Donor Strength Determination of Benzoxazolin-2 ... - ACS Publications

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Donor Strength Determination of Benzoxazolin-2-ylidene, Benzobisoxazolin-2-ylidene, and Their Isocyanide Precursors by NMR Spectroscopy of Their PdII and AuI Complexes

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Martin Meier,† Tristan Tsai Yuan Tan,† F. Ekkehardt Hahn,*,† and Han Vinh Huynh*,‡ †

Department of Inorganic and Analytical Chemistry, Westfälische Wilhelms-Universität Münster, Corrensstraße 28-30, 48149 Münster, Germany ‡ Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 S Supporting Information *

ABSTRACT: A series of mono- and dinuclear palladium(II) and gold(I) complexes containing the iPr2-bimy reporter ligand and monodentate or Janustype β-functionalized aryl (di)isocyanides have been prepared. Cyclization of the isocyanide ligands afforded complexes of monodentate or Janus-type N,Oheterocyclic carbenes (NOHCs) via a template-directed process. Using the iPr2bimy 13Ccarbene signals in these complexes, the electron-donating ability of NOHCs and their isocyanide precursors could be evaluated and compared for the first time. Overall, NOHCs were found to be rather weakly donating carbenes. Spectroscopic comparison of mono- with dinuclear complexes indicated that electronic communication between two metal centers occurs only across the Janus-type diNOHC, but not for the diisocyanide ligand. Support for this claim was also found in the solid-state molecular structures, which revealed coplanarity of all ligands only in the diNOHC complex as a prerequisite for π-electron delocalization.

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INTRODUCTION The coordination chemistry of N-heterocyclic carbenes has matured considerably in recent years, and these ligands are now ubiquitous in organometallic chemistry.1 The vast majority of publications in this field are concerned with classical NHCs, which are five-membered-ring compounds, in which the carbene donor is located adjacent to two nitrogen atoms. The substituents at these α-nitrogen atoms can be altered in order to tune the stereoelectronic properties of the NHC. A change in the α-heteroatoms is expected to have an even more drastic overall effect on the resulting NHC ligand and its metal complexes. For example, the formal substitution of one N−R group in stereotypical NHCs with an oxygen atom leads to N,O-heterocyclic carbenes (NOHCs), which have an obvious reduced steric impact upon binding to metals.2 Moreover, the more electronegative oxygen atom would exhibit different σ and π contributions toward the carbene atom in comparison to an N−R group changing the nature of the push−pull effect. The first complexes of NOHCs contained saturated oxazolidin-2-ylidenes, which were obtained via metal-template-controlled cyclization of β-functionalized isocyanides.3 Thereafter, different methodologies have been developed for the preparation of NOHC complexes, and interest in their organometallic chemistry is increasing. Some NOHC complexes have recently been explored for their photophysical properties.4 We have been particularly interested in the use of 2trimethylsiloxyphenyl isocyanide5 as a starting material to give various benzoxazolin-2-ylidene metal complexes.2a,6 Generally, © XXXX American Chemical Society

this synthesis involves a two-step process including (i) isocyanide coordination to a suitable metal center followed by (ii) cleavage of the oxygen−silicon bond. The fate of the resulting 2-hydroxyphenyl isocyanide complex is determined by the ability of the metal complex fragment to engage in backdonation to the isocyanide moiety, which is related to its Lewis acidity. Electron-rich metal centers normally stabilize the 2hydroxyphenyl isocyanide ligand,6,7 while electron-poor centers induce cyclization to the NOHC ligand. Apparently, this transformation can also be affected by metal-centered redox chemistry. Extension of the methodology to 2-azidophenyl or 2-nitrophenyl isocyanide has also been realized.8 Notably, these precursors allow for the preparation of complexes bearing Nheterocyclic carbene ligands featuring N−H substituents capable of hydrogen-bonding interactions, which are not accessible from azolium salts.9 In spite of the recent interest in NOHCs, comparison of their ligand donor strength with that of their isocyanide precursors and other N-heterocyclic carbenes has not been reported. Among the common ways to determine ligand donor strength,10 carbonyl-based methods (e.g., TEP)11 and electrochemical methods (e.g., LEP)12 appear unsuitable to assess the donor strength of NOHCs. The Ni0, RhI, and IrI metal centers commonly used for the former are low-valent and electron-rich, which would lead to stabilization of the intermediate isocyanide complex disfavoring the cyclization to the NOHC ligand. The Received: September 19, 2016

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DOI: 10.1021/acs.organomet.6b00736 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Synthesis of trans-[1], cis-[1], and [2]PF6

redox processes required for the latter will inherently interfere with the aforementioned reversible cyclization process as well. The most recent approach for the determination of ligand donor strength compares 13Ccarbene chemical shifts in trans[PdBr2(iPr2-bimy)L] complex probes, which are influenced by the ligand L of interest.13 In general, a downfield shift of the 13 Ccarbene resonance would indicate a more donating ligand L. Due to the more Lewis acidic PdII metal center in the complex probes, π-back-donation is less likely to occur. Therefore, primarily σ-donation of a ligand is measured, which is different from the inseparable net effect obtained by the carbonyl-based methods mentioned above. Various ligands including isocyanides, phosphines, normal and mesoionic NHCs, and Werner-type N-donors have been compared on a unified 13C NMR scale. For L = carbene, the range of complex probes has been successfully extended to cationic linear [Au(iPr2-bimy)L]+ complexes, and an excellent correlation between PdII and AuI complexes has been established.14 The increased Lewis acidity of these complex probes appears to be most compatible for heterocyclic carbenes obtained from the cyclization of βfunctionalized isocyanides. Herein, we report the preparation of mononuclear and dinuclear palladium(II) and gold(I) complexes containing monodentate and Janus-type protic N,O-heterocyclic carbene ligands from their respective aryl isocyanide precursors. Using these complexes, the donor strength of the NOHC ligand can be compared to other NHCs and its isocyanide parent. Moreover, potential electronic communication across the Janus-type ligands in the dinuclear complexes can be easily probed by 13C NMR spectroscopy as well.

As anticipated, the 13Ccarbene NMR signal of the iPr2-bimy reporter ligand in trans-[1] was observed at δ 176.5 ppm in CDCl3, while that for cis-[1] was detected at higher field at δ 174.8 ppm. It is well documented that trans-bis(NHC) complexes exhibit more downfield 13C carbene signals in comparison to their direct cis isomers.15 The 1H NMR signal for the N−H function in cis-[1] was detected downfield at δ 12.90 ppm, providing further support for the formation of a NH-benzoxazolin-2-ylidene complex. In addition to the 13C-labeling study mentioned above, attempts were made to prepare the cationic gold(I) complex [Au(iPr2-bimy)(NOHC)]PF6 [2]PF6 as an alternative 13C NMR probe for the donor strength evaluation of the NOHC ligand. Constitutional isomerism observed for the Pd II complexes is not possible for the linear d10 complex [2]PF6, and 13C NMR data collection should be more straightforward. Moreover, it was of interest to test if the reported PdII/AuI correlation could be extended to benzoxazolin-2-ylidenes. Mixing the isocyanide A with [AuCl(iPr2-bimy)] (C) and KPF6 in dichloromethane indeed led to the direct formation of [2]PF6 in good yield (Scheme 1). The intermediate isocyanide complex was not detected, apparently due to rapid cyclization of the isocyanide at the AuI center. In this case, deprotection of the trimethylsiloxy group most likely occurs by trace amounts of fluoride impurities from the KPF6 source. Notably, no formation of homobis(NHC) complexes was observed. The important 13Ccarbene resonance of the iPr2-bimy reporter ligand in [2]PF6 was detected at δ 186.2 ppm in CDCl3, while that for the NH-benzoxazolin-2-ylidene ligand resonates at δ 209.5 ppm, corroborating the identity of [2]PF6 as a heterobis(carbene) complex. The more downfield chemical shift for the NOHC carbene atom results from the deshielding effect of the electronegative, endocyclic oxygen atom. The rapid departure of the trimethylsilyl protection group during the synthesis of trans-[1]/cis-[1] and [2]PF6 did not allow the isolation of intermediate isocyanide complexes. These are also desired target compounds to study the change in donor ability upon ligand transformation from β-functionalized isocyanide to benzoxazolin-2-ylidene. Thus, 2-triisopropylsiloxyphenyl isocyanide (4) featuring the more stable O−Si(iPr)3 group was prepared in an attempt to generate complexes with

