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Closing the Gap: Preparation and Characterization of the First HalfOpen and Open Manganocene Complexes Matthias Reiners, Dirk Baabe, Matthias Freytag, Peter G. Jones, and Marc D. Walter* Institut für Anorganische und Analytische Chemie, Technische Universität Braunschweig, Hagenring 30, 38106 Braunschweig, Germany S Supporting Information *

ABSTRACT: The first preparations of half-open and open manganocenes were accomplished. Treatment of KPdl′ (Pdl′ = 2,4-(Me3C)2C5H5)) with [{(η5Cp”)Mn(thf)(μ-I)}2] (Cp″ = 1,2,4-(Me3C)3C5H2) and MnI2(thf)2 resulted in the formation of [(η5-Cp″)Mn(Pdl′)] (2) and [(Pdl′)2Mn] (4), respectively. Both compounds adopt a high-spin (S = 5/2) ground state. Maximum spin states are rather unusual for pentadienyl complexes, since these ligands generally stabilize transition metal complexes in their low-spin state. In addition, the electronic structure of 2 was compared to its closed analogue [(η5-Cp″)Mn(η5Cp′)] (Cp′ = 1,3-(Me3C)2C5H3), which also adopts the high-spin configuration because of steric hindrance destabilizing the electronically more favorable lowspin state. Reaction of KPdl′ with [(C5H5)2Mn] yields the trimetallic compound [{(η5-Pdl′)Mn(η5-C5H5)]}2Mn] (5) concomitant with the formation of 2,4,7,9tetra-tert-butyl-1,3,7,9-decatetraene (Pdl′2), indicating reduction of two Mn atoms. Solid-state magnetic susceptibility studies and density functional theory computations suggest that the electronic structure in 5 is best described as two diamagnetic (S = 0) [(η5-Pdl′)Mn(η5-C5H5)]− Mn(I) anions, each being coordinated to a central Mn(II) cation with a high-spin (S = 5/2) configuration.

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INTRODUCTION Manganocene, [(C5H5)2Mn], adopts a unique position among the 3d-transition metal metallocenes. Whereas it is monomeric in the gas phase,1 it forms a polymeric chain in the solid state, in which the manganese atoms are coordinated by one η5bonded and two bridging C5H5 ligands.2,3 Furthermore, associated with its high-spin (S = 5/2) electron configuration the e2g, a1g, and e1g* molecular orbitals (in D5d symmetry) are singly occupied, which leads to the lowest M−Cp bond dissociation energy of 51 kcal mol−1 of all the 3d-transition metal metallocenes.4 Besides this thermodynamic aspect, the Cp ligands are also kinetically labile and can readily be substituted by suitable nucleophiles.5 This property has been utilized for the synthesis of high-spin manganese half-sandwich complexes that are accessed via the substitution of one Cp ligand, either by the addition of nucleophiles or on protonolysis.6 Moreover, the spin state in manganocenes is readily modified by suitable substitutions at the Cp ring, leading either to high-spin, low-spin, or spin-crossover behavior.7 Consequently, many differently substituted manganocene derivatives have been prepared and studied in great detail.7 A ligand system closely related to the cyclopentadienyl ligand is the pentadienyl (Pdl) framework (Chart 1) pioneered by Ernst and co-workers.8 Because of this relationship the Pdl ligand is also often alluded to as “open-cyclopentadienyl”, and a large number of “open metallocenes” are known.8 However, despite numerous attempts, “open” or “half-open” manganocenes have so far proved elusive9 and instead “associated salts” © XXXX American Chemical Society

Chart 1

such as the trimetallic [(3-MeC5H6)4Mn3]9a,b or [(C5H5)2(2,4Me2C5H5)Mn2] and K[(3-Me-1,5-(Me3Si)2C5H4)3Mn]9c were obtained. This reactivity may be rationalized by the intrinsic property of the Pdl ligand to act in its η5-U conformation as a better π-donor and δ-acceptor ligand than the corresponding Cp system. Hence Pdl ligands are also known to stabilize metal atoms in low oxidation states and low-spin configuration.8 A nice example of this behavior is the manganese-centered coupling of two 2,4-dimethylpentadienyl ligands, resulting in the formation of the complex (η8-2,4,7,9-tetramethyl-1,3,7,9decatetraene)(trimethylphosphine)manganese(0).10 However, the isolation of K[(3-Me-1,5-(Me3Si)2C5H4)3Mn]9c also establishes the relationship between the Pdl and the allyl ligand in their ability to isomerize between different hapticities and to form −ate compounds (Chart 2). Homoleptic manganese allyl complexes have recently been prepared,11 but the base-free bis(allyl)manganese compounds remain elusive. In contrast, heteroleptic manganese allyl complexes such as [{η3-(1,3-Me3Si)2C3H3}Mn{C(SiMe3)3}Received: April 8, 2016

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DOI: 10.1021/acs.organomet.6b00281 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

in the Supporting Information, and the molecular structure with selected bond distances and angles is shown in Figure 1.

