Cover Feature: Dimorpholinoacetylene and Its Use for the Synthesis of Tetraaminocyclobutadiene Species (Chem. Eur. J. 65/2022)

Abstract

A stable cyclobutadiene dication was isolated and structurally characterized as a bis(tribromide) salt. Unlike the folded minimum structure calculated for the parent C4H42+ system, this dication exhibits a planar geometry in the solid state. However, calculations reveal a nearly isoenergetic folded isomer, and detailed studies employing advanced NICS methods show that, counterintuitively, cyclobutadiene dications are more aromatic when puckered rather than planar. For more information on this topic and related applications of diaminoacetylenes, see the Research Article by M. Tamm and co‐workers (DOI: 10.1002/chem.202202737).

Iridium(I) 1,3,4,4‐Tetraamino‐1,2‐cyclobutadiene Complexes

“…The reaction of two equivalents of 1,3,4,4‐tetrapiperidino‐3‐buten‐1‐yne ( 1 a ) with [Ir(μ‐Cl)(COD)] 2 in THF at room temperature overnight afforded the complex [( L4 )IrCl(COD)] ( 2 ) as a bright yellow powder in 86 % yield after evaporation and washing with n ‐hexane/benzene (Scheme 2). Ring closure and formation of the four‐membered CBA ligand L4 were confirmed by NMR spectroscopy, with the carbon atoms of the allene moiety giving 13 C NMR signals at 179.3 (for C1 and C3) and 145.1 ppm (for C2), which are in excellent agreement with the values reported for the iridium complex [( L2 )IrCl(COD)] (179.2 and 138.6 ppm) and also for the rhodium complex [( L5 )RhCl(COD)] (179.1 and 135.8 ppm) [4,11] . Treatment of a solution of 2 in THF with carbon monoxide at room temperature afforded the dicarbonyl complex cis‐ [( L4 )IrCl(CO) 2 ] ( 3 ) as a yellow solid after evaporation and washing with n‐ hexane (Scheme 2).…”

“…FTIR spectra were recorded on a Bruker Vertex 70 in solution (THF). The 1 H, 11 B, 13 C, 19 F and 31 P NMR spectra were recorded on a Bruker AVII300 (300 MHz), a Bruker AVIIHD400 (400 MHz), a Bruker AVIIHD500 (500 MHz), and a Bruker AVII600 (600 MHz). The chemical shifts are expressed in parts per million (ppm), utilizing the residual solvent signal as the internal standard for 1 H and 13 C NMR spectra.…”

“…Ring closure and formation of the four-membered CBA ligand L4 were confirmed by NMR spectroscopy, with the carbon atoms of the allene moiety giving 13 C NMR signals at 179.3 (for C1 and C3) and 145.1 ppm (for C2), which are in excellent agreement with the values reported for the iridium complex [(L2)IrCl(COD)] (179.2 and 138.6 ppm) and also for the rhodium complex [(L5)RhCl(COD)] (179.1 and 135.8 ppm). [4,11] Treatment of a solution of 2 in THF with carbon monoxide at room temperature afforded the dicarbonyl complex cis-[(L4)IrCl(CO) 2 ] (3) as a yellow solid after evaporation and washing with n-hexane (Scheme 2). The low-field 13 C NMR signals at 186.3 (CO), 181.8 (C1/C3), 172.4 (CO), and 137.7 ppm (C2) can be assigned to the allenic and carbonyl carbon atoms, respectively, and are similar to those reported for cis-[(L2)IrCl(CO) 2 ] in the same solvent (C 6 D 6 ) at 185.9, 180.2, 172.4, and 129.2 ppm.…”

“…Their reactions with main group and transition metal complexes afforded the complexes [(L4)ML n ] (X = CH 2 ) and [(L5)ML n ] (X = O), including ML n = AuCl, RhCl(COD) and RhCl(CO) 2 (Scheme 1). [10,11] DFT studies suggest that this process occurs via metal coordination to the triple bond in enynes 1, followed by ring closure and CÀ C bond formation between the terminal carbon atoms C1 and C4. In principle, one might conclude that this pathway is similar to early attempts to generate derivatives of L1 from conjugated enynes (see above), with the presence of amino substituents leading to a highly polarized 1,2-cyclobutadiene ligand that is further stabilized by metal coordination in the complexes [(L4)ML n ] and [(L5)ML n ].…”