Bi- and tridentate stannylphosphines and their coordination to low-valent platinum

Abstract

Semirigid bifunctional tin-substituted o-tolylphosphines of general formulae [Ph₂P(o-C₆H₄CH₂)SnR₃] (R = Ph, 1; R = Me, 2) and [{Ph₂P(o-C₆H₄CH₂)}₂SnPh₂] (3) were synthesized and isolated in good yields. The new compounds were fully characterized by single-crystal X-ray diffraction and multinuclear solution NMR spectroscopic techniques. The observed J(¹¹⁹Sn,³¹P) values in solution NMR spectroscopy as well as the P⋯Sn distances in the solid state and DFT calculations (B3LYP) on compounds 1 and 3 do not support the existence of intramolecular P → Sn bond interactions in either of the three compounds. 1 and 2 reacted with stoichiometric amounts of tristriphenylphosphine platinum(0) [Pt(PPh₃)₃] under toluene refluxing conditions leading to formation of Pt(II) distorted square-planar complexes [Ph₂P(o-C₆H₄CH₂)Pt(SnR₃)(PPh₃)], (R = Ph, 4; R = Me, 5), each bearing a five-membered carbometallated ring resulting from Pt coordination to P and the benzylic C sp³ atom of the ligand architecture rather than from activation of the terminal Sn–C carbon bonds of the phenyl or methyl substituents which would have rendered six-membered rings. Additionally, the fragment SnR₃ also binds to the metal centre disposing cis to the cyclometalated carbon atom and to the single remaining PPh₃. This carbometallation takes place affecting the integrity of the ligand skeleton. NBO calculations show the Sn fragment coordinates to the metal as X-type stannyl, SnR₃. The analogous reaction of [Pt(PPh₃)₃] towards the stannyldiphosphine 3 leads to the quantitative formation of complex [(Ph₂P-o-C₆H₄CH₂)Pt(Ph₂P-o-C₆H₄CH₂SnPh₃)], 6, which exhibits five- and six-membered metallacycles at the expense of the ligand frame. All compounds were characterized exhaustively by solution spectroscopic measurements and by single crystal X-ray diffraction analysis. DFT computations corroborate the higher stability of the observed products over those resulting from preservation of the ligand backbone.