Alkyne Insertion into the Pt−H Bond of Pt(H)(1-pentene)(β-diiminate) Initiates a Reaction Cascade That Results in C−H Activation or C−C Coupling

The lability of the 1-pentene ligand in (Cl-nacnac)Pt(H)(1-pentene) (1; Cl-nacnac = bis(N-aryl)-β-diiminate), which has been synthesized from the reaction of Me4Pt2(μ-SMe2)2 with Cl-nacnacH in pentane, allows the reactivity of the (Cl-nacnac)Pt(H) fragment to be probed. The reaction of 1 with alkynes was explored. Alkynes displace pentene and rapidly undergo insertion into the Pt−H bond of 1 to initiate a reaction cascade. The identity of the final platinum product depends on the substituents on the alkyne reagent. Reaction of 1 with 2-butyne results in hydrogen migration after insertion and yields an η3-allyl product, (Cl-nacnac)Pt(η3-C3H4Me) (2). When reacted with 1, terminal silyl alkynes R3SiCCH (R3Si = Me3Si, Ph3Si) undergo C−H activation of one of the silyl substituents after initial alkyne insertion into the Pt−H bond, resulting in net hydrogenation of the alkyne. The result is formation of (Cl-nacnac)Pt((o-C6H4)SiPh2(CHCH2)) (3) or (Cl-nacnac)Pt((CH2)SiMe2(CHCH2)) (5), which contains either an aryl or an alkyl ligand with a chelated vinylsilane substituent; complex 3 was characterized by X-ray crystallography. These products are the result of 2,1-insertion of the alkyne into the Pt−H bond, thus placing the silyl group and Pt on the same carbon, which positions the substituents on the silicon proximal to Pt and ripe for C−H activation. In contrast to the silylalkyne reagents, terminal alkynes lacking propargylic hydrogens, PhCCH and t-BuCCH, insert into the Pt−H bond in a 1,2- rather than a 2,1-fashion, which places the alkyne substituent β to the metal and precludes facile C−H activation. Instead, a second alkyne enters the coordination sphere and couples with the adjacent vinyl group in a head-to-tail fashion to form (Cl-nacnac)Pt(η1:η2-C(H)C(R)C(H)C(H)(R)) (7), which has a chelated η1:η2-butadienyl ligand with R groups at the 2- and 4-positions.