Zinc Complexes with the N-Donor-Functionalized Cyclopentadienyl

Article pubs.acs.org/Organometallics

Zinc Complexes with the N‑Donor-Functionalized Cyclopentadienyl Ligand C5Me4(CH2)2NMe2 Maren A. Chilleck, Thomas Braun,* Roy Herrmann, and Beatrice Braun Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany S Supporting Information *

ABSTRACT: The amino-functionalized zincocene [ZnCpN2] (4; CpN = C5Me4(CH2)2NMe2), which is the first mononuclear zincocene with two donor-functionalized cyclopentadienyl ligands, was prepared through treatment of KCpN with ZnCl2 in a 2/1 ratio. X-ray crystallography reveals the chelating binding mode of each CpN ligand via an η1-coordinated Cp ring and the amino group. The structural dynamics of 4 in solution were examined by variabletemperature 1H NMR spectroscopy, and decoalescence of the NMR signals was found at low temperatures. Reacting 4 with [ZnR2] (R = Et, Cp*) yields the monocyclopentadienyl and mixed-ring donorfunctionalized complexes [ZnEtCpN] (5) and [ZnCp*CpN] (6). In contrast to the reaction of KCpN with ZnCl2 in a 2/1 ratio, which yields the zincocene 4, the reaction in a 1/1 ratio results exclusively in the formation of the chlorido-bridged dimer [Zn(μCl)CpN]2 (7). This compound is comparable to [Zn(μ-Cl)Cp*(THF)]2 (8), which crystallizes from a THF solution of equimolar amounts of [ZnCp*2] (1) and ZnCl2.

■

INTRODUCTION Cyclopentadienyl (Cp) ligands with donor functions which are linked to the Cp ring by a side chain are promising tools for the intramolecular stabilization of otherwise unstable metal complexes.1 Indeed, the use of such donor-functionalized Cp ligands has enabled the isolation of many complexes in which the metal center may exhibit, for instance, a low coordination number or a cationic charge.2 The donor groups which have been employed so far include NR2, OR, PR2, and SR groups (R = H, alkyl, aryl) as well as N-heterocyclic carbenes and C−C multiple bonds and are linked to the Cp ring through a spacer unit, which is often an alkyl chain.1,3,4 In addition, complexes with donor-functionalized Cp ligands have been reported for a variety of different metal centers: i.e., main-group elements, transition metals, and lanthanides. In many cases the additional donor group coordinates to the metal center in a hemilabile coordination mode, which may prove useful for catalytic purposes.3 In recent years, zinc cyclopentadienyl complexes have attracted much interest, particularly because of their variable and often remarkable bonding motifs. For instance, decamethylzincocene [ZnCp*2] (1; Cp* = C5Me5) exhibits the unusual η5/η1 slipped-sandwich structure.5 The dizincocene [Zn2Cp*2] was the first stable molecular compound with a direct Zn−Zn bond.6 However, reports on zinc cyclopentadienyl complexes with donor-functionalized Cp ligands are rare. Darensbourg reported the dinuclear complexes [Cp′Zn(μ-Cl)]2 (Cp′ = Me2N(CH2)2C5HiPr3 (2)) and [Cp′Zn(μ-OAc)]2 (Cp′ = Me2N(CH2)2C5HiPr3 (3a), cyclo(C4H8N)(CH2)2C5Me4 (3b)) (Scheme 1), which are formed by the reaction of LiCp′ with an equimolar amount of ZnCl2 or Zn(OAc)2.7 In the solid-state structures of 2 and 3b the zinc atoms are bridged by chlorido or acetato ligands and each zinc © 2013 American Chemical Society

Scheme 1. Dinuclear Amino-Functionalized Cyclopentadienyl Zinc Complexes Which Were Reported by Darensbourg7

center is coordinated by a Cp′ ligand via the amino group as well as via one ring carbon atom. However, the corresponding homoleptic zincocenes [ZnCp′2] were not reported. An intramolecular donor−acceptor interaction between a zinc center and an aryl or heteroaryl group, which is attached to the Cp ligand via a spacer unit, was suggested by Bochmann et al. on the basis of NMR spectroscopic observations.8 SánchezBarba and Otero reported zinc complexes of a hybrid scorpionate/cyclopentadienyl ligand which coordinates to the Received: December 6, 2012 Published: February 11, 2013 1067

dx.doi.org/10.1021/om301181e | Organometallics 2013, 32, 1067−1074

Organometallics

Article

Scheme 2. Formation of [ZnCpN2] (4; CpN = C5Me4(CH2)2NMe2), the Monocyclopentadienyl Complex 5, and the Mixed-Ring Complex 6

zinc atom via two nitrogen donor functions and an η1-bound Cp ring.9 The fourth coordination site at the zinc center is occupied by an alkyl, chlorido, amido, or alkoxido ligand. Recently, zinc complexes with aminoalkyl-functionalized dicarbollyl (Dcab2−) ligands, which can be regarded as Cp analogues, were synthesized by Kang and co-workers.10 The pendant amino group does not coordinate to the central zinc atom but to an additional cationic {ZnEt}+ fragment for charge compensation. As donor-functionalized cyclopentadienyl zinc complexes promise to be interesting with regard to both their molecular structures and their chemical reactivities, we investigated this class of compounds in more detail. Herein, we report on the syntheses and structures of zinc complexes bearing the cyclopentadienyl ligand C5Me4(CH2)2NMe2 (=CpN).

