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1.
A tetra(o-tolyl) (μ-hydrido)diborane(4) anion 1, an analogue of [B2H5]− species, was facilely prepared through the reaction of tetra(o-tolyl)diborane(4) with sodium hydride. Unlike common sp2–sp3 diborane species, 1 exhibited a σ-B–B bond nucleophilicity towards NHC-coordinated transition-metal (Cu, Ag, and Au) halides, resulting in the formation of η2-B–B bonded complexes 2 as confirmed by single-crystal X-ray analyses. Compared with 1, the structural data of 2 imply significant elongations of B–B bonds, following the order Au > Cu > Ag. DFT studies show that the diboron ligand interacts with the coinage metal through a three-center-two-electron B–M–B bonding mode. The fact that the B–B bond of the gold complex is much prolonged than the related Cu and Ag compounds might be ascribed to the superior electrophilicity of the gold atom.A tetra(o-tolyl)(μ-hydrido)diborane(4) anion is facilely prepared via the reaction of tetra(o-tolyl)diborane(4) with NaH. It exhibits a σ-B–B bond nucleophilicity towards NHC-metal halides to give the corresponding η2-B–B bonded metal complexes. 相似文献
2.
Weiying He D. Dawson Beattie Hao Zhou Eric G. Bowes Laurel L. Schafer Jennifer A. Love Pierre Kennepohl 《Chemical science》2021,12(46):15298
Agostic interactions are examples of σ-type interactions, typically resulting from interactions between C–H σ-bonds with empty transition metal d orbitals. Such interactions often reflect the first step in transition metal-catalysed C–H activation processes and thus are of critical importance in understanding and controlling σ bond activation chemistries. Herein, we report on the unusual electronic structure of linear electron-rich d9 Ni(i) complexes with symmetric bis(C–H) agostic interactions. A combination of Ni K edge and L edge XAS with supporting TD-DFT/DFT calculations reveals an unconventional covalent agostic interaction with limited contributions from the valence Ni 3d orbitals. The agostic interaction is driven via the empty Ni 4p orbitals. The surprisingly strong Ni 4p-derived agostic interaction is dominated by σ contributions with minor π contributions. The resulting ligand–metal donation occurs directly along the C–Ni bond axis, reflecting a novel mode of bis-agostic bonding.Symmetric Ni(i) agostic complexes reveal an unusual mode of bonding that is dominated by direct carbon-to-metal charge transfer. 相似文献
3.
Mengxing Cui Ryuichi Murase Yongbing Shen Tetsu Sato Shohei Koyama Kaiji Uchida Tappei Tanabe Shinya Takaishi Masahiro Yamashita Hiroaki Iguchi 《Chemical science》2022,13(17):4902
Electrical conduction among metallocycles has been unexplored because of the difficulty in creating electronic transport pathways. In this work, we present an electrocrystallization strategy for synthesizing an intrinsically electron-conductive metallocycle, [Ni6(NDI-Hpz)6(dma)12(NO3)6]·5DMA·nH2O (PMC-hexagon) (NDI-Hpz = N,N′-di(1H-pyrazol-4-yl)-1,4,5,8-naphthalenetetracarboxdiimide). The hexagonal metallocycle units are assembled into a densely packed ABCABC… sequence (like the fcc geometry) to construct one-dimensional (1D) helical π-stacked columns and 1D pore channels, which were maintained under the liberation of H2O molecules. The NDI cores were partially reduced to form radicals as charge carriers, resulting in a room-temperature conductivity of (1.2–2.1) × 10−4 S cm−1 (pressed pellet), which is superior to that of most NDI-based conductors including metal–organic frameworks and organic crystals. These findings open up the use of metallocycles as building blocks for fabricating conductive porous molecular materials.Intrinsically electron-conductive metallocycle was synthesized. π-Radicals play a key role in constructing π-stacked columns among molecular hexagons and achieving high electrical conductivity over 10−4 S cm−1 in polycrystalline pellet. 相似文献
4.
Li-Hui Mou Yao Li Gong-Ping Wei Zi-Yu Li Qing-Yu Liu Hui Chen Sheng-Gui He 《Chemical science》2022,13(32):9366
The direct coupling of dinitrogen (N2) and methane (CH4) to construct the N–C bond is a fascinating but challenging approach for the energy-saving synthesis of N-containing organic compounds. Herein we identified a likely reaction pathway for N–C coupling from N2 and CH4 mediated by heteronuclear metal cluster anions CoTaC2−, which starts with the dissociative adsorption of N2 on CoTaC2− to generate a Taδ+–Ntδ− (terminal-nitrogen) Lewis acid–base pair (LABP), followed by the further activation of CH4 by CoTaC2N2− to construct the N–C bond. The N N cleavage by CoTaC2− affording two N atoms with strong charge buffering ability plays a key part, which facilitates the H3C–H cleavage via the LABP mechanism and the N–C formation via a CH3 migration mechanism. A novel Nt triggering strategy to couple N2 and CH4 molecules using metal clusters was accordingly proposed, which provides a new idea for the direct synthesis of N-containing compounds.A possible N–C bond formation directly from N2 and CH4 mediated by heteronuclear metal cluster anions CoTaC2− at room temperature was identified. 相似文献
5.
Mariusz Tasior Pawe Kowalczyk Marta Przyby Magorzata Czichy Patryk Janasik Manon H. E. Bousquet Mieczysaw apkowski Matt Rammo Aleksander Rebane Denis Jacquemin Daniel T. Gryko 《Chemical science》2021,12(48):15935
A two-step route to strongly absorbing and efficiently orange to deep red fluorescent, doubly B/N-doped, ladder-type pyrrolo[3,2-b]pyrroles has been developed. We synthesize and study a series of derivatives of these four-coordinate boron-containing, nominally quadrupolar materials, which mostly exhibit one-photon absorption in the 500–600 nm range with the peak molar extinction coefficients reaching 150 000, and emission in the 520–670 nm range with the fluorescence quantum yields reaching 0.90. Within the family of these ultrastable dyes even small structural changes lead to significant variations of the photophysical properties, in some cases attributed to reversal of energy ordering of alternate-parity excited electronic states. Effective preservation of ground-state inversion symmetry was evidenced by very weak two-photon absorption (2PA) at excitation wavelengths corresponding to the lowest-energy, strongly one-photon allowed purely electronic transition. π-Expanded derivatives and those possessing electron-donating groups showed the most red-shifted absorption- and emission spectra, while displaying remarkably high peak 2PA cross-section (σ2PA) values reaching ∼2400 GM at around 760 nm, corresponding to a two-photon allowed higher-energy excited state. At the same time, derivatives lacking π-expansion were found to have a relatively weak 2PA peak centered at ca. 800–900 nm with the maximum σ2PA ∼50–250 GM. Our findings are augmented by theoretical calculations performed using TD-DFT method, which reproduce the main experimental trends, including the 2PA, in a nearly quantitative manner. Electrochemical studies revealed that the HOMO of the new dyes is located at ca. −5.35 eV making them relatively electron rich in spite of the presence of two B−–N+ dative bonds. These dyes undergo a fully reversible first oxidation, located on the diphenylpyrrolo[3,2-b]pyrrole core, directly to the di(radical cation) stage.Ladder-type heterocycles encompassing two B−–N+ dative bonds possess intense green to red emission, large 2PA cross-sections and superb photostability. 相似文献
6.
