1,3‐Heterobinuclear Cyclobutenylidene Complexes with an Exocyclic CC bond — Synthesis, Structure, and Spectroscopic Properties

Pentacarbonyl(allenylidene)chromium and ‐tungsten, [(CO) 5 MCCCR 2 ] (M=Cr, R=C 6 H 4 CH 3 ‐ p ( 1a ), C 6 H 4 OCH 3 ( 1b ); M=W, R=C 6 H 4 CH 3 ‐ p ( 1c ), react with the alkynyl complexes [Cp(CO) 2 FeCCR′] (R′ = n Bu ( 2 ), Ph ( 4 )), [Cp * (CO) 2 FeCC n Bu] ( 6 ), [Cp(CO){P(OMe) 3 }‐FeCC n Bu] ( 8 ) and [Cp(PPh 3 )NiCC n Bu] ( 10 ) by cycloaddition of the CC bond of the alkynyl complexes to the C α =C β bond of the allenylidene ligand to give 3a–c ), ( 5 ), ( 7 ), and respectively. The cycloaddition is highly selective. The spectroscopic data of these 1,3‐heterobinuclear complexes indicate a delocalized π‐system (M=Cr, W; Y=[Fe(CO) 2 Cp], [Fe(CO)(P{OMe} 3 )Cp], [Fe(CO) 2 Cp * ], [Ni(PPh 3 )‐Cp]). X‐ray structural analyses of the compounds 3a, 9a , and 11c reveal a “butterfly” conformation of the four‐membered ring. Its puckering angle is 142° ( 3a ), 152° ( 9a ), and 151° ( 11c ). All Complexes exhibit a moderate to strong negative solvatochromic behavior. The extent of the solvatochromic effect depends on [(CO) 5 M] and Y and considerably decreases when C=CR 2 is replaced by e.g. CMe 2 .