Endohedral and Exohedral Complexes of T8-Polyhedral Oligomeric

Sep 18, 2008 - these species form endohedral complexes with the T8-POSS cage except ... nated T8-POSS) consists of silicon atoms occupying the vertice...
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J. Phys. Chem. C 2008, 112, 16070–16077

Endohedral and Exohedral Complexes of T8-Polyhedral Oligomeric Silsesquioxane (POSS) with Transition Metal Atoms and Ions Delwar Hossain,*,†,‡ Charles U. Pittman, Jr.,† Frank Hagelberg,§ and Svein Saebo† Department of Chemistry, Mississippi State UniVersity, Mississippi 39762, Chemistry Department, Jahangirnagar UniVersity, SaVar, Dhaka 1342, Bangladesh, and Department of Physics, Astronomy, and Geology, East Tennessee State UniVersity, Johnson City, Tennessee 37614 ReceiVed: January 23, 2008; ReVised Manuscript ReceiVed: June 23, 2008

The equilibrium geometries of the exohedral and endohedral complexes of the polyhedral oligomeric silsesquioxane (POSS) cage (HSiO3/2)8 containing the transition metal atoms or ions Sc0,+, Cr0,+, Fe0,+, Co0,+, Ni0,+, Cu0,+, Zn0,+, Mo0,+, W0,+, Ru0,+, Os0,+ have been investigated at the B3LYP/LanL2DZ levels. All these species form endohedral complexes with the T8-POSS cage except Sc0,+, Mo0, and W0,+. The Mo0 and W0,+ species as well as Cr0,+, Fe0,+, Co0,+, Ni0,+, Cu+, Zn+, Ru0,+, Os0 form stable exohedral complexes. Geometries, electronic properties and ionization potentials were computed. The Si-O and Si-H bond lengths in the cationic endohedral complexes are shorter than in the corresponding complexes of neutral transition metal atoms. The zero-point corrected inclusion energies of the endohedral species X@(SiHO3/2)8 (X ) Fe+, Co+, Ni+, Cu+, Os+) are all negative, suggesting that these complexes are more stable than their isolated components. All exohedral complexes have energies that are lower than their corresponding endohedral analogs. Transition metal atom encapsulation raised the HOMO and lowered the LUMO energies, reducing the HOMO-LUMO gaps of every complex compared to that of the pure cage. The HOMO-LUMO gap of the empty cage is 8.1 eV while the endohedral complexes exhibit gaps between 1.2 and 4.96 eV. Insertion of Cr, Fe, Co, Ni, Cu, or Zn into the POSS cage is more favorable in water than in the gas phase. However, insertion of Co+, Ni+, Cu+, or Zn+ into the POSS cage is less favorable in water than in the gas phase. Overall, both the neutral and ionic endohedral transition metal complexes, X@(SiHO3/2)10, (X ) Cr0,+, Fe0,+, Co0,+, Ni0,+, Cu0,+, Zn0,+, Ru0,+, Os0,+) appear to be viable synthetic targets. I. Introduction Polyhedral oligomeric silsesquioxanes (POSS) are cage molecules comprised of a silicon and oxygen core, exhibiting the composition (RSiO3/2)2n, where R denotes a hydrogen, organic, or inorganic ligand. Octahydridosilsesquioxane, (HSiO3/2)8, (designated T8-POSS) consists of silicon atoms occupying the vertices of a cube, oxygen atoms bridging each pair of silicon atoms and a single hydrogen atom attached to each silicon atom. T8POSS and its derivatives incorporated into organic polymers, dendrimers, and zeolites have received substantial attention due to their applications in materials science and catalysis.1-14 POSS has also emerged as a viable filler in high performance nanocomposites.15 POSS polymer nanocomposites are substantially harder than the unfilled polymers.16 POSS cage incorporation into the polymeric materials frequently enhances such properties glass transition temperatures, decomposition temperatures and mechanical strength.17-20 In POSS-PEO-based polymer electrolytes, POSS acts as an inhibitor of polyethylene oxide (PEO) crystallization.21 Cationic POSS units can serve as carriers and potential drug delivery agents.22 Because of their nanostructured nature and their ceramic like properties, POSS units are being used for synthesis of polymer-derived ceramics.23 Metal-containing siloxanes, and oligometalla-silosesquioxanes are used as catalytic carriers for both homogeneous and * To whom correspondence should be addressed. E-mail: hossaind2004@ yahoo.com. † Mississippi State University. ‡ Jahangirnagar University. § East Tennessee State University.

