Neutral and Cationic Zinc Complexes with N- and S-Donor

Article pubs.acs.org/Organometallics

Neutral and Cationic Zinc Complexes with N- and S‑DonorFunctionalized Cyclopentadienyl Ligands Maren A. Chilleck, Thomas Braun,* Beatrice Braun, and Stefan Mebs Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany S Supporting Information *

ABSTRACT: The synthesis of neutral and cationic zinc cyclopentadienyl (Cp) complexes with amino and thio donor groups that are attached to the Cp ring via a side chain is reported. The amino-functionalized zincocene [ZnCp3N2] (3; Cp3N = C5Me4(CH2)3NMe2) was prepared in a salt metathesis reaction from KCp3N and ZnCl2. In the solid-state structure of 3, which was determined by single-crystal X-ray diffraction, one of the Cp3N ligands is coordinated as a chelating ligand through a ring carbon atom and through the amino group, whereas the other Cp3N ligand is bound in a monodentate mode through the Cp ring only. A reaction of 3 with ZnEt2, [ZnCp*2] (Cp* = C5Me5), and [ZnCp2N2] (2; Cp2N = C5Me4(CH2)2NMe2) gave the heteroleptic complexes [ZnEtCp3N] (4), [ZnCp*Cp3N] (5), and [ZnCp3NCp2N] (6), respectively. The incorporation of a second amino group in the same side chain led to the formation of the mononuclear zinc chlorido complex [ZnClCptmeda] (7; Cptmeda = C5Me4(CH2)2NMe(CH2)2NMe2), in which the Cptmeda ligand is bound in a tridentate coordination mode. In addition, the thio-functionalized zincocene [ZnCp2S2] (8; Cp2S = C5Me4(CH2)2SMe) was obtained and shown to exhibit an intramolecular coordination by the sulfur atoms. Starting from the neutral complexes 2, 3, and 8, the cationic compounds [ZnCpD(py)2]+[BArF4]− (CpD = Cp2N (9), Cp3N (10), Cp2S (11); py = pyridine; BArF4 = B{3,5-(CF3)2C6H3}4) were obtained either by protonation or upon reaction with an electrophile in the presence of pyridine. In these cationic complexes the highly electrophilic zinc center is stabilized by intramolecular coordination through the donor groups.

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INTRODUCTION Zinc cyclopentadienyl (Cp) complexes are known for their ability to adopt unusual bonding motifs. A remarkable example is represented by the dizincocene [Zn2Cp*2] (Cp* = C5Me5), published by Carmona et al. in 2004, which exhibits a direct Zn−Zn bond.1 With regard to the Lewis acidic nature of the zinc center, cationic zinc cyclopentadienyl complexes are of special interest. In recent years, cationic organozinc compounds, which were either used as isolated complexes or generated in situ, have been shown to act as catalysts or precatalysts in catalytic transformations in which cationic zinccontaining intermediates are assumed to constitute the catalytically active species.2 However, there have been only a few reports on cationic zinc cyclopentadienyl complexes that were structurally characterized.2a,3,4 The formation of the cationic complex [ZnCp′(tmeda)]+[BEt(C6F5)3]− (Cp′ = (3,5Me2C6H3)CH2CMe2(C5H4); tmeda = N,N,N′,N′-tetramethylethylenediamine) in the reaction of [ZnEtCp′(tmeda)] with B(C6F5)3 was proposed by Bochmann et al. on the basis of NMR spectroscopic observations.2a We showed that the reaction of [ZnCp*2] with [H(OEt2)2]+[BArF4]− (BArF4 = B{3,5-(CF3)2C6H3}4) in a 2:1 ratio in 1,2-difluorobenzene leads to the formation of the cationic triple-decker complex [Zn2Cp*3]+[BArF4]− (1).3 The “slipped” triple-decker structure of 1 with η5-coordination of the terminal Cp* rings and η1© 2014 American Chemical Society

coordination of the bridging Cp* ring was confirmed by singlecrystal X-ray analysis. Reacting 1 with chelating ligand precursors L (L = Me2N(CH2)2PiPr2, Cy2P(CH2)2PCy2, C y 2 P C H 2 P Cy 2 ) a ff o r d s t h e c a t i o n ic c o m p o u n d s [ZnCp*L]+[BArF4]−. Similarly, Fischer and co-workers showed that the protonation of the dizincocene [Zn2Cp*2] with [H(OEt2)2]+[BArF4]− in fluorobenzene yields the cationic triple-decker complex [Zn4Cp*3(OEt2)2]+[BArF4]−, in which the Zn−Zn bond is preserved.4 Furthermore, treatment of [Zn2Cp*2] with [FeCp2]+[BArF4]− in diethyl ether also results in an ether-stabilized cationic complex, [Zn 2 Cp*(OEt2)3]+[BArF4]−. In contrast, previous studies by Schulz et al. showed that the protonation of [Zn2Cp*2(dmap)2] (dmap = 4-dimethylaminopyridine), which was obtained by reacting [Zn2Cp*2] with dmap, led to the elimination of HCp* under formation of the dicationic binuclear complex [Zn2(dmap)6]2+{[Al{OC(CF3)3}4]−}2.5 As it can be deduced from these examples, cationic zinc cyclopentadienyl complexes can be stabilized by additional neutral donor ligands. Therefore, the application of donorfunctionalized Cp ligands in which a neutral donor group is linked to the Cp ring via a side chain may represent a new Received: November 5, 2013 Published: January 7, 2014 551

dx.doi.org/10.1021/om401076g | Organometallics 2014, 33, 551−560

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C5Me4(CH2)3NMe2 (Cp3N), which represents a homologue to the Cp2N ligand, incorporating an additional CH2 group in the spacer unit. Herein we report on zinc complexes bearing amino- and thio-functionalized cyclopentadienyl ligands. By taking advantage of the stabilization by the intramolecular amino and thio donor groups, cationic zinc cyclopentadienyl complexes were obtained.

possibility of stabilizing such cationic zinc complexes without using additional ligands. A large variety of donor-functionalized cyclopentadienyl ligands have been described in the literature, which may differ in the donor atom (e.g., N, O, P, S, C), the length of the spacer unit between Cp ring and donor group, and the other substituents on the Cp ring.6 Donor-functionalized cyclopentadienyl ligands have proven to be useful for the synthesis of complexes exhibiting low-coordinated metal centers as well as for stabilizing cationic complexes.6 In addition, their potential to act as hemilabile coordinating ligands makes them of interest for catalytic transformations in which vacant coordination sites have to be provided at the metal center in a reversible way. However, zinc complexes with donor-functionalized Cp ligands have scarcely been investigated.7−10 Darensbourg et al. described the formation of the dinuclear complexes [Cp″Zn(μ-Cl)] 2 (Cp″ = Me 2 N(CH2)2C5HiPr3) and [Cp′″Zn(μ-OAc)]2 (Cp′″ = Me2N(CH2)2C5HiPr3, cyclo-(C4H8N)(CH2)2C5Me4) starting from the corresponding lithium cyclopentadienides and one equivalent of ZnCl2 or Zn(OAc)2.7 Recently, we reported the first zinc complex with two donor-functionalized chelating Cp ligands [ZnCp2N2] (2; Cp2N = C5Me4(CH2)2NMe2; Scheme 1),

