Elucidation of 1H NMR Paramagnetic Features of Heterotrimetallic

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Elucidation of 1H NMR Paramagnetic Features of Heterotrimetallic Lanthanide(III)/Manganese(III) 12-MC‑4 Complexes Corrado Atzeri,† Vittoria Marzaroli,† Martina Quaretti,† Jordan R. Travis,‡ Lorenzo Di Bari,*,§ Curtis M. Zaleski,*,‡ and Matteo Tegoni*,† †

Department of Chemistry, Life Sciences, and Environmental Sustainability, University of Parma, Parco Area delle Scienze 11A, 43124 Parma, Italy ‡ Department of Chemistry and Biochemistry, Shippensburg University, Shippensburg, Pennsylvania 17257-2200, United States § Department of Chemistry and Industrial Chemistry, University of Pisa, Via Risorgimento 35, I-56126 Pisa, Italy S Supporting Information *

ABSTRACT: The paramagnetic one-dimensional 1H NMR spectra of twelve LnIIINaI(OAc)4[12-MCMnIII(N)shi-4] complexes, where LnIII is PrIII−YbIII (except PmIII) and YIII, are reported. Their solid-state isostructural nature is confirmed in methanol-d4 solution, as a similar pattern in the 1H NMR spectra is observed along the series. Notably, a relatively well-resolved spectrum is reported for the GdIII complex. The chemical shift data are analyzed using the “all lanthanides” method, and the Fermi contact and pseudo-contact contributions are calculated for the lanthanide-induced shift (LIS). For the TbIII−YbIII complexes, the pseudo-contact contributions are typically 1 order of magnitude higher than the Fermi contact contributions; however, for the GdIII complex, the Fermi contact is the main contribution to the paramagnetic chemical shift. For the methyl protons of the axial acetate (−OAc) ligands, the LIS is opposite in sign, with respect to that of the aromatic salicylhydroximate (shi3−) protons, because of structural rearrangements that occur upon dissociation of the NaI cation in solution. The calculated crystal field parameters (BLn) for the TbIII (360 cm−1), DyIII (250 cm−1), HoIII (380 cm−1), ErIII (410 cm−1), TmIII (620 cm−1), and YbIII (380 cm−1) complexes are not constant, likely as a consequence of the inaccuracy of the Bleaney’s constants and, to a smaller extent, the small structural changes that occur in solution. Overall, the metallacrown scaffold retains structural integrity and similarity in solution for the entire series; however, small structural features, which do not affect the overall similarity, do likely occur.

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bonds. The pseudo-contact shift (δPC), which is also known as the dipolar contribution, is the result of the lanthanide unpaired electron density interacting with the nucleus being probed through space and is a consequence of the molecular structure of the complex. The pseudo-contact shift information provides both structural and magnetic anisotropy information about the complexes.22−26 Following Bleaney’s theory, the dipolar contribution (δPC Ln ) to the LIS for a proton i in an axially symmetrical complex (i.e., containing a symmetry axis of at least third order) can be described by the product of three terms as follows:

INTRODUCTION The use of lanthanide ions in coordination complexes is widely applicable to areas such as biomedical analysis, catalysis, magnetic resonance imaging, luminescence, and single-molecule magnetism.1−18 In the study of these complexes, approaches aimed at correlating their structural features in the solid state (e.g., from X-ray crystallography) with their properties in solution remains sometimes challenging. One common technique used to characterize lanthanide coordination complexes in solution is paramagnetic 1H NMR as the spin−orbit coupling of the 4f electrons leads to short electronic relaxation times and thus observable ligand proton peaks in the NMR spectrum.19,20 The paramagnetic shift (δpara) of the proton signal influenced by the presence of a lanthanide ion is known as the lanthanide-induced shift (LIS). The paramagnetic shift (δpara) is composed of two components: the Fermi contact shift and the pseudo-contact shift.19−21 The Fermi contact shift (δcon) is a result of the electron density due to the unpaired electrons of the lanthanide ion being delocalized onto the nucleus being probed.21 Since this delocalization must occur through bonds, it is considered to be insignificant beyond four © 2017 American Chemical Society

PC (i) = CJ(Ln) ·BLn · δ Ln

(3 cos2 θi − 1) ri 3

(1)

where CJ(Ln) is Bleaney’s constant, BLn is the sum of crystal field parameters of various orders, and (3 cos2 θ1 − 1)/ri3 is the geometric term, hereafter indicated as G(i).27 In the G(i) parameter, θi is the angle formed by the LnIII−proton direction Received: April 14, 2017 Published: July 5, 2017 8257