■

RESULTS AND DISCUSSION The intramolecularly donor-coordinated zincocene [ZnCpN2] (4; CpN = C5Me4(CH2)2NMe2) was obtained by reacting a mixture of the potassium salt K[C5Me4(CH2)2NMe2] and ZnCl2 in a 2/1 ratio in tetrahydrofuran (Scheme 2). Compound 4 represents the first mononuclear zinc complex which contains two donor-functionalized cyclopentadienyl ligands. The reaction proceeds smoothly at room temperature with formation of insoluble KCl. After workup of the reaction mixture, analytically pure 4 was isolated in 55% yield. Compound 4 exhibits a pronounced sensitivity toward air and moisture and is soluble in common aprotic organic solvents. The molecular structure of 4, which was obtained by single-crystal X-ray analysis, is shown in Figure 1. The asymmetric unit contains four crystallographically independent molecules of 4, which show only small differences in their bond lengths and angles. Therefore, only one molecule is discussed. The central zinc atom is coordinated by two essentially equivalent CpN ligands in a distorted-tetrahedral coordination geometry. Each CpN ligand binds through the amino group as well as one ring carbon atom, which is in a neighboring position (α position) to the carbon atom that bears the amino-functionalized side chain. Thus, a sixmembered metallacycle is formed. Both the Zn−N distances (Zn1−N1 = 2.2373(16) Å, Zn1−N2 = 2.2182(16) Å) and the Zn−C(η1) distances (Zn1−C1 = 2.1491(19) Å, Zn1−C14 = 2.1386(19) Å) are longer than the corresponding values in the aforementioned dinuclear complexes [{Me2N(CH2)2C5HiPr3}Zn(μ-Cl)]2 (2; Zn−N = 2.124(3) Å, Zn−C(η1) = 2.063(3) Å)

Figure 1. Molecular structure of 4. Only one of the four crystallographically independent molecules is shown. The crystal structure shows the presence of a racemic mixture of an enantiomeric pair. The enantiomer which is depicted here exhibits S configurations of both stereocenters and therefore an S configuration of the chiral axis. Ellipsoids are drawn at the 50% probability level, and the hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Zn1−N1 = 2.2373(16), Zn1−N2 = 2.2182(16), Zn1− C1 = 2.1491(19), Zn1−C14 = 2.1386(19); N1−Zn1−N2 = 103.17(6), N1−Zn1−C1 = 102.08(7), N2−Zn1−C14 = 101.35(7), N1−Zn1−C14 = 114.12(7), N2−Zn1−C1 = 114.78(7), C1−Zn1− C14 = 120.46(7), Zn1−C1−C13 = 108.05(13), Zn1−C14−C26 = 107.67(12).

and [{cyclo-(C4H8N)(CH2)2C5Me4}Zn(μ-OAc)]2 (3b; Zn−N = 2.172(2) Å, Zn−C(η1) = 2.110(2) Å) which were reported by Darensbourg et al.7 This may be due to the larger steric demand of two cyclopentadienyl ligands coordinated to one zinc center in 4. This trend is also reflected in the smaller N− Zn−C bite angles in 4 (N1−Zn1−C1 = 102.08(7)°, N2−Zn1− C14 = 101.35(7)°) in comparison to 108.70(13) and 106.72(8)° in 2 and 3b, respectively. The η1 bond between the ring carbon atom and the zinc center possesses predominantly σ character.11 This is reflected by Zn−C(η1)− Me angles of 108.05(13)° (Zn1−C1−C13) and 107.67(12)° (Zn1−C14−C26). Additionally, a distinct C−C bond length alternation is observed for the Cp ring, which indicates localized single and double bonds. With respect to the chelating η1(C)/κ1(N) bonding mode of the CpN ligands, the 1068


Organometallics

Article

the nitrogen atoms, shows decoalescence to give two new singlets of approximately equal intensities. At 182 K these new signals appear at chemical shifts of δ 1.85 and 1.18 ppm. In the same way, the other two singlets with chemical shifts of δ 2.12 and 1.94 ppm at room temperature show decoalescence to give four new singlets in the range δ 2.6−1.9 ppm, each representing one methyl group at the Cp rings. The decoalescence of these signals takes place at slightly lower temperatures of ca. 200 and 195 K, respectively. Unfortunately, the signals of the methylene protons cannot be found in the low-temperature spectra due to severe line broadening and signal overlap. The dynamic processes probably include sigmatropic rearrangements of the metal atom along the cyclopentadienyl rings. Thus, the carbon atoms at the α position (α to the donor-functionalized carbon atom) rapidly change positions. Additionally, the carbon atom which carries the side chain may participate in zinc coordination in the transition state or in an intermediate. This process can be regarded as a migration of the zinc atom along three carbon atoms of the Cp ring, which is depicted in Scheme 3.15,16

