Paramagnetic 18-Valence-Electron Alkylcyclopentadienylnickel (II

Article pubs.acs.org/Organometallics

Paramagnetic 18-Valence-Electron Alkylcyclopentadienylnickel(II) Bromide Dimers Marion Schar̈ ,† Dirk Saurenz,† Frank Zimmer,† Ina Schad̈ lich,† Gotthelf Wolmershaü ser,† Serhiy Demeshko,‡ Franc Meyer,‡ Helmut Sitzmann,*,† Oliver M. Heigl,§ and Frank H. Köhler*,§ †

Fachbereich Chemie, TU Kaiserslautern, Erwin-Schrödinger-Strasse 54, 67663 Kaiserslautern, Germany Institut für Anorganische Chemie, Georg-August-Universität, Tammannstrasse 4, 37077 Göttingen, Germany § Department Chemie, Technische Universität München, Lichtenbergstrasse 4, 85747 Garching, Germany ‡

S Supporting Information *

ABSTRACT: The paramagnetic behavior of 18-electron cyclopentadienylnickel(II) complexes of the [CpNi(μ-Br)]2 type with two unpaired electrons per metal ion has been investigated by 1H and 13C NMR spectroscopy of [Cp‴Ni(μBr)]2 (1; Cp‴ = 1,3,4-tBu3C5H2), [3CpNi(μ-Br)]2 (2; 3Cp = 1,3,4-iPr3C5H2), and [4CpNi(μ-Br)]2 (3; 4Cp = 2,3,4,5iPr4C5H). The tri-tert-butylcyclopentadienyl derivative 1 crystallizes in the triclinic space group P1̅ and has been investigated by X-ray crystallography. Solid-state magnetic susceptibility measurements of 3 revealed an effective magnetic moment at room temperature of 4.04 μB, confirming the presence of two d8 nickel(II) ions. While antiferromagnetic coupling via the bromo bridges is weak (J = −2.4 cm−1), zerofield splitting is substantial (D = +48.2 cm−1). NMR spectra of complexes 1−3 show signals with half-widths up to 3600 Hz within a spectral window exceeding 500 ppm (1H) or 2200 ppm (13C). An analysis of the spectra gave insight into the spin delocalization, the equilibrium orientation of the iPr substituents, and the presence of different conformers of compound 3. Paramagnetic behavior has also been observed for the Cp* derivative [Cp*Ni(μ-Br)]2 (Cp* = C5Me5) by 1H NMR spectroscopy. The presence of two unpaired electrons is discussed in terms of the weak ligand field originating from the combined interaction of poorly π accepting alkylcyclopentadienyl and π donating bromo ligands with the nickel(II) center.

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INTRODUCTION When bulky alkylcyclopentadienyl chemistry began to produce paramagnetic organometallic compounds, the reactivity of halfsandwich complexes such as [4CpM(μ-X)]2 (4Cp = C5HiPr4, MX = FeBr,1 CoCl,2 NiBr3,4) appeared quite attractive, but the large number of broad signals seen in proton NMR spectra within a large spectral window and with hardly interpretable integral ratios was puzzling. A collaborative effort using state of the art proton and 13C NMR spectroscopy of paramagnetic cyclopentadienylnickel complexes was therefore undertaken in order to derive a maximum amount of information from the spectra. This study includes SQUID magnetometry and X-ray crystallography and is aimed at full characterization of the nickel complex [Cp‴Ni(μ-Br)]2 (1; Cp‴= C5H2tBu3) and its C17H29BrNi isomer [4CpNi(μ-Br)]2 (3). Complex 2, the triisopropylcyclopentadienyl derivative [3CpNi(μ-Br)]2, is less bulky, not sufficiently stabilized against ligand dismutation and nickelocene formation, and could not be obtained as an analytically pure material. Complex 3 has been used as a starting compound for nucleophilic substitution reactions with dichalcogenide dianions3,5 and with other anionic nucleophiles such as dimethylphenolate or Grignard compounds.4 The © 2013 American Chemical Society

catalytic activity of cyclopentadienylnickel complexes in coupling reactions,6−8 olefin polymerization,9−11 or alkyne hydrothiolation12 warrants a further investigation of alkylcyclopentadienylnickel complexes. This work intends to establish the paramagnetic nature of the dinuclear 18-valence-electron complexes [CpRNi(μ-X)]2 against arguments to the contrary found in the chemical literature and elsewhere, as cited below. I n 1 9 8 5 K ö l l e r e p o r t e d t h e s y n t h e s i s o f pentamethylcyclopentadienylnickel(II) bromide as a thermally sensitive material in tetrahydrofuran solution at −10 °C, which was characterized as a dimer by mass spectrometry and could be converted in situ to nickel half-sandwich complexes.13 The first cyclopentadienylnickel(II) halide to be isolated was a pentaphenylcyclopentadienyl derivative.14 NMR spectra and magnetic behavior were not mentioned, but the complex [(C5Ph5)Ni(μ-Br)]2 was said to be diamagnetic.15 No information on magnetic properties was given regarding the closely related tris(trimethylsilyl)cyclopentadienylnickel chloride, which was isolated as a thermally sensitive oil and Received: June 24, 2013 Published: October 7, 2013 6298

