A Zwitterionic Transition-Metal Complex: Platinum−closo-Borate Coordination Synthesis, Structure, and DFT Calculations

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

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 PSn 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.

Stannaborate transition metal chemistry: ligand properties, reactivity, and density functional theory calculations of platinum and palladium complexes

Three stannaborate complexes of platinum(II) and a novel stannoborate palladium(II) derivative have been prepared in excellent yield. The tin transition metal bond is formed through nucleophilic substitution and the resulting complexes [Bu3MeN] [trans-[(Et3P)2Pt(SnB11H11)H]] (6), [trans-[(Et3P)2Pt(SnB11H11)(CNtBu)]] (7), [Bu3MeN]2[trans-[(Et3P)2Pt(SnB11H11)2-(CNtBu)]] (8), and [Bu3MeN][(dppe)-Pd(SnB11H11)Me] (12) (dppe = 1,2-bis-(diphenylphosphanyl)ethane) were characterized by NMR spectroscopy and elemental analysis. In the cases of the zwitterion 7, the pentacoordinated complex 9, the palladium salt 12 and [(triphos)Pt(SnB11H11)] (10) (triphos = 1,1,1-tris(diphenylphosphanylmethyl)ethane), their solid-state structures are determined by X-ray crystal structure analyses. The trans influence of the [SnB11H11] ligand is evaluated from the results of the IR spectroscopy and X-ray crystallographic structures of complexes 6, 7, and 12. The dipole moment of the zwitterion 7 is calculated by density functional theory (DFT) methods. The alignment of the dipole moments of the polar molecules 7 and 12 in the solid state is discussed.