The instability of cesium lead bromide (CsPbBr3) nanocrystals (NCs) in polar solvents has hampered their use in photocatalysis. We have now succeeded in synthesizing CsPbBr3–CdS heterostructures with improved stability and photocatalytic performance. While the CdS deposition provides solvent stability, the parent CsPbBr3 in the heterostructure harvests photons to generate charge carriers. This heterostructure exhibits longer emission lifetime (τave = 47 ns) than pristine CsPbBr3 (τave = 7 ns), indicating passivation of surface defects. We employed ethyl viologen (EV2+) as a probe molecule to elucidate excited state interactions and interfacial electron transfer of CsPbBr3–CdS NCs in toluene/ethanol mixed solvent. The electron transfer rate constant as obtained from transient absorption spectroscopy was 9.5 × 1010 s−1 and the quantum efficiency of ethyl viologen reduction (ΦEV+˙) was found to be 8.4% under visible light excitation. The Fermi level equilibration between CsPbBr3–CdS and EV2+/EV+˙ redox couple has allowed us to estimate the apparent conduction band energy of the heterostructure as −0.365 V vs. NHE. The insights into effective utilization of perovskite nanocrystals built around a quasi-type II heterostructures pave the way towards effective utilization in photocatalytic reduction and oxidation processes.The insights into effective utilization of perovskite nanocrystals built around a CsPbBr3–CdS heterostructure pave the way towards their utilization in photocatalytic reduction and oxidation processes. 相似文献
7.
Co-crystallization of the prominent Fe(ii) spin-crossover (SCO) cation, [Fe(3-bpp)2]2+ (3-bpp = 2,6-bis(pyrazol-3-yl)pyridine), with a fractionally charged TCNQδ− radical anion has afforded a hybrid complex [Fe(3-bpp)2](TCNQ)3·5MeCN (1·5MeCN, where δ = −0.67). The partially desolvated material shows semiconducting behavior, with the room temperature conductivity σRT = 3.1 × 10−3 S cm−1, and weak modulation of conducting properties in the region of the spin transition. The complete desolvation, however, results in the loss of hysteretic behavior and a very gradual SCO that spans the temperature range of 200 K. A related complex with integer-charged TCNQ− anions, [Fe(3-bpp)2](TCNQ)2·3MeCN (2·3MeCN), readily loses the interstitial solvent to afford desolvated complex 2 that undergoes an abrupt and hysteretic spin transition centered at 106 K, with an 11 K thermal hysteresis. Complex 2 also exhibits a temperature-induced excited spin-state trapping (TIESST) effect, upon which a metastable high-spin state is trapped by flash-cooling from room temperature to 10 K. Heating above 85 K restores the ground-state low-spin configuration. An approach to improve the structural stability of such complexes is demonstrated by using a related ligand 2,6-bis(benzimidazol-2′-yl)pyridine (bzimpy) to obtain [Fe(bzimpy)2](TCNQ)6·2Me2CO (4) and [Fe(bzimpy)2](TCNQ)5·5MeCN (5), both of which exist as LS complexes up to 400 K and exhibit semiconducting behavior, with σRT = 9.1 × 10−2 S cm−1 and 1.8 × 10−3 S cm−1, respectively.Co-crystallization of the cationic complex [Fe(3-bpp)2]2+ with fractionally charged TCNQδ− anions (0 < δ < 1) affords semiconducting spin-crossover (SCO) materials. The abruptness of SCO is strongly dependent on the interstitial solvent content. 相似文献
8.
Fengbo Liu Hai Yang Dongdong Sun Fang Gao Xiongzhi Zhang Zhiyong Zhao Xie Han Simin Liu 《Chemical science》2022,13(24):7247
Aqueous room temperature phosphorescence (aRTP) from purely organic materials has been intriguing but challenging. In this article, we demonstrated that the red aRTP emission of 2Br–NDI, a water-soluble 4,9-dibromonaphthalene diimide derivative as a chloride salt, could be modulated by anion–π and intermolecular electronic coupling interactions in water. Specifically, the rarely reported stabilization of anion–π interactions in water between Cl− and the 2Br–NDI core was experimentally evidenced by an anion–π induced long-lived emission (λAnion–π) of 2Br–NDI, acting as a competitive decay pathway against the intrinsic red aRTP emission (λPhos) of 2Br–NDI. In the initial expectation of enhancing the aRTP of 2Br–NDI by inclusion complexation with macrocyclic cucurbit[n]urils (CB[n]s, n = 7, 8, 10), we surprisingly found that the exclusion complexation between CB[8] and 2Br–NDI unconventionally endowed the complex with the strongest and longest-lived aRTP due to the strong intermolecular electronic coupling between the nπ* orbit on the carbonyl rims of CB[8] and the ππ* orbit on 2Br–NDI in water. It is anticipated that these intriguing findings may inspire and expand the exploration of aqueous anion–π recognition and CB[n]-based aRTP materials.The aqueous room temperature phosphorescence of 2Br–NDI is modulated by long-lived-emitting anion–π interactions and tremendously enhanced by intermolecular electronic coupling interactions with the ISC-boosting carbonyl rims of CB[8] host. 相似文献
9.
Matthew J. Evans Mathew D. Anker Claire L. McMullin Samuel E. Neale Nasir A. Rajabi Martyn P. Coles 《Chemical science》2022,13(16):4635
Multiply-bonded main group metal compounds are of interest as a new class of reactive species able to activate and functionalize a wide range of substrates. The aluminium sulfido compound K[Al(NONDipp)(S)] (NONDipp = [O(SiMe2NDipp)2]2−, Dipp = 2,6-iPr2C6H3), completing the series of [Al(NONDipp)(E)]− anions containing Al–E{16} multiple bonds (E{16} = O, S, Se, Te), was accessed via desulfurisation of K[Al(NONDipp)(S4)] using triphenylphosphane. The crystal structure showed a tetrameric aggregate joined by multiple K⋯S and K⋯π(arene) interactions that were disrupted by the addition of 2.2.2-cryptand to form the separated ion pair, [K(2.2.2-crypt)][Al(NONDipp)(S)]. Analysis of the anion using density functional theory (DFT) confirmed multiple-bond character in the Al–S group. The reaction of the sulfido and selenido anions K[Al(NONDipp)(E)] (E = S, Se) with CO2 afforded K[Al(NONDipp)(κ2E,O-EC{O}O)] containing the thio- and seleno-carbonate groups respectively, consistent with a [2 + 2]-cycloaddition reaction and C–E bond formation. An analogous cycloaddition reaction took place with benzophenone affording compounds containing the diphenylsulfido- and diphenylselenido-methanolate ligands, [κ2E,O-EC{O}Ph2]2−. In contrast, when K[Al(NONDipp)(E)] (E = S, Se) was reacted with benzaldehyde, two equivalents of substrate were incorporated into the product accompanied by formation of a second C–E bond and complete cleavage of the Al–E{16} bonds. The products contained the hitherto unknown κ2O,O-thio- and κ2O,O-seleno-bis(phenylmethanolate) ligands, which were exclusively isolated as the cis-stereoisomers. The mechanisms of these cycloaddition reactions were investigated using DFT methods.Reaction of Al–E (E = S, Se) multiple bonds with C O functionalities generates new C–E bonds. 相似文献
10.
