Complexes of p-tert-butylcalix[5]arene with lanthanides: synthesis,
structure and photophysical properties †
Loïc J. Charbonnière,a Christian Balsiger,a Kurt J. Schenk b and Jean-Claude G. Bünzli *,a
a Institute of Inorganic and Analytical Chemistry, BCH, University of Lausanne,
CH-1015 Lausanne, Switzerland
b Institute of Crystallography, BSP, University of Lausanne, CH-1015 Lausanne, Switzerland

Spectrophotometric pK determination for p-tert-butylcalix[5]arene (H L) in acetonitrile (pK = 11.5 ± 0.7, pK = 15.4 ± 1.0 at 298 K) evidenced both intra- and inter-molecular stabilisation of the deprotonated forms. Dimeric complexes [Ln (H L) (dmso) ] (Ln = EuIII, GdIII, or TbIII; dmso = dimethyl sulfoxide) were
isolated from tetrahydrofuran (thf) in the presence of NaH as base. A single-crystal analysis of [Eu (H L) (dmso) ]ؒ 10thf showed the deformation of the cone conformation of the calixarene upon complexation and co-ordination
of dmso molecules by inclusion through the hydrophobic cavity of the ligand. A photophysical investigation
revealed a total quenching of the metal luminescence by a ligand-to-metal charge-transfer state in the case of EuIII
while luminescence of TbIII is sensitised (quantum yield in thf: 5.1%). The temperature-dependent lifetime of
TbIII is analysed in terms of a potential metal-to-ligand back-transfer process.
The cyclic framework of calixarenes,1 associated with the
presence of phenol oxygen donor atoms, affords an interesting
platform for the complexation of metal atoms,2 while the
hydrophobic cavity allows the inclusion of charged 3 and
neutral 4 organic guests. The relative facility with which calix-
arenes can be partially or totally functionalised at the upper or lower rims, coupled with their easy large-scale synthesis, at least for the even members of the series (n = 4, 6 or 8), has opened
further perspectives for their use in supramolecular chemistry.5
In particular, the hard acid character of the lower rim makes calixarenes interesting potential ligands for the complexation
of trivalent lanthanide ions, either for extraction purposes 6
or for the design of efficient lanthanide-based luminescent
devices.7–9 Antenna effects 10 can be generated directly by the
reported by Steed et al.19 who isolated water-soluble inclusion
phenol units or by lower or upper rim substitution.
complexes between p-sulfonatocalix[5]arene with La, Gd, Eu, Previous work on the complexation of lanthanides with Tb and Yb and showed by X-ray crystallography that the calix[n]arenes has mainly focused on the four-, six- and eight- interaction is either outer sphere or occurs exclusively via one membered systems. While p-tert-butylcalix[8]arenes form sulfonato functionality, the metal centre acting often as a bridg- bimetallic complexes in which the ligand is six times deproton- ing brick. In this paper we present the isolation and the struc- ated,11 the six-membered parent calixarene is bonded to LnIII by
tural and photophysical properties of lanthanoid() (Ln = Eu, a single phenolate group.12 In the case of p-tert-butylcalix[4]-
Gd or Tb) phenolate-bonded inner-sphere complexes with arene 2 : 2 dimeric complexes were obtained, in which the two p-tert-butylcalix[5]arene (H L). The reported data add valuable metal atoms are separated by only 3.91 Å.13 In our laboratory
information on the interaction between lanthanoid() ions and we have mainly studied the photophysical properties of bi- p-tert-butylcalix[n]arenes and demonstrate that multiple bonds metallic complexes with p-tert-butylcalix[8]arene 14–16 and
between the ligand and the metal ions can be generated only in demonstrated the influence of the low-lying ligand-to-metal the presence of a strong base. In addition, the spectrophoto- charge-transfer state (LMCT) on the EuIII-containing
metric determination of the first two pK values of the ligand is assemblies, which enhances the f–f absorption probabilities through mixing with the 4f orbitals and quenches the 5D (Eu)
excited level. Moreover, the sensitisation of EuIII and/or TbIII
Results and Discussion
can be conveniently tuned by changing the nature of the para
Acid-base behaviour of H L in acetonitrile
Calix[5]arenes appear to be adequately suited for the com- The compound H L was titrated at 298 K by Et N in plexation of lanthanoid() ions in view of the oxygen-rich acetonitrile with NEt ClO as supporting electrolyte. The evo- array displayed by the five phenol groups in the cone conform- lution of the UV/VIS spectra during titration could be best ation,18 and the possibility of forming multiply charged
fitted with the model (standard deviation: 0.007 absorbance anions.19 To our knowledge, the only study on the interaction
unit between calculated and measured values) in equations (1)– between lanthanoid() ions and a calix[5]arene has been (3). The first two acidity constants of p-tert-butylcalix[5]arene, † Supplementary data available: UV/VIS and IR spectra, crystallo- H LϪ ϩ Et NHϩ log K = 7.0 ± 0.7 (1) graphic numbering scheme. For direct electronic access see, otherwise available from BLDSC (No. SUP 57328, 4 pp.) or the RSC Library. See Instructions for Authors, 1998, Issue 1 (
J. Chem. Soc., Dalton Trans., 1998, Pages 505–510 Species distribution in solutions of H L in acetonitrile vs. the H L2Ϫ ϩ 2 Et NHϩ
K and K , can be evaluated from the relationships K = K / Et NHϩ and K12
Et NHϩ)2. With pKEt NHϩ = 18.46 20 one
gets pK = 11.5 ± 0.7 and pK = 15.4 ± 1.0. The distribution curves as a function of the Et N : H L ratio are shown in Fig. 1.
