Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy
Vibrational spectroscopy of the double complex salt Pd(NH3)4(ReO4)2, a bimetallic catalyst precursor
Abstract
Tetraamminepalladium(II) perrhenate, a double complex salt, has significant utility in PdRe catalyst preparation; however, the vibrational spectra of this readily prepared compound have not been described in the literature. Herein, we present the infrared (IR) and Raman spectra of tetraamminepalladium(II) perrhenate and several related compounds. The experimental spectra are complemented by an analysis of normal vibrational modes that compares the experimentally obtained spectra with spectra calculated using DFT (DMol3). The spectra are dominated by features due to the ammine groups and the Re
O stretch in Td ReO4−; lattice vibrations due to the D4h Pd(NH3)42+ are also observed in the Raman spectrum. Generally, we observe good agreement between ab initio calculations and experimental spectra. The calculated IR spectrum closely matches experimental results for peak positions and their relative intensities. The methods for calculating resonance Raman intensities are implemented using the time correlator formalism using two methods to obtain the excited state displacements and electron-vibration coupling constants, which are the needed inputs in addition to the normal mode wave numbers. Calculated excited state energy surfaces of Raman-active modes correctly predict relative intensities of the peaks and Franck-Condon activity; however, the position of Raman bands are predicted at lower frequencies than observed. Factor group splitting of Raman peaks observed in spectra of pure compounds is not predicted by DFT.
Introduction
Addition of a second metal is a common strategy employed to improve the performance (e.g., activity, selectivity, and stability) of supported metal catalysts. Bimetallic particle morphology and metal-metal interactions (e.g., alloying) that affect catalyst performance ideally are controlled through the preparation method. A catalyst precursor containing both metals in a fixed stoichiometric ratio is often advantageous. In order to foster contact between the metals in bimetallic clusters, reduction at high temperature in hydrogen is often required [1]. One example of this approach is the use of bimetallic carbonyl compounds (e.g., Re2Pt(CO)12) [2]. PdRe catalysts have proven useful in the selective hydrogenolysis of esters, carboxylic acids and dicarboxylic acids [3], [4], [5], [6]. The PdRe binary phase diagram shows that these metals do not form a solid solution over a large range of composition [7]. Use of the double complex salt Pd(NH3)4(ReO4)2 provides a simpler way of increasing heterometallic interactions than other methods that have been applied to PdRe catalysts, such as catalytic reduction of Re by hydrogen on Pd [6] and deposition of Pd on reduced Re particles [8]. However, to our knowledge, the only report of the use of Pd(NH3)4(ReO4)2 in the preparation of PdRe catalysts is a patent from Schubert et al. [3].
In this report, we examine the vibrational spectra of Pd(NH3)4(ReO4)2 and related compounds. The double complex salt has not been widely characterized in the literature despite its potential value as a precursor for supported PdRe catalysts [9]. We have made infrared and Raman spectroscopy measurements and employed density functional theory (DFT) calculations to address the assignment of vibrational modes and their relationship to the structure of the compounds [10], [11]. The spectrum of Pd(NH3)4(ReO4)2 comprises the normal modes of vibration of the tetraamminepalladium cation, Pd(NH3)42+, obtained from IR and Raman spectra of Pd(NH3)4Cl2 and Pd(NH3)4(NO3)2, and those of the perrhenate anion, ReO4−, without any vibrations attributed to PdORe bonding. The high symmetry of the component ions in the bimetallic salt contributes to our confidence in the assignments of the normal modes.
Section snippets
Experimental
Infrared and Raman spectra for several Pd(NH3)42+ complexes and related compounds were measured: Pd(NH3)4Cl2 (99.99 + % Aldrich), Pd(NO3)2-H2O (99 + % Strem) and NH4ReO4 (99 + % Strem) were used as received; Pd(NH3)4(NO3)2 (10 wt% aq. solution, 99 + %, Sigma) was precipitated from solution with ethanol, filtered and dried. The double salt, Pd(NH3)4(ReO4)2, was prepared by mixing saturated solutions of NH4ReO4 and Pd(NH3)4(NO3)2. Upon mixing, a light yellow salt precipitated from solution; the
FTIR Spectroscopy
The infrared absorption spectrum of the double complex salt shows features which can be assigned to the two ions which it comprises: NH3 ligand modes and PdN lattice (skeletal) modes arising from the square planar [Pd(NH3)4]2+ ion and ReO stretching arising from the ReO4− ion. Infrared absorption spectra for the [Pd(NH3)4]2+ complexes [Pd(NH3)4](NO3)2, [Pd(NH3)4]Cl2, and [Pd(NH3)4](ReO4)2 share many features in common attributed to vibrational modes of the NH3 ligands (Fig. 1); indeed, the
Discussion
Absorption bands corresponding to normal modes of ammine ligands do not vary dramatically between complexes, as these modes are least dependent on metal-N bond character or counter ion identity [17], [18]. For reference, these six normal modes of the ammine ligands are depicted in Fig. S2. The band positions and intensities calculated using DMol3 generally agree with those observed in experiments conducted here and the literature; however, the ways in which the calculations differ from observed
Conclusions
A vibrational spectroscopy and normal mode study of a bimetallic PdRe complex and related [Pd(NH3)4]2+ and ReO4− compounds was conducted using both IR and Raman spectroscopy. Spectra of the double complex salt are composed of bands due to the two ions, without any unique bands that could be attributed to formal bonding between the two ions. The agreement between ab initio DFT calculations of the vibrational spectra of Pd(NH3)42+ and experimentally-obtained spectra was reasonable for the IR
Acknowledgements
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. We would like to thank Dr. Dennis McOwen for his assistance in acquiring Raman spectra and Dr. Roger Sommer for performing X-ray crystallography of the double complex salt.
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The interaction of perrhenate and acidic/basic oxygen-containing groups on biochar surface: A DFT study
2020, Chemical Engineering JournalCitation Excerpt :However, as to the aqueous, whether in an explicit solvent or an implicit solvent, the average bond distance of Re-O is 1.763 Å, which is in agreement with the results of Bridgeman et al (1.75–1.77 Å) [39]. The computed vibrational frequencies are all positive in the range of 286–921 cm−1 (see Fig. 4), it is in accordance with the study of Thompson et al [40]. Compared with the gaseous state, it can clearly be seen from Figs. 3 and 4 that the aqueous ReO4− has a slight discrepancy in the average bond distance of Re-O and the simulated vibrational spectra, the bond distance is reduced by 0.002 Å, for the simulated vibrational spectra of ReO4−, the gaseous O-Re-O degenerate bending mode and Re-O asymmetric stretching band shift from ~302/894 cm−1 to ~287/871 cm−1 in aqueous.
Catalysts for selective hydrogenation of furfural derived from the double complex salt [Pd(NH<inf>3</inf>)<inf>4</inf>](ReO<inf>4</inf>)<inf>2</inf> on Γ-Al<inf>2</inf>O<inf>3</inf>
2017, Journal of CatalysisCitation Excerpt :One such precursor, [Pd(NH3)4](ReO4)2, was prepared and characterized originally by Zadesenets et al. [33]. This slightly water-soluble double complex salt (DCS) consists of two perrhenate ions weakly coordinated to a square-planar tetraammine Pd2+ complex [34]. Reduction of the unsupported DCS has been reported to yield a Re:Pd solid solution having an hcp structure and 2:1 atomic ratio; however, the Pd-Re binary phase diagram indicates that a mixture of this composition should separate into Pd-rich (∼90 at.