Chemistry and catalysis advances in organometallic chemistry and catalysis
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experimental validation of a theory. Obviously, new serendipitous discoveries then need further experimental validations to become a general theory and to be universally useful. The first time a new result is obtained is very often by chance, but any unexpected, interesting result must be deeply studied and exploited. Serendipity is thus a very important issue but it is not enough if the results that it produces are not developed and utilized at the best. As stated by Louis Pasteur, “Dans les champs de l’observation le hasard ne favorise que les esprits prepar´es” [1]. The words “unexpected,” “surprising,” or “unusual” can be found in numerous scientific articles (mainly in the titles), conference lectures, or poster presentations and they are normally employed to emphasize the novelty of the reported
First Edition. Edited by Armando J. L. Pombeiro. © 2014 John Wiley & Sons, Inc. Published 2014 by John Wiley & Sons, Inc.
408 FROM SERENDIPITY TO POROSITY: SYNTHESIS AND REACTIVITY OF COORDINATION POLYMERS data. Nevertheless, numerous researchers are ashamed to declare that the results they are reporting have been obtained serendipitously and not by following a specifically designed procedure, because this seems to suggest the absence of a serious study that should have been carried out before the experiment. Here the author would like to reappraise the relevance of the results obtained serendipitously. In particular, the author reports how serendipity was a relevant issue and, to some extent, drove his specific researches in the last years in the field of coordination polymers (CPs). For the inspiration of this note, the author is largely indebted to R.E.P. Winpenny [2] for his article where he highlights some advantages of the serendipitous assembly of polynuclear cage compounds with respect to the “designed assembly.”
CPs, sometimes referred to as metal–organic frameworks (MOFs) [3], represent one of the most interesting class of hybrid (organic–inorganic) materials [4]. These crystalline compounds are characterized by peculiar polymeric structures that can be envisaged as the product of a rigorously regular copolymerization, where metal ions (or small assemblies of metal ions)—nodes—are joined through polytopic organic ligands— linkers. Depending both on the coordination geometries of the metals and the ligands structural features, CPs in 1-, 2-, or 3-dimensional scale can be obtained (Fig. 30.1). Polytopic ligands mainly contribute to define the connectivity between the nodes, generating repetitive entities also referred to as secondary building units (SBUs) [5] that form the extended structures of CPs. CPs can have interesting properties connected to their structures, such as porosity (with regular pores sizes), that may produce high surface area, sorption–desorption behavior of energetically or environmentally important gases, heterogeneous catalysis ability, etc. The porosity of CPs is mainly related to the rigidity of the linkers and to their specific structures [6], particularly to the structural characteristics of the spacer, that is, the fragment connecting the specific donor groups joining the metal centers. In Fig. 30.2, one possible structure of a 3D CP is sketched, where donor groups (D) and the spacer (S) are evidenced in the linkers, and the nodes M adopt an octahedral coordination geometry. 1D 2D
(a) (b)
(c) Figure 30.1 Schematic examples of possible (a) 1D, (b) 2D, and (c) 3D CP architectures. Nodes are indicated as black spheres, while linkers are in white.
Scheme of a 3D CP architecture where the octahedral metal nodes (M) are joined by ditopic linkers. The specific donor groups (D) and the spacer fragment (S) are evidenced.
TRINUCLEAR TRIANGULAR CU II MOIETIES TO BUILD CPS 409 O O O O O O O O O O O O O O O O 11.2 12.6 16.4
19.1 M M M M M M M M M = Zn 4 (O)– A B C D = dianions A-D Figure 30.3 Schematization of the IRMOFs obtained by Yaghi group [7] by connecting Zn 4 (O) with rigid dianions A–D. The number below each dianion indicates the size of the cubic structure sketched in the left, calculated as the diameter (in angstrom) of a sphere that can fit in it. In numerous works on CPs that appeared in the last years, the so-called designed synthesis has been often claimed to be employed in order to obtain interesting derivatives, even though the author thinks that in some cases the word “serendipitous” should be more appropriate and not shameful. On the other hand, a very elegant example of a “true” designed synthesis was pursued by Yaghi and coworkers [7], which obtained a series of iso-reticular metal–organic frameworks (IRMOFs). Actually, the Yaghi group, exploiting the octahedral geometry of basic copper acetate, [Zn 4 (O)(MeCOO) 6 ], connected the Zn 4
of porous IRMOF, which differ only in the dimensions of the spacer placed between the two carboxylates groups. The IRMOFs [Zn 4 O(OOC-(S)-COO) 3 ] thus obtained all have the same primitive cubic structure sketched in Fig. 30.3, with three series of identical perpendicular channels whose free diameters are directly related to the dimensions of the spacers, provided that no interpenetration occurred [7]. This was certainly one of the best results in the designed synthesis of MOFs, evidencing the value of the researcher’s imagination, pursued with the aim to obtain a series of derivatives having structural properties directly related to the dimensions of the spacers used. On the other hand, it is often useful to overcome also the researcher’s imagination and, according to Winpenny, “ . . .
design assembly approach ( . . . ) relies on a limited range of experience and on the imagination of the scientists involved.” [2], rely also on serendipity.
