CO-Induced Methyl Migration in a Rhodium Thiophosphoryl Pincer

Article pubs.acs.org/Organometallics

CO-Induced Methyl Migration in a Rhodium Thiophosphoryl Pincer Complex and Its Comparison with Phosphine-Based Complexes: The Divergent Effects of S and P Donor Ligands Michael Montag,*,† Irena Efremenko,‡ Gregory Leitus,§ Yehoshoa Ben-David,‡ Jan M. L. Martin,‡ and David Milstein*,‡ †

Department of Biological Chemistry, Ariel University, Ariel 40700, Israel Department of Organic Chemistry, Weizmann Institute of Science, Rehovot 76000, Israel § Department of Chemical Research Support, Weizmann Institute of Science, Rehovot 76000, Israel ‡

S Supporting Information *

ABSTRACT: A cationic Rh(III) aryl-methyl complex, bearing a thiophosphorylbased SCS-type pincer ligand, undergoes facile migratory insertion upon addition of CO, thereby affording a Rh(III) aryl-acetyl complex. This reactivity diverges from that of structurally analogous Rh(III) aryl-methyl complexes of phosphinebased PCP- and PCN-type pincer ligands, which have been previously shown to undergo C−C reductive elimination upon addition of CO, thereby giving C−C agostic Rh(I) complexes. A comparative DFT study of CO migratory insertion in the SCS and PCP complexes reveals that the difference in reactivity originates from the higher electrophilicity of the sulfur atoms of the SCS ligand relative to the phosphorus atoms of the PCP ligand. This leads to decreased Rh → CO π backdonation in the SCS system, resulting in CO labilization and facilitating the metalto-CO methyl migration. In the PCP system it leads to stronger Rh−CO bonding, which enhances OC → Rh → arene σ donation, thereby weakening the Rh−Cipso bond and facilitating metal-to-Cipso methyl migration.

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INTRODUCTION We have recently reported that the thiophosphoryl-based, SCStype ligand 1 (Scheme 1) undergoes room-temperature oxidative addition of an sp2−sp3 C−C bond upon reaction with the cationic Rh(I) precursor [Rh(COE)2(acetone)2]BF4 (COE = cyclooctene), affording the Rh(III) aryl-methyl complex 2 (Scheme 1).1,2 This reaction was found to be both facile and highly selective, yielding no C−H cleavage products, even though sp2 and sp3 C−H bonds in ligand 1 are more numerous and accessible for activation than the cleaved C−C bond. The experimental work was complemented by a DFT study, which showed that the chemistry of the SCS-Rh system is governed by significant π repulsion between occupied rhodium d orbitals and the sulfur lone-pair electrons and that

this is the root cause of the observed selectivity. It was also shown that the reaction pathways to both C−C and C−H cleavage originate from a common intermediate that features a novel η3 C−C−H agostic interaction. Herein, we continue to explore the effects of electron richness on the reactivity of the SCS-Rh system by introducing carbon monoxide as a ligand. CO is generally considered a strong π acceptor and would commonly be expected to lower π repulsion. However, we have previously demonstrated computationally that in cationic complexes CO can actually behave as both a σ and π donor, with only weak π acceptor character, and may therefore increase metal−ligand repulsion.3 These conclusions were supported by experimental observations involving complexes A and B (Scheme 2a),3,4 which are cationic Rh(III) complexes of phosphine-based, PCP-type pincer ligands that are structurally similar to 2, but exhibit a hydride instead of methyl ligand. Addition of one equivalent of CO to these complexes resulted in C−H reductive elimination, due to enhanced Caryl−Rh−CO π repulsion, yielding the corresponding C−H agostic Rh(I) complexes C and D. Earlier work by our group showed that the same type of reactivity is also observed for the cationic aryl-methyl PCP-Rh(III) complexes E and G (Scheme 2b),5 as well as PCN-Rh(III)

Scheme 1. sp2−sp3 C−C Cleavage upon Reaction of Ligand 1 with the Cationic Rh(I) Precursora 1,2

a

Received: August 29, 2013 Published: November 1, 2013

COE = cyclooctene. © 2013 American Chemical Society

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Scheme 2. Previously Reported Reactions of CO with Cationic PCP-Rh(III) Aryl-Hydrido3,4 (a) and Aryl-Methyl5 (b) Complexes, Affording the Corresponding C−H and C−C Agostic Complexes; Similar Reactivity Was Also Invoked for a Cationic PCN-Rh(III) Aryl-Methyl Complex (c),6 but the C−C Agostic Species Was Not Directly Observed

Scheme 3. Reaction of 2 with CO to Afford Acetyl Complexesa

a

solv = acetone, methanol, acetonitrile.

complex I (Scheme 2c),6 wherein addition of CO led to C−C reductive elimination, yielding the corresponding C−C agostic Rh(I) complexes F, H, and J.7,8 In light of the documented effect of CO on the phosphinebased pincer systems, the structurally analogous complex 2 was also expected to undergo C−C reductive elimination, affording a C−C agostic Rh(I) complex. However, such a reaction was not observed, and instead the methyl ligand of 2 was found to migrate to the newly introduced CO ligand, thereby giving a Rh(III) acetyl complex. This intriguing result prompted us to examine the reaction in greater detail, as well as conduct a computational comparison between the seemingly similar phosphine- and thiophosphoryl-based pincer systems. This study revealed that the outcome of methyl migration is strongly influenced by the nature of bonding between rhodium and the pincer ligand, such that for each type of ligand (SCS or PCP) a

different metal−ligand bond is activated upon CO coordination, and the methyl ligand migrates in different directions.

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EXPERIMENTAL RESULTS Reaction of 2 with CO at Room Temperature. Treatment of an acetone or methanol solution of 2 with one equivalent of CO at room temperature resulted in facile migratory insertion of CO, affording the new Rh(III) aryl-acetyl complex 3 within minutes, as shown in Scheme 3. This reaction also took place in acetonitrile, but required heating, e.g., a few minutes at 80 °C.9 The 31P{1H} NMR spectrum of the new complex in CD3CN features a doublet at 87.67 ppm (2JRhP = 1.4 Hz), and its 1H NMR spectrum exhibits a singlet at 2.50 ppm, which is due to the methyl moiety of the acetyl ligand. In the 13C{1H} NMR spectrum this methyl group gives rise to a singlet at 39.93 ppm, and the carbonyl moiety is represented by a doublet at 223.76 ppm (1JRhC = 27.5 Hz). Furthermore, the 7164


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Table 1. Crystallographic Data for Complexes 3 and 8 and Ligand 5 formula fw, g mol−1 space group cryst syst a, Å b, Å c, Å α, deg β, deg γ, deg cell volume, Å3 Z D (calc), g cm−3 μ, mm−1 temp, K radiation R1 (I > 2σ(I)), % R1 (all data), % CCDC no.

3

5

8

C24H40BF4N2OP2RhS2 688.36 Pca21 orthorhombic 17.737(4) 10.480(2) 16.782(3) 90.00 90.00 90.00 3119.5(11) 4 1.466 0.828 120(2) Mo Kα (λ = 0.71073 Å) 2.72 2.95 745235

C18H32P2S2 374.50 P21/c monoclinic 7.3990(15) 22.164(4) 13.295(3) 90.00 102.82(3) 90.00 2125.9(8) 4 1.170 0.397 120(2) Mo Kα (λ = 0.71073 Å) 5.73 10.13 745234

C38H68I2P4Rh2S4 1236.66 P21/n monoclinic 11.701(2) 16.033(3) 12.540(3) 90.00 96.54(3) 90.00 2337.2(8) 2 1.757 2.369 120(2) Mo Kα (λ = 0.71073 Å) 4.05 5.66 745233

infrared spectrum of 3 features a strong C−O stretch band at 1717 cm−1, which is characteristic of an acyl group. Finally, the proposed structure of this complex, as shown in Scheme 3, was confirmed by crystallographic analysis, as detailed below. Crystals of 3 suitable for X-ray diffraction were grown at −20 °C from an acetonitrile solution overlaid with diethyl ether (crystallographic data are presented in Table 1). An ORTEP representation of the complex is depicted in Figure 1. As

Treatment of complex 2 with excess CO, instead of just one equivalent, afforded complex 4, which is simply a monocarbonyl adduct of complex 3, as shown in Scheme 3. The 31 1 P{ H} NMR spectrum of 4 in acetone-d6 exhibits a doublet at 100.29 ppm (2JRhP = 2.8 Hz), which is shifted significantly downfield of complex 3 (singlet at 82.93 ppm in acetone-d6). The 13C{1H} NMR spectrum of 4 features two carbonyl resonances, a doublet at 206.15 ppm (1JRhC = 26.5 Hz) for the acetyl group and a doublet-of-triplets at 185.63 ppm (1JRhC = 45.8 Hz, 3JPC = 17.1 Hz) for the terminal CO ligand. The cis configuration of the two carbonyl moieties was established by incorporating 13C labels at both carbonyls12 and noting that no significant 13C−13C coupling is observed between the two ligands.13 The remaining coordination site in complex 4 (trans to the acetyl group) is either empty or occupied by a solvent molecule, depending on the coordinating ability of the solvent.14 It is also noteworthy that the terminal CO ligand of 4 is labile, since it was easily removed by purging a solution of this complex with argon or treating the complex with excess acetonitrile. The latter reaction was found to result in the acetonitrile-stabilized complex 3 (see crystal structure in Figure 1), which could also be prepared directly from 2, by treating an acetonitrile solution of the latter with excess CO. Stepwise Synthesis of 2 for the Incorporation of a 13C Label at the Methyl Ligand. In order to facilitate the investigation of the migratory insertion reaction, we sought to introduce a 13C label at the methyl ligand of 2. To this end, we devised a stepwise synthesis of this complex that includes the oxidative addition of CH3I to a Rh(I) intermediate. In this manner, an isotopically labeled methyl ligand can be conveniently installed by using the readily available 13C-labeled alkyl halide. The complete preparative pathway is illustrated in Scheme 4 and is briefly described below. It should be noted that, unlike the reaction of 2 with CO, the reactivity of the SCSRh system that is reflected in the following sequence of reactions is analogous to previously reported phosphine-based pincer complexes of rhodium.15 The synthetic route to complex 2 began with a reaction between [Rh(COE)2(acetone)2]BF4 and the new thiophosphoryl ligand 5. This ligand is very similar to 1, but bears a

Figure 1. ORTEP drawing of complex 3 (50% probability level). All hydrogen atoms and the BF4− counterion were omitted for clarity. Selected bond distances (Å) and angles (deg) for complex 3: Rh1−C1, 2.021(4); Rh1−C23, 1.997(4); Rh1−S1, 2.3656(10); Rh1−S2, 2.3769(9); Rh1−N1, 2.259(4); Rh1−N2, 2.148(3); C1−Rh1−C23, 88.71(16); C1−Rh1−S1, 90.49(11); C1−Rh1−S2, 92.29(11); C1− Rh1−N1, 92.52(15); C1−Rh1−N2, 174.79(16).

expected, the metal ion is situated in an octahedral environment, defined by a meridionally coordinated SCS ligand, an axial acetyl moiety, and two acetonitrile ligands.10 The BF4− counterion is located outer-sphere, as was also indicated by the solution 19F{1H} NMR spectrum of complex 3 in either methanol, acetone, or acetonitrile.11 7165


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Scheme 4. Stepwise Synthesis of Complex 2: (a) sp2 C−H Cleavage upon Reaction of Ligand 5 with the Cationic Rh(I) Precursor; (b) Preparation of 2 by Deprotonation of 6b, Addition of CH3I, and Abstraction of the Iodide Ligands

such as KOtBu, thereby affording the corresponding Rh(I) species.17 The latter are stabilized by available ancillary ligands, such as N2 from the glovebox atmosphere, solvent molecules, or added phosphines. In order to apply this procedure to complex 6a, its acetone ligands were first substituted by acetonitrile to avoid unwanted reactions between acetone and the base. The obtained complex, 6b, was then treated with KOtBu in a THF solution containing excess acetonitrile, yielding the neutral, acetonitrile-stabilized Rh(I) complex 7 (Scheme 4).18 The 31P{1H} NMR spectrum of this complex in CD3CN features a sharp doublet at 77.29 ppm (3JRhP = 4.5 Hz), and its 1H NMR spectrum indicates a C2v molecular symmetry, which is consistent with a square-planar geometry around the rhodium ion.19 Complex 7 exhibited significant thermal instability in the absence of excess acetonitrile, undergoing extensive decomposition upon removal of the solvent under vacuum at room temperature. However, even in neat acetonitrile this complex underwent partial decomposition over prolonged periods at room temperature, and therefore full NMR characterization was accomplished at low temperature (−40 °C; see Experimental Section). Addition of CH3I to complex 7, which was generated in situ in a THF/CH3CN solution, afforded the neutral, dimeric Rh(III) complex 8, as shown in Scheme 4. Unlike its Rh(I) precursor, this coordinatively saturated complex was stable and isolable in pure form. Its 31P{1H} NMR spectrum in CD2Cl2 exhibits a slightly broadened singlet at 98.33 ppm (Δν1/2 = 19 Hz), and its 1H NMR spectrum features a singlet at 1.69 ppm, corresponding to the metal-bound methyl group. This methyl moiety also gives rise to a broad multiplet at −7.29 ppm in the 13 C{1H} NMR spectrum. Overall, the solution NMR spectra of complex 8 indicate a single mirror plane that bisects its molecular structure along the CAr−Rh bonds, perpendicular to the plane of the aromatic rings. Finally, the dimeric nature of the complex, as presented in Scheme 4, was corroborated by its crystal structure, which is shown in Figure 3. Crystals of complex 8 suitable for X-ray diffraction were isolated from a benzene solution that was heated at 80 °C (see Table 1 for the crystallographic data). The complex, which crystallized in the P21/n monoclinic space group, exhibits a dimeric structure of C2h symmetry. The two monomeric parts are symmetry-related through an inversion point located at the center of the Rh1−I1−Rh1a−I1a ring. Each monomer exhibits a rhodium atom in an octahedral environment, with the equatorial positions being occupied by the meridionally