DOI: 10.1021/acs.inorgchem.7b00970 Inorg. Chem. 2017, 56, 8257−8269

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Inorganic Chemistry with the molecular axis (along z), while ri is the LnIII−proton distance. As for Bleaney’s constant CJ, this parameter is different for each lanthanide and it is correlated with the anisotropic (oblate or prolate) 4f-shell electron distribution.16,27 Bleaney’s theory relies on several assumptions, foremost that the crystal field parameters are small, compared to kT, which may be at least questionable, when not incorrect. For this reason, Bleaney’s theory has been recently criticized.20,28 Therefore, the CJ(Ln) constants should be regarded just as first approximation values, although its use provides a powerful starting point for the interpretation of the NMR spectra of LnIII complexes. The structural information gleaned from the pseudo-contact shifts can confirm or refute if the solid-state structure is retained in solution or if a series of molecules made with the same ligand set but different lanthanide ions are isostructural in solution.22−26 Almost all studies that have investigated a series of lanthanide complexes rely on complexes that contain one LnIII ion with a diamagnetic, organic ligand. Thus, little is known regarding the 1H NMR behavior, and, in particular, the pseudo-contact shift properties of LnIII ions in the presence of paramagnetic 3d transition metals.29−35 Metallacrowns (MC) are a class of molecules that offer a window into such investigations, which may result in unpredicted properties. Metallacrowns are considered the functional and structural analogues of crown ethers, where the −[C−C−O]n− repeat unit of the crown ether is replaced by a −[M−N−O]n− repeat unit in the MC (Figure 1a).36−40 Typically, the metallacrown

investigation of the [12-MCMnIII(N)shi-4] framework regarding ligand exchange and alkali-metal preference. Recently, we reported the first heterotrimetallic metallacrowns (Figure 1b).47 These complexes were also the first example of the [12-MCMnIII(N)shi-4] framework binding LnIII ions in the central cavity, where LnIII is PrIII−YbIII (except PmIII) and YIII. The LnIIINaI(OAc)4[12-MCMnIII(N)shi-4] complexes also bind a Na ion on the opposite face of the MC central cavity. The solid-state structures of these isostructural complexes were extensively investigated through single-crystal X-ray diffraction, and, recently, we have shown that it is possible in the solid state to replace the acetate ligands, which bridge the central LnIII to the ring MnIII ions, with other carboxylate ligands (benzoate, salicylate, and trimethyl acetate).51,52 However, little is known about the solution-state behavior of the LnIIINaI(OAc)4[12-MCMnIII(N)shi-4] molecules including their solution integrity and if they retain their isostructural nature in the solution state. The controllable formation of heterotrimetallic systems is still a synthetic challenge; thus, a better understanding of a series of compounds can provide insight into their physical properties, since heterotrimetallic systems can have interesting applications in magnetism, luminescence, and catalysis.53−64 Herein, we report the 1H NMR characterization in solution of 12 isostructural metallacrowns having different LnIII ions in the cavity (and YIII as the diamagnetic reference) that provides a complement to the solid-state structural analysis. The present report provides a more complete understanding of the properties and characteristics of this class of molecules. The solution-state data indicate the integrity of the complexes upon dissolution, and the LIS information confirms the isostructural nature of these complexes. The bandwidth, chemical shifts, and longitudinal relaxation rates have been analyzed and interpreted on the basis of the presence of four MnIII ions and one LnIII ion. In addition, the series of isostructural MCs allows the use of the “all lanthanides” method when considering the LIS contribution to the 1H NMR chemical shift. We demonstrate below that (i) the pseudo-contact and Fermi contact contributions to the LIS can be determined with good precision, even in the presence of multiple paramagnetic centers, and (ii) the ability to separate the pseudo-contact contribution to the LIS from the overall paramagnetic shift allows the extraction of key structural information from the paramagnetic 1H NMR data. Both points are unprecedented for this class of metal-based supramolecules. Lastly, the 1H NMR spectrum of the GdIIINaI(OAc)4[12MCMnIII(N)shi-4] complex is reported, which displays the unprecedented observation of the ligand (acetate and shi3−) proton resonances in a small-sized GdIII complex.

Figure 1. (a) Representation of the [12-MCMnIII(N)shi-4] framework. The connectivity related to the central 12-MC-4 cavity is highlighted. (b) Single-crystal X-ray structure of YIIINaI(OAc)4[12-MCMnIII(N)shi4](H2O)4·6DMF (see ref 47). [Legend: green, YIII; purple, MnIII; and yellow, NaI.]

framework consists of a 3d transition-metal ion in the MC ring and an organic hydroximate ligand provides the N−O atoms of the repeat unit and fulfills the other coordination sites of the ring metal ion. The MC framework then produces a cavity to capture a central ion, as in a crown ether. One class of MCs that has been extensively studied is the [12-MCMnIII(N)shi-4] complexes, where 12 represents the number of atoms in the MC ring and 4 represents the number of oxygen atoms.41−49 In these complexes, the MC framework is composed of four salicylhydroximate ligands (shi3−) and four MnIII ions (Figure 1a). This framework has shown the ability to bind alkali-metal ions, alkaline-earth-metal ions, transition-metal ions, and, more recently, lanthanide-metal ions in the central cavity.41−49 The first 12-MC-4, MnII(OAc)2[12-MCMnIII(N)shi-4], where −OAc is acetate, was reported in 1989 by Pecoraro and Lah,41 and in 2011 it was reported that the molecule displayed singlemolecule magnet properties in both the solid and solution states.50 In addition, the solution-state integrity of this molecule and related centrally bound Li, Na, and K ions has been studied.44 However, this is the only detailed 1H NMR

