Formation of Heterobimetallic Complexes by Addition of d10-Metal

Aug 28, 2017 - We were curious to see whether similar heterobimetallic complexes containing cis-oriented μ-2-C6F4PPh2 groups could be generated from ...
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Formation of Heterobimetallic Complexes by Addition of d10-Metal Ions to cis-[(dppe)M(κC‑2‑C6F4PPh2)2] (M = Ni, Pd, and Pt) Robert Gericke,*,†,‡ Martin A. Bennett,§ Steven H. Privér,† and Suresh K. Bhargava*,† †

Centre for Advanced Materials and Industrial Chemistry (CAMIC), School of Applied Sciences, RMIT University, GPO Box 2476, Melbourne, Victoria 3001, Australia ‡ Institut für Anorganische Chemie, Technische Universität Bergakademie Freiberg, D-09596 Freiberg, Germany § Research School of Chemistry, Australian National University, Canberra, Australian Capital Territory 2601, Australia S Supporting Information *

ABSTRACT: The reactivity of [(dppe)M(κC-2-C6F4PPh2)2] (M = Ni (1Ni), Pd (1Pd), and Pt (1Pt); dppe = 1,2-bis(diphenylphosphino)ethane)) toward monovalent coinage metal ions (M′ = Cu, Ag, and Au) has been investigated. Two main isomers [(dppe)M(κC-2-C6F4PPh2)(μ-2-C6F4PPh2)M′Cl] (2PdAg, 2PdAu, 2PtCu, 2PtAg, and 2PtAu) and [(dppe)M(μ-2-C6F4PPh2)2M′Cl] (3PdCu, 3PdAg, and 3PtCu) could be detected. Quantum chemical calculations underpin the enhanced thermodynamic stability of [(dppe)M(κC-2-C6F4PPh2)(μ2-C6F4PPh2)M′Cl] for combinations of the heavier metals. NBO calculations reveal attractive Pt···Cu and Pt···Ag interactions in complexes 2PtCu and 2PtAg, respectively. From the reaction of [(dppe)M(κC-2-C6F4PPh2)2] with an excess of [AuCl(tht)], a series of trinuclear complexes [(dppe)M(μ-2-C6F4PPh2)2(AuCl)2] (4NiAu2, 4PdAu2, and 4PtAu2) was prepared. Reaction of [(dppe)M(κC-2-C6F4PPh2)(μ-2C6F4PPh2)M′Cl] with TlPF6 leads to the formation of cationic derivatives [(dppe)M(μ-2-C6F4PPh2)2M′]PF6 with P−M′−P (6PtAu) or P−M′−C (7PdAu, 7PtAu) bonding situations around the coinage metal.

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INTRODUCTION A range of homonuclear and heterobinuclear complexes containing a pair of transition elements with d8- and d10co n fig u r at io n s b r id g e d by li g an d s su ch a s bi s(diphenylphosphino)methane (Ph2PCH2PPh2, dppm) and bis(dicyclohexylphosphino)methane (Cy2PCH2PCy2, dcpm) is known.1−4 Single-crystal X-ray structural analyses have shown that the metal atoms in such complexes are in close contact, with separations of ca. 2.9−3.0 Å, and theoretical calculations have indicated the presence of weak attractive interactions,1,5−7 which arise mainly from dispersion forces that in the case of gold are enhanced by a relativistic effect. Fewer examples are known in which the bridging ligand is unsymmetrical. Starting from the P-coordinated precursors [Au(κP-Ph2PCH2SPh)2]OTf and [AuCl(κP-Ph2PCH2SPh)],5,6 Laguna et al. have synthesized heterobimetallic Au−Ag and Au−Pd complexes which exhibit attractive (metallophilic) interactions. The coordination behavior of carbanionic ligands of the type 2-C6R4PPh2 (R = H and F) is similar in some respects to that of dppm and dcpm.7 These ligands commonly form either chelate, four-membered ring complexes, such as cis-[Pt(κ 2 -2C6H4PPh2)2]8 and trans-[M(κ2-2-C6F4PPh2)2] (M = Ni, Pd, and Pt), 9 or binuclear complexes, such as [M 2 (μ-2C6H4PPh2)2(μ-OAc)2] (M = Rh10 and Pd11). The chelate rings of the bis(ortho-metalated) complex trans-[Pd(κ2-2C6F4PPh2)2] are opened by an excess of trimethylphosphine to give a mixture of syn- and anti-isomers of trans-[(Me3P)2Pd(κC-2-C6F4PPh2)2].12 The dangling PPh2 groups bind to © 2017 American Chemical Society

