versión impresa ISSN 0366-1644
Bol. Soc. Chil. Quím. v.45 n.3 Concepción set. 2000
(a)Facultad de Ciencias Químicas, Universidad de Concepción, Casilla 160-C,
Concepción , Chile.
(b)Departamento de Química, Facultad de Ingeniería, Universidad de La Frontera,
(Received: April 24, 2000 - Accepted July 6,2000)
In memoriam of Doctor Guido S. Canessa C.
The interaction of copper(II) halides and tetraalkylthiuram disulfides in organic solvents using stoichiometric amounts of starting materials at 10C yields copper(III) dialkyldithiocarbamates. The products are characterized by analyses, room-temperature magnetic measurements, IR and photoelectron spectroscopies, as well as cyclic voltammetry studies.
KEYWORDS: Copper(III), Coordination Chemistry, Metal Dithiocarbamates
La interacción de haluros de cobre(II) y tetraalquiltiurano disulfuros en solventes orgánicos usando cantidades estequiométricas de reactantes a temperatura ambiente produce dialquilditiocarbamatos de cobre(III). Los productos se caracterizan por análisis elemental, medidas magnéticas a temperatura ambiente, espectroscopía IR y fotoelectrónica, además de estudios de voltametría cíclica.
PALABRAS CLAVES: Cobre(III), Química de Coordinación, Ditiocarbamatos metálicos
Transition metals are characterized by the display of a wide variety of oxidation states. Some values are those typically exhibited in coordination compounds with classical ligands such as H2O, Cl- and OH-. We refer to these as «usual» oxidation states. Other possibilities are here and elsewhere termed «unusual». Such unusual oxidation states occur in many Main Group and Transition metals.
In the latter case, complexes which display unusual oxidation states have often been invoqued in mechanistic considerations dealing with living organisms 1and as short-lived species in catalytic systems.2 Hard evidence relating to such states is in many cases lacking, due to the very reactive nature of the intermediates postulated. This reactivity stems from exposed d orbitals on the metal. In the case of high oxidation states, the exposed orbitals are empty.
Several approaches have been utilized in dealing with the issue of stabilization of these unusual complexes. Thus the excess positive charge upon the metal has been neutralized with the strongly electronegative F ligand in a series of hypervalent metal complexes.3 Other researchers have attempted with moderate success the use of either bulky ligands or polychelating macrocycles to protect the metal center.4
As with many other metal systems, the chemistry of hypervalent copper complexes has been explored to a very limited extent. Copper(IV) compounds have not been realized in a systematic manner, while their Cu(III) counterparts have been the subject of only a handful of reports.5
A review covering the coordination chemistry of copper during 1991-1994 devoted a section to the limited advances published during the period on copper(III).6 This section was deleted in the review for the year 19957, indicating a diminishing interest on the subject. Interest picked up again as papers discussing theoretical models for the electron configuration of the controversial Cu(CF3)4- began to appear.8, 9 Also, the preparation and properties of tetrapeptide complexes of trivalent copper was reported.10 The first half of 1998 brought us the appearance of several interesting studies on hypervalent cupric derivatives. Salicylaldehyde benzoylhydrazone inhibits DNA synthesis and cell growth. It was found that the copper(II) complex shows significantly enhanced activity, which was attributed to the intermediacy of a trivalent derivative.11 Also, conveniently substituted oxamide and oxamate ligands were used to stabilize the trivalent state.12 A similar macrocycle ligand effect might be at play during the oxidation of Cu(I) oxyhemocyanin with oxygen, as revealed by an analysis of the bonding and electronic effects involved.13
Thiuram disulfides R2NC(S)S-SC(S)NR2 are the thiocarbamoyl esters of dithiocarbamic acids and their ability to afford metal dithiocarbamates in abnormally high oxidation states has been recognized for several years now. This capability stems from the presence of the potential dithiocarbamate ligands which can delocalize positive charge from the metal towards the periphery of the complex.14 The literature shows many examples of this.15 Some examples from recent years are the synthesis of [V2(m-S2)2(Et 2dtc)4] from VS43- and Et4tds16, and the reaction of thiuram disulfides and HB(Me2pz)3W(CO)3 to afford HB(Me2pz)3W(CO)2R 2dtc and [HB(Me2pz)3WII(CO) 2(µ-S)WIV(R2dtc) 2(thiocarboxamido)] (HB(Me2pz)3 =3,5-dimethylpyrazol-1-yl borate).17 Other products arising from this extremely complicated reaction are W(R2dtc)4+ and HB(Me2pz)3W(S)R2dtc. 18 In this paper we report a series of copper(III) dithiocarbamates, obtained by oxidation of copper(II) halides with tetraalkylthiuram disulfides (R4tds) R2NC(S)S-SC(S)NR2.
