Hg Exchange Reaction of 1,2-Fc(PPh2)

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Mechanistic Studies into the Sn/Hg Exchange Reaction of 1,2Fc(PPh2)(SnMe3) with HgCl2: Competitive Sn−Me over Sn−Fc Cleavage in Noncoordinating Solvents Alain C. Tagne Kuate,†,‡ Roger A. Lalancette,† and Frieder Jäkle*,† †

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

‡

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ABSTRACT: Tin−mercury exchange represents one of the most versatile and cleanest routes to arylmercuric halides. We found that reaction of the ferrocenylstannane 1,2-Fc(PPh2)(SnMe3) (1) with HgCl2 in acetone results in the unexpected spontaneous formation of 2·HgCl2, a diferrocenylmercury (Fc2Hg)-supported diphosphine chelate ligand as its HgCl2 complex. Mechanistic investigations into the generation of 2·HgCl2 reveal initial formation of an adduct of 1 with HgCl2, followed by competitive Sn−Me and Sn−Fc bond cleavage with formation of chloromercury and chlorodimethylstannyl-substituted ferrocene species. When the reaction is performed in chloroform as a noncoordinating solvent, formation of 2·HgCl2 is not observed, but instead 1,2-Fc(PPh2)(SnMe2Cl) (5) is generated as the major product. 5 is initially isolated as a complex with MeHgCl (generated as a byproduct), but the latter can be easily released by heating under high vacuum. When 5 is further reacted with 2 equiv of HgCl2 in acetone, the adduct 1,2-Fc(PPh2· HgCl2)(HgCl) (6·HgCl2) forms. An X-ray crystal structure of 6·HgCl2 shows two individual molecules that form Hg···Clbridged dimers, which in turn are linked by intermolecular Hg···Cl contacts to give a polymeric structure. In contrast, the equimolar reaction of 5 and HgCl2 results in initial complexation to give 5·HgCl2, which slowly transforms into the diferrocenylmercury species 2·HgCl2. These results confirm that both 1 and the byproduct 5 obtained by Sn−Me bond cleavage are competent intermediates in the formation of complex 2 in acetone. The preferential cleavage of the Sn−Me over the Sn−Fc bond in noncoordinating solvents is attributed to the presence of the diphenylphosphino group in an ortho position. These observations may have broader implications due to the formation of MeHgCl as a highly toxic and volatile byproduct and suggest that noncoordinating solvents are better avoided and extreme caution is necessary when performing Sn/Hg exchange reactions on donor-substituted substrates.

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INTRODUCTION Polyfunctional arylmercuric halides and diarylmercury species represent an interesting class of Lewis acidic hosts to anions and neutral guests.1 As a classic example, Hawthorne’s carboranylmercury macrocycle I (Figure 1) has been shown to bind anions with unusual coordination geometries.2 Trimeric o-phenylene mercury species II have been widely explored as building blocks of supramolecular materials.3 Lewis base functionalized diarylmercury species have also attracted interest as ambiphilic chelate ligands for metal complexation. The presence of the Lewis acidic Hg(II) in close proximity to another metal can give rise to Z-type ligand behavior, where the Hg atom is able to engage in metallophilic4 interactions. For instance, short M···Hg contacts have been reported for complexes of ligands III and IV with various d8 and d10 metal ions.5 The requisite diarylmercury species are frequently prepared by reaction of an aryllithium precursor with 0.5 equiv of HgCl2 or, alternatively, by redistribution of arylmercuric halides in the © XXXX American Chemical Society

presence of a soft base. For instance, while the synthesis of diarylmercury compounds can be achieved easily through the lithium chloride salt elimination reaction (for example in the synthesis of III and IV),5b,f alternative preparations involve heating at reflux an acetone solution of the corresponding arylmercury halide and ammonium thiocyanate, refluxing an ethanol solution with potassium cyanide, or, in the case of IV· HgCl2, heating the arylmercury halide in CH2Cl2.5b,f,6 Earlier work on the symmetrization of arylmercury halide mentions that the reaction can be achieved using ammonia in chloroform.7 The arylmercuric halides in turn are frequently obtained by an Sn−Hg exchange reaction between an aryltrialkylstannane and HgCl2 in acetone or tetrahydrofuran, sometimes followed by precipitation of the product by addition into water.6,8 Received: November 28, 2018

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DOI: 10.1021/acs.organomet.8b00862 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 2. Comparison of 31P NMR spectra of 1 (a) prior to and (b− d) after sequential addition of 2 × 0.5 equiv of HgCl2 (acetone) and (e) isolated 2·HgCl2 (CDCl3). Hg satellites are indicated with asterisks.

the soluble part revealed the 31P NMR signal for the product (+26.6 ppm, 22%) and that for the precursor 1, which became sharp (−16.4 ppm, 38%). The resonance at +32.4 ppm disappeared and a new broad resonance emerged at −12.8 ppm (40%). Addition of another 0.5 equiv of HgCl2 to the reaction mixture led to complete disappearance of the 31P NMR resonance for 1, while a broad resonance reemerged at +32.3 ppm (33%) and the other resonances at +25.6 ppm (2· HgCl2, previously at +26.6 ppm, 26%), and −11.3 ppm (previously at −12.8 ppm, 41%) remained. The presence of residual starting material in the reaction with 0.5 equiv of HgCl2 indicates that the HgCl2, once bound to the diphosphine chelate ligand in the final product 2·HgCl2, is not readily available for reaction with 1. The observation of 2·HgCl2 immediately after mixing of the reactants suggests that a fast Sn/Hg exchange reaction takes place, presumably with initial generation of Me3SnCl and an ophosphinoferrocenylmercuric chloride species. One possible reaction pathway to product 2·HgCl2 (PATH A, Scheme 2) then involves two successive Sn/Hg exchange reactions where the second step is favored by intramolecular attack of the ferrocenylmercury fragment on another ferrocenylstannane moiety. Another scenario is that the dimer 4 is generated, which then disproportionates into 2·HgCl2 (PATH B, Scheme 2). Coates et al. described that arylmercury halides rapidly disproportionate in the presence of tertiary phosphines to give bis(aryl)mercury and mercury(II) chloride phosphine complexes and that the rate of the disproportion increases with the dielectric constant of the solvent.11 The authors also proposed that the first step involves the formation of a salt, on the basis of the observation that the conductance of a phenylmercuric chloride solution increases when triphenylphosphine is added. The latter suggests that triphenylphosphine displaces the chloride, resulting in the formation of an ionic 1:1 complex.12 Indeed, compound IV·HgCl2 was reported to form by heating a DCM solution of the arylmercury halide in the absence of any additives, a process that was reversed in toluene as the solvent.5f We note, however, that a spontaneous disproportionation of isolated [HgCl(o-C6H4PPh2)]n in DCM into III· HgCl2 (Figure 1) was not observed but occurred only on heating in an aqueous−ethanolic solution of potassium cyanide.5a,b A key question arises: what is the identity of the species associated with the hitherto unassigned signals at +32.4 and

Figure 1. Diarylmercury species as hosts for anions and as Z-type ligands used in the preparation of complexes exhibiting metallophilic interactions.

We previously reported that stirring a mixture of ferrocenylstannane 1 (pS isomer) and HgCl2 in acetone over a period of 1 h9 afforded a yellow precipitate that was identified as 2·HgCl2 (pS,pS isomer) by single-crystal X-ray diffraction analysis (Scheme 1).10 The unexpected spontaScheme 1. Formation of 2·HgCl2 via Mercuriodestannylation10

neous formation of this diarylmercury species prompted us to further investigate the reaction mechanism with the aim of identifying possible intermediates or byproducts generated in the process, as well as gaining a better understanding of the role of the reaction conditions on the product distribution. We offer here detailed mechanistic studies into the formation of this unusual complex.

