Alkali and Alkaline-Earth Metal Ketyl Complexes: Isolation, Structural Diversity, and Hydrogenation/Protonation Reactions

Abstract

The use of hexamethylphosphoric triamide (HMPA) as a stabilizing ligand allowed successful isolation of a series of structurally characterizable alkali metal and calcium ketyl complexes. Reaction of lithium and sodium with one equivalent of fluorenone and reaction of sodium with one equivalent of benzophenone in THF, followed by addition of two equivalents of HMPA, yielded the corresponding ketyl complexes 1, 2, and 11, respectively, as microketyl-bridged dimers. If one equivalent of HMPA was used in the reaction of sodium with fluorenone, a further aggregated complex, the mu3-ketyl-bridged tetramer 3, was isolated, whereas analogous reaction of benzophenone with sodium afforded the trimeric ketyl complex 13, rather than a simple benzophenone analogue of 3. In the reaction of potassium with fluorenone, the use of two equivalents of HMPA gave the tetramer 4, rather than a dimeric complex analogous to 1 or 2. Compared to the tetrameric sodium complex 3, there is an extra HMPA ligand that bridges two of the four K atoms in 4. When 0.5 equiv of HMPA was used in the above reaction, complex 5, a THF-bridged analogue of 4, was isolated. In the absence of HMPA, the reaction of sodium with an excess of fluorenone yielded the tetrameric ketyl complex 6, in which two of the four Na atoms are each terminally coordinated by a fluorenone ligand, and the other two Na atoms are coordinated by a THF ligand. Two bridging THF ligands are also observed in 6. Reaction of 1,2-bis(biphenyl-2,2'-diyl)ethane-1,2-diol (7) with two equivalents of LiN(SiMe3)2 or NaN(SiMe3)2 in the presence of four equivalents of HMPA easily afforded 1 or 2, respectively, via C-C bond cleavage of a 1,2-diolate intermediate. The reaction of calcium with two equivalents of fluorenone or benzophenone in the presence of HMPA gave the corresponding complexes that bear two independent ketyl ligands per metal ion. In the presence of 3 or four equivalents of HMPA, the fluorenone ketyl complex was isolated in a six-coordinate octahedral form (10), while the benzophenone ketyl complex was obtained as a five-coordinate trigonal bipyramid (13). The radical carbon atoms in both benzophenone ketyl and fluorenone ketyl complexes are still in an sp2-hybrid state. However, in contrast with the planar configuration of the whole fluorenone ketyl unit, the radical carbon atom in a benzophenone ketyl species is not coplanar with any of the phenyl groups; this explains why benzophenone ketyl is more reactive than fluorenone ketyl. Hydrolysis of 2 or 11 with 2N HCI yielded the corresponding pinacol-coupling product, while treatment of 2 or 11 with 2-propanol, followed by hydrolysis, gave the pairs fluorenone and fluorenol or benzophenone and benzhydrol, respectively. A possible mechanism for these reactions is proposed.

Bulky Ytterbium Formamidinates Stabilise Complexes with Radical Ligands, and Related Samarium “Tetracyclone” Chemistry

“…Currently there is only one other diketyl species that displays such a cisoid ‐diketyl arrangement, namely [Ca(fn . ‐ O ) 2 (hmpa) 3 ] (hmpa=hexamethylphosphoramide), with an O‐Ca‐O angle of: 114.20°, more than 10° wider than that observed in 10⋅ Et 2 O. In addition, this calcium complex is only stable in the presence of hmpa, whereas 10⋅ Et 2 O is stable in Et 2 O, PhMe, or THF.…”

“…A broad absorption at λ max =614 nm was observed, which is near to that of the deep‐blue [Na 3 (bp . ‐ O ) 3 (hmpa) 4 ] ketyl (675 nm), and the deep‐blue potassium benzophenone polymer [K(bp . ‐ O )] ∞ (656 nm), suggesting the initial formation of a ketyl species.…”

Reduction of Carbonyl Groups by Uranium(III) and Formation of a Stable Amide Radical Anion

“…The reduction of carbonyl π‐bonds has been accomplished by alkali metals, transition metals, and f‐block complexes . The ketyl radical intermediates proposed in such chemistry, however, are often too reactive to isolate or observe.…”

Structural and Spectroscopic Characterization of a Charge-Separated Uranium Benzophenone Ketyl Radical Complex

“…17,[29][30][31][32][33] To date, the isolation and structural characterization of benzophenone ketyl radical complexes are limited to complexes of alkali/alkaline earth and samarium metals. 15,[34][35][36] Here we demonstrate the use of a triazacyclononane-anchored trisaryloxide ligand chelated to uranium to stabilize a charge-separated di-tert-butyl benzophenone ketyl radical species. In addition to X-ray diffraction, this complex has been characterized by electronic absorption spectroscopy, combustion analysis, magnetism, and DFT studies.…”