Structural and spectroscopic characterization of first-row transition metal(II) substituted blue copper model complexes with hydrotris(pyrazolyl) borate

Article abstract of DOI:10.1021/ic049814x

[CuL(SC₆F₅)] (1) (L = hydrotris(3,5-diisopropyl-1- pyrazolyl)borate anion) has been reported as a good model for blue copper proteins [Kitajima, N.; Fujisawa, K.; Tanaka, M.; Moro-oka, Y. J. Am. Chem. Soc. 1992, 114, 9232-9233]. To obtain more structural and spectroscopic insight, the first-row transition metal(II) substituted complexes of Cu(II) (1) to Mn(II) (2), Fe(II) (3), Co(II) (4), Ni(II) (5), and Zn(II) (6) were synthesized and their crystal structures were determined. These model complexes have a distorted tetrahedral geometry arising from the tripodal ligand L. The d value, which is defined by the distance from the N₂S basal plane to the metal(II) ion, and the bond angles such as N-M-N and S-M-N are good indicators of these structural distortions. The obtained complexes were characterized by UV-vis absorption, EPR, NMR, far-IR, and FT-Raman spectroscopies and electrochemical and magnetic properties. In UV-vis absorption spectra, the sulfur-to-metal(II) CT bands and the d-d transition bands are observed for 1 and 3-5. For 1, the strong sulfur to Cu(II) CT band at 663 nm, which is one of the unique properties of blue copper proteins, is observed. The CT energies of the Fe(II) (3), Co(II) (4), and Ni(II) (5) complexes are shifted to higher energy (308 and 355 nm for 3, 311 and 340 nm for 4, 357 and 434 nm for 5) and are almost the same as the corresponding Co(II)- and Ni(II)-substituted blue copper proteins. In the far-IR spectra, three far-IR absorption bands for 2-6 at ca. 400, ca. 350, and ca. 310 cm^-1 are also observed similar to those for 1. Other properties are consistent with their distorted tetrahedral geometries.

Full text of DOI:10.1021/ic049814x

Inorg. Chem. 2005, 44, 325−335
Structural and Spectroscopic Characterization of First-Row Transition
Metal(II) Substituted Blue Copper Model Complexes with
Hydrotris(pyrazolyl)borate
Yuki Matsunaga, Kiyoshi Fujisawa,* Naoko Ibi, Yoshitaro Miyashita, and Ken-ichi Okamoto
Department of Chemistry, UniVersity of Tsukuba, Tsukuba 305-8571, Japan
Received February 13, 2004
[CuL(SC6F5)] (1) (L
) hydrotris(3,5-diisopropyl-1-pyrazolyl)borate anion) has been reported as a good model for
blue copper proteins [Kitajima, N.; Fujisawa, K.; Tanaka, M.; Moro-oka, Y. J. Am. Chem. Soc. 1992, 114, 9232
−
9233]. To obtain more structural and spectroscopic insight, the first-row transition metal(II) substituted complexes
of Cu(II) (1) to Mn(II) (2), Fe(II) (3), Co(II) (4), Ni(II) (5), and Zn(II) (6) were synthesized and their crystal structures
were determined. These model complexes have a distorted tetrahedral geometry arising from the tripodal ligand L.
The d value, which is defined by the distance from the N2S basal plane to the metal(II) ion, and the bond angles
such as N
characterized by UV
magnetic properties. In UV
are observed for 1 and 3
−
M
−
N and S
vis absorption, EPR, NMR, far-IR, and FT-Raman spectroscopies and electrochemical and
vis absorption spectra, the sulfur-to-metal(II) CT bands and the d d transition bands
5. For 1, the strong sulfur to Cu(II) CT band at 663 nm, which is one of the unique
−M−N are good indicators of these structural distortions. The obtained complexes were
−
−
−
−
properties of blue copper proteins, is observed. The CT energies of the Fe(II) (3), Co(II) (4), and Ni(II) (5) complexes
are shifted to higher energy (308 and 355 nm for 3, 311 and 340 nm for 4, 357 and 434 nm for 5) and are almost
the same as the corresponding Co(II)- and Ni(II)-substituted blue copper proteins. In the far-IR spectra, three far-IR
1
absorption bands for 2
−
6 at ca. 400, ca. 350, and ca. 310 cm- are also observed similar to those for 1. Other
properties are consistent with their distorted tetrahedral geometries.
Introduction
resonance Raman spectrum.1-4 These properties arise from
a distorted tetrahedral geometry in the central Cu(II) ion,
which has been revealed by X-ray crystallography.5 For
native blue copper proteins, metal substitutions of the Cu(II)
center,6-15 as well as the mutations of amino acids around
the Cu(II) ion,16 have been studied to understand differences
in the interaction between the metal(II) ion and its coordi-
The active sites of blue copper proteins play important
roles in electron transfer between proteins, but they do not
carry out any catalytic reactions.1-4 Blue copper proteins have
unique spectral properties that include a strong sulfur to
Cu(II) CT bands (at ca. 600 nm) (which is the origin of its
blue color) in the UV-vis absorption spectrum, a small
hyperfine structure constant in the EPR spectrum, and a
number of bands in the 250-500 cm-1 region of the
(5) Handbook of Metalloproteins; Messerschmidt, A., Huber, R., Poulos,
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C. p 1215. (f) Nersissian, A. M.; Hart, P. J.; Valentine, J. S. p 1219.
(g) Shoham, M. p 1235.
* Author to whom correspondence should be addressed. E-mail:
kiyoshif@chem.tsukuba.ac.jp. Tel.: +81-29-853-6922. Fax: +81-29-853-
6503.
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10.1021/ic049814x CCC: $30.25
Published on Web 12/24/2004
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 2, 2005 325

Matsunaga et al.
Chart 1. Copper coordination Geometry: Azurin19 (Left),
nated atoms. These studies provide evidence for why a Cu(II)
ion is used as the central metal and how it controls electron-
transfer reaction.17 In this context, much research, including
X-ray crystal analysis,6-9 UV-vis absorption,10-11 CD and
MCD,12 fluorescence,13 NMR,10a,10d,11 EPR,6,10d and resonance
Raman11 spectroscopy, has been reported for azurins, which
were substituted by transition metal(II) ions such as Co(II)
(Pseudomonas aeruginosa6), Ni(II) (P. aeruginosa7), Zn(II)
(P. aeruginosa and Pseudomonas putida8), and Cd(II)
(Alcaligenes denitrificans9).18
Plastocyanin20 (Right)
The Cu(II) ion in azurin from P. aeruginosa has a nearly
trigonal arrangement with two imidazole N atoms from His46
and His117 and one thiolate S atom from Cys112 (Chart 1,
left).19 A sulfide S atom from Met121 is found along the
trigonal axis at a long distance, reflecting a weak interaction,
and a peptide carbonyl O atom (Gly45) is located on the
other side of the trigonal N2S plane at an even longer
distance. Thus, the structure of Cu(II) ion in azurin takes on
a CuIIN2S2 type distorted tetrahedral geometry. The Cu(II)
ion in plastocyanin from Populus nigra also has a nearly
trigonal-based, distorted tetrahedral geometry with two
Cu-N bonds (His37 and His87), a short Cu-S bond
(Cys84), and a long Cu-S bond (Met92) (Chart 1, right).20
To study the origin of the unique properties of blue copper
proteins, many model complexes have been reported.21
However, these model complexes do not completely repro-
duce the properties of the protein due to the fact that the
Cu(II) ion prefers an octahedral-based conformation, which
is stabilized by the Jahn-Teller effect over a tetrahedral
conformation. Furthermore, Cu(II)-S bonds dissociate easily
and are affected by the reduction of the Cu(II) ion and the
formation of a disulfide bond.1-4,21,22 Much research still
focuses on overcoming these synthetic challenges.
(10) (a) Salgado, J.; Jime´nez, H. R.; Moratal, J. M.; Kroes, S.; Warmerdam,
G. C. M.; Canters, G. W. Biochemistry 1996, 35, 1810. (b) Bilio, A.
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W. H.; Tennent, D. L.; McMillin, D. R. Biochem. Biophys. Res.