dx.doi.org/10.1021/om400606t | Organometallics 2013, 32, 6298−6305

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mmol) of NiBr2(DME) in 130 mL of THF was cooled to −20 °C, and a solution of 5.51 g (21.48 mmol) of sodium tetraisopropylcyclopentadienide in tetrahydrofuran (100 mL) was added dropwise at that temperature. The resulting red-brown suspension was stirred for 30 min at −20 °C, warmed to 10 °C, and then evaporated to dryness. After extraction of the dark residue with petroleum ether (150 mL), centrifugation, and reduction in volume to ca. 30 mL, the product was crystallized at −78 °C as a rust-red microcrystalline powder. A second fraction was obtained from the concentrated mother liquor at −78 °C: total yield 5.50 g (7.39 mmol, 68.8%). In an evacuated test tube sublimation with beginning decomposition occurred above 156 °C. Anal. Calcd (found) for C17H29BrNi (fw 372.024): C, 54.89 (53.40); H, 7.86 (7.60). Molecular weight in benzene solution: 690 (cryoscopy). IR (KBr, cm−1): 2958 (ss, νCH), 2928 (s, νCH), 2871 (s, νCH), 1458 (s, νCC, ring, according to ref 22), 1377 (m) and 1365 (s) (this almost symmetric double band is characteristic for isopropyl groups),23 1095 (s, νCC), 807 (s, CH deformation vibration of the single ring proton).22,23 Magnetic Measurements. Temperature-dependent magnetic susceptibilities were measured with a SQUID magnetometer (Quantum Design MPMS XL-5) in the range from 295 to 2.0 K at a magnetic field of 0.5 T. Magnetic data were simulated using the julX program.24 The powdered sample was contained in a Teflon bucket and fixed in a nonmagnetic straw. Each raw data file for the measured magnetic moment was corrected for the diamagnetic contribution of the sample holder. The molar susceptibilities were corrected using Pascal’s constants.25 NMR Spectroscopy. The NMR spectra were recorded on a Bruker AV 300 spectrometer with the solvent signals used as a reference. The signal shifts are quoted on the δ scale (high-frequency shifts are positive) and, in the case of cited diamagnetic compounds, relative to tetramethylsilane. The paramagnetic compounds were measured in tubes equipped with ground glass and stoppers. A high-power solenoid probe head with tubes described previously26,27 was used for recording the 13C NMR spectra. The paramagnetic signal shifts at the measuring temperature T, δTpara, were obtained from the experimental signal shifts, δTexptl, by referencing each signal relative to the signal of the corresponding diamagnetic compound as a secondary standard (see Table 1). Contamination by susceptibility shifts was avoided by using the signal shifts of the solvent toluene-d8 as primary references (δ(CH3) 2.49 ppm, δ(CH3) 39.5 ppm) in order to measure δTexptl The δTpara values were converted to the contact shifts, δTcon, by subtracting the dipolar shifts.28 The latter should be similar to those of the nickelocenes because in all cases (at 300 K magnetically independent, S = 1) (RnC5H5−n)Ni fragments with similar interatomic distances and angles are present. It has been shown29,30 that owing to a very small g factor anisotropy of the nickelocenes the dipolar shifts can be neglected so that δTcon ≈ δTpara. The temperature was determined by recording the proton signal shift of nickelocene in a separate sample before and after each measurement of the new compounds (T = −77132/(δTpara − 1.04)).31 Single-Crystal X-ray Analysis. The crystal structure analysis of complex 1 has been carried out on a Stoe IPDS diffractometer. Details regarding data collection and structure solution can be found in the Supporting Information.

converted to mononuclear nickel(II) complexes.16 In another early publication in this field the pentamethylcyclopentadienyl derivative [Cp*Ni(μ-Br)]2 (Cp* = C5Me5) was isolated and characterized.17 The paramagnetism of this compound escaped recognition, however. The tetraisopropylcyclopentadienylnickel(II) bromide dimer [4CpNi(μ-Br)]2 (3) was crystallographically characterized,3 but its magnetic behavior was not mentioned at that time. More than 25 years after the first report on complexes of the [CpNiX] type the findings presented in this paper raise questions regarding the electron configuration of the longknown pentamethylcyclopentadienylnickel(II) bromide, which has been addressed as well.

