Lanthanide N,N-Dimethylaminodiboranates as a New Class of Highly Volatile Chemical Vapor Deposition Precursors

Abstract

New lanthanide N,N-dimethylaminodiboranate (DMADB) complexes of stoichiometry Ln(H₃BNMe₂BH₃)₃ and Ln(H₃BNMe₂BH₃)₃(thf) have been prepared, where Ln = yttrium, lanthanum, cerium, praseodymium, neodymium, samarium, gadolinium, terbium, dysprosium, holmium, erbium, thulium, and lutetium, except that isolation of the desolvated complexes proved difficult for Eu and Yb. The tetrahydrofuran (thf) complexes are all monomeric, and most of them adopt 13-coordinate structures in which each DMADB group chelates to the metal center by means of four B–H···Ln bridges (each BH₃ group is κ²H; i.e., forms two B–H···Ln interactions). For the smallest three lanthanides, Tm, Yb, and Lu, the metal center is 12 coordinate because one of the DMADB groups chelates to the metal center by means of only three B–H···Ln bridges. The structures of the base-free Ln(H₃BNMe₂BH₃)₃ complexes are highly dependent on the size of the lanthanide ions: as the ionic radius decreases, the coordination number decreases from 14 (Pr) to 13 (Sm) to 12 (Dy, Y, Er). The 14-coordinate complexes are polymeric: each metal center is bound to two chelating DMADB ligands and to two “ends” of two ligands that bridge in a Ln(κ³H-H₃BNMe₂BH₃-κ³H)Ln fashion. In the 13-coordinate complexes, all three DMADB ligands are chelating, but the metal atom is also coordinated to one hydrogen atom from an adjacent molecule. The 12-coordinate complexes adopt a dinuclear structure in which each metal center is bound to two chelating DMADB ligands and to two ends of two ligands that bridge in a Ln(κ²H-H₃BNMe₂BH₃-κ²H)Ln fashion. The complexes react with water, and the partial hydrolysis product [La(H₃BNMe₂BH₃)₂(OH)]₄ adopts a structure in which the lanthanum and oxygen atoms form a distorted cube; each lanthanum atom is connected to three bridging hydroxyl groups and to two chelating DMADB ligands. One B–H bond of each chelating DMADB ligand forms a bridge to an adjacent metal center. Field ionization MS data, melting and decomposition points, thermogravimetric data, and NMR data, including an analysis of the paramagnetic lanthanide induced shifts (LIS), are reported for all of the complexes. The Ln(H₃BNMe₂BH₃)₃ compounds, which are highly volatile and sublime at temperatures as low as 65 °C in vacuum, are suitable for use as chemical vapor deposition (CVD) and atomic layer deposition (ALD) precursors to thin films.