Synthesis, characterization, and tautomerism of four novel copper(I) complexes from 3-hydroxy-3-(p-R-phenyl)-2-propenedithioic acids

Article abstract of DOI:10.1021/ic0488132

The paper describes the synthesis and structural characterization of four novel copper(I) complexes [CuL(PPh₃)₂] (L = 3-hydroxy-3-(p-R-phenyl)-2-propenedithioate). In addition, a tautomeric equilibrium in solution was found and Hammett correlations with ¹³C NMR parameters were studied. The structure of one complex was fully established by X-ray diffraction analysis.

Full text of DOI:10.1021/ic0488132

Inorg. Chem. 2004, 43, 8572−8576
Synthesis, Characterization, and Tautomerism of Four Novel Copper(I)
Complexes from 3-Hydroxy-3-(p-R-phenyl)-2-propenedithioic Acids
Ivan Garc´ıa-Orozco,*,† Jose´ G. Lo´pez-Corte´s,† M. Carmen Ortega-Alfaro,‡ Rube´n Alfredo Toscano,†
Guillermo Penieres-Carrillo,§ and Cecilio Alvarez-Toledano†
Instituto de Qu´ımica, UniVersidad Nacional Auto´noma de Me´xico, Circuito Exterior,
Ciudad UniVersitaria, Coyoacan, CP 04510 Mexico, DF, Lab. QEA, Facultad de Qu´ımica,
UniVersidad Nacional Auto´noma de Me´xico, Circuito Exterior, Ciudad UniVersitaria,
Coyoacan, CP 04510 Mexico, DF, and FES, Cuautitla´n, UNAM, Campo 1, AV. 1 de mayo s/n,
Cuautitla´n Izcalli, CP 54740 Estado de Me´xico, Mexico
Received August 26, 2004
The paper describes the synthesis and structural characterization of four novel copper(I) complexes [CuL(PPh3)2]
(L
) 3-hydroxy-3-(p-R-phenyl)-2-propenedithioate). In addition, a tautomeric equilibrium in solution was found and
Hammett correlations with 13C NMR parameters were studied. The structure of one complex was fully established
by X-ray diffraction analysis.
Scheme 1. Reaction of Methyl Ketones with CS2
Introduction
The reaction between methyl ketones and CS2 pro-
duces the corresponding dithio acid compounds, which can
present a keto-enol equilibrium in solution1 (Scheme 1).
This equilibrium is generally favored toward the chelating
enol form, when R ) phenyl, because of its energetical
stability.2
Chart 1. Possible Coordination Modes for 3-Hydroxy-3-(p-R-phenyl-
2-propenedithioic Acids
The 3-hydroxy-3-(p-R-phenyl)-2-propenedithioic acid has
shown different applications such as being the active moiety
in ionic resins for transition metal ion separations,3,4 in the
development of new Ag+-selective membrane electrodes,5,6
and as hair-growing agents using Zn as metallic center,7
among others. But in all cases, the complex structures have
not been determined.
Weigand,8 Coucouvanis,9,10 and Lih3 have proposed two
possible coordination modes (V and VI) for these kinds of
ligands (Chart 1); however, only a few examples displaying
S,S-coordination have been reported for Ni(II), Pd(II),
Pt(II),8,11,12 and Fe(0)13,14 complexes (Chart 2).
As a part of our studies with 1a-d and with the aim of
understanding the factors that govern the reactivity of these
ligands toward transition metals, we herein report the
* Author to whom correspondence should be addressed. E-mail: cecilio@
servidor.unam.mx. Fax: +52 5556162217.
† Instituto de Qu´ımica, Universidad Nacional Auto´noma de Me´xico.
‡ Facultad de Qu´ımica, Universidad Nacional Auto´noma de Me´xico.
§ FES, Cuautitla´n, UNAM.
(1) Dunn, A. D.; Rudorf, W.-D. Carbon disulphide in Organic Chemistry;
Ellis Harwood: Chichester, U.K., 1989; p 120.
(2) Saquet, M.; Thuillier, A. Bull. Soc. Chim. Fr. 1967, 2841.
(3) Liu, C.-Y.; Lih, J.-J. Fresenius’ J. Anal. Chem. 1988, 332, 171.
(4) Lih, J. J.; Yeh, K. Y.; Liu, C. Y. Fresenius’ J. Anal. Chem. 1990,
336, 12.
(5) Galan-Vidal, C. A.; Paez-Hernandez, M. E.; Cuellar-Cruz, M.; Perez-
Lopez, B.; Lopez-Martinez, E.; Pliego-Canton, T. C.; Alvarez
Toledano, C. Sens. Chemom. 2001, 39.
(8) Weignad, W.; Bosl, K.; Polborn, K. Chem. Ber. 1990, 123, 1339.
(9) Coucouvanis, D.; Fackler, J. P., Jr. J. Am. Chem. Soc. 1967, 89, 1346.
