Reversible Coordination of H2 by a Distannyne - ACS Publications

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Reversible Coordination of H2 by a Distannyne Shuai Wang, Tobias J Sherbow, Louise A Berben, and Philip P. Power J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11798 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 22, 2017

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Reversible Coordination of H2 by a Distannyne Shuai Wang†, Tobias J. Sherbow†, Louise A. Berben†, and Philip P. Power†* † Department of Chemistry, University of California Davis, 1 Shield Avenue, Davis, CA 95616, United States. Supporting Information Placeholder

The terphenyl tin(II) hydride, [AriPr4Sn(μ-H)] 2 (1) (AriPr4 = C6H3-2,6(C6H3-2,6i Pr2)2) was shown to form an equilibrium with the distannyne, AriPr4SnSnAriPr4 (2) and H2 in toluene at 80 oC. The equilibrium constant (Keq) and Gibbs free energy (ΔG) for the dissociation of H2 are 2.23 x 10-4 ± 4.9 % and 5.89 ± 0.68 % kcal/mol by 1H NMR spectroscopy, and 2.33 x 10-4 ± 6.2 % and 5.86 ± 0.73 % kcal/mol by UV-Vis spectroscopy, indicating that the hydride 1 is strongly favored. Further heating 2 at ca. 100 oC afforded the known pentagonal bipyramidal Sn7 cluster, Sn5(SnAriPr4)2 (3). Mechanistic studies show that 3 is formed from the distannyne 2 which is generated from 1. The order of the reaction for the conversion of 2 into 3 was found to be zero and the rate constant is 1.77 x 10-5 M s-1 at 100 oC. Hydride 1 was further characterized by cyclic voltammetry and its pKa is 18.8 (2) via titration with 1,8diazabicyclo[5.4.0]undec-7-ene (DBU). The bond dissociation free energy (BDFE ΔG) was estimated to be 51.1 ± 3.4 % kcal/mol on the basis of its pKa and reduction potential. Studies with deuterium indicate ready exchange of D2 with the hydrides in 1. ABSTRACT:

The interaction of hydrogen with transition metals is a key process in organometallic chemistry.1-6 Furthermore, reversible oxidative addition and reductive elimination reactions of H2 with transition metal complexes constitute fundamental steps in many catalytic cycles.7-8 Such processes have been widely studied not only in organometallic chemistry,9 but also in surface chemistry,10-12 hydrogenase enzymes.13-14 and in connection with their relevance to chemical hydrogen storage.15-16 In effect the binding, storage and release of H2 under mild conditions are of major importance,17-20 but despite the investigations of several H2 carrier species, e.g. amine boranes,21 Mg,22-23 or Al24-25 systems, there have been

relatively few instances where reversible absorption and release of hydrogen has been effected under mild conditions for main group compounds.26-30 Two examples involve use of metal free frustrated Lewis pair systems,26-29 and anti-aromatic boron containing organic rings.30 In addition, several main group compounds have been shown to react with H2 under ambient conditions to yield hydrides.31-36 These include low valent group 13 and 14 element compounds, which feature frontier orbitals with small energy separations and suitable symmetry to react with H2,37 and whose reactivity can mimic that of transition metal complexes.38 In 2008, it was shown that low valent dimeric tin(II) hydrides, [ArSn(μH)]2 (Ar = terphenyl), or their isomers, ArSnSn(H)2Ar, which had been synthesized earlier by various conventional reduction pathways,39-44 can be generated via reacting H2 with the corresponding distannynes, ArSnSnAr, under ambient conditions.31, 32

Herein, we establish reversibility for such reactions and show that the tin(II) hydride, [AriPr4Sn(μH)]2 (1), exists in equilibrium with the corresponding distannyne, AriPr4SnSnAriPr4 (2), and hydrogen in toluene solution at 80 oC. The equilibrium constant (Keq) and the Gibbs free energy (ΔG) were determined via 1H NMR and UV-Vis spectroscopy and shown to strongly favor the tin(II) hydride. In addition, the pKa of 1 was determined to be 18.8 (1) by titration of 1 with the strong base, 1,8diazabicyclo[5.4.0]undec-7-ene (DBU) in a THF solution and using electrochemistry demonstrate that the Sn-H bond strength is ca. 50 kcal mol-1. We also demonstrate that the thermolysis of 1 to give the cluster Sn5(SnAriPr4)2 (3) proceeds through the distannyne 2 which decomposes to give 3. The reaction of D2 with 1 affords a mixture of isotopomers incorporating both deuterium and hydrogen in bridging positions indicating ready H2/D2 exchange.

