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

Date

1994-08-01

Author

GREVELS, FW
KAYRAN, C
Özkar, Saim

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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 monodentate fashion. Complex 1 is isolated as red crystals and fully characterized (elemental analysis, IR, UV-vis, H-1 NMR and C-13 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 x 10(-5) s-1 under Ar at 23-degrees-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 DELTAH(double dagger) = 48 +/- 6 kJ mol-1 and DELTAS(double 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 additional Cr(CO)6 or Cr(CO)5(eta2-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.

Subject Keywords

Metal hexacarbonyl solutions, Reaction-kinetics, W(co)5l intermediate, Spectral characterization, Infrared-spectroscopy, Tungsten hexacarbonyl, Electronic absorption, Diimine ligands, Photolysis

URI

https://hdl.handle.net/11511/33130

Journal

ORGANOMETALLICS

DOI

https://doi.org/10.1021/om00020a008

Collections

Department of Chemistry, Article

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Citation Formats

F. GREVELS, C. KAYRAN, and S. Özkar, “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,” ORGANOMETALLICS, pp. 2937–2943, 1994, Accessed: 00, 2020. [Online]. Available: https://hdl.handle.net/11511/33130.