Phenylchromium(III) Chemistry Revisited 100 Years after Franz Hein

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Phenylchromium(III) Chemistry Revisited 100 Years after Franz Hein (Part I) Reinald Fischer, Helmar Görls, Regina Suxdorf, and Matthias Westerhausen* Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University Jena, Humboldtstraße 8, D-07743 Jena, Germany

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S Supporting Information *

ABSTRACT: On the basis of early studies of Franz Hein (1892−1976) on (poly)phenylchromium(III) compounds of the type PhxCrX3−x(L)n (1 ≤ x ≤ 3) and polyphenylchromate(III) derivatives of the type (L)nMx−3CrPhx (3 ≤ x ≤ 6) with L being an ether like tetrahydrofuran (thf), 1,2-dimethoxyethane (dme), or tetrahydropyran (thp), we reinvestigated the coordination chemistry depending on the s-block metal of the phenyl-transfer reagent (Li, Mg, Ca) and the solvent (denticity, donor strength). Thus, the following compounds have been prepared, and their molecular structures determined (the number of the complex relates to the number of σ-bound phenyl groups): [mer-I3Cr(thf)3], [trans-Cl2CrPh(thf)3] (1-Cl), [trans-I2CrPh(thf)3] (1-I), [(thf)5MgCl]+ [{Ph2Cr(thf)}2(μ-Cl)3]− ((2-Cl)2·MgCl2), [fac-Ph2ICr(thf)3] (2-I), [fac-Ph3Cr(thf)3] (3), [Li(dme)3]+ [Ph4Cr(dme)]− (4-Li), [PhMg(dme)2(thf)]+ [Ph4Cr(dme)]− (4Mg), [{(dme)Li}2CrPh5] (5-Li), [Li(dme)3]+ [{(dme)Li}CrPh5]− (5-Li′), [{(dme)(thf)Ca(μ-Ph)3}2Cr]+ [Ph4Cr(thf)]− (5Ca), [Li(thf)4]+ [{(thf)Li(μ-Ph)3}2Cr]− (6-Li), [Li(thp)4]+ [{(thp)Li(μ-Ph)3}2Cr]− (6-Li′), and [Li(dme)3]+ [{(thf)Li(μPh)3}2Cr]− (6-Li″). Especially the hexaphenylchromates(III) with lithium counterions are very reactive, and the ether degradation product [{(thf)3Li2O}CrPh3]2 (7) or other side products like [Li(thp)4]+ [{(thp)Li}2(μ-Ph)4(μ-C12H8)Cr]− (8) with a biphenyl-2,2′-diide ligand are observed. In phenylchromium(III) complexes, the octahedral environment is preferred. Highly nucleophilic phenyl reagents (like phenyl-lithium) are required to prepare hexaphenylchromate(III) complexes, whereas an excess of diphenyl calcium is not sufficient to synthesize pure derivatives with the general composition of Ca3(CrPh6)2. Furthermore, hexaphenylchromate(III) ions are only stable if the ipso-carbon atoms of the phenyl groups are bridged by electropositive cations (like lithium).

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and Na3CrPh6.14 The first sandwich complexes, ferrocene15 and dibenzenechromium,16 were discovered in the 1950s.3 Thereafter, reduction of chromium(I) in these π-arene complexes with dithionite allowed a straightforward synthesis of [(η6-C6H6)2Cr]17 and similar η6-biphenyl complexes.18 Zeiss and Tsutsui also recognized the sandwich-like structure of some polyphenylchromium complexes (e.g., Ph4CrI as bis(biphenyl)chromium iodide),19,20 whereas Hein studied truly σ-bound phenylchromium complexes.21 To establish this (poly)phenylchromium chemistry, he also developed procedures to safely handle and manipulate extremely air- and moisture-sensitive organometallics in an inert atmosphere of purified argon.3 Thereafter, phenylchromium(II) complexes have been studied by Seidel et al.,22 and they obtained yellow Li2CrPh4·THF via the metathetical approach of CrCl2·2THF with LiPh in THF or via the redox reaction of Ph2CH-CrPh2 with LiPh in THF.22 Thereafter, 1 and 2 equiv of LiMes were added to CrMes2·3THF yielding ether adducts of LiCrMes3 and Li2CrMes4, respectively.23 At the beginning of the pioneering work of Hein, phenylmagnesium bromide was routinely prepared in diethyl ether. Many years later, tetrahydrofuran (THF) was recognized

INTRODUCTION After early unsuccessful efforts to prepare phenylchromium complexes1 Franz Hein (1892−1976, Figure 1)2,3 established the (poly)phenylchromium chemistry and published in January 1919, exactly 100 years ago, the reaction of phenylmagnesium bromide with anhydrous CrCl3 obtaining a crude amorphous orange solid with the proposed formula of “Ph5CrBr”.4 During the synthesis of this compound, Hein observed the formation of substantial amounts of biphenyl and the yields of “Ph5CrBr” were always below 20%.5 Thereafter, this substance could be separated into three compounds that he addressed as “penta-“, “tetra-“, and “triphenylchromium halides”; remarkable were a very similar tendency to form salts, the similar orange color, and the same magnetic moment of 1.7 μB.6 Hein was engaged nearly his whole life in studying of the coordination chemistry and clarifying the bonding nature of polyphenylchromium complexes. In the beginning of (poly)phenylchromium chemistry, characterization of new compounds was mainly performed by elemental analysis, magnetic properties, and derivatization reactions. On the basis of these analytical methods, Hein studied and reported among others7,8 polyphenylchromium complexes (often as ether adducts) of the types CrPh3,9 CrPh 4 , 1 0 PhCrCl 2 , 1 1 Ph 2 CrCl, 1 1 Ph 3 CrI, 9 Ph 4 CrI, 9 Li2Cr2Ph6,12 Na2Cr2Ph6,12 LiCrPh4,13 NaCrPh4,13 Na2CrPh5,14 © XXXX American Chemical Society

Received: November 5, 2018

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DOI: 10.1021/acs.organomet.8b00811 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

hydrogenation,32 tetramerization of acetylene,33 preparation of substituted naphthalenes from 2-butyne,34 or synthesis of other chromium(III) complexes via metalation of H-acidic substrates.35 In polyarylchromium(II) complexes, distorted square planar coordination spheres are preferred. In tetraarylchromate(II) anions, Cr−C bond lengths of 225.5(5) pm for the mesityl complex and of 216.9(3) pm for the phenyl congener have been observed; the lithium ions occupy bridging positions between ipso-carbon atoms of the chromium(II)-bound aryl groups.36 In [(NHC)2CrPh2] (NHC = N-heterocyclic carbene) similar Cr−C distances of 217.3(5) and 220.8(4) pm have been determined.36 Oxidation to chromium(V) and formation of the nitridochromium(V) complex [(NHC)2Cr(N)Ph2] (Cr−C 212.5(2) and 212.7(2) pm) expectedly leads to a significant shortening of the Cr−C bonds.37 We reinvestigated the reaction of CrCl3(thf)3 with phenyl reagents of selected s-block metals in dependency of stoichiometry, s-block metal reagent, and solvent. Chromium(III) complexes are d3-transition metal systems which cannot be characterized sufficiently by NMR and EPR spectroscopy; therefore, X-ray structure determinations are mandatory to reliably characterize these complexes. To obtain a complete series of phenylated chromium complexes, we used reagents with different phenylation power such as phenyl-lithium, diphenylmagnesium, and diphenylcalcium, limiting our experiments on phenyl reagents without bulky or Lewis basic sidearms to prevent steric influence and additional stabilization. In the lifetime of Hein, arylcalcium reagents38 were not available and diphenylmagnesium had to be prepared via transmetalation of highly toxic diphenylmercury with magnesium.39 This systematic approach allows isolation and structurally characterization of (poly)phenylchromium(III) complexes of the type PhxCrX3−x(L)n (1 ≤ x ≤ 3) and polyphenylchromate(III) derivatives of the type (L)nMx−3CrPhx (3 ≤ x ≤ 6). The numbering of these chromium complexes follows the number x of σ-bound phenyl groups, followed by the counterions; thus, 2-I or 5-Li symbolizes derivatives of Ph2CrI or Li2CrPh5, respectively. Due to the rather poor quality of the room temperature structure of [(thf)3CrPh3] (3),24c we redetermined the X-ray structure at low temperature for comparison reasons.

