Triangular, Ferromagnetically-Coupled Cu II 3 -Pyrazolato Complexes as Possible Models of Particulate Methane Monooxygenase (pMMO) Roman Boc ˇa, † L’ubomı ´r Dlha ´n ˇ, † Gellert Mezei, ‡ Tamara Ortiz-Pe ´rez, ‡ Raphael G. Raptis,* ,‡ and Joshua Telser § Department of Chemistry, UniVersity of Puerto Rico, San Juan, Puerto Rico 00931-3346, Department of Inorganic Chemistry, SloVak Technical UniVersity, SK-812 37 BratislaVa, SloVakia, and School of Science & Mathematics, RooseVelt UniVersity, 430 S. Michigan AVenue, Chicago, Illinois 60605-1394 Received April 27, 2003 Magnetic susceptibility and EPR studies show that trinuclear Cu II - pyrazolato complexes with a Cu 3 (µ 3 -X) 2 core (X ) Cl, Br) are ferromagnetically coupled: J Cu-Cu ) + 28.6 cm -1 (X ) Cl), + 3.1 cm -1 (X ) Br). The orderly transition from an antiferromagnetic to a ferromagnetic exchange among the Cu centers of Cu 3 (µ 3 -X) complexes, X ) O, OH, Cl, Br, follows the change of the Cu- X-Cu angle from 120° to ∼80°. The crystal structures of [Bu 4 N] 2 - [Cu 3 (µ 3 -Br) 2 (µ-pz*) 3 Br 3 ] (pz* ) pz (1a) or 4-O 2 N-pz (1b), pz ) pyrazolato anion, C 3 H 3 N 2 1- ) are presented. Multimetallic systems with ferromagnetic ground states are attracting attention because of their potential application as single molecule magnets. 1 Ferromagnetically coupled trinuclear Cu II complexes in particular are of further interest because they may relate to the active sites of pMMO. The latter enzyme, in its fully oxidized form, has been proposed to have an S ) 3 / 2 ground state, attributed to ferromagneti- cally coupled trinuclear Cu II centers. 2 In the extensive Cu II chemistry, there are only four examples of ferromagnetic trinuclear copper complexes: one linear, two angular, and one triangular. 3 We have recently described a triangular Cu II 3 -pyrazolate that provides a stable metallacyclic framework for the pH- dependent exchange of µ 3 -O, µ 3 -OH, and (µ 3 -Cl) 2 bridging ligands. 4 We have further shown that the planar Cu 3 (µ 3 -O) species is strongly antiferromagnetic, while its protonated, pyramidal Cu 3 (µ 3 -OH) analogues are known weakly anti- ferromagnetic systems. 4,5 Continuing our investigation of this system, we describe here the synthesis and crystal structures of [Bu 4 N] 2 [Cu 3 (µ 3 -Br) 2 (µ-pz*) 3 Br 3 ] (pz* ) pz (1a) or 4-O 2 N- pz (1b), pz ) pyrazolato anion, C 3 H 3 N 2 1- ), as well as the magnetic susceptibility studies of 1b and the chloro complex [Bu 4 N] 2 [Cu 3 (µ 3 -Cl) 2 (µ-pz) 3 Cl 3 ], 2. Complexes 1 and 2 are rare examples of triangular, ferromagnetically coupled Cu II complexes. 3d This discovery demonstrates that the pH- controlled interconversion of the Cu 3 (µ 3 -O), Cu 3 (µ 3 -OH), and Cu 3 (µ 3 -X) 2 motifs (X ) Cl, Br) is accompanied by an orderly switch from antiferro- to ferromagnetic coupling of the three Cu II centers. Complexes 1a and 1b are prepared from CuBr 2 and pzH or 4-O 2 N-pzH, respectively, in the presence of a base, following the established procedure 4 for the synthesis of the chloride complex 2. 6 Single crystal X-ray structure deter- minations of 1a and 1b show them to consist of ap- proximately planar copper pyrazolato trimers capped on either side by two µ 3 -Br ligands (Figure 1). 8 The three Cu atoms of 1b define an approximately equilateral triangle with Cu-Cu distances of 3.428(1)-3.443(1) Å, while those of 1a are more disparate, 3.424(1) and 3.510(1) Å. Two axial pyrazolates and three equatorial bromides define the distorted trigonal-bipyramidal environment of the five-coordinate Cu atoms. Two capping bromides are loosely held by the three copper atoms of 1a and 1b at average Cu-(µ 3 -Br) distances of 2.763 and 2.698 Å, respectively, much longer than the bond lengths of the terminal bromide ligands. The Cu-(µ 3 - Br) distances fall into the range of the corresponding Cu- * To whom correspondence should be addressed. E-mail: raphael@ adam.uprr.pr. ² Slovak Technical University. ‡ University of Puerto Rico. § Roosevelt University. (1) (a) Sessoli, R.; Gatteschi, D.; Caneschi, A.; Novak, M. A. Nature 1993, 365, 141. (b) Gatteschi, D.; Caneschi, A.; Pardi, L.; Sessoli, R. Science 1994, 265, 1054. (2) (a) Nguyen, H.-H. T.; Nakagawa, K. H.; Hedman, B.; Elliott, S. J.; Lidstrom, M. E.; Hodgson, K. O.; Chan, S. I. J. Am. Chem. Soc. 1996, 118, 12766. (b) Lemos, S. S.; Yuan, H.; Perille Collins, M. L.; Antholine, W. E. Curr. Topics. Biophys. 2002, 26, 43. (3) (a) Gehring, S.; Astheimer, H.; Haase, W. J. Chem. Soc., Faraday Trans. 2 1987, 83, 347. (b) Gehring, S.; Fleischauer, P.; Paulus, H.; Haase, W. Inorg. Chem. 1993, 32, 54. (c) Meenakumari, S.; Tiwary, S. K.; Chakravarty, A. R. Inorg. Chem. 1994, 33, 2085. (d) Suh, M. P.; Han, M. Y.; Lee, J. H.; Min, K. S.; Hyeon, C. J. Am. Chem. Soc. 1998, 120, 3819. (4) Angaridis, P. A.; Baran, P.; Boca, R.; Cervantes-Lee, F.; Haase, W.; Mezei, G.; Raptis, R. G.; Werner, R. Inorg. Chem. 2002, 41, 2219. (5) (a) Liu, J.-C.; Guo, G.-C.; Huang, J.-S.; You, X.-Z. Inorg. Chem. 2003, 42, 235. (b) Ferrer, S.; Lloret, F.; Bertomeu, I.; Alzulet, G.; Borra ´s, J.; Garcı ´a-Granda, S.; Liu-Gonza ´lez, M.; Haasnoot, J. G. Inorg. Chem. 2002, 41, 5821 and references therein. Inorg. Chem. 2003, 42, 5801-5803 10.1021/ic0344416 CCC: $25.00 © 2003 American Chemical Society Inorganic Chemistry, Vol. 42, No. 19, 2003 5801 Published on Web 08/22/2003