Communication pubs.acs.org/IC
Upside Down! Crystallographic and Spectroscopic Characterization of an [FeIV(Osyn)(TMC)]2+ Complex Jai Prakash,† Gregory T. Rohde,† Katlyn K. Meier,‡ Eckard Münck,*,‡ and Lawrence Que, Jr.*,† †
Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, Minneapolis, Minnesota 55455, United States ‡ Department of Chemistry, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States S Supporting Information *
but no crystal structure for 3 could be obtained. In exploring the reaction of FeII(TMC)(OTf)2 with a different iodosoarene, we observed the formation of a different complex, [FeIV(O)(TMC)]2+ (2), and we report crystallographic and spectroscopic evidence that the O atom in 2 occupies the syn face of the TMC ligand (Scheme 1). The reaction of FeII(TMC)(OTf)2 in MeCN solvent with 1 equiv of PhIO (dissolved in CF3CH2OH) at 25 °C generates 1NCMe in 92% yield, as indicated by the appearance of its signature near-IR band at 824 nm (εM = 400; Figure 1, left, blue
ABSTRACT: FeII(TMC)(OTf)2 reacts with 2-tBuSO2− C6H4IO to afford an oxoiron(IV) product, 2, distinct from the previously reported [FeIV(Oanti)(TMC)(NCMe)]2+. In MeCN, 2 has a blue-shifted near-IR band, a higher ν(Fe O), a larger Mössbauer quadrupole splitting, and quite a distinct 1H NMR spectrum. Structural analysis of crystals grown from CH 2 Cl 2 reveals a complex with the formulation of [FeIV(Osyn)(TMC)(OTf)](OTf) and the shortest FeIVO bond [1.625(4) Å] found to date.
T
etramethylcyclam (TMC, 1,4,8,11-tetramethyl-1,4,8,11tetraazacyclotetradecane) has played an important role as a ligand in coordination complexes1 ever since its introduction in 1973.2 Thirty years later, this ligand was found to stabilize an oxoiron(IV) center in [FeIV(Oanti)(TMC)(NCMe)](OTf)2 (1NCMe, NCMe = acetonitrile), resulting in the first crystal structure of an oxoiron(IV) complex (Scheme 1).3 This and Scheme 1. Oxo Binding Modes of [FeIV(O)(TMC)(X)]n+
Figure 1. Left: UV−vis spectra of 1-NCMe (dotted blue line) and 2 (solid red line) obtained at RT by adding 1 equiv of PhIO and ArIO (in CF3CH2OH), respectively, to FeII(TMC)(OTf)2 in CH3CN. Right: FTIR spectra of 2 (precipitated solid from MeCN solution) generated using ArI16O (black line) and ArI18O (red line). The asterisk denotes the ν(FeO) of a minor fraction of 1-NCMe in the sample.
dotted trace),3 which arises from d−d transitions of the S = 1 FeIVO center.14 To our surprise, the same reaction performed with 1 equiv of 2-tBuSO2−C6H4IO, ArIO instead of PhIO, gave rise to a different species 2 with a very similar but distinct near-IR band at 815 nm (εM = 380; Figure 1, left, red solid trace). The slight variation in their near-IR bands suggests a very subtle structural difference between 1-NCMe and 2. Like 1-NCMe, 2 also has one prominent ion cluster peak at m/z 477 in its ESI-MS spectrum with an isotopic pattern corresponding to the {Fe(O)(TMC)(OTf)}+ ion, which is upshifted by 2 units to m/z 479 with ArI18O as the oxidant, showing the incorporation of 18O from ArI18O into 2 (Figure S1). The Mössbauer spectrum of a sample containing 2 in MeCN recorded in zero applied magnetic field (Figure S2) exhibits four quadrupole doublets. The majority component (red line), representing 72% total Fe (FeT), has ΔEQ = 1.55 mm/s and δ
