Po/yhedron Vol. 9, No. 1, pp. 119-124, 1990 Printed in Great Britain

0277-5387/90 $3.00+.00 0 1989 Pergamon Press plc

A STUDY OF THE REACTION BETWEEN CHROMIUM(III) HEXAAMMINE AND GLYCINE IN A MELT

PAUL O’BRIEN* and NIGEL B. SELLERS

Department of Chemistry, Queen Mary College, University of London, Mile End Road, London El 4NS, U.K.

(Received 23 June 1989 ; accepted 17 August 1989)

Abstract-Powdered mixtures of glycine and [Cr(NH&](NO,), melt and react at tem- peratures between 130 and 180°C to produce a glassy, purple material [Cr(NH,), (02CCH2NH3)3](N03)3 (I), which on dissolution in water, yields fat-[Cr(glyO)J (II) as a precipitate. The initial reaction between the ammine and glycine had previously been reported to occur in the solid state. The method has been used to prepare fully deuterated II.

For some time we have been interested, both in the reactions of various coordination complexes in the solid statelA4 and in the coordination chemistry of chromium(II1). 5-8 We were consequently intrigued by the report of an alleged solid state reaction between powdered mixtures of glycine and [Cr(NH,),](NO,),, which could be used to prepare fac-tris-glycinatochromium(III)9 and, by exten- sion, a series of related tris-amino acidates.” We have re-investigated this reaction and our results show that the reaction proceeds in a melt and appar- ently stops after three molecules of ammonia have been lost. The dissolution in water of the glass obtained on cooling the reaction mixture leads to the precipitation of red II.9 The method can be used to prepare this complex on a small scale ; such syntheses are often a problem with chromium(II1). 7 The procedure has been used to prepare the com- plex from fully deuterated glycine, which may be a useful compound for *H NMR studies. ’ ’

RESULTS AND DISCUSSION

Initial experiments and observations

Preliminary experiments rapidly confirmed that heating a 3 : 1 mixture of glycine and [Cr(NH,),] (NO,), (142”C, 20 min) yielded a purple material I.

I was glassy on cooling to room temperature and

* Author to whom correspondence should be addressed.

was readily soluble in cold water. On standing over- night a precipitate of II was obtained. The fact that I appeared glassy led us to observe the course of the reaction using a hot stage microscope.

Mixtures of glycine and [Cr(NH,),](NO,), (3 : 1) were prepared by direct weighing and ground for 5 min in an agate mill. On heating (ca 5°C min- ‘) the mixture clearly melted at ca 135”C, just after the decomposition of the ammine had started. This experiment was repeated using a range of glycine to hexaammine ratios and melting was observed for compositions between 40 and 90 mol % of glycine. These results are summarized in Fig. 1 and Table 1. Clearly some kind of a reactive eutectic mixture is formed by mixtures of glycine and [Cr(NH,),] (NO,),, which is surprising as both of the com- pounds are polar species which decompose, char (glycine) or deaminate ([Cr(NH,),](NO,),), before melting. In isothermal experiments, on the hot stage microscope, we observed that there was a slight induction time, during which some reaction occurred before melting and these results are sum- marized in Table 1.

Thermal studies

Thermogravimetric analysis (10°C min- ‘, static air) of various samples of glycine and [Cr(NH,),] (NO,), confirmed that the reaction proceeded with the loss of three equivalents of ammonia under a variety of reaction conditions. Typical results are shown in Fig. 2. The reaction in the melt occurs at

119

120 P. O’BRIEN and N. B. SELLERS

i

1401 I I I

40 60 80 100

Idol percentage of glycine

Fig. 1. Melting behaviour of various mixtures of [Cr(NH,),](NO,), and glycine: n initial appearance of

liquid ; l final disappearance of solid.

a lower temperature than the deammination of the hexaammine (170°C) ‘* or the decomposition of gly- tine (245”(Z), when heated separately under similar conditions. Differential thermal analysis of mix- tures of glycine and the hexaammine (1O’C min- ‘, static air) resulted in the traces shown in Fig. 2. Two endotherms are observed, the second of these (extrapolated onset temperature 154°C) is con- current with the reaction in the melt. The first (extrapolated onset temperature 106°C) occurs

