Heteroatom-Bridged ortho-Biferrocenes: Stereoselective Synthesis

Jun 1, 2016 - A versatile synthetic protocol is described for the synthesis of heteroatom-bridged biferrocene derivatives, including the symmetric spe...
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Heteroatom-Bridged ortho-Biferrocenes: Stereoselective Synthesis, Structural Features, and Electrochemical Properties Jiawei Chen,† Alain C. Tagne Kuate,†,‡ Roger A. Lalancette,† and Frieder Jak̈ le*,† †

Department of Chemistry, Rutgers University-Newark, 73 Warren Street, Newark, New Jersey 07102, United States Department of Chemistry, Faculty of Sciences, University of Dschang, P.O. Box 67, Dschang, Cameroon

‡

S Supporting Information *

ABSTRACT: A versatile synthetic protocol is described for the synthesis of heteroatom-bridged biferrocene derivatives, including the symmetric species 2-SnSn and the planar-chiral species 2-SnSi, 2-BSi, and 2-SnP. Treatment of (pR,SS)-2lithio-1-(p-tolylsufinyl)-ferrocene with 0.5 equiv of Me2SnCl2 afforded the tin-bridged biferrocene 1-Sn. A second Sn, Si, or P bridge was then incorporated by substitution of the sulfinyl group in the ortho-position with a dimethylstannyl, dimethylsilyl, or tert-butylphosphino group, respectively, to give the doubly bridged biferrocenes 2-SnSn, 2-SnSi, and 2-SnP. The dimethylstannyl moiety of 2-SnSi was subsequently replaced with a borane bridge via a two-step transmetalation procedure comprising treatment with HgCl2, followed by PhBCl2. The formation of ferrocene-fused six-membered heterocycles was confirmed by multinuclear NMR spectroscopy and highresolution MS analyses. The stereochemical configuration of the chiral biferrocenes 2-SnSi, 2-SnP, and 2-BSi was studied by single-crystal X-ray diffraction, chiral HPLC, and optical rotation measurements. The redox characteristics and absorption properties were investigated as well. The longest wavelength absorption experienced a bathochromic shift and an increase in intensity for 2-BSi relative to 2-SnSn and 2-SnSi, indicative of significant charge transfer character. The cyclic voltammograms of 2-SnSn and 2-SnSi displayed two separate one-electron oxidations as expected for the presence of two ferrocene units in close proximity to one another. The incorporation of boron in 2-BSi resulted in an anodic shift of both oxidation waves and an enlarged peak potential separation. The chemical oxidation of 2-BSi was carried out with [Ag(CH2Cl2)]{Al[OC(CF3)3]4}, and the reaction was monitored by NMR spectroscopy. Attempts to crystallographically characterize the corresponding doubly oxidized species proved unsuccessful.

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INTRODUCTION Since the first reports on its synthesis in the 1950s,1 ferrocene has been widely studied. While initial attention focused on the fundamental investigation of the sandwich-type structure2 and the redox activity of the iron center,3 much recent effort has been directed at the use of ferrocenes as ligands in chiral catalytic transformations4 and as building blocks for new functional materials.5 In the area of materials chemistry, the controlled ring-opening polymerization of strained ferrocenophanes generates metal-containing polymers (A, Chart 1) and corresponding block copolymers that serve as precursors to magnetic materials, self-assemble into nanostructured materials, and can be utilized as redox-switchable optical materials.6 The corresponding cyclic dinuclear [1.1]ferrocenophanes (B) have proved to be ideal model systems to investigate structural aspects and the electronic communication between metal centers in these organometallic polymers. Heteroatom-bridged [1.1]ferrocenophanes are readily obtained by salt metathesis of 1,1′-dilithiated ferrocenes with element halides from Group 137 (B, Al, Ga, In), Group 148 (Si, Ge, Sn, Pb), Group 159 (P, As), and Group 1210 (Zn, Hg). Comparatively less explored is the alternative approach of linking two ferrocenes via the 1,2positions of one of the Cp rings of ferrocene. Among the few © XXXX American Chemical Society

Chart 1. Representative Examples of Element-Bridged Polyferrocenes and Biferrocenes

Received: April 6, 2016

A

DOI: 10.1021/acs.organomet.6b00272 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Synthesis of Planar Chiral Tin-Bridged Biferrocene 1-Sn and the Unexpected Biferrocene 1-Si′

examples that have been reported thus far are disilacycles (C),11 diboracycles (D),12 and an unusual aluminum-bridged13 species. The Si-bridged compounds are obtained by addition of FeCl2 to a mixture of the disila-s-indacene dianion and CpNa (in excess). The desired products have to be isolated from a mixture of species, which typically consists of the anti-isomer (C), the corresponding syn-isomer, and other multinuclear species that contain more than two ferrocene units.11 In a different approach, compounds D were obtained by boron− mercury exchange of 1,2-bis(chloromercurio)ferrocene or via ortho-borylation of 1,1′-distannylated ferrocene with BCl3 or PhBCl2.12a−d Structural analyses of D reveal that the boryl moieties are bent out of the plane of the substituted Cp rings toward the Fe centers, which is generally attributed to a delocalized interaction14 between the electron-rich iron, the Cp ring, and the electron-deficient boron centers. As a consequence, a remarkable stimuli-responsive behavior of compounds D can be observed in that geometry changes to a more coplanar conformation are triggered by oxidation of ferrocene, reduction at boron, or Lewis base complexation to the Lewis acidic boron centers.12a,e While species C and D display inversion symmetry, the respective compounds with two different bridging elements are expected to be chiral.15 Unsymmetric biferrocenes of this type are extremely rare,16 and only very recently has the first enantiomerically pure heteroatom-bridged biferrocene been reported. Formal replacement of one of the borane moieties in D with phosphorus results in the unusual planar-chiral biferrocene E, which acts as an ambiphilic ligand whose electron-donating ability toward transition metals can be tuned by complexation of the Lewis acidic boryl moiety with a fluoride anion.17 In here, we report a general approach to this intriguing class of heteroatom-bridged biferrocenes. We describe synthetic routes to symmetric and unsymmetric (chiral) biferrocenes with Si, Sn, P, and B as bridging elements. We also discuss in detail the effects of different heteroatoms on the structural features, absorption spectra, and redox behavior.

