119
REACTIONS OF COORDINATION COMPOUNDS IN THE SOLIDSTATE "OFF-ON" process (Figure 3A), the current increased rapidly with increasing temperature, and a large peak appears at - 131', accompanied by a small shoulder a t - 142". Then other positive peaks appear a t - 122' and -112" and hence the current increases up to the melting point. I n the case of the "ON-ON" process (Figure 3B), positive pealis are also observed a t - 142, - 131, - 122, and - 112" as in the case of the "OFF-ON" process. The current then increases with increasing temperature, coinciding with that of the "OFF-ON" process near the melting point. I n the case of the "ON-OFF" process (Figure 3C), only negative peaks are observed a t -160, -131, -122, and -112". The peaks a t -131 and -112' are in corre-
spondence to the crystal-crystal transition points which have been identified by therma11*14-16and X-ray analysi~.*7!1~ The transition points a t -131 and -112' correspond to the transformations from the unstable phase to stable phase, namely a! to p a t - 131" in a lower temperature region and p to a! at -112' in a higher temperature region, respectively, as was reported earlier.10~17*18 A transition at -142' was also reported earlier;17*18 in the present study this peak was irreproducible and depended significantly on the purification process of AN. The peak a t -122" was irreproducible in both the appearance temperature and the intensity. The present study cannot tell whether this peak results from a transition phenomenon or not.
Reactions of Coordination Compounds in the Solid State. The Racemization of (+)-[ Co (en),]X, nH2O1&,b by Charles Kutal and John C. Bailar, Jr.* Department of Chemistry and Chemical Engineering, University of Illinois, Urbana, Illinois 61801 (Received August 2, 1971) Publication costs assisted by T h e Advanced Research Projects Agency
The solid-state racemization of (+)-[C0(en)~]X~.nH~0 (where X = C1-, Br-, I-, NCS- and n = 0, 1) has been investigated. The hydrated iodide and bromide complexes were found to racemize considerably faster than their anhydrous analogs. This rate enhancement results from the physical modification of the lattice during dehydration rather than a displacement of one end of an ethylenediamine ligand by a water molecule (aquation). The actual mechanism of racemization is attributed to a twist process, an assignment which is consistent with the observed sequence for the racemization rates of the anhydrous complexes, X = I- > Br- > NCS- > C1-. Taken together with previously published studies, the present results suggest that lattice water can play a dual role in the solid-state reactions of metal complexes,
Introduction The earliest r e p ~ r t s z -of~ the racemization of coordination compounds in the solid state concerned salts of the complex ion [iIf(C2O&]*-(If = (Co(II1) or Cr(II1)). It was noted that lattice water in a sample enhanced the loss of optical rotation upon heating; a more recent study5 of K3(+)-[Cr(Cz04)3] a2HzO has confirmed this result. A similar effect is found during the racemization of (+)- [Cr(en)2Clz]C1.Hz06 and isomerization of trans- [CO(NH&CI~]IO~.~H~O,' suggesting that lattice water generally plays an important role in the solid state reactions of metal complexes. One explanation of this effect is provided by the mechanism below (shown for the racemization of a trischelate complex).8
A
Jr
(+ 1- [Jf(AA)3 I .HzO (+)-[M(AA)3]
+ H2O (dehydration)
(1)
(1) (a) Based on the doctoral dissertation of C. Kutal, University of Illinois, 1970; (b) en = ethylenediamine. (2) C. H. Johnson and A . Mead, Trans. Faraday SOC., 29, 626 (1933). (3) C. H. Johnson, ibid., 31, 1612 (1935). (4) C. H. Johnson and A. Mead, ibid., 31, 1621 (1935). (5) D. M. Chowdhury and G. M . Harris, J . f h y s . Chem., 73, 3366 (1969). Under extreme pressure, however, the presence of lattice water retards racemization. Cf. C. D. Schmulbach, J. Brady, and F. Dachille, Inorg. Chem., 7, 287 (1968). (6) H. E. LeMay, Jr., and J. C. Bailar, Jr., J. A m e r . Chem. Sac., 90, 1729 (1968). (7) H. E. LeMay, Jr., and J. C. Bailar, Jr., ibid., 89, 5577 (1967). (8) A similar mechanism is applicable to [M(AA)Xz] complexes; in this case, however, water displaces a monodentate ligand (X) in eq 2.
