An Extraordinarily Distorted cyclo-Tetrastibine. Crystallographic Structure of a New Polymorph of cyclo-(σ-C₅Me₅Sb)₄

Abstract

A new crystalline polymorph of cyclo-(Cp*Sb) 4 has been crystallographically characterized that contains the mostly acutely angled four-membered, homoatomic ring system known (Sb-Sb-Sb ) 76.1(1)°). In the previously reported form, the Cp* and Sb 4 ring centroids (viewed along the C-Sb bond) are anti; in the new form the Cp* ring rotates over 100°to a more crowded gauche form. The new form melts at a temperature 72 °C higher than the one previously reported and is about 15% more dense. While the Sb-Sb ring distances are normal (2.836(1) Å), the transannular Sb‚‚‚Sb distances are the shortest known for antimony (3.518(1) Å) and apparently provide a stabilization that compensates for the extreme ring distortion.

Syntheses and chemistry of hypervalent cyclo-R4Sb4, cyclo-(RSbE)n[R = 2-(Me2NCH2)C6H4, E = O, S] and precursors

“…The irregular trigonal-pyramidal environment of the tetra-coordinate Sb atom is reflected in the corresponding bond angles (Table 2). The Sb-W bond lengths [2.7547 (10) crystallizes as a 1 : 1 mixture of R,R,S and S,S,R isomers. The molecular structure of the R,R,S isomer of 3 is shown in Fig.…”

“…Over the last decades the investigation of organoantimony oligomers, R n Sb n , has revealed an extraordinary diversity of structural features and of the chemical behaviour of cyclic and noncyclic systems under a strong influence of the organic substituents. [1][2][3][4][5][6] Bulky groups [R = t Bu, 7 Mes, 8 Cp*, 9,10 (Me 3 Si) 2 CH 11,12 ] protect four-membered rings, cyclo-R 4 Sb 4 , and preserve the ring size on phase transition. 3 In contrast, when aromatic groups, e.g.…”

Die ersten Organobismutringe: (RBi)3 und (RBi)4 (R=(Me3Si)2CH)

“…Einen entgegengesetzen Einfluû haben stabilisierende transanulare E´´´E-Wechselwirkungen. [15] Die beim Übergang zu 2 auftretende leichte Zunahme der Faltung kann auf transanulare Bi´´´Bi-Wechselwirkungen zurückgeführt werden. Die entsprechenden Bi-Bi-Abstände liegen mit 383.4 und 385.9 pm deutlich unter der Summe der van-der-Waals-Radien, die mit 480 pm angegeben wird.…”

The First Organobismuth Rings: (RBi)3 and (RBi)4, R=(Me3Si)2CH

“…An opposing effect is exerted by stabilising transannular E´´´E interactions. [15] The small increase of folding on the transition to the bismuth ring 2 may be related to transannular Bi´´´Bi contacts. The corresponding Bi À Bi distances are 383.4 and 385.9 pm.…”