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RESULTS AND DISCUSSION We first examined whether reaction of 1 with a deficiency of HgCl2 would lead to formation of the free ligand (without bound HgCl2) or give the complex 2·HgCl2 along with residual starting material. Thus, an NMR sample was prepared containing 1 and 0.5 equiv of HgCl2 in acetone-d6 and the reaction followed by 31P and 1H NMR spectroscopy (Figure 2 and Figures S1−S6). After 30 min, the 31P NMR spectrum showed a sharp signal with mercury satellites at +27.4 ppm (20%, 1JHg,P = 4970 Hz) assigned to the product 2·HgCl2, along with two broad resonances at +32.4 (22%) and −16.0 ppm (58%). The signal at −16.0 ppm matches well with the chemical shift of the precursor 1, except that it is strongly broadened. After the sample stood overnight, some precipitation occurred, indicative of product formation. Analysis of B


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Organometallics Scheme 2. Possible Pathways in the Formation of 2·HgCl2a

a

Acetone solvent coordination to Hg is expected for some of the proposed intermediates but is omitted for clarity. Intermediates with SnMe2Cl in place of SnMe3 groups can also form due to competing Sn−Me cleavage, as illustrated in Scheme 3.

−11.3 ppm in the 31P NMR spectra of the reaction mixture? Could they correspond to intermediates along the reaction path, or are they due to the formation of byproducts? Evidence for Phosphine−HgCl2 Complex Formation. The initial formation, disappearance over time, and subsequent reemergence upon addition of more HgCl2 suggests that the peak at +32.4 ppm corresponds to a reaction intermediate. It appears in a chemical shift range which is similar to that of the final product, indicating the coordination of HgCl2 to a phosphine moiety. It might therefore correspond to a 1:1 or 2:1 Lewis adduct of 1 and HgCl2 or to a later reaction intermediate such as 3 or 4 in Scheme 2. The model compounds FcPPh2·HgCl2 and [(FcPPh2)2·HgCl2] were synthesized by reaction of FcPPh2 with HgCl2 in a 1:1 and 1:2 ratio in acetone-d6 and structurally characterized by singlecrystal X-ray diffraction analysis after recrystallization from a CH2Cl2/hexanes mixture (Figure 3). FcPPh2·HgCl2 adopts a dimeric structure with one terminal and one bridging Cl substituent for each HgCl2 moiety. The Hg2Cl2 entity is slightly unsymmetrical with Hg−Cl distances of 2.6051(13)/ 2.7298(14) Å (molecule 1) and 2.6596(15)/2.6798(14) Å (molecule 2); they are significantly longer than those to the terminal Cl atoms of 2.3961(14) and 2.3772(15) Å. The terminal Hg−Cl distances of 2.5173(7) and 2.5170(7) Å for [(FcPPh2)2·HgCl2] are comparatively longer. In contrast, the Hg−P distances of 2.4069(15) Å (molecule 1) and 2.3941(14) Å (molecule 2) for FcPPh2·HgCl2 are shorter than those found for [(FcPPh2)2·HgCl2] (Hg1−P1 2.5045(7), Hg1−P2 2.4812(6) Å), indicative of stronger coordination of the phosphine ligands. FcPPh2·HgCl2 and [(FcPPh2)2·HgCl2] exhibit 31P NMR resonances at δ +31.5 ppm in CDCl3 (JHgP = 7740 Hz, +31.6 ppm in acetone-d6 at −50 °C) and δ +21.8 ppm (JHgP = 4610 Hz, +20.4 ppm in acetone-d6 at −50 °C), respectively (Table 1). The former is close to the resonance at +32.4 ppm, providing evidence for the formation of a 1:1 Lewis adduct, 1· HgCl2, as a reaction intermediate. The complex likely exists as a dimer, as seen in the X-ray structure of FcPPh2·HgCl2 or may be stabilized by acetone solvent coordination. The signal at δ +21.8 ppm for the 2:1 mixture of FcPPh2 and HgCl2 is at a chemical shift similar to that of 2·HgCl2, consistent with complexation of HgCl2 by two phosphine ligands. However, a corresponding species ((1)2·HgCl2) could not be detected in

Figure 3. (a) Molecular structure of one of two independent molecules of FcPPh2·HgCl2 (50% thermal ellipsoids; hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): molecule 1, Hg1−Cl1 2.3961(14), Hg1−Cl2 2.6051(13), Hg1−Cl2A 2.7298(14), Hg1−P1 2.4069(15), P1−C1 1.779(6), Hg1···Hg1A 3.935, Hg1−Cl2−Hg1A 95.01(4); molecule 2, Hg2−Cl3 2.3772(15), Hg2−Cl4 2.6596(15), Hg2−Cl4A 2.6798(14), Hg2−P2 2.3941(14), P2−C23 1.775(6), Hg2···Hg2A 3.938, Hg2−Cl4−Hg2A 95.05(5). (b) Molecular structure of (FcPPh2)2·HgCl2 (50% thermal ellipsoids; hydrogen atoms omitted for clarity). Selected bond lengths (Å) and angles (deg): Hg1−Cl1 2.5173(7), Hg1−Cl2 2.5170(7), Hg1−P1 2.5045(7), Hg1−P2 2.4812(6), P1−C1 1.788(3), P2−C23 1.790(3), P1−Hg1−P2 116.01(2).

the course of the reaction of 1 and HgCl2. It should be noted that low-temperature (−60 °C) 31P NMR solution studies of the related [PPh3·HgCl2] and [(PPh3)2·HgCl2] showed resonances at δ +33.6 ppm (JHgP = 7670 Hz) and at δ +27.7 C


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Organometallics Table 1. Comparison of Selected NMR Data for Isolated or in Situ Generated Compounds solvent, T/°C

δ(119Sn)/ppm

δ(31P)/ppm

δ(1H, Me)/ppm

−7.2 (JSn,P = 7.6 Hz) n.d. n.d.

125.9 (JSn,P = 26 Hz)

−16.2 (nr) −16.5 (nr) 32.1 28.0 (JHg,P = 5020 Hz) 26.4 (nr) −15.8

0.21 (Sn−Me, JSn,H = 55 Hz) 0.21 (Sn−Me, JSn,H = 56 Hz) 0.14 (Sn−Me, JSn,H = 56 Hz)

5

CDCl3, RT acetone-d6, RT acetone-d6, −50 CDCl3, RT acetone-d6, RT CDCl3, RT

5·MeHgCl

acetone-d6, RT CDCl3, RT

n.d. 112.3

−16.2 −10.9

acetone-d6, −50

n.d.

34.3 (br)

acetone-d6, RT

n.d.

−13.8

acetone-d6, −50 acetone-d6, RT CDCl3, RT acetone-d6, RT acetone-d6, −50 CDCl3, RT acetone-d6, RT acetone-d6, −50

n.d.

30.1 35.3 31.5 33.5 31.6 21.8 25.0 20.4

compound 1 1·HgCl2 2·HgCl2

5·HgCl2 6·HgCl2 FcPPh2·HgCl2

(FcPPh2)2·HgCl2

(JHg,P (JHg,P (JHg,P (nr) (nr) (JHg,P (JHg,P (JHg,P

= 7560 Hz) = 7560 Hz) = 7740 Hz)

1.00 0.53 0.83 1.15 0.79 0.96 0.66 0.98 0.79 0.63

(Sn−Me, JSn,H = 61 Hz) (Sn−Me, JSn,H = 59 Hz) (Sn−Me, br) (Hg-Me, JHg,H = 198 Hz) (Sn−Me, br) (Hg-Me, nr) (Sn−Me, br) (Hg-Me, JHg,H = 211 Hz) (Sn−Me, JSn,H = 63 Hz) (JSn,H = 73 Hz)

= 4610 Hz) = 2190 Hz) = 4560 Hz)

