摘要:
Neil Robertson,*a Simon Parsons,b Kunio Awagac and Wataru Fujitac a Department of Chemistry, Imperial College of Science, Technology and Medicine, London, UK SW7 2AY. E-mail: neilrobertson@ic.ac.uk b Department of Chemistry, University of Edinburgh, West Mains Road, Edinburgh, UK EH9 3JJ c Department of Basic Science, The Graduate School of Arts and Sciences, University of Tokyo, Komaba, Meguro, Tokyo 153, Japan Received 7th June 2000, Accepted 5th July 2000, Published 18th July 2000 Slow interdiffusion of acetonitrile solutions of [TBA][Ni(bdt)2] (bdt = 1,2-benzenedithiol) and [MPYNN][I] (MPYNN = 4,4,5,5- tetramethyl-2-(1-methyl-4-pyridinio)-3-oxide-4,5-dihydro-1H-1-imidazolyloxyl) resulted in two sets of crystals suitable for Xray analysis. One of these showed a 2 : 1 salt comprising 1-methyl-4(1H-4,5-dihydro-4,4,5,5-tetramethylimidazol-2- yl)pyridinium, resulting from double deoxygenation of [MPYNN]+, and [Ni(bdt)2]– with two oxygen atoms bonded to two of the sulfur atoms in a trans configuration suggesting an oxygen-transfer reaction between the two components. The salt packed with mixed stacks of anions and cations.The second structure showed the starting complex [TBA][Ni(bdt)2], only the second structural characterisation of the radical [Ni(bdt)2]–. Strong interactions between the complexes are prevented by the alkyl arms of the countercation. Structural characteristics of dithiolene and nitronyl nitroxide salts: a unique oxygenation– deoxygenation process Fig. 2 [Ni(bdt)2]–. Introduction The construction of magnetic materials by combination of selected paramagnetic molecular building blocks has received wide attention in recent years due to the opportunity to study structure–property relationships in such materials.1–3 This approach has led to such phenomena as ferromagnets,4 ferrimagnets,5 low-dimensional materials,6 spin-transition compounds7 and hybrid conducting/magnetic materials.8 Within this context, the nitronyl nitroxide family of compounds has been used to construct examples of low temperature ferromagnetic and weak ferromagnetic materials consisting only of organic molecules9,10 and has also been used to prepare unusual magnetic materials, such as spin ladder,11 or in hybrid organic–inorganic magnetic systems.12 Such hybrid systems can be prepared by the preparation of salts containing a nitronyl nitroxide cation (see Fig.1) and a paramagnetic counter anion composed of a metal bis-1,2- dithiolene complex (see Fig. 2). Such metal complexes are also well known as components of molecular magnetic materials.13 Several previous studies have sought to combine these building blocks into new magnetic materials and have demonstrated that attractive results can be obtained.12 In this context we have investigated the crystallisation of cation [p-MPYNN]+ (4,4,5,5-tetramethyl- 2-(1-methyl-4-pyridino)-3-oxide-4,5-dihydro-1H-1- imidazolyloxyl) (1) with the complex [TBA][Ni(bdt)2] (2) and have observed an unexpected instability of the components leading to chemical reaction in addition to salt formation.We also report only the second structural determination of a salt containing [Ni(bdt)2]– and discuss the potential of this complex in the preparation of novel magnetic materials. DOI: 10.1039/b004549g CrystEngComm, 2000, 22 Fig. 1 [p-MPYNN]+. Results and discussion The crystallisation of new mixed dithiolene–nitronyl nitroxide materials was attempted by slow interdiffusion of acetonitrile solutions of TBA[Ni(bdt)2] and [p-MPYNN][I]. From the reaction mixture, two sets of crystals suitable for X-ray structural investigation were obtained. First, the unreacted starting material TBA[Ni(bdt)2] (2) was obtained and although the paramagnetic, delocalised molecule [Ni(bdt)2]– has been known for 35 years, only one structural characterisation of a salt of this complex has previously been reported.This is despite extensive study of various salts of other families of metal bis-1,2-dithiolene complexes due to their magnetic and conducting properties.14 In addition, crystals were obtained of a salt (3) comprising both an unexpected anion and an unexpected cation. The anion consists of the parent [Ni(bdt)2]– component with two additional oxygen atoms attached to sulfur atoms arranged in a trans conformation (Fig. 3). The cation consists of theFig. 