Article pubs.acs.org/Organometallics
New Fe(III)(cyclam) Complexes Bearing Axially Bound geminalDiethynylethenes Zhi Cao, Phillip E. Fanwick, William P. Forrest, Yang Gao, and Tong Ren* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *
ABSTRACT: Reported herein are the preparation and characterization of trans[Fe(cyclam)(X-gem-DEE)2]OTf (where cyclam = 1,4,8,11-tetraazacyclotetradecane, gem-DEE = σ-geminal-diethynylethene, and OTf = trifluoromethanesulfonate) compounds 2a, 2b, and 2c (X = −Ph (2a), −SiiPr3 (2b), and −Fc (2c)), which are the first examples of redox-active 3d metal complexes containing gemDEE ligands. These compounds were prepared from the reaction between cis/ trans-[Fe(cyclam)(OTf)2]OTf (1) and X-gem-DEE-Li. Compounds 2a−2c were characterized by spectroscopic/voltammetric techniques. The trans-orientation of the gem-DEE ligands was established from the single-crystal X-ray diffraction study of 2a. Furthermore, the electronic structures of the model compounds 2a′+ and 2b′+ were analyzed with density-functional-theory calculations, which revealed significant dπ(Fe)−π(gem-DEE) interactions.
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INTRODUCTION Transition-metal alkynyl and alkenyl compounds have received intense interest as potential molecular wires and other electronic and optoelectronic materials.1 Facile charge transfer within transition-metal σ-alkynyl complexes containing linearly conjugated organic frameworks has been demonstrated for a number of metal centers, including polyyn-diyl ((C C)m) bridged bimetallic species of Re,2 Fe,3 and Ru.4 Many inspiring examples of iron monoalkynyl compounds as building blocks for molecular wires have been reported by the laboratories of Lapinte5 and Akita,6 where the piano-stool type Fe(II) centers, either CpFe(PP) or CpFe(CO)2, are prevalent (type Ia in Chart 1). In comparison, iron bis-alkynyl
ligands remain under-explored. Nevertheless, recent advances in the synthesis of nonlinear enyne and enediyne scaffolds make it possible to explore branched metal−alkynyl compounds.10,11 Nonlinearly conjugated organic compounds have drawn considerable interest due to their novel optoelectronic properties (i.e., push−pull NLO chromophores and optical sensors),11,12 and those containing the geminal-diethynylethene unit (also known as iso-triacetylene, and abbreviated as gemDEE) are of particular interest to physical organic chemists because of the cross-conjugation therein.11 Transition-metal complexes with enyne or enediynes as σ-alkynyl ligands are very rare, and the ones based on gem-DEE were restricted to Pt(II) before 2011.13 Our group reported the first examples of redoxactive transition metal complexes with the gem-DEE ligands in a trans-geometry, namely, trans-Ru2(DMBA)4(X-gem-DEE)2 type compounds (DMBA = N,N′-dimethylbenzamidinate).14 Subsequently, the first examples of 3d-metal complexes containing gem-DEE based on the CrIII(cyclam) (cyclam = 1,4,8,11tetraazacyclotetradecane) motif were reported.15 It is worth mentioning that a series of diruthenium compounds with various cross-conjugated bridging ligands were elaborated by Bruce and Low very recently.16 Described in this contribution are the synthesis, characterization, and computational studies of Fe(III) gem-DEE complexes 2a−2c, based on the Fe(cyclam) unit (Scheme 1).