Scheme 3. Formation of 5 and MeHgCl in CDCl3

B(R)Cl). Binding of the Lewis basic substituent to Sn is believed to weaken the Sn−Me bond trans to the coordinating group.8e,14 A similar mechanism likely operates here, where binding of the phosphine moiety itself, or the phosphinebound HgCl2, results in weakening of the apical Sn−Me group in a pentacoordinate Sn intermediate. A plausible intermediate that may also exist in equilibrium with a Hg2Cl2-bridged dimeric complex is shown in Scheme 3. The fact that the attack of HgCl2 at the Sn−Me bond is less favorable in acetone indicates that, unlike chloroform as a noncoordinating solvent,15 acetone competes with the phosphine moiety in the coordination to HgCl2, thereby affecting the reaction outcome. Compound 5 was fully characterized by multinuclear NMR spectroscopy, high-resolution mass spectrometry, and elemental analysis. In CDCl3, the 31P NMR signal was detected as a singlet at −15.8 ppm and the 119Sn NMR signal at δ +125.9 ppm appeared as a doublet due to coupling between the 119Sn and 31P nuclei with JSn,P = 26 Hz (Table 1). A similar signal splitting but with a lower coupling constant was reported previously for Fc(PPh2·BH3)(SnMe2Cl)8f (JSn,P = 12.3 Hz). In acetone-d6, the 31P signal of 5 is only slightly shifted to high field at δ −16.2 ppm, suggesting that the solvent does not coordinate to Sn to a significant extent (Figure 4). For the complex 5·MeHgCl, only a modest upfield shift of the 119Sn NMR resonance to δ +112.3 ppm and downfield shift of the 31 P NMR resonance to δ −10.9 ppm was observed in CDCl3. Similar data were recorded in acetone-d6 with a 31P NMR shift

ppm (JHgP = 4766 Hz), respectively, which corroborate those found for FcPPh2·HgCl2 and [(FcPPh2)2·HgCl2] above.13 Competitive Sn−Me Bond Cleavage with Formation of Phosphinoferrocenyl Chlorodimethylstannane 5. The broad signal at −11.3 ppm in the 31P NMR spectrum of the reaction mixture (Figure 2) could be assigned to a byproduct that results from competitive Sn−Me abstraction, which produces the phosphinoferrocenyl chlorodimethylstannane 5 and methylmercury(II) chloride. The product, 5· MeHgCl, was isolated after removal of solid 2·HgCl2 by filtration and purified by repeated crystallization from acetone at −23 °C. NMR spectroscopic data indicated that the compound cocrystallizes with methylmercury(II) chloride. A signal with Hg satellites in the 1H NMR spectrum in CDCl3 (1.15 ppm, 2JHg,H = 198 Hz) that integrates to 3 protons confirmed the 1:1 stoichiometry of 5 and MeHgCl (Table 1). Spectroscopically pure 5 was isolated upon heating the compound to 90 °C under high vacuum, whereby MeHgCl is decomplexed and removed by sublimation. Remarkably, 5 and methylmercury(II) chloride became the predominant products when the reaction between 1 and HgCl2 was performed in chloroform (Scheme 3 and Figures S7−S10). The product was once again decomplexed from MeHgCl by sublimation and 5 isolated as an orange solid in 78% overall yield. While not well established in Sn−Hg exchange chemistries, there is precedent for cleavage of Sn−Me groups in trimethylstannylferrocenes by boron halides in the presence of donor substituents in an ortho position (e.g., pyridyl, D


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(1.80) and Sn (2.17).16 However, this interaction only modestly influences the geometry, as the Sn atom resides in a distorted-monocapped-tetrahedral configuration. The extent of trigonal-bipyramidal character at Sn as judged by the sum of the equatorial angles of 339.9° is more pronounced than for the trimethylstannyl derivative FcPPh2SnMe310 (∑(equatorial angles) = 332.2°) but significantly less than for the BH3complexed species [Fc(PPh2·BH3)SnMe2Cl] (∑(equatorial angles) = 349.4°). A much shorter Sn1−Cl1 distance of 2.259(2) Å in comparison to that in [Fc(PPh2·BH3)SnMe2Cl]8f (2.4022(9) Å) further indicates that the P1···Sn1 interaction is rather weak. Low-Temperature NMR Studies. In order to further verify the involvement of complexes [1]n·HgCl2 (n = 1, 2) and to identify subsequent intermediates along the reaction pathway, a series of VT NMR experiments were performed. An NMR sample containing HgCl2 in acetone-d6 was frozen in liquid nitrogen. Then, a solution of 1 in acetone-d6 was added on top and immediately frozen. The sample was immersed in an acetone/dry ice bath for some time and then quickly introduced into the NMR spectrometer, which was precooled to −50 °C. The reaction processes were followed by 1H and 31 P NMR while the mixture was gradually warmed to room temperature (Figure 6 and Figures S11−S19).

Figure 4. 31P NMR spectra of 5·MeHgCl in acetone-d6 at room temperature (RT) and −50 °C in comparison to data for Hg-free 5.

of δ −13.8 ppm (υ1/2 = 200 Hz). However, when a solution of 5·MeHgCl in acetone-d6 was cooled to −50 °C, the 31P signal strongly broadened and low field shifted to δ +34.3 ppm (υ1/2 = 1490 Hz), indicating phosphorus coordination to Hg (Figure 4). These observations suggest that the binding equilibrium between 5 + MeHgCl and 5·MeHgCl is strongly temperature dependent with little formation of 5·MeHgCl at room temperature. Importantly, no rearrangement of 5·MeHgCl into the species 2·HgCl2 was observed in acetone, even after the solution was kept for several days, which implies that reaction of 5 with MeHgCl can be ruled out as a pathway to 2· HgCl2. The chirality of the pS isomer of 5 was established by measurement of the optical rotation in chloroform and calculation of the corresponding specific rotation, which is determined to be [α]20D = +128 (c 0.1). However, attempts to obtain single crystals of enantiomerically pure (pS)-5 or (pS)5·MeHgCl for X-ray diffraction analysis proved unsuccessful. In order to assess the molecular structures of these compounds in the solid state, the synthesis was repeated starting with rac-1, resulting in isolation of racemic 5 and 5·MeHgCl, respectively, as orange and yellow crystalline solids. X-ray diffraction data were collected on single crystals of 5 obtained from CH2Cl2/ hexanes solution by slow evaporation, and the molecular structure is depicted in Figure 5. The P1···Sn1 interatomic distance of 3.8522(12) Å in 5 is slightly shorter than the sum of the van der Waals radii of P

Figure 6. 31P (left) and 1H (right) NMR spectra of [1 + HgCl2] at −50 °C (bottom) and after warming to RT (top). Tin satellites are indicated with asterisks and peaks due to residual hexanes with solid squares.

Immediately after the sample was inserted into the spectrometer (T = −50 °C), 31P NMR resonances were observed at δ +36.7 ppm (minor) and at δ +32.1 ppm (major). As discussed above for the model systems, these signals are in the typical region of 1:1 complexes of HgCl2 and phosphine. The signals for the precursor 1 (δ −16.5 ppm in acetone-d6) and the final product 2·HgCl2 (δ +26.4 ppm in acetone-d6) were absent, demonstrating that 1 had reacted, but the formation of the product was not favorable at that temperature. Indeed, the 1H NMR spectrum at −50 °C exhibited in the high-field region a singlet at δ 0.13 ppm (JSn,H = 56 Hz) corresponding to the SnMe3 group, suggesting that the SnMe3 group remained largely unreacted. Minor signals at δ 0.59, 0.61, 0.91, and 1.20 ppm were observed and are tentatively assigned to formation of small amounts of Me3SnCl, 5, MeHgCl, and Me2SnCl2. These observations are in line with formation of the adduct 1·HgCl2 (+32.1 ppm) and a small amount of a byproduct due to Sn−Me cleavage (+36.7 ppm, vide infra). When the sample reached ambient temperature,17 the 31P NMR resonances shifted to δ +35.9 and +38.3 ppm but still remained in the region typical of phosphine adducts of HgCl2. The signal for the SnMe3 group in the 1H NMR

Figure 5. Molecular structure of 5 (racemate, only one enantiomer shown). Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Sn1−Cl1 2.259(2), Sn1−C1 2.131(4), Sn1−C11 2.136(5), Sn1−C12 2.143(5), P1−C2 1.813(5), P1···Sn1 3.8522(12), C1−Sn1−Cl1 100.47(13), C1−Sn1−C11 113.89(19), C1−Sn1−C12 112.72(19), C11−Sn1−C12 113.3(2), Cl1−Sn1−C11 106.27(17), Cl1−Sn1−C12 109.06(19), P1···Sn1−Cl1 140.93(7), Sn1−C1−C2−P1 0.3(6). E


