Chromium-and sulfur-containing complexes

Abstract: The complexes are tetranuclear sulfide-bridged complexes of Cr(III) having bridging organo-carboxylate ligands. One such complex has a cation of the general formula Cr₄S(RCO₂)₈(L)₄²⁺, wherein R represents alkyl groups having from 1 to 12 carbon atoms, L represents a ligand or ligands containing a donor atom selected from oxygen, nitrogen, carbon, and sulfur. The complexes have ferromagnetic properties.

Description: This invention relates to novel complexes containing chromium and sulfur. More especially, this invention relates to novel tetranuclear sulfido-bridged complexes of Cr(III) having strongly magnetic ground states and methods of making the same. Various complexes containing two, three and four chromium atoms are known. With respect to bi-and trinuclear chromium complexes, see, for example, Magneto-Structural Correlation in Exchange Coupled Systems , Willett et al., Ridel, Dordrecht, pp. 497-522 (1985). See also Dubicki et al., Inorganic Chemistry , 11, 1869 (1972) and Figgis et al., Nature (London), 205 , 694 (1964) who report a trinuclear chromium (III) acetate in which the three chromium atoms are linked to a central oxygen atom. Two complexes of chromium that have chromium atoms bound to a central sulfur atom have been reported in journal articles. For instance, Darensburg et al., Organometallics , 3, 1598-1600 (1984) report the synthesis of a sulfur-capped trinuclear chromium carbonyl cluster, S[Cr(CO)₄]₃²⁻; and Hoefler et al., J. Am. Chem. Soc. , 105, 6338-6339 (1983) report the synthesis of [PPN]₂(µ₂-CO)₃(CO)₉Cr₃(µ₄-S)Cr(CO)₅], in which the anion consists of chromium triangle capped with an SCr(CO)₅ group. Chromium complexes with bridging ligands formed from benzoic acid and containing three chromium atoms connected to a central sulfur atom have been reported by Eremenko et al., Inorg. Chim. Acta , 73, 225-229 (1983). None of these references disclose a tetranuclear sulfide bridged complex of Cr(III). Broadly stated, the present invention encompasses tetranuclear sulfide-bridged complexes of Cr(III) having bridging organo-carboxylate ligands. According to one embodiment, the present invention is directed toward a novel composition of matter having a complex cation of the general formula Cr₄S(RCO₂)₈(L)₄²⁺, wherein R represents alkyl groups having from 1 to 12 carbon atoms, L represents a ligand or ligands containing a donor atom selected from oxygen, nitrogen, carbon, and sulfur. In the complex of the invention, normally the four Cr(III) atoms are bound to a central tetrahedral sulfur atom. The chromium atoms normally are octahedrally coordinated with bridging carboxylates and the ligand molecule. Also, the bridging carboxylates, RCO₂ of the above formula, are the same or different carboxylates. The complexes and compositions of the present invention are especially unique in that they have ferromagnetic properties. As a result thereof, they are particularly useful as contrasting agents for nuclear magnetic resonance (NMR) imaging; shiftless relaxation reagents to aid the accumulation of ¹³C NMR data; orienting agents for attached polymers in a magnetic field; selective microwave absorbers for localized heating of a particular organelle, organ, organism or inanimate substructure; or as part of a thermometer for use above 0.2 degrees Kelvin. The novel compositions may be prepared by reacting a source of chromium, such as chromium metal or chromium carbonyl with sulfur and the appropriate source of coordinating bridging carboxylate, such as the corresponding carboxylic acid, acid anhydride or mixtures thereof at temperatures and for time sufficient to form the composition. The invention will now be illustrated in non-limiting fashion by the following example and discussion. BRIEF DESCRIPTION OF THE DRAWING The drawing is a depiction of the structure of a complex cation of the present invention. The invention encompasses tetranuclear sulfide-bridged complexes of Cr(III) including bridging ligands from carboxylates. Indeed, the complex cations of the