Chart 2

(L)] (L = thf, PMe3, N,N′-dimethylaminopyridine (dmap), quinuclidine) and [{η3-(1,3-Me3Si)2C3H3}Mn{C(SiMe3)3}] are accessible.12 In the course of our investigations on the sterically demanding pentadienyl ligand, 2,4-(Me3C)2C5H5 (Pdl′),13 we have prepared several open metallocenes [(η5-Pdl′)2M] (M = Ti−Cr) and the half-open ferrocene [(η5-Cp″)Fe(η5-Pdl′] (Cp″ = 1,2,4-(Me3C)3C5H2).13b This encouraged us to introduce this ligand system into organomanganese chemistry, to close the current gap in the organopentadienyl chemistry of the 3d-transition metals. These investigations are described below, together with a comparison to related manganese complexes [(η5-Cp′)2Mn], [(η5-Cp″)Mn(Pdl′)], and [(η5Cp″)Mn(η5-Cp′)] (Cp′ = 1,3-(Me3C)2C5H3).

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Figure 1. ORTEP of 2 with thermal displacement parameters drawn at 30% probability. Hydrogen atoms were omitted for clarity. Selected bond distances (Å) and angles (deg): C1−C2 1.390(2), C2−C3 1.420(2), C3−C4 1.385(2), C4−C5 1.445(2), C5···C1 2.944(2), Mn−C1 2.3181(16), Mn−C5 2.1943(16), Mn−C(Cp″) (range) 2.3533(15)−2.4280(14), Cp″(cent)−Mn 2.0543(7), Cp″(plane)− Mn 2.0532(3), Cp″(plane)−Pdl′(plane) 37.06(7).

RESULTS AND DISCUSSION Synthesis and Molecular Structure. Half-Open Manganocenes. Ernst and co-workers showed that the reaction of [(η5-C5H5)MnCl(L)]2 (L = thf, PMe3)14 with two equivalents of K(2,4-Me2C5H5) did not yield the desired half-open manganocene [(η5-C5H5)Mn(η5-2,4-Me2C5H5)], but rather the bimanganese complex [(C5H5)2(2,4-Me2C5H5)Mn2].9c This observation implied that the use of sterically more demanding Cp and Pdl ligands might pave the way to this elusive manganese compound. In analogy to our work on halfopen ferrocenes,13b we decided to start from the well-defined manganese half-sandwich complex [{(η5-Cp″)Mn(thf)(μ-I)}2] (1)15 (Scheme 1). The kinetic stability of this complex and of

The Cp″ ligand binds to the Mn(II) atom in η5-fashion with a Cp″(cent)−Mn distance of 2.0543(7) Å. Studies on several manganocenes have established that the Cp(cent)−Mn distance is a very sensitive indicator of the spin state. For low-spin manganocenes (S = 1/2) this distance is ca. 1.76 Å, whereas it increases by 0.3 Å to ca. 2.08 Å in high-spin complexes (S = 5/2).7b,g,i,17 Based on this precedence the Cp″(cent)−Mn distance in 2 may therefore indicate a high-spin configuration. Nevertheless, the half-open manganocene 2 does not exhibit a typical metallocene-like structure. The Pdl′ ligand adopts a U conformation and binds via the terminal C atoms, C1 and C5, to the Mn atom. However, the Mn−C1 and Mn− C5 distances of 2.3181(16) and 2.1943(16) Å, respectively, are very different, suggesting that the bonding may be better described as a κC-σ-bond between Mn and C5 together with a weaker interaction between Mn and C1. In solution a rapid exchange between these positions may take place, but the 1H NMR resonances are too broad to be informative. Consistent with this description, there is also a distinct alternation within the C−C bonds of the Pdl′ moiety (C1−C2 1.390(2) Å, C2− C3 1.420(2) Å, C3−C4 1.385(2) Å, and C4−C5 1.445(2) Å). Furthermore, Mn−C distances of 2.10−2.16 Å are common for Mn(II) σ-alkyl complexes.12,18 The synthesis of the first stable half-open manganocene 2 is of note, since Pdl ligands usually stabilize transition metals in low oxidation states and more importantly in their low-spin configuration.8 We then prepared the related closed manganocene [(η5Cp”)Mn(η5-Cp′)] (3) and explored its molecular and electronic structure. The preparations of mixed-ring manganocenes have not been accomplished so far because of competing

Scheme 1

related iron compounds16 is due to the very sterically encumbered Cp″ ligand. Indeed salt metathesis reaction between 1 and KPdl′ in THF proceeds cleanly, and the monomeric half-open manganocene 2 can be isolated as red crystals from a concentrated pentane solution at −24 °C in moderate yield (71%) (see Experimental Details section). Furthermore, once formed, compound 2 is stable toward ligand redistribution in THF, toluene, or hexane solution at ambient temperature. The crystallographic data are provided in Table S1 B


