Article pubs.acs.org/IC
A Modern Twist to a Classic Synthetic Route: Ph3Bi-Based Redox Transmetalation Protolysis (RTP) for the Preparation of Barium Metalorganic Species Yuriko Takahashi,† Anna O’Brien,‡ Glen B. Deacon,§ Philip C. Andrews,§ Melanie Wolf,† Ana Torvisco,† Miriam M. Gillett-Kunnath,† and Karin Ruhlandt-Senge*,† †
Department of Chemistry, 1-014 Center for Science and Technology, Syracuse University, Syracuse, New York 13244-4100, United States ‡ Department of Chemistry and Physics, Le Moyne College, Syracuse, New York 13214, United States § School of Chemistry, Monash University, Clayton, Melbourne, Victoria 3800, Australia S Supporting Information *
ABSTRACT: This paper reports advances in redox transmetalation/protolysis (RTP) utilizing the readily available Ph3Bi for the synthesis of a series of barium metal-organic species. On the basis of easily available starting materials, an easy one-pot procedure, and workup, we have obtained BaL2 compounds (L = bis(trimethylsilyl)amide, phenyl(trimethylsilyl)amide, pentamethylcyclopentadienide, fluorenide, 2,6-diisopropylphenolate, and 3,5-diphenylpyrazolate) quantitatively by sonication of an excess of barium metal with triphenylbismuth and HL in perdeuterotetrahydrofuran, as established by NMR measurements. Rates of conversion are affected by both pKa and bulk of HL. Competition occurs from direct reaction of Ba with HL, thereby enhancing the overall conversion, the effect being pronounced for the less bulky and more acidic ligands. Overall, the method significantly adds to the synthetic armory for barium metal-organic/organometallic compounds.
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INTRODUCTION Over the past decade, applications in fields as diverse as synthetic,2−6 catalytic,7−13 polymer,14−18 and materials chemistry19−23 have provoked increased attention to the organometallic chemistry of the heavy alkaline earth metals. A selection of alkaline earth metal sources including metal halides (salt metathesis) and metal amides (transamination) provide access to a wide array of alkaline earth metal based species including amides,24−27 pyrazolates,24,25 and cyclopentadienides,28,29,31−38 as delineated in several review articles.40,41 Extensive effort has been devoted toward the optimization of these synthetic pathways and the exploration of alternative routes to overcome various synthetic challenges.29,40−47 While remarkable progress has been made, significant restrictions and limitations including costly reagents, the prior synthesis of airsensitive reactants, work with condensed gases, and toxic starting materials, among other factors, limit synthetic access to the heavy alkaline earth metal compounds.43 An especially attractive source for the preparation of organometallic alkaline-earth compounds is based on the free metals, because of their ready availability and low cost compared with the purified alkaline earth dihalides and especially the more soluble diiodides.24,25,50−5253 Direct metalation (DM) involving the reaction of the metal with an acidic ligand remains the most straightforward synthetic © 2017 American Chemical Society
strategy, but it is limited to quite acidic ligand systems and/ or an elevated temperature syntheses.46,54,55 In general, for metal-based reactions, the limiting factor is surface area. Further, DM typically requires an oxide-free metal surface and the activation of the metal.47,56 This can be accomplished by the addition of mercury for amalgamation,57,58 the reduction of the metal iodides with potassium metal or lithium naphthalenide,59−61 the vacuum distillation of the metals between 850 and 1050 °C,62−64 co-condensation of the metals, or the solvation of the metals in anhydrous liquid ammonia.65−68 However, many of the methods are nontrivial, requiring either toxic reagents (Hg and its organometallic derivatives), high temperature, prior synthesis of highly airsensitive reagents, additional purification steps such as the removal of KI or naphthalene, or other special conditions. Consistent with the increased reactivity of the heavier alkaline earth metals, DM works best for the heavier metals, especially barium. The lighter congener strontium reacts significantly slower and requires extended reflux conditions, a trend further amplified for calcium. In fact, DM for calcium is not useful, unless highly acidic ligands are utilized. As a result, DM involving barium has been successful for the synthesis of Received: March 24, 2017 Published: September 21, 2017 11480
