Generally Applicable NMR Titration Methods for the Determination of

ARTICLE pubs.acs.org/Organometallics

Generally Applicable NMR Titration Methods for the Determination of Equilibrium Constants for Coordination Complexes: Syntheses and Characterizations of Metallacrown Ethers with α,ω-Bis(phosphite)polyether Ligands and Determination of Equilibrium Binding Constants to Li+ Justin T. Sheff, Aaron L. Lucius, Sam B. Owens, and Gary M. Gray* Department of Chemistry, University of Alabama at Birmingham, CHEM 201, 1530 Third Avenue South, Birmingham, Alabama 35294-1240, United States

bS Supporting Information ABSTRACT:

A new metallacrown ether, cis-PdCl2{(2,20 -C12H8O2)P(OCH2CH2)4OP(2,20 -O2H8C12)}, has been synthesized and fully characterized by NMR spectroscopy and X-ray crystallography. A new synthetic method and complete NMR characterization for the previously reported and closely related cis-Mo(CO)4(2,20 -C12H8O2)P(OCH2CH2)4OP(2,20 -O2H8C12) metallacrown ether are also reported. Both complexes are monomeric, exhibiting cis coordination geometries both in solution and in the solid state, and the cis-Mo(CO)4(2,20 -C12H8O2)P(OCH2CH2)4OP(2,20 -O2H8C12) metallacrown ether does not appear to undergo cistrans isomerization in the presence of HgCl2. These results are surprising because the closely related PdCl2(Ph2P(CH2CH2O)nCH2CH2PPh2) (n = 35) metallacrown ethers exhibit both cistrans and monomercyclic oligomer equilibria in solution, and cis-Mo(CO)4 (Ph2P(CH2CH2O)nCH2CH2PPh2) (n = 35) metallacrown ethers rapidly undergo cistrans isomerization in the presence of HgCl2. The coordination of Li+ cations to the cis-Mo(CO)4(2,20 -C12H8O2)P(OCH2CH2)4OP(2,20 -O2H8C12) metallacrown ether has been evaluated using 31P{1H} NMR spectroscopy. A 31P{1H} NMR continuous variation Job experiment of LiB(C6F5)4 3 2Et2O and cis-Mo(CO)4(2,20 -C12H8O2)P(OCH2CH2)4OP(2,20 -O2H8C12) indicates that the binding stoichiometry is 1:1. Accurate binding constants were determined by performing multiple 31P{1H} NMR titrations at varying concentrations of cis-Mo(CO)4(2,20 -C12H8O2)P(OCH2CH2)4OP(2,20 -O2H8C12) using a binding density and McGheevon Hippel analysis in an optimized mixture of dichloromethane-d2 and tetrahydrofuran. This yielded an observed binding affinity, Kobs, of 0.077 ( 0.005 mM1 under nonstoichiometric, equilibrium conditions. The observed binding affinity was then related to the concentration of tetrahydrofuran (log Kobs = 3.58 log [THF] + log Ka) using a solvent back-titration, where the negative slope, 3.58, represents the average number of tetrahydrofuran molecules displaced from the alkali cation upon coordination to the metallacrown ether. An FT-IR titration was also performed by monitoring ligand carbonyl stretching frequencies under high affinity, stoichiometric conditions. This study developed and demonstrated the use of solvent back-titrations and that cation binding to the metallacrown ether reduces π-back-bonding between the Mo metal and the CO ligands, increasing the CottonKraihanzel stretching force constants (kcis and ktrans) for all CO ligands by approximately 10%.

’ INTRODUCTION Organic crown ethers have been used as ligands in a variety of applications due to their excellent size selectivity for metal cations and some small molecules.13 The size selectivity of crown ethers has been explained as a function of both the number of oxygen donor sites and the ether cavity size.48 Incorporation of various functional groups into the crown ether ring has been used to modify solubilities, provide additional donor sites, or add a spectroscopic probe.9,10 r 2011 American Chemical Society

The incorporation of a metal center into a crown ether creates a metallacrown ether. Metallacrown ethers have a chemistry that can be significantly different from that of either the crown ether or the metal center.11 A great deal of the research on metallacrown ethers has focused on those that are formed by the chelation of α,ω-bis(phosphorus-donor)-polyether ligands to Received: July 1, 2011 Published: October 13, 2011 5695

dx.doi.org/10.1021/om200580t | Organometallics 2011, 30, 5695–5709

Organometallics transition-metal centers.1221 Metallacrown ethers, including cisMo(CO)4(Ph2P(CH2CH2O)nCH2CH2PPh2) (n = 4, 5), have been shown to bind hard metal cations and small molecules.1825 Hard/soft bimetallic complexes, such as those formed by the coordination of an alkali metal cation to a metallacrown ether, are of interest as catalysts for organic reactions involving carbon monoxide because the interaction of both metals with carbon monoxide may give rise to activities and/or selectivities that are different from those of the monometallic, transition-metal catalysts. We have recently reported that Rh(I) metallacrown ethers with α,ω-bis(phosphite)-polyether or α,ω-bis(phosphite)polyether/amide ligands are catalysts for the hydroformylation of styrene, and the production of the branched isomer (regioselectivity) could be increased by the addition of an alkali metal salt.2630 However, it was not clear if this effect was due to binding of the alkali metal cation by the metallacrown ether because it was also observed with bis(phosphite) ligands that did not contain a polyether group and required a large excess of the alkali metal salt in tetrahydrofuran. If the effects of the hard metal cations on the catalytic activities and selectivities of the metallacrown ether catalysts are to be understood and optimized, it is important to understand all the factors that affect hard metal cation binding by metallacrown ethers. NMR spectroscopy has been used to study the binding of alkali metal cations to both metallacrown ethers and crown ethers.3,4,8,2225,3135 NMR binding studies involving metallacrown ethers have demonstrated that these metallacrown ethers selectively bind alkali metal salts, and the binding selectivity is different from that of crown ethers containing the same number of ether oxygens.2225,3135 Unfortunately, the binding studies that have been carried out to date are of limited use for understanding the effects of the hard metal cations on the catalytic activities and selectivities of the metallacrown ether catalysts. One reason for this is that many of these titration studies were performed on metallacrown ethers with bis(phosphine)-polyether ligands and solvents (acetonitrile, carbon tetrachloride) that are not appropriate for alkene hydroformylation reactions and would be expected to effect alkali binding affinity. More importantly, all of the previous titrations were carried out under high affinity, stoichiometric binding conditions in which the macromolecule concentration exceeded the dissociation equilibrium constant, Kd, by ∼10 fold. Under such stoichiometric or nonequilibrium conditions, the ligand, cation, etc. are all bound and only the upper limit of the binding dissociation constant, Kd, can potentially be determined from any applied, binding model to experimental titration data.23,25 Because of these limitations in titration techniques, we have decided to study cation binding by cis-Mo(CO)4((2,20 -C12H8O2)PO(CH2CH2O)nP(2,20 -O2H8C12)) (n = 3, 4 (2), 5) metallacrown ethers in solvents that are used in our catalytic studies by adapting and developing a biophysical and biochemical approach for use with NMR titration experiements. In this paper, we report the results of a detailed, novel 31P{1H} NMR spectroscopic titration study adapted from biophysicial and biochemical techniques for Li+ binding by 2. This study includes a preliminary Job analysis of binding stoichiometry, an extensive binding density analysis of titrations of 2 with LiB(C6F5)4 3 2Et2O carried out under low affinity, equilibrium conditions in which the observed binding affinity, Kobs, is obtained from a global, nonlinear least-squares regression of multiple, experimental, NMR titration data sets, and the demonstration of a recently developed titration technique referred to a solvent

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back-titration using the work of McGhee, von Hippel, Manning, and Record to determine the role and effect of solvents on the observed binding association constant, Kobs, to the alkali salt by 2.3641 This work demonstrates a biochemical and biophysical adapted and developed technique performing solvent backtitrations where previous experiments have principally focused on the effects of pH and salt concentration. More importantly, this work demonstrates principles and generally applicable titration methods for obtaining accurate equilibrium association binding constants using NMR spectroscopy that could be applied to a variety of inorganic, organometallic, and related research fields where such biophysical techniques are not generally known.4245 Demonstration of global, nonlinear least-squares regression of spectroscopic titrations, binding density analysis with determination of signal response to metallacrown ether binding density, and solvent back-titration of ethereal solvents were all adapted for organometallic and inorganic chemistry directly from biophysical and biochemical techniques.3841 In addition, both 2 and cis-Mo(CO)4(P(OPh)3)2 were titrated with LiB(C6F5)4 3 2Et2O in dichloromethane under high affinity conditions by monitoring carbonyl ligand stretching frequencies by FT-IR to gain a better understanding of all the interactions of alkali salts with carbonyl ligands in metallacrown ethers.46 Finally, the product of the reaction of (2,20 -C12H8O2)PO(CH2CH2O)4P(2,20 -O2H8C12)}, 1, with Pd(PhCN)2Cl2 has been characterized by 31P{1H} NMR spectroscopy and X-ray crystallography to provide insight into cistrans and monomer oligomer equilibria in a labile metallacrown ether similar to the Rh(I) metallacrown ether hydroformylation catalysts.

