Communication pubs.acs.org/JACS
Cationic Copper Hydride Clusters Arising from Oxidation of (Ph3P)6Cu6H6 Shuo Liu,‡ Michael S. Eberhart,‡,§ Jack R. Norton,* Xiaodong Yin, Michelle C. Neary,† and Daniel W. Paley¶ Department of Chemistry, Columbia University, 3000 Broadway, New York, New York 10027, United States S Supporting Information *
We have been been unable to isolate the cation radical [(Ph3P)6Cu6H6]•+ (1•+). When generated in situ, 1•+ is intensely green, with a λmax at 655 nm that we assign to a dd transition from lower molecular orbitals (HOMO-5 and HOMO-6) to the SOMO orbital left by one-electron oxidation. (Details of the DFT calculations are given in SI.) When 1 equiv of 1 is oxidized with less than 1 equiv of Cp*2Fe+, proportionally less 1•+ is formed (Figure 1a). The absorbance
ABSTRACT: Transfer of the first electron from (Ph3P)6Cu6H6 to Cp*2Fe+ is fast (k > 106 L·mol−1·s−1). Transfer of a second electron to the same oxidant has a much lower thermodynamic driving force and is considerably slower, with k = 9.29(4) × 103 L·mol−1·s−1. The second oxidation leads to the formation of [(Ph3P)6Cu6H5]+. The structure of [(Ph3P)6Cu6H5]+ has been confirmed by its conversion back to (Ph3P)6Cu6H6 and by microanalysis; X-ray diffraction shows that the complex is a bitetrahedron in the solid state. [(Ph3P)6Cu6H5]+ can also be prepared by treating (Ph3P)6Cu6H6 with MeOTf. With less than 1 equiv of Cp*2Fe+ as oxidant, (Ph3P)6Cu6H6 gives [(Ph3P)7Cu7H6]+ as the major product; X-ray diffraction shows a Cu6 octahedron with one face capped by an additional Cu. [(Ph3P)7Cu7H6]+ can also be prepared by treating (Ph3P)6Cu6H6 with [Cu(CH3CN)4]+ (along with 1 equiv of Ph 3 P), and can be converted back to (Ph3P)6Cu6H6 with base/H2.
Figure 1. (a) Absorbance change at 655 nm upon addition of various amounts of Cp*2Fe+ to 0.87 mM of 1 in CH2Cl2 in a stopped-flow apparatus, with a light path of 1.5 mm. The reaction was monitored for 500 ms. (b) Plot of absorbances at 655 nm vs various concentrations of Cp*2Fe+ in CH2Cl2.
C
opper hydrides have become widely used in organic synthesis and catalysis.1 There were early suggestions that electron transfer was the initial step in the catalysis by [(Ar3P)6Cu6H6]2 of the hydrogenation of the CC in enones and α,β-unsaturated esters,3 but later experiments argued against that proposal.2e These clusters are, however, good oneelectron reducing agents, with reversible potentials around −1.1 V relative to Fc/Fc+; we have already shown that they transfer an electron to the 1-(phenoxycarbonyl)pyridinium cation.4 Furthermore, they can be prepared from hydrogen gas, which suggests the possibility that they can catalyze the generation of electrons from H2. Indeed, (Ph3P)6Cu6H6 slowly (12 turnovers/5 days) catalyzes the oxidation of H2 by Cp*2Fe+ in the presence of excess NEt3. Decamethylferrocenium (Cp*2Fe+), which has an E1/2 of −0.59 V vs Fc/Fc+ in CH2Cl2,5 carries out the first oxidation of (Ph3P)6Cu6H6 (1) within a few milliseconds in a stopped-flow apparatus. (The rate constant for electron transfer must be >1 × 106 L·mol−1·s−1.) In an NMR tube, 1 equiv of Cp2*Fe+ produces a quantitative yield of decamethylferrocene (by 1H integration), implying the stoichiometry in eq 1. (Ph3P)6 Cu6H6 + Cp*2 Fe+ → [(Ph3P)6 Cu6H6]•+ + Cp*2 Fe 1
1• +
© 2017 American Chemical Society
