In contrast to the four-coordinate tetrahedral {Fe(NO)2}9 DNICs with EPR g value as 2.03, the newly synthesized non-classical six-coordinate {Fe(NO)2}9 DNIC [(1-MeIm)2(□2-ONO)Fe(NO)2] (1-MeIm = 1-methylimidazole) (1-Me) displays an EPR signal g = 2.013. The temperature-dependent reversible transformation occurs between the six-coordinate chelating nitrito {Fe(NO)2}9 DNIC 1-Me and the four-coordinate monodentate nitrito {Fe(NO)2}9 DNIC [(1-MeIm)(ONO)Fe(NO)2] (2-Me). PPh3 promotes oxygen atom transfer of the chelating nitrito under mild condition to generate, describing as nitroxyl (NO—) coordinative addition to the {Fe(NO)x} (x = 1, 2) motif, {Fe(NO)2}9 DNIC [PPN][(ONO)2Fe(NO)2] (6) and {Fe(NO)2}10 DNIC [(1-MeIm)(PPh3)Fe(NO)2] (3). The biomimetic reaction cycle centered on the iron site may be useful for describing a potential mode of nitrite activation involving the dynamic equilibrium, the Fe-NO— (nitrite-to-nitroxyl conversion) step, and the histidine(s) serving as a role in modulating the activation properties of iron center. The result deciphers that the non-classical six-coordinate nitrite-containing {Fe(NO)2}9 DNIC 1-Me and {Fe(NO)}7 MNIC [PPN]2[(ONO)3(□2-ONO)Fe(NO)2] (5) may act as an active center to trigger the transformation of nitrite into nitric oxide in biological systems. The dynamic equilibrium between DNIC 1-Me and DNIC 2-Me implicates that the initial nitrite activation may take place via its reversible chelating of a nitrite, which may play a crucial role in subtly tuning NO homeostasis in hypoxia condition.

References
1. (a) Alderton, W. K.; Cooper, C. E.; Knowles, R. G. Biochem. J. 2001, 357, 593-615. (b) Li, H.; Poulos, T. L.; J. Inorg. Biochem. 2005, 99, 293-305.
2. (a) Palmer, R. M. J. ; Ferrige, A. G.; Moncada, S. Nature 1987, 327, 524-526. (b) Bulter, A. R. ; Megson, I. L. Chem. Rev. 2002, 102, 1155-1166. (c) Szaciłowski, K. ; Chmura, A.; Stasicka, Z. Coord. Chem. Rev. 2005, 249, 2408-2436.
3. (a) Murad, F. Angew. Chem. Int. Ed. 1999, 38, 1856-1868. (b) Furchgott, R. F. Angew. Chem. Int. Ed. 1999, 38, 1870-1880. (c) Ignarro, L. J. Angew. Chem. Int. Ed. 1999, 38, 1882-1892.
4. Lundberg, J. O.; Weitzberg, E.; Gladwin, M. T. Nat. Rev. Drug Disc. 2008, 7, 156-167.
5. (a) Ueno, T.; Susuki, Y.; Fujii, S.; Vanin, A. F.; Yoshimura, T. Biochem. Pharmacol. 2002, 63, 485-493. (b) Foster, M. W.; Cowan, J. A. J. Am. Chem. Soc. 1999, 121, 4093-4100. (c) Vanin, A. F. Biochem. (Moscow) 1998, 63, 782-793. (d) Frederik, A. C.; Wiegant, I. Y.; Malyshev, I. Y.; Kleschyov, A. L.; van Faassen, E.; Vanin, A. F. FEBS Lett. 1999, 455, 179-182. (e) Boese, M.; Keese, M. A.; Becker, K.; Busse, R.; Mulsch, A. J. Biol. Chem. 1997, 272, 21767-21773.
6. (a) Luchsinger, B. P.; Rich, E. N.; Gow, A. J.; Williams, E. M.; Stamler, J. S.; Singel, D. J. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 461-466. (b) Stamler, J. S.; Jia, L.; Eu, J. P.; McMahon, T. J.; Demchenko, I. T.; Bonaventura, J.; Gernert, K.; Piantadosi, C. A. Science 1997, 276, 2034-2037. (c) Pawloski, J. R.; Hess, D. T.; Stamler, J. S. Nature 2001, 409, 622-626.
