Article pubs.acs.org/Organometallics
trans-[Fe(cyclam)(C2R)2]+: A New Family of Iron(III) Bis-Alkynyl Compounds Zhi Cao, William P. Forrest, Yang Gao, Phillip E. Fanwick, and Tong Ren* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *
ABSTRACT: We previously communicated the preparation and characterization of two trans-[Fe(cyclam)(CCR)2]OTf compounds, 2b and 2c (where cyclam = 1,4,8,11-tetraazacyclotetradecane, R = −SiiPr3 (2b) or −Ph (2c), and OTf = trifluoromethanesulfonate), which were the first examples of Fe(III) bis-alkynyl complexes. In this work, the series has been expanded to include R = −H (2a), −C2SiMe3 (2d), −C4SiMe3 (2e), and −Fc (2f), which were prepared from the reaction between cis/trans-[Fe(cyclam)(OTf)2]OTf (1) and LiCCR (NaC CH for 2a). Compounds 2a−2f were characterized by spectroscopic/ voltammetric techniques as well as high-resolution mass spectrometry (HR-MS). The trans-orientation of the alkynyl ligands were established from the single-crystal X-ray diffraction studies of 2b−2d. Furthermore, the electronic structures of the model compounds 2a′+, 2d′+, and 2e′+ were analyzed with density-functional calculations, which revealed significant dπ−π(CC) interactions. flexibility.9−12 While these Ru 2 species are still being investigated with a focus on derivatives containing crossconjugated enediyne ligands, such as geminal-diethynylethene (gem-DEE),13 and their utility in the molecular passivation of semiconductor surfaces,14 we are seeking other structural motifs that would also favor the trans-orientation of two alkynyls. Of particular interest to us is the pseudo-octahedral motif based on 3d metal complexes of cyclam (1,4,8,11-tetraazacyclotetradecane). A number of Cr/Rh(III)-cyclam bis-arylethynyls (cis or trans) were reported by the laboratory of Wagenknecht, where the metal-centered emissions are noteworthy.15 Nishi and coworkers reported Cr(III)-cyclam compounds based on ethynyltetrathiafulvalenes and the weak ferromagnetism therein.16 Examples of M(cyclam) monoalkynyl complexes are limited to the cobalt(III) ethynylbenzene series by the laboratory of Shores.17 We recently communicated the preparation and characterization of two trans-[Fe(cyclam)(CCR)2]OTf-type compounds with R = −SiiPr3 (2b) and −Ph (2c), which were the first examples of bis-alkynyl compounds based on an Fe(III) core.18 In this contribution, the scope of the study has been expanded to include both the synthesis of new compounds, 2a and 2d−2f (Scheme 1), and the detailed characterizations of type 2 compounds via X-ray diffraction, DFT calculations, and voltammetric/spectroscopic studies.
T
ransition-metal alkynyl compounds have received intense interest as potential molecular wires and other electronic and optoelectronic materials since the pioneering work on the linear [M]-alkynyl polymers by Nast and Hagihara.1 Facile charge transfer along the [M]−(CC)m− linkage has been demonstrated for a number of metal centers in both bulk solution studies2,3 and nanojunction measurements.4 Many inspiring examples of iron monoalkynyl compounds as molecular wires have been reported by the laboratories of Lapinte3 and Akita,5 where the piano-stool-type Fe(II) centers, either CpFe(P−P)− or CpFe(CO)2−, are prevalent (type I in Chart 1). In comparison, iron bis-akynyl compounds, Chart 1. Common Fe Alkynyl Structures
