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

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.


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] 2À structure with one localized and one allylic delocalized negative charge, [5] a trapezoid structure with 1,2localized 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 crystallographically by Sekiguchi et al. in 2000 (I, Figure 1). [12]Subsequently in 2004, the same group isolated [CBD] 2À derivatives of the heavier group 14 elements ([R 4 Si 4 ] 2À , II, and [R 4 Si 2 Ge 2 ] 2À , III; R=SiMe t Bu 2 ) [13] in which the central four-membered ring is significantly folded, and features two η 2 -1,3-coordinated potassium cations.These systems are thought to be non-aromatic on the basis of nucleus-independent chemical shift (NICS) calculations.
We have recently been interested in the use of strongly σdonating and sterically encumbered boryl ligands, À B(NDippCH) 2 for the stabilization of main group systems featuring unusual electronic or geometric structure, and unprecedented modes of reactivity. [14]14j] Here we show that a superficially similar tin compound K 2 [Sn 4 {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] 2À , but in contrast to its indium counterpart, is nonaromatic in nature.

Results and Discussion
The reduction of {(HCDippN) 2 B}Sn(IPrMe)Br (1; Dipp = 2,6-i Pr 2 C 6 H 3 ; 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 11 B NMR resonance at δ B = 49 ppm.The 1 H NMR spectrum suggests a lower symmetry for the ligand scaffold, showing four i Pr CH 3 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 K 2 [Sn 4 {B(NDippCH) 2 } 4 ] (2) (Scheme 1 and Figure 2).
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(SnÀ Sn) = 2.9018(9), 2.9010(6) Å, d(SnÀ B) = 2.306(5) Å), and the internal SnÀ SnÀ Sn angles being close to 90°(88.41(3)°).The structure is completed by two K + counter-ions positioned above and below the Sn 4 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 3.088-3.413Å).The Sn 4 unit is puckered, with each tin atom lying 0.24 Å out of the least-squares plane, such that the Sn 4 centroid-SnÀ B angles are non-linear (132.8°), and alternate boron atoms are positioned above/below the approximate Sn 4 plane.As a consequence each tin centre is pyramidalized, with the sum of the SnÀ SnÀ Sn and SnÀ SnÀ 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]n a broader sense, 2 represents the formal dimer of the radical anion [Sn 2 {B(NDippCH) 2 } 2 ] *À , the terphenyl analogue of which, [Sn 2 Ar Dipp 2 ] *À , has been reported by Power and coworkers (Ar Dipp = C 6 H 3 Dipp 2 -2,6). [16]From a synthetic perspective, it is noteworthy that 2 can also be formed by one-electron oxidation of [Sn 2 {B(NDippCH) 2 } 2 ] 2À [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 SnÀ B bond (cf.SnÀ C) and five-(rather than six-) membered central heterocycle cause the pendant Dipp groups to exert a smaller steric profile at the tin centre.
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 (r 2 -scan def2-TZVP level) reveal that π-symmetry orbitals in 2 are primarily defined by the in-phase HOMO-5 (analogous to Π 1 for [CBD] 2À ) and by a near-degenerate pair (HOMO and HOMO-1) of essentially equivalent form reminiscent of Π 2 /Π 3 for [CBD] 2À , but twisted due to the non-planar nature of the Sn 4 unit (Figure 3. The highest energy molecular orbital possessing SnÀ Sn σ-bonding character is the HOMO-2.Nucleus independent chemical shift (NICS) values have been calculated for 2 (NICS(0) = À 0.28, NICS(1) = À 3.17) and can be compared with the values calculated using the same method for 'moderately aromatic' K 2 [In 4 {B-(NDippCH) 2 } 4 ] (IV; NICS(0) = À 7.52, NICS(1) = À 8.61), implying that the degree of aromatic character in 2 is minimal.This  finding is consistent with the non-aromatic nature of the 6πelectron 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 zcomponent of the electron density at the ring critical points (RCPs) in tetra-indium system IV (À 0.087 e Å À 5 ) and tetra-tin compound 2 (+ 0.024 e Å À 5 ) also provide further evidence for their aromatic/non-aromatic nature.
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-d 6 solution was investigated.The product (3) precipitates as deep purple crystals and can be shown by X-ray crystallography to consist of a tetrahedral Sn 4 cluster [Sn 4 {B(NDippCH) 2 } 2 ] 2À featuring two boryl ligands bridging opposite SnÀ 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 1 H NMR) as (HCDippN) 2 BD.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] 2À system, which suggest that electron loss should be extremely facile for the gas-phase (cation-free) species. [3]he oxidation of 2 was also examined with the possibility of accessing related neutral (or even cationic) Sn 4 systems.14f] 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 [Sn 4 {B(NDippCH) 2 } 4 ] 2À framework.Reaction of 2 with benzoic acid in benzene solution leads to the formation of dark purple crystals of the doubly protonated product Sn 4 {B(NDippCH) 2 } 4 H 2 (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 119 Sn and 117 Sn nuclei (J 119SnÀ H = 53.9Hz, J 117SnÀ H = 45.6 Hz).The solid state structure shows a short Sn(1)À Sn(2) distance (2.6737(7) Å) and three longer SnÀ 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 6), 2.8911(7), 2.8729(7), 2.8184(6), 2.8414(6), 3.1108(8); SnÀ B1 2.737(6), 2.370(6); SnÀ B2 2.618(5), 2.468(5).Scheme 3. Oxidation of 2 leading to formation of a 1 : 2 mixture of Sn 6 {B(NDippCH) 2 } 4 and Sn{B(NDippCH) 2 } 2 . [15]rom 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 Si 2 Ge 2 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)À 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 Sn II precursor {(HCDippN) 2 B}Sn-(IPrMe)Br with 1.5 equivalents of potassium graphite leads to the generation of the cyclic tetra-tin tetra-boryl system K 2 [Sn 4 {B-(NDippCH) 2 } 4 ], a homo-metallic heavier analogue of the cyclo-butadiene 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 Sn 4 unit, respectively, while protonation generates the neutral dihydride 1,2-Sn 4 {B(NDippCH) 2 } 4 H 2 in a manner consistent with the predicted charge distribution.Interestingly, although the 6π-system [Sn 4 {B(NDippCH  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.
)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 SnÀ H bonds either side of the Sn 4 plane, and with the overall C 2 symmetry implied by solution phase NMR measurements.