Synthesis, Characterization, and Biological Studies of a Piperidinyl Appended Dipicolylamine Ligand and Its Rhenium Tricarbonyl Complex as Potential Therapeutic Agents for Human Breast Cancer

A novel ligand bearing a central piperidinyl sulfonamide group, N(SO₂pip)dpa, and its corresponding Re tricarbonyl complex, [Re(CO)₃(N(SO₂pip)dpa)]⁺, have been synthesized in good yield. The methylene CH₂ signal seen as a singlet (4.54 ppm) in a ¹H NMR spectrum of the ligand in DMSO-d₆ appears as two doublets (5.39, 5.01 ppm) in a spectrum of the [Re(CO)₃(N(SO₂pip)dpa)]⁺ complex and confirms the presence of magnetically nonequivalent protons upon coordination to Re. Structural results revealed that the Re-N bond lengths fall within the normal range establishing coordination of ligand to metal. The presence of intraligand π → π^⁎ and n → π^⁎ transitions is indicated by the absorption peaks around 200-250 nm in UV-visible spectra. Absorption peaks in UV-visible spectra around 300 nm for metal complexes were identified as MLCT transitions. The S-N stretch observed as a strong peak at 923 cm^-1 for N(SO₂pip)dpa appeared at a shorter frequency, at 830 cm^-1 in an FTIR spectrum of the [Re(CO)₃(N(SO₂pip)dpa)]⁺. The intense fluorescence displayed by the N(SO₂pip)dpa ligand has quenched upon coordination to Re. Relatively low IC₅₀ values given by human breast cancer cells, MCF-7, (N(SO₂pip)dpa = 139 μM, [Re(CO)₃(N(SO₂pip)dpa)]⁺ = 360 μM) indicate that N(SO₂pip)dpa and [Re(CO)₃(N(SO₂pip)dpa)]⁺ are promising novel compounds that can be further investigated on their usage as potential anticancer agents.