Carbene Analogues of Boron Stabilized by Neighboring BB Moieties: Doubly Aromatic Bishomotriboriranides

Unverzagt, Markus; Subramanian, Govindan; Hofmann, Matthias; Schleyer, Paul von Ragué; Berger, Stefan; Harms, Klaus; Massa, Werner; Berndt, A.

doi:10.1002/anie.199714691

Cited by 54 publications

(48 citation statements)

References 21 publications

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“…On the other hand, for the latertransition-metal centers, and in particular for Fe and Co, the remarkable enhancement of both the s and the p aromaticity renders the N 4 ring doubly aromatic, a concept first introduced by Schleyer and co-workers [13] and then experimentally confirmed by Berndt and co-workers. [14,15] Nickel does not alter the s antiaromatic and p aromatic character of the N 4 2À ring upon complexation.…”

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confidence: 97%

Mono‐ and Multidecker Sandwich‐Like Complexes of the Tetraazacyclobutadiene Aromatic Ring

2004

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Dedicated to Professor Cecilia SarasolaSince the fortuitous discovery [1] of ferrocene, [(C 5 H 5 ) 2 Fe], the cyclopentadienyl anion has emerged as the most versatile aromatic ring for the synthesis of sandwich-like complexes. Indeed, it has been estimated that more than 80 % of all known organometallic complexes of transition metals contain a substituted or unsubstituted cyclopentadienyl ligand. [2] Nevertheless, new aromatic compounds have recently been identified whose incorporation into sandwich complexes opens new routes for chemical creativity. In particular, we have predicted [3] that the all-metal [Al 4 ] 2À aromatic ring [4] can complex Ti to form [Al 4 TiAl 4 ]2À . The tetraazacyclobutadiene dianion, N 4 2À , also fulfills the aromaticity requirements.[5] It has a square-planar structure with six delocalized p electrons, four of which occupy the nonbonding p highest occupied molecular orbitals (HOMOs); the other two p electrons participate effectively in the aromatic stabilization, in agreement with the (4 n+2) rule. The nucleus-independent chemical shift (NICS) [6] at 1.0 above the molecular plane is À4.09 ppm, [7] confirming further the p-aromatic character of the ring. Consequently, N 4 2À can, in principle, be incorporated into sandwich-like complexes although, as far as we know, no such structures have been reported to date.We have therefore theoretically characterized stable sandwich-type complexes of the N 4 2À aromatic ring with early (Ti, V, and Cr) and late (Fe, Co, and Ni) transition-metal centers, with charges 2À (Ti, Fe), 1À(V, Co), and 0 (Cr, Ni). The optimized structure of the Ti sandwich-like complex has D 4h symmetry; the V and Cr analogues have C 2v symmetry, although they can also be viewed as bent D 4h structures (Figure 1). The optimum geometries of the late-transitionmetal complexes have D 4d symmetry. Consequently, for the complexes of the early-transition metals the nitrogen atoms are eclipsed whereas they are staggered for those of the late transition metals.The calculations were carried out with the B3LYP functional, as implemented in Gaussian 98, [8] with the standard 6-311G basis set.

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Section: àmentioning

confidence: 97%

Mono‐ and Multidecker Sandwich‐Like Complexes of the Tetraazacyclobutadiene Aromatic Ring

2004

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“…The unexpected large resonance energy of Al 4 2À has been attributed to the multiple-fold aromaticity, [3,4] a concept first introduced by Schleyer [5] and later experimentally confirmed by Berndt. [6,7] Multiple-fold aromaticity applies to molecules that possess more than one independent delocalized bonding system, either s or p type, each of them satisfying the 4n+2 electron Hückel counting rule of aromaticity. This concept helps the understanding of simultaneous contributions to aromaticity within the same molecular structure as arising from various delocalized bonding systems, and should be of general applicability irrespective of whether the molecule is organic or inorganic.…”

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confidence: 99%

Sandwich Complexes Based on the “All‐Metal” Al₄²⁻ Aromatic Ring

Mercero

Formoso

Matxain

et al. 2006

Chemistry A European J

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We report on novel sandwichlike structures [Al(4)MAl(4)](q-) (q=0-2 and M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W) based on the recently synthesized all-metal aromatic Al(4)(2-) square ring. The sandwichlike structures have two aromatic tetraaluminum square rings which trap a transition-metal cation from either the first, second, or third row. The stability of the anionic sandwichlike complexes towards electron detachment is discussed, and addition of alkali cations is found to stabilize the 2- charged complexes, preventing spontaneous electron detachment. Once the sandwichlike complexes are formed, the Al(4)(2-) square properties remain nearly unchanged; this fact strongly supports the hypothesis that in these complexes the Al(4)(2-) square rings remain aromatic.

