A combined quantum-chemical and matrix-isolation study on molecular manganese ﬂ uorides †‡

Molecular manganese ﬂ uorides were studied using quantum-chemical calculations at DFT and CCSD(T) levels and experimentally by matrix-isolation techniques. They were prepared by co-deposition of IR-laser ablated elemental manganese or manganese tri ﬂ uoride with F 2 in an excess of Ne, Ar, or N 2 or with neat F 2 at 5 – 12 K. New IR bands in the Mn – F stretching region are detected and assigned to matrix-isolated molecular MnF x ( x = 1 – 3).


Introduction
Manganese is an essential element for all living creatures and plays a key role in the process of photosynthesis. 1 Beyond that it has gained importance on a laboratory scale as a reagent for quantitative analysis 2 and on an industrial scale as an alloy component. 3This is mostly due to its high bonding affinity towards oxygen that culminates in the stability of Mn 2 O 7 4 which contains manganese in its highest possible oxidation state +VII.For the stabilization of high positive oxidation states very electronegative ligands are necessary and besides oxygen also fluorine is used. 5The highest oxidation state of a manganese fluoride is +IV in solid MnF 4 .
In the binary system of manganese and fluorine MnF has gained attention due to the high multiplicity of its 7 Σ + ground state.MnF has been studied extensively by theoretical methods 6 as well as experimentally using mass spectrometry, 7 rotational, [8][9][10] matrix-infrared 11 and electron spin resonance spectroscopy. 12Commercially available MnF 2 and MnF 3 have been studied in detail in both solid and gaseous forms especially with regard to Jahn-Teller-distortion [12][13][14] using gasphase electron diffraction, 15,16 X-ray diffraction, 17,18 neutron scattering, 19 vibrational spectroscopy, [20][21][22] and mass spectrometry. 7,23MnF 2 and MnF 3 were thermally evaporated using a Knudsen cell and subsequently deposited in solid argon and neon matrices. 20,21Another approach to study molecular MnF x species is from thermally generated manganese atoms, which were treated with elemental fluorine.MnF, MnF 2 and MnF 3 were formed, and an IR band obtained in solid Ar at 768.7 cm −1 was tentatively assigned to "a molecule richer in fluorine than MnF 3 ". 11MnF 4 is a blue solid, first synthesized in 1961 24 and later employed in the chemical synthesis of elemental fluorine. 25It has been characterized in the solid state by X-ray diffraction, 26 vibrational spectroscopy 22,27 and magnetic susceptibility measurements. 24,28Since it cannot be evaporated without decomposition to MnF 3 and F 2 29 the investigation of molecular MnF 4 is challenging.In an earlier study solid MnF 3 was treated with TbF 4 at 700 K and an IR band at 794.5 cm −1 in the MnF stretching region as well as two weaker bands at 172.9 cm −1 and 176.6 cm −1 of the Ar-matrix isolated products were claimed to be molecular MnF 4 . 23From the appearance of a single band in the Mn-F stretching region a tetrahedral structure was deducted.The fact that up to now binary fluorides of manganese higher than MnF 4 have not been obtained is remarkable.The heavier homologue technetium forms TcF 6 , 30 while TcF 7 is predicted to be stable, 31 and ReF 7 is well-known. 32FeF 4 and CrF 5 have been characterized, 33,34 while earlier reports on the existence of CrF 6 were shown to be erroneous. 35A previous report about the possible existence of manganese pentafluoride taking into account thermochemical data and sterical considerations 36 stimulated us to reinvestigate the higher molecular Mn-F species using quantum-chemical calculations at DFT and CCSD(T) levels and to conduct matrix-isolation experiments in combination with IR spectroscopy.
Instead of thermally evaporating manganese, we have used the IR-laser ablation of elemental manganese to generate more reactive excited manganese atoms, which were cocondensed with fluorine diluted (0.25-3%) in neon, argon or nitrogen and neat fluorine.In addition we report about IR-laser ablation of manganese trifluoride, which has not been described previously.

