Planar-Chiral 1,1â²-Diboryl Metallocenes: Diastereoselective

Article pubs.acs.org/IC

Planar-Chiral 1,1′-Diboryl Metallocenes: Diastereoselective Synthesis from Boryl Cyclopentadienides and Spin Density Analysis of a Diborylcobaltocene Emmanuel Lerayer,† Patrice Renaut,† Stéphane Brandès,† Hélène Cattey,† Paul Fleurat-Lessard,*,† Ghenwa Bouhadir,‡ Didier Bourissou,*,‡ and Jean-Cyrille Hierso*,†,§ †

Université de Bourgogne Franche-Comté, Institut de Chimie Moléculaire de l’Université de Bourgogne (ICMUB UMR 6302 CNRS), 9 Avenue Alain Savary, 21078 Dijon, France ‡ Laboratoire Hétérochimie Fondamentale et Appliquée (LHFA), Université Paul Sabatier/CNRS UMR 5069, 118 Route de Narbonne, 31062 Toulouse, Cedex 09, France § Institut Universitaire de France (IUF), 103 Boulevard Saint Michel, 75005 Paris Cedex, France S Supporting Information *

ABSTRACT: The reaction of nonsubstituted alkali metal cyclopentadienides with haloboranes leads to ∼90:10 mixtures of isomeric diene products that can be deprotonated to give simple boryl cyclopentadienides. We extended this transformation to the sterically hindered lithium tert-butylcyclopentadienide 1 using FBMes2 (Mes = 2,4,6-trimethylphenyl) and ClBCy2 as electrophiles. The boryl group is selectively introduced in the remote position to minimize steric congestion. The new boryl dienes are obtained as mixtures of isomers, and subsequent deprotonation with MeLi or LiHMDS affords the lithium 1,3-disubstituted cyclopentadienides 5a,b in yields over 95%. Direct assembling of tertbutylated boryl cyclopentadienides with MCl2 (M = Fe, Co) selectively leads to 1,1′-planar chiral ferrocenes 6a,b and cobaltocene 7. To shed light into the diastereoselective formation of 6a, DFT calculations were performed. The potential energy surface was scrutinized so as to identify and compare its diastereoisomers and conformers. This stereoselectivity is attributed to minimized steric repulsions between the tert-butyl and the BMes2 groups in the eclipsed conformation of the racemic diastereoisomers. The X-ray structures of boryl diene 2a and diboryl ferrocene 6a are reported. The electronic structure of cobaltocene 7 was analyzed by EPR and DFT calculations. The spin density of this unique open-shell complex is mainly localized on the Co center, but significant spin density is also found on the boron atoms, indicating substantial delocalization of the unpaired electron over the Lewis acid moieties. Consistently, the singly occupied molecular orbital is a combination of a Cocentered 3d orbital with π(BC) orbitals on each CpBMes2 rings. There is only weak, if any, direct M···B interaction in 6 and 7.

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type A−D)5c,d,6,7b,9 or one Lewis acid site and a Lewis base (compounds of type E−H)10,11 related to the concepts of frustrated Lewis pairs and ambiphilic ligands. Another appealing feature of metallocenes is the planar chirality inherent to heterodisubstituted compounds.12,13 As illustrated in Figure 2, different planar-chiral structures can be envisioned for bifunctional derivatives: the Lewis acid moieties can be linked to the metallocene either in 1,2-,9,13−15 1,3-,16 or 1,1′-positions.5c,17 The planar chirality arises from different Lewis acid groups in the case of 1,2- and 1,3-substituted derivatives (E1 ≠ E2), while further substitution of the metallocene (R groups) is needed to generate planar-chiral 1,1′-diacids.

INTRODUCTION Boryl metallocenes, which combine the unique features of sandwich complexes with the Lewis acid properties of trivalent boron centers, attract considerable interest.1−4 Fundamental studies focus on the reciprocal influence of the Lewis acid moiety and metallocene platform in terms of geometry, electronic structure, and chemical and electrochemical behavior. In addition, the possible existence of direct M → B interactions has been pointed out and investigated thoroughly.5−7 Boryl metallocenes have also been applied in different areas, including acid catalysis of organic transformations,1 Ziegler−Natta polymerization catalysis,2 selective anion recognition,3,4 and boron neutron capture therapy (BNCT).8 Over the past few years, the field has progressed further with the development of bifunctional boryl metallocenes. Representative examples of such species are depicted in Figure 1. They combine two Lewis acid sites (compounds of © XXXX American Chemical Society

Received: October 20, 2016

A

DOI: 10.1021/acs.inorgchem.6b02510 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

preparation of planar-chiral polyfunctionalized ferrocenylphosphines.18 In this context, we report herein direct assembling of novel [t-BuCpBR2] anions with metal salts leading to planar-chiral 1,1′-diboryl metallocenes of type c. Racemic isomers of ferrocene and cobaltocene derivatives are selectively obtained. Their structures have been analyzed based on XRD and EPR data, as well as DFT calculations.