For a 1 : 2 ratio the major species is an adduct formed from Top: UV absorption spectra of [Ln (H L) (dmso) ] in thf.
two calixarene molecules with one proton removed [equation Bottom: visible absorption spectrum for Ln = Eu showing the LMCT (2)]. This can be related to the association process (4) with log and 5D ← 7F transitions
these conditions, crystalline complexes were obtained by mixing stoichiometric amounts of H L with dimethyl sulfoxide (dmso) Et NHϩ/K1
adducts of the lanthanoid() nitrates. The elemental analyses Adding more base leads to the monodeprotonated calixarene are compatible with [Ln (H L) (dmso) ] with x = 4 for Ln = Gd x
while the dianionic species appears for ratios larger than 1 : 1.
and Tb and 3 for Eu, which emphasises the tendency of the The observed pK values can be understood on the basis of the latter compound to lose 1–2 dmso molecules upon drying.
monoanion stabilisation by intramolecular hydrogen bonds These analyses confirm the absence of nitrate in the isolated forming cyclic arrays, as previously reported for pK data compounds. Infrared spectra of the three solids are very similar, measured in water-containing solvents 21 or in the same solvent
showing an absorption band at 1023 cmϪ1, not present in the
but with different calixarenes.22 The reconstructed electronic
spectrum of free H L, and assigned to the S᎐O stretching spectra (SUP 57328) display an increase of the intensity of the vibration of oxygen-bonded dmso.23 Upon complexation the
low-energy π → π* transition centred on the ligand at 288 nm broad ν(O᎐H) pattern shifts from 3305 cmϪ1 for free H L to a
upon deprotonation, together with the emergence of a shoulder band centred around 3409 cmϪ1 with a shoulder at 3305 cmϪ1
around 305 nm. We tried to demonstrate the formation of the postulated dimeric adduct [H LؒH L]Ϫ by measuring the elec- The π → π* absorption bands of the ligand are very simi- trospray (ES) mass spectrum of a solution with a 1 : 2 base : lar for the three complexes in thf (Fig. 2). They undergo a red calixarene ratio at which the dimer is the major species in solu- shift with respect to H L, with a maximum at 288 nm (34 200 tion (88%). The spectrum effectively displays a peak at m/z cmϪ1;ε = 25 620, 26 230 and 25 420 dm3 molϪ1 cmϪ1 respectively
1641.1 (15%), which can be attributed to a dimer [2H Lؒ for Eu, Gd and Tb) and a shoulder at 295 nm (33 900 cmϪ1).