(10) Caffery, M. L.; Coucouvanis, D. J. Inorg. Nucl. Chem. 1975, 37, 2081.
(11) Weigand, W.; Saumweber, R.; Schulz, P. Z. Naturforsch. 1993, 48b,
1080.
(12) Buchweitz, J.; Gompper, R.; Polborn, K.; Robl, C.; Sailer, M.-T.;
Weigand, W. Chem. Ber. 1994, 127, 23.
(6) Galan-Vidal, C. A.; Paez-Hernandez, M. E.; Cuellar-Cruz, M.; Perez-
Lopez, B.; Portillo-Ortega, G.; Alvarez-Toledano, C. ReV. Mex. Fis.
2000, 46 (Supl. 2), 132.
(7) Oda, Y.; Kubota, S.; Hiraga, K.; Koga, H.; Niwano, Y. Jpn. Kokai
Tokkyo Koho 1995, 14; Chem. Abstr. 1996, 124, 74794.
(13) Alvarez-Toledano, C.; Enr´ıquez, J.; Alfredo Toscano, R.; Mart´ınez-
Garc´ıa, M.; Corte´s-Corte´s, E.; Osornio, Y. M.; Garc´ıa-Mellado, O.;
Gutie´rrez-Pe´rez, R. J. Organomet. Chem. 1999, 38, 577.
(14) Alvarez-Toledano, C.; Delgado, E.; Donnadieu, B.; Herna´ndez, E.;
Mart´ın, G.; Zamora, F. Inorg. Chim. Acta 2003, 351, 119.
8572 Inorganic Chemistry, Vol. 43, No. 26, 2004
10.1021/ic0488132 CCC: $27.50
© 2004 American Chemical Society
Published on Web 12/03/2004

Cu(I) Complexes from 2-Propenedithioic Acids
cm-1): 2549.8 w, 2518.2 w, 1682.8 w, 1591.8 s, 1557.3 s, 1485.1
s, 1421.4 s, 1240.3 s, 1096.3 s, 968.5 s, 935.3 s, 774.2 vs, 736.9
m, 645.4 m, 545.3 m. 1H NMR (CDCl3): δ 5.46 (s, 1H, H-9), 6.90
(s, 1H, H-2), 7.44 and 7.81 (AA′BB′ system, 4H, J ) 8.72 Hz,
H-6, H-5), 15.32 (s, 1H, H-8). 13C NMR (CDCl3): δ 107.9 (C-2),
128.3 (C-5), 129.3 (C-6), 132.2 (C-4), 138.9 (C-7), 171.4 (C-3),
211.8 (C-1).
Chart 2. Reported Complexes for 3-Hydroxy-2-propenedithioate
Ligands
General Procedure. Synthesis of (3-Hydroxy-3-(p-methyl-
phenyl)-2-propenedithioate-S,S′)bis(triphenylphosphine-P)cop-
per(I) (2a). A 1 equiv (0.56 mmol) amount of Cu(PPh3)2NO3 in
CH2Cl2 was added to an aqueous solution of ligand 1a (0.56 mmol)
and NaOH (0.56 mmol). The original orange organic solution
became dark-red. The mixture was stirred for 15 min, and then the
organic layer was dried with Na2SO4. The solvent was distilled
under vacuum. Then, ethanol was added to precipitate the complex,
which was filtered out and dried under vacuum to obtain a red
solid. Yield: 0.15 g (34%). Mp: 182-183 °C. IR (KBr, cm-1):
3449.8 s, 3051.8 s, 1581.5 vs, 1556.3 vs, 1505.3 vs, 1433.5 vs,
1210.9 vs, 1093.9 s, 1038.7 vs, 981.2 s, 745.2 vs, 694.3 vs, 508.8
vs. FAB MS: m/z 859.0, [M + Cu]+. HR FAB (m/z): calcd for
C46H39Cu2OP2S2, 859.0510; found, 859.0472. Anal. Calcd for
C46H39CuOP2S2: C, 69.3; H, 4.9. Found: C, 67.1; H, 4.9. 1H NMR
(CDCl3): δ 2.37 (s, 3H, Me, keto), 2.40 (s, 3H, Me, enol), 4.59 (s,
2H, H-2, keto), 6.88 (s, 1H, H-2, enol), 7.20 and 7.72 (AA′BB′
system, 4H, J ) 8.07 Hz, H-6, H-5, enol), 7.22 and 8.06 (AA′BB′
system, 4H, J ) 8.07 Hz, H-6, H-5, keto,), 7.25 (m, PPh3), 13.01
(s, 1H, H-8, enol). 13C NMR (CDCl3): δ 21.6 (Me, keto), 21.7
(Me, enol), 66.1 (C-2, keto), 113.2 (C-2, enol), 126.3 (C-6, enol),
126.5 (C-5, enol), 128.5 (aromatic, PPh3 + C-5, keto), 129.2 (C-6,
keto), 129.5 (aromatic, PPh3), 132.9 (C-4, enol,), 134.7 (C-4, keto,),
140.8 (C-7, enol,), 143.7 (C-7, keto,), 163.2 (C-3), 193.4 (C-1).
