Reversible Addition of CO to Coordinatively Unsaturated High-Spin

Article pubs.acs.org/Organometallics

Reversible Addition of CO to Coordinatively Unsaturated High-Spin Iron(II) Complexes Christian Holzhacker,† Christina M. Standfest-Hauser,† Michael Puchberger,‡ Kurt Mereiter,§ Luis F. Veiros,∥ Maria José Calhorda,⊥ Maria Deus Carvalho,△ Liliana P. Ferreira,#,▽ Margarida Godinho,# František Hartl,□ and Karl Kirchner*,† †

Institute of Applied Synthetic Chemistry, ‡Institute of Materials Chemistry, and §Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria ∥ Centro de Quı ́mica Estrutural, Instituto Superior Técnico, Universidade Técnica de Lisboa, 1049-001 Lisboa, Portugal ⊥ Centro de Quı ́mica e Bioquı ́mica/DQB, △Centro de Ciências Moleculares e Materiais/DQB, #Centro de Fı ́sica da Matéria Condensada/DF, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal ▽ Departamento Fı ́sica, Faculdade Ciências e Tecnologia, Universidade de Coimbra, 3004-516 Coimbra, Portugal □ Department of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, United Kingdom S Supporting Information *

ABSTRACT: Several new coordinatively unsaturated iron(II) complexes of the types [Fe(EN-iPr)X2] (E = P, S, Se; X = Cl, Br) and [Fe(ON-iPr)2X]X containing bidentate EN ligands based on N-(2-pyridinyl)aminophosphines as well as oxo, thio, and seleno derivatives thereof were prepared and characterized by NMR spectroscopy and X-ray crystallography. Mössbauer spectroscopy and magnetization studies confirmed their high-spin nature with magnetic moments very close to 4.9 μ B, reflecting the expected four unpaired d-electrons in all these compounds. Stable low-spin carbonyl complexes of the types [Fe(PN-iPr)2(CO)X]X (X = Cl, Br) and cis-CO,cis-Br-[Fe(PN-iPr)(CO)2X2] (X = Br) were obtained by reacting cis-Fe(CO)4X2 with the stronger PN donor ligands, but not with the weaker EN donor ligands (E = O, S, Se). Furthermore, the reactivity of [Fe(PN-iPr)X 2] toward CO was investigated by IR spectroscopy. Whereas at room temperature no reaction took place, at −50 °C [Fe(PN-iPr)X2] added readily CO to form, depending on the nature of X, the mono- and dicarbonyl complexes [Fe(PN-iPr)(X)2(CO)] (X = Cl) and [Fe(PN-iPr)(CO)2X2] (X = Cl, Br), respectively. In the case of X = Br, two isomeric dicarbonyl complexes, namely, cis-CO,trans-Br-[Fe(PN-iPr)(CO)2Br2] (major species) and cis-CO,cis-Br-[Fe(PN-iPr)(CO)2Br2] (minor species), are formed. The addition of CO to [Fe(PN-iPr)X 2] was investigated in detail by means of DFT/B3LYP calculations. This study strongly supports the experimental findings that at low temperature two isomeric low-spin dicarbonyl complexes are formed. For kinetic reasons cis,trans-[Fe(PN-iPr)(CO)2Br2] releases CO at elevated temperature, re-forming [Fe(PN-iPr)Br2], while the corresponding cis,cis isomer is stable under these conditions.

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substrates are iron(II)−heme systems. The strong binding of CO to iron in hemoglobin, which renders the molecule incapable of absorbing oxygen, is particularly noteworthy.1 In recent years, however, the study of nonheme iron carbonyl compounds has increased since it was discovered that the FeCO moiety is present in the active site of hydrogenases, making it the first time that carbon monoxide coordination has been observed to occur in Nature.2 Another aspect of studying nonheme Fe(II) carbonyl compounds is to obtain insights into their electronic features that allow the coordination and functionalization of more challenging substrates, such as dinitrogen in the iron-catalyzed nitrogen fixation process3 or dihydrogen in [Fe]-hydrogenases4 such as the mono iron hydrogenase ([Fe]-H2ase).5 Notwithstanding this, such studies are important for the development of more environmentally friendly and sustainable reactions. Iron is

