Schiff base oligopyrrolic macrocycles as ligands for lanthanides and actinides

Journal of Alloys and Compounds 418 (2006) 171–177

Schiff base oligopyrrolic macrocycles as ligands for lanthanides and actinides Jonathan L. Sessler a,∗ , Patricia J. Melfi a , Elisa Tomat a , Wyeth Callaway a , Michael T. Huggins a , Pamela L. Gordon b , D. Webster Keogh b , Richard W. Date c , Duncan W. Bruce c,e , Bertrand Donnio d,∗ a

Department of Chemistry and Biochemistry, Institute for Cellular and Molecular Biology, 1 University Station A5300, The University of Texas at Austin, Austin, TX 78712-0165, USA b C-Chemistry and NMT-Nuclear Materials Technology Divisions, Los Alamos National Laboratory, Los Alamos, NM 87545, USA c Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, UK d Institut de Physique et Chimie des Mat´ eriaux de Strasbourg (IPCMS), Groupe des Mat´eriaux Organiques (GMO), CNRS-ULP (UMR 7504) 23 rue du Loess BP 43, F-67034 Strasbourg Cedex 2, France e Department of Chemistry, University of York, Heslington, YORK YO10 5DD, UK Received 28 May 2005; received in revised form 28 June 2005; accepted 30 June 2005 Available online 5 December 2005

Abstract The coordination of f-block cations with Schiff base oligopyrrolic macrocycles is discussed. Analysis of the mesophase of a uranyl 2,5diformylpyrrole-derived expanded porphyrin complex through temperature-dependent X-ray diffraction (XRD) methods has provided evidence for liquid-crystalline properties, and for molecular stacking into columns, arranged in a 2D hexagonal lattice. In separate studies, UV–vis spectral analysis has indicated the formation of three new f-block oligopyrrolic complexes. Addition of neptunyl ([NpO2 ]2+ ) or plutonyl ([PuO2 ]2+ ) chloride salts to the free base of a dipyrromethane-derived Schiff base macrocycle induces an immediate spectral change, namely the growth of a Q-like band at 630 nm. Such changes in the absorption spectra cause a dramatic color change from pale yellow to blue. It is postulated that oxidation of this macrocycle, stimulated by reduction of the metal center, leads to the observed spectral changes. An immediate visible and spectral change is also observed with the reaction of lutetium silylamide (Lu[N(Si(CH3 )3 )2 ]3 ), with a different, tetrapyrrole-containing Schiff base macrocycle. In this case, the formation of a complex with 1:1 metal-to-ligand binding stoichiometry is further supported by MALDI-TOF mass spectrometry. © 2005 Elsevier B.V. All rights reserved. Keywords: Liquid crystals; Chemical synthesis; X-ray diffraction

1. Introduction The use of actinide elements, namely uranium and plutonium, as a source of energy and in nuclear weapon production has caused increasing attention to be focused on the problem of fuel reprocessing and waste storage [1,2]. Additionally, with the threat of a ‘dirty bomb’ or a spill releasing radioactive elements into the environment, the desirability of being able to detect actinides in a fast and efficient manner is now widely appreciated. In order to solve these two problems, a better understanding of the coordination chemistry and behavior of

∗

Corresponding authors. Tel.: +1 512 471 5009; fax: +1 512 471 7550. E-mail address: [email protected] (J.L. Sessler).

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the actinide cations is necessary; it is essential to develop systems able to coordinate and/or sense actinide cations, such as uranyl ([UO2 ]2+ ), neptunyl ([NpO2 ]+ ) and plutonyl ([PuO2 ]+ ). One attractive approach involves the use of so-called expanded porphyrins. Expanded porphyrins are a broad class of macrocyclic compounds containing pyrrole, furan and/or thiophene subunits linked together (directly or via spacers) in such a way that the internal ring pathway contains a minimum of 17 atoms. The larger core present in expanded porphyrins relative to naturally occurring tetrapyrrolic macrocycles (e.g., porphyrins, chlorins, etc.) allows the exploration of a remarkably diverse metalation chemistry [3,4], involving the coordination of large cations, such as the actinides, or the development of multinuclear complexes. Ongoing work in our laboratories has focused on the synthesis

