A thiolate-bridged, fully delocalized mixed-valence dicopper(I,II) complex that models the CuA biological electron-transfer site

Full text of DOI:10.1021/ja953776m

J. Am. Chem. Soc. 1996, 118, 2101-2102
A Thiolate-Bridged, Fully Delocalized
2101
Mixed-Valence Dicopper(I,II) Complex That Models
the CuA Biological Electron-Transfer Site
Robert P. Houser, Victor G. Young, Jr., and
William B. Tolman*
Department of Chemistry, UniVersity of Minnesota
207 Pleasant Street SE, Minneapolis, Minnesota 55455
ReceiVed NoVember 9, 1995
Nitrous oxide reductase (N2OR) and cytochrome c oxidase
(CcO) catalyze the reductions of N2O to N2 during microbial
denitrification and of O2 to H2O during cellular respiration,
respectively.1,2 These disparate metalloenzymes have in com-
mon an unusual copper site, CuA, that functions analogously to
the ubiquitous type 1 monocopper centers3 to transfer electrons
during enzyme turnover. Controversy surrounding the structure
Figure 1. Synthesis of 1 and a representation of its X-ray crystal
structure showing one of the two chemically similar binuclear cations
generated from the two crystallographically independent half-dimers
present in the unit cell (50% ellipsoids, hydrogen atoms omitted for
clarity). Selected bond distances (Å) and angles (deg) for this cation:
Cu1-S1, 2.250(1); Cu1-S1′, 2.292(1); Cu1-N1, 2.115(3); Cu1-N2,
2.125(3); Cu1‚‚‚Cu1′, 2.9306(9); S1‚‚‚S1′, 3.480(2); S1-Cu1-S1′,
99.64(3); S1-Cu1-N1, 149.23(9); S1-Cu1-N2, 90.25(9); S1′-Cu1-
N1, 107.64(9); S1′-Cu1-N2, 122.20(9); N1-Cu1-N2, 87.2(1); Cu1-
S1-Cu1′, 80.36(3). The similar parameters for the other cation are
presented in the supporting information.
of CuA appears to have been resolved by recent X-ray crystal-
lographic results4 and spectroscopic and biochemical studies of
N2OR, CcO, and engineered CuA sites.5 The combined data
support a novel dithiolate-bridged, delocalized mixed-valence
(Cu1.5Cu1.5) resting-state formulation with four-coordinate,
distorted tetrahedral copper ions in close proximity ( 2.6 Å
apart). A key spectroscopic signature that supports the delo-
calized nature of the site is an EPR signal with seven-line
hyperfine coupling patterns in its low-field components arising
Herein we report the successful synthesis and characterization
of a unique molecule with a {Cu2(µ-SR)2}+ core that closely
mimics the resting-state CuA geometry, oxidation level, and high
degree of electron delocalization as reflected by EPR spectros-
copy.
Our synthesis of the title complex hinged on the use of the
new ligand NaLiPrdacoS, which was prepared from 1,5-bis(p-
toluenesulfonyl)diazacyclooctane by using precedented N-
macrocycle deprotection and alkylation strategies.10,11 Admix-
ture of a 3:2 ratio of NaLiPrdacoS and Cu(O3SCF3)2 in MeOH,
followed by removal of solvent, extraction with CH2Cl2 to
remove byproducts, and crystallization from MeOH/Et2O,
yielded (LiPrdacoSCu)2(O3SCF3) (1), analytically pure in 60%
yield as deep blue crystals (Figure 1).10 The 3:2 ligand-metal
stoichiometry is critical for obtaining the pure product, as the
3
from spin interactions with both I ) /2 copper ions.5h The
structure represents a fundamentally new type of electron-
transfer site in biology and is a novel entity from a purely
inorganic chemical point of view as well. Although several
complexes with bis(µ-thiolato)dicopper(I,I) cores are known,6
fully delocalized “class III” 7 mixed-valence dicopper(I,II)
complexes are extremely rare,8,9 and no example of such a
species bridged by thiolates has appeared in the literature.
(1) Babcock, G. T.; Wikstro¨m, M. Nature 1992, 356, 301-309.
(2) Kroneck, P. M. H.; Beuerle, J.; Schumacher, W. In Metal-dependent
ConVersion of Inorganic Nitrogen and Sulfur Compounds; Kroneck, P. M.
H., Beuerle, J., Schumacher, W., Eds.; Marcel Dekker: New York, 1992;
Vol. 28, pp 455-505.
