The Oxo Wall Prevails: Investigations of an Example of Pathological Science

Brian B. Trinh Literature Seminar December 4, 2012

Transition metal oxo complexes are involved in, or believed to be involved in, many oxygen transfer reactions, such as those carried out by cytochrome P450 enzymes.1-4 Terminal oxo complexes of the early transition metals are ubiquitous, and some are stable to concentrated acid solutions. However, there are very few reports of isolated terminal oxo complexes of the late transition metals.5 In 1962, Ballhausen and Gray developed a molecular orbital diagram from the vanadyl ion, VO(OH2)5

2+, for tetragonal transition metal complexes containing a terminal oxo ligand. Based on this molecular orbital diagram, they proposed that terminal oxo ligands will not exist for transition metals with a d-electron count greater than five in a tetragonal ligand field. In fact, no tetragonal terminal oxo complexes or their isoelectronic imido analogs have been isolated for any transition metal from group 9 onwards. This phenomenon is referred to as the oxo wall.5,6

In 1993, Wilkinson et al. synthesized the first isolable late transition metal terminal oxo complex, Ir(O)(mes)3.7 The Ir atom in this d4 Ir(V) complex is in a trigonal, not tetragonal, ligand field and therefore does not violate the oxo wall. Since then, there have been reports of a d6 distorted square planar Pt(IV) oxo in 2008 and a d8 linear Ni(II) imido complex in 2011.8,9 None of these complexes violate the oxo wall.

Between 2004 and 2007, Hill et al. reported the synthesis and characterization of air-stable Pt(IV) oxo, Pd(IV) oxo, and Au(III) oxo complexes, which were formulated as K7Na9[Pt(O)(OH2)(PW9O34)2]·21.5H2O, K10Na3[Pd(O)(OH)WO(OH2)(PW9O34)2], K15H2[Au(O)(OH2)(PW9O34)2]·25H2O, and K7H2[Au(O)(OH2)P2W20O70]·27H2O.10-12 These complexes utilized clamshell polyoxotungstate ligands, placing the late transition metal in a tetragonal ligand field. All of these complexes violate the oxo wall. The authors used between nine and fifteen techniques to characterize these complexes and often performed duplicate or triplicate analyses to confirm their results.10-12

Earlier this year, Hill et al. retracted these papers and published a reinterpretation of the data they collected, explaining how the data had been misinterpreted. The authors reformulated the Pt(IV) and Au(III) complexes as their W(VI) analogs, and they proposed that the Pd(IV) complex contains a domed, square planar Pd(II) center without a terminal oxo or hydroxo ligand.13

Hill et al. based their initial formulations mostly on the X-ray and neutron diffraction data. Errors in the data processing led the authors to believe these compounds were bona fide late transition metal oxo complexes. Upon investigating the parameters they used to initially process

Revisiting the Polyoxometalate-Based Late-Transition-Metal-OxoComplexes: The “Oxo Wall” StandsKevin P. O’Halloran,† Chongchao Zhao,† Nicole S. Ando,† Arthur J. Schultz,‡ Thomas F. Koetzle,‡

Paula M. B. Piccoli,‡ Britt Hedman,§ Keith O. Hodgson,§ Elena Bobyr,§ Martin L. Kirk,∥

Sushilla Knottenbelt,∥ Ezra C. Depperman,∥ Benjamin Stein,∥ Travis M. Anderson,† Rui Cao,†

Yurii V. Geletii,† Kenneth I. Hardcastle,† Djamaladdin G. Musaev,⊥ Wade A. Neiwert,† Xikui Fang,†

Keiji Morokuma,†,⊥ Shaoxiong Wu,† Paul Kogerler,#,! and Craig L. Hill*,†

†Department of Chemistry, Emory University, Atlanta, Georgia 30322, United States‡Intense Pulsed Neutron Source, Argonne National Laboratory, Argonne, Illinois 60439, United States§Department of Chemistry and Stanford Synchrotron Radiation Lightsource, SLAC, Stanford University, Stanford, California 94305,United Statess∥Department of Chemistry and Chemical Biology, The University of New Mexico, Albuquerque, New Mexico 87131-0001, UnitedStates⊥Cherry L. Emerson Center for Scientific Computation, Emory University, Atlanta, Georgia 30322, United States#Ames Laboratory, Iowa State University, Ames, Iowa 50011, United States