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EXPERIMENTAL SECTION

General Procedures. All synthetic procedures were performed under a dry dinitrogen atmosphere following conventional Schlenk or drybox techniques. Tetrahydrofuran and petroleum ether (boiling range 40−60 °C) were distilled from potassium metal. Elemental analyses have been obtained using a Perkin-Elmer 240 Elemental Analyzer. IR spectra of potassium bromide pellets containing complexes 1 and 3 have been recorded on a JASCO FT/IR-4100 spectrometer. The preparation of the nickel(II) bromide DME complex has been mentioned in a footnote of ref 13. Additionally, an extraction procedure with boiling dimethoxyethane was employed in order to separate the nickel bromide complex from residual nickel metal. Sodium tri-tert-butylcyclopentadienide was obtained from tritert-butylcyclopentadiene18 by metalation with sodium amide in boiling tetrahydrofuran and worked up by filtration, evaporation, washing with petroleum ether, and drying of the ivory powder as mentioned in ref 19. Isomerically pure sodium 1,3,4-triisopropylcyclopentadienide was obtained by double recrystallization of a 78:22 mixture of the 1,3,4- and 1,2,3-isomers20 from tetrahydrofuran. Sodium tetraisopropylcyclopentadienide was prepared according to ref 20. Synthetic pathways leading to tetraisopropylcyclopentadiene have been evaluated and updated in ref 21. Bromo{1,3,4-tri-tert-butylcyclopentadienyl}nickel(II) Dimer, [(1,3,4-tBu3C5H2)Ni(μ-Br)]2 (1). NiBr2(DME) (4.00 g, 15.60 mmol) was suspended in THF (80 mL) and cooled to −20 °C. A solution of sodium tri-tert-butylcyclopentadienide (4.82 g, 15.61 mmol) in THF (70 mL) was added dropwise. The resulting red-brown solution was stirred for 30 min and then warmed to 10 °C and evaporated to dryness. After extraction of the dark residue with petroleum ether (120 mL), centrifugation, and reduction to ca. 30 mL, the product was crystallized at −30 °C as red-brown microcrystals in 3.80 g (5.11 mmol, 65.5%) yield, mp 168−170 °C dec. Anal. Calcd (found) for C17H29BrNi (fw 372.024): C, 54.89 (54.10); H, 7.86 (7.80). EI-MS (70 eV, m/z (%)): 371.9 ([M/2]+, 100); 314.9 ([M/2 − C4H9]+, 3.9); 292.0 ([M/2 − Br]+, 40.1); 233.0 (C17H29+, 81.6); 176.0 (C13H20+, 5.9); 57.1 (C4H9+, 89.1). IR (KBr, cm−1): 2961 (s, νCH), 2912 (s, νCH), 2870 (s, νCH), 1485 and 1458 (both s, νCC, ring, according to ref 22), 1398 (s) and 1362 (m) (this double band is characteristic for tert-butyl groups),23 1243 (s, νCC), 841 (s, CH deformation vibration of two ring protons).22,23 Bromo(1,3,4-triisopropylcyclopentadienyl)nickel(II) Dimer, [(1,3,4-iPr3C5H2)Ni(μ-Br)]2) (2). A suspension of NiBr2(DME) (1.55 g, 5.02 mmol) in THF (50 mL) was cooled to −20 °C, and a solution of 1.08 g (5.04 mmol) of sodium triisopropylcyclopentadienide in THF (15 mL) was added dropwise at that temperature. The resulting red-brown solution was then stirred for 30 min at −20 °C, warmed to 10 °C, and then evaporated to dryness. After extraction of the dark residue with petroleum ether (50 mL), centrifugation, and solvent evaporation a dark brown oil was isolated and used for NMR spectroscopic investigation. This oil could not be crystallized from pentane or diethyl ether solutions. It turned green-black upon moderate heating to temperatures above 50 °C within a few minutes. Bromo(tetraisopropylcyclopentadienyl)nickel(II) Dimer, [(2,3,4,5- iPr4C5H)Ni(μ-Br)]2 (3). A suspension of 6.63 g (21.48

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RESULTS AND DISCUSSION The title compounds 1−3 (Scheme 1) have been prepared from the nickel(II) bromide−dimethoxyethane adduct and the corresponding sodium alkylcyclopentadienide salt in tetrahyScheme 1. Graphic Representation of Complexes 1−3