heterogeneous catalysts in olefin processing.24,25 Epoxidation of alkene is easily accomplished with certain POSS catalysts.26-28 POSS is also used as a support for Ziegler-Natta catalysts.29 Coupar et al.30 has reported dendrimer catalysts based on POSS structural cores. These materials are expected to combine the traits of homogeneous catalysts with the high activity and precise control of catalytic sites normally associated with homogeneous catalysts. Murfee et al.31 reported that metallodimers with a diphenylphosphino-POSS core and Ru-based chromophores exhibit unique advantages. A liquid crystalline silsesquioxane dendrimer exhibiting chiral nematic and columnar mesophases has been synthesized by Saez et al.32 The cage-like structures of POSS chemicals makes them useful for separating gas mixtures when present in siloxanes and silicon-based capillary membranes.33,34 Poly(dimethylsiloxane) is more permeable to oxygen35 than to nitrogen and is used to separate N2/O2 mixtures.36 Cationic polyhedral POSS units can serve as carriers and potential drug delivery agents.22 Most experimental and theoretical studies37-43 describe pure or metal-substituted POSS cages but not encapsulated species. However, a few studies of endohedral T8-POSS complexes have appeared.44-46 Tossell calculated the 19F and 29Si NMR shifts and stabilities of F-, encapsulated in silsesquioxanes, in aqueous and in toluene solutions using DFT.47 Kudo et al. studied the insertion of H2 into POSS compounds using Hartree-Fock (HF) and second order perturbation (MP2) methods.48 The parent T8-cage molecule (HSiO3/2)8 structure has been characterized by IR and NMR in solution as well as X-ray and neutron diffraction in the solid state, and mass spectrometry in

10.1021/jp8006798 CCC: $40.75  2008 American Chemical Society Published on Web 09/18/2008

Complexes of T8-Polyhedral Oligomeric Silsesquioxane the gas phase.49-55 Matsuda et al.56 and Pa¨ch et al.57 studied the double four-membered ring (D4R) silicate cage with an encapsulated hydrogen atom by ESR spectroscopy. Taylor et al.45 synthesized an endohedral fluoride ion complex, octaphenyl octasilsesquioxane fluoride, as its quaternary ammonium salt. This structure was confirmed by 1H NMR and 29Si NMR, negative-ion Fast Atom Bombardment (FAB) mass spectrometry, and X-ray diffraction.45 Endohedral analog complexes of octaphenyl octasilsesquioxane fluoride also exist. For example, Morris et al.58,59 prepared fluoride-encapsulated octaspherogermante, F-@[(OH)GeO3/2]8, confirming its structure by NMR and X-ray diffraction experiments. The properties of POSS and its derivatives and the reaction paths that lead to endohedral incorporation of atomic or ionic species can be obtained from computational studies which may predict novel complexes for future experimental examination. Mattori et al. reported computational studies of atomic hydrogen trapping and detrapping in an octasilsesquioxane host cage.60 Ion mobility studies and molecular mechanics (MM) calculations predicted that sodium cations form exohedral complexes with POSS.61,62 Encapsulation of Na+, F- or OH- inside [(OH)SiO3/2]8 has been investigated by local density functional techniques.46 The geometrical structures and the charge redistribution among the host cage and the endohedral ionic species were predicted. Computational results have been reported for the structures and encapsulation mechanisms of the endohedral complexes X@(HSiO3/2)8 (X)N2 and O2),62 H@(HSO3/2)860 and X@(HSiO3/ 2)844 (X ) Li+, Na+, K+, He, Ne, Ar, F-, Cl-, Br-). However, no studies on the structures of complexes of transition metal atoms or ions with (HSiO3/2)8 cages have been reported. In the present computational study, the following questions will be addressed. Is the T8-POSS cage stable when different transition metal or their cations are incorporated into the cage? What kinds of interactions occur between the incorporated ion (or atom) with the T8-POSS skeleton? In addition, future experiments might realize T8-POSS synthesis by cage assembly around a core consisting of a transition metal (TM) atom or its cation, as was found for F-(PhSiO3/2)8.45 This article addresses the extent to which the geometric, energetic, and electronic properties of (HSiO3/2)8 are influenced by addition of transition metal (TM) atoms or ions. Specifically, the endohedral X@(HSiO3/ 2)8 and exohedral X(HSiO3/2)8 (X ) Sc0,+, Cr0,+, Fe0,+, Co0,+, Ni0,+, Cu0,+, Zn0,+, Mo0,+, W0,+, Ru0,+, Os0,+) systems are reported. This is a total of 44 POSS complexes. The direct insertion is assumed to occur by the passage of impurities through a double four membered ring (D4R) face of the cubic POSS structure. This work may contribute to the understanding of the design and synthesis of these systems and in order to create new polyhedral sphero-metal-oxide materials with novel properties. II. Computational Details All calculations were performed using the Gaussian 0364 program. The geometries of all species were determined with density functional theory (DFT) calculations employing the B3LYP65,66 potential. Vibrational frequencies were calculated at the same level to ensure that all optimized structures were minima (zero imaginary frequencies). The frequency calculations also allowed the calculated energy differences to be corrected for differences in zero-point vibrational energies. The electronic properties and atomic charges were evaluated using natural bond orbital analysis (NBO).67 Initially, all structures were studied using the LanL2MZ 68-70 basis set. Subsequently, refined calculations were carried out