Figure 2. ORTEP drawing of SCS ligand 5 (50% probability level). All hydrogen atoms were omitted for clarity. Selected bond distances (Å) and angles (deg) for ligand 5: P1−S1, 1.9600(10); P2−S2, 1.9643(11); C6−P1, 1.828(3); C2−P2, 1.823(3); C1−C6−P1, 120.2(2); C1−C2−P2, 121.1(2); C6−P1−S1, 113.90(10); C2−P2− S2, 112.04(10).

hydrogen atom instead of a methyl moiety between the ligand arms. Its crystal structure is depicted in Figure 2, and its synthesis is described in the Experimental Section. In analogy to ligand 1, when 5 was mixed with an equivalent amount of the Rh(I) precursor in acetone at room temperature, facile sp2 C− H cleavage took place,16 affording the Rh(III) aryl-hydrido complex 6a. The 31P{1H} NMR spectrum of this complex in CD3OD features a broad singlet at 79.15 ppm (Δν1/2 ≈ 70 Hz), and its 1H NMR spectrum is consistent with a meridionally coordinated pincer ligand and an axial hydride ligand (Cs molecular symmetry). The 1H NMR signal for this hydride appears as a sharp doublet at −23.45 ppm (1JRhH = 22.5 Hz). Such a high-field resonance is characteristic of a hydride ligand trans to a vacant coordination site or to a loosely coordinated ligand, such as methanol. No significant coordination of the BF4− counterion was observed by 19F NMR spectroscopy,11 indicating that the two remaining coordination sites at the metal center are likely occupied by solvent molecules. It has been previously shown that Rh(III) aryl-hydrido complexes that are structurally similar to 6a, but bear phosphorus-donor-based pincer ligands instead of SCS, undergo hydride ligand deprotonation upon treatment with a base 7166


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at room temperature, reaching completion within only a few minutes and affording complex 3 as the only observed product. However, when the reaction was carried out at low temperatures, it was possible to identify and fully characterize two intermediates along the reaction pathway, as shown in Scheme 5. Thus, a methanol solution of 2 was treated with one equivalent of CO at −80 °C (dry ice/acetone), and the sample was then examined by NMR spectroscopy at −70 °C, revealing the formation of complex 9. At this temperature, the 31P{1H} NMR spectrum of 9 features a singlet at 81.36 ppm, and its 1H NMR spectrum exhibits a doublet at 1.26 ppm (2JRhH = 1.3 Hz), which is associated with the methyl ligand. This ligand also gives rise to a doublet at 21.03 ppm (1JRhC = 15.4 Hz) in the 13C{1H} NMR spectrum. Such a low-field resonance for the methyl ligand suggests that it is trans to the CO ligand,21 and this was corroborated by 13C-labeling at both the CO and methyl ligands.22 The doubly labeled complex exhibits a large 13 C−13C coupling between the two ligands, amounting to 38.5 Hz, and this is consistent with a trans configuration.13 It follows that these ligands are also positioned cis to the arene ipso carbon. The coordination site trans to the ipso carbon is most probably occupied by a solvent molecule (methanol), as was indicated by computational examination of this system (see below). The BF4− counterion is not coordinated to a significant extent, as indicated by the 19F{1H} NMR spectrum.11 When the solution containing complex 9 was warmed from −70 to −40 °C, ligand rearrangement took place to give isomeric complex 10, in which CO is cis to the methyl ligand and trans to the ipso carbon. The 31P{1H} NMR spectrum of this complex at −40 °C exhibits a singlet at 100.08 ppm, and its 1 H NMR spectrum features the methyl ligand signal as a doublet at 0.99 ppm (2JRhH = 2.2 Hz). This ligand is represented in the 13C{1H} NMR spectrum by a doublet at −7.77 ppm (1JRhC = 21.7 Hz). The cis configuration of the carbonyl and methyl ligands is clearly indicated by the absence of observable 13C−13C coupling in the doubly labeled complex 10.13 That the CO ligand is trans to the ipso carbon is supported by the fact that the 13C{1H} NMR signal associated with this ligand (doublet-of-triplets at 186.92 ppm) exhibits a large three-bond 13C−31P coupling of 17.9 Hz, whereas the methyl signal shows no such coupling. The coordination site trans to the methyl ligand is probably occupied by a methanol molecule, as was indicated by our computational results (see below). According to the 19F{1H} NMR spectrum the BF4− counterion is not coordinated to a measurable extent.11 Finally, when the solution containing complex 10 was allowed to warm to room temperature, facile CO migratory insertion took place, giving acetyl complex 3 within minutes. It is this last step, which can be viewed as the migration of CH3− from rhodium to the CO ligand, that sets the SCS system apart from its PCP and PCN analogues (Scheme 2). As mentioned

Figure 3. ORTEP drawing of complex 8 (50% probability level). All hydrogen atoms were omitted for clarity. Selected bond distances (Å) and angles (deg) for complex 8: Rh1−C1, 1.989(4); Rh1−C19, 2.185(5); Rh1−S1, 2.3679(13); Rh1−S2, 2.3728(13); Rh1−I1, 2.7966(8); Rh1−I1a, 2.8691(7); P1−S1, 2.0036(17); P2−S2, 2.0076(16); C1−Rh1−C19, 86.27(17); C1−Rh1−I1a, 93.53(12); C1−Rh1−I1, 178.02(13); C1−Rh1−S1, 92.14(13); C1−Rh1−S2, 92.39(13).

coordinated pincer ligand and an iodide ligand, and the axial positions being occupied by the methyl ligand and second iodide. It is noteworthy that each iodide ligand exhibits two markedly different Rh−I distances, such that the Rh−I bond trans to the aryl moiety (Rh1−I1 = 2.7966(8) Å) is shorter than the bond trans to the methyl ligand (Rh1a−I1 = 2.8690(7) Å). This reflects the stronger trans influence of the methyl ligand relative to the aryl.20 Finally, complex 2 was generated from 8 by abstraction of the iodide ligands with two equivalents of AgBF4 in acetone. As mentioned above, utilizing 13C-labeled CH3I allowed the installment of labeled methyl ligands in 8, and this ultimately led to the desired isotopically labeled complex 2. This, in turn, was highly instrumental in the low-temperature study of the migratory insertion reaction, expediting the structural elucidation of the observed reaction intermediates. By measuring the magnitude of the 13C−13C coupling between the methyl and CO ligands in the intermediates, it was possible to determine the relative positions of these ligands. The structural information obtained from these experiments was subsequently employed in our computational study of the migratory insertion reaction, as will be elaborated below. Low-Temperature Investigation of the Reaction of 2 with CO and Characterization of the Observed Migratory Insertion Intermediates. As described above, the reaction of 2 with CO in acetone or methanol is very facile

Scheme 5. Detailed Mechanism of the Reaction of 2 with CO in Methanola

a

The 13C labels incorporated into the CH3 and CO ligands are marked in bold. 7167


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above, addition of CO to the latter complexes resulted in Cipso− Calkyl reductive elimination to give C−C agostic Rh(I) complexes. This can be viewed as the migration of CH3+ from rhodium to the ipso carbon of the aryl ligand. The significant difference in reactivity between the SCS and phosphine-based systems, which is illustrated in Scheme 6,

Scheme 7. In the interest of computational brevity, the isopropyl substituents on all phosphorus atoms were replaced by methyl groups. In all of the carbonyl complexes the CO ligand is bound extremely strongly, as is clearly evident by comparing their energies with those of the parent complexes 11 and 15. Isomers 13 and 17, with CO trans to the aryl, are the most stable species in their respective complex groups, either with or without a coordinated solvent molecule. In all of the examined complexes methanol coordination was found to have a stabilizing effect. In the SCS system, methanol stabilization of 12 and 14 (by 11.2 and 11.5 kcal mol−1, respectively) is much larger than its effect on 13 (2.4 kcal mol−1). In the PCP system, inclusion of a solvent molecule in 16 and 17 stabilizes them to a nearly identical extent (9.1 and 8.5 kcal mol−1, respectively). Its influence on the stability of complex 18 was found to be critical, since in the absence of coordinated methanol this complex was not identified as a minimum on the potential energy surface (PES). The above results explain the sequence of reactions observed when a methanol solution of complex 2 was treated with CO at low temperature. In complex 13, the lowest energy isomer, the CO ligand is positioned trans to the aryl. However, in the initial complex 11 this position is occupied by a strongly bonded solvent molecule, with a binding energy of 31.3 kcal mol−1. Thus, CO addition to 11 should first lead to complex 12, the model analogue of complex 9. Indeed, at −70 °C complex 9 was the only observed product. The added CO strongly labilizes the coordinated methanol, facilitating its dissociation from 12, and the resulting unsaturated complex (with L = vacant site) readily rearranges to 13 with an energy gain of 10.0 kcal mol−1. This is followed by methanol coordination to the vacant site trans to the methyl group, leading to an additional decrease in energy of 2.4 kcal mol−1. Thus, in the experimental setting, raising the temperature of the solution from −70 to −40 °C resulted in rearrangement of complex 9 to its more

Scheme 6. Schematic Representation of the Net Methyl Migration in Cationic Rh(III) Carbonyl Complexes of SCSand PCP-Type Pincer Ligandsa

a

The SCS complex undergoes CO migratory insertion to yield a Rh(III) acyl species, whereas the PCP complex undergoes C−C reductive elimination to afford a C−C agostic Rh(I) species.

prompted us to examine these systems computationally, in an attempt to understand how the nature of the pincer ligand influences the electronic structure and chemical behavior of the complexes.

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COMPUTATIONAL RESULTS Stability of the SCS and PCP Aryl-Methyl-Carbonyl Complexes in Methanol. The relative energies of the CO adducts of the SCS and PCP aryl-methyl complexes, with and without a coordinated molecule of methanol, are shown in

Scheme 7. Relative Energies (ΔG298, kcal mol−1) of the Isomeric CO Adducts of the SCS and PCP Aryl-Methyl Complexes (L = vacant site, methanol)a

a

Complexes 11 and 15 without ancillary ligands (L = vacant site) are taken as the reference energy points. 7168


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Scheme 8. Computed Reaction Pathways for Methyl Migration in the SCS (top) and PCP (bottom) Cationic Pincer Systems

Figure 4. Calculated reaction profiles for methyl migration in the SCS (a) and PCP (b) systems in methanol solution, in the absence and presence of coordinated solvent molecules. Complexes 13 and 17 without ancillary ligands are taken as the reference energy points.

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Figure 5. Optimized geometries of the transition states for methyl migration in the SCS aryl-methyl-carbonyl complex 13, in the absence of coordinated solvent molecules. Atoms are color-coded as follows: Rh, green; S, yellow; P, orange; O, red; C, large gray spheres; H, small gray spheres.

