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Transition Metal Complexes of Calix[4]arene: Theoretical Investigations into Small Guest Binding within the Host Cavity Paul Murphy, Scott J. Dalgarno, and Martin J. Paterson J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.5b11758 • Publication Date (Web): 19 Jan 2016 Downloaded from http://pubs.acs.org on January 23, 2016

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The Journal of Physical Chemistry

Transition Metal Complexes of Calix[4]arene: Theoretical Investigations into Small Guest Binding within the Host Cavity Paul Murphy, Scott J. Dalgarno, Martin J. Paterson* Institute of Chemical Sciences, Heriot Watt University, Edinburgh EH14 4AS, United Kingdom

ABSTRACT: The ability to selectively detect or store small molecules, such as gases, is of enormous commercial potential. Calixarenes have been studied extensively as host molecules, however recent synthetic advances have seen the formation of new polymetallic calixarene clusters which have not yet been explored for such purposes. We therefore present a theoretical study, using Density Functional Theory, to thoroughly investigate the binding preferences of calix[4]arene, with a variety of transition metal cations coordinated to the calixarene tetraphenolic pocket, towards a series of important small molecules - H2S, SO2, H2O, O2, H2, N2, N2O, CO2, NH3 and HCN. It was found that the inclusion of a metal atom at the lower-rim of the calixarene caused significant strengthening of binding energy with all of the small molecules in our study compared to metal-free calixarene. The guests, SO2 and NH3 were found to bind strongest with H2 binding weakest. Our calculations predict that simply introducing metal coordination of any type to calix[4]arene will make the largest difference to the binding energies.

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Subsequently changing the type, oxidation state or the spin state of the metal coordinated to the calixarene tetraphenolic pocket was found to have a lesser effect on these.

Introduction Calixarenes1 are macrocyclic molecules formed by the condensation oligomerisaton of parasubstituted phenols in the presence of formaldehyde. The cone conformation of calix[4]arene, (Figure 1, hereafter referred to as C[4]), is stabilised by hydrogen bonding interactions, and this allows the molecule to act as a host for a wide range of suitably sized guests. The wide upper-rim generates a hydrophobic cavity due to the aromatic rings present, and it is noteworthy that significant effort has been invested in studying the storage of various small gas molecules using C[4]s with a variety of para-substituents2-24.

Figure 1. Calix[4]arene (C[4]) in the cone conformation

0Early attempts2 at gas phase complexation involving C[4] derivatives saw the use of bridging across the upper-rim of the molecular framework; examples include 2,4-hexadiynyl or xylyl moieties, both of which create cavitands. These bridging moieties served a dual purpose. Firstly, they provided additional binding sites for gaseous species to interact within the internal cavity of the cavitand host. Secondly, they hold the C[4] derivative molecules into the cone conformation


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to stabilise the encapsulation of the guest molecule where inversion through the annular ring might have otherwise allowed escape of the guest. In this way, endo-inclusion of a range of organic solvent molecules (alcohols, esters, ketones, benzene, MeCN etc.) was shown to be successful with preference shown for molecules exhibiting acidic methyl groups. p-tert-Butylcalix[4]arene (hereafter referred to as TBC[4]) was found to store vinyl bromide guests3 without subsequent severe fracturing of the host structure, as was commonplace in many other gas storage substrates. Fracturing of the host structure, was found to be avoided through a re-arrangement of the host structure to accommodate guests through a dynamic transport mechanism. Enright et al.4 subsequently extended the range of guests to include gas molecules such as Xe, NO, air and SO2, discovering that thermally controlled release of the guests was possible suggesting potential use in applications requiring “on-demand” release of reagents. Further research5 suggested selective sequestering of CO2 over H2 and provided evidence that guest molecules were being absorbed into the crystal lattice itself rather than adhering to the surface of the particles. Additionally, in this work, CO2 was found to be absorbed more rapidly than O2 and N2. Of course, in order to be useful as a practical gas storage medium, it is essential that reversible storage is provided which allows non-destructive release of stored guests. TBC[4] was found6-8 to display such characteristics when storing CH4, a known energy bridge towards the production of H2, with no discernible structural change within the host during guest uptake. Such stability was found to be particular useful for volatile gases such as H2 and acetylene. The Ripmeester group9-11 were able to further elucidate the nature of guest inclusion in TBC[4] using solid state NMR, XRD and 13C NMR. TBC[4] was found to behave in a similar manner to


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organic zeolites, with filling characteristics described by Langmuir isotherms when small guests were used. Contrary to the earlier work, above, which found no discernible host structure changes, XRD and

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C NMR studies showed some structural re-arrangement beyond 25%

Xe:TBC[4] loading and also when higher pressure CO2 was added. In the case of CO2, low pressures resulted in the expected CO2 inclusion within the TBC[4] upper cavity but higher pressures saw additional CO2 molecules occupy interstitial sites allowing a storage capacity of 2:1 CO2:TBC[4] in addition to consequent structural changes. Work by Zyryanow et al.12 saw the use of C[4] as a reversible transport agent for NO+ using Oalkylated lower-rim substitution. C[4] was seen to not only capture toxic and environmentally damaging NO2/N2O4 but also to interact with it using a charge transfer mechanism to produce the stable C[4]-encapsulated NO+. NO2/N2O4 could then be recovered by addition of aqueous methanol. Gas phase optical detectors13 were investigated in 2005 using nitrophenylazo substitutions at the upper-rim of C[4] resulting in a red shift in the visible absorption bands when exposed to amine vapours. This chromogenic effect was increasingly pronounced for amines displaying higher levels of basicity. It was found that the amines were bound in the exo-position and characterisation included some limited semi-empirical calculations using AM1 and the STO-3G* basis set. Interestingly, it was found that purging the amine-bound C[4] species with N2 removed all traces of amine from the calixarenes. The binding of small gas molecules14, such as SO2, N2, and H2, to TBC[4] was investigated using the approach of molecular dynamics simulations at 173K. Inclusion energies were calculated per molecule, which showed a preference towards SO2 (-2.0 kcal/mol) followed by N2 (-0.8


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kcal/mol) and finally H2 (-0.6 kcal/mol). No details were provided however regarding the location of the inclusion gas molecules although reference was made to the fact that H2 was not retained within the calixarene cavity. Further molecular dynamics simulations15 suggested a repulsive interaction between H2 and TBC[4] at room temperature with CO2 found to have a binding energy of around -4.0 kcal/mol at 250K. In addition to the issue of porosity within calixarenes, the ability of gas to diffuse through an extended calixarene structure was elucidated by Adams16 who postulated that repeated phenyl ring rotation within each calixarene could allow trapped gas to escape and move to another calixarene. This “dynamic storage” picture complicates the identification of the location of a trapped gas molecule, and in particular crystal structures are inconclusive due to the fact this only represents a snapshot of the real situation. Adams therefore used molecular modelling to characterise the dynamics of gases adsorbed in order to gauge preferential binding between molecules such as CO2, ethylene, ethane, acetylene and methanol. From this study, the binding energy of CO2 was estimated to be around -2.30 kcal/mol. Metal coordination to calixarenes with alkali metals cations Na+, Li+ and K+ was shown17 to improve the binding of H2 gas molecules. In this case, the metal cations bound to the phenyl rings of the calixarene in an exo-fashion with H2 molecules coordinating to the cation rather than being coordinated within the calixarene upper-rim cavity. Nevertheless, this represented a step forwards in attempting to store H2 within an extended calixarene structure. Further molecular modelling studies18 involved the bonding of calixarenes to a gold surface via lower-rim S atoms. Tethering of the calixarene to the gold surface was found to introduce stronger binding energies of around 1.58 kcal/mol for benzene molecules suggesting that some calibration may be required for practical use in quantitative gas sensing depending on the tethering substrate.


