Calixsmaragdyrin: A Versatile Ligand for Coordination Complexes

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Calixsmaragdyrin: A Versatile Ligand for Coordination Complexes Tamal Chatterjee,† Brian Molnar,‡ G. G. Theophall,‡ Way-Zen Lee,*,§ K. V. Lakshmi,*,‡ and Mangalampalli Ravikanth*,† †

Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India Department of Chemistry and Chemical Biology and The Baruch ′60 Center for Biochemical Solar Energy Research, Rensselaer Polytechnic Institute, Troy, New York 12180, United States § Instrumentation Center, Department of Chemistry, National Taiwan Normal University, 88 Section 4 Ting-Chow Road, Taipei 11677, Taiwan ‡

S Supporting Information *

ABSTRACT: The Ru(II) and BF2 complexes of calixsmaragdyrin were prepared under simple reaction conditions and characterized by HR-MS, 1D and 2D NMR spectroscopy, optical spectroscopy, and electrochemistry, and the structure of the Ru(II) complex of calixsmaragdyrin was elucidated by Xray crystallography. The crystal structure of the Ru(II) complex revealed that the Ru(II) ion is hexacoordinate with the three pyrrole nitrogen ligands from the tripyrrin unit of the calixsmaragdyrin macrocycle, and the remaining coordination sites of Ru(II) ion were occupied by two carbonyl groups and one hydroxyl (−OH) group. The calixsmaragdyrin macrocycle in the Ru(II) complex was distorted with a dome-like structure. In the BF2 complex of calixsmaragdyrin, the BF2 unit was bound to two pyrrolic nitrogens of the dipyrrin moiety of calixsmaragdyrin as deduced by detailed 1- and 2-dimensional NMR spectroscopy studies. The Ru(II) complex displayed a strong Soret-like absorption band at 449 nm with the absence of Q-bands, whereas the BF2 complex showed a Soret-like band at 475 nm with two well-defined Q-bands at 787 and 883 nm, respectively. Quantum mechanical DFT calculations yielded relaxed equilibrium structures that were similar to the X-ray crystal structures, and the related charge density distributions indicated that the d orbital of the Ru(II) ion was contributing to the HOMO and LUMO states. In addition, TD-DFT calculations successfully reproduced the large bathochromic shifts, oscillator strengths, and electronic transitions that were observed in the experimental absorption spectra of all three complexes. Both the Ru(II) and the BF2 complexes of calixsmaragdyrin were stable under redox conditions.

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INTRODUCTION Smaragdyrin is a pentapyrrolic expanded porphyrin where five pyrrole rings are connected through three meso-carbon atoms and two direct pyrrole−pyrrole linkages.1 Smaragdyrin and its derivatives are attracting attention from the scientific community because of their improved spectral and electrochemical properties. Since 1970, Johnson and co-workers,2 Sessler and co-workers,3 and others4 have worked independently on the preparation of stable smaragdyrin macrocycles. However, their efforts to achieve a stable azasmaragdyrin macrocycle 1 (Chart 1) were unsuccessful. Chandrashekar and co-workers reported the synthesis of the first stable 25oxasmaragdyrin and stable Ni(II) and Rh(I) complexes of 25oxasmaragdyrin.5 More recently, Ravikanth and co-workers reported the synthesis, reactivity, and electronic properties of BF2,6, B(OR)2,7, PO2,8, and mixed B(III)/P(V) complexes9 of 25-oxasmaragdyrin. However, the preparation of stable azasmaragdyrin has remained elusive, and this has prevented its potential application as a ligand to form metal complexes. Recently, we reported the first successful synthesis of stable meso-aryl calixsmaragdyrin 2, where introduction of one sp3 meso-carbon atom instead of a sp2 meso-carbon atom provides © 2017 American Chemical Society

Chart 1. Chemical Structure of Smaragdyrin (1) and Calixsmaragdyrin (2)

increased flexibility and stability to the smaragdyrin core.10−12 The calixsmaragdyrin macrocycle 2 has three ionizable inner NH atoms which indicate its potential for multidentate coordination chemistry. Given the limited stability of Received: September 16, 2016 Published: March 23, 2017 3763

DOI: 10.1021/acs.inorgchem.6b02225 Inorg. Chem. 2017, 56, 3763−3772

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Inorganic Chemistry Scheme 1. Synthetic Scheme for the Ru(II) and BF2 Complexes of Calixsmaragdyrin 3 and 4

Figure 1. Partial 1H NMR spectra of (a) compound 3 and (b) compound 4 which were recorded in CDCl3 (asterisk (*) indicates residual solvent peak) at room temperature (inner NH resonances are shown in the inset).

smaragdyrin 1 it is not currently possible to explore its role as a potential ligand in coordination chemistry. Therefore, we explored the coordination properties of calixsmaragdyrin 2. Herein, we report the first examples of Ru(II) and BF2 complexes of calixsmaragdyrin that exhibit wide structural diversity. In the former complex, the Ru(II) ion is coordinated to three pyrrolic nitrogen atoms of the calixsmaragdyrin macrocycle with three other ligands that leads to a hexacoordinate complex 3. In the latter complex, the BF2 unit is bonded to two pyrrolic nitrogen atoms of the calixsmaragdyrin macrocycle 4. Both the Ru(II) and the BF2 complexes of calixsmaragdyrin exhibit interesting spectral and electrochemical properties. The hypothesis of increased stability by incorporation of one sp3 meso-carbon atom instead of a sp2 meso-carbon atom was investigated by density functional theory (DFT) methods to elucidate the effect of the structure on the electronic properties of 2, 3, and 4. The relaxed equilibrium structures of the Ru(II)

(3) and BF2 (4) complexes conformed to distorted rings with partial planar character induced by inorganic chelation. The increase in the torsional strain was partially relieved by the presence of the flexible sp3 meso carbon atom as inferred from the distorted structure proximal to the carbon atom while retaining planarity near the inorganic chelate.

