Platinum

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Single versus Double Cu(I) Catalyzed [3 + 2] Azide/Platinum Diacetylide Cycloaddition Reactions Xi Yang, Sudarsan VenkatRamani, Christopher C. Beto, Trevor J. Del Castillo, Ion Ghiviriga, Khalil A. Abboud, and Adam S. Veige* Center for Catalysis, University of Florida, P.O. Box 117200, Gainesville, Florida 32611, United States S Supporting Information *

ABSTRACT: This report focuses on Cu(I) catalyzed cycloaddition reactions between organic azides and the platinum diacetylide complexes trans-(PR3)2Pt(CCR′)2 (where PR3 = P(OEt)3, PEt3, PnBu3, PPhMe2, PPh3, and PBn3; and R′ = H, Ph, and p-PhNO2). Pt(II)-Diacetylides supported by P(OEt)3, PEt3, PnBu3, and PPhMe2 react with benzyl azide to provide syn/anti isomers of double cycloaddition products. In contrast, Pt(II)-diacetylide complexes supported by PPh3 and PBn3 afford single cycloaddition products, exclusively. Steric congestion enforced by the larger phosphines PPh3 and PBn3 prevent the second cycloaddition. Ground state DFT computations provide some insight into the divergent reactivity and indicate that the π-acidity of the phosphine ligands is a variable in the single vs double cycloaddition outcome.

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metal-acetylide and organic azide and vice versa.14 From our previous work on iClick,22,24 it became apparent that the thermal cycloaddition between a M-azide and a M-acetylide is not general. In fact, so far, the M-azide/M-acetylide thermal cycloaddition only occurs when Au(I) is present as the Macetylide. For example, Rh(I)-azide will react with Au(I)acetylide to give a Rh(I)-triazolate-Au(I) complex; however, Rh(I)-acetylide does not react with Au(I)-azide.25 Considering some recent mechanistic data, it is now thought that the Au(I)acetylide plays the role of the Cu(I)-acetylide intermediate in CuAAC.24 Pt(II)-diacetylide complexes do not readily undergo thermal cycloaddition with organic azides, but we thought Cu(I) might catalyze the reaction. Thus, we sought a generalized synthetic protocol for the synthesis of metal triazolate complexes via CuAAC between metal-acetylides and organic azides. In that effort, we report here an efficient synthesis of Pt(II)-triazolate complexes via CuAAC between Pt(II)-diacetylides and organic azides. This study specifically focuses on how the steric and electronic properties of the phosphine ligand influence the number of cycloaddition reactions.

INTRODUCTION Metal-azides1 from all three rows of the transition metal series ([M]-N3 where M = Fe,2 Co,3−5 Ni,3,6 Ta,7 Ru,8−13 Mn,14 Os,15 Mo,16 and Au17) can undergo thermal (not Cu-catalyzed) azide−alkyne cycloaddition to provide M-triazolate complexes. In most cases, the alkynes are either electron deficient, such as dimethyl-but-2-ynedioate, or highly strained, such as cyclooctyne and its derivatives. In 2009, Gladysz and co-workers applied Cu(I)-catalyzed azide−alkyne cycloaddition (CuAAC) in the synthesis of triazole-capped Pt(II)-polyyne complexes.18,19 However, the cycloaddition process does not occur adjacent to the metal center but rather at the terminal alkyne. The Pt-ion is essentially a spectator, playing no apparent role in the cycloaddition process. Reactions between NHC-Au(I)-acetylides (NHC = N-heterocyclic carbene) and organic azides, by Gray and co-workers, demonstrate that cycloaddition can occur adjacent to the metal center.20,21 More recently, Gray was able to synthesize a Au3 complex bridged by a triazolate with Au(I) at the 1, 3, and 5 positions.20 iClick (inorganic click) is an inorganic synthetic methodology to yield triazolate bridged multinuclear organometallic complexes through the cycloaddition of a metal-azide with a metal-acetylide.22,23 Recently, the definition of iClick has been expanded to include organometallic click reactions involving a © XXXX American Chemical Society

Received: January 27, 2017

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DOI: 10.1021/acs.organomet.7b00067 Organometallics XXXX, XXX, XXX−XXX

