Article Cite This: Inorg. Chem. 2017, 56, 13029-13034
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Photochemistry and Anion-Controlled Structure of Fe(III) Complexes with an α‑Hydroxy Acid-Containing Tripodal Amine Chelate Jennifer E. Vernia,† Mary R. Warmin,† Jeanette A. Krause,† David L. Tierney,‡ and Michael J. Baldwin*,† †
Department of Chemistry, University of Cincinnati, Cincinnati, Ohio 45221-0172, United States Department of Chemistry and Biochemistry, Miami University, Oxford, Ohio 45056, United States
‡
S Supporting Information *
ABSTRACT: The tripodal amine chelate with two pyridyl groups and an α-hydroxy acid (AHA) group, Pyr-TPA-AHA, was synthesized. Different Fe(III) complexes form with this chelate depending upon the counterion of the Fe(III) source used in the synthesis. A dinuclear complex, Fe(III)2(Pyr-TPA-AHA)2(μ-O), 1, and mononuclear complexes Fe(III)(Pyr-TPA-AHA)X (X = Cl− or Br−, 2 and 3, respectively) were synthesized. 2 can be easily converted to 1 by addition of silver nitrate or a large excess of water. The structure of 1 was solved by X-ray crystallography (C32H34N6O7Fe2·13H2O, a = 14.1236(6) Å, b = 14.1236(6) Å, c = 21.7469(15) Å, α = β = γ = 90°, tetragonal, P42212, Z = 4). 2 and 3 each have simple quasireversible cyclic voltammograms with E1/2 (vs aqueous Ag/AgCl) = +135 mV for 2 and +470 for 3 in acetonitrile. The cyclic voltammogram for 1 in acetonitrile has a quasireversible feature at E1/2 = −285 mV and an irreversible cathodic feature at −1140 mV. All three complexes are photochemically active upon irradiation with UV light, resulting in cleavage of the AHA group and reduction of the iron to Fe(II). Photolysis of 1 results in reduction of both Fe(III) ions in the dinuclear complex for each AHA group that is cleaved, while photolysis of 2 and 3 results in reduction of a single Fe(III) for each AHA cleavage. The quantum yields for 2 and 3 are significantly higher than that of 1.
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3,5-diCl, all H, 3-OCH3, or 3,5-ditBu on the phenyl ring).14 These chelates form trinuclear Fe3(X-Sal-AHA)3(μ-OR)− (R = H or CH3) complex monoanions with Fe(III). These clusters, characterized by a Fe3O4 incomplete cubane-like core, have very high stability constants and are photochemically active,14,15 undergoing decarboxylation with iron reduction analogous to photolysis of the marine siderophores. The quantum yield of the Fe3(X-Sal-AHA)3(μ-OR)− clusters is modulated by the electron donating or withdrawing ability of the ring substituents.16 To expand the structure types and variety of ancillary metal binding groups available in the mixed-donor, AHA-containing chelates, new ligands were synthesized that incorporate the AHA group within a tripodal amine motif. In the example described here, a pair of pyridyl groups were employed as the ancillary functional groups to give a pentadentate aminobis(pyridyl)-α-hydroxy acid chelate referred to hereafter as PyrTPA-AHA. The structures of Fe(III) complexes of this chelate differ in nuclearity depending upon the presence of different
INTRODUCTION Many marine bacteria produce siderophores that sequester iron and use a photochemical mechanism to reduce the tightly bound Fe(III) and release it as Fe(II).1−4 These photoactive siderophores generally include an α-hydroxy acid (AHA) group among the Fe-binding moieties.5 An AHA → Fe(III) charge transfer transition is responsible for the photochemical decarboxylation of the ligand and reduction of the iron. Inspired by these photochemically active marine siderophores, new mixed-donor chelates were synthesized. These chelates are intermediate in complexity between the structurally complex siderophores and the all-carboxylate AHAs such as citrate and tartrate. Potential applications for this type of molecule include those analogous to metal delivery applications of the light sensitive chelates referred to as photocages.6−13 Like the AHAbearing chelates, the photocages bind metal ions and release them upon irradiation. In contrast to the photocages which are light sensitive on their own, though, the AHA chelates are only photochemically active when coordinated to an appropriate metal ion.5,14 We previously reported new potentially tetradentate salicylidene-AHA chelates (X-Sal-AHA, where X = 5-NO2, © 2017 American Chemical Society
