Article pubs.acs.org/IC
Inter- versus Intramolecular Structural Manipulation of a Dichromium(II) Pacman Complex through Pressure Variation Charlotte J. Stevens,† Alessandro Prescimone,† Floriana Tuna,‡ Eric J. L. McInnes,‡ Simon Parsons,† Carole A. Morrison,† Polly L. Arnold,*,† and Jason B. Love*,† †
EaStCHEM School of Chemistry, University of Edinburgh, David Brewster Road, Edinburgh EH9 3FJ, U.K. EPSRC National EPR Facility, School of Chemistry and Photon Science Institute, University of Manchester, Oxford Road, Manchester M13 9PL, U.K.
‡
S Supporting Information *
ABSTRACT: The effect of pressure on the intranuclear M···M separation and intermolecular secondary interactions in the dinuclear chromium Pacman complex [Cr2(L)](C6H6) was evaluated because this compound contains both a short Cr···Cr separation and an exogenously bound molecule of benzene in the solid state. The electronic structure of [Cr2(L)] was determined by electron paramagnetic resonance spectroscopy, SQUID magnetometry, and density functional theory calculations and shows a diamagnetic ground state through antiferromagnetic exchange, with no evidence for a Cr−Cr bond. Analysis of the solid-state structures of [Cr2(L)](C6H6) at pressures varying from ambient to 3.0 GPa shows little deformation in the Cr···Cr separation, i.e., no Cr−Cr bond formation, but instead a significantly increased interaction between the exogenous arene and the chromium iminopyrrolide environment. It is therefore apparent from this analysis that [Cr2(L)] would be best exploited as a rigid chemical synthon, with pressure regulation being used to mediate the approach and secondary interactions of possible substrates.
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INTRODUCTION Chemical reactions and processes often involve a subtle interplay of interactions in the primary and secondary coordination spheres of metal centers.1,2 For example, biological systems such as metalloenzymes not only use the primary coordination chemistry of the metal to facilitate catalytic chemical transformations but also exploit the secondary-sphere protein environment to stabilize intermediates and direct substrates and reagents to the active site.3 Similar control of combined primary and secondary interactions has also been exploited in small-molecule models, in particular the use of hydrogen bonding to stabilize reactive intermediates in oxidation reactions,4−6 and in the development of ligands or systems that provide a hydrophobic environment through encapsulation or sterically demanding substituents.7−9 Even so, the modulation of these variables in order to control the chemical reactivity is subtle and difficult without wholesale changes in a biological protein environment through site mutagenesis or through complex ligand design strategies in small-molecule chemistry. Systems in which several energetically competitive structures are related by quite subtle changes may be sensitive to externally applied conditions such as pressure. The primary bond distances and angles of organic materials are relatively insensitive to pressures up to about 10 GPa, and the effects of pressure are most prominent in the intermolecular contacts.10 Accordingly, high pressure has been used to explore alternative crystal-packing arrangements in the organic solid state, leading to new phases of compounds such as pyridine11 and energetic materials12 or new cocrystals and solvates.13 However, less © XXXX American Chemical Society
straightforward behavior has been found for coordination compounds, where beyond changes in intermolecular interactions the primary coordination sphere of the metal cation can also be changed by pressure. The pressure sensitivity of ligand binding has been used to explore magnetostructural relationships in single-molecule magnets such as [Mn12O12(O2CCH2tBu)16(H2O)4]·CH2Cl2·MeNO2, which has a Mn3+-centered Jahn−Teller axis, and pressure switches the compound between fast and slow magnetic relaxation14 and to tuning of the crystal-field splitting and color of salicylaldoximatonickel complexes. 15 The coordination number of unsaturated metal centers can also be increased with pressure. The dinuclear Cu centers in [HGu][Cu2(OH)(citrate)(Gu)2] (Gu = guanidine) have long interatomic contacts at ambient pressure that are converted into coordination bonds at 2.9 GPa, i.e., the coordination number of Cu2+ changes from 5 to 6,16 while in cis-[PdCl2([9]aneS3)], four-coordinate Pd forms long Pd···S contacts at ambient pressure that shorten at 4.6 GPa to generate a polymeric structure with six-coordinate Pd cations.17 Furthermore, a high-pressure crystallographic and QTAIM study on low-oxidation-state uranium amido complexes has shown that ambient-pressure C−H···U interactions are agostic at 3.2 GPa.18 Recently, we reported the synthesis of a dinuclear Pacman chromium(II) complex of a Schiff-base calixpyrrole, [Cr2(L)] (Figure 1).19 The X-ray diffraction study of the structure of [Cr2(L)] as its benzene solvate revealed that the complex Received: September 18, 2015
A
DOI: 10.1021/acs.inorgchem.5b02151 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. Schiff-base calixpyrrole H4L and its dinuclear chromium(II) Pacman complex [Cr2(L)].
adopts the classic wedge-shaped Pacman geometry in the solid state, with each CrII ion occupying a pseudo-square-planar N4 donor pocket. Molecules of [Cr2(L)](C6H6) are arranged in the crystal structure in alternating chains with benzene molecules that cap the exo faces of the macrocycles with a nearest Cr···C distance of 3.608(2) Å. The structure of [Cr2(L)] is also notable in that the Cr···Cr separation of 3.1221(1) Å is the shortest M···M separation observed in any [M2(L)] complex of this ligand.20 These features led us to question the electronic structure of [Cr2(L)](C6H6), the potential presence and nature of a Cr−Cr bonding interaction, and whether the Cr···Cr separation and/or the supramolecular interaction between the Cr and exogenous molecule of benzene could be modulated by a change in the pressure. Such modifications may lead to new chemical reactivity and bonding, such as those seen in the high-pressure formation of uranium dinitrogen complexes,21 multielectron reduction chemistry by d3-metal cations,22 or stabilization of rhodium alkane complexes in the crystalline state.23 Herein, we report the results of electron paramagnetic resonance (EPR) spectroscopy and SQUID magnetometry investigations and the effect of high pressures on single crystals of [Cr2(L)](C6H6), in combination with computational analyses.