31P NMR (CDCl3) δ 0.01 (broad signal).
synthesis of novel copper(I) complexes of type [CuL(PPh3)]
(L ) 3-hydroxy-3-(p-R-phenyl)-2-propenedithioate).
Experimental Section
Materials. FTIR spectra were recorded on either a Perkin-Elmer
283 B or a Perkin-Elmer 1420 spectrophotometer in KBr. Melting
points were measured on a Melt-Temp II apparatus and are
uncorrected. 1H, 13C, and 31P NMR spectra were obtained on a JEOL
Eclipse +300 using CDCl3 as solvent. Chemical shifts were reported
1
in δ relative to TMS for H and 13C and 30% H3PO4 for 31P. The
MS-FAB+ spectra were obtained on a JEOL JMS SX 102A. The
3-hydroxy-3-(p-R-phenyl)-2-propenedithioic acid 1a-d were syn-
thesized by Larsson’s method.15 Bis(triphenylphosphine)copper(I)
nitrate was obtained as reported in the literature.16 The keto-enol
tautomerism was studied by 1H, 13C, COSY, and HETCOR NMR
experiments.
Synthesis of 3-Hydroxy-3-(p-methylphenyl)-2-propenedithioic
Acid (1a). Yield: 5.3 g (75%). Mp: 85-86 °C (lit. mp: 85 °C).
IR (KBr, cm-1): 3433.0 w, 2920.1 w, 2544.0 w, 2497.7 w, 1605.1
m, 1581.7 m, 1551.1 s, 1502.7 m, 1426.7 m, 1251.1 s, 956.5 m,
836.5 m, 785.5 s, 674.6 m. 1H NMR (CDCl3): δ 2.41 (s, 3H, Me),
5.38 (s, 1H, H-9), 6.95 (s, 1H, H-2), 7.26 and 7.77 (AA′BB′ system,
4H, J ) 8.26 Hz, H-6, H-5), 15.38 (s, 1H, H-8). 13C NMR
(CDCl3): δ 21.8 (Me), 107.9 (C-2), 127.1 (C-5), 129.8 (C-6), 130.8
(C-4), 143.8 (C-7), 173.3 (C-3), 210.4 (C-1).
(3-Hydroxy-3-phenyl-2-propenedithioate-S,S′)bis(triphenylphos-
phine-P)copper(I) (2b). Yield: 0.12 g (27%). Mp: 166-167 °C.
IR (KBr, cm-1): 3432.2 w, 3052.1 w, 1681.1 w, 1585.9 m, 1558.9
s, 1484.1 m, 1432.2 m, 1206.3 m, 1093.6 m, 1044.3 m, 987.3 m,
745.5 m, 693.2 s, 497.2 m. FAB MS: m/z 845.0, [M + Cu]+. HR
FAB (m/z): calcd for C45H37Cu2OP2S2, 845.0353; found, 845.0369.
Anal. Calcd for C45H37CuOP2S2: C, 69.0; H, 4.8. Found: C, 68.7;
1
H, 4.9. H NMR (CDCl3): δ 4.62 (s, 2H, H-2, keto), 6.89 (s, 1H,
3-Hydroxy-3-phenyl-2-propenedithioic Acid (1b). Yield: 1.8
g (62%). Mp: 86-89 °C (lit. mp: 63 °C). IR (KBr, cm-1): 3430.8
w, 2512.1 w, 1591.1 m, 1550.2 s, 1486.6 m, 1452.8 m, 1244.5 s,
912.4 m, 822.9 m, 757.9 s, 678.9 m. 1H NMR (CDCl3): δ 5.42 (s,
1H, H-9), 6.96 (s, 1H, H-2), 7.46 (m, 2H, H-6), 7.55 (m, 1H, H-7),
7.87 (m, 2H, H-5), 15.36 (s, 1H, H-8). 13C NMR (CDCl3): δ 108.2
(C-2), 127.0 (C-6), 129.0 (C-5), 132.7 (C-7), 133.8 (C-4), 172.9
(C-3), 211.4 (C-1).
H-2, enol), 7.25 (m, PPh3), 7.47 (t, H-6, enol), 7.42 (m, 2H, H-6,
keto), 7.53 (m, 2H, H-7, keto + enol), 7.96 (m, 2H H-5, enol),
8.17 (m, 2H, H-5, keto), 13.04 (s, 1H, H-8, enol). 13C NMR
(CDCl3): δ 66.1 (C-2, keto), 113.6 (C-2, enol), 126.4 (C-5, enol),
128.2 (C-6, enol), 128.5 (aromatic, PPh3 + C-6, keto), 128.6
(aromatic, PPh3), 129.5 (C-5, enol), 130.4 (C-7, enol), 132.9 (C-7,
keto), 133.7 (aromatic, PPh3), 135.8 (C-4, enol), 136.9 (C-4, keto),
162.9 (C-3), 193.7 (C-1). 31P NMR (CDCl3): δ 0.31 (broad signal).
