CHEMPHYSCHEM ARTICLES DOI: 10.1002/cphc.201300119
Solid-State NMR Correlation Experiments and Distance Measurements in Paramagnetic Metalorganics Exemplified by Cu-Cyclam Shashi K. Kumara Swamy,[a] Agnieszka Karczmarska,[a] Malgorzata Makowska-Janusik,[b] Abdelhadi Kassiba,[a] and Jens Dittmer*[a] We show how to record and analyze solid-state NMR spectra of organic paramagnetic complexes with moderate hyperfine interactions using the Cu-cyclam complex as an example. Assignment of the 13C signals was performed with the help of density functional theory (DFT) calculations. An initial assignment of the 1H signals was done by means of 1H–13C correlation spectra. The possibility of recording a dipolar HSQC spectrum with the advantage of direct 1H acquisition is discussed. Owing to the paramagnetic shifting the resolution of such par-
amagnetic 1H spectra is generally better than for diamagnetic solid samples, and we exploit this advantage by recording 1H– 1 H correlation spectra with a simple and short pulse sequence. This experiment, along with a Karplus relation, allowed for the completion of the 1H signal assignment. On the basis of these data, we measured the distances of the carbon atoms to the copper center in Cu-cyclam by means of 13C R2 relaxation experiments combined with the electronic relaxation determined by EPR.
1. Introduction Metalorganic and organometallic complexes can be found in many natural systems and their applications reach from providing reactive sites in pharmacy and biology to catalysts in organic chemistry. One important method used for their characterization is nuclear magnetic resonance (NMR) spectroscopy. However, if the metal complex is paramagnetic, NMR characterization is hindered by the hyperfine interaction of the nuclear spin with the unpaired electron spin. Methodological progress in NMR led to solutions how to deal with other interactions that are small perturbations of the Zeeman interaction such as the chemical shift, J-, dipolar, and quadrupolar coupling. Particularly for solutions of biological macromolecules, methods have been developed to deal with the hyperfine interaction, which has been exploited to obtain structural information, for example, in the form of distances (by measuring nuclear relaxation),[1] angles (by measuring pseudocontact shifts),[2] or immersion into a membrane (by measuring relaxation).[3] Recently, these methods have been extended to biomolecules in the solid phase.[4] However, in solid samples with relatively small structural units, and thus short distances of the nuclei to the paramagnetic center, NMR analysis is often inhib-
[a] S. K. Kumara Swamy, A. Karczmarska, Prof. A. Kassiba, Prof. J. Dittmer Institut des Molcules et Matriaux du Mans (IMMM) UMR CNRS 6283, LUNAM, Universit du Maine Avenue Olivier Messiaen, 72085 Le Mans (France) Fax: (+ 33) 2 43 83 3518 E-mail:
[email protected] [b] Prof. M. Makowska-Janusik Insitute of Physics, Jan Dlugosz University Al. Armii. Krajowej 13/15, 42-200 Czestochowa (Poland) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cphc.201300119.
