Structures and stabilities of group 13 adducts [(NHC)(EX(3) )] and [(NHC)(2) (E(2) X(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 groupa??13 complexes [(NHC)(EX(3) )] and [(NHC)(2) (E(2) X(n) )] (E=B to In; X=H, Cl; n=4, 2, 0; NHC=N-heterocyclic carbene). The monodentate Lewis acids EX(3) and the bidentate Lewis acids E(2) X(n) bind N-heterocyclic carbenes rather strongly in donor-acceptor complexes [(NHC)(EX(3) )] and [(NHC)(2) (E(2) X(n) )]. The equilibrium structures of the bidentate complexes depend on the electronic reference state of E(2) X(n) , which may vary for different atoms E and X. All complexes [(NHC)(2) (E(2) X(4) )] possess C(s) symmetry in which the NHC ligands bind in a trans conformation to the groupa??13 atoms E. The complexes [(NHC)(2) (E(2) H(2) )] 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 Ei ?E pi bond. In contrast, the indium complex [(NHC)(2) (In(2) H(2) )] 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(2) Cl(2) )], but the boron complex [(NHC)(2) (B(2) Cl(2) )] deviates only slightly from C(s) symmetry. The B(2) fragment in the linear coordinated complex [(NHC)(2) (B(2) )] has a highly excited (3)(1) Sigma(g) (-) reference state, which gives an effective B=B triple bond with a very short interatomic distance. The heavier homologues [(NHC)(2) (E(2) )] (E=Al to In) exhibit a anti-periplanar arrangement of the NHC ligands in which the E(2) fragments have a (1)(1) Delta(g) reference state and an Ei ?E double bond. The calculated energies suggest that the dihydrogen release from the complexes [(NHC)(EH(3) )] and [(NHC)(2) (E(2) H(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=beta-diketiminate) with [(NHC)(EH(3) )] becomes increasingly exothermic with the trend B