Understanding the origins of Oyl–U–Oyl bending in the uranyl (UO22+) ion

Trevor W. Hayton
Department of Chemistry and Biochemistry, University of California Santa Barbara, Santa Barbara, CA 93106, USA. E-mail: hayton@chem.ucsb.edu

Received 1st November 2017 , Accepted 20th December 2017

First published on 20th December 2017


The structure of the uranyl ion (UO22+) has been the topic of investigation for almost a century. Since the first structural study of uranyl in 1935, over 4000 uranyl complexes have been characterized by X-ray crystallography. The vast majority of these complexes feature a linear uranyl group (e.g., Oyl–U–Oyl = 180°); however, there are a handful of complexes that feature much more acute Oyl–U–Oyl angles. In fact, the smallest experimentally observed Oyl–U–Oyl angles are ca. 161°. This Frontier Article catalogs every reported uranyl complex that features an Oyl–U–Oyl angle below 172°, and attempts to rationalize the origins of the observed Oyl–U–Oyl bending. In particular, I describe two distinct causes of Oyl–U–Oyl bending: (1) bending that occurs as a result of unfavourable steric interactions between the equatorial co-ligands and the uranyl oxo groups; and (2) bending that appears to have an electronic origin. In addition, I describe several possible avenues for future investigation. Understanding the effect that Oyl–U–Oyl bending has on uranyl electronic structure could ultimately provide insight into several unique aspects of the uranyl ion, such as the inverse trans influence and the involvement of the “pseudo-core” U6p orbitals in U–Oyl bonding.


History

The structure of the uranyl ion (UO22+) has intrigued chemists for decades. The earliest confirmation of the trans uranyl stereochemistry occurred in 1935, when Fankuchen determined the partial structure of [Na][UO2(OAc)3] by X-ray crystallography.1–3 Several years later, Zachariasen determined the structures of [Ca]2[UO23-O)2] and [K]3[UO2F5], placing the trans stereochemistry of the uranyl ion on even firmer experimental footing.4,5 Curiously, however, the solution phase structure of uranyl was still the subject of debate as late as 1958.6,7 This ambiguity was a consequence of the observation of weak ν1 and ν3 U[double bond, length as m-dash]O stretching modes in the IR and Raman spectra, respectively, of a variety of uranyl compounds.2,8–10 For example, Conn and Wu observed a weak band at 860 cm−1 in their IR spectrum of UO2Cl2,11 which they assign to the nominally forbidden ν1 mode. Similarly, Sacconi and co-workers reported the observation of a weak ν1 vibration at ca. 830 cm−1 for [UO2(acac)2(py)].7 Rabinowitch and Belford suggested that these observations were not evidence for a bent uranyl group, but were instead a consequence of ligation of co-ligands to the uranyl equatorial plane, which weakened the vibrational selection rules.2 Interestingly, in 1978, Alcock and co-workers appeared to confirm by X-ray crystallography that the Oyl–U–Oyl angle in [UO2(acac)2(py)] was, in fact, slightly bent (173.5(8)°).12 However, a crystallographic reinvestigation of [UO2(acac)2(py)] in 2006, by Kawasaki and co-workers, revealed a normal Oyl–U–Oyl angle (178.3(2)°) for this complex.13 Thus, it appears likely that observation of the ν1 mode in this example (assuming the original assignment is correct) is due to other factors.

In the 1980s, several research groups performed electronic structure calculations on uranyl in an effort to understand the preference for the trans stereochemistry.14–16 These efforts renewed interest in Oyl–U–Oyl bending and the synthesis of the cis isomer of uranyl. However, subsequent synthetic attempts to generate cis uranyl,17,18 most notably by Duval and co-workers at Los Alamos National Laboratory, were unsuccessful.19,20 Nonetheless, these efforts helped to illuminate the incredible stability of the trans uranyl fragment and highlighted the challenges inherent in the synthesis of a cis uranyl complex. Since these pioneering studies, little progress has been made toward the synthesis of an authentic cis uranyl species. A 2007 report on the isolation of a cis uranyl-containing ferrocenedicarboxylate complex21 came under scrutiny very soon after publication,22 while a 2015 example of a cis uranyl-containing succinate complex is also almost certainly erroneous.23

While efforts to generate a cis uranyl complex appear to have temporarily stalled, these past studies have greatly expanded our understanding of the uranyl ion. In particular, past computational and spectroscopic investigations have uncovered two remarkable phenomena that are operative in uranyl, namely, the inverse trans influence and the involvement of the “pseudo-core” U6p orbitals in the U–O bonding framework.24–28 Both phenomena appear to be unique to the actinyl ions, but the exact nature of these effects, and the role they play in dictating the chemical behavior of actinyls, is still not entirely clear. The study of Oyl–U–Oyl bending could provide answers to these questions.