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ited. The hyperfine shift superimposes the chemical shift and renders signal assignment by comparison with chemical shift tables impossible. The contact shift[5] is generally dominating for distances below 10 , while dipolar coupling between an unpaired electron and a nucleus[5] rather becomes manifest in line broadening through relaxation or through its anisotropy. In this article, we focus on this short distance range. As a model system, we chose the complex of the macrocyclic ligand cyclam coordinating a Cu(II) ion in a crystalline phase with Cl- anions. The Cu-cyclam complex shows various configurations in solution and has a stable trans-III configuration in the solid phase.[6] Metal-cyclam complexes have potential applications in pharmaceutical industries, for example, as raw material for anti-HIV and anti-cancer drugs.[6] They can also be used as active groups in mesoporous structures, thereby functional materials with applications in electronics and optics,[7] or in chemical sensing with high sensitivity.[8] The distances to the central ion are between 2.0 for the ligating nitrogens and 4.4 for two of the equatorial hydrogens. In this distance range, electron–nuclear dipolar coupling can cause an extremely fast relaxation which not only introduces additional line broadening, but also renders the conventional NMR experiments developed for solids unusable. The excited coherences relax already during the application of the pulse sequence before the acquisition of the time signal (FID). Pulse sequences for such systems must therefore not exceed a few hundred microseconds in the best case. The magnitude of nuclear relaxation covers a wide range. It depends on the electronic spin, electronic spin relaxation, and the distance to the paramagnetic center. Cu2 + ions have an effective electronic spin of Se = 1=2 , and represent examples for which the perturbation by hyperfine interactions is not too extreme.[9] For such ChemPhysChem 2013, 14, 1864 – 1870
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CHEMPHYSCHEM ARTICLES systems, hyperfine interactions cause even certain benefits, in particular for metalorganic complexes. In 2003 and 2005 Ishii and co-workers found that under very fast magic-angle spinning, typically well beyond 20 kHz, 13C and 1H signals, respectively, arise.[10, 11] This discovery meant a breakthrough in solidstate NMR of paramagnetic metalorganic systems. Under such spinning conditions decoupling of the 1H–1H dipolar network is facilitated by the larger differences in resonance frequencies due to the hyperfine shifts. In addition, the resolution benefits from the dispersion of the signals over a much larger range of shifts, which in many cases overcompensates for the additional broadening.[9] The range of 1H NMR spectra is some hundred ppm for paramagnetic systems compared to 15 ppm for diamagnetic ones. Thus, even direct 1H spectroscopy is possible for solid paramagnetic systems where it is impossible for analogous diamagnetic ones. In addition, the extremely fast longitudinal relaxation due to paramagnetic interaction allows for the experiment to be repeated orders of magnitudes more often within a given period of time than in the diamagnetic case.[10] This compensates partially for the signal loss through relaxation, in particular in 13C experiments. Nevertheless, the pulse sequences have to be adapted to the rapid relaxation conditions. For example, Kervern et al.[12] have shown that a 1H– 13 C polarization transfer by a TEDOR or INEPT type of sequence is often superior to cross-polarization (CP). In 1H–13C correlation experiments the CP block is therefore replaced accordingly.[12] One goal of the present work is the extension of such correlation experiments. From the 1H–13C correlation spectra, one can deduce some of the proton resonances, once the carbon resonance is known. However, the correlations are not always complete; in particular, cross peaks to 1H signals that are shifted very far tend to be missing. Moreover, there is ambiguity in all cases where more than one proton is bonded to a carbon. In NMR experiments of diamagnetic solutions one applies 1H–1H correlation experiments, but in the case of solids poor 1H resolution usually inhibits viable spectra. In this paper we explore the possibilities of exploiting the better spectral resolution of paramagnetic solids in order to obtain 1H–1H correlation experiments that give the missing information for 1H signal assignment. As for all interactions in NMR, hyperfine coupling does not only mean perturbation, complicating the experiment; it also contains useful information. The electron–nuclear dipolar interaction depends on the average distance of the electron to the nucleus. Herein, we investigate whether, and with what precision, one can determine copper–carbon distances by measuring the relaxation of 13C magnetization that is induced by this very interaction.
2. Results 1
H and 13C One-Dimensional Solid-State NMR, Spin Densities, and 13C Assignment Spinning the sample faster than 25 kHz results in relatively well-resolved 1H signals (Figure 1 A), as has been discovered by Wickramasinghe et al. for Cu-alanine.[11] The hyperfine coupling not only improves spectral resolution by better dispersing the 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 1. 1D 1H (A) and 13C (B) spectra of the Cu-cyclam complex (C). Stars denote spinning side bands.
