Fluxional Behavior of the Dinitrogen Ligand 2,9-Dimethyl-1,10-phenanthroline in Cationic Methyl Platinum(II) Complexes (original) (raw)

2001, Inorganic Chemistry

The ionic methylplatinum(II) complexes [Pt(Me)(L)(dmphen)]X (dmphen ) 2,9-dimethyl-1,10-phenanthroline, L ) Me 2 SO, X ) PF 6 -1a, BF 4 -1b, CF 3 SO 3 -1c, ClO 4 -1d, B(C 6 H 5 ) 4 -1e, [B(3, ) 2 C 6 H 3 ) 4 ] -1f; L ) n-Bu 2 SO, X ) CF 3 SO 3 -1g; L ) PPh 3 , X ) PF 6 -2a, BF 4 -2b, CF 3 SO 3 -2c, ClO 4 -2d, B(C 6 H 5 ) 4 -2e, [B(3, ) 2 C 6 H 3 ) 4 ] -2f; X ) CF 3 SO 3 -, L ) CyNH 2 3a, i-PrNH 2 3b, 2,6-Me 2 py 3c, EtNH 2 3d, AsPh 3 3e, dimethylthiourea (Me 2 th) 3f and the uncharged [Pt(Me)(X)(dmphen)] (X ) SCN -4a, SeCN -4b) complexes have been synthesized and fully characterized. In chloroform, as well as in acetone or methanol, complexes 1a-1g, 2a-2h (X ) Clg, NO 2h, formed "in situ"), and 3e show dynamic behavior due to the oscillation of the symmetric chelating ligand dmphen between nonequivalent bidentate modes. All the other compounds feature a static structure in solution. The crystal structure of 2a shows a tetrahedral distortion of the square planar coordination geometry, a loss of planarity of the dmphen ligand, and, most notably, a rotation of the dmphen moiety, around the N1-N2 vector, to form a dihedral angle of 42.64(8)°with the mean coordination plane. The hexafluorophosphate ion lies on the side of the phenanthroline ligand. The interionic structures of 2a, 2b, and 2f were investigated in CDCl 3 at low temperature by 1 H-NOESY and 19 F{ 1 H}-HOESY NMR spectroscopies. Whereas PF 6 -(2a) and BF 4 -(2b) show strong contacts with the cation [Pt(Me)(PPh 3 )(dmphen)] + , being located preferentially on the side of the phenanthroline ligand, the [B(3,5-(CF 3 ) 2 C 6 H 3 ) 4 ] -(2f) ion does not form a tight ion pair. The dynamic process was studied by variable-temperature NMR spectroscopy for 1a-1f and 2a-2h in CDCl 3 . The activation energies ∆G q 298 for the sulfoxide complexes 1a-1f are lower than those of the corresponding phosphine complexes 2a-2f by ≈10 kJ mol -1 . The nature of the counteranion exerts a tangible influence on the fluxionality of dmphen in both series of complexes 1 and 2. The sequence of energies observed for 2a-2h encompasses an overall difference of about 16 kJ mol -1 , increasing in the order Cl -≈ NO 2 -, CF 3 SO 3 -< ClO 4 -< B(C 6 H 5 ) 4 -< BF 4 -≈ PF 6 -< B(3,5-(CF 3 ) 2 C 6 H 3 ) 4 -. Acetone and methanol have an accelerating effect on the flipping. Concentration-dependent measurements, carried out in CDCl 3 for 2a with n-Bu 4 NPF 6 and the ligands dmphen, n-Bu 2 SO, sec-Bu 2 SO, and sec-Bu 2 S showed that the rate of the fluxional motion is unaffected by added n-Bu 4 NPF 6 , whereas in the other cases this increases linearly with increasing ligand concentration, according to a pattern of behavior typical of substitution reactions. Dissociative and associative mechanisms can be envisaged for the observed process of flipping. Dissociation can be prevalent within the ion pair formed by a "noncoordinating" anion with the metallic cationic complex in chloroform. Among the possible associative mechanisms, promoted by polar solvents or by relatively strong nucleophiles, a consecutive displacement mechanism is preferred to intramolecular rearrangements of five-coordinate intermediates. (1) (a) Fanizzi, F. P.; Intini, F. P.; Maresca, L.; Natile, G.; Lanfranchi, M.; Tiripicchio, A. J. Chem. Soc., Dalton Trans. 1991, 1007. (b) De Felice, V.; Albano, V. G.; Castellari, C.; Cucciolito, M. E.; De Renzi, A. J. Organomet. Chem. 1991, 403, 269. (c) Milani, B.; Alessio, E.; Mestroni, G.; Sommazzi, A.; Garbassi, F.; Zangrando, E.; Bresciani-Pahor, N.; Randaccio, L.