Synthesis and Physical Properties of Pentamethylmanganocene

Oct 3, 2016 - The inclusion complexes of composition (Cp*2Yb)2(Cp2M) (M = V, Cr, Fe, and Co; Cp* = η5-C5Me5; Cp = η5-C5H5) are isolated in the solid...
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Synthesis and Physical Properties of Pentamethylmanganocene, (C5Me5)Mn(C5H5), and the Inclusion Compounds [(C5Me5)2Yb]2[(C5H5)2M] (Where M = V, Cr, Fe, Co) Marc D. Walter,†,‡ Carol J. Burns,† Phillip T. Matsunaga,† Michael E. Smith,† and Richard A. Andersen*,† †

Department of Chemistry and Chemical Sciences Division of Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, United States ‡ Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106 Braunschweig, Germany S Supporting Information *

ABSTRACT: The inclusion complexes of composition (Cp*2Yb)2(Cp2M) (M = V, Cr, Fe, and Co; Cp* = η5-C5Me5; Cp = η5-C5H5) are isolated in the solid state. The crystal structure of one of them, M = Co, shows that the Cp2Co metallocene is sandwiched between two Cp*2Yb metallocenes with two long Yb···C bond distances of 2.914(6) Å, one from each of the Cp rings of Cp2Co. When M = Mn and Ni are used, the ring exchange products, Cp*MCp, are isolated along with Cp*YbCp, a hydrocarbon-insoluble green solid isolated as the thf adduct. This Cp for Cp* exchange reaction is the only currently available synthesis for the low-spin pentamethylmanganocene and the pentamethylytterbocene. The crystal structures and magnetic and related physical properties of Cp*MCp, M = Mn, Co, and Ni (Organometallics 1985, 4, 1680), are reported and analyzed. The origin of the different relative rates of Cp* for Cp ring exchange is traced to the kinetic lability resulting from the (e1g*)2 electronic structure of Cp2M, M = Mn and Ni.

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INTRODUCTION

A high-yield synthesis of Cp*MnCp is developed in this article, using Cp*2Yb as the Cp*-transfer reagent. The physical properties of Cp*MCp, M = Mn, Co, and Ni, are also reported.

The synthesis of heteroleptic metallocenes of the 3d-block transition metals continues to be of interest to the organometallic chemical community due to their role in stoichiometric and catalytic organic synthesis.1 The general synthetic strategy involves exchange of an X-ligand in a half-sandwich compound, Cp′MX, with a different cyclopentadienyl anion, a methodology that often results in the formation of the homo- and heteroleptic metallocenes that have to be separated. The specific methodologies that have been used for the preparation of the pentamethylmetallocenes, Cp*MCp, are illustrated in Scheme 1, from which two observations can be made: (i) the manganese derivative is absent,2 and (ii) the most general syntheses are those of Manriquez and co-workers for Fe, Co, and Ni.3

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RESULTS Strategy. The successful synthetic methodology for the preparation of Cp*MnCp was developed during our study of the coordination chemistry of base-free decamethylytterbocene, Cp*2Yb. This diamagnetic metallocene has a bent sandwich structure in the gas phase11 and in the solid state.12 In solution the ytterbocene forms coordination complexes with, for example, MeCCMe, 13 (C 2 H 4 )Pt(PPh 3 ) 2 and Me 2 Pt(PR3)2,13,14 and MeBeCp*,15 in which the electron density on the carbon atoms is acting as the donor to the bent sandwich Cp*2Yb in the solid-state structures. Inclusion compounds in which, for example, 1,2-C2B10H12 can be cocrystallized with Cp*2Yb are also known.12 These observations suggested that the first-row, d-block metallocenes, Cp2M, should also form molecular complexes with Cp*2Yb as the cyclopentadienyl rings in, for example Cp2Fe, carry a negative charge. The polarizability in Cp2Fe (and in Cp2Ru) is anisotropic; that is, the charge distribution is more polarizable along the molecular z-axis than along the x,y-axes, as are the molecular quadrupole and magnetic moments. Relative to the free-iron atom, the transfer of negative charge from the metal to

Scheme 1a

a (i) ref 4, (ii) ref 5, (iii) refs 3, 6, (iv) ref 7, (v) ref 8, (vi) ref 3a, (vii) ref 3a, (viii) ref 9, (ix) ref 10.

© XXXX American Chemical Society

Received: July 10, 2016

A

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Organometallics the ligands results in a net charge of +0.7 on the iron atom in ferrocene.16 The fact that the cyclopentadienyl rings in ferrocene, and presumably the other first-row metallocenes, carry negative charge density generates the question, can the bent-sandwich Cp*2Yb sense the negative charge density on the rings of the 3d-metallocene and, if so, what is the molecular structure of the resulting compounds? Answering this question resulted in a high-yield synthesis of Cp*MnCp, the missing link in Scheme 1, and Cp*YbCp. (Cp*2Yb)2(Cp2M). Synthesis and Physical Properties. Mixing Cp*2Yb and the 3d-metallocenes, Cp2M, where M is V, Cr, Fe, or Co, in either a 1:1 or 2:1 molar ratio in toluene yields solids of composition (Cp*2Yb)2(Cp2M). The solution 1H NMR spectra are essentially identical to those of the individual metallocenes that were examined, M = Fe and Co. For the diamagnetic (Cp*2Yb)2(Cp2Fe), two resonance signals are observed in C6D6 at 30 °C at δ 3.99 and 1.93 ppm. The chemical shift of Cp2Fe is 4.00 ppm and that of Cp*2Yb is 1.92 ppm in C6D6 at 30 °C. The 1H NMR spectrum of (Cp*2Yb)2(Cp2Co) also contains two resonance signals at δ 1.93 and −49.8 (ν1/2 = 130 Hz); the chemical shift of Cp2Co is −50.1 (ν1/2 = 145 Hz). In aromatic hydrocarbon solvents, the 1 H NMR spectra of the two solid-state compounds examined are identical to the individual metallocenes, implying that in solution essentially no interaction exists between them. In the solid state, the infrared spectra of all four compounds are essentially superimposable; the stretching frequencies for the vanadium species are listed in the Experimental Section. The melting behavior is similar for M = V and Fe (they melt at 202−206 and 199−202 °C, respectively) and for M = Cr and Co, both of which turn green over the temperature range 150− 185 °C, but neither melts to 300 °C. The pure 3d-metallocenes melt at 173 °C,17 and Cp*2Yb melts at 189−191 °C; it turns orange-red at ca. 130 °C, reversibly.12 Although these solidstate compounds behave differently on heating, the melting behavior is not consistent with a physical mixture and implies that some interactions exist in the solid. Further, the color change for the chromium and cobalt species suggests that a chemical reaction occurs near the melting point of the pure 3dmetallocenes. The solid-state magnetic susceptibility of the (Cp*2Yb)2(Cp2V) compound follows the Curie−Weiss law over the temperature range of 5−300 K with μeff = 3.93 and 3.85 μB with θ = −2.5 and −7.3 K at 5 and 40 kG, respectively (Figure 1). Pure Cp2V also follows Curie−Weiss behavior above ca. 15 K with μeff = 3.78 μB with θ = −9 K; the ground state is 4A1 (e2a1) (Figure 1).18 As pointed out by König and co-workers, the significant decrease in the effective magnetic moment at low temperature cannot be simply explained on the basis of zero-field splitting, but below 15 K, antiferromagnetic ordering occurs with TN = 8.3 K, which is due to intermolecular spin exchange.18b In (Cp*2Yb)2(Cp2V), the Cp2V molecules are isolated from each other, so no intermolecular electron spin coupling is observed to 5 K. The (Cp*2Yb)2(Cp2Cr) complex behaves similarly; from 5 to 40 K, μeff = 3.09 μB, and from 170 to 300 K, μeff = 3.69 μB. Although the shape of the χ−1 vs T plots is similar for (Cp*2Yb)2(Cp2Cr) and pure Cp2Cr,19 with a 3E2 ground state (a1e3), the values of μeff are about 6% higher for the diluted sample (Figure 2 and see Supporting Information for details). The deviation of the χ−1 vs T plot from Curie behavior is attributed to the Jahn−Teller effect,19c,20 which is altered by the diamagnetic host in the inclusion compound. Nevertheless, as

Figure 1. χ−1 and μeff vs T plots for (Cp*2Yb)2(Cp2V) and Cp2V recorded at 5−300 K and 5 kG.

