Bindungssituation in NHC- und Phosphin-Komplexen [(L)m(EmHn)] (m = 1, 2; n = 0 – 4) der Gruppen 13 – 15 und ihre Anwendbarkeit als lösliche Wasserstoffspeicher

In the present work the bonding situation and the reactivity of NHC and PMe3 complexes [(L)m(EmHn)] (m = 1, 2; n = 0 – 4) of group 13 to 15 was analyzed. The starting point was the synthetically known hydrogenation of the magnesium(I) dimer [(L)2(Mg2)] (L = [(MesNCMe)2CH]– with Mes = mesityl) via dimerization of two [(NHCDipp)(AlH3)] giving [(NHCDipp)2(Al2H4)] and some other synthetically known compounds [(L)n(EnHm)]. Furthermore, the study was expanded towards as yet unknown compounds [(L)n(EnHm)] of groups 13 to 15. Model systems of the ligands NHC with methyl groups on nitrogen and PMe3 were employed. In addition to the optimization of geometries and the calculations of reaction energies of the complexation, dimerization and hydrogen elimination various bonding analyses were carried out. This includes energy decomposition analysis with its extension natural orbitals for chemical valence (EDA-NOCV) of the donor-acceptor and E–E bonds, Wiberg bond indices (WBI) of the E–H, E–E and donor-acceptor bonds, NBO partial charges of the involved bond atoms and the investigation of the Lewis structure by NBO analysis. The investigation of the hydrogenation of the magnesium(I) dimer focused on the applicability of different group 13 monomers [(L)(EH3)] as hydrogen source yielding an energetic improvement of the reaction for heavier complexes and by utilization of the phosphine ligand. Both employed ligands exhibit a free electron pair as HOMO which is capable of σ donation. In the EDA-NOCV, the latter is found to be more pronounced for the NHC ligands than for PMe3. The LUMO+1 of NHC shows better π backdonation abilities than its orbital analogue of PMe3, however, in PMe3 there is a second orthogonal and energetically degenerate LUMO+1 which can participate in π backdonation if the symmetry of the complex is accordingly. Within the groups, the geometries of the dimer complexes [(L)2(E2Hn)] may have different structure motives. The complexes of the lighter atoms of the groups rather show a planar coordination on E and trans oriented ligands while for heavier E there is a tendency towards pyramidal coordination on E as well as towards gauche oriented ligands in some cases. The latter properties are more distinct in the PMe3 complexes than in those with NHC. With scarce exceptions, which already stand out by highly different bonding motives, the EDA-NOCV results in donor-acceptor bonds that are stronger for the NHC compounds than for the phosphine ligated for the analyzed group 13 to 15 complexes. In return, the latter show stronger E–E bonds. The WBI of the corresponding bonds supports these findings. Furthermore, the NBO analysis shows considerably higher negative partial charges for the EmHn moieties of the phosphine compounds. This is also due to the lower π backdonation to PMe3. The shape of the molecular orbitals and the analysis of the Lewis structure of the compounds show a trend towards stabilization of electron density on E and thus the formation of free electron pairs or strongly localized E–E π bonds for heavier complexes within the groups. This can be explained by the inert pair effect and is enforced by the use of the PMe3 ligand. This results in the deviation from planar coordination on E such as the trans orientation of the ligands in heavier complexes. Overall the complexes of the heteroatoms boron, carbon and nitrogen of the second period of the periodic table exhibit stronger donor-acceptor und E–E bonds than their heavier homologues. Also NBO analysis gives considerably higher partial charges in the centre moiety which may be explained by the higher electronegativity of these atoms. In contrast to the heavier homologues there is also a protic bonding situation found for the E–H bonds for the lighter complexes.