Synthesis, characterization and electrochemistry of rhodium(III) complexes of meso-substituted [14]tribenzotriphyrin(2.1.1)

Abstract

A thermal reaction using a series of [14]tribenzotriphyrins(2.1.1) (TriPs, 1a–d) with Rh₂(C₈H₁₂)Cl₂ provides Rh^III–TriP complexes (2a–d) in 40−52% yields. The complexes were characterized by mass spectrometry, UV-visible absorption and ¹H NMR spectroscopy. Single crystal X-ray analysis reveals that 2b adopts a dome-shaped conformation. The rhodium(III) ion is coordinated by the three pyrrole nitrogen atoms, two chloride ions and the nitrogen atom of an acetonitrile (CH₃CN) solvent molecule. The optical spectra can be assigned using Michl's perimeter model. The L and B bands of the 2a–d complexes lie at ca. 600 and 500 nm, respectively, and are markedly red shifted relative to those of 1a–d. A reversible one-electron oxidation and two reversible one-electron reductions are observed in the cyclic voltammograms of 2a–d in CH₂Cl₂. The redox potentials are consistent with the optical data and the relatively narrow HOMO–LUMO gaps that are predicted in DFT calculations. TD-DFT calculations have been used to assign a third intense spectral band at 375 nm to a higher energy π → π* transition of the [14]tribenzotriphyrin(2.1.1) π-system.

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Introduction

Porphyrins consist of four pyrrole units which are linked by four meso-carbon atoms in a planar arrangement due to their 18-π heteroaromatic character.¹ The rich chemistry of porphyrins and their derivatives, has led to a strong research focus on their synthesis and properties, since the 1920s.² As a result, porphyrins and their analogues have found applications in various fields, such as polymer materials, chemical catalysis, electroluminescent materials, and molecular targeted drugs.^3,4

Two approaches have commonly been used to form ring-contracted porphyrin analogues. The first is to remove one of the four meso-carbon atoms to form corroles⁵ and the other is to remove one of the four pyrrole rings to form a triphyrin (Chart 1).^6–9 The successful preparation of triphyrin macrocycles would provide novel flexible metal complexation environments, which would facilitate novel applications. A key research goal in the triphyrin field has been to form free-base triphyrin compounds, so that a richer coordination chemistry can be accessed through metal insertion reactions. Unfortunately to date, there have been no reports of the synthesis and properties of free base triphyrin analogue of the porphyrin ring. Instead, only boron(III)-contained triphyrin complexes have been prepared. The first example of a boron(III)-triphyrin analogue, boron tribenzosubporphyrin,¹⁰ was reported by the Osuka group in 2006, while Kobayashi and coworkers subsequently reported a series of meso-aryl-substituted boron subporphyrins (Chart 1). Since then, a number of meso-aryl substituted subporphyrins have been reported.^11–18 With the exception of core-modified subpyriporphyrins,¹⁹ and compounds with expanded macrocycle rings such as [n]triphyrins(m.1.1)²⁰ and [14]heterotriphyrins(2.1.1),^21,22 only boron complexes with highly nonplanar cone-shaped conformations have been reported in recent years.¹²

Chart 1 Structure of triphyrin and its derivatives.

In 2008, some of us reported the facile synthesis of meso-aryl-substituted free-base [14]tribenzotriphyrins(2.1.1) (TriPs, 1).^23,24 This provided the first example of a boron-free ring-contracted subporphyrin analogue containing three pyrrole moieties (Chart 1). Their formation is facilitated by the presence of an extra meso-carbon atom, because this means that only one inner NH proton to form a heteroaromatic π-system with 14-π electrons. Since then, meso-²⁵ and β-free²⁶ compounds have also been prepared, together with hetero-[14]triphyrins(2.1.1),²² through a series of different condensation reactions. TriPs can serve as tridentate nitrogen ligands and form monoanions upon deprotonation, and hence have an unusual coordination chemistry. Previously we have reported several kinds of transition metal [14]tribenzotriphyrins(2.1.1) complexes, such as metals from group 7 including Mn(I)²⁷ and Re(I);²⁴ group 8 including Fe(II)²⁹ and Ru(II)²⁴ and group 10 including Pd(II),³⁰ Pt(II)²³ and Pt(IV).²⁸ Most of the reported triphyrin metal complexes adopt octahedral comformations while in contrast Pd(II) and Pt(II) triphyrin complexes adopt deeply saddled structures. To date, no group 9 metal has been combined with a [14]tribenzotriphyrin(2.1.1) ligand.