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(12) Solomon, E. I.; Rawlings, J.; McMillin, D. R.; Stephens, P. J.; Gray,
H. B. J. Am. Chem. Soc. 1976, 98, 8046.
The first successful synthesis of a blue copper model
complex was [Cu{HB(3,5-Me2pz)3}(SC6H4NO2)], made by
Ibers and Thompson (HB(3,5-Me2pz)3 ) hydrotris(3,5-
dimethyl-1-pyrazolyl)borate anion, SC6H4NO2 ) 4-nitroben-
zenethiolate anion) in 1977.23 This complex reproduced the
intense blue color (λmax, 588 nm (3900 M-1 cm-1)) and the
complicated Cu-S stretching in the resonance Raman
spectrum. However, its molecular structure has not been
characterized by X-ray analysis. Only the structures of
copper(I) thiolato complex K[Cu{HB(3,5-Me2pz)3}(SC6H4-
NO2)]23 and Co(II)-substituted thiolato complex [Co{HB-
(3,5-Me2pz)3}(SC6F5)]24 were reported. In 1992, we reported
structurally characterized copper(II) thiolato complexes
[CuL(SC6F5)] (1) and [CuL(SCPh3)] (L ) hydrotris(3,5-
diisopropyl-1-pyrazolyl)borate anion), in which the Cu(II)
ion could have a highly distorted tetrahedral geometry due
to the tripodal ligand L which forces a distorted four-
coordinate configuration.25 The distance (d) from Cu(II) ion
to the N2S basal plane defined by the three coordinated atoms
confirmed this highly distorted tetrahedral configuration. The
d values of 1 and [CuL(SCPh3)] are 0.34 and 0.20 Å,
(13) Hutnik, C. M.; Szabo, A. G. Biochemistry 1989, 28, 3935.
(14) (a) Donaire, A.; Salgado, J.; Moratal, J.-M. Biochemistry 1998, 37,
8659. (b) Romero, C.; Moratal, J. M.; Donaire, A. FEBS Lett. 1998,
440, 93. (c) Vila, A. J.; Ramirez, B. E.; Bilio, A. J. D.; Mizoguchi, T.
J.; Richards, J. H.; Gray, H. B. Inorg. Chem. 1997, 36, 4567. (d)
Salgado, J.; Jime´nez, H. R.; Donaire, A.; Moratal, J. M. Eur. J.
Biochem. 1995, 231, 358. (e) Moratal, J.-M.; Salgado, J.; Donaire,
A.; Jime´nez, H. R.; Castells, J. Inorg. Chem. 1993, 32, 3587. (f)
Moratal, J.-M.; Salgado, J.; Donaire, A.; Jime´nez, H. R.; Castells, J.
J. Chem. Soc., Chem. Commun. 1993, 110. (g) Blaszak, J. A.; Ulrich,
E. L.; Markley, J. L.; McMillin, D. R. Biochemistry 1982, 21, 6253.
(h) Hill, H. A. O.; Smith, B. E.; Storm, C. B.; Ambler, R. P. Biochem.
Biophys. Res. Commun. 1976, 70, 783.
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Inorg. Chem. 1996, 35, 2737.
(16) (a) Farver, O.; Jeuken, L. J. C.; Canters, G. W.; Pecht, I. Eur. J.
Biochem. 2000, 267, 3123. (b) Kroes, S, J.; Hoitink, C. G.; Andrew,
C. R.; Ai, J. Y.; Sanders-Loehr, J.; Messerschmidt, A.; Hagen, W.
R.; Canters, G. W. Eur. J. Biochem. 1996, 240, 342. (c) van
Pouderoyen, G.; Andrew, C. R.; Loehr, T. M.; Sanders-Loehr, J.;
Mazumdar, S.; Hill, H. A. O.; Canters, G. W. Biochemistry 1996, 35,
1397. (d) den Blaauwen, T.; Canters, G. W. J. Am. Chem. Soc. 1993,
115, 1121. (e) Romero, A.; Hoitink, C. W. G.; Nar, H.; Huber, R.;
Messerschmidt, A.; Canters, G. W. J. Mol. Biol. 1993, 229, 1007. (f)
Nar, H.; Messerschmidt, A.; Huber, R.; van de Kamp, M.; Canters,
G. W. J. Mol. Biol. 1991, 218, 427. (g) den Blaauwen, T.; van de
Kamp, M.; Canters, G. W. J. Am. Chem. Soc. 1991, 113, 5050.
(17) (a) Crane, B. R.; Di Bilio, A. J.; Winkler, J. R.; Gray, H. B. J. Am.
Chem. Soc. 2001, 123, 11623. (b) Gray, H. B.; Malmstro¨m, B. G.;
Williams, R. J. P. J. Biol. Inorg. Chem. 2000, 5, 551 and references
therein. (c) Williams, R. J. P. Eur. J. Biochem. 1995, 234, 363. (d)
Malmstro¨m, B. G. Eur. J. Biochem. 1994, 223, 711. (e) Vallee, B. L.;
Williams R. J. P. Proc. Natl. Acad. Sci. U.S.A. 1968, 59, 498.
(18) The structures of other metal substituted blue copper proteins have
been reported such as copper-containing nitrite reductase (Zn: Murphy,
M. E. P.; Turley, S.; Kukimoto, M.; Nishiyama, M.; Horinouchi, S.;
Sasaki, H.; Tanokura, M.; Adman, E. T. Biochemistry 1995, 34,
12107).
(20) Guss, J. M.; Bartunik, H. D.; Freeman, H. C. Acta Crystallogr. 1992,
B48, 790.
(21) Kitajima, N. AdV. Inorg. Chem. 1992, 39, 1.
(22) Hellinga, H. W. J. Am. Chem. Soc. 1998, 120, 10055.
(23) (a) Thompson J. S.; Marks T. J.; Ibers J. A. J. Am. Chem. Soc. 1979,
101, 4180. (b) Thompson J. S.; Marks T. J.; Ibers. J. A. Proc. Natl.
Acad. Sci. U.S.A. 1977, 74, 3114.
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Soc. 1979, 101, 4193.
(19) Nar, H.; Messerschmidt, A.; Huber, R.; van de Kamp, M.; Canters,
G. W. J. Mol. Biol. 1991, 221, 765.
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Soc. 1992, 114, 9232.
326 Inorganic Chemistry, Vol. 44, No. 2, 2005

Metal-Substituted Blue Copper Model Complexes
respectively. These values are very similar with those found
for azurins (P. aeruginosa, 0.10 Å,5b,19 and A. denitrificans,
0.13 Å5b,26) and plastocyanins (P. nigra, 0.36 Å,5a,19,25 and
Enteromorpha prolifera, 0.37 Å5a,24,27). In addition, 1 and
[CuL(SCPh3)] show an intense blue color around 600 nm
(672 nm (5960 M-1 cm-1) and 625 nm (6600 M-1 cm-1)),25
a unique axial EPR signal (55 and 74 G),25 and a Cu-S
stretching frequency around 400 cm-1 (409 and 422 cm-1)
by 647 nm laser excitation,28-29 respectively. Thus, 1 and
[CuL(SCPh3)] reproduce the well-known properties of blue
copper proteins and serve as good structural model com-
plexes of the active site.30,31 Moreover, the properties of 1
in coordinating solvents such as DMF dramatically change
because the solvent molecule is coordinated to Cu(II) center
(EPR parameters: g| 2.33, A| 153 G, g 2.07 at 113 K).21,32,33
Therefore, the EPR spectrum of [Cu{HB(3,5-Me2pz)3}-
(SC6H4NO2)] was typical for a tetragonal complex, coordi-
nating THF at 77 K.23
Very recently, we determined the self-exchange rate
constants (3.14 × 105 M-1 s-1 at 25 °C) for the redox pair
of [CuIIL(SC6F5)] and K[CuIL(SC6F5)] by NMR line-
broadening techniques.34 This value is characteristic of the
most labile Cu(II/I) redox systems, including blue copper
proteins.35 Therefore, 1 is not only a good structural model
but also a good functional model for blue copper proteins.
Herein, we report the synthesis of metal(II)-substituted
complexes of 1 by using the first-row transition metals of
Mn(II) (2), Fe(II) (3), Co(II) (4), Ni(II) (5), and Zn(II) (6)
to obtain more insight into the structural and spectroscopic
properties of blue copper proteins. This research is also aimed
at understanding the relative rigidity of the hydrotris-
(pyrazolyl)borate ligand L and its ability to accommodate
first row transition metal(II) ions. For this reason, a series
of metal(II) complexes (1-6) with the same ligand has been
synthesized and studied. We have observed subtle angular
and torsional variations with the ligand framework to
accommodate the ionic radius of the first-transition metal-
(II) ions and their general coordination structure preference.36
On the basis of the crystal structures and UV-vis absorption,
NMR, EPR, far-IR, and FT-Raman spectral data and
electrochemical and magnetic properties, the influences of
different central metal(II) ions are discussed. Furthermore,
the structures and properties of the Co(II) (4) and Ni(II) (5)
complexes are compared with those of the corresponding
metal(II)-substituted azurins.
Experimental Section
Instrumentation. IR and far-IR spectra were recorded on KBr
pellets in the 4600-400 cm-1 region and on CsI pellets in the 650-
100 cm-1 region using a JASCO FT/IR-550 spectrophotometer.
FT-Raman spectra were recorded on KBr pellets in the 600-200
cm-1 region using a Perkin-Elmer Spectrum GX spectrophotometer.