’ EXPERIMENTAL SECTION All reactions and purification procedures were carried out under highpurity nitrogen. All starting materials were reagent grade and were used as received. Tetraethylene glycol was distilled under vacuum from activated Mg onto 3 Å molecular sieves using a short-path distillation apparatus and stored in a N2 atmosphere drybox. Solvents were distilled from the appropriate drying agent (tetrahydrofuran (THF) first from calcium hydride and then sodium/benzophenone; triethylamine from sodium/benzophenone; dichloromethane from calcium hydride), and then all were stored over 3 Å molecular sieves under a N2 atmosphere. All solvent and solution transfers were carried out using cannula techniques. Deuterated NMR solvents (chloroform-d, dichloromethane-d2, acetonitrile-d3) were treated with 3 Å, 812 mesh molecular sieves when first opened to remove residual water and subsequently were opened and handled under a N2 atmosphere at all times. LiB(C6F5)4 3 nEt2O was purchased from Bouldersci Co., and the number of diethyl ether molecules (n = 2) was determined using an NMR standard addition method. All glassware, Celite, and basic alumina were stored in the oven. All one-dimensional 31P{1H}, 13C{1H}, and 1H NMR spectra of the ligands and complexes were recorded using a Bruker DRX-400 MHz NMR spectrometer with a tunable 5 mm probe. The 31P{1H} NMR spectra were referenced to external 85% phosphoric acid, and both the 13 C and the 1H NMR spectra were referenced to internal TMS. Quantitative 31P{1H} NMR spectra were acquired using an inversegated 30° pulse sequence with a 10 s pulse delay and a 4.5 s acquisition time. The inverse-gated pulse sequence was employed to remove any possible NOE. All quantitative 31P{1H} NMR spectra had a signal-tonoise ratio greater than 200:1 for the most intense peak. Accurate 31 1 P{ H} NMR integrations were calculated from the 31P{1H} NMR spectra using the Bruker UXNMR software. The NMR spectrometer temperature was maintained at 301 K during all alkali salt and solvent 5696

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back-titration experiments. Numbering of atoms in the NMR assignments and X-ray crystal structures for 2 and 3 are consistent. Elemental analyses (C and H) were performed by Atlantic Microlabs, Norcross, GA. 2,20 -(C12H8O2)PO(CH2CH2O)4P(2,20 -O2H8C12), 1. This ligand was synthesized using a modification of the previously reported procedure.47 A solution of 10.3 g (41.3 mmol) of 2,20 -biphenylenephophochloridite ester dissolved in 125 mL of tetrahydrofuran was added to a solution of 3.56 mL (20.6 mmol) of tetraethylene glycol and 5.74 mL (41.3 mmol) of triethylamine in 50 mL of tetrahydrofuran over a 1 h period under a constant stream of N2(g). The solution was then cannula transferred in portions into a dry, 50 mL fritted funnel containing a mixture of Celite and basic alumina to remove the triethylammonium chloride byproduct and any phosphite hydrolysis byproducts. This solution was filtered via nitrogen pressure into a 150 mL Schlenk flask that was purged with nitrogen. After the filtration was completed, the residue in the filter was washed with two approximately 25 mL portions of tetrahydrofuran. Removal of the tetrahydrofuran under vacuum yielded 11.5 g (89.5%) of pure 1 as a colorless oil. 31P{1H} (chloroform-d1): δ 141.27 ppm (s). 1H NMR (chloroform-d1): δ 7.36 (m, 16H, ArH), 4.00 (m, 4H, CH2), 3.60 (m, 12 H, CH2). 0

0

cis-Mo(CO)4{2,2 -(C12H8O2)PO(CH2CH2O)4P(2,2 -O2H8C12)}, 2.

Compound 2 was prepared by a modification of a previously published method where a single addition method was substituted for the dilute double addition method previously published.47 A solution of 1.22 g (1.96 mmol) of 1 in 50 mL of dry, degassed CH2Cl2 was stirred at ambient temperature as a solution of 0.59 g (2.0 mmol) of Mo(CO)4nbd in 50 mL of dry, degassed CH2Cl2 was added dropwise over 30 min under a constant stream of N2(g). After the addition was completed, the reaction mixture was stirred for 1 h, and then the solvent was removed under vacuum to yield a foamy, white solid. Approximately 10 mL of dry dichloromethane was added to the solid, and the solution was heated under a stream of nitrogen as 5 mL of hexanes was slowly added. When the volume was reduced to 10 mL, the solution was filtered through a bed of silica gel in a 25 mL sintered glass funnel to remove nbd and polymerized nbd impurities. The filtrate was evaporated to dryness to yield 1.10 g (67.5%) of spectroscopically pure 2 as a white, foamy solid. Recrystallization from a dichloromethane/hexanes mixture yielded pure 2 as colorless crystals. 31P{1H} (chloroform-d1): δ 170.96 ppm (s). 1H NMR (chloroform-d1): δ 7.36 (m, 16H, Ar), 4.15 (m, 4H, CH2), 3.71 (m, 12 H, CH2). 13C{1H} (chloroform-d1): δ 66.7 (t, C1 and C8, |2J(PC) + 4J(P0 C)| 8 Hz), 70.3 (t, C2 and C7, |3J(PC) + 5J(P0 C)| 6 Hz), 70.8 (s, C3 and C6), 71.1 (s, C4 and C5), 149.9 (t, C32, C21, C20 and C9, |2J(PC) + 4J(P0 C)| 9 Hz), 130.2 (s, C26, C27, C15 and C14), 130.0 (s, C25, C28, C16 and C13), 129.5 (s, C23, C30, C18 and C11), 125.5 (s, C24, C29, C17 and C12), 122.4 (s, C22, C31, C19 and C10), 206.6 (AX2, cis CO, |2J(PC)| 14 Hz), 210.7 (AXX0 , trans CO, |2J(PC) + 2J(P0 C)| 33 Hz). 0

0

cis-PdCl2{2,2 -(C12H8O2)PO(CH2CH2O)4P(2,2 -O2H8C12)}, 3. A solution of 1.07 g (1.72 mmol) of 1 in 50 mL of dry, degassed CH2Cl2 was added dropwise over a period of 15 min to a solution of 0.665 g (1.73 mmol) of Pd(CNPh)2Cl2 in 40 mL of dry, degassed dichloromethane. This solution was stirred at room temperature for 1 h, and then the solvent was removed to yield 1.10 g (79.1%) of the crude product as a yellow solid. The crude product was recrystallized from dichloromethane/ hexanes to yield analytically pure 3 as small pale yellow crystals. 31P{1H} (dichloromethane-d2): δ 106.65 ppm (s). 1H NMR (dichloromethaned2): δ 7.26 (m, 16H, Ar), 4.53 (m, 4H, CH2), 3.83 (m, 16 H, CH2). 13 C{1H} (dichloromethane-d2): δ 70.56 (t, C1 and C8, |2J(PC) + 4 J(P0 C)| 6 Hz), 71.41 (s, C2 and C7), 71.4 (s, C3 and C6), 71.60 (s, C4 and C5), 148.97 (t, C32, C21, C20 and C9, |2J(PC) + 4J(P0 C)| 10 Hz), 129.69 (s, C26, C27, C15 and C14), 130.82 (s, C25, C28, C13 and C16), 127.48 (s, C23, C30, C18 and C11), 130.76 (s, C24, C29, C12 and C17), 122.85 (s, C22, C31, C10 and C19). Anal. Calcd. for C32H32Cl2O9P2Pd 3 0.25CH2Cl2: C, 47.17; H, 3.99. Found: C, 47.15; H, 4.00.

