β‐Octabromo‐ and β‐Octakis(trifluoromethyl)isocorroles: New Sterically Constrained Macrocyclic Ligands

Abstract

Presented herein is a study of the acid‐induced demetalation of two sterically hindered copper corroles, Cu β‐octabromo‐meso‐triphenylcorrole (Cu[Br₈TPC]) and β‐octakis(trifluoromethyl)‐meso‐tris(p‐methoxyphenyl)corrole (Cu[(CF₃)₈TpOMePC]). Unlike reductive demetalation, which affords the free‐base β‐octabromocorrole, demetalation of Cu[Br₈TPC] under non‐ reductive conditions (CHCl₃/H₂SO₄) resulted in moderate yields of free‐base 5‐ and 10‐hydroxy isocorroles. The isomeric free bases could be complexed to Co^II and Ni^II, affording stable complexes. Only reductive demetalation was found to work for Cu[(CF₃)₈TpOMePC], affording a highly saddled, hydrated corrole, H₃[5‐OH,10‐H‐(CF₃)₈TpOMePC], where the elements of water had added across C₅ and C₁₀. Interaction of this novel free base with Co^II resulted in Co[iso‐10‐H‐[CF₃)₈TpOMePC], a Co^II 10‐hydro isocorrole. The new metal complexes were all characterized by single‐crystal X‐ray diffraction analysis and, despite their sterically hindered nature, were found to exhibit almost perfectly planar isocorrole cores.

1. Introduction

Isocorroles are fascinating hybrid ligands that combine the dianionic character of porphyrins with the sterically constricted N₄ cavity of corroles (Figure 1). In addition, with significant absorption in the near‐IR, they are of considerable interest as near‐IR dyes for medical imaging. Traditionally, isocorroles have been synthesized from linear mono‐, di‐, and tetra‐pyrrolic starting materials.1 Recently, Paolesse et al. showed that they could also be directly accessed from corroles. An attempt to synthesize a free‐base β‐octabromocorrole through the interaction of free‐base meso‐triarylcorrole and N‐bromosuccinimide resulted instead in a free‐base β‐octabromoisocorrole, which, interestingly, aromatized to a corrole when complexed to Co^III.2 Subsequently, the same group synthesized β‐unsubstituted meso‐triarylisocorroles through DDQ oxidation of the corresponding free‐base corroles.3 Attempts to demetalate silver corroles4 and to selectively brominate a β‐nitrocorrole5 also afforded unexpected routes to isocorroles.

Figure 1.

Isocorroles as hybrid ligands.

Reductive demetalation of Cu β‐octabromocorroles with concentrated sulfuric acid and an excess of a reducing agent such as Fe^II or Sn^II finally provided a reliable route to free‐base β‐octabromocorroles.6 We also discovered that prolonged exposure to an acidic medium such as CHCl₃/H₂SO₄ without an added reductant resulted in moderate yields of 5‐ and 10‐hydroxyisocorroles. Details of such a protocol have recently been published for undecaarylisocorroles.7 Presented herein are the results of our continued studies on sterically hindered isocorroles, including optimized protocols for the synthesis of 5/10‐hydroxy‐β‐octabromo‐meso‐triphenylisocorrole, H₂[iso‐5/10‐OH‐Br₈TPC], and its complexation to Co^II and Ni^II. Also presented are our first results on the demetalation of a copper β‐octakis‐ (trifluoromethyl)‐meso‐triarylcorrole. The various products obtained were characterized as far as possible with single‐crystal X‐ray crystallography; as discussed below, the results, a total of five X‐ray structures (Table 1), provide substantial insights into the structural characteristics of isocorrole derivatives.

Table 1.

Crystallographic data for the compounds analyzed.

