ersion of the phosphorane intermediate to the cleavage product. However, the same plot for the isomerization reaction, Figure 2, shows that the rate of isomerization decreases when the [Im] is increased. We
HO.
T.3′ cyclic phosphate
+ R-OH nucleoside
Table I. Pseudo-First-Order Rate Constants ( X I O ) h-I) for the Cleavage and Isomerization of 3′,5″-ApA Catalyzed by Various Concentrations and Ratios of Imidazole Buffer at 80 OC, Corrected for the Buffer Independent Rate
Im/Im-HCI concn, M k(cleavage) k(isom) 100/0
0/100 100/0
O/lOO 100/0 40160 0/100
40160
40160
0.8 0.8 0.8 1.3 1.3 1.3 2.0 2.0 2.0
8.6 f 0.2 -0.31 f 0.26 8.6 f 2.0 -0.51 f 0.46 4.1 f 0.6 0.19 f 0.72
11.4 f 1.7 -1.26 f 0.34 13.9 f 0.3 -0.93 i 0.23
1.13 f 0.75 20.1 f 1.1 -1.20 f 0.09 21.5 f 1.0 -1.33 f 0.12
0.91 f 0.04
7.6 f 1.5
0.0 0.2 0.4 0.6 Uml (M)
Figure 2. Pseudo-first-order rate constant for isomerization of 3′,5″-ApA to 2′,5″-ApA at 80 ‘C as a function of imidazole concentration at two different fixed imidazolium ion concentrations. ., [ImH+] = 0.5 M; A, [ImHt] = 1.0 M. The values are interpolated from plots of the data i n Table I .
see a similar negative catalysis when we plot our previous data on UpU isomerization in this fashion. The negative rate constants in Table I also reflect a decrease in the rate of uncatalyzed (by buffer) isomerization when imidazole is added.
This negative catalytic term is expected for our mechanism, whose kinetics are expressed in e q s 1 and 2. For cleavage [Im]
J . Am. Chem. SOC. 1990, 1 1 2 , 9623-9624 9623
rate of cleavage =
k,k,[ApA][ImH+] + k , k-,[TmH+] + kz[lm] + k 3 + k , rate of isomerization = (2)
appears in both numerator and denominator of eq 1, but for isomerization [Im] appears only in the denominator of eq 2. An increase in [Im] a t constant [ImH+] diverts the common inter- mediate I toward cleavage; this decreases the steady-state con- centration of I and thus the rate of isomerization.
Such a negative kinetic effect would not be seen if the cleavage and isomerization paths did not branch from a common inter- mediate whose concentration is decreased when we catalyze one of the branches; the kinetic data are at early times, so Im does not appreciably decrease the concentration of the starting material itself. The effect is intellectually related to the demonstration that an isotope effect on one path affects the rate of another first used by Katz6 to show that two paths diverge from a common intermediate. Our kinetic version of it does not seem to be widely known or used. In any case, this work shows that the mechanistic conclusions from our previous studies of UpU reactions are indeed soundly based.
Acknowledgment. This work has been supported by the N I H and the O N R .
(6) Katz, T. J.; Cerefice, S . A. J. Am. Chem. SOC. 1969, 91,6519-6521. (7) Brenner. D. G . ; Knowles, J. R . Biochemisrry 1981, 20, 3680-3687.
A Planar Oxocuprate(I1) Array via Heterometallic Alkoxide Chemistry
John A. Samuels, Brian A. Vaartstra, John C. Huffman, Kathleen L. Trojan, William E. Hatfield, and Kenneth G. Caulton*
Department of Chemistry and Molecular Structure Center Indiana University, Bloomington, Indiana 47405
Department of Chemistry, University of North Carolina Venable and Kenan Laboratories
Chapel Hill, North Carolina 27599 Received September 26, 1990
Application of the molecular precursor methodl*2 to the synthesis of copper-based high-temperature superconductors3 rests on our ability to produce copper-containing heterometallic a l k o x i d e ~ . ~ We have reported r e ~ e n t l y ~ * ~ on the chemistry of the anion Zr,- (OiPr)9-, which is related to recent reports by the group of M e h r ~ t r a . ~ # * We report here our investigation of the coupling of this and related anions to CuCI, of relevance to hydrolytic routes to copper/oxo superconductors.