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RESULTS AND DISCUSSION Synthesis of 3 and 4. The calixsmaragdyrin (2) was prepared as reported previously.12 We optimized the metal insertion reactions of 2 by using a variety of metal salts under various experimental conditions for preparation of the Ru(II) calixsmaragdyrin (3). The Ru(II) complex of calixsmaragdyrin 3 was prepared by refluxing 2 with 2.5 equiv of Ru3(CO)12 in dry toluene for 2 h (Scheme 1). Following removal of solvent, the crude reaction mixture was purified by silica gel column chromatography by using a mixture of petroleum ether− dichloromethane (DCM) (3:1) to afford a green-colored solid 3764

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correlations in the 1- and 2-D NMR spectra, the resonances at 5.99 and 5.71 ppm were assigned as e- and d-type pyrrole protons, respectively. The b- and c-type pyrrole protons were assigned by NOE correlations with the meso-tolyl proton resonances. The methyl protons of the meso-tolyl group at 2.36 ppm were correlated with the ortho-protons of the meso-tolyl group in the ∼7.16−7.12 ppm region of the spectrum which in turn was correlated with the singlet and doublet resonances at 5.72 and 6.16 ppm, respectively (Supporting Information S9). Signals at 5.72 and 6.16 ppm were assigned to the a- and b-type pyrrole protons, respectively. The b-type pyrrole resonance at 6.16 ppm also displayed a correlation with the doublet at 6.03 ppm in the COSY spectrum which was assigned to c-type pyrrole protons (Supporting Information S7). The axial hydroxyl proton of 3 was not observed in the roomtemperature 1H NMR spectrum. The 1H NMR spectrum of 4 contained a larger number of resonances than the free base 2, which confirmed the unsymmetrical nature of 4 (Supporting Information S5). Compound 4 can exist in three tautomeric forms 4A, 4B, and 4C (Supporting Information S17), and NMR is interpreted by taking the most stable tautomer 4B which is shown as compound 4 in Scheme 1. All of the resonances were identified based on cross-peak correlations observed in COSY and NOESY spectra (Figure 1b). The spectrum of 4 displayed 10 resonances from the β-pyrrole protons. The five doublet resonances at 6.29, 6.51, 6.55, 6.72, and 6.84 ppm correspond to one β-pyrrole proton each, and the resonance at 6.98 ppm corresponds to two β-pyrrole protons. The resonances from the remaining three β-pyrrole protons were overlapping with the aryl resonances in the 7.23−7.42 ppm region. The inner NH protons of 4 were observed as singlet and triplet resonances at 13.53 and 11.27 ppm, respectively (Figure 1b). The triplet at 11.27 ppm was assigned to m′-type inner NH protons as it was hydrogen bonded with the two fluorine atoms of the BF2 moiety. Similar intramolecular hydrogen bonds between the inner NH protons and BF2 were previously observed in other expanded porphyrins.6,15 In addition, the 11B and 19F NMR spectra of 4 displayed a triplet at ∼2.00 ppm and a broad signal at ∼−133.36 ppm, respectively (Supporting Information S10). Although 4 was structurally asymmetric, we observed only two signals from the methyl groups: the resonance at 2.45 ppm was assigned to six tolyl-CH3, and the resonance at 1.80 ppm was assigned to six meso-CH3 protons. We anticipated that the mesomethyl protons and the tolyl-CH3 protons would display NOE connectivity with the adjacent β-pyrrole and aryl protons, respectively. The resonance at 1.80 ppm showed a NOE connectivity with the two β-pyrrole resonances at 6.72 and 6.85 ppm (Supporting Information S14) arising from either the f- or the g-type pyrrole protons, respectively. The resonances at 6.72 and 6.85 ppm in the 1H−1H COSY spectrum were correlated with the resonances at 7.31 and 7.23 ppm, respectively (Supporting Information S11). The signal at 7.31 ppm was in turn correlated with the inner NH proton, which appeared as a singlet at 13.53 ppm (m-type) (Supporting Information S12). Hence, on the basis of the COSY and NOESY spectra, the resonances at 6.85, 7.23, 6.72, and 7.31 ppm were assigned to the f-, e-, g-, and h-type pyrrole protons, respectively. However, no correlation was observed between the m-type and the g-type pyrrole protons (Supporting Information S11). The two doublets at 7.31 (h-type) and 7.23 ppm (e-type) in the NOESY spectrum were correlated with the doublets at 6.29 and 6.98 ppm, respectively (Supporting Information S13). On the