T h e Journal of Physical Chemistry, Vol. '76,ilia. 1 , 1972
120
CHARLES KUTALAND JOHN C. BAILAR, JR.
IL
'OH21
r A + +
(=t)-[N(AA),]
+ H 2 0 (ligation)
(3)
According to this mechanism, racemization occurs via eq 2 and/or eq 3, with lattice water assuming the role of a reactive species capable of coordinating to the metal atom. Since reactions like those shown in eq 2 and 3 are also thought to cause the racemization of (+)- [Cr(C204)3]3-g and (+)- [Cr(en)zClz]+lo in solution, it appears that for these complexes the same mechanism is operative in two phases. This situation undoubtedly does not exist in all cases, however. The mechanism shown above, for example, becomes less likely in the solid state relative to solution as the chargr on the complex increases, since the absence of solvation in the transition state should inhibit the breaking of the metalligand bond (eq 2). For this reason a twist mechanism'l might be favored. To explorc this possibility, the solid-state racemization of several (+)- [Co(en),]X3 complexes has been studied. Both anhydrous and hydrated samples were examined to note the effect of lattice water on the observed kinetics. Experimental Section Preparation of Complexes. a. (+)- [Co(enj3]13and (+)- [ C ~ ( e n ) ~.HzO. ] l ~ Racemic [ C ~ ( e n ) ~ ]was C l ~prepared by the procedure of Work12 and resolved as the chloro tartrate by the method of Busch.13 The optically active material was converted into the iodide salt by metathesis with sodium iodide. Recrystallization from an aqueous solution yielded crystals of the monohydrate, which could be dehydrated by heating in vacuo a t 55" for 2 hr.14 A sample of the anhydrous complex was doped with Co2+by being placed between pieces of filter paper moistened with a 0.1 M aqueous solution of CoC12. b. (+)-[Co(enj3]Br3 and (+)-[Co(en)3]B~3.HaO. The resolved chloro tartrate complex was dissolved in a dilute hydrobromic acid solution and precipitated with absolute ethanol, and the resulting solid was recrystallized from an approximately 6 Ad HBr solution. An aqueous solution of the solid was slowly evaporated in a vacuum desiccator, resulting in crystals of the monohydrate complex. A portion of this sample was dehydrated by heating a t 80" for 2 hr in vacuo.14 The Journal of Physical Chemistry, Vol. 76, No. 1, 1978
c. (+)-[Co(en)3lCl3 and (+)-[Co(en)3]CZ3.Hz0. The resolved chloro tartrate complex was dissolved in hot concentrated hydrochloric acid and reprecipitated with absolute ethanol. A portion of this material was dissolved in water and left to crystallize in a vacuum desiccator. Although the compound has been formulated with 2 mol of water of crystallization, l5 the present method of preparation yielded a complex containing only one. The remainder of the material precipitated from the hydrochloric acid solution was dehydrated by heating at 80" for 24 hr in a vacuum oven.14 d. (+)-[Co(en)3](NCX)3. This compound was prepared by the method of Lenlay. l6 The various samples of the optically active complexes are listed in Table I; also included are the particle size and specific rotation. The 20-40 mesh sample was prepared by gently breaking larger crystals with a pestle, passing these through a 20 mesh screen, and catching the suitably sized particles on a 40 mesh screen. Samples denoted as 200-325 mesh were passed through a 200 mesh screen with the aid of a pestle and caught on a 325 mesh screen. The general effect of the pestle was to crush the crystals. The (200-325 mesh ) > (40% racemization but no changr in its powder pattern. When (+)- [ C o ( ~ n ) ~ ] C H1O l ~ . was dehydrated, its powdrr pattern remained unchanged, suggesting that the prescnce of hydrate water was not a major factor in stabilizing the crystal lattice. Heating (+)-[Co-
REACTIONS OF COORDINATION COMPOUNDS IN THE SOLIDSTATE ~~
~~~~~
Table 111: T h e Prominent Lines iii the X-Ray Powder Photographs of Optically Active [Co(en)g]Br3.niHd3 95' vac