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RESULTS AND DISCUSSION The salt metathesis reaction of the potassium cyclopentadienide KCp3N and ZnCl2 in a 2:1 ratio in tetrahydrofuran led to the formation of a colorless to yellowish oil. NMR spectroscopic experiments revealed the product to be the desired amino-functionalized zincocene [ZnCp3N2] (3; Scheme 2), together with varying amounts (10−30 mol %) of HCp3N. Attempts to reduce the amount of HCp3N proved to be unsuccessful. Upon storage of the oily product for several days at −30 °C, it partially solidified, resulting in a product of waxlike consistency.11 In contrast, complex 2 is a colorless solid, which could be isolated in analytical purity.8 As it was observed for the other amino-functionalized cyclopentadienyl zinc complexes,8 3 is extremely reactive toward air and moisture. Compound 3 is highly soluble in aprotic organic solvents, even including cold (−90 °C) hexane. Fortunately, after storage at room temperature, a sample of the oily product was found to contain single crystals that were suitable for X-ray crystallography. In the solid-state structure of 3, which is shown in Figure 1, only one of the Cp3N ligands is coordinated as a chelating ligand with both the cyclopentadienyl ring and the amino function bound to the zinc center. The other Cp3N ligand is coordinated in a monodentate fashion, and the aminofunctionalized side arm is directed toward the periphery of the molecule. Thus, the coordination geometry at the zinc atom is distorted-trigonal. Notably, this is in contrast to the solidstate structure of 2, in which both of the Cp2N ligands are bound in a chelating mode.8 The fact that in complex 3 only one of the amino groups is coordinated to the zinc center can be attributed to steric reasons. Note that the wax-like consistency of 3 is caused possibly not only by contaminations with HCp3N, which is an oil at room temperature, but also by the reduced structural rigidity, as one of the side chains is not fixed to the metal center. In 3 the bond distance between the zinc center and the coordinated nitrogen atom (N1) is 2.185(2) Å, which is slightly shorter than the Zn−N distances in 2 (2.23 Å, average).8 With regard to the distances between the zinc atom and the ring carbon atoms of the chelating Cp3N ligand, there are one short (Zn1−C1 = 2.148(3) Å), one intermediate (Zn1−C2 = 2.348(3) Å), and three larger (2.66− 3.07 Å) contacts. On the basis of the results of previous AIM and ELI-D DFT studies,12 we consider only the shortest contact, Zn1−C1, as bonding interaction, which corresponds to an η1-bonding mode of the Cp ring. The zinc atom is located inside the perimeter of the Cp ring, as it is reflected in a Zn1− C1−Cg1 (Cg = center of gravity of the Cp ring; ring centroid) angle of only 83.67(17)°. Note that a Zn−C(η1)−Cg angle of about 90° or smaller is indicative of π contributions to the Zn− C(η1) bonding.13 Thus, the Cp ring of the chelating Cp3N ligand can be regarded as being η1(π)-coordinated. As the coordinated carbon atom C1 is the ring carbon atom carrying the side chain, a six-membered metallacycle is formed. The monodentate Cp3N ligand is clearly η1(σ)-coordinated, which is shown by the relatively short Zn1−C15 distance of 2.063(3) Å

Scheme 1. Amino-Functionalized Zincocene 2 and the Formation of [ZnRCp2N]a and of the Dinuclear Complex [Zn(μ-Cl)Cp2N]2

a

R = Et, Cp*; Cp2N = C5Me4(CH2)2NMe2.

which was formed by treatment of KCp2N with ZnCl2 in a 2:1 ratio.8 In addition, the monocyclopentadienyl and mixed-ring compounds [ZnEtCp2N] and [ZnCp*Cp2N] were obtained by reacting 2 with ZnEt2 and [ZnCp*2], respectively (Scheme 1). The beneficial effect of the intramolecular donor group was shown by the isolation of the dinuclear complex [Zn(μCl)Cp2N]2, whereas [Zn(μ-Cl)Cp*(THF)]2 (THF = tetrahydrofuran), which lacks a chelating Cp ligand, was not stable in vacuo due to loss of coordinated solvent molecules. Variation of the length of the side chain that bears the donating group is an important option for altering the coordination properties of donor-functionalized cyclopentadienyl ligands. This leads to changes in the bite angle of the chelating ligand, which is associated with differences in the ring strain of the metallacycle that is formed upon coordination. Thus, we aimed at the synthesis of zinc complexes with the amino-functionalized cyclopentadienyl ligand 552


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Scheme 2. Formation of the Amino-Functionalized Zincocene 3 and of the Heteroleptic Complexes 4 and 5 and the Proposed Structure for 6a

a

Cp2N = C5Me4(CH2)2NMe2, Cp3N = C5Me4(CH2)3NMe2.

characterized by NMR spectroscopic methods (Figures S3 and S4) as well as by single-crystal X-ray analysis. The solidstate structure, which exhibits an intramolecular coordination of the zinc atom by the amino group (Figure 2), is very similar to the structure of [ZnEtCp2N] and is therefore not discussed in detail. Likewise, the reaction of [ZnCp*2] with a mixture of KCp3N and ZnCl2 led to the formation of a colorless solid. The product was shown by single-crystal X-ray diffraction analysis to be the mixed-ring compound [ZnCp*Cp3N] (5; Scheme 2, Figure 3). Because of the poor quality of the X-ray diffraction data of 5 (see the Supporting Information), no bond parameters are given. However, 1H and 13C NMR spectroscopic data revealed that a solution of the crystalline product 5 in [D8]toluene contained not only complex 5. The NMR spectra recorded at 298 K show the signals of 5 together with the resonances of the homoleptic compounds [ZnCp3N2] (3) and [ZnCp*2] (Figures S5−S7). Integration of the 1H NMR signals gave a ratio of 5 to 3 to [ZnCp*2] of approximately 1:0.4:0.4. The three complexes appear to be in equilibrium, and the ratio can slightly be changed to ca. 1:0.5:0.5 by increasing the temperature to 333 K. Notably, the heteroleptic complexes 4, [ZnEtCp2N], and

and a Zn1−C15−Cg2 angle of 109.17(18)°. In contrast to the situation in the solid state, the 1H and 13C NMR spectra of 3 in C6D6 at room temperature indicate both Cp3N ligands to be equivalent on the NMR time scale (Figures S1 and S2). We assume a fast exchange between the chelating and the monodentate form of the ligands, which is associated with the reversible formation and cleavage of both Zn−N bonds. 1H NMR spectra (300 MHz) were measured in [D8]toluene in the temperature range from 298 to 203 K. At 203 K the signals showed a significant broadening, but no decoalescence was observed. The identity of 3 was also verified by mass spectrometric (LIFDI MS = liquid injection field desorption ionization mass spectrometry) measurements. With regard to the reactivity of [ZnCp2N2] (2) toward ZnEt2 and [ZnCp*2], which affords the monocyclopentadienyl and mixed-ring compounds [ZnEtCp2N] and [ZnCp*Cp2N], we also studied the reaction of 3 with ZnEt2 and [ZnCp*2]. As complex 3 could not be isolated free of contaminations by HCp3N, it was prepared in situ from KCp3N and ZnCl2 in the presence of ZnEt2 or [ZnCp*2]. After treatment of ZnEt2 with KCp3N and ZnCl2, [ZnEtCp3N] (4) was obtained as a colorless solid in analytical purity (Scheme 2). Complex 4 was 553


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Figure 1. Molecular structure of 3. Ellipsoids are drawn at the 50% probability level, and the hydrogen atoms are omitted for clarity. Selected distances [Å] and angles [deg]: Zn1−N1 = 2.185(2), Zn1− C1 = 2.148(3), Zn1−C2 = 2.348(3), Zn1−C15 = 2.063(3); N1− Zn1−C1 = 99.58(11), N1−Zn1−C15 = 116.27(10), C1−Zn1−C15 = 143.82(12), Zn1−C1−C6 = 116.9(2), Zn1−C15−C20 = 110.2(2).