DOI: 10.1021/acs.inorgchem.7b00764 Inorg. Chem. 2017, 56, 11480−11489
Article
Inorganic Chemistry Table 1. pKa, StCN Values of Ligands Used and t50%conv for Each Reactiona
a
All reactions were repeated several times and slowly monitored. bAverage 50% conversion time for the simultaneous reactions of RTP and direct metalation. Replicate reactions were conducted; averages reported here. cpKa and StCN values were obtained from several methods described in multiple references. dEstimated value. The substrate acidity of H2N(Ph) (pKa = 30.7), HN(Ph)2 (pKa = 25.93), and HN(SiMe3)2 (pKa = 26) and that of the secondary amines with two phenyl rings (HN(Ph)2) or two trialkylsilyl groups HN(SiMe3)2 (pKa 26) suggest that the pKa of a phenyl substituted silyl amine such as HN(Ph)(SiMe3) is between 26 and 30. eEstimated value. NPh2− has an StCN value of 1.7948 and the StCN value of N(SiMe3)2− is 2.17. Thus, the StCN of HN(Ph)(SiMe3) is around 2.00.
barium cyclopentadienide, but this work cannot be extended to the lighter metals. Likewise, barium hexamethyldisilazide may be prepared by DM using ammonia activation; but again, this route is not amendable to the lighter congeners.42 DM has also been used to prepare other amides and aryloxides,45,55 but organometallic examples remain rare.41,44,45 Alternative, but less explored, activation methods to supply metals suitable for DM rely on ultrasonication as the increase of the metals’ surface area is a key factor in this procedure.69−72 An alternative, metal-based synthetic methodology is redox transmetalation/protolysis (RTP) involving a reaction between metal filings and a weakly acidic ligand in the presence of a transmetalation agent. This method has been used extensively for the preparation of rare earth species,73−75 along with selected heavy alkaline earth compounds.27,29,43,76,77 Classical RTP reactions utilize an aryl (C6H5 or C6F5) mercurial as the redox transmetalation agent.27,29,73,75,78,79 The transmetalation agent, HgR2 (R = C5H5, C6F6), is considered to initially react with the metallic element to form a highly reactive diaryl intermediate, which then undergoes protolysis with the acid RH, to form the target compound under liberation of elemental mercury and benzene or pentafluorobenzene (eq 1).29,73,80 Ae(s) + HgR′2 → AeR′2 + Hg(s)
A major drawback of this route is the acute toxicity of mercury and its reagents. The desire to replace the organomercurial with a more benign transmetalation agent has been the major motivation of our studies. Our group has previously identified the alternative, environmentally friendly redox transmetalation agent Ph3Bi (as compared to Bi(C6F5)3 and Bi(2-Naph)3; see the Supporting Information), affording calcium, strontium, and barium bis(trimethylsilyl)amide in high yields and purity in a straightforward manner.82,87 Ph3Bi-based RTP is made possible by the significant negative redox potential of the s-block metals as compared with the lanthanides,83,84 allowing the use of a redox transmetalating agent with a less positive standard redox potential than that of the organomercurials (E° ≈ 0.851 V (HgII), E° ≈ 0.308 V (BiIII)).85 Further benefits of Ph3Bi-based RTP include (1) the reduced toxicity of Ph3Bi, as compared to the classic organomercurial (LD50 Ph3Bi: 180 g kg−1 (dog, oral); LD50 HgPh2: 50−400 mg kg−1 (rat, oral)),86 and (2) the inexpensive and commercial availability of the reagent. The compound is also air- and moisture-stable. Just as HgR2-based RTP requires the