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GeI−GeI Coupling Reaction Induced by a Mixture of CoBr2 and a Seven-Membered N‑Heterocyclic Carbene Xinmiao Wang,† Jingjing Liu,† Jiaxiu Yu, Lei Hou, Wei Sun, Yaoyu Wang, Sanping Chen, Anyang Li,* and Wenyuan Wang* Key Laboratory of Synthetic and Natural Functional Molecule Chemistry, Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’An 710127, People’s Republic of China S Supporting Information *

Driess group produced bis(germyliumylidene) diborate1b (BLGe-GeBL; BL = biscarbene borate anion) by irradiation of a phosphaketenyl germyliumylidene borate (BLGe-PCO). Here we report a novel route to obtaining a digermylene. Recently, we have successfully synthesized radical βdiketiminatogermylene 17 (Scheme 2), which possesses three

ABSTRACT: A new digermylene has been synthesized through the reaction of a six-membered cyclic radical germylene precursor with a mixture of CoBr2 and a sevenmembered N-heterocyclic carbene. The density functional theory simulation on the geometry of the digermylene agrees well with the experimental X-ray diffraction result. The digermylene possesses the shortest GeI−GeI bond [2.4853(6) Å] compared to the known singly bonded analogues, which can be attributed to σ → π* conjugation. In addition, it is revealed that a stable dative bond cannot form for the reductive radical germylene with the oxidative metal centers.

Scheme 2. Reactions of Radical Germylene 1 with CoBr2

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any cases of the digermylene with a single bond between GeI atoms have been achieved in the past 10 years.1 These molecules feature diverse trans-bent structures with lone electron pairs at each of the pyramidalized Ge centers. For the bonding of heavier group 14 elements in these carbene analogues, the trend of increasing s character of the lone-pair orbitals and p character of the E−X σ bonds (E = group 14 elements; X = connected atoms) down group 14 is clear-cut.2 Because of the electronic and steric effects of the ligand systems, the GeI−GeI distances of the reported digermylenes1b,3 vary by more than 0.2 Å, which is of great interest. Usually, many digermylenes were synthesized by the reduction of germylene chloride (LGeCl; L = monoanionic chelate ligands) with powerful reducing agents such as Li, K, KC8, and (NacNacMg)24 (Scheme 1). In our previous work, we published an asymmetrically substituted digermylene,3c which was assembled by a salt metathesis reaction between a germylidenide (germylene anion) and germylene chloride.5 Because those germylidenide salts are very rare,6 they are seldom used as precursors in inorganic synthesis. Quite recently, the

active nonbonding electrons. It is therefore proposed to generate multiply bonded Ge−M (M = transition metals) species using germylene 1. To probe its coordination behavior, the reaction of 1 with CoBr2 in the presence of seven-membered N-heterocyclic carbene8 (NHC) 2 [(CH2CH2N-Dip)2C; Dip = 2,6-iPr2C6H3] was investigated. Unexpectedly, the reaction furnished digermylene 3 and β-diketiminatogermylene(II) amide 4 (Scheme 2). Anhydrous CoBr2 and 2 (a 1:1 molar ratio) were dissolved in dry tetrahydrofuran (THF), and the resulting blue-green solution was stirred until CoBr2 disappeared. When the same amount of radical 1 was added to the solution at ambient temperature in a glovebox, the color of the solution changed immediately to dark green. Compounds 3 and 4 were isolated by

Scheme 1. Three Reported Routes Toward Digermylenesa

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L and L′ are monoanionic chelate ligands. BL is a biscarbene borate anion. © XXXX American Chemical Society