Figure 1. Franz Hein during his period at the Friedrich Schiller University Jena, Germany (reproduced with permission of the University Archive of the Friedrich Schiller University Jena).

as a solvent with strong donor capability and became commercially available. THF strongly coordinates to chromium ions, whereas diethyl ether is a weakly bonding Lewis base leading to free coordination sites that are responsible for the formation of π-arene complexes of chromium. Due to this reason, Herwig and Zeiss isolated the first chromium complex Ph3Cr(thf)3 with truly σ-bound phenyl groups.24 The rich redox chemistry and structural variability including sandwichtype complexes and formation of Cr−Cr multiple bonds (which were recognized much later) led to a variety of diverse polyphenylchromium complexes.7,8 Even though the (poly)phenylchromium chemistry received timely attention with respect to spectacular milestones like bis(benzene)chromium16 or bis(arylchromium(I)) with a quintuple Cr−Cr bonding stabilized by extremely bulky terphenyl substituents,25 a systematic structural elucidation of (poly)phenylchromium compounds is still lacking. Nevertheless, sporadic reports on structures of (poly)phenylchromium complexes hint toward a favored coordination number of six for chromium(III) but smaller coordination numbers are also feasible. [(thf)3CrPh3] crystallizes with a facial arrangement of the phenyl groups (Cr−C 206(1) pm),24c whereas in [(thf)3Cr(p-Tol)Cl2] (Cr− C 201.4(10), Cr−Cl 230.7(3), and 233.1(3) pm) a meridional coordination of the anionic ligands is observed.26 In bipyridyl complexes of diarylchromium(III) iodide, separated ion pairs of the type [(bpy)2Cr(Ar)2]I (Cr−C 208.7(4) (Ar = Ph);27 209.5(12) and 210.7(12) pm (Ar = C6H4-2-OMe)28 ) precipitate as cis-diarylchromium(III) complexes. In Na2Cr(C6H5)5·3OEt2·THF, a penta-coordinate chromium(III) center (Cr−C between 201(6) and 222(7) pm) has been found.29 A crystal structure of high quality has been reported for [(Et2OLi)3CrPh6] with a chromium(III) atom in an octahedral environment of six aryl groups (Cr−C between 222.2(3) and 226.6(3) pm).30 The phenylchromium(III) complexes, especially [(thf)3CrPh3], are important precursors for the preparation of catalysts for polymerization of ethene,31

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RESULTS AND DISCUSSION Phenylchromium(III) Halides with 1−3 Phenyl Groups at Chromium. Zeiss et al. prepared the first triphenylchromium complex via a metathetical approach of phenylmagnesium bromide with CrCl3(thf)3 in tetrahydrofuran, however, the yield was as low as 3% due to the challenging removal of the magnesium halide-containing byproducts Ph3CrMgX2(thf)6.24 Twenty-five years thereafter, Kahn et al. performed X-ray diffraction experiments and obtained a structural motif of [fac-Ph3Cr(thf)3] with a facial arrangement of the phenyl groups.24c To improve yield and purity as well as quality of the crystal structure determination, we reacted diphenylmagnesium in a solvent mixture of THF and 1,4dioxane (dx) with a stoichiometric amount of CrCl3(thf)3 (Scheme 1). After removal of insoluble MgCl2(dx) by filtration, red [fac-Ph3Cr(thf)3]·0.25dx (3·0.25dx) crystallized in a 71% yield. The crystal contains two crystallographically independent molecules A and B and a half dioxane molecule in gaps between the complexes without short contacts to the chromium atoms. Molecular structure and numbering scheme B


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Organometallics

transferred to the transition metal.41 We prepared 1-Cl in analogy to complex 3 and reacted CrCl3(thf)3 with Ph2Mg(dx) in the solvent mixture of tetrahydrofuran and 1,4-dioxane with a molar ratio of 2:1 (Scheme 2). After removal of precipitated

Scheme 1. Synthesis of [fac-Ph3Cr(thf)3] (3) via a Metathetical Approach

Scheme 2. Synthesis of [trans-Cl2CrPh(thf)3] (1-Cl) via a Metathetical Approach

of molecule B of [fac-Ph3Cr(thf)3] (3) are depicted in Figure 2.

magnesium chloride-dioxane adduct and cooling to 0 °C, yellow-green rhombic crystals of the product formed. Molecular structure and numbering scheme of this complex 1-Cl are shown in Figure 3. In this slightly distorted octahedral complex the chlorine atoms are trans-arranged, leading to a meridional arrangement of the thf ligands.

Figure 2. Molecular structure and numbering scheme of molecule B of [fac-Ph3Cr(thf)3] (3). H atoms are neglected for the sake of clarity. Selected bond lengths of molecule A [B] (pm): Cr1−C1 208.6(3) [208.9(3)], Cr1−C7 208.6(3) [207.2(3)], Cr1−C13 208.6(3) [210.2(3)], Cr1−O1 217.70(19) [219.63(18)], Cr1−O2 219.56(18) [220.15(19)], Cr1−O3 220.36(18) [218.06(19)]; bond angles (deg): C1−Cr1−C7 94.58(11) [92.39(11)], C1−Cr1−C13 92.37(11) [95.15(10)], C7−Cr1−C13 93.42(11) [92.14(11)], O1− Cr1−O2 83.54(7) [83.19(7)], O1−Cr1−O3 83.47(7) [83.67(7)], O2−Cr1−O3 84.02(7) [84.18(7)], C1−Cr1−O1 172.80(9) [172.80(9)], C7−Cr1−O2 171.64(9) [174.13(9)], C13−Cr1−O3 173.78(9) [171.85(9)], C2−C1−C6 113.9(3) [114.6(2)], C8−C7− C12 114.2(3) [114.2(3)], C14−C13−C18 114.2(3) [114.3(3)].

Figure 3. Molecular structure and numbering scheme of [Cl2CrPh(thf)3] (1-Cl). H atoms are omitted for clarity reasons. Selected bond lengths (pm): Cr1−C1 205.97(15), Cr1−Cl1 234.09(4), Cr1−Cl2 232.27(4), Cr1−O1 204.56(11), Cr1−O2 216.68(11), Cr1−O3 205.41(11); bond angles (deg): C1−Cr1−Cl1 91.28(4), C1−Cr1− Cl2 91.70(4), C1−Cr1−O1 94.00(5), C1−Cr1−O2 179.28(5), C1− Cr1−O3 93.76(5), Cl1−Cr1−Cl2 176.985(18), O1−Cr1−O2 86.01(4), O1−Cr1−O3 172.22(5), O2−Cr1−O3 86.22(4), C2− C1−C6 116.59(15).

The chromium atoms are in distorted octahedral coordination spheres of three ipso-carbon atoms and three ether ligands. The Cr−C bond lengths vary between 207.2(3) and 210.2(3) pm whereas the Cr−O distances are larger and values between 217.70(19) and 220.36(18) pm were observed. Due to the shorter bonds and the negative charge on the ipso-carbon atoms repulsion between the phenyl groups is larger than that between the ether ligands leading to larger C−Cr−C bond angles, whereas the O−Cr−O bond angles are smaller than 90°. The small C−C−C bond angles at the ipso-carbon atoms are characteristic for phenyl anions due to electrostatic repulsion between the negatively charged sp2 orbital at the ipso-C atom and the neighboring C−C bonds. It is noteworthy that the fac-configuration of the phenyl groups in 3 is in contrast to the meridional arrangement of the chlorine atoms in precursor complex [mer-Cl3Cr(thf)3].40 We were also interested in phenylchromium(III) complexes with a smaller number of phenyl groups. Light green [Cl2CrPh(thf)3] (1-Cl) had been prepared by Kurras via the reaction of CrCl3 with Ph3Al in THF; regardless of the stoichiometric ratio of the substrates, only one phenyl group is

The Cr−C bond length is smaller than that observed in complex 3. Again, the phenyl group provokes the largest repulsion, and all C1−Cr1−X bond angles are larger than 90°, whereas the O−Cr−O angles are smaller than 90°. The strong trans-influence of the phenyl group leads to an elongation of the Cr1−O2 bond (216.68(11) pm), whereas the Cr1−O1 and Cr1−O3 bonds in cis-position to the phenyl group are shorter by more than 10 pm. This structure shows that the first phenylation of [mer-Cl3Cr(thf)3] occurs at the middle chlorine functionality, and the configuration is maintained. This had been observed earlier also for the para-tolyl derivative [transCl2Cr(Tol)(thf)3].26 C


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Organometallics Crystal structures of the cationic diarylchromium(III) ions in [cis-Ph2Cr(bpy)]I27 and [cis-(4-MeO-C6H4)2Cr(bpy)]I28 had already been reported. The electroneutral complex Ph2CrCl(dme)1.5 (dme = 1,2-dimethoxyethane) had been prepared as cacao-brown complex via ligand exchange between the ate complexes Na2CrPh5(thf)(Et2O)3 and CrCl3(thf)3 in DME.11 We were curious if the use of Ph2Mg(dx) would allow the synthesis and structural characterization of a derivative of diphenylchromium(III) chloride. From the brown reaction mixture, containing equimolar amounts of Ph2Mg(dx) and CrCl3(thf)3 in a solvent mixture of THF and 1,4-dioxane, MgCl2(dx) was removed by filtration (Scheme 3), and the Scheme 3. Synthesis of [(thf)5MgCl]+ [{Ph2Cr(thf)}2(μCl)3]− ((2-Cl)2-MgCl2) via a metathetical approach

Figure 4. Molecular structure and numbering scheme of molecule B of [(thf)5MgCl]+ [{Ph2Cr(thf)}2(μ-Cl)3]− ((2-Cl)2·MgCl2). The asymmetric units contains two molecules A and B. H atoms and the intercalated THF molecule are omitted for clarity reasons. Selected bond lengths (pm) of molecule A [B]: Cr1−C1 207.6(10) [207.8(11)], Cr1−C7 210.4(11) [210.0(10)], Cr1−O1 208.1(8) [205.7(8)], Cr1−Cl1 251.8(3) [251.7(3)], Cr1−Cl2 251.1(3) [252.3(3)], Cr1−Cl3 237.0(3) [237.5(3)], Mg1−O3 210.4(9) [213.3(9)], Mg1−O4 210.5(9) [211.4(9)], Mg1−O5 210.0(8) [211.9(9)], Mg1−O6 213.3(9) [212.0(9)], Mg1−O7 214.3(8) [212.8(8)], Mg1−Cl4 239.5(4) [239.1(4)].

