Synthesis and Characterization of Palladium(II) Complexes of meso

Nov 25, 2015 - Synopsis. Two palladium(II) dichloride complexes of [14]tribenzotriphyrin(2.1.1) have been successfully synthesized. The complexes were...
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Synthesis and Characterization of Palladium(II) Complexes of mesoSubstituted [14]Tribenzotriphyrin(2.1.1) Zhaoli Xue,*,† Yemei Wang,† John Mack,*,‡ Yuanyuan Fang,†,§ Zhongping Ou,† Weihua Zhu,† and Karl M. Kadish*,§ †

School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, China Department of Chemistry, Rhodes University Grahamstown 6140, South Africa § Department of Chemistry, University of Houston, Houston, Texas 77204-5003, United States ‡

S Supporting Information *

ABSTRACT: Metalation of 6,13,20,21-tetrakis-aryl-22H-[14]tribenzotriphyrin(2.1.1) (TriPs) with PdCl2 provides PdII−TriP complexes in 45−56% yields. The complexes were characterized by mass spectrometry, and UV−visible absorption, magnetic circular dichroism, and 1H NMR spectroscopy. A single crystal X-ray analysis reveals that the PdII−TriPs adopts a deeply saddled conformation. The palladium(II) ion is coordinated by two pyrrole nitrogen atoms and two chloride ions to form the square-planar coordination environment. The redox properties of the PdII−TriPs were studied by cyclic voltammetry. Each compound undergoes one irreversible and two reversible oneelectron reductions. There is a marked red-shift of the main spectral bands, relative to those of the free-base TriP ligand, due to a marked relative stabilization of the LUMO upon coordination by PdCl2.

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INTRODUCTION Porphyrin and its derivatives have been studied extensively for decades, and a wealth of knowledge has been accumulated on their synthesis, structures, and electrochemical properties, which has provided the basis for research on a wide range of applications.1 Among the many porphyrin derivatives that are available, ring-contracted porphyrins represent a relatively new branch which has received considerable attention in recent years due to its potential application in a wide variety of hightechnology fields.2,3 The first example of a ring-contracted subporphyrin analogue containing three pyrrole moieties and three meso-carbons was reported by Osuka and co-workers in 2006.4 A number of meso-aryl substituted subporphyrins were then subsequently reported.5−14 With the exception of subp yriporphyrin, 1 5 [18]triphyrin(n.1.1), 1 6 [14]heterotriphyrin(2.1.1),17 and [15]triphyrin(1.1.3),18 only boron complexes with highly nonplanar cone-shaped conformations have been reported for triphyrin compounds and their core-modified analogues. In 2008, we reported a facile procedure to synthesize meso-aryl-substituted free-base [14]tribenzotriphyrin(2.1.1) (1), which is similar to subporphyrin but requires only one inner NH proton to form a heteroaromatic π-system, as there is an additional mesocarbon.19 Since then, meso-20 and β-substituent-free21 compounds have been prepared through modified condensation reactions. These novel contracted porphyrin systems provide a unique metal complexation behavior, which could lead to novel © XXXX American Chemical Society

applications. In contrast with most tetrapyrrole porphyrins, the [14]tribenzotriphyrin(2.1.1) ligand is rather flexible. Because of the presence of three nitrogen atoms on the inner ligand perimeter and the formation of a monoanion upon deprotonation, [14]tribenzotriphyrins(2.1.1) can serve as tridentate nitrogen ligands. Octahedral rhenium(I), 22 manganese(I),23 ruthenium(II), and platinum(IV)24 [14]tribenzotriphyrin(2.1.1) complexes have already been reported along with square-planar platinum(II)24 and η5-cyclopentadienyl-iron(II) sandwich complexes.25 Encouraged by the success of platinum(II) complexation, the palladium(II) ion was the obvious next target for synthetic work. Herein, the synthesis, characterization and properties of palladium(II) [14]tribenzotriphyrins(2.1.1) complexes with meso-phenyl and -tolyl rings are reported (Scheme 1). A detailed study of the optical and redox properties has been carried out using UV−visible and magnetic circular dichroism (MCD) spectroscopies, and a series of electrochemical measurements.