Received: July 19, 2017 Published: October 9, 2017 13029
DOI: 10.1021/acs.inorgchem.7b01799 Inorg. Chem. 2017, 56, 13029−13034
Article
Inorganic Chemistry
product was dried under vacuum for 1 h, giving a brown solid (0.99 g, 73% yield). ESI-MS (+), m/z: 435 (MH+) Instrumental Methods. UV/visible absorption spectra were obtained using a spectrophotometer with either a fiber optic dip probe with a 1 cm path length or a cuvette accessory with a 1 cm quartz cuvette (Spectral Instruments, Inc. model 420). Circular dichroism spectra were obtained on a Jasco J-715 spectropolarimeter. Electrospray ionization-mass spectrometry (ESI-MS) data were collected at the University of Cincinnati Mass Spectrometry Facility on a Micromass Q-TOF-2 spectrometer. NMR spectra were collected at 400 MHz (Bruker Avance). Samples for anaerobic experiments were prepared under an Ar atmosphere in a glovebox (M. Braun LabStar, [O2] < 0.1 ppm). Cyclic voltammograms were obtained using a BAS 50W electrochemical analyzer with a three-electrode configuration consisting of a glassy carbon working electrode, a platinum wire auxiliary electrode, and an aqueous Ag+/AgCl reference electrode that was soaked in an aqueous KCl solution between runs to keep the membrane hydrated. Electrochemical experiments were run with 0.20 M TBAPF6 in acetonitrile (cleaned with anhydrous CuSO4). Frozen X-band EPR spectra were recorded on a Bruker EMX EPR spectrometer equipped with an ER-4116DM dual mode resonator with temperature maintained at 4.5 K by an Oxford ESR900 flow cryostat. Photochemical Experiments. Irradiation of samples was accomplished using a Luzchem LZC 4 photoreactor with sets of 16 lamps provided by Luzchem Research, Inc. (Ottawa, Ontario, Canada) centered at 350 nm (UVA). Quantum yields were determined in two ways. For one determination, Fe(II) production was monitored spectrophotometrically under an Ar atmosphere using a bathophenanthroline disulfonate assay. For the second, chelate cleavage was monitored by loss of circular dichroism intensity under aerobic conditions. K3[Fe(oxalate)3] was used as the actinometer, as described previously.16 All samples for quantum yield determination were in methanolic solutions at concentrations such that the absorbance in the region of the photoactive chromophore (around 300−350 nm) was >1, so that >90% of the photons were absorbed. X-ray Crystallography. X-ray data were collected on a suitable orange-yellow dichroic rod-shaped crystal, 0.185 × 0.120 × 0.105 mm, on a Bruker APEX-II CCD diffractometer (equipped with a TRIUMPH curved-graphite monochromator). The structure was solved by a combination of direct methods and the difference Fourier technique and refined by full-matrix least-squares on F2. Nonhydrogen atoms were refined with anisotropic displacement parameters. Water H atoms were either located directly from the difference map or calculated based on H-bonding criteria; remaining H atoms were calculated. All H atoms were treated with a riding model.
anions during synthesis of the complex. While all of the Fe(III) complexes of Pyr-TPA-AHA are photochemically active, the quantum yield is significantly affected by the different anions used. In this paper, we describe the synthesis of the tripodal amineAHA chelate, Pyr-TPA-AHA, and its different iron complexes. The manner in which the anion controls the Fe complex structure type is examined, and the crystallographically derived structure of the dinuclear Fe(III) complex is reported. The photochemical reactions of these complexes are characterized, and the quantum yields for three different Fe(III)(Pyr-TPAAHA) complexes are determined from the points of view of both metal reduction and chelate cleavage.
