Kinetic studies on the reactions of some tetrakis(arylisocyanide)cobalt(II) complexes with pyridine in 2,2,2-trifluoroethanol

Abstract

The interaction of pyridine with four tetrakis(arylisocyanide)cobalt(II) complexes, [Co(CNR) 4 (ClO 4 ) 2 ] R = 2,6-Me 2 C 6 H 3 (A), 2,4,6-Me 3 C 6 H 2 (B), 2,6-Et 2 C 6 H 3 (C) and 2,6-iPr 2 C 6 H 3 (D), have been studied in 2,2,2-trifluoroethanol medium. The kinetics of the reactions were investigated over the 293-318 K temperature range. The reaction profile exhibited two distinct processes, proposed to be an initial fast substitution followed by a slow reduction, for each of the reactions. The pseudo first-order rate constants for both processes increased with increasing concentration of pyridine with the reduction processes exhibiting saturation kinetics at high pyridine concentrations. Steric hindrance plays a significant role in the rates of the reactions, as the rates decrease in the order k(A) > k(B) > k(C) > k(D). The activation enthalpies, DH à , increase from A to D while the activation entropies, DS à , are relatively similar for the four reactions, indicating similar transition states and hence similar mechanisms. Complex B was first synthesized and characterized in this study.

Substitution and Redox Properties of Some Organoisocyanide Cobalt(II) Complexes

“…Plots of k s 0 versus [L] gave curves typical of saturation kinetics ( Figure 4). Similar observations have been reported in our earlier studies [24] as well as in both the aminolysis of sulfamate esters in chloroform [41] and in the reactions of Co(II) protoporphyrins IX dimethyl ester with pyridine and related compounds [34,37]. Such saturation kinetics is also typical of the formation of a precursor complex prior to electron transfer as observed in the electron transfer reactions of halopentacyanocobaltate(III) complexes [42].…”

“…Characterization of the complexes was carried out using infrared and electronic spectra as well as elemental analyses. The results are in close agreement with those of earlier workers [4,22,24,28,33]. The ligands 1,1,3,3-tetramethylbutylisocyanide (t-octylisocyanide), CNC 8 H 17 -t, triphenylphosphine, PPh 3 , tris(4-methoxyphenyl)phosphine, P(C 6 H 4 OMe-p) 3 , tri (p-tolyl)phosphine, and P(C 6 H 4 Me-p) 3 were purchased from Aldrich Chemicals and used without further purification.…”

“…Cobaltemployed in this study are those that have been standing for about 24 hours. The shift in peak on addition of PPh 3 with the attendant decrease in absorbance is attributed to ligand substitution followed by reduction, an observation similar to that reported in earlier studies [24,[43][44][45]. Figure 5 shows the repeated UV/Vis spectra of the reaction mixture taken at intervals of time for 3 hours.…”

“…For pyridine, the corresponding k 1 and k 2 values for the substitution process have been reported earlier [24] and are shown in The data fit well to the rate law described by the following equation:…”

“…Apart from the many pentakis(arylisocyanide)cobalt(II) complexes that have been prepared, the synthesis and characterization of a number of tetrakis(arylisocyanide)bis(perchlorato) cobalt(II) complexes have also been reported [4,[22][23][24]. Earlier studies showed that these pentakis and tetrakis cobalt(II) complexes react with amines to undergo reduction to their 4 ] in the À1, 0, and +1 oxidation states were synthesized a few years back [29] using m-terphenyl isocyanide as the stabilizing ligand.…”