signals, but also helps to partially decouple the proton–proton dipolar network.[9, 11] Only peak Hd1 is already sharp at low spinning rates and is therefore assigned to diamagnetic cyclam units without copper center. Sidebands were distinguished from isotropic peaks by comparing spectra of different spinning speeds. Recording 13C NMR spectra by applying a simple echo pulse sequence without cross-polarization or 1H decoupling but very fast repetition[10] (48 ms interscan delay) yields three principal peaks, one in the normal chemical shift range and two shifted up-field, one of which being split and the other one relatively broad (Figure 1 B). The chemical structure of cyclam (Figure 1 C) contains likewise three different principal carbon sites. The contact shifts dfc depend on the electronic spin densities 1 at the positions of the nuclei ~ r, as defined in Equation (1): dfc
¼
ð Se þ 1 Þ
g2 m2B m0 1ð~ rÞ 9 kB T
ð1Þ
where dfc is given as relative frequency, Se denotes the total electronic spin, g the electronic spin g-factor, and the other variables have their usual meaning.[13] In order to assign the ChemPhysChem 2013, 14, 1864 – 1870
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Table 1. Spin densities at the carbon nucleus, hyperfine and chemical shifts calculated by DFT and compared to experimental shifts.
a
1 [103 au]
dcalc fc [ppm]
0.06
6
dcalc dia [ppm]
dcalc tot [ppm] 25
31
C1 b
0.03
3
a
1.84
196
dexpt [ppm] 25
28 141
142 173
55
C2 b
2.50
266
212
a
3.32
354
301
b
3.80
405
324
54
C3
350
carbon signals, the spin densities were calculated by means of density functional theory (DFT). They are shown in Table 1 for all carbon nuclei together with the corresponding contact shifts dfc. The contact shifts were added to the calculated chemical shifts. The total theoretical shifts allowed for an unambiguous assignment of the three 13C principal peaks. The bcarbons in the six-membered rings are furthest away from the Cu center and have a very small spin density, so that the chemical shift is dominating (C1). The signal between 150 and 200 ppm is assigned to the four a-carbons, and the calculations even reproduce its splitting which is caused by an asymmetry between the two six-membered rings in chair conformation.[14] This allows for a tentative assignment of the two sub-peaks (C2a, C2b in Figure 1 B and C). However, one must acknowledge that doped cyclam is a flexible molecule existing in different conformations possessing more than one local energy minimum. Other computational approaches yield a conformation where the diagonal opposite a-carbons have similar distances to Cu.[15] In all cases an asymmetry is obtained due to the binding of cyclam to only one Clion. The four carbons in the two five-membered rings also group to two sub-sites in the calculations. In the spectrum, however, they merge to a single broad peak C3. Overall, the calculations of the contact shifts allow for an unambiguous assignment that matches the expectations. Due to overlap and the number of signals, the assignment of the hydrogen spectrum by DFT would not be so easy to assess. DFTcalculated shifts can reproduce the experimental ones well, as it was the case above, but there can also be tremendous discrepancies.[16] We therefore try to assign the 1H signals by NMR techniques and compare the assignment in the end with the shifts obtained by DFT calculations. 13
C–1H Correlation Experiments for 1H Assignment
The basis of the assignment of the 1H signals was the transfer of the 13C assignment by correlation experiments. Most information stems from a refocused dipolar INEPT experiment which is similar to the TEDOR version of Kervern et al.[12] (Figure 2 A). As the fundamental obstacle for 1H NMR of solids— the loss of resolution due to large line widths—is overcome 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 2. A) Dipolar INEPT spectrum correlating 1H and 13C hyperfine shifts. B) and C) Dipolar HSQC sequence (acquisition on 1H) and spectrum of the correlations to C1.
