Daniel A. Hillesheim

Platinum-Containing Polyoxometalates†

Lee, Kortz, and their co-workers recently reported a Pt-containing polyoxovanadate, [H2Pt V9O28] 5 , in this journal. There are noteworthy features of this complex, and given the potential similarity of Ptand other noble metalcontaining polyoxometalates to a range of important metal-oxide supported noble-metal-based catalysts, electrodes, sensors, and other industrially significant materials, there is unquestionably interest in [H2Pt V9O28] 5 and other Ptcontaining polyoxometalates as possible tractable molecular representations of these materials. Nevertheless, there are some points many investigators will want to note. Lee, Kortz, and co-workers state: “Therefore, 1 [[H2Pt V9O28] 5 ] represents the first transition-metal-substituted decavanadate derivative...”. However, substituted decavanadates have been known for nearly two decades. See, for example, the work of Howarth, Pettersson, and co-workers on monoand di-molybdopolyvanadates or the study by Kamenar and colleagues on [(CH3)4N]4[H2MoV9O28]Cl. [3] Note that no {Mo10O28} “decavanadate” analogue exists; in the Mo-substituted decavanadates (contrary to several other polyoxometalate structure types) the Mo sites thus do not constitute addenda sites but represent genuine substituents. In addition, other substituted decavanadate motifs exist: Sasaki, Pope, and coworkers, for example, refer to their complex, [(MnV11O31)2O2] 10 , as “a manganese-substituted decavanadate” where “each half-unit may be dissected to reveal other familiar polyoxoanion structures, i.e., planar MnV6O24 (Anderson) or MnV9O28 (Mn-substituted decavanadate)”. There are also some technical concerns regarding the work reported in Ref. [1]. Lee, Kortz, and co-workers claim that the hydrogen atoms (Hb7 and Hb8) on two of the oxygen atoms (Ob7 and Ob8) bound to Pt were “actually located in the difference Fourier map and refined with the O···H separations restrained to 0.85(10) .” This is an unusual claim because hydrogen atoms on polyanion oxygen atoms bound to 5d metal centers (i.e. W or Pt) have seldom been located by X-ray analysis. The fact that the X-ray diffraction dataset was collected at high temperature (T= 293 K) makes this claim even more suspect. Without some independent confirmation of this claim, the community will be very skeptical about it. A single-crystal neutron diffraction study of this complex would provide such confirmation. We felt compelled to conduct a single-crystal neutron diffraction study of our terminal platinum oxo complex [O=Pt(H2O)(PW9O34)2] 16 [5] for precisely this reason. Without such a study, knowledgeable colleagues would have rightly questioned whether that report involved a terminal platinum oxo or platinum hydroxo unit (i.e. whether there was a hydrogen on that critical oxygen atom or not). Another serious objection with regard to the crystallographic identification of hydrogen, however, is that Lee, Kortz, and coworkers state they “placed the H atoms of the water molecules in calculated positions.” Of all the HFIX subroutines in SHELX, there is no command that will do this. In addition, Lee, Kortz, and coworkers report a Rint of “0.000” that indicates they did not collect any equivalent reflections in their data set; it is always better to have equivalent reflections, even in space group P1̄. Moreover, they state “We also prepared derivatives of 1 with different degrees of protonation, such as [HxPt V9O28] (7 x) (x = 2.5, 3, 4, 5), as shown by single-crystal X-ray analysis.” How does the latter analysis show a protonation state of 2.5 in a single molecular unit? You can have fractional occupancies (such disorder, a common phenomenon, was not mentioned in the article), but you cannot have a fractional hydrogen. Lee, Kortz, and co-workers, in reference [1] addressing our 2004 report of a terminal platinum oxo compound, state we did not show W or Pt NMR spectra. However, these investigators do not explain why we did not report these spectra (the reason was that these investigations are not possible given the lability of the platinum oxo complex in solution), but more importantly Lee, Kortz, and co-workers fail to mention that this platinum oxo complex was characterized by 10 techniques including single-crystal neutron diffraction that confirmed a terminal platinum oxo and not a platinum hydroxo unit is present. Thus W and Pt NMR, the only techniques Lee, Kortz, and co[*] Dr. R. Cao, Dr. T. M. Anderson, D. A. Hillesheim, Dr. K. I. Hardcastle, Prof. Dr. C. L. Hill Department of Chemistry Emory University Atlanta, GA 30322 (USA) Fax: (+ 1)404-727-6076 E-mail: chill@emory.edu

Controlled synthesis of a functionalized polytungstate ligand and a {MaMbMc(PW9)2} sandwich complex

The synthesis of an unprecedented vacant polytungstate ligand and its use to obtain a crystallographically characterized {MaMbMc(PW9)2}-type sandwich structure, the first example of the purposeful and controlled metal incorporation into a derivatized POM, are reported.

Mono-substituted Keggin, Wells-Dawson and {P2W21}-type polyoxometalates without positional disorder

Three new phenyltin-substituted polyoxometalates, ((CH3)2NH2)4[Sn(C6H5)PW11O39] (1), K7[Sn(C6H5)P2W17O61] (2), and ((CH3)2NH2)6K[Sn(C6H5)(H2O)P2W20O70(H2O)2] (3), have been synthesized and characterized by single crystal X-ray diffraction, 31P and 1H NMR, and FT-IR spectroscopy. Significantly, the mono-substituted phenyltin group in all three complexes is not positionally disordered. The Sn(C6H5) group is unambiguously determined and refined anisotropically with full occupancy, a result that is very unusual for mono-substituted polyoxometalates, and in particular polytungstates. Three factors account for these disorder-free mono-phenyltin-substituted polyanion structures: the steric bulk and rigidity of the phenyl group, hydrogen bonding and cation–π interactions between the phenyl ring and countercations. These results demonstrate the ability of a phenyl group to remove the crystallographically imposed positional disorder typically seen in mono-substituted polyoxometalates, an attribute that usually renders structural and structure-related reactivity studies of this large class of polyoxometalates quite difficult.