Figure 2. χ−1 and μeff vs T plots for (Cp*2Yb)2(Cp2Cr) and Cp2Cr recorded at 5−300 K and 5 kG.

in the case of (Cp*2Yb)2(Cp2V), the ytterbocene does not change the magnetic ground state of the d-metallocene. The cobalt derivative is similar; (Cp*2Yb)2(Cp2Co) follows Curie−Weiss behavior over the temperature range of 5 to 130 K, with μeff = 1.94 μB and θ = −4 K (Figure 3). This compares well with a sample of pure Cp2Co, 2E ground state, which also follows Curie−Weiss behavior from 20 to 300 K with μeff = 1.84 μB and θ = −8 K.19 Pure Cp2Co shows substantial curvature in the χ−1 vs T plot at temperatures below 20 K, which Kö nig and co-workers attribute to the thermal depopulation of the excited state,19c but this curvature is not apparent in the χ−1 vs T plot of (Cp*2Yb)2(Cp2Co). As in the other two examples described above, Cp*2Yb appears to act as a magnetic diluent, magnetically isolating the 3d-metallocenes, but does not change their spin states. The similarity of the infrared spectra for the compounds described in this section implies that they have similar structures. However, the cobalt metallocene is perhaps the most interesting one to study by X-ray crystallography since it has a 2E ground state and therefore is subject to a Jahn−Teller distortion. It is conceivable that the structure will show the effect of the lattice, Cp*2Yb, on the distortions of the Cp2Co, which have been thoroughly studied by Ammeter.20 In B

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Figure 4. ORTEP of (Cp*2Yb)2(Cp2Co) (30% probability ellipsoids). Selected bond distances (Å): Yb−C1 2.721(5), Yb−C2 2.685(5), Yb− C3 2.660(4), Yb−C4 2.634(5), Yb−C5 2.685(5), Yb−C6 2.642(4), Yb−C7 2.639(5), Yb−C8 2.654(4), Yb−C9 2.644(4), Yb−C10 2.650(4), Yb−C(Cp*) av 2.67 ± 0.02; Yb−Cpcent,1 2.395; Yb−Cpcent,2 2.356; Co−C21 2.060(6), Co−C22 2.095(6), Co−C23 2.039(5), Co−C24 2.095(6), Co−C25 2.075(7), Co−Cpcent 1.719, C21−C22 1.384(10), C23−C24 1.359(9), C24−C25 1.314(11), Yb···C22 2.914(6), Yb···C21 3.388(8).

Figure 3. χ−1 and μeff vs T plots for (Cp*2Yb)2(Cp2Co) and Cp2Co recorded at 5−130 K and 5 kG.

addition, Cp2Co has been investigated in the solid state by Xray crystallography21 and in the gas phase by electron diffraction,22 so comparisons between the structure in these phases could be meaningful. (Cp*2Yb)2(Cp2Co) crystallizes in the tetragonal crystal system with the space group I41/a. The data were collected at −110 °C, the heavy atoms were refined anisotropically, and the hydrogen atoms were placed in their idealized positions using a riding model, but not refined. The crystal data are shown in Table 1.

defined by the other Cp*-ring carbons (C6−C10 and C21− C25) is 30°. These intersection angles would be about 71° if the Cp centroids of the cobaltocene approached the ytterbocene fragment along the C2-axis. The off-axis approach means that the Yb···C(Cp2Co) contact distances are unequal; the Yb···C22 of 2.914(6) Å is substantially shorter than the Yb···C23 and Yb···C21 distances of 3.870(6) and 3.388(8) Å, respectively. The shortest Yb···C contact distance is ca. 0.05 Å longer than the Yb···C distance in the carbon adducts,13 but ca. 0.03 Å shorter than the shortest intermolecular Yb···C(Cp*) distance in the polymeric modification of base-free Cp*2Yb.12 The geometry of the Cp*2Yb fragment is essentially not perturbed relative to that found in the polymeric modification of Cp*2Yb (black in color).12 In addition, the bond lengths for Cp2Co are listed in order to show that the geometry of Cp2Co is not greatly changed when it is cocrystallized with Cp*2Yb. Table 2 compares the Co−C distances to the ones obtained for Cp2Co determined by X-ray crystallography at 100 K21 and gasphase electron diffraction (GED).23

Table 1. Crystallographic Data empirical formula fw temp (K) wavelength λ (Å) cryst syst space group a [Å] b [Å] c [Å] α [deg] β [deg] γ [deg] volume [Å3] Z indep reflns Goodness of fit on F2 ρcalcd [g cm−3] μ [mm−1] R(Fo) [I > 2σ(I)] Rw(Fo2) Δρ [e Å−3]

(Cp*2Yb)2(Cp2Co)

Cp*MnCp

C50H70CoYb2 1076.07 163(2) 0.710 73 tetragonal I41/a 21.826(2) 21.286(2) 19.703(3) 90 90 90 8927.3(18) 8 2906 1.114 1.601 4.555 0.0214 0.0518 0.309/−0.682

C15H20Mn 255.25 183(2) 0.710 73 triclinic P1̅ 7.865(2) 8.2036(15) 12.163(2) 101.645(2) 96.985(18) 118.493(18) 653.5(2) 2 1696 1.088 1.297 0.981 0.0350 0.0912 0.239/−0.536

Table 2. Comparison of the Co−C Distances Cp2Co (100 K)a Co−C1 Co−C2 Co−C3 Co−C4 Co−C5 Co−C(av) a

2.099(2) 2.101(2) 2.079(2) 2.095(3) 2.112(2) 2.097 ± 0.012

Cp2Co (GED)b

(Cp*2Yb)2(Cp2Co)

2.119(3)

2.060(6) 2.095(6) 2.039(5) 2.095(6) 2.075(7) 2.073 ± 0.024

Ref 21. bRef 23. cThis work.

Nickelocene, Cp2Ni, behaves differently from the metallocenes mentioned above. Stirring Cp2Ni with Cp*2Yb in a 1:1 molar ratio in toluene for 0.5 h at 25 °C gives a light green precipitate and a green solution from which the heteroleptic metallocene Cp*NiCp is isolated in 80% yield, eq 1, M = Ni:

As shown in Figure 4, the Cp2Co metallocene is sandwiched between two Cp*2Yb molecules. Selected bond distances are listed in the figure caption. The rings of Cp2Co approach the ytterbium atoms oblique to the C2-axis of the Cp*2Yb fragment. The intersection of the plane defined by the Cp*ring carbon atoms (C1−C5) and that defined by the Cp-ring carbons (C21−C25) is 112°. The intersection of the plane

Cp* 2Yb + Cp2 M → Cp*MCp + Cp*YbCp↓

(1)

This heteroleptic metallocene is a known compound prepared in a straightforward synthesis by ligand substitution from either Cp*NiBr(PPh3)9 or Cp*Ni(acac).3a Thus, the C

