Thermal and Catalytic Dehydrogenation of the Guanidine–Borane Adducts H₃B·hppH (hppH = 1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a]pyrimidine) and H₃B·N(H)C(NMe₂)₂: A Combined Experimental and Quantum Chemical Study

Abstract

Herein thermal and catalytic dehydrogenation of the guanidine–borane adducts H3B·hppH (hppH = 1,3,4,6,7,8‐hexahydro‐2H‐pyrimido[1,2‐a]pyrimidine) and H3B·N(H)C(NMe2)2 are analysed. Thermal decomposition of H3B·hppH at 80 °C leads to [HB(μ‐hpp)]2 and a second boron hydride, which is tentatively identified as [(κ2N‐hpp)BH2]. Decomposition in boiling toluene (110 °C) leads to a mixture of [H2B(μ‐hpp)]2 and [HB(μ‐hpp)]2, from which [H2B(μ‐hpp)]2 can be separated and crystallised. In the presence of a catalyst (with Cp2TiCl2/nBuLi or [Rh(1,5‐cod)Cl]2 as precatalysts) dehydrogenation at 80 °C leads predominantly to [H2B(μ‐hpp)]2. In the case of H3B·N(H)C(NMe2)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 Cp2Ti‐catalysed dehydrogenation are analysed for the model compound H3B·N(H)C(NH2)2 by means of quantum chemical (DFT) calculations.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The Flexible Coordination Modes of Guanidine Ligands in Zn Alkyl and Halide Complexes: Chances for Catalysis

“…One such example is provided by thermal decomposition of the tetramethylguanidine-borane adduct sketched in Equation (1). [11] The BH 3 group migrates to one of the amino N atoms. Then a 1,4-dehydrogenation reaction follows leading to decomposition to give finally (HBNMe) n oligomers.…”

The Borane Complexes Htbo·BH3 and Htbn·BH3 (Htbo = 1,4,6‐Triazabicyclo[3.3.0]oct‐4‐ene, Htbn = 1,5,7‐Triazabicyclo[4.3.0]non‐6‐ene): Synthesis and Dehydrogenation to Dinuclear Boron Hydrides

“…Thermal as well as catalytic dehydrogenation were studied. [9] For reversible dihydrogen addition the thermodynamic properties of the reaction are calculations, and we also analyse the possibility of further reductive dehydrogenation to give the double base-stabilized diboranes (4) [BH(µ-tbo)] 2 and [BH(µ-tbn)] 2 or oligomeric H 2 B(µ-tbn) 2 BH{BH(µ-tbn) 2 BH} n BH(µ-tbn) 2 quite important. The H 2 addition should be associated with a standard Gibbs energy close to zero to allow addition and elimination at low temperatures and pressures (of course subject to a sufficiently low activation barrier or the possibility to decrease the barrier with the aid of a catalyst).…”

Synthesis of a Stable B2H5+ Analogue by Protonation of a Double Base‐Stabilized Diborane(4)

“…The initial thermal dehydrogenation of 1 proved not to be the ideal route to 3; however, we recently showed that 1 can be catalytically dehydrogenated with [{Rh(1,5-cod)Cl} 2 ] in toluene at 80 8C to give 2 (with a B···B separation of 306.5 pm). [8] This species, dissolved in toluene, can be further dehydrogenated at 114 8C in the presence of catalytic amounts of [{Rh(1,5-cod)Cl} 2 ] to yield 3 [Eq. (1)], with a BÀB bond distance of 177.2 pm, in a clean reaction.…”

“…The molecular structures of 1, 2, and 3 from X-ray diffraction results. [7,8] [a] The energy-minimum structures were calculated at the B3LYP/6-31 + + G** level. The d( …”