Bimetallic N-Heterocyclic Carbene Rh(I) Complexes: Probing the

Article pubs.acs.org/Organometallics

Bimetallic N‑Heterocyclic Carbene Rh(I) Complexes: Probing the Cooperative Effect for the Catalyzed Hydroelementation of Alkynes Vera Diachenko,† Michael J. Page,† Mark R. D. Gatus,† Mohan Bhadbhade,‡ and Barbara A. Messerle*,†,§ †

School of Chemistry and ‡Mark Wainwright Analytical Centre, University of New South Wales, Sydney, Kensington, New South Wales 2051, Australia

Downloaded by UNIV OF NEBRASKA-LINCOLN on September 15, 2015 | http://pubs.acs.org Publication Date (Web): September 14, 2015 | doi: 10.1021/acs.organomet.5b00594

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ABSTRACT: A series of bimetallic N-heterocyclic carbene Rh(I) complexes (13, 14, 16, 18, 19, and 21) were prepared, in which two metal fragments are linked to a central organic scaffold. These complexes were investigated as catalysts for the hydroelementation of alkynes, and their efficiency as catalysts was compared to that of a series of monometallic analogues (12, 15, 17, and 20). To determine the catalyst properties that result in a cooperative, bimetallic enhancement of the reaction rate, systematic variation of the intermetallic distance and the ligand donor properties of the bimetallic complexes was explored. Two related catalyzed reactions were investigated: the dihydroalkoxylation and hydrosilylation of alkynes. Nanoparticle formation was observed during the hydrosilylation reaction using complexes 17−19.

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INTRODUCTION The use of bimetallic complexes as catalysts to enhance the rate and selectivity of catalyzed reactions is of significant interest. A bimetallic catalyst is often substantially more efficient than a monometallic catalyst of similar structure, with the increase in catalyst performance attributed to “cooperative” interactions between the two metals and the substrate of the reaction.1 How the two metals of a bimetallic catalyst interact with each other and/or the reaction substrate during the catalytic cycle is difficult to determine from direct methods, and whether or not a new bimetallic complex will promote a cooperative increase in catalyst performance is hard to predict. One of the more versatile approaches to bimetallic catalyst design is to link two discrete metal complexes of proven catalytic efficiency by a covalently bound tether.2 Such an approach has been used with great effect for the synthesis of bimetallic Co2, Cu2, and Zn2 catalysts for the hydrolysis of phosphate esters;3−5 Cr2, Al2, and Co2 catalysts for the asymmetric ring opening of epoxides;6 and various Ni2, Ti2, and Zr2 polymerization catalysts.7,8 A number of bimetallic complexes have been reported that contain a di-N-heterocyclic carbene ligand bridging the two metal fragments.9,10 N-Heterocyclic carbenes (NHCs) form exceptionally strong metal−ligand bonds of high thermodynamic stability and low kinetic lability.11 They are therefore ideally suited for studying bimetallic-catalyzed reaction mechanisms by ensuring the structural integrity of the bimetallic unit. NHC ligands have also proven to be spectacularly successful in yielding metal complexes of high catalytic efficiency for a variety of reactions.12 Despite these advantages, few bimetallic complexes containing two NHC ligands bridged by a phenylene scaffold have been investigated as catalysts.10 Selected examples include the Ru2 complex © XXXX American Chemical Society

(Figure 1a), which was found to be a poor catalyst for the ringopening polymerization metathesis of alkenes,10b and the Rh2

Figure 1. Bimetallic catalysts containing NHC ligands.

complex (Figure 1b), which was found to be an efficient catalyst for the hydrosilylation of selected unsaturated functional groups, including alkenes.10f,g The Pd2 complex (Figure 1c), however, was found to decompose to metallic nanoparticles during the catalyzed Heck reaction.10e Of particular note was that the Heck reaction was not catalyzed by the bis-Pd complex but was catalyzed by palladium nanoparticles which were a result of the complex decomposing under the reaction conditions. It has been shown that carbene ligands can assist in the stabilization of nanoparticles,13 which may have led to the Pd2 (Figure 1c) complex decomposing. To date, however, there have been no reports of rhodium complexes with NHC ligands Received: July 9, 2015

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DOI: 10.1021/acs.organomet.5b00594 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

arylimidazoles 1−4 were first prepared by a copper-catalyzed Ullmann coupling of the corresponding aryl bromide with imidazole, following modified literature procedures.17 Alkylation of the imidazole nitrogen of 1−4 with methyl iodide in MeCN afforded the imidazolium salts 5−8, which have been reported previously.18 Alkylation of the imidazole nitrogen of 1−3 with 1-(2-bromoethyl)pyrazole in refluxing DMF, followed by anion exchange of the bromide with NaBPh4, afforded the imidazolium salts 9−11. The monodentate NHC ligands derived from 5−8 are denoted κ1-C-mono, κ1-C-ortho, κ1-C-meta, and κ1-C-para, respectively, and the bidentate NHC-pyrazole chelates derived from 9−11 are denoted κ2-C,N-mono, κ2-C,N-ortho, and κ2C,N-meta, respectively. The synthesis of Rh(I) 1,5-cyclooctadiene (COD) complexes was attempted by reaction of the imidazolium salts 5−11 with NaOEt and [Rh(COD)(μ-Cl)]2 in methanol (Scheme 1). This procedure worked well for the synthesis of the monometallic complexes 12 and 15 and the bimetallic complexes 13, 14, and 16, where the two Rh fragments are disposed in a meta or para position on the phenylene ring. The complexes 12−16 were isolated in good yields (40−80%) as air-stable yellow powders. For the neutral complexes 12−14 the halide ligand was unequivocally identified to be iodide through HRMS and/or elemental analysis of the compound, indicating complete metathesis of the chloride ligand initially present in [Rh(COD)(μ-Cl)]2. In each of the 1H and 13C NMR spectra of the bimetallic complexes 13, 14, and 16, two separate products of similar composition were identified. This is characteristic of the formation of two atropisomers resulting from restricted rotation of the Ph−N bonds. The formation of such isomeric mixtures has been reported previously by Hollis and co-workers with similar bimetallic Rh(I) complexes.10a Reaction of the ortho-substituted imidazolium salts 6 and 10 with NaOEt and [Rh(COD)(μ-Cl)]2 in methanol gave a complex mixture of products. To improve this outcome, a variety of base, solvent, and temperature regimes were explored: however, with no benefit. The difficulty in coordinating two metals to the ortho-disposed ligands 6 and 10 is perhaps unsurprising considering the steric hindrance an ortho disposition would impose on such a bimetallic structure. The reaction of analogous ortho-bis-carbene ligands with [Rh(COD)(μ-Cl)]2 has previously been shown to yield complexes where the two NHCs chelate to the one metal


decomposing into nanoparticles under the reaction conditions for catalysis. We have previously reported the use of mono- and bimetallic Rh complexes as catalysts for the intramolecular dihydroalkoxylation of alkynediols (Figure 2, previous work14,15). The

Figure 2. Previous work: mono-/bimetallic complexes containing N,N-bidentate donor ligands on a phenylene scaffold. This work: mono-/bimetallic complexes containing a monodentate NHC ligand or a bidentate NHC-pyrazole donor ligand on a phenylene scaffold.