for paramagnetic systems, one could envisage the performance of correlation experiments by 1H acquisition, optimally as a dipolar version of the HSQC.[17] One would, as in liquidstate NMR, benefit from the higher sensitivity of 1H signal detection. Figure 2 C shows a dipolar HSQC of the H1/C1 signals. In comparison to a conventional HSQC in liquid-state NMR, which exploits J-coupling, the transfer times are only one rotor period (33.3 ms), which is thus shorter by two orders of magnitude, and the 13C p pulses are omitted in order to allow for a recoupling of the (1 wr) modulated term of the dipolar Hamiltonian (Figure 2 B). This experiment however suffers from a stochastic breakthrough of signals from hydrogens attached to 12C, so that the theoretical advantage in sensitivity could not be achieved. In contrary, a high number of repetitions was necessary to observe cross peaks of the two H1 signals, while other signals could not be observed at all. The correlation spectra allow for an attribution of two hydrogen signals to signal C1 and one each to C2a, C2b, and C3. The far-shifted signal at 177 ppm and the signal at 6.4 ppm do not correlate with any of the carbon signals. The latter, showing no correlation signals in any experiment, is identified as yet another diamagnetic signal from non-coordinating cyclam units (Hd2). There is a different hyperfine shift expected for axial and equatorial protons, as the hyperfine coupling follows a Karplus-type dependence of the dihedral angle q defined by the dihedron CuNCH [Eq. (2)]:[18, 19] dfc
¼
a cos2 q þ b cos q þ c
ð2Þ
As coefficient a @ b and c, equatorial hydrogen atoms should in general display a stronger hyperfine shift. We therefore assign the signal at 18 ppm to the equatorial hydrogen of C1 and the signal at 11 ppm to the axial one. C2a, C2b, and ChemPhysChem 2013, 14, 1864 – 1870
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CHEMPHYSCHEM ARTICLES C3 each correlate with only one 1H signal rather than two. Furthermore, there is the amine HN. Thus, there are four more 1H signals expected whereas only one 1H signal is completely unassigned (177 ppm).
www.chemphyschem.org shifts with the NMR assignment in general shows a good agreement. The discrimination of the sub-sites a and b could however be reproduced in only one case. The HN of site b is probably too much covered by side bands.
Completion of 1H Assignment by 1H–1H Correlation In liquid-state NMR, 1H–1H correlation spectra, such as COSY, TOCSY and NOESY are the most frequently applied 2D experiments. In solid-state NMR such spectroscopy is rarely possible due to signal broadening. With the better 1H signal resolution in paramagnetic NMR, correlation spectra by means of strong dipolar coupling should be possible as long as relaxation is not too fast with respect to the 1H–1H magnetization transfer. We applied a simple 2D sequence without any recoupling for a transfer by the strong homonuclear dipolar couplings, followed by a rotor synchronized echo sequence (Figure 3 B). Diamagnetic signals are diminished by saturation through weak continuous irradiation during interscan delay and mixing. Figure 3 A shows the spectrum for a mixing time of tmix = 140 ms. In order to avoid too much overlap by spectral aliasing, the t1 increments are reduced to one quarter of the rotation time (spectral width 120 kHz). The spectrum is dominated by diagonal and secondary diagonal peaks (due to spinning side bands), but there are several cross peaks visible: axH2b correlates with eqH1, but not with axH1, and axH2a correlates with axH3. Most interesting is the strong cross peak of the so far unassigned 177 ppm signal to H3, indicating that it stems from the equatorial H3. Cross peaks between axH1 and eqH1, confirming that they are attached to the same carbon, are manifested only as shoulders of the diagonal peaks. There is however no indication of a cross peak between the signals of the two axH2, even at longer mixing times. Although reaching the limit of resolution, one would at least expect a sign in the form of a broadening if the two different C2a and C2b were bonded to the same C1, as the distances between two axial H2 would be about 2.5 . Its absence means that H2a and H2b are relatively far from each other, that is, in different six-membered rings, as already concluded from the DFT calculations of the hyperfine shifts. When increasing the mixing time towards 1 ms, spin diffusion leads to mutual cross peaks of all signals except for the diamagnetic Hd1 and Hd2. Figure 3 C concludes the dipolar connectivities (without symmetrical redundancies). The lacking 1H signals for eqH2 and HN could be found among the spinning side bands by a stepwise variation of the temperature down to 281 K. As the contact shift is inverse proportional to the temperature (Curie’s law), signals with strong contact coupling can be distinguished from spinning side bands by their strong temperature dependence (see the Supporting Information). Two further signals were found this way, one at 370 and one at 240 ppm (at 58 8C). According to Equation (2), eqH2 is not to be expected to have negative shifts if axH2 is positive, and thus the signal at 370 ppm is assigned to this proton (without any distinction of sites a and b), whereas the signal at 240 ppm is assigned to HN. This is confirmed by DFT calculations (listed in Table 2) showing a negative spin density at HN due to spin polarization. Comparison of the other calculated 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Figure 3. A) 1H–1H correlation spectrum obtained with the pulse sequence shown in (B) with a mixing time of 140 ms (cross-peaks of axH1 to eqH1 are superimposed by the diagonal peaks and not visible in this representation). C) Conclusion of the correlations/vicinities observed by the 1H–1H experiment.
