Unprecedented stabilisation of the Ag₂²⁺-ion by two hydrido-iridium(III) complexes

Abstract

The complex [{(η⁵-C₅Me₅)(Ph₃P)Ir(μ-H)₂}₂Ag₂(OSO₂CF₃)₂], containing the Ag₂²⁺-ion, has been synthesized; crystallographic and spectroscopic data are described.

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There is considerable interest in theory and evidence of d¹⁰–d¹⁰ closed-shell attractions.¹ Such metallophilic attractions are found both in organometallic and inorganic compounds.² Therein, Ag⁺–Ag⁺ distances below 288.9 pm as in metallic silver³ are of particular interest. So far, the shortest contact of 267 pm was found in the bis[1,3-diphenyltriazenido-silver(I)].⁴ Ag⁺-ions are known to form adducts with transition-metal fragments, such as in [{(η⁵-C₅Me₅)(CO)₂Ir}₂Ag][BF₄]⁵ with Ag–Ir interactions and, more often as in [(Ph₃P)₃Ir(μ₃-H)(μ-H)₂Ag₂(OSO₂CF₃)(H₂O)](CF₃SO₃)⁶ or [(Ph₃P)Ag(μ-H)IrH₂(PPh₃)₃](CF₃SO₃)⁷ with hydrido-bridged Ag–(μ-H)–Ir linkages. The capability of many transition-metal hydrides to combine with electron-deficient species forming bimetallic complexes of the type M–(μ-H)–M′ is well established.⁸ However, stabilisation of d¹⁰–d¹⁰ closed shell units by transition-metal hydrides has as yet not been reported. Herein, we describe the synthesis, crystal structure analysis and spectroscopic characterisation of the first example of a Ag₂²⁺-ion coordinated by two hydrido-iridium(III) complexes in [{(η⁵-C₅Me₅)(Ph₃P)Ir(μ-H)₂}₂Ag₂(OSO₂CF₃)₂] (2).

Addition of 1 equiv. of AgOSO₂CF₃ to the iridium(III) complex (η⁵-C₅Me₅)(Ph₃P)Ir(OEt)(H)⁹ (1) in dichloromethane solution readily affords [{(η⁵-C₅Me₅)(Ph₃P)Ir(μ-H)₂}₂Ag₂(OSO₂CF₃)₂] (2), which is isolated as a greenish-yellow crystalline solid in 85% yield (Scheme 1).† Formation of 2 occurs by β-elimination of ethoxide forming intermediate (η⁵-C₅Me₅)(Ph₃P)IrH₂ (3) which reacts with AgOSO₂CF₃. Evidence for the formation of acetaldehyde was given by small resonances in the NMR spectra. Solutions of 2 in dry dichloromethane remain unchanged for several days, but upon contact with moisture decomposition occurs indicated by deposition of a black residue.