Table 1. Characterization of reaction in melt and zero-order rate constants

Time to Time to Temperature completion melt % Reaction 10’k

KW : W-1 (“C) (10’S) (s) in melt (s- ‘)

3:l 134 8.4 365 95.6 0.197 139 5.4 260 95.2 0.240 148 2.4 140 94.2 0.690 159 1.02 - 1.618

5:l 142 2.52 240 89.5 0.544 147 1.50 220 85.4 0.814 153 1.02 125 87.5 1.315

8:l 141 5.04 148 1.62 - 151 1.32 -

before melting and is independent of any mass loss. This may be associated with some sintering of the fine powders of glycine and the hexaammine ; no changes in the IR spectra of such samples are detected (4000400 cn- ‘).

Isothermal studies of the decomposition were then carried out to investigate the effect of varying the temperature and the composition of the reaction mixture on the reaction rate. The progress of the reaction was plotted as a vs time (~1, fraction of reaction, c1 = 1, 100% reaction, corresponding to the loss of three molecules of ammonia from each hexaammine). Figure 3 shows some typical results for various temperatures and compositions. The reaction profile is approximately linear (0.15 > a > 0.75) and attempts to fit the whole profile to successive first-order reactions produced ill- defined results. Reaction profiles were similar at temperatures between 134 and 15 1 “C and estimated zero-order rate constants for the reaction are sum- marized in Table 1. Rate constants derived from experiments under a wide range of conditions are shown on the Arrhenius plot, Fig. 4. The activation energy derived from this plot is ca 115 kJ mol- ‘. The reaction of the hexaammine with alanine was exactly similar. These results indicate :

(1) a zero-order rate, relatively independent of the amino acid concentration in the melt (Table 1, 1: 3-l : 8 ratios of chromium complex to glycine, Fig. 4) ;

(2) a zero-order rate, relatively independent of the amino acid used (Table 2) ;

(3) a fairly high enthalpy of activation and an entropy of activation close to zero.

Taken together the results suggest that the reaction in the melt may proceed by a dissociative reaction

Chromium(II1) hexaammine and glycine 121

I I I I I I 60 100 140 180

Temp. ( o C )

Fig. 2. Typical DTA and TGA, for a 3 : 1 [gly] : [Cr] mixture.

in which the rate-determining step is the loss of ammonia molecules from the ammine complex. Thus it resembles, in being dissociative, the reaction of glycine with [Cr(NH&H2013+, as studied by Sykes et a1.,13 in which water is lost dissociatively. Given the markedly different conditions of the reac- tion (melt vs aqueous solution) the activation pa- rameters are remarkably similar to those for the aquopentammine, an enthalpy of activation of 106 kJ mol- ‘, compared to our value of ca 115 kJ mol- ’ and an entropy of activation close to zero (Table 2). The linearity of plots of a vs time for the majority of the reaction suggests the ammonia molecules are lost at a similar rate from each of the intermediate complexes in the reaction.

IR and electronic spectra

The IR and electronic spectra are summarized in Tables 3 and 4. The IR spectra of the mixture before heating is simply an addition of the spectra of free glycine and the hexaamrm ‘ne nitrate, all of the bands can be assigned by reference to the literature (Table 3). After heating, the spectrum is considerably modified. The antisymmetric stretch of the COO- group is shifted from 1604 cm-’ in the mixture to 1659 cm-’ in the product and the new position is consistent with coordination of the carboxylate function to the metal. l4 New bands are also observed at 534, 518 and 412 cm-’ and may be assigned tentatively to M-O stretches. l4 The

0 1200 2400 3600

Time(s)

Fig. 3. Typical results of isothermal gravimetric experiments (calculated with cc = 1.00 for the loss of three mols of ammonia per hexaammine); (a) [gly] : [Cr] 3 : 1 139°C; (b) [gly]: [Cr] 3 : 1 145°C; (c)

[gly] : [Cr] 3 : 1 148°C; (d) [gly] : [Cr] 5 : 1 142°C.

122 P. O’BRIEN and N. B. SELLERS

arations (see Experimental) were feasible and deut- eration was confirmed by IR spectroscopy.

