Synthesis, Structure, And Characterization Of Rare Earth(Iii) Transition Metal Cyanides; Lanthanide(Ii) And Metallocene Amidotrihydroborates

by Wilson, Duane C.

Abstract (Summary)
PART I Water-free rare earth(III) hexacyanoferrate(III) complexes, {Ln(DMF)6(ยต-CN)2Fe(CN)4}? (DMF= N,N-dimethylformamide; Ln= rare earths excluding Pm), were synthesized in dry DMF through the metathesis reactions of [(18-crown-6)K]3Fe(CN)6 with LnX3(DMF)n (X= Cl or NO3). Anhydrous DMF solutions of LnX3(DMF)n were prepared at room temperature from LnCl3 or LnX3*nH2O under a dynamic vacuum. Compounds were characterized by IR, X-ray powder diffraction, elemental analysis, and single crystal X-ray diffraction. Infrared spectra reveal that a monotonic, linear relationship exists between the ionic radius of the Ln and the ??-CN stretching frequency. X-ray powder diffraction data are in agreement with powder patterns calculated from single crystal X-ray diffraction results indicating that each compound consists of one pure crystalline phase. This agreement is a useful alternative for bulk sample confirmation when elemental analyses are difficult to obtain. Eight-coordinate Ln(III) metal centers are observed for all structures. Trans-cyanide units of [Fe(CN)6]3- form isocyanide linkages with Ln(III) resulting in one-dimensional polymeric chains. Rare earth(III) tetracyanometalate(II) complexes [Ln(DMF)n]2[M(CN)4]3 (M= Ni, Pd, Pt) have been synthesized and structurally characterized. The assumption that only the size (identity) of rare earth element dictates the observed structure type has been proven false. The coordination number of the rare earth metal (n= 5 or 6) was found to depend on the identity of the group-10 metals as well as the identity of the rare earth metal. Complexes [LnX(DMF)n][M(CN)4] (where X= Cl or NO3) with a Ln:M ratio of 1:1 have been synthesized and structurally characterized. The anion [Pt(CN)4]2- was not able to replace the nitrate ligand of [Ce(NO3)(DMF)5][Pt(CN)4]; however, the anion [Pt(CN)4]2- was able to replace the chloride ligand to produce [Ce(DMF)5]2[Pt(CN)4]3. PART II An investigation into the coordination chemistry of the amidotrihydroborate as a ligand was made. The compounds Cp2Ti[NH2BH3], Cp2Hf[NH2BH3]X (X= H, Cl), and (DME)2Yb[NH2BH3]2 were synthesized and characterized. Metathesis reactions of Cp2MCl2 with NaNH2BH3 produced the for mentioned metallocene compounds. A direct reduction of NH3BH3 by Yb metal was used to synthesize (DME)2Yb[NH2BH3]2. The single crystal X-ray structures indicate that the ?^2-[NH2BH3] coordinates with its hydrogen atoms in an eclipsed formation. Both the single crystal X-ray data and IR spectra indicate that the B-H bond is weakened upon forming a 3-center 2-electron bond. The electron donating ability of the other coordinated ligands (H- versus Cl-) has been observed to affect the B-H bond strengths. The amidotrihydroborate ligand is a reducing agent capable of reducing Ti(IV) to Ti(III). The formation of Cp2Hf[NH2BH3]Cl indicates that the [NH2BH3]- ligand prefers to coordinate to the metallocene unless sterically prohibited, in which case it donates an H-. The ligand is an H- donor as observed by the formation of Cp2Hf[NH2BH3]H. This may be part of the mechanism for the reduction of Ti(IV) by [NH2BH3]-. The IR data confirms that the ligand remains coordinated to the metal center when dissolved in a THF solution; however, the BH3 hydrogen atoms are fluxional as indicated by NMR data. The study of the structural and spectral properties of these compounds allows for a better understanding of the amidotrihydroborate as a ligand. This knowledge is essential for the development of a catalyst for the dehydrogenation of amidotrihydroborate at low temperatures and a method for the regeneration of ammonia borane from BNHx, possibly involving a transition metal catalyst.
Bibliographical Information:


School:The Ohio State University

School Location:USA - Ohio

Source Type:Master's Thesis

Keywords:rare earth lanthanide transition metal cyanide iron amidotrihydroborate hydrogen storage metallocene nh2bh3


Date of Publication:01/01/2009

© 2009 All Rights Reserved.