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the way these molecules pack into their respective crystal lattices. The homoleptic metallocenes pack more efficiently than do the heteroleptic ones; see below. Electronic Structure. The electronic structure of the heteroleptic nickelocene is the same as its homoleptic analogues, viz., 3A2 (C5v, e24 a12 e12). The χ−1 vs T plot is not linear in the temperature range from 5 to 300 K (Figure 5); the

exchange reaction shown in eq 1 is not an improved synthesis. The green, toluene-insoluble residue is Cp*YbCp; see later. The Cp*/Cp ring exchange in the nickelocene suggests a possible synthesis for Cp*MnCp. The products that are formed in the reaction of Cp*2Yb and Cp2Mn (0.12 g) in a molar ratio of 1:1 in toluene at 25 °C are studied in considerable detail. Mixing these two metallocenes gives a green powder and an orange-red solution that are separated by filtration. Evaporation of the filtrate to dryness and sublimation of the residue gives Cp*2Mn (0.05 g) and Cp*MnCp (0.08 g), identified by mass spectroscopy. Decamethylmanganocene, Cp*2Mn, is a known metallocene,24 but pentamethylmanganocene is not known; its full characterization is described below. The green, tolueneinsoluble residue gives a purple solution in tetrahydrofuran, which yields a purple solid when evaporated to dryness. Addition of toluene to the purple solid yields a red solution and an orange solid. Separation of the red solution by filtration from the orange solid and evaporating the filtrate to dryness yields a green solid (0.13 g), which yields red crystals of Cp*YbCp(thf)2 upon crystallization from tetrahydrofuran. This is an unknown ytterbocene, and a partial characterization is given in the Experimental Section. The orange residue (0.1 g) crystallizes from tetrahydrofuran and is identified as Cp2Yb(thf).25 Thus, for Cp2Mn, the 1:1 reaction gives all possible products as shown in eq 2, where the product coefficients are approximate values.

Figure 5. χ−1 and μeff vs T plots for Cp*NiCp recorded at 5−300 K and 5 and 40 kG.

2Cp* 2Yb + 2Cp2 Mn

nonlinearity is ascribed to the population of the first excited state as observed in the homoleptic metallocenes. However, it obeys the Curie−Weiss law at temperatures greater than 25 K with μeff = 3.05 μB (θ = −48 K) and μeff = 3.07 μB (θ = −49 K) at 5 and 40 kG, respectively (Figure 5). At temperatures below 25 K, the curve is independent of temperature, a behavior that was observed for Cp2Ni and Cp*2Ni and ascribed to a large zero-field splitting (D). Using the procedure developed by Prins,26 the D value of Cp*NiCp is determined to be +36 cm−1. The D value in Cp2Ni and Cp*2Ni is +34 and +31 cm−1, respectively.24a,27 The 1H NMR spectrum recorded at 20 °C in toluene-d8 consists of two broad resonance signals at δ 230 (ν1/2 = 350 Hz) and δ −208 (ν1/2 = 430 Hz) due to Me5C5 and C5H5 hydrogens, respectively. The resonance signals follow Curie−Weiss behavior since a plot of δ vs T−1 is linear for both resonance signals from −90 to +100 °C (see Supporting Information for details). The chemical shifts of the heteroleptic compound are close but distinct from those of the homoleptic compounds from which it was derived, Cp*2Ni, δ 235, and Cp2Ni,28 δ −245 at 20 °C. The heteroleptic cobaltocene has a 2E1 (C5v, e24 a12 e11) ground state, the same as in the homoleptic compounds.24a,29 A plot of χ−1 vs T is linear from 5 to 300 K and μeff = 1.78 μB (θ = −7.8 K) and μeff = 1.77 μB (θ = −7.3 K) at 5 and 40 kG applied field, respectively (Figure 6). The EPR spectrum is not observed in a methylcyclohexane solution at 25 °C, but a spectrum is observed in a frozen solution at 4.5 K (see Supporting Information for details). The signal is broad, but the axial g-values are observed at g⊥ = 1.946 and g∥ = 1.818 and A⊥ = 163.5 G, and the hyperfine coupling on g∥ cannot be resolved; thus giso = 1.861. The g-values for Cp*2Co and Cp2Co in various host lattices have similar values.24a,29 No 1H NMR spectrum is observed at 30 °C. The electronic structure of pentamethylmanganocene, Cp*MnCp, is perhaps the most interesting, since Cp*2Mn is

→ Cp*MnCp + Cp* 2Mn + Cp*YbCp ↓ +Cp2Yb↓ (2)

However, using the 1:2 stoichiometry of Cp*2Yb and Cp2Mn gives Cp*MnCp as the only identified product from the toluene mother liquor, isolated in 56%. The latter reaction constitutes a cleaner synthesis of pentamethylmanganocene; indeed the ytterbocene ring exchange reaction is the only known synthesis of this metallocene. The light green precipitate that forms in the reaction shown in eq 1, M = Mn, and the red crystals formed from the THF extract in the reaction illustrated in eq 2 are base-free Cp*YbCp and Cp*YbCp(thf) 2, respectively. The most satisfactory synthesis of Cp*MnCp employs the stoichiometry indicated in eq 1. The pale green solid is insoluble in pentane, toluene, and diethyl ether, but it dissolves in THF, forming a red-purple solution from which Cp*YbCp(thf)2 crystallizes on cooling (see Experimental Section for details). It is noteworthy, in the context of ring exchange in the firstrow metallocenes, that the 2:1 complexes (Cp*2Yb)2(Cp2M) are isolated by crystallization from toluene after stirring for a short time, ca. 30 min. If the metallocenes of V, Cr, or Co are stirred for longer periods of time, on the order of hours to weeks, green precipitates slowly form, presumably due to the formation of Cp*YbCp and therefore ring exchange. However, the ferrocene, M = Fe, is inert under these conditions. Pentamethylcyclopentadienylmetallocenes of Mn, Co, and Ni. Physical Properties. The pentamethylmetallocenes of Mn, Co, and Ni are soluble in aliphatic hydrocarbons. They sublime at room temperature under diffusion pump vacuum; thus their volatility is similar to Cp2M17 and higher than Cp*2M.24a,b This trend extends to their melting points of 110 ± 2 °C; the heteroleptic compounds melt about 60 °C lower that their Cp2M analogues and about 200 °C lower than their Cp*2M analogues.17,24a,b This presumably is a reflection of D

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Figure 6. χ−1 and μeff vs T plots for Cp*CoCp recorded at 5−300 K and 5 and 40 kG.

Figure 8. X-band EPR spectrum of Cp*MnCp recorded at 2 K (9.21963 GHz).

low-spin with an electronic ground state of 2E2 (e23a12).24b In contrast, Cp2Mn is high-spin 6A1 (e22a11e12) above temperatures of 140 K. Below this temperature the spins couple antiferromagnetically.30 A plot of χ−1 vs T for Cp*MnCp is linear from 5 to 300 K with μeff = 2.35 μB (θ = −7.86 K) (Figure 7). Thus, the electronic structure of Cp*MnCp is

