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Journal of the American Chemical Society
earlier-ethane (-600%) is obtained. The nature of the ethylene hydrogenation catalyst found in this reaction remains to be determined, but, under similar conditions, C2H4Fe(C0)4, Fe2(C0)9, and Fe3(CO)12 are separately found to be ineffective catalysts for this hydrogenation. Acknowledgment. This work was supported by the National Science Foundation and the Robert A. Welch Foundation (Grants No. F-067 and F-233). We are also indebted to the National Science Foundation for the purchase of the Syntex P21 diffractometer (Grant No. GP-37028). Supplementary Material Available: Table of fractional coordinates and anisotropic thermal parameters ( 1 page) for both crystallographic data sets. Ordering information is given on any current masthead page. References and Notes
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February 27, 1980
Table I. Lifetime of ( ' E ) C r ( ~ h e n ) 3 ~in + Deaerated Aqueous 1 M HCI Solutions at 22 " C [ C r ( ~ h e n ) 3 ~ +M] , 9.2 X 1.2 x 2.3 X 2.7 X 4.4 x 6.9 X 8.1 x I .6 x 2.3 x
10-5
IO-5 10-5 10-5
I 0-4 10-4
7,
msa
0.36 0.34 0.37 0.33 0.36 0.29 0.30 0.3 I 0.22
[ C r ( ~ h e n ) 3 ~ +M] , 2.7 x 5.6 x 6.7 x 1.2 x 2.0 x 5.0 x 7.5 x 1.0 x
10-4 10-4
10-4
10-3 10-3 10-3
10-3 10-2
T , msa
0.23 0.22 0.1 5 0.1 3 0.12 0.067 0.050 0.037
Each value represents the average of three-ten individual runs.
Table 11. Lifetime of (2E)Cr(bpy)33+in Deaerated Aqueous 5 M HCI Solutions at 22 OC
(1) Hofmann, P. Angew. Chem., Int. Ed. Engl. 1979, 18, 554. (2) Calvert, R. E.; Shapley, J. R. J. Am. Chem. SOC.1977, 99,5225. Jones, R. A.: Maiik, K. M. A,; Wilkinson, G. J. Am. Chem, (3) Hursthouse, M. 0.; SOC.1979, 101.4128. (4) Hermann, W. A,; Kruger, C.; Goddard, R.; Bernal. I. Angew. Chem., Int. Ed. Engl. 1977, 16, 334. (5) Hermann. W. A,; Reiter, E.; Biersack, H. J. Organomet. Chem. 1975, 97, 245. (6) Fischer, E. 0.; Kiener, V.; Fischer, R. D. J. Organomet. Chem. 1989, 16, P60. (7) Blankenship. L. T. M.S. Thesis, The University of Texas at Austin, 1973. (8)Collins, R. L., personal communication. (9) Two data sets were acquired with the same crystal of 1: one at room temperature (22 "C) for comparison with the room temperature structure Of Fez(C0)g(see ref 10) and another at -35 O C . Crystallographic experimental procedures were essentially as previously delineated: Riley, P. E.; Davis, R. E. Acta Crystallogr., Sect. B 1976, 32, 381. (10) Cotton. F. A , ; Troup. J. M. J. Chem. SOC.,Dalton Trans. 1974, 800. (11) Powell, H. M.; Ewens, R. V. G. J. Chem. SOC. 1939, 286. (12) That is, each bridging position is occupied, statistically, 2 / 3 by CO and '13 by CH2. Since the two presumably slightly different C positions would be indistinguishable by this experiment, the crystallographic description is ' 1 3 ) and thus one with a full-weight C (i.e.. 2/3 ' I s ) , a,z/~-weight0 ('/3 two '/3-weight H atoms at each bridging position. (13) Full details of the crystal structures of 1 will be published subsequently. (14) Tebbe, F. N.: Parshall. G. W.; Ovenall. D. W. J. Am. Chem. SOC.1979, 101, 5074.
[ C r ( b ~ y ) ~ ~M +], 1.2 2.8 4.6 4.7 5.7 9.4
x
10-5
X
x x x x 1.0 x 2.0 x 2.8 x 4.7 x 6.0 x 9.0 x
10-5 10-5 10-5 10-4 10-4 10-4
10-4 10-4 10-4
T , msa
[ C r ( b ~ y ) 3 ~ +M 1,
0.1 1 0.10 0.1 1 0.12 0.1 1 0.1 1 0.1 I 0.096 0.093 0.085 0.087 0.086
9.4 x 10-4 1.0 x 10-3 1.2 x 10-3 1.5 x 10-3 1.8 x 10-3 3.6 x 10-3 4.8 x 10-3 6.0 x 10-3 1 .o x 10-2 1.1 x 10-2 1.2 x 10-2
T,
msa
0.084 0.078 0.08 1 0.082 0.077 0.060 0.073 0.047 0.041 0.044 0.042
Each value represents the average of three-ten individual runs.