developed primarily in the laboratory of Field, are rare and limited to Fe(II) centers with the alkynyl ligands in a transgeometry and bidentate chelating phosphines as the auxiliary ligands (type II in Chart 1).6,7 Recently, the electronic and magnetic interactions among Fe(III)(P−P)2 units connected by a 1,3,5-triethynylbenzene bridge were elegantly elaborated by the laboratory of Shores.8 Work from several laboratories including ours revealed that Ru2(II,III) and Ru2(III,III) alkynyl compounds are excellent building blocks for molecular wires due to their unrivaled redox © 2012 American Chemical Society
Received: June 9, 2012 Published: August 13, 2012 6199
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Scheme 1. Synthesis of Bis-alkynyl Fe(III) Cyclam Complexes
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RESULTS AND DISCUSSION As shown in Scheme 1, the synthesis of trans-[Fe(cyclam)(C CR)2]OTf-type compounds began with the preparation of cis/ trans-[Fe(cyclam)Cl2]Cl using the literature method,19 affording the cis- (yellow) and trans-isomers (brown) in yields of 86% and 8%, respectively. Subsequently, the reaction between cis[Fe(cyclam)Cl2]Cl and trifluoromethanesulfonic acid (HOTf) in large excess resulted in a cis/trans-[Fe(cyclam)(OTf)2]OTf (1) mixture (with the cis- isomer being predominant) in excellent yield (89%). The reaction between compound 1 and 2.2 equiv of NaCCH yielded compound 2a (45%) in 10 h, while those between 1 and 2.2 equiv of LiCCR led to compounds 2b (R = −SiiPr3), 2c (R = −Ph), 2d (R = −C2SiMe3), 2e (R = −C4SiMe3), and 2f (R = −Fc) in yields of 57%, 65%, 33%, 11%, and 63%, respectively. Both the type 1 and 2 compounds are paramagnetic and cannot be unambiguously characterized with 1H NMR spectroscopy. Instead, these compounds were authenticated by HR-nESI-MS and single-crystal X-ray diffraction studies (2b, 2c, and 2d). The room-temperature magnetic susceptibility measurement of 1 revealed an effective magnetic moment of 5.32 μB, which is slightly below the spin-only value for an S = 5/ 2 species (5.92 μB). The room-temperature effective magnetic moments of compounds 2a, 2b, 2c, 2d, 2e, and 2f are 1.90, 1.94, 1.89, 1.96, 1.93, and 1.92 μB, respectively, which are in agreement with an S = 1/2 ground state (1.73 μB). The contrast in the magnetic properties between 1 and type 2 compounds is attributed to the fact that alkynyls are strong-field ligands and stabilize a low-spin Fe(III) center, while −OTf ligands are weak-field ligands giving rise to a high-spin Fe(III) center. Single-crystal X-ray diffraction studies were performed for compounds 2b, 2c, and 2d, and the ORTEP plots of the cations are shown in Figures 1−3, respectively, with selected bond lengths and angles listed in Table 1. Compounds 2b, 2c, and 2d were crystallized in triclinic, monoclinic, and orthorhombic settings, respectively. The asymmetric unit of crystal 2b contains the halves of two independent cations. Since the two independent cations are very similar in geometries, the structural plot and selected key metric parameters are presented for only one of them herein. Each of the asymmetric units of crystals 2c and 2d contains one-half of the compound, which is related to the other half through an inversion center situated at the Fe(III) site. Similar to [Cr(cyclam)(CCAr)2]+-type compounds, the pseudo-octahedral coordination mode is adopted by all [Fe(cyclam)(CCR)2]+ species with the
Figure 1. ORTEP plot of [2b]+ at the 30% probability level. H atoms were omitted for clarity.
Figure 2. ORTEP plot of [2c]+ at the 30% probability level. H atoms were omitted for clarity.
Figure 3. ORTEP plot of [2d]+ at the 30% probability level. H atoms were omitted for clarity.
