Stabilization of Trans Disilyl Coordination at Square-Planar Platinum Complexes

H2 and carbon-heteroatom bond activation mediated by polarized heterobimetallic complexes

The field of heterobimetallic chemistry has rapidly expanded over the last decade. In addition to their interesting structural features, heterobimetallic structures have been found to facilitate a range of stoichiometric bond activations and catalytic processes. The accompanying review summarizes advances in this area since January of 2010. The review encompasses well-characterized heterobimetallic complexes, with a particular focus on mechanistic details surrounding their reactivity applications.

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.

σ-Silane Platinum(II) Complexes as Intermediates in C-Si Bond-Coupling Processes

Platinum complexes [Pt(NHC')(NHC)][BAr^F ] (in which NHC' denotes a cyclometalated N-heterocyclic carbene ligand, NHC) react with primary silanes RSiH₃ to afford the cyclometalated platinum(II) silyl complexes [Pt(NHC-SiHR')(NHC)][BAr^F ] through a process that involves the formation of C-Si and Pt-Si bonds with concomitant extrusion of H₂ . Low-temperature NMR studies indicate that the process proceeds through initial formation of the σ-SiH complexes [Pt(NHC')(NHC)(HSiH₂ R)][BAr^F ], which are stable at temperatures below -10 °C. At higher temperatures, activation of one Si-H bond followed by a C-Si coupling reaction generates an agostic SiH platinum hydride derivative [Pt(H)(NHC'-SiH₂ R)(NHC)][BAr^F ], which undergoes a second Si-H bond activation to afford the final products. Computational modeling of the reaction mechanism indicates that the stereochemistry of the silyl/hydride ligands after the first Si-H bond cleavage dictates the nature of the products, favoring the formation of a C-Si bond over a C-H bond, in contrast to previous results obtained for tertiary silanes. Furthermore, the process involves a trans-to-cis isomerization of the NHC ligand before the second Si-H bond cleavage.