DALTON COMMUNICATION J. Chem. Soc., Dalton Trans., 1999, 659–661 659 Vanadium(V) complexes based on a bis(pyridine)-imine ligand (HL); synthesis and crystal structure of a dioxovanadium(V) complex involving a ligand cyclisation Alette G. J. Ligtenbarg,a Anthony L. Spek,†b Ronald Hage *c and Ben L. Feringa *a a Department of Organic and Inorganic Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands. E-mail: feringa@chem.rug.nl b Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, University of Utrecht, Padualaan 8, 3584 CH Utrecht, The Netherlands c Unilever Research Laboratory, Olivier van Noortlaan 120, 3133 AT, Vlaardingen, The Netherlands Received 4th December 1998, Accepted 18th January 1999 Synthesis and characterisation of two novel dioxovanadium( V) complexes based on (2-hydroxybenzylidene)- di(2-pyridin-2-yl)methylamine (HL) and 3-(2-hydroxyphenyl)- 1-pyridin-2-yl-imidazo-[1,5-a]-pyridine (HL9), which is the result of a vanadium-mediated oxidative cyclisation of HL, are reported.The notion that vanadium plays an important role in various biological systems has invoked great interest in the coordination chemistry of vanadium.1 Vanadium(V) complexes have been found to act as catalysts in oxidation reactions of various substrates using peroxides.2 The catalytic oxidation by SchiV- base complexes of vanadium with dioxygen has also been reported.3 During our investigation on the catalytic properties of V(V) complexes, we synthesised novel dipyridylmethyl amine SchiV base dioxovanadium(V) complexes and found a vanadium-mediated oxidative cyclisation.4 In the course of this work, a related iron mediated oxidative cyclisation of a polypyridine ligand has been published by Meunier et al.5 However, in our case the obtained cyclised product was not released from the metal, but remained coordinated resulting in a stable dioxovanadium( V) complex which was completely characterised by X-ray analysis, ES-MS spectrometry, 1H and 51V NMR, UV-Vis and IR measurements.(2-Hydroxybenzylidene)di(2-pyridin-2- yl)methylamine (HL) was prepared starting from dipyridin-2- yl-methylamine 6 and salicylaldehyde (Scheme 1) in 93% yield. Complexation of HL with triisopropoxyvanadium(V) oxide [VO(OiPr)3] in MeOH under reflux aVorded complex 1 in 39% yield.7 Yellow crystals suitable for X-ray analysis were obtained by evaporation of a MeOH–EtOH solution (1: 1) of 1.‡ An ORTEP plot is shown in Fig. 1. Complex 1 contains a dioxovanadium( V) moiety. The vanadium center is pentacoordinated by two nitrogen atoms [2.173(3) and 2.090(3) Å with a bond angle between N(1)–V(1)–N(3) of 72.87(12)8], one oxygen atom of the deprotonated phenolic moiety [1.888(3) Å and a bond angle between N(1)–V(1)–O(1) of 81.448] and two oxo groups [1.610(4), 1.632(4) Å]. The V]] O distances are nearly equal and typical for dioxovanadium(V) complexes in which the oxygens are not involved in hydrogen bonding.8 The bond angle between the oxo groups and the vanadium center is 109.02(16)8. We assumed that initially, upon addition of VO(OiPr)3 to a solution of L in MeOH, complex 2 is formed.In fact, vanadium(V) complex 2 of the uncyclised SchiV base ligand can be obtained in 60% yield when the reaction of L with VO(OiPr)3 is performed in EtOH at 0 8C under an atmosphere of argon. Unfortunately, no crystals suitable for X-ray analysis were obtained.The UV spectrum of 2 in acetonitrile exhibits three bands at † Address correspondence concerning crystallography to this author. 