In the Bottlebrush Garden: The Structural Aspects of Coordination

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In the Bottlebrush Garden: The Structural Aspects of Coordination Polymer Phases formed in Lanthanide Extraction with Alkyl Phosphoric Acids Ross J Ellis, Thomas Demars, Guokui Liu, Jens Niklas, Oleg G. Poluektov, and Ilya A. Shkrob J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b05679 • Publication Date (Web): 05 Aug 2015 Downloaded from http://pubs.acs.org on August 11, 2015

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The Journal of Physical Chemistry

In the Bottlebrush Garden: The Structural Aspects of Coordination Polymer Phases formed in Lanthanide Extraction with Alkyl Phosphoric Acids

Ross J. Ellis,* Thomas Demars, Guokui Liu, Jens Niklas, Oleg G. Poluektov, Ilya A. Shkrob* Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 S. Cass Ave, Argonne, IL 60439

Received: June 15, 2015 Revised: Corresponding authors *R.J.E.: tel. (630) 2523647; email [email protected] *I.A.S.: tel. (630) 252-9516; e-mail [email protected]

ACS Paragon Plus Environment

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ABSTRACT Coordination polymers (CPs) of metal ions are central to a large variety of applications, such as catalysis and separations. These polymers frequently occur as amorphous solids that segregate from solution. The structural aspects of this segregation remain elusive due to the dearth of the spectroscopic techniques and computational approaches suitable for probing such systems. Therefore there is lacking an understanding of how the molecular building blocks give rise to the mesoscale architectures that characterize CP materials. In this study we revisit a CP phase formed in the extraction of trivalent lanthanide ions by diesters of the phosphoric acid, such as the bis(2-ethylhexyl)phosphoric acid (HDEHP). This is a well-known system with practical importance in strategic metals refining and nuclear fuel reprocessing. A CP phase – referred to as a “third phase” – has been known to form in these systems for half a century, yet the structure of the amorphous solid is still a point of contention, illustrating the difficulties faced in characterizing such materials. In this study, we follow a deductive approach to solving the molecular structure of amorphous CP phases, using semi-empirical calculations to set up an array of physically plausible models and then deploying a suite of experimental techniques, including optical, magnetic resonance, and x-ray spectroscopies, to consecutively eliminate all but one model. We demonstrate that the “third phase” consists of hexagonally packed linear chains in which the lanthanide ions are connected by three O-P-O bridges, with the modifying groups protruding outwards, as in a bottlebrush. The tendency to yield linear polynuclear oligomers that is apparent in this system may also be present in other systems yielding the “third phase”, demonstrating how molecular geometry directs polymeric assembly in hybrid materials. We show that the packing of bridging molecules is central to directing the structure of CP phases and that by manipulating the steric requirements of ancillary groups one can control the structure of the assembly.


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1. INTRODUCTION Coordination polymers (CPs) are hybrid organometallic materials that consist of metal cation centers interconnected by organic ligands and extend indefinitely into one, two or three dimensions. This extended structure frequently causes their insolubility, and they tend to aggregate and precipitate. The unusual properties of these CP phases have been driving a surge of interest in recent years from a number of fields including catalysis, luminescence, electronics, magnetism and ion separations.

1

Structure, with

regard to both local environments around the metal ions as well as the higher-ordered architectures that define polymer material morphology, is crucial in understanding the emergent properties of these materials.

2

However, the structure of CP materials can be

challenging to establish, especially when they lack crystallinity. One of the longest-known and most well-documented CPs is that formed from association between lanthanide(III) ions and dialkyl phosphoric acids, most notably bis(2-ethylhexyl) phosphoric acid (HDEHP).

3, 4

This system was developed more than

half a century ago for the liquid-liquid extractive separation of lanthanide rare earths 5 but is limited by the formation of CPs, which reports from solution as an amorphous gelatinous phase at the oil-water interface. Although extraction of lanthanides using organophosphorus acids remains the paramount technique for separating the rare earths in numerous industrial applications,

6-9

the morphology of the CP phase that limits all of

these systems remains underexplored, and there is no consensus as to the molecular structure in the studies that have been published. The maturity of this open problem exemplifies the challenges faced in characterizing the structure of amorphous CP phases, which have ever increasing importance in a variety of emerging applications. In this study, we seek to establish the structural motifs for the aggregates that are formed in the extraction of trivalent lanthanide (Ln3+) ions from the acidic solutions by diesters of the phosphoric acid, such as HDEHP (Scheme 1). In the following, we will use the contractions HD[R]P and D[R]P- (where R is the modifier group) for the acid and the corresponding base, respectively (Scheme 1).


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Scheme 1. Chemical Structure for the Extracting Agent HD[R]P, where R=EH (2Ethylhexyl).

HDEHP (that has pKa 1.8) readily ionizes in mildly acidic solutions (pH 2-4). The acid (HL) extracts Ln3+ ions into a hydrocarbon solvent via a concerted proton and ion exchange that involves the complexation of the Ln3+ ion by the hydrogen bound HL··HL dimers which are pre-formed in the nonpolar diluent 4, 10 Ln3+ aq +3 HL..HL org

Ln(L..HL)3 aq + 3 H+ aq

(1)

(The indexes “aq” and “org” in reaction 1 indicate the aqueous and organic phases, respectively). In the resulting octahedral complex the three L-..HL dimers form a tight shell around the Ln3+ ion, with the protonated HL units bound to the deprotonated Lanions through their free P=O groups, as shown in Figure 1a.

11, 12

This causes the

extreme sensitivity of the stability constant for the complex to the radius of the Ln3+ ion, which in turn accounts for the rapid increase in the distribution coefficient (the ratio of the equilibrium concentrations in the organic and aqueous phases) across the lanthanide period. 4, 10 As the concentration of the metal ion increases, the acidity decreases, or if a polar co-solvent is added to the organic phase, CP begins to form. SANS studies of Jensen et al. 13, 14 on Nd3+ complex aggregation suggested that the initial step was the formation of an Nd2L6 dimer, which is the largest complex that remains in a solution (Figure 1b).


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Figure 1. Structural motifs for (a) Ln(L..HL)3 monomers and (b-d) {LnL3}n complexes: (b) the 2-oxo 1-4-1 dimer (type a), (c) the 3-3 chain, (d) the 2-2-2 sheet (type a), and (e) the 2-2-2 ribbon (type b). Only the first carbon atom is shown in the R-groups.

Further aggregation yields a gel like solid residue with 1:3 stoichiometry.

11, 15-19

As shown in previous studies, this material is partially ordered: there are several welldefined peaks in their powder x-ray diffraction pattern (XRD). 15, 16, 20 Unfortunately, the poor quality of these x-ray data do not allow confident indexing of the unit cell, and different structural models proliferated (Figure 1). In these structures, the Ln3+ ion is


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believed to be octahedrally coordinated 20 (as was suggested by the luminescence spectra for the Eu3+ ions; we will demonstrate below that the crystal field symmetry is, in fact, trigonal). One of the earliest structures suggested for CPs 3+

three O-P-O bridges between the Ln

19, 21

was a linear chain with

ions, as shown in Figure 1c. 20 In the following we

will refer to this structure as the 3-3 chain. This CP has been identified crystallographically by Lebedev et al. 22 in the triclinic neodymium(III) diethylphosphate, and other chains of this type have been found in the related systems (e.g. 23-25). Suglobov et al.

20

suggested that these 3-3 chains for (EuL3)∞ pack into a hexagonal lattice, as

illustrated in Figure 2a, and estimate the Eu-Eu distance as 5.2 Å. An entirely different structure was suggested for (NdL3)∞.

26

The unit cell was

monoclinic and the crystal consisted of hexagonally tiled sheets (Figure 1d); each hexagon is composed of the trefoils made of Nd3+ ions bridged through two O-P-O bridges with a Nd-Nd distance of 6.5 Å, as shown in Figure 1d. This structure will be referred to as the 2-2-2 sheet. The authors speculated that extended aliphatic chains pack together in layers between the 2-2-2 sheets, as illustrated in Figure 2b.

26

This structure,

too, has the single-crystal analog in the subsequently studied 1:3 complexes of dimethylphosphate (R=Me) with La3+,

27

Sm3+,

28

and Eu3+.

