Comparative study of phosphine and N-heterocyclic carbene stabilized Group 13 adducts [L(EH3)] and [L2(E2H(n))]

Quantum chemical calculations using density functional theory at the BP86/TZ2P level have been carried out to determine the geometries and stabilities of Group 13 adducts [(PMe3)(EH3)] and [(PMe3)2(E2H(n))] (E = B-In; n = 4, 2, 0). The optimized geometries exhibit, in most cases, similar features to those of related adducts [(NHC(Me))(EH3)] and [(NHC(Me))2(E2H(n))] with a few exceptions that can be explained by the different donor strengths of the ligands. The calculations show that the carbene ligand L = NHC(Me) (:C(NMeCH)2) is a significantly stronger donor than L = PMe3. The equilibrium geometries of [L(EH3)] possess, in all cases, a pyramidal structure, whereas the complexes [L2(E2H4)] always have an antiperiplanar arrangement of the ligands L. The phosphine ligands in [(PMe3)2(B2H2)], which has Cs symmetry, are in the same plane as the B2H2 moiety, whereas the heavier homologues [(PMe3)2(E2H2)] (E = Al, Ga, In) have Ci symmetry in which the ligands bind side-on to the E2H2 acceptor. This is in contrast to the [(NHC(Me))2(E2H2)] adducts for which the NHC(Me) donor always binds in the same plane as E2H2 except for the indium complex [(NHC(Me))2(In2H2)], which exhibits side-on bonding. The boron complexes [L2(B2)] (L = PMe3 and NHC(Me)) possess a linear arrangement of the LBBL moiety, which has a B≡B triple bond. The heavier homologues [L2(E2)] have antiperiplanar arrangements of the LEEL moieties, except for [(PMe3)2(In2)], which has a twisted structure in which the PInInP torsion angle is 123.0°. The structural features of the complexes [L(EH3)] and [L2(E2H(n))] can be explained in terms of donor-acceptor interactions between the donors L and the acceptors EH3 and E2H(n), which have been analyzed quantitatively by using the energy decomposition analysis (EDA) method. The calculations predict that the hydrogenation reaction of the dimeric magnesium(I) compound L'MgMgL' with the complexes [L(EH3)] is energetically more favorable for L = PMe3 than for NHC(Me).