present invention are represented by the general formula Cr₄S(RCO₂)₈(L)₄²⁺, wherein R is an alkyl group having from 1 to 12 carbon atoms and L is a ligand containing a donor atom selected from oxygen, nitrogen, carbon, and sulfur. Preferably, R is methyl and L is selected from H₂O, or CH₃OH. It should be noted that the bridging carboxylates in the above formula are preferably all of the same type; however, such is not necessary and a mixture of different bridging carboxylates may be employed, if so desired. The ligand L is preferably selected from H₂O, CH₃OH and CH₃CN. Compounds containing the novel complex cation will contain an anion. Any anion which will render the compound stable is suitable. Examples of such anions are BF - / 4, NO₃-, B(C₆H₅)₄-, SO₄²-, MoO₄²-, C1⁻, and Br⁻. These complexes may be made by reacting Cr(CO)₆ or chromium metal with sulfur and an appropriate source of bridging organo-carboxylates such as organo-carboxylic acid and organo-carboxylic anhydrides at a temperature and for a time sufficient to form the complex cation. The reaction may be conducted in an inert solvent; however, when the source of the bridging carboxylate is a liquid a solvent is not necessary. Indeed, it is particularly preferred to react the Cr(CO)₆ or chromium metal with sulfur and with a mixture of an acid and an anhydride. For example, especially preferred is a one-to-one mixture of acetic acid and acetic anhydride. The reaction also is preferably conducted at elevated temperatures such as the refluxing temperature of that of the source of coordinating bridging carboxylate. The reaction yields a mixture of cationic species which can be separated by generally known techniques. For example, these species are separable by ion-exchange chromatography. Moreover, elution with an appropriate source of anion will yield the complex cation in a form which can be crystallized as a salt by slow evaporation of the solvent, or by recrystallization from a suitable solvent. The following is an example of one species of the present invention and its method of preparation. EXAMPLE Cr₄S(O₂CCH₃)₈(H₂O)₄[(BF₄)₂·H₂O] was prepared by adding Cr(CO)₆ (2 grams, 9.1 millimoles) and elemental sulfur (0.3 grams, 9.4 millimoles) to 50 ml of a one-to-one mixture of acetic acid/acetic anhydride and refluxing the mixture for 2.5 hours. The reaction mixture then was diluted with 100 ml H₂O and adsorbed on an ion-exchange column containing DOWEX 50W-X2, a cation exchange resin containing sulfonated polystyrene sold by Dow Chemical Company, Midland, Michigan (DOWEX is a trademark). Elution with 0.1 M HBF₄ solution removed a green band of [Cr₃O(O₂CCH₃)₆(H₂O)₃]⁺. The blue band of [Cr₄S(O₂CCH₃)₈(H₂O)₄]²⁺ was removed with 0.5 M HBF₄ solution and the blue solution was placed in a dessicator for evaporation of the water. Upon evaporation of the water, dark blue crystals were obtained. These were recrystallized from methanol by slowly evaporating that solvent. Elemental analysis was as follows: Calculated for [Cr₄S(O₂CCH₃)₈(H₂O)₄(BF₄)₂·H₂O; C, 19.67; H, 3.48; S, 3.28; Cr, 21.31; F, 15.57. Found: C, 18.93; H, 3.46; S, 3.50; Cr, 20.48; F, 15.39. The compound also was analyzed by single crystal X-ray diffraction analysis techniques. The key parameters for determining the structure of the complex obtained in accordance with the present invention is shown in Table I. Parameters from Structure Determination of [Cr₄S(O₂CCH₃)₈(H₂O)₄](BF₄)₂·H₂O a, Å 21.066(4) b, Å 19.086(3) c, Å 9.323 α, deg 90.0 β, deg 90.0 α, deg 90.0 V, Å³ 3,748(1) z 4.0 Calculated Density, g/cm³ 1.73 Reflections with F>3σ 2,939.0 µ, cm⁻¹ 1.27mn⁻¹ sinϑ/λ (max) 0.650 R 0.048 R w 0.049 GOF 2.03 Atomic coordinates for non-hydrogen atoms are given in Table II. As is shown in the drawing, the complex cation of this invention consists of a central four coordinate sulfur atom bound to four chromium atoms which are in an approximate tetrahedral array. The chromium-chromium distances average 3.83 Å (range 3.72-3.91 Å). Each chromium atom is six-coordinated with a near-octahedral arrangement of five oxygen donors