31

P{1H} NMR Continuous Variation Job Study of cis-Mo(CO)4{(2,20 -C12H8O2)PO(CH2CH2O)4P(2,20 -C12H8O2)}, 2, with LiBPh4 3 3dme. The stoichiometry of the binding of Li+ by 2 was 36,37

A 0.0101 M initially determined using a continuous variation Job plot. solution of 2 (0.0421 g, 0.0507 mmol) in 5.00 mL of dichloromethane-d2 in a volumetric flask and a 0.0097 M solution of LiBPh4 3 3dme (0.0288 g, 0.0483 mmol) diluted to 5.00 mL with dichloromethane-d2 in a volumetric flask were first prepared. These stock solutions were then used to prepare separate NMR solutions with mole fractions of LiBPh4 3 3dme ranging from 0.200 to 0.800 mol of Li+. In each case, the total number of moles and volume in the sample (0.50 mL) remained constant. A 31P{1H} NMR spectrum of each of the individually prepared solutions was taken. A second continuous variation Job experiment was then repeated using stock solutions of approximately 10 times the concentrations of the original solutions (0.167 g (0.201 mmol) of 2 was diluted to 2.00 mL with dichloromethane-d2 in a 2.00 mL volumetric flask to form a 0.100 M solution, and 0.120 g (0.201 mmol) of LiBPh4 3 3dme was diluted to 2.00 mL with dichloromethane-d2 in a volumetric flask to form a 0.100 M solution) to observe any changes as a function of the concentration of 2.48

P{1H} NMR Titration of a 1:1 Solution of cis-Mo(CO)4{(2,20 C12H8O2)PO(CH2CH2O)4P(2,20 -C12H8O2)}, 2, and LiB(C6F5)4 3 2Et2O in Dichloromethane-d2 with a 1:1 Solution of cisMo(CO)4{(2,20 -C12H8O2)PO(CH2CH2O)4P(2,20 -C12H8O2)}, 2, and LiB(C6F5)4 3 2Et2O in Tetrahydrofuran-d8. To determine the + 31

effect of solvent coordination on the observed binding affinity of Li to 2, and to find a single, suitable solvent mixture for alkali salt titrations under equilibrium and nonstoichiometric conditions, a dichloromethane-d2 solution containing equal molar amounts of 2 and LiB(C6F5)4 3 2Et2O was titrated with a tetrahydrofuran-d8 solution having the same concentrations of 2 and Li+, and the change in the 31P{1H} NMR chemical shift was monitored.49 The dichloromethane-d2 solution was prepared by adding 0.0050 g (6.0 103 mmol) of 2, 0.0045 g (5.4 103 mmol) of LiB(C6F5)4 3 2Et2O, and 0.60 mL of dichloromethane-d2 to a 5 mm NMR tube. The tetrahydrofuran-d8 solution was prepared by dissolving 0.0125 g (1.50 102 mmol) of 2 and 0.0116 g (1.39 102 mmol) of LiB(C6F5)4 3 2Et2O in 1.50 mL of tetrahydrofuran-d8. The titration was carried out by adding 5.0 μL injections of the tetrahydrofuran-d8 solution to the dichloromethane-d2 solution in the NMR tube using a Drummond micropipet. The NMR sample was then mixed to homogeneity and allowed to reach thermal equilibrium (∼3 min) before the 31P{1H} NMR spectrum of the solution was recorded. The spectrometer was maintained at a constant temperature just above room temperature, 301 K, to ensure that subtle temperature changes did not affect the chemical shift and/or observed equilibrium association constants.

P{1H} NMR Titration of cis-Mo(CO)4{(2,20 -C12H8O2)PO(CH2CH2O)4P(2,20 -C12H8O2)} with LiB(C6F5)4 3 2Et2O. The 31P 31

NMR titrations were carried out by adding aliquots of a solution of both LiB(C6F5)4 3 2Et2O and 2 in a mixture of 0.40 M tetrahydrofuran in dichloromethane-d2 (titrant solution) to a solution of 2 in the same mixture of 0.40 M tetrahydrofuran in 600 μL of dichloromethane-d2 (NMR solution). The alkali salt solution contained 2 as well, as described by biophysical techniques, to limit the change in metallacrown ether concentration as the titration proceeded. Initially, four titrations were carried out with different concentrations of 2 using the preparation procedure described above, and the compositions of the solutions are summarized in Table 1 below. The general procedure for the titration is as follows. A solution of the appropriate amount of 2 was prepared in 1.20 mL of dichloromethaned2. Half (0.60 mL) of this solution was transferred to a 5 mm NMR tube using a 1.0 mL gastight syringe. The remaining 0.60 mL of the solution was treated with the appropriate amount of LiB(C6F5)4 3 2Et2O and 1.4 mL of dry dichloromethane-d2. Dry tetrahydrofuran was added to 5697


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Table 1. Compositions of the Solutions Used in the NMR Titrations NMR titrant solution 2 (g/mol/M) 4.9 10

3

CD2Cl2 (mL) 0.60

6.0 106

alkali salt solution THF (μL/M) 19.5 0.40

9.6 103 9.7 103 1.2 10

0.60

5

19.5 0.40

1.9 102 2.7 102

0.60

5

3.3 10 5.3 102 5.0 102 6.0 10

5

9.7 102

19.5 0.40

0.60

19.5 0.40

LiB(C6F5)4 3 2Et2O (g/mol/M)

2 (g/mol/M) 3

4.9 10

0.14

6.0 106

1.7 104

2.9 103

8.3 102

9.7 103

0.33

1.2 105

4.0 104

5.7 103

1.94 101

2.7 102

0.83

3.3 105 1.6 102

9.9 104 4.8 101

5.0 102

1.6

6.0 105

2.0 103

2.9 102

9.5 101

both the solution in the NMR tube (19.5 μL) using a 25 μL gastight syringe and the solution in the 2.00 mL volumetric flask (65.0 μL) using a 100 μL gastight syringe to make both solutions 0.40 M in tetrahydrofuran. An initial 31P{1H} NMR spectrum of the solution in the NMR tube was then recorded. Next, an appropriate volume of the LiB(C6F5)4 3 2Et2O solution was removed from the volumetric flask and added to the NMR sample using either a gastight syringe or a Drummond micropipet (15 μL) depending on the particular relative concentrations of metallacrown ether and alkali salt. The NMR sample was then mixed to homogeneity, placed in the spectrometer, and allowed to reach thermal equilibrium before the 31P{1H} NMR spectrum was collected. The spectrometer was maintained at a constant temperature just above room temperature, 301 K. However, the alkali titration preparation procedure of 2 described above did allow the concentration of 2 to vary slightly during the NMR titration. To ensure that the variation of 12 mM in the concentration of 2 did not affect the results, another, fifth alkali titration in which the concentration of 2 remained constant at 9.7 mM was performed following the same titration procedure described above.50 The NMR solution for this titration was prepared by dissolving 5.0 103 g (6.0 103 mmol) of 2 in a mixture of 0.60 mL of dichloromethane-d2 and 19.5 μL of tetrahydrofuran. The alkali salt solution for this titration was prepared by dissolving 0.14 g (0.17 mmol) of LiB(C6F5)4 3 2Et2O and 1.7 102 g (0.02 mmol) of 2 in a mixture of 2.0 mL of dichloromethane-d2 and 65 μL of tetrahydrofuran.

FT-IR Titration of cis-Mo(CO)4{(P(OPh)3}2 with LiB(C6F5)4 3 2Et2O in CH2Cl2. A 1.0 103 M dichloromethane solution of cis-

Mo(CO)4{(P(OPh)3}2 was prepared by dissolving 0.025 g (3.0 102 mmol) of cis-Mo(CO)4{(P(OPh)3}2 in 30.0 mL of dry dichloromethane. A FT-IR spectrum of this solution was first taken, and then subsequent FT-IR spectra were taken after aliquots of the titrant, a solution of 0.082 M in LiB(C6F5)4 3 2Et2O and 1.0 103 M in cisMo(CO)4{(P(OPh)3}2) in dichloromethane, were added. The titrant solution was prepared by adding 0.14 g (1.6 101 mmol) of LiB(C6F5)4 3 2Et2O and 0.0017 g (2.0 103 mmol) of cis-Mo(CO)4{(P(OPh)3}2 to a 2.0 mL volumetric flask and adding 2.0 mL of dichloromethane using a 1.0 mL gastight syringe. The initial aliquots of the titrant solution, which were 74 μL, contained approximately 0.20 equiv of LiB(C6F5)4 3 2Et2O and were added using a 100 μL gastight syringe. The final two aliquots, which were 370 μL, contained approximately 1.0 equiv of the LiB(C6F5)4 3 2Et2O and were added using a 250 μL gastight syringe.

FT-IR Titration of cis-Mo(CO)4{(2,20 -C12H8O2)PO(CH2CH2O)4P(2,20 -C12H8O2)} with LiB(C6F5)4 3 2Et2O in Dichloromethane.