(4- )-[Co(en)a]Br. HzO -+ 1 hr 127'
(+)-[Co(en)dBrg
a. K 7.06 (m)b 6.78 (vw) 6.43 ( m ) 5 . 3 7 (w) 4.97 ( m ) 4.77 (m) 4 . 3 1 (w) 4.17 (s) 3.71 (w) 3.44 (w) 3.24 (s) 3 . 1 5 (w) 3 . 0 9 (w) 2.96 (w) 2.75 (w) 2 . 6 8 (w) 2.51 ( m ) 2.46 (w) 2.35 (vw) 2.18 (w) 2.12 (vw) a
d,
I
d,
6.76 (s) 6.49 (w) 5.97 (w) 5.02 ( V W ) 4.37 ( m ) 4.10 (s) 3 . 9 8 (w) 3.64 (w) 3 . 5 1 (w) 3.36 (m) 3.26 (m) 2.98 (m) 2 . 6 0 (w) 2.33 (vw) 2.25 (vw)
Sample >40% racemized.
s = strong, m = medium,
1217rac-[Co(en)3]Br~
w
6.76 (s) 5 . 9 8 (w) 5 . 0 3 (w) 4 . 3 6 (w) 4 . 1 1 (s) 3 . 9 9 (w) 3.66 (w) 3.53 (w) 3.38 ( m ) 3.28 (in) 2 . 9 9 (m) 2.61 (w) 2 . 3 4 (vw) 2.25 (VW)
* Intensities estimated = weak,
vw
visually :
= very weak.
(~II)~](NCS), at 146.3" for 11 hr caused no change in its powder pattern other than a slight line broadening, even though the sample lost 60% of its optical activity. Finally, the powder patterns taken before and after the dehydration of (.t)-[Coen,]X3-HzO (X = I- arid Br-) are different, while those of (A)-[Co(en)3]Cl3. 2.5H20 show no change. There is little correspondence between the powder patterns of analogous racemic and optically active complexes. This result is expected, since a racemate generally possesses the higher crystal symmetry. More significantly, the powder pattern of a sample of the anhydrous iodide or bromide complex which has lost >40% of its optical activity by heating contains no new lines characteristic of the corresponding racemic complex. This indicates a difference in crystal structure between a racemic sample crystallized from solution and one formed from the optically active complex by heating in the solid state.
Discussion The marked effect of lattice water on the rates of racemization of (+)- [Co(en)3]13.H 2 0 (Figure 1) and (+)- [Co(en)3]Br3.H2O (Figure 2) has several possible explanations. If the aquation-ligation mechanism (eq 1-3) mentioned earlier were responsible, increasing the concentration of water in the system should accel-
125
erate the loss of rotation. No effectwas observed, however, when a sample of the iodide salt was heated with excess water vapor in a sealed tube. In fact, an aqueous solution of the complex had a lower rate of racemization at 127" than the corresponding crystal hydrate. This is exactly opposite to the behavior observed for complexes reported to exhibit an aquation-ligation mechanism.21 For example, (+)- [Cr(en)&12]+ racemizes in aqueous solution a t 25' with a half-life of approximately 1 hr,1° whereas in the solid state only a few per cent racemization occurs a t 140" during the same interval.6 Likewise, solid K3(+)-[Cr(C204),] 2H20 must be heated at 23606before racemizing a t a rate comparable to that of an aqueous solution of the complex at E I O ' . ~ ~ The results for the iodide complex are therefore not readily explainable in terms of a mechanism involving metal-ligand bond rupture. Some similarities exist between the present results and those reported for the solid-state exchange of cobalt(II1) complexes doped with radioactive Co2+.2 3 The plots of per cent exchange os. time resemble the curves shown in Figures 1 and 2 ; in addition, lattice water accelerates the rate of exchange. It was suggested that free electrons in the crystal are captured by Co2+ to form excited Co+ (or Co), which instantaneously reacts with a neighboring complex resulting in exchange. The higher dielectric constant of the hydrated crystal presumably lowers the energy gap between the forbidden and conduction bands in the crystal and thus increases the supply of free electrons at a given temperature. It is conceivable that exchange of a catalytic amount of Co2+ could lead to steric rearrangement^^^ and thus be responsible for the racemization observed in the present case. In the absence of labeling studies, however, two pieces of evidence can be cited against this mechanism. First, crushing a sample, which is linown to produce electrons