Tetrametallic clusters (Ir₂Rh₂) through an ancillary ortho-carborane-1,2-dichalcogenolato ligands

Abstract

The tetrametallic cluster complexes {Cp*Ir[E₂C₂(B₁₀H₉)]}Rh₂(cod){Cp*Ir[E₂C₂ (B₁₀H₁₀)]} (E = S (3a); Se (3b)) have been synthesized by reactions of the 16-electron half-sandwich iridium complexes [Cp*Ir{E₂C₂(B₁₀H₁₀)}] [Cp* = η⁵-C₅Me₅, E = S (1a), Se (1b)] with [Rh(cod)(µ-OEt)₂] at room temperature in toluene solution. In the solid state, this tetrametallic cluster exhibits an irregular nearly planar metal skeleton with the two carborane dichalcogenolato ligands bridging the four metal centers from both sides of the tetrametallic plane. Even though all metal atoms coordinate bridging chalcogen atoms, they show different electronic and coordination environments. The molecular structures of 3a and 3b have been determined by X-ray crystallography.

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Introduction

The chemistry of mixed-metal clusters has been of enduring interest because of the unique chemistry resulting from having two or more metals with different chemistry properties in close proximity and also because of their potential catalytic applications.^1,2 Many dithiolene ligands have been employed for the preparation of heterometallic clusters.³ During the past decade, considerable attention has been devoted to metal complexes containing chelating 1,2-dicarba-closo-dodecaborane-1,2-dichalcogenolate ligands, to take advantage of their unique molecular structure.⁴ A number of mononuclear 16-electron Cp and Cp* half-sandwich complexes of Co, Rh and Ir have been described which contain a bidentate, chelating 1,2-dicarba-closo-dodecaborane-1,2-dichalcogenolato ligand, [(B₁₀H₁₀)C₂E₂]²⁻ (E = S, Se) and a “pseudo-aromatic” metalladichalcogenolene five-membered ring.⁴ These compounds have been used as models to study the insertion of alkynes into one of the metal–sulfur bonds; this may lead to the formation of a metal-to-boron bond or substitution of the carborane cage in the positions of B(3)/B(6).^5,6 In recently, we have developed a methodology to build up polynuclear frameworks, containing metal–metal bonds, by using the 16-electron mononuclear complexes of metals such as Cp*Ir[E₂C₂(B₁₀H₁₀)], Cp*Rh[E₂C₂(B₁₀H₁₀)] and Cp*Co[E₂C₂(B₁₀H₁₀)] (E = S, Se) (see Scheme 1 for selected examples A, B and C).⁵

Scheme 1 Examples of structurally characterized complexes.

The incorporation of the 1,2-dithiolato or 1,2-diselenolato structural unit into the bulky closo-dicarbadodecaborane ligand protects the inner coordination sphere and stabilized the metal–metal bond, which has occasionally been observed at dithiolene ligands.⁷ Such properties aid in the construction of different types of binuclear, trinuclear and other unprecedented hetero-metallic clusters.

In previous work, we have already described, treatment of Cp*Ir[E₂C₂(B₁₀H₁₀)] with [Rh(cod)(µ-Cl)]₂ resulted in the formation of a trimetallic cluster with the Ir₂Rh core.⁵ For comparison, now the reactions of 1a, 1b with analogous complexes [Rh(cod)(µ-OEt)]₂ have been investigated. Here, we present the results of these studied along with details of the synthesis and characterization of novel tetranuclear cluster complexes 3a, 3b.

