of the uvvisible absorption changes during the reactions of both I and I1 with the metal in degassed acetonitrile.
Results Brick-red crystals of dimethiodide I1 and zinc dust were covered in a closed system with acetonitrile previously degassed by several freeze-thaw cycles under high vacuum. Although compound I1 is not very soluble in acetonitrile, in the presence of zinc it dissolved to produce a magenta solution which gives the uv-visible absorption shown in Figure 1 (curve 1). The other curves in Figure 1 represent changes in the visible absorption of such a sample in a sealed quartz cell, after 524 nm reached maximum absorbance the band a t A, in curve 1 and no solid I1 remained. The initially formed compound with A,, at 524 nm changed progressively over several months into a new species with absorption maximum at 480 nm as the solution became red. Whereas the magenta solution was intensely paramagnetic and gave the esr spectrum shown in Figure 2, the red component proved t o be diamagnetic. The isosbestic point at 497 nm formed by the curves The Journal of Physical Chemistry, Vol. 74, No. 86, 1970
reflecting the dark reaction testifies to the absence of consecutive or competitive processes. Both colored species were oxygen sensitive; on admission of air the solution became either brown (from magenta) or gold (from red). When monomethiodide I and zinc dust were mixed with degassed acetonitrile under vacuum, a magenta of virtually the same color and visible absorption (A, 523 nm) as that from I1 was formed. However, this solution did not show the presence of free radicals. Furthermore, the color of the sealed sample remained unchanged indefinitely, to change rapidly to yellow only upon admission of air. Metal reduction of the parent dipyridyl ketone was not investigated, since the expected species, the corresponding ketyl radical anion, was previously examined.6J
Discussion The paramagnetic species formed on reduction of dimethiodide I1 with zinc in acetonitrile is 111, a previously unreported radical. Its identity was verified
L
I11
J
by comparison of electron densities calculated from (1) (a) Taken in part from work done by C. L. T. for the Ph.D. degree; (b) to whom correspondence should be addressed. (2) (a) P. Borger, C. C. Black, and A, San Pietro, Biochemistry, 6 , 80 (1967); (b) P. Borger and A. San Pietro, Arch. Biochem. Biophys., 120, 379 (1967); (0) 0.Rogne, Biochem. Pharmacol., 16, 1853 (1967). (3) C. 8 . Johnson and H. S. Gutowsky, J . Chem. Phys., 39, 58 (1963). (4) A. H. Corwin, R. R. Arellano, and A. B. Chivvis, Biochim. Bwphys. Acta, 162, 533 (1968). (5) (a) E.M.Kosower and E. J. Poziomek, J . Amer. Chem. Soc., 86, 5515 (1964); (b) E. M.Kosower and J. L. Cotter, ibid., 86, 5524 (1964); (0) E.M.Kosower and I. Schwager, ibid., 86, 5528 (1964). (6) J. C. M.Henning, J . Chem. Phys., 44, 2139 (1966). (7) F. L. Minn, C. L. Trichilo, C. R. Hurt, and N. Filipescu, J . Amer. Chem. Soc., 92, 3600 (1970).
4345
ESRO F ZINC-REDUCED DI(4-PYRIDYL) KETONEMETHIODIDES
3.01
A
2.5
(
'
-
u O = 9.48 GHz OBSERVED ESR SPECTRUM I aN =2.634 oe I I I
1
I
I 1
aCH3= 2.514 aiH =0.743 a2H =0.629
I
Figure 1. Changes in the uv-visible absorption spectrum of an w l O - 4 M solution of I1 in degassed acetonitrile in the presence of zinc: 1, after the initial reaction of I1 with Zn was complete; 2-9, spectra after 364, 603, 874, 1684, 2451, 2607, 3284, and 3837 hr, respectively, in the dark a t 298°K following complete reaction of I1 (numerous intermediate curves were omitted for clarity); 10, after admission of air.