ferrocene derivative, which was subsequently identified as 1-Si′ (Scheme 1) by multinuclear NMR and MALDI-MS analysis. We propose that, due to steric effects, attack of the lithiated ferrocene on Si is relatively slow at low temperature. As the temperature increases, isomerization to a less hindered benzyl anion occurs, which then readily undergoes nucleophilic substitution at silicon to give the unexpected biferrocene 1Si′. In the reaction with PhPCl2, several unidentified species formed, which could not be separated by chromatography. Fortunately, switching to Me2SnCl2 as a better electrophile provided the expected tin-bridged biferrocene 1-Sn, which was isolated in a yield of 80% after purification by column chromatography and subsequent crystallization from THF (Scheme 1). A single-crystal X-ray analysis of 1-Sn (Figure 1) unambiguously confirmed the desired biferrocene structure with the expected stereochemical environment. The dilithiation of 1-Sn proceeded readily upon treatment with 2 equiv of t-BuLi at −78 °C. The biferrocenes 2-SnSn, 2SnSi, and 2-SnP were obtained by metathesis of the dilithiated species with the corresponding element dichloride (Me2SnCl2, Me2SiCl2, and tBuPCl2). Compounds 2-SnSn and 2-SnSi

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RESULTS AND DISCUSSION Reaction of SS-FcSOTol with LDA according to Kagan’s method18 is known to furnish the ortho-lithiated ferrocene with excellent diastereoselectivity. The resulting lithioferrocene was reacted with Me2SiCl2, PhPCl2, or Me2SnCl2 at −78 °C in an attempt to link two chiral ferrocene units with different heteroatoms. With Me2SiCl2 as the electrophile, a yellow solid was isolated after chromatography. However, 1H NMR analysis of the product showed the typical pattern of a monosubstituted

Figure 1. ORTEP plot of 1-Sn (50% thermal ellipsoids); H atoms are omitted. Selected distances (Å) and angles (deg): Sn1−C1 2.136(4), Sn1−C11 2.169(4), Sn1−C21 2.144(4), Sn1−C22 2.133(4), S1−O1 1.499(3), S1−C2 1.769(4), S2−O2 1.501(3), S2−C12 1.758(3), C1− Sn1−C11 104.47(14). B

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The relatively low isolated yield for 2-SnP (10%) might be due to steric repulsion as the tBu group gets into close contact with the ferrocene units, thereby preventing efficient nucleophilic replacement of the chlorides to form the heterocycle. Such congestion is also reflected in the distorted geometry of the Xray structure of 2-SnP (vide inf ra). The 1H NMR spectra of 2-SnSn and 2-SnSi show only one set of signals for the substituted Cp rings, which is consistent with the expected C2 symmetry of the molecules (Figure 2). In contrast, the chemical environment of the individual ferrocene units is different in 2-SnP due to the presence of two different exocyclic substituents at phosphorus (tBu and BH3), resulting in splitting of the Cp protons into two sets of signals. The heteronuclear NMR resonances further confirm the attachment of the specific elements in 2-SnSn, 2-SnSi, and 2-SnP. For instance, compound 2-SnSn exhibits a 119Sn NMR signal at δ −22.1 ppm, whereas formal replacement of one of the tin bridges with silicon or phosphorus leads to an upfield shift of the 119Sn NMR signal in 2-SnSi (δ −24.3 ppm) and more so in 2-SnP (δ −28.0 ppm). Additional heteronuclear signals could be assigned to the corresponding elements in 2-SnSi (29Si NMR: δ −6.4 ppm) and 2-SnP (31P NMR: δ 29.0 ppm; 11B NMR: δ −39.0 ppm). We next investigated the replacement of the dimethylstannyl unit in 2-SnSi by a boryl group. Reaction of 2-SnSi with 2 equiv of HgCl2 in acetone (1 h) resulted in quantitative conversion to the corresponding dimercurated species. In the 1H NMR spectrum, the typical pattern for a 1,2-disubstituted ferrocene derivative was retained, while the absence of the Sn-Me signal suggested quantitative replacement of tin with mercury. The product was then reacted with PhBCl2 in toluene at 90 °C overnight to give a rare example of a ferrocene-fused silaborin,19 2-BSi, in a yield of 47% (Scheme 2). The incorporation of a tricoordinate borane bridge is reflected in a signal at δ = 53.9 in the 11B NMR, a downfield shift for the

(Scheme 2) were purified by silica gel column chromatography using hexanes as the eluent, followed by crystallization in Scheme 2. Synthesis of Heterocycles 2-SnSn, 2-SnSi, 2-BSi, and 2-SnP

methanol (64% yield for 2-SnSn, 62% yield for 2-SnSi). In the synthesis of 2-SnP, a solution of BH3 in THF was added following the addition of tBuPCl2 to avoid any potential oxidation of the phosphorus during workup and purification.

Figure 2. Comparison of the aliphatic and Cp region of the 1H NMR spectra for (a) 2-SnSn (C6D6), (b) 2-SnSi (CDCl3), (c) 2-SnP (CDCl3), and (d) 2-BSi (CDCl3). * denotes a trace amount of methanol and hexanes, and • denotes water residue in the NMR solvent. C

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Figure 3. ORTEP plots of (a) 2-SnSi, (b) 2-SnP, and (c) 2-BSi; 50% thermal ellipsoids, H atoms are omitted for clarity. Selected interatomic distances (Å) for 2-SnSi [2nd molecule]: Sn1−C1 2.115(9) [2.137(9)], Sn1−C11 2.149(9) [2.147(9)], Si1−C2 1.882(12) [1.878(11)], Si1−C12 1.856(12) [1.881(12)], Si1···Sn1 3.869(8) [3.829(6)]; 2-SnP: Sn1−C1 2.128(2), Sn1−C11 2.135(3), Sn1−C21 2.139(3), Sn1−C22 1.146(3), P1− C2 1.815(2), P1−C12 1.805(3), P1−C23 1.866(3), P1−B1 1.932(3), P1···Sn1 3.814(1); 2-BSi: Si1−C1 1.861(3), Si1−C11 1.851(3), Si1−C21 1.878(3), Si1−C22 1.879(3), B1−C2 1.543(4), B1−C12 1.555(4), B1−C23 1.570(4), Si1···B1 3.391(3).