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Compound 1 was synthesized via the reduction of the corresponding tin(II) halide, [AriPr4Sn(μ-Cl)]2 with the reducing agent diisobutylaluminum hydride (DIBAL) at ca. -78 oC in diethyl ether.41

Scheme 1. Summary of the previous32, 41, 45 and current work (panel) on [AriPr4Sn(μ-H)]2 (1)/AriPr4SnSnAriPr4 (2), and Sn5(SnAriPr4)2 (3).

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In parallel experiments, detailed 1H NMR spectroscopic monitoring of the reaction at 80 oC in a sealed tube indicated relatively rapid conversion of 1 to 2 in the first hour, after which time the rate of the conversion decreased and eventually ceased reaching equilibrium after ca. 2.5 h, where an integration ratio of 2.35:1 was observed for 1 and 2. The equilibrium constant (Keq) of this reaction was calculated (eq. 1) to be 2.23 x 10-4 ± 4.9 % at 80 oC, where the H2 concentration in toluene was calculated to be 0.549 mM (see supporting information),46 and the Gibbs free energy (ΔG) of the equilibrium at 80 oC was also determined to be 5.89 ± 0.68 % kcal/mol.

Keq =

AriPr4 SnSnAriPr4 [H2 ]

(1)

[AriPr4 SnμH]2

A further experiment which involved the heating of a toluene solution of 1 at ca. 80 oC with the head space of the NMR tube filled with 1 atm of H2 gas indicated no significant formation of 2 even after 16 h heating, and shows that the presence of H2 hinders conversion of 1 into 2, further supporting the reversible nature of the reaction.

1 K = 2.30 x 10-4 ± 4.9 % by 1H NMR 2.39 x 10-4 ± 6.2 % by UV-Vis

2

This work

Heating a d8-toluene solution of the hydride 1 at ca. 80 oC for ca. 2.5 h in a sealed NMR tube afforded two new doublet signals at 1.10 and 1.34 ppm and a new septet signal at 2.80 ppm in the 1H NMR spectrum and the disappearance of the bridging hydride signal at 9.06 ppm. These are indicative of the clean formation of the distannyne, AriPr4SnSnAriPr4 (2) (Figure 1). The integration ratio of 6:1 for 1 and 2 indicates a 14.2 % of conversion of the hydride species to the distannyne. Further heating the solution for 16 h led to no significant change in the extent of the conversion. If however the H2 in the head space is removed and replaced with N2, heating the solution for a further 2.5 h resulted in a new integration ratio for 1 and 2 of 2.34:1, indicating 30.0 % conversion of 1 to 2. Repetition of this process resulted in further dehydrogenation until all 1 is converted to 2.

Figure 1. The 1H NMR spectrum of the d8-toluene solution of 1 at 25 °C displaying the formation of 2 upon heating at 80 oC for 16 h. The addition of D2 to the head space of an NMR tube containing 0.6 mL of toluene solution of 1 (4.1 mM), and heating the solution for 16 h at 80 oC also afforded no evidence of the formation of 2 (cf. corresponding experiment with H2 above). However, the 119 Sn NMR spectrum displayed a broad singlet (full width at half maximum (FWHM) = 115 Hz) at 643.7 ppm, representing the formation of [AriPr4Sn(μ-D)]2 (1D).31 This is accompanied by the disappearance of the bridging hydride signal at 9.06 ppm in the 1H NMR spectrum. 1H NMR spectroscopy of a toluene solution of 1 with a D2 filled head space heated at 80 o C for the shorter time of 2 h, resulted in a spectrum that displayed a decrease in the intensity of the bridging hydride signal at 9.06 ppm by 52 % along

