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. 2026 Jan 20;65(4):2400–2409. doi: 10.1021/acs.inorgchem.5c05290

Selective Catalytic Styrene Dimerization through the Combined Action of a Strong Sn(IV)-Porphyrin Lewis Acid and a Weak Brønsted Base

Justin L Peterson 1, Arely M Guijarro 1, Timothy C Johnstone 1,*
PMCID: PMC12869503  PMID: 41556775

Abstract

Complexes of the form Sn­(TPP)­X2 (where TPP2– = tetraphenylporphyrinate and X– = Cl–, OTf–, ClO4 –, and NTf2 –) were investigated as catalysts for the activation of the nonpolar CC bond of styrene. Sn­(TPP)­(OTf)2 was able to dimerize styrene with 100% selectivity for 1,3-diphenyl-1-butene. Other catalysts based on p-block metals have been used to activate styrene in the past, but they produced mixtures of oligomerization products. Pd catalysts have been used to selectively yield 1,3-diphenyl-1-butene, and prior mechanistic studies suggest that those reactions proceed through a hydride transfer mechanism. In this present work, our data, particularly those from experiments with isotopically labeled molecules, suggest a mechanism that requires the combined action of the strong Lewis acidity of a transiently formed [Sn­(TPP)­(OTf)]+ and the weak Brønsted basicity of free OTf– to perform a rate limiting deprotonation step. The reaction does not proceed when X– = ClO4 – or NTf2 – suggesting that OTf– has a Brønsted basicity just strong enough to perform a deprotonation of an activated C–H bond while still being a weak enough Lewis base to easily dissociate from the Sn center.

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Introduction

Metalloporphyrin and metalloporphyrinoid complexes have played many important roles throughout the history of homogeneous catalysis. , Much of this work has drawn inspiration from the enzymatic catalysis of heme-containing proteins, but their reactivity has been extended far beyond that found in natural systems. , Investigation into the reactivity and properties of these compounds helped establish the fundamental organometallic chemistry of metalloporphyrin compounds, which has since been used to support efforts ranging from the development of elegant supramolecular catalysts to the directed evolution and rational design of biological macromolecules with new-to-nature reactivity and function. − This chemistry has largely focused on transition-metal porphyrin complexes, but porphyrin and porphyrinoid compounds with central p-block elements have been investigated as well. − Although there has been interest in getting compounds of p-block elements to exhibit chemistry characteristic of transition-metal complexes, the differences in the chemistry of these main-group elements can also serve as a rich source of chemical diversity. Sn­(IV) porphyrin complexes, in particular, have received attention because of their ease of synthesis, stability, and well-behaved photophysics and reactivity. Sn­(IV) complexes of 5,10,15,20-tetraphenylporphyrinate (TPP2–) are readily prepared by heating SnCl2 with H2TPP in the presence of air and a Brønsted base, and subsequently substituting the chloride ligands. Sn­(TPP)­X2 complexes where X– is a weakly coordinating anion, would be expected to exhibit Lewis acidic behavior and complexes where X– = BF4 –, OTf–, and ClO4 – have all demonstrated an ability to catalyze reactions that proceed via attack on a polar bond by further polarizing it. − As expected, in these examples, the more weakly coordinating the X– ligand of the Sn­(TPP)­X2 complex, the more potently the complex catalyzed the reaction.

As we report here, while investigating the ability of main-group porphyrin complexes to activate less polar bonds, we tested the ability of Sn­(TPP)­X2 (where X– = Cl–, OTf–, ClO4 –, and NTf2 –) to catalyze the polymerization of styrene. We were surprised by two outcomes of the initial investigation. First, Sn­(TPP)­(OTf)2 smoothly facilitates the dimerization of styrene but Sn­(TPP)­(NTf2)2 does not, even though NTf2 – is a more weakly coordinating ligand than OTf–. Second, the sole product of the reaction catalyzed by Sn­(TPP)­(OTf)2 was that of head-to-tail dimerization: 1,3-diphenyl-1-butene. In the past, such selectivity has typically been achieved using catalysts containing Pd or other noble metals, , although some examples of such selectivity with base transition metal catalysts are known. , In contrast, the use of main-group Lewis acids in such reactions has afforded mixtures of isomers or uncontrolled polymerization, if not altogether different reactivity (e.g., Friedel–Crafts alkylation). ,

We present here the results of our investigation into the origins of these unexpected outcomes, which indicate that the reaction proceeds through the combined action of the Lewis acid and Brønsted base that are generated upon dissociation of X– from Sn­(TPP)­X2. The released X– needs to be weakly Lewis basic enough to dissociate readily from the Sn­(IV) center, but strongly Brønsted basic enough to perform a rate-limiting deprotonation. This combined reactivity of the acid and base, which is reminiscent of the FLP-like reactivity that has been observed for stable acid–base adducts that nevertheless must access the dissociated state, presents a rich area for further development in main-group catalyst design.

Results

Synthesis of Sn-Porphyrin Complexes

5,10,15,20-Tetraphenylporphyrin (H2TPP) was readily metalated using a Sn­(II) precursor followed by refluxing in open air to oxidize the chelated Sn­(II) center to Sn­(IV) (Scheme ). The direct product of the reaction features a mixture of axial chloride and hydroxide ligands, but can be converted entirely to Sn­(TPP)­Cl2 by treatment with HCl(aq). The 1H NMR spectrum of the product is consistent with the D 4 symmetry expected of the complex in solution and the β pyrrole resonance at 9.21 ppm shows clear Sn satellites. Coupling to both 117Sn (I = 1/2) and 119Sn (I = 1/2) nuclei is expected but two distinct sets of satellites are not resolved and a single doublet with 4 J SnH = 15 Hz is observed. The 119Sn­{1H} NMR spectrum shows a single singlet at −586 ppm.

