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. 2023 Feb 27;29(20):e202300006. doi: 10.1002/chem.202300006

Synthesis of Homo‐Metallic Heavier Analogues of Cyclobutene and the Cyclobutadiene Dianion**

Xiongfei Zheng 1, Agamemnon E Crumpton 1, Andrey V Protchenko 1, Mathias A Ellwanger 1, Andreas Heilmann 1, Simon Aldridge 1,✉
PMCID: PMC10947146  PMID: 36594576

Abstract

The reduction of the boryl‐substituted SnII bromide {(HCDippN)2B}Sn(IPrMe)Br with 1.5 equivalents of potassium graphite leads to the generation of the cyclic tetratin tetraboryl system K2[Sn4{B(NDippCH)2}4], a homo‐metallic heavier analogue of the cyclobutadiene dianion. This system is non‐aromatic as determined by Nucleus Independent Chemical Shift Calculations (NICS(0)=−0.28, NICS(1)=−3.17), with the primary contributing resonance structures shown by Natural Resonance Theory (NRT) to involve a Sn=Sn double bond and 1,2‐localized negative charges. Abstraction of the K+ cations or oxidation leads to contraction or cleavage of the Sn4 unit, respectively, while protonation generates the neutral dihydride 1,2‐Sn4{B(NDippCH)2}4H2 (a heavier homologue of cyclobutene) in a manner consistent with the predicted charge distribution in the [Sn4{B(NDippCH)2}4]2− dianion.

Keywords: aromaticity, cyclobutadiene dianion, quantum chemical calculations, sub-valent compounds, tin

The reduction of {(HCDippN)2B}Sn(IPrMe)Br with 1.5 equivalents of KC8 generates the cyclic tetra‐tin tetra‐boryl system K2[Sn4{B(NDippCH)2}4], a homo‐metallic heavier analogue of the cyclobutadiene dianion. This system is non‐aromatic as determined by Nucleus Independent Chemical Shift Calculations, with the principal resonance structures shown to involve a Sn=Sn double bond and 1,2‐localized negative charges.

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Introduction

The concept of aromaticity is a key tool in understanding structure and reactivity in organic chemistry. [1] Extrapolation to the heavier elements of group 14 brings with it additional challenges relating not only to synthetic strategy, but also to the rationalization of geometric data in terms of electronic structure. [2] At a superficial level, Huckel's rule predicts that both the dianion and dication of cyclobutadiene (CBD) should be aromatic with 6π and 2π electrons, respectively. However, the potential aromaticity of these systems has attracted significant debate; doubly anionic 6π cyclic systems have been predicted to behave quite differently from neutral and singly charged analogues because of additional Coulombic repulsion.[ 3 , 4 ] A folded [CBD]2− structure with one localized and one allylic delocalized negative charge, [5] a trapezoid structure with 1,2‐localized negative charges and a C=C double bond, [6] and a planar delocalized structure stabilized by the coordination of two Li cations,[ 7 , 8 ] have all been evaluated via quantum chemical methods. Experimentally, there have been studies on the transient cyclobutadiene dianion [7] and its derivatives stabilized by ester or phenyl groups.[ 10 , 11 ] The first cyclic system featuring a delocalized 6π‐electron system was characterized crystallo‐graphically by Sekiguchi et al. in 2000 (I, Figure 1). [12] Subsequently in 2004, the same group isolated [CBD]2− derivatives of the heavier group 14 elements ([R4Si4]2−, II, and [R4Si2Ge2]2−, III; R=SiMe t Bu2) [13] in which the central four‐membered ring is significantly folded, and features two η2‐1,3‐coordinated potassium cations. These systems are thought to be non‐aromatic on the basis of nucleus‐independent chemical shift (NICS) calculations.

Figure 1.

Figure 1

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Dianionic derivatives of cyclobutadiene and their heavier analogues of relevance to the current study.

