Structure and dynamic NMR behavior of rhodium complexes supported by Lewis acidic group 13 metallatranes

James T. Moore , Nicholas E. Smith and Connie C. Lu *
Department of Chemistry, 207 Pleasant St SE, Minneapolis, MN, USA. E-mail: clu@umn.edu

Received 18th December 2016 , Accepted 14th February 2017

First published on 15th February 2017


Monovalent Rh was installed into the group 13 metallatranes, M[N(o-(NCH2P(iPr)2)C6H4)3] (where M = Al and Ga, abbreviated as ML) to generate Rh → M bonds in the parent complexes, Cl–RhAlL (1-Cl) and Cl–RhGaL (2-Cl). The electron-withdrawing nature of the group 13 metalloids was probed by cyclic voltammetry, and Rh–Ga was found to be more electron-deficient than Rh–Al (Epc = −2.07 and −1.95 V vs. Fc+/Fc for 1-Cl and 2-Cl, respectively). Both 1-Cl and 2-Cl were further functionalized through metathesis reactions using MeLi to generate 1-CH3 and 2-CH3, respectively, or using LiHBEt3 to form 1-H and 2-H, respectively. The solid-state structures of all Rh–M bimetallics feature Rh–M bond lengths that are less than the sum of the covalent radii of Rh and M (Rh–M: 2.50–2.54 Å for 1-X and 2.49–2.46 Å for 2-X, where X = Cl, CH3, and H). In the Rh–M structures, the Rh center is distorted from square pyramidal geometry due to steric interactions between X and the isopropyl substituents of L. Finally, all the Rh–M bimetallics exhibit fluxionality that involves phosphine exchange. Of note, the two phosphines cis to the X ligand become inequivalent at low temperature. The activation barrier to exchange these two phosphine donors is: 14.9, 14.2, 10.9, and 11.5 kcal mol−1 for 1-Cl, 2-Cl, 1-H, and 2-H, respectively. The activation barriers for 1-CH3 and 2-CH3 are both >15.2 kcal mol−1. At high temperature, 2-Cl was also found to exchange all three phosphine donors. Mechanisms for the different types of phosphine exchange are proposed.


Introduction

Chemical bonds between a transition metal (TM) and a σ-acceptor (Z-type) ligand reverse the traditional roles of the metal as a Lewis acid and of the ligand as a Lewis base.1,2 Ambiphilic ligands that tether strong donors to the σ-acceptor functionality can lend coordinative stability to TM → Z complexes. While the majority of ambiphilic ligands feature B as the σ-acceptor,3–6 the heavy main group congeners (e.g. Al,7–13 Ga,14,15 In,15,16 Tl,17,18 Sb,19–22 Bi,23–25 and Te26,27) are less explored. Beyond bonding, TM → Z complexes are also emerging as bifunctional catalysts that operate via metal–ligand cooperativity.28,29 For example, Z-type ligands benefit catalysis by directly assisting in heterolytic E–H activation,15,30–33 supporting redox transformations during N2 fixation,34 and tuning the electrophilicity of the transition metal to bind and activate alkyne substrates.21,35,36

The double-decker triphosphino(triamido)amine ligand, [N(o-(NCH2P(iPr)2)C6H4)3]3− (abbreviated as L), was shown to support TM → Z bonds between a late first-row metal (Fe, Co, and Ni) and AlIII.9,15 The coordination chemistry was subsequently extended to Ni–Ga and Ni–In.15 Of interest, the catalytic activity of these Ni bimetallic complexes in olefin hydrogenation depended significantly on the identity of the group 13 ion: Ni–Ga was competent, Ni–In was sluggish, and Ni–Al was inactive. Because of the widespread use of Rh in homogeneous catalysis,37,38 we began to investigate the coordination chemistry of Rh paired with group 13 ions.

Unlike Rh → BIII complexes, which are plentiful, few Rh → AlIII and Rh → GaIII examples have been structurally validated. A search of the Cambridge structure database39 resulted in three complexes containing a discrete Rh → AlIII or Rh → GaIII bonding interaction (Fig. 1).40,41 As an aside, other bonding interactions between Rh and the heavy group 13 metalloids, e.g. Rh ← MI (where M = Al or Ga), which are uncommon, have been reported.42–45


image file: c6dt04769f-f1.tif
Fig. 1 Structurally characterized Rh complexes featuring AlIII or GaIII ligands, including X–RhML complexes in this work. The TM → Z bonding interaction is highlighted in red.

We report a six-membered family of X–Rh–MIII complexes, where X is Cl, CH3, or H, and M is trivalent Al or Ga. A significant Rh → M interaction is present across all complexes. The geometry of the five-coordinate Rh center is distorted from ideal square pyramidal geometry, and the extent of the distortion depends on the steric bulk of the X ligand. Finally, these complexes undergo fluxional processes to exchange inequivalent phosphine donors. Based on variable temperature 1H and 31P NMR data, the thermodynamic parameters and potential mechanisms for the fluxional behavior are discussed.

Experimental section

General considerations

Unless otherwise stated, all manipulations were performed under an inert atmosphere in a glovebox or using standard Schlenk techniques. Standard solvents were deoxygenated by sparging with inert gas and dried by passing through activated alumina columns of a SG Water solvent purification system. Deuterated solvents were purchased from Cambridge Isotope Laboratories, Inc. or Sigma-Aldrich, degassed via freeze–pump–thaw cycles and stored over activated 4 Å molecular sieves. Elemental analyses were performed by Robertson Microlit Laboratories, Inc. (Ledgewood, NJ). All 1H and 31P NMR spectra were recorded on a Bruker 400 MHz spectrometer at ambient temperature unless otherwise stated. The temperature of the probe during the variable-temperature NMR experiments was calibrated against an external methanol or ethylene glycol standard at temperatures below and above 25 °C, respectively. The barrier that equilibrates the inequivalent phosphine donors, ΔG, is calculated using eqn (1)
 
ΔG = RTc[23.760 + ln(Tc/kc)](1)
where the coalescence temperature Tc is in Kelvin, R is the ideal gas constant, and image file: c6dt04769f-t1.tif, where Δν is the difference in frequency (Hz) between the exchanging phosphorus nuclei, and J is the coupling constant between the exchanging 31P nuclei.46,47 Note, this equation was originally derived for a simple AB spin system. Here, the equation is applied to an ABX spin system, where ABX refers to the three unique 31P nuclei and excludes 103Rh coupling, to study the coalescence of the A and B nuclei. Cyclic voltammetry was performed using a CH Instruments 600 electrochemical analyzer. The one-cell set-up used a glass carbon working electrode, Pt wire counter electrode, and Ag/AgNO3 reference electrode in CH3CN. Analyte solutions consisted of 0.1 M [nBu4N]PF6 and the voltammograms were referenced internally to the FeCp20/+ (abbreviated as Fc+/Fc) redox couple. RhCl3·3H2O was purchased from Pressure Chemicals and used as received. All other metal halides were purchased either from Strem or Sigma Aldrich and used without further purification. The protio-ligand, N(o-(NHCH2PiPr2)C6H4)3 (abbreviated as LH3), AlL, GaL, and {Rh(μ-Cl)(C2H4)2}2 were synthesized according to literature procedures.9,15,48