Solvents and starting materials: Commercial cupric chloride dihydrate (Merck) was oven-dried overnight at 115oC and stored in a dessicator over phosphorus pentoxide. Cupric bromide, tetraalkylthiuram disulfides and solvents were prepared or purified by standard methods already described.19
Photoelectron spectra were run on a Fissons Instruments Escalab 220 IXL spectrometer with a monochromatic X-ray source. Spectra were calibrated against C 1s line (284.8 eV). Samples were mounted on adhesive tape.
Preparation of the complexes: A general method of synthesis was followed and it is described in full for the case of the dichlorodimethyldithiocarbamatocopper(III) complex. All manipulations involved standard cannulation or Schlenk techniques. Other derivatives were prepared by trivial modifications of this procedure.
0.8819 g (2.50 mmol) of the organic disulfide was dissolved in 70 mL of warm hexane. The cooled solution was added dropwise to a magnetically stirred slurry of 0.6722 g (5.00 mmol) of anhydrous cupric chloride in 50 mL of THF, cooled with a slush of ice-salt. After the addition was completed, the solid product was filtered from the dark red solution and washed with two 5 mL portions of diethyl ether. The solid product was pumped dry to yield 1.25 g of crude product. Elementary analyses indicated a purity of better than 98% for this material. Small amounts (0.10 g) were recrystallized from 50 mL of fresh THF by addition of 25 mL of diethyl ether. When care was taken to layer the ether above the THF, shiny small crystals were observed after two hours. This crystalline material turned amorphous when the mother liquor was removed.
Dichlorodi-(isopropyl)dithiocarbamatocopper(III): microcrystalline dark red solid, yield:80%, mp: 145oC(d). Slightly soluble in water. These solutions decompose within minutes to a brown colloid. Soluble in 1,4-dioxane, THF and acetonitrile. Solutions in the latter solvent decompose to yellow-green solids not yet characterized. Anal. Calcd. for C7H14NS2CuCl 2: C,27.1; H, 4.6; Cu, 20.5; Cl, 22.8. Found for the recrystallized material: C, 27.0; H, 4.5; Cu, 20.4, Cl, 22.8. IR(KBr): 3164w, 2967m, 2901m, 1585vs, 1453m, 1400w, 1380m, 1361s, 1175w, 1144s, 1032w, 830w. (Nujol) 390m. 1H-NMR(CDCl3): 4.28(s,br,1), 1.32(s,br,6). LM(THF, 0.01mol/L):17 mho-L/mol. mEFF: 0.0 MB
The method described above has been used to obtain the full series of compounds X2CuS2CNR2 (X = Cl and Br; R = Me, Et and i-Pr).
Dichlorodiethyldithiocarbamatocopper(III): Amorphous dark red solid, yield 83%, mp: 132oC(d). Solubility properties similar to above. Anal. Calcd. for C5H10NS2CuCl2 : C, 21.2; H, 3.6; Cu, 22.5; Cl, 25.1. Found: C, 21.0; H, 3.5; Cu, 22.4, Cl, 24.8. IR(KBr): 2999w, 2934m, 2875m, 1604vs, 1460s, 1400w, 1364w, 1289m, 1193m, 1150w, 1100w, 1086w, 1071w, 848w, 788w. (Nujol) 400m. LM(THF, 0.01mol/L):13 mho-L/mol. mEFF: 0.0 MB
Dichlorodimethyldithiocarbamatocopper(III): Amorphous dark red solid, yield 89%, mp: 160oC(d). Solubility properties similar to above. Anal. Calcd. for C3H6NS2CuCl2 : C, 14.2; H, 2.4; Cu, 25.0; Cl, 27.8. Found: C, 14.4; H, 2.5; Cu, 25.4, Cl, 27.8. IR(KBr): 2929w, 1618vs, 1414m, 1249m, 1170w, 1058w, 848w, 788w. (Nujol) 397m. LM(THF, 0.01mol/L):15 mho-L/mol. mEFF: 0.1 MB
Dibromodi-(isopropyl)dithiocarbamatocopper(III): microcrystalline dark blue solid, yield 71%, mp: 130-133oC(d). Soluble in 1,4-dioxane, THF and acetonitrile to give bright blue solutions. Anal. Calcd. for C7H14NS2CuBr2 : C,21.0; H, 3.5; Cu, 15.9; Br, 40.0. Found: C, 20.8; H, 3.4; Cu, 15.7; Br, 39.8. IR(KBr): 2964m, 2924m, 2861m, 1574vs, 1461m, 1370m, 1350m, 1172w, 1141m, 1114sh, 1019w. (Nujol) 402m. 1H-NMR(CDCl3): 4.25(s,br,1), 1.30(s,br,6). LM(THF, 0.01mol/L):23 mho-L/mol. mEFF: 0.0 MB.
