The Gladysz Research Group

Research

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Research in the Gladysz group has traditionally been centered around organometallic chemistry, and from this core area branches into catalysis, organic synthesis, enantioselective reactions, stereochemistry, mechanism, and materials chemistry. Currently, about half of the group is involved with catalysis projects, divided 50:50 between structurally novel enantioselective catalysts and highly fluorinated recoverable catalyst systems. The other half designs organometallic building blocks for the synthesis of molecular wires, compasses, and gyroscopes.
One active area involves complexes in which wire-like sp-carbon chains span two transition metals. There is the obvious question of how long a polyalkyne or (C≡C)_n bridge can be synthesized. As shown in Scheme 1, PtC_xPt complexes with as many as twenty-eight carbons are easily isolated – shattering the record for polyynes previously held by carbon or silicon endgroups.¹

Scheme 1. Synthesis of Diplatinum Polyynediyl Complexes with Even Numbers of C≡C Bonds: (a) wet n-Bu₄N⁺ F^-/Acetone; (b) ClSiMe₃; (c) cat. CuCl/TMEDA, O₂, Acetone.

Because of the metal termini, all such species are redox-active. This in turns raises the possibility of a variety of charge-transfer phenomena involving the metal termini. In order to help stabilize certain charged redox states, we have sought to "insulate" the sp-carbon chains. The two approaches in Scheme 2 yield novel double-helical diplatinum complexes of the type I, in which sp³-carbon chains coil around the sp carbon chain.² Such compounds are now being tested as "molecular wires". Figure 1 illustrates the impressive degree of steric protection.

Scheme 2. Syntheses of Double-Helical Diplatinum Complexes

Figure 1. Space Filling Representation of a Typical Double-Helical Diplatinum Complex

In Scheme 2, it was a pleasant surprise that alkene metathesis could be applied within a metal coordination sphere. This led to the consideration of other possible uses. For example, gyroscopes have numerous technological applications, but until our work no molecules that mimic the symmetry, connectivity, and rotational abilities of common toy gyroscopes were known. In a synthetic tour-de-force, a three-fold alkene metathesis reaction was used as shown in Scheme 3 to access families of complexes with the structures III and IV.³ Views of the crystal structure of III are supplied in Figure 2.

Scheme 3. Syntheses of gyroscope-like molecules.

Figure 2. ORTEP (top) and space-filling (bottom) representations of the crystal structure of III (n = 6).

NMR measurements show that the Fe(CO)₂(NO) moiety in IV rotates within the methylene cage at an effective rate of 1000000 rpm at room temperature. This work has been extended to other metals and rotating groups, as well as much larger cage sizes. Some recent results with a similarly-synthesized platinum gyroscope are illustrated in Scheme 4. In collaborations with physicists, possible nanotechnological applications are being probed.

Scheme 4. Reactions of a Platinum-Containing Gyroscope-like Molecule

In enantioselective catalysis, ferrocene-containing chiral ligands play a major role. We have sought to incorporate a wider range of "spectator" metal fragments that feature unique architectural and electronic properties into donor ligands. Typical examples of ligands and/or their complexes are shown in Figure 3.⁴ These have provided exceptionally effective rhodium and palladium catalysts for enantioselective hydrogenations, hydrosilylations, and related processes. Furthermore, the rhenium-containing phosphine VII has recently been shown to be an effective "organocatalyst" for various transformations, two of which are depicted in Scheme 5.

Figure 3. Ligands that contain a chiral rhenium fragment, and/or complexes there of.

Scheme 5. Enantioselective Catalysis using Chiral, Rhenium-Containing Phosphines

Another area of current interest involves "fluorous" chemistry. When sufficient numbers of "ponytails" (CH₂)_mRf_n (Rf_n = (CF₂)_n-1CF₃) are added to catalysts or reagents, they acquire exceptional affinities for perfluoroalkanes and other fluorous phases. Furthermore, they also commonly exhibit highly temperature-dependent solubilities in organic solvents. As shown in Scheme 6, reactions can often be conducted under homogeneous conditions at elevated temperatures, and the catalyst recovered by a liquid/solid phase separation at lower temperature.

Scheme 6. Recovery of Fluorous Catalysts via Solid/Liquid Phase Separation.

For laboratory-scale reactions involving efficient catalysts, it is often only necessary to re­cover a few milligrams of material. To facilitate such separations, fluorous supports have been investigated. Of these, the most interesting proves to be common laboratory Teflon tape. In fact, the catalyst can even be precoated on the tape, allowing delivery to be controlled by length. As shown in Scheme 7, a rhodium complex containing fluorous phosphines (IX) desorbs at 55 °C in dibutyl ether, catalyzes the hydrosilylation of ketones, and reprecipitates onto the tape upon cooling.⁵

Scheme 7. Recycling of a Thermomorphic Fluorous Rhodium Hydrosilylation Catalyst using Teflon® Tape.

(1) Zheng, Q.; Gladysz, J. A. J. Am. Chem. Soc. 2005, 127, 10508.

(2) Stahl, J.; Bohling, J. C.; Bauer, E. B.; Peters, T. B.; Mohr, W.; Martín-Alvarez, J. M.; Hampel, F.; Gladysz, J. A. Angew. Chem., Int. Ed. 2002, 41, 1871; Angew. Chem. 2002, 114, 1951.

(3) Shima, T.; Hampel, F.; Gladysz, J. A. Angew. Chem., Int. Ed. 2004, 43, 5537; Angew. Chem. 2004, 116, 5653.

(4) Friedlein, F. K.; Hampel, F.; Gladysz, J. A. Organometallics 2005, 4013.

(5) Dinh, L. V.; Gladysz, J. A. Angew. Chem., Int. Ed. 2005, 44, 4095; Angew. Chem. 2005, 117, 4164.