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EXPERIMENTAL SECTION

Materials and Methods. All reagents and solvents were purchased from Sigma−Aldrich or Alfa Aesar, and used without further purification. The metallacrown complexes of formula LnIIINaI(OAc)4[12-MCMnIII(N)shi-4](H2O)4·6DMF were prepared following the synthetic procedure reported elsewhere.47 Monodimensional 1H and 23Na NMR spectra were recorded on a spectrometer (Bruker, Model Avance) operated at 400 MHz, using standard pulse sequences. CD3OD was used as the solvent. Chemical shifts (δ) are reported in parts per million (ppm) and referenced to residual solvent resonances. The spectral window ranged from −60 ppm to +100 ppm for 1H, and from −100 ppm to +50 ppm for 23Na. Inversion recovery experiments were performed through the standard sequence, using delay time values (τ) between the 180° and 90° pulses ranging from 10−4 s to 5 × 10−2 s. The integrals of each 8258


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Inorganic Chemistry resonance were treated using the relationship I = I0(1 − p exp[−(τ/ T1)]), where τ is the relaxation delay time, T1 the longitudinal relaxation constant, I the integral for a specific value of τ, I0 the integral at infinite τ, and p an empirical fitting parameter, ranging from 1 to 2, which resulted in being >1.5 for all resonances. NMR spectra processing and analysis (including deconvolution and bandwidth analyses) was performed using the MestreNova 8.0 program.65 All least-squares regression analyses (relaxation times and calculations through the “all lanthanides” method) were performed using SPSS software.66

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RESULTS Using monodimensional 1H NMR in deuterated methanol (CD3OD), we analyzed a series of 12 isostructural heterometallic metallacrowns, with a general formula of LnIIINaI(OAc)4[12-MCMnIII(N)shi-4](H2O)4·6DMF. The main difference in the 12 structures is the nature of the LnIII ion, which varies between PrIII, NdIII, SmIII, EuIII, GdIII, TbIII, DyIII, HoIII, ErIII, TmIII, YbIII, and YIII (the latter being a transitionmetal ion of comparable charge/radius ratio). Despite their paramagnetic nature, which results from the presence of five paramagnetic centers in the same molecule (four in the case of the YIII compound), we expected 1H NMR to be a suitable technique for the study of this class of compounds, since both manganese(III) and lanthanides(III) (except GdIII) are known to be ions that allow high-resolution NMR experiments, thanks to their short electronic relaxation times.21 With regard to the isostructural nature of these compounds in the solid state, a detailed discussion can be found in the recent literature.39,47 Here, we wish to focus on a visualization of the superimposed and aligned X-ray structures of the LnIIINaI(OAc)4[12-MCMnIII(N)shi-4](H2O)4·6DMF complexes with all 12 central cations (Figure 2). Further discussion and

Figure 3. 1H NMR spectrum of the species YIII-MC in CD3OD. Inset shows the molecular structure of YIII-MC (NaI ion omitted). NMR resonances are labeled with numbers, while protons in the structure are indicated with letters. Nonlabeled resonances are related to DMF and solvent.

In the solid state, the YIII-MC molecule does not exhibit a crystallographic symmetry axis. Therefore, the shi3− and acetate ligands are only pseudo-symmetrical (i.e., not related by a crystallographic symmetry element). In contrast, the pattern of the NMR spectrum is consistent with the presence of an actual 4-fold axial symmetry of the complex. Therefore, our NMR data suggest that, on the NMR time scale, the complex presents an average C4 symmetry in solution, as the result of small structural rearrangement and possible fluxionality. Overall, these spectral features are also consistent with the integrity of the MC scaffold in solution, the encapsulation of the LnIII, and the presence of coordinated acetate ions. However, no information is provided either on the presence of a coordinated NaI ion on the concave face of the metallacrown, or conversely on its possible dissociation. The one-dimensional 1 H NMR spectra for the LnIIINaI(OAc)4[12-MCMnIII(N)shi-4](H2O)4·6DMF (Ln-MC, LnIII = YIII, PrIII, NdIII, SmIII, EuIII, GdIII, TbIII, DyIII, HoIII, ErIII, TmIII, YbIII) are reported in Figure 4. The general pattern of the resonances observed for the YIII-MC spectrum is

Figure 2. Overlay of the 12 X-ray crystal structures of the LnIIINaI(OAc)4[12-MCMnIII(N)shi-4](H2O)4·6DMF (LnIII = PrIII, NdIII, SmIII, EuIII, GdIII, TbIII, DyIII, HoIII, ErIII, TmIII, YbIII, and YIII). [Legend: green, LnIII; purple, MnIII; and yellow, NaI. See ref 47.]

comparison of the X-ray structural features useful for the interpretation of the NMR data will be provided below. In addition, to simplify the nomenclature of the compounds, the symbol LnIII-MC will be used to refer to the appropriate LnIIINaI(OAc)4[12-MCMnIII(N)shi-4](H2O)4 complex. Because of the diamagnetic nature of the encapsulated metal, we started by collecting the monodimensional 1H spectrum of YIII-MC (Figure 3). In this spectrum, besides the solvent peaks (DMF at 2.86, 2.99, 7.97 ppm; MeOH at 3.31 ppm; and H2O at 4.87 ppm), only five relatively broad resonances are present. The four resonances in the spectrum at +10.4, −24.3, −20.6, and −16.9 ppm (resonances 1−4) account for one proton each, and they have been assigned to the four protons of the shi3− ligands (labeled as A−D; inset in Figure 3). Conversely, the broader peak at +27.9 ppm (Me) has been assigned to the acetate methyl group, having an integral value, which is three times higher than the former peaks.

