Alkane Coordination by a Neutral, Lewis Acidic Magnesium Complex

Three geometrically constrained, Lewis acidic magnesium diamides [Mg( R NON Ar )] ( R NON Ar = 4,5-bis(ArN)-2,7-dialkyl-9,9-dimethyl-xanthene; R = Et, Ar = 2,4,6-tricyclohexylphenyl (TCHP) 1; R = t Bu, Ar = TCHP 2; R = Et, Ar = 2,6-di(3-pentyl)phenyl (Dipep) 3 ) have been synthesized. X-ray crystal structures of 1 and 2 reveal them to be dimeric, with Mg centers intermolecularly coordinated by a cyclohexyl-CH 2 fragment from an opposing monomer unit. Treating 1 with n -pentane, n -hexane, or tetramethylsilane (TMS) gave the first examples of group 2 metal-alkane/TMS complexes, [Mg( Et NON TCHP )(κ 2 - H,H- pentane)] 4, [Mg( Et NON TCHP )(κ 2 - H,H- hexane)] 5 and [Mg( Et NON TCHP )(κ 3 - H,H,H- TMS)] 6 . While the coordinated alkane in 4 and 5 is displaced in cycloalkane solutions, compound 6 was spectroscopically observed to remain intact at low temperatures. Neutron and/or X-ray crystal structures of 4 – 6 confirm their formulations in the solid state. DFT analyses of 4 – 6 reveal the bonding between the alkane/TMS and Mg( Et NON TCHP ) fragment to be weak (Δ G ° 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 4 - 6 indicate that electrostatic interactions between the alkane/TMS and Mg( Et NON 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( Et NON TCHP ). Approximately, 40–60% of the orbital interactions between the alkane/TMS and Mg( Et NON TCHP ) units in 4 – 6 result from charge flow from the alkane/TMS CH 2 or CH 3 fragment to the magnesium center. This is supported by NCI and QTAIM analyses of the compounds.