Encapsulation of hydride by molecular main group metal clusters: manipulating the source and coordination sphere of the interstitial ion

Abstract

The sequential treatment of Lewis acids with N,N′-bidentate ligands and thereafter with Bu^tLi has afforded a series of hydride-encapsulating alkali metal polyhedra. While the use of Me₃Al in conjunction with Ph(2-C₅H₄N)NH gives Ph(2-C₅H₄N)NAlMe₂1 and this reacts with MeLi in thf to yield the simple ’ate complex Ph(2-C₅H₄N)NAlMe₃Li·thf 3, the employment of an organolithium substrate capable of β-hydride elimination redirects the reaction significantly. Whereas the use of Bu^tLi has previously yielded a main group interstitial hydride in which H⁻ exhibits µ₆-coordination, it is shown here that variability in the coordination sphere of the encapsulated hydride may be induced by manipulation of the organic ligand. Reaction of (c-C₆H₁₁)(2-C₅H₄N)NH with Me₃Al/Bu^tLi yields [{(c-C₆H₁₁)(2-C₅H₄N)N}₆HLi₈]⁺[(Bu^t₂AlMe₂)₂Li]⁻6, which is best viewed as incorporating only linear di-coordination of the hydride ion. The guanidine 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2-a]pyrimidine (hppH) in conjunction with Me₂Zn/Bu^tLi yields the µ₈-hydride [(hpp)₆HLi₈]⁺[Bu^t₃Zn]⁻·0.5PhMe 7. Formation of the µ₈-hydride [(hpp)₆HLi₈]⁺[Bu^tBEt₃]⁻8 is revealed by employment of the system Et₃B/Bu^tLi. A new and potentially versatile route to interstitial hydrides of this class is revealed by synthesis of the mixed borohydride–lithium hydride species [(hpp)₆HLi₈]⁺[Et₃BH]⁻10 and [(hpp)₆HLi₈]⁺[(Et₃B)₂H]⁻12 through the direct combination of hppLi with Et₃BHLi.