Periodic trends and easy estimation of relative stabilities in 11-vertex nido-p-block-heteroboranes and -borates

2006

Dalton Trans.

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The relative thermodynamic stabilities of ortho-, meta- and para-isomers of 12-vertex closo-heteroboranes and -borates with different p-block heteroatoms were determined using density functional theory. More electronegative (smaller) heteroatoms tend to occupy non-adjacent, whereas less electronegative (larger) heteroatoms tend to occupy adjacent vertices in the thermodynamically most stable closo-diheterododecaborane isomers. The computed relative stabilities agree perfectly with experimental observations. The energy differences of para- and meta- relative to ortho-isomers of 12-vertex closo-heteroboranes generally depend on the extent of electron localization by a given heteroatom and show highly periodic trends, i.e. increase along the period and decrease down the group. The energy penalties for the HetHet structural feature (two heteroatoms adjacent to each other) for the 12-vertex closo-cluster are apparently significantly different from those for the 11-vertex nido-cluster. Reformulating two 11-vertex nido-structural features, i.e. Het(5k)(2) and HetHet, in terms of connection increments along with the additional structural feature HetHet(m) give the relative stabilities of various isomeric 11-vertex nido- as well as 12-vertex closo-heteroboranes and -borates, using one unique set of increments.

Section: Redefining Het 5k (2) and Hethet In Terms Of Connection Incr...mentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

The relative stabilities of 11-vertex nido- and 12-vertex closo-heteroboranes and -borates: facile estimation by structural or connection increments

Kiani

2006

Dalton Trans.

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“…The derived energy penalty values are very close to zero for nitrogen, but lie between 13.3 and 20.6 kcal mol À1 for its higher homologs. In general, HetHet and HetC energy penalties decrease down one group and increase along the period, as was found by estimating the energy penalties for Group 14 (C, Si, Ge, Sn), Group 15 (N, P, As, Sb), and Group 16 (O, S, Se, Te) heteroboranes [8].…”

Section: Eleven-vertex Nido-clustermentioning

confidence: 91%

“…For all the other elements considered, however, the penalties were found to be less or even negative, which means that 1,2-is preferred over 1,12-substitution. Overall, the penalties for adjacent heteroatoms show nice periodic trends [8,9], as displayed graphically in Figure 25.1a. …”

Section: Twelve-vertex Closo-clustermentioning

confidence: 97%

Quantifying Building Principles of Borane Clusters

Modeling of Molecular Properties

2011

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Boranes show an intriguing structural variety; notably, in order to reduce their electron deficiency they form nonclassical multicenter bonds and adopt cluster structures. Structural relationships among borane clusters were realized after an increasing number of structures had became known. Wades rule [1] allows (hetero) boranes to be classified according to their number of skeletal electrons, and the shape of the corresponding cluster framework to be determined. Closo, nido, arachno, and hypho are used to label compounds with 2n þ 2, 2n þ 4, 2n þ 6, and 2n þ 8 skeletal electrons, respectively, where n equals the number of cluster atoms. Closo compounds adopt the most spherical deltahedral structures -that is, polyhedra with only trigonal faces. Nido, arachno, and hypho compounds have polyhedral structures in which one, two or three vertices of the corresponding closo deltahedra remain unoccupied. Examples of these are shown in Scheme 25.1.In heteroboranes, one or more boron atoms are replaced by heteroatoms such as C, N, and S. While the cluster type may be derived from the number of skeletal electrons, the placement of heteroatoms allows for various possible isomers. The preferred sites for heteroatom placement were derived from the arrangements found for structurally characterized compounds. For example, the most stable carborane isomer has carbon atoms at low coordinate sites and in nonadjacent positions [2,3]. Other disfavored arrangements, for example, endo terminal rather than bridging hydrogen atoms, were also identified. However, in general, not all of these may be avoided at the same time for a given formula. So, the question arises: Which structural conflict(s) may be tolerated best? Clearly, values are needed to specify the severity of the violation of a particular structural rule, and only if such quantitative rules are available can the best -that is, the thermodynamically most stable -structure be predicted, both easily and reliably. The derivation of meaningful energy penalties for important unfavorable structural features in heteroboranes, and how they may be applied, are described in the following subsections.Modeling of Molecular Properties, First Edition. Edited by Peter Comba.

“…acluster increments as well as the DFT computed relative stabilities are higher for nido(9):nido(9)-B 16 H 20 and nido(7):nido(11)-B 16 H 20 . For 17 and 18 vertexes, again the most stable isomer incorporates a 10-vertex nido-fragment in each case.…”

mentioning

confidence: 91%

Cluster increments for macropolyhedral boranes

Kiani

2006

Dalton Trans.

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Cluster increments derived for individual cluster fragments reproduce the DFT computed relative stabilities of macropolyhedral boranes usually within +/-6 kcal mol(-1). A simple summation procedure helps to select the best partner for a given cluster fragment in order to construct the thermodynamically most stable macropolyhedral borane. Cluster increments are considerably smaller for nido-cluster fragments with an even number of vertexes than for odd nido-cluster fragments pointing towards high thermodynamic stability of macropolyhedral boranes with even numbered nido-units.