Mixed-ligand copper complexes with 8-methylthioquinoline and triphenylphosphane or the o-semiquinone/catecholate redox system

Article abstract of DOI:10.1002/ejic.200500567

The complex [Cu(MTQ)(PPh₃)₂] (BF₄), MTQ = 8-methylthioquinoline, exhibits distorted tetrahedral coordination at the copper(I) center. One of two crystallographically independent molecules found in the unit cell exhibits a more pronounced inclination towards a (3+1) coordination arrangement. In comparison to the analogous complex with the related imine/thioether chelate ligand 1-methyl-2-(methylthiomethyl)-1H- benzimidazole, the cation [Cu(MTQ)(PPh₃)₂]⁺ shows stronger bonding of Cu^I to S and weaker interaction with N. With 3,5-di-tert-butyl-o-semiquinone as co-ligand instead of two PPh₃ ligands a valence-tautomer equilibrium situation involving the copper(II)-catecholate state can be observed by EPR spectroscopy, showing an unusually large isotropic ^63,65Cu hyperfine coupling of 2.1 mT and an untypically small isotropic g value of 1.975. Wiley-VCH Verlag GmbH & Co. KGaA, 2005.

Full text of DOI:10.1002/ejic.200500567

FULL PAPER
DOI: 10.1002/ejic.200500567
Mixed-Ligand Copper Complexes with 8-Methylthioquinoline and
Triphenylphosphane or the o-Semiquinone/Catecholate Redox System
Shengfa Ye,[a] Biprajit Sarkar,[a] Mark Niemeyer,[a] and Wolfgang Kaim*[a]
Keywords: Copper / Crystal structure / EPR spectroscopy / Heterocycles / Thioether ligands
The complex [Cu(MTQ)(PPh3)2](BF4), MTQ = 8-methylthio-
quinoline, exhibits distorted tetrahedral coordination at the
copper(I) center. One of two crystallographically indepen-
dent molecules found in the unit cell exhibits a more pro-
nounced inclination towards a (3+1) coordination arrange-
ment. In comparison to the analogous complex with the re-
lated imine/thioether chelate ligand 1-methyl-2-(methylthio-
methyl)-1H-benzimidazole, the cation [Cu(MTQ)(PPh3)2]+
shows stronger bonding of CuI to S and weaker interaction
with N. With 3,5-di-tert-butyl-o-semiquinone as co-ligand in-
stead of two PPh3 ligands a valence-tautomer equilibrium sit-
uation involving the copper(II)-catecholate state can be ob-
served by EPR spectroscopy, showing an unusually large iso-
tropic 63,65Cu hyperfine coupling of 2.1 mT and an untypi-
cally small isotropic g value of 1.975.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
Valence-tautomer (or redox isomer) equilibria are not
only of interest for the enzymatic mechanism of copper-
dependent amine oxidases;[3] (1) has also been implicated in
catechol-enhanced Fenton processes for wood decay.[5] In
general, valence-tautomer equilibria[6] involving cobalt,[7]
manganese, copper,[4,8] nickel,[9] and iron[10] have been dis-
cussed with respect to potential applications in molecular
electronics (“switching”).[6–11]
While thioether (methionine) coordination to biological
copper(i/ii) is observed for electron transfer proteins[1,2a]
and enzymes,[12] there has also been a recent report describ-
ing N–S five-membered ring chelate binding of copper in
methanobactin.[13]
Introduction
Imino and thioether donor atoms have different charac-
teristics for metal ions, especially for copper in its two bio-
relevant oxidation states +1 and +2.[1,2] While a “soft” thio-
ether S (such as in methionine) prefers copper(i), the
imino function (as in the imidazole ring of histidine) toler-
ates both the CuI and CuII states.[1,2] In an attempt to mimic
the biochemically relevant[3] valence tautomerism [Equa-
tion (1)] between the copper(i)-o-semiquinone and cop-
per(ii)-catecholate combinations, we have used a corre-
sponding mixed-donor ligand system in the form of 1-
methyl-2-(methylthiomethyl)-1H-benzimidazole (mmb) to
observe such a temperature-dependent valence-tautomer
equilibrium outside biological material.[4]
Extending our previous approach[4] to other N–S chelate
ligands we have focused on 8-methylthioquinoline
(MTQ),[14–16] which contains an azine (pyridyl) nitrogen do-
nor instead of the more basic but less π-accepting azole
(imidazole) N in mmb. Also, mmb contains a flexible dialk-
ylthioether substituent, whereas MTQ offers an arylalkythio-
ether sulfur in a more rigid chelate setting. Accordingly, a
comparison between mmb and MTQ compounds with d6-
configured metal complex fragments has revealed consider-
able differences in the metal–donor bond lengths within the
five-membered chelate ring.[15] A homoleptic complex of
MTQ with copper(i), [Cu(MTQ)2](ClO4), was also reported
recently,[16] showing relatively short bonds from the metal
to N (2.0165 Å) and S (2.3242 Å).