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RESULTS AND DISCUSSION The optimized reaction conditions for synthesizing Pt(II)triazolate complexes via CuAAC were determined by screening several Cu(I) sources and solvents in two model reactions. The model reactions involved treating trans-(PPhMe2)2PtII(C CH)2 with benzyl azide or phenyl azide (see Scheme S1). The screening results (Tables S1 and S2) indicate that Cu(I) acetate (CuOAc) provides the highest percent conversion to product and that the solvent plays no appreciable role. Thus, all of the reactions in this report utilize CuOAc in dichloromethane. The optimized conditions are consistent with previous results by the groups of Bertrand, Novák, and Straub on traditional CuAAC. Bertrand, Straub, and Novák conclude that the acetate anion promotes the metalation of the organic alkyne; Bertrand further concludes that formation of the Cu2 species, and the final protonation event to release Cu(I) is attributable to the acetate ion.26−28 Straub and Hu demonstrated that CuOAc and acetate containing Cu(I) clusters can enhance the rate of CuAAC.29,30 Treating complex 1-PR3 with two equivalents of benzyl azide provides a mixture of two isomers 3-PR3-syn/3-PR3-anti (PR3 = PEt3, 89%; PPhMe2, 82% yield) in a 65:35 ratio for PEt3 and 72:28 ratio for PPhMe2 (Scheme 1). For brevity, only the

crystals. A single-crystal X-ray diffraction experiment performed on one of the crystals confirmed the structure of the 3-PEt3-syn isomer. Complex 3-PEt3-syn (Figure 1) crystallizes in the P21/c

Figure 1. Molecular structure of 3-PEt3-syn with all hydrogen atoms except the two-triazolate protons H2A and H12A removed for clarity. Selected bond lengths (Å) and angles (deg): Pt1−C1 = 2.049(2), Pt1−C11 = 2.067(2), Pt1−P2 = 2.3060(6), Pt1−P1 = 2.3134(6); ∠C1−Pt1−C11 = 177.46(10), ∠C1−Pt1−P2 = 92.74(7), ∠C11− Pt1−P2 = 87.65(7), ∠C1−Pt1−P1 = 90.28(7), ∠C11−Pt1−P1 = 89.03(7), ∠P2−Pt1−P1 = 172.28(2).

Scheme 1. Cu(I) Catalyzed Cycloaddition Reaction between Benzyl Azide (2) and Terminal Pt(II)-Diacetylide Complexes 1-PR3 (Where PR3 = PEt3 and PPhMe2)

space group and has C2v symmetry in the solid state. The four coordinate Pt(II) metal ion adopts a square planar geometry comprising the trans-PEt 3 and triazolate ligands. The asymmetric unit comprises four chemically equivalent but crystallographically independent molecules. The plane created by the two triazolate rings is perpendicular to the square plane of the metal ion. The Pt−C1 (2.049(2) Å) and Pt−C11 (2.067(2) Å) bonds are significantly longer than the Pt−C bond (1.988(9) Å) of a previously reported Pt-triazolate complex.34 The mutual trans-influence of the triazolates leads to bond elongation in 3-PEt3-syn. Without Cu(I) catalyst, the reaction described in Scheme 1 does not occur, implying the need for Cu(I)-acetylide or Cu(I)π-alkyne intermediates to catalyze the cycloaddition. To probe the role of Cu(I), 1-PPhMe2 was added to D2O in the presence/absence of CuOAc. The formation of a Cu-alkyne πinteraction significantly lowers the pKa of terminal alkyne protons.35,36 In the presence of CuOAc, deuterium from D2O exchanges into the terminal proton of 1-PPhMe2 (Supporting Information, section 6). However, without CuOAc, no deuterium exchange occurs, suggesting at least the importance of Cu(I) in either lowering the pKa of the terminal proton on 1PPhMe2 or a Cu(I)-acetylide intermediate. In contrast to the common 1,4-triazole products synthesized from CuAAC, the Cu(I) catalyzed cycloaddition between 1-PR3 and benzyl azide provide a exclusively 5-Pt-triazolates. Consistent with the X-ray data, the 2D NMR data also confirms the 5-Pt triazolate structure of 3-PEt3-syn/anti in solution. The 1H−13C gHMBC spectrum of the mixture of 3-PEt3-syn/anti exhibits 3JHC coupling between benzyl methylene protons and the quaternary carbon coordinated to the Pt center (see Figure S7; the correlation of 5.69(1H) ppm−144.5(13C) ppm is attributable to the syn-isomer, and 5.65(1H) ppm−144.6(13C) is attributable to the anti-isomer). If the Pt(II) ion is considered to be an