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RESULTS AND DISCUSSION Synthesis of Complexes. The coordination of 2trimethylsiloxyphenyl isocyanide (A) to selected electronpoor metal centers followed by removal of the trimethylsilyl group to afford benzoxazolin-2-ylidene complexes in a template-controlled cyclization reaction is well-known.6a,e,h In an attempt to prepare the complex trans-[PdBr2(iPr2-bimy)(NOHC)] (trans-[1]) for donor strength evaluation of the benzoxazolin-2-ylidene (NOHC) ligand, the dinuclear complex [PdBr2(iPr2-bimy)]2 (B) was reacted with 2 equiv of A in dichloromethane at ambient temperature. This reaction was anticipated to afford the mixed carbene/isocyanide complex [PdBr2(iPr2-bimy)(A)] as an intermediate for a subsequent deprotection/cyclization. However, in situ deprotection of the trimethylsiloxyl protection group already took place under the selected reaction conditions, which is likely triggered by trace amounts of acid in the dichloromethane used. Moreover, the anticipated trans complex trans-[1] was not detected and only the heterobis(carbene) complex cis-[PdBr 2 ( i Pr 2 -bimy)(NOHC)] cis-[1] could be isolated after workup as the thermodynamically stable final product (Scheme 1). If the reaction was stopped after only 2 h, a mixture of complexes trans-[1] and cis-[1] was obtained. The observed complete isomerization to the cis isomer over a short reaction time indicates that the desired complex trans-[1] is only the kinetically preferred product, for which 13C NMR data acquisition under standard conditions is severely hampered. Nevertheless, it was possible to capture the desired carbene signal of trans-[1] by “fast-track” 13C NMR spectroscopy of the mixture trans-[1]/cis-[1] using the 13Ccarbene-labeled iPr2-bimy complex B as a starting material. B


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Organometallics Scheme 2. Synthesis of 2-Triisopropylsiloxyphenyl Isocyanide (4) in a Stepwise Procedure Starting from N-(2Hydroxyphenyl)formamide

stable β-functionalized isocyanide ligands which can still be converted into NOHC ligands. The synthesis of 4 (Scheme 2) differs from the synthesis of its 2-trimethylsiloxy analogue A and could not be accomplished by simple treatment of benzoxazole with n-butyllithium followed by the introduction of the protection group using triisopropylsilyl chloride.5 The latter requires harsher conditions, which are generally incompatible with organolithium species. Instead, a stepwise route was necessary starting from commercially available N-(2hydroxyphenyl)formamide. After the protection of the hydroxyl group with triisopropylsilyl chloride, the resulting N-formamide 3 was dehydrated with triphosgene to give the isocyanide 4. The formation of 4 was confirmed by IR spectroscopy, where the CN stretching vibration was detected at ν̃ 2123 cm−1. In the 13C NMR spectrum, the resonance of the isocyanide carbon atom was detected at δ 167.0 ppm. The signal of the silicon atom of the triisopropylsilyl protection group was observed at δ 19.5 ppm in the 29Si NMR spectrum. The subsequent preparation of the complex trans[PdBr2(iPr2-bimy)(4)] ([5]) occurred by reacting dipalladium complex B with isocyanide 4 in dichloromethane (Scheme 3).

complexes. These interactions involve electrons of the weakly antibonding 5σ* orbital and thus lead to an increase of the C N bond order. The lack of back-donation to the isocyanide also provides an explanation for the fast cyclization observed for the reactions involving the related trimethylsiloxyphenyl isocyanide A. The important 13Ccarbene NMR resonances of the iPr2-bimy ligands in the isocyanide complexes were observed at δ 169.3 and 181.0 ppm for [5] and [6]PF6, respectively. The resonances of the isocyanide carbon donors shift upfield upon coordination from δ 167.0 for the free ligand 4 to δ 153.3 and 153.9 ppm for [5] and [6]PF6, respectively. This observation is also in line with an almost pure σ-donor character of the isocyanide ligands. Recently, the ligand 2,5-bis(triisopropylsiloxy)phenyl 1,4diisocyanide (D) (Scheme 4), which allows for the preparation of linear, dinuclear metal complexes and molecular squares or rectangles with Janus-type diisocyanides and benzobisoxazolin2-ylidenes, has been introduced.17 In an attempt to test our 13C NMR based electronic parameter for the evaluation of bridging ligands, we applied the diisocyanide D for the preparation of dinuclear complex probes of palladium(II) and gold(I). The reaction of D with the dipalladium complex B in a 1:1 ratio leads exclusively to the formation of all-trans-[{PdBr2(iPr2bimy)}2(μ-D)] ([7]) among four theoretically possible isomers: i.e., all-trans, syn-all-cis, anti-all-cis, and cis-trans (Scheme 4). As observed for complex [5], the selectivity is a result of the steric repulsion between the bulky organic ligands. The formation of the all-trans isomer [7] was verified by 1H NMR spectroscopy, where only one septet was detected at 5.98 ppm for the CH(CH3)2 protons of the iPr2-bimy ligand. Moreover, only one signal each was observed in the 13C NMR spectrum for the carbon donors of the diisocyanide and carbene ligands at δ 147.0 and 168.5 ppm. The 13Ccarbene resonance is characteristic for trans-configured mixed iPr2-bimy/isocyanide complexes (trans, δ ∼168 ppm; cis, δ ∼164 ppm).16 In the same way, the reaction of D and 2 equiv of the gold(I) precursor C takes place with formation of the linear digold species [{Au(iPr2-bimy)}2(μ-D)](PF6)2 ([8](PF6)2). The 13C NMR spectrum of [8](PF6)2 shows signals for the iPr2-bimy carbene atoms at δ 181.1 ppm, and the isocyanide carbon donor resonance was found at δ 148.0 ppm. CN vibrational bands observed at ν̃ 2186 cm−1 for [7] and at ν̃ 2218 cm−1 for [8](PF6)2 provided IR spectroscopic evidence for the isocyanide coordination. Similar to the case for the mononuclear analogues, these bands are shifted to higher wavenumbers in comparison to that of the free ligand D (ν̃CN 2130 cm−1)16 indicating stronger CN bonds in the complexes. A comparison between mono- and dinuclear species revealed that the isocyanide CN stretches of the mononuclear complexes [5] and [6]PF6 are found at higher wavenumbers in comparison to their dinuclear analogues [7] and [8](PF6)2 ([5]/[7]m Δν̃CN = 17 cm−1; [6]PF6/ [8](PF6)2, Δν̃CN = 14 cm−1). These differences can be

Scheme 3. Synthesis of Complexes [5] and [6]PF6

Here, only the trans isomer was observed, although similar reactions with simpler aromatic or aliphatic isocyanides always led to isomer mixtures.16 Apparently, the trans arrangement is favored due to steric repulsion between the isopropyl groups of i Pr2-bimy and the 2-triisopropylsiloxy group of the isocyanide ligand. The gold(I) complex [Au(iPr2-bimy)(4)]PF6 ([6]PF6) was prepared in a similar fashion by exposure of [AuCl(iPr2bimy)] (C) to isocyanide 4 and KPF6 in dichloromethane at ambient temperature. The coordination of 4 to PdII and AuI was indicated by C N vibrational bands at ν̃ 2203 cm−1 for [5] and at ν̃ 2232 cm−1 for [6]PF6, which are in the typical range for such metal centers.2a,6f In comparison to the proligand 4 (ν̃CN 2123 cm−1), these values are shifted to higher wavenumbers by 80 and 109 cm−1, respectively. The observed CN bond strengthening upon complex formation indicates that the isocyanide ligand 4 acts primarily as a σ-donor in these C