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Organometallics spectrum at δ 0.13 ppm had almost completely disappeared. The signal previously observed at δ 0.59 ppm also disappeared, while other signals at δ 0.62 (JSn,H = 64 Hz), 0.98, and 1.22 ppm (JSn,H = 86 Hz) gained intensity. The NMR sample was left standing overnight to give an orange crystalline solid, which was confirmed to consist of 2·HgCl2. NMR analysis of the supernatant showed resonances at δ 0.61 (JSn,H = 62/65 Hz, Me3SnCl), 0.97 (JHg,H = 212 Hz, MeHgCl), and 1.17 ppm (JSn,H = 82/86 Hz, Me2SnCl2 ), all flanked by easily distinguishable Sn and Hg satellites. This again strongly suggests the formation of products arising from Sn−Me cleavage, especially under kinetic conditions when the reaction sequence was initiated at low temperature. Given that the reactivity of the Cp−Sn bond in the chlorodimethylstannyl species 5 is expected to be lower than in the trimethylstannyl species 1, making it potentially easier to detect further intermediates in the Sn−Hg exchange process, a similar study was performed with racemic 5 and HgCl2 (Scheme 4 and Figures S20−S23). This also allowed us to

The X-ray crystal structure identified the major product as [Fc(PPh2·HgCl2)(HgCl)] (6·HgCl2). The asymmetric unit contains two independent molecules, which are connected through multiple Hg···Cl contacts. The dimerization is primarily a result of formation of an unsymmetric Hg2Cl2 bridge between the phosphine-bound HgCl2 units with Hg2− Cl3/Cl5 distances of 2.642(6)/2.965(7) Å and Hg4−Cl5/Cl3 distances of 2.523(7)/2.686(7) Å; these are all relatively longer than the bonds to the chlorines that are not involved in the bridge: i.e. Hg2−Cl2 of 2.404(8) Å and Hg4−Cl6 of 2.475(6) Å (Figure 7a). The structure is different from that of a related arylmercury complex with an acenaphthyl backbone5f in that an additional weak contact is observed between a HgCl2 chlorine of one molecule and the Fc−Hg−Cl mercury atom of the other (Cl2···Hg3 3.088(7) Å). A similar contact, Cl5···Hg1

Scheme 4. Reaction of 5 with HgCl2 in Acetone

assess whether the initially formed byproduct 5, while unreactive toward MeHgCl as indicated by the successful isolation of 5·MeHgCl, may serve as a reaction intermediate when additional HgCl2 is present. The 31P NMR spectrum of a mixture of 5 and HgCl2 in a 1:2 ratio recorded at −50 °C in acetone-d6 showed a sharp major resonance with mercury satellites at δ +30.1 ppm (JHg,P = 7560 Hz), which is close to that of the model complex [FcPPh2·HgCl2] (δ +31.5 ppm in CDCl3, JHgP = 7740 Hz; + 31.6 ppm in acetone-d6 at −50 °C) and thus indicates the initial formation of a complex 5·HgCl2 (Table 1). The Sn−Me protons are observed at 0.63 ppm (JSn,H = 73 Hz) in the 1H NMR spectrum similar to those of 5· MeHgCl (δ 0.66 ppm in acetone-d6 at −50 °C). When the temperature was raised to room temperature, Sn/Hg exchange occurred, and the product then gradually crystallized as an orange solid. A similar reaction was performed directly at RT, and 1H and 31P NMR data were collected immediately, prior to precipitation. The 31P NMR spectrum exhibited a single resonance at δ +35.3 ppm (JHg,P = 7540 Hz), and the formation of Me2SnCl2 as a byproduct of the Sn/Hg exchange was evidenced by a signal at δ 1.23 ppm (JSnH = 82/85 Hz) in the 1H NMR spectrum (Table 1). The isolated pure product proved insoluble when we attempted to redissolve it in CDCl3 or acetone-d6, preventing 13C NMR analysis, but the product composition of the mercurated species 6·HgCl2 was verified by elemental analysis and MALDI-TOF MS, which showed the expected ion peak for [6]+ (Figure S24).

Figure 7. (a) Molecular structure of the dimercury compound 6· HgCl2 (pS isomer, 50% thermal ellipsoids, hydrogen atoms omitted for clarity). Selected interatomic distances (Å) and angles (deg): molecule 1, Hg1−C1 2.06(3), Hg1−Cl1 2.303(7), Hg2−Cl2 2.404(8), Hg2−Cl3 2.642(6), Hg2−P1 2.387(7), P1−C2 1.77(3), C1−Hg1−Cl1 169.0(8), Cl2−Hg2−Cl3 98.2(2); molecule 2, Hg3− C23 2.12(3), Hg3−Cl4 2.318(8), Hg4−Cl5 2.523(7), Hg4−Cl6 2.475(6), Hg4−P2 2.420(7), P2−C24 1.77(3), C23−Hg3−Cl4 172.0(8), Cl5−Hg4−Cl6 94.8(2). (b) Illustration of intermolecular contacts between molecule 1 and molecule 2, which lead to a polymeric structure. Selected interatomic distances (Å) and angles (deg): Hg1···Cl5 3.266(7), Hg2···Cl5 2.965(7), Hg3···Cl2 3.088(7), Hg4···Cl3 2.686(7), Hg2−Cl2···Hg3 83.4(2), Hg2−Cl3···Hg4 89.1(2), Hg2···Cl5−Hg4 85.5(2), Cl1−Hg1···Cl5 85.7(2), Cl2− Hg2···Cl5 85.7(2), Cl3−Hg2···Cl5 78.6(2), Cl4−Hg3···Cl2 82.2(2), Cl5−Hg4···Cl3 86.1(2), Cl6−Hg4···Cl3 100.2(2), Hg1···Cl5···Hg2 168.7(3). F


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Organometallics

especially in CHCl3 as a weakly coordinating solvent (see Scheme 3), potentially has broader implications due to formation of highly toxic and volatile MeHgCl. Sn−Hg exchange reactions are typically assumed to offer facile and selective access to arylmercury species. Our results suggest that noncoordinating solvents may be better avoided and extreme caution is necessary when these transmetalations are performed in the presence of neighboring donor substituent.

3.266(7) Å, leads to extension to a polymeric chain structure in the crystal lattice (Figure 7b). These results demonstrate that, although 5 does not undergo further reaction with MeHgCl at room temperature, it readily reacts with HgCl2 via mercuriodestannylation to give 1,2-Fc(PPh2)(HgCl), which is stabilized by complexation to HgCl2 in the presence of excess HgCl2. When the reaction of 5 with HgCl2 was performed in a 1:1 molar ratio at RT in acetone, again the adduct 5·HgCl2 was generated initially as the major product according to 1H and 31P NMR analyses (Figures S25 and S26). The 31P NMR spectrum showed a broad peak at δ +31.1 ppm, which is close to the chemical shifts found for the adducts [1 + HgCl2] (δ +32.1 ppm) and [5 + 2 HgCl2] (δ +30.1 ppm) at low temperature (−50 °C). However, in contrast to the reaction of 5 with 2 equiv of HgCl2 which ultimately gave 6·HgCl2, upon standing at room temperature slow conversion of 5·HgCl2 into the dimeric species 2·HgCl2 was observed (δ(31P) 27.3 ppm, 35% after 24 h, Figures S27 and S28). The latter crystallized from the reaction mixture, and its structure was confirmed by verification of the previously reported unit cell parameters10 using X-ray diffraction analysis. Importantly, we found no evidence for “free” Fc(PPh2)(HgCl) or the formation of its dimer [Fc(PPh2)(HgCl)]2 (4, Scheme 2) through an intermolecular Hg−P Lewis acid−base reaction. Finally, to ascertain that a species akin to the dimer 3 in PATH A (Scheme 2) is competent as a reaction intermediate, an excess of 1 (pS isomer) was added to a suspension of (poorly soluble) 6·HgCl2 (pS isomer) in acetone and the mixture heated to 50 °C for 24 h. Indeed, 1H and 31P NMR analysis in CD2Cl2 of the residue after solvent evaporation indicated the selective formation of 2·HgCl2 (Figures S29 and S30). It is reasonable to assume then that in the absence of precipitation of the intermediate, and with the more reactive trimethylstannyl substituent, rapid conversion of 3 to the product 2·HgCl2 would occur (Scheme 2). Collectively, these results offer further support for a mechanism proceeding via PATH A in the formation of 2·HgCl2 from 1.