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of novel expanded porphyrin macrocycles and studies of their metal complexation chemistry. In this paper, we illustrate two emerging directions in expanded porphyrin coordination chemistry, namely their use in stabilizing uranyl-containing liquid crystals (columnar hexagonal (Colh ) mesophases) and in the formation of new, stand-alone f-block complexes involving the uranyl, neptunyl and plutonyl cations. In all cases, the oligopyrrole macrocycles in question are novel Schiff base systems whose synthesis was specifically targeted towards the formation of hitherto unprecedented coordination complexes. 2. Experimental 2.1. General procedures Prior to use, all glassware was soaked in KOH-saturated isopropyl alcohol for ca. 12 h and then rinsed with water and acetone before being dried thoroughly. Tetrahydrofuran (THF) was dried by passage through two columns of activated alumina under argon. Hexane was distilled from sodium under argon. All solvents were deoxygenated using freeze–pump–thaw cycles (three times) before being transferred into an inert atmosphere drybox. Ligand 3 was prepared according to a procedure reported previously [5]. Ligand 5 was prepared according to the procedures reported for several analogues of this macrocycle [6,7]. Synthetic details for 5 will be described elsewhere. Lutetium silylamide (Lu[N(Si(CH3 )3 )2 ]3 ) was prepared according to a previously published procedure [8] and stored in the drybox. The [NpO2 ]+ , [NpO2 ]2+ and [PuO2 ]2+ chloride salts were prepared as aqueous solutions of known molarities, as previously reported [9].

2.2. Binding of Lu to macrocycle 5 Macrocycle 5 (10 mg, 0.017 mmol) was allowed to stand overnight under vacuum before being transferred into a drybox, where it was dissolved in THF (5 mL) under an inert argon atmosphere. Lu[N(Si(CH3 )3 )2 ]3 (14 mg, 0.021 mmol) was separately dissolved in THF (5 mL) and added drop-wise to a stirred solution of 5 over the course of 30 min. The reaction mixture was stirred overnight. An olive green powder was obtained in quantitative yield by slow vapor diffusion of hexane into the resulting reaction mixture. MALDI-TOF mass spectrometry: M+ m/z 742. UV–vis (THF, 296 K, Ar): λmax (nm) 391, 458, 594 and 702.

2.3. X-ray diffraction (XRD) analyses The XRD patterns were obtained using two different experimental setups. ˚ was obtained In all cases, a linear monochromatic Cu K␣1 beam (λ = 1.5405 A) using a sealed-tube generator (900 W) equipped with a bent quartz monochromator. In the first setup, the transmission Guinier geometry was used, whereas a Debye–Scherrer-like geometry was used in the second experimental setup. In all cases, the samples (as crude powders) were placed in Lindemann capillaries of 1 mm diameter. An initial set of diffraction patterns was recorded on an image ˚ to be measured and the sample template. This allowed periodicities up to 80 A perature to be controlled to within ±0.3 ◦ C. A second set of diffraction patterns was then recorded using a curved Inel CPS 120 counter gas-filled detector linked ˚ can be to a data acquisition computer; using this setup, periodicities up to 60 A measured while the sample temperature can be controlled to within ±0.05 ◦ C. In each case, exposure times were varied from 1 to 24 h.

