Pyrrolidinecarbodithioate as a planarity chuck in the search for cis-platin analogues of nickel: Spectral, single crystal X-ray structural, BVS, and CSM analysis of some planar nickel(II) mixed ligand complexes

Abstract

[Ni(pyrdtc)(PPh₃)(NCS)] (1), [Ni(pyrdtc)(4-MP)(NCS)] (2), [Ni(pyrdtc)(PPh₃)(CN)]·H₂O (3), [Ni(pyrdtc)(PPh₃)₂]ClO₄ (4), and [Ni(pyrdtc)(P͡P)]BPh₄·2H₂O (5) [where pyrdtc: Pyrrolidine carbodithioate/S͡S, PPh₃: Triphenylphosphine, 4-MP: Tri(4-methylphenyl)phosphine, dppe/P͡P: 1,2-Bis(diphenylphosphino)ethane] have been prepared from the parent bis-dithiocarbamate, [Ni(pyrdtc)]₂ (6). The prepared compounds were characterized by electronic, IR, ¹H, ¹³C, and ³¹P NMR spectra. In the IR spectra of the compounds, thioureide bands are observed at higher wavenumbers for the mixed ligand complexes 1-5 (1528-1540 cm^-1) than the parent compound (1490 cm^-1). Cyclic voltammetry showed an increasing order of reduction potentials: 5 << 1 ~ 2 < 3 < 4 << [Ni(pyrdtc)₂] indicating an alleviation of electron density on nickel in the mixed complexes compared to the parent compound. Single crystal X-ray structure of the complexes displayed planar geometry around nickel which is in keeping with their diamagnetism. Bond Valence Sums calculated with the corrected R_ij indicated the divalent nature of nickel with predominant covalent interactions. Continuous shape measure analysis of the mixed ligand chromophores stipulates a planar square environment around central nickel atom and deviation to tetrahedral or trigonal bipyramidal variants are absolutely negated. In this study, CSM analysis of cis-platin, a clinical anti-cancer agent, showed a comparable shape measure as those of the mixed ligand complexes 1-5. Hence, pyrrolidinecarbodithioate acts as a ‘chuck’ in compounds 1-5 to stabilize the planar square shape of the nickel chromophores and provides a suitable template to synthesize analogues of cis-platin.