We have recently been interested in the use of strongly σ‐donating and sterically encumbered boryl ligands, −B(NDippCH)2 for the stabilization of main group systems featuring unusual electronic or geometric structure, and unprecedented modes of reactivity. [14] These include a germanium analogue of vinylidene {(HCDippN)2B}2Ge=Ge, [14h] and a 2π‐electron tetra‐indium dianion [In4{B(NDippCH)2}4]2− (IV, as the dipotassium salt) which shows a moderate degree of aromatic character. [14j] Here we show that a superficially similar tin compound K2[Sn4{B(NDippCH)2}4] can be accessed via controlled (stoichiometric) reduction of a boryltin(II) precursor. This system represents the first tin‐containing analogue of [CBD]2−, but in contrast to its indium counterpart, is non‐aromatic in nature.

Results and Discussion

The reduction of {(HCDippN)2B}Sn(IPrMe)Br (1; Dipp=2,6‐ i Pr2C6H3; IprMe=C{(N i PrCMe)2}) [15] with 1.5 equiv. of potassium graphite leads to the formation of a diamagnetic product characterized by a single 11B NMR resonance at δB=49 ppm. The 1H NMR spectrum suggests a lower symmetry for the ligand scaffold, showing four i Pr CH3 doublets and two CH septets, in addition to two mutually coupled doublets for the CH groups of the boryl heterocycle. Black crystals grown from toluene/pentane solution allow definitive characterization of the product to be achieved by X‐ray crystallography, and shows that it is the cyclic tetratin tetraboryl system K2[Sn4{B(NDippCH)2}4] (2) (Scheme 1 and Figure 2).

Scheme 1.

Scheme 1

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Reduction of NHC‐supported boryl‐tin bromide 1 with 1.5 equiv. of potassium graphite to generate cyclic tetratin tetraboryl system K2[Sn4{B(NDippCH)2}4] (2). ([B]={B(NDippCH)2}; Dipp=C6H3 i Pr2‐2,6).

Figure 2.

Figure 2

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Molecular structure of K2[Sn4{B(NDippCH)2}4] (2) in the solid state as determined by X‐ray crystallography. Hydrogen atoms omitted and selected groups shown in wireframe format for clarity; thermal ellipsoids drawn at the 40 % probability level. Key bond lengths (Å) and angles (°): Sn−Sn 2.9018(9), 2.9010(6); Sn−B 2.306(5); Sn⋅⋅⋅K 3.915, 3.510; Sn−Sn−Sn 88.41(3); centroid‐Sn−B 132.8.

The solid‐state structure of 2 features an approximately square array of four (symmetry‐related) tin atoms, with each metal centre being additionally bound to a single terminal boryl ligand (d(Sn−Sn)=2.9018(9), 2.9010(6) Å, d(Sn−B)=2.306(5) Å), and the internal Sn−Sn−Sn angles being close to 90° (88.41(3)°). The structure is completed by two K+ counter‐ions positioned above and below the Sn4 unit (K⋅⋅⋅Sn distances: 3.915 and 3.510 Å), which are sandwiched between the flanking Dipp aryl rings of diagonally opposite boryl ligands (with K⋅⋅⋅arene contacts in the range 3.088–3.413 Å). The Sn4 unit is puckered, with each tin atom lying 0.24 Å out of the least‐squares plane, such that the Sn4 centroid‐Sn−B angles are non‐linear (132.8°), and alternate boron atoms are positioned above/below the approximate Sn4 plane. As a consequence each tin centre is pyramidalized, with the sum of the Sn−Sn−Sn and Sn−Sn−B angles being 318.6°. By means of comparison, the corresponding angles in tetra‐silicon system II reported by Sekiguchi and co‐workers sum to 341/326° (for the two distinct silicon centres). [13]

In a broader sense, 2 represents the formal dimer of the radical anion [Sn2{B(NDippCH)2}2]⋅−, the terphenyl analogue of which, [Sn2ArDipp 2]⋅−, has been reported by Power and co‐workers (ArDipp=C6H3Dipp2‐2,6). [16] From a synthetic perspective, it is noteworthy that 2 can also be formed by one‐electron oxidation of [Sn2{B(NDippCH)2}2]2−[15] using trityl oxidants. From a structural viewpoint, the underlying difference between tetranuclear 2, and the dimeric terphenyl systems presumably relates to the smaller steric profile of the boryl ligand. The longer Sn−B bond (cf. Sn−C) and five‐ (rather than six‐) membered central heterocycle cause the pendant Dipp groups to exert a smaller steric profile at the tin centre.