Synthesis of ClRhAl(N(o-(NCH2PiPr2)C6H4)3), 1-Cl

A solution of AlL (161.4 mg, 0.229 mmol) in THF (8 mL) was added to a solution of {Rh(μ-Cl)(C2H4)2}2 (44.5 mg, 0.115 mmol) in THF (1 mL). The mixture was stirred overnight, after which the solvent was removed in vacuo to yield a red-orange solid. The solid was then dissolved in benzene (4 mL) and filtered through a pad of Celite. The filtrate was concentrated in vacuo to yield red-orange powder (189.2 mg, 98% crude yield). Crystals suitable for X-ray diffraction were grown by slow diffusion of hexane into a concentrated benzene solution of 1-Cl. 1H{31P} NMR (400 MHz, CD2Cl2, −18 °C): δ 7.39 (app t, J = 7.0 Hz, 2H, aryl), 7.30 (d, J = 7.5 Hz, 1H, aryl), 7.05 (app t, J = 7.4 Hz, 1H, aryl), 7.00 (app t, J = 7.4 Hz, 2H, aryl), 6.52–6.43 (m, 4H, aryl), 6.36 (d, J = 7.7 Hz, 1H, aryl), 6.31 (d, J = 7.7 Hz, 1H, aryl), 3.40 (d, J = 11.7 Hz, 1H, methylene), 3.35 (d, J = 11.7 Hz, 1H, methylene), 3.33–3.20 (m, 3H, methylene, 2 methine), 3.11 (d, J = 12.8 Hz, 1H, methylene), 2.88 (sept, J = 7.3 Hz, 1H, methine), 2.68 (d, J = 13.3 Hz, 1H, methylene), 2.59 (d, J = 12.8 Hz, 1H, methylene), 2.33 (sept, J = 7.2 Hz, 1H, methine), 2.26 (sept, J = 6.8 Hz, 1H, methine), 1.93 (d, J = 7.3 Hz, 3H, methyl), 1.89 (overlapping, 1H, methine), 1.45 (d, J = 7.2 Hz, 3H, methyl), 1.38–1.33 (m, 9H, methyl), 1.30–1.28 (m, 6H, methyl), 1.19 (d, J = 7.3 Hz, 3H, methyl), 1.13 (d, J = 6.8 Hz, 3H, methyl), 0.90 (d, J = 6.5 Hz, 3H, methyl), 0.84 (d, J = 6.8 Hz, 3H, methyl), 0.49 (d, J = 6.5 Hz, 3H, methyl). 31P{1H} NMR (162 MHz, CD2Cl2, −18 °C): δ 80.7 (app dt, JRh–P = 157 Hz, 2JcisP–P = 25 Hz, 1P, Ptrans), 45.0 (ddd, 2JtransP–P = 317 Hz, JRh–P = 131 Hz, 2JcisP–P = 24 Hz, 1P, Pcis), 26.8 (ddd, 2JtransP–P = 317 Hz, JRh–P = 107 Hz, 2JcisP–P = 25 Hz, 1P, Pcis). Anal. Calcd for 1-Cl·(C6H6)0.5, C39H60N4P3AlRhCl·0.5(C6H6) (%): C, 57.18; H, 7.20; N, 6.35. Found: C, 58.03; H, 7.51; N, 6.31. UV–vis [THF; λmax, nm (ε, M−1 cm−1)]: 325 (16[thin space (1/6-em)]700), 400 (3000).

Synthesis of ClRhGa(N(o-(NCH2PiPr2)C6H4)3), 2-Cl

2-Cl was synthesized using the same procedure as 1-Cl, except using GaL (168.6 mg, 0.225 mmol) and {Rh(μ-Cl)(C2H4)2}2 (43.8 mg, 0.113 mmol). 2-Cl was isolated as a red solid (198.6 mg, 99% crude yield). Red crystals suitable for X-ray diffraction were grown by diffusion of hexane into a concentrated benzene solution of 2-Cl. 1H{31P} NMR (400 MHz, CD2Cl2, −49 °C): δ 7.41 (d, J = 7.6 Hz, 1H, aryl), 7.37 (d, J = 7.9 Hz, 1H, aryl), 7.30 (d, J = 7.4 Hz, 1H, aryl), 7.07–6.94 (m, 3H, aryl), 6.53–6.42 (m, 4H, aryl), 6.34 (d, J = 7.6 Hz, 2H, aryl), 3.45 (d, J = 11.7 Hz, 1H, methylene), 3.38–3.27 (m, 2H, methine, methylene), 3.27–3.10 (m, 3H, 2 methylene, methine), 2.92 (sept, J = 7.2 Hz, 1H, methine), 2.57 (d, J = 13.3 Hz, 1H, methylene), 2.52 (d, J = 13.0 Hz, 1H, methylene), 2.33 (sept, J = 7.0 Hz, 1H, methine), 2.21 (sept, J = 6.5 Hz, 1H, methine), 1.94 (d, J = 6.9 Hz, 3H, methyl), 1.74 (sept, J = 6.8 Hz, 1H, methine), 1.43 (d, J = 7.0 Hz, 3H, methyl), 1.39–1.33 (m, 9H, methyl), 1.32–1.26 (m, 6H, methyl), 1.18 (d, J = 7.1 Hz, 3H, methyl), 1.12 (d, J = 6.7 Hz, 3H, methyl), 0.84 (d, J = 6.0 Hz, 3H, methyl), 0.79 (d, J = 6.6 Hz, 3H, methyl), 0.45 (d, J = 6.4 Hz, 3H, methyl). 31P{1H} NMR (162 MHz, CD2Cl2, −18 °C): δ 93.3 (app dt, JRh–P = 146 Hz, 2JcisP–P = 23 Hz, 1P, Ptrans), 51.2 (ddd, 2JtransP–P = 325 Hz, JRh–P = 128 Hz, 2JcisP–P = 22 Hz, 1P, Pcis), 30.5 (ddd, 2JtransP–P = 325 Hz, JRh–P = 105 Hz, 2JcisP–P = 24 Hz, 1P, Pcis). Anal. Calcd for 2-Cl·(C6H6)0.5, [C39H60N4P3GaRhCl·0.5(C6H6)] (%): C, 54.54; H, 6.86; N, 6.06. Found: C, 53.99; H, 6.91; N, 5.93. UV–vis [THF; λmax, nm (ε, M−1 cm−1)]: 310 (19[thin space (1/6-em)]400), 425 (2600).