related complexes serve as models4,5 for oxoiron(IV) intermediates in the catalytic cycles of nonheme iron oxygenases responsible for many important metabolic transformations.6 The crystal structure of 1-NCMe shows that TMC adopts the trans-I (R,S,R,S) configuration with all four methyl groups arrayed on one side of the macrocycle and the dianionic oxide bound to the anti face of the TMC ligand (Scheme 1). This binding mode stands in stark contrast to the pattern found for all other [Fe(TMC)(anion)] complexes reported to date, the crystal structures of which show anion binding only at the syn face.7−10 Among these is the ScIII−O−FeIII(TMC) complex described by Fukuzumi and Nam, where the “Sc−O” unit is bound to the syn face, despite being obtained from the reaction of 1-NCMe with Sc(OTf)3.11,12 In addition, a [FeIV(Osyn)(TMC)(NCMe)]2+ complex (3) was proposed by Ray et al.13 and forms from the reaction of 1-NCMe with PhI(OAc)2 in the presence of BF4−, © XXXX American Chemical Society
Received: September 2, 2015
A
DOI: 10.1021/acs.inorgchem.5b02011 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry = 0.16 mm/s, consistent with an S = 1 FeIVO species, and we thus assign this species to 2. Interestingly, the minority component (blue line, 14% FeT) has ΔEQ = 1.24 mm/s and δ = 0.17 mm/s, which correspond to the values reported previously for species 1-NCMe. There are also two high-spin FeII contaminants: a remnant of the FeII precursor with ΔEQ = 3.06 mm/s and δ = 1.00 mm/s (green, 11% FeT) and a component ΔEQ ≈ 3.75 mm/s and δ ≈ 1.05 mm/s (yellow, 3% FeT). A sample of FeII(TMC)(OTf)2 in MeCN (Figure S3) contained ≈80% of its FeT associated with the ΔEQ = 3.06 mm/s species and ≈18% of a species with ΔEQ ≈ 3.9 mm/s and δ = 1.03 mm/s. Analysis of the applied field spectra of Figure S4 yields for 2 a zero-field-splitting parameter, D = 27 cm−1, identical with that for 1-NCMe; the 57Fe magnetic hyperfine tensors of 2 and 1NCMe are also quite similar.13 In fact, the larger ΔEQ value of 2 is what distinguishes it from 1-NCMe (Table S1). Complex 2 exhibits an ν(FeO) at 856 cm−1 in its Fourier transform infrared (FTIR) spectrum, which downshifts 36 cm−1 to 820 cm−1 upon 18O labeling (Figure 1, right). There is also a minor amount of 1-NCMe present in the sample, as indicated by the feature at 830 cm−1, which disappears upon 18O labeling. This frequency difference further emphasizes that 2 is distinct from 1NCMe. On the basis of the properties of a series of [FeIV(O)(TMC)(anion)]+ complexes,15 the higher ν(FeO) value for 2 suggests that 2 has a weaker axial ligand trans to the FeO unit than 1-NCMe. To gain insight into the nature of the axial ligand in 2, elemental analysis (C, H, N, and S) of 2 was obtained for a vacuum-dried sample that was precipitated from MeCN (see the SI for details) and found to be fully consistent with the formulation of Fe(O)(TMC)(NCMe)(OTf)2. Its 19F NMR spectrum in MeCN displays only one peak that has the chemical shift and line width corresponding to a free triflate ion and integrates to two triflates per mole (Figure S5), so triflate is definitely not bound to the FeO unit. Instead, we propose MeCN to be the axial ligand in 2 and hereafter refer 2 as 2NCMe. Thus, 1-NCMe and 2-NCMe have the same molecular formula and appear to be isomers. The difference between 1-NCMe and 2-NCMe is most starkly manifested in their 1H NMR spectra (Figure 2A,B and Table S1). The 1H NMR spectrum of 1-NCMe (Figure 2A) exhibits seven distinct paramagnetically shifted proton signals spread over a 200 ppm range,13 with a 1:1:2:2:2:2:6 intensity ratio that is consistent