1. General Approach to the Synthesis of Lewis Acidic Sn­(IV) Porphyrin Complexes.

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Suspension of a slight excess of AgOTf (2.5 equiv) in a DCM solution of Sn­(TPP)­Cl2 provided access to Sn­(TPP)­(OTf)2. There was no change in the apparent symmetry of the complex, but the 1H NMR resonances exhibited a general downfield shift. The coupling of the β pyrrole protons to the Sn nuclei increased slightly to 20 Hz. The 119Sn NMR signal shifted upfield to −628 ppm. A similar strategy was used to access Sn­(TPP)­(ClO4)2 and Sn­(TPP)­(NTf2)2. The 4 J SnH and 119Sn chemical shifts were broadly similar across all three of the complexes featuring weakly coordinating axial ligands, with all three exhibiting 119Sn chemical shifts upfield of Sn­(TPP)­Cl2. These variations in δ with ligand pK aH has been described previously, as has the anomalous behavior of the halide-bound species in this regard.

X-ray Crystallography

The structures of Sn­(TPP)­X2 for X– = Cl–, OTf–, ClO4 –, and NTf2 –, were determined by single-crystal X-ray diffraction and, in all cases, the Sn atom is hexacoordinate with both X– ligands in the inner coordination sphere. In the structure of Sn­(TPP)­Cl2 (Figure S33), which has been previously described, the molecule resides on a special position with site symmetry 4/m, allowing it to assume its full potential D 4h point-group symmetry in the crystal. The Sn atom lies rigorously in the plane of the porphyrin, with an Sn–N distance of 2.097(2) Å and an Sn–Cl distance of 2.4218(8) Å.

Sn­(TPP)­(OTf)2 crystallizes in space group P1̅ with Z = 2 but nevertheless exhibits crystallographically required centrosymmetry because the asymmetric unit features two half molecules, each centered on a crystallographically distinct inversion center (Figure a). The molecules have similar core structures but can be readily confirmed to be crystallographically distinct because of the differential canting of their meso phenyl groups. The Sn–O bond lengths are 2.1536(12) and 2.1478(12) Å and the Sn–N distances range from 2.0704(14) to 2.0814(14) Å. Both Sn­(TPP)­(ClO4)2 (Figure b) and Sn­(TPP)­(NTf2)2 (Figure c) also crystallize on crystallographic inversion centers in their respective space groups with similar Sn–O and Sn–N bond lengths. Given the 1̅ site symmetry in each case, the Sn atom rigorously lies in the plane of the four N atoms for each. A higher-temperature structure of Sn­(TPP)­(ClO4)2 was reported previously; no chemically significant differences exist between the previous structure and that reported here.

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Thermal ellipsoid plots (50% probability) of (a) Sn­(TPP)­(OTf)2, (b) Sn­(TPP)­(ClO4)2, and (c) Sn­(TPP)­(NTf2)2. In each case, the depicted molecule resides on a crystallographic inversion center. H atoms are omitted for clarity.

Solution-Phase Behavior of Sn­(TPP)­X2

Although the solution-phase spectroscopic data above are consistent with stable six-coordinate complexes of the type revealed crystallographically to be present in the solid state, they would also be consistent with dynamically interconverting mixtures of Sn­(TPP)­X2 and [Sn­(TPP)­X]­X (Scheme ). Addition of excess (NBu4)­(OTf) to a solution of Sn­(TPP)­(OTf)2 did not result in any significant change to the NMR signals, suggesting that, if there was an equilibrium of the type shown in Scheme , it lies far to the left-hand side. We did observe, however, that addition of 2 equiv of (NBu4)Cl to either Sn­(TPP)­(OTf)2, Sn­(TPP)­(ClO4)2, or Sn­(TPP)­(NTf2)2 resulted in rapid formation of Sn­(TPP)­Cl2, which was confirmed spectroscopically (Figures S16–S18). Consistent with the greater basicity of OTf– as compared to ClO4 – and NTf2 –, the transformation of Sn­(TPP)­(ClO4)2 and Sn­(TPP)­(NTf2)2 to Sn­(TPP)­Cl2 was very fast (full conversion within 10 min), but the conversion of Sn­(TPP)­(OTf)2 to Sn­(TPP)­Cl2 took longer (full conversion within 2.5 h). Additionally, combination of Sn­(TPP)­(NTf2)2 and (NBu4)­OTf resulted in a mixture of Sn­(TPP)­(NTf2)2, Sn­(TPP)­(OTf)2, and Sn­(TPP)­(OTf)­(NTf2), further highlighting the lability of these weakly coordinated ligands and their relative strengths of binding (Figure S19). These results collectively suggest that the right-hand side of the equilibrium in Scheme is readily accessible for X– = OTf–, ClO4 –, and NTf2 –.

2. Equilibrium to Access Electrophilic Cations of the Type [Sn­(TPP)­X]+, where X– is a Weakly Coordinating Anion.

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Investigation of Lewis Acidity

As a preliminary investigation of the Lewis acidity of these Sn­(IV) complexes, we investigated their reaction with the weak Lewis base OPPh3 (Figure S20). Sn­(TPP)­(OTf)2 (10 equiv) produced an 8.3 ppm downfield shift in the 31P NMR resonance of the phosphine oxide upon complexation. In the presence of 10 equiv of Sn­(TPP)­(ClO4)2 or Sn­(TPP)­(NTf2)2, the signal of OPPh3 was shifted downfield by an even greater amount (Δδ = 8.7 and 10.3 ppm, respectively). The Lewis acidities of the proposed [Sn­(TPP)­X]+ cations were further probed computationally. First, the free energy required to dissociate a single X– ligand from the Sn center in each of the complexes was calculated at the PBE0-D3BJ/def2-TZVPP-CPCM­(chloroform) level of theory. As expected, for all three complexes, an input of energy is required to dissociate one of the X– ligands, and the magnitude of this energy cost varied as ClO4 – > OTf– > NTf2 – (Table ). It should be noted that, although the donicity of the ligands systematically varies as OTf– > ClO4 – > NTf2 –, the electrophilicity of the Sn center varies as NTf2 – > ClO4 – > OTf– because of the variation in donation of the remaining X– ligand. The result of these competing trends is that OTf– and ClO4 – require a similar amount of energy for dissociation, while NTf2 – requires significantly less.