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Organometallics

parameters, complex 3 also adopts a high-spin configuration in the solid state similar to 2. This assumption is further substantiated by solid-state magnetic susceptibility studies. Figure 3 shows the μeff vs T plot for compounds 2 and 3

ligand redistribution. For example, addition of Li(C5Me5) to [η5-C5H5)MnI(thf)2] only forms [(C5H5)2Mn] and [η5C5Me5)2Mn].19 Similarly any attempt to prepare [η5-C5Me5)MnX(L)2] (X = halides, L = thf or PR3) only resulted in the formation of MnX2 and [η5-C5Me5)2Mn].20 This can be rationalized by the formation of low-spin manganocenes exhibiting stronger Mn−C(Cp) bonds than in their high-spin analogues. Furthermore, while [(η5-Cp″)2Mn] adopts a highspin configuration, [(η5-Cp′)2Mn] behaves as a spin-crossover molecule in the solid state and solution,7i so the magnetic behavior of 3 is expected to be revealing. The half-sandwich compound 1 converts cleanly with NaCp′ to the mixed manganocene 3, which can be isolated from concentrated pentane solutions at −24 °C as yellow crystals in moderate yield (64%). Complex 3 crystallizes in the monoclinic space group C2/c with two independent molecules in the asymmetric unit (see Table S1 in the Supporting Information). The structure of one of these molecules is shown in Figure 2, while

Figure 3. μeff and χ−1 vs T plots of complexes 2 and 3 recorded between T = 2.4 and 300 K with an external applied magnetic field of 1000 G. The adaptation of the Curie−Weiss law (solid lines, inset) was executed at temperatures above T = 50 K.

recorded between 2.6 and 300 K. The effective magnetic moments for 2 and 3 of 5.86 and 5.63 μB (at T = 300 K) are clearly consistent with high-spin (S = 5/2) compounds. However, they are slightly lower than expected from the spin-only value of 5.92 μB, which may be attributed to small weighing errors during sample preparation. The inverse magnetic susceptibility of compounds 2 and 3 indicates Curie−Weiss behavior with Curie constants of C = 4.327(1) and 3.950(4) cm3 K mol−1 and low Weiss temperatures of θ = −2.74(4) and −2.12(20) K, respectively (Figure 3, inset). For complex 2, the effective magnetic moment decreases from 5.74 μB at T = 50 K to 3.82 μB at T = 2.4 K, which we attribute to the presence of zero-field splitting (ZFS). A simulation on the basis of a simplified spin Hamiltonian with negligible rhombic ZFS parameter (E/D = 0) resulted in an estimate for the axial ZFS parameter of D = 0.6 cm−1 (see Supporting Information for details, Figure S7). This is in good agreement with D values found for other Mn(II) high-spin complexes.21 However, to further quantify the ZFS parameter D and E/D high-field EPR measurements would be helpful. For complex 3, the characteristic decrease of the effective magnetic moment at low temperatures, indicating ZFS, is not observed. Open Manganocenes. After the successful synthesis of the half-open manganocene 2 we proceeded to prepare the first open manganocene (4) (Scheme 2). On addition of two equivalents of KPdl′ to a THF solution of MnI2(thf)2 at −78 °C, a yellow solution is formed, but when the solvent is changed from THF to pentane, the solution color changes to orange-red, which might indicate weak THF coordination to 4.

Figure 2. ORTEP of 3 with thermal displacement parameters drawn at 30% probability. Hydrogen atoms were omitted for clarity. Two independent molecules are found in the asymmetric unit, but only one of them is shown. Selected bond distances (Å) and angles (deg): Mn− C1 2.438(2) [2.475(2)], Mn−C2 2.460(2) [2.426(2)], Mn−C3 2.370(2) [2.338(2)], Mn−C4 2.353(2) [2.365(2)], Mn−C5 2.353(2) [2.394(2)], Mn−C18 2.443(2) [2.496(2)], Mn−C19 2.467(2) [2.487(2)], Mn−C20 2.444(2) [2.411(2)], Mn−C21 2.331(2) [2.311(2)], Mn−C22 2.333(2) [2.354(2)], Cp″(cent)−Mn 2.0655(4) [2.0711(11)], Cp′(cent)−Mn 2.0809(12) [2.0904(11)], plane(Cp″)−Mn 2.0625(4) [2.0678(4)], plane(Cp′)−Mn 2.0742(4) [2.0804(4)], Cp″(cent)−Mn-Cp Cp′(cent) 176.48(5) [177.05(4)] plane(Cp″)−plane(Cp′) 5.66(14) [4.84(13)]. The values for the other molecule in the asymmetric unit are given in parentheses.

selected bond distances and angles are provided in the figure caption. In contrast to the half-open derivatives 2, the closed analogue 3 adopts the expected sandwich structure with two η5coordinate Cp ligands. The Cp(cent)−Mn distances are 2.0655(10) [2.0711(11)] Å and 2.0809(12) [2.0901(11)] Å for the Cp″ and Cp′ ligands, respectively. The slightly shorter Cp″(cent)−Mn distances may reflect the stronger donor capability of Cp″ relative to Cp′. For comparison, the Cp(cent)−Mn distance for the more sterically encumbered [(η5-Cp″)2Mn] is 2.11 Å.7i On the basis of the metric

Scheme 2

C


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Organometallics

in the solid state, which was also confirmed by solid-state magnetic susceptibility studies (Figure 5). The μeff vs T plot for

However, thermally sensitive, red crystals of 4 were isolated when the red pentane solutions were stored at −24 °C. X-ray diffraction data were collected at T = 100 K (see Table S1 in the Supporting Information), and the molecular structure of 4 is shown in Figure 4.