Article

Organometallics Scheme 4. Synthesis of [7], [8](PF6)2, and [9](PF6)2

attributed to a complicated interplay between additional ringsubstituent effects in the diisocyanide ligand. The transformation of the bridging diisocyanide ligands in [7] and [8](PF6)2 into bridging benzobisoxazolin-2-ylidenes was not successful for the dipalladium complex [7]. Upon cleavage of the O−Si(iPr)3 bonds under various reaction conditions only insoluble and intractable mixtures were obtained possibly containing complex “[10]”. Cleavage of the O−Si(iPr)3 bonds in [8](PF6)2, on the other hand, was achieved with a catalytic amount of KF in methanol and afforded the benzobisoxazolin-2-ylidene complex [{Au(iPr2bimy)}2(μ-diNOHC)] ([9](PF6)2). In agreement with the formation of [9](PF6)2, a peak at m/z 957.24667 (calcd 957.24655 [[9]2+ − H+]+) was observed in the HRMS (ESI, positive ions) spectrum. Moreover, the CN stretch was no longer observed in the IR spectrum of [9](PF6)2. In the 13C NMR spectrum, two carbene carbon signals were detected, as was anticipated for a symmetrical heterotetracarbene complex. The resonance at δ 205.3 ppm was assigned to the Janus-type diNOHC ligand, while the resonance for the carbene carbon atom of the iPr2-bimy ligand was recorded at δ 193.2 ppm. Notably, the diisocyanide−dicarbene transformation ([8](PF6)2 → [9](PF6)2) results in a significant downfield shift of Δδ = 12.1 ppm (δ 181.1 ppm → δ 193.2 ppm) for the diagnostic iPr2-bimy carbene signal in the 13C NMR spectrum. Molecular Structures. Single crystals of cis-[1]·CH2Cl2, [2]PF6, [5], and [9](PF6)2·2THF·EtOH were studied by X-ray diffraction. The thermodynamically stable arrangement of the ligands proposed for the PdII complex cis-[1] has been retained in the solid state (Figure 1). As commonly observed for squareplanar NHC complexes, the planes of both carbene ligands are oriented almost perpendicular to the essentially square-planar [PdBr2C2] coordination plane with dihedral angles of 83 and 77° for the iPr2-bimy and NOHC ligands, respectively. Notably, the C1−Pd−C14 angle of 87.96(8)° between these two bulkiest ligands in the complex is smaller than the ideal 90° angle for a perfect square-planar complex. The Pd−C1 distance of 1.983(2) Å is significantly longer than the Pd−C14 bond length of 1.958(2) Å, which would imply a stronger metal− carbon bond for the NOHC in comparison to the classical NHC ligand. This observation might be due to an increased π-

Figure 1. Molecular structure of cis-[1] in the solid state (hydrogen atoms have been omitted for clarity except for N3H, ellipsoids drawn at 50% probability). Selected bond lengths (Å) and angles (deg): Pd− Br1 2.4801(4), Pd−Br2 2.4773(4), Pd−C1 1.983(2), Pd−C14 1.958(2), N1−C1 1.343(3), N2−C1 1.351(3), O−C14 1.350(3), N3−C14 1.321(3); Br1−Pd−Br2 94.036(11) Br1−Pd−C1 90.08(6), Br1−Pd−C14 177.37(6), Br2−Pd−C1 174.10(6), Br2−Pd−C14 88.05(6), C1−Pd−C14 87.96(8), N1−C1−N2 108.1(2), O−C14− N3 108.7(2).

acceptor ability of the NOHC ligand. In agreement with this notion, the Pd−Br1 bond of 2.4801(5) Å trans to the NOHC was found to be longer than the Pd−Br2 distance of 2.4772(4) Å trans to the iPr2-bimy ligand, which may also point to a stronger trans influence of the NOHC ligand. The intra-ring N1−C1−N2 (108.1(2)°) and O−C14−N3 (108.7(2)°) angles at the carbene carbon atoms are identical within experimental error. Complex cis-[1] is one of the very rare examples featuring an NHC and an NOHC ligand bound to the same metal center, allowing a direct comparison of the metric parameters of two different NHC ligands. Next, we determined the molecular structure of the gold(I) NHC/NOHC complex [2]PF6 (Figure 2). Complex cation [2]+ features, as expected for d10 ions, an almost linear coordination geometry (angle C1−Au−C14 171.42(15)°). The two NHC ligands are oriented almost perpendicularly to each D


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reported so far, and complex [5] reveals the rather similar binding properties of these two different CII-donor ligands. The digold complex [9](PF6)2 contains two [Au(iPr2bimy)]+ complex fragments that are linearly bridged by a Janus-type benzobisoxazolin-2-ylidene ligand (Figure 4). The nonbonding Au···Au distance is 10.51 Å. In contrast to the related monogold species [2]PF6 bearing an NHC and an NOHC ligand, all ligand planes in cation [9]2+ are arranged in an almost coplanar fashion with a very small interplanar angle of ∼9°. All in all, the metal−CNHC (2.024(6) Å) and metal− CNOHC (2.007(5) Å) bond distances are similar to those in the monodentate NHC/NOHC complex [2]PF6 (Au−CNHC 2.025(4) Å, Au−CNOHC 1.998(4) Å). Discussion of the Relative Donor Strength. As reported previously, complex probes of the type trans-[PdBr2(iPr2bimy)L] and [Au(iPr2-bimy)L]PF6 can be used to evaluate the net donor ability of a ligand L of interest by 13C NMR spectroscopy.13,14 Generally, a more downfield shift of the iPr2bimy carbene resonance would indicate a stronger donor, while a weaker donor would give rise to a more upfield signal. For NHCs, an excellent PdII/AuI correlation was established, which allows the interconversion of values using the formula [Pd] = 1.19[Au] − 45.0 ppm, where [Pd] and [Au] are the 13Ccarbene chemical shifts of the iPr2-bimy CNHC resonance in the respective complex probes. The initially targeted complex probe trans-[1] is unstable and quickly undergoes isomerization to the cis form, which hampers capture of the diagnostic 13C NMR signal under standard conditions. Therefore, the alternative gold probe [2]PF6 was prepared, and its iPr2-bimy reporter signal was detected at δ 186.2 ppm (Table 1). Using the aforementioned equation and the recorded chemical shift for the gold complex [Au(iPr2-bimy)(NOHC)]PF6, a chemical shift of δ 176.6 ppm can be predicted for the i Pr2-bimy signal of the palladium complex trans-[1]. The subsequent detection of the reporter signal in trans-[1] at δ 176.5 ppm by a 13C-labeling experiment demonstrates the predictive power and the usefulness of the [Pd]/[Au] correlation. Overall, the donor strength of the NOHC ligand can be placed between those of the slightly stronger donor 1,4dibenzyltriazolin-5-ylidene13 and the slightly weaker 3-benzylbenzothiazolin-2-ylidene14 and tricyclohexylphosphine13 donor ligands (Figure 5). For the weaker donor isocyanide 4 in complex [5], the expected upfield-shifted resonance for the iPr2-bimy ligand was recorded at δ 169.3 ppm. The electron-releasing triisopropylsilyl substituent in 4 makes it a slightly stronger donor in comparison to 2,6-dimethylphenyl isocyanide (δ 168.8 ppm) and comparable in donor strength to cyclohexyl isocyanide (δ 169.1 ppm).13,16 Going from mononuclear isocyanide to dinuclear diisocyanide-bridged complexes does not lead to large changes in either the gold(I) or the palladium(II) system. In the mononuclear NHC/CNR complexes, the signals for the iPr2-bimy reporter ligand were observed at chemical shifts very close to those found for the dinuclear diisocyanide-bridged complexes (for example, [6] δ 181.0 ppm, [8]PF6 δ 181.1 ppm). It appears that the substantial modification of the isocyanide backbone with gold(I) and palladium(II) complex fragments can only be sensed as a small inductive effect, and electron delocalization across the diisocyanide D is minimal (Figure 6). Upon cyclization of the bridging diisocyanide D in [8]PF6 to the Janus-type diNOHC ligand in complex [9](PF6)2, a significant downfield shift of Δδ = 12.1 ppm for the iPr2-