Figure 6. Optimized geometries of the transition states for methyl migration in the PCP aryl-methyl-carbonyl complex 17, in the absence of coordinated solvent molecules. Atoms are color-coded as in Figure 5.

migration to either the CO ligand or arene exhibit prohibitively high activation barriers at room temperature (31.7 and 33.4 kcal mol−1, respectively). By contrast, in the absence of coordinated solvent both transition states are significantly lower in energy, as are the apparent activation barriers for the formation of acetyl complex 19 (18.4 kcal mol−1) and η3 C− C−H complex 20 (25.2 kcal mol−1). Thus, loss of coordinated methanol, which raises the energy of 13 by only 2.4 kcal mol−1, should precede the methyl migration step. Similarly, solvent dissociation is of critical importance to methyl migration in the PCP system. First, methanol coordination to 23 is decidedly unfavorable, leading to a highly unstable structure (see above). Second, in the presence of coordinated solvent the conversion of 17 to 22 has an activation barrier of 37.0 kcal mol−1, which is practically insurmountable under ambient conditions. However, upon solvent dissociation from 17, which requires a relatively low input of energy (8.5 kcal mol−1), the apparent activation barriers become accessible for the formation of both η3 C−C− H complex 23 (24.6 kcal mol−1) and acetyl complex 22 (24.1 kcal mol−1). It should be noted that the importance of solvent dissociation was manifested in the experimental observation that in methanol or acetone solutions CO migratory insertion (2 → 3) occurred readily at ambient temperature, but required heating when the more strongly coordinated acetonitrile was used as solvent (see above). Finally, to summarize the above results, the calculated reaction profiles indicate that in the SCS system formation of the CO-insertion product (acetyl complex) is favored both kinetically and thermodynamically, whereas in the PCP system formation of the C−C reductive-elimination product (η3 C−C−H complex) is essentially thermodynamically controlled. Moreover, solvent dissociation renders all of the above reactions possible at ambient or even lower temperatures.

stable isomer, 10. At no point was the experimental analogue of complex 14 observed, in accordance with its predicted instability. Reaction Profiles for Methyl Migration. With knowledge of the structure and composition of the lowest energy SCS and PCP aryl-methyl-carbonyl complexes, we are now at a position to model the methyl migration step. As shown in Scheme 8, for each aryl-methyl-carbonyl precursor (13 and 17) two possible directions for methyl migration were considered, as follows: (a) toward the aromatic ring, with formation of an η3 C−C−H agostic Rh(I) complex (20 and 23) or an η1 Rh−Cipso complex (21 and 24); and (b) toward the carbonyl ligand with formation of an acetyl Rh(III) complex (19 and 22). The calculated reaction profiles for methyl migration are shown in Figure 4. The most salient feature of these energetic results is the difference in the lowest energy species of the two systems. In the SCS system it is the acetyl complex 19 with a coordinated methanol molecule (19·MeOH),23 whereas in the PCP system it is the η3 C−C−H complex 23, without coordinated solvent. Attempts to optimize the geometry of 23 with coordinated solvent (23·MeOH) led to either methanol dissociation or breaking of the aryl-methyl C−C bond, with formation of 17·MeOH. Similarly, the η1 Rh−Cipso complex 24 was not found as a minimum on the corresponding PES, with or without a coordinated solvent molecule. Among the SCS complexes, only the η3 C−C−H complex 20·MeOH was not found as a minimum. It should be noted that in the absence of coordinated solvent complex 20 is virtually isergonic with 21,24 and the two can easily interconvert, being separated by a very small activation barrier of only 1.8 kcal mol−1.25 As discussed above, the most stable SCS aryl-methylcarbonyl species, 13, involves a coordinated solvent molecule. For this species, the calculated transition states for methyl 7170


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Figure 7. QTAIM atomic charges for selected atoms in the SCS complexes 11 and 13 (a) and the PCP complexes 15 and 17 (b), all without coordinated solvent. The numbers in parentheses pertain to the parent complexes in the absence of CO (11 and 15). Total charges are given for the methyl group. Bond critical points (BCP) are shown as small, light green spheres. Atoms are color-coded as in Figure 5.

Transition States for Methyl Migration. Figure 5 depicts the optimized geometries of the two transition states involved in methyl migration in the SCS carbonyl complexes shown in Figure 4, without coordinated solvent molecules. The most striking feature of these structures is that although the most stable aryl-methyl-carbonyl isomer, complex 13, exhibits a CO ligand trans to the aryl, transition states TS(13→19) and TS(13→20) exhibit CO ligands cis to the aryl. In fact, the geometries of the two transition states are close to those of 14 and 12, respectively, which are much less stable than 13 (Scheme 7). When a coordinated methanol ligand is present in the ground-state complex (13·MeOH), it occupies the position cis to the aryl, thereby hindering the movement of CO to this position and causing a strong rise in both activation barriers (see above).26 Similar results were obtained for methyl migration toward the CO ligand in the PCP complex 17, with and without coordinated methanol. In transition state TS(17→22) the methyl ligand is positioned trans to the aryl, whereas the CO ligand attains a cisoid configuration with respect to the aryl. As in the SCS system, the presence of coordinated methanol significantly increases the activation barrier for methyl migration toward CO (Figure 4). Interestingly, for methyl migration toward the aromatic ring we found two transition states with almost the same energy, TS(17→23)1 and TS(17→ 23)2 (Figure 6). TS(17→23)1 is similar to that of the SCS system, with the CH3 and CO ligands trans to each other, whereas in TS(17→23)2 the CO ligand is positioned trans to the aryl. Atomic Charge Distribution in the SCS and PCP Complexes. The different behavior of the SCS and PCP systems can be traced to the electronic properties of the pincer ligand. In our previous report1 we showed, using charge decomposition analysis, that the two sulfur atoms of the SCS ligand bring about significant π-repulsive interactions with the electron-rich Rh cation. In the present work we apply the quantum theory of atoms in molecules (QTAIM)27 for a deeper investigation of the electronic factors behind the different reactivity observed for the SCS and PCP systems. Although modern quantum chemical methods yield accurate and unequivocal values for various molecular properties, extraction of chemically relevant concepts from such calculations is usually challenging and ambiguous.28 These

difficulties arise from the fact that the basic concepts used in general chemistry often do not correspond to quantum mechanical observables. Thus, in quantum mechanics a molecule is treated as a set of nuclei placed in an electron cloud, whereas in general chemical theories a molecule is considered as a set of atoms linked by a network of bonds. QTAIM combines these two different approaches in a physically meaningful fashion. Based on topological analysis of electron density (ρb), QTAIM rigorously defines reliable, accurate, consistent, and stable properties of atoms and their bonding in a molecule.29 Notably, Bader and Matta showed that, in contrast to most traditional partitioning schemes, in QTAIM “atomic charges are measurable quantum expectation values”30 that reflect the principal electron transfer within a molecule. Atomic charges calculated using QTAIM for selected atoms of the PCP and SCS aryl-methyl complexes, in the absence and presence of CO, are shown in Figure 7. A comprehensive list of atomic properties for selected calculated SCS and PCP complexes can be found in the Supporting Information (SI). It is clearly apparent that in the PCP complexes the Rh ion is bound to strongly positively charged P atoms (+1.81 in 15, +1.85 in 17), whereas in the SCS complexes it is bound to S atoms with a high negative charge (−0.81 in 11, −0.69 in 13). This results in a significantly higher charge on Rh in the latter case (+0.53 in 11 vs +0.36 in 15). Another salient difference between the two ligands is that in the SCS complexes high negative charge accumulates on the Cortho atoms adjacent to Cipso (−0.69 in 11 vs −0.02 in 15) and, consequently, on the aryl group as a whole (−1.27 in 11 vs −0.16 in 15). The higher total electron density on the aryl is accompanied by a lower charge on Cipso (−0.07 vs −0.15, respectively). Natural population analysis (NPA), which was employed in our previous reports, supports these diverging charge distributions in the SCS and PCP systems, showing that the Cortho atoms bear charges of −0.55 in 11 vs −0.05 in 15, whereas the Cipso atoms of these complexes bear charges of −0.11 and −0.21, respectively. CO is usually regarded as a weak σ donor and strong π acceptor. This is certainty true for anionic and neutral carbonyl complexes, but, as we have previously shown,3 in cationic latetransition-metal complexes, such as the ones described herein, one should expect CO to exhibit significant donation, with only 7171


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Table 2. Topological Parameters of Bond Critical Points (Electron Density, ρb; Laplacian of Electron Density, ∇2ρb; Total Energy Density, Hb; ellipticity, ε) and Delocalization Indices DI(A,B) for Selected Bonds in the SCS and PCP Aryl-Methyl Complexes, in the Absence (11, 15) and Presence (13, 17) of the CO Liganda property ρbBCP

ligand type

complex

Rh−Cipso

Cipso−Cortho

Rh−P/S

Rh−CCH3

SCS

11 13 15 17 11 13 15 17 11 13 15 17 11 13 15 17 11 13 15 17

0.155 0.124 0.149 0.129 0.117 0.176 0.082 0.127 −0.079 −0.053 −0.075 −0.063 0.066 0.033 0.064 0.045 1.122 0.859 1.102 0.904

0.294 0.294 0.297 0.294 −0.671 −0.673 −0.688 −0.677 −0.262 −0.264 −0.266 −0.276 0.164 0.134 0.189 0.169 1.330 1.348 1.309 1.311

0.085 0.085 0.100 0.101 0.203 0.187 0.125 0.100 −0.024 −0.024 −0.039 −0.039 0.071 0.019 0.022 0.040 0.867 0.861 0.830 0.810

0.142 0.126 0.138 0.122 0.035 0.024 0.040 0.028 −0.069 −0.057 −0.066 −0.055 0.013 0.025 0.005 0.011 1.056 0.992 1.058 0.984

PCP ∇2ρbBCP

SCS PCP

HbBCP

SCS PCP

εBCP

SCS PCP

DI(A,B)

SCS PCP

a

Rh−CCO

C−O

0.132

0.484

0.133

0.479

0.455

1.210

0.461

1.125

−0.055

−0.872

−0.053

−0.866

0.022

0.002

0.026

0.005

0.996

1.595

1.057

1.575

All values are in atomic units. Coordinated solvent was excluded in all cases.

Figure 8. Laplacian of the electron density in complexes 11 (a, b) and 15 (c, d), viewed along the plane defined by the atoms Rh, Cipso, and P/S (a, c) and along the plane defined by the atoms Rh, CCH3, and P/S (b, d). The small green spheres indicate BCPs. Solid lines denote local charge concentration (∇2ρb 0). The charge magnitude is denoted by different colors. The range of Laplacian values is from −800 to 800 au.

of only 0.08 e− in the PCP complex 17 and virtually no net electron transfer in the SCS complex 13. For comparison, the M→CO electron transfer in the neutral complexes Cr(CO)6, Fe(CO)5, and Ni(CO)4 was found to range from 0.11 to 0.19 e−.31 Nonetheless, despite the minute net electron transfer in the SCS and PCP complexes, the charges on the C and O

weak electron-withdrawing ability. Indeed, applying NPA, as done in our previous reports, the CO ligand of both complexes 13 and 17 is found to bear a positive charge of +0.44, clearly indicating much stronger electron donation than electron withdrawal. A more subtle picture emerges with QTAIM, with atomic charges implying a small net Rh→CO electron transfer 7172


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atoms notably decrease in absolute value (with respect to qC = 1.22 and qO = −1.22 in free CO), indicating a coordinationinduced decrease of CO polarization. This results in accumulation of charge on Cipso, which in the PCP system is the main effect of CO on charge distribution, and also causes depletion of electron density on Rh and the P or S atoms, which is especially pronounced in the SCS system. Metal−Ligand Bonding in the SCS and PCP Complexes. In QTAIM, chemical bonds are characterized by their delocalization indices (DI) and the properties of the bond critical points (BCPs; Figure 7),32 namely, electron density (ρb), Laplacian of electron density (∇2ρb), total energy density (Hb), and ellipticity (ε). Each of these parameters was calculated for the representative SCS complexes 11 and 13, and the PCP complexes 15 and 17, and will be shortly addressed below. The values of these parameters at selected BCPs are summarized in Table 2. Additional information regarding BCPs in selected calculated complexes can be found in the SI. In general, an electron density of ρb ≈ 0.20 au at a BCP is typical of a single covalent bond, whereas ρb < 0.1 au characterizes “closed-shell bonding” (ionic, van der Waals, hydrogen).33 The ρb values at the BCPs of rhodium bonding with the aryl, methyl, and CO ligands fall in the range of covalent bonds with significant ionic character. The Rh−P interaction in the PCP complexes is consistent with ionic bonding with a small covalent character, whereas Rh−S bonding in the SCS complexes appears to be mostly ionic. In both types of complexes the ρb values for the Cipso−Cortho BCPs are typical of aromatic bonds. As for the effect of CO binding on electron density, it is clearly apparent in both the SCS and PCP systems that introduction of the CO ligand trans to the aryl decreases the electron density in the Rh−Cipso and Rh− CCH3 BCPs, whereas the influence of CO on the other metal− ligand bonds is negligible. The Laplacian of the electron density (∇2ρb) indicates the regions of electron density accumulation (∇2ρb < 0) and depletion (∇2ρb > 0) with respect to the immediate neighborhood and is usually deemed the most informative parameter of QTAIM.27 The maps of the Laplacian distribution in complexes 11 and 15, presented in Figure 8, exhibit bonding features that are typical of transition metal complexes. That is to say, these maps show charge polarization toward a region of locally increased Lewis acidity at the metal center, in accordance with the concept of “ligand-opposed” charge concentrations. In all of the complexes considered in this work, the BCPs for Rh bonding with its ligands are in regions of positive values of ∇2ρb, characteristic of significantly polarized bonding. The BCPs of the polar C−O bonds are shifted toward the carbon atoms, also resulting in ∇2ρb > 0. Similar behavior is commonly observed in other Rh complexes34 and in transition metal complexes in general.31 The sign and magnitude of the total energy density, Hb, reflect the mechanics of bonding.35 This index is the sum of the local potential and kinetic energies (Hb = Gb + Vb). For a covalent interaction the potential energy component dominates and Hb < 0, whereas for a mostly ionic interaction the kinetic component dominates and Hb > 0.35 The bonds involving Rh in the SCS and PCP complexes are characterized by small negative Hb values, confirming the conclusions drawn on the basis of ρb and ∇2ρb and indicating ionic bonding with slight covalent contribution, as is typical of transition metal complexes.31,34 It is noteworthy that in the SCS complexes