Figure 4. Spectra of the 12 heterometallic LnIII-MCs in CD3OD. The encapsulated LnIII ion is indicated on the left. All spectra were recorded in CD3OD at 298.2 K. 8259


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lanthanides (DyIII−YbIII) widely exhibit different chemical shifts, with respect to those observed for the earlier lanthanides, with signals shifted up to 15 ppm, with respect to YIII-MC. Despite these differences (in particular, after the GdIII break along the lanthanide series), all these spectra are consistent with the presence of a stable, undissociated 4-fold symmetrical MC scaffold in solution. The largest shifts, with respect to the signals of YIII-MC, were observed in the spectrum of TmIII-MC, which also presents two shi3− signals overlapped (signals 2 and 3, at ca. −27 ppm). A similar situation is observed in the spectrum of DyIII-MC where the Me signal and signal 1 are partially overlapped (ca. +18 ppm to +24 ppm). For both compounds, the chemical shift of the overlapped signals were obtained by deconvolution of the spectrum, using the MestReNova program (Figures S1 and S2 in the Supporting Information).65 In order to further characterize the MC complexes, we performed inversion recovery experiments to determine the longitudinal relaxation time (T1) of the protons (Figure S3 in the Supporting Information). Their values, along with the bandwidths of the signals, are reported in Table S1. The T1 value were determined to be 54.74°, see eq 1). Indeed, the θ angles evaluated from the X-ray structural data for the shi3− protons are actually ca. 110° (Table S2).47 Since the δPC(i) contribution is dependent on the geometric term G(i), it intrinsically contains structural information (eq 7). Therefore, we have used the Mi parameters in combination with the G(i) values calculated from the X-ray structural data for the shi3− protons (A−D; Figure 2) to confirm and support the assignment of the resonances in the NMR spectrum, as proposed above (protons A, D, B, and C for resonances 1, 2, 3, and 4, respectively).44 The first step was to calculate the G(i) terms from crystal data. In the crystal structure, the four shi3− ligands are not equivalent by symmetry and, therefore, the geometric terms for protons A−D are best represented as ⟨G(i)⟩, which correspond to the average values of the G(i) terms for each of the four pseudo-equivalent protons of the ligands. The values of the ⟨G(i)⟩ terms are reported in Table S3 in the Supporting Information. For the sake of completeness, the average Ln···H distances and angles, with respect to the molecular z-axis for the LnIII-MC complexes (LnIII = GdIII−YbIII), are also reported in Table S2. In Table 3, the ⟨G(i)⟩ parameters for the i protons (A−D) of the TbIII-MC are representatively reported, along with their normalized values (referred to the smallest ⟨G(i)⟩ value observed for proton B). As for the NMR data, the δPC(i) contributions could not be used to calculate the G(i) values as the BLn parameters are unknown (eq 7). However, the normalized ⟨G(i)⟩ values should be equal to the normalized Mi parameters for the same

However, from the data in Table 2, note that while the slopes (Mi) can be determined with significant precision, the same is not true for the intercepts (Qi). Under these circumstances, the δcon Ln (i) parameters can be better estimated, not from eq 4, but para rather by subtraction of the δPC Ln (i) from the δLn (i) values as in eq 5: con para PC δ Ln (i) = δ Ln (i) − δ Ln (i )

(6)

(5) 8262


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mean value of 400 cm−1 for this series of compounds. This aspect will be further discussed below. Acetate (Me) Protons. The Mi values for the methyl protons of the acetate groups (Me resonances) are positive (Table 2), which imply that ⟨G(Me)⟩ should be positive (eq 6). This, in turn, implies that the acetate CH3 protons should be located in solution at an angle θ smaller than the magic angle (54.74°), with respect to the principal magnetic axis of the lanthanide ion. In order to determine how this experimental observation fits with the X-ray structural data, we initially described the positions of the methyl protons as the calculated centroid of the three methyl hydrogen atoms from the X-ray crystal structure (averaged over the four acetates, ⟨θcentroid⟩; Table 5 and red spheres in Figure 7). Quite unexpectedly, these

Table 3. ⟨G(i)⟩ Values for the shi3− Protons in the TbIII-MC Complex and Their Normalized Values (Referred to Those of Proton B)a 1

X-ray Structure H

b

−3

⟨G(i)⟩ (× 10 cm )

Norm. ⟨G(i)⟩

signal

−9.75 −7.16 −8.22 −16.3

1.36 1 1.15 2.27

2 4 3 1

20

A B C D

H NMR

c

Mi

Norm. Mi

−2.72 −1.67 −1.90 −4.04

1.63 1 1.14 2.42

a

Mi values (Table 2), and their values normalized to signal 4 are also reported. bLabeling of protons as in Figure 3. cnumbering of signals as in Figure 3.

