Rhodium Complexes with the Chelating and Binucleating Ligands P(CH 2 CH 2 Py) n Ph 3-n (Py ) 2-Pyridyl; n ) 1, 2): Structures and Fluxional Behavior M. Ara ´ nzazu Alonso, Juan A. Casares, Pablo Espinet,* and Katerina Soulantica Departamento de Quı ´mica Inorga ´nica, Facultad de Ciencias, Universidad de Valladolid, E-47005 Valladolid, Spain Jonathan P. H. Charmant and A. Guy Orpen School of Chemistry, University of Bristol, Bristol BS8 1TS, United Kingdom ReceiVed June 2, 1999 Several rhodium(I) complexes of the type [RhX(CO)(PePy 2 )], [Rh(diene)(PePy)] + , and [Rh(diene)(PePy 2 )] + (PePy n ) P(CH 2 CH 2 Py) n Ph 3-n ; Py ) 2-pyridyl; n ) 1, 2) have been prepared. The two former are square planar; the latter are pentacoordinated for diene ) tetrafluorobenzobarrelene or norbornadiene (confirmed by X-ray diffraction), but an equilibrium of 4- and 5-coordinate isomers exists in solution for diene ) 1,5-cyclooctadiene. The fluxional behavior of all these complexes is studied by NMR spectroscopy. The complex [Rh(NBD)(PePy 2 )]PF 6 ‚Cl 2 CH 2 crystallizes in the monoclinic space group P2 1 /n with a ) 8.455(1) Å, b ) 18.068(3) Å, c ) 19.729(3) Å,  ) 99.658(3)°, and Z ) 4. The complexes [Rh(diene)(PePy 2 )] + react with CO to give the dimeric complex [Rh 2 (CO) 2 {P(CH 2 CH 2 Py) 2 Ph} 2 ](BF 4 ) 2 with the pyridylphosphine acting as P,N-chelating and P,N-bridging. Introduction There is interest in the chemistry of Rh(I) complexes with ligands containing pendant arms that can either coordinate or leave a vacant position, since this may facilitate a catalytic cycle. A common problem in the study of these complexes is that in square-planar complexes there is usually fast exchange of the pendant and coordinated arms of the multidentate ligand, a situation which is difficult to distinguish from pentacoordination. The problem has been extensively studied in substituted hydrotris(pyrazolyl)borate (Tp R ) complexes, for which many solid-state structures have been described, as well as the existence of equilibria between square-planar and pentacoor- dinated complexes, boat-to-boat conformational changes, and intramolecular substitution processes. 1 Recently, an extreme example has been reported: The compound Tp i Pr Rh(NBD) (NBD ) norbornadiene) crystallizes in two coordination geometries in the same unit cell. 2 This case provides structural information which is lost when the complexes are studied by NMR spectroscopy in solution because of the fast exchange between both structures and between coordinated and uncoor- dinated arms in the square-planar isomer. The authors have characterized the complexes using IR spectroscopy for the assignment of the solution structures. For other multidentate ligands, the situation is less known, since less structural and dynamic information is available. Related behavior is possible for 2-(phosphino)pyridines, which have been extensively used as homo- and heterometallic binucleating ligands. Some representative coordination modes are shown in Chart 1. Thus, 2-(diphenylphosphino)pyridine gives dimeric complexes of the type A, whereas 2,6-bis- (diphenylphosphino)pyridine allows the synthesis of linear compounds of higher nuclearity of the type B. 3 In these complexes, two binucleating ligands are coordinated to the metal center in mutually trans positions, with the remaining ligands also in trans positions. Budzelaar et al. have used a group of binucleating 2-pyridyldiphosphines capable of forming metal complexes in which the P and N atoms are in mutually cis positions, as in type C. 4 2-Pyridylphosphines with the P atom separated from the Py group by one or two methylene links, capable of forming five- and six-membered rings by chelation, have been much less studied and seem to show little tendency to behave as bridging ligands. 3,5 We report here the syntheses and structures of complexes of rhodium(I) with the ligands PePy n (PePy n ) P(CH 2 CH 2 Py) n Ph 3-n ; Py ) 2-pyridyl; n ) 1, 2). PePy can act as a bidentate ligand, whereas PePy 2 is capable of behaving either as a bidentate or as a tridentate chelating ligand, to give square-planar and pentacoordinated structures (D and E), and also as a bridging * Corresponding author. E-mail: espinet@qi.uva.es. (1) (a) Trofimenko, S. Chem. ReV. 1993, 93, 3. (b) Kitajima, N.; Tolman, W. B. Prog. Inorg. Chem. 1995, 43, 419. For recent examples, see: (c) Purwoko, A. A.; Lees, A. J. Inorg. Chem. 1996, 35, 675. (d) Keyes, M. C.; Young, V. G., Jr.; Tolman, W. B. Organometallics 1996, 15, 4133. (e) Chauby, V.; Le Berre, S.; Daran, J.-C.; Commenges, G. Inorg. Chem. 1996, 35, 6345. (f) Sanz, D.; Santa Marı ´a, M. D.; Claramunt, R. M.; Cano, M.; Heras, J. V.; Campo, J. A.; Ruiz, F. A.; Pinilla, E.; Monge, A. J. Organomet. Chem. 1996, 526, 341. (g) Connely, N. G.; Emslie, D. J. H.; Metrz, B.; Orpen, A. G.; Quaile, M. J. J. Chem. Soc., Chem. Commun. 1996, 2289. (h) Oldham, W. J.; Heinekey, D. M. Organometallics 1997, 16, 467. (2) Akita, M.; Ohta, K.; Takahasi, Y.; Hikichi, S.; Moro-Oka, Y. Organometallics 1997, 16, 4121. (3) (a) Newkome, G. R. Chem. ReV. 1993, 93, 2067-2089. (b) Sharp, P. R. In ComprehensiVe Coordination Chemistry; Abel, E. W., Stone, F. G. A., Wilkinson, G., Eds.; Pergamon Press: Oxford, U. K., 1995; Vol. 8, Chapter 2, pp 152-158. (4) Budzelaar, P. H. M.; Frijns, H. G.; Orpen, A. G. Organometallics 1990, 9, 1222. (5) Yang, H.; Lugan, N.; Mathieu, R. Organometallics 1997, 16, 2089. 705 Inorg. Chem. 2000, 39, 705-711 10.1021/ic990634a CCC: $19.00 © 2000 American Chemical Society Published on Web 01/27/2000