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EXPERIMENTAL SECTION

Measurements. Melting points were measured with a Yanaco M500D melting point apparatus. 1H 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, Received: September 10, 2015

A

DOI: 10.1021/acs.inorgchem.5b02093 Inorg. Chem. XXXX, XXX, XXX−XXX

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using CH2Cl2 as solvent. The first eluted yellow−green fraction was identified as the target complex 2. PdII−TriP (R = −CH3) 2a. Mp: >300 °C. Yield: 56%. 1H NMR (400 MHz, CDCl3, 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, −CH3), 2.47 (s, 6H, −CH3) ppm. 13C NMR (101 MHz, CDCl3): δ 156.9, 155.5, 153.1, 139.5, 139.3, 138.7, 137.1, 136.9, 136.6, 136.2, 134.9, 133.2, 131.1, 127.8, 127.3, 127.2, 126.9, 124.3, 123.7, 123.1, 21.8, 21.5 ppm. MS (MALDI-TOF): calcd for C56H40Cl3N3NaPt [M − Cl]+, 896.202; found, 896.044. UV−vis (in CH2Cl2) λ [nm] (ε [M−1cm−1]): 337 (34 700), 421 (52 700), 476 (87 100), 642 (25 400). PdII−TriP (R = −H) 2b. Mp: >300 °C. Yield: 45%. 1H NMR (400 MHz, CD2Cl2, 298 K): δ 8.47 (m, 2H, phenyl), 8.30 (d, J = 8.0 Hz, 2H, phenyl), 8.07 (m, 4H, phenyl), 7.95−7.97 (m, 4H, phenyl), 7.83− 7.85 (m, 4H), 7.68−7.70 (m, 2H), 7.62 (br, 1H, NH), 7.47−7.54 (m, 8H), 7.23−7.33 (m, 6H) ppm. 13C NMR (101 MHz, CD2Cl2): δ 157.8, 155.1, 142.1, 141.0, 138.7, 138.6, 137.9, 137.7, 136.9, 135.3, 134.9, 133.2, 133.0, 132.2, 132.0, 131.9, 131.2, 130.3, 130.1, 130.1, 129.0, 125.9, 124.8, 123.7 ppm. MS (MALDI-TOF): calcd for C56H41Cl2N3NaPd [M − Cl]+, 840.140;, found, 840.377. UV−vis (in CH2Cl2) λ [nm] (ε [M−1cm−1]): 334 (31 700), 423 (50 900), 471 (81 100), 630 (21 100).

Scheme 1. Synthesis of PdII−TriP Complexes 2a and 2b

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. IR and electronic absorption were recorded with Bruker Vector-22 and PerkinElmer Lambda 35 UV/vis spectrometers, respectively. Magnetic circular dichroism (MCD) spectra were recorded using a Jasco J-725 spectrodichrometer equipped with a Jasco permanent magnet (1.6 T). The conventions of Piepho and Schatz are used to describe MCD intensity and the Faraday terms.26 Cyclic voltammetry was carried out at 298 K using an EG&G Princeton Applied Research (PAR) 173 potentiostat/galvanostat. A homemade three-electrode cell consisting of a platinum button or glassy carbon working electrode, a platinum counter electrode, and a homemade saturated calomel reference electrode (SCE) was used for cyclic voltammetric measurements. 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. Thin-layer UV− visible spectroelectrochemical experiments were performed with a home-built thin-layer cell containing a light transparent platinum net working electrode. Potentials were applied and monitored with an EG&G PAR model 173 potentiostat. X-ray crystallographic analyses were carried out on a Bruker Smart Apex CCD diffractometer using monochromatic MoKα radiation (λ = 0.71073 Å) at 153 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-2000 program package.27 CCDC 1034192 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Theoretical Calculations. Geometry optimization calculations were carried out for the C13H13− parent hydrocarbon perimeter and 2a and 2b with the Gaussian09 software package28 by using the B3LYP functional with SDD basis sets. The B3LYP functional was also used with SDD basis sets to carry out TD-DFT calculations on the B3LYP geometries. Chemicals. Dichloromethane (CH2Cl2) was purchased from Aldrich Co. and used as received for electrochemistry. Tetra-nbutylammonium perchlorate (TBAP) was purchased from Sigma or Fluka, recrystallized from ethyl alcohol, and dried under vacuum at 40 °C for at least 1 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. PdCl2 was purchased commercially and used as received. Synthesis. Two different [14]tribenzotriphyrins(2.1.1) (1a and 1b) were prepared according to the published methods.19 A toluene solution (10 mL) containing 1a [14]tribenzotriphyrin(2.1.1) (0.013 mmol) and PdCl2 (24.3 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

