Addition of Ethylene or Hydrogen to a Main Group Metal Cluster under Mild Conditions

: Reaction the tin cluster Sn 8 (Ar Me6 ) 4 (Ar Me6 = C 6 H 3 -2,6-(C 6 H 3 -2,4,6-Me 3 ) 2 ) with excess ethylene or dihydrogen at 25 °C / 1 atmosphere yielded two new clusters that incorporated ethylene or hydrogen. The reaction with ethylene yielded Sn 4 (Ar Me6 ) 4 (C 2 H 2 ) 5 that contained five ethylene moieties bridging four aryl substituted tin atoms and one tin-tin bond. Reaction with H 2 produced a cyclic tin species of formula (Sn(H)Ar Me6 ) 4 which could also be synthesized by the reaction of {(Ar Me6 )Sn( μ -Cl)} 2 with DIBAL-H. These reactions represent the first instances of direct reactions of isolable main group clusters with ethylene or hydrogen under mild conditions. The products were characterized in the solid state by X-ray diffraction and IR spectroscopy and by multinuclear NMR and UV-Vis spectroscopy. Density functional theory calculations were performed to explain the reactivity of the cluster.

In 2005 it was shown that a main group molecule could react with dihydrogen at room temperature and atmospheric pressure. [1] Since then a wide variety of main group compounds have been investigated for their reactions with small molecules under mild conditions. [2][3][4][5] Such reactions are dependent on the existence of donor and acceptor orbitals of suitable symmetry and modest energy separation. [6] Thus, multiply bonded or unsaturated main group species have commonly been used. For example, heavier group 14 alkyne, Ar iPr 4EEAr iPr 4, and carbene, :E(Ar iPr 4) 2, analogues (E = Ge or Sn; Ar iPr 4 = C6H3-2,6-(C6H3-2,6-iPr2)2) react readily with hydrogen, ethylene and other small molecules under mild conditions. [7][8][9][10] Activation of small molecules can also be effected by stable carbenes such as :C(tBu)(iPr)2N [5,11] or frustrated Lewis pairs using a phosphine or related electron donor and B(C6F5)3 as the acceptor. [12] Several reactions have been shown to be reversible, which also has generated widespread interest. [13][14][15] The reactivity of these main group compounds toward small molecules can resemble that of transition metal complexes, and thus, may have use in catalytic applications. [16] The reactivity of main group clusters towards small molecules under ambient conditions has remained virtually unexplored. [17] There have been a few theoretical studies on dihydrogen activation by aluminum clusters [18] and the activation of ammonia-borane by a gallium nitrogen cage compound. [19] Main group molecular clusters are also of interest because the coordination of their constrained atoms may resemble that of atoms at elemental surfaces. [20][21][22][23] Herein we report the reactions of the tin cluster Sn8(Ar Me 6) 4 (Ar Me 6 = C6H3-2,6-(C6H2-2,4,6-Me3)2) [20] with excess ethylene or dihydrogen to afford the products 1 and 2 as depicted in Scheme 1. The product 1 shows that the initial cluster has absorbed five ethylene molecules. A reaction between the tin cluster and dihydrogen yields the tin hydride 2. Significantly, both reactions involve the loss of the unsubstituted tin atoms in the cluster. The syntheses of 1 and 2 are described as well as their characterization by NMR spectroscopy and X-ray crystallography. Computational investigations of model systems for 1 and 2 are also described. Compound 1 was synthesized by treating Sn8(Ar Me 6) 4 [20] in THF with ethylene under ambient conditions (Scheme 1). The initially dark purple mixture was stirred for 2.5 days at 25 °C to afford a dark