Results and discussion

In our previous report, the dimer [Rh(cod)(µ-Cl)]₂ was an acceptable material to produce the Ir₂Rh trinuclear metallic clusters 2a, 2b.⁵ However, the work of Vinas et al. shows that the reaction of carborane complexes with other types of rhodium complexes, such as [Rh(cod)(acac)], would lead to some interesting reactions.⁸ Taking into account the similarity of [Rh(cod)(µ-OEt)]₂ with [Rh(cod)(µ-Cl)]₂, which has been long known to be an useful precursor to prepare a variety of rhodium complexes, we consider whether this similar reaction can give different results. The experimental procedure is similar to the reaction of [Rh(cod)(µ-Cl)]₂. The 16-electron complexes 1a and 1b can be easily synthesized from the half-sandwich iridium dichloride complex [Cp*IrCl(µ-Cl)]₂ with dilithium 1,2-dicarba-closo-dodecaborane (12)-1,2-dithiolate in THF solution (Scheme 3).⁹1a and 1b were reacted with [Rh(cod)(µ-OEt)]₂ in the molar ratio 2 : 1 in toluene, and the green solution changed to red gradually. After the products were separated by column chromatography, two products: {Cp*Ir[S₂C₂(B₁₀H₉)]}Rh(cod) (2a) and {Cp*Ir[S₂C₂(B₁₀H₉)]}Rh₂(cod){Cp*Ir[S₂C₂(B₁₀H₁₀)]} (3a) were obtained in 18 and 30% yield, respectively (Scheme 2). In addition, analogous products {Cp*Ir[Se₂C₂(B₁₀H₉)]}Rh(cod) (2b) and {Cp*Ir[Se₂C₂(B₁₀H₉)]}Rh₂(cod){Cp*Ir[Se₂C₂(B₁₀H₁₀)]} (3b) were also obtained by the same method in yields of 16 and 30% respectively. Complexes 3a and 3b can be obtained in higher yields when longer reaction times are applied.

Scheme 2 Reaction of 1a and 1b with [Rh(cod)(µ-OEt)]₂.

Scheme 3 Synthesis of complexes 1a and 1b.

The complexes 3a and 3b are neutral, diamagnetic and air-sensitive in solution. The IR spectra of the products 3a and 3b in the solid state exhibit intense B–H of carborane stretching at about 2573 (vs) and 2588 (vs) cm⁻¹.

The molecule 2a is unsymmetrical, as confirmed by the ¹H NMR spectrum with two singlets of Cp* rings at δ 2.11 and 2.19 ppm.⁵ The ¹H NMR data for complexes 3a display two intense methyl singlets of pentamethylcyclopentandienyl ligands (2.07, 2.19 ppm) and a number of multiplets in the ranges 4.02–4.29 ppm (olefinic) and 1.58–1.83 ppm (aliphatic) which can be ascribed to the 1,5-cyclooctadiene ligand. Both molecules 2a and 3a contain an Ir–B bond, as indicated by the appearance of Ir–B resonances in the ¹¹B NMR spectrum at about −20 ppm.¹⁰ Excellent elemental analyses were obtained for all complexes and consistent with the proposed structures.

Crystals of 2a, 3a, 2b and 3b suitable for X-ray diffraction study were obtained by slow diffusion of hexane into a concentrated solution of the complexes in dichloromethane at low temperature. The molecular structures of 2a and 2b have been described in our previous papers,⁵ so they were only used to compare with 3a and 3b here. Selected bond distances and angles of 3a and 3b are listed in Table 1.

