Equilibrium studies on complexation of iron (III) by acet-, glycinium and betaine hydroxamic acids

Abstract

EquHtbri1um studies were performed to invest:iga.te ithe com- 1plexa1tion of aqueous high-spin irnn(!I!) by three synthetic monohydroxamic actds: acet-, glycin.ium, and be1ta1ne hydroxamic acids. Under neutral and acidic conditions studied, 1acethydroxamic acid is neutral (AH), while glycinium CGH2+) a.nd betaine CBH+) hydroxa.mic ac1ds have a positive charge on ·the nitro:gen atom. The equilibrium quotients ·for the formation of monoacethydroxamatoiron( I!!) complex Qi', A-= [FeA2+J/( [Fe3+] [A- J ), bisacethydroxamatoiroin(III) complex Q2', A-= [FeA2+J/( [Fe3+] [A-]2), a.nd .tirisacethydroxamatoiiron(!I!) comp;lex Q3', A-= = [FeA3]/([Fe3+J [A-]3) were found to be: lg Qi', A-= 10.38 (0.01), lg Q2', A-= 19.16 (0.14), and lg Q3', A- = 25.56 (0.70). Ana.logous equiUbrimn quotients for glycinium-hydrnxamic acid with protonated amino group (GH2+) and betaine-hydroxamic ac·id CBH+) ligands are: lg Qi', GH = [Fe(GH)3+J/([Fe3+J [GHJ) = 7.77 (0.08), lg Q2', GH = [Fe(GH)i3+J/([Fe3+J [GHJ2) = 13.71 (0.08), lg QJ', GH = [Fe.CGH)J3+]/([Fe3+J [GHJ3) = 17.63 (0.14), lgQi',B = [FeB3+]/([Fe3+) [BJ)= 7.28 (0.02), lgQ2',B = [FeB23+J/([Fe3+] [B]2) = 13.41 (0.05), and lgQ3'.B = [FeB33+]/([Fe3+) [B]3) = 16.46 (0.24). Determinations were made at 1.0 M ionic strength (NaCD and at 25 °C by spectrophotometric methods. The synthesis of a new compound beatine hydroxamic acid chloride i1s desc·ri:bed. INTRODUCTIO