Metal-Metal Cooperative Bond Activation by Heterobimetallic Alkyl, Aryl, and Acetylide Ptii/Cui Complexes Shubham Deolka,† Orestes Rivada-Wheelaghan,†,║,* Sandra L

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Metal-metal cooperative bond activation by heterobimetallic alkyl, aryl, and acetylide PtII/CuI complexes Shubham Deolka,† Orestes Rivada-Wheelaghan,†,║,* Sandra L. Aristizábal,† Robert R. Fayzullin,‡ Shrinwantu Pal,§ Kyoko Nozaki,§ Eugene Khaskin,† and Julia R. Khusnutdinova†*

†Coordination Chemistry and Catalysis Unit Okinawa Institute of Science and Technology Graduate University, 1919-1 Tan- cha, Onna-son, 904-0495, Okinawa, Japan. ‡Arbuzov Institute of Organic and Physical Chemistry, FRC Kazan Scientific Center, Russian Academy of Sciences, 8 Ar- buzov Street, Kazan, 420088, Russian Federation. §Department of Chemistry and Biotechnology, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan. ABSTRACT: We report selective formation of heterobimetallic PtII/CuI complexes that demonstrate how facile bond activation processes can be achieved by altering reactivity of common organoplatinum compounds through their interaction with another metal center. The interaction of the Cu center with Pt center and with a Pt-bound alkyl group increases the stability of PtMe2 towards undesired rollover cyclometalation. The presence of the CuI center also enables facile transmetalation from electron-deficient F - tetraarylborate [B(Ar )4] anion and mild C-H bond cleavage of a terminal alkyne, which was not observed in the absence of an electrophilic Cu center. The DFT study indicates that the role of Cu center acts as a binding site for alkyne substrate, while activat- ing its terminal C-H bond.

Metal-metal cooperation plays a crucial role in small mole- complexes, rigid ligand design often blocks access to coordi- cule activation in enzymes and synthetic systems, including nation sites suitable for cooperative substrate binding.26-30 homogeneous and heterogeneous catalysts.1-2 Many classical catalytic systems rely on thorough optimization of the ligand environment to induce the desired reactivity at a single metal center. However, there is currently a growing realization that the catalyst’s reactivity can be significantly altered through close communication with another metal center, either by design or via unexpected bimetallic processes, thus enabling new approaches to bond activation.3-4 The second metal may facilitate substrate binding and pre- activation or even stabilize the bond activation product (Scheme 1). The adoption of the bimetallic approach has led to many recent advances in stoichiometric and catalytic bond Scheme 1. Schematic representation of altering reactivity activation processes.3-6 Bimetallic cooperation is often proof a single metal center through heterobimetallic complex posed in many C-C coupling processes, e.g. the Cu-to-Pd formation. transmetalation step in the Sonogashira coupling.7-12 The accelerating effect of metal additives (e.g. Cu salts) in Pd- In this work, we report a new bifunctional soft/hard unsymmetrical ligand scaffold, which selectively incorporates catalyzed C-C coupling reactions such as Stille and Suzuki II I coupling is commonly referred to as “the copper effect”.13-16 both Pt and Cu centers. The close proximity between the two However, precise understanding of how the reactivity at the metals allows for coordination of alkyl, aryl, or acetylide lig- single metal center can be affected by communication with a ands to both metal centers, and for the dialkyl complexes ena- second metal is often lacking due to the synthetic challenges bles metal-metal interaction. These heterobimetallic complex- in selective synthesis of such reactive heterobimetallic comes allow us to directly observe the effect of the second metal plexes with well-defined structure. center on the reactivity of Pt, which is interesting given that Pt complexes with d10 metal additives are widely used in C-H While multiple symmetrical ligand platforms have been de- 31-33 17-24 bond activation and studied as models for Pd-catalyzed veloped for the construction of homobimetallic complexes, cross-coupling.34 Our findings demonstrate that even subtle examples of ligand scaffolds that could selectively support interactions between two different metals alter the reactivity of two different metals and at the same time contain available common organometallic species and ligand design to intro- reactive sites, are exceedingly rare. This is especially the case duce proximity significantly affects reactivity. when the combination of a 1st row and a late 2nd or 3rd row transition metal is targeted.25 Among known heterobimetallic We have previously reported reversible stepwise formation of homomultimetallic