bonding motif in complex 4 resembles that of the monocyclopentadienyl CpN complexes of some main-group metals (e.g., Al, Ga, In),1,12 whereas most other metals show a preference for η5 coordination through all five ring carbon atoms of the CpN ligand in addition to the η1 coordination of the amino group. For instance, the group 2 metal complexes [MCpN2] (M = Ca, Ba) exhibit the latter bonding mode with two η 5-coordinated cyclopentadienyl rings, despite the frequently observed similarities between alkaline-earth metals and zinc.13 Notably, the spiro structure of 4 is chiral because of the presence of two stereocenters (both η1 carbon atoms) as well as an axis of chirality running through the zinc atom. Out of the four crystallographically independent molecules in the unit cell, two possess an R configuration of both stereocenters and the chiral axis whereas the other two molecules exhibit an S configuration of the stereocenters and the axis. Thus, a pair of enantiomers is present in the solid state.14 In contrast, no diastereomers to this pair of enantiomers can be observed, presumably because they are energetically disfavored. The 1H NMR spectrum (400 MHz) of 4 in [D8]toluene at room temperature exhibits three singlets and two triplets for the methyl and methylene protons, respectively. Accordingly, the 13 C NMR spectrum (101 MHz) in C6D6 shows five signals for the aliphatic and three resonances for the ring carbon atoms (see the Supporting Information). Thus, complex 4 appears to be dynamic in solution at room temperature, which leads to three pairs of equivalent methyl groups at each chelating CpN ligand. Note that the CpN ligand has a marked tendency to exhibit dynamic behavior, which has been described in the literature before.15 To study the dynamic processes of 4 in solution, 1H NMR spectra were measured at variable temperatures. Some representative spectra, which were recorded in the temperature range from 182 to 298 K in [D8]toluene at a frequency of 400 MHz, are shown in Figure 2. At higher temperatures (above ca. 220 K) averaged signals can be observed, as has been described for the spectrum at ambient temperature. With decreasing temperature the signals become broader. At a temperature of ca. 208 K, the singlet at δ 1.79 ppm (the chemical shift refers to the spectrum recorded at room temperature), which corresponds to the methyl groups at

Scheme 3. Structural Dynamics of 4 in Solution

Migration from the asymmetric carbon atom α to position α′ leads to the formation of a new stereocenter at α′, whereas α loses its chirality. Inspection of the configurations reveals that the newly formed stereocenter has the opposite configuration to the old one. Conclusively, this leads to an inversion of configuration at the ligand. Since there are no signals in the NMR spectra which may be ascribed to diastereomers, the inversion of the configuration at one CpN ring appears to be simultaneous with the inversion at the second CpN ligand. At the same time, the axis of chirality changes its orientation. Note that a diastereomeric form resulting from the inversion of the configuration at only one of the CpN rings would probably suffer from severe steric strain. At temperatures below the coalescence point the two methyl groups at each amino group also become inequivalent on the NMR time scale because they will be diastereotopic. A reaction pathway which involves the reversible cleavage of the Zn−N bond cannot be excluded but seems to be less likely because of the low activation barrier of rearrangements at the Cp ring.15 Using an approximation formula,17 the activation energy of the dynamic process described above was estimated to be ca. ΔG⧧ = 40 kJ/mol. Compound 4 appears to be a suitable starting material for the generation of a variety of zinc complexes featuring the CpN ligand. Following a procedure which has been applied for the preparation of zinc half-sandwich complexes before, 18 [ZnEtCpN] (5) was obtained on treatment of 4 with a slight excess of ZnEt2 (Scheme 2). The reaction proceeds rapidly in hexane solution at room temperature. After removal of residual ZnEt2 in vacuo, complex 5 was isolated in analytical purity in very good yields. The product is crystalline, and a sample was found to contain single crystals which were suitable for X-ray

Figure 2. Variable-temperature 1H NMR spectra of 4 at 400 MHz (intensity not scaled). Conditions: solvent, [D8]toluene; temperature range, 182−298 K. Assignment of peaks: (a/a′) 3,4-CH3; (b/b′) 2,5CH3; (c/c′) NMe2; (d/e) methylene protons; (*) solvent. 1069


Organometallics

Article

Compound 4 was also used for the preparation of a complex featuring two different Cp ligands. Only a few examples of such zinc complexes have been described in the literature.11 An equimolar mixture of [ZnCpN2] (4) and [ZnCp*2] (1) in hexane at room temperature yielded the mixed-ring complex [ZnCp*CpN] (6) (Scheme 2). As in the case of 5, the heteroleptic complex is thermodynamically favored over the homoleptic starting materials. NMR spectra at high and low temperature do not indicate any equilibrium of 6 with the homoleptic compounds 1 and 4. The solid-state structure resembles the structure of 5 with a σ-bound Cp* ligand instead of an ethyl group (Figure 4).

diffraction analysis. The solid-state structure proves the presence of a monomeric zinc complex that exhibits a zinc center which is coordinated by a chelating CpN ligand and an ethyl group (Figure 3).

Figure 3. Molecular structure of 5 (only one of the two crystallographically independent molecules is shown). Ellipsoids are drawn at the 50% probability level, and the hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Zn1− N1 = 2.188(2), Zn1−C1 = 2.146(2), Zn1−C2 = 2.303(2), Zn1−C5 = 2.441(2), Zn1−C14 = 1.962(3); N1−Zn1−C1 = 83.12(9), N1−Zn1− C14 = 119.67(12), C1−Zn1−C14 = 151.19(12), Zn1−C1−C6 = 109.03(16).

Figure 4. Molecular structure of 6. Ellipsoids are drawn at the 50% probability level, and the hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Zn1−N1 = 2.1416(19), Zn1−C1 = 2.139(2), Zn1−C2 = 2.341(3), Zn1−C5 = 2.463(3), Zn1− C14 = 2.017(2); N1−Zn1−C1 = 84.89(9), N1−Zn1−C14 = 122.19(9), C1−Zn1−C14 = 152.61(10), Zn1−C1−C10 = 107.36(17), Zn1−C14−C19 = 111.21(17).