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with the appearance of another broad signal at 9.00 ppm. This new signal is due to the formation of the mono-deuterated hydride, AriPr4Sn(μ-HD)SnAriPr4 (1HD). To further investigate this hypothesis, the isotopmer 1D, was combined in a 1:1 ratio with compound 1 in a toluene solution. The 119Sn NMR spectrum revealed a roughly 1:2:1 triplet signal at 657.9 ppm (JSn-H = 86 Hz), a broad singlet at 643.7 ppm, representing 1 and 1D, respectively, as well as a doublet at 650.6 ppm (JSn-H = 84 Hz), indicating coupling to a single bridging hydrogen consistent with the formation of the 1HD isotopomer. The 1H NMR spectrum of the 1:1 mixture of 1 and 1D in d8-toluene showed an integration ratio of 1:1 for the signals at 9.06 and 9.00 ppm, which represent the 1:2 ratio for 1 containing two hydrogen atoms and 1HD containing one hydrogen and one deuterium atom, further supporting the ready stoichiometric scrambling of the isotopologues. In parallel with the NMR experiments, the equilibrium of 1 with 2 and H2 was also studied via UVvis spectroscopy. A dilute toluene solution of 1 (0.16 mM) features a weak absorption band at 600 nm (ε = 70 M-1 cm-1), in agreement with the previously reported value for 1.41 However, two new bands at 410 nm and 597 nm, corresponding to those of 2,47 appeared upon heating the sample at 80 oC (Figure 2). The extent of conversion was calculated based on the increasing intensity absorption at 410 nm. The equilibrium constant Keq and the Gibbs free energy ΔGo for the equilibrium were determined to be 2.33 x 10-4 ± 6.2 % and 5.86 ± 0.73 % kcal/mol by this method, which are in good agreement with Keq and ΔGo values determined by 1H NMR spectroscopy. The small Keq and the positive ΔGo again illustrate the strong preference for the hydride 1 over 2 in the equilibrium. 1.2 1 Absorption

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0.8

2

0.6

1

0.4 0.2 0 350

450

550

650

λ (nm)

750

850 o

Figure 2. UV-Vis spectrum of 1 heated at 80 C over 2.5 h (ca. 20 min intervals) in toluene (0.16 mM). The λmax of 1 at 595 nm overlaps with the low energy λmax of 2 at 597 nm.

Beyond the intermediate formation of 2, the mechanistic details of the formation of the cluster, Sn5(SnAriPr4)2 (3) from 1 are unknown. Upon raising the temperature of a d8-toluene solution of 1 to ca. 90 oC, a new species, featuring a septet signal at 2.85 ppm and two doublets at 0.46 and 1.42 ppm was observed along with 2 in the 1H NMR spectrum. These correspond to the spectrum of 3,45 which was originally obtained by refluxing (ca. 110 oC) a toluene solution of 1 for 16 h (or via the reduction of a 2:5 mixture of [AriPr4Sn(μ-Cl)]2, SnCl2 with KC8). An 1H NMR experiment involving the heating of a d8toluene solution of 1 at ca. 100 oC was carried out. The formation rate of 3 was slow during the first 10 h of heating, then increased rapidly during the following 5 h, and slowed again in the last 4 h as the conversion of 1 to 3 approaches completion (see Supporting Information). To throw further light on the formation pathway for 3, a similar experiment was carried out by heating a toluene solution of 1 at 100 o C for 16 h with a H2 filled head space. No evidence of the formation of either 2 or 3 was observed; instead, only a new septet at 2.85 ppm and two doublets at 1.09 and 1.11 ppm, which represent the formation of AriPr4H, were observed along with the tin metal deposition indicating the decomposition of 1. In contrast, heating a toluene solution of 2 at ca. 100 o C in the absence of H2 afforded compound 3 in ca. 25 % yield and other unidentified products. The reaction rate (k) and the rate of the formation of 3 were determined to be 6.11 x 10-5 ± 4.8 % M s-1 and 1.67 x 10-5 ± 7.2 % M s-1, respectively. These experiments show that 3 is not formed directly from 1; instead, it is formed from 2 which in this case is generated from 1. The hydride species 1 was further characterized by cyclic voltammetry (CV) performed on a 0.75 mM, 0.3 M Bu4NBF4 THF solution of 1 (Figure 3). 1 displays two reduction events at -1.63 and -2.00 V. The first is quasi-reversible and the second irreversible, and the second is barely observed at slow scan rates which suggests that species generated by the second reduction of 1 is fairly unstable (Figure S18). To determine the number of electrons associated with the reversible reduction at -1.63 V, we performed differential pulse voltammetry (DPV) experiments with a solution containing equimolar concentrations of 1 and FcCp2*. The area of each of the reduction observed using DPV are 4.12 × 10-7 and 5.65 × 10-7 VA, respectively (Figure S19). This implies that the reversible couple at -1.63 V is best assigned as a one-electron reduction event.