obtained chromium complex is extremely soluble and even at −40 °C no crystals could be obtained from a very concentrated mother liquor. Therefore, the filtrate was layered with anhydrous n-pentane leading to precipitation of growntogether red-brown single crystals of the formal composition of (Ph2CrCl)2·MgCl2(thf)7.5 ((2-Cl)2-MgCl2) besides a few yellow-green crystals of 1-Cl. The molecular structure and numbering scheme of [(thf)5MgCl]+ [{Ph2Cr(thf)}2(μ-Cl)3]− ((2-Cl)2-MgCl2) are depicted in Figure 4. The asymmetric unit contains two crystallographically independent, but very similar, molecules A and B. Intercalated THF and H atoms are omitted for clarity reasons. The salt-like nature of [(thf)5MgCl]+ [{Ph2Cr(thf)}2(μCl)3]− ((2-Cl)2-MgCl2) explains the very high solubility of this compound in THF. The complex anion consists of two distorted octahedrons of Ph2CrCl3(thf) with a common Cl3 face. The Cr−C bond lengths are comparable to those of [facPh3Cr(thf)3] (3). Again, a strong trans-influence leads to significantly different Cr−Cl distances. The Cr−Cl bonds trans-arranged to an oxygen atom are significantly shorter than those which are in trans-positions to phenyl groups. The counter cations contain distorted octahedrally coordinated magnesium atoms; in this cation all Mg−O distances are comparable, and the chlorine atom shows no trans-influence. The isolation of [(thf)5MgCl]+ [{Ph2Cr(thf)}2(μ-Cl)3]− ((2-Cl)2-MgCl2) showed that the equimolar reaction of Ph2Mg(dx) with CrCl3(thf)3 in a solvent mixture of THF and 1,4-dioxane proceeded differently from expected. Desired Ph2CrCl(thf)3 seemed to be an intermediate which reacted immediately with an equivalent of MgCl2 yielding salt-like (2Cl)2-MgCl2. The magnesium chloride component of this compound could not be removed with excess of 1,4-dioxane. Therefore, two pathways were proposed to prepare the desired Ph2CrX(thf)3 (X = halide): (i) Ligand exchange in an equimolar mixture of [fac-Ph3Cr(thf)3] (3) and [Cl2CrPh(thf)3] (1-Cl); this procedure had already successfully been e m p l o y e d t o p r e p a r e a p h o s p h a n e co m p l e x o f diphenylchromium(III) chloride.42 (ii) Cleavage of a Cr−C bond in [fac-Ph3Cr(thf)3] (3) with iodine.

During stirring of a concentrated equimolar suspension of [fac-Ph3Cr(thf)3] (3) and [Cl2CrPh(thf)3] (1-Cl) in THF the substrates slowly dissolved forming a brown solution. Layering of this reaction mixture with diethyl ether and cooling to −40 °C gave red crystals of 3 and then green crystals of 1-Cl precipitated. The product Ph2CrCl(thf)3 (2-Cl) seemed to be much more soluble than the substrate 3; therefore, [facPh3Cr(thf)3] (3) crystallized first, shifting the equilibrium toward 3 and 1-Cl (Scheme 4). Scheme 4. Equilibrium between [fac-Ph3Cr(thf)3] (3) and [trans-Cl2Ph(thf)3] (1-Cl) on the One Side and [facPh2ClCr(thf)3] (2-Cl) on the Other

The second variant shown in Scheme 5 was more successful. Treatment of [fac-Ph3Cr(thf)3] (3) with 1 equiv of iodine in THF at 0 °C and cooling to −20 °C gave light brown crystals of Ph2CrI(thf)3 (2-I). If starting material 3 contained MgCl2(dx) from the synthesis, a few colorless crystals of [Mg(thf)6]I243 were obtained. The molecular structure and Scheme 5. Synthesis of [fac-Ph2ICr(thf)3] (2-I) via Oxidation of [fac-Ph3Cr(thf)3] (3) with Iodine

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groups and the orange complex [Li3CrPh6(Et2O)3] with six σbound phenyl substituents were obtained.21 The molecular structures of these complexes show distorted square pyramidal29 and octahedral environments30 at the chromium(III) atoms. Structures of chromates(III) with four σ-bound phenyl groups are unknown as of yet; however, tetraarylchromate(III) anions with stabilizing ortho-chloro substituents at the aryl groups had been reported earlier. 4 5 Solutions of tetraphenylchromate(III) derivatives in diethyl ether or THF are thermally instable and readily react via reductive elimination of biphenyl to chromium(II) compounds of the type [M2Cr2Ph6L3].12 At room temperature the more stable complexes are those with the compositions of LiCr(C6H4-2OMe)4L46 and LiCrPh4(dme)4 (4-Li).47 We synthesized derivative 4-Li via an equimolar addition of LiPh onto [facPh3Cr(thf)3] (3) in DME according to Scheme 6. Molecular structure and numbering scheme of 4-Li are shown in Figure 6.

numbering scheme of 2-I are depicted in Figure 5. The chromium atom is in a distorted octahedral environment with a facial arrangement of the thf ligands as also observed in substrate [fac-Ph3Cr(thf)3] (3).

Scheme 6. Synthesis of [Li(dme)3]+ [Ph4Cr(dme)]− (4-Li) via Addition of LiPh onto [fac-Ph3Cr(thf)3] (3) Figure 5. Molecular structure and atom labeling scheme of [Ph2CrI(thf)3] (2-I). H atoms are omitted for clarity reasons. Selected bond lengths (pm): Cr1−I1 273.12(7), Cr1−C1 208.3(4), Cr1−C7 213.9(4), Cr1−O1 222.9(3), Cr1−O2 206.8(3), Cr1−O3 218.6(3); bond angles (deg): I1−Cr1−C1 94.71(12), I1−Cr1−C7 94.31(10), I1−Cr1−O1 93.05(7), I1−Cr1−O2 170.24(8), I1−Cr1− O3 86.71(8), C1−Cr1−C7 91.70(14), C1−Cr1−O1 171.79(14), C1−Cr1−O2 90.23(14), C1−Cr1−O3 91.71(12), C7−Cr1−O1 90.37(12), C7−Cr1−O2 93.95(12), C7−Cr1−O3 176.35(13), O1−Cr1−O2 81.69(10), O1−Cr1−O3 86.08(10), O2−Cr1−O3 84.74(11).

The Cr−C bond lengths have values of 208.5(4) and 213.9(4) pm and are on the same order of magnitude as those found in substrate 3. As observed for the complexes discussed above, a strong trans-influence of the phenyl groups leads to elongated Cr1−O1 and Cr1−O3 bonds, whereas the Cr1−O2 bond trans-arranged to the iodine atom is more than 10 pm shorter. The phenyl groups are more demanding than the thf ligands leading to O−Cr1−O bond angles smaller than 90°, whereas the C1−Cr1−C7 and C−Cr1−I1 bond angles are larger than 90°. In summary, the molecular structures of [facPh3Cr(thf)3] (3) and [Ph2CrI(thf)3] (2-I) are very similar to slight additional distortions induced by the iodine atom. The analogous reaction of [fac-Ph3Cr(thf)3] (3) with 2 equiv of iodine led to formation of diverse products. At first dark brown [mer-I3Cr(thf)3] precipitated from the reaction mixture which is only sparingly soluble in THF. The molecular structure is shown in the Supporting Information. This complex was accessible via time-consuming extraction of CrI3 with THF in the presence of catalytic amounts of zinc dust.44 All subsequent crystal fractions contained [mer-I3Cr(thf)3] besides [trans-I2CrPh(thf)3] (1-I). The molecular structure of this complex is depicted in the Supporting Information and is very similar to the structure of 1-Cl. Polyphenylchromates(III) with 4−6 Phenyl Groups at Chromium. It is remarkable that Hein et al. managed the first preparation of polyphenylchromates(III) via the reaction of NaPh (stabilized by 10% of LiPh) or LiPh with CrCl3(thf)3 in diethyl ether. Thus, the blue-green compounds of the type [M2CrPh5Lx] (M = Li, Na; L = thf, Et2O)14 with five phenyl

Figure 6. Molecular structure and numbering scheme of [Li(dme)3]+ [Ph4Cr(dme)]− (4-Li). H atoms are neglected for the sake of clarity. Selected bond lengths (pm): Cr1−C1 217.7(2), Cr1−C7 207.4(2), Cr1−C13 208.1(2), Cr1−C19 216.7(2), Cr1−O1 226.32(15), Cr1− O2 225.41(15), Li1−O3 215.5(4), Li1−O4 215.8(4), Li1−O5 222.9(4), Li1−O6 206.3(4), Li1−O7 212.3(4), Li1−O8 209.2(4); angles (deg): C1−Cr1−C7 90.49(8), C1−Cr1−C13 95.77(8), C1− Cr1−C19 170.27(8), C1−Cr1−O1 88.10(7), C1−Cr1−O2 83.85(7), C7−Cr1−C13 95.95(8), C7−Cr1−C19 94.64(8), C7−Cr1−O1 95.58(7), C7−Cr1−O2 169.73(7), C13−Cr1−C19 91.92(8), C13− Cr1−O1 167.80(7), C13−Cr1−O2 93.14(7), C19−Cr1−O1 83.18(7), C19−Cr1−O2 89.80(7), O1−Cr1−O2 75.73(6).

