Electrochemical behavior of an aminotrithioether ligand: Copper(II)-mediated oxidative C-C bond formation

Article abstract of DOI:10.1021/ic7009429

A neutral aminotrithioether interacts with Cu^I, generating a coordination polymer in the solid state. Electrochemical studies indicate that the ligand is prone to oxidation by Cu^II, which results in a novel C-C bond formation reaction.

Full text of DOI:10.1021/ic7009429

Inorg. Chem. 2007, 46, 9510−9512
Electrochemical Behavior of an Aminotrithioether Ligand:
Copper(II)-Mediated Oxidative C C Bond Formation
−
Rau´l Huerta,† Aaro´n Flores-Figueroa,† V´ıctor M. Ugalde-Sald´ıvar,‡ and Ivan Castillo*,†
Instituto de Qu´ımica and Facultad de Qu´ımica, DiVisio´n de Estudios de Posgrado, UniVersidad
Nacional Auto´noma de Me´xico, Ciudad UniVersitaria, Me´xico D.F., 04510 Me´xico
Received May 15, 2007
A
neutral aminotrithioether interacts with CuI, generating
a
the ethylene-bridged tripodal NS3 ligands employed by the
group of Rorabacher form stable complexes with copper,18-20
and their electronic properties resemble those of type 1
copper sites in metalloenzymes.21,22 Related anionic and
neutral tripodal trithioethers have been used to prepare
monomeric, oligomeric, and polymeric CuI complexes (Chart
1).10-17 In contrast, attempts to prepare cupric analogues have
been unsuccessful, and the observed reduction of CuII to CuI
indicates ligand-based oxidation.11 This raises the question
of whether the differences in coordination properties between
the ethylene- and methylene-bridged trithioethers toward CuII
are due to the lack of a bridgehead N donor in the latter or
to the intrinsic reactivity of the E-CH2-S linker (E ) B,
C, Si). In order to discriminate between the two scenarios,
we herein report the synthesis of a methylene-bridged
aminotrithioether ligand, its reactivity toward CuI and CuII,
and the spectroscopic and structural characterization of the
products.
coordination polymer in the solid state. Electrochemical studies
indicate that the ligand is prone to oxidation by CuII, which results
in a novel C−C bond formation reaction.
Thioether ligands have raised interest in recent years
because of their involvement in several types of metalloen-
zyme active sites.1-4 Among the inorganic models developed
to mimic the properties of these enzymes, polythioethers
provide a sulfur-rich environment around biologically rel-
evant metal centers.5-17 In the specific case of trithioethers,
* To whom correspondence should be addressed. E-mail: joseivan@
servidor.unam.mx. Fax: (+52) 55-56162217.
† Instituto de Qu´ımica.
‡ Facultad de Qu´ımica, Divisio´n de Estudios de Posgrado.
(1) Solomon, E. I.; Szilagyi, R. K.; George, S. D.; Basumallick, L. Chem.
ReV. 2004, 104, 419-458.
(2) Prigge, S. T.; Eipper, B.; Mains, R.; Amzel, L. M. Science 2004, 304,
864-867.
(3) Rorabacher, D. B. Chem. ReV. 2004, 104, 651-697.
(4) Klinman, J. P. J. Biol. Chem. 2006, 281, 3013-3016.
(5) Chatt, J.; Leigh, G. J.; Storace, A. P. J. Chem. Soc. A 1971, 1380-
1389.
The aminotrithioether (LMe) was obtained by acid-
catalyzed condensation of 2,4-dimethylbenzenethiol and
hexamethylenetetramine (eq 1). The reaction of LMe with
an equimolar amount of [Cu(CH3CN)4]PF6 results in a yellow
microcrystalline material formulated as [LMeCu]PF6 (1) based
(6) Sellmann, D.; Schwarz, J. J. Organomet. Chem. 1983, 241, 343-
361.
1
on combustion analysis. H NMR spectra of acetonitrile-d3
(7) Riley, D. P.; Oliver, J. D. Inorg. Chem. 1986, 25, 1814-1821.
(8) Barton, A. J.; Connolly, J.; Levason, W.; Mendia-Jalon, A.; Orchard,
S. D.; Reid, G. Polyhedron 2000, 19, 1373-1379.
(9) Boardman, B.; Hanton, M. J.; van Rensburg, H.; Tose, R. P. Chem.
Commun. 2006, 2289-2291.
(10) Ge, P.; Haggerty, B. S.; Rheingold, A. L.; Riordan, C. G. J. Am. Chem.