The vast majority (93%) of the 4000+ structurally characterized uranyl complexes feature Oyl–U–Oyl angles of 175° or greater, according to a search of the Cambridge Structural Database.29 Nonetheless, there are a handful of uranyl complexes that feature unusually bent uranyl fragments. Herein, I describe these complexes and attempt to identify the origins of the Oyl–U–Oyl perturbations. In particular, this survey has identified two distinct causes of Oyl–U–Oyl bending: (1) instances where Oyl–U–Oyl bending is the result of unfavourable steric interactions; and (2) instances where Oyl–U–Oyl bending has an electronic origin. I will also very briefly describe previous computational efforts to explore Oyl–U–Oyl bending. In an effort to keep this Article a reasonable length, I have restricted my discussion to complexes that feature Oyl–U–Oyl angles below 172°, with a few exceptions. For convenience, I am also restricting the discussion to uranyl(VI) ions (e.g., UVIO22+); however, I will note that there are several UVO2+ compounds that feature small Oyl–U–Oyl angles.19,30–34 Because of their weaker U[double bond, length as m-dash]O bonds, the energy required to perturb the Oyl–U–Oyl angle in UVO2+ is probably less than that required for UVIO22+. Also, a handful of compounds with small Oyl–U–Oyl angles have been excluded from this discussion because of disorder,35–39 or the use of crystallographic constraints on the uranyl fragment.40

Oyl–U–Oyl bending in silico

The bending of uranyl has been explored computationally by several different groups.15,41–45 In all cases, the trans isomer of uranyl was found to be more stable than the cis isomer. Interestingly, however, the relative energy difference of the trans and cis isomers appears to be strongly dependent on the identity of the equatorial co-ligands. For example, the cis isomer of [UO2(OH)4]2− is 19 kcal mol−1 higher in energy than trans isomer,42 whereas the cis isomer of [UO22-NO3)2(H2O)2] is 33 kcal mol−1 higher in energy than trans isomer.45 Indeed, the lowest trans/cis isomerization energies are found for [UO2(OH)4]2− and [UO2(H2O)22-ONHCH[double bond, length as m-dash]O)2],42,43,45 which both feature strongly donating, anionic equatorial co-ligands. This observation is perhaps not surprising, as strong equatorial donors are known to weaken the U–Oyl bonds.46 Also of note, adsorption of UO22+(aq) onto alumina is predicted to cause significant bending of the Oyl–U–Oyl angle (ca. 149°), according to DFT calculations.47,48 This bending is likely a consequence of the close approach of the two oxo ligands to the alumina surface. Comparable Oyl–U–Oyl bending is predicted upon adsorption of uranyl onto rutile.49,50 To my knowledge, however, Oyl–U–Oyl bending upon uranyl adsorption has not been confirmed experimentally.