Compound	H2[iso‐5‐OH‐Br8TPC]	CoII[iso‐10‐OH‐Br8TPC](PPh3)	NiII[iso‐5‐OH‐Br8TPC]	H2[5‐OH,10‐H‐(CF3)8TpOMePC]	CoII[iso‐10‐H‐(CF3)8TpOMePC]
Chemical formula	C81H49Br16Cl3N8O2	C73H49Br8N4OPCo	C153H69Br32Cl15N16O4Ni4	C48H26F24N4O4	C48H22F24O3N4Co
Formula mass	2551.19	1727.34	5518.95	1178.73	1217.62
Crystal system	Triclinic	Triclinic	Triclinic	Triclinic	Orthorhombic
Space group	P−1	P−1	P1	P−1	Pbca
λ [Å]	0.61992	0.7749	0.9537	0.93221	0.7749
a [Å]	14.4512(5)	12.2904(6)	14.4643(6)	12.3913(8)	12.2005(3)
b [Å]	14.5377(5)	13.8275(7)	14.5129(6)	13.3041(7)	25.9102(7)
c [Å]	21.5890(8)	19.5628(9)	22.0745(10)	15.3611(8)	28.2726(7)
α [°]	95.609(2)	76.253(3)	73.459(2)	67.137(3)	90
β [°]	101.915(2)	80.009(3)	76.456(2)	85.025(4)	90
γ [°]	113.646(2)	83.493(3)	66.8599(19)	82.537(4)	90
Z	2	2	1	2	8
V [Å3]	3981.9(3)	3171.4(3)	4045.2(3)	2311.8(2)	8937.5(4)
Temperature [K]	100(2)	100(2)	100(2)	100(2)	100(2)
Density [g cm‐3]	2.128	1.809	2.266	1.693	1.810
Meas. reflections	73 287	55 091	84 466	30 220	175 802
Unique reflections	19 760	18 561	19 417	10 371	17 022
Parameters	1075	793	2082	741	735
Restraints	240	0	799	0	7
R int	0.0697	0.0469	0.0653	0.0436	0.0429
θ range [°]	1.362–24.411	2.240–33.073	2.075–39.542	1.889–37.386	1.714–36.589
R 1, wR 2 all data	0.0584, 0.1512	0.0336, 0.0740	0.0636, 0.1770	0.0670, 0.2135	0.0331, 0.0889
S (GooF) all data	1.044	1.048	1.030	1.024	1.049
Max/min res. dens. [e Å‐3]	2.881/−1.980	0.810/−0.951	5.539/−2.087	0.361/−0.372	0.919/−0.764

2. Results and Discussion

2.1. β‐Octabromo‐meso‐triphenylisocorrole Derivatives

As shown in Figure 2, demetalation of Cu β‐octabromo‐meso‐triphenylcorrole, Cu[Br₈TPC], with CHCl₃/H₂SO₄ resulted in moderate yields of the isomers H₂[iso‐5‐OH‐Br₈TPC] and H₂[iso‐10‐OH‐Br₈TPC] in approximately 2 h.8 Both could be efficiently complexed to Co^II or Ni^II in about 1 h or less. Single‐crystal X‐ray structures were obtained for H₂[iso‐5‐OH‐Br₈TPC] (Figure 3), Co^II[iso‐10‐OH‐Br₈TPC](PPh₃) (Figure 4), and Ni^II[iso‐5‐OH‐Br₈TPC] (Figure 5). The structures exhibit short metal–nitrogen distances and essentially planar isocorrole cores. Like corroles, isocorroles thus appear to be strongly resistant to nonplanar distortions, which is thought to be a consequence of the rigidity of the C1–C19 pyrrole–pyrrole linkage and its resistance to both twisting and pyramidalization. Both metalloisocorrole structures exhibit intramolecular hydrogen bonding involving the meso‐OH groups and intermolecular metal‐β‐bromine interactions (Figure 4 and Figure 5). Finally, the new isocorroles exhibit fairly strong near‐IR absorption (Figure 6), which may promise application as a near‐IR dye in medical imaging and/or photodynamic therapy.

Figure 2.

Demetalation of Cu[Br₈TPC] and complexation of the resulting 5‐ and 10‐hydroxy isocorroles to Co^II and Ni^II.

Figure 3.

Two views of the X‐ray structure of the free‐base isocorrole H₂[iso‐5‐OH‐Br₈TPC].

Figure 4.

X‐ray structure of Co^II[iso‐10‐OH‐Br₈TPC](PPh₃): a) top view, b) side‐view showing 10‐OH⋅⋅⋅π(PPh₃) hydrogen bonding, and c) side view showing stacking and Co⋅⋅⋅Br interactions (Å). Selected distances: Co(1)–N(1) 1.887(2), Co(1)–N(2) 1.930(2), Co(1)–N(3) 1.927(2), Co(1)–N(4) 1.881(2), and Co(1)–P(1) 2.3837(7) Å.

Figure 5.

