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

Abstract

Intermetallic lithium−magnesium hexamethyldisilazide [{(Me 3 Si) 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 ring: the Mg center occupies a distorted trigonal planar environment of three N atoms, while the Li center occupies a distorted tetrahedral N 2 C 2 environment due to two additional C−Li interactions with the μ-N(SiMe 3 ) 2 substituents. This structure appears to be retained in arene solution on the basis of the evidence of 1 H NMR spectral data. In the presence of trace amounts of oxygen, the surprising peroxide−oxide variant [{(Me 3 Si) 2 N} 4 Li 2 Mg 2 (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(NSiMe 3 ) 2 } 4 Li 4 Mg(O)]. X-ray crystallography has revealed its remarkable cage structure built around an unprecedented distorted trigonal bipyramidal Li 4 MgO core, at the center of which lies a μ 5 -O atom. Interestingly, all the metal centers lie out of the benzamidinate NCN ligand planes to indicate a significant degree of π-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.