CuI linear chain complexes using an unsymmetrical napthyridinone-based ligand A (Scheme 2).35 Simple functionalization of the O-atom of this ligand by reaction with chlorobis(tert-butyl)phosphine leads to a new ligand L, in which two well-defined sites are created: a hard-donor site containing a picolylamine arm and a soft-donor site containing a phosphinite arm. We first tested if ligand L shows differentiation between soft and hard Lewis acids using II IV (NBD)Pt Me2 (NBD = norbornadiene) and [Pt Me3I]4 precursors, respectively. As expected, specific binding of the PtII center to the soft phosphinite site only is observed, giving complex 1. The PtIV center, on the other hand, specifically binds to the hard N-donor site (complex 2). Interestingly, upon reacting complex 1 with methyl iodide, immediate migration of the Pt center from the soft to the hard site is observed, pre- senting an alternative pathway for the formation of 2. The mono-metallic reactivity thus confirms a proof-of-concept for using this ligand platform further in designing heterobimetallic systems selectively via the soft/hard Lewis acid concept. Scheme 3. Formation of cationic heterobimetallic complexes 3·[X] and a dearomatized heterobimetallic complex 4.

F Figure 1. ORTEP of 3[B(Ar )4] (a) and 4 (b) at 50 % probability Scheme 2. a) Synthesis of ligand L; b) Selective binding to F PtII and PtIV centers via hard and soft sites. level. Hydrogen atoms, the counterion for 3[B(Ar )4], and solvent molecules for 4 are omitted for clarity. In the case of 4, only one Next we targeted formation of heterobimetallic complexes of three symmetrically independent molecules is shown. Herein- using PtII and CuI precursors as this combination is known to after coordination bonds are shown in accordance with AIM anal- show metallophilic closed-shell d8-d10 interactions. First, ysis for the gas-phase optimized structures. treatment of complex 1 with 2 equiv of CuCl led to the for- The structures of complexes 3[X] and 4 were confirmed by II I 42 mation of the heterobimetallic Pt Me2/Cu complex 3[CuCl2] single crystal XRD studies (Figure 1). The geometry of the (Scheme 3). To avoid the presence of a potentially non- Pt centers in 3[X] are distorted square planar, with PtII···CuI I innocent counter anion, [Cu (MeCN)4][X] (X = BF4 or interactions (2.6119(3)-2.6486(3) Å) being shorter than the F F 43 B(Ar )4; B(Ar )4 = tetrakis[3,5- sum of covalent radii (2.68 Å), but slightly longer than in the bis(trifluoromethyl)phenyl]borate) were used leading to com- II I 34 Pt Me2/Cu complex reported by Chen et al. (2.5275(7) Å). F I plexes 3[BF4] and 3[B(Ar )4], respectively. All complexes Interestingly, Cu in 3[X] has close interaction with the carbon were isolated in 65-71% yields, characterized by X-ray dif- of the proximal Me group with Cu1···C1 distances of fraction (XRD) (Figure 1), NMR, IR, UV-vis spectroscopies, 2.160(3)-2.362(9) Å. Thus, complexes 3[X] are rare examples ESI-MS, and elemental analysis. of a solution-stable heterobimetallic complex with an unsym- 44 The ligand platform also contains an acidic benzylic CH2 metrical bridging Me group. ESI-MS analysis also confirmed position, in order to take advantage of the known ability of that the bimetallic PtII/CuI species 3+ is present in polar sol- mononucleating PNN pincer ligands to undergo an N-bound vents (MeCN or THF), confirming its stability. 36-38 CH2-arm deprotonation coupled with dearomatization. Interestingly, the dearomatized complex 4, characterized by Accordingly, we attempted the dearomatization of our binu- three molecules in the asymmetric cell, features a noticeably cleating P,N-donor ligand L using a strong base. longer interaction between CuI and PtII, 2.6890(5)-2.7459(6) Å, t 43 Gratifyingly, treatment of 3[CuCl2] with KO Bu resulted in which is not much larger than the sum of covalent radii The a deep red solution, from which 4 could be isolated cleanly. distances from C of the proximal Me group to CuI are longer Complex 4 features a dearomatization of the naphthyridine (2.518(5)-2.559(5) Å) compared to complexes 3[X]. These ring owing to deprotonation of its CH2 arm and thus is the first structural changes are ascribed to the loss of electrophilicity at example of a dearomatized heterobimetallic complex, resem- a formally neutral CuI center in 4 leading to weakening inter- bling dearomatization in pincer-based mononucleating ligands actions of CuI with both the PtII and the bridging Me group. utilized for metal-ligand cooperation catalysis.22, 39-41 Dearomatization of the ligand is evident from X-ray diffrac- tion data featuring double bond character (1.359(6)-1.371(7) Å) in the deprotonated arms as opposed to C11–C12 of 1.512(3) Å in the non-dearomatized complex 3[CuCl2].

