The role of transition metal ions in hydrogen bonded networks: A density functional molecular orbital theory study

The capacity of biureto- (C2N3O2H 32-) and dithiobiureto- (C2N3S 2H41-) complexes of nickel, L2Ni(bt) and L2Ni(dbt) [L2 = (CO)2, (PH 3)2, (NH3)2, acac -,† (Cl-)2, (CN-) 2] to form hydrogen bonds with complementary probe molecules has been studied using density functional molecular orbital theory. The charge on the trans ligands, L, is found to be the dominant factor in determining the strength of the hydrogen bonding. The π donor and acceptor properties of the ligands are relatively unimportant. Increasing the negative charge on the ligands enhances hydrogen bonding in biureto complexes, where there is an excess of proton acceptor groups. In dithiobiureto complexes, where proton donor groups are in excess, the influence of the trans ligands is much smaller, and in the opposite direction; greater negative charge reducing the strength of the hydrogen bonds. These results suggest an approximately two-fold greater susceptibility of the proton acceptor groups to changes at the metal centre

Novel intermolecular C-H pi interactions: An ab initio and density functional theory study

The recent characterisation of short contacts between chloroform solvate molecules and the C-C triple bond of gold ethynides has prompted a theoretical investigation of the strength of C-H ⋯ π interactions. Extensive ab initio and density functional theory calculations have been performed on a variety of model systems displaying a T-shaped C-H ⋯ π motif. The interaction of ethyne, C2H2, with a variety of small proton donor molecules (HCN, CH4-nCln, n = 0-3) is invariably found to be weak (ΔEint < 10 kJ mol -1). Replacement of the two acetylenic protons with more electron-donating sodium atoms increases the electron density in the C-C π bond and results in a substantial increase in the interaction with the proton donor. The calculated interaction energies rise to as much as 60 kJ mol -1 in the case of C2Na2 ⋯ CHCl 3. The interaction of CHCl3 with a model gold ethynide, H3PAuCCAuPH3, is intermediate between these two extremes, ab initio and density functional calculations both giving estimates of ca.25 kJ mol-1 comparable to a reasonably strong hydrogen bond. The unusually strong C-H ⋯ π interactions in the gold ethynide arise directly as a consequence of the electron-donating properties of the AuPR3 fragment and are fundamentally different to the much weaker C-H ⋯ π interactions in purely organic systems

Comparison of the reactivity of [2.2]paracyclophane and p-xylene

The relative abilities of [2.2]paracyclophane (C16H16) and p-xylene (C6H4Me2-1,4) to form arene tricarbonyl complexes from chromium hexacarbonyl has been studied in dioxane using the Strohmeier reflux method, and the rate constants contrasted. The reactions are found to proceed more quickly with [2.2]paracyclophane by ca. 25%. Density functional molecular-orbital calculations have rationalised this observation, and indicate that the enhanced reactivity of the [2.2]paracyclophane system relative to p-xylene is a consequence of repulsive interactions between the two arene decks in the former, which are relieved to some extent by co-ordination of the electron-withdrawing Cr(CO)3 fragment

Comparison of the reactivity of [2.2]paracyclophane and p-xylene

The relative abilities of [2.2]paracyclophane (C16H16) and p-xylene (C6H4Me2-1,4) to form arene tricarbonyl complexes from chromium hexacarbonyl has been studied in dioxane using the Strohmeier reflux method, and the rate constants contrasted. The reactions are found to proceed more quickly with [2.2]paracyclophane by ca. 25%. Density functional molecular-orbital calculations have rationalised this observation, and indicate that the enhanced reactivity of the [2.2]paracyclophane system relative to p-xylene is a consequence of repulsive interactions between the two arene decks in the former, which are relieved to some extent by co-ordination of the electron-withdrawing Cr(CO)3 fragment