Tetracarbalane structures: nido polyhedra and non-spherical deltahedra

Abstract

Uhl and coworkers have synthesized and characterized structurally the 11-vertex tetracarbalanes (AlMe)7(CEt)4(μ-H)2 and (AlEt)7(CCH2Ph)4(C[triple bond, length as m-dash]CPh)(μ-H). In order to understand the nature of the unusual C4Al7 polyhedra in such systems, the complete series of permethylated tetracarbalanes C4Aln-4Men (n = 6 to 14) as model compounds have been investigated using density functional theory. An overriding factor in determining the polyhedra for the lowest energy structures of a wide range of tetracarbalanes C4Aln-4Men (n = 6 to 14) is the availability of vertices of degrees 3 and 4 for all four carbon atoms. For the 11-vertex C4Al7Me11 system the experimentally observed C4Al7 deltahedron with carbon atoms at the four degree 4 vertices is found in the lowest energy structure but severely distorted from ideal C2h to Cs symmetry. This distortion is removed by adding two bridging hydrogens to the C4Al7Me11 structure in the same locations as in the experimental structures. The lowest energy structures of the tetracarbalanes C4Aln-4Men (n = 12, 13, 14) with more than 11 vertices are deltahedra having the carbon atoms at degree 4 vertices similar to the lowest energy C4Al7Me11 structures. The lowest energy structures for the smaller tetracarbalanes C4Aln-4Men (n = 6, 7, 8) are nido structures with pentagonal or hexagonal open faces related to nido borane structures. The intermediate 9- and 10-vertex tetracarbalanes C4Aln-4Men (n = 9, 10) have relatively complicated energy surfaces.

Segregation of tetracarbon units in low‐energy tetracarbindane structures: Major differences from their aluminum and gallium analogs

“…The significantly larger covalent radius of indium (1.42 Å) relative to that of gallium (1.22 Å) [53] leads to major differences in the low-energy struc- The cyclic C 4 tetragon found in a few low-energy tetracarbindane clusters is a new structural motif not found in any of the previously studied low-energy tetracarbalane [12] and tetracarbagallane [13] structures. Such C 4 tetragons are essentially trapezoids with two nearly equivalent C C distances of 1.49 ± 0.02 Å, a significantly longer C C distance of 1.57 ± 0.03 Å, and a short C C distance of 1.37 ± 0.02 Å corresponding to a formal double bond.…”

“…The theoretical methods used in this study are similar to those in our recently reported studies on the analogous tetracarbalanes and tetracarbagallanes . Thus, the initial model structures were based on various In n Me n polyhedral frameworks in which a systematic substitution of four indium atoms by four carbon atoms led to 1365 starting structures of the type C 4 In n−4 Me n (n = 6‐14) (see the Supporting information).…”

“…A similar difference between the corresponding tetracarbalane and tetracarbagallane structures was found in theoretical studies. In many of the lowest energy tetracarbalane C 4 Al n−4 Me n structures, including C 4 Al 7 Me 11 closely related to the experimentally known (AlMe) 7 (CEt) 4 (μ‐H) 2 , the four carbon atoms were isolated leading to an absence of CC edges in the central C 4 Al n−4 polyhedron . This contrasted with the tetracarbagallanes for which all of the low‐energy C 4 Ga n−4 Me n structures, except for the C 4 Ga 6 Me 10 structure analogous to the experimentally known R' 4 C 4 Ga 6 R 6 derivatives, were found to contain at least one CC edge .…”

“…The theoretical methods used in this study are similar to those in our recently reported studies on the analogous tetracarbalanes and tetracarbagallanes. [10][11][12][13] Thus, the initial model structures were based on various In n Me n polyhedral frameworks in which a systematic substitution of four indium atoms by four carbon atoms led to 1365 starting structures of the type C 4 In n−4 Me n (n = 6-14) (see the Supporting information). Geometry optimizations were carried out on these starting structures using the B3LYP DFT functional coupled with the 6-31G(d) basis set [19][20][21][22] as implemented in the Gaussian 09 software package.…”

“…In many of the lowest energy tetracarbalane C 4 Al n−4 Me n structures, including C 4 Al 7 Me 11 closely related to the experimentally known (AlMe) 7 (CEt) 4 (μ-H) 2 , the four carbon atoms were isolated leading to an absence of C C edges in the central C 4 Al n−4 polyhedron. [12] This contrasted with the tetracarbagallanes for which all of the low-energy C 4 Ga n−4 Me n structures, except for the C 4 Ga 6 Me 10 structure analogous to the experimentally known R´4C 4 Ga 6 R 6 derivatives, were found to contain at least one C C edge. [13] In such C 4 Ga n−4 Me n structures the four carbon atoms were typically partitioned into two C 2 units or a C 3 unit plus an isolated carbon atom.…”