Pentaarylcyclopentadienyl Iron, Cobalt, and Nickel Halides - Inorganic

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Pentaarylcyclopentadienyl Iron, Cobalt, and Nickel Halides Uttam Chakraborty,† Moritz Modl,† Bernd Mühldorf,† Michael Bodensteiner,† Serhiy Demeshko,‡ Niels J. C. van Velzen,§ Manfred Scheer,*,† Sjoerd Harder,*,§,∥ and Robert Wolf*,† †

Institute of Inorganic Chemistry, University of Regensburg, D-93040 Regensburg, Germany Institute of Inorganic Chemistry, University of Göttingen, D-37077 Göttingen, Germany § Stratingh Institute for Chemistry, Nijenborgh 4, 9747 AG Groningen, Netherlands ∥ Inorganic and Organometallic Chemistry, Friedrich Alexander University Erlangen-Nürnberg, D-91058 Erlangen, Germany ‡

S Supporting Information *

ABSTRACT: The preparation of new stable half-sandwich transition metal complexes, having a bulky cyclopentadienyl ligand C5(C6H4-4Et)5 (CpAr1) or C5(C6H4-4-nBu)5 (CpAr2), is reported. The tetrahydrofuran (THF) adduct [CpAr1Fe(μ-Br)(THF)]2 (1a) was synthesized by reacting K[CpAr1] with [FeBr2(THF)2] in THF, and its molecular structure was determined by X-ray crystallography. Complex 1a easily loses its coordinated THF molecules under vacuum to form the solvent-free complex [CpAr1Fe(μ-Br)]2 (1b). The analogous complexes [CpAr1Co(μ-Br)]2 (2), [CpAr1Ni(μ-Br)]2 (3), and [CpAr2Ni(μ-Br)]2 (4) were synthesized from CoBr2 and [NiBr2(1,2-dimethoxyethane)]. The mononuclear, low-spin cobalt(III) and nickel(III) complexes [CpAr2MI2] (5, M = Co; 6, M = Ni) were prepared by reacting the radical CpAr2 with NiI2 and CoI2. The complexes were characterized by NMR and UV− vis spectroscopies and by elemental analyses. Single-crystal X-ray structure analyses revealed that the dimeric complexes 1a, 1b, and 3 have a planar M2Br2 core, whereas 2 and 4 feature a puckered M2Br2 ring.

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INTRODUCTION Half-sandwich complexes of the type [CpRMXn] (CpR = cyclopentadienyl (Cp) or substituted Cp derivative, X = halide) are useful starting materials for introducing cyclopentadienyl metal fragments into a large variety of transition metal compounds.1−3 For late first-row transition metals such as iron, cobalt, and nickel, the synthesis of such heteroleptic cyclopentadienyl metal halides is challenging due to their labile nature. The complexes tend to decompose rapidly into the homoleptic species MX2 and CpR2M. Stable heteroleptic complexes [CpRM(μ-X)]2 can be obtained by using sufficiently bulky cyclopentadienyl ligands.4,5 Some examples are displayed in Figure 1.4 In addition to the divalent iron, cobalt, and nickel halfsandwich complexes discussed above, very few trivalent metal complexes of the type [CpRMX2] have been reported.6 In case of cobalt, the complexes [CpCoX2]n (X = Cl, Br, I; n = 2 or more) were synthesized by decarbonylation of [CpCo(CO)X2], but attempts to isolate and fully characterize these compounds were impaired by their instability.6a By contrast, the pentamethyl cyclopentadienyl derivative [Cp*CoBr2]2 is stable and was shown to be dimeric in the solid state as determined by an X-ray diffraction study.6e Monomeric cobalt half-sandwich complexes of the type [CpRCoX2] are, so far, unknown. The nickel(III) cyclopentadienyl derivative [(η5tBu2C5H2)(tBu2C5H3)NiBr2] (Figure 1), reported by Jutzi and co-workers, is the only structurally characterized compound of this type known to date for any transition metal.6d © XXXX American Chemical Society

Figure 1. Examples of well-characterized divalent complexes [CpRM(μ-X)]2 (M = Fe, Co, Ni; X = Cl, Br, I) and the trivalent complex [(η5tBu2C5H2)(tBu2C5H3)NiBr2].4,6d

As an alternative to bulky alkylcyclopentadienyls, pentaarylcyclopentadienyls may provide substantial kinetic stabilization due to the steric bulk and promise to be less strongly electron donating.7 The pentaphenyl cyclopentadienyl ligand is wellinvestigated, and its metal complexes are accessible from the ligand precursors C5Ph5Br and C5Ph5−.1c However, a major problem is the poor solubility of these complexes in common organic solvents. In this respect, perarylated cyclopentadienyl ligands with alkyl groups in the para position of the aryl Received: December 27, 2015

A

DOI: 10.1021/acs.inorgchem.5b02896 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Scheme 1. Synthesis of Complexes 1a and 1b

Scheme 2. Synthesis of Complexes 2, 3, and 4

substituents have been developed.8 Ligands of the type C5(C6H3-4-R)5 with alkyl substituents such as ethyl or nbutyl groups form highly soluble complexes, which can be easily characterized in solution.9−12 To the best of our knowledge, there is no prior example of a structurally characterized transition metal complex of the type [CpRMX] with a perarylated cyclopentadienyl ligand. The compound [(C5Ph5)Ni(μ-Br)]2 was reported to be formed in the reaction of C5Ph5Br with Ni(CO)4, but no characterization was performed to confirm the nature of the compound.13 This paucity prompted us to investigate the utility of the modified ligands C5(C6H3-4-Et)5 (CpAr1) and C5(C6H3-4-nBu)5 (CpAr2), which have ethyl and n-butyl substituents in the periphery. The ligand C5(C6H3-4-Et)5 is introduced here for practical reasons. It was found that complexes with the C5(C6H3-4-Bu)5 ligand are often too soluble. Also crystal structures of complexes with C5(C6H3-4-Et)5 show generally less disorder than complexes with the C5(C6H3-4-nBu)5 ligand. Another motivation to use the ethyl-substituted ligand lies in the synthesis of the ligand itself: C5(C6H3-4-Et)5H can be purified by crystallization, whereas C5(C6H3-4-nBu)5H forms an oil that must be purified by chromatography. Here, we describe the high-yield synthesis and full characterization of the new complexes [CpAr1M(μBr)]2 (1b−3, M = Fe, Co, Ni) and [CpAr2Ni(μ-Br)]2 (4). Furthermore, we report the synthesis of the trivalent halfsandwich cobalt and nickel diiodides [CpAr2MI2] (5, M = Co; 6, M = Ni) having an unusual monomeric structure. We discuss the single-crystal X-ray structures, spectroscopic data, and magnetic properties of these new complexes.