29

, For the latter two Ln3+

ions the Ln-Ln distance is considerably longer than in the crystalline 3-3 polymers (ca. 5.9 Å 28, 29 vs. 5.1 Å 22). This 2-2-2 model has been accepted in most of the subsequent studies (e.g. refs. 15, 16

). The 2-2-2 motif for a ribbon like CP (Figure 1e) was also suggested by Mikulski et

al. 17-19 Neither one of these previously suggested structures for CP occurring in the third phase has been established experimentally or even modelled structurally. In a more recent development, Nifant’yev et al.,

30

isolated an Nd dimer (Nd2L6) with a short (4.02 Å)

distance between the Nd3+ ions and the structure shown in Figure 1b. In this dimer, the two Nd3+ ions are bridged by four O-P-O bridges arranges to give two -oxo bridges, while the two remaining ligands are pointing outwards (the 1-4-1 motif). The “secret” of stabilizing this 1-4-1 dimer in a crystal was in using the sterically crowded groups. 30 This 1-4-1 structure could be related to the Nd2L6 complex identified in the initial stages of the third phase formation.

13, 14

There are other known examples of such -oxo bridged

organophosphate dimers forming hybrid materials

31 32

As seen from this brief review,


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several structural possibilities for CP and its precursor exist, each having credible single crystal references.

Figure 2. Packing of the (a) 3-3 chains and (b) 2-2-2 sheets in Nd(DEtP)3 crystals (MOPAC Sparkle/RM1 models). Space-filling rendering is used for the ethyl groups.

In this study, we present a comprehensive deductive approach that establishes the elusive structure of amorphous lanthanide-organophosphate CP phases. With the aid of semi-empirical calculations, we set up an array of possible models (including those proposed in the previous studies) and then test these models using a variety of experimental

techniques

including

time-resolved

laser

induced

fluorescence

measurements (TRLF), x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), xray photoelectron spectroscopy (XPS), continuous wave (cw) and time-domain electron paramagnetic spectroscopy (EPR), electron spin echo envelope modulation (ESEEM)


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spectroscopy, solid-state magic angle spinning NMR (MAS NMR) spectroscopy. By using this vast arsenal of experimental probes, we demonstrate that the 3-3 polymer shown in Figure 1c forms the bulk of the CP matrix. To save space, many of the supporting schemes, tables, figures, and the list of abbreviations have been placed in the Supporting Information (SI). When referenced in the text, these materials have the designator "S", as in Figure 1S in SI.

2. METHODS 2.1. Sample preparation. All reagents were obtained from Sigma-Aldrich and used as supplied without further purification. Typically, one volume of 1 M HDRP (R=2-ethylhexyl (EH), n-octyl (Oc), n-butyl (Bu), and phenyl (Ph)) in toluene was put in contact with one volume of 0.1-0.2 M lanthanide(III) nitrate in nitric acid solution at pH=3. In some experiments, the aqueous solution contained 3 M NaNO3. This mixture was vortexed for 5 min and the organic layer was separated by centrifuging. For HDOcP and HDPhP, this procedure was sufficient to induce gel formation during the extraction. For mildly polar diluents (such as 1-octanol or hydrophobic ionic liquids), in which the internal hydrogen bonding required to stabilize the monomer is weakened, the formation of CP is also prompt, for all of the alkylphosphoric acids. In other systems, a polar solvent (1:10 v/v) needs to be added to the toluene solution to initiate the third phase formation. Almost any polar solvent, protic or aprotic, induced gelling; methanol and 2-propanol were most convenient due to rapid Ostwald ripening. After 10 min of stirring, the gel was isolated by centrifuging. The supernatant was removed, and the residue was washed with the methanol by repeated suspension of the gel in methanol followed by centrifuging. Traces of the solvent were subsequently removed in vacuum using a rotavapor, and the gel was dried in a vacuum oven for 12 h at 80 oC. To prepare doped samples, the appropriate aqueous mixtures of the lanthanide ions were used. Since the treatment considerably changes the relative concentration of the lanthanide ions in the solid matrix as compared to the aqueous solution, 11 the elementary analysis of the solid gel was required to determine the doping levels. To this end, weighed dry gel was placed in a glass vial, one volume of toluene-d8 containing 5 wt% cumene as an analytical standard and one volume of 3 M nitric acid


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were added, and the sample was sonicated and vortexed until the complete digestion. The organic layer was separated and used to determine the concentration of the L- anions by 1

H NMR using an Avance DMX 500-MHz spectrometer (Bruker Biospin), while the

aqueous phase was analyzed using a Perkin-Elmer model SCIEX ELAN DRC II inductively coupled plasma mass spectrometer. Within the experimental error, the dry gel samples corresponded exactly to the LnL3 stoichiometry. For XAS, the sonicated suspension of Eu(DEHP)3 gel in 2-propanol was filtered through Whatman No. 5 paper filter with 2.5 m pores. The filtrate was collected, the filter was replaced with a new one, and the operation was repeated several times, as shown in Figure 1S in SI. In this fashion, samples with uniform surface covering and different particle densities were obtained. The relative coverage was determined using fluorescence spectroscopy (Figure 2S in SI). The deposited wet gel was dried at 80 oC and the paper strips were sandwiched between the Al foil and Kapton film and flattened with a steel roller (Figure 1S in SI).

2.2. Time-resolved fluorescence spectroscopy (TRLF). For optical measurements, solid samples were placed in sealed Suprasil glass capillaries or deposited on filter paper as described above. The 5D0 → 7FJ emission (J=04) of the Eu3+ ion between 570 and 720 nm was induced using 355 nm photons (1 mJ, 6 ns fwhm, 10 Hz) from a Nd:YAG laser (Quantel Brilliant or Continuum). For kinetic measurements the emitted light was passed through a color glass filter and a bandpass interference filter (20 nm fwhm) with the transmission maximum at 610 nm (corresponding to the J=2 band). This light was sampled using a photomultiplier, and the signal was terminated into a 4 k load at the digitizing oscilloscope (Tektronix model 360). The photomultiplier was operated at 0.5-1 kV, depending on the emission yield. Figure 2S in SI gives the typical decay kinetics of the photoinduced luminescence. For spectroscopic measurements, the Eu-DEHP sample sealed in an evacuated glass capillary was mounted inside Optistat bath dynamic variable cryostat (Oxford Instruments) and cooled down to 77 K. The laser-induced emission was analyzed using a Spex Model 1704 spectrometer with the spectral resolution of 0.2 Å. The photomultiplier output terminated


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into 1 M load was sampled at a delay of 1-10 ms using a Stanford research Systems Model 250 boxcar integrator with 15 s gate. As shown later in section 3.3, the single line of the Eu3+ 5D0-7F0 transition at 580 nm indicates the presence of the single emission center, and the doublet of lines in the J=1 band suggests a trigonal or higher site symmetry for Eu3+. In case of trigonal symmetry, the crystal-field (CF) Hamiltonian HCF can be expanded as 33 H CF  B02 O02  B04 O04  B34 O34  B06 O06  B34 O34  B06 O66

(2)



where Bqk are real CF parameters and Oqk Jˆ are the conventional Stevens operators of rank k=2,4,6 and q=0,…k constructed of the components of the total moment Jˆ . In the superposition model,

34

the CF around the Ln3+ ion can be expanded into the Wybourne

normalized real spherical harmonics g k , q  i ,  i  (also called the coordination factors) using the atomic coordinates calculated in our model (section 3.1), and the coefficients

Bqk are estimated from Bqk   B k ri g k ,q  i ,  i 

(3)

i

For oxygen atoms in the first coordination shell we have



B k ri   b k r0 ri



tk

(4)

where b k are the intrinsic CF parameters at r  r0 , where r0 is the standard distance and tk are the exponential factors given in Table 1S in SI. For all other atoms, point charge approximation was used B k ri   e 2 r k q i ri  k 1

(5)

where r k are the averages calculated using the 4f radial functions and shielding factors (Table 1S), and qi are the Milliken atomic charges. The summation in eq. 3 was extended out to 20 nm; the electrostatic contribution given by eq. 5 is significant only for B02 . The estimated CF parameters can be compared to the ones obtained through the optimization


10

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of the computed energy levels for the 7FJ multiplets calculated using trial CF parameters in eq. 2. Appendix 1S in SI provides the details of these analyses.

2.3. Continuous wave EPR spectroscopy. For EPR measurements, the samples were placed in 3 or 4 mm outer diameter Suprasil tubes, evacuated and flame sealed. The first-derivative cw EPR spectra were recorded using a 9.44 GHz Bruker ESP300E spectrometer, equipped with a programmable flow helium cryostat (Oxford Instruments, model CF935). The magnetic field and the parameters of the spin-Hamiltonian are given in units of Gauss (1 G = 104 T). If not stated otherwise, the EPR spectra were obtained using 2 mW of microwave power and 5 G modulation at 100 kHz. The resonance line positions are given by the effective g-factor defined as g eff  hv /  B B0 , where  is the microwave frequency, B0 is the external magnetic field of the EPR spectrometer, and B is the Bohr magneton. Two paramagnetic Ln3+ ions where used as a probe, Gd3+ (4f 7, S=7/2) and Yb3+ (4f

13

, S=1/2), while closed shell Eu3+ and Lu3+ ions served as the “inert” matrix for

magnetic dilution experiments. Gd3+ is a convenient probe, as in the ground state it is an orbital singlet (8S7/2) that has the largest spin among the trivalent lanthanide ions (S=7/2), negligible hyperfine coupling constants, and isotropic g-tensor with giso≈2.