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RESULTS AND DISCUSSION Synthesis and Characterization. [14]Tribenzotriphyrin(2.1.1) (1) was treated with PdCl2 following the conventional thermal procedure that is generally used for the synthesis of metalloporphyrins.29 A dry toluene solution of 1 in a Schlenk flask was treated with 10 equiv of PdCl2 and refluxed for 24 h under 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 again removed and the residue was purified by short silica gel column chromatography using CH2Cl2 as an eluent. The first eluted yellow−green fraction was evaporated to afford the crude product. Crystallization from CH2Cl2 and CH3OH then gave the pure target compound in 56% yield for 2a (R = −CH3), and 45% for 2b (R = −H), respectively. The structures of 2a and 2b were characterized by mass spectrometry and 1H NMR spectroscopy. During the reaction, no PdIV−TriP was obtained. Unlike PtII−TriP complex, which transforms spontaneously in solution to form a PtIV−TriP complex24 to the open air, 2a and 2b were found to be much more stable, when solutions were left open to the air for 1 week. X-ray Diffraction Analysis. The structures were unambiguously determined through the X-ray diffraction analysis of a single crystal of 2a, which was obtained by slow diffusion of methanol into the CH2Cl2 solution over a period of 5 days. Details of the crystal structure of 2a are provided in Figure 1, Tables 1 and 2, and in the Supporting Information in Figures S1 and S2 and Table S1. The TriP macrocycle adopts a deep saddle conformation with Cs symmetry, which is similar to that of PtII−TriP complexes. The palladium(II) ion is coordinated by two pyrrole nitrogen atoms and two chloride ions with the following bond lengths: 2.009(4) Å for Pd−N and 2.297(2) Å for Pd−Cl in a cis configuration in a rigid square-planar arrangement. The Pd−N bond length is almost the same as that reported for PdII−TPP (TPP = tetraphenylporphyrin) complex.30 The distance between Pd(II) ion and the nonbonded nitrogen atom is 2.732(7) Å. The average distance between the three nitrogen atoms is 2.716 Å which is significantly longer than the value of 2.558 Å reported for previously free-base TriP,22 due to the saddling of the structure. B

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Inorganic Chemistry Table 2. Selected Bond Lengths and Angles for 2a

Table 1. Crystal Data and Data Collection Parameters for 2a 2a

a

bond length (Å)

angle

bond angle (deg)

2.297(2) 2.009(4) 1.448(7) 1.387(6) 1.422(9) 1.427(6) 1.343(6) 1.397(7) 1.352(6) 1.508(7) 1.469(6)

Cl1′−Pd1−Cl1 Cl1−Pd1−N1 N1′−Pd1−N1 C1′−C1−C2 C1−C2−N1 C9−C10−C11 C2−N1−C9 N1−C9−C10 C9−C10−C11 C11′−N2−C11 C1−C2−C3

94.6(6) 90.2(1) 84.9(2) 127.2(5) 123.6(4) 118.3(5) 110.5(4) 122.2(5) 118.3(5) 114.1(6) 127.4(5)