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drofuran. The deep red tri-tert-butylcyclopentadienyl derivative 1 and the red-brown tetraisopropylcyclopentadienyl derivative 3 have been isolated in 66% and 69% yields as microcrystalline powders from pentane extracts by crystallization at dry ice temperature. The triisopropylcyclopentadienyl derivative 2 was isolated as a dark brown oil and used without purification for characterization by NMR spectroscopy. This oil refused to crystallize, and its thermal stability was not sufficient for purification by distillation or sublimation. We must assume that it contained traces of solvents and small amounts of the corresponding nickelocene. We have therefore been unable to obtain satisfactory elemental analyses for 2. Although 3 and especially 1 are much easier to handle, they are sensitive toward air and moisture, which did not allow for satisfactory carbon values. The possibility of nickel carbide formation during the combustion process could also explain the low carbon values. 1−3 are readily soluble in all common organic solvents. Crystalline samples of 1 and 3 can be handled in air for short periods of time but decompose upon prolonged exposure to moist air. Solutions are very air sensitive. 1 and 3 have been heated in sealed capillaries and found to decompose above 182 °C (1) and 156 °C (3). EI mass spectra of complex 1 show the signal of the monomeric [(tBu3C5H2)NiBr]+ moiety with 100% intensity and the [(tBu3C5H2)Ni]+ cation with 40% intensity. Other prominent signals can be attributed to the (tBu3C5H2)+ cation and the trimethylcarbenium ion with 82 and 89% intensities. The isotope distribution of these signals is in accordance with the calculated signal pattern. Single-Crystal X-ray Analysis of Complex 1. Crystallization of complex 1 from a concentrated pentane solution at −40 °C gave dark red single crystals suitable for X-ray diffraction.32 In the triclinic unit cell two dinuclear molecules of 1 are arranged around a center of symmetry. The bromine atoms of the planar four-membered Ni2Br2 ring are 3.423(1) Å apart, which is close to the sum of the van der Waals radii of two bromide ions (3.64 Å for coordination number 6).33 The smaller nickel atoms are at an even larger distance of 3.445(3) Å, which is nonbonding. The differences in Ni−Br distances are so small (about 0.03 Å) that the bromo bridges are almost symmetric. The five-membered rings are planar; deviations from the calculated mean plane are all below 0.01 Å. The tertbutyl group bound to C9 exhibits rotational disorder; all methyl carbon atoms of this alkyl group and the other tBu group connected to C4 show large thermal ellipsoids (cf. C44 in Figure 1).The nickel−ring centroid distance of 1.787 Å for both (tBu3C5H2)Ni fragments is between the value found for the nickelocene parent (1.73 Å34) and the 1.827 Å value found for 1,1′,2,2′,4,4′-hexaisopropylnickelocene4 and is therefore in the range expected for cyclopentadienylnickel(II) compounds with two unpaired electrons.4 A view along the vector Ni1−Ni2 (Figure 2) shows that the bromo bridges use the space between the alkyl substituents of both five-membered rings, which is the reason for the eclipsed conformation of the two (tBu3C5H2)ligands. A similar but more pronounced behavior has been seen in the dicarbonyldimetal complexes [{(tBu3C5H2)M}2(μCO)2] (M = Ni,35 Co36) with the same ring ligand set, where the short metal−metal distances of 2.396(4) Å (M = Ni) and 2.3538(6) Å (M = Co) enhance steric strain and force both bridging carbonyl ligands in positions parallel to the two ring C−H vectors. Solid-State Magnetic Measurements. In order to investigate the electronic structure of 3, magnetic susceptibility data for a solid sample have been collected in the temperature

Figure 1. Molecular structure of complex 1 in the crystal state. Distances (Å) and angles (deg): Ni1···Ni2 = 3.445(3), Br1···Br2 = 3.423(1), Ni1−Br1 = 2.4334(9), Ni1−Br2 = 2.4303(9), Ni2−Br1 = 2.4428(9), Ni2−Br2 = 2.4075(9), Ni1−C1 = 2.174(5), Ni1−C2 = 2.170(5), Ni1−C3 = 2.148(5), Ni1−C4 = 2.157(4), Ni1−C5 = 2.150(5), Ni2−C6 = 2.174(4), Ni2−C7 = 2.159(4), Ni2−C8 = 2.137(5), Ni2−C9 = 2.162(5), Ni2−C10 = 2.152(5), Ni1−ring plane = 1.787, Ni2−ring plane = 1.787; Ni1−Br1−Ni2 = 89.92(3), Ni1− Br2−Ni2 = 90.82(3), Br1−Ni1−Br2 = 89.47(3), Br1−Ni2−Br2 = 89.79(3), angle between two five-membered-ring planes 3.2.

Figure 2. Projection of complex 1 along the Ni−Ni vector, showing the eclipsed conformation of the five-membered rings and the position of the bromo bridges.

range from 295 to 2.0 K, using a SQUID magnetometer. The μeff value at room temperature of 4.04 μB (Figure 3, left) is very close to the spin-only value expected for two S = 1 centers (4.00 μB for g = 2.00) and clearly reveals the high-spin state of the d8 nickel(II) ions in 3. The decrease of the μeff curve below 50 K might be due to the effect of zero-field splitting and/or intramolecular antiferromagnetic interactions. Indeed, analysis of the magnetic data using the isotropic Heisenberg−Dirac− van Vleck (HDvV) exchange Hamiltonian that includes additional terms for zero-field and Zeeman splitting (eq 1) leads to a good fit with values g = 2.04, J = −2.4 cm−1, and |D| = 46.2 cm−1. 2

Ĥ = −2JS1̂ S2̂ +

2

∑ (D(Szî − i=1

1 (Si(Si + 1)) + gμB⇀ B ·⇀ Si ) 3 (1)

