Nature of Polyoxometalate Intramolecular Coordination to Quaternary

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Nature of Polyoxometalate Intramolecular Coordination to Quaternary Ammonium Salts from Paramagnetic Relaxation Enhancement Jeffrey G. Lundin, Spencer L. Giles, James P. Yesinowski, Brian T. Rasley, and James H. Wynne J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b05122 • Publication Date (Web): 15 Jul 2016 Downloaded from http://pubs.acs.org on July 25, 2016

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The Journal of Physical Chemistry

Nature of Polyoxometalate Intramolecular Coordination to Quaternary Ammonium Salts from Paramagnetic Relaxation Enhancement

Jeffrey G. Lundin†, Spencer L. Giles†, James P. Yesinowski†, Brian T. Rasley‡, and James H. Wynne†*,

†Chemistry Division, Naval Research Laboratory, 4555 Overlook Avenue SW, Washington, D.C. 20375 ‡ Department of Chemistry & Biochemistry, University of Alaska Fairbanks, Fairbanks, AK 99775 USA

Abstract

Polyoxometalates (POMs) exhibit catalytic activity toward a variety of harmful chemicals such as chemical warfare agents, qualifying them as promising candidates as additives to create self1

ACS Paragon Plus Environment


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decontaminating surfaces and materials. However, POMs exhibit poor solubility and dispersion behavior in organic matrices, including polymeric coatings.

In an effort to improve

compatibility with polymer coatings and impart surface segregating behavior, we describe the encapsulation of a Ni(II)-containing POM, α2-K8P2W17O61(Ni2+·OH2)·17 H2O (Ni-POM), with a series of amphiphilic alkyl ethoxy dimethyl quaternary ammonium salts (QASs) and elucidate their structural coordination.

Diffuse reflectance infrared Fourier transform spectroscopy

(DRIFTS), differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA) were utilized to confirm that QASs are coordinated to the Ni-POM and that the average number of QAS coordinated to each Ni-POM increases with increasing alkyl moiety length. The 1H NMR spectra of the QAS(Ni-POM) complexes show marked site-specific broadening and reduced spin-lattice relaxation times T1 compared to either a non-paramagnetic QAS(POM) complex or the neat QAS ligand. These paramagnetic relaxation enhancement (PRE) effects were used to obtain structural and dynamical information about the binding of QASs to the Ni-POM. The single-exponential saturation recovery behavior observed in all cases indicated that all bound QAS molecules are rapidly moving about the entire (Ni-POM) surface on a time scale less than tens of milliseconds. Motionally-averaged distances of the QAS protons to the paramagnetic Ni2+ center were estimated using a modified Solomon-Bloembergen equation. Comparisons of relative distances for protons at different sites on the QAS molecule provide key insights into the structural nature of the bonding. Surprisingly, the ethylene oxide moiety of the amphiphilic QAS was found to coordinate more closely with the surface of the Ni-POM than the quaternary ammonium nitrogen cation, and the alkyl moieties extended outwards from the Ni-POM center. These results suggest that QAS(Ni-POM) complexes should behave effectively as a hydrophobic non-polar complexes in their desired roles as catalytic centers in coatings. 2


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I. Introduction Polyoxometalates (POMs) consist of a broad class of metal oxide compounds that exhibit catalytic activity towards a variety of substrates, including chemical warfare agents (CWAs) and their simulants.1

Their effective oxidative catalytic ability to decompose CWAs, including

chloroalkyl sulfides, makes POMs very promising candidates for self-decontaminating coating additives.2 There are various types of POMs, including Keggin, Sandwich, and Wells-Dawson structures, each differentiated by the geometries of the metal oxide polyhedra.3-4 Wells-Dawson polyoxometalates containing substituted heterometal defects have exhibited improved capability for catalytic oxidation.5 Recent work has demonstrated that the Wells-Dawson polyoxometalate α2-K8P2W17O61(Ni2+·OH2)·17H2O (Ni-POM) exhibits promising activity against CWA simulants when directly mixed into a commercial coating matrix.6 Despite the numerous benefits of POMs as catalysts, POMs often exhibit limited solubility in organic polymer matrices, such as commercial polyurethane coatings. Therefore, modification of POMs with non-polar ligands presents a reasonable approach to attempt to improve the solubility and compatibility of these species with organic polymer matrices, while at the same time preserving their catalytic behavior. POMs afford the potential for chemical modification through cation exchange due to their anionic metal oxide composition. Cation exchange can occur upon solvation of POM in mildly acidic aqueous solution containing suitable cations.7 Ionic encapsulation of POM molecules by surfactant molecules has been a topic of interest for several research groups recently.8-9

Layer-by-layer deposition10 and adsorption onto silica

templates11 of similar POM compounds have been performed; however, such approaches do not allow for as broad an application as a direct coating additive. It has been demonstrated that some 3



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amphiphilic coating additives, such as amphiphilic quaternary ammonium salts (QASs), automatically surface segregate to the coating-air interface during curing, thus increasing the surface concentration of additive.12-14 The same concept can be applied toward the encapsulation of POMs to result in a surface segregating amphiphilic molecule.

