RBONYL(1,4-DIISOPROPYL-1,4-DIAZABUTADIENE)CHROMIUM - ISOLATION AND REACTIVITY OF THE MONODENTATE INTERMEDIATE EN-ROUTE TO CR(CO)4(IPROP-DAB) CHELATE RING-CLOSURE

Exchange of the olefin ligand in Cr(CO)5(eta2-(Z)-cyclooctene) by 1,4- diisopropyl-1,4-diazabutadiene (iprop-DAB) yields Cr(CO)5(iprop-DAB) ( 1), where the potentially bidentate DAB ligand coordinates in a monode ntate fashion. Complex 1 is isolated as red crystals and fully charact erized (elemental analysis, IR, UV-vis, H-1 NMR and C-13 NMR spectra). In hydrocarbon solution at ambient temperature it decays via two comp etitive routes involving (a) chelate ring closure with CO extrusion to form Cr(CO)4(iprop-DAB) (2) and (b) loss of the iprop-DAB ligand and takeup of CO to form Cr(CO)6, as monitored by means of NMR, UV-vis, an d IR spectroscopy. Favorable conditions for the chelate ring closure l eading to 2 are the high concentration of 1, the presence of added ipr op-DAB, and the absence of CO, while the opposite is true for the form ation of Cr(CO)6. The decay of 1 is retarded in the presence of increa sing amounts of added iprop-DAB. It essentially follows pseudo-first-o rder kinetics with k(obs) approaching a lower limiting value of 2.7 x 10(-5) s-1 under Ar at 23-degrees-C, whereby Cr(CO)4(iprop-DAB) (2) an d Cr(CO)6 are formed in a ca. 20:1 ratio. Experiments at variable temp erature yield DELTAH(double dagger) = 48 +/- 6 kJ mol-1 and DELTAS(dou ble dagger) = -170 +/- 18 J K-1 mol-1. In the presence of added CO or (E)-cyclooctene (ECO) the decay of 1 is accelerated, whereby additiona l Cr(CO)6 or Cr(CO)5(eta2-ECO) are formed at the expense of Cr(CO)4(ip rop-DAB) (2) production. Complementary studies involving continuous ir radiation of Cr(CO)6 and iprop-DAB indicate that photogenerated 1 subs equently undergoes photolytic CO dissociation with formation of 2 in a ddition to the thermal chelate ring closure.