Source: https://pubs.rsc.org/en/content/articlehtml/2019/cp/c9cp01309a
Timestamp: 2019-04-21 12:06:28+00:00

Document:
Local energy decomposition (LED) analysis decomposes the interaction energy between two fragments calculated at the domain-based local pair natural orbital CCSD(T) (DLPNO-CCSD(T)) level of theory into a number of chemically meaningful contributions. Herein, this scheme is applied to the interaction between the transition metal (TM) and the alkane in σ-complexes. It is demonstrated that the often-neglected London dispersion (LD) energy is a fundamental component of the TM–alkane interaction for a wide range of experimentally characterized σ-complexes. LD effects determine the structure and the thermodynamic stability of σ-complexes and influence the selectivity of CH activation reactions. The magnitude of the LD energy can be modulated by increasing the size of the alkane and of the ancillary ligands on the TM. These results provide further evidence on the fundamental role that London dispersion plays in organometallic chemistry.
Despite the success of these simple interpretative frameworks, a unified understanding of intermolecular interactions in the various fields of chemical research would require models that explicitly include LD as one of the components of the chemical bond, irrespective of the intermolecular distance or of the interaction strength. Unfortunately, rigorous perturbative approaches like SAPT are not applicable to strong TM–L interactions.
Herein, the DLPNO-CCSD(T)/LED approach is used to quantify the LD component of TM–L bonds of various strength and to elucidate the factors that contribute to its magnitude in organometallic complexes. As a case study, we consider the bond between the TM and the CH bond of an alkane in σ-complexes. These complexes have been suggested as crucial intermediates for alkane C–H activation reactions,38–44 which are of paramount importance in various chemical industries. In particular, the development of chemical processes for the conversion of alkanes from fossil sources to more-valuable compounds has attracted great attention over past decades.44–48 In the simplest mechanistic scenario, the CH bond initially coordinates to the TM to form a σ-complex (intermediate 2 in Scheme 1a), eventually leading to the cleavage of the CH bond through oxidative addition to the TM.
Scheme 1 (a) Oxidative addition of an alkane to a TM. R denotes an alkyl group. (b) Simple orbital representation describing the donor acceptor interaction between the R–H bond and the TM in alkane σ-complexes.
Although alkane σ-complexes have been proposed for decades, their direct experimental characterization has constituted one of the major challenges for organometallic chemists.54,55 The few experimental characterized σ-complexes can be grouped into three families, differing for the complex structure and their synthetic route. Herein, we base our analysis on a series of σ-complexes that includes representative systems of each class.
The first class of σ-complexes investigated herein is a series of group VII TM complexes that were previously characterized mainly by means of IR spectroscopy. Their structure is shown in Scheme 2a. The series includes neutral complexes differing for the size of the alkane such as [CpMn(CO)2(alkane)] (alkane = C2H6, n-C7H16)56–59 and [CpRe(CO)2(alkane)] (alkane = CH4, n-C5H12, n-C7H16)56,57,60 as well as the positively charged [(HEB)Re(CO)2(n-C5H12)]+ (HEB = hexaethylbenzene).61 Note that for the longer alkanes many conformers are possible, depending on their relative orientation with the TM. As case studies, two conformers are investigated for each complex, in which the TM interacts with a CH bond located on either the methyl or the methylene carbon. Conformers of the first type are denoted hereafter as “head-on” structures, whilst the latter are denoted as “side-on” structures.
Scheme 2 TM–alkane complexes investigated and nomenclature used in this work.
Finally, the last class includes Rh complexes of norbornane bearing different diphosphine ligands [(L)Rh(norbornane)]+ (L = dpce, dippe, dibpe) as well as the [(dcpe)Rh(n-C5H12)]+ complex. All these complexes were fully characterized by X-ray crystallography and solid state NMR.49,64–66 Their structures are shown in Scheme 2c.
For all these complexes, the main factors contributing to their formation, the coordination preference of the alkane and the activation of the CH bond are discussed in the framework of the LED analysis and the role played by LD in all these situations is elucidated. Moreover, the factors that contribute to the magnitude of the LD energy are identified and designing principles for σ-complexes with tailored bonding features are given.
