Copper(I) and copper(II) complexes of the bidentate imidazole/thioether ligand 1-methyl-2-(methylthiomethyl)-1H-benzimidazole

Article abstract of DOI:10.1016/S0020-1693(99)00018-3

The mixed imidazole/thioether chelate ligand 1-methyl-2-(methylthiomethyl)-1H-benzimidazole (mmb) which has been shown earlier to effect biomimicking valence tautomerism in copper/o-quinone systems forms crystallographically characterized chelate complexes [(η²-mmb)Cu^I(PPh₃)₂](BF ₄) (1) and [(η²-mmb)₂Cu^II(η¹-ClO ₄)](ClO₄) (2). Compound 1 contains copper(I) in an approximately tetrahedral environment and complex 2, obtained as methanol solvate, exhibits an asymmetrically (trigonal bipyramidal/square pyramidal) coordinated Cu^II center. Whereas bidentate mmb forms shorter N-Cu^II (1.945 A) than N-Cu^I bonds (2.040 A), the S(thioether)-Cu bond lengths of about 2.43 A are independent of the metal oxidation state. Two relatively simple ligands mmb can effect similar spectroscopic responses as more complicated multidentate ligands.

Full text of DOI:10.1016/S0020-1693(99)00018-3

Inorganica Chimica Acta 287 (1999) 204–208
Copper(I) and copper(II) complexes of the bidentate
imidazole/thioether ligand
1-methyl-2-(methylthiomethyl)-1H-benzimidazole
Markus Albrecht, Klaus Hu¨bler, Thomas Scheiring, Wolfgang Kaim *
Institut fu¨r Anorganische Chemie, Uni6ersita¨t Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany
Received 23 September 1998; accepted 16 December 1998
Abstract
The mixed imidazole/thioether chelate ligand 1-methyl-2-(methylthiomethyl)-1H-benzimidazole (mmb) which has been shown
earlier to effect biomimicking valence tautomerism in copper/o-quinone systems forms crystallographically characterized chelate
complexes [(h2-mmb)CuI(PPh3)2](BF4) (1) and [(h2-mmb)2CuII(h1-ClO4)](ClO4) (2). Compound 1 contains copper(I) in an
approximately tetrahedral environment and complex 2, obtained as methanol solvate, exhibits an asymmetrically (trigonal
I
bipyramidal/square pyramidal) coordinated CuII center. Whereas bidentate mmb forms shorter N–CuII (1.945 A) than N–Cu
˚
˚
˚
bonds (2.040 A), the S(thioether)–Cu bond lengths of about 2.43 A are independent of the metal oxidation state. Two relatively
simple ligands mmb can effect similar spectroscopic responses as more complicated multidentate ligands. © 1999 Elsevier Science
S.A. All rights reserved.
Keywords: Crystal structures; EPR; Copper complexes; Imidazole complexes; Thioether complexes
1. Introduction
A large number of chelate ligands with sophisticated
construction of imine N donor and thiolate and/or
thioether S donor centers has been studied during the
last two decades [1,2], in connection with attempts to
mimic the structure, spectroscopy and electrochemical
behavior of ‘type 1’ copper centers in ‘blue’ copper
proteins [3–5]. In these centers of biological electron
transfer, the metal is usually coordinated by two his-
tidine, one cysteinate and one methionine side chain of
the polypeptide backbone [3–6]. Within a different
In combination with copper and 3,5-di-tert-butyl-o-
quinone (Q), we were able to establish for the first time
a CuI–semiquinone/CuII–catecholate valence tautomer
equilibrium (Eq. (1)) outside biology [7,8]. This success-
ful approach was based on the previous observation
project, we have recently studied [7,8] the spectroscopi-
cally-established [9] and biologically-functional [4,10]
valence tautomerism of copper/o-quinone complexes
using a simply constructed imidazole/thioether ligand 1-
methyl-2-(methylthiomethyl)-1H-benzimidazole (mmb)
which contains the donor characteristics of histidine
and methionine [7].
that all
N
ligands favor CuII–catecholate forms
whereas all-thioether ligands stabilize the CuI–
semiquinone state [11].