The solvent-separated ion pair 4-Li consists of the cation [Li(dme)3]+ and the anion [Ph4Cr(dme)]− with the metal atoms in distorted octahedral environments. In the chromate(III) ion two phenyl groups are in trans-, the two other ones in cis-positions to the oxygen atoms of the dme ligand. This arrangement leads to two phenyl groups which are transpositioned to each other. The strong trans-influence of the phenyl group disfavors this trans-orientation; hence, the Cr1− C1 and Cr1−C19 bond lengths to these phenyl groups are significantly larger than the Cr1−C7 and Cr1−C13 values of those phenyl groups trans to the oxygen atoms. Complex 4-Li E


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Organometallics dissolves in 1,2-dimethoxyethane with a deep red color and decomposes in this solution above 60 °C. From a solution of [fac-Ph3Cr(thf)3] (3) and LiPh with a molar ratio of 1:2 were obtained green cuboid crystals of the formal constitution Li2CrPh5(dme)2 (5-Li) as shown in Scheme 7. The molecular structure and numbering scheme Scheme 7. Synthesis of [{(dme)Li}2CrPh5] (5-Li) via TwoFold Addition of LiPh onto [fac-Ph3Cr(thf)3] (3)

Figure 7. Molecular structure and numbering scheme of [{(dme)Li}2CrPh5] (5-Li). The asymmetric unit consists of two molecules A and B. H atoms are neglected for the sake of clarity. Selected bond lengths (pm) of molecule A [molecule B]: Cr1−C1 202.0(5) [203.6(5)], Cr1−C7 216.2(5) [214.7(5)], Cr1−C13 215.0(5) [216.8(5)], Cr1−C19 216.8(5) [215.2(5)], Cr1−C25 214.8(5) [215.9(5)], Li1−C7 230.1(10) [220.7(10)], Li1−C25 219.9(10 [225.8(10)], Li1−O1 201.5(10) [199.8(11)], Li1−O2 192.6(10) [195.3(11)], Li2−C13 221.1(10) [229.9(10)], Li2−C19 228.9(9) [222.2(10)], Li2−O3 200.3(10) [198.3(10)], Li2−O4 196.0(9) [192.2(10)]; angles (deg): C1−Cr1−C7 101.2(2) [102.3(2)], C1− Cr1−C13 99.7(2) [100.2(2)], C1−Cr1−C19 101.4(2) [101.9(2)], C1−Cr1−C25 99.6(2) [98.3(2)], C7−Cr1−C13 85.66(18) [85.25(18)], C7−Cr1−C19 157.4(2) [155.9(2)], C7−Cr1−C25 90.51(19) [90.44(18)], C13−Cr1−C19 90.70(18) [90.17(18)], C13−Cr1−C25 160.7(2) [161.4(2)], C19−Cr1−C25 85.59(18) [86.40(18)].

of this homoleptic complex is depicted in Figure 7. The asymmetric unit contains two crystallographically independent molecules A and B. The chromium atoms are in slightly distorted square pyramidal coordination spheres. The τ-values represent index factors to determine the coordination polyhedron of penta-coordinate metal atoms according to the equation τ = (β − α)/60° (β largest angle, α second largest angle) with τ = 0 for the square pyramid and τ = 1 for the trigonal bipyramid.48 For molecules A and B, τ values of 0.05 and 0.09 were determined. The structure of [{(dme)Li}2CrPh5] (5-Li) represents a rare example of organometallic penta-coordinate chromium(III) complexes.29,31,49 The Cr1− C bond lengths to the ipso-carbon atoms of the phenyl groups in the square plane are significantly larger than the Cr1−C1 distance to the apical phenyl group. This finding is caused by additional interactions to the lithium atoms. Compound 5-Li was only sparingly soluble in DME but dissolved readily in a mixture of DME and THF. Cooling of this solution to −40 °C yielded dark green crystals of the composition Li2CrPh5(dme)4 (5-Li′, Scheme 7). The high solubility in THF suggested a salt-like structure with solventseparated ion pairs. The poor crystal quality forbids a detailed discussion of bond lengths and angles but the structure can unequivocally determined as [Li(dme)3]+ [{(dme)Li}CrPh5]− (5-Li′). In the anion the penta-coordinate chromium(III) atom remains in a square pyramidal environment. The structural motif of this complex is depicted in Figure 8. It seems to be important that at least one alkali metal atom remains in close contact with the chromate(III) ion. Therefore, we also studied the reaction of [fac-Ph3Cr(thf)3] (3) with

Figure 8. Structural motif of [Li(dme)3]+[{(dme)Li}CrPh5]− (5-Li′). H atoms are neglected for the sake of clarity. The atoms are drawn with arbitrary radii.

diphenyl complexes of the alkaline earth metals magnesium [Ph2Mg(dx)] and calcium [Ph2Ca(thf)4], and we were curious if penta-coordinate chromate(III) congeners were accessible via this strategy. The reaction of [fac-Ph3Cr(thf)3] (3) with an equimolar amount of [Ph2Mg(dx)] in a solvent mixture of THF and DME at 65 °C yielded a deep red compound of the composition MgCrPh5(dme)3(thf) (Scheme 8). Due to its color a pentaphenylchromate(III) could be excluded because the characteristic color of these anions is green as observed for 5-Li and 5-Li′. A deep red color was indicative for tetraphenylchromate(III) complexes as found for 4-Li; thereF


Article

Organometallics

of the Grignard reagent is present, and tetraphenylchromate(III) species might have formed which could be important for the formation of π-arene complexes. The reaction of 3 with Ph2Ca(thf)4 with a molar ratio of 1:5 in a solvent mixture of THF and DME gave a product of the formal composition of Ca(dme)(thf)1.5CrPh5 (Scheme 9). Due

Scheme 8. Synthesis of Solvent-Separated [PhMg(dme)2(thf)]+ [Ph4Cr(dme)]− (4-Mg) via Addition of Ph2Mg(dx) onto [fac-Ph3Cr(thf)3] (3)

Scheme 9. Synthesis of [{(dme)(thf)Ca(μ-Ph)3}2Cr]+ [Ph4Cr(thf)]− (5-Ca) via Addition Reactions fore, this magnesium chromate should also contain a tetraphenylchromate(III) anion. Indeed, the crystal structure verified this conclusion, and the solvent-separated ion pair [PhMg(dme)2(thf)]+ [Ph4Cr(dme)]− (4-Mg) was isolated as deep red crystals. The molecular structure and atom labeling scheme is depicted in Figure 9. The anion is very similar to the

to the green color of this compound, a pentaphenylchromate(III) was assumed; however, X-ray diffraction experiments at a single crystal led to a surprisingly different molecular structure of this complex. Molecular structure and atom labeling scheme of this ion pair [{(dme)(thf)Ca(μ-Ph)3}2Cr]+ [Ph4Cr(thf)]− (5-Ca) is depicted in Figure 10. The crystalline state consists of penta-coordinate tetraphenylchromate(III)-thf anions and centrosymmetric cations with central hexaphenylchromate(III) units with bridging phenyl groups to the calcium atoms. In the [Ph4Cr(thf)]− anions, the chromium atoms have square pyramidal coordination spheres with τ values of 0.11−0.26;

Figure 9. Molecular structure and atom labeling scheme of the solvent-separated ion pair [PhMg(dme)2(thf)]+ [Ph4Cr(dme)]− (4Mg). The ellipsoids represent a probability of 30%, H atoms are neglected for clarity reasons. Selected bond lengths (pm): Cr1−C1 209.01(17), Cr1−C7 217.50(16), Cr1−C13 208.75(16), Cr1−C19 218.70(17), Cr1−O1 221.98(11), Cr1−O2 226.56(12), Mg1−C29 218.15(18), Mg1−O3 210.04(13), Mg1−O4 218.73(13), Mg1−O5 214.41(13), Mg1−O6 215.51(12), Mg1−O7 217.12(13); angles (deg): C1−Cr1−C7 94.82(6), C1−Cr1−C13 93.54(6), C1−Cr1− C19 93.73(6), C1−Cr1−O1 94.23(5), C1−Cr1−O2 169.74(5), C7− Cr1−C13 92.96(6), C7−Cr1−C19 169.70(6), C7−Cr1−O1 86.68(5), C7−Cr1−O2 83.57(5), C13−Cr1−C19 92.23(6), C13− Cr1−O1 172.22(6), C13−Cr1−O2 96.66(6), C19−Cr1−O1 86.97(5), C19−Cr1−O2 87.00(6), O1−Cr1−O2 75.58(5).

chromate(III) of lithium congener 4-Li. The bond angles at the chromium atom deviate less than 5°. Deviations of the bonding parameters are insignificant and are a consequence of the packing in the solid state. In the cation, the magnesium atom is embedded in a distorted octahedral environment with a cis-arrangement of the phenyl group and the thf base. Due to the electrostatic repulsion between the negatively charged phenyl group and the oxygen atoms the C29−Mg1−O bond angles are larger than 90°. The deep red colored reaction mixture and the molecular structure verified that [Ph2Mg(dx)] was only able to transfer one phenyl group even if excess of this phenylation reagent was applied. Contrary to this finding, the stronger bases phenyllithium and -sodium were able to produce also chromate(III) anions with a higher content of σ-bound phenyl groups. Due to this fact, derivatives similar to 4-Mg must be considered during the original Hein reaction of CrCl3 with ethereal solutions of PhMgBr. Especially in the beginning of the reaction, an excess