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Investigations by Hontama et al. 19 discovered that water was predicted to bind to calix[4]arene in both exo- and endo- forms during MP2/aug-cc-VQZ calculations. Exo-binding occurred via the lower-rim and consisted of hydrogen bonding to the O atoms of the calixarene with endobinding occurring within the upper-rim cavity via OH-π bonding between the water molecule and the phenyl rings of the calixarene. Both experiment and theoretical calculations were performed leading to endobinding energy calculations of around -8.94 kcal/mol compared to 8.98 kcal/mol for experiment. Several attempts20,21 have been made to produce practical calixarene-based sensors via deposition of thin films of calixarenes on various substrates using the Langmuir-Blodgett method. Capan et al.20 prepared a stable monolayer of calix[8]arene on a gold plated substrate in an attempt to create a sensing device for detection of volatile organic vapours whereas Ozmen21 used calix[4]arene to detect species such as benzene derivatives, CHCl3 and various alcohols. The literature returns very few theoretical studies of the binding of small gas molecules with calixarenes although work by Kaneko22 predicted binding energies for NH3 and H2O using MP2/CBS of -11.09 kcal/mol and -8.10 kcal/mol respectively. These results were compared to experimental values of around -8.00 kcal/mol for NH3 and -8.98 kcal/mol for H2O. Horvat et al.23 investigated the binding characteristics of C[4] derivatives, containing secondary amide substitution at the lower-rim, towards MeCN. It was found that the inclusion of a metal cation at the lower rim, such as Na+ or Li+, resulted in stronger interactions between MeCN and the calixarene upper cavity compared to the metal-free case. It was shown that the metal cation induced pre-organization of the calixarene upper rim to a square cone conformation compared to the flattened cone conformation found with metal-free calixarene. This was shown to be an


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essential step in providing a more favourable geometric environment for the binding of the guest molecule. Finally, Ozbek et al.24 discovered that doping calix[4]arene with Fe atoms resulted in a vastly increased uptake of CO gas of around 28 times. Concurrent tests using a range of upper and lower-rim substituents displayed a considerably smaller effect, suggesting that metal doping of calixarenes was more important than substitutions at either rim. Recent developments25,26 in the field of calixarenes have seen the creation of a range of polymetallic clusters which involve the binding of transition metals such as Fe and Cu to the lower-rim of C[4] derivatives, some of which are shown in Figure 2 and Figure 3: these molecules being of particular interest as they display magnetic properties.


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Figure 2. Polymetallic TBC[4] cluster containing Fe(III) and Gd(III)25


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Figure 3. Polymetallic TBC[4] cluster containing Cu(II)26 The inclusion of a metal atom at the lower-rim of these complexes provides an additional possible binding site for small guest molecules stored in the upper-rim cavity. The present work investigates the effect of transition metals on the binding energies and preferences of C[4] towards a variety of important small guest molecules using Density Functional Theory. Several of these gas molecules are ambidentate and binding energies for both linkage isomers are presented.


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Computational Details All calculations were performed using Gaussian 0927. In the course of our calculations, the following DFT functionals were used: BLYP28,29, B3LYP29,30, CAM-B3LYP31, M0632, M06L33, wB9734, wB97X34 and wB97XD35. Except where stated, DFT gas phase geometry optimisations were performed using B3LYP/6-31G** with GD3BJ36,37 empirical dispersion for non-metal atoms. Energy values are internal reaction energies corrected for zero-point energy with no symmetry constraints applied. For metal atoms the Stuttgart-Dresden relativistic Electron Core Potential SDDALL38 method was used, which treats 10 electrons as core. Calculations using BLYP and CAM-B3LYP used GD3BJ empirical dispersion, M06 and M06L used GD3 empirical dispersion and finally wB97, wB97X and wB97XD used no empirical dispersion. BLYP was used to test the multireference nature of the systems (as described later). The remaining DFT functionals were used to compare more recent functionals with B3LYP in terms of their ability to model these systems. The Truhlar functionals M06 and M06L have been specifically designed to provide improved performance with non-covalent interactions and transition metal bonding over B3LYP – both of which are features of our system. M06 is particularly well suited for non-covalent interactions and M06L is better suited towards organometallic and inorganometallic systems. CAM-B3LYP is expected to add long-range corrections to B3LYP, which may be a feature of our system. Finally, the wB97, wB97X and wB97XD functionals are designed to better model non-covalent interactions with long-range corrections incorporated and are expected to improve on B3LYP. Whilst wB97 includes no short-range exact exchange, wB97X contains around 16% and


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wB97XD contains 22%. wB97XD also contains dispersion corrections and all three of these functionals use 100% long-range exact exchange. Geometry optimisation was checked by analysis of the analytical Hessian, which confirmed the nature of the critical points as minima via the absence of negative eigenvalues. After geometry optimisation was performed on each compound, wavefunction stability tests were carried out on all structures. In the presence of wavefunction instabilities, a new more stable wavefunction was found and was used in a subsequent geometry re-optimisation. Final wavefunction stability checks were carried out in all such circumstances to avoid internal instabilities, which would render all analytical frequency calculations invalid for that compound. Following geometry optimisation and wavefunction stability corrections, all calculations were checked to ensure absence of spin contamination. From the gas phase geometries described above, Basis Set Superposition Error corrections were made using the Counterpoise facility within the Gaussian program39,40 with subsequent basis set improvements to 6-311G**, 6-311+G** and 6-311++G** via single point calculations at the 6-31G** geometry. In order to calculate binding energies, the following is used:Ebind = Ecomplex – Egas – EC[4] In this equation, Ecomplex is the zero point energy corrected internal energy of the geometryoptimised, bound calixarene-gas molecule complex which already contains the counterpoise corrections as calculated by the Gaussian counterpoise calculation, Egas is the zero point energy corrected internal energy of the geometry-optimised free gas molecule and EC[4] is the zero point energy corrected internal energy of the geometry optimised free C[4] molecule in the cone conformation.


