High-Yield Synthesis of
PPh3-Ligated Decanuclear
Tl–Pd Cluster, Pd9[Tl(acac)](CO)9(PPh3)6: Comparative Analysis of Tl(I)–Pd(0)
Bonding Connectivities with Known Tl–Pd Clusters and Resulting
Insight Concerning Their Dissimilar Dynamic Solution Behavior
posted on 2016-02-21, 15:14authored byEvgueni G. Mednikov, Nicky Vo, Charles G. Fry, Lawrence F. Dahl
The new Tl(I)–Pd(0) cluster Pd9[μ3/3-Tl(acac)](μ2-CO)6(μ3-CO)3(PPh3)6 (1) was
prepared in high yields (over 90%), both by reaction of Pd10(CO)12(PPh3)6 (4),
PPh3, and TlPF6 in THF in the presence of acetylacetone
(Hacac) and base (NEt3) and by direct reaction of Pd10(CO)12(PPh3)6 with PPh3 and Tl(acac). The composition and molecular structure of 1 were unambiguously established from 100 K CCD X-ray diffractometry
studies of two solvated crystals, 1·1.5Hacac·0.5THF
(1A) and 1·0.3THF (1B),
which showed essentially identical geometries for the entire Pd9Tl(CO)9P6 fragment of pseudo-C3v symmetry; its composition
is in agreement with X-ray Tl/Pd field-emission microanalysis with
a scanning electron microscope for crystals of 1B. This
cluster can be viewed as a markedly deformed Pd6 octahedron (oct) with the three Pd(oct) atoms of one of its
eight triangular faces connected both by three edge-bridging wingtip
(wt) Pd(μ2-CO)2PPh3 fragments
and by a symmetrical capping Tl(I). Three triply bridging carbonyl
ligands asymmetrically cap the lower alternate 3-fold-related triangular
faces of the Pd6 octahedron, and the three other PPh3 ligands are each coordinated to Pd atoms in the geometrically
opposite staggered Pd(oct)3 face. The 6s25d10 Tl(I) is also equivalently attached to both chelating O
atoms of a bidentate acetylacetonate (acac) monoanion. Although the C2 axis of the pseudo-C2v planar Tl(acac) fragment is approximately
parallel to the pseudo-C3 axis of the
TlPd9 core, the orientation of the Tl(acac) plane relative
to the octahedral-based Pd9 geometry is considerably different
for each of the three independent nondisordered molecules of 1 in 1A and 1B; these different
planar Tl(acac) orientations may be mainly attributed to anisotropic
crystal-packing effects. Coordination of the Tl(I) atom to the three
Pd(oct) atoms of the Pd9 core presumably occurs via its
so-called “inert” 6s2 electron pair with
resulting three short Tl–Pd(oct) connectivities of mean distance
2.83 Å; these connectivities together with three longer Tl–Pd(wt)
ones of mean distance 3.15 Å give rise to a (crown-like)Pd6 sextuple (μ3/3-Tl) coordination mode. Of
particular stereochemical interest is a comparison of solution behavior
of 1 with that for the known structurally related analogue,
Pd9[μ3-TlCo(CO)3L](μ2-CO)6(μ3-CO)3L6 (2) (with L = PEt3 instead of PPh3). In 2 the Tl(I) is alternatively attached to
a trigonal-bipyramidal Co(CO)3L monoanion and primarily
coordinated to the three inner Pd(oct) atoms of a similar PR3/CO-ligated octahedron; corresponding Tl–Pd(oct) and Tl–Pd(wt)
mean distances for two independent molecules in 2 are
2.77 and 3.31 Å, respectively. Variable-temperature 31P{1H} NMR solution data of 1 indicate the
occurrence of presumed fast wobbling-like motion of the [μ3/3-Tl(acac)] entity about the pseudo-C3 axis of the Pd9(μ2-CO)6(μ3-CO)3P6 fragment without Pd–Tl detachment (i.e., the entire cluster
of 1 remains intact). In direct contrast, corresponding
temperature-dependent 31P and 13C NMR data of 2 instead are consistent with rapid, reversible dissociation/association
of the entire [μ3-TlCo(CO)3L]
ligand from the analogous Pd9(μ2-CO)6(μ3-CO)3P6 fragment
of 2. This highly dissimilar dynamic solution behavior
that points to a stronger Tl(I) attachment to the Pd9 core
in 1 than that in 2 may be attributed from
the above crystallographic evidence to greater involvement of the
outer three wingtip Pd(wt) atoms in bonding connectivities to the
Tl(I) in 1 compared to predominant bonding connectivities
of only the three inner Pd(oct) atoms to the Tl(I) in 2. 1H NMR solution spectra of 1 also suggest
significant covalent character in the bidentate Tl–O(acac)
bonding in 1 based upon the observation of H(acac)–Tl
coupling; this premise is consistent with its Tl–O distances
of 2.35 Å (av) being ca. 0.2 Å shorter than those of 2.52
Å (av) found in crystalline Tl(acac), which with no observed
H–Tl NMR coupling in solution implies ionicity of its bidentate
Tl–O bonding. Both 1 and 2 conform
to an 86 CVE count expected for an octahedral metal polyhedron based
upon the Tl(I) and each wingtip Pd(μ2-CO)2L fragment contributing 2 and 4 CVEs, respectively.