Planar Tetracoordinate Carbon Species Involving Beryllium Substituents

Abstract

Planar tetracoordinate carbon (ptC) arrangements can be achieved by employing multiple substituents based on beryllium, despite its rather weak pi-acceptor ability. A variety of ptC-containing examples, some with more than one ptC, have been designed computationally by elaborating the planar C(BeH) 4 (2-) prototype at B3LYP/6-311++G(3df,2p) and MP2/6-311++G(3df,2p) levels of theory for some small ptC representatives. The prototype prefers a D(2h) paramagnetic triplet ground state due to Hund's rule, rather than a singlet. The highly polarized C-Be bonding weakens the rigidity of the tetrahedral carbon in T(d)C(BeH) 4 enormously, and the enhancement of both C-Be and Be 4 peripheral covalent bonding exerted by the extra electrons stabilizes the ptC eventually. The delocalization of the two p pi electrons is only modest, but their density on the most electronegative carbon atom helps stabilize the ptC arrangement. This is in contrast to the conventional strategy to delocalize p(pi) lone pairs for stabilizing the ptC arrangement. Various strategies to achieve neutral derivatives with ptCs are demonstrated.

Four Decades of the Chemistry of Planar Hypercoordinate Compounds

“…[36] Radoms group [16] designed af amily of alkaplane molecules (for example,s pirooctaplane, 2c)w hich constrain the ptC candidate in hydrocarbon cages.A lthough approaching ap lanar C(C) 4 -type ptC more closely than ever before,s uch "mechanical" designs without "electronic" assistance must struggle to overcome the enormous strain of ap tC with a p lone-pair HOMO.More buttresses are needed to achieve planarity (for example,i nd imethanospiro[2,2]octaplane 2e). .…”

“…[87a] Remarkably,e ven larger all-boron clusters, including quasiplanar structures,n amely chiral B 30 À [91] and B 35 À [92] with adouble-hexagonal vacancy ( Figure 42) as well as aplanar hexagonal B 36 À [93] (Figure 43), have been detected in the gas-phase and characterized by photoelectron spectroscopy and ab initio calculations. À,2À , [81] B 9 À , [81] B 10 ,B 11 À ,a nd B 12 [90] can be viewed as analogues of benzene on the basis of their six p electrons, because they have p-molecular orbital patterns similar to those of benzene.Aperfectly planar B 16 2À (D 2h ) [94] with 10 p electrons can be considered an all-boron naphthalene.A lso, three neutral boron clusters B 11 ,B 16 ,a nd B 17 are shown to have planar or quasi-planar structures by vibrational spectroscopy and ab initio calculations.…”

“…Figure 43. Calculated structure of a) B 36 À , and b) B 36 ,and c) proposed extended boron sheets based on B 36 units. [93] (Reprinted with permission from Ref.…”

Computationally Designed Families of Flat, Tubular, and Cage Molecules Assembled with “Starbenzene” Building Blocks through Hydrogen‐Bridge Bonds

“…The intermolecular H-bridge bonds glue starbenzene pieces to- gether. Each hexagonal block in this type of flat molecules (see 1PPF 19 ) except for those on the verges links to six neighbors optimally, which arrange all the H-bridge bonds along the BeH directions and thus minimize the strain. Regardless of the size of the systems, the average bonding energy per H-bridge bond (E HBB ) remains nearly identical (see Table 3 and the Supporting Information SI3), being À33.7 (1PPF 2 ), À33.…”

“…We recently found that the BeH group is an appropriate ligand to stabilize ptC, [19] which, along with the isolobal relationship of BeH to Li, suggested our replacing the six Li atoms in C 6 Li 6 with BeH groups. We also expected that the electron deficiency of BeH group would result in intermolecular hydrogen-bridge (H-bridge) bonds similar to that in BeH 2 dimer to meet criterion 2.…”