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Figure 1. Schematic representation of host POSS cage species (HSiO3/2)8 with D4R units and guest species; (a) host cage with Oh symmetry (b) identity of X (c) endohedral species of X@(HSiO3/2)8.

employing the LanL2DZ basis set.68-70 Geometry optimizations and frequency computations for the host (HSiO3/2)8 were also performed at the B3LYP/6-311G(2d,p) level. Additional calculations using the cc-pVDZ and cc-pVTZ basis sets71,72 were carried out on the parent (HSiO3/2)8 cage for comparison. The LanL2DZ basis set slightly overestimates the bond lengths. However, considering the limited number of suitable basis sets for transition metals, we used LanL2DZ basis set throughout our calculations. The spin multiplicity for the M-POSS complexes was either singlet or doublet. However, for bare metal atoms or ions, the low energy spin multiplicity was chosen. Since we have calculated the energy for the ground-state endohedral system, we usually used low spin state of the metal. For instance, in Fe@T8-POSS complex the calculations was carried out for the singlet state, while for Fe itself the pentet state was chosen. Similarly, the Cr heptet, Ni triplet, Mo heptet, W pentet, Ru pentet and Os pentet states were used for cthe ground states of these atoms. The inclusion energy for endohedral complexes, Einc, the binding energy for exohedral complexes, Ebind, and the isomerization energy Eiso are defined as73

Einc)Eendo-(Ecage+Ex) + ∆EZP

(1)

Ebind)Eexo-(Ecage+Ex) + ∆EZP

(2)

Eiso)Eexo-Eendo + ∆EZP)Ebind-Einc + ∆EZP

(3)

respectively. Ex is the total energy of the guest species X. Ecage is the energy of the empty T8-POSS cage, and Eendo and Ebind are the total energies of the endohedral and exohedral complexes, respectively. ∆EZP represents the difference in zeropoint energies. Adiabatic ionization potentials (IP) were computed for the X@(HSiO3/2)8 complexes (where X ) Cr, Fe, Co, Ni, Cu, Zn, Ru, Os) as the difference between the total energies of the optimized cationic and the optimized neutral transition metal complexes of (HSiO3/2). Solvent effects were investigated using the PCM solvent model74 with water as the solvent for both the endohedral X@(HSiO3/2)8 (X ) Cr, Fe, Co, Ni, Cu, Zn) and X@(HSiO3/2)8 (X ) Co+, Ni+, Cu+, Zn+) complexes. Basis set superposition errors (BSSE) are often important in studies of weak interactions. Normally, the magnitude of the BSSE are estimated using the counter-poise method by Boys and Bernardi.75 In our previously reported studies of endohedral and exohedral complexes of charged an neutral atomic species and T8-POSS, T10-POSS, Si12, Si18, and Si20 cages44,73,76 counterpoise calculations were carried out. The BSSE’s were significant but they did not change any trends for the endohedral complexes while they were insignificant for the exohedral complexes. Since counter-poise calculations are quite expensive we chose not to include these in this study (Figure 1). III. Results and Discussion This section is organized in the following way: Geometrical features will be discussed first starting with the empty host cage

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TABLE 1: Selected Structural Parameters of T8-POSS, (HSiO3/2)8: Bond lengths (in Å) and Bond Angles (in deg) HF basis set