[CpRFe(μ-I)]2 (CpR = 1,2,4-tBu3C5H2).4e Complex 1b is highly sensitive to air and moisture, and the red-orange colored solution changes to a dark blue immediately on contact with air. The cobalt complex [CpAr1Co(μ-Br)]2 (2) was obtained by reacting K[Cp Ar1 ] with CoBr 2 in THF (Scheme 2). Analogously, treatment of K[CpAr1] and Na[CpAr2] (CpAr2 = C5(C6H4-4-nBu)5) with [NiBr2(DME)] (DME = 1,2-dimethoxyethane) yielded [CpAr1Ni(μ-Br)]2 (3) and [CpAr2Ni(μBr)]2 (4). Complex 2 (82% isolated yield) is a thermally robust olive-green solid. Complexes 3 and 4 were isolated as brown, thermally stable solids in high yields of 78% and 83%, respectively. Complexes 2 and 3 dissolve well in toluene, but they are insoluble in n-hexane, whereas the n-butyl-substituted derivative 4 has a good solubility in n-hexane. Analogous to 1b, 2−4 are air-sensitive as solids and in solution. X-ray Crystallography. Single-crystal X-ray diffraction on a crystal of 1a revealed a dimeric halide-bridged structure (Figure 2) with a nearly planar Fe2Br2 ring (Fe−Br range of 2.5688(6)−2.5870(6) Å) and one THF molecule coordinating to each iron atom. The Fe···Fe distance of 3.6858(7) Å indicates the absence of an iron−iron bond. The two Cp rings are eclipsed, having the phenyl rings in a “propeller”-like arrangement. A similar structure has been reported for the

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RESULTS AND DISCUSSION Synthesis and Properties of Divalent Fe, Co, and Ni Complexes (1−4). The reaction of the potassium salt K[CpAr1] with [FeBr2(THF)2] in tetrahydrofuran (THF) initially affords a yellow solution containing the THF adduct [CpAr1Fe(μ-Br)(THF)]2 (1a, Scheme 1). Upon isolation, complex 1a loses coordinated THF molecules very easily, forming the orange THF-free complex [CpAr1Fe(μ-Br)]2 (1b). The yellow solid slowly becomes red-orange even in an argonfilled glovebox over the course of 1 h and converts to orangebrown 1b under high vacuum (1 × 10−3 Torr). Complex 1b dissolves in toluene, giving a red-orange solution. A yellow solution is observed in THF due to the coordination of the THF molecules to iron (formation of 1a). Similar phenomena have been reported for the alkylcyclopentadienyl complex

Figure 2. Solid-state molecular structures of 1a (left) and 1b (right). Thermal ellipsoids are drawn at the 35% probability level. The H atoms and ethyl groups on the aryl rings are omitted for clarity. Selected distances (Å) and angles (deg) for 1a: Fe1···Fe2 3.6858(7), Fe1−Cp(center) 2.037(1), Fe2−Cp(center) 2.042(1), Fe1−Br1 2.5773(6), Fe1−Br2 2.5777(6), Fe2−Br1 2.5870(6), Fe2−Br2 2.5688(6), Fe1−O1 2.111(2), Fe2−O2 2.101(2); Br1−Fe1−Br2 88.712(1), Br1−Fe2−Br2 88.729(1); torsion angle Fe1−Br1−Fe2− Br2 0.25(2), interplanar angle between two C5Ar15 rings 0.61; for 1b: Fe1···Fe1′ 3.3847(1), Fe1/Fe1′−Cp(center) 1.93856(8), Br1−Fe1/ Fe1′−Br1′ 93.621(4). B


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Inorganic Chemistry

[Cp*Co(μ-Br)]2 show an even stronger puckering of the Co2Br2 unit, which displays a shorter Co···Co distance (2.77 Å) with a fold angle of 67° between the two Co2Br planes.4b By contrast, the chlorido complex [(C5iPr4H)Co(μ-Cl)]2 contains a nearly planar Co2Cl2 ring with a Co···Co distance of 3.183(2) Å.4b The Co−Cp(center) distances (1.696(1) and 1.697(1) Å) in 2 are comparable to that of the cobalt(I) dicarbonyl complex [(C5Ph5)Co(CO)2] (1.70 Å).18 The molecular structure of 3 (Figure 4) features a planar, centrosymmetric Ni2Br2 ring similar to the Fe2Br2 unit in the

related tri-tert-butyl cyclopentadienyl manganese complexes [CpRMn(μ-X) (THF)]2 (CpR = 1,2,4-tBu3C5H2; X = Cl, I).14 The X-ray structure determination of 1b reveals a similar halide-bridged arrangement (Figure 2). A η5-coordinated CpAr1 ligand and two bromine atoms surround each iron atom. Each molecule resides on a crystallographic inversion center. Compared with 1a, shorter Fe···Fe (3.3847(1) Å), Fe1− Cp(center) (1.9386(1) Å), and Fe−Br (2.4757(1) Å) distances are observed in agreement with the lower coordination number of iron (Figure 2). Alkylcyclopentadienyl iron(II) complexes of the type [CpRFe(μ-Br)]2 (CpR = 1,2,4-tBu3C5H2, C5iPr4H, C5iPr5) have similar bond distances of the Fe2Br2 core as complex 1b.4h The Fe1−Cp(center) distances in 1a and 1b are longer than in the complex [(C5Ph5)Fe(CO)2Br] (Fe− Cp(center) 1.738(5) Å)7b and the ferrocene derivatives, for example, [Fe(C5Ph5)2] and [Fe(η5-C5Ph5)(η5-C5H5)].15 The cyclopentadienyl rings in 1b are parallel to each other, having an almost staggered conformation. The electronic difference between aryl-substituted and alkyl-substituted Cp ligands is apparent from the redox potential of −0.04 V for [Fe(C5Ph5)2] versus −0.364 V for [Fe(1,2,4-tBu3C5H2)2] with respect to ferrocene/ferrocenium couple in dichloromethane.16,17 It is interesting to note that the crystal structure of 2 (Figure 3) markedly differs from that of 1b. While the latter complex

