Structures and stabilities of group 13 adducts [(NHC)(EX₃)] and [(NHC)₂(E₂X_n)] (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

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(E₂X_n)] (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 E₂X_n bind N-heterocyclic carbenes rather strongly in donor-acceptor complexes [(NHC)(EX₃)] and [(NHC)₂(E₂X_n)]. The equilibrium structures of the bidentate complexes depend on the electronic reference state of E ₂X_n, which may vary for different atoms E and X. All complexes [(NHC)₂(E₂X₄)] possess C_s 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 C_s 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 C_i form is calculated for all chloride systems [(NHC)₂(E ₂Cl₂)], but the boron complex [(NHC)₂(B ₂Cl₂)] deviates only slightly from C_s symmetry. The B₂ fragment in the linear coordinated complex [(NHC) ₂(B₂)] has a highly excited (3)¹Î£ _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)¹σ_g reference state and an E=E double bond. The calculated energies suggest that the dihydrogen release from the complexes [(NHC)(EH₃)] and [(NHC)₂(E₂H_n)] 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)(E₂X_n)] (see scheme) depend on the electronic reference state of E₂X_n (E=B-In; X=H, Cl; n=0-2).