Vol. 7, No. 4 , April 1968
AMINECOMPLEXES OF CHROMIUM(III)749 CONTRIBUTION No. 2176 FROM THE DEPARTMENT OF CHEMISTRY, UNIVERSITY O F CALIFORNIA, LOS ANGELES,CALIFORNIA 90024
Synthesis of a Monodentate Ethylenediamine Complex of Chromium(II1) and Kinetics of Hydrolysis of Tetraaquoethylenediaminechromium(II1)Cation and of Pen taaquo(2- aminoethy1ammonium)chromium(111) Cation'"," BY R. F. CHILDERS, JR., K. G. VANDER ZYL, J R . , ' ~D. A . HOUSE, R . G. HUGHES,
AND
C. S. GARNER
Received Nouember 15, 1967 The new complex Cr(enH)(OH2)b4+has been synthesized from Cr(en)(OH2)d3+and isolated in aqueous solution; the visible absorption spectrum has been obtained. This is apparently the first isolation of a chromium(II1) complex with a monoand of aquation of the dentate ethylenediamine ligand. The kinetics of hydrolysis of C ~ ( C ~ ) ( O Hto~Cr(enH)(0H2)h4+ )~~+ pentaaquo complex to Cr(OHz)e3+in the dark have been studied spectrophotometrically in 0.03-3 F HClO4 ( p = 0.15-3 M , XaC101) a t 60,80,85, and 90". At 59.90' the first-order rate constant kl for hydrolysis of Cr(en)(OH2)43+in 3.0 F HClO4 is 0.2) X loT6see-', E, = 27.7 f. 1.5 kcal mole-', and log PZ (see-1) = 13.7 f. 1.1; acid concentration and ionic (3.0 i strength dependencies are small. A t 59.90' the first-order rate constant kz for aquation of Cr(enH)(OHz)s*+to Cr(OHz)e3+ see-', E, = 25.4 & 2.3 kcal mole-', and log PZ (see-I) = 10.9 f. 1.5; a t 6O-9Oo, kz is in 3.0 F HClOa is (1.9 i:0.3) X essentially independent of HClOa concentration from 0.03 to 3 F a t y = 3 M a n d is independent of p from 0.15 to 3 M in 0.1 FHClOI.
Introduction dynamic interpretations referred to above. The availability of solutions of the pure tetraaquo complex The tetraaquoethylenedianiinechromium(II1) cation, prompted us to examine directly the kinetics of its Cr(en) (OHJb3+, has been h y p o t h e s i ~ e d ~ ~ as an hydrolysis and to attempt to isolate the intermediate intermediate in the hydrolysis of cis-diaquobis(ethy1conversion of the tetraaquo complex to the enediamine)chromium(III) cation, c i ~ - C r ( e n ) ~ ( O H ~ ) ~in ~ +the , hexaaquo complex. in acid solution to hexaaquochroniium(II1) cation, We report here the successful chromatographic isoCr(OHs)63+,based on interpretation of the changes in lation of the intermediate, pentaaquo(2-aminoethylamthe visible absorption spectrum of the reaction solumonium) chromium (I1I) cation, Cr (enH) (OH2)b4+, in tion. A second intermediate hypothesized in the hyaqueous solution and its visible absorption spectrum. drolysis was described2 as a complex in which one end So far as we know, this is the first isolation of a cornof the ethylenediamine ligand became free, the vacated plex of chromium(II1) with a monodentate ethylcoordination site of the central chromium atom then enediamine ligand. We also report the results of our becoming occupied by a water molecule. Neither kinetic studies of the aquation of this intermediate to intermediate was isolated, although attempts were the hexaaquo complex and of the hydrolysis of the made2i3 to deduce the visible absorption spectrum of tetraaquo complex. the tetraaquo complex from the spectral data. An attempt was made to obtain rate constants for aquaExperimental Section tion of the tetraaquo and the monodentate ethyleneTe traaquoethylenediamiechromium( 111) Cation .