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RESULTS AND DISCUSSION As reported by Herberich,19 the reaction of nonsubstituted alkali metal cyclopentadienides with haloboranes leads to ∼90:10 mixtures of isomeric diene products that can be deprotonated to give simple boryl cyclopentadienides. We extended this transformation to the sterically hindered lithium tert-butylcyclopentadienide 1 using FBMes2 (Mes = 2,4,6trimethylphenyl) and ClBCy2 as electrophiles (Scheme 1). The boryl group is selectively introduced in the remote position to minimize steric congestion. The boryl dienes are obtained as mixtures of isomers 2−4,20 but subsequent deprotonation with MeLi or LiHMDS affords the lithium 1,3-disubstituted cyclopentadienides 5a,b in yields over 95%. The lithium cyclopentadienides 5a,b were then treated with metal salts to assemble metallocenes (Scheme 2). The reaction

Figure 1. Representative bifunctional boryl ferrocenes.

Scheme 2. Metallocenes Synthesis by Boryl Cyclopentadienide Assembling

Figure 2. Planar-chiral bifunctional metallocenic Lewis acids.

Various 1,2-planar chiral ferrocenes of type a have been synthesized upon rearrangement of 1,1′-disubstituted precursors followed by selective ortho-borylation.9,13−16 1,3-Disubstituted compounds of type b are comparatively rare, and their selective formation requires one to mask one cyclopentadienyl (Cp) ring by permethylation.16 Planar-chiral compounds of type c are even rarer and to the best of our knowledge limited to the early contributions of Herberhold and Wrackmeyer.5c,17 Alkyl ferrocenes and 1,1′-dibromoferrocene reacted with excess BBr3 to give 1,1′-diboryl ferrocenes, among other products. All synthetic routes mentioned above involve multistep modification of the ferrocene platform. Another classical approach to metallocenes entails the assembling of adequately substituted cyclopentadienides. This strategy enables one to readily vary the central transition metal and proved very efficient for the

of 5a with iron dichloride led to the formation of a single compound 6a which was isolated in 61% yield after workup. The 1H NMR spectrum shows a unique set of signals at δ 5.13,

Scheme 1. Synthesis of tert-Butylated Boryl Lithium Cyclopentadienides 5a,b

B


Article

Inorganic Chemistry

Figure 3. Ortep view of the diboryl ferrocene 6a with 30% probability ellipsoids (top and side view). Hydrogen atoms and one molecule of hexane solvent have been omitted for clarity. Selected bond length (Angstroms) and angles (degrees): Fe−Ct1 = 1.6814(8); Fe−Ct2 = 1.6792(8); C1−B1 = 1.555(3); C6−B2 = 1.552(3); Ct1−Fe−Ct2 = 178.70(4); B1−Ct1−Ct2−B2 = 141.59(6); B···B = 6.476(3).

Table 1. Optimized Geometries (top views) and Free Energy (in kcal·mol−1) of the Three Stereoisomers of 6a

To shed light into the diastereoselective formation of 6a, DFT calculations were performed. The potential energy surface, which implicitly includes the toluene solvent, was scrutinized so as to identify and compare its diastereoisomers and conformers (SI for details). Three energy minima were located on the potential energy surface (Table 1). In line with experimental observations, the most stable form is an eclipsed conformer of the rac diastereoisomer (6a-rac1). The optimized geometry reproduces nicely the crystallographic data (Table S2, SI), substantiating the accuracy of the computational method. The other minima correspond to a staggered conformer of the rac diastereoisomer (6a-rac2) and to the meso diastereoisomer (6a-meso). They are located about 5 kcal·mol−1 higher in energy. In the eclipsed conformation of the rac diastereoisomer, steric repulsions between the bulky tBu and BMes2 groups are minimized. The introduction of sterically demanding t-Bu groups on the Cp rings of ferrocenyl polyphosphines has been previously documented by some of us. It enables conformation control, and these ligands have been valued in Pd-catalyzed C−C coupling reactions.18b,c Here, the preparation of the tert-butylated diboryl ferrocenes 6a,b demonstrates another leverage of the Cp substitution, namely, diastereoselective control upon formation of planar-chiral 1,1′difunctional ferrocenes. An additional practical interest of introducing these alkyl groups is the crystallinity provided to 6a. Indeed, the achiral cousin compound 1,1′-(BMes2)2Fc has been reported not to crystallize due to its extremely high solubility, even in nonpolar organic media.16b To further support the synthetic interest of assembling boron metallocenes from alkylated Cp boranes, the boryl cyclopentadienide 5a was reacted with cobalt dichloride. 22 Cobaltocene 7 was formed in 58% isolated yield as a deep