35

However,

there is zero-field spitting (ZFS, see below), 35 which complicates the analyses of cw EPR spectra 36 and even more so of ESEEM. 37, 38 In contrast, Yb3+ is an S=1/2 ion, for which there is no ZFS; however, it has two magnetic isotopes, 171Yb (I=1/2, 14.3 at%) and 173Yb (I=7/2, 16.1 at%) with significant hyperfine coupling in both of them, and large g-tensor anisotropy , so the resulting EPR spectra can also be quite complicated. 39 The ZFS contribution depends on CF around the probe ion. As the 8S7/2 state cannot contribute to CF, ZFS in Gd3+ ion originates entirely from the mixing of this ground state with the excited 6P7/2 and 6D7/2 states (that account for ca. 16% and 1%, respectively), and so it is rather weak (< 0.1 cm-1) .

40

This state mixing also makes the

anisotropy of ZFS very sensitive to the environment, and it can be large even in the Gd3+ complexes of the nominally high symmetry.

36

Neglecting the terms given by the tensor

operators with k>2, in the principal axes of rank-2 tensor this contribution is given by


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

 

H ZFS  D Sˆ Z2  13 S ( S  1)  E Sˆ X2  SˆY2

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

(6)

The second term in eq. 6 introduces nonaxial component that is characterized by the asymmetry parameter   3E D . In matrices other than single crystals, both D and (especially)  vary for different sites (which is referred to as strain); there is no unique set of these parameters.

36

In our EPR simulations, these ZFS parameters were sampled

independently from the corresponding Gaussian distributions with the centroids D and

 and the variances D and  , respectively. Powder EPR spectra were simulated using second-order perturbation theory

41 42

or the exact diagonalization,

43, 44

Carlo integration over the spherical angles and the strain parameters

using Monte

41, 42, 45

and the

contributions from the allowed ( M S  1 in a strong field) and forbidden transitions ( M S  1 ) were calculated separately. Typically, 5x104 of random field directions were averaged over the spherical quadrant and the resulting histograms were convoluted with the Gaussian line shapes with the peak-to-peak width Bpp of 20-50 G.

2.4. Magnetometry. The magnetometry was carried out using a Quantum Design Magnetic Property Measurement System 3. The effective magnetic moment eff of the Ln3+ ion was found from the equation

 H 0  N Ln  eff2  3 , where

  dM dH

is the magnetic

susceptibility, M is the magnetic moment, H is the field outside of the sample, NLn is the number of Ln3+ ions,   1 k B T , T is the temperature, and kB is the Boltzmann constant. 0 For the free Gd3+ ion,  eff  B  g J  J  1 ≈7.94. As Lu3+ is a close shell trivalent ion,

Lu-DEHP was used to determine the diamagnetic contribution (Figure 3S in SI). After the correction for temperature independent magnetism, the magnetic susceptibility for Gd-DEHP for T >20 K closely followed the Curie law with  eff =7.98 B. The details of the analyses are given in Appendix 2S in SI. At lower temperature, magnetic interactions between the ions become important. It can be shown (see Appendix 2S in SI) that the sufficiently high temperature and low field

 eff  eff0  1  2 3 J J  1 J ex  1   T 


(7)

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where J ex is the site-average isotropic component of the Heisenberg exchange coupling constants (that may also include dipole-dipole interactions of the ions) and  T  are the correction in second power of  and H.

2.5. ESEEM spectroscopy. Pulsed EPR spectra and kinetics were obtained using a Bruker ELEXSYS E580 EPR spectrometer (Bruker Biospin) equipped with a 1 kW traveling wave tube amplifier (Applied Systems Engineering). A Flexline dielectric ring resonator (Bruker EN 4118XMD4-W1) was used. Primary and stimulated electron spin echo (pESE and sESE, respectively) were obtained using /2-τ--τ and /2-τ-/2-T-/2-τ microwave pulse sequences using the appropriate phase cycling (Figure 4S in SI). Data processing was carried out using Xepr software (Bruker BioSpin). The magnetic dipole interaction of unpaired electrons with magnetic nuclei in the matrix causes modulation of the envelope (ESEEM) of the primary (p-) and stimulated (s-) echo as a function of the time intervals τ and T (for fixed τ) between the microwave pulses. Modulus Fourier transform (FT) of these time domain traces yields frequency domain ESEEM spectra that are an analog of NMR spectra for paramagnetic ions. For an S=1/2, I=1/2 system, there are two NMR frequencies,  and , corresponding to the up () and down () orientation of the electron spin relative to the magnetic field B0. For a distant (weakly coupled) nucleus in a matrix these frequencies are close to the Larmor frequency  for a free nucleus. In solid state, the decay rate of the pESE kinetics in the τ-domain is limited by the transverse relaxation time for the electron spin (T2), whereas the decay of the sESE kinetics in the Tdomain is determined by the slower longitudinal relaxation (T1 ≫ T2). A longer sampling interval for the latter (due to the slower decay of the echo signal) makes the sESEEM spectra better resolved in the frequency domain as compared to the pESEEM ones. The additional advantage of using sESE is in the ability to selectively suppress or enhance contributions from different types of nuclei (with NMR frequency ν), as the amplitude changes as 1-cos(2πντ). The standard formulae for the effective S  =1/2 system (the


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Kramers doublet corresponding to the  1 2    1 2 transition) 46 from ref. 38 where used to simulate and analyze the time-domain ESEEM traces.

2.6. Solid state magical angle spinning nuclear magnetic resonance (MAS-NMR). The 31P and 1H MAS-NMR experiments were performed at 7.02 T (300 MHz) on a Bruker Avance III HD spectrometer operating at a Larmor frequency of 121.5 MHz (for 31

P) using a 1.3 mm probe. All MAS NMR spectra were acquired at 50 kHz with single

pulse experiments at the room temperature. Pulse width of 1.5 s was used with pulse recycle delays of 15 s. Chemical shifts for 31P and 1H nuclei are given with respect to 85 wt% aqueous phosphoric acid and tetramethylsilane (TMS), respectively.

2.7. XPS. The gel samples were dried in a vacuum oven for 24 h and handled inside an argon filled glove box with the water and oxygen levels < 1 ppm. The gel was hand pressed onto a silicon holder that was transferred into a chamber with base pressure of 5x10-9 torr. The XPS spectra were obtained using a Physical Electronics VersaProbe II system equipped with a monochromated Al x-ray source and a hemispherical analyzer. A low energy Ar+ ion and an electron flood guns were used at pre-optimized conditions to neutralize the sample during the data acquisition in the fixed analyzer transmission mode. The survey spectra were acquired with the pass energy of 187.85 eV, the scan rate 0.80 eV/step, and the acquisition time of 0.450 s/step. Spectral regions were acquired with the pass energy of 2.95 eV or 11.75 eV, the scan rate of 0.05 eV/step or 0.10 eV/step, and the total acquisition time of 0.3 s/step or 4.8 s/step. The energies given in the text were calibrated using the C 1s line from the aliphatic arms of the ligand.

2.8. XAS. X-ray absorption fine structure (EXAFS) spectra were collected on the Materials Research Collaborative Access Team (MRCAT) bending magnet beam line, 10-BM, at Argonne National Laboratory’s Advanced Photon Source. The incident energy was selected using a double-crystal Si(111) monochromator with the second crystal detuned


14

Page 15 of 65

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


to 50% of the peak intensity. Eu L3 edge spectra were measured in fluorescence mode (Eu Lα emission line) using a Vortex-ME4 4-element silicon drift detector (Hitachi HighTechnologies Science America, Inc.). For measurement, samples were cooled to 40 K using a closed-cycle helium gas refrigerator to reduce the thermal fluctuations and to increase the signal from the more distant atoms. Many scans were summed for each sample for more than 107 photons per data point at higher k. The data were processed using the Athena interface to ifeffit. 47 The edge energy E0 was determined by finding the maximum of the first derivate of the  0 E  spectrum. Normalized data were obtained after subtracting a linear pre-edge background and three-term quadratic function for the atomic absorption background and normalized using the Lengeler-Eisenberger procedure. 48

The processed data were analysed using the Artemis suit and feff version 8.28.

