Article pubs.acs.org/Organometallics
Metal- and Ligand-Centered Reactivity of meta-Carboranyl-Backbone Pincer Complexes of Rhodium Bennett J. Eleazer, Mark D. Smith, and Dmitry V. Peryshkov* Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, South Carolina 29208, United States S Supporting Information *
ABSTRACT: We report the synthesis of the chelating phosphinite-arm carboranyl POBOP-H (POBOP = 1,7-OP(i-Pr)2-m-carboranyl) ligand precursor, preparation of its rhodium complexes, and their reactivity in oxidative addition/reductive elimination reactions. The oxidative addition of iodobenzene to the low-valent (POBOP)Rh(PPh3) resulted in the selective formation of the 16-electron complex (POBOP)Rh(Ph)(I), featuring a highly strained exohedral rhodium−boron bond. The complex (POBOP)Rh(Ph)(I) is the first example of a B-carboranyl aryl metal complex, which is a proposed intermediate in metal-promoted B−C coupling reactions. The complex (POBOP)Rh(Ph)(I) was selectively and directly converted, in the presence of acetonitrile, to (POB(BPh)OP)Rh(H)(I)(CH3CN) (POB(BPh)OP = 1,7-OP(i-Pr)2-2-Ph-m-carboranyl) through unprecedented cascade reductive elimination of the phenyl-B-carboranyl and the oxidative addition of a vicinal B−H bond of the boron cluster to the metal center, exhibiting both metal- and cluster-centered reactivity.
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both σ- and π-interactions with exohedral substituents, depending on both electronic and geometric arrangements.7c,e−j The steric bulk of the carborane cluster can be demonstrated by comparing the van der Waals volume of orthocarborane (148 Å3) with that of adamantane (136 Å3) and benzene (79 Å3).7k Unsupported exohedral metal−boron bonds have been reported only for several mercury, thallium, and iridium complexes.8 In contrast, late transition metals have been demonstrated to activate B−H bonds of carboranes upon coordination to directing P or N donors with formation of cyclometalated products, often with high regioselectivity.9 The use of m-carborane, functionalized with thioethers, selenoethers, pyridines, and oxazoline groups as donor arms, resulted in the isolation of B-metalated pincer-type complexes (Scheme 1).4f,10 Conspicuously absent from this list are phosphine and phosphinite ligands which enjoy a great popularity in traditional carbon−metalated pincers.
INTRODUCTION Rhodium complexes have attracted significant attention due to their remarkable reactivity in a multitude of organic transformations.1 The understanding of the oxidative addition/ reduction elimination cycle of the Rh(I)/Rh(III) pair led to the development of numerous catalytic cycles, including C−C, C− N, C−S coupling and H−H, C−H, C−C, and C−N activation reactions.2 A number of ligand platforms have been developed, including tridentate pincers. For instance, phosphorus-arm cyclometalated pincer complexes have been extensively studied due to the combination of stability and reactivity provided by a rigid ligand framework, which makes them attractive for catalytic applications.3 Exploration of a ligand influence on the reactivity led to the development of numerous carbonbackbone ligands and, in addition, a large number of heteroatom-containing systems, including complexes featuring metal−nitrogen, −silicon, −phosphorus, and −boron bonds.4 Boron-based pincer frameworks have recently attracted an increasing attention with the introduction of boryl and Bcarboranyl complexes.4c,f For instance, the participation of a tricoordinate metalated borolidine ligand in small molecule activation has been demonstrated for Co, Ni, and Ru complexes.5 Icosahedral closo-dicarbadodecaboranes, C2B10H12, are remarkably robust neutral boron clusters with two boron vertices replaced by carbon atoms.6 The delocalized electron density of the carborane cage is not uniform and gives rise to significant differences in the electronic effects of the cluster on an exohedral substituent located on the carbon or boron atoms.7 The C2B10H12 clusters are usually regarded as electronwithdrawing groups when connected through the cluster carbon atoms7a,b and electron-donating when connected through cluster boron-atoms.7b,c Carborane clusters can exhibit © XXXX American Chemical Society
Scheme 1. Carboranyl-Based Pincer Complexes
Received: September 22, 2015
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DOI: 10.1021/acs.organomet.5b00807 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Wilkinson’s catalyst in tetrahydrofuran at room temperature for 48 h afforded the (POBOP)Rh(H)(Cl)(PPh3) complex (2) as a yellow powder in high yield (92%, Scheme 2).