F NMR spectra of complexes 3[BF4] and 3[B(Ar )4] exhibit NBO analysis also shows that complex 3 exhibits strong well-resolved, sharp proton resonances. Diagnostic features of electron density donation from the proximal Pt-MeA fragment I (2) the NMR spectra corresponding to the proximity of a Cu cen- to the Cu-center [σ(Pt-CH3)→Cu(I); E = 80.4 kcal/mol]. In F (2) ter to the Pt–Me group in 3[X]; X = BF4 and B(Ar )4 are contrast, much lower [E = 26.8 kcal/mol] donation is ob- (2) compared to those of 1 and 4 in Tables 1 and S1. Position of served from the distal Pt-MeB fragment. A weak [E = 6.0 the Me groups was determined by selective nuclear Overhau- kcal/mol] and even weaker [E(2) = 3.0 kcal/mol] back-donation ser effect (NOE) experiments. The MeA group located between of electrons from Cu to the proximal Pt-MeA and distal Pt-MeB II I the Pt and Cu atoms shows a significant downfield shift of fragments respectively, are observed. 13 the C signal by ca. 21 ppm compared to the analogous MeA We first studied solution-state stability of complexes 3[X] group located trans to the phosphinite in the Cu-free analogue compared with its monometallic counterpart 1. The common 1. In comparison, almost no change in chemical shift was ob- I decomposition pathway for dimethyl Pt complexes with N,P- served for the MeB group distal from the Cu center of 3 as donor ligands involves rollover cyclometalation leading to compared to 1. Moreover, Pt–H and Pt–C coupling constants undesired C-H bond activation of the ligand.48-49 Heating show considerable decrease for the Me group compared to 1, A monometallic complex 1 at 40 °C in THF for 12 h (or 3 days while only minor changes are seen in the distal Me . As ex- B in benzene) led to the expected cyclometalation to form com- pected from crystallography data, neutral complex 4 features plex 5 characterized by XRD (Scheme 4a). In contrast, bime- an Me group with the 13C chemical shift and coupling con- A tallic complex 3[BF ] was considerably more stable towards stant values that are intermediate between those observed for 4 cyclometalation and remained unchanged upon heating in complexes 3[X] and 1, consistent with a weaker Cu/Pt-Me I I THF at 40 °C for 12 h. The presence of an electrophilic Cu interaction when compared to 3[X]. Coordination of the Cu 195 center coordinated by bridging Me group presumably offers center also leads to an upfield shift of the Pt signal, which some kinetic resistance to rollover cyclometalation. shows a larger coupling constant to the P-atom when short II I F Pt ···Cu contacts are present. Surprisingly, when complex 3[B(Ar )4] was heated in C6H6 at 80 C for 18 h, a new complex 6[B(ArF) ] was obtained in Table 1. Chemical shifts of complexes 3[X] and 4 in THF- ° 4 46% in situ yield resulting from an aryl group transfer from a d8. F – [B(Ar )4] counteranion to a Pt center (Scheme 4, Figure 3). Although such electron deficient aryl group transfer is known for some electrophilic monometallic complexes (Rh, Au, Pt)50- 52 24, 53 and homobimetallic Cu2 and Fe2 complexes, this is the first example of such transmetalation from a tetraarylborate anion by a heterobimetallic complex with a formally neutral Pt center. Although the fate of the Me group and B-containing product could not be determined, the less than 50% yield of F 2 1 complex 6[B(Ar )4] likely results from the necessity to sacri- Complex δH ( JH,Pt, Hz) δC ( JC,Pt, Hz) δPt F – [20] 1 fice a [B(Ar )4] counteranion for aryl group transfer. In- ( JP,Pt, deed, when the reaction was performed in the presence of 4.5 Hz) F F equiv of Na[B(Ar )4], the in situ yield of 6[B(Ar )4] increased MeA MeB MeA MeB Pt to 80%. 1 0.93 0.96 15.7 -21.9 -3894 (66) (94) (662) (805) (2006) F 3[B(Ar )4] 1.10 1.23 -5.9 -21.3 -3971 (44) (86) (491) (711) (2877)