Figure 4. Solid-state molecular structures of 3 (left) and 4 (right). Thermal ellipsoids are drawn at the 35% probability level. The H atoms and ethyl groups (for 3) or n-butyl groups (for 4) on the aryl rings are omitted for clarity. Selected distances (Å) and angles (deg) for 3: Ni1···Ni1′ 3.4391(8), Ni1/Ni1′−Cp(center) 1.803(1), Ni1/ Ni1′−Br1/Br1′ 2.412(1); Ni1−Br1/Br1′−Ni1′ 91.39(5), Br1−Ni1/ Ni1′-Br1′ 88.61(5); for 4: Ni1···Ni2 3.2818(6), Ni1−Cp(center) 1.803(3), Ni2−Cp(center) 1.807(3), Ni1−Br1 2.4182(5), Ni1−Br2 2.4290(5), Ni2−Br1 2.4243(5), Ni2−Br2 2.4112(5); Ni1−Br1−Ni2 85.33(2), Ni1−Br2−Ni2 85.38(2), Br1−Ni1−Br2 88.83(2), Br1− Ni2−Br2 89.10(2); Ni1−Br1−Ni2−Br2 torsion angle 25.083(1), fold angle along a line through the two Br atoms 36.4°, interplanar angle between two C5 rings of CpAr2 39.8°.

structure of 1b. The Ni···Ni distance in 3 of 3.4393(2) Å is in the range observed for the complexes [CpRNi(μ-Br)]2 (CpR = 1,2,4-tBu3C5H2 and C5iPr4H; Ni−Ni 3.41−3.48 Å).4c,g This large value indicates that a bonding interaction between the Ni atoms is very unlikely. The structure of the n-butyl-substituted analogue 4 is similar to that of the cobalt complex 2, featuring a puckered Ni2Br2 ring having a fold angle of 36.4° along a line through two bromine atoms. The Ni···Ni distance in 4 (3.2818(6) Å) is shorter than that of 3 by 0.16 Å. Planar and puckered conformers were observed in the same unit cell for the related isopropyl-substituted complexes [(C5iPr4H)M(μBr)]2 (M = Fe, Ni), which suggests that the energy difference between the planar and the puckered conformation is small.4h The Ni−Cp(center) distances in 3 and 4 (1.8029(4)−1.807(3) Å; see Figure 4) are comparable with those in the complexes [CpRNi(μ-Br)]2 (1.77−1.79 Å);4c,g however, these distances are slightly longer than in the monomeric diamagnetic complex [{C5iPr4CH(naphthyl)(NMe2)}NiBr] (Ni−Cp(center) 1.729− 1.757 Å).4f Nuclear Magnetic Resonance and UV−vis Spectroscopy. The 1H NMR spectra of the paramagnetic complexes 1b−4 feature one set of broad and considerably shifted signals (Figures S1−S5). The 1H NMR spectrum of 1b in C6D6 shows four broad signals in the range from −60 to 10 ppm for the C5(C6H4-4-Et)5 unit (Figure S1). The methyl and methylene protons of 1b give rise to broad singlets at −8.1 and 10.2 ppm, respectively. The very broad signal at −59.9 ppm and another broad signal at 0.5 ppm may be assigned to the ortho and meta aromatic protons. Addition of excess THF into the sample

Figure 3. Solid-state molecular structure of 2. Thermal ellipsoids are drawn at the 35% probability level. The H atoms and ethyl groups on the aryl rings are omitted for clarity. Selected distances (Å) and angles (deg): Co1···Co2 3.2071(6), Co1−Cp(center) 1.696(1), Co2− Cp(center) 1.697(1), Co1−Br1 2.376(2), Co1−Br2 2.3551(5), Co2−Br1 2.387(2), Co2−Br2 2.3705(5); Br1−Co1−Br2 89.501(2), Br1−Co2−Br2 88.852(2); Co1−Br2−Co2−Br2 torsion angle 25.30(4), fold angle along a line through two Br atoms 36.7°, 1

interplanar angle between the C5 rings of CpAr 34.6°.

displays a planar Fe2Br2 core (vide supra), 2 exhibits a butterfly structure with a Co···Co distance of 3.2069(6) Å and Co−Br distances in the range of 2.3551(5)−2.387(2) Å. The fold angle along a line through two Br atoms is 36.7°, and the two Cp rings in the dimeric unit are not parallel, having an interplanar angle of 34.6°. In comparison, reported preliminary structural data of the analogous pentamethylcyclopentadienyl derivative C


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Inorganic Chemistry

Figure 5. ΧMT vs T plot (left) and VTVH magnetization measurements as Mmol vs B/T (right) for 1b. Solid lines represent the calculated curve fit (see text).

similarity of the complexes 3 and 4 in solution is also evident from their UV−vis spectra in toluene, which show absorptions at ∼400, 450, and 560 nm (Figure S14). Magnetic Measurements. Complexes 1−6 are paramagnetic; their magnetic moments were determined in solution by the Evans NMR method. The solution magnetic susceptibility measurement of 1b in C6D6 gave a magnetic moment of 7.8(1) μB per dimer at 300 K. This value indicates the presence of high-spin Fe(II) centers with four unpaired electrons per metal atom. The solution magnetic moment of 2 in C6D6 was found to be 2.9(1) μB per dimer. This relatively low magnetic moment suggests that the dimer contains lowspin Co(II) centers having one unpaired electron each. The solution magnetic moments of 3 (4.1(1) μB) and 4 (4.3(1) μB) are similar and suggest the presence of two high-spin Ni(II) centers with two unpaired electrons each. Interestingly, even though the first half-sandwich complexes of the type [CpRMX]2 (M = Fe, Co, Ni) were reported more than 30 years ago,19,20 reports about the temperaturedependent magnetic properties in the solid state are still extremely rare. To the best of our knowledge, there are only two such reports for iron,4i,21 one for nickel4g complexes, and two reports for cobalt.4b,22 Thereby, some of these examples were analyzed with the Curie−Weiss law, which has a limited applicability as the most overworked formula in paramagnetism,23 or even without specifications how the analysis was done, rendering magnetic investigations of 1b, 2, 3, and 4 timely and important. To investigate the solid-state magnetism of 1b, the temperature-dependent magnetic susceptibility as well as variable-temperature variable-field (VTVH) magnetization measurements were performed (Figure 5). The χMT value of 6.7 cm3 K mol−1 at 210 K (corresponding to a μeff value of 7.32 μB) is somewhat higher than the spin-only value for two uncoupled high-spin Fe(II) ions (6.0 cm3 K mol−1 for g = 2.00), similar to the solution measurement. The decrease of the χMT curve upon cooling indicates anti-ferromagnetic exchange between iron centers and/or zero-field splitting (ZFS). Experimental χMT versus T data as well as the VTVH magnetization measurements were simultaneously modeled using the isotropic Heisenberg−Dirac−van Vleck (HDvV) exchange Hamiltonian that includes additional terms for ZFS and Zeeman splitting (eq 1).24

