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Ni−Fe and Pd−Fe Interactions in Nickel(II) and Palladium(II) Complexes of a Ferrocene-Bridged Bis(imidazolin-2-imine) Ligand Kristof Jess, Dirk Baabe, Thomas Bannenberg, Kai Brandhorst, Matthias Freytag, Peter G. Jones, and Matthias Tamm* Institut für Anorganische und Analytische Chemie Technische, Universität Braunschweig, Hagenring 30, 38106 Braunschweig, Germany S Supporting Information *

ABSTRACT: The bis(imidazolin-2-imine) ligand N,N′-bis(1,3diisopropyl-4,5-dimethylimidazolin-2-ylidene)-1,1′-ferrocenediamine, fc(NIm) 2 (1) was prepared. Its reaction with [NiCl2(dme)] (dme = 1,2-dimethoxyethane) or [PdCl2(MeCN)2] afforded the tetrahedral, paramagnetic complex [(1-κ2N,N′)NiCl2] (6a) or the diamagnetic, square-planar complex [(1-κ2N,N′)PdCl2] (6b), respectively. For the latter, slow rearrangement to the ionic complex [(1-κFe,κ2N,N′)PdCl]Cl, [7]Cl, was observed, which was followed by 1H NMR and UV/vis spectroscopy. Treatment of [7]Cl with NaBF4 afforded [7]BF4; the palladium atoms in both cations adopt square-planar environments with short Fe−Pd bonds (ca. 2.65 Å). In addition, a series of dicationic complexes of the type [(1-κFe,κ2N,N′)ML](BF4)2 (8a: M = Ni, L = MeCN; 8b: M = Pd, L = MeCN; 9a: M = Ni, L = PMe3; 9b: M = Pd, L = PMe3) was prepared from 6a (M = Ni) or [7]BF4 by chloride abstraction with NaBF4 or AgBF4 in the presence of acetonitrile or trimethylphosphine, respectively. In the presence of triphenylphosphine, the palladium(II) complex [(1-κFe,κ2N,N′)Pd(PPh3)](BF4)2 (10) was isolated. Iron−nickel and iron− palladium bonding in these complexes was studied experimentally by NMR, UV/vis, and Mössbauer spectroscopy and by cyclic voltammetry. Detailed DFT calculations were carried out for the cations [(1-κFe,κ2N,N′)M(MeCN)]2+ in the 8a/8b couple, with Bader’s atoms in molecules theory revealing the presence of noncovalent, closed-shell metal−metal interactions. Potential energy surface scans with successive elongation of the Fe−M bonds allow an estimation of the iron−metal bond dissociation energies (BDE) as BDE(Fe−Ni) = 11.3 kcal mol−1 and BDE(Fe−Pd) = 24.3 kcal mol−1.

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INTRODUCTION Ferrocene-bridged chelate ligands are among the most popular ancillary ligands in homogeneous catalysis because numerous methods are available to equip the ferrocene molecule with appropriate donor heteroatoms.1,2 While diphosphanes such as 1,1′-bis(diphenylphosphanyl)ferrocene (dppf) are certainly particularly important,3−10 transition metal complexes containing related N,N-, O,O-, and S,S-donor ligands have been studied extensively, as have 1,1′-unsymmetrical ferrocenes.11−15 The reactivity of the resulting metal chelate complexes can be influenced appreciably by the structural and electronic nature of the ferrocene backbone, and its redox properties, for instance, can be exploited for reversibly switching the activity of homogeneous catalysts.16−23 In addition, the ferrocene moiety is able to form dative iron−metal bonds, which are usually classified as weak donor−acceptor interactions.24 This structural motif was initially found by Seyferth et al. in 1983 for the palladium(II) dithiolate complex [(fcS2)Pd(PPh3)] (A, fc = 1,1′-ferrocenediyl);25 the solid-state structure revealed a κFe,κ2S,S′ coordination mode with an Fe−Pd distance of 2.878(1) Å (Figure 1).25,26 In 1987, Akabori and Sato reported similar ferrocene and ruthenocene complexes of type B featuring Fe−Pd, Fe−Pt, or Ru−Ni interactions,27,28 © XXXX American Chemical Society

whereas the Fe−Ni congener C was structurally characterized in 1998 by Hidai (Fe−Ni = 2.886(1) Å).29 Sato and Akabori also reported a series of dicationic thioether complexes D,30−35 along with the first diphosphane complex of type E, viz. [(dppf)Pd(PPh3)](BF4)2 (Fe−Pd = 2.877(2) Å).34 Besides reports on the potential relevance of Fe−Pd interactions in similar complexes for palladium-catalyzed cross-coupling reactions,36,37 this class was significantly extended by Nataro and co-workers in 2013, who established the structures of several 1,1′-bis(diarylphosphanyl)- and 1,1′-bis(alkylphosphanyl)ferrocene complexes E (L = PMe3, PPh3, n = 2; X = Cl, I, n = 1) with Fe−Pd distances in the range 2.893−3.017 Å.38 This paper also provides a thorough DFT analysis, indicating the presence of weak, noncovalent Fe−Pd interactions. Ferrocene-based N,N-donor ligands have also been employed for the preparation of complexes with Fe−Pd bonds. The bis(iminophosphorane) complex F reported by Metallinos in 2006 represents the first example with a κFe,κ2N,N′ coordination mode and exhibits a notably short Fe−Pd Received: October 23, 2015

A

DOI: 10.1021/acs.inorgchem.5b02457 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

we wish to report herein our independent findings regarding the preparation and characterization of related nickel(II) and palladium(II) complexes, together with a theoretical analysis of Fe−Ni and Fe−Pd bonding in these systems.

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RESULTS AND DISCUSSION Synthesis and Characterization of the FerroceneBridged Bis(imidazolin-2-imine) Ligand 1. The synthetic procedure developed by Siemeling for the preparation of ferrocene-bridged bis(guanidines)40 and also those for related Schiff-base diimine ligands67−69 usually commence from 1,1′diaminoferrocene, which is readily available from the corresponding diazide upon hydrogenation70 or from the Boc-protected derivative fc(NHBoc)2 (2) upon hydrolysis.68,71 Since the latter route, which can be performed on a large scale,71 proceeds via the diammonium salt 3, this was used as the starting material. Thus, treatment of fc(NHBoc)2 (2) with acetyl chloride in methanol afforded 2 as a yellow powder in 82% yield by simple precipitation with diethyl ether, filtration, and drying under vacuum (Scheme 2). Following a protocol Scheme 2. Synthesis of the Ferrocene-Bridged Bis(imidazolin-2-imine) fc(NIm)2 (1)

Figure 1. Representative examples of ferrocene-bridged chelate complexes with metal−metal interactions.

distance of approximately 2.67 Å.39 Likewise, Siemeling recently introduced ferrocene-based bis(guanidine) ligands, which formed complexes of type G with the Fe−Pd distances falling in the same range (av. 2.69 Å).40 1,1′-Ferrocene diamide ligands have also been widely used for the isolation of complexes with iron−metal interactions, involving, in particular, early transition and f-block metals.41−46 Recently, the parent diamide fc(NH)2 was employed by Diaconescu and Zink to support iron−ruthenium complexes such as H, and the Fe−Ru interaction was assessed by combined spectroscopic and computational studies.47 Our own group has a long-standing interest in imidazolin-2imine ligands,48 which can be regarded as particularly basic (superbasic) guanidine ligands.49−52 Thereby, bidentate ethylene-bridged and tridentate pyridine-bridged bis(imidazolin-2imine) ligands have found widespread use in organometallic and coordination chemistry.53−65 In view of the enormous interest in ferrocene-based ligands (vide supra), we set out to investigate ligands such as fc(NIm)2 (1) and their coordination chemistry. As indicated by the mesomeric forms 1A and 1B in Scheme 1, this ligand can be expected to act as a particularly strong N-donor toward transition metals because of the ability of the imidazole moiety to efficiently stabilize a positive charge.66 Accordingly, ligand 1 might tentatively be regarded as an even more basic analogue of Siemeling’s guanidines,40 and

introduced by Kunetskiy and Lyapkalo,72 the reaction of 2 with the chloroimidazolium tetrafluoroborate 4 in acetonitrile in the presence of excess potassium fluoride and triethylamine gave the bis(imidazolium) salt [fc(NHIm)2](BF4)2 (5). Deprotonation was performed with KOtBu in THF, affording the free ligand fc(NIm)2 as an orange solid in satisfactory yield (Scheme 2). Ligand 1 and its precursor 5 show similar 1H NMR spectra, with 5 giving rise to an additional signal at 6.66 ppm (in CDCl3) for the NH hydrogen atoms. In addition, the solid-state structures of 1 (Figure 2) and 5 (Figure 3) were determined by X-ray diffraction analyses. Both compounds crystallize in the monoclinic space group P21/n with half of a molecule (and one BF4 ion in the case of 5) in the asymmetric unit. The ferrocene molecules display crystallographic inversion symmetry, with the

Scheme 1. Resonance Structures of the Ferrocene-Bridged Bis(imidazolin-2-imine) fc(NIm)2 (1)

B


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Inorganic Chemistry

Scheme 3. Preparation of Nickel(II) and Palladium(II) Chloro Complexes

Figure 2. Molecular structure of 1 with thermal displacement parameters drawn at the 50% probability level; hydrogen atoms are omitted. Selected bond lengths (Å) and angles (deg): C1−N1 1.3868(19), C6−N1 1.305(2), C6−N2 1.3901(19), C6−N3 1.3794(19), C1−N1−C6 124.08(13), N1−C6−N2 132.73(14), N1− C6−N3 121.52(13).

ppm, revealing the presence of a paramagnetic nickel(II) complex. Accordingly, magnetic susceptibility measurements reveal a magnetic moment of μeff = 3.9μB at 300 K (Figure S43), and X-ray diffraction analysis affords a molecular structure with approximate 2-fold symmetry (rms deviation 0.19 Å). The geometry around the nickel atom is tetrahedral (Figure 4), albeit slightly distorted, as indicated by an angle of

Figure 3. Molecular structure of 5 with thermal displacement parameters drawn at the 50% probability level; counter-anions and hydrogen atoms (except at nitrogen) are omitted. Selected bond lengths (Å) and angles (deg): C1−N1 1.408(2), C6−N1 1.373(2), C6−N2 1.343(2), C6−N3 1.342(2), C1−N1−C6 120.93(13), N1− C6−N2 125.62(14), N1−C6−N3 125.90(14).