in the central sulfur atom. The one water ligand on each chromium lies trans to the sulfur atom. The remaining oxygen ligands of each chromium come from bridging acetate ligands. These are of two types: four acetate ligands are involved as two pairs of double bridges. These acetates are equivalent and related by an approximate S₄ axis. The overall symmetry of the cation is close to a D 2d while that of inner Cr₄S core is very close to T d . The structure shown is related to that of beryllium basic acetate, Be₄O(CH₃CO₂)₆ (see Charalombous et al., Inorg. Chim. Acta , 14 , [1975], although in this structure, the Be atoms are tetrahedrally four-coordinated rather than octahedrally six-coordinated, as are the Cr atoms in the chromium complex. Clearly, the complex of the present invention represents a new structural type containing a central four-coordinated sulfur atom, and four octahedral metal ions on the tetrahedral periphery. In addition to the unique structure of the complex of the present invention, it displays unusual magnetic behavior. The magnetic susceptibility χ versus temperature T was measured on polycrystalline samples from 13°mK to 300°K in magnetic fields, H, of 75 Gauss to 6.3k Gauss using SQUID, Faraday and vibrating sample magnetometers (VSM). In addition, magnetization versus H isotherms were measured at 1.3°K and 4.2°K using the VSM. These data prove that the ground state of the Cr₄ cluster has a spin S = 6, with a Landé g factor g = 2.00 ± 0.02. Electron spin resonance data at 15°K for the Cr complex frozen in solution yielded a very similar g value. The chromium compound of the present invention is useful, for example, as a thermometer at temperatures above 0.2°K. By measuring the magnetic susceptibility, χ, of the compound, the temperature can be determined from the following relationship: wherein χ o = -4.66 x 10⁻⁴ cm³/mole Cr₄, E = -JS(S+1) with J/k B = 14.5°K, G s = 4, 9, 11, 10, 6, 3 and 1 for S = 0, 1, ..., 6, respectively, N is Avogadro's number, g = 2.00 is the Landé g-factor, µ B is the Bohr magnetron, k B is Boltzmann's constant, and Z is the partition function: It should be understood that the foregoing disclosure, description and example are only illustrative of the invention. Various changes to the details of the invention will be apparent to the skilled worker, and may be made within the scope of the appended claims without departing from the spirit of the invention.

Claims: A tetranuclear sulfide-bridged complex of Cr(III) having bridging organo-carboxylate ligands. The complex of claim 1, wherein the four chromium atoms are bound to a central tetrahedral sulfur atom and are octahedrally coordinated with bridging carboxylate ligands. A composition of matter containing a cation having the formula: Cr₄S(RCO₂)₈(L)₄²⁺ wherein R is an alkyl group having from 1 to 12 carbon atoms; L is a ligand or ligands containing a donor atom selected from oxygen, nitrogen, carbon, and sulfur. The composition of claim 3, wherein RCO₂ groups represent the same organo-carboxylate. The composition of claim 3, wherein the RCO₂ groups represent a mixture of organo-carboxylates. The composition of any one of claims 3 to 5, wherein L is selected from H₂O, CH₃OH, and CH₃CN. The composition of any one of claims 3 to 6, further including a BF₄- anion. A method of forming a tetranuclear sulfur-bridged complex of Cr(III) having bridging organo-carboxylate ligands, which method comprises reacting Cr(CO)₆ or chromium metal with sulfur and a source of bridging organo-carboxylates. The method of claim 8, wherein chromium hexacarbonyl is employed and said source of organo-carboxylates is selected from organo-carboxylic acids, acid anyhydrides and mixtures thereof. The method of claim 9, wherein said source of organo-carboxylates is a mixture of carboxylic acid and its corresponding acid anhydride.

Inventors: BINO AVI, JOHNSTON DAVID CARL, STIEFEL EDWARD IRA, BINO, AVI, JOHNSTON, DAVID CARL, STIEFEL, EDWARD IRA
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1988
Main classification: C07F11
IPC codes: C07F11