Figure 3. Molecular structure of 5. Ellipsoids are drawn at the 50% probability level, and the hydrogen atoms are omitted for clarity.

ratio led to the selective formation of the heteroleptic complex [ZnCp3NCp2N] (6). A suggested structure of 6 is depicted in Scheme 2. Compound 6 was obtained as a yellowish oil, which solidified upon storage for several days at −30 °C.11 In the 1H and 13C NMR spectra of 6 in C6D6, a set of resonances for each a Cp2N and a Cp3N ligand was observed (Figures S8−S10). Notably, the chemical shifts of these signals were significantly different from the chemical shifts that were recorded for the resonances of the homoleptic complexes 2 and 3. The difference in the 1H NMR chemical shifts for the protons of the methyl groups bound at the nitrogen atoms in 6 (δ 2.13 and 1.48 ppm in C6D6) may indicate the existence of a coordinated and a noncoordinated amino group. The 1H NMR signals of 6 are rather broad, which suggests the occurrence of dynamic processes that might involve a reversible coordination of the nitrogen atoms. The identity of 6 was further verified by a LIFDI MS spectrum, which showed the molecular ion peak for 6, whereas no signals that could be assigned to 2 or 3 were observed. Obviously, the formation of the heteroleptic complex 6 is thermodynamically favored over the corresponding homoleptic compounds 2 and 3. Unfortunately, despite repeated attempts, no single crystals of 6 suitable for an Xray analysis could be obtained. Thus, it was not possible to establish if in the solid state one of the amino-functionalized Cp ligands is coordinated in a monodentate mode (like in the case of 3) or if both ligands are bound as chelating ligands (like in the case of 2). However, complex 6 tends to exhibit an oily consistency and is highly soluble in hexane, as was found for 3. Therefore, we propose that complex 6 attains an “open” structure in the solid state in which one of the ligands is coordinated in a monodentate mode. This would lead to a reduction of steric strain. From the results described so far, it is evident that the Cp3N ligand, despite being sterically more demanding than the Cp2N ligand, occupies sterically only a little more than one-half of the coordination sphere at the zinc center. Therefore, the potentially tridentate ligand precursor C5Me4(CH2)2NMe(CH2)2NMe2 (Cptmeda) was tested for its ability to coordinate at zinc. Starting from KCptmeda and ZnCl2 in tetrahydrofuran,

Figure 2. Molecular structure of 4. Ellipsoids are drawn at the 50% probability level, and the hydrogen atoms are omitted for clarity. Selected distances [Å] and angles [deg]: Zn1−N1 = 2.192(4), Zn1− C1 = 2.124(5), Zn1−C2 = 2.326(5), Zn1−C5 = 2.433(5), Zn1−C15 = 1.979(4); N1−Zn1−C1 = 98.22(17), N1−Zn1−C15 = 111.49(17), C1−Zn1−C15 = 149.8(2), Zn1−C1−C6 = 118.1(4).

[ZnCp*Cp2N] were formed quantitatively from the corresponding homoleptic precursor compounds 2 or 3 and ZnEt2 or [ZnCp*2] according to the NMR spectra.8 DFT calculations (B3LYP/cc-pVTZ (C, H, N); cc-pVTZ-PP (Zn)) 14−16 corroborate these results; the calculated Gibbs free reaction energies (ΔGR) for the formation of the heteroleptic complexes starting from the homoleptic compounds amount to ΔGR = −34, −65, and −32 kJ/mol for 4, [ZnEtCp 2N ], and [ZnCp*Cp2N], respectively. In contrast, for 5 the reaction is predicted to be slightly endergonic with ΔGR = +13 kJ/mol. It seems to be reasonable that steric factors contribute to these differences in ΔGR to a major extent. In addition to the mixed-ring compound 5, a complex bearing the two different amino-functionalized cyclopentadienyl ligands Cp2N and Cp3N was prepared. Interestingly, the reaction of the compounds [ZnCp2N2] (2), KCp3N, and ZnCl2 in a 1:2:1 554


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The 1H and 13C NMR spectra of 7 are remarkably different from the spectra of the other amino-functionalized cyclopentadienyl zinc compounds described so far (Figures S11 and S12). The latter complexes display resonances for Cp ligands with Cs symmetry or that appear to have Cs symmetry because of the fluxionality, which reduces the number of signals observed in the NMR spectra. In compound 7, however, each of the seven methyl groups gives rise to an individual resonance in the 1H and 13C NMR spectra. This observation suggests that there is no dynamic process on the NMR time scale that renders both sides of the Cptmeda ligand to be equal. Such a process, which could be equivalent to an equilibrium between two enantiomeric forms of the molecule, would presumably require substantial structural changes, including the reversible cleavage of Zn−N bonds, and is thus expected to possess a high activation energy. To the best of our knowledge, there have been no investigations on zinc complexes with cyclopentadienyl ligands that bear donor functions other than amino groups in the side chain. Since in biological systems zinc atoms are, in addition to nitrogen and oxygen donors, preferentially coordinated by sulfur-containing ligands (e.g., the amino acid cysteine)17 and as sulfur donors appear to be good ligands for zinc centers, the thio-functionalized ligand precursor C5Me4(CH2)2SMe (Cp2S) was used for reactivity studies. The reaction of two equivalents of the sodium cyclopentadienide NaCp2S with ZnCl2 in tetrahydrofuran gives [ZnCp2S2] (8; Scheme 4), which exhibits

the zinc monocyclopentadienyl chlorido complex [ZnClCptmeda] (7) was obtained as an off-white solid (Scheme 3). Scheme 3. Formation of [ZnClCptmeda] (7)a

a

Cptmeda = C5Me4(CH2)2NMe(CH2)2NMe2.

X-ray diffraction proved the molecular structure of 7 to be mononuclear with the zinc atom being coordinated by a tridentate Cptmeda ligand and a chlorido ligand in a distortedtetrahedral geometry (Figure 4). Comparison of 7 with [Zn(μ-

Scheme 4. Formation of Thio-Functionalized Complex [ZnCp2S2] (8)a

Figure 4. Molecular structure of 7. Ellipsoids are drawn at the 50% probability level, and the hydrogen atoms are omitted for clarity. Selected distances [Å] and angles [deg]: Zn1−Cl1 = 2.2543(7), Zn1− N1 = 2.144(2), Zn1−N2 = 2.0947(19), Zn1−C8 = 2.080(2); N1− Zn1−N2 = 87.21(9), N1−Zn1−C8 = 107.48(10), N2−Zn1−C8 = 129.51(10), Cl1−Zn1−N1 = 107.04(6), Cl1−Zn1−N2 = 105.48(7), Cl1−Zn1−C8 = 114.90(8), Zn1−C8−C15 = 104.03(17).

a

Cp2S = C5Me4(CH2)2SMe.

two thio-functionalized Cp ligands. Complex 8 is obtained as a colorless solid, which is highly reactive toward air and moisture. Regarding its physical and chemical properties, compound 8 is comparable to the amino-functionalized zincocene 2. The solid-state structure of 8, which was established by single-crystal X-ray diffraction, resembles the structure of 2 (Figure 5). There are two crystallographically independent entities in the asymmetric unit, for which the zinc atoms are located on special positions. However, as they are very similar, only one molecule will be discussed. The zinc center is coordinated by both Cp2S ligands in a chelating mode, with the Cp2S ligands being related by symmetry due to a C2 axis running through the zinc atom. The sulfur atoms are coordinated to the zinc center exhibiting a Zn1−S1 distance of 2.4892(16) Å, and the Cp rings are η1-coordinated (Zn1−C4 = 2.110(6) Å) via a carbon atom in a neighboring position to the carbon atom bearing the side chain. Similar to 2, complex 8 is a chiral molecule. Both η1-coordinated carbon atoms (C4) represent stereocenters, in addition to the sulfur atoms, which are stereogenic due to coordination to the zinc center.