Chart 1. Common Fe Alkynyl Structures
compounds are rare with most examples based on the transFeII(PP)2(CCR) type compounds (type Ib in Chart 1) developed in the laboratory of Field.7 Recently, our laboratory has expanded the scope of this type of compounds with the introduction of the series of trans-[Fe III (cyclam)(C CR)2]OTf (where R = H, SiiPr3, Ph, Fc, C2SiMe3, and C4SiMe3) complexes.8,9 Compared with transition-metal compounds with linearly conjugated alkynyl ligands, compounds of nonlinearly πconjugated (cross-conjugated and cruciform motifs) alkynyl © 2013 American Chemical Society
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RESULTS AND DISCUSSION As shown in Scheme 1, complexes 2a−2c were produced from the reactions between cis/trans-[Fe(cyclam)(OTf)2]OTf and 3 equiv of in situ generated Li-gem-DEE-X. The H-gem-DEE-X Received: July 11, 2013 Published: August 15, 2013 4684
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Fe(III)C bond lengths determined for the trans-[Fe(cyclam)(CCR)2]+ (1.94−1.97 Å)8,9 and the Fe(II)C trans-Fe(PP)2(CCR) type compounds (1.92−1.97 Å).7 Interestingly, the Fe center does not reside on an inversion center, as was previously observed in the structure of [Cr(cyclam)(Fc-gem-DEE)2]+.15 Instead, both of the phenyl substituents point in the same direction and the two Ph-gemDEE fragments are approximately coplanar. The redox activities of compounds 2a−2c are very relevant to the general interest in using trans-bis-gem-DEE metal complexes as the building blocks for molecular wires, and they were examined carefully using cyclic voltammetric (CV) and differential pulse voltammetric (DPV) techniques. The CVs recorded are shown in Figure 2, while the DPVs are
Scheme 1. Preparation of Compounds 2a, 2b, and 2c
was initially treated with 3.5 equiv (with respect to [Fe(cyclam)(OTf)2]OTf) of n-BuLi to afford Li-gem-DEE-X, which was then transferred by cannula to the Schlenk flask containing a THF solution of cis/trans-[Fe(cyclam)(OTf)2]OTf. Compounds 2a−2c were isolated as dark green solids in satisfactory yields (71% for 2a, 58% for 2b, and 42% for 2c). The type 2 complexes are paramagnetic and cannot be unambiguously characterized with 1H NMR spectroscopy. Instead, these compounds were authenticated by elemental analysis, HR-nESI-MS, and the single-crystal X-ray diffraction study of 2a. The room temperature effective magnetic moments of 2a−2c are 2.06, 1.95, and 1.98 μB, respectively, which is typical for a Fe(III) center with an S = 1/2 ground state. Similar to simple alkynyl ligands, gem-DEE is a strong-field ligand and stabilizes a low-spin Fe(III) center. A single-crystal X-ray diffraction study was performed for compound 2a, and the ORTEP plot of the cation is shown in Figure 1 along with the selected bond lengths and angles.
Figure 2. Cyclic voltammograms recorded for compounds 2a−2c in a 0.20 M THF solution of n-Bu4NPF6 at a scan rate of 100 mV/s.
provided in the Supporting Information (Figure S2). There is no detectable oxidation couple up to +1.0 V for compounds 2a and 2b, which reflects the electron deficiency of the Fe(III) species. All three compounds undergo a reversible 1 e− reduction (A), which is attributed to the reduction of Fe(III) to Fe(II). The formal potential of 2a (−0.582 V) is anodically shifted from that of 2b (−0.620 V), indicating that the reduction of the Fe(III) center becomes easier with the Ph-gemDEE ligand due to an extended π-delocalization pathway. Compared with those of trans-[Fe(cyclam)(CCR)2]OTf (−0.525 V for R = −Ph and −0.585 V for R = −SiiPr3),9 the reduction potentials for 2a and 2b are cathodically shifted by 57 and 35 mV, respectively. The cathodic shift clearly reflects that gem-DEE ligand is a better π electron donor in comparison with linear