Extensive recent studies of the reactivity of unsupported metal−boryl bonds toward unsaturated substrates led to a great expansion of metal-catalyzed hydroboration and diboration reactions.11a,b In contrast to tricoordinate boryls, the chemistry of B-metalated carboranyl complexes and the potential reactivity of the presumably electron-rich and nucleophilic metal−boron bond remain unexplored. While there are numerous examples of metal-catalyzed B−C, B−N, B−O, and B−S coupling reactions of carboranes, no B-metalated intermediates implicated in such transformations have been isolated.11d−j Furthermore, the unique three-dimensional geometry of carborane clusters, in addition to steric shielding, creates a possibility of an additional reactivity centered on the boron atom that is vicinal to the metalated cage boron (Scheme 1). Herein, we report the synthesis of the chelating phosphinite-arm carboranyl complexes of rhodium possessing remarkably strained exohedral rhodium−boron bonds and the formation of the first example of a stable intermediate in B−C coupling reaction of carboranyl clusters by a direct B−H activation. We observed the unprecedented cascade reductive elimination of the phenyl-B-carboranyl moiety followed by oxidative addition of the vicinal B−H bond of the boron cluster to the metal center, resulting in the reformation of the metal− boron bond.
Scheme 2. Synthesis of B-Carboranyl Phosphinite Complexes of Rh(III) and Rh(I)
The NMR spectroscopy data of the diamagnetic complex 2 were consistent with the expected pincer-type structure. Two phosphorus atoms in the complex are equivalent and give rise to a signal at 213.8 ppm (1JRhP = 122 Hz), which is within a typical range for the related Rh(III) POCOP complexes. The 11 1 B{ H} NMR pattern is composed of two distinct regions: one resonance at 5.85 ppm and a set of partially overlapping resonances in the range from −6.50 to −19.50 ppm. In the proton-coupled 11B NMR spectrum of 2, all signals convert to doublets except the resonance at 5.85 ppm, which remains a singlet. The latter is indicative of the metalated boron atom of the cage. The 1H NMR spectrum of 2 in C6D6 contains two sets of signals from the isopropyl groups due to their inequivalence for a given phosphorus atom. The signal corresponding to the hydride ligand is present as a multiplet at −17.0 ppm. Colorless plate-shaped single crystals of 2 formed upon slow evaporation of a saturated hexane solution at room temperature. The X-ray crystal structure confirmed the successful metalation of the boron cluster and the expected octahedral geometry around rhodium (Figure 2). The molecular structure of 2 is consistent with NMR spectroscopy data that show two equivalent phosphorus atoms and inequivalent sets of isopropyl groups on each ligand arm. The formation of the complex proceeds with B−H bond activation at one of the boron atoms adjacent to the two carbon atoms of the cluster. The boron−
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RESULTS AND DISCUSSION The reaction of 1,7-dihydroxy-m-carborane (the numbering scheme for the m-carborane cluster is given in Supporting Information (SI)) with diisopropylphosphine chloride in THF in the presence of excess triethylamine resulted in the formation of the phosphinite ligand precursor, 1,7-OP(i-Pr)2m-carborane (POBOP)H (1), as a colorless air- and moisturesensitive solid highly soluble in the majority of organic solvents, including hexanes. The (POBOP)H compound was isolated upon recrystallization from acetonitrile at −30 °C in 83% yield and in 99% purity according to 31P NMR data. The combination of NMR spectroscopy (1H, 31P, and 11B NMR) and single-crystal X-ray crystallography was employed for confirmation of the molecular structure of the product. The molecular structure of the ligand precursor is shown in Figure 1. It was expected that the metal center will be brought to the vicinity of the H1 and H2 hydrogen atoms on the boron cage upon coordination to the chelating phosphinite arms resulting in facile B−H activation. It was found that the (POBOP)H ligand precursor can be used without additional purification for the synthesis of metal complexes. The reaction of the ligand precursor 1 with
Figure 2. Displacement ellipsoid plot (50% probability) of the (POBOP)Rh(H)(Cl)(PPh3) complex (2). (a): a general view; (b): a view perpendicular to the (B1−Rh1−H1−P3) plane. Atoms belonging to isopropyl groups of the ligand and all hydrogen atoms, except for the metal hydride, have been omitted for clarity. Selected bond distances (Å) and angles (deg): Rh1−B1 = 2.054(2), Rh1−P1 = 2.335(1), Rh1−P3 = 2.445(1), Rh1−H1 = 1.46(2), B2−B1−Rh1 = 109.8(2), B1−Rh1−P3 = 178.7(1), Cl1−Rh1−H1 = 177(1).
