New Carbaalanes Containing Clusters of Aluminum and Carbon Atoms:  (AlEt)₈(CCH₂C₆H₅)₅H, (AlMe)₈(CCH₂C₆H₅)₅(C⋮C-C₆H₅), and (AlMe)₇(CCH₂CH₃)₄H₂. Quantum Chemical Characterization

Abstract

The open carbaalane cluster (AlEt)7(CCHC6H5)2(CCH2C6H5)3H (2), which was obtained in our group only recently, reacted with an excess of diethylaluminum hydride with hydroalumination of its CC double bonds and formation of the compound (AlEt)8(CCH2C6H5)5H (4). The central cluster of the product comprises a cube of eight Al atoms, five faces of which are occupied by C-benzyl groups; the sixth face is bridged by a hydrogen atom. The carbaalane (AlMe)8(CCH2C6H5)5(C⋮CC6H5) (5) was obtained by hydroalumination of Me2AlC⋮CC6H5 by Me2AlH at 60 °C. Its structure is similar to that of 3, but the bridging hydrido ligand has been replaced by an ethynyl group, which is coordinated to four Al atoms and shows a quite unusual C⋮C stretching vibration of 1956 cm-1. The first carbaalane possessing only aliphatic substituents, (AlMe)7(CCH2CH3)4H2 (6), was obtained by the reaction of Me2AlC⋮CCH3 with Me2AlH. Its structure contains an Al7C4 cluster, which is quite similar to that of the closo-borate [B11H11]2-. An intense emission of a cluster vibration characteristic for carbaalanes was assigned at about 325 cm-1 in the Raman spectra. The structure and bonding of arachno-type cluster compounds related to 4 were investigated by quantum chemical calculations. A delocalized multicenter bonding situation was verified with only weak direct Al−Al interactions.

New Carbaalanes − (AlMe)8(CCH2Me)5(C≡C−Me) and the THF Adduct (AlMe)8(CCH2Me)5H·2THF

“…A crystal structure determination of 5 revealed a structure similar to that of 1 [2,4] (Figure 1) with a cube of eight aluminium atoms. However, two opposite aluminium atoms of the hydrogen-bonded face are coordinated by a THF molecule.…”

“…[9] All these parameters were similarly determined in carbaalanes of type 1. [2,4] The most notable devi- ation from these recently published structures is a greater distortion of the hydrogen-bonded face with strongly differing transannular AlϪAl distances of 483.2 pm (Al2ϪAl2Ј, unsupported) and 331.8 pm (Al4ϪAl4Ј, hydrogen bonded). The latter distance resembles that in the trimeric compound [(Me 3 C) 2 AlH] 3 containing a six-membered Al 3 H 3 heterocycle with three µ 2 -bridging hydrogen atoms.…”

“…[4] As a main product, we isolated the heptaaluminium carbaalane (AlMe) 7 (CCH 2 -Me) 4 H 2 in 63% yield, which possesses the closo-structure 3 shown in Scheme 1. In all of these reactions, we observed the formation of a hitherto uncharacterized by-product [4; Equation (1)], which is almost insoluble in nonpolar solvents such as pentane or toluene and could not be purified by recrystallization.…”

“…[5] The bonding situation in the carbaalanes has been studied by quantum-chemical calculations. [4] They revealed a delocalized multi-centre bonding with only weak direct AlϪAl interactions. We report herein 3059 two opposite aluminium atoms of this face are coordinated by one THF ligand each.…”

Hydroalumination versus Deprotonation of Alkynes with Sterically Demanding Substituents

“…This is evident from a modest lowering of the ν(C≡C) stretching frequency in the IR spectrum (by 50 to 100 cm −1 ) of the bridging C≡C-R group as compared to terminal alkynyl groups (Table 1, c. f. compound 3), type A alkynides (Me 2 Al-C≡C-Ph 2089 cm −1 [11]) or terminal alkynes (Ph-C≡C-H 2119 cm −1 , [23]). The different structural motifs may be recognised in the 13 C NMR spectrum by a larger (except compound 1e) difference (≥ 30 ppm) between the chemical shifts of the carbon atoms of the C≡C triple bond as compared to type A compounds (≤ 30 ppm) with the signal of the metal bound carbon atom being shifted significantly to higher field in comparison to that of the carbon atom bound to the substituent R . In terminal fragments, monomeric compounds (c. f. compound 4 and ref.…”