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Figure 3. μeff versus T plot (left) and variable-temperature−variable-field (VTVH) magnetization measurements as Mmol versus B/T (right) for 3. Solid lines represent the calculated curve fits (see text).

approximately nonbonding or are even destabilized by the πdonor properties of the bromo bridges. The energy gap between the highest of the four nonbonding and the lowest of the five metal−ligand antibonding metal-centered orbitals is obviously smaller than the spin pairing energy, which leads to a triplet ground state as illustrated previously.42 The triplet ground state is associated with a remarkably large zero-field splitting (D ≈ 48 cm−1) which exceeds that of the related nickelocene (D ≈ 34 cm−1 43). The magnetic interaction within the dimer is weak because the Ni−Br−Ni angles are close to 90°, so that the magnetic orbitals are almost orthogonal. This behavior has long been known for halide-bridged copper(II) dimers44 featuring small J values, which may be positive or negative depending on the geometrical details. Weak antiferromagnetism has also been found for halide-bridged (cyclopentadienyl)manganese derivatives with Mn−X−Mn angles close to 90°,45 while this angle deviates a bit more in similar (cyclopentadienyl)chromium compounds so that the J values become larger.46,47 NMR Studies of the Paramagnetic Compounds. Basic Features. The NMR spectra of 1−3 in solution exhibit broad and strongly shifted signals which are characteristic of compounds having rapidly relaxing unpaired spins; examples are given in Figure 4. The 1H and 13C NMR signals cover ranges of 550 ppm and more than 2200 ppm, respectively, while the signal half-widths are 130 Hz up to about 3600 Hz. The full data are given in Table 1. These results suggest that the 1 H NMR signal shifts reported for [(Me5C5)NiBr]217 were those of diamagnetic impurities. To confirm this, solid (Me5C5) Li was added to a frozen suspension of an excess of NiBr2(THF)1.5 in THF-d8 in a NMR tube. All manipulations were carried out under an atmosphere of dinitrogen. Immediately after the reaction mixture was warmed to ambient temperature, the proton spectrum was recorded at 305 K. Two signals appeared at 235 and 286 ppm in addition to the solvent signal. The first signal is that of (Me5C5)2Ni, for which δpara 240.0 has been found for an analytically pure sample at 298 K,48 while the more strongly shifted signal must be assigned to [(Me5C5)NiBr]2. It is gratifying that the shift is similar to that found for the corresponding signal of freely rotating groups of 2 (see also below). When more (Me5C5)Li was added to the mixture, the signal of (Me5C5)2Ni increased at the expense of that of [(Me5C5)NiBr]2, as expected. Spin Delocalization and Signal Assignment. There is a noticeable similarity between the NMR features of 1−3 and those of nickelocenes.23−25,50 This is not surprising, as CpNi fragments with two unpaired electrons are concerned in all

While antiferromagnetic coupling mediated by the two bridging bromo ligands is only weak, the zero-field splitting in 3 is substantial. Since the reliability of D values calculated from such powder measurements cannot be guaranteed, magnetization measurements at variable temperature and variable field (VTVH; Figure 3, right) were performed. The values g = 2.03, J = −2.0 cm−1 and D = +48.2 cm−1 are in excellent agreement with those derived from susceptibility data and evidence the positive sign of the large zero-field splitting. The paramagnetism of an 18-valence-electron complex is not completely unexpected. In a review article on open-shell organometallic compounds37 Poli organized the bulk of the material according to metal d electron count and valence electron number. Open-shell organometallics with an 18valence-electron count at the central atom are not mentioned in this article. The sole example is [(C5Me5)Fe(dppe)(O CMe2)]+F3CSO3− with intermediate spin (two unpaired electrons); the C5H5 derivative has been found to be diamagnetic.38 Interestingly, the 18-valence-electron cyclopentadienylnickel derivative [(C5Me5)Ni(acac)] (acac = acetylacetonate) has a diamagnetic ground state and a lowlying triplet excited state, which exhibits a low-spin−high-spin equilibrium with an equilibrium constant of 0.47 at 303 K.39 Other paramagnetic 18-VE cyclopentadienyl complexes are known for manganese, which belong to the [(C5R5)Mn(CO)2L] type with R  H, Me and nitrogen donors L including pyridine, pyrazine, and tetracyanoethylene. These complexes also possess a diamagnetic ground state with easily accessible magnetic states of integer spin.40 There are actually two reasons 3 (and its congeners 1 and 2) are paramagnetic: due to steric reasons (the bulky tetraisopropylcyclopentadienide does not allow for optimal ligand approach toward the central atom) and electronic reasons (according to the spectrochemical series the ligand field exerted by bromo ligands should be even weaker than that of acetylacetonate), the nickel(II) central atoms of [(iPr4C5H)Ni(μ-Br)]2 (3) adopt triplet states at ambient temperature and the magnetic exchange interaction in the dimer 3 is small. A qualitative consideration accounts for the open-shell character of the nickel(II) atoms of 3. Five occupied ligand donor orbitals (three of the [iPr4C5H] ring and two of the bromo bridges) are energetically stabilized and become ligand-centered, metal− ligand bonding molecular orbitals. In turn, five metal orbitals are destabilized and become metal-centered, metal−ligand antibonding orbitals. Neither the cyclopentadienyl ligand41 nor the bromo bridges are good acceptors for back-donation from the metal; therefore, the remaining four metal d orbitals remain 6301