Single point energies at the DLPNO-CCSD(T) level were computed using TightPNO settings.31–33 The def2-TZVPP basis set was used in conjunction with the def2-TZVPP/C auxiliary basis set. Table S2 (ESI†) shows that the DLPNO-CCSD(T) results are at convergence with respect to the basis set size. The RIJCOSX approximation71 was used for the calculation of Coulomb and exchange integrals in the HF reference. The def2/J auxiliary basis set was used. The COSX grid was set to GridX5.
For the systems studied in this work EC-CCSDdisp amounts to about 90% of the overall Edisp contribution, as shown in Table S3 (ESI†).
The remaining part of the correlation contribution to the binding energy is denoted as ΔEC-CCSD(T)no-disp. The sum of this term with the overall HF contribution to the binding energy ΔEHF gives the overall non-dispersive contribution to the binding energy, i.e., ΔEno-disp.
In order to provide an in-depth analysis of the factors contributing to the LD energy, the Dispersion Interaction Density (DID) plot29,72 were employed. They provide a useful spatial analysis of the LD energy and were calculated as detailed in ref. 29.
We first describe here a detailed investigation of the Re⋯CH bond in the [CpRe(CO)2(CH4)] complex. The analysis will be then extended to the whole series of σ-complexes shown in Scheme 2 in the next section.
Fig. 1 (a) DLPNO-CCSD(T) binding energy decay with the Re⋯CH4 distance in [CpRe(CO)2(CH4)]. Its decomposition into dispersive (Edisp) and non-dispersive (ΔEno-disp) contributions is also shown. (b) Binding energy profiles with different computational methods. (c) Comparison of LED (Edisp) and -D3 estimate for the London dispersion. The vertical line marks the equilibrium geometry.
In order to compare different computational methodologies, the binding energy profile obtained with different functionals is compared with the DLPNO-CCSD(T) one in the central panel of Fig. 1. In the bottom panel of the same figure the decay of the -D3 correction for different functionals is compared with the Edisp values extracted from the LED scheme.
In the long range, all methods provide similar Re⋯CH4 binding energies. At the equilibrium geometry, the difference between the various functionals increases, with binding energies being −51.6, −55.8, −64.2 and −64.5 kJ mol−1 for BLYP-D3, B3LYP-D3, TPSS-D3 and PBE-D3, respectively. The corresponding -D3 correction follows the opposite trend, being −25.7, −21.6, −16.6 and −12.7 kJ mol−1. The differences between the -D3 estimates are mostly due to the effect of the damping functions, as also recently discussed by Grimme et al.13 By comparing these figures with the Edisp value (−46.7 kJ mol−1), one finds that the functionals that better reproduce the DLPNO-CCSD(T) binding energy (e.g. PBE) are those for which the -D3 correction is smaller and far from the accurate LED values. Hence, in the following only DLPNO-CCSD(T)/LED results will be discussed.
In summary our results clearly show that LD is fundamental for both the formation and the thermodynamic stability of [CpRe(CO)2(CH4)]. In fact, it dominates the TM–alkane interaction in the long range and significantly contributes to the magnitude of the binding energy at the equilibrium geometry.
In order to test the generality of our findings for the Re⋯CH4 interaction in [CpRe(CO)2(CH4)], geometries, binding energies and the corresponding LD contributions were calculated for the whole series of σ-complexes investigated in this work. The results obtained are summarized in Fig. 2. Note that, for the sake of simplicity, only electronic energies are discussed in this section, while the free energies at experimental conditions are reported in Table S4 (ESI†).
Fig. 2 Comparison of TM⋯alkane binding energies (ΔE) and the associated LD contribution (Edisp) for the systems investigated in this work. The distance between the TM and the closest carbon atom on the alkane r(TM–C) is also reported.
Binding energies span a broad range of values, going from −57.6 to −127.8 kJ mol−1 for [CpMn(CO)2(C2H6)] and [(dcpe)Rh(norbornane)]+, respectively. Similarly, the LD energy ranges from −46.7 to −111.3 kJ mol−1 for [CpRe(CO)2(CH4)] and [(dcpe)Rh(norbornane)]+, respectively.
Having established the fundamental importance of LD for the thermodynamic stability of σ-complexes, we aim at identifying the most important factors contributing to its magnitude in a view of aiding to the development of designing principle for stable TM–alkane adducts.
