Reinvestigation of a Ru(2)-incorporated polyoxometalate dioxygenase precatalyst, "[WZnRu(2)(III)(H(2)O)(OH)(ZnW(9)O(34))(2)](11-)": evidence for marginal, <or=0.2 equivalents of Ru incorporation plus faster catalysis by physical mixtures of [Ru(II)(DMSO)(4)Cl(2)] and the parent polyoxometalate [WZn(3)(H(2)O)(2)(ZnW(9)O(34))(2)](12-).
A 1997 Nature paper (Nature 1997, 388, 353-355) and subsequent 1998 J. Am. Chem. Soc. paper (J. Am. Chem. Soc. 1998, 120, 11969-11976) reported that a putative Ru(2)-substituted polyoxoanion, "[WZnRu(2)(III)(H(2)O)(OH)(ZnW(9)O(34))(2)](11-)", (1), is an all inorganic dioxygenase able to incorporate one O(2) into two adamantane CH bonds to yield 2 equiv of 1-adamantanol as the primary product. In a subsequent 2005 Inorg. Chem. publication (Inorg. Chem. 2005, 44, 4175-4188), strong evidence was provided that the putative dioxygenase chemistry is, instead, the result of classic autoxidation catalysis. That research raised the question of whether the reported Ru(2) precatalyst, 1, was pure or even if it contained two Ru atoms, since Ru is known to be difficult to substitute into polyoxoanion structures (Nomiya, K.; Torii, H.; Nomura, K.; Sato, Y. J. Chem. Soc. Dalton Trans. 2001, 1506-1521). After our research group had contact with three other groups who also had difficulties reproducing the reported synthesis and composition of 1, we decided to re-examine 1 in some detail. Herein we provide evidence that the claimed 1 actually appears to be the parent polyoxoanion [WZn(3)(H(2)O)(2)(ZnW(9)O(34))(2)](12-) with small amounts of Ru (</=0.2 atoms) either substituted into the parent complex or present as a small amount of a Ru(n+) impurity, at least in our and two other group's hands. The evidence obtained, on three independent samples prepared from two research groups including ours, includes elemental analysis on the bulk samples, single crystal X-ray diffraction, elemental analysis on single crystals from the same batch used for X-ray diffraction, (183)W NMR, and adamantane oxidation oxygen uptake and product determination studies. Also re-examined herein are the two previously reported crystal structures of 1 that appear to be very similar to the structure of the parent polyoxoanion, [WZn(3)(H(2)O)(2)(ZnW(9)O(34))(2)](12-). Furthermore, we report that trace Ru alone, in the form of [Ru(DMSO)(4)Cl(2)], or that the parent polyoxoanion [WZn(3)(H(2)O)(2)(ZnW(9)O(34))(2)](12-) alone, are capable of producing the same products. More significantly, a simple physical mixture of [WZn(3)(H(2)O)(2)(ZnW(9)O(34))(2)](12-) plus the average 0.13 equiv of Ru found by analysis added as the [Ru(DMSO)(4)Cl(2)] starting material is a ca. 2-fold kinetically more competent catalyst than is "[WZnRu(2)(III)(H(2)O)(OH)(ZnW(9)O(34))(2)](11-)", (1). In short, the evidence is strong that the putative "[WZnRu(2)(III)(H(2)O)(OH)(ZnW(9)O(34))(2)](11-)", (1), which underlies the previously reported all-inorganic dioxygenase catalysis claim, is probably not correct. That does not mean that 1 cannot or even does not exist, but just that (a) no reliable synthesis of it exists if it has actually been made before, and (b) that a simple mixture of the [Ru(DMSO)(4)Cl(2)] plus [WZn(3)(H(2)O)(2)(ZnW(9)O(34))(2)](12-) precursors gives about 2-fold faster catalysis of adamantane hydroxylation that occurs by, the evidence suggests, a radical-chain autoxidation mechanism rather than via the previously claimed, novel all-inorganic-based dioxygenase catalysis.
Morris
Aimee M
AM
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, USA.
Anderson
Oren P
OP
Finke
Richard G
RG
D002384:Catalysis; D049308:Dioxygenases / Q000737:chemistry*; D015394:Molecular Structure; D012428:Ruthenium / Q000737:chemistry*; D017973:Tungsten Compounds / Q000737:chemistry*; D000099294:Polyoxometalates
D049308:Dioxygenases; D012428:Ruthenium; D017973:Tungsten Compounds; C102296:polyoxometalate I; D000099294:Polyoxometalates