Cl)Cp2N]2, which is dinuclear in the solid state,8 shows that the occupation of a third coordination site by the Cptmeda ligand enables the isolation of a mononuclear instead of a dinuclear compound. The cyclopentadienyl ring in 7 shows an η1coordination via a ring carbon atom that is in a neighboring position to the carbon atom carrying the side chain, exhibiting a Zn1−C8 distance of 2.080(2) Å. Both amino functions are coordinated to the zinc center, featuring zinc nitrogen distances of 2.144(2) Å (Zn1−N1) for the inner amino group and 2.0947(19) Å (Zn1−N2) for the terminal amino group. This results in the formation of a six-membered and a fivemembered metallacycle. The molecule is chiral since the zinc atom, the zinc-coordinated carbon atom (C8), and the inner nitrogen atom (N1) represent stereocenters. The crystal from which the structure was determined by X-ray analysis contained only one enantiomer, which exhibits an R configuration at all three stereocenters. 555


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situ from [H(OEt2)2]+[BArF4]− and pyridine, in the presence of an excess of pyridine in 1,2-difluorobenzene to give the cationic compounds [ZnCp2N(py)2]+[BArF4]− (9; py = pyridine), [ZnCp3N(py)2]+[BArF4]− (10), and [ZnCp2S(py)2]+[BArF4]− (11), respectively (Scheme 5). The synthetic procedure includes washing the crude reaction product with hexane in order to remove neutral byproducts and impurities, e.g., the cyclopentadiene, residual zinc precursor complex, and the excess of pyridine, which affords the ionic compounds 9−11 as yellow powders in high yields. Whereas complexes 9 and 11 could be obtained in analytical purity, as shown by correct microanalyses, compound 10 still contained residual HCp3N (ca. 30 mol %) as an impurity according to the NMR spectra. Complexes 9−11 were characterized by 1H and 13C NMR spectroscopy as well as by LIFDI mass spectrometry. In the 1H and 13C NMR spectra of 9−11 (see Figures S15−S19 for the NMR spectra of 9 and Figures S20−S23 for the NMR spectra of 10), a set of resonances is observed for the donorfunctionalized cyclopentadienyl ligand, the pyridine ligands, and the [BArF4]− anion. Integration of the 1H NMR signals of the pyridine ligands, which are significantly broadened compared to the resonances of free pyridine, indicates the coordination of two equivalents of pyridine. A downfield shift of the 1H NMR signals is observed for the CH2N/CH2S (Δδ = 0.58−0.80 ppm) and NMe2/SMe (Δδ = 0.03−0.46 ppm) groups in 9−11 compared to the corresponding resonances of the neutral complexes 2, 3, and 8, which may be ascribed to the cationic nature of compounds 9−11. The LIFDI mass spectra of 9 and 11 display a peak for the cations as well as a fragment ion peak resulting from the elimination of one molecule of pyridine from the cations, whereas for 10 only the fragment ion [ZnCp3N(py)]+ was observed. Interestingly, an alternative route for the synthesis of 10 is represented by the reaction of 3 with one equivalent of [ZnCp*(py)n]+[BArF4]− (n = 1−3) in 1,2difluorobenzene. The latter complex is formed in situ by reacting the triple-decker complex 1 with pyridine.3 Presumably, [ZnCp*(py)n]+[BArF4]− abstracts a Cp3N ligand from 3, and the resulting cationic species is coordinated by pyridine under formation of 10. Except for 10 and [ZnCp*(py)n]+[BArF4]−, however, no other zinc-containing species could be identified in the reaction mixture. After workup the product still contained HCp3N (ca. 20−30 mol %) as well as minor amounts of residual [ZnCp*(py)n]+[BArF4]−. Single crystals of the cationic compounds 9−11 suitable for an X-ray analysis proved difficult to obtain, as the products tended to be polycrystalline. Despite repeated attempts, only crystals of low quality could be grown. Nonetheless, the X-ray crystal structure of 10 could be obtained. As the data are of poor quality (see the

Figure 5. Molecular structure of 8. Only one of two crystallographically independent molecules is shown. The Cp2S ligands are symmetry-related due to a C2 axis. In the crystal structure a racemic mixture is present. The enantiomer that is depicted here exhibits R configurations of the four stereocenters (both S1 and C4 atoms) and therefore an R configuration of the chiral axis. Ellipsoids are drawn at the 50% probability level, and the hydrogen atoms are omitted for clarity. Selected distances [Å] and angles [deg]: Zn1−S1 = 2.4892(16), Zn1−C4 = 2.110(6); S1−Zn1−S1′ = 86.54(8), S1− Zn1−C4 = 104.87(17), S1−Zn1−C4′ = 110.29(16), C4−Zn1−C4′ = 131.0(3), Zn1−S1−C1 = 99.0(2), Zn1−S1−C12 = 110.2(2), C1− S1−C12 = 101.8(3), Zn1−C4−C11 = 105.5(4).

Moreover, because of its spiro structure, 8 exhibits axial chirality. Complex 8 crystallizes as a racemic crystal, whereas in solution there are dynamic processes that presumably involve an interchange of the enantiomers. This process, which is fast on the NMR time scale at room temperature, leads to resonances in the 1H and 13C NMR spectra corresponding to Cp2S ligands that appear to have Cs symmetry (Figures S13 and S14). In addition to coordinating the zinc atom in neutral compounds, intramolecular donor groups attached to the Cp ring should also be able to stabilize the highly electrophilic zinc atom in cationic complexes. The neutral precursor complexes [ZnCp2N2] (2), [ZnCp3N2] (3), and [ZnCp2S2] (8) were protonated with [C5H5NH]+[BArF4]−, which was prepared in