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EXPERIMENTAL SECTION
Materials. All solvents and chemicals were obtained from Fisher or Aldrich and were used without further purification except where otherwise noted. 4-(N,N-Bis(pyridin-2-ylmethyl)amino)-2-hydroxybutyric acid (PyrTPA-AHA). S-(−)-4-Amino-2-hydroxybutyric acid (1.19 g) and sodium bicarbonate (4.21 g) were dissolved in 20 mL of DI water. This solution was cooled in an ice bath to 0 °C. 2-(Bromomethyl)pyridine hydrobromide (5.06 g) was dissolved in 80 mL of methanol. The methanol solution was added slowly to the aqueous solution, which was stirred at 0 °C for 15 min before being allowed to warm to room temperature. The solution was heated to reflux and stirred for 24 h. After cooling to room temperature, the solution was filtered, and the solvent was removed by rotary evaporation. The product was extracted with ethanol twice and filtered, and the ethanol was removed by rotary evaporation. Then, the product was extracted once with acetonitrile to exclude remaining salts, and the acetonitrile was removed by rotary evaporation. The product was further dried under vacuum for 1 h, producing a red-orange solid (2.83 g, 85% yield as the sodium salt). 1H NMR, ppm (in DMSO-d6): 8.5d, 7.7t, 7.5d, 7.2t, 3.7q, 3.5q, 2.6m, 1.9m, 1.5m; 13C NMR, ppm (in DMSO-d6): 171, 159, 148, 136, 122, 121, 69, 59, 51, 32; ESI-MS (+), m/z: 302 (MH+). Fe(III)2(Pyr-TPA-AHA)2O, 1. Pyr-TPA-AHA (0.52 g, 1.6 mmol) (Na+ salt) was dissolved in 50 mL of methanol, followed by addition of 0.58 g (6.9 mmol) of sodium bicarbonate. The mixture was stirred at room temperature for 15 min; then, 0.70 g (1.6 mmol) of Fe(NO3)3·9H2O was added, and the mixture was stirred at room temperature overnight, protected from light. The solution was filtered, and the solvent was removed by rotary evaporation. The product was extracted with ethanol twice and filtered, and solvent was removed by rotary evaporation. The product was further dried under vacuum for 1 h, producing a red-orange solid (0.38 g, 65% yield). Further purification for crystallization was accomplished by running an aqueous solution of the product through a Hypersep C18 cartridge. Crystals suitable for Xray crystallography were obtained by slow evaporation of a methanol solution. ESI-MS (+), m/z: 727 (MH+). Fe(III)(Pyr-TPA-AHA)Cl, 2. Pyr-TP-AHA (0.50 g, 1.5 mmol) (Na+ salt) was dissolved in 50 mL of acetonitrile. KCl (0.64 g, 8.6 mmol) was added, and the mixture was stirred for 15 min, followed by addition of 0.67 g (1.5 mmol) Fe(NO3)3·9H2O. The resulting mixture was protected from light and stirred at room temperature overnight. The mixture was then filtered, and the solvent was removed by rotary evaporation. The product was extracted with acetonitrile, and the solvent was removed by rotary evaporation. The product was dried under vacuum for 1 h, giving a red-brown solid (0.40 g, 62% yield). ESI-MS (+), m/z: 391 (MH+). Fe(III)(Pyr-TPA-AHA)Br. Pyr-TPA-AHA (0.94 g) was added to 50 mL of acetonitrile, followed by 1.89 g of potassium bromide. The solution was stirred for 15 min, and then 1.27 g of Fe(NO3)3·9H2O was added. The resulting mixture was protected from light and stirred at room temperature overnight. The mixture was then filtered, and the solvent was removed by rotary evaporation. The product was extracted with acetonitrile, and the solvent removed by rotary evaporation. The
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RESULTS The new ligand Pyr-TPA-AHA (4-(N,N-bis(pyridin-2ylmethyl)amino)-2-hydroxybutyric acid in its neutral, doubly protonated state), shown in Figure 1, was prepared as the
Figure 1. Line drawing of H2Pyr-TPA-AHA.