proton (eq 6). Therefore, by comparing the normalized ⟨G(i)⟩ (from X-ray data) with the normalized Mi parameters (from NMR data) we could unambiguously assign the NMR resonances to the four shi3− protons (representative data for the TbIII-MC are reported in Table 3). These assignment are, perhaps quite expectedly, in agreement with those reported in the literature for similar [12-MCMnIII(N)shi-4] scaffold but encapsulating NaI, LiI, KI, or MnII in their cavities.44 However, in the present work, we obtained this assignment without any a priori assumption, based on the resonances of the previously known 12-MC-4 species. Rather, our approach was based solely on the evaluation of the LIS, and therefore the assignments reported in Table 3 represent a proof of consistency of our approach and of the hypothesis that are at its basis. With the assignment of resonances now known, we calculated the value of the ligand field splitting parameter BLn for each LnIII-MC, limiting it to the B02 contribution and considering an axial symmetry of the complexes using eq 8.20,27,79 (Details of the calculations procedure are reported in the Supporting Information.) ⎛ C μ 2 ⎞ ⎛ 3 cos2 θ − 1 ⎞ J B PC 6 ⎟B 2 = ⎜⎜ δ Ln ⎟ × 10 2⎟ 0 ⎜ ⎠ r3 ⎝ 60(kT ) ⎠ ⎝

Table 5. Average θ Angles for the Centroids of the Methyl Groups for the Studied Complexesa and ⟨G(Me)⟩ Values for the Methyl Protons

Ln

⟨θcentroid⟩ (deg)

⟨G(Me)⟩ (× 1020 cm−3)

LnIII− OoxMP distanceb (Å)

Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb

59.28 58.69 57.91 57.47 57.12 56.88 56.64 56.23 55.89 55.65 55.55

−11.7 −9.43 −6.63 −4.95 −3.59 −2.70 −1.82 −0.0899 1.34 2.55 2.88

1.70 1.68 1.65 1.63 1.62 1.61 1.59 1.58 1.57 1.56 1.55

NaI− OoxMP distanceb (Å)

LnIII ionic radiusb (Å)

1.88 1.89 1.89 1.90 1.91 1.90 1.90 1.91 1.91 1.91 1.91

1.15 1.14 1.11 1.10 1.09 1.07 1.06 1.05 1.04 1.03 1.03

The ⟨G(Me)⟩ values reported are averaged over the 12 methyl protons of the structure. The distance between LnIII or NaI and the mean plane of the oxygen cavity (LnIII-OoxMP), and the LnIII ionic radii calculated from the X-ray structural data are also reported.47. b Data taken from ref 47. a

(8)

Here, μB is the Bohr magneton parameter, k the Boltzmann constant, and T the thermodynamic temperature. Chemical shift values are expressed in units of ppm. The B02 parameter is the second-order ligand field term, which is determined primarily by local symmetry and donor atom polarizability. The parameters δPC Ln , CJ, θ, and r are as already defined above. The calculated B02 parameters for the LnIII-MC complexes are in 250−620 cm−1 range, which are typical values for LnIII complexes (Table 4).20,28 However, perhaps unexpectedly for an isostructural series of complexes, the crystal field splitting B02 parameters oscillate quite largely (up to ca. 38%) around the

calculated average ⟨θcentroid⟩ angles are higher than the magic angle for all of the complexes of the PrIII−YbIII lanthanide series. The ⟨θcentroid⟩ angles are actually in the range of 55.5°− 59.3°, which should correspond to negative ⟨G(Me)⟩ geometric factors for all complexes.

Table 4. Calculated B02 Values for the LnIII-MC (LnIII = TbIII−YbIII)a Ln

B02 (cm−1)

rLnb (Å)

Tb Dy Ho Er Tm Yb

360(30) 250(20) 380(30) 410(30) 620(50) 380(30)

1.04 1.027 1.015 1.004 0.994 0.985

Figure 7. θ and r values for the centroid of the CH3 groups (red sphere), and for individual protons of the methyl groups (z-axis approximated coincident to the NaI···LnIII direction). [Legend: green, TmIII; purple, MnIII; yellow, NaI. See ref 47.]

a

The literature ionic radii rLn for the LnIII ions in a 8-coordinated environment are also reported.80 bIonic radii taken from ref 80. 8263


