Elektronenüberschuß‐Komplexe von Cobalt und Nickel mit Cyclobutadien‐Liganden: Synthese, Cyclovoltammetrie und Reaktivität

Electron‐Excess Complexes of Cobalt and Nickel with Cyclobutadiene Ligands: Syntheses, Cyclovoltammetry, and Reactivity [1] A cyclovoltammetric investigation of tetraphenylcyclobutadiene complexes [ML(C 4 Ph 4 )] + [ 1 : ML = NiCp; 2a : ML = Co(C 6 H 6 ); 2b : ML = Co(mesitylene)] reveals a reversible reduction to paramagnetic complexes 1 and 2a,b with a 19‐electron count, and a subsequent irreversible reduction to anions 1 ‐ and 2a,b ‐ with a 20‐electron shell. Preparative reduction by one equivalent of CpFe(HMB) (HMB = C 6 Me 6 ) produces the novel 19‐electron complexes 1 and 2b . Complex 1 can also be obtained by treatment of 1 · Br with one equivalent of CoCp 2 at room temperature while reduction at ‐20°C with two equivalents of CoCp 2 affords a novel C–C‐linked adduct CpNi[μ,η 3 :η 4 ‐(C 4 Ph 4 –C 5 H 5 )]CoCp ( 5 ). Above ‐10°C a slow equilibrium between 5 and its components 1 and CoCp 2 is observed with Δ H = 15 kJ/mol for the dissociation process. With an excess of the stronger reductant sodium amalgam the cations 1 + and 2a + are reduced to give cyclobutenyl complexes ML(η 3 ‐1,2,3, endo ‐4‐C 4 HPh 4 ) [ 6 : ML = NiCp; 7 : ML = Co(C 6 H 6 )]. 1 reacts with p ‐nitrobenzyl bromide to give 1 Br and a p ‐nitrobenzyl addition product CpNi[η 3 ‐exo‐4‐( p ‐O 2 NC 6 H 4 CH 2 )‐1,2,3, endo ‐4‐C 4 Ph 4 ] ( 11 ) while 2b reacts with MeI to give 2b · I and two isomeric methyl addition products (η 5 ‐1,3,5, exo ‐6‐C 6 H 3 Me 4 )Co(C 4 Ph 4 )] ( 12 ) and (mes)Co(η 3 ‐exo‐4‐Me‐1,2,3, endo ‐4‐C 4 Ph 4 ) ( 13 ). Sodium‐amalgam reduction of the tetramethylcyclobutadiene cations [(arene)Co(C 4 Me 4 )] + ( 4a : arene = C 6 H 6 ; 4b : arene = mesitylene) affords diamagnetic dinuclear compounds of type 8–10 with C–C linkages between C 4 Me 4 /C 4 Me 4 , arene/C 4 Me 4 , and arene/arene rings, respectively.