Figure 5. μeff and χ−1 vs T plots of complex 4 recorded between T = 2.4 and 270 K with an external applied magnetic field of 1000 G. The adaptation of the Curie−Weiss law (solid line, inset) was executed at temperatures above T = 50 K.

compound 4 reveals an effective magnetic moment of 5.59 μB at T = 270 K. The inverse magnetic susceptibility shows Curie− Weiss behavior with a Curie constant of C = 3.913(1) cm3 K mol−1 and a low Weiss temperature of θ = −1.91(4) K. In contrast, its closed analogue [(η5-Cp′)2Mn] exhibits spincrossover behavior in the solid state and in solution.7i Similar to the half-open manganocene 2, we also observed a declining effective magnetic moment at low temperatures for the open manganocene 4 caused by zero-field splitting, which leads to an μeff = 3.41 μB at T = 2.4 K. The adaptation of a simplified spin Hamiltonian with E/D = 0 yields only a rough estimate of D = 0.2 cm−1 (see Supporting Information for details, Figure S7), which is also concordant with D values reported for other Mn(II) high-spin complexes.21 Nevertheless, to further quantify the ZFS parameter, high-field EPR measurements would be necessary. We also noted that the open manganocene 4 is thermally unstable in the solid state, when stored for a prolonged period of time at ambient temperature. The thermal stability of 4 was also investigated by SQUID magnetometry, establishing rapid thermal degradation on heating of the sample to T = 350 K, which causes μeff to decrease from 5.56 μB to 2.23 μB within ca. 20 min (see Supporting Information for details, Figure S3). Substitution of Cyclopentadienyl by Pentadienyl in [(C5H5)2Mn]. The C5H5 ligand in [(C5H5)2Mn] is kinetically labile;5 we therefore expected that the addition of KPdl′ (1 equiv) would selectively substitute one Cp ligand to yield the half-open manganocene [(η5-Pdl′)Mn(η5-C5H5)], providing us with an alternative entry into this class of compounds. However, in contrast to our expectations we isolated 2,4,7,9tetra-tert-butyl-1,3,7,9-decatetraene (Pdl′2) and the paramagnetic trimetallic complex [{(η5-Pdl′)Mn(η5-C5H5)]}2Mn] (5), whose molecular composition was established by elemental analysis and EI-MS spectrometry (Scheme 3). The formation of the tetraene Pdl′2 implied that a reduction had occurred; by adjusting the stoichiometry to the correct stoichiometric ratio of 3:4 for [(C5H5)2Mn] and KPdl′ the

Figure 4. ORTEP of 4 with thermal displacement parameters drawn at 30% probability. Hydrogen atoms were omitted for clarity. Selected bond distances (Å): C1−C2 1.461(2), C2−C3 1.376(2), C3−C4 1.449(2), C4−C5 1.365(2), C5···C1 3.016(2), C14−C15 1.458(2), C15−C16 1.377(2), C16−C17 1.455(2), C17−C18 1.363(2), C18··· C14 3.025(2), Mn−C1 2.1665(16), Mn−C5 2.4710(16), Mn−C14 2.1792(17), Mn−C18 2.5132(16), τ4 = 0.91 (τ4: see text).

Both Pdl′ ligands in 4 adopt a U-conformation, while the bonding occurs via the termini of the Pdl′ ligand C1, C5, C14, and C18. The Mn−C distances can be divided into two classes, two short (Mn−C1 and Mn−C14 with 2.1665(16) and 2.1792(17) Å, respectively) and two long (Mn−C5 and Mn− C18 with 2.4710(16) and 2.5132(16) Å, respectively). Houser and co-workers developed a convenient analysis of the ligand arrangement in four-coordinate complexes by introducing the τ4 value, which is defined as τ4 = [360° − (α + β)]/141°, where α and β represent the largest angles within the four-coordinate species.22 As an example, for tetrahedral (Td) and square planar (D4h) structures the τ4 values are 1.0 and 0, respectively. Distortions from these idealized geometries, including those with a trigonal pyramidal (C3v) or seesaw (C2v) ligand arrangement, result in τ4 values of 0 < τ4 < 1.22 Applying these criteria to 4, a τ4 value of 0.91 is found for the coordination environment at the Mn atom, which is close to a trigonal pyramidal ligand arrangement with a τ4 value of 0.85 and in which C18 occupies the axial position. The different Mn−C distances along with the alternation in the C−C bonds of the Pdl′ ligands are consistent with two strong σ-bonds between Mn and C1/C14 combined with weak interactions between Mn and C5/C18 to increase the coordination number to four at the Mn atom. These structural features resemble those observed for the half-open manganocene 2, suggesting that 4 may also adopt a high-spin configuration in solution and D