dissolved in C6D6 led to slight shifts of the signals (Figure S2), probably due to the formation of the THF adduct 1a or the monomeric species [CpAr1FeBr(THF)2]. Interestingly, only one set of 1H NMR signals could be identified for the THF molecules. These signals are slightly downfield shifted compared to free THF in C6D6. The 1H NMR spectrum of 1b is similar in THF-d8 at 300 K (Figure S3). Variabletemperature 1H NMR measurements in THF-d8 down to 193 K revealed that the signals observed at room temperature (r.t.) broaden upon cooling, followed by the appearance of additional very broad signals (Figures S4 and S5). Although the nature of these species remains presently unclear, it is tempting to speculate whether they might arise from a monomer−dimer equilibrium. Coordination of THF is also apparent from the distinct UV− vis spectra of 1b in toluene and THF (Figure S12). The UV− vis spectrum of a toluene solution displays an intense band at 365 nm and weaker absorptions at 495, 575, and 614 nm, whereas the spectrum of a THF solution shows an intense featureless band at 351 nm tailing into the visible region. The 1H NMR spectrum of the 17 valence electron (VE) complex 2 displays three broad signals in the range from 1.1 to 8.4 ppm in C6D6 assigned to the C5(C6H4-4-Et)5 moiety (Figure S6). The methyl and methylene protons of 2 appear as broad singlets at 1.1 and 5.2 ppm, respectively. The ortho and meta protons give rise to a broad signal at 8.4 ppm and another very broad signal at 5.5 ppm, overlapping with the signal from methylene protons. The UV−vis spectrum of 2 in toluene exhibits strong shoulders at λ = 320 and 365 nm and weaker absorptions at λ = 465, 575, and 614 nm, which are similar to those of 1b in toluene (Figure S13). The 1H NMR spectroscopic patterns of 3 and 4 in C6D6 are similar to those of 1b and 2, but the signals are comparatively sharp. Complex 3 exhibits four signals in the range from −19 to 30 ppm with the intensity ratio of 10:15:10:10 as expected for the C5(C6H4-4-Et)5 ligand (Figure S7). The methyl and methylene protons appear as a triplet and quartet at 1.9 and 29.9 ppm, respectively. Two broad signals at −19.4 and 24.4 ppm may be assigned to the ortho and meta aromatic protons. The 1H NMR spectrum of 4 displays six broad signals in the range from −18 to 27 ppm with relative intensities expected for C5(C6H4-4-nBu)5 units (Figure S8). The CH3 protons appear as a broad signal at 1.6 ppm, and the CH2 protons give three broad signals at 1.3, 1.9, and 26.7 ppm. The ortho and meta protons of the aryl rings appear at −18.4 and 23.9 ppm. The D


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Inorganic Chemistry 2

Ĥ = −2JS1̂ S2̂ +

⎧

The temperature-dependent magnetic susceptibility measurements of solid 3 and 4 reveal the S = 1 high-spin state of individual Ni(II) ions, since the observed χMT values at r.t. of 2.45 cm3 K mol−1 (or 4.43 μB) for 3 and 2.57 cm3 K mol−1 (or 4.53 μB) for 4 are slightly higher than the spin-only value of 2.0 cm3 K mol−1 for two S = 1 spin carriers. Interestingly, the temperature dependences of χMT for 3 and 4 differ principally (Figure 7). While the χMT versus T plot for 3 remains nearly constant up to 50 K and then approaches zero at lower temperatures, the χMT curve for 4 clearly increases with decreasing temperature at first and then decreases. Therefore, the χMT curve shape indicates an anti-ferromagnetic exchange interaction and/or ZFS for 3 and ferromagnetic exchange with ZFS for 4. The nesting of VTVH magnetization curves (Figure 7) suggest a sizable ZFS contribution for both complexes. The simultaneous analysis of experimental χMT versus T as well as the VTVH data using eq 1 (assuming gx = gy = gz) led to the values g = 2.22, J = −1.4 cm−1, and D = +30.5 cm−1 for 3 and g = 2.16, J = +16.4 cm−1, and D = +36.8 cm−1 for 4. The parameters obtained for 3 are similar to those for recently reported [(C5iPr4H)Ni(μ-Br)]2 (J = −2.0 cm−1 and D = +48.2 cm−1), as one could expect for the structurally similar Ni2Br2 cores: planar in both cases and with almost identical Ni−Br−Ni angles of 91.4° for 3 and 89.9−90.8° for [(C5iPr4H)Ni(μBr)]2.4g The Ni−Br−Ni angles of 85.3−85.4° in 4 are also close to these values, but the magnetic exchange coupling is of the opposite sign. We explain this finding with the butterfly structure of 4, that is, with the folding of Ni2Br2 ring along a line through the two Br atoms. The corresponding fold angle between both NiBr2 planes is 36.4° for 4 and 0° for planar 3 and [(C5iPr4H)Ni(μ-Br)]2. This distortion can be also expressed through the Ni−Br−Br−Ni torsion angle, which is 25.1° for 4 and again 0° for 3 and [(C5iPr4H)Ni(μ-Br)]2. The deviation from the planarity results in the minimization of the overlap integral through the bridging ligands, and therefore, the anti-ferromagnetic exchange contribution (Jantiferro) to the overall coupling term (J = Jferro + Jantiferro) becomes smaller, providing a ferromagnetically coupled system. Although the influence of the deviation from the planarityusually in the form of the torsion angle dependenceon the sign and strength of magnetic exchange is well-established in molecular magnetism,25 a change of the sign of magnetic coupling in [CpRNiX]2-type complexes has been observed here for the first time. Trivalent Half-Sandwich Complexes of Ni and Co. Aiming at the synthesis of new trivalent cobalt and nickel complexes, we studied the reactivity of the CpAr2 radical toward CoI2 and NiI2. The CpAr2 radical was prepared from Na[CpAr2] and CuBr according to the literature procedure.11b The reaction of the radical with MI2 (M = Co, Ni) in n-hexane afforded the monomeric trivalent half-sandwich complexes [CpAr2CoI2] (5) and [CpAr2NiI2] (6) (Scheme 3). Complex 5 was obtained as a waxy dark green solid in 23% yield, while complex 6 was isolated as a dark red waxy solid in 58% yield. Compounds 5 and 6 dissolve well in n-hexane and can be purified by crystallization from that solvent. They give a blue solution in coordinating solvents such as THF and acetonitrile, indicating the formation of the blue CpAr2 radical due to decomposition. Similar decomposition phenomena have also been reported for [CpCoI2]2, which decomposes in coordinating solvents to the cobaltocenium cation and CoI2.6a The Cp* derivative [Cp*CoI2]2 does not undergo such decomposition.6a,e This stability of [Cp*CoI2] has been attributed to the steric

