Taking Solution Proton NMR to Its Extreme: Prediction and Detection

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Taking Solution Proton NMR to Its Extreme: Prediction and Detection of a Hydride Resonance in an Intermediate-Spin Iron Complex Jonas C. Ott, Hubert Wadepohl, Markus Enders, and Lutz H. Gade J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11330 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on November 29, 2018

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Journal of the American Chemical Society

Taking Solution Proton NMR to Its Extreme: Prediction and Detection of a Hydride Resonance in an Intermediate-Spin Iron Complex Jonas C. Ott, Hubert Wadepohl, Markus Enders,* and Lutz H. Gade* Anorganisch-Chemisches Institut, Universität Heidelberg, Im Neuenheimer Feld 276, 69120 Heidelberg Supporting Information Placeholder ABSTRACT: Guided by DFT based modeling the

chemical shift range of a hydride resonance in the proton nuclear magnetic resonance (NMR) spectrum of the intermediate-spin, square planar iron complex tBu(PNP)Fe-H was predicted and detected as a broad resonance at 3560 ppm (295 K) with a temperature dependent shift of approx. 2000 ppm between 223 K and 383 K. The first detection of a metal-bonded hydrogen atom by solution NMR in a complex with a paramagnetic ground state illustrates the interplay of theory and experiment for the characterization of key components in paramagnetic base metal catalysis.

While the mechanisms of the catalytic hydrogenations involving the closed shell heavier d metal complexes such as rhodium were established in the 1960s - 80s using NMR spectroscopy as key analytical method,1 identification of the corresponding reaction intermediates for the 3d metals has been hindered by a wide-spread paramagnetism.2 Paramagnetic 3d metal hydrides recently have attracted increasing attention due to their high reactivity and ubiquity in reduction catalysis, albeit without direct detectability by NMR in solution.3–5 While paramagnetic NMR spectroscopy is a wellestablished method, complete characterization and full signal assignment of the spectra of open shell paramagnetic compounds remains challenging.2,6 In particular, systems with S > ½ render a correct theoretical treatment challenging, as chemical shifts are determined by many additional contributions compared to simple radicals.7,8 The NMR shift (δexp) in paramagnetic compounds may be adequately understood and modeled by considering three major contributions: the diamagnetic orbital shift (orbs), the Fermi-contact shift (fcs) and the pseudocontact shift (pcs):9–13 (1)

The temperature dependent fcs is a measure for the spindensity of unpaired spin (ραβ) located at the nucleus of interest and can be readily calculated with the aid of quantum chemistry.7 In contrast, pcs contribution arises from dipolar interactions between the magnetic dipoles of the measured nucleus and the unpaired electrons. In a system, where the magnetic susceptibility is axially anisotropic, pcs is described in the point-dipole approximation by Eq. 2: (2) where Δax is the axial component of the diagonalized magnetic susceptibility tensor, r is the length of the vector connecting the nucleus of interest and the unpaired electron (usually located at the metal ion), and θ is the angle between the r vector and the magnetic field axis.6a The effect of the unpaired electron spin on the nuclear spin relaxation leads to a significant signal broadening in the NMR spectrum,7,14,15 a phenomenon which depends on the spatial distance of the nucleus to the paramagnetic center as well as the nature of the paramagnetic center itself.6d Although iron(II) usually gives rise to well resolved paramagnetic NMR spectra, signal broadening often leads to no signal being observed at all for nuclei in close proximity,2,16–20 in particular, nuclei of atoms directly bonded to a paramagnetic metal center.21

Figure 1. Iron(II) hydrido complexes bearing a carbazole based PNP pincer ligand.

We recently reported a dimeric ferrous complex with two bridging hydrides stabilized by a carbazole based PNP pincer ligand (Figure 1).22 Substitution of the

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phosphorus-bound isopropyl groups by tert-butyl substituents23 now gave access to a monomeric, squareplanar, intermediate-spin, ferrous hydride for which we report the first case of a directly detectable metal-bonded hydride with a paramagnetic ground state by solution 1H NMR spectroscopy. The ferrous hydrido complex 3 was synthesized by a similar route to that reported for related complexes in the literature (Scheme 1).22 Treatment of chlorido complex 123 with alkylating agents gave the high-spin24 ferrous alkyl complexes tBu(PNP)FeR [R = CH2SiMe3 (2a), R = CH2Ph (2b)] for which X-ray diffraction studies revealed distorted tetrahedral coordination spheres (see the Supporting Information). Scheme 1. Access to the terminal hydrido complex 3 by two synthetic routes.

