New
heterodonor NPPN tetradentate ligands, 2-PyCH2(Ph)P(CH2)
n
P(Ph)CH2-2-Py (meso- and rac-L
n
; n = 2–4, Py = pyridyl), were prepared and
reacted with [Cp*MCl2]2 (M = Ir, Rh; Cp* is
pentamethylcyclopentadienyl) in the presence of NH4BF4 to afford a series of dinuclear complexes [(Cp*MCl)2(meso-L
n
)](BF4)2 (M = Ir, n = 2 (2a),
3 (3a), 4 (4a); M = Rh, n = 2 (2c), 3 (3c), 4 (4c))
and [(Cp*MCl)2(rac-L
n
)](BF4)2 (M = Ir, n =
2 (2b), 3 (3b), 4 (4b); M =
Rh, n = 2 (2d), 3 (3d),
4 (4d)), which were characterized by IR, 1H and 31P{1H} NMR, and ESI mass spectroscopic
techniques and X-ray crystallography. The configurations around the
two metal centers were controlled by the configuration of the coordinated
P atoms so as to avoid repulsive interaction between the phenyl group
on P and the chloride ligand, resulting in the formation of stereospecific
isomers; a meso configuration of the metal centers
is induced from meso-L
n
(abbreviated as meso-P2/meso-M2), and in contrast, a rac configuration
is induced from rac-L
n
(rac-P2/rac-M2). Furthermore, inversion of metal centers for the Ir2 complexes occurred in DMSO at higher temperatures (60–100
°C), generating equilibrium mixtures of minor diastereomers (meso-P2/rac-M2 or rac-P2/meso-M2) in
low ratios together with the major isomers (meso-P2/meso-M2 or rac-P2/rac-M2). The equilibrium
constants, K = [minor isomer]/[major isomer], varied
appreciably depending on the lengths of the methylene chains as well
as configurations of the NPPN ligands; the overall propensity for
the K values was observed to be L2 <
L3 < L4 and meso-L
n
< rac-L
n
, while rac-L3, rac-L4, and meso-L4 showed almost
identical equilibrium constants, presumably resulting from no steric
influence between the two metal centers.