Geometrically, the structure of 2 bears a close resemblance to the corresponding indium compound K2[In4{B(NDippCH)2}4] (IV, Scheme 1), which possesses four electrons fewer within a less puckered [In4{B(NDippCH)2}4]2− unit. [14j] In the indium system, the four (symmetry‐equivalent) metal centres are less significantly displaced from the least‐squares plane (0.12 Å), and Nucleus Independent Chemical Shift calculations are consistent with moderately aromatic character attributable to the two π‐electrons within the cyclic manifold. The corresponding unit in 2 could also give rise to Hückel aromaticity on the basis of the six π‐electron count, and we therefore set out to examine the electronic structure of 2 by quantum chemical methods.

CASSCF calculations (see Supporting Information) are consistent with the singlet ground state implied experimentally for 2 (with a singlet‐triplet gap of 0.50 eV); the associated HOMO‐LUMO separation is 1.29 eV. DFT calculations (r2‐scan def2‐TZVP level) reveal that π‐symmetry orbitals in 2 are primarily defined by the in‐phase HOMO‐5 (analogous to Π1 for [CBD]2−) and by a near‐degenerate pair (HOMO and HOMO‐1) of essentially equivalent form reminiscent of Π2/Π3 for [CBD]2−, but twisted due to the non‐planar nature of the Sn4 unit (Figure 3. The highest energy molecular orbital possessing Sn−Sn σ‐bonding character is the HOMO‐2. Nucleus independent chemical shift (NICS) values have been calculated for 2 (NICS(0)=−0.28, NICS(1)=−3.17) and can be compared with the values calculated using the same method for ′moderately aromatic′ K2[In4{B(NDippCH)2}4] (IV; NICS(0)=−7.52, NICS(1)=−8.61), implying that the degree of aromatic character in 2 is minimal. This finding is consistent with the non‐aromatic nature of the 6π‐electron silicon and germanium systems II and III, and presumably reflects the markedly pyramidal nature of the tin centres in 2. The contrasting signs of the curvature of the z‐component of the electron density at the ring critical points (RCPs) in tetra‐indium system IV (−0.087 e Å−5) and tetra‐tin compound 2 (+0.024 e Å−5) also provide further evidence for their aromatic/non‐aromatic nature.

Figure 3.

Figure 3

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Electron density surfaces of key molecular orbitals for K2[Sn4{B(NDippCH)2}4] (2) calculated by DFT (r2‐scan def2‐TZVP level): (left) HOMO; (right) HOMO‐5.

Given the potential structural role of the K+ ions in 2 (and related systems such as II and III) we wanted to probe the consequences of cation abstraction. As such, the reaction of 2 with two equiv. of 2.2.2‐cryptand in benzene‐d6 solution was investigated. The product (3) precipitates as deep purple crystals and can be shown by X‐ray crystallography to consist of a tetrahedral Sn4 cluster [Sn4{B(NDippCH)2}2]2− featuring two boryl ligands bridging opposite Sn−Sn edges, together with two [K(2.2.2‐crypt)]+ counter‐cations (Scheme 2 and Figure 4). 3 can be viewed as a nido cluster based on the Wade‐Mingos cluster electron counting rules (6 PSEPs), and results from the loss of two (formally charge neutral) boryl ligands – which are observed in situ (by 1H NMR) as (HCDippN)2BD. The formation of 3 from 2 under these conditions implies that the inclusion of the K+ cations is essential to the structural integrity of 2. At a broader level, this finding is consistent with computational studies of the [CBD]2− system, which suggest that electron loss should be extremely facile for the gas‐phase (cation‐free) species. [3]

Scheme 2.

Scheme 2

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Cation abstraction from 2 by 2.2.2‐crypt leading to formation of the nido cluster [K(2.2.2‐crypt)]2[Sn4{B(NDippCH)2}2] (3).

Figure 4.