UV-vis spectra of 1 (CH2Cl2 solution in 260-1400 nm) or 2-6
(cyclohexane solution in 200-1700 nm) were measured with a
JASCO V-570 spectrophotometer at room temperature using a
quartz cell (0.10 cm path length). Low-temperature absorption
spectra of 1-6 were recorded in CH2Cl2 solution at -45 °C on an
Otsuka Electronics MCPD-2000 system with an optical fiber
attachment (300-1100 nm). The magnetic susceptibilities were
measured with a Sherwood MSB-AUTO susceptibility balance
apparatus at 296 K for 4, 293 K for 5, and 296 K for 6. Diamagnetic
corrections employed tabulated constants.37 EPR spectra were
recorded on a Bruker EMX-T EPR spectrometer as a frozen solution
(CH2Cl2/ClCH2CH2Cl (1:1)) at low temperature in quartz tubes
(diameter 5 mm) with an Oxford Instruments liquid-helium cryostat
1
EPR-10 or liquid-nitrogen temperature controller BVT 3000. H
NMR (600 MHz) and 13C NMR (150 MHz) spectra of 6 were
recorded on a Bruker AVANCE-600 at ambient temperature in
1
CD2Cl2. H NMR (270 MHz) spectra of 3-5 were recorded on a
JEOL-EX-270 at ambient temperature in CD2Cl2. Chemical shifts
were reported as δ values downfield from an internal standard
(CH3)4Si. Electrochemical measurements were carried out under
argon at 0 °C in solutions of acetone with (Bu4N)(ClO4) as a
supporting electrolyte using a BAS CV-50W voltammetric analyzer.
Nonaqueous Ag/AgCl electrode (BAS, RE-5) and platinum wire
were used as reference and auxiliary electrodes, respectively. A
platinum disk was used for the working electrode. The ferrocenium/
ferrocene couple was measured under the same conditions to correct
for the junction potentials both as an internal reference and as a
separate solution. The potential values were corrected by assigning
the ferrocenium/ferrocene couple a value of 0.50 V versus SCE.38
Elemental analysis (C, H, N) was performed by the Chemical
Analysis Center of the University of Tsukuba.
(26) Baker, E. N. J. Mol. Biol. 1988, 203, 1071.
(27) Collyer, C. A.; Guss, J. M.; Sugimura, Y.; Yoshizaki, F.; Freeman,
H. C. J. Mol. Biol. 1990, 211, 617.
(28) Qiu, D.; Kilpatrick, L.; Kitajima, N.; Spiro, T. G. J. Am. Chem. Soc.
1994, 116, 2585.
(29) Randall, D. W.; DeBeer George, S.; Hedman, B.; Hodgson, K. O.;
Fujisawa, K.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122, 11620.
(30) The structural characterizations of 1 and [CuL(SCPh3)] have been
published as a communication in ref 25. The M-S stretching analyses
of 1, [CuL(SCPh3)], and [CuL(SCMe3)] have been published by Spiro
in ref 28. The detailed electronic structural characterization of only
[CuL(SCPh3)] has been performed by Solomon in ref 29.
(31) Holland and Tolman have reported two structural models for blue
copper proteins with a highly hindered â-diketiminate. The complexes
reproduced the N2S plane separated from Cu(II) center having N2S
and N2SS′ ligand donor sets. (a) Holland, P. L.; Tolman, W. B. J.
Am. Chem. Soc. 1999, 121, 7270. (b) Holland, P. L.; Tolman, W. B.
J. Am. Chem. Soc. 2000, 122, 6331.
(32) Fujisawa, K. Unpublished results.
(33) Kitajima, N.; Fujisawa, K.; Moro-oka, Y. J. Am. Chem. Soc. 1990,
112, 3210.
(34) Fujisawa, K.; Fujita, K.; Takahashi, T.; Kitajima, N.; Moro-oka, Y.;
Matsunaga, Y.; Miyashita, Y.; Okamoto, K. Inorg. Chem. Commun.
2004, 7, 1188.
General Materials. Preparation and handling of all complexes
were performed under an argon atmosphere by employing standard
Schlenk line techniques. Dichloromethane was distilled from P2O5
prior to use. Diethyl ether, tetrahydrofuran, and heptane were
carefully purified by refluxing/distilling under an argon atmosphere
over sodium benzophenone ketyl.39 Ethanol, methanol, cyclohexane,
and acetone were spectroscopic grade and were used after bubbling
with argon gas. NaSC6F5 and KSC6F5 were prepared by the reaction
of HSC6F5 with 0.8 equiv of NaH in THF and with 0.8 equiv of
KOH in ethanol, respectively.
Preparation of Complexes. [CuL(SC6F5)] (1). This complex
was prepared by the same procedure as described in the literature.25
(37) O’Connor, C. J. Prog. Irog. Chem. 1982, 29, 203.
(35) Rorabacher, D. B. Chem. ReV. 2004, 104, 651 and references therein.
(36) It is a strong point for model complexes. Some transition metal(II)
substituted blue copper proteins have been only reported.6-9
(38) Connelly, N. G.; Geiger, W. E. Chem. ReV. 1996, 96, 877.
(39) Armarego, W. L. F.; Perrin, D. D. Purification of Laboratory
Chemicals, 4th ed.; Butterworth-Heinemann: Oxford, U.K., 1997.
Inorganic Chemistry, Vol. 44, No. 2, 2005 327

Matsunaga et al.
Table 1. Summary of Crystallographic Data for [ML(SC6F5)] (M ) Mn (2), Fe (3), Co (4), Ni (5), and Zn (6))
2‚CH2Cl2
3‚C7H16
4‚0.5C7H16‚0.25CH2Cl2
5‚0.5C7H16‚0.25CH2Cl2
6
formula
fw
cryst system
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
C34H48N6SF5BCl2Mn
804.50
monoclinic
P21/c (No. 14)
9.905(5)
16.032(7)
25.508(12)
C40H62N6SF5BFe
820.68
monoclinic
P21/n (No. 14)
21.843(11)
9.461(4)
C36.75H54.5N6SF5BCl0.5Co
794.90
triclinic
C36.75H54.5N6SF5BCl0.5Ni
794.66
C33H46N6SF5BZn
730.01
monoclinic
P21/n (No. 14)
13.983(10)
16.105(11)
18.54(1)
triclinic
P1h (No. 2)
9.417(2)
17.684(5)
27.748(8)
79.623(8)
80.841(9)
68.873(7)
4217(2)
4
P1h (No. 2)
9.378(3)
16.873(6)
27.892(10)
80.852(8)
79.621(9)
79.401(10)
4231(3)
4
21.893(11)
90.591(6)
99.350(7)
114.020(8)
4050(3)
4
1.319
4465(4)
4
1.221
3814(5)
4
1.271
Z
D
calc, g/cm3
1.248
1.251
cryst size, mm3
µ(Mo KR), cm-1
F(000)
0.25 × 0.12 × 0.12
0.30 × 0.25 × 0.05
0.50 × 0.50 × 0.10
0.50 × 0.30 × 0.20
0.35 × 0.20 × 0.25
5.64
4.39
5.41
5.96
7.53
1676
1744
1674
1678
1528
temp, °C
-61
-61
-89
-89
-61
2θ range, deg
reflcns collcd
unique reflcns
Rint
5-55
5-55
5-55
5-55
5-55
33 820
9246
0.049
36 931
10 199
0.060
34 161
19 051
0.050
34 566
19 108
0.050
30 469
8435
0.090
no. of observns
no. of variables
reflcn/param ratio
Ra
3220 (I > 3σ(I))
4487 (I > 3σ(I))
14 878 (I > 3σ(I))
13 409 (I > 3σ(I))
4640 (I > 3σ(I))
499
549
1002
1047
470
6.45
0.067
0.063
8.17
0.063
0.057
14.85
0.074
0.084
12.81
0.068
0.073
9.87
0.077
0.081
a
Rw
goodness of fit
1.55
0.92/-0.75
1.79
0.56/-0.37
3.92
1.41/-1.06
2.55
1.07/-1.00
2.65
0.65/-0.55
max/min peak, e/Å3
2
a R ) Σ||Fo| - |Fc||/Σ|Fo|; Rw ) [(Σw(|Fo| - |Fc|)2/ΣwFo )]1/2, w ) 1/σ2(|Fo|).
To the solution of the binuclear hydroxo complex [(CuL)2(µ-OH)2]40
(0.500 g, 0.46 mmol) in dichloromethane/heptane (50 cm3) (3:2,
v/v) was added pentafluorobenzenethiol (0.15 cm3, 1.12 mmol) with
cooling at -20 °C. The solution was stirred for 2.5 h, and then the
solvent was removed in vacuo at the same temperature. The
resultant blue powder was recrystallized from dichloromethane/
heptane at -30 °C, and blue crystals were formed. Single crystals
were obtained from dichloromethane/pentane/n-octane at -20 °C.
