Mn2° core complexes

For both elements the dominant types of complexes in these oxidation states, especially V, are those containing M03+, M02, M2C +, and MN2+ cores, and these have been covered in Sections 18-D-6 and 18-D-7. Thus there is only a little more to add here. 

Several conclusions can be drawn from the model studies. First, it is clear that Mn is just as versatile as Fe or Cu and that Mn complexes can, with appropriate design, be synthesized in a variety of oxidation states and coordination enviromnents. A coimnon feature of the model chemistry is the observation of (/u.-0)(/u.-carboxylate)2Mn2 and (fx-0)2(ii-carboxylate)Mn2 core structures similar to those suggested for Mn catalase. By judicious protonation of the oxo bridge(s), it is possible to vary the Mn reduction potential of these structures over a wide range. 

In Chapter 19, we described many examples of the use of sterically demanding ligands. Reaction 22.57 illustrates how an Mn(I) complex is stabilized by the use of a bulky P-diketiminato ligand. The product of this reaction is the first example of a 3-coordinate, Mn(I) compound. The molecule formally contains an Mn2 core (Mn-Mn = 272 pm) and magnetic data are consistent with a rare example of a high-spin Mn(I) (d ) complex in which there is antiferromagnetic coupling between the metal centres. 

Therefore, we concluded that i) in PSII preparations depleted in extrinsic polypeptides plus part of Mn + addition of exogenous Mn2+ enhances atrazine binding probably contributing partially the native conformation to the PSII core complex and ii) in preparation where 16 and 23 kDa polypeptides are depleted and Ca and Cl" are disorganized, addition of exogenous CaCl2 reduces atrazine inhibition, probably producing barrier in atrazine accessibility to its sites of action in the vicinity of the PSII complex. From the overall discussion... 

For the [Mn2(/i-alkoxo)2(/r-carboxylato)2] core (type II), only the benzoate analogues, [Mn2(spa 3-OMe-spa)2(OBz)2] (97) (where H2spa = 3-salicylideneamino-l-propanol), of the acetate complex reported many years ago has been synthesized and structurally characterized. The Mn "Mn separation in (97) is 2.872 A while the bridging Mn—Oaikoxo distances are 1.896 A and 1.939 A. 

The solution electronic spectra of these complexes are typical of the [Mn2(/u-0)(/u-0Ac)] core and the observed bands are due to d-d transitions. 

Mu2(3,5-Cl2(SALPN))2(OCH3)2] (149) is the first bis(//-alkoxo) dinuclear Mn(III) complex having John-Teller distortions along two of the Mn-OMe bonds. It contains a pseudo C axis with a highly distorted [Mn2(//-OCH3)2] core. The Mn "Mn separation in 149 is 3.19A."" ... 

All the above complexes, 9-12, are isolated from reactions involving gallium or indium metal and [Mn2(CO)10] or [Re2(CO)10] in an autoclave, although 11 has also been obtained by thermolysis of 6. Some details on the reactivity of 11 toward phosphonium halides (23) have been reported and indicate formation of the halo-indium complexes [PPh4]2[Mn2(CO)8 /j-lnX2 2]. A higher nuclearity cluster has also been obtained in the reaction of [Re2(CO)10] with indium metal, viz. [Re4(CO)12 /i3-InRe(CO)5 4], 13 (24,24a). This consists of a central tetrahedral Re4(CO)12 core with each face capped by an InRe(CO)s fragment as shown in Fig. 4. 

Transition metal complexes comprise a typical example. The lone pair orbitals of ligands like CO, H20, Cl-, and O2- experience significant repulsion from the upper core shells 3s and 3p (first transition series). The Pauli repulsion with these shells determines the repulsive wall in the metal-ligand -versus-R curve. Of course, there is also overlap with deeper core orbitals, and so their effect is less important. An illustration is provided elsewhere (Figures 2 and 3 of Ref. 35), where the behavior of the Pauli repulsion is demonstrated along the Mn—O bond distance in MnO. The fragments are Mn2+, which has 5 electrons with spin up in the 3d orbitals, and the 02 cage, which has 5 electrons... 

Examples of tetranuclear adamantane-shaped complexes (Type Iff) are [Mn4 06(bpea)4]" + and its corresponding one-electron rednced species, while cnbane cores (Type TV) are fonnd in [Mn2 Mn2 04(02PPh2)6] and its one-electron rednced species. A large family of tetrannclear clnsters is... 

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