There are two crystallographically independent molecules in the unit cell, which exhibit only small structural differences. Therefore, only one molecule is discussed. In contrast to the case for complex 4, the shortest Zn−C distance (Zn1−C1 = 2.146(2) Å) is found for the ring carbon atom which carries the amino-functionalized side chain. However, there are two other relatively short Zn−C distances (Zn1−C2 = 2.303(2) Å, Zn1− C5 = 2.441(2) Å). Thus, the bonding situation is between η1 and η2/η3 scenarios. Previous AIM and ELI-D DFT studies suggest that in zinc cyclopentadienyl complexes a Zn−Cring bond exists only if the Zn−Cring distance is shorter than about 2.3 Å.19 Therefore, we prefer the η1 notation for complex 5. This leads to a distorted-trigonal coordination geometry at the zinc atom. Interestingly, the Zn−C1−Cg (Cg = center of gravity of the Cp ring; ring centroid) angle (74.9°) is smaller than 90°, which means that the zinc atom is slightly slipped inside the ring perimeter. This might be indicative of some π contribution to the bonding between the zinc atom and the Cp ring.20 In addition, the C−C bond lengths between the ring carbon atoms range from 1.40 to 1.44 Å and thus show smaller variations than in 4 (1.37−1.47 Å), which can be associated with more delocalized Cp π electrons in 5. The Zn−N bond length of 2.188(2) Å is somewhat shorter than in 4 (2.228(2) Å, average). The five-membered metallacycle is characterized by an N−Zn−C1 bite angle of 83.12(9)°. Comparable to the case for compound 4, the 1H NMR spectrum of 5 in C6D6 at room temperature exhibits three singlets for the methyl groups and two triplets for the CH2 groups of the CpN ligand. A triplet at δ 1.29 ppm and a quartet at δ 0.12 ppm are characteristic of a zinc-bound ethyl group. As in the NMR spectra of 4, a dynamic behavior of the CpN ligand is evident in the 1H and 13C NMR spectra of 5 at room temperature. In contrast to the case for complex 4, no decoalescence of the signals could be observed in the 1H NMR spectra even at 183 K (300 MHz).

The Zn−N bond length (2.1416(19) Å) and the Zn−C distance which involves the σ-bound carbon atom of the Cp* ligand (Zn1−C14 = 2.017(2) Å) are relatively short. The structural data of the chelating binding mode of the CpN ligand are comparable to those in 5. The shortest Zn−C contact of 2.139(2) Å (Zn1−C1) is attained by the carbon atom carrying the amino-functionalized side chain. Again, there are two other relatively short Zn−C distances of 2.341(3) Å (Zn1−C2) and 2.463(3) Å (Zn1−C5).19 The 1H NMR spectrum of 6 in C6D6 at room temperature displays a singlet for the methyl groups of the Cp* ligand in addition to the resonances of the CpN ligand. Thus, the Cp* ligand is also fluxional in solution due to fast sigmatropic shifts. In addition to the reaction of K[C5Me4(CH2)2NMe2] and ZnCl2 in a 2/1 ratio, which yields [ZnCpN2] (4) exclusively, we also studied the reaction of an equimolar mixture of these reagents. Treatment of K[C5Me4(CH2)2NMe2] with 1 equiv of ZnCl2 in THF at room temperature afforded [Zn(μ-Cl)CpN]2 (7) as a colorless solid after workup of the reaction mixture (Scheme 4). Compound 7 is insoluble in hexane but exhibits good solubility in polar aprotic solvents such as THF or CH2Cl2. A more convenient route to 7 involves the treatment of 4 with an equimolar amount of ZnCl2 (Scheme 4). Notably, the formation of 4 can also be achieved by reacting 7 with K[C5Me4(CH2)2NMe2]. Single-crystal X-ray analysis proved complex 7 to be the chlorido-bridged dimer [Zn(μ-Cl)CpN]2 (Figure 5), which is structurally comparable to the dinuclear 1070


Organometallics

Article

Scheme 4. Formation of the Chlorido-Bridged Dimer [Zn(μ-Cl)CpN]2 (7)

