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Metallathiacrown Ethers: Synthesis and Characterization of Transition-Metal Complexes Containing α,ω-Bis(phosphite)Polythioether Ligands and an Evaluation of Their Soft Metal Binding Capabilities Justin R. Martin, Aaron L. Lucius, and Gary M. Gray* Department of Chemistry, University of Alabama at Birmingham, 901 14th Street South, Birmingham, Alabama 35294-1240, United States S Supporting Information *

ABSTRACT: The metallathiacrown ethers cis-Mo(CO)4{2,2′-(C12H8O2)PO(CH2CH2S)nCH2CH2OP(2,2′-(O2H8C12))} (n = 2, 3) and cis-Mo(CO)4{2,2′(C12H8O2)POCH2CH2S-1-(C6H4)-2-SCH2CH2OP(2,2′-(O2H8C12))} have been prepared as soft metal selective molecular receptors. Multinuclear NMR spectroscopy and X-ray crystallography have been used to show that byproducts formed during the syntheses of the metallathiacrown ethers cisMo(CO)4{2,2′-(C12H8O2)PO(CH2CH2S)nCH2CH2OP(2,2′-(O2H8C12))} (n = 2, 3) are homobinuclear complexes with cis-Mo(CO)4(P-donor)(S-donor) centers. The abilities of the metallathiacrown ethers to bind PdCl2 and PtCl2 have been assessed using 31P{1H} NMR spectroscopy and X-ray crystallography. The complexes showed null results with PdCl2; however, the PtCl2 experiments showed that the complexes cis-Mo(CO)4{2,2′-(C12H8O2)PO(CH2CH2S)nCH2CH2OP(2,2′-(O2H8C12))} (n = 2, 3) formed heterobinuclear cis,cis-{[Mo(CO)4{2,2′-(C12H8O2)PO(CH2CH2S)nCH2CH2OP(2,2′-(O2H8C12))}]PtCl2} (n = 2, 3) complexes. The 31P{1H} NMR spectra of these complexes suggest a cis-PtS2Cl2 coordination environment in each, which leads to asymmetric binding in the latter complex. Binding of HgCl2 by the complexes cis-Mo(CO)4{2,2′-(C12H8O2)PO(CH2CH2S)nCH2CH2OP(2,2′(O2H8C12))} (n = 2, 3) has been studied using 31P{1H} NMR spectroscopy. Each complex was titrated with HgCl2, and the quantitative shifts in the 31P{1H} NMR spectra were fit to a binding mechanism. The n = 2 metallathiacrown ether binds HgCl2 to form a 1:1 complex with Ka = 12.0(0.2) M−1. In contrast, interaction of HgCl2 with the n = 3 metallathiacrown ether results in isomerization to the trans complex, and HgCl2 binds to both isomers. A model has been adapted to fit the titration data of this complex and to extract equilibrium constants for each step in the cycle: the cis−trans equilibrium of the free, Kfree (0.570 (0.004)), and bound, Kbound (0.16 (0.03)), metallathiacrown ethers, as well as the association binding constants for the cis, Kcis ([4.1(0.3)] × 102 M−1) and trans, Ktrans ([1.1(0.2)] × 102 M−1), metallathiacrown ethers. The equilibrium constants demonstrate that the cis metallathiacrown ether binds more strongly to the HgCl2 than does the trans metallathiacrown ether and that the cis−trans equilibrium favors the cis metallathiacrown ether.

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INTRODUCTION Due to their excellent selectivities for binding small molecules and metal cations, crown ethers have been studied for many years by both organic and inorganic chemists alike.1−3 Modification of the crown ether ring with various functional groups has been used to provide additional donor sites, increase selectivity for specific substrates, or add a spectroscopic probe.4−10 The heteroatom donor sites in these compounds have also been varied to enhance selectivities for specific classes of metal cations and metal-containing small molecules. For instance, hard metals such as the alkali metals show an increased selectivity for hard donor atoms such as oxygen (crown ether)1 and nitrogen (azacrown ether),2,11 whereas metals such as those found in the fifth and sixth rows of the periodic table more strongly bind to soft donor atoms such as sulfur (thiacrown ether).12−28 Due to their soft donor © XXXX American Chemical Society

capability, thiacrown ethers, in particular, have found a wide variety of applications from toxic heavy metal sensing and removal agents to nanomaterials and anticancer agents.29−38 Metallacrown ethers are metallamacrocycles that are closely related to crown ethers. However, the difference is that they contain a transition-metal center in place of one of the organic bridging groups. These complexes are often formed by the bidentate chelation of α,ω-bis(phosphorus-donor)-polyether ligands to transition metals. Like the crown ethers, the metallacrown ethers prefer to bind hard metal cations, but the presence of a transition-metal center modifies their selectivity and also provides additional spectroscopic probes, such as the 31P{1H} NMR chemical shift, that allow the binding Received: August 11, 2015

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DOI: 10.1021/acs.organomet.5b00693 Organometallics XXXX, XXX, XXX−XXX

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Organometallics to be evaluated both qualitatively and quantitatively.39−55 Hard metal cation binding also has been reported to affect the activities and selectivities of metallacrown ether catalysts in alkene hydrogenation and hydroformylation reactions.54−58 Due to their preference for hard metal cations, only one example of metallacrown ethers binding metals from the fifth and sixth rows of the periodic table has been reported.59 To prepare metallacrown ether like compounds that are more capable of binding such soft metals, we have begun an investigation into metallathiacrown ethers: that is, transitionmetal complexes in which α,ω-bis(phosphite)-polythioether ligands are coordinated to the metal center through the phosphite groups. Only one example of a metallathiacrown ether, in which Ph2P(CH2CH2S)2CH2CH2PPh2 is chelated to a cis-tetracarbonylmolybdenum(0) center, has been reported in the literature.60 Reid and co-workers have suggested, solely on the basis of the 31P{1H} NMR spectra of the reaction mixture of cis-Mo(CO)4(nbd) (nbd = norbornadiene) and the ligand, that this complex and a binuclear complex, in which the ligand is coordinated to the metal center through both P and S donor groups, form simultaneously. In this paper, we report the synthesis and complete characterization, using multinuclear NMR spectroscopy and X-ray crystallography, of three metallathiacrown ethers containing α,ω-bis(phosphite)-polythioether ligands. One of the byproducts has also been isolated and characterized. Finally, an evaluation of the ability of these metallathiacrown ethers to bind palladium(II) and platinum(II) chloride has been conducted using multinuclear and multidimensional NMR spectroscopy and X-ray crystallography, and their abilities to bind mercury(II) chloride have been assessed using 31P{1H} NMR spectroscopic titration experiments. Association binding constants, Ka, have been determined by fitting the data from the titration experiments to appropriate binding models using nonlinear least-squares methods.

protons in the ligands. The four methylene protons adjacent to these are observed as a 1:2:1 triplet in the 2.70−3.10 ppm range. The remaining methylene protons in L1 and L3 are observed as a singlet at 2.65 ppm, whereas the phenylene protons in L2 give rise to a doublet of doublets in the aromatic region. Because no unexpected resonances were observed in either the 31P{1H} or 1H NMR spectra, the ligands were used as is in the syntheses of the metal complexes 1−3 without further purification. Synthesis and NMR Characterization of Metallathiacrown Ethers 1−3 and Binuclear Complexes 4 and 5. The metallathiacrown ether complexes were prepared by reacting the α,ω-bis(phosphite)-polythioether ligands L1−L3 with the cis-Mo(CO)4(nbd) precursor, as shown in Scheme 2. Scheme 2. Synthesis of Metallathiacrown Ethers 1−3 and Binuclear Complexes 4 and 5

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RESULTS AND DISCUSSION Synthesis and NMR Characterization of α,ω-Bis(phosphite)-Polythioether Ligands L1−L3. The ligands L1−L3 have been prepared by the reactions of the appropriate dialcohols with 2,2′-biphenylylenephosphochloridite ester in THF in the presence of triethylamine, as shown in Scheme 1. A

These reactions were carried out by initially mixing cisMo(CO)4(nbd) and the appropriate ligand in a 1:1 mol ratio in DCM and then adding sufficient cis-Mo(CO)4(nbd) to coordinate any remaining free phosphite, if necessary. The reaction of L2 with cis-Mo(CO)4(nbd) yielded 2 with a singlet 31 1 P{ H} NMR resonance at 174.56 ppm. The chemical shift of this resonance is consistent with the previously reported metallacrown ether analogue.57 The 13C{1H} NMR spectra of 2 displays the expected apparent quintet (A portion of an AXX′ spin system) and triplet (A portion of an AX2 spin system) resonances in the downfield carbonyl region consistent with cis coordination of two equivalent phosphorus-donor ligands to a Mo(CO)4 center. The reaction of L3 with cis-Mo(CO)4(nbd) yielded approximately equal amounts of two complexes that were separated by column chromatography. Each component was fully characterized. The complex giving rise to the upfield 31 1 P{ H} NMR resonance was assigned as the metallathiacrown ether 3 on the basis of the similarity of the chemical shift of its 31 1 P{ H} NMR resonance (171.44 ppm) to that of the metallacrown ether analogue (170.96 ppm).49,52 Its carbonyl 13 C{1H} resonances, shown in the lower spectrum of Figure 1, are an apparent quintet (A portion of an AXX′ spin system)