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bonding, which entails planar chirality at the respective substituted Cp carbon atoms. The effect has been used previously for the study of substituted benzenes.52−54 The spin transfer beyond the α carbons generates positive spin at the β nuclei and depends on the dihedral angle between the spin-carrying 2pz orbital of the ring carbon atom and the α−β bond.55 The quantitative relation is δT con(Nβ) = [δ0(Nβ) + B(Nβ)⟨cos2 θ ⟩]ρ(C)

(2)

Because the contact shift of any β nucleus, δT (Nβ), is a measure of the expectation value ⟨cos2 θ⟩ of the dihedral angle θ (and the spin density, ρ(C)), at the respective substituted Cp carbon atom, the orientation of the substituent relative to the Cp plane can be estimated. More details are given in the Supporting Information. The results are collected in Table 2. It turns out that the θ values found for the tBu groups and the enantiotopic iPr groups at C1 of 1 and 2, respectively, are not far from 45°. This angle is expected for alkyl groups which are bonded to a planar ligand and whose 2-fold rotational barrier is low (“free rotation”56). Actually, these groups have no adjacent substituent constraining their rotation. The remaining iPr groups of 2 and 3 provide richer information, owing to the diastereotopic methyl groups. Experimentally, three θ values for each iPr group are now available, one for the Cα−Hβ bond and two for the Cα−Cβ bonds, so that the projections in Figure 5 can be drawn. There are slight deviations from ideal 3-fold symmetry, as expected for an iPr group, but these are also due to the approximations used. According to Figure 5, the primed C atoms are those of the MeproS groups, while the nonprimed atoms belong to MeproR. Reflecting all α−β bonds at the broken line yields an equivalent assignment where MeproS and MeproR are interchanged. An independent confirmation for the distinction of the primed and unprimed methyl carbons can be derived from the signal halfwidths (for details see the Supporting Information). It should be recalled that equilibrium orientations of the iPr groups are determined in solution. The orientations reflect the average population of conformers with variably gear-meshed rotamers. Such conformers have been discussed for iPr groups bonded to five-membered57,58 and six-membered rings.52−54,59 Remarkably, however, in the case of 3 these orientations correspond to those found in the X-ray study, except that from the NMR work it cannot be concluded whether in the dominating conformer the iPr groups are all interlocked to the same side or not. The NMR results considerably extend the ESR study of the pentaisopropylcycopentadienyl radical.60 While the proton dihedral angles of the congested iPr groups obtained from ESR and NMR spectroscopy are similar (mean difference 13°), the dihedral angles of the Cα−Cβ bonds were not available from the ESR spectra. The signals of the γ protons confirm the presence of diastereotopic methyl groups in 2 and 3, since there are two for each iPr group in positions 3/4 and 2/5. All signal shifts are positive, which is consistent with spin transfer through second-order hyperconjugation.51 Further assignment follows the spin distribution in the Cp ring and the dihedral angle dependence found for the corresponding β carbons. The overall signal shifts are comparatively small because they depend on a second dihedral angle,51 which is 45° for the freely rotating methyl groups. The temperature-dependent 1H NMR spectra were recorded in order to block the rotation of the substituents at low temperature. For the tBu groups of 1 and the iPr groups of 2 (Figure 6) blocking was not possible down to 178 and 172 K, con

Figure 4. (top) 1H NMR spectrum of 1 in toluene-d8 at 305 K. (bottom) 13C NMR spectrum of 3 in toluene-d8 at 375 K, Inset: signals near −450 ppm expanded. Solvent signals (S) have been truncated in the 13C NMR spectrum. α, β, and γ designate nuclei separated from the corresponding Cp carbon atom (with symmetryadapted numbering) by one, two, and three bonds, respectively.