4.63, and 4.61 ppm for the cyclopentadienyl ring protons. Consistently, five 13C NMR signals at δ 108.8 (t-Bu−C), 75.8 (B−C), and 80.9, 77.6, 73.9 (CH) ppm are observed for the Cp rings. The 11B NMR resonance signal of 6a is found at 78 ppm, significantly shifted downfield than that of the boryl cyclopentadienide (5a 57 ppm) but in the typical range for triarylboranes.16b,19 Compound 6a was authenticated as the planar-chiral racemic (rac) stereoisomer by single-crystal X-ray diffraction structure (Figure 3). No trace of the corresponding meso diastereoisomer was detected by NMR, indicating that the ferrocene synthesis is totally diastereoselective. The extension to −BCy2 and −Bi-Pr2 alkylborane analogues lead to stability troubles. Boron−carbon bond lability in such cyclopentadienides and metallocenes has to be managed with caution to limit deborylation, especially in solution (for instance, −Bi-Pr2 cyclopentadienide purity was restricted to 80%). Minimal manipulation is required, and ideally Schlenk tubes should be conserved in a glovebox. However, we were pleased to confirm that BCy2-substituted cyclopentadienide 5b also reacts in a diastereoselective manner. Upon reaction with FeCl2, it selectively forms in a yield over 90% the ferrocene 6b in pure rac form supporting the generality of the diastereoselective process. The XRD structure of air-stable 6a confirms the trigonal planar geometry of the two boron centers, whose coordination planes are slightly tilted by 12−14° from the adjacent Cp planes.21 In agreement with previous observations,10c,16b steric hindrance prevents the establishment of Fe → B interactions in 6a, as typically observed with noncongested BX2-substituted ferrocenes.6 The BMes2 groups are tilted away from the metal center with dip angles α* = 180 − ∠[Cpcentroid−Cipso−B] of 8.02(17)° and 6.57(16)° and Fe···B distances of 3.353(2) and 3.371(2) Å. C


Article

Inorganic Chemistry Table 2. EPR: g-Tensor Main Values and Hyperfine Couplings A (in MHz)

green paramagnetic compound. This low-stability open-shell system could not be XRD characterized, but UV and EPR analyses were carried out to elucidate its structure. The UV−vis spectrum of 7 in hexane solution (Figure S1, SI) shows two main bands at λ 282 and 337 nm, in the typical range for cobaltocenes,23,24 and a broad band at λ 645 nm relating to a d−d transition. The EPR spectrum recorded at 100 K in frozen toluene at X-band is shown in Figure 4. This pattern is

experimental 7-rac1 7-rac2 7-meso

g1

g2

g3

A1

A2

A3

1.932 1.934 1.904 1.924

2.025 2.030 2.032 2.034

2.096 2.096 2.101 2.098

126 148 112 118

70 57 77 72

330 333 363 362

Figure 4. Experimental (black line), simulated (orange), and computed (red line) EPR spectra of 7-rac1 recorded in frozen toluene solution (100 K) at X-band (9.439 GHz).

characteristic of a Co(II) species (S = 1/2, I = 7/2) and immediately disappears upon exposure to air. The hyperfine structure was simulated. The best fit was obtained with the spin Hamiltonian parameters g1 = 1.932, g2 = 2.025, g3 = 2.096 and hyperfine couplings of the unpaired electron with the Co center of A1 = 126, A2 = 70, and A3 = 330 MHz (the EPR parameters are anisotropic24 for Jahn−Teller active cobaltocene derivatives).25 Note that the g-tensor values for 7 compare well with those reported for other substituted cobaltocenes and ansa derivatives.24,26 DFT calculations were then performed as for the diboryl ferrocene 6a. The different possible stereoisomers of the diboryl cobaltocene 7 were considered, and their geometries were optimized (Figure S3, SI). The eclipsed conformation of the planar-chiral isomer 7-rac1 was found again to be the most stable form (the corresponding meso structure is found 6.0 kcal/mol higher in free energy), supporting diastereoselective formation of the rac isomer. To confirm the structure of 7, the optimized geometries of the three isomers were used to compute the EPR g-tensors and the hyperfine coupling parameters A (see Table 2 for gi/Ai data). Then the EPR spectra were simulated with a spin Hamiltonian using the computed parameters and compared to the experimental EPR spectra recorded (Figure 5).27 DFT-computed parameters were found very convincing to discriminate the isomers since the best agreement between the experimental and the theoretical data was clearly found for the planar-chiral diastereoisomer 7-rac1, especially in the fitting of the magnetic resonance (mT).

Figure 5. Experimental (black curve) and simulated EPR spectra for isomers 7-rac1 (top, red curve), 7-rac2 (middle, green curve), and 7meso (bottom, blue curve).

