Tuning the regiospecificity of cleavage in Fe(III) catecholate complexes: Tridentate facial versus meridional ligands

Article abstract of DOI:10.1002/1521-3773(20001201)39:23<4284::AID-ANIE4284>3.0.CO;2-I

A facial tridentate ligand is a key feature of iron catecholate complexes that elicit extradiol cleavage of catechols (see picture). The facial geometry allows O₂ and the catecholate dianion to form a tridentate intermediate on the opposite face. Accordingly, complexes of a meridional tridentate ligand do not elicit any extradiol cleavage but instead yield quinone or result in intradiol cleavage.

Full text of DOI:10.1002/1521-3773(20001201)39:23<4284::AID-ANIE4284>3.0.CO;2-I

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Tuning the Regiospecificity of Cleavage in FeIII
Catecholate Complexes: Tridentate Facial
versus Meridional Ligands**
additional databases 18 and 19 <Scheme 4) to establish the
complete structure of the desertomycin/oasomycin class of
natural products.
Received: August 8, 2000 [Z15604]
Du-Hwan Jo and Lawrence Que, Jr.*
Bacterial catechol dioxygenases are a component of
natureꢁs strategy for degrading aromatic molecules to ali-
phatic products in the environment.[1] These enzymes catalyze
the oxidative cleavage of catechols, which leads to the scission
[1] Y. Kobayashi, C.-H. Tan, Y. Kishi, Angew. Chem. 2000, 112, 4449;
Angew. Chem. Int. Ed. 2000, 39, 4279.
[2] The degradation product 2b was also obtained from oasomycin B in
five steps: 1) O3/MeOH/ À 788C; 2) NaBH4/MeOH/RT; 3) Ac2O/Py;
4) EtSH/conc. HCl; 5) Ac2O/Py.
[3] We thank Dr. Gerhard Kretzschmar for samples of oasomycins A and
B.
[4] a) H. C. Brown, K. S. Bhat, J. Am. Chem. Soc. 1986, 108, 5919;
b) H. C. Brown, K. S. Bhat, R. S. Randad, J. Org. Chem. 1989, 54,
1570.
[5] Y. Kobayashi, J. Lee, K. Tezuka, Y. Kishi, Org. Lett. 1999, 1, 2177.
[6] The enantiomeric excess was estimated from the 19F NMR spectrum
of its <R)-Mosher ester.
[7] The minor product was removed by column chromatography on silica
gel.
[8] a) S. D. Rychnovsky, D. J. Skalitzky, Tetrahedron Lett. 1990, 31, 945;
b) D. A. Evans, D. L. Reiger, J. R. Gage, Tetrahedron Lett. 1990, 31,
7099.
[9] E. J. Corey, P. D. Jardine, S. Virgil, P.-W. Yuen, R. D. Connell, J. Am.
Chem. Soc. 1989, 111, 9243.
À
À
of the C1 C2 <intradiol) or C2 C3 <extradiol) bond. Intra-
diol-cleaving enzymes have an iron<iii) active site, while
extradiol-cleaving enzymes require FeII or MnII.[1, 2] To date
the factors that determine the regiospecificity of cleavage are
not well understood. Most biomimetic studies have focused
on iron<iii) catecholate complexes with tetradentate ligands,
all of which performed only intradiol cleavage.[3] However,
the few examples of iron<iii) catecholate complexes having
tridentate ligands result in at least some extradiol cleavage
products.[4] To further investigate the factors that determine
the cleavage site, we characterized a series of mononuclear
iron<iii) catecholate complexes [<L)Fe<DBC)Cl] [L ˆ Me3-
TACN <1), TPY <2), BnBPA <3)],[5] containing tridentate
ligands that can coordinate the metal center in a facial or
meridional fashion. Their reactivity towards O2 provides
insight into the factors that tune the regiospecificity of
cleavage.
[10] Dihydroxylation of 9 in the presence of the Corey <S,S)-ligand gave a
1:7mixture of the two diols.
[11] The dithiane 11 was prepared from l-<‡)-arabinose. By using Grayꢁs
procedure <M. Y. H. Wong, G. R. Gray, J. Am. Chem. Soc. 1978, 100,
3548), l-<‡)-arabinose was transformed to the diethylthioacetal,
corresponding to structure 4 in Grayꢁs paper. This diethylthioacetal
was then subjected to the following three steps: 1) TBDPSCl,
imidazole, DMF, RT; 2) HgO, HgCl2, acetone/H2O <10/1), RT;
3) 1,3-propanedithiol, BF3 ´ OEt2, CH2Cl2, 08C <26% overall yield
from l-<‡)-arabinose).
