Reactivity of alkynyl platinum complexes towards PPh2H and PPh2(O)H: Unexpected formation of alkynyl tetralithium diplatinum compounds stabilized by μ3-(κ3P,O,O-PPh2O-) ligands

Resumen

The reactivity of Li2-[Pt(C≡CR)4] and cis-[Pt(C≡CR)2COD] (R = tBu a, Ph b) towards PPh2H and PPh2(O)H has been investigated. The course of the reaction of Li2[Pt(C≡CR)4] with PPh2H strongly depends not only on the reaction conditions employed, but also on the alkyne subtituent R. Thus, treatment of Li2[Pt(C≡CtBu)4] with PPh2H (1:3) in an acetone/ethanol mixture affords trans-[Pt(C≡CtBu)2-(PPh2H)2] (1a) together with an unusual tetralithium diplatinum species [{Pt(C≡CtBu)2-x(PPh2O)2+xLi 2Sn}2] (x = 0, Sn = (H2O)3 2′a; x = 1, Sn = (H2O)2 3′a) in low yield. Complexes [{Pt(C≡CtBu)2(PPh2O)2Li 2(μ-H2O)-(Me2CO)2}2] (2a) and [{Pt(C≡CtBu)-(PPh2O)3Li2(H 2O)(thf)}2] (3a) have been characterized by X-ray diffraction. On the other hand, treatment of Li2[Pt(C≡CPh)4] with PPh2H (1:3 molar ratio) allows the synthesis not only of the analogous derivatives 1b and 2′b, but also of the unexpected, novel mononuclear compound [Pt(C≡CPh)2-(PPh2CHPhCH2PPh2)] (4b), which has been characterized by X-ray diffraction and contains cis-terminal alkynyl groups and the new chelating ligand 1-phenyl-1,2-bis(diphenylphosphino)ethane. Similar σ-alkynyl complexes cis-[Pt(C≡CR)2(PPh2H)2] 5 are easily prepared by displacement of the cyclooctadiene (COD) ligand from the precursor cis-[Pt(C≡CR)2COD] at low temperature (-30°C) by PPh2H. The unexpected diplatinum complex [{Pt(C≡CtBu)(μ-PPh2)(PPh2H)}2] (6a), also characterized by X-ray diffraction, is also formed (2% yield) during the synthesis of complex 5a. By contrast, cis-[Pt-(C≡CR)2COD] reacts with PPh2(O)H in CH2Cl2 at low temperature (-40°C), either in 1:2 or 1:3 molar ratio, to produce novel diphenylphosphinous acid/phosphinite complexes [{Pt(C≡CR)-{(PPh2O)2H}(PPh2OH)] 7, which are precursors of the related neutral compounds [Pt(C≡CR){(PPh2O)2H}L] (L = PEt3, PPh2H, CNtBu 8-10) and the ionic (NBu4)[Pt(C≡CR)(CN)-{(PPh2O)2H}] (11), which are formed through simple PPh2OH substitution reactions. Complexes 7 can also be doubly deprotonated by LiOH to give the corresponding tetralithium diplatinum species 3; this route is the most convenient one for the synthesis of 3a and the only one for complex 3b.