central cores provides a platform that can stabilize a wide range of coordination modes [10]. One of the most extensively studied of the Schiff base porphyrin analogues is texaphyrin 1 (Scheme 1), first reported by our group in 1988 [11]. To date, texaphyrin has proved to be an excellent ligand for a number of transition metal cations [12] and for those of the trivalent lanthanide series [13]. These complexes are potentially interesting as drugs. Indeed, water-solubilized lutetium(III) and gadolinium(III) texaphyrin derivatives are currently undergoing clinical testing for use in photoangioplasty and cancer therapy, respectively [14]. Unfortunately, to date it has not proved possible to stabilize an actinide complex using the texaphyrin platform. On the other hand, several actinide-containing Schiff base macrocycles have been reported by our group [4,15–18]. Among these, perhaps the best studied has been ‘alaskaphyrin’ (2), a 2,5-diformylpyrrolederived Schiff base system that is characterized by its ease of synthesis and its robustness [15]. In light of these attributes, we are working to develop this system as a platform for the generation of actinide-containing liquid crystals. In the first part of this paper, we describe the preparation and characterization of an initial set of uranyl-based alaskaphyrin liquid crystals [18]. In the second part of the paper, the synthesis of new actinide complexes derived from other Schiff base pyrrolic macrocycles is detailed. 3.1. Temperature-dependent XRD study of liquid crystalline alaskaphyrins The uranyl complex of the tetramethoxy substituted alaskaphyrin 2 (Scheme 1) [15] can be obtained readily via metal templation, i.e., through acid-catalyzed condensation of 3,4-diethylpyrrole-2,5-dicarbaldehyde with 4,5-diamino-1,2dimethoxybenzene in the presence of uranyl nitrate. Alternatively, the same uranyl complex can be prepared via the addition of the uranyl cation salt to the preformed macrocycle. It is important to note that this macrocycle is just one of a number of analogous Schiff base macrocycles derived from 2,5-diformylpyrrole. For instance, synthetic replacement of the o-phenylenediamine moiety by other diamines has allowed a number of related uranyl complexes to be prepared. For the most part, these systems were prepared via metal templation [10]. However, the presumed neptunyl and plutonyl complexes of alaskaphyrin have also been

3. Discussion Schiff base oligopyrrolic macrocycles are rather unique ligands. This is because the differing donor functionality, namely pyrrolic and iminic nitrogen atoms, present within the various

Scheme 1. Compounds 1 and 2.

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reported [17]. For these latter studies, it proved preferable to add the metal salt to the preformed macrocycle due to the handling restrictions associated with the use of radioactive cations. One attractive application of uranyl alaskaphyrins is in the area of liquid crystals. At present, uranium-containing liquid crystals are rare [19,20]. Likewise, expanded porphyrin liquid crystals are limited in number, with the first only being reported by our group in 2003 [21]. More recently, we reported that uranyl alaskaphyrins of general structure 2 (Scheme 1) display liquid crystalline properties when substituted with appropriate alkoxy chains [18]. In this initial study, complexes 2a–c were characterized by standard methods, as well as by polarizing optical microscopy (POM). On the basis of these latter analyses, it was concluded that complexes 2b and 2c, soft solids at room temperature, melted to form a mesophase upon heating to 106 and 108 ◦ C, and then cleared at 133 and 135 ◦ C, respectively. It was also inferred that 2a was devoid of mesomorphism. Unfortunately, the formation and the identity of a mesophase cannot always be assigned unequivocally from the optical texture alone. Further, analysis by differential scanning calorimetry (DSC) revealed broad peaks, and as a consequence, provided data that were unhelpful in confirming the temperature range of the mesophase. On the other hand, the presence of homeotropic domains in some preparations provided strong evidence that the phases in question were columnar hexagonal. In order to provide further evidence in support of this conclusion, compounds 2a–c were analyzed using temperature-dependent X-ray diffraction methods. The results of these new studies are detailed below. From room temperature up to the clearing point, the three uranyl macrocycles 2a–c show evidence of liquid crystalline organization, as deduced from temperature-dependent XRD (in this experiment an X-ray pattern was recorded every 20 ◦ C from 20 to 180 ◦ C). Thus, the melting temperatures deduced by microscopy reported in ref. [18] actually correspond to the glass transition temperatures (viscous-to-fluid); below this temperature, the materials exist as anisotropic glasses, i.e., mesophases frozen into a glassy state [22]. Experiments carried on samples subject to a second heating–cooling cycle confirmed the reversibility of the formation of the vitrified mesophase, but the correct glass transition temperatures could not be deduced accurately. The slight discrepancy in the mesophase-to-isotropic liquid transition temperatures reflects the fact that it is the onset temperature that is given by XRD, while it is the peak temperature that is given by POM. All of the samples gave similar X-ray patterns, with the resulting diffractograms being characteristic of a columnar phase. In particular, a set of sharp reflections is observed in the small-angle range, which is typical of a hexagonal columnar phase (Fig. 1). In the small-angle region, an intense and sharp reflection and two other small√signals√ were observed with a reciprocal spac√ ing ratio of 1, 3 and 4 (and 7 for 2a). These reflections were thus indexed satisfactorily in the hexagonal 2D lattice as (h k) = (1 0), (1 1), (2 0) and (2 1) for 2a (Table 1). A broad scattering, reflecting the organization of the paraffinic chains, and two other weak signals ascribed to the intramolecular organization of the molecular cores are also seen. Thus, a broad and diffuse scattering halo is visible in the wide-angle region. It is