DFT

bond length

bond angle

bond length

bond angle

6-31G**

Si-O 1.626 Si-H 1.453

Si-O 1.644 Si-H 1.464

6-311G**

Si-O 1.622 Si-H 1.453

cc-pVDZ

Si-O 1.650 Si-H 1.462

cc-pVTZ

Si-O 1.615 Si-H 1.456

Si-O-Si 149.4 O-Si-O 109.0 H-Si-O 110.0 Si-O-Si 150.7 O-Si-O 108.3 H-Si-O 110.6 Si-O-Si 149.0 O-Si-O 109.2 H-Si-O 109.8 Si-O-Si 149.3 O-Si-O 109.0 H-Si-O 109.9

Si-O-Si 148.1 O-Si-O 109.6 H-Si-O 109.3 Si-O-Si 149.4 O-Si-O 109.0 H-Si-O 110.0 Si-O-Si 147.2 O-Si-O 110.1 H-Si-O 108.9 Si-O-Si 148.1 O-Si-O 110.1 H-Si-O 109.3 Si-O-Si 152.3 O-Si-O 107.5 H-Si-O 109.2

LanL2DZ experimental

Si-O 1.642 Si-H 1.460 Si-O 1.668 Si-H 1.473 Si-O 1.637 Si-H 1.462 Si-O 1.668 Si-H 1.463

Si-O 1.625 Si-H 1.461

Si-O-Si 147.3 O-Si-O 109.4

and followed by the endohedral and exohedral complexes, respectively. This will be followed by a discussion of the energetics, and finally atomic charges and electronic properties will be discussed. Geometrical Features. Host Cage. In a study reported earlier, we found that the Oh and Th forms of the empty T8-POSS cage have very similar structures and almost the same energies with the Oh form slightly more stable than the Th form.77 In solution, both 1H and 29Si NMR spectra give signals indicating an octahedral structure for (HSiO3/2)877 and the structure in the gas phase was determined to possess Th symmetry.48 Tejrina and Gordon63 carried out calculations starting with the C4V, D4h, Td and Oh forms of the T8-POSS cage and all geometry optimizations converged to the same Oh structure. Based on these previously reported results, we assumed that the Oh form of the T8-POSS cage is its most stable structure. Additional calculations on the empty cage were performed with several basis sets including the ECP-basis sets used in the remainder of this study. The results of these calculations are summarized in Table 1. Theoretical studies of the empty cage yielded distances ranging from 1.62 to 1.68 Å. 78-82 These are consistent with our value of 1.668 Å calculated at the B3LYP/LANL2DZ level. Our calculated structure is also in good agreement with experimental results for hexamethyl disiloxane and (HSiO3/2)8.23,24 Endohedral Neutral and Cationic Complexes X@(SiHO3/2)8. The optimized structures of the neutral and cationic endohedral complexes of POSS-T8 and transition metals are shown in Figure 2. Despite considerable variations in the sizes of the embedded atoms or cations, stable endohedral geometries were obtained for all the neutral and ionic complexes, except Sc,0,+ Mo0 and W0,+ Geometric deformations of the cage induced by encapsulated Cr,0,+ Fe,0,+ Co,0,+ Ni,0,+ Cu,0,+ Zn,0,+, Mo,+ Ru,0,+ Os0,+ were small. All attempts to find endohedral Sc0,+ complexes resulted in destruction of the POSS-T8 cage, and these systems will not be further discussed. Table 2 contains the total energies, point groups, inclusion energies for the endohedral X@(HSiO3/2)8 complexes. The geometrical parameters for these structures are given in Table S-1 of the Supporting Information. When Oh symmetry was imposed, most of the endohedral complexes had several imaginary frequencies, and whenever a structure had imaginary frequencies the geometry was distorted and reoptimized until a structure without imaginary frequencies

