Chemical Science EDGE ARTICLE View Article Online View Journal | View Issue Metal–metal cooperative bond activation by heterobimetallic alkyl, aryl, and acetylide PtII/CuI Cite this: Chem. Sci., 2020, 11, 5494 complexes† All publication charges for this article have been paid for by the Royal Society a a a of Chemistry Shubham Deolka, Orestes Rivada-Wheelaghan, ‡* Sandra L. Aristizabal,´ Robert R. Fayzullin, b Shrinwantu Pal, c Kyoko Nozaki, c Eugene Khaskin a and Julia R. Khusnutdinova *a We report the selective formation of heterobimetallic PtII/CuI complexes that demonstrate how facile bond activation processes can be achieved by altering the reactivity of common organoplatinum compounds through their interaction with another metal center. The interaction of the Cu center with the Pt center and with a Pt-bound alkyl group increases the stability of PtMe2 towards undesired rollover Received 3rd February 2020 cyclometalation. The presence of the CuI center also enables facile transmetalation from an electron- Accepted 29th April 2020 F À deficient tetraarylborate [B(Ar )4] anion and mild C–H bond cleavage of a terminal alkyne, which was DOI: 10.1039/d0sc00646g Creative Commons Attribution-NonCommercial 3.0 Unported Licence. not observed in the absence of an electrophilic Cu center. The DFT study indicates that the Cu center rsc.li/chemical-science acts as a binding site for the alkyne substrate, while activating its terminal C–H bond. Introduction transmetalation step in the Sonogashira coupling.4 The accel- erating effect of metal additives (e.g. Cu salts) in Pd-catalyzed Metal–metal cooperation plays a crucial role in small molecule C–C coupling reactions such as Stille and Suzuki coupling is activation in enzymes and synthetic systems, including homo- commonly referred to as “the copper effect”.5 geneous and heterogeneous catalysts.1 Many classical catalytic However, a precise understanding of how the reactivity at the This article is licensed under a systems rely on the thorough optimization of the ligand envi- single metal center can be affected by communication with ronment to induce the desired reactivity at a single metal a second metal is oen lacking due to the synthetic challenges center. However, there is currently a growing realization that in selective synthesis of such reactive heterobimetallic ’ complexes with a well-dened structure. Open Access Article. Published on 02 May 2020. Downloaded 10/17/2020 7:05:15 AM. catalysts reactivities can be signi cantly altered through close communication with another metal center, either by design or While multiple symmetrical ligand platforms have been via unexpected bimetallic processes, thus enabling new developed for the construction of homobimetallic complexes,6 approaches to bond activation.2 examples of ligand scaffolds that could selectively support The second metal may facilitate substrate binding and pre- metal–metal interactions between two different metals and at activation or even stabilize the bond activation product the same time contain available reactive sites, are exceedingly (Scheme 1). The adoption of the bimetallic approach has led to rare.7 This is especially the case when the combination of a 1st many recent advances in stoichiometric and catalytic bond row and a late 2nd or 3rd row transition metal is targeted. Among activation processes.2,3 Bimetallic cooperation is oen proposed known heterobimetallic complexes, rigid ligand design oen in many C–C coupling processes, e.g. the Cu-to-Pd blocks access to coordination sites suitable for cooperative aCoordination Chemistry and Catalysis Unit, Okinawa Institute of Science and Technology Graduate University, 1919-1 Tancha, Onna-son, 904-0495, Okinawa, Japan. E-mail: [email protected]; [email protected] bArbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientic Center, Russian Academy of Sciences, 8 Arbuzov Street, Kazan, 420088, Russian Federation cDepartment of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan † Electronic supplementary information (ESI) available. CCDC 1975187–1975194. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d0sc00646g ‡ Current address: Orestes Rivada-Wheelaghan: Universit´e de Paris, Laboratoire d'Electrochimie Mol´eculaire, UMR 7591 CNRS, 15 rue Jean-Antoine de Ba¨ıf, Scheme 1 Schematic representation of the altering reactivity of F-75205 Paris Cedex 13, France. a single metal center through heterobimetallic complex formation. 