Soc. 1994, 116, 8406-8407.
solutions of LMe with increasing amounts of [Cu(CH3CN)4]-
PF6 (0.5-2.0 equiv) reveal only one set of resonances that
do not change significantly, even at -40 °C, consistent with
the presence of several coexisting and rapidly exchanging
species.23 Some of the complexes present in solution were
detected by mass spectrometry; electrospray ionization mass
spectrometry (ESI-MS) spectra of 1 contain peaks corre-
(11) Ohrenberg, C.; Saleem, M. M.; Riordan, C. G.; Yap, G. P. A.; Verma,
A. K.; Rheingold, A. L. Chem. Commun. 1996, 1081-1082.
(12) Schebler, P. J.; Mandimutsira, B. S.; Riordan, C. G.; Liable-Sands,
L. M.; Incarvito, C. D.; Rheingold, A. L. J. Am. Chem. Soc. 2001,
123, 331-332.
(13) DuPont, J. A.; Coxey, M. B.; Schebler, P. J.; Incarvito, C. D.;
Dougherty, W. G.; Yap, G. P. A.; Rheingold, A. L.; Riordan, C. G.
Organometallics 2007, 26, 971-949.
(18) Ambundo, E. A.; Deydier, M.-V.; Grall, A. J.; Aguera-Vega, N.;
Dressel, L. T.; Cooper, T. H.; Heeg, M. J.; Ochrymowycz, L. A.;
Rorabacher, D. B. Inorg. Chem. 1999, 38, 4233-4242.
(19) Ambundo, E. A.; Ochrymowycz, L. A.; Rorabacher, D. B. Inorg.
Chem. 2001, 40, 5133-5138.
(20) Ambundo, E. A.; Yu, Q.; Ochrymowycz, L. A.; Rorabacher, D. B.
Inorg. Chem. 2003, 42, 5267-5273.
(21) Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc. 2000, 122, 6331-
6332.
(14) Yim, H. W.; Tran, L. M.; Dobbin, E. D.; Rabinovich, D.; Liable-
Sands, L. M.; Incarvito, C. D.; Lam, K.-C.; Rheingold, A. L. Inorg.
Chem. 1999, 38, 2211-2215.
(15) Yim, H. W.; Tran, L. M.; Pullen, E. E.; Rabinovich, D.; Liable-Sands,
L. M.; Concolino, T. E.; Rheingold, A. L. Inorg. Chem. 1999, 38,
6234-6239.
(16) Weidner, T.; Kra¨mer, A.; Bruhn, C.; Zharnikov, M.; Shaporenko, A.;
Siemeling, U.; Tra¨ger, F. Dalton Trans. 2006, 2767-2777.
(17) Ohrenberg, C.; Liable-Sands, L. M.; Rheingold, A. L.; Riordan, C.
G. Inorg. Chem. 2001, 40, 4276-4283.
(22) Aboelella, N. W.; Gherman, B. F.; Hill, L. M. R.; York, J. T.; Holm,
N.; Young, V. G., Jr.; Cramer, C. J.; Tolman, W. B. J. Am. Chem.
Soc. 2006, 128, 3445-3458.
(23) See the Supporting Information.
9510 Inorganic Chemistry, Vol. 46, No. 23, 2007
10.1021/ic7009429 CCC: $37.00
© 2007 American Chemical Society
Published on Web 10/18/2007

COMMUNICATION
Figure 2. ORTEP diagram of a fragment of the polymeric structure of 3
at the 50% probability level. H atoms and a molecule of acetonitrile are
omitted for clarity. Selected bond lengths (Å) and angles (deg): Cu1-I1,
2.636(1); Cu1-I#1, 2.674(1); Cu1-S1, 2.304(1); Cu1-S2, 2.326(1); Cu2-
I2, 2.651(1); Cu2-I#2, 2.599(1); Cu2-S3, 2.330(1); Cu2-N2, 2.004(2);
I1-Cu1-I#1, 120.57(1); Cu1-I1-Cu#1, 59.43(1); S1-Cu1-S2, 103.12-
(3); I2-Cu2-I#2, 117.29(2); Cu2-I2-Cu#2, 62.72(2); S3-Cu2-N2,
109.55(7). Color code: C, gray; N, blue; Cu, green; I, pink; S, yellow.
V (III, Figure 1c). Oxidative degradation of LMe (see below)
precludes measurement of the LMe/CuII stability constant,
which determines the redox potentials in related tripodal
ligand/Cu systems.18,24-26 Nonetheless, the high redox po-
tential of the CuII/CuI couple of 1 can be attributed to the
thioether donors, which raise the potential relative to N
donors, and the S-Cu-S chelate ring size of 6.18 Because
the CuII/CuI redox potential of 0.84 V is higher than the
ligand-centered oxidation at 0.53 V, coordination of CuII to
LMe could induce an intramolecular redox reaction (see
below).