Figure 10. Molecular structure and atom labeling scheme of the ion pair [{(dme)(thf)Ca(μ-Ph)3}2Cr]+ [Ph4Cr(thf)]− (5-Ca). Symmetryrelated atoms (−x + 2, −y + 1, −z + 1) are marked with the letter “A”. The ellipsoids represent a probability of 30%, H atoms are omitted for clarity reasons. Selected bond lengths (pm) of anion: Cr1−C1 212.4(2), Cr1−C7 212.98(18), Cr1−C13 203.4(2), Cr1−C19 213.4(2), Cr1−O1 221.27(16), Cr2−C29 225.1(2), Cr2−C35 222.8(3), Cr2−C41 224.6(3), Ca1−C29A 263.6(2), Ca1−C35 262.7(3), Ca1−C41 264.6(3), Ca1−O2 237.9(2), Ca1−O3 238.40(19), Ca1−O4 241.7(2); angles (deg): C1−Cr1−C7 90.31(10), C1−Cr1−C13 102.79(10), C1−Cr1−C19 156.19(10), C1−Cr1−O1 87.71(7), C7−Cr1−C13 96.22(10), C7−Cr1−C19 92.75(9), C7−Cr1−O1 171.64(9), C13−Cr1−C19 100.34(9), C13− Cr1−O1 92.14(8), C19−Cr1−O1 85.85(7). G


Article

Organometallics

structural motif of [Li(thf)4]+ [{(thf)Li(μ-Ph)3}2Cr]− (6-Li) is depicted in Figure 11. These structures are strikingly different

the variance is caused by slight disordering of one phenyl group and of the thf ligand. The Cr−C bond lengths to the apical positions are significantly smaller than the Cr−C values to the basal plane. The Cr−C bonds in the cation with the central hexaphenylchromate(III) moieties are longer than in the anions due to the larger coordination number of the chromium atoms and due to the bridging nature of the ipsocarbon atoms between chromium and calcium. The Ca−C distances vary in a very narrow range between 260.2(2) and 266.6(2) pm, which are in the range of typical values for Ca−C bond lengths.38 It is remarkable that the reaction of 3 with the large excess of diphenylcalcium does not yield pure hexaphenylchromate(III) but complex 5-Ca. Instead, the obviously rather labile pentaphenylchromate(III) underwent a transfer of one phenyl group in DME/THF solution yielding hexaphenyl- and tetraphenylchromate(III) species. The following reactions demonstrate that the coordination gap at chromium can be filled with another phenyl group and that it is feasible to prepare hexaphenylchromates(III) if phenyl-lithium is used as phenylation reagent. Thus, addition of excess of phenyl-lithium to a suspension of [fac-Ph3Cr(thf)3] (3) in THF or in tetrahydropyran (THP) gave the yellow complexes [Li(L)4]+ [{(L)Li(μ-Ph)3}2Cr]− (L = thf (6Li) and thp (6-Li′)) as depicted in Scheme 10. Recrystalliza-

Figure 11. Structural motif of the solvent-separated ion pair [Li(thf)4]+ [{(thf)Li(μ-Ph)3}2Cr]− (6-Li). The atoms are shown with arbitrary radii, H atoms are neglected for clarity reasons.

than the molecular structure of the diethyl ether adduct leading to molecular [{(Et2O)Li(μ-Ph)2}3Cr].30 Obviously, the enhanced donor strength of the Lewis bases thf and dme enabled release of one solvated lithium cation. In THF solution, yellow hexaphenylchromates(III) 6 dissociated under elimination of phenyl-lithium in analogy to [{(Et2O)Li(μ-Ph)2}3Cr] (Scheme 10). This degradation reaction led to formation of green [{(thf)2Li(μ-Ph)2}2CrPh].50 This reaction can be reversed via addition of excess of phenyllithium. The yellow crystals of [Li(thf)4]+ [{(thf)Li(μ-Ph)3}2Cr]− (6-Li) are not stable in contact with THF even at temperatures as low as −40 °C. This solution slowly turned green and green crystals slowly precipitated. This conversion is complete after approximately 5 weeks under these conditions and could not be reversed by addition of phenyl-lithium. The X-ray structure determination verified that ether cleavage caused the formation of the oxide-containing centrosymmetric complex [{(thf)3Li2O}CrPh3]2 (7) that is shown in Scheme 11 and

Scheme 10. Synthesis of [Li(L)4]+ [{(L)Li(μ-Ph)3}2Cr]− via Addition of Phenyl-Lithium onto 3 and Reversible Loss of PhLi in THFa

Scheme 11. Formation of Green [{(thf)3Li2O}CrPh3]2 (7) via Ether Degradation in THF Solution

a

L = thf (6-Li) and thp (6-Li′).

Figure 12. Such tetrahydrofuran cleavage reactions had already been observed earlier by Mulvey and co-workers who also trapped the degradation intermediates.51 The molecule of 7 contains a central planar Cr2O2 ring. Three terminal σ-bound phenyl groups complete the square pyramidal coordination sphere of the penta-coordinate chromium atom. The Cr1−C13 bond to the apical position is significantly shorter than the

tion of 6-Li (L = thf) from a mixture of DME and THF led to an exchange of the ether ligands of the cation, whereas the anion remained unchanged. Thus, the crystalline complex [Li(dme)3]+ [{(thf)Li(μ-Ph)3}2Cr]− (6-Li″) was isolated. In all these ion pairs, disordering of the cation hampered the quality of the crystal structure determinations. Nevertheless, the structure was verified unequivocally in all these cases. The H


Article

Organometallics

Figure 13. Molecular structure and atom labeling scheme of [Li(thp)4]+ [{(thp)Li}2(μ-Ph)4(μ-C12H8)Cr]− (8). The ellipsoids represent a probability of 30%, H atoms are omitted for clarity reasons. Selected bond lengths (pm): Cr1−C1 223.3(3), Cr1−C7 220.2(2), Cr1−C13 223.7(3), Cr1−C19 221.4(3), Cr1−C25 214.5(3), Cr1−C36 215.0(3), Li1−C1 215.2(5), Li1−C19 223.5(5), Li1−C25 240.8(6), Li1−O1 192.5(5), Li2−C7 223.2(5), Li2−C13 217.9(5), Li2−C36 240.0(5), Li2−O2 192.4(5); angles (deg): C1−Cr1−C7 89.59(9), C1−Cr1−C13 174.78(10), C1−Cr1− C19 93.92(9), C1−Cr1−C25 87.64(9), C1−Cr1−C36 88.54(9), C7−Cr1−C13 93.38(9), C7−Cr1−C19 91.28(9), C7−Cr1−C25 174.99(10), C7−Cr1−C36 95.11(9), C13−Cr1−C19 90.32(10), C13−Cr1−C25 89.09(9), C13−Cr1−C36 86.91(9), C19−Cr1− C25 93.07(10), C19−Cr1−C36 173.17(10), C25−Cr1−C36 80.65(10).

Figure 12. Molecular structure and atom labeling scheme of [{(thf)3Li2O}CrPh3]2 (7). Symmetry-related atoms are marked with the letter “A”. The ellipsoids represent a probability of 30%, H atoms are neglected for clarity reasons. Selected bond lengths (pm): Cr1− C1 215.30(19), Cr1−C7 214.2(2), Cr1−C13 205.92(19), Cr1−O1 194.90(13), Cr1−O1A 194.87(13), Li1−C7A 246.4(4), Li1−O1 183.9(4), Li1−O2 197.0(4), Li1−O3 207.5(4), Li2−C1 252.5(4), Li2−O1 185.5(4), Li2−O3 223.5(4), Li2−O4 198.1(4); angles (deg): C1−Cr1−C7 88.68(7), C1−Cr1−C13 96.48(8), C1−Cr1− O1 90.16, C1−Cr1−O1A 162.93(7), C7−Cr1−C13 100.99(8), C7− Cr1−O1 162.71(7), C7−Cr1−O1A 89.79(6), C13−Cr1−O1 96.28(7), C13−Cr1−O1A 100.50(7), O1−Cr1−O1A 86.29(6).

Cr1−C1 and Cr1−C7 bonds to the basal phenyl groups. The lithium atoms bridge basal carbon and oxygen atoms. The formation of this product can be understood assuming that the primary product [{(thf)2Li(μ-Ph)2}2CrPh] after elimination of LiPh slowly attacks ether finally leading to Li2O which is trapped by the chromium complex. The used phenyl-lithium solution contained very small amounts of biphenyl which slowly reacted with the yellow complex [Li(thp)4]+ [{(thp)Li(μ-Ph)3}2Cr]− (6-Li′) leading to the formation of product 8 slowly precipitating from the mother liquor at −40 °C as depicted in Scheme 12.52 The

comparison to the hexaphenylchromate(III) ion of 6-Li′. The coordination of the rigid biphenyl-2,2′-diide chelate elongates the Cr−C bonds to the phenyl groups (220.3(2)−223.7(3) pm) and smaller distances of 214.5(3) and 215.0(3) pm to the chelating ligand are observed. In return, the Li−C bonds to the phenyl groups are shorter than those to the biphenyl-2,2′-diide ion. The bite C25−Cr1−C36 of the biphenyl-2,2′-diide ligand (80.85(10)°) is rather small, whereas all other C−Cr1−C bond angles are larger than 90°. This distortion leads to a slight bending of the Li1···Cr1···Li2 fragment of 175.52(15)°. These lithium atoms are in severely distorted tetrahedral coordination spheres, whereas Li3 of the [Li(thp)4]+ cation is regularly surrounded by four thp ligands. The yellow complexes [Li(L)4]+ [{(L)Li(μ-Ph)3}2Cr]− (L = thf (6-Li) and thp (6-Li′)) are very strong bases, but THP is significantly less reactive with respect to α-deprotonation and subsequent ether degradation sequences.53 Therefore, thp adduct 6-Li′ is significantly more stable in THP than 6-Li in THF. The reaction solution contains biphenyl as side-product from the synthesis of phenyl-lithium. Biphenyl is slowly metalated in the vicinity of chromium(III) leading to biphenyl-2,2′-diide and 2 equiv of benzene.