3 in 72% yield. Since the calixsmaragdyrin macrocycle contains both amino and imino nitrogen atoms, we expected that it would form a BF2 complex, and our attempts were successful in preparing the novel BF2 complex of the calixsmaragdyrin macrocycle 4. The BF2 complex of calixsmaragdyrin (4) was prepared by treating a solution of the free base (2) in DCM with 40 equiv of triethylamine followed by an excess of BF3.OEt2 at room temperature (Scheme 1). The crude compound was subjected to basic alumina column chromatography, and compound 4 was collected as a highly pure green solid in 30% yield. The identities of both 3 and 4 were confirmed by the corresponding molecular ion peak using HRMS (Supporting Information S1−S3). The axial ligand dissociation of complex 3 was also observed in the HRMS spectrum (Supporting Information S2). Further, the IR spectrum of 3 displayed the appearance of two strong absorption bands at 2040 and 1987 cm−1 that clearly indicated the presence of two carbonyl groups on the Ru(II) metal ion in 3 (Supporting Information S4). In addition, a strong absorption at 583 cm−1 is observed due to the Ru−O bond which is in agreement with previously published literature on similar Ru(II) porphyrinoid complexes.13,14 Thus, the IR studies also supported the presence of an axial hydroxyl group (−OH) at the Ru(II) metal center. NMR Spectroscopy of 3 and 4. The Ru(II) and BF2 complexes 3 and 4 were characterized in detail by 1- and 2-D NMR spectroscopy (Figure 1). In general, the free base calixsmaragdyrin (2) showed 4 sets of doublets and 1 singlet resonance in the 5.75−6.80 ppm region from the 10 β-pyrrole protons (Supporting Information S5). Upon formation of 3 and 4 there were both upfield and downfield shifts of the βpyrrole proton resonances which clearly suggested a change in the electronic properties of 2. The 1H NMR resonances of 3 and 4 were assigned based on the cross-peak correlations that were observed in the respective 1H−1H COSY and 1H−1H NOESY spectra (Supporting Information S6−S9). In the case of 3, the 10 β-pyrrole protons appeared as five similar sets of resonances in the 5.71−6.16 ppm region but with slight upfield shifts (∼0.2 ppm) compared with 2 (Figure 1a). The resonances of the methyl (−CH3) protons of meso-tolyl groups and the meso-sp3 carbon atom appeared as singlet resonances at 2.36, 2.57, and 1.13 ppm. Further, a downfield resonance was observed at 15.77 ppm that corresponded to the two inner NH protons of 3. In the 1H−1H COSY NMR spectrum of 3, the inner NH resonance at 15.77 ppm displayed correlations with the doublet resonances at 5.99 and 5.71 ppm (Supporting Information S6) which were assigned to the d-type and e-type β-pyrrole protons of 3, respectively. The inner NH proton resonance at 15.77 ppm also displayed a NOE correlation with the resonance at 2.57 ppm that was assigned to the protons of a methyl group (type I) present at the meso-sp3 carbon atom. However, the other set of methyl protons (type II) at the mesosp3 carbon atom displayed a NOE correlation with the resonance from the e-type pyrrole protons at 5.99 ppm (Supporting Information S8). The two sets of meso-methyl group resonances at 2.57 and 1.13 ppm also showed cross-peak correlations with each other in 1H−1H COSY and NOESY spectra. Since the meso-methyl groups are spatially oriented opposite to each other (Supporting Information S7), we anticipated that type I meso-methyl protons would be correlated with the resonance from the inner NH proton and the type II meso-methyl protons would exhibit a NOE correlation with etype pyrrole protons of 3. On the basis of the cross-peak 3765

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Figure 2. (a) Perspective view of 3. (b) Side view of 3 (meso-tolyl groups, meso-methyl groups, and hydrogen atoms are omitted for clarity). (c) Diagram showing the out-of-plane displacements (in units of 0.01 Å) of the Ru(II) ion in 3 with respect to the 28 atom mean plane. (d) Coordination geometry of the central Ru(II) ion in 3 (bond lengths are depicted in Angstroms).

of 3 and 4 were identified based on the COSY and NOESY spectra. X-ray Crystallography of 3. Structural identification of hexacoordinate Ru(II) dicarbonyl complex 3 was obtained using single-crystal X-ray crystallography. Single crystals suitable for crystallographic analysis of 3 were grown via vapor diffusion of n-hexane into a solution of 3 in DCM. The crystal structure of 3 is shown in Figure 3a, and the crystallographic data are presented in Table S1 (Supporting Information S15). As observed in Figure 2a, the Ru(II) ion is coordinated by the three pyrrole nitrogen atoms of the tripyrrane unit of 3 and the other three coordination sites of Ru(II) are occupied by two carbonyl groups and one hydroxyl (−OH) group. The Ru(II) ion is positioned ∼0.182 Å below the mean plane defined by three coordinating nitrogen atoms of calixsmaragdyrin (N2N3N4 plane), whereas it is ∼0.467 Å below the mean plane of the macrocycle defined by the five pyrrole rings and three meso-carbon atoms (28 atoms). Upon incorporation of Ru(II), the calixsmaragdyrin macrocycle was further distorted (Figure 2b). In order to obtain insight on the effects of metalation on the calixsmaragdyrin core, we compared the crystal structure of 3 with the structure of the free base calixsmaragdyrin (2). Both macrocycles exhibited some degree of distortion; however, 3 was less planar in comparison with 2. An out-of-plane displacement of 0.01 Å was

