NHC-Coordinated Diphosphene-Stabilized Gold(I) Hydride and Its Reversible Conversion to Gold(I) Formate with CO2.

An NHC-coordinated diphosphene is employed as ligand for the synthesis of a hydrocarbon-soluble monomeric AuI hydride, which readily adds CO2 at room temperature yielding the corresponding AuI formate. The reversible reaction can be expedited by the addition of NHC, which induces β-hydride shift and the removal of CO2 from equilibrium through the formation of an NHC-CO2 adduct. The AuI formate is alternatively formed by dehydrogenative coupling of the AuI hydride with formic acid (HCO2 H), thus in total establishing a reaction sequence for the AuI hydride mediated dehydrogenation of HCO2 H as chemical hydrogen storage material.

Tertiary phosphines I (Scheme 1) are ubiquitous ligands and thus play an important role in the organometallic chemistry of transition metals1 and thereby also in the area of homogenous catalysis.2 Celebrated examples of the use of phosphines include Wilkinson's catalyst,3 Noyori's catalyst,4 and the first generation Grubbs catalyst.5 Other phosphorus‐based species such as diphosphenes II 6 and base‐stabilized phosphinidenes III 7 (Scheme 1) have also found applications as ligands in transition‐metal complexes. In contrast, Bertrand's base‐free phosphino phosphinidene is electrophilic in nature.8 None of these P‐centered donors (I–III), however, can compete with carbenes in terms of the stabilization of reactive intermediates.9

Scheme 1

Chemical structures of I–IV (R=monoanionic ligand, NHC=N‐heterocyclic carbene).

Recently, we have reported the reversible coordination of an NHC to a diphosphene to yield IV (Scheme 1), which possesses two nonbonding electron pairs at the dicoordinate P‐center.10 Frontier orbital analysis revealed that the HOMO almost exclusively consists of a p‐orbital at the formally negatively charged dicoordinate phosphorus center. Theoretical calculations further suggest that the NHC‐coordinated diphosphene should be a stronger donor than Ph3P.11 Moreover, the calculated binding energy of IV to AuCl is −61.1 kcal mol−1 which is higher in comparison to I (−50.1 kcal mol−1), II (−38.5 kcal mol−1), and III (−55.8 kcal mol−1).11 Armed with this knowledge, we sought to utilize IV as a ligand towards gold(I) hydride. The monomeric parent AuI hydride (AuH) is kinetically unstable and has only been observed in cold matrices12 and considered as an intermediate in numerous gold‐catalyzed organic transformations.13 With an NHC as a stabilizing ligand, it was isolated as a room‐temperature stable compound.14

Herein, we thus disclose the use of IV as a new P‐centered neutral ligand that ultimately allowed for the isolation of a monomeric AuI hydride complex, which in turn is shown to undergo the first hydroauration reaction of CO2 to form the corresponding AuI formate. Surprisingly, the AuI formate spontaneously releases CO2 even at room temperature, a process that is facilitated by the presence of NHC. We further show that the AuI formate is also accessible by the dehydrogenation of HCO2H (a chemical hydrogen storage material)15 by the AuI hydride, thus stoichiometrically demonstrating its principal potential as dehydrogenation catalyst. Reversible hydrogenation of CO2 is known by the bacterial enzyme carbondioxide reductase.16 In an analogous manner we report a synthetic system that shows reversible hydroauration behavior of a AuI hydride.

The reaction of AuCl⋅SMe2 with a 1:1 solution of NHC 17 and diphosphene 1 18 in THF at −78 °C yields the NHC /diphosphene‐coordinated AuI chloride complex 2 (Scheme 2).11 Formation of 2 reveals the ability of the diphosphene motif to act simultaneously both as a Lewis acid and a Lewis base in analogy to compounds with heavier Group 14 multiple bonds.19 The 31P NMR spectrum of 2 exhibits two doublets at δ=1.34 and −31.46 ppm with 1 J PP=462 Hz, which is in between the values of 1⋅NHC (1 J PP=423 Hz)10 and the monoaurated adduct, Mes*(AuCl)P=PMes* (1 J PP=539 Hz).20

Scheme 2

Synthesis of NHC ‐coordinated diphosphene‐stabilized AuI‐Cl complex 2 (Ar=2,6‐Mes2C6H3, Mes=2,4,6‐Me3C6H2).