N-substituents thus adopting a perfectly antiperiplanar orientation. 1 has similar structural features to those of the bis(guanidines) reported by Siemeling.40 As expected, the C6− N1 bond (1.305(2) Å) is significantly shorter than C6−N2 (1.3901(19) Å) or C6−N3 (1.3794(19) Å), affording a ρ value of 0.95; the structural parameter is defined as ρ = 2a/(b + c), with a, b, and c representing the exo- (a) and endocyclic (b, c) distances within a CN3 guanidine or imidazolin-2-imine moiety.73,74 Protonation of the nitrogen atoms in 5 results in a ρ value of 1.02 that indicates elongation of the exocyclic and shortening of the endocyclic C−N bonds, in agreement with an efficient charge delocalization. Synthesis and Characterization of Nickel(II) and Palladium(II) Complexes. Complexation of NiCl2 was accomplished by addition of 1 to a THF suspension of [NiCl2(dme)] (dme = 1,2-dimethoxyethane) and furnished complex 6a as a brownish-green powder (Scheme 3). The 1H NMR spectrum shows signals in the range from −9 to +22

Figure 4. Molecular structure of 6a with thermal displacement parameters drawn at the 50% probability level; hydrogen atoms are omitted. Selected bond lengths and angles are assembled in Table 1.

81.58(3)° between the N1−Ni−N4 and Cl1−Ni−Cl2 planes. With regard to the nitrogen substituents, the cyclopentadienyl rings adopt a synclinal staggered conformation with a Ci−Ct− Ct′−Ci′ torsion angle τ of 43.4° (Ci = ipso-carbon atom, Ct = centroid of a Cp ring; Table 1). Thereby, the Cp rings deviate by 12.7(1)° from a perfectly coplanar orientation as found in precursors 1 and 5. The bond lengths and angles around the Ni atom are similar to those established for an ethylene-bridged bis(imidazolin-2-imine) nickel(II) complex.61 C


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Inorganic Chemistry Table 1. Structural Data for Several Ferrocene Complexesa 6a Fe−M M−Nb M−Lc Fe−Ctd N−M−N′ Fe−M−L αe θf τg

3.801(3) 2.0149(14)/ 1.9997(14) 2.2673(5)/ 2.2487(5) 1.645/1.647 103.47(6) 357.1(4)/ 357.6(4) −12.7(1) 43.4

6b 3.9966(6) 2.088(2) 2.3145(7) 1.647 96.46(14) 359.8(7) −11.8(1) 37.6

[7]BF4

8a

8b

9a

9b

10

2.6447(3) 2.0530(13)/ 2.0501(3) 2.3740(4)

2.6268(4) 1.8990(16)/ 1.8697(17) 1.8807(18)

2.6297(4) 2.060(2)/ 2.050(2) 2.094(3)

2.7376(4) 1.8534(19)/ 1.8841(19) 2.1582(6)

2.7475(4) 2.010(2)/ 2.0598(19) 2.2586(6)

2.7424(3) 2.0436(17)/ 2.0731(19) 2.3098(6)

1.685/1.686 165.42(5) 177.301(13) 333.0(3)/ 334.3(3) 17.6(1) 11.9

1.672/1.681 166.10(7) 177.65(5) 331.8(4)/ 348.5(4) 13.0(1) 16.1

1.683/1.686 164.38(9) 176.36(8) 331.8(5)/ 335.5(5) 18.3(2) 14.6

1.658/1.660 163.00(8) 163.48(2) 359.1(5)/ 338.7(5) 11.9(1) 8.6

1.675/1.679 160.56(8) 167.672(18) 359.2(5)/ 333.1(5) 16.5(1) 8.7

1.665/1.677 157.49(7) 162.492(17) 355.0(4)/ 333.7(5) 16.7(1) 17.2

a

Distances/Å, angles/°; values without error estimates were calculated using Mercury 3.5.1. bN: exocyclic imine-nitrogen. cL: coordinating atom of the monodentate ligand(s) Cl, MeCN, PMe3. dCt: centroid of C5H5 moiety, cf. Fe−Ct for 5 is 1.658 Å and for 1 is 1.664 Å. eα: sum of angles around exocyclic imine-nitrogen atoms. fθ: tilt angle of C5H5 ring planes; positive: opened toward coordinated metal. gτ: torsion angle: Ci−Ct−Ct′−Ci′ (Ci: ipso-carbon).

Addition of 1 to a THF suspension of [PdCl2(MeCN)2] gave the corresponding palladium(II) complex 6b as a dark gray powder. However, 1H NMR spectroscopy in CD2Cl2 solution revealed a mixture of two diamagnetic species (see Supporting Information, Figure S24). One set of signals comprises two doublets for diastereotopic isopropyl CH3 groups (δ = 1.71, 1.37 ppm), along with a singlet for the backbone CH3 groups (δ = 2.19 ppm), two pseudo triplets (δ = 3.89, 3.76 ppm) for the ferrocene α- and β- protons, and a septet (δ = 6.08 ppm) for the isopropyl CH hydrogen atoms. These signals can be assigned to the neutral species [{fc(NIm)2}PdCl2] (6b; Scheme 3). The second set of signals showed only one doublet for the isopropyl CH3 groups (δ = 1.54 ppm), along with a backbone methyl singlet (δ = 2.21 ppm) and a broad signal for the isopropyl CH group (δ = 5.52 ppm). Most notably, the signals assigned to the ferrocene α- and β-hydrogen atoms at 3.23 and 5.30 ppm show a large chemical shift difference of Δδ = 2.07 ppm, cf. Δδ = 0.13 ppm in 6b, which is characteristic for complexes with iron−metal bonds, e.g., F and G (Figure 1).39,40 Therefore, these signals can be assigned tentatively to a cationic complex [{fc(NIm)2}PdCl]Cl, [7]Cl (Scheme 3). It should be noted that the 1H NMR spectrum showed complete conversion into the latter compound after 16 h in CD2Cl2 solution, which was followed by 1H NMR spectroscopy. A first-order rate constant kobs ≈ k1 = 1.34·10−4 s−1 could be derived for this process, corresponding to a half-life of t1/2 = 86 min, which is associated with an activation barrier of ΔG⧧ = 22.7 kcal mol−1 for the forward reaction and the Gibbs free energy being ΔG = 3.4 kcal mol−1 (see Supporting Information pages S8−S12 for details). The conversion could also be followed by UV/vis spectroscopy: A 0.02 M solution of 6b in CH2Cl2 was analyzed over a period of 6 h. The spectrum in Figure 5 exhibits three isosbestic points at ∼275, ∼321, and ∼495 nm, as expected for the direct interconversion between 6b and [7]Cl. The most prominent changes in absorption are the decrease at λmax = 296 nm and the increase at λmax = 354 nm, with the latter band also being characteristic for other complexes with Fe−Pd bonds, e.g., [7]BF4 (vide infra; see Supporting Information, Figure S16). Analysis of the time dependence of the absorbance at these two wavelengths allowed to derive a first-order rate constant kobs ≈ k1 = 1.64·10−4 s−1, which corresponds to a half-life of t1/2 = 70 min. The derived values for the activation barrier and the Gibbs free energy of ΔG⧧ = 22.6 kcal mol−1 and ΔG = 2.9 kcal mol−1

Figure 5. UV/vis diagram of the conversion of 6b to [7]Cl.

are in good agreement with the values established by NMR spectroscopy (see Supporting Information pages S13−S19 for details). These observations indicate that complex 6b forms during the reaction and precipitates from THF. If it is dissolved in dichloromethane, slow rearrangement to [7]Cl takes place by chloride elimination. This reaction is reversible, since suitable crystals for X-ray diffraction analysis of 6b were obtained by layering a dichloromethane solution of [7]Cl with diisopropyl ether (Figure 6). 6b crystallizes in the orthorhombic space group P21212 with half of a molecule in the asymmetric unit. The complex displays crystallographic C2 symmetry and shows a slightly distorted square-planar geometry at the Pd atom with an angle of 11.0(2)° between the N1−Pd−N1′ and Cl1−Pd− Cl1′ planes. The Ci−Ct−Ct′−Ci′ torsion angle τ of 37.6° indicates a nearly perfect (36.0°) synclinal staggered conformation of the Cp rings (Table 1), which subtend an interplanar angle of 11.8(1)°. The bond lengths and angles around the Pd atom are similar to those established for ethylene-bridged bis(imidazolin-2-imine) palladium(II) complexes.62,75 In contrast, when a CDCl3 solution of 6b was layered with diisopropyl ether, single crystals of [7]Cl·3CDCl3 were formed, revealing that the chloride counterion and also the chloro ligand are involved in hydrogen bonding with CDCl3 solvate molecules (Figure 7). Similarly, a solvent-dependent equiliD