(3-Hydroxy-3-(p-fluorophenyl)-2-propenedithioate-S,S′)bis-
(triphenylphosphine-P)copper(I) (2c). Yield: 0.32 g (60%).
Mp: 176-178 °C. IR (KBr, cm-1): 3440.7 w, 3045.7 w, 1678.5
w, 1594.9 s, 1565.2 s, 1499.7 s, 1432.2 s, 1227.0 s, 1201.5 s, 1154.4
s, 1092.7 s, 1034.2 s, 981.3 m, 843.3 s, 743.4 s, 693.3 s, 510.1 s.
FAB MS: m/z 863.0, [M + Cu]+. HR FAB (m/z): calcd for
C45H36Cu2FOP2S2, 863.0259; found, 863.0270. Anal. Calcd for
C45H36CuFOP2S2: C, 67.4; H, 4.5. Found: C, 67.1; H, 4.7. 1H NMR
(CDCl3): δ 4.57 (s, 2H, H-2, keto), 6.83 (s, 1H, H-2, enol), 7.06
(m, 4H, H-6, keto + enol), 7.25 (m, PPh3), 7.79 (m, 2H, H-5, enol),
8.18 (m, 2H, H-5, keto), 13.04 (s, 1H, H-8, enol). 13C NMR
3-Hydroxy-3-(p-fluorophenyl)-2-propenedithioic Acid (1c).
Yield: 2.0 g (12%). Mp: 67-70 °C (lit. mp: 74 °C). IR (KBr,
cm-1): 2495.5 w, 1593.9 s, 1562.3 s, 1434.2 m, 1228.0 s, 1157.2
1
m, 960.3 m, 841.3 m, 773.5 m, 572.5 m. H NMR (CDCl3, 298
K): δ 5.42 (s, 1H, H-9), 6.89 (s, 1H, H-2), 7.14 (m, 2H, H-6),
7.88 (m, 2H, H-5), 15.35 (s, 1H, H-8). 13C NMR (CDCl3): δ 107.9
(s, C-2), 116.3 (d, 2JC-F ) 21.92 Hz, C-6), 129.4 (d, 3JC-F ) 9.23
1
Hz, C-5), 129.9 (C-4), 165.5 (d, JC-F ) 261.34 Hz, C-7), 171.7
(C-3), 211.4 (C-1).
3-Hydroxy-3-(p-chlorophenyl)-2-propenedithioic Acid (1d).
Yield: 2.5 g (14%). Mp: 88-91 °C (lit. mp: 99.5 °C). IR (KBr,
2
(CDCl3): δ 66.2 (C-2, keto), 113.4 (C-2, enol), 115.6 (C-6, JC-F
(15) Larsson, F. C. V.; Lawesson, S.-O. Tetrahedron 1972, 28, 5341.
(16) (a) Cotton, F. A.; Goodgame, D. M. L.; J. Chem. Soc. 1960, 5267.
(b) Jardine, F. H.; Vohra, A. G.; Young, F. J. J. Inorg. Nucl. Chem.
1971, 33, 2941.
) 21.8 Hz, enol + keto), 128.5 (aromatic, PPh3), 129.7 (aromatic,
PPh3), 131.9 (C-4, enol), 132.1 (C-5, 3JC-F ) 9.2 Hz, keto + enol),
133.4 (C-4, keto), 133.7 (aromatic, PPh3), 161.8 (C-3), 164.1 (C-
Inorganic Chemistry, Vol. 43, No. 26, 2004 8573

Garc´ıa-Orozco et al.
Scheme 2. Preparation of Copper(I) Coordination Compounds 2a-d
Table 1. X-ray Crystallographic Data for Complex 2a
formula
fw
cryst system
space group
Z
C46H39CuOP2S2
797.37
monoclinic
P21/c
4
a, Å
b, Å
c, Å
13.947(2)
10.828(2)
26.508(3)
92.750(1)
3998.6(1)
1.325
â, deg
V, Å3
Table 2. 1H and 13C NMR Chemical Shifts for Ligands 1a-d
d
calcd, g/cm3
temp, K
293
atom
1a
1b
1c
1d
wavelength, Å
abs coeff, mm-1
goodness of fit on F2
R,a %
0.710 73
0.765
1.014
5.87
10.28
δ(1H)
5.42
6.96
7.46
7.87
H-9
H-2
H-6
H-5
H-8
5.38
6.95
7.26
7.77
15.38
5.42
6.89
7.14
7.88
15.35
5.45
6.90
7.44
7.81
15.38
Rw,b %
a R ) Σ(|Fo| - |Fc|)/Σ|Fo|. b Rw ) {[Σw(Fo - Fc )2]/[Σw(Fo )2]}1/2
.