-9-

I I I I 2.30 2.34 2.38 2.42 2.44

IIT

Fig. 4. Arrhenius plot of the various zero-order rate constants: @ [gly]:[Cr] 3: 1; l [gly]:[Cr] 5: 1; 0

[gly]:[Cr] 8: 1.

The reaction between chromium hexaammine and glycine proceeds in a melt. Earlier suggestions that this was a solid state reaction are erroneous. The fact that this reaction proceeds in a homo- geneous phase explains why the reaction effectively stops after the reaction of three mols of glycine with the hexaammine, i.e. at the electrically neutral N303 chromophore (the overall complex is presumably charged due to pendant -NH: groups). This material I presumably ring closes in aqueous solu- tion to give substantial yields of the red fac- tris-glycinatochromium(II1) complex. The method is useful for preparing this complex on a small scale and has been used to prepare the fully deuterated complex.

modes of vibration associated with the NH: of glycine remain essentially unchanged ; 1587, 1142, 518 cm- ’ (Table 3).

The electronic spectra of various complexes with chromophores from CrNs to CrN303 are sum- marized in Table 3 ; the lowest energy d-d transition (4A ig + 4T,, in Oh symmetry) gives an approximate measure of the ligand field strength. I5 The initial product of the reaction, I, can be assigned as a CrN303 chromophore. The fact that the lowest energy transition is not split suggests a facial arrangement of the donor atoms. 5,‘6

Taken together with the thermo-analytical results the spectroscopic data support the formulation of the intermediate as a ring open facial complex of zwitterionic glycine and chromium(II1).

Preparation of deuterated [Cr(gly),]

The method proved to be a facile way of pre- paring fully deuteratedfac-[Cr(gly),] from solid DS- glycine and the hexaammine. Small scale prep-

CONCLUSIONS

EXPERIMENTAL

Materials

Reagent grade D,-glycine was purchased from Goss Chemicals, all other reagents were purchased from B.D.H. Chemicals Ltd. [Cr(NH&](NO& was prepared by the literature method. l7

Physical measurements

Electronic spectra were recorded with a Perkin- Elmer 330 spectrophotometer ; IR spectra were rec- orded on a Mattson Polaris FT-IR spectrometer; as Nujol mulls or KBr disks (ca 2%, 200 mg). Thermogravimetric studies were carried out using a Stanton Redcroft TG-750 (in static air). Differential thermal analysis was undertaken using a DTA 671 B, calibrated with high purity indium metal.

Table 2. Parameters derived from thermal analysis

AH” AHib ASzb

FlyI : KY (kJ mol- ‘) (kJ &-I, (kJ mol- ‘) (J K- ’ mol- ‘)

3:l 170 116 (12) 129 (11) - 1.33 5:l - 115 (12) 113 (8) - 38.33 8:l - 101 (40) 107 (15) -45.64

All data 115(16) - -

[Ala] : [Cr] 245 145 (23) 148 (19) 3:l

Errors are one standard deviation. s Per mol of chromium(II1). ‘Calculated as described in ref. 18.

Chromium(II1) hexaammine and glycine

Table 3. IR spectra

123

Literature Reaction Product glycine” mixture (glass) Assignment (PWIJ J’Y

1659 1604 1642

1627 > 1526 1587 1404 1406 1383 1385

1375 1362

1334 1355 1312 1312 1132 1142 1112 1132 1 1034 1038

949 911 916 893 832

826 758 754

717 698 694 608 668 504 599

578 534 ) 518 476 465 I 412

v,(COO-) 1610

&WI-V (1630)

&WG > 1585 v,(COO-) 1413

v.dNO; 1 13906

pw(CH 2) W’JH 3)

&NH:)

WCN) PACH J

VSWN)

PSNH 3) (748)

Pwwow 694 S(COO-) 607

PKOW 504

VOW (545)

ANH : ) 516

v@W (495,473,456)

v&W 415”

1333 (1307) 1131 1110 1033

910 893

‘For details of assignments see ref. 14, v, stretching, k, wagging, nr, rocking, h, twisting, 6, in-plane bending deformation.

b Band due to nitrate (see ref. 19), the reference ammine spectrum is the chloride salt, see a,