Calculated structures, thermochemistry and vibrational frequencies
Optimized structures for the binary manganese fluorides MnF x (x = 1-7) are shown in Fig. 1.
The binary manganese fluorides have a high-spin ground state.The bond length of diatomic MnF calculated at the CCSD(T) level by Nhat et al. was 185 pm, 6 which is fairly close to our value of 184.3 pm at the DFT level and that obtained experimentally by rotational spectroscopy (183.9 pm). 10 MnF 2 is computed to be linear with a CCSD(T) bond length of 181.6 pm.Gas phase electron diffraction gave a consistent value of 181.1 pm. 15 MnF 3 has a distorted trigonal planar structure.
The quintet ground state shows two longer Mn-F bonds (175.3 pm) and one shorter Mn-F bond (173.0 pm) at the CCSD(T) level, which is in very good agreement with the results from gas phase electron diffraction (175.4 pm and 172.8 pm). 16According to our CCSD(T) calculations MnF 4 has a distorted tetrahedral structure (C 2v symmetry) with Mn-F bond lengths of 173.0 pm and 169.2 pm.For MnF 5 a trigonal bipyramidal minimum structure (D 3h symmetry) with bond lengths of 174.1 pm (Mn-F ax ) and 167.4 pm (Mn-F eq ) was computed at the CCSD(T) level.The d 1 electron configuration of manganese in MnF 6 causes a distortion from a regular octahedral structure which leads to either a structure with D 4h -or D 3d -symmetry.Both isomers are minima on the potential energy surface with the trigonal antiprismatic structure (bond lengths of 172.7 pm) just 0.5 kJ mol −1 in energy above the tetragonal bipyramidal structure (Mn-F ax 172.1 pm, Mn-F eq 173.4 pm) at the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ level.For MnF 7 also two minimum structures were computed at the DFT level.The regular D 5h -symmetrical pentagonal bipyramidal isomer exhibits Mn-F ax bond lengths of 171.3 pm and Mn-F eq bond lengths of 180.4 pm.The monocapped trigonal prismatic isomer revealed four Mn-F bonds of 175.4 pm and two slightly shorter bonds of 175.3 pm, while the seventh fluorine atom is considerably weaker bound with 193.0 pm (Fig. 1).At the CCSD(T)/aug-cc-pVTZ//B3LYP/aug-cc-pVTZ level the latter isomer is favored in energy by 37.9 kJ mol −1 .This is in contrast to the computed minimum structure of TcF 7 31 and the crystal structure of ReF 7 37 which both are (slightly) distorted pentagonal bipyramids.The reason for this disparity could be the smaller radius of the manganese atom compared to technetium and rhenium atoms.
The observability of a transient molecule in a matrix-isolation experiment requires at least a marginal thermochemical stability.This can be assessed by calculating the enthalpy of possible decomposition reactions.Table 1   ponding reaction enthalpies for the optimized manganese fluorides.

lists the corres-
The decomposition of MnF 7 and MnF 6 is strongly exothermic at all considered pathways including concerted elimination of F 2 , bimolecular reaction and homolytic bond fission.For MnF 5 however the decomposition channels are computed to be endothermic at the CCSD(T) level.Thus, it should be possible to obtain molecular MnF 5 under appropriate conditions, for example in matrix-isolation experiments.However, ΔG (298.15K) for the gas phase reaction MnF 5 → MnF 4 + 1 2 F 2 is calculated to be −12.2kJ mol −1 at the CCSD(T)/aug-cc-pVTZ level, so molecular MnF 5 is not stable at ambient temperatures in the gas phase.Considering that sublimation enthalpies are usually smaller for the higher fluorides, MnF 5 is most likely also not stable in the condensed phase.The value of ΔH(0 K) = 107.0kJ mol −1 obtained for the decomposition MnF 4 → MnF 3 + 1 2 F 2 also suggests a low thermal stability of molecular MnF 4 , which is consistent with the reported vapor pressure of F 2 over solid MnF 4 at room temperature. 29ibrational spectra of the manganese fluorides were calculated at the DFT (MnF-MnF 7 ) and CCSD(T) levels (MnF-MnF 5 ).The corresponding Mn-F stretching wavenumbers for MnF-MnF 5 are listed and compared to experimentally observed IR bands in Table 2. Calculated IR wavenumbers of Mn-F stretching modes for MnF 6 and MnF 7 are listed in Table S2.‡