efficiency of these catalysts was found to be strongly dependent on the intermetallic Rh−Rh distance, the conformational freedom of the bimetallic structure and the relative alignment of the two metal centers.14a−c,15 The aim of this work was to synthesize a series of analogous bimetallic Rh NHC complexes bridged by a phenylene scaffold (Figure 2) and develop an understanding of which structural parameters affect the efficiency of the bimetallic catalysts. We investigated two types of metal binding pockets: a monodentate NHC and a bidentate NHC-pyrazole group and, by changing the relative positions of complex pairs on the phenylene ring, varied the distance between the two metal centers. This provided significant scope in the structure of the bimetallic catalysts. To explore which reaction properties benefited most from the particular nature of the bimetallic catalyst structure, we investigated two reaction types for catalysis: the dihydroalkoxylation and hydrosilylation of alkynes. As a basis for comparison, the substrates under consideration, in particular the alkynediol used for the dihydroalkoxylation reaction, have been examined extensively as substrates for the two reaction types.14d−f,16

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RESULTS AND DISCUSSION Synthesis. The imidazolium ligand precursors 5−11 were synthesized in two steps, according to Scheme 1. The Scheme 1. Synthesis of κ1 and κ2 Complexes

B


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Organometallics center.19 An alternative process involving dimerization of the NHCs has also been reported.20 The carbon monoxide containing complexes 17−21 were synthesized by exposing a solution of the respective COD containing complexes 12−16 to an atmosphere of CO (Scheme 1). Disappearance of coordinated COD resonances was immediately evident in the 1H NMR spectra of the products 17−21, consistent with displacement of the COD ligand with CO. No atropisomerism was observed in the 1H NMR spectra of the bimetallic complexes 19 and 21; however, complex 18 contained highly broadened 1H resonances, suggesting conformational exchange processes were occurring on a time frame similar to that of the signal acquisition. The 1H NMR spectrum of 18 at 250 K showed two atropisomers present in a 2:3 ratio. These observations indicate that rotation of the Ph− N bond is much more facile in complexes 18, 19, and 21 in comparison to complexes 13, 14, and 16 containing the bulkier COD coligand. The IR spectra of complexes 17−21 contained strong carbonyl stretching bands in the region 1960−2090 cm−1 (Table 1) characteristic of coordinated CO. The IR spectrum

Figure 3. X-ray structure displaying the cationic fragment of 15. Ellipsoids are shown at the 50% probability level.

Table 1. IR Stretching Frequencies for the CO Ligands of Complexes 17−21 complex 17 18 19 20 21

(κ1-mono-CO) (κ1-meta-CO) (κ1-para-CO) (κ2-mono-CO) (κ2-meta-CO)

ν(CO) stretching frequency/cm−1 2006, 1989, 1968, 2025, 2027,

2071 2002, 2069, 2084 1997, 2016, 2070 2087 2090

Figure 4. X-ray structure displaying the cationic fragment of 16, viewed parallel (a) and perpendicular (b) to the axis of the phenylene ring. Ellipsoids are shown at the 50% probability level.

of the monometallic Rh(I) complex 17 contained two CO stretching frequencies corresponding to symmetric and asymmetric CO stretching modes typical of such dicarbonyl complexes. However, in the IR spectra of the bimetallic complexes 18 and 19, four CO stretching frequencies are observed. This would indicate a strong coupling of the CO vibrational modes from each fragment, a result that is not typical of previously reported bimetallic carbonyl containing complexes.14a−c,15 In contrast, the IR spectrum of the bimetallic complex 21 contains only two CO stretching frequencies, indicating the loss of vibrational coupling between Rh(CO)2 fragments. The CO stretching frequencies of the monometallic complex 20 are nearly identical with those observed for the analogous bimetallic complex 21. This would suggest that the Rh−CO bonding in both complexes is very similar and that for complex 21 the electronics of each Rh(CO)2 fragment are not significantly perturbed by the neighboring Rh center. X-ray Crystallography. X-ray-quality single crystals of the monometallic complex 15 (κ2-C,N-mono-Rh(COD)) and the bimetallic complex 16 (κ2-C,N-meta-Rh(COD)) were grown by recrystallization of the complex from dichloromethane/ diethyl ether or from saturated acetone-d6, respectively. The structures of the complex cations are shown in Figure 3 (for complex 15) and Figure 4 (for complex 16) with selected bond lengths and angles given in Table 2. The structure of the bimetallic complex 16 contains the two Rh fragments disposed on opposite sides of the phenylene plane. This sterically favorable orientation results in a long Rh−Rh separation of 8.567(1) Å. The Rh−ligand bond lengths of each Rh fragment are similar but not quite equivalent and are either within error or are slightly larger than the corresponding bonds of the

Table 2. Selected Bond Lengths (Å) and Angles (deg) of 15 and 16a 16 15 Rh−CNHC Rh−N Rh−CODtransN Rh−CODcisN N−Rh−CNHC a

2.027(5) 2.090(4) 2.009 2.080 86.7(2)

2.028(6) 2.100(5) 2.019 2.078 86.9(2)

2.046(6) 2.113(5) 2.068 2.102 86.3(2)

Rh−COD distances are measured to the centroid of the CC bond.

monometallic complex 15. This similarity in coordination geometry between monometallic and bimetallic structures would suggest that the individual Rh fragments of 16 are also chemically very similar to the monometallic analogue 15. Catalysis. The monometallic and bimetallic complexes containing CO coligands 17−21 were investigated as catalysts for two different reactions. It has previously been demonstrated that Rh(I) complexes containing NHC and/or N-donor ligands in combination with CO or COD coligands can be highly efficient catalysts for the addition of O−H and Si−H bonds across unsaturated alkyne CC bonds.10f,14a This hydroelementation of alkynes provides an atom-economical route toward the synthesis of new O−C and Si−C bonds. The two reactions we chose to investigate were the intramolecular dihydroalkoxylation of alkynediols to yield spiroketals and the intermolecular hydrosilylation of alkynes to yield vinylsilanes. The two reactions proceed through different mechanisms, and C


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principle is governing the rate enhancement observed in this reaction and is not confined/unique to one particular type of catalyst design. The turnover frequency of complex 18 (52 h−1) is approximately 5 times faster than that of 17 (11 h−1), which compares favorably with the rate enhancements achieved by catalysts of related structure (Figure 2, previous work).14 To investigate catalysts 17−19 further, we performed the reaction in the presence of NaBArF4 (BArF4 = tetrakis[3,5bis(trifluoromethyl)phenyl]borate (1 equiv per Rh, Figure 5b). It was anticipated that the substitution of the iodide ligand for the weakly coordinating BArF4 anion would produce a significant improvement on the reaction rate. The catalytic efficiency for complexes 17−19 improved dramatically with TOF values 2.1−2.5 times greater than the catalyzed reactions that did not contain NaBArF4 and significantly reduced reaction times for the complexes (90% vs 50% conversion at 5 h) for complexes 17 and 19. The turnover frequency of complex 18 (κ1-C-meta-(Rh(CO)2)2; 135 h−1) is still approximately 4 times faster than that of 17 (κ1-C-mono-Rh(CO)2; 35 h−1), which would indicate that the cooperative rate enhancement achieved with the bimetallic catalyst 18 is independent of the chemistry of the iodide ligand. The cationic complexes 20 (κ2-C,N-mono-Rh(CO)2) and 21 2 (κ -C,N-meta-(Rh(CO)2)2) were far less effective catalysts for the dihydroalkoxylation of 22 in comparison to catalysts 17−19 (Figure 5c). The bidentate ligand coordination appears to inhibit the activity of these complexes, suggesting dissociation of the iodide coligand in complexes 17−19 allows the substrate to bind to the metal center more effectively. A notable increase in the catalytic efficiency of the bimetallic complex 21 in comparison to the monometallic analogue 20 is still apparent. The mechanism of the hydroalkoxylation reaction using monometallic complexes has been described previously and studied by a number of research teams using a variety of metal centers.14d,21,22 In order to obtain a mechanistic understanding of the nature of the bimetallic cooperativity in enhancing the reaction rates of the dihydroalkoxylation reaction, a significant investigation of the mechanism incorporating DFT calculations is currently underway in our research group. Hydrosilylation Reaction. The mechanism for the hydrosilylation of alkynes is well established.23 The hydrosilylation of diphenylacetylene with Et3SiH to yield the vinylsilane products

