Nucleophilic attack at the five-coordinate anion [Mn(CO)3(3,5-di-tert-butyl-catecholate)]- controlled by electronic and steric effects

Article abstract of DOI:10.1016/S0020-1693(97)05627-2

The five-coordinate anion [Mn(CO)₃(DBCat)]^- has been found to undergo nucleophilic CO substitution reactions with L = PR₃, P(OR)₃ producing the dicarbonyl derivatives [Mn(CO)₂(L)(DBCat)]^-. The reaction yield is higher for better nucleophiles such as L = PEt₃ and P(OEt)₃; the latter species reacts quantitatively even when added in an equivalent amount. Contrary to this, bulky phosphines with the cone angle θ≥ 145°, namely L = PPh₃, PCy₃, do not show this reaction but interact reversibly with [Mn(CO)₃(DBCat)]^- at low temperatures to give the six-coordinate adducts [Mn(CO)₃(DBCat) · L]^-. For L = pyridine the corresponding adduct is already formed at room temperature. Similarly, [Mn(CO)₂(P(OEt)₃)(DBCat)]^- takes up reversibly P(OEt)₃. The spectroscopic and redox properties of the adducts closely correspond with those of the invariably six-coordinate anions [Re(CO)_4-n(L)_n(DBCat)]^- (n = 1, 2). The uptake of a Lewis base by [Mn(CO)₃(DBCat)]^- is facilitated by an electrophilic attack at the oxygen lone pairs of the σ,π-donor DBCat ligand. Examples are the anions [Mn(CO)₃(L)(DBCat·BF₃)]^- (L = THF, PPh₃) and [Mn(CO)₃(PPh₃) (DBCat·CS₂)]^- which are inherently stable at ambient temperature. This reactivity is closely related to the DBCat-localized le oxidation of [Mn(CO)₃(DBCat)]^- producing six-coordinate radical s [Mn(CO)₃(L)(DBSQ)]. The electrophilic attack of CS₂ at the DBCat ligand has also been documented by the conversion of [Mn(CO)₃(DBCat)]^- in the absence of PPh₃ to the trithiocarbonate complex [Mn(CO)₄(S₂CS)]^- and uncoordinated 4,6-di-tert-butyl-1,3-benzdioxol-2-thione. 1998 Elsevier Science S.A.