I
(L)Cu (Q −) X (L)CuII(Q2−
)
(1)
’
In amine oxidases, ubiquitous enzymes for connective
tissue maturation and other biological roles [4,10,12],
* Corresponding author. Tel.:+49-711-6854170/4171; fax: +49-
711-6854165.
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00018-3

M. Albrecht et al. / Inorganica Chimica Acta 287 (1999) 204–208
205
Table 1
Crystal data for complexes 1 and 2 · 0.5CH3OH
1
2 · 0.5CH3OH
Empirical formula
Crystal color, form
Formula weight (g mol−1
Crystal system
C46H42BCuF4N2P2S
colorless, rods
867.17
monoclinic
P21/c
12.694(2)
15.480(2)
21.723(3)
C20H22Cl2CuN4O8S2 · 0.5CH3OH
blue–green, blocks
661.00
)
triclinic
¯
Space group
P1
˚
a (A)
10.364(3)
11.114(2)
12.227(3)
˚
b (A)
˚
c (A)
h (°)
i (°)
k (°)
90
86.15(2)
78.10(2)
75.30(2)
1332.7(6)
104.810(12)
90
4126.7(11)
3
˚
V (A )
Z
4
2
Dcalc (g cm−3
)
1.396
0.711
1792
1.647
1.231
676
Absorption coefficient, v (mm−1
F(000)
)
Crystal size (mm)
q Range (°)
0.2×0.2×0.4
1.66–25.08
0.2×0.2×0.3
1.70–26.00
Limiting indices
Reflections collected
Independent reflections
Rint
−75h515;−185k517; −255l524
6918
6917
0.2705
05h512; −125k513;−145l515
5253
5213
0.0337
Data/restraints/parameters
6567/0/515
1.0009
5212/9/380
1.064
GOF on F2
R1 (I\2|(I))
0.0560
0.0585
wR2 (all data)
Residual peaks: max., min. (e A
0.1357
0.733, −0.438
0.1652
1.010, −0.966
−3
˚
)
this process (Eq. (1)) serves to generate the copper(I)
state which is essential for binding and activation of
dioxygen [4,6,13].
ring is distinctly asymmetric with a longer CuI–S (2.434
I
˚
˚
A) and a shorter Cu –N (2.040 A) bond; at 2.249 and
I
˚
2.296 A the typically different [14] Cu –P bonds exhibit
Unfortunately, structural information is not available
for the system mmb/Cun+/Qn− [7,8]. We therefore set
out to obtain single crystals of conventional copper(I)
and copper(II) compounds with the ligand mmb and
arrived at compounds [(h2-mmb)CuI(PPh3)2](BF4) (1)
and [(h2-mmb)2CuII(h1-ClO4)](ClO4) (2). Their crystal
structure, spectroscopic and electrochemical characteri-
zation are reported in this contribution.
intermediate bond distances. As has been concluded
from EPR studies [11], the triorganophosphines are
better acceptor ligands towards CuI than thioether
functions.
The poorly soluble complex 2 contains two h2-mmb
ligands and one disordered O-coordinated perchlorate
around the copper(II) center (Figs. 2 and 3). Its coordi-
nation geometry falls between that of the trigonal
bipyramidal (tbp) and square planar (sp) alternative;
adopting the criteria of Reedijk and coworkers [2] (see
also Schmuck and Seppelt [15]), we obtain an angular
structural parameter ~=0.46 and thus a very similar
value to ~=0.48 that was obtained for complex
[Cu(bmdhp)(OH2)](ClO4)2 (3) with one tetradentate 1,7-
bis(N - methylbenzimidazol - 2% - yl) - 2,6 - dithiaheptane
(bmdhp) ligand [2].
