Pyridine

Article pubs.acs.org/Organometallics

Iron Complexes of a Macrocyclic N‑Heterocyclic Carbene/Pyridine Hybrid Ligand Iris Klawitter,† Markus R. Anneser,‡ Sebastian Dechert,† Steffen Meyer,† Serhiy Demeshko,† Stefan Haslinger,‡ Alexander Pöthig,‡ Fritz E. Kühn,*,‡ and Franc Meyer*,† †

Institute of Inorganic Chemistry, Georg-August-University Göttingen, Tammannstraße 4, D-37077 Göttingen, Germany Chair of Inorganic Chemistry/Molecular Catalysis, Department of Chemistry and Catalysis Research Center, Technische Universität München, Ernst-Otto-Fischer-Straße 1, D-85747 Garching bei München, Germany

‡

S Supporting Information *

ABSTRACT: The tetradentate ligand system L1 combines two Nheterocyclic carbene (NHC) and two pyridine donor functions in a macrocyclic scaffold. Its iron(II) complex [FeL1(MeCN)2](PF6)2 (1) has been synthesized and fully characterized. The macrocyclic ligand in 1 is puckered and shows a significant barrier for ring inversion (ΔH⧧ = 15.1 kcal mol−1, and ΔS⧧ = −4.7 cal mol−1 K−1). Axial ligands in 1 can be readily substituted to give heteroleptic [FeL1(CO)(MeCN)](PF6)2 (2) or neutral [FeL1(N3)2] (3). The strong ligand field of the NHC/pyridine hybrid ligand imparts low-spin states (S = 0) for all iron(II) complexes 1−3. Mössbauer data reflect the asymmetric electronic situation that results from the strongly covalent Fe−CNHC bonds in the basal plane constituted by the macrocyclic ligand L1. Oxidation of 1 has been monitored by UV−vis spectro-electro chemistry, and the resulting iron(III) complex [FeL1(MeCN)2](PF6)3 (4) has been isolated after chemical oxidation. SQUID and Mössbauer data have shown an S = 1/2 ground state for 4, and X-ray crystallographic analyses of 1 and 4 revealed that structural parameters of the {FeL1} core are basically invariant with respect to changes in the metal ion’s oxidation state. Density functional theory calculations support the experimental findings. The combined structural, spectroscopic, and electrochemical data for 1 with its {C2N2} hybrid ligand L1 allowed for useful comparison with the related iron(II) complex that has a macrocyclic {C4} tetracarbene ligand. respective imidazolium salts with the basic iron(II) precursor [Fe{N(SiMe3)2}2]2.24−27 The NHC unit has been combined advantageously with pyridine as the N-donor moiety in chelating scaffolds of various topologies and with different denticities. Selected known NHC/N-donor proligands are shown in Figure 1 and range from tri- and tetradentate macrocyclic systems, [H2L1]2+28 or [H2L2]2+,29 to pentadentate NHC/py4 ligands [HL3]+,30 tetradentate bis(NHC)s [H2L4]2+,31 tridentate NHC/py2 ligands [HL5]+,32 and pyridine- or pyrimidine-substituted imidazolium salts like [HL6]+.32b,33 As an example of the beneficial use of such hybrid ligands in iron chemistry, we note that the ferric complex of the open chain NHC/pyridine ligand L4b has been shown to serve as a catalyst for aromatic hydroxylation34 and olefin epoxidation35 reactions. The particular NHC/pyridine hybrid proligand [H2L1](PF6)2 combines each two pyridine-N and imidazol-2-yliden donors in mutual trans disposition in a macrocyclic ligand system; so far, [H2L1](PF6)2 has been used for only the synthesis of Ag(I),28 Au(I),36 Ni(II),37 Hg(II),38 and Pd(II)39 complexes. Considering that natural heme monooxygenases such as cytochrome P450 contain a rather rigid cyclic porphyrin

N-Heterocyclic carbenes (NHCs) have attracted a great deal of attention over the past two decades. NHCs, most of them imidazol-2-ylidenes, are now established as a prime ligand class in organometallic chemistry, with numerous applications in homogeneous catalysis.1−4 Among their favorable characteristics are the strong σ-donor ability and the high stability of the resulting complexes. Appending ancillary coordinating donor side arms to the imidazolium ring has been exploited to further support the NHC−metal ion binding, and for tuning the electronic structure of the NHC−metal unit.5−7 Furthermore, two or more imidazol-2-ylidene subunits have been joined to create various multidentate NHC ligands, ranging from chelating biscarbenes8 to macrocyclic tetracarbenes.9 In catalytic applications, NHC complexes of late transition metals such as rhodium and palladium are particularly abundant,10−13 though base metal NHC catalysts are now catching up rapidly. While first examples of Fe−NHC complexes were introduced during the 1970s,14,15 the first molecular structure was published in 1996.16 Fe−NHC complexes have received much attention in recent years17 because of their great potential in catalysis18,19 and in bioinspired chemistry20,21 as well as for the stabilization of reactive intermediates and unusual iron oxidation states.22−24 A common method for synthesizing iron(II)−NHC complexes is the reaction of the © XXXX American Chemical Society