Scheme 1. Synthesis of α,ω-Bis(phosphite)-Polythioether Ligands L1−L3

single 31P{1H} NMR resonance is observed for each ligand, and the chemical shifts are comparable to those of similar bis(phosphite) ligands derived from 2,2′-biphenylylenephosphochloridite ester.49,52,57,58 The 1H NMR spectra of the ligands exhibit a broad multiplet in the 4.00−4.10 ppm region that is reduced to a well-defined AB quartet when the spectra are decoupled from 31P, indicating that the protons of the O− CH2 groups are chemically inequivalent to the other methylene B


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Organometallics

Figure 1. 13C{1H} NMR stack plot outlining differences in the AXX′ and AX2 carbonyl splitting patterns in 3 and AX carbonyl splitting patterns in 5.

and triplet (A portion of an AX2 spin system) consistent with cis coordination of two equivalent phosphorus-donor ligands to a Mo(CO)4 center. The structure of the compound giving rise to the downfield singlet resonance at 177.62 ppm, 5, was assigned on the basis of its carbonyl 13C{1H} resonances (upper spectrum of Figure 1). Unlike the apparent quintet and 1:2:1 triplet observed in the 13 C{1H} NMR spectrum of 3, compound 5 exhibits three doublets in a 1:1:2 ratio. These resonances are consistent with a complex that has a cis-Mo(CO)4(P-donor)(S-donor) metal center, as previously pointed out by Reid and co-workers.60 Each doublet is the A portion of an AX spin system, with the most upfield doublet assigned to the four carbonyls that are trans to other carbonyls. Additionally, the |2JPC| coupling constant is indicative of 90° P−C coupling. The most downfield doublet has a |2JPC| coupling constant similar to that of the most upfield doublet but is approximately half as intense; therefore, it is assigned to the two carbonyls trans to sulfurs. The remaining doublet, which has the largest |2JPC| coupling constant, is assigned to the two carbonyls trans to phosphorus consistent with 180° P−C coupling. The reaction of L1 with cis-Mo(CO)4(nbd) also yielded one major and one minor product, and the minor product was removed during the purification of 1. The major product is the metallathiacrown ether 1 and has a singlet 1P{1H} NMR resonance whose chemical shift (171.94 ppm) is similar to that of the metallacrown ether analogue (169.30 ppm).69 The 13 C{1H} NMR spectra of 1 displays the expected apparent quintet (A portion of an AXX′ spin system) and triplet (A portion of an AX2 spin system) resonances in the downfield carbonyl region consistent with cis coordination of two equivalent phosphorus-donor ligands to a Mo(CO)4 center. The chemical shift of the 31P{1H} NMR resonance of the minor product is observed at 177.62 ppm. The similarity of this chemical shift to that of 5 suggests that this minor product is cis,cis-{[Mo(CO)4(2,2′-(C12H8O2)PO(CH2CH2S)]2(CH2CH2)} (4). X-ray Crystal Structures of Metallathiacrown Ethers 1−3 and the Binuclear Complex 5. To better understand the conformations of the metallathiacrown ethers, suitable single crystals of 1−3 were analyzed by X-ray crystallography. The solid-state molecular structures of 1−3 are shown in Figures 2−4, respectively, and selected bond distances and angles and torsion angles are given in Table 1. Compound 1 crystallizes in the triclinic space group P1̅, 2 crystallizes in the monoclinic space group P21/n, and 3 crystallizes in the monoclinic space group P21/c. The slightly distorted cis-

Figure 2. X-ray crystal structure of 1. Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms have been omitted for clarity.


octahedral coordination environments of the Mo(CO)4(Pdonor)2 centers in 1−3 are comparable to those of the previously reported metallacrown ethers.49,52,57,58 The torsion angle data for 1−3 (Table 1) provide some insight into the structural features and potential binding modes of these complexes. For instance, both O−C−C−S torsions of 1 adopt gauche conformations that more closely resemble those of the metallacrown ether complexes.53,69 This is in contrast to 2 and 3, which adopt one gauche and one anti O−C−C−S torsion angle. The O−C−C−S torsion angles of 2 and 3 more closely resemble those of organic oxathiacrown ethers, which arise due to a conformational compromise between the longer carbon−sulfur bond distances (approximately 1.8 Å for C−S vs 1.4 Å for C−O).17 Moreover, a common structural feature of organic oxathiacrown ethers is adopted in the flexible C


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Organometallics

ethers to bind soft metal cations and small molecules,12−38 the ability of the new metallathiacrown ethers 1−3 to bind group X soft metal chlorides of the type MCl2 (M = Pd, Pt) has been evaluated using 31 P{ 1 H} and 1 H NMR spectroscopy. Surprisingly, no changes in either the 31P{1H} or 1H NMR spectra were observed upon addition of PdCl2 to solutions of 1−3, suggesting that PdCl2 does not strongly bind to the metallathiacrown ethers. Similar experiments with PtCl2 and 2 produced similar results; however, the 31P{1H} and 1H NMR spectra of 1 and 3 indicated that PtCl2 binding was occurring when PtCl2 was added to solutions of these complexes. Unlike PdCl2, PtCl2 binds to both 1 and 3 (Scheme 3), as demonstrated by the 31P{1H} and 1H NMR spectra of the compounds. Symmetric PtCl2 binding to the two thioethers in 1 yields 6, which exhibits a singlet resonance in the 31P{1H} NMR spectrum that is shifted 2.02 ppm upfield relative to that of 1. Additionally, if a less than stoichiometric amount of the PtCl2 is added to a solution of 1, 31P{1H} NMR resonances for both 1 and 6 are observed, indicating that any exchange of the PtCl2 is slow on the NMR time scale. Thus, the amount of PtCl2 relative to the amount of 1 can be determined from the integrations of the two 31P{1H} NMR resonances as long as a quantitative 31P{1H} NMR spectrum (recycle time greater than 5T1 with no NOE) is run. In contrast to PtCl2 binding to 1, which yields a singlet 31 1 P{ H} NMR resonance, PtCl2 binding to 3 yields a complex, 7, that displays two doublets in an AX splitting pattern in its 31 1 P{ H} NMR spectrum. There is no observable platinum− phosphite coupling in the 31P{1H} NMR spectrum of 7, suggesting that the cis-Mo(CO)4(phosphite)2 portion of this complex is conserved. Therefore, the inequivalent phosphite donors must arise from an asymmetric PtCl2 coordination environment in which the platinum(II) is bound to two adjacent sulfur atoms of the thiacrown ring. On the basis of other complexes in which PtCl2 is bound to thiacrown ethers,23,70,71 the ligand environment about the platinum(II) in 6 and 7 is most likely cis. As is the case for 1, 31P{1H} NMR resonances for both 3 and 7 are observed when an excess of 3 is present, and the relative amount of PtCl2 to 3 can be determined from the integrations of the resonances in a quantitative 31P{1H} NMR spectrum. The contrasting PtCl2 binding modes in 6 and 7 are a function of the number of sulfur atoms available for complexation, with the trithiacrown ether in 7 offering a larger number of binding options than the dithiacrown ether in 6. The closest analogues of these complexes in the literature are those studied by Grant, Blake, and Lee. Grant and co-workers have performed extensive studies of the coordination of [9]aneS3 with group X transition metals containing a variety of spectator ligands. Results of these studies show that these transition metals bind two sulfur atoms in a cis configuration and that some of the complexes may also exhibit an apical interaction with the third sulfur atom to yield a distorted-square-pyramidal coordination geometry.13−16,22−24 The studies of Blake and Lee and co-workers show that group X transition metals bind only to the sulfur atoms of oxathiacrown ether ligands. When these ligands contain only two sulfur donors, the conformation of the crown will change such that the two sulfur atoms will coordinate cis to one another. Oxathiacrowns containing three sulfur atoms will coordinate to the transition metal in an asymmetric fashion in which two adjacent sulfur donors are cis coordinated, leaving one sulfur atom unbound.71−73 The PtCl2 binding in 7 most closely resembles that of the larger


metallathiacrown ethers 1 and 3 in that all of the S−C−C−S torsion angles of these complexes adopt anti conformations.17 This orients the lone pair electrons on the sulfur atoms in opposite directions from each other, indicating that additional soft metal binding will not occur without significant conformational changes within the thioether backbone of these complexes. The X-ray crystal structure of the bimetallic byproduct 5 was also determined and is shown in Figure 5. Selected bond distances and angles and torsion angles are given in Table 2. Compound 5 crystallizes in the monoclinic P21/c space group, and the coordination environment about the molybdenum centers is a slightly distorted octahedron. The crystallographic data confirm the bimetallic cis-Mo(CO)4(P-donor)(S-donor) coordination environment in which three chemically inequivalent carbonyl groups are present, which is consistent with the NMR spectra of this complex. The metal centers are bridged by a CH2CH2SCH2CH2 group, separating the molybdenum atoms by 6.175(10) Å. An interesting structural feature in this complex is the two six-membered chelate rings formed by the mixed P/S donation to the metal centers. Both chelates are present in distorted-twist-chair conformations that are defined by three least-squares planes (chelate 1, base O(9), C(30), S(1), P(1), foot O(9), C(29), C(30), head P(1), Mo(1), S(1); chelate 2, base O(10), C(35), S(3), P(2), foot O(10), C(36), C(35), head P(2), Mo(2), S(3)). The foot of each chelate is only slightly distorted (chelate 1, 68.53°; chelate 2, 69.51°) from ideal (60.00°), whereas the molybdenum-containing head lies flatter (chelate 1, 28.54°; chelate 2, 17.02°) than ideal (60.00°) because of longer P−Mo and S−Mo bond lengths. The basal atoms are also distorted from the least-squares plane, with the largest deviation being O(9) in chelate 1 (0.116 Å), and O(10) in chelate 2 (0.055 Å). Moreover, the signs of equivalent torsion angles in these respective areas of the complex are reversed, indicating that the opposite hands of the two chelate rings are present. Binding of MCl2 (M = Pd, Pt) by the Metallathiacrown Ethers. Due to the propensity of the sulfur atoms in thiacrown D