Herein, we describe the synthesis, characterization and properties of Rh(III) [14]tribenzotriphyrin(2.1.1) dichloride complexes (Scheme 1). A detailed analysis of the magnetic circular dichroism (MCD) spectral data and the trends identified in TD-DFT calculations, and electrochemical data, has been carried out to identify the key structure–property relationships.

Scheme 1 Synthesis of Rh^III–TriP complexes 2a–d.

Results and discussion

Synthesis and characterization

[14]Tribenzotriphyrin(2.1.1) (1) was treated with Rh₂(C₈H₁₂)Cl₂ following the conventional procedure that are generally used for the synthesis of metalloporphyrins. A dry toluene solution of 1 in a Schlenk flask was treated with 10 equiv. of Rh₂(C₈H₁₂)Cl₂ and refluxed for 24 h under a nitrogen atmosphere. After the elimination of the solvent, the residue was dissolved in CH₂Cl₂ and the solution was filtered to remove precipitates. The solvent was removed and the residue was purified by short silica gel column chromatography using CH₂Cl₂ as an eluent. The first eluted grey blue fraction was evaporated to afford the crude product. Crystallization from CH₂Cl₂ and CH₃OH gave the pure target compound in 52% yield for 2a (R = –CH₃), 48% for 2b (R = –H), 45% for 2c (R = –F) and 40% for 2d (R = –COOCH₃), respectively. The structures of 2a–d were characterized by mass spectrometry and NMR spectroscopy.

¹H NMR spectroscopy

The ¹H NMR spectra of 1a and 2a are shown in Fig. S2,† and those of 2b–d are provided as ESI, Fig. S3–S6.† The NH proton signal, which lies at 8.23 ppm in the spectrum of 1a, is no longer observed. 2a has a relatively simple set of signals in the aromatic region, due to the symmetric structure. The singlet peaks at 2.76 and 2.51 ppm are associated with the methyl protons of the tolyl groups. In a similar manner to what is observed for Pt(IV), Re(I), Mn(I) and Ru(II) [14]tribenzotriphyrin(2.1.1) complexes, the proton peaks of the meso-aryl groups are significantly broadened.^24,27,28

X-ray diffration analysis

The structure of 2b was unambiguously determined by single-crystal X-ray diffraction analysis, which was obtained by slow diffusion of acetonitrile into the dichloromethane solution over a period of 10 days. Details of the crystal structures of 2b are summarized in Fig. 1 and 2, Tables 1 and 2, and Fig. S1 and Table S1 in the ESI.† The triphyrin macrocycle adopts a bowl-shaped conformation which is similar to that reported previously for Re^I–TriP and Pt^IV–TriP complexes.^24,28 The bowl depth of the Rh^III–TriP complex, as defined by the distance from the metal ion to the mean plane of the six peripheral pyrrole β-carbon atoms, is 1.935 Å and is much more shorter than that reported for the Re^I–TriP complex (2.248 Å).²⁴ The Rh(III) ion is coordinated by three pyrrole nitrogen atoms, two chloride ions and a CH₃CN solvent molecule. The average bond length between the Rh(III) ion and the three pyrrole nitrogens is 1.974(5) Å (Fig. 2), which is significantly shorter than that reported for the Rh(III) complex of N-confused tetraphenylporphyrin (2.087(3) Å),³¹ in marked contrast with the longer bond length between the Rh(III) ion and the nitrogen atom of CH₃CN ligand (2.071(6) Å). The average bond length between the meso-carbon atom and the phenyl rings is 1.501(9) Å, ESI, Table S1.† The dihedral angles of the meso-aryl substituents (Φ₁–Φ₄) are larger than those reported previously for the Pt^IV–TriP and Re^I–TriP complexes (ESI Table S1†).^28,29

Fig. 1 ORTEP diagrams of 2b. (a) Top view and (b) side view. Thermal ellipsoids are drawn at 50% probability with the phenyl groups omitted. Solvent molecules are omitted for clarity.

