FULL PAPER DOI: 10.1002/chem.201103096 Assessing Metal–Metal Multiple Bonds in CrÀCr, MoÀMo, and WÀW Compounds and a Hypothetical UÀU Compound: A Quantum Chemical Study Comparing DFT and Multireference Methods Giovanni Li Manni,[a] Allison L. Dzubak,[b] Abbas Mulla,[b] David W. Brogden,[c] John F. Berry,*[c] and Laura Gagliardi*[b] Abstract: To gain insights into the has also been investigated. All of the and W compounds the electronic ab- trends in metal–metal multiple bonding compounds studied here show impor- sorption spectra have been studied, among the Group 6 elements, density tant multiconfigurational behavior. For combining density functional theory 2 4 2 functional theory has been employed the Mo2 and W2 compounds, the s p d and multireference methods to make in combination with multiconfigura- configuration dominates the ground- absorption feature assignments. In all tional methods (CASSCF and state wavefunction, contributing at cases, the main features observed in CASPT2) to investigate a selection of least 75%. The Cr2 compounds show a the visible spectra may be assigned as bimetallic, multiply bonded com- more nuanced electronic structure, charge-transfer bands. For all com- pounds. For the compound [Ar-MM- with many configurations contributing pounds investigated the Mayer bond Ar] (Ar =2,6-(C6H5)2-C6H3,M= Cr, to the ground state. For the Cr, Mo, order (MBO) and the effective bond Mo, W) the effect of the Ar ligand on order (EBO) were calculated by densi- the M core has been compared with ty functional theory and CASSCF 2 Keywords: bond theory · density the analogous [Ph-MM-Ph] (Ph = methods, respectively. The MBO and functional calculations · electronic phenyl, M=Cr, Mo, W) compounds. A EBO values share a similar trend states · metal–metal interactions · set of [M (dpa)ACHTUNGRE ] (dpa =2,2’-dipyridyla- toward higher values at shorter nor- 2 4 quantum chemistry mide, M=Cr, Mo, W, U) compounds malized metal–metal bond lengths. Introduction terest. For example, various groups have shown interest in oligothiophene compounds incorporating metal–metal mul- Ever since the discovery of the multiple metal–metal bond tiple bonds because of their potential applications in optoe- 2À [1,2] [4] in [Re2Cl8] , there has been a considerable amount of re- lectronic and magnetic devices. Burdzinski et al. recently ACHTUNGRE search dedicated to metal–metal multiple bonding. Elec- prepared oligomers of empirical formula [Mo2(TiPB)2- ACHTUNGRE ACHTUNGRE tron-rich metal–metal units are of general interest because (O2C(Th)-C4(n-hexyl)2S-(Th)CO2)] (TiPB =2,4,6-triisoprop- of their unique electronic and optical properties.[3] Several yl benzoate; Th= thiophene) and compounds of formulae ACHTUNGRE new examples of metal–metal multiply bonded compounds trans-[Mo2(TiPB)2L2] in which L= Th, BTh (Bth=2,2’-bi- incorporating the Group 6 metals have recently been of in- thiophene-5-carboxylate) and TTh (the corresponding thie- nylcarboxylate), which are considered as models for the ACHTUNGRE oligomers. The X-ray analysis of trans-[Mo2(TiPB)2BTh2](1; Figure 1) revealed the presence of Lp*–M2d–Lp* conjuga- [a] G. Li Manni tion, and density functional theory (DFT) calculations indi- Department of Physical Chemistry, University of Geneva cated that the HOMO is mainly a M2 d orbital and the 30, q. E. Ansermet, 1211 Genve (Switzerland) LUMO is mainly based on the thienylcarboxylate p* orbi- [b] A. L. Dzubak, A. Mulla, Prof. L. Gagliardi tals. Department of Chemistry, Burdzinski et al.[4] studied also the photophysical proper- University of Minnesota and Minnesota Supercomputing Institute 207 Pleasant St. SE, Minneapolis, MN 55455 (USA) ties of these oligomers, which showed relatively slow metal- E-mail: [email protected] to-ligand charge-transfer (MLCT) triplet intersystem cross- [c] D. W. Brogden, Prof. J. F. Berry ing compared to the majority of second- and third-row tran- Department of Chemistry, University of Wisconsin—Madison sition metal complexes. They noticed that the 1101 University Ave. Madison, WI 53706 (SA) 1MLCT–3MLCT gap is relatively small in the Mo complexes, E-mail: [email protected] suggesting a large mixing of the metal d and organic p sys- Supporting information for this article is available on the WWW tems. These Mo -based oligothiophenes have thus a unique under http://dx.doi.org/10.1002/chem.201103096. Additional details of 2 the calculations: cartesian coordinates; total electronic energies and metal-based triplet emission. molecular orbitals and electronic spectra. Chem. Eur. J. 2012, 18, 1737 – 1749 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1737 ACHTUNGRE ACHTUNGRE tances of 2.23 and 2.14 in [W2(dpa)4][BPh4] and [Mo2- ACHTUNGRE ACHTUNGRE (dpa)4][BPh4], respectively, are in agreement with metal– ACHTUNGRE metal bond orders of 3.5. The molecules [W2(dpa)4] and ACHTUNGRE ACHTUNGRE [Mo2(dpa)4] have been utilized along with the [Cr2(dpa)4] analogue (2a) to prepare linear, trinuclear heterometallic molecules with an MM···M’ chain, with M=Cr, Mo, or W, and M’=Cr, Mn, Fe, Co, Ni, and Zn.