KINETICS AND MECHANISM OF THE REACTIONS OF A QUASI-AROMATIC PLATINUM(II) COMPLEX, [PT(PNAO)-6H](0), WITH FORMALDEHYDE

The kinetics and mechanism of the reaction of formaldehyde with a quas i-aromatic platinum(II) complex [Pt(PnAO)-6H](0)(AH), has been studied spectrophotometrically. The Pt,2N,3C quasi-aromatic heterocyclic ring in AH is highly reactive at the central-aromatic-carbon atom, C(12), to most aldehydes. Under neutral conditions in 1/3(v/v) MeOH-H2O, the reaction scheme is as follows: AH + CH2O-->(k1) A-CH2-OH+AH-->(k2)A-CH 2-A CH2O + A-CH(2)A (k-3)reversible arrow(k3) 2A-CH2-OH The substances AH, A-CH2OH, and A-CH2-A have been isolated and identified by H-1 NMR , IR. The kinetic data support the above reaction sequence. All reacti ons were second-order overall. The rate constants determined at 35.0 d egrees C were k(1) = 5.94 x 10(-3) dm(3) mol(-1) s(-1), k(2) = 1.77 dm (3) mol(-1) s(-1), k(3) = 0.120 dm(3) mol(-1) s(-1), k(-3) = 14.0 dm(3 ) mol(-1) s(-1). The activation parameters found for each step are as follows: Delta H-l(not equal) = 56.0 +/- 2.8 kJ mol(-1), Delta S-l(not equal)= -105.8+/-9.1 J K-1 mol(-1), Delta H-2(not equal) = 45.0+/-2.9 kJ mol(-1), Delta S-l(not equal) = -94.4+/-9.2 J K-1 mol(-1); Delta H -3(not equal) = 52.9 +/- 1.8 kJ mol(-1), Delta S-3(not equal) = -91.0 +/- 5.7 J K-1 mol(-1); Delta H--3(not equal) -57.2 +/- 1.7 kJ mol(-1), Delta S--3(not equal) = -37.6 +/- 5.4 J K-1 mol(-1). The consequences of these reaction rate constants their activation parameters are disc ussed. (C) 1997 Elsevier Science Ltd.