basis of these correlations, we assigned the peaks at 6.29 and 6.98 ppm as i-type and d-type pyrrole protons, respectively. The i-type resonance at 6.29 ppm was correlated with the resonance at 6.55 ppm, identified as a j-type pyrrole proton, whereas the resonance at 6.98 ppm was correlated with the resonance at 6.51 ppm, assigned as a c-type pyrrole proton. Furthermore, c-type pyrrole proton was also correlated with m′-type inner NH protons (Supporting Information S12); hence, the c-type pyrrole proton appered as a doublet of doublets. The aryl protons of 4 appeared as three sets of resonances: a doublet at 7.41 ppm from two aryl protons and two multiplets at 7.26 ppm from six aryl protons. The tolylCH3 protons at 2.45 ppm were correlated with the orthoprotons of the aryl groups in the 7.23−7.31 ppm region (Supporting Information S14). Due to the asymmetric structure of 4, the doublet resonance at 7.42 ppm was assigned to the meta-protons of the aryl groups (x and x′-type), which were correlated with the signals at 6.98 and 6.55 ppm, respectively (Supporting Information S13), which were assigned to the a- and j-type pyrrole protons, respectively. The 6.98 ppm resonance in the 1H−1H COSY spectrum was correlated with a resonance at 7.30 ppm that overlapped with the aryl signals. The peak at 7.30 ppm was tentatively assigned to the b-type pyrrole proton. Thus, all of the proton resonances 3766

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The axial −OH ligand was oriented opposite to the N1 and N5 nitrogen atoms of the macrocyle 3 (Figure 2a). The presence of a direct hydroxyl ligand coordinated to the Ru(II) ion is rare in porphyrinoid literature. There have been several reports where hexacoordinate Ru(II) porphyrin complexes have an axial solvent ligand in solution (MeOH, EtOH, etc.) or have been crystallized with axial pyridyl ligand(s).16 To the best our knowledge, this is first example where the Ru(II) ion was coordinated with axial hydroxyl and carbonyl ligands (Figure 2d). All of the bond angles of the Ru(II) ligands were ∼90°. Furthermore, the two meso-tolyl groups have an average dihedral angle of ∼79.61° with the mean plane of the macrocycle, which suggests that the meso-tolyl groups were almost orthogonal to the macrocycle. Thus, analysis of the crystal structures reveals that the Ru(II) calixsmaragdyrin macrocycle (3) was more distorted compared to calixsmaragdyrin (2) and showed unique coordination features. Absorption and Electrochemical Properties of 3 and 4. The absorption properties of the Ru(II) (3) and BF2 (4) complexes of calixsmaragdyrin were compared with the free base 2. A comparison of absorption spectra and redox potentials of 2−4 is presented in Figure 3a and Table 2. The spectrum of 2 exhibits a broad Soret-like band at 425 nm and one ill-defined Q-type band at 685 nm. However, upon Ru(II) and BF2 complexation, the calixsmaragdyrin displayed a change in color from green to dull green which correlated with the 25− 50 nm red shifts that were observed in the Soret-type band absorption maxima. The spectrum of 3 exhibits one broad band at 449 nm and no other band, whereas 4 exhibits a sharp Soretlike band at 475 nm with two well-defined Q-type bands at 787 and 883 nm (Table 2). However, both 3 and 4 did not display fluorescence properties. The redox properties of 2 and 4 were studied by cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using tetrabutylammonium perchlorate as the supporting electrolyte in CH2Cl2. The representative oxidation and reduction waves and redox potentials of 3 are presented in Figure 3b and 3c and Table 2. While 2 exhibited four irreversible oxidation peaks (0.71, 0.95, 1.42, and 1.62 V) and three irreversible reductions (−0.25, −0.59, and −1.11 V),12 3 displayed two reversible reductions at −0.57 and −1.04 V and two quasi-reversible oxidations at 0.96 and 0.80 V. Further, three irreversible oxidations (0.75, 1.28 and 1.65 V) and three irreversible reductions (−0.22, −0.48, and −0.71 V) were observed for 4 (Table 2). However, the changes in redox

observed in 3 (Figure 2c), where the N2 and N4 atoms coordinating pyrrole nitrogen atoms were displaced below the mean plane of the macrocycle, while the N3 pyrrole atom was above the mean plane. Since the Ru(II) ion in 3 was coordinated to the three pyrrole nitrogen atoms of the tripyrrane moiety, the metal ion was located toward one end of the macrocyclic cavity, which constrains the three pyrrole nitrogen atoms, N2, N3, and N4, to remain in the same plane and leads to increased strain in the macrocycle (Figure 2b). As a result, the remaining pyrrole rings, N1 and N5, including the sp3 meso-carbon of the macrocyle are bent in comparison with the free base 2 (the distance of the sp3 meso-carbons from the mean plane of the macrocycle is ∼1.212 and ∼1.249 Å in 2 and 3, respectively). However, the other two sp2 meso-carbon atoms were slightly deviated by ∼0.071 Å with respect to the mean plane. The Ru(II) ion displayed octahedral geometry with three nitrogen ligands from the macrocyclic ring and two carbonyls and one hydroxyl ligand. The two carbonyl ligands were in a cis orientation (Figure 2d), whereas the hydroxyl group (−OH) ligand was ∼1.491 Å above the mean plane of the macrocycle. The distances between Ru(II) and three coordinating pyrrole nitrogen ligands was similar (∼2.101 Å), but the Ru−C bond distances of the axial carbonyl ligands were different (Table 1). Table 1. Selected Bond Lengths (Angstroms) and Angles (degrees) of 3a parameters

bond length [Å]

parameters

Ru1−N2 Ru1−N3 Ru1−N4 Ru1−C1 Ru1−C2 Ru1−O1

2.115(6) {0.002} 2.091(6) {0.003} 2.118(6) {0.014} 1.911(9) {0.009} 1.863(11) {0.018} 2.102(5) {0.005}