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form a nearly linear array (177.51(6)°).9a These Mn−Mn distances are short compared to those found between two Mn(II) ions of 2.7−2.8 Å,18a,23 and the Mn−Mn single-bond distance of 2.923 Å in [Mn2(CO)10].24 These values may also be related to the formal MnMn triple bond in [{(η5C5Me5)Mn}2(μ-CO)3], which is significantly shorter at 2.170 Å.25 Nevertheless, in contrast to complexes 2−4 the analysis of the electronic structure in 5 is less obvious. The Cp(cent)−Mn distance for the [(η5-Pdl′)Mn(η5-C5H5)] fragments gives values of 1.7490(15) and 1.7465(11) Å, respectively, whereas the Pdl′(cent)−Mn distances are 1.5082(11) and 1.5123(12) Å, respectively. An anionic [(η5-Pdl′)Mn(η5-C5H5)]− Mn(I) fragment would be isoelectronic to the known 18VE halfopen ferrocene [(η5-Pdl′)Fe(η5-Cp″)] (d6-system), for which Cp″cent−Fe and Pdl′cent−Fe distances of 1.73 and 1.52 Å were reported, respectively.13b This would lead to a central Mn2+ being coordinated by two [(η5-Pdl′)Mn(η5-C5H5)]− Mn(I) fragments. The rather long Mn3−C1, C5, C19, and C23 distances suggest a high-spin configuration for the central Mn2+ unit (vide supra). The temperature-dependent solid-state magnetic moment of 5 is shown in Figure 7. The effective

Scheme 3

overall yield of this reaction was improved (63%). The dark red complex 5 can be crystallized from concentrated pentane solutions at −24 °C, and its molecular structure is shown in Figure 6. Selected bond distances and angles are provided in the figure caption.

Figure 6. ORTEP of 5 with thermal displacement parameters drawn at 30% probability. Hydrogen atoms were omitted for clarity. Selected bond distances (Å) and angles (deg): C1−C2 1.451(3), C2−C3 1.419(3), C3−C4 1.422(3), C4−C5 1.455(3), C5···C1 2.782(3), C14−C15 1.408(5), C15−C16 1.403(6), C16−C17 1.398(4), C17− C18 1.417(4), C18−C14 1.417(4), C19−C20 1.454(3), C20−C21 1.424(3), C21−C22 1.422(3), C22−C23 1.454(3), C23···C19 2.781(3), C32−C33 1.415(3), C33−C34 1.406(4), C34−C35 1.411(3), C35−C36 1.434(3), C36−C32 1.432(3), Mn1−C1 2.129(2), Mn1−C2 2.071(2), Mn1−C3 2.0972(19), Mn1−C4 2.066(2), Mn1−C5 2.130(2), Pdl′(cent)−Mn1 1.5082(11), Pdl′(plane1)−Mn1 1.4662(10), Mn1−C14 2.110(3), Mn1−C15 2.126(3), Mn1−C16 2.138(3), Mn1−C17 2.124(2), Mn1−C18 2.103(2), Cp(cent)−Mn1 1.7490(15), Cp(plane1)−Mn1 1.7486(3), Mn2−C19 2.124(2), Mn2−C20 2.0746(19), Mn2−C21 2.1051(18), Mn2−C22 2.0716(19), Mn2−C23 2.136(2), Pdl′(cent)−Mn2 1.5123(12), Pdl′(plane2)−Mn2 1.4683(10), Mn2−C32 2.115(2), Mn2−C33 2.138(2), Mn2−C34 2.142(2), Mn2−C35 2.121(2), Mn2−C36 2.100(2), Cp(cent)−Mn2 1.7465(11), Cp(plane2)−Mn2 1.7459(3), Mn3−C1 2.388(2), Mn3−C5 2.3562(19), Mn3−C19 2.394(2), Mn3−C23 2.3437(19), Mn1−Mn3 2.5007(4), Mn2−Mn3 2.5091(4), Mn1−Mn3−Mn2 159.891(17), Cp(plane1)−Pdl′(plane1) 1.06(13), Cp(plane2)−Pdl′(plane2) 1.06(13), Pdl′(plane1)− Pdl′(plane2) 59.35(4), τ4 = 0.57.

Figure 7. μeff and χ−1 vs T plots of complex 5 recorded between 2.6 and 300 K with an external applied magnetic field of 1000 G and 40 kG, respectively. The adaptation of the Curie−Weiss law (solid line, inset) was executed at temperatures above T = 50 K.

magnetic moment of 5.89 μB at T = 300 K is completely consistent with a total spin of S = 5/2 ground state. It is also temperature invariant, and the value stays more or less constant down to T = 2.6 K; the χ−1 vs T plot obeys the Curie−Weiss law with C = 4.329(2) cm3 K mol−1 and θ = −1.05(10) K. This supports the assumption that complex 5 consists of two diamagnetic (S = 0) [(η5-Pdl′)Mn(η5-C5H5)]− Mn(I) anions, each being coordinated to a central Mn(II) cation with a highspin (S = 5/2) configuration. Furthermore, no indications of a significant zero-field splitting occur down T = 2.6 K. An additional measurement with an applied magnetic field of 40 kG reveals a declining effective magnetic moment with decreasing temperature below T = 20 K, suggesting that within this temperature regime and magnetic field range the Curie law approximation (i.e., B/T ≪ 1)26 is no longer valid. This interpretation is further confirmed by a field-dependent measurement with applied magnetic fields between 0.5 and