Scheme 12. Metalation of Biphenyl by 6-Li′ Yielding [Li(thp)4]+ [{(thp)Li}2(μ-Ph)4(μ-C12H8)Cr]− (8)

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CONCLUSION In the frame of this study we present a series of phenylchromium(III) complexes (see Table 1) with 1−6 σbound phenyl groups to better understand the coordination chemistry of organochromium(III) compounds. The color of the (oligo)phenylchromium(III) complexes with σ-bound phenyl groups is a convenient indicator of a successful conversion. Coordination number and nature of halogen significantly influence the color. Diverse additional conclusions can be derived from this investigation: (1) At

structure and labeling scheme of this subsequent product 8 are shown in Figure 13. Large similarities between starting 6-Li′ and the degradation product are obvious. Both compounds contain a chromium(III) center in an octahedral environment of six aryl groups. Although the cation remained unchanged, the anion of 7 contains one doubly deprotonated biphenyl ligand. Coordination of a biphenyl-2,2′-diide at chromium(III) in 8 leads to a characteristic distortion of the coordination sphere in I


Article

Organometallics

pentaphenylchromate(III) derivatives (214−217 pm). (8) The coordination of the phenyl groups at a metal atom leads to distortion of the aromatic ring in all chromium(III) complexes with rather small C−C−C bond angles between 111.3(5) and 118.1(4)° at the ipso-carbon atoms. (9) The hexaphenylchromate(III) ions are very strong bases and react with THF or biphenyl. THF is degraded, and bridging oxygen atoms are incorporated in the dinuclear complex [{(thf)3Li2O}CrPh3]2 (7). In contrast, biphenyl is metalated yielding biphenyl-2,2′-diide which is bound at chromium(III) in the complex [Li(thp)4]+ [{(thp)Li}2(μ-Ph)4(μ-C12H8)Cr]− (8). In summary, we could prepare and isolate neutral phenylchromium(III) compounds with 1−3 aryl ligands as well as oligophenylchromates(III) with 4−6 aryl ligands. Depending on the counter s-block ions and the solvent (donor strength, denticity, and chelating behavior) contact ion pairs or solvent-separated ion pairs were observed.

Table 1. Overview on the Phenylchromium Complexes with 1−6 σ-Bound Phenyl Groups compound 1-Cl 1-I (2-Cl)2-MgCl2 2-I 3 4-Li 4-Mg 5-Li 5-Li′ 5-Ca 6-Li 6-Li′ 7 8

formula [Cl2CrPh(thf)3] [I2CrPh(thf)3] [(thf)5MgCl]+ [{Ph2Cr(thf)}2(μ-Cl)3]− [Ph2CrI(thf)3] [fac-Ph3Cr(thf)3] [Li(dme)3]+ [Ph4Cr(dme)]− [PhMg(dme)2(thf)]+ [Ph4Cr(dme)]− [{(dme)Li}2CrPh5] [Li(dme)3]+ [{(dme)Li}CrPh5]− [{(dme)(thf)Ca(μ-Ph)3}2Cr]+ [Ph4Cr(thf)]− [Li(thf)4]+ [{(thf)Li(μ-Ph)3}2Cr]− [Li(thp)4]+ [{(thp)Li(μ-Ph)3}2Cr]− [{(thf)3Li2O}CrPh3]2 [Li(thf)4]+ [{(thf)Li}2(μ-Ph)4(μ-C12H8) Cr]−

color yellow-green brown red-brown light brown deep red deep red deep red bottle-green dark green green yellow yellow green yellow

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chromium(III), σ-bound phenyl groups introduce a strong trans-influence avoiding a trans arrangement and leading to [fac-Ph3Cr(thf)3] (3), whereas the weak halide ligands form [mer-X3Cr(thf)3] (X = Cl, I) due to electrostatic repulsion between the halide anions. (2) Chromium(III) prefers the coordination number of 6 with (distorted) octahedral coordination spheres. The mer-arrangement of the anionic ligands in starting CrCl3(thf)3 is maintained in singly phenylated derivatives [Cl2CrPh(thf)3] (1-Cl) and [I2CrPh(thf)3] (1-I). The doubly and triply phenylated chromium(III) complexes Ph2CrI(thf)3 (2-I) and [fac-Ph3Cr(thf)3] (3) show a facial arrangement of the anionic ligands. Exceptions from the hexa-coordinate environment of chromium(III) have only been observed for homoleptic pentaphenylchromate(III) with square pyramidal coordination spheres. The sixth coordination site can only be occupied by strong bases like phenyl-lithium or diphenylcalcium yielding hexaphenylchromates(III). (3) Ph2CrCl(thf)3 (2-Cl) is very soluble in THF and shows an equilibrium with [Cl2CrPh(thf)3] (1-Cl) and [fac-Ph3Cr(thf) 3 ] (3). Cooling of the mother liquor leads to crystallization of 3, followed by precipitation of 1-Cl. Despite this behavior crystalline [(thf)5MgCl]+ [{Ph2Cr(thf)}2(μCl)3]− ((2-Cl)2·MgCl2) and Ph2CrI(thf)3 (2-I) can be isolated. (4) Stabilization of penta- and hexaphenylchromate(III) ions requires bridging lithium and calcium cations between the ipso-carbon atoms of the phenyl groups. (5) The pentaphenylchromates(III) can crystallize as [{(dme)Li}2CrPh5] (5-Li) and [Li(dme)3]+ [{(dme)Li}CrPh5]− (5Li′) with two or four dme ligands, respectively. (6) The pentaphenylchromate(III) with calcium counterions is less stable and shows ligand exchange reactions leading to the formation of [{(dme)(thf)Ca(μ-Ph)3}2Cr]+ [Ph4Cr(thf)]− (5Ca) with hexaphenylchromate(III) and tetraphenylchromate(III) moieties. The synthesis of pure hexaphenylchromates(III) with calcium counterions is impossible via the reaction of [fac-Ph3Cr(thf)3] (3) with excess of diphenylcalcium. (7) The Cr−C bond lengths of the phenylchromium(III) species with 1−4 phenyl groups vary in the narrow range between 206 and 210 pm. Exceptions are those phenyl groups in trans-position to another phenyl group leading to significant elongation of the Cr−C bonds to 217−219 pm. Further elongation is observed for the hexaphenylchromate(III) species. The smaller coordination number leads to smaller Cr−C distances in

EXPERIMENTAL SECTION

General Remarks. All manipulations were carried out under strictly anaerobic conditions in an argon atmosphere using standard Schlenk techniques. The solvents were dried according to common procedures and distilled in an argon atmosphere prior to use. Deuterated solvents were dried over sodium, degassed, and saturated with argon. The yields given are not optimized. The chromium contents of the compounds were determined by complexometric titrations against xylenolorange.54 Anhydrous chromium(III) chloride and bromobenzene were purchased from ABCR GmbH, Alfa Aesar, or Merck; phenylmagnesium bromide solutions in THF and diethyl ether were supplied by Sigma-Aldrich. The starting compounds CrCl3(thf)3,55 Ph2Mg(dx),56 phenyl-lithium,57 and Ph2Ca(thf)458 were prepared according to literature protocols. The concentration of the solutions of phenyl-lithium in diethyl ether and of diphenylcalcium in THF were determined by acidimetric titrations of an aliquot with N/10 hydrochloric acid against phenolphthalein. Due to loss of ligated ether molecules during combustion and formation of carbonates, no reliable CHN analyses could be performed. Therefore, we determined the metal content and compared the colors of the complexes with those reported by Hein and co-workers. Titration of the metal content was indicative of the purity of these complexes. Synthesis of [fac-(C6H5)3Cr(thf)3·0.25 dx] (3). CrCl3(thf)3 (5.6 g, 15.0 mmol) was suspended in a mixture of 170 mL of THF and 5 mL of dioxane. This stirred suspension was cooled to −30 °C, and 6.67 g of Ph2Mg(dx) (25.0 mmol) dissolved in 60 mL of THF was added. Then, the reaction mixture was slowly warmed to r.t. During this time the precipitate dissolved, and a deep red solid formed, finally yielding a deep red solution with a colorless precipitate. The colorless solid was removed by filtration and the filtrate cooled to −40 °C yielding deep red crystals which were collected on a frit and briefly dried in vacuo. These crystals can be stored below 0 °C in the refrigerator. Beginning loss of coordinated thf can be recognized by a green powder forming on the surface of the crystals. Yield: 5.55 g of large deep red crystals of 3 (70.9% with respect to starting CrCl3(thf)3). Elemental analysis (C31H41CrO3.5, 521.6) calcd: Cr 9.97. Found: Cr 9.86. Synthesis of [trans-Cl2CrPh(thf)3] (1-Cl). CrCl3(thf)3 (1.0 g, 2.67 mmol) was suspended in a mixture of 30 mL of THF and 3 mL of 1,4-dioxane. The stirred suspension was cooled to −30 °C, and 3.1 mL of a 0.44 M solution of Ph2Mg(dx) (1.36 mmol) in THF was added. Then, the reaction mixture was slowly warmed to r.t. During this time a precipitate formed which changed the color from deep red over orange to yellow-green. Thereafter, the dull reaction mixture was warmed to 40 °C and filtered to remove all solid materials. The yellow-green filtrate was stored in a refrigerator at −20 °C. Precipitated yellow-green crystals were collected, washed with diethyl J


