Redox Induced Configurational Isomerization of Bisphosphine

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Redox Induced Configurational Isomerization of Bisphosphine− Tricarbonyliron(I) Complexes and the Difference a Ferrocene Makes Mark R. Ringenberg,*,† Florian Wittkamp,‡ Ulf-Peter Apfel,‡ and Wolfgang Kaim† †

Universität Stuttgart, Institut für Anorganische Chemie, Pfaffenwaldring 55, D-70569 Stuttgart, Germany Inorganic Chemistry 1/Bioinorganic Chemistry, Ruhr-University Bochum, Universitätsstrasse 150, D-44780 Bochum, Germany

‡

S Supporting Information *

ABSTRACT: The tricarbonyliron (TCFe) complexes Fe(CO)3(dppf) and Fe(CO)3(dppp), where dppf = 1,1′-bis(diphenylphosphino)ferrocene and dppp = 1,3-bis(diphenylphosphino)propane, exhibit redox activity that induces configurational isomerization. The presence of the ferrocenyl (Fc) group stabilizes higher oxidized forms of TCFe. Using spectroelectrochemistry (IR, UV−vis, Mössbauer, and EPR) and computational analysis, we can show that the Fc in the backbone of the dppf ligand tends to form a weak dative bond to the electrophilic TCFeI and TCFeII species. The open shell TCFeI intermediate was stabilized by the distribution of spin between the two Fe centers (Fc and TCFe), whereas lacking the Fc moiety resulted in highly reactive TCFeI species. The [Fe(CO)3(dppf)]+ cation adopts two possible configurations, square-pyramidal (without an Fe−Fe interaction) and trigonal-bipyramidal (containing an Fe−Fe interaction). The two configurations are in equilibrium with the trigonal-bipyramidal configuration being enthalpically favored (ΔH = −7 kJ mol−1). There is an entropic penalty (ΔS = −20 J mol−1) due to tilting of the Cp (cyclopentadienide) rings of the dppf moieties by ∼8°. Additionally, the terminal iron hydride [FeH(CO)3(dppf)]BF4 was formed by protonation with a strong acid (HBF4·Et2O).

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INTRODUCTION

Mimicking the structure and reactivity of metalloenzymes can be difficult; however, chemists are able to use organometallic compounds and ligands that are not found in the biosphere to adopt certain enzymatic properties. Among the numerous ligand systems, phosphine ligands are common in transition-metal chemistry because they can be easily modified to tune electronic and structural parameters by the variation of their substitution patterns.7 Ferrocenyl-based phosphines, especially 1,1′-bis(diphenylphosphino)ferrocene (dppf) and chelating alkylphosphine ligands such as 1,n-bis(diphenylphosphino)alkanes where n > 0, are common ligand motifs because they offer a range of bite angles.7,8 Even small changes in the bite angles and thus in the coordination environment may result in dramatic changes of the reactivity of transition-metal catalysts.9 Ferrocenyl-based ligands such as dppf are notable because they can adopt different coordinating modes (Scheme 1).8 Monodentate (κ1-dppf) and bidentate (κ2dppf) modes are straightforward, as well as bridging between two metal centers (μ-dppf)10 or in a cluster (μ′-dppf).11 Quite interesting is the tridentate (κ3-dppf) mode,12 where the cyclopentadienide (Cp−) ligands distort in the presence of an electrophilic metal allowing the iron atom to interact, forming an Fe→M dative bond.12a,13 Recently, we reported a κ3-dchpf ligation (dchpf = 1,1′-bis(dicyclohexylphosphino)ferrocene) in Fe(CO)3(dchpf) where the ferrocene (Fc) iron was not only

Bimetallic compounds and interactions between two metal centers in general are of interest because they can reveal how electrons are delocalized in complex systems, often stabilizing open shell (e.g., radical) species.1 The importance of M−M interactions is further highlighted by the presence of metal clusters in natural enzymes, many existing as homobimetallic, heterobimetallic, or even higher nuclei clusters.2 Such clusters consist primarily of first row transition metals, due in part to bioavailability and their abundance in the earth’s crust. Optimization of these enzymes over millions of years of evolution has produced catalysts capable of performing reactions often achieved by precious metal-based catalysts in the lab.3 Iron is fast becoming a pillar in catalysis,4 and it is present in many metalloenzymes as a cofactor essential to life. Iron complexes can occur in diverse redox (Fe−II to FeIV) and spin states (high, intermediate, and low spin). Metalloenzymes tune these redox and spin states through different ligands and structural variations of the protein environment, even employing CO and CN−, redox-active noninnocent ligands, and metal clusters. Redox-active moieties in close proximity to active sites allow for rapid separation or combination of charge, a key factor for performing multielectron transfers using base metals.5 Charge separation is also important in the processing of small molecules, such as H2, N2, O2, or CO2, which must overcome large kinetic barriers toward activation.6 © 2017 American Chemical Society