Received: February 5, 2015

A

DOI: 10.1021/acs.organomet.5b00103 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

in close analogy to previously described protocols for the preparation of iron(II)−NHC systems, including A.23−25 Treatment of [H2L](PF6)2 with [Fe{N(SiMe3)2}2]2 in MeCN gave [FeL1(MeCN)2](PF6)2 (1) in reasonable yield (65−70%). Its formation was indicated by ESI mass spectrometry showing a dominant signal for the dication [FeL1]2+ at m/z 199.0. Brownish-yellow crystals of 1·MeCN suitable for X-ray diffraction were obtained from MeCN/Et2O solutions. 1·MeCN crystallizes in triclinic space group P1̅; its molecular structure is shown in Figure 3, and selected distances and

Figure 3. Molecular structure of the cation of 1 (30% probability thermal ellipsoids). Anions and hydrogen atoms have been omitted for the sake of clarity.

Table 1. Selected Bond Lengths and Angles of 1 and 4 Figure 1. Selected NHC/pyridine hybrid ligands reported in the literature.

Bond Lengths (Å) 1

40

Fe−C1 Fe−C11 Fe−N1 Fe−N4 Fe−N7 Fe−N8

1

ligand system, iron complexes of macrocyclic ligand L represent a promising alternative to the more flexible open chain NHC/pyridine-based systems for applications in oxidation and oxygenation catalysis. Herein, we report on the synthesis and characterization of the first iron complexes based on [H2L1](PF6)2. The resulting complexes are analyzed by a variety of methods, including their molecular structure determination by X-ray diffraction in selected cases, and the potential of the L1 ligand for supporting metal ion redox chemistry is investigated. In particular, the iron(II) complex of the NHC/pyridine hybrid ligand L1 allows for useful comparison with the related macrocyclic tetracarbene complex [FeLMTC(MeCN)2](OTf)2 [A (Figure 2)], which recently gave rise to novel high-valent iron chemistry.24,41

N1−Fe−C1 C1−Fe−N4 N4−Fe−C11 C11−Fe−N1 C1−Fe−C11 N1−Fe−N4 N7−Fe−N8

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RESULTS AND DISCUSSION Synthesis and Characterization of Iron(II) Complexes. The syntheses of a first iron(II) complex of L1 were conducted

4

1.936(4) 1.939(4) 2.022(3) 2.032(3) 1.933(3) 1.941(3) Bond Angles (deg)

1.923(4) 1.929(4) 2.055(3) 2.044(3) 1.922(5) 1.921(5)

1

4

89.92(14) 89.91(14) 89.70(14) 90.47(15) 179.59(17) 177.68(13) 178.81(14)

90.16(17) 89.81(17) 90.04(14) 90.05(14) 178.6(2) 177.44(16) 177.79(16)

angles are listed in Table 1. The metal ion is found to be sixcoordinated by the tetradentate ligand L1, establishing a square planar {N2C2} coordination in the basal plane (the distance from the iron atom to the best plane of the coordinating atoms is 0.03 Å) and two axial MeCN ligands. The nearly perfect octahedral environment is reflected by the almost right-angle bonds (Table 1). Both Fe−CNHC distances are found to be around 1.94 Å and are thus slightly longer than those in the related NHC/pyridine systems [FeL 4 b (MeCN) 2 ] 2 + , [FeL6(MeCN)2]2+, and [Fe(L5)2]2+ (1.84−1.93 Å31c,42), albeit shorter than the Fe−CNHC bonds of the macrocyclic tetracarbene complex A (1.97−2.02 Å).24 The Fe−N py distances in complex 1 are 2.022 and 2.032 Å and in the range of the Fe−Npy bond lengths of the complexes mentioned

