Abstract
The reaction of 1,2‐dipiperidinoacetylene (1) with 0.5 equivalents of SnCl2 or GeCl2⋅dioxane afforded the 1,2,3,4‐tetrapiperidino‐1,3‐cyclobutadiene tin and germanium dichloride complexes 2 a and 2 b, respectively. A competing redox reaction was observed with excess amounts of SnCl2, which produced a tetrapiperidinocyclobutadiene dication with two trichlorostannate(II) counterions. Heating neat 1 to 110 °C for 16 h cleanly produced the dimer 1,3,4,4‐tetrapiperidino‐3‐buten‐1‐yne (3); its reaction with stoichiometric amounts of SnCl2 or GeCl2⋅dioxane furnished the 1,3,4,4‐tetrapiperidino‐1,2‐cyclobutadiene tin and germanium dichloride complexes 4 a and 4 b, respectively. Transition‐metal complexes containing this novel four‐membered cyclic bent allene (CBA) ligand were prepared by reaction of 3 with [(tht)AuCl], [RhCl(CO)2]2, and [(Me3N)W(CO)5] to form [(CBA)AuCl] (5), [(CBA)RhCl(CO)2] (6), and [(CBA)W(CO)5] (7). The molecular structures of all compounds 2–7 were determined by X‐ray diffraction analyses, and density functional theory (DFT) calculations were carried out to rationalise the formation of 3 and 4 a.
Highlights
Diaminoacetylenes, or ynediamines, have been known since 1964, when Viehe and Reinstein reported the synthesis of 1,2bis(diethylamino)acetylene from 1,1-dichloro-2-fluoroethylene and lithium diethylamide.[1]
The 13C{1H} NMR spectra of 2 a/2 b exhibit three signals each for the quaternary carbon atoms at 89.8/82.2 (C1), 112.6/114.0 (C2) and 173.7/172.9 ppm (C2 + C4), which is in good agreement with the chemical shifts reported for the tin(IV) adduct VI, 78, 113, and 172 ppm,[12] and confirms the formation of four-membered rings with Cs-symmetry
The observation of the signal for the tin-coordinated carbon atom in 2 a at lower field (Dd = 12 ppm) compared with VI is in agreement with the trend observed for other systems, for example N-heterocyclic carbene (NHC) tin(II) and tin(IV) complexes.[22]
Summary
Diaminoacetylenes, or ynediamines, have been known since 1964, when Viehe and Reinstein reported the synthesis of 1,2bis(diethylamino)acetylene from 1,1-dichloro-2-fluoroethylene and lithium diethylamide.[1]. The stannane adduct VI was described as a tin-stabilised cyclobutadienyl system formed by [2+2] cycloaddition in the presence of the Lewis acid Me2SnCl2,[12] which resembles the reactivity described much earlier by Viehe for the cyclisation of bis(dimethylamino)acetylene upon protonation or alkylation with triethylammonium bromide or methyl iodide, respectively.[13a] reaction of the former, the hydrobromide species, with elemental bromine gave the tetrakis(dimethylamino)cyclobutenediylium dibromide [C4(NMe2)]Br2, which contains a rare example of an authenticated, albeit not structurally characterised “cyclobutadiene dication”.[14] Independently of the work reported by the Braunschweig group on the isolation of the tin(IV) adduct VI,[12] we have studied the reaction of dipiperidinoacetylene (1) towards germanium(II) and tin(II) chloride, the original goal being to produce 1:1 complexes for subsequent preparation of heavier Group 14 diaminocyclopropenylidene species of the type [(R2NC)2E] (E = Ge, Sn).[15] Instead, as we wish to report the zwitterionic 2:1 adducts 2 were isolated in a manner analogous to the formation of VI (Figure 1). The ligand properties of this novel four-membered carbenoid will be assessed and compared to other acyclic and cyclic bent allene (CBA) systems,[17,18,19,20] which have emerged as powerful ancillary ligands in organotransition-metal chemistry and homogeneous catalysis.[21]
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