To estimate the effect of the boryl groups on the electronic structure of the cobaltocene, we also conducted EPR calculations (g-tensors and hyperfine couplings) on a model compound 7-H derived from 7-rac1 by replacing the BMes2 groups by hydrogen atoms (Table S3, Figures S4 and S5, SI). The main change was found for g1 whose value decreased from D


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Inorganic Chemistry

for transition metals and as Lewis acids and Lewis pairs in their own.30

1.934 to 1.777, a value similar to the one experimentally determined for decamethylcobaltocene [CoCp*2]. Thus, boron atom attachment deeply impacts EPR g-tensors. The electronic structure of 7-rac1 was further analyzed by DFT calculations. Accordingly, the spin density (Figure S4, SI) is mainly localized on the Co center (difference between net population of α and β electron = 0.972 e, Table S4, SI). Significant spin density is also found on the boron atoms (0.101 e on each), indicating substantial delocalization of the unpaired electron over the Lewis acid moieties (the net spin population on Co in 7-H is 1.072 e, Table S4, SI). Consistently, the singly occupied molecular orbital (SOMO, Figure 6) is a combination

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EXPERIMENTAL SECTION

General Conditions. All reactions were performed under argon atmosphere using Schlenk techniques or a glovebox. Toluene, hexane, THF, and DME were degassed and distilled from sodium benzophenone treatment under argon atmosphere prior to use. Toluene and THF have also been obtained from a Solvent Purifier System. Benzene-d6 and THF-d8 were dried over 3 Å molecular sieves. The boron reagents Cy2BCl and Mes2BF are commercially available.31 The products were characterized at the Chemical Analysis Platform and Molecular Synthesis, University of Burgundy. 1H NMR (δ in ppm) spectra (300.13 and 600.13 MHz), 11B NMR (δ in ppm) spectra (96.3 and 160.5 MHz), and 13C NMR (δ in ppm) spectra (75.5 and 150.9 MHz) (including identification by 2D NMR experiments COSY, HMQC, and HMBC sequences) were recorded at 300 K (unless otherwise mentioned) on a Bruker 300 Advance, Bruker 500 Advance DRX, or Bruker 600 Advance II spectrometer. Exact masses were obtained from a LTQ-Orbitrap XL (THERMO). Diboryl Metallocenes Synthesis. 1,1′-Bis(dimesitylboryl)-3,3′bis(tert-butyl)ferrocene (6a). A solution of lithium 1-dimesitylboryl-3tert-butylcyclopentadienide·DME (5a) (239 mg, 0.512 mmol) in 2 mL of THF was added dropwise over 2 min to a suspension of iron dichloride (32.9 mg, 0.260 mmol) in 2 mL of THF at reflux (66 °C). The reaction mixture was refluxed for 24 h. The solution was evaporated in vacuo, extracted with Et2O, and washed with water. After drying of organic phases over MgSO4, evaporation of the solvent gave 6a as a purple crystalline solid (124 mg, 0.156 mmol, 61% yield). 1 H NMR (600 MHz, benzene-d6): δ 6.81 (4 H, brs, m-CH), 5.12 (1 H, dd, J 2.6, 1.4, 4−CH), 4.62 (1 H, d, J 1.4, 2−CH), 4.60 (1 H, dd, J 2.5, 1.3, 5−CH), 2.43 (12 H, brs, o-CH3), 2.21 (6 H, s, p-CH3), 1.03(9 H, s, t-Bu). 11B NMR (160 MHz, benzene-d6): δ 78 ppm. 13C NMR (151 MHz, benzene-d6): δ (ppm) = 143.8 (C-B), 139.4 (o-C), 137.6 (p-C), 129.0 (m-CH), 108.8 (3-CH), 80.9 (5-CH), 77.6 (2-CH), 73.9 (4CH), 32.0 (C(CH3)3), 31.1 (C(CH3)3), 24.8 (o-CH3), 21.1 (p-CH3), CCp-B obscured. Single crystals of 6a were obtained from a hexane solution. A molecule of C6H14 is present in the unit cell with two of 6a (see SI). Anal. Calcd for C54H68B2Fe·1/2C6H14 (837.64): C, 81.65; H, 8.95. Found: C, 80.68; H, 8.97. C54H68B2Fe (794.69). Exact mass [M]+: m/z = 794.48511, simulated = 794.48336, σ = 0.039, err-[ppm] = −2.199. 1,1′-Bis(dicyclohexylboryl)-3,3′-bis(tert-butyl)ferrocene (6b). A solution of lithium 1-dicyclohexylboryl-3-tert-butylcyclopentadienide 5b (781 mg, 2.57 mmol) in 10 mL of THF was added at room temperature to a stirred suspension of iron dichloride in excess (0.32 g, 2.5 mmol) in 10 mL of THF. The reaction mixture was stirred for 24 h. The solution was evaporated in vacuo, and hexane was added to precipitate lithium chloride and the excess of FeCl2. After filtration and removal of solvent under vacuum, 833 mg of red oil was isolated as 6b with hexane traces (9 wt %) (1.16 mmol, 93% yield). 1H NMR (600 MHz, bz-d6): δ 4.71(1 H, s, Cp−CH), 4.52 (1 H, s, Cp−CH), 4.44 (1 H, s, Cp−CH), 2.00−1.31 (22 H, m, Cy), 1.26 (9 H, s, t-Bu). 11B (160 MHz, bz-d6): δ 75. Exact mass [M]+: m/z = 650.48747, simulated = 650.48658, δ[ppm] = 3.633. (Compound 6b is highly sensitive to deborylation, which occurs during extended time 13C NMR analysis.) 1,1′-Bis(dimesitylboryl)-3,3′-bis(tert-butyl)cobaltocene (7). A solution of 5a lithium 1-dimesitylboryl-3-tert-butylcyclopentadienide· DME (168 mg, 0.360 mmol) in 5 mL of DME was added at −50 °C to a suspension of cobalt dichloride (24 mg, 0.184 mmol) in 5 mL of DME at reflux (85 °C). The reaction mixture was stirred for 15 h, allowing the temperature slowly rise up to room temperature. The solution was evaporated in vacuo, and hexane was added to precipitate lithium bromide. After filtration and removal of solvent under vacuum, borylcobaltocene 7 was formed as an air-sensitive blue-green oil (100 mg, 0.105 mmol, 58% yield, purity is estimated at 85 wt % due to dienes traces). EPR and UV analysis confirmed the cobaltocene structure.

