UC Santa Barbara

Reaction of [Ce(NR2)3] (R = SiMe3) with LiNO3 in THF, in the presence of 2,2,2-cryptand, results in the formation of the Ce(III) “ate” complex, [Li(2,2,2-cryptand)][Ce(κ2-O2NO)(NR2)3] in 38% yield. Photolysis of this complex at 380 nm affords [Li(2,2,2-cryptand)][Ce(O)(NR2)3] in 33% isolated yield after reaction workup. This complex is the first reported example of a Ce(IV) oxo complex where the oxo ligand is not supported by hydrogen bonding or alkali metal coordination. Also formed during photolysis are [Li(2,2,2-cryptand)]2[(µ3-O){Ce(µ-O)(NR2)2}3] and [Li(2,2,2-cryptand)][Ce(OSiMe3)(NR2)3], whose identities were confirmed by X-ray crystallography. The latter complex can also be prepared independently via reaction of [Ce(NR2)3] with LiOSiMe3 in THF, in the presence of 2,2,2-cryptand. When synthesized in this fashion, it can be isolated in 47% yield.

Reaction of [Ce(NO3)3(THF)4] with 6 equiv of Li(N=CtBuPh), followed by addition of 0.5 equiv of I2, affords the homoleptic Ce(IV) ketimide, [Li]2[Ce(N=CtBuPh)6], which can be isolated in 44% yield after workup. Similarly, reaction of [ThCl4(DME)2] with 6 equiv of Li(N=CtBuPh) in THF affords the isostructural Th(IV) ketimide, [Li]2[Th(N=CtBuPh)6], which can be isolated in 53% yield after workup. Both complexes were fully characterized, including analysis by X-ray crystallography, allowing for a detailed structural and spectroscopic comparison. The electronic structures of both complexes were also explored with density functional theory (DFT) calculations. Additionally, the redox chemistry of [Li]2[Ce(N=CtBuPh)6], was probed by cyclic voltammetry, which revealed a highly cathodic Ce(IV)/Ce(III) reduction potential, providing evidence for the ability of the ketimide ligand to stabilize high oxidation states of the lanthanides.

Reaction of anhydrous CeCl3 with 2 equiv of [Li(Et2O)]2[1,8-DMC] (1,8-DMC = 1,8-dimethyl-1,4,8,11-tetraazacyclotetradecane) in THF at 65 °C for 2 d affords [Li][Ce(1,8-DMC)2] as yellow blocks in 75% yield, after crystallization from a concentrated Et2O solution. Similarly, reaction of anhydrous PrCl3 with 2 equiv of [Li(Et2O)]2[1,8-DMC] in THF at 65 °C for 2 d affords [Li][Pr(1,8-DMC)2] as pale blue blocks in 63% yield. Both complexes were fully characterized, including analysis by X-ray crystallography, UV-Vis/NIR spectroscopy and cyclic voltammetry. Oxidation of [Li][Ce(1,8-DMC)2] with 0.5 equiv of I2 in THF affords the Ce(IV) bis-cyclam complex [Ce(1,8-DMC)2] as purple plates in 34% yield. In contrast, reaction of [Li][Pr(1,8-DMC)2] with 0.5 equiv of I2 or 1 equiv of AgOTf in Et2O or THF only results in isolation of [Li(py)(1,8-DMCH2)][X] (X = I, OTf). No praseodymium containing material could be isolated from these reactions. Interestingly, reaction of [Li][Pr(1,8-DMC)2] with 0.5 equiv of I2, in the presence of 1 equiv of 2,2,2-cryptand, results in formation of the Pr(III) iodocyclam complex [Pr(1,8-DMC)(2,2,2-crypt)(I)], which was characterized by X-ray crystallography. Both attempts at oxidizing the Pr(III) center in [Li][Pr(1,8-DMC)2] are believed to result in either direct ligand protonation or ligand oxidation followed by hydrogen atom abstraction from solvent.

Reaction of Li2(tmtaa) (tmtaaH2 = dibenzotetramethyltetraaza[14]annulene) with 1 equiv of [UO2Cl2(THF)3], in an attempt to form cis-[UO2(tmtaa)], affords the bis(uranyl) complex, [Li(THF)3][Li(THF)2][(UO2Cl2)2(tmtaa)], as a red-brown crystalline solid in modest yield. This complex can be synthesized rationally by reaction of Li2(tmtaa) with 2 equiv of [UO2Cl2(THF)3]. Under these conditions, it can be isolated in 44% yield. In contrast to the Li2(tmtaa) reaction, addition of [K(DME)]2[tmtaa] to 1 equiv of [UO2Cl2(THF)3] results in formation of the 2e- oxidation products of (tmtaa)2-. Specifically, three isomers of C22H22N4 were isolated as a mixture of orange crystals in 41% combined yield. All three isomers were characterized by X-ray crystallography. We hypothesize that these ligand oxidation products are formed upon decomposition of the unobserved cis uranyl intermediate, cis-[UO2(tmtaa)], which undergoes a facile intramolecular redox reaction.

Reaction of [UO2(N(SiMe3)2)2(THF)2] with 1 equiv of Cy7Si7O9(OH)3 in THF affords [U(OSiMe3)3(Cy7Si7O12)] as orange plates in 24% isolated yield. We propose that the formation of this complex proceeds through a transient uranyl silsesquioxide intermediate, [{Cy7Si7O11(OH)}UO2], which undergoes rapid oxo silylation by HN(SiMe3)2, followed by silyloxy ligand scrambling, to form [U(OSiMe3)3(Cy7Si7O12)] and the U(VI) bis(silsesquioxane) complex, [U(Cy7Si7O12)2], among other products. The formation of [U(Cy7Si7O12)2] was confirmed by its independent synthesis and comparison of its 29Si{1H} NMR spectrum with that of the in situ reaction mixture. In contrast to the reaction in THF, the reaction of [UO2(N(SiMe3)2)2(THF)2] with Cy7Si7O9(OH)3 in hexanes, followed by recrystallization from Et2O/MeCN, results in formation of the uranyl cluster, [(UO2)3(Cy7Si7O12)2(Et2O)(MeCN)2], as yellow rods in 42% isolated yield. Overall, the conversion of [UO2(N(SiMe3)2)2(THF)2] to [U(OSiMe3)3(Cy7Si7O12)] and [U(Cy7Si7O12)2] is likely promoted by the strong electron donor ability of the silsesquioxane ligand, and suggests that the actinide coordination chemistry of mineral surface mimics, such as silsesquioxane, is a fruitful arena for the discovery of new reactivity.