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44904-AC3
Rigid Pincer Ligands: New Prospects in Hypercoordinate Main Group Chemistry

Oleg V. Ozerov, Brandeis University

Fafard, C. M.; Chen, C.-H.; Foxman, B. M.; Ozerov, O. V. "Covalent Palladium-Zinc Bonds and Their Reactivity", Chem. Commun. 2007, 4465.

Photolysis of (PNP)Pd-Et in the presence of Et₂Zn resulted in the formation of (PNP)Pd-Zn-Pd(PNP), containing a linear Pd-Zn-Pd linkage with approximately square-planar environments about the Pd centers.  This is the first example of a compound with a covalent Pd-Zn bond.  It displays unusual reactivity in the reaction with MeI: the kinetic products are (PNP)Pd-I and Me₂Zn(!).

Gatard, S.; Guo, C.; Foxman, B. M.; Ozerov, O. V. "Thioether, Dinitrogen, and Olefin Complexes of (PNP)Rh: Kinetics and Thermodynamics of Exchange and Oxidative Addition Reactions", Organometallics 2007, 26, 6066.

(PNP)Rh-L compounds where L = R₂S or bulky olefin are convenient synthons for the transient (PNP)Rh.  We determined that (PNP)Rh-SPrⁱ₂ (and likely other (PNP)Rh-L) reacts with PhBr via reversible dissociation of SPrⁱ₂ prior to addition of PhBr.  In the same study, we determined the relative affinity of (PNP)Rh for a series of L ligands.  Here are some of the findings: a) steric effects are dominant in the binding of dialkyl sulfides; b) SPrⁱ₂ binds more strongly than Me₃CCH=CH₂, more weakly than N₂; c) consistently with a dissociative mechanism, the rate of the PhBr OA reactions to (PNP)Rh-L decreases with the increased strength of binding of L; d) (PNP)Rh(SPh₂) is isoergic with its C-S OA isomer (PNP)Rh(Ph)(SPh): a 1:1 equilibrium is rapidly established at 70 °C.

Zhu, Y.; Fan, L.; Chen, C.-H.; Finnell, S. R.; Foxman, B. M.; Ozerov, O. V. "C-H Oxidative Addition to a (PNP)Ir Center and Ligand-Induced Reversal of Benzyl/Aryl Selectivity", Organometallics 2007, 26, 6701.

Complex (PNP)Ir(H)(Mes) is an attractive synthon for (PNP)Ir.  For instance, it activates benzene or toluene solvent via rate-limiting reductive elimination of mesitylene at a reasonable rate (t_1/2 < 30 min at 60 °C).  With pyridine, instead, isomerization to the benzylic isomer of mesityl was observed.  The drive for isomerization is primarily owing to steric factors: isomerization of mesityl to a less sterically imposing 3,5-dimethylbenzyl by itself is not favorable, but is compensated by the stronger binding of pyridine to the latter.  Additional conclusions from this study are that 1) the aryl-to-benzylic isomerization in (PNP)Ir(H)(Mes) is kinetically readily accessible and is faster than loss of mesitylene and 2) reductive elimination of mesitylene does not proceed from the 6-coordinate Ir, but only from the five-coordinate Ir (this echoes other studies on RE from d⁶ complexes).

Bontemps, S.; Bouhadir, G.; Gu, W.; Mercy, M.; Chen, C.-H.; Foxman, B. M.; Maron, L.; Ozerov, O. V.; Bourissou D. "Metallaboratranes Derived from a Triphosphanyl-Borane: Intrinsic C₃ Symmetry Supported by a Z-Type Ligand", Angew. Chem. Int. Ed. 2008, 47, 1481.

Our initial synthetic attempts at the preparation of PBP pincers led instead to the C₃-symmetric BP₃ ligand.  At that time, we discovered that BP₃ was also being investigated by the Bourissou group at Univ. Paul Sabatier (Toulouse, France).  We thought it most scientifically productive to combine efforts in a joint manuscript describing the synthesis, structure, and computational analysis of BP₃ complexes of Pt and Au.

Pursuit of five-coordinate boron.  We were able to synthesize (PNP)BF₂ which displays apparent C_2v symmetry in its NMR spectra down to –80 °C.  Strong coupling was observed among the two ³¹P, the ¹¹B, and the two ¹⁹F nuclei, consistent with a five-coordinate environment at boron.  However, an X-ray study revealed a solid-state structure in which boron is four-coordinate, with one phosphine arm of the PNP ligand being “off”.  It is possible that two degenerate four-coordinate structures are rapidly interconverting in solution with low activation barrier.  Our next step is to investigate analogous (^TPNP)BF₂ complexes with the “tied” PNP ligand.  The “tied” PNP restricts the conformational freedom of the phosphine arms and may serve better to enforce pentacoordination at boron.  

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