Figure 1. Displacement ellipsoid plot (50% probability) of the (POBOP)H ligand precursor (1). Atoms belonging to isopropyl groups of the ligand and all hydrogen atoms, except for H1 and H2 atoms on the cluster, have been omitted for clarity. Selected bond distances (Å) and angles (deg): C1−O1 = 1.391(2), O1−P1 = 1.704(1), C1−B1 = 1.705(2), B1−B2 = 1.788(3), B2−B1−H1 = 116.1(9), B1−C1−O1 = 114.1(1). B
DOI: 10.1021/acs.organomet.5b00807 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics metal distance in 2 is 2.054(2) Å, which is comparable to Rh−B distances in rhodium borolidine complexes (1.980(7)− 2.058(2) Å).4c The complex possesses a distorted octahedral geometry with the PPh3 ligand coordinated trans to the borane cluster and the hydride ligand coordinated trans to the chloride. The P1− Rh1−P2 angle of 149.3(1)° significantly deviates from the expected linearity. It is important to note that complex 2 is the first example of structurally characterized meta-C2B10H12 cluster containing the exohedral Rh−B bond. The molecular structure of 2 revealed a significant strain of the metal−boron bond enforced by the chelating ligand arms. The B2−B1−Rh1 angle is 109.8(2)°, whereas the unstrained exohedral bonds in the idealized icosahedral cluster form an angle of 120° (cf. the B2− B1−H1 angle of 116.1(9)° in the (POBOP)H ligand precursor 1). The distinct strain in the complex is caused by the unusual geometry of the carboranyl pincer ligand: the carbon atoms of the boron cluster, which serve as points of attachment of the phosphinite arms, are significantly above the metalated boron atom. In contrast, in typical aryl- and pyridyl-based pincers the metalated backbone atom and arm attachments are coplanar. As a consequence, the square planar fragment of carboranyl pincers possesses idealized Cs symmetry rather than the C2v symmetry typical for aryl-backbone pincers. Therefore, the axial positions, occupied by the hydride and chloride ligands in 2, are rendered inequivalent, i.e. two possible isomers with PPh3 trans to carboranyl can potentially exist, namely with the hydride syn or anti relative to the vicinal B2 cluster atom (Scheme 3). In our studies, only the syn isomer was observed spectroscopically for both Rh hydride complexes 2 and 4 (see below).
Figure 3. Displacement ellipsoid plot (50% probability) of the (POBOP)Rh(PPh3) complex (3). (a): a general view; (b): a view perpendicular to the (B2−B1−Rh1−P3) plane. Isopropyl groups of the ligand and hydrogen atoms, except for the carborane cluster, have been omitted for clarity. Selected bond distances (Å) and angles (deg): Rh1−B1 = 2.047(1), Rh1−P1 = 2.263(1), Rh1−P3 = 2.345(1), B2− B1−Rh1 = 104.5(1), B1−Rh1−P3 = 177.2(1).
linearity, contributing to the distortion from idealized squareplanar environment of the metal center. Compound 3 is formally a Rh(I) 16-electron complex, thus potentially reactive through oxidative addition to the metal center, especially if the PPh3 ligand is dissociated. According to 31 P NMR data, the reaction of 3 and excess PhI proceeds at room temperature with formation of a Rh(III) product with a signal at 188.0 ppm (1JRhP = 122 Hz) and a concomitant formation of 1 equiv of free PPh3. The product of the reaction was purified by silica gel chromatography and isolated after recrystallization from a CH3CN/Et2O solvent mixture as an orange-red crystalline powder in 83% yield (Scheme 4).