1. Calculated Lewis Acidity Metrics for the Studied Sn­(IV) Porphyrin Complexes .

  Sn–X Heterolysis Energy (kJ/mol) [Sn(TPP)X]+ FIA (kJ/mol)
Sn(TPP)(OTf)2 25.44 391
Sn(TPP)(ClO4)2 25.48 392
Sn(TPP)(NTf2)2 23.18 409
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Calculations performed at the PBE0-D3BJ/def2-TZVPP-CPCM­(chloroform) level of theory.

The Lewis acidity of the resulting [Sn­(TPP)­X]+ complexes, which we hypothesize will be able to function as active catalysts, were computationally assessed by determining their fluoride ion affinity (FIA) values (Table ). The solvent-corrected FIA values of all three complexes were approximately 400 kJ/mol, which places them well above the average value of 212 kJ/mol that has been calculated for a library of monocationic Lewis acids. This library included the cationic Sn species [SniPr3]+, which had a solvent-corrected FIA value of 263 kJ/mol. As expected, the FIA value for [Sn­(TPP)­(NTf2)]+ was the greatest of the three, in alignment with the comparatively more weakly donating nature of the NTf2 – ligand, resulting in a more electron deficient Sn­(IV) center. Alternatively, the variation in FIA can be understood by considering that the weaker Sn–O bond in [Sn­(TPP)­(NTf2)]+ affords a correspondingly lower-energy Sn–O antibonding orbital. Because this orbital would serve as the locus of Lewis acidity of the cation, a lowering of its energy would correspond to an increase in acidity.

Catalytic Dimerization of Styrene

Having confirmed the ability of the Sn­(TPP)­X2 complexes to form electrophilic [Sn­(TPP)­X]+ cations, we aimed to take advantage of their Lewis acidity to catalyze the polymerization/oligomerization of styrene. This reaction was chosen because main-group Lewis acids have previously been observed to facilitate such transformations, , and we envisioned that the modular porphyrin scaffold could be used to impart selectivity. For the initial proof-of-concept reactivity, however, the simple tetraphenylporphyrin complexes were used. When 10 mol % of Sn­(TPP)­(OTf)2 was combined with styrene at room temperature in CDCl3, we observed by 1H NMR spectroscopy the systematic consumption of the starting material and growth of a single new set of resonances corresponding to the formation of 1,3-diphenyl-1-butene. The clean formation of this single product was surprising, because the previously mentioned main-group catalysts had produced polymers or mixtures of various oligomeric products. , Transition-metal catalysts, typically containing Pd, are used to selectively catalyze formation of 1,3-diphenyl-1-butene from styrene. − In the reaction with Sn­(TPP)­(OTf)2, not only was a single product formed, but there were no signs of catalyst degradation over the course of the reaction. The catalyzed reaction proceeds readily at room temperature, but heating the reaction mixture to 50 °C resulted in the consumption of >90% of the styrene, with 1,3-diphenyl-1-butene as the sole product (Figure ; Table , entry 2). In the absence of catalyst, no reaction occurs under these conditions (Table , entry 3).

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Selective dimerization of styrene using Sn­(TPP)­(OTf)2 as the catalyst. Reaction progress is monitored by 1H NMR spectroscopy (500 MHz, CDCl3). Peaks corresponding to styrene, the catalyst, and 1,3-diphenyl-1-butene are marked with green circles, blue squares, and purple diamonds, respectively.

2. Catalytic Dimerization of Styrene to 1,3-Diphenyl-1-butene.

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Entry Catalyst Temp (°C) Time (h) Additional conditions Conversion (%)
1 Sn(TPP)(OTf)2 25 24   55
2 Sn(TPP)(OTf)2 50 24   91
3 none 50 24   0
4 Sn(TPP)(ClO4)2 50 24   0
5 Sn(TPP)(NTf2)2 50 24   0
6 Sn(TPP)Cl2 50 24   0
7 Sn(TPP)(OTf)2 25 24 Wet CDCl3 0
8 Sn(TPP)(OTf)2 25 24 30 mol % H2O at 10 min 19
9 Sn(TPP)(OTf)2 25 1 30 mol % H2O at 10 min 22
10 Sn(TPP)(OTf)2 25 1 30 mol % H2O at 30 min 33
11 Sn(TPP)(OTf)2 25 1 30 mol % H2O at 60 min 45
12 Sn(TPP)(OTf)2 25 24 20 mol % 2,6-lutidine 0
13 Sn(TPP)(OTf)2 25 24 20 mol % DIPEA 0
14 B(C6F5)3 25 24   0
15 B(C6F5)3 25 24 10 mol % (NBu4)OTf 0
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a

Conversion to 1,3-diphenyl-1-butene.

b

Polymerization.

Unlike Sn­(TPP)­(OTf)2, 10 mol % loading with either Sn­(TPP)­(NTf2)2 or Sn­(TPP)­(ClO4)2 did not produce any 1,3-diphenyl-1-butene over the course of 24 h at 50 °C (Table , entries 4, 5; Figures S14–S15). Not only was no 1,3-diphenyl-1-butene produced, but there were no changes in the 1H NMR spectra of the reactions containing either species, indicating that no transformation of the styrene to any product occurred. Similarly, when the reaction was performed with 10 mol % of Sn­(TPP)­Cl2, as expected, no transformation of the styrene occurred (Table , entry 6; Figure S15).