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Organometallics Table 1. Selected Bond Distances (Å) and Angles (deg) and Torsion Angles (deg) for 1−3 Mo−P(1) Mo−P(2) Mo−C(1) Mo−C(2) Mo−C(3) Mo−C(4) C(1)−O(1) C(2)−O(2) C(3)−O(3) C(4)−O(4) P(1)−O(9) P(2)−O(10) C(1)−Mo−P(1) C(1)−Mo−P(2) C(2)−Mo−P(1) C(2)−Mo−P(2) C(3)−Mo−P(1) C(3)−Mo−P(2) C(4)−Mo−P(1) C(4)−Mo−P(2) P(1)−Mo−P(2) Mo−P(1)−O(9) Mo−P(2)−O(10) P(2)−Mo−P(1)−O(9) P(1)−Mo−P(2)−O(10) Mo−P(1)−O(9)−C(29) P(1)−O(9)−C(29)−C(30) O(9)−C(29)−C(30)−S(1) C(29)−C(30)−S(1) − C(31) C(34)−C(33)−S(2)−C(32) C(33)−S(2)−C(32)−C(31) S(2)−C(32)−C(31)−S(1) C(32)−C(31)−S(1)−C(30) Mo−P(2)−O(10)−C(34) Mo−P(2)−O(10)−C(36) P(2)−O(10)−C(34)−C(33) P(2)−O(10)−C(36)−C(35) O(10)−C(34)−C(33)−S(2) O(10)−C(36)−C(35)−S(3) C(36)−C(35)−S(3)−C(34) C(35)−S(3)−C(34)−C(33) S(3)−C(34)−C(33)−S(2)

1

2

3

2.4363(6) 2.4242(6) 2.023(3) 2.022(2) 2.058(2) 2.046(2) 1.140(3) 1.140(3) 1.141(3) 1.133(3) 1.5923(16) 1.5918(16) 176.12(6) 89.70(7) 87.95(7) 173.32(6) 91.70(7) 94.76(6) 92.09(7) 88.61(6) 91.24(2) 115.89(7) 115.30(6) 65.80(6) −93.39(6) −173.86(15) 127.55(18) 67.2(2) −79.3(2) 117.34(19) −65.1(2) −179.9(1) −150.0(2) −157.89(16)

2.4398(5) 2.4455(5) 2.036(2) 2.008(2) 2.046(2) 2.048(2) 1.135(3) 1.145(3) 1.140(3) 1.138(3) 1.5970(15) 1.5890(15) 176.13(7) 93.83(6) 87.96(6) 175.66(6) 91.22(7) 89.16(6) 89.23(6) 93.59(6) 90.038(18) 122.33(6) 114.44(6) −31.04(6) −61.90(7) −64.08(17) −142.31(15) −174.12(14) −81.80(18) 67.99(18) −163.2(2) −0.4(3) 94.9(2) −179.32(15)

2.4216(6) 2.4272(6) 2.026(3) 2.019(3) 2.034(3) 2.046(3) 1.139(4) 1.141(3) 1.142(4) 1.138(4) 1.5848(18) 1.581(2) 178.21(9) 88.28(8) 90.23(7) 177.13(8) 89.91(8) 92.26(8) 88.07(8) 85.66(8) 91.33(2) 114.51(7) 113.05(8) 0.06(6) −43.34(6) −173.0(2) −172.35(19) −178.57(19) 79.6(2) 74.5(3) 78.8(2) −172.7(2) 81.6(2)

−135.73(18)

150.71(15)

−61.6(2)

53.6(2)

−164.8(2) −158.5(2) −69.8(3) −79.7(3) 156.4(3) −174.1(2)

of the complex. The fact that all of these protons can be observed in 7 indicates that there is no Pt−S exchange occurring on the NMR time scale. A very unusual property of 7 is the extreme sensitivity of the chemical shifts of the 31P{1H} NMR resonances of this complex to the nature of the solvent (Figure 6). The largest separation between the two resonances is observed in tetrahydrofuran-d8 and results in an AX pattern. This separation decreases, giving rise to an AM pattern in acetonitrile-d3, and continues to decrease in chloroform-d and dichloromethane-d2, giving rise to AB patterns. As would be expected, the |2JPP| coupling constant is unchanged in all of the solvents studied. This does not appear to be due to platinum(II) exchange among the sulfur donors, because the methylene carbons in 7 remain chemically inequivalent and the methylene protons remain diastereotopic. Perhaps most interesting is the fact that these effects also do not appear to be due to changes in dipole moment because

oxathiacrown ethers because of the asymmetric binding and lack of any apical interactions with the unbound sulfur atom in this complex. The 1H NMR spectra of 6 and 7 are more complex than those of their respective precursor complexes 1 and 3 due to significant conformational changes in the thiacrown ether rings that occur upon binding of PtCl2. Symmetrical PtCl2 binding in 6 causes the methylene protons in the OCH2CH2S portions of the thiacrown ring to become diastereotopic. This means that four resonances can now be observed in this portion of the spectrum, with each resonance integrating to two protons, corresponding to one of the two methylene protons on each of the symmetrical ethylene carbons in this portion of the ring. On the other hand, asymmetrical PtCl2 binding in 7 causes these same methylene resonances to become diastereotopic as well as chemically inequivalent, resulting in eight distinct resonances, each corresponding to one proton in the OCH2CH2S portions E


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Organometallics

Scheme 3. Reaction of Metallathiacrown Ethers 1 and 3 with PtCl2

ence occurs in tetrahydrofuran-d8. A possible explanation is that these changes in the 31P{1H} NMR spectrum are due to solvent-induced changes in the conformation of the chelate ring in 7. The asymmetric PtCl2 binding in 7 was confirmed by X-ray crystallography. The solid-state structure of 7 is shown in Figure 7, and selected bond distances and angles and torsion angles are given in Table 3. This compound crystallizes in the triclinic P1̅ space group with the asymmetric unit consisting of two molecules of 7 and two molecules of dichloromethane. The coordination environment about the molybdenum(0) center is a slightly distorted octahedron similar to those observed in 1− 3. The platinum(II) center in this complex is a slightly distorted


acetonitrile-d3 has a much larger dipole moment than does tetrahydrofuran-d8; however, the largest chemical shift differ-

Table 2. Selected Bond Distances (Å) and Angles (deg) and Torsion Angles (deg) for 5 Mo(1)−P(1) 2.398(2) Mo(2)−P(2) 2.439(2) Mo(1)−S(1) 2.538(2) Mo(2)−S(3) 2.544(2) Mo(1)−C(1) 2.026(9) Mo(2)−C(37) 2.002(9) Mo(1)−C(2) 1.991(9) Mo(2)−C(38) 1.982(10) Mo(1)−C(3) 2.059(9) Mo(2)−C(39) 2.037(10) Mo(1)−C(4) 2.022(10) Mo(2)−C(40) 2.045(10) C(1)−O(1) 1.129(11) C(1)−Mo(1)−P(1) 174.7(3) C(1)−Mo(1)−S(1) 90.7(3) C(2)−Mo(1)−P(1) 91.9(3) C(2)−Mo(1)−S(1) 175.1(3) C(3)−Mo(1)−P(1) 93.2(3) C(3)−Mo(1)−S(1) 86.6(3) C(4)−Mo(1)−P(1) 85.2(3) C(4)−Mo(1)−S(1) 92.3(3) C(37)−Mo(2)−P(2) 173.9(3) C(37)−Mo(2)−S(3) 95.8(3) C(38)−Mo(2)−P(2) 92.1(3) C(38)−Mo(2)−S(3) 174.3(3) Mo(1)−S(1)−C(30)−C(29) Mo(1)−P(1)−O(9)−C(29) P(1)−Mo(1)−S(1)−C(30) P(1)−O(9)−C(29)−C(30) S(1)−C(30)−C(29)−O(9) S(1)−Mo(1)−P(1)−O(9) Mo(2)−S(3)−C(35)−C(36) Mo(2)−P(2)−O(10)−C(36) P(2)−Mo(2)−S(3)−C(35) P(2)−O(10)−C(36)−C(35) S(3)−C(35)−C(36)−O(10) S(3)−Mo(2)−P(2)−O(10)