Scheme 3. Possible syn and anti Isomers of the Rh(III) Hydride Complex 2
Scheme 4. Oxidative Addition of PhI to the (POBOP)Rh(PPh3) Complex
Reductive elimination of HCl from complex 2 in the presence of NEt3 proceeds quickly at room temperature as indicated by the appearance of new signals in the 31P NMR spectrum at 222.9 ppm (1JRhP = 186 Hz) and the disappearance of the metal hydride signal in the 1H NMR spectrum. The triphenylphosphine ligand remained coordinated to the metal, according to 31P NMR data. The metalated boron atom signal in the 11B and 11B{1H} NMR spectra appears at 9.95 ppm as a singlet distinct from the rest of the cluster boron atom signals in the range from 2 to −20 ppm. The Rh(I) complex (POBOP)Rh(PPh3) (3) was isolated in high yield as an orange solid (98%, Scheme 2). Single crystals of 3 were grown from a THF/hexane mixture at room temperature. The X-ray diffraction data revealed a distorted square-planar geometry of the complex with the PPh3 ligand coordinated trans to the boron cluster (Figure 3). The Rh1−B1 distance is 2.047(1) Å, which is comparable to that in 2. The angle B2−B1−Rh1 in complex 3 is 104.5(1)°, which indicates a greater exohedral boron−metal bond strain than that in complex 2, presumably due to the absence of the apical hydride ligand in 3, allowing the metal center to be closer to B2 boron atom. The P1−Rh1− P2 chelate angle (151.1(1)°) is significantly deviated from
The single crystal X-ray diffraction study revealed the distorted square-pyramidal (POBOP)Rh(Ph)(I) (4) oxidative addition product with the phenyl group in the apical position and a slight deviation of the iodide ligand from the basal plane (Figure 4). The Rh1−B1 bond length in 4 is 2.030(2) Å. The unique feature of the molecular structure of 4 is a highly strained cage-metal B2−B1−Rh1 angle of 85.2(1)°, which demonstrates a drastic distortion from the expected 120° exohedral angle for the icosahedral cluster. The unusually acute B2−B1−Rh1 angle resulted in a short Rh1−B2 contact of 2.582(2) Å and an apparent short Rh1−H2A(B2) contact of 2.20(1) Å, which are likely too long for a strong three-center two-electron B−H···Rh interaction. For comparison, the majority of agostic B−H···Rh interactions in structurally characterized anionic closo-CB11 and nido-C2B9 carborane and metalacarborane clusters have been reported to occur in the range of 2.338(1)−2.467(3) Å for B···Rh distances.12 However, at least in two cases, B−H···Rh interactions have been observed at longer metal−boron distances within the range of 2.521(4)− C
DOI: 10.1021/acs.organomet.5b00807 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Scheme 5. Phenyl Group Migration and B−H Activation in Complex 4
the octahedral Rh(III) hydride iodide complex (POB(BPh)OP)Rh(H)(I) (CH3CN) 5, with the vicinal boron atom derivatized with the phenyl ring (Figure 5).
Figure 4. (a) Displacement ellipsoid plot (50% probability) of the (POBOP)Rh(Ph)(I) complex (4). (a): general view; (b): a view perpendicular to B2−B1−Rh1−C2 plane. Isopropyl groups of the ligand and hydrogen atoms, except for the carborane cluster and the phenyl group, have been omitted for clarity. Selected interatomic distances (Å) and angles (deg): Rh1−B1 = 2.030(2), Rh1−B2 = 2.582(2), Rh1−C2 = 2.028(2), Rh1−I1 = 2.774(1), Rh1−H2A = 2.20(1), Rh1−P1 = 2.325(1), B2−B1−Rh1 = 85.2(1), B1−Rh1−C2 = 98.3(1), B1−Rh1−I1 = 159.7(1).
