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).