Binuclear Platinum(II) Triazolopyrimidine Bridged Complexes. Preparation, Crystal Structure, NMR Spectroscopy, and ab Initio MO Investigation on the Bonding Nature of the Pt(II)‚‚‚Pt(II) Interaction in the Model Compound {Pt 2 [NHCHN(C(CH 2 )(CH 3 ))] 4 } Jorge A. R. Navarro, ² M. Angustias Romero, ² Juan M. Salas,* ,² Miguel Quiro ´ s, ² Jaouad El Bahraoui, ‡ and Jose ´ Molina ‡ Departamento de Quı ´mica Inorga ´nica and Laboratorio de Modelizacio ´n y Disen ˜o Molecular, Universidad de Granada, 18071 Granada, Spain ReceiVed May 8, 1996 X A condensation reaction between two cis-[PtCl 2 (Hmtpo) 2 ] (where Hmtpo) 4,7-H-5-methyl-7-oxo[1,2,4]triazolo- [1,5a]pyrimidine) molecules takes place in neutral aqueous media, giving [Pt 2 (μ-mtpo) 4 ]‚4H 2 O(1). The X-ray structure of the recrystallization product of 1 in DMSO:EtOH (1:1) (DMSO ) dimethyl sulfoxide), namely, [Pt 2 (μ-mtpo) 4 ]‚2DMSO (2) has been determined. Compound 2 crystallizes in the orthorhombic space group Pbcn with unit cell dimensions a ) 14.207(3) Å, b ) 15.187(3) Å, c ) 17.165(3) Å, and Z ) 4. The molecular structure shows that the two Pt atoms are bridged by four mtpo ligands. Thus, it presents two face to face PtN 4 units with a Pt‚‚‚Pt 2.744(2) Å separation. Compound 1 has also been characterized by 1 H and 195 Pt NMR. The very short Pt(II)‚‚‚Pt(II) contact suggests an interaction between the two metal centers, supported by the great deshielding observed for the platinum nuclei in the 195 Pt NMR spectrum (δ )-2005 ppm) compared to a Pt(II) in a typical N 4 environment. In order to make an approach to the possible bonding nature of the Pt(II)‚‚‚Pt(II) interaction, a theoretical analysis has been performed on the basis of the properties of the electronic charge density distribution, derived from ab initio MO calculations for the model compound {Pt 2 [NHCHN(C(CH 2 )(CH 3 ))] 4 } using RHF/LANL2DZ and B3LYP/LANL2DZ wave functions; both take into account relativistic effects and the second electronic correlation also. A significant directional interaction between the two metal centers has been found. Thus, a bond critical point appears between the two platinum nuclei, with a density of charge F b ) 0.056 e‚bohr -3 , which is half of that found for the platinum nitrogen bond. Moreover, a value of the energy density, E d (r b ) < 0(E d (r) )-0.0175 hartree‚bohr -3 ), at this point, shows the bonding nature of the interaction. Introduction Association of square-planar complexes of Pt(II) and other d 8 metal ions in pairs or stacks is a well-known phenomenon, both in the solid state 1,2 and in solution. 3,4 These compounds present M‚‚‚M contacts in the range 2.6-3.5 Å, which is shorter than the sum of van der Waals radii. This association appears to be a key step in the formation of one-dimensional materials such as Pt mixed valence complexes. 