Table 1 Selected bond distances (Å) and angles (°) for 3a and 3b

3a
Ir(1)–S(1)	2.332(3)	Ir(1)–S(2)	2.347(3)
Ir(1)–Rh(2)	2.5990(10)	C(1)–C(2)	1.610(14)
Ir(2)–Rh(2)	2.9814(10)	Rh(2)–S(3)	2.304(3)
Rh(2)–S(4)	2.295(3)	Ir(2)–Rh(1)	2.7862(11)
Ir(2)–S(3)	2.315(3)	Ir(2)–B(16)	2.139(11)
Rh(1)–S(3)	2.332(3)	Rh(1)–S(4)	2.365(3)
Rh(1)–C(3)	2.132(10)	C(3)–C(4)	1.627(14)
S(1)–Ir(1)–S(2)	57.69(7)	C(1)–S(1)–Ir(1)	104.1(4)
C(1)–S(1)–Rh(2)	103.3(3)	S(1)–Rh(2)–S(2)	79.25(9)
Ir(1)–S(1)–Rh(2)	66.76(7)	Ir(1)–S(2)–Rh(2)	66.48(7)
Ir(1)–Rh(2)–Ir(2)	168.99(3)	S(4)–Rh(2)–S(3)	83.43(10)
S(3)–Rh(2)–Ir(2)	49.95(7)	S(3)–Ir(2)–Rh(2)	49.64(6)
S(3)–Ir(2)–Rh(1)	53.45(7)	Rh(2)–S(4)–Rh(1)	95.40(9)
Rh(1)–Ir(2)–Rh(2)	73.32(3)	S(3)–Rh(1)–S(4)	81.31(9)
C(3)–Rh(1)–S(4)	75.0(3)	C(3)–Rh(1)–Ir(2)	83.8(3)
C(4)–S(4)–Rh(1)	81.6(3)	B(16)–Ir(2)–Rh(1)	66.6(3)

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3b
Ir(1)–Se(1)	2.4316(16)	Ir(1)–Se(2)	2.4433(16)
Ir(1)–Rh(2)	2.6281(12)	C(1)–C(2)	1.614(19)
Ir(2)–Rh(2)	3.0085(13)	Rh(2)–Se(3)	2.4038(18)
Rh(2)–Se(4)	2.3975(18)	Ir(2)–Rh(1)	2.8258(13)
Ir(2)–Se(3)	2.4435(16)	Ir(2)–B(16)	2.151(15)
Rh(1)–Se(3)	2.4578(18)	Rh(1)–Se(4)	2.4777(18)
Rh(1)–C(3)	2.138(15)	C(3)–C(4)	1.648(19)
Se(1)–Ir(1)–Se(2)	83.41(5)	C(1)–Se(1)–Ir(1)	102.6(4)
C(1)–Se(1)–Rh(2)	101.8(4)	Se(1)–Rh(2)–Se(2)	81.42(6)
Ir(1)–Se(1)–Rh(2)	64.63(4)	Ir(1)–Se(2)–Rh(2)	64.39(4)
Ir(1)–Rh(2)–Ir(2)	169.63(5)	Se(4)–Rh(2)–Se(3)	83.96(6)
Se(3)–Rh(2)–Ir(2)	52.23(4)	Se(3)–Ir(2)–Rh(2)	51.04(4)
Se(3)–Ir(2)–Rh(1)	55.03(4)	Rh(2)–Se(4)–Rh(1)	94.15(6)
Rh(1)–Ir(2)–Rh(2)	75.39(4)	Se(3)–Rh(1)–Se(4)	81.18(6)
C(3)–Rh(1)–Se(4)	76.2(4)	C(3)–Rh(1)–Ir(2)	83.5(4)
C(4)–Se(4)–Rh(1)	78.5(4)	B(16)–Ir(2)–Rh(1)	66.2(4)

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The molecular structure of 3a is shown in Fig. 1. The metal atoms of the core adopt a quite irregular nearly planar Ir₂Rh₂ arrangement (Ir(1), Ir(2), Rh(1) and Rh(2) metals are coplanar with maximum deviation at 0.177 Å). To our knowledge, this type of Ir₂Rh₂ core cluster is rare, only some pyramidal shape metal carbonyl clusters of Ir₂Rh₂(CO)₁₂ and its derivatives were reported previously.¹¹ The two dithiolato ligands of carboranes are situated at the both sides of the tetrametallic plane with three of the sulfur atoms acting as µ₂-bridging ligands and the other sulfide simultaneously connecting three metals. The Ir(2), Rh(1) and Rh(2) metals exhibit six-coordinate environments and Ir(1) is five coordinate if considering the metal–metal bonds, and their coordination spheres are clearly different (Fig. 2). Ir(2) coordinates a different set of ligands bonding to Ir(1), boron and two metal bonds with Rh(1) and Rh(2), except for the similar Cp* bridging thiolates ligands. By use of the thiolate ligands, Rh(2) connects the Ir(1) and Ir(2) atoms.