Figure 2. Macrocyclic tetracarbene iron(II) complex [FeLMTC(MeCN)2](OTf)2 (A). B


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Organometallics

the open chain ligand L4c, which has only a single ethylene bridge, undergoes a related conformational change, and a very similar barrier of ΔH ⧧ = 14.4 kcal mol −1 could be determined.31c This may indicate that the flip of the two ethylene bridges in 1 occurs largely independently. However, according to NMR spectroscopy, the macrocyclic nickel(II) complex [NiL1]2+, having a low-spin d8 metal ion in a squareplanar environment and puckered ligand conformation with apparent D2 symmetry similar to that of 1, is rigid and shows no ring flip on the NMR time scale even at 80 °C.37 Somewhat surprisingly, no obvious signal for the CN stretch of the axial MeCN ligands can be detected in the IR spectrum of 1; only an extremely weak band is discernible at 2253 cm−1 (see Figure S6 of the Supporting Information). The axial MeCN ligands can be readily substituted as shown by the reaction with CO that, at a CO pressure of 2.5 bar, gives [FeL1(MeCN)(CO)](PF6)2 (2), and by the reaction with [nBu4N]N3 giving the neutral bis(azido) complex [FeL1(N3)2] (3) (Scheme 1).

above (1.924−2.096 Å), but they are slightly longer than the apical Fe−NMeCN bonds. The two imidazol-2-ylidene rings and the two pyridine rings of L1 are not coplanar, but the macrocycle is severely puckered and adopts an overall saddlelike shape with approximate (non-crystallographic) D2 symmetry. NMR spectroscopy of 1 in d6-DMSO or d3-MeCN shows signals in the range expected for a diamagnetic compound. The zero-field 57Fe Mössbauer spectrum of solid 1 at 80 K (Figure 4) shows a quadrupole doublet with an isomer shift δ = 0.32

Scheme 1. Synthesis of [FeL1(MeCN)(CO)](PF6)2 (2) and [FeL1(N3)2] (3) Figure 4. Zero-field 57Fe Mössbauer spectrum of solid 1 at 80 K (natural abundance 57Fe). The red line represents a simulation with δ = 0.32 mm s−1 and ΔEQ = 3.12 mm s−1.

mm s−1 and with a large quadrupole splitting ΔEQ = 3.12 mm s−1 (Figure 4). The small isomer shift is in accordance with a low-spin (S = 0) iron(II) ion. While the quadrupole splitting for a low-spin d6 ion is expected to be small considering only the point charge distribution (crystal-field theory) in a nearly ideal octahedral ligand environment, in this particular case one clearly has to consider pronounced covalency effects originating from the very strong σ-donor character of the two carbenes as well as from additional contributions like axial or rhombic distortions.43 The large quadrupole splitting thus reflects the very different σ-donor character of the NHC, pyridine, and MeCN donors in 1. These Mössbauer parameters are indeed well reproduced by the density functional theory (DFT)calculated values for 1 [δcalc = 0.30 mm s−1, and ΔEQcalc = 2.99 mm s−1 (see the Supporting Information for details)]. The 13C NMR resonance of the coordinating carbene C atoms features a characteristic chemical shift at low field [204.1 ppm; compare δ (13C) = 197.8 ppm for the carbene C atom in A]. The presence of two 1H NMR doublets (J = 16.7 Hz) for the macrocyclic CH2 linkers, each doublet integrating to 4H, suggests that the saddle-shaped conformation of the macrocycle with apparent D2 symmetry is retained in solution and does not rapidly equilibrate via a planar (D2h) conformation on the NMR time scale at room temperature. However, dynamic behavior is evident from variable-temperature NMR spectra recorded at higher temperatures for d3-MeCN solutions of 1 (see Figure S1 of the Supporting Information) that show coalescence of the two 1H NMR doublets representing the CH2 linkers at a Tc of ∼350 K. A sizable Gibbs energy of activation (ΔG⧧ = 16.5 kcal mol−1) could be determined for the barrier of the conformational ring inversion (which possibly involves a planar D2h transition state). Via analysis of the temperature dependence of rate constant k of this ring flip, the following activation parameters could be determined: ΔS⧧ = −4.7 cal mol−1 K−1, and ΔH⧧ = 15.1 kcal mol−1 (see Figure S4 of the Supporting Information). The iron(II) complex [FeL4c]2+ of

