COMMUNICATIONS 2120  WILEY-VCH Verlag GmbH, D-69451 Weinheim, 2000 0570-0833/00/3912-2120 $ 17.50+.50/0 Angew. Chem. Int. Ed. 2000, 39, No. 12 Metal-Based NO Sensing by Selective Ligand Dissociation** Katherine J. Franz, Nisha Singh, and Stephen J. Lippard* The large and increasing number of biological processes for which nitric oxide is implicated necessitates the development of improved methods of NO detection. Currently available techniques often rely on the identification of NO metabolites, such as nitrite and nitrate, or lack sensitivity. [1] The application of ratiometric biosensors [2] and ultramicro amperometric sensors [3±5] affords one avenue to obtain selective, sensitive detection of NO in vivo. Fluorescent indicators also provide desirable properties that allow direct, real-time detection with both spatial and temporal resolution, [6] as has been amply demonstrated for Ca II sensors. [7] Although the invention of fluorescent NO sensors is an active research area, so far these methods rely on indirect detection of more reactive NO x species, [8±10] display decreased fluorescence intensities upon NO binding, [11] or require further chemistry to provide a positive fluorescence response. [12, 13] We report herein an approach in which the formation of a transition metal nitrosyl complex triggers a positive fluorescent signal in response to NO. The design of this NO sensor takes advantage of the fluorescence-quenching properties of transition metal ions with partly filled d shells. We prepared a ligand containing a fluorophore that is quenched by the metal center, in this case Co II , in the absence of NO to give little residual signal for the ªoffº response. In the presence of NO, however,the formation of the metal ± nitrosyl adduct selectively displaces a fluores- cent ligand, thereby removing it from the quenching environ- ment and turning the fluorescence ªonº. A similar approach was reported for an Fe II complex of a quinoline pendant cyclam but the fluorescence intensity decreased in the presence of NO. [11] An analogous ligand-displacement strat- egy has also been applied for a pH-sensitive fluorescent probe. [14] In the newly designed ligand H 2 DATI-4 (1), each aminotro- poniminate (ATI) ring is modified with a dansyl fluoro- phore on one of the imine nitrogen atoms and linked through the other nitrogen by a 4-methylene chain to a sec- ond such chelating unit. [15] The yellow ligand 1 is rather in- solved by direct methods with SHELXS-86 and refined with SHELXL-93. [12] The program XPMA was used for the graphical processing of the data. [13] The figures were produced with WINRAY- 32. [14] The refinement was carried out anisotropically against F 2 , hydrogen atoms were included in calculated positions. 1: space group P2 1 /c, a  1277.4(3), b  1956.0(4), c  2436.8(5) pm, b  103.63(3)8, V  5917  10 6 pm 3 , 1 calcd  1.641 gcm 3 ,2q max  52.18, Z  4, 71438 measured reflections, 11474 independent reflections, of which 5485 (I > 2s(I)) observed, 758 refined parameters, R  0.070, Rw  0.112, max. residual electron density 1.54  10 6 epm 3 . 2 : space group P1 Å , a  1422.2(3), b  1902.5(4), c  2609.1(5) pm, a  100.58(3), b  103.63(3), g  108.48(3)8, V  6490  10 6 pm 3 , 1 calcd  1.543 gcm 3 , 2q max  55.08, Z  4, 58282 measured reflections, 29826 independent reflections, of which 19004 (I > 2s(I)) observed, 1463 refined param- eters, R  0.077, Rw  0.204, max. residual electron density 2.39  10 6 epm 3 . The two crystallographically independent anions 2 are the same with regard to all the important structural features. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC-137594 (1), -137593 (2), and -137592 (Na 2 [{(CO) 5 Cr} 2 Pb- (NO 3 ) 2 ]). Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax: ( 44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk). [9] a) A. H. Cowley, D. M. Giolando, R. A. Jones, C. M. Nunn, J. M. Power, Polyhedron 1988, 7 , 1909 ± 1910; b) S. C. Goel, M. Y. Chiang, D. J. Rauscher, W. E. Buhro, J. Am. Chem. Soc. 1993, 115,160±169; c) A. Winkler, W. Bauer, F. W. Heinemann, V. G. Montalvo, M. Moll, J. Ellermann, Eur. J. Inorg. Chem. 1998, 437 ± 444; d) A. L. Balch, D. E. Oram, Organometallics 1986, 5, 2159±2161. [10] The compounds H[E{Fe(CO) 4 } 3 ] 2 (E  As, Sb) are in the broadest sense isoelectronic to 2, though their relationship to the unsaturated analogues [E{Fe(CO) 4 } 3 ]  is not chemically confirmed, particularly since these are unknown to date. a) R. E. Bachmann, S. K. Miller, K. H. Whitmire, Inorg. Chem. 1994, 33, 2075±2076; b) P. Henderson, M. Rossignoli, R. C. Burns, M. L. Scudder, D. C. Craig, J. Chem. Soc. Dalton Trans. 1994, 1641±1647; c) R. E. Bachmann, S. K. Miller, K. H. Whitmire, Organometallics 1995, 14, 796±803. [11] Collect, data collection software, Nonius, 1998 (http://www.nonius.- com). [12] a) G. M. Sheldrick, SHELXS-86 Program for Crystal Structure Solution, Universität Göttingen, 1986 ; b) G. M. Sheldrick, SHELXL-93 Program for Crystal Structure Refinement, Universität Göttingen, 1993 (http://www.shelx.uni-ac.gwdg.de/shelx/index.html); c) International Tables for X-Ray Crystallography , Vol. 4, Kynoch, Birmingham, 1974. [13] L. Zsolnai, G. Huttner, XPMA, Universität Heidelberg, 1998 (http:// www.rzuser.uni-heidelberg.de/  v54/xpm.html). [14] R. Soltek, G. Huttner, WINRAY-32, Universität Heidelberg, 1998 (http://www.rzuser.uni-heidelberg.de/  v54). [*] Prof. S. J. Lippard, K. J. Franz, N. Singh Department of Chemistry Massachusetts Institute of Technology 77 Massachusetts Avenue Cambridge, MA 02139 (USA) Fax: ( 1)617-258-8150 E-mail: lippard@lippard.mit.edu [**] This work was supported by a grant from the National Science Foundation and a fellowship to N.S. from the Undergraduate Research Opportunity Program (MIT). We thank Prof. Roger Tsien for valuable discussions at the inception of this project.