PREPARATION OF VOLATILE MOLECULAR LITHIUM-NIOBIUM ALLIOXIDES - CRYSTAL-STRUCTURES OF [NB(MU-OCH2SIME3)(OCH2SIME3)(4)](2) AND -OCH2SIME3)-(MU(2)-OCH2SIME3)(2)(OCH2SIME3)(3)](2)

New heterometallic alkoxides [LiNb(OCH2CMe3)(6)](2) (4), LiNb(OCH2CMe3 )(6)(Me3CCH2OH)(2) (5), and [LiNb(OCH2SiMe3)(6)](2), (6) have been pre pared from the corresponding homometallic alkoxides. All are moderatel y soluble in hydrocarbons and sublime in moderate to good recoveries. The structure of 6 determined crystallographically contains an Nb2Li2( mu-O)(4)(mu(3)-O)(2) central-core geometry. Compound 4 appears to be i sostructural on the basis of H-1 NMR data. Bulk pyrolysis of 4 gives c rystalline LiNbO3 under N-2 at 600 degrees C. However, LiNbO3 films ar e not obtained from 4 by spray MOCVD under conditions that give LiNbO3 films from [LiNb(OEt)(6)](infinity). Crystallographic structure deter mination for the precursor compound [Nb(mu-OCH2SiMe3) (OCH2SiMe3)(4)]( 2) (3) reveals an edge-shared bioctahedral geometry in which the mu-O atoms are distinctly pyramidal. This structure is maintained in soluti on; the pyramidal bridge configurations allow coexistence of syn and a nti dimeric isomers. Precursor 3 also exhibits a dimer-monomer equilib rium for which Delta H = 63(3) kJ/mol and Delta S = 180(10) J/(K mol). (C) 1998 Elsevier Science Ltd. All rights reserved.