In compound 2, CO coordination is reflected by a new intense IR absorption at 1974 cm−1 and a new 13C NMR resonance at 187.2 ppm that is slightly shifted downfield compared to non-coordinated carbon monoxide (185.2 ppm31c). These values suggest significant Fe(II) → CO πbackbonding from the relatively electron rich metal ion. The 13 C NMR signal for the carbene C atoms in 2 is significantly shifted to lower field compared to the signal of those of 1, which may reflect the fact that any Fe(II) → NHC πbackbonding is suppressed in the presence of the much better π-acceptor CO. Unfortunately, no single crystals for X-ray diffraction could be obtained, but the presence of one CO and one MeCN as axial ligands is corroborated by elemental analysis and NMR data. Assuming a puckered molecular structure akin to that observed for 1, the symmetry of 2 is lowered to C2. In accordance with a relatively high inversion barrier, four 1H NMR doublets (J = 17.2 Hz) are indeed observed for the macrocyclic CH2 linkers. Azide coordination in 3 is evidenced by a strong IR absorption at 2023 cm−1 that lies within the typical range for the antisymmetric N3− stretching vibrations of other octahedral ferrous complexes with trans azido ligands (2014−2051 cm−1 44−46). Furthermore, this value is in reasonable agreement C


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Organometallics

iron(II) state is efficiently stabilized by the NHC/pyridine hybrid macrocycle L1. To confirm that the observed redox process is metalcentered, a sample of 1 was oxidized chemically by adding 1.2 equiv of thianthrene cation radical perchlorate (Th•+ClO4−), a strong oxidant, in acetonitrile at 0 °C and stirring the mixture for 1 h. The zero-field 57Fe Mössbauer spectrum of the resulting solution of oxidized 1 in acetonitrile, frozen at 80 K, shows a doublet characteristic of a low-spin iron(III) center with δ = 0.13 mm s−1 and ΔEQ = 2.48 mm s−1 (see Figure S7 of the Supporting Information). Bulk material of dark blue [FeL1(MeCN)2](PF6)3 (4) could be isolated when using Th•+PF6−, and single crystals were obtained from MeCN/ Et2O solutions. The molecular structure of the cation of 4, determined by X-ray diffraction, is depicted in Figure S21 of the Supporting Information, and selected atom bond lengths and angles are listed in Table 1. An overlay of the cations of 1 and 4 is shown in Figure 6. Overall, the molecular structures of

with that calculated for a DFT-optimized molecular structure of 3 [2050 cm−1 (see Figures S13−S15 of the Supporting Information)]. Mössbauer parameters for neutral 3 [δ = 0.39 mm s−1, and ΔEQ = 3.24 mm s−1 (see Figure S6 of the Supporting Information); DFT-calculated values δcalc = 0.30 mm s−1, and ΔEQcalc = 3.20 mm s−1] resemble those of 1. The similarity of the Mössbauer parameters for 1 and 3 suggests that the axial ligands have only a minor effect on the charge density and distribution of the iron ion, which had also been observed for ferric cyclam complexes with different ligands in axial positions.44 In contrast, the related macrocyclic tetracarbene iron(II) complex A (low-spin, δ = 0.23 mm s−1, and ΔEQ = 2.10 mm s−1)24 and the four-coordinate square-planar iron(II) complex of Zlatogorsky et al. featuring two chelating bis(imidazol-2-yliden) ligands (δ = 0.18 mm s−1, and ΔEQ = 4.16 mm s−1)26 show isomer shifts significantly smaller than those of 1 and 3, because δ decreases with a higher covalency and shorter metal−ligand bonds,43,47 and 1 and 3 have only two strongly covalent Fe−CNHC bonds compared to four Fe−CNHC bonds in A. The observed large quadrupole splittings seem unusual for a formally symmetric low-spin d6 configuration, but they reflect the asymmetric electronic situation that results from the strongly covalent Fe−CNHC bonds in the basal plane. Redox Properties of 1 and Characterization of an Iron(III) Complex. The cyclic voltammogram (CV) of complex 1, measured in an acetonitrile solution containing 0.1 M [nBu4N]PF6, is shown in Figure 5 together with the CV

Figure 6. Overlay of the cations of 1 (blue) and 4 (red).

cations [FeIIL1(MeCN)2]2+ and [FeIIIL1(MeCN)2]3+ are very similar. Fe−CNHC distances are slightly shorter in the ferric complex [1.923(4)/1.929(4) Å in 4 vs 1.939(4)/1.938(4) Å in 1], while Fe−Npy distances are slightly longer [2.055(3)/ 2.044(3) Å in 4 vs 2.022(3)/2.032(3) Å in 1]. The 57Fe Mössbauer spectrum of crystalline material of 4 is essentially identical with the spectrum of the frozen MeCN solution of freshly oxidized 1 (Figure 7), indicating that the structure of 4 determined by X-ray diffraction is retained in solution. The asymmetric line broadening results from paramagnetic relaxation that is typical for many ferric compounds with moderate dipole and exchange interactions.43

