Anhydrous Chromium(III) Carboxylates:

Reactions of Cr03 with Carboxylic Acid Anhydrides

Ramesh Kapoor* and Ramneek Sharma Department of Chemistry, Panjab University, Chandigarh-160014, India Z. Naturforsch. 38b, 42-44 (1983); received August 28, 1982 Anhydrous Chromium(III) Carboxylates, Chromium(VI) Oxide, Carboxylic Acid Anhydrides, IR Spectra, Magnetic Moment

Chromium(VI) oxide reacts with excess of carboxylic acid anhydrides [(RC0)20 where R = CH3, C2H5, n-C3H7 and CHCI2] to give pure chromium(III) carboxylates. Their IR and 1H NMR spectra suggest the presence of two different types of carboxylate groups. Magnetic susceptibility and absorption spectra favour an octahedral geometry around Cr.

Introduction carried out in an identical manner. About 80% yield was recorded for each product. There have been very few reports in literature on Chromium was determined volumetrically by anhydrous chromium(III) carboxylates [1, 2]. The oxidizing Cr(III) to Cr(VI) with KBrOs- Any excess preparation of Cr(OOCCH3)3 was first reported in KBr03 was destroyed by heating the solution with 1911 [3] but very little information is available on (NÜ4)2S04 till the evolution of bromine vapours the nature of this compound. Solvolysis of OCI3 in completely ceased. Chlorine was estimated gravi- metrically as AgCl by fusing the compounds with anhydrous carboxylic acids gives basic trinuclear a mixture of Na2COs(AnalaR) and KOH. Carbon, chromium (III) carboxylates [4-6]. In this paper we hydrogen and nitrogen were determined micro- report the preparation of pure chromium (III) car- analytically. Infrared spectra were recorded as mulls in Nujol of hexachlorobutadiene between Csl boxylates, Cr(OOCR)3 (R = CH3, C2H5, n-C3H7 plates on a Perkin-Elmer 621 grating spectrophoto- and CHC12) by the reaction of Cr03 with appro- meter. The XH NMR spectrum was recorded on a priate carboxylic acid anhydride. Varian EM 390 spectrometer using TMS as external standard for solution in deuterated acetone. Thermogravimetric analysis was carried out on a Experimental MOM Derivatograph (type Paulik, Paulik and -1 Cr03(BDH, LR) was dried at 110 °C in vacuo for Erdey) at a heating rate of 10° min . 4 to 6 h. The carboxylic acid anhydrides were prepared by the reaction of sodium salt of carboxylic Results and Discussion acid [7] with the corresponding acyl chloride [8]. The anhydrides were distilled over P4O10 before use. The analytical results along with some of the physical properties are summarized in Table I. The Reactions of CrOz with carboxylic acid anhydrides compounds contain chromium in -f- III oxidation state which is indicated by their green colour and All reactions were carried out in a teflon lined steel pressure bomb** of 100 ml capacity. In a typical from their inability to oxidize acidic KI to I2. They experiment 0.1 mol of freshly distilled acetic an- are paramagnetic having room temperature mag- hydride was slowly added to 0.02 mol of dry OO3 in netic moments between 3.60 to 3.90 BM. These the reactor under dry N2 atmosphere. The reactor values lie close to the range 3.70 to 3.90 BM ob- was then placed in an oil bath and its temperature was slowly raised to about 140 °C. The temperature served for high spin chromium(III) compounds [9]. of the reactants was maintained around 100 °C for Chromium (III) acetate and propionate exhibit about 8 h. The contents of the vessel were then slightly low values and this may be attributed to allowed to cool to 0 °C. A green solid separated out the presence of some antiferromagnetic Cr-Cr inter- on the addition of CCI4 which was filtered, washed actions. with fresh CCI4 and dried in vacuo. Reactions of OO3 with other carboxylic acid anhydrides were These compounds are quite stable in air except for Cr(OOCCHCl2)3 which is sentitive to moist air. Chromium(III) acetate (1) is insoluble in all * Reprint requests to Dr. R. Kapoor. common organic solvents, whereas Cr(OOCC2Hs)3 ** These reactions are highly exothermic and are (2) and Cr(OOCC3H7)3 (3) dissolve only in methanol. accompanied by the evolution of gaseous products. 0340-5087/83/0100-42/$ 01.00/0 The dichloroacetate Cr(OOCCHCl2)3 (4) is soluble R. Kapcor and R. Sharma • Anhydrous Chromium(III) Carboxylates 43 in polar organic solvents such as nitrobenzene, bands present is much greater than in the spectrum nitromethane, acetonitrile, acetone and methanol. of the corresponding sodium salt. The increase in These compounds behave as non-electrolytes in the complexity of their spectra may be due to the methanol (Table I). The molecular weight of polymeric nature of these compounds. Bands at Cr(OOCCHCl2)3 in nitrobenzene agrees with its ca. 1610 and at ca. 1550 cm-1 may be attributed to monomeric formulation (Table I). va(COO~) whereas an absorption band at ca. Infrared spectra have been recorded and band 1410 cm-1 has been assigned to vs(COO~) stretching assignments for the carboxylate groups have been vibrations of the carboxyl group. The presence of made by comparison with the spectra of sodium more than one band which may be assigned to salts of the parent carboxylic acids [10. 11], (Table va(COO~) vibrations suggests that the three car- II). The carbon-oxygen stretching region of each boxylato groups are non-equivalent. A Av value of compound is complex as the number of absorption ca. 140 cm-1 is indicative of bidentate carboxylato

Table I. Analytical results and physical properties of anhydrous chromium(III) carboxylates.