equilibrating mixture of both. Variable-temperature 1H NMR spectra, which were recorded in the temperature range 183− 298 K at 300 MHz in [D8]THF, showed a decoalescence of all of the signals at approximately 200 K. The signal pattern is comparable to that which was found in the low-temperature 1H NMR spectra of compound 4. However, the spectra of 7 will not be discussed further because of the ambiguity in signal assignment arising from signal overlap, but it seems reasonable that the underlying dynamic processes are similar to those proposed for 4. Apparently, the outcome of the reaction of K[C5Me4(CH2)2NMe2] with ZnCl2 strongly depends on the ratio of the reagents. This raises the question if it is possible to obtain similar zinc cyclopentadienyl chlorido complexes starting from other zincocenes without a donor group attached to the Cp ligand. Decamethylzincocene (1) is a well-examined complex,5 but there have been no reports on a compound of the formal composition [ZnCp*Cl]. Note that the beryllium analogue [BeCp*Cl] was found to be a monomeric half-sandwich complex.23 An equimolar mixture of [ZnCp*2] (1) and ZnCl2 in THF at room temperature yielded a yellowish solid after evaporation of the solvent in vacuo. The 1H and 13C NMR spectra of this product in [D8]THF proved difficult to analyze. The spectra showed very broad signals for the Cp* ligand, which appeared approximately at the same chemical shifts as the signals of the starting material 1. Nonetheless, upon slow evaporation of the THF solvent at room temperature under 1 atm of argon, single crystals which were suitable for X-ray analysis were obtained. The solid-state structure consists of the chlorido-bridged dimer [Zn(μ-Cl)Cp*(THF)]2 (8), which is comparable to 7 (Figure 6). In 8 a THF molecule coordinates to the zinc atom instead of the intramolecular amino group in 7. The Zn−Cl distances (Zn1−Cl1 = 2.3640(4) Å, Zn1−Cl1i = 2.3630(4) Å) as well as the Zn−Cl−Zn and Cl−Zn−Cl angles (86.326(14) and 93.675(14)°, respectively) have nearly the same values as found for 7. The Cp* ligand is η1 coordinated with a Zn−C1 distance of 2.0291(15) Å, whereas the other Zn−C distances have to be considered nonbonding (2.578(2)−3.209(2) Å).19 Interestingly, the Zn−C1−Cg angle of only 95.8° points toward some π contribution to the Zn−C bonding,20 but the Zn−C1− Me angle (109.93(10)°) as well as the distinct C−C bond length alternation within the Cp* ring (1.38−1.47 Å) are characteristic of σ bonding. The fourth coordination site at the zinc center is occupied by the THF oxygen atom, featuring a Zn−O distance of 2.0057(11) Å. Attempts to isolate analytically pure samples of 8 for microanalysis proved unsuccessful. Apparently, prolonged drying of the crystals in vacuo leads to a loss of coordinated solvent molecules, which in turn results in a solid of unknown structure, which cannot completely be redissolved in THF. We propose a polymeric structure for this product in analogy to the solid-state structure of [EtZnCl]∞, which has been shown by Bochmann et al. to consist of infinite sheets of chlorido-bridged zinc atoms bearing terminal ethyl

Figure 5. Molecular structure of 7 (i indicates atoms which are symmetry related by inversion). Ellipsoids are drawn at the 50% probability level, and the hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Zn1−Cl1 = 2.3403(3), Zn1−Cl1i = 2.3525(3), Zn1−N1 = 2.1102(19), Zn1−C4 = 2.0972(11); Zn1−Cl1−Zn1i = 87.167(11), Cl1−Zn1−Cl1i = 92.832(11), Cl1−Zn1−N1 = 104.78(3), Cl1i−Zn1−N1 = 99.81(3), Cl1−Zn1−C4 = 123.38(3), Cl1i−Zn1−C4 = 123.21(3), N1−Zn1− C4 = 109.17(4), Zn1−C4−C11 = 114.02(8).

compounds 2 and 3a,b described by Darensbourg et al. (Scheme 1).7 Additionally, Bochmann et al. reported various dinuclear zinc complexes in which the zinc centers are bridged by Cl, N, or O atoms.21 A remarkable example is represented by the arene-coordinated compound [Zn(μ-Cl)(C 6 F 5 )(C6Me6)]2.21 Moreover, dinuclear zinc complexes with bridging alkoxido or peroxido groups can be obtained through treatment of zinc dialkyl compounds with molecular dioxygen.22 The asymmetric unit contains only half of the dimer because the molecule is located on an inversion center (the index i denotes atoms which are symmetry related by inversion). Each zinc atom is coordinated in a distorted-tetrahedral geometry by a chelating CpN ligand and two chlorido ligands which are in a bridging position to the second zinc atom. The Zn−Cl distances (Zn1−Cl1 = 2.3403(3) Å, Zn1−Cl1i = 2.3525(3) Å), the Zn−N distance (2.1102(19) Å), and the Zn−Cl−Zn and Cl−Zn−Cl angles of 87.167(11) and 92.832(11)°, respectively, are comparable to the corresponding values in [{Me2N(CH2)2C5HiPr3}Zn(μ-Cl)]2 (2).7 As in complex 4, the shortest Zn−C contact (Zn1−C4 = 2.0972(11) Å) is found for one of the α ring carbon atoms. The Zn−C distance for the side-chain-substituted carbon atom, however, is also relatively short (Zn1−C3 = 2.3602(11) Å). The 1H and 13C NMR spectra of 7 in [D8]THF or CD2Cl2 at room temperature display the signals for a fluxional CpN ligand. It has to be noted that it is not possible to deduce from the NMR spectra if the solution contains mononuclear or dinuclear species or a rapidly 1071


Organometallics

Article

compound 8 cannot be isolated by removing the solvent from the reaction mixture under vacuum. Thus, the presence of an intramolecular donor group, which is fixed to the cyclopentadienyl ring by a side chain, proves to be essential for the stability of complex 7. We assume that the concept of intramolecular stabilization by donor groups can be extended to the synthesis of other highly reactive zinc cyclopentadienyl complexes.