with the trans-I (R,S,R,S) configuration of TMC1 seen in the crystal structure of 1-NCMe. The most readily assigned signals are the intense N−CH3 signal at −42 ppm and a pair of sharper resonances at 27 and 45 ppm with unit relative intensity, which are associated with the diastereotopic β-CH2 protons of the propylene bridges. The four other resonances correspond to the diastereotopic α-CH2 protons adjacent to the amine donors. Such features are also found in the 1H NMR spectrum of 2NCMe (Figure 2B), but they correspond to the minor component of the sample, in corroboration with the FTIR and Mössbauer data (Figures 1, right, and S2). The major component in the sample of 2-NCMe exhibits a very different pattern of peaks (Figure 2B and Table S1). Nevertheless, seven resonances can also be associated with this component with a 1:1:2:2:2:2:6 intensity ratio, consistent with maintenance of the trans-I (R,S,R,S) TMC configuration. The diastereotopic β-CH2 protons of 2-NCMe are found in the diamagnetic region (7.3 and 8 ppm) compared to their downfield-shifted counterparts in 1-NCMe and are sharper than those of 1-NCMe by about a factor of 2. On the other hand, the N−CH3 signal at −51 ppm is more upfield-shifted than the corresponding signal in 1-NCMe at −42 ppm, but its line width is a factor of 2 broader than the corresponding N−CH3 signal of 1-NCMe. Integration of the respective N−CH3 resonances gives an estimated 5:1 ratio for 2NCMe to 1-NCMe. The observation that the TMC protons are affected differently by the paramagnetic FeIVO centers of 1NCMe and 2-NCMe suggests a difference in the geometric relationship between the FeO unit and the TMC protons. The simplest way to make 2-NCMe different from 1-NCMe while maintaining the trans-I (R,S,R,S) TMC configuration is to flip the orientation of the FeO unit such that the O atom in 2NCMe now binds to the syn face of TMC, instead of the anti face. To provide structural evidence for what distinguishes 2-NCMe from 1-NCMe, we tried to grow crystals of 2-NCMe from MeCN solution but failed. However, diffraction-quality crystals were obtained from the reaction of Fe II (TMC)(OTf)2 with stoichiometric ArIO at −80 °C in CH2Cl2. The crystal structure shows that the O atom does bind to the syn face of TMC (Figure 2C), with triflate as the axial ligand instead of CH3CN, which was expected because MeCN was not present in the crystallization medium. We thus designate this complex as [FeIV(Osyn)(TMC)(OTf)]+ (2-OTf). The FeO distance in 2-OTf is 1.625(4) Å, which is 0.02 Å shorter than that found in 1-NCMe [1.646(3) Å].3 The average Fe−NTMC distance in 2-OTf is 2.068 Å, 0.02 Å shorter than the corresponding distance of 2.091 Å found in 1NCMe.3 These differences among the five Fe(O)TMC bond distances represent a cumulative bond-length shortening of more than 0.1 Å, which is compensated for by the Fe−OTf bond of 2.146(4) Å, which is 0.09 Å longer than the Fe−NCMe distance of 2.058(3) Å found in 1-NCMe. These bond-length differences were in fact anticipated by density functional theory.13 The Fe atom in 2-OTf lies 0.07 Å above the TMC N4 mean plane and toward the methyl groups, slightly longer than the 0.03 Å distance for the Fe atom in 1-NCMe but much shorter than the 0.53 Å distance seen for the five-coordinate Fe atom of the (TMC)FeIII−O−ScIII complex.11,12 The structural differences between 2-OTf and 1-NCMe easily rationalize the distinct spectroscopic properties exhibited by 2NCMe in MeCN solution (Table S1) for which 19F NMR data strongly suggest that the bound triflate in 2-OTf is replaced by solvent. The shorter FeO bond found in 2-OTf indicates a stronger FeO bond that is reflected by the higher ν(FeO) of 2-NCMe and gives rise to a greater axial distortion, which leads to
