Alkane Coordination by a Neutral, Lewis Acidic Magnesium Complex

Three geometrically constrained, Lewis acidic magnesium diamides [Mg(^RNON^Ar)] (^RNON^Ar = 4,5-bis(ArN)-2,7-dialkyl-9,9-dimethyl-xanthene; R = Et, Ar = 2,4,6-tricyclohexylphenyl (TCHP) <b>1</b>; R = <i>t</i>Bu, Ar = TCHP <b>2</b>; R = Et, Ar = 2,6-di(3-pentyl)phenyl (Dipep) <b>3</b>) have been synthesized. X-ray crystal structures of <b>1</b> and <b>2</b> reveal them to be dimeric, with Mg centers intermolecularly coordinated by a cyclohexyl-CH₂ fragment from an opposing monomer unit. Treating <b>1</b> with <i>n</i>-pentane, <i>n</i>-hexane, or tetramethylsilane (TMS) gave the first examples of group 2 metal-alkane/TMS complexes, [Mg(^EtNON^TCHP)(κ²-<i>H,H-</i>pentane)] <b>4</b>, [Mg(^EtNON^TCHP)(κ²-<i>H,H-</i>hexane)] <b>5</b> and [Mg(^EtNON^TCHP)(κ³-<i>H,H,H-</i>TMS)] <b>6</b>. While the coordinated alkane in <b>4</b> and <b>5</b> is displaced in cycloalkane solutions, compound <b>6</b> was spectroscopically observed to remain intact at low temperatures. Neutron and/or X-ray crystal structures of <b>4</b>-<b>6</b> confirm their formulations in the solid state. DFT analyses of <b>4</b>-<b>6</b> reveal the bonding between the alkane/TMS and Mg(^EtNON^TCHP) fragment to be weak (Δ<i>G</i>°_{298 K} = -2.9 to -5.6 kcal mol^-1) but comparable to related values for transition metal σ-alkane complexes. ETS-NOCV and NBO analyses of the bonding in <b>4</b>-<b>6</b> indicate that electrostatic interactions between the alkane/TMS and Mg(^EtNON^TCHP) fragments dominate (ca. 65%), but dispersion (ca. 20%) and orbital (ca. 15%) interactions are significant. Dispersion forces predominantly result from interaction of the alkane/TMS unit with the TCHP flanked lipophilic coordination pocket of Mg(^EtNON^TCHP). Approximately, 40-60% of the orbital interactions between the alkane/TMS and Mg(^EtNON^TCHP) units in <b>4</b>-<b>6</b> result from charge flow from the alkane/TMS CH₂ or CH₃ fragment to the magnesium center. This is supported by NCI and QTAIM analyses of the compounds.