Shuaishuai Li Xiaomeng Liu Hongping Wu Zhongfu Song Hongwei Yu Zheshuai Lin Zhanggui Hu Jiyang Wang Yicheng Wu 《Chemical science》2021,12(41):13897
The nonlinear optical (NLO) crystals that can expand the wavelength of the laser to the deep-ultraviolet (DUV) region by the cascaded second harmonic generation (SHG) are of current research interest. It is well known that borates are the most ideal material class for the design of new DUV NLO crystals owing to the presence of good NLO genes, e.g., BO3 or B3O6 groups. However, the NLO pyro-borates with the B2O5 dimers as the sole basic building units are still rarely reported owing to their small SHG responses. In this communication, by constructing a planar pentagonal [Ca(B2O5)]∞ layer, the NLO pyro-borate Ba4Ca(B2O5)2F2 with a large SHG response (∼2.2 × KDP, or ∼7 × α-Li4B2O5) and a DUV transparent window has been designed and synthesized. The first-principles calculations show that the large SHG response of Ba4Ca(B2O5)2F2 mainly originates from the better π-conjugation of the coplanar B2O5 dimers in the [Ca(B2O5)]∞ layer. In addition, the planar pentagonal pattern in the [Ca(B2O5)]∞ layer provides an ideal template for designing the new DUV NLO crystals, apart from those in known DUV borates, e.g., the [Be2BO3F2]∞ layer in KBe2BO3F2 (KBBF).A new deep-UV NLO pyro-borate Ba4Ca(B2O5)2F2 was synthesized by solid-state reactions. The better π-conjugation of B2O5 dimers in the planar pentagonal layer achieves a large SHG response (∼2.2 × KDP), which is the largest among all the known DUV transparent borates with B2O5 units.Deep-ultraviolet (DUV, λ < 200 nm) coherent lights with high photon energy, high spatial resolution, and a small heat-affected zone are of significance for applications in photolithography, high-resolution spectroscopy, laser cooling, and scientific equipment.1–4 However, it is difficult or well-nigh impossible for solid-state lasers to directly radiate the DUV coherent lights. In contrast, relying on the process of second harmonic generation (SHG) of nonlinear optical (NLO) crystals is a more effective way to generate the DUV coherent lights and causes much attention.5,6 Therefore, the NLO crystal has become an important material basis of solid-state lasers, which seriously affects the development of all-solid-state laser technology. However, it is still a great challenge to rationally design and synthesize DUV NLO crystals because of the extremely rigorous requirements of structural symmetry and properties.7–10 Structurally, the DUV NLO crystals must crystallize in the noncentrosymmetric (NCS) space groups which are the prerequisite for the materials to exhibit SHG responses. Moreover, it should possess a broad transparency window, a largely effective NLO coefficient (deff ≥ 0.39 pm V−1), and a moderate birefringence (0.05–0.10@1064 nm) to achieve the phase-matching (PM) conditions in the DUV region.10 Based on these requirements, borates have been considered as the ideal material class for DUV NLO crystals because of their special structure and properties'' virtues, including the rich acentric structural types, large band gaps, and stable physical and chemical properties.8 To date, the commercialized borate-based UV NLO crystals consist of β-BaB2O4 (BBO), LiB3O5 (LBO), CsLiB6O10 (CLBO),9,10 and the practical DUV NLO crystal KBe2BO3F2 (KBBF). Especially for KBBF, it has become the sole material that can generate DUV coherent laser light (177.3 nm) by a direct SHG method.7 Other excellent borate-based UV NLO crystals also consist of K3B6O10Cl,11 SrB5O7F3,12 Li2B6O9F2,5 CsAlB3O6F,13 M2B10O14F6 (M = Ca, Sr),14 NH4B4O6F,15 NaSr3Be3B3O9F4,16 AB4O6F (A = K, Rb, and Cs),17etc.The above borate-based materials have achieved great success as UV and DUV NLO crystals, which are mainly attributed to the ability of boron atoms to coordinate with three or four oxygen anions forming trigonal-planar or tetrahedral building blocks.18,19 For example, the first borate-based NLO crystal, KB5O8·4H2O (KB5), has the basic building units (BBUs) of [B5O10], while the BBUs of β-BBO, LBO, and KBBF are [B3O6], [B3O7], and isolated [BO3], respectively.7,8 Remarkably, although various borate crystals with different types of borate groups have been explored during the past decades, the pyro-borate NLO crystals with B2O5 groups as the sole BBUs are rarely reported owing to their weak SHG responses.20–23 For example, the SHG response of the DUV transparent α-Li4B2O5 (ref. 23) is only ∼0.3 × KDP, which is far smaller than the expected value (0.39 pm V−1, 1 × KDP).Actually, the flexible B2O5 groups which are composed of two π-conjugated BO3 units through corner-sharing may also be capable of generating excellent optical performance if they have benign arrangements. In recent research, Pan''s group has indicated that the B2O5 dimers are perfect for the design of DUV birefringent crystals. By the synergistic combination, they have successfully designed a potential pyro-borate birefringent crystal, Li2Na2B2O5, with a short UV cut-off edge (181 nm) and large birefringence (0.095@532 nm).21 And they have also grown Ca(BO2)2 crystals exhibiting a short UV cut-off edge and larger birefringence (169 nm; 0.2471@193 nm). Based on the analysis of the structure–property relationship of Ca(BO2)2, they stated that the polymerized planar BnO2n+1 groups, e.g., B2O5, could generate a larger anisotropy than isolated BO3.22 However, their opposite arrangements of B–O groups make them crystallize in the centrosymmetric (CS) space groups, which limit their further development as NLO compounds. Thus, it is clear that pyro-borates exhibiting a large birefringence and a short UV cut-off edge would also be promising DUV NLO crystals if their SHG responses can be enhanced.Based on the above-mentioned ideas, a systematical investigation has been performed on DUV pyroborates. And finally, we successfully synthesized a new NCS pyro-borate, Ba4Ca(B2O5)2F2, which can exhibit not only a large SHG response (∼2.2 × KDP and ∼7 × α-Li4B2O5) but also a short UV cut-off edge (<190 nm). Analyzing its structure, one can find that its excellent NLO properties mainly originate from the unique planar pentagonal [Ca(B2O5)]∞ layer, where the B2O5 groups adopt the