Cationic Rh I Complexes of Azulenes and Their Catalytic Activity on the Formation of Heptalene‐1,2‐dicarboxylates from Dimethyl Acetylenedicarboxylate and Azulenes

[Rh 1 (η 5 ‐azulene)(cod)] + BF   4 −complexes 3a–g (cod = ( Z,Z )‐cycloocta‐1,5‐diene) have been synthesized by reaction of [Rh 1 (cod)] + BF   4 −in THF with the corresponding azulenes 1a–g ( Table 1 ). The structure of [Rh 1 (cod)(η 5 ‐guaiazulene)] + BF   4 −( 3a ) has been determined by X‐ray diffraction analysis ( Fig. 1 and 2 ). The Rh‐atom is oriented above the five‐membered ring of the azulene with almost equal RhC distances to all five C‐atoms of the ring. The ( Z,Z )‐cycloocta‐1,5‐diene ring occurs in two enantiomorphic distorted ( C 2 v → C 2 ) tub conformations in the crystals ( Fig. 3 ). In CDCl 3 solution, the cod ligand in the complexes 3 shows a dynamic behavior on the 1 H‐NMR time scale which is best explained by rotation of the cod ligand relative to the azulene ligands around an imaginary codRhazulene axis. The new complexes 3 catalyze the formation of heptalene‐1,2‐dicarboxylates 2 from dimethyl acetylenedicarboxylate (ADM) and the corresponding azulenes 1 just as effectively as [RuH 2 (PPh 3 ) 4 ] and the analogous [RhH(PPh 3 ) 4 ] complex in MeCN solution ( Table 3 ). On grounds of simplicity, 3 can be generated in situ , when [RhCl(cod)] 2 is applied as catalyst ( Table 3 ).