Synthesis of CH3RhAl(N(o-(NCH2PiPr2)C6H4)3), 1-CH3

A solution of MeLi in Et2O (0.0932 mL, 0.140 mmol) was added to THF (2 mL), and subsequently added dropwise to a solution of 1-Cl (98.3 mg, 0.117 mmol) in THF (5 mL) at −78 °C. The mixture was stirred for 3 h. The volatiles were removed in vacuo, and the resulting brown solid was washed with a cold (−20 °C) solution of 4[thin space (1/6-em)]:[thin space (1/6-em)]1 hexanes[thin space (1/6-em)]:[thin space (1/6-em)]diethyl ether (3 × 3 mL). The yellow solid was dissolved in benzene (5 mL) and filtered through a pad of Celite. The filtrate was condensed in vacuo to yield 1-CH3 as a light yellow powder (48.3 mg, 50% yield). Note, MeMgCl can also be used instead of MeLi. Crystals suitable for X-ray diffraction were grown by slow diffusion of pentane into a concentrated benzene solution of 1-CH3. 1H{31P} NMR (400 MHz, toluene-d8, −18 °C): δ 7.47–7.43 (m, 2H, aryl), 7.41 (dd, J = 7.6, 1.4 Hz, 1H, aryl), 7.20–7.10 (m, 3H, aryl CH), 6.56 (app dt, J = 7.6, 1.2 Hz, 1H, aryl), 6.55–6.49 (m, 3H, aryl), 6.44 (d, J = 7.8 Hz, 1H, aryl), 6.37 (dd, J = 7.9, 1.1 Hz, 1H, aryl), 3.37 (d, J = 10.9 Hz, 1H, methylene), 3.20 (d, J = 12.3 Hz, 1H, methylene), 3.11 (d, J = 10.9 Hz, 1H, methylene), 2.94 (d, J = 12.6 Hz, 1H, methylene), 2.78 (d, J = 12.8 Hz, 1H, methylene), 2.71 (sept, J = 7.5 Hz, 1H, methine), 2.60 (d, J = 12.8 Hz, 1H, methylene), 2.57 (sept, J = 7.4 Hz, 1H, methine), 2.39 (sept, J = 6.9 Hz, 1H, methine), 2.20 (sept, J = 7.0 Hz, 1H, methine), 2.15 (sept, J = 7.1 Hz, 1H, methine), 2.06 (sept, J = 7.3 Hz, 1H, methine), 1.52 (d, J = 7.5 Hz, 3H, methyl), 1.26 (d, J = 7.5 Hz, 3H, methyl), 1.20 (d, J = 7.3 Hz, 3H, methyl), 1.16–1.13 (m, 6H, methyl), 1.10 (d, J = 7.2 Hz, 3H, methyl), 0.99 (d, J = 7.3 Hz, 3H, methyl), 0.90 (d, J = 7.0 Hz, 3H, methyl), 0.86–0.80 (m, 9H, methyl), 0.64* (d, 2JRh = 1.1 Hz, 3H, Rh–CH3), 0.63 (d, overlapping with previous peak, 3H, methyl). 31P{1H} NMR (162 MHz, toluene-d8, 7 °C): δ 59.0 (app dt, JRh–P = 108 Hz, 2JcisP–P = 19 Hz, 1P), 45.1 (ddd, 2JtransP–P = 313 Hz, JRh–P = 155 Hz, 2JcisP–P = 19 Hz, 1P), 23.3 (ddd, 2JtransP–P = 313 Hz, JRh–P = 126 Hz, 2JcisP–P = 18 Hz, 1P). *Note: Coupling resolved at 25 °C. Anal. Calcd for 1-CH3·(C6H6)0.5, [C40H63N4P3AlRh·0.5(C6H6)] (%): C, 59.93; H, 7.72; N, 6.50. Found: C, 59.81; H, 7.93; N, 6.49.

Synthesis of CH3RhGa(N(o-(NCH2PiPr2)C6H4)3), 2-CH3

2-CH3 was synthesized using the same procedure as 1-CH3, except using 2-Cl (127.3 mg, 0.144 mmol) and MeLi (0.115 mL, 0.172 mmol). 2-CH3 was isolated as a dark orange solid (77.0 mg, 62% crude yield). Single crystals suitable for X-ray diffraction were grown by slow evaporation of a benzene solution of 2-CH3. 1H{31P} NMR (400 MHz, toluene-d8, −18 °C): δ 7.53 (dd, J = 7.6, 1.1 Hz, 1H, aryl), 7.52 (dd, J = 7.6, 1.2 Hz, 1H, aryl), 7.48 (dd, J = 7.8, 1.2 Hz, 1H, aryl), 7.21–7.11 (m, 3H, aryl), 6.61–6.51 (m, 4H, aryl), 6.45 (d, J = 7.7 Hz, 1H, aryl), 6.42 (d, J = 7.4 Hz, 1H, aryl), 3.44 (d, J = 10.9 Hz, 1H, methylene), 3.30 (d, J = 12.6 Hz, 1H, methylene), 3.07 (d, J = 11.0 Hz, 1H, methylene), 2.97 (d, J = 12.5 Hz, 1H, methylene), 2.74 (sept, J = 7.5 Hz, 1H, methine), 2.66 (d, J = 12.7 Hz, 1H, methylene), 2.51 (sept, J = 7.4 Hz, 1H, methine), 2.49 (d, J = 12.3 Hz, 1H, methylene), 2.32 (sept, J = 7.0 Hz, 1H, methine), 2.17 (sept, J = 7.0 Hz, 1H, methine), 2.00 (sept, J = 7.2 Hz, 2H, methine), 1.48 (d, J = 7.5 Hz, 3H, methyl), 1.24 (d, J = 7.5 Hz, 3H, methyl), 1.16 (d, J = 7.4 Hz, 3H, methyl), 1.13 (d, J = 7.3 Hz, 3H, methyl), 1.12–1.05 (m, 6H, methyl), 0.96 (d, J = 7.4 Hz, 3H, methyl), 0.88 (d, J = 7.0 Hz, 3H, methyl), 0.79 (d, J = 6.6 Hz, 3H, methyl), 0.79 (d, J = 6.6 Hz, 3H, methyl), 0.75 (d, J = 7.0 Hz, 3H, methyl), 0.72* (d, 2JRh = 1.0 Hz, 3H, Rh–CH3), 0.62 (d, J = 6.8 Hz, 3H, methyl). 31P{1H} NMR (162 MHz, toluene-d8, 3 °C): δ 70.8 (app dt, JRh–P = 99 Hz, 2JcisP–P = 17 Hz, 1P), 50.3 (ddd, 2JtransP–P = 317 Hz, JRh–P = 154 Hz, 2JcisP–P = 17 Hz, 1P), 27.3 (ddd, 2JtransP–P = 317 Hz, JRh–P = 124 Hz, 2JcisP–P = 17 Hz, 1P). *Note: Coupling resolved at 42 °C. Anal. Calcd for 2-CH3·(C6H6)0.5, [C40H63N4P3GaRh·0.5(C6H6)] (%): C, 57.10; H, 7.35; N, 6.19. Found: C, 57.19; H, 7.50; N, 6.15. Fig. S26 shows the corresponding 1H NMR spectrum for this sample.