almost coplanar configurations that favor the structure to generate large SHG response and birefringence,21 meanwhile the terminal O atoms of B2O5 groups are also linked by the Ca2+ cations, which eliminate the dangling bonds of B2O5 groups and further blue-shift the UV cut-off edge. More importantly, the adjacent [Ca(B2O5)]∞ layers in Ba4Ca(B2O5)2F2 are linked by other B2O5 groups to form a 3D framework, which will be favorable for the material to avoid the layer habit that KBBF suffers from. In this sense, the planar pentagonal [Ca(B2O5)]∞ layer is similar to the [Be2BO3F2]∞ layer in KBBF, and it can be seen as a new structure template for the design of new DUV NLO crystals, especially for the DUV pyro-borates. Herein, we will describe the synthesis, experimental and computational characterization as well as the functional properties of the new DUV NLO material, Ba4Ca(B2O5)2F2.A polycrystalline sample of Ba4Ca(B2O5)2F2 was synthesized by the conventional solid-state reaction and the purity was confirmed by powder X-ray diffraction (XRD) (Fig. S1†). With the polycrystalline sample, the thermal behavior of Ba4Ca(B2O5)2F2 was studied by the thermogravimetric (TG) and differential scanning calorimetry (DSC) measurements. The heating DSC curve shows a sharp endothermic peak at 815 °C with no obvious weight loss in the TG curve (Fig. S2†), suggesting that Ba4Ca(B2O5)2F2 has good thermal stability. To further investigate the thermal behavior of Ba4Ca(B2O5)2F2, the polycrystalline sample was calcined at 840 °C and the XRD analysis showed that the calcined sample was Ba4Ca(B2O5)2F2, Ba2Ca(BO3)2 (PDF #01-085-2268), Ba2CaB6O12 (PDF #01-075-1401) and other unknown phases (Fig. S3†). These results illustrate that Ba4Ca(B2O5)2F2 melts incongruently and the suitable flux is necessary for the crystal growth.With the Na2O–PbF2–B2O3 as the flux, millimeter-sized block crystals of Ba4Ca(B2O5)2F2 were grown for the single-crystal XRD structure determination. Ba4Ca(B2O5)2F2 crystallizes in the NCS and polar space group, P21 (Table S1†). In the asymmetric unit, there are four unique Ba, one Ca, four B, ten O, and two F atom(s), which all fully occupy the 2a Wyckoff positions (Table S2†). All B atoms are coordinated to three oxygen atoms to form the BO3 triangles with the B–O distances ranging from 1.312(17) to 1.460(16) Å and O–B–O angles varying from 108.0(13) to 130.2(15)°. The BO3 triangles are further connected to form two types of B2O5 dimers, i.e. plane B(1,3)2O5 and twisted B(2,4)2O5, which are the BBUs of Ba4Ca(B2O5)2F2. The Ca atoms are coordinated to six oxygen atoms to form CaO6 octahedra with the Ca–O distances ranging from 2.285(9) to 2.325(13) Å. For the Ba2+ cations, they exhibit three different coordination environments, Ba(1,2)O6F2, Ba(3)O8F2, and Ba(4)O7F2 (Fig. S4†) with the Ba–O distances ranging from 2.585(9) to 3.250(11) Å and the Ba–F bond lengths ranging from 2.635(8) to 2.736(8) Å. Remarkably, for the F− anions, each unique fluorine atom serves as a common vertex for four Ba atoms to form the FBa4 polyhedra (Fig. S5a†), which could be treated as fluorine-centered secondary building units (SBUs). The Ba–F–Ba angles vary from 99.0 (2) to 120.2 (3)°. The bond valence sum (BVS) calculations show the values of 1.67–1.97, 2.45, 2.88–3.10, 1.78–2.13, and 0.95–1.09, for Ba2+, Ca2+ B3+, O2−, and F−, respectively (Table S2†). The BVSs of atoms are consistent with their expected oxidation states except the one from the Ca2+ cations. The larger BVSs of Ca2+ cations can be attributed to six shorter Ca–O bond lengths, which are also observed in other Ca2+-containing borates, such as YCa3(VO)3(BO3)4 (2.44),24 Rb2Ca3B16O28 (2.29), and Cs2Ca3B16O28 (2.30).25The structure of Ba4Ca(B2O5)2F2 is shown in Fig. 1. In the structure, the plane B(1,3)2O5 dimer is first connected with four CaO6 octahedra, meanwhile, each CaO6 octahedron is also linked by four B(1,3)2O5 dimers through sharing their four equatorial O atoms to form a unique planar pentagonal [Ca(B2O5)]∞ layer in the b–c plane (Fig. 1a, b). Then, these [Ca(B2O5)]∞ layers are further linked by the twisted B(2,4)2O5 dimers to construct a 3D framework with Ba2+ cations maintaining the charge balance (Fig. 1c). Remarkably, for the arrangements of the Ba2+ cations and the F− anions, the fluorine-centered SBU FBa4 polyhedra are linked to construct the 2D [F2Ba4] infinite layer (Fig. S5b†) with the same orientation, which further fills the apertures in the [Ca(B2O5)2] framework (Fig. S5c†). The existence of fluorine-centered SBUs would certainly have a strong influence on the local coordinate environments, and finally on the whole structure.26Open in a separate windowFig. 1(a) The [Ca(B2O5)] layer is composed of B2O5 dimers and CaO6 octahedra. (b) The planar pentagonal topology layer. The comparison of structures between (c) Ba4Ca(B2O5)2F2 and (d) KBBF.It is very interesting that Ba4Ca(B2O5)2F2 contains a planar pentagonal [Ca(B2O5)]∞ layer, which is similar to the [Be2BO3F2]∞ layer in KBBF. The structural evolution from KBBF to Ba4Ca(B2O5)2F2 is also shown in Fig. 1c and d. In KBBF, the BBUs are the planar BO3 triangles, which are connected with BeO3F in the a–b plane by strong covalent bonds to form the [Be2BO3F2]∞ layers (Fig. S6c†) and the [Be2BO3F2]∞ layers have achieved excellent NLO properties of the KBBF crystal.7 However in Ba4Ca(B2O5)2F2, the BO3 triangles are changed into the B2O5 dimers, and the BeO3F tetrahedra are substituted by the CaO6 polyhedra. These B2O5 dimers are also connected by the CaO6 polyhedra to form the interesting planar pentagonal [Ca(B2O5)]∞ layer (Fig. S6d†). More importantly, in KBBF, the adjacent [Be2BO3F2]∞ layers are connected by the weak K+-F− ionic bonds that results in the strong layer habit of the KBBF crystals, whereas in Ba4Ca(B2O5)2F2, the [Ca(B2O5)]∞ layers are bridged by the strong covalent B–O bonds to form a stable 3D framework, which will greatly overcome the layering tendency of the KBBF crystal and facilitate the crystal growth.In addition, we also notice that the planar pentagonal [Ca(B2O5)]∞ layer maybe helpful for enhancing the SHG responses of pyro-borates because small SHG responses of pyro-borates are attributed to the typical twisted configurations of