sodium salt of its carboxylate by addition of two methylpyridine groups to the amine of S-(−)-4-amino-2-hydroxy-butyric acid. This produces a potentially pentadentate N3O2 chelate with a tripodal amine motif and a chiral center at the carbon adjacent to the carboxylic acid (the α-carbon). The available metal coordinating groups include the tertiary tripodal amine nitrogen, two pyridyl nitrogens, and the bidentate AHA moiety comprised of an alcohol oxygen and a carboxylate oxygen. 13030
DOI: 10.1021/acs.inorgchem.7b01799 Inorg. Chem. 2017, 56, 13029−13034
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Inorganic Chemistry Different Fe(III) complexes of Pyr-TPA-AHA were observed to form depending on the presence of different anions. This was initially observed based on differences due to the specific Fe(III) salt used in the synthesis. When FeCl3 is used, a monomeric Fe(Pyr-TPA-AHA)Cl complex is formed. In contrast, if Fe(NO3)3 is used in the absence of excess halide and without efforts to dry the solvent, the dinuclear, oxobridged complex Fe2(Pyr-TPA-AHA)2(μ-O) results. Alternatively, Fe(NO3)3 may be used in the presence of excess halide salt to give the monomeric complexes. In each case, the resulting complex is neutral with monocationic Fe(III)(PyrTPA-AHA) units balanced by different anionic ligands. The different complexes can be distinguished in solution by their mass spectra (see Supporting Information) as well as their UV/ visible absorption spectra (Figure 2).
Figure 3. UV/visible spectra of 2 (solid blue), 2 + AgNO3 (dotted green), and 2 + water (dashed red)
Figure 2. UV/visible absorption spectra of 1 (solid black), 2 (dotted blue), and 3 (dashed red) in acetonitrile, [Fe] = 0.06 mM. Figure 4. ORTEP diagram of 1 with 50% probability ellipsoids. H atoms omitted for clarity.
The dinuclear, oxo-bridged LFeOFeL complex (1) and mononuclear LFeX complexes (2, X = Cl and 3, X = Br) can be interconverted by changing the conditions or by addition of reagents. 2 can be cleanly converted to 1 by addition of silver nitrate, resulting in loss of the chloride ligand driven by precipitation of AgCl and formation of 1 (Figure 3). Addition of water to 2 also drives its conversion to 1. Once 1 is formed, however, its conversion to 2 is not easily accomplished. The structure of 1 was determined by X-ray crystallography (Figure 4). Crystallographic data are given in Table 1. The structure of 1 is generally similar to that of other oxo-bridged Fe(III) dimers with tripodal amine chelates, including complexes of tris(2-pyridylmethyl)amine 17 and bis(2pyridylmethyl)amine.17,18 The two Fe(III) ions are connected to each other by a single oxo bridge that is nearly linear (179°). Each of the two Fe(III) ions is coordinated by a single PyrTPA-AHA chelate in a pentadentate manner. As expected based on other structures, the tertiary amine group is coordinated trans to the oxo bridge, and most of the coordinating groups form the common five- and six-membered chelate rings. The tertiary amine forms a less common sevenmembered chelate ring with the carboxylate oxygen of the bidentate AHA portion of the chelate. The two FeL units are related to each other by a twofold proper symmetry axis
Table 1. Crystalographic Data for 1 CCDC deposition number formula formula weight temperature wavelength crystal system space group unit cell dimensions
volume Z final R indices [I > 2σ(I)] R indices (all data)
1561381 C32H34N6O7Fe2·13H2O 960.56 150(2) K 0.71073 Å tetragonal P42212 a = 14.1236(6) Å, α = 90° b = 14.1236(6) Å, β = 90° c = 21.7469(15) Å, γ = 90° 4338.0(5) Å3 4 R1 = 0.0230, wR2 = 0.0609 R1 = 0.0248, wR2 = 0.0620
perpendicular to the oxo bridge but no improper symmetry axis, making the dinuclear complex chiral like the isolated chelate. Selected bond lengths and angles are given in Table 2. 13031
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Inorganic Chemistry Table 2. Selected Bond Lengths (Å) and Angles (°) for 1 Fe−O(4) Fe−N(3) O(1)−C(16) O(4)−Fe−O(3) O(4)−Fe−N(3) O(1)−Fe−N(2) O(1)−Fe−N(1) O(4)−Fe−N(1)
