Article pubs.acs.org/IC
Gallium(III) Tetraphenylporphyrinates Containing Hydrosulfide and Thiolate Ligands: Structural Models for Sulfur-Bound Iron(III) Hemes Daniel J. Meininger, Max Chee-Garza, Hadi D. Arman, and Zachary J. Tonzetich* Department of Chemistry, University of Texas at San Antonio (UTSA), San Antonio Texas 78249, United States S Supporting Information *
ABSTRACT: Gallium(III) tetraphenylporphyrinates (TPP) containing anionic sulfur ligands have been prepared and characterized in the solid state and solution. The complexes serve as structural models for iron(III) heme sites containing sulfur coordination that otherwise prove challenging to synthesize due to the propensity for reduction to iron(II). The compounds prepared include the first well-characterized example of a trivalent metalloporphyrinate containing a terminal hydrosulfide ligand, [Ga(SH)(TPP)], as well as [Ga(SEt)(TPP)], [Ga(SPh)(TPP)], and [Ga(SSiiPr3)(TPP)]. The stability of these compounds toward reduction has permitted an investigation of their solid-state structures and electrochemistry. The structural features and reaction chemistry of the complexes in relation to their iron(III) analogs is discussed.
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spin Fe(III) due to its comparable ionic radius (rGa(III) ≈ 0.620 Å vs rFe(III) ≈ 0.645 Å),27 charge, and coordination chemistry.28 The lack of an accessible GaII/III redox couple ensures that the resulting complexes are not subject to metal-based reduction. Furthermore, the diamagnetism of gallium(III) renders solution characterization by NMR spectroscopy more facile. In consideration of these favorable attributes, several previous studies have employed gallium(III) as a structural substitute for iron(III) in both heme and non-heme biomimetic systems.29−36 The first structurally characterized examples of gallium(III) porphyrinates bearing thiolate ligands, [Ga(SAr)(OEP)] (OEP = octaethylporphyrinate; Ar = Ph or S-2-CF3CONHC6H4), were reported by Nakamura and co-workers in 1998.37 Comparison of the metric data for these complexes with those of the analogous iron(III) species demonstrated close structural homology. Subsequent work described several variations on compounds of the type [Ga(SR)(OEP)], including species with coordinated cysteine-containing peptides.31,38,39 Given the precedent for stable gallium(III) porphyrinates containing aryl thiolate ligands, we reasoned that isolation and structural characterization of the corresponding complex containing a hydrosulfide ligand would be straightforward. Such a complex would then serve as a structural model for hydrosulfide coordination to iron(III) hemes. We report here our findings concerning such a complex, as well as related Ga(III) tetraphenylporphyrinates (TPP) containing other sulfur-based ligands.
INTRODUCTION Since the discovery of a role for hydrogen sulfide in physiological signal transduction,1 a significant amount of attention has been directed toward understanding the interactions of H2S with various biological cofactors.2−5 Iron hemes in particular represent very intriguing targets given their propensity for coordination to related thiol/thiolate molecules.6,7 A number of synthetic iron porphyrinates bearing sulfur-derived ligands have been synthesized in order to model the interaction of heme sites with sulfur-based molecules.8,9 One significant challenge to modeling the chemistry of oxidized hemes with sulfurcontaining ligands, however, is the propensity of thiols/ thiolates to reduce iron(III). In this vein, the limited number of studies that have specifically examined the chemistry of synthetic Fe(III) porphyrinates with H2S/HS− have reported rapid reduction to Fe(II).10−16 Such reactivity stands in stark contrast to observations with native heme proteins, such as hemoglobin HbI from the mollusk Lucina pectinata, which requires the ferric state for binding and transport of H2S.17,18 In natural systems, factors such as hydrogen-bonding, coordination of proximal histidine residues, and distal pocket polarity may play a large role in stabilizing iron(III) centers toward reduction.19−21 These factors are challenging to account for in model systems and have therefore impeded the development of synthetic examples of ferric porphyrinates containing hydrosulfide ligands. Such synthetic examples are highly desirable because they permit structural studies, which are as yet unrealized with natural proteins. In contrast to the apparent instability of heme iron(III) hydrosulfide complexes, several examples of biologically relevant non-heme iron(III) complexes containing terminal hydrosulfide ligands have been reported.11,22−26 One strategy to circumvent the complications posed by the FeII/III redox couple is to examine the analogous complexes of gallium. The Ga(III) ion is a good structural substitute for high © XXXX American Chemical Society
