Theoretical analysis of the [Mn2(μ-oxo)2(μ-carboxylato)2]+ core

The first example of a dinuclear manganese complex containing two oxo and two carboxylate bridges, [Mn2(μ-O)2(μ-O2CArTol)2(bpy)2]+ (where bpy = 2,2′-bipyridine, and ArTolCO2 = 2,6-di(p–tolyl)benzoate), was reported recently (J. Am. Chem. Soc. 2003, 125, 13010).

X-ray crystallographic analysis performed on this complex reveals a trapped mix-valence species as evidenced, for example, by very different metal–ligand bond distances at the MnIII and MnIV centers.

The fact that there are rather bulky bridging carboxylate ligands present in this recently reported dinuclear species raises the question as to whether they affect the extent of valence trapping and the metrical parameters in general.

Specifically, it was thought that intramolecular nonbonded contacts could play an important role.

In the work reported here, density functional theory calculations were used to address this issue.

Structural parameters obtained from calculations on a model compound bearing sterically small bridging carboxylates, [Mn2(μ-O)2(μ-O2CH)2(bpy)2]+, are in good agreement with the experimentally determined single crystal X-ray structure.

Thus, the sterically large carboxylate bridges in [Mn2(μ-O)2(μ-O2CArTol)2(bpy)2]+ appear not to have a significant effect on the metal–ligand bond distances and angles.

There is calculated to be minimal Mn⋯Mn bonding despite contraction of the Mn⋯Mn distance relative to related complexes.

In addition to calculations on the mixed-valence MnIIIMnIV complex, various electronic configurations of the corresponding MnIIIMnIII and MnIVMnIV complexes are explored.

Although our calculations support assignment of [Mn2(μ-O)2(μ-O2CH)2(bpy)2]+ as a valence-trapped MnIIIMnIV configuration involving high-spin MnIII, a delocalized configuration arising from low-spin MnIII is calculated to lie very close in energy.

The energetic proximity of the delocalized configuration is attributed to an effective crossed-exchange mechanism, which permits mixing of an eg-based orbital (nominally on high-spin MnIII) with a vacant t2g-based orbital (nominally on MnIV).


Metalloenzymes containing manganese at their active sites are plentiful.

Manganese-dependent ribonucleotide reductase, C. ammoniagenes RRase1,2 and superoxide dismutase3 were shown to contain a single Mn ion at the reaction center, whereas dimanganese active sites are found in bacterial catalase,4,5 rat liver arginase, aminopeptidase P and in dinitrogen reductase-activating glycohydrolase.6

An oxo-bridged tetramanganese cluster (Mn4), present at the active site of Photosystem II (PSII) Water Oxidase (WO), catalyzes the light induced water-to-oxygen conversion, the process that is responsible for existence of aerobic life on the earth.7–13

In the catalytic process the Mn4 cluster cycles through five intermediate states, known as Si (i = 0–4) states, oxygen being evolved during the S3 to S0 state conversion via the transient S4 state.

Structural information regarding the Mn4 cluster is unclear and contentious at this point, mainly due to the lack of high resolution X-ray crystal data.

Electron density for metal ions, assigned as four Mn atoms, has been located recently in the crystal structures of the dark-adapted S1 state of Thermosynechococcus elongates and Thermosynechococcus vulcanus at resolutions of 3.5–3.8 Å.14–16

However, the assignments greatly differ in the most recent 3.5 Å structures,16 where a Mn3Ca cubane core, linked to a mono-oxo-bridged Mn center has been proposed for the WO catalyst.

In contrast, in the previously reported two structures, three Mn atoms were roughly placed at three corners of an isosceles triangle and the fourth Mn was found to be at the center of the triangle obtruding towards the lumenal surface of the membrane.14,15

Locations of the essential cofactors, calcium and chloride ions, have not been resolved beyond doubt in these latter structures.

In addition, these aforementioned studies do not address the questions regarding the structural properties of the Mn4 cluster at the higher S states.