coinage metal halides with formation of 1:1 adducts, [(Me3P)2Pd(μ-2-C6F4PPh2)2M′Cl] (M′ = Cu, Ag, and Au), in which the two bridging 2-C6F4PPh2 ligands are mutually trans (Scheme 1). The short Pd···M′ distances (M′ = Cu 2.8448(3) Å, Ag 2.9170(4) Å, and Au 2.8842(4) Å), together with theoretical calculations, are consistent with the presence of weak metallophilic interactions.12 Of current interest are transition metal complexes containing σ-acceptor (ambiphilic, Z-type) ligands13 in which a Lewis acidic main group atom like boron, silicon, or antimony is attached to one, two, or three 2-C6H4PR2 (R = Ph and iPr) units (Chart 1).14−22 Compounds of this type have been extensively studied and used as a probe to investigate metallophilic interactions. It is noteworthy that ligands containing two 2-C6H4PR2 units attached to a main group element behave like a pincer ligand and coordinate to a transition metal in which the PR2 groups are either mutually cis or trans. We were curious to see whether similar heterobimetallic complexes containing cis-oriented μ-2-C6F4PPh2 groups could be generated from the precursors [(dppe)M(κC-2-C 6 F 4 PPh 2 ) 2 ] (M = Ni, Pd, and Pt; dppe = Ph2PCH2CH2PPh2)9 and whether they too would provide evidence for metallophilic interactions. Received: February 23, 2017 Published: August 28, 2017 3178

DOI: 10.1021/acs.organomet.7b00145 Organometallics 2017, 36, 3178−3188

Article

Organometallics Scheme 1. Formation of trans-[(Me3P)2Pd(μ-2-C6F4PPh2)2M′Cl] (M′ = Cu, Ag, and Au)

Chart 1. Transition Metal Complexes Containing One, Two and Three 2-C6H4PR2 Fragments Bound to a Main Group Atom

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RESULTS As reported previously,9 the bis(ortho-metalated) complexes trans-[M(κ2-2-C6F4PPh2)2] (M = Ni, Pd, and Pt) react with 1,2-bis(diphenylphosphino)ethane (dppe) by opening of the chelate four-membered rings to give cis-[(dppe)M(κC-2C6F4PPh2)2] (M = Ni: 1Ni, Pd: 1Pd, and Pt: 1Pt; Scheme 2). In contrast to the trimethylphosphine analogue transScheme 2. Formation of [(dppe)M(κC-2-C6F4PPh2)2] (M = Ni, Pd, and Pt)

Figure 1. Molecular structure of 1Pd. Ellipsoids show 50% probability levels, and hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd1−P3 2.2998(3), Pd1−P4 2.3053(3), Pd1−C1 2.0646(10), Pd1−C7 2.0730(10), C1−Pd1−P4 93.84(3), C7−Pd1−P3 93.49(3), C1−Pd1−C7 90.15(4), P3−Pd1− P4 84.77(1), C1−Pd1−P3 166.97(3), C7−Pd1−P4 169.48(3).

[(Me3P)2Pd(κC-2-C6F4PPh2)2],12 only the anti-isomers of 1Ni, 1Pd and 1Pt could be detected by 31P NMR spectroscopy and X-ray diffraction; presumably, steric crowding of the two bulky PPh2 groups disfavors the formation of the syn-isomer (Scheme 2). The molecular structure of [(dppe)Pd(κC-2-C6F4PPh2)2] (1Pd) was not determined by X-ray diffraction in the earlier study, so it is included here (Figure 1). 1Pd is isostructural and isomorphous with its nickel and platinum analogues (triclinic, P1)̅ .9 As expected, the palladium atom exhibits close to square planar coordination geometry, and the bond lengths and angles about the metal atom are comparable to those in [(dppe)Pt(κC-2-C6F4PPh2)2]. DFT calculations on 1Ni, 1Pd, and 1Pt indicate that the energies of the syn- and anti-isomers are similar in all three cases. For 1Pd and 1Pt, the anti-isomer is predicted to be more stable by 1.7 and 2.0 kcal/mol, respectively, whereas for 1Ni the syn-isomer is predicted to be more stable by 1.2 kcal/mol. Although they are not observed experimentally, the syn- and anti- isomers should therefore be capable of coexistence in solution with a low barrier to interconversion.

