Complexes with Azo Compounds
Manganese(II) complexes with azo compounds R-N=N-R′ are known, where R and R′ are aromatic or heterocyclic groups. As shown by IR data, the nitrogen atoms of the azo groups are coordinated to manganese if additional donor atoms in sterically suited positi
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Complexes with Azo Compounds
Complexes with Azo Compounds
General Remarks Manganese(II) complexes with azo compounds R-N=N-R' are known, where R and R' are aromatic or heterocyclic groups. As shown by IR data, the nitrogen atoms of the azo groups are coordinated to manganese if additional donor atoms in sterically suited positions permit the formation of fused chelate rings with the metal ion:
N~ Q= ~_M~Ny X,
y=O,O or O,N
'y In the pyridylazo compounds, the pyridine nitrogen atom is coordinated to manganese forming a five-membered chelate ring:
O-~\9 N\ /N
Mn ____
\
/;
a
The azo group is always coordinated as the trans =(E)-isomer. Stabilization of the complexes by the hydrazo-keto tautomeric form was observed, especially for the pyrazolylazo complexes. Included also are complexes where the azo group is not involved in coordination, e.g. chelates of phenylazo derivatives of salicylaldehyde or salicylic acid. Generally, 1:1 and 1: 2 complexes of manganese(II) are formed with the ligand anions in mixed aqueous-organic solution at pH 8 to 12. The Irving-Williams order Znll-N=N-@--@-N=N-Q-OH OHC Ligand 3 (= C26H1SN405)
COOH
Complexes in Solution. The formation of 1: 1 compLexes with the sodium sulfonate form of ligand 1 was shown by spectrophotometric measurements. A mitd break in the titration curve near pH 8 was considered to show cheLation of both the deprotonated phenoLic and carboxyLic oxygen atoms. A chelate with coordinated but undissociated phenolic group is expected to be formed at lower pH vaLues. The stability constants of both species were determined potentiometrically at 28 a C and 1= 0.1 M (NaCL0 4): log K = 2.99 and 3.64, respectively [1]. Gmelin Handbook Mn Main VoI. D 5
283
Wjth Arylazo Compounds
With ligand 2 as its monosodium salt, the formation of a 1: 1 and a 1: 2 complex was evaluated potentiometrically (glass electrode) on aqueous solutions containing Mn 2+ ions at 25 ± 0.1°C and 1= 0.1 M (KNO s)' For the pH range from 7 to 9.3, where hydrolytic effects could be excluded, only the carboxyl proton of the ligand is involved in complex formation (pKa, = 2.38, pKa2 = 11.04). Stability constants were determined: log Ki = 4.94 ± 0.02 referring to the equilibrium Mn 2++HL2-:;=MnHL and log K2=3.5±0.1 referring to MnHL+HL2-:;= [Mn(HLh]2-. Bidentate chelation, with the phenolic group coordinated, is presumed. The stability of the 1:1 complex slightly decreased, compared with those of manganese 5-sulfosalicylate and the salicylate, due to the greater electron withdrawing effect of the parasulfophenylazo group [2]. [Mn(C26H,6N40S)(H20hJn' The dark colored polychelate, erroneously formulated in the paper as [Mn(C26H14N40S)(H20hl. was prepared from equimolar amounts of ligand 3 in refluxing dimethylformamide and manganese(lI) chloride in ethanol. After addition of CHsCOONa, the mixture was refluxed for 2 h, and the solid obtained was washed several times with hot water, with DMF, finally with ethanol, and dried at 45°C. Its IR spectrum displays characteristic bands in the regions 3560
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