∑ ⎨μB(SxigxiBx + SyigyiBy + SzigziBz ) i=1

⎩

⎡ 2 1 ⎤⎫ + D⎢Szî − Si(Si + 1)⎥⎬ ⎣ ⎦⎭ 3

(eq 1)

The best-fit values are gx = gy = 1.94, gz = 3.06, J = −7.2 cm−1, and D = −8.4 cm−1. Thus, the magnetic exchange is clearly anti-ferromagnetic with the resulting diamagnetic ST = 0 ground state. Similar antiferromagnetic coupling between two high-spin Fe(II) centers also has been demonstrated very recently for [(1,2,4tBu3C5H2)Fe(μ-I)]2.4i In contrast to this observation, the magnetic coupling in the previously reported [(C5iPr4H)Fe(μBr)]2 was found to be ferromagnetic.19 However, the detailed magneto-structural correlations and comparisons cannot be done here, since the structure of [(C5iPr4H)Fe(μ-Br)]2 was determined 18 years after the magnetic measurement, and crystals for X-ray analysis were obtained by a modified synthetic procedure; that is, the samples for the magnetic and X-ray studies are not necessarily the same polymorphic modification. This makes 1b a rare example of iron complex of this type to be characterized magnetically and structurally. The magnetic properties of 2 are consistent with two lowspin Co(II) ions, which are anti-ferromagnetically coupled (Figure 6).

Figure 6. ΧMT vs T plot for 2. Solid line represents the calculated curve fit (see text).

The χMT versus T measurement was analyzed using the HDvV Hamiltonian with isotropic exchange interaction and Zeeman splitting (eq 2). 2

Ĥ = −2JS1̂ S2̂ +

∑ gμBB⃗ ·Si⃗ i=1

(eq 2)

The fitting procedure led to the values g = 1.97 and J = −58 cm−1. A similar medium-strong anti-ferromagnetic exchange interaction of −30 cm−1 (for comparison, all literature known J values are adapted to the H = −2JS1S2 formalism and converted in cm−1) was observed for [(C5iPr4H)Co(μ-Cl)]2.4b In both complexes the Co−X−Co angles are similar and close to orthogonality, that is, 84.7−85.5° for 2 and 91.2° for [(C5iPr4H)Co(μ-Cl)]2. Much stronger coupling of J = −238 cm−1 was found for [Cp*Co(μ-Cl)]2; however, structural information is still missing for this compound synthesized in 1986.20 E


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Inorganic Chemistry

Figure 7. ΧMT vs T plots ((a) for 3 and (c) for 4) and VTVH magnetization measurements as Mmol vs B/T ((b) for 3 and (d) for 4). Solid lines represent the calculated curve fit (see text).

Scheme 3. Synthesis of Complexes 5 and 6

Figure 8. Solid-state molecular structures of 5 (left) and 6 (right). Thermal ellipsoids are drawn at the 35% probability level. The H atoms and nBu groups on the aryl rings are omitted for clarity. Selected bond distances (Å) and angles (deg) for 5: Co−Cp(center) 1.679(2), Co−I1 2.496(1), Co−I2 2.4859(7); I1−Co−I2 93.86(3); for 6: Ni−Cp(center) 1.765(1), Ni−I1 2.4747(5), Ni−I2 2.4836(6); I1−Ni−I2 96.33(2).

hindrance from the bulkier Cp* ligand and to the stronger Co− Cp* bonding compared to the Cp analogue.6a,e Complexes 5 and 6 were fully characterized by NMR spectroscopy, UV−vis spectroscopy, X-ray crystallography, and elemental analysis. The 1H NMR spectrum of the 16 VE diamagnetic d6 cobalt(III) complex 5 in C6D6 displays a set of signals for C6H4-4-nBu units in the range of 0.6−6.1 ppm (Figure S9). The aliphatic protons give four multiplets in the range of 0.6−1.8 ppm with the intensity ratio of 15:10:10:10, while the aromatic protons appear as two doublets at 5.73 and 6.10 ppm. In the case of the 17 VE paramagnetic complex 6, broad overlapping signals in the range of 0.9−1.2 ppm correspond to the methyl and methylene protons of C6H4-4nBu units, whereas the two very broad signals appearing at 11.0 and 13.7 ppm may be assigned to the aromatic protons (Figure S11). The solution magnetic moment of 6 in C6D6 was found to be 2.2(1) μB, suggesting a low-spin d7 configuration having one unpaired electron on the nickel center. The structures of 5 and 6 (Figure 8) are isomorphous and display a two-legged piano-stool motif with an η5-coordinated CpAr2 ligand and two iodine atoms bound to cobalt or nickel. A similar coordination environment has been observed for [(η5tBu2C5H2)(tBu2C5H3)NiBr2].6d The Co−Cp(center) distance

in 5 (1.679(2) Å) is comparable with those of 2 and the dimeric Cp* derivative [Cp*CoBr2]2 (Co−Cp* 1.688(9) Å).6e Complex 5 features Co−I distances of 2.496(1) and 2.4859(7) Å, which are shorter than those of the 18 VE complexes [CpCoI2(L)] (L = CO, PPh3; Co−I 2.565−2.607 Å)26 because of the lower coordination number of the cobalt atom in 5. The Ni−Cp(center) distance of 6 (1.765(1) Å) is similar to that of 3 and 4 (Table 1) and slightly longer than in the complex [(η5tBu2C5H2)(tBu2C5H3)NiBr2] (1.734 Å).6d The Ni−I distances of 2.4747(5) Å and 2.4836(6) Å in 6 are in the range observed for the Ni(I) NHC complexes [CpNiI(NHC)] (Ni−I: 2.334− 2.525 Å).27 The I1−M−I2 bond angles in 5 (93.86(3)°) and 6 (96.33(2)°) are slightly smaller than the Br−Ni−Br bond angle (99.3°) in the complex [(η5-tBu2C5H2)(tBu2C5H3)NiBr2].6d F