(3) (a) Sykes, A. G. AdV. Inorg. Chem. 1991, 36, 377-408. (b) Adman,
E. T. AdV. Protein Chem. 1991, 42, 145-197.
(4) (a) Tsukihara, T.; Aoyama, H.; Yamashita, E.; Tomizaki, T.;
Yamaguchi, H.; Shinzawa-Itoh, K.; Nakashima, R.; Yaono, R.; Yoshikawa,
S. Science 1995, 269, 1069-1074. (b) Iwata, S.; Ostermeier, C.; Ludwig,
B.; Michel, H. Nature 1995, 376, 660-669. (c) Wilmanns, M.; Lappalainen,
P.; Kelly, M.; Saver-Eriksson, E.; Saraste, M. Proc. Natl. Acad. Sci. U.S.A.
1995, 92, 11949-11951.
(5) (a) Malmstro¨m, B. G.; Aasa, R. FEBS Lett. 1993, 325, 49-52 and
references therein. (b) Lappalainen, P.; Saraste, M. Biochim. Biophys. Acta
1994, 1187, 222-225. (c) von Wachenfeldt, C.; de Vries, S.; van der Oost,
J. FEBS Lett. 1994, 340, 109-113. (d) Lappalainen, P.; Aasa, R.;
Malmstro¨m, B. G.; Saraste, M. J. Biol. Chem. 1993, 268, 26416-26421.
(e) Blackburn, N. J.; Barr, M. E.; Woodruff, W. H.; van der Oost, J.; de
Vries, S. Biochemistry 1994, 33, 10401-10407. (f) Henkel, G.; Mu¨ller,
A.; Weissgra¨ber, S.; Buse, G.; Soulimane, T.; Steffens, G. C. M.; Nolting,
H.-F. Angew. Chem., Int. Ed. Engl. 1995, 34, 1488-1492. (g) Farrar, J.
A.; Thomson, A. J.; Cheesman, M. R.; Dooley, D. M.; Zumft, W. G. FEBS
Lett. 1991, 294, 11-15. (h) Antholine, W. E.; Kastrau, D. H. W.; Steffens,
G. C. M.; Buse, G.; Zumft, W. G.; Kroneck, P. M. H.; Eur. J. Biochem.
1992, 209, 875-881. (i) Andrew, C. R.; Han, J.; Vries, S. d.; van der
Oost, J.; Averill, B. A.; Loehr, T. M.; Sanders-Loehr, J. J. Am. Chem. Soc.
1994, 116, 10805-10806. (j) Hulse, C. L.; Averill, B. A. Biochem. Biophys.
Res. Commun. 1990, 166, 729-735.
(8) Mixed-valence multicopper complexes usually are completely valence
localized (class I), but in some instances spectroscopic evidence for
delocalization of unpaired spin in a binuclear complex or dicopper protein
active site, usually at elevated temperatures (>77 K), has been observed
(class II behavior). For examples, see: (a) Dunaj-Jurco, M.; Ondrejovic,
G.; Melnik, M. Coord. Chem. ReV. 1988, 83, 1-28. (b) Long, R. C.;
Hendrickson, D. N. J. Am. Chem. Soc. 1983, 105, 1513-1521. (c) Gagne´,
R. R.; Koval, C. A.; Smith, T. J.; Cimolino, M. C. J. Am. Chem. Soc. 1979,
101, 4571-4580. (d) Westmoreland, T. D.; Wilcox, D. E.; Baldwin, M.
J.; Mims, W. B.; Solomon, E. I. J. Am. Chem. Soc. 1989, 111, 6106-
6123. (e) Aasa, R.; Deinum, J.; Lerch, K.; Reinhammar, B. Biochim.
Biophys. Acta 1978, 535, 287-298.
(9) Complete valence delocalization (class III behavior) mediated by a
copper-copper bond has been reported: (a) Harding, C.; McKee, V.;
Nelson, J. J. Am. Chem. Soc. 1991, 113, 9584-9685. (b) Barr, M. E.;
Smith, P. H.; Antholine, W.; Spencer, B. J. Chem. Soc., Chem. Commun.
1993, 1649-1652. (c) Harding, C.; Nelson, J.; Symons, M. C. R.; Wyatt,
J. J. Chem. Soc., Chem. Commun. 1994, 2499-2500.
(10) Cyclization of the disodium salt of N,N-bis(p-toluenesulfonyl)-
propane-1,3-diamine with 1,3-bis(p-toluenesulfonyloxy)propane in DMF
afforded 1,5-bis(p-toluenesulfonyl)diazacyclooctane.10a Single detosylation
with 30% HBr/HOAc,10b alkylation with isopropyl bromide, and subsequent
detosylation with H2SO4 afforded 1-isopropyl-1,5-diazacyclooctane. Func-
tionalization with thiirane followed by deprotonation with NaH yielded
NaLiPrdacoS 11
See supporting information for synthetic details and charac-
.
terization data for all new compounds, including 1. (a) Atkins, T. J.;
Richman, J. E.; Oettle, W. F. Organic Synthesis; John Wiley & Sons: New
York, 1988; Collect. Vol. 6, pp 652-662. (b) Sessler, J. L.; Sibert, J. W.