cases. It follows that direct delocalization transfers positive spin from the metal into the π orbitals of the Cp ligand so that the 13 C NMR signals of the five-membered-ring carbons are strongly shifted to high frequency20,50,51 (826 < δTexptl < 1265; Table 1). The splitting of these signals is determined by the substitution pattern via its impact on the spin distribution. This is outlined in the Supporting Information. It turns out that the signal sequence of the trialkylated Cp derivatives is δTcon(C1) > δTcon(C3/4) > δTcon(C2/5), whereas for the tetraalkylated analogue it is δTcon(C1) < δTcon(C3/4) < δTcon(C2/5). The signals of the α, β, and γ nuclei which are separated from the Cp ring by one, two, and three bonds, respectively, are also (more or less) strongly shifted because spin density is further delocalized to the substituents. In the first step the bond between the ring carbons and the α nuclei is polarized so that the signal shifts of the α protons and α carbons are negative. Their signal sequences are again determined by the spin pattern of the Cp ring. For example, compounds 1 and 2 have |δ T con (Cα1)| > |δ T con (Cα3/4)|, which corresponds to |δTcon(C1)| > |δTcon(C3/4)| (Table 1). In any case, the signal areas ensure the final signal assignment. The assignment of the β and γ nuclei is elucidated in the following. Stereochemistry and Dynamic Behavior. The methyl groups of 3 and those in positions 3/4 of 2 are diastereotopic.20 Therefore, two signals are found for the β carbons and γ protons of these methyl groups. Their assignment is not trivial and shall be considered below. It is worth noting that here diastereotopism can be observed owing to metal−ligand π 6302


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Table 1. NMR Data of Compounds 1−3 [(1,3,4-tBu3C5H2)NiBr]2 (1) nucleus, position H1 Hα2/5 Hβ1 Hβ2/5 Hβ3/4 Hγ1 Hγ2/5 Hγ′2/5 Hγ3/4 Hγ′3/4 C1 C2/5 C3/4 Cα1 Cα2/5 Cα3/4 Cβ1 Cβ2/5 Cβ′2/5 Cβ3/4 Cβ′3/4

[(1,3,4-iPr3C5H2)NiBr]2 (2)

[2,3,4,5-(iPr4C5H)NiBr]2 (3)

δTexptla

Δν1/2a

δrefb

δ298con

δTexptla

Δv1/2a

δrefc

δ298con

−299.3

560

4.20

−310.6

−231.9 244.5

380 440

3.83 2.82

−241.3 247.4

113.3 11.4

200 170

2.61 1.46

113.3 10.2

25.8

150

1.34

25.0

19.4

130

1.32

18.5

1255 923 1096 −458.8

2710 2220 2460 380

63.3 92.4d 92.4d 26.8

−451.3 628.0

330 470

26.8 23.9e

556.2

23.9e

470

1500 1045 1263 −611.1

26.2 14.7 1265 1027 1182 −553.0

150 150 280 1820 460 190

1.09 1.21 92.4d 63.3 92.4d 26.8

25.7 13.8 1476 1213 1371 −729.6

−601.6 760.2

−523.0 702.2

330 380

26.8 23.9e

−691.9 853.6

669.8

496.6 1183

380 1450

23.9e 23.9e

594.8 1459

δTexptla,f

Δv1/2a,f

δrefc

δ298con

−257.0

450

3.84

−293.2

101.5 32.8

1180 220

2.83g 2.83g

29.4 16.4 12.9 22.8 826 1102 1020

160 170 160 170 1650 2890 3570

1.53 1.31 1.05 1.38 59.4 91.0h 91.0h

31.3 17.0 13.3 24.1 965 1272 1169

−446.0 −452.9

420 380

26.1 26.1

−594.1 −602.8

1015.8 683.1 531.0 875.3

690 800 520 850

27.1i 24.1i 24.8i 22.3i

1244 829 637 1073

110.9 33.7

H and 13C NMR spectra were meastured at 305 and 375 K, respectively, unless stated otherwise. The experimental signal shifts, δTexptl, those of the diamagnetic reference compounds, δref, and the contact shifts at 298 K, δ298con, are in ppm; the signal half width, Δν1/2, is in Hz. b1H NMR: (1,3,4tBu3C5H2)2Fe.49 13C NMR: data not available. Reference shifts from (1,3,4-iPr3C5H2)2Fe.20 cReference 9. dMean shift of signals which were not distinguished. eMean shift of signals which were not distinguished. f1H NMR at 335 K. gMean shift of signals which were not distinguished. hMean shift of signals which were not distinguished. iTentative assignment by analogy to the corresponding 1H NMR signals. a1

Table 2. Average Dihedral Angles θ Associated with the β Nuclei of 1−3 [(1,3,4-tBu3C5H2)Ni(μ-Br)]2 (1)

[(1,3,4-iPr3C5H2)Ni(μ-Br)]2 (2)

δ298 (Nβ)

δ298 (Nβ)

θ (deg)

Hβ1

247.4

51

Hβ3/4 Cβ1

113.3 853.6

64 43

nucleus

Cβ1

Cβ3/4

con

760.2

669.8

θ (deg)

42

40

nucleus

Cβ3/4 Cβ′3/4

con

594.8 1459

[(2,3,4,5-iPr4C5H)Ni(μ-Br)]2 (3)

51 6

nucleus

δ298con(Nβ)

θ (deg)