Interestingly, in solution, the coordinated NHC does not dissociate unlike 1⋅NHC , which exists in equilibrium with 1 and NHC .10 The stability of 2 is probably due to the coordination of the diphosphene moiety to AuCl, which enhances the electrophilicity of the second P center resulting in stronger binding to NHC . This is also supported by DFT calculations on the highly endergonic dissociation of NHC from 2 (21.8 vs. 6.7 kcal mol−1 from 1⋅NHC ) in THF.11

The molecular structure of 2 was confirmed by single‐crystal X‐ray diffraction (Figure 1). The P−Au bond distance (2.2540(8) Å) in 2 is longer than those of Ph3P⋅AuCl (Au−P 2.235 Å)21 and the corresponding diaurated adduct of Mes*‐substituted diphosphene, Mes*(AuCl)P=P(AuCl)Mes* (2.201 Å).20 The P1−P2 bond distance is 2.219(1) Å and thus considerably longer than in free diphosphene 1 (2.029 Å)22 or 1⋅NHC (2.134 Å).10

Figure 1

Molecular structure of 2 with ellipsoids set at 50 % probability. All hydrogen atoms and one molecule of co‐crystallized toluene are omitted for clarity.33

The 1:1 reaction of 2 with N‐selectride at −78 °C affords the NHC/diphosphene‐stabilized AuI hydride, 3 in 90 % yield as light‐yellow crystals (Scheme 3). The 31P NMR spectrum of 3 exhibits two resonances at δ=1.6 ppm as doublet (1 J PP=470 Hz) and at δ=−14.5 ppm as a doublet of doublets (1 J PP=470 Hz, 2 J PH=138 Hz).

Scheme 3

Synthesis of NHC‐coordinated diphosphene‐stabilized AuI‐hydride 3 and the rearrangement to 4.

In the 1H NMR spectrum, the doublet at δ=4.60 ppm (2 J PH=138 Hz) can be unambiguously assigned to the Au‐H resonance, which is upfield shifted in comparison to that of NHCDip‐stabilized AuI‐hydride (5.11 ppm).14a The IR spectrum of 3 shows a strong sharp band at 1893 cm−1 for the Au‐H motif, in good agreement with the calculated value (1880.2 cm−1).11

The molecular structure of 3 (Figure 2) reveals a P−P bond distance of 2.197(1) Å, which is slightly shorter than the P−P bond distance in 2 due to less pronounced π‐back‐donation. Indeed, the P−Au bond distance in 3 of 2.3297(9) Å is larger than the 2.2540(8) Å in 2 suggesting a stronger trans influence of the hydride compared to the chloride ligand. The AuI hydride 3 is stable in presence of degassed water in toluene overnight; a solid‐crystalline sample even persists in open air at least for two days.

Figure 2

Molecular structure of 3 with ellipsoids set at 50 % probability. All H atoms except Au‐H and one co‐crystallized molecule of benzene are omitted for clarity.33

In solution, however, 3 slowly undergoes a 1,3‐hydrogen shift from the Au center to the β‐phosphorus atom, resulting in the AuI phosphinophosphide 4, as shown by the appearance of a 1H NMR doublet of doublets at δ=4.19 ppm (dd, 1 J PH=214. Hz, 2 J PH=9 Hz). The concomitant migration of the NHC ligand from the phosphorus to the gold center is evident from the significantly smaller coupling of the 13C{1H} signal at δ=193.6 ppm of the carbenic carbon atom to the nearest 31P nucleus (2 J CP=53 Hz for 4 vs. 1 J CP=99 Hz for 2). Conversion is completed by heating to 65 °C for one hour. According to our DFT results, the rearrangement of 3 to 4 is exergonic by 26.1 kcal mol−1.11 The structure of the NHC‐stabilized AuI phosphinophosphide 4 was finally confirmed by X‐ray diffraction on single crystals (Figure 3). The P−P bond distance of 4 is 2.218(1) Å and thus slightly longer than in the one of the reported boryl substituted lithium phosphinophosphide (2.1775 Å).23

Figure 3

Molecular structure of 4 with ellipsoids set at 50 % probability. All hydrogen atoms and one molecule of co‐crystallized hexane solvent molecule are omitted for clarity.33

To address the hydridic character of the Au‐H moiety of 3, we considered the hydroauration reaction with CO2. The hydrometalation of carbonyl compounds, in particular of CO2, is a key step of catalytic conversions to access C1‐feedstock materials.24 In fact, the formation of 5 from 3 and CO2 is computed to be exergonic by 11.6 kcal mol−1.11 Upon passing CO2 gas into a toluene solution of 3 at room temperature, the quantitative formation of AuI formate 5 was observed based on 31P NMR of the reaction mixture (Scheme 4).