CD2Cl2 (mL) 2.00

THF (μL/M) 65 0.40

2.00

65 0.40

2.00

65 0.40

2.00

65 0.40

A 1.07 103 M dichloromethane solution of 2 was prepared by dissolving 0.0269 g (3.20 103 mmol) of 2 in 30.0 mL of dichloromethane. A FT-IR spectrum of this solution was first taken, and then subsequent FT-IR spectra were taken after aliquots of the alkali titrant solution were added. The titrant solution was prepared by dissolving 0.18 g (2.2 101 mmol) of LiB(C6F5)4 3 2Et2O and 0.0020 g (2.4 103 mmol) of 2 in 2.0 mL of dichloromethane and was 1.1 102 M in LiB(C6F5)4 3 2Et2O and 1.2 103 M in 2. Each of the first 10 aliquots of the LiB(C6F5)4 3 2Et2O solution were added using a 100 μL gastight syringe and contained approximately 0.2 mol equiv in each 59 μL injection. Each of the last four aliquots of the titrant solution were added using a 250 μL gastight syringe and contained approximately 1.0 equiv of LiB(C6F5)4 3 2Et2O in each 295 μL injection. HgCl2-Catalyzed cistrans Isomerization of Complex 2. A 0.02 M chloroform-d solution of 2 was prepared in an NMR tube by dissolving 9.0 103 g (1.0 102 mmol) of 2 in 0.6 mL of chloroform-d. A catalytic amount of HgCl2 (1.0 mg, 4 104 mmol) was then added to the solution in the NMR tube, and a 31P{1H} NMR spectrum was immediately taken. Additional 31P{1H} NMR spectra were then taken over a period of several days, but no changes in the spectra were observed. This experiment was repeated with an excess of 0.0868 g (0.3197 mmol) of HgCl2, yielding the same null result.

Concentration Dependence of the 31P{1H} NMR Spectra of Chloroform-d Solutions of 3. Solutions containing three

different concentrations of 3 in chloroform-d (7 103, 0.01, and 0.02 M) were prepared and allowed to equilibrate before quantitative 31 1 P{ H} NMR spectra were taken. In each case, only a single 31P NMR resonance was observed. The experiment was repeated used using crude 3 with no changes observed between minor impurity peaks. X-ray Data Collection and Solution. A single crystal of 3 was glued on a glass fiber with epoxy and aligned upon an Enraf Nonius CAD4 single-crystal diffractometer under aerobic conditions. Unit cell parameters were determined by least-squares fits of 25 reflections. Systematic absences indicated that the space group was monoclinic P2(1)/c. Data collection was carried out using the CAD4-PC software, and details of the data collection are given in Table 2. The analytical scattering factors were corrected for both Δf0 and iΔf0 components of anomalous dispersion. All data were corrected for Lorentz and polarization effects. All crystallographic calculations were performed with the Siemens SHELXTL-PC program.61 The Pd and P positions were located using the Patterson method, and the other non-hydrogen atoms were located in difference Fourier maps. Full-matrix refinement of the positional and anisotropic thermal parameters for all non-hydrogen atoms versus F2 was carried out. All hydrogen atoms were placed in calculated positions with the 5698


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Table 2. Crystal Data and Structure Refinement Information for 3 empirical formula fw

C32H32Cl2O9P2Pd 799.82

refinement method data/restraints/parameters

full-matrix least-squares on F2 4395/0/415

temp

293(2) K

goodness-of-fit on F2

1.126

wavelength

0.71073 Å

final R indices [I > 2σ(I)]

R1 = 0.0350, wR2 = 0.0891

cryst syst space group

monoclinic P2(1)/c

R indices (all data) extinction coeff

R1 = 0.0432, wR2 = 0.0930

largest diff peak and hole

0.552 and 0.781 e 3 Å3

unit cell dimensions

0.1 0.1 0.1 mm3

a

12.994(3) Å

b

14.509(3) Å

θ range for data collection

2.1522.48°.

c

18.537(4) Å

index ranges

13 e h e 13

cryst size

1 e k e 15 19 e l e 19 α β

90° 105.20(3)°

γ

90°

completeness to θ = 22.48°

99.9%

volume

3372.4(12) Å3

abs correction

none

Z

4

abs coeff

0.856 mm1

F(000)

1624

reflns collected independent reflns

3

density (calculated)

1.575 Mg/m

Table 3. Selected Bond Lengths (Å) and Angles (°) for 3 atoms

length (Å)

atoms

angle (0)

PdP2 PdP1

2.2198(10) 2.2168(10)

P2PdP1 Cl1PdP1

92.97(4) 175.82(4)

PdCl2

2.3193(10)

Cl2PdP1

92.48(4)

PdCl1

2.3410(11)

Cl2PdP2

174.43(4)

P1O1

1.560(3)

Cl1PdP2

82.91(4)

P1O5

1.588(2)

Cl2PdCl1

91.65(4)

P1O6

1.592(3)

O1P1Pd

112.70(10)

P2O4

1.562(3)

C1O1P1

127.1(2)

P2O8 P2O7

1.583(3) 1.591(3)

O6P1Pd O8P2Pd

116.35(10) 117.92(11)

C8O5P2

121.0(3)

O9P2Pd

112.09(10)

O5P2Pd

115.55(11)

appropriate molecular geometry (δ(CH) = 0.96 Å, isotropic thermal parameter fixed at 1.2 times that of the atom to which the hydrogen is bound). Selected bond lengths and angles for 3 are given in Table 3.

’ RESULTS AND DISCUSSION Syntheses and NMR Spectroscopic Characterization of cisMo(CO)4((2,20 -(C12H8O2)PO(CH2CH2O)4P(2,20 -O2H8C12)), 2, and cis-Pd(Cl)2(2,20 -(C12H8O2)PO(CH2CH2O)4P(2,20 -O2H8C12)), 3. The

syntheses and the 1H and 31P{1H} NMR spectra of 2,20 -(C12H8O2)PO(CH2CH2O)4P(2,20 -O2H8C12), 1, and cis-Mo(CO)4((2,20 (C12H8O2)PO(CH2CH2O)4P(2,20 -O2H8C12)), 2, have been previously reported.47 The experimental procedures for these compounds in the Experimental Section are significant improvements on the previous literature procedures, giving both compounds in high yields with simplified synthetic procedures. The synthesis of 3 has not previously been reported. This compound was formed in high yield by the reaction of 1 with Pd(CNPh)2Cl2 in dichloromethane, as summarized in Scheme 1, using a simple addition reaction at moderate concentrations. The elemental analysis of the product is consistent

9604 4395 [R(int) = 0.0290]

Scheme 1. Synthesis of PdCl2{(2,20 -C12H8O2)PO(CH2CH2O)4P(2,20 -O2H8C12)}, 3

with the empirical formula PdCl2{(2,20 -C12H8O2)PO(CH2CH2O)4P(2,20 -O2H8C12)}. Only singlet resonances are observed in the 31P{1H} NMR spectra of 2 and 3. The 31P{1H} resonance shifts from 141.27 ppm for 1 to 170.96 ppm for 2 and to 106.65 ppm for 3. These coordination chemical shifts are consistent with those for previously reported, related complexes.47,5153 The observation of a single 31P{1H} NMR resonance for 2 confirms the expected equivalence of the two phosphorus atoms in the cis octahedral geometry. The observation of a single 31P{1H} NMR resonance for 3 at several different concentrations indicates that a single species with equivalent two phosphorus nuclei is also present for the square-planar coordination geometry over the concentration range that was studied. This is surprising because a number of singlet resonances are observed in the 31P{1H} NMR spectrum of the closely related PdCl2{Ph2P(CH2CH2O)4CH2CH2PPh2)} metallacrown ether due to cistrans and monomeroligomer equilibria.5153 Apparently, such equilibria are not present in solutions of 3, perhaps due to the differences in the phosphorusdonor groups in the two metallacrown ethers. The 1H NMR spectra for 2 and 3 are essentially identical in both coupling constants and chemical shifts with the largest chemical shift differences being approximately 0.3 ppm. This similarity is likely due to similar solution conformations of the bis(phosphite)polyether ligands in 2 and 3. Three distinct 5699


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Figure 1. ORTEP drawing of the molecular structure of 3. The thermal ellipsoids are drawn at the 25% probability level, and hydrogen atoms are omitted for clarity.