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RESULTS AND DISCUSSION Initial attempts to synthesize [Ga(SH)(TPP)] involved introduction of H2S gas to solutions of [Ga(OH)(TPP)] in benzene under nitrogen. Surprisingly, no reaction was found to take Received: December 4, 2015
A
DOI: 10.1021/acs.inorgchem.5b02822 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry place as judged by 1H NMR spectroscopy indicating that the gallium(III) hydroxide functional group lacks sufficient basicity to effect deprotonation of H2S. Based on this result, we next examined the more basic methoxide complex, [Ga(OMe)(TPP)]. Upon exposure of the complex to an atmosphere of H2S gas, a new 1H NMR resonance was observed at −6.3 ppm corresponding to the H atom of coordinated hydrosulfide (eq 1). The remainder of the NMR spectrum is consistent with
Table 1. Metric Parameters for Ga(III) and Related Fe(III) Tetraphenylporphyrinates Bond Metric
[Ga(SH) (TPP)]
[Ga(SEt) (TPP)]
[Ga(SPh) (TPP)]
[Fe(SPh) (TPP)]a
M−S (Å) M−Navg (Å) M−porb (Å) M−S−R (deg)
2.2612(19) 2.063(4) 0.551 93(5)
2.275(4) 2.058(12) 0.565 99.3(6)
2.2754(19) 2.071(5) 0.527 102.7(3)
2.312(3) 2.084 0.533 103
a
Reference 28. bDisplacement from mean porphyrin plane.
distance is closer to the value of 2.2688(10) Å found for the Fe−S bond in [FeIII(SSiiPr3)(TPP)], which bears the electronically distinct silanethiolate ligand.14 Given the successful isolation of [Ga(SH)(TPP)], we next attempted to synthesize the μ-sulfido digallium(III) complex, [Ga2(μ-S)(TPP)2]. Such a sulfide-bridged “dimer” is unprecedented in iron porphyrin chemistry despite the marked stability of the analogous μ-oxo complex, [Fe2(μ-O)(TPP)2]. Three different routes were pursued to the putative [Ga2(μ-S)(TPP)2] species as displayed in Scheme 1. Unfortunately, all three strategies failed to produce the desired bridging sulfide complex in our hands, resulting in each case in no reaction or formation of unidentified byproducts. Therefore, the Ru complex, [Ru2(μ-S)(OEP)2], remains the lone example of dimetal porphyrinate with a single μ-sulfido bridge.44 Other Ga(III) tetraphenylporphyrinates containing thiolate ligands were also examined for comparison to [Ga(SH)(TPP)] and known FeIII(TPP) analogues. As depicted in eq 3, reaction
a five-coordinate gallium(III) porphyrinate (see Supporting Information). The ortho and meta hydrogen atoms of the meso phenyl rings appear as single sets of broadened multiplets, consistent with slow rotation of the rings on the NMR time scale.40 Alternatively, we were able to synthesize the desired hydrosulfide compound directly from [GaCl(TPP)] by treatment with a slight excess of Bu4NSH in THF under nitrogen (eq 2).41 The [Ga(SH)(TPP)] complex proved to be air stable
and was obtained in yields greater than 70%. Attempts to observe the S−H stretch by IR spectroscopy were unsuccessful in both the solid state and solution. We note, however, that the absence of an assignable S−H stretch was also found for an iron(II) porphyrinate containing a terminal hydrosulfide ligand.13 The solid state structure of [Ga(SH)(TPP)] is displayed in Figure 1. The porphyrin bond metrics about the Ga(III) center are comparable to those published previously for [GaCl(TPP)],42 however the gallium atom displays a larger displacement from the N4 plane resulting in slightly elongated Ga−N contacts (Table 1). The Ga(1)−S(1) bond distance of 2.2612(19) Å is slightly shorter than the Ga−S and Fe−S distances of 2.274(2) and 2.312(3) Å reported for [Ga(SPh)(OEP)] and [Fe(SPh)(TPP)], respectively.37,43 Notably, this
of [GaCl(TPP)] with the sodium salts of EtS−, PhS−, and Pr3SiS−, afforded the new gallium porphyrinates, [Ga(SEt)(TPP)], [Ga(SPh)(TPP)], and [Ga(SSiiPr3)(TPP)], respectively. The complexes were isolated in good to excellent yields and displayed 1H NMR features similar to those of [Ga(SH)(TPP)] i
Figure 1. Thermal ellipsoid drawing (30%) of the solid state structure of [Ga(SH)(TPP)]. Hydrogen atoms, with the exception of the S−H, omitted for clarity. Bond metrics can be found in Table 1. B
DOI: 10.1021/acs.inorgchem.5b02822 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Scheme 1. Attempted Routes to [Ga2(μ-S)(TPP)2]
Figure 2. Thermal ellipsoid drawings (50%) of the solid state structure of [Ga(SEt)(TPP)] (left) and [Ga(SPh)(TPP)] (right). Hydrogen atoms, cocrystallized toluene molecules, and the minor components of the disorder omitted for clarity. Bond metrics can be found in Table 1.
