Biguanide metals

Complex compounds of biguanides and guanylureas with metallic elements. P. Ray, Chem. Rev., 1961,61,313-359(188). 

A discussion of metal complexes of biguanides has recently formed part of m extensive review (532) on co-ordination compounds, so that this subject need not be considered here in detail. As in the past, Indian laboratories continue to be the main source of publications on this aspect of biguanide chemistry (38, 193, 195, 281, 282, 300, 435, 527, 528, 529, 533, 586, 649, 650). 

The numerous highly coloured metal complexes obtainable from biguanides and elements of the transition series have posed yet another interesting structural problem. The accumulated evidence suggests that the structures agreeing most satisfactorily with the observations are of type (I o M = divalent metal). This aspect of the subject has been discussed fully by Ray [532). 

In the conventional extension 24) of this synthesis to S-alkyhsothio-ureas, low yields of the expected biguanides were obtained. Curd, Rose et al. 128, 131) have improved this synthesis by performing the condensation under conditions which first convert the S-alkylisothiourea into the corresponding cyanamide (i.e. in non-hydroxylic organic media boiling above 100°, and in the presence of an alkali metal alkoxide). 

Another variation of the synthesis of biguanides utilising the condensation of cyano-compoimds and amines is the reaction of dicyanimide and two moles of amine (457, 465, 581, 582). In practice (457) a dicyanimide metal salt is fused, or heated in water or in an organic solvent with an amine salt, preferably between 90 and 130°. Condensation may also be effected using free dicyanimide in aqueous acid media (581). 

Heterocyclic carboxylic esters (708) are as reactive as formate and acetate esters. Extended to 1-substituted biguanides, the reaction is promoted by metal alkoxides. Since the heterocyclic ring must be five-or six-membered and contains a conjugated system of double bonds, the reaction resembles formally that involving a,p-unsaturated esters (see below). Several successful examples are on record (599, 606). 

According to Thurston (702), the lower dibasic esters yield pure carbox5unonoguanamines so readily that this reaction is an excellent synthetic route, when confined to the lower esters. It has been apphed to 1-aryl- and alkyl-biguanides as usual, and to the alkali metal salts of half esters. The reaction involving diethyl malonate is represented below succinic acid esters (292, 699) react similarly. 

Carbamylguanamines (CXIV) may be prepared from biguanides and ester amides of dibasic acids in the usual manner (709), preferably in the presence of metal alkoxide. The rather difficultly accessible ester amides may be replaced by the more readily available imides of the dibasic acids (454). 

Biguanides have a marked tendency of stabilizing unusually high oxidation states of the metals, e.g. Ag111203 and Ni111.139... 

The bivalent metals, as usual, combine with two molecules of biguanide to form 4-coordinated planar complexes, while the trivalent cobalt and chromium combine with three molecules of the ligand to produce a 6-coordinated octahedral configuration. The only exception is the trivalent silver which yields, however, a 4-coordinated planar complex. The preparation of the free tris(biguanidato) chromium, Cr(C2N5H6)s, in the anhydrous state,6 as well as of the corresponding anhydrous cobalt(III),8 copper(II), cobalt(II), palladium(II), and nickel(II) compounds, provides indisputable evidence for the structure proposed. Similar anhydrous metallic complexes with numerous substituted biguanides also have been included in the above-mentioned studies. 

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