The reactions of [<L)FeCl3], DBCH2, and NaOCH3 in a
1:1:2 ratio in CH2Cl2 under N2 afforded complexes 1 ± 3 as
purple-black solids, which were recrystallized from THF/
hexane, acetone, and DMF/Et2O, respectively. All of these
complexes have high-spin iron<iii) centers and exhibit two
intense catecholate-to-iron<iii) charge transfer bands in the
spectral region of 400 ± 1000 nm, similar to [<TPA)-
FeIII<DBC)]BPh4 <4).[3c] The crystal structures of 1 and 2
<Figure 1) reveal a distorted octahedral geometry with a facial
<1) or meridional <2) tridentate ligand, a catecholate dianion,
and a chloride ligand at the sixth coordination site.[6] The
[12] D. A. Evans, K. T. Chapman, E. M. Carreira, J. Am. Chem. Soc. 1988,
110, 3560.
[13] Acetonization <MeC<OMe)2Me, acetone, CSA) of 13 yielded an
appromixately 1:1:1 mixture of the acetonides. The acetonide d was
isolated in a pure form by chromatography on silica gel.
[14] 2b <synthetic): 1H NMR <500 MHz, CDCl3): see Figure 1; 13C NMR
<100 MHz, CDCl3): d ˆ 9.4, 10.7, 20.8 ± 21.2 <overlapped), 34.8, 35.5,
36.6, 37.0, 37.3, 37.6, 39.1, 61.9, 65.1, 65.6, 67.1, 67.2, 67.6, 67.9, 71.5, 71.6,
71.8, 77.2, 169.9 ± 170.5 <overlapped); [a]2D0 ˆ ‡5.7< c ˆ 0.23 in
MeOH). 2b <natural): 1H NMR <500 MHz, CDCl3): see Figure 1;
13C NMR <100 MHz, CDCl3): identical to synthetic 2b; [a]2D0 ˆ ‡7.8
<c ˆ 0.25 in MeOH).
À
À
Fe N and Fe O bond lengths in both complexes are typical of
À
high-spin iron<iii) complexes. The fairly long Fe Cl bond
lengths <average 2.385<2) Š for 1 and 2.325<1) Š for 2)
suggest that the chloride ligand should be highly labile in
solution.
[15] Oasomycins A and B are known to exhibit significant differences in
the chemical shifts of the carbon atoms and protons at the C20 ± C23
region; see the structures in Scheme 1 and the references quoted in A.
Bax, A. Aszalos, Z. Dinya, K. Sudo, J. Am. Chem. Soc. 1986, 108, 8056
and S. Grabley, G. Kretzschmar, M. Mayer, S. Philipps, R. Thiericke, J.
Wink, A. Zeeck, Liebigs Ann. Chem. 1993, 573.
[16] Abbreviations: Ac ˆ acetyl; Bu ˆ butyl; Bn ˆ benzyl; CSA ˆ 10-cam-
phorsulfonic acid; DMAP ˆ 4-dimethylaminopyridine; DDQ ˆ 2,3-
dichloro-5,6-dicyano-1,4-benzoquinone; DMF ˆ N,N-dimethylform-
amide; DMP ˆ Dess ± Martin periodinane; DMSO ˆ dimethylsulf-
oxide; Ipc ˆ isopinocamphenyl; MPM ˆ 4-methoxyphenylmethyl;
NMO ˆ N-methylmorpholine N-oxide; Piv ˆ pivaloyl ˆ trimethyl-
acetyl; Py ˆ pyridine; TBS ˆ tert-butyldimethylsilyl; TBDPS ˆ tert-
butyldiphenylsilyl; THF ˆ tetrahydrofuran; Ts ˆ toluene-4-sulfonyl.
