Inorganica Chimica Acta 407 (2013) 91–97

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Inorganica Chimica Acta

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Synthesis, characterization, and photoinduced CO-release reactivity of aPb(II) flavonolate complex: Comparisons to Group 12 analogs

0020-1693/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.ica.2013.07.029

⇑ Corresponding author. Address: Department of Chemistry & Biochemistry, UtahState University, 0300 Old Main Hill, Logan, UT 84322-0300, USA. Tel.: +1 435 7971625; fax: +1 435 797 3390.

E-mail addresses: grubelk@gmail.com (K. Grubel), sushmasaraf4@gmail.com(S.L. Saraf), snanderson@aggiemail.usu.edu (S.N. Anderson), BrynnaL@clemson.edu(B.J. Laughlin), Rhett@clemson.edu (R.C. Smith), arif@chem.utah.edu (A.M. Arif),lisa.berreau@usu.edu (L.M. Berreau).

Katarzyna Grubel a, Sushma L. Saraf a, Stacey N. Anderson a, Brynna J. Laughlin b, Rhett C. Smith b,Atta M. Arif c, Lisa M. Berreau a,⇑a Department of Chemistry & Biochemistry, Utah State University, Logan, UT 84322-0300, USAb Department of Chemistry, Clemson University, Clemson, SC 29634, USAc Department of Chemistry, University of Utah, Salt Lake City, UT 84112, USA

a r t i c l e i n f o

Article history:Received 1 May 2013Received in revised form 11 July 2013Accepted 18 July 2013Available online 27 July 2013

Keywords:Lead compoundsPhotochemistryCarbon monoxideFlavonolZinc compoundsCatalysis

a b s t r a c t

The synthesis, characterization, and photoinduced CO-release reactivity of [(6-Ph2TPA)Pb(3-Hfl)]ClO4 (1;6-Ph2TPA = N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine; 3-Hfl = anion of 3-hydroxyflavone) is reported and compared with Group 12 analogues ([(6-Ph2TPA)M(3-Hfl)]ClO4

(M = Hg(II) (2), Cd(II) (3), Zn(II) (4)). The Pb(II) complex exhibits unique features within this group ofcomplexes in terms of its structural and spectroscopic features involving the coordinated flavonolateligand. Similar to the Group 12 compounds, irradiation of 1 at 300 nm in an O2-containing environmentresults in the quantitative release of CO and the formation of a Pb(II) O-benzoylsalicylate (O-bs, depside)complex via a photoinduced dioxygenase-type reaction. Comparison of the quantum yield for the reac-tion of the Pb(II) flavonolate complex (U = 0.21(6)) versus the reactions of the structurally-related Group12 metal complexes (6-Ph2TPA)M(3-Hfl)]ClO4 (M = Hg(II) (2, U = 0.31(2)), Cd(II) (3, U = 0.28(2)), Zn(II)(4), U = 0. 09(1)) revealed that flavonolate complexes of a heavy metal ion (Cd(II), Hg(II), Pb(II)) exhibita higher reaction quantum yield than the Zn(II) derivative. Both Pb(II) and Zn(II) flavonolate complexeswere found to be catalysts for the oxidative photoinduced degradation of 3-hydroxyflavone. The com-bined results of these investigations suggest that metal contaminants typically present in soil, includingtoxic heavy metal ions, might facilitate the oxidative decomposition of plant-derived flavonols via pho-toinduced reactions.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Lead is a widespread environmental contaminant, with soilbeing cited as a major point of deposition of this toxic metal ion[1–3]. As an approach toward investigating the influence of toxicPb(II) on the photochemistry of plant-derived humic substancesin soil, Cornard and co-workers recently reported studies of thephotochemistry of Pb(II)–flavonolate species, [Pb(3-Hfl)]+

(3-Hfl = 3-hydroxyflavonolate) [4–6]. To date, however, the struc-tural characterization of a Pb(II) flavonolate complex has not beenreported. Thus, information about the coordination properties ofPb(II) versus other heavy metal ions (e.g. Hg(II), Cd(II)) with flavo-nols is lacking. Additionally, there is currently a significant gap in

understanding of how Pb(II) coordination, versus that of other bio-logically relevant metal ions (e.g. Zn(II)), influences photochemicalreactions that could occur in nature [7].