Figure 6. Singly occupied molecular orbital of the diboryl cobaltocene 7-rac1 (Co in ochre, C in gray, and B in yellow; H atoms are omitted for clarity). Isovalue = 0.03.

of a Co-centered 3d orbital with π(BC) orbitals on each CpBMes2 ring. The density between the Co and the B centers is very small (0.049 e vs 0.001 to 0.212 e between the Co center and the Cp carbon atoms), indicating very weak, if any, direct Co···B interactions.28 The spin delocalization is in fact essentially mediated by the Cp rings. This open-shell complex boryl cobaltocene forms part of a series of boron-centered radicals bearing mesityl and related substituents (duryl) that have scant representation in the recent literature.29

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CONCLUSION This work describes the synthesis, characterization, and solidstate and solution structures of planar-chiral 1,1′-metallocenes. Planar-chiral compounds with a diboryl-1,1′,3,3′-substitution pattern were virtually unknown due to the synthetic challenges of functionalizing metallocenes cores in this manner. We proposed herein an alternative synthetic route that is fully diastereoselective. Thus, the presence of tert-butyl groups on boryl cyclopentadienides enables diastereoselective control and formation of planar-chiral 1,1′-difunctionalized sandwich complexes. For now, the chiral resolution of the racemate is not trivial due to the lability of the B−C bond. The ferrocene and cobaltocene derivatives do not feature significant direct M → B interactions, but we clearly show for the first time in an open-shell complex of this type that the spin density of the cobaltocene 7 is delocalized over the two boryl moieties, having a strong effect on the EPR spectrum anisotropy. Ongoing work in our laboratory aims at preparing a series of stable planarchiral 1,1′-difunctionalized metallocenes by varying the substituents at boron and at exploring their use both as ligands E


Article

Inorganic Chemistry Notes on the Preparation of Boryl Dienes and Ratio of Isomers. Lithium salts synthesis is detailed in SI. The lithium salt 1 is quantitatively converted into the 1-dimesitylboryl-3-tert-butylcyclopentadienes 2a and 4a (mixture 48:52, 3a is not observed) in DME. This mixture of dienes was isolated as a white crystalline powder and can be handled under air. The X-ray diffraction structure of isomer 2a was obtained after crystallization (Scheme 1). The B−C1 bond length of 2a = 1.5387(1) Å is in the range of reported values with slightly destabilized longer double bonds C3−C4 = 1.3559(1) Å and C1−C5 bond =1.3981(1) Å (vs 1.34−1.35 Å). Following the same synthetic protocol the dienes incorporating (dicyclohexyl)boryl fragments were also synthesized (2b and 3b = 54:46, 98%, 4b is not observed). Decreasing in yield was due to the marginal occurrence of deborylation. Lithium cyclopentadienide 5a was obtained in 96% isolated yield from MeLi addition in DME at −50 °C. 1H NMR of 5a after crystallization in DME indicates the presence of 1 equiv of DME which strongly interacts with 5a; the presence of DME is persistent even after vacuum treatment for several hours. Deprotonation of [2b/ 3b] diene mixture was realized by addition of LiHMDS in hexane at −50 °C; these conditions limit detrimental deborylation and lead to products 5b in 95% isolated yield with about 5% of deborylation to 1. EPR Studies. The measurements were made at the PACSMUB platform. Continuous wave (CW) EPR spectra were recorded on a Bruker ELEXSYS 500. The instrument was equipped with a 4122 SHQE/0405 X-band resonant cavity operating at 9.43 GHz, a X-band high-power dual-gun-oscillator bridge, and a quartz cryostat cooled at 100 K with a stream of nitrogen. The temperature was regulated with an ER 4131VT accessory. All apparatus as well as data acquisition were controlled using Xepr software. The magnetic field was swept from 250 to 360 mT through 2048 points. Spectra were recorded at 6 mW power, 100 kHz frequency modulation, 0.5 mT modulation amplitude, 10 ms time constant, and 40 ms conversion time (spectra recorded at 100 K in frozen toluene). EPR spectra were simulated using the EasySpin 4.5.5 software package, implemented as a toolbox in MATLAB. A Gaussian line width (0.5 mT) with a Lorentzian contribution (0.2 mT) was used. Computational Studies. Quantum mechanics calculations were performed with the Gaussian09 software package. Energy, forces, and harmonic frequencies were computed by density functional theory with the B3PW91 hybrid exchange-correlation functional. All calculations were conducted in toluene, the solvent used in the EPR experiments, described as a polarizable continuum model (PCM) as implemented in Gaussian09. Geometries were optimized with the def2-TZVP basis set for the transition metal atom (Fe and Co) and the 6-31+G(d,p) basis set for other atoms. Full details are available in the SI.