the ionic component is higher and the covalent component is lower than in the PCP complexes. This is also apparent from the positions of the BCPs in the first coordination sphere of the various complexes (see SI). In general, the more ionic the interaction, the closer the BCP is to the more electropositive atom in the bond. Thus, in the SCS complexes the BCPs relating to Rh and its immediate bonding partners are closer to the more electropositive atoms than in the PCP complexes. An independent measure of the covalency of a bond is the delocalization index, DI(A,B), which accounts for the electrons shared between atoms. For a purely covalent interaction this index yields a measure of the bond order.36 As the polarity of a bond increases, the delocalization index tends to decrease.37 The DI values presented in Table 2 show that all of the bonds involving the Rh atom have a significant level of sharing, with DI(Rh,X) values in the range 0.8−1.1. This degree of electron sharing is typical for covalent interactions. The calculated DI(Rh,CCO) values for 13 and 17 are 0.996 and 1.057, respectively, indicating weaker and/or more polar bonding in the SCS complexes than in their PCP analogues. The contribution of π interaction to the bonding can be assessed by the ellipticity (ε) at the BCP. This sensitive topological index gives a direct measure of the degree to which the electron density is distorted away from cylindrical (σ-type) symmetry along the bond axis.38 The low ellipticity of all rhodium−ligand bonds in the SCS and PCP complexes reflects their predominant σ character, with low to negligible π contribution, particularly in the case of the Rh−CH3 and Rh−CO bonds. The very low C−O ellipticity is consistent with the high cylindrical symmetry of this triple bond.39 It is important to note that εRh−P doubles in the presence of CO (Table 2), which implies increased π bonding, whereas εRh−S strongly decreases, implying a drop in π bonding. Furthermore, in both the SCS and PCP complexes the ellipticity of the Rh− Cipso bond decreases upon inclusion of CO, indicating a COinduced reduction in Rh−Cipso π-bonding, with this effect being much stronger in the SCS system than in the PCP one. The smaller εC−O and εRh−CO in the SCS system can also be attributed to reduced π bonding. All in all, the rhodium bonding interactions are characterized by low ρb, positive ∇2ρb, and negative Hb. In terms of Espinosa’s classification,40 such bonding belongs to the “transit closed shell” category, with some covalent contribution. All of the topological indices show that bonding in the SCS complexes is more ionic than in the PCP complexes. The weakening of the Rh−Cipso bond induced by CO, especially in the SCS complexes, points to the significant role of electron repulsion in these electron-rich systems. The smaller DI(Rh− CCO), DI(Rh−OCO), and ellipticity of the Rh−CO bond in the SCS complexes relative to the PCP ones, together with the absence of Rh→CO charge transfer in the SCS complexes, as revealed by the total atomic charges, strongly indicate that π back-donation is negligible in the SCS system. Among the selected atomic properties, the characteristics of the methyl group and of the Rh−CH3 bonding appear to be very similar in the SCS and PCP aryl-methyl complexes. It should be stressed that the general QTAIM features outlined above for each ligand type (SCS vs PCP) are common to all complexes of this ligand that are considered in the present work (see SI). Nature of Bonding and Reactivity Patterns of the SCS and PCP Carbonyl Complexes. The QTAIM analysis presented above allows us to draw a generalized, qualitative picture of bonding in the SCS and PCP complexes, as depicted 7173


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corresponding PCP complexes. This, in turn, is consistent with the smaller π back-donation, weaker Rh−CO bonding, and stronger Rh−Cipso bonding in the SCS system relative to the PCP one. The metal−ligand bonding in the SCS and PCP complexes, as outlined above, reveals the core electronic effects underlying their distinct chemical properties. The sulfur atoms exhibit higher electrophilicity than phosphorus, in line with the commonly accepted trend in electronegativity, so that the Rh-bound S atoms have large negative charges, whereas the Rhbound P atoms have large positive charges. This notable difference in donor-group electrophilicity affects the two pincer systems to such an extent that in the SCS system the oxidation state of rhodium is best described as Rh(V), whereas in the PCP system it is Rh(III). The enhanced positive charge on the metal center in the SCS complexes45 decreases Rh→CO π back-donation, and this, in turn, weakens the Rh−CO bonding and labilizes CO. By contrast, the stronger Rh−CO bonding in the PCP complexes results in enhanced OC→Rh→aryl σ donation, which weakens the Rh−Cipso bond due to repulsive interactions. The same bond in the SCS system is affected to a lesser extent. Therefore, breaking the Rh−Cipso bond in the PCP system is more favorable than in the SCS system, and, consequently, the methyl group in the PCP aryl-methylcarbonyl complex migrates to the ipso carbon, whereas in the analogous SCS complex the methyl migrates to the CO ligand.

in Scheme 9. This highlights the principal differences between the two systems and allows us to elucidate the electronic basis Scheme 9. Schematic Representation of the Bonding in the PCP (a) and SCS (b) Aryl-Methyl-Carbonyl Complexesa

a

Straight lines represent covalent bonds formed by one-electron donation from each participating atom, straight arrows represent Lewis-type dative bonds, and the curved arrow represents π backdonation.

for their divergent chemistry, as observed experimentally vis-àvis the metal-to-ligand methyl migration. The electron density distribution in the PCP complexes, as revealed by the topological analysis, is consistent with the commonly accepted view of bonding in such complexes (Scheme 9a). Thus, a Rh+ ion donates one electron to each of the covalent bonds it shares with the CH3 and aryl groups, resulting in a formal Rh(III) oxidation state. The phosphine groups and CO behave as classical Lewis bases and are bound to the metal ion by dative bonds, using the lone electron pairs of the P and C atoms. The Rh−CO interaction is further stabilized by back-donation of some π electron density. In the SCS system, on the other hand, a very different bonding scheme emerges from the topological analysis (Scheme 9b). First, in contrast to the commonly used notation (and the one used throughout this report), the P−S bond is best described as a single, polar covalent bond rather than a double bond. This is in line with previously reported results41,42 and reflects the low tendency of third-row elements, such as P and S, to engage in π bonding. Second, the Rh−S interaction is not dative but rather a single covalent bond, as are the Rh−CH3 and Rh−Cipso bonds. Third, the P−Cortho interaction appears to be dative, using the lone electron pairs on the thiophosphoryl P atoms, rather than a “regular” covalent bond. The same type of bonding also exists between CO and rhodium, in keeping with the classical role of CO as a Lewis base. All in all, the presence of four covalent bonds to rhodium (two Rh−S, one Rh−Cipso, and one Rh−CH3) implies a formal Rh(V) oxidation state in the SCS aryl-methyl complexes,43 regardless of the absence or presence of a CO ligand. In the carbonyl SCS complexes the high oxidation state of Rh renders Rh→CO π back-donation negligible, weakening the Rh−CO bonding. It is important to note that, in agreement with our QTAIM topological analysis, NPA also gives charges of opposite sign on the ligating atoms of the SCS and PCP ligands, namely, −0.21 for S and +1.66 for P. However, NPA gives no indication of the high oxidation state of the metal center and in fact yields the rather unreasonable charges of −0.35 and −0.46 for the Rh atom in the SCS and PCP complexes, respectively. Nevertheless, NPA does provide the correct relative properties of bonding in the two systems. Thus, the SCS complexes exhibit smaller Wiberg bond indices (WBIs)44 for Rh−CCO and Rh− OCO, as well as larger WBIs for Rh−Cipso, than the

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CONCLUSION We have shown that the cationic Rh(III) aryl-methyl complex 2, bearing a thiophosphoryl-based SCS-type pincer ligand, undergoes facile migratory insertion upon addition of CO, thereby affording the Rh(III) aryl-acetyl complex 3. This observed reactivity contrasts markedly with that of structurally analogous Rh(III) aryl-methyl complexes of phosphine-based PCP- and PCN-type pincer ligands, which have been previously observed to undergo C−C reductive elimination upon addition of CO, thereby giving C−C agostic Rh(I) complexes. In other words, both the thiophosphoryl- and phosphine-based systems exhibit 1,2-migration of the rhodium-bound methyl ligand upon CO coordination, but in the SCS system the methyl migrates to the newly introduced CO ligand, whereas in the PCP and PCN systems the methyl migrates to the arene ipso carbon atom. A comparative DFT study of CO migratory insertion in the SCS and PCP complexes demonstrated that the direction of methyl migration depends on the relative thermodynamic stability of the products, which differs for each type of ligand, whereas in both systems kinetic factors favor the formation of acetyl complexes. It was also found that coordinated solvent molecules kinetically stabilize the aryl-methyl-carbonyl complexes and that solvent dissociation is a mandatory prerequisite to methyl migration. A detailed analysis of the electronic structures of the SCS and PCP complexes, using the quantum theory of atoms in molecules, demonstrated that the difference in reactivity stems from the higher electrophilicity of the ligating sulfur atoms in the SCS ligand relative to the phosphorus atoms of the PCP ligand. Consequently, in the SCS system the electronic structure of the rhodium atom is consistent with a Rh(V) oxidation state, whereas in the PCP system the oxidation state is Rh(III). The higher metal oxidation state in the SCS system decreases Rh→CO π backdonation, and this weakens the Rh−CO bonding and labilizes the CO ligand, thereby facilitating metal-to-CO methyl migration. By contrast, the stronger Rh−CO bonding in the 7174


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PCP system results in enhanced OC→Rh→arene σ donation, and this weakens the Rh−Cipso bond and facilitates methyl migration to the ipso carbon atom.

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PCH(CH3)2), 1.11 (dd, 3JPH = 17.9 Hz, 3JHH = 6.8 Hz, 6H, PCH(CH3)2). Selected 13C{1H} NMR (101 MHz, acetone-d6): 36.57 (br s, COCH3). 19F{1H} NMR (376 MHz, acetone-d6): −152.42 (s, free BF4). 31 1 P{ H} NMR (162 MHz, acetone-d6, −30 °C): 82.77 (s). 1H NMR (400 MHz, acetone-d6, −30 °C): 7.67 (m, 3JHH = 7.7 Hz, 2H, Ar−H), 7.34 (m, 3JHH = 7.7 Hz, 1H, Ar−H), 3.00 (m, 2JPH = 3JHH = 6.9 Hz, 2H, PCH(CH3)2), 2.73 (m, 2JPH = 3JHH = 7.1 Hz, 2H, PCH(CH3)2), 2.69 (s, 3H, COCH3), 1.30 (dd, 3JPH = 18.3 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.19 (dd, 3JPH = 18.4 Hz, 3JHH = 6.8 Hz, 6H, PCH(CH3)2), 1.15 (dd, 3JPH = 18.0 Hz, 3JHH = 6.5 Hz, 6H, PCH(CH3)2), 1.12 (dd, 3JPH = 18.3 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2). Selected 13C{1H} NMR (101 MHz, acetone-d6, −30 °C): 36.89 (s, COCH3). 19F{1H} NMR (376 MHz, acetone-d6, −30 °C): −151.67 (s, free BF4). Synthesis of the Acetonitrile Adduct of Complex 3. 30.3 mg (0.044 mmol) of complex 2 were dissolved in 0.8 mL of acetonitrile, and the solution was loaded into an NMR tube. The tube was then fitted with a rubber septum, and CO gas was freely bubbled through the solution for 1 min (the system was kept open during this time by using a second syringe as a gas outlet). The solution was then heated at 80 °C, under excess CO, for 15 min. Excess CO was then pumped off, and the solution was added to 17 mL of diethyl ether, with stirring. The liquid phase was then decanted, and the product was washed with 4 mL of diethyl ether. Removal of residual solvent under vacuum afforded 24.4 mg (0.035 mmol, 81.2% yield) of the product as a light brown powder. 31 1 P{ H} NMR (162 MHz, CD3CN): 87.67 (d, 2JRhP = 1.4 Hz). 1H NMR (400 MHz, CD3CN): 7.37 (m, 3JHH = 7.7 Hz, 2H, Ar−H), 7.20 (m, 3JHH = 7.7 Hz, 1H, Ar−H), 2.62 (m, 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 2.56 (m, 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 2.50 (s, 3H, COCH3), 1.21 (dd, 3JPH = 17.8 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.21 (dd, 3JPH = 17.9 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), 1.10 (dd, 3 JPH = 17.7 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), 1.09 (dd, 3JPH = 18.0 Hz, 3JHH = 7.1 Hz, 6H, PCH(CH3)2). 13C{1H} NMR (101 MHz, CD3CN): 223.76 (d, 1JRhC = 27.5 Hz, COCH3), 183.03 (dt, 1JRhC = 37.7 Hz, 2JPC = 20.2 Hz, Cipso), 141.50 (dd, 1JPC = 92.2 Hz, 3JPC = 13.7 Hz, Arortho), 134.15 (d, 2JPC = 12.8 Hz, Armeta), 121.65 (t, 3JPC = 11.7 Hz, Arpara), 39.93 (s, COCH3), 29.42 (d, 1JPC = 43.8 Hz, PCH(CH3)2), 29.05 (d, 1JPC = 42.5 Hz, PCH(CH3)2), 16.89 (s, PCH(CH3)2), 16.82 (s, PCH(CH3)2), 15.90 (s, PCH(CH3)2). Assignment of the 13C{1H} NMR signals was confirmed by 13C−1H heteronuclear correlation. 19 1 F{ H} NMR (282 MHz, CD3CN): −152.60 (s, free BF4). ESI-MS (CH 3 CN/CH 2 Cl 2 ): M + , m/e 519.87 (calcd for C20H34OP2RhS2 519.06, complex -2 CH3CN), m/e 491.83 (complex -2 CH3CN - CO). M−, m/e 87.23, calcd for BF4 86.80. IR (film, NaCl): νCN = 2316 (m), 2284 cm−1 (m); νCO = 1717 cm−1 (s). Anal. Found: C 40.72, H 5.63, N 3.95. Calcd for C24H40BF4N2OP2RhS2: C 41.87, H 5.86, N 4.07. X-ray Structural Analysis of the Acetonitrile Adduct of Complex 3. Complex 3 was crystallized at −20 °C from an acetonitrile solution overlaid with diethyl ether. Crystal Data. C24H40N2OP2RhS2 + BF4, yellow prism, 0.5 × 0.4 × 0.2 mm3, orthorhombic, Pca21, a = 17.737(4) Å, b = 10.480(2) Å, c = 16.782(3) Å, V = 3119.5(11) Å3, Z = 4, fw = 688.36 g mol−1, Dc = 1.466 g cm−3, μ = 0.828 mm−1. Solution and Refinement. Rint = 0.0256, 346 parameters with 1 restraint, final R1 = 0.0272 (based on F2) for data with I > 2σ(I) and R1 = 0.0295 for 3394 reflections, goodness of fit on F2 = 1.026, largest electron density peak = 0.814 e Å−3. In Situ Preparation of Complex 4 in Acetone-d6 and CD2Cl2. 12.1 mg (0.019 mmol) of complex 2 were dissolved in 0.58 mL of acetone-d6, and the solution was loaded into an NMR tube. The tube was then fitted with a rubber septum, and CO gas was freely bubbled through the solution with a syringe for 1 min (the system was kept open during this time by using a second syringe as a gas outlet). The solution was kept under CO atmosphere during the NMR measurement. The same procedure was also carried out for a solution of complex 2 in CD2Cl2.