*S Supporting Information

ABSTRACT: Terminal oxo complexes of the late transition metals Pt, Pd, andAu have been reported by us in Science and Journal of the American ChemicalSociety. Despite thoroughness in characterizing these complexes (multipleindependent structural methods and up to 17 analytical methods in one case),we have continued to study these structures. Initial work on these systems wasmotivated by structural data from X-ray crystallography and neutron diffractionand 17O and 31P NMR signatures which all indicated differences from all previously published compounds. With significant newdata, we now revisit these studies. New X-ray crystal structures of previously reported complexes K14[P2W19O69(OH2)] and“K10Na3[Pd

IV(O)(OH)WO(OH2)(PW9O34)2]” and a closer examination of these structures are provided. Also presented are the17O NMR spectrum of an 17O-enriched sample of [PW11O39]

7− and a careful combined 31P NMR-titration study of thepreviously reported “K7H2[Au(O)(OH2)P2W20O70(OH2)2].” These and considerable other data collectively indicate thatpreviously assigned terminal Pt-oxo and Au-oxo complexes are in fact cocrystals of the all-tungsten structural analogues withnoble metal cations, while the Pd-oxo complex is a disordered Pd(II)-substituted polyoxometalate. The neutron diffraction datahave been re-analyzed, and new refinements are fully consistent with the all-tungsten formulations of the Pt-oxo and Au-oxopolyoxometalate species.

! INTRODUCTIONMany of the catalysts and O2-based technologies that are ofglobal importance are based on the late-transition-metalelements. Terminally bound oxo species of these metals areproposed to be key intermediates in these important and usefulreactions.1 A terminal oxo ligand is a doubly or triply bondedoxygen atom which acts as a multielectron donor to oneelectrophilic transition metal. Despite over 40 years of effortfrom the research community to isolate such complexes of thelate transition elements, they still remain elusive. The scarcity ofthese compounds is due to electronic repulsion between oxoligands (or other multiply bonded ligands) and high d-counttransition metals in tetragonal environments and was explainedby molecular orbital calculations,2 supported experimentally,3

and later extended to other geometries by Mayer et al.4 Holmhas reviewed a large number of metal oxo complexes andconcludes that “MO groups are stabilized at metal centers

with an oxidation state of no less than 4+ and no more thanfour d electrons.”5 Consequently, oxo complexes of the earlytransition elements (groups 3−6) are extremely stable andabundant. Those of the midtransition elements (groups 7−8)are generally more reactive and intensely investigated, andthose of the late transition elements (groups 9−11) are rarelyreported. This combination of theory and observations hasgiven rise to the defensible concept of the “Oxo Wall”, inparticular articulated by Gray and Winkler: transition metals tothe right of the Fe−Ru−Os group will not support a terminaloxo ligand in a tetragonal environment,6 since d electrons herebegin to populate orbitals that are antibonding in the MOunit.7 There are a handful of interesting examples that areexceptions to these guidelines: [FeIIIH3L(O)]

2− where L =

Received: April 28, 2011Published: June 13, 2012

Article

pubs.acs.org/IC

© 2012 American Chemical Society 7025 dx.doi.org/10.1021/ic2008914 | Inorg. Chem. 2012, 51, 7025−7031

Figure 1: The oxo wall divides group 8 and group 9.13

O(35)

Pt(1) O(30)

O(29)

O(6W)

O(30A)

O(29A)Pd(1)

O70

O71

O33

O32O37

O38

O(35)

Au O34O32A

O1W

O34A

O32

a) b) c)

Figure 2: Local environments of a) Pt(IV) oxo complex,10 b) Pd(IV) oxo complex,11 and c) Au(III) oxo complexes.12 The local environment of both Au(III) oxo complexes is nearly identical.  

