Manganese(ii): the black sheep of the organometallic family

Abstract

The organometallic chemistry of manganese in the +2 oxidation state is distinct from the organometallic chemistry of a 'typical' transition metal due to a significant ionic contribution to the manganese(II)-carbon bonds. The reduced influence of covalency and the 18-electron rule result in organomanganese(II) cyclopentadienyl, alkyl and aryl complexes possessing reactivity and structural diversity that is unique in organotransition metal chemistry. Recently, this unusual reactivity has resulted in a range of novel applications in selective organometallic and organic synthesis, and polymerization catalysis. This tutorial review summarizes key milestones in the development of manganese(II) organometallics and discusses how some of their current synthetic applications have evolved from many fascinating fundamental studies in the area.

Tetralithio Metalla‐aromatics with Two Independent Perpendicular Dilithio Aromatic Rings Spiro‐fused by One Manganese Atom

“…[11] Hence,d ilithio reagents can exhibit different valence states (A, B and C)i nt ransition metal complexes (Scheme 3), which has also been demonstrated in our previous work. In the upper valence region, one can identify five singly occupied spin-up MOs of predominant Mn 3d parentage (> 70 %), and two singly occupied spin-down MOs that are essentially the p*-orbitals of the two butadienyl ligands.T he electronic structure of 2a is thus best interpreted as ah igh-spin Mn II center (S Mn = 5/2) [13] antiferromagnetically coupled to two doublet trianionic radical ligand (S A,B = 1/2), thereby yielding an overall quartet ground state.T herefore,s ignificant electron density resides in the butadienyl p*-LUMO,w hich accounts for the CÀCbond distortions discussed above.More importantly,t he observed large spatial overlaps of the two antiferromagnetic-coupling pathways suggest that the spindown electrons occupied in the butadienyl p*-orbital of Ring A 2a and Ring B 2a strongly interact with the spin-up electrons in the Mn 3d xz and 3d xy orbitals,r espectively.T his notion is supported by the computed exchange coupling constant of as large as 1890 cm À1 .T om aximize such p-type interactions, ap lanar geometry of both rings is preferable.A saconsequence,t here are six electrons in the p-system of each manganacycle,o fw hich five electrons comes from the butadienyl moiety and one from Mn. TheMOdiagram of complex 2a is depicted in Figure 5.…”

Spin crossover in phosphorus- and arsenic-bridged cyclopentadienyl-manganese(ii) dimers

“…(14)-2.437(16) (average 2.400 Å ), and 2.29(3)-2.46(4) Å (average 2.38 Å ), respectively, and hence are similar to those found in high-spin manganese(II) cyclopentadienides. 7 The MnÁ Á ÁMn distances in 1a, 1b and 2 are 3.429(2), 3.431(2) and 3.641(1) Å , respectively (Tables 1, S2). Since the range of manganese-manganese bond lengths in the CSD is 2.170-3.291 Å (average 2.843 Å ), 8 it is unlikely that such bonding occurs in 1 and 2.…”

Silica‐Supported MnII Sites as Efficient Catalysts for Carbonyl Hydroboration, Hydrosilylation, and Transesterification

“…It is thus not surprising that a growing research effort has been directed at developing catalytic processes with non‐noble metals . Although manganese could fill this role due to its low toxicity and relatively high abundance (950 mg kg −1 in the Earth's crust), this element has been rarely used in catalysis because of its often complex coordination chemistry. The most prominent example of its use in catalysis is for the asymmetric epoxidation of alkenes .…”