As far as the author’s specific work in the CPs field is concerned, it certainly started from a serendipitous event, when, working with carbon suboxide, C 3 O 2 , he easily synthesized a series of diamidic derivatives, all having the malonate core and some azolate as the amidic moieties (Scheme 30.1), with the aim to use them as ligands, analogously to other extensively used azolate derivatives [8]. The malonyl diamides, easily obtained in good yields, are quite stable and, in some cases, it was possible to obtain their single-crystal (SC) X-ray diffraction (XRD) characterizations that were afterward reported [9]. To quickly check the possibility of these derivatives to act as ligands, the author treated an ethanol solution of the just-isolated Hpz-based diamide
410 FROM SERENDIPITY TO POROSITY: SYNTHESIS AND REACTIVITY OF COORDINATION POLYMERS N H
N H N N H N N N H N O C C C O + 2 Azole
O O N N N R Azole = N R Pyrazole (Hpz)
3,5-Dimethylpyrazole (3,5Me
2 Hpz)
Indazole (Hin)
Benzotriazole BHtz
Scheme 30.1 Figure 30.4 Ball-and-stick representation of the asymmetric unit of 1. Color codes: light gray (Cu), dark grey (O), black (N), gray (C), and white (H). with a water solution of copper(II) acetate that he happened to have on his work bench. The solution immediately turned deep blue, indicating that the coordination of some kind of nitrogen to copper(II) had occurred and, after some days, a few deep-blue hexagonal platelets formed. Owing to the scarcity of the obtained product, only an SC-XRD determination was carried out on the crystals, yielding a completely unexpected result. Instead of some kind of copper derivative of the malonyl diamide, the structure of the trinuclear triangular assembly [Cu 3 (
3 -OH)(
μ-pz) 3 (MeCOO) 2 (Hpz)], 1, shown in Fig. 30.4, was found. This synthesis was certainly serendipitous. In the presence of water, Cu II , and acetate ions, the quite stable diamide underwent hydrolysis forming Hpz that generated the observed trinuclear triangular derivative through a series of nontrivial processes. First of all, we ascertained that it was possible to obtain compound 1 in a quantitative yield by directly reacting a water solution of copper acetate with Hpz [10]. Moreover, continuing the studies, we realized that serendipity worked hard in this reaction. As a matter of fact, the trinuclear triangular assembly was obtained thanks to the fact that copper(II) acetate instead of copper chloride, nitrate, or sulfate had on hand. Actually, the basicity of the acetate ions is needed to favor the deprotonation of pyrazole and water to form pyrazolates and the capping μ 3
further confirmed by the obtaining of analogous trinuclear triangular derivatives by employing copper formate, propionate, and butyrate [11], while the reaction failed when copper trifluoroacetate was used [10], the basicity of the CF 3 COO
− anion
TRINUCLEAR TRIANGULAR CU II MOIETIES TO BUILD CPS 411 3 Cu(RCOO) 2 + m Hpz [Cu 3 ( μ 3 -OH)( μ-pz) 3 (RCOO) 2 L
L'
] . n H 2 O + 4 RCOOH 1 R = CH 3 , m = 4, L = Hpz, x = 1, y = 0, n = 0 2 R = H, m = 5, L = L' = Hpz, x = y = 1, n = 1 3a R = CH 3 CH
, m = 3, L = EtOH, x = 1, y = 0, n = 0 3b R = CH 3 CH
, m = 3, L = H 2 O, x = 1, y = 0, n = 0 3c R = CH 3 CH 2 , m = 3, L = H 2 O, x = 1, y = 0, n = 1 4a R = CH 2 =CH, m = 4, L = Hpz, L' = H 2 O, x = 1, y = 2, n = 0 4b R = CH 2 =CH, m = 3, L = MeOH, x = 1, y = 0, n = 0 5 R = CH 2 =C(CH 3 ), m = 3, x = 0, y = 0, n = 0 6 R = CH 3 (CH 2 ) 2 , m = 3, L = MeOH, L' = H 2 O, x = y = 1, n = 0 7 R = CH 3 (CH 2 ) 3 , m = 3, L = H 2 O, x = 1, y = 0, n = 0 8 R = CH 3 (CH 2 ) 4 , m = 3, L = EtOH, x = 1, y = 0, n = 0 9 R = CH 3 (CH
2 ) 5 , m = 3, L = EtOH, x = 1, y = 0, n = 0 . H 2 O, EtOH, or MeOH 1-9 N
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