However, in the absence of sufficient crystallographic information the Mn4 species has been conjectured in all of its kinetically accessible oxidation states (S0 to S3) with the aid of spectroscopic modes, most commonly X-ray absorption fine structure (EXAFS) and electron paramagnetic resonance (EPR) techniques.9,17–21

Based on the findings from these studies and manganese oxide mineral structures, several possible arrangements comprising oxo-bridged dinuclear and trinuclear structural motifs have been put forward for the different S states of the PSII Mn4 cluster.9,22

In contrast, the structures of non-heme manganese-containing catalases have been determined at high resolution.5,23

Catalase promotes the disproportionation of toxic H2O2 in aerobic organisms to H2O and O2.24

The dimanganese center in this enzyme exists in four oxidation states: reduced MnII2, mixed-valent MnIIMnIII, oxidized MnIII2, and super-oxidized MnIIIMnIV forms.

The Mn⋯Mn separations were found to be 3.13 Å in the reduced form,23 and 3.03 Å in the oxidized form,25 as deduced from X-ray crystallography.

The super-oxidized form has yet to be characterized crystallographically; however, EXAFS data suggests a 2.70 Å distance between the manganese centers.26

Additionally, X-ray absorption, EPR and electronic spectroscopic studies on the Mn2III and MnIIIMnIV states have garnered a great deal of knowledge that aids the biomimetic studies of this enzyme to a great extent.24,26–28

Most of these aforementioned multinuclear manganoenzymes harbor carboxylate-bridged Mn centers along with histidine N-donor ligands.

Many synthetic efforts aimed at obtaining suitable structural replicas for these enzyme active sites, especially catalase and the PSII WO, resulted in the isolation of a number of di-, tri-, and tetranuclear complexes supported largely by pyridine-based N-donor and commercially available carboxylate ligands.

Among them, dimanganese complexes containing [Mn2(μ-O)2], [Mn2(μ-O)2(μ-carboxylato)] and [Mn2(μ-O)(μ-carboxylato)2] cores are common, [Mn2(μ-O)2(bpy)4](ClO4)3 (1) being the first member in this family.13

Some of the synthetic complexes are closely relevant to the dinuclear manganese enzymes, yet none have been shown to replicate the spectroscopic features of the PSII tetramanganese cluster.

However, a dinuclear complex where the metal centers are bridged simultaneously by a pair of oxo groups and a pair of carboxylato groups was unknown in transition metal chemistry until recently.

We reported the first example of such a case in the complex, [Mn2(μ-O)2(μ-O2CArTol)2(bpy)2]+ (where bpy = 2,2′-bipyridine, and ArTolCO2 = 2,6-di(p–tolyl)benzoate) (2), which incorporates a sterically hindering carboxylate ligand.29

Complex 2 was obtained by ligand substitution on 1.

The di-oxo di-carboxylato core discussed herein could conceivably be present as a segment of the PSII active site.

Furthermore, since one of the glutamate residues (glu 178) is close to the dimanganese site in the Lactobacillus plantarum structure, there exists the possibility that, in another super-oxidized form, it joins glu 66 in spanning the dimanganese core.5

If this were the case, the native core would resemble that of 2.

Complex 2 has been structurally characterized with X-ray crystallography.

The [Mn2(μ-O)2(bpy)2] moiety in 2 is approximately planar, as shown in Fig. 1, while two carboxylate ligands bridge between Mn centers, one from above and one from below this plane.

The MnIII and MnIV centers are found to be valence-trapped in 2 in the solid state.

The Mn–Ocarboxylate bond lengths for the MnIII center are markedly longer than those of the MnIV center, av 2.317 Å vs. av 1.943 Å, respectively, as they are along the Jahn–Teller axis of elongation for the MnIII center.

Other dinuclear Mn(iii, iv) complexes exhibit valence trapping in the solid state, however the Mn-axial ligand bond distances are not so dramatically different.

Complex 2 has a Mn⋯Mn distance of 2.505(1) Å and Mn–O–Mn angles averaging 88.3°, the shortest distance and smallest angles reported for any dimanganese species containing the [Mn2(μ-O)2] core to date.