Tetrahedron 1993, 49, 8727-8738.
(6) (a) Strange, A.; Kaim, W. Z. Naturforsch. B 1995, 50, 115-122.
(b) Chadha, R. K.; Kumar, R.; Tuck, D. G. Can. J. Chem. 1987, 65, 1336-
1342.
(7) Robin, M. B.; Day, P. AdV. Inorg. Chem. Radiochem. 1967, 10, 247-
422.
(11) Houser, R. P.; Halfen, J. A.; Young, V. G., Jr.; Blackburn, N. J.;
Tolman, W. B. J. Am. Chem. Soc. 1995, 117, 10745-10746.
0002-7863/96/1518-2101$12.00/0 © 1996 American Chemical Society

2102 J. Am. Chem. Soc., Vol. 118, No. 8, 1996
Communications to the Editor
extra half equivalent of NaLiPrdacoS provides the needed reducing
equivalent to afford the mixed-valence complex plus disulfide.
An X-ray crystal structure determination of 1 confirmed the
presence of a single triflate counterion and a {Cu2(µ-SR)2}+
core, as shown in Figure 1 for one of the dimers generated from
one of the two crystallographically independent but chemically
similar half-dimers in the unit cell.12 The Cu2S2 unit is planar,
with an average Cu‚‚‚Cu distance of 2.92 Å that is slightly
longer than that suggested for CuA (2.5 Å by EXAFS,5e,f 2.5-
2.7 Å by X-ray crystallography).4 The geometry of each of
the metal ions is best described as distorted trigonal pyramidal,
with N1 and the bridging thiolate sulfur atoms defining the
trigonal plane, from which Cu1 is only slightly displaced (by
0.22 and 0.18 Å for the two crystallographically independent
molecules). Principal distortions away from idealized trigonal
pyramidal local symmetry involve displacement of N2 27°
from the vector normal to the N1,S1,S1′ plane and angles in
this plane that deviate from 120° (range 99-149°). The overall
coordination geometry is analogous to those of “trigonal” type
1 copper protein sites and model complexes,3,13,14 except the
basal plane in 1 incorporates a NS2 rather than a N2S donor
atom set.15 Also similar to type 1 centers, the ligand arrange-
ment in 1 can be construed to be intermediate between
geometries favorable for CuI or CuII oxidation levels, presumably
in order to facilitate redox reactions and, in this instance, to
stabilize a Cu1.5Cu1.5 form.
Figure 2. EPR data for 1 shown as (a) first-derivative simulated, (b)
first-derivative experimental, (c) second-derivative simulated, and (d)
second-derivative experimental spectra (9.213 GHz, 4.2 K, MeOH/
toluene glass). Parameters used to obtain the simulated spectra are g1
Cu
Cu
) 2.010, g2 ) 2.046, g3 ) 2.204, A2 ) 36.3 G, and A3 ) 49.9 G.
The delocalized mixed-valence description implied by the
X-ray structural data is supported by spectroscopic and physical
information obtained for 1 in MeOH solution.16 Most impor-
tantly, the X-band EPR spectrum of 1 in a MeOH/toluene glass
at 4.2 K contains a nearly axial signal with clearly recognizable
seven-line hyperfine splitting patterns in the two lower field
components (Figure 2). Copper hyperfine coupling constants
and g values were estimated via spectral simulation (assuming
interaction of the unpaired spin with both metal ions) to be g1
Cu
Cu
) 2.010, g2 ) 2.046, g3 ) 2.204, A2 ) 36.3 G, and A3
)
49.9 G. Comparison of this signal to those reported for the
CuA sites in CcO and N2OR reveals striking similarities,5h
notwithstanding differences in details such as ACu values for
the synthetic model 12-15 G larger than for the proteins. We
therefore assign analogous class III delocalized electronic
structures to 1 and the biological sites.