39.48° and 39.25°, respectively. The average bond length between the meso-carbon atom and the phenyl rings is 1.489 Å (Table S1). The dihedral angles of the meso-aryl substituents (Φ1−Φ4) are much larger than those reported previously for PtII−TriP complexes (Table S1).24 1 H NMR Spectroscopy. A detailed analysis of the spectra of 1a and 1b has been reported previously.22 The 1H NMR spectra of 1a and 2a are shown in Figure 2, and those of 1b and 2b are provided as Figures S3−S6. 2a has a relatively simple set of signals for the aromatic protons, due to the mirror plane of symmetry. The singlet peaks at 2.74 and 2.47 ppm are associated with the methyl protons of the meso-tolyl groups. The doublet peak at 6.31 ppm in the spectrum of 1a has been assigned to the benzo-rings based on 1H−1H COSY NMR data.22 The signals for these protons shift downfield in the spectrum of 2a in a manner similar to that reported previously for the analogous platinum(II) complexes.24 As has been reported for other metallotriphyrins, such as platinum(IV),24 rhenium(I),22 manganese(I),23 and ruthenium(II)22 [14]tribenzotriphyrin(2.1.1) complexes, the proton peaks of the meso-aryl and benzo groups are significantly broadened. Temperature dependence studies on the platinum(IV) complex have previously demonstrated that this is due to conformational flexibility related to the nonplanarity of the macrocycle, as significant band sharpening was observed when the temperature was lowered to 213 K.22 Optical Spectroscopy and Theoretical Calculations. The optical spectra of palladium(II) [14]tribenzotriphyrins(2.1.1) complexes (Figure 3), can be readily assigned by using Michl’s perimeter model to analyze the results of TD-DFT calculations on B3LYP optimized geometries.31−33 A C13H13− species can be viewed as the parent hydrocarbon perimeter for the 13 atom 14-π-electron associated with the inner perimeter of the TriP ligand, with an ML = 0, ±1, ±2, ±3, ±4, ±5, ±6 sequence for the π-MOs in ascending energy terms. The HOMO and LUMO of the parent perimeter have ML = ±3, ±4 angular nodal patterns, and this results in an allowed (ΔML = ±1) B band and a weaker forbidden (ΔML = ±7) L band at lower energy (Figure 3). Michl introduced an a, s, −a and −s terminology (Figure 4), to describe the four frontier π-MOs of aromatic cyclic polyenes based on whether there is a nodal plane or a significant MO coefficient on the atoms that lie on the y-axis, since this aids the comparison of trends in compounds with differing molecular symmetries. The frontier π-MOs of 2 can be readily identified on this basis (Figure 4 and Table 3), and are predicted to give rise to bands in the L and B band regions that are broadly comparable to those reported previously for the free base

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

formula fw crystal symmetry space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) T (K) dcalcd (g cm−3) Z F (000) μ (mm−1) indep reflns R1,a wR2b (I > 2σ (I)) R1,a wR2b (all data) GOF

bond Pd1−Cl1 Pd1−N1 C1−C2 C1−C1′ C3−C8 C12−C12′ C2−N1 C9−N1 C11−N2 C1−C15 C10−C21

C56H41Cl2N3Pd·2CH3OH 997.30 orthorhombic Pnma 21.018(4) 22.515(5) 9.907(2) 90.0000 90.0000 90.0000 4688.2(16) 150(2) 1.413 4 2056 0.558 4577 0.0789, 0.1390 0.1021, 0.1515 1.038

R1 = ∑∥F0| − |Fc∥/∑F0|. bwR2 = [∑w(F02 − Fc2)2/∑w(F02)]1/2.

The dihedral angle of the two coordinated pyrrole rings is 116.2°, which is slightly smaller than that of PtII−TriP complexes, and the bond distance C2−C2′ (3.139(6) Å) is almost the same.24 The noncoordinated pyrrole ring tilts downward from the mean plane defined by the four mesocarbon atoms, while the two coordinated pyrrole rings are tilted upward. The dihedral angles between the individual pyrrole rings and the mean plane of the four meso-carbon atoms are C

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Figure 2. 1H NMR spectra of (a) 1a (CDCl3, 298 K) and (b) 2a (CD2Cl2 298 K). An asterisk is used to denote the presence of a solvent peak.