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Fig. 1. X-ray pattern of the Colh phases at T = 100 ◦ C. (a) Complex 2a, (b) complex 2b and (c) complex 2c.

˚ h2 , and corresponds to the liquid-like order centered at 4.6 A, ˚ of the molten aliphatic chains. The signal observed at 3.8 A, h1 , sharp but weak, likely corresponds to the average distance between successive aromatic cores (taken hereafter as the stack˚ h3 , ing periodicity, h1 ). Finally, the feature seen at 6.0–6.2 A, broad for 2b and 2c and sharp for 2a, is likely due to regular spacing of the coordinated uranyl cation. This latter signal, presumably due to the heavy scatterer [UO2 ]2+ , is so intense that it masks in part the wide-angle region (i.e., by increasing the background scatter). Comparison with the crystalline structure of a homologous compound [15], leads us to propose that this ˚ may reflect reticular planes rich in uranium. In band at 6.0 A

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Table 1 Thermal behavior of the UO2 complexes and X-ray characterization of the mesophases Compound

Transition temperatures (◦ C)a

˚ dmeas (A)

I

Indexation (h k)

˚ dtheor (A)

Mesophase parameters measured at 100 ◦ C

Volmue mass ρ at 100 ◦ C (g cm−3 )

2a

g[Colh ] 110 I

21.0 12.15 7.8 6.1 4.6 3.8

VS M W W br br

10 11 21 h3 h2 h1

20.9 12.1 7.9

˚ a = 24.15 A ˚2 S = 507 A ˚3 Vcell = 1925 A

1.33

25.05

VS

10

25.05

˚ a = 28.9 A

1.21

14.5 12.5 6.2 4.6 3.8

M M br br br

11 21 h3 h2 h1

14.5 12.5

˚2 S = 725 A ˚3 Vcell = 2755 A

28.95

VS

10

28.95

˚ a = 33.4 A

16.7 14.5 8.0 6.0 4.6 3.8

M M m br br br

11 21 – h3 h2 h1

16.7 14.5

˚2 S = 965 A ˚3 Vcell = 3675 A

2b

g[Colh ] Tg Colh 115 I

2c

g[Colh ] Tg Colh 125 I

1.12

The terms
dmeas √ and dtheor refer to the measured and theoretical diffraction spacings (dtheor is deduced from the following mathematical expression: d1 0  = 1 ( d h2 + k2 + h k), where Nh k is the number of h k reflections for the Colh phase), I the intensity of the reflection (VS: very strong, M: medium, W: h k Nh k hk weak, m: massive of peak unresolved, br: broad), h k the assigned indices of the reflections corresponding to the Colh phases, a the lattice parameter of the Colh phase (a = √2 d1 0 ), S the lattice area (S = a d1 0 ) and Vcell is the volume of the hexagonal cell (h1 S), i.e., a slice of column h1 thick. The volume mass, ρ, is 3

defined as ρ = T0 a

= 25 ◦ C)

VCH2 (T ) M Vcell 0.6022 VCH2 (T0 ) ,

where M is the molecular weight, VCH2 (Ti ) the volume of one methylene group at Ti (VCH2 (T ) = 26.5616 + 0.02023T , T in ◦ C,

and 0.6022 is the Avogadro number. I: isotropic liquid, Colh : hexagonal columnar phase. g[Colh ] refers to the frozen Colh mesophase, or glassy Colh phase. Tg corresponds to the glass transition temperature, but could not be accurately measured.