C−Fe−C vector approximately orthogonal to the plane defined by the four N centers of cyclam, as shown in Figures 1−3. The Fe−C bond lengths in [2b]+ (1.961(3) Å), [2c]+ (1.967(5) Å), and [2d]+ (1.935(8) Å) are in agreement with the Fe(II)−C bond lengths determined for the trans-Fe(P− P)2(CCR)-type compounds (1.92−1.97 Å).6,20,21 It is 6200
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Table 1. Selected Bond Lengths (Å) and Angles (deg) for Compounds [2b]+, [2c]+, and [2d]+ [2b]+
[2c]+
[2d]+
Fe1−C1 Fe1−N1 Fe1−N2 C1−C2 C2−Si1
1.961(3) 2.013(2) 2.007(2) 1.210(4) 1.833(3)
Fe1−C1 Fe1−N1 Fe1−N2 C1−C2 C2−C3
1.967(5) 1.999(4) 2.008(4) 1.211(7) 1.437(7)
C1−Fe1− C1A C1−Fe1− N1 C1−Fe1− N2 N1−Fe1− N2 Fe1−C1− C2
180.0(2)
C1−Fe1− C1A C1−Fe1− N1 C1−Fe1− N2 N1−Fe1− N2 Fe1−C1− C2
179.99(1)
91.63(1) 89.69(1) 94.57(9) 174.1(2)
88.23(2) 90.86(2) 84.84(2) 172.8(4)
Fe1−C1 Fe1−N1 Fe1−N2 C1−C2 C2−C3 C3−C4 C1−Fe1− C1A C1−Fe1− N1 C1−Fe1− N2 N1−Fe1− N2 Fe1−C1− C2
1.935(8) 2.002(6) 2.011(6) 1.230(8) 1.38(1) 1.223(9) 179.99(1) 89.7(3) 89.5(3) 86.0(3) 177.5(6)
noteworthy that the shortening of the Fe−C bond in [2d]+ is accompanied by the elongation of the Fe-bound CC bond (1.230 Å, the longest among the three structurally characterized compounds). Such a structural feature is indicative of an enhanced contribution of the cumulenic resonance structure in addition to the nominal acetylenic resonance structure.10,22 The redox activity of compounds 2a−2f is very relevant to the general interest in using trans-bis-alkynyl metal complexes as the building blocks for molecular wires and was examined carefully using cyclic voltammetric (CV) and differential pulse voltammetric (DPV) techniques. The CVs recorded are shown in Figure 4, while the DPVs are provided in the Supporting Information (Figure S3). There is no detectable oxidation couple up to +1.0 V for compounds 2a−2e, which reflects the electron deficiency of the Fe(III) species. All compounds undergo a reversible 1e− reduction (A), which is attributed to the reduction of Fe(III) to Fe(II). The formal potential of 2c is anodically shifted from those of 2a and 2b, indicating that the reduction of the Fe(III) center becomes easier with arylethynyl ligands due to an extended π-delocalization. Compounds 2d and 2e exhibit voltammagrams very similar to those of 2a−2c albeit with the reduction potentials anodically shifted, namely, −0.364 V for −(CC)2TMS (2d) and −0.215 V for −(C C)3TMS (2e).12 This drastic anodic shift is due to the increased electron-withdrawing nature of the alkynyl ligand with each additional acetylene unit.23 Previously, the redox activity of trans-Fe(DMPE)(CCPh)2 (DMPE = bis(dimethylphosphine)ethane) was investigated by Field and co-workers using cyclic voltammetry, and two consecutive 1e− oxidation couples, Fe(+3/+2) and Fe(+4/ +3), were observed at +0.01 and +0.99 V, respectively, versus SCE.20 In comparison, the Fe(+3/+2) couple in 2a−2c and 2f appears between −0.525 and −0.594 V versus Ag/AgCl, which is ca. 0.60 V more cathodic (after the correction for the difference in reference electrodes) than that of trans-Fe(DMPE)(CCPh)2. This clearly demonstrates that the Fe center can be selectively stabilized at the formal oxidation state of +2 or +3 by employing soft (P) or hard (N) auxiliary ligands, respectively. The presence of the two ferrocenyl (−Fc) termini in compound 2f offers a unique opportunity to assess the efficiency of electronic delocalization across the −CC−Fe− CC− fragment. Previously, the stepwise Fc oxidations with substantial ΔE1/2 were observed for −CC−M−CC−
Figure 4. Cyclic voltammograms recorded for compounds 2a−2f in a 0.20 M THF solution of Bu4NPF6.
fragments with M as Ru,24 [Cu3(dppm)3]3+,25 Ru3(dpa)4 (dpa = dipyridylamidate),26 and Ru2(DMBA)4 (DMBA = N,N′-dimethylbenzamidinate).11,12 For the latter compounds, a remarkably large ΔE1/2 of ca. 300 mV was observed, and the mixed-valent [Fc−Fc]+ ion was identified as a class III Robin− Day ion on the basis of spectroelectrochemical analysis. In contrast, the oxidation waves of Fc (B) are apparently not stepwise in both the CV (Figure 4) and DPV (Figure 5)
Figure 5. DPVs of the Fc oxidation region measured for 2f under both the standard conditions (dashed line) and conditions according to the Richardson−Taube method (solid line).