Yield: 88% (0.59 g). Anal. Calcd for C33H46N6F5SBCu: C, 54.43;
H, 6.37; N, 11.54. Found: C, 54.74; H, 6.28; N, 11.61. IR (ν˜/
cm-1): 2967 vs, 2545 m, 1537 m, 1508 vs, 1478 vs, 1174 s, 1055
s, 976 s, 858 s, 791 m. Far-IR (ν˜/cm-1): 522 m, 406 s, 350 m, 311
m, 243 vs, 221 s, 199 s. (Symbols: vs, very strong; s, strong; m,
medium; w, weak.) UV-vis absorption [CH2Cl2, rt (room temper-
ature), λmax/nm (ꢀ/M-1 cm-1)]: 362 (1170), 414 (shoulder, 620),
663 (5980), 1040 (1170). UV-vis absorption [CH2Cl2, -45 °C,
H, 6.33; N, 11.33. IR (ν˜/cm-1): 2966 vs, 2557 m, 1536 m, 1506
vs, 1476 vs, 1168 s, 1053 s, 969 s, 862 s, 793 m. Far-IR (ν˜/cm-1):
516 m, 399 s, 339 m, 334 m, 308 m, 199 vs. FT-Raman (ν˜/cm-1):
511 vs, 489, 473, 444 s, 414 w, 385 s, 351 m, 332 m, 308 m, 238
s. UV-vis absorption [cyclohexane, rt, λmax/nm (ꢀ/M-1 cm-1)]: 218
(39 800), 264 (16 200). EPR (CH2Cl2/ClCH2CH2Cl, 20 K): g 3.91.
[FeL(SC6F5)] (3). The mononuclear chloro complex [FeCl(L)]43
(0.409 g, 0.73 mmol) and potassium salt of pentafluorobenzenethiol
(0.208 g, 0.87 mmol) were dissolved in THF (30 cm3) with cooling
at -15 °C. After being stirred for 2 h, the solution was filtered.
The filtrate was concentrated to dryness in vacuo. The resultant
slightly brown colored solid was recrystallized from dichlo-
romethane/heptane at -30 °C, and then white crystals were formed.
Yield: 49% (0.26 g). Anal. Calcd for C33H46N6F5SBFe: C, 55.01;
H, 6.43; N, 11.66. Found: C, 54.84; H, 6.55; N, 10.90. IR (ν˜/
cm-1): 2966 vs, 2558 m, 1536 m, 1505 vs, 1475 vs, 1169 s, 1051
s, 969 s, 858 s, 794 m. Far-IR (ν˜/cm-1): 519 s, 399 s, 347 m, 309
m, 216 s, 197 vs. UV-vis absorption [cyclohexane, rt, λmax/nm
(ꢀ/M-1 cm-1)]: 210 (28 050), 242 (14 740), 268 (9400), 308 (5940),
355 (1390), 599 (60), 1360 (95). UV-vis absorption [CH2Cl2, -45
°C, λmax/nm (ꢀ/M-1 cm-1)]: 309 (4290), 343 (shoulder, 1050). 1H
NMR (CD2Cl2, 270 MHz, 296 K): δ -11.76 (s, 1H, BH), 3.83
(br, 18H, CHMe2), 4.07 (br, 18H, CHMe2), 10.98 (br, 3H, CHMe2),
16.15 (s, 3H, CHMe2), 66.11 (s, 3H, pz).
λ
max/nm (ꢀ/M-1 cm-1)]: 362 (1320), 420 (shoulder, 590), 670
(5960), 1028 (1300). EPR (CH2Cl2/ClCH2CH2Cl, 140 K): g| 2.32,
A| 52 G, g 2.10. Cyclic voltammetry (acetone vs Fc+/Fc /mV):
-468 (∆E ) 109 mV).
[MnL(SC6F5)] (2). To the solution of the binuclear hydroxo
complex [(MnL)2(µ-OH)2]41-42 (0.471 g, 0.44 mmol) in dichlo-
romethane (50 cm3) was added pentafluorobenzenethiol (0.14 cm3,
1.05 mmol) with cooling at 0 °C. The solution was stirred for 2.5
h, and then the solvent was removed in vacuo. The resultant white
powder was recrystallized from dichloromethane at -15 °C, and
white crystals were formed. Yield: 32% (0.20 g). Anal. Calcd for
C33H46N6F5SBMn: C, 53.73; H, 6.24; N, 11.31. Found: C, 53.88;
[CoL(SC6F5)] (4). The synthesis was carried out by the same
method as 2 using [(CoL)2(µ-OH)2]42 (0.153 g, 0.14 mmol) and
pentafluorobenzenethiol (0.042 cm3, 0.31 mmol). Blue crystals were
obtained by recrystallization from dichloromethane/heptane at -30
°C. Yield: 40% (0.081 g). Anal. Calcd for C33H46N6F5SBCo: C,
54.78; H, 6.41; N, 11.62. Found: C, 54.87; H, 6.36; N, 11.40. IR
(ν˜/cm-1): 2967 vs, 2551 m, 1536 m, 1506 vs, 1476 vs, 1167 s,
1052 s, 970 s, 860 s, 792 m. Far-IR (ν˜/cm-1): 519 s, 399 vs, 346
(40) (a) Kitajima, N.; Fujisawa, K.; Fujimoto, C.; Moro-oka, Y.; Hashimoto,
S.; Kitagawa, T.; Toriumi, K.; Tatsumi, K.; Nakamura, A. J. Am.
Chem. Soc. 1992, 114, 1277. (b) Kitajima, N.; Fujisawa, K.; Moro-
oka, Y. Inorg. Chem. 1990, 29, 357.
(41) Kitajima, N.; Singh, U. P.; Amagai, H.; Osawa, M.; Moro-oka, Y. J.
Am. Chem. Soc. 1991, 113, 7757.
(42) Kitajima, N.; Hikichi, S.; Tanaka, M.; Moro-oka. Y. J. Am. Chem.
Soc. 1993, 115, 5496.
(43) Ito, M.; Amagai, H.; Fukui, H.; Kitajima, N.; Moro-oka, Y. Bull. Chem.
Soc. Jpn. 1996, 69, 1937.
328 Inorganic Chemistry, Vol. 44, No. 2, 2005

Metal-Substituted Blue Copper Model Complexes
Figure 1. ORTEP representations of (A) [MnL(SC6F5)] (2) and (B) [FeL-
(SC6F5)] (3) (50% probability thermal ellipsoids).
s, 310 m, 224 vs, 211 s. UV-vis absorption [cyclohexane, rt, λmax
/
nm (ꢀ/M-1 cm-1)]: 215 (22 070), 253 (11 300), 311 (5030), 340
(2930), 430 (650), 605 (broad, 1170), 812 (140), 1479 (115). UV-
vis absorption [CH2Cl2, -45 °C, λmax/nm (ꢀ/M-1 cm-1)]: 312
(5000), 338 (3149), 420 (shoulder, 452), 599 (1121), 809 (148).
Magnetic moment (µeff/µB, 296 K): 4.62. EPR (CH2Cl2/ClCH2-
1
CH2Cl, 20 K): g1 5.90, g2 2.66, g3 1.74. H NMR (CD2Cl2, 270
MHz, 296 K): δ -13.85 (br, 1H, BH), -3.26 (br, 3H, CHMe2),
3.19 (s, 18H, CHMe2), 3.84 (s, 18H, CHMe2), 8.14 (s, 3H, CHMe2),
73.67 (s, 3H, pz).
[NiL(SC6F5)] (5). The synthesis was carried out by the same
method as 2 using [(NiL)2(µ-OH)2]42 (0.161 g, 0.15 mmol) and
pentafluorobenzenethiol (0.044 cm3, 0.33 mmol). Orange crystals
were obtained by recrystallization from dichloromethane/heptane
at -15 °C. Yield: 25% (0.053 g). Anal. Calcd for C33H46N6F5-
SBNi: C, 54.80; H, 6.41; N, 11.62. Found: C, 54.53; H, 6.38; N,
11.41. IR (ν˜/cm-1): 2966 vs, 2549 m, 1537 m, 1506 vs, 1476 vs,
1176 s, 1054 s, 969 s, 862 s, 795 m. Far-IR (ν˜/cm-1): 526 s, 399
Figure 2. ORTEP representations of (A) [CoL(SC6F5)] (4A), (B) [NiL-
(SC6F5)] (5A), and (C) [ZnL(SC6F5)] (6) (50% probability thermal el-
lipsoids).
vs, 349 s, 310 m, 221 vs. UV-vis absorption [cyclohexane, rt, λmax
/
salt of pentafluorobenzenethiol (0.371 g, 1.67 mmol). White crystals
were obtained by recrystallization from dichloromethane/methanol
at -15 °C. Single crystals for X-ray analysis were obtained by the
slow evaporation of the dichloromethane/heptane solution at room
temperature. Yield: 53% (0.44 g). Anal. Calcd for C33H46N6F5-
SBZn: C, 54.29; H, 6.35; N, 11.51. Found: C, 54.20; H, 6.38; N,
11.36. IR (ν˜/cm-1): 2967 vs, 2557 m, 1538 m, 1508 vs, 1479 vs,
1172 s, 1053 s, 972 s, 862 s, 795 m. Far-IR (ν˜/cm-1): 520 s, 402
s, 343 s, 310 m, 191 vs. FT-Raman (ν˜/cm-1): 512 vs, 444 s, 413
w, 401 s, 385 s, 366 w, 339 m, 311 w, 263 s, 219 s, 192 s. UV-
nm (ꢀ/M-1 cm-1)]: 210 (32 100), 246 (11 310), 357 (1850), 434
(3100), 804 (145), 1400 (35). UV-vis absorption [CH2Cl2, -45
°C, λmax/nm (ꢀ/M-1 cm-1)]: 354 (2150), 427 (3360), 798 (broad,
160). Magnetic moment (µeff/µB, 293 K): 3.30. 1H NMR (CD2Cl2,
270 MHz, 296 K): δ -11.24 (br, 1H, BH), -4.20 (br, 3H, CHMe2),
-0.12 (s, 18H, CHMe2), 3.21 (s, 18H, CHMe2), 4.53 (s, 3H,
CHMe2), 81.09 (s, 3H, pz).