Table 2. Spin densities at the hydrogen nucleus, hyperfine and chemical shifts calculated by DFT compared to experimental shifts.
ax
H1 H1 ax H2 eq H2 ax H3 eq H3 HN eq
1 [103 au]
dcalc fc [ppm]
dcalc dia [ppm]
dcalc tot [ppm]
dexpt [ppm]
0.09 0.18 0.47/0.58[a] 2.97/3.58[a] 0.14 1.69/1.94[a] 1.90/0.94[a]
10 19 50/62[a] 316/381[a] 15 180/206[a] 202/100[a]
1.5 2.0 3.0 3.2 2.9 2.9 2.2
8.5 17 53/65[a] 319/384[a] 18 183/209[a] 200/98[a]
-11 -18 54/58[a] 370 15 176 240
[a] Site a/b.
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Copper–Carbon Distances Measured by R2 Relaxation
Table 3. Cu–C distances from 13C NMR relaxation experiments compared to distances obtained from XRD.
Dipolar relaxation rates of nuclei close to a paramagnetic center depend in first approximation (point-dipole approximation) on the distance r between nucleus and central atom and therefore should allow for a measurement of this distance. In the following we test the suitability of this approach with the transverse relaxation rates R2 of the carbon nuclei. The contribution of the dipolar coupling between unpaired electron and nucleus to the transverse relaxation rate in a rigid solid is shown in Equation (3):[18, 20] 1 m0 2 2 2 2 1 3tc 13tc R2 ¼ mB g g Se ðSe þ 1Þ 6 4tc þ þ 15 4p r 1 þ w2n t2c 1 þ w2e t2c ð3Þ where wn and we are the Larmor frequencies of the nucleus and the electron. The rate depends on two external parameters, the distance to the central atom and the correlation time tc of the fluctuation of the electron–nuclear dipolar interaction. For most metal ions in solids, the dominating process causing fluctuations of the interaction is the flipping of the spin orientation of the electron, described by the relaxation time(s), with the approximation tc T1e .[18, 21, 22] We tentatively set T1e T2e which we estimated from the line width DB of the electron paramagnetic resonance (EPR) signal according to Equation (4):[23]
ðT2e Þ1 ¼
pffiffiffi 3 mB ^ DB g 2 h
ð4Þ
We find DB = 100 G and the average Land g factor [Eq. (5)]: _
g ¼ ðgII þ 2g? Þ=3 ¼ ð2:180 þ 2 2:055Þ=3 ¼ 2:10
ð5Þ
thus tc T2e = 0.6 ns. The transverse relaxation rates of the four distinguishable C signals have been measured by means of CPMG (Carr–Purcell–Meiboom–Gill)[24] experiments. Due to the low sensitivity of these experiments data processing is crucial. Peak positions and line widths were determined from the echo spectrum shown in Figure 1 B so that only intensities remained as fit parameters. After subtraction of an estimated non-paramagnetic contribution (mainly 1H–13C dipolar relaxation) of 300 Hz, we obtained paramagnetic relaxation rates of the signal decays between 2000 and 6000 s1. The measurements were reproducible within about 5 %. Table 3 compares the distances derived from these values in conjunction with electronic spin relaxation times with values obtained by X-ray crystallography.[25] As there is no crystal structure of Cu-cyclam with a Cl ion, we chose the structure of Cu-cyclam coordinating isothiocyanate.[25] The NMR relaxation distances match the XRD distances within a precision of 0.1 . The estimated experimental precision is in the same range.
