Hydrogen storage in dinuclear Pt(II) metallacycles

https://doi.org/10.1016/j.ijhydene.2009.05.018Get rights and content

Abstract

Hydrogen gas physisorption in two distinct dinuclear Pt(II) metallacycles was investigated with 2H nuclear magnetic resonance (NMR) spectroscopy. Hydrogen adsorption isotherm data and 2H NMR spectroscopy indicated that hydrogen storage occurs via condensation within the cavities of one of the macrocycles and at the interstitial sites in the other. In addition, this study further supports the notion that heat of adsorption and pore size play critical roles in hydrogen storage.

Introduction

The development for an effective hydrogen storage medium that is safe, efficient, and light-weight has been a major impediment to the advent of a hydrogen-based industry and economy. The DOE 2010 target for a storage medium is 6 wt% at a density of 45 g/L [1], and to our knowledge no material has been shown to exhibit these criteria at STP. Carbon nanotubes (CNT) and metal organic framework (MOF) compounds have been considered good candidates for hydrogen storage due to their high surface areas and large pore volumes; however, it has been shown that surface area and pore volume do not necessarily correlate with uptake capacity [2], primarily because the heat of adsorption is generally too low to give rise to significant storage at ambient temperature [3]. Current experimental and theoretical work has focused on understanding the fundamental molecular features that are responsible for hydrogen physisorption in the solid-state. Because MOF compounds offer many molecular design opportunities and their structure may be well-characterized, they are good platforms for studying hydrogen physisorption as a function of the nature of the molecular components. For example, one can analyze hydrogen storage as a function of the identity and oxidation state of the metal ions and the nature of the organic linkers in structurally characterized MOFs.

Metal ions have been observed to give rise to higher binding energies with hydrogen. Sun et al. predicted that early representatives of the transition metal series such as Sc, Ti, and V give rise to such higher binding energies [1]. Similarly, Pt(0) nanoparticles have been shown to increase hydrogen absorption via the spillover effect in MOFs and CNTs [4], and the metal sites presumably increased the heat of adsorption of the gas [5]. Furthermore, Liu et al. demonstrated a strong correlation between exposed, coordinatively unsaturated metal centers and enhanced hydrogen surface density [6], a result that is in agreement with data reported by Rowsell and Yaghi [7]. Hydrogen surface packing density has been used as a metric to model the strength of the H2-surface interaction that is required to reduce the intramolecular H2–H2 distance. Similarly, Lee et al. proposed a cooperative mechanism in which the metal sites initiate the propagation of hydrogen adsorption [8]. Rowsell and Yaghi concluded that large charge gradients on the metal oxide units and restricted pore dimensions are key features for high storage capacity materials [7], [9]. Belof et al.'s Monte Carlo simulations [10] support these findings by emphasizing the importance of polarization interactions of MOFs with hydrogen in conjunction with narrow pores.

In this contribution, we present hydrogen storage data on two discrete Pt(II) dimer complexes, which represent a well-characterized motif of typical larger MOF networks. Compound 1 (compound 7 in Chatterjee et al.) [11] is a dinuclear Pt(II) metallacycle that combines cis-(Me3P)2Pt(OTf)2 and 1,2-ethanediyl di-4-pyridinecarboxylate (3) to form the discrete metallacycle [(Me3P)2Pt(3)]2(OTf)4 (1), Fig. 1. Compound 2 (5a in Chatterjee et al.) [11] differs from 1 in that the two trimethylphosphine ligands have been replaced by 1,3-bis(diphenylphosphino)propane, and form [(dppp)-Pt(3)]2(OTf)4 (2), which exhibits extended π–π interactions. These complexes were characterized with X-ray crystallography which revealed that the metallacycles in both complexes form supramolecular one-dimensional channels of 12 Å in diameter. We employed nuclear magnetic resonance (NMR) using 2H2 as a probe and adsorption isotherms in order to study hydrogen physisorption in 1 and 2 under varying temperature and pressure conditions.

Section snippets

Experimental

For NMR experiments, the samples were weighed into 5 mm outer diameter Pyrex glass tubes (23.9 mg for 1, and 9.86 mg for 2). The open end of the tube was connected via Swagelok fittings to a long piece of PFA tubing, which was attached to a vacuum rack equipped with a turbo-molecular pump. A tank of deuterium gas (99.8%, Cambridge Isotope Laboratories, Andover, MA) was attached to this assembly via a 3-way stopcock, so that the sample could be evacuated and exposed to deuterium gas while inside

Results and discussion

The structures of 1 and 2 differ in the external phosphino ligands which are aliphatic in 1 (trimethylphosphine) and aromatic in 2 (1,3-bis(diphenylphosphino)propane). The aromaticity of these ligands leads to increased π–π stacking interactions through dppp and the pyridyl moieties in the linkers, which causes the N-Pt-N angle to open up slightly in 2 (86° vs. 81°). [11] Consequently, the P-Pt-P angle is smaller in 2 (92° vs. 96°). The increased π–π interactions lead to a smaller distance

Conclusions

We have studied hydrogen adsorption in two crystalline, microporous Pt(II)-containing materials using macroscopic and microscopic techniques. The samples consist of dinuclear Pt(II) metallacycles formed via metal-directed self-assembly which exhibit channeled structures in the crystalline solid-state. The metal centers are in a square planar configuration, although the triflate counter ions interact with the Pt(II) at the axial sites, and the dipyridyl-based organic linkers form a discrete

Acknowledgements

This material is based upon work supported by the National Science Foundation under Grant No. EPS-0447691, ACS-PRF 44703-GB3 and the UTEP SEED Grant for M.H. We gratefully acknowledge a grant by Cambridge Isotope Laboratories, Inc. providing us with 2H2 used in this study.

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Present address: Biochemistry Department, Wake Forest University School of Medicine, Medical Center Boulevard, Winston-Salem, NC 27157, USA.

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