Tuning the Reactivity in Classic Low-Spin d6 Rhenium(I) Tricarbonyl Radiopharmaceutical Synthon by Selective Bidentate Ligand Variation (L,L′-Bid, L,L′ = N,N′, N,O, and O,O′ Donor Atom Sets) in fac-[Re(CO)3(L,L′-Bid)(MeOH)]n Complexes

A range of fac-[Re(CO)(3)(L,L'-Bid)(H2O)](n) (L,L'-Bid = neutral or monoanionic bidentate ligands with varied L,L' donor atoms, N,N', N,O, or O,O': 1,10-phenanthroline, 2,2'-bipydine, 2-picolinate, 2-quinolinate, 2,4-dipicolinate, 2,4-diquinolinate, tribromotropolonate, and hydroxyflavonate; n = 0, +1) has been synthesized and the aqua/methanol substitution has been investigated. The complexes were characterized by UV-vis, IR and NMR spectroscopy and X-ray crystallographic studies of the compounds fac-[Re(CO)(3)(Phen)(H2O)]NO3 center dot 0.5Phen, fac-[Re(CO)(3)(2,4-dQuinH)-(H2O)]center dot H2O, fac-[Re(CO)(3)(2,4-dQuinH)Py]Py, and fac-[Re(CO)(3)(Flav)-(CH3OH)]center dot CH3OH are reported. A four order-of-magnitude of activation for the methanol substitution is induced as manifested by the second order rate constants with (N,N'-Bid) < (N,O-Bid) < (O,O'-Bid). Forward and reverse rate and stability constants from slow and stopped-flow UV/vis measurements (k(1), M-1 s(-1); k(-1), s(-1); K-1, M-1) for bromide anions as entering nucleophile are as follows: fac-[Re(CO)(3)(Phen)(MeOH)](+) (50 +/- 3) X 10(-3), (5.9 +/- 0.3) X 10(-4), 84 +/- 7; fac-[Re(CO)(3)(2,4-dPicoH)(MeOH)] (15.7 +/- 0.2) x 10(-3), (6.3 +/- 0.8) X 10(-4), 25 +/- 3; fac-[Re(CO)(3)(TropBr(3))(MeOH)] (7.06 +/- 0.04) X 10(-2), (4 +/- 1) X 10(-3), 18 +/- 4; fac-[Re(CO)(3)(Flav)(MeOH)] 7.2 +/- 0.3, 3.17 +/- 0.09, 2.5 +/- 2. Activation parameters (Delta H-k1(double dagger), kJmol(-1); Delta S-k1(double dagger) J K-1 mol(-1)) from Eyring plots for entering nucleophiles as indicated are as follows: fac-[Re(CO)(3)(Phen)(MeOH)](+) iodide 70 +/- 1, -35 +/- 3; fac-[Re(CO)(3)(2,4-dPico)(MeOH)] bromide 80.8 +/- 6, -8 +/- 2; fac-[Re(CO)(3)(Flav)(MeOH)] bromide 52 +/- 5, -52 +/- 15. A dissociative interchange mechanism is proposed.