2.643(8) Å.12e,i The 1H{11B} spectrum of 4 at room temperature in CD2Cl2 exhibits a broadened singlet signal corresponding to one boron-bound hydrogen atom at 0.24 ppm, which is relatively close to the range of chemical shifts from 0 ppm to −4 ppm usually associated with the presence of B−H···Rh interactions in solution.12 The position of this upfield H(B) signal is shifted to 0.16 ppm upon cooling the sample to 208 K; however, no apparent 103Rh−1H coupling could be discerned. All the other B−H resonances in the 1 H{11B} spectrum of 4 were observed in the range from 1.4 to 2.8 ppm. In addition, no new 11B signals were observed in the spectra recorded below room temperature. The infrared spectra of a solid sample of 4 exhibited a broad absorption band typical for the carborane cluster B−H stretches at 2600 cm−1 and no band in the region from 2020 to 2050 cm−1 typical for strong B−H···M interactions.12 The phenyl ring rotation around the Rh1−C2 bond is slow at room temperature as indicated by a set of broad signals in the aromatic region of the 1H NMR spectrum and hindered at −60 °C as demonstrated by the appearance of the set of five sharp C−H signals. The metalated boron atom gives rise to a resonance at 1.37 ppm in the 11B NMR and 11B{1H} spectra at room temperature, distinct from the other overlapping cluster boron atom signals in the range from −12.5 ppm to −19.3 ppm. The close contact between the Rh metal center and the vicinal B2 atom in 4 led us to a hypothesis that the cluster B2− H2A bond can be readily activated if reductive elimination from Rh(III) were to occur. Upon addition of a donor solvent, acetonitrile, no interaction was observed in the 31P NMR spectrum of a solution of 4 in C6D6 at room temperature, indicating that the coordination of acetonitrile does not occur under these conditions. However, heating (POBOP)Rh(Ph)I in the presence of 10 equiv of CH3CN in C6D6 solution at 80 °C led to the appearance of a new signal in the 31P NMR spectrum and a broad metal hydride signal at −17.46 ppm. The 11 B NMR spectrum of the product contained two signals at 6.79 and −3.37 ppm belonging to two boron atoms not directly connected to hydrogen atoms, indicative of the activation of the boron atom adjacent to the B−Rh bond (Scheme 5). The product of the reaction was isolated as a pale yellow powder upon recrystallization from CH3CN/Et2O at −30 °C. The single-crystal X-ray diffraction revealed the formation of
Figure 5. Displacement ellipsoid plot (50% probability) of the (POB(BPh)OP)Rh(H)(I)(CH3CN) complex (5). (a): a general view; (b): a view perpendicular to the (B1−B2−Rh1−I1) plane. Isopropyl groups of the ligand and hydrogen atoms, except for the carborane cluster, the metal hydride, and acetonitrile ligand, have been omitted for clarity. Selected bond distances (Å) and angles (deg): Rh1−B2 = 1.997(2), Rh1−P1 = 2.299(1), Rh1−I1 = 2.795(1), Rh1−H1 = 1.41(2), Rh1−N1 = 2.167(2), B1−B2−Rh1 = 115.5(1), B2−Rh1−I1 = 175.8(1), H1−Rh1−N1 = 178(1), B2−B1−C3 = 129.4(2).
The B2−Rh1 bond length in 5 is 1.997(2) Å. Analogously to complex 2, the hydride ligand is in syn orientation relative to the carborane cluster B1 atom. The iodide is located trans to the carboranyl backbone; the acetonitrile molecule is coordinated trans to the hydride. The B1−B2−Rh1 angle is 115.5(1)°, which is larger that that in the octahedral Rh(III) complex 2 possibly due to the increase in the steric repulsion between the hydride H1 and phenyl ring atom C3 on the boron cluster in comparison with the repulsion between hydride H1 and hydrogen atom H2 on the boron cage in 2. Due to this steric hindrance, the B2−B1−C3 angle