13
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13
C R2 [s1]
rCuC from R2 []
rCuC from XRD []
C1
1990
3.42
3.34 3.35
C2a C2b
4540 4900
2.98 2.94
2.99 3.01
C3
5920
2.85
2.81 2.82
3. Discussion The r6 dependence of the relaxation rates implies both advantages and disadvantages for the measurement of distances. On the one hand, it limits the range of application to a relatively thin shell, the boundaries of which depend on the electronic spin and its relaxation properties. If the atom is too far, the nuclear relaxation is too weak compared to other relaxation contributions; if it is too close, the high R2 causes extensive line broadening. If in the latter case the signals are at the limit of detectability, it may be preferable to determine the relaxation rate directly from the line width rather than by a CPMG experiment. On the other hand, the r6 dependence allows for a relatively high precision of the determined distance, even if the relaxation rates themselves are affected by a poor signal-to-noise ratio and by other relaxation contributions which have to be estimated. The estimated precision of < 0.1 does not reach the level of X-ray diffraction or EXAFS, but compared to the latter technique the method has the advantage that the coordination shell is better resolved. The situation is more ambiguous if a carbon atom has a similar distance to two or more metal centers, as there is more than one source of relaxation. For Cu-cyclam(Cl), the relaxation rates of all intermolecular Cu– C interactions are negligible compared to the intramolecular ones. The r6 dependence is also in this context advantageous. The specification of the experimental error in the Cu–C distances would have been more precise if there were an X-ray structure available. We chose Cu-cyclam coordinating isothiocyanate[25] as reference (Table 3) because it is also five-coordinated and adopts the same square-pyramidal symmetry as found for Cu-cyclam coordinating Cl in DFT calculations.[15] The Cu–C distances of Cu-cyclam coordinating (ClO4)2,[26] CuCl4,[27] or Br2[28] deviate maximally by 0.15 from the tabulated X-ray distances, but in most cases not more than by 0.02 . Our DFT calculations on Cu-cyclam(Cl) and those shown in ref. [15] differ from these X-ray values by maximal 0.08 . For the determination of the electronic relaxation times we applied the simplest approach, which in other cases may not be appropriate. T1e may not be approximated by T2e, and one might have to discriminate the dependence of R2 on T1e and T2e in Equation (3) (see Eq. (3.17) in ref. [18]). The electronic relaxation time T1e could be determined by power saturation techniques or, if a pulsed EPR spectrometer is available, a saturation recovery experiment. On the other hand, there are arguments that T2e alone represents the correct correlation time tc for the ChemPhysChem 2013, 14, 1864 – 1870
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fluctuation of the dipolar interaction.[22] Wickramasinghe et al.[29] compared the electronic correlation time determined from nuclear relaxation with the coupling between electronic spins of the metal centers of different molecular units. Furthermore, the EPR line width can contain other contributions than the actual relaxation T2e . Inhomogeneous broadening and experimental artifacts as saturation would lead to an underestimation of T2e . The procedure for the determination of the Cu–C distances implies several approximations: the model uses the pointdipole approximation, averaging the positions of the delocalized electrons to the metal nucleus. The electronic relaxation is measured at a lower magnetic field typical for EPR experiments. The electronic relaxation time was measured by the EPR linewidth, and, as outlined above, it was assumed that transverse and longitudinal electronic relaxations are similar. Furthermore, relaxation by Fermi contact interaction which could become significant for high-spin densities was neglected (its correlation time is not necessarily the same as that for the dipolar relaxation). The good precision of the distances determined by R2 relaxation rates might appear surprising in this regard; moreover, as a similar approach using longitudinal (R1) relaxation was not successful. The determination of the Cu–C distances by means of transverse relaxation rates R2 rather than longitudinal relaxation rates R1 might appear inconvenient. For other related purposes, such as so-called paramagnetic relaxation enhancement (PRE) experiments typically applied to biological macromolecules,[1, 3, 4] an R1 experiment is used. It is technically easier and normally less prone to artifacts. These experiments however usually measure longer distances by small rate increases which are usually interpreted in a semiquantitative way yielding relative distance information. For shorter distances, the longitudinal relaxation should as the transverse one be dominated by electron–nuclear dipolar couplings, and the R1 rate depends in a similar way on the distance [Eq. (6)]:[18, 20] 2 m0 2 2 2 2 1 3tc 7tc R1 ¼ mB g g Se ðSe þ 1Þ 6 þ 15 4p r 1 þ w2n t2c 1 þ w2e t2c ð6Þ We determined these rates for the four 13C signals by means of saturation recovery and inversion recovery experiments. However, they are in the average about one order of magnitudes too low compared to the theory. The fact that the experimental rates are too low (rather than too high) excludes the possibility that the discrepancy is due to additional relaxation contributions such as inter-nuclear dipolar or Fermi contact interaction. Interestingly, the rates appear to be consistent with the rates found by Wickramasinghe et al.