it was our aim to discover what reaction properties would benefit from the use of a bimetallic catalyst structure. Dihydroalkoxylation Reaction. The dihydroalkoxylation of the alkynediol 22 to yield spiroketals 23a,b is a two-step tandem process (Scheme 2). The Rh(I) complex catalyzes the


Scheme 2. Dihydroalkoxylation Reaction Pathways of 22 To Form either the 6,5-Spiroketal 23a or the 5,6-Spiroketal 23b

reaction via coordination of the alkyne to the Lewis acidic metal center.14d,21 This facilitates the addition of the first OH group to the alkyne triple bond which can occur with either alcohol to yield either a five- or six-membered heterocycle. The addition of the second OH group to the vinyl ether intermediate is rapid (Scheme 2), and the mono-hydroalkoxylated product is typically not observed.14d,21 The results obtained using complexes 17−21 to catalyze the dihydroalkoxylation of 22 are shown in Figure 5. The reactions were performed in an NMR tube in tetrachloroethane-d2 (TCE-d2) at 100 °C using 1 mol % Rh concentration (i.e., 0.5 mol % of the bimetallic catalyst). The reaction progress was monitored in situ using 1H NMR spectroscopy, with conversions determined by comparison of the substrate and product integrals. Isomer 23a was found to be the major product in each case (23a:23b ≈ 2:1). Figure 5a clearly shows that the bimetallic catalyst 18 (κ1-C-meta-(Rh(CO)2)2) is a superior catalyst in comparison to either the monometallic analogue 17 (κ1-C-mono-Rh(CO)2) or the bimetallic complex 19 (κ1-C-para-(Rh(CO)2)2), with both 17 and 19 showing very similar reaction profiles. This confirms conclusions drawn from our previous studies, which showed a short spatial separation between metals is necessary to achieve effective cooperative rate enhancements for this reaction.14,15 This work provides another example where the dihydroalkoxylation of 22 is cooperatively enhanced by use of a bimetallic catalyst with optimum Rh−Rh separation. This in turn suggests a more general cooperativity

Figure 5. Time course plots of the catalyzed dihydroalkoxylation reaction at 100 °C in TCE-d2: (a) κ1 catalysts 17−19; (b) κ1 catalysts 17−19 with the addition of 1 mol % NaBArF4, and (c) κ2 catalysts 20 and 21. D


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Scheme 3. Proposed Hydrosilylation Reaction Pathway of Diphenylacetylene with Triethylsilane using a Rh(I) Complex

nanoparticles are present, the mercury will form an amalgam with the nanoparticles and the reaction will be inhibited. Under these conditions, less than 1% conversion was obtained after 1 h at 50 °C, suggesting that Rh nanoparticles were indeed responsible for the catalytic activity originally observed. A TEM image of the reaction mixture using catalyst 18 was also obtained (Figure 6b), providing clear evidence for the formation of nanoparticles during the reaction. It is curious to note that complexes 17−19 display varying induction times for this reaction. This would indicate that the bimetallic structure has an effect on the rate at which the complexes decompose to form nanoparticles. Most notably, for complex 19 (κ1-C-para-(Rh(CO)2)2), the p-phenylene ligand scaffold appears to inhibit the degradation of the complex in comparison to both 17 (κ1-C-mono-Rh(CO)2) and 18 (κ1-Cmeta-(Rh(CO)2)2). The mercury poisoning test was subsequently performed for the dihydroalkoxylation of 22 using catalyst 17. The reaction was not affected by the addition of mercury, suggesting that the reducing potential of Et3SiH is responsible for nanoparticle formation in the hydrosilylation reaction. The hydrosilylation of diphenylacetylene with Et3SiH was also performed using catalysts 20 and 21, containing the chelating (κ2) NHC-pyrazole ligands. For catalyst 20 (κ2-C,Nmono-Rh(CO)2) the reaction was complete within 3 min at 50 °C. The rapid reaction rate at 50 °C did not allow a clear understanding of the reaction profile; thus, the temperature of the catalyzed reaction was reduced to 25 °C. Both catalysts promote the reaction at an extremely rapid rate at this temperature, affording complete conversion of the substrate within 25 min (Figure 7a). Very little difference in reaction rate was observed between the catalysts, showing no cooperative effect of the bimetallic catalyst 21. A mercury poisoning test was also performed, but no effect on the reaction rates was observed, which suggests that the reactions are catalyzed by a homogeneous metal complex in this case. The chelating NHCpyrazole ligand group in 20 and 21 therefore appears to stabilize the complexes against degradation to metallic nanoparticles compared to complexes 17−19, which contain monodentate NHC ligands. Rhodium(I) complexes containing COD coligands have also been used to effectively catalyze the hydrosilylation of alkynes previously.10a,24 We therefore investigated the catalytic activity of complexes 12, 13, 15, and 16 for the hydrosilylation of diphenylacetylene at 50 °C over 1.5 h (Figures 7b,c). A surprising difference in reactivity was observed between the monometallic catalysts 12 and 15 in comparison to their bimetallic counterparts 13 and 16, respectively. For example, for the complexes containing the bidentate (κ2) ligand group (Figure 7c), a large increase in the reaction rate was observed with the bimetallic catalyst 16 (κ2-C,N-meta-(Rh(COD))2, 68% conversion) in comparison to the monometallic catalyst 15 (κ2C,N-mono-Rh(COD), 0% conversion). Curiously, the opposite trend was true for the complexes containing the monodentate (κ1) ligand group (Figure 7b), where the monometallic catalyst 12 (κ1-C-mono-Rh(COD)) achieved 20% conversion of the


25-Z and 25-E proceeds via the initial oxidative addition of the silane Si−H bond to Rh(I) (Scheme 3). This is followed by insertion of the alkyne into the Rh−Si bond and reductive elimination of the vinyl silane product 25-Z, which slowly isomerizes to 25-E. The reaction profiles obtained using complexes 17−19 as catalysts for the hydrosilylation of diphenylacetylene with Et3SiH are shown in Figure 6a. The reactions were performed