The optical absorption spectrum of blue 1 in MeOH (Figure
3) contains features at λmax ) 358 (ꢀ 2700 M-1 cm-1), 602
(800), 786 (sh), and 1466 (1200) nm. The visible absorption
bands are red-shifted and 2-3 times less intense relative to the
purple CuA sites from various sources,17 possibly due to
differences in donor atom sets, metal-metal distances (shorter
in CuA), or copper geometries (the metal ions in CuA have been
suggested to be more akin to the tetrahedrally distorted class
of type 1 copper sites than to the trigonal planar set better
mimicked by those in 1).5i The intriguing, intense band evident
in the near-infrared region of the absorption spectrum of 1 has
Figure 3. Optical absorption spectrum of 1 in MeOH (250-800 nm)
or CD3OD (800-1600 nm). See text for λmax and ꢀ values.
not yet been assigned. Finally, in its cyclic voltammogram,
waves corresponding to one-electron reduction or oxidation of
1 in MeOH are apparent (Epc ) -0.50 V and E1/2 ) +0.35 V
vs SCE with 0.1 M TBAPF6), but our finding that the former
is completely irreversible and the latter becomes chemically
reversible only at high scan rates (>1.0 V s-1) attests to
significant stabilization of the mixed-valence state.
In sum, we have prepared a novel compound with a bis(µ-
thiolato)dicopper(I,II) core that replicates essential elements of
the structural and spectroscopic features of the CuA metallo-
protein active site. The combination in 1 of a low coordination
number (<5), thiolate bridges, and an unusual distorted trigonal
pyramidal metal ion geometry that is supported by LiPrdacoS
contributes to the stabilization of the delocalized mixed-valence
form of the {Cu2(µ-SR)2}+ unit that heretofore was accessible
only in dicopper(I,I)6 or -(II,II)11 oxidation states. By analogy,
similar factors would appear to be responsible for the physi-
cochemical properties of the biological CuA centers, for which
numerous questions regarding correlation of structure and
function remain unanswered.
(12) Crystal data for (LiPrdacoSCu)2(O3SF3) (1): MW ) 706.90, C23H46-
Cu2F3N4O3S3, crystal dimensions 0.40 mm
0.15 mm
0.13 mm,
monoclinic, space group P21/n, a ) 12.4726(3) Å, b ) 17.9093(5) Å, c )
13.0768(3) Å, â ) 91.786(1)°, V ) 2919.6(1) Å3, Z ) 4, Fcalcd ) 1.608 g
cm-3, 2θmax ) 50.14°, Mo KR radiation (λ ) 0.710 73 Å), T ) 173 K.
Data were collected using a Siemens SMART system, and the structure
was solved via direct methods. Full-matrix least-squares refinement on F2
using SHELXTL V5.0 converged with final R1 ) 0.0444 and wR2 ) 0.0867
for 5149 independent reflections with I > 2σ(I) and 393 parameters.
(13) (a) Han, J.; Loehr, T. M.; Lu, Y.; Valentine, J. S.; Averill, B. A.;
Sanders-Loehr, J. J. Am. Chem. Soc. 1993, 115, 4256-4263. (b) Penfield,
K. W.; Gay, R. R.; Himmelwright, R. S.; Eickman, N. C.; Norris, V. A.;
Freeman, H. C.; Soloman, E. I. J. Am. Chem. Soc. 1981, 103, 4382-4388.
(14) Kitajima, N.; Fujisawa, K.; Tanaka, M.; Moro-oka, Y. J. Am. Chem.
Soc. 1992, 114, 9232-9233.
Acknowledgment. We thank the National Institutes of Health
(GM47365), the National Science Foundation (NYI award to W.B.T.
and CHE-9413114 for the purchase of the diffractometer), and the
Alfred P. Sloan and Camille and Henry Dreyfus Foundations (fellow-
ships to W.B.T.) for financial support.
(15) Al-Karadaghi, S.; Cedergren-Zeppezaur, E. S.; Danter, A.; Wilson,
K. S. Acta Crystallogr. 1995, 117, 10745-10746.
Supporting Information Available: Experimental procedures and
characterization data for all new compounds and full details of the X-ray
crystal structure determination of 1 (17 pages). This material is
contained in many libraries on microfiche, immediately follows this
article in the microfilm version of the journal, can be ordered from the
ACS, and can be downloaded from the Internet; see any current
masthead page for ordering information and Internet access instructions.
(16) Electrospray MS (MeOH): [M - O3SCF3]+ ) 556. Molar
conductivity (MeOH): Λ ) 81 cm-1 mol-1 Ω-1. Magnetic moment (per
complex, Evans method, CD3OD, 298 K, 500 MHz): µeff ) 1.7(1) µB.
(17) Visible absorption bands reported for CuA in N2OR from Achro-
mobacter cycloclastes:5j λmax ) 350 (sh), 481 (ꢀ 5200 M-1 cm-1), 534
(5300), 630 (sh), 780 (2900) nm. For the soluble CuA binding domain
from Pseudomonas denitrificans CcO:5d λmax ) 363 (1200), 480 (3000),
532 (3000), 808 (1600) nm.
JA953776M