Alkali and alkaline-earth metal ketyl complexes: Isolation, structural diversity, and hydrogenation/protonation reactions

The use of hexamethylphosphotic triamide (HMPA) as a stabilizing ligand all owed successful isolation of a series of structurally characterizable alkal i metal and calcium ketyl complexes. Reaction of lithium and sodium with on e equivalent of fluorenone and reaction of sodium with one equivalent of be nzophenone in THF, followed by addition of two equivalents of HMPA, yielded the corresponding ketyl complexes 1, 2, and 11, respectively, as mu-ketyl- bridged dimers. If one equivalent of HMPA was used in the reaction of sodiu m with fluorenone, a further aggregated complex, the mu(3)-ketyl-bridged te tramer 3, was isolated, whereas analogous reaction of benzophenone with sod ium afforded the trimeric ketyl complex 13, rather than a simple benzopheno ne 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 eq uiv of HMPA was used in the above reaction, complex 5, a THF-bridged analog ue 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 tw o of the four Na atoms are each terminally coordinated by a fluorenone liga nd, and the other two Na atoms are coordinated by a THF ligand. Two bridgin g THF ligands are also observed in 6. Reaction of 1,2-bis(biphenyl-2,2'-diy l)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, respe ctively, via C-C bond cleavage of a 1,2-diolate intermediate. The reaction of calcium with two equivalents of fluorenone or benzophenone in the presen ce 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, t he fluorenone ketyl complex was isolated in a six-coordinate octahedral for m (10), while the benzophenone ketyl complex was obtained as a five-coordin ate trigonal bipyramid (13). The radical carbon atoms in both benzophenone ketyl and fluorenone ketyl complexes are still in an spl-hybrid state. Howe ver: in contrast with the planar configuration of the whole fluorenone kety l unit, the radical carbon atom in a benzophenone ketyl species is not copl anar with any of the phenyl groups; this explains why benzophenone ketyl is more reactive than fluorenone ketyl. Hydrolysis of 2 or 11 with ZN HCl yie lded the corresponding pinacol-coupling product, while treatment of 2 or 11 with 2-propanol. followed by hydrolysis, gave the pairs fluorenone and flu orenol or benzophenone and benzhydrol, respectively. A possible mechanism f or these reactions is proposed.