Theory and Calculations of Molecules Containing Heavier Main Group Elements and Fullerenes Encaging Transition Metals: Interplay with Experiment

Topochemical transformations of layered materials CaX2 (X=Si, Ge) are the method of choice for the high-yield synthesis of pristine, defect-free two-dimensional systems silicane and germanane, which have advanced electronic properties. Based on solid-state dispersion-corrected calculations, mechanisms for such transformations are elucidated that provide an in-depth understanding of phase transition in these layered materials. While formation of such layered materials is highly favorable for silicane and germanane, a barrier of 1.2 eV in the case of graphane precludes its synthesis from CaC2 topochemically. The energy penalty required for distorting linear acetylene into a trans-bent geometry accounts for this barrier. In contrast it is highly favorable in the heavier analogues, resulting in barrierless topochemical generation of silicane and germanane. Photochemical generation of the trans-bent structure of acetylene in its first excited state (S1 ) can directly generate graphane through a barrierless condensation. Unlike the buckled structure of silicene, the phase-h of CaSi2 with perfectly planar silicene layers exhibits the Dirac cones at the high symmetry points K and H. Interestingly, topochemical acidification of the cubic phase of calcium carbide is predicted to generate the previously elusive platonic hydrocarbon, tetrahedrane.

Section: Resultsmentioning

confidence: 99%

Topochemical Transformations of CaX₂ (X=C, Si, Ge) to Form Free‐Standing Two‐Dimensional Materials

Pratik

Nijamudheen

Datta

2015

“…[1][2][3][4] Such molecules can be categorizedi nto two groups according to the formal charge of the encaged metal atoms. [1][2][3][4] Such molecules can be categorizedi nto two groups according to the formal charge of the encaged metal atoms.…”

Section: Introductionmentioning

confidence: 99%

The Unanticipated Dimerization of Ce@C_2v(9)‐C₈₂ upon Co‐crystallization with Ni(octaethylporphyrin) and Comparison with Monomeric M@C_2v(9)‐C₈₂ (M = La, Sc, and Y)

Suzuki

Yamada

Maeda

et al. 2016

Self Cite

We report that Ce@C (9)-C forms a centrosymmetric dimer when co-crystallized with Ni(OEP) (OEP = octaethylporphyrin dianion). The crystal structure of {Ce@C (9)-C } ⋅2[Ni(OEP)]⋅4 C H shows that a new C-C bond with a bond length of 1.605(5) Å connects the two cages. The high spin density of the singly occupied molecular orbital (SOMO) on the cage and the pyramidalization of the cage are factors that favor dimerization. In contrast, the treatment of Ni(OEP) with M@C (9)-C (M = La, Sc, and Y) results in crystallization of monomeric endohedral fullerenes. A systematic comparison of the X-ray structures of M@C (9)-C (M = Sc, Y, La, Ce, Gd, Yb, and Sm) reveals that the major metal site in each case is located at an off-center position adjacent to a hexagonal ring along the C axis of the C (9)-C cage. DFT calculations at the M06-2X level revealed that the positions of the metal centers in these metallofullerenes M@C (9)-C (M = Sc, Y, and Ce), as determined by single-crystal X-ray structure studies, correspond to an energy minimum for each compound.

“…Also, the remaining 3p y orbitali se mpty.T herefore, two p orbitals (i.e.,3 p x and3 p y )c an play the role of the acceptor orbitalt op articipate in the CT from the Pd atoms in the equatorial plane,w hich supports the bondingi nteraction shown in Figure 7. We wish to mentiont hat the 3s orbital of the Si atom participates in the bonding interaction little due to the difference in size and energy from those of the 3p orbitals [29] (for ad etailed discussion see page S2 in the Supporting Information).…”

Section: Bonding Interactions Of the Seven-coordinated Si Speciesmentioning

confidence: 99%

“…[32] One more factor to be considered here are the PdÀSi and PdÀGe distances;a s mentioned above, the PdÀSi distance is shorter than the PdÀ Ge one because the atomicr adius of Ge is larger than that of Si. [29] The Pd-Si pairwith ashorter interatomic distance stabilizes the negative chargeb etween them more than the Pd-Ge pair with al ongeri nteratomic distance. As ar esult,t he electron density is more accumulatedi nt he region between the Pd and Si atoms than in the region between the Pd and Ge atoms, which leads to the larger formation energies of compounds 1 Si and 2 Si compared to that of compounds 1 Ge and 2 Ge .…”

Section: Differences Of This Seven-coordinatedmentioning

confidence: 99%

Theoretical Study of Pd₁₁Si₆ Nanosheet Compounds Including Seven‐Coordinated Si Species and Its Ge Analogues

Chen

Sunada

Nagashima

et al. 2015

Nanosheet compounds Pd11 (SiiPr)2 (SiiPr2 )4 (CNtBu)10 (1) and Pd11 (SiiPr)2 (SiiPr2 )4 (CNMes)10 (2), containing two Pd7 (SiiPr)(SiiPr2 )2 (CNR)4 plates (R=tBu or Mes) connected with three common Pd atoms, were investigated with DFT method. All Pd atoms are somewhat positively charged and the electron density is accumulated between the Pd and Si atoms, indicating that a charge transfer (CT) occurs from the Pd to the Si atoms of the SiMe2 and SiMe groups. Negative regions of the Laplacian of the electron density were found between the Pd and Si atoms. A model of a seven-coordinated Si species, that is, Pd5 (Pd-SiMe), is predicted to be a stable pentagonal bipyramidal molecule. Five Pd atoms in the equatorial plane form bonding overlaps with two 3p orbitals of the Si atom. This is a new type of hypervalency. The Ge analogues have geometry and an electronic structure similar to those of the Si compounds. But their formation energies are smaller than those of the Si analogues. The use of the element Si is crucial to synthesize these nanoplate compounds.