[ZnL(SC6F5)] (6). The synthesis was carried out by the same
method as 3 using [ZnBr(L)]42 (0.692 g, 1.13 mmol) and the sodium
Inorganic Chemistry, Vol. 44, No. 2, 2005 329

Matsunaga et al.
Table 2. Selected Bond Distances (Å) and Angles (deg) for Complexes 1-6 and Blue Copper Proteins and Metal-Substituted Ones
M/N11N31S
plane
M-S M-N11 M-N21 M-N31 S-M-N11 S-M-N21 S-M-N31 N11-M-N31
Cu (1)a
Mn (2)
Fe (3)
Co (4A)
Co (4B)
Ni (5A)
Ni (5B)
Zn (6)
2.176(4) 2.037(9) 2.119(8) 1.930(9)
2.385(3) 2.121(6) 2.125(6) 2.119(6)
2.288(2) 2.045(4) 2.073(4) 2.055(4)
2.267(1) 2.001(3) 2.026(3) 2.025(3)
2.265(1) 2.013(4) 2.039(3) 2.034(3)
2.259(2) 1.995(3) 2.004(3) 1.981(4)
2.259(1) 1.990(4) 2.009(3) 2.004(3)
2.248(2) 2.019(6) 2.032(4) 2.024(6)
122.7(3)
128.3(2)
131.5(1)
129.6(1)
129.6(1)
128.9(1)
132.8(1)
127.4(2)
138.3(6)
133(2)
112.7(2)
118.3(2)
107.2(1)
107.82(9)
107.8(1)
106.8(1)
106.40(8)
114.3(2)
107.7(6)
134.6(3)
129.6(2)
129.3(1)
128.31(9)
127.8(1)
132.7(1)
128.4(1)
126.4(2)
121.9(6)
123(1)
90.9(3)
90.2(2)
94.5(2)
94.9(1)
95.1(1)
92.8(1)
93.2(1)
94.4(2)
93.3(7)
103(2)
105
0.34
0.43
0.26
0.32
0.33
0.28
0.28
0.41
[Co(SC6F5){HB(3,5-Me2pz)3}]b 2.26(1)
1.97(2)
2.11(5)
2.08
2.01(2)
1.98(2)
2.03(7)
2.00
azurin (P. aeruginosa)c
azurin (A. denitrificans)d
Co (P. aeruginosa)e
Ni (P. aeruginosa)g
2.25(3)
2.14
0.10
0.13
0.20f
0.18
0.15i
135
119
2.20
2.32
2.25
2.39(7)
2.30
2.23(9)
2.01
2.22(12)
2.07
127(5)
128
136
132(1)
132
125
123(5)
121
118
121(1)
121
120
108(1)
110
103
106(2)
97
104
Zn (P. aeruginosa)h
Zn (P. putida)j
2.12
1.94
1.94
Cd (A. denitrificans)k
plastocyanin (P. nigra)l
plastocyanin (E. prolifera)m
2.39(5)
2.07
2.25(2)
1.91
2.21(1)
2.06
0.05(1)
0.36a
2.12
1.89
2.17
0.37a
a Reference 25. b Reference 24. c Reference 19. d Reference 26. e Reference 6. f Reference 11a. g Reference 7. h Reference 8b. i Reference 5b. j Reference
8a. k Reference 9. l Reference 20. m Reference 27.
vis absorption [cyclohexane, rt, λmax/nm (ꢀ/M-1 cm-1)]: 210
(31 790), 214 (32 720), 250 (11 990). Magnetic moment (µeff/µB,
296 K): diamagentic. H NMR (CD2Cl2, 600 MHz, 296 K): δ
1EZ, U.K., and copies can be obtained on request, free of charge,
by quoting the publication citation and the deposition nos. 230707-
230711.
1
1.13 (d, 18H, CHMe2), 1.26 (d, 18H, CHMe2), 2.95 (m, 3H,
CHMe2), 3.46 (m, 3H, CHMe2), 5.90 (s, 3H, pz). 13C NMR (CD2Cl2,
150 MHz, 296 K): δ 23.50 (CHMe2), 23.53 (CHMe2), 26.54
(CHMe2), 27.80 (CHMe2), 97.76 (pz-C-H), 117-147 (C6F5), 157.09
(pz-CdN), 160.70 (pz-CdN).
Results and Discussion
General Aspects. Complexes 1, 2, 4, and 5 were
synthesized by using binuclear hydroxometal(II) complexes,
[(ML)2(µ-OH)2], as starting materials. Complexes 3 and 6
were synthesized by using mononuclear halogenometal(II)
complexes, [FeCl(L)] and [ZnBr(L)], as starting materials
because [(FeL)2(µ-OH)2] is unstable toward dioxygen49 and
[Zn(L)(OH)]n (n ) 1 or 2) cannot be obtained in a pure
form.42 Complexes 1-3 are unstable toward dioxygen and/
or temperature, and therefore were handled under an argon
atmosphere and kept at low temperature. All complexes are
easily soluble in noncoordinating solvents, such as dichlo-
romethane and heptane.
Crystal Structures. The structures of the neutral [ML-
(SC6F5)] complexes (2-6) are shown in Figures 1 and 2.
Important bond distances, bond angles, and structural
parameters are summarized in Table 2 including those of 1.
All of the complexes 1-6 contain four-coordinate metal(II)
ions, having a highly distorted tetrahedral configuration. In
the case of 4 and 5, two independent molecules exist in the
asymmetric unit. For the tables and the geometric compari-
sons in this paper, the independent molecules in 4 and 5 are
denoted as 4A, 4B and 5A, 5B, respectively. The difference
X-ray Data Collection and Structural Determination. The
single crystals of X-ray quality were obtained by recrystallization
from dichloromethane solution at -15 °C for 2, from dichlo-
romethane/heptane solution at -30 °C for 3 and 4, and from
dichloromethane/heptane solution at -15 °C for 5. The crystals of
6 were obtained by the slow evaporation of the dichloromethane/
heptane mixed solution at room temperature. Crystal data and
refinement parameters for 2-6 are given in Table 1. The diffraction
data for all complexes were measured on a Rigaku/MSC Mercury
CCD system with graphite-monochromated Mo KR (λ ) 0.710 69
Å) radiation at low temperature for 2, 3, and 6 at -61 °C and for
4 and 5 at -89 °C. All crystals were mounted on the tip of glass
fiber by heavy-weight oil. The unit cell parameters of each crystal
were determined using CrystalClear44 from 6 images. The crystals
to detector distance were ca. 45 mm. Data were collected using
0.5° intervals in φ and ω for 30 s/frame of 2 and 4, for 60 s/frame
of 3, for 35 s/frame of 5, and for 20 s/frame of 6 to a maximum 2θ
value of 55.0°. A total of 744 oscillation images were collected.
The highly redundant data sets were reduced using CrystalClear
and corrected for Lorentz and polarization effects.44 An empirical
absorption correction was applied for each complex. Structures were
solved by direct methods using the program SIR-92.45 The position
of the metal atoms and their first coordination sphere were located
from a direct method E-map; other non-hydrogen atoms were found
in alternating difference Fourier syntheses46 and least-squares
refinement cycles and during the final cycles were refined aniso-
tropically (CrystalStructure).47-48 Hydrogen atoms were placed in
calculated positions. The chlorine atoms of dichloromethane
molecule and the carbon atoms of heptane molecule in 4 were not
refined anisotropically because of high disorder. The GOF values
of 4 and 5 are not good due to disordered solvents. Crystallographic
data and structure refinement parameter including the final dis-
crepancies (R and Rw) are listed in Table 1. Crystallographic data
have been deposited at the CCDC, 12 Union Road, Cambridge CB2
(44) Pflugrath, J. W. CrystalClear, ver. 1.3. Acta Crystallogr. 1999, D55,
1718.