Fig. 2 Bond lengths [Å] around the metal center of 2b.

Table 1 Crystal data and data collection parameters of 2b

Parameter	2b
a R1 = Σ||Fo| − |Fc||/ΣFo|.b wR2 = [Σw(Fo^2 − Fc^2)^2/Σw(Fo^2)]^1/2.
Formula	C108H72Cl4N8ORh2·6CH3CN·H2O
fw	2109.70
Crystal symmetry	Triclinic
Space group	P1̄
a (Å)	12.498(3)
b (Å)	14.522(3)
c (Å)	14.653(3)
α (deg)	92.88(3)
β (deg)	101.13(3)
γ (deg)	109.11(3)
V (Å^3)	2447.3(11)
T (K)	153(2)
dcalcd (g cm^−3)	1.431
Z	1
F (000)	1084.0
μ (mm^−1)	0.509
Indep reflns	8506
R1^a, wR2^b (I > 2σ(I))	0.0870, 0.1598
R1^a, wR2^b (all data)	0.1290, 0.1406
GOF	1.116

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Table 2 Selected bond lengths and angles of 2b

Bond	Bond length (Å)	Angle	Bond angle (deg)
Rh–N1	1.962(5)	Cl1–Rh–N1	94.2(2)
Rh–N2	1.985(5)	Cl1–Rh–N2	90.8(2)
Rh–N3	1.975(6)	Cl1–Rh–N4	84.2(2)
Rh–N4	2.071(6)	Cl2–Rh–N1	94.9(2)
Rh–Cl1	2.373(2)	Cl2–Rh–N3	90.7(2)
Rh–Cl2	2.373(2)	Cl2–Rh–N4	85.3(2)
C17–N2	1.365(8)	N1–Rh–N2	83.2(2)
C20–N3	1.376(8)	N1–Rh–N3	83.5(2)
C17–C18	1.424(8)	N2–Rh–N3	88.6(2)
C18–C19	1.456(9)	C17–C18–C19	130.5(6)
C19–C20	1.409(9)	C18–C19–C20	132.7(6)

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Optical spectroscopy and theoretical calculations

The optical spectra of [14]tribenzotriphyrin(2.1.1) complexes (Fig. 3), can be readily assigned by using Michl's perimeter model for aromatic and heteroaromatic cyclic polyenes.^32–34 A D_13h symmetry C₁₃H₁₃⁻ hydrocarbon species can be viewed as the parent cyclic polyene perimeter for the inner perimeter of the TriP ligand. The π-MOs are arranged in an M_L = 0, ±1, ±2, ±3, ±4, ±5, ±6 sequence in ascending energy terms, where M_L is the magnetic quantum number. The HOMO and LUMO have M_L = ±3, ±4 angular nodal patterns. The four spin allowed one-electron transitions between these MOs result in an allowed (ΔM_L = ±1) B band and a weaker forbidden (ΔM_L = ±7) L band at lower energy. The frontier π-MOs of 2a–d can be readily identified on this basis (Fig. 4 and Table S2†), and the L(0,0) and B(0,0) transitions can be assigned to the bands at ca. 600 and 500 nm (Table S2†), respectively, significantly to the red of the corresponding bands in the spectrum of 1b, which lie at 414 nm, and 523 and 578 nm, respectively.²⁴ Magnetic circular dichroism (MCD) spectroscopy provides further support for this assignment. The three Faraday [scr A, script letter A]₁, [scr B, script letter B]₀ and [capital script C]₀ terms can provide extra information that cannot easily be derived from the electronic absorption spectrum.^35,36 Since [14]tribenzo-triphyrins(2.1.1) lack a three-fold or higher axis of symmetry, the visible region of the MCD spectrum of 2b is dominated by the coupled pairs of oppositely-signed Gaussian-shaped Faraday [scr B, script letter B]₀ terms associated with the L and B bands. A third intense band at 375 nm is assigned to a third transition of the ligand π-system with significant ligand-to-metal charge transfer (LMCT) character (Table S2†).

Fig. 3 The energies and angular nodal patterns at an isosurface value of 0.04 a.u. of the a, s, -a and -s MOs of C₁₃H₁₃⁻ (TOP) and 2b (CENTER). The energies of the frontier MOs of 1a–d and 2a–d (BOTTOM). Thicker dark gray lines are used to highlight the a, s, -a and -s MOs. And triangles and circles are used a/-a and s/-s MOs, respectively. The HOMO–LUMO gaps are denoted with large green triangles and are plotted against a secondary axis.