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Organometallics ether, and dried in vacuo. Yield: 0.82 g (74% with respect to CrCl3(thf)3). Elemental analysis (C18H29Cl2CrO3, 416.3) calcd: Cr 12.49. Found: Cr 12.59. Reaction of 3 with Iodine in a Molar Ratio of 1:2. Complex 3 (1.0 g, 1.92 mmol) was recrystallized twice from THF and then dissolved in THF at 0 °C. Freshly sublimed iodine (0.98 g, 3.86 mmol) was added as a solid. A yellow precipitate formed which slowly converts to brown-black crystals of [mer-I3Cr(thf)3] upon heating to 40 °C. Yield: 0.34 g (27% with respect to starting 3) of brown-black crystals of [mer-I3Cr(thf)3]. Elemental analysis (C12H24CrI3O3, 449.0) calcd: Cr 8.01. Found: Cr 8.15. The mother liquor was cooled to −40 °C. Several fractions of mixtures of crystalline [mer-I3Cr(thf)3] and brown crystals of [trans-I2CrPh(thf)3] (1-I) were obtained. Synthesis of [(thf)5MgCl]+ [{Ph2Cr(thf)}2(μ-Cl)3]− ((2-Cl)2MgCl2). CrCl3(thf)3 (1.0 g, 2.67 mmol) was suspended in a mixture of 30 mL of THF and 3 mL of dioxane. The stirred suspension was cooled to −30 °C, and 6.2 mL of Ph2Mg(dx) (2.73 mmol) were added as 0.44 M THF solution. Thereafter the reaction mixture was slowly warmed to r.t. During this time a precipitate formed which changed color from deep red over orange to brown. At r.t. a nearly colorless solid remained which was removed by filtration over diatomaceous earth. The brown filtrate was layered with 10 mL of npentane and stored at 0 °C. A mixture of fascicular red-brown crystals of Ph2CrCl(thf)3 and a few yellow-green crystals of Cl2CrPh(thf)3 (1Cl) formed. The mother liquor was decanted and separated from this fascicular crystal mixture at the wall of the glass flask. The mother liquor was concentrated yielding a residue which was washed with diethyl ether and briefly dried in vacuo. Yield: 0.46 g of red-brown crystals of (2-Cl)2-MgCl2 which appear violet in transmitted light (31% with respect to starting CrCl3(thf)3). Synthesis of [cis-Ph2CrI(thf)3] (2-I). Complex 3 was recrystallized twice from THF (1.0 g, 1.92 mmol) and dissolved in THF at 0 °C. Then, 0.49 g of freshly sublimed iodine (1.93 mmol of I2) was slowly added as solid. During the time when the iodine reacted, a yellow precipitate formed. The suspension was warmed to 40 °C, and the precipitate nearly completely dissolved. All solids were removed by filtration over a frit with diatomaceous earth. The filtrate which smelled like iodobenzene was stored at −20 °C. Light brown crystals precipitated and were collected on a frit, washed with cold THF, and briefly dried in vacuo. Yield: 0.48 g of light brown crystals 2-I (45% with respect to starting 3). Elemental analysis (C24H34CrIO3, 549.4) calcd: Cr 9.46. Found: Cr 9.54. Synthesis of [Li(dme)3]+ [(C6H5)4Cr(dme)]− (4-Li). Complex 3 (1.85 g, 3.55 mmol) was suspended in 10 mL of DME and poured into a Schlenk flask, covered with a septum, and cooled to −20 °C. Then, 4.4 mL of a 0.88 M solution of phenyl-lithium (3.87 mmol) in diethyl ether was added within 15 min via a hollow needle. During this procedure the red crystals of the starting material yielded a cherry-red solution, and colorless and red crystals precipitated. The colorless solid was removed at 0 °C by filtration and the filtrate stored at −20 °C. The deep red crystals were collected on a frit, washed with diethyl ether, and briefly dried in vacuo. Yield: 2.10 g of 4-Li (81% with respect to starting 3). This compound readily dissolved in toluene and THF with a green color and in DME with a red color. This complex is only sparingly soluble in diethyl ether. Elemental analysis (C40H60CrLiO8, 727.8) calcd: Cr 7.14. Found: Cr 7.07. Synthesis of [{(C6H5)Mg(dme)2(thf)}+(C6H5)4Cr(dme)}−] (4Mg). Complex 3 (0.80 g, 1.53 mmol) was suspended in 10 mL of DME. Solid Ph2Mg(dx) (0.45 g, 1.69 mmol) was added at once. In the stirred reaction mixture, the substrates reacted yielding a deep red precipitate. Addition of 20 mL of THF and warming to 60 °C gave a red solution. Then, the reaction solution was cooled to r.t., and deep red crystals formed which were collected on a Schlenk frit, washed with a few milliliters of diethyl ether and briefly dried in vacuo. Yield: 0.82 g of 4-Mg (85% with respect to 3). Synthesis of [{Li(dme)}2Cr(C6H5)5] (5-Li). Compound 3 (1.50 g, 2.87 mmol) was suspended in 15 mL of DME at −20 °C in a Schlenk flask sealed with a septum. Within 15 min, 6.75 mL of a 0.88 M solution of phenyl-lithium (5.94 mmol) in diethyl ether was added to the stirred suspension. During this process the red crystals reacted to a

suspension containing colorless and green microcrystals. After a 1 h reaction period, the mixture was warmed to r.t. Overnight large colorless crystals and small green crystals precipitated. The large colorless crystals were removed by filtration with a G1 frit, and the filtrate contained the majority of the green compound. The green microcrystalline material was collected on a G3 frit, washed with diethyl ether, and briefly dried in vacuo. Yield: 1.44 g of 5-Li as bottlegreen crystals (79% with respect to 3). This compound was soluble in THF but was only sparingly soluble in DME and diethyl ether. Elemental analysis (C38H45CrLi2O4, 631.6) calcd: Cr 8.23. Found: Cr 8.08. Preparation of [{Li(dme)3}]+ [{Li(dme)}Cr(C6H5)5}−] (5-Li′). Complex 5-Li (1.26 g, 2.0 mmol) was suspended in 15 mL of DME and stirred at 40 °C. Then, approximately 15 mL of THF were added dropwise until the starting material dissolved completely. After filtration of the dark green solution, the filtrate was stored overnight at −40 °C. The precipitated crystals were collected on a frit, washed with diethyl ether, and briefly dried in vacuo. Yield: 0.65 g of Li2Cr(C6H5)5(dme)4 (5-Li′, 40% with respect to 5-Li) as dark green crystals. Elemental analysis (C46H65CrLi2O8, 811.8) calcd: Cr 6.40. Found: Cr 6.31. Synthesis of [{{Ca(dme)(thf)}2Cr(C6H5)6]+ [(thf)Cr(C6H5)4]− (5Ca). Complex 3 (1.0 g, 1.92 mmol) was suspended with 10 mL of DME at −20 °C in a Schlenk flask sealed with a septum. Then, 27 mL of a 0.16 M solution of diphenylcalcium in THF was added via a hollow needle. During warm up to r.t., the red crystals of 3 reacted. The solution was filtered over a frit covered with diatomaceous earth, and the filtrate was stored in a refrigerator. Overnight green crystals formed which were collected on a frit, washed with diethyl ether, and briefly dried in vacuo. Yield: 0.33 g of 5-Ca as green crystals (25% with respect to starting 3). Synthesis of [{Li(thf)4}+{(Li(thf))2Cr(C6H5)6}−] (6-Li). Complex 3 (0.52 g, 1.00 mmol) was suspended in 10 mL of THF at −20 °C in a Schlenk flask sealed with a septum. Within 15 min, 4.1 mL of a 0.75 M solution of phenyl-lithium (3.07 mmol)) in diethyl ether were added dropwise via a cannula. Thereafter, the reaction mixture was stirred at 0 °C. During this time the red crystals of 3 converted to a light yellow solid which was collected on a frit, washed with diethyl ether, and briefly dried in vacuo. Yield: 0.82 g of 6-Li as yellow crystals (85% with respect to 3). Elemental analysis (C60H78CrLi3O6, 968.0) calcd: Cr 5.37. Found: Cr 5.28. Single crystals suitable for Xray diffraction experiments were grown overnight at −40 °C from the filtrate which was layered with 5 mL of diethyl ether. Synthesis of [{Ph3Cr(μ-O)}2{{(thf)Li}2(μ-thf)}2] (7). A suspension of 0.82 g of 6-Li (0.85 mmol) in 10 mL of THF was kept at −40 °C. The yellow color of the suspension slowly changed to a green color. After a few days, the first crystals of 7 precipitated. After approximately 5 weeks, the transformation was nearly quantitative. The crystals were collected on a frit and briefly dried in vacuo. Yield: 0.33 g of 7 as green crystals (73% with respect to complex 6-Li). Elemental analysis (C60H78Cr2Li4O8, 1059.0) calcd: Cr 9.82. Found: Cr 9.95. Synthesis of [{Li(thp)4}+ {{(thp)Li}2Cr(C6H5)6}−·3THP] (6-Li′) and Preparation of a Few Crystals of [{Li(thp)4}+ {(Li(thp))2Cr(o,o′-C12H8)(C6H5)4}−·3THP] (8) in the Mother Liquor. Complex 3 (0.53 g, 1.02 mmol) was dissolved in 12 mL of THP in a Schlenk flask with a septum. The red crystals completely dissolved at r.t. yielding a green solution. At −40 °C the solution turned red. Then, 4.5 mL of a 0.75 M solution of phenyl-lithium (3.37 mmol) in diethyl ether were added dropwise via a cannula within 15 min. During this process the solution turned green and finally brown-yellow. Next, stirring was continued at 0 °C. Light yellow microcrystalline precipitate of [Li(thp)4]+ [{(thp)Li}2Cr(C6H5)6]− (6-Li′) formed. This precipitate was collected on a frit, washed with diethyl ether, and briefly dried in vacuo. The mother liquor also contained product 8; therefore, optimization of the yield by workup of the mother liquor was neglected. Yield: 0.52 g of [{Li(thp)4}+ {{(thp)Li}2Cr(C6H5)6}−· 3THP] as yellow microcrystals (ca. 40% with respect to 3). Elemental analysis (C81H120CrLi3O9, 1310.6) calcd: Cr 3.97. Found: Cr 4.03. Storage of the mother liquor, which also contained biphenyl from the K