Steric perturbation of the Oyl–U–Oyl angle

Of the thousands of uranyl complexes that appear in the chemical literature, only about 30 feature Oyl–U–Oyl angles smaller than 172°. Of these complexes, unfavourable steric interactions between the equatorial co-ligands and the uranyl oxo groups appear to be the primary cause of the Oyl–U–Oyl bending. For example, Wilkerson and co-workers suggest that the small Oyl–U–Oyl angle (167.8(4)°) in UO2(O-2,6-tBu2C6H3)2(THF)2 (1) is a consequence of the steric repulsion between the tert-butyl substituents of the aryloxide ligands and the uranyl oxo groups (Table 1, Chart 1).51 Steric repulsion between the SiMe3 groups in UO2(N(SiMe3)2)2(py)2 and UO2(NCN)2(THF) (2) and their uranyl oxo ligands, also likely explains the small Oyl–U–Oyl angles in these complexes (170.5(3) and 169.7(2)°, respectively) (Table 1).52,53 A handful of uranyl pentamethylcyclopentadienyl complexes are known (Table 1), and they all feature small Oyl–U–Oyl angles, ranging from 168.40(9)° for [NEt4]2[Cp*UO2(CN)3] (3) to 167.4(4)° for Cp*UO2(tBu-MesPDIMe) (4).54,55 The deviation away from linearity is a consequence of steric repulsion between the uranyl oxo ligands and methyl groups of the η5-Cp* ring. Interestingly, none of these complexes were made by salt metathesis between M[Cp*] (M = alkali metal) and a uranyl(VI) precursor. Instead, they were each made by O-atom transfer to a lower valent uranium pentamethylcyclopentadienyl complex.
image file: c7dt04123c-c1.tif
Chart 1 Selected structural representations of uranyl complexes with small Oyl–U–Oyl angles as a result of steric and/or electrostatic repulsion.
Table 1 Uranyl complexes with small Oyl–U–Oyl angles as a result of steric and/or electrostatic repulsion
Complex Oyl–U–Oyl angle Ref.
Abbreviations: MeN4 = N,N′-dimethyl-2,11-diaza[3,3](2,6) pyridinophane; HN4 = 2,11-diaza[3,3](2,6) pyridinophane; nPr-btp = 2,6-bis(dipropyl-1,2,4-triazin-3-yl)pyridine; H4CS4 = p-sulfonatocalix[4]arene; tBu-MesPDIMe = 2,6-((Mes)-N[double bond, length as m-dash]CMe)2-p-C(CH3)3C5H2N; MesPDIMe = 2,6-((Mes)N[double bond, length as m-dash]CMe)2C5H3N; NCN = (Me3SiN)CPh(NSiMe3); Me-btp = 2,6-bis(dimethyl-1,2,4-triazin-3-yl)pyridine; CyMe4btbp = 6,6′-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-2,2′-bipyridine; Et(p)TDPA = N,N‘-diethyl-N,N‘-bis(4-methylphenyl)pyridine-2,6-dicarboxamide; LH8 = p-tert-butyloctahomotetraoxacalix[8]arene; C2mim = 1-ethyl-3-methylimidazole.
[UO2(OTf)(THF)(MeN4)][OTf] 161.7(5) 56
[UO2Cl2(phen)2] (7) 161.8(1) 57
UO2(OTf)2(HN4) (6) 162.8(3) 56
UO2Cl2(HN4) 164.1(3) 56
UO22-NO3)2(nPr-btp) (5) 166.2(1) 58
[NMe4]2[UO2(CS4)]·0.5H2O 167.4(6) 59
Cp*UO2(tBu-MesPDIMe) (4) 167.4(4) 55
UO2(O-2,6-tBu2C6H3)2(THF)2 (1) 167.8(4) 51
UO2Cl2(MeN4) 168.2(3) 56
[NEt4]2[Cp*UO2(CN)3] (3) 168.40(9) 54
Cp*UO2(MesPDIMe) 168.4(2) 55
UO2(NCN)2(THF) (2) 169.7(2) 53
[UO2(MeOH)(salen-calix[4]arene)] 170.0 60
UO22-NO3)2(Me-btp) 170.49(9) 58
[U(CyMe4btbp)2(μ-O)UO22-NO3)3][OTf] 170.4(3) 58
UO2(N(SiMe3)2)2(py)2 170.5(3) 52
UO22-NO3)2(Et(p)TDPA) 171.1(1) 61
UO21-O2CC6F5)(κ2-O2CC6F5)(Ph3PO)2 171(2) 62
[(UO2)2(py)4(LH4)] 171.1(9) 63
[C2mim]3[UO2(NCS)5] 171.2(9) 64
[UO2(salphen(C6H4-o-OMe)2)(NC5H4-p-tBu)] 171.8 65
UO22-O2CC6F5)2(bipy) 172(2) 62