(1)
Results and Discussion
The chelate ligand MTQ forms a stable complex with
[Cu(PPh3)2]+ [Equation (2)]. The results of the structural
analysis as shown in Figure 1 and summarized in Table 1
[a] Institut für Anorganische Chemie, Universität Stuttgart,
Pfaffenwaldring 55, 70550 Stuttgart, Germany
E-mail: kaim@iac.uni-stuttgart.de
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S.Ye, B. Sarkar, M. Niemeyer, W. Kaim
FULL PAPER
Table 1. Selected bond lengths [Å] and angles [°] for [Cu(MTQ)(PPh3)2](BF4) and calculated values for [Cu(MTQ)(PH3)2]+.
Exp.
Calcd.[a]
Molecule 1
Molecule 2
Cu–N
Cu–S
Cu–P
Cu–P
N–Cu–S
N–Cu–P
N–Cu–P
S–Cu–P
S–Cu–P
P–Cu–P
2.101(7)
2.363(3)
2.256(3)
2.289(3)
84.2(2)
111.0(2)
103.0(2)
119.88(9)
104.84(10)
125.19(9)
(Cu–N1)
(Cu1–S1)
(Cu1–P2)
2.077(8)
2.376(2)
2.255(3)
2.287(2)
84.9(2)
108.3(2)
107.2(2)
117.08(10)
107.51(9)
124.34(9)
(Cu2–N2)
(Cu2–S2)
(Cu2–P3)
(Cu2–P4)
(N2–Cu2–S2)
(N2–Cu2–P3)
(N2–Cu2–P4)
(P3–Cu2–S2)
(P4–Cu2–S2)
(P3–Cu2–P4)
2.051
2.387
2.281
2.304
87.1
115.9
112.4
116.7
107.8
114.0
(Cu1–P1)
(N1–Cu1–S1)
(N1–Cu1–P2)
(N1–Cu1–P1)
(P2–Cu1–S1)
(P1–Cu1–S1)
(P2–Cu1–P1)
[a] Calculation for [Cu(MTQ)(PH3)2]+.
for the two crystallographically independent molecules re- and shorter Cu–S bonds; the difference ΔNS [d(Cu–S) –
veal a slight asymmetry in the binding of the two tri- d(Cu–N)] is 0.394 Å for [Cu(mmb)(PPh3)2]+[18] but only
phenylphosphane ligands as is typical for many such cop- about 0.28 Å for the two molecules of [Cu(MTQ)(PPh3)2]+.
per(i) compounds.[17]
Apparently, the higher degree of rigidity conferred by the
aromatic chelate ligand MTQ enforces a more balanced co-
ordination by the two donor atoms, whereas mmb, which
has a higher flexibility due to the CH2 group in the chelate
ring, allows for a more disparate binding of N and S to the
metal.
(2)
The optimized geometry of [Cu(MTQ)(PH3)2]+ is in rea-
sonable agreement with the experimental results. Some de-
viations (Table 1) result from the approximation of PPh3 by
PH3. The highest occupied molecular orbital (HOMO) of
this complex is mainly formed by a combination between d
orbitals of Cu (42%) and p orbitals of S (18%). The next
lower lying orbital consists of 45% d orbitals from the Cu
center and 12% p orbitals on the two P atoms. The lowest
unoccupied molecular orbital (LUMO) comprises π* orbit-
als of the ligand with about 2% contribution from Cu orbit-
als. The contribution of d orbitals of Cu to the LUMO can
be viewed as metal-to-ligand π-back-bonding. HOMO and
LUMO diagrams of [Cu(MTQ)(PH3)2]+ are depicted in
Figure 2.