Received: April 18, 2017 Published: June 9, 2017 7501

DOI: 10.1021/acs.inorgchem.7b00957 Inorg. Chem. 2017, 56, 7501−7511

Article

Inorganic Chemistry Scheme 1. Binding Modes of 1,1′Bis(diphenylphosphino)ferrocene

Scheme 2. Synthesis of Bisphosphine Tricarbonyliron

acting as a mere donor but also behaving noninnocently by sharing some spin density on the Fc iron atom, leading to unusual class II mixed-valent species.14 However, redox activity of dppf is seldom invoked in catalytic transformation due to relatively high anodic potential when found in metal complexes such as Mo(CO)4(dppf) (+0.42 V vs Fc0/+),15 although it is clear from our previous report that ferrocenyl-based ligands can behave noninnocently when bound to a redox-active metal such as iron.14 In this work, we present the example of prototypical dppf16 in Fe(CO)3(dppf) (1) behaving not just as an innocent spectator. The iron of the ferrocenyl moiety is shown to stabilize higher charge states of the tricarbonyliron, i.e., TCFeI and TCFeII. The corresponding Fe(CO)3(dppp) (2), where dppp = 1,3-diphenylphosphinopropane with a similar ligand arrangement as in 1, was used as a purely “innocent” ligand to further highlight the function of the ferrocene group.17

Figure 1. Molecular structure of 2 in the crystal state. Thermal ellipsoids are shown at 50% probability, and H atoms are omitted for clarity.

Table 1. Selected Experimental and DFT Calculated Bond Distances and Angles of 1 and 2

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1 X-ray19

RESULTS AND DISCUSSION Synthesis. Compounds 1 and 2 were synthesized in a twostep process that resulted in the exclusive formation of Fe(κ2dppf)(CO)3 and Fe(κ2-dppp)(CO)3.14 The addition of a bidentate phosphine ligand to a slurry of Fe2(CO)9 in tetrahydrofuran (THF) resulted in the formation of a dark yellow solution. The solvent and Fe(CO)5 were subsequently removed under reduced pressure. The resulting dark yellow solid was dissolved in THF and exposed to high-intensity light from a low-pressure Hg-arc lamp (50 W) to afford a light yellow solution of 1 or 2. This method is preferred to the previous report by Hor et al.18 where Fe(CO)5 is used as the iron carbonyl source or as the displacement of MA (MA = maleic acid) from (MA)Fe(CO)429 because the desired tricarbonyliron complex is formed exclusively and does not require chromatography to separate it from side products (Scheme 2).10 The molecular structure 1 was previously reported;10,19 the cyclopentadienyl rings in dppf were rotated by 28° from an eclipsed conformation in order to coordinate the P atoms axial and equatorial to an approximate trigonal-bipyramidal Fe center. The axial ligand’s OCax−Fe−P angle was 167.2°. This differs from our previous report where Fe(dchpf)(CO)3, dchpf = 1,1′-bis(dicyclohexylphosphino)ferrocene, shows the P atoms occupying two equatorial positions.14 Complex 2 has a similar ligand arrangement as 1 in the crystal (Figure 1). The Fe is pentacoordinated with approximate trigonal-bipyramid geometry (Table 1). The P atoms occupy one equatorial and one axial position with a quasi-facial arrangement of carbonyls.

