Stepwise Expansion of Pd Chains from Binuclear Palladium(I)
Complexes Supported by Tetraphosphine
Ligands
- Publication date
- Publisher
Abstract
Reaction of [Pd<sub>2</sub>(XylNC)<sub>6</sub>]X<sub>2</sub> (X = PF<sub>6</sub>, BF<sub>4</sub>) with a
linear tetraphosphine, <i>meso</i>-bis[(diphenylphosphinomethyl)phenylphosphino]methane
(dpmppm), afforded binuclear Pd<sup>I</sup> complexes, [Pd<sub>2</sub>(μ-dpmppm)<sub>2</sub>]X<sub>2</sub> ([<b>2</b>]X<sub>2</sub>), through an asymmetric dipalladium complex, [Pd<sub>2</sub>(μ-dpmppm)(XylNC)<sub>3</sub>]<sup>2+</sup> ([<b>1</b>]<sup>2+</sup>). Complex [<b>2</b>]<sup>2+</sup> readily reacted
with [Pd<sup>0</sup>(dba)<sub>2</sub>] (2 equiv) and an excess of
isocyanide, RNC (R = 2,6-xylyl (Xyl), <i>tert</i>-butyl
(<sup><i>t</i></sup>Bu)), to generate an equilibrium mixture
of [Pd<sub>4</sub>(μ-dpmppm)<sub>2</sub>(RNC)<sub>2</sub>]<sup>2+</sup> ([<b>3</b>′]<sup>2+</sup>) + RNC ⇄ [Pd<sub>4</sub>(μ-dpmppm)<sub>2</sub>(RNC)<sub>3</sub>]<sup>2+</sup> ([<b>3</b>]<sup>2+</sup>), from which [Pd<sub>4</sub>(μ-dpmppm)<sub>2</sub>(XylNC)<sub>3</sub>]<sup>2+</sup> ([<b>3a</b>]<sup>2+</sup>) and [Pd<sub>4</sub>(μ-dpmppm)<sub>2</sub>(<sup><i>t</i></sup>BuNC)<sub>2</sub>]<sup>2+</sup> ([<b>3b</b>′]<sup>2+</sup>) were isolated. Variable-temperature UV–vis and <sup>31</sup>P{<sup>1</sup>H} and <sup>1</sup>H NMR spectroscopic studies
on the equilibrium mixtures demonstrated that the tetrapalladium complexes
are quite fluxional in the solution state: the symmetric Pd<sub>4</sub> complex [<b>3b</b>′]<sup>2+</sup> predominantly existed
at higher temperatures (>0 °C), and the equilibrium shifted
to the asymmetric Pd<sub>4</sub> complex [<b>3b</b>]<sup>2+</sup> at a low temperature (∼−30 °C). The binding constants
were determined by UV–vis titration at 20 °C and revealed
that XylNC is of higher affinity to the Pd<sub>4</sub> core than <sup><i>t</i></sup>BuNC. In addition, both isocyanides exhibited
higher affinity to the electron deficient [Pd<sub>4</sub>(μ-dpmppmF<sub>2</sub>)<sub>2</sub>(RNC)<sub>2</sub>]<sup>2+</sup> ([<b>3F</b>′]<sup>2+</sup>) than to [Pd<sub>4</sub>(μ-dpmppm)<sub>2</sub>(RNC)<sub>2</sub>]<sup>2+</sup> ([<b>3</b>′]<sup>2+</sup>) (dpmppmF<sub>2</sub> = <i>meso</i>-bis[{di(3,5-difluorophenyl)phosphinomethyl}phenylphosphino]methane).
When [<b>2</b>]X<sub>2</sub> was treated with [Pd<sup>0</sup>(dba)<sub>2</sub>] (2 equiv) in the absence of RNC in acetonitrile,
linearly ordered octapalladium chains, [Pd<sub>8</sub>(μ-dpmppm)<sub>4</sub>(CH<sub>3</sub>CN)<sub>2</sub>]X<sub>4</sub> ([<b>4</b>]X<sub>4</sub>: X = PF<sub>6</sub>, BF<sub>4</sub>), were generated
through a coupling of two {Pd<sub>4</sub>(μ-dpmppm)<sub>2</sub>}<sup>2+</sup> fragments. Complex [<b>2</b>]<sup>2+</sup> was
also proven to be a good precursor for Pd<sub>2</sub>M<sub>2</sub> mixed-metal complexes, yielding [Pd<sub>2</sub>Cl(Cp*MCl) (Cp*MCl<sub>2</sub>)(μ-dpmppm)<sub>2</sub>]<sup>2+</sup> (M = Rh ([<b>5</b>]<sup>2+</sup>), Ir ([<b>6</b>]<sup>2+</sup>), and
[Au<sub>2</sub>Pd<sub>2</sub>Cl<sub>2</sub>(dpmppm–H)<sub>2</sub>]<sup>2+</sup> ([<b>7</b>]<sup>2+</sup>) by treatment with
[Cp*MCl<sub>2</sub>]<sub>2</sub> and [AuCl(PPh<sub>3</sub>)], respectively.
Complex [<b>7</b>]<sup>2+</sup> contains an unprecedented PC(sp<sup>3</sup>)P pincer ligand with a PCPCPCP backbone, dpmppm–H
of deprotonated dpmppm. The present results demonstrated that the
binuclear Pd<sup>I</sup> complex [<b>2</b>]<sup>2+</sup> was
a quite useful starting material to extend the palladium chains and
to construct Pd-involved heteromultinuclear systems