Mechanistic Studies: Hydrolysis and Brønsted Bases

To rule out the possibility that trace hydrolysis was generating HX that could act as the catalyst, the catalytic dimerization with Sn­(TPP)­(OTf)2 was carried out in “wet” CDCl3 that had not been dried over molecular sieves. The presence of additional water did not increase the rate of the reaction, but rather completely prevented conversion to the product (Table , entry 7; Figure S21). The reaction was repeated with a defined amount of water (30 mol % in the form of water-saturated CDCl3) added via syringe into a septum-sealed NMR tube containing a solution of the other reagents that had been prepared under inert conditions. The reaction was greatly inhibited although a small amount of product was formed (Table , entry 8; Figure S22). To confirm that this small amount of product was simply arising from the extent of reaction that proceeded between combination of the reagents, removal from the glovebox, and addition of the water-saturated CDCl3, we systematically increased the amount of time between the combination of catalyst and substrate, and the addition of water. The systematic increase in the amount of product formed following the increase in the amount of time before the addition of water indicated that reaction is inhibited upon addition of the water (Table , entries 9–11; Figure S24). Analogous water-addition experiments with Sn­(TPP)­(NTf2)2 and Sn­(TPP)­(ClO4)2 similarly yielded no product (Figure S23). In the case of Sn­(TPP)­(NTf2)2, addition of the water-saturated CDCl3 to the reaction mixture led to the rapid precipitation of red crystals. Single-crystal X-ray diffraction showed this material to be [Sn­(TPP)­(OH2)2]­(NTf2)2 (Figure ) and provides an explanation for the water-mediated abrogation of catalytic activity: the water molecules coordinate strongly to the Lewis acidic Sn­(IV) centers, preventing them from activating styrene.

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Thermal ellipsoid (50% probability level) plot of [Sn­(TPP)­(OH2)2]­(NTf2)2, which was formed upon addition of water-saturated chloroform to a chloroform solution of Sn­(TPP)­(NTf2)2. C-bound H atoms are omitted, and meso phenyl rings and bistriflimide CF3 groups are shown as sticks for clarity. O-bound H atoms are shown as spheres of arbitrary radius.

As a parallel control to rule out the possible role of adventitious hydrolysis generating Brønsted acid catalysts, we performed the catalytic dimerization of styrene with Sn­(TPP)­(OTf)2 in the presence of 20 mol % of the noncoordinating Brønsted base 2,6-lutidine. Unexpectedly, 2,6-lutidine completely prevented the dimerization reaction (Table , entry 12; Figure S25). This same result was also observed in the presence of 20 mol % of DIPEA (Table , entry 13; Figure S26). The experiments with controlled amounts of water indicate that the dimerization reaction is not being catalyzed by hydrolysis-generated Brønsted acids, but the impact of 2,6-lutidine/DIPEA indicated that an on-cycle protic intermediate is likely being generated during the catalytic reaction.

Comparative Reactivity with B­(C6F5 ) 3

To assess whether the Lewis acidity of [Sn­(TPP)­(OTf)]+ alone is sufficient to catalyze the dimerization, the reaction was performed with 10 mol % of the strong Lewis acid B­(C6F5)3 (Table , entry 14; Figures S26–S27). Under conditions identical to those of the reactions with Sn­(TPP)­(OTf)2, there was no conversion of the styrene. Performing the reaction with a combination of 10 mol % of B­(C6F5)3 and 10 mol % of (NBu4)­OTf, however, resulted in the rapid consumption of the alkene (Table , entry 15). In contrast to the well-defined product obtained with the Sn-porphyrin complex, NMR spectra of the B­(C6F5)3/(NBu4)­OTf reaction mixture indicated the formation of a dispersion of oligomers/polymers (Figures S28–S29).

Catalytic Dimerization of (β,β-2H2)­Styrene and (α-2H)­Styrene

Isotopic labeling studies were performed to follow the fate of the H atoms in the reaction. When the reaction was carried out with (β,β-2H2)­styrene, the only product formed was (2,4,4,4-2H4)­1,3-diphenyl-1-butene and when the reaction was carried out with (α-2H)­styrene, the only product formed was (1,3-2H2)­1,3-diphenyl-1-butene (Scheme , Figures S30–S31). The lack of any 1H/2H scrambling or partial 1H incorporation at sites that were otherwise deuterated further corroborates the absence of reactivity mediated by adventitious hydrolysis. Notably, the reaction proceeds at a comparable rate if styrene or (α-2H)­styrene is used, but is significantly slower if (β,β-2H2)­styrene is used (Figure ). Using either the initial rates of formation or the relative proportions of the products formed, the kinetic isotope effect is estimated to be k H/k D ≈ 2.5, which indicates that a C–H bond breaking step is rate-determining in this reaction.

3. Isotopic Labeling Studies Using Either (α-2H)­Styrene or (β,β-2H2)­Styrene Indicate That Deuterons Are Incorporated Only at the Positions Explicitly Shown in the Corresponding Final Products.

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Comparison of the catalytic dimerization of styrene, (α-2H)­styrene, and (β,β-2H2)­styrene catalyzed by 10 mol % of Sn­(TPP)­(OTf)2.

Discussion

The synthesis of the Sn­(TPP)­X2 (X– = OTf–, NTf2 –, ClO4 –) complexes proceeded readily from Sn­(TPP)­Cl2 via treatment with 2 equiv of the appropriate AgX salt. The crystallographically determined structures of these molecules reveal that they are six-coordinate with both X– ligands bound to the central Sn atom in the solid state. The solution-phase NMR spectroscopic characterization data for these compounds are consistent with these species remaining predominantly six-coordinate in solution but with a dynamic equilibrium of the type Sn­(TPP)­X2 ⇌ [Sn­(TPP)­X]+ + X–. Addition of (NBu4)Cl to solutions of these complexes results in their rapid conversion to Sn­(TPP)­Cl2, which is consistent with the right-hand side of this equilibrium being accessible. DFT calculations indicate that dissociation of the X– ligand requires a comparable amount of energy for all three complexes. Calculated FIA values for the three resulting [Sn­(TPP)­X]+ cations indicate that all three form strong Lewis acids.

