Mixed Ligand Complexes of Iron(h) as Nonaqueous Acid-13as-e and Aqueous Oxidation-reduction Indicators

Studies conducted on the lxeparation and properties of the complexes of iron(I1) and (III) wit11 cyanide ancl aromatic cliimine miscd ligands *r-:r indicate that they may be iiscful in a variety of ways in chemical analysis. Tlms far analytical use has been made only of tire neutral clicyano-bis-(x,ro-l~i~~x~a~~tliroline) -iron complex and of its 4,7-cliphenyl-x,zo-phcnanthrolinc homolog. A highly selective, sensitive colorimetric mctlrod for tlic clctcrmination of cyanide !x~2cl on formation and extraction of the former has been clcscribec! 4. ‘The homologous phenyl-substituted comples was shown to he a suitable form in wlriclr trace amounts of iron can be quantitatively separated from copper ant! measured spectropl~otomctricr!!_v~. In the present study it is found that dicy~lno-l,is-(x,x0-l~!~enant!~ro!inc)-irorl(II) ant1 clicyano-bis(z,z’-bipyricline) -iron(I I) arc useful as acid-base and oxidation-recluction indicators. Future stuclics are nnticipatccl which may lcacl to the use of the charged species in tins series of misec! ligand complcses as precipitation reagents. Tlie tctrikcyano-mono-(r,Io-phcnant!rroline)-fcrratc(I1) anion, for esamplc, which forms insoluble salts with a number of tile heavy metal ions, may prove to bc ;L practical and sclcctivc precipitation reagent. The tctracyano comples and its salts are highly colored; thus smxll amounts coulc! be mcnsurcc! colorimctrically. For simplicity and convenicncc, the trivial names ferrocyphen and fcrrocypyr are proposed for clicyano-his-(r,xo-phcnanthro!inc)-iron(I1) and c!icyano-bis-(z,z’!>ipyric!ine)-iron( rcspcctivcly; the cationic iron(II1) species, obtained on osidation of these neutral complcses, may then bc refcrrecl to respectively as fcrricyplien and fcrricypyr. It is believed that this marks the first time a metal complex of any kind has been found to be effective as an acid-base indicator. In any case, the use of a metal complex for such a purpose is sufficiently novel to prompt some elaboration as to its conception. During a recent investigation 3 it was found tlrat fcrrocyphcn and ferrocypyr react rapiclly and reversibly with strong acids to give stable mono-and diprotonatec! species, Acid eschnngc measurements, carried out in glacial acetic acid, indicated that fcrrocyphen nncl ferrocypyr are similar to o-chloronnilinc in basicity. In acetic acid solvent. the monoprotonatccl species provccl to be very much weaker than o-ch!oro:mi!inc, somewhat wcnker than thiourea, yet considerably stronger than the solvent in comlxtition for pcrclrloric acid. Since the visible absorption characteristics, as well as the lxlsicitics, of the protonated and neutral species cliffer markedly, the neutral complexes s!~oulcl be able to function as acid-base indicators. This