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Kinetics of Hydride Transfers from CH SiH GeH and SnH Groups to Carbenium Ions.

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and carbon-centered "multiple warhead" tetraradical 10, a
potentially powerful species for DNA damage. In the event,
however, none of the products expected to arise from 10 were
formed. Instead, the two novel dihydroperoxides 8 and 9
were isolated in 22 and 10% yield, respectively. The same
transformations were also effected, although less cleanly, by
thermolysis in the presence of hydrogen atom donors. Quite
intriguing is the possibility of the intermediacy of a dioxetener'] which is presumed to eject the equivalent of ethenedione,"] perhaps as two carbon monoxide molecules.[91
Compounds2 and 3 caused significant DNA cleavage
when incubated with +XI74 supercoiled DNA at pH 8.5 and
50 "C (Fig. 2). Interestingly, dione 4 caused only slight DNA
damage under the same conditions, whereas hydroperoxides 8 and 9 showed, as expected, quite strong DNA-cleaving
activities. Based on the chemistry of these systems we presume that whereas the ten-membered ring enediynes 2 and 3
may exert their DNA-cleaving power by cyclization to an
aromatic diradical, the open-chain enediynes 8 and 9 may
operate by an entirely different mechanism involving the
generation of oxygen-centered radicals.
1
2
3
L
5
6
Fig. 2. Interaction of supercoiled DNA with enediynes 2-4, 8, and 9. 4x174
DNA was incubated for 48 h at 50 "C with the enediynes in buffer ( S O mM
Tris-HCI, pH 8.5) and analyzed by electrophoresis (1 % agarose gel, ethidium
bromide stain). Lane 1 : DNA control; Lane 2: DNA+ cis diol 3 (SOOO VM);
Lane 3 : DNA + t i a m diol 2 (SOOO PM); Lane 4: DNA + dione 4 (5000 VM);
Lane 5: DNA + trans enediyne 8 (5000 FM); Lane 6: DNA + cis enediyne 9
(SOOO p ~ )1-111
.
stand for form I, form 11, and form III DNA, respectively.
The described chemistry demonstrates a highly convergent route to conjugated cyclic enediynes, some fascinating
stereochemical effects of rigid ten-membered ring systems,
the introduction of new "locking devices" to stabilize labile
enediynes, and the DNA-cleaving action of a number of new
enediynes.
Received: March 19. 1992 [Z 5250 IE]
German version: Angew. Chem. 1992, 104, 1094
[l] For recent reviews, see: K. C. Nicolaou, W.-M. Dai, Angew. Chem. 1992,
103, 1453; Angen,. Chem. Int. Ed. Engl. 1991, 30, 1387; M. D. Lee, G. A.
Ellestad, F. B. Borders, Arc. Chem. Res. 1991, 24, 235.
[2] For the design and synthesis of the first monocyclic ten-membered ring
enediynes with DNA-cleaving properties, see: K. C. Nicolaou, G. Zuccarello, Y. Ogawa, E. J. Schweiger, T. Kumazawa, 1 Am Chem. SOC.1988,110,
4866; K. C. Nicolaou, Y. Ogawa, G. Zuccarello, H. Kataoka, ibid. 1988,
110, 7247.
[3] For a theoretical discussion of the effect of substituents at the bridge connecting enediyne moiety, see: J. P. Snyder, I Am. Chem. Soc. 1990, ff2,
5267.
[4] Dialdehyde 1 was prepared by two methods starting from either 2,2dimethyl-l,3-propdnediolor methyl acetoacetate and (.Z-1,2-dichloroethene.
[ 5 ] R. Kaptein, I Chrm. SOC.1971, 732.
I
Am Chem. Soc. 1981, 103,4091.
[6] T. P. Lockhart, R. G. Bergman, .
[7] A similar dioxetene system has been previously postulated: N. J. Turro, V.
Ramamurthy, K.-C. Liu, I Am. Chem. Soc. 1976, 98, 6758.
[8] The existence of ethenedione is debatable; see: M. B. Rubin, A. Patyk, W.
Sander Tetrahedron Letr. 1988,29, 6641; D. M. Birney, 1.A. Berson, Tetrahedron, 1986,42, 1561; G. P. Raine, H. F. Schaefer, R. C. Haddon, I Am.
Chem. Soc. 1983, 105, 194 and references cited therein.
[9] For references on the photochemistry of a-diketones, see: M. B. Rubin,
Fortschr. Chem Forsch. 1969, 13, 251; G. E. Gream, J. C. Paice, C. C. R.
