Pentacarbonyl(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

Article abstract of DOI:10.1021/om00020a008

Exchange of the olefin ligand in Cr(CO)₅(η²-(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 monodentate fashion. Complex 1 is isolated as red crystals and fully characterized (elemental analysis, IR, UV-vis, ¹H NMR and ¹³C NMR spectra). In hydrocarbon solution at ambient temperature it decays via two competitive 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, and IR spectroscopy. Favorable conditions for the chelate ring closure leading to 2 are the high concentration of 1, the presence of added iprop-DAB, and the absence of CO, while the opposite is true for the formation of Cr(CO)₆. The decay of 1 is retarded in the presence of increasing amounts of added iprop-DAB. It essentially follows pseudo-first-order kinetics with k_obs approaching a lower limiting value of 2.7 × 10^-5 s^-1 under Ar at 23 °C, whereby Cr(CO)₄(iprop-DAB) (2) and Cr(CO)₆ are formed in a ca. 20:1 ratio. Experiments at variable temperature yield ΔH^≠ = 48 ± 6 kJ mol^-1 and ΔS^≠ = -170 ± 18 J K^-1 mol^-1. In the presence of added CO or (E)-cyclooctene (ECO) the decay of 1 is accelerated, whereby additional Cr(CO)₆ or Cr(CO)₅(η²-ECO) are formed at the expense of Cr(CO)₄(iprop-DAB) (2) production. Complementary studies involving continuous irradiation of Cr(CO)₆ and iprop-DAB indicate that photogenerated 1 subsequently undergoes photolytic CO dissociation with formation of 2 in addition to the thermal chelate ring closure.