POMs are capable of

modification with organic moieties that range in polarity and result in an amphiphilic catalyst.15 Well-ordered phases of anion clusters and cationic surface lipid bilayers have been observed upon coordination of cetyltrimethylammonium bromide (CTAB) to Keggin-type POMs.16 However, the effects of encapsulation of POM with a class of amphiphilic quaternary ammonium salts (QAS) that exhibit a range of amphiphilic character has not previously been investigated, nor has the structural coordination between QAS and POM been definitively elucidated. In this study we utilize the paramagnetic nature of the high-spin d8 Ni2+ moiety (two unpaired electrons, S = 1) in the octahedral bonding environment of the POM. NMR spectroscopy has been used in the past to discern intermolecular and intramolecular distances in molecules that contain paramagnetic species such as Ni2+.17-19

Specifically, the effects of

1

H NMR

paramagnetic relaxation enhancement (PRE) on T1 relaxation have been utilized to determine distances between protons and paramagnetic species or heterometals in the range of 10-35 Å.20-21 Application of PRE has been successful in determining intramolecular distances in polypeptides22 and for fullerene-peptide interactions.23 Distances calculated from PRE, which represent the conformation of molecules in a solvent, can differ significantly from those calculated from crystal structures that may involve different conformations.22 As such, noncrystalline compounds, such as QAS-POMs, require new approaches to provide detailed structural analysis.

4


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Herein, we report here the synthesis and characterization of novel QAS encapsulating WellsDawson polyoxometalates containing a Ni heterometal substitution, including the use of PRE to obtain semi-quantitative structural information about the coordination. We propose a structural model of the intramolecular binding of QAS to POM, and in doing so, present a method to elucidate the structures of amorphous organic-inorganic hybrid complexes containing paramagnetic species.

II. EXPERIMENTAL Materials All solvents and reagents were reagent grade and used without further purification. Phosphorus

tribromide,

2-methyoxyethanol,

N,N-dimethylhexadecylamine,

and

cetyltrimethylammonium bromide (CTAB) were purchased from Sigma Aldrich (St. Louis, MO) and used as received. All common laboratory solvents and salts were purchased from Fisher Scientific.

Synthesis of QAS A series of QASs (Figure 1) was prepared following a previously reported procedure.12 Briefly, for the QAS C16EO1, 7.8 mmol phosphorus tribromide was added drop-wise to 15.6 mmol of 2-methyoxyethanol maintained at 0 °C. The solution was allowed to equilibrate to room temperature, and then heated to 90 °C, at which point the solution turned yellow. After the solution was allowed to cool to room tempereature, 6 mL of 10 % NaHCO3 was added. Extraction with diethyl ether was followed by drying over MgSO4. The product, 1-bromo-2-(2(2-methoxy-ethoxy)-ethoxy)

ethane,

was

reacted

at

1:1

5


molar

ratio

with

N,N-


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dimethylhexadecylamine under nitrogen in ethanol at 83 °C for 24 h. The resulting C16EO1 product was recrystallized from diethyl ether to form an off-white powder. Product identity and purity was confirmed by 1H NMR.

Synthesis of QAS-POM conjugates The QASs utilized in this study were synthesized following a previously reported procedure.12 Ni containing polyoxometalates (Ni-POMs) were synthesized by the application of procedures modified from two previous reports, in which a single nickel atom was substituted into the α2 position of a Wells-Dawson polyoxometalate.8,

24

Coordination of QAS to Ni-POM was

achieved via a modification of an ion exchange reaction (Figure 1).8

Briefly, either α2-

K8P2W17O61(Ni2+·OH2)·17H2O (Ni-POM) or α2-K8P2W18O62·17H2O (POM-0) and QAS were dissolved in deionized water at pH 6-7 at mole ratios of QAS to POM of 8:1. The solution was stirred for 10 min, during which time the product precipitated. The product was subsequently isolated by liquid-liquid extraction with dichloromethane and ether. The organic phase was filtered and dried, resulting in a solid green/yellow product. Products were characterized via diffuse reflectance FTIR (DRIFTS), 1H NMR, 31P NMR, TGA, and DSC.

Figure 1. General synthetic scheme of QAS-(Ni-POM) complexes. C8EO1(Ni-POM) (m=1, n=6); C8EO2(Ni-POM) (m=2, n=6); C8EO3(Ni-POM) (m=3, n=6); C8EO4(Ni-POM) (m=4, n=6); C10EO1(Ni-POM) (m=1, n=8); C12EO1(Ni-POM) (m=1, n=10); C12EO2(Ni-POM) (m=2, n=10); C16EO1(Ni-POM) (m=1, n=14); C16EO2(Ni-POM) (m=2, n=14). 6


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Thermogravimetric analysis (TGA) was performed with a TA Instruments Discovery TGA at a heating rate of 10 °C/min under nitrogen. Mass loss at less than 600 °C was correlated to the amount of QAS due to the relatively lower temperatures at which QASs degrade compared to Ni-POM. By conversion of the percent mass loss to initial mass, a molecular binding ratio of QAS to Ni-POM was determined.

A TA Instruments Discovery Differential Scanning

Calorimeter (DSC) was employed to investigate thermal properties of the complexes. The DSC was first equilibrated to -50 °C under a nitrogen flow of 50 mL/min.

