PNNL

Abstract

X-ray absorption fine structure (XAFS) is used to determine the structure of the rhodium cluster present during the catalyzed dehydrocoupling of amine boranes under operando conditions. Analysis of the in-situ XAFS spectra using a series of amine boranes (NH3BH3, L2NBH3 and LNHBH3 where L = methyl, isopropyl, t-butyl and cyclohexyl) and rhodium catalyst precursor compounds (including di-?-chloro-bis[cis,cis-?4-1,5-cyclo-octadiene)rhodium(I)], ?5-(pentamethylcyclopentadienyl) rhodium dichloride dimer, hexarhodium hexadecacarbonyl, tetrarhodium dodecacarbonyl and bis(1,5-cyclooctadiene)rhodium(I) trifluoromethane sulfonate) strongly suggest that the active catalyst species for this reaction is a homogeneous rhodium complex. Rhodium clusters containing four or six rhodium atoms (Rh4-6) bound to amine boranes are observed as the major (>98%) rhodium containing species during and after the catalyzed anaerobic dehydrocoupling. During the later stages of the reaction, a non-metallic rhodium complex precipitate forms in which individual Rh4-6 clusters likely form polymer chains ligated by the reaction products that have two or more ligating sites. The best FEFF fits of the XAFS data show that the major rhodium species (80%) has each rhodium atom directly bound to three rhodium atoms with an observed bond distance of 2.73 Å and to two boron atoms at 2.095 Å. A minor (20%) rhodium species has each rhodium atom bound to four rhodium atoms with a bond distance of about 2.73 Å and a single rhodium atom at a non-bonding distance of 3.88 Å. No metallic rhodium was observed at any time during the anaerobic reaction. This work was supported by the Office of Basic Energy Sciences of the Department of Energy, in part by the Chemical Sciences program and in part by the Engineering and Geosciences Division. The Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.