Table 1. Selected Bond and Interplanar Angles (deg) for 2-SnSi, 2-SnP, 2-BSi and Reference Compounds α (deg)a

β (deg)b

4.0(4) [2.0(7)]

2.1(5) [1.9(8)]

2-SnP

11.9(1), 3.8(1)

43.8(1) (boat)

2-BSi

0.7(1), 1.0(1)

14.2(1)

C (R = R′ = Me)e D (R = Ph)f D(DMAP)2 (R = Ph)g

7.4, 11.3 1.2 6.1

44 (boat) 0 0

2-SnSi

d

γ (deg)c E E E E E E E E E

= = = = = = = = =

Sn 101.7(5) [102.4(6)] Si 108.7(8) [111.2(7)] Sn 96.16(10) P 107.18(11) B 118.4(2) Si 102.60(11) Si 104.1(2), 103.7(2) B 115.8(1) B 109.6(2)

a c

Interplanar angle between Cp rings of the same ferrocene moiety. bInterplanar angle between substituted Cp rings of different ferrocenes. Endocyclic bridge angle C−E−C (E = Si, Sn, P, B). d2nd molecule in brackets. eFrom ref 11d. fFrom ref 12a. gFrom ref 12e.

substituted Cp protons in the 1H NMR (Figure 2), and the presence of a new set of aromatic B-Ph signals. All the biferrocene derivatives with asymmetric bridging groups exhibit planar chirality. The planar chiral configuration was examined by a combination of optical rotation, chiral HPLC, and single-crystal XRD measurements (see the Experimental Section and the SI). The optical rotation values for 2-SnSi, 2-SnP, and 2-BSi measured in CH2Cl2 solution are [α]20 D = −88, +332, and −2122° (c = 0.10), respectively. The corresponding chiral HPLC traces feature a single major trace, which could be assigned unequivocally to the respective biferrocenes by comparison of the UV−vis detector response with the absorption spectra of the isolated species. The presence of a single isomer in corresponding crystals is evidenced by refinement of the Flack parameter from the single-crystal X-ray diffraction data (vide inf ra). While the datain the absence of corresponding racemic mixturesdo not offer absolute proof, they suggest that it is likely that the compounds are obtained as single enantiomers. The structural/conformational features of the ferrocenefused heterocycles are very interesting. According to literature precedents, for biferrocenes C with their tetrahedral silane bridging moieties, a boat conformation is typically observed, in which the substituted cyclopentadienyl rings show a large dihedral angle (e.g., β = 44° for C with R = Me, R′ = Me).11d For these disilacycles, the endocyclic C−Si−C bridge angles (γ) are small and range from 103.7(2)° to 104.1(2)°. In contrast, the trigonal borane moieties in compounds D give rise to much larger endocyclic C−B−C bridge angles of γ = 114.9(4)− 116.9(2)° and the Cp rings adopt (almost) coplanar conformations. 12a,c Some deviations from planarity are

observed due to interactions between the electron-deficient tricoordinate boryl unit and the electron-rich iron atoms, which result in dip angles of boron toward the metal centers of 7.4− 15.6°. Interestingly, even for the Lewis base complex D(DMAP)2 (DMAP = 4-dimethylaminopyridine), in which boron is in a pseudotetrahedral environment (γ = 109.6(2)°), a planar conformation is found by X-ray crystallography.12e Single crystals of the new biferrocenes that are suitable for Xray analysis were obtained by recrystallization of the corresponding compounds from solutions in methanol (for 2SnSn and 2-SnSi) or hexanes (for 2-SnP and 2-BSi). The crystallographic characterization of 2-SnSn was hampered by unsolved disorder of the ferrocenyl units; minor disorder was also present for 2-SnSi (C2 space group, see the Figure S1), whereas the structures of 2-SnP (P212121) and 2-BSi (P21) could be readily solved. For all compounds, the ferrocene units are positioned in an antiarrangement relative to the bridging ligand and adopt the expected pR configuration [(pR,pR)-2SnSi, (pR,pR)-2-SnP, and (pR,pR)-2-BSi] according to the CIP20 protocol (Figure 3). However, the conformation of the ferrocene moieties varies significantly with the bridging elements. For 2-SnSi, which contains two independent molecules in the unit cell, the tin and silicon atoms are essentially in one plane with the substituted Cp rings. The endocyclic C−E−C (E = Si, Sn) bridge angles are γ = 101.7(5)° [102.4(6)°] at the larger Sn atom and 108.7(8)° [111.2(7)°] at Si (Table 1). For 2-SnP, the bridge angle is similar at P with γ = 107.2(1)°, but significantly smaller at Sn with γ = 96.2(1)°. The smaller angle at Sn can be traced to a distortion of the SnP heterocycle toward a boat conformation (β = 43.8(1)°) that may be triggered by the large discrepancy D

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Figure 4. Cyclic (left, 25 mV/s) and square-wave (right, 100 mV/s) voltammograms for 2-SnSn, 2-SnSi, 2-SnP, and 2-BSi (CH2Cl2, 0.05 M Bu4N[B(C6H3(CF3)2)4] as electrolyte, reported vs Fc/Fc+, which is taken as +610 mV vs decamethylferrocene (Fc*) as an internal reference).

