Cyclometallated Platinum(II) Complexes with Small Crown Ether Rings: Appropriate Choice of the Bridging Diphosphane to Coordinate Potassium Cations

This account reports the synthesis and structural characterization
of the first cyclometallated platinum(II) complex that coordinates
a potassium cation in a sandwich arrangement via two 15-crown-5 ether
rings within the same molecule. The cooperation of the two small crown
ether moieties allows the entrapment of the non-ideal potassium ion.
The reaction of the parent thiosemicarbazone ligand 3,4-(C8H16O5)C6H3C(Me)NN–(H)C(S)NHMe, 1, containing the crown ether ring, with K2[PtCl4], or alternatively with PtCl2(DMSO)2, and subsequent treatment with the diphosphanes Ph2PCH2PPh2 (dppm) and Ph2PC(CH2)PPh2 (vdpp) produced the double nuclear platinacycles 3a, 3b, and 4, probably via formation
of the 2a and 2b intermediates. Complex 3a with the K+ cation in a sandwich coordination
was slightly mixed with 3b lacking any K+.
Alternatively, reaction of 1 with K2[PtCl4]
or with PtCl2(DMSO)2 followed by the diphosphane
Ph2PC(CH2)PPh2 (vdpp) only
gave the dinuclear phosphane-bridged compound 4; this
highlights the importance of choosing the right diphosphane ligand.
Density functional theory calculations (B3LYP-D3/LANL2DZ-ECP-6.311++G**)
revealed similar affinities for both dppm and vdpp derivatives to
coordinate potassium cations. Crystal structure analysis was performed
for compounds 3a and 4.