˚ the case of 2c, an additional weak multi-line signal, at ca. 8.0 A (designated ‘m’) was also seen; it is likely that this corresponds to a set of additional poorly resolved reflections indexed as (2 2) and (3 1). The X-ray characterization data for the mesophases of compounds 2a–c are summarized in Table 1. From S, the columnar cross-section, and h1 , the stacking periodicity, and assuming that one molecule occupies a columnar slice (as seen for metalloporphyrins and metallophthalocyanines [23]), a density ρ in the range 1.33–1.12 g cm−3 can be deduced for compounds 2a–c (Table 1). The values obtained in this way are slightly greater than those usually observed in organic liquid crystals; however, they are reasonable in the present case. Assuming a molecular volume that is likely to be similar to that of the free-base macrocycle, the overall density is expected to increase substantially as the result of the heavy metal atom present in the uranyl complexes. Additionally, it is observed that the density ρ decreases as the alkyl chains are elongated from 2a to 2c. In fact, the effect of the uranyl species on the density is decreased as the relative volume contribution of the aliphatic chains becomes enhanced. This is entirely as expected. Due to the molecular shapes involved, the formation of the observed columnar structure is most logically ascribed to the fact that these macrocycles stack in one direction, along the small molecular axis. The apparent cross-section of the cylindrical column (measured by XRD and necessarily circular) is,

however, slightly larger than the orthogonal projection of the compound onto the 2D plane. This means that the complex is free to rotate about its small axis or that the stacking of the molecules is not strictly collinear with the columnar axis, consistent with the broad nature of h1 (cf. Fig. 2). 3.2. UV–vis spectral evidence for the coordination of f-block cations Recently, we reported the synthesis of the Schiff base oligopyrrolic macrocycle 3 (Scheme 2); it was obtained via

Fig. 2. Spatial orientation of uranyl alaskaphyrins 2a–c as determined by temperature-dependent X-ray diffraction measurements.

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175

Scheme 2. Oxidation of macrocycle 3.

the [2 + 2] condensation of a diformyl dipyrromethane and ophenylenediamine [5]. The versatile metalation chemistry of this macrocycle and its analogues was quickly established, both in our group [24] and that of Love and co-workers [25]. The large core, capable of coordinating two transition metals, was considered a candidate for coordination of the actinide cations. Indeed, Arnold et al. reported the synthesis of the monouranyl complex of this class of macrocycle in 2004 [26]. Interestingly, however, the formation of a binuclear uranyl species was not observed, even when the reaction was carried out in the presence of an excess of the uranyl cation and at elevated temperatures. In light of the above findings, we were eager to explore further the metalation chemistry of ligand 3. As one part of this program, efforts have been made to extend the studies of f-block elements to include the [NpO2 ]2+ and [PuO2 ]2+ cations. For this, the free base form of macrocycle 3 was dissolved methanol and reacted with 2 equiv. of either neptunyl(VI) chloride in 2 M HCl or plutonyl(VI) chloride in 1 M HCl. With the addition of either cation, an immediate color change from pale yellow to blue was observed. UV–vis spectral analysis of the presumed neptunyl complex revealed a slight blue-shift in the position of the Soret-like band relative to the starting macrocycle (i.e., λmax shifts from 343 to 320 nm). The growth of strong Q-like bands at λmax = 636 and 593 nm was also observed (Fig. 3). The

Fig. 3. Changes in the UV–vis spectra observed in the presence of Np(IV) and Pu(VI). Solutions contain the free base of 3 in methanol with: (—) no metal, (- - -) 1.5 equiv. Np(VI) and (· · ·) 1.5 equiv. Pu(VI).