red solution and a metallic precipitate, assumed to be elemental tin. After work-up, compound 1 was isolated in 14 % yield. Colorless crystals were grown from diethyl ether at 6 °C overnight. Compound 1 crystallizes in a triclinic P1̅ space group with two diethyl ether solvent molecules. [24] Figure 1 depicts the solid state structure of 1 and it can be seen that the initial tin cluster, Sn8(Ar Me 6) 4, has incorporated five ethylene molecules and the four unsubstituted tin atoms have been eliminated. One C2M4 moiety is disordered over two positions, each with 50 % occupancy and only one of these sites (C7A and C8A) is shown. Each tin is tetrahedrally coordinated and carries one Ar Me 6 ligand. Two tins, Sn(1) and Sn (2), are bound also to three carbon atoms from different ethylenes whereas Sn(3) and Sn(4) are bonded to two carbon atoms from bridging ethylenes as well as to each other. Thus there is one intact Sn-Sn bond (i.e. Sn(3)-Sn(4), 1, 2.8549(5) Å), similar in length to the ethylene bridged tin-tin single bond in Ar iPr 4Sn(μ 2:η 1 :η 1 -C2H4)2SnAr iPr 4, formed by reversible addition of ethylene to the distannyne Ar iPr 4SnSnAr iPr 4. [3] The average Sn-C(ethylene) and ethylene C-C bond lengths are 2.177 Å and 1.527 Å, respectively. These are normal for Sn-C and C-C single bonds. [25,26] Interligand angles at the tin atoms are near 109.5° indicating tetrahedral geometry although disorder at the C7-C8 moiety and Sn-Sn bond cause deviation from the ideal value. The 1 H and 13 C NMR spectra of compound 1 indicate the presence of symmetry in the molecule in solution as only two unique sets of signals corresponding to Ar Me 6 ligand environments are observed. The 119 Sn NMR spectrum reveals two signals at 336.0 [Sn(3) and Sn(4)] and 1.1 ppm [Sn(1) and Sn (2)], indicating two tin environments whose chemical shifts are consistent with reported values. [8] The reaction of Sn8(Ar Me 6) 4 with excess of H2 was conducted similarly and yielded the tetrameric tin(II) hydride 2 ( Figure 2) in low yield. It was characterized by single crystal X-ray diffraction. [24] The low yield is due probably to the reaction conditions which required mild heating in THF, and which decompose the majority of the starting cluster with deposition of elemental tin. Hence, the tin hydride 2 was synthesized in a more straightforward manner by the reaction of the precursor {(Ar Me 6)Sn(μ-Cl)} 2 [27] with diisobutylaluminum hydride (DIBAL-H) (Scheme 1). A solution of DIBAL-H in hexanes was added dropwise to a diethyl ether solution of {(Ar Me 6)Sn(μ-Cl)} 2. The initially yellow solution turned orange and after workup compound 2 was collected as yellow crystals in 21 % yield.
Compound 2 crystallizes in an orthorhombic Fddd space group. The asymmetric unit consists of a Sn(H)(Ar Me 6) moiety and 0.5 Et2O, with the rest of the molecule generated by symmetry. It consists of a puckered ring of four tin atoms each σbonded to a hydrogen and an Ar Me 6 ligand in addition to the neighboring tins. The tin atoms are tetrahedrally coordinated. The overall tetrameric structure may be contrasted with that of the more sterically encumbered hydrogen bridged dimer {Ar iPr 4Sn(μ-H)} 2 [28] or that of the asymmetric stannylstannylene Ar iPr 8SnSn(H) 2Ar iPr 8 [29] and related species. [30] The orientation of the hydrogens alternate on either side of the Sn4. The Sn-Sn bond lengths (2.8433(4) and 2.8050(3) Å) are slightly shorter than the Sn-Sn single bonds in the other terminal tin hydrides, [29,30] but within the range of typical Sn-Sn single bonds.
