Striking Differences between the Solution and Solid-State Reactivity of

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Organometallics 2009, 28, 6902–6914 DOI: 10.1021/om900816c

Striking Differences between the Solution and Solid-State Reactivity of Iron PNP Pincer Complexes with Carbon Monoxide )

David Benito-Garagorri,† Luis Gonc-alo Alves,† Michael Puchberger,‡ Kurt Mereiter,§ Luis F. Veiros, Maria Jose Calhorda,^ Maria Deus Carvalho,6¼ Liliana P. Ferreira,#,z Margarida Godinho,# and Karl Kirchner*,† Institute of Applied Synthetic Chemistry, ‡Institute of Materials Chemistry, §Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria, Centro de Quı´mica Estrutural, Instituto Superior T ecnico, 1049-001 Lisboa, Portugal, ^Centro de Quı´mica e 6¼ Bioquı´mica/DQB, Centro de Ci^ encias Moleculares e Materiais/DQB, and #Centro de Fı´sica da Mat eria Condensada/DF, Faculdade de Ci^ encias, Universidade de Lisboa, 1749-016 Lisboa, Portugal, and z Departamento Fı´sica, Faculdade Ci^ encias e Tecnologia, Universidade de Coimbra, 3004-516 Coimbra, Portugal

)

†

Received September 18, 2009

Several new iron(II) complexes of the types [Fe(PNP)X2] (X = Cl, Br) containing tridentate PNP pincer-type ligands based on 2,6-diaminopyridine and 2,6-diaminopyrimidine have been prepared. They all exhibit intermolecular Fe-X 3 3 3 H-N hydrogen bonds, forming supramolecular networks in the solid state. In the case of X = Cl these compounds react readily with gaseous CO both in the solid state and in solution to give selectively the octahedral complexes cis- and trans-[Fe(PNP)(CO)(Cl)2], respectively, whereas with X = Br mixtures of cis and trans isomers are obtained. These transformations are accompanied by color and spin-state changes. CO binding is fully reversible in all cases, and heating solid samples of either cis- or trans-[Fe(PNP)(CO)(X)2] leads to complete ossbauer spectroscopy confirmed the high-spin regeneration of analytically pure [Fe(PNP)(X)2]. M€ nature of the parent five-coordinate Fe(II) complex (δ = 0.80(1) mm s-1) and the shift to two different low-spin octahedral species after reaction with CO in the solid (δ = 0.13(1) mm s-1) or in solution (δ = 0.15(1) mm s-1). Magnetization studies led to a magnetic moment close to 4.9 μB, reflecting the expected four unpaired d-electrons in [Fe(PNP)Cl2], which were confirmed by DFT calculations. The DFT study of the reaction pathway for CO capture led to low energy barriers (e3.4 kcal mol-1). The cis-trans isomerization reaction was found to take place with a low energy barrier (10.8 kcal mol-1), after initial loss of chloride, and involves also spin-state changes.

Introduction One of the ways of modifying and controlling the properties of transition metal complexes is the use of so-called “pincer” ligands. This class of compounds has found numerous applications in various areas of chemistry, including materials chemistry and catalysis, due to their combination of stability, activity, and variability.1 Focusing on the chemistry of transition metal pincer complexes we have recently developed a modular synthetic strategy for PNP ligands where the steric, electronic, and stereochemical properties of the ligands can be varied systematically by introducing dialkyl and diaryl as well as various P-O bond containing phosphine units to N-heterocyclic diamines.2 This has resulted in the preparation of a range of new pincer complexes *Corresponding author. E-mail: [email protected]. (1) (a) van der Boom, M.; Milstein, D. Chem. Rev. 2003, 103, 1759. (b) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750. (c) Singleton, J. T. Tetrahedron 2003, 59, 1837. (d) van Koten, G. Pure Appl. Chem. 1989, 61, 1681. (2) Benito-Garagorri, D.; Becker, E.; Wiedermann, J.; Lackner, W.; Pollak, M.; Mereiter, K.; Kisala, J.; Kirchner, K. Organometallics 2006, 25, 1900. pubs.acs.org/Organometallics

Published on Web 12/04/2009

of molybdenum, iron, ruthenium, nickel, palladium, and platinum,2-5 and applications in catalysis.3 A common structural feature of all these new types of pincer complexes (in comparison to PNP ligands where the PR2 substituents are connected to the central pyridine backbone by CH2 groups or oxygen atoms instead of -NH moieties) is that they all show pronounced hydrogen bonds between the NH groups and available acceptors such as counteranions, solvent molecules, or substituents of neighboring complexes (e.g., Cl, Br), thereby forming insular, 1D, 2D, and 3D assemblies. In certain cases these compounds may be promising for the design of solid-state structures via controlled self-assembly to give products that in a sense resemble metal organic frameworks. Self-assembly induced by weak noncovalent forces, particularly hydrogen bonding, has proven especially effective, largely because of the design of tailored recognition elements in the form of hydrogen (3) Benito-Garagorri, D.; Wiedermann, J.; Pollak, M.; Mereiter, K.; Kirchner, K. Organometallics 2007, 26, 217. (4) Benito-Garagorri, D.; Mereiter, K.; Kirchner, K. Eur. J. Inorg. Chem. 2006, 4374. (5) Benito-Garagorri, D.; Kirchner, K. Acc. Chem. Res. 2008, 41, 201. r 2009 American Chemical Society

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Scheme 1

Scheme 2

bond donor-acceptor arrays that direct the assembly process.6-8 In this context we have recently reported the synthesis of the coordinatively unsaturated iron PNP pincer complex [Fe(PNP-iPr)Cl2] (PNP-iPr = N,N0 -bis(diisopropylphosphino)2,6-diaminopyridine).3,9 This compound features metal-bound chloride atoms, as hydrogen bond acceptors, and secondary amino groups on the pincer ligand, as hydrogen donor groups, and self-assembles in the solid state via intermolecular Fe-Cl 3 3 3 H-N hydrogen bonds, to form three-dimensional networks with solvents such as THF or diethyl ether incorporated. These solvent molecules are readily liberated in the solid state within a few minutes, leaving channels for gas uptake behind. For instance, a solid-state reaction can then take place with CO generating [Fe(PNP-iPr)(CO)Cl2] of red color and a cis configuration of the Cl ligands. In contrast, the same reaction in solution leads selectively to the blue [Fe(PNP-iPr)(CO)Cl2] complex, where the Cl ligands are now trans to one another. CO binding in the solid state is fully reversible, and heating solid samples of cis- or trans-[Fe(PNP-iPr)(CO)Cl2] under vacuum leads to the complete regeneration of [Fe(PNP-iPr)Cl2], which can react again with CO (Scheme 1). In the course of this reaction, the supramolecular connectivities (6) (a) Comprehensive Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., V€ogtle, F., Eds.; Pergamon Press: Oxford, 1996. (b) Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinheim, 1995. (c) Lehn, J.-M. Angew. Chem. 1990, 102, 1347. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (7) For examples on hydrogen-bond-mediated self-assembly, see: (a) Sherrington, D. C.; Taskinen, K. A. Chem. Soc. Rev. 2001, 30, 83. (b) Krische, M. J.; Lehn, J.-M. Struct. Bonding (Berlin) 2000, 94, 3. (c) Prins, L. J.; Huskens, J.; de Jong, F.; Timmerman, P.; Reinhoudt, D. N. Nature 1999, 398, 498. (d) Desiraju, G.; Steiner, T. In The Weak Hydrogen Bond: Applications to Structural Chemistry and Biology; Oxford University Press: Oxford, 1999. (e) Brunet, P.; Simard, M.; Wuest, J. D. J. Am. Chem. Soc. 1997, 119, 2737. (f) Zaworotko, M. J. Nature 1997, 386, 220. (g) Burrows, A. D.; Chan, C.-W.; Chowdry, M. M.; McGrady, J. E.; Mingos, D. M. P. Chem. Soc. Rev. 1995, 329. (h) MacDonald, J. C.; Whitesides, G. M. Chem. Rev. 1994, 94, 2383. (8) (a) Lehn, J.-M. Science 2002, 295, 2400. (b) Hollingsworth, M. D. Science 2002, 295, 2410. (9) Benito-Garagorri, D.; Puchberger, M.; Mereiter, K.; Kirchner, K. Angew. Chem., Int. Ed. 2008, 47, 9142. Angew. Chem. 2008, 120, 9282.

between single molecules are maintained and the reaction proceeds without loss of crystallinity. Such reactions are extremely rare because chemical reactions at the molecular level are likely to destroy the architecture of the supermolecule and its properties. Another remarkable example of this type is the reversible binding of gaseous SO2 to square-planar platinum(II) pincer complexes in the solid state.10 In this article, we extend our investigations on iron PNP pincer complexes and report a combined synthetic, structural, spectroscopic, and DFT computational study aimed at exploring and understanding the electronic structure of iron PNP pincer complexes. Striking differences between the solution and solid-state reactivity of iron PNP pincer complexes with carbon monoxide are discussed.

Results and Discussion Synthesis and Characterization of [Fe(PNP)X2] (X = Cl, Br) Complexes. Similarly to the known ligand PNP-iPr (1a), the new PNP ligands PNPClpym-iPr (1b) and PNPEtOpym-iPr (1c) are prepared conveniently in 68-85% yield by treatment of 2 equiv of PiPr2Cl with the respective 2,6-diaminopyrimidine in the presence of a base (NEt3) in THF as solvent. The ligands were isolated as air-stable solids and were characterized by 1H, 13 C{1H}, and 31P{1H} NMR spectroscopy. Treatment of anhydrous FeCl2 with 1 equiv of the respective PNP ligands in THF at room temperature afforded the pentacoordinated coordinatively unsaturated complexes [Fe(PNPClpym-iPr)Cl2] (2b) and [Fe(PNPOEtpym-iPr)Cl2] (2c) in 92% and 89% isolated yields (Scheme 2). The synthesis of [Fe(PNP-iPr)Cl2] (2a) has been reported elsewhere.3 The analogous bromide complexes [Fe(PNP-iPr)Br2] (3a), [Fe(PNPClpym-iPr)Br2] (3b), and [Fe(PNPOEtpym-iPr)Br2] (3c) were obtained in similar fashion by straightforward complexation of the respective free PNP ligands with anhydrous ferrous (10) (a) Albrecht, M.; van Koten, G. Adv. Mater. 1999, 11, 171. (b) Albrecht, M.; Lutz, M.; Spek, A. L.; van Koten, G. Nature 2000, 406, 970. (c) Albrecht, M.; Lutz, M.; Schreurs, A. M. M.; Lutz, E. T. H.; Spek, A. L.; van Koten, G. J. Chem. Soc., Dalton Trans. 2000, 3797.