characterization data for 3-PEt3-syn/3-PEt3-anti is discussed (see Supporting Information characterization data for 3PPhMe2-syn/3-PPhMe2-anti). The two isomers 3-PEt3-syn/ 3-PEt3-anti are distinguishable via NMR spectroscopy. For example, 3-PEt3-syn contains equivalent protons on the ethyl groups of the PEt3 ligands, whereas in 3-PEt3-anti they are diasteriotopic. Specifically, the quartet of triplets at 1.14 ppm is attributable to 3-PEt3-syn, and for 3-PEt3-anti, the protons resonate as two doublets of quartets of triplets at 1.11 and 1.01 ppm. The triplet multiplicities come from virtual coupling31−33 to the 31P nucleus, thus providing substantial support for the assignment of a P−Pt−P coordination environment. Consistent with the 1H NMR spectrum of the product mixture, the 31 1 P{ H} NMR spectrum exhibits two signals at 6.92 ppm (JPtP = 2413.95 Hz) and 8.59 ppm (JPtP = 2411.53 Hz) for 3-PEt3syn and 3-PEt3-anti, respectively. By monitoring the reaction between 1-PEt3 and benzyl azide via 1H and 31P{1H} NMR spectroscopy (see Supporting Information), it was clear that products 3-PEt3-syn/3-PEt3-anti form via two consecutive cycloaddition steps. A single cycloaddition intermediate forms initially (10.87 ppm in 31P{1H} NMR spectra), and then, a second cycloaddition produces the double-clicked products (8.95 and 6.92 ppm in 31P{1H} NMR spectra). Layering a concentrated dichloromethane solution of 3-PEt3syn/3-PEt3-anti with hexanes induces precipitation of colorless B

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Scheme 2. Cu(I) Catalyzed Cycloaddition Reaction between Benzyl Azide (2) and Internal Pt(II)-Diacetylide Complexes

products 7-PR3 (PR3 = PPh3, 92%; PBn3, 88%), exclusively. Only one equivalent of benzyl azide participates in each cycloaddition process, and one unreacted acetylide fragment remains in complexes 7-PR3. Elevation of the reaction temperature and elongating the reaction time yield the same single cycloaddition product. Multinuclear (1H and 31P) NMR, 2D-NMR (see Supporting Information), and combustion analysis (see Supporting Information) support the identity of 7-PR3. Because of the similarity in the spectroscopic features of the single cycloaddition products, only the characterization data for 7-PPh3 will be discussed (see Supporting Information characterization data for 7-PBn3). The 31P{1H} NMR spectrum of 7-PPh3 reveals only one major resonance at 18.08 ppm (1JPtP = 2761.68 Hz). In the 1H NMR spectrum, the two benzyl methylene protons appear as a singlet at 4.61 ppm. The two ortho-protons in the phenyl acetylide resonate as a doublet at 6.12 ppm (3JHH = 6.6 Hz). Consistent with previously reported M-triazolates bearing a phenyl group, the ortho-protons in the phenyl resonate downfield as a conspicuous doublet at 7.63 ppm (3JHH = 7.7 Hz).22 The cycloaddition products presented thus far are mononuclear Pt-complexes. To demonstrate the structural diversity of products accessible via the Cu(I) catalyzed cycloaddition, benzyl azide was replaced with a diazide capable of linking two metal ions. As depicted in Scheme 4, treating two equivalents of 6-PPh3 with 4,4′-diazido-3,3′-dimethyl-1,1′-biphenyl (8) at ambient temperature for 3 h produces the dinuclear complex 9-PPh3 in 60% yield. In the 1H NMR spectrum, the orthoprotons of the phenyl ring on the triazolate appear as a doublet resonance downfield at 8.24 ppm. The protons ortho to the triazolate ring in the biphenyl moiety also exhibit a doublet resonance downfield at 8.12 ppm. Further upfield, a doublet resonance at 6.16 ppm is attributable to the ortho-protons of the phenyl ring in the unreacted acetylide. Finally, a singlet resonance at 1.44 ppm corresponds to the methyl protons in the biphenyl linker. Unambiguous structural assignment of 9-PPh3 as a dinuclear complex comes from an X-ray diffraction experiment performed on single crystals of 9-PPh3. Complex 9-PPh3 crystallizes in the P21/c space group with 4 asymmetric units in one unit cell. Both the Pt(II) centers in 9-PPh3 adopt a distorted square planar geometry and connect via the triazolate rings and the 3,3′-biphenyl linker (Figure 2). The bond angles ∠P1−Pt1−P2 (165.94(7)°) and ∠P1′−Pt1′−P2′ (166.04(8)°) deviate significantly from the expected 180° to minimize the steric repulsion between the phosphine ligands coordinated on adjacent Pt(II) centers. The solid-state structure of 9-PPh3 reveals that only one acetylide group within each Pt(II) coordination environment undergoes a cycloaddition reaction (single cycloaddition). Interestingly, in the solid state, the