■

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. HC5Me4(CH2)2NMe2 and [ZnCp*2] (1) were prepared according to literature procedures.25,5 K[C5Me4(CH2)2NMe2] was obtained by heating an equimolar mixture of KH and HC5Me4(CH2)2NMe2 in THF at 70 °C for 30 min, followed by evaporation of the volatiles and washing the solid product with hexane. The NMR spectra were recorded at a Bruker Avance 400 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; C6D5CD2H, δ 2.09 ppm; [D7]THF, δ 1.73 and 3.58 ppm). 13 C{1H} NMR shifts were referenced to the 13C NMR signal of the solvent (C6D6, δ 128.06 ppm; [D8]THF, δ 25.3 and 67.4 ppm). The 1 H and 13C{1H} NMR spectra of compounds 4−7 are depicted in the Supporting Information. Microanalyses were measured at a HEKAtech Euro EA 3000 elemental analyzer. Synthesis of [Zn{C5Me4(CH2)2NMe2}2] (4). A solution of ZnCl2 (0.66 g, 4.8 mmol) in THF (10 mL) was added to a suspension of K[C5Me4(CH2)2NMe2] (2.24 g, 9.7 mmol) in THF (20 mL) at room temperature. The resulting turbid mixture was stirred for 1 h. After the volatiles were removed in vacuo, the residue was extracted with hexane (5 × 10 mL). The solvent was evaporated from the extract under vacuum to give 4 as a colorless solid. Yield: 1.19 g, 55%. Minor impurities which may lead to a yellowish, sticky product can be removed by washing with very small amounts of cold (−30 °C) hexane. Single crystals which were suitable for X-ray analysis were obtained upon cooling a solution of 4 in hexane to −30 °C. 1H NMR (400.1 MHz, [D8]toluene, 298 K): δ 2.28 (t, 3J(H,H) = 6.6 Hz, 4H, CH2), 2.12 (s, 12H, CH3), 1.94 (s, 12H, CH3), 1.86 (t, 3J(H,H) = 6.6 Hz, 4H, Me2NCH2), 1.79 (s, 12H, NMe2). 13C{1H} NMR (100.6 MHz, C6D6, 298 K): δ 124.9 (Cring), 110.1 (Cring), 108.3 (Cring), 62.4 (NCH2), 46.3 (NMe2), 24.5 (CH2), 13.0 (CH3), 12.4 (CH3). Anal. Calcd for C26H44N2Zn: C, 69.39; H, 9.85; N, 6.22. Found: C, 69.23; H, 10.06; N, 6.20. Synthesis of [ZnEt{C5Me4(CH2)2NMe2}] (5). ZnEt2 (1.3 mL of a 1 M solution in hexane, 1.3 mmol) was added to a solution of [Zn{C5Me4(CH2)2NMe2}2] (4; 400 mg, 0.89 mmol) in hexane (15 mL) at room temperature. After the clear solution was stirred for 20 min, the volatiles were removed under vacuum to give 5 as a colorless solid. Yield: 490 mg, 96%. 1H NMR (400.1 MHz, C6D6, 298 K): δ 2.30 (s, 6H, CH3 + t, 3J(H,H) = 6.5 Hz, 2H, CH2), 2.15 (s, 6H, CH3), 2.01 (t, 3J(H,H) = 6.5 Hz, 2H, Me2NCH2), 1.60 (s, 6H, NMe2), 1.29 (t, 3J(H,H) = 8.1 Hz, 3H, CH2CH3), 0.12 (q, 3J(H,H) = 8.1 Hz, 2H, CH2CH3). 13C{1H} NMR (100.6 MHz, C6D6, 298 K): δ 119.4 (Cring), 103.6 (Cring), 98.9 (Cring), 59.6 (NCH2), 44.3 (NMe2), 23.2 (CH2), 13.7 (CH2CH3), 11.6 (CH3), 11.4 (CH3), −6.2 (CH2CH3). Anal. Calcd for C15H27NZn: C, 62.82; H, 9.49; N, 4.88. Found: C, 62.79; H, 9.54; N, 4.40. Synthesis of [ZnCp*{C5Me4(CH2)2NMe2}] (6). A mixture of [Zn{C5Me4(CH2)2NMe2}2] (4; 400 mg, 0.89 mmol) and [ZnCp*2] (1; 299 mg, 0.89 mmol) was dissolved in hexane (15 mL) at room temperature, and the resulting solution was stirred for 30 min. After the volatiles were removed under vacuum, the solid residue was

Figure 6. Molecular structure of 8 (i indicates atoms which are symmetry related by inversion). Ellipsoids are drawn at the 50% probability level, and the hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Zn1−Cl1 = 2.3640(4), Zn1−Cl1i = 2.3630(4), Zn1−O1 = 2.0057(11), Zn1−C1 = 2.0291(15); Zn1−Cl1−Zn1i = 86.326(14), Cl1−Zn1−Cl1i = 93.675(14), Cl1−Zn1−O1 = 99.43(4), Cl1i−Zn1−O1 = 98.03(4), Cl1−Zn1−C1 = 119.56(4), Cl1i−Zn1−C1 = 118.36(4), O1−Zn1− C1 = 122.10(6), Zn1−C1−C6 = 109.93(10).

groups.24 Notably, significant amounts of [ZnCp*2] (1) can be extracted by washing the product with hexane. This leaves a solid residue of the composition {ZnCp*1−xCl1+x} (0 < x < 1). On the basis of these results, we propose that in THF solution [ZnCp*2] (1) and ZnCl2 exhibit a Schlenk-type equilibrium including the dimer [Zn(μ-Cl)Cp*(THF)]2 (8) among other unidentified species (Scheme 5). So far it has not been possible Scheme 5. Formation of Dinuclear [Zn(μ-Cl)Cp*(THF)]2 (8) in THF Solution

to isolate the unsolvated, structurally well-defined molecular species [ZnCp*Cl]2 or [ZnCp*Cl] upon removal of the solvent. This demonstrates the importance of the intramolecular donor group in the structurally related complex [Zn(μ-Cl)CpN]2 (7). The occupation of the fourth coordination site at the zinc atom enables the isolation of a structurally well-defined dinuclear species.