Complexes 1 ± 3 react with O2 in the presence of one
equivalent of AgOTf to afford oxidative cleavage products of
DBC. The addition of silver salt removes the chloride ligand
to generate an empty coordination site on the metal center
and enhances the reactivity of the complex toward O2.[7]
Complex 1, with the fac-Me3TACN ligand, affords only
[*] Prof. Dr. L. Que, Jr., Dr. D.-H. Jo
Department of Chemistry and Center for Metals in Biocatalysis
University of Minnesota, Minneapolis, MN 55455 <USA)
Fax : <‡1)612-624-7029
E-mail: que@chem.umn.edu
[**] This work was supported by a grant from the National Institutes of
Health <GM-33162). D.-H.J. is grateful to the Korean Science and
Engineering Foundation <KOSEF) for a postdoctoral fellowship. We
thank Dr. Victor G. Young, Jr. and Dr. Maren Pink of the University
of Minnesota X-ray Crystallographic Laboratory for determining the
crystal structures of 1 and 2.
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that [<L)FeIII<DBC)] complexes with tetradentate ligands
gave high yields of intradiol cleavage products <e.g., 98%
for 4).[3]
Table 1 summarizes the results for [<L)FeIII<DBC)] com-
plexes reported thus far. The accumulated results show that
the coordination geometry imposed by the ancillary ligand
controls the regiospecificity of the oxidative cleavage reac-
tion. Figure 2 shows a mechanistic scheme we propose to
Figure 1. ORTEP plots for 1A <a) and 2 <b) with hydrogen atoms omitted
for clarity. The structure of 1B, the other molecule in the asymmetric unit of
1, is not shown. Selected bond lengths [Š] for 1A and corresponding values
À
À
À
for 1B in brackets: Fe1 N1 [Fe2 N4] 2.226<6) [2.232<6)], Fe1 N2
Figure 2. Proposed mechanism for the reaction of O2 with [<L)Fe<DBC)]
complexes.
À
À
À
[Fe2 N5] 2.202<6) [2.195<6)], Fe1 N3 [Fe2 N6] 2.276<6) [2.278<6)],
À
À
À
À
Fe1 O1 [Fe2 O3] 1.941<4) [1.954<4)], Fe1 O2 [Fe2 O4] 1.920<5)
À
À
À
À
[1.914<4)], Fe1 Cl1 [Fe2 Cl2] 2.381<2) [2.388<2)], O1 C1 [O3 C24]
À
À
1.361<8) [1.369<8)], O2 C2 [O4 C25] 1.366<8) [1.359<8)]. Selected bond
rationalize the results. As discussed previously, the low energy
of the catecholate-to-FeIII charge-transfer transitions confers
an enhanced iron<ii) semiquinonate character on the com-
plexes, which is reflected in their spectroscopic proper-
ties.[3b, 3c] Thus two potential sites exist for interaction with
O2: the metal center and the bound catecholate, and the
nature of the products is determined by the initial site of O2
attack. For [<L)FeIII<DBC)] complexes with tetradentate
ligands, the metal center is coordinatively saturated, so O2
attacks the catecholate to give rise to a bidentate peroxo
intermediate that leads to intradiol cleavage, consistent with
the mechanism of substrate activation proposed for intradiol-
cleaving dioxygenases <Figure 2d).[1b,c] For complexes with
tridentate ligands, the metal center has an available site for O2
attack. For 1, with the fac-Me3TACN ligand, only extradiol
cleavage is observed. We propose that O2 exclusively binds to
the metal center and becomes activated to attack the bound
À
À
À
lengths [Š] for 2: Fe1 N1 2.137<2), Fe1 N2 2.143<2), Fe1 N3 2.173<3),
Fe1 O1 1.990<2), Fe1 O2 1.913<2), Fe1 Cl1 2.325<1), O1 C1 1.329<3),
À
À
À
À
À
O2 C2 1.343<9).
extradiol cleavage products <3,5- and 4,6-di-tert-butyl-2-py-
rone) in nearly quantitative yield <97%). Previously, we
reported that the TACN analogue of 1 also afforded extradiol
cleavage products in nearly quantitative yield, but the
addition of pyridine was needed to direct the reaction from
simple auto-oxidation to form quinone to the desired
cleavage.[4b, 8] Complex 2, with the mer-TPY ligand, gives
3,5-di-tert-butyl-benzoquinone <78%) and the intradiol cleav-
age product 3,5-di-tert-butyl-1-oxacyclohepta-3,5-diene-2,7-
dione <20%). Interestingly, 3 yields both extradiol <72%)
and intradiol <25%) cleavage products. These results must be
viewed in light of previously published papers which showed
Table 1. Cleavage products obtained upon reaction of [<L)FeIII<DBC)] with O2.