Figure 2. Molecular structure of [2]+ in [2]PF6 in the solid state (hydrogen atoms have been omitted for clarity except for N3H, ellipsoids drawn at 50% probability). Selected bond lengths (Å) and angles (deg): Au−C1 2.025(4), Au−C14 1.998(4), N1−C1 1.356(5), N2−C1 1.353(5), O−C14 1.351(5), N3−C14 1.322(5); C1−Au− C14 171.42(15), N1−C1−N2 106.8(3), O1−C14−N3 107.9(3).

other with an interplanar angle of 89°. As was observed for the palladium(II) complex cis-[1], the metal−CNOHC bond length (Au−C14 1.998(4) Å) is slightly shorter than the metal−CNHC distance (Au−C1 2.025(4) Å). To the best of our knowledge, complex cation [2]+ constitutes the first example of a gold(I) complex bearing an NHC and an NOHC ligand. Complex [5] features an NHC ligand and a β-functionalized phenyl isocyanide ligand (as a precursor for an NOHC carbene) occupying trans positions in a square-planar PdII complex (Figure 3). The two organic ligands are arranged in

Figure 3. Molecular structure of [5] in the solid state (hydrogen atoms have been omitted for clarity, ellipsoids drawn at 50% probability). Selected (Å) and angles (deg): Pd−Br1 2.4283(3), Pd−Br2 2.4202(3), Pd−C1 1.989(2), Pd−C14 1.994(2), N1−C1 1.343(3), N2−C1 1.350(3), N3−C14 1.146(3), N3−C15 1.396(3), O1−C20 1.346(3), Si−O1 1.670(2); Br1−Pd−Br2 175.429(13), Br1−Pd−C1 88.36(7), Br1−Pd−C14 91.52(7), Br2−Pd−C1 89.01(7), Br2−Pd−C14 91.32(7), C1−Pd−C14 176.84(9), N1− C1−N2 108.2(2), Pd−C14−N3 176.5(2), C14−N3−C15 171.7(2), C20−O−Si 139.0(2).

an almost linear fashion with an angle of 176.84(9)°. As is clearly visible, the steric bulk of the two organic ligands prevents the formation of the cis-configured complex. The angle C14−N3−C15 (171.7(2)°) is close to linearity, as would be expected for an isocyanide coordinated to an electron-poor metal center. The Pd−CNHC (Pd−C1 1.989(2) Å) and Pd− CCNR (Pd−C14 1.994(2) Å) separations are almost equidistant. The Pd−C1 separation corresponds also well to the equivalent distance in cis-[1] (Pd−C1 1.983(2) Å). Complexes bearing an NHC and NHC precursor isocyanide have not been E


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Figure 4. Molecular structure of [9]2+ in [9](PF6)2·2THF·EtOH in the solid state (hydrogen atoms are omitted for clarity except for N3H, ellipsoids drawn at 50% probability). Atoms labeled with asterisks were generated by the symmetry operator −x, −y, −z. Selected bond lengths (Å) and angles (deg): Au−C1 2.024(5), Au−C14 2.007(5), N1−C1 1.339(7), N2−C1 1.344(7), O2−C14 1.352(7), N3−C14 1.315(7); C1−Au−C14 179.1(2), N1−C1−N2 107.9(4), O2−C14−N3 109.1(4); Au···Au distance 10.51 Å.

Table 1. Diagnostic 13C NMR iPr2-bimy Carbene Chemical Shifts of Gold(I) and Palladium(II) Complexes complex

formula

δ(13Ccarbene) i Pr2-bimya

trans-[1] [2]PF6 [5] [6]PF6 [7] [8](PF6)2 [9](PF6)2 “[10]”c

trans-[PdBr2(iPr2-bimy)(NOHC)] [Au(iPr2-bimy)(NOHC)]PF6 trans-[PdBr2(iPr2-bimy)(4)] [Au(iPr2-bimy)(4)]PF6 all-trans-[{PdBr2(iPr2-bimy)}2(μ-D)] [{Au(iPr2-bimy)}2(μ-D)](PF6)2 [{Au(iPr2-bimy)}2(μ-diNOHC)](PF6)2 “all-trans-[{PdBr2(iPr2-bimy)}2(μ-diNOHC)]”c

176.5b 186.2 169.3 181.0 168.5 181.1 193.2 184.9c

Figure 6. Schematic representation of the formal backbone modification of complexes [5] and [6]PF6 to give the dinuclear complexes [7] and [8](PF6)2.

the isocyanide carbon atom from sp to sp2 for the NOHC carbene carbon atom, which occurs during the cyclization process, possibly enables an extension of the pπ-orbital system, which allows the iPr2-bimy reporter ligand to detect remote electronic changes. Overall, one can conclude that modification of an NOHC ligand with a gold(I) complex fragment increases its donating ability through electronic communication. Only in one case has electronic communication of a more elaborate pyridine-functionalized, Janus-type diNHC been established by more complicated electrochemical means.18 The ability to simply detect such a phenomenon by the 13Ccarbene NMR signal extends the potential of the 13C NMR based electronic parameter.

a

Samples were measured in CDCl3 and referenced to the solvent signal at δ 77.7 ppm. bDetermined by using the 13Ccarbene-labeled complex mixture cis-[1]/trans-[1]. cComplex [10] could not be prepared. The 13Ccarbene signal was calculated from that of [9](PF6)2 using the equation [Pd] = 1.19[Au] − 45.0 ppm.

bimy CNHC resonance was observed. This downfield shift is much more pronounced in comparison to the mononuclear couple [6]PF6 → [2]PF6 (Figure 7). Direct comparison between benzoxazolin-2-ylidene and the ditopic benzobisoxazolin-2-ylidene on the AuI system revealed a great difference of 7.0 ppm (8.4 ppm on the PdII system, Figure 5), which cannot be purely attributed to the different ligand donor strengths. We believe that this significant change can be taken as an indication for electron delocalization across the bridging dicarbene ligand. The coplanar arrangement of all ligands observed in the solid-state molecular structure of [9](PF6)2 provides further support for this notion. The rehybridization of

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CONCLUSION We have prepared mixed β-functionalized isocyanide/NHC complexes. The isocyanide ligands were subsequently cyclized to give unique mononuclear NOHC/NHC and dinuclear

Figure 5. Donor strength comparison among the NOHC ligands, other NHCs, and PCy3. F