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Validation of the use of DFT functionals which contain HF exchange, was performed by analyzing the qualitative multireference nature of the system. This was achieved using a modified version of Truhlar’s B1 diagnostic test41 where B3LYP was used instead of B1LYP. This modified B1 test, allows the use of GD3BJ empirical dispersion, which is not explicitly defined for B1LYP in the Gaussian program. It is the amount of HF exchange contained within the DFT functional which is important for this test. B1LYP contains 28% HF exchange with B3LYP containing 20% and thus substitution of B1LYP with B3LYP is thus not unreasonable. Because of the fact that DFT exchange contains some static correlation via the exchange functional with the correlation functional recovering dynamic correlation energy only42,43, any invariance of the binding energy calculations to the amount of HF exchange included in the functional is a very good indicator that the system is largely single reference. This in turn provides a measure of validation for our use of the popular and successful B3LYP functional in describing our system. This work describes and compares binding energies of C[4] towards small gas molecules using a range of different metals, in a variety of oxidation states and spin states, coordinated at the lower-rim. The core metals examined in this work are Fe3+ (quartet spin) and Mn3+ (quintet). These metals have been specifically chosen because crystal structures of polymetallic calixarene clusters containing these metals exist in the literature as described earlier 25,44 and are also based on calculations from our previous work45, which examined the lower rim binding energy preferences of C[4]. This helps to provide some benchmarking data for our calculations as well as forming a potential basis of future experimental work to check the validity of our theoretical predictions. Fe3+ (sextet spin), Cu2+ (doublet spin), Cu3+ (singlet spin), Cu3+ (triplet spin), Ni2+ (singlet spin), Ni2+ (triplet spin), Ni3+ (quartet spin), Co2+ (quartet spin), Co3+ (quintet spin) and


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metal-free C[4] binding energies are included in order to investigate the effect of change of metal, oxidation state and spin state on binding energies. For our calculations, the coordination of each metal is derived from crystal structures with the upper-rim coordinated molecule removed (where necessary) to allow binding of the guest molecule. In our case, the unsaturated Fe and Mn complexes are modelled as 5 coordinate with the 5th coordination site comprising one water molecule binding underneath the calixarene. Unsaturated Cu, Ni and Co complexes are modelled as 4 coordinate. Model complexes are shown in Figure 4 using CO2 as an example of guest inclusion within the cavity.

Figure 4. Model complexes for binding energy calculations.

For the multireference B1 diagnostic tests, each species is geometry optimised at the BLYP/631G** level. B3LYP single point calculations are then performed on these optimised structures. The following calculation then gives an energy value, B1, which provides an approximate feel for the level of multireference character of the complex. B1 = (BEBLYP – BEB3LYP//BLYP) / n


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where BEBLYP is the binding energy for the complex calculated using BLYP/6-31G**, BEB3LYP//BLYP is the binding energy for the complex calculated when a B3LYP/6-31G** single point calculation is made at the BLYP /6-31G** optimised geometry, and n is the number of bonds to be broken in order to remove the metal fully from the calixarene (in this work, n is always 1). All B1 tests are run using CO2 as the guest molecule. More details of how to run the B1 test can be found in our earlier work45. A closely related multireference test is the Aλ test, which also uses DFT functionals with differing levels of HF exchange. Recent literature46 has shown that this type of test can provide a reasonably good indication of the multireference nature of a system. Spin density calculations were performed using isovalue of 0.002 e/au3 on all complexes consistent with our previous work on similar systems45. Solvent calculations were performed using the CPCM solvent model47,48 using B3LYP/6311++G**/GD3BJ empirical dispersion. Because synthesis of C[4]-supported clusters can be carried out in a variety of mixed solvent systems that routinely contain alcohols, water was used as a suitable alcohol-analog solvent in our calculations. NBO3.0 calculations were performed at the B3LYP/6-311++G**/GD3BJ empirical dispersion level using Gaussian 09. Results and Discussion The results of the B1 tests are shown in Table 1. As can be seen, in all cases, the systems are shown to be largely single reference with values significantly less than the recommended 10 kcal/mol upper limit and therefore the provisional choice of B3LYP is justified in terms of consideration of the amount of HF exchange used.


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Choice of Basis Set Basis set comparisons were then made by investigating the variation of binding energy of Fe3+coordinated C[4] (quartet spin) towards CO2 in order to direct the choice of an appropriate basis set for the calculations. Results are shown in Table 2. As can be seen from the Pople basis set results, the expansion from double zeta to triple zeta has the largest impact on the binding energy. The inclusion of diffuse functions makes relatively little difference to the binding energy. The increase in the number of contracted basis functions is relatively small however and therefore 6-311++G** is the preferred basis set, from the Pople collection, for our calculations. Comparing this basis set to the correlation consistent Dunning basis sets, we see that there is relatively consistent agreement between 6-311++G** and cc-pVTZ binding energies. Because cc-pVTZ shows only a small improvement over cc-pVDZ we conclude that the cc-pVTZ basis set represents the basis set limit value of binding energy for this system. Although there is some difference between the 6-311++G** and cc-pVTZ results, this is very small (0.33 kcal/mol). 6311++G** uses significantly fewer basis functions than cc-pVTZ and we therefore conclude that the use of the 6-311++G** basis set is justified in terms of computational cost without significant loss of accuracy.


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Table 1. Results of B1 Diagnostic Tests on all Complexes within which a CO2 gas molecule is bound in endo fashion within the upper-rim. Values are in kcal/mol and are calculated per bond broken. In all cases, one calixarene to CO2 molecule bond requires breaking. C[4]coordinated metal

Spin

B1 (kcal/mol)

No Metal

N/A

-1.21

Mn3+

quint

-0.70

Fe3+

sextet

-0.77

quartet

-2.17

doublet

-1.93

Cu2+

doublet

-0.37

Cu3+

triplet

-0.23

singlet

-0.31


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Table 2. Effect of basis set on binding energy of Fe3+ (quartet spin) lower-rim co-ordinated C[4] towards endo-bound CO2 molecule. Values are in kcal/mol. All calculations use B3LYP and GD3BJ empirical dispersion with SDDALL on the Fe3+ metal and are based on the 6-31G** (for Pople) or cc-pVDZ geometries (for Dunning) basis sets with BSSE corrections. Basis Set

Binding Energy (kcal/mol)

6-31G**

-6.31

6-311G**

-9.00

6-311+G**

-8.87

6-311++G**

-8.85

cc-pVDZ

-8.48

cc-pVTZ

-8.52

Choice of DFT Functional Following the justification of the basis set used in our calculations, and the provisional choice of B3LYP (via the B1 tests), we present a comparison between a variety of commonly used DFT functionals in order to select the most appropriate for our system. The results of these binding energy calculations on Fe3+-coordinated C[4] (quartet spin) towards CO2, using the 6-311++G** basis set are shown in Table 3.


