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

Abstract

The five-coordinate anion [Mn(CO) 3(DBCat)] has been found to undergo nucleophilic CO substitution reactions with L  PR 4, P(OR) 4 producing the dicarbonyl derivatives [Mn(CO) 2(L)(DBCat)]. The reaction yield is higher for better nucleophiles such as L  PEt 4 and P(OEt) 4: the latter species reacts quantitatively even when added in an equivalent amount. Contrary to this, bulky phosphines with the cone angle 0 ≥ 145°, namely L  PPh 4, PCy 3, do not show this reaction but interact reversibly with [Mn(CO) 4(DBCat)] at low temperatures to give the six-coordinate adducts [Mn(CO) 4(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) 4(L)(DBCat · BF 4] (L  THF, PPh 4) and [Min(CO) 4(PPh 4)(DBCat · CS 2)] which are inherently stable at ambient temperature. This reactivity is closely related to the DBCat-localized le oxidation of [Mn(CO) 4(DBCat)] producing six-coordinate radicals [Mn(CO) 4(L)(DBSQ)]. The electrophilic attack of CS 2 at the DBCat ligand has also been documented by the conversion of [Mn(CO) 4(DBCat)] in the absence of PPh 4 to the trithiocarbonate complex [Mn(CO) 4(S 2CS)] and uncoordinated 4,6-di-tert-butyl-1,3-benzdioxol-2-thione.