Actinide and Alkali Metal Complexes of Rigid Monoanionic Chalcogenoether-Donor Ligands

Abstract

The rigid acridan-backbone phenyl-substituted thioether- and selenoether-containing pro-ligands H[AS2Ph2] (1) and H[ASe2Ph2] (2), and the bulky 2,4,6-triisopropylphenyl-substituted selenium and tellurium analogues, H[ASe2Tripp2] (3) and H[ATe2Tripp2] (4) were synthesized via a palladium catalyzed cross-coupling approach {for H[AS2Ph2] (1)} or a nBuLi-mediated synthesis strategy {for H[ASe2Ph2] (2), H[ASe2Tripp2] (3) and H[ATe2Tripp2] (4)}. Pro-ligands 1 and 2 were deprotonated with one equiv. of nBuLi to afford dimeric lithium complexes [Li(AE2Ph2)]2 (E = S (5), Se (6)) or with one equiv. of KCH2Ph to afford the potassium complexes [K(AS2Ph2)(dme)]x (7) and [K(ASe2Ph2)(dme)2] (8). Pro-ligands 3 and 4 were also deprotonated using KCH2Ph to afford [K(AE2Tripp2)(dme)2] {E = Se (9), Te (10)}. Compounds 1−10 were characterized by multinuclear NMR spectroscopy, where applicable, and single-crystal X-ray structures were obtained for all lithium and potassium complexes (5, 6 and 7−10), revealing the first examples of Li–SeR2 interactions in 6 and rare examples of K–SeR2 bonding in 8 and 9. DFT calculations were performed to assess the nature of bonding between the hard group 1 cations and the soft chalcogenoether donors. Dissolution of the potassium complex [K(ATe2Tripp2)(dme)2] (10) in THF, layering with hexanes, and cooling to –30 °C afforded X-ray quality crystals of [K(ATe2Tripp2)(THF)3] (11). The K–TeR2 distances in 11 are substantially shorter than those in 10, and DFT and QTAIM calculations support the presence of K–TeR2 interactions, providing the first unambiguous examples of s-block–telluroether bonding. Attempts to prepare bulk quantities of 11 afforded [K(ATe2Tripp2)(THF)2] (12), and further drying yielded [K(ATe2Tripp2)(THF)] (13) and [K(ATe2Tripp2)]x (14). The selenium analogues of 11, 12 and 13 (15, 16 and 17, respectively) were also prepared, and 11, 14, 15 and 16 were crystallographically characterized. Deprotonation of H[ATe2Tripp2] (4) with nBuLi in dme followed by recrystallization from dme/hexanes furnished crystals of [Li(dme)3][ATe2Tripp2] (18), whereas deprotonation of 4 with tBuLi in hexanes in the presence of only one equiv. of dme afforded [Li(ATe2Tripp2)(dme)] (19). The X-ray structure of 18 features lithium coordinated to three κ2-dme ligands along with a ‘naked’ ATe2Tripp ligand anion featuring no coordination to the metal centre. By contrast, the solid-state structure of 19 features coordination of lithium to one κ2-dme and one κ2-ATe2Tripp2 ligand via the nitrogen and one telluroether. Reaction of one equiv. of nBuLi with 4 in hexanes, followed by recrystallization from toluene/hexanes furnished crystals of the lithium cluster complex [Li8(ATeTripp)4(TenBu2)] (20), where ATeTripp (4-(2,4,6-triisopropylphenyltellurido)-2,7,9,9-tetramethylacridan-N,5-diide) is a dianionic CNTe-donor ligand generated in-situ by loss of a ‘TeTripp’ fragment from the ATe2Tripp2 monoanion. The complex features bridging and terminal Li–TeR2 linkages, in addition to an unsupported lithium–TenBu2 interaction. The structures of 19 and 20 contain the first examples of lithium–telluroether coordination, and complex 20 features the first unsupported s-block–telluroether interaction. DFT and QTAIM calculations on models of 19 and 20 were employed to gain insight into the nature of the Li–TeR2 interactions and probe the extent of covalency present in the bonds. Reaction of two equiv. of 7 or 8 with [UI4(dioxane)2] afforded the uranium(IV)–thioether complex [(AS2Ph2)2UI2] (21) and the first example of a uranium–selenoether complex, [(ASe2Ph2)2UI2] (22). X-ray structures revealed distorted square antiprismatic geometries in which the AE2Ph2 ligands are κ3-coordinated. The nature of the U–ER2 bonding in 21 and 22, as well as methyl-free analogues of 21 and 22 and a hypothetical ether analogue (complex 0*), was investigated computationally (including NBO, AIM, and ELF calculations), illustrating appreciable covalency increasing from O to S to Se. Reactions of the lithium complexes [Li(AE2Ph2)]2 {E = S (5) or Se (6)} with [ThCl4(dme)2] or UCl4 (for E = Se) afforded the actinide(IV) chalcogenoether chloro complexes [(AE2Ph2)2ThCl2] (E = S (23), Se (24)), and [(ASe2Ph2)2UCl2] (25). X-ray crystal structures of 23–25 revealed tetravalent actinide cations complexed to two κ3-coordinated AE2Ph2 ligands, with short Th−ER2 and U−ER2 distances (i.e. below the respective sums of the covalent radii). Complexes 23–25 provide extremely rare examples of thorium−thioether, thorium−selenoether, and uranium−selenoether bonds, and 23 and 24 contain the shortest known Th−SR2 and Th−SeR2 distances. DFT and QTAIM calculations confirm the presence of significant An(IV)−ER2 interactions in 23–25 and provide insight into the extent of covalency in the An−ER2 bonds. The THF-coordinated uranium(IV) trichloro complex [(ASe2Tripp2)UCl3(THF)] (26), featuring a single ASe2Tripp2 ligand was prepared by the reaction of [K(ASe2Tripp2)(dme)2] with one equiv. of UCl4 in THF. Two different crystal structures of 26 (26·o-DFB and 26·0.4 THF) gave structures with U–Se distances that differ by ~0.1 Å relative to each other, pointing to a shallow potential energy surface for the perturbation of U–Se bonds in 26. Treatment of 26 with LiCH2SiMe3 or LiCH2SiMe2Ph afforded the trialkyl species [(ASe2Tripp2)U(CH2SiMe3)3] (27) and [(ASe2Tripp2)U(CH2SiMe2Ph)3] (28), respectively, marking the first organouranium selenoether complexes. The X-ray crystal structure of 27 reveals substantially elongated U–Se distances relative to those in 26 likely due to steric repulsion between the relatively bulky CH2SiMe3– ligands on uranium and the flanking 2,4,6-triisopropylphenyl substituents at selenium, and quantum chemical calculations point to lower (but appreciable) covalency in the longer U–SeR2 interactions of 27 compared to the shorter ones in 26. Additional research contributions are presented, including the synthesis of a semi-bulky H[ASe2Ph2] pro-ligand analogue H[ASe2Mes2] (29; Mes = mesityl) and its deprotonation to afford the potassium–selenoether complexes [K(ASe2Mes2)(dme)] (30) and [K(ASe2Mes2)(dme)2] (31), (ii) variable temperature NMR characterization of a trivalent uranium AS2Ph2 complex [(AS2Ph2)2UI] (32) and trivalent neodymium AE2Ph2 (E = S, Se) complexes, and CV studies of complex 32, (iii) the development of new or modified syntheses for actinide starting materials, including the identification of the previously unknown etherate of UCl4 [UCl4(Et2O)2] (33), and (iv) the determination of the structure of a cationic yttrium dialkyl complex by NMR spectroscopy.