Figure 4

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Molecular structure of the dianionic component of [K(2.2.2‐crypt)]2[Sn4{B(NDippCH)2}2] (3) in the solid state as determined by X‐ray crystallography. Cations and hydrogen atoms omitted and selected groups shown in wireframe format for clarity; thermal ellipsoids drawn at the 40 % probability level. Key bond lengths (Å): Sn−Sn 3.1279(6), 2.8911(7), 2.8729(7), 2.8184(6), 2.8414(6), 3.1108(8); Sn−B1 2.737(6), 2.370(6); Sn−B2 2.618(5), 2.468(5).

The oxidation of 2 was also examined with the possibility of accessing related neutral (or even cationic) Sn4 systems. However, in line with other reports of charge neutral systems of the stoichiometry [Sn{B(NDippCH)2}] n [15] (and in contrast to the related germanium systems, for which the digermavinylidene, {(HCDippN)2B}2GeGe, can be isolated), [14h] disproportionation is observed in the presence of a range of oxidizing agents, leading to the formation of a 1 : 2 mixture of the cluster Sn6{B(NDippCH)2}4 and the bis(boryl)stannylene (Scheme 3). [14f]

Scheme 3.

Scheme 3

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Oxidation of 2 leading to formation of a 1 : 2 mixture of Sn6{B(NDippCH)2}4 and Sn{B(NDippCH)2}2. [15]

Finally, we examined the behaviour of 2 in the presence of protic reagents, with a view to providing a route to the parent neutral cyclobutene analogue, and also as a probe of sites of electron density within the dianionic [Sn4{B(NDippCH)2}4]2− framework. Reaction of 2 with benzoic acid in benzene solution leads to the formation of dark purple crystals of the doubly protonated product Sn4{B(NDippCH)2}4H2 (4) in good (ca. 60 %) yield. 4 has been characterized by NMR spectroscopy and X‐ray crystallography (Scheme 4 and Figure 5). The (single) hydride signal appears at 4.25 ppm with two sets of satellites being resolved due to coupling to the 119Sn and 117Sn nuclei (J 119Sn−H=53.9 Hz, J 117Sn−H=45.6 Hz). The solid state structure shows a short Sn(1)−Sn(2) distance (2.6737(7) Å) and three longer Sn−Sn distances (2.8045(7), 2.8232(5), 2.8333(5) Å), consistent with a structure featuring a formal Sn=Sn double bond and three single bonds, respectively (Figure 5). The B(1)Sn(1)Sn(2)B(2) unit approaches coplanarity (torsion angle=28.1°), while the B(3)Sn(3)Sn(4)B(4) unit features a much wider torsion angle of 125.0°. While the location of the tin‐bound hydrogen atoms by X‐ray crystallography must be viewed critically, the alignment of the boryl groups at Sn(3) and Sn(4) is consistent with the projection of the two Sn−H bonds either side of the Sn4 plane, and with the overall C 2 symmetry implied by solution phase NMR measurements.

Scheme 4.

Scheme 4

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Top) Protonation of the tin core of K2[Sn4{B(NDippCH)2}4] (2), yielding Sn4{B(NDippCH)2}4H2 (4). Bottom) Major contributing resonance structures for [Sn4{B(NDippCH)2}4]2−.

Figure 5.

Figure 5

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Molecular structure of Sn4{B(NDippCH)2}4H2 (4) in the solid state as determined by X‐ray crystallography. Most hydrogen atoms omitted and selected groups shown in wireframe format for clarity; thermal ellipsoids drawn at the 40 % probability level. Key bond lengths (Å), angles (°) and torsions (°): Sn−Sn 2.6737(7), 2.8232(5), 2.8045(7), 2.8333(5); Sn−B 2.280(4), 2.273(3), 2.269(3), 2.262(4); Sn−Sn−Sn 91.01(2), 89.12(2), 88.18(2), 86.77(2); B1−Sn1−Sn2−B2 28.1; B3−Sn3−Sn4−B4 125.0.

From a synthetic perspective, protonation in this (1,2‐trans) fashion is consistent (i) with a 1,2‐localization of negative charge in 2 in a manner similar to that proposed by Sekiguchi and co‐workers for Si2Ge2 system III,[ 12 , 13 ] and (ii) with the steric bulk of the boryl substituents favouring an anticlinal rather than eclipsed conformation about the Sn(3)−Sn(4) bond. Consistently, Natural Resonance Theory (NRT) calculations show that the primary resonance contributions (>80 %) involve a Sn=Sn double bond and 1,2‐localized negative charges (Scheme 4); this model of electronic structure also provides a rationale for the non‐aromatic character of 2 determined by the NICS calculations.