X‐ray structure of Ni^II[iso‐5‐OH‐Br₈TPC]: a) top view and b) side view showing stacking and Ni⋅⋅⋅Br interactions (Å). Selected distances: Ni(1A)–N(1A) 1.851(11), Ni(1A)–N(2A) 1.904(10), Ni(1A)–N(3A) 1.920(10), and Ni(1A)–N(4A) 1.863(11) Å.

Figure 6.

UV/Vis spectra of iso‐5/10‐OH‐Br₈TPC derivatives.

2.2. β‐Octakis(trifluoromethyl)‐meso‐triarylisocorrole Derivatives

Although copper9 and gold10 β‐octakis(trifluoromethyl)‐meso‐tris(p‐X‐phenyl)corrole derivatives, M[(CF₃)₈TpXPC] (M=Cu, Au), were synthesized in one of our laboratories a few years ago, a useful demetalation procedure for the complexes has so far proved elusive. Reported herein is the first such demetalation, carried out under reductive conditions on the complex Cu[(CF₃)₈TpOMePC], and the complexation of the resulting free base to cobalt(II). As shown in Figure 7, the demetalation occurs with a twist: the metal‐free product obtained in high yield is not an isocorrole, but rather a hydrated corrole, with the elements of water added across C₅ and C₁₀, whereas the Co^II complex is a 10‐hydro isocorrole. The X‐ray structure of the free‐base product, denoted here as H₃[5‐OH,10‐H‐(CF₃)₈TpOMePC], revealed a strongly saddled macrocyclic core, clearly a result of exceptional steric crowding owing to the three central NH groups and the peripheral substituents (Figure 8 a).11 The X‐ray structure of the Co^II complex, denoted here as Co[iso‐10‐H‐[CF₃)₈TpOMePC], on the other hand, was found to exhibit a planar isocorrole core (Figure 8 b), with intermolecular Co⋅⋅⋅OMe interactions (Figure 8 c). Like other isocorrole derivatives, Co[iso‐10‐H‐[CF₃)₈TpOMePC] was also found to exhibit a strong near‐IR feature (λ _max=707 nm) (Figure 9).

Figure 7.

Reductive demetalation of Cu[(CF₃)₈TpOMePC] and complexation of the resulting free base to Co^II.

Figure 8.

Thermal ellipsoid plots of a) H₃[5‐OH,10‐H‐(CF₃)₈TpOMePC] and b) Co[iso‐10‐H‐[CF₃)₈TpOMePC]. c) Intermolecular interactions of Co[iso‐10‐H‐[CF₃)₈TpOMePC]. Selected distances for Co[iso‐10‐H‐[CF₃)₈TpOMePC]: Co(1)–N(1) 1.8730(8), Co(1)–N(2) 1.9053(8), Co(1)–N(3) 1.9071(8), Co(1)–N(4) 1.8730(8), and Co(1)–O(2) 2.2921(8) Å.

Figure 9.

UV/Vis spectra of partially saturated (CF₃)₈TpOMePC derivatives.

3. Conclusions

Optimized protocols have been developed for the demetalation of the sterically hindered copper corroles Cu[Br₈TPC] and Cu[(CF₃)₈TpOMePC]. Although 5‐ and 10‐hydroxyisocorroles were obtained as the major products of demetalation of Cu[Br₈TPC] under nonreductive conditions, Cu[(CF₃)₈TpOMePC] could only be demetalated under reductive conditions and the major product turned out to be a unique 5‐hydroxy‐10‐hydro corrole, that is, a free‐base hydrated corrole. The free‐base ligands could all be complexed to Co^II and/or Ni^II to afford stable metalloisocorroles. X‐ray structures of the metal complexes exhibited short metal–nitrogen bonds and essentially isocorrole cores. The stability and robustness of both the free‐base ligands and metalloisocorroles reported here appear to foreshadow a bright future for isocorroles as transition‐metal ligands. The strong near‐IR absorption of isocorroles also promises applications as near‐IR dyes, notably in medicinal applications such as photodynamic therapy.

Experimental Section

Materials

All reagents and solvents were used as purchased. Silica gel 60 (0.04–0.063 mm particle size; 230–400 mesh, Merck) was used for flash chromatography. Silica gel 60 preparative thin‐layer chromatographic plates (20×20 cm; 0.5 mm thick, Merck), were used for further purification where relevant. The starting materials Cu[Br₈TPC]12 and Cu[(CF₃)₈TpOMePC]9a were synthesized as previously described.