was found. Most of these minima have C1 symmetry except for the structure of Cu@(HSiO3/2)8 and Ru@(HSiO3/2)8 which both have Th symmetry. However, in all cases the distortions from Oh symmetry were relatively small. The Si-O distances for all the endohedral neutral metal or cation T8-POSS complexes are longer than the respective distances in the pure host cage. The Si-O distances lengthen as the size of the endohedral transition metal or their cations increases. All endohedral metal cationic complexes have shorter Si-O and Si-H distances than their corresponding neutral metal complexes. In the X ) Fe, Fe+, Co and Co+ complexes, four of the oxygen atoms in the D4R faces moved toward the metal, while the remaining eight moved away from the metal when compared to the parent empty cage. The differences between the short and the long metal oxygen bond lengths are substantial (2.080 and 2.992 Å for the Fe complex and 2.259 and 3.180 Å for the Co complex). In the cationic complexes, the metal oxygen distances are shorter by a few tenths of an Ångstrom. The structure of Ni@(HSiO3/2)8 is different than that of Ni+@(HSiO3/2)8. The Ni atom in Ni@(HSiO3/2)8 is joined to eight Si atoms and two-oxygen atoms. These two-oxygen atoms are drawn inward (Ni-O ) 2.021 Å) and the remaining ten oxygen atoms move outward (2.887 Å). This deformation created two distinct Ni-Si distances (four at 2.732 Å and the remaining four at 2.848 Å). Further details are given in Table S-1. All of the silicon to metal distances of each Zn, Cu, and Ru complex are equal: 2.823 Å (Ru), 2.819 Å (Cu), 2.835 (Cu+) and 2.841 Å (Zn). The X-O distances are 2.777 Å (Ru) 2.746 Å (Cu), 2.305 Å (Cu+) and 2.768 Å (Zn), respectively, the Si-O distances are 1.697 Å (Ru), 1.746 Å (Cu), 1.697-1.676 Å (Cu+) and 1.695 Å (Zn) complexes, respectively, and the Si-H distances are 1.461 Å (Ru), 1.461 Å (Cu), 1.457 Å (Cu+), and Zn (1.461 Å), respectively. The optimized structure of Ru+@(HSiO3/2)8 is reminiscent of the structure of the endohedral Fe, Co, Co+ and Ni+ complexes with four short (2.195 Å) and eight longer Ru-O distances (2.841 Å). In the Zn+@(HSiO3/2)8 complex all eight Si-Zn+ distances are equal (2.875 Å), and all 12 O-Zn+ distances are 2.677Å. The Si-O distances are 1.687 Å and the Si-H distances are 1.457 Å. The shape of the T8-POSS cage is significantly distorted from the original Oh symmetry in Os@(HSiO3/2)8 and all the bond angles and lengths are different from each other. In contrast, in

Complexes of T8-Polyhedral Oligomeric Silsesquioxane

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Figure 2. Optimized Geometries of X@(HSiO3/2)8 and X+@(HSiO3/2)8 Complexes at the B3LYP/LanL2DZ Level.

Os+@(HSiO3/2)8, six oxygen atoms are pulled inward toward Os+ (Os+-O ) 2.202-2.665 Å) and the remaining six oxygen atoms move outward (Os+-O ) 2.921-3.063 Å. Exohedral Complexes. Table 3 summarizes total energies, zero point energies, lowest vibrational frequencies, binding energies, and isomerization energies for exohedral X(HSiO3/2)8 complexes of the neutral transition metals and their +1 cations. The host cage’s high symmetry (Oh) is removed upon exohedral metal coordination. The Si-X distances for the neutral X(HSiO3/2)8 exohedral complexes (X ) Cr, Fe, Co, Ni, Mo, W, Os, Ru) are 3.034, 3.021, 3.995, 3.026, 3.133, and 3.076 Å, respectively. These Si-X distances are significantly longer than their endohedral counterparts. The O-X lengths for the these exohedral complexes (X ) Cr, Fe, Co, Ni, Mo, W, Os, Ru) are 2.207, 2.084, 3.564, 2.252, 2.191, and 2.209 Å, respectively. The exohedral Cu and Zn complexes exhibit very long Si-X and O-X distances from the 4DR surfaces of the host cages (for Cu Si-X ) 4.113 and O-X ) 3.674 and for Zn, Si-X ) 6.329 Å and O-X ) 5.931 Å). Thus, they are hardly distinguishable from the separated species and the apparent

bonding in these complexes is probably due to basis set superposition errors. The exohedral complexes of X(HSiO3/2)8 (X ) Cr+, Fe+, Co+, Ni+, Cu+, Zn+, Mo+, W+, Ru+, Os+) are also shown in Figure 3. The structure of exohedral complexes involving Cr+, Cu+ and Zn+ differs from their neutral exohedral counterparts. In Cr0(HSiO3/2)8, the Cr0 atom forms bridging bonds between two oxygen atoms on one of the D4R surfaces. In contrast, the exohedral Cr+ in Cr+(HSiO3/2)8 is attached to the four oxygen and the four silicon atom of the D4R surface. The Cr+-O and Cr+-Si distances are slightly shorter than the corresponding Cr0-O and Cr0-Si distances (Table 3). Although Cu0 and Zn0 do not form exohedral complexes, both Cu+ and Zn+ do form exohedral complexes with (HSiO3/2)8. Exohedral Cu+(HSiO3/2)8 and Cr+(HSiO3/2)8 have almost the same structure. Zn+ is attached to four oxygen atoms on one of the D4R surfaces in Zn+(HSiO3/2)8. These O-Zn+ distances are 2.339 Å and while the Si-Zn+ distances to the same D4R face are 3.143 Å. The exohedral complexes of (HSiO3/2)8 with Fe0,+, Ni0,+ and Ru0,+ are all structurally similar. In these exohedral complexes,