Ramsay, Austr. J. Chem. 1967,20, 1671.
1046
0 VCH Verlagsgesellschaft mbH, W-6940 Weinhelm. 1992
Kinetics of Hydride Transfers from CH, SiH,
GeH, and SnH Groups to Carbenium Ions**
By Herbert Mayr* and Nils Basso
Dedicated to Professor George A . Olah
on the occasion of his 65th birthday
Hydrides of the group IV elements are important hydrogen sources in radical and ionic reductions." - 4 1 While homolytic fissions of these element-hydrogen bonds have been
studied intensively,['] the current kinetic data on ionic hydride-transfer reactionsl6- 1' do not allow a direct comparison of the reactivity of these compounds toward hydride
acceptors. We now report on the kinetics of the reactions of
triphenylmethane and of trisubstituted germanes and stannanes with diarylcarbenium ions, and we compare these data
with the recently published rate constants for the hydride
abstractions from silanes.[61Because of the huge differences
in reactivity, not all experiments could be carried out with
the same hydride acceptor; We used linear free energy relationships for a quantitative comparison of these reducing
agents.
The recently described photometric method for determining the kinetics of the reactions of diarylcarbenium ions with
silanes[6d1was also employed here for the investigation of
element hydrides with the general formula HER,. Since the
nature of the counterion (trifluoromethanesulfonate (OTf-)
or TiC1;) was found not to affect the rates of the reactions
of the bis-para-methoxy-substituted benzhydryl cation (pMeOC,H,),CH+ with tributylgermane and triphenylstannane, we conclude that under the selected reaction conditions, the counterion is not involved in the rate-determining
step, as previously reported for silanes (Scheme 1).
Y
Y
Y
Scheme 1
In analogy to the reactions of diarylcarbenium ions with
alkenes, the reactions of free and paired carbenium ions have
identical rates, resulting in a second-order rate law [Eq. (a)],
where [Aryl,CH+] is the sum of the concentrations of free
and paired carbenium ions.
d[Aryl,CH']/dt
=-
k,[Aryl,CH+][HER,]
(4
When the rate constants listed in Table 1 are plotted
against the rate constants of the reference reaction previously used, Aryl,CH+ + 2-methyl-I -pentene,['] almost parallel
lines are obtained (Fig. l), indicating that the relative reac-
["I
Prof. Dr. H. Mayr,'" Dip].-Ing. N. Basso
Institut fur Chemie der Medizinischen Universitat
Ratzeburger Allee 160, D-W-2400 Lubeck (FRG)
['I New address:
Institut fur Organische Chemie der Technischen Hochschule
Petersenstrase 22, D-W-6100 Darmstadt (FRG)
[**I This work work was supported by the Deutsche Forschungsgemeinschaft
and the Fonds der Chemischen Industrie.
0570-0833192/0808-l046$3.500+ ,2510
Angew. Chem. Int. Ed. Engl. 1992, 31, N o . 8
Table 1. Rate constants of the reactions of p,p'-substituted benzhydryl cations (pXC,H,)@-YC,H,)CH+ (counterion MZ, I ) with element hydrides HER, in CH,Cl, at
- 70 "C.
X
HER,
Y
MZ,,
c1
HCPh,
HSiPh,
CI
OMe
OMe
OMe
OPh
Me
HSinBu, OMe
OMe
OMe
OMe
HGePh, OMe
OMe
OPh
Me
HGenBu, NMe,
OMe
OMe
OMe
OMe
HSnPh, NMe,
OMe
OMe
OMe
OMe
HSnnBu, NMe,
GaCI;
OTfTiCI;
TiCI;
TiCI;
TiCI;
TiCI;
TiCl;
TiCl;
TiCI;
TiCl; /OTf
TiCI;
TiCI;
TiCI;
OTfTiCl;/OTfTiCI;
TiCl;
TiCI;
OTfOTf-/TiCI;
TiCl;
TiCl;
TiCl;
OTf-
OMe
Me
H
H
Me
OMe
OPh
Me
H
OMe
H
H
Me
NMe,
OMe
OPh
Me
H
NMe,
OMe
OPh
Me
H
NMe,
AH*
k,
AS'
[K'S-'] [kJmol-'1
[Jmol-'K-']
6.57 10-3
6.52 x lo-'
2.22
8.27
4.52 x 10'
3.47 x 102
4.30
2.15 x 10'
1.22 x 102
3.85 x 10,
1.59
1.14 x 10,
5.86 x 10,
2.96 x 103
8.8 x 10-6
3.19 x 10,
2.11 x lo3
4.57 x 103
2.8 104
1.01 x 10-4
1.13 x 10'
2.65 x 10'
1.78 103
4.98 x 103
4.22 x lo-'
37.9
- 97
22.2
-115
51.2
- 86
38.2
-130
37.2
Lit.
