Dinuclear Rhodium Complexes Containing the Diyne 1,4-C6H4(C≡CH)2 and the Isomeric Bisvinylidene 1,4-C6H4(CH=C:)2 as Bridging Units

The reaction of RhCl(PiPr₃)₂] ( 1 ) with 1,4-C₆H₄(C≡CH)₂ at 0°C leads almost quantitatively to the formation of the bis(alkyne) complex (PiPr₃)₂ClRh-(HC≡C₆H₄-C≡CH)RhCl(PiPr₃)₂] (2). At elevated temperatures (THF, 60°C) it rearranges to give the isomeric bis(vinylidene) complex (PiPr₃)₂ClRh-(=C=CH-C₆H₄-CH=C=)RhCl(PiPr₃)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 (PiPr₃)₂(py)Cl(H)Rh(-C≡C-C₆H₄-C≡C-)Rh(H)Cl(py)(PiPr₃)₂] ( 4 ) in which both metal centers are octahedrally coordinated. Whereas the reaction of 2 with NaC₅H₅ produces the complex C_sH₅(PiPr₃)Rh(HC≡C-C₆H₄-C≡CH)Rh(PiPr₃)C₅H₅ ( 7 ), the bis(vinyl-idene) isomer C₅H₅(PiPr₃)Rh(=C=CH-C₆H₄-CH=C=)Rh(PiPr₃)C₅H_s ( 8 ) is obtained from 4 and NaC₅H₅. Electrophiles preferably attack the Rh=C bonds of 8 and thus on protonation with CF₃CO₂H the bis(vinyl) complex C₅H₅(PiPr₃)(CF₃CO₂)-Rh(Z,Z-CH=CH-C₆H₄-CH=CH)Rh(O₂CCF₃)(PiPr₃)C₅H_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 C₅H₅(PiPr₃)Rh(≡²-C,S; η²-C,S-S=C=CH-C₆H₄-CH=C=S)-Rh(PiPr₃)C₅H₅ ( 12 ), for which only one diastereomer is observed. All attempts to prepare mononuclear rhodium compounds containing the diyne HC≡C-C₆H₄-G≡CH or the isomeric vinylidene: C=CH-C₆H₄-G≡CH as ligand failed.