Synthesis of HRhAl(N(o-(NCH2PiPr2)C6H4)3), 1-H

A solution of LiHBEt3 in THF (0.0734 mL, 0.0734 mmol) was added to THF (2 mL), and subsequently added dropwise to a solution of 1-Cl (56.3 mg, 0.0668 mmol) in THF (4 mL) at −78 °C. The mixture was stirred for 3 h. The volatiles were removed in vacuo, and the product was extracted using a cold (−20 °C) solution of 4[thin space (1/6-em)]:[thin space (1/6-em)]1 hexanes[thin space (1/6-em)]:[thin space (1/6-em)]diethyl ether (3 × 3 mL) and filtered through a pad of Celite. The solvent was removed in vacuo to yield 1-H as a light yellow powder (38.0 mg, 70% crude yield). Single crystals suitable for X-ray diffraction were grown by slow evaporation of hexanes into a concentrated benzene solution of 1-H. 1H{31P} NMR (400 MHz, CD2Cl2, 25 °C): δ 7.29–7.22 (m, 3H, aryl), 6.95 (app t, J = 7.5 Hz, 3H, aryl), 6.37 (app t, J = 7.5 Hz, 3H, aryl), 6.34–6.29 (m, 3H, aryl), 3.16 (d, J = 12.0 Hz, 2H, methylene), 3.10 (d, J = 12.0 Hz, 2H, methylene) 3.03 (s, 2H, methylene), 2.41 (sept, J = 7.2 Hz, 2H, methine), 2.33 (sept, J = 6.7 Hz, 2H, methine), 2.16 (sept, J = 7.2 Hz, 2H, methine), 1.23–1.17 (m, 24H, methyl), 1.10 (d, J = 6.7 Hz, 6H), 1.05 (d, J = 6.7 Hz, 6H), −8.38 (d, JRh–H = 12.4 Hz, 1H, Rh–H; with 31P coupling: dtd, 2JtransP–H = 107.1 Hz, 2JcisP–H = 24.2 Hz, JRh–H = 12.4 Hz). 31P{1H} NMR (162 MHz, CD2Cl2, −73 °C): δ 59.1 (dd, 2JtransP–P = 265 Hz, JRh–P = 138 Hz, 1P), 51.6* (app dt, JRh–P = 119 Hz, 2JcisP–P = 20 Hz), 43.4 (dd, 2JtransP–P = 265 Hz, JRh–P = 125 Hz, 1P). *Note: Coupling resolved at −49 °C. Anal. Calcd for 1-H·(C6H6)0.5, [C39H61N4P3AlRh·0.5(C6H6)] (%): C, 59.50; H, 7.61; N, 6.61. Found: C, 60.41; H, 7.56; N, 6.66. Repeated attempts to obtain satisfactory elemental analysis failed for 1-H, presumably due to its extreme air sensitivity.

Synthesis of HRhGa(N(o-(NCH2PiPr2)C6H4)3), 2-H

2-H was synthesized using the same procedure as 1-H, except using 2-Cl (35.6 mg, 0.0402 mmol) and LiHBEt3 (5.6 mg, 0.0402 mmol). 2-H was isolated as a light yellow/orange solid (30.0 mg, 87% crude yield). Single crystals suitable for X-ray diffraction were grown by slow diffusion of hexanes into a concentrated benzene solution of 2-H. 1H NMR (400 MHz, toluene-d8, 25 °C): δ 7.51–7.47 (m, 3H, aryl), 7.13–7.08 (m, 3H, aryl), 6.54–6.44 (m, 6H, aryl), 3.19 (d, J = 11.6 Hz, 2H, methylene), 3.09 (d, J = 11.6 Hz, 2H, methylene), 3.06 (s, 2H, methylene), 2.16 (br, 2H, methine), 2.02 (br, 4H, methine), 1.07–1.01 (m, 18H, methyl), 0.96 (d, J = 6.8 Hz, 12H, methyl), 0.87 (d, J = 6.6 Hz, 6H, methyl), −8.54 (dtd, JtransP–H = 109.3 Hz, JcisP–H = 23.0 Hz, JRh–H = 12.3 Hz, 1H, Rh–H). 31P{1H} NMR (162 MHz, toluene-d8, −73 °C): δ 64.4 (ddd, 2JtransP–P = 273 Hz, JRh–P = 142 Hz, 2JcisP–P = 17 Hz, 1P), 58.3* (app dt, JRh–P = 114 Hz, 2JcisP–P = 19 Hz), 49.2 (dd, 2JtransP–P = 273 Hz, JRh–P = 123 Hz, 1P).*Note: Coupling resolved at −49 °C. Anal. Calcd for 2-H·(C6H6)0.5, [C39H61N4P3GaRh·0.5(C6H6)] (%): C, 56.65; H, 7.24; N, 6.29. Found: C, 52.59; H, 6.64; N, 5.95. Repeated attempts to obtain satisfactory elemental analysis failed for 2-H, presumably due to its extreme air sensitivity. The analysis is close to the expected ratios for an oxidized sample, [C39H61N4O3P3GaRh] (%): C, 52.08; H, 6.84; N, 6.23.

X-ray crystallography and structure refinement details

A red-orange block of 1-Cl (0.48 × 0.45 × 0.20 mm), a yellow block of 1-CH3 (0.10 × 0.11 × 0.20 mm), a yellow plate of 1-H (0.21 × 0.18 × 0.1 mm), a red block of 2-Cl (0.3 × 0.2 × 0.1 mm), an orange block of 2-CH3 (0.35 × 0.21 × 0.1 mm), and a yellow plate of 2-H (0.19 × 0.20 × 0.24 mm) were mounted on a 200 μm MiTeGen microloop and placed on a Bruker APEX-II Platform diffractometer or a Bruker PHOTON-II CMOS diffractometer (for 1-CH3 only) for data collection at 173(2) K or 123(2) K. The data collection was carried out using Mo Kα (graphite monochromator) or Cu Kα radiation with normal parabolic mirrors. The data intensities were corrected for absorption and decay with SADABS.49 Final cell constants were obtained from least-squares fits from all reflections. A direct-methods solution (SHELXS-97) provided most non-hydrogen atoms from the electron-density map (E map).50 Full matrix least-squares/difference Fourier cycles were performed to locate the remaining non-hydrogen atoms. All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters, with the exception of the Rh–H moieties in 1-H and 2-H, which were located in the Fourier difference maps. The crystal of 1-CH3 was found to contain substitutional disorder between the methyl group and the chloride starting material, 1-Cl. The Rh–Cl bond distance was constrained to the experimentally measured bond distance found in 1-Cl, allowing for the individual methyl and chloride components to be refined and quantified, yielding a 11% composition of the 1-Cl impurity in the crystal. Complex 1-H was refined as a non-merohedral twin with the main component contributing 85%; the appropriate twin law was applied. Absorption correction for both components of the twin was accomplished using TWINABS.511-Cl, 1-CH3, 1-H, 2-Cl, and 2-CH3 contained benzene solvent molecules that were disordered over an inversion center. Each carbon atom was refined with 50% occupancy and treated as an ideal hexagon using the SHELXL AFIX 66 restraint. In addition, the anisotropic displacement parameters were restrained using the RIGU geometrical restraint, and the carbon atoms related by inversion symmetry were treated with the EADP constraint. Complex 2-H contained a disordered benzene molecule that was modeled using the SAME and DELU geometrical restraints and EADP constraint. Finally, a disordered isopropyl group in complex 1-H was modeled using the SAME restraint and EADP constraint. One reflection in 2-H was found to have been affected by the beamstop and was removed in the final refinement. Crystallographic data are summarized in Table 1.
Table 1 Crystallographic details for 1-Cl, 2-Cl, 1-CH3, 2-CH3, 1-H, and 2-H
  1-Cl 2-Cl 1-CH3
Chemical formula C39H60N4P3AlRhCl·0.5(C6H6) C39H60N4P3GaRhCl·0.5(C6H6) C39H60N4P3AlRh(CH3)0.89Cl0.11·0.5(C6H6)
Formula weight 882.21 924.95 863.98
Crystal system Triclinic Monoclinic Triclinic
Space group P[1 with combining macron] P21/n P[1 with combining macron]
a (Å) 12.3569(19) 12.2670(19) 12.3599(3)
b (Å) 13.627(2) 26.703(4) 13.5401(3)
c (Å) 14.305(2) 13.837(2) 14.2951(3)
α (°) 66.376(2) 90 67.2469(11)
β (°) 81.629(2) 108.349(2) 81.6400(11)
γ (°) 75.031(2) 90 76.0822(12)
V3) 2129.6(6) 4302.0(12) 2137.74(9)
Z 2 4 2
D calcd (g cm−3) 1.376 1.428 1.342
λ (Å), μ (mm−1) 0.71073, 0.632 0.71073, 1.218 1.54178, 4.811
T (K) 173(2) 173(2) 123(2)
θ range (°) 1.556–27.483 1.525–25.782 3.358–74.637
Reflns collected 24[thin space (1/6-em)]898 36[thin space (1/6-em)]954 44[thin space (1/6-em)]289
Unique reflns 8696 5294 7966
Data/restraints/params 9614/36/479 8209/36/478 8694/37/489
R 1, wR2 (I > 2σ(I)) 0.0255, 0.0645 0.0492, 0.0987 0.0446, 0.1134

