Transition metal complexes of the PPO/POP ligand: variable coordination chemistry and photo-luminescence properties

In the current work the tautomeric equilibrium between tetraphenyldiphosphoxane (Ph$_2$P–O–PPh$_2$, POP) and tetraphenyldiphosphine monoxide (Ph$_2$P–P([double bond, length as m-dash]O)Ph$_2$, PPO) in the absence and presence of transition metal precursors is investigated. Whereas with hard transition metal ions, such as Fe(II) and Y(III), PPO-type complexes, such as [FeCl$_2$(PPO)$_2$] and [YCl$_3$(THF)$_2$(PPO)], are formed, softer transition metals ions tend to form so-called coordination stabilised tautomers of the POP ligand form, such as [Cu$_2$(MeCN)$_3$(μ$_2$-POP)$_2$](PF$_6$)$_2$, [Au$_2$Cl$_2$(μ$_2$-POP)], and [Au$_2$(μ$_2$-POP)$_2$](OTf)$_2$. The photo-optical properties of the PPO- and POP-type transition metal complexes are investigated experimentally using photo-luminescence spectroscopy, whereby the presence of metallophillic interactions was found to play a crucial role. The dinuclear copper complex [Cu$_2$(MeCN)$_3$(μ$_2$-POP)$_2$](PF$_6$)$_2$ shows a very interesting thermochromic behavior and intense photo-luminescence with remarkable phosphoresence lifetimes at 77 K, which can probably be attributed to short intramolecular Cu–Cu distances.