ARTICLE pubs.acs.org/JPCC
[As7M(CO)3]3 M = Cr, Mo, W: Bonding and Electronic Structure of Cluster Assemblies with Metal Carbonyls Sukhendu Mandal,† Meichun Qian,‡ Arthur C. Reber,‡ Hector M. Saavedra,† Paul S. Weiss,*,†,§ Shiv N. Khanna,*,‡ and Ayusman Sen*,† †
Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802, United States Department of Physics, Virginia Commonwealth University, Richmond, Virginia 23284, United States § California NanoSystems Institute and Departments of Chemistry & Biochemistry and Materials Science & Engineering, University of California, Los Angeles, Los Angeles, California 90095, United States ‡
bS Supporting Information ABSTRACT: Understanding the factors controlling the band gap energy of cluster-assembled materials is an important step toward nanoassemblies with tailored properties. To this end, we have investigated the band gap energies of cluster assemblies involving arsenic clusters bound to carbonyl charge-transfer complexes, M(CO)3, M = Cr, Mo, W. The binding of a single charge-transfer complex is shown to have a small effect on the band gap energy because the arsenic lone pair orbital and metal carbonyl orbitals are closely aligned in energy, resulting in a gap similar to the original cluster. The band gap energy is also found to be insensitive to the architecture of the assembled material. In the case where two charge-transfer complexes are bound to the cluster, the bottom of the conduction band is shown to be localized on a solvent molecule bound to the metal carbonyl.
1. INTRODUCTION Atomic clusters may be linked through ionic or covalent linkers into hierarchical cluster-assembled materials.110 The electronic structure and related properties of these clusterassembled materials evolve from those of the cluster building blocks but may be altered significantly by the components, architecture, and nature of linking within the assembly. For example, the HOMOLUMO gap of the isolated Li3As7 cluster is 2.86 eV, but the band gap energy of the Li3As7 clusterassembled material is 0.52 eV.2 This highlights the need to understand how the interactions marking different classes of cluster-assembled materials affect the electronic structure. Previous studies in our group on assemblies involving As73 Zintl ions formed by linking As7 clusters with different alkali atoms have shown that the valence band is full and that the conduction band edge is marked by the valence electronic states derived from the counterion. This is because the alkali atom donates its valence electron to the As7 motif and its valence state becomes unoccupied during the charge transfer. As the position of the valence state changes with the identity of the alkali atom, the identity of the counterion controls the position of the conduction band edge and hence the band gap of the ionic cluster-assembled materials.2 Our studies also showed that the architecture of the assembled material can affect the band gap in ionic cluster assemblies. The effect of the architecture on the band gap energy is primarily through short-range interactions in which the counterions generate an internal electric field that shifts the position of the valence band.3 Another class of assemblies involves metal clusters r 2011 American Chemical Society
covalently linked by atoms. In such covalently linked cluster assemblies, a cluster orbital theory for covalent interactions in which states from the cluster form electronic orbitals with their linker results in a modified building motif with specifically altered electronic structure.4 Our previous research has focused on bonding between main group and noble metals with arsenic clusters; however, the incorporation of transition metals on the band gap energy has not been extensively studied. One method of incorporating transition metals into cluster assemblies is to bind charge-transfer complexes such as M(CO)3 molecules to the electronegative cluster.11,12 In this Article, we examine the effects on the band gap energy of cluster assemblies from an As73 bound to one or two M(CO)3 molecules, in which M is the group VIb elements, Cr, Mo, and W. Zintl clusters enable a comprehensive study of the effects of architecture, linker, and counterion on the band gap energy. Whereas the Zintl phases were originally synthesized by dissolving main-group metals in reducing ammoniaalkali metal solutions, new synthetic methods offer greater control of the assembly.1216 The dimensionality of these assemblies can be controlled by choosing the ratio of alkali to cryptated alkali cations that stabilizes the polyanions. The crypt molecule (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) encapsulates the potassium ion and its large size reduces the Received: July 29, 2011 Revised: October 14, 2011 Published: October 24, 2011 23704
dx.doi.org/10.1021/jp207268x | J. Phys. Chem. C 2011, 115, 23704–23710
The Journal of Physical Chemistry C
ARTICLE
Table 1. Synthesis Conditions of Compounds 13a synthesis conditions a
molar ratio
time (d)
yield (%)
composition
(C9H12)Cr(CO)3:K3As7:3(2,2,2-crypt)
34
85
[K-crypt]3[CrAs7(CO)3], 1
(C5H5)Mo(CO)3:K3As7:3(2,2,2-crypt)
34
80
[K-crypt]3[(en)Mo(CO)3MoAs7(CO)3], 2
(C9H12)W(CO)3:K3As7:3(2,2,2-crypt)
34
80
[K-crypt]3[(en)W(CO)3WAs7(CO)3], 3
(C9H12)Cr(CO)3:0.5K3As7:0.5Cs3As7:3(2,2,2-crypt)
45
70
[K-crypt]2[As7Cr(CO)3Cs].en.THF, 1a
(C5H5)Mo(CO)3:0.5K3As7:0.5Cs3As7:3(2,2,2-crypt)
45
75
[K-crypt]2[As7Mo(CO)3Cs].en.THF, 2a
(C9H12)W(CO)3:0.5K3As7:0.5Cs3As7:3(2,2,2-crypt)
45
75
[K-crypt]2[As7W(CO)3Cs].2en, 3a
a
All reactions were done at room temperature in ethylendiamine solvent and either toluene or tetrahydrofurane (THF) were used for crystallizations. (See the Supporting Information for details.)
