Polyhedral Oligomeric Silsesquioxane (POSS) cages with endohedral metal hydrides

Motivation

Polyhedral Oligomeric Silsesquioxane (POSS) cages are of major interest as building blocks for nano-structured hybrid materials and nanocomposites [1-3]. POSS molecules may be employed to increase the interfacial area and to tune domain distances in solar cells based on conjugated polymers and fullerenes [1].

In this contribution, we investigate encapsulation of metal hydrides as a means to alter the energetic and electronic properties of three basic POSS cages ( Tn with n = 8, 10, 12) in a controlled way. As an additional benefit, POSS cages with internal metal hydrides might prove to be efficient as novel media of hydrogen storage.

AlH3 , a light metal-hydride

(left: Al crystals, right: AlH3 crystals)

Fullerene CagesL.Gagliardi performed a computational study of metal-hydrides encapsulated in within fullerenes, such as ZrH16 @ C60 and 2TiH16 @ C114 [4].

Method

The interaction between metal hydride cores and POSS cages was explored by using various ab initio and density functional theory (DFT) techniques [5], with ab initio procedures ranging from HF (Hartree-Fock) to MP2 (Moeller-Plesset perturbation theory at second order).

A variety of metal-hydrides could be stored in T12 cages. However, the moreconventional T8 cage is more easily fabricated.

PtH4 within T12

Optimization performedusing frequency analysis at

the B3LYP/CEP-31G level

FeH2 within T8

Results were found at the HF/CEP-121G level. Geometric optimizations were confirmed

using MP2 and B3LYP.

Results

Equilibrium structures were obtained for systems of the form MHm@Tn [(SiO3/2H)n],

with m = 2, 3, 4, n = 8, 10, 12, and M = transition metal elements in Group IVB, VIB, VIII, IIB.

The potential surface inside the less studied T12 cage turned out to be relatively flat, and equilibrium geometries were obtained for a wide range of large metal hydrides.

Zero-point corrected stabilization energies ΔE for T10 based systems were found to be negative, corresponding to an exothermal encapsulation process, in 4 cases:

For MHm molecules inside the T8 and T10 cages, no stability was found for m>2 . For m=2, cage symmetry change associated with elongation along the MH2 axis was found to occur in T8 cage.

Some common structures of MH2@T10 complexes

Due to the size restriction associated with T8, no systems of the form MHm@T8 with m > 2 were found to converge.Some periodic arrangements of T8 and MH2@T8 were included and proven to be stable:

Summary

• A wide variety of POSS cages with enclosed metal hydrides were shown to be stable by ab initio computation. For RuH2, PdH2, OsH2 , PtH2 encapsulation into T10 was found to be an exothermal process.

• An effort to analyze POSS polymers with endohedral metal hydrides was initialized.

Xiqiao Wang, John Corn, Frank HagelbergDepartment of Physics and Astronomy

East Tennessee State UniversityJohnson City, TN 37614

References:[1] F.Wang, X.Lu, C.He, J.Mat.Chem. 21, 2775 (2011)[2] D. Hossain, F.Hagelberg, C.Pittman, S.Saebo, J.Inorg. Org.Pol.Mat. 20, 1574 (2010)[3] D. Hossain, C. Pittman Jr., F. Hagelberg, S. Saebo, J.Phys.Chem.C, 112, 16070 (2008) [4] L.Gagliardi, J.Chem.Theo.Comp. 1, 1172 (2005)[5] M.J.Frisch et al. Gaussian, Rev. B.01, Gaussian Inc., Wallingford, CT (2004)

Acknowledgment:Support by TN-SCORE (NSF EPS 1004083)is gratefully acknowledged.

System Method ΔE [Hartree]RuH2@T10 MP2/CEP-121G -0.024 ~ -0.030PdH2@T10 MP2/CEP-121G -0.007OsH2@T10 MP2/CEP-121G -0.012PtH2@T10 MP2/CEP-121G -0.006