Matrix-isolation experiments
Molecular manganese fluorides were generated by IR-laser (λ = 1064 nm) ablation of a rotating elemental manganese target and co-condensed with fluorine diluted (0.25-3%) in neon or argon.After deposition of the sample new IR absorptions in the Mn-F stretching region were observed.Fig. 2 shows spectra obtained after deposition of the reaction products of manganese atoms with varying concentrations of F 2 in Ar.The most intense band obtained with fluorine concentrations up to 1%, observed at 699.6 cm −1 in Ar and 721.6 cm −1 in Ne, Fig. 2 IR spectra in the 450-850 cm −1 region of the Ar matrix-isolated (12 K) reaction products of laser ablated manganese atoms in the presence of F 2 (0.25-3%) after 1 h of deposition.The region of the bending mode of atmospheric CO 2 (660-670 cm −1 ) was omitted for a better representation.respectively (ESI; Fig. S1 ‡), is associated with the Σ − u -mode of MnF 2 .Bands of MnF 3 were observed at 758.7 cm −1 and 711.2 cm −1 in Ar and at 773.4 cm −1 and 726.9 cm −1 in Ne, respectively.These values agree very well with those from experiments of thermally evaporated manganese difluoride and trifluoride, respectively. 20,21While the intensity of the band due to MnF 2 increased going from 0.25% to 0.5% F 2 and then decreased with higher fluorine content, the intensity of the bands due to MnF 3 increased steadily.Also the antisymmetric stretching band of [F 3 ] − has been observed at 510.1 cm −1 .This anion has been shown to be formed from F − and a F 2 molecule during the deposition process, where the F − anion arises from a fluorine atom by electron capture in the plasma plum of an IR-laser ablated metal. 38,39A decrease in intensity with increasing fluorine concentration was observed for the band at 589.2 cm −1 , which is assigned to diatomic MnF, which is in excellent agreement with a previous report. 11ts counterpart in Ne was observed at 608.5 cm −1 (ESI; Fig. S1 ‡).This wavenumber is consistent with an estimated value of 612.0 cm −1 obtained by rotational spectroscopy 40 considering the red-shift of transition metal monofluoride bands experienced in solid noble gases. 41To the best of our knowledge this is the first report about MnF isolated in a neon matrix.It is noteworthy that IR-laser ablation of elemental manganese in the presence of fluorine did not produce any higher fluoride than MnF 3 .Irradiation of Ar-isolated F 2 molecules with λ = 455 ± 10 nm is proved to produce reactive F-atoms, which are subject to a restricted movement within the solid Ar matrix, and thus may react with adjacent molecules such as MnF 3 .However, even this photolysis of the deposit failed to produce higher fluorides of manganese.
When neat fluorine was used as matrix gas, several metal dependent absorptions could also be observed within Mn-F stretching range between 730 cm −1 and 780 cm −1 (ESI; Fig. S2 ‡).However, these absorptions were broad and relatively weak in intensity, and their assignment appeared to be difficult.Band broadening and overlapping features were also observed using N 2 or 1% F 2 in N 2 together with IR-laser ablated manganese atoms (ESI; Fig. S3 ‡).This result is most likely due to the presence of several matrix sites.However, based on its behavior upon annealing and photolysis a band at 585.8 cm −1 in solid N 2 could be unambiguously assigned to molecular MnF.
We have also conducted IR-laser ablation experiments using a pressed target pellet made of MnF 3 .Very recently we reported about the laser ablation of alkali metal fluorides, 42 but, to the best of our knowledge, IR-laser ablation of solid transition metal fluorides has not yet been explored.Molecular MnF 3 generated from a solid MnF 3 target was first co-condensed with an excess of Ne at 5 K (Fig. 3) and produced bands at 773.4 cm −1 and 726.9 cm −1 .The presence of MnF 2 (Σ − u : 721.6 cm −1 ) indicates some decomposition of MnF 3 in the plasma plum, probably by F-atom elimination.
When Ar was used as matrix gas (Fig. 4), the band for MnF 2 shifts to 699.6 cm −1 and those of MnF 3 to 758.7 cm −1 and 711.2 cm −1 .Additionally, the weaker third Mn-F stretching band of MnF 3 was observed at 643.7 cm −1 .A sharp band observed at 735.7 cm −1 could not be assigned to a mononuclear MnF 2 or MnF 3 species.This band was previously also observed by the thermal evaporation of solid MnF 3 and tentatively assigned to a fluorine bridged dimer. 11In addition, traces of oxygen containing species such as OF 2 and MnO 3 F [43][44][45] were detected (ESI; Fig. S4 and S5 ‡).Although the target was prepared and fixed to the target holder under inert conditions, some superficial hydrolysis could probably not be Fig. 3 IR spectrum in the 700-850 cm −1 region of the Ne matrix-isolated (5 K) products of laser ablated solid MnF 3 after 15 min of deposition.prevented during the preparation of the target.The oxygen species OF 2 and MnO 3 F might then have been formed in the plasma plum from the products of this hydrolysis.IR-laser ablation of solid MnF 3 was also carried out in the presence of fluorine (1% F 2 in Ar).Apart from some line broadening, the spectrum (Fig. 5a and ESI; Fig. S6 ‡) is very similar to that obtained without fluorine (Fig. 4).After photolysis (λ = 455 ± 10 nm) a weak feature at 795 cm −1 appeared, which did not grow further with prolonged photolysis (Fig. 5b).
The carrier of this weak band is difficult to assign.A single IR band in the Mn-F stretching region at 794.5 cm −1 was previously obtained in solid Ar from the vaporization of a mixture of MnF 3 and TbF 4 at 700 K and assigned to molecular MnF 4 . 23rom their observation, the authors inferred a tetrahedral structure of molecular MnF 4 .However, the presence of a single Mn-F stretching band is not in agreement with the calculated C 2v structure of MnF 4 (Fig. 1), for which at least three strong Mn-F stretching bands are predicted (Table 2).The previous assignment of molecular MnF 4 to a single Mn-F stretching band at 794.5 cm −1 is therefore questionable, and the carrier of the strong IR band obtained in these experiments remains unknown.