EXPERIMENTAL SECTION

General Procedures. All experiments were carried out under an atmosphere of purified nitrogen in an MBraun MB 150B-G glovebox or under purified argon in an MBraun Unilab glovebox. The compounds LiP(iPr)246 and [Rh(COE)2(acetone)2]BF447 were prepared according to literature procedures. Ligand 1 and complex 2 were prepared as previously reported.1 All solvents were reagent grade or better. All nondeuterated solvents were refluxed over sodium/ benzophenone ketyl and distilled under argon. Deuterated solvents were used as received, degassed with argon, and kept in the glovebox over 3 or 4 Å molecular sieves (except for acetone, which was dried with Drierite). Commercially available reagents were used as received. Crystal structures were drawn using the program ORTEP-3.48 Analysis. NMR spectra (1H, 13C, 19F, and 31P) were recorded using Brüker Avance-400 and Brüker Avance-500 NMR spectrometers. All measurements were done at 20 °C, unless noted otherwise. 1H and 13 C NMR chemical shifts are reported in ppm relative to tetramethylsilane. 1H NMR chemical shifts are referenced to the residual hydrogen signal of the deuterated solvent, and the 13C NMR chemical shifts are referenced to the 13C signal(s) of the deuterated solvent. 19F NMR chemical shifts are reported in ppm relative to CFCl3 and referenced to an external solution of C6F6 in CDCl3. 31P NMR chemical shifts are reported in ppm relative to H3PO4 and referenced to an external 85% solution of phosphoric acid in D2O. Abbreviations used in the description of NMR data are as follows: Ar, aryl; br, broad; s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet. Infrared spectra were recorded using Nicolet Protégé 460 and Nicolet 6700 FT-IR spectrometers. Electrospray (ES) mass spectrometry was performed at the Chemical Analysis Laboratory (Department of Chemical Research Support), Weizmann Institute of Science, using Micromass Platform LCZ 4000 (Micromass, Manchester, UK) with a cone voltage of 43 V, extractor voltage of 4 V, and desolvation temperature of 150 °C. Elemental analyses were performed at the Chemical Analysis Laboratory (Department of Chemical Research Support), Weizmann Institute of Science, or at H. Kolbe Mikroanalytisches Laboratorium, Mülheim an der Ruhr, Germany. X-ray Crystallographic Analysis. Data were collected on a Nonius KappaCCD diffractometer at 120(2) K, with Mo Kα radiation (λ = 0.71073 Å) and a graphite monochromator. Data processing was carried out with Denzo−Scalepack.49 Structures were solved by direct methods with SHELXS-97 and refined with SHELXL-97 using the fullmatrix least-squares method based on F2.50 In Situ Synthesis of Complex 3 in CD3OD and Acetone-d6. 31.3 mg (0.049 mmol) of 2 were dissolved in 0.56 mL of CD3OD, and the solution was loaded into an NMR tube, which was then fitted with a rubber septum. Then 1.1 mL (0.046 mmol) of CO was added to the solution with a syringe. The same procedure was also carried out for a solution of complex 2 in acetone-d6. 31 1 P{ H} NMR (202 MHz, CD3OD): 79.23 (s). 1H NMR (500 MHz, CD3OD): 7.46 (m, 3JHH = 7.8 Hz, 2H, Ar−H), 7.23 (m, 3JHH = 7.7 Hz, 1H, Ar−H), 2.72 (m, 3JHH = 7.1 Hz, 2H, PCH(CH3)2), 2.70 (m, 3JHH = 7.1 Hz, 2H, PCH(CH3)2), 2.51 (s, 3H, COCH3), 1.32− 1.12 (m, 24H, PCH(CH3)2). 13C{1H} NMR (126 MHz, CD3OD): 224.27 (br m, COCH3), 179.03 (br m, Cipso), 143.92 (dd, 1JPC = 92.6 Hz, 3JPC = 12.2 Hz, Arortho), 134.92 (m, Armeta), 122.02 (t, 3JPC = 11.9 Hz, Arpara), 38.36 (s, COCH3), 29.53 (d, 1JPC = 42.8 Hz, PCH(CH3)2), 29.08 (d, 1JPC = 43.8 Hz, PCH(CH3)2), 17.28 (s, PCH(CH3)2), 17.08 (s, PCH(CH3)2), 16.14 (s, PCH(CH3)2), 16.00 (s, PCH(CH3)2). 19 1 F{ H} NMR (376 MHz, CD3OD): −155.36 (s, free BF4). 31 1 P{ H} NMR (162 MHz, acetone-d6): 82.93 (s). 1H NMR (400 MHz, acetone-d6): 7.66 (m, 3JHH = 7.5 Hz, 2H, Ar−H), 7.35 (m, 3JHH = 7.5 Hz, 1H, Ar−H), 2.98 (m, 2H, PCH(CH3)2), 2.74 (m, 2H, PCH(CH3)2), 2.74 (s, 3H, COCH3), 1.34 (dd, 3JPH = 18.3 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.24 (dd, 3JPH = 18.7 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), 1.19 (dd, 3JPH = 18.6 Hz, 3JHH = 6.9 Hz, 6H, 7175


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P{1H} NMR (202 MHz, acetone-d6): 100.29 (d, 2JRhP = 2.8 Hz). H NMR (500 MHz, acetone-d6): 7.96 (m, 3JHH = 7.6 Hz, 2H, Ar−H), 7.57 (m, 3JHH = 7.6 Hz, 1H, Ar−H), 3.12 (m, 2JPH = 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 2.81 (m, 2JPH = 8.5 Hz, 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 2.49 (s, 3H, COCH3), 1.39 (dd, 3JPH = 18.2 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.28 (dd, 3JPH = 18.5 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), 1.19 (dd, 3JPH = 18.2 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), 1.13 (dd, 3JPH = 18.2 Hz, 3JHH = 7.1 Hz, 6H, PCH(CH3)2). 13C{1H} NMR (126 MHz, acetone-d6): 206.15 (d, 1JRhC = 26.5 Hz, COCH3, signal overlaps with the acetone carbonyl signal), 189.15 (dt, 1JRhC = 32.7 Hz, 2JPC = 25.9 Hz, Cipso), 185.63 (dt, 1JRhC = 45.8 Hz, 3JPC = 17.1 Hz, CO), 141.02 (dd, 1JPC = 90.5 Hz, 3JPC = 14.9 Hz, Arortho), 135.08 (d, 2JPC = 16.2 Hz, Armeta), 124.46 (t, 3JPC = 11.4 Hz, Arpara), 43.10 (s, COCH3), 29.96 (d, 1JPC = 42.33 Hz, PCH(CH3)2), 27.77 (d, 1JPC = 40.5 Hz, PCH(CH3)2), 17.06 (s, PCH(CH3)2), 16.71 (s, PCH(CH3)2), 16.14 (s, PCH(CH3)2), 15.68 (s, PCH(CH3)2). 31 1 P{ H} NMR (162 MHz, CD2Cl2): 99.98 (d, 2JRhP = 2.8 Hz). 1H NMR (400 MHz, CD2Cl2): 7.71 (m, 3JHH = 7.8 Hz, 2H, Ar−H), 7.56 (m, 3JHH = 7.7 Hz, 1H, Ar−H), 2.89 (m, 2JPH = 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 2.58 (m, 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 2.42 (s, 3H, COCH3), 1.40 (dd, 3JPH = 18.4 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), 1.31 (dd, 3JPH = 18.7 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), 1.10 (dd, 3 JPH = 18.2 Hz, 3JHH = 6.6 Hz, 6H, PCH(CH3)2), 1.08 (dd, 3JPH = 18.2 Hz, 3JHH = 6.6 Hz, 6H, PCH(CH3)2). 13C{1H} NMR (101 MHz, CD2Cl2): 200.30 (d, 1JRhC = 25.4 Hz, COCH3), 187.63 (dt, 1JRhC = 33.2 Hz, 2JPC = 25.3 Hz, Cipso), 184.11 (dt, 1JRhC = 46.4 Hz, 3JPC = 17.3 Hz, CO), 140.58 (ddd, 1JPC = 88.6 Hz, 3JPC = 15.8 Hz, 2JRhC = 2.7 Hz, Arortho), 134.48 (dq, 2JPC = 16.1 Hz, Armeta), 124.89 (t, 3JPC = 11.3 Hz, Arpara), 42.76 (d, 2JRhC = 2.8 Hz, COCH3), 29.34 (d, 1JPC = 41.8 Hz, PCH(CH3)2), 27.09 (d, 1JPC = 40.1 Hz, PCH(CH3)2), 16.80 (s, PCH(CH3)2), 16.20 (s, PCH(CH3)2), 16.09 (s, PCH(CH3)2), 15.41 (s, PCH(CH3)2). Assignment of the 13C{1H} NMR signals was confirmed by 13C−1H heteronuclear correlation. 19F{1H} NMR (376 MHz, CD2Cl2): −153.37 (s, free BF4). In Situ Synthesis of 13CO-Labeled Complex 4 in Acetone-d6. 13 CO-labeled complex 4 was prepared in a manner identical to the nonlabeled complex (see above), albeit using 13C-labeled CO gas. 31 1 P{ H} NMR (162 MHz, acetone-d6): 100.32 (dd, 3JCP = 17.2 Hz, 2 JRhP = 2.7 Hz). 1H NMR (400 MHz, acetone-d6): 7.96 (m, 3JHH = 7.6 Hz, 2H, Ar−H), 7.57 (m, 3JHH = 7.6 Hz, 1H, Ar−H), 3.12 (m, 2JPH = 3 JHH = 6.9 Hz, 2H, PCH(CH3)2), 2.81 (m, 2JPH = 8.5 Hz, 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 2.50 (d, 2JCH = 5.9 Hz, 3H, COCH3), 1.40 (dd, 3 JPH = 18.2 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), 1.29 (dd, 3JPH = 18.4 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), 1.20 (dd, 3JPH = 18.2 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.14 (dd, 3JPH = 18.2 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2). Selected 13C{1H} NMR (101 MHz, acetone-d6): 206.04 (d, 1JRhC = 26.5 Hz, COCH3, signal overlaps with the acetone carbonyl signal), 185.64 (dt, 1JRhC = 45.7 Hz, 3JPC = 17.1 Hz, 13CO). Synthesis of Ligand 5. This new ligand was prepared in two steps, as described below. Step 1: Synthesis of the Bisphosphine Precursor 1,3-Bis(diisopropylphosphino)benzene.51 A glass pressure vessel equipped with a magnetic stirring bar was loaded, in the nitrogen glovebox, with 5.06 g (40.8 mmol) of LiP(iPr)2, 5.60 g (17.0 mmol) of 1,3diiodobenzene, 0.12 g (0.2 mmol) of Pd(dba)2 (dba = dibenzylideneacetone), and 130 mL of dry, distilled dioxane. The pressure vessel was then tightly closed, removed from the glovebox, and heated at 148 °C, with stirring, for 60 h. The solvent was then removed under high vacuum, and the obtained liquid residue was reintroduced into the glovebox, dissolved in diethyl ether, and filtered through Celite. The resulting filtrate was then concentrated under vacuum to again afford a liquid residue, which was distilled under vacuum (140 °C, 0.1 mmHg) to yield 2.60 g (8.4 mmol, 49.4% yield) of the product as a viscous, yellowish liquid. 31 1 P{ H} NMR (162 MHz, CDCl3): 12.12 (s). 1H NMR (400 MHz, CDCl3): 7.54 (m, 1H, Ar−H), 7.42 (m, 2H, Ar−H), 7.28 (m, 1H, Ar− H), 2.08 (m, 3JHH = 7.0 Hz, 4H, PCH(CH3)2), 1.05 (dd, 3JPH = 15.1 Hz, 3JHH = 7.0 Hz, 12H, PCH(CH3)2), 0.87 (dd, 3JPH = 11.3 Hz, 3JHH