Figure 6. (a) Time course plots of the catalyzed hydrosilylation reaction of diphenylacetylene with Et3SiH using the κ1 complexes 17− 19 in THF-d8 at 50 °C and (b) TEM image of the resultant mixture for the hydrosilylation reaction using complex 18.

in an NMR tube at 50 °C in THF-d8, using 4 mol equiv of Et3SiH and 2 mol % of catalyst relative to substrate. Isomer 25Z was found to be the major product in each case (Z:E ≈ 97:3). The most notable feature of this catalysis is the sigmoidal reaction profile, which is due to the significant induction period before conversion of the substrate proceeds. A comparison of the slope of the graphs between 10% and 90% conversion reveals that all three catalysts have very similar activities once the reaction is initiated. These features are indicative of complexes 17−19 decomposing to yield Rh nanoparticles as the active catalytic species. To confirm this hypothesis, a mercury poisoning test was performed by exposing the reaction solution to a large excess of elemental mercury at time 0. If Rh E


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Figure 7. Time course plots of the catalyzed hydrosilylation reaction in THF-d8: (a) κ2 catalysts 20 and 21 at 25 °C; (b) κ1 catalysts 12 and 13 at 50 °C; (c) κ2 catalysts 15 and 16 at 50 °C.

substrate and the bimetallic catalyst 13 (κ1-C,N-meta-(Rh(COD))2) was entirely unreactive. These results highlight how the design of cooperative bimetallic catalysts can be substantially affected by small changes in the coordination sphere of the two complex fragments, often in ways that cannot be predicted from the reactivity of the corresponding monometallic complexes. The two active catalysts 12 and 15 were both unaffected by the presence of mercury for the hydrosilylation reaction of 24, consistent with a homogeneous complex structure being retained during the reaction. The resistance of complex 12 (κ1-C-mono-Rh(COD)) to degradation indicates that the COD coligand plays an important role in stabilizing the complex during the hydrosilylation reaction. As described above, the analogous CO containing complex 17 (κ1-C-mono-Rh(CO)2) rapidly decomposes to metallic nanoparticles under the same reaction conditions.

with these nanoparticles being responsible for the catalytic activity observed. The integrity of the homogeneous catalyst structure was retained in the case that a bidentate (κ2) NHCpyrazole ligand group was present (i.e., for 20 and 21) or if a COD coligand was used in place of CO (i.e., for 12, 13, 15, and 16). The formation of nanoparticles from the Rh-NHC complexes 17−19 is unusual and was not observed with similar NHC complexes reported by Huckaba et al.10f for catalyzed hydrosilylation reactions. A cooperative increase in the hydrosilylation reaction rate was observed in one case, with the bimetallic complex 16 (κ2-C,N-meta-(Rh(COD))2) being a far superior catalyst to its monometallic analogue 15 (κ2-C,Nmono-Rh(COD)). This highlights the sensitivity of the cooperative phenomenon to small changes in the coordination environment of the metal center.

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EXPERIMENTAL SECTION

All manipulations of metal complexes and air-sensitive reagents were carried out using standard Schlenk techniques or in a nitrogen-filled glovebox. Reagents were purchased from Aldrich Chemicals or Alfa Aesar and were used without further purification unless otherwise stated. Rhodium(III) chloride hydrate was obtained from Precious Metals Online, and all deuterated solvents were obtained from Cambridge Isotopes Laboratories. Diethyl ether, n-pentane, and dichloromethane were obtained dry under nitrogen from a PuraSolv solvent purification system. Methanol was distilled under nitrogen from dimethoxymagnesium. The compounds 1−4,16 5−8,17 and [Rh(μ-Cl)(COD)]225 were synthesized using reported methods. 1H and 13C{1H} NMR spectra were recorded on Bruker DPX300, DMX400, and DMX600 spectrometers at 298 K unless otherwise specified. Chemical shifts (δ) are quoted in ppm and are referenced to internal solvent resonances. The abbreviations pz (pyrazole) and im (imidazole) are used to identify NMR signals. Infrared spectra were measured using a Nicolet 380 Avatar FTIR spectrometer. Elemental analyses were carried out at the Elemental Analysis Unit, Australian National University, Canberra, Australia. Single-crystal X-ray analyses were carried out at the Mark Wainwright Analytical Centre, University of New South Wales, Sydney. X-ray diffraction measurements were carried out on a Bruker Kappa APEXII CCD diffractometer using graphite-monochromated Mo Kα radiation (λ = 0.710723 Å). All structures were solved by direct methods, and the full-matrix leastsquares refinements were carried out using SHELXL.26 Absorption correction was performed using Multiscan SADABS, and H atom parameters were treated as constrained. The structure of 16 showed disorder in two moieties: one of the COD groups coordinated to

CONCLUSIONS A series of structurally related monometallic and bimetallic Rh NHC complexes 12−21 were prepared that vary according to the degree of separation between metals (meta vs para), the donor properties of the ligand (κ1-NHC donor vs an κ2N,NHC donor), and the coligand identity (COD vs CO). On promotion of the dihydroalkoxylation of 22 using catalysts 17−21, a cooperative rate enhancement was observed on using both meta-disposed bimetallic complexes 18 (κ1-Cmeta-(Rh(CO)2)2) and 21 (κ2-C,N-meta-(Rh(CO)2)2) as catalysts; however, the para-disposed bimetallic complex 19 (κ1-C-para-(Rh(CO)2)2) had a reactivity identical with that of the monometallic analogue 17 (κ1-C-mono-Rh(CO)2). These results indicate that a cooperative rate enhancement is achieved most effectively by a short spatial separation between the pairs of metal centers and any electronic influence of the neighboring metal center has a minimal impact on this reaction. The impact of spatial configuration on the bimetallic cooperativity mirrors our previous observations with N,N-donor bimetallic catalysts.14a This suggests that the reaction mechanisms are similar for both Rh complexes containing NHC donors or N,Ndonors. Complexes 17−19 led to the formation of metallic nanoparticles during the catalyzed hydrosilylation reaction, F