Attempts to prepare complexes of the type [(dppe)Ni(μ-2C 6 F 4 PPh 2 ) 2 M′Cl] or [(dppe)Ni(κC-2-C 6 F 4 PPh 2 )(μ-2C6F4PPh2)M′Cl] by treating [(dppe)Ni(κC-2-C6F4PPh2)2] (1Ni) with 1 equiv of M′Cl (M′ = Cu and Ag) were unsuccessful; abstraction of the dppe ligand occurred to give the bis(chelate) complex trans-[Ni(κ2-2-C6F4PPh2)2]9 and [(dppe)M′Cl],23 as shown by 31P NMR spectroscopy. The reaction of 1Ni with [AuCl(tht)] was more complex, giving a mixture of unreacted nickel starting material, known digold(I) complex [Au2(μ-2-C6F4PPh2)2],24 and unidentified byproducts. Analogous to the behavior of trans-[(Me3P)2Pd(κC-2C6F4PPh2)2], treatment of solutions of 1Pd with M′Cl (M′ = Cu and Ag) afforded heterobinuclear complexes [(dppe)Pd(μ2-C6F4PPh2)2M′Cl] (3PdCu and 3PdAg) in which the coinage metal atom is coordinated by the two PPh2 groups (Scheme 3). The 31P NMR spectra of 3PdCu and 3PdAg each show a pair of multiplet resonances at ca. 40 ppm, assignable to the two equivalent phosphorus nuclei in coordinated dppe, and a 3179

DOI: 10.1021/acs.organomet.7b00145 Organometallics 2017, 36, 3178−3188

Article

Organometallics Scheme 3. Reactions of [(dppe)M(κC-2-C6F4PPh2)2] (M = Ni, Pd, and Pt) with Coinage Metal Halide Complexes

second resonance at lower frequency (3PdCu: 6.5, 3PdAg: 20.4 ppm) for the PPh2 groups bound to the coinage metal atom; the upfield resonance in 3PdAg is further split into a doublet of 420 Hz due to 1J coupling with 107/109Ag. In addition to the two signals of 3PdAg, the 31P NMR spectrum of the crude reaction mixture of 1Pd with AgCl after 2 h at room temperature showed a set of four signals which can be assigned to proposed intermediate [(dppe)Pd(κC-2-C6F4PPh2)(μ-2C6F4PPh2)AgCl] (2PdAg), in which only one of the available PPh2 groups is coordinated to AgCl. The chemical shifts of two signals at 48.1 and 43.5 ppm are consistent with an unsymmetrically bound dppe ligand, and two signals at 24.5 and 0.1 ppm can be assigned to the Ag-bound and dangling PPh2 groups, respectively. The signal at 24.5 ppm shows a doublet splitting of about 680 Hz due to 1J coupling with 107/109 Ag. Attempts to isolate complex 2PdAg in a pure form were unsuccessful, and the samples were contaminated with impurities of 1Pd or 3PdAg. The molecular structure of [(dppe)Pd(μ-2C6F4PPh2)2CuCl] (3PdCu) was confirmed by single-crystal X-ray diffraction analysis (Figure 2 and Table 1). The geometries about the palladium and copper atoms are approximately square planar and trigonal planar, respectively. As a consequence of the cis-orientation of the 2-C6F4PPh2 groups, the P1−Cu1−P2 angle (125.23(2)°) and P−Cu (2.2214(5) and 2.2367(5) Å) and Cu−Cl (2.2439(5) Å) bond lengths in 3PdCu are significantly smaller than the corresponding parameters in [(Me3P)2Pd(μ-2C6F4PPh2)2CuCl] (143.42(2)°, 2.3071(4) and 2.2843(6) Å, respectively). Conversely, the Pd···Cu separation in 3PdCu (3.1989(5) Å) is significantly larger than that in [(Me3P)2Pd(μ2-C6F4PPh2)2CuCl] (2.8448(3) Å), indicative of little or no metallophilic interaction between the metal atoms. In sharp contrast to the formation of 3PdCu and 3PdAg, treatment of 1Pd with [AuCl(tht)] afforded the heterobimetallic complex [(dppe)Pd(κC-2-C6F4PPh2)(μ-2-C6F4PPh2)AuCl] (2PdAu), in which only one of the PPh2 groups is coordinated to the gold atom. This behavior is also in contrast to that observed for its PMe3 analogue which gave the 1:1

Figure 2. Molecular structure of 3PdCu. Ellipsoids show 50% probability levels. Hydrogen atoms and disordered solvent of crystallization have been omitted for clarity.