2
2
2
15.35
δ(13C)
108.2
127.0
133.8
129.0
132.7
172.9
211.4
1
1
7, JC-F ) 251.5 Hz, enol), 165.7 (C-7, JC-F ) 251.5 Hz, keto),
192.1 (C-1). 31P NMR (CDCl3): δ 0.50 (broad signal).
C-2
C-6
C-4
C-5
C-7
C-3
C-1
107.9
129.8
130.8
127.1
143.8
173.3
210.4
107.9
116.3
129.9
129.4
165.5
171.7
211.4
107.9
129.3
132.2
128.3
138.9
171.4
212.1
(3-Hydroxy-3-(p-chlorophenyl)-2-propenedithioate-S,S′)bis-
(triphenylphosphine-P)copper(I) (2d). Yield: 0.50 g (54%).
Mp: 150-154 °C. IR (KBr, cm-1): 3442.1 w, 3051.8 w, 1680.0
w, 1584.0 s, 1553.6 s, 1483.4 s, 1434.1 s, 1238.2 m, 1209.8 s,
1093.0 s, 1036.6 s, 985.0 m, 819.9 m, 744.7 s, 694.4 s, 510.1 s.
FAB MS: m/z 879.0, [M + Cu]+. HR FAB (m/z): calcd for
C45H36ClCu2OP2S2, 878.9963; found, 878.9956. Anal. Calcd for
acid bis(triphenylphosphine)copper(I) leads to a decrease the
CdS bond strength.18 A similar behavior has been observed
in analogous Ni(II), Pd(II), and Pt(II) complexes8,9 and
copper(I) thiourea complexes.19 The characteristic tri-
phenylphosphine vibrations were observed at 1433, 1094,
745, and 694 cm-1.
The molecular ion is not observed in the FAB+ mass
spectrum of 2a; however, a peak that corresponds to the
adduct of the expected molecular ion with a copper atom
[M + Cu]+ appeared at m/z 859. The isotopic distribution
of this species is in agreement with the calculated composi-
tion. The second copper ion may be bonded through the
already coordinated sulfur atoms of the complex by means
of a weak interaction, as it was observed in other sulfur
complexes.14,20 Likewise, a peak at m/z 534, assigned to the
fragment [M+ - PPh3], was observed.
1
C45H36ClCuOP2S2: C, 66.1; H, 4.4. Found: C, 64.9; H, 4.5. H
NMR (CDCl3): δ 4.55 (s, 2H, H-2, keto), 6.82 (s, 1H, H-2, enol),
7.25 (m, PPh3), 7.35 and 8.08 (AA′BB′ system, 4H, J ) 8.07 Hz,
H-6, H-5, keto), 7.35 and 7.72 (AA′BB′ system, 4H, J ) 7.67 Hz,
H-6, H-5, enol), 13.02 (s, 1H, H-8, enol). 13C NMR (CDCl3): δ
66.2 (C-2, keto), 113.6 (C-2, enol), 127.6 (C-6, keto), 128.5
(aromatic, PPh3), 128.8 (C-5, enol), 129.6 (aromatic, PPh3), 130.8
(C-5, enol), 130.9 (C-6, keto), 133.8 (aromatic, PPh3), 134.3 (C-7,
enol), 135.3 (C-4, enol), 136.3 (C-4, keto), 139.3 (C-7, keto), 161.3
(C-3), 192.5 (C-1). 31P NMR (CDCl3): δ 0.03 (broad signal).
X-ray Crystallography. Red crystals of compound 2a were
obtained by slow diffusion of dichloromethane/methanol system
at room temperature. Details of crystal data collected are provided
in Table 1. The structure was solved using direct methods.
Anisotropic structure refinements were achieved using full-matrix
least-squares techniques on all non-hydrogen atoms. All hydrogen
atoms (except H-1) were placed in ideal positions on the basis of
hybridization, with isotropic thermal parameters fixed at 1.2 times
the value of the attached atom. Structure solution and refinements
were performed using SHELXTL V 6.10.17 Selected bond distances
and bond angles for compound 2a are given in Tables 5 and 6.
The 1H NMR spectrum of 2a presents several differences
in comparison to 1a (Table 2). The enolic proton signal H-8
in 1a is shifted upfield from δ 15.38 to 13.01, which is due
to the coordination of sulfur to the copper and hence a
decrease of electronic density on the sulfur atom. The
corresponding signal for the vinylic proton is observed at δ
6.88.