the B2O5 groups, which are unfavorable for forming the π-conjugation and the superposition of the microscopic SHG response. For example, α-Li4B2O5, a DUV transparent pyro-borate with sole B2O5 units as the BBUs, has a weak SHG response, which may be derived from the twisted B2O5 groups and non-planar arrangements (Fig. S7a†). However, in Ba4Ca(B2O5)2F2, the planar configuration of the pentagonal layers can assist the B2O5 groups to adopt a nearly coplanar arrangement (Fig. S7b†) and effectively enhance the π-conjugation of B2O5 groups. The better π-conjugation of the planar B2O5 groups in the planar pentagonal [Ca(B2O5)]∞ layer has also been confirmed by the electron orbital calculation based on the first-principles calculations.27 The calculated result is shown in Fig. 2. Clearly, the prominent conjugated interactions are observed in the nearly coplanar B(1,3)2O5 dimers of Ba4Ca(B2O5)2F2 (Fig. 2a), whereas it does little in the twisted B(2,4)2O5 dimers of Ba4Ca(B2O5)2F2 (Fig. 2b) and two types of twisted B2O5 dimers in α-Li4B2O5 (Fig. 2c and d). It can be expected that the nearly coplanar B2O5 dimers are more conducive to the large SHG response than the twisted B2O5 dimers. Remarkably, the similar pentagonal layers are also observed in other pyro-phosphates, such as Ba2NaClP2O7, K2Sb(P2O7)F, Rb3PbBi(P2O7)2, and Rb3BaBi(P2O7)2. Clearly, as pyro-phosphates are the non-π-conjugated systems, the planar pentagonal layers are only helpful for the orientation of anion groups.28–31 However, they cannot form the better π-conjugation. Therefore, the better π-conjugation of the nearly coplanar B2O5 groups in planar pentagonal layers of pyro-borate Ba4Ca(B2O5)2F2 would have a different contributing mechanism to the SHG effect with other non-π-conjugated pyro-phosphates.Open in a separate windowFig. 2The orbitals of the nearly coplanar B(1,3)2O5 (a) and twisted B(2,4)2O5 dimers (b) in Ba4Ca(B2O5)2F2. The orbitals of two twisted B2O5 dimers (c and d) in α-Li4B2O5.The presence of BO3 triangles in Ba4Ca(B2O5)2F2 is confirmed by the IR spectral measurements (Fig. S8†). The peaks at 1362 cm−1 and 1208 cm−1 can be attributed to the asymmetric stretching of BO3 groups.32 A strong band at 1069 cm−1 in the IR spectrum may be associated with the stretching vibration of B–O–B in B2O5.33,34 The weak absorption bands at 950, and 810 cm−1 correspond to the symmetrical stretching vibrations of BO3 and B–O–B in B2O5, respectively. The peaks at 751 and 615 cm−1 can be attributed to the out-of-plane bending of the BO3 groups.34 Further, the UV-vis-NIR diffuse reflectance spectrum was also measured (Fig. S9†), which shows that Ba4Ca(B2O5)2F2 is transparent down to the DUV region with a UV cut-off edge less than 190 nm (corresponding to a large band gap of 6.2 eV), which is comparable to the newly developed NLO-active borates, such as RbB3O4F2 (<190 nm), CsZn2BO3X2 (X2 = F2,Cl2, and FCl)) (<190 nm) and so on.35–38 The short cut-off edge demonstrates the potential application of Ba4Ca(B2O5)2F2 as a DUV NLO crystal.As Ba4Ca(B2O5)2F2 crystalizes in the NCS space group P21, it possesses the SHG response, which was measured by the Kurtz-Perry method with the well-known NLO material KH2PO4 (KDP) as a reference.39 As shown in Fig. 3, the SHG intensities of Ba4Ca(B2O5)2F2 increase with the increase of particle sizes, indicating that Ba4Ca(B2O5)2F2 is type-I phase-matchable. The SHG intensity of Ba4Ca(B2O5)2F2 at the particle size of 150–212 μm is about 2.2 times that of KDP, and is larger than that of KBBF (1.2 × KDP) or comparable with those newly reported UV NLO crystals, i.e. γ-Be2BO3F (2.3 × KDP),6 β-Rb2Al2B2O7 (2 × KDP),40 Li4Sr(BO3)2 (2 × KDP),41 CsB4O6F(∼1.9 × KDP).2 In addition, as we know, the SHG response of Ba4Ca(B2O5)2F2 is the largest among all the known DUV transparent borates with B2O5 units (Table S4†). Its SHG response (2.2 × KDP) is about seven times larger than that of α-Li4B2O5 (0.3 × KDP), another DUV transparent pyro-borate with sole B2O5 units.Open in a separate windowFig. 3(a) Phase-matching curve, i.e., particle size vs. SHG intensity, data for Ba4Ca(B2O5)2F2 and KH2PO4 (KDP) as reference. The solid curve is a guide for the eye, not a fit to the data. (b) Oscilloscope traces showing SHG intensities for Ba4Ca(B2O5)2F2 and KDP.To understand the origin of the excellent optical properties of Ba4Ca(B2O5)2F2, we also carried out the first-principles calculations.27 It shows that Ba4Ca(B2O5)2F2 has an indirect band gap of 6.34 eV (Figures S10a†), which is in accordance with the experimental results. The valence band maximum (VBM) of Ba4Ca(B2O5)2F2 is mainly composed of the orbitals in Ba, and O atoms, while the conduction band minimum (CBM) is dominantly composed of the orbitals in Ba, B, and O atoms. Therefore, the band gap of Ba4Ca(B2O5)2F2 is mainly determined by Ba atoms and B2O5 groups. Based on the calculated electron structure, the NLO coefficients of Ba4Ca(B2O5)2F2 are also calculated. The largest NLO coefficient of Ba4Ca(B2O5)2F2 is d22 = −0.524 pm V−1, which is about 5 times lower than that of α-Li4B2O5 (d24 = −0.102 pm V−1) (Table S5a†), which is matched with the experimental one. Further, the SHG-weighted density maps of Ba4Ca(B2O5)2F2 are shown in Fig. 4. These reveal that B2O5 dimers make the dominant contribution (72.7%) to the total SHG effect (Table S5b†). The band-resolved SHG analysis can also conclude that B–O orbitals in Ba4Ca(B2O5)2F2 contribute more to the SHG response than those in α-Li4B2O5 (Fig. S10b, S10c†), indicating that the arrangements of B2O5 dimers in Ba4Ca(B2O5)2F2 is more beneficial for the large SHG response. And different from α-Li4B2O5, F-centered secondary building units (SBUs) exist in the structure of Ba4Ca(B2O5)2F2, and they are further linked to construct 2D [F2Ba4]∞ infinite layers, which could help B2O5 groups arrange in a planar pattern (Fig. S5†).26 So, based on the above analysis, we can conclude that the nearly coplanar B2O5 dimers in the planar pentagonal layer and the SBU FBa4 tetrahedra make a significant contribution to the SHG response of Ba4Ca(B2O5)2F2.Open in a separate windowFig. 4The SHG-weighted density maps of the virtual electron process (a) and virtual hole process (b) of d22 for Ba4Ca(B2O5)2F2. 相似文献
11.