Two successive

temperature ramps were then performed from -50 °C to 200 °C at a rate of 20 °C/min, between which the sample was equilibrated to -50 °C. TA Instruments Trios software was utilized for both TGA and DSC analyses. A Thermo Scientific Nicolet 6700 FTIR was utilized for diffuse reflectance infrared Fourier transform (DRIFTS) analysis of neat QAS, neat Ni-POM, and QAS(Ni-POM) conjugates. Samples were first mixed with oven dried KBr, finely ground with a mortar and pestle, and placed in an oven at 120 °C for 1 hour prior to analysis. TGA and DSC samples were performed in triplicate and values were averaged. 1

H and 31P NMR spectra were obtained with samples prepared at concentrations of 50 mM in

CDCl3 at 298 K on a Bruker 300 MHz nuclear magnetic resonance spectrometer. For 1H NMR, a TMS internal chemical shift reference was employed. Spin-lattice, or T1, relaxation values were determined by inversion recovery experiments. The recovery delay time between the 180° inversion pulse (23.2 µs) and the 90° measurement pulse (11.6 µs) was progressively increased over 40 intervals between 0.1 ms and 5 seconds in order to observe exponential relaxation. Data analysis and peak deconvolution of NMR spectra were performed using Spinworks version 3.1.7. Peak deconvolution was first performed on the longest delay time spectra to obtain peak shifts 7



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and half-height line widths (HHLW). Chemical shifts and HHLW thus obtained were then held constant and used in the deconvolution of spectra across the array of delay times. The signal intensity was plotted versus recovery delay time and fit to a single exponential using non-linear curve fitting in OriginPro 2016 to determine T1 relaxation values.

III. RESULTS AND DISCUSSION

Precipitation of QAS(Ni-POM) from the aqueous reaction solution provided an initial confirmation of a successful ion exchange reaction. However, deducing the relative amounts of QAS coordinated to each Ni-POM and the directional orientation of the coordination between QAS and Ni-POM was non-trivial. As such, analyses were performed to both confirm successful synthesis and to elucidate the nature of QAS(Ni-POM) binding.

IR Analysis Neat QAS and Ni-POM were analyzed by DRIFTS for reference and compared to spectra of QAS(Ni-POM) complexes (Supporting information). Overall, significant absorptions from both Ni-POM and QAS were observed in all QAS(Ni-POM) DRIFTS spectra, which are demonstrated for C16EO2(Ni-POM) in Figure 2. Specifically, the aliphatic stretching modes from 2800-3000 cm-1 corresponding to CH2 of QAS and the ether stretching vibrations at approximately 1120 cm-1 were present in the QAS(Ni-POM) spectra.

As expected, the

absorbance of the ether stretching modes at approximately 1200 and 1120 cm-1 that were observed in the QAS containing ether moieties were absent in CTAB spectrum (Supporting information). The successful coordination of QAS to Ni-POM was indicated by a narrowing of 8


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the region in which the ether stretching at 1120 cm-1, as seen in the representative spectral comparison in Figure 2, suggesting that the ether moieties were in similar chemical environments upon their coordination to the Ni-POM.

Thus, the ether moieties are most likely in close

proximity to the Ni-POM cage. Vibrational contributions from Ni-POM, such as the absorbance due to the asymmetric stretching of W=O and W-O-W at 943 and 907 cm-1, respectively, were observed in the IR spectrum of C16EO2(Ni-POM) in Figure 2, as well as other QAS(Ni-POM) complexes. Taken together, these observations provide evidence of the expected electrostatic coordination of QAS and Ni-POM.

Figure 2. DRIFTS spectra of Ni-POM, C16EO2(Ni-POM), and C16EO2.

Thermal Analysis TGA The relative quantities of QAS bound to both Ni-POM and POM-0 were interrogated by TGA. Thermal properties of each QAS(POM) complex from TGA are presented in Table 1. QAS(NiPOM) conjugates were stable at temperatures up to 200 °C, above which a range of 24 – 41 % 9



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mass loss was observed upon increased heating to 600 °C. TGA of neat Ni-POM indicated stability of the inorganic cage to temperatures greater than 600 °C. Thus the mass loss between 200 and 600 °C was attributed to the thermal degradation of the organic QAS component of the complex. Conversion of percentage of mass loss to initial mass afforded the molar ratio of QAS to Ni-POM for each molecule to be determined. A range of approximately 5 to 8 QAS were coordinated to each Ni-POM, depending on QAS composition, which corresponds well with the formal charge of -8 for the POM cage.8 Other formal charges could result from impurities; however, recrystallization steps were performed to limit the presence of impurities within the synthesized POM.

During the ion exchange reaction to synthesize QAS-POM, potassium

counter cations are replaced with QAS, which causes the QAS-POM molecules to precipitate. For the QAS-POM with less than 8 QAS per POM as determined by TGA, it is possible that precipitation occurred prior to exchange of all 8 QAS, while full coordination was still maintained through combination of QAS and potassium counter-ions.

Table 1. TGA data of QAS(POM)s. mass loss (mg)

moles QAS

C8EO1(Ni-POM)

initial mass (mg) 7.437

final mass moles (mg) POM

1.773

8.19E-06 5.664

1.32E-06

6.2 ± 0.2

C8EO2(Ni-POM)

10.528

3.109

1.19E-05 7.419

1.72E-06

6.9 ± 0.1

C8EO3(Ni-POM)

11.312

3.020

9.92E-06 8.292

1.93E-06

5.2 ± 0.4

C8EO4(Ni-POM)