Spectroscopic studies of the anions of 3-hydroxyflavone (3-HflH)and quercetin (Fig. 1(b)) in the presence of Pb(II) have suggested theformation of 1:1 cationic species of the formula [Pb(3-Hfl)]+

(Fig. 1(a)) and [Pb(quercetin)]+ (Fig. 1(b)), with coordination of themetal ion at the 3-hydroxy-4-pyrone chelate motif. A similar 1:1Pb(II)–flavonolate interaction has been proposed to occur in a ruthe-nium acetylide fluorescent chemosensor that includes a 3-hydrox-yflavonol appendage for metal coordination (Fig. 1(c)) [8]. We notethat a complex of the formulation Pb(3-Hfl)2�2H2O (Fig. 1(d)) hasbeen reported based on elemental analysis and spectroscopic data,but was not characterized by X-ray crystallography [9]. As part ofour ongoing studies of divalent metal flavonolate complexes[10–12], in this contribution we report the synthesis and firststructural characterization of a mononuclear Pb(II) 3-hydroxyfl-avonolate complex, [(6-Ph2TPA)Pb(3-Hfl)]ClO4 (1, 6-Ph2TPA =N,N-bis((6-phenyl-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine;3-Hfl = anion of 3-hydroxyflavone). The structural and spectroscopicproperties of this compound, as well as its photoinduced CO-release

Fig. 1. Pb(II)–flavonol interactions proposed on the basis of spectroscopic andanalytical data.

92 K. Grubel et al. / Inorganica Chimica Acta 407 (2013) 91–97

reactivity, are compared with those of Group 12 analogs,[(6-Ph2TPA)M(3-Hfl)]ClO4 (M = Hg(II) (2), Cd(II) (3), Zn(II) (4)).

2. Experimental

2.1. Materials and methods

All reagents and solvents were obtained from commercialsources and were used as received unless otherwise noted. Anaer-obic procedures were performed in a Vacuum Atmospheres glove-box. Solvents for glovebox use were dried according to publishedmethods and distilled under N2 [13]. The 6-Ph2TPA ligand was pre-pared using methods that have been previously described [14,15]but was purified by column chromatography using 1:2 metha-nol:ethyl acetate as the eluent. 1H and 13C NMR spectra of thePb(II) complexes were obtained in CD3CN using a Bruker ARX-400 spectrometer and are referenced to the residual solvent signal(1H NMR 1.94 ppm; 13C NMR 1.39 ppm). IR spectra were collectedas KBr pellets using a Shimadzu FTIR-8400 spectrometer. UV–Visspectra were recorded at ambient temperature using a Hewlett–Packard 8453A diode array spectrophotometer. Fluorescence emis-sion spectra were collected using a Shimadzu RF-530XPC spec-trometer in the range of 250–900 nm, with the excitationwavelength corresponding to the absorption maximum of thecomplex above 400 nm. The excitation and emission slit widthswere set at 5 mm for all experiments. The fluorescence quantumyield and lifetime for 1 were determined as previously described[10]. A Rayonet photoreactor equipped with RPR-3000A lampswas used for all photochemical reactions. Quantum yields weredetermined by ferrioxalate actinometry, by using an integrativeanalysis method [16]. The output measured during the quantum

yield determination for 1 was 4.44128 � 1016 photons/s. CO wasquantified using an Agilent 3000A Micro GC with helium carriergas, a molecular sieve column, and a TCD detector. The instrumentconditions for the CO quantification were: sample inlet: 100 �C;injector temp: 90 �C; column temp: 80 �C; column head pressure:25 psi. Mass spectral data was collected at the Mass SpectrometryFacility, University of California, Riverside. MALDI mass spectrawere recorded on a PE Biosystems DE-STR MALDI TOF system(2000) using a WinNT data system. ESI/APCI mass spectra were re-corded on a Agilent LCTOF (2006) with a Windows XP based oper-ating system. Elemental analyses were performed by AtlanticMicrolab, Inc., Norcross, GA, using a PE2400 automatic analyzer.

Caution! Perchlorate salts of metal complexes with organic li-gands are potentially explosive. Only small amounts of materialshould be prepared, and these should be handled with great care[17].