Table 2. Redox Potentials Derived from Cyclic or Square-Wave Voltammetry Datad 2-SnSna 2-SnSia 2-SnPa 2-BSia D (R = Ph)b

E1/2 (1) (mV)

ΔEp (1) (mV)

E1/2 (2) (mV)

ΔEp (2) (mV)

E1/2 (2) − E1/2 (1) (mV)

−101 −88 118 10 60

94 86 82 100

273 306 534 557 570

90 78 n.r. 129

374 394 416 547 510

c

c

E1/2 values are calculated from square-wave voltammetry data, ΔEp (Epa − Epc) values from cyclic voltammetry data. bCyclic voltammetry data from ref 12a. cData not available. dCH2Cl2, 0.05 M Bu4N[B(C6H3(CF3)2)4] as electrolyte, reported vs Fc/Fc+ redox couple. a

in steric bulk between the tBu group and the BH3 substituent on phosphorus. As another consequence, the Cp rings on the same ferrocene units are significantly tilted as reflected in interplanar angles α = 11.9(1)° and 3.8(1)°. For 2-BSi, the introduction of a trigonal borane moiety (Σ∠C−B−C = 360.0(2)°) results in a much larger angle of γ = 118.4(2)° at B, whereas the angle of γ = 102.6(1)° at Si is quite small. A considerable interaction between the electron-deficient tricoordinate boryl unit and the electron-rich iron atoms can be deduced from the dip angle of boron toward the metal centers (13.8(2)°). As a result of these distortions, the substituted Cp rings in 2-BSi are positioned at a relatively large interplanar angle of β = 14.2(1)°. The structural features of 2-BSi resemble those of the 4-t-butylpyridine monoadduct of D (R = Ph), with both a tricoordinate and a tetracoordinate boron center.12e For the latter, the tricoordinate boron atom remains trigonal and shows an interaction with the iron atoms (Σ∠C−B−C = 359.0(4)°, γ = 115.6(4)° and dip angles of 11.1 and 18.3°), but the tetracoordinate boron atom adopts a tetrahedral geometry with γ = 110.1(4)° and small dip angles of 5.8° and 2.9°. The interplanar angle of β = 19.7° is comparable to that of 2-BSi (β = 14.2(1)°). We conclude that the arrangement of the ferrocene units is strongly influenced by the coordination geometry of the bridging atoms and also the steric bulk of the substituents on the corresponding atoms. This might provide opportunities to tailor the chiral environment for future applications of 2-SnP as a chiral Lewis base/ligand and 2-BSi as a chiral Lewis acid.21 The incorporation of boron as a linker also leads to significant changes of the electronic structure and photophysical properties. The longest wavelength UV−vis absorption maximum for compound 2-BSi is greatly intensified and redshifted when compared to those of compounds 2-SnSn, 2-SnSi, and 2-SnP. This phenomenon is attributed to an enhancement of the charge transfer character of the lowest energy transition

upon introduction of the electron-deficient borane (Figure S5). The redox properties of the biferrocenes were examined by cyclic and square-wave voltammetry (Figure 4, Table 2). The cyclic voltammograms show quasi-reversible independent oxidation processes for the ferrocene units with redox splittings of 374 (2-SnSn), 394 (2-SnSi), 416 (2-SnP), and 574 mV (2BSi). This shows that the redox behavior of compounds 2SnSn and 2-SnSi is similar, suggesting that switching between tin and silicon bridges has just a minor influence on the geometry and electronic coupling between the ferrocene units. A slight increase in the redox splitting and a significant anodic shift for 2-SnP may be related to the presence of the electronwithdrawing BH3 protecting group on P. Incorporation of boron in the bridging position of 2-BSi results not only in a more dramatic anodic shift of the oxidation potentials but also an enlarged peak separation, which is attributed to more effective electronic communication due to involvement of the empty p-orbital on boron. A peculiar feature is that 2-SnSn and 2-SnSi exhibit two redox waves with essentially equal intensities, but 2-SnP and 2BSi show a considerably smaller current for the second oxidation. When studying the double reduction behavior of anthraquinone in low electrolyte support, Compton and coworkers observed a similar phenomenon.22 They attribute this observation to a fast comproportionation process that leads to accumulation of the monoanion, making the transport of the monoanion rate-limiting. The comproportionation is thermodynamically more favorable when the peak separation for two redox processes is larger. In the case of low electrolyte support, the diffusion of the monoanion is reduced by repulsion from the negatively charged reaction layer. As a result, the magnitude for the second reduction is decreased. Although we used a relatively well-supported system (ca. 50 fold amount of the electrolyte compared to the substrates), we still observe this phenomenon due to ineffective screening of the electric field. A E

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similar observation was made for the diborabiferrocenes C, which also show very large redox splittings.12a,c,d We also explored the chemical oxidation of 2-BSi by treatment with 2 equiv of the Ag derivative of Krossing’s23 salt, [Ag(CH2Cl2)]{Al[OC(CF3)3]4}. Upon addition, the initial orange-red color of 2-BSi changed to green, concomitant with precipitation of element silver. 1H NMR analysis of the filtrate revealed downfield shifted resonances at 44.4 ppm for the protons of the unsubstituted Cp ring and at 28.9, 22.5, 18.1 ppm for those of the substituted Cp ring, which is consistent with formation of the expected doubly oxidized species. Attempts to further characterize the product by crystallography failed, because suitable crystals could not be obtained. We found that the oxidized product undergoes relatively rapid hydrolysis even in controlled atmosphere environments. A small amount of a hydrolyzed species with the Ph group replaced with an OH group was isolated from a CH2Cl2 solution and analyzed by single-crystal X-ray diffraction (see Figure S2). Oxidation of both ferrocene units was evident from the distances between the centroids of the Cp rings (3.420 Å), which fall into the typical range for oxidized ferrocenes. After extended exposure of a solution of the oxidized species to moisture, only crystals of the fully degraded Cp2Fe{Al[OC(CF3)3]4} could be isolated. These observations clearly demonstrate that, although neutral 2-BSi is quite stable toward air and moisture, the decomposition of oxidized 2-BSi is rapid and likely facilitated by the enhanced Lewis acidity of boron and possibly silicon upon ferrocene oxidation.12b