recorded under standard conditions, but broad due to the coalescence of two 1e− waves. To estimate the separation of the two 1e− waves, the Richardson−Taube method was employed.27 The DPV of 2f, recorded under the conditions stipulated by Richardson and Taube, yielded a half-width of 146 mV (Figure 5), which corresponds to a ΔE1/2 of 74 mV. Clearly, the {Fc---Fc}+ species in 2f is a class II Robin−Day mixed-valent ion at best,28 indicating that the −CC− Fe(cyclam)−CC− fragment is not as efficient as the 6201
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aforementioned Ru and Ru2 species in mediating electronic delocalization. As shown in Figure 6 for 2a, 2d, 2e, and 2f, type 2 compounds generally display structured d−d bands in their
Figure 7. Energies of the lowest energy absorption maxima of 2a, 2d, and 2e in CH3CN solutions and their reciprocal fit function with n as the number of (CC) units.
organic polyynes (expressed in the form of the power law E ≈ n−x).32 Compounds 2a−2f are quite different from the previously studied iron bis-alkynyl compounds in both the oxidation state (III versus II) and the auxiliary ligands (cyclam versus bidentate phosphines). In addition, the unusually intense d−d transitions observed for 2a−2f imply significant dπ−π(CC) interactions that typify CpFeL2(CCR) compounds.3,33 These observations prompted us to explore the electronic structure of type 2 compounds using density-functional theory calculations at the B3LYP/LanL2DZ level (Gaussian03 program).34 The calculations were performed on the model cations 2a′+, 2d′+, and 2e′+, where the number of −CC− units in the alkynyl ligand increases from 1 to 3. The optimized bond lengths and angles for 2d′+ are in good agreement with the crystal structural data of 2d+. The optimized bond lengths and angles for the first coordination sphere of 2a′+ and 2e′+ are comparable to those determined for 2b+, 2c+, and 2d+. The computed energies and counter plots of the most relevant MOs for the model compounds 2a′+, 2d′+, and 2e′+ are given in Figure 8, while a detailed list of the optimized geometries and MOs are given in the Supporting Information. The optimized Fe−C bond lengths of 2a′+ (1.939 Å) and 2d′+ (1.915 Å) are slightly shorter than the X-ray experimental values of 2b+ (1.961 Å) and 2d+ (1.935 Å). The C1C2 bond length of 2a′+ is elongated to 1.266 Å (versus 1.210 Å, X-ray value for 2b+), and the C1C2 and C3C4 bond lengths of 2d′+ are elongated to 1.270 and 1.254 Å (versus 1.230 and 1.223 Å, X-ray values for 2d+), respectively, due to the tendency of overestimation of electronic delocalization by the B3LYP method.35 With the additional −CC− units in the backbone, the optimized Fe−C bond length in 2e′+ is shortened to 1.905 Å due to the extended electronic delocalization. The DFT results of 2a′+, 2d′+, and 2e′+ are in agreement with the ligand-field theory prediction for a d5 species 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. Those closely related MOs, as well as energy levels, are plotted in Figure 8. The loss of orbital degeneracy for the eg and t2g sets is due to both the low symmetry of the cyclam ligand (C2 only) and Jahn−Teller instability (in both Oh and D4h settings). The two highest occupied MOs, namely, HOMO−1 and SOMO, consist of the antibonding combinations of dπ and π(CC), which are typical for transition-metal alkynyls.36 The contribution from the π(CC) orbitals becomes more prominent in 2d′+ and 2e′+ as the π-orbitals
Figure 6. UV−vis absorption spectra of compounds 2a, 2d, 2e, and 2f recorded in CH3CN.