2.2. Preparation and characterization of [(6-Ph2TPA)Pb(3-Hfl)]ClO4 (1)

In a N2-filled glovebox, Pb(ClO4)2�3H2O (35 mg, 8.6 � 10�2

mmol) was dissolved in methanol (5 mL). This solution was addedto solid 6-Ph2TPA (38 mg, 8.6 � 10�2 mmol) and the resultingmixture was stirred for 2 h until all of the 6-Ph2TPA had dissolved.This mixture was added to a methanol solution (5 mL) containing3-hydroxyflavonol (3-HflH) (20 mg, 8.6 � 10�2 mmol) and Me4

NOH�5H2O (16 mg, 8.6 � 10�2 mmol). The resulting mixture wasstirred for 4 h at ambient temperature. The solvent was then re-moved under reduced pressure. The residual yellow solid was dis-solved in dichloromethane and filtered through a Celite/glass woolplug. The filtrate was concentrated under reduced pressure and theproduct was precipitated by the addition of excess hexanes. Yield:62 mg, 73% (yellow powder). Single crystals suitable for X-ray dif-fraction were obtained via diethyl ether diffusion into a methanol/isopropanol/dichloromethane (2:0.5:1) solution of the complex.Elemental Anal. Calc. for C45H35ClN4O7Pb: C, 54.77; H, 3.58; N,5.68. Found: C, 54.17; H, 3.59; N, 5.64%. UV–Vis kmax (CH3CN)/nm(e/M�1 cm�1) 406 (16000); FTIR (KBr, cm�1) 1558 (mC@O); 1HNMR (CD3CN, 400 MHz): d 8.33 (d, J = 2.4 Hz, 1H), 8.26 (m, 2H),7.75–7.28 (m, 25H), 4.47 (br s, 6H). 13C{1H} NMR (CD3CN,400 MHz): d 181.9, 158.9, 157.3, 156.9, 155.4, 148.5, 140.7, 140.1,139.1, 140.7, 140.1, 139.1, 133.9, 130.8, 130.4, 130.1, 129.3,128.6, 128.4, 125.5, 125.4, 125.3, 125.1, 124.1, 124.0, 123.0,119.1, 64.3, 64.1; MALDI-MS, m/z (relative intensity) 887.2431([(6-Ph2TPA)Pb(3-Hfl)]+, 100%). Calc. m/z 887.2475.

2.3. Photoinduced CO release reactivity of 1. Isolation andcharacterization of [(6-Ph2TPA)Pb(O-bs)]ClO4 (5)

An aerobic acetonitrile solution (90 mL) of 1 (48 mg,4.8 � 10�2 mmol) was exposed to sunlight until the yellow solu-tion was observed to lose color. The solvent was then removed un-der reduced pressure and the residue was dissolved indichloromethane. This solution was filtered through a Celite/glasswool plug and the filtrate was then brought to dryness under re-duced pressure to give 5. Yield: 38 mg (80%, colorless solid). Ele-mental Anal. Calc. for C44H35ClN4O8Pb: C, 53.36; H, 3.56; N, 5.66.Found: C, 53.39; H, 3.67; N, 5.41%. 1H NMR (CD3CN, 400 MHz): d8.36 (s, 1H), 7.91–7.78 (m, 4H), 7.67–7.24 (m, 2H), 7.49–7.35 (m,15H), 7.30–7.26 (t, J = 8.0 Hz, 4H), 7.13–7.09 (m, 3H), 4.62 (s, 2H),4.32 (br s, 4H); 13C{1H} NMR (CD3CN, 100 MHz): d 172.7, 166.1,158.7, 157.0, 156.5, 151.0, 148.5, 140.8, 140.3, 138.4, 134.4,133.1, 132.4, 132.1, 130.7, 130.6, 130.3, 129.5, 128.3, 126.7,125.4, 124.4, 123.9, 123.3, 64.7, 64.3 (28 signals expected forequivalent phenyl-appended pyridyl donors; 26 observed); MAL-DI-MS, m/z (relative intensity) 891.2457 ([(6-Ph2TPA)Pb(O-bs)]+,100%). Calc. m/z 891.2424. Performing the reaction using 18O2

K. Grubel et al. / Inorganica Chimica Acta 407 (2013) 91–97 93

resulted in quantitative incorporation of two labeled oxygen atomsas determined by ESI/APCI m/z (relative intensity) 895.2521([6-Ph2TPA)Pb(18O2–O-bs)]+, 2%). Calc., m/z 895.2504.