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calculate the lanthanide-induced shift (LIS = δpara Ln ) for each resonance in the spectra of the different LnIII-MC complexes by adopting two initial approximations. First, we considered the [MnIII]4 and LnIII paramagnetic systems as independent (i.e., weakly coupled). Under this condition, the paramagnetic contribution to the observed chemical shift δobs Ln can be treated para as the sum of the two δpara Mn and δLn paramagnetic contributions, related to the [MnIII]4 and LnIII systems, respectively. The second approximation is that the δparaMn contribution should remain constant along the series by virtue of the isostructural nature of the compounds. Therefore, the chemical shifts in the NMR spectrum of YIII-MC, affected only by the [MnIII]4 system, were used to calculate the δpara Ln contributions for the remaining LnIII-MC complexes using eq 2. The LIS (δpara Ln ) contributions were subsequently treated using the “all lanthanides” method, eventually leading to the δFC Ln PC and δLn contributions for the GdIII−YbIII series of MC complexes (Table 2). We were able to separate these contributions only for the heavier lanthanides, since, for these nuclides, the Fermi contact δcon Ln (i) contributions were, in most of the cases, 1 order of magnitude smaller than the dipolar δPC Ln (i) contributions, in turn, validating the linear trends of Figure 6. It is noteworthy that the data treatment carried out including the GdIII complex is unprecedented and implicitly represents a check of the “all lanthanides” method and theory for this almost NMR-inaccessible metal. Indeed, the data in Table 2 show that GdIII is the only examined lanthanide for which calculated δcon(i) contributions are dominant while δPC(i) contributions are practically zero (Table 2), as expected for its isotropic 4f 7-electron distribution. This is very remarkable, because our “all lanthanide” procedure does not introduce this information a priori;19 therefore, this result constitutes an experimental confirmation of the high-symmetry of GdIII paramagnetism and of the validity of our data treatment. Our approach allowed eventually the assignment of shi3− ligand proton resonances (Table 3), and to estimate the B02 parameter for the TbIII−YbIII series of metallacrowns. The calculated B02 parameters were in the range of 250−620 cm−1, which is perhaps an unexpectedly large interval for the heavier lanthanides, as they experience a very limited decrease of the ionic radius (Table 4).27 Anomalies in the trends of the pseudo-contact shift values (and, in turn, the B02 values) in homogeneous and possibly isostructural series of LnIII complexes have been previously reported.19,82 These anomalies have been assigned either to variation of the crystal field parameter(s) due to changeable axial coordination or to a decrease in lanthanide radii.19,82 Alternatively, the validity of Bleaney’s constants CJ(Ln) has been challenged, because of the fact that Bleaney’s theory is based on the assumption that crystal field splitting should be smaller than kT, which is obviously not true in most cases, such as in the present investigation (at room temperature, kT is ca. 200 cm−1),20,83 and it neglects contributions from crystal field parameters higher than the second order, in the case of axial symmetry (B02). We believe that, for the current series of MCs, both the inaccuracy of the Bleaney’s theory and the small structural differences in solution may be at the origin of the wide range of calculated B02 parameters, with the former possibly being the predominant factor.79,84 On one hand, small structural adaptations along the series may affect, for instance, the coupling between the [MnIII]4 and LnIII series (and therefore

A deeper analysis of the solid-state structure reveals that the use of the centroid to describe the position of the CH3 protons is not a sufficient approximation to interpret the NMR data. The reason is the fact that the ⟨θcentroid⟩ angles are very close to 54.7° (Table 5, Figure 7), and that the CH3 group rotates around the C−C axis. Therefore, when the LnIII and a considered proton are syn to each other (with respect of the acetate C−C bond), the angle θ is smaller than the magic angle (e.g., 46.1°, HMe(1); Figure 7). In contrast, hydrogens that are in an anti position, with respect to LnIII, are located at an θ angle that is significantly greater than the magic angle (e.g., 63.0°, HMe(2); Figure 7). This, in turn, implies that, upon rotation of the CH3 cone, the hydrogen atoms are located at angles θ in the range of 45°−65°, and each position provides different θ and r contributions to the overall average ⟨G(Me)⟩ terms. Thus, protons in syn positions provide the average ⟨G(Me)⟩ with a large positive G(Me) contribution, while a small negative contribution is given by hydrogens in anti positions. We decided to consider this aspect by calculating the ⟨G(Me)⟩ term as an average of the G(Me) terms for each of the 12 nonequivalent hydrogen positions in the crystal structures. These ⟨G(Me)⟩ values are reported in Table 5. Through this analysis, positive geometric terms (i.e., consistent with the NMR observation) were indeed obtained, at least for the three smallest lanthanide ions examined (ErIII, TmIII, and YbIII). We put forward the hypothesis that a change in the interaction between the LnIII ions and the oxygen atoms of the MC cavity could arise from the possible dissociation in solution of the NaI ion, since this process has been observed in methanol also for the [NaI ⊂ (12-C-4)] adduct.81 To gain insight on the occurrence of this dissociation process, we have collected the 23Na NMR spectra of YIII-, PrIII-, TbIII-, and YbIIIMC complexes in methanol-d4, along with that of NaCl as the reference for a solvated NaI. The spectra are reported in Figure S5 in the Supporting Information. The spectrum of NaCl in methanol shows a 23Na signal with a line width of ca. 17 Hz. Those of the YIII-, PrIII-, TbIII-, and YbIII-MCs show signals with line widths of ca. 28, 30, 30, and 27 Hz, respectively. The discussion of these data is reported in the next section, although we can anticipate here that they show that the dissociation of the NaI cation from the MC scaffold is likely.