(45) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A.; Burla,
M. C.; Polidori, G.; Camalli, M. J. Appl. Crystallogr. 1994, 27, 435.
(46) Beurskens, P. T.; Admiraal, G.; Beurskens, G.; Bosman, W. P.; de
Gelder, R.; Israel, R.; Smits, J. M. M. The DIRDIF-99 program system;
Technical Report of the Crystallography Laboratory; University of
Nijmegen: Nijmegen, The Netherlands, 1999.
(47) CrystalStructure 3.51: Crystal Structure Analysis Package; Rigaku
and Rigaku/MSC: The Woodlands, TX, 2003.
(48) Crystal Issue 10; Watkin, D. J.; Prout, C. K.; Carruthers, J. R.;
Betteridge, P. W. Chemical Crsytallography Laboratory: Oxford, U.K.,
1996.
(49) Kitajima, N.; Tamura, N.; Tanaka, M.; Moro-oka, Y. Inorg. Chem.
1992, 31, 3342.
330 Inorganic Chemistry, Vol. 44, No. 2, 2005

Metal-Substituted Blue Copper Model Complexes
Figure 3. Model of a highly distorted tetrahedral configuration.
between 4A and 4B is the orientation of methyl groups on
the next carbon of N22. In the case of 5A and 5B, the
orientation of methyl groups on the next carbon of N21 and
the orientation of each benzene ring are different. The
geometries of both 4 and 5 are consistent with those of 1-3
and 6. The N-M-N angles (N11-M-N21, N11-M-N31,
and N21-M-N31) are about 90°, which are more acute than
an ideal value of 109.7°, while the S-M-N angles (S-
M-N11, S-M-N21, and S-M-N31) are ca. 100-130°,
which are more obtuse than an ideal one. Thus, the ligand L
attributes to the structural distortions of these complexes.
This is the general tendency of other Cu(II) complexes with
hydrotris(pyrazlyl)borate ligands.50
Figure 4. Plot of d values, which was defined as the distance between
metal ion and N11N31S plane, for highly distorted tetrahedral compounds
1-6.
In comparison with the structurally characterized cobalt(II)
thiolato complex [Co{HB(3,5-Me2pz)3}(SC6F5)], the dis-
tances of Co-S bond and all Co-N bonds are almost the
same. However, this complex is slightly distorted, having
the larger S-Co-N11 angle and the smaller N11-Co-N31
angle, because of the smaller ligand substitutions as com-
pared with L.24
For complexes 1-6 the distance (d) between the N11N31S
plane and metal(II) ion varies, depending on the structure,
as shown in Figure 3. The d value for an ideal tetrahedral
configuration is ca. 0.7 Å, and that for a general trigonal
one is 0 Å. Deviations from an ideal tetrahedral configuration
are evident from the plot of the d value vs the complex
number as shown in Figure 4. The d value decreases from
0.43 Å in Mn(II) (2) to 0.28 Å in Ni(II) (5) and again
increases to 0.41 Å in Zn(II) (6). The exception is Fe(II) (3)
which has a d value of 0.26 Å and is thus smaller than the
d values for either Co(II) (4) or Ni(II) (5). The reason for
the deviation in this trend in 3 is not clear, as changes in the
Fe(II)-N bond distances of 3 paralleled those of the other
complexes (see below). These results reflect that Mn(II) and
Zn(II) ions prefer a tetrahedral geometry as opposed to a
highly distorted tetrahedral one, while Fe(II), Co(II), Ni(II),
and Cu(II) ions can take on either geometry that is suitable
for the ligands.51
Figure 5. Plots of M-N and M-S distances for highly distorted
tetrahedral compounds 1-6: M-N distances which contain the N11N31S
plane (O); M-N21 distance which does not contain the N11N31S plane
(0); M-S distances (2).
The values obtained by taking an average of M-N11 and
M-N31 distances show a decreasing trend on going from
Mn(II) (2) at 2.12 Å to Fe(II) (3) at 2.05 Å, to Co(II) (4) at
2.02 Å, to Ni(II) (5) at 1.99 Å and then an increase on going
to Zn(II) (6) at 2.03 Å, as shown in Figure 5. The M-N21
distances in 2-6 also show a similar trend. The exception
in 1 arises from the Jahn-Teller effect against Cu-N21 axis,
resulting in the long Cu-N21 distance compared with other
two Cu-N distances. In addition, the Cu-S distance in 1 is
the shortest M-S bond distance in the series because of the
high covalency of the Cu-S bond.29 From these results, the
d values and the bond angles are good indicators for these
structural distortions, depending on the central metal(II) ions.
Similar tendencies have been reported by Stack et al. for
(50) For recent book and reviews: (a) Parkin, G. Chem. Commun. 2000,
1971. (b) Trofimenko, S. Scorpionates-The Coordination Chemistry
of Polypyrazolylborate Ligands; Imperial College Press: London,
1999. (c) Kitajima, N.; Tolman, W. B. Prog. Inorg. Chem. 1995, 43,
419.
(51) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; John Wiley & Sons: New York, 1999.
Inorganic Chemistry, Vol. 44, No. 2, 2005 331

Matsunaga et al.
Table 3. Absorption Spectral Data (λ/nm (ꢀ/M-1 cm-1)) for
Complexes 1 and 3-5 at Room Temperature
sulfur-to-metal(II)
complex
charge-transfer bands (ꢀ)
d-d bands (ꢀ)
1040 (1170)
1360 (95)
605 (broad, 1170), 812 (140),
1479 (115)
Cu (1)a
Fe (3)b
Co (4)b
663 (5980)
308 (5940), 355 (1390)
311 (5030), 340 (2930)
Ni (5)b
357 (1850), 434 (3100)
804 (145), 1400 (35)
a Dichloromethane solution. b Cyclohexane solution.
the changes of M-N distances in [M(Cl)(PY5)] (PY5 ) 2,6-
bis(bis(2-pyridyl)methoxymethyl)pyridine; M ) Mn(II),
Fe(II), Co(II), Ni(II), Cu(II), and Zn(II)).52
The structures of blue copper proteins such as azurin,5b
plastocyanin,5a pseudoazurin,5c amicyanin,5d cucumber basic
protein,5e stellacyanin,5f rusticyanin,5g and many others have
been reported. The Cu-S distance in 1 is almost the same
as in those of blue copper proteins.25 Metal(II)-substituted
azurins to Ni(II), Co(II), Zn(II), and Cd(II) from Cu(II) have
been reported in terms of their crystal structures,6-9,18 UV-
vis absorption,10-11 and other spectroscopic properties.12-15
The M-S bond lengths of 4-6 are almost the same as
the corresponding metal(II)-substituted azurins (Table 2). The
d values of 4-6 suggest that their coordination geometries
are more tetrahedrally distorted than those of native azurin
or metal(II)-substituted azurins, and the bond distances from
metal(II) to N21, which is in the axial position, do not show
any large differences. To discuss this factor, the bond angles
of 1-6 were compared with the corresponding metal(II)-
substituted azurins. Although the S-M-N11 and N11-M-
N31 bond angles in 1-6 are smaller than those of corre-
sponding azurins, it is clear that the S-M-N11 and S-M-
N21 bond angles are more obtuse than the N11-M-N31
bond angle. The two imidazole rings of histidines in azurins
are more flexible than the tripodal N3 ligand in 1-6, and
for this reason, there are some differences between the bond
angles in azurins and the model complexes (see below). The
d values of 1-6 are similar to that of plastocyanin, because
there are no interactions of the fifth coordination atom (the
opposite position to Met92) of plastocyanin.
UV-Vis Spectrscopy. The UV-vis absorption spectra
of 1-6 were measured using cyclohexane (200-1700 nm)
or dichloromethane (260-1400 nm) solution under an argon
atmosphere. The assigned absorption bands are listed in Table
3. Their spectra in the region of 300-900 nm are shown in
Figure 6. The low-temperature spectra using dichloromethane
solution are plotted in Figure S6 (Supporting Information).
The spectrum of 1 is unique and is in good agreement with
that of native azurin. The assignments of the observed bands
at 663 and 1040 nm in 1 were made by comparing to those
of [CuL(SCPh3)]29 and the mononuclear copper(II) cumyl-
peroxo complex [CuL(OOCMe2Ph)].53
Figure 6. UV-vis absorption spectra of [ML(SC6F5)] (M ) Cu (1) (s),
Fe (3) (‚‚‚), Co (4) (---), Ni (5) (- - -) at room temperature.
band was observed at 1360 nm, which is assigned as a d-d
transition band, since the analogous complex [FeL(SC6H4-
4-tBu)] has a similar band at ca. 1320 nm.54 In addition, the
molar extinction coefficient of 3 is consistent with a d-d
transition band.55 In general, Co(II) complexes with a
pseudotetrahedral geometry have d-d transition bands in the
ca. 600-2200 nm region.55 Three bands were observed at
605, 812, and 1479 nm for complex 4. Moreover, a shoulder
band was also observed at 575 nm. The analogous complex,
[Co{HB(3,5-Me2pz)3}(SC6F5)], shows three d-d transition
bands at 570 (270), 605 (350), and 660 nm (95 M-1 cm-1).24
Both complexes have a band at 605 nm, but the ꢀ value of
4 is much larger than that of [Co{HB(3,5-Me2pz)3}(SC6F5)].