Scheme 5. Formation of the Donor-Functionalized Cationic Complexes 9−11a

a

Solvent: 1,2-difluorobenzene. 556


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washing the solid product with hexane. Na[C5Me4(CH2)2SMe] (NaCp2S) was prepared according to a literature procedure.19 [ZnCp*2], [Zn2Cp*3]+[BArF4]− (1), [ZnCp2N2] (2), and [H(OEt2)2]+[B{3,5-(CF3)2C6H3}4]− were synthesized according to the literature.20,3,8,21 The NMR spectra were recorded at a Bruker Avance 400, at a Bruker Avance III 500, or at a Bruker Avance III 300 spectrometer. NMR chemical shifts are reported in ppm. 1H NMR chemical shifts were referenced to the residual proton signals of the deuterated solvents (C6D5H, δ 7.15 ppm; CDHCl2, δ 5.32 ppm). 13 C{1H} NMR shifts were referenced to the 13C NMR signal of the solvent (C6D6, δ 128.06 ppm; CD2Cl2, δ 53.84 ppm). 1H,13C HMQC and HMBC NMR spectra were used to confirm assignments. The 1H and 13C{1H} NMR spectra of the compounds 3−10 are depicted in the Supporting Information; the spectra of 11 are comparable to those of 9 and 10. Liquid injection field desorption ionization mass spectra were measured at a Micromass Q-TOF 2 mass spectrometer equipped with a LIFDI 700 (Linden CMS) ion source. Microanalyses were measured at a HEKAtech Euro EA 3000 elemental analyzer. Synthesis of [Zn{C5Me4(CH2)3NMe2}2] (3). A solution of ZnCl2 (278 mg, 2.0 mmol) in THF (15 mL) was added to a suspension of K[C5Me4(CH2)3NMe2] (1.00 g, 4.1 mmol) in THF (15 mL) at room temperature. The resulting turbid mixture was stirred for 45 min. After removing the volatiles in vacuo, the residue was extracted with hexane (30 mL). The solvent was evaporated from the extract under vacuum to yield 713 mg of a pale yellow oil, which contained [Zn{C5Me4(CH2)3NMe2}2] (3) and variable amounts (10−30 mol %) of HC5Me4(CH2)3NMe2. Upon storage for several days at −30 °C, the product partially solidified, resulting in a waxy yellowish to colorless solid. 1H NMR (400.1 MHz, C6D6, 298 K): δ 2.29 (m, 4H, CH2), 2.13 (m, 4H, NCH2), 2.02 (s, 12H, CH3), 1.98 (s, 12H, NMe2), 1.98 (s, 12H, CH3), 1.36 (m, 4H, CH2). 13C{1H} NMR (100.6 MHz, C6D6, 298 K): δ 118.1 (Cring), 117.0 (Cring), 104.1 (Cring), 61.3 (NCH2), 45.8 (NMe2), 29.2 (CH2), 25.8 (CH2), 12.0 (CH3), 11.8 (CH3). LIFDI TOF MS (toluene): m/z 476 [Zn{C5Me4(CH2)3NMe2}2]•+, 270 [Zn{C5Me4(CH2)3NMe2}]+. Synthesis of [ZnEt{C5Me4(CH2)3NMe2}] (4). K[C5Me4(CH2)3NMe2] (308 mg, 1.25 mmol) was suspended in THF (10 mL). ZnEt2 (0.94 mL of a 1 M solution in hexane, 0.94 mmol) was added to this suspension at room temperature, followed by the addition of a solution of ZnCl2 (86 mg, 0.63 mmol) in THF (10 mL). The resulting turbid mixture was stirred for 45 min. After removing the volatiles in vacuo, the residue was extracted with hexane (30 mL). The solvent was evaporated from the extract under vacuum to give a colorless solid. Yield: 296 mg, 78%. Minor amounts of impurities can be removed by washing with very small amounts of cold (−90 °C) hexane. Single crystals that were suitable for X-ray analysis were obtained upon slow evaporation of a solution of 4 in hexane at −30 °C. 1H NMR (400.1 MHz, C6D6, 298 K): δ 2.41 (t, 3J(H,H) = 6.4 Hz, 2H, CH2), 2.23 (s, 6H, CH3), 2.18 (s, 6H, CH3), 2.05 (m, 2H, NCH2), 1.66 (s, 6H, NMe2), 1.41 (m, 2H, CH2), 1.28 (t, 3J(H,H) = 8.1 Hz, 3H, CH2CH3), 0.09 (q, 3J(H,H) = 8.1 Hz, 2H, CH2CH3). 13 C{1H} NMR (100.6 MHz, C6D6, 298 K): δ 117.3 (Cring), 107.2 (Cring), 104.4 (Cring), 62.0 (NCH2), 45.5 (NMe2), 28.3 (CH2), 25.9 (CH2), 13.9 (CH2CH3), 11.6 (CH3), 11.3 (CH3), −6.4 (CH2CH3). LIFDI TOF MS (toluene): m/z 270 [Zn{C5Me4(CH2)3NMe2}]+. Anal. Calcd for C16H29NZn: C, 63.88; H, 9.72; N, 4.66. Found: C, 63.46; H, 9.71; N, 4.66. Synthesis of [ZnCp*{C5Me4(CH2)3NMe2}] (5). A solution of ZnCl2 (278 mg, 2.0 mmol) in THF (15 mL) was added to a suspension of K[C5Me4(CH2)3NMe2] (1.00 g, 4.1 mmol) and [ZnCp*2] (684 mg, 2.0 mmol) in THF (15 mL) at room temperature. The resulting turbid mixture was stirred for 1 h. After removing the volatiles in vacuo, the residue was extracted with hexane (50 mL). The solvent was evaporated from the extract under vacuum to give an oily residue, which was washed with hexane (10 mL + 2 × 3 mL) at −90 °C. Upon drying in vacuo, 537 mg of a colorless solid was obtained. As indicated by NMR spectroscopy at room temperature, a solution of the product in C6D6 or [D8]toluene consists of a mixture of [ZnCp*{C5Me4(CH2)3NMe2}] (5), [Zn{C5Me4(CH2)3NMe2}2] (3), and [ZnCp*2] in a ratio of ca. 1:0.4:0.4. Crystals of 5 for X-ray analysis

Supporting Information), the bond parameters will not be discussed further. However, the results are useful to verify the identity of 10. The solid-state structure of the cation in 10 is depicted in Figure S24. The asymmetric unit comprises also the [BArF4]− anion as well as two cocrystallized molecules of 1,2difluorobenzene. The zinc atom is located in a distortedtetrahedral coordination environment. The Cp3N ligand is coordinated in a chelating mode with both the amino group and one of the ring carbon atoms bound to the zinc atom, which results in a seven-membered metallacycle. The other coordination sites are occupied by two pyridine ligands. Because the complexes 9 and 11 are very similar to 10, it is reasonable to assume an intramolecular coordination by the donor groups in 9 and 11 as well.

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CONCLUSION We have reported on zinc cyclopentadienyl complexes with amino and thio functions linked to the Cp ring via a side chain. Both nitrogen and sulfur donor atoms exhibit intramolecular coordination to the zinc center. When comparing the smaller Cp2N (Cp2N = C5Me4(CH2)2NMe2) with the larger Cp3N (Cp3N = C5Me4(CH2)3NMe2) ligand, the steric demand of the ligand increases, which has the effect that in [ZnCp3N2] (3) only one of the Cp3N ligands is coordinated to the zinc atom via the amino group. This illustrates the ability of aminofunctionalized Cp ligands to bind to the zinc atom in a hemilabile coordinating mode. The tendency of organozinc compounds toward ligand scrambling reactions was exploited in the synthesis of the complexes [ZnRCp3N] (R = Et (4), Cp* (5), Cp2N (6)). Minor alterations in the structure of the ligands may have drastic effects on the equilibrium between the homoand heteroleptic complexes, as it can be seen in the case of 5. We also described the synthesis of a zinc cyclopentadienyl complex featuring two amino functions in the same side chain [ZnClCptmeda] (7; Cptmeda = C5Me4(CH2)2NMe(CH2)2NMe2). Note that the incorporation of the second amino group allows for the isolation of a mononuclear complex, whereas [Zn(μCl)Cp2N]2 is dinuclear in the solid state. In addition, the thiofunctionalized complex [ZnCp2S2] (8; Cp2S = C5Me4(CH2)2SMe) was shown to exhibit a close resemblance to the amino-functionalized complex 2. Starting from 2, 3, and 8, the cationic complexes [ZnCpD(py)2]+[BArF4]− (CpD = Cp2N (9), Cp3N (10), Cp2S (11); py = pyridine; BArF4 = B{3,5(CF3)2C6H3}4) were obtained. In these cations the highly electrophilic metal center is stabilized by intramolecular coordination of the donor group. Although the presence of additional pyridine ligands is essential for the stability of complexes 9−11, it is conceivable that the stabilization of cationic zinc cyclopentadienyl complexes may also be achieved exclusively by neutral donor groups linked to the Cp ligand.

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EXPERIMENTAL SECTION

General Methods. All experiments were performed with a Schlenk line under an atmosphere of argon or in an argon-filled glovebox with oxygen levels below 10 ppm. All solvents were purified and dried by conventional methods and distilled under an atmosphere of argon. Anhydrous ZnCl2 was purchased from Acros Organics and stored in an argon-filled glovebox. ZnEt2 was obtained from Sigma-Aldrich as a 1 M solution in hexane. The synthesis of HC5Me4(CH2)3NMe2 (HCp3N) and HC5Me4(CH2)2NMe(CH2)2NMe2 (HCptmeda), which was done in analogy to a method in the literature,18 is described in the Supporting Information. KCp3N and KCptmeda were obtained by heating an equimolar mixture of KH and HCp3N or HCptmeda in THF at 70 °C for 30 min, followed by evaporation of the volatiles and 557


