Synthesis of Intramolecularly Coordinated Aluminum and Gallium Compounds for the Preparation of [1]Ferrocenophanes

Two different ligands equipped with pyridine donor moieties,
(2-H4C5N)­(Me3Si)2C (R′)
and (2-H4C5N)­(Me3Si)­(Me)C (R″),
were applied in the preparation of aluminum and gallium dihalides
that could be employed in salt metathesis reactions with 1,1′-dilithioferrocene.
R′GaCl2 (1) was accessible from LiR′
and GaCl3 (21%), whereas the respective aluminum compound
R′AlCl2 (3Cl) and its bromine analogue R′AlBr2 (3Br) could only be prepared through the intermediate
species R′AlMe2 (2) by the addition
of Me3SnCl and Br2, respectively. An improved
synthesis of the ligand precursor R″H, (2-H4C5N)­(Me3Si)­(Me)­CH), is described. Attempted syntheses
of R″AlX2 starting from LiR″ and AlCl3 or ClAlMe2 gave the bis-ligand compounds R″2AlCl (4) and R″2AlMe (6), respectively. As deduced from proton NMR spectroscopy,
the formation of 6 proceeded through the intermediate
R″AlMe2 (5) and was facilitated in
the presence of tmeda. The formation of 4 and 6, respectively, is diastereospecific, as only rac isomers were formed (R,R-Λ
and S,S-Δ). Molecular structures
of compounds 2, 3Br, and 6 were determined by single-crystal X-ray analysis.
Salt metathesis of the dihalides 1, 3Cl, and 3Br with 1,1′-dilithioferrocene gave the respective galla- and
alumina[1]­ferrocenes (7 and 8). Neither
compound could be isolated and were only identified by 1H NMR spectroscopy in reaction mixtures. Analytically pure polymers
(7n) of low
molecular weight were found and investigated by DLS (Mw = 8.3 ± 2.5 kDa; DPw = 17 ± 5).