3 Molecular structure of the salt 3. nitronyl nitroxide component with two oxygen atoms missing with respect to the parent compound 1 (Fig. 3). The structure of 2 is unusual in that the metal anions do not form face-to-face interactions with neighbouring complexes but are separated by the alkyl arms of the counterion (Fig.4). The complexes are arranged in side-by-side chains (Fig. 5), however no intermolecular interactions between the complexes are shorter than a C···S distance of 3.67 Å between the coordinating sulfur and an ortho carbon on the aromatic ring. This behaviour contrasts with other planar, monoanionic Ni–bis-1,2-dithiolene complexes, such as [TBA][Ni(mnt)2]15 (mnt2– = maleonitriledithiolate) and [PBu4][Ni(tdas)2]16 (tdas = 1,2,5-thiadiazole-3,4-dithiolate) where strong dimerisation of the metal complexes occurs with strong intermolecular Ni···S interactions. [TBA][Ni(bdt)2] shows an arrangement more akin to that of TBA[Ni(diod)2] (diod = 3,6-dithia-1-oxacyclohept-4-ene- 4,5-dithiolate) with anions and cations alternately arranged, however in the latter case this was readily rationalised in terms of the non-planarity of the complex impeding intermolecular close approach.It is unclear why the structure of [TBA][Ni(bdt)2] should follow this pattern, however it should be noted that the phase reported was only obtained from acetonitrile solution. A different, uncharacterised phase was obtained from ethanol (as determined by powder X-ray diffraction) indicating that more than one phase is readily achieved, with presumably comparable energies. Fig. 4 X-ray structure of [TBA][Ni(bdt)2] (2) showing anion and cation arrangement within the unit cell.Click on the image or here to access a 3D representation. Fig. 5 Side-by-side arrangement of [Ni(bdt)2]– sheets in 2. Click on the image or here to access a 3D representation. In contrast to other monoanionic nickel–bis-1,2-dithiolene complexes such as those above, only one other structure of a salt of [Ni(bdt)2]– has been previously reported, with the cation NMP (N-methyl phenazinium).17 Molecular parameters are comparable between the two structures, both showing a C–C bond between the two thiol groups which is comparable to the other C–C bonds in the aromatic ring. The C–C bond in other monoanionic Ni–bis-1,2-dithiolene complexes is typically around 1.32–1.37 Å18 and the slightly longer values in 1 and 2 would be anticipated due to the reduced double bond character expected between the carbons of the benzene ring.The packing within the [NMP][Ni(bdt)2] salt showed a mixed-stack with a CACCAC (C = cation, A = anion) arrangement and with the other anions arranged adjacent at an angle of 67°. Again, this illustrates a tendency for the [Ni(bdt)2]– anion to avoid the typical face-to-face dimerisation seen in related monoanionic, planar dithiolene salts. Magnetic susceptibility measurements were carried out on 2 recrystallised from CH3CN and confirmed by powder diffraction to possess the same structure as the single crystal determination. Over the temperature range 2–300 K, 2 showed Curie–Weiss behaviour with C = 0.3786 emu kmol–1 and Q = –4.5 K. This is consistent with one unpaired spin per Ni complex and the low Weiss constant is in keeping with the lack of strong intermolecular interactions displayed in the X-ray structure.The apparently reduced tendency of [Ni(bdt)2]– to form strong face-to-face dimers prevents the formation of a strongly antiferromagnetically coupled ground state, typical in other planar metal–bis-1,2-dithiolene radical complexes, and allows the possibility that more interesting magnetic states could be stabilised. The crystallisation of [Ni(bdt)2]– with small counterions to promote stronger intermolecular interactions may be an attractive and straightforward route towards magnetic materials such as those previously observed using [Ni(mnt)2]– salts which display ferromagnetic interactions.4 