Intramolecular Rotation through Proton Transfer: [Fe(η5-C5H4CO2−)2] versus [(η5-C5H4CO2−)Fe(η5-C5H4CO2H

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  Intramolecular Rotation through Proton Transfer: [Fe(η5-C5H4CO2−)2] versus [(η5-C5H4CO2−)Fe(η5-C5H4CO2H
  Ferrocenes DOI: 10.1002/ange.200501564 Intramolecular Rotation throughProton Transfer: [Fe( h 5 -C 5 H 4 CO 2  ) 2 ] versus[( h 5 -C 5 H 4 CO 2  )Fe( h 5 -C 5 H 4 CO 2 H)]**  Xue-Bin Wang, Bing Dai, Hin-Koon Woo, andLai-Sheng Wang* Intramolecular rotation is important in molecular dynamics;it is a fundamental molecular property and can be used tounderstand the concept of molecular motors. Inspired byelegant biological and macroscopic analogues, [1–3] the fabri-cation of molecular machines has beenvery actively pursued recently, [4] resultingin a variety of molecular devices, such asrotors, [5–8] shuttles, [9–11] and ratchets. [12] The central theme of molecular devicesis to control molecular motions and bind-ings upon external stimuli. The smallestmolecular machine can downscale to asingle molecule, and such a single molec-ular rotor has been observed. [13] Herein,we report the observation of intramolec-ular rotations of a single ferrocene derivative through protontransfer.Ferrocene is a prototypical sandwich complex with an ironatom symmetrically situated between two C 5 H 5  (cyclopenta-dienyl, Cp) rings and has been the subject of many exper-imental and theoretical studies since its discovery. [14–19] It hasmany applications in both fundamental research and materi-als science. In the solid phase the two Cp rings are in astaggered confirmation ( D 5 d ), [14] whereas in the gas phase theyare eclipsed ( D 5 h ). [20] The rotational barrier of the two ringsalong the  C  5  axis is very small in the gas phase ( E   1.1 kcalmol  1 ). [20] Therefore, ferrocene would be an ideal candidatefor a molecular rotor if the rotational freedom can becontrolled. Proton transfer, that is, acid–base chemicalreaction, is one of the most commonly used external stimulialong with photoinduced processes and electrochemicalreactions in realizing molecular devices. [4,21] Proton transferis also ubiquitous and plays a vital role in biological motors. [22] It changes the charge state of a molecule to result inalternation of electrostatic interactions. Such effects mayinfluence molecular binding, particularly in the gas phase orhydrophobic environments. Here, we study the energetics andconformations of two ferrocene derivatives, the doublycharged [Fe( h 5 -C 5 H 4 CO 2  ) 2 ] ([FeCp ’ 2 ],  1 ) and the singlycharged [( h 5 -C 5 H 4 CO 2  )Fe( h 5 -C 5 H 4 CO 2 H)] ( 2 ; seeScheme 1), and demonstrate that they can be viewed toform a model molecular rotor system controlled by protontransfer.Complexes  1  and  2  were produced using electrosprayionization, and their geometric and electronic structures wereprobed by photoelectron spectroscopy (PES) and theoreticalcalculations. The experiments were carried out on a newlydeveloped low-temperature PES apparatus coupled with anelectrospray source. [23] We detected abundant  1  and  2  byelectrospray of a 1 m m  solution of 1,1 ’ -ferrocenedicarboxylicacid in a water/methanol mixed solvent system. The 193-nm(6.424 eV) PES spectrum of   1  (Figure 1a) reveals two broadspectral bands, which likely contain many overlappingdetachment transitions. No transitions were observedbeyond binding energies of 3 eV owing to the cut-off by therepulsive coulomb barrier, unique to photodetachment of multiply charged anions. [24–26] We also measured the photo-electron spectrum of   1  at 266 nm (4.661 eV) with slightlybetter resolution. Molecular orbital analysis based on theoptimized structure (see below) indicates that the lower-binding-energy features (0.2–1 eV) are due to detachmentsfrom primarily lone-pair electrons of the carboxylate groups,whereas