Several other ligands can also cause perturbations of the Oyl–U–Oyl angle. For example, the two κ2-bound nitrate ligands in UO22-NO3)2(nPr-btp) (5) bind perpendicular to the equatorial plane (Fig. 1), which causes them to closely approach the uranyl oxo ligands, resulting in Oyl–U–Oyl bending (166.2(1)°) (Table 1). This unusual binding mode is a consequence of steric crowding caused by coordination of the bulky tridentate nPr-btp ligand to the uranyl equatorial plane.58 [U(CpMe4btbp)2(μ-O)UO22-NO3)3][OTf], UO22-NO3)2(Me-btp), and UO2(NO3)2(Et(p)TDPA) feature relatively small Oyl–U–Oyl angles, as well (170.4(3), 170.49(9), and 171.1(1)°, respectively), for the same reason (Table 1).58,61 Similarly, the bipy ring in UO22-O2CC6F5)2(bipy) is somewhat displaced out of the equatorial plane because of steric crowding.62 This results in deflection of the uranyl oxo ligands (172(2)°) because of the close approach of the hydrogens at the 3 position of the bipy ring.


image file: c7dt04123c-f1.tif
Fig. 1 Ball-and-stick diagram of UO22-NO3)2(nPr-btp) (5). Oyl–U–Oyl = 166.2(1)°. Green = uranium, red = oxygen, blue = nitrogen, grey = carbon.

Drawing on these results, my research group recently ligated the relatively rigid 12-membered pyridinophane macrocycles HN4 (2,11-diaza[3,3](2,6) pyridinophane) and MeN4 (N,N′-dimethyl-2,11-diaza[3,3](2,6) pyridinophane) to the uranyl fragment. We successfully isolated four uranyl pyridinophane complexes, including UO2Cl2(RN4) (R = H, Me), UO2(OTf)2(HN4) (6), and [UO2(OTf)(THF)(MeN4)][OTf].56 We hypothesized that the rigid backbone of the pyridinophane macrocycle would force the coordination of its four N-donor atoms to the uranyl ion. Because all four N-donor atoms cannot occupy the uranyl equatorial plane, we expected to observe a considerable steric interaction between the macrocycle backbone and the uranyl oxo groups.

Gratifyingly, the solid-state structures of UO2Cl2(RN4) (R = H, Me) reveal small Oyl–U–Oyl angles (ranging from 164.1(3)° to 168.2(3)°), on account of the steric clash between the oxo ligands and the macrocycle backbone (Table 1), as we initially hypothesized.56 Even smaller Oyl–U–Oyl angles were observed for the triflate derivatives. For example, we observe Oyl–U–Oyl angles of 162.8(3)° for 6 (Fig. 2) and 161.7(5)° for [UO2(OTf)(THF)(MeN4)][OTf], which are amongst the smallest Oyl–U–Oyl angles yet recorded. We rationalized that the triflate derivatives feature smaller Oyl–U–Oyl angles than their chloride cousins on account of their shorter U–N bonds (a consequence of the presence of electron-withdrawing triflate ligands), which increases the steric clash between the pyridinophane ligand and the oxo groups. Finally, it is interesting to note that Oyl–U–Oyl bending does not cause any apparent lengthening of the U–Oyl bonds, which is likely because the Oyl–U–Oyl bending is still quite modest within this series of complexes. It is possible that a larger amount of Oyl–U–Oyl bending would result in an observable lengthening of the U–Oyl bonds.


image file: c7dt04123c-f2.tif
Fig. 2 Ball-and-stick diagram of UO2(OTf)2(HN4) (6). Oyl–U–Oyl = 162.8(3)°. Green = uranium, yellow = sulfur, pink = fluorine, red = oxygen, blue = nitrogen, grey = carbon.

Very recently, Ikeda-Ohno and co-workers reported the structure of a uranyl bis(1,10-phenanthroline) complex, [UO2Cl2(phen)2] (7), which also features a significantly distorted Oyl–U–Oyl angle (161.8(1)°) (Fig. 3, Table 1).57 Indeed, the Oyl–U–Oyl angle in this complex is comparable to the Oyl–U–Oyl angles reported by us for [UO2(OTf)(THF)(MeN4)][OTf].56 Similar to [UO2(OTf)(THF)(MeN4)][OTf], 7 also exhibits normal U–Oyl distances. The X-ray structure of 7 reveals that one of its phen ligands coordinates to the uranyl ion in a manner that is orthogonal to the equatorial plane. The authors argue that this unusual coordination mode is a consequence of an intermolecular π-stacking network, which overrides the preference of uranyl to bind its co-ligands within the equatorial plane. As a result of this binding mode, the hydrogen atoms at the 2 and 9 positions of the phen ligand closely approach the uranyl oxo ligands, causing a large decrease the Oyl–U–Oyl angle. It should be noted that [UO2Cl2(phen)2] (7) was not characterized in solution, so it is not clear if this unusual phen binding mode is maintained upon dissolution. Nonetheless, the observation that intermolecular networks can be harnessed to effect O–U–O bending suggests a new strategy for uranyl manipulation.


image file: c7dt04123c-f3.tif
Fig. 3 Ball-and-stick diagram of [UO2Cl2(phen)2] (7). Oyl–U–Oyl = 161.8(1)°. Green = uranium, aquamarine = chlorine, red = oxygen, blue = nitrogen, grey = carbon.