Results and Discussion
The 13C NMR data provided further evidence supporting
this coordinating pattern. The signal attributed to C-1 and
C-3 in the ligand 1a are shifted upfield in the complex 2a
from δ 210.4 and 173.3 to 193.4 and 163.2, respectively,
while the assigned signal to the vinylic carbon atom C-2 is
shifted downfield from δ 107.9 to 113.2. These shifts are
The syntheses of copper(I) complexes were carried out
by following Scheme 2. The reaction of 3-hydroxy-3-(p-
methylphenyl)-2-propenedithioic acid (1a) with bis(tri-
phenylphosphine)copper(I) nitrate, at room temperature,
yields a stable red crystalline solid. The structure for complex
2a was assigned according to spectroscopy data.
IR spectrum of 2a shows the ν(CdS) vibration (1251
cm-1) shifted substantially to lower wavenumber than those
of the free ligand [ν(CdS) 1210 cm-1, ∆ν ) 41 cm-1]. This
means that coordination of ligand 1a to the cationic Lewis
(18) Nakamoto, K. Infrared and Raman spectra of inorganic and coordina-
tion compounds. Part B, 5th ed.; J. Wiley: New York, 1997; p 1.
(19) Bombicz, P.; Mutikainen, I.; Krunks, M.; Leskela¨, T.; Madara´sz, J.;
Miinistro¨, L. Inorg. Chim. Acta 2004, 357, 513.
(20) Coucouvanis, D.; Piltingsrud, D. J. Am. Chem. Soc. 1973, 95, 5556.
(b) Vicente, J.; Chicote, M. T.; Huertas, S.; Bautista, D.; Jones, P. G.
Inorg. Chem. 2001, 40, 2051. (c) Vicente, J.; Chicote, M. T.; Huertas,
S.; Jones, P. G.; Fischer, A. K. Inorg. Chem. 2001, 40, 6193.
(17) SHELXTL, version 6.10; Bruker Analytical X-ray Systems: Madison,
WI, 2000.
8574 Inorganic Chemistry, Vol. 43, No. 26, 2004

Cu(I) Complexes from 2-Propenedithioic Acids
Table 3. 1H and 31P NMR Chemical Shifts and Keq at Tamb for
Complexes 2a-d
2a
2b
2c
2d
δ(1H) keto (3) H-2
4.59
7.22
8.06
4.62
7.42
8.17
4.57
7.06
8.18
4.55
7.35
8.08
H-6
H-5
phosphines 7.15-7.45 7.15-7.35 7.10-7.40 7.10-7.40
enol (2) H-2
H-6
6.88
7.20
7.72
13.01
0.10
1.13
6.89
7.47
7.96
13.04
0.31
1.52
6.83
7.06
7.79
13.05
0.50
1.78
6.82
7.35
7.72
13.02
0.03
1.76
H-5
H-8
δ(31P)
Keq(Tamb
)
Table 4. 13C NMR Chemical Shifts for Complexes 2a-d
2a
2b
2c
2d
δ(13C)
keto (3)
C-2
C-5
C-6
C-4
C-7
C-3
C-1
C-2
C-5
C-6
C-4
C-7
C-3
C-1
66.1
128.5
129.2
134.7
143.7
163.2
193.4
113.2
126.5
126.3
132.9
140.8
163.2
193.4
66.1
129.5
128.2
136.9
132.9
162.9
193.7
113.6
126.4
128.5
135.8
130.4
162.9
193.7
66.2
132.1
115.6
133.4
165.7
161.8
192.1
113.4
132.1
115.6
131.9
164.1
161.8
192.1
66.2
130.9
127.6
136.3
139.3
161.3
192.5
113.6
128.8
130.8
135.3
134.3
161.3
192.5
Figure 1.
Table 5. Selected Bond Distances (Å) for 2a
Cu-P1
Cu-P2
Cu-S1
Cu-S2
C1-S1
2.263(1)
2.268(2)
2.381(1)
2.511(2)
1.701(6)
C1-S2
C1-C2
C2-C3
C3-O1
C3-C4
1.707(5)
1.448(8)
1.366(8)
1.338(7)
1.466(8)
enol (2)
Table 6. Selected Bond Angles (deg) for 2a
P-Cu-P
123.78(6)
113.76(6)
109.00(6)
117.65(5)
S-Cu-S
73.20(6)
106.16(6)
117.8(3)
attributed to the coordination of the bis(triphenylphosphine)-
copper(I) moiety to ligand 1a through the sulfur atoms. These
observations are in agreement with previously reported
palladium(II) coordination compounds8,10 and copper(I)
clusters.21
P1-Cu-S1
P1-Cu-S2
P2-Cu-S1
P2-Cu-S2
S1-C-S2
Table 7. Bond Distances (Å) and Angles (deg) of Intramolecular
Hydrogen Bonds for 2a
The 31P NMR spectrum exhibits a single broad signal
centered at δ 0.10. This signal appears at a lower field in
comparison to that of the free triphenylphosphine (δ 4.0),
confirming the coordination of phosphine to the atom copper.
