Crystalline-state reaction of cobaloxime complexes by X-ray exposure. XIV. Uneven racemization at the independent reaction sites

Abstract

Introduction Crystals of [(R)1-cyanoethyl][tri(p-chlorophenyl)phosphine]cobaloxime [cobaloxime = bis(2,3butanedione dioximato)cobalt(I I I) ], C29H30C13 CoNsO4P, [Co(C3H4N)(C4H7N202)2(C~sHI2C13P)] , Mr=7088, reveal crystalline-state racemization of the optically active cyanoethyl group on exposure to X-rays (Mo Ka, A = 0.71069 A). At the initial stage, the crystal is triclinic, space group P1, with a = 11335(4), b=11.331(4), c=13.717(4) A, a = 94.96(8) , /3=83.73(5) , y=114.38(4) °, V=1593(1) /~3, Z = 2 , D x = l . 4 7 8 g c m -3, / z=8 .17cm -I, F(000) = 728 and T = 293 K. Changes in the unit-cell dimensions with exposure time are small, and after about 700 h the values become constant. From the intensity data collected for five stages during the change, the structure at each stage was determined, including the occupancy factors of the original and inverted cyanoethyl groups (R =0.042, 0.048, 0.048 and 0.053 for stages I, II, III and IV, respectively). The structure analysis showed that the rate of inversion of the cyanoethyl group in one of the independent molecules is much greater than that of the other. With advancing racemization, a pseudo glide plane between the two independent molecules becomes a crystallographic one and in the final stage the crystal is monoclinic, space group Cc, with a = 19.033(8), b = 12.308(3), c= 13.718(3) A, /3=96.64(4) °, V= 3192(2) ,~3, z = 4, Dx = 1.475 g cm -3 and R = 0.052. At the two symmetry-related sites, the population ratios of (R)and (S)-cyanoethyl groups are 74(1):26(1) and vice versa. This characteristic racemization is discussed in terms of cavities [Ohashi, Uchida, Sasada & Ohgo (1983). Acta Cryst. B39, 54-61]. *To whom correspondence should be addressed. Present address: Department of Chemistry, Faculty of Science, Ochanomizu University, Otsuka, Bunkyo-ku, Tokyo 112, Japan. Serial studies on the crystals of [(R)-l-cyanoethyl]cobaloxime complexes with different base ligands have proved that in these crystals the inversion of the optically active cyanoethyl group (hereafter abbreviated to cn) occurs without degradation of the crystallinity (Ohashi & Sasada, 1977; Ohashi, Yanagi, Kurihara, Sasada & Ohgo, 1981, 1982; Ohashi, Uchida, Sasada & Ohgo, 1983; Uchida, Ohashi, Sasada, Ohgo & Baba, 1984; Tomotake, Uchida, Ohashi, Sasada, Ohgo & Baba, 1985; Ohashi, Tomotake, Uchida & Sasada, 1986). The lattice structure kept during the reaction makes possible detailed investigations of the movement of the reactive group and the effect of the surrounding molecules on it. Considering these advantages, we call such a characteristic solid-state reaction a 'crystalline-state reaction'. Depending on the molecular arrangement in the crystal, several complicated reaction modes have been observed (Kurihara, Uchida, Ohashi, Sasada, Ohgo & Baba, 1983; Tomotake et al., 1985; Ohashi et al., 1986). For these modes, we have defined, as a first approximation, a free space for movement of the reactive group as a 'cavity' and found a positive correlation between the cavity volume and the reaction rate (Ohashi et al., 1983). The present paper deals with the crystalline-state reactions of [(R)-l-cyanoethyl][tri(p-chlorophenyl)phosphine]cobaloxime, the crystals of which belong to space group P1 and in which two independent molecules in an asymmetric unit are related to each other by a pseudo glide operation. The reaction mechanism is discussed in terms of cavities.