-The diamine complexes2 and the equilibrium constants method of Hoffman,' as modified by House and Garner,b was used for what were assumed to be equilibria involving the to prepare [ C r ( e n ) ( O H ~ ) ( 0.HzO. ~ ) ~ ] The diperoxo compound, used fresh or after storage in the dark a t 1-5" for up hexaaquo, tetraaquo, and diaquo complexes. to 3 weeks, was converted to Cr(en)(OH~)4~+ by a modification I n 1965, House and Garner4)6synthesized tetra* of the method given e a r l i e r . * ~ ~Cu. 1.9 g of [Cr(en)(OHz)(Oz)~!] aquoethylenediaminechrornium(II1) cation by decomHzO was dissolved in small portions (cautionb)in 80 ml of 2 F position of diperoxoaquoethylenediaminechrornium(1V) HC104 (or sometimes 0.5 g in 50 ml of 1 F HC1O4)and heated a t monohydrate, [Cr(en)(OHn)(0z)2] .HzO, in 0.5-3 F 50" for 2 hr with occasional stirring; then the pink-red solution was used fresh or stored up to 3 weeks in the dark a t 1-5' before perchloric acid and obtained the visible absorption dilution a t 0" with water, NaOH, and/or KaC104 solutions for spectrum. This was similar to the spectra hypothkinetic runs in 1 F HCIOl or lower acidity. For kinetic runs of esized earlier2t3 but was sufficiently different to acidity greater than 1 F HC104, each Cr(en)(OH~)4~+ solution raise doubts of the validity of the kinetic and thermowas chromatographed as follow^.^ Ca. 10 ml of C r ( e n ) ( O H ~ ) 4 ~ + (I) (a) Work partly supported under Contract AT(ll-1)-34, Project No. 12, between the U. S. Atomic Energy Commission and the university. This paper constitutes Report No. UCLA-34P12-67 t o the AEC. (b) Presented a t the 155th National Meeting of the American Chemical Society, San Francisco, Calif., March-April 1968. (c) National Science Foundation Undergraduate Summer Research Participant, 1965. ( 2 ) H. L. Schlafer and R. Kollrack, Z.Physik. Chem. (Frankfurt), 18, 348
(1958). (3) M. Ohta, H. Matsukawa, a n d R. Tsuchiya, Bull. Chem. SOC.J a p a n , 57, 692 (1964). (4) D. A. House a n d C. S. Garner, Inorg. Nucl. Cheni. L e l l e v s , 1, 137 ( I 9 65). (5) 13. A. House and C. S. Garner, I n o v l . Chcm., 6 , 840 (1966).
(6) The abbreviation enH is used to denote a monodentate ethylenediamine ligand with its free N atom protonated: -NH2CHL!H2NH3+. (7) K. A. Hoffman, B e v . , S B , 3181(1906). (8) TKe diperoxo compound is light sensitive and explodes a t 96-97O if heated at Zo/min.' Considerable effervescence occurs in the dissolution in HCIOI. Shielding should be used as a precaution despite the fact t h a t we have never had any difficulty with the procedure. (9) I n preliminary experiments with chromatographed and nonchromatographed Cr(en)(OH&3+ small differences (up to & 2 mp and ca. 10% in molar absorbancy indices of the absorption bands) were observed in their visible absorption spectra, and chromatography was used subsequently where possible as a routine precaution to obtain pure Cr(en) (0Hi)aa +.
750 CHILDEKS,VANDERZYL, HOUSE,HUGHES, AND GARNER (ca. 0.1 F in complex and 1-2 F i n HClO,) was diluted with 10 ml of water and charged onto a n 8-cm X 1-cm diameter column of H+ Dowex BG50W-X8 cation-exchange resin (100-200 mesh) operated at 20-25’ with flow rates of ca. 5 ml/min. To remove any Cr(OHz)s3+possibly present, 50 ml of 1.5 F HCI04 was run through the column and discarded. Ca. 50 ml of 2 F HC104 was used next to bring the Cr(en)(OHy)43+ to the bottom of the resin column. An additional 125 ml of 2 F HCI04eluted perhaps 75% of the C r ( e n ) ( O H * ) P ; a center cut obtained after discarding the first 25 ml of effluent was used a t once for kinetic runs, after adjustment of acidity and ionic strength. The visible absorption spectrum was recorded routinely as a purity check. Pentaaquo( 2-aminoethylammonium)chrornium( 111) Cation.This complex was synthesized and isolated as follows. Nonchromatographed Cr(en)(OH2)a3+,prepared as above in 2 F HCIO4, was aged ca. 15 hr (ca. 2 half-lives) at 80” in the dark; 25 nil of the aged solution was diluted with 25 ml of water and charged onto a fresh cation-exchange column of the kind described above. Perhaps 80% of the Cr(OHz)s3+produced (grayblue band) was eluted with 50 ml of 1.5 F HCIO