Figure 5. Cyclic voltammograms (left) of 1 (green) and A (red) in acetonitrile (0.1 M nBu4NPF6) at 100 mV/s. Current of the anodic peak for 1 vs the square root of the scan rate (right).

of the related tetracarbene iron(II) complex A whose electrochemical properties have not yet been reported. For 1, a one-electron redox process is observed at Eox = 1.4 V versus NHE and assigned to the [FeIIL1]2+/[FeIIIL1]3+ couple; the linear dependence of the current on the square root of the scan rate and a peak-to-peak separation of 70 mV are indicative of an electrochemically reversible process (see the Supporting Information for details). However, the low current ratio between the reverse and forward peaks (irp/ifp = 0.4) suggests that the electrode reaction is not fully chemically reversible. The [FeIIL1]2+/[FeIIIL1]3+ peak potential for 1 is slightly higher than those of the [FeL4(MeCN)2]2+-type complexes (∼1.1 V31c), likely reflecting the larger structural flexibility of the open chain ligand L4. Tetracarbene complex A is already oxidized at a much lower potential of 0.79 V versus NHE, and the process is fully reversible (ΔEp = 95 mV). Compared to 1, the change of the macrocyclic ring size, together with the replacement of two NHC units by pyridine, leads to a large anodic shift of the oxidation potential of >500 mV. This reflects the fact that the

Figure 7. Zero-field 57Fe Mössbauer spectrum of solid 4 at 80 K (natural abundance 57Fe). The red line represents a simulation with δ = 0.13 mm s−1 and ΔEQ = 2.47 mm s−1, line width Γ = 0.76 mm s−1, ratio of intensity left (Il) to intensity right (Ir) (Il:Ir) = 0.87, and ratio of line width left (Γl) to line width right (Γr) (Γl:Γr) = 1.46. D


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Organometallics DFT calculations for the cation of 4, [L1Fe(MeCN)2]3+, reveal that a doublet ground state is only slightly lower in energy than the quartet state (S = 3/2), likely within the error margin of the calculation. However, Mössbauer parameters calculated for the low-spin species (S = 1/2; δcalc = 0.08 mm s−1, and ΔEQcalc = 2.96 mm s−1) agree much better with experiment than those calculated for the high-spin case (S = 3/2; δcalc = 0.29 mm s−1, and ΔEQcalc = 4.24 mm s−1). Variable-temperature magnetic susceptibility data for solid 4 finally confirmed the low-spin (S = 1/2) state; χMT is almost constant at 0.6 cm3 mol−1 K over the whole temperature range (see the Supporting Information for details). Despite the very different potentials, UV−vis spectroscopic changes upon electrochemical oxidation of complexes 1 and A bear some similarity (Figure 8). Complex 1 features a

and subsequent reduction leads back to the initial complex A with λ = 339 nm (ε = 9100 L mol−1 cm−1). These differences in optical absorption again underline the significantly stronger ligand field of the macrocyclic tetracarbene ligand LMTC and its stronger tendency to stabilize high metal oxidation states, compared to that of the NHC/pyridine hybrid ligand L1. In line with these findings, complex 1 with its {C2N2} hybrid ligand upon reaction with iodosobenzene does not give an isolable high-valent oxoiron(IV) species (see the Supporting Information), in contrast to the macrocyclic tetracarbene complex A.24

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CONCLUSIONS The macrocyclic tetradentate NHC/pyridine hybrid ligand L1, which combines two NHC-C and two pyridine-N donors in mutual trans disposition, has been shown to form organometallic iron(II) complexes. The metal ion is hosted in the basal plane of the macrocyclic scaffold and can bind various axial ligands, such as two neutral MeCN ligands in 1, two different ligands (one MeCN and one CO) in 2, or two anionic azido ligands in 3. These complexes are structurally similar to complexes of related macrocyclic tetracarbene ligands, but the macrocyclic {C2N2} hybrid ligand is severely puckered with a rather high barrier for ring inversion. Just like the tetracarbene complex A, though despite the weaker in-plane ligand field of L1, complexes 1−3 adopt a low-spin (S = 0) state. However, Mössbauer parameters reflect the lower covalency of 1 compared to that of A, and the FeII/FeIII redox potential of 1 is shifted anodically by >500 mV compared to that of A. The oxidized ferric complex [FeL1(MeCN)2](PF6)3 (4) has been isolated, and X-ray crystallographic characterization of both ferrous 1 and ferric 4 revealed that structural parameters of the {FeL1} core are basically invariant with respect to changes in the metal ion’s oxidation state. The robust macrocyclic ligand L1 evidently allows for chemically reversible metal-based redox processes, though spectroscopic signatures differ significantly for the iron(II) and iron(III) species. New complexes 1−4 add to the rapidly increasing number of iron−NHC systems, and they provide valuable information in comparison to the more abundant complexes of macrocyclic {N4} tetraaza and {C4} tetra-NHC ligands as well as open chain NHC/pyridine hybrid ligands.