- 85
tivities of these element hydrides are almost independent of
the electrophilicities of the hydride abstractors (constant selectivity relationship according to Ritchie).IgC3
lo] Intersections of these lines may be observed, however, when a very
wide range of reactivity is considered: The biskara-dimethyl, which is
aminopheny1)carbenium ion (p-Me,NC,H,),CH
5 to 10 orders of magnitude less electrophilic than the other
carbenium ions examined in this study, reacts 11.5 times
more slowly with HGenBu, than with HSnPh,, while HGenBu, is three to eight times more reactive than HSnPh, toward the carbenium ions shown in Figure 1.
+
para-methoxy-substituted benzyhydryl cation AnPhCH'
(An = p-MeOC,H,) (Table 2). The k , value for HSnnBu, is
calculated from the relative reactivities of HSnnBu, and
HSnPh, toward (p-Me,NC,H,),CH+. The corresponding
rate constant for HCPh, is derived from the relative
reactivities of HCPh, (Table 1) and HSiPh, toward (pClC,H,),CH+.
The latter value, IgkJHSiPh,
(pCIC,H,),CH+, 70 "C] = 5.39, is extrapolated from the correlation of lg k , (HSiPh,) with pK,+ of the particular hydride
abstractors.lfidl
Table 2 indicates that the decrease of the enthalpy of homolytic bond dissociation (C - H 2 Si - H > Ge - H >
Sn - H) is associated with an acceleration of the hydride and
of the hydrogen radical abstractions. Whereas in radical reactions the triphenyl-substituted compounds are generally
somewhat more reactive than the trialkyl-substituted ones,
in ionic reactions the ratio k,(HEBu,)/k,(HEPh,) grows
with the size of the core atom from less than 1 for HCR, to
a value of approximately 400 for HSnR,. This trend may be
+
Table 2. Comparison of the rates of hydride abstractions to the rates of H abstraction
from HER, and the corresponding bond dissociation energies (BDE).
R
k,(AnPhCH+) [a] nBu
Ph
k,,lk,,
nBu
k,(tBuO) [d]
Ph
k,,lkm
BDE
nBu
[kJmol- '1
Ph
HCR,
(2 x lo-') [b]
< 1 [cl
2.7 x lo5
2 . 6 ~lo6
0.1
390[f]
339LjJ
HER,
HSiR,
3.9 x 102
8.3
4.7 x 10'
5.7 x lo6 [el
1.1 x 107
0.5
377 [gl
% 356 [k]
HGeR,
HSnR,
2.8 x 10,
i.ixio2
2.5 x 10,
8.0 107
8.9 x 10'
0.9
346 [h]
336 [h]
(2 x lo6) [b]
s.oX103
2 4 . 2 x 10,
2.2 x 108
4.0 x 108
0.5
308 [i]
[a] This work; determined at 203 K. [b] Extrapolated, see text. [c] Approximation
based on the different stabilization of carbenium ions by phenyl and alkyl substituents.
[d] Determined at 300 K ; ref. [ l l a]. [el Et3SiH. [fl Me,CH, ref. [ l l b]. [g] Et,SiH, ref.
[Ilc]. [h] Ref. [lld]. [i] Ref. [llc]. ti] Ref. [lle]. [k] K. B. Clark, D. Griller, National
Research Council of Canada, personal communication.
explained by the decreasing ability of the phenyl group to
donate x electrons as the size of the central atom increases.
The considerably greater influence of the nature of the core
atom on the rates of the ionic reactions (compared with the
radical reactions) may be caused by the decrease of the ionization potential in the series R,C' to R,Sn' as well as by a
more productlike transition state of the ionic reactions.
OMe, OMe OMe, Me
OPh, H
j OMe,OPh
; 0Me.H i Me,Me
6.0-
.. . ..
HSinBu3
Received: March 5,1992 I25227 IE]
German version: Angew. Chem. 1992, 104, 1103
-2.0
-I
L
-2.0
.
.
:
I
..
.
..
:
I
I
I
I
0.0
<
...
..
