Two Different, Metal-Dependent Coordination Modes of a Dicarbene

Oct 14, 2013 - Two Different, Metal-Dependent Coordination Modes of a Dicarbene Ligand ... Formation and Site-Selective Reactivity of a Nonsymmetric ...
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Two Different, Metal-Dependent Coordination Modes of a Dicarbene Ligand Ramananda Maity, Arnab Rit, Christian Schulte to Brinke, Jutta Kösters, and F. Ekkehardt Hahn* Institut für Anorganische und Analytische Chemie and NRW Graduate School of Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstraße 30, D-48149 Münster, Germany S Supporting Information *

ABSTRACT: The bisimidazolium salt H2-1(PF6)2 featuring a bridging 1,4-phenylene group reacts with 0.5 equiv of [PdCl(allyl)]2 in the presence of Cs2CO3 to give the dinuclear complex [2](PF6)2, whereas the reaction of the same bisimidazolium salt with 0.5 equiv of [Ir(Cl)2(Cp*)]2 yields the mononuclear orthometalated complex [3]PF6 with one remaining imidazolium unit. The unreacted imidazolium group in complex [3]PF6, however, can also be metalated with RhIII to yield the doubly orthometalated heterobimetallic complex [4]. In complex [4], each MIII center (M = IrIII and RhIII) is coordinated by one NHC unit and orthometalates the central aryl ring of the ligand.

N

-Heterocyclic carbenes (NHCs) have become an intensively studied class of compounds due to their multiple applications as ligands in organometallic chemistry.1 Different N-heterocyclic carbenes have been used as spectator ligands in catalytically active metal complexes,2 organocatalysis,3 and biologically active compounds.4 A large number of synthetic transformations such as the ruthenium-catalyzed olefin metathesis5 or palladium-catalyzed C−C coupling reactions6 (Heck or Suzuki coupling), as prominent examples, have been catalyzed by complexes bearing monodentate NHCs. Applications for NHCs may be further enhanced by incorporation of the NHC donor groups into poly-NHC ligands.7 Very recently poly-NHC ligands have also been used to fabricate new two- and three-dimensional metallosupramolecular structures.8 The synthesis of heterobimetallic NHC complexes from poly-NHC ligands has attracted considerable attention due to their application in cooperative catalysis.9 Heterobimetallic NHC complexes have been prepared by the oxidative addition of a transition metal to a ruthenocene-fused imidazolium cation10 and by the stepwise metalation of 1,2,4-triazolyl-3,5diylidenes9a,c,d or of a tri-NHC ligand derived from a Y-shaped 4,5-di-NHC-substituted imidazolin-2-ylidene.9b Recently, the synthesis of heterobimetallic complexes of type A (Figure 1) in a one-pot site-selective metalation has been reported.11 This type of selectivity was not observed with the symmetrically 1,3,5-substituted phenylene-bridged tri-NHC ligand, where the reaction with linearly coordinated metals (AgI, AuI, CuI) leads to cyclinder-like three-dimensional structures,8e,f while orthometalated homodi- and homotrinuclear complexes were obtained when the phenylene-1,3,5tricarbene ligand was reacted with RhIII or IrIII.12 We became interested in 1,4-phenylene-bridged di-NHC ligands, which have been shown to form dinuclear molecular © XXXX American Chemical Society

Figure 1. Heterobimetallic (A) and homobimetallic (B) NHC complexes.

rectangles of type B (Figure 1) with AgI or AuI.8a,13 Here we describe the reaction a 1,4-bisimidazolium-substituted phenylene with PdII or IrIII to yield either a dinuclear PdII complex or a mononuclear orthometalated IrIII complex. The mononuclear IrIII complex can subsequently react with RhIII with metalation/ orthometalation of the second imidazolium group and the central phenylene ring. Reaction of the bisimidazolium salt H2-1(PF6)213 with 0.5 equiv of [PdCl(allyl)]2 in the presence of Cs2CO3 yielded the dinuclear palladium(II) complex [2](PF6)2 in 60% yield (Scheme 1). Complex [2](PF6)2 was fully characterized by 1 H and 13C{1H} NMR spectroscopy, mass spectrometry, and X-ray crystallography. The 1H NMR spectrum of [2](PF6)2 in CD3CN exhibited resonances for the dicarbene ligands, which are generally Received: September 16, 2013