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ORCID

Didier Bourissou: 0000-0002-0249-1769 Jean-Cyrille Hierso: 0000-0002-2048-647X Author Contributions

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the ANR (ANR-11-INTB-1008 MENOLEP), the Région Bourgogne (PARI IME SMT08), and the CNRS. Calculations were performed using HPC resources from DSI-CCUB (Université de Bourgogne).

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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.6b02510. Crystal data (CIF) Lithium 1-dimesitylboryl-3-tert-butylcyclopentadienide (5a), lithium 1-dicyclohexylboryl-3-tert-butylcyclopentadienide (5b), UV−vis data, computational details, geometry optimization of 6a and 7, EPR g tensor and hyperfine parameters calculation, crystal data, NMR spectra, and HR-mass and EA for 6a (PDF)

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REFERENCES

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Inorganic Chemistry

2009, 28, 3152. (c) Mom, S.; Beaupérin, M.; Roy, D.; Royer, S.; Amardeil, R.; Cattey, H.; Doucet, H.; Hierso, J.-C. Congested ferrocenyl polyphosphanes bearing electron-donating or electronwithdrawing phosphanyl groups: assessment of metallocene conformation from NMR spin couplings and use in palladium-catalyzed chloroarenes activation. Inorg. Chem. 2011, 50, 11592. (d) Wechsler, D.; Rankin, M. A.; McDonald, R.; Ferguson, M. J.; Schatte, G.; Stradiotto, M. New Racemic Planar-Chiral Metalloligands Derived from Donor-Substituted Indenes: A Synthetic, Structural, and Catalytic Investigation. Organometallics 2007, 26, 6418. (19) Herberich, G. E.; Fischer, A. Borylcyclopentadienides. Organometallics 1996, 15, 58. (20) See the note in Experimental Section for the ratio of isomers. (21) The angles between the mean planes are of 13.62(12)° for C1C2C3C4C5/B1C1C11C20 and 12.54(12)° for C6C7C8C9C10/ B2C6C33C42. (22) Diboryl cobaltocenes have been generated by reaction of unsubstituted boryl cyclopentadienides with CoCl2. They have been characterized by mass spectrometry and elemental analyses, but their electronic structure has not been analyzed, see: (a) Herberich, G. E.; Fischer, A.; Wiebelhaus, D. Bis(boryl)metallocenes. 1. 1,1′-bis(diisopropylboryl)cobaltocenium cation: A novel anion-binding ligand. Organometallics 1996, 15, 3106. (b) Herberich, G. E.; Englert, U.; Fischer, A.; Wiebelhaus, D. Bis(boryl)metallocenes. 2. Syntheses of 1,1′-bis(boryl)cobaltocenium complexes. Organometallics 1998, 17, 4769. (23) (a) Gordon, K. R.; Warren, D. K. Spectroscopic and magnetic studies of the 3d metallocenes. Inorg. Chem. 1978, 17, 987. (b) Weber, J.; Goursot, A.; Penigault, E.; Ammeter, J. H.; Bachmann, J. Electronic structure of metallocene compounds. 3. Comparison of the results of multiple-scattering Xα calculations with various electronic observables of cobaltocene. J. Am. Chem. Soc. 1982, 104, 1491. (24) (a) Braunschweig, H.; Breher, F.; Kaupp, M.; Gross, M.; Kupfer, T.; Nied, D.; Radacki, K.; Schinzel, S. Synthesis, crystal structure, EPR and DFT studies, and redox properties of [2]tetramethyldisilacobaltocenophane. Organometallics 2008, 27, 6427. (b) Pammer, F.; Sun, Y.; Sieger, M.; Fiedler, J.; Sarkar, B.; Thiel, W. R. A cobaltocenium complex of dibenzo[c,g]fluorenide and its structural and electrochemical Properties. Organometallics 2010, 29, 6165. (25) Symmetry breaking by the t-Bu/BMes2 groups in cobaltocene 7 allows EPR measurement at 100 K. More symmetrical cobaltocenes have to be analyzed at lower temperatures (4 K): Ammeter, J. H. EPR of orbitally degenerate sandwich compounds. J. Magn. Reson. 1978, 30, 299. (26) Pagels, N.; Prosenc, M. H.; Heck, J. An ansa-cobaltocene with a naphthalene handle: synthesis and spectroscopic and structural characterization. Organometallics 2011, 30, 1968. (27) For selected examples of experimental/theoretical comparison of EPR data, see ref 24a and: (a) Hadt, R. G.; Nemykin, V. N.; Olsen, J. G.; Basu, P. Comparative calculation of EPR spectral parameters in [MoVOX4]−, [MoVOX5]2−, and [MoVOX4(H2O)]− complexes. Phys. Chem. Chem. Phys. 2009, 11, 10377. (b) Hall, N.; Orio, M.; Gennari, M.; Wills, C.; Molton, F.; Philouze, C.; Jameson, G. B.; Halcrow, M. A.; Blackman, A. G.; Duboc, C. Multifrequency cw-EPR and DFT studies of an apparent compressed octahedral Cu(II) complex. Inorg. Chem. 2016, 55, 1497. (c) Rosenthal, A. J.; Devillard, M.; Miqueu, K.; Bouhadir, G.; Bourissou, D. A phosphine-coordinated boron-centered Gomberg-type radical. Angew. Chem., Int. Ed. 2015, 54, 9198. (28) One-electron M···B interactions remain rare and are mainly associated with cage complexes deriving from tris[2-(phosphino) phenyl]boranes ligands. For a Cu complex, see: Moret, M.-E.; Zhang, L.; Peters, J. C. A Polar copper−boron one-electron σ-bond. J. Am. Chem. Soc. 2013, 135, 3792. (29) For neutral radicals, see: (a) Chiu, C.-W.; Gabbai, F. P. A 9Borylated Acridinyl Radical. Angew. Chem., Int. Ed. 2007, 46, 1723. (b) Matsumoto, T.; Gabbai, F. P. A Borenium Cation Stabilized by an N-Heterocyclic Carbene Ligand. Organometallics 2009, 28, 4252. (c) Bissinger, P.; Braunschweig, H.; Damme, A.; Krummenacher, I.; Phukan, A. K.; Radacki, K.; Sugawara, S. Isolation of a Neutral Boron-

