Alkane Coordination by a Neutral, Lewis Acidic Magnesium Complex

Three geometrically constrained, Lewis acidic magnesium diamides [Mg(RNONAr)] (RNONAr = 4,5-bis(ArN)-2,7-dialkyl-9,9-dimethyl-xanthene; R = Et, Ar = 2,4,6-tricyclohexylphenyl (TCHP) 1; R = tBu, 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-CH₂ 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(EtNONTCHP)(κ²-H,H-pentane)] 4, [Mg(EtNONTCHP)(κ²-H,H-hexane)] 5 and [Mg(EtNONTCHP)(κ³-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(EtNONTCHP) fragment to be weak (ΔG°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(EtNONTCHP) 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(EtNONTCHP). Approximately, 40-60% of the orbital interactions between the alkane/TMS and Mg(EtNONTCHP) units in 4-6 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.