Fe1−P1 Fe1−P2 Fe1−C1 Fe1−C2 Fe1−C3 P2−Fe1−P1 C1−Fe1−P1 C1−Fe1−P2 C2−Fe1−C1 C3−Fe1−C1 C3−Fe1−C2

2 DFTa

Bond (Å) 2.243(2) 2.243 2.256(3) 2.252 1.777(10) 1.760 1.778(11) 1.759 1.760(11) 1.761 Angles (deg) 99.7 96.39 168.2 176.55 87.9 87.6 92.0 90.43 87.0 90.92 130.7 137.35

X-ray

DFTa

2.2240(6) 2.2266(6) 1.772(2) 1.773(2) 1.772(2)

2.233 2.231 1.760 1.760 1.761

91.27(2) 175.36(7) 90.30(7) 88.31(10) 93.43(10) 121.23(11)

89.31 172.96 88.67 89.00 96.61 119.51

a

TPSS/def2-TZVP level of theory, details and full coordinates in the Supporting Information.

The molecular structures as determined crystallographically were used as the basis for the DFT calculations, and the comparison of selected bond lengths are shown in Table 1. There is a good agreement between the reported structures and the DFT calculations.10 Both experimental and calculated angles are within the expected range for the dppf and dppp ligands.7 The infrared (IR) spectra of 1 and 2 contain three bands (ν) in the carbonyl stretching region, νCO(1) = 1987, 1911, 1882 cm−1 (Figure S9) and νCO(2) = 1982, 1908, 1882 cm−1 (Figure S17). The vibrational spectra of 1 and 2 confirm the similar configurations of both TCFe0 complexes as well as similar donor abilities of −PPh2 groups. The νCO values of 1 7502

DOI: 10.1021/acs.inorgchem.7b00957 Inorg. Chem. 2017, 56, 7501−7511

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Inorganic Chemistry

Electrochemistry. The cyclic voltammogram (CV) of 1 contains two redox couples, which differs from the CV of 2 with a single wave (Figure 3). The redox couples for 1 occur at E1/2[1]0/+ = −0.15 V vs FcH0/+ (the standard reference for all potentials reported herein) and at E1/2[1]+/2+ = 0.03 V. The Cottrell plot is linear (Figure S4), consistent with diffusion controlled processes.21 The Epa for both couples, E1/2[1]0/+ and E1/2 [1]+/2+, drifts with respect to scan rate (ν) with slopes ΔEpa/Δlog10(ν) of 110 and 136 mV, respectively (Figure S5), signifying electron transfers followed by chemical reactions.22 The presence of a second oxidation wave is unusual for bisphosphine tricarbonyl species because until recently a ferrous TCFe state was unknown.23 The redox couple for 2 occurs at E1/2[2]0/+ = −0.16 V. The Cottrell plot is linear, indicating diffusion controlled (Figure S7);21 the Epa drifts with respect to scan rate with a slope ΔEpa/ Δlog10(ν) of 83 mV (Figure S8), consistent with an electron transfer followed by a chemical reaction.22a No further oxidation waves were observed in the cyclic voltammograms within the solvent limits (CH2Cl2/0.1 M Bu4NPF6). The electrochemical behavior of 2 is consistent with other reported bisphosphine TCFe complexes.24 Iron(I) Species. Complex [1]BArF4, BArF4− = tetrakis[3,5bis(trifluoromethyl)phenyl]borate, was synthesized by the oxidation of 1 in Et2O with FcBArF4.25 As the reaction progressed, the solution became green, and the yellow solid was consumed. The green reaction product, despite analytical purity, showed several CO bands in the IR spectrum νCO of 2071s, 2044w, 2013m, and 1988br cm−1 (Figure S10). The UV−vis spectrum contains a broad low-energy band at λmax = 756 nm (Figure S18). These data are thought to be due to a mixture of two configurational isomers (Scheme 3) in which one configuration is predominant at low temperatures, as will be discussed in greater detail below. The Mössbauer spectrum at −80 °C of [1]BArF4 indicates two Fe sites in a 1:1 ratio with isomer shifts of δ = 0.49 (ΔEq 1.86) and 0.08 (ΔEq 0.40) mm s−1 (Figure 2). The isomer shift and quadrupole splitting of the ferrocene iron in [1]BArF4 show only a small change relative to the Mössbauer spectrum of unoxidized 1. A larger change is observed for the proposed TCFe site, and the quadrupole splitting decreases, consistent with decreased electron density. A single configuration is thought to be present at low temperatures, and the isomer shift and quadrupole splitting are consistent with one dominant configuration. To better understand the oxidation process of [1]0/+, it was studied by IR and UV−vis−NIR (near IR) spectroelectro-

and 2 obtained from the DFT numerical frequency analysis are summarized in Table 2 and further support the trigonalbipyramidal ligand arrangement found in the crystals. Table 2. Summary of IR in νCO Region for Complexes 1 and 2 charge