alkynyl ligands, which is in agreement with the trend previously established for [Cr(cyclam)(gem-DEE)2]OTf15 and Ru2(DMBA)4(gem-DEE)2.14 Unlike compounds 2a and 2b, compound 2c displays a 2e− wave (B) in the window of 0−1.0 V, which is attributed to the oxidation of the two ferrocenyl substituents. The ΔEp value (87 mV) of wave B is significantly larger than the theoretical value for a reversible 2e− process (29 mV), signaling that wave B is likely the result of convolution of two closely spaced 1e− waves. To explore this possibility, the Richardson−Taube method was employed.18 The DPV of 2c recorded under the conditions stipulated by Richardson and Taube yielded a half-width of 127 mV (Figure 3), which corresponds to a ΔE1/2 value of 63 mV. The presence of the two ferrocenyl (Fc) capping groups in FcCCMCCFc may enable the assessment of the degree of electronic delocalization across the CC
Figure 1. ORTEP plot of [2a]+ at 20% probability level. Hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and angles (deg): Fe1−C1, 1.978(4); Fe1−C15, 1.966(4); Fe1−Nav, 2.010(4); C1−C2, 1.202(5); C2−C3, 1.447(5); C3−C4, 1.349(6); C3−C5, 1.453(6); C5−C6, 1.191(6); C15−C16, 1.197(6); C16−C17, 1.472(7); C17−C18, 1.288(7); C17−C19, 1.455(7); C19−C20, 1.192(7); C1−Fe1−C15, 179.38(16); Fe1−C1−C2, 172.6(3); Fe1− C15−C16, 171.2(4).
Compound 2a crystallized in a monoclinic setting, and the asymmetric unit contains a complete [Fe(cyclam)(Ph-gemDEE)2]+ and a OTf anion. Similar to [Cr(cyclam)(C CR)2]+,17 [Cr(cyclam)(X-gem-DEE)2]+,15 and [Fe(cyclam)(CCR)2]+,8,9 the pseudo-octahedral coordination mode is observed in [2a]+ with the CFeC vector approximately orthogonal to the plane defined by the four N centers of cyclam, as shown in Figure 1. The FeC bond lengths in [2a]+ (1.966(4) and 1.978(4) Å) are in agreement with the 4685
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C)} 2 (C 6 H 4 )][PF 6 ] 2 and [1,3,5-{Cp*(dppe)Fe(C C)}3(C6H3)][PF6] reported by Lapinte.26,27 For Lapinte’s compounds, two maxima in the visible region were found at 573 and 662 nm, and both were ascribed to ligand-to-metal charge transfer (LMCT) transitions. Similarly, the two distinct absorption bands observed at low energy (550−700 nm) for 2a are probably due to ligand-to-metal charge transfer (LMCT) transitions from the low-lying ligand based molecular orbitals to the partially filled SOMO. There is one intense peak at 563 nm and a shoulder at ca. 510 nm for 2b that are also assigned as LMCT transitions. The maxima in 2a are red-shifted from those of 2b due to the extensive π-conjugation in the former. In contrast to the aforementioned Fe(III)-acetylide compounds by Lapinte, compounds 2a and 2b also display two distinct highenergy peaks at 410, 440 nm and 430, 470 nm, respectively. These transitions are probably metal-to-ligand charge transfer (MLCT) transitions from the d orbital dominated SOMO to the low-lying ligand based empty orbitals. Similar scenarios were reported previously for [{Fe(η5-C5Me5)(η2-dppe)(C C)}2{2,5-C4H2S}]+1/0.28 The nature of the orbitals involved in these LMCT and MLCT transitions will be elaborated on the basis of DFT studies below. In order to gain insight into the electronic interactions between the axial gem-DEE ligands and the Fe(III) center, the spin-polarized density functional calculations at the B3LYP/ LanL2DZ level (Gaussian 03 program)29 were performed on the model cations [2a′]+ (based on crystal structure without truncation) and [2b′]+ (where the Ph units were replaced by SiiPr3 groups and then followed by full optimizations). Comparable bond lengths and angles for the first coordination sphere of the Fe center were obtained for [2a′]+ and [2b′]+. The computed energies and contour plots of the most relevant MOs for the model cations [2a′]+ and [2b′]+ are given in Figure 5. The average optimized FeC bond length of [2a′]+ (1.966 Å) is slightly shorter than the X-ray experimental average values of [2a]+ (1.972 Å). The coordinated C1 C2 and C15C16 averaged bond length in [2a′]+ is increased to 1.246 Å (versus 1.199 Å, averaged X-ray value for [2a]+), while the averaged bond length of the free CC units of gem-DEE (C5C6 and C19C20) in [2a′]+ has been increased to 1.229 Å (versus 1.191 Å, averaged X-ray value for [2a]+). Similarly, the C3C4/C17C18 averaged bond length in [2a′]+ is increased to 1.380 Å (versus 1.319 Å, averaged X-ray value for [2a]+). These bond elongations are likely due to the tendency of overestimation of electronic delocalization by the B3LYP method.30 The DFT results for [2a′]+ and [2b′]+ are in agreement with the ligand-field theory prediction for a d5 center in a strong field: an empty eg set as LUMO (dx2−y2) and LUMO+1 (dz2) and an occupied t2g set as HOMO−2, HOMO−1, and SOMO. Closely related MOs as well as energy levels are plotted in Figure 5. The loss of orbital degeneracy is caused by both the low symmetry of the cyclam ligand (C2 only) and Jahn−Teller instability (in Oh settings).8,9 It is clear from Figure 4 that the HOMO−1 is the antibonding combination of dyz (Fe) and two π∥ (DEE) (in-plane π orbital of gem-DEE), while the SOMO is the antibonding combination of dxz (Fe) and two π⊥ (DEE) (out-of-plane π orbital of gem-DEE).31 The HOMO−2 is dominated by dxy with a minimum contribution from the gemDEE ligands. The LUMO is dominated by the antibonding combination of dx2−y2 and p orbitals of the surrounding N atoms, with no contribution from the alkynyls due to the orbital orthogonality. The LUMO+1 is best described as σ* (Fe−C)
Figure 3. DPVs of the Fc oxidation region measured for 2c under both the standard conditions (dash) and conditions according to the Richardson−Taube method (solid).
MCC on the basis of the potential difference (ΔE1/2) of the stepwise Fc oxidations. Previously, stepwise Fc oxidations with substantial ΔE1/2 were observed in the cases with M as Pt,19 Ru,20 Ru3(dpa)4 (dpa = dipyridylamidate),21 and Ru2(DMBA)4 (DMBA = N,N′-dimethylbenzamidinate).22 For the latter compounds, a remarkably large ΔE1/2 of ca. 300 mV was observed and the mixed-valent [FcFc]+ ion was identified as a class III Robin−Day mixed valent ion23 on the basis of spectroelectrochemical analysis. In contrast, the ΔE1/2 value of 2c is much smaller and may indicate that the {Fc-gemDEE-FeIII(cyclam)-gem-DEE-Fc}2+ is a weak-coupled class II mixed valent species. It should be noted, nevertheless, that there are documented examples for both delocalized mixed valent species without a discernible ΔE1/224 and compounds with sizable ΔE1/2 but localized mixed valent state.25 The electronic absorption spectra of compounds 2a−2c are shown in Figure 4, and those of 2a and 2b display several
Figure 4. Visible absorption spectra of compounds 2a−2c recorded in CH3CN.
intense absorptions in the visible region. Previously, multiple absorptions observed in the visible region for [Fe(cyclam)(CCR)2]1+ were ascribed to the d−d transitions because of high effective symmetry therein (D4h).8,9 One might expect coalescence and broadening of these d−d transitions in the 2 compounds because the reduced symmetry imposed by gemDEE ligands (Cs). The persistence of the feature of multiple absorptions in 2 hints at a different origin. In fact, the feature of multiple intense absorptions is reminiscent of the spectra recorded for other Fe(III)-acetylides, notably the bi- and trimetallic iron complexes [1,3-{Cp*(dppe)Fe(C 4686
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alkynyl−phenyl based orbitals, and hence, HOMO−7 → SOMO and HOMO−6 → SOMO are designated as the LMCT transitions.