= 1640.3). The presence of this species em- In the case of EuIII an additional absorption band around 409
phasises the possibility of intermolecular hydrogen bonds nm (24 445 cmϪ1, ε = 720 dm3 molϪ1 cmϪ1) is attributed to
stabilising the monoanion, a way found by the system to mini- the ligand-to-metal charge-transfer transition, as previously mise its energy in the absence of highly solvating molecules reported for bimetallic complexes with p-substituted-calix[8]- such as MeOH or water. Further deprotonation leads to arene.15,17 The presence of the LMCT transition relaxes
Coulombic repulsion between the two parts of the dimer and Laporte’s rule, so that the 5D ← 7F transition is observed at
577.7 nm (17 310 cmϪ1), with an unusually large absorption
coefficient (ε = 2.4 dm3 molϪ1 cmϪ1, after correction for MLCT
Synthesis and characterisation of the complexes (Ln ؍ Eu, Gd or
absorption). The ES mass spectra of thf solutions of the com- plexes of GdIII and TbIII display base peaks at m/z = 1932.6 and
1929.5 respectively, which are typical of the dimeric cations
Spectrophotometric investigation of the interaction between [Ln (H L) ]ϩ. No such peak is observed for the europium() H L and lanthanoid() ions in acetonitrile and in the presence complex, possibly as a result of reductive reactions on the of Et N revealed a weak interaction leading to the formation metal, in accordance with the low-lying LMCT state.
of 1 : 1 and 2 : 1 species, but the analysis of the data did notconverge properly. Given the values of pK reported above, Crystal structure of [Eu (H L) (dmso) ]ؒ10thf
triethylamine is not a strong enough base to produce the anionic species necessary for a stable association with the lan- Slow concentration of a mixture of the europium() complex thanoid() ions. We therefore switched to NaH to deprotonate in thf afforded yellow-orange platelets suitable for X-ray the calixarene and to tetrahydrofuran (thf) as solvent. Under diffraction analysis, with formula [Eu (H L) (dmso) ]ؒ10thf.
J. Chem. Soc., Dalton Trans., 1998, Pages 505–510 Selected averaged distances (Å) and angles (Њ) for the co-ordination sphere around the two europium() ions in [Eu (H L) (dmso) ]ؒ10thf (a and b refer to the dmso O atoms, b corresponding to the molecule included in the calixarene cavity) O(x)᎐Eu᎐O(y) angles y Stereoscopic view of a dimeric [Eu (H L) (dmso) ] unit Details of the experimental conditions, cell data, structure solu-
tion and refinement are given in the Experimental section.
Selected bond lengths and angles for the co-ordination sphere
around EuIII are given in Table 1 (the complete numbering
scheme is described in SUP 57328).
The unit cell contains two crystallographically independent and neutral [Eu (H L) (dmso) ] dimers comprised of two H L3Ϫ anions, two europium() ions and four dmso molecules
co-ordinated to the metal ions. Within a dimer, an inversion
centre is located halfway from the two EuIII. Each eight-co-
ordinated EuIII is bonded to the five oxygen atoms of a calix-
[5]arene trianion, two dmso oxygen atoms and a eighth oxygen
from the second calix[5]arene anion (see the ORTEP 24 view
in Fig. 3). The latter, O(5Ј), bridges the two europium ions,
as does O(5), O(5) and O(5Ј) being related by the inversion
centre. The co-ordination polyhedra of the two ions (SUP 57328)
Conformation of the ligand in [Eu (H L) (dmso) ] as drawn are severely distorted as the analysis of the Eu᎐O bond dis- with the PACHA program 28
The Eu᎐O(2) and Eu᎐O(3) bond lengths (2.54 and 2.56 Å, these rings points outward, in contrast to the four other bridg- respectively) are longer than the other Eu᎐O (calixarene) bonds.
ing methylene groups. The phenol rings A, D and E remain in Since two of the five phenol groups remain protonated, we the cone conformation and the whole configuration can be think this lengthening is typical of Eu᎐OH bonds similarly to described as distorted toward a 1,2-partial cone conformation what was observed for the parent complex with a calix[4]- arene.13 The average Eu᎐O distance is 2.41(10) Å resulting in an
This X-ray analysis confirms that the reaction of H5
effective ionic radius, according to Shannon’s definition,25 of
dmso adduct of a nitrate salt of europium in the presence of a strong base leads to the isolation of a dimeric complex, as for r = 1.31 Å), in reasonable agreement with the literature value of 1.07 Å for eight-co-ordination.26 The two
the parent p-tert-butylcalix[4]arene complexes with EuIII 13
bridging oxygen atoms form a parallelepiped O(5)᎐Eu(1)᎐ or with ZnII, AlIII, FeIII and TiIV.29 The Eu ؒ ؒ ؒ EuЈ distances in
O(5Ј)᎐Eu(1Ј) in which the metal ions are separated by 3.89 Å.
the two dimeric moieties with calix[4]- and calix-[5]arenes are A special feature of the structure is the bonding of one very similar, 3.91 Å for the former and 3.89 Å for H5
distances are also close to those measured in the bimetallic
dmso molecule to EuIII through the hydrophobic cavity of the
calixarene, thus combining co-ordination to the metal ion and complexes [Eu LЉ(dmf) (dmf = dimethylformamide): 3.69 inclusion in the calixarene, in a similar way to that reported for (LЉ = hexaanion of p-tert-butylcalix[8]arene 11) and 3.81 Å
(LЉ = hexaanion of p-nitrocalix[8]arene 30).
