Interionic Structure of Ion Pairs and Ion Quadruples of Half-Sandwich Ruthenium(II) Salts Bearing r-Diimine Ligands Daniele Zuccaccia, ² Gianfranco Bellachioma, ² Giuseppe Cardaci, ² Gianluca Ciancaleoni, ² Cristiano Zuccaccia, ² Eric Clot,* ,‡ and Alceo Macchioni* ,² Dipartimento di Chimica, UniVersita` degli Studi di Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy, and Institut Charles Gerhardt UniVersite´ Montpellier 2 and CNRS, Case Courrier 1501, Place Euge´ ne Bataillon, 34095 Montpellier Cedex 5, France ReceiVed April 2, 2007 The interionic structure of complexes [Ru(η 6 -Arene){(2-R-C 6 H 4 )NdC(Me)-C(Me)dN(2-R-C 6 H 4 )}- Cl]X was investigated by an integrated experimental (PGSE diffusion and NOE NMR spectroscopy and X-ray single-crystal studies) and theoretical (DFT and ONIOM calculations) approach. PGSE NMR experiments indicated that ion pairing is the main aggregative process in CD 2 Cl 2 and solvents with higher relative permittivity. They also showed that the tendency to ion pairing for isodielectric solvents is higher when the latter are protic. NOE interionic contacts were observed in 2-propanol-d 8 even for BARF - salts. Ion pairing was favored by more coordinating counterions and an increase in concentration. An equilibrium between ion pairs and ion quadruples was observed by PGSE measurements in chloroform-d and benzene-d 6 . Such equilibrium is shifted toward ion quadruples by an increase in the concentration or when least coordinating counterions are used. For small fluorinated counterions, NOE studies located the anion in ion pairs above the plane containing the CdN imine moieties. ONIOM calculations found that this anion-cation orientation was at least 35.9 kJ/mol lower in energy than a second orientation with the anion close to cymene, which, in some cases, was observed in the solid state. NOE investigations on complexes with BPh 4 - counterion did not allow a single orientation capable of explaining the observed NOEs to be found. X-ray studies showed that one cation is surrounded by two anions. ONIOM calculations found that these two anion-cation orientations have similar energies. X-ray and NOE NMR data strongly suggest that ion quadruples with BPh 4 - anions are constituted by an alternation of cations and anions. Interionic NOE intensities are almost invariant on passing from ion pairs to ion quadruples with small fluorinated counterions. X-ray studies suggested at least four possible structures of ion quadruples differing in both disposition and orientation of the ionic moieties. Three structures considered by ONIOM calculations were similar in energy, but more stable than the separated ion pairs. Introduction The ion-pairing 1 phenomenon plays a crucial role in transi- tion-metal chemistry. 2,3 Many chemical reactions are mediated by ionic (very often cationic) transition-metal complexes, and a proper choice of counterion and solvent is critical in order to maximize activity and selectivity. 2 A particularity of transition- metal ion pairs is that the counterion can occupy one of the coordinating sites or remain in the second coordination sphere, affording inner-sphere ion pairs (ISIPs) or outer-sphere ion pairs (OSIPs), respectively. 2 In favorable conditions, i.e., elevated concentration in solvents with low relative permittivity, OSIPs may aggregate, forming ion quadruples. 4,5 The formation of the latter can be facilitated by the establishment of “inter-ion-pair” hydrogen bonds 6 or π-π stacking interactions. 7,8 Looking at ion pairs as globally neutral species, the association of two ion pairs to form an ion quadruple differs little from the association of two neutral and polarized molecules to form a dimer. 9 It would be extremely important to correlate the structure of ion pairs and ion quadruples in solution with their reactivity. In recent years the interionic structure of several transition-metal complex ion pairs has been determined by means of NOE (nuclear Overhauser effect) 10 and PGSE (pulsed field gradient spin-echo) 11 NMR experiments, but a clear correlation with their reactivity has been found only in a few cases. 12,13 On the other hand, almost nothing is known about the interionic structure of ion quadruples in solution. Herein we report the interionic structure of ion pairs and ion quadruples of [Ru(η 6 -Arene){(2-R-C 6 H 4 )NdC(Me)-C(Me)d N(2-R-C 6 H 4 )}Cl]X complexes 14 investigated through an inte- * Corresponding authors. E-mail: clot@univ-montp2.fr; alceo@unipg.it. ² Universita` degli Studi di Perugia. ‡ Universite´ Montpellier 2. (1) Marcus, Y.; Hefter, G. Chem. ReV. 2006, 106, 4585-4621. (2) Macchioni, A. Chem. ReV. 2005, 105, 2039-2073, and references therein. (3) Chen, E. Y.-X.; Marks, T. J. Chem. ReV. 2000, 100, 1391. Bochmann, M. J. Organomet. Chem. 2004, 689, 3982. (4) Beck, S.; Geyer, A.; Brintzinger, H.-H. Chem. Commun. 1999, 2477- 2478. (5) Zuccaccia, C.; Stahl, N. G.; Macchioni, A.; Chen, M.-C.; Roberts, J. A.; Marks, T. J. J. Am. Chem. Soc. 2004, 126, 1448-1464. Song, F.; Lancaster, S. J.; Cannon, R. D.; Schormann, M.; Humphrey, S. M.; Zuccaccia, C.; Macchioni, A.; Bochmann, M. Organometallics 2005, 24, 1315-1328. (6) Zuccaccia, D.; Macchioni, A. Organometallics 2005, 24, 3476-3486. (7) Macchioni, A.; Romani, A.; Zuccaccia, C.; Guglielmetti, G.; Querci, C. Organometallics 2003, 22, 1526. Zuccaccia, D.; Bellachioma, G.; Cardaci, G.; Zuccaccia, C.; Macchioni, A. Dalton Trans. 2006, 1963. (8) Hamidov, H.; Jeffery, J. C.; Lynam, J. M. Chem. Commun. 2004, 1364. (9) Zuccaccia, D.; Clot, E.; Macchioni, A. New J. Chem. 2005, 29, 430- 433. (10) Macchioni, A. Eur. J. Inorg. Chem. 2003, 195, and references therein. 3930 Organometallics 2007, 26, 3930-3946 10.1021/om7003157 CCC: $37.00 © 2007 American Chemical Society Publication on Web 06/23/2007