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EXPERIMENTAL SECTION

Reagents and General Methods. Me2SnCl2, BH3·THF (1.0 M in THF), t-butyl lithium (1.7 M in hexanes), PhBCl2, PhPCl2, and tBuPCl2 were purchased from Aldrich and used without further purification. Me2SiCl2 was purchased from Aldrich and distilled over CaH2 prior to use. Lithium diisopropylamide (LDA) was freshly prepared by addition of n-butyl lithium (1.6 M in hexanes) to a THF solution of diisopropylamine at 0 °C, and the Ag derivative of Krossing’s salts, [Ag(CH2Cl2)]{Al[OC(CF3)3]4},23 was prepared according to literature procedures. All reactions and manipulations were carried out under an atmosphere of prepurified nitrogen using either Schlenk techniques or an inert-atmosphere glovebox (MBraun). 499.9/599.7 MHz 1H NMR, 125.7/150.8 MHz 13C NMR, 160.4 MHz 11 B NMR, 470.4 MHz 19F NMR, 99.3/119.2 MHz 29Si NMR, 202.4 MHz 31P, and 186.4 MHz 119Sn NMR spectra were recorded on Varian INOVA NMR spectrometers (Varian Inc., Palo Alto, CA) equipped with a boron-free 5 mm dual broadband gradient probe (Nalorac, Varian Inc., Martinez, CA). 11B NMR spectra were acquired with boron-free quartz NMR tubes. 1H and 13C NMR spectra were referenced internally to solvent signals and all other NMR spectra externally to SiMe4 (0 ppm). The following abbreviations are used for signal assignments: Ph = phenyl, Fc = ferrocenyl, Cp = cyclopentadienyl, tBu = tert-butyl, s = singlet, d = doublet, t = triplet, pst = pseudotriplet, br = broad, nr = not resolved. High-resolution MALDIMS data (benzo[α]pyrene or anthracene as matrix) were obtained in positive mode on an Apex Ultra 7.0 Hybrid FTMS (Bruker Daltonics). UV/vis absorption data were acquired on a Varian Cary 500 UV/vis/ NIR spectrophotometer. Electrochemistry measurements were carried out on a BAS CV-50W analyzer. The three-electrode system consisted of a Au disk as working electrode, a Pt wire as secondary electrode, and a Ag wire as the pseudo-reference electrode. The scans were referenced after the addition of a small amount of decamethylferrocene or ferrocene as internal standard. The potentials are reported relative to the ferrocene/ferrocenium couple (−610 mV for Cp*2Fe/Cp*2Fe+ in CH2Cl2/0.05 M (n-Bu4)N[B{C6H3(CF3)2}4]). Optical rotation analyses were performed on an Autopol III polarimeter (Rudolph Research Analytical) using a tungsten-halogen light source operating at λ = 589 nm. Chiral HPLC analyses were performed on a Waters Empower system equipped with a 717plus autosampler, a 1525 binary HPLC pump, and a 2998 photodiode array detector; a CHIRALPAK IA-3 column was used for separation. Elemental analyses were performed by Quantitative Technologies Inc., Whitehouse, NJ. X-ray diffraction intensities were collected on a Bruker SMART APEX II CCD diffractometer using Cu Kα (1.54178 Å) radiation at 100(2) K. The structures were refined by full-matrix least-squares based on F2 with all reflections (SHELXTL V5.10; G. Sheldrick, Siemens XRD, Madison, WI). Non-hydrogen atoms were refined with anisotropic displacement coefficients, and hydrogen atoms were treated as idealized contribution. SADABS (Sheldrick, 12 G.M. SADABS (2.01), Bruker/Siemens Area Detector Absorption Correction Program; Bruker AXS: Madison, WI, 1998) absorption correction was applied. Crystallographic data for the structures of 1-Sn, 2-SnSi, 2SnP, and 2-BSi have been deposited with the Cambridge Crystallographic Data Center as supplementary publications CCDC 1470561− 1470564. Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (+44) 1223-336-033; email: [email protected]). Synthesis of 1-Si′. To a precooled (−78 °C) solution of SSFcSOTol (1.94 g, 5.98 mmol) in THF (40 mL) was added a premade LDA solution (1.0 M in THF, 6.0 mL, 6.0 mmol, 1.0 equiv) dropwise. The mixture was allowed to warm up and stir at room temperature for 0.5 h. Me2SiCl2 (0.387 g, 3.00 mmol, 0.50 equiv) in THF (10 mL) was then added slowly at −78 °C. The reaction mixture was kept stirring at the same temperature for 1 h and then allowed to warm up overnight. After the addition of water, the mixture was extracted with CH2Cl2 (3 × 25 mL). The combined organic layers were washed with brine and water, dried over sodium sulfate, and concentrated. The residue was chromatographed on a silica gel column using THF/hexanes/CH2Cl2

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CONCLUSIONS In summary, we have established a practical protocol for the synthesis of several unprecedented heteroatom-linked biferrocenes via a stepwise stereoselective dimerization/cyclization process. Following such a strategy, heterocycles 2-SnSi, 2-SnP, and 2-BSi with different bridging elements were obtained and characterized by multinuclear NMR, high-resolution mass spectrometry, and elemental analyses. Unsymmetric heterocycles of this type are very rare,19,24 but the three-dimensional chiral structure of the fused ferrocene moieties makes these molecules truly unique. The successful isolation of the products as single enantiomers is suggested by a combination of singlecrystal X-ray diffraction, optical rotation, and chiral HPLC measurements. We also demonstrate that the electronic structure and steric features of this family of planar chiral biferrocenes can be tuned by varying the bridging elements. Electrochemical studies reveal that all the biferrocenes undergo two separate quasi-reversible one-electron oxidations for the Fc units. However, the introduction of boron results in anodic shifts of both redox potentials, as well as an enlargement of the peak separation. In addition, the characteristic long wavelength UV−vis absorption is significantly red-shifted and intensified for 2-BSi, as a result of enhanced charge transfer character. On the other hand, the single-crystal X-ray crystallography data of 2-SnSi, 2-SnP, and 2-BSi indicate that the relative orientation of the ferrocene units is strongly influenced by the coordination geometry of the bridging atoms and the size of the substituents on the corresponding atoms. Collectively, these results suggest that judicious choice of bridging heteroatoms offers a unique opportunity to tailor both the electronic structure and the chiral environment of Lewis acidic or Lewis basic chiral biferrocenes. F