1,2 One of the most interesting subjects of research in this field has been the chemistry of cisplatin and related compounds. Apart from the anticarcinogenic properties of these compounds, 5 they have been used with the help of secondary bridging ligands such as R-pyridone, R-pyrrolidone, 1-methyluracil, 1-methylcitosine, and analogous compounds, to prepare homo- 1,6 and heteronuclear 7 complexes. The resulting polynuclear compounds display interesting properties due to the short M-M contacts, such as photochemical properties, 8 stabilization of rare states of oxida- tion for platinum 9 and palladium, 10 or the utilization of the platinum nucleus as a weak σ-donor ligand in the coordination chemistry of metal ions. 11 The present paper reports the ability of a cisplatin analogue, namely, cis-[PtCl 2 (Hmtpo-N(3)) 2 ] 12 (where Hmtpo ) 4,7-H-5- methyl-7-oxo[1,2,4]triazolo[1,5a]pyrimidine) for generating plati- num binuclear complexes. This compound presents the inter- esting feature that the nitrogen donor ligand is able to participate as a bridging ligand. Thus, it is possible to get a tetrabridged platinum binuclear complex [Pt 2 (μ-mtpo-N(3),N(4)) 4 ] (Figure 1a), with an intermetallic distance (d Pt-Pt ) 2.744(2) Å) much shorter than the doubly bridged compounds achieved from cisplatin. 1 ² Departamento de Quı ´mica Inorga ´nica. ‡ Laboratorio de Modelizacio ´n y Disen ˜o Molecular. X Abstract published in AdVance ACS Abstracts, November 15, 1996. (1) (a) Hollis, L. S.; Lippard, S. J. J. Am. Chem. Soc. 1981, 103, 1230. (b) Tro ¨tscher, G.; Micklitz, W.; Scho ¨llhorn, H.; Thewalt, U.; Lippert, B. Inorg. Chem. 1990, 29, 2541. (2) Miller, J. S.; Epstein, A. J. Prog. Inorg. Chem. 1976, 20, 1. (3) Bailey, J. A.; Hill, M. G.; Marsh, R. E.; Miskowski, V. M.; Schaefer, W. P.; Gray, H. B. Inorg. Chem. 1995, 34, 4591. (4) Jennette, K. W.; Gill, J. T.; Sadownick, J. A.; Lippard, S. J. J. Am. Chem. Soc. 1976, 98, 6159. (5) Rosenberg, B.; vanCamp, L.; Trosko, J. F.; Mansour, V. H. Nature 1969, 222, 385. (6) (a) Scho ¨llhorn, H.; Thewalt, U.; Lippert, B. Inorg. Chim. Acta 1984, 93, 19. (b) Faggiani, R.; Lippert, B.; Lock, C. J. L.; Speranzini, R. A. J. Am. Chem. Soc. 1981, 103, 1111. (c) Matsumoto, K.; Harashima, K.; Moriyama, H.; Sato, T. Inorg. Chim. Acta 1992, 197, 217. (7) (a) Micklitz, W.; Riede, J.; Huber, B.; Mu ¨ller, G.; Lippert, B. Inorg. Chem. 1988, 27, 1979. (b) Lippert, B.; Scho ¨llhorn, H.; Thewalt, U. Inorg. Chem. 1987, 26, 1736. (8) Yip, H.-K.; Che, C. M.; Zhou, Z.-Y.; Mak, T. C. W. J. Chem. Soc., Chem. Commun. 1992, 1369. (9) (a) Barton, J. K.; Rabinowitz, H. N.; Szalda, D. J.; Lippard, S. J. J. Am. Chem. Soc. 1977, 99, 2827. (b) Sakai, K.; Matsumoto, K. J. Am. Chem. Soc. 1989, 111, 3074. (c) Matsumoto, K.; Takabashi, H.; Fuwa, K. J. Am. Chem. Soc. 1984, 106, 2049. (d) Renn, O.; Albinati, A.; Lippert, B. Angew. Chem., Int. Ed. Engl. 1990, 29, 84. (e) Lippert, B.; Scho ¨llhorn, H.; Thewalt, U. J. Am. Chem. Soc. 1986, 108, 525. (10) Micklitz, W.; Mu ¨ller, G.; Huber, B.; Riede, J.; Rashwan, F.; Heinze, J.; Lippert, B. J. Am. Chem. Soc. 1988, 110, 7084. (11) (a) Goodgame, D. M. L.; Hitchman, M. A.; Lippert, B. Inorg. Chem. 1986, 25, 2191. (b) Duckworth, D. M.; Goodgame, D. M. L.; Hitchman, M. A.; Lippert, B.; Murray, K. S. Inorg. Chem. 1987, 26, 1823. (12) Rodrı ´guez, J.; Salas, J. M.; Romero, M. A.; Quiro ´s, M.; Faure, R. J. Inorg. Biochem. 1995, 59, 223. 7829 Inorg. Chem. 1996, 35, 7829-7835 S0020-1669(96)00496-X CCC: $12.00 © 1996 American Chemical Society