  2-CH3 1-H 2-H
Chemical formula C40H63N4P3GaRh·C6H6 C39H61N4P3AlRh·0.5(C6H6) C39H61N4P3GaRh·0.5(C6H6)
Formula weight 943.59 847.77 890.51
Crystal system Triclinic Triclinic Monoclinic
Space group P[1 with combining macron] P[1 with combining macron] P21
a (Å) 12.3557(13) 11.611(7) 12.300(2)
b (Å) 13.1544(14) 14.168(8) 27.928(5)
c (Å) 14.8942(16) 14.280(8) 12.784(3)
α (°) 72.2110(10) 69.082(7) 90
β (°) 79.6010(10) 77.640(7) 106.028(2)
γ (°) 81.3190(10) 82.728(7) 90
V3) 2255.3(4) 2140(2) 4220.8(14)
Z 2 2 4
D calcd (g cm−3) 1.390 1.315 1.401
λ (Å), μ (mm−1) 0.71073, 1.106 0.71073, 0.565 0.71073, 1.177
T (K) 173(2) 173(2) 173(2)
θ range (°) 1.451–26.448 1.541–25.757 1.458–27.228
Reflns collected 24[thin space (1/6-em)]717 8104 48[thin space (1/6-em)]954
Unique reflns 7427 6320 16[thin space (1/6-em)]031
Data/restraints/params 9254/72/539 8104/39/486 18[thin space (1/6-em)]804/37/970
R 1, wR2 (I > 2σ(I)) 0.0335, 0.0726 0.0489, 0.1076 0.0386, 0.0598


Results and discussion

Synthesis of Rh–M complexes

Addition of 0.5 equiv. {Rh(μ-Cl)(C2H4)2}2 to the metalloligands, AlL and GaL, generated Cl–RhAlL (1-Cl) and Cl–RhGaL (2-Cl), respectively (Scheme 1). Both 1-Cl and 2-Cl can be further functionalized through metathesis reactions. Methylation of 1-Cl and 2-Cl was effected using a slight excess of methyllithium, yielding CH3–RhAlL (1-CH3) and CH3–RhGaL (2-CH3). Alternatively, 1-Cl and 2-Cl can be treated with 1.1 equiv. LiHBEt3, providing the hydride complexes, H–RhAlL (1-H) and H–RhGaL (2-H). Solutions of 1-Cl and 2-Cl are orange and red, respectively. The colors of the Rh–M methyl and hydride complexes are yellow and yellow-orange for M = Al and Ga, respectively.
image file: c6dt04769f-s1.tif
Scheme 1 Synthesis of X–RhAlL (1-X) and X–RhGaL (2-X) complexes, where X = Cl, CH3, and H.

Electrochemistry

Cyclic voltammograms (CVs) of the chloride complexes, 1-Cl and 2-Cl, were collected at various scan rates (Fig. 2). In addition to irreversible oxidation processes (ESI, Fig. S1 and S2), the CVs of 1-Cl and 2-Cl show an irreversible one-electron reduction that is coupled to subsequent electrochemical reactions (vide infra). The peak cathodic potential (Epc) of the reduction process is ∼120 mV more mild for 2-Cl (−1.95 V vs. Fc+/Fc, 25 mV s−1) than for 1-Cl (−2.07 V, same conditions). The milder Epc of 2-Cl suggests that GaIII is a better σ-acceptor for RhI than AlIII.
image file: c6dt04769f-f2.tif
Fig. 2 Proposed mechanistic scheme (top) for the EC steps in the cyclic voltammogram (CV) of 1-Cl and 2-Cl (bottom) in 0.1 M TBAPF6 in THF referenced to the Fc/Fc+ redox couple.

For both complexes, the initial irreversible reduction is followed by an oxidation event at a peak anodic potential (Epa) of −1.59 and −1.64 V for 1-Cl and 2-Cl, respectively (vs. Fc+/Fc, 25 mV s−1). The large peak-to-peak separation (|Epc− Epa| ≥ 310 mV) indicates an intermediary chemical reaction that is kinetically fast. A reasonable proposal is the dissociation of the Cl ligand upon reduction, where the EC steps are labeled as E1 and C1 in Fig. 2. The resulting product, RhML, is electrochemically active and undergoes one-electron oxidation (E2) to [RhML]+. Comparing the two single-electron transfers, E2 is more positive than E1, in part, because of the differences in overall charge between the redox pairs: [RhML]+/RhML and Cl–RhML/[Cl–RhML]. Of note, the [RhML]+ cation appears to be more stable for AlIII than GaIII since the E2 step exhibits quasi-reversibility at fast scan rates (≥250 mV, image file: c6dt04769f-t2.tif) for 1-Cl. The lack of reversibility for [RhGaL]+ indicates that the Rh center is much more electrophilic than that of [RhAlL]+, resulting in faster chloride association for Rh–Ga than Rh–Al.

