DIOXYGEN ADDUCTS OF LACUNAR COBALT(II) CYCLIDENE COMPLEXES

The factors controlling the reversible dioxygen binding in lacunar cob alt(II) cyclidene complexes (Figure 1) have been examined. Extensive s tructural variations reveal that the dioxygen affinity can be controll ed by both steric and electronic means, The dioxygen affinity decrease s monotonically with the length of a polymethylene bridging group (R1, Figure 1) from octamethylene to tetramethylene; no binding occurs wit h the still shorter trimethylene bridge. From the analysis of a large array of experimental data, the effects of the R2 and R3 substituents on dioxygen affinity are found to be mainly electronic; for example, e lectron-withdrawing groups at the R2 and R3 positions decrease the aff inity. The various substituent effects are cumulative, but they am not additive. The X-ray crystal structure of the dioxygen adduct [Co(MeMe C6[16]cyclidene)(1-MeIm)(02)](PF6)2 provides significant insight into the structural relationships. The space group is Pnma with a = 23.226( 2) angstrom, b = 18.981(2) angstrom, and c = 10. 124(1) angstrom. The Co-O-O bond angle is 121(1)-degrees, and the 0-O distance is 1.32(2) a ngstrom. The conformation of the ligand in this six-coordinate cobalt complex is different from that of the rive-coordinate complex, as expe cted on the basis of structures in which isothiocyanate occupies the c avity. The structural influence of the coordination of a small molecul e within the cyclidene cavity is further explored by examination of th e crystal structure of the cobalt(III) complex [Co(MeMeC8[16]cyclidene )(SCN)2](PF6). The space group is P1BAR with a = 12.020(3) angstrom, b = 12.379(3) angstrom, c = 14.159(4) angstrom, alpha = 111.73(2)-degre es, beta = 99.49(2)-degrees, and gamma = 94.57-degrees. Again, a predi cted conformational change accompanies occupation of the lacuna. For a series of complexes having R1 = (CH2)n R2 = CH3, and various R3 subst ituents, the dioxygen complexes have been examined by ESR experiments and simulations. From these, the dependence of the Co-O-O bond angles upon the ligand substituents has been evaluated.