resonances are observed in the 1H NMR spectra for both 2 and 3, a complex multiplet for the aromatic biphenol protons at approximately 7.26 ppm, a doublet of triplets for the methylene protons on the polyether ring closest to the P atoms (PO CH2CH2O) at approximately 4.3 ppm, and a complex multiplet for the remaining methylene protons (POCH2CH2 OCH2CH2O), centered around 3.7 ppm. The doublet of triplets for the methylene protons at 4.3 ppm collapses to a broad triplet when the phosphorus is decoupled, indicating that the multiplet is due to three bond couplings to both the phosphorus and the protons of the adjacent methylene. The relative integrations for the three resonances (4:1:3) are the same in the spectra of 1, 2, and 3, indicating that the integrity of the ligand is maintained upon coordition to the metal centers. The 13C{1H} NMR spectra of 2, reported in the Experimental Section, is more completely interpreted than in the original report of the metallacrown ether.47 The low-field portion of the 13 C{1H} NMR spectrum of 2 provides definitive information about the coordination geometry of the Mo center in solution. The two resonances, an apparent quintet (A portion of an AXX0 spin system) at 210.80 ppm and a triplet (A portion of an AX2 spin system) at 206.26 ppm, are consistent with carbonyl ligands in a cis coordination geometry for the phosphorus-donor groups of 2. X-ray Crystal Structure of cis-Pd(Cl)2(2,20 -(C12H8O2)PO(CH2CH2O)4P(2,20 -O2H8C12)), 3. Crystals of 3 were grown after several days under low-temperature conditions from a saturated dichloromethane/hexanes solution. An ORTEP drawing of the molecular structure of 3 is shown in Figure 1, and selected bond lengths and angles are given in Table 3. The two phosphites are coordinated to the palladium center in a distorted cis-square-planar

coordination geometry with the P1PdP2, P1PdCl2, and Cl1PdCl2 angles slightly larger than 90° (92.97(4), 92.48(4), 91.65(4)°), and the P2PdCl1 angle significantly smaller (82.91(4)°). Because a single PdCl2-metallacrown ether is observed in solutions of 3, it seems likely that this is also the cis-square-planar complex shown in Figure 1. A similar coordination geometry has been previously reported for the phosphite ligands in the closely related cis-PdCl2{2,20 -(C12H8O2)POCH2CH2O-1-C6H4-2-OCH2CH2OP(2,20 -O2H8C12)} metallacrown ether, but the cis coordination in this complex could have been due to steric constraints of the1,2-phenylene group and/or to the smaller metallacrown ether ring in this complex.2830 Observation of the same coordination geometry in 3 suggests that the cis coordination geometries in the complexes are instead due to more stable coordination of the phosphites trans to the chloro ligands. The solid-state conformation of the metallacrown ether ring in 3 is quite different from that of the metallacrown ether ring in 2, as shown by the different torsion angles in Table 4.47 The metallacrown ether ring conformation of 3 results in some of the ether oxygens pointing away from the metallacrown ether ring cavity (O2, O5) and the remaining three oxygens pointing inward (O1, O3, O4) in the solid state shown in Figure 2. Although the metallacrown ether rings in 2 and 3 have quite different conformations in the solid state, they have the same approximate average crown radius (the average crown radius is the average distance between a centroid generated at the center of the metallacrown ether ring and an oxygen atom of the polyether portion of the ligand and is 2.673(7) Å for 3 and 2.679(7) Å for 2). In addition, one oxygen in a phosphepin ring in each structure is approximately the same distance from the centroid (3, O9, 2.656(3) Å; 2, O8, 2.732(2) Å) and is thus 5700


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potentially capable of coordinating to a metal cation that is bound to the ether oxygens of the metallacrown ether ring.54 The molecular structures of both compounds 2 and 3 both appear suitable for binding cations despite the orientations of the O2 and O5 atoms of 3. Given the similarities of the 13C{1H} and 1H NMR spectra of the two metallacrown ethers, it seems likely that the conformational differences in the solid state between 2 and 3 are due to crystal packing forces and will not exist in solution. The conformations of the metallacrown ether rings in both 2 and 3 in the solid state allow intramolecular T-type ππ stacking Table 4. Comparison of Torsion Angles around the Metallacrown Ether Rings in 2 and 3 torsion angle P2MP1O1

complex 2 66.59(12)

MP1O1C1

180.0(3)

complex 3 57.12(11) 140.0(3)

P1O1C1C2

161.2(3)

169.8(3)

O1C1C2O2

73.8(4)

162.2(3)

C1C2O2C3

170.9(3)

173.1(4)

C2O2C3C4

77.0(5)

86.8(5)

O2C3C4O3 C3C4O3C5

63.3(5) 168.0(4)

67.3(6) 176.5(4)

C4O3C5C6

172.5(4)

175.3(4)

O3C5C6O4

74.2(5)

59.7(6)

C5C6O4C7

172.9(3)

179.5(5)

C6O4C7C8

89.8(4)

164.6(5)

O4C7C8O5

66.1(4)

62.2(6)

C7C8O5P2

164.2(3)

143.7(4)

C8O5P2M O5P2MP1

169.4(3) 115.70(11)

31.1(3) 120.94(13)

of adjacent biphenoxy rings (distance from H10 to the centroid of the C21C26 ring in 3, 2.6186(6) Å; distance from H19 to the centroid of the C27C32 ring in 2, 2.8966(4) Å), which could stabilize the conformations in both metallacrown ethers.5560 Such stabilization would favor the cis solid-state coordination geometry that is observed in both 2 and 3. cistrans and MonomerOligomer Equilibria in Metallacrown Ether Complexes of 1. Both 2 and 3 demonstrate a clear preference for cis, monomeric coordination of the phosphite groups of 1, and the X-ray crystal structures of the metallacrown ethers suggest that this is the best coordination geometry for the formation of a crown ether-like pocket. Thus, it is of interest to determine why this geometry is strictly preferred in both metallacrown ethers. The major factor that appears to determine the coordination preference of bis(phosphorus-donor)/polyether ligands in PdCl2 metallacrown ethers seems to be the coordination preference of the phosphorus-donor group in the ligands. Studies of PdCl2 metallacrown ethers containing Ph2P(CH2CH2O)nCH2CH2PPh2 (n = 35) ligands have demonstrated that both cistrans and monomercyclic oligomer equilibria are observed when these phosphine-donor ligands are coordinated.2123,5153 Similar results have also been obtained with closely related RhCl(CO) metallacrown ethers containing these ligands.11,14 In contrast, there is no evidence for either cistrans or monomer oligomer equilibria with either PdCl2 metallacrown ethers of 1 or of the closely related, but sterically constrained, (2,20 -C12H8O2)POCH2CH2O-1-C6H4-2-OCH2CH2OP(2,20 -O2H8C12).30 For each PdCl2 metallacrown ether, a single 31P{1H} NMR resonance is observed regardless of the concentration or the solution temperature. Further, PdCl2 complexes of related phosphite ligands, (2,20 -C12H8O2)POMe and (2,20 -C12H8O2)POCH2CH2OMe, also exhibit a single 31P{1H} NMR resonance.2830

Figure 2. ORTEP drawing of the metallacrown ether ring in 3. The thermal ellipsoids are drawn at the 25% probability level, and phosphorus substituents and hydrogen atoms have been omitted from the figure for clarity. 5701


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Table 5. cis T trans Equilibrium Constants for cis-Mo(CO)4(ligand)n Complexes ligand

na

Kctb

(2,20 -C12H8O2)P(OCH3)

2

1.2

(2,20 -C12H8O2)P(OCH2CH2OCH3) Ph2P(CH2CH2O)3CH2CH2PPh2

2 1

1.2 1

Ph2P(CH2CH2O)4CH2CH2PPh2

1

1

(2,20 -C12H8O2)POCH2CH2O-1-C6H4

1

0

1

0

-2-OCH2CH2OP(2,20 -O2H8C12) (2,20 -C12H8O2)P(OCH2CH2)4OP(2,20 -C12H8O2)

“n” represents the number of ligands coordinated to the Mo(CO)4 group. b Kct is the cistrans equilibrium constant defined as equal to [trans]/[cis].22,2830 a