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TABLE 2: Total Energies (in Hartrees), Zero-Point Energies (ZPE, kcal/mol), Molecular Point Groups, Lowest Vibrational Frequencies ω1 (cm-1) and Zero-Point Corrected Inclusion Energies (Einc, kcal/mol) for Endohedral Complexes of X@(HSiO3/2)8 X

energy

ZPE

symmetry

pure Cr Cr+ Fe Fe+ Co Co+ Ni Ni+ Cu Cu+ Zn Zn+ Mo+ Ru Ru+ Os Os+

-939.514309 -1025.585447 -1025.409870 -1062.782056 -1062.592079 -1084.461681 -1084.201992 -1108.716166 -1108.551032 -1135.563057 -1135.392573 -1004.957308 -1004.749652 -1006.531462 -1033.222697 -1033.028462 -1030.307446 -1030.085401

87.4 84.8 86.2 85.6 85.9 85.2 86.0 86.1 83.0 85.6 86.1 85.7 85.6 83.9 84.3 106.0 84.3 89.9

Oh C1 C1 C1 C1 C1 C1 C1 C1 Th C1 C1 C1 C1 Th C1 C1 C1

ω1

Einc

92.0 64.3 136.7 120.7 127.6 50.6 43.1 100.0 122.2 124.0 66.6 144.1 112.8 73.6 112.5 98.9 103.9 98.9

120.7 72.8 54.6 -6.6 41.6 -6.8 27.5 -88.4 40.9 -32.1 94.2 13.0 85.9 42.4 13.4 73.28 11.9

TABLE 3: Total Energies (in Hartrees), Zero-Point Energies (ZPE, kcal/mol), Molecular Point Groups, Lowest Vibrational Frequencies ω1 (cm-1), Zero-Point Corrected Binding Energies (Ebind, kcal/mol), and Isomerization Energies (Eisom, in kcal/mol) for Exohedral Complexes of X(HSiO3/2)8 X

energy

ZPE

ω1

Ebind

Eiso

W W+ Mo Mo+ Cr Cr+ Fe Fe+ Co Co+ Ni Ni+ Cu Cu+ Zn Zn+ Ru Ru+ Os Os+

-1007.222640 -1006.955441 -1006.921186 -1006.670974 -1025.588718 -1025.452088 -1062.784176 -1062.642449 -1084.524106 -1084.274789 -1108.749458 -1108.580715 -1135.634018 -1135.440625 -1005.110110 -1004.857550 -1033.244701 -1033.075463 -1030.353360 -1030.155475

85.6 85.4 85.5 85.4 86.4 87.2 86.7 87.4 87.3 87.6 87.5 87.3 87.5 87.3 87.4 87.2 86.5 87.3 86.6 87.2

60.3 74.5 57.7 65.7 72.9 93.1 54.6 45.5 7.4 95.2 160.1 91.7 14.9 48.9 54.6 68.8 9.8 78.2 39.1 62.9

-48.8 -67.6 -7.7 -0.2 118.8 -47.2 -55.2 -36.7 4.6 -50.9 113.9 -87.8 -1.7 -61.0 0.0 -53.1 30.8 -34.7 -10.5 -90.7

-86.1 -0.9 -25.5 0.6 -30.2 -37.1 -44.1 86.4 0.6 -42.6 -29.0 -94.2 -66.1 -11.6 -48.2 -27.5 -46.7

the metal atoms or their ions each form a bridging bond with two oxygen atoms on one of the D4R surfaces. The exohedral complexes for Mo and W atoms and their ions have similar structures. The Si-X and O-X distances in the cationic transition metal exohedral complexes of (HSiO3/2)8 are shorter than those of their exohedral neutral counterparts (Table S-2). Energetics. The endohedral inclusion energies, Einc (eq 1), for the X@(HSiO3/2)8 and X+@(HSiO3/2)8 complexes are summarized in Table 2. The inclusion energy is defined as the zero-point corrected difference between the energy of a complex and the sum of the energies of isolated X and the isolated empty host cage. A negative Einc value indicates that the endohedral complex is more stable than the isolated components. The calculated inclusion energies for the neutral Fe@(HSiO3/2)8, Ni@(HSiO3/2)8, and Ru@(HSiO3/2)8 species are all negative suggesting that these complexes are thermodynamically stable.