N2−Ru1−N3 N3−Ru1−N4 N4−Ru1−C1 C1−Ru1−N2 C1−Ru1−C2 C1−Ru1−O1

bond angle [deg] 89.1(2) 89.1(2) 90.1(3) 93.3(3) 86.0(4) 97.5(3)

{−0.49} {−0.49} {2.09} {−1.11} {0.58} {−0.46}

a Values in {} indicate the difference between the values that were calculated by DFT and those obtained from the X-ray crystal structure.

The Ru−C1 and Ru−C2 bond distances were 1.911 and 1.863 Å, respectively. The Ru−C1 bond was longer compared to the Ru−C2 bond, whereas the C1−O2 bond distance (1.137 Å) was shorter than the C2−O3 bond distance (1.148 Å). This observation clearly suggests that the π-back bonding, involving the metal center and carbonyl ligand, was more effective in the case of the C2 carbonyl compared with the C1 carbonyl ligand.

Figure 3. (a) Comparison of absorption spectra of 2 with 3 and 4 (1 × 10−6 M) recorded in CHCl3 at room temperature. (b) Oxidation and (c) reduction waves in the cyclic (solid trace) and differential pulse voltammogram (dotted trace) of 3 recorded in CH2Cl2 with 0.1 M TBAP as a supporting electrolyte and saturated calomel electrode (SCE) as a reference electrode at scan rates of 50 mV s−1. 3767

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Inorganic Chemistry Table 2. Absorption and Electrochemical Data of 2−4 redox data absorption data compound

Soret band (nm)

2 3 4

425 449 475

oxidation (V)

Q-bands (nm) 685 787, 883

reduction (V)

I

II

III

IV

I

II

III

0.71 0.80 0.75

0.95 0.96 1.28

1.42

1.62

−0.25 −0.57 −0.22

−0.59 −1.04 −0.48

−1.11

1.65

−0.71

The absorption spectra of 2 and 3 were calculated from first principles using the TD-DFT formalism to corroborate the relaxed equilibrium structures that are discussed above and understand the large bathochromic shifts that were observed in the experimental studies. As can be seen in Figure 4, the

potentials did not follow any particular trend, and more studies are required to understand the redox behavior of 2−4. Quantum Mechanical Calculations of 2, 3, and 4. The free-base (2), Ru(II) (3), and BF2 (4) calixsmaragdyrin complexes were investigated by quantum mechanical DFT calculations. The relaxed equilibrium structures and corresponding dihedral angles of the pyrrole and tolyl groups of each complex are presented in Figure S18 and Tables S2 and S3 (Supporting Information S19 and S20), respectively. The relaxed structure of the free-base calixsmaragdyrin (2) showed a distribution of dihedral angles of the pyrrole groups, N1−N5, with respect to the mean plane defined by 28 atoms (Supporting Information S19) where all of the nitrogen atoms, except N2, were positioned on one side of the plane (Supporting Information S18). Despite the mirror plane through the sp3 meso-carbon atom and nitrogen atom of the N3 pyrrole, the pyrrole rings N1 and N5 displayed vastly different dihedral angles due to steric interactions (Supporting Information S19). However, it is expected that the dihedral angles of the pyrrole groups of 2 fluctuate rapidly in solution, leading to the observation of an average planar structure at room temperature. The dihedral angles of the tolyl groups, A and B, in the relaxed structure of 2 were observed to be ∼50° with respect to the mean plane (Supporting Information S20). The atomic displacements of select atoms with respect to the mean plane of 2 are shown in Table S4 (Supporting Information S21). The relaxed DFT structure of complex 3 displayed dihedral angles of the pyrrole and tolyl groups that were symmetric about a plane along Ru(II) and N3 (Supporting Information S18) which is in agreement with the X-ray crystal structure. A comparison of selected bond lengths and angles of complex 3 in the DFT and X-ray crystal structures is presented in Table 1. The pyrrole groups, N1 and N5, displayed the largest dihedral angles of ∼37°, whereas those of N2, N3, and N4 were smaller (∼3−15°) (Supporting Information S19). The dihedral angles of the tolyl substituents were ∼80° (Supporting Information S20), and the pyrrole nitrogen atoms of N1, N3, and N5 were displaced above the mean plane, while those of N2 and N4 were below the plane (Supporting Information S21). The Ru(II) ion was also positioned below the mean plane (∼0.486 Å), leading to a strained octahedron that was formed by the tripyrrane moiety (Supporting Information S18). This distortion was enhanced by hydrogen bonding between the pyrrole groups, N1 and N5, with the hydroxyl group of Ru(II) and steric repulsion with the proximal carbonyl group. The distances between the methyl group I and the N−H moiety of the pyrroles, N1 and N5, were in agreement with the peaks that were observed in the NOESY spectrum of 3, which confirmed the presence of a distorted macrocyclic structure with a domelike structure (Figure 1). Moreover, the carbonyl geometry revealed a short and long C−O bond for carbonyl groups, 1 and 2, respectively, where the Ru−C1−O bond angle was ∼169°, which was in agreement with the X-ray crystal structure.