Article

Organometallics

Figure 7. Donor strength comparison among mono- and diisocyanide versus mono- and dicarbene ligands in complexes of the type [Au(iPr2bimy)L](PF6) and [{Au(iPr2-bimy)}2(μ-L2)](PF6)2. Coupling constants are expressed in hertz. Mass spectra were obtained with MicroTof (Bruker Daltonics, ESI), Reflex IV MALDI TOF (Bruker), and Orbitrap LTQ XL (Thermo Scientific, ESI) spectrometers. IR spectra were recorded with a Bruker Vektor 22 spectrometer. CHN analyses were obtained with Elementar Vario MICRO cube and Vario EL III CHNS elemental analyzers. 2Trimethylsiloxyphenyl isocyanide (A),5 bis(μ-bromido)bis(1,3-diisopropylbenzimidazolin-2-ylidene)dibromidodipalladium(II) (B),19 [(1,3-diisopropylbenzimidazolin-2-ylidene)chloridogold(I)] (C),20 and 2,5-bis(triisopropylsiloxy)phenyl 1,4-diisocyanide (D)17 were prepared according to literature procedures. cis-[Dibromido(benzoxazolin-2-ylidene)(1,3-diisopropylbenzimidazolin-2-ylidene)palladium(II)] (cis-[1]). 2-Trimethylsiloxyphenyl isocyanide (A; 42 mg, 0.22 mmol) and bis(μ-bromido)bis(1,3diisopropylbenzimidazolin-2-ylidene)dibromidodipalladium(II) (B; 93 mg, 0.10 mmol) were stirred together in dichloromethane (10 mL) at ambient temperature for 3 h. Subsequently, the mixture was filtered through Celite and the solvent was removed in vacuo. The obtained solid was dissolved in dichloromethane (1 mL). Evaporation of the solvent gave light yellow crystals of cis-[1]. Yield: 92 mg (0.078 mmol, 78%). 1H NMR (300 MHz, CDCl3): δ 12.90 (br s, 1H, NH), 7.82− 7.74 (m, 1H, Ar-H), 7.71−7.67 (m, 2H, Ar-H), 7.46 (t, 3JH,H = 7 Hz, 1H, Ar-H), 7.43−7.37 (m, 2H, Ar-H), 7.37−7.32 (m, 2H, Ar-H), 6.15 (sept br, 3JH,H = 7 Hz, 2H, CH(CH3)2), 1.78 (d, 3JH,H = 7 Hz, 6H, CH(CH3)2), 1.61 (d, 3JH,H = 7 Hz, 6H, CH(CH3)2). 13C{1H} NMR (75 MHz, CDCl3): δ 174.8 (NCN), 152.4, 151.6, 134.2, 132.7, 129.0, 123.9, 122.5, 120.8, 113.7 (Ar-C), 55.2 (CH(CH3)2), 21.6 (CH(CH3)2), 21.2 (CH(CH3)2). The signal for the NCO carbene carbon atom was not detected. Anal. Calcd for C20H23N3Br2OPd: C, 40.88; H, 3.95; N, 7.17. Found: C, 40.54; H, 3.82; N, 7.05. trans-[Dibromido(benzoxazolin-2-ylidene)(1,3-diisopropylbenzimidazolin-2-ylidene)palladium(II)] (trans-[1]). trans-[1] was detected in a mixture of cis-[1]/trans-[1] prepared analogously to complex cis-[1] but using a shorter reaction time of only 2 h. In addition, bis(μ-bromido)bis(1,3-diisopropylbenzimidazolin-2-ylidene)dibromidodipalladium(II) with a 13C-labeled carbene carbon atom was used. The complex mixture exhibited two NCN resonances for the cis and trans complexes in an intensity ratio of 0.6:0.4. Since the resonance for the cis complex is known, the second NCN resonance was assigned to the trans complex. 13C{1H} NMR (75 MHz, CDCl3): δ 174.8 (NCN for cis-[1]), 176.5 (NHC for trans-[1]). Only the 13 C{1H} resonances of 13C-labeled atoms were recorded.

[(NHC)−Au−(μ-diNOHC)−Au(NHC)] complexes. Using the complexes [5], [6]PF6, [7], and [8](PF6)2 we have determined the donor strength of β-functionalized isocyanides. Cleavage of the O−SiR3 bond of the coordinated β-functionalized isocyanide ligand gave complexes with NOHC or diNOHC ligands. Reaction of 2-trimethylsiloxyphenyl isocyanide A with [PdBr2(iPr2-bimy)]2 (B) or [AuCl(iPr2-bimy)] (C) in CH2Cl2 led, after cleavage of the O−SiMe3 bond, directly to the NHC/NOHC complexes cis-[1] and [2]PF6. The 13C NMR chemical shift for the desired complex trans-[1] was detected in a cis-[1]/trans-[1] mixture after 13C labeling of the iPr2-bimy ligand. The determined donor strength of the benzoxazolin-2-ylidene (NOHC) ligand is at the high-field edge of classical NHCs (equal to a less strong donor). Generally, the donor strength increases upon conversion of the β-functionalized isocyanide to the NOHC ligand. For the first time, the donor strengths of selected ligands in homobinuclear metal complexes have been tested by the 13C NMR method. The β,β′-OSi(iPr)3-protected diisocyanide ligand 4 in dinuclear complex [8](PF6)2 leads to a similar 13C NMR chemical shift for the reporter ligand in comparison to the 2-triisopropylsiloxyphenyl isocyanide ligand in the mononuclear complex [6]PF6. KF-initiated O−Si(iPr)3 bond cleavage in [8](PF6)2 gave the (NHC)Au−(diNOHC)−Au(NHC) complex [9](PF6)2. The relative donor strength measured for [9](PF6)2 is significantly increased in comparison to the mononuclear complex [2]PF6. This might be caused by a cooperative effect between the two AuI metal centers in the dinuclear complex [9](PF6)2. The ability to evaluate electronic communications between metal centers using 13C NMR spectroscopy as an alternative to electrochemical methods is promising, and we will explore its full potential in the near future.

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EXPERIMENTAL SECTION

All reagents and solvents were used as received without further purification. NMR spectra were recorded on Bruker AVANCE I 300, Bruker AVANCE I 400, Bruker AVANCE III 400, and Bruker AVANCE I 500 spectrometers. Chemical shifts (δ) are expressed in ppm using the residual protonated solvent as an internal standard. G