49-51

Pseudo-radial distribution functions were obtained by FT of k 3  k  between 2.5 and 12 Å-1 using the Hanning window. Phases and amplitudes were calculated using the test structural models as the initial input. The data were first fit using the ab initio calculation neglecting Eu-Eu scattering, and then the fit was refined in a model that included this scattering (with the exception of the 2-2-2 model, where this contribution was negligible, see below).

2.9. XRD. X-ray diffraction data were obtained over the angular range 5° to 150° at 295 K using Cu K (1.5405 and 1.5443 Å) radiation from a Scintag X1 theta-theta diffractometer. Approximately 5 mg powdered samples placed on a low background flat holder were used for these measurements. The peak positions were analyzed using the CMPR suit 52 and the unit cell was indexed using TREOR program. 53

2.10. Modeling. The structures were modeled at the semiempirical level using the MOPAC2012 suit (version 14.083, Stewart Computational Chemistry) 54, 55 with Sparkle/RM1 Hamiltonian. 56, 57


15


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 16 of 65

3. RESULTS 3.1. Structural modeling for isolated coordination polymers. To gain structural insight that is required for testing different models, we turned to the semiempirical Sparkle/RM1 modeling of the (LnL3)n structures. While such models are lacking in rigor as compared to ab initio and density functional theory methods, they gain in the ability to tackle systems consisting of 102-103 atoms and have been extensively tested for a large variety of lanthanide complexes and solid compounds. 56, 57 We first consider isolated chains, ribbons, and sheets; the crystal structures will be further examined in section 4. Table 1 summarizes estimated standard heats Em for the formation of the CPs per GdL3 monomer with R=EH or R=Ph. The structural motifs indicated in Table 1 are shown in Figures 1, 5S, and 6S in SI. We carried out such calculations for other substituting groups (Table 2S); the trends were general. Only species with six oxygen atoms in the first coordination shell were considered, as suggested by our experiments (see below). The dimers, which are the tentative precursors of CPs (section 1), can be classified according to the number of the O-P-O bridges between the Ln3+ ions (see Table 1 and Figure 5S), which can be two (the 12-2-12 motif), three (the 1-3-12 motif), or four (the 1-4-1 motifs). In the latter, the four O-P-O bridges can be equivalent (type a, structure iii in Figure 5S in SI) or nonequivalent (type b, Figure 2b). There are precedents for the four equivalent O-C-O bridges in certain Gd3+ binuclear complexes, e.g.

58-61

The

12-2-12 dimers were the least stable, the 1-3-12 and 2-oxo 1-4-1 species were nearly equal in energy, and the 1-4-1 dimer with the four equivalent O-P-O bridges had still lower energy. Thus, in the absence of hydration, the 1-4-1 dimers would be the most stable complexes (which was the case for sterically hindered groups used by Nifant’ev et al.

30

to stabilize such 1-4-1 dimers). When the aqua ligands are included, the energy

ordering reverses as the number nw of coordinated water molecules per Ln3+ ion exceeds two (see Table 3S and Figures 7S and 8S in SI): the hydrated 1-3-12 isomer has the lowest energy, with the 12-2-12 isomer coming the close second (within ≈2 kcal/mol.eq of the 1-3-12 isomer for nw=4, see Figure 8S). In these dimers, the coordinated water molecules break the Ln-OP bonds in nonbridging organophosphate ligands, forming strong hydrogen bonds with their P=O groups (Figure 7S in SI). As the formation of CP


16

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is typically initiated through the addition of water or a polar solvent, the bias towards the 3-3 chains is already seen in the dimer, which becomes the structural unit of CP, as shown below. For linear polymers, several structures with two or three bridging O-P-O groups are possible in addition to the ones discussed in the literature (Figures 1 and 6S in SI). All of the structures with the two connecting O-P-O bridges were significantly higher in energy than the 3-3 chains shown in Figure 1c. This includes the 2-4 chains that can be obtained by connecting the 1-4-1 dimers.

59, 60

In fact, the energy cost of this

isomerization would be so steep (ca. 47 kcal/mol.eq) that it appears unlikely that defects with short Ln-Ln distances can be present in the 3-3 chains at all. For small substituting groups, the 2-2-2 ribbon (designated as 2-2-2, type b in Table 1) shown in Figure 1e has higher energy than the 2-2-2 sheet; it also has low crystal field symmetry, as there are two short and one long Gd-Gd distances (Table 1), which is inconsistent with our data (see below). In Table 2, we compare the structural parameters for O and P atoms in the first two coordination shells around Gd3+ ions in 3-3 chain of Gd-DEHP (designated as type a in Table 1) and 2-2-2 sheet of Gd-DEtP (designated as 2-2-2, type a in Table 1). These two structures are the two main contenders for the building unit of the CP. In the 3-3 chains, the Gd3+ ion has the local S6 symmetry, while in the 2-2-2 sheet it has (nearly perfect) C3 symmetry. As seen from this comparison, the Ln-O and Ln-P distances are comparable, which explains the difficulty of distinguishing between these two motifs using spectroscopic techniques like EXAFS and ESEEM that are not directionally sensitive. The major differences are in the Gd-Gd distances (5.53 vs. 6.32 Å for 3-3 and 2-2-2 motifs, respectively) and the mean angles  between the symmetry axes and the Gd-O bonds (50.8o vs. 56.9o). In the 2-2-2 sheet, the two groups of rotationally equivalent oxygen atoms in the first coordination shell of Gd3+ ion are azimuthally shifted relative to each other by the angle  (so =0o corresponds to the D3h symmetry and =180o corresponds to the S6 symmetry). While polar angle  remains 56-57o in all of the calculated 2-2-2 models regardless of the substituting (R) groups, the azimuthal angle

decreases as the R-groups become bulkier, from 41o for R=Me to 35o for R=Bu to 25o for R=Ph. Since for small angle the q=3 terms disappear, the anisotropy of rank-4 and


17


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Page 18 of 65

rank-6 crystal field tensors for the 2-2-2 sheet is reduced as compared to the 3-3 chains, which can be used to distinguish these two motifs spectroscopically (section 3.3 and Appendix 1S). Interestingly, in the reported structure of single crystal Eu-DMeP, which consists of the layered 2-2-2 sheets,

29

this near perfect C3 symmetry predicted by our

computational model is broken, and the mean angles  (ca. 55o) and  (ca. 50o) are quite different from our estimates. The calculations for Gd-DPhP indicate that the 2-2-2 sheet has Em that is ≈ 20 kcal/mol.eq higher in energy than the corresponding 3-3 chain. The cause for this strong preference is steric hindrance and crowding of the substituting phenyl groups. The fully extended n-alkyl chains of any length (Figure 9S in SI) can be arranged normally to the plane of the 2-2-2 sheet which minimizes the congestion, and for R=Me and Et the formation energies for isolated 3-3 chains and 2-2-2 sheets are very close indeed (see Table 2S). For the phenyl groups, which have a larger footprint, this mode of packing becomes impossible; however, as shown in Figure 10S in SI, this strain can be relieved by bending one of the phenyl groups towards the hexagonal ring opening, breaking the local C3 symmetry of the crystal field. The same was observed for the 2-methylbutyl groups (Figure 11S in SI), for which the 2-2-2 sheet has Em ≈ 63 kcal/mol.eq higher energy than the corresponding 3-3 chain (Table 2S).

The 2-2-2 sheet can also be

decorated by 2-propyl and iso-butyl groups at the increasing energy cost vs. the 3-3 chain (Table 2S). Actually, our modeling suggests that for iso-butyl and 2-methylbutyl groups, the 2-2-2 ribbon would be lower in energy than the 2-2-2 sheet (Table 2S), although this 2-2-2 ribbon is still > 27 kcal/mol.eq higher in energy than the 3-3 chain. For the bulkier 2-ethylbutyl groups (and even more so for the 2-ethylbutyl groups), no possibility exists to place these groups on the 2-2-2 scaffolding. While the 22-2 motif is plausible for the n-alkyl groups and the smaller 1- and 2-methyl alkyl groups, it appears to be excluded for the branched alkyl groups having a larger cross section. The similar yet more relaxed constraint also applies to the 2-2-2 ribbons: while the phenyl, nalkyl and 2-methyl-n-alkyl groups can decorate this scaffolding, the 2-ethyl-n-alkyl chains cannot fit. Thus, according to our simulations, the dominant driver for the emergent structure of the amorphous CP phase is the steric requirements of the substituent R groups. In this


18

Page 19 of 65

way, only the 3-3 chain scaffolding can accommodate all of the substituting groups. Our inference, therefore, is that the 3-3 chain polymer rather than the currently favored 2-2-2 sheet is the likely structural unit of the CP. This is the assertion that we seek to verify or falsify below. As the previous researchers put much emphasis on x-ray crystallography as a means of characterizing the CP structure, we first critically examined the XRD data.

