Formation of a metal-to-nitrogen bond of normal length by a neutral sufonamide group within a tridentate ligand. A new approach to radiopharmaceutical bioconjugation

Article abstract of DOI:10.1021/ic302180t

We demonstrate that a tertiary sulfonamide group, N(SO₂R) R′₂, can rehybridize to form a M-N bond of normal length even when the group is in a linear tridentate ligand, such as in the new tridentate N(SO₂R)dpa ligands derived from di-(2-picolyl)amine (N(H)dpa). N(SO₂R)dpa ligands were used to prepare fac-[Re(CO) ₃(N(SO₂R)dpa)](PF₆ or BF₄) complexes. Structural characterization of the new complexes established that the tertiary sulfonamide nitrogen atom binds to Re with concomitant sp ²-to-sp³ rehybridization, facilitating facial coordination. The new fac-[Re(CO)₃(N(SO₂R)dpa)]X structures provide the only examples for any metal with the sulfonamide as part of a noncyclic linear tridentate ligand and with a normal metal-to- nitrogen(tertiary sulfonamide) bond length. Rare previous examples of such normal M-N bonds have been found only in more constrained situations, such as with tripodal tetradentate ligands. Our long-term objectives for the new tridentate N(SO₂R)dpa ligands are to develop the fundamental chemistry relevant to the eventual use of the fac-[M^I(CO) ₃]⁺ core (M = ^99mTc, ^186/188Re) in imaging and therapy. The sulfonamide group uniquely contributes to two of our goals: expanding ways to conjugate the fac-[M^I(CO)₃] ⁺ core to biological molecules and also developing new symmetrical tridentate ligands that can coordinate facially to this core. Tests of our conjugation method, conducted by linking the fac-[Re^I(CO) ₃]⁺ core to a new tetraarylporphyrin (T(N(SO ₂C₆H₄)dpa)P) as well as to a dansyl (5-(dimethylamino)naphthalene-1-sulfonyl) group, demonstrate that large molecular fragments can be tethered via a coordinated tertiary sulfonamide linkage to this core.