It was, therefore, initially surprising that, when the ability of these molecules to catalyze the dimerization of styrene was assayed, (i) Sn­(TPP)­(OTf)2 was the only one that formed a product, (ii) only dimerization occurred, and (iii) 1,3-diphenyl-1-butene was the only isomer formed. Control reactions with B­(C6F5)3 and styrene indicated that a strong Lewis acid alone is not sufficient to induce reactivity under these conditions. Addition of both B­(C6F5)3 and the weak Brønsted base OTf–, however, resulted in rapid consumption of the styrene starting material, highlighting that the combined action of a strong Lewis acid and weak Brønsted base could bring about a reaction, albeit an uncontrolled one. Given this result, we hypothesized that the OTf– ligand released from the Sn­(TPP)­(OTf)2 complexes was not simply functioning as a spectator ion, but was participating in the reaction. Control reactions demonstrated that the non-nucleophilic Brønsted bases 2,6-lutidine and DIPEA abrogated the catalytic activity of the Sn­(TPP)­(OTf)2 complexes, which led us to suspect that the released OTf– was functioning as a Brønsted base. Although OTf– is a very weak base, the conjugate acid HOTf is consequently a strong Brønsted acid. The weak strength of OTf– would likely result in any deprotonation that it effected being the rate limiting step of the cycle. The distribution of deuterons in the products formed when either (α-2H)­styrene or (β,β-2H2)­styrene was used, the presence of a kinetic isotope effect with (β,β-2H2)­styrene, and the absence of a kinetic isotope effect with (α-2H)­styrene collectively indicate that a β C–H bond is broken during the rate-limiting step of the cycle. The correspondence between the Brønsted basicities of the axial ligands and the observed reactivity also supports a protonation/deprotonation process as opposed to a radical-mediated process.

We use these data collectively to propose the following mechanism (Scheme ). The complex Sn­(TPP)­(OTf)2 is in equilibrium with dissociated OTf– and [Sn­(TPP)­(OTf)]+. This Lewis acidic cation activates a styrene, with the formal positive charge being best stabilized at the benzylic α position. A second styrene molecule then attacks the activated α position on the catalyst-bound styrene, forming the new C–C bond. The released OTf– ligand then deprotonates the Sn-bound diphenylbutyl intermediate at one of the C–H bonds that was originally a β C–H on the styrene that came in to attack the initially activated styrene. The released OTf– is a very weak Brønsted base, which causes this step to be rate limiting, giving rise to the observed kinetic isotope effect when using (β,β-2H2)­styrene. The resultant strong acid HOTf is then able to protonate the C atom interacting with the Lewis acidic Sn-porphyrin complex to release the final product, 1,3-diphenyl-1-butene.

4. Proposed Mechanism for the Selective Dimerization of Styrene to 1,3-Diphenyl-1-butene Catalyzed by Sn­(TPP)­(OTf)2 .

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a Two H atoms are colored green to allow them to be followed through the reaction more easily.

As noted above, the influence of the axial ligands is complex. HOTf, HClO4, and HNTf2 all have strong Brønsted acidities, which is needed to protonate the Sn-bound intermediate to release product. But the conjugate base also needs to be sufficiently basic to deprotonate the activated C–H bond. Moreover, the more weakly coordinating (less basic) the conjugate base, the easier the initial ligand dissociation reaction will be, giving more ready access to the active Lewis acid [Sn­(TPP)­X]+. Given that the deprotonation step is rate-limiting, Sn­(TPP)­(OTf)2 is able to catalyze the reaction the most effectively despite accessing the right-hand side of the equilibrium in Scheme the least readily (based on computational thermochemical parameters) and forming the least Lewis acidic [Sn­(TPP)­X]+ cation upon axial ligand dissociation.

Conclusions

This series of experiments revealed that Sn­(TPP)­(OTf)2 can serve as a catalyst for the formation of 1,3-diphenyl-1-butene from styrene. In contrast to previous main-group catalysts, the reaction proceeds with 100% selectivity for this product, which is otherwise accessed using transition-metal catalysts that proceed through a hydride transfer mechanism. , Our work suggests that the presently investigated system relies not only on the Lewis acidity of a transiently formed [Sn­(TPP)­(OTf)]+ cation, but also on the Brønsted basicity of the released OTf– ligand. It is noteworthy that a single product is formed given the multiplicity of species generated when other main-group catalysts are used in this reaction. , We believe that the characteristic steric profile of the porphyrin ligand encourages the selective dimerization reaction, although more work remains to be done to explore the impact of this scaffold on Lewis acid catalysis. Moreover, the balance between the electrophilicity of the Lewis acidic Sn center and the Brønsted basic released ligand provides controlled reactivity. The ease with which the steric profile of porphyrin ligands can be tuned and the ease with which axial substituents can be varied suggest that this platform will serve as a productive foundation upon which to build future catalytic systems.

Experimental Section

General Methods

No uncommon hazards are noted. Reagents and solvents were purchased from commercial vendors and used as received unless otherwise specified. Tetraphenylporphyrin (H2TPP) was synthesized according to a literature protocol. All reactions were performed in an OMNI-lab glovebox under a N2 atmosphere unless otherwise stated. All solvents were dried over 3-Å molecular sieves. NMR spectra were collected using a Bruker Avance III HD 500 spectrometer equipped with a multinuclear Smart Probe. Signals in the 1H and 13C NMR spectra are reported in ppm as chemical shifts from tetramethylsilane and were referenced using the CHCl3 (1H, 7.26 ppm) and CDCl3 (13C, 77.16 ppm) solvent signals for samples analyzed in CDCl3. The frequencies of 19F NMR signals are reported in ppm as chemical shifts from CFCl3 (referenced to BF3·OEt2 at −152.8 ppm). Signals in 119Sn NMR spectra are reported in ppm as chemical shifts from Me4Sn (referenced to Ph4Sn at −127 ppm). Signals in 31P NMR spectra are reported in ppm as chemical shifts from H3PO4 (referenced to OPPh3 at 29.9 ppm).