Figure 1. Molecular structures of two independent complex ions in
the crystal of [Cu(MTQ)(PPh3)2](BF4).
The tendency of the 3d10 system for (3+1) instead of 4
coordination may be further enhanced by π–π interactions
between different coordinated ligands.[17]
In comparison to the homoleptic [Cu(MTQ)2]+ ion[16]
the Cu–N and Cu–S distances in [Cu(MTQ)(PPh3)2]+ are
significantly lengthened from about 2.02 to 2.09 Å and
from about 2.32 to 2.37 Å, respectively. The Cu–N and Cu–
S bond length data represent the average bond lengths of
the two crystallographically independent molecules. Rela-
tive to several reported d6 metal complexes[15] of MTQ, the
copper(i) compound described here exhibits a balanced
Figure 2. Composition of the HOMO (left) and LUMO (right) of
[Cu(MTQ)(PH3)2]+.
The successful use[4] of the mmb ligand in combination
n–
binding of the metal to N and S. In contrast, RuII prefers with the Cun+/Qx valence-tautomer system (n = 1 or 2;
to bind to N and PtIV exhibits strong bonding to S.[15] Qx = 3,5-di-tert-butyl-o-benzoquinone) has prompted us to
In
comparison
with
[Cu(mmb)(PPh3)2]+,[18]
the employ MTQ in a similar set of EPR experiments. The EPR
[Cu(MTQ)(PPh3)2]+ ion is distinguished by longer Cu–N results from mixing equimolar amounts of MTQ and
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Eur. J. Inorg. Chem. 2005, 4735–4738

Mixed-Ligand Copper Complexes with Triphenylphosphane or Semiquinone Catecholate
FULL PAPER
Qx with excess activated copper[4] [Equation (3)] are shown chelate ligand enforces a strictly four-coordinate copper(i)
in Figure 3.
arrangement with significant contribution from the thio-
ether S center of MTQ, whereas more flexible chelate li-
gands such as mmb [see Equation (2)] with partially satu-
rated chelate ring centers allow for a 3+1 coordination. This
interpretation is supported by the shorter Cu–S bond ob-
served in [Cu(MTQ)(PPh3)2](BF4) relative to the mmb ana-
logue. The electronic effect alone would favor the CuI-semi-
quinone alternative because MTQ is a better π acceptor
than mmb.[15,16]
(3)
Experimental Section
Instrumentation: EPR spectra were recorded in the X band with a
Bruker System ESP 300 equipped with a Bruker ER035M gauss-
1
meter and a HP 5350B microwave counter. H NMR spectra were
recorded with a Bruker AC 250 spectrometer.
Synthesis:
A
solution containing 130 mg (0.172 mmol)
[Cu(CH3CN)2(PPh3)2]BF4 and 30.5 mg (0.172 mmol) MTQ[15,16]
was heated to reflux in 20 mL of dry CH2Cl2 for 4 h. After evapo-
rating half of the solvent volume, the precipitate was collected by
filtration and washed with cold diethyl ether to yield 117 mg (80%)
of light yellow microcrystals. Single crystals were obtained from a
CH2Cl2 solution layered with diethyl ether at –10 °C.
C46H39BCuF4NP2S: calcd. C 64.99, H 4.62, N 1.65; found C 64.65,
1
H 4.65, N 1.63. H NMR (250 MHz, 300 K, CDCl3): δ = 8.70 (dd,
3J = 4.6 Hz, 4J = 1.6 Hz, 1 H, H2), 8.62(dd, 3J = 8.4 Hz, 4J =
1.6 Hz, 1 H, H4), 8.11 (d, 3J = 8.0 Hz, 1 H, H7), 8.06 (d, 3J =
3
8.0 Hz, 1 H, H5), 7.77 (t, J = 8.0 Hz, 8.0 Hz, 1 H, H6), 7.65 (dd,
3J = 8.4 Hz, 4.6 Hz, 1 H, H3), 7.38–7.04 (m, 15 H, aromatic), 2.15
(s, 3 H, SCH3) ppm.