2. Results and discussion
Compounds 1 and 2 were synthesized via conven-
tional routes using [Cu(PPh3)4](BF4) and Cu(ClO4)2 as
metal-containing precursors. Compound 1 forms color-
less, air-stable, non-luminescing crystals, and the blue–
green complex 2 crystallizes with 0.5 equivalent of
methanol. Crystal structure analyses were possible for
both these compounds; however, no significant inter-
molecular interactions were found. The results are sum-
marized in Tables 1 and 2 and Figs. 1 and 2.
Complex 1 exhibits a well-established [14] pattern for
bis(triorganophosphine)copper(I) complexes of biden-
tate ligands, i.e. an approximately tetrahedral coordina-
tion of copper(I) (Fig. 1). The five-membered chelate

206
M. Albrecht et al. / Inorganica Chimica Acta 287 (1999) 204–208
Table 2
˚
Selected bond lengths (A) and angles (°) for complexes 1 and
2 · 0.5CH3OH
Complex 1
Cu(1)–N(1)
Cu(1)–S(1)
2.040(3)
2.4336(13) Cu(1)–P(2)
Cu(1)–P(1)
2.2491(12)
2.2961(12)
N(1)–Cu(1)–P(1)
N(1)–Cu(1)–P(2)
N(1)–Cu(1)–S(1)
114.53(10) P(1)–Cu(1)–P(2)
111.42(10) P(1)–Cu(1)–S(1)
84.25(10) P(2)–Cu(1)–S(1)
118.30(4)
117.32(5)
105.96(5)
2 · 0.5CH3OH
Cu(1)–N(3)
Cu(1)–N(1)
Cu(1)–S(1)
1.941(3)
1.949(3)
2.4193(13) Cu(1)–O(2A)
Cu(1)–S(2)
Cu(1)–O(2)
2.4436(13)
2.236(9)
2.287(10)
N(3)–Cu(1)–N(1)
S(1)–Cu(1)–S(2)
N(3)–Cu(1)–O(2)
N(1)–Cu(1)–O(2)
N(3)–Cu(1)–O(2A)
N(1)–Cu(1)–O(2A)
N(3)–Cu(1)–S(1)
172.53(14) N(1)–Cu(1)–S(1)
83.07(11)
102.9(5)
145.18(5)
99.8(3)
86.9(3)
98.0(4)
89.5(4)
O(2)–Cu(1)–S(1)
O(2A)–Cu(1)–S(1) 122.7(4)
N(3)–Cu(1)–S(2)
N(1)–Cu(1)–S(2)
O(2)–Cu(1)–S(2)
Fig. 2. Molecular structure of the cation [(h2-mmb)2Cu(h1-ClO4)]+
in the crystal of 2 · 0.5CH3OH (only one orientation of the disordered
perchlorate ligand shown).
83.56(11)
96.86(11)
111.9(5)
92.1(4)
92.31(10) O(2A)–Cu(1)–S(2)
understand the function of type 1 copper electron trans-
fer centers of ‘blue’ copper proteins [5]; they may also
be used to interpret the electron transfer function of
methionine-binding heme groups which occur e.g. in
cytochrome c [16].
~=(i−h)/60°
(2)
where ~ is the angular structural parameter (index of
trigonality) and h,i are basal angles.
~=0 for ideal sp (h=i=180°); ~=1 for ideal tbp
(h=120°, i=180°)
Data from absorption and EPR spectroscopy sup-
port the structural results: the copper(I) center in 1 has
a d10 configuration and displays no EPR activity, long-
wavelength metal-to-ligand charge transfer absorption
[14,17] or strong luminescence [14] after irradiation in
the UV region. On the other hand, the d9 copper(II)
center in 2 exhibits an EPR spectrum with gꢀꢀ=2.358,
gÞ=2.074 and Aꢀꢀ=13.4 mT; these parameters are
rather close to those of [Cu(bmdhp)(OH2)](ClO4)2 (3)
[2] because of similar geometrical configuration. At
750(sh) and 666 nm in acetonitrile solution the ligand-
field transitions are also similar to those of 3 [2].