Article

Inorganic Chemistry Table 1. Comparison of Selected Bond Lengths (Å) and Angles (deg) of 1−6

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1a 1b 2 3 4 5 6

M···M

M−Cp(center)

M−X

X−M−X

3.6858(7) 3.3847(1) 3.2069(6) 3.4393(2) 3.2818(6)

2.037(1), 2.042(1) 1.9386(1) 1.696(1), 1.697(1) 1.8029(4) 1.803(3), 1.807(3) 1.679(2) 1.765(1)

2.5688(6)−2.5870(6) 2.4757(1) 2.3551(5)−2.387(2) 2.3941(1) 2.4112(5)−2.4290(5) 2.496(1), 2.4859(7) 2.4747(5), 2.4836(6)

88.712(1), 88.729(1) 93.621(4) 89.501(2), 88.852(2) 88.266(4) 88.827(1), 89.097(1) 93.86(3) 96.33(2)

red solid remained. This residue was extracted with toluene (15 mL). Complete evaporation of the solvent gave a dark red solid that was treated with n-hexane (20 mL) and stirred overnight, giving an orange suspension. Complex 1b was isolated as an orange-brown solid, which was sufficiently pure for subsequent transformations, by filtration and subsequent drying in vacuo. The compound can be purified by recrystallization from toluene. Yield: 429 mg (0.594 mmol, 74%). mp > 287 °C (decomp); UV−vis (toluene): λmax/nm (εmax/L·mol−1·cm−1) = 365 (26 403), 495 (4860), 575 (6551), 614 (7886); UV−vis (THF): λmax (nm, (λmax/L·mol−1·cm−1)): 351 (22 395). 1H NMR (400.13 MHz, C6D6, 300 K): −59.9 (s vbr, Δυ1/2 = 2971 Hz, 10 H, 5 × o/m− CH), −8.06 (s, Δυ1/2 = 18 Hz, 15 H, 5 × CH3), 0.5 (s br, Δυ1/2 = 115 Hz, 10H, o/m−CH), 10.15 (s, Δυ1/2 = 23 Hz, 10H, 5 × CH2). Magnetic moment in C6D6 (Evans method): 7.8(1) μB for the dimer. Elemental analysis was obtained on a recrystallized sample from toluene; calcd. for C90H90Fe2Br2·C7H8 (Mw = 1535.35 g/mol) C 75.88, H 6.43; found C 75.91, H 6.58%. Complex 1a was obtained as a yellow crystalline solid upon diffusion of n-hexane into the THF solution of 1b and by cooling a saturated THF/n-heptane solution at −35 °C overnight. The yellow crystalline solid was isolated by decanting the supernatant or by filtration. The color of this solid slowly turned to orange over 1 h and then to orange-brown in vacuo, giving the THF-free complex 1b. [CpAr1Co(μ-Br)]2 (2). A solid mixture of K[CpAr1] (1.00 g, 1.60 mmol) and CoBr2 (350 mg, 1.60 mmol) was treated with THF (50 mL) and stirred at room temperature for 18 h, affording a brownishgreen solution. The mixture was completely evaporated to dryness, and the dark brown solid was extracted with toluene (25 mL). The toluene extract was evaporated to dryness, and the sticky dark brown solid was treated with n-hexane (20 mL) and stirred for 12 hours giving an olive-green suspension. Complex 2 was isolated as an olivegreen solid by filtration and subsequently dried in vacuo for 1 h. Yield: 952 mg (82%, 1.313 mmol). mp > 283 °C (decomp); UV−vis (toluene): λmax/nm (εmax/L·mol−1·cm−1) = 320 (sh, 14 022), 365 (sh, 9828), 465 (2199), 575 (1633), 614 (1913). 1H NMR (400.13 MHz, C6D6, 300 K): 1.1 (s br, Δυ1/2 = 23 Hz, 15 H, 5 × CH3), 5.2 (s br, Δυ1/2 = 32 Hz, 10H, 5 × CH2), 5.5 (very broad signal overlapping with the signal of the CH2 protons, 10H, 5 × o/m−CH), 8.4 (s br, Δυ1/2 = 39 Hz, 10H, 5 × o/m−CH). Magnetic moment in C6D6 (Evans method): 2.9(1) μB for the dimer. Anal. Calcd for C90H90Co2Br2 (Mw = 1449.38 g/mol) C 74.58, H 6.26; found C 74.36, H 6.38%. [CpAr1Ni(μ-Br)]2 (3). A mixture of K[CpAr1] (2.00 g, 3.20 mmol) and [NiBr2(DME)] (1.00 g, 3.24 mmol) was treated with THF (25 mL) while stirring. The brown solution was stirred for 18 h. The resulting slightly turbid red-brown solution was filtered, and the filtrate was evaporated to dryness. A brown solid remained that was extracted with toluene (25 mL). The extracted solution was evaporated to dryness. The resulting brown solid was washed once with n-hexane (15 mL) and dried in vacuo for 2 h. Yield: 1.80 g (2.48 mmol, 78%). mp > 285 °C (decomp); UV−vis (toluene): λmax/nm (εmax/L·mol−1·cm−1) = 340 (22 142), 394 (sh, 10 237), 450 (8724), 565 (2400). 1H NMR (400.13 MHz, C6D6, 300 K): −19.4 (s br, Δυ1/2 = 56 Hz, 10 H, 5 × CH2), 1.91 (t, 3JH,H = 7.0 Hz, 15 H, 5 × CH3), 24.44 (s, Δυ1/2 = 8 Hz, 10H, 5 × o/ m−CH), 29.94 (m, 10H, 5 × o/m−CH). Magnetic moment in C6D6 (Evans method): 4.1(1) μB for the dimer. Anal. Calcd for C90H90Ni2Br2 (Mw = 1448.90 g/mol) C 74.61, H 6.26; found C 75.03, H 6.43%.