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Table 3. Results of variation of DFT functional on binding energy of Fe3+ (quartet spin) lowerrim co-ordinated C[4] towards endo-bound CO2 molecule. Values are in kcal/mol. All calculations use the 6-311++G** basis set on non-metal atoms, SDDALL on the Fe3+ metal and are performed at 6-31G** geometries with BSSE corrections. Empirical dispersion GD3BJ is included with B3LYP and CAM-B3LYP calculations, GD3 is used on the Truhlar functionals M06, M06L and M062X and no empirical dispersion is included with the wB97 functional set. DFT Functional


B3LYP

-8.85

CAM-B3LYP

-8.69

M06

-11.84

M06L

-10.82

wB97

-7.87

wB97X

-6.29

wB97XD

-8.17

In order to make sense of the validity of these binding energies it is worthwhile comparing the ability of each of these functionals to correctly predict the geometry of the Fe3+-coordinated C[4] crystal data25 shown in Figure 2. The key geometric parameters along with DFT functional predictions are shown in Figure 5 and Table 4.


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Figure 5. Geometric parameters for Fe3+ coordinated C[4].

Considering the upper-rim geometry first, it appears that the Truhlar functionals M06 and M06L best predict the average C-C upper-rim distal (A and B). The crystal structure however confirms that the upper-rim is largely square. The Truhlar functionals describe a more rectangular arrangement, which is hidden when considering just average distances and surface areas. B3LYP, on the other hand, captures the square nature of the upper-rim. The other functionals also predict an upper-rim shape which is rectangular in nature, and in all of these cases no improvement on B3LYP is found. The lower-rim geometry in the crystal is again square around the tetraphenolic pocket although the presence of the extended crystal structure shows the Fe atom displaced out of square planar arrangement. B3LYP shows very good agreement with the crystal data for the square geometry around the O atoms. All other functionals predict a rectangular arrangement. As with the upperrim, the Truhlar functionals show fortuitous cancelling of errors when considering the average O-O lower-rim distal. Finally, the Truhlar functionals are best for predicting the Fe-O average distances but are the worst functionals for predicting the average C-O bond lengths at the lower-


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rim. B3LYP is in good agreement with both. No other functional shows an improvement on either B3LYP or the Truhlar set. In summary therefore, it is clear that the choice of which functional to use in our subsequent calculations comes down to B3LYP, M06 or M06L. Because of poorer modelling of the square geometry at both upper and lower-rims by the Truhlar functionals, B3LYP is considered a better choice in this instance than the Truhlar set and a higher confidence is therefore placed on the subsequent B3LYP energetic results listed in Table 3. This validation of the use of B3LYP is consistent with our use of this functional in other work on similar systems45,49. The B3LYP/6-31G** system is known to exhibit systematic problems which have been raised in the literature50 with recommended solutions. There are two shortcomings with this DFT system: the neglect of BSSE, which causes over-binding and the neglect of dispersion effects, which causes under-binding. In some instances these two effects cancel out, giving results which appear to give a correct computational answer, but for the wrong reason. In many systems however, the two effects do not perfectly cancel each other and this cannot be predicted in advance. It is therefore recommended that B3LYP calculations be corrected in three ways. Firstly, to include dispersion effects with the DFT-D3 scheme and Becke-Johnson correction (GD3BJ), secondly, to include counterpoise correction to deal with the neglect of BSSE and finally to use at least triple-zeta basis sets for energy calculations. Our calculations include all of these corrections for energy calculations although double-zeta calculations were considered adequate for geometry optimisations.


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Table 4. Comparison of the geometric predictions of Fe3+-coordinated C[4] (quartet spin), with no bound gas molecule, of various DFT functionals. All calculations use 6-31G** basis set on non-metal atoms and SDDALL on metal centres. Empirical dispersion GD3BJ is used with B3LYP and CAM-B3LYP, GD3 is used for M06 and M06L, no empirical dispersion is defined for wb97, wB97X or wB97XD. Values in brackets are % error with respect to crystal structure. Distances are in Å, areas in Å2 and angles are in degrees. Geometric parameter

Crystal Data25

B3LYP

CAMB3LYP

M06

M06L

wB97

wB97X

wB97XD

C-C upper-rim distal - A

7.8212

8.3301 (6.5%)

8.1309 (4.0%)

7.8929 (0.0%)

8.1100 (3.7%)

8.1331 (4.0%)

8.1630 (4.4%)

8.1121 (3.7%)

C-C upper-rim distal - B

8.0462

8.3613 (3.9%)

8.6475 (7.5%)

8.6125 (7.0%)

8.5133 (5.8%)

8.6682 (7.7%)

8.6547 (7.6%)

8.5854 (6.7%)

Average C-C upper-rim distal

7.9337

8.3457 (5.2%)

8.3892 (5.7%)

8.2527 (4.0%)

8.3117 (4.8%)

8.4007 (5.9%)

8.4089 (6.0%)

8.3488 (5.2%)

Upper-rim surface area (C-C upperrim distal A x C-C upper-rim distal B)

62.930

69.650 (10%)

70.312 (12%)

67.978 (8.0%)

69.043 (9.7%)

70.499 (12%)

70.648 (12%)

69.646 (11%)

Fe-O distance average (lower-rim)

1.9763

1.9226 (2.7%)

1.8850 (4.6%)

1.9522 (1.2%)

1.9387 (1.9%)

1.8902 (4.4%)

1.8952 (4.1%)

1.9033 (3.7%)

C-O distance average (lower-rim)

1.3633

1.3327 (2.2%)

1.3351 (2.1%)

1.3165 (3.4%)

1.3243 (2.9%)

1.3409 (1.6%)

1.3364 (2.0%)

1.3311 (2.4%)

O-O lower-rim distal - A

3.9111

3.8459 (1.7%)

3.8154 (2.4%)

3.9703 (0.0%)

3.9354 (1.3%)

3.8267 (2.2%)

3.8358 (1.7%)

3.8537 (1.5%)

O-O lower-rim distal - B

3.9034

3.8437 (1.5%)

3.7212 (4.7%)

3.8335 (1.8%)

3.8144 (2.3%)

3.7304 (4.4%)

3.7418 (4.1%)

3.7568 (3.8%)

Average O-O lower-rim distal

3.9073

3.8448 (1.6%)

3.7683 (3.6%)

3.9019 (0.1%)

3.8749 (0.8%)

3.7786 (3.3%)

3.7888 (3.0%)

3.8053 (2.6%)

O-Fe-O angle average

162.6

178.2 (9.6%)

177.0 (8.9%)

178.3 (9.7%)

177.8 (9.4%)

176.9 (8.8%)

177.1 (9.0%)

177.3 (9.1%)


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Effect of metal type, oxidation state and spin state Having therefore provided justification for the choice of B3LYP/6-311++G** for our calculations, we now investigate the effect of the metal centre on the binding energy of C[4] towards the CO2 molecule. Here we consider a range of metals, coordinated to the C[4] lowerrim, in various oxidation states and spin states - Fe3+ (quartet and sextet), Cu3+ (singlet and triplet), Cu2+ (doublet), Ni2+ (singlet and triplet), Ni3+ (quartet), Co2+ (quartet), Co3+ (quintet) and Mn3+ (quintet). For comparison, calculations are also performed with no metal attached to C[4]. Results are shown in Table 5.


