N4 Complexes

Submitted by A. MARTIN TAIT and DARYLE H. BUSCH Checked by J. CRAGEL, JR.  

The condensation of 2V,A -bis(3-aminopropyl)ethylenediamine (7V,A -ethylene-bis[l,3-propanediamine]) as its acid salt with 2,3-butanedione (biacetyl) in the presence of cobalt(II) or nickel(II) acetate gives complexes of 2,S-Mej [14] - 1,3-diene-l,4,8,11-N4 containing one a-diimine linkage. Experiments have shown that the presence of H ion determines whether or not a macrocyclic complex forms and that, in the presence of H, the time at which the metal acetate is added to the reaction mixture influences the yield of the complex. Unlike the reaction between biacetyl and 1,3-propanediamine to form 2,3,9,10-Me4[14]-1,3,8,10-tetraene-l,4,8,11-N4 (Sec. 4), the condensation of biacetyl withTVA -bis(3-aminopropyl)ethylenediamine is particularly sensitive to excess acetate so that the procedures given use the optimized conditions. 

To 1600 mL of methanol in a 3-L flask is added 58 g (0.33 mole) of A, iV -bis-(3-aminopropyl)ethylenediamine and, dropwise, 32.8 g (0.33 mole) of concentrated hydrochloric acid. The solution is cooled to 5° and stirred while 28.7 g (0.33 mole) of 2,3-butanedione is added. After 30 minutes the flask is removed from the ice bath and allowed to stand at room temperature. After a further 20 minutes the solution is pale orange and 82.6 g (0.33 mole) of nickel(II) acetate tetrahydrate is added. The mixture is stirred for 4 hours and 35 mL of concentrated hydrochloric acid is added to the deep red-brown solution. Addition of 45.4 g (0.33 mole) of zinc chloride produces an immediate red-brown precipitate, which is collected by filtration and washed thoroughly wtih diethyl ether. The yield is 110-120 g (68-74%). Anal Calcd. for Ci2H24N4Cl4ZnNi Ci 29.39, H, 4.90 N, 11.43 Cl, 28.95. Found C, 29.25 H, 5.10 N, 11.30 Cl, 28.41. 

The complex [Ni(2,3-Me2[14]-l,3-diene-l,4,8,lI-N4)] [ZnCU] is square planar and low-spin. The visible spectra show bands near 21.3 kK (characteristic of square planar nickel(II)), near 26.1 kK (due to the imine functions), and near 35.1 kK. The infrared spectra of all of the nickel complexes prepared show absorptions near 3195 and 1595 cm assignable to the N—H stretching vibration and to the symmetric imine vibration, respectively. A strong sharp band also occurs near 1210 cm and is characteristic of the a-diimine function. The NMR spectrum of the perchlorate complex in nitromethane shows a methyl singlet at 2.33 ppm. The ligand can be hydrogenated on nickel(II) with Raney nickel and hydrogen to produce the fully saturated macrocyclic complex [Ni(2,3-Me2[14]-ane-1,4,8,1 1-N4]  

DIBROMO(2,3-DIMETHYL-l,4,8,11-TETRAAZACYCLOTETRADECA-1,3-DIENE)COBALT(III) PERCHLORATE 

---

This classification is ratfier arbitrary, since different reaction products may form at the same electrode, depending on the reaction conditions. Nonmetallic substances such as oxides, semiconductors, and orgaihc N4 complexes are used as electrode materials as well. 

At the N4 complexes, cathodic oxygen reduction has been studied in the greatest detail. These systems are of great practical value inasmuch as these complexes are practically the only nonplatinum catalysts that can be used for oxygen reduction in acidic solutions. 

The catalytic activity of the N4 complexes depends both on the nature of the central metal ion and on the nature of the ligand and aU substituents. It was found that the metal ion is the active site where the electrocatalytic process is accomplished. During its adsorption, an oxygen molecule forms a stable complex (adduct) with the... 

The N4 complexes are rather stable in acidic solutions. However, sometimes the stability is not high enough, particularly so at higher temperatures. It was quite unexpected, therefore, to hud that after pyrolysis at temperatures of 600 to 800°C the catalytic activity of these compounds not only failed to decrease but in some cases even increased. The major result of pyrolysis is a drastic increase in catalyst stability. Tests have been reported where after pyrolysis such catalysts have worked for 4000 to 8000 h without activity loss. The reasons for the conservation of high activity after pyrolysis are not entirely clear. The activity evidently is associated with the central ion that has attained a favorable enviromnent of pyrolysis products. 

See related AMMINEMETAL OXOSALTS, [14] DIENE-N4 COMPLEXES A-(2-Pyridyl)acylacetamides... 

See related amminemetal oxosalts, [14] diene-n4 complexes See other metal perchlorates... 

Influence of the organic skeleton. The example of the Pfeiffer complexes makes it clear that the influence of the organic skeleton is of subordinate importance. Figure 20 shows this influence for a number of Co—N4 complexes. Here too, the differences in activity are much less than those produced by varying the central atom or the coordination. 

Fig. 20. Co-N4-Complexes. Influence of the organic skeleton on the oxygen activity of various Co-N4-complexes. (Electrolyte Carbonate/bicarbonate buffer solution)...

The oxygen activity of various Co—N4 complexes in sulfuric acid can be considerably improved by subjecting the carbon/chelate sample to thermal pretreatment in an atmosphere of a protective gas 7>. Such an effect is also reported by other authors for CoTMPP 22>. Figure 21 shows the influence of pretreatment on CoTMPP, pCoPc, and CoTAA. The given loadings (in mA/g catalyst) are all related to chelate plus carbon. The activity of CoTPP and CoTPAP is so low that,... 

Fig. 21. Influence of thermal activation on the electrocatalytic activity of Co-N4-complexes...

Complete A-alkylation of tren to give Me6- or Et6-tren stabilizes the Co11 oxidation state with respect to Co111 and monomeric N4 complexes with Cu11,767 Niu, Zn11,768 Cr11, Co11 and Fe" are known.742... 

---