Fig. 4 The electronic absorption and MCD spectra of 2b are plotted with black and purple lines, respectively. The calculated TD-DFT spectrum is plotted against a secondary axis. The L and B bands are highlighted with large red diamonds. Green and yellow diamonds are used for π → π* and charge transfer transitions from the chloride ions to the ligand π-system, respectively. The details of the calculation are provided in Table S2.†

Michl introduced an a, s, -a and -s terminology (Fig. 3), to describe the four frontier π-MOs, so that trends in their energies could be analyzed for series of porphyrinoid structures based on a consideration of the alignment of their angular nodal planes.³⁴ The a and -a MOs have nodal planes aligned with the y-axis, while the s and -s MOs have significant MO coefficients at these positions. Upon coordination by Rh(III) dichloride, the unoccupied -a and -s MOs are stabilized to a greater extent than the a and s MOs (Fig. 3). No significant changes are observed in the absorption spectra of 2a–d (Table 3), since inductive rather than mesomeric effects dominate and affect the a, s, -a and -s MOs in a similar manner (Fig. 3) through the σ-bonding framework.³⁴ Michl has demonstrated that the separation of the a and s MOs derived from the HOMO of the C₁₃H₁₃⁻ parent perimeter (referred to as ΔHOMO value, 0.66 eV for 2b) and that of the -a and -s (referred to as the ΔLUMO value, 0.53 eV for 2b) determines the sign sequences observed for the Faraday [scr B, script letter B]₀ terms in the MCD spectrum.^33,34 The −/+/−/+ sign sequence for the L(0,0) and B(0,0) bands in ascending energy terms (Fig. 4) is the pattern that is normally observed when ΔHOMO > ΔLUMO as is the case with 2b (Fig. 3), and this is what is observed in the MCD spectrum of 2b, which is typical of those observed for 2a–d. As has been demonstrated previously during studies on the optical properties of structurally similar series of porphyrinoids, this helps to validate the results of the TD-DFT calculations.^24,37,38

Table 3 UV-visible spectral data (λ_max, nm) of 2a–d in CH₂Cl₂ with added pyridine and the coordination constants

	λmax nm	Coordination constant
2a	375, 496, 593	log β2 = 2.8
2b	373, 497, 594	log β2 = 2.7
2c	372, 499, 597	log β2 = 2.6
2d	375, 502, 599	log β2 = 2.6

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Titration with pyridine

Rh(III) porphyrin complexes usually adopt a square pyramidal conformation and certain small solvent molecules, such as MeOH and THF, can coordinate to the Rh(III) center to form a pseudo-octahedral conformation. In a similar manner, the central Rh(III) of Rh^III–TriP complexes is still square pyramidal conformation and can be readily coordinated by small solvent ligand.^38,39 In order to check the spectral changes observed before and after coordination, a titration was carried out for 2d with pyridine in CH₂Cl₂ (Fig. 5). The final spectrum of the pyridine coordinated Rh^III–TriP complex is characterized by a new B band at 502 nm, together with a reduced broad band at 375 and 599 nm (Fig. 5). The slope of the Hill plot analyzing the spectral changes as a function of Py concentration is 1.0, indicating that one pyridine molecule coordinates the Rh(III) center to form an octahedral complex. The coordination constant was calculated as log β₂ = 2.6 for compound 2d in CH₂Cl₂ (Table 3). Similar values were calculated for 2a–c.

Fig. 5 Changes in the UV-visible absorption spectra observed during the titration of 2d with pyridine.