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DISCUSSION In this paper, we present the analysis of the chemical shifts observed in the 1H NMR spectra of twelve LnIIINaI(OAc)4[12MCMnIII(N)shi-4] complexes (LnIII-MC), where LnIII is PrIII−YbIII (except PmIII). YIII-MC was also studied as a reference compound that contains a central diamagnetic metal ion, having been unsuccessful in the isolation of LaIII- and LuIII-MC complexes. An important observation is that the general pattern of the NMR spectra of these complexes does not change dramatically along the series of the encapsulated LnIII ions, especially as it concerns the shi3− signals. Also, if we compare the chemical shift of the proton signals in this series of complexes with those of similar MCs bearing the same [MnIII]4 system and LiI, NaI, KI, or MnII coordinated to the oxygens cavity, we can observe that the NMR pattern results are also remarkably similar.44 This behavior suggests that the paramagnetic features of the [MnIII]4 system dominate, and that the presence of a paramagnetic ions coordinated to the cavity can be treated as a perturbation of the paramagnetic behavior. Also, this observation allowed us to 8264


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Inorganic Chemistry the calculated δPC Ln contributions). However, the structural rearrangements are expected to be small in solution for these relatively rigid molecular MC frameworks. Therefore, we present the hypothesis that the inaccuracy of the Bleaney’s theory is mostly at the origin of the B02 outliers (i.e., DyIII- and TmIII-MC) reported in Table 4. Actually, this might be especially true for the TmIII-MC, since the chemical shifts of other complexes such as TmIII(DOTP) complex were found to not be fully predicted using Bleaney’s theory (where DOTP = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrakis(methylenephosphonate)).79,85−87 We previously discussed that, on the NMR time scale, the 1H NMR patterns are consistent with the presence in solution of a 4-fold axial symmetrical MC species. Since the metallacrowns do not possess true 4-fold symmetry in the solid state, this behavior suggests that the MC shi3− scaffold has certain degree of adaptability. The occurrence of such structural adjustments between crystal and solution is particularly evident for the acetate CH3 groups of the coordinated (Me signal). In the crystal structures the ⟨G(Me)⟩ values (Table 5) start very large and negative for the early elements (PrIII−DyIII) and become progressively smaller. After passing through 0 for HoIII, the ⟨G(Me)⟩ values become progressively positive for the very late elements (ErIII−YbIII). On the other hand, the solution-state NMR data would suggest a positive and possibly constant term throughout the LnIII series. It is interesting to note that the trend toward positive values of ⟨G(Me)⟩ in the crystal structures (Table 5) arises principally because the CH3 groups get closer to the molecular pseudo-4-fold axis (i.e., the C−C bonds of the acetates become more aligned to the molecular axis; Table S4). These differences along the LnIII-MC series are accompanied by a decrease of the LnIII distance to the oxime oxygen mean plane of the MC (evaluated from the crystal structure; Table 5).47 Interestingly, the LnIII−cavity distance and the angle θ associated with the methyl groups (centroids) are also linearly correlated, as shown in Figure S4 in the Supporting Information. In methanol solution, we should take into account a partial dissociation of the NaI ion, which may leave the LnIII ion better interacting with the oxime oxygens, eventually resulting in the presence of a LnIII ion more encapsulated in the cavity than what is inferred from the X-ray data. As previously described, we have collected the 23Na NMR spectra of YIII-, PrIII-, TbIII-, and YbIII-MC complexes in methanol-d4. The spectrum of NaCl in the same solvent was collected as the reference for a solvated NaI. The spectra are reported in Figure S5 in the Supporting Information. All four LnIII-MCs present 23Na signals in the range from −2.7 ppm to 0.3 ppm, with line widths in the range of 27−30 Hz. Since 23Na is quadrupolar, the line broadening should be sensitive upon NaI binding to macrocycles. Indeed, line widths in the range of 120−500 Hz have been observed for the coordination of NaI to derivatives of 18-C-6 and 15-C-5 ligands, while line widths of ca. 20 Hz were observed for the solvated NaI ion.88−91 Our 23Na measured line widths for the LnIII-MC complexes are remarkably similar to that measured for NaCl (Figure S5), suggesting that the dissociation of NaI from the MC cavity is likely. This is not surprising, because NaI dissociation in methanol has also been observed for the [NaI ⊂ (12-C-4)] adduct.81 For this latter compound, the dissociation of the NaI ion in methanol originates from the small cavity radius of the ligand into which the cation does not fit properly. Not only is the cavity radius of the 12-MC-4 scaffold is similar to that of the 12-C-4 organic ligand,36 but the