Conclusions

In conclusion, we have shown that the controlled reduction of the boryl‐substituted SnII precursor {(HCDippN)2B}Sn‐(IPrMe)Br with 1.5 equivalents of potassium graphite leads to the generation of the cyclic tetra‐tin tetra‐boryl system K2[Sn4{B(NDippCH)2}4], a homo‐metallic heavier analogue of the cyclobutadiene dianion. This system is non‐aromatic, with the primary contributing resonance structures involving a Sn=Sn double bond and 1,2‐localized negative charges. Attempted abstraction of the K+ cations or oxidation lead to contraction or cleavage of the Sn4 unit, respectively, while protonation generates the neutral dihydride 1,2‐Sn4{B(NDippCH)2}4H2 in a manner consistent with the predicted charge distribution. Interestingly, although the 6π‐system [Sn4{B(NDippCH)2}4]2−, and 2π‐electron systems [In4{B(NDippCH)2}4]2− and (hypothetical) [Sn4{B(NDippCH)2}4]2+ conform to the Hückel 4n+2 stipulation for aromaticity, only the two‐electron systems appear to show aromatic character (for the [Sn4{B(NDippCH)2}4]2+ dication: NICS(0)=−9.46; NICS(1)=−11.76 (1)). [17] Such a finding is consistent with the key electronic structure‐defining role predicted for enhanced Coulombic repulsions in 6π electron systems bearing a double negative charge.[ 3 , 18 ]

Experimental Section

Selected synthetic and characterizing data are given here. Complete experimental data, representative spectra, details of quantum chemical calculations and CIFs relating to the X‐ray crystal structures can be found in the Supporting Information.

K2[Sn4{B(NDippCH)2}4] (2): A mixture of {(HCDippN)2B}Sn‐(IPrMe)Br (0.20 g, 0.26 mmol) and KC8 (0.055 g, 0.41 mmol) was dissolved/suspended in toluene (5 mL) at room temperature. The reaction mixture was stirred and monitored by NMR until most of the starting material turned into the desired product. The reaction mixture was then filtered into a Schlenk tube and concentrated. Pentane (5 mL) was then added and the tube was stored at 4 °C overnight to give black crystals suitable for X‐ray crystallography, which were isolated, washed with small amount of cold (−20 °C) pentane and dried in vacuo. Yield: 0.051 g, 36.4 %. Anal. Calc. for C52H72B2KN4Sn2: C 59.41 %, H 6.90 %, N 5.33 %; Meas.: C 59.58 %, H 7.27 %, N 5.34 %. 1H NMR (400 MHz, C6D6, 298 K): δH 0.64 (d, J HH=6.8 Hz, 12H, CH(CH3 )2 of Dipp), 1.15 (d, J HH=6.8 Hz, 12H, CH(CH3 )2 of Dipp), 1.39 (d, J HH=6.8 Hz, 12H, CH(CH3 )2 of Dipp), 1.62 (d, J HH=6.8 Hz, 12H, CH(CH3 )2 of Dipp), 3.45 (sept, J HH=6.8 Hz, 4H, CH(CH3)2 of Dipp), 3.60 (sept, J HH=6.8 Hz, 4H, CH(CH3)2 of Dipp), 6.04 and 6.36 (d, J HH=2.2 Hz, 4H, CH of boryl), 6.73–6.81 (m, 6H, ArH of Dipp⋅⋅⋅K), 7.20–7.32 (m, 6H, ArH of Dipp). 11B{1H} NMR (128 MHz, C6D6, 298 K): δB 49.4. 13C NMR (126 MHz, C6D6, 298 K) δC 24.8, 25.3, 25.8 and 26.2 (CH( C H3)2 of Dipp), 28.6 and 28.8 ( C H(CH3)2 of Dipp), 122.6 ( C H of boryl), 123.5 and 123.6 (m‐Ar of Dipp), 123.7 and 123.8 (p‐Ar of Dipp), 144.1 and 144.2 (o‐Ar of Dipp), 147.0 and 148.7 (i‐Ar of Dipp). UV‐Vis (methylcyclohexane): λmax=432 nm, ϵ=10,100 L mol−1 cm−1.