Figure 4. Comparison of absorption spectra of 2 with 3 and 4 that were calculated using TD-DFT to predict the electronic transitions.

calculated spectra that were obtained for 2 and 3 are in excellent agreement with the experimental spectra shown in Figure 3a. Both the experimental and the calculated spectra of 3 confirmed the presence of a highly symmetric structure as evidenced by the simpler UV−vis spectrum in comparison with 2. It was observed that the broad absorption peak of 2 at 685 nm was absent in the experimental and calculated spectrum of 3. This was due to the increased rigidity caused by insertion of the Ru(II) ion that enhanced the torsional strain of the macrocycle. Moreover, the large bathochromic shift that was observed in the experimental and calculated spectrum of 3 accounted for the highly distorted pyrrole groups, N1 and N5, in the relaxed DFT structure. The relaxed structure of 3 was also in agreement with the results that were obtained from IR spectroscopy. The geometry of the carbonyl ligands of the Ru(II) ion revealed that the carbonyl 1 ligand had a longer Ru− C bond length and shorter C−O bond length and displayed a bent geometry which was in agreement with the experimental IR band at higher wavenumbers due to weaker π-back bonding. As discussed in previous sections and shown in Supporting Information S17, there are three possible tautomers of complex 4; however, only tautomer 4B was observed in the experimental studies. To better understand the preference for tautomer 4B, we used DFT to calculate the relaxed equilibrium structures of all three tautomers, 4A, 4B, and 4C (Supporting Information S22). The corresponding geometric parameters are presented in Tables S5 and S6 (Supporting Information S23 and S24). 3768

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Figure 5. Charge density distribution in the HOMO and LUMO states for (a and b) 2, (c and d) 3, and (e and f) 4, respectively.

angles of the pyrrole groups, N1and N5, were ∼18° with respect to the mean plane of 4, which was lower than the average angles of ∼27° and ∼37° that were observed in the relaxed structures of 2 and 3, respectively (Supporting Information S19). This was likely due to the hydrogen bonding between the two rings (Supporting Information S23). The pyrrole group, N2, had the largest dihedral angle of ∼28°, while the pyrrole groups, N3 and N4, displayed the smallest dihedral angles (∼9−12°) in 4. The nitrogen atoms of all of the pyrrole groups, except for N4, were above the mean plane, and the boron atom was positioned ∼0.02 Å below the mean plane (Supporting Information S21). The average bond angle of the boron unit was ∼109.4° ± 2°, which suggested the presence of a slight strain. The chelation of the BF2 group resulted in a more distorted macrocycle as evidenced by the large dihedral

The trend of planarity that was observed in the relaxed DFT structures was 4A < 4B < 4C, which suggested that tautomer 4C would be more stable. However, analysis of the smaragdyrin core revealed that tautomer 4B was most favorable due to the presence of the largest N(H)···N and N(H)···F and smallest N···F and N(H)···N(H) distances, which minimized the lone pair repulsion and maximized the hydrogen bond donor− acceptor interactions within the core. In comparison with the relaxed DFT structures of 2 and 3, complex 4 was highly asymmetric due to the chelation of the BF2 group to the imino nitrogen atom of the pyrrole group, N3, and the amine of N4 (Supporting Information S18). The increased asymmetry was most readily observed in the increased complexity of the 1H NMR and UV−vis spectra that are shown in Figures 1 and 2a, respectively. The dihedral 3769

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Inorganic Chemistry

observed previously in metal complexes of pyrrole-based expanded porphyrinoids. The presence of an axial hydroxyl ligand is uncommon and warrants exploration of the chemistry of such complexes. In contrast, the BF2 unit of 4 was bound to the dipyrrin moiety of the calixsmaragdyrin macrocycle with completely different structural features in comparison with 3. Thus, the features in the absorption spectra of 3 and 4 were quite different from each other. The relaxed equilibrium structures that were obtained from DFT calculations were in excellent agreement with the X-ray structures and the experimental 1H NMR, IR, and absorption spectra. Further, the charge density distribution in the HOMO and LUMO states of 2, 3, and 4 that were calculated by DFT elucidated the chelation effects of Ru(II) and BF2 binding where 3 was a highly symmetric system with contributions from the Ru(II) d orbitals in both the HOMO and the LUMO and 4 was highly asymmetric. The TD-DFT calculations of the electronic transitions of 2, 3, and 4 were also in good agreement with the experimental absorption spectra, corroborating the bathochromic shifts. Both 3 and 4 were observed to be stable under redox conditions.