Instrumentation

UV/Vis spectra were recorded on an HP 8453 spectrophotometer in CH₂Cl₂. All NMR spectra were obtained on a Mercury Plus Varian spectrometer at 298 K. ¹H NMR spectra were recorded in CD₂Cl₂ (referenced to 5.30 ppm) or in 1,1,2,2‐[D₂]tetrachloroethane [(CDCl₂)₂], referenced to 5.91 ppm at 400 MHz. ¹⁹F NMR spectra were referenced to 2,2,2‐trifluoroethanol‐d₃ (δ=−77.8 ppm) at 376 MHz. High‐resolution electrospray ionization (HR‐ESI) mass spectra were recorded on an LTQ Orbitrap XL spectrometer.

Synthesis of H₂[iso‐5‐OH‐Br₈TPC] and H₂[iso‐10‐OH‐Br₈TPC]

To a pre‐sonicated and stirred solution of Cu[Br₈TPC] (62 mg,0.051 mmol) in CHCl₃ (10 mL) was added concentrated H₂SO₄ (95–97 %, 6 mL) in a dropwise manner over 6 min. The resulting suspension was stirred and sonicated alternately over 2 h. The green suspension obtained was shaken with distilled H₂O (100 mL × 2) and extracted with CHCl₃. The organic phase was washed with saturated aqueous NaHCO₃, dried over anhydrous Na₂SO₄, and filtered. The filtrate was concentrated to a minimum volume and chromatographed on a silica gel column with n‐hexane/CH₂Cl₂ (7:3) to yield impure green H₃[Br₈TPC] (14 mg) and bright green H₂[iso‐10‐OH(Br₈TPC)], closely followed by the 5‐isomer. Crystallization from 2:1 CH₃OH/CHCl₃ yielded the pure 10‐isomer (13.6 mg, 22.7 %), whereas crystallization from 2:1 n‐hexane/CHCl₃ yielded the pure 5‐isomer (17.4 mg, 29.1 %). H₂[iso‐5‐OH‐Br₈TPC]: UV/Vis (CH₂Cl₂): λ _max, nm (ϵ × 10⁻⁴, M⁻¹ cm⁻¹): 431 (4.52), 677 (0.71). ¹H NMR {(CDCl₂)₂}: δ 13.91 (s, 1 H, NH); 12.63 (bs, 1 H, NH ); 7.70–7.60 (m, 2 H); 7.52–7.16 (m, 15 H,); 3.44 (s, 1 H, OH). HRMS (ESI⁺, major isotopomer): [M + H]⁺=1174.4923 (expt), 1174.4944 (calcd). Elemental analysis: Found (calcd) : C, 36.07 (37.86); H, 1.55 (1.54), N, 4.34 (4.77).

H₂[iso‐10‐OH‐Br₈TPC]: UV/Vis (CH₂Cl₂): λ _max, nm (ϵ × 10⁻⁴, M⁻¹ cm⁻¹): 444 (5.94), 669 (0.53), 707 (0.51). ¹H NMR {(CDCl₂)₂}: δ 13.60 (s, 2 H, NH); 7.67–7.61 (m, 2 H); 7.51–7.45 (m, 2 H); 7.45–7.38 (m, 4 H,); 7.29–7.22 (m, 7 H); 3.75 (s, 1 H, OH). HRMS (ESI⁺, major isotopomer): [M + H]⁺=1174.4961 (expt), 1174.4944 (calcd).

Synthesis of Ni^II[iso‐5‐OH‐Br₈TPC]

To a solution of H₂[iso‐5‐OH‐Br₈TPC] (20 mg, 0.017 mmol) in CHCl₃ (20 mL) maintained at 50 °C, was added Ni(OAc)₂⋅4 H₂O (23 mg, 5 equiv) dissolved in CH₃OH (3 mL) in a dropwise manner over 5 min, whereupon the mixture turned from bottle green to olive green. After stirring for 1 h at 60 °C, TLC (with 3:2 n‐hexane/CH₂Cl₂) indicated full consumption of the starting material. The mixture was then evaporated to dryness and the residue chromatographed on a silica gel column. Initial elution with 3:2 n‐hexane/CH₂Cl₂ resulted in the removal of a pale‐yellow impurity. Gradual increase of solvent polarity to pure CH₂Cl₂ yielded the nickel isocorrole product as a brown band. The pure complex (18 mg, 85.6 %) was obtained by crystallization from 1:1 CHCl₃/CH₃OH. UV/Vis (CH₂Cl₂): λ _max, nm (ϵ × 10⁻⁴, M⁻¹ cm⁻¹): 459 (2.68), 892 (0.31), 982 (0.44). ¹H NMR {(CDCl₂)₂}: δ 7.88–7.83 (m, 2 H, ); 7.44–7.24 (m, 10 H); 7.19–7.04 (m, 3 H,); 3.29 (s, 1 H, OH). HRMS (ESI⁺, major isotopomer): [M]⁺=1229.4049 (expt), 1229.4056 (calcd). Elemental analysis: Found (calcd): C, 35.61 (36.11); H, 1.66 (1.31), N, 4.16 (4.55). Cubic crystals of X‐ray quality were grown by slow evaporation of a chloroform solution layered with an equal volume of n‐hexane.