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Inorganic Chemistry

[7]Cl are shorter, whereas the Pd−Cl bond is significantly longer, i.e., 2.3972(5) vs. 2.3145(7) Å, which suggests a considerable trans-influence of the ferrocene moiety. During attempted purification of 6b by chromatography on alumina with acetonitrile as eluent, single crystals of [7]Cl·1/2 CH2Cl2· CH3CONH2 were isolated, with the cocrystallized acetamide formed by hydrolysis of CH3CN. The structural parameters of the cation 7 are very similar, e.g., Fe−Pd = 2.6509(5) Å (Figure S1). Chloride exchange by addition of NaBF4 to 6b/[7]Cl in THF irreversibly formed the tetrafluoroborate salt [7]BF4, which contains the same cation [{fc(NIm)2}PdCl]+ (7) (Scheme 3). Consequently, the 1H NMR spectrum is fully consistent with the assignments made for [7]Cl. Single crystals of [7]BF4·2CH2Cl2 were subjected to X-ray diffraction analysis, confirming again very similar structural parameters for the cation despite the observation of an even shorter Fe−Pd bond length of 2.6447(3) Å (Table 1). The chloro ligand and the BF4 counterion display hydrogen bonding with the solvate molecules, and a presentation of the X-ray crystal structure can be found in the Supporting Information (Figure S2). It should be noted that the reaction of 1 with an excess of [PdCl2(MeCN)2], as demonstrated by Siemeling for complexes G,40 is also a viable method for the preparation of the cationic complex [{fc(NIm) 2 }PdCl] + , which crystallizes as a tetrachloropalladate(II) salt of the composition [{fc(NIm)2}PdCl]2[PdCl4]·2CH2Cl2 (Supporting Information, Figure S3). To establish a complex with an iron−nickel bond, chloride abstraction from 6a was also attempted with NaBF4. In THF solution, however, no reaction occurred even at elevated temperature, whereas decomposition was observed in methanol. In contrast, treatment of 6a with a large excess of NaBF4 (10 equiv) in acetonitrile afforded the dicationic complex 8a as greenish-brown crystals in 85% yield (Scheme 4). The 1H

Figure 6. Molecular structure of 6b with thermal displacement parameters drawn at the 50% probability level; hydrogen atoms are omitted. Selected bond lengths and angles are assembled in Table 1.

Scheme 4. Preparation of the Ni(II) and Pd(II) Acetonitrile Complexes 8 Figure 7. Molecular structure of [7]Cl·3CDCl3 with thermal displacement parameters drawn at the 50% probability level; hydrogen atoms (except at chloroform) are omitted. Selected bond lengths (Å) and angles (deg): C1−N1 1.382(2), C6−N4 1.383(2), N1−C11 1.366(2), N4−C22 1.366(2), C11−N2 1.359(2), C11−N3 1.344(2), C22−N5 1.345(2), C22−N6 1.356(2), Pd−N1 2.0546(15), Pd−N4 2.0439(15), Pd−Cl1 2.3972(5), Fe−Pd 2.6695(3), C1−N1−C11 120.49(15), C6−N4−C22 121.96(15), N1−Pd−N4 163.94(6), Fe− Pd−Cl1 176.625(14).

brium was established by Hartwig for [{fc(PtBu2)2}Pd(Br)(pC6H4CN)], which dissociates in polar solution (THF, CDCl3) to form [{fc(PtBu2)2}Pd(p-C6H4CN)]Br with an Fe−Pd bond length of 2.9988(8) Å.37 [7]Cl exhibits an Fe−Pd distance of 2.6695(3) Å that is identical to the value reported for F and slightly shorter than the distances in complexes G.39,40 The Pd atom resides in a square-planar environment with the Fe atom occupying one coordination site. Whereas the Fe−Pd−Cl1 angle of 176.625(14)° is close to linearity, a significantly smaller N1−Pd−N4 angle of 163.94(6)° is observed. The Cp rings are close to a synperiplanar arrangement (τ = 14.0°) and subtend a tilt angle of 16.7(1)°, indicating that Fe−Pd interaction requires a significant opening of the ferrocene sandwich structure. The metal-bound nitrogen atoms are markedly pyramidalized, with angle sums of 331.8(4)° (at N1) and of 335.8(4)° (at N4). Compared to 6b, the Pd−N bond lengths in

NMR spectrum in CD2Cl2 exhibits two characteristic signals for the Hα and Hβ protons with a large chemical shift difference of Δδ = 1.60 ppm, confirming the presence of a diamagnetic complex with an Fe−Ni bond. The preparation of the corresponding palladium complex 8b from [7]BF4 proved to be difficult, since even a 100-fold excess of NaBF4 did not lead to complete chloride substitution with an acetonitrile ligand. Therefore, an acetonitrile solution of [7]BF4 was treated with AgBF4, which could not be used in the case of 6a because of its oxidation, as indicated by formation of elemental silver. The desired Pd complex 8b was isolated as a brown solid in 84% E


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Inorganic Chemistry isolated yield by filtration, removal of the solvent, and crystallization (Scheme 4). The molecular structures of 8a (Figure 8) and 8b (see Supporting Information, Figure S4) were confirmed by X-ray

Scheme 5. Preparation of the Ni(II) and Pd(II) Trimethylphosphine Complexes 9

two complexes suitable for X-ray diffraction analysis were grown by liquid diffusion (9a: acetonitrile/diethyl ether; 9b: dichloromethane/diethyl ether). The complexes are isotypic and crystallize in the monoclinic space group P21/c; the molecular structure of 9a is presented in Figure 9 (see

Figure 8. Molecular structure of the cation in 8a with thermal displacement parameters drawn at the 50% probability level. The structure of the palladium complex 8b is analogous. Hydrogen atoms, counteranions, and disordered groups are omitted. Selected bond lengths and angles are assembled in Table 1.

diffraction analyses; 8a crystallized in the orthorhombic space group Pbca, whereas 8b·CH3CN crystallized in the triclinic space group P1̅. In both complexes, the metal atom resides in a square-planar environment with the iron atom occupying one coordination site. Surprisingly, the resulting Fe−Ni and Fe−Pd bond lengths are almost identical, viz. 2.6268(4) and 2.6297(4) Å. The former is considerably shorter than the Fe−Ni distance of 2.886(1) Å in [(fcS2)Ni(PMe2Ph)] (C; Figure 1), whereas the latter represents, to the best of our knowledge, the shortest bond found for ferrocene-palladium systems to date. In contrast to the iron−metal bonds, the metal−nitrogen bonds show the expected trend, with the Ni−N bonds being significantly shorter than the Pd−N bonds. They are also shorter than the Ni−Nimine bonds in the paramagnetic, tetrahedral NiCl 2 complex 6a. The overall structural parameters are similar to those established for the complexes [7]X (X = Cl, BF4), with the chloro ligand replaced by acetonitrile, affording almost linear Fe−Ni−N7 and Fe−Pd−N7 angles of 177.65(5)° and 176.36(8)°. Introduction of trimethylphosphine could also be achieved by the reactions of the nickel(II) and palladium(II) complexes 6a and [7]BF4 with PMe3 in THF solution in the presence of NaBF4 (Scheme 5). The complexes 9a (M = Ni) and 9b (M = Pd) were isolated in satisfactory yield as dark brown crystalline solids. NMR spectra were recorded in CD2Cl2; the 1H NMR spectra display a pronounced chemical shift difference for the Hα and Hβ Cp hydrogen atoms, i.e., Δδ = 1.34 (Ni), 1.88 (Pd) ppm, that indicates the presence of iron−metal interactions. The 31P NMR spectra give rise to singlet signals at −0.4 (Ni) and −7.3 (Pd) ppm, which is at slightly higher field compared to the values reported for Pd complexes of type E (approximately 16.5 ppm for L = PMe3).38 Crystals of the

Figure 9. Molecular structure of the cation in 9a with thermal displacement parameters drawn at the 50% probability level. The molecular structure for the palladium complex 9b is isotypic. Hydrogen atoms, counteranions, and disordered groups are omitted. Selected bond lengths and angles are assembled in Table 1.

