This paper is published as part of a Dalton Transactions themed issue on: The Synergy between Theory and Experiment Guest Editor John McGrady University of Glasgow, UK Published in issue 30, 2009 of Dalton Transactions Image reproduced with permission of Christophe Coperet Papers published in this issue include: A combined picture from theory and experiments on water oxidation, oxygen reduction and proton pumping Per E. M. Siegbahn and Margareta R. A. Blomberg, Dalton Trans., 2009, DOI: 10.1039/b903007g Mechanisms of C–H bond activation: rich synergy between computation and experiment Youcef Boutadla, David L. Davies, Stuart A. Macgregor and Amalia I. Poblador-Bahamonde, Dalton Trans., 2009, DOI: 10.1039/b904967c Are tetrathiooxalate and diborinate bridged compounds related to oxalate bridged quadruply bonded compounds of molybdenum? Malcolm H. Chisholm and Namrata Singh, Dalton Trans., 2009 DOI: 10.1039/b901734h Molecular recognition in Mn-catalyzed C–H oxidation. Reaction mechanism and origin of selectivity from a DFT perspective David Balcells, Pamela Moles, James D. Blakemore, Christophe Raynaud, Gary W. Brudvig, Robert H. Crabtree and Odile Eisenstein, Dalton Trans., 2009 DOI: 10.1039/b905317d Visit the Dalton Transactions website for more cutting-edge inorganic and organometallic research www.rsc.org/dalton PAPER www.rsc.org/dalton | Dalton Transactions Crystal structure of octabromoditechnetate(III) and a multi-configurational quantum chemical study of the d → d* transition in quadruply bonded 2- [M2X8] dimers (M = Tc, Re; X = Cl, Br)† Frederic Poineau,*a Laura Gagliardi,b,c Paul M. Forster,a Alfred P. Sattelbergera,d and Kenneth R. Czerwinskia Received 2nd February 2009, Accepted 3rd April 2009 First published as an Advance Article on the web 8th May 2009 DOI: 10.1039/b902106j The technetium(III) compound (n-Bu4N)2[Tc2Br8] was prepared by metathesis of (n-Bu4N)2[Tc2Cl8] with concentrated aqueous HBr in acetone and recrystallized from acetone–diethyl ether solution (2 : 1 v/v). The acetone solvate obtained, (n-Bu4N)2[Tc2Br8]·4[(CH3)2CO] (1), crystallizes in the monoclinic space ◦ group P21/n with a = 13.8959(8) A˚ , b = 15.2597(9) A˚ , c = 15.5741(9) A˚ , b = 109.107(1) , R1 = 0.028, and Z = 4. The Tc–Tc distance (2.1625(9) A˚ ) and the average Tc–Br distances (2.4734(7) A˚ )arein excellent agreement with those previously determined by EXAFS spectroscopy. These and other 2- experimental data on quadruply metal–metal bonded group 7 [M2X8] dimers (M = Tc, Re; X = Cl, Br) are compared to the results of a set of multi-configurational quantum chemical studies. The calculated molecular structures of the ground states are in very good agreement with the structures determined experimentally. The theory overestimates the d → d* transition energies by some 1 000 cm-1, but mimics the trends in d → d* energies across the series. Introduction Exploration of the fundamental chemistry of technetium re- mains limited due to the relatively small number of laboratories An understanding of molecular compounds that contain metal– equipped to work with suitable quantities of this fascinating metal bonds is fundamental to interpreting structural and bonding radioelement. Over the past three years, we have established the 1 properties, catalysis, metal surface chemistry, and magnetism. capability to work with synthetic quantities (i.e., multi-milligram The discovery of dirhenium(III) complexes with metal–metal amounts) of 99Tc in the radiological laboratories of the Harry quadruple bonds foreshadowed a revolution in the study of metal– Reid Center for Environmental Studies at UNLV and have been 2 metal bonds and the identification of novel types of bonding. The exploring several aspects of its inorganic and organometallic importance of quantum chemistry to this area of science is not only chemistry. One of our interests is multiply metal–metal bonded based on its ability to solve the quantum-mechanical equations to technetium complexes for which there is a paucity of data in the a good degree of approximation for complex molecules, but also primary literature. As one striking comparison with its heavier on the fact that the field can now perform theoretical simulations congener, more than one hundred quadruply metal–metal bonded of real benefit to the experimental community. Ab initio quantum rhenium(III) dimers have been crystallographically characterized, chemistry has made so many advances in the last 40 years that while only five such examples are known for technetium.3,4 In fact, it now allows the study of molecular systems containing any of it has been more than 15 years since the last crystal structure the atoms in the periodic table. Technetium chemistry represents 5 of a Tc(III)dimer,thatofTc2(O2CCH3)4(TcO4)2, was reported. a challenge for experimentalists and the interplay between theory 2- Recently, we presented structural data for the [Tc2X8] ions and experiment is of extreme importance in such a case. In