Cyclic voltammetry reveals that both the oxidation of
1 (at Epa=0.90 V) and the reduction of 2 (at Epc=0.01
[(h2-mmb)2CuII(h1-ClO4)](ClO4) · 0.5CH3OH:
h(N3–Cu–N1)=172.53(14)°;
145.18(5)°; ~=0.46.
The higher charge on the metal in 2 relative to 1
i(S1–Cu–S2)=
results in the expected shortening of the Cu–N bonds
˚
from 2.040 to about 1.945 A. However, at 2.419 and
˚
2.444 A, the Cu–S bond lengths in 2 are virtually
˚
identical to those in 1 (2.434 A), supporting the notion
of relatively little change in the copper–thioether bond
on metal oxidation. Such effects have been used to
Fig. 1. Molecular structure of [(h2-mmb)Cu(PPh3)2](BF4) (1) in the
crystal.
Fig. 3. Coordination of the copper(II) center in 2 · 0.5CH3OH with
both orientations of the disordered perchlorate ligand.

M. Albrecht et al. / Inorganica Chimica Acta 287 (1999) 204–208
207
V, all peak potentials versus Fc+/o in CH3CN/0.1 M
(16 000), 273 (17 000), 268(sh), 254 (15 000 M−1 cm−1
)
Bu4NPF6) are irreversible processes due to extensive
structural changes, including dissociation. The large
difference between these potentials reflects the influence
of two p-acidic triorganophosphine ligands in 1 which
stabilize the copper(I) state (anodic shift).
Summarizing, we have confirmed that mmb can bind
as chelate ligand to both CuI and CuII centers with
surprisingly little variation of the metal–S bond length.
The structural and spectroscopic results for the cop-
per(II) complex are very similar to those obtained with
corresponding tetradentate ligands [2] which suggests
the use of this simple ligand in combination with other
metal complex fragments.
nm.
3.2. Instrumentation
EPR spectra were recorded in the X-band on a
Bruker System ESP 300 equipped with a Bruker
ER035M gaussmeter and a HP 5350B microwave coun-
ter. 1H and 13C NMR spectra were recorded on a
Bruker AC 250 spectrometer. UV–Vis absorption spec-
tra were recorded on a Bruins Instruments Omega 10
spectrophotometer. Cyclic voltammetry was carried out
in acetonitrile/0.1 M Bu4NPF6 using a three-electrode
configuration (glassy carbon electrode, Pt counter elec-
trode, Ag/AgCl reference) and a PAR 273 potentiostat
and function generator. The ferrocene/ferrocenium cou-
ple served as internal reference.
3. Experimental
3.1. Synthesis
3.3. Crystallography
The preparation of the ligand mmb via Phillips con-
densation has been previously described [7].
Single crystals of 1 were obtained through slow cool-
ing to 4°C of a chloroform/n-pentane solution (1:1).
Single crystals of 2 · 0.5CH3OH were synthesized by
slow interdiffusion of cold (4°C), methanolic solutions
of Cu(ClO4)2 · 6H2O and mmb in methanol through a
porous glass frit.
3.1.1. [(p2-mmb)Cu(PPh3)2](BF4) (1)
A
suspension of 200 mg (0.167 mmol)
[Cu(PPh3)4](BF4) and 32 mg (0.166 mmol) mmb in
toluene/chloroform (10:1) was reacted at 80°C for 1 h.
Filtration of the hot solution and washing of the solid
residue with toluene (2 ml) yielded the crude product
which was dissolved in chloroform and reprecipitated
with n-pentane to yield 85 mg (59%) of colorless air-sta-
ble 1. Calc. for C46H42BCuF4N2P2S (867.22): C, 63.71;
H, 4.88; N, 3.23. Found: C, 63.64; H, 4.88; N, 3.26%.
