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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 (t1/2 < 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 C2v 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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