Abstract
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.
Highlights
Metal complexes containing metal-ligand multiple bonds are of significance due to their applications in atom transfer and metathesis reactions.[1]
Whereas an acetate complex was isolated from the acylation of 1 with acetic anhydride, further reactions were found for the electrophilic attack of 1 with the stronger electrophiles methyl triflate and B(C6F5)[3], which led to the formation of Ce(IV)
Density functional theory (DFT) calculations indicated that 6 contains a polarized Ce=O multiple bond and the oxo group exhibits nucleophilic behavior
Summary
Metal complexes containing metal-ligand multiple bonds are of significance due to their applications in atom transfer and metathesis reactions.[1] Of particular importance are metal-oxo complexes that are involved as active intermediates in metal-catalyzed oxidations and biological transformations.[2,3] transition-metal complexes with terminal oxo, nitrido, imido, and carbene ligands are well documented,1a,4 very few analogues of the 4f elements have been isolated. The paucity of stable isolated lanthanide (Ln) compounds with multiply bonded ligands can be attributed to the mismatch between the d (Ln) and p (X) (X = O, N, etc.) orbital energy that results in polar Ln-X bonds (i.e. Ln+-X-).[7] As such, Ln=X complexes are kinetically unstable and liable to electrophilic/nucleophilic attack. Early attempts to prepare lanthanide-imido complexes led to isolation of dimeric/polymeric bridged imido complexes.[8]
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