1.7961(2) 2.1479(16) 1.283(2) 106.90(6) 95.16(7) 84.35(6) 89.80(6) 166.47(4)
Fe−O(3) Fe−N(2) O(2)−C(16) O(4)−Fe−O(1) O(3)−Fe−N(3) N(3)−Fe−N(2) N(3)−Fe−N(1) Fe−O(4)−FeA
1.9314(14) 2.1502(16) 1.242(2) 100.42(6) 93.82(6) 94.59(5) 74.91(6) 178.93(12)
Figure 5. Cyclic voltammograms of 1 (black), 2 (blue), and 3 (red) in acetonitrile.
quasireversible features for each. The reduction potential for the chloride complex 2 is lower than that of the bromide complex 3 by just over 300 mV. The voltammogram of the dinuclear complex 1 includes two cathodic peaks and one clear anodic return peak. The electrochemical parameters for the three complexes are given in Table 3.
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DISCUSSION The new potentially pentadentate ligands X-TPA-AHA introduce the popular tripodal amine structural motif for mixed-donor, AHA-containing chelates. Unlike the previously reported mixed-donor, AHA-containing Sal-AHA series of chelates,14 the X-TPA-AHA chelates provide the opportunity to include many more ancillary metal-coordinating functional groups. The Pyr-TPA-AHA version of these tripodal amine chelates incorporates two pyridyl groups along with the AHA group. This results in an otherwise all-nitrogen coordination environment combined with the structurally and photochemically interesting AHA group. The different Fe(III) complexes of Pyr-TPA-AHA all include an LFe(III) unit (where L is Pyr-TPA-AHA) with an additional anion bound to the Fe. The complexes 1−3 demonstrate the use of differences in anion availability and other conditions to manipulate the nuclearity of the resulting Fe(III) complexes. This in turn guides important properties, including electron stoichiometry in the photochemical reactions. Once the monomeric complexes are formed, changing select conditions
Table 3. Electrochemical Parameters from Cyclic Voltammetry of 1−3 in Acetonitrile
2 3 1
anodic peak (mV vs Ag/AgCl)
E1/2 (mV vs Ag/AgCl)
ΔE (mV)
+35 +350 −430 −1170
+245 +590 −145
+140 +470 −285
210 240 285
2.0898(13) 2.3319(14) 1.406(2) 81.18(6) 96.19(7) 83.28(6) 75.85(6)
during or after irradiation or whether they are kept under anaerobic conditions, as shown in Scheme 1. Under anaerobic conditions, the main organic product includes the intact bis(pyridyl)amino portion of the original chelate, with the carbon chain that had included the AHA moiety truncated by one carbon and converted into a methyl ester and/or methyl hemiacetal. Under air, the carbon chain that had included the AHA is truncated by two or more carbons. EPR spectra of 2 and 3 (Figures S16 and S17 in the Supporting Information) are consistent with high-spin monomeric Fe(III) complexes. The EPR intensity is lost upon irradiation, consistent with conversion to integer spin Fe(II). The photochemical reactions of 1, 2, and 3 can be monitored from the point of view of either metal reduction or chelate cleavage. Production of Fe(II) due to the photochemical reduction of Fe(III) can be monitored spectrophotometrically using the phenanthroline derivative BPDS, which has a strong absorption at 535 nm when coordinated to Fe(II) but little visible absorption in the presence of Fe(III). Loss of the chiral AHA group on the chelate upon photolysis can be monitored by loss of the circular dichroism signal that is present in the intact chelate and its iron complexes. Quantum yields determined by both of these methods for the photochemical reactions of 1, 2, and 3 in methanol with irradiation by UVA light are given in Table 4. The quantum yields for the LFeX complexes 2 and 3 are essentially the same whether measured based on Fe(II) production or loss of CD signal. In contrast, the LFeOFeL complex 1 has a quantum yield significantly lower than those of 2 and 3, and its quantum yield determined for Fe(II) production is about twice that of its quantum yield determined for loss of the chiral center on the chelate.