angle of the tolyl groups, A and B (Supporting Information S20). Further, as can be seen in Figure 4, the calculated absorption spectrum of 4 was in excellent agreement with the experimental spectrum shown in Figure 3a. Both the experimental and the calculated spectra of 4 displayed a bathochromic shift in comparison with 2 and 3 and also contained additional peaks at ∼790 and 880 nm. The calculated Soret bands of 4 displayed bathochromic shifts that were larger than 2 and 3, which is in agreement with the experimental spectra. This was evidenced from the increased distortion of the macrocyclic in the relaxed DFT structure of 4. The relaxed DFT structure of compound 4, tautomer 4B was also in good agreement with the chemical shifts of the amine protons of the pyrrole groups, N1 and N5, which were observed by NMR spectroscopy (Supporting Information S22). A hydrogen bond distance of 1.92 Å was observed between fluorine and the proton of the N5 pyrrole group of 4B, which is in agreement with the NMR spectrum that had a shielded triplet signal from m′ compared with the downfield singlet signal of m from the amine hydrogen of the N1 pyrrole group. As shown in Figure 5, the charge density distribution in the HOMO and LUMO state of 2, 3, and 4 was calculated from the corresponding relaxed DFT structures. The HOMO of 2 displayed symmetric charge density relative to the plane of symmetry on the nitrogen atoms of the pyrrole rings, N2 and N4, the sp2 meso carbon atoms, and the Cα−Cβ bond of the pyrroles. The LUMO state also exhibited a similar symmetric charge density distribution on all of the nitrogen atoms of the pyrrole groups and the sp2 meso carbon atoms with some of the charge density delocalized on the tolyl substituents. The metalation of complex 2 with Ru(II) to yield 3 appeared to have greatly increased the symmetry of both the HOMO and the LUMO states while retaining similar charge density distributions. In addition, the HOMO displayed charge density on the oxygen atom of the carbonyl 1 ligand and a dxz or dyz contribution from Ru(II) ion which was out of phase with the pyrrole groups, N2 and N4. Moreover, the LUMO state displayed a contribution of the dz∧2 orbital on the Ru(II) ion which was off axis with respect to its axial ligands. In contrast to complexes 2 and 3, the calculated charge density distribution in the HOMO and LUMO states of 4 was asymmetric, which was in agreement with the larger number of calculated and experimental electronic transitions in the absorption spectrum (Figures 3a and 4). The charge density on the pyrrolic nitrogen atoms was preserved in both the HOMO and the LUMO states of 4 in comparison with 2 and 3. The LUMO state of 4 also displayed a small amount of charge density on the fluorine atom that was hydrogen bonded to the pyrrole group, N5.

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EXPERIMENTAL SECTION

General Synthesis. The calixsmaragdyrin (2) was prepared by following our reported method.12 Synthesis of 3. Ru3(CO)12 (147 mg, 0.230 mmol) was added to a solution of calixsmaragdyrin (2) (50 mg, 0.092 mmol) in 10 mL of dry toluene in one portion, and the resulting mixture was refluxed for 2 h under a nitrogen atmosphere. The progress of the Ru(II) insertion reaction was monitored by UV−vis spectroscopy and TLC analysis. After completion of the reaction as indicated by TLC, the solvent was removed under reduced pressure. The crude solid was subjected to flash silica chromatography, and the yellowish-green band of 3 was collected using a CH2Cl2/petroleum ether (1:3) solvent mixture as eluent. The solvent was removed by rotary evaporation that afforded a dark green solid. The compound was recrystallized from a mixture of CHCl3/n-hexane. This afforded the pure compound 3 as a brownish solid in 72% yield (∼48 mg). 1H NMR (500 MHz, CDCl3, 25 °C): δ 15.77 (s, 2H, inner NH), 7.22−7.20 (m, 2H, J (H,H) = 7.8 Hz, Ar), 7.16−7.12 (m, 4H, Ar), 7.03−7.05 (m, 2H, J (H,H) = 7.6 Hz, Ar), 6.16 (d, 2H, J (H,H) = 4.8 Hz, β-pyrrole H), 6.03 (d, 2H, J (H,H) = 4.8 Hz, β-pyrrole H), 5.99 (m, 2H, β-pyrrole H), 5.72 (s, 2H, β-pyrrole H), 5.71 (m, 2H, β-pyrrole H), 2.57 (s, 3H, CH3), 2.36 (s, 6H, CH3 tolyl), 1.13 (s, 3H, CH3) ppm. UV−vis (λmax nm (log ε), CHCl3): 449 (5.34). HR-MS calcd for C41H32N5O2Ru (M − OH)+ m/z 728.1622, obsd 728.1606 (M − OH)+. Synthesis of 4. The calixsmaragdyrin (2) (50 mg, 0.092 mmol) was dissolved in dry DCM, and triethylamine (3.68 mmol) was added to the mixture at room temperature. After 15 min BF3·OEt2 (4.60 mmol) was added, and the mixture was stirred at room temperature for 30 min. The reaction was monitored by TLC analysis, which showed the disappearance of calixsmaragdyrin (2) and the appearance of the less polar BF2 complex of calixsmaragdyrin (4). The reaction mixture was diluted with CH2Cl2 and washed thoroughly with 0.1 M NaOH solution and water. The organic layers were combined, dried over Na2SO4, and filtered, and solvent was removed on a rotary evaporator under vacuum. The resulting crude product was purified by column chromatography on alumina using petroleum ether/dichloromethane (70:30) and afforded pure compounds as a green powder in 30% yield (∼17 mg). 1H NMR (400 MHz, CDCl3, 25 °C): δ 13.53 (s, 1H, NH), 11.27 (t, 1H, J (H,F) = 17.2 Hz, NH), 7.42−7.41 (m, 2H, Ar), 7.32− 7.21 (m, 9H, Ar, β-pyrrole H), 6.99 (d, 1H, J (H,H) = 4.8 Hz, βpyrrole H), 6.97 (d, 1H, J (H,H) = 4.4 Hz, β-pyrrole H), 6.85 (d, 1H, J (H,H) = 4.0 Hz, β-pyrrole H), 6.73 (d, 1H, J (H,H) = 4.1 Hz, βpyrrole H), 6.56 (d, 1H, J (H,H) = 4.4 Hz, β-pyrrole H), 6.51 (dd, 1H, J (H,H) = 5.1 Hz, β-pyrrole H), 6.29 (d, 1H, J (H,H) = 4.5 Hz, βpyrrole H), 2.45 (s, 6H, CH3 tolyl), 1.80 (s, 6H, CH3) ppm. UV−vis

