Synchronized Fluxional Motion and Cyclometalation of Ligands in Platinum(II) Complexes

Oscillation of the 2,9-dimethyl-1,10-phenanthroline (dmphen) ligand between nonequivalent exchanging sites in [Pt(Me)(dmphen)(P(o-tolyl)3)]+ and phosphane rotation around the Pt-P bond take place at the same rate. Thus, this cationic complex behaves as a molecular gear, exhibiting a fascinating synchronism between two otherwise independent fluxional motions. The process (G333 = 68.5 ± 0.2 kJ mol-1) was found to be unaffected by (i) the nature of various counteranions (X = PF6- 1, SbF6- 2, CF3SO3- 3, BF4- 4, BArf- 5), (ii) the polarity or the electron-donor properties of the solvent and, (iii) the addition of weak nucleophiles. Restricted phosphane rotation around the Pt-P bond impedes free dmphen oscillation in a 14-electron three-coordinate T-shaped intermediate, containing 1-coordinated dmphen, generated by easy Pt-N bond dissociation from [Pt(Me)(dmphen)(P(o-tolyl)3)]+. 1-5 undergo easy orthoplatination, leading to new [Pt(dmphen){CH2C6H4P(o-tolyl)2-C,P}]X cyclometalated Pt(II) compounds (X = PF6- 1, SbF6- 2, CF3SO3- 3, BF4- 4, BArf- 5). The kinetics of the cyclometalation of 3 and 4 were followed in tetrachloroethane by both 1H NMR and spectrophotometric techniques (kobs = 1.7 × 10-4 s-1 at 333 K, H = 59.3 ± 3 kJ mol-1, and S = -141 ± 8 J K-1 mol-1). Ring opening of dmphen is again a prerequisite for C-H bond activation, which takes place through a multistep oxidative-addition reductive-elimination pathway. The molecular structure of cyclometalated 10 shows a butterfly shape with two o-tolyl rings projected above and below the coordination plane. Variable-temperature 1H NMR spectra revealed hindered rotation around the P-Cipso(o-tolyl) bonds at rather mild temperatures (G333 = 55.2 ± 0.4 kJ mol-1). Dmphen oscillation results very slowly and is dependent on the nature of the counteranions, of the solvents, and is strongly accelerated by the presence of weak nucleophiles that act as catalysts, according to an associative mode of activation.