the neutron diffraction data for the Pt(IV) complex, the authors discovered they had used a site occupancy of 0.50 for the Pt, although their originally published article claimed that the platinum atom had full occupancy. Half of the Pt scattering length (9.60 fm) is approximately the W scattering length (4.86 fm), which explains why the W atom actually present in the crystal could be successfully (though incorrectly) refined as a half Pt atom. The Pd(IV) complex suffered from disorder in which the Pd atom is replaced by two K atoms; this disorder was modeled incorrectly under the unwarranted assumption that a terminal oxo ligand was present. The Au(III) complexes were incorrectly refined with mixed (i.e. W and Au) occupancy at the “Au” sites, but Hill et al. now report that the data sets refine well if the site occupancy is assumed to be 100% W.13 Hill et al. published new data in a retraction paper that support the new formulations of the complexes.13 However, there are still discrepancies between this retraction paper and the original papers. If the data from the original papers are genuine and only the identities of the complexes were mistaken, then the data from the original papers should be consistent with the current formulations. In some cases (e.g. 17O NMR studies and elemental analyses), they are not.

17O NMR spectra were collected for the supposed Pd(IV) complex and its W(VI) analog. Peaks at δ 330 and 570 were assigned to Pd-OH and Pd=O, respectively, due to their absence in the 17O NMR spectrum of the W(VI) analog.11 17O NMR spectra were collected for both Au(III) complexes. A peak at δ 590 was assigned to Au=O for one complex and a peak at δ 605 was assigned to Au=O for the other complex based on the Pd=O chemical shift from the previous paper.12 In the retraction paper, Hill et al. claim that the peak at δ 570 in the 17O NMR spectrum of the Pd(IV) complex is due to [PdPW11O39]5-, a suspected decomposition product. The origin of the peak at δ 330 is still unexplained.13 The peaks at δ 590 and 605 in the 17O NMR spectra of the Au(III) complexes are proposed to be due to [PW11O39]7-, a decomposition product with Keggin-type structure.13

Figure 3: a) 17O NMR spectrum of the palladium(IV) complex.

b) 17O NMR spectrum of the tungsten(VI) analog.11 Hill et al. titrated the second Au(III) complex with PPh3 to study oxygen transfer

of the complex and observed the disappearance of the peak at δ 605 in the 17O NMR spectrum.12 In the retraction paper, they claim that the PPh3 is actually reacting with HAuCl4 and H2O.13 The reinterpretation does not attempt to explain the disappearance of the peak at δ 605 in the original paper. These are but a few of the inconsistencies between the original papers and the retraction paper.

!

S18

Figure S4. 17O NMR spectra of (A) 2 and (B) P2W20.

450 400 350 300 PPM

620 580 540 PPM

780 740 700 PPM

W=O

Pd=O

W-O-WPd-OH

A

B

800 750 700 650 PPM

550 450 350 PPM

o!

! 780 740 700 PPM

!

!

S18

Figure S4. 17O NMR spectra of (A) 2 and (B) P2W20.

450 400 350 300 PPM

620 580 540 PPM

780 740 700 PPM

W=O

Pd=O

W-O-WPd-OH

A

B

800 750 700 650 PPM

550 450 350 PPM

oS18

Figure S4. 17O NMR spectra of (A) 2 and (B) P2W20.

450 400 350 300 PPM

620 580 540 PPM

780 740 700 PPM

W=O

Pd=O

W-O-WPd-OH

A

B

800 750 700 650 PPM

550 450 350 PPM

o!

! 780 740 700 PPM

!

550 450 350 PPM

a) b)

References 1. Alsters, P. L.; Teunissen, H. T.; Boersma, J.; Spek, A. L.; van Koten, G.

Complexes by Inorganic and Organic Peroxides: Oxygen Insertion into the Palladium-Carbon Bond. Organometallics 1993, 12, 4691-4696.

2. Khan, M. M. T.; Chatterjee, D.; Merchant, R. R.; Bhatt, A.; Kumar, S. S. Synthesis of the Complex [RuV=O(edta)]- and its Application in the Oxidation of Saturated Substrates by Oxygen Atom Transfer Reaction. Journal of Molecular Catalysis 1991, 66, 289-293.