INTRODUCTION The reactivity of transition metal complexes toward small molecules is an increasingly important topic in organometallic chemistry. With energy prices on the rise and limited supplies of fossil fuels, transformations that involve incorporating small molecules such as N2, H2, CO2, or CO into complex chemicals can prove valuable in developing more sustainable chemistry. In order to achieve more efficient processes, it is important to understand the bonding and coordination properties of transition metals to such small molecules. This is typically done by examining the interactions of specific ligands with specific metal centers in different oxidation and spin states. In this regard, an important small molecule is carbon monoxide. In combination with iron, it is linked to a number of processes in industrial chemistry, from the water−gas shift reaction that converts it to carbon dioxide and hydrogen to the Fischer− Tropsch reaction, in which it is incorporated into liquid linear hydrocarbons. In addition, carbon monoxide coordination to iron plays an important role in Nature. Perhaps the best known © 2011 American Chemical Society

Received: August 1, 2011 Published: November 17, 2011 6587

dx.doi.org/10.1021/om200711q | Organometallics 2011, 30, 6587−6601

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the most abundant transition metal in Earth's crust, essentially nontoxic, inexpensive, and ubiquitously available. Accordingly, such investigations will be important for the rational design of well-defined iron catalysts. Surprisingly there is a large range in reactivity of iron complexes toward CO.6 In many cases iron fragments bind CO very strongly, others reversibly, and others not at all. Substrate binding requires not only a formal vacant coordination site, i.e., a coordinatively unsaturated (≤16e) species, but also an accessible empty orbital. In contrast to related second- and third-row transition metals, coordinatively unsaturated iron complexes are usually high-spin species where orbitals for substrate binding are half-occupied or empty and high-lying. Moreover, both geometry and coordination number will be important to determine the success of the reaction, the key factor being the overall ligand field of the ensemble of ligands in the reactant. We have recently reported7 that the 16e high-spin complexes [Fe(PNP)X2] readily add CO to afford, depending on the reaction conditions, either cis-[Fe(PNP)(X2)CO] or trans[Fe(PNP)(X2)CO] (PNP are tridentate pincer-type ligands based on 2,6-diaminopyridine and 2,6-diaminopyrimidine8). These transformations are accompanied by color and spin-state changes. CO binding is fully reversible in these cases upon heating under vacuum. As an extension of this work, we report here a combined synthetic, structural, spectroscopic, and DFT computational study aimed at exploring and understanding the reactivity of a series of new high-spin 14e and 16e iron(II) complexes toward carbon monoxide. The iron complexes described here feature bidentate heterodifunctional EN ligands (Chart 1) where the ligand field strengths qualitatively follow the order ON-iPr < SN-iPr ≈ SeN-iPr < PN-iPr.

The analogous bromide complexes [Fe(PN-iPr)Br2] (5b) and [Fe(PN-tBu)Br2] (6b) were obtained in similar fashion by straightforward complexation of the respective free PN ligands with anhydrous ferrous dibromide (90% and 82% yields). On the other hand, the same reaction with ON-iPr (2) proceeded differently, affording cationic pentacoordinated complexes of the type [Fe(ON-iPr)2Cl]Cl (7a) and [Fe(ON-iPr)2Br]Br (7b) in 96% and 81% yield, respectively, where two ON-iPr ligands are coordinated (Scheme 2). The formation of 7 is independent Scheme 2

of whether 1 or 2 equiv of ligand is used. However, in the first case substantial amounts of FeX2 remained unreacted. There was no evidence for the formation of tetracoordinate [Fe(ONiPr)X2]. With the ligands SN-iPr (3) and SeN-iPr (4), in analogy to the PN ligands 1a and 1b, the tetrahedral complexes [Fe(SN-iPr)Cl2] (8a), [Fe(SN-iPr)Br2] (8b), and [Fe(SeNiPr)Cl2] (9) were obtained in isolated yields up to 83% (Scheme 3). All complexes are thermally robust, white to pale Scheme 3

Chart 1

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RESULTS AND DISCUSSION yellow solids that are air sensitive in the solid state and particularly in solution. The magnetic properties of 5a, 7a, 7b, 8a, and 9 were investigated by SQUID magnetometry and Mössbauer spectroscopy. From the temperature dependence of the inverse molar magnetic susceptibility, which is well described by a Curie law above 50 K, iron effective magnetic moments of 4.96(1), 5.2(1), 5.1(1), 4.94(1), and 5.07(1) μ B, respectively, were obtained. These values are in good agreement with the effective magnetic moment of HS Fe(II) in the spin-only approximation (4.9 μ B). Figure 1 shows the thermal variation of the inverse molar magnetic susceptibility and χ mT for 5a. Mössbauer spectra of 5a, 7a, 7b, 8a, and 9 are depicted in Figure 2. Isomer shifts (IS) and quadrupole splittings (QS) obtained from the fitting procedure are shown in Table 1. All Mössbauer spectra are mainly resolved by one quadrupole doublet with IS ≥ 0.68 mm·s−1 and QS > 2 mms−1, which are characteristic of high-spin Fe(II) compounds9−11 in accordance

