Observation of highly stable and symmetric lanthanide octa-boron inverse sandwich complexes Wan-Lu Lia,b,1, Teng-Teng Chenc,1, Deng-Hui Xinga,b, Xin Chena,b, Jun Li (李 隽)a,b,2, and Lai-Sheng Wangc,2 aDepartment of Chemistry, Tsinghua University, Beijing 100084, China; bKey Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education, Tsinghua University, Beijing 100084, China; and cDepartment of Chemistry, Brown University, Providence, RI 02912 Edited by Christopher C. Cummins, Massachusetts Institute of Technology, Cambridge, MA, and approved June 14, 2018 (received for review April 14, 2018) While boron forms a wide range of metal borides with important PES industrial applications, there has been relatively little attention The experiment was performed using a magnetic-bottle PES ap- devoted to lanthanide boride clusters. Here we report a joint paratus equipped with a laser vaporization supersonic cluster source photoelectron spectroscopy and quantum chemical study on two SI Appendix Methods − (10) (see , for more details). Briefly, lanthanide = 11 11 octa-boron di-lanthanide clusters, Ln2B8 (Ln La, Pr). We found boron clusters were generated by laser ablation of a La/ BorPr/ B that these clusters form highly stable inverse sandwich structures, mixed target, followed by supersonic expansion using a helium car- – – − [Ln B8 Ln] , with strong Ln and B8 bonding via interactions be- rier gas seeded with 5% argon. Negatively charged clusters were σ π tween the Ln 5d orbitals and the delocalized and orbitals on extracted from the cluster beam and analyzed by a time-of-flight the B ring. A (d–p)δ bond, involving the 5dδ and the antibonding − − 8 mass spectrometer. Clusters with different LaxBy or PrxBy com- π orbital of the B8 ring, is observed to be important in the Ln–B8 − − positions were produced. The La2B8 and Pr2B8 clusters of current interactions. The highly symmetric inverse sandwich structures are interest were mass-selected and photodetached by the 193-nm ra- overwhelmingly more stable than any other isomers. Upon elec- diation of an ArF excimer laser (6.424 eV). PES data at 193 nm were tron detachment, the (d–p)δ orbitals become half-filled, giving rise − first obtained for the La B cluster, which was found to exhibit a to a triplet ground state for neutral La B . In addition to the two 2 8 2 8 particularly simple spectral pattern (Fig. 1A), suggesting a highly unpaired electrons in the (d–p)δ orbitals upon electron detach- − symmetric structure. Subsequently, the spectrum of Pr2B8 was ment, the neutral Pr2B8 complex also contains two unpaired 4f − measured (Fig. 1B), revealing a similar spectral pattern as La2B8 electrons on each Pr center. The six unpaired spins in Pr2B8 are – ferromagnetically coupled to give rise to a septuplet ground state. and indicating that the two di-Ln octa-boron clusters should have The current work suggests that highly magnetic Ln…B …Ln in- similar structures and bonding. 8 The two spectra each displayed four well-resolved bands labeled verse sandwiches or 1D Ln…B8…Ln nanowires may be designed with novel electronic and magnetic properties. as X, A, B, and C. PES involves electron detachment from the anions, resulting in neutral species. The lowest binding energy band (X) corresponds to detachment transition from the ground lanthanide boride clusters | inverse sandwich | photoelectron − spectroscopy | chemical bonding | magnetism state of the Ln2B8 anion to that of the corresponding neutral, whereas bands A, B, and C correspond to detachment transitions to excited states of neutral Ln B .Thebroadwidthsofsomeofthe oron forms a wide variety of boride materials, ranging from 2 8 the superconductor MgB to superhard transition metal bo- B 2 Significance rides (1, 2). In particular, lanthanide borides represent a class of highly valuable magnetic, optical, superconducting, and thermo- Lanthanide borides constitute an important class of materials electric materials (3–6). In the past two decades, extensive exper- with wide industrial applications, but clusters of lanthanide bo- imental and theoretical studies have been conducted to elucidate rides have been rarely investigated. It is of great interest to the structures and chemical bonding of size-selected boron clusters study these nanosystems, which may provide molecular-level – (7 11), resulting in the discoveries of novel graphene-like understanding of the emergence of new properties and pro- (borophene), fullerene-like (borospherene), and nanotubular vide insight into designing new boride materials. We have pro- structures (12–16). However, there has been relatively less at- duced lanthanide boride clusters and probed their electronic − tention devoted to metal boride clusters, even though a number structure and chemical bonding. Two Ln2B8 clusters are pre- of transition metal-doped clusters have been characterized (11, sented, and they are found to possess inverse sandwich struc- D 17–19). In particular, there have been few experimental studies tures. The neutral Ln2B8 complexes are found to possess 8h – – δ on lanthanide boron clusters. symmetry with strong Ln B8 bonding. A unique (d p) bond is − − found to be important for the Ln–B8–Ln interactions. The Ln2B8 The SmB6 and PrB7 clusters represent the first lanthanide inverse sandwich complexes broaden the structural chemistry of (Ln) boron clusters reported (20, 21), each featuring a B or B 6 7 the lanthanide elements and provide insights into bonding in cluster coordinated to the Ln atom. Here we present a joint lanthanide boride materials. photoelectron spectroscopy (PES) and quantum chemical study − − of the first di-Ln–doped boron clusters, La2B8 and Pr2B8 .We Author contributions: J.L. and L.