Revisiting B20H16 by means of a joint computational/experimental NMR approach

Abstract

Dedicated to Professor Bohumil Štíbr on the occasion of his 70th birthday as our gratitude to his achievements in boron cluster chemistry.A new synthesis of the fused macropolyhedral boron cluster B 20 H 16 is described and its molecular structure in solution discussed, based on multi-nuclear NMR spectra, including COSY measurements, in relation to its previously elucidated solid-state structure. To verify the conclusions from the NMR study, experimentally determined chemical shifts are compared with calculated values at the GIAO-B3LYP level with a TZP basis set by Huzinaga. There is a very good agreement between the experimental and computed δ( 11 B) values, suggesting that the MP2/6-31G* internal coordinates are a reasonable representation of the molecular geometry of this twenty-vertex cluster in solution that is essentially the same as its solid-state structure. A computational analysis of the FMO orbitals of B 20 H 16 , in particular of the LUMO, reveals that the four naked boron atoms, common for two shared icosahedral subclusters, are the reactive sites of this D 2d -symmetrical molecule.The icosahedron is the most symmetrical way to arrange twelve atoms into a polyhedral cluster and is the quintessential structural motif in boron hydride cluster chemistry, classically represented by the [closo-B 12 H 12 ] 2-dianion (Fig. 1, structure a). This dianion belongs to a point group of I h symmetry and is considered to be a three-dimensional aromatic system 1 . Such structural architectures are also found in elemental boron itself. Replacement of individual {BH} 2-vertices in this twelve-vertex polyhedral cluster by various isoelectronic and isolobal 2 main-group element units

Photochromic System among Boron Hydrides: The Hawthorne Rearrangement

“…The minima (I and II) and transition state (TS) were optimized by means of the DFT/B3LYP method as implemented in the GAUSSIAN 09 software package, 11 imposing symmetry constraints (C2h point group) and using the standard 6-31G* basis set. The DFT/B3LYP/6-31G* level of theory has described properly the structures of related polyhedral borane hydrides.…”

“…12 2.3. NMR measurements 11 B NMR spectra were measured in CD3 CN using a Varian XL-500 instrument at 11.75 T. The chemical shifts were referenced to BF3.OEt2.…”

“…Among the polyhedral [ closo -B n H n ] 2– ions ( n = 5–12), those with n = 10, 12 are commonly used in boron cluster chemistry − and are characterized in terms of a 3c–2e multicenter bonding scheme . The icosahedral [ closo -B 12 H 12 ] 2– , with all boron atoms being equivalent, dimerizes without retaining the individual icosahedra, and consequently, three or four boron vertices are shared, yielding the macropolyhedrons B 20 H 16 and [B 21 H 18 ] − , respectively. , In contrast, the biccapped-square antiprismatic [ closo -B 10 H 10 ] 2– ion is quite reactive and undergoes dimerization in which two [ closo -B 10 H 10 ] 2– ions formally lose a hydride ion; the resulting [ closo -B 10 H 9 ] − ions produce the [ trans -B 20 H 18 ] 2– species . The two [ closo -B 10 H 9 ] − fragments are linked together by a pair of unique 3c–2e B–B–B bonds in a transoid configuration.…”

A computational analysis of the apparent nido vs. hypho conflict: are we dealing with six- or eight-vertex open-face diheteroboranes?

“…When these structures were allowed to relax further by optimising the B-H r distances, 3c-2e bonding persisted for all five species 1a/H-1e/H. 36d,38b Armed with these results, we applied the mno rule to nido-[B 6 H 11 ] + as well as to 1a-1f. Similar analyses for 1a-1f (where the heteroatoms are present) showed a different picture, where all B-C/N/S distances were classified as 2c-2e bonds.…”

“…The unusually high stability of closo-[B 12 H 12 ] 2− , as well as the existence of condensed B 12 units [for example, in the socalled macropolyhedral boron clusters B 20 H 16 6 and B 21 H 18 − (ref. 7)], meant that a generalisation of the electron-counting rules was required since such structures are beyond the scope of the Williams-Wade formalism.…”