Synthesis, characterisation, nuclease and cytotoxic activity of phosphate-free and phosphate-containing copper $$4^{\prime }$$ 4 ′ -(N-methylpyridinium)- $$2{,}2^{\prime }{:}6^{\prime }{,}2^{\prime \prime }$$ 2 , 2 ′ : 6 ′ , 2 ″ terpyridine complexes