Intermolecular steric effects may also explain the small Oyl–U–Oyl angle (171.2(9)°) in [C2mim]3[UO2(NCS)5] (Table 1), which is a challenge to explain otherwise given the small steric profile of the thiocyanate ligand.64 As with [UO2Cl2(phen)2], this effect is likely confined to the solid state. A similar intermolecular steric effect probably explains the small Oyl–U–Oyl angle (167.4(6)°) found in [NMe4]2[UO2(CS4)]·0.5H2O (Table 1). In this case, the close approach of a sulfonate oxygen atom in the solid state may cause an electrostatic repulsion of a uranyl oxo ligand, decreasing the Oyl–U–Oyl angle.59

Our X-ray crystallographic analysis of UO2(OTf)2(HN4) (6) and its analogues revealed that the uranyl–pyridinophane interaction is quite weak (as evidenced by the long U–N bond lengths). Thus, attempts to cause further perturbation of the Oyl–U–Oyl angle in uranyl will likely require a stronger interaction between the uranium center and the co-ligands. In this regard, we recently attempted to ligate the 14-membered dianionic tmtaa (H2tmtaa = dibenzotetramethyltetraaza[14]annulene) macrocycle to the uranyl fragment,66 which, because of its anionic charge, should feature stronger (and shorter) U–N bonds. However, reaction of [UO2(N(SiMe3)2)2(THF)2] with H2tmtaa, in an effort to form cis-[UO2(tmtaa)], resulted in only partial protonolysis and formation of [UO2(tmtaaH)(N(SiMe3)2)(THF)] (8) (Scheme 1), which features an essentially linear uranyl fragment (174.0(2)°). This result highlights the thermodynamic stability of the trans uranyl configuration, as well as the surprising flexibility of the tmtaa ligand. The latter point is significant because unanticipated reaction outcomes have proven to be a major impediment in the effort to synthesize a cis uranyl complex.19,20


image file: c7dt04123c-s1.tif
Scheme 1 Attempted synthesis of cis-[UO2(tmtaa)].

Electronic perturbation of the O–U–O angle

In contrast to the abovementioned examples, the uranyl methanediide complex, UO2(BIPMTMS)(dmap)2 (9), reported in 2014 by Liddle and co-workers, does not feature any obvious steric repulsion between its equatorial co-ligands and its two oxo groups (Fig. 4). Yet, it features a relatively small Oyl–U–Oyl angle (167.16(9)°) (Table 2, Chart 2).67 The related methanediide complex, UO2(SCS)(py)2·0.5py (10), which was reported by Ephritikhine and co-workers in 2011, also features a small Oyl–U–Oyl angle (168.5(1)°).68 Moreover, in both examples the two oxo ligands appear to be bending away from the strongly donating methanediide carbon. DFT calculations on 10 nicely reproduce the Oyl–U–Oyl bending observed in the solid-state. According to calculations, the U–C σ- and π-bonds feature some U–O antibonding character and Ephritikhine and co-workers argue that this antibonding character is likely the cause of the Oyl–U–Oyl bending in this complex.68 Presumably, by bending the Oyl–U–Oyl angle, the amount of U–O antibonding in these bonds is reduced, strengthening the U–C and U–O interactions.
image file: c7dt04123c-f4.tif
Fig. 4 Ball-and-stick diagram of UO2(BIPMTMS)(dmap)2 (9). Oyl–U–Oyl = 167.16(9)°. Green = uranium, gold = phosphorus, plum = silicon, red = oxygen, blue = nitrogen, grey = carbon.

