1757
assigned, the chemical-shift assignments of Table I were consistent in that the C3 chemical shift-the shift expected to be least affected by the substituent-was always closest to that for benzene (except for benzyl chloride, where the chemical shifts for C2 and C3 were almost the same). The most obvious trend from Table I is the correlation between the magnitude of 517 and the hybridization of C7. Increasing the s character of C7 causes 517 to increase from 44.19 Hz in the sp2-sp3 system of toluene to 80.40 Hz in the sp2-sp system of benzonitrile. This trend is quite consistent with previously proposed empirical expressions3 which have been used to predict bond hybridization in strained systems.8 Another trend involving the C-1 carbon atom is the increase in 517 as the electronegativity of the C-7 substituent increases, for both the carbonyl and the sp3 cases. However, this substituent effect is several times larger for the carbonyl series (e.g., as -OH changes to -C1, JI7 changes by 0.06 and 2.48 Hz for the sp3 and sp2 cases, respectively). This observation is consistent with earlier work, where it was shown that 'Jc-C values for acetyl compounds4 vary to over 50z, whereas this variation for tert-butyl compounds7 is much more modest, Perhaps this large difference between carbonyl and sp3 systems is due to the polarizability of the carbonyl group. As expected, the magnitude of the J values in Table I is attenuated dramatically beyond the first bond. For these long-range Jc-c values, again hybridization and substituent trends are observed. As the s character of C-7 increases, again the magnitude of Jc-c increases for J37 and J47 (but remains the same, or decreases slightly, for 5 2 7 ) . As the substituent becomes more electronegative, again the magnitude of Jc-c increases for both the sp2 and sp3 cases for all the longrange values JZ7,J37, and J47 (except for the sp3 J47 values, where the trend may be reversed). As in the case of JI7 values, the substituent effect for the longrange J values is much more pronounced for the sp2 case than for the sp3case. The observation that the three-bonded coupling constant JS7 is larger than the two-bonded coupling constant J27 is most striking. In view of this remarkable observation, ' T - 7 p-nitrotoluene was synthesized to verify the coupling pattern. For this molecule, whose chemical-shift assignments have been rigorously established,16 again J37 was larger than J27 (3.88 and 3.47 Hz, respectively). The observation that J37 > J27 in the present system, 1, is quite analogous to the geometrically identical JCH system in benzene (2) where J H 1 - C 3 > JH,-C?(7.4 Hz > 1.0 Hz),17 but is in contrast to the geometrically identical JCF system in fluorobenzene (3)
P 1
2
3
where JF-Ca < J F - c ~(7.7 HZ < 21.0 Hz)." Acknowledgment. We should like to acknowledge support for this work by the Robert A. Welch Founda(16) P. C. Lauterbur, J . Chem. Phys., 38, 1432 (1963). (17) F. J. Weigert and J. D. Roberts, J . Amer. Chem. Soc., 89, 2967 (1967). (18) F . J. Weigert and J. D. Roberts, ibid., 93, 2361 (1971).
tion, Grant No. B-325, and by North Texas State University Faculty Research. (19) Robert A. Welch Postdoctoral Fellow, 1970-1972. Arthur M. Ihrig,Ig James L. Marshall* Department of Chemistry, North Texas State University Denton, Texas 76203 Received October 27, 1971
Ligand-to-Metal Intramolecular Electron Transfer in the Reduction of pNitrobenzoatopentaamminecobalt(II1) Ion' Sir: The hydrated electron, eaq-, generated in the radiolysis of aqueous solutions, reacts with coordination complexes of cobalt(II1) with specific rates very near to the diffusion-controlled limit quantitatively generating C O ~ + However, .~ in all the cases reported, no transient species have been detected by pulse radiolysis from the reaction of eaq- with simple pentaammine c0mplexes.~-6 These negative results imply either that the electron initially attacks the ligands followed by very rapid intramolecular electron transfer to the metal center or that attack is uiu tunnelling directly into the orbitals of the tripositive metal. We wish to report that the reaction of ,eaq- and the reducing radicals .C02and (CH3),COH with p-02NC~H4C02Co'1'(NH3)52+ (PNBPA) generates a transient intermediate (PNBPA-) in which the transferred electron is localized on the coordinated p-nitrobenzoato ligand; PNBPA- decays cia ligand-to-metal intramolecular electron transfer to form C O ~ + . ~ The radiolysis of aqueous solutions generates eaq-, OH, and H radicals with G values (number of molecules produced per 100 eV of energy absorbed) of 2.8, 2.8, and 0.6, respectively.8 In a N20-saturated solution (2.5 X M ) , e, is efficiently scavenged eaq-
+ N:O +OH + Nz + OHk = 5.6 X 100 M-' sec-1
The reducing radicals .C02- and (CH3)&OH can be generated conveniently
+
+
OH/H HCOz- + +COz- HzO/Hz k = 2.5 X 1O9/2.5 x lo8 M-Isec-1 OH/H
+ (CH&CHOH
+
(CH&eOH HzO/Hz k = 1.3 X 109/5.0X lo7 M-l sec-1