Figure 2. 1H-NMR spectra of 1-NCMe (A) and 2-NCMe (B) in CD3CN at RT. Peaks marked by ‘*’ in the spectrum of 2-NCMe arise from residual FeII(TMC)(OTf)2. B inset: Expanded 0−10 ppm region; unmarked peaks at 7−8 ppm derive from the ArI byproduct. (C) ORTEP plot of 2-OTf, [FeIV(Osyn)(TMC)(OTf)]+ drawn at 50% probability with H-atoms omitted for clarity. Selected bond lengths (Å): Fe−O, 1.625(4); Fe−Navg, 2.068 (9); Fe−OTf, 2.146(4). B
DOI: 10.1021/acs.inorgchem.5b02011 Inorg. Chem. XXXX, XXX, XXX−XXX
Inorganic Chemistry
■
the larger Mössbauer ΔEQ observed for 2-NCMe in a frozen CH3CN solution.15 The stronger equatorial Fe−NTMC bonds found in 2-OTf destabilize the Fe dx2−y2 orbital, resulting in the ∼10 nm blue shift observed for the near-IR ligand-field band of 2NCMe relative to that of 1-NCMe (Figure 1). Last, the opposite orientations of the FeO units in 2-OTf and 1-NCMe engender the distinct paramagnetic shift patterns observed in their 1H NMR spectra in MeCN solution. Given these results, it is clear that the reaction of FeII(TMC)(OTf)2 with ArIO generates a complex distinct from 1-NCMe. We speculate that the syn isomer forms because the steric bulk of ArIO disfavors its attack at the anti face of FeII(TMC)(OTf)2. However, this hypothesis needs further corroboration by studying the reactions of FeII(TMC)(OTf)2 with different oxotransfer agents. The results presented here invalidate our earlier claim for the formation of a syn isomer of 1-NCMe in 2008.13 This complex was obtained from the reaction of 1-NCMe with PhI(OAc)2 in the presence of Bu4NBF4 to produce 3. A comparison of spectroscopic properties in Table S1 shows that 1-NCMe, 2NCMe, and 3 are distinct species. Unlike the first two complexes, 3 has two near-IR bands that are characteristic of [FeIV(Oanti)(TMC)(anion)]+ complexes15 and are, in fact, identical with those of the fluoride adduct of 1, [FeIV(Oanti)(TMC)(F)]+ (Figure S6). Furthermore, their 1H NMR spectra match (Table S1 and Figure S6).18 Therefore, 3 cannot be the syn isomer of 1-NCMe as claimed. In light of the results in this paper, a reinvestigation of the earlier results will be required to clarify what misled us. In summary, we have found reaction conditions for the synthesis of a distinct [FeIV(Osyn)(TMC)(X)]+/2+ complex, where X is OTf or MeCN depending on the solvent used. The crystal structure of 2-OTf shows the oxo moiety to occupy the syn face of the TMC ligand in a trans-I (R,S,R,S) configuration (Scheme 1). Moreover, 2-OTf has the shortest FeO bond by 0.014 Å of any S = 1 FeIVO complex crystallized thus far.3,4,16,17 This complex in MeCN solution is formulated as 2NCMe, which can be distinguished from 1-NCMe by its blueshifted near-IR band, its higher ν(FeO), its larger Mössbauer quadrupole splitting, and a 1H NMR spectrum with a distinct paramagnetic shift pattern. Our studies reveal new aspects of the chemistry of the FeIV(O)(TMC) unit that may also apply to complexes of other cyclam-related ligands.18−20
■
ACKNOWLEDGMENTS This work was supported by the National Science Foundation (Grant CHE-1361773 to L.Q. and Grant CHE-1305111 to E.M.). G.T.R. thanks the University of Minnesota for a dissertation fellowship. We thank Ruixi Fan for experimental assistance.
