A Tetra-Amido-Protected Ge5‑Spiropentadiene

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Cite This: J. Am. Chem. Soc. 2019, 141, 19252−19256 pubs.acs.org/JACS

‑ A Tetra-amido-Protected Ge5 Spiropentadiene ⊥ † ⊥ † † † ‡ § § Yan Guo, , Zhengqiang Xia, , Jingjing Liu, Jiaxiu Yu, Shenglai Yao, Weiqun Shi, Kongqiu Hu, † † † ‡ † Sanping Chen, Yaoyu Wang, Anyang Li,*, Matthias Driess,*, and Wenyuan Wang*, † Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, College of Chemistry and Materials Science, Northwest University, Xi’an, Shaanxi 710069, China ‡ Metalorganics and Inorganic Materials, Department of Chemistry, Technische Universitaẗ Berlin, Straße des 17, Juni 135, Sekr. C2, 10623 Berlin, Germany § Laboratory of Nuclear Energy Chemistry, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China

*S Supporting Information

spiropentadiene B (spiropentasiladiene) was successfully fi ABSTRACT: The rst isolable Ge5-spiropentadiene 1 7 i isolated by Kira and co-workers. Up to now, isolation of was synthesized via the reduction of ( Pr3Si)2NGeCl (3) other E -spiropentadienes (E = Ge, Sn, Pb) has not been with potassium. The crystal structure of 1 reveals a 5 spirocyclic Ge skeleton containing two Ge−Ge double reported. 5 During the past decade, extremely bulky monodentate amido bonds (avg. 2.34 Å), which are fettered in two Ge3 rings with a dihedral angle of 70.193°. The DFT calculations protective groups were more applied in the synthesis of low- and orbital analysis show that the σ-delocalization of the coordinate complexes for main-group elements,8 transition π 9 10 Ge5 skeleton and the 2 -delocalized aromatic Ge3 rings metals, and f-block metals. Herein, we report the synthesis enhance the stability of molecule 1. fi and structure of the rst Ge5-spiropentadiene 1 (Figure 1), which is stabilized by four extremely bulky amido groups (-N(SiiPr ) ). ecause of its highly strained structure and poor 3 2 B thermodynamic stability, synthesis of spiropentadiene represents one of the most attractive organic synthetic challenges. The two derivatives1 of spiropentadiene reported so far (Scheme 1, A) are thermally very unstable and decompose − ° fi even at 100 C. By fettering ole ns in C3 rings, these molecules have higher strain energy than the saturated one.2

Scheme 1. Analogues of Spiropentadiene Downloaded via UPPSALA UNIV on July 18, 2020 at 10:56:24 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

It is now well-understood that chemical bonding rules for Figure 1. Molecular structure of 1. Thermal ellipsoids are drawn at the carbon chemistry cannot be directly generalized to the heavier i 3 50% probability level (except the C atoms of the Pr groups). A hexane group 14 elements. Since the core electronic structures of the molecule and all H atoms are omitted for clarity. Selected bond lengths latters involve more atomic orbitals than carbon, the p valence (Å): double bonds, Ge1−Ge2 2.332(3), Ge4−Ge5 2.349(5); single atomic orbitals of these heavier elements are more extended in bonds, Ge1−Ge3 2.491(4), Ge2−Ge3 2.511(5), Ge3−Ge4 2.490(6), space, leading to a larger energy separation between the s and p Ge3−Ge5 2.487(5), Ge1−N1 1.930(5), Ge2−N2 1.887(5), Ge4−N4 orbitals.4 Thus, the s and p orbitals of heavier p-block elements 1.921(5), Ge5−N3 1.885(5). Selected bond angles (deg): Ge2−Ge1− are separately used to form their low-valent compounds.3,5 Ge3 62.65(13), Ge1−Ge2−Ge3 61.76(11), Ge1−Ge3−Ge2 6 σ − 55.58(12), Ge3−Ge4−Ge5 61.78(14), Ge3−Ge5−Ge4 61.90(14), Moreover, the special electronic phenomena of -bonded E − − − − − − E−E units (σ-delocalization; E = Si, Ge, Sn) and the low strain Ge4 Ge3 Ge5 56.32(14), N1 Ge1 Ge2 151.7(2), N1 Ge1 Ge3 145.7(2). energy of E3 rings should make the E5 analogues of spiropentadiene more stable than spiropentadiene itself. There- fore, the heavier spiropentadienes are a fascinating topic for Received: October 10, 2019 ﬁ research in main-group chemistry. In 2000, the rst Si5- Published: November 24, 2019