In addition, there are two higher wavelength bands (812 and
1497 nm) in 4 and the corresponding bands in the spectrum
of [Co{HB(3,5-Me2pz)3}(SC6F5)] cannot be assigned due to
smaller molar extinction coefficient. Accordingly, the three
bands of 4 are assigned to d-d transition bands. In general,
Ni(II) complexes with a pseudotetrahedral geometry contain-
ing sulfur ligand have d-d transition bands in the ca. 800-
1500 nm region.55 In this context, the two bands at 804 and
1400 nm for 5 may be assigned to the d-d transition bands.
The spectra of 2 and 6 exhibit no absorption bands above
300 nm. For 2, the reason is that in high-spin d5 Mn(II)
complexes the d-d bands are spin forbidden as well as
Laporte forbidden; thus, they are generally very weak.55 In
6, Zn(II) ion has a stable d10 electron configuration, and thus,
no d-d transitions are possible.
The one or two bands, which are assigned to the sulfur-
to-metal(II) CT bands, should be observed for the spectra
of 3-5 in the 300-500 nm region. In the case of 3, two
bands were found at 308 and 355 nm. The reported model
complexes such as (Et4N)2[Fe(S2-o-xyl)2] (S2-o-xyl ) bis-
(o-xylyl-R,R′-dithiolato anion))56 and (Et4N)2[Fe(SEt)4]57
have two bands, one at ∼320 nm with very strong extinction
(54) Pavel, E. G.; Kitajima, N.; Solomon, E. I. J. Am. Chem. Soc. 1998,
120, 3949.
(55) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier
Science Publishing Co., Inc.: New York, 1984.
(56) Lane, R. W.; Ibers, J. A.; Frankel, R. B.; Papaefthymiou, G. C.; Holm,
R. H. J. Am. Chem. Soc. 1977, 99, 84.
The d-d transition band patterns of 3-5 are consistent
with those of a distorted tetrahedral geometry. For 3, one
(52) Gebbink, R. J. M. K.; Jonas, R. T.; Goldsmith, C. R.; Stack, T. D. P.
Inorg. Chem. 2002, 41, 4633.
(53) Chen, P.; Fujisawa, K.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122,
10177.
(57) Hagen, K. S.; Watson, A. D.; Holm, R. H. J. Am. Chem. Soc. 1983,
105, 3905.
332 Inorganic Chemistry, Vol. 44, No. 2, 2005

Metal-Substituted Blue Copper Model Complexes
coefficient (7100-7750 M-1 cm-1) and another band as a
shoulder at ∼350 nm. For (Et4N)2[Fe(S2-o-xyl)2], both bands
are assigned as CT bands.56 The spectral behavior and band
positions of 3 are very similar to those of these complexes.
Thus, the assignments of both bands for 3 are the sulfur to
Fe(II) CT bands. In the spectra of 4, two bands were observed
at 311 and 340 nm. The sulfur to Co(II) CT bands of
(Et4N)2[Co(S2-o-xyl)2]56 and {P(C6H5)4}2[Co(SC6H5)4]58 are
observed at 355 (3450) and 420 nm (3000 M-1 cm-1),
respectively. Moreover, the CT band of [Co{HB(3,5-Me2pz)3}-
(SC6F5)] is calculated at 289 nm.24 Thus, the two bands in 4
are assigned to the sulfur to Co(II) CT bands. In addition,
the band at the high-energy side at 311 nm is much stronger
than that at the low-energy side at 340 nm, which is in good
agreement with the Co(II) derivative of azurin,10 in which
both bands at 330 (3020) and 375 nm (1180 M-1 cm-1) are
assigned to the sulfur to Co(II) CT bands.10b For 5, two bands
were observed at 357 and 434 nm. The band of (Et4N)2[Ni2-
(S2-o-xyl)3] is observed at 486 nm (3760 M-1 cm-1), which
is assigned to sulfur to Ni(II) CT bands.56 Moreover, Ni-
(II)-substituted azurin has two bands at 358 (3098) and 440
nm (3060 M-1 cm-1),10b which are very similar with those
of 5. Therefore, both bands are assigned to the sulfur to Ni-
(II) CT band. These results confirm that the complexes 4
and 5 are good structural models for metal(II)-substituted
blue copper proteins and the relationship of the metal to the
N2S basal plane is a very important factor in determining
d-d transition energies and sulfur-to-metal(II) CT energies.
Moreover, the CT energies of Fe(II) (3), Co(II) (4), and Ni(II)
(5) are each shifted to higher energy relative to that of 1.
Magnetic and Electrochemical Properties. Magnetic
susceptibility was discussed only for 4 and 5 because they
are not oxygen and/or temperature sensitive.59 The Co(II)
complex 4 exhibits an effective magnetic moment µeff ) 4.62
µB at 296 K. In general, the magnetic moments of high-spin
tetrahedral Co(II) complexes are in the range from 4.6 to
4.8 µB.51 Therefore, this supports a high-spin configuration
for 4. For the Ni(II) complex 5, µeff ) 3.30 µB was obtained
at 293 K. This value is similar to the reported effective
magnetic moment of Ni(II)-substituted azurin, 3.2 µB by
Balzack et al.14g and 2.8 µB by Jime´nez et al.15 In contrast,
the magnetic moment of tetrahedral Ni(II) complexes includ-
ing distortions varies from 3.00 to 4.0 µB.51 Therefore, this
result shows good agreement with those of the Ni(II)-
substituted azurin.
Figure 7. 1H NMR spectra of [ML(SC6F5)] (M ) Fe (3) (a), Co (4) (b),
Ni (5) (c), Zn (6) (d)) at room temperature (expansion from 20 to -20
ppm).
For 2, the EPR spectrum shows a signal of g ) 3.91. The
spectral pattern of 2 is similar to that of Mn(II) superoxide
dismutase60 or Mn(II) catechol dioxygenase.61 Unfortunately,
the EPR signals of the Mn(II)-substituted blue copper
proteins have not been published.10d However, this report
has been made that indicates the Mn(II) ion is spontaneously
released from Mn(II)-substituted azurin.10d This result indi-
cates that the active sites of the native blue copper proteins
have a semirigid ligand environments compared with the N3
tripodal ligand. The spectral behavior of 4 (g1 ) 5.90, g2 )
2.66, and g3 ) 1.74) is similar with that of Co(II)-substituted
azurin.6,10d In addition, the temperature dependence and
spectral pattern indicate 4 is high spin.10d
The paramagnetic 1H NMR spectra were measured for the
Fe(II), Co(II), and Ni(II) complexes (3-5). These results are
shown in Figure 7 (expansion from 20 to -20 ppm) and
Figure S8 (all region) with the spectrum of the Zn(II)
complex 6. The previous assignments of Fe(II) complexes
are used.43,62-63 In general, the protons near to the metal(II)
ions suffer the largest chemical shift and the greatest line
broadening. However, the 4-H protons of the pyrazolyl ring
were observed to have the largest downfield shift in 65-80
ppm region, having an integration of 3 protons (Figure S8).
This is a general tendency for complexes containing the
hydrotris(pyrazolyl)borate ligand.43,62-63 The large shift
comes from the deshielding effects of the aromatic pyrazolyl
ring current.64 The signals are gradually shifted downfield,
going from 66.11 ppm for 3, to 73.67 ppm for 4, and to
81.09 ppm for 5. Furthermore, the methyl protons of its
isopropyl group are observed in the narrow region from -2
While EPR spectra of all complexes were measured, only
1, 2, and 4 showed EPR signals (Figure S7). No signal is
anticipated for 3 or 5, as these are high-spin distorted
tetrahedral Fe(II) and Ni(II) complexes with integer spins
of S ) 2 and S ) 1, respectively. While EPR signals were
observed for 1 and 2 at both 140 and 20 K, the signals of 4
only appeared below 20 K. The EPR spectrum of 1 is axial
with g| ) 2.32, g ) 2.10, and A| ) 52 G.25 These EPR
parameters are very similar to those of native azurin5b and
plastocyanin5a and reflecting the d values as described above.
(60) Whittaker, J. W.; Whittaker, M. M. J. Am. Chem. Soc. 1991, 113,
5528.
(61) Whiting, A. K.; Boldt, Y. R.; Hendrich, M. P.; Wackett, L. P.; Que,
L., Jr. Biochemistry 1996, 35, 160.
(62) Kitajima, N.; Tamura, N.; Amagai, H.; Fukui, H.; Moro-oka, Y.;
Mizutani, Y.; Kitagawa, T.; Mathur, R.; Heerwegh, K.; Reed, C. A.;
Randall, C. R.; Que, L., Jr.; Tatsumi, K. J. Am. Chem. Soc. 1994,
116, 9071.