activation of the metals, we have shown that metal activation is essential for the Ph3Bibased RTP route. Specifically, we found that the Ph3Bi-based RTP synthesis of hexamethyldisilazides required extended reaction times when metal pieces, Ph3Bi, and ligand were combined under reflux conditions.87 This reaction afforded low to moderate yields for strontium, slightly increased yields for barium, and no product formation for calcium. Employment of metal filings to increase the metal surface area in conjunction with ultrasound allowed the preparation of calcium, strontium, and barium amides in short reaction times and excellent yields,82,87 demonstrating that the increase and activation of the surface area by ultrasound is a simple, inexpensive, effective, and environmentally friendly alkaline earth metal activation technique. In order to establish Ph3Bi-based RTP as an attractive synthetic route to alkaline earth metalorganic compounds, it is essential to explore the reaction variables. In our initial investigation,82,87 we examined the roles of reagent concen-
(1)
AeR′2 + 2RH → AeR 2 + 2R′H
where Ae = Ca, Sr, Ba R′ = C6H5, C6F5 R = cyclopentadienide,75 pyrazolate,24,27,29,73 formamidinate,77 triazenide,81 aryoxide43,73,74,76 The difference in acidity between the resulting benzene (pKa ∼ 43) or pentafluorobenzene (pKa ∼ 26) and the respective ligands drives the second reaction step. The process is conducted as a one-pot procedure in the presence of a polar solvent. The workup is facile. RTP has been most widely used for the synthesis of lanthanoid complexes73,74,80 but also for the preparation of alkaline earth pyrazolates, formamidinates, cyclopentadienides, triazenides, and aryloxides.24,27,43,75−77,81 11481
DOI: 10.1021/acs.inorgchem.7b00764 Inorg. Chem. 2017, 56, 11480−11489
Article
Inorganic Chemistry
Figure 1. 1H NMR spectra of the reaction of Ba with Ph3Bi and HN(SiMe3)2 in D8-THF. Spectra were recorded at regular time intervals. Spectra shown here (from bottom to top) were taken after 0, 2, and 10 h, representing the overall course of the reaction. Spectra taken at other time intervals are consistent with the reported results. The peaks are labeled as follows: Δ: Ph3Bi; ○: HN(SiMe3)2; ☆: {N(SiMe3)2}−; ◊: C6H6; ◇: C6H12 (cyclohexane, internal standard).
had been prepared previously by other synthetic procedures,24,26,29,35,76,88,89 and NMR data were accessible for comparison.
tration and solvent polarity. Here, we turn our focus to a range of ligand systems to demonstrate the versatility and also map some limitations of Ph3Bi-based RTP. Since reactions with barium proceed the fastest, we here focus on the reactions with barium metal, summarized in eq 2. A possible explanation for the faster conversion rates for barium is that the Ba metal can be activated by finely divided precipitated Bi through surface abrasion.83 3Ba(s) + 2Ph3Bi → 3BaPh 2 + 2Bi(s)
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MATERIALS AND GENERAL PROCEDURES
All reactions were carried out under strict inert gas conditions using a glovebox and modified Schlenk techniques. Commercially available 1,1,1,3,3,3-hexamethyldisilazane (HN(SiMe3)2), N-(trimethylsilyl)aniline (HN(Ph)(SiMe3), 1,2,3,4,5-pentamethylcyclopentadiene (HCp*), and cyclohexane were dried over CaH2 and distilled prior to use. 2,6-Diisopropylphenol (HO(dipp)) was obtained from a commercial source and purified by azeotropic distillation using benzene. Fluorine (Hflu) was obtained commercially and sublimed prior to use, while 3,5-diphenylpyrazole (Ph2pzH) was synthesized according to literature procedures.90 Deuterated tetrahydrofuran (D8THF) was obtained from a commercial source and dried over sodium prior to use, followed by pipetting into the NMR tube. 1H NMR spectra were recorded with a Bruker DPX-300 spectrometer at 25 °C with [D]8-THF as solvent and referenced to residual solvent peaks. The reaction mixtures were sonicated using a Bransonic Ultrasonic Cleaner 2510. General Procedure for 1H NMR Studies. Each one-pot reaction mixture was prepared in the glovebox (nitrogen atmosphere) to prevent exposure to moisture. 0.5 mmol (0.07 g) of barium filings, 0.15 mmol of Ph3Bi (0.07 g), and 0.45 mmol of the desired ligand (HL) were placed into a NMR tube. Anhydrous cyclohexane (0.02 g) as an internal integration standard and [D]8-THF (0.6 mL) were added to this reaction mixture and sealed with a J-Young tap. The resulting suspensions were sonicated in the NMR tubes and monitored by 1H NMR spectroscopy at regular time intervals until HL was consumed. 1H NMR peaks representing HL, Ph3Bi, and deprotonated-
