[(CrGe9)Cr2(CO)13]4-: A disubstituted case of ten-vertex closo cluster with spherical aromaticity
English
[(CrGe9)Cr2(CO)13]4-: A disubstituted case of ten-vertex closo cluster with spherical aromaticity
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The chemistry of Group 14 zintl ions covers extensive studies of nine-atom deltahedral clusters, whereas those of ten-atom closo clusters remain relatively limited [1-10]. Albeit two homoatomic clusters E102- (E = Ge, Pb) have been isolated from solution reactions [11, 12], most of the structurally characterized ten-atom clusters are doped with at least one metal atom. At variance with most heteroatomic clusters capped by transition-metal fragments, such as [(η4-E9)M(CO)3]4- (E = Sn, Pb; M = Cr, Mo, W) [13, 14], [E9Zn-Ph]3- (E = Si, Ge, Sn, Pb) [15], [NHCDippMSn9]3- (M = Cu, Ag, Au) [16], [E9Cd(C6H5)]3- (E = Sn, Pb) [17] and [Ge9Ni-CO]3- [18], alternative coordination modes have been observed, for example, one Mo(CO)3 fragment in the [(η5-Pb9)Mo(CO)3]4- occupying a waist vertex [19]. Recently, another ten-vertex cluster [Ge8(Mo(CO)3)2]4- with two waist vertices replaced by Mo(CO)3 fragments has also been reported [20]. The inherent stability of closo architecture imparts to elusive substitution on such type of clusters. Not much has been known about the functionalized clusters, since only one monosubstituted ten-vertex cluster [Ge10Mn(CO)4]3- was isolated [21]. Herein, we report the isolation of a disubstituted hetero ten-vertex closo cluster derivative [K(2.2.2-crypt)]4[(CrGe9)Cr2(CO)13]·tol (1). Structural characterization and calculations revealed an unusual arrangement of three adjacent Cr(CO)n fragments, wherein both η5 and η1 coordination modes jointly interpret the sophisticated bonding pattern which further leads to multiple local aromaticity in the title cluster.
The title cluster [(CrGe9)Cr2(CO)13]4- (1a, Fig. 1) was obtained from the reaction of "KGe1.67" with (MeCN)3Cr(CO)3 and Cr(CO)6 in en in the presence of 2.2.2-crypt. Single-crystal X-ray diffraction reveals that the complex crystallizes in the triclinic space group P1 and the asymmetric unit contains one crystallographically distinct 1a with four [K(2.2.2-crypt)]+ cations as well as one solvent molecule of toluene, and an interaction was observed between one carbonyl group of 1a and [K(2.2.2-crypt)]+ (Fig. S2 in Supporting information).
Figure 1
Strong signals of the parent anions {H[K(2.2.2-crypt)]2[Ge9Cr3 (CO)13]}- and {H2[K(2.2.2-crypt)][Ge9Cr3(CO)13]}- (Fig. 2 and Fig. S6 in Supporting information) were observed in the electrospray ionization mass spectra (ESI-MS) of 1 in DMF. In addition, the ESI-MS revealed a signal of {H2[K(2.2.2-crypt)][Ge9Cr2(CO)8]}- (Fig. S5 in Supporting information) ion losing one Cr(CO)5 fragment. The successful isolation of the [(η5-Pb9)Mo(CO)3]4- reminded us the possible existence of [(η5-Ge9)Cr(CO)3]4-. However, all attempts to obtain [Ge9Cr2(CO)8]4- and [(η5-Ge9)Cr(CO)3]4- by changing the precursor and reaction conditions proved unsuccessful.
Figure 2
The structure of [(CrGe9)Cr2(CO)13]4- is best described as a hetero-ten-atom closo cluster [CrGe9] with a waist vertex occupied by the Cr(CO)3 and two coordinated Cr(CO)5 fragments on the positions of Ge2 and Ge9 in η1: η1-coordination modes. Not only the shape of [CrGe9] defines it as a closo cluster but its charge and electron count. The anionic cluster 1a with a total of 22 skeleton electrons, provided by 9 Ge-atoms (2 electrons each) and one Cr(CO)3 fragment (0 electron) as well as the charge (4 additional electrons), is in accordance with the Wade-Mingos rules for a ten-atom closo cluster [22, 23].
Insertion of the Cr(CO)3 fragment results in the distorted ten-vertex cage with a compressed Ge4 square (2.6659(11)-2.8670(11) Å) and an extended CrGe3 square (2.8104(12)-3.0324(12) Å). The Ge-Ge distances range from 2.5073(11) Å to 2.8670(11) Å and are comparable to those observed in other germanium clusters [24-31]. The very long Cr-Ge bond distances (Cr1-Ge6/Cr1-Ge7: 2.8814(13) Å /2.9061(14) Å) are located in the extended CrGe3-square, while the shortest (Cr1-Ge9: 2.5199(14) Å) is the bond between the Cr-vertex and the Ge-atom exo-bonded to Cr(CO)5, only 0.02-0.06 Å shorter than other distances at the exo-bonded germanium atoms (Cr3-Ge9/Cr1-Ge2/Cr2-Ge2: 2.5413(14) Å/2.5571(14) Å/2.5824(14) Å). Overall, the Cr-Ge bond distances in 1a span a wider range compared with those in the reported cluster Ge9[Si(SiMe3)3]3Cr(CO)3- (2.6462(13)-2.6957(14) Å) [32].
