10.1002/anie.201813419
Angewandte Chemie International Edition
COMMUNICATION
The geometrical evolution discussed above, in which two ligands
in trans (CO and Rf) remain as mere spectators, is reminiscent
of the classical oxidative addition of H2 to the Vaska's type
complex trans-[MCl(CO)(PPh3)2] (M = Rh, Ir).18 In the latter,
however, the L–M–L axis remains passive while the more
electron poor Cl–M–CO axis undergoes angle bending;19 the
process involves electron donation of the electron pair of the H–
H bond to Rh,20 eventually leading to a different isomer when
back-donation from Rh cleaves the H–H bond.21
DJEI/DES/SFI/HEA Irish Centre for High-End Computing
(ICHEC) is also acknowledged for the provision of computational
facilities and support. M. N. P.-D. gratefully acknowledges the
Spanish MECD for a FPU scholarship.
Keywords: bimetallic catalysis • oxidative addition • gold •
rhodium • transmetalation mechanism
Some concepts used in the text deserve some comment. We
are using the uncommon expression asymmetric oxidative
insertion of Rh into the Au–C bond to define the intimate
oxidation/reduction mechanism of the process, at variance with
the symmetric oxidative addition occurring with H2 on Vaska’s
complexes. The rules to assign formal oxidation numbers
impose that the M–M' bonds should not count because the
electron pair in that bond is supposed to be equally shared
between the two metals (non-polarized bond); with this rule,
complexes I2 would be assigned RhII and Au0 oxidation states,
corresponding to an overall one electron oxidation of RhI by AuI.
In the latter section we have considered the anionic moiety
[(Ph3As)Au:]– because classifying ligands in a trans-influence list
requires to consider them as 2e– donors; within this approach,
the complexes would be assigned RhIII and Au–I. Finally, in
apparent contradiction, in TS1 the Pf–AuL bond is eventually
asymmetrically cleaved providing the Rh center with the
fragments LAu+ and Pf–. This illustrates how assigning oxidation
states may be misleading about the real electron density on the
metal, and about bond polarizations. This is better examined by
looking at the NPA (natural population analysis) charges on Rh
in TS1 and I2 (–1.15 and –1.22 respectively), which show that
RhIII has more negative charge than in the RhI complex 2 (–0.70).
Overall, Rh gains electron density while it is formally being
oxidized. The gold atom shows only minor differences between
the linear molecule and the octahedral intermediate, with similar
slightly positive charges in the range +0.27 to +0.20.
[1]
[2]
a) M. H. Pérez-Temprano, J. A. Casares, P. Espinet, Chem. –Eur. J.
2012, 18, 1864–1884; b) D. R. Pye, N. P. Mankad, Chem. Sci. 2017, 8,
1705–1718.
a) Y. Shi, S. A. Blum, Organometallics, 2011, 30, 1776–1779; b) J.
delPozo, J. A. Casares, P. Espinet, Chem. – Eur. J. 2016, 22, 4274–
4284; c) R. J. Oeschger, P. Chen, J. Am. Chem. Soc. 2017, 139, 1069–
1072.
[3]
[4]
J. delPozo, D. Carrasco, M. H. Pérez-Temprano, M. García-Melchor, R.
Álvarez, J. A. Casares, P. Espinet, Angew. Chem. Int. Ed. 2013, 52,
2189–2193.
For more details and formation of homocoupling products R2–R2, see:
J. del Pozo, G. Salas, R. Álvarez, J. A. Casares, P. Espinet,
Organometallics, 2016, 35, 3604–3611; and references therein.
M. N. Peñas-Defrutos, C. Bartolomé, M. García-Melchor, P. Espinet,
Chem. Commun. 2018, 54, 984–987.
[5]
[6]
D. Carrasco, M. García-Melchor, J. A. Casares, P. Espinet, Chem.
Commun. 2016, 52, 4305–4308.
[7 ]
[8]
A. L. Casado, P. Espinet, Organometallics, 1998, 17, 3677–3683.
The reaction rate when the products formed reaches 10% is
proportional to (0.90)2 – (0.10)2 ≈ 0.8. At that point the rate has
decreased to 80% of r0. The rate constant error within this limit, when
translated to ∆G‡ values, is < 1 kcal mol–1
.
[9]
These experiments were carried out at 298 K due to the lower rate in
the presence of high concentrations of free arsine.
[10] For a related case of arsine dependence in Pd see: M. H. Pérez-
Temprano, J. A. Casares, A. R. de Lera, R. Álvarez, P. Espinet, Angew.
Chem. Int. Ed. 2012, 51, 4917–4920.
[11] S. Kozuch, S. Shaik, Acc. Chem. Res. 2011, 44, 101–110.
[12] S. Hoops, S. Sahle, R. Gauges, C. Lee, J. Pahle, N. Simus, M. Singhal,
L. Xu, P. Mendes, U. Kummer, Bioinformatics, 2006, 22, 3067–3074.
[13] To be more precise (see SI) COPASI produces changes in I2 and I3
(less than 1 kcal mol–1) when the reversibility condition is imposed.
[14] a) C. L. McMullin, J. Jover, J. N. Harvey, N. Fey, Dalton Trans. 2010,
39, 10833–10836; b) C. L. McMullin, N. Fey, J. N. Harvey, Dalton Trans.
2014, 43, 13545–13556.
In conclusion, the RhIAr/AuIAr' transmetalation studied here
does not follow the traditional Ar/Ar' double-bridged mechanism,
but involves an oxidation/reduction mechanism. Moreover this is
also an unusual one in that it is initiated by donation of an
electron pair from Rh to Au, which eventually triggers the
transfer of [Ar:]– to Rh. Interestingly, the different electronic
characteristics of the polar Ar–AuL and the non-polar H–H
bonds on Vaska's type complexes induce formation of different
octahedral isomers.
[15] R. Asatryan, E. Ruckenstein, J. Hachmann, Chem. Sci. 2017, 8, 5512–
5525.
[16] For a related case see: C. Bartolomé, Z. Ramiro, M. N. Peñas-Defrutos,
P. Espinet, ACS Catal. 2016, 6, 6537–6545.
[17] a) S. I. Gorelsky, D. Lapointe, K. Fagnou, J. Am. Chem. Soc. 2008, 130,
10848. b) M. García-Melchor, S. I. Gorelsky, T. K. Woo. Chem. – Eur. J.
2011, 17, 13847–13853; c) F. M. Bickelhaupt, K. N. Houk. Angew.
Chem. Int. Ed. 2017, 56, 10070–10086.
[18] L. Vaska, J. W. DiLuzio, J. Am. Chem. Soc., 1962, 84, 679–680.
[19] C. E. Johnson, R. Eisenberg, J. Am. Chem. Soc. 1985, 107, 3148–
3160.
Experimental Section
[20] A. Didieu, A. Strich, Inorg. Chem. 1979, 18, 2940–2943.
[21] J. J. Low, W. A. Goddard III, J. Am. Chem. Soc. 1984, 106, 6928–6937.
Experimental and computational details are given in the SI.
Acknowledgements
The authors thank the financial support from the Spanish
MINECO (projects CTQ2016-80913-P and CTQ2017-89217-P),
the Junta de Castilla
y León (project VA051P17). The
This article is protected by copyright. All rights reserved.