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Organometallics Rh1B is disordered over two sites with occupancies of 0.75 and 0.25. Two of the phenyl rings attached to B1F also exhibited orientational disorder over two sites. TEM samples were prepared by applying a droplet of the reaction solution onto a carbon plus Formvar coated copper grid. The droplet was allowed to sit on the grid for 1 min, and then the excess fluid was wicked off using filter paper. The grids were examined in a JEOL 1400 TEM instrument operating at 100 kV. Synthesis of the Ligand Precursors 9−11. The ligand precursors 9−11 were all synthesized following the same procedure. A solution of the appropriate mono- or bis(imidazolyl)benzene 1−3 (2.60 mmol) and 1-(2-bromoethyl)pyrazole (470 mg, 2.69 mmol for 1 or 950 mg, 5.43 mmol for 2 and 3) in N,N-dimethylformamide (20 mL) was refluxed for 48 h. The solvent was then removed by distillation under vacuum and the residue dissolved in methanol (10 mL). Sodium tetraphenylborate (890 mg, 2.6 mmol for 1 or 1.78 g, 5.2 mmol for 2 and 3) was added to the solution, resulting in the immediate formation of a pale brown precipitate. The precipitate was filtered and recrystallized from dichloromethane/methanol for ligand precursor 9 or from acetone/methanol for ligand precursors 10 and 11. κ2-C,N-mono (9). Yield: 62%. 1H NMR (DMSO-d6, 300 MHz): δ 9.56 (t, 1H, J = 1.6 Hz, NCHN), 8.25 (t, 1H, J = 1.9 Hz, imH), 7.72 (t, 1H, J = 1.9 Hz, imH), 7.70−7.55 (m, 6H, PhH and pzH), 7.49 (dd, 1H, J = 1.9 and 0.7 Hz, pzH), 7.17 (m, 8H, o-BPh4), 6.92 (t, 8H, J = 7.3 Hz, m-BPh4), 6.78 (t, 4H, J = 7.3 Hz, p-BPh4), 6.26 (t, 1H, J = 2.1 Hz, pzH), 4.68 (s, 4H, CH2) ppm. 13C{1H} NMR (DMSO-d6, 75 MHz): δ 163.4 (q, JBC = 49.5 Hz, BC), 139.8 (pzC), 135.8 (NCN), 135.6 (m, o-BPh4), 134.6 (PhC), 130.9 (pzC), 130.4 (PhC), 130.1 (PhC), 125.4 (q, JBC = 2.8 Hz, m-BPh4), 123.5 (imC), 121.8 (PhC), 121.6 (p-BPh4), 121.2 (imC), 105.8 (pzC), 50.3 (CH2), 49.7(CH2) ppm. Anal. Found (calcd for C38H35BN4·0.5CH3OH): C, 80.77 (80.48); H, 6.31 (6.49); N, 10.06 (9.75). ESI-HRMS, found (calculated): m/z 239.1289 (239.1291, [M − BPh4]+). κ2-C,N-ortho (10). Yield: 66%. 1H NMR (acetone-d6, 300 MHz): δ 8.97 (t, 2H, J = 1.5 Hz, NCHN), 7.91 (m, 2H, PhH), 7.77 (m, 2H, PhH), 7.66 (m, 4H, imH and pzH), 7.57 (t, 2H, J = 1.9 Hz, imH), 7.51 (d, 2H, J = 1.9 Hz, pzH), 7.33 (m, 16H, o-BPh4), 6.91 (t, 16H, J = 7.3 Hz, m-BPh4), 6.77 (t, 8H, J = 7.3 Hz, p-BPh4), 6.32 (t, 2H, J = 2.1 Hz, pzH), 4.82 (m, 4H, CH2), 4.70 (m, 4H, CH2) ppm. 13C{1H} NMR (acetone-d6, 75 MHz): δ 164.9 (q, JBC = 49.6 Hz, BC), 141.1 (pzC), 138.8 (NCN), 137.0 (m, o-BPh4), 133.9 (PhC), 131.6 (pzC), 130.9 (PhC), 129.3 (PhC), 126.1 (q, JBC = 2.8 Hz, m-BPh4), 125.1 (imC), 124.5 (imC), 122.3 (p-BPh4), 107.0 (pzC), 51.5 (2 × CH2) ppm. ESIHRMS, found (calculated): m/z 200.1053 (200.1057, [M − (BPh4)2]2+), 719.3771 (719.3777, [M − BPh4]+), κ2-C,N-meta (11). Yield: 52%. 1H NMR (acetone-d6, 300 MHz): δ 8.93 (t, 2H, J = 1.6 Hz, NCHN), 7.96 (m, 3H, imH and PhH), 7.91 (m, 1H, PhH), 7.83 (m, 2H, PhH), 7.65 (t, 2H, J = 1.9 Hz, imH), 7.61 (dd, 2H, J = 2.3 and 0.6 Hz, pzH), 7.47 (dd, 2H, J = 1.9 and 0.6 Hz, pzH), 7.32 (m, 16H, o-BPh4), 6.91 (t, 16H, J = 7.3 Hz, m-BPh4), 6.76 (m, 8H, p-BPh4), 6.26 (dd, 2H, J = 2.3 and1.9 Hz, pzH), 4.84 (m, 4H, CH2), 4.73 (m, 4H, CH2) ppm. 13C{1H} NMR (acetone-d6, 75 MHz): δ 164.9 (q, JBC = 49.6 Hz, BC), 140.9 (pzC), 137.0 (m, o-BPh4), 136.8 (PhC), 136.7 (NCN), 133.4 (PhC), 131.5 (pzC), 126.1 (q, JBC = 2.6 Hz, m-BPh4), 125.2 (PhC), 125.0 (imC), 122.9 (imC), 122.3 (p-BPh4), 118.1 (PhC), 106.9 (pzC), 51.4 (CH2), 51.3 (CH2) ppm. ESI-HRMS, found (calculated): m/z 200.1054 (200.1057, [M − (BPh4)2]2+), 719.3779 (719.3777, [M − BPh4]+). Anal. Found (calcd for C70H64B2N8): C, 80.61 (80.93); H, 6.33 (6.21); N, 10.89 (10.79). Synthesis of COD-Containing Complexes 12−16. The rhodium COD complexes 12−16 were synthesized by following the same procedure as above. The appropriate ligand precursor 5−11 (0.88 mmol), [Rh(COD)Cl]2 (217 mg, 0.44 mmol for 12 and 15 or 434 mg, 0.88 mmol for 13, 14, and 16) and sodium ethoxide (112 mg, 1.64 mmol for 12 and 15 or 224 mg, 3.28 mmol for 13, 14, and 16) were suspended in methanol (25 mL) and refluxed for 1 h (for complex 16, refluxing the solution for only 30 min afforded a cleaner product). Upon cooling to room temperature a yellow precipitate of the product formed. Complexes 12, 13, and 15 were filtered and recrystallized from dichloromethane/pentane to form a clean