νCO (exp)a

νCO (DFT)b

1 [1]+

1987s, 1911m, 1883s 2071s, 2044w, 2013m, 2000sh, 1988s 2070w, 2040w, 2011w, 1989s (−30°) 2104w, 2056s, 2036sh 1983, 1908, 1882 2072, 2017, 1998

1883s, 1909m, 1984s p2075m, 2015m, 1999m i2046w, 1984m, 1983m 2105w, 2047s, 2044w 1987, 1913, 1888 2076, 2015, 1994

[1]2+ 2 [2]+ a

In CH2Cl2/0.1 M Bu4NPF6. bVibrational modes determined by TPSS functional with CH2Cl2 COSMO model.

The Mössbauer spectrum of 1 indicates two Fe sites in a 1:1 ratio with isomer shifts of δ = 0.44 (ΔEq 1.86) and −0.01 (ΔEq 1.67) mm s−1, revealing no obvious interaction between the two iron centers (Figure 2). The higher isomer shift for 1 is

Figure 2. Mössbauer spectra of 1, [1]BArF, [1](OTf)2, and [1H]BF4. The red line is the proposed Fc site, and the blue line is the proposed TCFe site.

consistent with a low-spin Fc backbone, whereas the lower isomer shift indicates decreased electron density on the TCFe nucleus, a result of π-back-bonding on the ligands. These values are consistent with those for trans-[Fe(CO)3(PPh3)2] and similar TCFe complexes.14,20

Figure 3. Cyclic voltammogram (solid) at 100 mV/s of 1 mM 1 (left) and 3 mM 2 (right) and differential pulse voltammogram (dashed) in CH2Cl2/0.1 M Bu4NPF6 of the supporting electrolyte referenced versus FcH0/+. 7503

DOI: 10.1021/acs.inorgchem.7b00957 Inorg. Chem. 2017, 56, 7501−7511

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Inorganic Chemistry Scheme 3. Simplified Molecular Orbital Description of the Different Oxidation States of 1

spectrum shows two weak equal intensity bands at 2075 and 2011 cm−1 associated with this configuration (Figure 4); the third vibration is expected around 1999 cm−1 but is likely obscured by the broad band around 1989 cm−1.

chemistry (SEC).26 The IR SEC for [1]0/+ recorded at room temperature was complicated and exhibited nonisosbestic behavior (Figure S14). Therefore, the IR SEC was performed at −30 °C, which yields isosbestic behavior (Figure 4). Upon oxidation, new νCO bands emerged at 2070, 2040, and 2011 cm−1, and the band at 1989 cm−1 broadened while the two lower energy bands consistent with 1 at 1911 and 1882 cm−1 were completely consumed, which indicates the formation of [1]+. However, more bands appear than may be expected for a single TCFe species because the monocation [1]+ adopts two configurations (Scheme 3). Calculating the structure of [1]+ was complicated by the presence of two configurations; however, the energy minima that could be determined are consistent with spectroscopic observations. Complex [1]+ adopts either a square-pyramidal configuration with no TCFe/Fc iron−iron interaction (dFe−Fe(calc) = 4.34 Å, 1p+, Figure S38) or a trigonalbipyramidal configuration exhibiting a TCFe/Fc iron−iron interaction (dFe−Fe(calc) = 3.70 Å, 1i+, Figure S39), where the Cp rings of dppf twist by 36° from an eclipsed configuration and tilt by 8.64°. The assignment of the CO vibrations was aided by vibrational analysis performed during the calculations of 1p+ and 1i+. The room-temperature IR spectrum of [1]BArF4 contains several bands (Figure S10). The IR SEC at −30 °C for the redox couple [1]0/+ (Figure 4) suggests 1i+ as the major species. Structure 1i+ also appears as the favored configuration in the low-temperature EPR (discussed below) and Mössbauer (discussed above) analyses of [1]BArF4. Three vibrational modes are expected from the DFT for 1i+, a weak high-energy vibration (2043 cm−1) and two close vibrations at 1984 and 1983 cm−1, which are observed as a single band in the IR SEC around 1989 cm−1 (Figure 4). Three equally intense medium vibrations are expected by the DFT for 1p+, and the IR

Figure 5. IR SEC of [2]0/+ in the νCO region in CH2Cl2/0.1 M Bu4NPF6 at room temperature. The end spectrum is shown in red.