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1

Figure 1. H NMR spectrum of paramagnetic [Fe(PNPiPr)Cl2] (2a) (inset: 1H-13C HMQC spectrum of 2a). Asterisks indicate solvent resonances (acetone-d6, THF).

dibromide (75-85% yields). All complexes are thermally robust solids that are air stable both in the solid state and in solution for several days. These pale yellow (chlorides) to orange (bromides) complexes are paramagnetic and display contact-shifted 1H NMR spectra with relatively narrow line widths at room temperature. Thus, despite the paramagnetic nature of these complexes all expected ligand resonances can be readily assigned on the basis of integration. In the case of 2a also 13C{1H}, 1H-1H COSY, and 1H-13C HMQC spectra have been recorded. The 1H and 1H-13C HMQC spectra are shown in Figure 1. From the COSY spectrum, correlations were found between the signal at 137.8 ppm (H7) and the two signals at 12.7 and 6.7 ppm (H8/H9), which were assigned to the signals of the isopropyl group, as well as between the signals at 47.1 ppm (H3/H5) and -20.6 ppm (H4), which were assigned to the pyridine hydrogen atoms. The signal at 71.9 ppm corresponds to the NH protons. From the HMQC spectrum (insert of Figure 1), four correlations were found, viz., H3/5-C3/5 (438.2 ppm), H8/9-C8/9 (190.6 and 153.6 ppm), and H4-C4 (265.8 ppm). No correlation was observed between H7-C7 presumably due to the close proximity to the paramagnetic Fe center. The remaining two carbon signals could be assigned by integration of the carbon spectra (C6 at 660.9 ppm and C7 at -251.7 ppm) and are also depicted in Figure 1. The magnetic properties of 2a-c were further investigated by SQUID magnetometry, and 2a was also studied by M€ ossbauer spectroscopy (vide infra). Figure 2 shows the thermal variation of the inverse molar magnetic susceptibility and χmT of 2a. The variation of χm-1 is well described by a Curie-Weiss law over the whole temperature range, with values for the molar Curie constant (Cm) and paramagnetic Curie temperature (θp) of 2.88(2) cm3 K mol-1 and -2.30(2) K, respectively. An iron effective magnetic moment of 4.8(1) μB was extracted from Cm. Similar values were found for the iron effective moments of 2b and 2c, being 4.7(1) and 4.6(1) μB, respectively. These values are in good agreement with the effective magnetic moment of HS Fe(II) in the spin-only approximation (4.9 μB). In addition, the structures of 2b and 3b have been determined by X-ray crystallography. The molecular structure of 2b 3 THF is depicted in Figure 3, with selected bond distances

Benito-Garagorri et al.

Figure 2. Temperature dependence of the inverse molar magnetic susceptibility (open symbols) and χmT (solid symbols) for 2a. The straight line was obtained from the Curie-Weiss law fitting to the experimental values. A very small temperatureindependent paramagnetic fraction was also found from the fitting, and it was subtracted in the χmT plot.

Figure 3. Structural view of [Fe(PNPClpym-iPr)Cl2] 3 THF (2b 3 THF) showing 50% thermal ellipsoids (THF and C-bound H atoms omitted for clarity). Selected bond lengths (A˚) and angles (deg): Fe1-Cl1 2.4200(12), Fe1-Cl2 2.2601(14), Fe1-P1 2.4904(12), Fe1-P2 2.5188(12), Fe1-N1 2.270(3), Cl1-Fe1-Cl2 110.21(6), P1-Fe1-P2 146.20(4).

and angles given in the caption. A packing diagram of 3b 3 THF is depicted in Figure 4. In both compounds the coordination geometry of the iron center is a distorted square pyramid with N(1), P(1), P(2), and Cl(1)/Br(1) (for 2b 3 THF/3b 3 THF) defining the basal plane and Cl(2)/ Br(2) defining the apex. The comparatively large bond lengths suggest that the iron(II) is in the high-spin state. For both compounds the Fe-Cl/Br bond distances to the apical halide are significantly shorter (by 6.6%) than to the basal halide. The basal planes of the square pyramids have rms aplanarities of 0.178 A˚ (2b 3 THF) and 0.111 A˚ (3b 3 THF) with the iron atom lying 0.782(2) A˚ (2b 3 THF) and 0.734(1) A˚ (3b 3 THF) above these planes in the direction of the apical atoms Cl(2) and Br(2). The X-Fe-X angles in 2b 3 THF and 3b 3 THF are 110.21(6) and 108.33(3)o, respectively (cf. 111.25(1)o in 2a 3 THF).9 All these geometric features are similar to those reported for the five-coordinate complexes Fe(PNP-iPr)Cl2 (2a),3 [Fe(2,6-bis(di-isopropylphosphinomethyl)pyridine)(Cl)2],11 (11) Zhang, J.; Gandelman, M.; Herrman, D.; Leitus, G.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Inorg. Chim. Acta 2006, 359, 1955.

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Scheme 3

Figure 4. Packing diagram of [Fe(PNPClpym-iPr)Br2] 3 THF (3a 3 THF) at T = 100 K in a view down the a-axis (Fe-Br 3 3 3 H-N and OTHF 3 3 3 H-N hydrogen bonds as dashed lines). Selected bond lengths (A˚) and angles (deg): Fe1-Br1 2.5620(8), Fe1-Br2 2.3770(9), Fe1-P1 2.4919(13), Fe1-P2 2.5014(13), Fe1-N1 2.249(3), Br1-Fe1-Br2 108.33(3), P1-Fe1-P2 144.93(5).

[Fe(2,6-bis(dimethylaminomethyl)pyridine)(Cl)2],12 [Fe(2,6-bis(2,6-diisopropylphenylaminomethyl)pyridine)(Cl)2],13 and [Fe(2,6-bis(1-(2,6-diisopropylphenylimino)ethyl)pyridine)(Cl)2].14 As has been already observed in our previous investigations, PNP complexes based on 2,6-diaminopyridines and other related N-heterocyclic diamines3 contain relatively acidic NH protons that are available for hydrogen bond formation with anions and/or suitable solvent molecules. The present compounds reflect this phenomenon. Complexes 2b and 3b crystallize as solvates with one THF molecule per Fe complex, where the THF molecules are N-H 3 3 3 O hydrogen bonded to one of the two NH groups of each complex. The second NH group of complexes undergo intermolecular hydrogen bonding with the metal-bonded halides Cl(1) and Br(1) of neighboring complexes with N 3 3 3 Cl = 3.239 A˚ for 2b 3 THF and N 3 3 3 Br = 3.457 A˚ for 3b 3 THF. Thereby they form 1D hydrogen-bonded continuous zigzag chains with 21-symmetry and all chains parallel to the b-axis (monoclinic lattice of space group symmetry P21/n, b = 14.64 A˚) in the case of 2b 3 THF and with glide plane symmetry and all chains parallel to the c-axis (monoclinic lattice of space group symmetry P21/c, c = 15.96 A˚) in the case of 3b 3 THF. This proves that the two solids 2b 3 THF and 3b 3 THF are not isostructural, despite their architectural similarities. Synthesis and Characterization of cis- and trans-[Fe(PNP)(CO)X2] (X = Cl, Br) Complexes. Despite being coordinatively unsaturated, complexes 2 and 3 do not react with simple σ-donor ligands such as THF, MeOH, acetone, DMSO, pyridine, and tertiary phosphines (PPh3 and PMe3), but react readily with the strong π-acceptor ligand CO both in the solid state and in solution. When exposed to 1 atm of (12) O’Reilly, R. K.; Gibson, V. C.; White, A. J. P.; Williams, D. J. Polyhedron 2004, 23, 2921. (13) Britovsek, G. J. P.; Gibson, V. C.; Mastroianni, S.; Oakes, D. C. H.; Redshaw, C.; Solan, G. A.; White, A. J. P.; Williams, D. J. Eur. J. Inorg. Chem. 2001, 431. (14) Small, B. L.; Brookhart, M.; Bennett, A. M. A. J. Am. Chem. Soc. 1998, 120, 4049.