Organometallics

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(SCH2), 23.7 (CH2), 14.5 (SMe), 12.5 (CH3), 12.2 (CH3). LIFDI TOF MS (toluene): m/z 454 [Zn{C5Me4(CH2)2SMe}2]•+, 259 [Zn{C5Me4(CH2)2SMe}]+. Anal. Calcd for C24H38S2Zn: C, 63.20; H, 8.40; S, 14.06. Found: C, 63.00; H, 8.41; S, 13.73. Synthesis of [Zn{C 5 Me 4 (CH 2 ) 2 NMe 2 }(C 5 H 5 N) 2 ] + [B{3,5(CF3)2C6H3}4]− (9). Pyridine (179 μL, 2.22 mmol) was added to a freshly prepared solution of [H(OEt2)2]+[B{3,5-(CF3)2C6H3}4]− (750 mg, 0.74 mmol) in 1,2-difluorobenzene (4 mL) at room temperature. The resulting solution was added dropwise to a stirred solution of [Zn{C5Me4(CH2)2NMe2}2] (2; 400 mg, 0.89 mmol) in 1,2difluorobenzene (2 mL). The bright yellow solution was stirred for 15 min at room temperature. Then the volatiles were removed in vacuo, and the resulting oily residue was washed with hexane (10 mL + 3 × 5 mL). After drying under vacuum, 9 was obtained as a yellow powder. Yield: 850 mg, 90%. 1H NMR (500.1 MHz, 1,2-F2C6H4, C6D6 capillary, 298 K): δ 8.09 (s, br, 8H, o-H, BArF4−), 7.96 (m, br, 4H, ArH, pyridine), 7.73 (m, br, 2H, ArH, pyridine), 7.45 (s, br, 4H, p-H, BArF4−), 7.34 (m, br, 4H, ArH, pyridine), 2.65 (apparent s, 4H, NCH2, CH2), 2.24 (s, 6H, NMe2), 1.78 (s, 6H, CH3), 1.48 (s, 6H, CH3). 13 C{1H} NMR (125.8 MHz, 1,2-F2C6H4, C6D6 capillary, 298 K): δ 162.5 (q, 1J(C,11B) = 50 Hz, i-C, BArF4−), 148.3 (s, CAr, pyridine), 141.2 (s, CAr, pyridine), 135.1 (s, o-C, BArF4−), 129.7 (q, br, 2J(C,F) = 32 Hz, m-C, BArF4−), 127.5 (s, Cring), 126.1 (s, CAr, pyridine), 124.9 (q, 1J(C,F) = 272 Hz, CF3, BArF4−), 121.0 (s, Cring), 117.6 (m, p-C, BArF4−), 99.2 (s, Cring), 64.4 (s, NCH2), 46.1 (s, NMe2), 22.5 (s, CH2), 11.8 (s, CH3), 10.3 (s, CH3). LIFDI TOF MS (1,2-F2C6H4): m/z 414 [Zn{C 5 Me 4 (CH 2 ) 2 NMe 2 }(C 5 H 5 N) 2 ] + , 335 [Zn{C5Me4(CH2)2NMe2}(C5H5N)]+. Anal. Calcd for C55H44BF24N3Zn: C, 51.64; H, 3.47; N, 3.29. Found: C, 51.75; H, 3.41; N, 3.00. Synthesis of [Zn{C5Me4(CH2)3NMe2}(C5H5N)2]+[B{3,5-(CF3)2C6H3}4]− (10). Method A. The synthesis is similar to that of 9. Starting from [Zn{C5Me4(CH2)3NMe2}2] (3; 250 mg; containing residual HC5Me4(CH2)3NMe2), [H(OEt2)2]+[B{3,5-(CF3)2C6H3}4]− (265 mg, 0.26 mmol), and pyridine (84 μL, 1.04 mmol), compound 10 was obtained as a yellow powder (261 mg) which still contained residual HC5Me4(CH2)3NMe2 (ca. 30 mol %). Method B. Pyridine (307 μL, 3.80 mmol) was added to a solution of [Zn2Cp*3]+[B{3,5-(CF3)2C6H3}4]− (1; 1.33 g, 0.95 mmol) in 1,2difluorobenzene (5 mL) at room temperature. The resulting solution was added dropwise to a stirred solution of [Zn{C 5 Me 4 (CH 2 ) 3 NMe 2 } 2 ] (3; 910 mg; containing residual HC5Me4(CH2)3NMe2) in 1,2-difluorobenzene (5 mL). The bright yellow solution was stirred for 20 min at room temperature. Then the volatiles were removed in vacuo, and the resulting oily residue was washed with hexane (20 mL + 3 × 10 mL). After drying in vacuo, 10 was obtained as a yellow powder (1.14 g), which contained residual HC5Me4(CH2)3NMe2 (ca. 20−30 mol %) and in some cases also minor amounts of [ZnCp*(C5H5N)n]+[B{3,5-(CF3)2C6H3}4]− (n = 1−3). Crystals for X-ray analysis were obtained by slow evaporation of a solution of 10 in 1,2-difluorobenzene at −30 °C. 1H NMR (400.1 MHz, 1,2-F2C6H4, C6D6 capillary, 298 K): δ 8.07 (s, br, 8H, o-H, BArF4−), 8.05 (m, br, 4H, ArH, pyridine), 7.70 (m, br, 2H, ArH, pyridine), 7.44 (s, br, 4H, p-H, BArF4−), 7.31 (m, br, 4H, ArH, pyridine), 2.71 (m, 2H, NCH2), 2.48 (t, 3J(H,H) = 6.2 Hz, 2H, CH2), 2.01 (s, 6H, NMe2), 1.92 (m, 2H, CH2), 1.66 (s, 6H, CH3), 1.46 (s, 6H, CH3). 13C{1H} NMR (100.6 MHz, 1,2-F2C6H4, C6D6 capillary, 298 K): δ 162.5 (q, 1J(C,11B) = 50 Hz, i-C, BArF4−), 148.5 (s, CAr, pyridine), 141.6 (s, CAr, pyridine), 135.1 (s, o-C, BArF4−), 129.7 (q, br, 2 J(C,F) = 32 Hz, m-C, BArF4−), 127.1 (s, Cring), 126.5 (s, CAr, pyridine), 124.9 (q, 1J(C,F) = 272 Hz, CF3, BArF4−), 117.6 (m, p-C, BArF4−), 102.5 (s, Cring), 64.9 (s, NCH2), 47.4 (s, NMe2), 26.8 (s, CH2), 26.1 (s, CH2), 12.2 (s, CH3), 10.4 (s, CH3). The third signal of the ring carbon atoms (δ ca. 124 ppm) is obscured by solvent resonances. LIFDI TOF MS (1,2-F 2 C 6 H 4 ): m/z 349 [Zn{C5Me4(CH2)3NMe2}(C5H5N)]+. Synthesis of [Zn{C 5 Me 4 (CH 2 ) 2 SMe}(C 5 H 5 N) 2 ] + [B{3,5(CF3)2C6H3}4]− (11). The synthesis is similar to that of 9. Compound 11 was obtained as a yellow powder. Yield (starting from 250 mg (0.55 mmol) of [Zn{C5Me4(CH2)2SMe}2] (8), 462 mg (0.46 mmol) of [H(OEt 2)2]+[B{3,5-(CF3)2C6H3}4]−, 111 μL (1.38 mmol) of