Electrochemistry

The electrochemical properties of 2a–d were evaluated in CH₂Cl₂ containing 0.1 M TBAP as the supporting electrolyte at room temperature. Examples of cyclic voltammograms are shown in Fig. 6 while the reduction/oxidation potentials are summarized in Table 4. Two reversible one-electron reductions and a reversible one-electron oxidations are observed for 2a–d. The difference in half-wave potentials between the first and second reduction (ΔE_1/2(1r−2r)) ranges from 0.27–0.29 V (Table 4). As expected, the E_1/2 values vary with the electron-withdrawing and -donating properties of the substituents at the para-positions of the four phenyl rings and there is a linear relationship between the E_1/2 values and the sum of the Hammett constants^41,42 (Fig. 7). The slope of the correlation in the plots of Fig. 7 is defined by the equation ΔE_1/2 = Σσρ, where Σσ represents the sum of the substituent constants and ρ is the reaction constant.^41,42 The values of ρ for the first reduction and oxidation steps of 2a–d were calculated as 59 and 48 mV, respectively. The absolute potential differences between the first reduction and oxidation (the HOMO–LUMO gap) ranged from 2.00 to 2.05 V in CH₂Cl₂ (Table 4) with an average value of 2.02 ± 0.02 V (Fig. 7). This HOMO–LUMO gap is much smaller than that of the corresponding free base [14]tribenzotriphyrin(2.1.1) which generally averages 2.20 ± 0.03 V.²⁹

Fig. 6 Cyclic voltammograms of 2a–d in CH₂Cl₂ containing 0.1 M TBAP.

Table 4 Half-wave potentials (V vs. SCE) of 2a–d in CH₂Cl₂, 0.1 M TBAP

	4σ^a	Oxidation	Reduction			HOMO–LUMO gap^c (V)
		1^st	1^st	2^nd	ΔE1/2(1r−2r)^b
a Hammett substituent constant, see ref. 39 and 40.b Potential difference between the first and second reduction.c The potential difference between the first oxidation and first reduction.
2a	0.68	+1.15	−0.87	−1.14	0.27	2.02
2b	0.00	+1.21	−0.84	−1.11	0.27	2.05
2c	0.24	+1.24	−0.76	−1.05	0.29	2.00
2d	1.80	+1.28	−0.72	−1.00	0.28	2.00

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Fig. 7 Plots of half-wave potentials for the first reduction and first oxidation of compounds 2a–d in CH₂Cl₂ containing 0.1 M TBAP vs. the Hammett substituent constants (Σσ). The E_1/2 and Σσ values are provided in Table 4.

Conclusion

A series of rhodium(III) dichloride complexes of [14]tribenzotriphyrin(2.1.1) have been successfully prepared, and their optical and electrochemical properties have been investigated. An X-ray crystallograpic analysis reveals that 2b adopts a bowl-shaped conformation. The L and B bands of the 2a–d complexes lie at ca. 600 and 500 nm, respectively, and are markedly red shifted relative to those of 1a–d. A reversible one-electron oxidation wave and two reversible one-electron reduction waves are observed in the electrochemical measurements for 2a–d. Further studies on the metal complexes of [14]tribenzotriphyrins(2.1.1) are in progress. TD-DFT calculations have been used to assign a third intense spectral band at 375 nm to a higher energy π → π* transition of the [14]tribenzotriphyrin(2.1.1) π-system.

Experimental

Chemicals

Dichloromethane (CH₂Cl₂) was purchased from Aldrich Co. and used as received for electrochemical measurements. Tetra-n-butylammonium perchlorate (TBAP) was purchased from Sigma Chemical or Fluka Chemika Co., recrystallized from ethyl alcohol, and dried under vacuum at 40 °C for at least one week prior to use.

Materials

All solvents and chemicals were reagent grade quality, obtained commercially and used without further purification except as noted. For spectral measurements, spectral grade dichloromethane was purchased from J&K Scientific Ltd. Thin-layer chromatography (TLC), flush column chromatography, and gravity column chromatography were performed on Art. 5554 (Merck KGaA), Silica Gel 60 (Merck KGaA), and Silica Gel 60N (Kanto Chemical Co.), respectively. A series of four different [14]tribenzotriphyrins(2.1.1) were prepared according to the published method.²³ Rh₂(C₈H₁₂)Cl₂ was purchased commercially and used as received.