2.4. Reaction quantum yield

In a dark room under minimal red light, 1 (10.2 mg,1.03 � 10�5 mol) was dissolved in acetonitrile (5 mL,2.10 � 10�3 M). A 3 mL sample was then transferred to a quartzcuvette, purged with O2, inserted into a photoreactor (Rayonet),and was then exposed to 300 nm irradiation for 30–45 s. After thistime, the reaction progress was evaluated using UV–Vis spectros-copy by measuring the intensity of the band I absorption(406 nm) of the metal-bound flavonolate. The lamp output wasmeasured using ferrioxalate as a chemical actinometer [16,18].The quantum yield was determined to be 0.21(6) based on threereplicate runs.

2.5. Quantification of CO production

Quantification of CO production was achieved by comparison tocalibration curves determined for gas mixtures of CO and O2

(Fig. S1). The gas mixtures were prepared by loading a knownnumber of moles of gas into an evacuated 50 mL round bottomflask containing acetonitrile (5 mL) and sealed with a septum.The round bottom flask was then shaken for 30 s and 10 mL ofthe headspace was withdrawn via a gas tight syringe and trans-ferred to the injection port of the GC. The amount of CO generatedin the photoinduced reaction of 1 under O2 was found to be oneequivalent based on two replicate runs.

2.6. Catalytic degradation of 3-hydroxyflavone

Studies of the catalytic degradation of 3-HflH by the Pb(II) com-plex 1 and the Zn(II) analog 4 were performed as previously de-scribed [11]. However, the concentration of 3-HflH and metalcomplexes used in the current experiments were 3-HflH/1:2.0 � 10�2 M/2.0 � 10�4 M, and 3-HflH/4: 1.2 � 10�2 M/1.2 � 10�4 M, respectively.

2.7. X-ray single crystal diffraction analysis of 1

A yellow prism shaped single crystal of 1 was mounted on aglass fiber with traces of viscous oil and then transferred to a Non-ius Kappa CCD diffractometer equipped with Mo Ka radiation

Scheme

(k = 0.71073) for data collection. For unit cell dimensions, 10frames of data were collected at 150(1) K with an oscillation rangeof 1 deg/frame and an exposure time of 20 s/frame. Indexing andunit cell refinement based on all observed reflection from thoseten frames, indicated a triclinic P lattice. A total of 18959 reflec-tions were indexed, integrated, and corrected for Lorentz, polariza-tion, and absorption effects using DENZO-SMN and SCALEPAC [19].

The structure was solved by a combination of direct methodsand heavy atom using SIR97 [20]. All of the non-hydrogen atomswere refined with anisotropic displacement coefficients. Hydrogenatoms were assigned isotropic displacement coefficientsU(H) = 1.2U(C) or 1.5U(Cmethyl), and their coordinates were allowedto ride on their respective carbons using SHELXL97 [21]. Complex 1crystallizes in the space groupP�1. There are two disordered solventmolecules, Et2O and CH3CN, each with half occupancy per asym-metric unit, located near an inversion center.

3. Results and discussion

3.1. Synthesis and isolation of 1

Admixture of the 6-Ph2TPA chelate ligand with an equimolaramount of Pb(ClO4)2�3H2O in methanol, followed by the additionof an equimolar amount of [Me4N][3-Hfl] (generated in situ), re-sulted in the formation of [(6-Ph2TPA)Pb(3-Hfl)]ClO4 (1, Scheme 1)which was isolated as a yellow powder following deposition fromCH2Cl2/hexanes. Single crystals suitable for X-ray diffraction wereobtained by recrystallizing this powder from methanol/isopropa-nol/dichloromethane via diethyl ether diffusion.

3.2. X-ray crystallography

Details of the X-ray data collection and structure solution for 1are given in Table 1. Selected bond distances and angles are givenin Table 2. A drawing of the cationic portion of 1 is shown in Fig. 2.The Pb(II) center exhibits a coordination number of six in a hemi-directed geometry [22]. Hemidirected means that all of the bondsto the ligand atoms are directed to only a portion of the overallcoordination sphere. This type of structure can be contrasted witha holodirected ligand environment wherein the bonds to the li-gands are directed throughout the entire region encompassingthe coordination sphere. Hemidirected ligand coordination is com-mon in Pb(II) complexes having coordination numbers of 6–8, andin the case of 1 results in an open site in the coordination spheretrans to the tertiary amine nitrogen of the coordinated chelate

1.

Table 1Details of X-ray data collection and refinement for 1.