Methods

Mixtures ([gly]:[Cr], 3: 1; 5: 1; 8: 1; and [ala] : [Cr], 3 : 1) were prepared by direct weighing of the components, glycine or alanine and the hexa-

Table 4. Electronic spectra”

Compound Energy

[CrW-U13’ WN-I&13+/gly~ne

(1: 3 before heating) Purple glass (I)

(cooled to RT) fat-[Cr(glyO) 4

21.6 28.3 21.7 28.6

19.6 26.1

19.2 25.3

LI IO3 x cm-‘, by diffuse reflectance, for the two lowest energy bands.

ammine. The mixture was preground in an agate mortar and ground to a fine mixture in an agate mill (Speci-Mill, Specac) for 5-10 mm. This pro- cedure led to reproducible results in kinetic and other experiments. All mixtures were used immedi- ately and reaction was followed by thermo- gravimetry. In general, the mixtures used lost the equivalent of 3 mols of ammonia per mol of chro- mium. There was some slight evidence of further reaction in the 8 : 1 [gly] : [Cr] mixtures. Activation parameters were calculated as described by Edwards et al. ’ *

Preparation offully deuteratedfac-[Cr(gly,)]

[Cr(NH&](NO,), (0.43 g) and deuterated gly- tine (0.3 g) were ground together with pestle and mortar and then ground in an agate mill for 10 min.

124 P. O’BRIEN and

The mixture was placed in an oven for 45 min at 135°C. The glass formed was dissolved in deuterium oxide and the solution left overnight, after which time red crystals of deuterated fac-[Cr(gly)3] had precipitated. These were filtered off, washed with D20, and air dried. Yields, based on chromium, were 3&40%.

Acknowledgements-N.B.S. thanks the SERC for a research studentship. We also thank the University of London, Central Research Fund for their support of our work.

1. 2. 3. 4. 5.

6.

I.

REFERENCES

P. O’Brien, Polyhedron 1983,2,233. P. O’Brien, Transition Met. Chem. 1980,5, 314. P. O’Brien, Znorg. Chim. Acta 1981, 55, L27. P. O’Brien, Polyhedron 1986,5, 659. M. Abdullah, J. Barrett and P. O’Brien, J. Chem. Sot., Dalton Trans. 1984, 1647. J. Barrett, P. O’Brien and J. Pedrosa de Jesus, Poly- hedron 1985, 1,4. L. F. Larkworthy, K. B. Nolan and P. O’Brien, Com- prehensive Coordination Chemistry (Edited by G.

N. B. SELLERS

8.

9.

10. 11.

12.

13.

14.

15.

16.

17.

18.

19.

Wilkinson, R. D. Gillard and J. A. McCleverty), Chap. 35. Pergamon Press, Oxford (1987). M. Abdullah, J. Barrett and P. O’Brien, J. Chem. Sot., Dalton Trans. 1985,2085. H. Oki and K. Otsuka, Bull. Chem. Sot. Jpn. 1976, 49, 1841. H. Oki, Bull. Chem. Sot. Jpn. 1977, 50,680. W. D. Wheeler and J. I. Legg, Znorg. Gem. 1984, 23, 3798. W. W. Wendlandt and C. Y. Chou, J. Znorg. Nucl. Chem. 1964,26,943. T. Tamasami, R. S. Taylor and A. G. Sykes, Znorg. Chem. 1976,15,2318. K. Nakamoto, Infrared And Raman Spectra of Znor- ganic And Coordination Compounds, 4th edn, pp. 192, 234, and 245. John Wiley, Interscience, New York (1972). R. G. Wilkins and M. G. Williams, Modern Coor- dination Chemistry (Edited by J. Lewis and R. G. Wilkins), p. 187. Interscience, New York (1960). M. Watabe, H. Yano, Y. Odaka and H. Kobayashi, Znorg. Chem. 1981,20,3623. A. L. Oppegard and J. C. Bailar Jr, Znorg. Synth. 1950, 3, 153. J. 0. Edwards, F. Mona&Ii and G. Ortaggi, Znorg. Chim. Acta 1974, l&47. B. M. Gatehouse, S. E. Livingstone and R. S. Nyhohn, J. Chem. Sot. 1957,4222.