is 129.4(2)°, larger than the expected angle of 120°. The pincer chelation angle P1− Rh1−P2 is 152.5(1)°, which is comparable with that in 2 and 3. Thus, heating of the mixture of 4 and CH3CN in C6D6 at 80 °C resulted in the reductive elimination of the phenyl group and B-carboranyl ligand. The intermediate low-valent metal center is presumably kept in the vicinity of B1 and B2 cluster atoms by the coordination to the phosphinite ligand arms. As a consequence, the B(2)−H bond of the cluster is readily activated with the formation of B-coordinated Rh(III) hydride complex. The net outcome of this cascade process is the transfer of the phenyl ring to the boron atom and B−H activation of the cluster. This activation of the boron atom vicinal to the metalated boron of the carborane cage is unprecedented. D
DOI: 10.1021/acs.organomet.5b00807 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
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P(CH(CH3)2), 17.69 (d, 2JCP = 10 Hz, P(CH(CH3)2), 17.03 (d, 2JCP = 10 Hz, P(CH(CH3)2). 31P{1H} (C6D6): δ 169.9 (s). Found: C, 40.94; H, 9.37. Calcd for C14H38B10O2P2: C, 41.16; H, 9.38 Synthesis of (POBOP)Rh(H)(Cl)(PPh3) (2). A portion of Rh(PPh3)3Cl (1.36 g; 1.46 mmol) was added to a solution of (POBOP) H (0.600 g; 1.46 mmol) in tetrahydrofuran (8 mL). The mixture was stirred at room temperature for 48 h. The resulting yellow-orange reaction mixture was dried under vacuum and triturated with hexanes. The suspension was filtered, and the yellow powder was washed with hexane (30 mL). Yield: 1.09 g, 92%. 1H NMR (C6D6): δ 8.05 (m, 5H, P(C6H5)3), 7.01 (m, 10H, P(C6H5)3), 4.00−2.00 (br, 9H, C2B10H9), 2.92 (m, 2H, P(CH(CH3)2), 1.81 (m, 2H, P(CH(CH3)2) 1.30 (dd, 6H, P(CH(CH3)2)), 1.00 (dd, 6H, P(CH(CH3)2)), 0.79 (dd, 6H, P(CH(CH3)2)), 0.35 (dd, 6H, P(CH(CH3)2)), − 17.0 (m, 1H, RhH). 11 1 B{ H} (C6D6): δ 5.85 (s, BRh), − 6.9 to −19.4 (br, BH). 13C (C6D6): δ 136.6 (d, P(C6H5)3), 135.9 (d, P(C6H5)3), 130.4 (d, P(C6H5)3), 104.4 (C2B10), 32.4 (d, P(CH(CH3)2), 30.9(d, P(CH(CH3)2), 19.1 (P(CH(CH3)2), 18.6 (P(CH(CH3)2), 14.8 (P(CH(CH3)2). 31P{1H} (C6D6): δ 213.8 (dd, 1JPRh = 122 Hz, 2JPP = 7.2 Hz, 2P, P(CH(CH3)2), 13.55 (br, 1P, P(C6H5)3). Selected bands in the IR spectrum, cm−1: 30588−2880 (CH), 2611, 2586 (BH), 2048 (RhH). Found: C, 47.98; H, 6.58. Calcd for C32H53B10O2RhP3Cl: C, 47.50; H, 6.60. Synthesis of (POBOP)Rh(PPh3) (3). Triethylamine (194 mg; 1.91 mmol) was added to a solution of (POBOP)Rh(H)(Cl)(PPh3) (310 mg; 0.383 mmol) in tetrahydrofuran (5 mL). The reaction mixture was stirred at room temperature for 48 h. The reaction mixture was evaporated under vacuum, and the product was extracted to toluene (10 mL). The toluene solution was filtered, and the filtrate was dried yielding an orange solid. Yield: 290 mg, 98%. 1H NMR (C6D6): δ 7.70 (m, 5H, P(C6H5)3), 6.97 (br, 10H, P(C6H5)3), 3.80−1.70 (br, 9H, C2B10H9), 1.90 (br, 2H, P(CH(CH3)2), 1.27 (dd, 6H, P(CH(CH3)2), 1.10 (dd, 6H, P(CH(CH3)2), 0.97 (dd, 6H, P(CH(CH3)2), 0.91 (dd, 6H, P(CH(CH3)2). 11B{1H} (C6D6): δ 9.95 (s, BRh), − 9.4 to −18.8 (br, BH). 13C (C6D6): δ 139.8 (d, P(C6H5)3), 134.5 (d, P(C6H5)3), 129.2 (P(C6H5)3), 103.0 (C2B10), 32.2 (d, P(CH(CH3)2), 20.4 (d, P(CH(CH3)2), 17.4 (P(CH(CH3)2), 16.8 (P(CH(CH3)2). 