The IR SEC of [2]0/+ suggests that oxidation induces a configurational change and shows three νCO vibrational bands blue-shifted by ∼100 cm−1 (Figure 5), consistent with a metalbased oxidation.27 The νCO pattern for [2]+ shows three medium intense vibrations that are expected, on the basis of a square-pyramidal configuration with the chelate ligand in the basal plane also determined by the DFT calculations (Scheme 4). It has been well established that TCFeI undergoes a Scheme 4. Geometry Change upon Oxidation [2]0/+ and Calculated Structures of 2 and [2]+

trigonal-bipyramidal to square-pyramidal change upon oxidation.17,23,28 The calculated structure of [2]+ and the vibrational analysis support the square-pyramidal geometry depicted in Scheme 4.

Figure 4. IR SEC of [1]0/+ and [1]+/2+ in the νCO region in CH2Cl2/0.1 M Bu4NPF6 at −30 °C. The end spectrum is shown in red. 7504

DOI: 10.1021/acs.inorgchem.7b00957 Inorg. Chem. 2017, 56, 7501−7511

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Inorganic Chemistry

Figure 6. UV−vis−NIR SEC of [1]0/+ and [1]+/2+ in CH2Cl2/0.1 M Bu4NPF6 at room temperature. The end spectrum is shown in red.

electron density difference maps for these transitions are depicted in Figures S28−S30. Electron Paramagnetic Resonance. The room temperature EPR spectrum of [1]BArF4 in CH2Cl2 contains two triplets in a 1:3 ratio (Figure 8), with two distinctly different g

The UV−vis−NIR SEC response for the redox process [1]0/+ is accompanied by the appearance of two new long wavelength absorptions (Figure 6). The corresponding transitions were assigned according to TDDFT calculations performed on 1p+ and 1i+. The transition at λmax = 512 nm was found to correspond to 1p+ and is described as Fc to TCFe charge transfer; a similar transition for 1i+ was found at a lower energy (λmax = 1008 nm). Both these transitions appear in the spectra of [1]+ (Figures 6 and S18). These transitions are remarkable because they are “inverse” intervalence chargetransfer processes, from formally high oxidation state Fc with FeII to formally lower oxidation state TCFeI. Interestingly, this inverse charge transfer appears to be maintained in both configurations. The lower energy transition corresponds to the 1i+ configuration and is consistent with the close M−M distance (dFe−Fe = 3.70 Å). Such counterintuitive transitions are made possible through the very different ligation, i.e., donating cyclopentadienyl for Fc and π-accepting carbonyl ligands for TCFe. The higher energy transitions (below 400 nm) involve LMCT, i.e., π (ligand) to SOMO (metal) charge transfer. The electron density difference maps for the two low-energy transitions can be found in Figures S19−S23. The UV−vis−NIR SEC response for [2]0/+ results in a new long wavelength absorption band at λmax = 761 nm and a shoulder at λmax = 374 nm (Figure 7). These bands are also

Figure 8. EPR of [1]BArF4 in CH2Cl2 at room temperature (black) with simulated spectrum (blue/green).

values. This observation is consistent with the presence of a mixture of two species. The spectrum was simulated under the assumption of two products (1p+ and 1i+) using the Bruker WinEPR SimFonia.29 Two species occur at giso = 2.05 and A(31P) = 68 MHz (24 G) and at giso = 2.02 and A(31P) = 40 MHz (13 G) (Figure 8). Both signals show hyperfine coupling to two equivalent 31P nuclei. Lowering the temperature to 213 K results in the signal at giso = 2.02 to remain the same while the signal at giso = 2.05 decreases (Figure 9). The signal ratio in the EPR are the same whether starting from room temperature and going to lower temperature or vice versa. The signal at giso = 2.05 is attributed to the square-pyramidal structure 1p+ (Scheme 3). The DFT spin analysis of 1p+ reveals more than 94% of the spin residing at the TCFe and