| Abstract
| - In the cation complex [Pt(Me)(dmphen)(P(o-tolyl)3)]+, the rates of dmphen flipping and those of phosphane rotation around the Pt−P bond exhibit identical values. The system behaves as a molecular gear. The connection between the ligand motions appears to be due to the impediment to free dmphen oscillation by restricted phosphane rotation in a three-coordinate intermediate, generated by Pt−N bond dissociation. Coordinative unsaturation promotes also intramolecular C−H activation, leading to cycloplatination through an oxidative addition-reductive elimination reaction pathway.
- 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 otherwiseindependent fluxional motions. The process (ΔG333⧧ = 68.5 ± 0.2 kJ mol-1) was found to be unaffected by (i) thenature of various counteranions (X = PF6-1, SbF6-2, CF3SO3-3, BF4-4, BArf-5), (ii) the polarity or theelectron-donor properties of the solvent and, (iii) the addition of weak nucleophiles. Restricted phosphane rotationaround the Pt−P bond impedes free dmphen oscillation in a 14-electron three-coordinate T-shaped intermediate,containing η1-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-κC,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 4were followed in tetrachloroethane by both 1H NMR and spectrophotometric techniques (kobs = 1.7 × 10-4 s-1 at333 K, ΔH⧧ = 59.3 ± 3 kJ mol-1, and ΔS⧧ = −141 ± 8 J K-1 mol-1). Ring opening of dmphen is again aprerequisite for C−H bond activation, which takes place through a multistep oxidative-addition reductive-eliminationpathway. The molecular structure of cyclometalated 10 shows a butterfly shape with two o-tolyl rings projectedabove and below the coordination plane. Variable-temperature 1H NMR spectra revealed hindered rotation aroundthe P−Cipso(o-tolyl) bonds at rather mild temperatures (ΔG333⧧ = 55.2 ± 0.4 kJ mol-1). Dmphen oscillation resultsvery slowly and is dependent on the nature of the counteranions, of the solvents, and is strongly accelerated bythe presence of weak nucleophiles that act as catalysts, according to an associative mode of activation.
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