Figure 3. Temperature-dependent EPR spectra of the solution ob-
tained by treating MTQ and 3,5-di-tert-butyl-o-semiquinone (1:1)
with excess copper in toluene: derivative spectra (left) and inte-
grated (nonderivative) spectra (right).
Crystallography: Data for [Cu(MTQ)(PPh3)2](BF4) were collected
at 173 K with a Siemens P3 four-circle diffractometer using graph-
ite-monochromated Mo-Kα radiation (λ = 0.71073 Å). Formula
C46H39BCuF4NP2S; formula weight = 850.13 g·mol–1; monoclinic
space group Pc; a = 21.520(4), b = 10.607(2), c = 19.993(4) Å; β =
116.61(3)°; V = 4080.3(14) Å3; Z = 4 (2 independent molecules);
F(000) = 1752; ρcalcd. = 1.384 g·cm–3; μ = 0.717 mm–1, T =
173(2) K; θ range 1.92 – 25.02°; number of unique reflections 7376
(Rint = 0.064), number of parameters 1011; Flack parameter = –
0.009(18); R1 = 0.0566, wR2 = 0.0988 [for, 5436 reflections with I
Ͼ 2σ (I)]; R1 = 0.0955, wR2 = 0.1145 (all data); GOF = 1.073. The
structure was solved by direct methods and refined by full-matrix
least-squares against F2 of all data using the SHELXTL software
package.[22] Anisotropic thermal factors were assigned to the non-
hydrogen atoms. The hydrogen atoms were included in calculated
positions (riding model) and refined with fixed Uiso = 1.2 Uiso of
the carbon atoms to which they are bonded.
Although a uniform product could not be isolated from
this reaction, an EPR pattern corresponding to a tempera-
ture-dependent solution equilibrium [Equation (1)] was ob-
served with a major catecholato-copper(ii) component
characterized by broad signals with the typical CuII features
of aiso(
63,65Cu) = 8.5 mT and giso = 2.077 (Aʈ = 19.0 mT, gʈ
= 2.16, AЌ = 3.0 mT, gЌ = 2.036 at 110 K; 63Cu: 69.2%, I
= 3/2; 65Cu: 30.8%, I = 3/2). The minor component, emerg-
ing to an appreciable extent at T Ͼ 270 K, can be expected
to exhibit semiquinone/copper(i) features [aiso(
63,65Cu) Ͻ
1.2 mT, giso Ϸ 2.005],[19] as for [CuI(mmb)(Qx·–)],[4] however,
the parameters observed here are very unusual: The re-
solved quartet from the 63,65Cu hyperfine coupling shows a
large spacing of 2.1 mT which lies between typical values
of 0.5–1.2 mT for semiquinone/copper(i) species[19] and
8 mT for copper(ii) systems.[20] A semiquinone–H (H4) hy-
perfine coupling is not observed here because of the rela-
tively broad lines (peak-to-peak distance ca. 2.0 mT). Even
more unusual is the isotropic g factor at 1.975 for this quar-
tet signal which has no precedent in either copper(ii) or
semiquinone/copper(i) EPR spectroscopy.[19,20] The un-
usually low value indicates[21] the presence of a low-energy
excited state, possibly involving the π* orbital of MTQ, ly-
ing close to the doublet ground state. At this point we can
CCDC-275097 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Computational Details: Ground-state electronic structure calcula-
tions on [Cu(MTQ)(PH3)2]+ have been done by density functional
theory (DFT) methods, using the Gaussian 03 program package.[23]
Within the Gaussian program, the quasirelativistic effect core po-
tential basis sets[24] were employed for the Cu atom; for other atoms
Dunning’s DZP basis sets[25] were used. The geometry optimization
was performed by using the pure density BP86 functional[26] at the
spin-restricted level; the single point energy was calculated using
only speculate that the rigidity of the unsaturated MTQ the hybrid density B3LYP functional[27] with optimized geometry.
Eur. J. Inorg. Chem. 2005, 4735–4738
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4737

S.Ye, B. Sarkar, M. Niemeyer, W. Kaim
FULL PAPER
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Received: June 27, 2005
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