C(37)−O(11) C(2)−O(2) C(38)−O(12) C(3)−O(3) C(39)−O(13) C(4)−O(4) C(40)−O(14) P(1)−O(9) P(2)−O(10) S(1)−C(30) S(1)−C(31) S(3)−C(35) S(3)−C(34) C(39)−Mo(2)−P(2) C(39)−Mo(2)−S(3) C(40)−Mo(2)−P(2) C(40)−Mo(2)−S(3) S(1)−Mo(1)−P(1) S(3)−Mo(2)−P(2) Mo(1)−P(1)−O(9) Mo(1)−S(1)−C(30) Mo(1)−S(1)−C(31) Mo(2)−P(2)−O(10) Mo(2)−S(3)−C(35) Mo(2)−S(3)−C(34)

1.144(11) 1.130(11) 1.152(12) 1.131(11) 1.145(12) 1.143(12) 1.113(13) 1.596(7) 1.605(6) 1.832(11) 1.806(9) 1.827(9) 1.810(9) 97.7(3) 87.5(3) 87.1(3) 95.5(3) 90.14(8) 86.62(7) 119.5(3) 111.1(4) 106.4(3) 120.3(2) 109.4(3) 113.9(3) −49.6(11) 42.6(9) −33.5(1) −80.9(13) 83.9(13) 24.5(1) 66.2(7) −44.4(7) 19.5(2) 74.7(9) −86.6(8) −15.3(1)

F


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Organometallics

Table 3. Selected Bond Distances (Å) and Angles (deg) and Torsion Angles (deg) for 7 Mo−P(1) 2.4417(12) Mo−P(2) 2.4430(12) Mo−C(1) 2.011(4) Mo−C(2) 2.028(5) Mo−C(3) 2.060(5) Mo−C(4) 2.023(5) C(1)−O(1) 1.141(5) C(2)−O(2) 1.133(6) C(1)−Mo−P(1) 175.12(13) C(1)−Mo−P(2) 88.97(13) C(2)−Mo−P(1) 89.45(13) C(2)−Mo−P(2) 177.51(13) C(3)−Mo−P(1) 93.66(13) C(3)−Mo−P(2) 93.22(14) C(4)−Mo−P(1) 88.17(13) C(4)−Mo−P(2) 87.96(3) P(2)−Mo−P(1)−O(9) P(1)−Mo−P(2)−O(10) Mo−P(1)−O(9)−C(29) P(1)−O(9)−C(29)−C(30) O(9)−C(29)−C(30)−S(1) C(29)−C(30)−S(1)−C(31) Mo−P(2)−O(10)−C(36) P(2)−O(10)−C(36)−C(35) O(10)−C(36)−C(35)−S(3) C(36)−C(35)−S(3)−C(34) C(34)−C(33)−S(2)−C(32) S(1)−Pt−S(2)−C(32) S(2)−Pt−S(1)−C(31) S(1)−Pt−S(2)−C(33) S(2)−Pt−S(1)−C(30) S(2)−C(33)−C(34)−S(3) S(1)−C(31)−C(32)−S(2)

Figure 6. Depiction of changes in the δ(31P{1H}) NMR spectrum of 7 with respect to solvent.

Figure 7. X-ray crystal structure of 7. Only one of the two independent molecules is shown. Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms and dichloromethane molecules have been omitted for clarity.

C(3)−O(3) C(4)−O(4) P(1)−O(9) P(2)−O(10) Pt−Cl(1) Pt−Cl(2) Pt−S(1) Pt−S(2) P(1)−Mo−P(2) Mo−P(1)−O(9) Mo−P(2)−O(10) Cl(1)−Pt−Cl(2) Cl(1)−Pt−S(1) Cl(2)−Pt−S(2) S(1)−Pt−S(2)

1.146(5) 1.125(5) 1.598(3) 1.585(3) 2.3035(12) 2.3180(11) 2.2478(11) 2.2588(11) 91.67(4) 113.84(12) 112.39(13) 91.13(4) 176.76(4) 178.14(4) 89.87(4)

−88.53(3) 91.86(8) −162.9(3) −139.5(3) −86.2(4) 61.7(4) 178.4(3) 132.6(3) 74.3(4) 175.4(3) −79.5(4) 19.1(2) 6.0(2) 125.7(2) −101.6(1) 71.8(4) 57.0(4)

showed no interaction between the soft metal and the metallathiacrown ether. HgCl2 Binding with 1 Followed by 31P{1H} NMR Spectroscopy. The choice of solvent is crucial to accurately measuring binding constants.52 Toluene was first evaluated for titration experiments involving 1 and HgCl2, due to the fact that it does not strongly interact with HgCl2. However, the solubility of HgCl2 in toluene is limited (16 mM at ambient temperature).74 This severely hindered the titration experiments because the inflection point of the titration curve could not be reached at concentrations of 1 as low as 0.8 mM. To overcome this problem, a solvent mixture of 8.5 M ACN in toluene was employed to increase the solubility of HgCl2. ACN was chosen due to its ability to sufficiently solubilize 1 and also because addition of this solvent resulted in less quenching of the observed binding signal than other solvents that were studied. In contrast to the binding of PtCl2 to 1, a single 31 1 P{ H} NMR resonance is observed regardless of the ratio of HgCl2 to 1 present and the chemical shift of this resonance is dependent on the ratio of HgCl2 to 1 when insufficient HgCl2 is present to saturate 1. This behavior is consistent with fast exchange between HgCl2 and 1 on the NMR time scale. A continuous variation Job study was first carried out at [1] = 3 mM in the solvent system described above in order to demonstrate the stoichiometry of any observed binding events.67,68 Figure 8B shows that the Job curve has a well-

square plane in which the ligands are arranged in a cis configuration. This results in the formation of a five-membered chelate ring via the two Pt−S bonds. The five-membered ring is oriented in a twist-envelope conformation in which the sulfur atoms are approximately gauche to one another, resulting in C(32) positioned 0.213 Å above the S(1)−Pt−S(2) leastsquares plane and C(31) positioned 0.554 Å below it. The torsion angles associated with this confirmation are comparable to those reported for similar S2PdCl2 coordination environments in organic oxathiacrown ethers.71,72 Perhaps most interesting are the extensive conformational changes observed in the thioether backbone of 7 relative to its precursor complex, 3. It is rather apparent through comparison of both crystal structures that the anti S−C−C−S torsion angles of 3 (Table 1) have adopted gauche torsion angles in 7 to allow for the binding of PtCl2. This is even observed in the S(2)−C(33)− C(34)−S(3) torsion angle in 7, which is not directly involved in chelating the platinum(II) metal center. Binding of HgCl2 by the Metallathiacrown Ethers. On the basis of the ability of 1 and 3 to bind PtCl2, it seemed likely that these metallathiacrown ethers could also bind HgCl2. The abilities of 1 and 3 to bind HgCl2 were evaluated using 31P{1H} NMR titration experiments similar to those recently reported by our group.52 Solutions of 2 were also exposed to HgCl2 but G


Article

Organometallics defined maximum at a 0.5 mol fraction of HgCl2, indicating one binding event with a 1:1 stoichiometry in this solvent.52,68

Figure 9. 31P{1H} NMR spectral changes of 3 (5 mM) upon addition of HgCl2 in toluene-d8, displaying signals corresponding to the trans (downfield) and cis (upfield) isomers.

Figure 8. (A) Nonlinear least-squares fit of normalized 31P{1H} signal change vs total HgCl2 concentration. Blue data points are experimental titration values, and the solid line represents a nonlinear least-squares regression of the titration data. (B) 31P{1H} NMR continuous variation Job plot of 1 vs mole fraction of HgCl2.

the observed interactions of 3 with HgCl2 must be due to direct interactions of HgCl2 with the sulfur atoms. The fact that the equilibrium trans:cis 31P{1H} NMR integral ratio decreases as the HgCl2 concentration is increased indicates that HgCl2 binds more strongly to the cis isomer of 3. Assuming that only 1:1 3·HgCl2 adducts are formed under the conditions studied, the 31P{1H} NMR spectra (Figure 9) suggest a binding equilibrium involving at least four species.79 As depicted in Scheme 4, the equilibria among these four