(63) Mehn, M. P.; Fujisawa, K.; Hegg, E. L.; Que, L., Jr. J. Am. Chem.
Soc. 2003, 125, 7828.
(58) Swenson, D.; Baenziger, N. C.; Coucouvanis, D. J. Am. Chem. Soc.
1978, 100, 1932.
(59) The fatal color changes of 1-3 were observed during grinding.
(64) Ming, L.-J. In Physical Methods in Bioinorganic Chemistry: Spec-
troscopy and Magnetism; Que, L., Jr., Ed.; University Science Book:
Sausalito, CA, 2000.
Inorganic Chemistry, Vol. 44, No. 2, 2005 333

Matsunaga et al.
as 1 in the 300-500 cm-1 region. However, all bands are
slightly shifted from 1 in this region, depending on their
structures.
The largest shifts were observed for bands at ca. 400 cm-1
(403 ( 4 cm-1), which has the most M-S stretching
character.28 It reflects the changes in M-S bond distances,
from 2.176(4) Å in 1 to 2.248(2)-2.385(3) Å in 2-6,
reflecting the higher covalency of the Cu-S bond as
compared to the other metal(II)-substituted complexes.
Similar shifts are observed in the resonance Raman spectra
of native and Ni(II)-substituted azurin.11 Complexes 1-6 all
have the same bands at ca. 310 cm-1 (from 308 to 311 cm-1).
However, the bands at ca. 350 cm-1 may be divided into
two groups, 2 and 6 (average, 341 cm-1) and 3-5 (average,
347 cm-1). These differences arise from the central metal-
(II) geometries. Complexes 2 and 6 have larger d values
(average, 0.42 Å), whereas complexes 3-5 have smaller d
values (average, 0.29 Å). These two bands are therefore very
sensitive to the coordination geometry.
As mentioned before, the Raman shifts in 128 are consistent
with the far-IR spectra of 1. Therefore, the far-IR spectra
are good indicators for all metal(II)-substituted model
complexes (1-6).67 Moreover, we observed the Raman shifts
of 2 and 6: Mn (2), 385, 332, and 308 cm-1; Zn (6), 401,
339, and 311 cm-1 (Figure S10). Therefore, we can obtain
fundamental information about the M-S stretching vibration
by comparing far-IR spectra (1-6) and FT-Raman spectra
(2 and 6). The detailed assignment will be performed for 4
and 5 by resonance Raman spectroscopy.
Figure 8. Far-IR spectra of 1-6.
to 5 ppm. The methyl protons near the boron atom are almost
not shifted, Fe(II) 3.83, Co(II) 3.19, and Ni(II) 3.21 ppm.
However, the other methyl protons near the metal(II) ion
are broadened and shifted without order of the d electron
numbers, Fe(II) 4.07, Co(II) 3.84, and Ni(II) -0.12 ppm.
Other broad signals of the methine protons of its isopropyl
group and B-H proton are also shifted without order.
There have been many reports concerning the paramag-
1
netic H NMR spectra of Co(II)- and Ni(II)-substituted
azurin. However, the maximum chemical shifts in the spectra
of both metal substituted azurins are found at much lower
magnetic field than those of 4 and 5. Larger shifts and
broader line widths are observed compared in the spectra of
Co(II)-substituted azurin as compared to Ni(II)-substituted
azurin.5b,10a,14a,14d,14h Moreover, the protons of His46 and
His117 are in the -10 to 100 ppm region for Co(II)-
substituted azurin and the -15 to 65 ppm region for Ni(II)-
substituted azurin. However, the protons of Cys112 show
the largest chemical shift (ca. 240 ppm) in both spectra. The
chemical shifts of the protons on pyrazolyl rings (4 and 5)
fall into the same region with those on imidazole rings.
Cyclic voltammetric examinations of 2-6 in acetone under
argon were carried out. However, they showed no clear
peaks. Only 1 in the same condition shows a single reversible
redox signal at -468 mV versus Fc+/Fc as shown in Figure
S9. On the basis of this reversibility, it corresponds to Cu(II)/
Cu(I) process. The redox signal of [CuL(SMeIm)] (HSMeIm
) 2-mercapto-1-methylimidazole) was clearly shifted to
-912 mV versus Fc+/Fc.34,65 This means that 1 is very easily
reduced to Cu(I) state rather than [CuL(SMeIm)].
Summary and Biological Relevance The metal-substi-
tuted complexes [ML(SC6F5)] were synthesized using Mn(II)
(2), Fe(II) (3), Co(II) (4), Ni(II) (5), and Zn(II) (6) as models
for metal(II)-substituted azurins. All the complexes were
characterized by UV-vis absorption and far-IR spec-
troscopies and other properties. Moreover, the complexes of
4 and 5 were compared with metal(II)-substituted azurins.
All complexes have a highly distorted tetrahedral geometry,
which is the same as the corresponding metal(II)-substituted
azurins. In the UV-vis absorption spectra, the d-d bands
of 3-5 are assigned and have the same energy as distorted
tetrahedral geometry. Moreover, the sulfur-to-metal(II) CT
bands are shifted to higher energy in 3-5 rather than in 1.
Their M-S stretching vibration energies and other physi-
cochemical properties are consistent with their structural
distortions. The properties of metal(II)-substituted azurins
are very similar to those of model complexes of 4 and 5.
Therefore, the relationship of the metal(II) to the N2S basal
plane is a very important factor for these properties as
indicated in 1. We can summarize the structural and
spectroscopic behavior upon increasing the number of d
electrons as follow: (1) Distance d is consistent with the
M-S Stretching Vibration. The far-IR spectra of com-
plexes 1-6 are shown in Figure 8. All three far-IR bands in
1 at 406, 350, and 311 cm-1 are assigned to Cu-S stretching
vibration from resonance Raman results of 128 and native
azurin.66 The spectra in 2-6 show almost the same behavior
(65) Basumallick, L.; DeBeer George, S.; Randall, D. W.; Hedman, B.;
Hodgson, K. O.; Fujisawa, K.; Solomon, E. I. Inorg. Chim. Acta 2002,
337, 357.
(66) Andrew, C. R.; Han, J.; den Blaauwen, T.; van Pouderoyen, G.;
Vijgenboom, E.; Canters, G. W.; Loehr, T. M.; Sanders-Loehr, J. J.
Biol. Inorg. Chem. 1997, 2, 98.
(67) Since the complexes with Mn(II) (2), Fe(II) (3), and Zn(II) (6) have
no color, we cannot collect any M-S stretching frequencies by
resonance Raman spectroscopy.
334 Inorganic Chemistry, Vol. 44, No. 2, 2005

Metal-Substituted Blue Copper Model Complexes
ionic radius and the general coordination preference of each
metal(II) ion except for 3 (Figure 4). (2) From UV-vis
absorption spectra of 1, 4, and 5, the sulfur-to-metal(II) CT
bands are shifted to higher energy (Figure 6). (3) The
downfield-shifted proton signals of the pyrazolyl 4th position
in 1H NMR spectrum of 3-5 are observed with order (Figure
S8). Moreover, the electron configurations around the metal-
(II) ions of 4 and 5 are suggested to be similar to those of
the corresponding metal(II)-substituted azurin from 1H NMR
and EPR spectra (Figures 7 and S7). Only 1 shows a single
reversible redox signal in cyclic voltammograms (Figure S9).
This indicates that the copper(II) ion is most suitable for
electron-transfer reactions in this distorted tetrahedral ge-
ometry, so that in nature the proteins select this Cu(II) ion
for fast electron-transfer reaction. However, we need more
detailed experiments for their assignments and roles of metal-
(II) ions in the electron-transfer reaction mechanism including
the relationship between structure and function of electron
transfer.
Acknowledgment. This research was supported in part
by the Japan Society of the Promotion of Science (Grant
Nos. 13555257 and 14350471) and by the Ministry of
Education, Culture, Sports, Science, and Technology (the
21st Century COE program (Promotion of Creative Inter-
disciplinary Materials Science for Novel Function)). Finally,
we appreciate the kind editing of this manuscript by Dr.
Serena DeBeer George of the Stanford Synchrotron Radiation
Laboratory.
Supporting Information Available: Complete listings of
atomic coordinates for 2-6 (Figures S1-S5), temperature factors,
bond lengths, bond angles, and torsion angles for all structures 2-6
(Tables S1-S5), UV-vis absorption spectra of 1 and 3-5 at -45
1
°C (Figure S6), EPR spectra of 1, 2, and 4 (Figure S7), H NMR
spectra of 3-6 (all regions) (Figure S8), cyclic votammograms of
1 (Figure S9), FT-Raman spectra of 3 and 6 (Figure S10), and
crystallographic data in CIF format. This material is available free
of charge via the Internet at http://pubs.acs.org.
IC049814X
Inorganic Chemistry, Vol. 44, No. 2, 2005 335