at 655 nm is linear with the amount of oxidant used (Figure 1b), implying an extinction coefficient ε = 8.3(2) × 103 L· mol−1·cm−1 for 1•+. The cation radical 1•+ is stable for over 500 ms. Further oxidation of the cation radical 1•+ occurs when 1 is treated with more than 1 equiv of Cp*2Fe+. (Cyclic voltammetry on 1 in CH2Cl2 shows a second, irreversible, oxidation at −0.6 V relative to Fc/Fc+.6) If 3, 4, or 5 equiv of Cp*2Fe+ are used with 1 in a stopped-flow experiment, the additional equivalents oxidize the 1•+ that is formed initially. (We added, as has been common,2a,7 an excess of free PPh3 to stabilize the copper hydride clusters.) The disappearance of 1•+ can be monitored from the decay of λmax at 655 nm (Figure 2), and the rate constants can be determined by fitting the plots to the equation expected (eq 2) for a second-order rate law.8,9 (A(655) represents absorbances at 655 nm; Δ0 = [Cp*2Fe+]0 − [1•+]0.) All these experiments give approximately the same value of k, about 9.29(4) × 103 L·mol−1·s−1, confirming that the rate law is indeed second order. Received: March 6, 2017 Published: May 30, 2017
(1) 7685
DOI: 10.1021/jacs.7b02183 J. Am. Chem. Soc. 2017, 139, 7685−7688
Communication
Journal of the American Chemical Society
Figure 2. Time profiles of absorbance at 655 nm upon addition of various amounts Cp*2Fe+ to 0.83 mM 1 in CH2Cl2 in a stopped-flow apparatus, with a light path of 1.5 mm. (a) 2.50 mM of Cp*2Fe+ (b) 3.33 mM Cp*2Fe+ (c) 4.15 mM Cp*2Fe+. (Red lines: curve fittings using eq 2.) A(655)∞ + A(655)t =
{
(
A(655)0 1 − 1−
[1•+ ]0 [Fc* +]0
[1•+ ]0 [Fc* +]0
) − A(655) }e ∞
Figure 3. Core (Cu6/P6) ellipsoid plots (50% probability) of 3+, [(Ph3P)6Cu6H5]+. In 3a, the Cu(1)−Cu(3) distance is 2.560(3) Å; Cu(5)···Cu(11), 3.333(3) Å; Cu(7)···Cu(9), 3.987(3) Å. Average Cu−Cu distance: 2.552 Å. In 3b, the Cu(13)−Cu(15) distance is 2.642(3) Å; Cu(19)−Cu(21), 2.586(2) Å; Cu(17)···Cu(23), 4.102(2) Å. Average Cu−Cu distance: 2.571 Å.
−k Δ 0 t
[Ir6(NHC)8(CO)2H14]2+ dication.13 A metal-cluster geometry like that of 3b (two tetrahedra sharing an edge but also linked by another metal−metal interaction) has been reported for Ir4(CO)11(AuPPh3)2.14 The formation of 3+ from 12+ implies the loss of a proton. That proton is likely to be absent in 2: the addition of Et3N does not affect the rate of conversion of 2 to 3+. We postulate 12+ protonates a hydride ligand (of 1, or 1•+) to generate H2 (Scheme 1).15
e −k Δ 0 t
(2) [(Ph3P)6 Cu6H6]• + + Cp*2 Fe+ 1• +
2nd
⎯⎯⎯⎯⎯⎯⎯⎯⎯→ [(Ph3P)6 Cu6H6]2 + + Cp*2 Fe+ Oxidation
12 +
(3)
Removal of a second electron from 1 should give the dication [(Ph3P)6Cu6H6]2+ (12+) (eq 3). However, the irreversibility of the second oxidation of 1 in the CV implies a fast following reaction. Our stopped-flow experiments showed no noteworthy UV−vis features after the second oxidation, so we looked at the reaction by NMR spectroscopy. When 2 equiv of Cp*2Fe+ was added (as the [B(C6F5)4]− salt) to a solution of 1 and excess free ligand (PPh3) in C6D6 at room temperature, the first spectrum (10 min after mixing) showed a new hydride singlet at δ 2.92 from an unidentified species 2.10 Over 10 h, the signal of 2 was largely replaced by a septet (δ 4.28, JH−P = 7.2 Hz, 3+), from hydride ligands coupled equally (as a result of intramolecular rearrangement) to six phosphine ligands. Treatment of 1 with 1 equiv of MeOTf in C6D6 gave methane and a pure sample of the triflate salt of 3+ (eq 4). After the isolated salt was dissolved in CD2Cl2 and examined by 1H NMR (Figure S6), integration of the hydride signal vs the aromatic signals (from six PPh3) suggested that 3+ contained five hydride ligands. A formula of [(Ph3P)6Cu6H5]+ was inferred, and verified by microanalysis of the triflate salt. Metathesis with [K][B(C6F5)4] afforded [3][B(C6F5)4], with δ 4.28, JH−P = 7.2 Hz, in C6D6.