and 4.42 in [Cp*FeCp][PF6],34 respectively. The difference between the isoelectronic neutral manganocene and the cationic ferrocenium metallocene is traced to the orbital reduction factor k′ and the distortion parameter δ′, which is related to the extent of distortion from axial symmetry (Table 3).35 The slightly larger value of δ′ obtained from the EPR spectrum of Cp*MnCp relative to that for [Cp*FeCp]+ indicates that the former is a more “plastic” structure. The orbital reduction factor k′ is a measure of the degree of the mixing between the SOMO of 3dx2−y2 parentage with the empty e2*-ligand-based orbital; when k′ is 1, the unpaired electron is localized on the metal, and when k′ is 0, it is localized on the ligand and therefore a measure of the degree to which the electron is shared in the molecular orbital, i.e., covalence. The relative values of k′ for Cp*MnCp and [Cp*FeCp]+ suggest that the larger effective nuclear charge on the cationic ferrocenium increases the energy between the metal 3dx2−y2 orbital and the ligand e2*-orbital, resulting is less charge transfer in the cationic metallocene. The 1H NMR spectrum at 20 °C in C6D6 consists of two resonance signals at δ −2.00 (ν1/2 = 570 Hz) and δ −4.88 (ν1/2 = 310 Hz) due to C5Me5 and C5H5, respectively. These resonance signals are close to those in the low-spin homoleptic metallocenes; for (C5Me5)2Mn, δH −4.7 (ν1/2 = 320 Hz at 320 K)24a and for (C5D5)2Mn, δD −0.79 (ν1/2 = 82 Hz at 199 K).36 Crystal Structures. In the solid state, the pentamethylmetallocenes crystallize in the space group P1̅ (C5v), Table 4. Thus, all three heteroleptic metallocenes are isostructural and isomorphous with Cp*FeCp, whose crystal structure was reported several years ago.6b,37 An ORTEP of Cp*MnCp is shown in Figure 9. The rings in Cp*MnCp adopt an eclipsed conformation, and a packing diagram for Cp*MnCp is shown in Figure 10. Table 4 lists important bond distances for Cp*MCp (M = Mn, Fe, Co, Ni). The carbon atoms in these metallocenes do not suffer the positional disorder found in Cp2M compounds.39 The diffraction data are therefore of high quality, as shown by the well-described anisotropic thermal parameters for the nonhydrogen atoms (see Supporting Information for details). The high quality of the X-ray diffraction data for all four metallocenes affords us an ideal opportunity to search for any structural irregularities in the molecular structures as the electron count increases from 17 to 20. Examination of Table 5

Figure 7. χ−1 and μeff vs T plots for Cp*MnCp recorded at 5−350 K and 5 kG.

identical to that of Cp*2Mn in the solid state. The magnetic moments of these two low-spin metallocenes are higher than the spin-only moment for Cp*2Mn μeff = 2.18 μB (4−117 K) due to the orbital contribution to these orbitally degenerate ground states, as shown by the ligand field calculations and spectroscopy.24b,29b,31 The EPR spectrum of a powdered sample at 2.0 K consists of two broadened resonance signals with g⊥ = 1.702 and g∥ = 3.347 (Figure 8). These values are similar to those observed at low temperatures for Cp*2Mn in various host lattices (Table 3)24b and Cp2Mn in Cp2Fe at 4.3 K.32 The μeff of Cp*MnCp (calculated from the EPR spectrum at 3.0 K) of 2.23 μB agrees with the μeff (from SQUID) of 2.06 μB at 5.0 K. The isoelectronic ferrocenium compound [Cp*FeCp]+ (recorded as a powder at 4 K) shows a significantly larger g-value anisotropy with g⊥ and g∥ of 1.24 and 4.36 in [Cp*FeCp][BF4]33 and 1.28 E

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Organometallics Table 3. Comparison of the EPR Spectroscopic Data for Cp*MnCp and [Cp*FeCp]+ Obtained in the Solid State complex a

Cp*MnCp (powder, T = 2 K) Cp*2Mn (in toluene glass, T = 12 K)b Cp*2Mn (in methylcyclohexane glass, T = 12 K)b Cp*2Mn (in Cp*2Fe, T = 12 K)b [Cp*FeCp][BF4] (powder, T = 4 K)c [Cp*FeCp][PF6] (powder, T = 4 K)d [Cp*2Fe][PF6] (powder, T = 12 K)e a

g∥

g⊥

k′

δ′/cm−1

ξ/cm−1

ΔE/cm−1

3.347 3.26 3.36 3.51 4.36 4.42 4.433

1.702 1.68 1.42 1.17 1.24 1.28 1.350

0.64 0.58 0.48 0.47 0.79 0.75 0.82

317 274 149 102 266 241 305

196 177 147 147 319 305 334

745 652 417 350 830 776 905

This work. bRef 24b. cRef 33. dRef 34. eRef 43.

average M−C(Me5C5) distance is shorter than the average M−C(Cp) distance by 0.007 to 0.014 Å. This trend is also observed in the homoleptic compounds, presumably reflecting the stronger interaction between the metal and the C5Me5 ligand. In contrast to the distortions found in Cp*2Mn in the solid state, ascribed to a static Jahn−Teller distortion,24c Cp*MnCp is not distorted. The only hint of “something unusual” comes from the inspection of the data in Table 4. The five Mn− C(Cp*) distances can be separated into two sets, two at 2.093(3) and 2.097(3) Å that average to 2.095 ± 0.002 Å and three at 2.114(3), 2.118(3), and 2.110(3) Å that average to 2.116 ± 0.003 Å, indicating a slight slip of the CpMn fragment off the C5-axis of the Mn−Cp* ring centroid. The Co−C(Cp*) distances also show a similar pattern; three Co−C(Cp*) distances of 2.082(2), 2.091(2), and 2.086(2) Å that average to 2.085 ± 0.004 Å and two of 2.111(2) Å are significantly (>3σ) different. The Co−C(Cp) distances also separate into a three short, two long pattern, and the average distances are 2.095 ± 0.001 and 2.114 ± 0.003 Å, respectively. In contrast the five M−C(Cp*) distances in Cp*FeCp and Cp*NiCp are identical within 3σ. While the pattern of distortions is small, it is anticipated for the Jahn−Teller ions, Mn2+ and Co2+, shown to undergo dynamic Jahn−Teller behavior by the EPR studies of Ammeter.20

Table 4. Summary of Crystal Data for Cp*MCp (M = Mn, Fe, Co, Ni) M

Mna

Feb

Coc

Nic

formula fw (g/mol) space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z temp (°C)

C15H20Mn 255.26 P1̅ 7.865(2) 8.204(2) 12.163(2) 101.65(2) 96.99(2) 118.49(2) 653.5(6) 2 −90

C15H20Fe 256.17 P1̅ 7.720(1) 8.178(1) 12.143(1) 101.19(1) 95.33(1) 118.21(1) 639.0(1) 2 −120

C15H20Co 259.26 P1̅ 7.713(2) 8.197(2) 12.210(2) 101.58(2) 96.94(2) 118.18(2) 645.4(3) 2 −114

C15H20Ni 259.04 P1̅ 7.860(3) 8.204(2) 12.285(7) 101.19(2) 97.79(3) 118.57(2) 658.1(9) 2 −96

a

This work. bThe unit cell parameters for the reported structure of Cp*FeCp6b,37 were transformed to match the cell setting of the other three structures. cSee CCDC-1405852 and CCDC-1405853 for Cp*CoCp and Cp*NiCp, respectively.

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DISCUSSION The successful synthesis of Cp*MnCp is related to the kinetic lability of the Cp ligands in Cp2Mn and the insolubility of Cp2Yb and Cp*YbCp in hydrocarbon solvents.42 Manganocene is high-spin in solution,30c,36 and the HOMO contains two electrons in the doubly degenerate Mn−Cp antibonding molecular orbitals, (e1g*)2, as does nickelocene. The qualitative rates of Cp* for Cp ligand exchange between Cp*2Yb and Cp2M are in the order Cp2Mn ∼ Cp2Ni > Cp2Co ∼ Cp2Cr ∼ Cp2V > Cp2Fe. This order qualitatively follows from the number of electrons in the (e1g*) molecular orbital. The relative rates of C5H5 for C5D5 exchange in the reaction between (C5H5)2M and LiC5D5 in tetrahydrofuran at 25 °C are Cp2Mn ∼ Cp2Cr > Cp2Ni > Cp2V ≫ Cp2Co ≫ Cp2Fe,44 which roughly correlates with the qualitative observations of Cp*2Yb with Cp2Mn, Cp2Ni, and Cp2Fe. The rate law for the C5H5 for C5D5 exchange between (C5H5)2Mn and (C5D5)2Ni in benzene-d6 is second order, first order in each metallocene, and the activation energies are ΔH⧧ = 15 kcal mol−1, ΔS⧧ = −30 cal (mol·K)−1, and ΔG⧧(25 °C) = 24 kcal mol−1.45 A bimolecular transition state in which the different (C5H5)Mn and (C5D5)Ni groups are bridged by the C5H5 and C5D5 ligands is a reasonable postulate. A similar bimolecular process is likely for the exchange between the kinetically labile metallocenes Cp2Mn/Ni and Cp*2Yb; the solid-state crystal