Titrations of 1 with HgCl2 were then carried out by adding aliquots of a solution containing both 1 (3 mM) and HgCl2 (0.5 M) in a solvent mixture of 8.5 M acetonitrile-d3 in anhydrous toluene to a solution containing an identical solvent mixture and only 1 (3 mM) in a 5 mm gastight J-Young NMR tube. Figure 8A shows the nonlinear least-squares fit of the titration data using eq 3 (vide infra). The binding analysis is consistent with the Job plot in that it is indicative of one binding event with a 1:1 stoichiometry. The observed binding affinity, Ka, is 12.0(0.2) M−1. Given the oxophobicity of mercury(II),34 the binding of HgCl2 with 1 most likely involves bidentate chelation of a single mercury(II) center, resulting in a tetrahedral Cl2HgS2 coordination environment.75,76 HgCl2 Binding with 3 Followed by 31P{1H} NMR Spectroscopy. The binding of HgCl2 to 3 is much more complicated than is its binding to 1. This is apparent upon addition of a solution of HgCl2 to a solution of 3, which results in a new downfield 31P{1H} NMR resonance. This resonance was assigned as that of the trans isomer of 3 due to the appearance of a new 1:2:1 triplet resonance in the carbonyl region of the 13C{1H} NMR spectrum which exhibited a |2JCP| coupling constant that is similar to those reported for other trans-Mo(CO)4(P-donor)2 complexes.77,78 The rate of this isomerization was very slow (reached equilibrium in 48 h) at low concentrations of HgCl2 (0.50 mM) and increased as the concentration of HgCl2 was increased to the point at which the isomerization reached equilibrium in less than 100 s at the highest HgCl2 concentration (15 mM). In addition, the 31 1 P{ H} NMR singlet resonances of both the trans and cis isomers of 3 were observed to shift upfield as the concentration of HgCl2 increased, as depicted in Figure 9. This indicates that simultaneous HgCl2 binding was occurring with both isomers of 3 and is consistent with fast exchange between HgCl2 and both isomers on the NMR time scale. The observation of cis−trans isomerization for 3 and binding of HgCl2 to both isomers was somewhat surprising, due to the fact that no isomerization or HgCl2 binding by the metallacrown ether analogue of 3 were observed.49 This implies that

Scheme 4. Proposed Binding Equilibria between 3 and HgCl2

species can be separated into two groups: (1) cis−trans isomerization equilibria of both the free, Kfree, and bound, Kbound, species and (2) binding association constants for both the cis, Kcis, and trans, Ktrans, isomers. The 31P{1H} NMR spectroscopic data was evaluated in terms of the observed equilibrium constant, Kobs, which was taken from the NMR integration values and is representative of the ratio of the total equilibrium concentrations of both the cis, [cis-3]T, and trans, [trans-3]T, isomers, and the total amount of HgCl2 added, [HgCl2]T. The Kobs value was then related to the parameters Kfree, Kcis, and Ktrans by eq 1.79 Kobs =

K free(1 + K trans[HgCl 2]free ) 1 + Kcis[HgCl 2]free

(1)

Figure 10 depicts the fitting of the P{ H} NMR data according to Scheme 4 and eq 1 (vide infra). Nonlinear leastsquares regression analysis of the experimental data using eq 1 yielded the parameters Kfree, Kcis and Ktrans as 0.570 (0.004), 31

H

1


Article

Organometallics

relation to the free compound (Kbound < Kfree). These observations indicate that, while the interaction of HgCl2 with 3 results in a cis−trans isomerization of 3 to form the trans isomer, the cis material is a superior binding agent, and further addition of HgCl2 to the point of excess results in a reduction of trans isomer formation that approaches the bound state equilibrium, Kbound.

■

CONCLUSION The first metallathiacrown ether complexes in which the thioether moiety is coordinated to a molybdenum(0) center through α,ω-bis(phosphite) donors have been prepared by methods similar to those used for the corresponding metallacrown ethers. A bimetallic side product in which a sixmembered ring is formed through S−Mo−P chelation to the cis-tetracarbonylmolybdenum(0) center has also been isolated and characterized. X-ray crystallographic studies of two of the metallathiacrown ethers with aliphatic thioether chains, 1 and 3, demonstrated that the sulfur atoms in each of the thiacrown rings are positioned quite differently from the oxygen atoms in the corresponding metallacrown ethers. The phosphorus nuclei in 1 and 3 provide a very useful and quantitative NMR spectroscopic probe for soft metal binding to the sulfur atoms in the thiacrown ether ring, and 31P{1H} NMR has been used to study the binding of PdCl2, PtCl2, and HgCl2 to two of the metallathiacrown ethers. The differing interactions of these complexes with the three metal chloride

Figure 10. Nonlinear least-squares fit of the change in the observed cis−trans equilibrium constant values, Kobs, of 3 with increasing total HgCl2 concentration, [HgCl2]T. Blue data points are experimental values, and the solid line represents a nonlinear least-squares regression of the data according to eq 1.

[4.1(0.3)] × 102 M−1, and [1.1(0.2)] × 102 M−1, respectively. The excellent fitting of the experimental data confirms the binding stoichiometry of HgCl2 to 3 to be 1:1, as well as the validity of Scheme 4. The parameter Kbound was later calculated as 0.16(0.03) using the fitted parameters Kfree, Kcis, and Ktrans. The parameter values calculated from the nonlinear leastsquares regression analysis are consistent with the 31P{1H} NMR data in that they show a reduced association constant for the trans isomer in relation to the cis isomer (Ktrans < Kcis), as well as a lower equilibrium constant for the bound species in

Table 4. Data Collection and Structure Refinement Details for 1−3, 5, and 7 1 empirical formula formula wt temp, K wavelength, Å cryst syst space group unit cell dimens a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z calcd density, g/cm3 abs coeff, mm−1 F(000) θmax, deg cryst size, mm3 exposure time, s no. of rflns collected [Rint, %] no. of indep rflns completeness to θmax, % min/max transmissn refinement method abs cor goodness of fit final R indices (I > 2σ(I)), % R indices (all data), % largest diff peak and hole, e/Å3

2

3

7

C34H28MoO10P2S2 818.56 100(2) 1.54178 triclinic P1̅

C38H28MoO10P2S2 866.60 100(2) 1.54178 monoclinic P21/n

8.4804(3) 11.0034(5) 19.7233(10) 84.0370(10) 77.997(2) 68.9490(10) 1679.20(13) 2 1.619 5.766 832 60.01 0.079 × 0.203 × 0.212 20 11524 [1.87] 4788 95.8 0.760

12.9707(3) 15.7709(3) 14.889(5) 20.5722(4) 12.5117(3) 33.464(11) 13.4438(3) 20.0818(4) 9.693(4) 90 90 90 94.8640(10) 106.4190(10) 104.850(11) 90 90 90 3574.37(13) 3800.96(14) 4668.0(3) 4 4 4 1.610 1.532 1.656 5.458 5.637 7.383 1760 1784 2304 68.36 70.50 70.17 0.085 × 0.164 × 0.262 0.278 × 0.315 × 0.472 0.062 × 0.111 × 0.207 10 10 20 31466 [3.68] 29736 [3.33] 37078 [3.39] 6479 7062 8599 98.8 96.9 97.0 0.570 0.458 0.580 full-matrix least squares on F2 numerical 1.032 1.074 1.064 2.81 3.40 3.54 7.36 9.07 9.26 0.499, −0.450 0.675, −0.679 1.208, −0.493

1.081 2.44 6.15 0.507, −0.249

C36H30MoO10P2S3 876.66 173(2) 1.54178 monoclinic P21/c

5

I

C42H32ClMo2O14P2S2 1147.13 173(2) 1.54178 monoclinic P21/c

C78H68Mo2O20P4Pt2 S6 2459.18 100(2) 1.54178 triclinic P1̅ 10.8001(3) 19.3783(5) 22.6139(5) 69.6010(10) 80.1060(10) 88.8760(10) 4365.87(19) 2 1.871 13.012 2408 70.73 0.073 × 0.074 × 0.119 20 85233 [7.37] 15976 95.1 0.648