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“…The reaction of 8 with phenyllithium and methyllithium yields 5 a and 5 d, respectively. Compounds 7 a ± c and 8 were obtained from 7 d, which can be synthesized from 11 [10] via 10, 9, and 7 e. The structures of the new compounds are deduced from their 11 B, 1 H, and 13 C NMR spectra (Tables 1 and 2). Similarities of the NMR data with those of the known [11] isomers with cis-trimethylsilyl substituents allows identification of 7 d, 7 e, and 8.…”

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“…Compounds 7 a ± c and 8 were obtained from 7 d, which can be synthesized from 11 [10] via 10, 9, and 7 e. The structures of the new compounds are deduced from their 11 B, 1 H, and 13 C NMR spectra (Tables 1 and 2). Similarities of the NMR data with those of the known [11] isomers with cis-trimethylsilyl substituents allows identification of 7 d, 7 e, and 8. The crystal structures [12] of 5 a2 Li´2 Et 2 O and 5 b´2 Li´3 Et 2 O are shown in Figure 1, their relevant spectroscopic and physical data and that of 5 c´2 Li2…”

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Bishomoaromatic 1,2,4-Triboracyclopentane Dianions: Strong Three-Center, Two-Electron Bonds between Three Boron Atoms

Scheschkewitz

Ghaffari

Amseis

et al. 2000

Angew. Chem. Int. Ed.

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The three-center, two-electron (3c,2e) bonding of bishomoaromatic cations of type 1 [1±4] are known to be weakened by R C 6 H 5 [2b, 3a] and displaced by R p-MeOC 6 H 4 and R OH.[4] Evidently, cations 1 are stabilized by the 3c,2e bonds, whereas cations 2 are stabilized by donor substituents R. According to computational estimates, 1 u (R H) is more stable than 2 u (R H) by 15 kcal mol À1. [2a] The energy difference between the prototypes 3 u and 4 u, neither of which is known experimentally, is only 10.1 kcal mol We now report on bishomoaromatic dianions 5 of 1,2,4-triboracyclopentanes that have skeletons isoelectronic to that of 3 u. [6,7] The 3c,2e bonds of dianions of type 5 remain intact even in the presence of strong donor substituents like R NR 2 . Hence, the energy difference between 5 and 6 must be larger than the 26 kcal mol À1 stabilization afforded to a tricoordinate boron atom by a dialkylamino substituent. [8] Calculations at the MP2/6-31G* level [9] confirm this conclusion.The dianions 5 a ± c (Dur 2,3,5,6-tetramethylphenyl) are obtained by reduction of 7 a ± c with lithium in diethyl ether. Diederich, Chem. Eur. J. 1998, 4, 1351 ± 1361 [15] The steric influences of the dendrons are also mirrored in the long reaction times: The mixtures had to be stirred for up to four weeks before all of the starting materials had reacted while usually the reaction is complete after a few days. Furthermore, between 77 % (in case of 9 b) and 94 % (in case of 10 b) of the free axles were formed here.[16] To make sure that under these severe conditions the spectral changes were caused by mechanical deslipping and not by chemical decomposition, we exposed the free components of the rotaxanes, that is the tetralactam 1 and the axles, to the same conditions. Indeed, there was a slow color change of the solutions and slight changes in the spectra were observed, however, these processes were significantly slower than the changes observed in the spectra of the rotaxanes.

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Carbene Analogues of Boron Stabilized by Neighboring BB Moieties: Doubly Aromatic Bishomotriboriranides

Cited by 54 publications

References 21 publications

Mono‐ and Multidecker Sandwich‐Like Complexes of the Tetraazacyclobutadiene Aromatic Ring

Mono‐ and Multidecker Sandwich‐Like Complexes of the Tetraazacyclobutadiene Aromatic Ring

Sandwich Complexes Based on the “All‐Metal” Al₄²⁻ Aromatic Ring

Bishomoaromatic 1,2,4-Triboracyclopentane Dianions: Strong Three-Center, Two-Electron Bonds between Three Boron Atoms

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Carbene Analogues of Boron Stabilized by Neighboring BB Moieties: Doubly Aromatic Bishomotriboriranides

Cited by 54 publications

References 21 publications

Mono‐ and Multidecker Sandwich‐Like Complexes of the Tetraazacyclobutadiene Aromatic Ring

Mono‐ and Multidecker Sandwich‐Like Complexes of the Tetraazacyclobutadiene Aromatic Ring

Sandwich Complexes Based on the “All‐Metal” Al42− Aromatic Ring

Bishomoaromatic 1,2,4-Triboracyclopentane Dianions: Strong Three-Center, Two-Electron Bonds between Three Boron Atoms

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Sandwich Complexes Based on the “All‐Metal” Al₄²⁻ Aromatic Ring