Die riintgenographisch bestimmten Molekiilstrukturen des Dioxadiboretans 1 sowie des Diselenadiboretans 3 werden mit dem Dithiadiboretan 2 verglichen. Alle drei Vierringe besitzen ein kristallographisch bedingtes Inversionszentrum. Das Chalcogen beeinfluBt die BN-Bindungsliinge praktisch nicht; ferner sprechen auch die Ringinnenwinkel kaum auf eine Anderung des Chalcogen-Atoms an. Dies legt nahe, daD die elektronische Absattigung der Bor-Atome weitgehend von der BN-Bindung bestimmt wird. Der B. * * B-Abstand in 1 ist nur um z 10% grol3er als die B -B-Einfachbindung.Contributions to the Chemistry of Boron, 171') Crystal and Molecular Structure of 2,4-Bis(2,2,6,6-tetramethylpiperidino)-1,3,2,4-dichalcogendiboretanes The molecular structure of the dioxadiboretane 1 and the diselenadiboretane 3, as determined by X-ray structure analysis, is compared with the structure of the dithiadiboretane 2. All three four-membered ring systems possess a crystallographically imposed center of inversion, and are therefore planar. The group VI atom has no marked influence on the BN bond length; also, the internal ring angles are not affected by a change of these atoms. This indicates that the boron atoms are electronically saturated via the BN bonds. The intraannular B . . . B distance exceeds the B-B single bond length by approximately 10%.2,2,6,6-Tetramethylpiperidino-Derivate der 1,3,2,4-Dichalcogendiboretane erhalt man durch Thermolyse oder Photolyse der [2 + 21-Cycloaddukte von Dichalcogenkohlenstoffen mit (tert-Butylimino)(2,2,6,6-tetramethylpiperidino)boran nach (l)2). Die strukturelle Charakterisierung dieser Derivate beschrankte sich bisher auf 2. Da wir nun von 1 und 3 Einkristalle erhielten, bestimmten wir die Struktur dieser Verbindungen mittels Rontgenstrahlenbeugung, urn den EinfluI3 der Ringglieder X auf die Ringgeometrie einerseits und die BN-Bindung andererseits kennenzulernen.

The reaction of borazines, (-BRNR'-)3, with pyrazole, Hpz, gives (µ-pyrazolato)(#i-amido)diboron species of the type R(pz)B-(µ-)(µ-') ( ) (3) containing a central B2N3 ring system. Four compounds of type 3 with R = C2H5 or C6H5 and R' = H or CH3, respectively, were prepared. Identical species can be obtained on interaction of bis((dimethylamino)boryl)amines of the type R'N[BRN(CH3)2]2 with Hpz; they are closely related to the pyrazaboles, 2 (µ-)2 2, which contain a central B2N4 ring. A pathway for the formation of 3 is postulated and supported by NMR data. Conformational isomers of 3 could be identified but not separated. In contrast, RB[N(CH3)2]2 (R = C2H5) reacts with Hpz at room temperature to give (CH3)2HNBR(pz)2 and in refluxing toluene to yield the pyrazabole R(pz) (µ-)2 ( ). The latter compound was also obtained from (-BRNR'-)3 (R' = H, CH3) or 3 (R = C2H5, R' = H) and boiling Hpz (at temperatures near 180-200 °C). The compound could be separated into conformational isomers. The lower melting one with the terminal pz groups in cis (equatorial) position crystallizes tetragonally in space group 14{/acd, a = b = 14.876 A, c = 34.932 A, and Z = 16, with the central B2N4 ring in a boat conformation. The higher melting trans isomer crystallizes monoclinically in space group P2fc, a = 8.

No abstract

No abstract

No abstract