Dimerisation of Dipiperidinoacetylene: Convenient Access to Tetraamino‐1,3‐Cyclobutadiene and Tetraamino‐1,2‐Cyclobutadiene Metal Complexes

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.

It should be noted that attempts to improve the yield of 2a by variation of the stoichiometry and reaction time furnished grey metallicp recipitates,i np articularw hen an excess of tin(II) chloridea nd long reaction times (about 60 h) were employed. This indicatesthe formation of elemental tin through ac oncurrent redox process. Workupa fforded ay ellow solid and single crystalsw ere obtained by diffusion of n-hexane into as aturated THF solution. X-ray diffraction analysis revealed the formation of the tetrapiperidinocyclobutadiene dicationw ith two trichlorostannate(II) counterions (compound S1,F igures S7 and S8, Supporting Information). To the best of our knowledge,this compound represents the first structurally characterised salt containing ac yclobutadiene dication; [13] however,i ts isolation in pure form was hampered by the formation of am ixture of salts containing extendedc hloride-bridged polystannate anions of the type [Sn n Cl 2n + 1 ] À . [25] When an excess of GeCl 2 was used, another side product S2 was isolated and structurally characterized ( Figures S9 and S10), revealing trans-addition of two GeCl 3 units across the CÀCt riple bond in 1.T hese findings show that the stoichiometry and reaction conditions need to be carefully balanced to obtaino ptimum yields of 2a and 2b and to avoid ac ompeting one-or two-electron oxidation of the electron-rich diaminoacetylene 1.
SnCl 2 -a nd GeCl 2 -stabilised 1,2-cyclobutadienes Another side reactionw as also observed by serendipity:w hen diaminoacetylene 1,u suallys tored at À40 8Cu nder argon atmosphere, was left at room temperature for severaldays, as ignificant increase in viscosity of the liquid was observed. 1 HNMR spectroscopy revealed the formation of an ew,l ess symmetric species. Subsequently,w et ested different reaction conditions to isolatet he new compound in pure form;h eating 1 to 110 8Cw ithouta ny solvent for 16 hp rovedt ob et he best method. An orange-brownish resin was obtained and suspended by stirring in hexamethyldisiloxane to afford 1,1,2,4-tetrapiperidino-1-buten-3-yne( 3)a sabeige powder after filtrationi n high yield (92 %, Scheme1). Further investigations,i ncluding 2D-NMR spectroscopy and mass spectrometry,c onfirmed the formation of enyne 3 as the dimerisation product of 1.F or instance,t he carbon atoms along the C=CÀCCc hain give rise to four 13 CNMR signals at 157.8,1 02.6, 59.9, and 101.8 ppm, reflecting the asymmetric charge distribution within the C 4 chain.I na ddition, the molecular structure was determined unequivocally by X-ray diffraction analysiso fs ingle crystals obtained from cooling as aturated THF solution to À40 8C   Figure 3). Given that the piperidine ring at N1 is disordered over two positions, the structural parameters must be interpreted with caution. Nevertheless,t he expected connectivity is confirmed;t he molecule is slightly twisted,w ith the strongest deviation observed for the N3-C4-N4 unit, which subtends an interplanar angle of 25.68 with the enyne plane containing the carbon atoms C1-C4.
The reactions of enyne 3 with one equiv of SnCl 2 or GeCl 2 ·dioxane in THF at room temperature afforded yellowishwhite precipitates, and the dichlorostannylene and dichlorogermylene complexes 4a and 4b,r espectively,w ere isolated in good yield (approx. 77 %) by filtration, extraction with dichloromethane,a nd evaporation (Scheme 1). NMR spectroscopic characterisation indicated the formationo fC 2v -symmetric compounds with two different types of piperidine units. Notably,t he 13 CNMR spectra (in CDCl 3 )o f4a/4b show three signals each at 180.1/179.2 (C2 + C4), 148.1/137.2 (C1), and 97.7/95.9 ppm (C3) for the ring carbon atoms. The two lowfield signals can be assigned to the central NC 3 Nm oiety,w hichi si n excellent agreement with the chemical shifts of 185.7 and 151.6 ppm reported for the cyclic allene VII,o rr ather its lithium adduct. [16] Similar ranges are also found for transition-metal complexes of VII (see below). Strong p-conjugation across the diaminoallene NC 3 Nu nit and consequenth indered rotation aroundt he exocyclic CÀNb onds gives rise to broad 1 HNMR signals fort he 2,6-CH 2 hydrogen atoms of the two flankingp iperidyl substituents.
Single crystals of 4a·CHCl 3 and 4b were subjected to X-ray diffraction analysis; the molecular structure of the tin compound is presented in Figure 4, whereas that of the germanium derivativei ss hown in the Supporting Information (Figure S4). The four-membered rings are planar to within 0.03 .
Pertinents tructural data are given in Ta ble 1. With lengths of 2.2444(2)a nd 2.039(3) ,t he SnÀC1 and GeÀC1 bonds are shorter than those of N-heterocyclic carbenea dducts of the type (NHC)SnCl 2 and (NHC)GeCl 2 , [22] and are also clearly less than the values reportedf or similarc omplexes containing ab-normalN HC, cyclic alkyl(amino)c arbene (CAAC), and carbodiphosphorane ligands. [29] The carbon-carbon and carbon-nitrogen bond lengths within the N1-C2-C1-C4-N4 unit lie between the expected values for single and double bonds, indicating a high degree of p-conjugation and electron delocalisation. The tin and germanium atoms reside in acute trigonal-pyramidal environments with Cl-Sn-Cl and Cl-Ge-Cl angles of 93.310(17)8 and 97. 18(3)8,r espectively;t hese units are oriented in as taggered fashion towards the allene moieties in 4a and 4b.H owever,absolute N1-C2-C1-E and N4-C4-C1-E torsion angles of approximately 308 reveal twisted arrangements, in which the Ge and Sn atoms are displaced by 0.62 and 0.70 respectively from the C2-C1-C4 place.

Conclusions
The reaction of dipiperidinoacetylene (1)w ith SnCl 2 and GeCl 2 ·dioxane afforded the expected 1,3-cyclobutadiene tin and germanium dichloride complexes 2a and 2b.T he clean conversion of 1 into its dimer tetrapiperidino-3-buten-1-yne (3), as discoveredi nt his contribution, provideda ccess to the 1,2-cyclobutadiene isomers 4a and 4b.T hese complexes feature the tetrapiperidino-1,2-cyclobutadiene ligand as an ew four-membered addition to the family of cyclic bent allenes (CBA), and the cyclisationo f3 in the presence of Lewis acidic main-group and transition-metal complex fragments represents ag eneral entry to this class of CBA complexes,a sd emonstrated by the successful preparation of the gold(I), rhodium(I) and tungsten(0) complexes 5-7.T he strong electron-donating ability indicated by IR spectroscopy hints at the potential of this carbenoid ligand for applications in homogeneous catalysis, with the obviousp ossibilities of tuning this system by variation of the diaminoacetylene precursor or by postfunctionalisation at the terminal aminal group.