CCDC deposition number 898824Molecular formula (C45H35N4O7ClPb)2�C4H10O�CH3CNFormula weight (g/mol) 2088.00Crystal size (mm) 0.23 � 0.20 � 0.10Crystal form prismCrystal color yellowCrystal system triclinicSpace group P�1Radiation Mo KaT (K) 150(1)Dcalc (Mg/L) 1.596k (Å) 0.71073a (Å) 11.05570(10)b (Å) 12.0437(2)c (Å) 17.4239(2)a (�) 79.6210(7)b (�) 72.1890(8)c (�) 85.5493(9)V (Å3) 2172.45(5)Z 1l (mm�1) 4.004F(000) 10402h Range 2.036–54.97Reflection measured 18959Independent reflections (Rint) 9009 (0.0180)Completeness to h = 27.50� 99.6%Goodness-of-fit (GOF) on F2 1.040Data/restraints/parameters 9956/70/568Final R indices [I > 2rI] R1 = 0.0266, wR2 = 0.0593R indices (all data) R1 = 0.0323, wR2 = 0.0615Largest difference in peak and hole

(e �3)1.456 and �0.941

Table 2Selected bond distances (Å) and angles (�).

Pb(1)-O(1) 2.2270(19) Pb(1)–N(2) 2.605(2)Pb(1)–N(1) 2.539(2) Pb(1)–N(3) 2.924(2)Pb(1)–O(2) 2.595(2) Pb(1)–N(4) 2.684(2)

O(1)–Pb(1)–N(1) 89.95(8) O(2)–Pb(1)–N(4) 133.48(7)O(1)–Pb(1)–O(2) 68.14(7) N(2)–Pb(1)–N(4) 65.07(8)N(1)–Pb(1)–O(2) 73.31(8) O(1)–Pb(1)–N(3) 143.67(7)O(1)–Pb(1)–N(2) 76.78(8) N(1)–Pb(1)–N(3) 76.89(7)N(1)–Pb(1)–N(2) 67.08(8) O(2)–Pb(1)–N(3) 135.85(7)O(2)–Pb(1)–N(2) 126.21(7) N(2)–Pb(1)–N(3) 66.89(7)O(1)–Pb(1)–N(4) 72.95(7) N(4)–Pb(1)–N(3) 90.66(7)N(1)–Pb(1)–N(4) 131.63(8)

94 K. Grubel et al. / Inorganica Chimica Acta 407 (2013) 91–97

ligand. This type of structure energetically minimizes antibondinglead 6s-ligand np (6s/np) interactions [23]. Typically, Pb–L dis-tances are elongated near the open site, whereas a shortening ofthe Pb–L bond lengths is found for ligands located opposite theopen position. In 1 this results in significantly different Pb–O bondlengths (Pb(1)–O(1) 2.2270(19) Å; Pb(1)–O(2) 2.595(2) Å) involv-ing the coordinated flavonolate ligand, with the DM–O value of0.37 Å significantly exceeding the value found for 3-hydroxyfl-avonolate coordination in structurally-related 6-Ph2TPA-ligatedcomplexes of Group 12 metals (Table 3) [10,12]. It should be notedthat the longer of the two M–O(flavonolate) distances in all ofthese complexes is associated with the neutral ketone donor.

Three Pb(II) complexes of structurally related N4-donor chelateligands have been previously reported [24–26]. The range ofPb–N(chelate) distances found in 1 (2.539(2)–2.924(2) Å) islarger than the ranges found in [(H2tpaa)Pb–Cl] (2.646(6)–2.803(6) Å) [25], [(TQA)Pb(NO3)2] (2.549(3)–2.680(3) Å) [24], and[(6-Me3TPA)Pb(NO3)]NO3 (2.546–2.677 Å) [26]. The shortestPb–N distances in 1 are found for the pyridyl and tertiary aminedonors of the supporting chelate ligand (Pb(1)–N(1) 2.539(2) andPb(1)–N(2) 2.605(2) Å, respectively). One of the phenyl-appended

pyridyl donors coordinates to the Pb(II) center with only a slightlylonger distance (Pb(1)–N(4) 2.684(2) Å). However, the other phe-nyl-appended pyridyl appendage is positioned trans to the coordi-nated deprotonated hydroxyl oxygen of the flavonolate ligand andadjacent to the open position. Both of these factors likely contrib-ute to the elongated Pb–N interaction (Pb(1)–N(3) 2.924(2) Å).While this distance is within the sum of the van der Waals radii[27], it suggests a weak interaction. We note that the coordinationmotif of the 6-Ph2TPA chelate ligand in 1 somewhat resembles thatfound in [(6-Ph2TPA)Zn(3-Hfl)]OTf , albeit this complex exhibitsj3-coordination of the chelate ligand [12].