Analytical Results* [%] M.p. Molar Magnetic Molecular Compound Colour Conductance** moment weight Cr Cl C H [°C] [ohm-1 cm2 mole-1] [BM]

Cr(OOCCH3)3 (1) Green 22.2 - 31.20 4.01 - - 3.58 (22.7) (31.44) (3.93)

Cr(OOCC2H5)3 (2) Green 19.4 - 38.46 5.12 145- 146 14.4 (CH3OH) 3.68 (19.2) (39.85) (5.53)

Cr(OOCC3H7)3 (3) Green 16.1 - 45.42 6.40 129- 130 19.7 (CH3OH) 3.70 (16.6) (46.01) (6.71)

Cr(OOCCHCl2)3 (4) Green 11.3 48.0 16.18 1.10 119- 120 14.0 (CH3NO2) 3.94 420 (11.9) (48.8) (16.51) (0.69) 33.6 (CH3CN)

* Required values are given in parentheses; ** required values for 1:1 electrolytes in CH3OH, CH3NO2 and CH3CN are in the ranges 80-115, 75-95 and 120-160 ohm-1 cm2mole"1, respectively [15].

Table II. Infrared spectral bands* (cm4) of anhydrous chromium(III) carboxylates.

Assignments Cr(OOCCH3)3 Cr(OOCC2H5)3 Cr(OOCC3H7)3 Cr(OOCCHCl2)3

COO asym. str. 1610 s, 1520 sb 1605 s, 1565 sb 1605 s, 1550 s 1650 s, 1560 s COO sym. str. 1410 s 1420 s 1420 s 1410 s CH2 Sym. bend. def. - 1460 m 1455 sh - -CH2COO (Adj. to COO) - 1440 sh 1440 sh - CH3 bend. def. 1445 sh, 1430 s, 1370sh 1375sh — 1340 s -CH2 wagging - 1290 m 1305 m, 1255 w, - 1200 w CH bend. - - - 1230 m CH3 rock 1040 sh, 1010 m 1010 w 960 w, 940 w - C-C skeletal 950 m, 855 w 1070 m,885 w 1090 m,1050 w, 970 m 890 m CH2 rock - 805 m 790 m, 750 w - CCI2 asym. str. - - - 830 s CCI2 sym. str. - - - 790 m COO def. 650 s 645 m 640 sb 680 w CCOO in plane bend. 720 m 720 m 720 m 720 m 1 672 s, 605 m 640 s - - CH or COO out of plane 620 w 620 m - 620 w CH def. or COO rock 475 m — 460 w - v and ö (Cr-O) modes 505 m, 430 s, 440 s, 405 s 440 m, 285 w, 540 m, 450 m, 400 s, 315 s 250 m 250 m 265 m

* Sb = strong and broads; s = strong; m = medium; w = weak; sh = shoulder. 44 R. Kapoor and R. Sharma • Anhydrous Chromium(III) Carboxylates group, while Av value of ca. 200 cm-1 is larger than high value of the va(COO~) band. The other two that expected for bidentate carboxylato group and carboxylato groups may act as symmetrical may indicate the presence of unidentate carboxy- bidentate ligands probably exercising chelating or lato group [12]. Two different structural possibilities bridging action. may be considered: The possibility that Cr in these compounds is (i) Chromium in these compounds may be tetra- hexacoordinate seems more likely as Cr (III) is coordinate, wherein four of the six oxygen atoms always known to be hexacoordinate [13]. This is of the three carboxylato groups are used to form supported by the UV visible spectra of these com- bonds. The three carboxylato groups would then pounds. Three bands at ca. 16,300 (4T2g(F)*-4A2g), be non-equivalent (one bidentate and two uniden- 23,300(4Tig(F) <-4A2g) and 41,700(4Tig(P)^4A2g) are tate) and thus lead to the splitting of the va(COO-) observed which are expected for Oh complexes for band. Cr(III) [14]. (ii) Alternatively, chromium in these compounds The possibility that there are two types of car- is hexacoordinate provided one of the carboxylate boxylato groups is also consistent with the XII NMR group is asymmetric with one Cr-0 distance normal spectrum of Cr(OOCCHCl2)3- It shows two peaks in and the other significantly longer. the ratio 1:2 at r 10.1 and 12.1 ppm, respectively.

The proton in Cr(OOCCHCl2)3 is highly shielded R which may be attributed to the presence of un- I paired electrons on Cr. /c\ A 0 0. TG-DTG curves of Cr(OOCCH3)3 and / \ / Cr(OOCC2H5)3 show that the compounds decom- Cr Cr pose in a single step to Cr203. They begin to lose The carbon-oxygen bonds can show different weight on heating above 45-50° and their residue degree of bond order which is consistent with the attains constant weight around 450 °C.

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