As mentioned in the introduction, LD increases with the polarizability of the interacting fragments and decreases with the intermolecular distance. Hence, the size of the ancillary ligands and of the alkane, the nature and the oxidation state of the TM, and the coordination mode of the alkane (i.e. whether it forms head-on or side-on structures) are all expected to contribute to its magnitude.
Fig. 3 Dispersion interaction density (DID) plots for selected TM–alkane σ-complexes experimentally characterized (see Scheme 2). The density isosurfaces were generated for a contour value of 0.1 e Bohr−3. The DID color scheme ranges from highest value (red) to zero dispersion energy (blue).
The first eye-catching feature of these plots is that the most significant contribution to the LD energy originates from short TM⋯H–C contacts, as evidenced by the red color of the density isosurfaces around the C–H groups in close proximity of the TM. In Mn and Re complexes, only one C–H group strongly interacts with the TM, whilst all Rh complexes (Fig. 2b and c) are stabilized by two short TM⋯H–C contacts. For this reason, the LD energy in Rh complexes is typically larger than in the other σ-complexes investigated in this work.
The size of the alkane also plays an important role in affecting the magnitude of the LD contribution. By comparing [CpRe(CO)2(CH4)], [CpRe(CO)2(n-C5H12)] (I), and [CpRe(CO)2(n-C7H16)] (I), in which alkanes of different size interact through their methyl carbon with the same TM fragment, it can be found that the LD increases with the size of the alkane, in line with the above considerations. However, [CpRe(CO)2(n-C5H12)] (I) and [CpRe(CO)2(n-C7H16)] (I) have essentially the same amount of LD energy. This occurs because the tail of the heptane chain in [CpRe(CO)2(n-C7H16)] is too far away from the Re atom to affect the LD energy, as shown by the blue color that the DID plot assumes in this region. The same trend can also be observed for other pair of complexes, i.e., [CpMn(CO)2(C2H6)] vs. [CpMn(CO)2(n-C7H16)] (I) and [(PONOP)Rh(CH4)]+ vs. [(PONOP)Rh(C2H6)]+. As these variations in the LD energy correlate well with the corresponding variations in the binding energies, it can be concluded that the size of the alkane does not significantly affect the other components of the interaction.
Similarly, the use of bulky ligands on the TM also increases the LD contribution to the binding energy. In fact, going from [(dippe)Rh(norbornane)]+ to [(dbpe)Rh(norbornane)]+, Edisp increases from 103 to 110 kJ mol−1 as a consequence of the substitution on the four iPr groups on the diphosphine ligand with the more polarizable tBu groups. However, these figures do not correlate with the corresponding variations in the binding energy. This is probably due to steric effects associated with the inclusion of bulkier tBu groups, as shown by the fact that r(TM–C) distance slightly increases going from [(dippe)Rh(norbornane)]+ (2.40 Å) to [(dbpe)Rh(norbornane)]+ (2.42 Å).
As mentioned above, different coordination modes (e.g. side-on vs. head-on) are possible for the longer alkanes. To investigate the effect of LD on the coordination preference of the alkane, it is useful to compare binding and LD energies for the pairs [CpMn(CO)2(n-C7H16)] (I/II), [CpRe(CO)2(n-C5H12)] (I/II), [CpRe(CO)2(n-C7H16)] (I/II) and [(HEB)Re(CO)2(n-C5H12)]+.
In all cases, LD is larger for side-on than for head-on structures. In fact, the former are more compact and feature many TM⋯HC contacts. Consistent with this observation, the TM–alkane binding energy is always slightly larger for side-on structures, with the only exception being the positively charged [(HEB)Re(CO)2(n-C5H12)]+. In this case, the positive charge on the metal probably increases the relative importance of the non-dispersive interaction components, such as polarization and TM ← CH σ-donation. Indeed, by subtracting the LD energy from the binding energy, one finds that the non-dispersive contributions favour head-on structures over side-on structures in all cases. In fact, due to the less congested environment around the TM center, the head-on structures feature slightly smaller r(TM–C) values (Fig. 2), and charge transfer and polarization effects decay quickly with the intermolecular distance.29,73 This aspect will be further discussed in Section 3.3 while investigating the physical components affecting the selectivity of CH activation reactions.