precipitate of the desired product. Complex 16 appeared to degrade during recrystallization from dichloromethane, and the poor solubility of 14 prevented recrystallization; therefore, both 14 and 16 were washed with methanol (3 × 10 mL) and used without further purification. κ1-C-mono-Rh(COD) (12). Yield: 50%. 1H NMR (CDCl3, 400 MHz): δ 8.21 (d, 2H, J = 7.6 Hz, PhH), 7.52 (t, 2H, J = 7.6 Hz, PhH), 7.43 (t, 1H, J = 7.6 Hz, PhH), 7.16 (d, 1H, J = 1.7 Hz, imH), 7.00 (d, 1H, J = 1.7 Hz, imH), 5.27 (m, 1H, COD-CH), 5.14 (m, 1H, CODCH), 4.12 (s, 3H, CH3), 3.43 (m, 1H, COD-CH), 2.73 (m, 1H, CODCH), 2.26 (m, 2H, COD-CH2), 2.06 (m, 1H, COD-CH2), 1.77 (m, 3H, COD-CH2), 1.43 (m, 2H, COD-CH2) ppm. 13C{1H} NMR (CDCl3, 100 MHz): δ 183.1 (d, JRhC = 49.3 Hz, NCN), 140.3 (PhC), 128.9 (PhC), 128.0 (PhC), 124.6 (PhC), 123.0 (imC), 121.6 (imC), 96.1 (d, JRhC = 7.5 Hz, COD-CH), 96.0 (d, JRhC = 7.5 Hz, COD-CH), 71.6 (d, JRhC = 14.3 Hz, COD-CH), 71.3 (d, JRhC = 14.3 Hz, CODCH), 39.4 (CH3), 33.4 (COD-CH2), 30.9 (COD-CH2), 30.1 (CODCH2), 28.9 (COD-CH2) ppm. Anal. Found (calcd for C18H22IN2Rh): C, 43.63 (43.57); H, 4.55 (4.47); N, 5.67 (5.65). κ1-C-meta-(Rh(COD))2 (13). Yield: 77%. 1H NMR (CDCl3, 300 MHz), two atropisomers identified as Ha and Hb (a/b = 0.4/0.6): δ 9.95 (t, 0.4H, J = 2.1 Hz, PhHa), 9.02 (t, 0.6H, J = 2.1 Hz, PhHb), 8.72 (dd, 0.8H, J = 8.1 and 2.1 Hz, PhHa), 8.56 (dd, 1.2H, J = 8.1 and 2.1 Hz, PhHb), 7.70 (t, 1H, J = 8.1 Hz, PhHa,b), 7.61 (d, 1.2H, J = 2.0 Hz, imHb), 7.52 (d, 0.8H, J = 2.0 Hz, imHa), 7.08 (d, 0.8H, J = 2.0 Hz, imHa), 7.06 (d, 1.2H, J = 2.0 Hz, imHb), 5.21 (m, 4H, COD-CHa,b), 4.21 (s, 2.4H, CH3a), 4.16 (s, 3.2H, CH3b), 3.49−3.40 (m, 2H, CODCHa,b), 2.96−2.82 (m, 2H, COD-CHa,b), 2.52−1.35 (m, 16H, CODCH2a,b) ppm. 13C{1H} NMR (CDCl3, 75 MHz), atropisomers a and b could not be distinguished between the closely spaced resonance pairs: δ 183.4 (d, J = 49.2 Hz, NCN), 183.2 (d, J = 49.2 Hz, NCN), 140.2 (PhC), 140.0 (PhC), 129.1 (2 × PhC), 123.6 (imC), 123.5 (imC), 122.8 (PhC), 121.8 (imC), 121.4 (imC), 121.2 (PhC), 120.5 (PhC), 116.9 (PhC), 96.8 (d, J = 7.0 Hz, COD-CH), 96.2 (d, J = 7.0 Hz, COD-CH), 95.8 (d, J = 7.0 Hz, COD-CH), 95.6 (d, J = 7.0 Hz, CODCH), 72.5 (d, J = 14.0 Hz, COD-CH), 72.2 (d, J = 14.0 Hz, CODCH), 72.1 (d, J = 14.0 Hz, COD-CH), 71.6 (d, J = 14.0 Hz, CODCH), 39.9 (CH3a), 39.4 (CH3b), 33.5 (COD-CH2), 32.7 (COD-CH2), 31.7 (COD-CH2), 30.8 (COD-CH2), 30.7 (COD-CH2), 29.7 (CODCH2), 29.6 (COD-CH2), 28.8 (COD-CH2) ppm. Anal. Found (calcd for C30H38I2N4Rh2·H2O): C, 38.50 (38.65); H, 4.17 (4.32); N, 5.96 (6.01). κ1-C-para-(Rh(COD))2 (14). Yield: 80%. 1H NMR (CDCl3, 400 MHz), two atropisomers identified as Ha and Hb (a/b = 0.45/0.55): δ 8.61 (s, 2.2H, PhHb), 8.46 (s, 1.8H, PhHa), 7.28 (d, 1.1H, J = 2.0 Hz, imHb), 2.24 (d, 0.9H, J = 2.0 Hz, imHa), 7.08 (d, 1.1H, J = 2.0 Hz, imHb), 7.07 (d, 0.9H, J = 2.0 Hz, imHa), 5.26 (m, 4H, COD-CHa,b), 4.20 (s, 3.3H, CH3b), 4.17 (s, 2.7H, CH3a), 3.49 (m, 2H, COD-CHa,b), 2.87 (m, 2H, COD-CHa,b), 2.44−1.37 (m, 16H, COD-CH2a,b) ppm. The low solubility of 14 prevented acquisition of the 13C{1H} NMR spectrum. Anal. Found (calcd for C30H38I2N4Rh2): C, 40.18 (39.87); H, 4.02 (4.01); N, 6.23 (6.18). κ2-C,N-mono-Rh(COD) (15). Yield: 80%. 1H NMR (acetone-d6, 400 MHz): δ 8.12 (d, 2H, J = 7.3 Hz, PhH), 8.04 (d, 1H, J = 2.3 Hz, pzH), 7.87 (d, 1H, J = 2.3 Hz, pzH), 7.71 (t, 2H, J = 7.3 Hz, PhH), 7.62 (t, 1H, J = 7.3 Hz, PhH), 7.52 (d, 1H, J = 2.0 Hz, imH), 7.35 (m, 9H, imH and o-BPh4), 6.94 (m, 9H, NCH2 and m-BPh4), 6.78 (t, 4H, J = 7.3 Hz, p-BPh4), 6.47 (t, 1H, J = 2.3 Hz, pzH), 5.21 (m, 2H, NCH2 and COD-CH), 5.11 (ddd, 1H, J = 14.6, 4.6, and 3.2 Hz, NCH2), 4.68 (ddd, 1H, J = 14.6, 11.2, and 5.2 Hz, NCH2), 4.44 (q, 1H, J = 7.4 Hz, COD-CH), 4.36 (m, 1H, COD-CH), 2.80 (m, 1H, COD-CH), 2.57 (m, 2H, COD-CH2), 2.25 (m, 1H, COD-CH2), 2.11 (m, 2H, CODCH2), 1.80 (m, 1H, COD-CH2), 1.56 (m, 2H, COD-CH2) ppm. 13 C{1H} NMR (acetone-d6, 100 MHz): δ 176.4 (d, JRhC = 50.8 Hz, NCN), 165.2 (q, JBC = 49.3 Hz, BC), 141.9 (pzC), 141.5 (PhC), 137.1 (m, o-BPh4), 134.9 (pzC), 130.3 (PhC), 129.8 (PhC), 126.0 (q, JBC = 2.6 Hz, m-BPh4), 125.6 (PhC), 125.3 (imC), 123.8 (imC), 122.3 (pBPh4), 109.2 (pzC), 97.2 (d, JRhC = 7.9 Hz, COD-CH), 95.1 (d, JRhC = 7.9 Hz, COD-CH), 80.3 (d, JRhC = 13.0 Hz, COD-CH), 74.7 (d, JRhC = 13.0 Hz, COD-CH), 51.4 (NCH2), 50.3 (NCH2), 34.6 (COD-CH2), G


