Bismuthine adduct

The acid-base interaction in group 13-stibine and -bismuthine adducts seems to be very weak as is indicated by mass spectroscopic studies, which never showed the molecular ion peak but only the respective Lewis acid and Lewis base fragments. The extreme lability in the gas phase may also account for the fact that there are only very few reports on thermodynamic data of group 13-stibine or bismuthine adducts in the literature. Therefore, multinuclear NMR spectroscopy and single crystal X-ray diffraction are the most important analytical tools for the characterization of such adducts. 

In contrast to these trends observed for the stibine adducts, the H-NMR spectra of the bismuthine adducts R3M—BiR without exception show almost the same chemical shifts due to the organic groups as the starting trialkylalanes, -gallanes and -bismuthines, again indicating very weak acid-base interactions in solution. 

Reliable information on the thermodynamic stability of group 13/15 adducts is usually obtained by gas phase measurements. However, due to the lability of stibine and bismuthine adducts in the gas phase toward dissociation, temperature-dependent H-NMR studies are also useful for the determination of their dissociation enthalpies in solution [41b], We focussed on analogously substituted adducts t-BusAl—E(f-Pr)3 (E = P 9, As 10, Sb 11, Bi 12) since they have been fully characterized by single crystal X-ray diffraction, allowing comparisons of their thermodynamic stability in solution with structural trends as found in their solid state structures. 

Additional studies on R3AI—Bi(Tms)3 (R = Me 13, Et 14) showed the extreme lability of alane-bismuthine adducts toward dissociation in solution. Their H-NMR spectra at ambient temperature only show one resonance due to the Al—R groups, while at -70 °C two resonances of the A1—R groups in a... 

Up to now, fifteen group 13-stibine R3AI—SbR and four group 13-bismuthine adducts R3AI—BiR3 have been structurally characterized by single crystal X-ray diffraction studies. Their central structural parameters are summarized in Table 5. Structures 1-4 show the solid state structures of four representative adducts. 

Scheme 3. Synthesis of group 13-stibine and bismuthine adducts.

Computational calculations were performed in order to quantify the role of the substituents on the adduct stability of stibine and bismuthine adducts. These studies were completed by calculations of the corresponding phosphine and arsine adducts in order to reveal the influence of the group 15 element, as shown in Table V.47... 

It should also be stated that the M-E bond distances within the Et3M <— E(SiMe3)3 and t-Bu3M <- E(/-Pr)3 adduct families increase by about 40 pm according to the increase of the covalent radii of the group 15 element (rCOv(P) 110 pm, rcov(Bi) 150 pm). Within the sterically overcrowded t-Bu3M <— E(z-Pr)3 adduct families the arsine and bismuthine adducts show... 

As already was observed for hypercoordinated adducts MX3(ER 3)2, no stibine and bismuthine adducts of low-valent alanes, gallanes or indanes have been prepared, to date. According to the lability of low-valent group 13 compounds toward disproportionation into M(III) and elemental M, stibines and bismuthines are expected to be too weak as Lewis bases, preventing them from the stabilization of such compounds. 

Table 2 summarizes important structural parameters for alane-stibine and -bismuthine adducts R3A1-ER 3, while Table 3 features those for distibines and dibismuthines precursors R2E-ER 2 (E = Sb, Bi) and the corresponding alane adducts. 

Table 2 Selected bond lengths (A) and angles (°) for alane-stibine and alane-bismuthine adducts...

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