(H O) LЈ ] (LЈ = p-sulfonatocalix[5]arene).19
Luminescent properties
In the acetone and acetate clathrates of H L the macro- cycle adopts a typical cone conformation,18,27 which is not the
Upon excitation through the ligand π → π* transition, solu- case in the presently described structure. The phenol rings B tions of the three complexes in thf display a sizeable emission and C are strongly bent, with the oxygen atom pointing inside band in the range 33 000–27 800 cmϪ1, with a maximum at
the hydrophobic cavity, while the methylene group bridging 31 150 cmϪ1, assigned to luminescence from the 1ππ* state of
J. Chem. Soc., Dalton Trans., 1998, Pages 505–510 Arrhenius plot of the Tb(5D ) lifetime in [Tb (H L) (dmso) ] vs.
Luminescence spectra of 10Ϫ3  [Ln (H L) (dmso) ] in thf, at
the ligand (Fig. 5). In the case of GdIII the triplet-state emission
can also be seen as a weaker band in the range 27 000–20 000
cmϪ1 and with a maximum at 23 640 cmϪ1. Assignment to the
3ππ* state relies on its decay time (a few tenths of ms). For
the europium() compound no metal-centred luminescence is
observed. Previous studies on p-tert-butylcalix[8]arene com-
plexes with EuIII 15,17 have pointed to a low-lying LMCT state
being responsible for the almost complete quenching of the
EuIII-centred luminescence in these compounds and the same
explanation probably holds for the calix[5]arene dimeric
complex. In contrast to EuIII, the complex of TbIII displays
a luminescence pattern characteristic of the metal-centred
5D → 7F (J = 0–6) transitions (Fig. 5), revealing a ligand-
to-metal energy-transfer process. The quantum yield of the
dimeric complex 10Ϫ3  in thf amounts to 5.1%. Given the
absence of specifically designed chromophoric groups attached
to the calix[5]arene, this figure appears to be quite encourag-
ing for the development of calix[5]arene-based luminescent
In the solid state the lifetime of the Tb(5D ) excited state,
obtained by direct laser excitation to 5D , amounts to 1.12 ±
0.04, 0.90 ± 0.04, and 0.21 ± 0.01 ms at 10, 70 and 295 K, Schematic energy-level diagram for H L3Ϫ and its dimeric com-
respectively. This relatively short lifetime may be partially due plexes with EuIII and TbIII (I.C. = intersystem crossing)
to the complexation of the two hydroxyl groups O(2)H andO(3)H. In [Tb LЉ(dmf) ] (LЉ = hexaanion of p-tert-butyl- Conclusion
calix[8]arene) where each TbIII is co-ordinated to one phenoxyl
group only, τ(5D ) = 1.52 ms at 77 K 14 and similar lifetimes have
The data presented here point to p-tert-butylcalix[5]arene being been reported at room temperature for complexes with lower- an interesting ligand for lanthanide complexation. In a low- rim substituted calix[4]arenes with carbamoyloxy groups (1.5 polarity solvent such as acetonitrile, stabilisation of its ms in water for R = OCH CONEt 31 and 1.79 ms in methanol
anionic forms is achieved both by intra- and inter-molecular for R = OCH CONH 32). The large temperature dependence
hydrogen bonds. In the presence of a strong base, such as NaH, observed between 10 and 295 K and leading at room temper- deprotonation is favoured and the macrocyclic anion reacts ature to a lifetime shorter than that observed for the aqua-ion with lanthanide trivalent ions to form dimeric complexes, both (0.42 ms 33) is noteworthy. Such dependence has been assigned
in solution and in the solid state, as demonstrated by ES mass to back energy transfer from the terbium() ion to the ligand triplet state,34 a phenomenon often observed for TbIII included
The energy-level scheme reproduced in Fig. 7 summarises in supramolecular edifices.35,36
the photophysical properties of the isolated complexes. If the Analysis of the temperature-dependent τ(5D ) lifetime
europium dimer proves to be inefficient as a luminescent between 50 and 295 K according to an Arrhenius relation of the probe, the metal-centred luminescence being completely type ln(τϪ1 Ϫ τ Ϫ1) = A ϩ (E /RT ),34 where τ is the lifetime at
quenched by the LMCT state (cf. the respective energy of temperature T, τ that in the absence of quenching (taken here this state and of the ligand 3ππ* state), the terbium assembly