Article


Organometallics

(PhC), 39.9 (2 × CH3) ppm. FTIR (Nujol): ν 2084 (m, CO), 2069 (s, CO), 2002 (s, CO), 1989 (m, CO) cm−1. Anal. Found (calcd for C18H14I2N4O4Rh2): C, 27.15 (26.69); H, 1.63 (1.74); N, 7.05 (6.92). κ1-C-para-(Rh(CO)2)2 (19). Yield: 48%. 1H NMR (CDCl3, 300 MHz): δ 7.89 (s, 4H, PhH), 7.39 (br s, 2H, imH), 7.22 (d, 2H, J = 2.0 Hz, imH), 3.97 (s, 6H, CH3) ppm. 13C{1H} (CDCl3, 75 MHz): δ 187.0 (d, JRhC = 53.4 Hz, CO), 180.9 (d, JRhC = 77.0 Hz, CO), 173.9 (d, JRhC = 42.5 Hz, NCN), 139.7 (PhC), 126.1 (PhC), 124.3 (imC), 122.7 (imC), 39.9 (CH3) ppm. FTIR (Nujol): ν 2070 (s, CO), 2016 (s, CO), 1997 (s, CO), 1968 (m, CO) cm−1. Anal. Found (calcd for C18H14I2N4O4Rh2): C, 27.15 (26.69); H, 1.60 (1.74); N, 6.99 (6.92). κ2-C,N-mono-Rh(CO)2 (20). Yield: 65%. 1H NMR (acetone-d6, 400 MHz): δ 7.98 (d, 1H, J = 2.5 Hz, pzH), 7.95 (d, 1H, J = 2.3 Hz, pzH), 7.80 (m, 2H, PhH), 7.60 (m, 4H, imH and PhH), 7.39 (m, 8H, oBPh4), 7.28 (d, 1H, J = 2.0 Hz, imH), 6.95 (t, 8H, J = 7.3 Hz, m-BPh4), 6.80 (t, 4H, J = 7.3 Hz, p-BPh4), 6.56 (t, 1H, J = 2.4 Hz, pzH), 5.28 (br t, 2H, J = 4.9 Hz, CH2), 4.50 (t, 2H, J = 5.6 Hz, CH2) ppm. 13C{1H} NMR (acetone-d6, 100 MHz): δ 185.4 (d, JRhC = 55.8 Hz, CO), 185.3 (d, JRhC = 71.4 Hz, CO), 167.7 (d, JRhC = 45.5 Hz, NCN), 164.9 (q, JBC = 49.3 Hz, BC), 144.3 (pzC), 141.1 (PhC), 137.0 (m, o-BPh4), 135.3 (pzC), 130.6 (PhC), 130.3 (PhC), 127.0 (PhC), 126.1 (q, JBC = 2.6 Hz, m-BPh4), 125.9 (imC), 124.5 (imC), 122.3 (p-BPh4), 110.0 (pzC), 50.9 (CH2), 50.6 (CH2) ppm. FTIR (Nujol): ν 2087 (s, CO), 2025 (s, CO) cm−1. Anal. Found (calcd for C40H34BN4O2Rh): C, 67.24 (67.06); H, 4.90 (4.78); N, 8.09 (7.82). κ2-C,N-meta-(Rh(CO)2)2 (21). Yield: 41%. 1H NMR (acetone-d6, 400 MHz): δ 8.33 (dd, 2H, J = 8.0 and 2.1 Hz, PhH), 8.25 (m, 3H, pzH and PhH), 7.98 (d, 2H, J = 2.3 Hz, pzH), 7.93 (t, 1H, J = 8.0 Hz, PhH), 7.86 (d, 2H, J = 2.0 Hz, imH), 7.62 (d, 2H, J = 2.0 Hz, imH), 7.34 (m, 16H, o-BPh4), 6.91 (t, 16H, J = 7.3 Hz, m-BPh4), 6.77 (t, 8H, J = 7.3 Hz, p-BPh4), 6.66 (t, 2H, J = 2.4 Hz, pzH), 5.65 (br s, 4H, CH2), 4.89 (t, 4H, J = 5.7 Hz, CH2) ppm. 13C{1H} NMR (acetone-d6, 100 MHz): δ 185.6 (d, JRhC = 71.3 Hz, CO), 185.3 (d, JRhC = 55.8 Hz, CO), 168.3 (d, JRhC = 44.8 Hz, NCN), 165.0 (q, JBC = 49.1 Hz, BC), 144.5 (pzC), 141.9 (PhC), 137.1 (m, o-BPh4), 135.7 (pzC), 132.4 (PhC), 128.7 (PhC), 126.5 (imC), 126.0 (q, JBC = 2.6 Hz, m-BPh4), 125.0 (imC), 123.8 (PhC), 122.3 (p-BPh4), 110.2 (pzC), 51.3 (CH2), 50.9 (CH2) ppm. FTIR (Nujol): ν 2090 (s, CO), 2027 (s, CO) cm−1. ESI-HRMS, found (calculated): m/z 1035.1537 (1035.1527, [M − BPh4]+). Catalyzed Dihydroalkoxylation of Alkynediol (22). The dihydroalkoxylation reaction was performed under nitrogen in NMR tubes fitted with a Young’s concentric Teflon valve. The alkynediol 22 (38 mg, 200 μmol), the catalyst (2 μmol of the monometallic complexes or 1 μmol of the bimetallic complexes), and NaBArF4 if desired (2.0 mg, 2.3 μmol) were dissolved in TCE-d2 (0.6 mL). The tube was inserted into the NMR spectrometer preheated to 100 °C. The reaction progress was monitored by 1H NMR spectroscopy, with conversions determined by comparison of characteristic substrate and product integrals (see Figure S27 in the Supporting Information). Catalyzed Hydrosilylation of Diphenylacetylene (24). The hydrosilylation reaction was performed under nitrogen in NMR tubes fitted with a Young’s concentric Teflon valve. Diphenylacetylene (36 mg, 200 μmol), the catalyst (4 μmol of the monometallic complexes or 2 μmol of the bimetallic complexes), and NaBArF4 if desired (4.0 mg, 4.5 μmol) were dissolved in THF-d8 (0.6 mL). Triethylsilane (93 mg, 800 μmol) was then injected into the solution and the tube inserted into the NMR spectrometer preheated to the desired temperature (50 or 25 °C). The reaction progress was monitored by 1H NMR spectroscopy with conversions determined by comparison of characteristic substrate and product integrals (see Figure S28 in the Supporting Information).