The salt 3, contains anions and cations arranged in a mixed stacking arrangement with the anion and cation molecules slipped so that the pyridine ring of the cation lies over one of the aromatic rings of the anionic complex (Fig.6). Both oxygen atoms of the anionic complex participate in H-bond interactions. One of these is with the NH of the cation in the adjacent stack showing an N···O distance of 2.876 ÅFig. 6 Packing of anions and cations within the unit cell of 3 viewed along the a-axis. Click on the image or here to access a 3D representation. as tBuOOH19 or via photochemical reaction in the presence of O2.20 In our experiments, however, recrystallisation of 2 from acetonitrile did not lead to any oxygenated product except in the presence of 1.This suggests that the loss of oxygen from 1 and oxygenation of [Ni(bdt)2]– are not coincidental but involve a reaction between these components. Deoxygenation of nitronyl nitroxide radicals has previously been achieved via heating with PPh3 in benzene21 or unexpectedly, such as in reaction with [Mn(F6acac)2].22 However these reactions led to loss of only one oxygen from the radical. We are unaware of any example where both oxygens have been removed from this type of radical and no previous structural characterisation of the doubly deoxygenated fragment in 3 has been reported.23 Powder X-ray diffraction was carried out on precipitated product from the reaction of 1 with 2 over several weeks in acetonitrile. However, by comparison with the simulated powder pattern, no indication that 3 was present in the sample was obtained and therefore no bulk sample of 3 was available for magnetic measurements.and N···H···O angle of 150.34°. The other shows a distance of 2.537 Å to the oxygen of the included water. It is unsurprising that the inclusion of such oxygen atoms in the complex leads to hydrogen bonding becoming a controlling factor in the packing of the salt. The anionic metal complex shows, in general, similar structural parameters to the [Ni(bdt)2]– complex but with the additional two oxygen atoms attached to one of the donor S atoms of each ligand. Curiously, the S···Ni distance of this sulfur (2.155 Å) is comparable to those of [Ni(bdt)2]–, whereas the other S···Ni distance is significantly longer at 2.181 Å.The oxygenation of metal 1,2-dithiolene complexes has previously been observed via oxidation with a reagent such Value Property 1 P Formula Crystal system Space group a/Å b/Å c/Å V/Å3 a/° b/° g/° ZDc/mg m–3 F(000) T/K l/Å wR Unique reflections Independent reflections 1928 GOF R 1.017 0.0690 Experimental TBA[Ni(bdt)2]24 and [p-MPYNN][I]25 were prepared according to literature procedures. The crystals of [p- MPYNNH]2[Ni(bdt-SO2)2]·0.25H2O and the crystals of TBA[Ni(bdt)2] were grown by slow interdiffusion of a solution of [p-MPYNN][I] (30 mg) in 1 ml CH3CN and TBA[Ni(bdt)2] (35 mg) in 1 ml CH3CN over several weeks.The magnetic measurements were performed on a Quantum Design MPMS XL SQUID susceptometer. We carried out the correction for the diamagnetism, assuming the paramagnetic component followed the Curie law at high temperatures. Table 1 Crystal data, data collection and refinement parameters for [p- MPYNNH]2[Ni(bdt-SO2)2]·0.25H2O (3) and TBA[Ni(bdt)2] (2)a 3 2 C38H48.5N6NiO4.25S4 Triclinic C28H44NNiS4 Orthorhombic Pna2 16.483(5) 19.715(4) 18.613(8) 6049(4) ——— 7.233(4) 8.162(7) 17.091(8) 1007.4(11) 91.80(5) 91.65(5) 92.02(7) 1 8 1.392 445 220 1.54184 0.1960 2928 1.277 2488 220 1.54178 0.1235 5647 2958 0.942 0.0596 a Data were collected using Cu-Ka radiation. The structure was solved by Patterson (3) and direct (2) methods (SIR92) and refined on F2 by full-matrix least squares (SHELXL97). Click here for full crystallographic data (CCDC no.1350/27).Acknowledgements We thank Dr Gavin Whittaker, University of Edinburgh, for powder diffraction measurements, The Royal Society of Edinburgh/BP for a Research Fellowship (NR) and the British Council for a collaborative award with Japan. 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ISSN:1466-8033
DOI:10.1039/b004549g
出版商:RSC
年代:2000
数据来源: RSC