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Table 5. Effect of variation of metal on binding energy of C[4] towards endo-bound CO2 molecule. Values are in kcal/mol. All calculations use B3LYP/6-311++G** basis set on nonmetal atoms, SDDALL on metal centres including GD3BJ empirical dispersion and BSSE corrections. Metal Centre


No metal

-6.76

Fe3+ (quartet spin)

-8.85

Fe3+ (sextet spin)

-8.84

Cu2+ (doublet spin)

-8.83

Cu3+ (singlet spin)

-8.07

Cu3+ (triplet spin)

-8.18

Ni2+ (singlet spin)

-8.01

Ni2+ (triplet spin)

-8.41

Ni3+ (quartet spin)

-7.84

Co2+ (quartet spin)

-9.74

Co3+ (quintet spin)

-9.08

Mn3+ (quintet spin)

-9.05

As can be seen, there is relatively little variation between the binding energies as the nature of the metal is altered with an average binding energy of -8.63 kcal/mol. From this subset of transition metals, we predict that it is the presence of a metal, which largely affects the binding energy rather than the oxidation state or the spin state of the metal. This may have implications where a practical C[4] gas sensor is being designed. As has been described earlier, metals such as gold have been used to tether the C[4] entity to a surface. It is our prediction that cheaper metal surfaces may work equally well. As can be seen however, a little variation is seen and this


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suggests that, in line with the findings of Ytreberg18, some calibration would be required to take account of the specific metal used in the tethering process if quantitative results were required. Interestingly, in this work we find a similar strengthening of binding energies of between 1.08 and 2.98 kcal/mol once a transition metal is included with an average strengthening of binding energy of around 1.97 kcal/mol. This is in excellent agreement with the work of Ytreberg who discovered that gold strengthens the binding energies of gas molecules by 1.58 kcal/mol. Of course the relative invariance of binding energy to the type of metal coordinated within the C[4] lower-rim will require further analysis in order to be certain of this fact, and this will be the subject of further investigation.

Binding energy calculations We initially present results of calculations using metal-free C[4] with H2O and NH3 in order to compare with experimental results. We consider the effect of including BSSE corrections to gas phase calculations and also the effect of including solvent corrections. We also compare to MP2 where results are available. Results are shown in Table 6. It is noteworthy that whilst MP2 calculations give good agreement with experiment for H2O, particularly with the aug-cc-pVQZ basis set, this is not the case for NH3 with over-binding of 3.09 kcal/mol for the CBS case. Gas phase B3LYP calculations, on the other hand, predict binding energies for NH3 which are in good agreement with experiment but show under-binding of H2O. It should be noted however that without BSSE corrections, these results suffer from the problem of over-binding. BSSE corrected results show a more realistic picture. Good agreement is still seen with experiment for NH3 binding although H2O is seen to under-bind by 3.71 kcal/mol. Solvent calculations show severe under-binding in both cases.


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As clearly seen, MP2 does not necessarily improve on our B3LYP results and appears to be sensitive to the choice of basis set.

Table 6. Results for binding of H2O and NH3 with metal-free C[4]. Values are in kcal/mol. All calculations use B3LYP/6-311++G** basis set on non-metal atoms and SDDALL on metal centres with GD3BJ empirical dispersion. Experimental and MP2 values are provided where available. Guest Molecule

Calculated Binding Energy (B3LYP/6-311++G**/GD3BJ Empirical Dispersion)

H 2O Gas phase

-7.19

Gas phase + BSSE

-5.27

Solvent Phase

-3.61

Exp.19,22

-8.98

MP2/CBS2

-8.10

2

MP2/augcc-pVQZ19

-8.94

NH3 Gas phase

-8.65

Gas phase + BSSE

-7.22

Solvent Phase

-4.07

Exp.19,22

-8.00

MP2/CBS2

-11.09

2

MP2/augcc-pVQZ19

-


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Following the investigations above we now present the findings of the binding energies of both Mn3+ (quintet spin) and Fe3+ (quartet spin)-coordinated C[4] towards a range of important small molecules. We include the case where no metal is coordinated to C[4] for comparison with results shown in Table 7. Graphs showing binding energy trends are shown in Figure 6 (for metal-free C[4]), Figure 7 (Fe3+-coordinated C[4]) and Figure 8 (Mn3+-coordinated C[4]). As can be seen once again, the inclusion of any metal to C[4] produces the largest effect on the binding energy. For most of the molecules in our study, we note that the presence of a metal atom results in binding energies which are strengthened by around 0.29 to 2.89 kcal/mol which is in keeping with our findings above for CO2 and also finds excellent agreement with Ytreberg with SO2 and H2S experiencing strengthened binding energy a little above this range. It is noteworthy however that two guest molecules experience substantial strengthening of binding energy once Fe3+ is added: H2O and NH3 which see binding energy strengthening of 5.44 kcal/mol and 9.70 kcal/mol respectively. This represents a strengthening in binding energy of around 103% and 134% for H2O and NH3 respectively, in contrast with strengthening of 12% to 45% for the other guest molecules. By comparison, replacing Fe3+ with Mn3+ results in changes in binding energy of just 1.0% to 6.8% with most of the gases in our study experiencing differences of less than 4.7% (NH3, H2O and the two linkage isomers of HCN being between slightly higher than this). The inclusion of a metal can therefore be said to be the main cause of binding energy changes with the nature of that metal being of significantly less importance. Inclusion of the metal atom causes the calixarene to adopt a cone geometry which is more square at the upper rim compared to metal-free C[4]. In addition to providing a reduced upper-rim surface area, this pre-organisation process provides a more favourable binding environment for


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the subsequent inclusion of guest molecules, in terms of electronic factors, through reduced proximity between guest and host moieties. We discuss this further later in this paper.

As a result of the substantial strengthening of binding energies for some of the study molecules, once metal coordination to C[4] is introduced, the overall trend in binding energy is altered. The three trends are as shown below:-

For metal-free C[4]:H2 < O2 < N2 < H2O < N2O (via O) < CO2 < N2O (via N) < NH3 < HCN (via C) < HCN (via N) < H2S < SO2 (via S) < SO2 (via O)

For Fe3+-coordinated C[4]:H2 < O2 < N2 < N2O (via O) < CO2 < N2O (via N) < HCN (via N) < HCN (via C) < H2O < H2S < SO2 (via S) < NH3 < SO2 (via O)

For Mn3+-coordinated C[4]:H2 < O2 < N2 < N2O (via O) < CO2 < HCN (via C) < N2O (via N) < HCN (via N) < H2O < SO2 (via S) < H2S < SO2 (via O) < NH3


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Figure 6. Binding Energies of C[4] with a variety of guest molecules. No metal coordinated to C[4]. All calculations B3LYP/6-311++G** with GD3BJ empirical dispersion and BSSE corrections.