Synthesis and Characterization of Iron(II) Complexes with EN Ligands. Treatment of anhydrous FeCl2 with 1 equiv of the PN ligands PN-iPr (1a) and PN-tBu (1b) in THF at room temperature afforded the tetracoordinated coordinatively unsaturated 14e complexes [Fe(PN-iPr)Cl2] (5a) and [Fe(PN-tBu)Cl2] (6a) in 93% and 79% isolated yields (Scheme 1).

Scheme 1

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Table 1. Mössbauer Parameters Extracted from the Fitting Procedure of the Spectra Collected at Room Temperature and Geometry of the Local 57Fe Coordination and Electronic Density at the Nucleus (EDN) Calculated by DFT (IS, isomer shift; QS, quadrupole splitting) compound

geometry

[Fe(PN-iPr)Cl2 (5a) tetrahedral [Fe(SN-iPr)Cl2 (8a) [Fe(SeN-iPr)Cl2 (9) [Fe(ON-iPr)2Cl]Cl (7a) square [Fe(ON-iPr)2Br]Br (7b) pyramidal

IS (mm·s−1)

EDN (au)

QS (mm·s−1)

0.680(2) 12 862.780 13 2.871(4) 0.796(2) 12 862.254 50 2.998(4) 0.790(2) 12 862.287 18 2.996(5) 1.049(2) 2.008(4) 1.029(3) 2.583(7)

The quadrupole splitting arises from the interaction of the noncubic extranuclear electric fields (from ligand charges and iron valence electrons) with the quadrupole moment of the iron nucleus.9 From Table 1 it can be seen that larger QS values (around 3.0 mm·s−1) were found for the tetrahedral complexes, while lower QS values were obtained for the square-pyramidal ones. While similar quadrupole splittings were found for the tetrahedral complexes, the significant difference in the QS values for the square-pyramidal compounds may reflect a larger distortion from the square-pyramidal symmetry induced by the larger bromide when compared to the chloride ligand. On the other hand, the quadrupole splitting depends on the symmetry of the β electron, as the five α electrons lead to a half-occupation of the d levels. Indeed, the occupied β orbitals differ significantly upon going from the Cl to the Br derivative, while differences for the three tetrahedral complexes are much smaller (see Figure S1 in the Supporting Information). As expected from the magnetic data, complexes 5−9 display large paramagnetic shifted 1H and 13C{1H} NMR spectra. However, at room temperature the line widths are relatively narrow, and thus, most ligand resonances could be readily assigned on the basis of integration and 1H−1H and 1 13 H, C{1H} COSY spectra. The only exception is the pyridine hydrogen atom H6, which could not be observed presumably due to the close proximity to the paramagnetic Fe center. As a representative example, the 1H and 13C{1H} NMR spectra of 5a are depicted in Figure 3. The diastereotopic isopropyl

Figure 1. Temperature dependence of the inverse molar magnetic susceptibility (open symbols) and χ mT (solid symbols) for [Fe(PNiPr)Cl2] (5a). The straight line was obtained from the Curie law fitting to the experimental values. A very small temperature-independent magnetic fraction was also found from the fitting, and it was subtracted in the χ mT plot.

Figure 2. Room-temperature 57Fe Mössbauer of [Fe(PN-iPr)Cl2] (5a), [Fe(SN-iPr)Cl2] (8a), [Fe(SeN-iPr)Cl2] (9), [Fe(ON-iPr)2Cl] Cl (7a), and [Fe(ON-iPr)2Br]Br (7b). A second doublet of small intensity ( 2σ(I)] no. of params R1 [I > 2σ(I )] R1 [all data] wR2 [all data] min./max. resid electron density [e Å−3] formula fw cryst size [mm] color, shape cryst syst space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] V [Å3] T [K] Z ρ calc [g cm−3] μ [mm−1] (Mo Kα) F(000) θ max [deg] no. of rflns measd Rint no. of rflns unique no. of rflns [I > 2σ(I)] no. of params R1 [I > 2σ(I )] R1 [all data] wR2 [all data] min./max. resid electron density [e Å−3]

