Structures and stabilities of group 13 adducts [(NHC)(EX3)] and [(NHC)2(E2Xn)] (E=B to In; X=H, Cl; N=4, 2, 0; NHC=N-heterocyclic carbene) and the search for hydrogen storage systems: A theoretical study

Nicole Holzmann, Andreas Stasch, Cameron Jones, Gernot Frenking

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Abstract

Quantum chemical calculations using density functional theory at the BP86/TZVPP level and ab initio calculations at the SCS-MP2/TZVPP level have been carried out for the group13 complexes [(NHC)(EX3)] and [(NHC) 2(E2Xn)] (E=B to In; X=H, Cl; n=4, 2, 0; NHC=N-heterocyclic carbene). The monodentate Lewis acids EX3 and the bidentate Lewis acids E2Xn bind N-heterocyclic carbenes rather strongly in donor-acceptor complexes [(NHC)(EX3)] and [(NHC)2(E2Xn)]. The equilibrium structures of the bidentate complexes depend on the electronic reference state of E 2Xn, which may vary for different atoms E and X. All complexes [(NHC)2(E2X4)] possess Cs symmetry in which the NHC ligands bind in a trans conformation to the group13 atoms E. The complexes [(NHC)2(E2H2)] with E=B, Al, Ga have also Cs symmetry with a trans arrangement of the NHC ligands and a planar CE(H)E(H)C moiety that has a E-E π bond. In contrast, the indium complex [(NHC)2(In2H2)] has C i symmetry with pyramidal-coordinated In atoms in which the hydrogen atoms are twisted above and below the CInInC plane. The latter Ci form is calculated for all chloride systems [(NHC)2(E 2Cl2)], but the boron complex [(NHC)2(B 2Cl2)] deviates only slightly from Cs symmetry. The B2 fragment in the linear coordinated complex [(NHC) 2(B2)] has a highly excited (3)1Σ g - reference state, which gives an effective B≡B triple bond with a very short interatomic distance. The heavier homologues [(NHC)2(E2)] (E=Al to In) exhibit a anti-periplanar arrangement of the NHC ligands in which the E2 fragments have a (1)1σg reference state and an E=E double bond. The calculated energies suggest that the dihydrogen release from the complexes [(NHC)(EH3)] and [(NHC)2(E2Hn)] becomes energetically more favourable when atom E becomes heavier. The indium complexes should therefore be the best candidates of the investigated series for hydrogen-storage systems that could potentially deliver dihydrogen at close to ambient temperature. The hydrogenation reaction of the dimeric magnesium(I) compound [LMgMgL] (L=β-diketiminate) with [(NHC)(EH3)] becomes increasingly exothermic with the trend B<Al<Ga<In. Finding the balance: Quantum chemical calculations show that the equilibrium structures of Group13 donor-acceptor complexes [(NHC)(E2Xn)] (see scheme) depend on the electronic reference state of E2Xn (E=B-In; X=H, Cl; n=0-2).

Original languageEnglish
Pages (from-to)13517-13525
Number of pages9
JournalChemistry - A European Journal
Volume17
Issue number48
DOIs
StatePublished - 25 Nov 2011
Externally publishedYes

Keywords

  • chemical bonding
  • density functional calculations
  • donor-acceptor systems
  • group13 elements
  • hydrogen storage

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