L
Solvent
Intradiol
[%]
Extradiol[a]
[%]
Quinone
[%]
Ref.
TACN
CH2Cl2
±
3
82
[4b]
TACN
CH3CN
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
±
±
±
20
25
33
84 ± 98
35 <30/5)
98 <78/20)
97<83/14)
±
72 <62/10)
67<39/28)
±
65
trace
±
78
±
[4a]
[4b]
this work
this work
this work
[4c]
TACN ‡ 20 equiv 4-picoline
Me3TACN
TPY
BnBPA
TpiPr2
tetradentate ligands
toluene
DMF, CH3CN
±
±
[3a ± e, h]
[a] Ratio in parentheses is that of 3,5- to 4,6-di-tert-butyl-2-pyrone.
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metal complexes and extracted with diethyl ether <3 Â 5 mL). The
combined diethyl ether extracts were dried over anhydrous Na2SO4,
concentrated, subjected to GC analysis <Hewlett-Packard 6890 Series gas
chromatograph with a DB-5 column), and quantified by using 1,1'-biphenyl
as internal standard.
catecholate. The facial nature of the ancillary ligand allows
the two reactants to occupy the opposite face and form a
tridentate peroxo intermediate that leads to extradiol cleav-
age <Figure 2a). This ideal geometry cannot be attained for 2,
a complex of the tridentate mer-TPY, which does not afford
extradiol cleavage. Although O2 can bind to the metal center
in this complex, it is constrained to be in the same plane as the
catecholate dianion, which it hence cannot attack <Figure 2b).
Instead, the metal center acts as a conduit for electrons from
substrate to O2, and this generates quinone <78%) as the
major product. The minor amount of intradiol cleavage
<20%) observed is due to the attack of O2 on the bound
substrate <Figure 2c). In the case of 3, in which the tridentate
BnBPA has the flexibility to act as a facial or meridional
ligand, both intradiol and extradiol cleavage products are
observed.[4c] The major extradiol cleavage product is formed
by a mechanism analogous to that proposed for 1, while the
minor intradiol cleavage product results from attack of O2 on
bound substrate, as for the complexes with tetradentate
ligands.
The above model studies show that a facial tridentate
ancillary ligand is critical to elicit extradiol cleavage. In our
proposed mechanism, the facial ligand allows O2 and substrate
to occupy the opposite face and form an intermediate that
leads to the desired extradiol products. Such an intermediate
may resemble the crystallographically characterized triden-
tate peroxo species derived from the reaction of catecholate
complexes of iridium and rhodium with O2.[9] The facial
tridentate ligand would then correspond to the common
2-His-1-carboxylate facial triad[10] found in the active sites of
several extradiol dioxygenases, despite their having differing
tertiary structures.[11] Our current model complexes are
imperfect since they contain iron<iii), not iron<ii), centers;
however, they do elicit the desired extradiol cleavage. We also
do not yet understand how extradiol cleavage occurs once the
key peroxo intermediate is formed. Attempts are in progress
to trap this intermediate to gain further insight into the
mechanism of this novel class of dioxygenase enzymes.
Received: June 20, 2000 [Z15304]
[1] a) D. T. Gibson, Microbial Degradation of Organic Molecules, Marcel
Dekker, New York, 1984; b) J. D. Lipscomb, A. M. Orville, Metal Ions
Biol. Syst. 1992, 28, 243 ± 298; c) L. Que, Jr. in Bioinorganic Catalysis,
2nd ed. <Eds.: J. Reedijk, E. Bouwman), Marcel Dekker, New York,
1999, pp. 269 ± 321; d) H.-J. Krüger in Biomimetic Oxidations Cata-
lyzed by Transition Metal Complexes <Ed.: B. Meunier), Imperial
College, London, 2000, pp. 363 ± 413.
[2] L. Que, Jr., M. F. Reynolds, Metal Ions Biol. Syst. 2000, 37, 505 ± 525.
[3] a) L. Que, Jr., R. C. Kolanczyk, L. S. White, J. Am. Chem. Soc. 1987,
109, 5373 ± 5380; b) D. D. Cox, L. Que, Jr., J. Am. Chem. Soc. 1988,
110, 8085 ± 8092; c) H. G. Jang, D. D. Cox, L. Que, Jr., J. Am. Chem.