CONCLUSION Salt elimination reactions of a bulky pentaarylcyclopentadienide with suitable metal(II) bromides afforded a series of new stable divalent iron, cobalt, and nickel half-sandwich complexes [CpAr1M(μ-Br)]2 (1b−3) and [CpAr2M(μ-Br)]2 (4) in high yields. The THF complex [CpAr1Fe(μ-Br)(THF)]2 (1a) was identified as the initial reaction product of the synthesis of complex 1b in THF. The dimeric complexes 1a, 1b, and 3 have a planar M2Br2 core, whereas 2 and 4 display a puckered M2Br2 ring. Magnetic measurements in solution and in the solid state revealed the magnetic properties of the new complexes. Complex 2 features weakly anti-ferromagnetically coupled low-spin cobalt(II) atoms, whereas 3 and 4 display two Ni centers each having spin S = 1, but show weak antiferromagnetic coupling in the case of 3 and ferromagnetic coupling in the case of 4. This different magnetic behavior corresponds to the different structural arrangements observed in the solid state. Trivalent cobalt and nickel complexes [CpAr2MI2] (5, M = Co; 6, M = Ni), which have isomorphous mononuclear structures, are accessible via the radical addition of C5(C6H4-4nBu)5 to NiI2 and CoI2, respectively. The well-defined and readily accessible complexes 1−6 provide a convenient entry point for the future synthesis of pentaarylcyclopentadienyl metal chemistry with unprecedented structures and reactivities, for example, unprecedented dimetallocenes of the type [CpArMMCpAr]28 and novel polyphosphorus compounds for supramolecular chemistry.29,30 Efforts toward these goals are underway in our laboratories.

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EXPERIMENTAL SECTION

General Considerations. All experiments were performed under an atmosphere of dry argon, by using standard Schlenk and glovebox techniques. Solvents were purified, dried, and degassed with an MBraun SPS800 solvent purification system. NMR spectra were recorded on Bruker Avance 300 and Avance 400 spectrometers at 300 K and internally referenced to residual solvent resonances. The 1H NMR signals of complexes 1−4 were assigned by 1H,1H COSY experiments and the relative integration of the signals. Melting points were measured in sealed capillaries on a Stuart SMP10 melting point apparatus. UV−vis spectra were recorded on a Varian Cary 50 spectrometer. Elemental analyses were determined by the analytical department of Regensburg University. The pentaarylcyclopentadienyl salts K[CpAr1] and Na[CpAr2] were prepared according to known procedures for similar compounds.9c [FeBr2(THF)2]31 and [NiBr2(DME)] were synthesized according to the literature procedures. Anhydrous CoBr2 and CoI2 were purchased from Sigma-Aldrich and used as received. [CpAr1Fe(μ-Br)]2 (1b). K[CpAr1] (500 mg, 0.800 mmol) and [FeBr2(THF)2] (292 mg, 0.811 mmol) were mixed as solids and treated with THF (40 mL). After the mixture was stirred for 4 h at ambient temperature, the yellowish-green turbid solution was filtered. The filtrate was evaporated completely to dryness, and an oily, dark G


