Pentacarba-arachno-tridecaalane (AlMe)8(CCH2Ph)5H with an Al8C5 Skeleton—The First Polyhedral Carbaalane

Abstract

Hydroalumination of an aluminum alkynide led to the formation of a polyhedral carbaalane as the first example of a new class of aluminum derivatives analogous to carbaboranes. A pentacarbatridecaalane was obtained, which has an Al C skeleton and which, owing to the number of electrons in the cluster, belongs to the arachno-type group of compounds (see structure).

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

“…Compound 2 (Scheme 1) may be derived from this cubic arrangement, but it has a unique structure with one vertex unoccupied and possesses only seven aluminium atoms. The three remaining Al 4 faces are still occupied by CϪCH 2 ϪR groups, while the open site of the cluster is coordinated by one hydrogen atom and two alkenido ligands containing CϭC double bonds. This structure results from an incomplete hydroalumination reaction, and the formal addition of one equivalent of ethylaluminium dihydride to complete the Al 8 cube is missing.…”

“…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.…”

“…[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. [10] The THF molecules are bound to the aluminium atoms Al2 and Al2Ј with relatively long AlϪO ''dative'' distances of 197.9 pm.…”

“…[2Ϫ4] Two completely different structure types have so far been observed. One may be described by a cube of eight aluminium atoms (1, Scheme 1), the six faces of which are occupied by five CϪCH 2 ϪR groups and one hydrido or alkynido ligand. Compound 2 (Scheme 1) may be derived from this cubic arrangement, but it has a unique structure with one vertex unoccupied and possesses only seven aluminium atoms.…”

Hydroalumination versus Deprotonation of Alkynes with Sterically Demanding Substituents

“…In the related compounds 2 and 3 one of the alkynide groups is replaced by a bridging hydride anion. 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 .…”