The fourth bridging group between Mn centers is likely responsible for the resulting unusual distance and angles.

Several previous theoretical studies on oxygen-bridged MnIIIMnIV complexes, with a variety of bridging motifs, have been reported.

For purposes of structural comparison, we have previously reported Becke–Perdew (BP) calculations on such complexes featuring the cores Mn2(μ-O)2,30 Mn2(μ-O)2(O2CH), and Mn2(μ-O)(O2CH)2,31 for which Mn–Mn distances of 2.783, 2.667, and 3.269 Å were obtained.

Results from studies by other groups,32,33 on di-μ-oxo-bridged MnIIIMnIV complexes, have yielded broadly similar structural results to our own,30 while a study by Noodleman and co-workers34 on a complex featuring a Mn2(μ-O)2(O2CH) core with an additional bridging RHNCH2CH2NHR ligand, possessed a Mn–Mn distance ranging from 2.55 to 2.62 Å depending on the level of theory employed.

Here, we report detailed density functional theory calculations performed on this newly formed dimanganese core in order to determine the electronic and steric effects of the sterically encumbering carboxylate ligands.

The MnIII and MnIV centers are found to be strongly antiferromagnetically coupled in all other dinuclear complexes.13,35–40

We report the outcome of calculations on the [Mn2(μ-O)2(μ-O2CH)2(bpy)2]+ core to predict the nature of the intramolecular exchange interaction between the manganese centers.

Theoretical methods

Density functional theory calculations were performed on Linux-based Pentium IV computers using the Amsterdam Density Functional (ADF) program, version ADF 2002.03,41 developed by Baerends et al.42,43

Calculations on [Mn2(μ-O)2(μ-O2CH)2(bpy)2] structures, in various charge states, were performed in C2v symmetry (or, in some cases as identified, in higher symmetry) using the Becke–Perdew (BP) gradient-corrected density functional approach.44,45

Slater orbital basis sets used in all calculations were of triple-ξ quality (TZP).

Electrons in orbitals up to and including 1s {C, N, O} and 2p {Mn} were treated in accordance with the frozen-core approximation.

Optimized geometries were obtained using the gradient algorithm of Versluis and Ziegler.46

All calculations were performed in a spin-unrestricted fashion.

For calculations on antiferromagnetically coupled structures, the broken symmetry (BS) approach of Noodleman47 was affected using the ModifyStartPotential key to introduce an initial spin inhomogeneity between the two metal atoms.

Results and discussion

Density functional theory calculations on the model compound [Mn2(μ-O)2(μ-O2CH)2(bpy)2]+ reproduce the crystallographic structure very satisfactorily.

Our geometry optimization on the S = 1/2 ‘broken symmetry’ (BS) state of the model compound, in C2v symmetry, yields a structure for which all of the key bond lengths and bond angles, summarized in Table 1 and Fig. 2, agree with the crystallographic data to within ±2.5%.

In particular, the BS calculation demonstrates that the asymmetry in bond lengths between Mn atoms and the carboxylate O atoms arises through electronic effects rather than through steric considerations, since the bulky carboxylate substituent of the crystallographically isolated species is replaced with H in our model compound.

The important structural features of the BS geometry are also evident in the geometry optimized for the S = 7/2 state in C2v symmetry, although the Mn–Mn bond length found in this ferromagnetically coupled structure is over 0.1 Å longer than the experimental value.

In addition to these C2v symmetry optimizations, calculations were also pursued on moderately distorted geometries in Cs symmetry (with the mirror plane coincident with the carboxylate ligands and Mn atoms).

These optimizations commenced in Cs symmetry but spontaneously converged towards the C2v optimized geometries for both the S = 1/2 and S = 7/2 states.

In contrast, calculations on the S = 7/2 state in D2h symmetry led to a structure with enforced delocalization of all of the Mn-based electrons.