Figure 2. χT(T) (squares) and χ(T) (triangles) data for [Cr2(L)](C6H6) measured in a 0.5 T magnetic field and simulated using the model and parameters in the text.
where gi and di are the g-factor and local zero-field-splitting (ZFS) tensors of the individual spins si, J is the isotropic exchange interaction, and B is the applied magnetic field. Good agreements25 with χT(T) and χ(T) are found with J = −45 cm−1, assuming si = 2 and using a fixed isotropic gi = 1.98. The low-temperature tail in χ(T) is modeled by inclusion of a 2% S = 3/2 impurity, given that the impurity is likely to involve CrIII; a half-integer spin is consistent with low-temperature EPR and a very low but non-zero saturation magnetization at 1.8 K (ca. 0.05 μB). Q-band (34 GHz) EPR spectra of [Cr2(L)](C6H6) at 200 K have a broad feature around g = 2.0, with much sharper features across the entire lower field range (Figures 3 and S1 in the
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RESULTS AND DISCUSSION The straightforward and high-yielding (∼70%) preparation of [Cr2(L)](C6H6) afforded an opportunity to conduct a detailed study into the molecular and electronic structures of a dinuclear early-transition-metal Pacman complex. Magnetic Susceptibility and EPR Measurements. The solution magnetic moment of [Cr2(L)](C6H6) determined by the Evans’ method24 was measured in a tetrahydrofuran (THF)/C6D6 solution as 6.34 μB, a value approaching the theoretical spin-only value of 6.93 μB for two noninteracting S = 2 ions. In the solid state, the χT product, where χ is the molar magnetic susceptibility and T is the temperature, is ca. 2.0 cm3 K mol−1 at 300 K and decreases monotonically to near nil below ca. 30 K (Figure 2). Over this temperature range, χ(T) is only weakly temperature-dependent down to ca. 50 K, below which it collapses more rapidly. These data are only consistent with a strong antiferromagnetic coupling between the CrII ions. At very low temperatures, there is a sharp increase in χ(T), which is characteristic of a paramagnetic impurity. These data can be modeled by the first two terms in the spin Hamiltonian (1) Ĥ = −2Js1̂ ·s2̂ +
∑ i = 1,2
μB B ·gi·sî +
∑ i = 1,2
Figure 3. Q-band EPR spectrum of a polycrystalline sample of [Cr2(L)](C6H6) at 200 K (0−1.1 T region; top) and calculated spectrum using the parameters in the text and a line width of 200 G (bottom).
Supporting Information). Upon cooling, both the sharp and broad features decrease in intensity, while a sharp resonance grows in at g = 2.0 (Figure S1 in the Supporting Information). [The sharp resonance at g = 2.0 is the only feature observed at low temperature, consistent with a residual half-integer spin impurity. It is possible that this and the broad feature that grows around g = 2.0 upon warming are both due to a partially oxidized CrII,III form of the complex (which would have an S = 1 /2 ground state assuming antiferromagnetic coupling).] The features across the 0−10 kG field range (Figure 3) are indicative of large ZFS interactions. In order to probe this, we attempted to simulate25,26 the spectra with Hamiltonian (1). In
sî ·d i·sî (1) B
DOI: 10.1021/acs.inorgchem.5b02151 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry order to minimize the number of parameters, J and gi were fixed to the values above. The two CrII ions are symmetry-equivalent; hence, their di tensors are equivalent, and we assumed them to be near axial with their principal (z) axes normal to the {N4} planes given the approximate square-planar {N4} coordination environments. Calculating the best {N4} planes from the crystal structure gives an angle of 35° between the two principal ZFS axes. The major features of the spectrum can be reproduced with an axial ZFS term of di = −1.7 cm−1 and a small rhombicity of ei/di = 0.02 (these are the only free variables in the model). With these parameters, the observed transitions are largely due to the total spin S = 2 and 3 excited states, which have axial ZFS parameters (D) of ca. +0.52 and −0.31 cm−1, respectively, resulting from the projection of the local ZFSs. The S = 1 state has a much larger ZFS of +6.2 cm−1 (and only one transition in this field window), and the S = 4 state has insignificant population at 200 K (29, 46, 21, 4, and 2σ(I). Because of the low completeness of the data sets, some thermal similarities were applied (details are available in the CIFs in the Supporting Information). Unit cells and refinement parameters are reported in Table S1 in the Supporting Information.
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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.5b02151. Full EPR and computational data (PDF) X-ray crystallographic data in CIF format (ZIP) Animated GIF file showing the path of compression of [Cr2(L)](C6H6) (GIF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. Tel: 0044 131 6504762. Fax: 0044 131 6504743. *E-mail:
[email protected]. Tel: 0044 131 6504762. Fax: 0044 131 6504743. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the EPSRC and the University of Edinburgh for funding and the Diamond Light Source for access to beamline I19 (proposal number MT9205) that contributed to the results presented here. We also acknowledge the University of Edinburgh Computer and Data Facility.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.5b02151 Inorg. Chem. XXXX, XXX, XXX−XXX