I
I
I
1
I
I
I
2.0
I
I
4.0
Fig. 1. Correlation of the reactivities (CH,CI,, - 70 "C) of substituted diaryl(para substituents X and Y on the
carbenium ions @-XC,H,)(p-YC,H,)CH'
upper edge of the graph) versus several hydride donors HER, and 2-rnethyl-lpentene (reference reaction [Sc]).
When these small differences in slopes are neglected, a
reactivity scale of hydride donors can be given, which is
conveniently based on the reactivities toward the monoAngew. Chem. Int. Ed. Engl. 1992,31, No. 8
0 VCH
[1] General reviews: a) Reduction (Ed.: R. L. Augustine), Dekker, New York,
1968; b) M. Hudlicky, Reductions in Organic Chemistry, Horwood,
Chichester, 1984; c) A. Hajos, Methoden Org. Chem. fHouben-Weyl) 4th
ed. 1952-, Vol. I V / l d , 1981, pp. 1-486; d) F. G. A. Stone, Hydrogen Compounds of the Group IV Elements. Prentice Hall, New York, 1962.
[2] Silanes: a) D. N. Kursanov, 2. N. Parnes, Russ. Chem. Rev. (Engl.
Trans/.) 1969, 38, 812; b) F. A. Carey, Intra-Sri. Chem. Rep. 1973, 7 , 55;
c) D. N. Kursanov, Z. N. Parnes, N. M. Loim, Synthesis 1974, 633; d) Y.
Nagai, Org. Prep. Proced. fnt. 1980.12.13; e) E. Keinan, Pure Appl. Chem.
1989, 61, 1737; f) E. W. Colvin, Silicon Reagents in Orgonic Synthesis,
Academic Press, London, 1988; g) S. Pawlenko, Methoden Org. Chem.
(Houben- Wey!) 4th ed. 1952-, Vol. XIIIj5, 1980, pp. 350-360.
[3] Germanes: a) D. Quane, R. S. Bottei, Chem. Rev. 1963, 63, 403; b) G.
Bahr, H.-0. Kalinowski, Methoden Org. Chem. (Houben- Weyl) 4th ed.
1952-, Vol. XIII/6, 1978, pp. 61-67.
[4] Stannanes: a) R. K. Ingham, S . D. Rosenberg, H. Gilman, Chem. Rev.
1960, 60, 459; b) H. G. Kuivila, Synthesis 1970, 499; c) W. P. Neumann,
ibid. 1987, 665; d) M. Pereye, J.-P. Quintard, A. Rahm, Tin in Organic
Synthesis, Buttenvorth, Londen, 1987; e) I. Omae, Orgonotin Chemistry,
Elsevier, Amsterdam, 1989; f) Chemistry of Tin (Ed.: P. G . Harrison),
Yerlagsgeseiischaff mbH, W-6940 Weinheim, 1992
0570-0833/92/0808-l047 8 3.50+ ,2510
1047
Blackie & Son, Glasgow, 1989; g) G. Bahr, S. Pawlenko, Merhoden Org.
Chem. (Houben-Wevl) 4th ed. 1952-, Vol. XIII/6, 197'8, pp. 451-470.
[S] a) J. M. Kanabus-Kaminska, J. A. Hawari, D. Griller, C. Chatgilialoglu, J.
Am. Chem. Soc. 1987,109,5267;b) M. Lesage. J. A. Martinho Simoes, D.
Griller, J. Org. Chem. 1990, 55, 5413; c) P. Arya, C. Samson, M. Lesage,
D. Griller, ibid. 1990, 55, 6248; d) M. Ballestri, C. Chatgilialoglu, K. B.
Clark, D. Griller, 9. Giese, B. Kopping, ibid. 1991, 56. 678; e) U. Muller,
E. Popowski, Z . Phys. Chem. (Leipzigj 1990, 271, 703; f) J. Lusztyk, B.
Am. Chem. Soc. 1983,105,3578;
Maillard, D. A. Lindsay, K. U. Ingold, .l
g) J. Lusztyk, B. Maillard, S. Deycard, D. A. Lindsay, K. U. Ingold. J.
Org. Chem. 1987, 52, 3509; h) K. U. Ingold, J. Lusztyk, J. C. Scaiano, J.
Am. Chem. SOC.1984, 106, 343; i)C. Chatgilialoglu, K. U. Ingold, J. C.
Scaiano, ibid. 1981, 103, 7739; j) J. 0. Metzger, Methoden Org. Chem.