Under an atmosphere of 10 bar of H2, 2a and 2b were readily converted to hydrido complex 3 at ambient temperature, as did the addition of sodium triethylborohydride to a solution of 1 in toluene. For complex 3 a monomeric structure was established by single crystal X-ray diffraction (Figure 2), with a distorted square-planar coordination sphere (τ4’ (3) = 0.15).25 The hydrido ligands of the four crystallographically independent molecules were located and fully refined [d(Fe–H) 1.45(3) - 1.53(3) Å, see the Supporting Information]. All lie within the range of previously reported terminal iron-hydrogen bonds.20,26

NMR measurements of a toluene-d8 solution of 3 at 295 K (Evans method) revealed a magnetic susceptibility of 2.8 μB, indicating an intermediate spin ground state (μSO,S=1 = 2.83 μB). Complex 3 gives rise to a well resolved 13C NMR spectrum with eleven paramagnetically shifted resonances between 250 and 1000 ppm, a signal pattern which implies a molecule with an effective C2v symmetry in solution. We attribute this higher symmetry to a rapid interconversion of the two tilted carbazole backbone orientations on the NMR timescale and the rearrangement that entails. Density functional theory (DFT) calculations were employed for a full assignment of the observed resonances. The spin densities ραβ were evaluated at the B3LYP27 level of theory, with the def2-TZVP28 basis set for the iron atom only and the 6-311G(d,p)29 basis set for all other atoms and then employed to calculate the Fermi-contact shifts. The respective orbital shift was calculated with the Gauge-Independent Atomic Orbital (GIAO)30 method. In the 13C NMR spectrum, the sum of the calculated orbs and fcs correlates well with the experimentally obtained values and allows an unambiguous assignment of all resonances without the inclusion of a pcs (see the Supporting Information, Figure S4). In addition, a 13C1H HETCOR NMR experiment of 3 confirmed the signal assignment of the 13C and 1H NMR spectra (vide infra).

Figure 3. Correlation between the experimental (295 K, 600.13 MHz, toluene-d8) and calculated [B3LYP/6-311(d,p) + def2-TZVP(Fe)] 1H NMR resonances of complex 3, considering orbs and fcs only (red squares) as well as orbs, fcs and pcs (green dots). The green dashed line represents the best linear fit (slope m = 1.06; offset b = 2.11 ppm; R2 = 0.99987). Solvent resonances are indicated by asterisks.

Figure 2. Molecular structure of 3 (displacement ellipsoids drawn at 50 % probability). Carbon-bound hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg] for one of the four independent molecules in the asymmetric unit: Fe1–H 1.48(3), Fe1–P1 2.2263(8), Fe1–P2 2.2351(8), Fe1–N1 1.995(2); N1–Fe1–H 169.7(12), P1–Fe1–P2 167.81(3), N1–Fe1–P1 95.66(7), N1–Fe1–P2 96.29(7).

The 1H NMR spectrum of 3 displayed five resonances between 25 and 70 ppm and therefore represented the same effective C2v symmetry. However, the assignment of the resonances proved to be more difficult: In general, the sum of orbs and fcs did not correlate well with the experimental values (Figure 3, red squares).


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-2500

363 K 343 K

-3000

323 K

δ para [ppm]

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303 K 283 K 263 K 243 K 223 K

-3500 -4000 -4500 -5000

-3000

-3500

-4000

-4500

ppm

0.003

0.004

1/T [K-1]

Figure 4. (left) 1H NMR spectrum (399.89 MHz, toluene-d8) of the hydride resonance of complex 3 at various temperatures. (right) Curie plot of the hydride resonance of complex 3. The dashed line represents the best linear fit (slope = 1047261 ppmK; offset = 16.62 ppm; R2 = 0.9996). Table 1. Calculated paramagnetic 1H NMR shifts of complex 3 in comparison with the experimental data. DFT(B3LYP) δorba

δfc

δpc,pdb

δpc,intc

Exp δcalcd

δexpe

HCarb4/5

9.0

1.6

12.4

14.2

24.8

21.1

HCarb2/7

7.5

1.6

1.0

0.9

10.0

7.8

C(CH3)3

1.6

0.5

2.6

2.8

4.9

4.0

PC(CH3)3

0.7

4.0

9.4

–10.6

5.9

7.0

CH2

2.4

48.2

18.6

–22.4

68.2

68.3

Fe–H



5026.6

970.5



3772.1

3560

aδ orb = σTMS – σcomplex. The data is reported with respect to the calculated isotropic shielding constant σ(1H) of 32.03 ppm for tetramethylsilane (TMS) by the GIAO method. bDetermined by point-dipole approximation. cObtained from the fitted effective  tensor and the spin density using the software SPINACH.31 dδ e calc = δorb  δfc  δpc,int. 295 K, 600.13 MHz, toluene-d8.

With a magnetic axis that incorporates the iron-nitrogen bond, the effective axial magnetic anisotropy (Δax) to be used in the calculation of the pcs (Eq. 2) of complex 3 was determined to be 7.22 × 1032 m3 by the pointdipole approximation. This anisotropy is within the range of that reported for iron(II) compounds.32,33 However, as this simplified model is less consistent for nuclei in close proximity to the paramagnetic center, integration over the spin density was used to evaluate the data.31a Addition of pcs contributions to all 1H NMR signals led to a significantly better agreement with the experiment (Figure 3, green circles & Table 1). The comparably sharp resonances as well as the excellent correlation of the modeled and the experimentally observed chemical shifts encouraged us to systematically look for a potential hydride resonance. Calculation of the total chemical shift predicted the corresponding resonance at around  ppm at 295 K (Table 1). Notably, a proton spectrum in the respective region revealed a broad resonance at 3560 ppm at 295 K that shifted by approx. 2000 ppm over the temperature range from 223 K to 383 K (Figure 4, left).