gaseous CO at room temperature, solid (and solvent-free) 2a-c rapidly convert quantitatively into solid cis-[Fe(PNPiPr)(CO)(Cl)2] (4a), cis-Fe(PNPClpym-iPr)(CO)Cl2 (4b), and cis- [Fe(PNPOEtpym-iPr)(CO)Cl2] (4c) as the sole product, as indicated by a color change in the material from light yellow to deep red (Scheme 3). However, when single crystals in the form of 2 3 THF were exposed to CO, the formation of 4 takes place more slowly. It seems therefore reasonable to assume that desolvation of 2 3 THF takes place with at least partial preservation of the network structure with the sites formerly occupied by solvent molecules now serving as channels through which CO can readily diffuse into the solid. The exclusive formation of 4 in the solid state was confirmed by 13 C{1H}, 15N{1H}, and 31P{1H} solid-state NMR (in the case of 4a and 4b), 1H and 31P{1H} solution NMR spectroscopy (in the case of 4a-c), and IR spectroscopy. The recording of 13 C{1H} solution NMR spectra was precluded, since the red cis isomers 4 transform rapidly (e.g, within ca. 90 min in dmso-d6 as the solvent) and quantitatively into the blue trans complexes 5 (vide infra). It has to be noted that quantitative isomerization takes also place in solvents such as acetone, nitromethane, and THF. In sharp contrast, when CO was bubbled into acetone or nitromethane solutions of 2a-c for 2 min, blue solids were formed in excellent yields (92-95%), which were identified as the corresponding trans complexes [Fe(PNP-iPr)(CO)(Cl)2] (5a), [Fe(PNPClpym-iPr)(CO)Cl2] (5b), and [Fe(PNPOEtpym-iPr)(CO)Cl2] (5c) (Scheme 3). These compounds are air-stable both in the solid state and in solution. Complexes 5 were fully characterized by a combination of solution and solid-state 1H, 13 C{1H}, and 31P{1H} NMR, IR spectroscopy, and elemental analysis. In addition to the spectroscopic characterization, the solid-state structure of 5b was determined by single-crystal X-ray diffraction. An ORTEP diagram is depicted in Figure 5, with selected bond distances given in the captions. Accordingly, the reaction of the chloride complexes 2 with CO proceeds selectively, yielding either cis- or trans-[Fe(PNP-iPr)(CO)(Cl)2] depending on the reaction conditions employed. Both transformations are accompanied by changes in the color of the products, the coordination geometry around the iron center, and the iron spin states. CO binding is fully reversible, and heating solid samples of either 4 or 5 at 100 C for 5 min under vacuum leads to the complete regeneration of analytically pure crystalline 2,

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Figure 5. Structural view of trans-[Fe(PNPClpym-iPr)(CO)Cl2] 3 THF (5b 3 THF) showing 30% thermal ellipsoids (THF and C-bound H atoms omitted for clarity). Selected bond lengths (A˚) and angles (deg): Fe1-Cl1 2.3304(5), Fe1-Cl2 2.3067(5), Fe1-P1 2.2546(5), Fe1-P2 2.2469(5), Fe1-N1 1.9890(14), Fe1-C17 1.7659(18), P1-Fe1-P2 166.74(2), Cl1-Fe1-Cl2 175.36(2), N1-Fe1-C17 178.98(7).

which can react again with CO either in the solid state or in solution to give 4 or 5. At room temperature loss of CO is slow, needing about two weeks to completely regenerate 2. This “on” and “off” process can be repeated for at least five cycles without any noticeable decomposition of 2. The reversibility of this reaction in the solid state has been elucidated by time-resolved infrared spectroscopy, where the stretching vibration of coordinated CO in either 4 or 5 was monitored. The general effect of spin-state changes upon CO coordination to transition metal complexes is well described in the literature.15,16 With respect to Fe(II) several examples of reversible and irreversible carbonylation reactions are reported.17-20 Most sensitive to configurational changes at the metal center are the coordinated pyridine and pyrimidine nitrogen atoms. In the solid-state 15N-CP/MAS NMR spectrum these atoms give rise to signals at 119.7 and 102.6 ppm for 4a and 4b and 167.5 and 151.0 ppm for 5a and 5b, respectively. The resonances of the second pyrimidine nitrogen atoms are hardly affected, exhibiting signals at 183.8 and 186.1 ppm for 4b and 5b. The NH nitrogen atoms are observed in the range 49.1 to 64.0 ppm. Solid-state 15N-CP/MAS NMR spectra of 4a and 5a are shown in Figure 6. Likewise, 31P{1H} NMR solution (and solid-state) spectra of 4a-c exhibit resonances at 111.5, 119.8/102.0, and 113.4/101.9 ppm, while those of 5a-c are observed at 122.5, 134.9/117.4, and 128.3/ 115.7 ppm, respectively. In addition, complexes 4b,c and 5b,c show the expected doublet pattern with large JPP coupling constants of 173-234 Hz in the solution 31P{1H} NMR spectrum, consistent with a trans-P,P configuration. The IR spectra of 4a-c exhibit strong absorption bands at (15) (a) Keogh, D. W.; Poli, R. J. Am. Chem. Soc. 1997, 119, 2516. (b) Carreon, J.-L.; Harvey, J. N. J. Am. Chem. Soc. 2004, 126, 5789. (c) Strickland, N.; Harvey, J. N. J. Phys. Chem. B 2007, 111, 841. (16) For a review of open-shell complexes see: Poli, R. Chem. Rev. 1996, 96, 2135. (17) Hardman, N. J.; Fang, X.; Scott, B. L.; Wright, R. J.; Martin, R. L.; J. Kubas, G. J. Inorg. Chem. 2005, 44, 8306. (18) Stynes, D. V.; Hui, Y. S.; Chew, V. Inorg. Chem. 1982, 21, 1222. (19) Ellison, J.; Nienstedt, A.; Shoner, S. C.; Barnhart, D.; Cowen, J. A.; Kovacs, J. A. J. Am. Chem. Soc. 1998, 120, 5691. (20) (a) Danopoulus, A. A.; Pugh, D.; Smith, H.; Sassmanhshausen, J. Chem.;Eur. J. 2009, 15, 5491. (b) Henry, R. M.; Shoemaker, R. K.; Newell, R. H.; Jacobsen, G. M.; DuBois, D. L.; Rakowski DuBois, M. Organometallics 2005, 24, 2481. (c) Breuer, J.; Fruhauf, H.-W.; Smeets, W. J. J.; Spek, A. L. Inorg. Chim. Acta 1999, 291, 438.


Figure 6. Solid-state 15N-CP/MAS NMR spectra of (a) 4a and (b) 5a.

Figure 7. Room-temperature M€ ossbauer spectra of 2a, 4a, and 5a.

1947, 1952, and 1955 cm-1, while those of 5a-c are observed at 1956, 1974, and 1958 cm-1, being shifted slightly to higher wave numbers. This may be attributed to the fact that pyridine and pyrimidine are somewhat stronger σ-donors than the chloride ligand, thus having a stronger trans influence. Since pyridine is a stronger electron donor than pyrimidine, the CO absorptions of the pyridine-based complexes 4a and 5a are observed at lower wave numbers than the respective absorptions of the pyrimidine-based complexes 4b,c and 5b,c. M€ ossbauer measurements of 2a, 4a, and 5a were performed at room temperature, and the obtained spectra are depicted in Figure 7. For the 2a sample, the spectrum is mainly resolved by a quadrupole doublet with hyperfine parameters characteristic of high-spin (HS) Fe(II) in a rather asymmetric environment (isomer shift (δ) = 0.80(1) mm s-1 and quadrupole splitting (Δ) = 2.56(1) mm s-1). A second

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Scheme 4

small component (∼15%) is also detected and is attributed to a Fe(III) decomposition product, most probably formed during M€ ossbauer sample preparation and measurement under air. In the case of 4a and 5a, the results are significantly different. The spectrum of 5a is well fitted to a quadrupole doublet with δ = 0.15(1) mm s-1 and Δ = 1.56(1) mm s-1, and, for 4a, the main contribution corresponds to a quadrupole doublet with similar δ (0.13(1) mm s-1) and Δ = 1.09(1) mm s-1. These parameters were assigned to LS (lowspin) Fe(II). The isomer shift decrease observed from 2a to 4a and 5a is explained by the HS-LS change induced by the reaction with CO. Accordingly, the decrease in the quadrupole splitting values is explained by the much less asymmetric environment of Fe in coordination six (4a and 5a) than in five coordination (2a). A higher quadrupole splitting value is obtained for the trans complex (5a) than for the cis one (4a), which seems to be usual for Fe(II) low-spin complexes.21 Indeed, when the very simple point charge model for sixcoordinate LS Fe(II) complexes described by Berret and Fitzsimmons22 is applied, taking into account the formal charge of the ligands and the bond distances obtained by DFT calculations for complexes 4a and 5a, a relation of Δtrans/Δcis = 1.9 is deduced. Considering that in the present case the symmetry is rather different from the original ML4X2 model and the angles deviate from those characteristic of the perfect octahedral environment, the agreement with the experimental relation obtained (1.4) seems quite reasonable. In the 4a sample, a smaller fraction (26%) of a HS Fe(II) fraction is also detectable arising from spontaneous loss of CO re-forming slowly the parent compound 2a. In contrast to the chloride systems, the reaction of 3a and 3b with CO both in the solid state and in solution (acetone, nitromethane) is not selective and mixtures of cis- and trans-[Fe(PNP-iPr)(CO)(Br)2] (6a/7a) and [Fe(PNPClpym-iPr)(CO)Br2] (6b/7b) are formed (Scheme 4). These complexes could not be isolated in pure form, and characterization was accomplished by means of solution and solid-state 1H, 13 C{1H}, and 31P{1H} NMR and IR spectroscopy. Crystallization of a mixture of 6b/7b resulted in the formation of crystals that contained both the cis and trans form of [Fe(PNPClpym-iPr)(CO)Br2] overlaid in the crystal lattice in a single X-ray diffraction averaged moiety with a common pincer ligand and one axial ligand position exclusively occupied by Br (Br(3)), whereas the two remaining ligand positions showed a mixed occupation by ca. 50% Br and ca. 50% CO. Structural views of 6b 3 THF and 7b 3 THF are shown in Figure 8 with selected bond distances and angles given in the caption. The synthesis of iron complexes bearing tridentate PNP ligands in which the central pyridine-based ring donor (21) Bancroft, G. M., M€ ossbauer Spectroscopy-An Introduction for Inorganic Chemists and Geochemists; McGraw-Hill: New York, 1973. (22) Berrett, R. R.; Fitzsimmons, B. W. J. Chem. Soc. (A) 1967, 525.