UV−vis spectra that have extinction coefficients higher than values expected for centrosymmetric species. Analysis presented in the prior communication precluded vibronic sidebands as the origin of the observed fine structure.18 Instead, the observed transitions can be assigned using the strong-field limit of the Tanabe−Sugano diagram for a d5 ion:29 the spin-allowed excitations from the 2T2g ground state to the 2 T1g, 2A2g, 2Eg, and 2A1g excited states result in peaks at 425/ 559/637, 399/503/563, 374/457/503, and 333/414/454 nm in 2a/2d/2e, respectively. Structured d−d bands were also observed for the series of trans-[Cr(cyclam)(CCAr)2]+ arylethynyl complexes (where Ar = −C6H5, −C6H4CH3, −C6H4CF3, and −C6H5F), albeit with extinction coefficients about an order of magnitude smaller than those of compounds 2a−2f.15 The spectrum of 2f is also structured with absorptions at 444, 509, and 570 nm, with distinctly broadened peaks. While the exact cause of the broadening is undetermined, it is plausible that the presence of Fc reduces the effective symmetry and hence the further splitting of ligand field terms. The availability of trans-[Fe(cyclam)(C2nR)2]+-type complexes with n = 1, 2, and 3 allows for an examination of the energy dependence on the number of −CC− units and possible inference with the degree of π-delocalization. Specifically, the optical transition energy corresponding to the lowest energy absorption maxima in 2a, 2d, and 2e was plotted versus 1/n (Figure 7), and a linear relationship is apparent. It could be inferred by extrapolation that the lowest electron excitation energy (E(n)) for trans-[Fe(cyclam)(C2nR)2]+ would be about 1.44 eV as n approaches infinity. However, the πconjugation tends to reach saturation long before n approaches infinity in practice, and the saturation point n is often called the effective conjugation length.30 With a limited range of n, the πconjugation in trans-[Fe(cyclam)(C2nR)2]+-type complexes is far from saturation. It is worth noting that similar correlations were established for both the Re capped polyyn-diyls31 and 6202
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Figure 8. Molecular orbital diagrams for model compounds 2a′+, 2d′+, and 2e′+ (from left to right). The directions pointed toward the N atoms of the cyclam ring are designated as the x and y axes.
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extend over a longer framework, as shown in the HOMO−1 and SOMO. Similar trends were noted for the piano stool type Fe(II) monoalkynyl compounds3 and diplatinum bridged by polyyne-diyls.37 The LUMO is dominated by the dx2−y2 and N lone pairs with no contribution from the acetylenic units due to the orbital orthogonality. The LUMO+1 is best described as σ*(Fe−C) with the dominant contribution from the dz2 orbital. The SOMO−LUMO gaps calculated for 2a′+ (2.86 eV), 2d′+ (2.33 eV), and 2e′+ (2.05 eV) are fairly close to the d−d transition optical gaps for 2a (425 nm, or 2.92 eV), 2d (559 nm, or 2.22 eV), and 2e (637 nm, or 1.95 eV), respectively. It is clear from Figure 8 that the SOMOs become progressively stabilized with the increasing number of −CC− units. This trend is in agreement with the observed anodic shift of reduction potentials from 2a to 2e. In addition, the pronounced π(CC) contributions to both the SOMO and the HOMO−1 offer the possibility of d−d bands “stealing” intensity. It should be noted that the data presented here are based on ground-state calculations, and more rigorous comparison should be based on TD-DFT-type calculations.
EXPERIMENTAL SECTION
General Procedures. cis-[Fe(cyclam)Cl2]Cl was prepared as previously described.19 1,4,8,11-Tetraazacyclotetradecane was purchased from Strem Chemicals, and anhydrous iron(II) chloride, triflic acid, sodium acetylide (suspended in mineral oil), and n-BuLi (2.5 M in hexanes) were from Sigma-Aldrich. The alkynyl ligands, triisopropylsilylacetylene, 1,4-bis(trimethylsilyl)-2,3-butadiyne, and phenylacetylene were purchased from GFS Chemicals. Ethynylferrocene and 1,6-bis(trimethylsilyl)-1,3,5-hexatriyne were prepared according to the literature procedures.38 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 CH2Cl2/MeOH (50/50, v/v) (Q-Star Pulsar XL; Applied Biosystems/Sciex, Concord, ON, Canada). Masses were calculated by isotopic distribution utilizing Analyst 1.5 software (Applied Biosystems/Sciex, Concord, ON, Canada). 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 is always 1.0 mM. The ferrocenium/ ferrocene couple was observed at 0.570 V (vs Ag/AgCl) at the noted experimental conditions. Preparation of cis/trans-[Fe(cyclam)OTf2]OTf (1). A flask fitted with a nitrogen bubbler was charged with cis-[Fe(cyclam)Cl2]Cl (0.453 g, 1.17 mmol) and 10 g (66.6 mmol) of triflic acid. Nitrogen was bubbled through the green solution for 40 h. The resulting light yellow solution was transferred to a large beaker, to which was added 150 mL of anhydrous ether. Upon stirring and scratching, a light yellow powder formed, which was collected via filtration. Yield: 0.70 g (0.995 mmol), 89% based on Fe. HR-nESI-MS (m/e, based on 56Fe): 554.041, corresponding to [M]+ (C12H24F6S2O6N4Fe1+, calcd.