Soc. 1991, 113, 9200 ± 9204; d) W. O. Koch, H.-J. Krüger, Angew.
Chem. 1995, 107, 2928 ± 2931; Angew. Chem. Int. Ed. Engl. 1995, 34,
2671 ± 2674; e) M. Duda, M. Pascaly, B. Krebs, Chem. Commun. 1997,
835 ± 836; f) T. Funabiki, T. Yamazaki, A. Fukui, T. Tanaka, S.
Yoshida, Angew. Chem. 1998, 110, 527± 530; Angew. Chem. Int. Ed.
1998, 37, 513 ± 515; g) R. Viswanathan, M. Palaniandavar, T. Balasu-
bramanian, T. P. Muthiah, Inorg. Chem. 1998, 37, 2943 ± 2951; h) P.
Mialane, L. Tchertanov, F. Banse, J. Sainton, J.-J. Girerd, Inorg. Chem.
2000, 39, 2440 ± 2444.
[4] a) A. Dei, D. Gatteschi, L. Pardi, Inorg. Chem. 1993, 32, 1389 ± 1395;
b) M. Ito, L. Que, Jr., Angew. Chem. 1997, 109, 1401 ± 1403; Angew.
Chem. Int. Ed. Engl. 1997, 36, 1342 ± 1344; c) T. Ogihara, S. Hikichi, M.
Akika, Y. Moro-oka, Inorg. Chem. 1998, 37, 2614 ± 2615.
[5] Abbreviations: BnBPA ˆ N-benzyl N,N-bis<2-pyridylmethyl)amine;
DBCH2 ˆ 3,5-di-tert-butylcatechol; Me3TACN ˆ 1,4,7-trimethyl-1,4,7-
triazacyclononane; TACN ˆ 1,4,7-triazacyclononane; TPA ˆ tris<2-
pyridylmethyl)amine; TpiPr2 ˆ hydrotris<3,5-diisopropyl-1-pyrazolyl)-
borate; TPYˆ 2,2':6',2''-terpyridine.
[6] Crystal structure analysis: The data were collected on a Siemens
SMART platform diffractometer equipped with a CCD detector
<MoKa radiation; l ˆ 0.71073 Š). The structures were solved by direct
methods and refined by full-matrix least-squares methods on F 2 by
using the SHELXTL Plus program package <Version 5.1, Bruker
Analytical X-Ray Systems, Madison, WI). All non-hydrogen atoms
were refined with anisotropic thermal parameters, and all hydrogen
atoms were placed in ideal positions and refined as riding atoms with
individual isotropic displacement parameters. Crystal data for 1 at
173<2) K: C25H45ClFeN3O2.5
,
Mr ˆ 518.94, purple-black crystal of
Å
dimensions 0.6  0.38  0.08 mm, triclinic, space group P1, a ˆ
Experimental Section
13.0395<2), b ˆ 15.5233<2), c ˆ 16.7486<1) Š, a ˆ 113.703<1), b ˆ
111.192<1), g ˆ 92.132<1)8, Vˆ 2827.35<6) Š3, Z ˆ 4, 1calcd
ˆ
Syntheses of complexes: All FeIII catecholate complexes were prepared
under nitrogen atmosphere. [<L)FeCl3] <1.0 mmol)[12] was dissolved in
CH2Cl2 <30 mL), and a solution of DBCH2 <1.0 mmol) and NaOCH3
<2.2 mmol) in CH2Cl2 was slowly added. The mixture was stirred for 2 h,
dried by evaporation in vacuo, then recrystallized from an appropriate
solvent to give purple-black crystals. 1: recrystallized from THF/hexane
<yield 90%); elemental analysis calcd for 1 ´ 0.5THF <C25H45ClFeN3O2.5): C
57.86, H 8.74, N 8.10, Cl 6.83; found: C 57.96, H 8.65, N 8.20, Cl 7.03;
electronic spectrum <CH3CN): lmax <e) ˆ 496 <1700), 820 nm <2100). 2:
recrystallized from CH2Cl2/Et2O and then DMF/Et2O <yield 60%);
elemental analysis calcd for 2 ´ DMF <C32H38ClFeN4O3): C 62.19, H 6.20,
N 9.07, Cl 5.90; found: C 61.85, H 6.23, N 8.98, Cl 5.44; electronic spectrum
<CH3CN): lmax <e) ˆ 524 <1500), 830 nm <2100). 3: recrystallized from
acetone <yield 80%); elemental analysis calcd for 3 <C33H39ClFeN3O2): C
65.95, H 6.54, N 6.99, Cl 5.90; found: C 65.88, H 6.44, N 6.84, Cl 5.78;
electronic spectrum <CH3CN): lmax <e) ˆ 520 <1200), 850 nm <2300).