dimensionality of the ionically linked assembly, whereas the Cs atom is too large to be cryptated and ionically links the clusters. Zintl clusters may also be covalently linked to form assemblies of interest including homoatomic clusters linked by heteroatoms.1730 Zintl clusters can be functionalized by main group organometallic compounds and early transition-metal organometallics.3137 Most recently, organo-Zintl clusters have been synthesized.3843 The control over architecture possible with Zintl phases makes them prototypes for understanding the interplay of interactions between clusters with their linkers, and the global properties of the cluster assembly. The [As7]3 cage is an electronically precise structure in which the formal charge resides on the three two-coordinated equatorial atoms making the cluster an excellent electron donor. The [As7]3 cluster may serve as a donor of electrons in oxidative coupling reactions including the formation of [As7Au2As7]4, [As7ZnAs7]4, [R2As7]4, and numerous others,2,3,28,44,45 which are marked by the increase in the oxidation state of the complex from 3 to 2. [As7]3 may donate charge to form ligand-tometal charge-transfer complexes, which have the same oxidation state as the parent species. Charge-transfer complexes are wellknown as strong absorbers of visible light,46 so the incorporation of such complexes within a cluster-assembled material may affect the optical properties of the material. In this work, we examine zero-dimensional (0D) assemblies of [As7Cr(CO)3]3, 1D assemblies of [As7Cr(CO)3]3 linked by Cs atoms, 0D assemblies of [(en)Mo(CO)3As7(Mo(CO)3)2]3 and [(en)W(CO)3As7(W(CO)3)2]3, and 1D assemblies of [As7Mo(CO)3]3 and [As7W(CO)3]3 linked by Cs atoms. To understand the effects of charge-transfer ligands on the band gap energy, we have engaged in a synergistic study combining solution synthesis, X-ray diffraction, diffuse reflectance spectroscopy, experimental band gap energy, and theoretical investigations to understand the origins of the variations in the band gap energy.
2. METHODS Synthetic Methods. All manipulations were performed in an argon-filled glovebox. The detailed synthetic procedures of the precursors K3As7 and compounds 13a are described in the Supporting Information and summarized below in Table 1. The precursor K3As7 was synthesized by mixing As (∼700 mg) with a preheated mixture of K (∼120 mg) and ethylenediamine (en) (3 mL) in a scintillation vial, and the solution was stirred overnight at room temperature. The red suspension was filtered, and the resulting dark red solution was used for further reactions.
Single-Crystal Structure Determination. A suitable crystal for all compounds was carefully selected under a polarizing microscope and glued to a loop. Details of the single crystal structure determination are described in the Supporting Information. Pertinent experimental details of the structure determination of 13 and 1a3a are presented in Table S1 of the Supporting Information. Theoretical Methods. First-principles electronic structure studies within a gradient-corrected density functional framework47 were carried out to probe the nature of electronic bonding and to understand the experimental findings on the band gaps. Two kinds of theoretical studies were performed. Gradient-corrected electronic structure calculations47 were performed on periodic solids using the experimentally determined crystal structures to understand the nature of electronic bands and the origins of the band gap energy. These calculations were performed using the Vienna ab initio simulation package48 (VASP). The projector-augmented wave (PAW) pseudopotentials were used to describe the electronion interaction. The exchange interactions and correlations effects were incorporated using the generalized gradient corrected functional proposed by Perdew et al.47 Brillouin zone integrations were carried out on Monkhorst-Pack grid k points using the tetrahedral method. The kinetic energy cutoff of 300 eV was found to give converged results and was used for the plane wave basis. The geometries in these studies used the experimentally determined crystal structures; geometry optimization was found to have a small,