8.458

2.696

7.74E-06 5.762

1.34E-06

5.8 ± 0.3

C10EO1(Ni-POM) 9.486

2.809

1.15E-05 6.677

1.55E-06

7.4 ± 0.5

C12EO1(Ni-POM) 12.182

4.132

1.52E-05 8.050

1.87E-06

8.1 ± 0.2

10


molar ratio QAS:POM

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C12EO2(Ni-POM) 19.052

6.689

2.11E-05 12.363

2.87E-06

7.4 ± 0.3

C16EO1(Ni-POM) 10.638

4.119

1.25E-05 6.519

1.52E-06

8.3 ± 0.2

C16EO2(Ni-POM) 21.011

8.604

2.31E-05 12.407

2.88E-06

8.0 ± 0.1

CTAB(Ni-POM)

5.196

1.762

6.19E-06 3.434

7.98E-07

7.8 ± 0.3

C16EO1(POM-0)

10.193

3.221

3.22E-03 6.972

1.62E-06

6.1 ± 0.5

C16EO2(POM-0)

14.177

5.217

5.22E-03 8.960

2.08E-06

6.7 ± 0.2

A decrease in the QAS:POM binding ratio was expected to correlate with an increase in alkyl moiety length from 8 to 16 carbons, since increased nonpolar regions of the longer alkyl moiety might experience increased repulsion forces from the polar surface of the POM. However, TGA data indicated that an increase in alkyl moiety length x of the QAS resulted in a continuous increase in QAS binding per POM molecule for the two series CxEO1 and CxEO2.

This

unexpected result may have occurred due to the effect that increased alkyl tail length had on the solubility of QAS in aqueous solution during the complexation reaction. A longer alkyl tail would impart increased non-polar character on the QAS which may shift the reaction equilibrium from dissolution in water to favor its binding to POM. For the shorter alkyl tail lengths, it may be that the equilibrium of those molecules lies in a regime which allowed for a certain portion of QAS to remain soluble in the aqueous solution after coordination of QAS to POM caused the complexes to precipitate.

DSC Each neat QAS and neat Ni-POM was subjected to DSC analysis. Neat C12EO1 and C16EO1 each exhibited only a single phase transition, while the QAS species containing ethoxy moieties 11



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of two units in length, C12EO2 and C16EO2, both exhibited two thermal phase transitions (Figure 3, left). Thermal transitions for C12EO2 and C16EO2 occurred at 85 and 93 °C, respectively, in addition to the primary transition that occurred for all QAS in the region of 114 - 118 °C. The primary endothermic transition observed for all QAS corresponds to a reversible crystalline melting transition (Tm). Thus, the neat QASs exist in a crystalline phase until the transition at heating through 114 – 118 °C. A transition at 114 °C of comparable europium substituted Keggin polyoxmetalates coordinated to neat QAS species has been attributed to lamellar mesophases.25 Furthermore, exotherms in the cooling ramps, indicating recrystallization (Figure 3, right), occur over a similar temperature range, thereby supporting the Tm endotherm.

Figure 3. Heating (left) and cooling (right) ramps of QAS and Ni-POM, uncoordinated.

The thermal phase behaviors of QAS(Ni-POM)s were also investigated (Figure 4). DSC analysis of QAS(Ni-POM)s of carbon moiety lengths 8-10 did not demonstrate any notable thermal behavior upon heating from -50 to 200 °C (data not shown). However, the DSC analysis of C16EO1(Ni-POM), C16EO2(Ni-POM), and CTAB(Ni-POM) indicated reversible thermal 12


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transitions occurring between temperatures -50 and 25 °C.

Furthermore, the endothermic

transition was determined to be dependent on the alkyl and ethylene oxide moiety lengths since an endotherm was not observed until the alkyl moiety reached 16 carbons in length. The endotherm of C16EO1(Ni-POM) was sharper than that of C16EO2(Ni-POM), the difference between which is only a single ethylene oxide unit. QAS(Ni-POM) exhibited a much lower Tm than that of the respective neat QAS.

Thus, the interfacial binding energy of molecules in

crystalline phase of QAS(Ni-POM) was much less than in neat QAS. In similar crystalline QAS encapsulated Keggin polyoxometalates, lamellar to isotropic phase transitions have been observed.26 Therefore, due to both the absence of Tm in QAS(Ni-POM) species of carbon moiety lengths less than 12 and the low Tm for C16EOx(Ni-POM), it is proposed that the alkyl moieties between separate QAS(Ni-POM) molecules were coordinated together through cohesive nonpolar interactions.

The QAS(Ni-POM) of shorter alkyl moieties did not exhibit such

crystalline behavior since their electrostatic repulsion would hinder interaction at such interPOM radii.

Figure 4. DSC 2nd heating (left) and 1st cooling (right) ramps of QAS(Ni-POM)s.

13



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Exothermic transitions were observed in Figure 4, right during the DSC cooling cycles that corresponded with the same temperature regions as the endothermic transitions during heating. Considered together, these results confirm the crystallinity of QAS(Ni-POM) conjugates of 16 carbon moiety length at temperatures below the observed endotherms. Previous studies of branched QAS encapsulating Keggin POMs observed a secondary high temperature transition that increased from 129 to 168 °C as the number of QAS coordinated to POM increased.27 Yet, only single broad transitions were observed at low temperatures for QAS(Ni-POM) with alkyl moieties greater than 10 carbons in length. Narrow sharp transitions would be expected if the chemical environment of all the crystalline phases were identical, yet broad transitions were observed that were attributed to heterogeneity within the system causing crystallization of many alkyl moieties in varied chemical environments. Comparison of the thermal behavior between neat QAS and the QAS(Ni-POM) complexes yielded several insights. Whereas each neat QAS exhibited significant and sharp endothermic transitions upon heating at 114 - 118 °C associated with a Tm, the corresponding QAS(Ni-POM) complexes exhibited contrastingly broad low temperature (-50 – 25 °C) endothermic transitions. Thus, coordination of QAS to Ni-POM eliminated the crystalline Tm at approximately 110 °C. Indeed, the QAS(Ni-POM) conjugates were found to reside in crystalline form, similar to lamellar behavior that has been observed in previous literature for similar compounds.25-26 Such intermolecular behavior suggests hydrophobic character of the QAS(Ni-POM) molecules dominated by exterior oriented alkyl tails. Therefore, it was concluded that the crystalline phase transition that occurred between QAS(Ni-POM)s at approximately -10 °C resulted from the disordering of entwined alkyl moieties of separate QAS(Ni-POM) molecules. 14