(Houben- Weylj 4th ed. 1952-, Vol. E XIXa, Part 1, 1989, p. 91; k) C.
Chatgilialoglu, Acr. Chem. Res. 1992, 2.5, 188.
[6] Silanes: a) J. Chojnowski, W. Fortuniak, W. Stanczyk, J. Am. Chem. SOC.
1987, 109, 7776; b) J. Chojnowski, L. Wilczek, W. Fortuniak, J.
Organomet. Chem. 1977, 135, 13; c) F. A. Carey, C.-L. Wang Hsu, ibid.
1969, 19, 29; d) H. Mayr, N. Basso, G. Hagen, J. Am. Chem. Soc. 1992,
114, 3060.
[7] Stannanes; a) J. Lusztyk, E. Lusztyk, B. Maillard, K. U. Ingold, J. Am.
Chem. Soc. 1984,106,2923;b) J.-P. Quintard, M. Pereye, Bull. SOC.Chim.
Fr. 1972, 5 , 1950; c) A. J. Leusink, H. A. Budding, W. Drenth, J.
Organomet. Chem. 1968, 13, 163; d)A. J. Leusink, H. A. Budding, W.
Drenth, ibid. 1967, 9, 295; e) G. L. Grady, J. R. Saucier, W. J. Foley 111,
D. J. O'Hern, W. J. Weidmann, ibid. 1972,35, 307; f) D. D. Tanner, H. K.
Singh, J. Org. Chem. 1986, 51, 5182.
[8] Hydrocarbons: a) N. Deno, G. Saines, M. Spangler, J. Am. Chem. SOC.
1962,84,3295; b) M. M. Kreevoy, 1 . 4 . H. Lee, ibid. 1984,106,2550;c) D.
Ostovic, 1.4. H. Lee, R. M. G. Roberts, M. M. Kreevoy, J. Org. Chem.
1985,50,4206; d) M. M. Kreevoy, D. Ostovic, 1.3. H. Lee, D. A. Binder,
G. W. King, J. Am. Chem. Sac. 1988, 110, 524; e) M. M. Kreevoy, A. T.
Kotchevar, ibid. 1990, 112, 3579; f) A. Ohno, T. Shio, H. Yamamoto. S.
Oka, ibid. 1981, 103, 2045; g) A. van Laar, H. J. van Ramesdonk, J. W.
Verhoeven, Reel. Trav. Chim. Pays-Bas 1983, 102, 157; h) T. T. Romoff,
N. S. Sampson, P. van Eikeren, J. Org. Chem. 1987, 52, 4454; i) J. W.
Bunting, M. A. Luscher, Can. J. Chem. 1988, 66, 2524; j) J. W. Bunting,
M. M. Conn, ibid. 1990, 68, 537; k) M. Ishikawa, S. Fukuzumi, J. Chem.
Soc. Chem. Commun. 1990, 1353; I) S . J. Hannon, T. G. Traylor, J. Org.
Chem. 1981, 46, 3645; m)T. G. Traylor. G. S. Koenner, ibid. 1981, 46,
3651.
[9] H. Mayr, R. Schneider, C. Schade, J. Bartl, R. Bederke, J. Am. Chem. Soc.
1990, 112, 4446; b) H . Mayr, Angew. Chem. 1990, 102, 1415; Angew.
Chem. Inl. Ed. Engl. 1990,29,1371; c) H. Mayr, R. Schneider, U. Grabis,
J. Am. Chem. SOC.1990, 112,4460.
[lo] a) C. D. Ritchie, Ace. Chem. Res. 1972, 5 , 348; b) C. D. Ritchie, Can. J.
Chem. 1986,64,2239.
1111 a) C. Chatgilialoglu, K. U. Ingold, J. Lusztyk, A. S. Nazran, J. C. Scaiano,
Organomerallics 1983, 2, 1332; h) D. F. McMillen. D. M. Golden, Annu.
Rev. Phys. Chem. 1982, 33, 493; c) D. Griller, D. D. M. Wayner, Pure
Appl. Chem. 1989, 61, 717; d) K. 9. Clark, D. Griller, Organomefaliirs
1991, 10, 746; e) F. G. Bordwell, J.-P. Cheng, G.-Z. Ji, A. V. Satish, X.