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CONCLUSION The work described herein demonstrated the feasibility of preparing Fe(III) gem-DEE complexes based on the Fe(cyclam) motif. Strong π interactions between the occupied dπ(Fe) and π(gem-DEE) orbitals are evident from both the absorption spectra and the DFT calculations. The cathodic shift of the formal potential of the Fe(+3/+2) couple clearly reflects that the gem-DEE ligand is a better π electron donor in comparison with linear alkynyl ligands. Type 2 compounds generally display intense and structured bands in the visible absorption spectra, assigned as dipole allowed charge transfer (both MLCT and LMCT) transitions. Ongoing efforts in our laboratory focus on the preparation of unsymmetric bis-alkynyl complexes based on Fe(III)-cyclam, such as trans-[Fe(cyclam)(C2nR)L]+, trans-[Fe(cyclam)(C2nR)(C2mR′)]+, and bis-[Fe(cyclam)(R-gem-DEE)(R′-gem-DEE)2]+, and the elaboration of the π-conjugation therein.
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EXPERIMENTAL SECTION
General. cis/trans-[Fe(cyclam)OTf2]OTf (1) was prepared as previously described.8,9 X-gem-DEE ligands were prepared according to the literature procedures.32 1,4,8,11-Tetraazacyclotetradecane and Pd(PPh3)4 were purchased from Strem Chemicals, and bis(trimethylsilyl)acetylene, iso-butyryl chloride, anhydrous aluminum chloride, triflic anhydride (1 M in CH2Cl2), anhydrous iron(II) chloride, triflic acid, and n-BuLi (2.5 M in hexanes) were purchased from Sigma-Aldrich. The alkynyl ligands, triisopropylsilylacetylene and phenylacetylene, were purchased from GFS Chemicals. 2,6-Di-t-butyl4-methylpyridine was purchased from Fluka. All reagents were used as received with no further purification. THF was distilled under a nitrogen atmosphere over Na/benzophenone. UV−vis spectra were obtained with a JASCO V-670 spectrophotometer in CH3CN solutions. FT-IR spectra were measured on a Jasco FT/IR-6300 as neat samples via a ZnSe ATR. All HR-nESI-MS spectra were performed on a prototype version of a QqTOF tandem mass spectrometer in 1/1 CH2Cl2/MeOH. Masses were calculated by isotopic distribution utilizing Analyst 1.5 software (Applied Biosystems Sciex). Magnetic susceptibility was measured at 294 K with a Johnson Matthey Mark-I magnetic susceptibility balance. Cyclic and differential voltammograms were recorded in 0.2 M (n-Bu)4NPF6 solution (THF, N2-degassed) on a CHI620A voltammetric analyzer with a glassy carbon working electrode (diameter = 2 mm), a Pt-wire auxiliary electrode, and a Ag/AgCl reference electrode. The concentration of the Fe species was always 1.0 mM. The ferrocenium/ferrocene couple was observed at 0.570 V (vs Ag/AgCl) at the noted experimental conditions. trans-[Fe(cyclam)(Ph-gem-DEE)2]OTf (2a). A THF solution (5 mL) of 3 equiv of the Ph-gem-DEE (0.153 g, 0.852 mmol) was cooled to −78 °C (dry ice/acetone), to which was added 3.5 equiv of n-BuLi (0.40 mL, 0.994 mmol). The reaction mixture was stirred in the cold bath for 1 h and then removed and allowed to warm to room temperature with stirring for an additional 1 h. The in situ formed Phgem-DEE-Li solution was transferred to a THF (75 mL) solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.200 g, 0.284 mmol). The reaction mixture was stirred for 1 h to yield a dark green solution, and then, the reaction was quenched with the addition of a few drops of water. The solution was filtered through silica eluting with 30% CH3CN in CH2Cl2. Upon removal of the solvent, the oily residue was added to a cold Et2O (ca. 100 mL), yielding a dark green precipitate. Yield: 0.154 g (0.202 mmol), 71% based on Fe. Data for 2a: Anal. found (calcd) for C45H78N4Fe3SO3F3·H2O (2a·H2O): C, 57.68 (57.36); H, 8.42 (8.56); N, 6.07 (5.95). HR-nESI-MS (m/z, based on 56Fe): 614.306, corresponding to [M]+ (C38H46N4Fe+, calcd
Figure 5. Molecular orbital diagrams for [2a′]+ (left) and [2b′]+ (right) based on α-spin orbitals; MO levels of β spin are provided in the Supporting Information (Figure S5). The directions pointed toward the N atoms of the cyclam ring are designated as the X and Y axes.
