Reactions of AlR3 (R = Me, Et) with H2NCH2CH2NMe2:: Synthesis and characterization of amido- and imidoalanes

The reactions of R3Al (R = Me, Et) with N,N-dimethylethylenediamine (DMEDA) in a 1:1 molar ratio produce adducts, R3Al . NH2C2H4NMe2 (1a, R = Me; Ib, R = Et), which upon heating in refluxing toluene result in alkane elimination to afford the chelated monomers R2AlNHC2H4NMe2 (2a, R = Me; 2b, R = Et), respectively. Compounds 2a,b form adducts with 1 equiv of R3Al through the 3-coordinate nitrogen atom to produce R3AlN(H)C2H4NMe2AlR2 (3a, R = Me; 3b, R = Et), which can be alternatively prepared by the reaction of R3Al with DMEDA in a 2:1 molar ratio by alkane elimination at room temperature. Thermolysis of 3a,b at 140 degrees C in sealed ampules produces a mixture of cis and trans isomers of tetrametallic imidoalanes, [R2Al(mu-NC2H4NMe2AlR2)](2) (4a, R = Me; 4Cb, 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 H-1 NMR spectroscopy. The equilibrium has been observed to follow reversible first-order kinetics with Delta H degrees = 1.67 +/- 0.60 kJ mol(-1) and Delta S degrees = 9.07 +/- 2.80 J mol(-1) K-1. The activation parameters for the cis --> trans conversion are Delta H-1 = 125.8 +/- 9.3 kJ mol(-1) and Delta S-1 = 89.7 +/- 1.2 J mol(-1) K-1, and those for the reverse process are Delta H-1 = 124.2 +/- 9.3 kJ mol(-1) and Delta 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 degrees C in sealed ampules gives hexameric imidoalanes, (RAlC2H4NMe2)(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 Al2N2 ring and two five-membered AlN2C2 rings of cis conformation, and it has approximate C-2 symmetry with the C-2 axis going through the center of the Al2N2 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.