Scheme 5

contains -CH2PiPr2 substituents in the two ortho positions has been recently reported by Milstein and co-workers. Since these types of ligands are sterically similar but electronically rather different, we wondered whether iron complexes of that type would also undergo CO reactions similar to those of the iron PNP complexes 3 and 4. As a model complex we chose [Fe(PNPCH2-iPr)Cl2] (8).11 Accordingly, exposure of solid 8 (which is a yellow complex) to 1 atm of gaseous CO for 2 min yielded a light brown compound identified as cis-[Fe(PNPCH2-iPr)(CO)Cl2] (9) on the basis of multinuclear NMR and IR spectroscopy and combustion analysis (Scheme 5). The reaction proceeds without any significant color change. In analogy to complexes 2 and 3, the reaction was accompanied by a change of the spin state from a quintet (S = 2) to a singlet ground state (S = 0). In contrast to complexes 2 and 3, the outcome of the reaction of 8 with gaseous CO is independent of whether the reaction is carried out in the solid state or in solution. Moreover, the reaction is not reversible, and heating a solid sample of 9 resulted in decomposition to give only intractable materials. The 13C{1H} NMR spectrum of 9 gives rise to two sets of signals for the isopropyl methine and methyl carbon atoms, which is consistent with a complex of Cs symmetry with a cisdichloro arrangement. In the case of a trans-dichloro arrangement (C2v symmetry) only one set of signals for these carbon atoms would be observed. In the IR spectrum of 9 the CO stretching frequency is observed at 1943 cm-1, indicating only a slightly stronger π-interaction than in the case of 4a than in 5a (cf. 1947 and 1956 cm-1 in cis- and trans-[Fe(PNPiPr)(CO)(Cl)2], respectively). DFT Calculations. DFT calculations were performed in order to understand the electronic structure of the parent Fe(II) five-coordinate complexes and their limited reactivity. The structure of complex Fe(PNP-iPr)Cl2 (2a) was fully optimized using a DFT23 approach (ADF program).24 The (23) Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989. (24) (a) te Velde, G.; Bickelhaupt, F. M.; Baerends, E. J.; Guerra, C. F.; van Gisbergen, S. J. A.; Snijders, J. G.; Ziegler, T. J. Comput. Chem. 2001, 22, 931–967. (b) Guerra, C. F.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor. Chem. Acc. 1998, 99, 391–403. (c) ADF2005.01, SCM; Theoretical Chemistry, Vrije Universiteit: Amsterdam, The Netherlands.

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Figure 8. Structural views of cis-[Fe(PNPClpym-iPr)(CO)Br2] (6b) and trans-[Fe(PNPClpym-iPr)(CO)Br2] (7b) in cis/trans-[Fe(PNPClpym-iPr)(CO)Br2] 3 THF (6b/7b 3 THF) showing 50% thermal ellipsoids (THF and C-bound H atoms omitted for clarity). Selected bond lengths (A˚) and angles (deg): for 6b, Fe1-Br1 2.4272(13), Fe1-Br3 2.4808(7), Fe1-P1 2.2709(11), Fe1-P2 2.2642(11), Fe1-N1 1.973(3), Fe-C18 1.750(12) P1-Fe1-P2 166.85(4), Br1-Fe1-N1 178.78(10), Br3-Fe1-C18 176.4(3); for 7b, Fe1-Br2 2.3960(13), Fe1-Br3 2.4808(7), Fe1-P1 2.2709(11), Fe1-P2 2.2642(11), Fe1-N1 1.973(3), Fe-C17 1.770(12), P1-Fe1-P2 166.85(4), Br2-Fe1-Br3 176.58(4), N1-Fe1-C17 178.5(3).

Figure 9. Relative energy and nature of the R and β frontier orbitals of [Fe(PNP-iPr)Cl2] (2a). Five R-orbitals (all d) and one β-orbital (dxy) are occupied; the highest energy β-orbital (dx2-y2) is not shown.

agreement between the calculated and X-ray structures is relatively good, the most difficult parameter to reproduce being the Cl-Fe-Cl angle, which was calculated as 119, compared to an experimental value of ∼108. The frontier orbitals are shown in Figure 9, with their relative energy. Since the symmetry is low, the iron d-orbitals are mixed, so the labeling is indicative. There are five occupied d-levels with R-spin and only one with a β-spin. The fifth d-orbital with β-spin has a very high energy and is not shown. These results fit the magnetic properties described above and the magnetic moment corresponding to four unpaired electrons. A single-point calculation on this structure with a spin state S = 0 (LS) showed that the corresponding energy was 29.2 kcal mol-1 higher. Reactions of complex 2a will yield octahedral complexes, which may be high- or low-spin species, depending on the new ligand entering the coordination sphere of 2a. Since d6 high-spin octahedral complexes are not particularly stable, it is likely that reactions with strong ligands will be more successful. As shown

above, CO reacts with 2a, both in the solid and in solution. The most stable form of the octahedral complex is the LS 5a (trans), followed by the LS cis 4a (3.1 kcal mol-1 higher), the HS trans (36.6 kcal mol-1), and the HS cis (44.7 kcal mol-1). This reaction converts a HS d6 five-coordinate complex into a LS octahedral one, and although the reaction is exothermic and there should be a very small barrier for CO binding, the spin crossover barrier will determine the barrier. The reaction of CO addition to 2a was investigated by means of DFT calculations (Gaussian 03/PBE), and the energy profile obtained is presented in Figure 10. Carbon monoxide addition to the dichloro complex [Fe(PNP-iPr)Cl2] (2a) is a “spin-forbidden” or “nonadiabatic” reaction since there is a change in spin state, from the reagent to the product. In fact, while 2a has a spin-quintuplet (S = 2) ground state, the products, [Fe(PNP-iPr)(CO)Cl2] (4a and 5a), exist as spin-singlet molecules (S = 0). The energy profile associated with such a reaction goes through a minimum-energy crossing point (MECP) of the two potential

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Figure 10. Energy profile (PBE/VDZP) for the addition of CO to [Fe(PNP-iPr)Cl2] (2a), yielding trans-[Fe(PNP-iPr)(CO)Cl2] (left side) and cis-[Fe(PNP-iPr)(CO)Cl2] (right side). The energy values (kcal/mol, no solvent correction) are referred to the separated reagents. The solid curve corresponds to the spin-quintuplet PES (S = 2), and the dashed curve to the spin-singlet PES (S = 0). The Fe-C(CO) distance (A˚) along the reaction coordinate is indicated.

energy surfaces (PES) involved.25 In this MECP both the energy and the geometry of the molecule are the same in the two surfaces. Once that point (MECP) is reached, following the reaction coordinate, there is a given probability for the system to change spin state and hop from one PES to the other, thus giving rise to the “spin-forbidden” reaction.26 The energy profile obtained for the formation of the trans isomer of [Fe(PNP-iPr)Cl2(CO)] (5a) is presented on the left side of Figure 10. Starting with the separated reactants and following the S = 2 PES, there is formation of a van der Waals pair between the two reacting molecules, CO and [Fe(PNP-iPr)Cl2] (I5a), with a rather long Fe-C(CO) distance (3.43 A˚), and the corresponding small stabilization of the system (3.1 kcal mol-1). From here, the high -spin isomer of the product (55a) is formed in a single step, going over a negligible energy barrier (0.1 kcal mol-1). The “5” superscript in the label denotes the spin multiplicity (2Sþ1). In the corresponding transition state (TS5a), formation of the new Fe-C(CO) is only starting, as stated by a long separation (3.10 A˚), still more than 1 A˚ longer than the corresponding bond length in the product, 55a (2.03 A˚). From 55a, the minimum-energy crossing point (MECP) between the two potential energy surfaces (CP5a) is easily reached, with an associated energy barrier of only 0.9 kcal mol-1. This negligible barrier denotes the minimal geometry distortion associated with the process, which requires a simple 0.03 A˚ shortening in the Fe-C(CO) bond distance, to go from 55a to CP5a. Once the crossing point (CP5a) is reached and the hopping between surfaces is accomplished, the system follows the S = 0 PES downhill until the formation of the final product, i.e., low-spin (S = 0) trans-[Fe(PNP-iPr)(CO)Cl2] (5a). The overall process is very favorable, from the thermodynamic point of view, since 5a is 25.0 kcal/mol more stable than its high-spin isomer (55a) and 37.6 kcal mol-1 more stable than the initial reagents. The energy profile calculated for the formation of the cis isomer of [Fe(PNP-iPr)(CO)Cl2] (4a), represented on the (25) For excellent reviews on MECP and their location for transition metal complexes see: (a) Harvey, J. N.; Poli, R.; Smith, K. M. Coord. Chem. Rev. 2003, 238-239, 347. (b) Poli, R.; Harvey, J. N. Chem. Soc. Rev. 2003, 32, 1. (26) Harvey, J. N. Phys. Chem. Chem. Phys. 2007, 9, 331.

right side of Figure 10, is equivalent in its general features to the one obtained for 5a and discussed above. Thus, no further discussion is needed. Interestingly, small values were obtained for all energy barriers involved in the mechanism of CO addition, the highest barrier being 3.4 kcal mol-1, considering both profiles in Figure 10. This is in excellent agreement with the experimental observations, namely, with the prompt formation of the low-spin CO complexes from 2a and CO. When the reaction is carried on in solution, the most stable isomer (5a) is produced. However, if the reaction is performed in the solid state the cis isomer (4a) results, given the mobility restrictions associated with the crystal packing. In fact, formation of 5a requires more extreme geometry changes, compared with the formation of 4a, in particular the opening of the Cl-Fe-Cl angle, which reaches 174 in 5a. In solution (acetone, nitromethane, dmso) at room temperature, the red cis isomers 4 are unstable and transform rapidly into the blue trans complexes 5. In the absence of detectable intermediates we again performed DFT (Gaussian 03/PBE) calculations on the corresponding mechanism. As model systems we have chosen cis-[Fe(PNPiPr)(CO)(Cl)2] (4a) and trans-[Fe(PNP-iPr)(CO)(Cl)2] (5a). In agreement with the experiment, 5a is thermodynamically slightly favored over 4a by 2.6 kcal mol-1 (cf. 3.6 kcal mol-1 without solvent correction). For comparison, the calculated energy difference between cis-[Fe(PNP-iPr)(CO)(Br)2] (6a) and trans-[Fe(PNP-iPr)(CO)(Br)2] (7a) is even smaller (0.2 kcal mol-1). This explains why in contrast to [Fe(PNP-iPr)(CO)(Cl)2] both isomers are present in solution and presumably also in the solid state. The isomerization mechanism most likely involves chloride dissociation and the formation of a transient cationic intermediate [Fe(PNP)(CO)(Cl)]þ. This is supported experimentally by the fact that addition of a chloride anion source (e.g., nBu4NCl) to a solution of 4a leads to longer isomerization times, whereas performing the reaction under CO atmosphere has no effect. The pentacoordinated nature of intermediate [Fe(PNP)(CO)(Cl)]þ allows for the existence of two conformations, one with the CO in the apical and the Cl in the basal position (A) and vice versa (B), as shown in Figure 11. In the case of A, the singlet ground state 1A is