with the dominant contribution being from the dz2 orbital, and some additional contribution from both the p orbitals of the surrounding N atoms and minimum of π⊥ (C (gem) of the vinyl group). It is worth noting that the π interaction is the most extensive within HOMO−1 rather than SOMO, where both the vinyl and free ethynyl groups of the gem-DEE ligand contribute significantly to the π∥ orbital that mixed intimately with the dxz orbital of the Fe core. The π interaction between gem-DEE and Fe in the SOMO includes the contribution from vinyl and only the ethynyl coordinated to Fe but not the phenylethynyl groups further away from the Fe center. Quantitative assignment of electronic spectra may be achieved with TD-DFT calculation, which, however, is less reliable for open-shell molecules such as 2a and 2b reported herein. While performing TD-DFT is beyond the scope of this manuscript, qualitative assignment could be made on the basis of the ground-state DFT calculations. The SOMO in 2a is a dxz (Fe) based molecular orbital, while the LUMO+2 and LUMO +3 are nearly degenerate alkynyl−phenyl π* MOs. Hence, the most probable MLCT transitions are SOMO → LUMO+2 and SOMO → LUMO+3 (as shown in Figure S6 in the Supporting Information). The HOMO−6 and HOMO−7 are filled 4687
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614.307). UV−vis (λmax, nm (ε, M−1 cm−1)): 624 (5200), 562 (4900), 438 (9000), 410 (9200). FT-IR (neat, υ(CC)/cm−1): 2207, 2163, and 2063. Cyclic voltammogram [E1/2/V, ΔEp/V, ibackward/iforward]: A, −0.582, 0.064, 0.93. μeff = 2.06 μB. trans-[Fe(cyclam)(iPr3Si-gem-DEE)2]OTf (2b). A THF solution (5 mL) of 3 equiv of the iPr3Si-gem-DEE (0.222 g, 0.852 mmol) was cooled to −78 °C (dry ice/acetone), to which was added 3.5 equiv of n-BuLi (0.40 mL, 0.994 mmol). The reaction mixture was stirred in the cold bath for 1 h and then removed and allowed to warm to room temperature with stirring for an additional 1 h. The in situ formed i Pr3Si-gem-DEE-Li solution was transferred to a THF (75 mL) solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.200 g, 0.284 mmol). The reaction mixture was stirred for 1 h to yield a dark green solution, and then, the reaction was quenched with the addition of a few drops of water. The solution was filtered through silica eluting with 30% CH3CN in CH2Cl2. Upon removal of the solvent, the oily residue was added to a cold Et2O (ca. 100 mL), yielding a dark green precipitate. Yield: 0.152 g (0.165 mmol), 58% based on Fe. Data for 2b: Anal. found (calcd) for C39H46N4Fe3SO3F3 H2O (2b·H2O): C, 60.26 (59.92); H, 5.98 (6.19); N, 7.32 (7.17). HR-nESI-MS (m/z, based on 56 Fe): 774.511, corresponding to [M]+ (C44H78N4Si2Fe+, calcd 774.511). UV−vis (λmax, nm (ε, M−1 cm−1)): 563 (6100), 514 (4500), 467 (8000), 432 (6100). FT-IR (neat, υ(CC)/cm−1): 2216, 2168, and 2065. Cyclic voltammogram [E1/2/V, ΔEp/V, ibackward/ iforward]: A, −0.620, 0.063, 0.96. μeff = 1.95 μB. trans-[Fe(cyclam)(Fc-gem-DEE)2]OTf (2c). A THF solution (5 mL) of 3 equiv of the Fc-gem-DEE (0.245 g, 0.852 mmol) was cooled to −78 °C (dry ice/acetone), to which was added 3.5 equiv of n-BuLi (0.40 mL, 0.994 mmol). The reaction mixture was stirred in the