Figure 9. ORTEP of Cp*MnCp2 (30% probability ellipsoids). The non-hydrogen atoms are refined anisotropically, and the hydrogen atoms are placed in calculated positions using a riding model.

shows no evidence of any significant distortions of any kind. The M−C(av) bond length decreases by ca. 0.06 Å from Mn (e23a12) to Fe (e24a12). This is expected since the radius of the low-spin Fe(II) is ca. 0.06 Å smaller than that of low-spin Mn(II) in six coordination.40 The M−C(av) bond distances get longer on proceeding from Fe to Co to Ni by 0.05 and 0.06 Å, respectively. The reason for this lengthening, which compensates for the natural contraction of the metal radii across the 3dseries, is that the electrons are added to e1-symmetry orbitals that are metal to ring antibonding. This trend is also observed in the homoleptic compounds.24c,41 In the four structures, the F

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Figure 10. Packing diagrams for Cp*MnCp (183 K, triclinic crystal lattice, space group P1̅) and Cp*2Mn (125 K, orthorhombic crystal lattice, space group Cmca38).

Table 5. Selected Bond Distances for Cp*MCp (M = Mn, Fe, Co, Ni) Cp* M−C1 M−C2 M−C3 M−C4 M−C5 M−Cp*(av) M−Cp*(cent) C1−C2 C2−C3 C3−C4 C4−C5 C5−C1 C−C(av) Cp M−C11 M−C12 M−C13 M−C14 M−C15 M−Cp(av) M−Cp*(cent) C11−C12 C12−C13 C13−C14 C14−C15 C15−C11 C−C(av) a

Cp*MnCpa

Cp*FeCpb

Cp*CoCpc

Cp*NiCpc

2.093(3) 2.097(3) 2.114(3) 2.118(3) 2.110(3) 2.106 ± 0.011 1.725 1.422(5) 1.422(4) 1.418(4) 1.419(4) 1.429(4) 1.422 ± 0.004

2.039(3) 2.039(2) 2.038(4) 2.039(4) 2.040(3) 2.039 ± 0.001 1.642 1.427(4) 1.425(5) 1.423(4) 1.418(4) 1.411(4) 1.421 ± 0.006

2.080(2) 2.111(2) 2.091(2) 2.086(2) 2.111(2) 2.096 ± 0.014 1.711 1.423(3) 1.419(3) 1.434(4) 1.411(4) 1.424(3) 1.422 ± 0.008

2.161(4) 2.159(4) 2.164(4) 2.160(4) 2.164(4) 2.162 ± 0.002 1.795 1.402(4) 1.407(4) 1.397(4) 1.416(4) 1.402(4) 1.405 ± 0.007

2.105(3) 2.118(3) 2.128(3) 2.132(3) 2.122(3) 2.121 ± 0.010 1.751 1.417(5) 1.408(5) 1.399(4) 1.406(4) 1.407(4) 1.407 ± 0.006

2.054(4) 2.040(4) 2.040(3) 2.045(3) 2.050(3) 2.046 ± 0.006 1.658 1.410(4) 1.407(5) 1.416(5) 1.404(4) 1.406(4) 1.409 ± 0.005

2.096(2) 2.111(2) 2.095(2) 2.094(2) 2.117(2) 2.103 ± 0.011 1.730 1.402(4) 1.397(4) 1.416(4) 1.402(4) 1.407(4) 1.405 ± 0.007

2.183(4) 2.178(4) 2.167(4) 2.169(4) 2.181(4) 2.176 ± 0.007 1.821 1.392(6) 1.397(6) 1.398(6) 1.402(6) 1.404(6) 1.399 ± 0.005

This work. bRef 6b. cSee CCDC-1405852 and CCDC-1405853 for Cp*CoCp and Cp*NiCp, respectively.

to their melting points show that the reported synthetic reactions that result in mixtures of hetero- and homoleptic metallocenes are the result of exchange reactions between the reactants rather than dismutation of the heteroleptic products. For example, CpMnI(thf)2 is stable to dismutation, but when LiCp* is added, only the homoleptic maganocenes are isolated, which implies that Cp* for Cp exchange and Cp* for iodide exchange rates are similar.47

structure of the inclusion complex (Figure 4) indicates how a coordination complex can develop into a bimolecular transition state. Although the rates of self-exchange between the ytterbocenes have not been measured, it is noteworthy that the rates of water exchange in Mn2+(aq) and Yb3+(aq) are 2 × 107 and 5 × 107 s−1, respectively; the rate for water exchange for Yb2+(aq) is not known, but it is likely to be faster than that for Yb3+(aq).46 In addition to the relative rates, the insolubility of Cp2Yb and Cp*YbCp in hydrocarbon solvents removes the ytterbocenes from the reaction pathway. The isolation of Cp*MCp, M = Mn, Fe, Co, Ni, and the demonstration that they do not dismutate at temperatures up

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SUMMARY AND CONCLUSIONS The successful synthesis and characterization of low-spin pentamethylmanganocene, along with characterization of the G