Scheme 1. Electron Transfer Reactions of 1 and Formation of 3+
The cation 3+ is the product of the loss of H− from 1. The neutral 1 can be regenerated from 3+, H2 and base: excess NaOPh in THF, under H2, gives 1 in 70% yield (eq 5). Phenol does not produce appreciable H2 loss from 1 (the equilibrium in eq 6 lies to the left), but Et3NH+ does (the equilibrium in eq 7 lies to the right). We have been unable to perform the same experiments in CH3CN (because of low solubility), but if we assume that the positions of these equilibria are the same in that solvent we can estimate the hydricity of 1. For a hydride to release H2 with Et3NH+ requires a ΔGoH− (hydricity of the metal hydride) < 50 kcal/mol in CH3CN, whereas the release of H2 with phenol requires a ΔGoH− > 36 kcal/mol in CH3CN.16 The assumed positions of our equilibria in CH3CN suggest a ΔGoH− between 36 and 50 kcal/mol for 1 in that solvent, and suggest 1 is capable of transferring H− to CO2 (the ΔGoH− of HCO2− is 44 kcal/mol in CH3CN).1j This reasoning supports the conclusion in Appel’s recent paper1e that copper hydrides are powerful hydride donors.
MeOTf (1equiv)
(Ph3P)6 Cu6H6 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ (Ph3P)6 Cu6H5[OTf] 1
C6H6
[3][OTf]
(4)
Crystallization of [3][OTf] proved challenging because of its high solubility, but after many attempts we succeeded.11 X-ray diffraction showed two independent but closely related Cu6 cluster cations in the asymmetric unit (3a and 3b in Figure 3); their heavy-atom structures (Cu and P only) are depicted in Figure 3. Both cations contain two tetrahedra sharing an edge, but have slightly different Cu−Cu distances; the single 1H NMR peak shows that they give the same species in solution. DFT calculations converge on a structure (Figure S11) of C2v symmetry, with four symmetry-related face-bridging hydrides and one edge-bridging hydride. Similar edge-sharing bitetrahedra (3a) have been found in the structures of the [Au6(PPh3)6]2+ dication12 and the
H2(80 psi), NaOPh (excess)
(Ph3P)6 Cu6H5+OTf ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ (Ph3P)6 Cu6H6 [3][OTf]
(Ph3P)6 Cu6H6 + PhOH 1
7686
Overnight,THF ‐ d8
⥃ THF ‐ d8
1
(5)
+
(Ph3P)6 Cu6H5 OPh + H 2 [3][OPh]
(6)
DOI: 10.1021/jacs.7b02183 J. Am. Chem. Soc. 2017, 139, 7685−7688
Communication
Journal of the American Chemical Society (Ph3P)6 Cu6H6 + [HNEt3]+ ⥂ (Ph3P)6 Cu6H5+ + H 2 3+
THF ‐ d8
1
In summary, removal of the first electron from (Ph3P)6Cu6H6 (1) by Cp*2Fe+ is extremely fast. Removal of a second electron by Cp*2Fe+ is much slower, with a rate constant 9.29(4) × 103 L·mol−1·s−1. Loss of a proton from 12+ gives a hexanuclear copper cation [(Ph3P)6Cu6H5]+ (3+) with an edge-shared bitetrahedral structure. Pure 3+ can be prepared by removing a hydride from 1 with MeOTf. With
