Intermetallic lithium-magnesium hexamethyldisilazide:: Synthesis and structure, discovery of an oxygen-centered variant, and a reaction with benzonitrile that produces a novel amidinate cage compound with a trigonal bipyramidal Li4MgO core

Intermetallic lithium-magnesium hexamethyldisilazide [{(Me3Si)(2)N}(3)LiMg]; 1, has been synthesized from a nonstoichiometric 2:1 ratio of LHMDS and Mg(HMDS)(2) in hexane/toluene solution. Its crystal structure reveals a simple dinuclear arrangement based on a near-planar NMgNLi ring: the Mg center occupies a distorted trigonal planar environment of three N atoms, while the Li center occupies a distorted tetrahedral N2C2 environment due to two additional C-Li interactions with the mu-N(SiMe3)(2) :substituents. This structure appears to be retained in arene solution on the basis of the evidence of H-1 NMR spectral data. In the presence of trace amounts-of oxygen, the surprising peroxide-oxide variant [{(Me3Si)(2)N}(4) Li2Mg2(O-2)(x)(O)(y)] preferentially crystallizes from solution. X-ray crystallography has established that its structure contains peroxide predominantly [x = 0.715(7); y = 0.285(7)], the presence of which induces an aggregation state increase relative to 1: the four indistinguishable metal centers, held in a square-planar arrangement, side-on-coordinate to-the peroxide molecule and bind to two N atoms. The oxygen-scavenging ability of this system has been exploited further in a reaction with benzonitrile. This has yielded a third intermetallic lithium-magnesium crystalline compound, having the composition [{PhC(NSiMe3)(2)}(4)Li4Mg(O)]. X-ray crystallography has revealed its remarkable cage structure built around an unprecedented distorted trigonal bipyramidal Li4MgO core, at the center of which lies a mu(5)-O atom. Interestingly, all;the metal centers lie out of the benzamidinate NCN ligand planes to indicate a significant degree of pi-bonding. One NCN ligand also adopts an unusual face-capping mode to a Li-3 triangle, the like of which has previously only been observed in transition metal cluster compounds.