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Figure 11. Energy profile (PBE/VDZP) for the isomerization of cis-[Fe(PNP-iPr)(CO)Cl2] (4a) to trans-[Fe(PNP-iPr)(CO)Cl2] (5a). The energy values (kcal/mol, solvent corrected, solvent = CH3NO2) are referred to the cationic intermediate [Fe(PNP-iPr)(CO)Cl]þ (1A). The solid curve corresponds to the spin-singlet PES (S = 0), and the dashed curve to the spin-triplet PES (S = 1).

energetically favored by 9.8 kcal mol-1 over the triplet state 3 A, whereas in the case of B the stability order is reversed, with the triplet state 3B being more stable by 8.2 kcal mol-1. The superscript in the label denotes the spin multiplicity (2Sþ1). The energetically most favorable pathway proceeds from 1A to 3A, which is accompanied by a spin-state change from S = 0 to S = 1. The MECP between these two PES (CPA) is easily accessible, lying 10.3 kcal mol-1 above 1A. The actual isomerization reaction takes place via transition state 3TSAB to give 3B, which is thermodynamically more stable than 3A by 5.3 kcal mol-1. The energy barrier for this step is merely 1.2 kcal mol-1. Finally, coordination of a chloride atom to 3B yields, in agreement with experimental findings, the thermodynamic product 5a. The last step is again associated with a spin-state change (S = 1 to S = 0). An alternative isomerization pathway on the S = 0 PSE from 1A to 1B via 1TSAB is kinetically disfavored, with an energy barrier of 16.8 kcal mol-1 (cf. 1.2 kcal mol-1 on the S = 1 PSE), as depicted in Figure 11. Experimentally, however, the formation of paramagnetic species could not be detected when the isomerization process was monitored by 1H and 31P{1H} NMR spectroscopy.

Conclusions In summary, we have shown that pentacoordinated and thus formally coordinatively unsaturated iron(II) pincer complexes of the type [Fe(PNP)X2] (X = Cl, Br) are readily obtained by reacting anhydrous FeX2 with PNP ligands based on 2,6-diaminopyridine and 2,6-diamionopyrimidine. A common structural feature of these complexes in the solid state is that they all show pronounced hydrogen bonds between the NH groups and available acceptors such as solvent molecules and/or Cl and Br ligands of neighboring complexes, thereby forming various supramolecular assemblies. Surprisingly, the reactivity of these compounds toward CO in the solid state and solution turned out to be quite different. While in the solid state with X = Cl, regardless of

the nature of the central aromatic ring, i.e., pyridine or pyrimidine, cis-[Fe(PNP)(CO)(Cl)2] is selectively formed, in solution the corresponding trans isomer is exclusively formed. This reaction is accompanied by a color change (from yellow to red or blue). M€ ossbauer spectroscopy showed that the highspin pentacoordinate parent complex (δ = 0.80(1) mm s-1; Δ = 2.56(1) mm s-1) is transformed into low-spin octahedral complexes (δ around 0.14 mm s-1) with different quadrupole splittings corresponding to the cis (Δ = 1.09(1) mm s-1) and the trans (Δ = 1.56 mm s-1) isomers. DFT calculations also showed that the open-shell configuration with four unpaired d electrons is the most stable one, in agreement with the magnetic moments close to 4.9 μB obtained from magnetization data. A DFT study showed that CO coordinates to the parent HS complex [Fe(PNP-iPr)Cl2], forming the highspin octahedral derivative [Fe(PNP-iPr)(CO)(Cl)2], which then undergoes a spin crossover. The small calculated activation barriers (e3.4 kcal mol-1) are in excellent agreement with the experimental observations, i.e., fast formation of low-spin CO complexes from high-spin [Fe(PNP-iPr)Cl2] and CO. CO binding is fully reversible, and heating solid samples of either cis or trans isomer under vacuum leads to the complete regeneration of analytically pure Fe(PNP-iPr)Cl2, which can react again with CO. In solution, the cis chloro complexes readily isomerize to give the thermodynamically favored trans complexes. The isomerization reaction involves chloride dissociation via transient cationic intermediates [Fe(PNP-iPr)(CO)(Cl)]þ, which, depending on the geometry, adopt either a singlet or a triplet ground state. A corresponding pathway was also obtained from DFT. On the other hand, with X = Br, the reaction is no longer selective and yields mixtures of both cis and trans isomers in solution and in the solid state. The selectivity and full reversibility of CO uptake and release in these materials, which can be detected and monitored by a variety of techniques such as solid-state and solution NMR, UV/vis, IR spectroscopy, M€ ossbauer spectroscopy, SQUID magnetometry, and X-ray powder diffraction, as demonstrated,

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may suggest potential applications both as efficient CO sensors and as crystalline switches.

Experimental Section General Procedures. All manipulations were performed under an inert atmosphere of argon by using Schlenk techniques. The solvents were purified according to standard procedures.27 N, N0 -Bis(diisopropylphosphino)-2,6-diaminopyridine (PNP-iPr) (1a),2 [Fe(PNP-iPr)Cl2] (2a),3 cis-[Fe(PNP-iPr)(CO)Cl2] (4a),9 trans-[Fe(PNP-iPr)(CO)Cl2] (5a),9 and [Fe(PNPCH2-iPr)Cl2] (8)11 were prepared according to the literature. The deuterated solvents were purchased from Aldrich and dried over 4 A˚ molecular sieves. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on a Bruker AVANCE-250 spectrometer and were referenced to SiMe4 and H3PO4 (85%), respectively. 1H and 13 C{1H} NMR signal assignments were confirmed by 1 H-COSY, 135-DEPT, and HMQC(1H-13C) experiments. The solid-state NMR spectra were measured at room temperature at a Bruker AVANCE 300 spectrometer using a 4 mm MAS broadband probe head. The rotor spinning speed for all performed experiments was 11 kHz. The 13C spectra were measured with ramped-CP/MAS experiments at a resonance frequency of 75.40 MHz. The 31P spectra were measured with HPDEC (high-power decoupled) experiments at a resonance frequency of 121.38 MHz, and the spectra were referenced externally against phosphoric acid (31P: 0 ppm). The 15N spectra were measured with ramped-CP/MAS experiments at a resonance frequency of 30.38 MHz, and the spectra were referenced against NH4Cl (15N: 0 ppm). Magnetization measurements as a function of temperature were performed on powder samples using a SQUID magnetometer (Quantum Design MPMS). The curves were obtained at 5 and 100 mT for temperatures ranging from 2 to 300 K. The susceptibilities values were corrected for diamagnetism of the constituent atoms using Pascal constants. The M€ ossbauer spectra were recorded at room temperature in transmission mode using a conventional constant-acceleration spectrometer and a 25 mCi 57Co source in a Rh matrix. The velocity scale was calibrated using an R-Fe foil. The spectra were fitted to Lorentzian lines using the WinNormos software program, and the isomer shifts reported are relative to metallic R-Fe at room temperature. Synthesis. N,N0 -Bis(diisopropylphosphino)-2,6-diamino-4chloropymidine (PNPClpym-iPr) (1b). Triethylamine (3.9 mL, 27.7 mmol) was added to a solution of 2,6-diamino-4-chloropymidine (2.0 g, 13.8 mmol) in toluene (50 mL). The mixture was cooled to 0 C, and PiPr2Cl (27.7 mmol, 4.4 mL) was added dropwise. The reaction was allowed to reach room temperature and refluxed overnight. After that, the solution was filtered off and the solvent was removed under vacuum to give a white solid. Yield: 4.6 g (88%). Anal. Calcd for C16H31ClN4P2: C, 51.00; H, 8.29; N, 14.87. Found: C, 51.07; H, 8.25; N, 14.96. 1H NMR (δ, CDCl3, 20 C): 6.45 (d, J = 2.1 Hz, 1H, pym5), 5.01 (s, 1H, NH), 4.90 (s, 1H, NH), 1.77-1.71 (m, 4H, CH(CH3)2), 1.04-0.98 (m, 24H, CH(CH3)2). 13C{1H} NMR (δ, CDCl3, 20 C): 167.7 (d, J = 21.3 Hz, pym6), 163.8 (d, J = 13.8 Hz, pym4), 159.8 (pym2), 95.4 (d, J = 20.7 Hz, pym5), 26.1 (d, J = 10.3 Hz, CH(CH3)2), 26.0 (d, J = 11.0 Hz, CH(CH3)2), 18.5 (d, J = 19.5 Hz, CH(CH3)2), 18.4 (d, J = 19.5 Hz, CH(CH3)2), 17.2 (vt, J = 8.6 Hz, CH(CH3)2). 31P{1H} NMR (δ, CDCl3, 20 C): 52.4, 49.7. N,N0 -Bis(diisopropylphosphino)-2,6-diamino-4-ethoxypymidine (PNPEtOpym-iPr) (1c). This ligand has been prepared analogously to 1b with 2,6-diamino-4-ethoxypymidine (400 mg, 2.6 mmol), triethylamine (0.75 mL, 5.2 mmol), and PiPr2Cl (0.83 mL, 5.2 mmol) as the starting materials. Yield: 794 mg (79%). Anal. Calcd for C18H36N4OP2: C, 55.94; H, 9.39; N, 14.50. Found: C, (27) Perrin, D. D.; Armarego, W. L. F. Purification of Laboratory Chemicals, 3rd ed.; Pergamon: New York, 1988.