Cyclic voltammograms of the three complexes are shown in Figure 5. The mononuclear complexes 2 and 3 show single,
cathodic peak (mV vs Ag/AgCl)
Fe−O(1) Fe−N(1) O(3)−C(15) O(3)−Fe−O(1) O(4)−Fe−N(2) O(3)−Fe−N(1) N(2)−Fe−N(1)
Irradiation of all three of the complexes with UVA light results in spectral changes in the UV/visible absorption spectrum and loss of the circular dichroism spectrum. In the presence of a phenanthroline derivative, BPDS, a strong visible absorption at 535 nm grows in upon irradiation, corresponding to formation of a Fe(II)BPDS complex. Analysis of the photolysis products in methanol by ESI-MS shows essentially the same products for 1, 2, and 3, with the exception of the presence of Cl or Br ligands on the Fe complexes. The products differ depending upon whether the samples are exposed to air 13032
DOI: 10.1021/acs.inorgchem.7b01799 Inorg. Chem. 2017, 56, 13029−13034
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Inorganic Chemistry Scheme 1. Observed Photolysis Products for 1−3
Table 4. Quantum Yields (% ± 2σ) for 1, 2, and 3 for Iron Reduction and Chelate Cleavage complex
Fe reduction
chelate cleavage
1 2 3
19 ± 3 40 ± 9 40 ± 3
12 ± 3 40 ± 6 36 ± 10
products include the intact N(-Pyr)2 portion of the chelate with loss of two or more carbons from the four-carbon arm that had included the AHA group. These products are consistent with those observed previously for the Fe(III) complex of 3,5-diClSal-AHA.16 The loss of multiple carbons under aerobic conditions from the Sal-AHA chelate was shown to be due to initial decarboxylation to give the aldehyde with one fewer carbon, followed by Fe-promoted air oxidation of the aldehyde to a carboxylate and further photochemical decarboxylation. Under anaerobic conditions, the aldehyde with one fewer carbon was observed as the main product of photolysis of the Sal-AHA chelate with Fe(III). In the case of Pyr-TPA-AHA, a product with one fewer carbon consistent with initial decarboxylation is also observed. However, the observed product is the methyl ester or, sometimes, the methyl hemiacetal, rather than the aldehyde. Photolysis of the uranyl complex of 3,5-ditBu-Sal-AHA results in a mixture of the aldehyde with the methyl hemiacetal and the methyl ester and also with the carbon chain truncated by one carbon.20 This suggests that the methyl ester results from attack by methanol on an initial aldehyde product to form the hemiacetal, followed by oxidation (by two hydrogen atoms) of the hemiacetal to the methyl ester, as shown in Scheme 2. The oxidant may be another equivalent of the aldehyde, although the alcohol which would result as the reduction product of the aldehyde was not observed in the mass spectra. An alternative pathway to the methyl ester from the aldehyde could be oxidation of the aldehyde to the acid, followed by esterification of the acid. However, it is unlikely that an oxidant competent to oxygenate the aldehyde is present under anaerobic conditions. The quantum yield for the oxo-bridged 1 is much lower than that of the halides 2 and 3. The oxo bridge stabilizes the Fe(III)
can lead to conversion to the dimer. This is consistent with the generally high propensity of Fe(III) to form oxo-bridged dinuclear complexes. A search of the Cambridge Crystallographic Database gives nearly 500 entries for crystal structures of dinuclear Fe(III) complexes with a single oxo bridge. However, not one of these structures includes an AHA moiety in the iron coordination sphere. The X-ray crystal structure of 1 shows the manner of Fe(III) chelation by Pyr-TPA-AHA. In the absence of crystal structures of 2 or 3, it is considered likely that the chelate coordination is the same for those complexes as well. In 1, all five potentially coordinating atoms in the chelate coordinate to a single Fe(III) in a manner that is typical of oxo-bridged Fe(III) dimers coordinated to tripodal amine ligands,17,18 with the exception of the seven-member chelate ring between the tertiary amine and a carboxylate oxygen. This arrangement contrasts with complexes between the Sal-AHA chelates and Fe(III),14 Ga(III),15 Mn(III),19 and U(VI).20 In those structures, the tetradentate chelate spans two metals with the alkoxy oxygen of the AHA moiety bridging a pair of metal ions in a manner reminiscent of the dimeric Fe(III)-citrate complex,21 avoiding formation of the seven-member chelate ring. The photolysis products of the Pyr-TPA-AHA chelate are essentially the same for all three complexes 1−3. Under air, the