Fig. 1 Molecular structure of the tetranuclear complex 3a.

Fig. 2 Central skeleton of 3a showing the different metal coordination spheres.

Compared with structures of 2a and 3a, the difference lies in the [(cod)Rh] fragment in 3a. We can presume that in the whole reaction the [(cod)Rh] fragment exists in some reactive form and inserts into the original S(3)–C(3) bond, accompanying the cleavage of S(3)–C(3) bond, and 3a is formed. The detailed mechanism of the insertion reaction of [(cod)Rh] into S–C bonds is still not clear, and further investigations of these aspects are in progress.

The structure of 3a shows three short intermetallic separations: Ir(1)–Rh(2) 2.5990(10), Rh(2)–Ir(2) 2.9814(10), Ir(2)–Rh(1) 2.7862(11) Å. All these lie in the expected range for a metal–metal bond. Compared with the analogous fragment in 2a (2.685(3) Å), the Ir(1)–Rh(2) metal bond become much shorter. Slightly longer Rh–Ir separations were found in many cases, such as [Cp^tt₂Zr(µ-S)₂Ir(CO)₂Rh(cod)₂] (2.8205(10) Å)¹² (with a µ-S supported metal–metal bond), and the heterodinuclear cations RhIr(CO)₃(µ-dppm)₂]⁺ (2.7722(7) Å)¹³ and [RhIr(CH₃)(CO)₃(µ-dppm)₂]⁺ (2.743(1) Å).¹⁴ Much longer metal–metal distances were observed in the cluster [(Cp*Ir)₂(µ₃-S)₂Rh(cod)][RhCl₂(cod)] (2.906(1) and 2.913(9) Å).¹⁵ The presence of a shorter d⁸–d⁸ Ir(1)–Rh(2) interaction in 3a is also supported by the Ir(1)–S–Rh(2) angles (66.76(7) and 66.48(7)°), which are smaller than the analogous Rh–S–Ir angles in 2a (69.59(18) and 69.8(2)°). The Ir–S distances (2.332(3)–2.347(3) Å) and Rh–S distances (2.295(3)–2.365(3) Å) in 3a are in the normal range of Ir–S and Rh–S single bonds, but the Ir–S distances are significantly longer than those in the “pseudo-aromatic” iridadithiolene ring in Cp*Ir[S₂C₂(B₁₀H₁₀)] (1a) (2.252(5) and 2.251(6) Å).⁴ Due to the coordination of the S atoms of carborane to Rh, the pseudo-aromatic iridadithiolene heterocyclic system (Ir(1)–S(1)–C(1)–C(2)–S(2)) is destroyed and bent with a dihedral angle of 135.3° along the S(1)⋯S(2) vector.

The complex 3b crystallizes in the form of dark-red block in the monoclinic space group P2₁/n with four molecular in the unit cell. Its molecular structure is similar to 3a. Fig. 3 depicts the molecular structure of 3b. Complex 3b also contain four metal atoms, which form an irregular nearly planar Ir₂Rh₂ core (The Ir(1), Ir(2), Rh(1) and Rh(2) metals are coplanar with maximum deviation at 0.167 Å) similarly with 3a. There are three metal bonds: Ir(1)–Rh(2) 2.6281(12), Rh(2)–Ir(2) 3.0085(13), Ir(2)–Rh(1) 2.8258(13) Å. The Ir(1)–Se–Rh(2) angles (64.63(4) and 64.39(4)°) are smaller than the analogous Ir(1)–S–Rh(2) angles in 3a (66.76(7) and 66.48(7)°). The planar pseudo-aromatic system of the iridadiselene heterocycle 1b is also no longer present in 3b, the dihedral angle at the Se⋯Se vector in the IrSe₂C₂ ring is 135.8°. The Ir–Se bond distances (2.4316(16)–2.4435(16) Å) are similar to those in normal selenolate complexes, but are significantly longer than those in the “pseudo-aromatic” iridadiselene ring in Cp*Ir[Se₂C₂(B₁₀H₁₀)] (1b) (2.3753(9) and 2.3656(9) Å).⁹