■

CONCLUSION We have described the synthesis and characterization of the amino-functionalized zincocene [ZnCp N 2 ] (4; Cp N = C5Me4(CH2)2NMe2). Treatment of 4 with [ZnR2] (R = Et, Cp*) yields the monocyclopentadienyl complex [ZnEtCpN] (5) and the mixed-ring zincocene [ZnCp*CpN] (6), whereas the addition of ZnCl2 to 4 leads to the formation of the dinuclear compound [Zn(μ-Cl)CpN]2 (7). The solid state structures of the dinuclear complexes 7 and [Zn(μ-Cl)Cp*(THF)]2 (8) exhibit coordination by nitrogen and oxygen donor groups, respectively. In contrast to the case for 7, 1072


Organometallics

Article

washed with hexane (3 × 2 mL) at −30 °C. Upon drying in vacuo, 6 was obtained as a very pale yellow solid. Yield: 508 mg, 73%. Single crystals which were suitable for X-ray analysis were obtained upon cooling a solution of 6 in hexane to −30 °C. 1H NMR (400.1 MHz, C6D6, 298 K): δ 2.32 (s, 6H, CH3), 2.12 (s, 6H, CH3), 2.09 (t, 3 J(H,H) = 6.6 Hz, 2H, CH2), 1.85 (t, 3J(H,H) = 6.6 Hz, 2H, Me2NCH2), 1.84 (s, 15H, C5Me5), 1.48 (s, 6H, NMe2). 13C{1H} NMR (100.6 MHz, C6D6, 298 K): δ 122.1 (Cring), 116.3 (C5Me5), 110.5 (Cring), 89.5 (Cring), 61.0 (NCH2), 45.3 (NMe2), 23.8 (CH2), 12.4 (C5Me5), 12.2 (CH3), 12.0 (CH3). Anal. Calcd for C23H37NZn: C, 70.30; H, 9.49; N, 3.56. Found: C, 69.49; H, 9.46; N, 3.16.26 Synthesis of [Zn(μ-Cl){C5Me4(CH2)2NMe2}]2 (7). A mixture of [Zn{C5Me4(CH2)2NMe2}2] (4; 700 mg, 1.56 mmol) and ZnCl2 (177 mg, 1.30 mmol) was dissolved in THF (10 mL) at room temperature, and the resulting solution was stirred for 1 h. Subsequently, the solvent was removed under vacuum and the solid residue was washed with hexane (4 × 5 mL). After drying in vacuo, 7 was obtained as a colorless solid. Yield: 695 mg, 91%. Cooling of a CH2Cl2 solution of 7 to −30 °C produced single crystals that were suitable for X-ray analysis. 1H NMR (400.1 MHz, [D8]THF, 298 K): δ 2.61 (m, 8H, CH2CH2), 2.40 (s, 12H, NMe2), 1.91 (s, 12H, CH3), 1.76 (s, 12H, CH3). 13C{1H} NMR (100.6 MHz, [D8]THF, 298 K): δ 126.4 (Cring), 121.1 (Cring), 99.3 (Cring), 63.5 (NCH2), 46.7 (NMe2), 23.7 (CH2), 12.8 (CH3), 11.9 (CH3). Anal. Calcd for C26H44N2Cl2Zn2: C, 53.26; H, 7.56; N, 4.78. Found: C, 52.29; H, 7.32; N, 4.66.26 Formation of [Zn(μ-Cl)Cp*(THF)]2 (8). In an NMR tube a mixture of [ZnCp*2] (1; 100 mg, 0.30 mmol) and ZnCl2 (41 mg, 0.30 mmol) was dissolved in deuterated THF (0.6 mL) at room temperature. The resulting pale yellow solution was examined by NMR spectroscopy. The 1H and 13C{1H} NMR spectra exhibited broad signals for a Cp* ligand (1H NMR (400.1 MHz) δ 1.82; 13 C{1H} NMR (100.6 MHz) δ 113.1, 11.4). Slow evaporation of the solvent under 1 atm of argon gave single crystals of [Zn(μCl)Cp*(THF)]2 (8), which were suitable for X-ray diffraction analysis.