The reaction of K4ZrzO(OiPr),,,~ CuCI2, and water (2:4:1 mole ratio) in a refluxing T H F solution produces a deep olive green solution. Workup (Le., removal of solvent, extraction with pentane,
( 1 ) Hubert-Pfalzgraf, L. G. New J . Chem. 1987, 1 1 , 663. (2) Bradley, D. C. Chem. Reu. 1989, 89, 1317. (3) Bednorz, J. G.; Mgller, K. A.; Takashige, M. Science 1987, 236, 73. (4) Caulton, K. G.; Hubert-Pfalzgraf, L. G . Chem. Reo. 1990, 90, 969. (5) Vaartstra, B. A.; Huffman, J. C.; Streib, W. E.; Caulton, K. G. J .
(6) Vaartstra, B. A.; Streib, W. E.; Caulton, K. G . J . Am. Chem. Soc., i n Chem. SOC., Chem. Commun., i n press.
press.
1988, 341, 569. (7) Dubey, R . K.; Anirudh, S.; Mehrotra, R. C. J . Orgunomet. Chem.
(8) Dubey, R. K.; Singh, A.; Mehrotra, R. C. Polyhedron 1987, 6, 427.
0002-7863/90/15 12-9623$02.50/0
LJ
Figure 1. ORTEP drawing of the non-hydrogen atoms of Cu,Zr403- (top) viewed perpendicular to the C u 4 0 3 plane and (bottom)
viewed along the edge of the C u 4 0 3 plane. Oxygen atoms are stippled. Lines between metals are for clarity and are not bonds. Primes indicate atoms related by a center of symmetry. Selected structural parameters (distances, A; angles, deg): Cu3-06, 1.968 (3); Cu4-06, 1.966 (2);
039′, 1.901 (12); Cu3-031, 1.965 ( 1 1 ) ; Cu4-035, 1.966 ( 1 1 ) ; Cu3’-Cu4 = C u 3 C u 4 , 2.781 (8); cis angles M u – 0 range from 84.5 ( 5 ) O to 104.0
Cu3-05, 1.880 ( 1 8); Cu4-05, 1.896 ( 1 I ) ; Cu3-039, 1.892 ( 1 2); Cu4-
( 5 ) O .
concentration, and layering with 2-propanol) yields a blue-green solid ( 2 5 % yield), which was established9 to have the formula C U ~ ” Z ~ ~ ‘ ~ O ~ ( O ~ P ~ ) , ~ ( l ) , eq I . The centrosymmetric structure
2K4Zr20(OiPr),, + 4CuC1, + H 2 0 – C ~ ~ Z r , 0 ~ ( 0 ~ P r ) , ~ + 8KCI + 2HO’Pr ( I )
is shown in Figure 1 . The molecule contains a planar central
(9) Crystal data “C) for Z ~ , C U ~ C ~ ~ H , , O ~ , – C ~ H , ~ : u = 12.673 (8) A, b = 17.482 (13) d:?= 10.877 (8) A, a = 1_04.85 (3)O, = 113.52 ( 3 ) O , y = 75.65 (3)O with Z = 1 i n space group P I . R ( F ) = 0.0798, R J F ) = 0.0777 for 3101 reflections with F > 2.33a(F)
—————
The phosphorane intermediate is ersioned to the cleavage product. However, the same figure for the isomerization reaction, Figure 2, reveals that when [Im] increases, the rate of isomerization falls. We
HO.
T.3′ cyclic phosphate
+ R-OH nucleoside
Table I. Pseudo-First-Order Rate Constants ( X I O ) h-I) for the Cleavage and Isomerization of 3’,5”-ApA Catalyzed by Various Concentrations and Ratios of Imidazole Buffer at 80 OC, Corrected for the Buffer Independent Rate
Im/Im-HCI concn, M k(cleavage) k(isom) 100/0
0/100 100/0
O/lOO 100/0 40160 0/100
40160
40160
0.8 0.8 0.8 1.3 1.3 1.3 2.0 2.0 2.0
8.6 f 0.2 -0.31 f 0.26 8.6 f 2.0 -0.51 f 0.46 4.1 f 0.6 0.19 f 0.72
11.4 f 1.7 -1.26 f 0.34 13.9 f 0.3 -0.93 i 0.23
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