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Organometallics

Synthesis of 2-SnSi. A solution of 1-Sn (400 mg, 0.503 mmol) in THF (40 mL) was cooled to −78 °C, and a solution of t-butyl lithium in hexanes (0.62 mL, 1.05 mmol, 2.1 equiv) was added dropwise under stirring. The reaction mixture was kept stirring for 10 min at the same temperature, and a solution of Me2SiCl2 (0.091 g, 0.705 mmol, 1.4 equiv) in THF (1.5 mL) was added via syringe. The resulting solution was stirred for 1 h, and the temperature was slowly raised to 25 °C. After the addition of water, the mixture was extracted with diethyl ether (3 × 20 mL). The combined organic layers were washed with brine solution, followed by water, dried over sodium sulfate, and concentrated. The residue was subjected to silica gel column chromatography with hexanes/triethylamine (100:1) as the eluent to give the product as a red solid. Dissolution in hot methanol and subsequent crystallization at −37 °C gave X-ray quality crystals. Yield: 1 179 mg (62%). [α]20 D (c = 0.10, CH2Cl2) = −88°. H NMR (499.9 MHz, CDCl3, 25 °C): δ = 4.55 (nr, 2H; Cp), 4.42 (nr, 2H; Cp), 4.36 (nr, 2H; Cp), 3.96 (s, 10H; free Cp), 0.52 (s/d, 2J(117/119Sn,H) = 56 Hz, 6H; SnMe2), 0.48 (s, 6H; SiMe2). 13C NMR (125.69 MHz, C6D6, 25 °C): δ = 78.5 (ipso-Cp), 77.4 (s/d, J(117/119Sn,C) = 59 Hz; Cp), 76.5 (ipso-Cp), 76.1 (s/d, J(117/119Sn,C) = 43 Hz; Cp), 73.2 (s/d, J(117/119Sn,C) = 41 Hz; Cp), 69.2 (free Cp), 2.3 (SiMe2), − 6.7 (s/d, 1 117/119 J( Sn,C) = 349, 370 Hz; SnMe2). 29Si NMR (99.3 MHz, C6D6, 25 °C) δ = −6.4. 119Sn NMR (186.4 MHz, C6D6, 25 °C): δ = −24.3. UV−vis (CH2Cl2): λmax = 461 nm (280 M−1 cm−1). High-resolution MALDI-MS (+ mode, benzo[α]pyrene): m/z 575.9671 ([M]+, 100%, calcd for 12C241H2828Si56Fe2120Sn 575.9680). Elemental analysis for C24H28Fe2SiSn, calcd C 50.14, H 4.91, found C 49.79, H 4.72%. Synthesis of 2-SnP. A solution of 1-Sn (500 mg, 0.629 mmol) in THF (40 mL) was cooled to −78 °C, and a solution of t-butyl lithium in hexanes (0.76 mL, 1.29 mmol, 2.05 equiv) was added dropwise under stirring. The reaction mixture was kept stirring for 5 min at the same temperature, and a solution of tBuPCl2 (0.105 g, 0.660 mmol, 1.05 equiv) in THF (2.0 mL) was added dropwise via syringe, followed by addition of a solution of BH3 in THF (1.0 M, 0.75 mL, 0.75 mmol, 1.20 equiv). The resulting solution was stirred for 30 min, and the temperature was slowly raised to 25 °C. After addition of saturated NaHCO3 aqueous solution, the mixture was extracted with diethyl ether (3 × 20 mL). The combined organic layers were washed with brine solution, followed by water, dried over sodium sulfate, and concentrated. The residue was subjected to silica gel column chromatography with hexanes/triethylamine (100:1) as the eluent to give the product as a red solid. Dissolution in hot hexanes and subsequent crystallization at −37 °C gave X-ray quality crystals. Yield: 1 40 mg (10%). [α]20 D (c = 0.10, CH2Cl2) = +332°. H NMR (499.9 MHz, CDCl3, 25 °C): δ = 5.10 (nr, 1H; Cp), 4.83 (nr, 1H; Cp), 4.79 (d, J = 2 Hz, 1H; Cp), 4.57 (d, J = 2 Hz, 1H; Cp), 4.54 (nr, 1H; Cp), 4.34 (nr, 1H; Cp), 4.26 (s, 5H; free Cp), 3.83 (s, 5H; free Cp), 1.35 (d, 3J = 14.5 Hz, 9H, tBu), 0.8 (br, 3H, BH3), 0.60 (s/d, 2 117/119 J( Sn,H) = 58 Hz, 3H; SnMe), 0.59 (s/d, 2J(117/119Sn,H) = 56 Hz, 3H; SnMe). 13C NMR (125.69 MHz, CDCl3, 25 °C): δ = 79.2 (d, J(31P,C) = 20 Hz; Cp), 77.7 (d, J(31P,C) = 19 Hz; Cp), 77.3 (d, J(31P,C) = 6.3 Hz; Cp), 76.6 (d, 1J(31P,C) = 63 Hz; ipso-Cp-P), 76.5 (d, J(31P,C) = 5.0 Hz; ipso-Cp-Sn), 76.2 (d, J(31P,C) = 8.8 Hz; Cp), 73.8 (d, 1J(31P,C) = 63 Hz; ipso-Cp-P), 73.4 (d, J(31P,C) = 8.8 Hz; Cp), 72.2 (d, J(31P,C) = 8.8 Hz; Cp), 71.7 (d, J(31P,C) = 3.8 Hz; Cp), 70.1 (free Cp), 69.9 (free Cp), 30.7 (d, 1J(31P,C) = 37 Hz, CMe3), 26.3 (d, 2J(31P,C) = 2.5 Hz, CMe3), −3.6 (SnMe), −6.9 (SnMe). 11B NMR (160.4 MHz, CDCl3, 25 °C): δ = −39.0 (m, w1/2 = 270 Hz). 31P NMR (202.4 MHz, CDCl3, 25 °C): δ = 29.0. 119Sn NMR (186.4 MHz, CDCl3, 25 °C): δ = −28.0 (d, 3J(31P,Sn) = 5.2 Hz). UV−vis (CH2Cl2): λmax = 469 nm (800 M−1 cm−1). High-resolution MALDIMS (+ mode, benzo[α]pyrene): m/z 548.9182 ([M − tBu − BH3]+, 100%, calcd for 12C221H2256Fe231P120Sn 548.9179). Elemental analysis for C26H34BFe2PSn, calcd C 50.47, H 5.54, found C 50.31, H 5.55%. Synthesis of 2-BSi. A solution of 2-SnSi (0.129 g, 0.224 mmol) in acetone (10 mL) was added dropwise to a solution of HgCl2 (0.122 g, 0.448 mmol, 2.0 equiv) in acetone (10 mL). The mixture was stirred for 1 h and then added dropwise to water (50 mL) while stirring. Upon addition, a yellow precipitate was formed, which was collected on a filter paper, washed with hexanes (10 mL), and dried under