1.031 3.25 8.25 1.370, −0.926


Article

Organometallics

nitrogen before use. All reagents were of sufficient purity as received from the supplier. Literature procedures were used to prepare 3,6,9trithiaundecane-1,11-diol,4 1,2-bis(2-hydroxyethylthio)benzene,20 cisMo(CO)4(nbd) (nbd = norbornadiene),61 and 2,2′-diphenylylenephosphochlorodite ester.62 Characterization. NMR spectra were recorded on either a Bruker DRX 400 MHz or Avance 700 MHz spectrometer. Solutions of the complexes in deuterated solvents were prepared under a stream of N2 gas, and all NMR experiments were performed at room temperature. The 31P{1H} NMR spectra were referenced to external 85% H3PO4 in a coaxial tube that also contained CDCl3, and the 13C{1H} and 1H NMR spectra were referenced to internal SiMe4 (TMS). Some assignments were based on 2D NMR spectra (1H−13C HSQC, 1H−1H COSY). Elemental analyses (C and H) were performed by Atlantic Microlabs, Inc. Numbering of C and H assignments in the subsequent experimental descriptions are consistent with the numbers in the X-ray crystal structures of the complexes. Free ligands are numbered in the same manner as are the ligands in the complexes. 2,2′-(C12H8O2)PO(CH2CH2S)2CH2CH2OP(2,2′-(O2H8C12)) (L1). A solution of 2.53 g (10.1 mmol) of 2,2′-diphenylylenephosphochlorodite ester dissolved in 17 mL of dry THF was added dropwise to a stirred solution containing 0.700 g (3.84 mmol) of 3,6-dithia-1,8octanediol and 1.10 mL (7.69 mmol) of dry triethylamine in 50 mL of dry THF. When the addition was completed, the dropping funnel was washed with 10 mL of dry THF, and the reaction mixture was stirred for 2 h at room temperature under a constant stream of N2(g). The solution was then cannula-transferred in portions into a dry, 200 mL fritted funnel containing a mixture of Celite and basic alumina to remove the triethylammonium chloride precipitate byproduct and any phosphite hydrolysis byproducts. The solution was then filtered via positive N2 pressure into a 100 mL Schlenk flask, and the residue in the filter was washed with two 10 mL portions of dry THF. Removal of the solvent through rotary evaporation and vacuum drying yielded 2.10 g (89.5%) of spectroscopically pure L1 as a faint yellow oil. 31 1 P{ H} NMR (chloroform-d): δ 139.43 ppm (s). 1H NMR (chloroform-d): δ 7.48−7.46 ppm (m, 4H, Ar-H), δ 7.39−7.36 ppm (m, 4H, Ar-H), δ 7.29−7.24 ppm (m, 8H, Ar-H), δ 4.07 ppm (td, 4H, HC29, HC34, |3JPH| = |3JHH| = 7 Hz), δ 2.74 ppm (t, 4H, HC30, HC33, |3JHH| = 7 Hz), δ 2.66 ppm (s, 4H, HC31, HC32). 2,2′-(C 12 H 8 O 2 )POCH 2 CH 2 S-1-(C 6 H 4 )-2-SCH 2 CH 2 OP(2,2′(O2H8C12)) (L2). Using the procedure for L1, 1.65 g (6.56 mmol) of 2,2′-diphenylylenephosphochlorodite ester, 0.756 g (3.28 mmol) of 1,2-bis(2-hydroxyethylthio)benzene, and 0.919 mL (6.57 mmol) of dry triethylamine yielded 2.01 g (93.1%) of spectroscopically pure L2 as a colorless oil. 31P{1H} NMR (chloroform-d): δ 139.93 ppm (s). 1H NMR (chloroform-d): δ 7.44 ppm (dd, 4H, HC15, HC16, HC27, HC28, |3JHH| = 8 Hz, |4JHH| = 2 Hz), δ 7.33 ppm (td, 4H, HC11, HC12, HC23, HC24, |3JHH| = 8 Hz, |4JHH| = 2 Hz), δ 7.26 ppm (td, 4H, HC13, HC14, HC25, HC26, |3JHH| = 8 Hz, |4JHH| = 2 Hz), δ 7.23 ppm (dd, 2H, HC35, HC36, |3JHH| = 6 Hz, |4JHH| = 3 Hz), δ 7.16 ppm (dd, 4H, HC9, HC10, HC21, HC22, |3JHH| = 8 Hz, |4JHH| = 2 Hz), δ 7.07 ppm (dd, 2H, HC37, HC38, |3JHH| = 6 Hz, |4JHH| = 3 Hz), δ 4.06 ppm (dt, 4H, HC29, HC34, |3JPH| = |3JHH| = 7 Hz), δ 3.11 ppm (t, 4H, HC30, HC33, |3JHH| = 7 Hz). 2,2′-(C12H8O2)PO(CH2CH2S)3CH2CH2OP(2,2′-(O2H8C12)) (L3). Using the procedure for L1, 4.65 g (18.6 mmol) of 2,2′diphenylylenephosphochlorodite ester, 2.25 g (9.28 mmol) of 3,6,9trithiaundecane-1,11-diol, and 2.60 mL (18.6 mmol) of dry triethylamine yielded 5.47 g (87.8%) of spectroscopically pure L3 as a colorless oil. 31P{1H} NMR (chloroform-d): δ 139.50 ppm (s). 1H NMR (chloroform-d): δ 7.31 ppm (m, 16H, Ar-H), δ 4.05 ppm (m, 4H, HC29, HC36), δ 2.71 ppm (t, 4H, HC30, HC35, |3JHH| = 7 Hz), δ 2.65 ppm (s, 8H, HC31, HC32, HC33, HC34). cis-Mo(CO) 4 {2,2′-(C 12 H 8 O 2 )PO(CH 2 CH 2 S) 2 CH 2 CH 2 OP(2,2′(O2H8C12))} (1). A solution of 0.280 g (0.920 mmol) of cisMo(CO)4(nbd) in dry DCM was added dropwise with stirring to a solution of 0.350 g (0.570 mmol) of L1 dissolved in dry DCM. After the addition was completed, the dropping funnel was washed with more dry DCM, and the reaction mixture was stirred for 2 h under a constant stream of N2(g). Removal of the solvent in vacuo yielded the

salts allow clear differentiation of these salts in solution. The lack of any shift in the 31P{1H} NMR resonances of either 1 or 3 indicates that they do not form complexes with PdCl2. In contrast, the upfield shifts in the 31P{1H} NMR resonance of 1 when either PtCl2 or HgCl2 is added indicates that complexes with these salts are being formed. Interaction of the metallathiacrown ethers with these two metals is also clearly differentiated by the presence of two resonances for the slowly exchanging free and bound PtCl2 complexes, which contrasts with that of a single resonance representing the weighted average of rapidly exchanging free and bound HgCl2. Therefore, the amount of PtCl2 relative to that of either 1 or 3 can be readily quantitatively determined through integration of the two 31P{1H} resonances, while the amount of HgCl2 relative to that of either 1 or 3 can be quantitatively determined from the single 31P{1H} NMR resonance using the 1:1 binding models discussed. Furthermore, the nature of the 31P{1H} NMR resonance of the PtCl2 complex of 3 also provides insight into the interaction of the solvent with this complex, while the cis− trans isomerization observed with the HgCl2 complex of 3 provides additional confirmation of the presence of HgCl2. These results demonstrate that metallathiacrown ethers are an interesting new class of molecular receptors for heavy-metal compounds. Obviously, a great deal of work remains unperformed in order to fully characterize the range of the metal salt selectivity of these complexes, but the preliminary studies described in this paper suggest that they are a much more flexible class of sensor molecules for heavy-metal salts than are either their thiacrown ether or crown ether counterparts. Thus, further study of complexes of the type presented in this paper may lead to increased efficacy and specificity in the area of heavy-metal sensing.

■

EXPERIMENTAL SECTION

Materials and Methods. Tetrahydrofuran (THF) was initially dried over MgSO4 for at least 12 h and then distilled from CaH2 and

Table 5. Compositions of the Solutions Used in the Binding and Cis−Trans Isomerization Experiments with 3 [3] (mM)

[HgCl2] (mM)

HgCl2 stock soln (μL)

toluene-d8 (μL)

5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0 5.0

0.50 1.50 2.25 3.00 4.00 5.00 6.00 6.75 7.50 10.0 12.0 15.0

18.8 56.3 84.4 112.5 150.0 187.5 225.0 253.1 281.3 375.0 450.0 562.5

581.2 543.7 515.6 487.5 450.0 412.5 375.0 346.9 318.7 225.0 150.0 37.5

finally distilled from Na/benzophenone. It was stored over molecular sieves (3 Å; 8−12 mesh) and used within a few hours. Toluene was dried by distillation from Na and stored over molecular sieves (3 Å; 8−12 mesh). Dichloromethane (DCM) was dried by distillation from CaH2 and stored over molecular sieves (3 Å; 8−12 mesh). Acetonitrile (ACN) was dried by distillation from CaH2 and was stored over molecular sieves (3 Å; 8−12 mesh). Triethylamine was initially dried over KOH for a minimum of 12 h, distilled from Na/benzophenone, and stored over molecular sieves (3 Å; 8−12 mesh). Other solvents were reagent grade and were degassed using high-purity (99.998%) J


