Chemistry of tricarbonylrhenium(I) chelates of azopyrimidines, azoimidazoles and anion radicals thereof

Panda, Bikash Kumar ; Sengupta, Suman ; Chakravorty, Animesh (2004) Chemistry of tricarbonylrhenium(I) chelates of azopyrimidines, azoimidazoles and anion radicals thereof Journal of Organometallic Chemistry, 689 (10). pp. 1780-1787. ISSN 0022-328X

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Official URL: http://linkinghub.elsevier.com/retrieve/pii/S00223...

Related URL: http://dx.doi.org/10.1016/j.jorganchem.2004.02.034

Abstract

The azopyrimidine and azoimidazole ligands (general abbreviations, RL) used in the present work are 2-(p-R-C6H4NN)C4H3N2, RLpm (R=H, Cl) and 2-(p-R-C6H4NN)-1-(Me)C3H2N2, RLim (R=Me, Cl), respectively. The reaction of Re(CO)5Cl with a slight excess of RL in boiling benzene has furnished blue-violet complexes of type Re(CO)3Cl(RL) which have been spectrally characterized. In Re(CO)3Cl(HLpm) and Re(CO)3Cl(ClLim) the Re-Nh, Re-Na distances are 2.173(6), 2.136(6) Å and 2.150(5), 2.166(5) Å, respectively (Nh and Na are heterocyclic and azo N atoms, respectively). Their N-N lengths (1.271(8), 1.281(7) Å) implicate relatively weak Re-azo(π) back-bonding. In the lattice of Re(CO)3Cl(HLpm), pair-wise C-HO hydrogen bonding between symmetry related molecules is present (CO; 3.264(9) Å, HO; 2.460(10) Å; C-HO; 130.6(5)°). The lattice of Re(CO)3Cl(ClLim) also consists of centrosymmetric dimers held by aromatic π-π stacking between parallely placed pendant aryl rings (centroidcentroid distance, 3.781(9) A). Extended Huckel calculations reveal that the LUMO of Re(CO)3Cl(RL) is ~60% azo in character. One-electron quasireversible electrochemical reduction occurs near -0.1 and -0.4 V vs. SCE in the cases of Re(CO)3Cl(RLpm) and Re(CO)3Cl(RLim), respectively. The redox orbital is believed to be to the above noted LUMO. Electrogenerated Re(CO)3Cl(RL∗-) underwent spontaneous solvolytic chloride displacement in MeCN furnishing Re(CO)3(MeCN)(RL∗-) which has been isolated. The latter in turn reacted with imidazole and triphenyl phosphine furnishing Re(CO)3(C3H4N2)(RL-) and Re(CO)3(PPh3)(RL-), respectively. The pattern of carbonyl stretching frequencies of these radical anion complexes is similar to that of Re(CO)3Cl(RL) but for shifts to lower frequencies by 10-40 cm-1. All the three radical anion systems are one-electron paramagnets (1.7-1.8 μB). The unpaired electron is primarily localized in a predominantly azo-π orbital. A small metal contribution (185, 187Re, I=5/2) is present and both Re(CO)3(MeCN)(RL∗-) and Re(CO)3(C3H4N2)(RL∗-) display six-line EPR spectra (A~28 G). The line shapes and intensities are characteristic of the presence of g-strain. In the case of Re(CO)3(PPh3)(RL∗-) seven nearly equispaced lines are observed due to virtually equal coupling with metal and 31P (I=1/2) nuclei. The g values of the radical species span the range 2.0033-2.0066. Reaction of Re(CO)5Cl and 2-(arylazo)pyrimidine/2-(arylazo)-1-methylimidazole, RL, has afforded Re(CO)3Cl(RL) (such as A) which has been structurally characterized. Electroreduction in presence of D has furnished the EPR-active azo anion radical system (S=1/2) Re(CO)3(D)(RL∗-) (D=MeCN, C3H4N2 and PPh3) as in B.

Item Type:Article
Source:Copyright of this article belongs to Elsevier Science.
Keywords:Tricarbonylrhenium(I) Chelates; Azoligands and Radical Anions Thereof; Azopyrimidines and Azoimidazoles
ID Code:6844
Deposited On:26 Oct 2010 05:24
Last Modified:29 Jan 2011 11:46

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