2154. (d) Braunschweig, H.; Fernández, I.; Frenking, G.; Kupfer, T. Structural evidence for antiaromaticity in free boroles. Angew. Chem., Int. Ed. 2008, 47, 1951. (8) Weissfloch, L.; Wagner, M.; Probst, T.; Senekowitsch-Schmidtke, R.; Tempel, K.; Molls, M. A new class of drugs for BNCT? Borylated derivatives of ferrocenium compounds in animal experiments. BioMetals 2001, 14, 43. (9) Morgan, R.; Di Paolo, A.; Vidovic, D.; Fallis, I. A.; Aldridge, S. Facile syntheses of dissymmetric ferrocene-functionalized Lewis acids and acid−base pairs. Chem. Commun. 2009, 7288. (10) (a) Kimmich, B. F. M.; Landis, C. R.; Powell, D. R. Synthesis and characterization of boron-containing ferrocenyl ligands for asymmetric catalysis. Organometallics 1996, 15, 4141. (b) Vinci, D.; Mateus, N.; Wu, X.; Hancock, F.; Steiner, A.; Xiao, J. Oxazaphospholidine-oxide as an efficient ortho-directing group for the diastereoselective deprotonation of ferrocene. Org. Lett. 2006, 8, 215. (c) Bebbington, M. W. P.; Bontemps, S.; Bouhadir, G.; Hanton, M. J.; Tooze, R. P.; van Rensburg, H.; Bourissou, D. A 1,1′-ferrocenyl phosphine-borane: synthesis, structure and evaluation in Rh-catalyzed hydroformylation. New J. Chem. 2010, 34, 1556. (d) Chen, J.; Murillo Parra, D. A.; Lalancette, R. A.; Jäkle, F. Diferrocenophosphaborin: a planar-chiral, redox-active and anion-responsive ambiphilic ligand. Angew. Chem., Int. Ed. 2015, 54, 10202. (11) (a) Wang, X.; Kehr, G.; Daniliuc, C. G.; Erker, G. Internal adduct formation of active intramolecular C4-bridged frustrated phosphane/borane Lewis pairs. J. Am. Chem. Soc. 2014, 136, 3293. (b) Stephan, D. W.; Erker, G. Frustrated Lewis pairs: metal-free hydrogen activation and more. Angew. Chem., Int. Ed. 2010, 49, 46. (12) Schaarschmidt, D.; Lang, H. Selective syntheses of planar-chiral ferrocenes. Organometallics 2013, 32, 5668. (13) Chen, J.; Lalancette, R. A.; Jäkle, F. Synthesis and Lewis acid properties of a ferrocene-based planar-chiral borenium cation. Chem. Commun. 2013, 49, 4893. (14) (a) Morgan, I. R.; Broomsgrove, A. E. J.; Fitzpatrick, A. E. J.; Vidovic, D.; Thompson, A. L.; Fallis, I. A.; Aldridge, S. Anion recognition by highly sterically encumbered 1,2-diborylferrocenes. Organometallics 2010, 29, 4762. (b) Siewert, I.; Vidovic, D.; Aldridge, S. Syntheses of homochiral 1,2-ferrocene-functionalized Lewis acids and acid/base pairs. J. Organomet. Chem. 2011, 696, 2528. (15) (a) Jäkle, F.; Lough, A. J.; Manners, I. Selective synthesis of 1stanna-2-boraferrocenes: novel bidentate Lewis acids via an unexpected rearrangement reaction. Chem. Commun. 1999, 453. (b) Gamboa, J. A.; Sundararaman, A.; Kakalis, L.; Lough, A. J.; Jäkle, F. Ferrocene-based heteronuclear bidentate Lewis acids via highly selective ortho-borylation of 1,1′-bis(trimethylstannyl)ferrocene. Organometallics 2002, 21, 4169. (16) (a) Eckensberger, D. U.; Kunz, K.; Bolte, M.; Lerner, H.-W.; Wagner, M. Synthesis and structural characterization of the diborylated organometallics 1,3-bis(dibromoboryl)-1′,2′,3′,4′,5′(pentamethyl)ferrocene and 1,3-bis(dibromoboryl)cymantrene. Organometallics 2008, 27, 764. (b) Kelly, M. J.; Broomsgrove, A. E. J.; Morgan, I. R.; Siewert, I.; Fitzpatrick, P.; Smart, J.; Vidovic, D.; Aldridge, S. Syntheses and anion binding capabilities of bis(diarylboryl) ferrocenes and related systems. Organometallics 2013, 32, 2674. (17) (a) Wrackmeyer, B.; Dörfler, U.; Herberhold, M. Synthesis of polyborylated ferrocenes. Z. Naturforsch., B: J. Chem. Sci. 1993, 48, 121. (b) Aldridge, S.; Bresner, C. The coordination chemistry of boryl and borate substituted cyclopentadienyl ligands. Coord. Chem. Rev. 2003, 244, 71. (18) (a) Hierso, J.-C.; Smaliy, R.; Amardeil, R.; Meunier, P. New concepts in multidentate ligand chemistry: effects of multidentarity on catalytic and spectroscopic properties of ferrocenyl polyphosphines. Chem. Soc. Rev. 2007, 36, 1754. (b) Smaliy, R. V.; Beaupérin, M.; Cattey, H.; Meunier, P.; Hierso, J.-C.; Roger, J.; Doucet, H.; Coppel, Y. Conformational control of metallocene backbone by cyclopentadienyl ring substitution: a new concept in polyphosphane ligands evidenced by “through-space” nuclear spin−spin coupling. Application in heteroaromatics arylation by direct C−H activation. Organometallics G