Synthesis of Sn­(TPP)­Cl2

Sn­(TPP)­Cl2 was prepared using a variation of a previously reported procedure. Under ambient conditions, tetraphenylporphyrin (1.00 g, 1.63 mmol) and SnCl2·H2O (1.10 g, 4.87 mmol) were dissolved in pyridine (100 mL). The resulting dark purple solution was stirred at reflux for 3 h while open to air. The pyridine was removed under vacuum to leave a tacky black solid. This solid was dissolved in DCM (200 mL), washed with water (3 × 200 mL), and then acid (concentrated HCl diluted 1:9 in water, 200 mL). One additional wash with water (200 mL) was then performed. The organic layer was collected and dried with anhydrous Na2SO4. The solvent was removed under vacuum to afford a purple powder. Yield: 1.23 g (94%). The spectroscopic properties of the product match those previously reported.42 1H NMR (500 MHz, CDCl3) δ 9.21 (s, satellite d, J SnH = 15.24 Hz, 8H), 8.33 (d, 8H), 7.88–7.81 (m, 12H). 119Sn­{1H} NMR (186 MHz, CDCl3) δ −586.46. Crystals suitable for single-crystal X-ray diffraction were obtained by layering a concentrated solution of the product in DCM under hexanes. Purple crystals were grown over the course of 2 days.

Synthesis of Sn­(TPP)­(OTf)2

Sn­(TPP)­Cl2 (500 mg, 0.623 mmol) and AgOTf (400 mg, 1.56 mmol) were dissolved and suspended in DCM (15 mL total volume), respectively. The reaction was stirred overnight at ambient temperature. Over this time, the reaction turned magenta. The mixture was filtered through Celite and the solvent was removed from the filtrate under vacuum, which afforded a purple powder. The product was dissolved in DCM (2 mL) and layered under pentane. Purple crystals grew from the mixture over the course of 1 day. The supernatant was decanted, and the crystals were washed with pentane (3 × 2 mL) and dried under vacuum. Crystals suitable for X-ray diffraction were grown similarly. Yield: 520 mg (81%). The spectroscopic properties of the product match those previously reported. 1H NMR (500 MHz, CDCl3) δ 9.38 (s, satellite d, J SnH = 19.52 Hz, 8H), 8.30 (d, 8H), 7.92–7.84 (m, 12H). 13C­{1H} NMR (125 MHz, CDCl3) δ 147.70, 139.73, 134.70, 133.80, 129.27, 127.52, 122.68. 119Sn­{1H} NMR (186 MHz, CDCl3) δ −627.59.

Synthesis of Sn­(TPP)­(ClO4)2

SnTPPCl2 (250 mg, 0.312 mmol) and AgClO4 (323 mg, 1.56 mmol) were dissolved and suspended in DCM (10 mL), respectively. The reaction was stirred overnight at ambient temperature. Over this time, the reaction turned magenta. Purple solid had precipitated from the solution over the course of the reaction. The mixture was filtered through Celite and the solvent was removed from the filtrate under vacuum, which afforded a purple powder. Yield: 60 mg (21%). The spectroscopic properties of the product match those previously reported. 1H NMR (500 MHz, CDCl3) δ 9.39 (s, satellite d, J SnH = 19.36 Hz, 8H), 8.30 (d, 8H), 7.91–7.83 (m, 12H). 13C­{1H} NMR (125 MHz, CDCl3) δ 148.19, 139.75, 134.72, 133.86, 129.27, 127.51, 122.91. 119Sn­{1H} NMR (186 MHz, CDCl3) δ −612.53. Crystals suitable for X-ray diffraction were grown by dissolving the product in DCM (1 mL) and then layering that solution under pentane. Purple crystals grew from the mixture over the course of about 3 days.

Synthesis of Sn­(TPP)­(NTf2)2

SnTPPCl2 (500 mg, 0.623 mmol) and AgNTf2 (483 mg, 1.25 mmol) were dissolved in DCM (15 mL). The reaction was stirred for 1 h at ambient temperature. Over this time, the reaction turned magenta. The mixture was filtered through Celite and the solvent was removed from the filtrate under vacuum, which afforded a purple powder. The product was dissolved in DCM (2 mL) and layered under pentane. Purple crystals grew from the mixture over the course of 1 day. The supernatant was decanted, and the crystals were washed with pentane (3 × 2 mL) and dried under vacuum. Crystals suitable for X-ray diffraction were grown similarly. Yield: 628 mg (78%). 1H NMR (500 MHz, CDCl3) δ 9.47 (s, satellite d, J SnH = 20.94 Hz, 8H), 8.30 (d, 8H), 7.95–7.86 (m, 12H). 13C­{1H} NMR (125 MHz, CDCl3) δ 147.89, 139.25, 134.63, 134.19, 129.54, 127.72, 123.23. 119Sn­{1H} NMR (186 MHz, CDCl3) δ –611.20. Bulk purity was confirmed by PXRD.

X-ray Crystallography

Crystals of Sn­(TPP)­Cl2, Sn­(TPP)­(OTf)2, Sn­(TPP)­(ClO4)2, and Sn­(TPP)­(NTf2)2 were grown as described above. Crystals of [Sn­(TPP)­(H2O)2]­(NTf2)2 were grown as described below. All crystals were selected under a microscope, loaded onto a MiTeGen polyimide sample loop using Type NVH Cargille immersion oil and mounted onto a Rigaku XtaLAB Synergy-S single-crystal X-ray diffractometer. Each crystal was cooled to 100 K under a stream of nitrogen. Diffraction of Cu Kα radiation from a PhotonJet-S microfocus source was detected using a HyPix6000HE hybrid photon counting detector. Screening, indexing, data collection, and data processing were performed with CrysAlisPro. The structures were solved using SHELXT and refined using SHELXL following established strategies. − All non-H atoms were refined anisotropically. C-bound H atoms were placed at calculated positions and refined with a riding model and coupled isotropic displacement parameters (1.2 × Ueq for nonmethyl C–H atoms and 1.5 × Ueq for methyl groups).