3.3. Spectroscopic properties

Compound 1 is yellow due to an intense absorption band cen-tered at 406 nm (e = 16,000 M�1 cm�1; Fig. S2). This absorption istermed band I and is associated with a p?p⁄ transition of theflavonolate ligand. This band is hypsochromically-shifted relativeto that found in the flavonolate salt [Me4N][3-Hfl] (kmax = 458 nm(CH3CN))[12] as well as other divalent metal 3-hydroxyflavonolatecomplexes supported by the 6-Ph2TPA ligand (Table 3). Notably,within the d10 compounds listed in Table 3 there is a correlationbetween the DM–O value and the observed wavelength of the bandI absorption (Fig. S3), with increasing asymmetric metal flavono-late coordination corresponding to a higher energy band I absorp-tion feature. The absorption maximum of 1 matches thatpreviously reported for the [Pb(3-Hfl)]+ cation formed in situ inmethanol, but is shifted from that reported for the neutralPb(3-Hfl)2�2H2O (Table 4) [4–6,9]. The emission spectrum of 1shows a Stokes shift of 59 nm (Fig. S2). The excited state associatedwith this emission has a lifetime of 2.2(1) ns and a fluorescencequantum yield of 0.02. The values are similar to those exhibitedby [(6-Ph2TPA)Hg(3-Hfl)]ClO4 [10].

The room temperature 1H NMR spectra of 1–4 are shown inFig. S3. The spectral features of 2–4 have been previously re-ported [10]. Most notable in the spectra of the Cd(II) and Hg(II)complexes (2 and 3), relative to the Zn(II) complex (4), are: (a)upfield-shifted meta and para proton resonances for the phenylappendages of the 6-Ph2TPA ligand, and (b) a downfield-shiftedC(1)–H proton of the unsubstituted pyridyl ring. On the basis ofX-ray crystallographic characterization of all three compounds,these features in 2 and 3 are attributed to p-stacking involvingthe flavonolate, and a short contact between the C(1)–H of theunsubstituted pyridyl ring and the carbonyl oxygen atom of thecoordinated flavonolate, respectively. The 1H NMR spectrum of1 is notably different than that of 2–4, with the signals of the aro-matic region being less dispersed. The chemical shift rangeencompassed by the aromatic signals of 1 (�7.2–8.5 ppm) is sim-ilar to that found for the free 6-Ph2TPA ligand. The broad natureof a portion of the aromatic resonances suggests possible fluxio-nality in chelate ligand coordination. To examine this issue, vari-able temperature 1H NMR studies of 1 were performed for atemperature range of �35 to +65 �C (in CD3CN). Over this range,subtle differences were evident within the aromatic region, how-ever the spectral features remained generally similar to those ob-served at room temperature. Notably, the benzylic resonance in 1is found �0.55 ppm downfield relative to same signal in free 6-Ph2TPA. Combined, these NMR spectral characteristics suggestthat in the solution form of 1, coordination of the 6-Ph2TPA ligandinvolves binding of the tertiary amine nitrogen, and possibly theunsubstituted pyridyl donor, but likely involves weak or transientinteractions with the phenyl-appended pyridyl donors. This isconsistent with the 13C NMR data for 1, which indicates equiva-lent phenyl-appended pyridyl donors and relates to the longPb(1)–NPhPy distances identified in the X-ray structure of 1.

Fig. 2. (Top) Thermal ellipsoid representations of the cationic portion of 1. Ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. (Bottom)View of the hemidirected Pb(II) coordination sphere of 1.

Table 3Comparison of selected structural and spectroscopic features of 6-Ph2TPA-supported3-Hfl complexes [(6-Ph2TPA)M(3-Hfl)]X (X = OTf or ClO4).

M DM–Oa DM–NPhPy kabs (nm) e (M�1 cm�1) Refs.