Synthesis of Co^II[iso‐10‐OH‐Br₈TPC](PPh₃)

To a solution of H₂[iso‐10‐OH‐Br₈TPC] (20 mg, 0.017 mmol) in CHCl₃ (10 mL), was added Co(OAc)₂⋅4 H₂O (84 mg, 10 equiv) dissolved in CH₃OH (2 mL) in a dropwise manner, whereupon the mixture changed from bright green to brown. After stirring for 30 min, the mixture was evaporated to dryness and the residue was chromatographed on a silica gel column with 3:7 n‐hexane/CH₂Cl₂ as the eluent to yield Co^II[iso‐10‐OH‐Br₈TPC] as a brown band (16.8 mg, 80.2 %). UV/Vis (CH₂Cl₂): λ _max, nm (ϵ × 10⁻⁴, M⁻¹ cm⁻¹): 443 (5.14), 574 (0.87), 736 (0.72), 938 (0.29). HRMS (ESI⁺, major isotopomer): [M]⁺=1230.4076 (expt), 1230.4037 (calcd). To the Co^II[iso‐10‐OH‐Br₈TPC] (15 mg, 0.012 mmol) dissolved in CHCl₃ (10 mL), was then added triphenylphosphine (16 mg, 5 equiv) and the mixture was stirred for 10 min. The mixture was filtered and evaporated to dryness to yield Co^II[iso‐10‐OH‐Br₈TPC](PPh₃) as a brown solid (9 mg, 50 %). UV/Vis (CH₂Cl₂): λ _max, nm (ϵ × 10⁻⁴, M⁻¹ cm⁻¹): 451 (2.15), 697 (0.32). HRMS (ESI⁺, major isotopomer): [M]⁺=1492.4956 (expt), 1492.4959 (calcd). Vapor diffusion of n‐hexane into a saturated benzene solution of the product led within 10 days to rectangular dark‐red crystals suitable for X‐ray analysis.

Synthesis of H₃[5‐OH,10‐H‐(CF₃)₈TpOMePC]

To a solution of Cu[(CF₃)₈TpOMePC] (30 mg, 0.024 mmol) in CH₂Cl₂ (5 mL), was added anhydrous SnCl₂ (46 mg, 10 equiv), followed by dropwise addition of concentrated HCl (37 %, 1 mL). After stirring for 1 h, the purple suspension obtained was washed twice with distilled water and once with saturated aqueous NaHCO₃. The orange CH₂Cl₂ phase was dried over Na₂SO₄, filtered, evaporated to a minimum volume, and chromatographed on a silica gel column. Elution with 1:1 n‐hexane/CH₂Cl₂ led to a brown band identified with HR‐ESI as the detrifluoromethylated product H₃[(CF₃)₇TpMeOPC] (1.2 mg), whereas 2:3 n‐hexane/ CH₂Cl₂ resulted in H₃[5‐OH,10‐H‐(CF₃)₈TpOMePC] as an orange band (25 mg, 88 %). UV/Vis (CH₂Cl₂): λ _max, nm (ϵ × 10⁻⁴, M⁻¹ cm⁻¹): 486 (2.57). ¹H NMR (CD₂Cl₂): δ 11.44 (s, 1 H, NH); 8.75 (s, 1 H, NH); 7.57 (s, 1 H, NH); 7.43 (d, 2 H, 5,15‐o or m); 7.29 (d, 2 H,10‐o or m); 7.07 (d, 2 H, 5,15‐o or m); 7.02   6.95 (overlapping d, 4 H, 5,15‐o or m); 6.88 (d, 2 H, 10‐o or m); 6.30 (s, 1 H, 10‐meso, H); 3.91 (s, 3 H, 10‐p‐OMe, Ph); 3.80 (s, 6 H, 5,15‐p‐OMe, Ph); 3.40 (s, 1 H, 5‐meso, OH). ¹⁹F NMR: δ−51.70 (q, 3F); −52.64 to −52.89 (m, 9F); −54.78 (q, 3F); −55.91 to −56.11 (m, 3F); −56.80 to −57.0 (m, 3F); −57.12 (q, 3F). HRMS (ESI⁺, major isotopomer): [M + H]⁺=1179.1660 (expt), 1179.1644 (calcd). Elemental analysis. Found (calcd): C, 49.93 (48.91); H, 2.52 (2.22), N, 4.65 (4.75). Diffusion of a saturated CH₂Cl₂ solution of the latter product into n‐hexane yielded orange needles suitable for X‐ray analysis.