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EXPERIMENTAL SECTION

General Methods. All reactions were carried out under an atmosphere of dry nitrogen using high-vacuum Schlenk-line techniques or an inert-atmosphere glovebox (MBraun). Commercial-grade solvents (toluene, hexanes) were purified by a solvent purification system from Innovative Technologies, degassed, and stored over sodium−potassium (NaK) alloy prior to use. Tetrahydrofuran was distilled from sodium/benzophenone and dichloromethane from CaH2, degassed, and stored under a nitrogen atmosphere. 1H NMR (499.9/599.7 MHz), 13C NMR (125.7/150.8 MHz), 31P NMR (202.5 MHz), and 119Sn NMR (223.6 MHz) spectra were recorded on a Bruker Avance III HD NMR spectrometer (Bruker BioSpin, Billerica, MA) equipped with a 5 mm broad-band gradient SmartProbe (Bruker, Billerica, MA) or a 600 INOVA NMR spectrometer (Varian Inc., Palo Alto, CA) equipped with a boron-free 5 mm dual broadband gradient probe (Nalorac, Varian Inc., Martinez, CA). Chemicals shifts (δ) are given in ppm and were referenced internally to deuterated solvent signals (CDCl3 7.26 (1H), 77.36 ppm (13C); C6D6 7.15 (1H), 128.62 ppm (13C); acetone-d6 2.05 ppm (1H)). Coupling constants (J) are reported in hertz (Hz), splitting patterns are indicated as s (singlet), d (doublet), pt (pseudo triplet), t (triplet), br s (broad singlet), nr (nonresolved), and m (multiplet), and the following abbreviations are used for signal assignments: Ph = phenyl, Fc = ferrocenyl, Cp = cyclopentadienyl, Me = methyl. Optical rotation analyses were performed on an Autopol III polarimeter from Rudolph Research Analytical, using a tungsten−halogen light source operating at λ 589 nm. High-resolution matrix-assisted laser desorption ionization-mass spectrometry (MALDI-MS) data were obtained on an Apex Ultra 7.0 Hybrid FTMS instrument and MALDI-TOF (time-of-flight) MS data on a Bruker Ultraflextreme instrument. Elemental analyses were performed by Quantitative Technologies Inc., Whitehouse, NJ. Caution! Mercury compounds are highly toxic! Particularly the exchange reaction between diphenylphosphinoferrocenyl trimethylstannane and mercury(II) chloride generates methylmercury(II) chloride, which is highly toxic. These reactions have to be handled with extreme caution, and the use of engineering barriers including nonpermeable and resistant gloves is essential. Organolithium reagents and tert-butyllithium in particular are highly reactive and need to be handled accordingly. Materials. HgCl2, n-butyllithium (1.6 M in hexanes), and tertbutyllithium (1.7 M in pentane) were purchased from commercial sources and used without further purification. The syntheses of the pS isomer of 110 and the pS,pS isomer of 2·HgCl210 have been previously reported. X-ray Diffraction Analysis. Reflections were collected on a Bruker SMART APEX II CCD diffractometer using Cu Kα (1.54178 Å) radiation. Data processing, Lorentz−polarization, and face-indexed numerical absorption corrections were performed using SAINT, APEX, and SADABS computer programs.18 The structures were solved by direct methods and refined by full-matrix least squares based on F2 with all reflections using the SHELXTL V6.14 program package.19 Non-hydrogen atoms were refined with anisotropic displacement coefficients. All H atoms were found in electron-density difference maps and treated as idealized contributions. Experimental details are provided in Table S1 of the Supporting Information. CCDC 1880976−1880979 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

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CONCLUSIONS The information gained from 1H and 31P NMR analysis (Table 1) and VT NMR experiments suggests that addition of HgCl2 to (pS)-1 induces coordination of the phosphine moiety to the mercury atom and the resulting complex (31P NMR, δ +32.4 ppm) undergoes Sn/Hg exchange with cleavage of either a Sn−Fc or Sn−Me bond to give 2·HgCl2, 5, Me3SnCl, and MeHgCl. The broad nature of the resonances points to complex equilibria among 1, [1]·(HgCl2)n (n = 0.5, 1), [1]· MeHgCl, 5, [5]·(HgCl2)n (n = 0.5, 1), and [5]·MeHgCl. Not only 1 but also 5 can undergo mercuriodestannylation, as demonstrated by the isolation of the adduct [Fc(PPh2· HgCl2)(HgCl)] (6·HgCl2) when 5 was treated with an excess of HgCl2. The structure of 6·HgCl2 was verified by X-ray analysis. In the presence of only 1 equiv of HgCl2, 5 initially formed a HgCl2 adduct, followed by rearrangement into the dimeric species 2·HgCl2. The fact that we were not able to isolate the dimeric species 4 leads us to conclude that PATH A in Scheme 2 presents the more likely mechanism, with 3 as a plausible intermediate in the formation of 2·HgCl2. This assessment is further supported by the finding that mixing 6· HgCl2 with an excess of 1 selectively furnishes 2·HgCl2. The discovery that donor substituents may promote Sn−Me over Sn−Fc bond cleavage in Sn−Hg exchange reactions, G