the (X = Cl, Br) derived from analysis of EXAFS spectra of the following, we present a combined experimental and theoretical tetra-n-butyl ammonium (TBA) salts, as well as the electronic study in which we elucidate some key features of the Tc–Tc spectra of the two octahaloditechnetate ions in solution.6 The quadruple bond. close similarity of the optical spectra with those of their rhenium analogues permitted an assignment of the electronic transitions in a Department of Chemistry, University of Nevada Las Vegas, Las Vegas, 2- the [Tc2X8] dimers, including the ubiquitous d → d* transitions. NV 89154, USA. E-mail: [email protected] → bDepartment of Physical Chemistry, Sciences II University of Geneva, Of particular note was the near coincidence of the d d* 2- 2- = 30 Quai Ernest Ansermet, CH-121, Geneva, Switzerland. E-mail: laura. transitions in the [M2Cl8] and [M2Br8] complexes (M Tc, [email protected] Re). It occurred to us that calculations of the electronic spectra cUniversity of Minnesota Department of Chemistry and Supercomputing of these dimers had never been pursued simultaneously using Institute, 207 Pleasant St., SE Minneapolis, MN MN 55455-0431, USA state-of-the-art methods. This situation is partially remedied dEnergy Sciences and Engineering Directorate, Argonne National Labora- tory, Argonne, IL 60439, USA in the present work. We have performed multi-configurational † Electronic supplementary information (ESI) available: Additional crys- quantum chemical calculations, using the Complete Active Space tallographic tables and X-ray crystallographic data for 1,aswellasthe SCF (CASSCF) method, followed by second order perturbation 2- CASPT2 total energies for [M2X8] (M = Tc, Re; X = Cl, Br) and the 2- theory (CASPT2) to determine: (1) the structures of the [M2X8] coordinates of the optimized structures. CCDC reference number 720059. 1 1 For ESI and crystallographic data in CIF or other electronic format see complexes in their A1g ground states and A2u excited states, (2) DOI: 10.1039/b902106j the energies and trends associated with the d → d* transitions, 5954 | Dalton Trans., 2009, 5954–5959 This journal is © The Royal Society of Chemistry 2009 · and (3) the bond order of the metal–metal bonds in each complex. Table 1 Crystallographic parameters for (n-Bu4N)2[Tc2Br8] 4[(CH3)2CO] 2- Finally, the structure of the [Tc2Br8] ionasanacetonesolvate, · Formula C 44H96Br8N2O4Tc2 (n-Bu4N)2[Tc2Br8] 4[(CH3)2CO] (1), was determined by single- -1 Mr/g mol 1554.49 crystal X-ray diffraction and compared to the geometry derived Crystal system Monoclinic from the aforementioned EXAFS data. Space group P21/n a/A˚ 13.8959(8) b/A˚ 15.2597(9) Experimental c/A˚ 15.5741(9) a/◦ 90 ◦ Synthesis b/ 109.107(1) g /◦ 90 V/A˚ 3 3120.5(3) Caution. Techetium-99 is a weak beta emitter (Emax = 292 keV). All manipulations were performed in a radiochemistry Z 4 r/g cm-3 1.652 laboratory designed for chemical synthesis using efficient HEPA- T/K 150(2) filtered fume hoods, Schlenk and glove box techniques, and Radiation Mo Ka following locally approved radioisotope handling and monitor- Wavelength 0.71073 Reflections collected 5630 ing procedures. The compound (n-Bu4N)2[Tc2Br8] was prepared Independent reflections 2907 according to the procedure previously reported.7 Single crystals Parameters 280 suitable for X-ray diffraction were grown over the course of 1 week Largest diffraction peak/e A˚ -3 0.612 2 2 R(F o)(F o > 2s(F o ) 0.0280 from a concentrated acetone solution of the salt that was carefully 2 ◦ R(F o ) (all data) 0.0365 layered with diethyl ether (2 : 1 v/v) at -25 C. 2 Rw(F o ) 0.0696 Goodness-of-fit 1.008 Crystal structure determination a poorly defined crystallographic position for the molecule itself, Initial attempts to determine the structure of (n-Bu4N)2[Tc2Br8] as is chemically sensible based on the crystal structure, rather utilized samples that had been removed from the mother liquor than a low quality crystal structure determination. An attempt for a few hours. While the samples appeared crystalline to the at refining the acetone with the largest displacement parameters naked eye, only weak diffraction peaks were observed. Suspecting using partial occupancy lead to a refined occupancy parameter desolvation, a clear crystal was selected directly from the mother slightly above unity, indicative of disorder rather than partial liquor and quickly moved by needle into a drop of paratone oil. occupancy. Selected crystallographic information is presented in The paratone-covered crystal was then mounted on a glass fiber Table 1, full crystallographic information and a cif file are supplied and immediately cooled in a nitrogen cold stream to 150 K. A in the ESI.† full hemisphere of data was then collected on a Bruker APEX II diffractometer. The structure was solved by direct methods Computational details and refined against F 2 using SHELX-97.8 Hydrogen atoms were added geometrically and refined using the riding model.