Figure 7. Binding Energies of C[4] with a variety of guest molecules. Fe3+ coordinated to C[4]. All calculations B3LYP/6-311++G** with GD3BJ empirical dispersion and BSSE corrections.


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Figure 8. Binding Energies of C[4] with a variety of guest molecules. Mn3+ coordinated to C[4]. All calculations B3LYP/6-311++G** with GD3BJ empirical dispersion and BSSE corrections.


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Table 7. Results of binding energy of Fe3+(quartet) and Mn3+(quintet) coordinated C[4] towards endo-bound gas molecules. Values are in kcal/mol. All calculations use B3LYP/6-311++G** basis set on non-metal atoms and SDDALL on metal centres with GD3BJ empirical dispersion and BSSE corrections. No Metal

Fe3+ (quartet)

Mn3+ (quintet)

H2

-1.51

-1.83

-1.80

Triplet O2

-3.69

-4.13

-4.32

N2

-5.12

-7.01

-7.34

H 2O

-5.27

-10.71

-11.35

N 2O (bound via O atom)

-6.30

-7.93

-8.10

N 2O (bound via N atom)

-6.78

-9.19

-9.59

CO2

-6.76

-8.85

-9.05

HCN (bound via C atom)

-7.44*

-9.86

-9.26

HCN (bound via N atom)

-7.44

-9.48

-10.12

NH3

-7.22

-16.92

-17.83

H 2S

-8.86

-12.42

-12.78

SO2 (bound via S atom)

-10.91

-14.47

-13.80

SO2 (bound via O atom)

-11.79

-17.08

-17.25

Gas Molecule

* HCN (via C) molecule reverts to N binding when no metal atom is present

Figure 9 shows the change in geometry of each C[4]-bound guest molecule as a metal atom is added to the lower-rim of C[4] and Figure 10 shows the guest position within the upper-rim


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cavity for guests which do not change their position upon metal cation inclusion. At the lowerrim of both Mn3+ and Fe3+ coordinated C[4] is a water molecule, which binds to the metal at bond lengths of around 2.3516Å to 2.5076Å for Mn3+ and 2.3111Å to 2.4800Å for Fe3+. The H atoms of the water molecule also form hydrogen bonds to the tetraphenolic ring causing an M-OH angle of around 77-84 degrees regardless of the coordinated metal or the bound guest molecule. As can be seen, the addition of a metal atom significantly changes the binding position of H2O, HCN (both linkage isomers) and NH3. HCN in particular, when bound via the C atom, is fully rotated within the cavity when the metal atom is removed from C[4] resulting in binding with the N atom pointing down towards the lower-rim. The presence or absence of a metal atom therefore dictates which of these linkage isomers is preferentially bound within the cavity. The addition of a metal atom results in all three of these guest molecules being drawn further into the cavity, which would explain their stronger binding when the metal is added. Much smaller changes in binding position change are seen with the other bound guest molecules. The effect on the binding position within the cavity of changing the type of metal is demonstrated in Table 8. The addition of a metal atom causes binding distance changes of around 1% to 8% for most of the guest molecules. As can be seen, H2O and NH3 are most affected by the inclusion of a metal atom with changes of 14% and 20% respectively and this is unsurprisingly reflected in stronger binding energies. The large change for HCN (via C atom) was explained earlier and is related to the fact that the guest molecule changes to preferentially binding via the N atom when no metal is present. In most cases, there is a negligible difference in bond length between the metal and the guest molecule when Mn3+ is used instead of Fe3+ with differences in the range 0.07% to 2.9%.


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Figure 9. Geometry of C[4] bound guests showing change of guest position within upper-rim as metal atoms are coordinated to the C[4] lower-rim. All geometry optimisation calculations


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performed with B3LYP/6-31G** and GD3BJ empirical dispersion. Guest molecules which change position upon metal inclusion are shown.


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Figure 10. Geometry of C[4] bound guests showing guest position within upper-rim. These guests do not change position within the cavity on inclusion of metal cations. All geometry optimisation calculations performed with B3LYP/6-31G** and GD3BJ empirical dispersion for the Fe3+ metal cation.


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General nature of C[4] as a ligand for Fe3+ and Mn3+, In order to discuss the binding of small guest molecules to metal-coordinated C[4], we must first investigate the nature of a general M-L bond to determine whether C[4] actively engages in the binding of these guests to the metal. To do this we consider the binding of both NH3 and H2O to Fe3+ and Mn3+ coordinated C[4]. We compare the C[4] results with the binding energy of one ligand in the complexes [Fe(H2O)6]3+, [Fe(NH3)6]3+, [Mn(H2O)6]3+ and [Mn(NH3)6]3+. We additionally consider the energy required to stretch the M-L bond length to that of the corresponding M-L bond length seen when using C[4], in order to compare the effect of the metal in both cases. In this latter case the M-L bonds length is fixed and the remainder of the molecule is allowed to relax. Similarly the resulting 5 coordinate species is allowed to fully geometry optimise after bond breaking as part of our calculations in line with the principle established for our C[4] binding energy calculations which are described earlier. Results are shown in Table 9.


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Table 8. Effect of metal coordination on binding position of guest molecules within C[4]. All calculations use B3LYP/6-311++G** basis set on non-metal atoms, SDDALL on metal centres including with BSSE corrections and GD3BJ empirical dispersion. Comparison is made for the same gases when no metal is present. Experimental values are provided where available and are performed on metal-free C[4]. Tetraphenolic pocket centroid to lowest point of upper-rim bound gas distance (Å)

Surface area at upper-rim of calixarene (Å2)

Gas Molecule

No Metal*

Fe3+ (quartet)

Mn3+ (quintet)

No Metal*

Fe3+ (quartet)

Mn3+ (quintet)

No gas bound

N/A

N/A

N/A

71.1

69.6

69.7

H2

3.0910

2.7986

2.8355

70.6

69.2

68.4

Triplet O2

2.8533

2.9277

2.9769

70.1

68.4

70.0

N2

2.8445

2.8079

2.8426

69.7

67.9

68.3

H 2O

2.6947

2.4008

2.4278

66.9

69.1

69.0

N 2O (bound via O atom)

2.7214

2.7458

2.7000

70.3

68.4

68.7

N 2O (bound via N atom)

2.8487

2.7186

2.7335

69.9

67.8

67.8

CO2

2.7305

2.6652

2.6326

64.8

67.9

68.6

HCN (bound via C atom)

3.4951

3.8029

3.7894

69.8

68.1

68.3

HCN (bound via N atom)