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55.88; H, 9.42; N, 14.66. 1H NMR (δ, CDCl3, 20 C): 5.84 (d, 2.1 Hz, 1H, pym5), 4.74 (vt, J = 9.6 Hz, 2H, NH), 4.24 (q, J = 7.1 Hz, 2H, CH2), 1.81-1.70 (m, 4H, CH(CH3)2), 1.33 (t, J = 7.1 Hz, 3H, CH3), 1.11-0.98 (m, 24H, CH(CH3)2). 13C{1H} NMR (δ, CDCl3, 20 C): 170.8 (pym2), 167.7 (dd, J = 2.7 Hz, J = 20.3 Hz, pym6), 163.5 (dd, J = 2.3 Hz, J = 11.9 Hz, pym4), 79.0 (pym5), 62.0 (CH2), 26.2 (d, J = 13.1 Hz, CH(CH3)2), 26.1 (d, J = 11.5 Hz, CH(CH3)2), 18.7 (d, J = 15.0 Hz, CH(CH3)2), 18.5 (d, J = 14.6 Hz, CH(CH3)2), 17.4 (d, J = 8.8 Hz, CH(CH3)2), 17.1 (d, J = 8.0 Hz, CH(CH3)2), 14.6 (CH3). 31P{1H} NMR (δ, CDCl3, 20 C): 49.9, 49.7. [Fe(PNPClpym-iPr)Cl2] (2b). FeCl2 (336 mg, 2.65 mmol) was added to a solution of 1b (1.0 g, 2.65 mmol) in 50 mL of THF, and the mixture was stirred at room temperature overnight. The solvent was removed under reduced pressure, and the remaining pale yellow solid was washed twice with Et2O (10 mL) and dried under vacuum. Yield: 1.2 g (92%). Anal. Calcd for C16H31Cl3FeN4P2: C, 38.16; H, 6.20; N, 11.13. Found: C, 38.22; H, 6.23; N, 11.19. 1H NMR (δ, acetone-d6, 20 C): 139.72 (s, 2H, CH(CH3)2), 138.86 (s, 2H, CH(CH3)2), 75.96 (s, 1H, NH), 75.59 (s, 1H, NH), 35.04 (s, 1H, pym5), 12.34 (s, 6H, CH(CH3)2), 11.52 (s, 6H, CH(CH3)2), 7.07 (s, 6H, CH(CH3)2), 6.54 (s, 6H, CH(CH3)2). μeff = 4.7(1) μB. [Fe(PNPEtOpym-iPr)Cl2] (2c). This complex has been prepared analogously to 2a with 1c (1 g, 2.66 mmol) and FeCl2 (338 mg, 2.66 mmol) as the starting materials. Yield: 1.2 g (89%). Anal. Calcd for C18H36Cl2FeN4OP2: C, 42.13; H, 7.07; N, 10.92. Found: C, 42.05; H, 7.12; N, 10.98. 1H NMR (δ, acetone-d6, 20 C): 146.54 (s, 2H, CH(CH3)2), 144.72 (s, 2H, CH(CH3)2), 75.92 (s, 1H, NH), 68.80 (s, 1H, NH), 32.01 (s, 1H, pym5), 13.11 (s, 6H, CH(CH3)2), 12.95 (s, 6H, CH(CH3)2), 7.12 (s, 12H, CH(CH3)2), 5.89 (s, 2H, CH2), -0.22 (s, 3H, CH3). μeff = 4.6(1) μB. [Fe(PNP-iPr)Br2] (3a). This complex has been prepared analogously to 2a with 1a (370 mg, 1.08 mmol) and FeBr2 (233 mg, 1.08 mmol) as the starting materials. Yield: 511 mg (85%) Anal. Calcd for C17H33Br2FeN3P2: C, 36.65; H, 5.97; N, 7.54. Found: C, 36.70; H, 6.04; N, 7.60. 1H NMR (δ, acetone-d6, 20 C): 148.98 (s, 4H, CH(CH3)2), 59.47 (s, 2H, NH), 56.49 (s, 2H, py3,5), 16.02 (s, 12H, CH(CH3)2), 7.71 (s, 12H, CH(CH3)2), -20.15 (s, 1H, py4). [Fe(PNPClpym-iPr)Br2] (3b). This complex has been prepared analogously to 2a with 1b (465 mg, 1.23 mmol) and FeBr2 (266 mg, 1.23 mmol) as the starting materials. Yield: 583 mg (80%). Anal. Calcd for C16H31Br2ClFeN4P2: C, 32.43; H, 5.27; N, 9.46. Found: C, 32.44; H, 5.31; N, 9.44. 1H NMR (δ, acetoned6, 20 C): 151.13 (s, 4H, CH(CH3)2), 70.37 (s, 1H, NH), 66.50 (s, 1H, NH), 41.31 (s, 1H, pym5), 15.89 (s, 6H, CH(CH3)2), 14.76 (s, 6H, CH(CH3)2), 9.44 (s, 6H, CH(CH3)2), 8.51 (s, 6H, CH(CH3)2). [Fe(PNPEtOpym-iPr)Br2] (3c). This complex has been prepared analogously to 2a with 1c (520 mg; 1.34 mmol) and FeBr2 (290 mg, 1.34 mmol) as the starting materials. Yield: 581 mg (72%). Anal. Calcd for C18H36Br2FeN4OP2: C, 35.91; H, 6.03; N, 9.31. Found: C, 36.05; H, 6.10; N, 9.25. 1H NMR (δ, acetoned6, 20 C): 156.26 (s, 2H, CH(CH3)2), 154.06 (s, 2H, CH(CH3)2), 69.90 (s, 1H, NH), 58.93 (s, 1H, NH), 38.67 (s, 1H, pym5), 16.40 (s, 6H, CH(CH3)2), 15.95 (s, 6H, CH(CH3)2), 9.00 (s, 6H, CH(CH3)2), 8.55 (s, 6H, CH(CH3)2), 6.67 (s, 2H, CH2), 0.12 (s, 3H, CH3). cis-[Fe(PNPClpym-iPr)(CO)Cl2] (4b). Carbon monoxide was passed over 2b (300 mg, 0.60 mmol) for about 2 min, whereupon the solid changed its color from yellow to red. Yield: 319 mg (quantitative). Anal. Calcd for C17H31Cl3FeN4OP2: C, 38.41; H, 5.88; N, 10.54. Found: C, 38.49; H, 5.77; N, 10.58. 1H NMR (δ, dmso-d6, 20 C): 9.49 (s, 1H, NH), 9.32 (s, 1H, NH), 6.22 (s, 1H, pym5), 2.46 (s, 4H, CH(CH3)2), 1.63-1.00 (m, 24H, CH(CH3)2). 31 P{1H} NMR (δ, dmso-d6, 20 C): 116.8 (d, J = 195 Hz), 102.5 (d, J = 195 Hz). IR (ATR, attenuated total reflection, cm-1): 1952 (νCdO). Solid-state NMR: 13C NMR (δ, 20 C): 212.7 (CO), 160.4 (pym4,6), 153.1 (pym2), 88.7 (pym5), 12.2 (CH(CH3)2 and