Figure 8. Bulk electrochemical oxidations of 1 (top) and A (bottom) in acetonitrile (0.1 M nBu4NPF6) monitored by UV/vis spectroscopy. Isosbestic points are marked with asterisks.

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EXPERIMENTAL SECTION

All reactions and investigations of air-sensitive compounds were performed under a dry and oxygen-free nitrogen atmosphere in a glovebox (MBRAUN Labmaster) or by using standard Schlenk techniques. The used solvents were degassed and dried according to standard methods. [Fe{N(SiMe 3 )2 }2 ] 2,48 [H2 L1 ](PF6) 2,28 and Th•+ClO4−49 were synthesized according to literature procedures. Caution: Th•+ClO4− is potentially explosive and should be handled with proper safety precautions. 1H and 13C{1H} NMR spectra were recorded on Bruker Avance DRX 500 or Bruker Avance 300 spectrometers. Chemical shifts (δ) are given in parts per million relative to TMS, using the residual proton signal of the solvent as an internal standard. Mass spectrometry was performed with a Bruker HCT Ultra (ESI-MS) or with a Finnigan MAT LCQ (ESI-HRMS) instrument. 57Fe Mössbauer (MB) measurements were performed with a 57Co source in a rhodium matrix using an alternating constant-acceleration Wissel Mössbauer spectrometer operated in transmission mode and equipped with a Janis closed-cycle helium cryostat. Isomer shifts δ, quadrupole splitting ΔEQ, and full width at half-maximum Γ are given in millimeters per second. The isomer shift δ is given relative to elemental iron at ambient temperature. Simulations of the experimental data were performed with the MFIT program.50 Temperature-dependent magnetic susceptibilities were measured by

prominent absorption at 413 nm (ε = 2810 L mol−1 cm−1) that can be assigned to an FeII → pyridine MLCT transition based on TD-DFT calculations (Figures S10−S12 of the Supporting Information; calculated λmax = 360 nm; see the Experimental Section for details of the TD-DFT calculations). During bulk oxidation of iron(II) complex 1 at 1.3 V in a MeCN solution containing 0.1 M nBu4NPF6, this absorption at 413 nm vanishes while a broad asymmetric band at 660 nm (ε = 1350 L mol−1 cm−1) appears to finally give the deep blue solution characteristic of 4 (Figure 8, top). According to TDDFT, the low-energy absorption largely originates from a NHC(π) → FeIII(d) LMCT transition [calculated λmax = 585 nm (Figures S17, S19 and S20 of the Supporting Information)]. Subsequent bulk electrochemical reduction fully restores the original spectrum of 1, confirming that the redox process is chemically reversible. A spectro-electrochemical experiment for complex A shows similar features, but significantly blue-shifted: oxidation yields a violet solution with a broad band around 540 nm (ε = 2170 L mol−1 cm−1), E


