Differences Between Amine- and Phosphine-Boranes: Synthesis, Photoelectron Spectroscopy, and Quantum Chemical Study of the Cyclopropylic Derivatives

Abstract

Borane complexes of aziridine, phosphirane, cyclopropylamine, cyclopropylphosphine, cyclopropylmethylamine, and cyclopropylmethylphosphine have been prepared by the reaction at low temperatures of a borane complex or diborane on the free phosphine or amine. The products characterized by NMR spectroscopy and mass spectrometry have then been investigated by photoelectron spectroscopy and B3LYP/aug-cc-pVTZ quantum chemical study. The complexation led to rotamers with structures similar to the ones of the corresponding free systems. The main geometry change with the complexation is the P-C bond elongation and the N-C bond shortening, which can be rationalized by the charge transfer attached to the electron donation. The calculated relative stability order of the conformers changes with the complexation only in the case of cyclopropylamine. The calculated complexation energies are higher for the amines, in accord with the differences observed in the flash vacuum thermolysis of methylamine-, methylphosphine-, and aziridine-borane. The photoelectron spectra indicate essential differences between the amines and phosphines toward borane complexation. The dative bond is more stable in the studied amine-boranes than in phosphine-boranes, while the sigma(B-H) orbitals are more stable in the latter compounds. The enthalpy of the hydrogen release reaction of aziridine-borane is almost thermoneutral, indicating the potential of this complex as recyclable hydrogen storage material.

Direct Experimental Observation of in situ Dehydrogenation of an Amine–Borane System Using Gas Electron Diffraction

“…The dehydrogenation reaction is closer to thermoneutral than ammonia borane and dimethylamine borane, for which the dehydrogenation reaction enthalpies at CCSD(T)/CBS level are -21.3 kJ mol -1 69 and -7.5 kJ mol -1 . 26 The ΔG r value at the CCSD(T)/CBS level is comparable to that calculated for aziridine-BH 3 (-35.8 kJ mol -1 ) at B3LYP/aug-cc-pVTZ level 70 while the ΔS r is slightly lower than that calculated for NH 3 BH 3 (+125.0 J K -1 mol -1 ) at MP2/cc-pVTZ level. 71 The entropy and free energy change for the dehydrogenation of A shows that the reaction is exergonic, energetically feasible and will proceed spontaneously in the forward direction to form B and H 2 under standard conditions.…”

Structural and thermochemical studies of pyrrolidine borane and piperidine borane by gas electron diffraction and quantum chemical calculations

“…These values are larger than the calculated value for aziridine borane (-35.8 kJ mol -1 ) at B3LYP/aug-cc-pVTZ level. 4 The ΔSr for NH3BH3 (+125.0 J/mol/K) calculated at MP2/cc-pVTZ level 38 is comparable to the values calculated for PYB and PIB but slightly higher than that of AZB (117.0 J K -1 mol -1 ). This may indicate that NH3BH3, PYB and PIB, being solids, are likely to have their dehydrogenation taking place in the gas-phase similar to AZB which exists as a liquid in its pure form.…”

“…PYB and PIB were synthesised according to literature methods 4,35 and the purity was checked using 1 H, 13 C and 15 B NMR spectroscopy. The samples were provided for use in the GED apparatus without further purification.…”

“…They have found application as precursors in ceramic production 1,2 and as candidates for onboard chemical hydrogen storage. [3][4][5][6][7] Although closelyrelated linear analogues, such as ammonia borane (NH3BH3) [8][9][10][11][12][13][14][15][16] and alkyl amine boranes, [17][18][19][20][21][22][23][24][25][26][27][28] are well characterised, very few CABs have been studied both in terms of their structure and chemical properties. The first CAB to be synthesised was the three-membered aziridine borane; [29][30][31] the crystal structure was determined subsequently, 32 and the complex was later characterised by NMR and IR techniques 33 as well as low-level ab initio and semi-empirical methods.…”

Boron, aluminium, gallium, indium and thallium

“…The products have been studied using NMR, mass spectrometry and photoelectron spectroscopy and computationally. 44…”