All remaining neutral endohedral complexes were less stable than the separated species. The inclusion energies of the cationic complexes X+@(SiHO3/2)8 (X ) Cr+, Fe+, Co+, Ni+, Cu+, Zn+, Mo+, Ru+ and Os+) are all negative (Table 2) while the inclusion energy of Zn+ is positive. Table 3 summarizes the total energies, binding energies, Ebind (eq 2), and isomerization energies, Eisom (eq 3), for the exohedral X(HSiO3/2)8 complexes. All exohedral complexes are energetically favorable except Zn0 and Cu0 which do not bind to the cage. The exohedral ionic transition metal complexes are energetically more favorable compared to their neutral counterparts. With the exception of the Ni, Fe and Ni+ complexes which have positive isomerization energies, all exohedral neutral and ionic transition metal complexes are energetically favorable compared to their endohedral analogs. However, the isomerization energies are very small for Cr (-0.9 kcal/mol), Fe (0.6 kcal/mol) and Ni+(-0.6 kcal/mol). The small isomerization energies indicate that the endohedral and exohedral complexes of these metals have almost equal energy. Table 4 summarizes the inclusion energies for X@(SiHO3/2)8 (X ) Cr, Fe, Co, Co+, Ni, Ni+, Cu, Cu+, Zn, Zn+) systems calculated with water as solvent using the PCM solvent model.74 The solvent effects on the inclusion energies are negative for all neutral complexes indicating that the insertion of metal atom into the POSS cage is more favorable in water than in the gas phase. However, the solvent effects on the inclusion energies for metal ions are positive for all ionic complexes indicating that the insertion of metal ion into the POSS cage is less favorable in water than in the gas phase. Electronic Properties. The calculated atomic charges suggest that transfer of charge between the cage and endohedral species is small (Table S3). In the neutral endohedral complexes. a small transport of charge occurs from the cage to the Fe0, Ru0 and Os0 atoms while for the Co0, Ni0, Cu0, and Zn0 containing complexes charge is transferred from the atoms to the cage. The largest charge on an endohedral atom is +0.7 for the Co atom. The endohedral cationic complexes can be formally considered as a M+ ion inside a neutral cage. For all these complexes, however, the positive charge on the endohedral ion is smaller than the formal +1 charge suggesting charge transfer from the cage to the metal ion. The calculated HOMO-LUMO gap for the empty cage is large, 8.1 eV. Endohedral transition metals reduce the HOMOLUMO gaps significantly. In all cases, the metal atom raised the HOMO energy level and lowered the LUMO energy level. The largest change was found for the Ru complex for which theHOMO-LUMOgapwasreducedto1.2eV.TheHOMO-LUMO gaps for the other complexes were between 3.5 and 7.0 eV. These large HOMO-LUMO gaps indicate that the these endohedral complexes X@(SiHO3/2)8 are stable. Endohedral transition metal cations also reduce the HOMO-LUMO gaps relative to the empty T8-POSS cage. The changes in the HOMO-LUMO gaps were largest for Fe+ and Os+ complexes and smallest for Ni+ and Cu+ complexes. Ionization Potentials. The ionization potentials for the neutral X@(SiHO3/2)8 (X) Co, Ni, Cu, Zn, Ru, and Os) complexes calculated at the B3LYP/LanL2DZ level are summarized in Table 5. The calculated ionization potentials for the bare transition metal atoms are, except for Ni and Co, in satisfactory agreement with the experimental values. The ionization potential for a given endohedral complex is considerably lower than that of the free metal. This is consistent with the computational predictions for alkali metals encapsulated into the T10-POSS73 cages and also with

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Figure 3. Optimized geometry of X(HSiO3/2)8 and X+(HSiO3/2)8 complexes computed at the B3LYP/LanL2DZ level.

dodecahedrane endrohedral complexes of alkali and alkaline earth metals.83 As expected the ionization potentials for endohedral transition metals are considerably higher than for alkali and alkaline earth metals.

IV. Conclusions The calculations reported herein, predict that the cavity of the T8-POSS cage is sufficiently large (cavity radius 0.986 Å)

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Hossain et al.