Dibromodiethyldithiocarbamatocopper(III): Amorphous dark blue solid, yield 79%, mp: 160-164oC(d). Solubility properties similar to above. Anal. Calcd. for C5H10NS2CuBr2 : C, 16.6; H, 2.7; Cu, 17.1; Br, 43.0. Found: C, 16.3; H, 2.5; Cu, 17.4, Br, 42.8. IR(KBr): 2993m, 2927w, 2868w, 1592vs, 1440s, 1379w, 1348m, 1282m, 1196, 1143w, 1093w, 1066sh, 1006w, 846w, 787w. (Nujol) 400m. LM(THF, 0.01mol/L):16 mho-L/mol. mEFF: 0.0 MB
Dibromodimethyldithiocarbamatocopper(III): Amorphous dark blue solid, yield 92%, mp: 182-184oC(d). Solubility properties similar to above. Anal. Calcd. for C3H6NS2CuBr2 : C, 10.5; H, 1.8; Cu, 18.5; Br, 46.5. Found: C, 10.4; H, 1.5; Cu, 18.4, Br, 46.8. IR(KBr): 2981w, 1615vs, 1431m, 1405s, 1237m, 1167s, 1058w, 848w, 788w. (Nujol) 407m. LM(THF, 0.01mol/L):15 mho-L/mol. mEFF: 0.1 MB
RESULTS AND DISCUSSION
Interaction of copper(II) halides CuX2 (X=Cl or Br) with tetraalkylthiuram disulfides R4tds (R= Me, Et or iPr) in THF at ambient temperature yields the copper(III) dithiocarbamates X2Cu(R2dtc) as deeply colored, relatively air-stable microcrystalline solids, plus solutions which degrade rapidly in air to produce the copper(II) bisdithiocarbamate complex.
In keeping with the magnetic properties reported for most of the copper(III) derivatives known so far,5 it is found that the solid complexes reported here are diamagnetic at ambient temperature. This observation is consistent with a d8 ion and a strong argument in favor of the oxidation state proposed.
Analysis of the IR spectral data follows a standard pattern already established for dithiocarbamate-metal complexes.19 The "thiureide" band at ca. 1500 cm-1 in many metal dithiocarbamates, is found at an abnormally high value in the present complexes. This observation is indicative of extensive delocalization of electron density from the sulfur atoms and therefore supports the contention of the metal in a high oxidation state. Furthermore the Cu-S stretch is found at ca. 400 cm-1 in all the complexes studied, compared to typical values of 350 cm-1 in most copper dithiocarbamates, again indicative of Cu(III).
Cu(II) states in the photoelectron spectra are generally recognized by a main peak at binding energy (BE) of 933-934 eV, in addition to significantly intense shake-up satellites at 8-10 eV higher values from the main peak.20 Cu(I) and Cu(0) states have BE values around 932.5 eV, while Cu(III) states are not common, but are expected to have higher BE values than Cu(II). The values available for X2CuR2dtc samples ( X = Cl and Br; R = iPr) show features at BE of 934.4 and 935.2, which are fully consistent with their formulation as Cu(III) species.
Cyclic voltammetry studies for the Cu(III) complexes were carried out in CH2Cl2 solution. The voltammographs were compared with those of Cu(Me2dtc)2 and Zn(Me2dtc)2. The latter is inert from a redox point of view in the window observed. The Cu(III) complexes show a reversible one-electron oxidation wave (Cl2CuiPr2dtc: E1/2 = 0.055 V; Br2CuEt2dtc: E1/2 = 0.098V) assignable to a CuIII/CuIV process. The reduction side for both Cu(III) complexes is characterized by a quasi-reversible one-electron wave (Cl2CuiPr2dtc: Ep = -1.298 V; Br2CuEt2dtc: Ep = -1.143 V) attributable to the couple CuIII/CuII.
The present work furnishes a further example of the ability of dithiocarbamate ligands to stabilize hypervalent states in transition metals in general and in copper chemistry in particular. Cuprous halides and tetraalkylthiuram disulfides yield materials which contain the cation CuIII(dtc)2+. These are however insoluble, intractable materials21 which do not lend themselves to understanding the reaction or the structural chemistry of this oxidation state.
Support from Universidad de Concepción (DIUC 9721081) and CONICYT (FONDECYT 1990494) is gratefully acknowledged.
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