The structure of 5 resembles that of [(3-MeC5H6)4Mn3]9a,b and consists of two [(η5-Pdl′)Mn(η5-C5H5)] fragments and a single Mn atom. The two [(η5-Pdl′)Mn(η5-C5H5)] fragments are twisted by ca. 57° relative to each other, and the termini of Pdl′ ligands C1, C5, C19, and C23 coordinate to the central Mn3 atom with bond distances of 2.388(2), 2.3562(18), 2.394(2), and 2.3437(18) Å, respectively. The τ4 value (vide supra) for the Mn3 atom in 5 is 0.57, which suggests a severely distorted trigonal pyramidal ligand arrangement (with C19 in axial position). The atoms Mn1 and Mn2 are separated by ca. 5 Å overall (2.5007(4) and 2.5091(4) Å for Mn1−Mn3 and Mn2−Mn3, respectively), and the Mn1−Mn3−Mn2 angle is 159.891(17)°. For comparison, in [(3-MeC5H6)4Mn3] the average Mn−Mn distance is 2.516(1) Å and the Mn atoms E


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Organometallics 70 kG at T = 2.6 K (see Supporting Information for details, Figure S5). Computational Studies. Density functional theory (DFT) computations might also provide some additional insights into the bonding situation and the spin preference. In previous studies we have employed either B3LYP or dispersioncorrected B97D functionals for this purpose,27 but a detailed evaluation of the adequate DFT functional remains always indispensable. Therefore, we started our evaluation by computing the energies of the different spin states (S = 1/2 and 5/2 for 2 and 3, S = 1/2, 3/2, and 5/2 for 4) (Table 1) and the minimum structures.

We then turned our attention to the trimetallic compound [{(η5-Pdl′)Mn(η5-C5H5)]}2Mn] (5). In a first step the structure of 5 was computed at the B3LYP/6-311G(d,p) level of theory. Several spin states (i.e., the total spin of the molecule) such as S = 1/2, 5/2, 9/2, 11/2, and 15/2 were considered, but the metric parameters and relative energies were most consistent with an S = 5/2 ground state, in agreement with the experiment (see Supporting Information for details, Figure S10 and Table S4). A closer inspection of the spin distribution and the NBO charges reveals that compound 5 is best described as containing two diamagnetic (S = 0) [(η5Pdl′)Mn(η5-C5H5)]− Mn(I) anions, each being coordinated to a central Mn(II) cation with a high-spin (S = 5/2) configuration (see Supporting Information for details, Table S5). A similar picture also emerges for [(3-MeC5H6)4Mn3] originally reported by Ernst and co-workers.9a,b

Table 1. Relative Free Energies ΔG0 (in kcal/mol) for Manganocenes 2−4a S = 1/2 S = 3/2 S = 5/2

2

3

4

20.5 [6.7]

25.4 [−1.9]

0.0 [0.0]

0.0 [0.0]

24.0 [7.1] 16.9 [4.0] 0.0 [0.0]

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CONCLUSIONS

Reaction of KPdl′ with [{(η5-Cp″)Mn(thf)(μ-I)}2] and MnI2(thf)2 allowed us to isolate the first half-open (2) and open managnocenes (4). Despite the preference of pentadienyl ligands for stabilizing transition metal atoms in low oxidation and spin states, complexes 2 and 4 adopt a high-spin state (S = 5/2), in which the Pdl′ ligand binds only via the terminal C atoms, C1 and C5, to the Mn but with very different Mn−C1 and Mn−C5 bond distances. Nevertheless, compounds 2 and 4 show significantly different thermal stabilities; whereas the halfopen derivative 2 is indefinitely stable at ambient temperature under an N2 atmosphere, the open-manganocene 4 slowly degrades under these conditions. A trimetallic compound, [{(η5-Pdl′)Mn(η5-C5H5)]}2Mn] (5), is obtained when KPdl′ is reacted with [(C5H5)2Mn]. Solid-state magnetic susceptibility studies confirm that this compound also exhibits an S = 5/2 ground state, which can be rationalized in terms of two diamagnetic (S = 0) [(η5-Pdl′)Mn(η5-C5H5)]− Mn(I) anions being coordinated to a central Mn(II) cation with a high-spin (S = 5/2) configuration. This proposal is further supported by DFT computations. Further reactivity studies of the Pdl′ ligand with d- and f-block transition metals are in progress and will be reported in due course.

a

Computed at the B3LYP/6-311G(d,p) level of theory. Values given in parentheses were obtained by B97D/6-311G(d,p).

Although there is a methodological uncertainty on the order of a few kcal/mol associated with DFT methods and open-shell systems, the comparison between B3LYP and B97D for complexes 2−4 is very instructive. The dispersion-corrected Grimme B97D functional apparently overestimates dispersion and noncovalent interactions, and therefore the low-spin (i.e., more crowded) complexes are energetically overstabilized. This becomes very obvious for complex 3, for which B97D incorrectly predicts a low-spin ground state, which is clearly inconsistent with our experimental data. In view of these discrepancies associated with the B97D functional, we will focus in the following discussion only on the B3LYP results. Table 2 compares the structural parameters of the lowest energy, high-spin gas-phase structure (computed at the B3LYP level theory) with the experimental solid-state structure. The experimental and computed structures are in good agreement with each other, strengthening our arguments in favor of B3LYP to analyze these manganese compounds further.