(1:2:1) with 1% v/v triethylamine as the eluent. The product was redissolved in hot acetone and isolated as a yellow powdery solid after cooling to room temperature. Yield: 1.10 g (52%). 1H NMR (599.7 MHz, CDCl3, 25 °C): δ = 7.46 (d, J = 8.4 Hz, 4H; tolyl), 7.01 (d, J = 8.4 Hz, 4H; tolyl), 4.60 (nr, 2H; Cp), 4.34 (nr, 2H; Cp), 4.33 (s, 10H; free Cp), 4.32 (nr, 2H; Cp), 4.31 (nr, 2H; Cp), 2.10 (s, 4H; CH2), −0.10 (s, 2J(29Si,H) = 13.0 Hz, 6H; SiMe2). 13C NMR (150.8 MHz, CDCl3, 25 °C): δ = 143.4, 141.7 (tolyl, ipso-C-S and ipso-C-methyl), 128.8, 125.0 (ortho- and meta-tolyl), 94.6 (ipso-Cp-S), 70.1 (free Cp), 68.0 (Cp), 65.8 (Cp), 25.7 (CH2), −3.6 (SiMe2). 29Si NMR (119.2 MHz, CDCl3, 25 °C): δ = 2.4. High-resolution MALDI-MS (+ mode, anthracene): m/z 705.0689 ([M + H]+, 100%, calcd for 12C361H3756 Fe216O232S228Si 705.0651), 704.0627 ([M]+, 80%, calcd for 12 C361H3656Fe216O232S228Si 704.0621). Elemental analysis for C36H36Fe2O2S2Si, calcd C 61.37, H 5.15, found C 60.94, H 4.91% (the consistently slightly low carbon content may be due to incomplete combustion related to SiC formation). Synthesis of 1-Sn. To a precooled (−78 °C) solution of SSFcSOTol (4.00 g, 12.3 mmol) in THF (60 mL) was added a premade LDA solution (0.50 M in THF, 27.2 mL, 13.6 mmol, 1.10 equiv) dropwise, and the mixture was kept stirring for 1 h. A solution of Me2SnCl2 (1.62 g, 7.37 mmol, 0.60 equiv) in THF (10 mL) was added slowly. The reaction mixture was kept stirring at −78 °C for 1 h and then allowed to warm up overnight. After the addition of water, the mixture was extracted with CH2Cl2 (3 × 50 mL). The combined organic layers were washed with brine and water, dried over sodium sulfate, and concentrated. The residue was chromatographed on a silica column with THF/hexanes/triethylamine (25:75:1) as the eluent. X-ray quality crystals were obtained by recrystallization from 1 hot THF. Yield: 3.9 g (80%). [α]20 D (c = 0.10, CH2Cl2) = −34°. H NMR (499.9 MHz, CDCl3, 25 °C): δ = 7.44 (d, J = 8.0 Hz, 4H; tolyl), 7.10 (d, J = 8.0 Hz, 4H; tolyl), 4.48 (nr, 2H; Cp), 4.39 (nr, 2H; Cp), 4.37 (nr, 2H; Cp), 4.29 (s, 10H; free Cp), 2.33 (s, 6H; Me), 0.80 (s/d, 2 117/119 J( Sn,H) = 60 Hz, 6H; SnMe2). 13C NMR (125.69 MHz, CDCl3, 25 °C): δ = 142.5, 140.8 (tolyl, ipso-C-S and ipso-C-methyl), 129.3, 125.2 (ortho- and meta-tolyl), 97.6 (s/d, J(117/119Sn,C) = 39 Hz; ipsoCp-S), 77.7 (s/d, J(117/119Sn,C) = 41 Hz, Cp), 73.2 (s/d, J(117/119Sn,C) = 38 Hz, Cp), 70.6 (ipso-Cp-Sn), 69.9 (free Cp), 69.0 (s/d, J(117/119Sn,C) = 27 Hz; Cp), 21.4 (Me), −3.8 (s/d, 1J(117/119Sn,C) = 422/442 Hz; SnMe2). 119Sn NMR (186.4 MHz, C6D6, 25 °C): δ = −25.5. Elemental analysis for C36H36Fe2O2S2Sn, calcd C 54.37, H4.56, found C 54.56, H 4.21%. Synthesis of 2-SnSn. A solution of 1-Sn (100 mg, 0.126 mmol) in THF (10 mL) was cooled to −78 °C, and a solution of t-butyl lithium in hexanes (0.156 mL, 0.265 mmol, 2.10 equiv) was added dropwise under stirring. The reaction mixture was kept stirring for 10 min at the same temperature, and a solution of Me2SnCl2 (0.029 g, 0.132 mmol, 1.05 equiv) in THF (1.5 mL) was added via syringe. The resulting solution was stirred for 1 h, and the temperature was slowly raised to 25 °C. After the addition of water, the mixture was extracted with diethyl ether (3 × 5 mL). The combined organic layers were washed with brine solution, followed by water, dried over sodium sulfate, and concentrated. The residue was subjected to silica gel column chromatography with hexanes/triethylamine (100:1) as the eluent to give the product as a red solid. Yield: 54 mg (64%). 1H NMR (499.9 MHz, C6D6, 25 °C): δ = 4.51 (t, J = 2.5 Hz, 2H; Cp), 4.30 (d, J = 2.5 Hz, 4H; Cp), 3.99 (s, 10H; free Cp), 0.48 (s/d, 2J(117/119Sn,H) = 54/ 56 Hz, 12H; SnMe2). 13C NMR (125.69 MHz, C6D6, 25 °C): δ = 77.1 (s/d, 1J(117/119Sn,C) = 513/535 Hz; ipso-Cp-Sn), 77.1 (s/d, 2/3 117/119 J( Sn,C) = 46/63 Hz; Cp), 73.2 (s/d, 3J(117/119Sn,C) = 40 Hz; Cp), 69.1 (free Cp), −6.5 (s/d, 1J(117/119Sn,C) = 350/366 Hz; SnMe2). 119Sn NMR (186.4 MHz, C6D6, 25 °C): δ = −22.1(s/d, 3 117 J( Sn, 119Sn) = 122 Hz). UV−vis (CH2Cl2): λmax = 460 nm (310 M−1 cm−1). High-resolution MALDI-MS (+ mode, benzo[α]pyrene): m/z 667.8920 ([M]+, 100%, calcd for 12C241H2856Fe2120Sn2 667.8930), 652.8686 ([M − Me]+, 40%, calcd for 12C231H2556Fe2120Sn2 652.8699). Elemental analysis for C24H28Fe2Sn2, calcd C 43.31, H 4.24, found C 43.32, H 4.10%. G