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Inorganic Chemistry Containing Radical Stabilized by a Cyclic (Alkyl) (Amino)Carbene. Angew. Chem., Int. Ed. 2014, 53, 7360. For anionic radicals, see: (d) Olmstead, M. M.; Power, P. P. First structural characterization of a boron-centered radical: X-ray crystal structure of [Li(12-crown4)2]+[BMes3]•−. J. Am. Chem. Soc. 1986, 108, 4235. (e) Feng, P.-Y.; Liu, Y.-H.; Lin, T. S.; Peng, S.-M.; Chiu, C.-W. Redox Chemistry of a Hydroxyphenyl-Substituted Borane. Angew. Chem., Int. Ed. 2014, 53, 6237. (f) Yuan, N.; Wang, W.; Wu, Z.; Chen, S.; Tan, G.; Sui, Y.; Wang, X.; Jiang, J.; Power, P. P. A boron-centered radical: a potassiumcrown ether stabilized boryl radical anion. Chem. Commun. 2016, 52, 12714. (30) Lerayer, E.; Renaut, P.; Roger, J.; Hierso, J.-C. Fr. Demande, FR 3014871A1 20150619, 2015. (31) (a) Brown, H. C.; Dodson, V. H. Studies in Stereochemistry. XXII. The Preparation and Reactions of Trimesitylborane. Evidence for the Non-localized Nature of the Odd Electron in Triarylborane Radical Ions and Related Free Radicals. J. Am. Chem. Soc. 1957, 79, 2302. (b) Ito, A.; Kang, Y.; Saito, S.; Sakuda, E.; Kitamura, N. Photophysical and Photoredox Characteristics of a Novel Tricarbonyl Rhenium(I) Complex Having an Arylborane-Appended Aromatic Diimine Ligand. Inorg. Chem. 2012, 51, 7722.

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