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CH(CH3)2). 31P NMR (δ, 20 C): 119.8, 102.0. 15N NMR (δ, 20 C): 183.8 (pym3), 102.6 (pym1), 64.0 (NH). cis-[Fe(PNPEtOpym-iPr)(CO)Cl2] (4c). This complex has been prepared analogously to 2b with 2c (250 mg, 0.79 mmol) as the starting material. Yield: 264 mg (quantitative). Anal. Calcd for C19H36Cl2FeN4O2P2: C, 42.17; H, 6.70; N, 10.35. Found: C, 42.22; H, 6.80; N, 10.26. 1H NMR (δ, dmso-d6, 20 C): 8.72 (bs, 2H, NH), 5.46 (s, 1H, pym5), 4.02 (s, 2H, CH2), 2.45 (s, 4H, CH(CH3)2), 1.33-1.01 (m, 27H, CH(CH3)2 and CH3). 31P{1H} NMR (δ, dmso-d6, 20 C): 113.4 (d, J = 234 Hz), 101.9 (d, J = 234 Hz). IR (ATR, cm-1): 1955 (νCdO). trans-[Fe(PNPClpym-iPr)(CO)Cl2] (5b). Carbon monoxide was bubbled into a solution of 2b (300 mg, 0.60 mmol) in acetone (20 mL) for about 2 min, whereupon the color of the reaction mixture turned from yellow to violet. After removal of the solvent under reduced pressure, the remaining solid was washed twice with Et2O (10 mL) and dried under vacuum. Yield: 303 mg (95%). Anal. Calcd for C17H31Cl3FeN4OP2: C, 38.41; H, 5.88; N, 10.54. Found: C, 38.50; H, 5.91; N, 10.46. 1H NMR (δ, dmso-d6, 20 C): 8.58 (s, 1H, NH), 8.29 (s, 1H, NH), 6.46 (s, pym5), 2.99 (s, 4H, CH(CH3)2), 1.50-1.34 (m, 24H, CH(CH3)2). 13 C{1H} NMR (δ, dmso-d6, 20 C): 221.6 (vt, J = 22.7 Hz, CO), 167.0 (dd, J = 5.2 Hz, J = 13.2 Hz, pym6), 165.2 (dd, J = 5.2 Hz, J = 20.1 Hz, pym4), 158.2 (pym2), 93.9 (d, J = 7.5 Hz, pym5), 16.8 (dd, J = 4.0 Hz, J = 16.7 Hz, CH(CH3)2), 15.6 (d, J = 23.6 Hz, CH(CH3)2). The signals of CH(CH3)2 are obscured by residual solvent. 31P{1H} NMR (δ, dmso-d6, 20 C): 133.3 (d, J = 180 Hz), 118.1 (d, J = 180 Hz). IR (ATR, cm-1): 1974 (νCdO). Solid-state NMR: 13C NMR (δ, 20 C): 214.8 (CO), 160.2 (pym4,6), 150.3 (pym2), 88.6 (pym5), 11.6 (CH(CH3)2 and CH(CH3)2). 31P NMR (δ, 20 C): 134.9, 117.4. 15N NMR (δ, 20 C): 186.1 (pym3), 151.0 (pym1), 62.2 (NH). trans-[Fe(PNPEtOpym-iPr)(CO)Cl2] (5c). This complex has been prepared analogously to 5b with 2c (250 mg, 0.79 mmol) as the starting material. Yield: 243 mg (92%). Anal. Calcd for C19H36Cl2FeN4O2P2: C, 42.17; H, 6.70; N, 10.35. Found: C, 42.12; H, 6.58; N, 10.29. 1H NMR (δ, dmso-d6, 20 C): 9.11 (s, 1H, NH), 8.98 (s, 1H, NH), 5.94 (s, 1H, pym5), 4.33 (q, J = 6.7 Hz, 2H, CH2), 2.81-2.73 (m, 4H, CH(CH3)2), 1.46-1.03 (m, 27 H, CH3 and CH(CH3)2). 13C{1H} NMR (δ, dmso-d6, 20 C): 224.5 (dd, J = 22.6 Hz, J = 33.9 Hz, CO), 170.6 (pym2), 169.4 (dd, J = 2.2 Hz, J = 12.9 Hz, pym6), 167.2 (dd, J = 3.2 Hz, J = 16.2 Hz, pym4), 80.5 (d, J = 8.6 Hz, pym5), 62.7 (CH2), 25.5 (CH(CH3)2), 25.1 (CH(CH3)2), 19.3 (dd, J = 3.2 Hz, J = 8.6 Hz, CH(CH3)2), 18.1 (bs, CH(CH3)2). 31P{1H} NMR (δ, dmso-d6, 20 C): 128.3 (d, J = 173 Hz), 115.7 (d, J = 173 Hz). IR (ATR, cm-1): 1958 (νCdO). cis/trans-[Fe(PNP-iPr)(CO)Br2] (6a/7a). Method 1: Carbon monoxide was bubbled into a solution of 3a (150 mg, 0.27 mmol) in acetone (5 mL) for ca. 2 min, whereupon the color of the reaction mixture turned from yellow to brown. After removal of the solvent under reduced pressure, the remaining solid was washed twice with Et2O (10 mL) and dried under vacuum. A 38:62 mixture of the cis/trans isomers 6a and 7a was obtained. Yield: 157 mg (quantitative). Method 2: Carbon monoxide was passed over solid 3a (150 mg, 0.27 mmol) for 2 min, whereupon the solid changed color from yellow to brown. A 35:65 mixture of the cis/trans isomers 6a and 7a was obtained. Yield: 158 mg (quantitative). cis-[Fe(PNP-iPr)(CO)Br2] (6a). 1H NMR (δ, dmso-d6, 20 C): 8.21 (s, 2H, NH), 7.02 (t, J = 7.4 Hz, 1H, py4), 6.02 (d, J = 6.1 Hz, 2H, py3,5), 2.49 (s, 4H, CH(CH3)2), 1.39-1.05 (m, 24H, CH(CH3)2). 13C{1H} NMR (δ, dmso-d6, 20 C): 227.3 (t, J = 20.1 Hz, CO), 163.1 (t, J = 9.2 Hz, py2,6), 139.6 (py4), 98.8 (py3,5), 29.4 (vt, J = 20.1 Hz, CH(CH3)2), 28.6 (vt, J = 10.1 Hz, CH(CH3)2), 19.4-18.9 (m, CH(CH3)2). 31P{1H} NMR (δ, dmso-d6, 20 C): 105.6. IR (ATR, cm-1): 1928 (νCdO). trans-[Fe(PNP-iPr)(CO)Br2] (7a). 1H NMR (δ, dmso-d6, 20 C): 8.45 (s, 2H, NH), 7.36 (t, J = 6.4 Hz, 1H, py4), 6.47 (d, J = 5.9 Hz, 2H, py3,5), 2.96 (s, 4H, CH(CH3)2), 1.40-1.06

Benito-Garagorri et al. (m, 24H, CH(CH3)2). 13C{1H} NMR (δ, dmso-d6, 20 C): 223.2 (t, J = 23.6 Hz, CO), 162.7 (t, J = 7.2 Hz, py2,6), 138.7 (py4), 97.8 (py3,5), 27.5 (vt, J = 12.4 Hz, CH(CH3)2), 18.4 (bs, CH(CH3)2). 31P{1H} NMR (δ, dmso-d6, 20 C): 122.5. IR (ATR, cm-1): 1956 (νCdO). cis/trans-[Fe(PNPClpym-iPr)(CO)Br2] (6b/7b). Method 1: Carbon monoxide was bubbled into a solution of 3b (150 mg, 0.25 mmol) in acetone (5 mL) for ca. 2 min, whereupon the color of the reaction mixture turned from yellow to brown. After removal of the solvent under reduced pressure, the remaining solid was washed twice with Et2O (10 mL) and dried under vacuum. A 30:70 mixture of the cis/trans isomers 6b and 7b was obtained. Yield: 158 mg (quantitative). Method 2: Carbon monoxide was passed over solid 3b (160 mg, 0.27 mmol) for 2 min, whereupon the solid changed its color from yellow to brown. A 25:75 mixture of the cis/trans isomers 6b and 7b was obtained. Yield: 166 mg (quantitative). cis-[Fe(PNPClpym-iPr)(CO)Br2] (6b). 1H NMR (δ, acetone-d6, 20 C): 8.32 (s, 1H, NH), 8.11 (s, 1H, NH), 6.25 (s, 1H, pym5), 3.09 (s, 4H, CH(CH3)2), 1.57-1.30 (m, 24H, CH(CH3)2). 13C{1H} NMR (δ, acetone-d6, 20 C): 225.8 (t, J = 21.8 Hz, CO), 169.2 (dd, J = 6.3 Hz, J = 12.7 Hz, pym2), 167.0 (dd, J = 5.8 Hz, J = 19.0 Hz, pym6), 160.2 (pym4), 96.4 (d, J = 6.3 Hz, pym5), 27.6, 27.5, 27.3, 27.2 (CH(CH3)2), 19.1-18.9 (m, CH(CH3)2). 31P{1H} NMR (δ, acetone-d6, 20 C): 116.7 (d, J = 181 Hz), 102.6 (d, J = 181 Hz). IR (ATR, cm-1): 1947 (νCdO). trans-[Fe(PNPClpym-iPr)(CO)Br2] (7b). 1H NMR (δ, acetoned6, 20 C): 8.62 (s, 1H, NH), 8.42 (s, 1H, NH), 6.70 (s, 1H, pym5), 3.26 (s, 4H, CH(CH3)2), 1.57-1.30 (m, 24H, CH(CH3)2). 13C{1H} NMR (δ, acetone-d6, 20 C): 224.2 (t, J = 27.6 Hz, CO), 168.7 (dd, J = 5.2 Hz, J = 11.5 Hz, pym2), 166.9 (d, J = 19.5 Hz, pym6), 159.3 (pym4), 95.6 (d, J = 2.9 Hz, pym5), 26.3, 26.1 (CH(CH3)2), 18.5-18.0 (m, CH(CH3)2). 31P{1H} NMR (δ, acetone-d6, 20 C): 133.4 (d, J = 166 Hz), 118.5 (d, J = 166 Hz). IR (ATR, cm-1): 1976 (νCdO). cis-[Fe(PNPCH2-iPr)(CO)Cl2] (9). Method 1: Carbon monoxide was bubbled into a solution of 8 (200 mg, 0.42 mmol) in acetone (5 mL) for ca. 2 min, whereupon the color of the reaction mixture turned from yellow to brown. After removal of the solvent under reduced pressure, the remaining solid was washed twice with Et2O (10 mL) and dried under vacuum. Yield: 182 mg (86%). Method 2: Carbon monoxide was passed over solid 8 (200 mg, 0.42 mmol) for 2 min, whereupon the solid changed color from yellow to light brown. Yield: 212 mg (quantitative). 1H NMR (δ, acetone-d6, 20 C): 7.76 (s, 1H, py4), 7.59 (s, 2H, py3,5), 3.88 (s, 4H, CH2), 2.62 (s, 4H, CH(CH3)2), 1.32-1.26 (m, 24H, CH(CH3)2). 13 C{1H} NMR (δ, acetone-d6, 20 C): 205.1 (CO), 165.2 (py2,6), 138.9 (py4), 121.8 (py3,5), 40.0 (CH2), 23.8 (CH(CH3)2), 23.6 (CH(CH3)2), 19.7 (CH(CH3)2), 18.8 (CH(CH3)2). 31P{1H} NMR (δ, acetone-d6, 20 C): 74.1. IR (ATR, cm-1): 1943 (νCdO). X-ray Structure Determination. X-ray data for [Fe(PNPClpymiPr)Cl2] 3 THF (2b 3 THF), [Fe(PNPClpym-iPr)Br2] 3 THF (3b 3 THF), trans-[Fe(PNPClpym-iPr)(CO)Cl2] 3 THF (5b 3 THF), and cis/trans[Fe(PNPClpym-iPr)(CO)Br2] 3 THF (6b/7b 3 THF) were collected on a Bruker Smart APEX CCD area detector diffractometer using graphite-monochromated Mo KR radiation (λ = 0.71073 A˚) and 0.3 ω-scan frames covering complete spheres of the reciprocal space with θmax = 27-30. After data integration with the program SAINT corrections for absorption, λ/2 effects, and crystal decay were applied with SADABS.28 The structures were solved by direct methods (SHELXS97) and refined on F2 with the program SHELXL97.29 All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were inserted in idealized positions and were refined riding with the atoms to which they were bonded. (28) Bruker programs: SMART, version 5.629; SAINTþ, version 6.45; SADABS, version 2.10; SHELXTL, version 6.14; Bruker AXS Inc.: Madison, WI, 2003. (29) Sheldrick, G. M. SHELX97: Program System for Crystal Structure Determination; University of G€ottingen: G€ottingen, Germany, 1997.