3.4954

2.5468

2.5769

69.8

68.8

68.5

NH3

2.9021

2.4563

2.4995

70.1

69.0

68.7

H 2S

3.5314

3.2753

3.2616

69.7

68.6

68.5

SO2 (bound via S atom)

3.4184

3.4045

3.4302

67.3

66.9

67.3

SO2 (bound via O atom)

2.6853

2.5327

2.5517

66.4

64.3

67.3


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Table 9. Results of binding energy of one ligand from complexes [Fe(H2O)6]3+, [Mn(H2O)6]3+ [Fe(NH3)6]3+, [Mn(NH3)6]3+. Values are in kcal/mol. All calculations use B3LYP/6-311++G** basis set on non-metal atoms and SDDALL on metal centres with GD3BJ empirical dispersion and BSSE corrections based on geometries established with 6-31G** basis set and GD3BJ empirical dispersion. The energy required to break the longest axial bond is reported in each case. M-L bond length

Comparative M-L bond length when metalcoordinated C[4] is used as the ligand

M-L Binding energy

Comparative binding energy when C[4] ligand is used

Energy Required to stretch M-L bond length to that observed when metalcoordinated C[4] is used as the ligand.

[Fe(NH3)6]3+

2.323

2.328

-61.04

-16.92

0.00

[Fe(H2O)6]3+

2.088

2.324

-50.46

-10.71

0.73

[Mn(NH3)6]3+

2.359

2.396

-37.29

-17.83

0.00

[Mn(H2O)6]3+

2.156

2.374

-44.80

-11.35

1.35

From these results, it is quite clear that C[4] is not simply a passive ligand supporting normal ML bonding. C[4] actively and radically alters the binding energy of the guest molecule to the metal and this is true even when the M-L bond length is largely the same in both cases (as with NH3). As can be seen, stretching the M-L bond length to mimic that when the C[4] ligand is used accounts for very little of the M-L bond energy. The fact that C[4] reduces the binding energy by such a large amount is testament to the complex set of interactions taking place when binding H2O and NH3: the metal itself will attract a lone pair of electrons from the host and the aromatic ring will also provide some attraction. There will additionally, however, be a repulsive


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interaction between the guest lone pair and the O atoms of the tetraphenolic pocket. With no metal present in C[4], the tetraphenolic pocket interaction with the guest was attractive via Hbonding but with the introduction of metal coordination, the pocket is deprotonated in preference for the metal-O bonding and now the interaction between the tetraphenolic pocket and the guest molecule becomes repulsive. In order to be useful as a renewable detector or guest molecule capture system, the metalcoordinated C[4] must provide a balance between the M-L binding of simple systems such as [M-(NH3)6], which is too strong for practical systems, and under-binding which would make the system useless for secure guest molecule capture. Our binding energy results clearly illustrate that this compromise is in fact what we observe with this system. We now discuss the nature of the binding of each gas in more detail.

SO2 binding SO2 is an ambidentate ligand and can bind with either the S atom or an O atom pointing downwards into the cavity. In the absence of a metal cation, both forms are stabilised by interactions between the aromatic rings and the S-O bonds. An additional stabilisation is achieved when SO2 binds with O pointing into the cavity, which takes the form of an interaction between the O atom of the guest and the tetraphenolic H atoms of the calixarene. This helps explain the increased stability of this particular binding form. Upon metal cation inclusion, both forms contain dominant interactions between the S atom and the metal, however when SO2 binds with O pointing into the cavity, there is an additional interaction between the metal and the O atom of the guest, again explaining the increased stability of this binding form.


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NH3 and H2O binding Upon inclusion of a metal cation, both NH3 and H2O bind more strongly through interactions between the lone pair on the guest and the metal with both metals favouring bonding with NH3 as a result of the greater availability of the lone pair. Binding is also effected between the guest and the metal via X-H (X = O, N) σ -bonding. The absence of a metal atom causes a shortening of the X-H bond lengths and introduces H-bonding between the guest lone pair and the tetraphenolic H atoms. H2O binds via H-bonding to three of the aromatic rings. NH3 on the other hand, binds via H-bonding to all four aromatic rings and therefore provides more secure Hbonding than H2O. Additional X-H σ -bonding to the atoms of the aromatic ring, including the methylene and aromatic C-H groups, fixes the orientation of the guest molecules.

CO2 binding Without metal coordination, the main component of CO2 binding is that of the interaction with the aromatic ring π-system. Other lesser effects include interactions between the lone pairs of the tetraphenolic O atoms with the carbon atom of the guest and σ -bond donation from the methylene bridge to the guest. Upon metal inclusion, the position of the guest shifts away from the methylene bridge position removing that particular interaction. Furthermore the donation of the guest oxygen lone pair into the metal atom dominates with weak binding between the guest and the π-system of the aromatic ring.

H2S binding H2S binds to the metal free calixarene in two ways. Firstly the lone pairs on the tetraphenolic O atoms interact with the S atom of the guest. A second stronger interaction is that between the S


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atom and the π-system of the aromatic ring. The addition of a metal cation introduces additional interactions between the lone pairs on the S atom and the metal further strengthening the binding.

N2 binding N2 binds to the cavity via π-π interactions with the aromatic rings. All four aromatic rings are involved in the process and hence the molecule sits in a central vertical position within the cavity. Addition of metal coordination introduces an additional interaction between the lower N atom lone pair and the metal. This effect strengthens the binding energy of the gas bringing it further into the cavity, although this is significantly more apparent for Fe3+ compared to Mn3+. Both metals cause the lower-rim tetraphenolic pocket to open, which closes the upper-rim and increases the π-π interactions as a result of closer proximity.

N2O binding From the binding energy results, it is clear that N2O will preferentially bind with the outer N atom pointing downwards into the cavity. Both arrangements, bind to metal-free C[4] via π-π interactions with the aromatic rings and the N-N multiple bond. These interactions are stronger when the N2O binds with N pointing into the cavity as a result of the lower siting of the multiple bond. Inclusion of a metal atom, results in a dominant interaction between the X (X = O, N) lone pair and the metal. When N2O binds with O pointing into the cavity, this effect is weaker.

H2 binding H2 is the weakest binding gas in our study. With no metal atom, weak dispersion forces keep the guest within the cavity through donation of one of the meta- positioned C-H σ -bonds on the


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aromatic ring towards the guest and most importantly between the aromatic π-system and the guest. Addition of a metal atom sees binding of the H2 molecule through the donation of electron density from the H2 σ -bond to the metal. This binds the gas tighter within the cavity and strengthens the binding energy although the interaction is clearly still relatively weak.