In summary, LD is fundamental for the thermodynamic stability of σ-complexes. It favors the formation of σ-complexes of long alkanes, influences the alkane coordination preference and favors the formation of compact structure with many TM⋯CH contacts.
To provide an insight into this aspect, we investigated a model oxidative addition step in which n-pentane interacts with CpRe(CO)2 to form the oxidative addition product via the formation of the σ-complex intermediate [CpRe(CO)2(n-C5H12)] (Scheme 2a). This reaction has been recently studied by means of DFT calculations, but the role played by the LD energy in this context was not discussed.53 The computed mechanism at the DLPNO-CCSD(T) level of theory is shown in Fig. 4.
Fig. 4 Oxidative addition of pentane to the CpRe(CO)2 fragment. Electronic energies (ΔE) and free energies (ΔG) at 190 K are reported relative to the isolated fragments. The distance between the TM and the closest carbon atom on the alkane r(TM–C) is also reported.
These results can be explained by considering that the CH bond has to be in close proximity to the TM in order to be activated towards the oxidative addition. However, the equilibrium TM⋯C distance in [CpRe(CO)2(n-C5H12)] (II) is significantly longer than that of [CpRe(CO)2(n-C5H12)] (I) (2.68 vs. 2.63 Å) and this difference is partially retained also in the corresponding transition states (2.43 vs. 2.41 Å) and products (2.30 vs. 2.28 Å). This effect probably originates from significant steric effects in the side-on conformation, due to the congested environment around the TM center.
Consistent with these observations, the non-dispersive contribution to the binding energy amounts to −22.4 and −26.8 kJ mol−1 for [CpRe(CO)2(n-C5H12)] (II) and [CpRe(CO)2(n-C5H12)] (I), respectively, and to +63.4 and +52.6 kJ mol−1 for the corresponding transition states. Hence, polarization and charge transfer effects seem to favor the activation of the CH bond at the methyl carbon.
In summary, even though LD determines to a large extent the thermodynamic stability of σ-complexes, the activation barrier associated with the oxidative addition step is determined by an interplay of LD, steric and charge transfer effects.
The recently introduced DLPNO-CCSD(T)/LED scheme was used to compute TM⋯alkane binding energies for a broad range of experimentally characterized σ-complexes and to provide an accurate quantification of the London dispersion component of the coordination bond.
It was found that London dispersion significantly contributes to the TM⋯alkane interaction and is largely responsible for the thermodynamic stability of σ-complexes. Its magnitude can be modulated by modifying the size of the substituents on the TM fragment or by increasing the number of carbons in the alkane.
Moreover, LD influences the coordination preference of the alkane to the TM. As LD increases with the number of TM⋯CH contacts, side-on structures are generally favored over head-on structures, consistent with previous experimental findings. On the other hand, due to the steric effects, head-on structures feature a shorter distance between the TM and the coordinated carbon, leading to larger polarization and charge transfer interactions.
As charge transfer effects weaken the CH bond, activating it towards the cleavage, the oxidative addition of methyl CH bonds to the TM is expected to be kinetically preferred, as shown in a prototype oxidative addition reaction and consistent with previous experimental findings.
As LD is ubiquitous in nature, it is expected that our findings concerning its importance in organometallic chemistry can be applied to other TM–L interactions, such as the wide range of σ-complexes that serve as intermediates in CH activation reactions.
We acknowledge the Priority Program “Control of Dispersion Interactions in Molecular Chemistry” (SPP 1807) of the Deutsche Forschungsgemeinschaft for financial support. Open Access funding provided by the Max Planck Society.
G. E. Ball, C. M. Brookes, A. J. Cowan, T. A. Darwish, M. W. George, H. K. Kawanami, P. Portius and J. P. Rourke, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 6927 CrossRef CAS PubMed.
B. Jeziorski, R. Moszynski and K. Szalewicz, Chem. Rev., 1994, 94, 1887 CrossRef CAS.
A. Stone, The theory of intermolecular forces, OUP Oxford, 2013 Search PubMed.
J. P. Wagner and P. R. Schreiner, Angew. Chem., Int. Ed., 2015, 54, 12274 CrossRef CAS PubMed.
A. A. Fokin, T. S. Zhuk, S. Blomeyer, C. Perez, L. V. Chernish, A. E. Pashenko, J. Antony, Y. V. Vishnevskiy, R. J. F. Berger, S. Grimme, C. Logemann, M. Schnell, N. W. Mitzel and P. R. Schreiner, J. Am. Chem. Soc., 2017, 139, 16696 CrossRef CAS PubMed.