X-ray structures also reveal that the NaI ion is only bound to the four oxime oxygens of the MC, rather than being held in the site through bridging ligands like the LnIII. In addition, in all MCs the NaI-oxime oxygen mean plane distance (NaI−OoxMP, Table 5) is longer than the LnIII−OoxMP distance. The average difference between these two distances over the series of complexes is 0.30 Å, indicating that the NaI is less strongly held by the MC as the heavier Ln ions are bound in the central cavity (Table 5).47 All these structural changes are expected to allow the LnIII ion to become more encapsulated in the cavity and, therefore, the acetate groups to become more aligned with the z-axis, making the ⟨G(Me)⟩ values more positive, as indeed we observed by NMR. Noteworthy, going from TbIII to YbIII, the LnIII−OoxMP distance decreases by only 0.06 Å but the associated changes in the ⟨G(Me)⟩ values are very significant (Table 5). This supports the hypothesis that small changes in the distance between the LnIII and the LnIII−OoxMP reflect into large changes in the effective θ angles for the acetate protons. This feature is greatly amplified by the fact that θ oscillates very close to the magic angle, where ⟨G(Me)⟩ crosses 0, leading to a profound impact on the NMR features. In contrast, the shifts of the four shi3− protons, which are much greater than the magic angle, undergo much less significant variations. Finally, further support to the hypothesis that small structural adaptations originate the observed positive ⟨G(Me)⟩ values comes from the comparison between the structures of the DyIIIMC bearing either a NaI or a KI bound to the cavity (Figure S7).47 While the alkali metal−OoxMP is 1.90 Å for the NaI ion, the same distance in the DyIII-MC with KI is 2.21 Å. As a consequence, the ⟨θcentroid⟩ of the CH3 protons in the KI analogue result in an angle of 53.41°, whereas, in the NaIcontaining MC, the angle is 56.64° (Table 5). Consequently, the ⟨G(Me)⟩ values were determined to be 12.1 × 1020 cm−3 for the KI-containing MC, compared to −1.82 × 1020 cm−3 for the NaI analogue (the method to calculate these values was reported in the previous section). These data demonstrate that the increase of the distance between the alkali metal from the oxygen cavity results in a profound change of the alignment of the coordinated acetates of ca. 3°. We suspect that this effect would be even greater for the complete dissociation of the NaI ion in solution. Since the H atoms of the acetate ions are almost equal to the magic angle, the overall effect is to push the ⟨G(Me)⟩ values toward positive values, which is consistent with the experimental NMR observation.

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CONCLUSION We applied the “all lanthanides” method in the treatment of the chemical shift data obtained from one-dimensional 1H NMR spectra of a series of heterotrimetallic metallacrown complexes containing LnIII, MnIII, and NaI ions. These complexes, which are isostructural in the solid state, present 1H NMR spectra with similar patterns of resonances. Because of the diamagnetic nature of the encapsulated metal, the spectrum of the YIII-MC was taken as a reference to calculate the lanthanide-induced shift (LIS) from the spectra of the remaining LnIII-MCs. Quite expectedly, the largest LIS values were observed for heavier lanthanide ions. The observation of a reasonable well-resolved 1H spectrum for the GdIII-MC is noteworthy in the field of gadolinium complexes. Here, possibly because of a magnetic coupling between the MnIII and GdIII ions, we could not only record the spectrum of the GdIII-containing MC, but also treat the 8265


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Education program at Shippensburg University for financial support. M.T., C.A., and V.M. thank the Italian Ministry of Foreign Affairs and International Cooperation for financial support through a bilateral Italy−USA project. The research leading to these results have received funding from the European Community’s Seventh Framework Programme (FP7/2007-2013) under Grant Agreement No. 611488 (FP7Marie Curie IRSES “Metallacrowns” project). Dr. Domenico Acquotti (Centro Interdipartimentale Misure “G. Casnati”, University of Parma) is gratefully acknowledged for his assistance in the collection of 23Na NMR spectra. The authors thank Prof. Claudio Luchinat, Prof. Vincent L. Pecoraro, Prof. Giuseppe Amoretti, and Prof. Stefano Carretta for useful discussions.

chemical shifts together with those of the TbIII−YbIII species. Calculations performed with the “all lanthanides” method, which avoids any assumption on the specific properties of single lanthanides, showed that GdIII induces negligible pseudocontact terms. This result is in perfect agreement with the vanishing anisotropy of magnetic susceptibility expected for GdIII. On the other hand, the contact terms induced by this ion are very small, which is somewhat surprising, because the tabulated value of ⟨Sz⟩ for this ion is large.92 The calculation of the pseudo-contact contribution to the LIS allowed us to assign the resonances in the NMR spectra, which resulted in agreement with previous reports.44 This represents a result worthy of note, since it confirms the robustness of our approach. Finally, with the use of the X-ray structural data, we estimated the crystal field splitting parameter (B02). Through our results, we conclude that, although the MC assembly is expected to have some degree of rigidity, small structural adaptations occur in solution, as a consequence of the different ionic radii of the LnIII ions and the dissociation of the NaI ion. Perhaps more important these conformational adaptations, although small, still result in significant changes in the geometric terms of the protons examined. Possibly, these structural changes affect also the crystal field parameter, although we should admit that their calculated values are possibly a numeric pitfall where the precision of the parameters calculated through the “all lanthanides” method and the inaccuracies of Bleaney’s theory actually converge. These results are unprecedented in the field of the characterization of metallacrowns, and may open the future use of paramagnetic NMR methods for structural determination of these species in solution. We now intend to make use of the magnetic properties of these compounds to develop materials or probes which are responsive to stimuli that produce small conformational changes on their structure and, in turn, are responsive to the (para)magnetic properties of the materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00970. Calculations of LIS and crystal field splitting parameter B02, tables of derived NMR and crystallographic data, and figures of deconvolution of NMR data, inversion recovery data, and crystallographic theta angles (PDF)

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L. Di Bari). *E-mail: [email protected] (C. M. Zaleski). *E-mail: [email protected] (M. Tegoni). ORCID

Lorenzo Di Bari: 0000-0003-2347-2150 Matteo Tegoni: 0000-0002-9621-0410 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS C.M.Z. and J.R.T. thank the Undergraduate Research Grant Program and the CFEST Faculty Training and Continued 8266


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