TABLE 4: Total Free Energies of Solvation (in kcal/mol) for X@T8-POSS Complexes, Empty POSS Cage, Metal Atoms and Einclusion in a Solvent Media H2O total change of energies in kcal/mol in solvent (H2O) X

X@T8-POSS

empty cage

X atom

Einclsion in H2O in kcal/mol

Cr Fe Co Co+ Ni Ni+ Cu Cu+ Zn Zn+

19.39 13.66 16.30 -18.90 15.20 -16.08 19.14 -10.65 19.33 -10.04

19.29 19.29 19.29 19.29 19.29 19.29 19.29 19.29 19.29 19.29

2.01 1.74 1.95 -108.45 1.60 -109.81 4.57 -73.46 0.77 -111.31

-1.91 -7.37 -4.94 70.26 -5.69 74.44 -4.72 43.52 -0.73 81.92

Supporting Information Available: Bond lengths of the endohedral and exohedral metal complexes, the charge on the central metal atoms and the HOMO-LUMO gaps. This material is available free of charge via the Internet at http://pubs.acs.org.

TABLE 5: Ionization Potentials (IP, kcal/mol) of Free Atoms and Endohedral (X@T8-POSS) Complexes Calculated at B3LYP/LanL2DZ Level ionization process Cr f + Cr@(SiHO3/2)8 f Cr+@(SiHO3/2)8 + eFe f Fe+ + eFe@(SiHO3/2)8 f Fe+@(SiHO3/2)8 + eCo f Co+ + eCo@(SiHO3/2)8 f Co+@(SiHO3/2)8 + eNi f Ni+ + eNi@(SiHO3/2)8 f Ni+@(SiHO3/2)8 + eCu f Cu+ + eCu@(SiHO3/2)8 f Cu+@(SiHO3/2)8 + eZn f Zn+ + eZn@(SiHO3/2)8 f Zn+@(SiHO3/2)8 + eOs f Os+ + eOs@(SiHO3/2)8 f Os+@(SiHO3/2)8 + eRu f Ru+ + eRu@(SiHO3/2)8 f Ru+@(SiHO3/2)8 + eCr+

a

e-

IP (calcd) IP (exptl)a 159.2 110.2 181.5 119.2 212.2 163.0 220.1 103.6 180.5 110.2 211.4 130.3 206.3 139.3 172.5 121.9

Acknowledgment. This work was supported by a grant from Air Force Office of Scientific Research Grant No. F49620-021-026-0, by the National Science Foundation Grants No EPS 0132618, HRD-9805465 and DMR-0304036, by the National Institute of Health through Grant No. S06-GM008047, Department of Defense through the U.S. Army/Engineer Research and Development Center (Vicksburg, MS), Contract No. W912HZ06-C-0057. Most of the calculations were carried out on computers at the Mississippi Center for Supercomputer Research.

∆IP

156.1 49.0 182.2 62.3 187.7 49.2 176.2 116.5 178.2 70.3 216.6 81.1 200.8 67.0 169.8 50.6

References 84 and 85.

to accommodate a broad range of transition metal atoms or their cations. Several energetically favorable endohedral host-guest combinations were found. The inclusion energies of the endohedral X@(SiHO3/2)8 (X ) Cr+, Fe0,+, Co+, Ni0,+, Cu+, Ru0,+, Os+) complexes suggest that they are each more stable than their separated components. Exohedral species are formed with the T8-POSS cage by (X ) Cr0,+, Fe0,+, Co+, Ni0,+, Cu0,+, Mo0,+, W0,+, Ru0,+, Os0,+). These complexes were found to be more stable than their corresponding endohedral T8-POSS complexes. The cage properties change quite distinctly upon encapsulating a transition metal atom or ion. For example, upon encapsulating Cr0, Ni0, Cu0, and Zn0 into T8-POSS, electron density is transferred from the metal to the cage. In contrast, encapsulation of Fe0, Ru0 and Os0 leads to donation of electron density from the cage to the encapsulated metal. The ionization potentials of endohedral X@(SiHO3/2)10 (X ) Cr, Fe, Co, Ni, Cu, Zn, Mo, W, Ru, Os) species are significantly lower than those of the isolated metals. The HOMO-LUMO gaps of the endohedral transition metal T8POSS complexes are smaller than that of the pure cage. In aqueous solution insertion of neutral metal ions into a T8-POSS cage is energetically favorable while insertions of the corresponding ions are unfavorable. No experimental data are currently available for any T8-POSS complex with transition metals. The endohedral cage complexes X@(SiHO3/2)8 that were predicted here to be more stable than their isolated components should to be viable synthetic targets.

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