■

B.; Peris, E. Eur. J. Inorg. Chem. 2012, 1309−1318. (d) Pontes da Costa, A.; Viciano, M.; Sanaú, M.; Merino, S.; Tejeda, J.; Peris, E.; Royo, B. Organometallics 2008, 27, 1305−1309. (5) (a) Blom, R.; Boersma, J.; Budzelaar, P. H. M.; Fischer, B.; Haaland, A.; Volden, H. V.; Weidlein, J. Acta Chem. Scand. A 1986, 40, 113−120. (b) Blom, R.; Haaland, A.; Weidlein, J. J. Chem. Soc., Chem. Commun. 1985, 266−267. (c) Fischer, B.; Wijkens, P.; Boersma, J.; van Koten, G.; Smeets, W. J. J.; Spek, A. L.; Budzelaar, P. H. M. J. Organomet. Chem. 1989, 376, 223−233. (6) (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. (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. (7) Darensbourg, D. J.; Wildeson, J. R.; Yarbrough, J. C. Organometallics 2001, 20, 4413−4417. (8) Walker, D. A.; Woodman, T. J.; Schormann, M.; Hughes, D. L.; Bochmann, M. Organometallics 2003, 22, 797−803. (9) 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. (10) Lee, J.-D.; Han, W.-S.; Kim, T.-J.; Kim, S. H.; Kang, S. O. Chem. Commun. 2011, 47, 1018−1020. (11) 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) Cowley, A. H.; King, C. S.; Decken, A. Organometallics 1995, 14, 20−23. (13) (a) Jutzi, P.; Dahlhaus, J.; Kristen, M. O. J. Organomet. Chem. 1993, 450, C1−C3. (b) Hatanpäa,̈ T.; Vehkamäki, M.; Mutikainen, I.; Kansikas, J.; Ritala, M.; Leskelä, M. Dalton Trans. 2004, 1181−1188. (14) Note that the symmetry operation of inversion, which is inherent to the nonchiral P1̅ space group, generates the corresponding enantiomer for each crystallographically independent molecule in the asymmetric unit. (15) (a) Jutzi, P. Chem. Rev. 1986, 86, 983−996. (b) Jutzi, P.; Dahlhaus, J.; Neumann, B.; Stammler, H.-G. Organometallics 1996, 15, 747−752. (16) (a) Lopez del Amo, J. M.; Buntkowsky, G.; Limbach, H.-H.; Resa, I.; Fernández, R.; Carmona, E. J. Phys. Chem. A 2008, 112, 3557−3565. (b) Margl, P.; Schwarz, K.; Blöchl, P. E. J. Chem. Phys. 1995, 103, 683−690. (17) Friebolin, H. Basic One- and Two-Dimensional NMR Spectroscopy, 5th ed.; Wiley-VCH: Weinheim, Germany, 2011. (18) (a) Jastrzebski, J. T. B. H.; Boersma, J.; van Koten, G.; Smeets, W. J. J.; Spek, A. L. Recl. Trav. Chim. Pays-Bas 1988, 107, 263−266. (b) Resa, I.; Á lvarez, E.; Carmona, E. Z. Anorg. Allg. Chem. 2007, 633, 1827−1831. (19) Mebs, S.; Chilleck, M. A.; Grabowsky, S.; Braun, T. Chem. Eur. J. 2012, 18, 11647−11661. (20) A π-bonding interaction between zinc and a cyclopentadienyl ligand mainly involves the empty 4p orbitals of appropriate symmetry, rather than the completely filled 3d orbitals of zinc. The latter are too low in energy, which precludes a classical π back-bonding scenario as is 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. (21) Sarazin, Y.; Wright, J. A.; Harding, D. A. J.; Martin, E.; Woodman, T. J.; Hughes, D. L.; Bochmann, M. J. Organomet. Chem. 2008, 693, 1494−1501. ́ (22) (a) Lewiński, J.; Sliwiń ski, W.; Dranka, M.; Justyniak, I.; Lipkowski, J. Angew. Chem., Int. Ed. 2006, 45, 4826−4829. (b) Schulz, S.; Flörke, U. J. Chem. Crystallogr. 2010, 40, 888−891.

ASSOCIATED CONTENT

S Supporting Information *

A table and CIF files giving crystallographic data for compounds 4−8 and figures giving the 1H and 13C{1H} NMR spectra of compounds 4−7. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We thank Dipl.-Chem. Anna Eißler and Dr. Stefan Mebs for the X-ray crystallographic analyses of complexes 4, 5, and 8. We are grateful to Dr. Ann-Katrin Jungton, Dipl.-Chem. Cathérine Mitzenheim, M.Sc. Lada Zamostna, and Dipl.-Chem. Sabrina I. Kalläne for the variable-temperature NMR measurements of complexes 4 and 7. M.Sc. Darina Heinrich is acknowledged for experimental support.

■

REFERENCES

(1) For a review on complexes with donor-functionalized cyclopentadienyl ligands see: Müller, C.; Vos, D.; Jutzi, P. J. Organomet. Chem. 2000, 600, 127−143. (2) See for example: Wang, T.-F.; Lee, T.-Y.; Chou, J.-W.; Ong, C.W. J. Organomet. Chem. 1992, 423, 31−38. (3) Jutzi, P.; Redeker, T. Eur. J. Inorg. Chem. 1998, 663−674. (4) (a) Siemeling, U. Chem. Rev. 2000, 100, 1495−1526. (b) Butenschön, H. Chem. Rev. 2000, 100, 1527−1564. (c) Royo, 1073


Organometallics

Article

(23) (a) Burns, C. J.; Andersen, R. A. J. Organomet. Chem. 1987, 325, 31−37. (b) Pratten, S. J.; Cooper, M. K.; Aroney, M. J. J. Organomet. Chem. 1990, 381, 147−153. (c) del Mar Conejo, M.; Fernández, R.; Carmona, E.; Andersen, R. A.; Gutiérrez-Puebla, E.; Monge, M. A. Chem. Eur. J. 2003, 9, 4462−4471. (24) Guerrero, A.; Hughes, D. L.; Bochmann, M. Organometallics 2006, 25, 1525−1527. (25) Jutzi, P.; Dahlhaus, J. Synthesis 1993, 684−686. (26) Although these results are outside the range viewed as establishing analytical purity, they are provided to illustrate the best values obtained to date. The deviations from the theoretical values may be caused by the extreme sensitivity of the samples towards air and moisture.

1074


Zinc Complexes with the N-Donor-Functionalized Cyclopentadienyl

Recommend Documents