Supporting Information, Figure S5 for 9b). The Fe−Ni and Fe−Pd distances are 2.7376(4) and 2.7475(4) Å, respectively. These values are again very similar, but they are considerably longer than those found for the corresponding acetonitrile complexes 8. The Fe−M−P angles of 163.48(2)° (M = Ni) and 167.672(18)° (M = Pd) deviate more strongly from linearity than is found for the Fe−M−N angles in 8a/8b, which might be ascribed to unfavorable steric interaction between PMe3 and the diimine ligand. It is noteworthy that the metal-coordinated nitrogen atoms in 9a/9b display significantly different environments, with the angle sums indicating trigonal-planar (359.1(5)°/359.2(5)°) or trigonal-pyramidal (338.7(5)°/ 333.1(5)°) conformations at N1 and N4, respectively (Table 1). Attempts to prepare the corresponding triphenylphosphine complexes afforded the palladium(II) complex [{fc(NIm)2}Pd(PPh3)](BF4)2 (10), whereas its nickel(II) congener could not be isolated. The molecular structure (Figure 10) features F


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Inorganic Chemistry

and 9a, the quality of the fit could be improved by use of a Voigt line profile with minor contributions of a Gaussian distributed component (cf. Supporting Information, Tables S5 and S6).77 This can be physically interpreted in terms of a Gaussian distribution of isomer shifts, which we associate with the presence of minor but negligible sample inhomogeneities that might form in the course of the complex crystallization process. The resulting Mössbauer parameters are summarized in Table 2. The observed line widths (half-width at halfmaximum, ΓHWHM) range between 0.13 and 0.16 mm s−1 and are slightly larger than the experimental line width of approximately 0.12 mm s−1, which also substantiates the overall good sample homogeneity of the investigated compounds. The Mössbauer spectra measured at T = 100 K for the nickel complexes 6a, 8a, and 9a and for the free ligand 1 are presented in Figure 11, and the spectra for the analogous palladium Figure 10. Molecular structure of the cation in 10 with thermal displacement parameters drawn at the 50% probability level. Hydrogen atoms, counteranions, and disordered atoms are omitted. Selected bond lengths and angles are assembled in Table 1.

similar characteristics to those of 9b, e.g., Fe−Pd = 2.7424(3) Å. However, the smaller Fe−Pd−P angle of 162.492(17)° and the longer Pd−P bond length (2.3098(6) Å in 10 vs. 2.2586(6) Å in 9b) indicate that the introduction of the larger PPh3 ligand affords a sterically more congested system, which might also account for the inability to isolate the analogous nickel complex. Mössbauer Studies. To establish further experimental evidence for the presence of weak Fe−M interactions, we carried out a systematic Mössbauer study on polycrystalline specimens of the Ni and Pd complexes 6a/6b, 8a/8b, and 9a/ 9b. Mössbauer spectroscopy provides the opportunity to directly analyze ligand- and metal-induced effects on the electronic and structural properties of the 57Fe nucleus environment. As a reference, we compared the Mössbauer parameters derived for 6a/b, 8a/b, and 9a/b with those obtained for the free diimine ligand 1 and for ferrocene.76 The Mössbauer measurements were performed at temperatures T = 200, 100, and 20 K, and the spectra were consistently fitted with a Lorentzian doublet. For complexes 1

Figure 11. Mössbauer spectra of the free ligand 1 and the nickelcontaining complexes 6a, 8a, and 9a at T = 100 K.

complexes 6b, 8b, and 9b along with 1 are shown in Figure 12. For a better overview, the experimental isomer shifts δ and quadrupole splittings ΔEQ listed in Table 2 are illustrated in the correlation diagram in Figure 13. All compounds revealed doublets with isomer shifts ranging between δ = 0.46 and 0.56 mm s−1 and quadrupole splittings ranging from ΔEQ = 2.0 to

Table 2. Data from Mössbauer Spectroscopy for Compounds 1, 6a/b, 8a/b, and 9a/ba T/K 200

100

20

δ ΔEQ ΓHWHM A−/A+ δ ΔEQ ΓHWHM A−/A+ δ ΔEQ ΓHWHM A−/A+

1

6a

6b

8a

8b

9a

9b

0.513(3) 2.465(6) 0.132(4) 0.948(29) 0.552(3) 2.474(7) 0.134(5) 0.967(34) 0.558(1)b 2.435(2)b 0.137(1)b 0.984(4)b

0.482(3) 2.446(6) 0.167(5) 0.945(25) 0.520(2) 2.464(5) 0.158(3) 0.963(19) 0.531(2) 2.470(3) 0.153(2) 0.962(14)

0.484(3) 2.375(6) 0.134(4) 1.128(33) 0.519(2) 2.374(4) 0.138(3) 1.137(24) 0.527(2) 2.367(4) 0.143(3) 1.131(21)

0.461(4) 2.077(8) 0.148(6) 0.945(36) 0.496(2) 2.125(4) 0.161(3) 0.932(18) 0.508(2) 2.126(4) 0.167(3) 0.944(17)

0.490(4) 2.189(9) 0.156(6) 0.978(39) 0.524(3) 2.204(6) 0.161(5) 0.989(28) 0.535(2) 2.203(5) 0.159(3) 0.985(21)

0.464(3) 2.077(7) 0.160(5) 0.913(14) 0.500(2) 2.099(4) 0.166(3) 0.922(9) 0.514(2) 2.107(4) 0.167(3) 0.921(7)

0.491(6) 2.134(12) 0.134(9) 0.913(59) 0.525(3) 2.139(7) 0.137(5) 0.901(31) 0.538(3) 2.158(5) 0.138(4) 0.902(23)

a δ: isomer shift; ΔEQ: quadrupole splitting; ΓHWHM: half-line width at half-maximum. Values were obtained by least-squares fitting with doublets of Lorentzian lines and are given in mm s−1. The isomer shift is specified relative to metallic iron at room temperature. A−/A+ denotes the ratio of the spectral areas of the low-velocity peak to the high-velocity peak. bMössbauer parameters were obtained from a measurement on an identically prepared sample at T = 20.2(2) K.

G


Article

Inorganic Chemistry

element interactions.76 Albeit unnoticed, this trend can also be confirmed for the recently published complexes of type G and H, viz. ΔEQ [mm s−1]: G vs. its free ligand: 2.34/2.55,40 H vs. its precursor: 1.81(1)/2.42(1). 47 On the basis of our Mössbauer study on closely related compounds, therefore, the quadrupole splitting can be regarded as a sensitive parameter for the identification of iron−metal bonding. Electrochemistry. The ferrocene diimine ligand 1 and its palladium complexes 6b, [7]BF4, 8b, and 9b were also analyzed by electrochemical methods, whereas reliable data were not obtained for the more sensitive nickel complexes. For the latter, fast decomposition was observed as indicated by a successive decrease of the peak current and sometimes by formation of a metallic layer on the working electrode. Figure 14 shows the

Figure 12. Mössbauer spectra of the free ligand 1 and the palladium containing complexes 6b, 8b, and 9b at T = 100 K.

Figure 14. Cyclic voltammograms of 1 and 6b in 10−3 M CH2Cl2 solution, 0.1 M nBu4NPF6, 0.1 V s−1 scan rate, referenced vs. Fc0/+.

cyclic voltammogram (CV) of the free ligand 1 in dichloromethane solution. As previously established for guanidinesubstituted ferrocenes, the quasi-reversible redox process at E1/2 = −1.18 V (vs. Fc0/+) can be assigned to the Fe(II)/Fe(III) couple, whereas the irreversible oxidation at significantly higher potential (EOx = −0.05 V) is based on a guanidine-centered oxidation process.40,82 It should be noted that 1 is among the most electron-rich ferrocene derivatives and considerably easier to oxidize than the guanidine ligands present in complexes G,40,83 which reflects the particularly strong electron-donating properties of the imidazolin-2-imine moiety.66 The cyclic voltammogram of 6b is also shown in Figure 14. Since this compound converts into [7]Cl in solution (vide supra), the measurement was performed within 5 min after preparation of the sample solution. Quasi-reversibility is found for the iron-centered oxidation process, which occurs at a significantly more anodic potential (E1/2 = −0.41 V) compared to 1 because of a decrease of electron density at the ferrocene moiety upon Pd(II) coordination. The complexes with Fe−Pd bonds gave rise only to irreversible oxidation processes (Supporting Information, Figure S42); however, quasi-reversible CV traces could be recorded if the potential range was limited to the first oxidation process (Table 3, Figure 15). The resulting E1/2 values of [7]BF4 (0.54 V), 8b (0.78 V), and 9b (0.67 V) reveal an enormous anodic shift upon Fe−Pd bond formation. It must be emphasized, however, that the mono- and dicationic nature of these complexes also accounts for their higher stability toward oxidation in comparison with the neutral complex 6b. High oxidation potentials were also reported for

Figure 13. Correlation diagram of isomer shift and quadrupole splitting values of Table 2.

