Density Functional Theory Analysis of Carboxylate-Bridged Diiron Units in Two-Dimensional Metal-Organic Grids Ari P. Seitsonen,* ,² Magalı ´ Lingenfelder, ‡ Hannes Spillmann, ‡ Alexandre Dmitriev, ‡ Sebastian Stepanow, ‡ Nian Lin, ‡ Klaus Kern, ‡,§ and Johannes V. Barth* ,§,¶ IMPMC, CNRS & UniVersite ´ Pierre et Marie Curie, 4 place Jussieu, case 115, F-75252 Paris, France, Max-Planck-Institut fu ¨r Festko ¨rperforschung, Heisenbergstr. 1, D-70569 Stuttgart, Germany, Institut de Physique des Nanostructures, EPF Lausanne, CH-1015 Lausanne, Switzerland, and Departments of Chemistry and Physics & Astronomy, UBC VancouVer, BC V6T 1Z4, Canada Received January 10, 2006; E-mail: ari.p.seitsonen@iki.fi; jvb@chem.ubc.ca Supramolecular engineering of metal-organic architectures on well-defined substrates is currently emerging as a promising route toward novel nanoscale systems and functional surfaces. 1 Investiga- tions in this field, relying on scanning tunneling microscopy (STM) observations under ultrahigh vacuum conditions, revealed that on atomically clean metal surfaces an intriguing variety of low- dimensional metallosupramolecular arrangements can be synthe- sized by controlled deposition of polytopic linker molecules and transition metal centers. In particular, the assembly of nanoporous Fe-carboxylate metal-organic coordination networks (MOCNs) was achieved. 2 Such 2-D MOCNs comprise regular arrays of coordinatively unsaturated metal centers and distinct nanocavities which proved to be suitable as versatile and robust templates for the organization of appropriate guest species. However, while the STM investigations allow for molecular-level elucidation of morphology, dynamics, and host-guest chemistry of low-dimen- sional metal-organic grids, there is a clear need to rationalize the corresponding atomic structure, electronic properties, and chemical bonding in depth. Here we report an ab initio theoretical analysis for an exemplary MOCN, which was modeled using density functional theory (DFT) calculations. We notably address the diiron center key feature encountered in a series of fully 2-D reticulated Fe-carboxylate networks, which is reminiscent of catalytically active carboxylate-bridged diiron sites in metalloproteins. 4 Specif- ically, we modeled the nanoporous Fe-diterephthalate array fabricated from Fe atoms and 1,4-benzenedicarboxylic acid (tereph- thalic acid, tpa) molecules on Cu(100). 3 The constant current STM image depicted in Figure 1A shows the R-isomer of the 2-D Fe-terephthalate MOCN with its distinct nanocavities. On the basis of the topographic data, the schematic model superimposing the image was derived, suggesting diiron centers connected by both unidentate and chelating bidentate bonds to the adjacent diterephthalate linkers. 2,3 The STM observations clearly indicate a (6 × 4) metal-organic array fully commensurate with the underlying copper square lattice, where the protrusions associated with the closest Fe atoms are found at a distance corresponding to roughly two Cu(100) lattice units (a ) 2.55 Å). These structural features were employed as a starting point for the present DFT study, where we assumed that the Fe centers reside near the energetically favorable 4-fold hollow sites. The calculations were performed with the Generalized Gradient Approximation as the exchange-correlation functional of the Kohn- Sham equations in the code VASP. 5 The electronic wave functions were expanded in a plane wave basis set up to a cutoff energy of 37 Ry, and the core-valence interaction was modeled with the projected augmented wave method. Three substrate layers (first fully relaxed with adsorbate layer, second only laterally) together with almost 20 Å of vacuum between the two surfaces of the slab were employed, and a (2 × 2) grid of Monkhorst-Pack k-points in the first Brillouin zone was used to approximate the integration over the reciprocal space. A top view of the fully relaxed MOCN geometry is reproduced in Figure 1C. The corresponding STM image simulation (approximated as contours of constant electron density in a small energy window, specified by two Fermi functions with a width of 0.1 eV, around the Fermi energy) agrees well with the details of the experimental finding. Notably the shape of the nanocavities and the features associated with the Fe centers and the tpa backbone are nicely reproduced. This match corroborates the original interpretation of the STM results. 3 Moreover it agrees with earlier findings reporting that carboxylate groups tend to couple to the top sites of copper substrates. 6 A close-up view of the carboxylate diiron coupling motif is reproduced in Figure 2. The different bonding is reflected in the opening angle defined by the carboxylate oxygen, which amounts to 124.5 and 117.3° for bridging equatorial and chelating axial bonds, respectively, whereby the molecular backbone is slightly bent and residing above the plane defined by the Fe atoms. ² IMPMC, CNRS & Universite ´ Pierre et Marie Curie. ‡ Max-Planck-Institut fu ¨r Festko ¨rperforschung. § EPF Lausanne. ¶ UBC Vancouver. Figure 1. Fully reticulated nanoporous Fe-diterephthalate grid assembled on a Cu(100) substrate. (A) Constant current mode scanning tunneling microscopy image showing the R-isomer comprising a (6 × 4) unit cell (image size 40 × 30 Å 2 ). The arrangement of the tpa backbone indicates that a given molecule is engaged in either two bidentate or four unidentate carboxylate bonds to the diiron centers (marked as two blue spheres). (B) STM image simulation showing contours of constant LDOS at the sample Fermi level derived from the DFT model of the optimized structural arrangement depicted in model (C). Published on Web 04/07/2006 5634 9 J. AM. CHEM. SOC. 2006, 128, 5634-5635 10.1021/ja060180y CCC: $33.50 © 2006 American Chemical Society