Activation of the manganese(i) tricarbonyl core by selective variation of bidentate ligands (L,L′-Bid = N,N′ and N,O donor atom sets) in fac-[Mn(CO)3(L,L′-Bid)(CH3OH)]n complexes

Abstract

A range of fac-[Mn(CO)₃(L,L′-Bid)(H₂O)]ⁿ (L,L′-Bid = neutral or monoanionic bidentate ligands with varied L,L′ donor atoms, N,N′ and N,O, 1,10-phenanthroline, 2,2′-bipyridine, 2-picolinate, 2,4-quinolinate; n = 0, +1) has been synthesized and the methanol substitution has been investigated for the first time. The complexes were characterized by UV/vis, IR and NMR spectroscopy and X-ray crystallographic studies of the compounds fac-[Mn(CO)₃(Bipy)(H₂O)][CF₃SO₃] (1) and fac-[Mn(CO)₃(Phen)(H₂O)][CF₃SO₃] (6) are reported. A two 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). Forward and reverse rate and stability constants from slow and stopped-flow UV/vis measurements (k₁, M⁻¹ s⁻¹; k₋₁, s⁻¹; K₁, M⁻¹) for pyridine as entering nucleophile are as follows: fac-[Mn(CO)₃(Phen)(CH₃OH)]⁺ (2.39 ± 5) × 10⁻³, (1.5 ± 0.3) × 10⁻⁵, 159 ± 32; fac-[Mn(CO)₃(2,4-QuinH)(CH₃OH)] (4.5 ± 0.2), (4 ± 1) × 10⁻², 113 ± 29. Activation parameters (ΔH^≠_k1, kJ mol⁻¹; ΔS^≠_k1, J K⁻¹ mol⁻¹) from Eyring plots for entering nucleophiles as indicated are as follows: fac-[Mn(CO)₃(Phen)(CH₃OH)]⁺ (bromide ions) 66.7 ± 0.6, −27 ± 2; (pyridine) 80 ± 3, −25 ± 11; fac-[Mn(CO)₃(Pico)(CH₃OH)] (bromide ions) 68 ± 2, −24 ± 5. A dissociative interchange mechanism is proposed.