2.5 mm s−1. These values are in good agreement with those established for Fe(II) low-spin centers and for related ferrocene compounds (cf. δ/ΔEQ [mm s−1]: FeCp2 (78 K): 0.52(1)/ 2.37(1),78 dppf (77 K): 0.433(2)/2.304(5),79 [(dppf)PdCl2] (77 K): 0.408(2)/2.224(4),79 free ligand in complex G (one example, 80 K): 0.56/2.5540). The isomer shift of all investigated complexes 1, 6a/6b, 8a/ 8b, and 9a/9b shows only small variations; the observation of an increasing isomer shift with decreasing temperature can be ascribed predominantly to the second-order Doppler shift (Figure 13).80,81 The variation of the isomer shift (cf. Δδ ≈ 0.05 mm s−1 at T = 100 K) for all measured complexes is less than the observed line width. Therefore, the changes are too small to be interpreted reliably. In contrast, a clear trend can be identified for the quadrupole splitting, namely, that the complexes 8a/8b and 9a/9b with Fe−M bonds afford consistently smaller values (ΔEQ = 2.1 to 2.2 mm s−1) than the complexes 1 and 6a/6b (ΔEQ = 2.4 to 2.5 mm s−1), which contain unperturbed ferrocene moieties. The systematical observation of approximately 0.3 mm s−1 smaller ΔEQ values for the complexes with short Fe−M distances is in good agreement with the interpretation of Mössbauer data reported by Silver for ferrocene complexes with iron−metal and iron− H


Article

Inorganic Chemistry Table 3. Quasi-Reversible Oxidation Processes for Ferrocene Compounds 1, 6b, [7]BF4, 8b, and 9ba complex

E1/2 (ΔEp)b

1 6b [7]BF4 8b 9b

−1.18 −0.41 0.54 0.78 0.67

(0.16) (0.14) (0.10) (0.11) (0.12)

EOxc 0.04 0.60, 0.59, 0.83, 0.73,

0.78 1.12 1.14 1.14

10−3 M CH2Cl2 solution, 0.1 M nBu4NPF6, 0.1 V s−1 scan rate, referenced vs. Fc0/+. bE1/2: half-wave potential. For [7]BF4, 8b, and 9b only observed, when a small potential range was scanned. ΔEp: separation of anodic and cathodic peak. cAdditional irreversible oxidation processes. In the case of [7]BF4, 8b, and 9b, if a large potential range was scanned. a

Figure 16. Contour plots of the Laplacian ∇2ρ(r) in the Fe−M−Nimine planes for the cations [{fc(NIm)2}M(NCMe)]2+ in 8a (M = Ni, top) and 8b (M = Pd, bottom). Charge concentrations (black lines) indicate negative values; charge depletions (red lines) indicate positive values. The solid lines connecting the atomic nuclei are the bond paths; the green solid lines separating the atomic nuclei indicate the zero-flux surfaces in the plane.

Figure 15. Cyclic voltammograms of [7]BF4, 8b, and 9b in 10−3 M CH2Cl2 solution, 0.1 M nBu4NPF6, 0.1 V s−1 scan rate, referenced vs. Fc0/+.

tions.86,87 For hydrogen bonds, a proportionality between the bond dissociation energy (BDE) and the potential energy density V(r) at the BCP was proposed with −BDE = 1/ 2V(rBCP),88 which affords BDEs of 5.3 and 9.6 kcal mol−1 for the iron−nickel and iron−palladium bonds, respectively. These values indicate that the Fe−Pd bond is appreciably stronger than the Fe−Ni bond; however, the absolute values should not be regarded as reliable. Nevertheless, this trend was confirmed by calculation of compliance constants and their reciprocals, relaxed force constants,89 which afforded values of 77.0 N m−1 (Fe−Ni) and 99.2 N m−1 (Fe−Pd) for the latter constant. The stretching along the Fe−Ni and Fe−Pd bond axes associated with these vibrations was additionally studied by a potential energy surface (PES) scan with incremental increase of the iron−metal bonds. For the Pd system, a second minimum structure was identified at Fe−Pd = 3.849 Å, in which the Pd atom adopts a slightly distorted T-shaped geometry (Figure 17). This minimum lies 24.3 kcal mol−1 above the global minimum, and this value might be regarded as a good measure for the Fe−Pd BDE. Gas-phase addition of acetonitrile to the cation [{fc(NIm)2}Pd(NCMe)]2+ in 8b produces the square-planar bis(acetonitrile) complex [{fc(NIm)2}Pd(NCMe)2]2+ (see Supporting Information Table S7 for structural information). This reaction is calculated to be almost thermoneutral, i.e., ΔEel = −0.3 kcal mol−1 (Scheme 6), revealing that the BDE of the Fe−Pd bond is in the same order of magnitude as the BDE of the palladium−acetonitrile bond. For the Ni system, elongation of the Fe−Ni distance did not afford another minimum structure with a singlet ground state

complexes of types E and G, and it was also concluded for these systems that the interaction between the Fe and Pd atoms and the accompanying depletion of electron density from iron contribute significantly to the observed anodic shifts of the Fe(II)/Fe(III) redox potentials.38,40 Computational Studies. The bonding in complexes of types E and H was recently analyzed by DFT methods.38,47 Since the pairs 8a/8b and 9a/9b represent the first examples of isostructural nickel(II) and palladium(II) complexes featuring Fe−Ni and Fe−Pd interactions, comparative DFT calculations were performed for the acetonitrile complexes 8 using the B97D functional, which properly models noncovalent and longrange dispersion interactions.84 The fully optimized gas-phase structures of the cations [{fc(NIm)2}M(NCMe)]2+ in 8a (M = Ni) and 8b (M = Pd) display structural characteristics similar to those experimentally determined by X-ray diffraction (see Supporting Information, Table S8). Thus, the calculated Fe−Ni (2.667 Å) and Fe−Pd (2.696 Å) bond lengths are very similar, which was also found for the experimental values, i.e., Fe−Ni = 2.6268(4) Å and Fe−Pd = 2.6297(4) Å. The topology of the electron density was further analyzed by means of Bader’s atoms in molecules (AIM) theory,85 and Figure 16 shows the contour plots of the Laplacian ∇2ρ(r) in the Fe−M−Nimine plane. Fe−Ni and Fe−Pd bonding could be confirmed by identification of (3,−1) bond critical points (BCP); the small positive values of the Laplacian ∇2ρ(r) at the BCP of 0.03 e Å−5 (M = Ni) and 0.05 e Å−5 (M = Pd) indicate the presence of weak, noncovalent metal−metal interacI


Article

Inorganic Chemistry

in 8a is found to be exothermic, with ΔEel = −8.3 and −5.6 kcal mol−1 calculated for the formation of the bis(acetonitrile) complex [{fc(NIm)2}Ni(NCMe)2]2+ in the triplet or singlet electronic ground state, respectively (Scheme 6). In addition, this indicates that the Fe−Ni bond is weaker than the nickel− acetonitrile bond, albeit not to a large extent.

■

CONCLUSIONS The ferrocene-bridged bis(imidazolin-2-imine) ligand fc(NIm)2 (1) was successfully employed for the preparation of a series of cationic iron−nickel and iron−palladium complexes, viz. [(1)PdCl]+, [(1)ML]2+ (M = Ni, Pd; L = MeCN, PMe3), and [(1)Pd(PPh3)]2+. In these complexes, the ferrocene ligand acts as a tridentate ligand with a κFe,κ2N,N′ coordination mode, affording diamagnetic, square-planar complexes with short Fe−Ni and Fe−Pd bonds. It should be noted that the nickel complexes [(1)NiL]2+ (L = MeCN, PMe3) represent the first examples of Fe−Ni bonds in complexes with a neutral ferrocene-based ligand. In addition to X-ray diffraction analysis, metal−metal interaction in these complexes can be experimentally substantiated by means of 1H NMR, UV/vis, and Mössbauer spectroscopy and by cyclic voltammetry. Theoretical studies reveal the presence of noncovalent, closed-shell interactions; however, notably high BDEs of 11.3 kcal mol−1 (Fe−Ni) and 24.3 kcal mol−1 (Fe−Pd) were calculated for the complexes [(1)M(MeCN)]2+ (M = Ni, Pd). The latter value falls in the same order of magnitude as the BDE of the palladium−acetonitrile bond, indicating that the ferrocene moiety in ferrocene-bridged chelate ligands can be involved as a strong donor in dative metal−metal bonding,24 with obvious implications for the use of ferrocene-based chelate ligands in homogeneous catalysis.90,91

Figure 17. Potential energy surface (PES) scan at the B97D/6311G(d,p) level for the elongation of the Fe−Pd distance in the cation [{fc(NIm)2}Pd(NCMe)]2+ in 8b; the ball-and-stick drawings represent the minimum structures with hydrogen atoms and imidazole moieties omitted for clarity.

Scheme 6. Calculation of the Gas-Phase Addition of Acetonitrile to the Cations [{fc(NIm)2}M(NCMe)]2+ in Complexes 8

■

(S = 0), but an intersystem crossing to a triplet state (S = 1) is observed at ca. 3.2 Å. Two minimum structures were found on this triplet PES at Fe−Ni = 3.135 Å and Fe−Ni = 3.706 Å, in which the environments around the Ni atom are best described as Y- or T-shaped geometries, respectively (Figure 18). Both

EXPERIMENTAL SECTION

General Remarks. Standard Schlenk-line techniques under an inert gas atmosphere (Ar or N2) were used. P.A. grade methanol was used for the synthesis of 3 without further treatment. Other solvents were purified by Braun Solvent Purification System and stored over 3 or 4 Å molecular sieves. Protected diamine 2,71 chloroimidazolium salt 4,72 NiCl2(dme),92 PdCl2(MeCN)2,93 and PMe3,94,95 were synthesized according to literature methods. Triethylamine was distilled and stored over molecular sieves. Potassium fluoride was purchased from Acros and dried at 100 °C in vacuo; NaBF4 was purchased from SigmaAldrich and dried at 170 °C in vacuo. KOtBu and AgBF4 were purchased from Alfa Aesar and stored in a glovebox. NMR spectra were recorded on Bruker DPX-200, AV II-300, or AV III-400. The chemical shifts (δ) are expressed in ppm and are given relative to internal TMS (δ 0.00 ppm), to residual solvent 1H signals (DMSO-d5, δ 2.50 ppm; C6HD5, δ 7.16 ppm), or to the 13C resonance of the solvents (CDCl3, δ 77.16 ppm; CD2Cl2, δ 53.84 ppm; C6D6, δ 128.06 ppm). Elemental analyses were succeeded by combustion and gas chromatographic analysis on a VarioMICRO Tube instrument at the Technische Universitat Braunschweig. Cyclic voltammetry was performed on platinum working and counter electrodes and silver quasi-reference electrode, which were polished in an aqueous Al2O3 suspension by aid of an ultrasonic bath for at least 20 min. After that, the electrodes were subsequently cleaned with water, conc. HCl, dil. NH3, water, and acetone. Furthermore, the platinum electrodes were annealed in a gas flame. The cell was prepared in a glovebox under an argon atmosphere. Measurements were carried out in 10−3 M dichloromethane solutions with 0.1 M nBu4NPF6 at a 0.1 V s−1 scan rate. The measurements were referenced internally to ferrocene (E = 0.00 V; 0.46 V vs. SCE). UV/vis data were recorded on a Varian Cary 50 spectrometer. X-ray Diffraction Studies. Data were recorded on various diffractometers of the firm Oxford Diffraction, using monochromated