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1

H NMR Spectral Analysis

1

H NMR analyses were performed on QAS and QAS(Ni-POM) complexes and compared. The

presence of ethoxy protons of neat QAS were confirmed by second order coupling observed between the proximal protons (C16EO1 δ = 3.88 and 4.00 ppm). Further peak assignments for C16EO1 and C16EO2 are shown in Figure 5.

Figure 5. 1H NMR spectra of neat C16EO1 (left) and C16EO2 (right) and peak assignments.

Upon coordination to Ni-POM, NMR spectra for each QAS(Ni-POM) sample exhibit significant peak broadening compared to the respective neat QAS counterpart of each. This was observed to the greatest extent for the NMR signals in the range of 3.4 – 4.0 ppm; an example is presented in Figure 6. These signals corresponded to the protons of the ethylene oxide moiety as well as the protons bound to carbons proximal to the nitrogen atom. Not only are the signals 15



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broadened, but the signals in the range of 3.8 – 4.0 ppm, which correspond to ethylene oxide protons, are also shifted downfield into the broadened peak roughly centered at 4.1 ppm in the C16EO1(Ni-POM) spectrum. Figure 6 also shows that the QAS ligand peaks are also broadened for the diamagnetic (POM-0) complex, albeit to a lesser extent. The marked shortening of T1 relaxation times in the complex containing the Ni2+ ion described in the next section will be shown in the subsequent section to be most useful in obtaining structural distance-dependent information about the complex.

Figure 6. Example of 1H NMR peak broadening of the QAS C16EO2 due to coordination to the diamagnetic POM-0 and paramagnetic Ni-POM.

1

H NMR T1 Relaxation: Qualitative Analysis

The inversion-recovery behaviors of two peaks at different sites on the QAS molecule coordinated to Ni-POM are shown in Figure 7. It is clear that a single-exponential recovery satisfactorily fits these experimental data, as well as all other data not shown, over a wide range

16


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of 40 recovery times. Table 2 gives the measured T1 values for the assigned peaks in neat QAS and QAS(Ni-POM) complexes having different alkyl chain and EO chain lengths; Table 3 shows a comparison of T1 values for the diamagnetic and paramagnetic C16EO2 complexes. It is immediately clear from both tables that the paramagnetic complex has markedly shorter T1 values for the same peaks, with a greatly-differing relative magnitude of T1 reduction depending upon the peak position.

1.2

1.2

1.0

1.0

0.8

0.8

0.6

1.28 ppm 4.20 ppm Fit Curve of 1.28 ppm Fit Curve of 4.20 ppm

0.4 0.2

Relative Intensity

Relative Intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0 -0.2

0.6

0.2 0.0 -0.2

-0.4

-0.4

-0.6

-0.6

-0.8

1.28 ppm 4.20 ppm Fit Curve of 1.28 ppm Fit Curve of 4.20 ppm

0.4

-0.8 0

500

1000

1500

2000

2500

3000

0

20

Delay Time (ms)

40

60

80

100

Delay Time (ms)

Figure 7. Entire inversion recovery data (left), and expanded (right), for the EO (4.20 ppm) and alkyl tail (1.28 ppm) sites of C16EO2(Ni-POM) fit to single exponential.

To interpret the significance of these observations, we will consider the complexes to consist of 8 QAS molecules intramolecularly coordinated to a single diamagnetic or paramagnetic POM. The POM portion has dimensions of 16 x 12 x 12 Å,28 but will be approximated as a sphere with radius 7 Å. Given the clear evidence from Table 2 that the paramagnetic Ni2+ ion in the Ni-POM complex is responsible for dramatically shortening T1, we can make an additional important conclusion about the nature of the intramolecular coordination involved. 17


The electron-nuclear dipolar


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Page 18 of 31

contribution to T1 relaxation from paramagnetic ions is expressed by a 1/r6 dependence of the relaxation rate 1/T1,29 where r is the distance between the electron spins on the Ni2+ and the 1H nucleus in the QAS.