Given the temperature dependence of the paramagnetic shift, an increase of the temperature should lead to a limiting experimental shift, which is expected to be close to the orbs of the molecule. Extrapolation of the corresponding Curie plot (Figure 4, right) revealed an offset of only 16 ppm, which is in remarkably good agreement with the calculated orbs taking the large shift range into account. Therefore, we assign this resonance to the iron-coordinated hydrido ligand of complex 3. Interestingly, the calculated orbital shift of 59.4 ppm is at remarkably low field, and although such low-field shifts have been observed for some main group metal hydrides,34 diamagnetic iron hydrides are usually detected in the range from 0 to 20 ppm.35 We attribute this unusual orbital shift to an unquenched orbital contribution due to the open d shell of the iron atom.35,36 The negative nature of the fcs can be explained by the π symmetry of the singly occupied molecular orbital (SOMO) with respect to the iron-hydrogen bond, leading to spin polarization and hence results in negative spin density at the hydrido ligand. Although the absolute deviation from the calculated chemical shift is about 210 ppm, the simplified model represented in Eq. 1 correctly predicted the chemical shift region of the resonance and has allowed for the first time the detection and assignment of a hydride resonance in a complex with a paramagnetic ground state. The full width at half maximum (FWHM) of the resonance was found to be 16.8 kHz at 295 K which corresponds to a spin-spin relaxation T2* of 0.02 ms. We were unable to determine the spin-lattice relaxation T1 for this specific nucleus as it was too fast for the detection timescale. DFT modeling of the electronic structure of 3 revealed a spin isomer with a high-spin configuration, which is only 8.41 kcal/mol higher in energy than the intermediate spin ground state. The optimized geometry of the high spin isomer converged to a tetrahedrally distorted structure, indicated by the N–Fe–H angle of 136.4° (173.5° for the optimized intermediate spin



ground state). The plot of the relative energies as a function of the nitrogen-iron-hydrogen bond angle (Figure 5) revealed an intersection of both spin states close to the minimum of the high-spin function. The proximity of a spin-crossover could provide a potential channel for fast relaxation of the unpaired electrons, hence leading to the relatively slow nuclear relaxation, which is the precondition for the observation of the hydride resonance in 3.

Text, figures, and tables giving full experimental procedures, representative NMR spectra, DFT data, and crystallographic data.

AUTHOR INFORMATION Corresponding Author *(L.H.G.) Fax: (+49) 6221–545609. E-Mail: [email protected]. (M.E.) Fax: (+49) 6221–54161–6247. E-Mail: [email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We are grateful to Marko Damjanović for computational advice and support. We thank Anna Ditter for experimental support and the University of Heidelberg for funding. The authors acknowledge support by the state of Baden-Württemberg through bwHPC and the German Research Foundation through Grant INST 40/467-1 FUGG.

REFERENCES Figure 5. Relative energies [B3LYP/6-311(d,p) + TZVP (only for iron)] of the different spin configurations of 3 upon variation of the nitrogen-iron-hydrogen angle. The equilibrium geometries of the spin isomers are starred. A third S = 0 low spin isomer was found with a relative energy of 29.11 kcal/mol (see the Supporting Information).

The deuterated isotopologue tBu(PNP)FeD (3-D) was prepared in the same way as 3 from 2b under an atmosphere of 10 bar D2 gas. The 1H NMR spectrum of 3-D displayed the same resonances for the PNP ligand, with small differences in the chemical shifts which are attributed to a paramagnetic isotope effect on chemical shift (PIECS).37,38 Furthermore, the 2H NMR spectrum of 3-D revealed a deuterium resonance at 3504 ppm at 295 K with the same temperature dependent shift as the hydride resonance discussed above. We attribute this shift difference of 56 ppm compared to the corresponding hydride to a large PIECS as a result of the close proximity to the paramagnetic center. The observed resonance exhibits a FWHM of 0.5 kHz at 295 K, corresponding to a T2* relaxation of 0.62 ms, which reflects the smaller gyromagnetic ratio of the 2H nucleus compared with 1H. In conclusion, we provide a first example metal hydride with a paramagnetic ground state for which the hydrido ligand is directly detectable via solution 1H NMR spectroscopy. To the best of our knowledge, the observed hydride resonance of complex 3 represents by far the most shifted proton signal recorded to date, highlighting the challenge in targeting the appropriate chemical shift range when aiming to detect such nuclei. Moreover, this study illustrates the crucial interplay of theory and experiment for the characterization of such species in solution. ASSOCIATED CONTENT Supporting Information

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Taking Solution Proton NMR to Its Extreme: Prediction and Detection

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