| Abstract
| - The fluxional motion that exchanges the coordination site of the two phenanthroline nitrogen atoms in [Pt(Me)(L)(dmphen)]+X- complexes is strongly affected by the nature of the coordinated ligand L and, when L = PPh3, by the coordinating properties of the solvent, of the counterion X- and of nucleophiles added in solution. The mechanism is switchable between dissociative and associative pathways, depending on the structural features of the complexes and the action of potential nucleophiles.
- The ionic methylplatinum(II) complexes [Pt(Me)(L)(dmphen)]X (dmphen = 2,9-dimethyl-1,10-phenanthroline,L = Me2SO, X = PF6-1a, BF4-1b, CF3SO3-1c, ClO4-1d, B(C6H5)4-1e, [B(3,5-(CF3)2C6H3)4]-1f; L =n-Bu2SO, X = CF3SO3-1g; L = PPh3, X = PF6-2a, BF4-2b, CF3SO3-2c, ClO4-2d, B(C6H5)4-2e, [B(3,5-(CF3)2C6H3)4]-2f; X = CF3SO3-, L = CyNH23a, i-PrNH23b, 2,6-Me2py 3c, EtNH23d, AsPh33e,dimethylthiourea (Me2th) 3f and the uncharged [Pt(Me)(X)(dmphen)] (X = SCN-4a, SeCN-4b) complexeshave been synthesized and fully characterized. In chloroform, as well as in acetone or methanol, complexes1a−1g, 2a−2h (X = Cl-g, NO2-h, formed “in situ”), and 3e show dynamic behavior due to the oscillation ofthe symmetric chelating ligand dmphen between nonequivalent bidentate modes. All the other compounds featurea static structure in solution. The crystal structure of 2a shows a tetrahedral distortion of the square planarcoordination geometry, a loss of planarity of the dmphen ligand, and, most notably, a rotation of the dmphenmoiety, around the N1−N2 vector, to form a dihedral angle of 42.64(8)° with the mean coordination plane. Thehexafluorophosphate ion lies on the side of the phenanthroline ligand. The interionic structures of 2a, 2b, and 2fwere investigated in CDCl3 at low temperature by 1H-NOESY and 19F{1H}-HOESY NMR spectroscopies. WhereasPF6- (2a) and BF4- (2b) show strong contacts with the cation [Pt(Me)(PPh3)(dmphen)]+, being located preferentiallyon the side of the phenanthroline ligand, the [B(3,5-(CF3)2C6H3)4]- (2f) ion does not form a tight ion pair. Thedynamic process was studied by variable-temperature NMR spectroscopy for 1a−1f and 2a−2h in CDCl3. Theactivation energies ΔG⧧298 for the sulfoxide complexes 1a−1f are lower than those of the corresponding phosphinecomplexes 2a−2f by ≈10 kJ mol-1. The nature of the counteranion exerts a tangible influence on the fluxionalityof dmphen in both series of complexes 1 and 2. The sequence of energies observed for 2a−2h encompasses anoverall difference of about 16 kJ mol-1, increasing in the order Cl- ≈ NO2- ≪ CF3SO3-< ClO4-< B(C6H5)4-< BF4- ≈ PF6-< B(3,5-(CF3)2C6H3)4-. Acetone and methanol have an accelerating effect on the flipping.Concentration-dependent measurements, carried out in CDCl3 for 2a with n-Bu4NPF6 and the ligands dmphen,n-Bu2SO, sec-Bu2SO, and sec-Bu2S showed that the rate of the fluxional motion is unaffected by added n-Bu4NPF6,whereas in the other cases this increases linearly with increasing ligand concentration, according to a pattern ofbehavior typical of substitution reactions. Dissociative and associative mechanisms can be envisaged for the observedprocess of flipping. Dissociation can be prevalent within the ion pair formed by a “noncoordinating” anion withthe metallic cationic complex in chloroform. Among the possible associative mechanisms, promoted by polarsolvents or by relatively strong nucleophiles, a consecutive displacement mechanism is preferred to intramolecularrearrangements of five-coordinate intermediates.
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