Article

Organometallics

(chloroform-d): δ 177.62 ppm (s). 1H NMR (chloroform-d): δ 7.49 ppm (d, 4H, Ar-H), δ 7.40 ppm (t, 4H, Ar-H), δ 7.32 ppm (t, 4H, ArH), δ 7.25 ppm (d, 4H, Ar-H), δ 4.45 ppm (dd, 4H, HC29, HC36, |3JPH| = 16 Hz, |3JHH| = 7 Hz), δ 3.03 ppm (t, 4H, HC30, HC35,|3JHH| = 7 Hz), δ 2.88−2.95 ppm (m, 8H, HC31, HC32, HC33, HC34). 13 C{1H} NMR (chloroform-d): δ 212.82 ppm (d, C2, C38,|2JPC|= 14 Hz), δ 211.71 ppm (d, C1, C37, |2JPC| = 51 Hz), δ 206.67 ppm (d, C3, C4, C39, C40, |2JPC| = 14 Hz), δ 148.89 ppm (t, C5, C6, C17, C18, |2JPC + 4JPC| = 9 Hz), δ 130.73 ppm (d, C7, C8, C19, C20, |3JPC| = 2 Hz), δ 130.54 ppm (s, Ar-CH), δ 129.90 ppm (s, Ar-CH), δ 126.35 ppm (s, Ar-CH), δ 122.67 ppm (d, C15, C16, C27, C28, |3JPC| = 2 Hz), δ 63.84 ppm (d, C29, C36, |2JPC + 4JPC| = 6 Hz), δ 43.78 ppm (s, C32, C33), δ 35.54 ppm (d, C30, C35, |3JPC| = 2 Hz), δ 30.78 ppm (s, C31, C34). Anal. Calcd for C40H32Mo2O14P2S3·0.2CDCl3: C, 43.47; H, 2.94. Found: C, 43.19; H, 2.94. cis,cis-{[Mo(CO)4{2,2′-(C12H8O2)PO(CH2CH2S)2CH2CH2OP(2,2′(O2H8C12))}]PtCl2} (6). A solution of 0.052 g (0.063 mmol) of 1 and 0.017 g (0.064 mmol) of PtCl2 in dry ACN was stirred at room temperature for approximately 24 h. Removal of the solvent in vacuo yielded a yellow solid as the crude product. The desired product 6 was then extracted by dissolving in dry toluene. The dissolved material was then recrystallized by slow diffusion of hexanes into a concentrated dichloromethane solution to yield 0.063 g (92%) of analytically pure 6 as a yellow powder. 31P{1H} NMR (chloroform-d): δ 169.92 ppm (s). 1 H NMR (chloroform-d): δ 7.47−7.22 ppm (m, 16H, Ar-H), δ 4.92 ppm (t, 2H, HC29, HC34, |2JHH+ 3JHH| = 11 Hz), δ 4.33 ppm (dd, 2H, HC29, HC34, |2JHH+ 3JHH| = 12 Hz), δ 3.70−3.67 ppm (m, 2H, HC31, HC32), δ 3.39−3.37 ppm (m, 2H, HC30, HC33), δ 3.00−2.95 ppm (m, 4H, HC30, HC31, HC32, HC33). 13C{1H} NMR (chloroform-d): δ 209.57 ppm (aq, C1, C2, |2JPC + 2JP′C| = 17 Hz) δ 208.38 ppm (t, C3, |2JPC + 2JPC| = 12 Hz), δ 203.61 ppm (t, C4, |2JPC + 4JPC| = 13 Hz), δ 150.10 ppm (t, C5, C6, |2JPC + 4JPC| = 5 Hz), δ 148.89 ppm (t, C17, C18, |2JPC + 4JPC| = 5 Hz), δ 130.56 ppm (s, Ar-CH), δ 130.51 ppm (s, Ar-CH), δ 130.42 ppm (s, Ar-CH), δ 130.26 ppm (s, C7, C8), δ 129.88 ppm (s, Ar-CH), δ 129.29 ppm (s, C19, C20), δ 126.47 ppm (s, Ar-CH), δ 126.15 ppm (s, Ar-CH), δ 122.21 ppm (s, Ar-CH), δ 122.09 ppm (s, Ar-CH), δ 67.55 ppm (s, C29, C34), δ 40.06 ppm (s, C31, C32), δ 37.44 ppm (s, C30, C33). Anal. Calcd for C34H28MoO10P2S2PtCl2: C, 37.65; H, 2.60. Found: C, 35.66; H, 2.45. cis,cis-{[Mo(CO)4{2,2′-(C12H8O2)PO(CH2CH2S)3CH2CH2OP(2,2′(O2H8C12))}]PtCl2} (7). A solution of 0.078 g (0.088 mmol) of 3 and 0.025 g (0.093 mmol) of PtCl2 in dry ACN was stirred at room temperature for approximately 5 h. Removal of the solvent in vacuo yielded a yellow solid as the crude product. The solid was recrystallized by slow diffusion of hexanes into a concentrated dichloromethane solution to yield 0.090 g (89%) of analytically pure 7 as yellow crystals. 31 1 P{ H} NMR (acetonitrile-d3): δ 171.98 ppm (d, |2JPP| = 49 Hz), δ 170.70 ppm (d, |2JPP| = 49 Hz). 1H NMR (acetonitrile-d3): δ 7.52− 7.20 ppm (m, 16H, Ar-H), δ 4.70−4.66 ppm (m, 1H, HC29), δ 4.36− 4.32 ppm (m, 1H, HC29), δ 4.21−4.17 ppm (m, 1H, HC36), δ 4.13− 4.09 ppm (m, 1H, HC36), δ 3.43−3.39 ppm (m, 1H, HC30), δ 3.30− 3.15 ppm (m, 6H, HC31, HC32, HC33, HC34), δ 2.98−2.93 ppm (m, 2H, HC33, HC34), δ 2.89−2.85 ppm (m, 1H, HC34), δ 2.78 ppm (t, 2H, HC35, |2JHH| = 6 Hz). 13C{1H} NMR (acetonitrile-d3): δ 210.02 ppm (aq, C1, C2, |2JPC + 2JPC′| = 34 Hz) δ 207.98 ppm (t, C4, |2JPC + 2 JPC| = 14 Hz), δ 204.79 ppm (t, C3, |2JPC + 2JPC| = 13 Hz), δ 149.90 ppm (d, C5, |2JPC+ 4JPC| = 13 Hz), δ 149.50 ppm (d, C6, |2JPC+ 4JPC| = 11 Hz), δ 149.00 ppm (q, C17, C18, |2JPC+ 4JPC| = 8 Hz), δ 130.58 ppm (s, Ar-CH), δ 130.50 ppm (s, Ar-CH), δ 130.32 ppm (s, Ar-CH), δ 130.19 ppm (s, Ar-CH), δ 130.12 ppm (s, Ar-CH), δ 130.02 ppm (s, C7), δ 129.93 ppm (s, C8), δ 129.60 ppm (s, C19), δ 129.44 ppm (s, C20), δ 126.46 ppm (s, Ar-CH), δ 126.34 ppm (s, Ar-CH), δ 126.30 ppm (s, Ar-CH), δ 126.13 ppm (s, Ar-CH), δ 122.40 ppm (d, C15, |3JPC| = 2 Hz), δ 122.30 ppm (d, C16, |3JPC| = 2 Hz), δ 122.20 ppm (d, C27, |3JPC| = 2 Hz), δ 122.00 ppm (d, C28, |3JPC| = 2 Hz), δ 67.04 ppm (d, C29, |2JPC + 4JPC| = 8 Hz), δ 66.87 ppm (d, C36, |2JPC + 4JPC) = 8 Hz), δ 38.64 ppm (s, C33), δ 37.88 ppm (s, C34), δ 36.82 ppm (d, C30, |3JPC| = 5 Hz), δ 36.07 ppm (s, C31), δ 32.99 ppm (d, C35, |3JPC| = 6 Hz), δ 30.70 ppm (s, C32). Anal. Calc. for C40H32MoO14P2S3PtCl2: C, 37.77; H, 2.82. Found: C, 37.43; H, 2.96.