S. Rosel, H. Quanz, C. Logemann, J. Becker, E. Mossou, L. Canadillas-Delgado, E. Caldeweyher, S. Grimme and P. R. Schreiner, J. Am. Chem. Soc., 2017, 139, 7428 CrossRef PubMed.
S. Grimme and P. R. Schreiner, Angew. Chem., Int. Ed., 2011, 50, 12639 CrossRef CAS PubMed.
C. N. Pace, J. M. Scholtz and G. R. Grimsley, FEBS Lett., 2014, 588, 2177 CrossRef PubMed.
S. Parveen, S. Rana and R. Fangueiro, J. Nanomater., 2013, 19 Search PubMed.
D. J. Liptrot and P. P. Power, Nat. Rev. Chem., 2017, 1, 0004 CrossRef.
M. Strauss and H. A. Wegner, Eur. J. Org. Chem., 2019, 295 CrossRef CAS.
Q. Lu, F. Neese and G. Bistoni, Angew. Chem., Int. Ed., 2018, 57, 4760 CrossRef CAS PubMed.
M. Bursch, E. Caldeweyher, A. Hansen, H. Neugebauer, S. Ehlert and S. Grimme, Acc. Chem. Res., 2019, 52, 258 CrossRef CAS PubMed.
P. Sliwa and J. Handzlik, Chem. Phys. Lett., 2010, 493, 273 CrossRef CAS.
M. S. G. Ahlquist and P. O. Norrby, Angew. Chem., Int. Ed., 2011, 50, 11794 CrossRef CAS PubMed.
Y. Minenkov, G. Occhipinti, W. Heyndrickx and V. R. Jensen, Eur. J. Inorg. Chem., 2012, 1507, DOI:10.1002/ejic.201100932,.
A. Hansen, C. Bannwarth, S. Grimme, P. Petrovic, C. Werle and J. P. Djukic, ChemistryOpen, 2014, 3, 177 CrossRef CAS PubMed.
L. P. Wolters, R. Koekkoek and F. M. Bickelhaupt, ACS Catal., 2015, 5, 5766 CrossRef CAS.
G. Lu, R. Y. Liu, Y. Yang, C. Fang, D. S. Lambrecht, S. L. Buchwald and P. Liu, J. Am. Chem. Soc., 2017, 139, 16548 CrossRef CAS PubMed.
T. R. Cundari, B. P. Jacobs, S. N. MacMillan and P. T. Wolczanski, Dalton Trans., 2018, 47, 6025 RSC.
E. Detmar, V. Muller, D. Zell, L. Ackermann and M. Breugst, Beilstein J. Org. Chem., 2018, 14, 1537 CrossRef CAS PubMed.
A. A. Thomas, K. Speck, I. Kevlishvili, Z. H. Lu, P. Liu and S. L. Buchwald, J. Am. Chem. Soc., 2018, 140, 13976 CrossRef CAS PubMed.
J. Dewar, Bull. Soc. Chim. Fr., 1951, 18, C71 Search PubMed.
J. Chatt and L. Duncanson, J. Chem. Soc., 1953, 2939 RSC.
S. Kristyán and P. Pulay, Chem. Phys. Lett., 1994, 229, 175 CrossRef.
S. Grimme, A. Hansen, J. G. Brandenburg and C. Bannwarth, Chem. Rev., 2016, 116, 5105 CrossRef CAS PubMed.
R. Peverati and D. G. Truhlar, Philos. Trans. R. Soc., A, 2014, 372, 20120476 CrossRef PubMed.
W. B. Schneider, G. Bistoni, M. Sparta, M. Saitow, C. Riplinger, A. A. Auer and F. Neese, J. Chem. Theory Comput., 2016, 12, 4778 CrossRef CAS PubMed.