Measurements

Melting points were measured with a Yanaco M-500D melting point apparatus. ¹H and ¹³C NMR spectra were recorded in CDCl₃ on a JEOL JNM-AL 400 spectrometer. Chemical shifts are reported in units of ppm relative to the solvent residue peaks (CDCl₃, δ = 7.26 ppm for ¹H, 77.16 ppm for ¹³C). MALDI-TOF mass spectra were recorded on a Bruker Daltonics autoflexII MALDI-TOF MS spectrometer. Electronic absorption spectra were recorded with Perkin Elmer Lambda 35 UV/vis spectrometers. A homemade three-electrode cell was used for cyclic voltammetric measurements, consisting of a platinum button or glassy carbon working electrode, a platinum counter electrode and a homemade saturated calomel reference electrode (SCE). The SCE was separated from the bulk of the solution by a fritted glass bridge of low porosity which contained the solvent/supporting electrolyte mixture. Time-resolved UV-visible absorption spectra were recorded with a Hewlett-Packard Model 8453 diode array spectrophotometer. High purity N₂ from Trigas was used to deoxygenate the solution and was kept over the solution during each electrochemical experiment. X-ray crystallographic analyses were carried out on a Bruker Smart Apex CCD diffractometer using monochromatic Mo K_α radiation (λ = 0.71073 Å) at 293 K using the ω–2θ scan mode. The data were corrected for Lorenz and polarization effects. The structures were solved by direct methods and refined on F² using the full-matrix least-squares technique of the SHELXTL-2014 program package.⁴³ CCDC no. 1432460 contain the supplementary crystallographic data for this paper.

Theoretical calculations

Geometry optimization calculations were carried out for 1a–d and 2a–d using the B3LYP functional of the Gaussian09 software package⁴⁴ with SDD basis sets. An acetonitrile atom was added to the structures of 2a–d in a similar manner to that observed in Fig. 1. The B3LYP functional was also used with SDD basis sets to carry out TD-DFT calculations for the B3LYP geometries.

Determination of coordination equilibrium constants

A series of CH₂Cl₂ solutions containing pyridine (Py) at different concentrations were prepared, so that Py could be used as a coordination ligand-titration reagent. Microliter quantities of Py in CH₂Cl₂ were added gradually to a 5.5 ml CH₂Cl₂ solution of the Rh^III–TriP in a home-made 1.0 cm cell and the spectral changes were monitored after each addition. The coordination reaction is given by the following equation:

Changes in the UV-visible absorption spectra were analyzed as a function of the concentration of the added ligand, using both the mole ratio method and the Hill equation^36,37 to calculate equilibrium constants for the addition of Py in CH₂Cl₂, selected as a typical non-aqueous solvent.

Synthesis

A toluene solution (10 ml) containing a series of [14]Tribenzotriphyrins(2.1.1) (1a–d) (0.013 mmol) and Rh₂(C₈H₁₂)Cl₂ (38.4 mg, 0.137 mmol) was refluxed for 24 h under N₂. After solvent evaporation, the residue was dissolved in CH₂Cl₂ and filtered to remove the precipitates. The solvent was again evaporated and the residue was purified by silica gel column chromatography using CH₂Cl₂ as the eluent. The first grey blue fraction was identified as 2.

Rh^III–TriP (R = –CH₃) 2a

Mp: > 300 °C. ¹H NMR (400 MHz, CD₂Cl₂ 298 K) δ = 8.57 (br, s, 2H), 7.82–7.75 (m, 6H), 7.49 (br, s, 2H), 7.40–7.37 (m, 2H), 7.24–7.21 (m, 14H), 6.72–6.69 (m, 2H), 2.76 (s, 6H, –CH₃), 2.51 (s, 6H, –CH₃) ppm; ¹³C NMR (101 MHz, CDCl₃ 298 K) δ = 157.9, 155.4, 152.5, 140.1, 139.8, 139.0, 138.5, 137.6, 136.8, 135.7, 135.4, 133.1, 129.9, 127.8, 126.8, 126.6, 126.2, 123.7, 123.2, 122.5, 21.8, 21.4; MS (MALDI-TOF): calcd for C₅₆H₄₀ClN₃Rh [M − Cl]⁺: 892.197, found: 892.283; UV-vis (in CH₂Cl₂) λ [nm] (ε [M⁻¹ cm⁻¹]): 375 (27 000), 493 (25 500), 587 (12 100), 615 (12 300).