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Synthesis of (Cp*2Yb)2(Cp2Co). Cp*2Yb (0.28 g, 0.63 mmol) was dissolved in toluene (20 mL) and added to a stirred solution of Cp2Co (0.06 g, 0.32 mmol) in toluene (10 mL). The solution remained dark orange. The volume was reduced to 3 mL, and 5 mL of pentane was added to the solution. The mixture was then cooled to −25 °C, producing brown blocks. The yield was 0.26 g (77%). The crystals gradually turned bright green as they were heated above ca. 150 °C, but they did not melt below 300 °C. IR (Nujol): 3107sh, 3067m, 2725w, 2035vwbr, 1814vw, 1656wbr, 1448s, 1419sh, 1404w, 1336sh, 1214vw, 1164sh, 1153w, 1103m, 1061sh, 1046m, 1020m, 1007s, 994sh, 911w, 887vw, 832m, 802vs, 780sh, 723w, 695w, 664vw, 633vwbr, 623vwbr, 590vw, 522vw, 428wbr, 367mbr, 326sh, 308sh, 282vs cm−1. Anal. Calcd for C50H70Yb2Co: C, 55.8; H, 6.57. Found: C, 55.5; H, 6.59. 1H NMR (C6D6, 30 °C): δ 1.93 (s, 60H, C5Me5), −49.8 (10H, ν1/2 = 130 Hz, C5H5) ppm. 1H NMR of Cp2Co (C6D6, 30 °C): δ −50.1 (ν1/2 = 145 Hz, C5H5) ppm. The product was independent of the reaction stoichiometry; a 1:1 reaction gave the same species upon crystallization. It was found that if the solution was stirred for more than 10−12 h, a green precipitate began to appear, indicating slow ring exchange between the Yb and Co centers. Reaction of Cp*2Yb with Cp2Mn. 1:1 Stoichiometry. Cp*2Yb (0.29 g, 0.65 mmol) was dissolved in toluene (15 mL) and added to a solution of Cp2Mn (0.12 g, 0.65 mmol) in toluene (10 mL) with stirring. The solution immediately precipitated a bright green powder. The mixture was allowed to stir for 24 h. The green powder was isolated by filtration. The solvent was removed from the filtrate under reduced pressure, and the resulting orange residue was heated to 100 °C under vacuum. Some dark orange crystals (ca. 0.08 g) sublimed out of the residue, followed more slowly by yellow-orange crystals (ca. 0.05 g). The dark orange crystals melted at 82−84 °C. IR (Nujol): 3090m, 2730m, 2680w, 2671vw, 2058vw, 1733w, 1671w, 1618m, 1602sh, 1522m, 1424w, 1414w, 1309vw, 1167vw, 1156vw, 1110m, 1106m, 1067w, 1025m, 1005vs, 895vw, 850vw, 801s, 774vs, 723sh, 710sh, 666vw, 629vw, 586vw, 540s, 483s, 442w, 414w, 278m cm−1. The EI mass spectrum showed Cp*2Mn at 325, as well as a small-intensity ion for Cp*MnCp at 255. The mass spectrum of the yellow-orange complex showed only the fragmentation pattern for Cp*2Mn. The green solid from the reaction was dissolved in THF, giving a purple solution. The solution was filtered and the solvent was removed under reduced pressure, leaving a red-purple residue. The residue was extracted with toluene, leaving behind an orange solid (ca. 0.10 g). The red toluene solution was reduced in volume to 3 mL and cooled to −25 °C, precipitating a green solid from the solution (ca. 0.13 g). Both solids were recrystallized from THF. The orange solid recrystallized from THF as purple blocks that lost solvent and turned orange under reduced pressure. These orange blocks did not melt below 320 °C and are identified as Cp2Yb(thf)x (where x = 0.75− 1.00). IR (Nujol): 3088m, 3072m, 2707vw, 1739vwbr, 1700vwbr, 1639wbr, 1573wbr, 1465s, 1367w, 1340w, 1312vw, 1294vw, 1177w, 1033vs, 1008vs, 928w, 878s, 844sh, 789s, 770vs, 748vsbr, 670w, 485wbr, 396mbr cm−1. Anal. Calcd for C13H16O0.75Yb: C, 43.7; H, 4.51. Found: C, 43.8; H, 4.74. 1H NMR (thf-d8, 30 °C): δ 5.68 (s, 10 H, C5H5), 3.58 (m, 4H, thf α-CH), 1.73 (m, 4H, thf β-CH). The green compound was recrystallized from THF as red crystals. These crystals turned green when heated in toluene or when heated above 120 °C (did not melt below 320 °C) and are identified as Cp*YbCp(thf)x (where x = 1.5−2.0). IR (Nujol): 3087w, 3072w, 3056w, 2720w, 1581vw, 1449s, 1340w, 1311vw, 1294vw, 1173w, 1034vs, 1009s, 917m, 880s, 820sh, 799w, 774mbr, 742vs, 723sh, 670w, 589vw, 266m cm−1. Anal. Calcd for C21H32O1.5Yb: C, 52.4; H, 6.70. Found: C, 51.9; H, 6.84. 1H NMR (C7D8, 30 °C): δ 6.02 (s, 5 H, C5H5), 3.44 (m, 8H, thf α-CH), 2.15 (s, 15H, C5Me5), 1.40 (m, 8H, thf β-CH). Cp*2Yb:2Cp2Mn Stoichiometry. A solution of Cp*2Yb (0.26 g, 0.59 mmol) in toluene (10 mL) was added to a solution of Cp2Mn (0.22 g, 1.2 mmol) in toluene (5 mL). Upon mixing, an orange-brown precipitate formed. After stirring at room temperature for 6 h, the toluene was removed from the suspension under reduced pressure. The residue was extracted with pentane (15 mL) and filtered. The filtrate was concentrated to ca. 5 mL and cooled to −80 °C, affording red crystals (0.17 g, 0.67 mmol, 56%). Mp: 108−111 °C. IR (Nujol):

cobaltocene and nickelocene derivatives, show that metallocenes from manganese to nickel are stable to dismutation to the homoleptic metallocenes. The use of Cp*2Yb as a Cp*transfer reagent for pentamethylmanganocene suggests that this f-block metal metallocene may find a useful role in the synthesis of additional organometallic compounds that are currently unknown.

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EXPERIMENTAL SECTION

General Comments. All reactions and product manipulations were carried out under dry nitrogen using standard Schlenk and drybox techniques. Dry, oxygen-free solvents were employed throughout. The elemental analyses were performed by the analytical facility at the University of California at Berkeley. Magnetic measurements were conducted in a 7 T Quantum Design MPMS magnetometer utilizing a superconducting quantum interference device (SQUID). Measurements were performed in either KEL-F buckets or quartz tubes. For the investigations using quartz tubes between 10 and 25 mg of sample was sealed in evacuated quartz tubes held in place with ∼5 mg of quartz wool. This method provided a very small and reliable container correction, typically of about −2 × 10−5 emu/mol. The data were also corrected for the overall diamagnetism of the molecule using Pascal constants.48 For a more detailed description see ref 49. Synthesis of (Cp*2Yb)2(Cp2V). Cp*2Yb (0.73 g, 1.7 mmol) in toluene (25 mL) was added to a solution of Cp2V (0.15 g, 0.83 mmol) in toluene (25 mL). After mixing, the brown solution was concentrated to ca. 30 mL and pentane (10 mL) was added. The solution was filtered and the filtrate was cooled to −20 °C, producing dark brown prisms. Yield: 0.48 g (0.45 mmol, 54%). The crystals turned pale at ca. 150 °C, and the solid melted at 202−206 °C. IR (Nujol): 3065m, 2725m, 1805w, 1653w, 1348vw, 1263w, 1162w, 1105m, 1052w, 1001m, 948vw, 903vw, 888vw, 840w, 812m, 790s, 749vw, 730w, 696vw, 629vw, 592w, 428w, 387w, 365m, 327vw, 283s cm−1. Magnetic susceptibility (5−300 K): 5 kG, μeff = 3.83 μB, θ = −2.53 K. For Cp2V (5−300 K): 5 kG, μeff = 3.85 μB, θ = −7.26 K. Anal. Calcd for C50H70Yb2V: C, 56.2; H, 6.61. Found: C, 55.9; H, 6.85. If the product was allowed to sit in solution for extended periods of time, a green solid appeared. The IR spectrum of this product indicated that it was a ring-exchanged product similar to that observed for other reactions of Cp*2Yb with first-row metallocenes (except Cp2Fe). Synthesis of (Cp*2Yb)2(Cp2Cr). A solution of Cp*2Yb (0.44 g, 0.99 mmol) in toluene (10 mL) was added to a solution of Cp2Cr (0.09 g, 0.49 mmol) in toluene (10 mL). After mixing, the brown solution was concentrated to ca. 5 mL. The solution was filtered and pentane (5 mL) was added to the filtrate. Cooling the solution to −20 °C produced red-brown crystals. Yield: 0.24 g (0.22 mmol, 45%). The crystals took on a greenish coloration above 185 °C, but did not melt below 330 °C. IR (Nujol): 3062s, 2725m, 2035vw, 1652w, 1445s, 1420sh, 1262vw, 1163w, 1090m, 1021w, 985m, 898vw, 831w, 798m, 770m, 725w, 590w, 363w, 284s cm−1. Magnetic susceptibility (5 kG): 5−40 K, μeff = 3.09 μB, θ = −1.01 K; 120−300 K, μeff = 3.69 μB, θ = −43 K. Anal. Calcd for C50H70Yb2Cr: C, 56.2; H, 6.60. Found: C, 56.2; H, 6.58. If the product was left in solution for prolonged periods of time, a green precipitate formed presumably due to ring exchange. Synthesis of (Cp*2Yb)2(Cp2Fe). Cp*2Yb (0.22 g, 0.50 mmol) was dissolved in toluene (10 mL) and added to a solution of Cp2Fe (0.09 g, 0.48 mmol) in toluene (10 mL). The volume of the solution was reduced to 3 mL, and 5 mL of pentane was added. The mixture was cooled to −25 °C, producing orange-red blocks. The yield was 0.15 g (58%). Mp: 199−202 °C. IR (Nujol): 3101vw, 3069m, 2723w, 1653vwbr, 1495vs, 1448s, 1418m, 1407m, 1313vw, 1164sh, 1150w, 1103s, 1019vs, 1005s, 908vw, 886sh, 857w, 846sh, 826s, 815s, 752vw, 728w, 693w, 665w, 629vw, 591vw, 496m, 476m, 393brsh, 368mbr, 309sh, 280vs cm−1. Anal. Calcd for C50H70Yb2Fe: C, 56.0; H, 6.59. Found: C, 55.5; H, 6.55. 1H NMR (C6D6, 30 °C): δ 3.99 (s, 10H, C5H5), 1.93 (s, 60H, C5Me5) ppm. 1H NMR of Cp2Fe (C6D6, 30 °C): δ 4.00 (s, C5H5) ppm. H

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(90 °C, 10−2 mm Hg), producing a green, crystalline solid (0.12 g, 0.46 mmol, 80%). Mp: 108−110 °C (lit.9 116 °C). EI-MS: 258 (100,100), 259 (19,17), 260 (40,40), 261 (9,8), 262 (6,6). The IR and mass spectra were identical to those of the previously reported compound.9 The green precipitate was shown to be Cp*YbCp; full details will be published when they are available.

