Unusual Reduction of Ammonium Heptamolybdate to Novel Molybdenum(IV)-Stabilized Azo Anion Radical Complexes Anasuya Sanyal, ² Priyabrata Banerjee, ² Gene-Hsiang Lee, ‡ Shie-Ming Peng, ‡ Chen-Hsiung Hung, § and Sreebrata Goswami* ,² Department of Inorganic Chemistry, Indian Association for the CultiVation of Science, Kolkata 700 032, India, Department of Chemistry, National Taiwan UniVersity, Taipei, Taiwan, Republic of China, and Department of Chemistry, National Changhua UniVersity of Education, Changhua, Taiwan 500, Republic of China Received March 15, 2004 In an unusual reaction of the polyacid, ammoniumheptamolybdate tetrahydrate ((NH 4 ) 6 [Mo 7 O 24 ]‚4H 2 O), and the ligand, 2-[(arylamino)phenylazo]pyridine (general abbreviation HL), in the presence of PPh 3 afforded the brown oxo free molybdenum complexes of type [Mo(L) 2 ] in high yields (ca. 80%). The reaction occurs smoothly in ethanol. It is slow on a steam bath (25 h) but is complete in about an hour in a microwave oven. X-ray structures of two representatives are reported. In these complexes the ligand acted as a tridentate ligand using its pyridyl(N), azo(N), and the deprotonated amine(N), respectively. The geometry is meridional, and the relative orientations within the coordinated pairs of nitrogens are cis, trans, and cis, respectively. Bond length data of the coordinated ligands are consistent with a Mo IV [L • ] 2 ([L • ] 2- ) azo dianion radical formed by one electron reduction of the deprotonated anionic ligand, [L] - ) description. For example, the N-N lengths (1.349(5)-1.357(2) Å) in these complexes are appreciably longer than that (1.246(3) Å) in the uncoordinated and protonated salt of a representative ligand, [H 2 L d ]ClO 4 . The N-N lengths, however, correspond well with metal complexes of the ligand containing azo ion radical. The complexes are diamagnetic and showed highly resolved 1 H NMR and 13 C NMR spectra. The two coordinated ligands in these are magnetically equivalent, and resonances for only one ligand were observed in their spectra. Diamagnetism in the present molybdenum complexes is attributed to strong antiferromagnetic coupling between Mo IV (4d 2 ) and the two planar radical [L • ] 2- ligands. The complexes display multiple redox responses. The ESR spectrum of electrogenerated [1a] - showed a characteristic spectrum for Mo(III) with weak hyperfine lines due to the presence of molybdenum isotopes having nonzero nuclear spin. Visible range multiple charge transfer transitions in these complexes are ascribed to ligand-to-metal transitions. Introduction There has been considerable interest in the chemistry of polyoxomolybdates 1 (molybdenum-oxygen cluster anions) for several reasons including their structural diversity 2 and catalytic action. 3 These compounds contain oxygen atoms that are multiply bonded to molybdenum in the highest oxidation state and, in principle, are capable of transferring oxygen atom(s) to reductants such as phosphines, sulfides, and alkenes, etc., as these occur in several monometallic oxo-molybdenum(VI) complexes. 4,5 The above reactions are important in the context of industrial processes and biological processes such as enzyme catalysis. Our literature survey has revealed that examples of the chemical reduction * To whom correspondence should be addressed. E-mail: icsg@ mahendra.iacs.res.in. Fax: +91 33 2473 2805. ² Indian Association for the Cultivation of Science. ‡ National Taiwan University. § National Changhua University of Education. (1) (a) Pope, M. T.; Mu ¨ller, A. Angew. Chem., Int. Ed. Engl. 1991, 30, 34. (b) Pope, M. T. Isopolyanions and Heteropolyanions. In Compre- hensiVe Coordination Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds.; Pergamon Press: New York, 1987; Vol. 3, Chapter 38. (c) Day, V. W.; Klemperer, W. G. Science 1985, 228, 533. (2) Liu, T.; Diemann, E.; Li, H.; Dress, A. W. M.; Mu ¨ller, A. Nature 2003, 426, 59. (3) Piepgrass, K.; Pope, M. T. J. Am. Chem. Soc. 1989, 111, 753 and references therein. (4) (a) Holm, R. H. Chem. ReV. 1987, 87, 1401. (b) Holm, R. H.; Kennepohl, P.; Solomon, E. I. Chem. ReV. 1996, 96, 2239. (c) Hille, R. Chem. ReV. 1996, 96, 2757. (d) Seymore, S. B.; Brown, S. N. Inorg. Chem. 2000, 39, 325. (e) Tucci, G. C.; Donahue, J. P.; Holm, R. H. Inorg. Chem. 1998, 37, 1602. Inorg. Chem. 2004, 43, 7456-7462 7456 Inorganic Chemistry, Vol. 43, No. 23, 2004 10.1021/ic049671e CCC: $27.50 © 2004 American Chemical Society Published on Web 10/16/2004 Downloaded by NATIONAL TAIWAN UNIV on August 19, 2009 Published on October 16, 2004 on http://pubs.acs.org | doi: 10.1021/ic049671e