NzO-saturated neutral aqueous solutions of PNBPA (as the c104-salt)s in the presence of 0.1 M formate (1) Supported in part by NSF Grant GP 11213 and by NIH Grant GM 13557. (2) E. J. Hart and M. Anbar, "The Hydrated Electron," WileyInterscience, New York, N. Y.,1970. (3) D. Katakis and A. 0 . Allen, J . Phys. Chem., 68, 1359 (1964); N. Zevos, ibid., 72, 1506 (1968). (4) J. H . Baxendale. E. M. Fielden. and J. P. Keene. Proc. ROY.Soc.. Ser. A , 286,320(1965): (5) M. 2. Hoffman and M. Simic, J. Amer. Chem. Soc., 92, 5533 (1970). (6) The transient observed in the reaction of eag- with Co(bipy)ss+ has been attributed to low-spin Co(I1): W. L. Waltz and R. G. Pearson, J . Phys. Chem.,73,1941 (1969). (7) Co(NH&3+ is also reduced to Co*+ by . Con- and (CH&COH, as well as by eaq-: E. P. Vanek and M. Z. Hoffman, manuscript in preparation. (8) M. Anbar in "Fundamental Processes in Radiation Chemistry," P. Ausloos, Ed., Wiley-Interscience, New York, N. Y., 1968, p 651. (9) The complex was synthesized using the procedure of E. S. Could
Communications to the Editor
1758 Table I. Decay of PNBPAReactive sDecies
Solute present
Gas uuree
1 .O M tert-butyl alcohol 1 .O M tert-butyl alcohol 1.0 M tert-butyl alcohol 1 .O M tert-butyl alcohol 0 . 2 M terf-butyl alcohol 0 . 2 M tert-butyl alcohol 0.1 M HCOa0.1 M 2-propanol 0.1 M 2-propanol
Ar Ar Ar Ar Ar Ar NzO NzO Nz0
IPNBPA1. M
x x x x 8.3 x 8.3 x 5.0 x 1.0 x 1.0 x
eaq-
5.0 5.0 5.0 5.0
ucoz- . (CH&COH
10-4 10-4 10-4 10-4 10-5
10-5 10-4 10-4 10-4
-
I
DH
Doselpulse, krads 10M3k.sec-l
7.7 7.7 7.7 7.7 7.7 7.7 7.1 5.5
2.4 2.4 4.0 4.0 2.4 2.4 2.4 0.75 0.75
5.5
2.9 2.7 3.1 2.9 2.5 2.5 2.0 2.4 2.5
Av 2 . 6 X 103sec-l
were subjected to a 30-nsec pulse of 2.3-MeV electrons and the resulting transient optical absorption spectrum was recorded. lo Figure 1 shows the transient spectrum from .COzPNBPA; for comparison, the transient p-nitrobenzoate (free ligand ; spectrum from COzPNB) is also shown. It is obvious that the spectra of the transient species from the reduction of PNBPA and PNB by .COz- are identical. Furthermore, the
+
I 2 T
+
1
1
I
lo4
E
7
I
The disappearance of the absorption spectrum of PNBPA-, monitored at 350 nm, is uia first-order kinetics independent of [complex], radiation dose, or method of generation (Table I) with an average value of k = 2.6 X lo3 sec-1 (zk2073. The decay of the transient does not generate any permanent product absorbing at X >300 nm. In comparison, the electron adduct to the free ligand, PNB-, decays via secondorder kinetics and generates a strongly absorbing permanent product, presumably via disproportionation-combination processes. The steady-state radiolysis1 of an 02-free solution M PNBPA in 0.2 M formate at pH 3.8 of 1 X (acetic acid buffer) generates Co2+ l 2 with a value of G(.COn-)] = G(Co2+) = 6.3 f 0.3 = [G(eaq-) [G(e,,-) G(0H) G(H)]. There can be absolutely no question that PNBPA- decays via ligand-to-metal intramolecular electron transfer with the concomitant release of the ligands into solution.
+
r; 1x 1 ~ 4
+
+
+ 5NH3
+
0
250
O Z N C ~ H ~ C O ~ C O ~ ~+ ~ ( NCoZf H ~ ) ~ +02NC6H4C02k = 2.6 x lo3 sec-l
350
This appears to be the first reported observation of the intermediate produced when an electron is initially Figure 1. Transient absorption spectrum generated from the transferred to a ligand of a metal complex and the reaction of .COz- with (a) 2 X M p-OzNC~H4C0~Co111- measurement of the rate of transmission of that electron (NH3)&2+,pH 5.6, 0.1 M HCOz- ( 0 ) ; (b) 2 X M p-OnNCsH4through the ligand into the metal center. In our Cot-, pH 7.0, 0.1 M HCO2- (0). Dose/pulse = 1 krad. B values CBHSCO2Co1''previous communication on eaqare corrected for the consumption of the substrate. (NH3)62+(BPA), we pointed out5 that no transient was observed within the time resolution of the pulse instruidentical absorptiqn spectrum was obtained from the mentation (-0.5 psec). l 3 Inasmuch as the electron PNBPA and eaq- PNBPA reaction of (CH&COH adduct to CeH5COZ-shows a very intense absorption (in the presence of tert-butyl alcohol as an OH radical maximum at 310 nm (€310 2.5 X IO4 M-' cm-l),14 it scavenger). All the evidence indicates that both is certain that if the electron were localized on the external electron transfer from . COZ- and (CH&COH benzoato ligand for a period of time in excess of the and direct e,, attack generate the reduced form of the instrumental time limitation, the transient would be complex, PNBPA-, in which the added electron is if initial electron attack on BPA observed. Thus, localized on the p-nitrobenzoato ligand, OZNC6H4CO2occurs at the benzoato ligand, the specific rate of CO"'(NH~)~+. By following the formation of the intramolecular electron transfer must be > lo' sec- l. transient spectrum, the rate constants for these oneIf the nature of electron attack on BPA were similar electron transfer reactions were determined to that on CeHsC02-, then the incoming electron .COz- + PNBPA +PNBPA- + COa would add to a ir orbital conjugated throughout the k = 1.9 X lo8 M-lsec-l ligand.14 The overlap of the metal orbitals with those of the carboxylate ligand would provide an extremely (CHa)&0H + PNBPA +PNBPA- + (CH&CO + H+ k = 2.6 X lo0 M-lsec-l (11) Total dose up to 7 x 1017 eV; maximum decomposition of the complex