X-ray crystallography

Molecular structures of the Rh–M complexes were elucidated using single-crystal X-ray diffraction (Fig. 3). Nearly all the complexes crystallize in the triclinic space group P[1 with combining macron], with the exception of 2-Cl and 2-H, which crystallize in the monoclinic space groups P21/n and P21, respectively. The bimetallic complexes all feature five-coordinate Rh centers that are significantly distorted between square pyramidal and trigonal bipyramidal (vide infra). The exact geometry about Rh depends largely on the identity of X (Cl, CH3, or H), rather than the group 13 ion, MIII (Al or Ga). Hence, the Rh–M bimetallic complexes are best categorized as isostructural pairs, 1-X and 2-X, for identical X ligands. Selected geometric parameters for the Rh–M complexes are shown in Table 2.
image file: c6dt04769f-f3.tif
Fig. 3 Molecular structures of 1-X and 2-X with thermal ellipsoids set at 50% probability. For N- and P-atom labels, see 1-Cl. With respect to the X ligand, P2 is located trans, while P1 and P3 are cis. Non-coordinating solvent molecules and H atoms were omitted for clarity (except for the hydride atoms). Atom color guide: Rh, red; P, orange; Al, cyan; Ga, pink; Cl, green; C, gray; H (hydride), gold.
Table 2 Geometrical parameters, including selected bond lengths (Å) and angles (°) for 1-Cl, 2-Cl, 1-CH3, 2-CH3, 1-H, and 2-H. M is Al for 1-X and Ga for 2-X, where X represents Cl, H, or CH3
  1-Cl 2-Cl 1-CH3 2-CH3 1-H 2-H
a r = (Rh–M bond distance)/(sum of Rh and M covalent radii).52 b Interplanar angle between P1–Rh–P2 and P3–Rh–X.53 c Two independent molecules in the asymmetric unit. d Geometrical parameters involving the hydride may vary from the actual values due to the inaccuracy in placing a hydrogen atom near a heavy metal.
Rh–M 2.5400(6) 2.5003(7) 2.5460(9) 2.4925(4) 2.4974(14) 2.4793(8) 2.4459(8)
r 0.97 0.95 0.97 0.94 0.95 0.94 0.93
Rh–P1 2.3194(5) 2.3288(14) 2.2951(8) 2.2978(7) 2.2949(14) 2.2911(16) 2.2900(16)
Rh–P2 2.2296(5) 2.2368(14) 2.2804(8) 2.2966(7) 2.3511(14) 2.3625(16) 2.3266(15)
Rh–P3 2.3832(5) 2.3947(14) 2.3632(8) 2.3757(7) 2.3056(15) 2.3127(15) 2.3017(15)
Rh–X 2.3983(5) 2.4151(14) 2.188(19) 2.118(10) 1.623d 1.510d 1.437d
M–N1 1.8826(15) 1.929(4) 1.881(3) 1.939(2) 1.889(3) 1.939(4) 1.945(4)
M–N2 1.8823(15) 1.939(4) 1.889(3) 1.949(2) 1.895(3) 1.932(4) 1.945(4)
M–N3 1.8789(15) 1.942(4) 1.892(3) 1.935(2) 1.892(3) 1.939(4) 1.941(4)
M–N4 2.1227(14) 2.226(4) 2.145(3) 2.272(2) 2.140(3) 2.263(4) 2.259(4)
Rh–(P3-plane) 0.328 0.353 0.341 0.37 0.236 0.263 0.170
X–(P3-plane) 1.815 1.980 1.623 1.620 0.498d 0.426d 0.231d
M–(N3-plane) 0.261 0.354 0.287 0.391 0.300 0.377 0.362
P1–Rh–P2 101.206(18) 101.72(5) 102.01(3) 103.97(3) 110.52(5) 109.04(6) 107.53(5)
P2–Rh–P3 99.286(19) 99.32(5) 100.76(3) 102.08(3) 106.76(5) 106.98(5) 104.95(5)
P1–Rh–P3 151.711(17) 150.25(5) 149.16(3) 145.16(3) 139.39(5) 139.84(6) 145.65(5)
P1–Rh–X 86.048(19) 86.99(5) 83.8(4) 82.68(8) 75.54 77.22 71.45
P2–Rh–X 150.122(18) 146.70(5) 152.7(4) 153.06(8) 173.39 173.64 176.13
P3–Rh–X 85.828(19) 86.86(5) 130.0(4) 84.51(8) 68.27 66.72 75.74
s-angleb 37.6 41.3 36.5 38.3 18.3d 11.0d 18.4d
M–Rh–X 130.84(2) 134.11(4) 130.0(4) 130.88(8) 100.61d 97.13d 93.97d
N4–M–Rh 172.63(4) 172.84(10) 172.55(8) 173.51(6) 176.90(10) 177.62(11) 177.51(11)
N1–M–N2 115.85(6) 111.09(17) 112.07(12) 112.08(9) 118.14(15) 117.49(19) 117.27(18)
N2–M–N3 112.07(6) 114.38(17) 115.98(11) 113.90(9) 114.14(15) 113.37(18) 112.65(18)
N3–M–N1 126.32(6) 124.65(17) 125.06(12) 122.08(9) 120.33(16) 118.06(18) 119.94(18)


The Rh–M bond length are nearly invariant across the subfamilies 1-X and 2-X. The Rh–Al and Rh–Ga bond lengths vary slightly from 2.50–2.54 Å for 1-X and 2.45–2.50 Å for 2-X, respectively. The Rh–Ga bond lengths in 2-X are comparable to those in the two RhI → GaIII structures discussed previously (2.42 and 2.45 Å). To gauge the strength of the Rh–M interaction, the Rh–M bond distances were normalized to the sum of the metals’ covalent radii to obtain the covalent ratio (r) for these metal–metal interactions.52 The r value ranges from 0.93 to 0.97 across all six bimetallics, indicating a significant Rh → MIII dative interaction. Of note, the Rh–M bond distances between the M = Al and M = Ga complexes are similar (within 0.05 Å), though Rh–Ga bonds are slightly contracted compared to their Rh–Al counterparts. Similar Rh–M bond distances are consistent with the similar covalent radii of Al and Ga (1.21 and 1.22 Å, respectively).52 The slightly shorter Rh–Ga bonds may arise from the GaIII ion residing higher above the N3–plane than AlIII by 0.06 to 0.10 Å in this family.15

The methyl complexes 1-CH3 and 2-CH3 have Rh–C bond lengths of 2.19 and 2.12 Å, respectively. A search of the Cambridge Structural Database produced only 7 structurally characterized RhI methyl complexes: the average Rh–C bond length is 2.141 Å with a standard deviation of 0.038 Å.54–58 Thus, the Rh–C bond lengths in 1-CH3 and 2-CH3 compare well to this limited set. In 1-H and 2-H, the crystals were of sufficient quality such that the hydride atom was located in the Fourier difference map. However, the Rh–H bond lengths are unreliable because of the uncertainty associated with placing a weak-scattering hydrogen atom in the Fourier difference map, especially in the vicinity of a heavy metal.59 The Rh–P2 bond elongates in the order of Cl < CH3 < H, which is in accordance with the expected order based on the trans-influence of these ligands. For the bulkier Cl and CH3 ligands, a notable distortion is the elongation of one Rh–Pcis bond (cis refers to the phosphine position relative to X), such that the two Rh–Pcis bonds are notably inequivalent in 1-Cl, 2-Cl, 1-CH3, and 2-CH3. The differences between the Rh–P1 and Rh–P3 bond lengths range from 0.066 to 0.079 Å in these bimetallic complexes. Such a large deviation is not observed in 1-H, 2-H, or other four-53,60–67 and five-coordinate68–71 Rh complexes which contain two identical phosphine donors trans to one another with one exception (vide infra). For instance, the Rh–Pcis bonds in Rh(PPh3)3X complexes differ by 0.008 to 0.046 Å, with the greatest deviation observed for Rh(PPh3)3CN.

The similar sizes of the bulkier Cl and CH3 ligands also result in large angular distortions about the Rh center in 1-Cl, 2-Cl, 1-CH3, and 2-CH3. The P1–Rh–P3 and P2–Rh–X angles, where X is Cl or CH3, both approach 150°, deviating significantly from the expected 180° of an ideal square pyramid. The corresponding τ5 parameters (0.03 to 0.13) would denote nearly perfect square pyramidal geometry (τ5 = 0),72 which is certainly misleading in these cases. Instead, the s-angle, or the interplanar angle between the P1–Rh–P2 and P3–Rh–X (see Table 2), is a better way to describe the distortion of the Rh center from square planar/pyramidal geometry.53 In the Cl and CH3 complexes, the s-angles are all close to 40°. The distortion reduces the steric repulsion between the isopropyl groups of the cis-phosphines (relative to X) and the X ligand by raising the bulky X group out of the triphosphine plane.