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Organometallics

CH(CH3)2), 1.45 (sept, 3JH,H = 7 Hz, 3H, Si−CH(CH3)2), 1.20 (d, JH,H = 7 Hz, 18H, Si-CH(CH3)2). 13C{1H} NMR (101 MHz, CDCl3): δ 169.3 (NCN), 153.3 (CN), 134.1, 132.2, 128.6, 123.5, 123.2, 123.1, 121.6, 120.4, 113.6 (Ar-C), 55.1 (N-CH(CH3)2), 21.4 (N−CH(CH3)2), 18.8 (Si−CH(CH3)2), 13.6 (Si-CH(CH3)2). 29Si{1H} NMR (80 MHz, CDCl3): δ 20.6 (O-Si). IR (KBr): ν̃ 2203 cm−1 (s, CN). HR-MS (ESI, positive ions): m/z 766.04675 (calcd for [[5] + Na]+ 766.04675). Anal. Calcd for C29H43N3 Br2OPdSi: C, 46.82; H, 5.83; N, 5.65. Found: C, 46.51; H, 5.95; N, 5.66. (2-Triisopropylsiloxyphenyl isocyanide)(1,3-diisopropylbenzimidazolin-2-ylidene)gold(I) Hexafluorophosphate ([6]PF6). The gold complex C (32.8 mg, 0.0755 mmol), compound 4 (23 mg, 0.083 mmol), and KPF6 (55.6 mg, 0.302 mmol) were stirred together at ambient temperature in dichloromethane (10 mL) for 1 h. The solution was then filtered through Celite, and the solvent was removed in vacuo. The solid obtained was washed two times with npentane (10 mL each), giving [6]PF6 as a light yellow solid. Yield: 59 mg (0.072 mmol, 96%). 1H NMR (400 MHz, CDCl3): δ 7.75−7.70 (m, 3H, Ar-H), 7.49−7.40 (m, 3H, Ar-H), 7.06 (t, 3JH,H = 8 Hz, 1H, Ar-H), 6.98 (d, 3JH,H = 8 Hz, 1H, Ar-H), 5.32 (sept, 3JH,H = 7 Hz, 2H, N−CH(CH3)2), 1.82 (d, 3JH,H = 7 Hz, 12H, N-CH(CH3)2, 1.37 (sept, 3 JH,H = 7 Hz, 3H, Si-CH(CH3)2), 1.16 (d, 3JH,H = 7 Hz, 18H, SiCH(CH3)2). 13C{1H} NMR (101 MHz, CDCl3): δ 181.0 (NCN), 153.9 (CN), 133.9, 133.1, 129.6, 127.9, 125.8, 122.6, 120.1, 114.0, 114.1 (Ar-C), 55.1 (N-CH(CH3)2), 23.3 (N−CH(CH3)2), 18.6 (Si− CH(CH3)2), 13.4 (Si-CH(CH3)2). 29Si{1H} NMR (80 MHz, CDCl3): δ 21.4 (O-Si). 31P{1H} NMR (202 MHz, CDCl3): δ 144.2 (sept, 1JP,F = 713 Hz, PF6). IR (KBr): ν̃ 2232 cm−1 (s, CN). HR-MS (ESI, positive ions): m/z 674.28370 (calcd for [6]+ 674.28409). trans-Bis{dibromido(1,3-diisoproyplbenzimidazolin-2ylidene)}(2,5-bis(triisopropylsiloxy)phenyl 1,4-diisocyanide)dipalladium(II) ([7]). Complex B (40 mg, 0.043 mmol) and diisocyanide D (20.3 mg, 0.043 mmol) were stirred in dichloromethane at ambient temperature for 16 h. The amount of solvent was then reduced to a volume of 1 mL, and this solution was added dropwise to diethyl ether (10 mL). Complex [7] precipitated as a light yellow powder, which was isolated by filtration. Yield: 50 mg (0.036 mmol, 83%). 1H NMR (300 MHz, CDCl3): δ 7.60 (dd, 3JH,H = 6 Hz, 4 JH,H = 3 Hz, 4H, Ar-H), 7.26−7.22 (m, 4H, Ar-H), 6.97 (s, 2H, ArH), 5.98 (sept, 3JH,H = 6 Hz, 4H, N-CH(CH3)2), 1.75 (d, 3JH,H = 6 Hz, 24H, N−CH(CH3)2), 1.52−1.38 (m, 6H, Si-CH(CH3)2), 1.20 (d, 3 JH,H = 7 Hz, 36H, Si-CH(CH3)2). 13C{1H} NMR (75 MHz, CDCl3): δ 168.5 (NCN), 147.0 (CN), 134.0, 123.2, 120.7, 118.7, 113.5, 113.2 (Ar-C), 55.1 (N-CH(CH3)2), 21.4 (N-CH(CH3)2), 18.8 (SiCH(CH3)2), 13.6 (Si-CH(CH3)2). IR (KBr): ν̃ 2186 cm−1 (s, CN). MALDI-MS: m/z 1331 (calcd for [[7] − Br]+ 1331). HR-MS (ESI, positive ions): m/z 1449.03210 (calcd for [[7] + K]+ 1449.03062). Anal. Calcd for C52H80N6Br4O2Pd2Si2·CH2Cl2: C, 42.59; H, 5.53; N, 5.62. Found: C, 42.42; H, 5.19; N, 5.48. Bis(1,3-diisopropylbenzimidazolin-2-ylidene)(2,5-bis(triisopropylsiloxy)phenyl 1,4-diisocyanide)digold(I) Hexafluorophosphate ([8](PF6)2). Complex C (50 mg, 0.115 mmol), compound D (27.2 mg, 0.058 mmol), and KPF6 (90 mg, 0.5 mmol) were stirred in dichloromethane for 16 h at ambient temperature. The solution was then filtered through Celite, and the solvent was removed in vacuo. The solid obtained was washed three times with diethyl ether (10 mL each). Compound [8](PF6)2 was obtained as a colorless solid. Yield: 78 mg (0.50 mmol, 86%). 1H NMR (300 MHz, CDCl3): δ 7.80−7.65 (m, 4H, Ar-H), 7.52−7.36 (m, 6H, Ar-H), 5.37 (sept, 3JH,H = 7 Hz 4H, N-CH(CH3)2), 1.79 (d, 3JH,H = 7 Hz, 24H, N-CH(CH3)2), 1.47 (sept, 3JH,H = 7 Hz, 6H, Si-CH(CH3)2), 1.16 (d, 3JH,H = 7 Hz, 36H, Si-CH(CH3)2-TIPS). 13C{1H} NMR (75 MHz, CDCl3): δ 181.1 (NCN), 148.0 (CN), 133.1, 125.6, 125.4, 119.2, 114.1, 113.8 (ArC), 66.5 (N-CH(CH3)2), 23.0 (N-CH(CH3)2), 18.5 (Si-CH(CH3)2), 13.3 (Si-CH(CH3)2). IR (KBr): ν̃ 2218 cm−1 (s, CN). MALDI-MS: m/z 1415 (calcd for [[8]2+ + PF6−]+ 1415). trans-Bis(1,3-diisopropylbenzimidazolin-2-ylidene)(benzobisoxazolin-2-ylidene)digold(I) Hexafluorophosphate ([9](PF6)2). Complex [8](PF6)2 (75 mg, 0.048 mmol) was dissolved in methanol (10 mL) together with a catalytic amount (5 mg) of KF.

(Benzoxazolin-2-ylidene)(1,3-diisopropylbenzimidazolin-2ylidene)gold(I) Hexafluorophosphate ([2]PF6). Compound A (50 mg, 0.25 mmol), (1,3-diisopropylbenzimidazolin-2-ylidene)chloridogold(I) (C; 83 mg, 0.19 mmol), and KPF6 (100 mg, 0.54 mmol) were dissolved in dichloromethane (10 mL). The mixture was stirred for 4 h at ambient temperature and subsequently filtered trough Celite. The solvent was then removed in vacuo, and the crude reaction product was washed three times with diethyl ether (10 mL each) to yield [2]PF6 as a colorless powder. Yield: 95 mg (0.14 mmol, 74%). 1 H NMR (500 MHz, CDCl3): δ 12.53 (br s, 1H, NH), 7.85−7.81 (m, 1H, Ar-H), 7.75−7.68 (m, 3H, Ar-H), 7.58−7.49 (m, 2H, Ar-H), 7.43 (dd, 3JH,H = 6 Hz, 3JH,H = 6 Hz, 2H, Ar-H), 5.45 (sept, 3JH,H = 7 Hz, 2H, CH(CH3)2), 1.83 (d, 3JH,H = 7 Hz, 12H, CH(CH3)2). 13C{1H} NMR (126 MHz, CDCl3): δ 209.5 (NCO), 186.2 (NCN), 151.2, 133.3, 128.5, 127.8, 127.7, 125.2, 115.6, 113.9, 112.6 (Ar-C), 54.9 (CH(CH3)2), 23.0 (CH(CH3)2). 31P{1H} NMR (202 MHz, CDCl3): δ 143.2 (sept, 1JP,F = 714 Hz, PF6). ESI-MS: m/z 518 (calcd for [2]+ 518). Anal. Calcd for C20H23N3AuF6OP: C, 36.21; H, 3.49; N, 6.33. Found: C, 36.36; H, 3.83; N, 6.19. N-(2-(Triisopropylsilyloxy)phenyl)formamide (3). Commercially available N-(2-hydroxyphenol)formamide (455 mg, 3.32 mmol), imidazole (903 mg, 13.3 mmol), and triisopropylsilyl chloride (1280 mg, 6.64 mmol) were dissolved in toluene (15 mL). The reaction mixture was heated under reflux for 18 h. The solvent was then removed in vacuo, and the solid obtained was purified by column chromatography (SiO2, dichloromethane). Compound 3 was obtained as a mixture of the syn and anti isomers in a ratio of about 2:1. Yield: 711 mg (2.42 mmol, 73%) of a colorless powder. 1H NMR (400 MHz, CDCl3): δ 8.70 (d, 3JH,H = 12 Hz, 0.3H, NHanti), 8.44 (d, 3JH,H = 2 Hz, 0.7H, NHsyn), 8.39−8.28 (m, 0.7H, Ar-H), 7.79 (br s, 0.7H, CHOsyn), 7.72−7.57 (m, 0.3H, CHOanti), 7.16 (d, 3JH,H = 8 Hz, 0.3H, Ar-H), 7.01−6.81 (m, 3H), 1.39−1.24 (m, 3H, Si−CH(CH3)2), 1.21−0.98 (m, 18H, Si−CH(CH3)2). 13C{1H} NMR (101 MHz, CDCl3): δ 161.2, 158.4 (CHO syn and anti), 145.3, 144.1, 128.3, 127.8, 125.0, 124.0, 121.5, 121.4, 120.7, 118.6, 117.3, 117.1 (Ar-C syn and anti), 17.8 (Si−CH(CH3)2), 12.7 (Si-CH(CH3)2). 29Si{1H} NMR (80 MHz, CDCl3): δ 18.9 (O−Si). IR (KBr): ν̃ 1700, 1689 cm−1 (amide). HRMS (ESI, positive ions): m/z 316.17055 (calcd for [3 + Na]+ 316.17088). Anal. Calcd for C16H27NO2Si: C, 65.48; H, 9.27; N, 4.77. Found: C, 64.96; H, 9.41; N, 4.63. 2-Triisopropylsiloxyphenyl Isocyanide (4). Compound 3 (667 mg, 2.27 mmol), triphosgene (506 mg, 1.70 mmol), and triethylamine (918 mg, 9.08 mmol) were dissolved in dichloromethane (15 mL) at −78 °C. The solution was stirred and warmed to ambient temperature over 18 h. The solution was then washed two times with water (10 mL each). The organic solution was separated and dried over MgSO4, and the solvent was removed in vacuo to give a brown oil. The oil was purified by column chromatography (SiO2, ethyl acetate) to give 4 as a brown oil. Yield: 425 mg (1.54 mmol, 68%). 1H NMR (400 MHz, CDCl3): δ 7.30 (dd, 3JH,H = 8 Hz, 4JHH = 1 Hz, 1H, Ar-H), 7.25−7.20 (m, 1H, Ar-H), 6.93−6.87 (m, 2H, Ar-H), 1.39−1.28 (m, 3H, SiCH(CH3)2), 1.14 (d, 3J = 7 Hz, 18H, Si-CH(CH3)2). 13C{1H} NMR (101 MHz, CDCl3): δ 167.0 (CN), 161.3, 152.0, 130.1, 127.8, 121.1, 119.7 (Ar-C), 17.9 (Si-CH(CH3)2), 12.9 (Si-CH(CH3)2). 29 Si{1H} NMR (80 MHz, CDCl3): δ 19.5 (O−Si). EI-MS: m/z 275 (calcd for [4]+ 275), 232 (calcd for [4 − C3H7]+ 232). IR (KBr): ν̃ 2123 cm−1 (s, CN). HR-MS (ESI, positive ions): m/z 298.1600 (calcd for [4 + Na]+ 298.1603). Anal. Calcd for C16H25NOSi: C, 69.76; H, 9.15; N, 5.08. Found: C, 69.01; H, 8.84; N, 5.09. trans-Dibromido(2-triisopropylsiloxyphenyl isocyanide)(1,3-diisopropylbenzimidazolin-2-ylidene)palladium(II) ([5]). Dipalladium complex B (20.0 mg, 0.0214 mmol) and compound 4 (13.0 mg, 0.047 mmol) were stirred at ambient temperature for 1 h in dichloromethane (10 mL). The solution was then filtered through Celite, and the solvent was removed in vacuo, giving a yellow solid. The solid was washed two times with n-pentane (10 mL each) to give [5] as a yellow solid. Yield: 30 mg (0.04 mmol, 93%). 1H NMR (400 MHz, CDCl3): δ 7.63−7.56 (m, 3H, Ar-H), 7.45−7.37 (m, 1H, Ar-H), 7.26−7.21 (m, 2H, Ar-H), 6.95 (d, 3JH,H = 8 Hz, 2H, Ar-H), 6.04 (sept, 3JH,H = 7 Hz, 2H, N-CH(CH3)2), 1.77 (d, 3JH,H = 7 Hz, 12H, N-