the higher-binding-energy features (1.5–2.5 eV) aredue to detachments from the ferrocene framework, whichcontains closely spaced molecular orbitals from both the Fe3d orbitals and the Cp ’  rings. The surprising observation is theextremely low electron-binding energy for  1 . We measured anadiabatic detachment energy (ADE) of 0.25  0.05 eV for  1 (Figure 1a). This low electron-binding energy is a result of thestrong intramolecular coulomb repulsion due to the twonegative charges in  1 . To assess the influence of the coulombrepulsion, we performed a control experiment on a mono-carboxylated ferrocene anion, [( h 5 -C 5 H 5 )Fe( h 5 -C 5 H 4 CO 2  )]( 3 , Scheme 1), as shown in Figure 1c. Three spectral bandswere observed, all with very high electron-binding energies, in Scheme 1.  Structures of the ferrocene derivatives  1 – 3 .[*] Dr. X.-B. Wang, Dr. B. Dai, Dr. H.-K. Woo, Prof. Dr. L.-S. WangDepartment of PhysicsWashington State University2710 University DriveRichland, WA 99352 (USA)andW. R. Wiley Environmental Molecular Sciences Laboratory andChemical Sciences DivisionPacific Northwest National LaboratoryP.O. Box 999, Richland, WA 99352 (USA)Fax: (   1)509-376-6066E-mail:[**] We thank Dr. Jun Li for valuable discussions and help with thetheoretical calculations. This work was supported by the U.S.National Science Foundation (CHE-0349426) and performed at theW R.WileyEnvironmental MolecularSciencesLaboratory,a nationalscientific user facility sponsored by the DOE’s Office of Biologicaland Environmental Research and located at Pacific NorthwestNational Laboratory, which is operated for DOE by Battelle. Allthe calculations were performed using supercomputers at theMolecular Sciences Computing Facility of EMSL. Zuschriften 6176   2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim  Angew. Chem.  2005 ,  117  , 6176–6178  contrast to  1 . The ADE of   3  was measured to be 3.31  0.03 eV, which is almost identical to that of acetate andother long-chain aliphatic carboxylate groups. [27] Thus, theextremely low ADE of   1  is purely due to the intramolecularcoulomb repulsion, which can be approximately estimated asthe difference in ADE between  3  and  1 ; a very large value of approximately 3 eV is obtained.Figure 1b displays the 193-nm spectrum of   2 , whichreveals spectral patterns that are similar to those for  1  butat much higher binding energies. We expected that  2  wouldhave binding energies similar to  3  because the first electrondetachment in both systems should be from the CO 2  group.However, the ADE of   2  was measured to be 3.80  0.03 eV,about 0.5 eV higher than that of   3 . This observation suggeststhat the CO 2  group in  2  is stabilized relative to that in  3 . Theonly mechanism for this stabilization is through an intra-molecular hydrogen-bonding interaction between the CO 2  group and the COOH group on the second Cp ’  ligand. This ispossible if the two Cp ’  ligands in  2  would adopt a staggeredconformation, that is, if the two Cp ’  ligands would be rotated180   relative to each other, as shown in Scheme 1.To obtain more definitive information about the struc-tures and energetics, we performed theoretical calculations onthe three species. [28] The optimized structure of   1  indeedshows  C  2 h  symmetry (Figure 2), in which the two CO 2  groupsare opposite each other in a  trans  conformation. For  2 , anintramolecular hydrogen bond is observed between the-COOH and -COO  groups, as expected, that lock the twoCp ’  ligands in a  cis  conformation. The two Cp ’  ligands are notprecisely staggered. Instead, there is a 68   angle betweenthem to optimize the intramolecular hydrogen-bond forma-tion. The  trans  conformation of   2  lies 0.62 eV higher in energyrelative to the  cis  form (also see Figure 3a). The calculatedADEs for  1 ,  2 , and  3  are compared with the