Received: January 15, 2018

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DOI: 10.1021/acs.inorgchem.8b00086 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry fractional crystallization in hexane in the form of yellow and darkgreen crystals. Crystalline grains of 3 and 4 were gleaned artificially in 19% and 32% yield, respectively. Additionally, a tetrabromocobaltate dianion9 ([CoBr4]2−) in the form of its carbocation salt 5 was isolated from the further extracted THF solution with 26% yield [see the Supporting Information (SI) for the characterization of 5]. Three new compounds were characterized by single-crystal X-ray diffraction (XRD) analysis, as well as NMR spectroscopy and elemental analysis. The original β-diketimine 67 as a byproduct was found in the remaining hexane solution indicated by the 1H NMR spectrum. The carbene cobalt [2 → Co] complex and the expected Ge−Co complex were not observed during the processing procedures. To investigate the nature of this reaction, four other parallel experiments using different molar ratios of CoBr2 or in the absence of the NHC 2 were conducted. The direct reaction of 1 with CoBr2 in a 1:1 molar ratio at ambient temperature led to the germylene bromide 7 as the primary product with a small amount of 6, which was removed by a first extraction with hexane. 7 crystallized from a hexane solution by a second extraction in 41% yield and was characterized by multinuclear NMR spectroscopy and XRD. Similar chlorination of a GeI radical, employing C2Cl6, to the corresponding germylene chloride was reported in 2011.10 Compound 1 with 0.5 equiv of [CoBr2 + 2] formed a complicated mixture with some nonpolar products, including 3, 4, and 6, which were extracted in hexane and determined by 1H NMR spectroscopy. The reaction of 1 with 0.1 equiv of CoBr2 or 0.1 equiv of [CoBr2 + 2] was markedly slow, and the red-purple color of radical 1 disappeared after 5 h. The 1H NMR spectra of the resulting solution showed that ligand 6 was the main product and no 3 and 4 were detected. Obviously, the formation of 3 and 4 is based on the disproportional recombination of 1. The sixmembered C3N2Ge ring in 1 undergoes a ring contraction to give a five-membered C3NGe ring, the homocoupling of which gives the digermylene 3. The split arylamino group (Dip−N) migrates to another molecule of 1, followed by protonation at nitrogen, affording compound 4. According to our experiments, neither [CoBr2 + 2] nor CoBr2 works as the catalyst for this reaction. The stoichiometric [CoBr2 + 2] induced the formation of 3 and 4, while CoBr2 alone worked as a bromination reagent. In addition, the color of the [CoBr2 + 2]/THF solution (blue-green) is quite different from that of the CoBr2/THF solution (bright blue), indicating the effect of ligand coordination. Thus, the NHC 2 plays a key role in this cobalt-induced GeI−GeI homocoupling reaction. Unfortunately, we could not successfully isolate cobalt complexes other than 5 from the residue, which possibly reveals the mechanism of the formation of 3 and 4. The 1H NMR spectrum of 3 shows four sets of doublets at 1 δ( H) 0.43, 0.91, 1.07, and 1.27 for eight methyl groups and two sets of multiples at δ(1H) 2.74 and 3.60 for four CH groups of the isopropyl groups. Resonances for 36 aromatic protons of the Dip and phenyl groups appear in the range of δ(1H) 6.68−7.61. The ratio of three parts of H atoms is in accordance with evaluation of the spectral integral. The molecular structure of 3 is shown in Figure 1. 3 crystallizes in the orthorhombic space group P212121 and reveals a torsional trans-bent conformation. This is consistent with other digermylene molecules but not the case in digermylenes supported by very bulky amidinate or guanidinate ligands3a,e with a symmetric trans-bent conformation. The interior angle summations of two five-membered C3NGe rings in 3 are 538.41° and 537.49°, respectively, which is almost equal to the value of the planar pentagonum (540°), suggesting the existence of aromatic 6π resonance stabilization.

Figure 1. Molecular structure of 3 (50% ellipsoid probability). H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ge1−Ge2 2.4853(6), Ge1−N1 1.973(3), Ge1−C3 1.947(3), N1−C1 1.343(4), C1−C2 1.447(5), C2−C3 1.384(5), Ge2−N2 1.947(3), Ge2−C6 1.937(4), N2−C4 1.344(5), C4−C5 1.450(5), C5−C6 1.392(5); N1−Ge1−Ge2 96.08(9), C3−Ge1−Ge2 110.51(10), N1− Ge1−C3 83.51(14), C6−Ge2−Ge1 115.75(11), N2−Ge2−Ge1 99.80(9), C6−Ge2−N2 84.79(14).

The stability of the C3NGe ring is the driving force of the ring contraction of 1. The same transformations were described in our previous work,5a as well as the formation of a lithium germylidenide salt.6a The most important structural feature in 3 is the central Ge−Ge distance of 2.4853(6) Å, which is the shortest bond length in all of the reported Ge I −Ge I digermylenes1a,3 (2.506−2.709 Å) but still longer than those in a doubly bonded digermylene11 [NHC→GeGe←NHC; NHC = (HCN-Dip)2C; Ge0−Ge0 distance = 2.349 Å] and a triply bonded digermyne12 (2.285 Å). Actually, the Ge−Ge distance in 3 is close to the Ge−Ge single bonds associated via sp3 hybrid orbitals, such as those in iPr3Ge−GePh313 (2.464 Å) and Ph3Ge−GePh314 (2.446 Å). The latter two compounds have shorter Ge−Ge bonds because their orbitals have quite a lot of s character (over 20%). According to the low-coordinated environment of GeI atoms in 3, the GeI−GeI bonding should mainly make use of 4p orbitals2 and feature a longer distance. To evaluate the GeI−GeI bond situation of 3, density functional theory calculations have been performed. Geometry optimization using the hybrid functional M06-2X15 with Ahlrichs’def2TZVP basis set16 gives excellent agreement with the molecular structure determined by XRD analysis, such as the calculated GeI−GeI distance of 2.496 Å versus the experimental value of 2.485 Å, and the Wiberg bond index for the Ge−Ge bond is 1.02, which is slightly larger than those of other reported digermylenes.3c,h The highest occupied molecular orbital (HOMO) of the optimized 3 presents a typical σ-bonding interaction between the two Ge atoms (Figure 2a), and the natural bond orbital analysis indicates that the Ge−Ge bond has high p character (89.6%), which is similar to that of the previously reported Ge−Ge compounds.3b,f,h The structures of the two five-membered C3NGe rings are almost the same, and the bond lengths and angles are very close to the experimental mean results (Table S5). The aromaticity of the five-membered C3NGe rings has been confirmed by the strongly negative nucleus-independent chemical shift (NICS) values in both B