7. Hess, D. T.; Matsumoto, A.; Kim, S.-O.; Marshall, H. E.; Stamler, J. S. Nat. Rev. Mol. Cell Biol. 2005, 6, 150-166.
8. (a) Stamler, J. S.; Jaraki, O.; Osborne, J.; Simon, D. I.; Keaney, J.; Vita, J.; Single, D.; Valeri, C. R.; Loscalzo, J. Proc. Natl. Acad. Sci. U.S.A. 1992, 89, 7674-7677. (b) McMahon, T. J.; Stone, A. E.; Bonaventura, J.; Singel D. J.; Stamler, J. S. J. Biol. Chem. 2000, 275, 16738-16745.
9. (a) Cesareo, E.; Parker, L. J.; Pedersen, J. Z.; Nuccetelli, M.; Mazzetti, A. P.; Pastore, A.; Federici, G.; Caccuri, A. M.; Ricci, G..; Adam, J. J.; Parker, M. W.; Bello, M. L. J. Biol. Chem. 2005, 280, 42172-42180. (b) Kennedy, M. C.; Antholine, W. E.; Beinert, H. J. Biol. Chem. 1997, 272, 20340-20347. (c) Boese, M.; Mordvintcev, P. I.; Vanin, A. F.; Busse, R.; Mulsch, A. J. Boil. Chem. 1995, 207, 29244-29249. (d) Lee, M.; Arosio, P.; Cozzi, A.; Chasteen, N. D. Biochemistry 1994, 33, 3679-3687. (e) D’Autréaux, B.; Horner, O.; Oddou, J.-L.; Jeandey, C.; Gambarelli, S.; Berthomieu, C. ; Latour, J.-M. ; Michaud-Soret, I. J. Am. Chem. Soc. 2004, 126, 6005-6016.
10. (a) Roger, P. A. ; Eide, L.; Klungland, A.; Ding, H. DNA Repair 2 2003, 809-817. (b) Sellers, V. M.; Johnson, M. K.; Dailey, H, A. Biochemistry 1996, 35, 2699-2704. (c) Ding, H.; Demple, B. Proc. Natl. Acad. Sci. U.S.A. 2000, 97, 5146-5150.
11. (a) Roger, P. A.; Ding, H. J. Biol. Chem. 2001, 276, 30980-30986. (b) Yang, W.; Roger, P. A.; Ding, H. J. Biol. Chem. 2002, 277, 12868-12873.
12. (a) Lewandowska, H.; Męczyʼnska, S.; Sochanowicz, B.; Sadło, J.; Kruszewaji, M. J. Biol. Inorg. Chem. 2007, 12, 345-352. (b) Kim, Y.-M.; Chung, H.-T.; Simmons, R. L.; Billiar, T. R. J. Biol. Chem. 2000, 275, 10954-10961. (c) Kim, S.; Ponka, P. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 12214-12219.
13. Watts, R. N.; Hawkins, C.; Ponka, P.; Richardson, S. R. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 7670-7675.
14. (a) Bryan, S. N.; Fernandez, O. B. ; Bauer, M. S. ; Garcia-Saura, F. M. ; Milsom, B. A. ; Rassaf, T. ; Maloney, E. R. ; Bharti, A. ; Rodriguez, J.; Feelish, M. Nat. Chem. Boil. 2005, 1, 290-297. (b) Gladwin, M. T. Nat. Chem. Boil. 2005, 1, 245-246. (c) Donzelli, S.; Switzer, H. C.; Thomas, D. D.; Ridnour, A. L.; Espey, G. M.; Isenberg, S. J.; Tocchetti, G. C.; King, B. S.; Lazzarino, G.; Miranda, M. K.; Roberts, D. D.; Feeliah, M.; Wink, A. D. Antiox. Redox Signaling 2006, 8, 1363-1371. (d) Zhiqiang, C.; Stamler, J. S. Trends Cardiovasc. Med. 2006, 16, 259-265.
15. Cosby, K.; Partovi, K. S.; Crawford, J. H.; Patel, R. P.; Reiter, C. D.; Martyr, S.; Yang, B. K.; Waclawiw, M. A.; Zalos, G.; Xu, X.; Huang, K. T.; Shield, H.; Kim-Shapiro, D. B.; Schechter, A, N,; Cannon III, R. O.; Gladwin, M. T. Nat. Med. 2003, 9, 1498-1504.
16. (a) Ignarro. L. J. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 7816-7817. (b) Zhiqiang, C.; Zhang, J.; Stamler, J. S. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 8306-8311. (c) Wenzel, P.; Hink, U.; Oelze, M.; Schuppan, S.; Schaeuble, K.; Schildknecht, S.; Ho, K. K.; Weiner, H.; Bachschmid, M.; Münzel, T.; Daiber, A. J. Biol. Chem. 2007, 282, 792-799. (d) Artz, J. D.; Toader, V.; Zavorin, S. I.; Bennett, B. M.; Thatcher, G. R. J. Biochemistry 2001, 40, 9256-9264.