= 6.9 Hz, 12H, PCH(CH3)2). 13C{1H} NMR (126 MHz, CDCl3): 139.78 (t, 2JPC = 16.1 Hz, Ar), 135.32 (dd, 2JPC = 21.2 Hz, 4JPC = 0.8 Hz, Ar), 134.04 (dd, 1JPC = 19.0 Hz, 3JPC = 5.5 Hz, CAr−P), 127.17 (t, 3 JPC = 7.7 Hz, Ar), 22.72 (d, 1JPC = 11.6 Hz, PCH(CH3)2), 19.84 (d, 2 JPC = 18.4 Hz, PCH(CH3)2), 18.67 (d, 2JPC = 8.4 Hz, PCH(CH3)2). Assignment of the 13C{1H} NMR signals was confirmed by 13C DEPT 135. Anal. Found: C 69.58, H 10.45. Calcd for C18H32P2: C 69.65, H 10.39. Step 2: Sulfurization of 1,3-Bis(diisopropylphosphino)benzene. A solution of 309.9 mg (1.00 mmol) of 1,3-bis(diisopropylphosphino)benzene in 3.6 mL of THF was added to a suspension of 64.1 mg (2.00 mmol) of elemental sulfur in 4.2 mL of THF, and the resulting clear solution was stirred at room temperature for 21 h. The solvent was then removed under vacuum overnight to yield 369.5 mg (0.99 mmol, 98.8% yield) of the product as a white solid. 31 1 P{ H} NMR (202 MHz, acetone-d6): 67.80 (s). 1H NMR (500 MHz, acetone-d6): 8.41 (tt, 3JPH = 10.7 Hz, 4JHH = 1.5, 1H, Ar−H), 8.19 (m, 3JHH = 7.7 Hz, 4JHH = 1.6 Hz, 2H, Ar−H), 7.75 (m, 3JHH = 7.6 Hz, 4JPH = 2.5 Hz, 4JHH = 0.5, 1H, Ar−H), 2.67 (m, 3JHH = 6.9 Hz, 4H, PCH(CH3)2), 1.16 (dd, 3JPH = 17.3 Hz, 3JHH = 6.9 Hz, 12H, PCH(CH3)2), 0.99 (dd, 3JPH = 17.3 Hz, 3JHH = 7.0 Hz, 12H, PCH(CH3)2). 13C{1H} NMR (126 MHz, acetone-d6): 136.17 (dd, 2 JPC = 8.6 Hz, 4JPC = 2.7 Hz, Ar), 135.89 (t, 2JPC = 9.0 Hz, Ar), 130.14 (dd, 1JPC = 65.8 Hz, 3JPC = 9.3 Hz, CAr−P), 129.03 (t, 3JPC = 10.2 Hz, Ar), 28.24 (d, 1JPC = 51.5 Hz, PCH(CH3)2), 16.82 (d, J = 1.5 Hz, PCH(CH3)2), 16.15 (s, PCH(CH3)2). Assignment of the 13C{1H} NMR signals was confirmed by 13C−1H heteronuclear correlation. Anal. Found: C 57.67, H 8.53. Calcd for C18H32P2S2: C 57.72, H 8.61. X-ray Structural Analysis of Ligand 5. Ligand SCS was crystallized from acetone at −20 °C. Crystal Data. C18H32P2S2, colorless prism, 0.6 × 0.6 × 0.6 mm3, monoclinic, P21/c, a = 7.399(2) Å, b = 22.164(4) Å, c = 13.295(3) Å, β = 102.82(3)°, V = 2125.9(8) Å3, Z = 4, fw = 374.50 g mol−1, Dc = 1.170 g cm−3, μ = 0.397 mm−1. Solution and Refinement. Rint = 0.067, 207 parameters with no restraints, final R1 = 0.0573 (based on F2) for data with I > 2σ(I) and R1 = 0.1013 for 4744 reflections, goodness of fit on F2 = 1.032, largest electron density peak = 0.764 e Å−3. Synthesis of Complex 6 in Acetone. To a solution of 115.4 mg (0.219 mmol) of [Rh(COE)2(acetone)2]BF4 in 1.3 mL of acetone was added a solution of 82.2 mg (0.219 mmol) of ligand 5 in 1.6 mL of acetone, and the resulting solution was stirred at room temperature for 1 h. This solution was then added to 18 mL of pentane, with stirring. The liquid phase was then decanted, and the product was washed with 4 mL of pentane. Removal of residual solvent under vacuum yielded 72.9 mg (0.117 mmol, 53.4% yield) of the product as a dark brown powder. 31 1 P{ H} NMR (162 MHz, acetone-d6): 81.74 (br s). 1H NMR (400 MHz, acetone-d6): 7.54 (m, 3JHH = 7.6 Hz, 2H, Ar−H), 7.23 (m, 3JHH = 7.6 Hz, 1H, Ar−H), 2.89 (m, 3JHH = 6.9 Hz, 2H, PCH(CH3)2), 2.65 (m, 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 1.31 (dd, 3JPH = 17.8 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), 1.24 (dd, 3JPH = 17.7 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.20 (dd, 3JPH = 17.8 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), 1.16 (dd, 3JPH = 17.5 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), −23.96 (br s, Rh−H). 13C{1H} NMR (126 MHz, acetone-d6): 178.40 (br m, Cipso), 143.63 (d, 1JRhC = 94.7 Hz, CAr−P), 133.82 (m, Armeta), 121.28 (m, Arpara), 30.47 (d, 1JPC = 44.7 Hz, PCH(CH3)2), 27.19 (d, 1JPC = 44.1 Hz, PCH(CH3)2), 16.91 (s, PCH(CH3)2), 16.64 (s, PCH(CH3)2), 16.02 (s, PCH(CH3)2), 15.57 (s, PCH(CH3)2). 19F{1H} NMR (376 MHz, acetone-d6): −152.52 (s, free BF 4 ). Anal. Found: C 39.31, H 6.17. Calcd for C21H38BF4OP2RhS2: C 40.53, H 6.15. Synthesis of the Acetonitrile Adduct of Complex 6. To a solution of 142.0 mg (0.270 mmol) of [Rh(COE)2(acetone)2]BF4 in 1.9 mL of acetone was added a solution of 101.5 mg (0.271 mmol) of ligand 5 in 1.9 mL of acetone. The resulting solution was stirred at room temperature for 1 h, during which its color changed from orange to dark brown. At this stage, 0.6 mL of acetonitrile was added to the solution, which was again stirred at room temperature for an additional 4 h. The acetone/acetonitrile solution was then concentrated under

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Synthesis of Complex 8. To a solution of 50.0 mg (0.077 mmol) of 6 (acetonitrile adduct) in 0.6 mL of acetonitrile was added a solution of 9.5 mg (0.085 mmol) of KOtBu in 0.6 mL of THF. The resulting solution was stirred at room temperature for 30 min, and then 11.0 mg (0.077 mmol) of neat CH3I were added. Stirring was then resumed, this time in the dark, for an additional 1 h. At this stage the solvents were removed under vacuum, and the product was extracted with dichloromethane. The obtained mixture was then filtered through Celite to afford a brown solution. Removal of the solvent under vacuum yielded 43.7 mg (0.035 mmol, 91.8% yield) of the product as a brown solid mass. 31 1 P{ H} NMR (162 MHz, CD2Cl2, 20 °C): 98.33 (s). 1H NMR (400 MHz, CD2Cl2, 20 °C): 7.33 (m, 3JHH = 7.6 Hz, 2H, Ar−H), 7.12 (m, 3JHH = 7.6 Hz, 1H, Ar−H), 2.54 (m, 2JPH = 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 2.43 (m, 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 1.69 (s, 3H, Rh−CH3), 1.32 (dd, 3JPH = 17.5 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.24−1.09 (m, 18H, PCH(CH3)2). 13C{1H} NMR (101 MHz, CD2Cl2, 20 °C): 194.08 (bm, Cipso), 141.70 (dd, 1JPC = 93.6 Hz, 3 JPC = 14.7 Hz, Arortho), 132.44 (d, 2JPC = 15.5 Hz, Armeta), 120.13 (t, 3 JPC = 11.1 Hz, Arpara), 29.37 (d, 1JPC = 42.5 Hz, PCH(CH3)2), 27.55 (d, 1JPC = 40.2 Hz, PCH(CH3)2), 17.05 (s, PCH(CH3)2), 16.66 (s, PCH(CH3)2), 16.25 (s, PCH(CH3)2), 15.72 (s, PCH(CH3)2), −7.29 (br m, Rh−CH3). Assignment of the 13C{1H} NMR signals was confirmed by 13C−1H heteronuclear correlation. 31 1 P{ H} NMR (162 MHz, CD2Cl2, −20 °C): 98.36 (s). 1H NMR (400 MHz, CD2Cl2, −20 °C): 7.31 (m, 3JHH = 7.6 Hz, 2H, Ar−H), 7.10 (m, 3JHH = 7.6 Hz, 1H, Ar−H), 2.50 (m, 2H, PCH(CH3)2), 2.43 (m, 2H, PCH(CH3)2), 1.61 (d, 2JRhH = 2.3 Hz, 3H, Rh−CH3), 1.24 (dd, 3JPH = 17.7 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.21−1.08 (m, 18H, PCH(CH3)2). 13C{1H} NMR (101 MHz, CD2Cl2, −20 °C): 193.36 (br m, Cipso), 141.34 (ddd, 1JPC = 93.2 Hz, 3JPC = 14.0 Hz, 2JRhC = 2.0 Hz, Arortho), 132.60 (d, 2JPC = 15.9 Hz, Armeta), 120.32 (t, 3JPC = 11.0 Hz, Arpara), 29.17 (d, 1JPC = 42.6 Hz, PCH(CH3)2), 27.67 (d, 1JPC = 41.4 Hz, PCH(CH3)2), 17.01 (s, PCH(CH3)2), 16.71 (s, PCH(CH3)2), 16.18 (s, PCH(CH3)2), 15.75 (s, PCH(CH3)2), −6.26 (br m, 1JRhC = 26.2 Hz, Rh−CH3). 31 1 P{ H} NMR (162 MHz, CD3CN, 20 °C): 87.74 (br s). 1H NMR (400 MHz, CD3CN, 20 °C): 7.34 (m, 3JHH = 7.5 Hz, 2H, Ar−H), 7.12 (m, 3JHH = 7.5 Hz, 1H, Ar−H), 2.58 (m, 2JPH = 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 2.52 (m, 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 1.23 (dd, 3 JPH = 17.7 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.22 (dd, 3JPH = 17.7 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.14 (dd, 3JPH = 17.5 Hz, 3JHH = 7.1 Hz, 6H, PCH(CH3)2), 1.07 (dd, 3JPH = 17.7 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), 0.91 (d, 2JRhH = 2.4 Hz, 3H, Rh−CH3). Selected 13 C{1H} NMR (101 MHz, CD3CN, 20 °C): −1.64 (d, 1JRhC = 21.8 Hz, Rh−CH3). Assignment of the 13C{1H} NMR signals was confirmed by 13C−1H heteronuclear correlation. Anal. Found: C 36.93, H 5.61. Calcd for C38H68I2P4Rh2S4: C 36.90, H 5.54. X-ray Structural Analysis of Complex 8. Complex 8 was crystallized from benzene at 80 °C. Crystal Data. C38H68I2P4Rh2S4, yellow prism, 0.5 × 0.5 × 0.2 mm3, monoclinic, P21/n, a = 11.701(2) Å, b = 16.033(3) Å, c = 12.540(3) Å, β = 96.54(3)°, V = 2337.2(8) Å3, Z = 2, fw = 1236.73 g mol−1, Dc = 1.757 g cm−3, μ = 2.369 mm−1. Solution and Refinement. Rint = 0.0321, 235 parameters with 6 restraints, final R1 = 0.0405 (based on F2) for data with I > 2σ(I) and R1 = 0.0566 for 5327 reflections, goodness of fit on F2 = 1.057, largest electron density peak = 2.177 e Å−3. Synthesis of 13CH3-Labeled Complex 8. 13CH3-labeled complex 8 was prepared in a manner identical to the nonlabeled complex (see above), albeit using 13CH3-labeled CH3I. 31 1 P{ H} NMR (162 MHz, CD3CN): 87.70 (br s). 1H NMR (400 MHz, CD3CN): 7.34 (m, 3JHH = 7.5 Hz, 2H, Ar−H), 7.12 (m, 3JHH = 7.5 Hz, 1H, Ar−H), 2.58 (m, 2JPH = 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 2.52 (m, 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 1.23 (dd, 3JPH = 17.7 Hz, 3 JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.22 (dd, 3JPH = 17.8 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.14 (dd, 3JPH = 17.5 Hz, 3JHH = 7.1 Hz, 6H, PCH(CH3)2), 1.07 (dd, 3JPH = 17.7 Hz, 3JHH = 6.9 Hz, 6H,