Alexander Okorn Arumugam Jayaraman Lukas Englert Merle Arrowsmith Theresa Swoboda Jeanette Weigelt Carina Brunecker Merlin Hess Anna Lamprecht Carsten Lenczyk Maximilian Rang Holger Braunschweig 《Chemical science》2022,13(25):7566
The classical route to the PMe3-stabilised polycyclic aromatic hydrocarbon (PAH)-substituted diborenes B2Ar2(PMe3)2 (Ar = 9-phenanthryl 7-Phen; Ar = 1-pyrenyl 7-Pyr) via the corresponding 1,2-diaryl-1,2-dimethoxydiborane(4) precursors, B2Ar2(OMe)2, is marred by the systematic decomposition of the latter to BAr(OMe)2 during reaction workup. Calculations suggest this results from the absence of a second ortho-substituent on the boron-bound aryl rings, which enables their free rotation and exposes the B–B bond to nucleophilic attack. 7-Phen and 7-Pyr are obtained by the reduction of the corresponding 1,2-diaryl-1,2-dichlorodiborane precursors, B2Ar2Cl2(PMe3)2, obtained from the SMe2 adducts, which are synthesised by direct NMe2–Cl exchange at B2Ar2(NMe2)2 with (Me2S)BCl3. The low-lying π* molecular orbitals (MOs) located on the PAH substituents of 7-Phen and 7-Pyr intercalate between the B–B-based π and π* MOs, leading to a relatively small HOMO–LUMO gap of 3.20 and 2.72 eV, respectively. Under vacuum or at high temperature 7-Phen and 7-Pyr undergo intramolecular hydroarylation of the B B bond to yield 1,2-dihydronaphtho[1,8-cd][1,2]diborole derivatives. Hydrogenation of 7-Phen, 7-Pyr and their 9-anthryl and mesityl analogues III and II, respectively, results in all cases in splitting of the B–B bond and isolation of the monoboranes (Me3P)BArH2. NMR-spectroscopic monitoring of the reactions, solid-state structures of isolated reaction intermediates and computational mechanistic analyses show that the hydrogenation of the three PAH-substituted diborenes proceeds via a different pathway to that of the dimesityldiborene. Rather than occurring exclusively at the B–B bond, hydrogenation of 7-Ar and III proceeds via a hydroarylated intermediate, which undergoes one B–B bond-centered H2 addition, followed by hydrogenation of the endocyclic B–C bond resulting from hydroarylation, making the latter effectively reversible.In contrast to classical B–B bond-centred diborene hydrogenation, polycyclic aromatic hydrocarbon-substituted diborenes first undergo thermal intramolecular hydroarylation, followed by hydrogenation of the remaining B–B and endocyclic B–C bonds. 相似文献
12.
Shiori Ogawa Swarup Chattopadhyay Yuya Tanaka Tatsuhiko Ohto Tomofumi Tada Hirokazu Tada Shintaro Fujii Tomoaki Nishino Munetaka Akita 《Chemical science》2021,12(32):10871
Control of charge carriers that transport through the molecular junctions is essential for thermoelectric materials. In general, the charge carrier depends on the dominant conduction orbitals and is dominantly determined by the terminal anchor groups. The present study discloses the synthesis, physical properties in solution, and single-molecule conductance of paddle-wheel diruthenium complexes 1R having diarylformamidinato supporting ligands (DArF: p-R-C6H4-NCHN-C6H4-R-p) and two axial thioanisylethynyl conducting anchor groups, revealing unique substituent effects with respect to the conduction orbitals. The complexes 1R with a few different aryl substituents (R = OMe, H, Cl, and CF3) were fully characterized by spectroscopic and crystallographic analyses. The single-molecule conductance determined by the scanning tunneling microscope break junction (STM-BJ) technique was in the 10−5 to 10−4G0 region, and the order of conductance was 1OMe > 1CF3 ≫ 1H ∼ 1Cl, which was not consistent with the Hammett substituent constants σ of R. Cyclic voltammetry revealed the narrow HOMO–LUMO gaps of 1R originating from the diruthenium motif, as further supported by the DFT study. The DFT-NEGF analysis of this unique result revealed that the dominant conductance routes changed from HOMO conductance (for 1OMe) to LUMO conductance (for 1CF3). The drastic change in the conductance properties originates from the intrinsic narrow HOMO–LUMO gaps.Dominant conduction orbitals of paddle-wheel organodiruthenium complexes can be facilely controlled by the substituents embedded in the amidinato ligands. 相似文献
13.
Zhi-Rong Tan Yu-Qin Xing Jing-Zhao Cheng Guang Zhang Zhao-Qi Shen Yu-Jie Zhang Guangfu Liao Long Chen Shi-Yong Liu 《Chemical science》2022,13(6):1725
3,4-Ethylene dioxythiophene (EDOT), as a monomer of commercial conductive poly(3,4-ethylene dioxythiophene) (PEDOT), has been facilely incorporated into a series of new π-conjugated polymer-based photocatalysts, i.e., BSO2–EDOT, DBT–EDOT, Py–EDOT and DFB–EDOT, through atom-economic C–H direct arylation polymerization (DArP). The photocatalytic hydrogen production (PHP) test shows that donor–acceptor (D–A)-type BSO2–EDOT renders the highest hydrogen evolution rate (HER) among the linear conjugated polymers (CPs) ever reported. A HER up to 0.95 mmol h−1/6 mg under visible light irradiation and an unprecedented apparent quantum yield of 13.6% at 550 nm are successfully achieved. Note that the photocatalytic activities of the C–H/C–Br coupling-derived EDOT-based CPs are superior to those of their counterparts derived from the classical C–Sn/C–Br Stille coupling, demonstrating that EDOT is a promising electron-rich building block which can be facilely integrated into CP-based photocatalysts. Systematic studies reveal that the enhanced water wettability by the integration of polar BSO2 with hydrophilic EDOT, the increased electron-donating ability by O–C p–π conjugation, the improved electron transfer by D–A architecture, broad light harvesting, and the nano-sized colloidal character in a H2O/NMP mixed solvent rendered BSO2–EDOT as one of the best CP photocatalysts toward PHP.The excellent reactivity toward C–H direct arylation, water wettability and O–C p–π conjugation endow EDOT to be an attractive electron donor unit for CP photocatalysts, yielding an unprecedented hydrogen evolution rate up to 0.95 mmol h−1/6 mg catalyst. 相似文献
14.
Pablo Ríos Rocío Martín-de la Calle Pietro Vidossich Francisco Jos Fernndez-de-Crdova Agustí Lleds Salvador Conejero 《Chemical science》2021,12(5):1647
A reversible carbon–boron bond formation has been observed in the reaction of the coordinatively unsaturated, cyclometalated, Pt(ii) complex [Pt(ItBuiPr′)(ItBuiPr)][BArF], 1, with tricoordinated boranes HBR2. X-ray diffraction studies provided structural snapshots of the sequence of reactions involved in the process. At low temperature, we observed the initial formation of the unprecedented σ-BH complexes [Pt(HBR2)(ItBuiPr′)(ItBuiPr)][BArF], one of which has been isolated. From −15 to +10 °C, the σ-BH species undergo a carbon–boron coupling process leading to the platinum hydride derivative [Pt(H)(ItBuiPr–BR2)(ItBuiPr)][BArF], 4. Surprisingly, these compounds are thermally unstable undergoing carbon–boron bond cleavage at room temperature that results in the 14-electron Pt(ii) boryl species [Pt(BR2)(ItBuiPr)2][BArF], 2. This unusual reaction process has been corroborated by computational methods, which indicate that the carbon–boron coupling products 4 are formed under kinetic control whereas the platinum boryl species 2, arising from competitive C–H bond coupling, are thermodynamically more stable. These findings provide valuable information about the factors governing productive carbon–boron coupling reactions at transition metal centers.A reversible carbon–boron bond formation has been observed in the reaction of the coordinatively unsaturated, cyclometalated, Pt(ii) complex [Pt(ItBuiPr′)(ItBuiPr)][BArF], 1, with tricoordinated boranes HBR2. 相似文献
15.