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CONCLUSION A new family of Fe(III) bis-alkynyl compounds 2a−2f was prepared under mild conditions. Strong π-interactions between the occupied dπ and π(CC) orbitals are evident from both the absorption spectra and the DFT calculations. The formal potential of the Fe(+3/+2) couple is highly dependent on the number of acetylenic (−CC−) units in the −(CC)n−R ligand. Type 2 compounds generally display intense and structured bands in the visible absorption spectra, which are assigned as spin-allowed d−d transitions. Ongoing efforts in our laboratory focus on the preparation of both trans-[Fe(cyclam)(C2nR)2]+ (n > 3) and trans-[Fe(cyclam)(gem-DEE)2]+-type compounds and the elaboration of the π-conjugation therein. 6203
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Table 2. Crystal Data for Compounds 2b, 2c, and 2d 2b·C2H6O molecular formula fw space group a, Å b, Å c, Å α, deg β, deg γ, deg V, Å3 Z ρcalcd, g cm−3 μ, mm−1 T, K no. of rflns collected no. of indep rflns final R indices (I > 2σ(I)) GOF on F2
2c
C35H72F3FeN4O4SSi2 814.07 P1(̅ No. 2) 9.2957(5) 16.0961(6) 17.2103(9) 106.581(5) 105.399(4) 104.772(4) 2221.8(1) 2 1.217 4.096 150 40 150 6834 (R(Int) = 0.038) R1 = 0.051 wR2 = 0.140 1.038
2d
C27H34F3FeN4O3S 607.50 P21/c (No. 14) 9.5167(6) 11.8798(6) 12.2650(9)
C25H42F3FeN4O3SSi2 647.72 Pbca (No. 61) 12.0900(11) 11.6199(11) 24.842(3)
90.108(5) 1386.6(1) 2 1.455 5.556 150 13 139 1940 (R(Int) = 0.069) R1 = 0.076 wR2 = 0.216 1.038
554.039). UV−vis, λmax (nm, ε (M−1 cm−1)): 511 (640), 498 (770), 458 (800), 372 (4100), 334 (9900). FT-IR (ν, cm−1): 887 (N−H bending and CH2 rocking region for trans) and 858 (N−H bending and CH2 rocking region for cis). μeff = 5.32 μB. Anal. Found (calcd) for C13H24N4Fe1S3O9F9·CF3SO3 (1·CF3SO3H): C, 19.78 (19.63); H, 3.02 (2.94); N, 6.91 (6.54). Preparation of trans-[Fe(cyclam)(C2H)2]OTf (2a). To an 80 mL THF solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.250 g, 0.355 mmol) was added 2.2 equiv of NaC2H (0.781 mmol in THF) at room temperature. The reaction mixture was stirred for 10 h to yield a redorange 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 into cold Et2O (ca. 100 mL), yielding a pink precipitate. Yield: 0.072 g (0.158 mmol), 45% based on Fe. HR-nESIMS (m/e, based on 56 Fe): 306.152, corresponding to [M] + (C14H26N4Fe1+, calcd. 306.150). UV−vis, λmax (nm, ε (M−1 cm−1)): 425 (4800), 399 (4200), 374 (2800), 333 (2900). FT-IR (ν(CC), cm−1): 2038 and 889 (N−H bending and CH2 rocking region for trans). Cyclic voltammogram [E1/2/V, ΔEp/V, ibackward/iforward]: A, −0.594, 0.052, 0.85. μeff = 1.90 μB. Anal. Found (calcd) for C15H26N4Fe1SO3F3 (2a): C, 39.51 (39.31); H, 5.69 (5.72); N, 12.35 (12.22). Preparation of trans-[Fe(cyclam)(C2SiiPr3)2]OTf (2b). To an 80 mL THF solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.250 g, 0.355 mmol) was added 2.2 equiv of LiC2SiiPr3 (0.781 mmol in THF, prepared in situ from n-BuLi and HC2SiiPr3) at room temperature. The reaction mixture was stirred for 1 h to yield a dark red 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 into cold Et2O (ca. 100 mL), yielding a dark yellow precipitate. Yield: 0.156 g (0.203 mmol), 57% based on Fe. HR-nESI-MS (m/e, based on 56Fe): 618.421, corresponding to [M]+ (C32H66N4Si2Fe1+, calcd 618.417). UV−vis, λmax (nm, ε (M−1 cm−1)): 470 (4800), 433 (3000), 404 (1300), 364 (650). FT-IR (ν(CC), cm−1): 2015 and 883 (N−H bending and CH2 rocking region for trans). Cyclic voltammogram [E1/2/V, ΔEp/V, ibackward/iforward]: A, −0.585, 0.062, 0.96. μeff = 1.94 μB. Anal. Found (calcd) for C33H66N4Si2Fe1SO3F3·H2O (2b·H2O): C, 50.04 (50.24); H, 7.93 (8.69); N, 7.41 (7.10). Preparation of trans-[Fe(cyclam)(C2Ph)2]OTf (2c). To an 80 mL THF solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.250 g, 0.355 mmol) was added 2.2 equiv of LiC2Ph (0.781 mmol in THF, prepared in situ from n-BuLi and HC2Ph) at room temperature. The reaction
3489.9(6) 4 1.233 5.076 150 15 312 2166 (R(Int) = 0.103) R1 = 0.095 wR2 = 0.252 1.039
mixture was stirred for 1 h to yield a dark red 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 into cold Et2O (ca. 100 mL), yielding a magenta precipitate. Yield: 0.139 g (0.229 mmol), 65% based on Fe. HR-nESI-MS (m/e, based on 56Fe): 458.211, corresponding to [M]+ (C26H34N4Fe1+, calcd 458.213). UV− vis, λmax (nm, ε (M−1 cm−1)): 555 (4300), 500 (3200), 443 (2000), 407 (1500). FT-IR (ν(CC), cm−1): 2077 and 889 (N−H bending and CH2 rocking region for trans). Cyclic voltammogram [E1/2/V, ΔEp/V, ibackward/iforward]: A, −0.525, 0.035, 0.930. μeff = 1.89 μB. Anal. Found (calcd) for C27H34N4Fe1SO3F3 (2c): C, 53.24 (53.12); H, 5.44 (5.61); N, 9.16 (9.18). Preparation of trans-[Fe(cyclam)(C4TMS)2]OTf (2d). To an 80 mL THF solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.250 g, 0.355 mmol) was added 2.2 equiv of LiC4TMS (0.781 mmol in THF, prepared in situ from n-BuLi and Me3SiC4SiMe3) at room temperature. The reaction mixture was stirred for 1 h to yield a dark red 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 into cold Et2O (ca. 100 mL), yielding a magenta precipitate. Yield: 0.075 g (0.116 mmol), 33% based on Fe. HR-nESI-MS (m/e, based on 56Fe): 498.229, corresponding to [M]+ (C24H42N4Si2Fe1+, calcd 498.229). UV−vis, λmax (nm, ε (M−1 cm−1)): 559 (4000), 503 (3300), 457 (2100), 414 (1700). FT-IR (ν(CC), cm−1): 2162, 2125, 2080, 2010, and 889 (N−H bending and CH2 rocking region for trans). Cyclic voltammogram [E1/2/V, ΔEp/V, ibackward/iforward]: A, −0.364, 0.035, 0.98. μeff = 1.96 μB. Anal. Found (calcd) for C25H42N4Fe1Si2SO3F3 (2d): C, 46.18 (46.15); H, 6.61 (6.51); N, 8.61 (8.61). Preparation of trans-[Fe(cyclam)(C6TMS)2]OTf (2e). To an 80 mL THF solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.250 g, 0.355 mmol) was added 2.2 equiv of LiC6TMS (0.781 mmol in THF, prepared in situ from n-BuLi and Me3SiC6SiMe3) at room temperature. The reaction mixture was stirred for 1 h to yield a dark red 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 into cold Et2O (ca. 100 mL), yielding a dark green precipitate. Yield: 0.027 g (0.039 mmol), 11% based on Fe. HR-nESI-MS (m/e, based on 56Fe): 546.228, corresponding to [M]+ (C28H42N4Si2Fe1+, calcd 546.229). UV−vis, λmax (nm, ε (M−1 cm−1)): 637 (3700), 563 (1800), 503 (870), 444 (720). FT-IR (ν(CC), cm−1): 2142, 2117, 2034, 2000, and 889 (N−H bending and CH2 rocking region for 6204
