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Dihydrogen and Acetylene Activation by a Gold(I)/ Platinum(0) Transition Metal Only Frustrated Lewis Pair Jesus Campos J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b00491 • Publication Date (Web): 10 Feb 2017 Downloaded from http://pubs.acs.org on February 10, 2017
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Journal of the American Chemical Society
Dihydrogen and Acetylene Activation by a Gold(I)/Platinum(0) Transition Metal Only Frustrated Lewis Pair Jesús Campos* Instituto de Investigaciones Químicas (IIQ), Departamento de Química Inorgánica and Centro de Innovación en Química Avanzada (ORFEO-CINQA). Universidad de Sevilla and Consejo Superior de Investigaciones Científicas (CSIC). Avenida Américo Vespucio 49, 41092 Sevilla (Spain).
Supporting Information Placeholder ABSTRACT: The first example of a Frustrated Lewis Pair (FLP) solely constructed around transition metal centers is described in this work. We have focused on the established capacity of Au(I) and Pt(0) complexes to act as Lewis acidic and basic fragments, respectively, while employing suffit ciently bulky P Bu3 and terphenyl phosphine ligands. This avoids formation of metallic Lewis adducts and confers the Au(I)/Pt(0) pair a remarkable capacity to activate dihydrogen and acetylene molecules in a fashion that closely resembles that of traditional main group FLP systems. As a consequence, unusual heterobimetallic Au(I)/Pt(II) complexes containing hydride (-H), acetylide (-C≡CH) and vinylene (HC=CH-) bridges have been isolated.
The recent development of Frustrated Lewis Pairs (FLPs) has catapulted the chemistry of main group elements beyond its well-established limits, achieving reactivity patterns that 1 were once restricted to transition metals (TMs). A prominent example with profound implications in metal-free cata2 lytic hydrogenations, is the activation of dihydrogen by phosphine/borane pairs. Despite their importance, the catalytic usefulness of FLPs is nevertheless limited to mainly 3 hydrogenation and a few other reactions, possibly as a consequence of the reluctance of main group elements to participate in electron-transfer and other fundamental organometallic reactions, which are key elementary steps in transition metal catalysis. Incorporation of TMs into FLPs is therefore a promising 4 objective and, as such, it has attracted recently considerable attention, as exemplified by the pioneering work of the 5 6 groups of Wass and Erker on Zr/phosphine pairs and related systems. Although some TMs, in particular Pt(0) com7 plexes, have been employed as Lewis base (LB) partners in FLP-like designs, no examples of transition metal-only FLPs (TMOFLPs), where both the acidic and the basic functions 8 are constructed on TMs, seem to be known, despite the existence of a large number of heterobimetallic systems capable of activating small molecules in a fashion reminis9,10 cent of FLPs.
Cationic Au(I) and neutral Pt(0) complexes have a wellknown capacity to behave as LA and LB fragments, respectively. Hence, they were envisioned as suitable building blocks to frame an intermolecular TMOFLP. To attain frusDipp2 )] tration, triflimide Au(I) complex [Au(NTf2)(PMe2Ar 11 Dipp2 (1), containing the bulky terphenyl phosphine PMe2Ar Dipp2 i 12 (Ar = C6H3-2,6-(C6H3-2,6- Pr2)2), and the Pt(0) species t [Pt(P Bu3)2] (2), were utilized as FLP constituents. The present contribution reports the results of our preliminary investigation into the activation of H2 and C2H2 by the resulting cationic Au(I)/Pt(0) TMOFLP. 1
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Although the main H and P{ H} NMR features of complexes 1 and 2 remained unaltered when their C6D6 solutions were combined, upon mixing, the initially colorless solutions of 1 and 2 distinctly became bright yellow. Furthermore, in 31 1 CD2Cl2 as the solvent, the P{ H} NMR resonances of 1 (-11.5 ppm) and 2 (99.6 ppm) become broad, with ω1/2 values (25 ºC) of 35 and 280 Hz, respectively. It should be remarked that 31 1 upon cooling at -30 ºC, the P{ H} signals of 1 and 2 became sharp, and that no NMR line broadening with temperature was noted for separate solutions of 1 and 2. The observed changes may be attributed to establishment of the equilibrium depicted in Scheme 1, in which adduct formation became slightly favored in the more polar, weakly coordinating solvent CD2Cl2, in comparison with C6D6. The reduced thermodynamic stability of adduct A relative to the individual LA and LB components can be ascribed to steric frustration and it is in sharp contrast with the reversible formation of Au/Pt LPs by insertion of [Pt(PCy3)2] into gold-halogen bonds, 13 disclosed recently by Braunschweig and coworkers, as well 14 as to related systems based on [Pt(PR3)2] fragments.
Scheme 1. Proposed solution TMOFLP 1/2 (A was not isolated)
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i
resemble closely those reported for a related complex based 16 on P(tBu)2(o-biphenyl). In addition, complex 3 was independently synthesized by reaction of 1 with SiHEt3, that is to + say, following a general approach to [Au2(μ-H)L2] structural 16,17 motifs (see the Supporting Information for details).
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For complex 5 two strongly shielded H NMR resonances at 195 -1.67 and -11.39 ppm, with Pt satellites of 503 and 1053 Hz, respectively, can be confidently assigned to the bridging and 195 terminal hydride ligands. Observation of Pt satellites for Dipp2 2 the gold-bound PMe2Ar ligand (7.2 ppm, JPPt = 200 Hz) implies the existence of the heterobimetallic Au-Pt core, that 1 31 was further confirmed by observation in the H- P NMR correlation spectrum of cross-peaks between the bridging hydride and the two different kinds of phosphine ligands. This type of structure finds ample precedent in the litera9 + ture, though only two [Au(μ-H)Pt] frameworks were au18,19 thenticated by X-ray crystallography. Figure 1 contains the solid-state molecular structure of complex 5, characterized by a Pt-Au distance of 2.848(1) Å that is somewhat longer 17,18 than found for analogous compounds (ca. 2.70 – 2.80 Å) and it is significantly elongated when compared to others 13,20 The bulkiness of the with direct Pt-Au bonds (2.5-2.6 Å). Dipp2 H-AuPMe2Ar unit bonded to the Pt(II) center is probably responsible for the relatively reduced P2-Pt1-P3 angle of 159.26(9)º.
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Scheme 2. Reaction of Au(I)/Pt(0) TMOFLP with H2. To confirm that an equimolar mixture of complexes 1 and 2 could exhibit FLP behavior, the reactivity towards H2 and C2H2 was investigated. Neither complex 1 nor complex 2 yielded a reaction observable by NMR when their C6D6 solutions were treated with H2 (0.5 bar), even after a week of exposure. In marked contrast, a smooth, quick reaction took place (