With the series of [Ga(SR)(TPP)] complexes in hand, we next examined the electrochemistry of two of the derivatives by cyclic voltammetry. Displayed in Figure 3 is the CV of [Ga(SH)(TPP)] and [Ga(SPh)(TPP)] in CH2Cl2. Of the two compounds, the electrochemical events of the benzenethiolate complex are more straightforward, displaying two reversible oxidation and reduction events. These events are typical of the tetraphenylporphyrinate scaffold and have been observed previously with gallium porphyrinates.49 In addition to the two reversible ligand-based processes, [Ga(SPh)(TPP)] displays an irreversible oxidation at +0.395 V (vs ferrocene/ferrocenium). We assign this event to the oxidation of the PhS− ligand, which results in formation of PhS• and [Ga(TPP)]+. Similar behavior was noted by Kadish for a series of alkyl gallium(III) porphyrinates, [Ga(R)(TPP)].50 In the case of these alkyl complexes, the irreversible anodic event observed by CV was also ascribed to oxidation of the axial ligand. Although broadly similar to [Ga(SPh)(TPP)], the CV of [Ga(SH)(TPP)] appears more complex displaying several irreversible and quasi-reversible events. Most notably, the second cathode event at −2.191 V in [Ga(SH)(TPP)] is irreversible and appears to correspond to a two-electron process. The nature of this event is unknown at this time, although further reduction of the S−H bond to produce H2 is an attractive hypothesis.51
(see Supporting Information). Both [Ga(SEt)(TPP)] and [Ga(SPh)(TPP)] demonstrated sensitivity to ambient conditions, most likely as a result of their proclivity toward protonolysis. Solutions of both [Ga(SEt)(TPP)] and [Ga(SPh)(TPP)] were found to transform to the corresponding hydroxide complex when exposed to the ambient atmosphere for several minutes as judged by UV−vis spectroscopy. Crystals of [Ga(SEt)(TPP)] and [Ga(SPh)(TPP)] suitable for X-ray diffraction were obtained from concentrated toluene solutions. The solid-state structures of each complex are depicted in Figure 2, and the corresponding bond metrics appear in Table 1. As found for [Fe(SPh)(TPP)],43 a positional disorder of the coordinated thiolate ligand and a cocrystallized toluene molecule complicates the structure refinement for both compounds (see Supporting Information). The Ga−S contacts in both complexes are nearly identical and slightly elongated from that in [Ga(SH)(TPP)] (Table 1). The remaining bond metrics are similar to those of [Ga(SPh)(OEP)] and [Fe(SPh)(TPP)].37,43 Interestingly, [Ga(SEt)(TPP)] represents the first example of a structurally characterized analog of an iron(III) porphyrinate containing an alkanethiolate ligand.31,38,45−48 Despite our best efforts, we could not obtain crystals of [Ga(SSiiPr3)(TPP)] suitable for X-ray diffraction. C
DOI: 10.1021/acs.inorgchem.5b02822 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 3. Cyclic voltammograms of [Ga(SH)(TPP)] (left) and [Ga(SPh)(TPP)] (right) in CH2Cl2 at a Pt disk electrode. Scan rate is 50 mV/s, and the supporting electrolyte is 0.1 M Bu4NPF6.