As well as (understandably) failing to reproduce the observed intermetallic asymmetry, the S = 7/2 D2h calculations yielded a total energy markedly higher than either the S = 1/2 or S = 7/2 C2v optimized geometries (see Table 2), thereby underlining the energetic preference for a ‘valence-trapped’ d3d4 (MnIVMnIII) configuration rather than a ‘delocalized’ d3.5d3.5 configuration.

The S = 7/2 C2v structure is necessarily high-spin on MnIII; we have investigated also a S = 5/2 C2v configuration, which corresponds to ferromagnetically-coupled MnIV with low-spin MnIII.

The latter calculation spontaneously converges to a structure of D2h symmetry, indicating that in this mode of coupling a delocalized configuration is preferred.

The total energy for this configuration is only 14 kJ mol−1 above that of the S = 1/2 C2v (BS) optimized geometry.

We have also explored a D2h configuration for an unnaturally symmetric S = 1/2 species with a delocalized α-spin electron combined with α-spin versus β-spin dominance of the two metal centers.

While the latter species does not describe a tenable solution to the MnIIIMnIV complex’s electronic configuration, its relative energy (only 19 kJ mol−1 above that of the S = 1/2 C2v (BS) optimized geometry) emphasizes that the energetic preference for the C2v geometry, with distinctly different coordination environments around the two metal atoms, is really rather slight.

In addition to calculations on the mixed-valence MnIIIMnIV complex, we have also explored various electronic configurations of the corresponding MnIIIMnIII and MnIVMnIV complexes.

These calculations allow direct comparison with the results of DFT studies on Mn2 complexes with bridging motifs such as (μ-O)3, (μ-O)2(μ-O2), and (μ-O)(μ-O2) which have been investigated in the MnIIIMnIII and /or MnIVMnIV oxidation states, but not in the MnIIIMnIV configuration.48–50

Analysis of the optimized structures from those studies and from our earlier examples of μ-oxo-bridged MnIIIMnIV complexes,30,31 leads to the conclusion that the (μ-O)2(μ-O2CR)2 bridging combination leads innately to a Mn–Mn distance which is markedly smaller than that found for any of the bridging motifs yet explored except for that of MnIV(μ-O)3MnIV,49 for which our calculated metal–metal separation of 2.320 Å is only 0.1 Å shorter than the corresponding distance in the MnIVMnIV structure explored in the present work.

In particular, it is apparent that removal of either one μ-oxo, or of one μ-carboxylato, bridge from the Mn(μ-O)2(μ-O2CH)2Mn core leads to a considerable lengthening of the metal–metal axis regardless of the oxidation states on the two Mn atoms.

This tendency is fully consistent with comparisons between the crystalline geometry of 2 and of related Mn2(μ-O)2 and Mn2(μ-O)i(μ-O2CR)j (i + j = 3) complexes, as noted above.

Total bond energies and relative energies, for the various electronic configurations of [Mn2(μ-O)2(μ-O2CH)2(bpy)2]n+ (n = 0, 1, 2) are given in Table 2.

For the d4d4 (MnIIIMnIII) dinuclear complex, valid single-determinant descriptions are possible for BS (MS = 0) and for S = 1, 2, 3, and 4.

The S = 4 configuration explored corresponds to a ferromagnetically coupled, high-spin MnIII dimer, while the S = 2 configuration consists of two low-spin MnIII centers which are ferromagnetically coupled.

Mixed high-spin/low-spin dimers are represented in the S = 1 and S = 3 configurations which are, respectively, antiferromagnetically and ferromagnetically coupled, while the MS = 0 (broken symmetry, BS) configuration is an antiferromagnetic coupling of equivalent MnIII centers.

Within the broken symmetry framework, each MnIII center in the BS configuration can be either high-spin or low-spin.

Our calculations show that there is a considerable energetic preference for low-spin MnIII, which contrasts with the results of our previous calculations on di-μ-oxo-bridged (and mixed oxo-/carboxylato-bridged) MnIII dimers for which high-spin MnIII is preferred.30,31

For the d3d3 (MnIVMnIV) dinuclear complex, we have explored only the MS = 0 (BS) and S = 3 configurations which correspond, respectively, to antiferromagnetic and ferromagnetic coupling of two MnIV centers.

