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

Abstract

A cyclovoltammetric investigation of tetraphenylcyclobuta-With an excess of the stronger reductant sodium amalgam the diene complexes [ML(C4Ph4)]+ [1: ML = NiCp; 2a: ML = cations 1+ and 2a+ are reduced to give cyclobutenyl com-Co(C,H,); 2b: ML = Co(mesitylene)] reveals a reversible replexes ML(q3-1,2,3,endo-4-C4HPh4) [S: ML = NiCp; 7: ML = duction to paramagnetic complexes 1 and 2a.b with a 19-elec-CO(C&,)]. 1 reacts with p-nitrobenzyl bromide to give 1 + Br tron count, and a subsequent irreversible reduction to anions and a p-nitrobenzyl addition product CpNi[q3-exo-4-(p-1-and 2a, b-with a 20-electron shell. Preparative reduction 02NC6H4CH2)-1,2,3,endo-4-C4Ph4] (11) while 2b reacts with by one equivalent of CpFe(HMB) (HMB = C6Me6) produces Me1 to give 2b . I and two isomeric methyl addition products the novel 19-electron complexes 1 and 2b. Complex 1 can also (q5-1,3,5,exo-6-C6H3Me4)Co(C4Ph4)] (12) and (mes)Co(q3-exo-4be obtained by treatment of 1 . Br with one equivalent of Me-1,2,3,endo-4-C4Ph4) (13). Sodium-amalgam reduction of the CoCp, at room temperature while reduction at -20 "C with tetramethylcyclobutadiene cations [(arene)Co(C4Me4)] + (4a: two equivalents of CoCp, affords a novel C -C-linked adduct arene = C6H,; 4 b arene = mesitylene) affords diamagnetic CpNi[p,q3: q4-(C4Ph4-C5H5)]CoCp (5). Above -10°C a slow dinuclear compounds of type 8 -10 with C-C linkages beequilibrium between 5 and its components 1 and CoCp, is tween C4Me4/C4Me4, arene/C4Me4, and arene/arene rings, observed with AH = 15 kJ/mol for the dissociation process. respectively. Cobaltocen[*], CoCp,, ist mit 19 Valenzelektronen das alteste Beispiel eines ElektronenuberschuB-Komplexes. Die Synthese und Reaktivitat weiterer ElektronenuberschuD-Komplexe ist Gegenstand aktueller ForschungL3]. In zahlreichen Arbeiten wird die reduktive Erzeugung von 19e-und

Electronic Spectra of the Tetraphenylcyclobutadienecyclopentadienylnickel(II) Cation and Radical

“…The parent 1 is known to be reversibly reduced to a 19-electron neutral radical at −740 mV vs NHE in DMF and −730 mV vs NHE in CH 2 Cl 2 . Our measurement yielded a similar half-wave reduction potential vs NHE for the bromide salt in CH 3 CN/0.1 M n -Bu 4 N + PF 6 – , −0.765 V. For the bromide of 1a in CH 3 CN/0.1 M n -Bu 4 N + PF 6 – , the value was −0.662 V.…”

“…We attempted to observe photoluminescence, but were unable to detect any. The parent is known to be reversibly reduced to a 19-electron neutral radical at -740 mV vs NHE in DMF 7 regions reacted with and 1a in CH CN with a bimolecular rate constant of 1 × 10 11 M -1 s -1 to produce the reduced species in ~10 ns, after which the spectra were stable for longer than 1 ms.…”

Electron-Transfer Properties of Cp*FeP5: Evidence for Dimerization Reactions following both Oxidation and Reduction

“…A considerable number of analogues of 3 are known. Sandwich complexes having 19 valence electrons have been shown to form similar structures through coupling at one of their cyclic hydrocarbons, representative examples being Cp 2 Rh, CpFe(arene), and (η 4 -C 4 R 4 ) complexes of CpNi and (arene)Co . There is also precedent, however, for the P 5 −P 5 coupled structure in the complex [(Cp‘ ‘Rh) 2 (P 5 −P 5 )(RhCp‘ ‘) 2 ] (Cp‘ ‘ = 1,3-di- tert -butylcyclopentadienyl).…”

Toward a Square Grid Polymer: Electrochemistry of Tentacled Tetragonal Star Connectors, C4R4−Co−C5(HgY)5, on Mercury

“…A derivative with four alkyl substituents on the Cb ring shows a reversible oxidation wave at +0.46 V vs SCE, and some more complicated structures with two Co centers have also been investigated, but once again, the reduction seems not to have been examined. A related cation, tetraphenylcyclobutadiene(benzene)cobalt(I), is reduced reversibly at −1.22 V vs SCE, then irreversibly at −1.75 V and finally again reversibly at −2.06 V, and the product is oxidized irreversibly at −0.23 V. A replacement of the four phenyl substituents with four methyl groups changes the reduction pattern; only one irreversible reduction at −1.92 V and one irreversible oxidation at −0.80 V are observed. The formation of a protonated cyclobutenyl complex was postulated as a reason for the irreversibility and for the appearance of an oxidation wave …”