Abstract
Despite the importance of bulk lanthanide borides, nanoclusters of lanthanide and boron have rarely been investigated. Here we show that lanthanide-boron binary clusters, La2B x -, can form a new class of inverse-sandwich complexes, [Ln(η x -B x )Ln]- (x = 7-9). Joint experimental and theoretical studies reveal that the monocyclic B x rings in the inverse sandwiches display similar bonding, consisting of three delocalized σ and three delocalized π bonds. Such monocyclic boron rings do not exist for bare boron clusters, but they are stabilized by the sandwiching lanthanide atoms. An electron counting rule is proposed to predict the sizes of the B x ring that can form stable inverse sandwiches. A unique (d-p)δ bond is found to play important roles in the stability of all three inverse-sandwich complexes.
Highlights
IntroductionJoint experimental and quantum chemistry investigations have shown that small boron clusters form planar structures, consisting of localized two-center-twoelectron (2c-2e) s bonds on the periphery of the clusters and delocalized s and p bonding in the interior of the cluster plane due to the electron de ciency of the boron atom.[1,2,3,4,5,6] The delocalized p bonding in the planar boron clusters has been shown to be analogous to aromatic hydrocarbons (arenes), giving rise to concepts of aromaticity and hydrocarbon analogues of boron clusters.[7,8,9,10,11,12,13,14,15,16,17] while arenes can form sandwich compounds with transition metals due to strong d–p interactions,[18,19,20] no similar sandwich complexes have been observed for the aromatic planar boron clusters
These resolved photoelectron spectroscopy (PES) features serve as electronic ngerprints, which can be compared with theoretical calculations (Fig. 1b, vide infra) to provide insights into the structures and bonding of La2B7À and its corresponding neutral species
We report the discovery of a new class of di-lanthanide boron inverse sandwich complexes: [Ln(hx-Bx)Ln]À (x 1⁄4 7–9)
Summary
Joint experimental and quantum chemistry investigations have shown that small boron clusters form planar structures, consisting of localized two-center-twoelectron (2c-2e) s bonds on the periphery of the clusters and delocalized s and p bonding in the interior of the cluster plane due to the electron de ciency of the boron atom.[1,2,3,4,5,6] The delocalized p bonding in the planar boron clusters has been shown to be analogous to aromatic hydrocarbons (arenes), giving rise to concepts of aromaticity and hydrocarbon analogues of boron clusters.[7,8,9,10,11,12,13,14,15,16,17] while arenes can form sandwich compounds with transition metals due to strong d–p interactions,[18,19,20] no similar sandwich complexes have been observed for the aromatic planar boron clusters. Tubular-type or half-sandwich metal complexes of boron clusters have been observed.[21,22,23,24,25,26] A interesting class of transition metal doped boron clusters is the metalcentered borometallic molecular wheels, M©BxÀ (x 1⁄4 8–10), in which the monocyclic boron rings are stabilized by the central metal atoms,[27,28,29,30,31,32,33] even though such boron rings are not stable by themselves.[7,8] The largest ring size observed is B10 with M 1⁄4 Nb and Ta,[29] while the smallest ring size is B8 for M 1⁄4 3d transition metals.[27,30] an even smaller B7 ring exists in the planar B8 cluster (D7h),[7] which can be viewed as B©B7. The borometallic molecular wheels follow an electronic design principle, which requires double aromaticity in the s and p frameworks.[27,31,33]
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