vacuum to 0.6 mL and added to 18 mL of diethyl ether, with stirring. The liquid phase was then decanted, and the product was crushed to a powder and washed twice with 4 mL of diethyl ether. Removal of residual solvent under vacuum yielded 150.6 mg (0.249 mmol, 92.2% yield) of the product as a brown powder. 31 1 P{ H} NMR (202 MHz, CD3CN): 87.72 (s). 1H NMR (500 MHz, CD3CN): 7.33 (m, 3JHH = 7.5 Hz, 2H, Ar−H), 7.16 (m, 3JHH = 7.7 Hz, 1H, Ar−H), 2.69 (m, 3JHH = 7.2 Hz, 2H, PCH(CH3)2), 2.47 (m, 3JHH = 7.2 Hz, 2H, PCH(CH3)2), 1.28 (dd, 3JPH = 17.6 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.17 (dd, 3JPH = 11.1 Hz, 3JHH = 7.3 Hz, 6H, PCH(CH3)2), 1.14 (dd, 3JPH = 10.7 Hz, 3JHH = 6.8 Hz, 6H, PCH(CH3)2), 1.05 (dd, 3JPH = 17.3 Hz, 3JHH = 7.2 Hz, 6H, PCH(CH3)2), −20.31 (dt, 1JRhH = 15.5 Hz, 3JPH = 1.6 Hz, 1H, Rh−H). 13 C{1H} NMR (126 MHz, CD3CN): 181.25 (dt, 1JRhC = 33.7 Hz, 2JPC = 20.3 Hz, Cipso), 142.25 (ddd, 1JRhC = 92.2 Hz, 3JPC = 13.4 Hz, 2JRhC = 2.3 Hz, CAr−P), 133.42 (m, Armeta), 121.31 (t, 3JPC = 11.5 Hz, Arpara), 30.92 (d, 1JPC = 43.6 Hz, PCH(CH3)2), 27.39 (d, 1JPC = 44.4 Hz, PCH(CH3)2), 16.67 (s, PCH(CH3)2), 16.44 (s, PCH(CH3)2), 15.79 (s, PCH(CH3)2), 15.40 (s, PCH(CH3)2). Assignment of the 13C{1H} NMR signals was confirmed by 13C−1H heteronuclear correlation. 19 1 F{ H} NMR (376 MHz, CD3CN): −152.59 (s, free BF4). ESI-MS (CH3CN): M+, m/e 518.47 (calcd for C20H35NP2RhS2 518.49, complex with one acetonitrile ligand), 477.47 (calcd for C18H32P2RhS2 477.44, complex without acetonitrile ligands); M−, m/e 87.11, calcd for BF4 86.80. Anal. Found: C 39.03, H 6.05, N 2.84. Calcd for C20H35BF4NP2RhS2: C 39.69, H 5.83, N 2.31. In Situ Synthesis of Complex 6 in CD3OD. To a solution of 7.0 mg (0.013 mmol) of [Rh(COE)2(acetone)2]BF4 in 0.17 mL of CD3OD was added a solution of 5.0 mg (0.013 mmol) of ligand 5 in 0.45 mL of CD3OD, and the resulting solution was allowed to stand at room temperature overnight. 31 1 P{ H} NMR (162 MHz, CD3OD): 79.15 (br s). 1H NMR (400 MHz, CD3OD): 7.34 (m, 3JHH = 7.6 Hz, 2H, Ar−H), 7.13 (m, 3JHH = 7.6 Hz, 1H, Ar−H), 2.84 (m, 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 2.46 (m, 3JHH = 7.0 Hz, 2H, PCH(CH3)2), 1.35 (dd, 3JPH = 17.6 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.25 (dd, 3JPH = 17.8 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2), 1.22 (dd, 3JPH = 17.3 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), 1.11 (dd, 3JPH = 17.3 Hz, 3JHH = 7.1 Hz, 6H, PCH(CH3)2), −23.45 (d, 1JRhH = 22.5 Hz, 1H, Rh−H). 19F{1H} NMR (376 MHz, CD3OD): −155.58 (s, free BF4). Synthesis of Complex 7. To a solution of 15.0 mg (0.025 mmol) of 6 (acetonitrile adduct) in 0.4 mL of acetonitrile was added a solution of 3.0 mg (0.027 mmol) of KOtBu in 0.5 mL of THF, and the resulting solution was stirred at room temperature for 30 min. The solvent was then removed under vacuum for 20 min, and the resulting residue was extracted with benzene. The afforded solution was then filtered through Celite, combined with 0.2 mL of acetonitrile, and evaporated under vacuum for 30 min. At this point additional acetonitrile (0.3 mL) was added to the concentrated solution, which was then evaporated to dryness for 30 min. This afforded 10.4 mg (0.020 mmol, 81.1% yield) of the product as a dark brown solid. Note that prolonged exposure of complex 7 to vacuum conditions results in its decomposition. 31 1 P{ H} NMR (202 MHz, CD3CN, 20 °C): 77.29 (d, 3JRhP = 4.5 Hz). 1H NMR (500 MHz, CD3CN, 20 °C): 6.82 (m, 3JHH = 7.5 Hz, 2H, Ar−H), 6.65 (m, 3JHH = 7.4 Hz, 1H, Ar−H), 2.40 (m, 4H, PCH(CH3)2), 1.30 (dd, 3JPH = 16.5 Hz, 3JHH = 7.1 Hz, 12H, PCH(CH3)2), 1.10 (dd, 3JPH = 16.6 Hz, 3JHH = 7.0 Hz, 12H, PCH(CH3)2). 31 1 P{ H} NMR (202 MHz, CD3CN, −40 °C): 77.26 (d, 3JRhP = 4.3 Hz). 1H NMR (500 MHz, CD3CN, −40 °C): 6.83 (m, 3JHH = 7.5 Hz, 2H, Ar−H), 6.65 (m, 3JHH = 7.5 Hz, 1H, Ar−H), 2.39 (m, 4H, PCH(CH3)2), 1.25 (dd, 3JPH = 16.1 Hz, 3JHH = 6.7 Hz, 12H, PCH(CH3)2), 1.05 (dd, 3JPH = 16.7 Hz, 3JHH = 7.0 Hz, 12H, PCH(CH3)2). 13C{1H} NMR (126 MHz, CD3CN, −40 °C): 199.48 (dt, 1JRhC = 38.1 Hz, 2JPC = 27.3 Hz, Cipso), 142.08 (ddd, 1JPC = 100.6 Hz, 3JPC = 14.9 Hz, 2JRhC = 3.2 Hz, CAr−P), 130.17 (m, Armeta), 115.57 (t, 3JPC = 12.0 Hz, Arpara), 27.18 (d, 1JPC = 44.4 Hz, PCH(CH3)2), 16.37 (s, PCH(CH3)2), 15.91 (s, PCH(CH3)2). 7177


Organometallics

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PCH(CH3)2), 0.91 (dd, 1JCH = 133.8 Hz, 2JRhH = 2.4 Hz, 3H, Rh− CH3). Selected 13C{1H} NMR (101 MHz, CD3CN): −1.64 (d, 1JRhC = 21.8 Hz, Rh−CH3). Synthesis of 13CH3-Labeled Complex 2 by Iodide Abstraction from 13CH3-Labeled 8. To a solution of 49.6 mg (0.040 mmol) of 13CH3-labeled complex 8 in 0.6 mL of acetone was added a solution of 15.6 mg (0.080 mmol) of AgBF4 in 0.9 mL of acetone, and the resulting mixture was stirred in the dark, at room temperature, for 30 min. This mixture was then filtered several times through Celite, with addition of several milliliters of acetone, to remove the fine AgI particles. The resulting clear solution was then concentrated under vacuum to 1.9 mL and added to 17 mL of pentane, with stirring, to precipitate the product. The liquid phase was then decanted, and the product placed under vacuum to remove residual solvent. This afforded 46.6 mg (0.073 mmol, 91.3% yield) of the product as a brown powder. 31 1 P{ H} NMR (202 MHz, CD3OD): 79.01 (s). 1H NMR (500 MHz, CD3OD): 7.37 (m, 3JHH = 7.6 Hz, 2H, Ar−H), 7.11 (m, 3JHH = 7.6 Hz, 1H, Ar−H), 2.64 (m, 2H, PCH(CH3)2), 2.58 (m, 2H, PCH(CH3)2), 1.29 (dd, 3JPH = 17.6 Hz, 3JHH = 6.9 Hz, 12H, PCH(CH3)2), 1.18 (dd, 3JPH = 17.6 Hz, 3JHH = 7.1 Hz, 6H, PCH(CH3)2), 1.16 (dd, 3JPH = 17.8 Hz, 3JHH = 7.1 Hz, 6H, PCH(CH3)2), 1.16 (dd, 1JCH = 137.0 Hz, 2JRhH = 2.1 Hz, 3H, Rh− CH3). Selected 13C{1H} NMR (126 MHz, CD3OD): −8.71 (d, 1JRhC = 25.7 Hz, Rh−CH3). Reactions of Complex 2 with CO at Low Temperatures. In Situ Generation of Complexes 9 and 10 in CD3OD. A 15.0 mg (0.024 mmol) amount of complex 2 was dissolved in 0.6 mL of CD3OD, and the solution was loaded into an NMR tube, which was then fitted with a rubber septum. The solution was then cooled to −80 °C by dipping the NMR tube in a dry ice/acetone bath. At this point, 0.6 mL (0.025 mmol) of CO was added to the solution with a syringe, and the sample was quickly transferred into an NMR spectrometer that had been precooled to −70 °C. The NMR spectra of complex 9 were then recorded at −70 °C. After completing the NMR data acquisition for complex 9 the solution was warmed to −40 °C to facilitate its conversion to complex 10. The NMR spectra of the latter complex were then recorded at −40 °C. Complex 9. 31P{1H} NMR (202 MHz, CD3OD, −70 °C): 81.36 (s). 1H NMR (500 MHz, CD3OD, −70 °C): 7.57 (m, 3JHH = 7.8 Hz, 2H, Ar−H), 7.30 (m, 3JHH = 7.7 Hz, 1H, Ar−H), 2.86 (m, 2JPH = 3JHH = 6.7 Hz, 2H, PCH(CH3)2), 2.73 (m, 2JPH = 3JHH = 7.2 Hz, 2H, PCH(CH3)2), 1.35−1.21 (m, 18H, PCH(CH3)2), 1.26 (d, 2JRhH = 1.3 Hz, 3H, Rh−CH3), 1.14 (dd, 3JPH = 18.4 Hz, 3JHH = 6.9 Hz, 6H, PCH(CH3)2). 13C{1H} NMR (126 MHz, CD3OD, −70 °C): 182.29 (d, 1JRhC = 38.2 Hz, CO), 173.12 (dt, 1JRhC = 36.3 Hz, 2JPC = 16.0 Hz, Cipso), 141.38 (ddd, 1JPC = 88.0 Hz, 3JPC = 12.1 Hz, J = 1.7 Hz, Arortho), 136.22 (m, Armeta), 122.81 (t, 3JPC = 11.9 Hz, Arpara), 31.13 (d, 1JPC = 44.5 Hz, PCH(CH3)2), 27.39 (d, 1JPC = 44.1 Hz, PCH(CH3)2), 21.03 (d, 1JRhC = 15.4 Hz, Rh−CH3), 17.19 (s, PCH(CH3)2), 16.65 (s, PCH(CH3)2), 16.10 (s, PCH(CH3)2), 15.47 (s, PCH(CH3)2). Assignment of the 13C{1H} NMR signals was confirmed by 13C−1H heteronuclear correlation. 19F{1H} NMR (376 MHz, CD3OD, −70 °C): −154.35 (s, free BF4). Complex 10. 31P{1H} NMR (202 MHz, CD3OD, −40 °C): 100.08 (s). 1H NMR (500 MHz, CD3OD, −40 °C): 7.81 (m, 3JHH = 7.9 Hz, 2H, Ar−H), 7.44 (m, 3JHH = 7.7 Hz, 1H, Ar−H), 2.85 (m, 2JPH = 3JHH = 7.1 Hz, 2H, PCH(CH3)2), 2.70 (m, 2JPH = 3JHH = 7.2 Hz, 2H, PCH(CH3)2), 1.31 (dd, 3JPH = 17.7 Hz, 3JHH = 7.1 Hz, 6H, PCH(CH3)2), 1.30 (dd, 3JPH = 17.9 Hz, 3JHH = 7.0 Hz, 6H, PCH(CH3)2), 1.20 (dd, 3JPH = 18.0 Hz, 3JHH = 6.7 Hz, 6H, PCH(CH3)2), 1.18 (dd, 3JPH = 18.3 Hz, 3JHH = 6.6 Hz, 6H, PCH(CH3)2), 0.99 (d, 2JRhH = 2.2 Hz, 3H, Rh−CH3). 13C{1H} NMR (126 MHz, CD3OD, −40 °C): 192.21 (q, 1JRhC = 2JPC = 28.3 Hz, Cipso), 186.92 (dt, 1JRhC = 43.4 Hz, 3JPC = 17.9 Hz, CO), 141.65 (dd, 1 JPC = 90.7 Hz, 3JPC = 15.4 Hz, Arortho), 135.10 (s, Armeta), 123.96 (t, 3 JPC = 11.3 Hz, Arpara), 29.72 (d, 1JPC = 41.8 Hz, PCH(CH3)2), 29.27 (d, 1JPC = 41.7 Hz, PCH(CH3)2), 17.29 (s, PCH(CH3)2), 17.26 (s, PCH(CH3)2), 16.21 (s, PCH(CH3)2), 15.87 (s, PCH(CH3)2), −7.77 (d, 1JRhC = 21.7 Hz, Rh−CH3). Assignment of the 13C{1H} NMR