Figure 18. Potential energy surface (PES) scan at the B97D/6311G(d,p) level for the elongation of the Fe−Ni distance in the cation [{fc(NIm)2}Ni(NCMe)]2+ in 8b; blue bottom line for the singlet spin state (S = 0) and red line for the triplet spin state (S = 1); the ball-andstick drawings represent the minimum structures with hydrogen atoms and imidazole moieties omitted for clarity.

triplet structures have similar energies, and the slightly more stable Y-shaped isomer lies 11.3 kcal mol−1 above the energy of the singlet ground state. Again, this value represents the BDE for the Fe−Ni bond, which is significantly smaller compared to that of the Fe−Pd bond, viz. BDE(Fe−Ni) = 11.3 kcal mol−1 vs. BDE(Fe−Pd) = 24.3 kcal mol−1. Accordingly, gas-phase addition of acetonitrile to the cation [{fc(NIm)2}Ni(NCMe)]2+ J


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Inorganic Chemistry

the purity was sufficient for further reaction. 1H NMR (200 MHz, DMSO-d6, 300 K): δ [ppm] 10.08 (br. s, 6 H, NH3), 4.57 (m, 4 H, CHα/β), 4.33 (m, 4 H, CHα/β). 13C NMR (75 MHz, DMSO-d6, 299 K): δ [ppm] = 88.6 (Ci), 67.4 (Cα/β), 64.9 (Cα/β). Anal. Calcd for C10H14Cl2FeN2 (748.23): C 41.56, H 4.88, N 9.69. Found: C 42.54, H 5.07, N 9.90. [fc(NHIm)2](BF4)2 (5). A 250 mL Schlenk flask was charged with fc(NH3Cl)2 (2) (5.00 g, 17.3 mmol, 1 equiv), chloroimidazolium salt 4 (10.47 g, 34.6 mmol, 2 equiv), and potassium fluoride (9.03 g, 155.7 mmol, 9 equiv). Acetonitrile (100 mL) and triethylamine (14.4 mL, 103.8 mmol, 6 equiv) were added subsequently, and the mixture was stirred for 4 days at room temperature (until NMR indicated full conversion of the starting materials). After that time, it was treated with degassed CHCl3 (100 mL). The mixture was filtered through Celite directly into a Schlenk separation funnel containing NaBF4 (19.00 g, 173.0 mmol, 10 equiv) in degassed water (150 mL). The filtration residue was washed with CHCl3 (2 × 10 mL). After removal of the frit, the mixture was vigorously shaken. The organic layer was washed three times with degassed water before it was dried over MgSO4. After this, the red-brown suspension was filtered and the solvent was removed to yield the title compound as red-brown foam (11.51 g, 15.4 mmol, 89%). Crystals suitable for X-ray diffraction were obtained by layering a THF solution with n-pentane. 1H NMR (300 MHz, CDCl3, 298 K): δ [ppm] 6.56 (s, 2 H, NH), 4.74 (sept, 4 H, 3 JH,H = 7.1 Hz, CH(CH3)2), 4.17 (m, 4 H, CHα/β), 4.13 (m, 4 H, CHα/β), 2.32 (s, 12 H, CCH3), 1.51 (d, 24 H, 3JH,H = 7.0 Hz, CH(CH3)2). 13C NMR (75 MHz, C6D6, 299 K): δ [ppm] = 138.8 (NCN), 123.8 (CCH3), 103.1 (Ci), 65.9 (Cα/β), 61.2 (Cα/β), 50.1 (CH(CH3)2), 20.8 (CH(CH3)2), 9.8 (CCH3). Anal. Calcd for C32H50B2F8FeN6 (748.25): C 51.37, H 6.74, N 11.23. Found: C 51.73, H 6.69, N 11.23. fc(NIm)2 (1). In a 250 mL Schlenk flask, 5 (5.00 g, 6.7 mmol) was dissolved in THF (120 mL) and treated with KOtBu (1.80 g, 16.0 mmol, 2.4 equiv). After stirring for 30 min at room temperature, this mixture was dried in vacuo at 50 °C. The resulting dark residue was extracted with a mixture of toluene (30 mL) and n-hexane (20 mL) by assistance of an ultrasonic bath. The solution was filtered through Celite to remove a black solid, and the residue was washed three more times (each comprising 7.5 mL of toluene + 5 mL of n-hexane). After removal of the volatiles in vacuo at 50 °C, the sticky dark mass was extracted with n-hexane (20 mL), and the solution was filtered and crystallized at −34 °C. After crystallization, the product was isolated by decantation, washed with n-hexane (2 × 2 mL), and dried in vacuo. The solid was ground to an orange powder. The mother liquor yielded an additional product, with an isolated yield of 2.07 g (3.6 mmol, 54%) of the desired product. The dark extraction residue still contained considerable amounts of the product (as indicated by 1H NMR), but it could not be purified satisfactorily. Crystals suitable for X-ray diffraction were obtained from a mixture of diethyl ether and nhexane at −34 °C. 1H NMR (400 MHz, C6D6, 296 K): δ [ppm] = 4.72 (sept, 4 H, 3JH,H = 6.8 Hz, CH(CH3)2), 4.47 (m, 4 H, CHα/β), 4.29 (m, 4 H, CHα/β), 1.66 (s, 12 H, CCH3), 1.20 (d, 24 H, 3JH,H = 7.0 Hz, CH(CH3)2). 13C NMR (100 MHz, C6D6, 300 K): δ [ppm] = 150.1 (NCN), 116.3 (CCH3), 113.7 (Ci), 64.8 (Cα/β), 62.1 (Cα/β), 46.2 (CH(CH3)2), 21.1 (CH(CH3)2), 10.5 (CCH3). Anal. Calcd for C32H48FeN6 (572.62): C 67.12, H 8.45, N 14.63. Found: C 67.25, H 8.54, N 14.56. [{fc(NIm)2}NiCl2] (6a). A suspension of NiCl2(dme) (88 mg, 0.4 mmol) in THF (approximately 5 mL) was treated with a solution of 1 (230 mg, 0.4 mmol) in THF (approximately 2 mL), whereupon the color of the suspension changed from yellow to dark green and a dark solid precipitated. After stirring overnight, the product was completely precipitated by addition of n-hexane (25 mL). The almost colorless solution was removed via cannula, and the precipitate was washed three times with n-hexane (5 mL). After drying in vacuo, the desired product was isolated as a dark green, brownish powder (258 mg, 0.37 mmol, 92%). Crystals suitable for X-ray diffraction were obtained by layering a THF solution with diethyl ether. 1H NMR (200 MHz, CD2Cl2, 300 K): δ [ppm] 21.31 (br. s, 4 H), 16.52 (s, 12 H), 8.88 (s, 4 H), 3.45 (br. s, 24 H), −8.15 (br. s, 4 H). Anal. Calcd for