[7–11] The heterometallic molecules show rich optical and redox properties, and a better understanding of these properties can be greatly fa- cilitated by a quantum chemical analysis of the Cr2,Mo2, and W2 precursor molecules. We have studied metal–metal multiple bonds in the Cr2, Mo2, and W2 dimers by making use of the concept of effec- tive bond order (EBO)[14,15] that arises from a multiconfi- gurational complete active space-SCF (CASSCF) wavefunc- tion.[16] We have demonstrated that a sextuple bond exists in [14, 17] Mo2 and W2, but hardly in Cr2. The weakness of the CrÀCr bond is related to the difference in size between the Figure 1. Experimentally determined structure of compound 1. Color 3d and 4s orbitals. The 4s–4s interaction occurs at a consid- code: C =gray-capped stick, H= white-capped stick, S=black-capped erably longer distance than the 3d–3d interaction. This un- stick, Mo =black ball, O=gray ball. balance weakens the 3d bonds and makes the 4s–4s interac- tion repulsive at equilibrium geometry. Another important [3] ACHTUNGRE Alberding et al. prepared the [MM’(TiPB)4] compounds, factor is the repulsive interaction between the closed 3p in which M=Mo or W and M’=W and characterized them shells, which have about the same radial extension as the 3d with various techniques. Electronic absorption, steady-state orbitals. The unbalance between the s and d orbitals de- emission and transient absorption spectroscopy indicate that creases for second-row transition metals and even more for these compounds have strong absorptions in the visible the third row. Moreover, relativistic effects play an impor- region that are assigned to MM’ d to arylcarboxylate p* tant role in making the two sets of orbitals more equal in transitions, 1MLCT. Luminescence from two excited states size, which overall enhance the bond strength of the diatom- also occurs, which are assigned as the 1MLCT and 3MM’ d– ics. Various low-valent CrÀCr complexes recently synthe- d* states. sized present a multiple bond that, despite changes in the [5] ACHTUNGRE Nippe et al. reported the synthesis of [W2(dpa)4] (dpa= nature of the ligand or with the oxidation state of the Cr 2,2’-dipyridylamide) (2c) (Figure 2) and its characterization atom, yield EBO values in the relatively narrow range be- by X-ray crystallography and cyclic-voltammetry. They com- tween 3.4 and 3.9 that correlate roughly with the CrÀCr pared it with its earlier reported molybdenum analogue, bond length.[18] In order to protect the dimetallic unit from ACHTUNGRE [6] [Mo2(dpa)4](2b). They also synthesized one-electron oxi- possible oxidation or oligomerization, terphenyl ligands, the ACHTUNGRE ACHTUNGRE dation products of [W2(dpa)4] and [Mo2(dpa)4], namely [W2- skeleton structure of which is 2,6-(C6H5)2-C6H3 (Ar), have ACHTUNGRE ACHTUNGRE ACHTUNGRE ACHTUNGRE (dpa)4][BPh4] and [Mo2(dpa)4][BPh4] (BPh4 =tetraphenylbo- been employed to embed the metal dimer. Experimental rate). The crystallographically determined metal–metal dis- and theoretical works have also shown that [Ar’-CrCr-Ar’] (3a, Scheme 1) (Ar’=2,6-(2,6-iPr2-C6H3)2-C6H3) features a trans-bent geometry.[15, 19–21] Since the Ar ligand successfully stabilizes dimers of main group elements and the Cr dimer, its capabilities in protecting dimers of Fe and Co were also investigated.[22, 23] The flanking aryl/metal h6 interaction makes the FeÀFe and CoÀCo bonds longer than in other compounds. In order to quantify the influence of the flank- Figure 2. The experimental structure of compound 2c. Color code: C= gray-capped stick, H= white-capped stick, W=black ball, N= gray ball. Scheme 1. Compounds studied in this work. 1738 www.chemeurj.org 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2012, 18, 1737 – 1749 Metal–Metal Multiple Bonds FULL PAPER ing aryl ring on the MÀM bond, several simplified model ployed to re-optimize selected bond lengths, namely the MÀM(M= Cr, systems containing a CoÀCo and FeÀFe core unit, but with- Mo and W) and MÀC bonds. A numerical optimization procedure was employed, which consisted of varying the MÀM and MÀC distances, opti- out ligands capable of giving h6 interactions, were studied by mizing the structures at the DFT level while keeping the MÀM and MÀC DFT and CASSCF followed by perturbation theory to distances fixed, and performing CASPT2 calculations at these geometries. second order (CASPT2) and compared to the complexes Numerical gradients and hessians on the CASPT2 potential-energy surfa- featuring the h6 interaction computed at the same level of ces were then computed to check the nature of the stationary points.