Scheme 4

Synthesis of NHC‐coordinated diphosphene‐stabilized AuI‐formate 5.

The 1H NMR spectrum of 5 exhibits a doublet centered at δ=9.50 ppm (4 J HP=7 Hz), in line with the suggested formate as is a prominent IR band at 1884 cm−1 for the C=O stretching frequency. The molecular structure of 5 was confirmed by X‐ray crystallography (Figure 4). The Au−O bond distance in the AuI formate 5 of 2.140(4) Å is slightly longer than that of a reported AuIII formate (2.102 Å).25 To the best of our knowledge, the formation of 5 represents the first example of any gold formate obtained by direct hydroauration of CO2.

Figure 4

Molecular structure of 5 with ellipsoids set at 50 % probability. All hydrogen atoms and one molecule of toluene are omitted for clarity.33

We had noted that the 31P NMR spectrum of the residue after removal of the solvent shows the presence of about 5 % of the starting AuI hydride, 3. This observation prompted us to further investigate a possible spontaneous release of CO2 from 5. The release of CO2/HCO2 − from transition‐metal formates is well‐known26 and the reductive elimination of CO2 from a binuclear AuII/CO2 complex has been reported.27 Indeed, the application of 0.12 mbar vacuum for 15 h results in the original AuI‐hydride 3 in about 50 %. Decarboxylation of 5 above 65 °C proceeds to complete conversion, but also affords 4, the thermal isomerization product of 3 as side product.

To facilitate the release of CO2, we added NHC 14 in the anticipation that it might induce the required 1,3‐H shift (β‐hydride elimination)28 by coordination to the carbonyl group and removal of CO2 from equilibrium as NHC ‐CO2 adduct 6.29 Addition of one equivalent of NHC to a solution of 5 at room temperature indeed resulted in the immediate formation of 3 (Scheme 5).

Scheme 5

NHC ‐mediated release of CO2 from AuI‐formate 3.

To verify the CO2 release at lower temperatures and to check for intermediates, we carried out a VT‐NMR study of a 1:1 [D8]toluene solution of NHC and 5. At −78 °C, the 31P NMR spectrum does not show any indication for the release of CO2. At −10 °C, we observed one new set of peaks at δ=−35.2 and 0.9 ppm (1 J PP=465 Hz). These resonances disappear while approaching room temperature with the concomitant appearance of the resonances of 3. The occurrence of an intermediate suggests that the reaction may indeed proceed through the initial coordination of NHC to the carbonyl carbon center of 5 to give the thermally unstable adduct 7; in analogy to the nucleophilic coordination of NHC to aldehydes.30 Subsequent hydride migration (β‐hydride elimination) would lead to the NHC adduct of CO2 6 and AuI hydride 3 (Scheme 5). The calculated Gibbs free energy values confirm that the reaction 5+NHC →3+6 is endergonic by 7.4 kcal mol−1.11

Finally, we contemplated the use of the NHC/diphosphene coordinated AuI hydride 3 for the dehydrogenation of HCO2H. The stoichiometric reaction of 3 and HCO2H indeed results in the AuI formate 5 with elimination of H2 (Scheme 6). Computationally, the formation of 5 from 3 and HCO2H is thermodynamically favourable by 13.5 kcal mol−1.11

Scheme 6

AuI‐hydride 3 mediated release of H2 and CO2 from HCO2H (inset: Reaction of HCO2H to CO2 and H2).

In conclusion, we have disclosed a water‐stable monomeric terminal AuI‐hydride coordinated by an NHC/diphosphene adduct. Like other heavier Group 14 multiple bonds, the diphosphene can simultaneously act as a Lewis acid and as a Lewis base. The AuI hydride exhibits pronounced hydridic character and thus reacts with CO2 to the corresponding AuI formate, which spontaneously releases CO2 at room temperature, a feature that typically requires much higher temperatures.31 The alternative formation of formate from AuI hydride and HCO2H with release of H2 suggests that a thermally more stable AuI hydride might indeed be a competent catalyst for the release of H2 from HCO2H, a chemical hydrogen storage material at ambient conditions.32

Conflict of interest

The authors declare no conflict of interest.

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