Scheme 1

The infrared spectrum exhibits very weak bands in the typical region of transition-metal hydride absorptions.¹⁰ Absorption bands at 2036 and 2021 cm⁻¹ for hydrides bonded terminal to iridium appear together with a broad band at 1917 cm⁻¹ attributable to bridging Ir–(μ-H)–Ag linkages. The ¹H NMR spectrum of 2 shows the resonance for the protons of the η⁵-C₅Me₅ groups at δ 1.90 (br, d, J_H–P 2.3 Hz, 30H) and one for the four hydrides at δ −14.57 (d, J_H–P 23.5 Hz, 4H). The comparison with the dihydrido complex (η⁵-C₅Me₅)(Ph₃P)IrH₂ (3) (δ 1.90, br d, J_H–P 1.0 Hz, C₅Me₅; δ −16.5, d, J_H–P 31.7 Hz, IrH₂)¹¹ shows that the electron densities at the iridium centres are similar and less affected by the coordination to the silver atoms. In contrast, the resonance for the four hydrides in 2 is shifted to lower field in comparison to 3, consistent with a decreased electron density as a result of the interaction with the silver atoms. This is also in good agreement with a stronger acidity of bridging hydrides in comparison to terminal ones at the same transition-metal centre.¹² NMR analysis shows the hydrides in 2 to be magnetically equivalent probably due to dynamic processes in solution. Variable temperature ¹H NMR spectroscopy was performed on a sample of crystalline 2 in CD₂Cl₂ between 283 and 188 K (Fig. 1). The doublet for the hydrides in the ¹H NMR spectrum at 301 K splits on cooling. At 203 K the resonances at δ −14.79 (J_H–P 25.6, J_H–Ag 59.6 Hz) and at δ −14.32 (J_H–P 24.4, J_H–Ag 35.8 Hz) are compatible with an A₂MX and an A₂A₂′MM′XX′ spin system, respectively, together with a broad signal at δ −14.06. At lower temperatures the resonances did not resolve further. The relative intensities are of 0.7, 2.9 and 0.4 versus 30 protons of the η⁵-C₅Me₅ groups. The second resonance is consistent with the solid-state structure of 2 showing the coupling to phosphorous and to two equivalent silver nuclei. The first resonance suggests the presence of a phosphorous-containing species with one silver atom such as 2a, whereas the broad resonance is probably due to polymeric material (Scheme 2). Unambiguous determination of the H–Ag couplings occurred by phosphorus-decoupling of the ¹H NMR spectrum at 203 K, causing selective collapse of the signals to the expected A₂X and A₂A₂′XX′ spin systems, respectively. The broad signal remains unchanged. Separate ¹H–¹⁰⁹Ag and ¹H–¹⁰⁷Ag couplings were not resolved, similar to comparable structural fragments reported elsewhere.^13,14 At 301 K the ³¹P{¹H} NMR spectrum shows a singlet at δ 8.8 (δν_1/2 9 Hz), which splits into three broad resonances at δ 14.1 (δν_1/2 25 Hz), 11.4 (δν_1/2 165 Hz) and 9.4 (δν_1/2 110 Hz) on cooling. Phosphorous–silver couplings were not observed consistent with previously described examples, such as [AgIr(μ-H)₂(bpy)(PPh₃)₂]²⁺.¹³

Fig. 1 Variable temperature ¹H NMR spectra and ¹H{³¹P} NMR spectrum at 203 K of 2 showing the hydride region recorded in CD₂Cl₂.

Scheme 2

Single crystals suitable for X-ray diffraction analyses of 2 were obtained by slow diffusion of diethyl ether into a concentrated dichloromethane solution.‡Fig. 2 shows the molecular structure of [{(η⁵-C₅Me₅)(Ph₃P)Ir(μ-H)₂}₂Ag₂(OSO₂CF₃)₂] (2) along with selected bond lengths and angles. The complex consists of centrosymmetric dimers. The Ir–Ag₂–Ir core exhibits a planar rhombic geometry with Ir–Ag–Ir and Ag–Ir–Ag angles of 126.95(2)° and 53.05(2)°, respectively. The most important feature of the structure is the very short silver–silver bond length of 265.53(12) pm, which is comparable to the so far shortest of 266.86(1) pm in [Ag(PhNNNPh)]₂.⁴ The silver ions are additionally involved in weak contacts to anionic oxygen atoms of the CF₃O₂SO⁻ groups forming an O–Ag–Ag–O chain deviated from linearity by approximately 15°. The Ag–O bond length of 239.2(7) pm is in good agreement with that found in [Ag₃(OSO₂CF₃)₃(PPh₃)₃].¹⁵ The iridium–silver distances are 291.07(11) pm and 303.03(9) pm. These values are comparable to previously determined Ag–(μ-H)–Ir linkages of 280.8(4) pm and 276.4(4) pm in [(Ph₃P)₃Ir(μ₃-H)(μ-H)₂Ag₂(OSO₂CF₃)(H₂O)](CF₃SO₃).⁶ A significantly shorter bond length of 265.9(1) pm was determined for a direct Ag–Ir bond interaction in [{(η⁵-C₅Me₅)(CO)₂Ir}₂Ag][BF₄],⁵ which is close to the sum of covalent radii of iridium and silver (261 pm).¹⁶ The hydride ligands could not be positioned on the basis of the data obtained by single crystal diffraction methods, due to the absence of rest electron density.