3093s, 2725m, 1739w, 1689w, 1670w, 1596w, 1417m, 1380s, 1257vw, 1155vw, 1105s, 1067m, 1028s, 998s, 800s, 723vw, 585m, 351mbr, 315m cm−1. EPR (powder, 2.0 K): g∥ = 3.347 and g⊥ = 1.702. Magnetic susceptibility (5−300 K): 5 kG, μeff = 2.35 μB, θ = −7.86 K. Anal. Calcd for C15H20Mn: C, 70.6; H, 7.90. Found: C, 70.6; H, 7.93. 1 H NMR (C6D6, 20 °C): −2.00 (s, 15H, C5Me5, ν1/2 = 570 Hz), −4.88 (s, 5H, C5H5, ν1/2 = 310 Hz). Reaction of Cp*2Yb with Cp*2Mn. Cp*2Yb (0.27 g, 0.61 mmol) was dissolved in 20 mL of hexane and added to a solution of Cp*2Mn (0.20 g, 0.61 mmol) in hexane (15 mL), and the mixture was allowed to stir for 14 h. No reaction was apparent. The solution was then filtered, concentrated to 10 mL of volume, and cooled to −25 °C; the two starting materials crystallized separately. Attempted Syntheses of Cp*MnCp. Several attempts were made to prepare Cp*MnCp using more traditional synthetic routes, all of which gave Cp*2Mn. Some attempts included (i) Cp*Mn(acac)(L) was not obtained from Mn(acac)2 or Mn(acac)2(tmeda) using NaCp* or Cp*2Mg; all gave Cp*2Mn. The known half-sandwich, CpMnCl(tmeda), 50 with NaCp* also yielded Cp* 2 Mn. (ii) Mn[N(SiMe3)2]2(thf)51 did not react with Cp*H. (iii) Heating mixtures of Cp*2Mn with Cp2Mn did not yield any ring-exchanged metallocenes. Synthesis of Cp*CoCp. This complex was synthesized according to a published procedure, but only an elemental analysis was reported.3a To a solution of Cp*Co(acac)52 (0.58 g, 2.0 mmol) in THF (70 mL) was added 1.05 mL of a 1.88 M THF solution of NaCp (1.97 mmol). Upon mixing, the solution immediately changed color from deep red to dark brown and a white precipitate formed. After stirring at room temperature for 4 h, volatile materials were completely removed under reduced vacuum and the residue was extracted with pentane (60 mL). The dark brown solution was filtered, and the filtrate was concentrated to ca. 20 mL. Cooling to −80 °C afforded green-black crystals. Concentration of the mother liquor provided an additional crop of crystals, which were combined and sublimed at 30 °C under dynamic vacuum (oil diffusion pump) for a total yield of 0.38 g (1.5 mmol, 74%). Mp: 106−107 °C. IR (Nujol): 3100m, 2721w, 1725m, 1639m, 1548m, 1465s, 1415m, 1355sh, 1260w, 1104m, 1068m, 1026s, 998s, 779vs, 725sh, 581m, 425m, 395w, 300m. No resonance signals were observed in the 1H NMR spectrum in C6D6 at room temperature. EI-MS: 259 (100, 100); 260 (16, 17). Magnetic susceptibility (5−300 K): 5 kG, μeff = 1.76 μB, θ = −7.78 K; 40 kG, μeff = 1.77 μB, θ = −7.34 K. EPR (methylcyclohexane glass, 4.5 K): g⊥ = 1.946, A⊥ = 163.5 G, g∥ = 1.818, A∥ was not observed due to line width. Anal. Calcd for C15H20Co: C, 69.5; H, 7.78. Found: C, 69.8; H, 8.22. Synthesis of Cp*NiCp. Method 1. This complex was synthesized according to a published procedure.3a A solution of Cp2Mg (0.42 g, 2.6 mmol) in THF (70 mL) was added to a solution of Cp*Ni(acac)52 (1.50 g, 5.11 mmol) in THF (100 mL) at 0 °C. Upon mixing, the solution immediately changed color from red to bright green and a white precipitate formed. After stirring at 0 °C for 4 h, volatile materials were completely removed under dynamic pressure and the residue was extracted with pentane (80 mL). The bright green solution was filtered, and the filtrate was concentrated to ca. 50 mL. Cooling to −80 °C afforded bright green plates. Concentration of the mother liquor provided additional crops of crystals, which were combined and sublimed at 35 °C under dynamic vacuum (oil diffusion pump) for a total yield of 1.14 g (4.40 mmol, 86%). Mp: 111−112 °C (lit.9 116 °C). IR (Nujol): 3101m, 2723w, 1734m, 1634m, 1539m, 1466m, 1423s, 1263w, 1067w, 1023s, 1003vs, 869w, 773vs, 731w, 587w, 398sh, 376s. 1H NMR (C6D6, 293 K): δ 230 (s, 15H, ν1/2 = 350 Hz, C5Me5), −208 (5H, ν1/2 = 430 Hz, C5H5) ppm. EI-MS: 258 (100,100), 259 (19,17), 260 (40,40), 261 (9,8), 262 (6,6). Magnetic susceptibility (25−300 K): 5 kG, μeff = 3.05 μB, θ = −48.3 K; 40 kG, μeff = 3.07 μB, θ = −49.3 K. No EPR signal was observed in methylcyclohexane glass (4 K). Method 2. A solution of Cp*2Yb (0.26 g, 0.59 mmol) in toluene (5 mL) was added to a solution of Cp2Ni (0.11 g, 0.58 mmol) in toluene (5 mL). After stirring for 24 h at ambient temperature the precipitate was allowed to settle and filtered. The toluene liquor was removed from the filtrate under reduced pressure, and the residue was sublimed

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00554. χ−1 and μeff vs T plots for (Cp*2Yb)2(Cp2Co) and Cp2Co, δ vs T−1 plot for Cp*NiCp, X-band EPR spectrum of Cp*CoCp, and μeff vs T plots for Cp*MnCp recorded in quartz tubes or KEL-F buckets (PDF) Crystallographic data for [Cp*2 Yb]2(Cp2 Co) and Cp*MnCp (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Director, Office of Science, Office of Basic Energy Sciences (OBES), of the U.S. Department of Energy under Contract No. DE-AC0205CH11231. We thank Fred Hollander (at CHEXRAY, the U.C. Berkeley X-ray diffraction facility) for assistance with the crystallography. C.J.B. thanks the Fannie and John Hertz Foundation for a fellowship and M.E.S. thanks the NSF for a fellowship. We thank Wayne W. Lukens for the EPR spectrum of Cp*MnCp and many discussions. R.A.A. acknowledges the Alexander von Humboldt Foundation for a reinvitation grant within the Humboldt Senior Research Award program.

