Preparation and structures of some new pyrrolidinido- and piperidinidoalanes and -aluminates

Abstract

Alane and lithium tetrahydroaluminate each reacted with an excess of pyrrolidine or piperidine in tetrahydrofuran (thf) to give the new dinuclear compounds {Al[[graphic omitted]H₂]₃}₂[n= 3 (from pyrrolidine)1 or 4 (from piperidine)2] and L₂Li[µ-[graphic omitted]H₂]₂Al[[graphic omitted]H₂]₂[n= 3, L = thf 3 or H[graphic omitted]H₂4;n= 4, L = thf 5]. The compounds were characterised by elemental analysis, multinuclear NMR spectroscopy, mass spectrometry and, for 1, 4 and 5, X-ray crystallography. In 1, which contains an Al₂N₂ ring, average distances are Al–N(terminal) 1.797(2) and Al–N(bridging) 1.963(2)Å. Exocyclic N–Al–N angles are in the range 112.6–114.4°, the endocyclic N–Al–N angle is 86.69(9)° and Al–N–Al is 93.31(9)°. The sums of the angles at the terminal nitrogens are close to 360°. In 4, which contains an LiN₂Al ring, distances are Al–N(terminal) 1.824(8), Al–N(bridging) 1.880(7), Li–N(terminal) 2.035(20) and Li–N(bridging) 2.149(20)Å. The sums of the angles at terminal nitrogens are 349.1 and 355.4°. Ring angles are N–Al–N 100.0(3), Al–N–Li 82.0(4) and N–Li–N 84.2(5)°. In 5 average distances are Al–N(terminal) 1.828(3), Al–N(bridging) 1.895(3), Li–O 1.987(6)Å and Li–N(bridging) 2.135(6)Å. The sums of the angles at terminal nitrogens are 358.1 and 357.4°. Ring angles are N–Al–N 100.69(13), Al–N–Li 84.0(2) and N–Li–N 86.2(2)°. Exchange between bridging and terminal amido groups is slow on the NMR time-scale at 250 MHz in 1 and 2 but fast in 3–5. Separate signals for axial and equatorial protons are observed from cooled samples of 2 and 5.