This behavior suggests that the phosphite groups in these ligands prefer to be trans to the chloro ligands rather than to another phosphite. As was previously observed with the bis(phosphine) polyether ligands, the cis coordination geometry minimizes any tendency for oligomerizations to occur.11,14,2123,5153 The intramolecular ππ stacking between the biphenoxy substitutents on the two phosphite groups, discussed in the X-ray crystallography section, could also favor the cis coordination geometry. The coordination behavior of 1 and related phosphite ligands in cis-Mo(CO)4 complexes is quite different. The cis-Mo(CO)4{Ph2P(CH2CH2O)nCH2CH2PPh2} (n = 35) metallacrown ethers are inert to ligand exchange, but do undergo cistrans equilibria in solution upon addition of solid HgCl2.22 The equilibrium mixtures contain almost equal amounts of the two isomers regardless of the ring size (Table 5). Both cis-Mo(CO)4{(2,20 -C12H8O2)POCH2CH2OMe}2 and cis-Mo(CO)4{(2,20 C12H8O2)POMe}2 also isomerize in solution in the presence of HgCl2 to give approximately equal molar amounts of the cis and trans isomers.2830 In contrast, neither 2 nor cis-Mo(CO)4(2,20 -C12H8O2)POCH2CH2O-1-C6H4-2-OCH2CH2OP(2,20 -O2H8C12)} isomerizes in solution in the presence of any amount of HgCl2 and are among the first reported to not undergo reaction with HgCl2.2830 It seems unlikely that the lack of isomerization of these metallacrown ethers is due to the coordination preferences of the phosphite groups, as seems to be the case for the cis-PdCl2(phosphite)2 complexes, because cisMo(CO)4 complexes of the monodentate phosphites do isomerize in solution.2830A possible explanation is that subtle conformational preferences of the polyether groups in the metallacrown ethers are affecting the coordination geometries, but this does not explain why cistrans isomerization is observed with cis-Mo(CO)4{Ph2P(CH2CH2O)nCH2CH2PPh2} (n = 35) metallacrown ethers in which both the metallacrown ether ring sizes and the bridging polyether groups are similar. It is possible that the lack of isomerization in 2 and cis-Mo(CO)4(2,20 -C12H8O2)POCH2CH2O-1-C6H4-2-OCH2CH2OP(2,20 O2H8C12)}is due to HgCl2 being a poor catalyst for the isomerization reactions, although these would be the first cisMo(CO)4(phosphorus-donor ligand)2 complexes for which this is the case. Li+ Binding Studies of 2. The X-ray crystal structure of 3, discussed above, and the previously reported X-ray structure of 2 suggest that cation binding to the metallacrown ether oxygens could occur.47 To determine if cation binding to 2 could be followed by NMR spectroscopy, as was previously done with the closely related cis-Mo(CO)4(Ph2P(CH2CH2O)nCH2CH2PPh2)

(n = 4, 5) metallacrown ethers, a preliminary titration of 2 with solid LiBPh4 3 3dme was carried out. Significant changes were observed in the 31P{1H} NMR chemical shift of 2 upon addition of the LiBPh4 3 3dme. In contrast, similar studies that were previously carried out and reported with both cis-Mo(CO)4{(2,20 -C12H8O2)POCH2CH2OMe}2 and cis-Mo(CO)4{(2,20 C12H8O2)POMe}2 exhibited no shift or change in the 31P{1H} NMR spectra of these complexes upon addition of the LiBPh4 3 3 dme salt to solutions of the complexes.2830 31 1 P{ H} NMR Continuous Variation Job Studies of LiBPh4 3 3dme with 2. A continuous variation Job study was carried out at two different concentrations (10 and 100 mM) in dichloromethane-d2 to determine whether 2 could bind Li+ as well as to demonstrate the stoichiometry of any observed binding events, and the results are shown in Figure 3.36,37 The individual Job curves have well-defined maxima at a 0.50 mol fraction of LiBPh4 3 3dme, indicating a high alkali salt affinity in this solvent, and one binding event with a 1:1 stoichiometry.48 Changing the concentration of 2 does not appear to shift the maximum peak position or overall shape. These results are consistent with those from the 31P{1H} NMR titrations of 2 with Li+, discussed below, as well as with the X-ray crystallography results, discussed previously. The binding stoichiometry from these Job studies is also consistent with all previous results from titrations of cisMo(CO)4{(Ph2P(CH2CH2O)nCH2CH2PPh2-P,P0 } (n = 4, 5) metallacrown ethers with LiBPh4 and NaBPh4 in which one binding event with Li+ at a 1:1 stoichiometry was observed in a 1:1 acetonitrile/CCl4 mixture.23 31 1 P{ H} NMR Titration of a 1:1 Mixture of 2 and LiB(C6F5)4 3 2Et2O in Dichloromethane-d2 with a 1:1 Mixture of 2 and LiB(C6F5)4 3 2Et2O in Tetrahydrofuran. To obtain an accurate equilibrium binding constant, it is essential to carry out titrations under equilibrium conditions where both free metallacrown ethers and free Li+ are present throughout the titration (nonstoichiometric binding conditions). However, if this is not done, high affinity, stoichiometric binding conditions dominate and any calculated binding affinity only represents an upper limit of the Kd.3841 Stoichiometric, high affinity binding titration curves are often characterized by linear titration curves with hard breaks at the ligand concentration equal to the macromolecule concentration. Additional problems with stoichiometric binding in spectroscopic titrations are further complicated by the fact that the observed signal is often a weighted average of the free metallacrown ether and the bound metallacrown ether. Thus, the free [Li+] or free ligand concentration is not directly observable and should not be assumed or mathematically substituted for in any applied model of titration data and is a common mistake. The choice of the Li+ salt for the titration is important because all Li+ salts with noncoordinating anions have some solvent associated with the Li+ cation. We chose to use LiB(C6F5)4 3 2Et2O rather than LiBPh4 3 3dme because the diethyl ether coordinates more weakly to the alkali cation than does the dme.68 However, even with LiB(C6F5)4 3 2Et2O, nonstoichiometric binding conditions could not be achieved in either dichloromethane-d2 or chloroform-d, even at the lowest concentrations of 2 that were detectable by 31P{1H} NMR (∼1 mM) for accurate and timely signal observation. In contrast, titrations of 2 with LiB(C6F5)4 3 2Et2O in only tetrahydrofuran or acetonitrile did not exhibit a sufficiently large change in the chemical shifts of the 31P{1H}, 1H, or 13C{1H} NMR resonances between titration points to allow an accurate calculation of any binding affinity due to the coordination ability and competitive nature of the solvent. 5702


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Figure 3. 31P{1H} NMR continuous variation Job plot of 2 (Δσ [2]) vs mole fraction of LiBPh4 3 3dme. Square data points were run at [2] = 0.1 M, and diamond-shaped data points were carried out at [2] = 0.01 M.

Figure 4. 31P{1H} NMR titration of a 1:1 mixture of 2 and LiB(C6F5)4 3 2Et2O in dichloromethane-d2 with a 1:1 mixture of 2 and LiB(C6F5)4 3 2Et2O in tetrahydrofuran-d8. An observed, normalized signal value of 1 corresponds to 2 with a stoichiometric amount of LiB(C6F5)4 3 2Et2O bound; an observed, normalized signal value of 0 corresponds to all free 2.

To achieve nonstoichiometric binding conditions, the observed association binding constant, Kobs, was lowered by using a mixture of tetrahydrofuran-d8 and dichloromethane-d2. Tetrahydrofuran was chosen because it is very similar to the diethyl ether that is coordinated to the Li+ in the LiB(C6F5)4 3 2Et2O salt and because it is commonly used as a solvent in the styrene hydroformylation reactions catalyzed by Rh metallacrown ethers.2630 To determine a concentration of tetrahydrofurand8 in dichloromethane-d2 at which both free alkali metal cations and free 2 are present, and at which some coordination of the Li+ to 2 is detectable, a tetrahydrofuran solvent back-titration was carried out and plotted in Figure 4. Aliquots of a solution of tetrahydrofuran-d8 containing a 1:1 ratio of 2 and LiB(C6F5)4 3 2Et2O were added to a dichloromethane-d2 solution that also contained a 1:1 ratio of 2 and LiB(C6F5)4 3 2Et2O at the same

concentration at 301 K. The 31P{1H} NMR chemical shifts were converted to observed, normalized signal changes using the form (σo σI)/(σF σI), where σo is the observed chemical shift, σI is the initial chemical shift of 2 in this case, and σF is the final chemical shift of bound 2 3 Li+, and were plotted versus the concentration of tetrahydrofuran-d8 present, as shown in Figure 4. When no tetrahydrofuran-d8 was present, the observed, normalized signal was equal to 1, indicating that all of the Li+ is bound to 2 and there were no free Li+ cations in solution. As tetrahydrofuran-d8 was added, the normalized 31P{1H} signal decreased in a sigmoidal fashion until it approached zero at approximately 1 M in tetrahydrofuran-d8 concentration. When the signal is equal to zero, very little of the Li+ remains coordinated to 2. The titration curve in Figure 4 suggested that a 5703


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Figure 5. Plot of observed, normalized signal (σo σI)/(σF σI) vs [Li+]/[2] at [2] = 97, 53, 18.8, and 9.6 mM. The curves representing [2] = 97 and 53 mM overlap, indicating that these two titrations are under stoichiometric binding conditions and no free Li+ is present. The titration model was applied to only the [2] = 18.8 and 9.6 mM titration curves.