3[BF4] 1.04 1.18 -5.6 -21.3 -3971 (36) (82) (n.d.)[a] (719) (2866) 4 0.82 0.88 3.0 -20.2 -3980 (56) (84) (n.d.)[b] (n.d.)[b] (2467) [a] Not determined due to low intensity caused by insufficient 1 solubility; the corresponding JC,Pt for MeA in CD3CN solution was determined to be 505 Hz (see Table S1). [b] Not determined due to low intensity caused by insufficient solubility.

Atoms in Molecules (AIM) analyses for DFT-optimized structures of complexes 3 and 4 revealed that bond critical points (bcp) were located between Pt and Cu atoms with characteristics typical for closed-shell, metal-metal interactions 2 (positive value for ∇ 휌푏, low 휌푏, negative 푉푏 and 퐻푏, with 퐻푏 value close to zero).45-47 Interestingly, the bcp was also located between Cu and carbon of proximal MeA group in complex 3 with characteristics indicative of metal-ligand interactions (휌푏 ∇2 Scheme 4. (a) Cyclometalation of 1; (b) aryl group transfer 0.059 a.u.; ρb 0.218 a.u.), but not in complex 4, consistent F – with longer Cu···C distance observed by XRD. from [B(Ar )4] counterion to give 6; (c) terminal alkyne activation. 3

F Figure 3. ORTEP of 6[B(Ar )4] (a) and 7[BF4] (b) at 50 % probability level. Hydrogen atoms, counterions, and solvent molecules together with the minor disorder component for 7[BF4] are omitted for clarity. F The X-ray structure of 6[B(Ar )4] reveals close contacts of a I Cu center with the ipso-carbon of an aryl group (2.098(3) Å) Figure 4. Calculated energy profile for alkyne activation and and an adjacent ortho-carbon (2.335(3) Å), while the distance DFT-optimized structures for intermediates and transition states. between PtII and CuI atoms (2.7745(4) Å) is now longer than the sum of their covalent radii, indicating no metal-metal In summary, we designed and developed a reactive hetero- bonding when compared to 3[X] and also consistent with the bimetallic Pt/Cu species that conclusively demonstrates that lack of bcp according to AIM analysis.54 proximal interactions with a Cu center alters the reactivity of Pt. We found that a bridging alkyl group between the two We then examined the reactivity of 3 with a terminal alkyne metals prevents undesired rollover cyclometalation. The pres- as this substrate contains a reactive C–H bond and a π-system I I ence of a Cu center also induces facile transmetalation from that can potentially interact with a cationic Cu center. The F - I an electron-deficient [B(Ar ) ] anion and enables facile C-H synergistic effect of Cu salts has been previously implicated 4 7-9 bond activation of a terminal alkyne. DFT studies elucidate in bimetallic alkyne activation. Importantly, monometallic the role of the copper center in coordination and activation of complex 1 did not show any reaction with 4-ethynylanisole at an alkyne substrate which otherwise remains unreactive in the RT for at least 24 h. On the other hand, when 3[BF ] was re- 4 presence of a Pt-only monometallic complex. Considering the acted with 2 equiv of 4-ethynylanisole at RT, acetylide com- increased interest towards the utilization of bimetallic catalysis, plex 7[BF ] was cleanly