image file: c7dt04123c-c2.tif
Chart 2 Selected structural representations of uranyl complexes with small Oyl–U–Oyl angles as a result of electronic perturbation.
Table 2 Uranyl complexes with small Oyl–U–Oyl angles as a result of electronic perturbation
Complex Oyl–U–Oyl angle Ref.
Abbreviations: BIPMTMS = C(PPh2NSiMe3)2; dmap = 4-(dimethylamino)pyridine; SCS = C(PPh2S)2; CyMe4BTBP = 6,6′-bis(5,5,8,8-tetramethyl-5,6,7,8-tetrahydro-1,2,4-benzotriazin-3-yl)-2,2′-bipyridine.
UO2(BIPMTMS)(dmap)2 (9) 167.16(9) 67
UO2(SCS)(py)2·0.5py (10) 168.5(1) 68
[(UO2)23-O)(C8H12O4)(C10H8N2)(H2O)]2 171.1(3), 172.3(3), 176.6(2) 69
[{UO2(CyMe4BTBP)}2(μ-O)][I]2 (11) 171.4(2) 70
[{UO2(py)4}2(μ-O)][OTf]2 171.5(2), 173.1(2) 71
UO2(SCS)(py)2 171.8(2) 68


Oyl–U–Oyl bending appears to have an electronic origin in several uranyl bridged-oxo clusters, as well. For example, the two Oyl–U–Oyl angles in [{UO2(CyMe4BTBP)}2(μ-O)][I]2 (11) both deviate from linearity (171.4(2)°) (Table 2, Chart 2).70 Intriguingly, both uranyl units are deflected away from the strongly donating μ2-oxo ligand. Several other oxo-bridged uranyl clusters also feature uranyl groups with slightly bent Oyl–U–Oyl angles (Table 2).69,71 This bending is presumably electronic in origin, although in some cases steric perturbation may also be operative.71 That said, it appears that an equatorial oxo ligand cannot induce as large a deviation in uranyl Oyl–U–Oyl angles as can a methandiide ligand, which may be a consequence of the stronger donating ability of the latter.

Conclusions and outlook

Thus far, the degree of bending observed within the uranyl unit is still relatively small. The smallest observed Oyl–U–Oyl angles are ca. 161°, and they appear to be primarily a result of steric and/or electrostatic repulsion between the equatorial ligands and the uranyl oxo groups. While these changes are small, this review highlights several areas where further work could result in the observation of much larger degrees of Oyl–U–Oyl bending. For example, computational studies suggest that careful equatorial ligand tuning could make Oyl–U–Oyl bending more facile. In particular, it appears that ligation of strongly σ-donating ligands, such as alkoxide, amide, or alkyl, to the uranyl equatorial plane should reduce the energy penalty required to bend the uranyl group. Building on this idea, the inclusion of these strongly electron donating groups into a ligand with a large steric footprint could be a particularly valuable approach for effecting Oyl–U–Oyl bending. The use of unfavourable intermolecular steric interactions can also effect Oyl–U–Oyl bending, as was observed for [UO2Cl2(phen)2]. Future work should focus on implementing this promising (but underexplored) effect via the design of new ligands that can connect multiple uranyl units, but also place steric bulk next to an oxo ligand. That said, given the difficulty of predicting ligand binding modes upon coordination to uranyl, as revealed in the sections above, implementing this strategy in practice could be a challenge.

The coordination of strongly electron-donating equatorial ligands to uranyl can also perturb the Oyl–U–Oyl angle, although the effect is usually not as large, nor as common, as the perturbations caused by steric repulsion. This effect is largest for the uranyl methanediide complexes, UO2(BIPMTMS)(dmap)2 and UO2(SCS)(py)2, but its origins are still not well understood. Future work should attempt to better unravel the orbital interactions that result in Oyl–U–Oyl bending. In addition, there are several synthetic avenues of investigation that could prove fruitful. For instance, it would be interesting to synthesize a uranyl complex that featured a terminal imido ligand coordinated to its equatorial plane. Like the methanediide ligand, imidos are also strongly-donating with a 2- charge, and a uranyl imido species would likely feature a bent uranyl fragment, as well. A bridged-nitrido uranyl cluster could also feature bent uranyl groups; however, such a complex would likely be a challenge to synthesize.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Chemical Sciences, Biosciences, and Geosciences Division under Award Number DE-SC-0001861.

Notes and references

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Footnote

Dedicated to Prof. Phil Power on the occasion of his 65th birthday.

This journal is © The Royal Society of Chemistry 2018