30.8 (COD-CH2), 30.3 (under acetone-CD3, COD-CH2), 28.4 (CODCH2) ppm. Anal. Found (calcd for C46H46BN4Rh): C, 71.98 (71.88); H, 5.95 (6.03); N, 7.35 (7.29). κ2-C,N-meta-(Rh(COD))2 (16). Yield: 40%. 1H NMR (acetone-d6, 400 MHz), two atropisomers identified as Ha and Hb (a/b = 0.5/0.5): δ 9.21 (d, 1H, J = 7.2 Hz, PhHb), 9.10 (d, 1H, J = 7.2 Hz, PhHa), 8.24 (br s, 0.5H, PhHb), 8.21 (t, 0.5H, J = 8.0 Hz, PhHb), 8.17 (t, 0.5H, J = 8.0 Hz, PhHa), 8.14 (d, 1H, J = 2.5 Hz, pzHb), 8.10 (d, 1H, J = 2.5 Hz, pzHa), 8.00 (br s, 0.5H, PhHa), 7.90 (d, 2H, J = 2.0 Hz, pzHa,b), 7.82 (d, 1H, J = 1.8 Hz, imHa), 7.71 (d, 1H, J = 1.8 Hz, imHb), 7.49 (d, 1H, J = 1.8 Hz, imHa), 7.44 (d, 1H, J = 1.8 Hz, imHb), 7.31 (m, 16H, oBPh4), 7.10−6.72 (m, 2H, NCH2a,b), 6.91 (t, 16H, J = 7.3 Hz, mBPh4), 6.77 (t, 8H, J = 7.3 Hz, p-BPh4), 6.52 (t, 1H, J= 2.3 Hz, pzHb), 6.51 (t, 1H, J = 2.3 Hz, pzHa), 5.40−5.05 (m, 4H, NCH2a,b and 2H, COD-CHa,b), 4.85−4.35 (m, 2H, NCH2a,b and 4H, COD-CHa,b), 3.10−2.75 (m, 2H, COD-CHa,b), 2.70−1.50 (m, 16H, COD-CH2a,b) ppm. 13C{1H} NMR (acetone-d6, 100 MHz), atropisomers a and b could not be distinguished between the closely spaced resonance pairs, quaternary resonances PhC and NCN were not defined due to insufficient signal strength: δ 164.9 (q, JBC = 49.1 Hz, BC), 142.1 (pzC), 141.8 (pzC), 137.0 (m, o-BPh4), 135.5 (pzC), 135.2 (pzC), 131.2 (PhC), 130.9 (PhC), 126.0 (q, JBC = 2.6 Hz, m-BPh4), 125.8 (imC), 125.7 (2 × PhC), 125.6 (imC), 123.9 (imC), 123.7 (imC), 122.3 (p-BPh4), 120.1 (PhC), 119.5 (PhC), 109.2 (pzC), 109.1 (pzC), 97.6 (d, JRhC = 7.8 Hz, COD-CH), 97.0 (d, JRhC = 7.8 Hz, COD-CH), 96.6 (d, JRhC = 7.8 Hz, COD-CH), 95.8 (d, JRhC = 7.8 Hz, COD-CH), 80.8 (d, JRhC = 13.2 Hz, COD-CH), 80.5 (d, JRhC = 13.2 Hz, CODCH), 75.2 (d, JRhC = 13.2 Hz, COD-CH), 74.8 (d, JRhC = 13.2 Hz, COD-CH), 51.5 (NCH2), 51.4 (NCH2), 50.3 (NCH2), 50.1 (NCH2), 34.6 (COD-CH2), 33.7 (COD-CH2), 31.4 (COD-CH2), 31.1 (CODCH2), 30.5 (under acetone-CD3, COD-CH2), 30.3 (under acetoneCD3, COD-CH2), 29.1 (COD-CH2), 28.3 (COD-CH2) ppm. ESIHRMS found (calculated): m/z 410.0977 (410.0972, [M − (BPh4)2]2+), 1139.3607 (1139.3608, [M − BPh4]+). Synthesis of CO-Containing Complexes 17−21. The Rh(I) CO complexes 17−21 were synthesized by following the same procedure. CO gas was bubbled through a solution of the appropriate COD containing complex 12−16 (0.26 mmol) in dichloromethane (15 mL) for 10 min, resulting in a color change to pale yellow. In the case of complexes 19 and 21, the reactions were performed in chloroform and acetone, respectively. The resulting solution was added to a large volume of pentane (125 mL), presaturated with a CO atmosphere, resulting in precipitation of the desired product. For complex 17 concentration of the dcm/pentane solution to 20 mL under reduced pressure was necessary to precipitate the complex from solution. The precipitates were filtered, washed with pentane, and dried under vacuum. κ1-C-mono-Rh(CO)2 (17). Yield: 47%. 1H NMR (CDCl3, 600 MHz): δ 7.71 (d, 2H, J = 7.6 Hz, PhH), 7.49 (t, 2H, J = 7.6 Hz, PhH), 7.45 (t, 1H, J = 7.6 Hz, PhH), 7.27 (br s, 1H, imH), 7.16 (br s, 1H, imH), 3.95 (s, 3H, CH3) ppm. 13C{1H} (CDCl3, 150 MHz): δ 187.2 (d, JRhC = 53.9 Hz, CO), 181.1 (d, JRhC = 77.8 Hz, CO), 173.3 (d, JRhC = 41.7 Hz, NCN), 139.6 (PhC), 129.3 (PhC), 129.0 (PhC), 125.4 (PhC), 123.9 (imC), 122.8 (imC), 39.8 (CH3) ppm. FTIR (Nujol): ν 2071 (s, CO), 2006 (s, CO) cm−1. Anal. Found (calcd for C12H10IN2O2Rh): C, 32.65 (32.46); H, 2.41 (2.27); N, 6.31 (6.31). κ1-C-meta-(Rh(CO)2)2 (18). Yield: 52%. 1H NMR (CDCl3, 400 MHz, 250 K), two atropisomers identified as Ha and Hb (a/b = 0.4/ 0.6): δ 8.32 (t, 0.4H, J = 2.0 Hz, PhHa), 8.22 (t, 0.6H, J = 2.0 Hz, PhHb), 8.02 (dd, 1.2H, J = 8.0 and 2.0 Hz, PhHb), 7.93 (dd, 0.8H, J = 8.0 and 2.0 Hz, PhHa), 7.65 (t, 0.6H, J = 8.0 Hz, PhHb), 7.64 (t, 0.4H, J = 8.0 Hz, PhHa), 7.55 (d, 1.2H, J = 2.0 Hz, imHb), 7.41 (d, 0.8H, J = 2.0 Hz, imHa), 7.25 (d, 1.2H, J = 2.0 Hz, imHb), 7.23 (d, 0.8H, J = 2.0 Hz, imHa), 3.98 (s, 2.4H, CH3a), 3.96 (s, 3.6H, CH3b) ppm. 13C{1H} NMR (CDCl3, 100 MHz, 250 K), atropisomers a and b could not be distinguished between the closely spaced resonance pairs: δ 186.8 (d, JRhC = 53.4 Hz, 2 × CO), 180.5 (d, JRhC = 77.8 Hz, 2 × CO), 172.5 (d, JRhC = 41.6 Hz, NCN), 172.4 (d, JRhC = 41.6 Hz, NCN), 139.7 (PhC), 139.5 (PhC), 130.1 (PhC), 130.0 (PhC), 124.8 (PhC), 124.6 (PhC and imC), 124.5 (imC), 122.7 (imC), 122.5 (imC), 121.9 (PhC), 121.3

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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.5b00594. Crystallographic data (without structure factors) for the structures of 15 and 16 reported in this paper have also H


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been deposited with the Cambridge Crystallographic Data Centre (CCDC) as supplementary publication numbers 1408512 and 1408513. Copies of the data can be obtained free of charge from the CCDC (12 Union Road, Cambridge CB21EZ, U.K.; tel, +44-1223-336408; fax, +44-1223-336003; email, [email protected]. NMR spectra of ligands and complexes 9−21 and catalyzed reaction mixtures (PDF) Crystallographic data for 15 and 16 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for B.A.M.: [email protected]. Present Address §


Department of Chemistry and Biomolecular Sciences, Macquarie University, North Ryde 2109, Australia.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported under Australian Research Council’s Discovery Projects funding scheme (project number DP110101611). Financial support from the University of New South Wales and Macquarie University is gratefully acknowledged, and the Australian Government is acknowledged for a Ph.D. stipend (V.D.). We thank Katie Levick for the TEM image and Professor Andrew Weller for helpful discussions.

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