concentration of 0.4 M tetrahydrofuran-d8, which yielded an observed normalized signal change between that of free 2 and bound 2 3 Li+, and should allow 31P{1H} NMR titrations to be carried out under equilibrium conditions without high affinity, stoichiometric binding to LiB(C 6F5)4 3 2Et2O. According to Figure 4, tetrahydrofuran is able to compete with 2 for binding to the alkali metal salt as a negative, heterotropic ligand.3841 31 1 P{ H} NMR Titrations of 2 with LiB(C6F5)4 3 2Et2O. Titrations of 2 were carried out by adding aliquots of a solution containing LiB(C6F5)4 3 2Et2O and 2 to a solution that contained only 2 in a screw-top, NMR sample tube in dichloromethane-d2/ 0.4 M tetrahydrofuran. Four different concentrations of 2 (9.6, 18.8, 53, and 97 mM) were titrated to span all possible binding events over the 101102 M concentration range in 2. Plots of the observed, normalized signal versus the ratio of [Li+] to [2] are shown in Figure 5. The overlap of the plots for the titrations performed at concentrations of 53 and 97 mM in 2 indicates that stoichiometric binding is observed when the concentration of 2 is significantly larger than 19 mM in 2 in the dichloromethane-d2/ 0.4 M tetrahydrofuran solvent mixture.3841 The observed association binding constant, Kobs, was then determined only from the titration data at 9.6 and 18.8 mM in 2. The simple hyperbolic shapes of the titration curves in Figure 5 imply that 2 binds to Li+ in a 1:1 manner, consistent with the Job analysis. For each point in the titration with a 1:1 binding model, Kobs was related to the free Li+ concentration, [Li+]F, and to the binding density, υ, which is the ratio of the bound Li+ to the total

concentration of 2, as shown in eq 1.3841 υ¼

Kobs ½Liþ F 1 þ Kobs ½Liþ F

ð1Þ

A binding density analysis is used to generate υ and [Li+]F values.3841 The binding density, υ, is calculated at each point over the observed signal range between roughly 20% and 80% saturation using eq 2. The values of [Li+]T2 and [Li+]T1 are the total [Li+] at the same normalized signal for the [2]T2 = 18.8 mM and [2]T1 = 9.6 mM titrations, respectively. This calculation assumes that the binding density is the same for all species at the same signal regardless of the total concentration of 2 because the signal change is strictly determined from the amount of bound 2.3841 υ¼

½Liþ T2 ½Liþ T2 ½2T2 ½2T1

ð2Þ

The [Li+]F is related to υ by the conservation of mass in eq 3 shown below. ½Liþ T ¼ ½Liþ F þ v½2

ð3Þ

A global, nonlinear least-squares data fit of the 9.6 and 18.8 mM alkali titration data using eq 1 was performed in Scientist 3.0 and gave an observed binding affinity, Kobs, of 0.077 ( 0.005 mM1 (R = 0.98), as shown in Figure 6. The observed binding affinity is consistent with stoichiometric binding observed at the two higher concentrations in Figure 5 (53 and 97 mM). 5704


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Figure 6. Global fit of normalized signal vs [Li+]T values for titrations run at [2] = 9.6 and 18.8 mM. Data points are experimental titration values, while the solid lines represent the global, nonlinear least-squares regression of titration data. The global regression yields a binding affinity of Kobs = 0.077 mM1.

Because the metallacrown ether concentration for the titrations shown in Figure 5 and global, nonlinear least-squares titration fits in Figure 6 varied slightly, the method for the 9.6 mM titration was modified to ensure a constant metallacrown ether concentration of 9.7 mM remained throughout this repeated titration. A global, nonlinear least-squares fit of the 9.7 and 18.8 mM titrations was carried out as described above and gave a Kobs of 0.077 + 0.005 mM1. This is identical to the Kobs previously calculated using the 9.6 and 18.8 mM titrations, indicating that the slight variance in the metallacrown crown ether concentration during titration experiments had no effect on the determination of the observed binding affinity.50 These results are consistent with the expectation that the binding constant should be independent of the concentration of the metallacrown ether, but were repeated to ensure this result.3841 The observed equilibrium binding association constant determined for 2 with Li+ is valid only for the 0.4 M solution of tetrahydrofuran in dichloromethane-d2. It would be more useful to be able to calculate an observed binding association constant for any ratio of tetrahydrofuran to dichloromethane-d2. The linear relationship between the observed signal and binding density, shown in Figure 7, allows simplified relationships to be used to determine the average binding density and free alkali salt concentration from the single, solvent back-titration curve shown in Figure 4.3841 The simplified expression for the free ligand or free [Li+] is given by eq 4, and the expression for the binding density, υ, is given by eq 5, where Sobs is the observed normalized signal change and Smax is the maximum, observed signal change when the binding density is equal to 1. The value for Smax is obtained from a linear extrapolation of the normalized, observed signal

value to a binding density, υ, equal to 1. Sobs ½Liþ T ½Liþ F ¼ 1 Smax υ¼

Sobs ½Liþ T Smax ½2T

ð4Þ

ð5Þ

The noncooperative binding model of McGeevon Hippel for a 1:1 binding system can then be used to calculate the observed association binding constant, Kobs, at each tetrahydrofuran concentration in the solvent back-titration in Figure 4 from υ and [Li+]F using eq 6.40 υ ¼ K obs ð1 υÞ ½Liþ F

ð6Þ

The observed equilibrium association binding constants, Kobs, calculated using eq 6 at different tetrahydrofuran concentrations, can then be related to the intrinsic association binding constant, Ka. To do this, the Li+ is assumed to be coordinated only to tetrahydrofuran molecules and not to either diethyl ether or dichloromethane-d2 molecules. This assumption is reasonable because both diethyl ether and dichloromethane-d2 bind considerably more weakly to Li+ than does tetrahydrofuran, and diethyl ether is also present in much lower concentrations (approximately 1/10 at the equivalence point). If this is the case, the binding equilibrium reaction is MCE þ Liþ ðTHFÞn h MCE 3 Liþ þ nTHF 5705

ð7Þ


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Figure 7. Observed normalized signal vs υ, degree of binding, obtained from titrations at [2] = 9.6 and 18.8 mM. As the signal approaches the end point or 1, υ approaches 1.0, indicating 1:1 binding and demonstrating the linear correlation between signal dependence with the degree of binding with complex 2.

The intrinsic association equilibrium constant, Ka, for the 1:1 binding reaction of 2 to Li+ then becomes Ka ¼

½MCE 3 Li½THFn ½MCE½Liþ

ð8Þ

Because the observed association equilibrium constant, Kobs, is determined using only [MCE], [MCE 3 Li+], and [Li+], Kobs and Ka are related by Kobs ¼

½MCE 3 Li Ka ¼ ½MCE½Liþ ½THFn

ð9Þ

Equation 9 can then be rearranged into a more useful form log Kobs ¼ log Ka n log ½THF

ð10Þ

A plot of log Kobs versus log [THF] using the Kobs calculated at various tetrahydrofuran concentions using the data from Figure 4 and eqs 46 is shown in Figure 8.41 The data from Figure 4 was restricted to tetrahydrofuran concentrations that give saturations between 20% and 80% to avoid any inaccuracies at titration extremes.3841 A linear regression analysis of the data in Figure 8 obtained from the tetrahydrofuran solvent back-titration shown in Figure 4 allows both the average number of tetrahydrofuran molecules displaced upon binding of Li+ to 2 from the slope, n, and the intrinsic association binding constant, Ka, to be calculated.3841 In addition, Kobs values were obtained at every concentration of tetrahydrofuran by performing a single solvent back-titration experiment. The linear regression analysis of Figure 8 yields log Kobs = 0.446 3.58 log [THF] with R2 = 0.998 in slope-intercept form. The intrinsic association binding constant, Ka, of 2 to Li+, of 2.79(+ 0.16) M1 is obtained from the y intercept, and the

average number of tetrahydrofuran molecules displaced upon binding of Li+(THF)nB(C6F5)4 to 2, n = 3.58, is obtained from the slope. As a check of the accuracy of this approach, eq 10 was used to calculate Kobs for the 0.4 M solution of tetrahydrofuran in dichloromethane-d2. The calculated Kobs of 0.072 mM1 is the same as that obtained from the global, nonlinear least-squares fits of the experimental NMR alkali salt titration data within a standard deviation to measurements. The number of tetrahydrofuran molecules that are displaced, 3.58(+0.05), suggests that the metallacrown ether is displacing nearly all of the solvent molecules from the Li+ cation upon binding. Only a small fraction of the bound Li+ would, therefore, be expected to still be coordinated to any tetrahydrofuran once bound to 2. Consistent with the results from Figure 4, the release of tetrahydrofuran molecules from the alkali salt demonstrates that tetrahydrofuran acts as a negative, hetetropic ligand that competes for binding with 2 to the alkali salt. This conclusion is consistent with the favorable conformations of the ether oxgens in the X-ray crystal structure and expected from the use of tetrahydrofuran to lower the observed association constant of 2 to Li+ and to allow nonstoichiometric binding conditions.47,49 IR Titrations of 2 and cis-Mo(CO)4{P(OPh)3}2 with LiB(C6F5)4 3 2Et2O. The 31P{1H} NMR titrations described above clearly demonstrated that 2 exhibits strong binding to Li+ borate salts in pure dichloromethane. To further confirm the results of NMR titration studies and to ensure that no additional interactions occur between 2 and LiB(C6F5)4 3 2Et2O, FT-IR titrations of both 2 and a nonmetallacrown ether complex, cis-Mo(CO)4{P(OPh)3}2, were performed in dichloromethane with LiB(C6F5)4 3 2Et2O. The FT-IR titrations were followed by monitoring changes in the CO stretching bands of the FT-IR spectra 5706


Organometallics

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Figure 8. Plot of log Kobs vs log [THF]. Kobs values were calculated from tetrahydrofuran solvent back-titration data in Figure 4 using eqs 49. A linear regression yields log Kobs = 3.5766 log [THF] + 0.4464 with R2 = 0.998.