A. Altun, F. Neese and G. Bistoni, J. Chem. Theory Comput., 2019, 15, 215 CrossRef CAS PubMed.
A. Altun, M. Saitow, F. Neese and G. Bistoni, J. Chem. Theory Comput., 2019, 15, 1616–1632 CrossRef CAS PubMed.
C. Riplinger and F. Neese, J. Chem. Phys., 2013, 138, 034106 CrossRef PubMed.
C. Riplinger, B. Sandhoefer, A. Hansen and F. Neese, J. Chem. Phys., 2013, 139, 134103 CrossRef PubMed.
C. Riplinger, P. Pinski, U. Becker, E. F. Valeev and F. Neese, J. Chem. Phys., 2016, 144, 024109 CrossRef PubMed.
G. Bistoni, A. A. Auer and F. Neese, Chem. – Eur. J., 2017, 23, 865 CrossRef CAS PubMed.
A. Altun, F. Neese and G. Bistoni, Beilstein J. Org. Chem., 2018, 14, 919 CrossRef CAS PubMed.
F. Neese, M. Atanasov, G. Bistoni, D. Maganas and S. Ye, J. Am. Chem. Soc., 2019, 141, 2814–2824 CrossRef CAS PubMed.
V. M. Schmiedel, Y. J. Hong, D. Lentz, D. J. Tantillo and M. Christmann, Angew. Chem., Int. Ed., 2018, 57, 2419 CrossRef CAS PubMed.
C. L. Gross and G. S. Girolami, J. Am. Chem. Soc., 1998, 120, 6605 CrossRef CAS.
D. D. Wick, K. A. Reynolds and W. D. Jones, J. Am. Chem. Soc., 1999, 121, 3974 CrossRef CAS.
W. H. Bernskoetter, S. K. Hanson, S. K. Buzak, Z. Davis, P. S. White, R. Swartz, K. I. Goldberg and M. Brookhart, J. Am. Chem. Soc., 2009, 131, 8603 CrossRef CAS PubMed.
T. A. Mobley, C. Schade and R. G. Bergman, J. Am. Chem. Soc., 1995, 117, 7822 CrossRef CAS.
E. A. Cobar, R. Z. Khaliullin, R. G. Bergman and M. Head-Gordon, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 6963 CrossRef CAS PubMed.
S. E. Bromberg, H. Yang, M. C. Asplund, T. Lian, B. K. McNamara, K. T. Kotz, J. S. Yeston, M. Wilkens, H. Frei, R. G. Bergman and C. B. Harris, Science, 1997, 278, 260 CrossRef CAS.
M. Lersch and M. Tilset, Chem. Rev., 2005, 105, 2471 CrossRef CAS PubMed.
B. G. Hashiguchi, S. M. Bischof, M. M. Konnick and R. A. Periana, Acc. Chem. Res., 2012, 45, 885 CrossRef CAS PubMed.
W. D. Jones, Inorg. Chem., 2005, 44, 4475 CrossRef PubMed.
Y. Qin, L. H. Zhu and S. Z. Luo, Chem. Rev., 2017, 117, 9433 CrossRef CAS PubMed.
T. Brent Gunnoe, in Alkane C-H Activation by Single-Site Metal Catalysis, ed. P. Pedro, Springer, Dordrecht, Heidelberg, New York, London, 2012, pp. 1–15 Search PubMed.
F. M. Chadwick, T. Kramer, T. Gutmann, N. H. Rees, A. L. Thompson, A. J. Edwards, G. Buntkowsky, S. A. Macgregor and A. S. Weller, J. Am. Chem. Soc., 2016, 138, 13369 CrossRef CAS PubMed.
M. L. H. Green and G. Parkin, Struct. Bonding, 2017, 171, 79 CrossRef CAS.
J. M. Buchanan, J. M. Stryker and R. G. Bergman, J. Am. Chem. Soc., 1986, 108, 1537 CrossRef CAS.
W. Lamanna and M. Brookhart, J. Am. Chem. Soc., 1981, 103, 989 CrossRef CAS.
M. Thenraj and A. G. Samuelson, J. Comput. Chem., 2015, 36, 1818 CrossRef CAS PubMed.
D. Balcells, E. Clot and O. Eisenstein, Chem. Rev., 2010, 110, 749 CrossRef CAS PubMed.
C. Hall and R. N. Perutz, Chem. Rev., 1996, 96, 3125 CrossRef CAS PubMed.
A. J. Cowan, P. Portius, H. K. Kawanami, O. S. Jina, D. C. Grills, X. Z. Sun, J. McMaster and M. W. George, Proc. Natl. Acad. Sci. U. S. A., 2007, 104, 6933 CrossRef CAS PubMed.