DFT calculations at the PBE0/ma-TZVP level revealed distinct bonding modes of η5-Cr(CO)3 and η1-Cr(CO)5 in 1a [33-36]. The valence-electron count for hetero cage [Cr(Ge9)]4- is 40, ascribed to 36 electrons from nine Ge atoms and 4 negative charges. An adaptive natural density partitioning (AdNDP) scheme consisting of 7 lone pairs (LPs) of Ge atoms [37, 38], 2 two-center two-electron (2c-2e) Cr-Ge bonds formed by the exo-bonded Ge2/Ge9 and the η1-Cr2/Cr3, and multi-center units ([Ge5], [CrGe7], [CrGe4]) that partition the cage into three aromatic regions (Fig. 3). The [CrGe4] is capped by [Ge5] and [CrGe7] at opposite positions. Both [Ge5] and [CrGe4] have three five-center two-electron bonds that account for 5c-6e σ aromaticity, while [CrGe7] is 8c-10e σ-aromatic with 5 eight-center two-electron bonds [39, 40]. All three units follow the Hückel 4n + 2 rule (n = 1 for 5c-6e and n = 2 for 8c-10e). Taking into account the external resonance effects, the electron density of delocalized bonds (EDDB) of the [CrGe9]4- cage is 15.62e which is 71% of the total 22e from two 5c-6e and one 8c-10e aromatic systems and comparable to the degree of delocalization in all-metal aromatics such as 3Li3Al4- and 3Be2B6 [41, 42]. Considering only local resonances (Fig. 3), the EDDBF calculated for [Ge5] (2.53e, 0.51e per atom), [CrGe7] 5.81e (0.73e per atom) and [CrGe4] (1.72e, 0.34e per atom) indicate the delocalization is stronger within the [CrGe7] unit and weaker for the [CrGe4] where the atomic contribution (Fig. S9 in Supporting information) of Cr1 (0.20e) is smaller than those of four Ge atoms (0.28-0.61e).
Figure 3
Hence, the [Cr(Ge9)]4- can be considered an overall spherical aromatic hetero cage featuring three local σ-aromatic units [43, 44]. The iso-chemical-shielding surfaces (ICSS) demonstrate magnetic shielding cones along three canonical directions (x, y, z) regardless of the irregular shapes caused by the interference of η1-Cr atoms and CO ligands (Fig. 4), similar to the typical response of spherical aromatic species under an external magnetic field [45, 46]. Nucleus-independent chemical shift (NICS) at the geometry center is considerably negative (-49.2 ppm) with components NICSxx, NICSyy, and NICSzz being -62.1 ppm, -51.5 ppm and -34.0 ppm [47], respectively, further supporting the significant aromaticity of [Cr(Ge9)]4-.
Figure 4
Although the AdNDP scheme for 1a suggests that the η5-Cr1 participates in the multi-center bonding in both [CrGe7] and [CrGe4] units, the interaction between Cr1 and five adjacent Ge atoms can be represented by three principle interacting molecular orbitals (PIMOs) (Figs. 5a-c) which are also well delocalized within the corresponding [Cr1Ge5] unit which could be considered 6c-6e σ-aromatic with 2.31e of EDDBF (0.38e per atom) [48, 49]. The multi-center bonds of [Cr1Ge5] do not belong to the proposed AdNDP scheme, however, neither do they contradict the latter because the different manifestations of local aromaticity actually root in the same system with complicated resonance effects. On the other hand, the η1-Cr2/Cr3 forms only localized 2c-2e bonds with Ge atoms (Fig. 5d).
Figure 5
In conclusion, we report the synthesis and characterization of [(CrGe9)Cr2(CO)13]4-, a difunctionalized ten-vertex Zintl cluster where one η5-Cr(CO)3 and two η1-Cr(CO)5 adopt entirely different bonding modes in the interaction with the Ge9 moiety. Within an AdNDP scheme with well-partitioned valence-electrons, the η5-Cr1 serves as a member of both the 5c-6e [CrGe4] and the 8c-10e [CrGe7] σ-aromatic systems while the 5c-6e [Ge5] accounts for the rest multi-center bonds in 1a. Analyses of magnetic response and electron delocalization confirmed the spherical aromatic character of 1a. Focusing on the bonding between η5-Cr(CO)3 and the remaining moiety of 1a, we found three 6c-2e bonds that could result in another local σ-aromatic unit which is the Cr[Ge5] containing the η5-Cr1. In contrast, the η1-Cr2 and η1-Cr3 are attached to the hetero ten-atom cage [CrGe9]4- via only localized 2c-2e Cr-Ge bonds. Our findings demonstrate the first synthesis of a disubstituted hetero-ten-vertex closo cage, [(CrGe9)Cr2(CO)13]4-, with the sophisticated bonding patterns and aromatic characters disclosed.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Natural Science Foundation of China (No. 21971118 to Z.M. Sun and21573179 to J. Zhu) and the Natural Science Foundation of Tianjin City (No. 20JCYBJC01560)
Supplementary materials
Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.08.038.
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[1]
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Figure 5 Principle interacting orbital (PIO) analyses on 1a using η5-Cr(CO)3 and Ge9Cr2(CO)10 as fragments in (a-c), and η1-Cr2(CO)5 and Cr(CO)3Ge9Cr(CO)5 as fragments in (d). The top three PIO pairs contribute 30% + 27% + 14% = 71% of total interaction between the η5-Cr1 and the rest part of the cage. PBI: PIO-based bond index. Isovalue for surfaces: 0.05 a.u.
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