Probing the Reactivity of the Ce═O Multiple Bond in a Cerium(IV) Oxo Complex.

The reactivity of the cerium(IV) oxo complex [(LOEt)2CeIV(═O)(H2O)]·MeC(O)NH2 (1; LOEt- = [CoCp{P(O)(OEt)2}3]-, where Cp = η5-C5H5) toward electrophiles and Brønsted acids has been investigated. The treatment of 1 with acetic anhydride afforded the diacetate complex [CeIV(LOEt)2(O2CMe)2] (2). The reaction of 1 with B(C6F5)3 yielded [CeIV(LOEt)2(Me2CONH2)2][B(C6F5)3(OH)]2 (3), in which the [B(C6F5)3(OH)]- anions are H-bonded to the O-bound acetamide ligands. The treatment of 1 with HCl and HNO3 afforded [CeIV(LOEt)2Cl2] and [CeIV(LOEt)2(NO3)2], respectively. Protonation of 1 with triflic acid (HOTf) gave the diaqua complex [CeIV(LOEt)2(H2O)2](OTf)2 (4), in which the triflate anions are H-bonded to the two aqua ligands. The treatment of 1 with phenol afforded the phenoxide complex [CeIV(LOEt)2(OPh)2] (5). The oxo-bridged bimetallic complex [(LOEt)2(Me2CONH2)CeIV(O)NaLOEt] (6) with the Ce-Ooxo and Na-Ooxo distances of 1.953(4) and 2.341(4) Å, respectively, was obtained from the reaction of 1 with [NaLOEt]. Density functional theory calculations showed that the model complex [(LOMe)2CeIV(Me2CONH2)(O)NaLOMe] (6A; LOMe- = [CoCp{P(O)(OMe)2}3]-) contains a polarized Ce═O multiple bond. The energy for dissociation of the {NaLOMe} fragment from 6A in acetonitrile was calculated to be +33.7 kcal/mol, which is higher than that for dissociation of the H-bonded acetamide from [(LOMe)2CeIV(═O)(H2O)]·MeC(O)NH2 (1A) (calculated to be +17.4 kcal/mol). In hexanes containing trace water, complex 1 decomposed readily to a mixture of a tetranuclear cerium(IV) oxo cluster, [CeIV4(LOEt)4(μ4-O)(μ2-O)4(μ2-OH)2] (7), and a cerium(III) complex, [CeIII(LOEt)2(H2O)2][LOEt] [8(LOEt)], whereas the cerium/sodium oxo complex 6 is stable under the same conditions. The crystal structures of 3, 4·H2O, 6, and 8(LOEt) have been determined.