Abstract

Reaction of 2-mercapto-1-methylimidazole (methimazole) with tris(dimethylamino)borane, B(NMe₂)₃, provides the tetrahedral dimethylamine adduct of tris(methimazolyl)borane, [(Me₂HN)B(methimazolyl)₃]. By contrast, imidazole, 2-methylimidazole, 2-chloroimidazole and benzimidazole provide the homoleptic tetra-azolyl systems H[B(azolyl)₄], and the same product is obtained even when a substoichiometric quantity of the heterocyle is employed. The change in reaction outcome is correlated with the variation of basic pK_a for the heterocycles. A simple acid-base reaction with elimination of HNMe₂ is proposed for the reaction with the weakly basic, but more strongly acidic, methimazole. However, for the more strongly basic imidazoles, initial coordination of the heterocycle imine nitrogen to the weakly Lewis acidic boron centre in B(NMe₂)₃ to form the tetrahedral adduct [(azole)B(NMe₂)₃] is proposed. The greater availability of the NMe₂ lone pairs in this species results in increased basicity and a rapid reaction with further heterocycle to provide the observed H[B(azolyl)₄] products. For 2-nitroimidazole, the low basicity (and increased NH acidity) results in the formation of [(HNMe₂)B(2-nitroimidazolyl)₃] on reaction with B(NMe₂)₃, analogous to the product formed with methimazole. Both [(HNMe₂)B(methimazolyl)₃] and H[B(benzimidazolyl)₄] have been structurally characterised by single crystal X-ray crystallography. This chemistry has been exploited to provide a new synthesis of borate-centred tripod ligands, whereby N-methylimidazole is used to activate B(NMe₂)₃ to reaction with methimazole to form the new ligand [(N-methylimidazole)B(methimazolyl)₃] in good yield and a complex of this ligand with Ru^II has been structurally characterised.

A new high-yield synthesis of [(PhCH₂)₂Mg(thf)₂] and [{(PhCH₂)CH₃Mg(thf)}₂] via benzylpotassium has allowed a simple entry into benzylmagnesium coordination chemistry. The syntheses and X-ray crystal structures of both [(η²-Me₂NCH₂CH₂NMe₂)Mg(CH₂Ph)₂] and [η²-HC{C(CH₃)NAr′}₂Mg(CH₂Ph)(thf)] (Ar′=2,6-diisopropylphenyl) are reported. The latter β-diketiminate complex reacts with dioxygen to provide a 1:2 mixture of dimeric benzylperoxo and benzyloxo complexes. The benzylperoxo complex [{η²-HC{C(CH₃)NAr′}₂Mg(μ-η²:η¹-OOCH₂Ph)}₂] is the first example of a structurally characterised Group 2 metal–alkylperoxo complex and contains the benzylperoxo ligands in an unusual μ-η²:η¹-coordination mode, linking the two five-coordinate magnesium centres. The OO separation in the benzylperoxo ligands is 1.44(2) Å. Reaction of the benzylperoxo/benzyloxo complex mixture with further [η²-HC{C(CH₃)NAr′}₂Mg(CH₂Ph)(thf)] results in complete conversion of the benzylperoxo species into the benzyloxo complex. This reaction, therefore, establishes the cleavage of dioxygen by this system as a two-step process that involves initial oxygen insertion into the MgCH₂Ph bond followed by OO/MgC σ-bond metathesis of the resulting benzylperoxo ligand with a second MgCH₂Ph bond. The formation of a 1:2 mixture of the benzylperoxo and benzyloxo species indicates that the rate of the insertion is faster than that of the metathesis, and this is shown to be consistent with a radical mechanism for the insertion process.

The cyclic hexanuclear species [{HC[C(tBu)NAr′]₂Mg(C₃H₅)}₆] (see structure) in which the allyl group exhibits μ-η¹:η¹ bonding is formed on thermal treatment of [HC{C(tBu)NAr′}₂Mg(C₃H₅)(thf)] under vacuum. Ar′=2,6-diisopropylphenyl.

Die cyclische sechskernige Verbindung [{HC[C(tBu)NAr′]₂Mg(C₃H₅)}₆] (siehe Struktur) mit einer μ-η¹:η¹-gebundenen Allylgruppe erhält man durch Erhitzen von [HC{C(tBu)NAr′}₂Mg(C₃H₅)(thf)] unter Vakuum. Ar′=2,6-Diisopropylphenyl.