Table 2. Comparison of Experimental vs Computed Bond Distances (Å) for Complexes 2−4

Cp Moiety Cp″(cent)−Mn Cp′(cent)−Mn Cp″(plane)−Mn Cp′(plane)−Mn Pdl′ Moiety Mn−C1 Mn−C5 C1−C2 C2−C3 C3−C4 C4−C5

[Cp″Mn(Pdl)] (2)

[Cp″Mn(Cp′)] (3)

expa

calcb

expa

calcb

2.0543(7)

2.08

2.0532(3)

2.08

2.0683(39) 2.0857(67) 2.0652(37) 2.0773(44)

2.10 2.10 2.08 2.09

2.3181(16) 2.1943(16) 1.390(2) 1.420(2) 1.385(2) 1.445(2)

2.24 2.22 1.43 1.40 1.41 1.42

[(Pdl′)2Mn] (4) expa

calcb

2.1665(16)/2.1792(17 2.4710(16)/2.5132(16) 1.461(2)/1.458(2) 1.376(2)/1.377(2) 1.449(2)/1.455(2) 1.365(2)/1.363(2)

2.185/2.187 2.432/2.434 1.460/1.460 1.383/1.383 1.442/1.442 1.379/1.379

a

Derived from the solid-state crystallographic data. For complex 3 two independent molecules are present in the asymmetric unit, and therefore the values for both molecules were averaged. For complex 4 the values for the two Pdl′ moieties are provided. bComputed at the B3LYP/6-311G(d,p) level of theory. For equivalent ligands the value for the corresponding distances were averaged. F


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with pentane (10 mL). The dark red extracts were filtered, concentrated, and cooled to −24 °C. Initially a crop of colorless crystals of Pdl′2 (2,4,7,9-tetra-tert-butyl-1,3,7,9-decatetraene) was collected, and the product was then obtained as red blocks from the mother liquor. Alternatively, the tetraene Pdl′2 could also be removed by sublimation (50 °C/1.0 × 10−3 mbar). The red residue was then dissolved in a minimum amount of pentane and stored at −24 °C to give complex 5 as red crystals. Yield: 0.141 g (0.216 mmol, 63%). Mp: 208 °C (dec). Anal. Calcd for C36H56Mn3 (653.66 g/mol): C, 66.15; H, 8.64. Found: C, 66.20; H, 8.94. The EI mass spectrum (70 eV) showed a molecular ion at m/z = 653 amu with the following isotopic cluster distribution (in %): 652 (1), 653 (100), 654 (39), 655 (8), 656 (0.4). Simulated distributions (in %) for C36H56Mn3: 653 (100), 654 (39), 655 (7), 656 (1). UV/vis (n-pentane, 22 °C, nm): λ (ε, L mol−1 cm−1) = 276 (sh, 5240), 326 (sh, 3320), 424 (810). X-ray Diffraction Studies. Single crystals of each compound were examined under inert oil. Data collection was performed on various Oxford Diffraction diffractometers using monochromated Mo Kα or mirror-focused Cu Kα radiation (see Table S1 in the Supporting Information). Absorption corrections were performed on the basis of multiscans. Hydrogen atoms were included using rigid methyl groups or a riding model. The data were analyzed using the SHELXL97 program.32 Special features: The hydrogen atoms at the following carbons were refined freely; compound 2, C1, 3, 5; compound 4, C1, 3, 5 14, 16, 18; compound 5, C1, 3, 5, 19, 21, 23. Compounds 3, 4, and 5 were examined at slightly higher temperatures because they shattered on further cooling. For compound 3, the tert-butyl group at C14′ is disordered. Computational Details. All calculations employed the B3LYP33 or long-range dispersion-corrected Grimme’s functional (B97D)34 and were carried out with Gaussian 09.35 No symmetry restrictions were imposed (C1). C, H, and Mn were represented by an all-electron 6311G(d,p) basis set. The nature of the extrema (minima) was established with analytical frequency calculations. The zero point vibration energy and entropic contributions were estimated within the harmonic potential approximation. The Gibbs free energy, ΔG, was calculated for T = 298.15 K and 1 atm. Geometrical parameters were reported within an accuracy of 10−3 Å and 10−1 degrees.