Catalytic Styrene Dimerization Using Sn­(TPP)­(OTf)2

Sn­(TPP)­(OTf)2 (10 mg, 0.01 mmol) and styrene (11.5 μL, 10.4 mg, 0.1 mmol) were dissolved in CDCl3 (0.7 mL). The reaction progress was monitored via 1H NMR spectroscopy. An initial NMR spectrum was collected <10 min after the combination of reagents. The reaction was aged at 50 °C thereafter. After 24 h, 91% of the styrene was converted to 1,3-diphenyl-1-butene. 1H NMR (500 MHz, CDCl3) δ 7.37–7.18 (m, 10H), 6.45–6.37 (m, 2H), 3.65 (p, 1H), 1.47 (d, 3H). No evidence of other styrene oligomerization products was observed.

Catalytic Styrene Dimerization Using Sn­(TPP)­(ClO4)2

Sn­(TPP)­(ClO4)2 (9 mg, 0.01 mmol) and styrene (11.5 μL, 10.4 mg, 0.1 mmol) were dissolved in CDCl3 (0.7 mL). The reaction progress was monitored via 1H NMR spectroscopy. An initial NMR spectrum was collected <10 min after the combination of reagents. The reaction was aged at 50 °C thereafter. After 24 h, 0% of the styrene was converted to 1,3-diphenyl-1-butene.

Catalytic Styrene Dimerization Using Sn­(TPP)­(NTf2)2

Sn­(TPP)­(NTf2)2 (13 mg, 0.01 mmol) and styrene (11.5 μL, 10.4 mg, 0.1 mmol) were dissolved in CDCl3 (0.7 mL). The reaction progress was monitored via 1H NMR spectroscopy. An initial NMR spectrum was collected <10 min after the combination of reagents. The reaction was aged at 50 °C thereafter. After 24 h, 0% of the styrene was converted to 1,3-diphenyl-1-butene.

Catalytic Styrene Dimerization Using Sn­(TPP)­Cl2

Sn­(TPP)­Cl2 (8 mg, 0.01 mmol) and styrene (11.5 μL, 10.4 mg, 0.1 mmol) were dissolved in CDCl3 (0.7 mL). The reaction was aged at ambient temperature, and reaction progress was monitored via 1H NMR spectroscopy. After 24 h, 0% of the styrene was converted to 1,3-diphenyl-1-butene.

Ligand Exchange Studies with (NBu4)Cl and Sn­(TPP)­(OTf)2

Sn­(TPP)­(OTf)2 (5 mg, 0.005 mmol) and (NBu4)Cl (3 mg, 0.010 mmol) were dissolved in CDCl3 (0.7 mL). The reaction was aged at ambient temperature, and reaction progress was monitored via 1H NMR spectroscopy. Full conversion of Sn­(TPP)­(OTf)2 to Sn­(TPP)­Cl2 was observed after 2.5 h.

Ligand Exchange Studies with (NBu4)Cl and Sn­(TPP)­(ClO4)2

Sn­(TPP)­(ClO4)2 (5 mg, 0.005 mmol) and (NBu4)Cl (3 mg, 0.011 mmol) were dissolved in CDCl3 (0.7 mL). The reaction was aged at ambient temperature, and reaction progress was monitored via 1H NMR spectroscopy. Full conversion of Sn­(TPP)­(ClO4)2 to Sn­(TPP)­Cl2 was observed after <10 min.

Ligand Exchange Studies with (NBu4)Cl and Sn­(TPP)­(NTf2)2

Sn­(TPP)­(NTf2)2 (5 mg, 0.004 mmol) and (NBu4)Cl (2 mg, 0.008 mmol) were dissolved in CDCl3 (0.7 mL). The reaction was aged at ambient temperature, and reaction progress was monitored via 1H NMR spectroscopy. Full conversion of Sn­(TPP)­(NTf2)2 to Sn­(TPP)­Cl2 was observed after <10 min.

Ligand Exchange Studies with (NBu4)­OTf and Sn­(TPP)­(NTf2)2

Sn­(TPP)­(NTf2)2 (10 mg, 0.008 mmol) and (NBu4)­OTf (3 mg, 0.008 mmol) were dissolved in CDCl3 (0.7 mL). The reaction was aged at ambient temperature, and reaction progress was monitored via 1H NMR spectroscopy. A mixture of Sn­(TPP)­(NTf2)2, Sn­(TPP)­(OTf)2, and Sn­(TPP)­(OTf)­(NTf2) was observed within 10 min and the spectrum did not change over the course of 12 h.

Lewis Acid Strength Assessment Using OPPh3

Sn­(TPP)­(OTf)2 (56 mg, 0.05 mmol), Sn­(TPP)­(ClO4)2 (50 mg, 0.05 mmol), or Sn­(TPP)­(NTf2)2 (70 mg, 0.05 mmol) and OPPh3 (1.5 mg, 0.005 mmol) were dissolved in CDCl3 (0.7 mL). Solutions were made containing 10 equiv of the Sn complex in relation to OPPh3. The solutions were analyzed using 31P NMR spectroscopy.

Catalytic Styrene Dimerization Using Nonanhydrous Solvents

Under ambient aerobic conditions, Sn­(TPP)­(OTf)2 (10 mg, 0.01 mmol) and styrene (11.5 μL, 10.4 mg, 0.1 mmol), were dissolved in CDCl3 (0.7 mL, nonanhydrous). The reaction was aged at ambient temperature, and reaction progress was monitored via 1H NMR spectroscopy. After 24 h, 0% of the styrene was converted to 1,3-diphenyl-1-butene.

Catalytic Styrene Dimerization in the Presence of 30 mol % of Water Using Sn­(TPP)­(OTf)2

Sn­(TPP)­(OTf)2 (10 mg, 0.01 mmol) and styrene (11.5 μL, 10.4 mg, 0.1 mmol) were dissolved in CDCl3 (0.4 mL, anhydrous). The reaction was spiked with water-saturated CDCl3 (0.4 mL, 0.03 mmol of water). The reaction was aged at ambient temperature and reaction progress was monitored via 1H NMR spectroscopy. The initial spectrum, which was collected <10 min after the water-saturated CDCl3 spike, showed 19% of the styrene was converted to 1,3-diphenyl-1-butene, but no further changes were observed over the following 24 h.