A

dx.doi.org/10.1021/om400921c | Organometallics XXXX, XXX, XXX−XXX

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Scheme 1. Synthesis of the Dinuclear Palladium(II) Complex [2](PF6)2

upfield shifted in comparison to the parent bisimidazolium salt. The imidazolium C2−H signal observed in H2-1(PF6)2 at δ = 8.97 ppm13 was, as expected, absent in the 1H NMR spectrum of [2](PF6)2. The resonances for the allyl hydrogen atoms (δ = 5.44−5.65, 4.05−4.18, and 2.97 ppm) appeared in the range reported previously for [Pd(allyl)(NHC)2] complexes.12,14 The 13 C{1H} NMR spectrum of [2](PF6)2 showed the characteristic resonance for the CNHC carbon atoms at δ = 176.3 ppm, significantly downfield shifted from the C2 resonance of the parent bisimidazolium salt H2-1(PF6)2 (δ = 137.0 ppm).13 The remaining resonances fall in the range typical for palladium(II) dicarbene complexes with a cis-arragement of the carbene donors.14,15 The ESI mass spectrum (positive ions) of [2](PF6)2 shows peaks at m/z = 973.1568 (calcd for [[2]PF 6 ] + 973.1578) and 414.0955 (calcd for [2] 2+ 414.0968) as the strongest signals. Single crystals of [2](PF6)2 were obtained by slow vapor diffusion of Et2O into a saturated acetonitrile solution of [2](PF6)2 at ambient temperature. An X-ray diffraction study with these crystals confirmed the formation of the dinuclear palladium(II) complex cation [2]2+, which is depicted in Figure 2. The asymmetric unit contains two independent halves of the dication [2]2+ each related to the other half by a crystallographic inversion center. Metric parameters in the two independent halves of [2]2+ are almost identical. The two cispositions at each palladium atom are occupied by two NHC donors from two different dicarbene ligands, and the remaining coordination sites at the palladium center are occupied by an η3-coordinated allyl ligand. To accommodate the palladium atoms, the planes of the NHC donors are rotated out of the plane of the central phenyl ring by 35.1(5)−48.2(6)°. The two phenyl rings of the dicarbene ligands are arranged in a parallel fashion with a centroid to centroid separation of 3.755 Å, which is indicative of weak π···π interactions between these aromatic rings.16 The coordination geometry around the palladium(II) centers is distorted square-planar, and the metric parameters fall in the range previously described for cis-dicarbene PdII complexes.12,14,15 Treatment of the bisimidazolium salt H2-1(PF6)2 with 0.5 equiv of [Ir(Cl)2(Cp*)]2 in the presence of Cs2CO3 and NaOAc at 60 °C for 18 h yielded the orthometalated complex [3]PF6 in 65% yield (Scheme 2). No indications for the formation of a dimetalated complex were found. Even the addition of one or more equivalents of [Ir(Cl)2(Cp*)]2 does not yield the dimetalated complex but exclusively the mononuclear complex [3]PF6. The reaction of H2-1(PF6)2

Figure 2. Molecular structure of [2]2+ in [2](PF6)2 (50% displacement ellipsoids, hydrogen atoms have been omitted, and only the first atom of the N-ethyl substituents is shown for clarity). One of the N-ethyl substituents is disordered over two positions, and only one set of positions is depicted. Selected bond distances (Å) and angles (deg), ranges: Pd1−CNHC 2.043(4)−2.069(4), Pd1−Callyl 2.143(5)− 2.189(4), N−C NH C 1.341(6)−1.362(5); C NH C −Pd1−C N H C 91.71(15)−94.64(16), N−CNHC−N 104.1(3)−105.1(4).

