A Soluble Alkoxide of Barium with a Cubane Structure

There has been recent interest in alkaline earth metal alkoxides and aryloxides in an effort to make volatile precursors for the preparation of superconducting thin films using organometallic chemical vapor deposition (OMCVD). This has been especially true of barium, as it is an important metal in may of the high-temperature superconductors (YBa[sub 2]Cu[sub 3]O[sub 7], HgBa[sub 2]CuO[sub 4], HgBa[sub 2]Ca[sub 2]Cu[sub 3]O[sub 8]). A problem in the attempted synthesis of a well-defined [Ba(OR)[sub 2]][sub n] species has been the formation of oxo complexes as in [H[sub 4]Ba[sub 6]([mu][sub 6]-O)(OCH[sub 2]CH[sub 2]OCH[sub 3])[sub 14]], [Ba[sub 4]([mu][sub 4]-O)([mu][sub 2]-OC[sub 6]H[sub 2](CH[sub 2]NMe[sub 2])[sub 3]-2,4,6)[sub 6]], and [H[sub 2]Ba[sub 8]([mu][sub 5]-O)[sub 2](OPh)[sub 14]-(HMPA)[sub 6]]. The high nuclearity of these compounds decreases their volatility and thus their usefulness for OMCVD. The origin of the oxo ligand has not yet been determined with certainty but is unlikely to stem from adventitious water. The absence of an oxo ligand appears to decrease the nuclearity of the complexes synthesized. More recently, the synthesis of oxo-free silyloxides, [sup 8-10] aryloxides, and alkoxides was reported. The nuclearity of these compounds is usually not more than 3 with the exception of [Ba[sub 6](OPh)[sub 12](TMEDA)[sub 4]]. Another approach has been the use of multidentate ligands such as in [Ba(OAr)[sub 2](18-crown-6)] and [N(C[sub 2]H[sub 4]O)(C[sub 2]H[sub 4]OH)[sub 2]][sub 2]Ba. The high coordination number required by the large barium has thus far always been achieved by added neutral donor ligands such as THF, TMEDA, or the pendant arm of an amine alkoxide. The authors now report the first structurally-characterized homoleptic alkoxide with an unusual cubic structure.