In order to test the stability of [Ga(SH)(TPP)] in the presence of other thiols, its reactivity with both EtSH and PhSH was examined by 1H NMR spectroscopy. Introduction of several equivalents of EtSH to [Ga(SH)(TPP)] in benzene-d6 resulted in formation of only very minor amounts of [Ga(SEt)(TPP)]. The concentration of the ethanethiolate complex did not increase with additional equivalents of EtSH suggesting that its formation is not due to reactivity with the hydrosulfide complex but rather to the presence of trace impurities of [GaCl(TPP)] or [Ga(OH)(TPP)]. In contrast to EtSH, 1 equiv of benzenethiol reacted readily with [Ga(SH)(TPP)] leading to formation of [Ga(SPh)(TPP)] as judged by 1 H NMR spectroscopy (eq 4). Such reactivity is consistent with the reported pKa values of each thiol.52
relevant ethanethiolate ligand. A solution of [Ga(SEt)(TPP)] in benzene-d6 was exposed to 1 atm of H2S(g). The presence of dissolved H2S could easily be confirmed by a singlet resonance at 0.19 ppm. However, no evidence for the formation of [Ga(SH)(TPP)] was observed. This result is surprising given that [Ga(SEt)(TPP)] is not stable in the presence of ambient moisture and that H2S has a much lower pKa than that of H2O. Thiol pKa is therefore likely not the sole determinant in the outcome of thiolate exchange reactions with gallium tetraphenylporphyrinates.
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CONCLUSION In this contribution, we have presented the first reproducible synthesis of a trivalent metalloporphyrinate bearing the terminal HS− ligand. We have also demonstrated that a variety of thiolate complexes of gallium(III) tetraphenylporphyrinate can be prepared in straightforward fashion through salt metathesis reactions with [GaCl(TPP)]. The structural features of the [Ga(SR)(TPP)] complexes provide a set of metric parameters that can be used to approximate bond distances in the unstable iron(III) analogs (R = H, Et). Electrochemical measurements on the gallium tetraphenylporphyrinates demonstrate an irreversible thiolate-centered oxidation near +0.4 V along with a series of ligand-based events, several of which become irreversible in the case of [Ga(SH)(TPP)]. Of note, the potential of the thiolate-centered event is quite similar to the first anodic event found for the iron(III) silanethiolate complex, [Fe(SSiiPr3)(TPP)].14 This similarity suggests that thiolate oxidation is a common feature of both systems, further underscoring the homology between gallium(III) and iron(III) porphyrinate chemistry.
The reactivity of [Ga(SH)(TPP)] with the strong acid (H{Et2O}2)[B(3,5-{CF3}2C6H3)4] (HBArf4), was also probed to determine whether a cationic Ga−H2S complex could be prepared. Coordination of intact H2S to iron(III) hemes has been proposed to serve as a key step prior to reduction of the metal center.20 Binding of intact H2S also has precedent in the chemistry of ruthenium(II).53−58 Use of the Ga(III) porphyrinate affords an opportunity to investigate whether coordination of intact H2S to a trivalent metalloporphyrinate is in fact a facile process. Reaction of [Ga(SH)(TPP)] with HBArf4 in dichloromethane-d2 resulted in smooth protonation of the hydrosulfide ligand as judged by 1H NMR spectroscopy (see Supporting Information). However, resonances for the resulting gallium porphyrinate were inconsistent with a five coordinate species, and a peak for dissolved H2S(g) was clearly discernible. The identity of the gallium product is most consistent with the solvent adduct [Ga(Et2O)2(TPP)](BArf4) (eq 5), as judged by an upfield shift and broadening for the diethyl ether resonances and single sets of sharp multiplet resonances for the ortho and meta hydrogen atoms of the meso phenyl rings. Therefore, H2S appears to be a poor ligand for hard metal centers such as Ga(III). As a final experiment, we examined the reaction of H2S with the gallium(III) complex containing the biologically