■
REFERENCES
(1) Barefield, E. K. Coord. Chem. Rev. 2010, 254, 1607−1627. (2) Barefield, E. K.; Wagner, F. Inorg. Chem. 1973, 12, 2435−2439. (3) Rohde, J.-U.; In, J.-H.; Lim, M. H.; Brennessel, W. W.; Bukowski, M. R.; Stubna, A.; Münck, E.; Nam, W.; Que, L., Jr. Science 2003, 299, 1037−1039. (4) McDonald, A. R.; Que, L., Jr. Coord. Chem. Rev. 2013, 257, 414− 428. (5) Ray, K.; Pfaff, F. F.; Wang, B.; Nam, W. J. Am. Chem. Soc. 2014, 136, 13942−13958. (6) Krebs, C.; Galonić Fujimori, D.; Walsh, C. T.; Bollinger, J. M., Jr. Acc. Chem. Res. 2007, 40, 484−492. (7) Fiedler, A. T.; Halfen, H. L.; Halfen, J. A.; Brunold, T. C. J. Am. Chem. Soc. 2005, 127, 1675−1689. (8) Sastri, C. V.; Park, M. J.; Ohta, T.; Jackson, T. A.; Stubna, A.; Seo, M. S.; Lee, J.; Kim, J.; Kitagawa, T.; Münck, E.; Que, L., Jr.; Nam, W. J. Am. Chem. Soc. 2005, 127, 12494−12495. (9) McDonald, A. R.; Bukowski, M. R.; Farquhar, E. R.; Jackson, T. A.; Koehntop, K. D.; Seo, M. S.; De Hont, R. F.; Stubna, A.; Halfen, J. A.; Münck, E.; Nam, W.; Que, L., Jr. J. Am. Chem. Soc. 2010, 132, 17118− 17129. (10) Cho, J.; Jeon, S.; Wilson, S. A.; Liu, L. V.; Kang, E. A.; Braymer, J. J.; Lim, M. H.; Hedman, B.; Hodgson, K. O.; Valentine, J. S.; Solomon, E. I.; Nam, W. Nature 2011, 478, 502−505. (11) Fukuzumi, S.; Morimoto, Y.; Kotani, H.; Naumov, P.; Lee, Y.-M.; Nam, W. Nat. Chem. 2010, 2, 756−759. (12) Prakash, J.; Rohde, G. T.; Meier, K. K.; Jasniewski, A. J.; Van Heuvelen, K. M.; Münck, E.; Que, L., Jr. J. Am. Chem. Soc. 2015, 137, 3478−3481. (13) Ray, K.; England, J.; Fiedler, A. T.; Martinho, M.; Münck, E.; Que, L., Jr. Angew. Chem., Int. Ed. 2008, 47, 8068−8071. (14) Decker, A.; Rohde, J.-U.; Klinker, E. J.; Wong, S. D.; Que, L., Jr.; Solomon, E. I. J. Am. Chem. Soc. 2007, 129, 15983−15996. (15) Jackson, T. A.; Rohde, J.-U.; Seo, M. S.; Sastri, C. V.; DeHont, R.; Stubna, A.; Ohta, T.; Kitagawa, T.; Münck, E.; Nam, W.; Que, L., Jr. J. Am. Chem. Soc. 2008, 130, 12394−12407. (16) Meyer, S.; Klawitter, I.; Demeshko, S.; Bill, E.; Meyer, F. Angew. Chem., Int. Ed. 2013, 52, 901−905. (17) England, J.; Bigelow, J. O.; Van Heuvelen, K. M.; Farquhar, E. R.; Martinho, M.; Meier, K. K.; Frisch, J. R.; Münck, E.; Que, L. Chem. Sci. 2014, 5, 1204−1215. (18) Hong, S.; So, H.; Yoon, H.; Cho, K.-B.; Lee, Y.-M.; Fukuzumi, S.; Nam, W. Dalton Trans. 2013, 42, 7842−7845. (19) Wilson, S. A.; Chen, J.; Hong, S.; Lee, Y.-M.; Clemancey, M.; Garcia-Serres, R.; Nomura, T.; Ogura, T.; Latour, J.-M.; Hedman, B.; Hodgson, K. O.; Nam, W.; Solomon, E. I. J. Am. Chem. Soc. 2012, 134, 11791−11806. (20) Kim, D.; Cho, J.; Lee, Y.-M.; Sarangi, R.; Nam, W. Chem. - Eur. J. 2013, 19, 14112−14118.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02011. Experimental details, crystallographic and structural refinement data for 2-OTf, ESI-MS and Mössbauer data for 2-NCMe, 19F NMR spectrum of 2-NCMe, and near-IR and 1H NMR spectra of [FeIV(Oanti)(TMC)(F)]+ (PDF) Crystallographic file in CIF format for 2-OTf (CIF)
■
Communication
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. Notes
The authors declare no competing financial interest. C
DOI: 10.1021/acs.inorgchem.5b02011 Inorg. Chem. XXXX, XXX, XXX−XXX