Article


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Table 1. Details for the Crystal Structure Determinations of Complexes [Fe(PNPClpym-iPr)Cl2] 3 THF (2b 3 THF), [Fe(PNPClpym-iPr)Br2] 3 THF (3b 3 THF), trans-[Fe(PNPClpym-iPr)(CO)Cl2] 3 THF (5b 3 THF), and cis/trans-[Fe(PNPClpym-iPr)(CO)Br2] 3 THF (6b/7b 3 THF) Fe(PNPClpym-iPr)Cl2 3 THF (2b 3 THF)

Fe(PNPClpym-iPr)Br2 3 THF (3b 3 THF)

trans-Fe(PNPClpym-iPr)(CO)Cl2 3 THF (5b 3 THF)

cis/trans-Fe(PNPClpym-iPr)(CO)Br2 3 THF (6b/7b 3 THF).

)

)

)

formula C20H39Cl3FeN4OP2 C20H39Br2ClFeN4OP2 C21H39Cl3FeN4O2P2 C21H39Br2ClFeN4O2P2 fw 575.69 664.61 603.70 692.62 cryst.size, mm 0.40 0.14 0.10 0.30 0.20 0.12 0.54 0.32 0.28 0.40 0.20 0.15 cryst syst monoclinic monoclinic monoclinic orthorhombic P21/c (no. 14) P21/c (no. 14) P212121 (no. 19) space group P21/n (no. 14) a, A˚ 9.1114(16) 14.1196(6) 13.0910(9) 9.9382(4) b, A˚ 14.638(3) 13.2696(6) 9.9454(7) 13.1119(5) c, A˚ 21.026(4) 15.9627(7) 22.7475(16) 22.4446(9) R, deg 90 90 90 90 β, deg 93.279(3) 91.258(1) 104.545(1) 90 γ, deg 90 90 90 90 3 2799.7(9) 2990.1(2) 2866.7(3) 2924.7(2) V, A˚ Z 4 4 4 4 1.366 1.476 1.399 1.573 Fcalc, g cm-3 100(2) 100(2) T, K 100(2) 297(2)b -1 0.959 3.392 0.942 3.473 μ, mm (Mo KR) F(000) 1208 1352 1264 1408 27.0 27.0 30.0 30.1 θmax, deg no. of rflns measd 24 200 43 884 60 545 39 812 no. of rflns unique 6088 6512 8345 8484 no. of rflns I > 2σ(I) 3327 4181 7220 7943 no. of params 280 280 298 307 0.0494 0.0456 0.0371 0.0465 R1 (I > 2σ(I))a 0.1147 0.0821 0.0458 0.0496 R1 (all data) 0.1215 0.1560 0.1008 0.1182 wR2 (all data)a -0.78/0.49 -0.43/0.86 -0.71/1.13 diff Fourier peaks min./ -0.65/0.75 max., e A˚-3 P P P P a R1 = Fo| - |Fc / Fo|; wR2 = { [w(Fo2 - Fc2)2]/ [(w(Fo2)2]}1/2. b Measurement at 100 K attempted, but crystals cracked on cooling.

Compound 6b/7b 3 THF was found to contain both the cis and the trans form of [Fe(PNPClpym-iPr)(CO)Br2] overlaid in a single moiety with a common pincer ligand and one axial ligand position exclusively occupied by Br (Br(3)), whereas the two remaining ligand positions showed a mixed occupation by ca. 50% by Br and ca. 50% CO. The final refinement gave 53% Br(1) and 47% C(17)-O(1) for the meridional ligand position and 47% Br(2) and 53% C(18)-O(2) for the axial position trans to Br(3) using an unrestrained x,y,z, Uiso refinement of carbonyl C and O positions. These calculations proved the simultaneous presence of both the cis form (6b) and trans form (7b) of the complex Fe(PNPClpymiPr)(CO)Br2 in the crystal lattice of 6b/7b 3 THF. Basic crystallographic data are given in Table 1. Important geometric data are given in the figure captions of the corresponding complexes. Computational Details. Calculations were performed using the Gaussian 03 software package30 and the PBE functional31 without symmetry constraints. Unrestricted calculations were performed for the open-shell complexes. The optimized geometries (30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (31) (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865. (b) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78, 1396.

were obtained with the Stuttgart/Dresden ECP (SDD) basis set32 to describe the electrons of the iron atom. For all other atoms the 6-31G** basis set was employed.33 Transition-state optimizations were performed with the synchronous transit-guided quasi-Newton method (STQN) developed by Schlegel et al.34 Frequency calculations were performed to confirm the nature of the stationary points, yielding one imaginary frequency for the transition states and none for the minima. Each transition state was further confirmed by following its vibrational mode downhill on both sides and obtaining the minima presented on the energy profiles. Solvent effects (CH3NO2) were considered in the cis/trans isomerization profile (Figure 11) using the polarizable continuum model (PCM) initially devised by Tomasi and co-workers35 as implemented in Gaussian 03.36,37 The molecular cavity was based on the united atom topological model applied on UAHF radii, optimized for the HF/6-31G(d) level. In the case of the crossing (32) (a) Haeusermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 78, 1211. (b) Kuechle, W.; Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1994, 100, 7535. (c) Leininger, T.; Nicklass, A.; Stoll, H.; Dolg, M.; Schwerdtfeger, P. J. Chem. Phys. 1996, 105, 1052. (33) (a) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639. (b) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650. (c) Wachters, A. J. H. Chem. Phys. 1970, 52, 1033. (d) Hay, P. J. J. Chem. Phys. 1977, 66, 4377. (e) Raghavachari, K.; Trucks, G. W. J. Chem. Phys. 1989, 91, 1062. (f) Binning, R. C.; Curtiss, L. A. J. Comput. Chem. 1995, 103, 6104. (g) McGrath, M. P.; Radom, L. J. Chem. Phys. 1991, 94, 511. (34) (a) Peng, C.; Ayala, P. Y.; Schlegel, H. B.; Frisch, M. J. J. Comput. Chem. 1996, 17, 49. (b) Peng, C.; Schlegel, H. B. Isr. J. Chem. 1994, 33, 449. (35) (a) Cances, M. T.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032. (b) Cossi, M.; Barone, V.; Mennucci, B.; Tomasi, J. Chem. Phys. Lett. 1998, 286, 253. (c) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151. (36) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999. (37) Cossi, M.; Scalmani, G.; Rega, N.; Barone, V. J. Chem. Phys. 2002, 117, 43.

6914


point, CPA, single-point PCM calculations with CH3NO3 as the solvent were performed for both spin states, yielding energy values differing less than 0.1 kcal/mol. The minimum-energy crossing points (MECP) between the spin-singlet (S = 0), spin-triplet (S = 1), and the spin-quintuplet (S = 2) potential energy surfaces were determined using a code developed by Harvey et al.38 This code consists of a set of shell scripts and Fortran programs that uses the Gaussian results of energies and gradients of both spin states to produce an effective gradient pointing toward the MECP. The energy values presented in the energy profile associated with the “spin-forbidden” reaction (Figure 10) correspond to electronic energies, since MECP are not stationary points and, hence, a standard frequency analysis is not applicable. The geometry optimization and frontier orbital analysis of the dichloro complex (2a) was also studied using the Amsterdam Density Functional (ADF) program package.24 Gradient-corrected geometry optimizations,39 without symmetry constraints, (38) Harvey, J. N.; Aschi, M.; Schwarz, H.; Koch, W. Theor. Chem. Acc. 1998, 99, 95. (39) (a) Versluis, L.; Ziegler, T. J. Chem. Phys. 1988, 88, 322. (b) Fan, L. Y.; Ziegler, T. J. Chem. Phys. 1991, 95, 7401. (40) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. (41) Becke, A. D. J. Chem. Phys. 1987, 88, 1053. (42) (a) Perdew, J. P. Phys. Rev. B 1986, 33, 8822. (b) Perdew, J. P. Phys. Rev. B 1986, 34, 7406. (43) van Lenthe, E.; Ehlers, A.; Baerends, E. J. J. Chem. Phys. 1999, 110, 8943. (44) Portmann, S.; L€ uthi, H. P. Chimia 2000, 54, 766.

Benito-Garagorri et al. were performed using the local density approximation of the correlation energy (Vosko-Wilk-Nusair)40 and the generalized gradient approximation (Becke’s exchange41 and Perdew’s correlation functionals).42 The core orbitals were frozen for Fe ([1-2]s, 2p), P ([1-2]s, 2p), and C and N (1s). Triple-ζ Slatertype orbitals (STO) were used to describe the valence shells of C, N (2s and 2p), P (3s, 3p), and Fe (3d, 4s). A set of two polarization functions was added to C, N (single ζ, 3d, 4f), P, Cl (single ζ, 3d, 4p), and Fe (single ζ, 4p, 4f). Triple-ζ Slater-type orbitals (STO) were used to describe the valence shells of H (1s) with two polarization functions (single-ζ 2s,2p). Relativistic effects were treated with the ZORA approximation.43 Unrestricted calculations were performed for the open-shell complexes. Threedimensional representations of the orbitals were obtained with Molekel.44

Acknowledgment. D.B.-G. thanks the Basque Government (Eusko Jaurlaritza/Gobierno Vasco) for a doctoral fellowship. M.J.C. acknowledges FCT, POCI, and FEDER (project PPDCT/QUI/58925/2004) for financial support. Supporting Information Available: Complete crystallographic data and technical details in CIF format for [Fe(PNPClpym-iPr)Cl2] 3 THF (2b 3 THF), [Fe(PNPClpym-iPr)Br2] 3 THF (3b 3 THF), trans-[Fe(PNPClpym-iPr)(CO)Cl2] 3 THF (5b 3 THF), and cis/ trans-[Fe(PNPClpym-iPr)(CO)Br2] 3 THF (6b/7b 3 THF). This material is available free of charge via the Internet at http:// pubs.acs.org.

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