1.219 gcmÀ3. On the basis of 9513 unique reflections <Rint ˆ 0.0572)
and 633 variable parameters, final R values <I > 2s<I)): R1 ˆ 0.0787,
wR2 ˆ 0.1821. Crystal data for 2 at 173<2) K: C32H38ClFeN4O3, Mr ˆ
617.96, purple crystal of dimensions 0.40Â 0.12 Â 0.05 mm, monoclin-
ic, space group P21/c, a ˆ 12.3347<9), b ˆ 13.552<11), c ˆ 19.179<1) Š,
a ˆ 90, b ˆ 97.426<2), g ˆ 908, Vˆ 3197.1<4) Š3, Z ˆ 4, 1calcd
ˆ
1.291 gcmÀ3. On the basis of 7190 unique reflections <Rint ˆ 0.0393)
and 409 variable parameters, final R values <I > 2s<I)): R1 ˆ 0.0512,
wR2 ˆ 0.1450. Crystallographic data <excluding structure factors) for
the structures reported in this paper have been deposited with the
Cambridge Crystallographic Data Centre as supplementary publica-
tion no. CCDC-145996 <1) and CCDC-145997< 2). Copies of the data
can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB21EZ, UK <fax: <‡44)1223-336-033; e-mail:
deposit@ccdc.cam.ac.uk).
[7] Without AgOTf, the reactions require at least 24 h for completion but
afford similar cleavage products.
[8] Given that 1 affords nearly quantitative extradiol cleavage without the
addition of pyridine, we do not currently understand the role that
pyridine plays in the cleavage reaction with the TACN complex.
Reactions with O2: 0.05 mmol of complex and AgOTf were dissolved in
CH2Cl2 <10 mL) and exposed to O2 for 3 h. The solution was concentrated,
and the organic products were extracted with diethyl ether <3 Â 5 mL). The
solid residue was then acidified with 3n HCl to pH 3 to decompose the
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Clearly more work needs to be done on the apparently more complex
TACN system.
[9] a) P. Barbaro, C. Bianchini, K. Linn, C. Mealli, A. Meli, F. Vizza,
Inorg. Chim. Acta 1992, 198 ± 200, 31 ± 56; b) S. Dutta, S.-M. Peng, S.
Bhattacharya, Inorg. Chem. 2000, 39, 2231 ± 2234.
[10] a) S. Han, L. D. Eltis, K. N. Timmis, S. W. Muchmore, J. T. Bolin,
Science 1995, 270, 976 ± 980; b) T. Senda, K. Sugiyama, H. Narita, T.
Yamamoto, K. Kimbara, M. Fukuda, M. Sato, K. Yano, Y. Mitsui, J.
Mol. Biol. 1996, 255, 735 ± 752; c) A. Kita, S.-I. Kita, I. Fujisawa, K.
Inaka, T. Ishida, K. Horiike, M. Nozaki, K. Miki, Structure 1999, 7, 25 ±
34; d) K. Sugimoto, T. Senda, H. Aoshima, E. Masai, M. Fukuda, Y.
Mitsui, Structure 1999, 7, 953 ± 965.
This realization presents a particular problem with regard to
the construction of long bridges from conjugated subunits
where, as happens for polyacetylenes,[6] the triplet energy of
the connector decreases with increasing length. In such cases,
long-range triplet energy transfer will be replaced by sequen-
tial short-range steps that give a deceptively small b value.[7]
Herein we report that superexchange-type behavior is re-
tained over more than 50 Š when the polyacetylene is doped
with phenylene groups. This system provides an unusually
small attenuation factor for triplet energy transfer.[8, 9]
[11] a) E. L. Hegg, L. Que, Jr., Eur. J. Biochem. 1997, 250, 625 ± 629; b) L.
Que, Jr., Nature Struct. Biol. 2000, 7, 182 ± 185.
