Dinuclear Rhodium Complexes Containing the Diyne 1,4‐C 6 H 4 (C≡CH) 2 and the Isomeric Bisvinylidene 1,4‐C 6 H 4 (CH=C:) 2 as Bridging Units

The reaction of [RhCl(P i Pr 3 ) 2 ] ( 1 ) with 1,4‐C 6 H 4 (C≡CH) 2 at 0°C leads almost quantitatively to the formation of the bis(alkyne) complex [(P i Pr 3 ) 2 ClRh‐(HC≡C 6 H 4 ‐C≡CH)RhCl(P i Pr 3 ) 2 ] (2). At elevated temperatures (THF, 60°C) it rearranges to give the isomeric bis(vinylidene) complex [(P i Pr 3 ) 2 ClRh‐(=C=CH‐C 6 H 4 ‐CH=C=)RhCl(P i Pr 3 ) 2 ] (3). A one‐pot synthesis of 3 is also described. Treatment of either 2 or 3 with pyridine affords the bis(alkynyl)dihydrido compound [(P i Pr 3 ) 2 (py)Cl(H)Rh(‐C≡C‐C 6 H 4 ‐C≡C‐)Rh(H)Cl(py)(P i Pr 3 ) 2 ] ( 4 ) in which both metal centers are octahedrally coordinated. Whereas the reaction of 2 with NaC 5 H 5 produces the complex C s H 5 (P i Pr 3 )Rh(HC≡C‐C 6 H 4 ‐C≡CH)Rh(P i Pr 3 )C 5 H 5 ( 7 ), the bis(vinyl‐idene) isomer C 5 H 5 (P i Pr 3 )Rh(=C=CH‐C 6 H 4 ‐CH=C=)Rh(P i Pr 3 )C 5 H s ( 8 ) is obtained from 4 and NaC 5 H 5 . Electrophiles preferably attack the Rh=C bonds of 8 and thus on protonation with CF 3 CO 2 H the bis(vinyl) complex C 5 H 5 (P i Pr 3 )(CF 3 CO 2 )‐Rh( Z,Z ‐CH=CH‐C 6 H 4 ‐CH=CH)Rh(O 2 CCF 3 )(P i Pr 3 )C 5 H s ( Z‐9 ) is formed. In acetone solution, it rearranges to give the E isomer. Reaction of 8 with sulfur affords the bis(thioketene) complex C 5 H 5 (P i Pr 3 )Rh(≡ 2 ‐ C,S ; η 2 ‐ C,S ‐S=C=CH‐C 6 H 4 ‐CH=C=S)‐Rh(P i Pr 3 )C 5 H 5 ( 12 ), for which only one diastereomer is observed. All attempts to prepare mononuclear rhodium compounds containing the diyne HC≡C‐C 6 H 4 ‐G≡CH or the isomeric vinylidene: C=CH‐C 6 H 4 ‐G≡CH as ligand failed.