Hβ2/5 Hβ3/4

110.9 33.7

63 75

Cβ2/5 Cβ′2/5 Cβ3/4 Cβ′3/4

1244 829 637 1073

18 39 45 23

/298, should be independent of the temperature when the compound is magnetically simple. As can be seen in Figure 6, the normalized contact shifts are not constant. The values of δTcon(H2/5)T /298 of 1 and 2 pass from −181 to −535 ppm and from −207 to −278 ppm, respectively, although the temperature ranges are virtually the same. The shapes of the curves are also different, and the same applies to Hγ1 and Hγ3/ 4. Actually, there are two effects which may operate. First, there is antiferromagnetic interaction which has been established for 3. The experimental exchange coupling constant of ∼−2.2 cm−1, however, is much too small to be visible in the temperature window accessible by solution NMR spectroscopy. Second, the rotation of the substituents may render the normalized contact shifts temperature dependent when the population of the rotamers changes. This has been shown to occur for radicals by EPR spectroscopy.51 Indeed, the shapes of the curves of the propyl and isobutyl radicals are similar to those of 1 in Figure 6.56 In this context it is helpful to recall that the normalized contact shifts (NMR) are proportional to the

Figure 5. NMR-derived orientations of the iPr groups (left to right): iPr-3/4 of 2, iPr-2/5 of 3, and iPr-3/4 of 3 (see also Figure 4). The Cp planes and the Ni atoms in the background are gray.

respectively, as indicated by the number of signals, which did not change. While this corresponds to the behavior of the iron analogue of 1,1 the limit of observation is now pushed to lower barriers, as the signal shift spread is much larger for the paramagnetic than for the diamagnetic compounds. The NMR temperature behavior of 1 and 2 departs from the Curie law and is strikingly different. Thus, the contact shifts of nucleus N normalized to the standard temperature of 298 K, δTcon(N)T 6303


Organometallics

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Article

CONCLUSION Dinuclear alkylcyclopentadienylnickel(II) complexes with bromo bridges are paramagnetic with two unpaired electrons, despite their 18-valence-electron count. SQUID data reveal weak antiferromagnetic coupling between two S = 1 ions but a large positive zero-field splitting (D = +48.2 cm−1) in the solid state. NMR spectra resemble those of nickelocenes, with positive spin density being transferred from the metal to the ring carbon atoms, resulting in strong shifts to positive δ values. Alternating shifts for α and β carbon atoms are due to spin polarization. Owing to the large signal shift ranges the study of the stereochemistry in solution is significantly improved. In the solid state the lattice energy may force the central Ni2Br2 moiety to adopt a planar or puckered structure and the orientation of the Cp ligands is directed by the orientation of their substituents relative to the bromo bridges.

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ASSOCIATED CONTENT

■

AUTHOR INFORMATION

S Supporting Information *

Text, figures, tables, and a CIF file giving crystallographic data for complex 1, spin distribution in the five-membered ring of compounds 1−3 and NMR signal assignment, dihedral-angle dependence of the spin delocalization in iPr-substituted [CpNi]+ fragments, signal widths and signal assignment, and a full series of temperature-dependent 1H NMR spectra of compound 3. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 6. Normalized 1H contact shifts as a function of temperature for 1 (right) and 2 (left).

hyperfine coupling constants (EPR). However, in compounds 1 and 2 the ensemble of the barriers to rotation is more complicated than in the cited radicals. This might be the reason why the shapes of the curves change on going from compound 1 to compound 2. When a solution of 3 in toluene-d8 is cooled, the 1H NMR spectrum experiences drastic changes. This is illustrated for the γ proton signals in Figure 7. In addition to the expected shifts

Corresponding Authors

*E-mail for H.S.: [email protected]. *E-mail for F.H.K.: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS H.S. expresses his gratitude to Professor O. J. Scherer for many years of generous support. We also thank Professor G. Hornung for taking mass spectra of complex 1 and W. Martin for a cryoscopic determination of the molecular mass of complex 3. Dedicated to Professor O. J. Scherer on the occasion of his 80th birthday.

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Figure 7. Selected temperature-dependent 1H NMR spectra of 3 dissolved in toluene-d8, γ proton range. Solvent signals (S) have been truncated. At 284 K the signals of Hβ2/5 and Hγ2/5 coincide. See the Supporting Information for the full series of spectra.

REFERENCES

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to high frequency, the signals broaden until the first signal (Hγ′2/5) has disappeared around 260 K. When the temperature is lowered further, new signals appear which are best resolved at 188 K. The varying signal areas of more than 20 signals suggest that the signal sets of differently populated conformers with interlocked iPr groups have been frozen out. We have previously established activation barriers from paramagnetic NMR data.57,58 In the present case, attempts to deconvolute excessive signal overlap in the spectra of Figure 7 and their signal assignments were too speculative and frustrated the analysis. However, the signal of Hα2/5 split below 220 K and the signals of three conformers appeared at 273, 267, and 263 ppm (199 K; approximate abundance 1/3/5). 6304


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Paramagnetic 18-Valence-Electron Alkylcyclopentadienylnickel (II

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