adduct similar to 3PdCu and 3PdAg. The 31P NMR spectrum for 2PdAu shows four multiplet resonances at 47.5, 44.1, 38.8, and 0.23 ppm. The upfield peak is assigned to the dangling PPh2 group, and the remaining resonances are tentatively assigned to the two phosphorus nuclei in the chelate dppe ligand and the PPh2(AuCl) group, respectively. Reaction of 1Pt with M′Cl (M′ = Cu and Ag) or [AuCl(tht)] gave the series of related complexes 2PtCu, 2PtAg, and 2PtAu (Scheme 3). The 31P NMR spectra for 2PtCu, 2PtAg, and 2PtAu each show four resonances, corresponding to the four inequivalent phosphorus nuclei. Two resonances appear at ca. 40 and 37 ppm, flanked by 195Pt satellites of ca. 2000 Hz, assignable to the phosphorus nuclei in coordinated dppe, and a multiplet at ca. −4 ppm due to the uncoordinated PPh2 group. The chemical shifts and coupling constants are comparable to those in 1Pt. The fourth multiplet resonance, assigned to the phosphorus nuclei bound to M′Cl, appears at 13.6 (2PtCu), 18.4 (2PtAg), and 34.9 ppm (2PtAu), each with 195Pt satellites of ca. 300−400 Hz; additionally, the resonance for 2PtAg is split into a doublet of 670 Hz due to 3180

DOI: 10.1021/acs.organomet.7b00145 Organometallics 2017, 36, 3178−3188

Article

Organometallics Table 1. Selected Interatomic Separations (Å) and Angles (deg) in 2PdAu, 2PtCu, 2PtAg, 2PtAu, 3PdCu, and 3PtCu 2PdAu M1···M′1 M1−P3 M1−P4 M1−C1 M1−C7 M′1−P1 M′1−P2 M′1−Cl1 C1−M1−P4 C7−M1−P3 C1−M1−C7 P3−M1−P4 C1−M1−P3 C7−M1−P4 P1−M′1−Cl1 P2−M′1−Cl1 P1−M′1−P2

3.3565(3) 2.3255(7) 2.3073(7) 2.105(3) 2.105(3) 2.2306(7) 2.3021(7) 90.63(7) 93.81(7) 92.41(10) 83.30(3) 173.36(7) 175.50(8) 172.22(3)

2PtCu

2PtAg

2PtAu

3.0964(4) 2.3168(7) 2.3019(7) 2.094(3) 2.110(2) 2.3543(8)

3.2896(4) 2.3081(7) 2.2931(6) 2.0902(2) 2.103(2) 2.2265(7)

2.3490(9) 91.80(7) 95.18(7) 89.75(10) 83.32(3) 174.89(7) 177.54(8) 155.47(3)

2.2978(8) 91.04(6) 95.10(6) 90.38(8) 83.51(2) 174.30(6) 178.25(7) 167.77(3)

2.9466(3) 2.3147(3) 2.2979(3) 2.0901(11) 2.0988(11) 2.1678(4) 2.1354(4) 91.51(3) 95.04(3) 90.09(4) 83.395(12) 174.66(3) 178.00(3) 151.170(16)

3PdCu

3PtCu

3.1989(5) 2.3463(5) 2.3065(5) 2.0934(17) 2.0942(16) 2.2214(5) 2.2367(5) 2.2439(5) 91.81(5) 90.47(5) 92.46(7) 82.02(2) 168.78(5) 160.87(4) 111.75(2) 119.24(2) 125.23(2)

3.1605(6) 2.3201(5) 2.2904(5) 2.0977(18) 2.0935(17) 2.2166(5) 2.2334(5) 2.2452(5) 91.98(5) 90.40(5) 92.20(7) 82.32(2) 169.13(5) 161.30(5) 111.32(2) 118.54(2) 125.09(2)

coupling with 107/109Ag, the magnitude of which is similar to that observed for the intermediate 2PdAg. In the solid state, 2PtCu, 2PtAg, and 2PtAu are isostructural and isomorphous (monoclinic P21/n). The molecular structure of 2PtCu is shown in Figure 3, and selected bond lengths and

Figure 4. LP1(Pt) → LP*(M′) donor−acceptor interactions derived by NBO analyses of 2PtCu (left, M′ = Cu) and 2PtAg (right, M′ = Ag). NBOs are displayed with an isosurface value of 0.05.

Cu) and 42.9 kcal/mol for 2PtAg (M′ = Ag) with the donor moiety clearly being a lone pair located on Pt and contributions from M′ being only 0.42% in 2PtCu and 0.36% in 2PtAg. Because of the energy involved and the location of the lone pair, we interpret the situation as weak attractive donor− acceptor interactions between the platinum and coinage metal atoms, the former clearly being the donor moiety, causing the relatively pronounced bending of the P1−M′1−Cl1 angle (M′ = Cu and Ag). The observation of donor−acceptor interactions for Pt···M′ distances