at 10 K) and E the activation energy for the quenching process, conveniently absorbs UV light and transfers its energy from is shown on Fig. 6. A linear regression leads to E = 180 ± 20 the ligand 3ππ* state to the terbium 5D excited state. The
cmϪ1, smaller than the values reported 34 for triacetate chelates
overall efficiency of this transfer remains modest for two with 1,10-phenanthroline (900–2000 cmϪ1). If back energy
reasons. (i) The occurrence of a back-transfer process: Fig. 7 transfer from the 5D excited state of terbium to the triplet state
shows the near overlap between the Tb(5D ) level and the
occurs in [Tb (H L)(dmso) ], such a small activation barrier can low-energy tail of the 3ππ* state, so that a low-energy
be related to vibrational motion in the complex and the phonon-assisted back transfer will be easily achieved (cf. the deactivation pathway may be phonon assisted, for instance by activation energy of 180 cmϪ1 found for this process). (ii)
Ln᎐O vibrations, which occur at around 220 cmϪ1.37
More significantly, the intersystem 1ππ* → 3ππ* conversion
J. Chem. Soc., Dalton Trans., 1998, Pages 505–510 has a poor yield, as exemplified by the persistence of the Luminescence spectra were recorded on a Perkin-Elmer singlet-state ligand-centred luminescence in the spectrum of LS-50 spectrofluorimeter, using a 300 nm excitation filter for the dimeric metal complex. The ligand used in this study does the terbium-centred emission. The quantum yield was deter- not bear specifically designed chromophoric groups and it mined in degassed thf (90 ppm water) at room temperature as may be envisaged that the relatively small quantum yield previously described 42 using [Tb(terpy) ][ClO ] (terpy =
might be improved by grafting such groups on the calix- 2,2Ј : 6Ј,2Љ-terpyridine) in degassed acetonitrile (100 ppm water) as a secondary standard {absolute quantum yield 4.7%, as Finally, the inclusion-complexation properties of the calix- determined by the same method with [Ru(bipy) ][ClO ] [5]arene evidenced for dmso may be of interest to probe the (bipy = 2,2Ј-bipyridine) in air-saturated water as standard 43}.
inclusion of organic molecules in the calixarene cavity, since The concentrations used were 10Ϫ3  for both the terbium
co-ordination to the lanthanoid() ions will change the photo- dimer and the reference (to avoid decomplexation), with λ = 320.0 (sample) and 365.0 nm (reference). The 5D → 7F
transitions with J = 3–6 only were considered to obtain the Experimental
integrated luminescence intensity. Neglecting the weaktransitions to J = 0, 1 and 2 (<5% of the total integrated Synthesis and characterisation of the complexes
intensity) introduces negligible error and avoids correctionsfor the Rayleigh diffusion band interfering with these transi- Solvents and starting materials other than p-tert-butylcalix[5]- tions. The luminescence spectra were corrected for the arene (Acros) were from Fluka (Buchs, Switzerland) and used residual non TbIII-centred emission. The Tb(5D ) lifetimes
without further purification unless otherwise stated. Tetra- were determined on a previously described instrumental hydrofuran was distilled over Na and acetonitrile was treated set-up 44 using microcrystalline samples and selective laser
with CaH and P O .38 The dmso adducts of the lan-
excitation to the 5D level (486 nm); the reported lifetimes
thanoid salts were prepared from the oxides (Rhône- are averages of at least three determinations; biexponential Poulenc, 99.99%) 39 and their lanthanoid content determined
fitting of the curves was used, revealing a residual short life- by titration with Titriplex III (Merck) in the presence of uro- time due to an instrumental artefact, as confirmed by blank tropine (1,3,5,7-tetraazatricyclo[,7]decane) and xylene
The complexes were synthesized according to the follow- ing general procedure. The compound H L (1.3 × 10Ϫ4 mol, 1
equivalent) was dissolved in dry thf (0.12 dm3) under a nitrogen
The compound [Eu (H L) (dmso) ]ؒ10thf crystallised as atmosphere. Sodium hydride (4.55 × 10Ϫ4 mol, 3.5 equivalents,
yellow-orange platelets which were incorporated into a drop of 60% in oil) was added and the solution stirred for 2 h before Hostinert 216 oil and frozen to 170 K (Oxford Cryostream).