DOI: 10.1021/acs.organomet.6b00272 Organometallics XXXX, XXX, XXX−XXX

Organometallics

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airflow. Yield for Me2Si(FcHgCl)2: 185 mg (71%). 1H NMR (499.9 MHz, CDCl3, 25 °C): δ = 4.75 (pst, J = 2.0 Hz, 2H; Cp), 4.54 (nr, 2H; Cp), 4.37 (nr, 2H; Cp), 4.23 (s, 10H; free Cp), 0.64 (s, 6H; SiMe2). 13C NMR (125.69 MHz, C6D6, 25 °C): δ = 90.9 (ipso-CpHg), 78.1 (Cp), 76.5 (ipso-Cp-Si), 76.4 (Cp), 74.5 (Cp), 69.6 (free Cp), 1.0 (SiMe2). 29Si NMR (99.3 MHz, C6D6, 25 °C): δ = −6.8. To a solution of Me2Si(FcHgCl)2 (0.225 g, 0.250 mmol) in toluene (20 mL) was added a solution of PhBCl2 (0.047 g, 0.300 mmol, 1.2 equiv) in toluene (2 mL) at −37 °C inside a glovebox. The mixture was then heated to 90 °C overnight in a sealed reaction tube. A gray solid formed, which was removed by filtration. The filtrate was concentrated, taken back up in hot hexanes, and kept at −37 °C for crystallization. Yield: 60 mg (47%). [α]20 D (c = 0.10, CH2Cl2) = −2122°. 1H NMR (499.9 MHz, CDCl3, 25 °C): δ = 7.90 (m, 2H; oPh), 7.48 (m, 3H; m-Ph, p-Ph), 4.85 (pst, J = 2.0 Hz, 2H; Cp), 4.74 (nr, 2H; Cp), 4.61 (nr, 2H; Cp), 3.97 (s, 10H; free Cp), 0.46 (s, 6H; SiMe2). 13C NMR (125.69 MHz, CDCl3, 25 °C): δ = 132.6 (Ph), 128.2 (Ph), 127.3 (Ph), ipso-Ph-B not observed, 80.3 (ipso-Cp-B), 78.8 (Cp), 78.7 (Cp), 77.5 (ipso-Cp-Si), 77.1 (Cp), 69.5 (free Cp), 2.2 (SiMe2). 11B NMR (160.4 MHz, CDCl3, 25 °C): δ = 53.9 (w1/2 = 750 Hz). 29Si NMR (99.3 MHz, CDCl3, 25 °C): δ = −9.4. UV−vis (CH2Cl2): λmax = 367 nm (5460 M−1 cm−1), 471 nm (7060 M−1 cm−1). High-resolution MALDI-MS (+ mode, benzo[α]pyrene): m/z 514.0683 ([M]+, 100%, calcd for 12C281H2711B28Si56Fe2 514.0669). Elemental analysis for C28H27BFe2Si, calcd C 65.42, H 5.29, found C 64.76, H 5.15% (the consistently low carbon content may be due to incomplete combustion related to SiC formation). Preparative Oxidation of 2-BSi. To a solution of 2-BSi (0.010 g, 0.020 mmol) in CD2Cl2 (1 mL) was added dropwise a solution of [Ag(CH2Cl2)]{Al[OC(CF3)3]4} (0.046 g, 0.039 mmol, 2.0 equiv) in CD2Cl2 (1 mL) at −37 °C in a glovebox. An immediate color change from orange to green was observed, concomitant with the generation of a gray precipitate. The reaction mixture was stirred for an additional 20 min at room temperature, followed by filtration to remove elemental Ag. The filtrate was concentrated under vacuum. The resulting residue was redissolved in 1.5 mL of CD2Cl2 and filtered once more. The filtrate was placed at −37 °C to yield a blue-green microcrystalline solid. Yield: 18 mg (62%). 1H NMR (499.9 MHz, CD2Cl2, 25 °C): δ = 44.4 (br, 10H; free Cp), 28.9 (br, 2H; Cp), 22.5 (br, 2H; Cp), 18.1 (br, 2H; Cp), −3.41, −5.22, −10.4 (5H; Ph), −22.6 (br, 6H; SiMe2). 11B NMR (470.4 MHz, CD2Cl2, 25 °C): δ = 19.1 (w1/2 = 1700 Hz). 19F NMR (470.4 MHz, CD2Cl2, 25 °C): δ = −76.0.

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ACKNOWLEDGMENTS We thank the Petroleum Research Fund administered by the American Chemical Society (45648.01-AC3), the National Science Foundation (CHE-1308517), and Rutgers University for financial support. A.C.T.K. thanks the Deutsche Forschungsgemeinschaft (DFG) for a postdoctoral fellowship. One of the 500 MHz NMR spectrometers used in these studies was purchased with support from the National Science Foundation (NSF-MRI 1229030). The X-ray diffractometer was purchased with support from the National Science Foundation (NSFCRIF 0443538) and Rutgers University (Academic Excellence Fund). We are grateful to Fei Cheng for assistance with HPLC measurements.

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00272. Copies of NMR and mass spectra, UV−vis spectra, chiral HPLC traces, plots of supramolecular structures of 2SnSi and 2-BSi, and X-ray diffraction analysis parameters (PDF) Crystallographic data for (pR,pR,SS,SS)-1-Sn, (pR,pR)-2SnSi, (pR,pR)-2-SnP, and (pR,pR)-2-BSi (CIF)

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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. H

DOI: 10.1021/acs.organomet.6b00272 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.6b00272 Organometallics XXXX, XXX, XXX−XXX