212 nm (e = 1.89 × 104 M21 cm21), 266 (1.23) and 384 (0.23). The spectrum resembles the one recorded in acetonitrile of HL [213 nm (e = 2.43 × 104 M21 cm21), 262 (1.95) and 319 (0.45)]. The UV spectrum of 1 in acetonitrile, however, exhibits four bands at 212 nm (e = 0.89 × 104 M21 cm21), 304 (0.25), 343 (0.36) and 417 (0.24).The distinction between complexes 1 and 2 was further corroborated using electron spray ionisation mass spectrometry (ES-MS), UV-Vis, IR, 1H and 51V NMR measurements. ES-MS spectra for 1 in acetonitrile show peaks at m/z 369.8 and 739.3 which can be attributed to Scheme 1 Synthesis of HL and complexes 1 and 2. Reagents and conditions: (i) salicylaldehyde, rt, MeOH, 93%; (ii) VO(OiPr)3, MeOH, reflux, air, 5 min, 39%; (iii) VO(OiPr)3, EtOH, 0 8C, argon, 2 h, 60%.660 J.Chem. Soc., Dalton Trans., 1999, 659–661 {VO2L9 1 1H} and {2VO2L9 1 1H}, respectively. In addition, electron ionisation mass spectrometry (EI-MS) measurements show, besides the parent peak at m/z 369, a peak at m/z 287 which corresponds to the mass of free HL9. In the ES-MS spectrum of 2 in acetonitrile peaks were observed at m/z 372.0 {VO2L 1 H} and 743.4 {2VO2L 1 H}. Also the 1H NMR spectra of complexes 1 and 2 clearly indicate the diVerence between the two ligand systems.§ Complex 1 lacks two protons compared with 2, i.e.the characteristic imine hydrogen signal (Ha, Scheme 1) has disappeared as well as the signal for the double benzylic proton (Hb). 51V NMR spectra of these two complexes, however, are very similar. Complex 1 dissolved in DMSO-d6 has a single resonance at 2543 ppm (band width, b.w. = 1303 Hz), whereas 2 shows a single resonance at 2540 ppm (b.w. = 965 Hz). This is a strong indication that the coordination sphere around the vanadium(V) center is quite similar in both structures. 9 In the case of 2, the vanadium ion is surrounded by one pyridine nitrogen, the imine nitrogen, the deprotonated phenolic oxygen and two oxo groups as depicted in Scheme 1. As a result, the second pyridine entity is non-coordinating. Heating of a solution of 2 in MeOH results in the formation of 1 as was also shown by 1H NMR measurements. Infrared spectra reveal two V]] O absorptions for both complexes, at 926 and 949 cm21 for 1 and at 918 and 953 cm21 for 2, respectively.These observations correspond to the data known from the literature that stretching frequencies of V]] O bonds with a length of 1.607– 1.621 Å are found in the 930–960 cm21 region.10 The relatively low position of the V]] O stretching band may indicate the existence of intermolecular interactions via O bridges, but no evidence for such interactions was found in the crystal structure. The oxidative cyclisation of the coordinated SchiV base ligand L leads to the formation of an imidazo[1,5-a]pyridine 11 type of ligand (L9).Some imines of di(2-pyridyl)methylamine are known to cyclise spontaneously to imidazo[1,5-a]pyridines by air oxidation.12 However, HL itself remained intact even after refluxing in MeOH solution in the air for 3 h. Therefore, the vanadium(V) ion is likely to play an active role in the cyclisation process. Based on these results, a reaction mechanism is proposed for the ligand cyclisation reaction resulting in the formation of 1 starting from 2 (Scheme 2).13 First the SchiV base ligand is oxidised by vanadium(V) giving a vanadium(IV) species containing one oxo ligand and one hydroxy group (A and B).The radical in resonance structure B then attacks the non-coordinating pyridine nitrogen resulting in the formation of C, which is subsequently oxidised by air to form 1. The involvement of a vanadium(IV) species is supported by results of EPR experiments with crude 1.An EPR signal was observed (g = 1.97) which was attributed to a vanadium(IV) species by comparison with literature data.14 In conclusion, SchiV base ligand HL is found to be eVective in the