Synthesis of Co^II[iso‐10‐H‐[CF₃)₈TpOMePC]

To an orange solution of H₃[5‐OH,10‐H‐(CF₃)₈TpOMePC] (20 mg, 0.017 mmol) in absolute ethanol (5 mL), was added anhydrous sodium acetate (78.4 mg, 22 equiv) and the suspension stirred for 5 min, upon which it turned orange–red. Cobalt acetate (31.6 mg, 7.5 equiv) was then added and, after stirring for 30 min, the resulting green suspension was rotary evaporated to dryness. The obtained green residue was chromatographed on a silica gel column. Elution with pure CH₂Cl₂ resulted in the pure cobalt isocorrole product as an orange‐brown band. Subsequent elution with 5:1 CH₂Cl₂/MeOH resulted in several green bands. These were combined and rotary evaporated to yield a brown residue, which, according to HR‐ESI, was largely the impure product. Preparative TLC of the combined pure and impure fractions with 2:3 n‐hexane/CH₂Cl₂ yielded the pure product Co^II[iso‐10‐H‐[CF₃)₈TpOMePC] (8 mg, 39 %) as the first brown band. UV/Vis (CH₂Cl₂): λ _max, nm (ϵ × 10⁻⁴, M⁻¹ cm⁻¹): 496 (2.21), 707 (0.49). HRMS (ESI⁺, major isotopomer): [M]⁺=1217.0685 (expt), 1217.0635 (calcd). Diffusion of a saturated benzene solution of the complex into n‐hexane yielded brown needles suitable for X‐ray analysis.

X‐ray Crystallographic Analysis

X‐ray data were collected on beamline 11.3.1 at the Advanced Light Source, Lawrence Berkeley National Laboratory. Samples were mounted on MiTeGen^® Kapton loops and placed in a 100(2) K cold nitrogen stream provided by an Oxford Cryostream 700 Plus low temperature apparatus on the goniometer head of a Bruker D8 diffractometer. An APEXII CCD detector was generally used, except for Co^II[iso‐10‐OH‐Br₈TPC](PPh₃), where a PHOTON100 CMOS detector operating in shutterless mode was employed. Diffraction data were collected by synchrotron radiation monochromated using silicon(111) to wavelengths of 0.7749(1) Å for the two Co complexes, 0.9537(1) Å for Ni^II[iso‐5‐OH‐Br₈TPC], 0.61992(1) Å for H₂[iso‐5‐OH‐Br₈TPC], and 0.93221(1) Å for H₃[5‐OH,10‐H‐(CF₃)₈TpOMePC]. In all cases, an approximate full‐sphere of data was collected by using 0.3° ω scans. The structures were solved by intrinsic phasing (SHELXT) and refined by using full‐matrix least squares on F ² (SHELXL‐2013/4). All non‐hydrogen atoms were refined anisotropically. Hydrogen atoms on all carbon atoms were geometrically calculated and refined as riding atoms. Any hydrogen atoms located on oxygen or nitrogen atoms were found in the Fourier difference map, their distances fixed, and allowed to refine with a riding model. Additional crystallographic information has been summarized in Table 1 and full details can be found in the crystallographic information files provided as Supporting Information.

Conflict of interest

The authors declare no conflict of interest.

Supporting information

As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re‐organized for online delivery, but are not copy‐edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors.

Supplementary

K. E. Thomas, C. M. Beavers, K. J. Gagnon, A. Ghosh, ChemistryOpen 2017, 6, 402.