31P{1H} (C6D6): δ 223.0 (dd, 1JPRh = 186 Hz, 2JPP = 15, 2P, P(CH(CH3)2), 25.2 (br d, 1JPRh = 146 Hz 1P, P(C6H5)3). Selected bands in the IR spectrum, cm−1: 3079−2873 (CH), 2588 (BH). Found: C, 49.63; H, 6.52. Calcd for C32H52B10O2RhP3: C, 49.74; H, 6.78. Synthesis of (POBOP)Rh(Ph)(I) (4). Iodobenzene (60 mg; 0.30 mmol) was added to a solution of (POBOP)Rh(PPh3) (100 mg; 0.130 mmol) in tetrahydrofuran (5 mL). The reaction mixture was stirred overnight at room temperature. The volatiles were removed under vacuum yielding a red residue. The residue was extracted to diethyl ether (20 mL) and filtered through a glass microfiber filter paper. Volatiles were removed from the filtrate yielding the dark orange crystals, which were washed with acetonitrile (3 × 10 mL). The resulting solids were purified on silica gel chromatography column with diethyl ether as an eluent. The product was isolated as a crystalline orange-red solid. Yield: 77 mg; 83%. 1H NMR (C6D6): δ 8.38 (br, 2H, RhC6H5)), 6.72 (m, 3H, RhC6H5), 3.70−0.2 (br, 9H, C2B10H9), 2.70 (m, 2H, P(CH(CH3)2), 2.52 (m, 2H, P(CH(CH3)2), 1.38 (dd, 6H, P(CH(CH3)2), 1.05 (dd, 6H, P(CH(CH3)2), 0.75 (dd, 6H, P(CH(CH3)2), 0.43 (dd, 6H, P(CH(CH3)2). 11B{1H} (C6D6): δ 1.37 (s, BRh), − 12.8 to −14.1 (br, BH), − 19.2 (s, BH). 13C (C6D6): δ 149.6 (d, RhC6H5), 124.0 (RhC6H5), 106.6 (C2B10), 31.2 (d, P(CH(CH3)2), 29.6 (d, P(CH(CH3)2), 18.8 (P(CH(CH3)2), 17.9 ((P(CH(CH3)2), 15.6 ((P(CH(CH3)2), 14.4 (P(CH(CH3)2). 31P{1H} (C6D6): δ 188.0 (d, 1JPRh = 122 Hz). Selected bands in the IR spectrum, cm−1: 3081−2875 (CH), 2598 (BH). Found: C, 33.77; H, 5.96. Calcd for C20H42B10IO2P2Rh: C, 33.62; H, 5.93. Synthesis of (POB(BPh)OP)Rh(H)(I)(CH3CN) (5). To a solution of (POBOP)Rh(Ph)(I) (50 mg; 0.070 mmol) in deuterated benzene (1 mL) was added acetonitrile (20 mg; 0.45 mmol). The reaction mixture was heated to 80 °C for 3 h. Volatiles were removed under vacuum to yield a slightly yellow residue. Slow evaporation of a solution of acetonitrile and diethyl ether yielded colorless crystals. Yield: 52 mg, 98%. 1H NMR (C6D6): δ 7.70 (m, 2H, BC6H5), 7.11 (m,
CONCLUSION Complex 5 is the first example of a selective derivatization of the B1(B2) boron atom of the m-carborane cage by an aryl group. The related reductive elimination of alkyl and benzyl groups from a Rh(III) center to aryl pincers has been studied in great detail by Milstein and co-workers;13 however, there have been no reports of any similar intramolecular transfer of an aryl ring within a pincer complex, likely due to the great structural distortions required for such a group shift in planar pincer backbones. In the case of our three-dimensional carboranylbased pincer system, the presence of the vicinal B−H bond is a unique feature, which allowed for the sequential reductive elimination/oxidative addition process and the reformation of the Rh(III) metal center. The formation of complex 5 demonstrated that the carboranyl unit can act not only as a supporting spectator ligand but also in cooperative ligand− metal reactivity. Furthermore, the unique geometry of carborane-based pincers imposes a significant strain on metal−boron bonds, which likely contributed to the enhanced reactivity in reductive elimination with a formation of the arylB-carboranyl fragment. The related direct functionalization of B−H bonds of boron clusters is an emergent field of organic chemistry, in part motivated by unique photophysical properties of polyaromatic hydrocarbons containing carboranyl substituents.14 The recently reported examples include Pd- and Ir- promoted arylation and alkenylation of o-carborane with the use of carboxylate as a directing group and invoke metalated cluster intermediates.7c,e,f In this work, we report the first example of a double B−H activation cascade sequence with an intramolecular aryl group transfer and the structural characterization of isolated intermediates containing Rh−B bonds. Our investigation of other metal−boron bond transformations in these strained boron cluster systems is underway.