There are a few structures in the literature that incorporate a Sn4 ring, but none have Sn-H bonds. The known species have formulas Sn4L8 (L = alkyl ligand) or Sn4L′4 (L′ = bidentate ligand). [31][32][33]   The reactivity of the tin cluster towards H2 and ethylene was investigated computationally using the PBE0 hybrid functional [35][36][37][38] with def2-TZVP basis sets. [38] A model compound Sn8Ph4 was used to lower computational cost. The calculations show that the reaction of dihydrogen with the Sn8Ph4 cluster yields an addition product Sn8(H2)Ph4 (Figure 3) initially, whose formation is thermodynamically disfavored in the gas phase (∆H = 4 kJ mol -1 ; ∆G = 39 kJ mol -1 ). The Gibbs energy or activation was found to be 134 kJ mol -1 , consistent with the fact that the reaction with H2 required mild heating and an excess of hydrogen gas to proceed. A similar addition product was located for the reaction of Sn8Ph4 with ethylene (see Figure 3). However, the gas phase reaction is exothermic and only slightly disfavored by entropy (∆H = -33 kJ mol -1 ; ∆G = 22 kJ mol -1 ). The Gibbs energy of activation is also significantly smaller, 86 kJ mol -1 , consistent with experimental observations. The frontier orbitals of Sn8Ph4 have both electron donating and accepting features (see SI), which rationalizes the relatively facile formation of Sn8(H2)Ph4 and Sn8(C2H4)Ph4. The modelling of mechanisms for the formation of {Sn(H)Ph}4, Sn4Ph4(C2H4)5, and elemental tin is beyond the scope of this work. However, the peculiar structure of 1 with an unreacted Sn-Sn bond prompted us to investigate the possible insertion of a sixth ethylene to Sn4Ph4(C2H4)5 to form Sn4Ph4(C2H4)6. The structures of ethylene, Sn4Ph4(C2H4)5 and Sn4Ph4(C2H4)6 were optimized (see SI) and their energies compared, which revealed that the addition of a sixth equivalent of ethylene is thermodynamically favored (∆G = -12 kJ mol -1 ) though the reaction could be prevented by kinetic factors. Unfortunately, we could not locate a transition state for the addition of the sixth ethylene molecule, thus the magnitude of the activation barrier remains unknown. In summary, we have described the syntheses and characterization data for two new insertion products of small molecules to a tin cluster under mild conditions. Further studies on the reactivity of Sn8(Ar Me 6) 4 towards other small molecules and attempts to prepare Sn4(Ar Me 6) 4(C2H4)6 are on-going.

Experimental Section
All manipulations were carried out under anaerobic and anhydrous conditions by using modified Schlenk line techniques under a dinitrogen atmosphere or in a Vacuum Atmospheres HE-43 drybox. Solvents were dried and stored over sodium. Physical measurements were performed under anaerobic and anhydrous conditions. 1 H, 13 C{ 1 H} and 119 Sn{ 1 H} NMR spectra were obtained on a Varian 400 or 600 MHz spectrometers and referenced to known standards. IR spectra were recorded as Nujol mulls between CsI plates on a Perkin-Elmer 1430 Infrared Spectrometer. UV-visible spectra were recorded as dilute toluene solutions in 3.5 mL quartz cuvette using an Olis 17 Modernized Cary 14 UV/Vis/NIR Spectrophotometer. Melting points were determined on a Meltemp II apparatus using glass capillaries sealed with vacuum grease, and are uncorrected. All starting materials were obtained from commercial sources and used as received. Sn8(Ar Me 6)4 [20] and {(Ar Me 6)Sn(μ-Cl)}2, [27] were prepared by literature procedures.

Synthesis of {Sn(H)(Ar Me 6)} 4 (2). Method A.
Sn8(Ar Me 6)4 (0.661g) was dissolved in ca. 25 mL of toluene and warmed to 60 °C. The mixture was stirred under an H2 atmosphere for 3.5 h and then cooled to room temperature. Stirring was continued overnight without any color change but a significant amount of grey precipitate (elemental tin) was formed. The precipitate was allowed to settle and the solution was filtered. The volume was reduced to ca. 10 mL and 5 mL of THF was added. The mixture was placed in a fridge. Crystals of compound 2 were collected from the mixture which contained mainly unreacted Sn8(Ar  [32] Melting point: >300 °C. IR in Nujol mull (cm -1 ) with CsI plates: 1845 (Sn-H stretching), 730 (Sn-H bending). UV-Vis (toluene, nm): 296 and 308.