Scheme 2. Proposed Reaction Pathway To Obtain the Observed Methyl Ester Photoproduct
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DOI: 10.1021/acs.inorgchem.7b01799 Inorg. Chem. 2017, 56, 13029−13034
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Inorganic Chemistry ORCID
oxidation state relative to the Fe(II) photoproduct more effectively than the halides. This is borne out by the lower electrochemical reduction potential of 1 compared to 2 and 3. A more interesting difference, though, is the difference in the ratio of the quantum yields for iron reduction vs chelate cleavage. For 1, the ratio is nearly 2:1, just as it is in the Fe(III)3(Sal-AHA)3 complexes and Fe(III)2(cit)2.22 This is consistent with the reduction of two Fe(III)s with each twoelectron oxidative decarboxylation of the AHA. In each of these complexes, at least two Fe(III)s are available to accept the electrons. In complexes 2 and 3, only one Fe(III) is available to accept an electron, yet the two-electron decarboxylation still occurs. This can be explained by formation of a charge transfer excited state characterized as an Fe(II)-carboxyl radical23 that would dump a second electron onto the most convenient electron acceptor available.16 If a second Fe(III) is present in the complex, it accepts the second electron. The electron is transferred elsewhere if only one Fe(III) is present. The latter case results in the 1:1 Fe reduction to chelate decarboxylation ratio that is observed for 2 and 3 and is likely consistent with what happens upon photolysis of the marine siderophores which also have only one iron available. In summary, the new, pentadentate, mixed-donor, AHAcontaining tripodal amine chelate Pyr-TPA-AHA represents the X-TPA-AHA motif, which allows variation of the ancillary functional groups. This will provide chelates with broad variation in charge, coordinating atoms, chelate ring size, and other factors in the metal coordination environment to be combined with the useful structural and photochemical characteristics of the AHA moiety. The coordination properties are further tuned by the presence of different anions during formation of Pyr-TPA-AHA complexes to manipulate their electrochemical reduction potentials, quantum yields, and nuclearities. This control of the nuclearity in turn facilitates different metal−ligand stoichiometries in the photochemical redox chemistry. Although the reactions of the monomers and dimers result in the same organic photochemical products, the iron reduction stoichiometry is altered. This difference in metal reduction to ligand oxidation ratios provides new insight into the photochemical excited state and photolysis mechanism as well as new opportunities for directing transition-metal availability and oxidation state using light.
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Mary R. Warmin: 0000-0002-2953-1935 Michael J. Baldwin: 0000-0001-9912-9717 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS Funding for this work was provided in part by NSF Grant CHE-0955603. Funding for the X-ray diffractometer was provided by NSF-MRI Grant CHE-0215950.
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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.7b01799. Spectral characterizations of all new compounds and photolysis products (PDF) Accession Codes
CCDC 1561381 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.7b01799 Inorg. Chem. 2017, 56, 13029−13034
