Is Mn(I) More Promising Than Fe(II)—A Comparison of Mn vs Fe Complexes for Olefin Metathesis

Olefin metathesis is one of the most significant transformations in organic chemistry and is an excellent example for efficient homogeneous catalysis. Although most currently used catalysts are primarily based on 4d and 5d metals, cycloaddition and cycloreversion reactions can also be attributed to first-row transition metals, such as Fe. Surprisingly, the potential of Mn(I)-based catalysts for olefin metathesis has been unexplored despite their prominence in homogeneous catalysis and their diagonal relationship to Ru(II). In the present study, we have investigated the prospective capabilities of Mn complexes for cycloaddition and reversion reactions using density functional theory. Therefore, we have initially compared the literature known iron model systems and their isoelectronic Mn counterparts regarding their reactivity and electronic structure. Next, we constructed potential Mn complexes derived from synthetically accessible species, including carbonyl ligands and obeying octahedral geometry. Based on thermodynamic parameters and the calculation of electronic descriptors, we were able to validate the isodiagonal relationship. Our study serves as guidance for the experimental chemist.

The metallacycle is the most crucial intermediate in first row transiAon metal olefin metathesis.To study the influence of the exchange correlaAon funcAonal on the relaAve spin state spliqng, single point calculaAons were performed using the def2-TZVP basis set on the previously obtained geometries.It is relevant to noAce that implicit solvaAon (PCM, dichloromethane) used has a significant effect on the singlet-triplet spliqng as was already reported by Shirazi et al. [1] and Gonzalez et al. [2] for other systems.
In the present case a reducAon of ΔEST by approx. 2 kcal/mol can be observed.On the other hand, solvaAon is necessary to stabilize the caAonic systems in our study.
For reference purposes, single point calculaAons were also carried out with DLPNO-CCSD(T) uAlizing an unrestricted BP86 reference wave funcAon.The "T1" diagnosAc values were in both cases < 0.018.Clearly, TPSSh yielded the best agreement with the DLPNO-CCSD(T) reference data.
Table S2.RelaAve spin-state spliqng for the metallacycle of complex MECP were located using the algorithm implemented in ORCA using the SurfCrossOpt and NoTRAH keywords.The singlet metallacycle geometry was used as a starAng point.The nature of the MECP was addiAonally verified by a numerical Hessian calculaAon invoked using the SurfCrossNumFreq keyword.
To obtain free energies corrected for low frequencies, an approximaAon was used adding one half of the calculated thermal and entropic correcAons of the singlet and triplet metallacycle, each.

NICS and NMR shielding
For the calculaAon of NMR shi\s the GIAO (Gauge Including Atomic Orbitals) method was used as implemented in ORCA by default.Isotropic 13-Carbon shi\s (in ppm) are given relaAve to TMS.Values for the reference molecule were obtained at the same level of theory (TPSSh/def2-TZVP).To calculate the NICS values to invesAgate the aromaAcity of the TS, a ghost atom (" H: ") was placed in the geometric center of the TS ring structure (see Fig. S6).

Table S3 .
The energy values are given in kcal/mol. RelaAve spin-state spliqng ΔEST for the metallacycle of complex 2-Mn and 2-Fe.The energy values are given in kcal/mol.