Figure 3. Electronic absorption and MCD spectra of 2a and 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, yellow, and black diamonds are used for ππ*, metal-to-ligand charge transfer, and d → d transitions, respectively. The details of the calculation are provided as Supporting Information.

ligands (Table 4).22 Magnetic circular dichroism (MCD) spectroscopy aids the assignment of the main electronic L and B bands, since the three Faraday ( 1, ) 0, and * 0 terms provide extra information that cannot be easily derived from the electronic absorption spectrum.34,35Because palladium(II) [14]tribenzotriphyrins(2.1.1) complexes lack a 3-fold or higher axis of symmetry, the MCD spectra are dominated by coupled pairs of oppositely signed Gaussian-shaped Faraday ) 0 terms. In the spectra of 2a and 2b, the L and B bands lie in the 600− 650 and 450−500 nm regions (Figure 3 and Table 4), 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.22 The coordination of a metal ion by the lone pairs of the two nonprotonated pyrrole nitrogens of the TriP ligand upon the formation of a palladium(II) dichloride complex has an electron withdrawing effect on the π-MOs, since the third nitrogen atom is not deprotonated (Scheme 1). The −a and −s MOs are stabilized to a greater extent than the a and s MOs (Figure 4). The methyl para-substituents of 2a result in a destabilization of the frontier π-MOs, but there is no significant effect on the optical properties, since inductive rather than mesomeric effects affect the a, s, −a, and −s MOs in a similar manner.33 Michl has demonstrated that in the context of planar heteroaromatic cyclic polyenes, a ∓/∓ sign sequence in D

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Table 4. Calculated Electronic Absorption Spectra for 2a and 2b Based on TD−DFT Calculations Using the CAM-B3LYP Functional banda

no.b

calcdc

exptld

wave functione =

L L

1 2 3

16.8 18.4

595 543

0.22 0.01

16.0 16.9

624 590

B

13

25.0

399

0.89

20.6

485

B

14

25.5

393

0.11

21.6

462

L L

1 2 3

16.8 18.4

596 544

0.22 0.00

15.9 16.9

627 591

B

13

25.3

396

0.75

20.7

484

B

14

25.5

393

0.07

21.6

462

2a ground state 91% a → −s; ... 58% s → −s; 31% a → −a; ... 33% a → −a; 21% HOMO−5PdCl2 → −s; 20% HOMO−4PdCl2 → −s; 12% s → −s; ... 53% s → −a; 15% HOMO−6Pd → −a; ...

2b ground state 91% a → −s; ... 54% s → −s; 33% a → −a; ... 31% HOMO−5PdCl2 → −s; 28% a → −a; 11% HOMO−4PdCl2 → −s;... 44% s → −a; 20% HOMO−6Pd → −a; ...

a

Band assignment described in the text. bThe number of the state assigned in terms of ascending energy within the TD-DFT calculation. c Calculated band energies (103·cm−1), wavelengths (nm), and oscillator strengths in parentheses (f). dObserved energies (103· cm−1) and wavelengths (nm) based on assignments of the Faraday ) 0 terms in the MCD spectrum that are described in the text. eWave functions based on the eigenvectors predicted by TD-DFT. Oneelectron transitions associated with the four frontier π-MOs of Michl’s perimeter model31−33 are highlighted in bold. PdCl2 and Pd as superscripts denote MOs associated primarily with the chlorine atoms of the PdCl2 moiety, and a 4d orbital of the Pd(I) ion.

Figure 4. Energies and angular nodal patterns at an isosurface value of 0.04 au of the a, s, −a, and −s MOs of C13H13− (top) and 2b (center). Energies of the frontier MOs of 1 and 2 (bottom). Thicker dark gray lines are used to highlight the a, s, −a, and −s MOs. Triangles and circles are usedfor a/−a and s/−s MOs, respectively. MOs associated with the PdCl2 moiety are offset to the right. The HOMO−LUMO gaps are denoted with large red triangles and are plotted against a secondary axis.

ascending energy in the Faraday ) 0 terms observed for the L and B bands in the MCD spectrum, when the separation of the a and s MOs derived from the HOMO of the C13H13− parent perimeter (the ΔHOMO value in Michl’s terminology) is greater than that of the −a and −s MOs that are derived from the LUMO (the ΔLUMO value).32,33 A ±/± sign sequence is normally observed when ΔLUMO > ΔHOMO (Table 3), but this is not the case in the MCD spectra of 2a and 2b (Figure 3). Anomalous sign sequence has been reported previously for deeply saddled tetraphenyltetraacenaphthoporphyrins,36 so this may be related to the nonplanarity of the ligand. Electrochemical Properties. The electrochemical properties of the PdII−TriPs 2a−b were evaluated at room temperature in CH2Cl2 containing 0.1 M TBAP as supporting electrolyte. Examples of cyclic voltammograms of 2a and 2b are shown in Table 5 and Figure S7 (CH2Cl2). Because of the change of the structural conformation (planar to saddle), 2a and 2b have complex cyclic voltammograms. In the voltammogram of 2a, there are two irreversible reduction waves [−0.62