signals was confirmed by 13C−1H heteronuclear correlation. 19F{1H} NMR (376 MHz, CD3OD, −40 °C): −154.83 (s, free BF4). In Situ Generation of 13CO-Labeled Complexes 9 and 10 in CD3OD. 13CO-labeled complexes 9 and 10 were prepared in a manner identical to the nonlabeled complexes (see above), albeit using 13Clabeled CO gas. 13 CO-Labeled Complex 9. 31P{1H} NMR (202 MHz, CD3OD, −70 °C): 81.35 (s). Selected 13C{1H} NMR (126 MHz, CD3OD, −70 °C): 182.29 (d, 1JRhC = 38.2 Hz, 13CO). 13 CO-Labeled Complex 10. 31P{1H} NMR (202 MHz, CD3OD, −40 °C): 100.06 (d, 3JCP = 17.4 Hz). Selected 13C{1H} NMR (126 MHz, CD3OD, −40 °C): 186.92 (dt, 1JRhC = 43.4 Hz, 3JPC = 17.9 Hz, 13 CO). In Situ Generation of 13CO,13CH3-Labeled Complexes 9 and 10 in CD3OD. 13CO,13CH3-labeled complexes 9 and 10 were prepared in a manner identical to the nonlabeled complexes (see above), albeit using 13CH3-labeled complex 2 as the precursor, as well as 13C-labeled CO gas. 13 CO,13CH3-Labeled Complex 9. 31P{1H} NMR (202 MHz, CD3OD, −70 °C): 81.35 (s). Selected 1H NMR (500 MHz, CD3OD, −70 °C): 1.26 (dm, 1JCH = 132.1 Hz, 3H, Rh−CH3). Selected 13C{1H} NMR (126 MHz, CD3OD, −70 °C): 182.29 (t, 1 JRhC = 2JCC = 38.5 Hz, 13CO), 21.01 (dd, 2JCC = 38.5 Hz, 1JRhC = 15.5 Hz, Rh−13CH3). 13 CO,13CH3-Labeled Complex 10. 31P{1H} NMR (202 MHz, CD3OD, −40 °C): 100.06 (d, 3JCP = 17.9 Hz). Selected 1H NMR (500 MHz, CD3OD, −40 °C): 0.98 (dm, 1JCH = 138.9 Hz, 3H, Rh− CH3). Selected 13C{1H} NMR (126 MHz, CD3OD, −40 °C): 186.90 (dt, 1JRhC = 43.5 Hz, 3JPC = 18.0 Hz, 13CO), −7.78 (d, 1JRhC = 21.5 Hz, Rh−13CH3). Computational Methods. All calculations were carried out using the Gaussian 03 software package.52 Geometry optimizations and evaluation of harmonic frequencies were performed at the DFT level,53 using the PBE054 hybrid density functional in conjunction with the PC-1 basis set. The latter consists of the SDD basis set55 with an added f function for rhodium (exponent 1.062, the geometric mean of the two f exponents given by Martin and Sundermann56), together with Jensen’s polarization consistent PC-1 basis set for the remaining elements.57 This combination is of double-ζ plus polarization quality. The model SCS and PCP structures used for the calculations featured CH3 groups instead of the iPr substituents on the phosphorus atoms. All structures were fully optimized in the gas phase and characterized as minima or transition states by calculating the harmonic vibrational frequencies. Unless stated otherwise, energetic data are presented in this work as free energies (ΔG) at 298.15 K and include corrections for solvation and dispersion (see below). Bulk solvent effects of the experimental methanol medium have been taken into account by the self-consistent reaction field (SCRF) method, using the continuum solvation model COSMO (conductor-like screening model) as it is implemented in Gaussian 03.58 Gas-phase optimized geometries were used in single-point calculations at the COSMO level. Dispersion interactions in the present work were included by adding Grimme’s empirical dispersion correction term59 with the cutoff function of Becke and Johnson60 (D3BJ). Full topological analysis was performed using the program AIMALL.61 Natural bond orbital (NBO) calculations were performed using NBO5.62

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* Supporting Information S

Crystallographic data (CIF), NMR spectra, and QTAIM analysis data. This material is available free of charge via the Internet at http://pubs.acs.org.

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*E-mail: [email protected]. Fax: (+972)3-906-6634. *E-mail: [email protected]. Fax: (+972)8-9344142. 7178


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site trans to the acetyl as a function of the coordinating ability of the solvent. (15) We have previously described reaction sequences whereby a Rh(III) aryl-hydrido pincer complex is converted into a Rh(I) dinitrogen complex, which is then treated with an alkyl halide to afford a Rh(III) aryl-alkyl complex. For examples, see: (a) Van der Boom, M. E.; Higgitt, C. L.; Milstein, D. Organometallics 1999, 18, 2413−2419. (b) Salem, H.; Ben-David, Y.; Shimon, L. J. W.; Milstein, D. Organometallics 2006, 25, 2292−2300. (c) Frech, C. M.; Milstein, D. J. Am. Chem. Soc. 2006, 128, 12434−12435. (16) For initial concentrations [5]0 = [[Rh(COE)2(acetone)2]BF4]0 ≈ 20 mM, about 80% conversion was observed in acetone within one hour. The degree of conversion was estimated on the basis of 1H NMR signal integration, using the aromatic hydrogen signals. (17) For examples, see: (a) Reference 15b. (b) Kossoy, E.; Iron, M. A.; Rybtchinski, B.; Ben-David, Y.; Shimon, L. J. W.; Konstantinovski, L.; Martin, J. M. L.; Milstein, D. Chem.Eur. J. 2005, 11, 2319−2326. (c) Salem, H.; Shimon, L. J. W.; Leitus, G.; Weiner, L.; Milstein, D. Organometallics 2008, 27, 2293−2299. (d) Polezhaev, A. V.; Kuklin, S. A.; Ivanov, D. M.; Petrovskii, P. V.; Dolgushin, F. M.; Ezernitskaya, M. G.; Koridze, A. A. Russ. Chem. Bull. Int. Ed. 2009, 58, 1847−1854. (18) The reaction of 6b with KOtBu was initially carried out without excess acetonitrile, but this resulted in an intractable mixture of products. (19) The existence of coordinated acetonitrile in 7 was indirectly inferred from its observed properties (molecular symmetry and acetonitrile-dependent stability), since it could not be directly detected by spectroscopic techniques, due to the unavoidable presence of excess acetonitrile. (20) See, R. F.; Kozina, D. J. Coord. Chem. 2013, 66, 490−500. (21) A previously reported Rh(III) complex featuring a methyl ligand trans to CO also exhibits such a low-field methyl 13C resonance. See: Hay-Motherwell, R. S.; Wilkinson, G.; Hussain-Bates, B.; Hursthouse, M. B. Polyhedron 1990, 9, 2071−2080. (22) The methyl ligand was labeled by using the 13C-labeled complex 2, whereas the CO ligand was labeled by simply employing 13C-labeled CO gas. (23) Complex 19 is stabilized by methanol coordination to the extent of 8.9 kcal mol−1. (24) In complex 21, wherein no bonding exists between Rh and the CH3 group, the interaction of Rh with the arene is best described as η1 Rh−Cipso. In the absence of CO this complex would adopt an η2-arene configuration, wherein significant bonding exists between Rh and two adjacent C atoms in the aromatic ring, as described in our previous report regarding the formation of complex 2 (see ref 1). (25) Complex 21 is stabilized by 4.7 kcal mol−1 upon solvent coordination, and so the effective activation barrier for its conversion to 20 increases from 1.8 to 6.7 kcal mol−1. (26) The geometry of TS(20→21) is close to that of the η3 C−C−H complex 20, with an elongated Rh−CCH3 bond (from 2.45 to 2.56 Å) and Rh−HCH3 bond (from 1.92 to 2.05 Å). This enables the subsequent rotation of the methyl group, with formation of the η1 Rh− Cipso complex 21. The angle Rh−Cipso−CCH3 increases from 72.0° in the η3 C−C−H complex to 76.6° in TS(20→21), to 88.4° in the η1 Rh−Cipso complex. (27) Bader, R. F. W. Atoms in Molecules: A Quantum Theory; Oxford University Press: Oxford, 1990. (28) Geerlings, P.; De Proft, F.; Langenaeker, W. Chem. Rev. 2003, 103, 1793−1873. (29) Popelier, P. L. A. Mol. Phys. 1996, 87, 1169−1187. (30) Bader, R. F. W.; Matta, C. F. J. Phys. Chem. A 2004, 108, 8385− 8394. (31) (a) Cortés-Guzmán, F.; Bader, R. F. W. Coord. Chem. Rev. 2005, 249, 633−662. (b) Macchi, P.; Sironi, A. Coord. Chem. Rev. 2003, 238−239, 383−412. (c) Macchi, P.; Sironi, A. In The Quantum Theory of Atoms in Molecules: From Solid State to DNA and Drug Design; Matta, C. F., Ed.; Boyd, R. J., Ed.; Wiley-VCH: Weinheim, 2007; pp 343− 374.

M. Montag and I. Efremenko contributed equally to this article. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the Israel Science Foundation, by the Lise Meitner-Minerva Center for Computational Quantum Chemistry, and by the Helen and Martin Kimmel Center for Molecular Design.

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REFERENCES

(1) Montag, M.; Efremenko, I.; Diskin-Posner, Y.; Ben-David, Y.; Martin, J. M. L.; Milstein, D. Organometallics 2012, 31, 505−512. (2) Throughout this report the bond between phosphorus and sulfur in the SCS ligand is depicted as a double bond (PS), in keeping with conventional practice. However, as our calculations will show below, this bond is best described as a polar single bond (P−S). (3) Montag, M.; Efremenko, I.; Cohen, R.; Shimon, L. J. W.; Leitus, G.; Diskin-Posner, Y.; Ben-David, Y.; Salem, H.; Martin, J. M. L.; Milstein, D. Chem.Eur. J. 2010, 16, 328−353. (4) Montag, M.; Schwartsburd, L.; Cohen, R.; Leitus, G.; Ben-David, Y.; Martin, J. M. L.; Milstein, D. Angew. Chem. 2007, 119, 1933−1936; Angew. Chem., Int. Ed. 2007, 46, 1901−1904. (5) (a) Vigalok, A.; Rybtchinski, B.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Organometallics 1999, 18, 895−905. (b) Frech, C. M.; Milstein, D. J. Am. Chem. Soc. 2006, 128, 12434−12435. (6) Gandelman, M.; Shimon, L. J. W.; Milstein, D. Chem.Eur. J. 2003, 9, 4295−4300. (7) To the best of our knowledge, the cationic PCP- and PCNRh(III) aryl-methyl complexes shown in Scheme 2b,c are the only reported structural analogues of the present SCS system that have been treated with CO. (8) Complex J was not directly observed under the reaction conditions depicted in Scheme 2c, but its involvement was supported through its independent synthesis and subsequent treatment with CO to yield complex K (see ref 6). (9) The differences in the temperature required to obtain 3 is linked to the coordinating ability of the solvents. Acetonitrile is bound more strongly to the Rh(I) center than either methanol or acetone, and hence CO cannot easily displace CH3CN at room temperature. The varying binding affinities of these solvents are reflected in their calculated binding energies to complex 2: 17.2 kcal mol−1 for CH3CN vs 12.2 kcal mol−1 for acetone. (10) It is noteworthy that the Rh−N distances associated with the acetonitrile ligands differ significantly, i.e., the bond trans to the aryl group (Rh1−N2 = 2.148(3) Å) is much shorter than the bond trans to the acetyl moiety (Rh1−N1 = 2.259(4) Å). This difference may reflect the stronger trans influence of the acetyl moiety relative to the aryl. (11) The 19F{1H} NMR spectrum features a sharp singlet within the range −152 to −156 ppm, indicative of a noncoordinated BF4−. (12) 13C labeling at both carbonyl moieties of 4 was accomplished by simply treating 2 with excess 13C-labeled CO. (13) 13C−13C coupling constants are expected to be significant for carbonyl ligands in a mutually trans configuration, but very small to negligible in the cis configuration. For examples of 13C−13C coupling constants across metal centers, see: (a) Tachikawa, M.; Richter, S. I.; Shapley, J. R. J. Organomet. Chem. 1977, 128, C9−C14. (b) Silvio, A.; Domenico, O. J. Chem. Soc., Chem. Commun. 1981, 300−302. (c) Lindner, E.; Norz, H. Chem. Ber. 1990, 123, 459−465. (d) Noveski, D.; Braun, T.; Neumann, B.; Stammler, A.; Stammler, H.-G. Dalton Trans. 2004, 4106−4119. (14) The 13C{1H} NMR carbonyl signal for the acetyl ligand exhibits significant solvent dependence, i.e., an upfield shift of almost 6 ppm on going from acetone to dichloromethane, whereas all other signals are much less solvent-dependent (Δδmax = 1.5 ppm). This selective solvent effect is consistent with varying occupancy of the coordination 7179


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CO-Induced Methyl Migration in a Rhodium Thiophosphoryl Pincer

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