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EXPERIMENTAL SECTION
General Comments. Manipulations of air- and moisture-sensitive materials were performed in a glovebox under an atmosphere of purified nitrogen. Tetrahydrofuran, diethyl ether, methylene chloride, D
DOI: 10.1021/acs.inorgchem.5b02822 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
[Ga(SPh)(TPP)]. To 51.0 mg (71.0 μmol) of [GaCl(TPP)] in 5 mL of THF was added 11.3 mg of NaSPh (85.5 μmol). The reaction mixture was allowed to stir for 12 h at ambient temperature in the glovebox. The mixture was then filtered through a plug of Celite, and all volatiles were removed in vacuo. The remaining solid was washed with pentane and collected by filtration to afford 44.3 mg (79% yield) of a purple solid. Crystals suitable for X-ray diffraction were obtained from toluene/pentane. 1H NMR: δ 9.04 (s, 8 pyr−CH), 8.06 (br d, 8 o-ArH), 7.47 (m, 12 m/p-ArH), 6.49 (t, JHH = 7.2 Hz, 1 p-SPh), 6.19 (t, JHH = 7.5 Hz, 2 m-SPh), 4.36 (d, JHH = 7.5 Hz, 2 o-SPh). UV−vis: λmax, cm−1 (ε, M−1 cm−1) 22 900 (172 000), 23 700 (sh), 26 500 (21,800), 17 700 (12 900), 16 500 (5880). Anal. Calcd for C50H33GaN4S·1/2C6H6: C, 76.63; H, 4.37; N, 6.74. Found: C, 76.70; H, 4.47; N, 6.54. [Ga(SSiiPr3)(TPP)]. To 50.0 mg (69.6 μmol) of [Ga(TPP)(Cl)] in 3 mL of THF was added 18.9 mg of NaSSiiPr3 (89.0 μmol). The solution was allowed to stir for 12 h and then filtered through a plug of Celite. The THF was removed in vacuo, and the compound was washed with pentane and collected by filtration. Mass: 36.0 mg (59% yield). 1H NMR(C6D6): δ 9.08 (s, 8 pyr−CH), 8.35 (br, 4 o-ArH), 8.01 (br, 4 o-ArH), 7.48 (m, 12 m/p-ArH), −0.08 (d, JHH = 7.5 Hz, 18 SiCHMe2), −1.31 (sep, 3 SiCHMe2). UV−vis: λmax, cm−1 (ε, M−1 cm−1) 23 100 (433 000), 29 400 (25 400), 17 800 (18 100), 16 600 (6170), 15 800 (1489). Anal. Calcd for C53H49GaN4SSi: C, 73.01; H, 5.66; N, 6.43. Found: C, 72.57; H, 5.72; N, 6.24.
pentane, and toluene were purified by sparging with argon and passage through two columns packed with 4 Å molecular sieves. Benzene and benzene-d6 were dried over 4 Å molecular sieves prior to use. 1H NMR spectra were recorded in benzene-d6 on a Varian INOVA spectrometer operating at 500 MHz and referenced to the residual C6D5H peak of the solvent (δ 7.16 ppm vs TMS). UV−vis spectra were recorded at ambient temperature in toluene on a Cary-60 spectrophotometer in Teflon-capped quartz cells. Cyclic voltammetry measurements were performed in dichloromethane in a single compartment cell under a nitrogen atmosphere (in the glovebox) at 25 °C using a CH Instruments 620D electrochemical workstation. A three-electrode setup was employed comprising a 1 mm diameter Pt disk working electrode, platinum wire auxiliary electrode, and Ag quasi-reference electrode. Triply recrystallized Bu4NPF6 was used as the supporting electrolyte. All electrochemical data were referenced internally to the ferrocene/ ferrocenium couple at 0.00 V. Elemental analyses were performed by University of Rochester CENC facility. X-ray Crystallography. Crystals suitable for X-ray diffraction were mounted in Paratone oil onto a glass fiber. Diffraction data for [Ga(SR)(TPP)] were collected using a Rigaku AFC12/Saturn 724 CCD fitted with Mo Kα radiation (λ = 0.71073 Å) at temperatures