[12] K. Wieghardt, K. Pohl, W. Gebert, Angew. Chem. 1983, 95, 739 ± 740;
Angew. Chem. Int. Ed. Engl. 1983, 22, 727.
The required set of heterodinuclear complexes <Scheme 1),
comprising ruthenium<ii) and osmium<ii) bis<2,2':6',2''-terpyr-
idine) terminals separated by a 1,4-diethynylene-2,5-dialkoxy-
benzene connector, was prepared in a two-step procedure.
An Unusually Shallow Distance-Dependence
for Triplet-Energy Transfer**
Anthony Harriman,* Abderrahim Khatyr,
Raymond Ziessel,* and Andrew C. Benniston
There have been several reports of how the rate of
intramolecular electron transfer[1] or triplet-energy transfer
by electron exchange[2] decreases with increased separation of
the reactants and, in many cases, an attenuation factor <b)
describing an exponential drop-off has been noted. The
attenuation factor, which is often better expressed in terms of
the number of interspersed repeat units rather than distance,[3]
Scheme 1. The dinuclear RuII-based complexes and the corresponding
mixed-metal RuII/OsII complexes.
À1
has values ranging from around 0.05 to 1.7Š according to
the nature of the connecting framework and the process under
investigation.[4] It is recognized that the concept of an
exponential-type attenuation factor applies only to those
reactions that occur by way of superexchange[3] and, in other
cases, the distance dependence might take on a different
form.[5] With regard to triplet-energy transfer, this latter
requisite demands that the transfer process involves electron
exchange rather than dipole ± dipole interactions. It is also
important to ensure that the triplet energy of the interspersed
connector unit does not fall below that of the donor triplet.
First, the soluble polytopic ligands were treated with [Ru-
<terpy)<dmso)Cl2][10] <terpy ˆ 2,2':6',2''-terpyridine), dehalo-
genated with silver, in a mixture of dichloromethane and
methanol to give the corresponding mono-RuII complexes in
54 to 81% yield. These complexes, which contain an
uncoordinated terpy fragment, were subsequently treated
with [Os<terpy)<O)2<OH)]<NO3) ´ 2H2O in aqueous tetrahy-
drofuran using excess hydrazine hydrate as reducing agent.[4]
The resultant mixed-metal Ru/Os hybrids were precipitated
À
as PF6 salts and purified by column chromatography on
[*] Prof. A. Harriman
alumina prior to recrystallization from dichloromethane/
hexane. Characterization was made by full spectroscopic
and elemental analyses, including FAB mass spectra which
Department of Chemistry
University of Newcastle
‡
Newcastle-upon-Tyne, NE1 7RU <UK)
Fax : <‡44)191-222-8664
exhibited a molecular ion peak with the expected isotopic
distributions and no significant peaks at higher mass. The
corresponding dinuclear ruthenium<ii) complexes were pre-
pared according to a similar protocol using a slight excess of
the metal precursor. Synthesis of the rationally designed
polytopic ligands requires an iterative strategy based on
palladium-promoted cross-coupling reactions to extend the
central spacer framework outwards from the 1,4-diethyny-
lene-2,5-diiododecyloxybenzene core.[11]
Photophysical properties for the dinuclear ruthenium<ii)
complexes were recorded in deoxygenated acetonitrile at
208C following excitation at 532 nm. In particular, the
luminescence maxima <lMAX), quantum yields <FLUM), and
lifetimes <tLUM) were as expected for an alkynylene-substi-
E-mail: Anthony.Harriman@newcastle.ac.uk
Dr. R. Ziessel, A. Khatyr
Laboratoire de Chimie
dꢁElectronique et de Photonique Moleculaires, Ecole Chimie
Â
Á
Â
Polymeres, Materiaux <ECPM)
UPRES-A 7008 au CNRS 25 rue Becquerel
67087 Strasbourg Cedex <France)
Fax : <‡33)388-13-68-95
E-mail: ziessel@chimie.u-strasbg.fr
Dr. A. C. Benniston
Department of Chemistry, University of Glasgow, Glasgow G12 8QQ
<UK)
[**] This work was supported by the Centre National de la Recherche
Scientifique and by the Engineering School of Chemistry <ECPM).
We thank Prof. George Wippf <ULP) for many helpful discussions.
Angew. Chem. Int. Ed. 2000, 39, No. 23
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000
1433-7851/00/3923-4287 $ 17.50+.50/0
4287