Formation of one dimensional linear chains by Ir–Ir bonds in cis-dicarbonyldichloroiridate (I)
Graphical abstract
By means of EXAFS, electronic structure calculations and stochastic predictions, we obtained the structural parameters of the material K[IrCl2(CO)2] which comprises linear chains of Ir–Ir bonds.
Research highlights
► Electronic properties of K[IrCl2(CO)2]H2O related to supramolecular structure. ► Formation of one dimensional linear chains of Ir–Ir with at least 12 Ir atoms. ► The Ir–Ir distance is 2.82 ± 0.01 Å, short enough to assume a metal–metal bond. ► Monomers placed parallel in helical arrangement making 360° turn after 8 units.
Introduction
One of the most interesting properties of square-planar metal transition complexes is that the metal atoms may stack along the axis perpendicular to the ligand plane. This stacked structure forms linear chains of metal–metal bonded atoms, which allows, when only weak interaction exists between adjacent chains, a potential one-dimensional conduction. The conduction properties, high anisotropy and restricted dimensionality of these kinds of materials make them very attractive for nanotechnology, as they could allow developing devices of small dimensions [1], [2], [3].
There are several examples of compounds with Pt or Ir, that contains metal–metal bonds [4], [5], [6], [7], [8], [9], [10]. Singular anionic iridium carbonyl halides have been reported some decades ago [11], [12]. Ginsberg et al. [13] synthesized a set of compounds with an approximated formula of Kx[IrCl2(CO)2], from which the most intriguing observations made by the authors was the metallic copper luster of these compounds. By means of Mössbauer spectroscopy, magnetic susceptibility, electrical conductivity, optical anisotropy and elemental analysis, they concluded that the compounds must comprise Ir(I) planar linear-chain conductors. Using IR information, the authors suggested that the structure of the compound was stacked cis-[IrCl2(CO)2]0.6− planar linear-chains connected by Ir–Ir bonds. By comparison with other materials, they suggested an Ir–Ir bond distance of 2.86 Å, but no structure has been reported in the literature for these compounds. In a recent work, using electrochemical oxidation of AsPh4[IrCl2(CO)2] in tetraethylammonium perchlorate, it was shown that the growth of the crystal was compatible with a one-dimensional conductor [14]. By means of Rietveld analysis [15], [16] in X-ray powder diffraction studies, it was estimated the Ir–Ir bond to be 2.881 Å for N(C2H5)4[IrCl2(CO)2] [17]. Hexanuclear iridium compounds, which also have a copper luster, were recently described by Tejel et al. [18]. Their structure presents an almost linear chain of six iridium atoms with Ir–Ir bonds ranging 2.68–2.79 Å. In view of the interesting properties of this kind of Ir(I) complexes as a nanomaterial, and the lack of certainty about some structural parameters in the solid state, we investigated its molecular and supramolecular structure by the use of a variety of powerful experimental and theoretical techniques such as IR, NMR, ESI-MS, EXAFS, DFT calculations and MGAC/CPMD stochastic predictions. We present information which confirms the previous linear Ir–Ir chains suggestions and Ir–Ir bond estimations, and new findings about the estimated average length and helical twist of the Ir–Ir chain.
Section snippets
Synthesis of K[IrCl2(CO)2]H2O
An amount of 298 mg of K3IrCl6 (5.7 × 10−4 mol) were dissolved in a reaction flask with 3 mL of concentrated HCl and 3 mL of concentrated formic acid (85%). The solution was refluxed by using a vaseline bath at 150 °C for 3 h and stirred magnetically (if the solution is refluxed by using a lower temperature bath for only 25 min, the product obtained is [IrCl5CO]2− as described by Cleare et al. [19]). The solution’s color changed from purple to orange-brown. Upon cooling, a brown solid precipitated. The
Formation of [IrCl2(CO)2]−
As described in Section 2, the preparation procedure follows the route used previously to obtain [IrCl5CO]2− from [IrCl6]3− by formic acid dehydration (Eq. (1)). It seems plausible to assume that after longer reaction times a second CO can substitute another chloride ligand, presumably the one trans to the CO, which is labilized due to trans-effect. This would produce an Ir(III) octahedral cis-dicarbonyl complex, which could loss chlorine by reductive elimination (RE) to produce an Ir(I)
Generation of the initial population
The initial population was generated with locally optimized individuals, for which all the energy calculations have been done using the CPMD code, using the LDA exchange through PADE approximation and the Goedecker et al. pseudopotential [43], [44]. The calculations were done employing an energy cutoff of 80 Ry, and a cell length of 8 Å plus the largest dimension of the cluster. The individuals are “trimers” built by “stacking” anions [IrCl2(CO)2]− on planes randomly rotated about the axis
Conclusions
In this work we have used different techniques in order to obtain structural information for K[IrCl2(CO)2]·H2O. In particular, we have shown that despite no single crystal X-ray diffraction structure has been obtained to our knowledge, probably due to the microcrystalline characteristics of the solid, the use of EXAFS in combination with computational and experimental techniques allow us to confirm the suggested linear chain structure for the anions. Moreover, the Ir–Ir distance of 2.82 ± 0.01 Å
Acknowledgements
This work has been partially supported by generous computer time allocations from the CHPC at the University of Utah on the Arches cluster partially funded by NIH – National Center for Research Resource (# 1S10RR017214-01). We greatly acknowledge financial support from Universidad de Buenos Aires (UBACYT X065), Universidad Nacional de La Plata, ANPCyT (PICT 25515, 2008-00038, and 2006-2396), CONICET (PIP 112-200801-03079 and PIP 112-200901-00369) and LNLS (project D04B-XAFS1-5754).
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radicals, have been detected in situ by photoelectron photoion coincidence spectroscopy with vacuum ultraviolet synchrotron radiation (PEPICO-VUV). Our findings imply a strong relationship between I radicals’ as well as CO molecule’s roles in the breakage of NPs metal–metal bond. NPs with size larger than about 5 nm are difficult to be dispersed due to strong Ir–Ir bond together with the kinetic stability. Besides, 5 wt% Ir HSMSCs deactivated by metal agglomeration can be regenerated repeatedly and regain their initial activity for heterogeneous methanol carbonylation.