Article

Organometallics

mixture was allowed to stand overnight. During this period, a yellow solid precipitate formed, which is typical of the formation of 2·HgCl2. The solution was left standing for another night, and the yellow precipitate was then collected by filtration and identified to be 2· HgCl2 by 1H and 31P NMR NMR in CDCl3. The 1H and 31P NMR data of the supernatant were also recorded. 1H NMR (499.9 MHz, acetone-d6, 25 °C; only characteristic signals are assigned): δ 0.98 (s, 2 JHg,H = 211 Hz, MeHgCl), 0.78 (br, Me2SnCl of 5), 0.62 (s, 2JSn,H = 64 Hz, Me3SnCl). 31P{1H} NMR (202.5 MHz, acetone-d6, 25 °C): δ −11.3 (br, 41%), 25.6 (br, 26%, 2·HgCl2), 32.3 (br, 33%). The solvent was removed from the filtrate under vacuum and the residue washed several times with hexanes. Recrystallization from acetone at −23 °C afforded a yellow crystalline solid that was identified to be the complex 5·MeHgCl according to NMR analysis. Yield: 12.0 mg (36%). Compound 5 free of MeHgCl was obtained by sublimation of the latter during heating under high vacuum. Yield: 8.2 mg (36%). Reaction of 1 with HgCl 2 in CDCl 3 : Synthesis of Chlorodimethylstannyl Diphenylphosphinoferrocene (5) and Its MeHgCl Complex. In an NMR tube were mixed CDCl3, 1 (pS isomer, 0.031 g, 0.058 mmol) and HgCl2 (0.016 g, 0.059 mmol, 1 equiv), and the reaction was followed by 1H and 31P NMR. Data after 60 min standing are as follows. 1H NMR (499.9 MHz, CDCl3, 25 °C; only characteristic signals are assigned): δ 1.14 (s, 2JHg,H = 200 Hz, MeHgCl), 0.17 (br s, overlapping Sn−Me signals of 5·MeHgCl and 1). 31P{1H} NMR (202.5 MHz, CDCl3, 25 °C): δ −16.1 (br, 36%, 1), −11.7 (br, 30%, 5·MeHgCl), 25.9 (br, 6%, 2·HgCl2), 32.5 (br, 28%, 1 JHg,P = 5060 Hz, 1·HgCl2). Data after standing for 72 h are as follows. 1 H NMR (499.9 MHz, CDCl3, 25 °C; only characteristic signals are assigned): δ 1.25 (br, Me2SnCl2), 1.15 (s, 2JHg,H = 198 Hz, MeHgCl), 0.88 (br, Me2SnCl of 5·MeHgCl), 0.77 (br, SnMe2 of 5·MeHgCl), 0.20 (s, 2JSn,H = 54 Hz, SnMe3 of 1). 31P{1H} NMR (202.5 MHz, CDCl3, 25 °C): δ −15.9 (s, 10%, 1), − 5.7 (s, 85%, 5), + 32.5 (br, 5%, 1·HgCl2). A small amount of a precipitate formed throughout this process. The solvent was removed from the mixture under vacuum and the residue washed several times with hexanes. Recrystallization from acetone at −30 °C afforded 5·MeHgCl as a yellow crystalline solid. Yield: 0.037 g (79%). The purity of 5·MeHgCl (>97%) was established by 1H NMR and elemental analysis. Compound 5 was isolated free of MeHgCl by sublimation of the latter during heating under high vacuum. Overall yield: 0.025 g (78%). The purity of 5 (>97%) was established by 1H NMR and elemental analysis. Data for 5 are as follows. 1H NMR (599.7 MHz, CDCl3, 25 °C): δ 7.58 (m, 2H, Ph), 7.42 (m, 3H, Ph), 7.23 (m, 3H, Ph), 7.10 (m, 2H, Ph), 4.61 (nr, 1H, Cp), 4.60 (nr, 1H, Cp), 4.13 (nr, 1H, Cp), 4.05 (s, 5H, free Cp), 1.00 (br s, 2JSn,H = 61 Hz, 3H, SnMe), 0.53 (br s, 2JSn,H = 59 Hz, 3H, SnMe). 1H NMR (499.9 MHz, acetone-d6, 25 °C): δ 7.66 (m, 2H, Ph), 7.49 (m, 3H, Ph), 7.26 (m, 3H, Ph), 7.12 (m, 2H, Ph), 4.70 (pt, 3JH,H = 2.5 Hz, 1H, Cp), 4.57 (nr, 1H, Cp), 4.21 (nr, 1H, Cp), 4.01 (s, 5H, free Cp), 0.83 (br, 6H, SnMe2Cl). 13C{1H} NMR (150.8 MHz, CDCl3, 25 °C): δ 140.1 (d, 1JC,P = 6.8 Hz, i-Ph), 137.3 (d, 1JC,P = 6.8 Hz, i-Ph), 135.1 (d, 2JC,P = 20.5 Hz, o-Ph), 132.3 (d, 2JC,P = 16 Hz, o-Ph), 129.9 (s, p-Ph), 128.7 (d, 2JC,P = 8.3 Hz, mPh), 128.6 (d, 2JC,P = 5.4 Hz, m-Ph), 128.3 (s, p-Ph), 82.6 (s, i-CpSn), 80.9 (d, 1JC,P = 54 Hz, i-Cp-P), 77.5 (s, Cp), 74.2 (s, JC,Sn = 51 Hz, Cp), 74.1 (d, JC,P = 2.7 Hz, Cp), 69.8 (s, free-Cp), 1.8 (d, 2JC,P = 9.7 Hz, SnMe), 1.1 (br, SnMe). 31P{1H} NMR (202.5 MHz, CDCl3, 25 °C): δ −15.8 (s). 31P{1H} NMR (202.5 MHz, acetone-d6, 25 °C): δ −16.2 (s). 119Sn{1H} NMR (223.6 MHz, CDCl3, 25 °C): δ 125.9 (d, JSn,P = 26 Hz). High-resolution MALDI-MS (positive mode, anthracene): m/z 553.9644 ([M]+, calcd for 12 C 2 4 1 H 2 4 3 5 Cl 5 6 Fe 3 1 P 1 1 9 Sn 553.9669). Anal. Calcd for C24H24ClFePSn: C, 52.09; H 4.47. Found: C, 52.16; H, 4.13. Although these results are slightly outside the range viewed as establishing analytical purity, they are provided to illustrate the best values obtained to date. Data for 5·MeHgCl are as follows. 1H NMR (599.7 MHz, CDCl3, 25 °C): δ 7.58 (m, 2H, Ph), 7.44 (m, 3H, Ph), 7.26 (m, 3H, Ph), 7.12 (m, 2H, Ph), 4.65 (nr, 1H, Cp), 4.62 (nr, 1H, Cp), 4.11 (nr, 1H, Cp), 4.07 (s, 5H, free Cp), 1.15 (s, 2JHg,H = 198 Hz, 3H, MeHgCl), 0.79 (br, 6H, SnMe2Cl). 1H NMR (499.9 MHz, acetone-d6, 25 °C): δ 7.67

Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: + 44 1223 336033. NMR Data for 1 in Acetone-d6. 1H NMR (499.9 MHz, acetoned6, 25 °C): δ 7.58 (m, 2H, Ph), 7.44 (m, 3H, Ph), 7.26 (m, 3H, Ph), 7.11 (m, 2H, Ph), 4.55 (pt, 3JH,H = 2.5 Hz, 1H, Cp), 4.36 (nr, 1H, Cp), 4.02 (s, 5H, free Cp), 3.94 (nr, 1H, Cp), 0.21 (s, 2JSn,H = 56 Hz, 9H, SnMe3). 31P{1H} NMR (202.5 MHz, acetone-d6, 25 °C): δ −16.5 (s). NMR Data for 2·HgCl2 in Acetone-d6 (Poorly Soluble). 31 1 P{ H} NMR (202.5 MHz, acetone-d6, 25 °C): δ 26.4 (s). Synthesis of Model Compound [FcPPh2·HgCl2]. To a solution of HgCl2 (77.2 mg, 0.284 mmol) in acetone (3 mL) was added dropwise a solution of FcPPh2 (105.2 mg, 0.284 mmol, 1 equiv) in acetone (3 mL). The mixture was stirred for 1 h, whereupon a yellow solid precipitate formed that was collected by filtration, washed several times with hexanes, and dried under vacuum. Yield: 50.0 mg (27%). The purity of [FcPPh2·HgCl2] (>97%) was established by 1H NMR and elemental analysis. Crystals for X-ray analysis were obtained by slow evaporation of a solution of the compound in CH2Cl2/hexanes at room temperature. 1H NMR (599.7 MHz, CDCl3, 25 °C): δ 7.79 (m, 4H, Ph), 7.56 (m, 6H, Ph), 4.66 (nr, 4H, Cp), 4.30 (s, 5H, free Cp). 13C{1H} NMR (150.8 MHz, CDCl3, 25 °C): δ 133.9 (d, 3JC,P = 12.4 Hz, o-Ph), 132.8 (s, p-Ph), 129.8 (d, 2 JC,P = 12.4 Hz, m-Ph), 128.1 (d, 1JC,P = 57.6 Hz, i-Ph), 74.3 (d, JC,P = 13.7 Hz, Cp), 73.7 (d, JC,P = 9.7 Hz, Cp), 70.9 (s, free Cp), 66.1 (d, 1 JC,P = 67.3 Hz, i-Cp-P). 31P{1H} NMR (202.5 MHz, CDCl3, 25 °C): δ 31.5 (s, 1JHg,P = 7740 Hz). 31P{1H} NMR (202.5 MHz, acetone-d6, 25 °C): δ 33.5 (s). 31P{1H} NMR (202.5 MHz, acetone-d6, − 50 °C): δ 31.6 (s). High-resolution MALDI-MS (positive mode, anthracene): m/z 607.0118 ([M − Cl]+, calcd for 12C221H1935Cl56Fe200Hg31P 606.9958). Anal. Calcd for C22H19Cl2FeHgP: C, 41.18; H, 2.98. Found: C, 40.76; H, 3.05. Synthesis of Model Compound [(FcPPh2)2·HgCl2]. To a solution of HgCl2 (37.4 mg, 0.138 mmol) in acetone (3 mL) was added dropwise a solution of FcPPh2 (102.1 mg, 0.276 mmol, 2 equiv) in acetone (3 mL). The mixture was stirred for 1 h, whereupon a yellow solid precipitate formed that was collected by filtration, washed several times with hexanes, and dried under vacuum. Yield: 90.0 mg (65%). The purity of [(FcPPh2)2·HgCl2] (>90%) was established by 1H NMR analysis. The bulk material is contaminated by an unidentified byproduct that could not be removed even after repeated recrystallizations. Crystals for X-ray analysis were obtained by slow evaporation of a solution of the compound in CH2Cl2/ hexanes. 1H NMR (599.7 MHz, CDCl3, 25 °C): δ 7.69 (br, 8H, Ph), 7.46 (br, 4H, Ph), 7.38 (br, 8H, Ph), 4.42 (br, 4H, Cp), 4.24 (br, 4H, Cp), 4.14 (s, 10H, free Cp). 13C{1H} NMR (150.8 MHz, CDCl3, 25 °C): δ 133.9 (br, o-Ph), 131.6 (br, p-Ph), 129.2 (br, m-Ph), i-Ph not observed, 74.0 (nr, Cp), 72.7 (nr, Cp), 70.7 (s, free Cp), 68.2 (br, iCp-P). 31P{1H} NMR (202.5 MHz, CDCl3, 25 °C): δ 21.8 (s, 1JHg,P = 4610 Hz). 31P{1H} NMR (202.5 MHz, acetone-d6, 25 °C): δ 25.0 (s, 1 JHg,P = 2190 Hz). 31P{1H} NMR (202.5 MHz, acetone-d6, −50 °C): δ 20.4 (s, 1JHg,P = 4560 Hz). High-resolution MALDI-TOF MS (positive mode, DHB): m/z 606.9972 ([M − FcPPh2 − Cl]+, calcd for 12C221H1935Cl56Fe200Hg31P 606.9958). Sequential Reaction of 1 with Different Amounts of HgCl2 in Acetone. In an NMR tube were mixed acetone-d6, 1 (pS isomer, 22.0 mg, 0.041 mmol), and HgCl2 (6.0 mg, 0.022 mmol, 0.5 equiv), and the reaction was followed by 1H and 31P NMR. Data after 30 min standing are as follows. 1H NMR (499.9 MHz, acetone-d6, 25 °C; only characteristic signals are assigned): δ 0.98 (s, 2JHg,H = 211 Hz, MeHgCl), 0.66 (br s, SnMe2Cl of 5), 0.62 (s, 2JSn,H = 64 Hz, Me3SnCl), 0.16 (s, 2JSn,H = 56 Hz, Me3Sn of 1). 31P{1H} NMR (202.5 MHz, acetone-d6, 25 °C): δ −16.0 (br, 58%), 27.4 (s, 1JHg,P = 4970 Hz, 20%, 2·HgCl2), 32.4 (br, 22%). Data after overnight standing are as follows.1H NMR (499.9 MHz, acetone-d6, 25 °C; only characteristic signals are assigned): δ 0.98 (s, 2JHg,H = 209 Hz, MeHgCl), 0.79 (br s, SnMe2Cl of 5), 0.63 (s, 2JSn,H = 64 Hz, Me3SnCl), 0.22 (s, 2JSn,H = 55 Hz, SnMe3 of 1). 31P{1H} NMR (202.5 MHz, acetone-d6, 25 °C): δ −16.4 (s, 38%, 1), − 12.8 (br, 40%, 5), 26.6 (s, 1JHg,P = 4960 Hz, 22%, 2·HgCl2). Another 0.5 equiv of HgCl2 was added, and the H


Article

Organometallics

P{1H} NMR (202.5 MHz, acetone-d6, 25 °C): δ 35.3 (s, 1JHg,P = 7560 Hz). After further standing at RT an orange solid crystallized out. The crystals were collected and characterized by single-crystal Xray diffraction analysis, revealing the formation of the complex 6· HgCl2. Yield: 5.5 mg (87%). The purity of 6·HgCl2 (>97%) was established by elemental analysis. High-resolution MALDI-TOF MS (positive mode, anthracene): m/z 605.9855 ([M − HgCl2]+, calcd for 12 C 2 2 1 H 1 8 3 5 Cl 5 6 Fe 1 1 9 Hg 3 1 P 605.9880). Anal. Calcd for C22H18Cl3FeHg2P: C, 30.14; H, 2.07. Found: C, 30.53; H, 2.03. The product proved to be insoluble in nonpolar or moderately polar organic solvents, and in DMSO further reactions occurred, leading to unidentified species. Reaction of 5 with 1 equiv of HgCl2 in Acetone. A solution of 5 (pS isomer, 21.1 mg, 0.038 mmol) in acetone-d6 (1 mL) was added to HgCl2 (10.4 mg, 0.038 mmol, 1 equiv). The solution was well mixed to ensure complete dissolution of HgCl2 and then transferred to an NMR tube. The 1H and 31P NMR spectra of the sample were recorded after 1/2 h. 1H NMR (499.9 MHz, acetone-d6, 25 °C; only major signals are assigned): δ 7.89 (br m, 2H, Ph), 7.76 (m, 1H, Ph), 7.70 (m, 3H, Ph), 7.66 (br m, 2H, Ph), 7.57 (br m, 2H, Ph), 4.99 (nr, 1H, Cp of 5·HgCl2), 4.87 (nr, 1H, Cp of 5·HgCl2), 4.62 (s, 5H, free Cp of 5·HgCl2), 4.03 (nr, 1H, Cp of 5·HgCl2), 0.67 (s, 2JSn,H = 71 Hz, Me2Sn of 5·HgCl2). 31P{1H} NMR (202.5 MHz, acetone-d6, 25 °C): δ 31.1 (s, 1JHg,P = 7410 Hz, 5·HgCl2). The sample was kept at room temperature for 24 h and analyzed again by 1H and 31P NMR. 31 1 P{ H} NMR (202.5 MHz, acetone-d6, 25 °C): δ 30.7 (br s, 1JHg,P = 5038 Hz, 5·HgCl2, 47%), 27.3 (br s, 2·HgCl2, 35%). After standing for a few days, a yellow solid crystallized out, which was separated and dried under vacuum. Determination of the unit cell using X-ray diffraction analysis unambiguously confirmed the formation of 2· HgCl2. Yield: 0.015 g (65%). Reaction of 6·HgCl2 with an Excess of 1. In an NMR tube containing 6·HgCl2 (11.0 mg, 0.0126 mmol, 1 equiv) was placed an excess of 1 (8.1 mg, 0.0151 mmol, 1.2 equivs) in acetone-d6 (1 mL). The mixture was heated at 50 °C for 24 h, whereupon an insoluble material formed. After the sample was cooled to room temperature, CH2Cl2 (2 mL) was added, leading to a homogeneous solution. All volatile components were removed under vacuum, and the residue was redissolved in CD2Cl2 (0.9 mL) and investigated by NMR, revealing the formation of 2·HgCl2. 1H NMR (599.9 MHz, CD2Cl2, 25 °C): δ 7.84 (br m, 4H, Ph), 7.72 (br m, 4H, Ph), 7.60 (br m, 2H, Ph), 7.45 (br m, 10H, Ph), 4.73 (nr, 2H, Cp), 4.59 (nr, 2H, Cp), 4.30 (nr, 2H, Cp), 3.92 (s, 10H, free Cp). 31P{1H} NMR (202.5 MHz, CD2Cl2, 25 °C): δ 28.9 (82%, 2·HgCl2), −16.4 (11%, 1). The residue was then recrystallized from CH2Cl2/decane by partial solvent evaporation at room temperature to give 2·HgCl2 (12.2 mg, 80%) as a yellow crystalline solid. 31