If the QAS molecules were binding to specific sites on the Ni-POM

surface, and not moving from these sites, the result would be a very wide range of distances r for each observed nucleus on each QAS molecule. Dynamical flexibility of the alkyl chain on the QAS for the complex dissolved in CHCl3 would be expected to lead to a dependence for the relaxation rate 1/T1, as discussed below, but would not alter the expectation of widely different values of for each of the QAS molecules permanently bound to a specific surface site on the POM. Such a set of eight different values, one for each of the 8 QAS molecules on the surface, would directly lead to eight widely-different T1 values. These would manifest themselves as clearly multi-exponential inversion-recovery behavior, capable of being described by a stretched exponential.30 The clearly single-exponential behavior observed for inversion-recovery experiments in Figure 7 shows that a model involving QAS molecules bound in a stationary sense at specific surface sites of the Ni-POM cannot be correct. Instead, each of the 8 QAS molecules must be interchanging surface binding positions in order to result in an equivalence (for the 8 molecules) of all dynamically-averaged values at specific positions in the QAS structure. Such an interchange may take place if QAS molecules have significant surface mobility, e.g. are able to swap binding sites or move to adjacent vacant binding sites, or even to exchange with QAS molecules on another complex in solution. Regardless of the detailed mechanism, the end result would be to produce a single value of for each type of proton in the QAS molecule, and hence single-exponential T1 relaxation for that type. An upper bound for the time scale over which such interchange of QAS surface binding sites takes place can be determined from the set of recovery delay values used to infer single18


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exponential recovery for the shortest T1 value in Figure 7. We conclude that this interchange must be occurring on a time scale faster than about 10 ms in order for the recovery to be singleexponential.

Table 2. Chemical shifts (δ ppm) and T1 (ms) relaxation times for several QAS protons in diamagnetic (neat QAS) and paramagnetic (QAS(Ni-POM)) environments. C12EO1

C12EO1(NiPOM)

C16EO1

C16EO1(NiPOM)

C16EO2

C16EO2(NiPOM)

δ

δ

δ

δ

δ

δ

T1

T1

T1

T1

T1

T1 b

0.89 2285

0.90 526

0.88 2377

0.89 838

0.88 2301

0.89 1271

1.26 1076

1.28 233

1.26 999

1.28 311

1.26 972

1.28 465

3.42 590

3.42 19

3.39 445

3.43 16

3.36 813

3.37 86

-a

-a

-a

-a

-a

-a

-a

-a

3.54 631

3.54 42

-a

-a

-a

-a

-a

-a

-a

-a

3.68 927

3.71 25

4.08 8

3.99 376

4.20 10

3.95 323

4.07 9

4.00 503

a

No peak observed; bT1 relaxation times measured for C16EO2(Ni-POM) at 600 MHz (14.1 T) were, in respective descending order, 901, 378, 89, 44, 25, and 15 ms.

Relaxation by the electron-nuclear dipolar interaction is described by the modified SolomonBloembergen equation shown below in simplified form,31,29,32,33 in which we substituted for the fixed distance r a new variable r* = (-1/6) in order to take into account the effects of the aforementioned motional averaging.

ିଵ ܶଵ(௣௔௥௔) =

ௌ(ௌାଵ) ஜబ ଶ ଵହ

଺ఛ

ଵସఛ

ቀସగቁ ߛଵଶ ݃ଶ ߚ ଶ ‫ ଺ି∗ ݎ‬൬ଵାఠభమ ఛమ + ଵାఠమమఛమ ൰ ಺ భ

19


ೄ మ

(1)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

The above equation employs the nuclear Larmor frequency (ωl), electron Larmor frequency (ωs), permeability of free space (µ0), Bohr magneton (β), electron spin (S), electron g-tensor (g), the proton gyromagnetic ratio (γ1) and distance between the proton and the paramagnetic species (r) to determine T1(para)-1, the contribution to spin-lattice relaxation from the paramagnetic center. The correlation times τ1 and τ2 characterizing the fluctuating dipolar fields from the electron spin in our system can replaced by T1e and T2e, respectively, the relaxation times of the electron spins. This is because the typical T1e of Ni2+ is in the range of 10-10 to 10-13 s,33 shorter than either the ca. 10 ns estimated rotation correlation time (Supporting information) or other relaxation processes such as chemical exchange. With the assumption that τ1 = τ2 = T1e, Equation 1 can be rearranged to solve for r*:

∗

ଶ ଶ ଶ

ஜబ ଶ ௌ(ௌାଵ)

‫ = ݎ‬൬݃ ߚ ߛ ቀସగቁ

ଵହ

଺்భ೐

ଵସ்భ೐

ଵ/଺

൬ଵାఠమ ்మ + ଵାఠమ ்మ ൰ ܶଵ(௣௔௥௔) ൰ ೗ భ೐

ೞ భ೐

(2)

Semi-Quantitative Analysis of Paramagnetic Relaxation Enhancement As seen in Figure 6, the 1H NMR peaks in C16EO2(POM-0) are less broadened by formation of the complex than those in C16EO2(Ni-POM); the T1 relaxation times are also much longer for the peaks in the diamagnetic complex. Therefore, C16EO2(POM-0) was considered to be the diamagnetic counterpart of C16EO2(Ni-POM), differing only in the absence of a paramagnetic (Ni2+) site, in order to quantitatively analyze the paramagnetic relaxation enhancement. Inversion recovery experiments on solutions of C16EO2(POM-0) and C16EO2(Ni-POM) and other complexes with different QAS groups were performed separately and the resultant single-

20


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exponential T1 values compared in order to determine the relative motionally-averaged intramolecular distances of protons to the paramagnetic Ni2+ heterometal.

