polyhedral oligomeric silsesquioxane (poss) cages with endohedral metal hydrides

Polyhedral Oligomeric Silsesquioxane (POSS) cages with endohedral metal hydrides

MotivationPolyhedral 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].

MethodThe 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.

ResultsEquilibrium 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