Matrix-isolation experiments
CAUTION: Appropriate safety precautions should be taken into account when handling pure fluorine and metal fluorides.All equipment has to be adequately pretreated with fluorine prior to use.Note that sudden freezing of samples in metal cylinders to liquid nitrogen temperatures is potentially dangerous.The suitability of metal cylinders for such a cooling procedure should be proven and the cylinders should always be leak tested at liquid nitrogen temperature prior to filling with poisonous samples.The setup for matrix-isolation and IR-laser ablation experiments has been described in detail previously. 35he 1064 nm fundamental of a Nd:YAG laser was focused onto a freshly sanded rotating manganese (chemPUR, 99.99%) target or target pellets made of MnF 3 (Alfa Aesar, 98%).Gaseous mixtures of various concentrations (0.25-100%) of fluorine (Solvay Fluor AG, 99.998%) in neon (Air Liquide, 99.999%), argon (SWF, 99.999%) and nitrogen (Linde, 99.999%) were prepared using a fluorine cylinder (V = 1000 cm 3 , thick-walled stainless steel, p 298 K < 1 bar) cooled to 77 K to freeze out less volatile impurities such as traces of HF and CO 2 .These mixtures were than co-condensed together with the IR-laser ablated species for 45-60 min onto a CsI cold window cooled to 5 K (Ne matrices) or 12 K (Ar and F 2 ).Pure fluorine matrices were deposited onto a protective layer of argon to prevent oxidation of the cold window material.FTIR spectra were recorded at a resolution of 0.5 cm −1 on a Bruker Vertex 70 spectrometer equipped with a KBr beam splitter using a liquid nitrogen cooled mercury cadmium telluride (LN-MCTB) detector.A common IR band observed in all spectra of such experiments using IR-laser ablated metal atoms and fluorine is due to the [F 3 ] − anion at 510.1 cm −1 (Ar), 517.2 cm −1 (N 2 ), or 524.6 cm −1 (Ne), respectively. 46As no isotopic splitting of the IR bands of both the monoisotopic manganese and fluorine elements can be observed, any assignment of bands of binary manganese and fluorine species mainly relies on reliably calculated spectra and meaningful variations of experimental conditions.

Quantum-chemical calculations
Structures of the manganese fluorides MnF x (x = 1-7) displayed in Fig. 1 were fully optimized at the DFT level using the B3LYP functional [47][48][49] in conjunction with the aug-cc-pVTZ basis sets for fluorine 50 and manganese, 51 and considering all reasonable spin multiplicities.Subsequent optimizations at the CCSD(T) level were carried out for the ground states of MnF x (x = 2-5).Harmonic frequencies were calculated for stationary points on the potential energy surface.The DFT calculations were performed with Gaussian09 52 and CFOUR 53 was used for the coupled-cluster calculations.

Conclusion
For more than 50 years solid MnF 4 has been the highest experimentally known manganese fluoride, but experimental data about its molecular structure are still missing.Quantumchemical calculations at the CCSD(T) level predict molecular MnF 4 to be stable against its decomposition according to MnF 4 → MnF 3 + 1 2 F 2 by ΔH(0 K) = 107 kJ mol −1 .The corresponding decomposition route for the unprecedented MnF 5 (MnF 5 → MnF 4 + 1 2 F 2 ) is found to be only marginally endother-

Fig. 4
Fig. 4 IR spectrum in the 640-850 cm −1 region of Ar matrix-isolated (12 K) products of laser ablated solid MnF 3 after 15 min of deposition.The region of the bending mode of atmospheric CO 2 (660-670 cm −1 ) was omitted for a better representation.

Fig. 5
Fig. 5 IR spectra in the 640-850 cm −1 region of Ar matrix-isolated (12 K) products of laser ablated solid MnF 3 in the presence of F 2 (1%).(a) After 15 min of deposition.(b) After 15 min of λ = 455 ± 10 nm irradiation.The region of the bending mode of atmospheric CO 2 (660-670 cm −1 ) was omitted for a better representation.

Table 2
Calculated and experimentally observed IR wavenumbers of Mn-F stretching modes of molecular manganese fluorides a a Values in cm −1 , IR intensities in km mol −1 in parentheses.b Ref. 6. c This work.d Ref. 11. e Ref. 20. f Ref. 23. g Ref. 21. | Dalton Trans., 2016, 45, 5038-5044 This journal is © The Royal Society of Chemistry 2016