were obtained by cooling a solution of the product mixture in hexane to −30 °C. 1H NMR (400.1 MHz, C6D6, 298 K): δ 2.06 (s, CH3, 5), 2.05 (s, CH3, 5), 2.02 (s, CH3, 3), 1.99 (s, CH3, 3), 1.98 (s, CH3, 3), 1.87 (s, Cp*, [ZnCp*2]), 1.87 (s, CH3, 5), 1.85 (s, Cp*, 5). Due to signal overlap, the signals of the methylene protons could not be assigned with certainty. 13C{1H} NMR (100.6 MHz, C6D6, 298 K): δ 118.2 (Cring, 3), 117.6 (Cring, 5), 117.0 (Cring, 3), 114.9 (C5Me5, 5), 114.4 (Cring, 5), 111.7 (C5Me5, [ZnCp*2]), 104.0 (Cring, 3), 102.2 (Cring, 5), 62.0 (NCH2, 5), 61.3 (NCH2, 3), 45.8 (NMe2, 5), 45.8 (NMe2, 3), 29.5 (CH2, 5), 29.1 (CH2, 3), 25.8 (CH2, 3), 25.6 (CH2, 5), 12.1 (C5Me5, 5), 12.0 (CH3, 3), 11.8 (CH3, 3), 11.7 (CH3, 5), 11.5 (CH3, 5), 11.1 (C5Me5, [ZnCp*2]). LIFDI TOF MS (toluene): m/z 476 [Zn{C5Me4(CH2)3NMe2}2]•+, 405 [ZnCp*{C5Me4(CH2)3NMe2}]•+, 270 [Zn{C5Me4(CH2)3NMe2}]+. Synthesis of [Zn{C5Me4(CH2)3NMe2}{C5Me4(CH2)2NMe2}] (6). A solution of ZnCl2 (226 mg, 1.7 mmol) in THF (10 mL) was added to a suspension of K[C5Me4(CH2)3NMe2] (812 mg, 3.3 mmol) and [Zn{C5Me4(CH2)2NMe2}2] (2; 745 mg, 1.7 mmol) in THF (10 mL) at room temperature. The resulting turbid mixture was stirred for 45 min. After removing the volatiles in vacuo, the residue was extracted with hexane (40 mL). The solvent was evaporated from the extract under vacuum to give an oily residue. Upon storage for several days at −30 °C, the product solidified yielding an off-white solid. Yield: 940 mg, 61%. 1H NMR (400.1 MHz, C6D6, 298 K): δ 2.35 (s, 6H, CH3), 2.27 (m, 4H, CH2, NCH2), 2.13 (s, 12H, NMe2, CH3), 2.08 (t, 3 J(H,H) = 6.6 Hz, 2H, CH2), 1.93 (s, 6H, CH3), 1.88 (s, 6H, CH3), 1.84 (t, 3J(H,H) = 6.6 Hz, 2H, NCH2), 1.48 (s, 6H, NMe2), 1.38 (m, 2H, CH2). 13C{1H} NMR (100.6 MHz, C6D6, 298 K): δ 122.3 (Cring), 119.4 (Cring), 118.2 (Cring), 110.9 (Cring), 109.8 (Cring), 89.0 (Cring), 61.0 (NCH2), 60.4 (NCH2), 45.7 (NMe2), 45.4 (NMe2), 28.9 (CH2), 26.0 (CH2), 23.9 (CH2), 12.3 (CH3), 12.2 (2 CH3), 12.1 (CH3). LIFDI TOF MS (toluene): m/z 462 [Zn{C5Me4(CH2)3NMe2}{C5Me4(CH2)2NMe2}]•+, 270 [Zn{C5Me4(CH2)3NMe2}]+. Anal. Calcd for C27H46N2Zn: C, 69.88; H, 9.99; N, 6.04. Found: C, 69.59; H, 10.06; N, 5.92. Synthesis of [ZnCl{C5Me4(CH2)2NMe(CH2)2NMe2}] (7). A solution of ZnCl2 (472 mg, 3.5 mmol) in THF (20 mL) was added to a suspension of K[C5Me4(CH2)2NMe(CH2)2NMe2] (1.20 g, 4.2 mmol) in THF (20 mL) at room temperature. The resulting turbid mixture was stirred for 1 h. After centrifugation the mixture was filtered, and the solvent was removed from the filtrate in vacuo. The residue was washed with hexane (3 × 10 mL) and dried under vacuum to give an off-white solid. Yield: 1.15 g, 95%. Single crystals that were suitable for X-ray analysis were obtained by overlaying a solution of 7 in CH2Cl2 with hexane and subsequent storage of the biphasic mixture at 0 °C. 1H NMR (400.1 MHz, CD2Cl2, 298 K): δ 2.77−2.16 (m, 8H, CH2), 2.62 (s, 3H, NMe), 2.29 (s, 3H, NMe), 2.01 (s, 3H, CH3), 1.93 (s, 3H, CH3), 1.91 (s, 3H, NMe), 1.87 (s, 3H, CH3), 1.66 (s, 3H, CH3). 13C{1H} NMR (100.6 MHz, CD2Cl2, 298 K): δ 129.2 (Cring), 124.7 (Cring), 122.3 (Cring), 117.4 (Cring), 77.9 (Cring), 59.4 (NCH2), 58.1 (NCH2), 55.1 (NCH2), 48.4 (NMe), 48.3 (NMe), 45.5 (NMe), 23.3 (CH2), 13.8 (CH3), 11.7 (CH3), 11.6 (CH3), 11.2 (CH3). LIFDI TOF MS (THF): m/z 349 [ZnCl{C 5 Me 4 (CH 2 ) 2 NMe(CH2)2NMe2H}]+. Anal. Calcd for C16H29ClN2Zn: C, 54.86; H, 8.34; N, 8.00. Found: C, 54.18; H, 8.21; N, 7.78. Synthesis of [Zn{C5Me4(CH2)2SMe}2] (8). Na[C5Me4(CH2)2SMe] (1.00 g, 4.6 mmol) was dissolved in THF (10 mL), and a solution of ZnCl2 (312 mg, 2.3 mmol) in THF (30 mL) was added at room temperature. After stirring for 1 h, the turbid mixture was centrifuged and filtrated. The volatiles were removed from the filtrate under vacuum, and the residue was extracted with hexane (20 mL + 3 × 10 mL). The solvent was removed from the extract under vacuum to give a colorless solid. Yield: 778 mg, 75%. Minor amounts of impurities can be removed by washing with very small amounts of cold (−90 °C) hexane. Single crystals that were suitable for X-ray analysis were obtained upon cooling a solution of 8 in hexane to 0 °C. 1H NMR (400.1 MHz, C6D6, 298 K): δ 2.38 (t, 3J(H,H) = 6.5 Hz, 4H, CH2), 2.14 (s, 12H, CH3), 2.10 (t, 3J(H,H) = 6.5 Hz, 4H, SCH2), 1.89 (s, 12H, CH3), 1.57 (s, 6H, SMe). 13C{1H} NMR (100.6 MHz, C6D6, 298 K): δ 124.1 (Cring), 112.8 (Cring), 107.4 (Cring), 35.6 558


Organometallics

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pyridine): 506 mg, 86%. 1H NMR (400.1 MHz, 1,2-F2C6H4, C6D6 capillary, 298 K): δ 8.09 (s, br, 8H, o-H, BArF4−), 8.00 (m, br, 4H, ArH, pyridine), 7.67 (m, br, 2H, ArH, pyridine), 7.45 (s, br, 4H, p-H, BArF4−), 7.28 (m, br, 4H, ArH, pyridine), 2.80 (t, 3J(H,H) = 5.9 Hz, 2H, SCH2), 2.68 (t, 3J(H,H) = 5.9 Hz, 2H, CH2), 1.89 (s, 3H, SMe), 1.69 (s, 6H, CH3), 1.68 (s, 6H, CH3). 13C{1H} NMR (100.6 MHz, 1,2-F2C6H4, C6D6 capillary, 298 K): δ 162.5 (q, 1J(C,11B) = 50 Hz, i-C, BArF4−), 148.2 (s, CAr, pyridine), 140.6 (s, CAr, pyridine), 135.0 (s, oC, BArF4−), 129.7 (q, br, 2J(C,F) = 32 Hz, m-C, BArF4−), 125.9 (s, CAr, pyridine), 125.2 (s, Cring), 124.8 (q, 1J(C,F) = 272 Hz, CF3, BArF4−), 117.6 (m, p-H, BArF4−), 116.0 (s, Cring), 105.8 (s, Cring), 37.7 (s, SCH2), 22.8 (s, CH2), 13.0 (s, SMe), 11.4 (s, CH3), 10.6 (s, CH3). LIFDI TOF MS (1,2-F2C6H4): m/z 417 [Zn{C5Me4(CH2)2SMe}(C5H5N)2]+, 338 [Zn{C5Me4(CH2)2SMe}(C5H5N)]+. Anal. Calcd for C54H41BF24N2SZn: C, 50.58; H, 3.22; N, 2.18. Found: C, 50.62; H, 3.31; N, 2.04.