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synthesis of phenyl-lithium, for 2 weeks at −40 °C in the refrigerator led to a mixture of crystals of [{Li(thp)4}+ {{(thp)Li}2Cr(C6H5)6}−· 3THP] and a few large yellow crystals of 8. Structure Determination. Intensity data for the compounds were collected on a Nonius KappaCCD diffractometer using graphitemonochromated Mo Kα radiation. Data were corrected for Lorentz and polarization effects; absorption was taken into account on a semiempirical basis using multiple-scans.59−61 The structures were solved by Direct Methods (SHELXS)62 and refined by full-matrix least-squares techniques against Fo2 (SHELXL-9762 and SHELXL2014).63 All hydrogen atoms of the compound 1-Cl were located by difference Fourier synthesis and refined isotropically. All other hydrogen atoms were included at calculated positions with fixed thermal parameters. The crystals of (2-Cl)2-MgCl2 and 5-Li were nonmerohedral twins. The twin laws were determined by PLATON64 to (0.001 0.000 −0.999) (0.000 −1.000 0.000) (−1.001 0.000 −0.001) and (−1.00 0.000 −0.240) (0.000 −1.000 −0.006) (0.000 0.000 1.000), respectively. The contributions of the major components were refined to 0.708(1) and 0.737(3), respectively. The crystals of 3, (2-Cl)2-MgCl2, and 4-Li contain large voids, filled with disordered solvent molecules. The size of the voids are 174, 208, and 197 Å3/unit cell, respectively. Their contributions to the structure factors were secured by back-Fourier transformation using the SQUEEZE routine of the program PLATON64 resulting in 55, 52, and 60 electrons/unit cell, respectively. All nondisordered, nonhydrogen atoms were refined anisotropically.64 The crystals of 5-Li′, 6-Li, 6-Li′, and 6-Li″ were extremely thin and/or of low quality, resulting in a substandard data set. However, the structures were sufficient to show connectivity and geometry despite the high final R values. We publish only the conformation of the molecule and the crystallographic data. We have not deposited this data in the Cambridge Crystallographic Data Centre. XP65 and POV-Ray66 were used for structure representations.

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REFERENCES

(1) Bennett, G. M.; Turner, E. E. The action of chromic chloride on the Grignard reagent. J. Chem. Soc., Trans. 1914, 105, 1057−1062. (2) Franz Hein was a prominent chemist, and his achievements were already recognized quite early and honors granted in several articles.3 He was born in Grötzingen, Baden, near Karlsruhe (Germany), and studied chemistry at the University Leipzig from 1912 to 1917. His dissertation was supervised by the inorganic and organic professors K. Schäfer and A. Hantzsch. During his habilitation, he remained in the boundary region between inorganic and organic chemistry, and in 1921, he finished his habilitation with the title “Polyphenylchrombasen and ihre Salze” (polyphenylchromium bases and their salts) at the Laboratorium für Angewandte Chemie and became a professor at the University Leipzig. In 1942, he moved to Jena, Germany, and was professor for inorganic chemistry at the Friedrich Schiller University Jena till his retirement in 1959. Hein recognized the importance and relevance of organochromium compounds and contributed significantly to the development of the organometallic chemistry of transition metals in general. (3) (a) Rienäcker, G.; Klemm, W.; Schwaz, R.; Meiner, A. Prof. Dr. Franz Hein. Z. Anorg. Allg. Chem. 1952, 268, 201−201. (b) Oesper, R. E. Franz Hein. J. Chem. Educ. 1953, 30, 313−314. (c) Beyer, L.; Hoyer, E. Franz Hein, Arthur Schleede, Hans Kautsky und die Anorganische Chemie in Leipzig. Nachr. Chem. 2000, 48, 1493−1497. (d) Seyferth, D. Bis(benzene)chromium. 1. Franz Hein at the University of Leipzig and Harold Zeiss and Minoru Tsutsui at Yale. Organometallics 2002, 21, 1520−1530. (e) Seyferth, D. Bis(benzene)chromium. 2. Its discovery by E. O. Fischer and W. Hafner and subsequent work by the research groups of E. O. Fischer, H. H. Zeiss, F. Hein, C. Elschenbroich, and others. Organometallics 2002, 21, 2800−2820. (f) Behrends, R.; Beyer, L. Eine Familiengeschichte zwischen bildender Kunst und Naturwissenschaften; Passage-Verlag: Leipzig, Germany, 2012. (4) Hein, F. Notiz über Chromorganoverbindungen. Ber. Dtsch. Chem. Ges. B 1919, 52B, 195−196. (5) Hein, F. Chromorganische Verbindungen, I. Mitteilung: Pentaphenyl-chromhydroxyd. Ber. Dtsch. Chem. Ges. B 1921, 54B, 1905−1938. (6) Klemm, W.; Neuber, A. Magnetochemische Untersuchungen. XXII. Das magnetische Verhalten der Chromphenylverbindungen. Z. Anorg. Allg. Chem. 1936, 227, 261−271. (7) Cotton, F. A. Alkyls and aryls of transition metals. Chem. Rev. 1955, 55, 551−594. (8) Koschmieder, S. U.; Wilkinson, G. Homoleptic and related aryls of transition metals. Polyhedron 1991, 10, 135−173. (9) Hein, F.; Bähr, G. Zum thermischen Abbau des Tetraphenylchromjodids im Hochvakuum. Chem. Ber. 1953, 86, 1171−1182. (10) Hein, F.; Eißner, W. Ü ber das Tetraphenylchrom (C6H5)4Cr (VI. Mitteilung über chromorganische Verbindungen). Ber. Dtsch. Chem. Ges. B 1926, 59B, 362−366. (11) Hein, F.; Schmiedeknecht, K. Ü ber die Darstellung von σPhenylchromchloriden. Z. Anorg. Allg. Chem. 1967, 352, 138−144. (12) Hein, F.; Schmiedeknecht, K. Umsetzungen von Natriumpentaphenylochromat(III) und Lithiumhexaphenylochromat(III) mit Chromtrichloridtristetrahydrofuranat in Diäthyläther: Darstellung von Na2Cr2(C6H5)6·3O(C2H5)2, Li2Cr2(C6H5)6·3O(C2H5)2 und Chromaromaten-Komplexen. J. Organomet. Chem. 1966, 6, 45−52. (13) Hein, F.; Schmiedeknecht, K. Ü ber die Darstellung von Natrium- und Lithiumtetraphenylochromaten(III). J. Organomet. Chem. 1967, 8, 503−509. (14) (a) Hein, F.; Heyn, B.; Schmiedeknecht, K. Ein neuer Typ von Chromorganokomplexen: MeI2[Cr(C6H5)5Ae]·2Ae. Monatsber. Dtsch. Akad. Wiss. Berlin 1960, 2, 552−553. (b) Hein, F.; Heyn, B. Umsetzung von Lithium-Chrom-Phenyl mit “aciden” Kohlenwasserstoffen. Monatsber. Dtsch. Akad. Wiss. Berlin 1962, 4, 220−223. ( c ) H e i n , F . ; S c h m i e d e k n e c h t , K . Ü b e r Natriumpentaphenylochromat(III) und

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00811. Crystal parameters and refinement details of the X-ray crystal structures, molecular representations of [CrI3(thf)3], 1-I, 6-Li′, and 6-Li″ (PDF) Accession Codes

CCDC 1875588−1875599 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*Homepage: http://www.lsac1.uni-jena.de. E-mail: m.we@ uni-jena.de. Fax: +49 3641-9-48110. ORCID

Matthias Westerhausen: 0000-0002-1520-2401 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We commemorate this manuscript in memoriam Professor Franz Hein. We acknowledge the valuable support of the NMR service platform (https://www.nmr.uni-jena.de/login/) of the Faculty of Chemistry and Earth Sciences of the Friedrich Schiller University Jena, Germany. L


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Phenylchromium(III) Chemistry Revisited 100 Years after Franz Hein

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