(2)
BaPh 2 + 2RH → BaR 2 + 2PhH(l)
where RH = amine, cyclopentadiene, pyrazole, phenol We have chosen six ligands with varying degrees of acidity (pKa) and steric bulk (StCN: steric coordination number) to provide insight into the influence of acidity and steric bulk on the RTP reaction (Table 1). One factor guiding our ligand choice was pKa. As outlined above, ligands require a pKa value lower than that of benzene (pKa ∼ 43) to be a candidate for metalation by the Ph3Bi-based RTP route. Another factor was selecting ligands for which the steric bulk had been quantified. One method to quantify ligand bulk is the use of steric coordination numbers (StCN),48 defined as “the ratio of the solid angle comprising the van der Waals’ spheres of the atoms of the ligand relative to that of Cl− as 1.0, when placed at a typical mean distance from the central metal atom.”48 Furthermore, we chose ligands whose barium compounds 11482
DOI: 10.1021/acs.inorgchem.7b00764 Inorg. Chem. 2017, 56, 11480−11489
Article
Inorganic Chemistry
Figure 2. Net % Conversions of free ligand (HN(SiMe3)2): -●-, formation of deprotonated ligand ({N(SiMe3)2})−: -◆-, and Ph3Bi: -□- during the course of the reaction at different time intervals. The values were averaged across the duplicate runs; error bars represent the standard deviation of averaged values. ligand (L−) were integrated against the internal standard cyclohexane, allowing the monitoring of reagent and product concentration. Each experiment was repeated three to five times to test for reproducibility at a given time point. Due to the presence of excess metal, shimming was often made difficult, and therefore, some peaks were poorly resolved or broad in these NMR studies. Determination of Percent Conversion Values from NMR Data. NMR scale studies were conducted to monitor the progress of the RTP reactions. Reagent, product, and Ph3Bi integration values relative to the inert internal standard (cyclohexane) were measured at regular time intervals. Percentage conversions were calculated on the basis that the integration value of a representative peak of HL or Ph3Bi at time = 0 corresponded to 100%. The integration value of the peak of deprotonated-ligand (L−) once it reached its maximum and constant values corresponded to 100% of the product. For each of the species monitored (HL, Ph3Bi, and L−), percent conversions were calculated at each time point by dividing the integration value for a representative peak by the original integration value (at the start of the reaction) for HL and Ph3Bi or by the final integration value (at the end of the reaction) for L−.
(☆ at −0.01 ppm) and benzene (◊ at 7.28 ppm) was observed, indicating the product formation. The % conversion values for each of the species (HL, Ph3Bi, deprotonated-L) were calculated, as explained earlier, at each time point. The averaged % conversion values for each of the three species are plotted as a function of time in Figure 2. As expected, based on the heterogeneous nature of the reaction,92,93 the standard deviation of each data point is significant, but the observed reaction trend is clear. Nevertheless, experiments to improve and verify reproducibility were undertaken. The largest variable in metal-based chemistry is metal surface area. In an attempt to correlate the effect of particle size/surface area on the overall reaction, metal filings were sifted through screens of various mesh size (