3

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Article

Organometallics

2σ(I)), refinement of 317 parameters against |F2| of all measured intensities with hydrogen atoms on calculated positions. R = 0.0359, Rw = 0.0919, Rall = 0.0377, Rw,all = 0.0934. The asymmetric unit contains 1/2 formula unit of [9]PF6·2THF·Et2O. The PF6− anion is disordered over two positions, and the EtOH positions in the asymmetric unit were refined with SOF = 1/2.

The solution was stirred for 16 h at ambient temperature and subsequently filtered through Celite. The solvent was then removed in vacuo to give a colorless powder. The powder was washed three times with diethyl ether (10 mL each) to give [9](PF6)2 as a colorless solid. Yield: 57.4 mg (0.046 mmol, 95%). 1H NMR (400 MHz, CDCl3): δ 7.72 (s, 2H, Ar-H), 7.65 (dd, 3JH,H = 6 Hz, 4JH,H = 3 Hz, 4H, Ar-H), 7.36 (dd, 3JH,H = 6 Hz, 4JH,H = 3 Hz, 4H, Ar-H), 5.59 (sept, 3JH,H = 7 Hz, 4H, N-CH(CH3)2), 1.79 (d, 3JH,H = 7 Hz, 24H, N-CH(CH3)2). 13 C{1H} NMR (101 MHz, CDCl3): δ 205.3 (NCO), 193.2 (NCN), 149.4, 139.7, 133.2, 124.4, 113.7, 99.0 (Ar-C), 54.5 (N-CH(CH3)2), 22.8 (N-CH(CH3)2). 31P NMR (162 MHz, CDCl3): δ 144.3 (sept, 1 JP,F = 712 Hz, PF6). HR-MS (ESI, positive ions): m/z 957.24667 (calcd for [[9]2+ − H+]+ 957.24655). X-ray Diffraction Studies. X-ray diffraction data for compounds cis-[1]·CH2Cl2, [2]PF6, [5], and [9](PF6)2·2THF·EtOH were recorded with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å) at T = 100(2) K (cis-[1]·CH2Cl2, [2]PF6, [5]) and at T = 153(2) K ([9](PF6)2·2THF·EtOH). Diffraction data were collected over the full sphere and were corrected for absorption. Structure solutions were found with the SHELXS21 package using direct methods and were refined with SHELXL21 against |F2| using first isotropic and later anisotropic thermal parameters for all nonhydrogen atoms. Hydrogen atoms were added to the structure models on calculated positions and refined as riding atoms. If not noted otherwise, no hydrogen positions were calculated for disordered atoms (see the individual refinement details). Crystal data and structure refinement details for cis-[1]·CH2Cl2: C21H25N3Br2Cl2OPd, Mr = 672.56, colorless plate, 0.22 × 0.15 × 0.08 mm3, triclinic, space group P1̅, Z = 2, a = 9.0934(13) Å, b = 9.9794(15) Å, c = 14.098(2) Å, α = 81.042(5)°, β = 78.122(4)°, γ = 81.507(5)°, V = 1227.8(3) Å3, ρcalcd = 1.819 g cm−3, μ = 4.246 mm−1, ω and φ scans, 52208 measured intensities (4.6° ≤ 2θ ≤ 55.0°), semiempirical absorption correction (0.746 ≤ T ≤ 0.623), 5635 independent (Rint = 0.0364) and 5100 observed intensities (I ≥ 2σ(I)), refinement of 279 parameters against |F2| of all measured intensities with hydrogen atoms on calculated positions. R = 0.0223, Rw = 0.0561, Rall = 0.0272, Rw,all = 0.0580. The asymmetric unit contains one formula unit of cis-[1]·CH2Cl2. Crystal data and structure refinement details for [2]PF6: C20H23N3AuF6OP2, Mr = 663.35, yellow block, 0.26 × 0.16 × 0.08 mm3, triclinic, space group P1̅, Z = 2, a = 9.3977(13) Å, b = 11.5074(16) Å, c = 12.2369(16) Å, α = 108.238(4)°, β = 106.890(5)°, γ = 107.292(4)°, V = 1087.4(3) Å3, ρcalcd = 2.026 g cm−3, μ = 6.907 mm−1, ω and φ scans, 47790 measured intensities (4.1° ≤ 2θ ≤ 55.0°), semiempirical absorption correction (0.563 ≤ T ≤ 0.415), 4980 independent (Rint = 0.0310) and 4717 observed intensities (I ≥ 2σ(I)), refinement of 331 parameters against |F2| of all measured intensities with hydrogen atoms on calculated positions. R = 0.0255, Rw = 0.0542, Rall = 0.0285, Rw,all = 0.0559. The asymmetric unit contains one formula unit of [2]PF6. One of the isopropyl groups is disordered over two positions with the occupancy ratio 67:33. Crystal data and structure refinement details for [5]: C29H43N3Br2OPdSi, Mr = 743.97, yellow plate, 0.35 × 0.16 × 0.15 mm3, monoclinic, space group P21/n, Z = 4, a = 11.0405(2) Å, b = 19.4798(4) Å, c = 14.9316(3) Å, β = 98.3290(10)°, V = 3177.42(11) Å3, ρcalcd = 1.555 g cm−3, μ = 3.163 mm−1, ω and φ scans, 75232 measured intensities (7.3° ≤ 2θ ≤ 61.4°), semiempirical absorption correction (0.597 ≤ T ≤ 0.746), 9830 independent (Rint = 0.0713) and 7450 observed intensities (I ≥ 2σ(I)), refinement of 344 parameters against |F2| of all measured intensities with hydrogen atoms on calculated positions. R = 0.0341, Rw = 0.0700, Rall = 0.0565, Rw,all = 0.0771. The asymmetric unit contains one formula unit of [5]. Crystal data and structure refinement details for [9]PF6·2THF· EtOH: C44H62N6Au2F12O5P2, Mr = 1438.87, yellow block, 0.40 × 0.26 × 0.16 mm3, triclinic, space group P1̅, Z = 1, a = 10.5600(2) Å, b = 10.6675(2) Å, c = 12.8860(3) Å, α = 69.6340(10)°, β = 88.1030(10)°, γ = 77.3010(10)°, V = 1326.07(5) Å3, ρcalcd = 1.802 g cm−3, μ = 5.676 mm−1, ω and φ scans, 17831 measured intensities (6.2° ≤ 2θ ≤ 52.7°), semiempirical absorption correction (0.397 ≤ T ≤ 0.746), 5404 independent (Rint = 0.0421) and 5138 observed intensities (I ≥