C11H19Cl2FeN2P 337.00 0.40 × 0.36 × 0.25 colorless block monoclinic P21/n (no. 14) 7.7679(2) 14.8489(3) 14.0520(3) 90 96.370(1) 90 1610.82(6) 100 4 1.390 1.349 696 30.0 34601 0.0255 4680 4360 162 0.0234 0.0257 0.0643 −0.31/0.61 8a C11H19Cl2FeN2PS 369.06 0.42 × 0.34 × 0.21 colorless prism triclinic P1̅ (no. 2) 8.2674(1) 10.1295(1) 10.3420(1) 85.641(1) 69.484(1) 82.180(1) 803.24(2) 100 2 1.526 1.485 380 30.0 15 136 0.0178 4671 4398 170 0.0235 0.0249 0.0651 −0.20/0.69

5b C11H19Br2FeN2P 425.92 0.48 × 0.23 × 0.18 colorless prism monoclinic P21/c (no. 14) 14.637(3) 8.0898(18) 14.103(3) 90 102.268(3) 90 1631.8(6) 100 4 1.734 5.902 840 30.0 22797 0.0461 4757 4077 154 0.0248 0.0328 0.0620 −0.75/0.97 8b C11H19Br2FeN2PS 457.98 0.21 × 0.14 × 0.11 colorless block triclinic P1̅ (no. 2) 8.5734(2) 10.1295(1) 10.3010(3) 86.539(1) 70.255(1) 81.905(1) 833.19(4) 100 2 1.826 5.907 452 30.0 14 890 0.0264 4836 4092 170 0.0281 0.0369 0.0731 −0.40/1.18

MECP. The energy values presented in the energy profiles (Figures 9 and 10) correspond to electronic energy, since MECPs are not

6a C13H23Cl2FeN2P 365.05 0.40 × 0.30 × 0.18 colorless prism monoclinic P21/c (no. 14) 16.3517(10) 7.5191(5) 16.2743(10) 90 117.373(1) 90 1776.9(2) 100 4 1.365 1.229 760 30.0 20145 0.0226 5144 4757 178 0.0319 0.0348 0.0871 −0.37/1.51 9 C11H19Cl2FeN2PSe 415.96 0.38 × 0.18 × 0.09 colorless prism triclinic P1̅ (no. 2) 8.3127(5) 10.1519(7) 10.4485(7) 85.290(4) 69.281(3) 82.508(3) 817.06(9) 173 2 1.691 3.560 416 30.0 14 726 0.0219 4738 4187 170 0.0315 0.0367 0.0889 −0.74/1.28

7a C22H38Cl2FeN4O2P2 579.25 0.48 × 0.32 × 0.24 colorless prism monoclinic P21 (no. 4) 10.5712(2) 10.7944(2) 12.3258(2) 90 90.405(1) 90 1406.46(4) 100 2 1.368 0.865 608 30.0 35974 0.0227 8144 7998 312 0.0199 0.0205 0.0536 −0.22/0.54 10a C23H38Cl2FeN4OP2 575.26 0.45 × 0.30 × 0.28 orange prism tetragonal I4̅ (no. 82) 25.2681(9) 25.2681(9) 9.3535(3) 90 90 90 5972.0(4) 100 8 1.280 0.812 2416 27.0 22 999 0.0475 6395 4900 307 0.0659 0.0846 0.1908 −0.69/0.76

stationary points and, hence, a standard frequency analysis is not applicable. 6599


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ASSOCIATED CONTENT S Supporting Information * β-Electron orbitals for complexes 5a, 8a, 9, 7a, and 7b, difference absorbance UV−vis spectrum of 5b in CO-saturated CH2Cl2, and complete crystallographic data and technical details in CIF format for 5a, 5b, 6a, 7a, 8a, 8b, 9, and 10a. This material is available free of charge via the Internet at http:// pubs.acs.org.

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ACKNOWLEDGMENTS Financial support by the “Fonds zur Fö r derung der wissenschaftlichen Forschung” is gratefully acknowledged (project no. P24202). M.J.C. acknowledges FCT, POCI, and FEDER (project POCI/QUI/58925/2004) for financial support. L.F.V. acknowledges UTL/Santander for partial funding, and Fundaçaõ para a Ciência e Tecnologia (FCT, project PEstOE/QUI/UI0100/2011).

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