EXPERIMENTAL DETAILS

General Comments. All reactions and product manipulations were carried out under an atmosphere of dry, oxygen-free argon using standard high-vacuum, Schlenk, or drybox techniques. Dinitrogen (N2) was purified by passage through BASF R3-11G catalyst (Chemalog) and 4 Å molecular sieves. Solvents were distilled from sodium benzophenone ketyl, degassed, and stored over 4 Å molecular sieves. Elemental analyses (C, H) by combustion and gas chromatography were carried out at the analytical facilities at the TU Braunschweig using an Elementar varioMICRO. EI-MS and UV/ vis spectra were recorded on a Thermofinnigan MAT 95 XL and on a Varian Cary 50 Scan, respectively. Solid-state magnetic susceptibility studies were performed in quartz tubes as previously described,28 and the data were collected at a Cryogenic Ltd. closed-cycle SQUID magnetometer between T = 2.4 and 300 K with an external applied magnetic field of B = 1000 G and for complex 5 with an additional applied magnetic field of 40 kG. The background signal of the empty sample holder was experimentally determined and subtracted from the raw magnetization data. The data were also corrected for the overall diamagnetism of the investigated molecules using tabulated Pascal constants.29 [{(η5-Cp”)Mn(thf)(μ-I)}2],15 MnI2(thf)2,30 KPdl′,13a and [(C5H5)2Mn]31 were prepared according to literature procedures. Synthesis. [(η5-Cp″)Mn(Pdl′)] (2). A suspension of KPdl′ (0.090 g, 0.41 mmol) in THF (10 mL) was added to a stirred solution of [{(η5Cp″)Mn(thf)(μ-I)}2] (0.205 g, 0.21 mmol) in THF (15 mL) at ambient temperature. The mixture was stirred for 1 h. During this time the solution became yellow-brown, and a colorless precipitate formed. The solvent was removed in dynamic vacuum, and the brown residue was extracted with pentane (10 mL). The orange extracts were filtered, concentrated, and cooled to −24 °C. After 24 h red crystals were isolated. Yield: 0.137 g (0.293 mmol, 71%). Mp: 65−66 °C (dec). Anal. Calcd for C30H52Mn (467.68 g/mol): C, 77.05; H, 11.21. Found: C, 77.02; H, 11.27. UV/vis (n-pentane, 22 °C, nm): λ (ε, L mol−1 cm−1) = 288 (sh, 5900), 338 (3340), 422 (sh, 1080), 485 (sh, 670). [(η5-Cp″)Mn(η5-Cp′)] (3). A solution of NaCp′ (0.123 g, 0.62 mmol) in THF (5 mL) was added to a stirred solution of [{(η5Cp″)Mn(thf)(μ-I)}2] (0.308 g, 0.31 mmol) in THF (15 mL) at ambient temperature. The reaction was stirred for 1 h, and during this time the solution turned yellow. The solvent was removed in dynamic vacuum, and the yellow residue was extracted with pentane (10 mL). The extracts were filtered, concentrated, and cooled to −24 °C. After 24 h yellow crystals were isolated. Yield: 0.183 g (0.393 mmol, 64%). Mp: 121−123 °C. Anal. Calcd for C30H50Mn (465.67 g/mol): C, 77.38; H, 10.82. Found: C, 77.31; H, 10.79. UV/vis (n-pentane, 22 °C, nm): λ (ε, L mol−1 cm−1) = 332 (2020). [(Pdl′)2Mn] (4). A suspension of KPdl′ (0.200 g, 0.916 mmol) in THF (5 mL) at −78 °C was added to a stirred solution of MnI2(thf)2 (0.208 g, 0.458 mmol) in THF (15 mL). The reaction mixture was stirred and allowed to warm to room temperature for 1 h. During this time the reaction mixture turned yellow-orange and a colorless precipitate formed. It should be noted that when the reaction mixture was stirred for a prolonged period of time (>3 h) at ambient temperature, the solution turned brown, which resulted in a reduced yield. The solvent was removed in dynamic vacuum, and the redorange residue was extracted with pentane (15 mL) to give red extracts that were filtered, concentrated, and cooled to −24 °C. The next day red crystals were isolated that were thermally sensitive and decomposed to a brown-black amorphous solid when stored at ambient temperatures for a prolonged period of time. For long-term storage, complex 4 should be kept at −24 °C under an N2 atmosphere. Yield: 0.169 g (0.409 mmol, 89%). Mp: 91−92 °C (dec). Anal. Calcd for C26H46Mn (413.59 g/mol): C, 75.51; H, 11.21. Found: C, 75.60; H, 11.06. UV/vis (n-pentane, 22 °C, nm): λ (ε, L mol−1 cm−1) = 366 (3430). [{(η5-Pdl′)Mn(η5-C5H5)]}2Mn] (5). A suspension of KPdl′ (0.300 g, 1.37 mmol) in THF (10 mL) was added to a stirred solution of [(C5H5)2Mn] (0.190 g, 1.03 mmol) in THF (20 mL). The reaction mixture was stirred at ambient temperature for 12 h. The solvent was removed in dynamic vacuum, and the red-brown residue was extracted

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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.6b00281. Crystallographic details on compounds 2−5 and [(C5H5)2Mn], decomposition studies on complex 4, field-dependent magnetic studies on complexes 2−5, evaluation of the axial zero-field splitting parameter D for compounds 2 and 4, solution UV/vis spectra of compounds 2−5, computational details (PDF) Optimized structure (MOL) Optimized structure (MOL) Crystallographic information file (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. G


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ACKNOWLEDGMENTS We thank the Deutsche Forschungsgemeinschaft (DFG) for an Emmy-Noether and Heisenberg fellowship (WA 2513/2 and WA 2513/6) (M.D.W.) and financial support through grant WA 2513/7. We acknowledge Prof. Dr. M. Bröring (Institut für Anorganische und Analytische Chemie, TU Braunschweig) for providing access to the SQUID magnetometer.

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