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EXPERIMENTAL SECTION
All synthetic manipulations were carried out either in a nitrogen-filled drybox or on an air-free dual-manifold Schlenk line, unless stated otherwise. The solvents were sparged with nitrogen, passed through activated alumina, and stored over activated 4 Å Linde-type molecular sieves. Dichloromethane-d2 and benzene-d6 were degassed and stored over activated 4 Å Linde-type molecular sieves or used as received. NMR spectra were recorded using Varian spectrometers at 400 (1H), 100 (13C), 162 (31P), 128 (11B) MHz, reported in δ (parts per million) and referenced to the residual 1H/13C signals of the deuterated solvent or an external 85% phosphoric acid (31P (δ): 0.0 ppm) and BF3(Et2O) (11B(δ): 0.0 ppm) standards. J values are given in Hz. Midwest Microlab, Indianapolis, Indiana provided the elemental analysis results. 1,7-Dihydroxy-m-carborane was prepared according to the reported procedure.15 Rh(PPh3)3Cl, m-carborane (Katchem), P(CH(CH3)2)Cl, and PhI were used as received. Synthesis of 1,7-OP(i-Pr)2-m-Carborane, (POBOP)H (1). A storage tube was charged with a solution of 1,7-dihydroxy-m-carborane (1.10 g; 6.23 mmol) and chlorodiisopropylphosphine (2.00 g; 13.1 mmol) in tetrahydrofuran (10 mL). Triethylamine (4.41 g; 43.6 mmol) was added to the solution at room temperature, which led to immediate formation of a white precipitate. The mixture was stirred at 85 °C for 48 h, cooled to room temperature, and filtered through Celite. Volatiles were removed from the filtrate under vacuum at room temperature to give viscous pale yellow oil. The product was isolated as a colorless solid by recrystallization from acetonitrile at −30 °C. Yield: 2.11 g, 83%. 1H NMR (C6D6): δ 4.60−1.60 (br m, 10H, BH), 1.45 (m, 4H, P(CH(CH3)2), 0.96 (dd, 12H, P(CH(CH3)2), 0.84 (dd, 12H, P(CH(CH3)2). 11B{1H} (C6D6): δ − 13.9 (br, 6B), − 15.9 (br, 4B). 13C (C6D6): δ 104.8 (C2B10H10), 28.27 (d, 1JCP = 20 Hz, E
DOI: 10.1021/acs.organomet.5b00807 Organometallics XXXX, XXX, XXX−XXX
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Organometallics 3H, BC6H5), 2.90 (br, 2H, P(CH(CH3)2), 2.62 (br, 2H, P(CH(CH3)2), 3.8−2.0 (br, 8H, C2B10H8), 1.43 (m, 12H, P(CH(CH3)2), 1.30 (m, 6H, P(CH(CH3)2), 1.22 (m, 6H, P(CH(CH3)2), 0.77 (s, 3H, CH3CN), − 17.46 (br, 1H, RhH). 11B{1H} (C6D6): δ 6.79 (s, BRh), − 3.37 (s, BC6H5), − 11.0 (s, BH), −13.4 (s, BH), −16.9 (s, BH), − 19.6 (s, BH). 13C (C6D6): δ 133.1 (BC6H5), 116.6 (CH3CN), 104.9 (C2B10), 34.9 (P(CH(CH3)2), 34.4 ((P(CH(CH3)2)), 20.6 (P(CH(CH3)2), 19.9 (P(CH(CH3)2), 18.9 (P(CH(CH3)2), 18.2 (P(CH(CH3)2), 0.13 (CH3CN). 31P{1H} (C6D6): δ 215.6 (d, 1JPRh = 130 Hz). Selected bands in the IR spectrum, cm−1: 2979−2876 (CH), 2593 (BH), 2303, 2291 (CN), 2070 (RhH). Found: C, 35.15; H, 6.22; N, 1.58. Calcd for C22H46B10INO2P2Rh: C, 34.98; H, 6.01; N, 1.85.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00807. Crystallographic information (CIF) Experimental details and characterization data (PDF)
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AUTHOR INFORMATION
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Acknowledgment is made to the donors of the American Chemical Society Petroleum Research Fund (Award 54504DNI3) for the partial support of this research and to ASPIRE-I funding granted through the University of South Carolina, Office of the Vice President for Research.
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