Lanthanide Coordination with r-Amino Acids under Near Physiological pH Conditions: Polymetallic Complexes Containing the Cubane-Like [Ln 4 (μ 3 -OH) 4 ] 8+ Cluster Core Ruiyao Wang, 1a Hui Liu, 1a Michael D. Carducci, 1a Tianzhu Jin, 1b Chong Zheng, 1c and Zhiping Zheng* ,1a Department of Chemistry, University of Arizona, Tucson, Arizona 85721, State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking University, Beijing 100871, China, and Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115 ReceiVed December 22, 2000 Tetranuclear lanthanide-hydroxo complexes of the general formula [Ln 4 (µ 3 -OH) 4 (AA) x (H 2 O) y ] 8+ (1, Ln ) Sm, AA ) Gly, x ) 5, y ) 11; 2, Ln ) Nd, AA ) Ala, x ) 6, y ) 10; 3, Ln ) Er, AA ) Val, x ) 5, y ) 10) have been prepared by R-amino acid controlled hydrolysis of lanthanide ions under near physiological pH conditions (pH 6-7). The core component of these compounds is a cationic cluster [Ln 4 (µ 3 -OH) 4 ] 8+ whose constituent lanthanide ions and triply bridging hydroxo groups occupy the alternate vertexes of a distorted cube. The amino acid ligands coordinate the lanthanide ions via bridging carboxylate groups. Utilizing L-glutamic acid as the supporting ligand, a cationic cluster complex (4) formulated as [Er 4 (µ 3 -OH) 4 (Glu) 3 (H 2 O) 8 ] 5+ has been obtained. Its extended solid-state structure is composed of the cubane-like [Er 4 (µ 3 -OH) 4 ] 8+ cluster building units interlinked by the carboxylate groups of the glutamate ligands. All compounds are characterized by using a combination of spectroscopic techniques and microanalysis (CHN and metal). Infrared spectra of the complexes suggest the coordinated amino acids to be zwitterionic. The presence of mass (MALDI-TOF) envelopes corresponding to the [Ln 4 (µ 3 -OH) 4 ] 8+ (Ln ) trivalent Sm, Nd, or Er) core containing fragments manifests the integrity of the cubane- like cluster unit. Magnetic studies using Evans’ method suggest that exchange interactions between the lanthanide ions are insignificant at ambient temperature. The structural identities of all four compounds have been established crystallographically. The tetranuclear cluster core has been demonstrated to be a common structural motif in these complexes. A mechanism responsible for its self-assembly is postulated. Introduction Recently, the catalytic roles of lanthanide ions and their complexes in hydrolytic cleavage of phosphate diesters have received considerable attention. 2 It is generally agreed that the catalysis occurs via substrate activation by the electropositive lanthanide ions, followed by nucleophilic attack by the activated hydroxo ligands (Figure 1). 2b,d,e,g,l However, the ease of lan- thanide ion hydrolysis and the tendency of lanthanide-hydroxo species to form polynuclear aggregates in solution render unambiguous identification of the catalytically active species extremely difficult, if at all possible. 2b-i On the other hand, although there are a few reports of structurally characterized dinuclear 3 and tetranuclear 4 lanthanide-hydroxo complexes (Figure 2), the catalytic potential of these species has not yet been assessed. Furthermore, these compounds are either syn- thetic serendipities or accidents, and general synthetic guidelines are lacking. With the ultimate goal of achieving nonenzymatic hydrolysis of phosphate diesters using lanthanide catalysts, our initial efforts 5 focus on the design and synthesis of structurally well-defined lanthanide-hydroxo complexes with biologically relevant ligands. (1) (a) University of Arizona. (b) Peking University. (c) Northern Illinois University. (2) For leading references on lanthanide-catalyzed hydrolysis of phosphate diester substrates, see: (a) Williams, N. H.; Takasaki, B.; Wall, M.; Chin, J. Acc. Chem. Res. 1999, 32, 485-493. (b) Schneider, H.-J.; Rammo, J.; Hettich, R. Angew. Chem., Int. Ed. Engl. 1993, 32, 1716- 1719. (c) Rammo, J.; Hettich, R.; Roigk, A.; Schneider, H.-J. Chem. Commun. 1996, 105-107. (d) Oh, S. J.; Song, K. H.; Whang, D.; Kim, K.; Yoon, T. H.; Moon, H.; Park, J. W. Inorg. Chem. 1996, 35, 3780-3785. (e) Oh, S. J.; Choi, Y.-S.; Hwangbo, S.; Bae, S. C.; Ku, J. K.; Park, J. W. Chem. Commun. 1998, 2189-2190. (f) Roigk, A.; Hettich, R.; Schneider, H.-J. Inorg. Chem. 1998, 37, 751-756. (g) Jurek, P. E.; Jurek, A. M.; Martell, A. E. Inorg. Chem. 2000, 39, 1016- 1020. (h) Ragunathan, K. G.; Schneider, H.-J. Angew. Chem., Int. Ed. Engl. 1996, 35, 1219-1221. (i) Takasaki, B. K.; Chin, J. J. Am. Chem. Soc. 1993, 115, 9337-9338. (j) Komiyama, M. J. Biochem. 1995, 118, 665-670. (k) Takasaki, B. K.; Chin, J. J. Am. Chem. Soc. 1995, 117, 8582-8585. (l) Hurst, P.; Takasaki, B. K.; Chin, J. J. Am. Chem. Soc. 1996, 118, 9982-9983. (m) Bracken, K.; Moss, R. A.; Ragu- nathan, K. G. J. Am. Chem. Soc. 1997, 119, 9323-9324. (3) (a) Baraniak, E.; Bruce, R. St. L.; Freeman, H. C.; Hair, N. J.; James, J. Inorg. Chem. 1976, 15, 2226-2230. (b) Grillone, M. D.; Benetollo, F.; Bombieri, G. Polyhedron 1991, 10, 2171-2177. (c) Wang, R.; Zhao, J.; Jin, T.; Xu, G.; Zhou, Z.; Zhou, X. Polyhedron 1998, 17, 43-47. (4) (a) Dube ´, T.; Gambarotta, S.; Yap, G. Organometallics 1998, 17, 3967-3973. (b) Chen, X. M.; Wu, Y. L.; Tong, Y. X.; Sun, Z.; Hendrickson, D. N. Polyhedron 1997, 16, 4265-4272. (c) Plakatouras, J. C.; Baxter, I.; Hursthouse, M. B.; Abdul Malik, K. M.; McAleese, J.; Drake, S. R. J. Chem. Soc. Chem. Commun. 1994, 2455-2456. Figure 1. A possible mechanism of the activation and cleavage of a phosphate diester substrate by a dinuclear lanthanide-hydroxo species. 2743 Inorg. Chem. 2001, 40, 2743-2750 10.1021/ic001469y CCC: $20.00 © 2001 American Chemical Society Published on Web 05/11/2001