Synthesis and Reactivity of Cobalt Complexes with Pendant Nitrogen Functional Groups

Abstract

Several derivatives of cyclopentadienylcobalt complexes having a pendant amino-functionalised side chain (CpЈ) are described, the cyclopentadienyl ligands being (2-aminoethyl)cyclopentadienyl, (2-piperidinoethyl)cyclopentadienyl, 1-(2-piperidinoethyl)-2,3,4,5-tetraisopropylcyclopentadienyl and 2-picolylcyclopentadienyl. Chelation by the amino-functionalised side chain occurred when the Cp*-cobalt(I) dicarbonyl complexes 5, 6, 7 and 8 were oxidised by iodine, thereby forming the corresponding Cp*-cobalt(III) chelates 9, 10, 11 and 12, respectively, through the diiodocarbonyl intermediates 5Ј, 6Ј, 7Ј and 8Ј, respectively. The stabilities of the intermediates differ greatly due to the different amino functions in the pendant side chain. Structural studies of 9, 10 and 12 were carried out and the correlation of the I-Co-I

Chelate ring closing and opening behavior in cyclopentadienyl cobalt(III) complexes with pendant nitrogen functional group

“…The value of the enthalpy of activation for the chelate ring closure found for 2b (54.1 ± 5.8 kJ/mol) was still a little bit larger than that of 2a (45.2 ± 1.6 kJ/mol), indicating a larger energy barrier during the process of forming the corresponding chelator (g 5 :g 1 -C 5 H 4 C 2 H 4 NC 5 H 10 )CoI 2 (3b) compared to (g 5 :g 1 -C 5 H 4 C 2 H 4 NH 2 )CoI 2 (3a). This result agrees with the conclusion that 3b has a larger steric resistance compared to 3a deduced from their crystal structures and the COSY spectrum [15][16][17]. Our results are also consistent with the thermal property of some other d 6 metal carbonyl complexes during chelate ring closure [18][19][20].…”

“…And the lifetime or the stability of such corresponding monocarbonyl reaction intermediates 2a, 2b and 2c were compared with the persistence of this CO absorption band [15][16][17]. The CO stretching frequencies in tetrahydrofuran were similar to that obtained in dichloromethane reported previously [15]. The decay of this absorption band showed the pattern of the first-order reaction, and the plots of lnA(m CO ) versus time showed linear relationship (Fig.…”

“…The changing of this characteristic IR-active CO absorption band is a sign of the intramolecular displacement of the remaining carbonyl ligand by the pendant amino functional group. And the lifetime or the stability of such corresponding monocarbonyl reaction intermediates 2a, 2b and 2c were compared with the persistence of this CO absorption band [15][16][17]. The CO stretching frequencies in tetrahydrofuran were similar to that obtained in dichloromethane reported previously [15].…”

“…The N-functional groups in the pendant side chain of the metal complexes involved in this process are amine, piperidine and pyridine, respectively. The synthesis of the corresponding cobalt dicarbonyl compounds (g 5 -C 5 H 4 C 2 H 4 NH 2 )Co(CO) 2 (1a), (g 5 -C 5 H 4 C 2 H 4 NC 5 H 10 )Co(CO) 2 (1b) and (g 5 -C 5 H 4 CH 2 NC 5 H 4 )Co-(CO) 2 (1c) were described in elsewhere [15]. It is known that the chelators were formed via the diiodomonocarbonyl complexes (vide infra) when its precursor dicarbonyl metal complexes were oxidized by iodine.…”

“…Thus bidentate ligands of C 5 H 4 C 2 H 4 NH 2 , C 5 H 4 C 2 H 4 NC 5 H 10 and C 5 H 4 CH 2 NC 5 H 4 were synthesized. It was found that with such bidentate ligands, the binding ability of the donor nitrogen to the metal center was greatly altered and the chemical activity of these complexes varied greatly [15]. This might be attributed to small changes in electronic and steric effects around the donor by different substituent on it.…”