REACTIONS OF ALDEHYDES WITH CARBON-DIOXIDE AT NICKEL(0) CENTERS - A DETAILED KINETIC-ANALYSIS

The reactions of the nickel(0) complex [Ni(bipy)(COD)] (COD = cyclooct adiene, bipy = 2,2'-bipyridine) with benzaldehyde, propionaldehyde, an d carbon dioxide/propionaldehyde were investigated. A detailed kinetic analysis revealed that [Ni(bipy)(COD)] reacts reversible with aldehyd es and activation parameters for the forward and back reactions were c alculated from a temperature dependence study: forward reaction (benza ldehyde) Delta H double dagger = 47.8 +/- 0.4 kJ/mol, Delta S double d agger = -47 +/- 1 J/(mol K); back reaction (COD) Delta H double dagger = 45 +/- 6 kJ/mol, Delta S double dagger = -61 +/- 19 J/(mol K) and f orward reaction (propionaldehyde) Delta H double dagger = 55 +/- 3 kJ/ mol, Delta S double dagger = -58 +/- 9.0 J/(mol K); back reaction (COD ) Delta H double dagger = 39 +/- 0.5 kJ/mol, Delta S double dagger = - 58 +/- 2 J/(mol K). It could be shown that carbon dioxide did not reac t with [Ni(bipy)(COD)] directly but with the nickel propionaldehyde co mplex to form a five-membered cyclic nickel complex. The reaction proc eeded according to an associative mechanism, during which the carbon d ioxide was inserted into a Ni-O bond by an oxidative coupling step. Th e activation parameters for this process were Delta H double dagger = 43 +/- 6 kJ/mol and Delta S double dagger = -129 +/- 19 J/(mol K).