[30] on Cu(DL-Ala)2, a comparable system. The authors of this study did not directly determine the distances by Equation (6), but calibrated them with one single distance determined by X-ray diffraction. A reason for the unexpected rates might lie in using the pointdipole approximation[20] rather than integrating the inverse sixth power of the nuclear distances to the unpaired electron over space, weighted by the spin densities.[31] But even with this procedure one would expect that the corrected calcula 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
tions predict higher rather than lower rates. The low rates rather indicate an ambiguity in the correlation times of the fluctuation of the hyperfine interaction that induces a certain relaxation.
4. Conclusions We have shown how to reach a nearly complete assignment of 1 H signals of a solid paramagnetic metalorganic system by means of 1H–13C dipolar INEPT and a 1H–1H correlation experiments. Furthermore, the general feasibility of a dipolar HSQC experiment was demonstrated. The bottleneck for NMR analysis of paramagnetic systems is the need for DFT calculations for the prior assignment of the 13C signals. The metal–carbon distances could be measured with a precision of 0.1 by means of transverse relaxation (R2) experiments. It remains to be understood why the more common R1 experiments give rates that are by far too low compared to the theory.
Experimental Section Sample Preparation The copper-cyclam complex chloro-1,4,8,11-tetraazacyclotetradecane copper (II) was prepared as previously described.[15] The complexation of cyclam by copper was achieved in an ethanolic solution containing CuCl2. Heating the solution, filtering, and ethanol washing of the final reaction purified the product from excess of copper and ensured the metal location inside the cyclam rings. The fraction of cyclam molecules that did not chelate a copper ion was estimated by means of a one pulse 1H NMR spectrum with a long interscan delay to 5–10 % (data not shown). A room-temperature powder X-ray diffractogram was acquired; its sharp signals are characteristic of a well-crystallized phase.
Solid-State NMR Spectroscopy Solid-state NMR spectra were acquired on a Bruker Avance III 300 MHz spectrometer equipped with a double-resonance 2.5 mm VTN probe. All experiments were performed under MAS with a frequency of 30 kHz at room temperature. The sample temperature was 58 8C due to friction. The spectra were referenced to tetramethylsilane (TMS). Both 1H and 13C one-dimensional (1D) spectra were acquired by means of a Hahn echo experiment with hard pulses with nutation frequencies of 136 kHz for 1H and 143 kHz for 13 C (excitation pulse p90 = 1.84 ms for 1H and 1.75 ms for 13C). For all experiments an interscan delay between 40 and 55 ms was set. No decoupling was applied during the indirect evolution or during the acquisition period of any 1D or 2D experiment. 13C transverse relaxation rates (R2) were measured by means of the CPMG[24] experiment. For 2D 1H–13C correlation, a variation of dipolar INEPT or TEDOR[12, 32, 33] was applied with a mixing time of two rotor periods (66.7 ms). Pulse sequences for dipolar HSQC and 1H–1H correlation are described in Section 2. For 1D 1H and 13C spectra 128 K and 1024 K scans were accumulated, respectively, for the 13C relaxation spectra 128 K scans, for the dipolar INEPT 64 complex data points with 32 K scans each, incremented by 16.67 ms, for the dipolar HSQC 16 complex data points with 16 K scans, incremented by 33.33 ms, and for the 1H-1H correlation experiment 148 increments with 512 scans, incremented by 8.33 ms. For all 2D experiments, the States-TPPI quadrature detection scheme was applied. ChemPhysChem 2013, 14, 1864 – 1870
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CHEMPHYSCHEM ARTICLES EPR Spectroscopy Continuous-wave (CW)-EPR experiments were performed on a Bruker EMX spectrometer working with X-band (9.5 GHz). The spectra were recorded at room temperature by using low microwave power (20 mW), a modulation field of 5 G, and 100 kHz as a standard modulation frequency. The spectral parameters (gtensor, line width) were determined with the Bruker software SimFonia.