experimentalvalues in Table 1. Although the theoretical values areconsistently lower than the experimental values by approx-imately 0.2 eV, they reproduce the experimental trend verywell. In particular, a significantly smaller ADE is predictedfor the  trans  confirmation of   2 . In fact, the predicted ADE forthe  trans  form of   2  is in remarkable agreement with the ADEof   3 , and confirms unequivocally that the hydrogen-bonded cis  form is more stable and is the structure that was observedexperimentally.Therefore, we see that the different electrostatic inter-actions lock  1  and  2  in two different conformations. The Figure 1.  Photoelectron spectra at 193 nm (6.426 eV) of a) the [Fe( h 5 -C 5 H 4 CO 2  ) 2 ] dianion ( 1 ), b) its protonated singly charged anion  2 , andc) the monocarboxylated ferrocene anion, [( h 5 -C 5 H 5 )Fe( h 5 -C 5 H 4 CO 2  )]( 3 ). Figure 2.  Optimized structures of the [Fe( h 5 -C 5 H 4 CO 2  ) 2 ] dianion ( 1 )and its protonated singly charged anion  2  (O dark gray, C pale gray,H white). Note that the two Cp ’  ligands in  2  are staggered by 68   . Figure 3.  Calculated energies as a function of rotational angle definedby the two carboxylate groups for a)  2  and b)  1 . The two minimalocated at 68   and  68   for  2  are identical. At these angles, the intra-molecular hydrogen bond is optimized. Deviation from these anglesdisrupts the hydrogen bond to result in a non-hydrogen-bonded formof   2 . The sharp minima in (a) reflect the narrow angles for the hydro-gen bonding, outside which the rotational potential is relatively flat.The potential for  1  is primarily coulombic and is simpler: the mini-mum at 180   corresponds to the  trans  structure, whereas the maximaat 0   and 360   correspond to the  cis  form.  Angewandte Chemie 6177  Angew. Chem.  2005 ,  117  , 6176–6178  2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim  intramolecular coulomb repulsion dictates that the twonegative charges in  1  lie as far apart as possible, locking itin the  trans  conformation, whereas intramolecular hydrogenbonding locks  2  in the  cis  conformation. Deviation from theoptimal angle of 68   disrupts the hydrogen bond and results innon-hydrogen-bonded forms of   2 . The rotational barriers for 1  and  2  are 1.4 and 0.6 eV, respectively (see Figure 3), in thegas phase, which are much higher than thermal energies atambient temperatures. As shown in the Figure 2, structures  1 and  2  are controlled by a proton-transfer process. Protonationof   1  induces a 112   rotation that leads to  2 , while deproto-nation of   2  results in  1  by also involving a 112   intramolecularrotation. Received: May 8, 2005Revised: June 21, 2005Published online: August 12, 2005 . Keywords:  anions · cyclopentadienyl ligands · iron ·photoelectron spectroscopy · protonation [1] R. D. Vale, R. A. Milligan,  Science  2000 ,  288 , 88.[2] H. Noji, R. Yasuda, M. Yoshida, K. Kinosita,  Nature  1997 ,  386 ,299.[3] I. Rayment, H. M. Holden, M. Whittaker, C. B. Yohn, M.Lorenz, K. C. Holmes, R. A. Milligan,  Science  1993 ,  261 , 58.[4] V. Balzani, A. Credi, F. M. Raymo, J. F. Stoddart,  Angew. Chem. 2000 ,  112 , 3484–3530;  Angew. Chem. Int. Ed.  2000 ,  39 , 3348–3391.[5] T. R. Kelly, H. De Silva, R. A. Silva,  Nature  1999 ,  401 , 150.[6] N. Koumura, R. W. J. Zijlstra„ R. A. van Delden, N. Harada,B. L. Feringa,  Nature  1999 ,  401 , 152.[7] V. Bermudez, N. Capron, T. Gase, F. G. Gatti, F. Kajzar, D. A.Leigh, F. Zerbetto, S. Zhang,  Nature  2000 ,  406 , 608.[8] D. A. Leigh, J. K. Y. Wong, F. Dehez, F. Zerbetto,  Nature  2003 , 424 , 174.[9] R. A. Bissell, E. Cordova, A. E. Kaifer, J. F. Stoddart,  Nature 1994 ,  369 , 133.[10] A. S.Lane, D. A.Leigh,A.Murphy,  J. 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Chem.  1959 ,  1 , 86.[20] R. K. Bhn, A. Haaland,  J. Organomet. Chem.  1966 ,  5 , 470.