Pb(II) (1) 0.37 0.24 406 (16,000) This workHg(II) (2) 0.21 0.069 415 (14,000) [10]Cd(II) (3) 0.066 0.044 430 (13,500) [10]Zn(II) (4) 0.17 NAb 420 (20,100) [10,12]

a The M–O(1) (alkoxide oxygen) distance is always shorter than the M–O(2)(ketone oxygen) distance in this series of complexes.

b Complex contains only one M–NPhPy bond.

Table 4Spectroscopic properties of Pb(II) 3-Hfl complexes.

Complex Solvent kabs (nm) kem (nm) Refs.

[(6-Ph2TPA)Pb(3-Hfl)]ClO4 CH3CN 406 465a This work[Pb(3-Hfl)]+ MeOH 405 [4–6]Pb(3-Hfl)2�2H2O EtOH 424 [9]

a kex = 406 nm, CH3CN.

Table 5Reaction quantum yields and CO release quantification.

M Ua eq. CO

Pb(II) (1) 0.21(6) 1.0b

Hg(II) (2) 0.31(2) ndCd(II) (3) 0.28(2) ndZn(II) (4) 0.09(1) 0.93c

nd = not determined.a Reaction quantum yield with kirr = 300 nm, CH3CN solution, with standard error

given.b Average of 2 runs.c Average of 3 runs.

K. Grubel et al. / Inorganica Chimica Acta 407 (2013) 91–97 95

3.4. Photoinduced CO-release reactivity of 1

Irradiation of a CH3CN solution of 1 at 300 nm, or with sunlight,results in a bleaching of the yellow color and the formation of oneequivalent of CO (as determined by GC; Table 5) and the O-ben-zoylsalicylate (O-bs, depside) complex [(6-Ph2TPA)Pb(O-bs)]ClO4

(5, Scheme 2). We note that a similar reaction occurs using irradi-ation of a CH3CN solution of 1 at 419 nm, as evidenced by the cleangeneration of 5 (identified by 1H NMR) and the identification of COproduction. Use of 18O2 in this reaction results in the quantitativeincorporation of two labeled oxygen atoms in 5, which is consis-tent with dioxygenase-type reactivity. A dark control reactiondemonstrated that no reaction occurs in the absence of irradiation,providing evidence that the reaction is photoinduced and not a

thermal process. The quantum yield for the reaction of 1(U = 0.21(6); Table 5) is similar to that found for the Cd(II) andHg(II) analogs, and is higher than is found for the Zn(II) analog.

Complex 5 has been characterized by elemental analysis, 1H and13C NMR, FTIR, and mass spectrometry. To date, X-ray crystallo-graphic data has been obtained for the Cd(II) and Zn(II) depsidecomplexes (7 and 8), with the latter exhibiting monodentate car-boxylate coordination and the former bidentate carboxylate coor-dination [10]. As shown in Fig. S4, the Pb(II) depside complex 5has 1H NMR features that are somewhat similar to those of theCd(II) derivative 7.

3.5. Evaluation of photoinduced catalytic activity

Light driven catalytic reactions are of significant current inter-est [28]. A previous report from our laboratory showed that opend shell divalent metal flavonolate complexes can serve as catalystsfor the photoinduced oxidative decomposition of 3-hydroxyflav-one (3-HflH). Specifically, we previously found that irradiation(k = 300 nm, 18 h) of oxygen-purged acetonitrile solutions of com-plexes of the general formula [(6-Ph2TPA)M(3-Hfl)]ClO4

(M = Mn(II), Co(II), Ni(II), and Cu(II)) containing 20 equivalents of

Scheme 2.

96 K. Grubel et al. / Inorganica Chimica Acta 407 (2013) 91–97

3-HflH resulted in the complete consumption of the 3-HflH and thecatalytic production of two products, O-benzoylsalicylic acid (O-bsH, �50–65% yield) and a phthalide (7–17% yield) (Scheme 3)[11]. We have investigated the reactivity of [(6-Ph2TPA)M(3-Hfl)]ClO4 (M = Pb(II) (1) and Zn(II) (4)) under similar conditionsalbeit in the presence of 100 eq of 3-HflH and with irradiation at300 nm for only 3 h. For both metal ions, the 3-HflH is again com-pletely consumed, with the yield of O-benzoylsalicylic acid beingsimilar for the two metal ions (76–82%), as is the yield of phthalide(6–12%). The shorter irradiation period required for the catalyticreactivity of 1 and 4 versus that employed for the open d-shellcontaining metal complexes is an indication of the higher quantumyield for the photoinduced dioxygenase type reactions in thediamagnetic complexes versus the paramagnetic analogs(U = 0.005–0.008) [11].