Catalytic Styrene Dimerization in the Presence of 30 mol % of Water Using Sn­(TPP)­(NTf2)2

Sn­(TPP)­(NTf2)2 (13 mg, 0.01 mmol) and styrene (11.5 μL, 10.4 mg, 0.1 mmol) were dissolved in CDCl3 (0.4 mL, anhydrous). The reaction was spiked with water-saturated CDCl3 (0.4 mL, 0.03 mmol of water). The reaction was aged at ambient temperature and reaction progress was monitored via 1H NMR spectroscopy. The initial spectrum, which was collected <10 min after the water-saturated CDCl3 spike, showed significant reduction in the presence of the catalyst with almost complete loss of catalyst after 6 h. No further changes were observed over the course of 24 h, and 0% of the styrene was converted to 1,3-diphenyl-1-butene. Large red crystals had grown from the reaction mixture over the course of about a day. They were confirmed to be [Sn­(TPP)­(OH2)2]­(NTf2)2 by single crystal X-ray diffraction.

Catalytic Styrene Dimerization in the Presence of 30 mol % of Water at Time Intervals

Three separate samples were prepared by dissolving Sn­(TPP)­(OTf)2 (10 mg, 0.01 mmol) and styrene (11.5 μL, 10.4 mg, 0.1 mmol) in CDCl3 (0.4 mL, anhydrous). The samples were spiked with water-saturated CDCl3 (0.4 mL, 0.03 mmol of water) at specific time intervals. The first sample was spiked after aging at ambient temperature for 10 min, the second sample was spiked after 30 min, and the third sample was spiked after 1 h. Reaction progress of all three samples was checked via 1H NMR spectroscopy 1 h after the initial combination of styrene and catalyst. Spiking the water-saturated CDCl3 after 10 min, 30 min, and 1 h converted 22%, 33%, and 45% of the styrene to 1,3-diphenyl-1-butene, respectively.

Catalytic Styrene Dimerization with Addition of 2,6-Lutidine

Sn­(TPP)­(OTf)2 (10 mg, 0.01 mmol), styrene (11.5 μL, 10.4 mg, 0.1 mmol), and 2,6-lutidine (3 μL, 0.02 mmol) were dissolved in CDCl3 (0.7 mL). The reaction was aged at ambient temperature, and reaction progress was monitored via 1H NMR spectroscopy. After 24 h, 0% of the styrene was converted to 1,3-diphenyl-1-butene.

Catalytic Styrene Dimerization with Addition of DIPEA

Sn­(TPP)­(OTf)2 (10 mg, 0.01 mmol), styrene (11.5 μL, 10.4 mg, 0.1 mmol), and diisopropylethylamine (4 μL, 0.02 mmol) were dissolved in CDCl3 (0.7 mL). The reaction was aged at ambient temperature, and reaction progress was monitored via 1H NMR spectroscopy. After 24 h, 0% of the styrene was converted to 1,3-diphenyl-1-butene.

Catalytic Styrene Dimerization with B­(C6F5)3

B­(C6F5)3 (5 mg, 0.01 mmol) and styrene (11.5 μL, 10.4 mg, 0.1 mmol) were dissolved in CDCl3 (0.7 mL). The reaction was aged at ambient temperature, and reaction progress was monitored via 1H NMR and 19F NMR spectroscopic techniques. After 24 h, 0% of the styrene was converted to 1,3-diphenyl-1-butene.

Catalytic Styrene Dimerization Using B­(C6F5)3 and (NBu4)­OTf

B­(C6F5)3 (5 mg, 0.01 mmol), (NBu4)­OTf (4 mg, 0.01 mmol), and styrene (11.5 μL, 10.4 mg, 0.1 mmol) were dissolved in CDCl3 (0.7 mL). The reaction was aged at ambient temperature, and reaction progress was monitored via 1H NMR and 19F NMR spectroscopic techniques. After 24 h, 0% of the styrene was converted to 1,3-diphenyl-1-butene. The 1H NMR spectra were consistent with the formation of styrene oligomerization products.

Catalytic Dimerization of (β,β-2H2)­Styrene

Sn­(TPP)­(OTf)2 (10 mg, 0.01 mmol) and (β,β-2H2)­styrene (11.5 μL, 10.4 mg, 0.1 mmol) were dissolved in CDCl3 (0.7 mL). The sample was aged at room temperature and reaction progress was monitored via 1H NMR spectroscopy. After 25 h, 23% of the (β,β-2H2)­styrene was converted to (2,4,4,4-2H4)­1,3-diphenyl-1-butene.

Catalytic Dimerization of (α-2H)­Styrene

Sn­(TPP)­(OTf)2 (10 mg, 0.01 mmol) and (α-2H)­styrene (11.5 μL, 10.4 mg, 0.1 mmol) were dissolved in CDCl3 (0.7 mL). The sample was aged at room temperature and reaction progress was monitored via 1H NMR spectroscopy. After 25 h, 56% of the (α-2H)­styrene was converted to (1,3-2H2)­1,3-diphenyl-1-butene.

Supplementary Material

ic5c05290_si_001.pdf (2.4MB, pdf)
ic5c05290_si_002.xyz (56.1KB, xyz)

Acknowledgments

This work was supported by the Arnold and Mabel Beckman Foundation through a Beckman Young Investigator Award to T.C.J., by the NSF through a Graduate Research Fellowship to J.L.P. and MRI grant 2018501, and by the NIH though training grant T34GM­140956 to A.M.G.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.5c05290.

  • NMR data; thermal ellipsoid plots; supplementary tables (PDF)

  • Computationally optimized Cartesian coordinates (XYZ)

The authors declare no competing financial interest.

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ic5c05290_si_001.pdf (2.4MB, pdf)
ic5c05290_si_002.xyz (56.1KB, xyz)

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