Chen Chen Meng-hui Wang Lin-Yan Feng Lian-Qing Zhao Jin-Chang Guo Hua-Jin Zhai Zhong-hua Cui Sudip Pan Gabriel Merino 《Chemical science》2022,13(27):8045
The occurrence of planar hexacoordination is very rare in main group elements. We report here a class of clusters containing a planar hexacoordinate silicon (phSi) atom with the formula SiSb3M3+ (M = Ca, Sr, Ba), which have D3h (1A1′) symmetry in their global minimum structure. The unique ability of heavier alkaline-earth atoms to use their vacant d atomic orbitals in bonding effectively stabilizes the peripheral ring and is responsible for covalent interaction with the Si center. Although the interaction between Si and Sb is significantly stronger than the Si–M one, sizable stabilization energies (−27.4 to −35.4 kcal mol−1) also originated from the combined electrostatic and covalent attraction between Si and M centers. The lighter homologues, SiE3M3+ (E = N, P, As; M = Ca, Sr, Ba) clusters, also possess similar D3h symmetric structures as the global minima. However, the repulsive electrostatic interaction between Si and M dominates over covalent attraction making the Si–M contacts repulsive in nature. Most interestingly, the planarity of the phSi core and the attractive nature of all the six contacts of phSi are maintained in N-heterocyclic carbene (NHC) and benzene (Bz) bound SiSb3M3(NHC)6+ and SiSb3M3(Bz)6+ (M = Ca, Sr, Ba) complexes. Therefore, bare and ligand-protected SiSb3M3+ clusters are suitable candidates for gas-phase detection and large-scale synthesis, respectively.The global minimum of SiSb3M3+ (M = Ca, Sr, Ba) is a D3h symmetric structure containing an elusive planar hexacoordinate silicon (phSi) atom. Most importantly, the phSi core remains intact in ligand protected environment as well.Exploring the bonding capacity of main-group elements (such as carbon or silicon) beyond the traditional tetrahedral concept has been a fascinating subject in chemistry for five decades. The 1970 pioneering work of Hoffmann and coworkers1 initiated the field of planar tetracoordinate carbons (ptCs), or more generally, planar hypercoordinate carbons. The past 50 years have witnessed the design and characterization of an array of ptC and planar pentacoordinate carbon (ppC) species.2–14 However, it turned out to be rather challenging to go beyond ptC and ppC systems. The celebrated CB62− cluster and relevant species15,16 were merely model systems because C avoids planar hypercoordination in such systems.17,18 In 2012, the first genuine global minimum D3h CO3Li3+ cluster was reported to have six interactions with carbon in planar form, although electrostatic repulsion between positively charged phC and Li centers and the absence of any significant orbital interaction between them make this hexacoordinate assignment questionable.19 It was only very recently that a series of planar hexacoordinate carbon (phC) species, CE3M3+ (E = S–Te; M = Li–Cs), were designed computationally by the groups of Tiznado and Merino (Fig. 1; left panel),20 in which there exist pure electrostatic interactions between the negative Cδ− center and positive Mδ+ ligands. These phC clusters were achieved following the so-called “proper polarization of ligand” strategy.Open in a separate windowFig. 1The pictorial depiction of previously reported phC CE3M3+ (E = S–Te; M = Li–Cs) clusters and the present SiE3M3+ (E = S–Te and N–Sb; M = Li–Cs and Ca–Ba) clusters. Herein the solid and dashed lines represent covalent and ionic bonding, respectively. The opposite double arrows illustrate electrostatic repulsion.The concept of planar hypercoordinate carbons has been naturally extended to their next heavier congener, silicon-based systems. Although the steric repulsion between ligands decreases due to the larger size, the strength of π- and σ-bonding between the central atom and peripheral ligands dramatically decreases, which is crucial for stability. Planar tetracoordinate silicon (ptSi) was first experimentally observed in a pentaatomic C2v SiAl4− cluster by Wang and coworkers in 2000.21 Very recently, this topic got a huge boost by the room-temperature, large-scale syntheses of complexes containing a ptSi unit.22 A recent computational study also predicted the global minimum of SiMg4Y− (Y = In, Tl) and SiMg3In2 to have unprecendented planar pentacoordinate Si (ppSi) units.23 Planar hexacoordinate Si (phSi) systems seem to be even more difficult to stabilize. Previously, a C2v symmetric Cu6H6Si cluster was predicted as the true minimum,24 albeit its potential energy surface was not fully explored. A kinetically viable phSi SiAl3Mg3H2+ cluster cation was also predicted.25 However, these phSi systems24,25 are only local minima and not likely to be observed experimentally. In 2018, the group of Chen identified the Ca4Si22− building block containing a ppSi center and constructed an infinite CaSi monolayer, which is essentially a two-dimensional lattice of the Ca4Si2 motif.26 Thus, it is still an open question to achieve a phSi atom to date.Herein we have tried to find the correct combination towards a phSi system as the most stable isomer. Gratifyingly, we found a series of clusters, SiE3M3+ (E = N, P, As, Sb; M = Ca, Sr, Ba), having planar D3h symmetry with Si at the center of the six membered ring, as true global minimum forms. Si–E bonds are very strong in all the clusters, and alkaline-earth metals interact with the Si center by employing their d orbitals. However, electrostatic repulsion originated from the positively charged Si and M centers for E = N, P, and As dominates over attractive covalent interaction, making individual Si–M contacts repulsive in nature. This makes the assignment of SiE3M3+ (E = N, P, As; M = Ca, Sr, Ba) as genuine phSi somewhat skeptical. SiSb3M3+ (M = Ca, Sr, Ba) clusters are the sole candidates which possess genuine phSi centers as both electrostatic and covalent interactions in Si–M bonds are attractive. The d orbitals of M ligands play a crucial role in stabilizing the ligand framework and forming covalent bonds with phSi. Such planar hypercoordinate atoms are, in general, susceptible to external perturbations. However, the present title clusters maintain the planarity and the attractive nature of the bonds even after multiple ligand binding at M centers in SiSb3M3(NHC)6+ and SiSb3M3(Bz)6+. This would open the door for large-scale synthesis of phSi as well.Two major computational efforts were made before reaching our title phSi clusters. The first one is to examine SiE3M3+ (E = S–Po; M = Li–Cs) clusters, which adopt D3h or C3v structures as true minima (see Table S1 in ESI†), being