obtained (Scheme 4). The product 4 much of which comes from greater scrutiny of what were pre- was isolated in pure form in 59% yield and fully characterized. II viously considered monometallic systems, this study provides Single crystal XRD study reveals a Pt center with a σ-bound I a ligand design blueprint for the study of metal-metal coopera- acetylide ligand, which coordinates to a Cu center through the tion effect in organometallic reactivity. triple bond π-system (Cu1···C1 and Cu1···C2 distances of II I 1.982(5) Å and 2.141(4) Å). The distance between Pt and Cu ADDITIONAL INFORMATION is 3.0934(8) Å, indicative of a lack of interaction between two metals after coordination of the Cu center to the carbon atom Supporting Information is available including experimental of acetylide and consistent with AIM analysis. The facile acti- procedures and characterization, X-ray structure determination details (CCDCs 1975187-1975194), and computational details. vation demonstrated by 3[BF4] as compared to completely unreactive 1 is exactly indicative of a synergistic effect caused by a cationic CuI center in terminal alkyne C–H bond activa- AUTHOR INFORMATION tion. This closely resembles the accelerating effect of Cu salts Corresponding Author or other metal additives in C-C coupling reactions.7-9 * J.R.K. ([email protected]) and O.R.W. ([email protected]). Based on the above reactivity, we hypothesize that a cationic CuI center plays a role in coordinating the alkyne, thus bring- Present Addresses ing the substrate in proximity to the Pt center, while increasing ║Orestes Rivada-Wheelaghan: Université de Paris, La- 8 the acidity of the terminal C–H bond. We conducted DFT boratoire d'Electrochimie Moléculaire, UMR 7591 CNRS, studies using propyne as a truncated model substrate, and were 15 rue Jean-Antoine de Baïf, F-75205 Paris Cedex 13, able to find an energetically accessible pathway, which in- France. volves initial π-coordination of an alkyne to CuI of 3’ to ini- tially give π-complex A (Figure 4). NBO calculations reveal Competing interests enhancement in the polarization of the terminal C-H bond in A The authors declare no competing interests. (charges at C -0.369 and H +0.273) as compared to free propyne (C -0.265 and H +0.239). Oxidative addition to the Funding Sources PtII center leads to the PtIV hydride acetylide species B, where JSPS Kakenhi Grant Number 16F16038 the triple bond of the alkyne is still π-coordinated to Cu. Due to the weak coordination of the naphthyridine N-donor,55-56 low barrier C-H (methane) reductive elimination results in the ACKNOWLEDGMENT acetylide complex C with a net exergonicity of 28 kcal mol-1.54 O.R.-W. was a JSPS International Research Fellow. This work was supported by JSPS Kakenhi Grant Number 16F16038. The authors thank Mr. A. Villar-Briones and Dr. M. Roy for perform- ing HR-MS analysis and acknowledge OIST for funding. We 4 thank Dr. K. Eguchi (JEOL RESONANCE Inc.) for advice re- 23. Davenport, T. C.; Tilley, T. D. Dinucleating garding 195Pt NMR. Naphthyridine-Based Ligand for Assembly of Bridged Dicopper(I) Centers: Three-Center Two-Electron Bonding Involving an REFERENCES Acetonitrile Donor. Angew. Chem., Int. Ed. 2011, 50, 12205-12208. 24. Ziegler, M. S.; Levine, D. S.; Lakshmi, K. V.; Tilley, T. D. Aryl Group Transfer from Tetraarylborato Anions to an Electrophilic 1. Buchwalter, P.; Rose, J.; Braunstein, P. 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