In contrast, the hydride ligands in 1-H and 2-H reside within the Rh-triphosphine plane. The τ5 parameters for 1-H and 2-H are also misleading at 0.51 and 0.62, respectively. Instead the small distortion from square pyramidal geometry is reflected by the smaller s-angles, which range between 10 and 20° for 1-H and 2-H. The P2–Rh–H angle approaches 180°, but the Pcis–Rh–H bond angles deviate significantly from the ideal 90° (range from 67 to 76°). Such acute bond angles have been previously observed in RhH(PR3)3 structures, and are a result of greater repulsion between phosphine donors than phosphine-hydride ligands.64,73,74

Another close structural analog to the complexes 1-X and 2-X is the rhodaboratrane complex [{o-(Ph2P)C6H4}3BRh(CO)]+ synthesized by Nakazawa and co-workers.69 This cationic complex has a square pyramidal Rh center and an s-angle of 15°, which is comparable to 1-H and 2-H.69 Of interest, the Rh–Pcis bond lengths differ by 0.067 Å, which is nearly on par with the chloride and methyl complexes of 1 and 2. Hence, the steric forces of this rhodaboratrane cation featuring diphenyl phosphine groups and a CO ligand fall in between these hydride complexes and their methyl/chloride counterparts.

For the Rh–M bimetallics with bulky X ligands (Cl, CH3), a close contact of ∼2 Å is observed between the methine proton belonging to P1 and a methylene proton of the adjacent P2 ligand arm. This short contact, which is not observed in the hydride species, correlates with an elongated Rh–P1 bond compared to Rh–P3. Presumably, the Rh–P1 bond stretches to decrease steric congestion with the X ligand, but at the cost of increasing steric repulsion with the adjacent P2 ligand arm.

NMR spectroscopy

31P VT-NMR studies

The Rh–M complexes reported here also exhibit different extents of fluxionality at room temperature, and so, both low and high-temperature 31P and 1H NMR studies were performed. For the Rh–M bimetallics with bulky X ligands (X = Cl, CH3), three distinct phosphorus resonances were observed at room temperature, which sharpen upon cooling (Fig. 4, Table 3). Typically, trans-disposed phosphines in square pyramidal/planar Rh triphosphine complexes are chemically equivalent by a mirror plane of symmetry.53,61–63,65–67,69,75–77 However, the propeller-like geometry enforced by the ligand backbone of L78,79 makes it so that the complex does not possess a mirror plane of symmetry. While the solid-state structure suggests that the Pcis nuclei in 1 and 2-X should be chemically unique (cis refers to the phosphine position relative to X), the observation of two distinct Pcis nuclei by NMR spectroscopy is only rarely observed in five-coordinate triphosphine Rh complexes.80 In the rhodaboratrane complex [{o-(Ph2P)C6H4}3BRh(CO)]+ (vide supra), the trans-disposed phosphine donors are equivalent by 31P NMR spectroscopy, most likely due to some fluxional motion that exchanges the two phosphines. However, low-temperature NMR data for this complex were not reported.
image file: c6dt04769f-f4.tif
Fig. 4 31P{1H} NMR spectra of 1-X and 2-X complexes (162 MHz). Unless otherwise noted, the spectra were recorded in CD2Cl2. For 1-H, 2-H, and 2-CH3, spectra were recorded in toluene-d8. For 2-H, the −73 °C spectrum is shown, but the three 31P nuclei become resolved from −39 °C.
Table 3 31P{1H} NMR chemical shifts and coupling constants (in Hz) of 1-X and 2-X complexes (refer to Fig. 4 caption for conditions unless otherwise noted)
Complex P1 P2 P3
δ J Rh–P 1 J P 1 –P 3 J P 1 −P 2 δ J Rh–P 2 J P 1,3 –P 2 δ J Rh–P 3 J P 2 –P 3
a 103Rh: S = 1/2, 100% abundant. b Data obtained at −73 °C for P1 and P3. P2 coupling values were obtained at −49 °C due to better resolution. c Coupling was not resolved at the low T limit for the respective solvents.
1-Cl 45.0 131 317 24 80.7 157 25 26.8 107 25
2-Cl 51.2 128 325 22 93.3 146 23 30.5 105 24
1-CH3 45.0 155 313 19 59.0 108 19 23.3 126 18
2-CH3 50.3 154 317 17 70.8 99 17 27.3 124 17
1-H 59.1 138 265 c 51.6 119 20 43.4 125 c
2-Hb 64.4 142 273 17 58.3 114 19 49.2 123 c


For the Rh-M bimetallic complexes with bulky X ligands (1-Cl, 2-Cl, 1-CH3, and 2-CH3), the most downfield 31P resonance is a doublet of triplets (dt, JRh–P2 = 99 to 157 Hz, JP1,3–P2 = 17 to 25 Hz), which corresponds to P2, the phosphorus trans to X. The P2 signal is highly sensitive to the nature of X, shifting ∼30 ppm within the 1-X and 2-X subgroups. As the trans influence of X increases, the P2 resonance moves upfield, which is consistent with increased shielding of the 31P nucleus as the Rh–P2 bond elongates. The two trans-phosphorus nuclei that are cis to X, namely P1 and P3, give rise to two distinct “doublet of doublet of doublets” (ddd, JP1–P3 = 313 to 325 Hz, JRhP1,3 = 105 to 155 Hz, JP2P1,3 = 17 to 25 Hz).81,82 Analogous to the solid-state structures, the coupling values for the Rh–M bimetallics are similar when the X ligand is identical, regardless of the different group 13 MIII ions. On the other hand, the identity of M impacts the relative 31P NMR chemical shifts, where Rh–GaIII complexes display more downfield signals than their Rh–AlIII analogues.

Fluxionality that exchanges P1 and P3

For both 1-H and 2-H, the room temperature 31P NMR spectra display a single resonance corresponding to P2 (dt, avg. JRhP2 = 117 Hz, JP1,3P2 = 19 Hz, ESI Fig. S38 and S45). The P1 and P3 resonances are notably absent; presumably, the P1 and P3 peaks are broadened into the baseline due to coalescence. Upon lowering the temperature, the P1 and P3 peaks appear and resolve close to −70 and −40 °C for 1-H and 2-H, respectively (ddd, avg JP1P3 = 269 Hz, JRhP1,3∼132 Hz, JP2Pcis∼18 Hz). At the high-temperature probe limit (97 °C, toluene-d8), the P1 and P3 peaks of 2-H appear as a single doublet at 57 ppm, which is consistent with fast exchange (ESI, Fig. S46). In addition to 1-H and 2-H, coalescence of the P1 and P3 resonances were also observed for 1-Cl and 2-Cl, though at more elevated temperatures (Table 4). At the probe temperature limit of 370 K, the P1 and P3 peaks of 1-CH3, and 2-CH3 were approaching coalescence (Fig. S24 and S32, respectively).
Table 4 Coalescence temperature (Tc), rate constant (kc), and activation energy (ΔG) for the equilibration of the trans-disposed phosphines, P1 and P3, in Rh–M bimetallic complexes
  1-Cl 2-Cl 1-CH3 2-CH3 1-H 2-H
T c (K) 359 345 >370 >370 265 279
k c (s−1) 6700 7600 >7900 >8500 5900 5700
ΔG (kcal mol−1) 14.9 14.2 >15.2 >15.2 10.9 11.5


The motion that equilibrates the two trans-disposed P nuclei likely takes place through a mechanism by which all three ligand arms flip in between two different propeller-type geometries, exchanging the relative orientations of the two diisopropyl groups. Such a motion may be accompanied by an asymmetric stretching of the two Rh–P bonds along the P1–Rh–P3 axis that toggles between P1; Rh–P3 and P1–Rh; P3. Bourissou and coworkers have observed fluxional behavior in a variety of metallaboratranes, which involves flipping between two propeller geometries and concomitant exchange of the phosphine R groups. They propose a mechanism of phosphine dissociation and reassociation, which also cannot be discounted.83 The energy barrier (ΔG) for this fluxional process can be estimated from the coalescence temperature and chemical shift difference between P1 and P3 (see Experimental, Table S1). The activation energies increase with the size of the X ligand, such that the lowest barriers are found for the hydrides, followed by the chlorides, and then, the methyl complexes.