Even though these ligands have been known since 1910,22
there are only few examples of transition metal coordination
compounds8-12 where the ligands are bonded through both
sulfur atoms in 1,1-dithiolate fashion (Chart 1). To the best
of our knowledge, complex 2a is the first example where
this ligand is bonded as a dithio acid resulting in a
mononuclear complex instead of polynuclear compounds as
reported for sulfur ligand analogues.13,14
The syntheses of complexes 2b-d were carried out using
the same procedure. All compounds displayed features
similar to those of 2a. 1H, 13C, and 31P NMR data are given
in Tables 3 and 4.
Crystal Structure of Compound 2a. The proposed
structure for 2a was fully established by means of an X-ray
diffraction analysis (Figure 1 and Table 5). The metal center
is bonded to the ligand through both sulfur atoms in an
anisobidentate fashion forming a four-member ring. The
central copper atom forms a CuS2P2 core with approximately
tetrahedral geometry. Geometry distortion arises from the
bite S-C-S angle (ca. 73°) and the steric hindrance of
triphenylphosphine at P-Cu-P angle (Table 6). This is the
O-H
S‚‚‚H
0.94(7)
2.08(7)
O‚‚‚S
2.956(5)
155(6)
O-H‚‚‚S
coordination mode commonly presented by the dithio acid
ligand.23 The C2-C3, C1-C2, and C3-O bond distances
and a strong intramolecular hydrogen bond24 (Table 7)
confirmed that the enol form is the more stable one in the
crystal solid state.
On the other hand, the average Cu-P distances are
comparable to those reported in the literature for copper (I)-
phospine complexes.25
Equilibrium in Solution. In CDCl3, these copper com-
plexes exist in two tautomeric forms, enol (A) and keto (B)
tautomers (Scheme 3), which can be identified on the basis
of their NMR spectra (Tables 3 and 4). The NMR spectra
of copper(I) complexes 2a-d have shown characteristic
1
chemical shifts for each tautomeric species. In H NMR
spectra, the most salient signals for structure A (H-8 and
H-2) were described; vide supra. In all the complexes 2a-d
(23) Livingstone, S. E. In ComprehensiVe Coordination Chemistry; Willkin-
son, G., Gillard, R. D., McCleverty, J. A., Eds.; Pergamon: Oxford,
U.K., 1987; p 584.
(24) (a) Jeffrey, G. A. An introduction to Hydrogen Bonding; Oxford
University Press: Oxford, U.K., 1997; p 12. (b) Steiner, T. Angew.
Chem., Int. Ed. 2002, 41, 48.
(25) (a) Camus, A.; Marsich, N.; Nardin, G. J. Organomet. Chem. 1980,
188, 389. (b) Nakahodo, T.; Horn, E.; Tiekink, E. R. T. Acta
Crystallogr. 2000, C56, 1316. (c) Jian, F.; Bei, F.; Lu, X.; Yang, X.;
Wang, X.; Razak, I. A.; Raj, S. S. S.; Fun, H.-K. Acta Crystallogr.
2000, C56, e288. (d) Deivaraj, T. C.; Vittal, J. J. Acta Crystallogr.
2001, E57, m566.
(21) Coucouvanis, D.; Swenson, D.; Baezinger, N. C.; Pedelty, R.; Caffery,
M. L.; Kanodia, S. Inorg. Chem. 1989, 28, 2829.
(22) Kelber, C. Ber. Dtsch. Chem. Ges. 1910, 43, 1252.
Inorganic Chemistry, Vol. 43, No. 26, 2004 8575

Garc´ıa-Orozco et al.
Scheme 3. Tautomeric Equilibrium in Solution
Table 8. Hammett Linear Correlation of δ and ∆δ for Compounds
2a-d
atom
eq
r
S
eq no.
C-3
C-1
C-1a
δ ) 172.7 - 5.379σ
δ ) 211.1 + 3.886σ
∆δ ) -17.9 - 7.531σ
0.981
0.930
0.986
0.918
0.699
1.252
1
2
3
a ∆δ ) δcmpx - δlig has been considered.
a singlet in the region δ 4.55-4.62 was assigned to the
methylene protons H-2 for structure B.
the acquired slope for eq 2 suggests that electron-donor
substituents increase the electron density over C-1 and the
donor ability of sulfur is enhanced. This is in agreement with
the data reported for the Pd(II) complexes of these ligands.8,10
In the case of copper(I) complexes 2a-d a shielding effect
on C-1 (∆δ) was observed with respect to the free ligand. A
linear correlation of such ∆δ was established (eq 3). From
the estimated slope, we conclude that electron-withdrawing
substituents enhance the shielding effect. However, the
increment of the electron density on C-1 in the complexes
is opposite to the free ligand behavior (eq 1). This effect
could be explained by π back-bonding from the bis-
(triphenylphosphine)copper(I) fragment to the ligand,29 and
this can also be observed in the IR data for ν(CdS) in the
complexes 2a-d (vide supra). To our knowledge, this is the
first time where a π back-bonding effect is observed by NMR
techniques and Hammett correlation analysis.