Scheme 2. Synthesis of Complexes [3]PF6 and [4]

with 1.0 equiv of [Rh(Cl)2(Cp*)]2, however, yields the dinuclear rhodium(III) complex (see SI, Figure S8). We have currently no explanation for this behavior, which has been observed previously,12 and can only attribute it to differences in the reactivity of the metals. Complex [3]PF6 features an iridacycle with one unreacted imidazolium unit. This remaining imidazolium unit, however, can be metalated in a subsequent reaction. Reaction of complex [3]PF6 with 0.5 equiv of [Rh(Cl)2(Cp*)]2 in the presence of Cs2CO3 and NaOAc at 60 B

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°C for 18 h gave the doubly orthometalated heterobimetallic complex [4] in 54% yield. The formation of both complexes [3]PF6 and [4] was confirmed by 1H, 13C{1H}, and 2D correlation NMR spectroscopy in addition to mass spectrometry. The 1H NMR spectrum of complex [3]PF6 in CD3CN showed two sets of signals, attributed to the formation of a fluxional complex (most likely resulting from rotation about the CPh−Nimidazolium bond) in solution. The ratio of the two fluxional isomers was found to be 85:15 immediately after the reaction, but interconversion of the isomers was detected as a function of time. After 50 h at ambient temperature an equilibrium with a 50:50 ratio of two isomers was reached. The 1H NMR spectrum of complex [3]PF6 (mixture of isomers) revealed the signals for the unreacted imidazolium C2−H protons at δ = 8.82 and 8.86 ppm. The resonances for the Ph−H proton next to the orthometalated carbon atom (δ = 7.79 and 7.81 ppm) appear significantly downfield shifted compared to the other Ph−H proton resonances (δ = 7.45 and 7.57 for the Ph−H proton next to the metalated NHC; δ = 7.16 and 7.35 ppm for the Ph−H proton next to the imidazolium group). The 13C{1H} NMR spectrum of complex [3]PF6 exhibits the imidazolium NCHN carbon signals for the two isomeric complexes at δ = 135.1 and 135.2 ppm, respectively. The CNHC resonances were recorded at δ = 166.6 and 159.7 ppm, well within the range previously reported for orthometalated iridium(III) NHC complexes.11,12,17 Orthometalation of the phenylene ring was confirmed by 2D correlation NMR spectroscopy, where the resonance of the metalated phenylene carbon atom were found at δ = 147.3 and 140.2 ppm for the two isomeric complexes. These values are shifted significantly downfield from the resonances recorded for the nonmetalated phenylene carbon atoms (range δ = 112.1− 131.1 ppm), with the most downfield located resonance recorded for the Ph−C carbon atom located next to the orthometalated ring carbon atom. The ESI mass spectrum (positive ions) of complex [3]PF6 shows peaks at m/z = 629.2011 (calcd for [3]+ 629.2016) and 297.1163 (calcd for [[3]−Cl]2+ 297.1168). Single crystals of compound [3]PF6·CH3CN suitable for an X-ray diffraction study have been obtained by the slow diffusion of diethyl ether into a saturated acetonitrile solution of [3]PF6 at ambient temperature. The molecular structure analysis confirmed the formation of the mononuclear complex cation [3]+, featuring a five-membered iridacycle (Figure 3). The coordination geometry around the iridium(III) center in [3]+ is best described as three-legged piano stool, where the three legs are one NHC donor, one Ph−C carbon atom, and one chloride donor.17c The separation between the iridium atom and the centroid of the Cp* ring measures 1.848 Å. The Ir−CPh bond length (2.053(2) Å) is slightly longer than the Ir− CNHC bond length (2.014(2) Å). The bite angle of the CNHC− Ir−CPh chelate ring measures 77.81(8)° in [3]+, which is slightly smaller than observed for the six-membered chelate ring obtained after Ir-orthometalation of a benzyl-substituted NHC ligand.17c Additional bond angles and distances fall in the typical range for orthometalated iridium(III) NHC complexes.11,12,17c The torsion angle of the essentially planar iridacycle (C1−N2−C6−C7) measures 0.66°, whereas the torsion angle involving the free imidazolium unit (C12−N3−C9−C10) measures −6.67°. This value indicates that the free imidazolium unit is capable of forming another planar metallacycle when reacted with a suitable metal precursor. The molecular structure