Ln(NO ) ؒxdmso (x = 3.18, 2.97 or 3.25 for Ln = Eu, Gd or Such a manipulation prevented observation of the crystals Tb) (1.3 × 10Ϫ4 mol, 1 equivalent) was added and the mixture
under orthoscopic conditions but an episcopic inspection stirred for 18 h at room temperature. A white precipitate of revealed thin boundaries subdividing the platelets into rect- NaNO slowly appeared and was filtered off. The solution angular blocks which were warped with respect to each other.
was concentrated under reduced pressure until a precipitate Data were collected on a Stoe IPDS system equipped with Mo- formed, which was separated from the mother-liquor by Kα radiation (λ 0.710 73 Å); 200 images in φ intervals of 1Њ centrifugation, washed with cold thf (2 cm3), centrifuged,
were exposed for 6 min each. The crystal–image plate distance and separated from the supernatant liquid. The resulting (80 mm) corresponded to a 1.13 ÅϪ1 resolution. Other experi-
efflorescent solid was dried under vacuum [12 h, 40 ЊC, 10Ϫ2
mental details are reported in Table 2. The indexing program Torr (ca. 1.33 Pa)] to yield 66, 68 and 58% (Eu, Gd and Tb) of of the IPDS system found the cell parameters from 2000 reflec- [Ln (H L) (dmso) ] as a white (Tb and Gd) or orange (Eu) x
tions (20 images), but only 1– of the peaks belonged to the cell,
powder [Found: C, 65.33; H, 7.12. Calc. for C as a result of the poor quality of the crystals (several attempts (x = 3): C, 64.67; H, 7.11. Found: C, 63.17; H, 7.16. Calc. for to produce better crystals failed). The defects of the crystals Gd O S (x = 4): C, 63.18; H, 7.10. Found: C, 62.93; materialised in diffraction patterns containing large spots and in a large background noise all over the reciprocal space, which 7.09%]. No nitrogen was found in any compound. Elemental made it difficult to evaluate a reasonable effective mosaic analyses were performed by Dr. H. Eder (Microchemical spread for the data. Following a referee’s suggestion, we Laboratory, University of Geneva). Infrared spectra were attempted to solve the structure in a more symmetrical mono- measured on a Mattson Alpha Centauri FT spectrometer as clinic cell. This resulted in a slightly worse agreement factor KBr pellets. The absorption maxima (in cmϪ1) were identi-
and did not change the topology of the dimer. We therefore cal, within experimental error, for all three complexes: 3305, think that the cell proposed in Table 2 soundly describes the important features of the diffraction pattern, despite the large Data were corrected for Lorentz-polarisation effects (the Physicochemical measurements
intensity decay during the measurement was negligible) and Electrospray mass spectra were measured on a Finnigan SSQ the structure was solved with SHELXTL.45 A first solution
710C spectrometer with 10Ϫ4  solutions in thf or acetonitrile.
was found in space group P1, but after successful refinement Spectrophotometric titrations were performed at 298 ± 0.2 K the program MISSYM 46 revealed an inversion centre between
on a UV/VIS Perkin-Elmer Lambda 7 spectrometer with 1 cm the Eu atoms of the dimer. Therefore, we refined two half quartz cells. In a typical experiment, a 10Ϫ4  solution (10 cm3)
dimers related by a pseudo-translation (¹¯ c*). One isotropic dis- of H L in dry acetonitrile containing 7.9 × 10Ϫ3  NEt ClO as
placement parameter was used for all atoms of the phenol rings, inert salt was titrated by increasing amounts of a 10Ϫ3  solu-
one for the atoms from the tert-butyl groups and one for the tion of Et N in the same solvent, delivered by a Metrohm atoms of the methylene bridges. Benzene rings were restrained Dosimat E 535 and recorded for Et N : H L ratios between 0 to be flat and to have literature C᎐C bond lengths,47 and tert-
and 40 : 1. The spectra were fitted using the SPECFIT pro- butyl groups and dmso molecules were restrained to con- gram.41 Factor analysis revealed the presence of four different
ventional bond lengths and angles.47 The dmso molecules
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