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

The reaction of 1,2‐dipiperidinoacetylene ( 1 ) with 0.5 equivalents of SnCl 2 or GeCl 2 ⋅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 SnCl 2 , 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 SnCl 2 or GeCl 2 ⋅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 [(Me 3 N)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 .