isoelectronic to the previous phC CE3M3+ (E = S–Po; M = Li–Cs) clusters. In the SiE3M3+ (E = S–Po; M = Li–Cs) clusters, the Si center always carries a positive charge ranging from 0.01 to +1.03|e|, in contrast to the corresponding phC species (see Fig. 1). Thus, electrostatic interactions between the Siδ+ and Mδ+ centers would be repulsive (Fig. 1). Given that the possibility of covalent interaction with an alkali metal is minimal, it would be a matter of debate whether they could be called true coordination. A second effort is to tune the electronegativity difference between Si and M centers so that the covalent contribution in Si–M bonding becomes substantial. Along this line, we consider the combinations of SiE3M3+ (E = N, P, As, Sb; M = Be, Mg, Ca, Sr, Ba). The results in Fig. S1† show that for E = Be and Mg, the phSi geometry has a large out-of-plane imaginary frequency mode, which indicates a size mismatch between the Si center and peripheral E3M3 (E = N–Bi; M = Be, Mg) ring. On the other hand, the use of larger M = Ca, Sr, Ba atoms effectively expands the size of the cavity and eventually leads to perfect planar geometry with Si atoms at the center as minima. In the case of SiBi3M3+, the planar isomer possesses a small imaginary frequency for M = Ca. Although planar SiBi3Sr3+ and SiBi3Ba3+ are true minima, they are 2.2 and 2.5 kcal mol−1 higher in energy than the lowest energy isomer, respectively (Fig. S2†). Fig. 2 displays some selected low-lying isomers of SiE3M3+ (E = N, P, As, Sb; M = Ca, Sr, Ba) clusters (see Fig. S3–S6† for additional isomers). The global minimum structure is a D3h symmetric phSi with an 1A1′ electronic state for all the twelve cases. The second lowest energy isomer, a ppSi, is located more than 49 kcal mol−1 above phSi for E = N. This relative energy between the most stable and nearest energy isomer gradually decreases upon moving from N to Sb. In the case of SiSb3M3+ clusters, the second-lowest energy isomer is 4.6–6.1 kcal mol−1 higher in energy than phSi. The nearest triplet state isomer is very high in energy (by 36–53 kcal mol−1, Fig. S3–S6†) with respect to the global minimum.Open in a separate windowFig. 2The structures of low-lying isomers of SiE3M3+ (E = N, P, As, Sb; M = Ca, Sr, Ba) clusters. Relative energies (in kcal mol−1) are shown at the single-point CCSD(T)/def2-TZVP//PBE0/def2-TZVP level, followed by a zero-energy correction at PBE0. The values from left to right refer to Ca, Sr, and Ba in sequence. The group symmetries and electronic states are also given.Born–Oppenheimer molecular dynamics (BOMD) simulations at room temperature (298 K), taking SiE3Ca3+ clusters as case studies, were also performed. The results are displayed in Fig. S7.† All trajectories show no isomerization or other structural alterations during the simulation time, as indicated by the small root mean square deviation (RMSD) values. The BOMD data suggest that the global minimum also has reasonable kinetic stability against isomerization and decomposition.The bond distances, natural atomic charges, and bond indices for SiE3Ca3+ clusters are given in † for M = Sr, Ba). The Si–E bond distances are shorter than the typical Si–E single bond distance computed using the self-consistent covalent radii proposed by Pyykkö.27 In contrast, the Si–M bond distance is almost equal to the single bond distance. This gives the first hint of the presence of covalent bonding therein. However, the Wiberg bond indices (WBIs) for the Si–M links are surprisingly low (0.02–0.04). We then checked the Mayer bond order (MBO), which can be seen as a generalization of WBIs and is more acceptable since the approach of WBI calculations assumes orthonormal conditions of basis functions while the MBO considers an overlap matrix. The MBO values for the Si–M links are now sizable (0.13–0.18). These values are reasonable considering the large difference in electronegativity between Si and M, and, therefore, only a very polar bond is expected between them. In fact, the calculations of WBIs after orthogonalization of basis functions by the Löwdin method gives significantly large bond orders (0.48–0.55), which is known to overestimate the bond orders somewhat. The above results indicate that the presence of covalent bonding cannot be ruled out only by looking at WBI values.Bond distances (r, in Å), different bond orders (WBIs) {MBOs} [WBI in orthogonalized basis], and natural atomic charges (q, in |e|) of SiE3Ca3+ (E = N, P, As, Sb) clusters at the PBE0/def2-TZVP level
Open in a separate windowOur following argument regarding the presence of covalent Si–M bonding is based on energy decomposition analysis (EDA) in combination with natural orbital for chemical valence (NOCV) theory. We first performed EDA by taking Ca and SiE3Ca2 in different charge and electronic states as interacting fragments to get the optimum fragmentation scheme that suits the best to describe the bonding situation (see Tables S6–S9†). The size of orbital interaction (ΔEorb) is used as a probe.28 For all cases, Ca+ (D, 4s1) and SiE3Ca2 (D) in their doublet spin states turn out to be the best schemes, which give the lowest ΔEorb value. Energy term Interaction Ca+ (D, 4s1) + SiN3Ca2 (D) Ca+ (D, 4s1) + SiP3Ca2 (D) Ca+ (D, 4s1) + SiAs3Ca2 (D) Ca+ (D, 4s1) + SiSb3Ca2 (D) ΔEint −192.9 −153.0 −144.9 −129.9 ΔEPauli 139.8 115.2 115.7 110.9 ΔEelstata −162.0 (48.7%) −116.4 (43.4%) −113.0 (43.4%) −100.9 (41.9%) ΔEorba −170.7 (51.3%) −151.8 (56.6%) −147.6 (56.6%) −140.0 (58.1%) ΔEorb(1)b SiE3Ca2–Ca+(s) electron-sharing σ-bond −89.2 (52.3%) −79.4 (52.3%) −74.3 (50.3%) −66.9 (47.8%) ΔEorb(2)b SiE3Ca2 → Ca+(d) π‖-donation −32.9 (19.3%) −32.0 (21.1%) −31.8 (21.5%) −30.8 (22.0%) ΔEorb(3)b SiE3Ca2 → Ca+(d) σ-donation −13.1 (7.7%) −11.9 (7.8%) −12.0 (8.1%) −11.9 (8.5%) ΔEorb(4)b SiE3Ca2 → Ca+(d) π⊥-donation −12.3 (7.2%) −12.2 (8.0%) −12.5 (8.5%) −12.5 (8.9%) ΔEorb(5)b SiE3Ca2 → Ca+(d) δ-donation −8.1 (4.7%) −9.9 (6.5%) −10.9 (7.4%) −11.8 (8.4%) ΔEorb(rest)b −15.1 (8.8%) −6.4 (4.2%) −6.1 (4.1%) −6.1 (4.4%)
r Si–E | r Si–Ca | r E–Ca | q Si | q E | q Ca | |
---|---|---|---|---|---|---|
E = N | 1.669 | 2.555 | 2.246 | 1.57 | −1.93 | 1.74 |
(1.14) {1.23} [1.84] | (0.02) {0.13} [0.51] | (0.22) {0.67} [0.84] | ||||
E = P | 2.180 | 2.935 | 2.640 | 0.25 | −1.42 | 1.67 |
(1.34) {1.11} [1.52] | (0.03) {0.14} [0.54] | (0.27) {0.74} [1.05] | ||||
E = As | 2.301 | 3.004 | 2.721 | 0.07 | −1.34 | 1.65 |
(1.33) {1.10} [1.45] | (0.03) {0.15} [0.55] | (0.29) {0.71} [1.12] | ||||
E = Sb | 2.538 | 3.155 | 2.896 | −0.39 | −1.16 | 1.62 |
(1.29) {1.01} [1.33] | (0.04) {0.18} [0.48] | (0.30) {0.78} [1.14] |