Again, the antiferromagnetic broken-symmetry solution is the preferred (lower-energy) description of this dinuclear complex.

It is also worthwhile to compare the geometries of the homonuclear MnIIIMnIII and MnIVMnIV complexes with that of the broken-symmetry MnIIIMnIV minimum; the central bond lengths and bond angles for all of these structures are included in Table 1.

There is very good agreement between the calculated MnIIIMnIV geometry and the optimized MnIVMnIV structures, insofar as the coordination around the MnIV center (MnB) of the mixed-valence complex is concerned.

However, the agreement between geometric parameters featuring MnA (the identified MnIII center of the mixed-valence complex) and the MnIII centers in the broken-symmetry MnIIIMnIII geometry is much poorer.

Better general agreement with the geometry surrounding MnA is seen for the ferromagnetically coupled, high-spin (S = 4) MnIIIMnIII structure also summarized in Table 1.

Note, however, that all of the ferromagnetically-coupled structures detailed in Table 1 have metal–metal separations which significantly exceed those of the antiferromagnetically-coupled (broken-symmetry) minima, and of the crystallographically-isolated complex.

How can we rationalize the contraction in the Mn–Mn separation when the coupling mode is switched from ferromagnetic to antiferromagnetic?

Furthermore, what can be established for the lowest-energy electronic configuration for the MnIIIMnIV species, namely the BS (S = 1/2) C2v geometry?

A detailed perusal of the composition of the metal-based molecular orbitals (MOs) in the BS geometry of the MnIIIMnIV complex (see Tables 3–5) reveals that none of the magnetic electrons is strongly delocalized between both metal centers.

Instead, the 39a1 (↑), 17b2 (↑), and 10a2 (↑) MOs show strong mixing between MnA and the oxo bridges, with the corresponding spin-down MOs similarly mixed between MnB and the oxo bridges.

The remaining occupied valence metal-based orbital, 40a1 (↑), shows a modest degree (14%) of MnB character but is principally identifiable as MnA with mixing from the immediately attached oxygens of the carboxylate ligands.

The intermetallic interaction here is an example of ‘crossed exchange’ involving the Jx2y2/z2 pathway, resulting in transfer of charge from a majority-spin, eg-derived orbital on MnIII (here dx2y2) to a minority-spin, t2g-derived orbital on MnIV (here dz2).

It is tempting to alternatively interpret the mixture of α-spin MnA and MnB content in the 40a1 orbital, with MnA dominant, as an indication of the balance between the ‘high-spin MnIII’ d4d3 (MnIIIMnIV) description of this state, in which the excess α-spin electron is localized on α-spin-dominant MnA, and the ‘low-spin MnIII’ d3d4 (MnIVMnIII) configuration with the excess α-spin electron localized on otherwise β-spin-dominant MnB.

We conclude that the BS minimum for the MnIIIMnIV complex is best described as an antiferromagnetic coupling between high-spin MnIII and MnIV, with a modest degree of delocalization between metal atoms being evident in only one occupied valence molecular orbital.

This is broadly consistent with previous studies, both experimental8,13 and theoretical,31–34,48–50 which generally find MnIIIMnIV complexes featuring oxo bridges to adopt antiferromagnetically coupled configurations with some degree of valence trapping.

This analysis of the MO composition from the S = 1/2 structure suggests that the direct Mn–Mn bonding contribution to the bond length is minor compared to the constraints of the oxo and carboxylato bridges.

Composition of the occupied 39a1, 10a2, and 17b2 orbitals, as detailed in Table 3, clearly shows that the magnetic exchange between metal atoms is dominated by superexchange via the oxo bridges, in keeping with the results of our earlier investigations of di-30 and tri-μ-oxo-49 and mixed oxo- and carboxylato-bridged31 MnIIIMnIV dinuclear complexes.

