Oxidation of the 14-Membered Macrocycle Dibenzotetramethyltetraaza[14]annulene upon Ligation to the Uranyl Ion.

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)] (1) as a red-brown crystalline solid in modest yield. Complex 1 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 the solid state, complex 1 features two [UO2Cl2] fragments that are bridged by a highly puckered (tmtaa)2- ligand. Both uranyl fragments feature normal uranyl metrical parameters (U-O (av.) = 1.78 Å, O-U-O = 176.8(3)° and 178.0(3)°). The most notable structural feature of 1, however, is the presence of a lithium cation that coordinates to an oxo ligand from each uranyl fragment. 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-. Three isomers of C22H22N4 (compounds 2, 3, and 4) 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.

Uranyl Coordination by the 14-Membered Macrocycle Dibenzotetramethyltetraaza[14]annulene.

Reaction of [UO2(N(SiMe3)2)2(THF)2] with 1 equiv of dibenzotetramethyltetraaza[14]annulene (tmtaaH2) affords the uranyl complex [UO2(tmtaaH)(N(SiMe3)2) (THF)] (1) (THF = tetrahydrofuran) as red blocks in 83% yield. Similarly, thermolysis of a mixture of [UO2(N(SiMe3)2)2(THF)2] and 2 equiv of tmtaaH2 affords [UO2(tmtaaH)2] (2), which can be isolated as red-orange crystals in 67% yield after workup. Both 1 and 2 were fully characterized, including analysis by X-ray crystallography. The tmtaaH ligands in 1 and 2 are only coordinated to the uranium center via one ß-diketiminate fragment, while the protonated ß-diketimine portion of the ligand remains uncoordinated. Reaction of [UO2(N(SiMe3)2)2(THF)2] with 1 equiv of Li2(tmtaa) in C6H6 results in the formation of [Li(THF)]2[UO2(N(SiMe3)2)2(tmtaa)] (3), which can be isolated in 55% yield as a red-brown crystalline solid. The tmtaa ligand in complex 3 supports a dative interaction between an oxo ligand in the uranyl fragment and a lithium cation, suggesting that tmtaa could be a useful ligand for developing the oxo ligand functionalization chemistry of the uranyl ion.

Perturbation of the O-U-O Angle in Uranyl by Coordination to a 12-Membered Macrocycle.

Reaction of [UO2Cl2(THF)2]2 (THF = tetrahydrofuran) with 2 equiv of (H)N4 ((H)N4 = 2,11-diaza[3,3](2,6) pyridinophane) or (Me)N4 ((Me)N4 = N,N'-dimethyl-2,11-diaza[3,3](2,6) pyridinophane), in MeCN, results in the formation of UO2Cl2((R)N4) (R = H; 1; Me, 2), which were isolated as yellow-orange solids in good yields. Similarly, reaction of UO2(OTf)2(THF)3 with (H)N4 in MeCN results in the formation of UO2(OTf)2((H)N4) (3), as an orange powder in 76% yield. Finally, reaction of UO2(OTf)2(THF)3 with (Me)N4 in THF results in the formation of [UO2(OTf)(THF)((H)N4)][OTf] (4), as an orange powder in 73% yield. Complexes 1-4 were fully characterized, including characterization by X-ray crystallography. These complexes exhibit the smallest O-U-O bond angles measured to date, ranging from 168.2(3)° (for 2) to 161.7(5)° (for 4), a consequence of an unfavorable steric interaction between the oxo ligands and the macrocycle backbone. A Raman spectroscopic study of 1-4 reveals no correlation between O-U-O angle and the U═O νsym mode. However, complexes 1 and 2 do feature lower U═O νsym modes than complexes 3 and 4, which can be rationalized by the stronger donor strength of Cl(-) versus OTf(-). This observation suggests that the identity of the equatorial ligands has a greater effect on the U═O νsym frequency than does a change in O-U-O angle, at least when the changes in the O-U-O angles are small.

Synthesis, structure and bonding of hexaphenyl thorium(IV): observation of a non-octahedral structure.

We report herein the synthesis of the first structurally characterized homoleptic actinide aryl complexes, [Li(DME)3]2[Th(C6H5)6] (1) and [Li(THF)(12-crown-4)]2[Th(C6H5)6] (2), which feature an anion possessing a regular octahedral (1) or a severely distorted octahedral (2) geometry. The solid-state structure of 2 suggests the presence of pseudo-agostic ortho C-H···Th interactions, which arise from σ(C-H) → Th(5f) donation. The non-octahedral structure is also favoured in solution at low temperatures.

Oxo Ligand Substitution in a Cationic Uranyl Complex: Synergistic Interaction of an Electrophile and a Reductant.

Reaction of [U(VI)O2(dppmo)2(OTf)][OTf] (dppmo = Ph2P(O)CH2P(O)Ph2) with 4 equiv of Ph3SiOTf and 2 equiv of Cp2Co generates the U(IV) complex U(IV)(OTf)4(dppmo)2 (1), as a yellow-green crystalline solid in 83% yield, along with Ph3SiOSiPh3 and [Cp2Co][OTf]. This reaction proceeds via a U(IV) silyloxide intermediate, [U(IV)(OSiPh3)(dppmo)2(OTf)2][OTf] (2), which we have isolated and structurally characterized. Similarly, reaction of [U(VI)O2(TPPO)4][OTf]2 (TPPO = Ph3PO) with 6 equiv of Me3SiOTf and 2 equiv of Cp2Co generates the U(IV) complex, [Cp2Co][U(IV)(OTf)5(TPPO)2] (3), as a yellow-green crystalline solid in 76% yield, concomitant with formation of Me3SiOSiMe3, [Ph3POSiMe3][OTf], and [Cp2Co][OTf]. Complexes 1 and 3 have been fully characterized, including analysis by X-ray crystallography. The conversion of [U(VI)O2(dppmo)2(OTf)][OTf] and [U(VI)O2(TPPO)4][OTf]2 to complexes 1 and 3, respectively, represents rare examples of well-defined uranyl oxo ligand substitution.

Reductive silylation of the uranyl ion with Ph3SiOTf.

The reaction of 2 equiv of Ph3SiOTf with UO2(dbm)2(THF) (dbm = OC(Ph)CHC(Ph)O) and UO2((Ar)acnac)2 ((Ar)acnac = ArNC(Ph)CHC(Ph)O; Ar = 3,5-(t)Bu2C6H3) results in the formation of U(OSiPh3)2(dbm)2(OTf) (1) and [U(OSiPh3)2((Ar)acnac)2][OTf] (2), respectively, in good yield.

Comparison of the reactivity of 2-Li-C6H4CH2NMe2 with MCl4 (M=Th, U): isolation of a thorium aryl complex or a uranium benzyne complex.

Why do U react like that? Reaction of 2-Li-C6H4CH2NMe2 with [MCl4(DME)n] (M=Th, n=2; M=U, n=0) results in the formation of a thorium aryl complex, [Th(2-C6H4CH2NMe2)4] or a uranium benzyne complex, [Li][U(2,3-C6H3CH2NMe2)(2-C6H4CH2NMe2)3]. A DFT analysis suggests that the formation of a benzyne complex with U but not with Th is a kinetic and not thermodynamic effect.