Synthesis and Ligand‐Exchange Kinetics of the Solvated Trigonal‐Prismatic Clusters [W₆CCl₁₂L₆]²⁺ (L=dmf, py)

Abstract

A variety of hexanuclear transition-metal clusters have found use in solution-based syntheses of porous coordination solids, certain of which behave as molecular sieves, chemical sensors, or ion-exchange materials.[1] These clusters possess rigid, metal-metal-bonded core structures, typically consisting of an octahedron of metal atoms with eight face-capping or twelve edge-bridging ligands. Each metal center is additionally coordinated by an outer terminal ligand, for which the exchange chemistry and bridge-forming capability are of critical importance in the generation of solid frameworks. The recent discovery of high-temperature routes to the Z-centered clusters [W 6 ZCl 18 ] nÀ (Z = C, N; n = 1-3) has provided a new, highly redox-active building unit, wherein six terminal chloride ligands are arranged in a trigonal prism instead of an octahedron.[2] As a step toward utilizing these [W 6 ZCl 18 ] nÀ species in the synthesis of extended-framework materials, we herein report some initial ligand-substitution reactions, together with a detailed study of the self-exchange kinetics for a pyridine-solvated cluster. A triflate-terminated (triflate = CF 3 SO 3 À ) variant of the trigonal-prismatic cluster was sought, since prior work had demonstrated the versatility of octahedral clusters such as [W 6 Cl 8 (CF 3 SO 3 ) 6 ] 2À in ligand-substitution reactions.[3] Solid (Bu 4 N) 2 [W 6 CCl 18 ] was heated in neat triflic acid at 100 8C under a stream of nitrogen. After 8 h, the resulting black solution was reduced to dryness under an increased nitrogen flow. Recrystallization from a 1:25 mixture of nitromethane and diethyl ether at À28 8C afforded black, distorted tetrahedron-shaped crystals of (Bu 4 N) 2 [W 6 CCl 12 (CF 3 SO 3 ) 6 ] (1) in 91 % yield. X-ray analysis of a crystal of 1 indeed revealed a structure in which all six terminal chloride ligands have been replaced with oxygen-bound triflate anions (Figure 1). [4] With an unperturbed count of 24 W 6 C-based valence electrons, the geometry of the cluster core is essentially the same as that observed in (Bu 4 N) 2 [W 6 CCl 18 ], [2a] consisting of a central carbon atom surrounded by a regular trigonal prism of tungsten atoms and twelve edge-bridging chloride anions. + (Cp À = C 5 H 5 À ), and two reversible oneelectron reduction processes at E 1/2 = 0.117 V (DE p = 74 mV) and À0.460 V (DE p = 78 mV). In DMF solution, the couples for the oxidation and first reduction events are shifted to the more negative potentials of E 1/2 = 0.149 V (DE p = 66 mV) and À0.565 V (DE p = 78 mV), while the second reduction at À0.949 V now shows no reversibility. This last result suggests that, upon reduction of the cluster by two electrons, the triflate ligands become labile and are readily displaced by the strongly coordinating DMF solvent. In distinct contrast, the cyclic voltammogram of [W 6 CCl 18 ] 2À in DMF gives no indication of such lability.

Carbon Centered Trigonal Prismatic Tungsten Clusters [W6CCl18]n– (n = 1, 2) containing Copper(I) and Copper(II)

“…Copper ions in the structure occupy two distinct split-positions [refined occupancy 0.493 (7) and 0.507 (7)], each of them coupled though the inversion symmetry of the space group P1. A symmetry reduction to the space group P1 gave the same [13] 2.632(2) 3.037 (5) a) Δ -edges of triangles in trigonal prismatic cluster. b) Խ -edges between adjacent triangles in trigonal prismatic cluster.…”

“…A 13 C-NMR spectrum of Cu(C 2 H 6 OS) 6 [W 6 CCl 18 ] was recorded with a Bruker AvanceII+400 NMR spectrometer equipped with a 5 mm QNP (quad nucleus) probe head operating at 100.13 MHz ( 13 C), presented in Figure 2. Chemical shift is given in δ value in ppm relative to external SiMe 4 ( 1 H, 13 C, 29 Si). 13 ZCl 18 ] compounds which were reported with the hexagonal space group P6 3 /m.…”

“…Chemical shift is given in δ value in ppm relative to external SiMe 4 ( 1 H, 13 C, 29 Si). 13 ZCl 18 ] compounds which were reported with the hexagonal space group P6 3 /m. [12] The main motive in the crystal structure of I is the carbon-centered trigonal prismatic tungsten cluster anion [W 6 CCl 18 ] -.…”

“…[7] In 2003 the first carbon centered trigonal prismatic tungsten clusters W 6 CCl 16 and W 6 CCl 18 were published by Zheng, Schnering and collaborators. [8] The first observation of this type of cluster was due to a carbon impurity; but the identity of this type of cluster was confirmed by reduction of WCl 4 with tungsten metal powder and graphite in a temperature gradient around 700°C, and the presence of carbon was evidenced by 13 C-NMR spectroscopy. A few years later a new synthesis route was described for the cluster series of W 6 Cl 18 , W 6 CCl 15 , W 6 CCl 16 , W 6 CCl 18 , and W 30 C 2 (Cl,Br) 68 , in which more reactive carbon sources were employed.…”

“…[11,12] Among A[W 6 ZCl 18 ] compounds there exists examples with A = Li, Na, Ca, Ag with Z = C, N, [12] and ligand exchange reactions of terminal chlorides by means of solution chemistry were reported to yield [(W 6 ZCl 12 )L 6 ] 2clusters. [13] The bonding situation of these clusters including the interstitial atom have been discussed in several ways. [14] Recently several new tungsten chalcogenide compounds were obtained by a combined thermal scanning (DSC or DTA) -X-ray diffraction method.…”