Mo Kα or mirror-focused Cu Kα radiation. Absorption corrections were applied on the basis of multiscans. Structures were refined anisotropically on F2 using the program SHELXL-97 (G. M. Sheldrick, University of Göttingen, Germany). Treatment of hydrogen atoms: NH hydrogens were refined freely, methyls as idealized rigid groups allowed to rotate but not tip, other H using a riding model starting from calculated positions. Numerical details are given as Supporting Information (Table S1 and S2). Special features and exceptions: The anion of compound 5 and one anion of 8a have high U values. For 9a, both anions and one isopropyl group are disordered; for 10, one anion and one isopropyl group are disordered. Despite the use of appropriate restraints, dimensions of disordered groups should be interpreted with caution. Additionally, for 10, solvent site disorder, apparently consisting of superimposed dichloromethane and diethyl ether sites, could not be satisfactorily resolved, and the effects of the solvent were therefore removed mathematically using the routine SQUEEZE (part of the program suite PLATON; A. L. Spek, University of Utrecht, Netherlands). Derived parameters such as the formula weight correspond to an idealized composition with two molecules of diethyl ether and two molecules of dichloromethane per cell. For compound 6b, which crystallizes by chance in a chiral (Sohncke) space group, the Flack parameter refined to −0.023(7). See also the Supporting Information for details of the structures presented there. Complete data have been deposited at the Cambridge Crystallographic Data Centre under the numbers CCDC-1432706 (1), CCDC-1432707 (5), CCDC-1432708 (6a), CCDC-1432709 (6b), CCDC-1432710 ([7]Cl· 3CDCl3), CCDC-1432713 ([7]BF4), CCDC-1432712 ([7]2[PdCl4]), CCDC-1432711 ([7]Cl·1/2 CH2 Cl2 ·CH3CONH 2·1/4C 6 H14 O), CCDC-1432714 (8a), CCDC-1432715 (8b), CCDC-1432719 (9a), CCDC-1432716 (9b), and CCDC-1432720 (10). These data can be obtained free of charge from www.ccdc.cam.ac.uk/data_request/cif. Mö ssbauer Spectroscopy. 57Fe Mössbauer spectroscopic measurements have been performed on a conventional transmission spectrometer with sinusoidal velocity sweep. The activity of the Mössbauer source was about 4 mCi (for polycrystalline powder samples of complexes 8a, 8b, 9a, and 9b) and about 10 mCi (for polycrystalline powder samples of complexes 1, 6a, 6b, and [7]BF4) of 57 Co in a rhodium matrix kept at room temperature. The measurements were done with a Janis closed-cycle cryostat with helium exchange gas. The temperature was measured with a calibrated Si diode located close to the sample container made of Teflon. The isomer shift is specified relative to metallic iron at room temperature and was not corrected for second-order Doppler shift. Additional measurements on a sample of complex 1 were carried out on a CryoVac continuous-flow cryostat with comparable specifications, geometry, and sample environments as those described above. The activity of the Mössbauer source used here was about 8 mCi of 57Co in a rhodium matrix. SQUID Magnetometry. Magnetic susceptibility measurements were performed on a Cryogenic Ltd. closed-cycle SQUID magnetometer between T = 2.4 and 300 K with an applied external magnetic field of H = 1000 Oe. A polycrystalline powder sample of 6a (12.6 mg) was weighed in a gelatin capsule and then fixed in a polyethene sample holder. The background signal of an empty gelatin capsule and the sample holder were experimentally determined and subtracted from the experimental raw data set. The correction of the diamagnetic susceptibility of complex 6a (χcorr = −2.62 × 10−4 cm3 mol−1) was done by use of tabulated Pascal parameters. fc(NH3Cl)2 (3). 1,1′-Bis[(tert-butoxycarbonyl)amino]-ferrocene, fc(NHBoc)2 (1) (10.00 g, 24.0 mmol), was suspended in methanol (100 mL), degassed, and pulverized by assistance of an ultrasonic bath. In a second vessel, methanol (50 mL) was cooled to 0 °C and treated with acetyl chloride (25.8 mL, 360.3 mmol, 15 equiv) in portions of 1 mL. This solution was transferred to the suspension of fc(NHBoc)2, which was stirred at room temperature until no more gas formation was observed. Within 150 min, fc(NH3Cl)2 precipitated as a yellow powder and was completely precipitated by addition of diethyl ether (200 mL). The air-sensitive title compound was isolated after filtration, washing with diethyl ether, and drying in vacuo as a yellow powder (5.71 g, 19.8 mmol, 82%). Elemental analysis was unsatisfactory, but K


Article

Inorganic Chemistry

[{fc(NIm)2}Pd(NCMe)](BF4)2 (8b). A solution of [7]BF4 (80 mg, 0.1 mmol) in acetonitrile (3 mL) was treated with a solution of AgBF4 (20 mg, 0.1 mmol, 1 equiv) in acetonitrile (2 mL). A clouding of the solution was immediately observed. The mixture was stirred for approximately 5 min and filtered, and the solvent was removed in vacuo. The residue was redissolved in a small amount of acetonitrile and layered with diethyl ether. After decantation of the solution and drying in vacuo, the desired, air-stable product was obtained as a brown solid (75 mg, 0.084 mmol, 84%). Crystals suitable for X-ray diffraction were obtained by layering an acetonitrile solution with diethyl ether. 1H NMR (200 MHz, CD2Cl2, 300 K): δ [ppm] 5.46 (br. m, 4 H, CHβ), 5.40 (br. m, 4 H, CH(CH3)2), 3.37 (m, 4 H, CHα), 2.33 (s, 3 H, NCCH3), 2.26 (s, 12 H, CCH3), 1.54 (d, 24 H, 3JH,H = 7.0 Hz, CH(CH3)2). 13C NMR (50 MHz, CD2Cl2, 300 K): δ [ppm] = 146.1 (NCN), 123.5 (CCH3), 105.1 (Ci), 101.2 (NCCH3), 80.8 (Cβ), 67.7 (Cα), 50.2 (CH(CH3)2), 21.7 (CH(CH3)2), 10.3 (CCH3), 3.1 (NCCH3). Anal. Calcd for C34H51B2F8FeN7Pd (893.70): C 45.69, H 5.75, N 10.97. Found: C 45.30, H 6.00, N 11.29. [{fc(NIm)2}Ni(PMe3)](BF4)2 (9a). To a suspension of 6a (70 mg, 0.1 mmol) and NaBF4 (22 mg, 0.2 mmol, 2 equiv) in THF (5 mL) was added a solution of PMe3 (approximately 8 mg, 0.1 mmol, 1 equiv) in THF (1 mL), whereupon the green suspension turned brown immediately. This mixture was stirred for 3 days. After that time, stirring was stopped and the brown precipitate was allowed to settle. The almost colorless solution was removed via syringe, and the precipitate washed with THF (3 × 2 mL). After drying, the powder was dissolved in a minimal amount of acetonitrile (approximately 1 mL), filtered, and layered with a mixture of diethyl ether and n-hexane (approximately 2 mL). The resulting very dark crystals were washed with ether and dried in vacuo, ground to a powder, and further dried in vacuo. The desired product could be isolated in a yield of 54 mg (0.061 mmol, 61%) as an air-stable, brown powder. Crystals suitable for X-ray diffraction were obtained by layering an acetonitrile solution with diethyl ether. 1H NMR (200 MHz, CD2Cl2, 300 K): δ [ppm] 5.63 (sept, 4 H, 3JH,H = 7.0 Hz, CH(CH3)2), 5.10 (m, 4 H, CHβ), 3.76 (m, 4 H, CHα), 2.28 (s, 12 H, CCH3), 1.69 (d, 12 H, 3JH,H = 7.0 Hz, CH(CH3)2), 1.45 (d, 12 H, 3JH,H = 7.0 Hz, CH(CH3)2), 1.20 (d, 12 H, 2 JH,P = 10.5 Hz, P(CH3)3). 13C NMR (75 MHz, CD2Cl2, 299 K): δ [ppm] = 146.2 (NCN), 123.6 (CCH3), 81.9 (Ci), 77.8 (Cβ), 70.3 (Cα), 49.9 (CH(CH3)2), 22.4 (CH(CH3)2), 21.9 (CH(CH3)2), 14.6 (d, 1JC,P = 27.8 Hz P(CH3)3) 10.6 (CCH3). 31P{1H} NMR (81 MHz, CD2Cl2, 300 K): δ [ppm ] −0.4 (P(CH 3 ) 3 ). Anal. Calcd for C35H57B2F8FeN6NiP (880.99): C 47.72, H 6.52, N 9.54. Found: C 47.76, H 6.31, N 9.85. [{fc(NIm)2}Pd(PMe3)](BF4)2 (9b). To a suspension of [7]BF4 (80 mg, 0.1 mmol) and NaBF4 (11 mg, 0.1 mmol, 1 equiv) in THF (5 mL) was added a solution of PMe3 (approximately 8 mg, 0.1 mmol, 1 equiv) in THF (1 mL). This brown mixture was stirred for 3 days. After that time, stirring was stopped and the orange-brown suspension was dried in vacuo. After drying, the powder was dissolved in a minimal amount of acetonitrile (approximately 1 mL), filtered, and layered with a mixture of diethyl ether and n-hexane (approximately 2 mL). The resulting crystals were washed with ether and dried in vacuo, ground to a powder, and further dried in vacuo. The desired product could be isolated in a yield of 54 mg (0.058 mmol, 58%) as an airstable, orange-brown powder. Crystals suitable for X-ray diffraction were obtained by layering a dichloromethane solution with diethyl ether. 1H NMR (200 MHz, CD2Cl2, 300 K): δ [ppm] 5.24 (m, 4 H, CHβ), 5.16 (sept, 4 H, 3JH,H = 6.9 Hz, CH(CH3)2), 3.46 (m, 4 H, CHα), 2.30 (s, 12 H, CCH3), 1.61 (d, 12 H, 3JH,H = 7.1 Hz, CH(CH3)2), 1.45 (d, 12 H, 3JH,H = 7.1 Hz, CH(CH3)2), 1.35 (d, 12 H, 2 JH,P = 10.8 Hz, P(CH3)3). 13C NMR (75 MHz, CD2Cl2, 298 K): δ [ppm] = 146.3 (NCN), 123.8 (CCH3), 94.0 (Ci), 78.3 (Cβ), 68.0 (Cα), 49.8 (CH(CH3)2), 22.2 (CH(CH3)2), 22.0 (CH(CH3)2), 16.4 (d, 1JC,P = 28.3 Hz P(CH3)3) 10.5 (CCH3). 31P{1H} NMR (81 MHz, CD2Cl2, 300 K): δ [ppm ] −7.3 (P(CH 3 ) 3 ). Anal. Calcd for C35H57B2F8FeN6PPd (928.72): C 45.26, H 6.19, N 9.05. Found: C 45.20, H 6.31, N 9.84. [{fc(NIm)2}Pd(PPh3)](BF4)2 (10). A solution of [7]BF4 (80 mg, 0.1 mmol) and PPh3 (52 mg, 0.2 mmol, 2 equiv) in dichloromethane (3