elaborate R-group on an alkyne, then the expected products from the Cu(I) cycloaddition reaction should be 4-Pt,5-Htriazolate complexes. However, the products form with the reverse 5-Pt,4-H regiochemistry, exclusively. Several examples that demonstrate replacing terminal alkynes with metal acetylides in CuAAC consistently provide 5-metal triazolate click products. For example, Cu(I) catalyzed cycloaddition between Bi(III)-acetylide and Al(III)-acetylide complexes and organic azides provide 5-Bi(III) and 5-Al(III) triazolate click products,37,38 and Gray21 demonstrated the synthesis of 5Au(I)-triazolates. Also, thermal cycloaddition between Macetylides and RN3 produces 1,5-triazolates (where M = Li,37a Na,37 Mg,37 Zn,37 Si,37 Ge,37 and Sn37). Substituted Pt(II)-diacetylide complexes 4-PR3-Ar (PR3 = PMe2Ph, P(OEt)3, and PnBu3; Ar = p-PhNO2, Ph) also undergo Cu(I) catalyzed cycloaddition with benzyl azide according to Scheme 2. Treating the internal diacetylide complex 4PPhMe2-PhNO2 with two equiv of benzyl azide at ambient temperature provides the two isomers 5-PPhMe2-PhNO2-syn and 5-PPhMe2-PhNO2-anti in a 88:12 ratio. The 31P{1H} NMR spectrum of the mixture exhibits two resonances at −16.79 (JPtP = 2347.40 Hz) and −17.01 ppm (JPtP = 2342.56 Hz), attributable to 5-PPhMe2-PhNO2-syn and 5-PPhMe2PhNO2-anti, respectively. The assignment is consistent with the 1H spectrum of the mixture, which displays one signal for the methyl groups in the syn-isomer and two in the anti-isomer. These signals are triplets due to the virtual coupling with 31P, as expected for the P−Pt−P moiety. Following the same synthetic procedure, treating the complexes 4-PR3-Ph (PR3 = P(OEt)3, PnBu3) with benzyl azide provides double cycloaddition products 5-PR3-Ph-syn/5-PR3-Ph-anti (PR3 = P(OEt)3, PnBu3) (Scheme 2). The spectroscopic features of 5-PR3-Phsyn/5-PR3-Ph-anti are similar to those of 5-PPhMe2-PhNO2syn/5-PPhMe2-PhNO2-anti and therefore will not be discussed (see Supporting Information for the characterization data). 3-PR3-syn/3-PR3-anti and 5-PR3-Ar-syn/5-PR3-Ar-anti form via double cycloaddition. In contrast, as depicted in Scheme 3, treating (PR3)2Pt(CCPh)2 6-PR3 (PR3 = PPh3 and PBn3) with two equiv of benzyl azide at ambient temperature for 3 h provides only the single cycloaddition Scheme 3. Cu(I) Catalyzed Cycloaddition Reaction between 6-PR3 and Benzyl Azide (2)

