Thermal and Catalytic Dehydrogenation of the Guanidine–Borane Adducts H 3 B·hppH (hppH = 1,3,4,6,7,8‐hexahydro‐2 H ‐pyrimido[1,2‐ a ]pyrimidine) and H 3 B·N(H)C(NMe 2 ) 2 : A Combined Experimental and Quantum Chemical Study

Herein thermal and catalytic dehydrogenation of the guanidine–borane adducts H 3 B · hppH (hppH = 1,3,4,6,7,8‐hexahydro‐2 H ‐pyrimido[1,2‐ a ]pyrimidine) and H 3 B · N(H)C(NMe 2 ) 2 are analysed. Thermal decomposition of H 3 B · hppH at 80 °C leads to [HB(μ‐hpp)] 2 and a second boron hydride, which is tentatively identified as [(κ 2 N‐hpp)BH 2 ]. Decomposition in boiling toluene (110 °C) leads to a mixture of [H 2 B(μ‐hpp)] 2 and [HB(μ‐hpp)] 2 , from which [H 2 B(μ‐hpp)] 2 can be separated and crystallised. In the presence of a catalyst (with Cp 2 TiCl 2 / n BuLi or [Rh(1,5‐cod)Cl] 2 as precatalysts) dehydrogenation at 80 °C leads predominantly to [H 2 B(μ‐hpp)] 2 . In the case of H 3 B · N(H)C(NMe 2 ) 2 uncatalysed dehydrogenation turns out to be a very slow process even at 110 °C. Interestingly, the ultimate product of this process is oligomeric methylimino borane, [HBNMe] n . This pathway can be modelled and understood with the aid of quantum chemical calculations. Faster dehydrogenation can be initiated by addition of a catalyst. Finally, the possible mechanisms for thermal and Cp 2 Ti‐catalysed dehydrogenation are analysed for the model compound H 3 B · N(H)C(NH 2 ) 2 by means of quantum chemical (DFT) calculations.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)