To probe further the donor properties of LMe toward CuI,
LMe was treated with 1 equiv of CuI to yield [LMeCuI] (2),
as determined by combustion analysis. ESI-MS spectra of 2
display one copper-containing species [LMeCu]+ at m/z 530.23
Despite repeated attempts to structurally characterize 1 and
2, X-ray-quality crystals were obtained from the reaction of
LMe with 2 equiv of CuI, which afforded {[LMeCu2I2(CH3-
CN)]‚CH3CN}n (3). Its solid-state structure consists of a
coordination polymer with bridging iodides and LMe acting
in a µ-κ1,κ2 fashion (Figure 2).27 The independent Cu1 and
Cu2 centers are related to additional CuI ions by a crystal-
lographic inversion center, generating rhomboidal Cu2I2 cores
with average Cu-S and Cu-I bond lengths similar to those
of reported complexes of trithioethers with CuI.15,28 The lack
of bonding between the nearly planar N1 atom (sum of angles
) 358°) and the Cu centers can be attributed to the highly
strained four-membered chelate ring that would result. In
addition, the presence of the strongly coordinating iodo
ligands could also preclude Cu-N1 bonding.
Figure 1. Cyclic voltammograms of (a) LMe, (b) 1, and (c) [Cu(CH3-
CN)4]PF6 in CH3CN with (Bu4N)PF6 as the supporting electrolyte.
Chart 1
sponding to copper-containing species at m/z 997 [LMe2Cu]+,
and the base peak at m/z 530 [LMeCu]+, as well as ligand-
derived fragments.23
The solution behavior of 3 is similar to that of 1 and 2,
although the presence of heavier copper-containing species
was evidenced by mass spectrometry.23 Thus, the peaks at
The electrochemical behavior of 1 in acetonitrile is
characterized by six redox processes (I′-VI′ in Figure 1b),
four of which are ligand-centered, based on cyclic voltam-
metry studies of 1 and LMe. All but one of the processes of
1 are quasi-reversible, with the irreversible anodic peak
corresponding to ligand-based oxidation at 0.53 V (II′a,
Figure 1b; IIa at 0.56 V for LMe, Figure 1a). The other ligand-
centered processes are also displaced to lower potentials upon
coordination of CuI.23 The redox process at E1/2 ) 0.84 V
(III′, Figure 1b), which is absent in LMe, was assigned to
the CuII/CuI couple and is shifted to a considerably higher
potential compared to that of [Cu(CH3CN)4]PF6, E1/2 ) 0.56
(24) Karlin, K. D.; Hayes, J. C.; Juen, S.; Hutchison, J. P.; Zubieta, J. Inorg.
Chem. 1982, 21, 4106-4108.
(25) Karlin, K. D.; Yandell, J. K. Inorg. Chem. 1984, 23, 1184-1188.
(26) Lee, D.-H.; Hatcher, L. Q.; Vance, M. A.; Sarangi, R.; Milligan, A.
E.; Narducci Sarjeant, A. A.; Incarvito, C. D.; Rheingold, A. L.;
Hodgson, K. O.; Hedman, B.; Solomon, E. I.; Karlin, K. D. Inorg.
Chem. 2007, 46, 6056-6068.
(27) Crystal data for 3: C31H39Cu2I2N3S3, MW ) 930.71 g‚mol-1, crystal
0.34 × 0.18 × 0.17 mm3, triclinic, space group P1h, a ) 10.686(2) Å,
b ) 13.399(3) Å, c ) 14.177(3) Å, R ) 84.473(3)°, â ) 68.200(3)°,
γ ) 73.740(3)°, V ) 1809.2(7) Å3, Z ) 2, T ) 173(2) K, R1 ) 0.0221
for I > 2σ(I), wR2 ) 0.0566 for 6646 reflections.
(28) Yim, H. W.; Rabinovich, D.; Lam, K.-C.; Golen, J. A.; Rheingold,
A. L. Acta Crystallogr., Sect. E: Struct. Rep. Online 2003, E59,
m556-m558.
Inorganic Chemistry, Vol. 46, No. 23, 2007 9511

COMMUNICATION
Scheme 1
Figure 3. Schematic diagram and ORTEP view of the cation of 4 at the
50% probability level. Selected bond lengths (Å) and angles (deg): N3-
C2, 1.479(5); N3-C21, 1.467(5); N3-C4, 1.284(5); S1-C2, 1.767(5); S2-
C21, 1.791(4); C2-N3-C4, 119.7(4); C2-N3-C21, 117.6(4); C4-N3-
C21, 122.6(4).
aromatic rings and a methylene group (Figure 3).29 Few
examples of copper-mediated C-C bond formation have
been reported, and it appears that the presence of an
electrochemically active methylene group is essential.30
The low oxidation potential of LMe allows intramolecular
electron transfer upon coordination of CuII. Subsequent
heterolytic C(methylene)-S bond cleavage would yield a
sulfur-based radical and a nitrogen-stabilized carbocation.