Table 3. T1 relaxation times (T1) of inversion recovery and estimated motionally averaged interatomic distances r* calculated from Equation 2. C16EO2(POM-0)

C16EO2(Ni-POM)

PRE

δ (ppm)

assigned 1 a H

T1 (ms)

HHLW (Hz)

T1 (ms)

HHLW (Hz)

T1(para) (ms-1)b

r* (Å)c

0.88

a

2276

10.5

1271

12.9

3.47E-04

20.1

1.26

b

771

2.8

465

9.8

8.53E-04

17.3

3.35

e&i

675

2.3

86

15.3

1.02E-02

11.5

3.54

d&f

532

14.6

42

29.6

2.22E-02

10.1

3.70

g

412

14.3

25

47.2

3.81E-02

9.2

4.15

h

270

12.6

10

120.4

9.74E-02

7.9

a

Peak assignments are presented in Figure 5; b1/T1(para) = 1/T1paramagnetic – 1/T1diamagnetic, where T1paramagnetic = T1 of C16EO2(Ni-POM) and T1diamagnetic = C16EO2(POM-0); cRelative distances calculated from Equation 2 with assumption that T1e = 10-10

Protons vicinal to the quaternary ammonium N (δ = 3.35, 3.54, 3.70) and along the ethylene oxide moiety (δ = 3.35, 4.15) exhibited the greatest relaxation enhancement and thus their distances r* to the paramagnetic Ni2+ were calculated to be in the range of 7.9 – 20.1 Å. The radius of a Wells-Dawson polyoxotungstate polyhedra was determined to be approximately 7 Å by application of a spherical approximation based upon previous calculations.28 Although the paramagnetic Ni2+ heterometal was present in the terminal alpha position of the POM molecule, not in the direct center, the measured NMR spectra and subsequent T1 relaxation times result 21



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Page 22 of 31

from the motionally averaged signal with equal contributions from all coordinated QAS for the reasons discussed earlier.

Therefore, the 7.9 – 20.1 Å inter-nuclear distances that were

calculated for the ammonium and ethylene glycol protons represented averaged distances to the paramagnetic heterometal. These data suggest that those protons vicinal to the ammonium N atom and along the ethylene oxide moiety are directly coordinated or very close to the surface of the Ni-POM molecule. The averaged distances are reasonable in view of the dimensions of the Ni-POM core given above. A more accurate determination of r* values based upon a more extensive analysis involving 31P NMR measurements is in progress. The diamagnetic POM-0 with coordinated QAS molecules is a counterpart to the paramagnetic QAS(Ni-POM) complex, and thus offers a way to distinguish paramagnetic effects upon T1 relaxation from other mechanisms. However, neat QAS could also adequately represent this diamagnetic case, despite its lack of coordination to POM-0.

This approach would also

circumvent the further need to synthesize QAS(POM-0) conjugates. Therefore, the T1 relaxation rates of neat C16EO2 were measured by inversion recovery and incorporated into Equation 5 along with C16EO2(Ni-POM) to calculate intramolecular distances. The distances calculated from the comparison of C16EO2(Ni-POM) to neat C16EO2 were found to correlate well with those that were calculated from comparison of C16EO2(Ni-POM) to C16EO2(POM-0) (Figure 8). This result confirmed the validity of employing neat QAS, in place of QAS(POM-0), as the corresponding diamagnetic species to determine PRE intramolecular distances of QAS(Ni-POM) molecules.

22


Page 23 of 31

25 C16EO2(Ni-POM)/C16EO2 C16EO2(NiPOM)/C16EO2(POM-0)

20 r*

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


15 10 5

1

2

3

4

Chemical Shift (ppm)

Figure 8. Comparison of averaged inter-nuclear distances r* calculated from QAS(POM-0) and neat QAS.

T1 inversion recovery experiments were performed on C12EO1, C16EO1, and C16EO2 and also their respective Ni-POM complexes. This selection represented QAS of varying alkyl and ethoxy moiety lengths, such that their effect on T1 relaxation, and ultimately their binding to NiPOM, were able to be determined. The calculated intramolecular distances r* of QAS(Ni-POM) are presented in Table 4. Coordination of QAS to Ni-POM resulted in reduced relaxation times for all of the protons in the system and the protons that exhibited the smallest distance to the paramagnetic Ni were those along the ethylene oxide moiety. As such, the ethylene oxide moiety, possibly attracted through partial positive and negative charges, appears to be electrostatically coordinated to the anionic exterior of the Ni-POM.

This coordination

orientation also implies that the alkyl moieties are extended outwards from the Ni-POM core, presenting a hydrophobic exterior to the surroundings. The calculated r*for the terminal methyl groups (δ = 0.89 ppm) of each QAS(Ni-POM) coordinate correspond with the extended trans 23



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length of each respective alkyl moiety length estimated by employing a C-C bond length of 1.54 Å.

Table 4. PRE-derived parameter r* of resolved protons calculated from comparison of T1 relaxation times between neat QAS and QAS(Ni-POM). r* (Å) a 0.89 ppm

1.28 ppm

3.37 ppm

4.20 ppm

C12EO1(Ni-POM)

15.8

13.8

8.8

7.7

C16EO1(Ni-POM)

17.6

14.8

8.6

7.6

C16EO2(Ni-POM)