Huckels and the more refined McLachlane NIO's with those derived from the interpreted esr spectrum shown in Figure 2. The hyperfine splitting constants were found to be uH1 = 0.743 0.005, U H ~= 0.629 f 0.005, U N = 2.634 0.020, and UGH3 = 2.514 =J= 0.020 Oe. The excellent accuracy of these values was derived from fits with numerous computer simulations. Figure 2 also displays a computer-reconstructedlo spectrum using these hfs constants, a Lorentzian line shape, and a line width of 65 mOe. Our HMO calculation for molecule 111used (YN = ac hp, C Y C ~= (YC 0.lhP for the carbon atoms adjacent to the nitrogens13ao = ac 3- 2P, aco = ac 0.3P for the carbonyl carbon, PCN = Pcc = P, and PcO = 1.41 P.* A plot of T spin densities pw calculated a t the CI, Cz, and N sites for different values of the parameter h gave reasonable agreement with experimental values around h = 1.4, a value comparable to that used for the viologen radical^.^ Such plots also tended to verify the monocation nature of the radical, since other possibilities, species that may form either upon further reduction of 111 or upon fission of the molecule, give unacceptable correlation with experimental pw values. For example, a reasonable possibility that had to be removed was anion radical IV, which
*
+
+
+
0CH3-N3d==(=+3, -
-
lv
could be produced by three-electron reduction of dicat-
CALCULATED ESR SPECTRUM
Figure 2. Experimental and simulated esr spectra of di(4-pyridyl) ketone dimethiodide monocation radical 111.
ion 11. In this case the two types of aromatic hydrogens are theoretically expected to have widely different splitting constants within the range 0.5 I hI 1.5; this contradicts experimental findings. From the spectral changes shown in Figure 1we were able to examine the kinetics of the reaction in which radical 111was slowly converted into a diamagnetic red product. Plots of the reciprocal of the absorbance and the logarithm of the absorbance vs. time gave curves far from straight lines. This ruled out both second- and first-order reaction in free radical, respectively. On the other hand, the graph of optical density os. time was a nearly perfect straight line, which indicates that the radical-disappearance reaction is zero order with rate constant -2 X lods mol/(l. hr). Since virtually all zero-order transformations are heterogeneous, that is, surface reactions, the value of the rate constant may depend on the condition of the zinc surface. There are only three obvious reactions of I11 that may occur a t the zinc-liquid interface to generate a diamagnetic product, namely, further reduction, disproportionation, or dimerization. Because dimerization is not expected to take place preferentially on the metal surface and disproportionation is essentially equivalent to the I11 (8) As suggested, for example, by A . Streitwieser, Jr., "Molecular Orbital Theory for Organic Chemists," Wiley, New York, N. Y., 1961, Chapter 5 . (9) A. D. McLachlan, Mol. Phys., 3, 233 (1960). (10) Modifications to a program by A. Inzaghi and L. Mongini, European Atomic Energy Commission-Euratom, Report EUR4064e, 1968.
The Journal of Physical Chemistry, Vol. 74, N o . a6, 1070
4346
K.FILIPESCU, F. GEIGER,C. TRICHILO, AND F. MINN
V one-electron reduction, the latter is the most probable pathway. Enolic zwitterion V is expected to be -+
*
cation I11 and anion VI should have similar electronic spectra.
Experimental Section
./-
:O,
I11
;bV
oxygen sensitive and visible absorbing, as observed. The magenta diamagnetic species formed from the monomethiodide I with zinc is verx probably the anion V I formed by the disproportionation of a neutral radi-
VI1
N>c=(=J+cH,
P-
+ r
VI
cal intermediate VII. This latter species, unlike the charged radical 111from dimethiodide 11,is expected to be too short-lived for detection by our methods. Elementary MO theory predicts, as observed, that radical
The Journal of Physical Chemistry, Vol. 74, No. 86,1970
Di(4-pyridyl) ketone and its methiodides were prepared by methods described previ~usly.~Degassed solutions in spectrograde acetonitrile (Eastman) were prepared by repeated freeze-thaw cycles under vacuum in either silica absorption cellsll or 2-mm i.d. esr tubes provided with side reservoirs and constrictions for flame sealing. Degassing was carried out with the solvent in the side bulb and the solids in the sample compartment; they were mixed only after oxygen removal was complete. Uv-visible absorption spectra were recorded on Cary spectrophotometer Model 15in double-beam mode. Esr spectra were recorded on a modified Varian V-4502 spectrometer with 100-kHz modulation. The microwave bridge of the spectrometer consisted of a circulator in the sample arm and a precision attenuator and phase shifter in the bucking arm. Computer simulations were carried out on an IBM 360/91 computer and drawn by the Cal-Comp Associates plotter; other calculations were performed on an IBM 360/50 computer.
Acknowledgment. We thank the University Computer Center for availability of computer time and the University Research Committee for some financial assistance. Part of this work was supported by the Atomic Energy Commission under Contract AT-(40-1)
3797. (11) N. Filipescu and F. L. Minn, J. Chem. Soc., B, 84 (1969).