Sn4{B(NDippCH)2}4H2 (4): To a mixture of 2 (20 mg, 0.019 mmol) and PhCO2H (2.3 mg, 0.019 mmol) was added benzene (5 mL). The solution was stirred for 10 min and dried under vacuum to remove all volatiles. The residues were then extracted into pentane and concentrated to about half of the original volume, stored at −30 °C to afford dark purple small crystals which were suitable for crystallography. Yield: 12 mg, 62.3 %. 1H NMR (500 MHz, C6D6, 298 K): δH 0.40 (d, J HH=6.8 Hz, 6H, CH(CH3 )2 of Dipp), 0.57 (d, J HH=6.8 Hz, 6H, CH(CH3 )2 of Dipp), 0.99 (d, J HH=6.8 Hz, 6H, CH(CH3 )2 of Dipp), 1.10 (dd, J HH=6.8, 2.0 Hz, 12H, CH(CH3 )2 of Dipp), 1.21 (m, 48H, CH(CH3 )2 of Dipp), 1.31 (d, J HH=6.8 Hz, 6H, CH(CH3 )2 of Dipp), 1.38 (dd, J HH=6.8, 2.0 Hz, 12H, CH(CH3 )2 of Dipp), 2.53, 2.76, 2.78, 2.93, 3.09, 3.14, 3.31 and 3.48 (sept, J HH=6.8 Hz, 2H, CH(CH3)2 of Dipp), 4.25 (t, J HSn119=53.9 Hz, J HSn117=45.6 Hz, 2H, SnH), 6.03, 6.15, 6.16 and 6.24 (d, J HH=2.0 Hz, 2H, CH of boryl), 6.85 (d, J HH=7.6 Hz, 2H, p‐ArH), 7.07–7.19 (m, 6H, p‐ArH), 7.21–7.35 (m, 16H, m‐ArH). 11B{1H} NMR (128 MHz, C6D6, 298 K): δB 32.2 ([B]SnH), 44.9 ([B]Sn). 13C{1H} NMR (126 MHz, C6D6, 298 K): δC 23.8, 23.9, 24.4, 24.5, 25.0, 25.2, 25.4 and 25.6 (CH( C H3)2 of Dipp), 25.9, 26.2, 26.6, 26.7, 26.8, 27.0, 27.3, 27.5, 27.9, 27.9, 28.1, 28.4, 28.5, 28.6, 28.7 and 29.3, ( C H(CH3)2 of Dipp), 119.8, 122.8, 123.1 and 123.4 ( C H of boryl), 123.6, 123.7, 123.8, 123.9, (p‐Ar of Dipp), 124.0, 124.1, 124.2 and 125.1, (m‐Ar of Dipp), 139.8, 140.0, 140.2, 141.4, (o‐Ar of Dipp), 145.5, 145.7, 145.9, 146.2, 146.6, 146.8, 147.1 and 147.1 (i‐Ar of Dipp). UV‐Vis (methylcyclohexane): λmax=529 nm, ϵ=2,240 L mol−1 cm−1.

Conflict of interest

The authors declare no conflict of interest.

1.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supporting Information

CHEM-29-0-s001.pdf (2MB, pdf)

Acknowledgments

We thank the EPSRC Centre for Doctoral Training in Inorganic Chemistry for Future Manufacturing (OxICFM CDT, EP/S023828/1, studentship to A. E. C.), the Leverhulme Trust (RP‐2018‐246, studentship to A. H.) and the Alexander von Humboldt Stiftung (Feodor Lynen Fellowship to M. E.).

Zheng X., Crumpton A. E., Protchenko A. V., Ellwanger M. A., Heilmann A., Aldridge S., Chem. Eur. J. 2023, 29, e202300006.

**

A previous version of this manuscript has been deposited on a preprint server (https://chemrxiv.org/engage/chemrxiv/article‐details/636284efca86b8f5dccdbfce).

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

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Supporting Information

CHEM-29-0-s001.pdf (2MB, pdf)

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.

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