in a solid,23decreased the rate of racemization. Second, doping Go2+ into a sample did not enhance the loss of rotation (in fact, the rate decreased). The most probable mechanism for the solid state racemization of the iodide, bromide, and thiocyanate complexes is an intramolecular twist." The high charge on the cation, the large ligand field stabilization energy (LFSE) possessed by cobalt(III), and the lack of solvation energy make this a particularly favorable (21) Although care must be exercised when comparing rates between two phases, the preference for a bond rupture mechanism in aqueous solution appears reasonable. The mole ratio of water molecules to complex ions is considerably higher in solution than in the crystal lattice, thus leading to a higher probability of reaction. Also, in solution the possibility exists for solvation of the incipient charge separation in the transition state caused by the breaking of the metal-ligand bond. (22) E. Bushra and C. H. Johnson, J . Chem. Soc., 1937 (1939). (23) A. Nath, 8 . Khorana, P. K. Mathur, and S. Sarup, Indian J . Chem., 4, 51 (1966). (24) N. Saito, H. Sano, T. Tominaga, F. Ambe, and T. Fujino, Bull. Chem. SOC.Jap., 35, 744 (1962). T h e Journal of Physical Chemistry, Vol. 76, No. 1, 1072
CHARLES KUTALAND JOHN C. BAILAR,JR.
126
pathway.25 The lower rate of racemization in solution may mean either that a different mechanism (presumably bond rupture) is occurring, or the twisting motion is inherently slower than in the solid. There is insufficient evidence to decide which is more likely, but the former explanation receives some support from the recently reported isolation of a monodentate ethylencdiaminecobalt (111)complex.26 Three factors contribute to the activation energy of the twisting motion in the solid state: (a) changes in LFSE, (b) changes in ligand-ligand repulsion, and (c) changes in the interaction between [ C ~ ( e n ) ~and ] ~ +the lattice anions. If a and b are considered essentially constant within the present series of complexes, differences in the rates of racemization must arise from e. To understand fully thc nature of the interactions between the ions in the lattice, an accurate crystal structure is needed for each of the hydrated and anhydrous complexes. Except for ( + ) - C ~ ( e n ) ~ ] BHz0,27J8 r~' however, this information is lacking. Nevertheless, it is interesting to note that the observed sequence for the rates of racemization, X = I- > Br- > NCS- > C1-, lies in the order expected for decreased hydrogen bonding between the protons on the nitrogen atoms and the anions in the lattice29(assuming N-H . . . N bonding for thiocyanate). As the strength of the hydrogen bonds which must be broken decreases, the energy of activation for a twist process will similarly decrease. The importance of hydrogen bonding in determining the conformations of metal-chelate rings in a crystal has recently been discussed.3O Since the influence of lattice water on the rate of racemization does not result from chemical combination with the complex (aquation), it must instead involve the physical modification of the crystal lattice during dehydration. Visual observations on single crystals of ( +)-[Co (en)3 ] 1 3 .HnO and (+)-Cl(en) 3]Br3 HzO reveal that the loss of lattice water is accompanied by extensive cracking, indicating the introduction of a large number of defects into the solid. Since the complex ions near a defect are known to experience enhanced reactivity,31their inversion is expected to be rapid. The rate accelerating effect continues until the cessation of dehydration, a t which point the rapid production of defects also ends. Further racemization is then effected a t a lower rate by the remaining optically active ions. This latter process lvould be cxpectcd to obey a firstmorder rate expression if the ions had an equal probability of inversion, Such bejlavior has been reported for the racemization of I