Fig. 2 Molecular structure of 2. Selected bond lengths (pm) and angles (°): Ag1–Ag1A 265.53(12), Ir1–Ag1 291.07(11), Ir1–Ag1A 303.03(9), Ag1–O3 239.2(7), Ir1–P1 226.3(2); Ir1–Ag1–Ir1A 126.95(2), Ag1–Ir1–Ag1A 53.05(2), O3–Ag1–Ag1A 165.5(2), P1–Ir1–Ag1 112.72(5), P1–Ir1–Ag1A 107.71(5). ). Symmetry transformations used to generate equivalent atoms A: −x + 1, −y + 2, −z + 1.

The crystal structure of 2 provides the first structural evidence of argentophilic§ attraction in closed shell systems stabilised by transition metal fragments. The formation of the (η⁵-C₅Me₅)(Ph₃P)IrH₂ units in 2 is clearly demonstrated by IR and NMR spectroscopy. However, the nature of the hydride bonding in 2 is not certain. Spectroscopic data suggest bridging Ir–(μ-H)–Ag interactions rather than mixed bridging Ir–(μ-H)–Ag and terminal Ir–H bonds. The stabilisation of the Ag₂²⁺-ion would be possible by interaction with these hydrides. Substitution of iridium by other group 9 metal centres stabilising further d¹⁰–d¹⁰ systems might be helpful for the understanding of this kind of metallophilic attraction. Research in this field is in progress.¶

The Deutsche Forschungsgemeinschaft, the Degussa-Hüls AG, and the Fonds der Chemischen Industrie supported this work.

Notes and references

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Footnotes

† Selected analytical data of 2: Calc. for C₅₈H₆₄Ag₂F₆Ir₂O₆S₂P₂: C, 41.04; H, 3.80; found: C, 41.34; H, 3.97%. MS (EI) m/z: 1024 (100%) [M − 2AgOSO₂CF₃ − 2C₆H₅]⁺, 512 (20%) [M − 2AgOSO₂CF₃ − 2C₆H₅]²⁺. ¹H NMR (200.13 MHz, CD₂Cl₂): δ 7.6–7.2 (m, 15H, P(C₆H₅)₃), 1.90 (br d, 15H, J_H–P 2.3 Hz, C₅(CH₃)₅), −14.57 (d, 2H, J_H-P 23.4 Hz, IrH₂). ¹³C NMR (125.77 MHz, CD₂Cl₂): δ 133.56 (d, J_C–P 10.6 Hz, P(C₆H₅)₃), 131.30 (d, J_C–P 2.5 Hz, P(C₆H₅)₃), 128.97 (d, J_C–P 10.7 Hz, P(C₆H₅)₃), 121.01 (q, J_C–F 320.2 Hz, SO₃CF₃), 96.51 (s, C₅(CH₃)₅), 11.03 (d, J_C–P 0.5 Hz, C₅(CH₃)₅).¹⁹F NMR (188.30 MHz, CD₂Cl₂, ext. C₆F₆): δ 84.7. ³¹P NMR (81.01 MHz, CD₂Cl₂): δ 8.77 (br s). IR (KBr) 2036, 2021, 1917 (cm⁻¹).

‡ Crystal data for 2: C₅₈H₆₄Ag₂F₆Ir₂O₆S₂P₂, M = 1697.29, triclinic, space group P1̄, a = 1013.0(2) pm, b = 1118.6(3) pm, c = 1447(4) pm, α = 71.179(19)°, β = 72.004(11)°, γ = 89.714(11)°, V = 1.4686(7) nm³, T = 200(2) K, Z = 1, F(000) = 824. Data were collected in the range 3.65 to 25.02° (θ-scan), 5154 independent reflections (R_int = 0.1175), final R₁ = 0.0404, with allowance for thermal anisotropy of all non-hydrogen atoms. CCDC reference number 194589. See http://www.rsc.org/suppdata/cc/b2/b209884a/ for crystallographic data in CIF or other electronic format.

§ In the case of gold, the term aurophilic attraction for intra- and intermolecular Au⁺–Au⁺ contacts was used.¹⁷

¶ Note added in proof: an Ag–Ag distance of 265.44(11) pm has been reported in [MeSi{SiMe₂N(p-Tol)}₃SnAg]₂: B. Findeis, L. H. Gàde, I. J. Scowen and M. McPartlin, Inorg. Chem., 1997, 36, 960.

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