In fact, the di-μ-oxo-bridged species [(NH3)4MnIII(μ-O)2MnIV(NH3)4]3+, which we have previously studied,30 is seen to bear a very close resemblance to the species under investigation in the present work.

This resemblance is reflected both structurally, in the elongation of bonds from the MnIII atom to the μ-oxo bridges, and the more pronounced elongation of bonds from MnIII to the axially-coordinated atoms (respectively, the N of NH3, or the O of the carboxylate bridge), and in the electronic configuration determined for the S = 1/2 broken symmetry state.

Consistent trends are also seen when we compare the present results with those of our study on MnIII(μ-O)2(μ-O2CH)MnIV and MnIII(μ-O)(μ-O2CH)2MnIV complexes:31 in each case, the BS (S = 1/2) complex can be described as a largely valence-trapped structure consisting of high-spin MnIII antiferromagnetically coupled to MnIV, although the MnIII(μ-O)(μ-O2CH)2MnIV core is rather more electronically complex and displays configurational mixing from low-spin MnIII.

The mixed μ-oxo/μ-carboxylato bridged complexes also invariably display markedly longer bonds for MnIII–O than for MnIV–O (whether to oxo or carboxylato O atoms), but the elongation of the MnIII–O (carboxylato) bonds within the present S = 1/2 complex (bond lengths of 2.27 Å are 0.3 Å longer than the corresponding MnIV–O distances) is rather greater than is seen in the earlier study,31 presumably because in the present work these carboxylato O atoms lie on the Jahn–Teller axis.

Nevertheless, for all of the mixed μ-oxo/μ-carboxylato bridged complexes which we have studied,31 the disparity between MnIII–O and MnIV–O is more extreme in the ferromagnetically coupled S = 7/2 configuration than for S = 1/2.

This tendency for ‘milder’ Mn-ligand bond lengths in the broken-symmetry configuration than in S = 7/2 presumably indicates that the valence trapping within the BS state is less extreme than in S = 7/2, implying that pathways for overlap or mixing of Mn-based orbitals exist for S = 1/2 which are not viable (or which are less effective) for S = 7/2.

The ‘crossed exchange’ pathway discussed above fulfils this description, since the mixing of an occupied eg-derived high-spin-MnIII-based orbital with a vacant t2g-derived MnIV-based orbital is possible only if the spins on MnIII and MnIV are (antiferromagnetically) opposed.

Another consequence of the ‘crossed exchange’ interaction is that mixing of the occupied MnIII eg-derived orbital with a t2g-derived MnIV orbital leads to a lessening in the Jahn–Teller distortion of the MnIII center.

The carboxylic oxygens lie on the nominal Jahn–Teller axis in this instance, and the shorter MnIII-Ocarb distance of 2.27 Å seen for the S = 1/2 MnIIIMnIV dimer than that found for the S = 7/2 structure (2.32 Å) is consistent with the structural implications of the crossed-exchange mechanism.

Further structural analysis—for example, of the variation in Mn–O–Mn angles as a function of the bridging combination within different modelled complexes—can be given: not surprisingly, the Mn–O–Mn angles found here, of 87.4° (BS) and 91.7° (S = 7/2), are uniformly less than those which we have previously reported for the complexes containing MnIII(μ-O)2MnIV (97.7° (BS)), MnIII(μ-O)2(μ-O2CH)MnIV (94.2° (BS); 93.4° (S = 7/2)), and MnIII(μ-O)(μ-O2CH)2MnIV (130.5° (BS); 124.7° (S = 7/2)) cores , reflecting the more compact Mn–Mn distance in the present case.

The Mn2O2 core geometry may well not hold a significant direct influence over the crossed-exchange pathway, since this appears to operate through mixing of near-degenerate orbitals rather than through overlap, but the antiferromagnetic pathways Jxy/xy and Jyz/yz (which are, respectively, analogous to the Jxz/xz and Jyz/yz pathways explored in our study of the di-μ-oxo-bridged complexes)30 are dependent on the overlap of Mn-centered and O-centered orbitals and thus should be affected by the Mn–Mn and Mn–O distances and the Mn–O–Mn angles.