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00736. Nuclear magnetic resonance (NMR) spectra for selected compounds (lacking microanalytical data) (PDF) X-ray crystallographic data for cis-[1]·CH2Cl2, [2]PF6, [5] and [9](PF6)2·2THF·EtOH (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail for F.E.H: [email protected]. *E-mail for H.V.H: [email protected]. ORCID

Han Vinh Huynh: 0000-0003-4460-6066 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge financial support from the Deutsche Forschungsgemeinschaft (SFB 858 and IRTG 2027). The authors also thank the National University of Singapore and the Singapore Ministry of Education for financial support (WBS R143-000-609-112). Technical support from staff at the CMMAC of our department is appreciated.

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REFERENCES

(1) (a) Bourissou, D.; Guerret, O.; Gabbaï, F. P.; Bertrand, G. Chem. Rev. 2000, 100, 39−91. (b) Hahn, F. E.; Jahnke, M. C. Angew. Chem., Int. Ed. 2008, 47, 3122−3172. (c) Schuster, O.; Yang, L.; Raubenheimer, H. G.; Albrecht, M. Chem. Rev. 2009, 109, 3445− 3478. (d) Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. Nature 2014, 510, 485−496. (2) (a) Tamm, M.; Hahn, F. E. Coord. Chem. Rev. 1999, 182, 175− 209. (b) Jahnke, M. C.; Hahn, F. E. Coord. Chem. Rev. 2015, 293−294, 95−115. (3) (a) Fehlhammer, W. P.; Plaia, U. Z. Naturforsch., B: J. Chem. Sci. 1986, 41, 1005−1010. (b) Fehlhammer, W. P.; Hoffmeister, H.; Stolzenberg, H.; Boyadjiev, B. Z. Naturforsch., B: J. Chem. Sci. 1989, 44, 419−428. (c) Fehlhammer, W. P.; Hoffmeister, H.; Boyadjiev, B.; Kolrep, T. Z. Naturforsch., B: J. Chem. Sci. 1989, 44, 917−922. (d) Kernbach, U.; Fehlhammer, W. P. Inorg. Chim. Acta 1995, 235, 299−305. (e) Fehlhammer, W. P.; Bartel, K.; Plaia, U.; Völkl, A.; Liu, A. T. Chem. Ber. 1985, 118, 2235−2254. (f) Plaia, U.; Stolzenberg, H.; Fehlhammer, W. P. J. Am. Chem. Soc. 1985, 107, 2171−2172. (4) (a) Ng, C.-O.; Yiu, S.-M.; Ko, C.-C. Inorg. Chem. 2014, 53, 3022−3031. (b) Ko, C.-C.; Ng, C.-O.; Yiu, S. M. Organometallics 2012, 31, 7074−7084. (c) Velmurugan, G.; Ramamoorthi, B. K.; Venuvanalingam, P. Phys. Chem. Chem. Phys. 2014, 16, 21157−21171. (5) Jutzi, P.; Gilge, U. J. Organomet. Chem. 1983, 246, 159−162. (6) (a) Hahn, F. E.; Imhof, L. Organometallics 1997, 16, 763−769. (b) Hahn, F. E.; Tamm, M. J. Organomet. Chem. 1993, 456, C11−C14. (c) Hahn, F. E.; Hein, P.; Lügger, T. Z. Anorg. Allg. Chem. 2003, 629, 1316−1321. (d) Tamm, M.; Hahn, F. E. Inorg. Chim. Acta 1999, 288, 47−52. (e) Kembach, U.; Lügger, T.; Hahn, F. E.; Fehlhammer, W. P. J. Organomet. Chem. 1997, 541, 51−55. (f) Hahn, F. E.; Lügger, T. J. Organomet. Chem. 1994, 481, 189−193. (g) Tamm, M.; Lügger, T.; I


Article

Organometallics Hahn, F. E. Organometallics 1996, 15, 1251−1256. (h) Hahn, F. E.; Tamm, M. J. Chem. Soc., Chem. Commun. 1993, 842−844. (i) Hahn, F. E.; Tamm, M. J. Chem. Soc., Chem. Commun. 1995, 569−570. (7) (a) Hahn, F. E.; Tamm, M. Organometallics 1995, 14, 2597− 2600. (b) Dumke, A. C.; Pape, T.; Kösters, J.; Feldmann, K.-O.; Schulte to Brinke, C.; Hahn, F. E. Organometallics 2013, 32, 289−299. (8) (a) Hahn, F. E.; Langenhahn, V.; Meier, N.; Lügger, T.; Fehlhammer, W. P. Chem. - Eur. J. 2003, 9, 704−712. (b) Hahn, F. E.; Plumed, C. G.; Münder, M.; Lügger, T. Chem. - Eur. J. 2004, 10, 6285−6293. (c) Hahn, F. E.; Langenhahn, V.; Pape, T. Chem. Commun. 2005, 5390−5392. (d) Hahn, F. E.; Langenhahn, V.; Lügger, T.; Pape, T.; Le Van, D. Angew. Chem., Int. Ed. 2005, 44, 3759−3763. (9) (a) Meier, N.; Hahn, F. E.; Pape, T.; Siering, C.; Waldvogel, S. R. Eur. J. Inorg. Chem. 2007, 2007, 1210−1214. (b) Hahn, F. E. ChemCatChem 2013, 5, 419−430. (10) Nelson, D. J.; Nolan, S. P. Chem. Soc. Rev. 2013, 42, 6723. (11) Tolman, C. A. Chem. Rev. 1977, 77, 313−348. (12) Lever, A. B. P. Inorg. Chem. 1990, 29, 1271−1285. (13) Huynh, H. V.; Han, Y.; Jothibasu, R.; Yang, J. A. Organometallics 2009, 28, 5395−5404. (14) Guo, S.; Sivaram, H.; Yuan, D.; Huynh, H. V. Organometallics 2013, 32, 3685−3696. (15) Huynh, H. V.; Ho, J. H. H.; Neo, T. C.; Koh, L. L. J. Organomet. Chem. 2005, 690, 3854−3860. (16) Han, Y.; Huynh, H. V. Dalton Trans. 2009, 2201−2209. (17) (a) Conrady, F. M.; Fröhlich, R.; Schulte to Brinke, C.; Pape, T.; Hahn, F. E. J. Am. Chem. Soc. 2011, 133, 11496−11499. (b) Schmidtendorf, M.; Pape, T.; Hahn, F. E. Angew. Chem., Int. Ed. 2012, 51, 2195−2198. (18) Nussbaum, M.; Schuster, O.; Albrecht, M. Chem. - Eur. J. 2013, 19, 17517. (19) Huynh, H. V.; Han, Y.; Ho, J. H. H.; Tan, G. K. Organometallics 2006, 25, 3267−3274. (20) Jothibasu, R.; Huynh, H. V.; Koh, L. L. J. Organomet. Chem. 2008, 693, 374−380. (21) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122.

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