3. Limberg, C. What Does it Really Take to Stabilize Complexes of Late Transition Metals with Terminal Oxo Ligands? Angew. Chem. Int. Ed. 2009, 48, 2270-2273.

4. Mukerjee, S.; Skogerson, K.; DeGala, S.; Caradonna, J. P. Skirting the oxo-wall: characterization and catalytic reactivity of binuclear Co2+/3+ 1,2-bis(2-hydroxybenzamido)benzene complexes with comparison to their isostructural Fe2+/3+ analogs. Implications of d-electron count on oxygen atom transfer catalysis. Inorganica Chemica Acta 2000. 297, 313-329.

5. Ballhausen, C. J.; Gray, H. B. The Electronic Structure of the Vanadyl Ion. Inorg. Chem. 1962, 1, 111-122.

6. Winkler, J. R.; Gray, H. B. Electronic Structures of Oxo-Metal Ions. Struct Bond. 2012, 142, 17-28

7. Hay-Motherwell, R. S.; Hussain-Bates, B.; Hursthouse, M. B.; Wilkinson, G. Synthesis and X-Ray Crystal Structure of Oxotrimesityliridium(V). Polyhedron 1993, 12, 2009-2012.

8. Poverenov, E.; Efremenko, I.; Frenkel, A. I.; Ben-David, Y.; Shimon, L. J. W.; Leitus, G.; Konstantinovski, L.; Martin, J. M. L.; Milstein, D. Evidence for a Terminal Pt(IV)-Oxo Complex Exhibiting Diverse Reactivity. Nature 2008, 455, 1093-1096.

9. Laskowski, C. A.; Miller, A. J. M.; Hillhouse, G. L.; Cundari, T. R. A Two-Coordinate Nickel Imido Complex That Effects C-H Amination. J. Am. Chem. Soc. 2011, 133, 771-773.

10. Anderson, T. M.; Neiwert, W. A.; Kirk, M. L.; Piccoli, P. M. B.; Shultz, A. J.; Koetzle, T. F.; Musaey, D. G.; Morokuma, K.; Cao, R.; Hill, C. L. A Late-Transition Metal Oxo Complex: K7Na9[O=PtIV(H2O)L2], L = [PW9O34]9-. Science 2004, 306, 2074-2077

11. Anderson, T. M.; Cao, R.; Slonkina, E.; Hedman, B.; Hodgson, K. O.; Hardcastle, K. I.; Neiwert, W. A.; Wu, S.; Kirk, M. L.; Knottenbelt, S.; Depperman, E. C.; Keita, B.; Nadjo, L.; Musaev, D. G.; Morokuma, K.; Hill, C. L. A Palldium-Oxo Complex. Stabilization of This Proposed Catalytic Intermediate by an Encapsulating Polytungstate Ligand. J. Am. Chem. Soc. 2005, 127, 11948-11949

12. Cao, R.; Anderson, T. M.; Piccoli, P. M. B.; Schultz, A. J.; Koetzle, T. F.; Geletii, Y. V.; Slonkina, E.; Hedman, B.; Hodgson, K. O.; Hardcastle, K. I.; Fang, X.; Kirk, M. L.; Knottenbelt, S.; Kögerler, P.; Musaev, D. G.; Morokuma, K.; Takahashi, M.; Hill, C. L. Terminal Gold-Oxo Complexes. J. Am. Chem. Soc. 2007, 129, 1118-11133

13. O’Halloran, K. P.; Zhao, C.; Ando, N. S.; Schultz, A. J.; Koetzle, T. F.; Piccoli, P. M. B.; Hedman, B.; Hodgson, K. O.; Bobyr, E.; Kirk, M. L.; Knottenbelt, S.; Depperman, E. C.; Stein, B.; Anderson, T. M.; Cao, R.; Geletii, Y. V.; Hardcastle, K. I.; Musaev, D. G.; Neiwert, W. A.; Fang, X.; Morokuma, K.; Wu, S.; Kögerler, P.; Hill, C. L. Revisiting the Polyoxometalate-Based Late-Transition-Metal-Oxo Complexes: The “Oxo Wall” Stands. Inorg. Chem. 2012, 51, 7025-7031