A more detailed analysis of the structural/magnetic interplay in the Mn2(μ-O)2(μ-O2CR)2 core is in preparation but is beyond the scope of the present work.

In the study on [(NH3)4MnIII(μ-O)2MnIV(NH3)4]3+ in the BS (S = 1/2) state, we found that the composition of the metal-dominated, occupied valence molecular orbitals showed strong mixing between one or other Mn atom and the oxo bridges in all but the highest occupied, α-spin, a1-symmetry metal-based orbital.

The composition of this highest occupied orbital is particularly pertinent.

Using a notation scheme similar to that employed in Table 3, our earlier study reported percentage compositions of the 17a1 orbital of 31% Naxial (attached to MnA), 48% MnA (principally dz2 character), 2% Obr, 12% MnB (principally dx2y2 character), and 0% Naxial (attached to MnB).

As with the HOMO 40a1 in the S = 1/2 (BS) [Mn2(μ-O)2(μ-O2CH)2bpy2]+ complex which forms the focus of the present study, the 17a1 HOMO in the di-μ-oxo-bridged complex studied previously30 is therefore a crossed exchange pathway.

Note that, due to differences in the axis assignments of the two studies (see Fig. 2 for the axis assignment used here), the occupied majority-spin eg-derived orbital on MnIII is identified, in the earlier study, as dz2 and the vacant minority-spin t2g-derived orbital on MnIV is dx2y2.

There are also indications that the strength of the crossed exchange pathway depends upon the energy spacing between the atomic orbitals involved in this pathway.

If we assess the ratio of ‘MnIV’ to ‘MnIII’ character in the highest-occupied a1-symmetry orbital of di-μ-oxo-bridged complexes featuring, respectively, 0,30 2 (this work), and 131 carboxylato bridge(s), we find that this ratio consistently increases from 0.25 to 0.3 to 0.55 as the energy gap between the highest occupied and the lowest unoccupied α-spin a1-symmetry orbitals is reduced from 1.35 to 1.29 to 0.85 eV.

While a more detailed analysis of this trend appears unwarranted since the species being compared do not possess identical terminal ligands in all cases, the results of our present study certainly strengthen the grounds for hypothesizing that the crossed exchange pathway is most efficient when the respective majority-spin eg-derived MnIII and minority-spin t2g-derived MnIV orbitals are most nearly degenerate.


Our density functional theory calculations on the complex [(bpy)Mn(μ-O)2(μ-O2CH)2Mn(bpy)] show excellent structural agreement with the crystallographic results which have previously been reported for a closely analogous complex featuring bulky carboxylate substituents.29

Accordingly, we can confidently assign the experimentally observed asymmetric structure, with markedly canted carboxylate bridges, as arising from an innate electronic preference of the Mn(μ-O)2(μ-O2CR)2Mn core for a valence-trapped MnIIIMnIV configuration (with MnIII identifiably dominated by high-spin character, although an idealized low-spin structure is only 14 kJ mol−1 higher in energy according to our calculations) and not from steric effects originating from the bulky substituents.

This valence-trapped distortion of the metal/bridge core, in turn, appears to be heavily influenced by the crossed-exchange pathway, which leads to a lessening of the Jahn–Teller distortion experienced by the nominal MnIII centre.

The extent of configurational mixing evident between high-spin MnIII (contaminated by MnIV character) and MnIV (contaminated, in turn, by low-spin MnIII character) is greater than that seen in our previous calculations on complexes containing the Mn(μ-O)2Mn core, but less than in complexes featuring a Mn(μ-O)2(μ-O2CH)Mn centre.

It is tempting to suggest that the strength of this crossed-exchange pathway is acutely sensitive to the proximity of the singly occupied and unoccupied a1-symmetry orbitals involved in this mixing, but further study is required to substantiate this notion.