[21] R. Ballardini, V. Balzani, A. Credi, M. T. Gandolfi, M. Venturi,  Acc. Chem. Res.  2001 ,  34 , 445.[22] C. A. Schalley, K. Beizai, F. Vgtle,  Acc. Chem. Res.  2001 ,  34 ,465.[23] The experiments were carried out on a new homebuilt instru-ment, which couples electrospray ionization with a magnetic-bottle time-of-flight photoelectron spectrometer and the capa-bility of controlling ion temperatures. The electrospray sourceand the magnetic-bottle photoelectron spectrometer are similarto that described previously (L. S. Wang, C. F. Ding, X. B. Wang,S. E. Barlow,  Rev. Sci. Instrum.  1999 ,  70 , 1957). Briefly, theanions of interest were produced from a solution of thecorresponding acids under slightly basic conditions in a mixedsolvent system of methanol/water (3:1 v/v). After their desolva-tion from the electrospray source, anions were guided by a RF-only (RF = radiofrequency) octopole device and transportedthrough a quadrupole mass filter (operated at RF-only mode),before they entered a 3D Paul trap. The trap was attached to acryostat, which consists of a closed-cycle helium refrigerator anda feedback heater. Temperatures of the trap can be controlledfrom 18–400 K. Ions were trapped for a period of 20–100 msand collisionally cooled by using a 10  4 Torr He background gascontaining about 10% H 2 . The cold anions were then unloadedfrom the trap and were analyzed using a time-of-flight massspectrometer. The ions of interest were selected and deceleratedbefore they were intercepted with a laser beam in the interactionzone: 266 nm from a Nd:YAG laser and 193 nm from an ArFexcimer laser. The lasers were operated at 20 Hz repetition rate,with the ion beam off at alternate laser shots for backgroundsubtraction. The photodetached electrons were collected withnearly 100% efficiency by a magnetic bottle and analyzed in a 5-m long electron-flight tube. The electron-energy resolution of the apparatus is  D E  / E   2%, that is, 20 meV for 1 eV electrons.Photoelectron time-of-flight spectra were collected and thenconverted to kinetic-energy spectra, calibrated by the knownspectra of ClO 2  and I  . The electron-binding-energy spectrapresented were obtained by subtracting the kinetic-energyspectra from the detachment photon energies.[24] M. K. Scheller, R. N. Compton, L. S. Cederbaum,  Science  1995 ,  270 , 1160.[25] L. S. Wang, X. B. Wang,  J. Phys. Chem. A  2000 ,  104 , 1978.[26] X. B. Wang, X. Yang, L. S. Wang,  Int. Rev. Phys. Chem.  2002 ,  21 ,473.[27] L. S. Wang, C. F. Ding, X. B. Wang, J. B. Nicholas,  Phys. Rev.Lett.  1998 ,  81 , 2667.[28] We optimized the structures of   1  and  2  at the density functionaltheory level using the B3LYP hybrid functional (A. D. Becke,  J.Chem.Phys.  1993 ,  98 , 1372;A. D. Becke,  J.Chem.Phys. 1993 , 98 ,5648) and the standard Ahlrichs VTZ basis set (A. Schafer, H.Horn, R. Ahlrichs,  J. Chem. Phys.  1992 ,  97  , 2571). Vibrationalfrequencies were calculated by numerical differentiation meth-ods to confirm the ground states. All the calculations wereperformed using the NWChem. 4.6 program (High PerformanceComputational Chemistry Group,  NWChem, A Computational Chemistry Package for Parallel Computers, Version 4.6 , PacificNorthwest National Laboratory, Richland, Washington 99352,USA.  2003 ) and the Molecular Science Computing Facility(MSCF) located at the Environmental Molecular SciencesLaboratory.[29] E. R. Lippincott, R. D. Nelson,  Spectrochimica Acta  1958 ,  10 ,307–329. Table 1:  Experimental adiabatic detachment energies (ADEs [eV]) of   1 ,  2 ,and  3 , compared to calculated values.Species Experimental Calculated 1  0.25  0.05   0.03 2  ( cis ) 3.80  0.03 3.64 2  ( trans ) 3.32 3  3.31  0.03 3.18 Zuschriften 6178   2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim  Angew. Chem.  2005 ,  117  , 6176–6178
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