of 293(2) K (R = H) or 98(2) K (R = Ph, Et). Low temperature data collection was accomplished with a nitrogen cold stream maintained by an X-Stream low-temperature apparatus. Data collection and unit cell refinement were performed using the Crystal Clear software.59 Data processing and absorption correction, giving minimum and maximum transmission factors, were accomplished with Crystal Clear and ABSCOR,60 respectively. All structures were solved by direct methods and refined on F2 using full-matrix, least-squares techniques with SHELXL-97.61 All non-hydrogen atoms were refined with anisotropic displacement parameters. All carbon bound hydrogen atom positions were determined by geometry and refined by a riding model. The sulfur bound hydrogen atom of [Ga(SH)(TPP)] was located in the difference map, and its position was refined isotropically. Crystallographic data and refinement parameters for each structure can be found in the Supporting Information. Materials. [GaCl(TPP)], [Ga(OH)(TPP)], [Ga(OMe)(TPP)], (H{Et2O}2)[B(3,5-{CF3}2C6H3)4] (HBArf4), and Bu4NSH were prepared by literature methods or slight modifications thereof.37,41,42,49,62 PhSH, EtSH, iPr3SiSH, and (Me3Si)2S were purchased from commercial suppliers and used as received. NaSPh, NaSEt, and NaSSiiPr3 were prepared by treatment of the corresponding thiols with NaH in THF. [Ga(SH)(TPP)]. To 50.0 mg (69.6 μmol) of [GaCl(TPP)] in 5 mL of THF was added 22.8 mg of Bu4NSH (82.8 μmol). The solution was allowed to stir for 12 h at ambient temperature in the glovebox. The solvent was removed in vacuo, and the resulting residue was extracted into 5 mL of toluene and filtered through a plug of Celite. The toluene was removed in vacuo, and the remaining solid was washed with pentane and collected by filtration to afford 38.1 mg (77% yield) of the desired material as a purple solid. 1H NMR: δ 9.08 (s, 8 pyr−CH), 8.02 (br d, 8 o-ArH), 7.48 (app t, 4 p-ArH), 7.41 (br m, 8 m-ArH), −6.32 (s, SH). UV−vis: λmax, cm−1 (ε, M−1 cm−1) 23 300 (483 000), 30 100 (26 900), 17 900 (17 600), 16 700 (6350). Anal. Calcd for C44H29GaN4S: C, 73.86; H, 4.09; N, 7.83. Found: C, 73.50; H, 3.90; N, 7.51. [Ga(SEt)(TPP)]. To 50.5 mg (70.3 μmol) of [GaCl(TPP)] in 5 mL of THF was added 6.6 mg of NaSEt (79 μmol). The resulting solution was allowed to stir for 12 h in the glovebox and then filtered through a plug of Celite. The THF was removed in vacuo, and the remaining residue was dissolved in benzene and lyophilized to afford 49.0 mg (94% yield) of a purple solid. Crystals suitable for X-ray diffraction were obtained from toluene/pentane. 1H NMR: δ 9.09 (s, 8 pyr−CH), 8.07 (br d, 8 o-ArH), 7.48 (app t, 4 p-ArH), 7.42 (br m, 8 m-ArH), −0.97 (t, JHH = 7.5 Hz, 3 SCH2CH3), −1.55 (br q, 2 SCH2CH3). UV−vis: λmax, cm−1 (ε, M−1 cm−1) 23 100 (335 000), 29 400 (27 800), 17 700 (14 300), 16 600 (6450). Anal. Calcd for C46H33GaN4S· 1/2C6H6: C, 75.20; H, 4.64; N, 7.16. Found: C, 75.48; H, 4.59; N, 6.55.
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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.5b02822. 1 H NMR and electronic absorbance spectra, thermal ellipsoid drawings, and crystallographic data and refinement parameters (PDF) Crystallographic structures (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the Welch Foundation (Grant AX-1772 to Z.J.T.) for supporting this work. M.C.-G. was supported by the ACS Project SEED and the UTSA College of Sciences.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.5b02822 Inorg. Chem. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.inorgchem.5b02822 Inorg. Chem. XXXX, XXX, XXX−XXX
