Reactions of AlR₃ (R = Me, Et) with H₂NCH₂CH₂NMe₂:  Synthesis and Characterization of Amido- and Imidoalanes

Abstract

The reactions of R 3 Al (R = Me, Et) with N,N -dimethylethylenediamine (DMEDA) in a 1:1 molar ratio produce adducts, R 3 Al·NH 2 C 2 H 4 NMe 2 ( 1a, R = Me; 1b, R = Et), which upon heating in refluxing toluene result in alkane elimination to afford the chelated monomers ( 2a, R = Me; 2b, R = Et), respectively. Compounds 2a, b form adducts with 1 equiv of R 3 Al through the 3-coordinate nitrogen atom to produce ( 3a, R = Me; 3b, R = Et), which can be alternatively prepared by the reaction of R 3 Al with DMEDA in a 2:1 molar ratio by alkane elimination at room temperature. Thermolysis of 3a, b at 140 °C in sealed ampules produces a mixture of cis and trans isomers of tetrametallic imidoalanes, ( 4a, R = Me; 4b, R = Et). Slow recrystallization of the isomeric mixture affords crystals of only the cis isomer for both 4a and 4b, but in solution cis - 4a and cis - 4b equilibrate with the respective trans isomers. The cis → trans isomerization of 4a has been studied by 1 H NMR spectroscopy. The equilibrium has been observed to follow reversible first-order kinetics with Δ H ° = 1.67 ± 0.60 kJ mol - 1 and Δ S ° = 9.07 ± 2.80 J mol - 1 K - 1 . The activation parameters for the cis → trans conversion are Δ H 1 = 125.8 ± 9.3 kJ mol - 1 and Δ S 1 = 89.7 ± 1.2 J mol - 1 K - 1, and those for the reverse process are Δ H - 1 = 124.2 ± 9.3 kJ mol - 1 and Δ S - 1 = 80.7 ± 1.2 J mol - 1 K - 1 . Pyrolysis of 4a, b in the presence of 2 equiv of DMEDA at 190 °C in sealed ampules gives hexameric imidoalanes, (RAlNC 2 H 4 NMe 2 ) 6 ( 5a, R = Me; 5b, R = Et), by alkane elimination. Compounds 5a, b can be also prepared by pyrolysis of 2a, b under similar conditions, but in lower yields. The molecular structures of cis - 4b and 5b have been determined by single-crystal X-ray diffraction. The molecular geometry of cis - 4b involves one four-membered Al 2 N 2 ring and two five-membered AlN 2 C 2 rings of cis conformation, and it has approximate C 2 symmetry with the C 2 axis going through the center of the Al 2 N 2 ring. The hexameric imidoalane 5b consists of a hexagonal prism of the (AlN) 6 cage formed by two flat six-membered (AlN) 3 rings linked together by six transverse Al−N bonds.