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Organometallics trans). Cyclic voltammogram [E1/2/V, ΔEp/V, ibackward/iforward]: A, −0.215, 0.040, 0.78. μeff = 1.93 μB. Anal. Found (calcd) for C29H42N4Si2Fe1SO3F3·2CF3SO3H·CH2Cl2 (2e·2CF3SO3H·CH2Cl2): C, 35.68 (35.46); H, 4.00 (4.28); N, 5.40 (5.17). Preparation of trans-[Fe(cyclam)(C2Fc)2]OTf (2f). To an 80 mL THF solution of cis/trans-[Fe(cyclam)(OTf)2]OTf (0.250 g, 0.355 mmol) was added 2.2 equiv of LiC2Fc (0.781 mmol in THF, prepared in situ from n-BuLi and HC2Fc) at room temperature. The reaction mixture was stirred for 1 h to yield a dark red 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 into cold Et2O (ca. 100 mL), yielding a dark purple precipitate. Yield: 0.185 g (0.225 mmol), 63% based on Fe. HR-nESI-MS (m/e, based on 56 Fe): 674.145, corresponding to [M]+ (C34H42N4Fe3+, calcd 674.145). UV−vis, λmax (nm, ε (M−1 cm−1)): 570 (3000), 509 (3000), 444 (3100). FT-IR (ν(CC), cm−1): 2071 and 887 (N−H bending and CH2 rocking region for trans). Cyclic voltammogram [E1/2/V, ΔEp/V, ibackward/iforward]: A, −0.580, 0.052, 0.71.; B, 0.663, 0.017, 0.70. μeff = 1.92 μB. Anal. Found (calcd) for C35H42N4Fe3SO3F3 (2f): C, 50.82 (50.51); H, 4.96 (5.09); N, 6.85 (6.73). X-ray Data Collection, Processing, Structure Analysis, and Refinement for Crystals 2b, 2c, and 2d. Single crystals of compounds 2b, 2c, and 2d were grown via slow diffusion of a saturated CH3CN solution of the desired compound into an Et2O/hexanes (9:1, v/v) solution. X-ray 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 DIRDIF200839 and refined using SHELXTL.40 Relevant information on the data collection and figures of merit of final refinement are listed in Table 2.
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REFERENCES
We gratefully acknowledge the financial support from both the National Science Foundation (CHE 1057621) and the Purdue Research Fund. T.R. acknowledges the IR/D support from the National Science Foundation during the preparation of the manuscript.
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By using density functional theory method, the calculations were based on the model cations 2a′+, 2d′+, and 2e′+. The model compound 2d′+ was optimized on the basis of the crystal structure of cation 2d+ without truncation. The model 2a′+ was built on the basis of the crystal structure of 2b+ with the −SiiPr3 groups of 2b+ being replaced with −H. The model 2e′+ was optimized from the crystal structure of 2d+ with the insertion of one −CC− unit between the −SiMe3 and Fe−CC−CC− fragments. Both B3LYP and BP8641 were applied as exchange−correlation functionals, and the former produced more reliable results compared with experimental data. In the calculations, quasi-relativistic pseudo-potentials of the Fe valence electrons were employed and the LanL2DZ basis sets associated with the pseudopotential are adopted. All the calculations were performed using the Gaussian03 program package.34 No negative frequency was observed in the vibrational frequency analysis, which indicates that these Fe(III) bis-alkynyl complexes are stable equilibrium structures.
S Supporting Information *
High-resolution mass spectra of all compounds, UV−vis spectra of compounds 1, 2b, and 2c, DFT calculations for model compounds 2a′+/2d′+/2e′+, and DPVs for compounds 2a−2e; X-ray crystallographic details (CIF) of 2b, 2c, and 2d. 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. 6205
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Organometallics
Article
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