This invention relates to a process for the preparation of olated chromium (III) complexes and to certain novel complexes prepared thereby.
Formation of fumarato chromium (III) nitrate complexes from chromium (III) hexaaquo nitrate has been disclosed in the applications of A. J. Deyrup, Ser. Nos. 25,097, filed Apr. 2, 1970 and now abandoned, 119,608, filed Mar. 1, 1971, and 210,833, filed Dec. 22, 1971, all assigned to the assignee of the present application. It is recognized in the Deyrup applications that dimeric or polymeric complexes can be formed in the process of his invention, which involves repeated boiling in water of fumaric acid and hexaaquo chromium nitrate, together with a careful pH adjustment at each stage. While dimeric or polymeric complexes undoubtedly are formed in that process, the technique is lengthy and cumbersome. The chromium complexes prepared by Deyrup are useful agents for coupling glass, metals, or metal oxides to resins.
Dimerization of chromic salts by boiling in aqueous solutions is explained by Stiasny ("Gerbereichemie", Steinkopff, Dresden, 1931) as resulting from "olation", as follows: EQU 2Cr(H.sub.2 O).sub.6 .sup.+.sup.+.sup.+ .revreaction. 2H.sup.+ + 2Cr(H.sub.2 0).sub.5 OH.sup.+.sup.+ .fwdarw. (1) ##EQU1##
Hall and Eyring (J. Am. Chem. Soc. 72, 782-90, 1956) observed that the formation of oxygen bridges is facilitated by addition of ethanol to a solution of chromic nitrate before refluxing. Hall and Eyring favor formulas such as (3) or (4), below, for the plated chromium compounds: ##EQU2##
Although the above authors studied quite thoroughly the kinetics of olation, their standard procedure involved refluxing aqueous chromic salt solutions for 60 hours to obtain the maximum number of oxygen bridges. In some cases, Hall and Eyring refluxed chromic salt solutions in 90% ethanol and noticed a change in color as well as increase in acidity after only a few minutes. Rollinson observes in Bailar "Chemistry of the Coordination Compounds", 1956, p. 458, that Hall and Eyring's paper suggests that the alcohol competes with chromium for the hydroxo groups as well as the aquo groups, thereby leading to the loss of protons from the ol bridges and the formation of oxo bridges. This is oxolation at the expense of olation.
It can be shown that at room temperature, only a very small amount of olation occurs in a solution containing even as little as 10% of water. Moreover, at elevated temperatures, e.g. at reflux, undersired reactions, such as reduction of the nitrate by the alcohol, can occur. It is, therefore, undesirable to prepare olated chromium (III) nitrate complexes under the conditions used by Hall and Eyring.