formation of a dioxovanadium(V) complex. However when HL is heated in the presence of vanadium(V), imidazo- Fig. 1 An ORTEP plot of complex 1 (50% probability level). pyridine type ligand L9 is formed due to an oxidative cyclisation reaction. Catalytic oxidation experiments with these complexes are in progress.We gratefully acknowledge Dr M. Lubben for initial synthesis and Mrs C. M. Jeronimus-Stratingh for performing the ES-MS measurements. This work was supported in part (A. L. S.) by the Netherlands Foundation of Chemical Research (SON) with financial aid from the Netherlands Organisation for Scientific Research (NWO) and in part (A. G. J. L., R. H., B. L. F.) by Unilever Research Vlaardingen, The Netherlands. Notes and references ‡ Crystal data for 1: C18H12N3O3V, Mr = 369.25, yellow needle (0.10 × 0.12 × 0.43 mm), monoclinic, space group P21/c (no. 14) with a = 7.0787(16), b = 16.833(5), c = 13.573(5) Å, b = 115.21(2)8, V = 1463.3(8) Å3, Z = 4, Dc = 1.676 g cm23, F(000) = 752, m(Mo-Ka) = 7.0 cm21, 6346 reflections measured, 3006 independent, R(int) = 0.106, qmax = 26.58, w scan, T = 150 K, Mo-Ka radiation, graphite monochromated, l = 0.71073 Å, Enraf-Nonius CAD4T on rotating anode. Data were corrected for absorption (PLATON/DELABS).The structure was solved by direct methods (SHELXS86) and refined on F 2 using SHELXL97. Hydrogen atoms were taken into account at calculated positions. Convergence was reached at R = 0.0579 for 1984 reflections with I > 2s (I) and 226 parameters. CCDC reference number 186/1318. See http://www.rsc.org/suppdata/ dt/1999/659 for crystallographic files in .cif format. § 1H NMR (300 MHz, DMSO-d6) for 1: d 9.24 (m, 1H), 9.05 (m, 1H), 8.49 (m, 2H), 8.28 (m, 2H), 7.54 (m, 3H), 7.27 (m, 1H), 7.09 (m, 2H); and for 2: 9.22 (s, 1H), 8.86 (s, 2H), 8.06 (m, 2H), 7.70 (m, 3H), 7.57 (m, 3H), 6.93 (m, 3H). 1 D. Rehder, Angew. Chem., Int. Ed. Engl., 1991, 30, 148; A. Butler and C. J. Carrano, Coord. Chem. Rev., 1991, 109, 61. Scheme 2 Proposed mechanism for the formation of complex 1 (R = H, CH3).J. Chem. Soc., Dalton Trans., 1999, 659–661 661 2 K. B. Sharpless and R. C. Michaelson, J. Am. Chem. Soc., 1973, 95, 6136; K. B. Sharpless and T. R. Verhoeven, Aldrichim.Acta, 1979, 12, 63; A. Butler, M. J. Clague and G. E. Meister, Chem. Rev., 1994, 94, 625. 3 K. Yamamoto, K. Oyaizu and E. Tsuchida, J. Am. Chem. Soc., 1996, 118, 12665; K. Oyaizu, K. Yamamoto, K. Yoneda and E. Tsuchida, Inorg. Chem., 1996, 35, 6634. 4 Recently, Dr M. Döring (University of Jena) informed us about the copper mediated cyclisation of L. 5 M. Renz, C. Hemmert, B. Donnadieu and B. Meunier, Chem. Commun., 1998, 1635. 6 E. Niemers and R. Hiltmann, Synthesis, 1976, 593. 7 Complex 1 was stable for several weeks when stored under an atmosphere of dry argon. 8 C. A. Root, J. D. Hoeschele, C. R. Cornman, J. W. Kampf and V. L. Pecoraro, Inorg. Chem., 1993, 32, 3855. 9 V. Conte, F. Di Furia and S. Moro, J. Mol. Catal. A, 1995, 104, 159. 10 G. Asgedom, A. Sreedhara, J. Kivikoski, J. Valkonen and C. P. Rao, J. Chem. Soc., Dalton Trans., 1995, 2459. 11 D. D. Davey, J. Org. Chem., 1987, 52, 1863. 12 A. P. Krapcho and J. R. Powell, Tetrahedron Lett., 1986, 27, 3713; M. T. Edgar, G. R. Pettit and T. S. Krupa, J. Org. Chem., 1979, 44, 396; R. Grigg, P. Kennewell, V. Savic and V. Sridharan, Tetrahedron, 1992, 48, 10423. 13 J. Fossey, D. Lefort and J. Sorba, Free radicals in organic chemistry, John Wiley & Sons, New York, 1995; T. Fukuda, F. Sakamoto, M. Sato, Y. Nakano, X. S. Tan and Y. Fujii, Chem. Commun., 1998, 1391. 14 M. Bonchio, V. Conte, F. Di Furia, G. Modena, S. Moro and J. O. Edwards, Inorg. Chem., 1994, 33, 1631. Communication 8/09476D