Figure 3. Molecular structure of the complex cation [3]+ in [3]PF6· CH3CN (50% displacement ellipsoids, hydrogen atoms except for H12 have been omitted, and only the first atom of each of the N-ethyl substituents is shown for clarity). Selected bond distances (Å) and angles (deg): Ir1−Cl1 2.4035(5), Ir1−C1 2.014(2), Ir1−C7 2.053(2), N1−C1 1.355(3), N2−C1 1.358(3), N3−C12 1.335(3), N4−C12 1.325(3); Cl1−Ir1−C1 88.82(6), Cl1−Ir1−C7 86.76(6), C1−Ir1−C7 77.81(8), N1−C1−N2 104.3(2), N3−C12−N4 109.2(2).

determination of cation [3]+ allows for a direct comparison of the metric parameters of a coordinated imidazolin-2-ylidene and its imidazolium precursor present in the same molecule. As noted previously for NHC complexes featuring free imidazolium groups,8i,12 the transformation of an imidazolium cation into a coordinated NHC leads to an expansion of the N−C bond distances within the heterocycle (Δd ≈ 0.025 Å) and a decrease of the N−C−N angles (∼5°) in accord with previous DFT-based predictions.18 The reaction of [3]PF6 with [Rh(Cl)2(Cp*)]2 giving the doubly orthometalated heterobimetallic complex [4] (Scheme 2) was monitored by 1H and 13C{1H} NMR spectroscopy. The 1 H NMR spectrum of complex [4] no longer features a resonance for the imidazolium NCHN proton. The protons of both N−CH2 groups become diastereotopic after the second metalation, and two multiplets were detected in the range δ = 4.17−4.30 and 4.43−4.53 ppm. The 13C{1H} NMR spectrum of complex [4] revealed the characteristic resonances for the carbene carbon atoms at δ = 181.7 (d, 1JC,Rh = 55.5 Hz) ppm for the Rh−CNHC carbon atom and at δ = 163.9 ppm for the Ir−CNHC carbon atom. These resonances compare well to the equivalent resonances reported for the M−CNHC atoms in orthometalated rhodium(III) and iridium(III) complexes.11,12,17 The double orthometalation of the phenylene ring was confirmed by the 2D correlation NMR spectroscopy. The resonances for the metalated ring carbon atoms appeared at δ = 148.7 (d, 1JC,Rh = 35.7 Hz, Rh−CPh) and 133.5 ppm (Ir− CPh) and are thus recorded significantly downfield from the resonances for the remaining CPh−H carbon atoms (δ = 119.1 and 118.5 ppm). Both the downfield shift of the M−CPh resonances and the 1JC,Rh coupling clearly indicate the double metalation to the phenylene ring of the ligand. Formation of complex [4] was also confirmed by ESI mass spectrometry. The ESI mass spectrum (positive ions) showed the strongest peaks at m/z = 906.2360 (calcd for [[4]−Cl+CH3CN]+ 906.2355), 865.2090 (calcd for [[4]−Cl]+ 865.2089), and 415.1201 (calcd for [[4]−2Cl]2+ 415.1204). We have demonstrated the formation of two different types of complexes, [2](PF6)2 and [3]PF6, starting from the same bisimidazolium ligand precursor H2-1(PF6)2. The complex formation is significantly influenced by the metal precursor used. Reaction of H2-1(PF6)2 with IrIII (ratio 1:0.5) leads not only to metalation of the NHC donor but also to C

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orthometalation of the phenylene ring. No dinuclear complexes (metalation of the second imidazolium group) were detected even if an excess of iridium precursor was used. The stepwise metalation of phenylene-bridged polyimidazolium salts19 described here generally allows the stepwise preparation of heterobimetallic NHC complexes. The monometalated complex [3]PF6, possessing an imidazolium unit, reacts with 0.5 equiv of [Rh(Cl)2(Cp*)]2 to yield the heterobimetallic complex [4], featuring a doubly orthometalated phenylene bridge. Further studies will be directed toward exploring the scope of the stepwise metalation of ligands such as H2-1(PF6)2 for the synthesis of heterobimetallic complexes and the use of these complexes in tandem catalytic processes.

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ASSOCIATED CONTENT

S Supporting Information *

Preparation of all new compounds, reaction procedures, spectra, and crystallographic details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Deutsche Forschungsgemeinschaft (SFB 858) and the NRW Graduate School of Chemistry, Münster, for a predoctoral grant for R.M.

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REFERENCES

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dx.doi.org/10.1021/om400921c | Organometallics XXXX, XXX, XXX−XXX