The invention provides new methods for synthesis of large boron hydride clusters e.g., boron hydride molecules of the formula BnHm where 5≦n≦96 and m≦n+8, wherein m and n satisfy the electron counting rules of macropolyhedral boranes. The invention is particularly useful for synthesis of octadecaborane (B18H22). Preferred methods of the invention include iteratively generating a conjugate acid from a salt of the [BaHb]c− or [Bn+2Hm−4]2− anion followed by degradation under conditions conducive to concentrating and drying of the conjugate acid to provide a borane BnHm and residual salt of the [BaHb]c− or [Bn+2Hm−4]2− anion which is reused in the method of synthesis. The invention further provides isotopically enriched boranes, particularly isotopically enriched 10B18H22 and 11B18H22.