ISSN:
0947-6539
Schlagwort(e):
carbenes
;
CH activation
;
dinuclear complexes
;
sandwich complexes
;
spin equilibria
;
Chemistry
;
General Chemistry
Quelle:
Wiley InterScience Backfile Collection 1832-2000
Thema:
Chemie und Pharmazie
Notizen:
The dinuclear μ-cyclopentadienylidene complexes [(C5R5)Co(μ-C5H4)-Co(C2H4)(C5R'5)] (5) [5aa (R = R' = H), 5ab (R = H, R' = Me), 5ac (R = H, R'5 = H4Me), 5ba (R = Me, R' = H), 5bb (R = R' = Me) and 5da (R5 = Me4Et, R' = H)] were synthesised from [(C5R5)Co(η4-C5H6)] (4) [4a (R = H), 4b (R = Me), 4d (R5 = Me4Et)] and [(C5R'5)Co(C2H4)2] (1) [1a (R' = H), 1b (R' = Me) and 1c (R'5 = H4Me)]. In these reactions, both CH bonds of the methylene group of coordinated cyclopentadiene are activated under mild conditions. Substitution of the ethylene ligand in 5 by L leads to the carbonyl, isocyanide, phosphine, and phosphite derivatives [(C5R5)Co(μ-C5H4)Co(L)-(C5R'5)] [6aa, 6ab, 6ba, 6bb (L = CO), 7aa (L = tBuNC), 8aa, 8ab (L = PMe3), 9aa (L = PMe2Ph), 10aa (L = PMePh2) and 11aa (L = P(OMe)3)]. The crystal structures of 5aa, 5ab, 5ba, 6aa and 8aa have been determined. The experimental geometry is rationalised in terms of two limiting structures with μ-η4:η1 and μ-η5:η1 coordination of the bridging cyclopentadienylidene ligand. On the basis of the 18 valence electron rule, zwitterionic character is assigned to latter. This structure is preferred when L is an acceptor ligand, as in 6. In solution, 5-11 are fluxional with rapid rotation about the very short cobalt-carbene C bond. In addition the hindered rotation of the ethylene ligand in 5ab was studied by DNMR spectroscopy. In solution, a singlet-triplet equilibrium was established by variable-temperature NMR spectroscopy for 8aa. The temperature-dependent 1H NMR line shifts were analysed by means of an isotropic shift model to give 24≤ΔH° ≤ 32 kJ mol-1 and 45≤ΔS° ≤74 J mol-1 K-1 with the triplet state being preferred by entropy at higher temperatures.
Zusätzliches Material:
9 Ill.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1002/chem.19960021119
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