Fluxionality differences between 2-Cl and 2-H

Of note, 2-Cl is the sole Rh–M bimetallic for which fast exchange of all three 31P nuclei was observed within the probe temperature limit, which occurs at 97 °C (Fig. S16). The exchange between Ptrans and Pcis (trans and cis refer to the phosphine position relative to X) likely requires initial dissociation of one phosphine, followed by phosphine re-association at a different position, as shown in Fig. 5. The Ptrans resonance in 1-Cl also appears to broaden and begin to coalesce with Pcis, however coalescence is not achieved even at 115 °C (Fig. S9). In the case of the hydride complex, 2-H, P2 showed no propensity to exchange with P1,3 even at 97 °C (Fig. S47). This difference in fluxional behavior between 2-H and 2-Cl could reflect the favorability of dissociating a phosphine donor from a sterically encumbered Rh center in 2-Clversus2-H. Since the temperatures (and, therefore the energy barriers) for first equilibrating P1 and P3, and then equilibrating P2 and P1,3 are reasonably close for 2-ClTc = 25 K), it is possible that some Rh–P bond breaking character is already present in the exchange of P1 and P3. In contrast, the energy barrier for equilibrating P1 and P3 in 2-H is low at 11.5 kcal mol−1, and no sign of equilibrating P2 and P1,3 was detected. Hence, equilibrating the two Pcis donors in the non-bulky hydride 2-H is facile and does not involve Rh–P bond cleavage.
image file: c6dt04769f-f5.tif
Fig. 5 Proposed mechanism to exchange the phosphine trans to Cl (P2) with the phosphine cis to Cl (e.g. P1) that is observed for 2-Cl at high temperature (Tc = 369 K). Only donors around the Rh center are shown.

1H VT-NMR studies

The fluxionality of the Rh–M bimetallics was interrogated by variable temperature 1H NMR spectroscopy to complement the 31P NMR studies. At room temperature, the bimetallics with bulky X ligands display broad, ill-defined resonances, especially in the aliphatic region. The peaks sharpen upon cooling, and the solution-state structure becomes fully asymmetric, or C1. As a representative example, the 1H NMR spectrum of 1-CH3 at −20 °C is shown in Fig. 6. The six methylene and six methine protons of the L ligand were all observed as individual peaks. For the Rh–methyl group, a small 2JRh–H coupling constant of 1.0 Hz could also be discerned at higher temperatures.
image file: c6dt04769f-f6.tif
Fig. 6 1H{31P} NMR spectrum of 1-CH3 (400 MHz, toluene-d8, −20 °C). Peaks that are labeled with a symbol correspond to ligand methylene (■), ligand methine (•), and Rh–CH3 (◆) protons.

The 1H NMR spectra of 1-Cl and 2-Cl at high temperature (115 and 75 °C, respectively) simplify to 8 broad peaks for the L ligand. The 8 broad peaks can be assigned to the 4 unique aryl protons, methylene, methine, and “up” and “down” methyl groups.78 Hence, the chloride complexes approach C3v symmetry,78 which is consistent with the (near) coalescence of all three phosphorus atoms (P1 = P2 = P3) described above. At similarly high temperatures, the 1H NMR spectra of 1-CH3 and 2-CH3 exhibit Cs symmetry, where 2 sets of the four aryl protons each are observed with a relative integration of 2[thin space (1/6-em)]:[thin space (1/6-em)]1. This is also consistent with the observed coalescence of the two trans-disposed phosphorus nuclei, such that P1 = P3 ≠ P2.

The room-temperature 1H NMR spectra of 1-H and 2-H are consistent with Cs symmetry, where the distinctive methylene protons on the ligand backbone are observed as 3 unique peaks with equal integrations: a broad singlet corresponding to PtransCH2, and a set of doublets for the diastereotopic protons, PcisCHH′ and PcisCHH′ for P1 and P3. At −77 °C, both 1-H and 2-H display broad, ill-defined peaks, especially in the aliphatic region. Hence, unlike the chloride and methyl complexes, the C1 solution structures are not observed for the hydride complexes above the freezing point of toluene-d8. The different temperature profile of the hydride complexes compared to the other bimetallics is consistent with the significantly lower barrier to equilibrate P1 and P3 in the hydrides. The hydride resonance is a doublet of triplet of doublets, as shown in Fig. 7. The 2JRh–H coupling constant of 12.3 and 12.4 Hz can be observed for 1-H and 2-H, respectively, and is well within the norm for RhI hydrides trans to a trialkyl phosphine.84–86


image file: c6dt04769f-f7.tif
Fig. 7 Stacked plots of the 1H and 1H{31P} NMR spectra (400 MHz) of the Rh hydride resonance in 1-H (left) and 2-H (right). Coupling constants (2JP(trans)–H, 2JP(cis)–H, and JRh–H): 107.1, 24.2, and 12.4 Hz for 1-H in CD2Cl2 at −8 °C; 109.3, 23.0, 12.3 Hz for 2-H in toluene-d8 at 7 °C.

Conclusions

Six complexes that feature significant Z-type Rh → Al or Rh → Ga interactions were synthesized and structurally characterized. The cyclic voltammograms of 1-Cl and 2-Cl exhibit an irreversible reduction process at Epa of −2.07 and −1.95 V (vs. Fc+/Fc), respectively. The milder reduction potential observed for 2-Cl suggests that GaIII is a stronger σ-acceptor for Rh than AlIII. The ratio (r) of the Rh–M bond length to the sum of the metals’ covalent radii are less than unity for all Rh–M complexes: 0.97 to 0.95 for 1-X and 0.95 to 0.93 for 2-X. The lower r values for Ga compared to Al is consistent with the Ga being the stronger σ-acceptor.

Though Rh → M interactions do not vary across the different X groups (chloride, methyl, or hydride), the steric bulk of the X group determines the extent of geometric distortion at Rh. Because of the steric clash between X and the isopropyl substituents on the ligand, larger X groups (Cl, CH3) exhibit greater distortions than the hydrides. Related to the distortion, all three phosphine donors are chemically inequivalent by NMR spectroscopy, which is unusual for triphosphine Rh complexes with identical donors. Two different fluxional processes that exchange the different phosphines in 1-X and 2-X were scrutinized. The lower barrier process involves equilibration of two trans-disposed phosphine donors (cis to the X) via the asymmetric stretching of the Rh–P bonds. The second process involves equilibration of all three phosphine donors, which likely requires Rh–P bond dissociation and reassociation. Future efforts will focus on examining the reactivity of these Rh complexes, with a focus on whether the identity of the Lewis acid can lead to differing reactivity profiles for these Rh metallatranes.

Acknowledgements

We thank Dr Victor Young, Jr. and Ryan Cammarota for experimental assistance. JTM would also like to thank Dr Varinia Bernales, Dr Dale Pahls, and Dr Letitia Yao for helpful discussions. This work was supported as part of the Inorganometallic Catalyst Design Center, an EFRC funded by the DOE, Office of Basic Energy Sciences (DE-SC0012702).

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Footnote

Electronic supplementary information (ESI) available: Additional physical and spectroscopic data, and CIF files. CCDC 1523218–1523223. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6dt04769f

This journal is © The Royal Society of Chemistry 2017