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REFERENCES

(1) (a) Togni, A.; Haltermann, R. L. Metallocenes: Synthesis, Reactivity and Applications; Wiley-VCH: Weinheim, 1998. (b) Okuda, J. Top. Curr. Chem. 1992, 160, 97−145. (c) Jonas, K. Angew. Chem., Int. Ed. Engl. 1985, 24, 295−311. (d) Jonas, K.; Häselhoff, C.-C.; Goddard, R.; Krüger, C. Inorg. Chim. Acta 1992, 198−200, 533−541. (2) Burkart, J. (Ph.D. Thesis, Ruhr-Universität Bochum, Bochum, 1985) reports that addition of LiCp* to isolated CpMnI(thf)2 results in the formation of the homoleptic manganocenes. (3) (a) Bunel, E. E.; Valle, L.; Manriquez, J. M. Organometallics 1985, 4, 1680−1682. (b) Manríquez, J. M.; Bunel, E. E.; Oelckers, B.; Vásquez, C.; Miller, J. S. Inorg. Synth. 1997, 31, 214−217. (4) Rüsseler, W. Ph.D. Thesis, Ruhr-Universität Bochum, Bochum, 1986. (5) Dawkins, G. M. Chromium-containing complex polymerization catalyst. EP416785A2, 1991. (6) (a) Paciello, R. A.; Manriquez, J. M.; Bercaw, J. E. Organometallics 1990, 9, 260−265. (b) Bildstein, B.; Hradsky, A.; Kopacka, H.; Malleier, R.; Ongania, K.-H. J. Organomet. Chem. 1997, 540, 127−145. (7) Phillips, L.; Lacey, A. R.; Cooper, M. K. J. Chem. Soc., Dalton Trans. 1988, 1383−1391. I

DOI: 10.1021/acs.organomet.6b00554 Organometallics XXXX, XXX, XXX−XXX

Article

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2NμB2 9kT

(31) (a) Walter, M. D.; Sofield, C. D.; Booth, C. H.; Andersen, R. A. Organometallics 2009, 28, 2005−2019. (b) Warren, K. D. Inorg. Chem. 1974, 13, 1317−1324. (32) Ammeter, J. H.; Bucher, R.; Oswald, N. J. Am. Chem. Soc. 1974, 96, 7833−7835. (33) Miller, J. S.; Glatzhofer, D. T.; Vazquez, C.; McLean, R. S.; Calabrese, J. C.; Marshall, W. J.; Raebiger, J. W. Inorg. Chem. 2001, 40, 2058−2064. (34) Rittinger, S.; Buchholz, D.; Delville-Desbois, M. H.; Linares, J.; Varret, F.; Boese, R.; Zsolnai, L.; Huttner, G.; Astruc, D. Organometallics 1992, 11, 1454−1456. (35) (a) Maki, A. H.; Berry, T. E. J. Am. Chem. Soc. 1965, 87, 4437− 4441. (b) Prins, R. Mol. Phys. 1970, 19, 603−620. The relevant parameters shown in Table 3 can be accessed directly from EPR m e a s u r e m e n t s a cc or di n g to t he f ol l ow i n g e qu a t i o n s :

g z = g = k′

and gx = gy = g⊥ =

2 1 + ξ 2 / δ ′2

, where ξ is

the spin−orbit coupling constant and δ′ is the low-symmetry distortion parameter. Other useful parameters extractible from these equations are the orbital reduction factor k′, which is a measure of covalence (k′ = (g∥ − 2)/{2g⊥[(2/g⊥)2 − 1]0.5}), and the energy difference, ΔE, between the dx2−y2- and dxy-orbitals is ΔE = 2(ξ2 + δ′2)0.5. (36) Köhler, F. H.; Schlesinger, B. Inorg. Chem. 1992, 31, 2853− 2859. (37) Zanin, I. E.; Antipin, M. Y.; Struchkov, Y. T.; Kudinov, A. R.; Rybinskaya, M. I. Metalloorg. Khim. 1992, 5, 579−89. (38) Augart, N.; Boese, R.; Schmid, G. Z. Anorg. Allg. Chem. 1991, 595, 27−34. (39) Dunitz, J. D.; Orgel, L. E.; Rich, A. Acta Crystallogr. 1956, 9, 373−375. (40) Shannon, R. Acta Crystallogr., Sect. A: Cryst. Phys., Diffr., Theor. Gen. Crystallogr. 1976, A32, 751−767. (41) Clark, M. M.; Brennessel, W. W.; Holland, P. L. Acta Crystallogr., Sect. E: Struct. Rep. Online 2009, 65, m391. (42) Apostolidis, C.; Deacon, G. B.; Dornberger, E.; Edelmann, F. T.; Kanellakopulos, B.; MacKinnon, P.; Stalke, D. Chem. Commun. 1997, 1047−1048. (43) Duggan, D. M.; Hendrickson, D. N. Inorg. Chem. 1975, 14, 955−970. (44) Switzer, M. E.; Rettig, M. F. J. Chem. Soc., Chem. Commun. 1972, 687−688. (45) Switzer, M. E.; Rettig, M. F. Inorg. Chem. 1974, 13, 1975−1981. (46) (a) Cossey, C.; Merbach, A. E. Pure Appl. Chem. 1988, 60, 1785−1796. (b) Helm, L.; Merbach, A. E. Chem. Rev. 2005, 105, 1923−1960. (47) Burkart, J. Ph.D. Thesis, Ruhr-Universität Bochum, Bochum, 1985. (48) O’Connor, C. J. Prog. Inorg. Chem. 1982, 29, 203−285. (49) Walter, M. D.; Schultz, M.; Andersen, R. A. New J. Chem. 2006, 30, 238−246. (50) Heck, J.; Massa, W.; Weinig, P. Angew. Chem., Int. Ed. Engl. 1984, 23, 722−723. (51) Andersen, R. A.; Faegri, K.; Green, J. C.; Haaland, A.; Lappert, M. F.; Leung, W. P.; Rypdal, K. Inorg. Chem. 1988, 27, 1782−1786. (52) Smith, M. E.; Andersen, R. A. J. Am. Chem. Soc. 1996, 118, 11119−11128.

(g 2 + 2g⊥2), where N is Avogadro’s number, μB

is the Bohr magneton, k is the Boltzmann’s constant, T is the temperature, and g∥ is assumed to have the free electron value (2.0023). The low-temperature (temperature-independent) magnetic susceptibility data were then used to calculate the zero-field splitting 4Nμ

(−ξ / δ ′) 1 + ξ 2 / δ ′2

2

(D, in cm−1) from the following equation: χ = 3DB (g⊥2). (27) Baltzer, P.; Furrer, A.; Hulliger, J.; Stebler, A. Inorg. Chem. 1988, 27, 1543−1548. (28) Rettig, M. F. Spin Distribution in Organometallic Compounds. In NMR of Paramagnetic Molecules: Princliples and Applications; LaMar, G. N.; Horrocks, W. D. J.; Holm, R. H., Eds.; Academic Press: New York, 1973; pp 217−242. (29) (a) Ammeter, J. H.; Swalen, J. D. J. Chem. Phys. 1972, 57, 678− 698. (b) Ammeter, J. H. J. Magn. Reson. 1978, 30, 299−325. (30) (a) Wilkinson, G.; Cotton, F. A.; Birmingham, J. M. J. Inorg. Nucl. Chem. 1956, 2, 95−113. (b) Kö nig, E.; Desai, V. P.; Kanellakopulos, B.; Klenze, R. Chem. Phys. 1980, 54, 109−113. (c) Walter, M. D.; Sofield, C. D.; Andersen, R. A. J. Organomet. Chem. 2015, 776, 17−22. J

DOI: 10.1021/acs.organomet.6b00554 Organometallics XXXX, XXX, XXX−XXX