presumed plutonyl complex displayed a similar UV–vis spectra with absorptions being observed at λmax = 317, 580 and 628 nm (Fig. 3). The sapphire blue color of methanolic solutions of the putative neptunyl and plutonyl complexes of 3 stands in contrast to the khaki green color of the monouranyl complex prepared by Arnold et al. [26]. We believe that this difference in color reflects the small, but substantial differences in the actual ligands used by the two groups. In the case of the macrocycle used by Arnold et al., the system is ‘locked in’ an sp3 -hybridized dipyrromethane oxidation state both in the absence and presence of the metal cation. By contrast, conversion from the dipyrromethane to the corresponding sp2 -hybridized dipyrromethene is possible in the case of 3. In other words, the substitution pattern of macrocycle 3 allows for oxidation of the ring via reaction with O2 or an oxidizing metal species (Scheme 2). This produces an extended ␲-chromophore that likely accounts for the darker colors seen for the actinide complexes prepared from 3 as compared to that synthesized by Arnold et al. To put the above conclusion on a firmer experimental footing, Np(V) (as opposed to Np(VI); vide supra) was added to the free base form of 3 in methanol. UV–vis spectra were then recorded at various time intervals. Immediately following metal addition, no shift in the Soret band and no Q-like band was evident. However, over time, the Q-like band was seen to grow in and the color of the solution changed from its initial yellow to the sapphire blue color observed previously (cf. Fig. 4 for the associated spectra). On the basis of these observations, we propose that ligand oxidation occurs concurrent with cation coordination (Scheme 2). Specifically, we suggest that, when used as the metal source, the Np(VI) cation undergoes rapid reduction to the corresponding pentavalent state, thus acting as an oxidant that promotes oxidation of the macrocycle. When Np(V) is used as the metal source, there is no built-in oxidant and the ligand is likely oxidized by O2 over a period of time, thus accounting for the more gradual spectral changes. While not yet established unequivocally in the present instance, the idea of ligand oxidation in the context of actinide cation complexation is not new; it has been previously reported by our group in the case of isoamethyrin, 4 (Scheme 3), a formally 24 ␲-electron expanded porphyrin that undergoes oxidation upon coordination with the [UO2 ]2+ or the [NpO2 ]+ cations to produce the corresponding 22 ␲-electron derivative [16]. The result is an easy-to-visualize color change that makes

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Fig. 4. Changes in UV–vis spectra over time. Solutions contain the free base of 3 in methanol and 1 equiv. of Np(V): (—) spectrum recorded immediately after addition and (- - -) after 2 h, (· · ·) after 1 week.

isoamethyrin attractive as a possible colorimetric sensor for the actinide cations. We believe that this is also likely to prove true in the case of 3. Accordingly, efforts to develop this promise are currently underway. A different Schiff base oligopyrrole macrocycle developed in our group, grandephyrin 5 [6,7] (Scheme 3), is also attractive as a potential ligand for high valent actinide cations. Grandephyrin contains an inner core that is similar in size to that present in isoamethyrin 4, mentioned above [16]. In light of this, salts of [UO2 ]2+ , [NpO2 ]2+ or [PuO]2 2+ cations were added to methanolic solutions of the free base form of grandephyrin. In all cases, these additions caused significant spectral changes in the UV–vis region that were fully consistent with actinide cation coordination. In addition, crystals of the uranyl complex suitable for X-ray diffraction analysis were obtained [17]. The resulting structure revealed that the macrocycle is nearly planar and that the uranyl cation is displaced slightly in the direction of the dipyrromethene unit. This stands in contrast to what is seen in the metal-free methanol adduct, where a ‘ruffled’ conformation is observed. The reactivity of grandephyrin towards actinide cations prompted us to investigate whether it would stabilize coordination complexes with trivalent lanthanide cations. As a first step towards exploring this chemistry, the free-base form of grandephyrin 5, as a dark purple solution in THF, was treated with an excess of Lu(NO3 )3 . This treatment did not induce any appreciable spectral changes in the UV–vis region.

Fig. 5. Changes in the UV–vis spectrum of macrocycle 5 seen upon the addition of Lu[N(Si(CH3 )3 )2 ]3 in THF at room temperature.