The reaction of potassium amide 210a (Scheme 2) with 1 amido group (N1), the N2−Ge1−Cl1 angle (84.82(4)°)is · ° molar equiv of GeCl2 dioxane led to the formation of amido obviously squeezed and smaller by 5 than the intersection angle of the p orbitals (90°). The distances of Ge1−N1 (1.9324(14) Scheme 2. Synthesis of Compounds 1, 2, and 3 and Proposed Å) and Ge1−Cl1 (2.3366(5) Å) in 4 are elongated in contrast to Mechanism for the Formation of 3 those of 3. This is probably caused by the interaction of the “empty” 4p-orbital of the Ge atom with the lone-pair electrons of pyridine. The Ge1−N2 distance (2.1849(18) Å) is 0.25 Å longer than the Ge1−N1 distance, indicating that a weak dative bond12a is established between pyridine and germanium. The reduction of 3 with 1.5 equiv of potassium chips in toluene caused the formation of compound 1,which precipitated from hexane solution at −20 °C as dark red crystals in 58% yield. But we could not isolate 1 in the same way starting with compound 4. Because of the unusual formation of 1, its mechanism is of great interest. We propose the formation of digermyne 58b as an intermediate product during the reduction. The subsequent cycloaddition of 5 with 3 could give the transient cyclotrigermene 6.15 The latter is transformed into radical intermediate 716 under reductive environment. Finally, the reductive deamination of 7 and the continuous cyclo- germylene chloride 3 (Scheme 2) in 68% yield. The Lewis addition with 5 could afford spiropentagermadiene 1. acidity of 3 can be quenched by coordination with pyridine, Compound 1 was fully characterized by means of spectros- offering a Lewis acid−base adduct as product, that is, pyridine copy and X-ray crystallography. The 1H NMR spectrum of 1 coordinate germylene chloride 4. The composition and shows one set of doublets at δ = 1.42 ppm for methyl moieties constitution of 3 and 4 were fully characterized by 1H, 13C, and one set of multiplet at δ = 1.59 ppm for CH protons of and 29Si NMR spectroscopy, elemental analysis, and X-ray isopropyls. A sharp singlet resonance in the 29Si NMR spectrum crystallographic analysis. of 1 was found at δ = 7.7 ppm. The UV−vis spectrum of 1 was λ The molecular structure of 3 (Figure 2, left) reveals a recorded in n-hexane and exhibits an absorption band at max = monomeric two-coordinate amido germylene chloride with a 464 nm, which is red-shifted in comparison to the absorption maximum (428 nm) of B. The EPR measurement of 1 confirmed that there are no unpaired electrons in this molecule. The molecular structure of 1 (Figure 1) has the anisotropic displacements on the internal Ge atoms (Ge6−Ge10, see the Supporting Information), not on the peripheral substituents. This experimental observation suggests that the four extremely i ffi bulky amido groups (-N(Si Pr3)2) are su ciently large and cause a slightly asymmetric structure of 1. The central part of molecule 1 features a spirocyclic Ge5 skeleton, in which the Ge3 atom is at the node position of the bicyclic rings. The sum of bond angles Figure 2. Molecular structure of compounds 3 (left) and 4 (right). around the vertex Ge atoms (Ge1, Ge2, Ge4, and Ge5) is exactly Thermal ellipsoids are drawn at the 50% probability level. H atoms and 360°, indicating that N1 and N2 with the Ge1−Ge2−Ge3 ring the C atoms of the iPr groups at the silyl groups in 4 are omitted for − − − as well as N3 and N4 with the Ge3 Ge4 Ge5 ring are coplanar, clarity. Selected bond lengths (Å) and angles (deg): for 3, Ge1 Cl1 respectively. However, these two planes are not perpendicular to 2.2689(6), Ge1−N1 1.8838(11), N−Si 1.768−1.769, N1−Ge1−Cl1 ° − − − each other and twist with a dihedral angle of 70.193 , which is 102.78(4); for 4, Ge1 Cl1 2.3366(5), Ge1 N1 1.9324(14), Ge1 N2 fi ° 2.1849(18), N1−Ge1−Cl1 106.54(4), N1−Ge1−N2 100.00(6), N2− signi cantly smaller than that (78.26 ) in spiropentasiladiene B. Ge1−Cl1 84.82(4). The optimized geometry by density functional theory (DFT) calculations at the M06-2X/def2-SVP level17 shows strong resemblance to the XRD structure, such as the corresponding N1−Ge1−Cl1 angle of 102.78(4)°. Similar structures were dihedral angle of 68.36°. The Ge1−Ge2 (2.332(3) Å) and observed in other ultra-bulky amido germylene chlorides.8b,11 Ge4−Ge5 (2.349(5) Å) distances of 1 lie in the middle of the The possible dimerized products of 3, like digermene with a Ge−Ge double bond range (2.212−2.509 Å) in the reported Ge−Ge double bond3,12 or digermylene formed via double digermenes,3,8b,18 but they are somewhat longer than the Ge− 13 μ i − chlorine bridges (R2N-Ge( -Cl)2Ge-NR2,R=SiPr3), were Ge double bond lengths (2.239 2.274 Å) in the cyclo- not observed in the course of our experiment. The Ge1−N1 trigermene molecules,15,19 and slightly shorter than the average distance (1.8838(11) Å) and the Ge1−Cl1 distance (2.2689(6) Ge−Ge distance (2.35 Å) in a cyclotrigermenyl radical.16 The Å) in 3 are shortened in comparison with the related distances in Mayer bond index (MBI)20 of 1.50 for the two bonds suggests three-coordinate germylene,14 presumably due to the inter- that they belong to the normal Ge−Ge double bond. With action of the empty 4p-orbital with the lone pair electrons of calculated MBI of 0.92, the four Ge−Ge distances (2.487−2.511 adjacent N1 and Cl1 atoms. But the Ge1−N1 bond length here Å) around Ge3 in 1 belong to single bonds. They are slightly is ca. 0.18 Å longer than the doubly bonded Ge−N distances, longer than those in some (poly)germanes21 (2.43−2.51 Å), 3 t 22 − which are normally shorter than 1.70 Å. In the molecular ( Bu3Si)4Ge4 cluster (2.43 2.45 Å), and the elemental structure of 4 (Figure 2, right), the GeII center adopts a trigonal germanium with a diamond structure (2.445 Å), and almost − 23 pyramidal geometry. Owing to the steric repulsion of the huge equal to the Ge Ge single bond lengths of a Ge9Si2 cluster,