cold bath for 1 h and then removed and allowed to warm to room temperature with stirring for an additional 1 h. The in situ formed Fcgem-DEE-Li solution was transferred to a THF (75 mL) solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.200 g, 0.284 mmol). The reaction mixture was stirred for 1 h to yield a dark green solution, and then, the reaction was quenched with the addition of a few drops of water. The solution was filtered through silica eluting with 30% CH3CN in CH2Cl2. Upon removal of the solvent, the oily residue was added to a cold Et2O (ca. 100 mL), yielding a dark green precipitate. Yield: 0.117 g (0.119 mmol), 42% based on Fe. Data for 2c: Anal. found (calcd) for C47H54N4Fe3SO3F3·H2O (2c·H2O): C, 57.11 (56.89); H, 5.49 (5.66); N, 5.62 (5.62). HR-nESI-MS (m/z, based on 56Fe): 830.239, corresponding to [M]+ (C46H54N4Fe3+, calcd 830.240). UV−vis (λmax, nm (ε, M−1 cm−1)): 605 (4100), 436 (8700). FT-IR (neat, υ(CC)/cm−1): 2203, 2158, and 2054. Cyclic voltammogram [E1/2/V, ΔEp/V, ibackward/iforward]: A, −0.650, 0.075, 0.99; B, 0.595, 0.087, 0.79. μeff = 1.98 μB. X-ray Data Collection and Structure Refinement for Crystal 2a. A single crystal of compound 2a was grown via slow diffusion of hexanes into a saturated CH2Cl2 solution of the desired compound. Xray diffraction data were collected on a Rigaku RAPID-II image plate diffractometer using Cu Kα radiation (λ = 1.54184 Å) at 150 K, and the structures were solved using the structure solution program DIRDIF200833 and refined using SHELX-TL.34 Crystal data of 2a: C39H46F3FeN4O3S, fw = 763.71, monoclinic, P21/n, a = 12.2898(4) Å, b = 11.5783(5) Å, c = 26.8427(10) Å, β = 94.162(3)°, V = 3809.5(3) Å3, Z = 4, Dcalc = 1.332 g cm−3, R1 = 0.066, wR2 = 0.172. Computational Details. By using the density functional theory method, the calculations were based on the model cations [2a′+] and [2b′+]. The model compound [2a′+] was fully optimized on the basis of the crystal structure of cation [2a+] without truncation. The model [2b′+] was built based on the crystal structure of [2a+] with the −Ph groups of [2a+] being replaced with −SiiPr3, and then followed by a full optimization. Both B3LYP35 and BP8636 were applied as exchange functionals, and the former produced more reliable results compared with experimental data. In the calculations, quasi-relativistic pseudopotentials of the Fe valence electrons were employed and the LanL2DZ basis sets associated with the pseudo-potential are adopted. All the calculations were performed using the Gaussian 03 program package.29 No negative frequency was observed in the vibrational
frequency analysis, which indicates that these iron(III) bis-gem-DEE complexes are metastable equilibrium structures.
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ASSOCIATED CONTENT
S Supporting Information *
X-ray crystallographic details (CIF) of 2a, high-resolution mass spectra of all compounds, DFT calculations for model compounds [2a′+] and [2b′+], and DPVs for compounds 2a−2c. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from both the National Science Foundation (CHE 1057621) and the Purdue Research Fund.
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