3.2. XRD. Although amorphous, the CP phase has a crystalline component that defines the inorganic core of the material. Collecting powder XRD can therefore be used to index the signature Bragg peaks.

2

4x10

Ln-DEHP 295 K

Nd counts

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


2 Lu

Eu 0 10

20 

30

40



Figure 3. Powder diffraction patterns for Ln-DEHP at 295 K (Ln=Na, Eu, Lu). The 5o peaks are removed to facilitate the comparison between the samples. The vertical lines indicate the persistent peaks whose positions are identical in all of these samples.

Figure 3 exhibits powder x-ray diffraction patterns obtained for Ln-DEHP gels, where Ln=Nd, Lu, and Eu. It is seen that the five strongest Bragg peaks are conserved, and the comparison with the data in the literature

15, 16, 20


suggests that all of the 19


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Page 20 of 65

lanthanide compounds yield essentially the same XRD pattern, although the resolution may vary considerably from sample to sample. Nothing in our data suggests that the structural motifs for Nd-DEHP and Eu-DEHP are different, as was implied by the studies of Trifonov and co-workers, who postulated 2-2-2 sheet structure for Nd-DEHP and 3-3 chain structure for Eu-DEHP. 20, 26 These five conserved peaks, however, are insufficient to index the unit cell reliably. As seen from Figure 3, Lu-DEHP polymer exhibits the highest degree of crystal ordering. Taking into account the best defined 15 peaks that are listed in Table 4S in SI, the best match (Table 5S in SI), was given by a triclinic unit cell with a=19.06 Å, b=18.09 Å, c=9.12 Å, =99o, =100o, and =112.6o (cell volume 2770 Å3). For Eu-DEHP, it was a triclinic cell with a=18.57 Å, b=18.12 Å, c=8.13 Å, =99.4o, =90.4o, and =118.9o (cell volume 2350 Å3). For Nd-DEHP, which exhibited the greatest disorder and yielded the smallest set of peaks, the “best” unit cell was hexagonal with a=b=18.41 Å, c=15.12 Å, ==90o, and =120o. For Eu-DEHP, Suglobov et al.

20

reported a hexagonal unit cell with a=18.6 Å

and c=10.4 Å. This reasonably agrees with our computational estimates for a and b (see section 4 and Table 5S in SI) but the agreement for c is poor, and there is significant trigonal distortion. For Nd-DEHP, Trifonov et al.

26

determined a monoclinic cell with

a=19.5 Å, b=11.26 Å, c=16.04 Å, and =93o that did not fit our XRD pattern at all. We surmise that poor resolution and disorder that is inherent to this material precludes reliable unit cell indexing. Even with our improved resolution, there is no guarantee that the unit cell was correctly and uniquely defined, and the structural inferences made exclusively on the basis of such analyses need to be viewed cautiously. Complementary methods were therefore sought.

3.3. TRLF. Time-resolved and laser-induced fluorescence spectroscopy provides a means to probe the crystal field (CF) around the probe Ln3+ ion. Rank-4 and rank-6 CF parameters yield the directional information about the six oxygens in the first coordination shell (see section 2.2 and Appendix 1S in SI), as their coordination factors (eq. 3) steeply depend on the spherical angles and so are quite different in different structural models. For


20

Page 21 of 65

convenience, we chose Eu3+ ions that exhibit strong 5D0 → 7FJ (J=0-4) emission in the visible. Figure 12S in SI exhibits the dynamics of 610 nm fluorescence from the roomtemperature Eu-DEHP; very similar kinetics were observed at 77K (see Figure 13S in SI). These decay kinetics are exponential for t>2 ms, while at a shorter delay time the kinetics become dispersive; the same was observed at 77 K. For t>80 s, these roomtemperature kinetics (Figure 4) can be approximated using two exponential components with the life times of 7.3 and 1.4 ms (±0.02 ms) and the relative weights of 67 and 33%, respectively (Figure 4).

6 4 2

100 I(t), scaled

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


8 6 4

Eu-DEHP 2

10

(i) (ii) (iii)

8 6 4 2

1 2

100µs

4 6

2

1ms

4 6

2

4

10ms

Figure 4. The decay kinetics of 610 nm luminescence from photoexcited Eu-DEHP matrix at 295 K (trace i) analyzed by decomposition into one (trace ii) or two (trace iii) exponential components.

In a disordered EuL3 matrix, there is apparently Förster-type of energy transfer 62 between the Eu3+ ions at distorted sites that leads to inhomogeneous broadening and excitation energy migration. Because energy transfer depends on Eu3+ concentration as


21


well as on site distance, we tried to suppress this transfer using 6 at% Eu3+-doped GdDEHP (Figure 14S). The resulting decay kinetics were still biexponential, with 32% of the 6.8 ms component and 68% of the 1.03 ms component. The same behavior was also observed for Ln-DPhD solids, as shown in Figure 15S in SI. More accurate analysis of the non-exponential decay curves requires using Inokuti-Hirayama model

62

of donor-

donor energy transfer. However, to serve the purpose of the present work in characterization of the Eu3+ luminescence dynamics, we limited ourselves to using the long-range part of the fluorescence decay (> 2 ms) in which energy transfer is absent (i.e. resulting from Eu3+ ions more isolated from each other).

J=1

I(t), normalized

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 22 of 65

J=2

Eu-DEHP, 77 K 355 excitation

i ii iii calc. J=3

J=4

X10 0

1000 2000 3000 -1 relative energy, cm

Figure 5. The emission spectrum of Eu-DEHP observed at 77 K (355 nm excitation). Zero energy corresponds to the 7F0 level. Normalized traces i, ii, and iii were obtained at the delay times of 1, 2, and 9 ms, respectively. The vertical bars are calculated energy levels. Note the multiplication factor of ten for the higher energy transitions. The total moment J corresponding to these 7FJ bands is indicated in the plot.

The emission lines for Eu-DEHP were poorly resolved at the room temperature (Figure 16S in SI); however, the asymmetrical J=1 band clearly indicates splitting and


22

Page 23 of 65

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


excludes the octahedral CF symmetry postulated by Suglobov et al.

20

The structure

becomes fully resolved at 77 K (Figure 5). The J=0 band exhibits the single peak at 579.8 nm, suggesting the existence of the single type of the emission Eu3+ center, which excludes most of the structures in Table 1 that would give more than one CF environment. The J=1 band has two resolved lines at 591.3 and 593.4 nm, which indicates a trigonal or hexagonal type of CF (or site) symmetry. Appendix 2S, Tables 6S to 10S and Figure 17S in SI give step-by-step account as to how the emission spectrum was analyzed to obtain CF parameters and compare them with the CF parameters computed using our semiempirical models. The main conclusion is that only the 3-3 chain structure is compatible with the observed CF parameters without assuming unrealistic atomic parameters. The experimentally inferred polar angle

 of 51.5o for the S6 symmetrical crystal field (Table 10S) is close to 50.6-50.8o given by our semiempirical models (Table 2 and 8S). The main contender is the 2-2-2 sheet that in our model has nearly perfect C3 symmetry at Eu3+ sites (Table 7S in SI). While it is possible to fit the spectra with this model, too, unrealistic assumptions concerning the spin-orbital coupling and the Slater-Gordon parameters were needed to achieve the agreement. Furthermore, the inferred =52.3o and =32.6o significantly differ from these parameters predicted for the 2-2-2 sheet (cf. Tables 2 and 8S in SI) and obtained in the Eu-DMeP crystals (section 3.1 and Tables 7S and 8S in SI). These results strongly imply that the 3-3 chain provides a better structural model for the Eu3+ center that is responsible for the long-lived luminescence in Eu-DEHP. 3.4. XPS and 31P MAS NMR. The difference between the proposed structural models considered in section 3.1 originates in part from the role of the organophosphate ligand that can adopt different bridging and non-bridging modes. These modes are expected to result in varying chemical environments around the O and P atoms that can be observed using x-ray photoemission spectroscopy (XPS), as the binding energies for the x-ray photoemission peaks correspond to the ligand adopting different positions. While TRLF measurements discussed in the previous section suggested that Eu3+ emission centers had trigonal


23


symmetry and favored the 3-3 chain model, such centers may not be representative of the bulk material, given the inherent disorder and the occurrence of the energy transfer in the polymer matrix. To mitigate this shortcoming of the TRLF approach, we sought an independent verification of the structural uniformity.