Similarly, in the 13C NMR spectra of 2a-d, a signal at δ
66.0 assigned to methylene group C-2 (structure B) was
observed. Chemical shifts of both dithio acid carbon C-1
and enol carbon C-3 atoms are common for both the forms
(Table 4). The tautomeric equilibrium was also confirmed
by means of variable-temperature NMR experiments. The
intensity relationship of each equilibrium species varies with
temperature; at a high temperature the keto tautomer
concentration increases, as in acetylacetone derivatives.23
To calculate the equilibrium constants for 2a-d com-
pounds, both the enol H-8 and methylene H-2 signals were
integrated to determine the population of each tautomer on
equilibrium and the methylene integral value was normalized.
The value of the tautomeric equilibrium constants were
calculated by taking the ratio Keq ) [enol]/[keto] (Table 3).
In general, Keq’s values reveal that the enol tautomer is the
main species in equilibrium and that the electron-withdrawing
substituents favor this same tautomer.26
Conclusions
It is important to note that ligands 1a-d do not present
tautomerism,2 which may be due to the formation of a formal
intramolecular hydrogen bond hence stabilizing the enol
species27 However, their copper(I) complexes have the ability
to undergo a keto-enol equilibrium. Thus, we propose that
copper(I) coordination reduces the strength of the intra-
molecular hydrogen bond, prompting the tautomeric equi-
librium. Moreover, the shift to lower wavenumbers of
ν(CdS) in IR spectrum and the enolic proton signal H-8
shifting upfield in 1H NMR spectrum support this behavior.
Hammett Correlation Studies. In a previous study of free
ligands, Larsson and Lawesson observed a linear correlation
between NMR chemical shifts of enol protons and Hammett
constants.15 To define the electronic influence of the sub-
stituent on the aromatic ring over the coordination centers,
13C NMR was chosen to identify a possible linear correlation
with Hammett constants28 for both ligands 1a-d and
complexes 2a-d. As can be seen in Table 8, the substituent
displays a cooperative effect in the ligand coordination
behavior.
Four new mononuclear copper(I) complexes (2a-d)
derived from ligands 1a-d have been synthesized and
characterized, and the dithioate-coordination fashion of these
ligands is observed for the first time. In solution, complexes
2a-d present keto-enol tautomerism where the enol tau-
tomer was predominant at room temperature and favored by
electron-withdrawing groups, as was demonstrated by NMR.
Some linear correlations on 13C NMR data and Hammett
substituent constants were found for the ligand (1a-d) and
the complexes (2a-d), and a shielding effect (∆δ) on C1
could be related to a π back-bonding effect. This is the first
report of a π back-bonding effect being observed by 13C
NMR and Hammett correlation analysis.
Acknowledgment. We are grateful to Rocio Patin˜o and
Luis Velasco for their technical assistance. The authors thank
the CONACyT, DGEP-UNAM for fellowships (145129), and
DGAPA-PAPIT IN202404.
Supporting Information Available: Complete details of the
1
X-ray diffraction study (CIF), an H NMR spectrum of complex
In the 13C NMR spectra of ligands 1a-d, a correlation of
the chemical shift for C-3 with Hammett substituent constants
was found (eq 1). According to the obtained slope, we
deduced that electron-donating substituents decrease electron
density that leads C-3 to show carbonyl behavior. Moreover,
2a, and plots of Hammett correlations. This material is available
free of charge via the Internet at http://pubs.acs.org.
IC0488132
(29) This could be better explained by means of resonance-assisted
hydrogen-bonding theory: (a) Steiner, T. Chem. Commun. 1998, 411.
(b) Gilli, G.; Bellucci, F.; Ferretti, V.; Bertolasi, V. J. Am. Chem.
Soc. 1989, 111, 1023. (c) Bertolasi, V.; Gilli, P.; Ferretti, V.; Gilli, G.
J. Am. Chem. Soc. 1991, 113, 4917. (d) Gilli, P.; Bertolasi, V.; Ferretti,
V.; Gilli, G. J. Am. Chem. Soc. 1994, 116, 909. (e) Gilli, P.; Bertolasi,
V.; Ferretti, V.; Gilli, G. J. Am. Chem. Soc. 2000, 122, 10405.
(26) Toullec, J. In The Chemistry of the Enol; Rappoport, Z., Ed.; Wiley:
Chichester, England, 1990; pp 323-398.
(27) March, J.; Smith, M. B. March’s AdVanced Organic Chemistry, 5th
ed.; J. Wiley: New York, 2001; p 370.
(28) May, W. R.; Jones, M. M. J. Inorg. Nucl. Chem. 1962, 24, 517.
8576 Inorganic Chemistry, Vol. 43, No. 26, 2004