Communication pubs.acs.org/IC
Self-Assembled, Iron-Functionalized Polyoxovanadate Alkoxide Clusters Feng Li, Lauren E. VanGelder, William W. Brennessel, and Ellen M. Matson* Department of Chemistry, University of Rochester, Rochester, New York 14627, United States S Supporting Information *
ABSTRACT: Herein we report the synthesis of a series of iron-functionalized, mixed-valent, polyoxovanadate alkoxide clusters, [V5O6(OCH3)12Fe]X (X = Cl, Br, SO3CF3) comprised of a hexanuclear Lindqvist (M6O19n−) core. By substituting a VO moiety from the well-defined hexavanadate clusters [VIVnVV6−nO7(OR)12]4−n (R = Me, Et) with a metal cation, we have developed a novel template for investigation of the organometallic properties of these systems. Characterization of the clusters was performed by 1H NMR, Fourier transform infrared, and electron absorption spectroscopies and electrospray ionization mass spectrometry. The IR and UV−vis spectra suggest substantial electronic delocalization, similar to the previously reported cluster, V6O7(OCH3)12.
Figure 1. Previously reported hexanuclear polyoxovanadate alkoxide clusters11 and the iron-functionalized derivative reported herein.
a wide variety of organic solvents and possess core structures analogous to those of the Lindqvist hexametalate anions, [M6O19]n−. The unique nature of the extensively delocalized d electrons in mixed-valent clusters led us to hypothesize that these systems could function as three-dimensional redox-active reservoirs.9 Herein, we report the synthesis and characterization of the first heterometallic POV alkoxide complex, [V5O6(OCH3)12FeX]. Seeking to develop procedures for the self-assembly of ironfunctionalized POV clusters, we began to explore the synthesis of [VnOn+1(OCH3)12(FeCl)6−n]. FeCl2 [20 mol % relative to VO(OtBu)3] was added to the reaction vessel containing VO(OtBu)3 and methanol (MeOH) and exposed to conditions identical to those described for the assembly of the hexavanadate species (24 h, 125 °C).10 Analysis of the reaction mixture by electrospray ionization mass spectrometry (ESI-MS) and 1H NMR spectroscopy revealed the selective formation of [V6O7(OCH3)12] (1-V6O7). This is distinct from the published synthesis, which yields a mixture of 1-V 6 O 7 and [V6O8(OCH3)11] (2-V6O8).11 While the heterometallic cluster was not observed, the selective formation of 1-V6O7 suggested that iron might be involved mechanistically in the generation of the hexavanadate structure. Given the apparent involvement of the ferrous salt, we hypothesized that an intermediate en route to the formation of 1V6O7 could be an iron-containing heterometallic cluster. As such, we sought to access these species through variation of the reaction conditions. Room temperature reactions yielded a complicated mixture of products; however, ESI-MS confirmed the existence of several vanadium/iron clust ers, [V4O5+n(OMe)12−n(FeCl)2] and [V3O4+n(OMe)12−n(FeCl)3] (n = 0, 1; Figures S1 and S2). Raising the temperature of the reaction to 60 °C resulted in the generation of principally the
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eterometallic polyoxometalates (POMs) have seen increased attention in recent years because of their fascinating electronic properties and their ability to serve as homogeneous models for catalysts immobilized on metal oxide supports.1 Despite their novel properties, the controlled synthesis of transition-metal-functionalized POMs remains an underdeveloped field. Systems are typically accessed through the isolation of lacunary molybdate-2 or tungstate-based3 clusters, whose incomplete structures enable subsequent heterometallic functionalization. Successful transition-metal functionalization has been observed only for anionically charged, polyoxovanadate (POV) derivatives, where metal cations are coordinated electrostatically to the surface of the assembly.4 The earliest reports of heterometallic POVs explored the reactivity of [V4O12]4− with first-row transition metals, resulting in the formation of [MCl(V4O12)]3− (M = Co, Zn).5 More recently, Streb and co-workers reported a chloride-centered dodecavanadate template for the controlled synthesis of transition-metalfunctionalized POVs.6 This measured approach to heterometal incorporation results in the coordination of the metal cation to the surface of the POV. Streb and co-workers have since demonstrated the electronic consequences of transition-metal deposition, reporting enhanced catalytic and photochemical properties of these systems upon manganese incorporation.7 Our research group has recently initiated explorations into the synthesis of heterometallic transition-metal cluster complexes as we envision using these metal oxide structures as redox-active electron reservoirs. Interested in exploring small, neutral derivatives of these inorganic structures, we focused on POV alkoxide clusters (Figure 1).8 Comprised of six vanadyl moieties bridged by alkoxide ligands, these POV assemblies are soluble in © XXXX American Chemical Society
Received: June 8, 2016
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DOI: 10.1021/acs.inorgchem.6b01349 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry
exclusive formation of 3-V5FeX. Employment of the methoxide precursor, VO(OMe)3, allows for the use of alternative solvents, as the self-assembly of clusters with bridging methoxide moieties is no longer dependent on solvent for ligand substitution. Toluene was selected because of its high boiling point and limited ability to bind to metal centers. Reactions at 65 °C with FeX2 (X = Cl, Br) proceeded slowly and formed a mixture of 3V5FeX and 4-V4(FeX)2. Despite prolonged heating, large amounts of VO(OMe)3 and FeX2 remained unreacted. Heating a mixture of VO(OMe)3 and FeX2 (3:1) at 85 °C for 4.5 days resulted in full consumption of VO(OMe)3, with minimal formation of the hexavanadate congeners (Figures S9,S10). Analysis of the reaction mixture by ESI-MS and 1H NMR spectroscopy confirmed the formation of a simple product mixture composed of 3-V5FeBr (80%) and 4-V4(FeBr) 2 (