1H NMR (CD3CN) l: 1.91 (s, 3H, SCH3), 3.62 (s,
2H, CH2SCH3), 3.74 (s, 3H, NCH3), 7.13–7.56 (m,
34H, CH) ppm. 13C NMR (CD3CN) l: 19.1 (SCH3),
31.3 (NCH3), 32.8 (CH2SCH3), 111.8, 119.2 (C-5,6
mmb), 123.8, 124.4 (C-4,7 mmb), 129.9 (d, JCP=9.0
Hz, PPh3), 131.3 (s, PPh3), 133.7 (d, JCP=28.1 Hz,
PPh3), 134.3 (d, JCP=14.9 Hz, PPh3), 137.9, 140.7
(C-3a,7a mmb), 153.7 (C-2 mmb) ppm.
Reflections were collected on a Siemens P3 diffrac-
tometer at 173 K with graphite-monochromated Mo
Ka radiation (ꢀ-scans). The structures were solved by
direct methods using the SHELXTL-PLUS package [18],
the refinement was carried out with SHELXL-93 [19]
employing full-matrix least-squares methods. In the
center of the unit cell of the CuII complex the space
allows for occupation by only one methanol molecule
which is disordered on two positions. For
2 · 0.5CH3OH, the coordinated perchlorate was also
found to be disordered (ratio 51:49). Anisotropic ther-
mal parameters were refined for all non-hydrogen
atoms. The hydrogen atoms were added to the structure
model on calculated positions with isotropic tempera-
ture factors 20% (CH) or 50% (CH3) higher than those
of the corresponding carbon atoms.
UV–Vis (CH3CN) umax(m): 287(sh), 270(sh), 259
(27 000 M−1 cm−1) nm.
3.1.2. [(p2-mmb)2Cu(p1-ClO4)](ClO4) (2)
4. Supplementary material
A solution of 83 mg (0.432 mmol) mmb in 6 ml
CH3OH was slowly added to 80 mg (0.216 mmol)
Cu(ClO4)2 · 6H2O, dissolved in 6 ml CH3OH. Within 24
h a greenish precipitate is formed which is collected by
filtration, washed with 20 ml of n-pentane and dried
under vacuum to yield 115 mg (82%) of green 2. The
compound is soluble in DMF, CH3CN and CH3NO2
but insoluble in other conventional solvents. Calc. for
C20H24Cl2CuN4O8S2 (647.01): C, 37.13; H, 3.74; N,
8.66. Found: C, 37.13; H, 3.74; N, 8.58%. UV–Vis
(CH3CN) umax(m): 750(sh), 666 (560), 358 (3200), 281
A full list of data collection and refinement details,
tables of positional parameters for all atoms, bond
distances and angles, anisotropic temperature factors,
as well as calculated and observed structure factor
amplitudes, have been deposited and can be obtained
from the Fachinformationszentrum Karlsruhe, Gesell-
schaft fu¨r wissenschaftlich-technische Information
mbH, D-76344 Eggenstein-Leopoldshafen, Germany,
on quoting the deposition number CSD-410009 for
[Cu(mmb)(PPh3)2](BF4) (1) and CSD-410010 for

208
M. Albrecht et al. / Inorganica Chimica Acta 287 (1999) 204–208
[8] J. Rall, M. Wanner, M. Albrecht, F.M. Hornung, W. Kaim,
submitted for publication.
[Cu(mmb)2](ClO4)2 · 0.5CH3OH (2) and the full journal
citation.
[9] D.M. Dooley, M.A. McGuirl, D.E. Brown, P.N. Turowski,
W.S. McIntire, P.F. Knowles, Nature 349 (1991) 262.
[10] J.P. Klinman, D. Mu, Annu. Rev. Biochem. 63 (1994) 299.
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2905.
Acknowledgements
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This work was supported by Deutsche Forschungsge-
meinschaft (DFG) and Volksagenstiftung. The authors
would also like to thank Dr Jochen Rall for the com-
munication of preliminary experiments.
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