5494 | Chem. Sci.,2020,11,5494–5502 This journal is © The Royal Society of Chemistry 2020 View Article Online Edge Article Chemical Science substrate binding.8 Although several other classes of binu- immediate migration of the Pt center from the so to the hard cleating bridging ligands have been developed, many of these site is observed, presenting an alternative pathway for the ligands do not allow for close metal–metal interactions in formation of 2. The mono-metallic reactivity thus conrms homo- or heteropolymetallic systems.9 a proof-of-concept for using this ligand platform further in In this work, we report a new bifunctional so/hard designing heterobimetallic systems selectively via the so/hard unsymmetrical ligand scaffold, which selectively incorporates Lewis acid concept. both PtII and CuI centers. The close proximity between the two Next we targeted the formation of heterobimetallic metals allows for coordination of alkyl, aryl, or acetylide ligands complexes using PtII and CuI precursors as this combination is to both metal centers, and for the dialkyl complexes, enables known to show metallophilic closed-shell d8–d10 interactions.13 metal–metal interaction. These heterobimetallic complexes First, treatment of complex 1 with 2 equiv. of CuICl led to the ff II I I allow us to directly observe the e ect of the second metal center formation of the heterobimetallic Pt Me2/Cu complex 3[Cu Cl2] on the reactivity of Pt, which is interesting given that Pt (Scheme 3). By comparison, the reaction with 1 equiv. of CuICl complexes with d10 metal additives are widely used in C–H bond gave a mixture of products with the major species characterized 10 1 I activation and studied as models for Pd-catalyzed cross- by an H NMR spectrum similar to that of complex 3[Cu Cl2] coupling.11 Our ndings demonstrate that even subtle interac- indicative of the formation of a similar CuI/PtII core; however, tions between two different metals alter the solution reactivity the reaction was generally less clean and did not proceed with of common organometallic species, particularly in the C–H high yield. To avoid the presence of a potentially non-innocent I ¼ F F ¼ bond activation and boron-to-metal transmetalation reactions, counter anion, [Cu (MeCN)4][X] (X BF4 or B(Ar )4; B(Ar )4 and ligand architecture that induces proximity signicantly tetrakis[3,5-bis(triuoromethyl)phenyl]borate) was used ff F a ects reactivity. leading to complexes 3[BF4] and 3[B(Ar )4], respectively. All complexes were isolated in 65–71% yields, characterized by X-ray diffraction (XRD) (vide infra), NMR, IR, and UV-vis Creative Commons Attribution-NonCommercial 3.0 Unported Licence. Results and discussion spectroscopies, ESI-MS, and elemental analysis. Ligand design and synthesis of monometallic and Noting that the ligand platform also contains an acidic heterobimetallic complexes benzylic CH2 position and the previously reported ability of the mononucleating PNN pincer ligands to undergo an N-bound We have previously reported the reversible stepwise formation 14 CH2-arm deprotonation coupled with dearomatization, we of homomultimetallic CuI linear chain complexes using an attempted the dearomatization of our binucleating P,N-donor unsymmetrical napthyridinone-based ligand L (Scheme 2).12 0 ligand L using a strong base. Simple functionalization of the O-atom of this ligand by reac- t Gratifyingly, treatment of 3[CuCl2] with KO Bu resulted in tion with chlorobis(tert-butyl)phosphine leads to a new ligand a deep red solution, from which 4 could be isolated cleanly. This article is licensed under a L, in which two well-de ned coordination sites are created: Complex 4 features a dearomatized naphthyridine ring owing to a hard-donor site containing a picolylamine arm and a so- the deprotonation of its CH2 arm and thus is the rst example of donor site containing a phosphinite arm. We rst tested if a dearomatized heterobimetallic complex, resembling dear- ligand L shows differentiation between so and hard Lewis Open Access Article. Published on 02 May 2020. Downloaded 10/17/2020 7:05:15 AM. omatization in pincer-based mononucleating ligands utilized acids using (NBD)PtIIMe (NBD ¼ norbornadiene) and 2 for metal–ligand cooperation catalysis.15 Dearomatization of the [PtIVMe I] precursors, respectively. As expected, the specic 3 4 naphthyridine-based binucleating PNNP (“expanded pincer”) binding of the PtII center to the so phosphinite site only is IV ligand has also been recently reported by Broere and co-workers observed, giving complex 1. The Pt center, on the other hand, 6f in a homobimetallic Cu2 complex. specically binds to the hard N-donor site (complex 2). Inter- estingly, upon reacting complex 1 with methyl iodide, Scheme 2 (a) Synthesis of ligand L; (b) selective binding to PtII and PtIV Scheme 3 Formation of cationic heterobimetallic complexes 3[X] and centers via hard and soft sites. a dearomatized heterobimetallic complex 4. This journal is © The Royal Society of Chemistry 2020 Chem. Sci.,2020,11,5494–5502 | 5495 View Article Online Chemical Science Edge Article Characterization of heterobimetallic complexes in the solid Table 1 Selected interatomic and bond distances in complexes 3[X] s state and solution and analysis of metal–metal interactions determined by XRD and 4 values for the Pt center a ˚ F The structures of complexes 3[X] and 4 were con rmed by Bond distance (A) 3[CuCl2] 3[BF4] 3[B(Ar )4] single crystal XRD studies (Fig.