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Organometallics Scheme 4. Cu(I) Catalyzed Cycloaddition Reaction between 6-PPh3 and 4,4′-Diazido-3,3′-dimethyl-1,1′-biphenyl (8)

the click reactions between Pt(II)-diacetylide complexes with relatively large phosphine ligands (PPh3, PBn3) yield only monocycloaddition products. Using 5,5′-dimethyl-2,2′-bipyridine as the ancillary ligand, Yam et al. reported in 201434 exclusive single cycloaddition when [Pt(5,5′-Me2bpy)(CCPh)2] was treated with organic azides. Since the bipyridine is not sterically bulky, it is odd that only one cycloaddition event occurred. Considering Yam’s result, we decided to examine the potential underlying electronic factors by employing DFT computational analysis. To assess the factors that determine the fate of the second cycloaddition event, four single clicked Pt(II)-acetylide model complexes (10−13) with varying phosphine ligands were chosen for the DFT study. The common motifs in the model complexes include a Ph-substituted acetylide and a phenyl and benzyl substituted triazolate, as depicted in Figure 3. The

Figure 2. Molecular structure of 9-PPh3 with all hydrogen atoms and phenyl rings in PPh3 removed for clarity. Selected bond lengths (Å) and angles (deg): Pt1−C16 = 2.016(8), Pt1−C8 = 2.028(8), Pt1−P1 = 2.307(2), Pt1−P2 = 2.318(2), Pt1′−C16′ = 2.011(9), Pt1′−C8′ = 2.047(8), Pt1′−P1′ = 2.299(2), Pt1′−P2′ = 2.311; ∠C16−Pt1−C8 = 175.6(3), ∠C16−Pt1−P1 = 85.5(3), ∠C8−Pt1−P1 = 95.7(2), ∠C16−Pt1−P2 = 86.9(3), ∠C8−Pt1−P2 = 92.9(2), ∠P1−Pt1−P2 = 165.94(7), ∠C16′−Pt1′−C8′ = 174.7(3), ∠C16′−Pt1′−P1′ = 85.5(3), ∠C8′−Pt1′−P1′ = 96.0(2), ∠C16′−Pt1′−P(2′) = 86.0(2), ∠C8′−Pt1′−P2′ = 93.5, ∠P1′−Pt1′−P2′ = 166.04(8).

Figure 3. Model Pt monocycloaddition products chosen for the DFT study. The phosphine ligand was varied in each of the complexes while retaining the triazolate and the internal phenyl acetylide motifs.

unreacted acetylides orient on one side of the complex (syn) rather than on opposite sides (anti). Single vs Double Cycloaddition: Computational Insights. The Cu(I)-catalyzed cycloaddition reactions between Pt(II)-diacetylides and organic azides, as depicted in Scheme 2 and Scheme 3, exhibit divergent reactivity. Pt(II)-diacetylide complexes supported by P(OEt)3, PEt3, PnBu3, and PPhMe2 react with benzyl azide to provide the corresponding double cycloaddition products. Whereas PPh3 and PBn3 coordinated Pt(II)-diacetylide complexes afford single cycloaddition products, exclusively. The size of the phosphine ligand, based on Tolman’s cone angle,39 provides a straightforward rationale for the observed reactivity. Table 1 summarizes the outcome of the cycloaddition reaction (single vs double-cycloaddition) relative to the cone angle of the phosphine ligand. Sterically unencumbered phosphine ligands P(OEt)3, PPhMe2, PEt3, and PnBu3 promote cycloaddition reactions on each Pt(II)acetylide arm, producing double clicked products. In contrast,