(a)

counts

1200

O 1s 535eV

525

(b)

counts

800

530

P 2p

140eV 250

135

130

(c)

counts

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 24 of 65

Lu 4f 0 20eV

15 10 5 binding energy, eV

0

Figure 6. XPS of Lu-DEHP gel for (a) O 1p, (b) P 2p, and (c) Lu 4f lines (open circles). The solid lines are the Gaussian fits on a quadratic slope; in panel c the line is decomposed into two components each fit using 3:4 doublets of the 4f7/2 line.

The XPS spectra of Lu-DEHP polymer are shown in Figure 6 (the full scan is shown in Figure 18S in SI). For the O 1s and P 2p lines centered at 531.2 eV and 133 eV (panels a and b in Figure 6), respectively, there is a single Gaussian component with the full width at half magnitude (fwhm) corresponding to 3.16 and 2.3 eV, respectively. This result suggests the existence of the single environment for these atoms. For the Lu 4f line at 9.22 eV, 63 there is also a single environment (panel c in Figure 6), but there is a little shoulder centered at 5.2 eV (that integrates to 3% of the main line) which may originate


24

Page 25 of 65

from a different environment, although this line would overlap with a weak line from Lu

-16.76 1.1%

-10.8 1.9%

31

P MAS NMR signal

(a)

-15 -20 31 ( P), ppm vs. H3PO4 1.33 CH, CH2 0.88 CH3

-10

-19.49 6.7%

-18.70 90.4%

4d. These results imply that almost all of the Ln3+ ions have an identical environment.

3.85 POCH2

H MAS NMR signal

(b)

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


5

1

( H), ppm vs. TMS

0

Figure 7. The (a) 31P and (b) 1H MAS NMR spectra of Lu-DEHP (solid lines). In panel a, the dotted line indicates the decomposition into Lorentzian components, with the centroids and weights of these components indicated in the plot. In panel b, the chemical shifts and attributions for the resonance lines of the protons in the EH groups are given in the plot.

A complementary technique to XPS is MAS NMR that is used to probe the local environments of magnetic nuclei and their interactions with the (paramagnetic) metal ion. Figure 7a exhibits

31

P MAS NMR spectrum of the same compound (with the chemical

shifts  in ppm given vs. the aqueous phosphoric acid). We remind that Lu3+ is a closeshell (4f14) ion, i.e. there are no paramagnetic shifts induced in the magnetic nuclei (1H,


25


31

P). Ca. 91% of the signal (with the spinning side bands taken into the account) is

comprised of a single narrow line (260 Hz fwhm) at -18.7 ppm, while three smaller resonance lines at -10.8, -16.7 ppm and -19.5 ppm account for, respectively 1.9%, 1.1% and 6.7% of the 31P nuclei (it was established that these ratios are undistorted by the spin relaxation). The material appears to be well-ordered, as we also obtained fully resolved 1

H MAS NMR spectrum from the 2-ethylhexyl arms (Figure 7b).

-145.0

P MAS NMR signal

-8.5

-289.7

Lu Yb Eu

31

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 26 of 65

400

0

-400

-800

31

( P), ppm vs. H3PO4

Figure 8. The 31P MAS NMR spectra of Ln-DEHP for Ln=Lu, Yb, and Eu (see the legend given in the plot). The spinning side bands are indicated with the arrows.

These observations (as well as XPS data examined above) indicate that in the major component of CP there is only one type of the phosphorus atom, which excludes many of the structural possibilities, such as 2-4 chains and 2-2-2 ribbons. The additional structural


26

Page 27 of 65

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


insight (indirectly bearing the directional information) can be gained through observation of the paramagnetic shifts induced by Ln3+ ions, as described below. For Yb-DEHP and Eu-DEHP in Figure 8) the resonance lines for

31

P were

considerably broader and their positions were shifted by ~300 ppm. We remind that Yb3+ ion is a spin-1/2 ion in the ground 2F7/2 state with the effective magnetic moment eff ≈4.5 B and Eu3+ ion exhibits temperature-dependent Van Vleck paramagnetism due to the involvement of the 7F1 state, typically having eff≈3.3 B around 300 K (vs. 7.94B for Gd3+).

64

In solid-state MAS NMR, the chemical shift  induced by a paramagnetic

metal ion can be separated into the Fermi contact and (pseudo contact) dipolar coupling terms. 65 As demonstrated in section 3.5 using ESEEM spectroscopy, the contact term for 31

P is negligible, and so (assuming fast spinning and thermal averaging),

66

,

  B 0  g  g  d , where d is the tensor for magnetic dipole interaction between the Ln3+ ion and the



31

P nucleus. In first approximation, this pseudo contact shift is



proportional to 3 cos 2   1 r , where r is the distance from the Ln3+ ion to the

31

P

nucleus and is the angle between the radius-vector r and the z-axis of the axially symmetrical g-tensor. As there is only a single strongly shifted resonance line for both of the paramagnetic ions, all of the phosphorus atoms must be at approximately the same distance from the Ln3+ ion and the corresponding radius-vectors should make approximately the same angle with the long axis of the g-tensor. This suggests the high local symmetry of the Ln3+ ion in the CP, which is consistent with the computational models and TRLF data indicating trigonal symmetry for the Ln3+ ions. Indeed, every structure but the 3-3 and 2-2-2 polymers considered in section 3.1 would have either more than one type of the

31

P nuclei (in a Lu3+ compound) or more than one set of

paramagnetic shifts in these 31P nuclei (in paramagnetic Ln3+ compounds). Thus, our XPS and NMR experiments indicate that the emission centers examined in section 3.3 are indeed representative of the majority of the Ln3+ ions in the CP matrix.


27


Ln-DPhP, 7 K Gd/Eu 1:8 =88 ns

pESE

(a)

(i) (ii)

2

3

3

4

5x10

B0 , G

1

(b)

(c)

modulus FT

sESE

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 28 of 65

31

0

P

1

H

10 20 MHz

30

B0=3.48 kG 0 0.0

0.5

1.0 T, s

1.5

2.0

Figure 9. Pulsed X-band EPR on Gd3+-doped Eu-DPhP (Gd/Eu =1:8) at 7 K (9.686 GHz) (a) Primary electron spin echo spectrum (i) and the first derivative of this spectrum (ii). (b) sESEEM trace (solid line) obtained at the signal maximum at 3.48 kG and =88 ns (corresponding to the optimum conditions for 31P modulation). The dashed line is the simulation for the S’=1/2 system. (c) Fourier transform of the modulation pattern shown in panel b. The two prominent lines correspond to the Larmor frequencies of 31P and 1H nuclei in the DPhP ligands.

3.5. ESEEM and EXAFS. While the Ln-31P distances can be crudely estimated from these paramagnetic NMR shifts, ESEEM spectroscopy provides a direct method that is free from the uncertainties and simplifications inherent in such analyses. Like EXAFS and unlike TRLF, this technique does not give the directional structural information; however, the combination of EXAFS and ESEEM provides estimates for Ln-O and Ln-P distances in


28

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the first two coordination shells around the Ln3+ ions. While EXAFS is most sensitive to the positions of the oxygens in the first coordination shell, ESEEM is sensitive only to the 31

P nuclei in the second coordination shell. To avoid rapid spin relaxation and line broadening due to the magnetic dipole and

Heisenberg exchange interaction of the paramagnetic ions (see sections 3.6 and 3.7), magnetically dilute samples were prepared for these ESEEM studies. Figure 9a depicts the X-band spin-echo detected EPR spectrum of Gd3+-doped Eu-DPhP, Figure 9b shows sESEEM trace obtained for =88 ns, and Figure 9c exhibits the modulus FT spectrum of the modulation pattern (section 2.5). It is seen that both 1H (>99.9 at%; I=1/2) and

31

P

(100 at%; I=1/2) nuclei contribute to this modulation with the frequencies that are close to the Larmor frequencies for free nuclei, suggesting that the isotropic Fermi contact hyperfine interactions a P for the

31

P nuclei are weak (< 0.5 MHz, which justifies our

interpretation of 31P MAS NMR spectra in section 3.4). These NMR and TRLF results, in turn, suggest that a shell model (in which N magnetically equivalent nuclei are placed at equal distance rP from Ln3+ ion) should be adequate for the CP. We limited the consideration to N=6 phosphorus nuclei, given the stoichiometry and the trigonal symmetry for the Ln3+ site (sections 3.1 and 3.3). The simulated trace in Figure 9b is for

rP  3.71 Å corresponding to Gd-P distances (the simulation parameters are given Table 3). For the protons, only the ensemble averaged distance r , defined as r