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the equilibrium (ΔG) indicated a strong preference for the hydride. The Sn7 cluster, Sn5(SnAriPr4)2 is the thermolysis product of AriPr4SnSnAriPr4 rather than of the Sn(II) hydride. Further characterization of 1

by cyclic voltammetry (E / = -1.63 V vs SCE), and the pKa value of 18.8 (2) allowed the BDFE of the SnH to be determined as ca. 50 kcal mol-1. ASSOCIATED CONTENT Supporting Information

Figure 3. Cyclic voltammogram of 0.35 mM solution of 1 and 0.35 mM decamethylferrocene (FcCp2*), in 0.3 M Bu4NBF4 THF. The irreversibility of the second reduction event is more consistent with the reduction at a single metal center, such as the divalent tin in the isomer AriPr4SnSn(H)2AriPr4, which differs in energy from 1 by less than 3 kcal mol-1.35 Also, reduction of a small equilibrium concentration of monomeric AriPr4SnH species, which was calculated to be preferred energetically over 1,35 cannot be completely ruled out and its presence is consistent with previous work involving the reaction of 1 and related species with Lewis bases.49 After an unsuccessful attempt to obtain the pKa of 1 with a mild base, Et3N (pKa of Et3NH+ = 14.9 in THF),50 one equivalent of the strong base, 1,8diazabicyclo[5.4.0]undec-7-ene (DBU), was used to titrate a THF solution of 1 (0. 75 mM) (eq. 2).

Experimental procedures for all compounds and their spectral data, as well as details of the determination of the equilibrium constants (Keq), ΔG values, the titration of 1 with DBU, electrochemical measurements and the calculation of the Sn-H bond dissociation free energy. Supporting information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author

*P.P.P.: fax, +1-530-732-8995; tel, +1-530-752-6913; email, [email protected]. ORCID Philip P Power: 0000-0002-6262-3209 Louise Berben: 0000-0001-6461-1829

ACKNOWLEDGMENT We wish to acknowledge the U.S. Department of Energy (DE-FG02-07ER4675) Office of Basic Energy Sciences for support of this work. LAB thanks the UCD academic senate.

REFERENCES

The 1H NMR spectrum, which was recorded under equilibrium conditions, displays a new signal at 10.37 ppm, representing the protonated DBUH+, the equilibrium constant Keq for eq. 2 was determined to be 2.16(3). The known pKa of DBUH+ (pKa = 19.1 in THF)48 yielded a pKa of 18.8(2) in THF. The bond dissociation free energy (BDFE ΔG) was then calcu lated with the pKa and the reversible E / in THF, to be 51.1 ± 3.4 % kcal/mol (see supporting information).51,52 This value is somewhat lower than the ca. 75 kcal mol-1 reported for some trialkyltin(IV) hydrides.53 In summary, the equilibrium between the low valent tin(II) hydride, (AriPr4SnH)2 (1), the distannyne, AriPr4SnSnAriPr4, and hydrogen was studied. The equilibrium constant (Keq) and Gibbs free energy of

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