Halogen Bonding in Stereoselective Metal Chloride (M-Cl) Bond Activation and Transformation to Metal Triiodide (M-I₃)

The isolation of isomeric complexes trans-[(RuRuLPyHPQ)-L-II(CO)(PPh3)Cl] 2 and cis-[(RuLPyHPQ)-L-II(CO)(PPh3)Cl] 3, (Cl and PPh3 are trans in 2 and they are cis in 3 respectively) as well as [(RuLPyHPQ)-L-II(CO)(PPh3)2Cl] 4 and [(RhLPyHPQ)-L-III(PPh3)Cl-2] 5 disclose the flexidentate nature of the multifunctional ligand, HLPyHPQ 2-pyridylhydrazinophenanthrenequinone, 1. These complexes are maneuvered as molecular templates for understanding the role of halogen bonds (XB) in M-Cl bond activation and subsequent cleavage processes. Both isomeric forms 2 and 3 furnish trans-[(RuLPyHPQ)-L-II(CO)(PPh3)(I-3)] 6 (where PPh3 and I-3(-) in trans position) when they are treated separately with I-2 in a hydrocarbon solvent. Nonetheless, complex 5 is converted to 7 upon similar treatment but 4 remains unreacted. Crystal structure of 7 revealed that it is composed of 62% trans-[(RhLPyHPQ)-L-III(PPh3)Cl(I-3)] and 38% trans-[(RhLPyHPQ)-L-III(PPh3)Cl(ClI2)] (where PPh3 and I-3 -/ClI2- are trans). The I-3 -/ClI2- gets coordinated to metal centers exclusively in an end-on fashion in all cases. Since the valence state of metal centers remains unaltered in due course of the reaction, the emergence of M-I-3/M-ClI2 from M-Cl in the absence of any reductant in solution is indicative of prior activation of M-Cl bond via the formation of Halogen Bond (XB) of type [M-Cl-I-I] in an intermediate step. The geometrical fluctuations around the coordination sphere during the transformation of cis-[(RuLPyHPQ)-L-II(CO)(PPh3)Cl] 3 to trans-[(RuLPyHPQ)-L-II(CO)(PPh3)(I-3)] 6 is reminiscent of the active influence of XB, triggering the isomerization of 3 to 2 owing to steric encumbrance, before it is being converted to 6. Electrochemical behavior and optoelectronic properties of the complexes have been complimented by theoretical analysis and the structures were characterized by SCXRD. Repeated pi-stacking motifs in the crystalline phase architecture of the complexes have been scrutinized using DFT calculations.