17. Hitchman, M. A.; Rowbottom, G. L. Coord. Chem. Rev. 1982, 42, 55-130.
18. (a) Franz, K. J.; Lippard, S. J. J. Am. Chem. Soc. 1999, 121, 10504-10512. (b) Hauser, C.; Glaser, T.; Bill, E.; Weyhermüller T.; Wieghardt, K. J. Am. Chem. Soc. 2000, 122, 4352-4365. (c) Arulsamy, N.; Bohle, D. S.; Hansert, B.; Powell, A. K.; Thomson, A. J.; Wocaldo, S. Inorg. Chem. 1998, 37, 746-750.
19. Hedberg, L.; Hedberg, K.; Satija. S. K.; Swanson, B. I. Inorg. Chem. 1985, 24, 2766-2771.
20. Sheldrick, G. M. SADABS, Siemens Area Detector Absorption Correction Program; University of Göttingen: Germany, 1996.
21. North, A. C. T.; Phillips, D. C.; Mathews, F. S. Acta Crystallogr. 1968, A24, 351-359.
22. Sheldrick, G. M. SHELXTL, Program for Crystal Structure Determination; Siemens Analytical X-ray Instruments Inc.: Madison, WI, 1994.
23. (a) Afshar, R. K.; Eroy-Reveles, A. A.; Olmstead, M. M.; Mascharak, P. K. Inorg. Chem. 2006, 45, 10347-10354. (b) Patra, A. K.; Afshar, R. K.; Rowland, J. M.; Olmstead, M. M.; Mascharak, P. K. Angew. Chem. 2003, 115, 4655-4659. (c) Cheng, L.; Powell, D. R.; Khan, M. A.; Richter-Addo, G. B. Chem. Commun. 2000, 2301-2302.
24. (a) Tsou, C.-C.; Lin, Z.-S.; Lu, T.-T.; Liaw, W.-F. J. Am. Chem. Soc. 2008, 130, 17154-17160. (b) Lu, T.-T.; Tsou, C.-C.; Huang, H.-W.; Hsu, I-J.; Chen, J.-M.; Kuo, T.-S.; Wang, Y.; Liaw, W.-F. Inorg. Chem. 2008, 47, 6040-6050. (c) Huang, H.-W.; Tsou, C.-C.; Kuo, T.-S.; Liaw, W.-F. Inorg. Chem. 2008, 47, 2196-2204. (d) Tsou, C.-C.; Lu, T.-T.; Liaw, W.-F. J. Am. Chem. Soc. 2007, 129, 12626-12627. (e) Tsai, M.-L.; Hsieh, C.-H.; Liaw, W.-F. Inorg. Chem. 2007, 46, 5110-5117. (f) Lu, T.-T.; Chiou, S.-J.; Chen, C.-Y.; Liaw, W.-F. Inorg. Chem. 2006, 45, 8799-8806. (g) Tsai, M.-L.; Liaw, W.-F. Inorg. Chem. 2006, 45, 6583-6585. (h) Hung, M.-C.; Tsai, M.-C.; Liaw, W.-F. Inorg. Chem. 2006, 45, 6041-6047. (i) Chen, T.-N.; Lo, F.-C.; Tsai, M.-L.; Shih, K.-N.; Chiang, M.-H.; Lee. G.-H,; Liaw, W.-F. Inorg. Chimi. Acta 2006, 359, 2525-2533. (j) Tsai, F.-T.; Chiou, S.-J.; Tsai, M.-C.; Tsai, M.-L.; Huang, H.-W.; Chiang, M.-H.; Liaw, W.-F. Inorg. Chem. 2005, 44, 5872-5881. (k) Tsai, M.-L.; Chen, C.-C.; Hsu, I.-J.; Ke, S.-C.; Hsieh, C.-H.; Chiang, K.-A.; Lee, G.-H.; Wang, Y.; Liaw, W.-F. Inorg. Chem. 2004, 43, 5159-5167.
25. (a) DuBois, J. L.; Mukherjee, P.; Stack, T. D. P.; Hedman, B.; Solomon, E. I.; Hodgson, K. O. J. Am. Chem. Soc. 2000, 122, 5775-5787. (b) Westre, T. E.; Kennepohl, P.; Dewitt, J. G.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J. Am. Chem. Soc. 1997, 119, 6297-6314.