-S.W. designed research; W.-L.L., T.-T.C., D.-H.X., and X.C. have found that these clusters have highly stable and symmetric performed research; W.-L.L. and T.-T.C. analyzed data; and W.-L.L., T.-T.C., J.L., and L.-S.W. wrote the paper. inverse sandwich structures: a B8 ring sandwiched by the two Ln − The authors declare no conflict of interest. atoms, [Ln−B8−Ln] . Both anionic complexes are found to have This article is a PNAS Direct Submission. D4h symmetry, whereas upon electron detachment the neutral Published under the PNAS license. La2B8 and Pr2B8 complexes are found to possess perfect D8h 1W.-L.L. and T.-T.C. contributed equally to this work. symmetry. The interactions between the Ln atoms and the B8 π δ 2To whom correspondence may be addressed. Email: [email protected] or Lai- ring are derived from strong Ln5d and B2p and interactions. [email protected]. The bonding in the Ln2B8 inverse sandwich complexes can help This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. us understand the bonding properties of bulk systems and the 1073/pnas.1806476115/-/DCSupplemental. design of novel lanthanide boride materials. Published online July 9, 2018. E6972–E6977 | PNAS | vol. 115 | no. 30 www.pnas.org/cgi/doi/10.1073/pnas.1806476115 Downloaded by guest on September 30, 2021 − − − − Fig. 1. Photoelectron spectra of (A)La2B8 and (B)Pr2B8 at 193 nm and comparison with the simulated spectra for the (C) D4h La2B8 and (D) D4h Pr2B8 .See − − SI Appendix, Tables S4 and S5 for the detachment energies of the observed bands and their assignments for La2B8 and Pr2B8 , respectively. detachment bands suggest that they may contain multiple de- 36.37 kcal/mol above the global minimum at the PBE0/TZP level. tachment transitions. The X band yielded the first vertical de- The isomer with a pseudoC7v B©B moiety (the most stable − 7 CHEMISTRY tachment energy (VDE) of 1.76 eV for La2B8 and 1.75 eV for structure for bare B ) (26), sandwiched by the two La atoms, is − 8 Pr2B8 . The adiabatic detachment energy (ADE) for band X was even higher in energy (67.07 kcal/mol at the PBE0/TZP level). evaluated from its onset to be 1.64 ± 0.05 eV and 1.59 ± 0.05 eV, − which represent the electron affinities (EAs) of neutral La2B8 and Pr2B8 . On the basis of their similar PE spectra, we expected that − − − Pr2B8, respectively. The A band at 2.91 eV (La2B8 )and2.94eV Pr2B8 should have a similar global minimum as La2B8 . Indeed, − − (Pr2B8 ) was broad and intense in both spectra. At around 4 eV, we found that Pr2B8 has a D4h structure with a sextet ground state 6 both spectra displayed a relatively sharp band B, closely followed ( B2u). Neutral Pr2B8 upon electron detachment was found to have 7 by a weak band C. The signal/noise ratios above 5 eV were poor, aperfectD8h symmetry with six unpaired electrons ( A2g). The and no specific detachment bands could be definitively identified. structural parameters for the D8h Pr2B8 arealsoshowninFig.2, Some weak features appeared around bands A and B and also together with those of La2B8, and its coordinates are given in the SI possibly above band C (labeled by *). These weak features were Appendix,TableS2. We have obtained preliminary PES data for a − likely due to multielectron or shakeup processes, as a result of late lanthanide, Tb2B8 (SI Appendix,Fig.S2), which also gives a − − strong electron correlation effects expected for these systems (22). similar spectral pattern as those of La2B8 and Pr2B8 .Hence,we The well-resolved photoelectron spectral features served as elec- expect that many lanthanide elements may form the highly sym- tronic fingerprints to allow analyses of their structures and metric Ln−B8–Ln inverse sandwich structures, as confirmed for bonding by comparing with theoretical calculations. − − La2B8 and Pr2B8 by comparison between experiment and theory. Global Minimum Structure Searches Comparison Between the Experimental and Computational − − La2B8 . The global minimum structure for La2B8 was searched Results − − using the Tsinghua Global Minimum (TGMin) package (23, 24) To validate the global minima of La B and Pr B ,wecalcu- SI Appen- 2 8 2 8 with a constrained basin-hopping algorithm (25) (see lated their ADEs and VDEs using the ΔSCF–TDDFT formalism.