G. I. Childs, C. S. Colley, J. Dyer, D. C. Grills, X. Z. Sun, J. X. Yang and M. W. George, J. Chem. Soc., Dalton Trans., 2000, 1901–1906, 10.1039/b001987i.
O. Torres, J. A. Calladine, S. B. Duckett, M. W. George and R. N. Perutz, Chem. Sci., 2015, 6, 418 RSC.
F. P. A. Johnson, M. W. George, V. N. Bagratashvili, L. N. Vereshchagina and M. H. Palmer, Mendeleev Commun., 1991, 1, 26 CrossRef.
X. Z. Sun, D. C. Grills, S. M. Nikiforov, M. Poliakoff and M. W. George, J. Am. Chem. Soc., 1997, 119, 7521 CrossRef CAS.
H. M. Yau, A. I. McKay, H. Hesse, R. Xu, M. He, C. E. Holt and G. E. Ball, J. Am. Chem. Soc., 2016, 138, 281 CrossRef CAS.
W. H. Bernskoetter, C. K. Schauer, K. I. Goldberg and M. Brookhart, Science, 2009, 326, 553 CrossRef CAS.
M. D. Walter, P. S. White, C. K. Schauer and M. Brookhart, J. Am. Chem. Soc., 2013, 135, 15933 CrossRef CAS PubMed.
S. D. Pike, A. L. Thompson, A. G. Algarra, D. C. Apperley, S. A. Macgregor and A. S. Weller, Science, 2012, 337, 1648 CrossRef CAS PubMed.
F. M. Chadwick, N. H. Rees, A. S. Weller, T. Kramer, M. Iannuzzi and S. A. Macgregor, Angew. Chem., Int. Ed., 2016, 55, 3677 CrossRef CAS PubMed.
S. D. Pike, F. M. Chadwick, N. H. Rees, M. P. Scott, A. S. Weller, T. Kramer and S. A. Macgregor, J. Am. Chem. Soc., 2015, 137, 820 CrossRef CAS PubMed.
J. M. Tao, J. P. Perdew, V. N. Staroverov and G. E. Scuseria, Phys. Rev. Lett., 2003, 91, 146401 CrossRef.
S. Grimme, J. Antony, S. Ehrlich and H. Krieg, J. Chem. Phys., 2010, 132, 154104 CrossRef.
A. D. Becke and E. R. Johnson, J. Chem. Phys., 2005, 123, 154101 CrossRef.
F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys., 2005, 7, 3297–3305 RSC.
F. Neese, F. Wennmohs, A. Hansen and U. Becker, Chem. Phys., 2009, 356, 98 CrossRef CAS.
A. Wuttke and R. A. Mata, J. Comput. Chem., 2017, 38, 15 CrossRef CAS.
M. P. Mitoraj, A. Michalak and T. Ziegler, J. Chem. Theory Comput., 2009, 5, 962 CrossRef CAS PubMed.
W. Scherer and G. S. McGrady, Angew. Chem., Int. Ed., 2004, 43, 1782 CrossRef CAS PubMed.
D. J. Lawes, S. Geftakis and G. E. Ball, J. Am. Chem. Soc., 2005, 127, 4134 CrossRef CAS.
R. D. Young, D. J. Lawes, A. F. Hill and G. E. Ball, J. Am. Chem. Soc., 2012, 134, 8294 CrossRef CAS.
A. Sivaramakrishna, P. Suman, E. V. Goud, S. Janardan, C. Sravani, T. Sandeep, K. Vijayakrishna and H. S. Clayton, J. Coord. Chem., 2013, 66, 2091 CrossRef CAS.
J. A. Labinger and J. E. Bercaw, Nature, 2002, 417, 507 CrossRef CAS.
M. C. Asplund, P. T. Snee, J. S. Yeston, M. J. Wilkens, C. K. Payne, H. Yang, K. T. Kotz, H. Frei, R. G. Bergman and C. B. Harris, J. Am. Chem. Soc., 2002, 124, 10605 CrossRef CAS.

References: V. 
 V. 
 V. 
 V. 

V. 
 V. 
 V. 
 V.