Article

Inorganic Chemistry [CpAr2NiBr]2 (4). A solid mixture of Na[CpAr2] (500 mg, 0.66 mmol) and [NiBr2(DME)] (206 mg, 0.66 mmol) was treated with THF (10 mL) under stirring. The brown solution was stirred for 2 h, and the solvent was removed in vacuum. The brown residue was triturated with n-hexane (10 mL) and filtered through Celite. The brown filtrate was concentrated to 3 mL and cooled to −35 °C. Brown crystals were isolated by decanting the mother liquor and dried in vacuum. Another portion of crystals was obtained from the mother liquor upon cooling to −35 °C. Yield: 473 mg (0.274 mmol, 83%). mp > 89 °C (decomp); UV−vis (toluene): λmax/nm (εmax/L·mol−1·cm−1) = 345 (24 605), 400 (sh, 10 588), 455 (9751), 575 (3268), 615(2773). 1H NMR (400.13 MHz, C6D6, 300 K): −18.4 (s br, Δυ1/2 = 70 Hz, 10H, 5 × o/m−CH), 1.28 (s br, Δυ1/2 = 26 Hz, 10H, 5 × CH2), 1.62 (s br, 15H, 5 × CH3), 1.92 (s br, Δυ1/2 = 27 Hz, 10H, 5 × CH2), 23.85 (s, Δυ1/2 = 18 Hz, 10H, 5 × o/m−CH), 26.69 (s, Δυ1/2 = 23 Hz, 10H, 5 × CH2). Magnetic moment in C6D6 (Evans method): 4.3(1) μB for the dimer. Anal. Calcd for C110H130Ni2Br2 (Mw = 1729.44 g/mol) C, 76.40; H, 7.58; found C, 76.22; H, 7.59%. [CpAr2MI2] (5, M = Co; 6, M = Ni). A solution of Na[CpAr2] (250 mg, 0.33 mmol) in THF (3 mL) was added to a suspension of CuBr (47 mg, 0.33 mmol) in THF (3 mL). The mixture turned into blue/ purple, and a dark precipitate formed. After removal of the solvent, nhexane (5 mL) was added. The CpAr2 radical solution was filtered off via cannula and added directly to a suspension of MI2 (CoI2: 113 mg, 0.36 mmol; NiI2: 113 mg, 0.36 mmol) in n-hexane (10 mL). The reaction mixture was stirred for 18 h and filtered through Celite. The obtained solution was reduced to 5 mL; needle-shaped crystals formed during storage at 4 °C. Further concentration of the mother liquor and storage at 4 °C yielded a second crop of crystals. Yield: 78 mg of 5 (0.076 mmol, 23%); 95 mg of 6 (0.092 mmol, 58%). Analytical data for 5: mp > 188 °C (decomp); UV−vis (toluene): λmax/nm (εmax/L·mol−1·cm−1) = 365 (8508), 434 (5890), 528 (4902), 616 (1744), 693 (br, 560). 1H NMR (300.13 MHz, C6D6, 296.5 K): 0.60 (t, 3JHH = 7.2 Hz, 15H, 5 × CH3), 0.87 (m, 10H, 5 × CH2CH3), 1.00 (m, 10H, 5 × CH2CH2CH2), 1.76 (t, 3JHH = 7.7 Hz, 10H, 5 × C6H4CH2CH2), 5.73 (d, 3JHH = 7.9 Hz, 10H, 5 × o/m−CH), 6.10 (d, 3 JHH = 7.9 Hz, 10H, 5 × o/m−CH). 13C{1H} NMR (300.13 MHz, C6D6, 300 K): 13.8 (nBu), 22.3 (nBu), 32.4 (nBu), 35.5 (nBu), 119.4 (C5(C6H4-4-nBu)5), 128.2 (C5(C6H4-4-nBu)5), 129.1 (C5(C6H4-4nBu)5), 140.6 (C5(C6H4-4-nBu)5), 143.0 (C5(C6H4-4-nBu)5). Anal. Calcd for C55H65CoI2 (Mw = 1038.87) C, 63.59; H, 6.31; found C, 63.93; H, 6.30%. Analytical data for 6: mp > 192 °C (decomp); UV−vis (toluene): λmax/nm (εmax/L·mol−1·cm−1) = 365 (7612), 463 (9927), 515 (9610), 618 (br, 1652). 1H NMR (300.13 MHz, C6D6, 296.5 K): 0.92−1.19 (m br, 45H, 5 × CH3 + 5 × C6H4−CH2CH2CH2CH3), 11.0 (s br, Δυ1/2 = 194 Hz, 10H, 5 × o/m−CH), 13.7 (s br, Δυ1/2 = 202 Hz, 10H, 5 × o/m−CH). Magnetic moment in C6D6 (Evans method): 2.2(1) μB. Anal. Calcd for C55H65NiI2 (Mw = 1038.63) C 63.60, H 6.31; found C 63.53, H 6.24%. X-ray Crystallography. Yellow single crystals of 1a were obtained from THF/n-hexane solution. Suitable dark red crystals for 1b were grown upon slow evaporation of a toluene solution. Dark orange single crystals of 2 were obtained by layering a THF solution with n-hexane. Dark red-brown single crystals of 3 were obtained by slow evaporation of a toluene solution. Single crystals of 4 (dark red-brown), 5 (dark brown), and 6 (brown-red) suitable for X-ray structure determinations were grown from n-hexane solutions at −35 °C. The crystals were processed at an Agilent Technologies SuperNova Atlas CCD diffractometer with microfocus Cu radiation (1a, 2, 3, 6), an Agilent Technologies SuperNova Eos CCD device employing microfocus Mo radiation (1b), or a GV50 TitanS2 CCD device with microfocus Cu radiation (4, 5). The CrysAlisPro software was used to apply analytical (1b, 2, 4, 5, 6) or multiscan absorption corrections (1a, 3).32 Using Olex2,33 the structures were solved with direct methods by ShelXS or ShelXT and refined with ShelXL using least-squares minimization.34 The selected crystal of 1a was a nonmerohedral twin. The twin law was determined with CrysAlisPro software, and the structure was refined with the corresponding HKLF5 file. The Br1 position in 1b is disordered over four sites (Br1, Br2, Br3, and Br4). In addition, one

aryl ring is disordered. For complex 2, two methyl groups and a THF solvate molecule are disordered. In addition, Br1 in 2 is disordered over two sites (Br1 and Br1a). In complex 3, one methyl group was disordered. In addition, Br1 is disordered over three sites (Br1, Br1a, and Br1b). PLATON SQUEEZE was used for the refinement of 4.35 Geometrical and displacement restraints were applied to the disordered parts of the structures where necessary. In case of disordered Br atoms (1b, 2, and 3), the structural parameters are given for the part associated with higher occupancy. Details of the structure determinations and additional crystallographic information are given in Table S1 of the Supporting Information. Magnetic Moments Measurements. Temperature-dependent magnetic susceptibility measurements were performed with a Quantum-Design MPMS-XL-5 SQUID magnetometer equipped with a 5 T magnet in the range from 210 to 2 K for 1b and from 295 to 2 K for 2−4 in a magnetic field of 0.5 T. The polycrystalline samples were contained in a gel bucket, in the case of 1b additionally covered with a drop of low viscosity perfluoropolyether-based inert oil Fomblin Y45 to fix the crystals, and fixed in a nonmagnetic sample holder. The maximum measuring temperature of 210 K for 1b was chosen because of the pour point of the oil, to keep the oil in the frozen state, and to avoid therefore the orientation of the crystals parallel to the magnetic field. Each raw data file for the measured magnetic moment was corrected for the diamagnetic contribution of the gel bucket and of the inert oil according to Mdia = χg·m·H, with the experimentally obtained gram susceptibility of the gel bucket (χg = −5.70 × 10−7 emu/(g·Oe) and of the oil (χg = −3.82 × 10−7 emu/(g· Oe)). The molar susceptibility data were corrected for the diamagnetic contribution according to χMdia(sample) = −0.5·M × 1 × 10−6 cm3· mol−1.36 A Curie-behaved paramagnetic impurity (PI) with spin S = 5/ 2 for 1b (0.4%) and S = 1/2 for 2 (2%) as well as temperatureindependent paramagnetism (TIP) were included according to χcalc = (1 − PI)·χ + PI·χmono + TIP. Before simulation, the experimental data were corrected for TIP (TIP = 90 × 10−6 cm3·mol−1, 730 × 10−6 cm3· mol−1, and 290 × 10−6 cm3·mol−1 for 1b, 2, and 3, respectively).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02896. The crystallographic information files (CIF) have been deposited at the CCDC, 12 Union Road, Cambridge, CB21EZ, U.K., and can be obtained on request free of charge, by quoting the publication citation and deposition numbers CCDC 1441736−1441742. NMR and UV−vis spectra, crystallographic data of 1−6. (PDF) X-ray crystallographic information. (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (M.S.) *E-mail: [email protected]. (S.H.) *E-mail: [email protected]. Website: http://www.uniregensburg.de/chemistry-pharmacy/inorganic-chemistry-wolf/ index.html. (R.W.) Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the Deutsche Forschungsgemeinschaft is gratefully acknowledged. H


Article

Inorganic Chemistry

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