DFT Calculations The calculations of Fermi contact shifts and diamagnetic chemical shifts of Cu-cyclam monochloride were performed with DFT using the generalized gradient approximation (GGA) applying the hybrid potential implemented in the program Gaussian 09.[34] Two Opteron 2.3 GHz processors with 16 GB RAM of a Beowulf-type parallel computing facility were used. An initial X-ray structure of the Cucyclam complex was obtained from ref. [25]. The structure was adapted (SCN was replaced by Cl) manually by means of the program GaussView 5.0. A full geometry optimization of this structure was performed with the unrestricted Hartree–Fock method (UHF) at the B3LYP[35] level of theory with the 6–311G basis set. The self-consistent field (SCF) convergence was equal to 106 Hartree or up to 64 iterations. The vibration frequencies showed no imaginary modes. Upon calculations in the UHF approximation, Gaussian 09 generates a list of unpaired electron spin densities and Fermi contact couplings at the positions of all nuclei of NMR interest, out of which the Fermi contact shifts were determined. The optimized structure of Cu-cyclam was furthermore used for the calculation of the (diamagnetic) chemical shifts with the 6–311 + G(2d,p) basis set. These shifts were referenced to TMS (calculated with the same theory and basis set) and added to the hyperfine shifts.
Acknowledgements The authors acknowledge the Region Pays de la Loire for financial support (Convention No. 2007–11860). We would like to thank Prof. Ahmad Mehdi for the Cu-cyclam sample, Dr. Uday Ravella for the X-ray crystallography test, and Ms. Rene Duquette, Dr. Morwenna Pearson-Long, and Dr. Cindie Kehlet for critically revising the manuscript. Keywords: density functional calculations · nuclear magnetic resonance spectroscopy · hyperfine coupling · metalorganics · solid state [1] a) P. A. Kosen, Methods Enzymol. 1989, 177, 86 – 121; b) L. Banci, C. Luchinat, Inorg. Chim. Acta 1998, 275, 373 – 379. [2] L. Banci, I. Bertini, K. L. Bren, M. A. Cremonini, H. B. Gray, C. Luchinat, P. Turano, J. Biol. Inorg. Chem. 1996, 1, 117 – 126. [3] a) M. Respondek, T. Madl, C. Gçbl, R. Golser, K. Zangger, J. Am. Chem. Soc. 2007, 129, 5228 – 5234; b) J. Dittmer, L. Thøgersen, J. Underhaug, K. Bertelsen, T. Vosegaard, J. M. Pedersen, B. Schiøtt, E. Tajkhorshid, T. Skrydstrup, N. C. Nielsen, J. Phys. Chem. B 2009, 113, 6928 – 6937. [4] R. Linser, U. Fink, B. Reif, J. Am. Chem. Soc. 2009, 131, 13703 – 13708. [5] a) N. Bloembergen, W. C. Dickinson, Phys. Rev. 1950, 79, 179 – 180; b) H. M. McConnell, R. E. Robertson, J. Chem. Phys. 1958, 29, 1361 – 1365; c) A. Nayeem, J. P. Yesinowski, J. Chem. Phys. 1988, 89, 4600 – 4608. [6] M. Liang, M. Weishupl, J. A. Parkinson, S. Parsons, P. A. McGregor, P. J. Sadler, Chem. Eur. J. 2003, 9, 4709 – 4717.
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