Electrophilic aromatic substitution at the 6 position would
then lead to C-C bond formation to afford L′Me (Scheme
1). L′Me/CuI complexes detected by ESI-MS could not be
isolated, and [Cu(CH3CN)4]ClO4 was recovered. On the basis
of the redox potentials determined for the electrochemically
generated L′Me, further oxidation by a second equiv of CuII
is prohibited. Instead, H-atom abstraction (by the limited
amount of ArS• available) followed by oxidation by excess
CuII would yield the observed product 4.
In summary, LMe/CuI represents a dynamic system with
rapidly interconverting complexes in solution. The chelate
ring size of 4 does not support CuII complexation, in line
with previously reported stability trends for related complexes
of tripodal ligands.18-20,24-26 Instead, oxidation of LMe by
CuII results in an intramolecular C-C bond formation
reaction. Efforts to adapt this novel CuII-promoted transfor-
mation to other organic compounds with electrochemically
active methylene groups are currently underway.
m/z 1189 and 997 were assigned to the dicopper species
[LMe2Cu2I]+ and the previously identified [LMe2Cu]+.
Complexes 1-3 are stable toward oxidation by O2 in the
solid state: after exposure to air for 7 days, no sign of
oxidation to CuII was found by electron paramagnetic
resonance (EPR) spectroscopy. In an aerobic acetonitrile
solution, 1 decomposes over a period of 5 days, yielding
ArSH (m/z 138, [ArSH2]+), ArSSAr (m/z 274), and unidenti-
fied copper-containing products.23 Similarly, exposure of
acetonitrile solutions of 3 to air led to decomposition, based
on UV-vis spectra acquired over 3 weeks.23 Spectral changes
are consistent with ligand-based oxidation, with no bands
in the visible region that could be ascribed to d-d transitions
of CuII.
The reduction of CuII to CuI by LMe was evidenced by
the bleaching of solutions of [Cu(H2O)6](ClO4)2 and [Cu-
(H2O)6]Cl2. Thus, CuII complexes of LMe are not accessible,
as is inferred from electrochemical data, in contrast with the
reported CuII complexes of ethylene-bridged NS3 ligands.18,19
This behavior had previously been observed in the reaction
of trithioether borates with CuII,11 which leads to unidentified
CuI products. The reaction of LMe with 1 equiv of [Cu(H2O)6]-
(ClO4)2 in acetonitrile by ESI-MS shows peaks at m/z 883,
tentatively assigned to [Cu(ArSSAr)3]+, at m/z 802 for the
copper-containing species [L′Me2Cu(CH3CN)2]+, at m/z 747
for an unidentified complex, and at m/z 721 for [L′Me2Cu]+;
fast atom bombardment MS revealed the presence of ArSH
and ArSSAr.23
Acknowledgment. We thank Dr. Rube´n A. Toscano for
crystallographic work, Virginia Go´mez-Vidales for EPR
measurements, Carmen Ma´rquez and Luis Velasco for mass
spectrometry measurements, and DGAPA-UNAM (IN247402
and IN216806) and Conacyt (195791) for financial support.
Supporting Information Available: Crystal structure informa-
tion (CIF) and experimental details (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
Although the ligand derivative L′Me was not isolated
(Scheme 1),23 the reaction of LMe with 1-2 equiv of [Cu-
(H2O)6](ClO4)2 afforded a new compound (4) with two
inequivalent aromatic groups, based on the four ArMe
singlets in the δ 2.28-2.50 ppm region of 1H NMR spectra
(ESI-MS: m/z 328). The solid-state structure of 4 revealed
a novel C-C bond formation reaction between one of the
IC7009429
(29) Crystal data for 4: C19H22ClNO4S2, MW ) 427.95 g‚mol-1, crystal
0.20 × 0.06 × 0.05 mm3, monoclinic, space group P21/c, a ) 8.887-
(3) Å, b ) 14.107(4) Å, c ) 16.121(5) Å, â ) 98.751(5)°, V ) 1997.6-
(10) Å3, Z ) 4, T ) 173(2) K, R1 ) 0.0605 for I > 2σ(I), wR2 )
0.1119 for 3643 reflections.
(30) Kumar Padhi, S.; Manivannan, V. Inorg. Chem. 2006, 45, 7994-
7996.
9512 Inorganic Chemistry, Vol. 46, No. 23, 2007