(m, 2H, Ph), 7.51 (m, 3H, Ph), 7.28 (m, 3H, Ph), 7.13 (m, 2H, Ph), 4.71 (pt, 3JH,H = 2.0 Hz, 1H, Cp), 4.59 (nr, 1H, Cp), 4.20 (nr, 1H, Cp), 4.09 (s, 5H, free Cp), 0.98 (s, 2JHg,H = 211 Hz, 9H, MeHgCl), 0.79 (br, 2JSn,H = 63 Hz, SnMe2Cl). 1H NMR (499.9 MHz, acetoned6, − 50 °C): δ 7.80 (m, 2H, Ph), 7.65 (m, 3H, Ph), 7.49 (m, 5H, Ph), 5.01 (nr, 1H, Cp), 4.77 (nr, 1H, Cp), 4.34 (s, 5H, free Cp), 3.92 (nr, 1H, Cp), 0.96 (s, 9H, MeHgCl), 0.66 (br, SnMe2Cl). 13C{1H} NMR (150.8 MHz, CDCl3, 25 °C): δ 139.3 (nr, i-Ph), 136.6 (nr, iPh), 135.0 (d, 2JC,P = 19.3 Hz, o-Ph), 132.4 (d, 2JC,P = 16.4 Hz, o-Ph), 130.1 (s, p-Ph), 128.8 (s, p-Ph), 128.7 (d, 3JC,P = 8.1 Hz m-Ph), 128.6 (d, 3JC,P = 6.9 Hz, m-Ph), 81.6 (d, 1JC,P = 35.7 Hz, i-Cp-Sn), 81.3 (d, 1 JC,P = 83.8 Hz, i-Cp-P), 78.0 (d, JC,P = 13.7 Hz, Cp), 74.3 (s, Cp), 74.1 (s, Cp), 69.9 (s, free-Cp), 8.2 (s, MeHgCl), 2.1 (br s, 1JC,Sn = 423 Hz, SnMe2Cl). 31P{1H} NMR (202.5 MHz, CDCl3, 25 °C): δ −10.9 (s). 31P{1H} NMR (202.5 MHz, acetone-d6, 25 °C): δ −13.8 (s). 31 1 P{ H} NMR (202.5 MHz, acetone-d6, −50 °C): δ 34.3 (br). 119 Sn{1H} NMR (223.6 MHz, CDCl3, 25 °C): δ 112.3 (br s, υ1/2 = 125 Hz). Anal. Calcd for C25H27Cl2FeHgPSn: C, 37.32; H, 3.38. Found: C, 37.47; H, 3.27. Reaction of (pS)-1 with 1 equiv of HgCl2 in Acetone-d6 followed by VT 1H and 31P NMR. In an NMR tube were mixed acetone-d6 (1 mL) and HgCl2 (1.9 mg, 0.0070 mmol), and the mixture was frozen with liquid nitrogen. Then, a solution of 1 (pS isomer, 3.7 mg, 0.0069 mmol, 1 equiv) in acetone-d6 (1 mL) was added on top and immediately frozen. The sample was mixed in an acetone/dry ice bath and then quickly introduced into an NMR spectrometer precooled to −50 °C. The reaction was followed by VT 1 H and 31P NMR while the sample was gradually warmed to room temperature. Data at −50 °C are as follows. 1H NMR (499.9 MHz, acetone-d6; only characteristic signals are assigned): δ 1.20 (br, Me2SnCl2), 0.91 (br, MeHgCl), 0.61 (br, SnMe2Cl of 5), 0.59 (br, Me3SnCl), 0.14 (br, 2JSn,H = 56 Hz, SnMe3 of 1). 31P{1H} NMR (202.5 MHz, acetone-d6): δ 32.1 (br), 36.7 (br). At −30 °C: 1H NMR (499.9 MHz, acetone-d6; only characteristic signals are assigned): δ 1.23 (br, Me2SnCl2), 0.93 (br, MeHgCl), 0.62 (br, SnMe2Cl of 5), 0.60 (br, Me3SnCl), 0.14 (br s, 2JSn,H = 56 Hz, SnMe3 of 1). 31P{1H} NMR (202.5 MHz, acetone-d6): δ 33.0 (br), 37.2 (br). Data at −10 °C are as follows. 1H NMR (499.9 MHz, acetone-d6; only characteristic signals are assigned): δ 1.23 (br, Me2SnCl2), 0.94 (br, MeHgCl), 0.63 (br, 2JSn,H = 71.5 Hz, SnMe2Cl of 5), 0.60 (br, 2 JSn,H = 63.5 Hz, Me3SnCl), 0.15 (br s, 2JSn,H = 57 Hz, SnMe3 of 1). 31 1 P{ H} NMR (202.5 MHz, acetone-d6): δ 33.7 (br), 36.5 (br). At 25 °C: 1H NMR (499.9 MHz, acetone-d6; only characteristic signals are assigned): δ 1.22 (s, 2JSn,H = 86 Hz, Me2SnCl2), 0.98 (br, MeHgCl), 0.62 (br, 2JSn,H = 63.5 Hz, Me3SnCl), 0.13 (br s, SnMe3 of 1). 31P{1H} NMR (202.5 MHz, acetone-d6): δ 35.9 (br), 38.3 (br). After the sample stood overnight at room temperature, an orange solid crystallized out, which was confirmed by NMR analysis as 2·HgCl2. The supernatant was also analyzed. 1H NMR (499.9 MHz, acetoned6; only characteristic signals are assigned): δ 1.18 (s, 2JSn,H = 87 Hz, Me2SnCl2), 0.97 (s, 2JHg,H = 213 Hz, MeHgCl), 0.61 (s, 2JSn,H = 65 Hz, Me3SnCl). Reaction of (pS)-5 with 2 equiv of HgCl2 in Acetone-d6 followed by VT 1H and 31P NMR: Isolation of HgCl2 Complex of Chloromercurio Diphenylphosphinoferrocene (6·HgCl2). In an NMR tube were mixed acetone-d6 (0.1 mL) and HgCl2 (3.9 mg, 0.014 mmol), and the mixture was frozen with liquid nitrogen. Then, a solution of 5 (pS isomer, 4.0 mg, 0.0072 mmol, 0.5 equiv) in acetone-d6 (0.1 mL) was added on top and immediately frozen. The sample was mixed in an acetone/dry ice bath and then quickly introduced into an NMR spectrometer precooled to −50 °C. Data for 5·HgCl2 are as follows. 1H NMR (499.9 MHz, acetone-d6, −50 °C): δ 4.93 (nr, Cp), 4.87 (nr, Cp), 4.60 (s, free Cp), 3.91 (nr, Cp), 0.63 (s, 2 JSn,H = 73 Hz, SnMe2Cl). 31P{1H} NMR (202.5 MHz, acetone-d6, − 50 °C): δ 30.1 (s, 1JHg,P = 7560 Hz). A similar reaction was performed at RT and NMR data were acquired after 30 min. Data for 6·HgCl2 are as follows. 1H NMR (599.9 MHz, acetone-d6, 25 °C): δ 7.86 (m, 2H, Ph), 7.72−7.62 (m, 8H, Ph), 4.98 (nr, Cp), 4.93 (nr, Cp), 4.71 (nr, Cp), 4.52 (s, free Cp), 1.22 (s, 2JSn,H = 82/85 Hz, Me2SnCl2).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00862. NMR and MS characterization and X-ray crystallographic data (PDF) Accession Codes

CCDC 1880976−1880979 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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Corresponding Author

*E-mail for F.J.: [email protected]. I


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Organometallics ORCID

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Roger A. Lalancette: 0000-0002-3470-532X Frieder Jäkle: 0000-0001-8031-9254 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.C.T.K. thanks the Deutsche Forschungsgemeinschaft (DFG) for a postdoctoral fellowship. A 500 MHz NMR spectrometer used in these studies was purchased with support from the NSF-MRI program (1229030) and Rutgers University. The Xray diffractometer was purchased with support from the NSFCRIF program (0443538) and Rutgers University (Academic Excellence Fund).

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REFERENCES

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Hg Exchange Reaction of 1,2-Fc(PPh2)

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