Communication
Remarkably Large Geometry Dependence of 57Fe NMR Chemical Shifts†
Michael Bühl Dr.,
Frank T. Mauschick Dipl.-Chem.,
Frank Terstegen Dr.,
Bernd Wrackmeyer Prof. Dr.,
Michael Bühl Dr.
Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany) Fax: (+49) 208-306-2996
Search for more papers by this authorFrank T. Mauschick Dipl.-Chem.
Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany) Fax: (+49) 208-306-2996
Search for more papers by this authorFrank Terstegen Dr.
Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany) Fax: (+49) 208-306-2996
Search for more papers by this authorBernd Wrackmeyer Prof. Dr.
Universität Bayreuth Universitätsstrasse 30, 95440 Bayreuth (Germany)
Search for more papers by this authorMichael Bühl Dr.,
Frank T. Mauschick Dipl.-Chem.,
Frank Terstegen Dr.,
Bernd Wrackmeyer Prof. Dr.,
Michael Bühl Dr.
Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany) Fax: (+49) 208-306-2996
Search for more papers by this authorFrank T. Mauschick Dipl.-Chem.
Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany) Fax: (+49) 208-306-2996
Search for more papers by this authorFrank Terstegen Dr.
Max-Planck-Institut für Kohlenforschung Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany) Fax: (+49) 208-306-2996
Search for more papers by this authorBernd Wrackmeyer Prof. Dr.
Universität Bayreuth Universitätsstrasse 30, 95440 Bayreuth (Germany)
Search for more papers by this authorFirst published: 01 July 2002
Citations: 46
†
This work was supported by the Deutsche Forschungsgemeinschaft. M.B. thanks Prof. W. Thiel for his continuing support. Computations were performed on Compaq XP1000 and ES40 workstations at the MPI Mülheim.
Abstract
What's new about complexes 1 and 2, textbook examples of coordination compounds? Quantum-chemical simulations reveal an exceptionally strong sensitivity of their 57Fe NMR spectroscopy chemical shifts to the FeC bond length, which, in turn, changes noticeably on going from the gas phase to aqueous solution.
References
- 1 For instance:
- 1a B. Wrackmeyer, A. Ayazi, H. E. Maisel, M. Herberhold, J. Organomet. Chem. 2001, 630, 263–265;
- 1b C. Janiak, T. Dorn, H. Paulsen, B. Wrackmeyer, Z. Anorg. Allg. Chem. 2001, 627, 1663–1668;
- 1c B. Wrackmeyer, O. L. Tok, M. Herberhold, Organometallics 2001, 20, 5774–5776.
- 2 For instance: N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, 2nd ed., Butterworth-Heinemann, Oxford, 1997.
- 3
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M. Bühl, M. Parrinello, Chem. Eur. J. 2001, 7, 4487–4494.
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- 5 M. Bühl, F. T. Mauschick, unpublished results.
- 6 The Fe complexes were calculated at the RI-BP86/AE1 level, that is, using the gradient-corrected functionals of Becke (A. D. Becke, Phys. Rev. A 1988, 38, 3098–3100) and Perdew ( J. P. Perdew, Phys. Rev. B 1986, 33, 8822–8824; J. P. Perdew, Phys. Rev. B 1986, 34, 7406), the extended Wachters basis on Fe ( A. J. H. Wachters, J. Chem. Phys. 1970, 52, 1033–1036; P. J. Hay, J. Chem. Phys. 1977, 66, 4377–4384), 6–31G* basis on the ligands, and making use of the resolution-of-identity (RI) approximation as implemented in the TURBOMOLE program ( R. Ahlrichs, M. Bär, M. Häser, H. Korn, M. Kölmel, Chem. Phys. Lett. 1989, 154, 165–169), together with a medium-sized grid (grid 3) and suitable auxiliary basis sets ( K. Eichkorn, O. Treutler, H. Öhm, M. Häser, R. Ahlrichs, Chem. Phys. Lett. 1995, 240, 283; K. Eichkorn, F. Weigend, O. Treutler, R. Ahlrichs, Theor. Chem. Acc. 1997, 97, 119–124). Water was described by the CHARMm force field of the MSI-CHARMm 25b2 program ( QUANTA98, Molecular Simulations, Inc., 9685 Scranton Rd. San Diego, CA 92121. The published results were generated by using the program CHARMm. This program is distributed by Molecular Simulations Inc.). For the coupling between QM and MM parts, a polarized embedding scheme was used (corresponding to model B in: D. Bakowies, W. Thiel, J. Phys. Chem. 1996, 100, 10 580–10 594). MD simulations were performed by using the ChemShell program ( P. Sherwood, A. H. deVries, ChemShell - A Shell for Computational Chemistry, CCLRC Daresbury Laboratory, 1999, see http://www.dl.ac.uk) for NVT ensembles at approximately 300 K. For the aqueous species, the complexes were placed at the center of a spherical water cluster with a diameter of 30 Å, containing a total of 449 water molecules. After a short minimization (100 steps), the outmost layer to a depth of 5 Å from the surface was completely frozen to avoid evaporation of the minidroplet in the subsequent MD run. The aqueous species were simulated for 4 ps with a time step of 1 fs. Data sampling was started after the first 3 ps, which were taken for equilibration. In addition, the OH distances in the water monomers were frozen with the SHAKE algorithm. Isolated complexes were simulated (after 0.5 ps of equilibration) for 1 ps with a time step of 0.5 fs.