HCN binding Without metal coordination, HCN only stays in the cavity as one arrangement: that of N atom pointing downwards slightly and binding via the dominant interaction between the aromatic πsystem at the top rim of the calixarene cavity and the C-H σ -bond of the guest, with a lesser interaction between the aromatic π-system and the C and N atoms of the guest. Once metal coordination is introduced however, the situation changes dramatically. From the considerable difference in geometric results for both metals, as regards cavity binding position, it appears that HCN preferentially binds with the N atom pointing downwards into the cavity as opposed to the C atom pointing downwards, but a glance at the energetic binding results shows that the bindings of both arrangements are of roughly equally magnitude. The binding of HCN via the N atom changes to incorporate the direct interaction between both the C atom and the C-N π-bond of the guest and the metal cation. The has the effect of bringing HCN further into the cavity. At this point, favourable interactions are initiated between the C-N atoms of the guest and the π-system of opposing aromatic rings, which further strengthens the binding of the gas. The strongest interaction is seen between the σ -bond of the methylene bridge of the calixarene and the C-N π bond of the guest. Mn3+ causes a greater effect than Fe3+ as a result of a more pronounced opening of the lower-rim which then results in a smaller upper-rim distance between the two opposing aromatic rings affected by this interaction. Mn3+ thus presents a stronger binding


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energy for HCN storage when the N atom is pointing down into the cavity. When the C atom is pointing downwards into the cavity, the bonding here is caused by interactions between the metal and both C and H atoms of the guest, supplemented by metal to C-N π interactions and very weak σ -π interactions between the C-H bond of HCN and the aromatic rings.

O2 binding O2 binds to all of the C[4] complexes via interactions between the guest oxygen atoms and the carbons of the aromatic rings in addition to some bonding to the tetraphenolic H atoms. Metal insertion causes an interaction between the lone pair of one of the O2 oxygen atoms and the metal and this interaction dominates, all other interactions becoming negligible in comparison.

Spin density calculations Spin density calculations provide details of the location of unpaired electrons in our open shell systems. Comparative results are shown for both Fe3+ (quartet) and Mn3+ (quintet) coordinated C[4] in Table 10. As can be seen, the 3dyz orbital appears to be largely doubly occupied in Fe3+ quartet species with the three unpaired electrons (overall spin density of +3.23 to +3.32 on Fe) relatively localised within the 3dz2, 3dxz and 3dxy orbitals. Some “leakage” of spin density into dx2-y2 is observed. When HCN is the bound gas (binding via N atom), one of the paired electrons in the 3dyz orbital switches to the 3dxz orbital leaving the 3dyz orbital with an unpaired electron suggesting that these two orbitals are close in energy as would be expected for an octahedral complex. Mn3+ quintet (overall spin density of +4.20 to +4.27 on Mn) results show that 3


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unpaired electrons largely reside within the 3dz2, 3dxz and 3dyz orbitals with the fourth varyingly found in either the 3dx2-y2 or 3dxy orbitals. Again it is not unexpected to find the latter two orbitals close in energy in an octahedral complex. All of these spin density calculations show some spin polarisation through the extended π-system presented by the calixarene although this effect is minor. As expected, this results in a lack of spin contamination as confirmed by our calculations. Other than triplet O2, no spin density is induced in the bound guest molecules and it appears that the presence of a guest molecule doesn’t unduly alter the spin density within the metal either, regardless of whether the bound gas binds directly to the metal or not. This is true for both metals investigated.

Table 10. Spin density values and unpaired electron location within Mn3+ and Fe3+ coordinated C[4]. Positive values refer to areas of excess α electron density. Negative values refer to areas of excess β electron density. Values quoted are the significant contributions to spin density. Bound Gas

No gas bound

H2

Mn3+ (quintet) coordinated C[4] Spin Densities

Fe3+ (quartet) coordinated C[4] Spin Densities

(spin density = 2 * < SZ >)

(spin density = 2 * < SZ >)

+0.77 on Mn 3dz2 +0.82 on Mn 3dxz +0.82 on Mn 3dyz +0.71 on Mn 3dx2-y2 +0.36 on Mn 3dxy

+0.66 on Fe 3dz2 +0.80 on Fe 3dxz +0.15 on Fe 3dyz +0.36 on Fe 3dx2-y2 +0.77 on Fe 3dxy

Total +4.20 on Mn

Total +3.26 on Fe



Total +4.23 on Mn

Total +3.28 on Fe


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Triplet O2

N2

H 2O

N 2O (bound atom)

N 2O (bound atom)

via

via

O

N

CO2

HCN (bound atom)

via

C

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+1.00 and +0.97 on O2 (px and py) +0.78 on Mn 3dz2 +0.82 on Mn 3dxz +0.82 on Mn 3dyz +0.67 on Mn 3dx2-y2 +0.41 on Mn 3dxy

+0.99 and +0.97 on O2 (px and py) +0.67 on Fe 3dz2 +0.81 on Fe 3dxz +0.13 on Fe 3dyz +0.37 on Fe 3dx2-y2 +0.79 on Fe 3dxy

Total +4.27 on Mn

Total +3.31 on Fe



Total +4.25 on Mn

Total +3.30 on Fe



Total +4.24 on Mn

Total +3.26 on Fe



Total +4.24 on Mn

Total +3.28 on Fe



Total +4.25 on Mn

Total +3.29 on Fe



Total +4.26 on Mn

Total +3.29 on Fe




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HCN (bound atom)

via

N

NH3

H 2S

SO2 (bound atom)

SO2 (bound atom)

via

via

S

O

Total +4.24 on Mn

Total +3.32 on Fe



Total +4.26 on Mn

Total +3.28 on Fe



Total +4.23 on Mn

Total +3.23 on Fe



Total +4.26 on Mn

Total +3.27 on Fe



Total +4.24 on Mn

Total +3.31 on Fe



Total +4.27 on Mn

Total +3.31 on Fe

Summary and Conclusions Calix[4]arene complexes were investigated for their ability to bind a variety of important small guest molecules. Coordination to the lower-rim of the C[4] with both Mn3+ (quintet) and Fe3+ (quartet) was shown to induce strengthened binding energies of C[4] towards these guest


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molecules although the nature of the metal was shown to be considerably less important than the presence of any metal. A range of metal types, oxidation states and spin states were investigated with binding energies showing relative invariance once any single metal was in place, offering the prospect of using cheaper and more readily available metals than commonly used metals such as gold. Some calibration was predicted to be required. We found that SO2 (binding via the O atom) bound more strongly than other gas molecules in our study when no metal was present in C[4], with SO2 (binding via the O atom) and NH3 binding most strongly when metal coordination was introduced. Our calculations also predict that in a mixture of two or more gases, metal coordinated C[4] could be useful in the preferential detecting or separating out the most strongly binding gas.

Corresponding Author * Martin J. Paterson: phone, +44 (0)131 451 8035; email, [email protected]

Acknowledgements M.J. Paterson would like to thank the European Research Council (ERC) for funding under the European Union’s Seventh Framework Programme (FP7/2007-2013)/ERC Grant No. 258990. P. Murphy would like to thank the EPSRC for DTP Studentship funding. References 1.

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Transition Metal Complexes of Calix[4]arene - American Chemical

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