DOI: 10.1021/acs.inorgchem.8b00086 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

indicates that a stable coordination behavior cannot exist between the highly reductive radical 1 and the slightly oxidative CoBr2. In summary, the reaction of the radical germylene 1 with a mixture of CoBr2 and the seven-membered NHC 2 was investigated, delivering the new digermylene 3, germylene amide 4, and tetrabromocobaltate dianion salt 5. It is noteworthy that stoichiometric cobalt is needed to effectively induce the onestep conversion of 1 to 3 and 4. Importantly, the shortest GeI− GeI bond [2.4853(6) Å] is found in 3 for all singly bonded digermylenes to date. The optimized geometry of 3 at the M062X/def2-TZVP level is in excellent agreement with the structure from the XRD result. The two planar C3NGe rings present 6π aromatic stabilization in 3, and the antibonding orbitals π* distributed in the ring plane interact with the GeI−GeI σ orbital as a σ → π* conjugation, which stabilizes the digermylene 3 and reduces the GeI−GeI bond length. In addition, this work unravels that the reductive radical germylene 1 cannot form a stable dative bond with the oxidative metal centers.

Figure 2. Frontier molecular orbitals of 3.

C 3 NGe rings [NICS(1) = −9.3; NICS(−1) = −8.1]. Interestingly, the lowest unoccupied molecular orbital (LUMO) is distributed in both C3NGe rings and shows antibonding character as π* (Figure 2b). Because the σ-bonding orbital of Ge−Ge in 3 is nearly perpendicular to the two planar rings, the σ-bonding HOMO is symmetry-adapted with the πantibonding LUMO, which realizes a positive σ → π*conjugative coupling (Figure S11). This conjugation expands the delocalization in C3NGe rings and a Ge−Ge bond, which should enhance the stability of compound 3 and shorten the Ge−Ge bond length. Another reported digermylene molecule3g with a similar conjugative ring system also has a very short GeI−GeI distance (2.506 Å). Germanium dibromide was not commonly used as a starting material; thus, germylene bromides are also scarce species. The reaction of 1 with CoBr2 furnishes the germylene bromide 7 as the main product. However, this reaction is not the optimal method for the preparation of 7 because it is very difficult to prepare radical 1. The structure of 7 is shown in Figure 3. 7

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00086. General remarks, chemical equations, experimental section, crystal data and structure refinement parameters for compounds 3−5 and 7, NMR spectra, and computational details (PDF) Accession Codes

CCDC 1580648−1580650 and 1582451 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Figure 3. Molecular structure of 7 (50% ellipsoid probability). H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Br1−Ge1 2.5060(6), Ge1−N1 1.944(3), Ge1−N2 2.011(3), N1−C1 1.372(5), N2−C3 1.337(5), C1−C2 1.386(6), C2−C3 1.435(6); N1− Ge1−N2 89.23(14), N1−Ge1−Br1 96.17(10), N2−Ge1−Br1 95.17(9).

Lei Hou: 0000-0002-2874-9326 Yaoyu Wang: 0000-0002-0800-7093 Sanping Chen: 0000-0002-6851-7386 Wenyuan Wang: 0000-0003-0898-3278 Author Contributions †

These authors contributed equally. All authors have given approval to the final version of the manuscript.

crystallizes in the monoclinic space group P21/n. The structure of 7 is different from the known β-diketiminatogermylene bromides.17 The central six-membered C3N2Ge ring is strongly distorted. Consequently, two N−Ge bonds are divided into a single bond [1.944(3) Å] and a dative bond [2.011(3) Å]. The Br1−Ge1 distance of 2.5060(6) Å in 7 is equal to that in the βdiketiminato germylene bromide17a [HC(MeCN-Dip)2GeBr] but slightly longer than that of 2.472 Å in the dimeric one.17b The bond angles around the Ge atom in 7 are 89.23(14), 96.17(10), and 95.17(9)°, respectively. These are close to the intersection angle of the p orbitals, suggesting an almost direct bonding via 4p orbitals, just like the observation in 4. The formation of 7

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSF of China (Grants 21371141 and 21727805), Key Science and Technology Innovation Team of Shaanxi Province (Grant 2017KCT-37), and Education Committee of Shaanxi Province (Grants 14JS091 and 15JS114). C

DOI: 10.1021/acs.inorgchem.8b00086 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.8b00086 Inorg. Chem. XXXX, XXX, XXX−XXX