- 7 See for instance: M. Orozco, F. J. Luque, Chem. Rev. 2000, 100, 4187–4225.
- 8 Magnetic shieldings were computed for equilibrium geometries and for snapshots taken from the MD simulations at the GIAO-B3LYP level (see ref. [3a] for details), by employing a medium-sized grid and DZ basis, that is, the augmented Wachters basis on Fe and DZ basis (W. Kutzelnigg, U. Fleischer, M. Schindler in NMR Basic Principles and Progress, Vol. 23, Springer, Berlin, 1990, pp. 165–262) on all other atoms. The use of the smaller DZ basis, rather than the larger basis II (that has been employed, for instance, in ref. [3a]) resulted in relatively small changes of the computed δ values, for instance by Δδ=40 ppm for 2, but makes the large number of NMR computations for the snapshots more tractable. In the chemical-shift calculations, the 20 nearest solvent water molecules were included explicitly. Representative snapshots were taken every 20 fs. Chemical shifts are reported relative to [Fe(CO)5] (3), optimized or simulated at the same respective level (σe and σ300 K values −3036.6 and −3282.3, respectively, see Figure 1). Neat 3 is the accepted standard in 57Fe NMR spectroscopy. We have only performed a simulation for the gas phase, not for the liquid, which would be a formidable task in itself, but have used the averaged σ300 K value from the gas-phase simulation as a reference value also for the solution studies. It should be kept in mind that this procedure could introduce a systematic error for the computed gas-to-liquid shifts, and that more attention should be paid to the trends in the δ values between 1 and 2, rather than to their actual values.
- 9 It should be noted that this value only represents the “classical” thermal effect and is not averaged over the zero-point motion. More than qualitative agreement with experiment is not to be expected anyway, due to the rather limited basis sets employed in the computations.
- 10 Obtained by performing NMR computations for a total of four static structures with elongated or compressed FeC distances (in steps of 0.02 Å) and a linear fit of the resulting σ values.
- 11 A similar value, 35 200 ppm Å−1, is obtained when the larger basis II is used for the ligands.
- 12 C. J. Jameson, D. Rehder, M. Hoch, J. Am. Chem. Soc. 1987, 109, 2589–2594; this empirical value has been qualitatively reproduced by using theoretical methods similar to those employed in the present study: N. Godbout, E. Oldfield, J. Am. Chem. Soc. 1997, 119, 8065–8069.
- 13 In typical hydrogen-bonded systems containing OH⋅⋅⋅O moieties, the O⋅⋅⋅O distance is smaller than 3.5 Å and the OH bond is directed towards the second oxygen atom such that the OH⋅⋅⋅O angle is larger than 140° (see for instance: E. Schwegler, G. Galli, F. Gygi, Phys. Rev. Lett. 2000, 84, 2429–2432); the same criteria have been applied to the OH⋅⋅⋅X moieties in 1 and 2.
- 14 Counterions, which have not been considered explicitly, are likely to affect the fine structure of the solvation shell, but would probably not change the qualitative picture.
- 15 M. Bühl, T. Steinke, P. v. R. Schleyer, R. Boese, Angew. Chem. 1991, 103, 1179–1180; Angew. Chem. Int. Ed. Engl. 1991, 30, 1160–1161.
- 16 Simple optimization of 2 in a polarizable continuum, as has been done for BH3NH3 in ref. [15], also leads to FeC bond contraction, but gives much too short FeC bond lengths, below 1.90 Å.
- 17 For similar pictorial rationalizations of paramagnetic contributions see, for instance: Y. Ruiz-Morales, T. Ziegler, J. Phys. Chem. A 1998, 102, 3970–3976.
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