20.1

16.6

11.4

7.9

a

Motionally averaged distance, see text

The distances r* of the terminal methyl protons (δ = 0.89 ppm) from the paramagnetic Ni2+ increase in the progression C12EO1(Ni-POM), C16EO1(Ni-POM), and C16EO2(Ni-POM). The increase in r* between the terminal methyl protons (0.89 ppm) from C12EO1(Ni-POM) to C16EO1(Ni-POM) was 1.8 Å, which was on the order of the increase in distance due to increased carbon moiety length expected when considering an average distance from trans-gauche isomerizations. Additionally, r* of the terminal methyl protons (0.89 ppm) increased 2.5 Å from C16EO1(Ni-POM) to C16EO2(Ni-POM), despite both QAS exhibiting equal length alkyl moieties. Therefore, the increase in r* can be attributed to the coordination of QAS to Ni-POM through the ethylene oxide moiety. Furthermore, r* of the methyl protons vicinal to N (δ 3.37 ppm) for C16EO2(Ni-POM) was greater than r* for both C12EO1(Ni-POM) and C16EO1(Ni-POM) by at least 2.6 Å, which corresponds to approximately the length of a single ethylene oxide moiety unit. The r* of the methylene protons of the ethylene oxide moiety (δ = 4.20 ppm) were both the shortest and the most similar between all three QAS(Ni-POM), thereby indicating that the 24


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ethylene oxide moiety protons were coordinated to the Ni-POM. Furthermore, the calculated distance r* of the ethylene oxide protons to the paramagnetic Ni (~7.7 Å) correspond reasonably well with the approximated molecular radius of the Ni-POM cage (7 Å). These results suggest that the ethylene oxide moiety, not the quaternary ammonium nitrogen, was directly coordinated to the polyoxometalate surface. A dipolar attraction between the electron deficient methylene groups of the ethylene oxide moiety and the electron dense Ni-POM cage may account for the observed coordination.

Structural Model for QAS(Ni-POM) Complex Overall, the results from each of the analytical techniques discussed allow us to propose a structural model for the QAS(Ni-POM) complex. Precipitation of the product out of aqueous solution clearly indicate that QAS and POM are coordinated. It had been previously assumed that coordination occurs between the cationic nitrogen of the QAS and the anionic surface of the POM, yet IR analysis provided initial evidence that the QAS and POM were coordinated through the ethylene oxide moiety as a narrowing of the region in which the ether stretching modes resided, indicating similar chemical environments. Furthermore, thermal behavior of QAS(NiPOM) complexes through DSC indicated that the alkyl tail lengths were oriented away from the POM center, able to interact with alkyl chains of proximal complexes and form crystalline phases with low transition temperatures. The average quantity of QAS bound to each POM for each QAS composition was determined to be in the range of 5 – 8 QAS per POM by TGA. It can be postulated that the number of QAS molecules bound to each POM is limited by the physical space available. However, considering the surface area of POM according to Dawson’s dimensions28 there would be an excess of surface area for each QAS, and this could then 25



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Page 26 of 31

theoretically allow for more QAS to coordinate. Since this was not observed, it is concluded that QAS interacts with POM through electrostatic charge, limiting the quantity to the formal charge of the POM cage (-8). While the relatively short interatomic distance of the ammonium cation to the anionic Ni-POM surface was expected, the direct coordination of the ethylene glycol moiety to the Ni-POM surface was unexpected. Through NMR Inversion recovery and PRE, the distances between the ethylene oxide protons and the Ni heterometal were less than that of the protons vicinal to the ammonium nitrogen.

The inter-nuclear distances determined from the modified Solomon-

Bloembergen equation between the terminal methyl protons of the alkyl moiety and the paramagnetic center corresponded well with calculated distances of the alkyl moiety using average C-C bond lengths of 1.54 Å and taking into account trans-gauche isomerizations. This result suggests that alkyl moiety was directed outward, away from the paramagnetic Ni center of the Ni-POM. Therefore, the intermolecular behavior of the entire QAS(Ni-POM) complex would be dominated by alkyl moiety exterior, effectively a hydrophobic molecule (Figure 9). From these results, the dispersive behavior of QAS(Ni-POM) in polymer matrices could be predicted. Coordination with QAS resulted in increased hydrophobic character of the complex molecule due to dipolar interactions between the anionic Ni-POM surface and the ethylene glycol moiety of the QAS.

26


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Figure 9. Conceptualized coordination in the QAS(Ni-POM) complex.

IV. CONCLUSION This work is the first report to elucidate the binding orientation of QAS to Ni-POMs through an application of modified Solomon-Bloembergen theory of PRE by NMR inversion recovery experiments. It was expected that the quaternary ammonium nitrogen of amphiphilic QAS would coordinate directly to the surface of Ni-POM. However, it was shown that the ethylene oxide moieties, not the quaternary ammonium nitrogen, were coordinated most closely with the Ni-POM surface.

PRE, specifically the application of a modified Solomon-Bloembergen

equation, proved effective in determining intramolecular distance between protons and the paramagnetic Ni center.

As this is the first known use of PRE to elucidate the relative

orientation of QAS bound to POM, this approach will be able to be utilized as a valuable tool to characterize the binding orientation of organic species to POMs containing paramagnetic heterometals in future work. Furthermore, predictions about the behavior of dispersions of such molecules in a variety of environments, such as polymer solutions and matrices, are now possible.

27



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Supporting Information DRIFTS spectra of neat QAS (Figure S1), DRIFTS spectra of QAS(Ni-POM) complexes (Figure S2), terms and values of constants used in MSB equations (Table S3), explanation of determination of rotational correlation time τc (S4), and Single exponential fit of C16EO2(NiPOM) inversion recovery at 600 MHz (Figure S5).

Acknowledgements This work was funded by the Office of Naval Research (ONR), the Naval Research Laboratory, and the Defense Threat Reduction Agency (DTRA).

*Corresponding author. Tel.: (202)404-4010, Email address: [email protected]

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Nature of Polyoxometalate Intramolecular Coordination to Quaternary

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