Redox Site Confinement in Highly Unsymmetric Dimanganese Complexes

Abstract

A set of highly preorganized pyrazolate-bridged dimanganese complexes L(Mn)MnX have been prepared and structurally characterized. They can be described as hybrid organometallic/Werner-type systems that consist of a low-spin CpMn(I)(CO)2 subunit (Mn1) and a proximate tripodal tetradentate {N4} binding pocket accommodating a high-spin Mn(II) ion (Mn2), with Mn...Mn distances of approximately 4.3 A and different coligands bound to Mn2. Density functional theory (DFT) calculations (both the hybrid B3LYP and the pure BP86 functionals and the all-electron basis sets 6-311G and 6-311G*) confirm that the valence alpha and beta Kohn-Sham molecular orbitals (MOs) of these mixed-valent Mn(I)Mn(II) compounds have predominant Mn(3d) character and an almost perfectly localized nature: all five unpaired electrons are essentially localized at the Werner-type Mn2, whereas Mn1 possesses an effective closed-shell structure with the MOs of highest energy centered there. One-electron oxidation occurs in a clean process at approximately E(1/2) = -0.6 V (versus ferrocene/ferrocinium), giving the low-spin/high-spin Mn(II)Mn(II) species. UV/vis and IR spectroelectrochemistry as well as a detailed theoretical analysis reveal that the redox process takes place with strict site control at the organometallic subunit, while it does not significantly influence the spin and charge distribution on the Werner-type site. Positions and shifts of the nu(C[triple bond]O) absorptions are largely reproduced by the DFT calculations. These systems thus represent an exceptional example of the effect the unsymmetry of a dinucleating ligand scaffold has on the spin and charge distribution in homobimetallic complexes and might offer interesting prospects for the study of the cooperative effects of bimetallic arrays.

Organometallic and Classical Coordination Sites in Highly Preorganized Pyrazolate‐Based Hybrid Systems: The Mn/Ni Case

“…IR spectroscopic changes during the gradual oxidation of 3 in dce/nBu 4 Figure 8 and S3, S4). The rise of a similar band at about 400 nm has also been observed upon oxidation of the corresponding Mn/Zn, Mn/Co and Mn/Mn systems [6][7][8] and can thus be safely assigned to the π(pyrazolate) Ǟ CpMn II -(CO) 2 LMCT transition, which is obviously relatively insensitive to the identity of the metal ion within the adjacent Werner-type {N 4 } compartment.…”

“…A comparison of the Mn-Cg (Cg defines the centroid of the ring atoms of the cyclopentadienido-anion), Mn-CO, Mn-N, and CϵO bond lengths of 3-5 and related Mn/Zn, Mn only [6] Mn···Mn [7] Mn···Co [8] Mn···Zn [6] Mn···Ni Mn-Cg Mn/Co, and Mn/Mn complexes, as well as complexes where the {N 4 } site is unoccupied, reveals that the choice of metal in the classical Werner-type site seems to have only a small influence on the bonding parameters of the organometallic CpMn(CO) 2 site (Table 1). Only the CO-Mn-N angles appear to be somewhat flexible and span a range from 94.8°t o 104.1° (Table 1).…”

“…[18] The shift of about 125 cm -1 to higher frequencies is very similar to the shift observed in the corresponding Mn/Zn, Mn/Co and Mn/Mn complexes. [6][7][8] It is typical for a oneunit increase in the oxidation state of the metal ion and reflects the diminished backbonding ability in the oxidized CpMn II (CO) 2 subunit. This confirms that the anodic process in the present Mn/Ni systems is fully localized at the organometallic site, which means that nickel remains in the +II oxidation state.…”

“…The effects at the organometallic CpMn(CO) 2 We have recently reported a series of unsymmetrical homo-and heterobimetallic complexes of type A that formally combine an organometallic CpMn(CO) 2 fragment B and a classical subunit of type C (Scheme 1). [6][7][8] The latter is reminiscent of the tripodal tetradentate tris(pyridylmethyl)amine systems that are well established in mononuclear coordination chemistry. [9] Manganese(II), cobalt(II), or zinc(II) ions with different co-ligands X can be accommodated in the Werner-type compartment of complexes A, with only subtle changes of the spectroscopic and redox properties of the organometallic site.…”

“…[9] Manganese(II), cobalt(II), or zinc(II) ions with different co-ligands X can be accommodated in the Werner-type compartment of complexes A, with only subtle changes of the spectroscopic and redox properties of the organometallic site. [6][7][8] It appears that the two metal ions in these complexes are electronically quite independent. In contrast, strong electronic coupling was observed in the symmetric dimanganese complex D, which was shown to undergo two sequential metal-centered oneelectron oxidations with fast intramolecular thermal electron transfer (k ET ≈ 2.6 ϫ 10 10 s -1 at 298 K) in the mixedvalent Mn I Mn II species, which has a formal low-spin d 5 d 6 electronic configuration.…”