19253 DOI: 10.1021/jacs.9b10946 J. Am. Chem. Soc. 2019, 141, 19252−19256 Journal of the American Chemical Society Communication which has four symmetric spiro-linked Ge−Ge single bonds with atom in A cannot allow the electron delocalization with the a distance of 2.485 Å. The average Ge−Ge bond lengths in neighboring π-bonds, and that is out of the question to alleviate 2− 24 − germanium cluster anions (Ge6 ) are a little bit longer than the ring strain. Six other low-lying orbitals (Figure 3, HOMO 2, the Ge−Ge single bonds in 1. The N−Ge bond lengths in 1, −3, −4, −8, −20, and −21) were found along Ge−Ge bonds and ranging from 1.885(5) to 1.930(5) Å, are similar to those in 3 the adjacent atoms connected with Ge atoms. These canonical and 4. The N−Si bond lengths (1.762(5)−1.777(5) Å) are very molecular orbitals exhibit clearly σ-delocalized characters across close to those in N-silyl compounds, for instance, the N−Si the spiro ring. We believe that the σ-delocalization effect here is − 25 fi π bonds in Ge[N(SiMe3)2]2 (1.749 1.757 Å). That means the responsible for the xedness of the Ge5 skeleton, while the 2 - ff steric congestion of triisopropylsilyl has less e ect on the delocalization in Ge3 aromatic rings should obviously reduce the elongation of N−Si bonds. magnitude of the ring strain. These σ and π double By upon further investigation of the molecular stability of 1, delocalizations in 1 enhance the stability of the entire molecule. we found altogether eight occupied molecular orbitals (MOs) In brief, this work provides an understanding of chemical fi related to the central Ge5 skeleton (Figure 3), which should be bonding and stability of the rst spiropentagermadiene 1, which was successfully synthesized via a reduction procedure of amido germylene chloride 3 by potassium. The structural characterization of 1 reveals a spiro-linked Ge5 skeleton with the singly bonded Ge−Ge distances of 2.50 Å (avg.), the doubly bonded Ge−Ge distances of 2.34 Å (avg.), and dihedral angle of 70.193°. This peculiar structure and the molecular stability of 1 were perfectly explained by DFT calculations and orbital analysis. Due to the σ delocalization of six MOs among all Ge atoms of 1, fi the Ge5 skeleton has the rm cluster structure. Additionally, the 2π-delocalized aromatic systems decrease the strain energy of two spiro Ge3 rings and increase the stability of molecule 1. ■ ASSOCIATED CONTENT *S Supporting Information The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/jacs.9b10946. Experimental procedures, characterizations, crystallographic analyses, and computational data, including Figures S1−S13 (PDF) X-ray crystallographic data for 1 (CCDC 1955890), 3 (CCDC 1955888), and 4 (CCDC 1955889) (CIF)

■ AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected] *[email protected] ORCID Zhengqiang Xia: 0000-0002-9342-8422 0000-0001-9929-9732 Figure 3. Selected canonical occupied molecular orbitals related to the Weiqun Shi: Sanping Chen: 0000-0002-6851-7386 central Ge5 skeleton of 1. Yaoyu Wang: 0000-0002-0800-7093 Anyang Li: 0000-0001-6634-842X filled with 16-skeleton electrons. The two highest occupied MOs Matthias Driess: 0000-0002-9873-4103 − (HOMO and HOMO 1) are quasi-degenerated and widely Wenyuan Wang: 0000-0003-0898-3278 delocalize on both sides of Ge3 rings with nodal planes Author Contributions − − ⊥ containing Ge1 Ge2 and Ge4 Ge5 bond axes. With the help Y.G. and Z.X. contributed equally. of the adaptive natural density partitioning (AdNDP) method,26 we performed an orbital analysis to obtain the distribution of Notes fi orbital localization. The analyzed result shows two delocalized The authors declare no competing nancial interest. π three-center two-electron (3c-2e) bonds on the two Ge3 rings in 1, respectively (Figure S13). In addition, the computed ■ ACKNOWLEDGMENTS nucleus-independent chemical shift (NICS)27 values for both This work was supported by the NSF of China (Grants − Ge3 rings are negative ( 13.8 ppm), which suggests the 21727805 and 21371141) and the Key Science and Technology π considerable inorganic aromaticity in two independent Ge3 2 Innovation Team of Shaanxi Province (Grant 2017KCT-37). systems. This is essentially different from spiropentadiene A,in The calculations were performed at the National Demonstration which the bonding orbitals arise from the mutual overlap of the Center for Experimental Chemistry Education (Northwest hybridized orbitals and are treated as localized. The spiro C University).

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19256 DOI: 10.1021/jacs.9b10946 J. Am. Chem. Soc. 2019, 141, 19252−19256