crude product as a foamy white solid. This was purified first by column chromatography using silica gel as the stationary phase and a 4/1 hexanes/ethyl acetate mixture as the eluent followed by recrystallization from DCM/hexanes to yield 0.256 g (63.5%) of analytically pure 1 as colorless crystals. 31P{1H} NMR (chloroform-d): δ 171.94 ppm (s). 1H NMR (chloroform-d): δ 7.48−7.50 ppm (m, 4H, Ar-H), δ 7.35−7.39 ppm (m, 4H, Ar-H), δ 7.28−7.32 ppm (m, 8H, Ar-H), δ 4.46−4.29 ppm (m, 4H, HC29, HC34), δ 3.08 ppm (s, 4H, HC30, HC33), δ 2.82 ppm (t, 4H, HC31, HC32, |3JHH| = 7 Hz). 13C{1H} NMR (chloroform-d): δ 210.86 ppm (aq, C3, C4, |2JPC + 2JP′C| = 33 Hz), δ 206.48 ppm (t, C1, C2, |2JPC| = 27 Hz), δ 150.18 ppm (t, C5, C6, C17, C18, |2JPC + 4JPC| = 9 Hz), δ 130.62 ppm (s, Ar-CH), δ 130.55 ppm (s, C7, C8, C19, C20), δ 126.13 ppm (s, Ar-CH), δ 122.68 ppm (s, Ar-CH), δ 122.41 ppm (s, Ar-CH), δ 69.13 ppm (t, C29, C34, |2JPC + 4JPC| = 7 Hz), δ 34.63 ppm (s, C31, C32), δ 33.58 ppm (t, C30, C33, |3JPC + 5JPC| = 6 Hz). Anal. Calcd for C34H28MoO10P2S2: C, 49.88; H, 3.45. Found: C, 49.35; H, 3.19. c i s - Mo ( CO ) 4 {2 ,2 ′ - ( C 1 2 H 8 O 2 ) P O CH 2 C H 2 S - 1 - ( C 6 H 4 ) - 2SCH2CH2OP(2,2′-(O2H8C12))} (2). A solution of 0.228 g (0.760 mmol) of cis-Mo(CO)4(nbd) in dry DCM was added dropwise with stirring to a solution of 0.500 g (0.759 mmol) of L2 in dry DCM. After the addition was completed, the dropping funnel was washed with more dry DCM, and the reaction mixture was stirred for 2 h under a constant stream of N2(g). Removal of the solvent in vacuo and recrystallization of the crude product from DCM/hexanes yielded 0.431 g (65.4%) of analytically pure 2 as colorless crystals. 31P{1H} NMR (chloroform-d): δ 174.56 ppm (s). 1H NMR (chloroform-d): δ 7.36 ppm (d, 4H, HC15, HC16, HC27, HC28, |3JHH| = 7 Hz), δ 7.30− 7.31 ppm (m, 2H, HC35, HC36), δ 7.16−7.21 ppm (m, 10H, Ar-H, HC37, HC38), δ 6.92 ppm (d, 4H, HC9, HC10, HC21, HC22, |3JHH| = 8 Hz) δ 4.26 ppm (bs, 4H, HC29, HC34), δ 3.08 ppm (t, 4H, HC30, HC33, |3JHH| = 6 Hz). 13C{1H} NMR (chloroform-d): δ 210.50 ppm (aq, C3, C4, |2JPC + 2JP′C| = 16 Hz), δ 205.90 ppm (t, C1, C2, |2JPC| = 14 Hz), δ 149.63 ppm (bs, C5, C6, C17, C18), δ 137.95 ppm (s, C7, C8, C19, C20), δ 131.76 ppm (s, Ph-CH), δ 130.21 ppm (s, Ar-CH), δ 129.58 ppm (s, Ar-CH), δ 127.76 ppm (s, Ph-CH), δ 125.65 ppm (s, Ar-CH), δ 122.10 ppm (s, Ar-CH), δ 63.83 ppm (s, C29, C34), δ 35.36 ppm (s, C31, C32). Anal. Calcd for C38H28MoO10P2S2: C, 52.66; H, 3.26. Found: C, 52.83; H, 3.21. cis-Mo(CO) 4 {2,2′-(C 12 H 8 O 2 )PO(CH 2 CH 2 S) 3 CH 2 CH 2 OP(2,2′(O 2 H 8 C 12 ))} (3) and cis,cis-{[Mo(CO) 4 (2,2′-(C 12 H 8 O 2 )PO(CH2CH2SCH2CH2)]2S} (5). Two solutions, the first consisting of 0.0900 g (0.290 mmol) of cis-Mo(CO)4(nbd) and the second consisting of 0.190 g (0.280 mmol) of L3, dissolved in dry DCM were simultaneously added dropwise to a small amount of the stirred solvent. After the addition was completed, the dropping funnels were washed with small amounts of dry DCM and the reaction mixture was stirred for 4 h under a constant stream of N2(g). Removal of the solvent in vacuo yielded the crude product as a foamy light yellow solid. Column chromatography using silica gel as the stationary phase and a 4/1 hexanes/ethyl acetate mixture as the eluent yielded an initial fraction containing 3 and a second fraction containing 5. Removal of the solvent from the 3 fraction in vacuo yielded a fluffy white solid that was recrystallized from DCM/hexanes to yield 0.0890 g (35.8%) of analytically pure 3 as colorless crystals. 31P{1H} NMR (chloroform-d): δ 171.44 ppm (s). 1H NMR (chloroform-d): δ 7.49−7.51 ppm (m, 4H, Ar-H), δ 7.38−7.41 ppm (m, 4H, Ar-H), δ 7.25−7.33 ppm (m, 8H, Ar-H), δ 4.15 ppm (bs, 4H, HC29, HC36), δ 2.82 ppm (t, 4H, HC30, HC35, |3JHH| = 7 Hz), δ 2.77 ppm (s, 8H, HC31, HC32, HC33, HC34). 13C{1H} NMR (chloroform-d): δ 210.24 ppm (aq, C3, C4, |2JPC + 2JP′C| = 17 Hz), δ 206.27 ppm (t, C1, C2, |2JPC| = 14 Hz), δ 149.75 ppm (t, C5, C6, C17, C18, |2JPC + 4JPC| = 5 Hz), δ 130.27 ppm (s, Ar-CH), δ 130.18 ppm (s, C7, C8, C19, C20), δ 129.63 ppm (s, ArCH), δ 125.80 ppm (s, Ar-CH), δ 122.18 ppm (s, Ar-CH), δ 66.59 ppm (t, C29, C36, |2JPC + 4JPC) = 4 Hz), δ 33.12 ppm (s, C32, C33), δ 32.37 ppm (t, C30, C35, |3JPC + 5JPC| = 3 Hz), δ 32.05 ppm (s, C31, C34). Anal. Calcd for C36H32MoO10P2S3: C, 49.21; H, 3.67. Found: C, 50.13; H, 4.01. Evaporation of the 5 fraction in vacuo and recrystallization of the residue from DCM/hexanes yielded 0.0730 g (29.3%) of analytically pure 5 as light yellow crystals. 31P{1H} NMR K


Article

Organometallics X-ray Data Collection and Solution. Compounds 1−3, 5, and 7 were recrystallized as stated in their respective syntheses above. A suitable single crystal of each was mounted on a MicroLoop (MiTeGen, Inc.) after coating in Paratone-N oil (Hampton Research Corp.). Data were collected at the specified temperature with a Bruker CCD diffractometer (SMART APEX2) fitted with a low-temperature device (Oxford Cryostream). The diffractometer used graphitemonochromated Cu Kα (λ = 1.54178 Å) radiation with a detector distance of 5 cm. The program SAINT was used to collect and reduce the data.63 Unit cell constants are based upon refinement of the XYZ centroids found using a standard indexing routine (APEX2).64 The details of the data collection are given in Table 4. All data were corrected for Lorentz and polarization effects and were scaled using the numerical method SADABS.65 The structures were solved and refined using the Bruker SHELXTL software package.66 Some of the heavy atoms were found using direct methods, and the remainder were located in difference Fourier maps. Hydrogen atoms were placed in calculated positions with the appropriate molecular geometry δ (C−H = 0.96 Å). The isotropic thermal parameter associated with each hydrogen atom was fixed equal to 1.2 times the Ueq value of the atom to which it is bound. Full-matrix least-squares refinement on F2 was performed on positional and anisotropic parameters for these atoms. Crystallographic data have been deposited with the Cambridge Crystallographic Database (1, CCDC 1415841; 2, CCDC 1415839; 3, CCDC 1415840; 5, CCDC 1415838; 7, CCDC 1415837). Compound 5 crystallized with disordered dichloromethane solvent molecules. The disorder was modeled with fragmented dichloromethane molecules containing one carbon atom at full occupancy, two carbon atoms each at 1/2 occupancy, and two chlorine atoms each at 1/2 occupancy. Compound 7 crystallized with two molecules in the asymmetric unit and one dichloromethane solvent molecule per molecule of 7. 31 1 P{ H} NMR Continuous Variation Job Plot of 1. Continuous variation Job studies of 1 were carried out in a solvent mixture of 8.5 M acetonitrile-d3 in anhydrous toluene. Each experiment was carried out by first preparing 10 mM stock solutions of 1 and HgCl2 in the solvent mixture described previously. These stock solutions were then used to prepare five separate NMR solutions with mole fractions of HgCl2 ranging from 0.25 to 0.75. The total number of moles and sample volume remained constant throughout each experiment. The data were plotted as ΔSobs*[1] vs mole fraction of HgCl2,67,68 in which the observed signal (ΔSobs) term was calculated using eq 3.

ΔSobs = 31

was held constant throughout each experiment, and compositions of the solutions are summarized in Table 5. The equilibrium data were fit using Scientist 3.0 software. The mechanism and equations are detailed in the Results and Discussion.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b00693. Statistics for all data presented and data sets plotted for Job plots of 1, HgCl2 titration of 1, and simultaneous binding and cis−trans isomerization of 3 with HgCl2 (PDF) Crystallographic data for 1−3, 5, and 7 (CIF)

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1

*E-mail for G.M.G.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge the National Science Foundation (NSF) for support under grant EPS-1158862. J.R.M. gratefully thanks the NSF Alabama EPSCoR for funding under a Graduate Research Scholars Program (GRSP) graduate fellowship. Special thanks to Dr. Charles Lake of the Department of Chemistry at Indiana University of Pennsylvania for his assistance in solving the disorder in the X-ray crystal structure of 5.

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1

P{ H} NMR Titration of 1 with HgCl2. The P{ H} NMR titrations were carried out by adding aliquots of a solution containing both 1 (3 mM) and HgCl2 (0.5 M) in a solvent mixture of 8.5 M acetonitrile-d3 in anhydrous toluene to a solution containing 1 (3 mM) in an identical solvent mixture in a 5 mm gastight J-Young NMR tube. All titrant additions were performed under a stream of N2 and the NMR tube was back-filled with N2 after each addition to ensure that the experiment remained under an inert atmosphere. Titration data were plotted as ΔSobs vs total HgCl2 concentration, [HgCl2]T, in which the observed signal (ΔSobs) term was calculated using eq 3. The titration data were then fit using Scientist 3.0 and a nonlinear leastsquares regression analysis to a 1:1 binding model eq 3 in which the extent of binding term, X̅ , is a function of the equilibrium constant, K, and the free ligand concentration, [HgCl2]free.

X̅ =

K[HgCl2]free 1 + K[HgCl2]free

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(2) 31

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δ(31P{1H})observed − δ(31P{1H})initial δ(31P{1H})initial

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(3)

Equilibrium Experiments Monitoring the Simultaneous Binding and Cis−Trans Isomerization of 3 with HgCl2. The HgCl2 catalyzed cis−trans isomerization of 3 was performed by adding the appropriate amount of a stock solution containing 16 mM HgCl2 in anhydrous toluene to an NMR tube containing a solution of 3 (5 mM) in the appropriate amount of toluene-d8. The concentration of 3 L


Article

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