and −1.05 V (vs SCE)] together with a reversible wave [−1.31 V (vs SCE)], and two reversible oxidation waves [+1.16 and +1.53 V (vs SCE)] (Table 4). It is particularly noteworthy that the separations between the first reduction and oxidation steps (which is normally anticipated to closely match the trend in the HOMO−LUMO gap) of 1.76 ± 0.02 V are significantly smaller than the values of 2.02 ± 0.02 V that have been reported for free-base TriPs,22 as would be anticipated based on the red shift of the spectral bands. There is a significantly larger shift of the first reduction potential to more positive values than for the first oxidation potential. This is consistent with the predicted relative stabilization of the LUMO in the TD-DFT calculation (Figure 4).

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CONCLUSION In conclusion, two palladium(II) dichloride complexes of [14]tribenzotriphyrin(2.1.1) (2a and 2b) have been success-

Table 3. MO Energies of the Frontier π-MOs of 1 and 2 in TD-DFT Calculations at the CAM-B3LYP/SDD Level of Theory

1a 1b 2a 2b

s

a

ΔHOMO

−s

−a

ΔLUMO

ΔHOMO−ΔLUMO

−6.71 −6.81 −7.05 −7.19

−6.04 −6.13 −6.53 −6.63

0.67 0.68 0.52 0.56

−1.48 −1.58 −2.50 −2.61

−1.07 −1.17 −1.79 −1.92

0.41 0.41 0.71 0.69

+0.26 +0.27 −0.19 −0.13

E

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Table 5. Half-Wave Potentials (V vs SCE) of TriP (1a and 2a) and PdII−TriP (2a and 2b) in CH2Cl2 Containing 0.1 M TBAP oxidation f

1a 1bf 2a 2b

reduction

4σa

2nd

1st

ΔEOc

1st

2nd

ΔERd

H−L(V)e

0.68 1.80 0.68 1.80

+1.03 +1.02 +1.53 +1.57

+0.53 +0.50 +1.16 +1.19

0.50 0.52 0.37 0.38

−1.63 −1.67 −0.62b −0.56b

−1.74 −1.75 −1.05 −1.00

0.11 0.08 0.43 0.44

2.16 2.17 1.78 1.75

a Hammett substituent constant.37,38 bIrreversible peak potential at a scan rate of 0.10 V/s. cPotential difference between the second and first oxidations. dPotential difference between the first and second reductions. eThe potential difference between the first oxidation and first reduction (HOMO−LUMO gap). fData reported previously for the free base ligands.22

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fully prepared, and their optical and electrochemical properties have been investigated. PdII−TriP complexes are found to be much more stable to the open air than the analogous PtII−TriP complexes. X-ray crystallography reveals that 2a adopts a deeply saddled structure. During electrochemical measurements both complexes undergo two reversible oxidation waves together with an irreversible and two reversible reduction waves. The electrochemically measured HOMO−LUMO gaps are significantly smaller than those of the free-base TriPs. This results in a significant red-shift of the main spectral bands, due primarily to a significant relative stabilization of the LUMO.

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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.5b02093. NMR spectra, cyclic voltammograms, crystallographic data (PDF) X-ray crystallographic data in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by grants from the Natural Science Foundation of China (21301074 and 21071067), the Natural Science Foundation of Jiangsu Province (BK20130483), China Postdoctoral Science Foundation (2013M540415 and 2014T70474), and the Robert A. Welch Foundation (K.M.K., Grant E-680). We thank Prof. Zhen Shen of Nanjing University for the MCD spectral measurements. The theoretical calculations were carried out at the Centre for High Performance Computing in Cape Town.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.5b02093 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.5b02093 Inorg. Chem. XXXX, XXX, XXX−XXX