Rh^III–TriP (R = –H) 2b

Mp: > 300 °C. ¹H NMR (400 MHz, CD₂Cl₂ 298 K) δ = 8.72 (br, s, 2H), 8.03 (br, s, 2H), 7.93–7.87 (m, 6H), 7.69 (br, s, 2H), 7.44–7.33 (m, 10H), 7.21–7.16 (m, 8H), 6.63 (br, s, 2H) ppm; ¹³C NMR (101 MHz, CDCl₃ 298 K) δ = 155.7, 153.3, 149.9, 149.2, 141.9, 141.8, 139.4, 136.9, 136.7, 136.4, 136.4, 133.9, 133.6, 133.6, 132.5, 131.8, 130.8, 130.5, 129.8, 128.9, 128.8, 128.8, 127.8, 127.5, 127.5, 126.9, 126.9, 124.0, 123.9, 123.4, 123.1 ppm; MS (MALDI-TOF): calcd for C₅₂H₃₂ClN₃Rh [M − Cl]⁺: 836.134, found: 836.240; UV-vis (in CH₂Cl₂) λ [nm] (ε [M⁻¹ cm⁻¹]): 375 (25 800), 493 (25 500), 587 (12 200), 615 (12 400).

Rh^III–TriP (R = –F) 2c

Mp: > 300 °C. ¹H NMR (400 MHz, CD₂Cl₂ 298 K) δ = 8.68 (br, s, 2H), 7.83–7.76 (m, 6H), 7.47–7.45 (m, 8H), 7.31–7.30 (m, 2H), 7.20–7.16 (m, 8H), 6.74–6.73 (m, 2H) ppm; ¹³C NMR (101 MHz, CDCl₃ 298 K) δ = 165.1, 163.7, 162.6, 161.3, 161.2, 153.35, 153.3, 153.2, 150.2, 149.4, 149.4, 139.2, 137.9, 137.8, 136.6, 136.4, 135.2, 135.0, 134.9, 134.4, 134.3, 133.04, 132.59, 132.20, 129.9, 129.82, 128.0, 127.9, 127.8, 127.3, 127.1, 123.8, 123.8, 123.4, 123.1 ppm; MS (MALDI-TOF): calcd for C₅₂H₂₈ClF₄N₃Rh [M − Cl]⁺: 908.096, found: 908.147; UV-vis (in CH₂Cl₂) λ [nm] (ε [M⁻¹ cm⁻¹]): 375 (24 400), 494 (21 500), 589 (11 000), 615 (11 200).

Rh^III–TriP (R = –COOCH₃) 2d

Mp: > 300 °C. ¹H NMR (400 MHz, CD₂Cl₂ 298 K) δ = 8.31–8.33 (m, 2H), 8.15–8.17 (m, 2H), 7.95–7.96 (m, 4H), 7.88 (br, 2H), 7.74–7.76 (m, 2H), 7.64–7.66 (m, 4H), 7.53–7.55 (m, 2H), 7.45 (br, 1H, NH), 7.23–7.37 (m, 10H), 2.74 (s, 6H, –COOCH₃), 2.47 (s, 6H, –COOCH₃) ppm; ¹³C NMR (101 MHz, CDCl₃ 298 K) δ = 166.9, 166.7, 157.1, 155.8, 153.0, 147.3, 143.0, 139.3, 138.4, 137.4, 133.2, 131.3, 129.3, 128.6, 127.5, 127.4, 126.9, 123.5, 123.3, 122.5, 121.1, 119.3, 114.1, 52.6, 52.3; MS (MALDI-TOF): calcd for C₆₀H₄₀ClN₃O₈Rh [M − Cl]⁺: 1068.156, found: 1068.040; UV-vis (in CH₂Cl₂) λ [nm] (ε [M⁻¹ cm⁻¹]): 375 (26 600), 496 (22 900), 592 (11 400), 614 (10 900).

Acknowledgements

This work was supported by grants from the Natural Science Foundation of China (No. 21301074 and 21071067), the Natural Science Foundation of Jiangsu Province (No. BK20130483), China Postdoctoral Science Foundation (No. 2013M540415 and 2014T70474), Jiangsu Provience Universities Natural Sciences Fund (No. 13KJB150010), the Robert A. Welch Foundation (KMK, Grant E-680), and a CSUR grant from the National Research Foundation of South Africa (UID 93627). The theoretical calculations were carried out at the Centre for High Performance Computing in Cape Town.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1432460. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra03028a

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