Fig. 3 Molecular structure of the tetranuclear complex 3b.

Conclusion

In this work, we carried out the reactions of 16-electron coordinatively unsaturated iridium dichalcogenolato complexes 1a, 1b with [Rh(cod)(µ-OEt)₂]. Two novel tetranuclear cluster complexes were prepared and fully characterized. The resultant clusters 3a, 3b feature an Ir₂Rh₂ core containing three Ir–Rh metal bonds.

Experimental

General information

All manipulations were performed under an atmosphere of nitrogen using standard Schlenk techniques. Solvents were dried by refluxing over sodium/benzophenone ketyl (toluene, hexane, dichloromethane) and distilled just before use. Cp*Ir[S₂C₂(B₁₀H₁₀)] (1a),^4,9 Cp*Ir[Se₂C₂(B₁₀H₁₀)] (1b)⁹ and [Rh(cod)(µ-OEt₂)]¹⁶ were prepared by methods reported previously. IR spectra were recorded on a Nicolet AVATAR-360IR spectrometer, whereas ¹H (500 MHz) and ¹¹B (160 MHz) NMR spectra were obtained on a Bruker DMX-500 spectrometer in CDCl₃ solution. Elemental analyses were performed on an Elementar vario EI Analyzer.

Synthesis of {Cp*Ir[S₂C₂(B₁₀H₉)]}Rh (2a) and {Cp*Ir[S₂C₂(B₁₀H₉)]}Rh₂(cod){Cp*Ir[S₂C₂ (B₁₀H₁₀)]} (3a). [Rh(cod)(µ-OEt)]₂ (51 mg, 0.1 mmol) was added to a green solution of 1a (106 mg, 0.2 mmol) in toluene (30 mL). The mixture was stirred for 24 h at room temperature; and the color changed from green to dark-red. After removal of the solvent, the residue was chromatographed on silica gel. Elution with hexane gave 2a (42 mg, 18%) as a violet zone. Recrystallization from hexane afforded air-stable dark-violet crystals. Elemental analysis. Calc. for C₂₄H₅₀B₂₀S₄Ir₂Rh (%): C, 24.63; H, 4.31. Found (%): C 24.59, H 4.30. Spectroscopic data as reported previously.⁵ Elution with toluene–hexane (1: 2) gave 3a (91 mg, 30%) as a dark-red band. Recrystallization from CH₂Cl₂–hexane afforded red crystals. Elemental analysis. Calc. for C₃₂H₆₁B₂₀S₄Ir₂Rh₂S₄ (%): C 25.47, H 4.08. Found (%): C 25.16, H 3.89. ¹H NMR (25 °C, TMS): δ 1.58 (m, 4H, CH₂), 1.83 (m, 4H, CH₂), 2.07 (s, 15H, C₅Me₅), 2.19 (s, 15H, C₅Me₅), 4.02 (m, 2H, CH=), 4.29 (m, 2H, CH=). ¹¹B NMR (25 °C, BF₃·Et₂O): δ −3.2, −6.7, −9.0, −11.1, −13.6, −15.8, −20.2; IR (KBr disk): 2573, 2588 cm⁻¹ (ν_B–H).