6

  ri 6 , i

where the summation is over all weekly coupled protons, can be estimated (giving

rH  3.08 Å). Using the structural model discussed in section 3.1 (see Table 2), for an isolated 3-3 chain we estimated rP  3.82 Å and rH  3.2 Å, which is fairly close to the ESEEM estimates in Table 3. In the 2-2-2 sheet model, we estimated rP  3.87 Å and

rH  2.91 Å (Table 2); the latter estimate (reflecting the crowding of the phenyl groups) is significantly smaller than our ESEEM estimate. For DEHP polymers, the spin relaxation was more rapid (see Figure 19S in SI), and to slow it down we used Gd3+-doped Lu-DEHP with the doping ratio of 1:300. Figure 10a shows the pESE spectrum and Figures 10b and 10c summarize the effect of the delay time  between the pulses on the modulus FT spectrum, with the time interval chosen to


29


maximize or minimize the

31

P or 1H modulation, in order to improve the accuracy of

measuring the Ln-P and Ln-H distances. The optimized model parameters obtained through the direct fitting of the time-domain traces are given in Table 3. The fit quality

Ln-DEHP, 7 K Gd/Lu 1:300

pESE

(a)

(i) (ii)

modulus FT

2

modulus FT sESE

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 30 of 65

B0=3480 G

(b) 31

3 B ,G 4 0

5x10 1

, ns P

31

P

H

88 108 136

=88 ns

(c)

3

1

H

B0, kG 2.45 3.08 3.48 3.87

10

MHz

20

Figure 10. Pulsed X-band EPR on Gd3+-doped Lu-DEHP (Gd/Lu =1:300) at 7 K (9.686 GHz) (a) Primary ESE spectrum (i) and the first derivative (ii). The vertical bars indicate the magnetic fields B0 used to obtain thee sESEEM traces whose Fourier transforms are shown in panel b. (b) The variation of the modulus FT sESE spectrum obtained at 3.48 kG with the delay time  between the microwave pulses (indicated in the plot). (c) The variation of the modulus FT sESE spectrum obtained for =88 ns with the field B0 (indicated in panel a).

was improved by assuming a small isotropic hyperfine constant a P on 31P nuclei of ≈0.4 MHz. The estimates for the Gd-P distance in the first coordination shell are 3.9-4.0 Å, and rH

is 3.3-3.35 Å (pESE analysis favored a lower estimate of 2.85 Å). In the 3-3

chain model (section 3.1 and Table 2), rP  3.82 Å and


rH  2.67 Å (we cannot 30

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compare with the 2-2-2 model, as the corresponding structure for R=2EH does not exist). Judging from the extended Gd-P distances, the CP exhibits more axial distortion than is given by our models for isolated structures, as is also suggested by TRLF,

31

P MAS

NMR and cw EPR. As the Gd3+ ion has nonzero ZFS (section 2.3), the allowed spin transitions ( M S  1 ) corresponding to different spin projections MS overlap in frequency, complicating ESEEM analyses.

37

In the analyses presented above, we neglected this

complication by regarding the spin system as the Kramers doublet with S   1/2 and choosing the  1 2    1 2 transition that is least affected by the ZFS interaction. Given the possible complications introduced by such simplification, we sought to complement these results by using the Yb3+ ion as a probe, as this is an S=1/2 species (section 2.2). Figure 20S, panel a in SI exhibits the ESE spectrum of Yb3+-doped Lu-DEHP. The sESEEM traces were obtained in the fields corresponding to the axial and equatorial components of the g-tensor, and Figure 20S, panel b in SI shows the corresponding FT sESEEM spectra. Tuning both the external magnetic field and the interval  between the pulses allowed us to find the conditions where the 31P modulation was greatly suppressed, which gave very accurate estimate for rH

(Table 3) of 2.50-2.65 Å, which is in

excellent agreement with the computational model for the 3-3 chain (Table 2). The Yb-P distances ( rP  4.1-4.16 Å) are even longer than Gd-P distances in the Gd3+ doped LuDEHP, suggesting still greater degree of the axial distortion. To further validate and rectify these structural models, we used low-temperature EXAFS. Figures 11 and 21S in SI exhibit k- and R-space k3-weighted EXAFS spectra obtained for Eu-DEHP at 40 K that are compared with the traces obtained by the constrained geometry optimization using our 3-3 and 2-2-2 models as the input geometries. The optimized parameters for the first two coordination shells obtained using these models are summarized in Tables 4 and 11S in SI, respectively. Table 4 gives the Eu-O and Eu-P distances, the coordination numbers and the Debye-Waller factors obtained by optimizing and rectifying the structural parameters for the model of the 3-3 chain polymer (section 3.1 and Figure 1c). The coordination numbers and the distances are in fair agreement with this model; the greatest discrepancy is in the estimates for the


31


Ln-OP bond lengths, as the Sparkle/RM1 model does not include covalent interactions between the Ln3+ ions and the oxygen ligands. For the 2-2-2 sheet (Figure 21S and Table 11S) the fit quality was almost as good as that for the 3-3 chains, so it is impossible to choose one model with the threefold symmetry over the other from these EXAFS data alone. exp. (i) (ii)

(a)

3

k (k)

5

0

-5

0

4

-1

8

12

k, Å 10 modulus FT

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 32 of 65

exp. (i) (ii)

(b)

5

0 0

2

4

6

R, Å

Figure 11. Comparisons between (a) the experimental k3-weighed EXAFS data and (b) the corresponding modulus FT to the R-space for Eu-DEHP at 40 K (solid line) and the best least squares optimized fit assuming the 3-3 chain geometry shown in Figure 1c. Traces i and ii correspond to the patterns obtained in the models without/with Eu-Eu scattering, respectively.

3.6. Cw EPR. Through the dilution of paramagnetic probe ions (such as Gd3+ and Yb3+) with diamagnetic ions (such as Eu3+ and Lu3+), it is possible to suppress the spin exchange and


32

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magnetic dipole interactions between the adjacent paramagnetic ions and in this fashion study the magnetically-decoupled probe ions. EPR studies on such magnetically dilute CPs yield the structural information about isolated paramagnetic ion, whereas at high concentration one probes magnetic interactions between these ions. We sought to use both of the approaches to interrogate the structure of Ln-DRP solids. In the first-derivative cw EPR spectra of a room-temperature aqueous solution, the Gd3+ ion appears as a wide Lorentzian line with peak-to-peak distance Bpp (between the points of the maximum slope) of 286 G (Figure 22S, panel a in SI). The EPR spectrum of the Gd(L..HL)3 complex in a toluene solution is more complex; it can be best described as a superposition of two Lorentzian lines with Bpp of 87 and 328 G and the relative weights of 23 and 77%, respectively (Figure 22S, panel b in SI). This characteristic EPR line is observed in all of the liquid samples, regardless of the HL concentration, the subsequent dilution of the complex in toluene, the presence of sodium cations, the pH of the aqueous phase, the Gd3+ concentration (1 to 200 mM) and the Gd/Eu dilution in the aqueous sample. All of these results indicate the formation of a well-defined single-type Gd3+ complex in the solution. Jensen et al.

13, 14

reported the

formation of Nd2L6 complexes in the organic phase when 3 eq KOH was added to the aqueous phase during the extraction. We have repeated this experiment, but observed only the much reduced EPR signal of the residual Gd(L..HL)3 complexes; the shape of these EPR spectra was identical to the species extracted from pH=3 solutions. This was the first indication that the dimer complex can have different magnetic properties from the monomer, as this EPR observation suggested that this dimer did not contribute to solution EPR. When the solvent was changed from toluene to 1-octanol, a gel suspension instantly formed during the extraction, yielding an entirely different EPR spectrum (see below). The same EPR signal was observed when 2-propanol was added to the toluene solution; actually, the features observed in the suspended gel were also observed in the solid material, suggesting that the polymerization was rapid. Figure 12a exhibits the room-temperature EPR spectra of dry Gd-D[R]P, which are wide signals centered at g≈2. Similar EPR spectra were obtained for different substituting groups R=Oc, Bu, Ph, and


33


EH in the D[R]P- ligands (Figure 12a), and there is little temperature variation in these

EPR signal, 1st derivative

EPR spectra between 4 and 300 K (Figure 23S in SI).


