Group 13/15 heterocycles of the heavier homologues
Group 13/15 heterocycles of the heavier homologues
Group 13/15-compounds have received growing interest due to their potential use as single-source precursors for the synthesis of the corresponding binary materials. While nitrogen- and phosphorus-containing compounds have been studied in detail for a long time, compounds of the heavier homologues of group 15 were almost unknown. Only two Ga-Sb and In-Sb heterocycles, which were synthesized by the reaction of t-Bu2SbLi with Me2GaCl or Me2InCl by LiCl-elimination (salt elimination), as well as t-Bu2SbSiMe3 with GaCl3 or InCl3 by elimination of Me3SiCl (Dehalosilylation), were reported in the literature.
Compounds of the type [R2MER'2] are available as four- (x = 2) or six-membered (x = 3) rings, and can formally be described as intermolecular stabilized "head-to-tail adducts".
Compounds containing the heaviest group 15 elements, which typically show a low thermal stability and high reactivity, couldn't be prepared by conventional synthetic methods. As a consequence, alternate synthetic methods were developed in our group and we further investigated their structures and reactivity in detail.
R2MCl react with R'2Sb(SiMe3) with elimination of Me3SiCl as was shown by Wells et al. and our group. Compared with the salt elimination reaction, this reaction avoids filtration under inert gas conditions since the resulting trimethylchlorosilane can be easily removed in vacuo.
Ga-Sb and In-Sb heterocycles are easily accessible by the dehalosilylation reaction, while this method generally failed to give the corresponding Al-Sb and M-Bi heterocycles.
Dialkylalanes and -gallanes R2MH (M = Al, Ga) react with tris(trimethylsilyl)penteles E(SiMe3)3 (E = P, As, Sb, Bi) with elimination of trimethylsilane and formation of the desired M-E-heterocycles. This reaction allowed for the first time the synthesis of both Al-Sb and M-Bi heterocycles (M = Al, Ga).[3,4].
The degree of oligomerization (ring size) of the resulting heterocycles is determined by the steric bulk of the substituents: small substituents lead to the formation of six-membered heterocycles, while sterically demanding substituents generate preferably four-membered rings.
Dialkylsilylstibines of the type R2SbSiMe3 can also be used as was shown in the reaction of t-Bu2SbSiMe3 with R2AlH, yielding completely alkyl-substituted heterocycles [R2AlSb(t-Bu)2]x, which are promising precursors in material synthesis due to the lack of any further heteroatom.
Distibine cleavage reaction
Distibine bis-adducts of the type [R3M]2[Sb2R'4] (M = Ga, In) are stable in substance, but they rapidly react in solution with Sb-Sb and M-C bond breakage and subsequent formation of the desired M-Sb heterocycles as well as the mixed-substituted stibine RSbR'2.
This reaction pathway generally allows the synthesis of completely alkyl-substituted M-Sb heterocycles. In contrast, dibismuthine adducts [R3M]2[Bi2R'4] (M = Al, Ga) react under disproportionation and formation of elemental bismuth and the simple Lewis acid-base adducts R3M-BiR'3.
Base-stabilized monomers of the type dmap–Al(Me2)E(SiMe3) (dmap = 4-dimethylamino-pyridine) react with group 13-trialkylene MR3 (M = Ga, In, Tl) at very low temperatures with formation of [Me2ME(SiMe3)2]x and dmap-AlMe3 (metathesis reaction).
This reaction occurs through initial formation of the adducts dmap–Al(Me2)E(SiMe3)-MR3, which could be isolated and structurally characterized with sterically demanding trialkylalanes and -gallanes. In contrast, analogous MMe3 adducts react under Al/M exchange to the described heterocycles. This specific metathesis reaction for the first time allowed the synthesis of extremely thermolabile compounds such as [Me2InBi(SiMe3)2]3 as well as previously unknown Tl-E-heterocycles [Me2TlE(SiMe3)2]x (E = P, As, Sb). Analogous reactions were observed with base-stabilized organogalliumchlorides and-hydrides.
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