Figure 9. FT-IR titration study of the effect on 2 with LiB(C6F5)4 3 2Et2O in dichloromethane on the A1, A2, B1, and B2 carbonyl stretching modes. A1 bands are observed for both 2 and the 1:1 adduct of 2 with Li+ until the stoichiometry reaches a 1:1 molar ratio of Li+ to 2, as indicated by arrows inserted into the spectrum.

as aliquots of a dichloromethane solution of LiB(C 6 F 5 )4 3 2Et2O were added to a dichloromethane solution of either 2 or cis-Mo(CO)4{P(OPh)3}2. The addition of LiB(C6F5)4 3 2Et2O to cis-Mo(CO)4{P(OPh)3}2 caused no changes in the CO ligand peak positions or shapes throughout the titration. This result indicates that there is no interaction or binding observed between the Li+ cation and either the carbonyl or the phosphite oxygens of cis-Mo(CO)4{P(OPh)3}2 in the dichloromethane solution. In contrast, the plot of the titration data for 2 with LiB(C6F5)4 3 2Et2O, shown in Figure 9, demonstrates that cation binding to 2 affects the bonding of the CO ligands to the Mo

metal center. Under the stoichiometric binding conditions demonstrated from the NMR titration studies, the addition of LiB(C6F5)4 3 2Et2O causes the IR stretches of the carbonyl ligands of 2 to shift to higher frequencies (∼5 cm1) upon coordination of the alkali salt. This was most clearly seen for the totally symmetric A1 band at 2045 cm1. As LiB(C6F5)4 3 2Et2O is added, a new symmetric A1 band, presumably due to the 1:1 adduct based on the NMR titration data, appears at 2050 cm1, as indicated by the arrow in Figure 9. Continuing addition of LiB(C6F5)4 3 2Et2O causes the 2045 cm1 band to gradually decrease in intensity and the 2050 cm1 band to increase in 5707


Organometallics intensity until only the 2050 cm1 band is present after 1 equiv of LiB(C6F5)4 3 2Et2O had been added. An isosbestic point between the 2045 and 2050 cm1 A1 bands indicates that 2 is being converted into a new species upon addition of LiB(C6F5)4 3 2Et2O. Addition of LiB(C6F5)4 3 2Et2O beyond a 1:1 ratio of Li+ to 2 has little effect on the A1 band intensity and position, only broadening the A1 band. Similar shifts to higher energy also appear to be occurring for the A2, B1, and B2 bands, but these are more difficult to quantify because of the overlap between these bands in a cis-C2v metal complex, such as 2. The shift of the carbonyl absorption A1 band to higher energy corresponds to an increase in the stretching force constants between the carbon and oxygen of these carbonyl ligands. The stretching force constants for both the lithium adduct of 2 and free 2 were calculated using the CottonKraihanzel procedure.46 These calculations indicate that both kcis (cis-coordinated CO ligand stretching force constant) and ktrans (trans-coordinated CO ligand stretching force constant) increase upon binding of the alkali metal salt and that the increase is approximately the same (9.9% for kcis increased vs 9.8% for ktrans). The significant, but relatively small, change in the carbonyl stretching frequencies is most likely the result of a Li+ cation being bound near the cisMo(CO)4 metal center. This would decrease the ability of Mo to donate metal d electrons into the π* orbitals of the carbonyl ligands, thereby increasing the bond orders of the CO ligands. The fact that no effects are observed with the CO ligand stretching frequencies or band shape of cis-Mo(CO)4{P(OPh)3}2 upon the addition of LiB(C6F5)4 3 2Et2O strongly suggests that binding of Li+ to 2 is responsible for the inductive effect observed with the CO ligands in the FT-IR spectrum when LiB(C6F5)4 3 2Et2O is added to the metallacrown ether, 2.

’ CONCLUSIONS Both Mo(CO)4, 2, and PdCl2, 3, metallacrown ethers derived from (2,20 -C12H8O2)PO(CH2CH2O)4P(2,20 -O2H8C12), 1, have a cis coordination geometry, and neither 2 nor 3 shows any evidence of cistrans or monomeroligomer equilibrium in solution. The X-ray crystal structures of 2 and 3 suggest that the conformations of their metallacrown ether rings would allow the coordination of cations, such as Li+. Job and equilibrium titration studies of 2 with LiB(C6F5)4 3 2Et2O have been carried out using 31P{1H} NMR spectroscopy. Both the titration data and the Job studies indicate that only a 1:1 binding stoichiometry with Li+ is observed. Complex 2 binds Li+ too strongly to allow equilibrium binding constants to be measured in weakly coordinating solvents, such as dichloromethane or chloroform. However, the addition of tetrahydrofuran (0.4 M) to dichloromethane allows the accurate measurement of equilibrium binding constants for the binding of Li+ by 2 at concentrations of 2 below approximately 20 mM by lowering the observed equilibrium association constant, Kobs. In this solution, the observed association equilibrium constant, Kobs, for the binding of Li+ by 2 was calculated to be 0.077 mM1 by global, nonlinear least-squares regression analysis of titration data using Scientist 3.0.62 Using data obtained from a tetrahydrofuran solvent back-titration, a plot of log Kobs vs log [THF] was created, and a linear regression analysis of this data yielded log Kobs = 3.58 log [THF] + 0.446. Through the use of the solvent back-titration data, the intrinsic association binding constant was calculated from the y intercept and allowed Kobs to be calculated for any dichloromethane/tetrahydrofuran mixture. The slope,

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3.58, gave the average number of tetrahydrofuran molecules that are displaced from the Li+ upon coordination to the metallacrown ether and suggested that the metallacrown ether fills nearly all of the coordination sites on the Li+. The Kobs calculated from the tetrahydrofuran solvent back-titration data was consistent with the Kobs calculated from the global, nonlinear regression of titration data in Figure 6. Evaluation of the effect of Li+ binding to 2 on the CO ligands using FT-IR titrations demonstrated that this increased the cis (kcis) and trans (ktrans) stretching force constants of the CO ligands by approximately 10% in pure dichloromethane under high affinity, stoichiometric binding conditions. Such a decrease of the donor ability of the Mo center could increase the susceptibility of the carbonyl ligands to nucleophilic attack and migratory-insertion reactions. Perhaps the most significant aspect of this research is the development of this methodology that allows the accurate determination of inorganic and organometallic equilibrium binding association constants under nonstoichiometric (low affinity) conditions from NMR titration data. The solvent back-titration allows the best conditions for accurate, equilibrium binding constants under nonstoichiometric conditions, independent of the intrinsic binding affinity, to be quickly determined in a single titration as a function of solvent as well. The global, nonlinear least-squares regression of multiple titration data sets that was demonstrated is far superior to and just as easily performed as nonlinear leastsquares regression analyses of individual titration curves. Further, when any observed signal change is linear with the binding density, υ, the dependence of the observed equilibrium binding constants on the concentration of the coordinating and competitive solvent can be determined from a single, solvent back-titration. This allows the effects of solvent coordination, pH, or other nonspecific interactions to be determined in one titration experiment. The approach developed and demonstrated in this work should be applicable to a wide variety of organometallic and inorganic systems in which binding can be followed by NMR spectroscopy or other spectroscopic methods but in which the concentrations of free species of the ligand cannot be directly monitored.

’ ASSOCIATED CONTENT

bS

Supporting Information. Individual Job data and titration data sets plotted at each concentration of metallacrown ether, data sets plotted for solvent back-titration, data sets plotted for FT-IR titration, and crystallographic information files (CIF) for compound 3. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors acknowledge the support of the National Science Foundation under Grant EPS-0447675. J.T.S. thanks the GRSP Scholarship for providing 36 months of funding for a graduate fellowship as well as Samantha Hastings for her contributions. ’ REFERENCES (1) Pedersen, C. J. J. Am. Chem. Soc. 1967, 89, 7017. (2) Pedersen, C. J. J. Am. Chem. Soc. 1970, 92, 386. 5708


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Generally Applicable NMR Titration Methods for the Determination of

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