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Organometallics

[FeL1(MeCN)2](PF6)3 (4). Complex 1 (0.10 g, 0.13 mmol) and thianthrenylhexafluorophosphate (47 mg, 0.13 mmol) were dissolved in 5 mL of acetonitrile and stirred for 1 h at room temperature. A solid was precipitated by the addition of Et2O to the dark blue solution, until the supernatant solution appeared to be only slightly violet. The violet supernatant was filtered off, and the dark blue residue was washed twice with Et2O. After the sample had been dried in a vacuum, a dark blue powder was obtained (89.0 mg, 75%): 1H NMR (400 MHz, CD3CN, 292 K) δ 65.49 (s, 4H, CH2), 17.10 (s, 4H, HPy3), 0.52 (s, 2H, HPy4), −4.71 (s, 4H, CH2), −14.79 (s, 4H, HIm); 13C NMR (101 MHz, CD3CN, 292 K) δ 173.0 (CPy4), 125.9 (d, 1JCH = 201 Hz, CIm), 101.0 (CPy2), 61.3 (CPy3), −13.5 (CH2); CIm2 was not observed. Anal. Calcd for C24H24N8F18P3Fe: C, 28.83; H, 2.18; N, 10.09. Found: C, 29.04; H, 2.30; N, 9.91. DFT Calculations. ORCA (version 3.0.1 or 3.0.2) was used for all calculations.52 TD-DFT calculations for the cation of 1 [low-spin iron(II) state; S = 0] were performed with the coordinates obtained from the solid state structure (restricted DFT calculations with the B3LYP hybrid functional, RIJCOSX approximation, def2-tzvp and def2-tzvp/j basis sets;53 the hydrogen atom positions were recalculated because bond lengths involving hydrogen atoms obtained by X-ray single-crystal structure determination are systematically too short). Solvent effects were considered by invoking the conductor-like screening model (COSMO) for acetonitrile as a solvent. Eighty excited states were calculated; the maximal dimension of the expansion space in the Davidson procedure (MaxDim) was 800. Geometry optimizations of [FeL1(N3)2] (3, S = 0) and [FeL1(MeCN)2]3+ (cation of 4, in the S = 1/2 and S = 3/2 states) were performed by restricted {for [FeL1(N3)2]} and unrestricted (for all other species) DFT calculations using the BP86 functional, RI approximation, def2tzvp and def2-tzvp/j basis sets, dispersion correction D3ZERO,54 and COSMO (acetonitrile). For the calculation of the final single-point energies, the functional was changed to B3LYP and the RIJCOSX approximation was used. Subsequent spin-unrestricted TD-DFT calculations of the cationic iron(III) complexes were performed with the B3LYP hybrid functional, RIJCOSX approximation, def2-tzvp and def2-tzvp/j basis sets, dispersion correction D3ZERO, and COSMO (acetonitrile). Eighty excited states were calculated; the maximal dimension of the expansion space in the Davidson procedure (MaxDim) was 800. Mössbauer parameters were calculated at the DFT level of theory [restricted for 1 and FeL1(N3)2 and unrestricted DFT calculations for 4] with the B3LYP functional, def2-tzvp basis set and enlarged CP(PPP) basis set for the Fe atom, dispersion correction D3ZERO, and COSMO (acetonitrile). The isomer shifts were obtained using the correlation formula and parameters reported by Neese et al.55