phosphine ligands were varied to examine their influence on the second cycloaddition event. All of the model complexes were geometry optimized using M0640/LANL2DZ41 level of theory. Single point and vibrational frequency analyses were performed on the optimized structures. It should be noted that a similar experimental study was conducted by Gray et al. employing Au(I)-acetylides,21 where they probe cycloaddition as a function of incoming azide sterics. Their data clearly demonstrate that tertiary azides such as adamantyl azide fail to undergo cycloaddition due to steric congestion. Au(I)acetylides are linear, and therefore much of the steric congestion from the ancillary ligand resides on the opposite side of attack from the azide. Within Pt(II)-acetylides, however, due to the presence of phosphines cis to the incoming azide, steric congestion is expected to be more pronounced. Given the current mechanistic model for the action of Cu(I) in CuAAC, Cu(I) must bind to the Pt(II)-acetylide to induce the second cycloaddition event. Pt(II)-acetylide π* orbitals within the LUMO are necessary for Cu(I) binding. The model complex 10 (L = PEt3) exhibits π* acetylide orbitals within its LUMO (as probed by DFT calculations) (Figure 4), suggesting that it should facilitate the double-clicked product, and it does. The model complex 11 with PMe2Ph deviates from this trend. In 11, the acetylide π* orbitals appear within the LUMO(+1) level. The LUMO instead features orbital contributions from the aromatic framework of the phenyl group on the PMe2Ph

Table 1. Summary of the Cycloaddition Reaction Results choice of PR3 in (PR3)2Pt(C CPh)2

cone angles of PR3

extent of cycloaddition

P(OEt)3 PPhMe2 PEt3 PnBu3 PPh3 PBn3

108° 122° 132° 132° 145° 165°

double cycloaddition double cycloaddition double cycloaddition double cycloaddition single cycloaddition single cycloaddition D

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Figure 4. LUMO and LUMO(+1) of model complexes 10−12 (M06/LANL2DZ level of theory). Complex 10 exhibits π*-acetylide orbitals within the LUMO, a necessary electronic requirement for Cu-binding. Complex 11 exhibits the π*-acetylide orbitals within the LUMO(+1). The LUMO(+1) within complex 12 exhibits π*-orbitals centered on the phosphine aromatic groups.

ligand. Despite the π* acetylide orbitals being in the LUMO(+1) level, successful double cycloaddition occurs suggesting that the acetylide π* orbitals are accessible. By increasing the number of phenyl groups on the phosphine ligand, such as in PPh3, the lowest unoccupied orbitals are progressively dominated by the aromatic π-framework of the phenyl group rather than acetylide π* orbitals. Thus, it may not be sterics alone that prevents the second cycloaddition. Further evidence for the strong interplay between sterics and electronics of the ancillary phosphine ligand is evident in 13, featuring PBn3. Experimental data confirm that only one cycloaddition occurs (refer Scheme 3). However, the DFT calculation indicates that the presence of the requisite acetylide π* orbitals are within the LUMO, suggesting a second cycloaddition should occur. Instead, presumably due to the increased steric congestion enforced by the bulky PBn3 ligand, only one cycloaddition event occurs.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]fl.edu. ORCID

Adam S. Veige: 0000-0002-7020-9251 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-SC0010510 and DESC0016526. K.A.A. thanks UF and the NSF for funds to purchase X-ray equipment (CHE-0821346).

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CONCLUSION The synthetic chemistry explored in this work provides a new protocol for generating diverse Pt(II)-triazolate complexes. By tuning the steric and electronic properties of the ancillary phosphine ligands, selective formation of single or doublecycloaddition products can be achieved. Supported by less sterically bulky phosphine ligands, such as P(OEt)3, PPhMe2, or PEt3, the Cu(I)-catalyzed cycloaddition between Pt(II)diacetylide complexes and benzyl azide provides double cycloaddition products 3-PEt3-syn/3-PEt3-anti and 5-PR3-Arsyn/5-PR3-Ar-anti. Single cycloaddition products 7-PR3 are accessible via the same reaction scheme but using Pt(II)diacetylide reagents supported by more sterically bulky phosphine ligands such as PBn3 or by ensuring the π* orbitals on the acetylide are destabilized. One variable that could not be probed due to synthetic limitations is the example of a Pt(II)acetylide with a large phosphine (between 132° and 145°) that undergoes double cycloaddition. Future studies include applying this new knowledge to the synthesis of Pt(II)triazolate metallopolymers for photo physical interrogation.

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H, 13C{1H}, 31P{1H}, and 2-D NMR spectra (PDF) Atomic coordinates for the geometry optimized structure of 10 (PDF) Crystallographic data (CIF)

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REFERENCES

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DOI: 10.1021/acs.organomet.7b00067 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.7b00067 Organometallics XXXX, XXX, XXX−XXX