C32H48Cl2FeN6Ni (702.21): C 54.73, H 6.89, N 11.97. Found: C 54.78, H 7.00, N 11.76. The effective magnetic moment was determined by magnetic susceptibility measurements: μeff = 3.9 μB (300 K) (Figure S43). [{fc(NIm)2}PdCl2] (6b). A suspension of PdCl2(MeCN)2 (52 mg, 0.2 mmol) in THF (approximately 5 mL) was treated with a solution of 1 (115 mg, 0.2 mmol) in THF (3 mL) and stirred for 2 days. After that, the black precipitate was isolated by filtration, washed two times with minimal amounts of THF, and dried in vacuo. This yielded 117 mg (0.16 mmol, 78%) of 6b as a dark gray powder. Black crystals, suitable for X-ray diffraction, could be obtained from a dichloromethane solution, layered with diisopropyl ether. Elemental analysis of the gray powder was not sufficient in the carbon value (vide infra), but a powder diffractogram was in accordance with the calculated pattern from the crystal structure of 6b (Figure S6) and therefore indicated its high purity. The 1H NMR showed two sets of signals due to chloride abstraction from 6b, forming [7]Cl. 1H NMR (300 MHz, CD2Cl2, 298 K): 6b: δ [ppm] 6.08 (sept, 4 H, 3JH,H = 7.1 Hz, CH(CH3)2), 3.89 (m, 4 H, CHα/β), 3.76 (m, 4 H, CHα/β), 2.19 (s, 12 H, CCH3), 1.71 (d, 12 H, 3JH,H = 7.1 Hz, CH(CH3)2), 1.37 (d, 12 H, 3JH,H = 7.1 Hz, CH(CH3)2); [7]Cl: δ [ppm] 5.52 (br. m, 4 H, CH(CH3)2), 5.30 (br. m, 4 H, CHβ), 3.23 (br. m, 4 H, CHα), 2.21 (s, 12 H, CCH3), 1.54 (d, 24 H, 3JH,H = 7.0 Hz, CH(CH3)2). 13C NMR (75 MHz, CD2Cl2, 299 K): 6b: δ [ppm] = 152.2 (NCN), 119.5 (CCH3), 113.9 (Ci), 67.4 (Cα/β), 65.6 (Cα/β), 48.5 (CH(CH3)2), 21.4 (CH(CH3)2), 21.1 (CH(CH3)2), 10.4 (CCH3); [7]Cl: δ [ppm] = 147.6 (NCN), 122.4 (CCH3), 101.5 (Ci), 78.7 (Cβ), 67.0 (Cα), 49.9 (CH(CH3)2), 21.8 (CH(CH3)2), 10.4 (CCH3). Anal. Calcd for C32H48Cl2FeN6Pd (749.94): C 51.25, H 6.45, N 11.21. Found: C 50.30, H 6.49, N 11.42. [{fc(NIm)2}PdCl]BF4 ([7]BF4). A suspension of Pd(MeCN)2Cl2 (130 mg, 0.5 mmol) and NaBF4 (55 mg, 0.5 mmol, 1 equiv) in THF (5 mL) was treated with a solution of 1 (286 mg, 0.5 mmol, 1 equiv) in THF (3 mL). This mixture was stirred at room temperature for 2 days, during which time a brown powder formed and precipitated. The product was completely precipitated by addition of n-hexane (25 mL), the almost colorless solution was removed, and the powder was dried in vacuo. The powder was dissolved in a small amount of dichloromethane, filtered, and dried. After recrystallization by layering a dichloromethane solution (5 mL) with n-hexane (approximately 20 mL), the crystals were dried at 70 °C and 3·10−3 mbar for a few hours to remove the crystallized dichloromethane. The product could be isolated as air stable, red-brown crystals (365 mg, 0.46 mmol, 91%). Crystals suitable for X-ray diffraction were obtained by layering a dichloromethane solution with diethyl ether. 1H NMR (400 MHz, CDCl3, 296 K): δ [ppm] 5.62 (br. m, 4 H, CH(CH3)2), 5.36 (m, 4 H, CHβ), 3.27 (m, 4 H, CHα), 2.21 (s, 12 H, CCH3), 1.55 (d, 24 H, 3JH,H = 7.0 Hz, CH(CH3)2). 13C NMR (100 MHz, CDCl3, 298 K): δ [ppm] = 147.4 (NCN), 121.8 (CCH3), 101.3 (Ci), 78.2 (Cβ), 66.7 (Cα), 49.5 (CH(CH3)2), 21.6 (CH(CH3)2), 10.1 (CCH3). Anal. Calcd for C32H48BClF4FeN6Pd (801.29): C 47.97, H 6.04, N 10.49. Found: C 47.99, H 6.15, N 10.58. [{fc(NIm)2}Ni(NCMe)](BF4)2 (8a). A solution of 6a (70 mg, 0.1 mmol) in acetonitrile (5 mL) was treated with NaBF4 (109 mg, 1.0 mmol, 10 equiv) and stirred for 4 h. After that time, the mixture was filtered through a filter pipet and the solvent was removed in vacuo. The residue was dissolved in dichloromethane and filtered, and the solution was layered with a 1:1 mixture of diethyl ether and n-hexane. After crystallization, decantation, washing with diethyl ether, and drying in vacuo, the product could be isolated as dark green to brown crystals (72 mg, 0.085 mmol, 85%). Crystals suitable for X-ray diffraction were obtained by layering an acetonitrile-d3 solution with diethyl ether and n-hexane. 1H NMR (300 MHz, CD2Cl2, 298 K): δ [ppm] 6.33 (br. m, 4 H, CH(CH3)2), 5.23 (m, 4 H, CHβ), 3.63 (m, 4 H, CHα), 2.31 (s, 3 H, NCCH3), 2.31 (s, 12 H, CCH3), 1.63 (d, 24 H, 3 JH,H = 7.1 Hz, CH(CH3)2). 13C NMR (75 MHz, CD2Cl2, 299 K): δ [ppm] = 146.3 (NCN), 128.1 (NCCH3), 123.2 (CCH3), 93.8 (Ci), 79.9 (Cβ), 68.5 (Cα), 50.4 (CH(CH3)2), 21.9 (CH(CH3)2), 10.2 (CCH3), 3.8 (NCCH3). Anal. Calcd for C32H51B2F8FeN7Ni (845.98): C 48.27, H 6.08, N 11.59. Found: C 48.37, H 6.42, N 11.35. L


Article

Inorganic Chemistry

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mL) was treated with solid AgBF4 (20 mg, 0.1 mmol, 1 equiv). The mixture was stirred for approximately 5 min and filtered through Celite, and the solvent was removed in vacuo. The residue was redissolved in a small amount of dichloromethane and treated with diethyl ether, and the solution was decanted to remove excess PPh3. Recrystallization by layering a dichloromethane solution with diethyl ether, decantation, and drying the crystals in vacuo yielded the desired, air-stable product as a red-brown crystalline solid (65 mg, 0.058 mmol, 58%). 1H NMR (200 MHz, (CD3)2CO, 300 K): δ [ppm] 7.66−7.45 (m, 15 H, PPh3), 5.48 (m, 4 H, CHβ), 5.34 (sept, 4 H, 3JH,H = 7.0 Hz, CH(CH3)2), 3.91 (m, 4 H, CHα), 2.18 (s, 12 H, CCH3), 1.31 (d, 12 H, 3JH,H = 7.0 Hz, CH(CH3)2), 1.09 (d, 12 H, 3JH,H = 7.0 Hz, CH(CH3)2). 13C NMR (50 MHz, (CD3)2CO, 300 K): δ [ppm] = 146.9 (NCN), 135.1 (d, JC,P = 12.1 Hz, CPh), 132.3 (d, JC,P = 0.1 Hz, CPh), 131.5 (d, JC,P = 38.9 Hz, C-1Ph), 130.0 (d, JC,P = 10.2 Hz, CPh), 123.9 (CCH3), 99.0 (Ci), 80.6 (Cβ), 68.9 (Cα), 50.2 (CH(CH3)2), 22.3 (CH(CH3)2), 21.1 (CH(CH3)2), 10.2 (CCH3). 31P{1H} NMR (81 MHz, (CD3)2CO, 300 K): δ [ppm] 19.9 (PPh3). Anal. Calcd for C35H57B2F8FeN6PPd (1114.94): C 53.86, H 5.70, N 7.54. Found: C 53.53, H 5.84, N 7.44.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02457. Structure files (MOL) Crystallographic data (CIF) Crystallographic data for all structures, including figures and text for the compounds [7]Cl·1/2CH2Cl2·CH3C(O)NH2, [7]BF4, [7]2[PdCl4], 8b, and 9b; details of the kinetic study of the decomposition of 6b, including NMR and UV/vis spectra and detailed computation; UV/vis spectra of [7]BF4, 8b, and 9b; NMR spectra of all compounds; comparison of Mössbauer parameters for Lorentz plot vs. Voigt plot for 1 (at 20 K) and 9a; wide range CV for [7]BF4, 8b, and 9b; computational details, including energies of all optimized structures, bond lengths, and angles and singlet−triplet splittings; and details for magnetic susceptibility measurements (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+49) 531 391 5387. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge Prof. Dr. M. Bröring (Institut für Anorganische und Analytische Chemie) and Prof. Dr. F. J. Litterst (Institut für Physik der Kondensierten Materie) at TU Braunschweig for providing access to the SQUID magnetometer and 57Fe Mössbauer equipment.

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REFERENCES

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Inorganic Chemistry

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