Zhang, J. Am. Chem. Soc. 1991, 113,9790.
forts at investigating functional metal complexes that initiate
DNA cleavage by unusual mechanisms,141
and have reported
efficient cleavage of DNA by the dicationic complex lL5I
(bpy = 2,2'-bipyridine, tpy = terpyridine). 1 was generated
via oxidation of 2 [Eqs. (a) and (b)J.["J
[Ru"OHz(bpy)(tpy)12+
[Ru"'OH(bpy)(tpy)12+
By Nishi Gupta, Neena Grover, Gregory A . Neyhart,
Weigen Liang, Phirtu Singh, and H . Holden Thorp*
The development of new DNA cleavage agents is of interest for the designing of synthetic restriction enzymes,"] for
an understanding of the structure of DNA,['] and for pharmaceutical application^.^^] We have recently directed our ef[*I
Prof. H. H. Thorp, Dr. N. Gupta, N. Grover, Dr. G. A. Neyhdrt,
W. Liang, Dr. P. Singh
Department of Chemistry
North Carolina State University
Raleigh, NC 27695 (USA)
[**I This work was supported by the National Science Foundation (Presidential Young Investigator Award), the David and Lucile Packard Foundation (Fellowship in Science and Engineering), and the North Carolina
Biotechnology Center. Helpful discussions with Prof. M.-H. Whangho are
gratefully acknowledged.
1048
0 VCH
Verlagsgesellschaft mbH, W-6940 Weinheim, 1992
+ e- + H +
(a)
+ +H+
[ R ~ ' ~ O ( b p y ) ( t p y ) ] ~ +e 1
(b)
The DNA cleavage with 1 can be effected either stoichiometrically by addition of 1 to the DNA or electrocatalytically by application of a potential of 0.8 V to a solution of DNA
and 2.['I In attempts to increase the binding affinity of these
agents, we have now prepared the dicationic complex 3 (dppz = dipyridophenazine, see Scheme 1).
The planar dppz ligand imparts high DNA affinity to the
complex 4."] Studies with topoisomerase showed that 4 unwinds DNA by 30°, strongly implicating an intercalative
binding mode. Complex 4 has very attractive photophysical
properties, but does not exhibit DNA cleavage reactivity.
Complex 3 also exhibits a high DNA affinity while retaining
the DNA cleavage reactivity of the Ru"OH,/RU'"O functionality. Furthermore, the X-ray crystal structure of this
complex shows extensive x-stacking of the planar dppz ligand (see Fig. 3), which may provide insight into the high
DNA affinity of dppz complexes.
Reaction of Ru(tpy)Cl, with dppz in the presence of triethylamine affords the complex [RuCl(dppz)(tpy)]CI. Treatment of an aqueous solution of this complex with AgCIO,
leads to the precipitation of 3-(C104), . H,O, in the form of
black crystals suitable for X-ray structure analysis. The complex exhibits a typical Ru" --t py(x*) metal-ligand charge
transfer (MLCT) absorption band with A,,,,, = 482 nm ( E =
12000 M-lcm-'). In aqueous solution, the complex is oxidized sequentially to the dicationic Ru" complex [Eqs. (c)
and (d)].
3
[RuO(dpp~)(tpy)]~':a DNA Cleavage Agent with
High DNA Affinity**
[Ru"'OH(bpy)(tpy)12+
2
[Ru'"OH(dppz)(tpy)]*+
[Ru"'(dppz)(tpy)12+
+ e - + H'
(C)
+ + H+
[ R ~ ' ~ O ( d p p z ) ( t p y ) ] ~ +e 5
( 4
At pH 7, E,,,(Ru~"/Ru") = 0.56 V and E,,,(Ru'"/Ru"') =
0.62 V (SCE), compared to 0.49 V and 0.62 V in the case of
the complex 2.[61The pK, value of the aqua ligand in 3 is 8.6
compared to a pK, of 9.7 in 2.16]The structure of 3 is shown
in Figure 1 .[81 The coordination about the Ru center is very
similar to that in both [R~"OH,(tmen)(tpy)]~+(tmen =
N,N,N',N',-tetramethylethylenediamine)lgland 2." Thus,
incorporation of the strongly intercalating dppz ligand has
not altered the electronic properties of the Rul'OH, functionality that are vital to the DNA cleavage reactivity.
Complex 3 binds very strongly to DNA. We have determined the binding constants for a series of complexes related
to 2 using an electrochemical method based on the cyclic
voltammetric currents for the Ru"'/Ru" redox pairs of the
complexes with and without DNA.191When the complex is
0570-0833/92/0808-l048 $3.50+ .25/0
Angew. Chem. Ini. Ed. Engl. 1992, 31, N o . 8
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