Nor, did it lead to the production of a lutetium complex, as inferred from mass spectrometric analyses. The subsequent addition of triethylamine likewise failed to produce any evidence of complex formation. We thus turned our attention to the bis(trimethylsilyl)amide of lutetium(III). This reagent has the advantage of containing a ‘built-in’ deprotonating agent in the form of the bis(trimethylsilyl)amido groups which, after deprotonation of the pyrrolic nitrogen atoms of the ligand, can be removed from the reaction mixture as hexamethyldisilazane. Since bis(trimethylsilyl)amido compounds are generally airand moisture-sensitive, the metalation reaction was carried out in a glove-box as described in Section 2 above. Under these conditions the drop-wise addition of a THF solution of Lu[N(Si{CH3 }3 )2 ]3 to a stirred THF solution of the free-base macrocycle resulted in an immediate color change from purple to olive green. The accompanying UV–vis spectral changes are shown in Fig. 5. The formation of a Lu–grandephyrin adduct was confirmed by MALDI-TOF mass spectrometry. The species observed at m/z 742 corresponds to a lutetium complex in which all of the pyrrolic nitrogens of the macrocycle have undergone deprotonation such that they bind to the metal center as anionic donors. Unfortunately, the olive green lutetium adduct appears to be unstable to hydrolysis, undergoing slow decomposition upon exposure to air and moisture. This is hampering efforts to characterize this complex more fully. Current efforts are thus focused on finding other metal centers, including ones of the lanthanide(III) series, that might be able to form more stable complexes with grandephyrin 5 and its analogues. 4. Conclusions

Scheme 3.

Temperature-dependent X-ray diffraction analysis of molecules 2a–c has shown that, in contrast to what was inferred from the optical microscopy experiments, all three samples display liquid crystalline behavior over an accessible temperature range. These same studies revealed that the liquid crystalline species, 2a–2c, have volume masses of 1.33, 1.21 and 1.12 g cm−3 , respectively, in their Colh mesophases.

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These values, higher than those typically observed for organic liquid crystalline systems, are attributed to the presence of the heavy uranyl cation. Additionally, it was shown that while the molecules stack in one direction along the small molecular axis (resulting in the observed columnar structure), they are free to rotate about this axis. Spectroscopic evidence consistent with the formation of fblock cation complexes with other oligopyrrolic complexes has also been presented. For instance, it was demonstrated that neptunyl or plutonyl solutions react with macrocycle 3 to produce a visible color change that is fully consistent with complex formation. Similarly, reaction of the lutetium reagent Lu[N(Si{CH3 }3 )2 ]3 with macrocycle 5 (under an argon atmosphere) causes a dramatic color change from purple to green and the formation of an air-sensitive lutetium complex as confirmed by MALDI-TOF mass spectrometry. Acknowledgements B.D. would like to thank the CNRS for support and Dr. Benoˆıt Heinrich for his assistance in the X-ray analysis. D.W.B. thanks the Leverhulme Trust for a grant to R.W.D. Support for this work was also provided by the Department of Energy, Office of Basic Energy Sciences (grant DEFG03-01ER-15186 to J.L.S.). E.T. wishes to thank Jennifer A. Moore for providing the lutetium silylamide and for helpful discussions. P.J.M. expresses her appreciation to the Seaborg Institute for Transactinium Science for a Summer Fellowship. The work at Los Alamos National Laboratory was funded by the Department of Energy, Office of Basic Energy Sciences and the Defense Programs Education Office. Los Alamos National Laboratory is operated by the University of California under contract W-7405-ENG-36. References [1] G.R. Draganic, Z.D. Draganic, J.-P. Adloff, Radiation and Radioactivity on Earth and Beyond, CRC, Boca Raton, FL, 1990. [2] D.L. Clark, D.E. Hobart, M.P. Neu, Chem. Rev. 95 (1995) 25–48. [3] K.M. Kadish, K.M. Smith, R. Guilard (Eds.), The Porphyrin Handbook, vols. 2–3, Academic Press, San Diego, 2000.

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