3.6. Discussion and conclusions

Pb(II) is a common trace metal ion that is deposited into soilsfrom many sources, including from industrial pursuits and urbanrefuse [3]. The light induced reactivity of Pb(II) complexes of or-ganic molecules relevant to those found in the soil is therefore ofinterest. In this contribution we report the first structural charac-terization of a Pb(II) flavonolate complex, [(6-Ph2TPA)Pb(3-Hfl)]ClO4 (1) and its photoinduced reactivity. The results of

Scheme

these studies are compared to those of Group 12 analogs. This typeof comparison is important, as Group 12 metals are also commonsoil contaminants (particularly Zn(II)) [3] and very few studieshave been reported wherein the coordination chemistry andreactivity of Pb(II) species versus Group 12 analogs have been re-ported [29].

In the absence of divalent metal ions, 3-HflH has been shown toundergo photorearrangement to give 3-hydroxy-3-phenyl-1,2-indandione in CH3OH or CH3CN (Scheme 3, bottom) [30,31]. Pro-longed irradiation results in a subsequent reaction wherein CO isreleased and a phthalide is formed [11,32]. Cornard and co-work-ers have previously reported that methanol solutions of Pb(II) hin-der the visible-light (kirr � 400 nm) induced photodegradation of3-HflH, whereas solutions containing Zn(II) have no effect [6].Addition of water to these reactions was reported to enhance therate of photodecomposition in the presence of Pb(II), but had onlya very minor effect on the Zn(II)-containing reaction. The presenceof O2 was found to enhance the rate of photodecomposition of3-HflH in the presence of metal ions, particularly for Zn(II). Nota-bly, for all of these photoreactions, the primary organic productidentified was 3-hydroxy-3-phenyl-1,2-indanedione. The resultsof our reactivity studies differ with those reported by Cornard[6], most notably in that we observe photoinduced oxidativedegradation of the Pb(II) flavonolate (producing a depside (O-bs)product) as the primary reaction under both stoichiometric and

3.

K. Grubel et al. / Inorganica Chimica Acta 407 (2013) 91–97 97

catalytic conditions. Initially we hypothesized that we might beobserving differing reactivity due to the wavelength of lightemployed (kirr � 300 nm (results presented herein) versus400 nm (Cornard work)). However, we found that irradiation ofan CH3CN solution of 1 at 419 nm also results in dioxygenase-typereactivity and the formation of [(6-Ph2TPA)Pb(O-bs)]ClO4 (5) andCO. Alternatively, the observed difference in reactivity between[Pb(II)(3-Hfl)]+ and 5 may be due to the different solventsemployed in the photochemical studies of these complexes.Differences in polarity and aprotic/protic nature of the solventhave been previously reported to influence the photochemistry of3-hydroxyflavonol [31,33,30,34]. The reactivity observed byCornard for [Pb(3-Hfl)]+ in methanol is similar to the chemistryof free 3-HflH in that the same photoproduct (3-hydroxy-3-phenyl-1,2-indandione) is generated under all conditions, indicat-ing that the metal ion does not influence the reaction pathway.This suggests that an excited state [Pb(3-Hfl)]+ species in methanolmay undergo loss of the flavonolate ligand from the metal center.We have found no evidence for flavonolate dissociation in the[(6-Ph2TPA)M(3-Hfl)]ClO4 family of complexes in CH3CN [10,11],with clean dioxygenase type reactivity being identified. We havealso found that several complexes of this family, including thePb(II) complex 1 and the Zn(II) analog 4, can serve as catalystsfor the oxidative degradation of 3-HflH to give a carboxylic acidand CO. In these catalytic reactions, only a minor amount ofphthalide product resulting from flavonol isomerization isgenerated.

Overall, our results demonstrate that Pb(II), like the Group 12metal ions, can form a flavonolate complex that exhibits photoin-duced CO release reactivity. Such reactivity is relevant to possiblephotoinduced decomposition processes of organic matter in soilscontaminated with this toxic heavy metal ion.

Acknowledgements

We thank the National Science Foundation (CHE-0848858 toLMB and CHE-0847132 to RCS) for funding.

Appendix A. Supplementary material

CCDC 898824 contains the supplementary crystallographic datafor this paper. These data can be obtained free of charge from TheCambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article

can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2013.07.029.

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