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CONCLUSIONS The Ru(II) and BF2 complexes of calixsmaragdyrin 3 and 4 were synthesized under simple reaction conditions indicating that calixsmaragdyrin is a potential macrocyclic ligand that forms both metal and nonmetal complexes. The 1H NMR spectra of the Ru(II) complex 3 reflected the upfield shifts of the β-pyrrole protons, whereas the BF2 complex 4 displayed downfield shifts of the β-pyrrole protons of calixsmaragdyrin macrocycle compared to the free base calixsmaragdyrin (2). The X-ray crystal structure of 3 revealed that the Ru(II) ion was coordinated by three nitrogen ligands from the macrocycle, two axial carbonyl ligands, and one hydroxyl ligand. The structural features of 3 were unique and have not been 3770

DOI: 10.1021/acs.inorgchem.6b02225 Inorg. Chem. 2017, 56, 3763−3772

Inorganic Chemistry

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(λmax nm (log ε), CHCl3: 475 (6.01), 787 (4.86), 883 (5.21). HR-MS calcd for C39H33BF2N5 (M + H)+ m/z 620.2796, obsd 620.2798. Computational Methods. Geometric relaxation and electronic structure calculations were carried out using density functional theory (DFT) at the B3LYP/TZVP 17,18 level of theory and the RIJCOSX19−22 approximation using the ORCA electronic structure package.23−25 The initial coordinates of 2 and 3 were obtained from crystal structure data, while 4 was constructed from 2 using the molecular builder and visualization tool, Avogadro.26 To accurately describe solution conditions and transition metal electrons, corrections were applied to the standard B3LYP functional and TZVP energies. The solvent effects from chloroform were approximated by the COSMO solvation model,27and the relativity of Ru(II) d-electrons employed the zeroth-order regular approximation (ZORA). Timedependent-DFT (TD-DFT)28−32 was used to predict the electronic transitions and absorption spectra of 2−4. The simulated absorption spectra were generated by using the wavelengths of absorption predicted in the TD-DFT calculations by superimposing Gaussian functions with arbitrary widths and peak heights. Visualization of electronic orbitals and molecular structures was performed with the software program Chemcraft (http://www.chemcraftprog.com).

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ACKNOWLEDGMENTS M.R. thanks the Department of Science & Technology, Government of India, for funding the project, and T.C. thanks CSIR for the SRF fellowship. K.V.L. thanks the Center for Computational Innovations (CCI) at Rensselaer Polytechnic Institute (Troy, NY) for computational time.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02225. HR-MS mass spectrum of compound 3, mass spectrum of different components of compound 3, HR-MS mass spectrum of compound 4, IR spectrum of compound 3, comparison of the partial 1 H NMR spectra of compounds 2, 3, and 4, partial 1H−1H COSY NMR spectra of 3, 1H−1H COSY NMR spectra of 3, partial 1 H−1H NOESY NMR spectra of 3, 1H−1H NOESY NMR spectra of 3, 11B and 19F NMR spectra of 4, partial 1 H−1H COSY NMR spectra of 4, 1H−1H COSY NMR spectra of 4, partial 1H−1H NOESY NMR spectra of 4, partial 1H−1H NOESY NMR spectra of 4, crystal data and data collection parameters of compound 3, comparison of cyclic voltamograms of compounds 2, 3, and 4, structure of possible tautomers of compound 4, relaxed equilibrium DFT structures of 2, 3, and 4, dihedral angles of the pyrrole rings of 2, 3, and 4 relative to the mean plane, dihedral angles of the tolyl substituents of 2, 3, and 4 relative to the mean plane, atomic displacements of select atoms of 2, 3, and 4 from the mean plane, relaxed equilibrium DFT structures of 4A, 4B, and 4C, comparison of geometric parameters of the relaxed equilibrium DFT structures of 4A, 4B, and 4C, and atomic displacements of select atoms from the mean plane of tautomers 4A, 4B, and 4C (PDF) Crystallographic information file (CIF)

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Article

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Mangalampalli Ravikanth: 0000-0003-0193-6081 Notes

The authors declare no competing financial interest. 3771

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