The complexes as C104- salts were available from our previous studies.* Solutions were prepared from reagent grade (Fisher) HCI ( 7% Fe and heavy metals) and water that had been distilled from KMn04 and/or purified by passage through a Millipore train; solutions were deaerated by bubbling with prepurified N2 or Ar for 30 min. Values of T were deterCharles E. Sumner, Jr., Paul E. Riley Raymond E. Davis,* R. Pettit* mined at 22 f 1 OC by the monitoring of the first-order decay of the emission from 2E a t 727 nm excited by pulsed laser ilDepartment of Chemistry, The Unicersity of Texas at Austin lumination. The apparatus in Montreal consisted of a I-kW Austin, Texas 78712 N2 laser providing 4-11s pulses a t 337 nm as previously deReceiued October 1. 1979 scribed by Demas and Flynn.Il The apparatus in Boston utilized a frequency-doubled ruby laser providing 30-ns pulses at 347 nm. The values of 7 (= 1/k&sd) within a single experimental set were reproducible with < 10%variation; the stanGround-State Quenching of the *E Excited State dard deviation of values from replicate experiments were of of C r ( b ~ y ) 3 ~and + Cr(phenh3+ the order of IO- 15%. Sir: The data for C r ( ~ h e n ) 3 ~in+I M HCI given in Table I show that T decreases by an order of magnitude as [ C r ( ~ h e n ) 3 ~ + ] The lowest metal-centered excited state (2E) of polypyridyl is increased from 1 X IOT5 to 1 X M. The data produce complexes of Cr(ll1) [ C I - ( N N ) ~ ~has + ] been shown to be a a h e a r plot of k&sd vs. [ C r ( ~ h e n ) 3 ~ (Figure +] 1) with a slope powerful oxidizing agent capable of engaging in excited-state electron-transfer reactions* and photoelectrochemical a ~ t i o n . ~ of 2.3 X I O h M-I s-I and an intercept of 3.0 X I O 3 s-'. By comparison, the average of ten individual experiments yields Knowledge of the lifetime ( 7 ) of 2E is necessary in order to 7 = 0.25 ( f 0 . 0 3 ) ms over the substrate concentration range evaluate the photophysics of the state, calculate rate constants 1.2 X to 1.2 X 1 0-3 M in the absence of HCI; solubility ( k q ) from Stern-Volmer luminescence quenching, and engirestrictions prevent the extension of the range in neat H20. The neer potential solar energy conversion schemes. Because of the quenching phenomenon is the same in either 1 M HCI or 1 M thermal and photochemical stability of Cr(NN)33+ in acidic NaCI. In contrast, 7 values for (2E)Cr(bpy)33+ in 1 M HC1 aqueous s ~ l u t i o n 1, ~M~HCI ~ has been the medium of choice and neat H2O are virtually indistinguishable: T = 0.073 for many lifetime measurements. Examination of data from (&0.008) ms in 1 M HCI and 0.068 (&0.008)ms in H20 over many l a b o r a t ~ r i e sreveals ~ - ~ ~ variations in the values of T and the substrate concentration range 1.4 X to 2.5 X M. k , for ostensibly identical systems and experimental conditions. I n order to observe the ground-state quenching phenomenon We note, however, that the concentrations of the ground-state for C r ( b ~ y ) 3 ~5+ M , HCI was used; thedata (Table 11) result complexes are often not specified and can vary over many orin a linear quenching plot (Figure 2) with aslope of 1.3 X IO6 ders of magnitude depending upon the experimental technique M-l s-' and an intercept of 1 .O X 1 O4 s-l. or the application. In this communication we report the deThe use of different samples of the recrystallized complexes, pendence of T of (2E)Cr(phtn)33+ (phen = I,lO-phenananalytical reagent grade acid, and purified water had no effect throline) and ( * E ) C r ( b ~ y ) 3 ~(bpy + = 2,2'-bipyridine) upon on the results. I n order to exclude free ligand within the subthe concentration of the ground-state substrates.
+
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0002-7863/80/ 1502-1 754$01 .OO/O
0 1980 American Chemical Society
Communications to the Editor I
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Figure 1. Quenching plot of the data for (2E)Cr(phen)33f in deaerated aqueous 1 M HCI solutions at 22 f 1 OC; points at [substrate]