using a SQUID magnetometer (Quantum Design MPMS XL-5). Magnetic properties were simulated using the julX program.51 Cyclic voltammograms (CVs) were recorded using a PerkinElmer (263A) potentiostat and a three-electrode arrangement with a glassy carbon working electrode (0.3 cm diameter), a Ag/AgNO3 reference electrode, and a Pt auxiliary electrode. Coulometric experiments followed by UV−vis (spectro-electrochemistry) were performed using a Princeton Applied Research Versastat 3 potentiostat/galvanostat (Ag wire as a reference electrode, Pt wire as a counter electrode, and Pt wire gauze as a working electrode), arranged together with an allquartz immersion probe (1 mm optical path, Hellma Analytics) in a custom-made reaction tube, connected to a Cary 50 Bio spectrophotometer. All electrochemical experiments were performed in MeCN containing 0.1 M [nBu4N]PF6. Elemental analyses were conducted by the analytical laboratory of the Institute of Inorganic Chemistry at the Georg-August-University Göttingen using an Elementar Vario EL III instrument. [FeL1(MeCN)2](PF6)2 (1). To a solution of [H2L1](PF6)2 (508 mg, 0.8 mmol, 1.0 equiv) in acetonitrile (20 mL) was added [Fe{N(SiMe3)2}2]2 (300 mg, 0.8 mmol, 1.0 equiv), and the reaction mixture was stirred for 15 h. After addition of diethyl ether (20 mL), brown crystals of the product [FeL1(MeCN)2](PF6)2·MeCN suitable for Xray crystallography were obtained (400 mg, 0.52 mmol, 65%): 1H NMR (300 MHz, DMSO-d6, 298 K) δ 7.96 (s, 4H, HIm2), 7.75 (t, 4JHH = 7.7 Hz, 2H, HPy4), 7.43 (d, 4JHH = 7.7 Hz, 4H, HPy3), 5.80 (d, 2JHH = 16.7 Hz, 4H, CH2), 5.41 (d, 2JHH = 16.7 Hz, 4H, CH2); 1H NMR (400 MHz, CD3CN, 296 K) δ 7.96 (s, 4H, CH), 7.76 (t, 3JHH = 7.7 Hz, 2H, HPy), 7.44 (d, 3JHH = 7.7 Hz, 4H, HPy), 5.81 (d, 2JCH = 16.9 Hz, 4H, CH2), 5.42 (d, 2JCH = 16.9 Hz, 4H, CH2); 13C{1H} NMR (125 MHz, DMSO-d6, 25 °C) δ 204.1 (CIm2), 162.3 (CPy2), 139.0 (CPy4), 126.3 (CPy3), 124.0 (m, CIm), 54.6 (CH2); 13C{1H} NMR (101 MHz, CD3CN, 296 K) δ 204.0 (CIm2), 162.2 (CPy), 139.0 (CPy), 126.3 (CPy), 124.0 (CH), 54.5 (CH2); MS (ESI, MeCN) m/z (%) 199.0 (100) [FeL1]2+, 219.4 (16) [FeL1(MeCN)]2+, 424.0 (28), 460.0 (20); UV/ vis λmax [ε (L mol−1 cm−1)] 260 (11480), 285 (2820), 413 (2810) nm; IR (KBr, 298 K) ν 1480 (w), 1446 (w), 1427 (w), 824 (vs), 771 (s), 734 (m), 715 (m), 697 (m), 688 (m), 555 (vs) cm−1. A very weak band at 2253 cm−1 may possibly be assigned to the CN stretch of the MeCN ligands (see Figure S6 of the Supporting Information). 57Fe MB: δ = 0.32 mm s−1, ΔEQ = 3.12 mm s−1. Anal. Calcd for C24H24N8F12P2Fe: C, 37.42; H, 3.14; N, 14.55. Found: C, 37.39; H, 3.21; N, 14.71. [FeL1(CO)(MeCN)](PF6)2 (2). Complex 2 (300 mg, 0.39 mmol) was dissolved in acetonitrile (30 mL), and the dark solution was frozen in liquid nitrogen. After evacuation, 2.5 bar of CO was added, and the mixture was stirred overnight at 25 °C. The bright yellow solution was concentrated to a volume of 10 mL, and addition of diethyl ether (50 mL) resulted in the formation of a yellow precipitate. Filtration gave a yellow residue that was washed twice with Et2O. After the sample had been dried under vacuum, the product was obtained as a pale yellow powder (253 mg, 86% yield): 1H NMR (400 MHz, CD3CN, 296 K) δ 7.91 (t, 3JHH = 7.7 Hz, 2H, HPy), 7.74 (m, 4H, HPy), 7.55 (m, 4H, HCH), 5.75 (d, 2JCH = 17.2 Hz, 2H, CH2), 5.72 (d, 2JCH = 17.2 Hz, 2H, CH2), 5.55 (d, 2JCH = 17.2 Hz, 2H, CH2), 5.42 (d, 2JCH = 17.2 Hz, 2H, CH2); 13C{1H} NMR (101 MHz, CD3CN, 296 K) δ 217.9 (CIm2), 187.2 (CCO), 185.2 (dissolve CO), 160.1 (CPy2), 159.3 (CPy2′), 140.5 (CPy4), 128.0 (CPy3), 127.2 (CPy3′), 124.4 (CIm), 123.9 (CIm), 54.9 (CH2), 54.2 (CH2); IR (ATR, 296 K) ν(CO) 1974 cm−1. Anal. Calcd for C23H22N7F12OP2Fe: C, 36.43; H, 2.92; N, 12.93. Found: C, 36.87; H, 3.04; N, 13.13. [FeL1(N3)2] (3). A solution of 2 (50 mg, 65 μmol, 1.0 equiv) in MeCN (5 mL) was treated with (nBu4N)N3 (37 mg, 130 μmol, 2.0 equiv) and stirred for 15 h. To the violet suspension was added Et2O (5 mL), and the solid product was obtained by filtration and washing with Et2O (31 mg, 65 μmol, quantitative): IR (KBr, 298 K) ν 2027 (vs), 1666 (w), 1437 (s), 1263 (m), 1181 (m), 1002 (w), 969 (w), 774 (m), 702 (m) cm−1. 57Fe MB: δ = 0.39 mm s−1, ΔEQ = 3.42 mm s−1. Anal. Calcd for C20H18FeN12: C, 49.81; H, 3.76; N, 34.85. Found: C, 49.19; H, 4.03; N, 34.14.

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ASSOCIATED CONTENT

S Supporting Information *

CV data at different scan rates, SQUID data, additional spectroscopic data, crystallographic details, and DFT coordinates. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/ acs.organomet.5b00103.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Georg-August-University Göttingen (F.M.) and the King Abdullah University of Science and Technology (KAUST) (F.E.K.) is gratefully acknowledged. S.H. and M.R.A. are thankful for financial support by the TUM F


Article

Organometallics

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graduate school. We thank A. Bretschneider (Institute for Inorganic Chemistry, Georg-August-University Göttingen) for the preparation of Th•+ClO4− and Dr. H. Frauendorf (Institute for Organic and Biomolecular Chemistry, Georg-AugustUniversity Göttingen) for collecting HRMS spectra.

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