Synthesis of {Cp*Ir[Se₂C₂(B₁₀H₉)]}Rh (2b) and {Cp*Ir[Se₂C₂(B₁₀H₉)]}Rh₂(cod){Cp*Ir[Se₂C₂(B₁₀H₁₀)]} (3b). [Rh(cod)(µ-OEt)]₂ (51 mg, 0.1 mmol) was added to a green solution of 1b (125 mg, 0.2 mmol) in toluene (30 mL). The mixture was stirred for 48 h at room temperature; and the color changed from green to dark-red. After removal of the solvent, the residue was chromatographed on silica gel. Elution with hexane gave 2b (46 mg, 16%) as a red zone. Recrystallization from hexane afforded air-stable dark-red crystals. Elemental analysis. Calc. for C₂₄H₅₀B₂₀Se₄Ir₂Rh (%): C, 29.06; H, 5.08. Found (%): C, 28.99; H, 5.07. Spectroscopic data as reported previously.⁵ The second dark-red zone was close to the first band. Elution with toluene–hexane (1 : 2) gave 3a (91 mg, 30%) as a dark-red band. Recrystallization from CH₂Cl₂–hexane afforded red crystals. Elemental analysis. Calc. for C₃₂H₆₁B₂₀S₄Ir₂Rh₂S₄ (%): C 25.47, H 4.08. Found (%): C 25.16, H 3.89; ¹H NMR 25 °C, TMS): δ 1.66 (m, 2H, CH₂), 1.81 (m, 4H, CH₂), 2.02 (s, 15H, C₅Me₅), 2.13 (s, 15H, C₅Me₅), 4.10 (m, 2H, CH=), 4.43 (m, 2H, CH=); ¹¹B NMR (25 °C, BF₃·Et₂O): δ −2.9, −6.1, −8.8, −12.5, −14.7, −19.8; IR (KBr disk): 2577, 2585 cm⁻¹ (ν_B–H).

Crystallographic analysis

Dark-red crystals of 3a and 3b were grown by slow diffusion from dichloromethane–hexane solution. The data crystals of both compounds were mounted by gluing onto the end of a thin glass fiber. X-Ray intensity data were collected on the CCD-Bruker SMART APEX system. All the determinations of unit cell and intensity data were performed with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). All the data were collected at room temperature using the ω scan technique. These structures were solved by direct methods, using Fourier techniques, and refined on F² by a full-matrix least-squares method. All non-hydrogen atoms were refined with anisotropic thermal parameters. All the hydrogen atoms were included but not refined. All the calculations were carried out the SHELXTL program.¹⁷ Crystal data, data collection parameters, and the results of the analyses of compounds 3a, 3b are listed in Table 2.

Table 2 Crystallographic data for complexes 3a and 3b

	3a	3b
Formula	C32H61B20S4Ir2Rh2S4·CH2Cl2	C32H61B20S4Ir2Rh2Se4·CH2Cl2
M	1465.39	1652.99
Crystal system	Monoclinic	Monoclinic
Space group	P21/n	P21/n
a/Å	11.066(4)	11.104(3)
b/Å	26.140(8)	26.517(6)
c/Å	18.062(6)	18.003(4)
β/°	98.633(5)	98.155(4)
V/Å^3	5165(3)	5247(2)
Z	4	4
D c/g cm^−3	1.884	2.092
µ(Mo-Kα)/mm^−1	6.057	8.578
Collected reflections	21604	22009
Unique	9083	9264
Parameters	597	595
Goodness of fit	1.020	1.016
R 1 (I > 2σ(I))	0.0499	0.0549
wR 2 (I > 2σ(I))	0.1174	0.1424
Max., min. residual density/e Å^−3	1.917, −0.986	1.399, −1.083

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CCDC reference numbers 281883 and 281884.

For crystallographic data in CIF or other electronic format see DOI: 10.1039/b512027f

Acknowledgements

This work was supported by the National Science Foundation of China for Distinguished Young Scholars (29925101, 20421303) and by the National Basic Research Program of China (2005CB623802), and is gratefully acknowledged.

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