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42806-B3
Compounds of Bidentate Phosphines with Metallocene Backbones: Electrochemistry and Catalysis

Chip Nataro, Lafayette College

����������� The undergraduates in my lab have continued to investigate the properties of bidentate phosphines that have metallocene backbones.� To date our work has focused on the synthesis, electrochemistry and structural analysis of compounds containing either ���

1,1'-bis(diphenylphosphino)metallocene (metallocene = ferrocene (dppf), ruthenocene (dppr) or osmocene (dppo)) of 1,1'-bis(dialkylphosphino)ferrocene (alkyl = i-propyl (dippf) or t-butyl (dtbpf)).� With the exception of dtbpf, the oxidative electrochemistry of the free ligands is complicated by a chemical reaction.

����������� We recently investigated the oxidative electrochemistry of the free ligand 1,1'-bis(dicyclohexylphosphino)ferrocene (dcpf) and complexes containing this ligand.¹� As with the similar ligands, the oxidative electrochemistry of the free ligand is followed by a chemical reaction while oxidation of compounds containing dcpf is typically reversible.� In addition, the structure of [(dcpf)PdCl₂] was determined (Figure 1).� The P-Pd-P angle in [(dcpf)PdCl₂] was determined to be 102.45^o which is larger than the dppf analogue and smaller than the dippf analogue.

����������� The use of the [(P-P)PdCl₂] (P-P = dppf, dppr, dippf, dcpf or dtbpf) compounds as catalyst precursors in Buchwald-Hartwig catalysis was also investigated.¹� Previous studies had employed several of the compounds, but there have been no reports comparing all of these ligands for the same catalytic reactions.� We investigated the coupling of a primary amine with an aryl bromide (Scheme 1).� In general, we found the dppf compound to be the most effective catalyst, although with the electron withdrawing

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Scheme 1.� Buchwald-Hartwig reaction.

-CF₃ group, the dtbpf gave the most efficient catalyst.� Additional catalytic studies are currently underway.�

����������� We have also developed an interest in the closely related chiral bidentate phosphines with metallocene backbones.� Our initial work was with the BoPhoz ligands.²� More recently, we investigated the oxidative electrochemistry of a series of Josiphos ligands (Figure 3).³� The oxidative electrochemistry typically displays multiple waves with varying amounts of reversibility.� The initial oxidation seems to occur at the phosphorus atoms bound directly to the C₅ ring. �Subsequent oxidations occur likely occur in the π-system and at the iron center.� In addition to the free ligands, we investigated the oxidative electrochemistry of the [(Josiphos)PdCl₂] and [(Josiphos)PtCl₂].� Upon coordination the electrochemistry simplifies to one reversible peak.

Figure 3.� Josiphos ligands.

1)                  Hagopian, L. E.; Campbell, A. N.; Golen, J. A.; Rheingold, A. L.; Nataro, C. J. Organomet. Chem. 2006, 691, 4890.

2)                  Ghent, B. L.; Sites, L. A.; Rheingold, A. L.; Nataro, C. Organometallics 2005, 24, 4788.

3)                  Ghent, B. L.; Martinak, S. L.; Sites, L. A.; Golen, J. A.; Rheingold, A. L.; Nataro, C. J. Organomet. Chem. 2007, 692, 2365.

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