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(3) Chilleck, M. A.; Braun, T.; Braun, B. Chem.Eur. J. 2011, 17, 12902−12905. (4) Freitag, K.; Banh, H.; Ganesamoorthy, C.; Gemel, C.; Seidel, R. W.; Fischer, R. A. Dalton Trans. 2013, 42, 10540−10544. (5) Schulz, S.; Schuchmann, D.; Krossing, I.; Himmel, D.; Bläser, D.; Boese, R. Angew. Chem., Int. Ed. 2009, 48, 5748−5751; Angew. Chem. 2009, 121, 5859−5862. (6) See for example: (a) Müller, C.; Vos, D.; Jutzi, P. J. Organomet. Chem. 2000, 600, 127−143. (b) Jutzi, P.; Siemeling, U. J. Organomet. Chem. 1995, 500, 175−185. (c) Jutzi, P.; Redeker, T. Eur. J. Inorg. Chem. 1998, 663−674. (d) Siemeling, U. Chem. Rev. 2000, 100, 1495− 1526. (e) Butenschön, H. Chem. Rev. 2000, 100, 1527−1564. (f) Krut′ko, D. P. Russ. Chem. Bull., Int. Ed. 2009, 58, 1745−1771. (g) Royo, B.; Peris, E. Eur. J. Inorg. Chem. 2012, 1309−1318. (7) Darensbourg, D. J.; Wildeson, J. R.; Yarbrough, J. C. Organometallics 2001, 20, 4413−4417. (8) Chilleck, M. A.; Braun, T.; Herrmann, R.; Braun, B. Organometallics 2013, 32, 1067−1074. (9) (a) Garcés, A.; Sánchez-Barba, L. F.; Alonso-Moreno, C.; Fajardo, M.; Fernández-Baeza, J.; Otero, A.; Lara-Sánchez, A.; López-Solera, I.; Rodríguez, A. M. Inorg. Chem. 2010, 49, 2859−2871. (b) Honrado, M.; Otero, A.; Fernández-Baeza, J.; Sánchez-Barba, L. F.; LaraSánchez, A.; Tejeda, J.; Carrión, M. P.; Martínez-Ferrer, J.; Garcés, A.; Rodríguez, A. M. Organometallics 2013, 32, 3437−3440. (10) Lee, J.-D.; Han, W.-S.; Kim, T.-J.; Kim, S. H.; Kang, S. O. Chem. Commun. 2011, 47, 1018−1020. (11) It has been reported previously that zincocenes bearing sterically demanding cyclopentadienyl ligands may tend to exhibit a wax-like or oily consistency: (a) Burkey, D. J.; Hanusa, T. P. J. Organomet. Chem. 1996, 512, 165−173. (b) Fernández, R.; Grirrane, A.; Resa, I.; Rodríguez, A.; Carmona, E.; Á lvarez, E.; Gutiérrez-Puebla, E.; Monge, Á .; López del Amo, J. M.; Limbach, H.-H.; Lledós, A.; Maseras, F.; del Río, D. Chem.Eur. J. 2009, 15, 924−935. (12) Mebs, S.; Chilleck, M. A.; Grabowsky, S.; Braun, T. Chem.Eur. J. 2012, 18, 11647−11661. (13) A π-bonding interaction between zinc and a cyclopentadienyl ligand mainly involves the empty 4p orbitals of appropriate symmetry, rather than the filled 3d orbitals of zinc. The latter are too low in energy, which precludes a classical π-back-bonding scenario as observed for transition metals with partially filled d orbitals. For a molecular orbital scheme of the ZnCp fragment see: Robles, E. S. J.; Ellis, A. M.; Miller, T. A. J. Phys. Chem. 1992, 96, 3247−3258. Note that π contributions have also been discussed for cadmium cyclopentadienyl complexes: Cummins, C. C.; Schrock, R. R.; Davis, W. M. Organometallics 1991, 10, 3781−3785. (14) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; 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.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision A.02; Gaussian, Inc.: Wallingford, CT, 2009. (15) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785−789. (c) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200−1211. (d) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623−11627. (16) (a) Dunning, T. H., Jr. J. Chem. Phys. 1989, 90, 1007−1023. (b) Peterson, K. A.; Puzzarini, C. Theor. Chem. Acc. 2005, 114, 283−

ASSOCIATED CONTENT

S Supporting Information *

Synthesis of the ligand precursors HCp3N and HCptmeda; details on the DFT calculations including the Cartesian coordinates of the optimized geometries of 2−5, [ZnEtCp2N], [ZnCp*Cp2N], ZnEt2, and [ZnCp*2]; figures of the 1H and 13C{1H} NMR spectra of the compounds 3−10; crystallographic data of the compounds 3, 4, 5, 7, and 8 in tabular form and in CIF format; figure of the solid-state structure of the cation in complex 10; details on the crystal structure determinations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dipl.-Chem. Roy Herrmann for the X-ray crystallographic analysis of complex 5. We are grateful to Dipl.-Chem. Theresia Ahrens and Dipl.-Chem. Sabrina I. Kalläne for the variable-temperature NMR measurements of complexes 3 and 5, respectively.

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REFERENCES

(1) (a) Resa, I.; Carmona, E.; Gutierrez-Puebla, E.; Monge, A. Science 2004, 305, 1136−1138. (b) del Río, D.; Galindo, A.; Resa, I.; Carmona, E. Angew. Chem., Int. Ed. 2005, 44, 1244−1247; Angew. Chem. 2005, 117, 1270−1273. (c) Grirrane, A.; Resa, I.; Rodriguez, A.; Carmona, E.; Alvarez, E.; Gutierrez-Puebla, E.; Monge, A.; Galindo, A.; del Río, D.; Andersen, R. A. J. Am. Chem. Soc. 2007, 129, 693−703. (d) Li, T.; Schulz, S.; Roesky, P. W. Chem. Soc. Rev. 2012, 41, 3759− 3771. (2) For some examples see (only cationic complexes exhibiting a Zn−C bond have been included): (a) Walker, D. A.; Woodman, T. J.; Schormann, M.; Hughes, D. L.; Bochmann, M. Organometallics 2003, 22, 797−803. (b) Sarazin, Y.; Schormann, M.; Bochmann, M. Organometallics 2004, 23, 3296−3302. (c) Hannant, M. D.; Schormann, M.; Bochmann, M. J. Chem. Soc., Dalton Trans. 2002, 4071−4073. (d) Wheaton, C. A.; Ireland, B. J.; Hayes, P. G. Organometallics 2009, 28, 1282−1285. (e) Lichtenberg, C.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed. 2012, 51, 8101−8105; Angew. Chem. 2012, 124, 8225−8229. (f) Pissarek, J.-W.; Schlesiger, D.; Roesky, P. W.; Blechert, S. Adv. Synth. Catal. 2009, 351, 2081−2085. (g) Lühl, A.; Nayek, H. P.; Blechert, S.; Roesky, P. W. Chem. Commun. 2011, 47, 8280−8282. (h) Lühl, A.; Hartenstein, L.; Blechert, S.; Roesky, P. W. Organometallics 2012, 31, 7109−7116. (i) Wehmschulte, R. J; Wojtas, L. Inorg. Chem. 2011, 50, 11300−11302. 559


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Neutral and Cationic Zinc Complexes with N- and S-Donor

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