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 34 of 65

(a)

Gd-D[R]P 295 K

Oc

Bu Ph EH

3+

Gd -doped Ln-DEHP

(b) (ii) Gd/Lu 1:530

(i) Gd/Eu 1:80

2

4

6kG

Figure 12. (a) The first-derivative cw X-band EPR spectra obtained from the room-temperature dry Gd-H[R]P gels, where R is (from top to bottom) n-octyl (Oc), n-butyl (Bu), phenyl (Ph), and 2-ethylhexyl (EH). (b) The first-derivative cw X-band EPR spectra of Gd3+ -doped (i) Eu-DEHP and (ii) Lu-DEHP. The arrow indicates the spin forbidden transition in the low field.

On the other hand, when EuL3 and LuL3 gels were lightly doped with Gd3+ ions (Figures 12b and 24S in SI), quite different EPR spectra were observed. This implies strong Gd3+-Gd3+ magnetic interactions, which become suppressed when the paramagnetic Gd3+ ions around the probe ion are substituted with diamagnetic Ln3+ ions.


34

Page 35 of 65

3+

Gd /3M NaNO3 1 M HDEHP (1:1 v/v) 50 K

(iii) (iv) (v)


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


(i) (ii)

5.66 4.00

2.85 2.32

1.74

2

geff

4kG

Figure 13. Normalized first-derivative cw X-band EPR spectra obtained from the Gd(L..HL)3 monomers in the frozen toluene glass at 50 K. The complexes were extracted using 1 M (traces i to iii) or 0.1 M (trace iv) HDEHP (1:1 v/v) from the aqueous phase (pH=3) containing 3M NaNO3. The composition of the aqueous phase was (i) 2.5 mM Gd3+, (ii) 0.8 mM Gd3+ with 9.2 mM Eu3+, (iii) 12.5 mM Gd3+, and (iv) 54 mM Gd3+. Trace v is the numerically computed first derivate ESE spectrum (=200 ns) obtained at 7 K from sample i. The forbidden transitions at geff > 3 are not observed in the spin-echo spectrum.

As in the case of Gd-D[R]P, the EPR spectra observed in Gd3+-doped Ln-D[R]P matrices only weakly depend on the substituent groups R- in the ligands, suggesting the structural similarity for all such compounds. At strong magnetic dilution, the EPR spectrum originates mainly from the 3-3 chain polymer, and the Gd3+ dopant can be used as a structural probe for this polymer. In Figure 13 we show the EPR spectrum of the extracted Gd(L..HL)3 complex in the toluene glass at 50 K. In addition to the prominent feature at geff≈2, this EPR spectrum exhibits the features at geff≈2.85 and 5.7, which are typical for the so-called Uspectra of the Gd3+ ions in amorphous matrices.

36

The current consensus is that these

features originate through the rhombic contribution to the ZFS interaction (eq. 6) in the strong coupling limit ( D ~500-600 G (0.050-0.055 cm-1)) in conjunction with the broad


35


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Page 36 of 65

distribution of the asymmetry parameter  between 0 and 1 (section 2.3). This EPR spectrum does not change with the Gd3+ concentration and magnetic dilution of Gd3+ ions with Eu3+ ions in solution suggesting that it originates entirely from isolated Gd3+ ions. Acting together, the strong D coupling and the asymmetry result in the efficient spinforbidden transitions in the low field

36

that are observed in cw EPR experiments (when

the microwave field continuously pumps such spin-forbidden transitions), but are not observed in spin-echo experiments, where this pumping does not occur. From the EPR perspective, the crystal field is predominantly rhombic (i.e., the corresponding rank-2 tensor terms in eq. 6 greatly exceed the higher rank terms). Comparison of the cw EPR spectra for Gd3+ ion in Gd(L..HL)3 complex and Gd3+doped EuL3 (for L=DEHP) reveals the striking differences between the environment of this probe ion (Figures 14). In the polymer matrix, the forbidden transitions are observed only at great magnification, suggesting weak ZFS coupling regime (i.e., weak axial distortion). These transitions can still be observed at low temperature, high gain and high microwave power, as shown in Figure 25S in SI. From the positions of the resonance lines at geff ≈ 10.9, 5.7, and 3.67 (Figure 14) we estimated giso≈1.98, D ≈ 190 G (0.018 cm-1), D < 20 G, while the envelope of the central geff≈2 component is consistent with

    0.3-0.5. As the sample temperature increases, the forbidden transitions become progressively weaker (Figure 26S in SI), and the EPR spectrum resulting from the allowed spin transitions undergoes the characteristic change shown in Figure 27S in SI, revealing the additional features. The similar low-temperature EPR spectra (Figure 26S in SI) and their temperature evolution (Figure 28S in SI) were observed for Gd3+-doped EuDPhP suggesting very similar environments around the Gd3+ ion in these two LnL3 matrices. Our simulations indicate that the EPR spectra obtained at 200-300 K can be described by an ensemble with D ≈370 G (0.034 cm-1) and D ~ 30-40 G, with

    0.25. These transformations can be attributed to the growing axial distortion of the crystal field at a higher temperature. The same axial distortion was also observed for Yb3+ ions, as shown in Figure 29S in SI. At 4-20 K, the EPR spectra for Yb-DEHP and Yb3+-doped Lu-DEHP are strikingly asymmetric, indicating the strongly axial g-tensor (note that there is no ZFS


36

Page 37 of 65

since this is a S=1/2 ion); there is also a hint of the hyperfine structure on the magnetic Yb nuclei (the 490 G triplet indicated in the plot). As the temperature increases to 70 K, the line broadening sets in, suggesting the growing distortion. The shape of the lowtemperature EPR spectrum weakly depends on the Yb3+ ion concentration, suggesting weak magnetic interaction between the adjacent spin-1/2 ions.

X20 EPR signal, 1st derivative

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


(i) (ii) 3.67

10.9 5.89

2.31

geff

2.85

2

4kG

Figure 14. Comparison of first-derivative cw X-band EPR spectra obtained from the Gd3+-doped Eu-DEHP (trace i) and the Gd3+ monomer complex extracted by HDEHP in toluene from Figure 13 (trace ii). Both EPR spectra were obtained at 50 K and 2 mW. The effective g-factors for the spin transitions are indicated in the plot. The geff=2.85 feature is absent in trace i, while the geff=10.9 feature is much more distinct in trace i. The geff=2 feature is clipped from trace ii to facilitate comparison; note the magnification factor for the low field part of trace i. The forbidden transitions are more prominent in trace ii.

We turn now to the EPR spectra of the Gd-D[R]P samples at low magnetic dilution. Figure 30S in SI exhibits the EPR spectra (normalized by their double integral) obtained for a series of Ln-DEHP samples in which the mole ratio x of Eu to Gd was varied from 0 to 80. As seen from Figure 30S in SI, at the intermediate Gd3+ concentrations, the EPR spectra can be represented as the sums of weighted traces i for x=0 and ii for x→∞, and the ratio of the corresponding weights changes linearly with the ratio of the ion concentrations (Figure 31S in SI). In the caption to Figure 31S in SI we


37


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Page 38 of 65

present a simple statistical argument, suggesting that just such a behavior would be expected in a situation when there is equal probability for the Gd3+ and Eu3+ ions to occupy the sites in the 3-3 chain. It appears that the characteristic EPR spectrum of GdD[R]P samples emerges when a Gd3+ ion is coupled magnetically to another Gd3+ ion. The resulting EPR spectrum becomes strongly broadened. From our simulations, we estimated that the second moment M2 for this broadening is ca. 4.2x104 G2. Assuming that the latter originates through magnetic dipole interactions involving the adjacent Gd3+ ions, this broadening can be crudely estimated using the Van Vleck formula, M2 

3

5

 eff2 rGd

6

,

67

which gives the estimate of rGd  6.5 Å (vs. 5.5 Å in Table 2).

Given the difficulty of interpreting this highly anisotropic EPR spectrum in the absence of a developed theory of magnetic resonance in high-spin chains, we turned to static magnetometry, as it averages over the anisotropic interactions.

3.7. Magnetometry. While there are several techniques that probe the first and the second coordination shells around the Ln3+ ion, obtaining an estimate for the Gd-Gd distance remained an experimental challenge. The EXAFS estimate given in Table 4 should be viewed with caution, as the fit quality improves incrementally when Eu-Eu scattering is included in the model (traces i and ii Figure 11), which may be merely a statistical effect of increasing the number of fitting parameters. Magnetometry offers a possible solution, as the strength of the magnetic interaction between the ions is distance dependent. The isotropic Heisenberg spin exchange (  2 J ex Sˆ a Sˆb term in the spin-Hamiltonian) between the adjacent Gd3+ ions can result in their ferromagnetic (Jex>0) or antiferromagnetic (Jex1/2 Centers in Orientationally Disordered Systems. J. Magn. Reson. 2002, 158, 126-142.

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In the Bottlebrush Garden: The Structural Aspects of Coordination

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