Neutral Adducts of Molybdenum Hexafluoride: Structure and Bonding in MoF₆(NC₅H₅) and MoF₆(NC₅H₅)₂

Abstract

The first Lewis acid base adducts of MoF₆ and an organic base have been synthesized, i. e., MoF₆(NC₅H₅) and MoF₆(NC₅H₅)₂. These adducts are structurally characterized with X‐ray crystallography, showing that both adducts adopt capped trigonal prismatic structures. The MoF₆(NC₅H₅) and MoF₆(NC₅H₅)₂ adducts are fluxional on the NMR time scale at room temperature. Two different fluorine environments could be resolved by ¹⁹F NMR spectroscopy at −80 °C for the 1 : 2 adduct, MoF₆(NC₅H₅)₂, whereas MoF₆(NC₅H₅) remains fluxional at that temperature. Density functional theory (DFT) calculations aide the assignment of the infrared and Raman spectra. Natural Bond Order and Molecular Electrostatic Potential analyses elucidate the structures and properties of the MoF₆ pyridine adducts. Regions of significantly higher molecular electrostatic potential, i. e., σ‐holes, in trigonal prismatic compared to octahedral MoF₆ rationalize the capped trigonal prismatic geometry of the adducts. Whereas MoF₆(NC₅H₅) is stable at room temperature under exclusion of moisture, MoF₆(NC₅H₅)₂ decomposes at 60 °C in pyridine solvent, and the solid slowly decomposes at room temperature after 24 h.

Molybdenum hexafluoride was shown to form Lewis acid base adduct with the organic base pyridine, the first neutral transition metal adducts beyond those of WF₆. The MoF₆(NC₅H₅) and MoF₆(NC₅H₅)₂ adducts adopt capped trigonal prismatic geometries. The trigonal prismatic MoF₆ moiety in the adducts can be explained by the high electrostatic potentials of the square faces, where pyridine can coordinate.

Introduction

The 16 confirmed hexafluorides range in properties from the inert SF₆ to the extremely strong oxidizer PtF₆. ^[1] A subgroup of hexafluorides are transition metal hexafluorides, which adopt octahedral geometries. Among the transition metal hexafluorides, MoF₆, WF₆, TcF₆, and ReF₆ exhibit Lewis‐acid properties as displayed by their adducts with fluoride ions. Among the transition metal hexafluorides, molybdenum hexafluoride and tungsten hexafluoride are the least oxidizing ones. The initial claim of the preparation of the other possible group‐6 hexafluoride, CrF₆, has not been substantiated, and CrF₆ has been predicted to be unstable under ambient pressure conditions, but has been predicted to be possibly accessible at high pressures.[ ¹ , ² , ³ ] Molybdenum hexafluoride is a somewhat weaker Lewis acid (fluoride‐ion affinities: MoF₆ 316 kJ/mol, WF₆ 325 kJ/mol), but a stronger oxidizer than WF₆. ^[4] The latter property is exemplified by MoF₆ being able to oxidize NO, whereas WF₆ cannot.[ ⁵ , ⁶ ] The trigonal prismatic structures of MoF₆ and WF₆ are transition states for the interconversion between the octahedral ground‐state structures, called Bailar‐twist, having energy barriers of 28 and 46 kJ/mol, respectively.[ ⁷ , ⁸ ] On the other hand, Mo(CH₃)₆ and W(CH₃)₆ have trigonal prismatic ground states. Molecular orbital and valence bond models suggest that the octahedral geometry is favoured for six‐coordinate transition metal compounds over the trigonal prismatic geometry when the ligands can act as π‐donors, like the fluorido ligand. ^[9]

Derivatization of MoF₆ via substitution of fluorido ligands has been studied. ^[10] Alkoxide‐fluoride substitution using (CH₃)₂Si(OCH₃)₂ and (CH₃)Si(OCH₃)₃ yield [MoF(OCH₃)₅] and [MoF₂(OCH₃)₄], respectively, although under certain conditions explosions were observed. ^[11] Full substitution of the fluorido ligands of MoF₆ was accomplished with (CH₃)₃SiN₃ producing the shock‐sensitive Mo(N₃)₆. ^[12] Chloride‐fluoride substitution is observed when MoF₆ reacts with BCl₃ forming MoCl₆. ^[13] The Mo^VI oxide fluorides MoOF₄ and MoO₂F₂ are Lewis acids and show extensive coordination chemistry with organic ligands.[ ¹⁰ , ¹⁴ ] For example, MoOF₄ forms the hexacoordinate MoOF₄(NC₅H₅) and heptacoordinate MoOF₄(NC₅H₅)₂ adducts. ^[15]

Among the transition metal hexafluorides, WF₆ is the only one that has been reported to form neutral adducts, consistent with it having the lowest oxidizing strength. Neutral adducts of WF₆ with nitrogen bases, phosphines, sulfides, and even arsines have been reported.[ ¹⁴ , ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹ ]

The WF₆ adducts with pyridine, i. e., WF₆(NC₅H₅) and WF₆(NC₅H₅)₂, were synthesized by reacting WF₆ with NC₅H₅ in a 1 : 1 and 1 : 2 molar ratios, respectively, in dichloromethane. ^[20] Single‐crystal X‐ray diffraction showed that both adducts have capped trigonal prismatic geometries.[ ²⁰ , ²¹ ] Fluorine‐19 NMR spectroscopy at room temperature show a singlet resonance for each adduct even though two fluorine environments are expected for the rigid structures, suggesting rapid exchange of fluorine environments. The singlet splits into quintets and triplets at low temperatures. ^[20] For WF₆(NC₅H₅) and WF₆(NC₅H₅)₂, these two multiplets are well resolved at 140 and 193 K, with approximate energy barriers for the exchange of 47 and 142 kJ mol⁻¹, respectively. ^[20]

Other notable adducts of WF₆ are those with P(CH₃)₃, P(CH₃)₂(C₆H₅), AsR₃ (R=CH₃, C₂H₅), and bidentate nitrogen bases.[ ¹⁷ , ¹⁸ , ²² , ²³ ] The structure of WF₆{P(CH₃)₃} is capped trigonal prismatic as is the case for most WF₆ 7‐coordinate adducts, while WF₆{P(CH₃)₂(C₆H₅)} is the only reported WF₆ adduct that adopts a different geometry, being capped octahedral. ^[17]

Whereas adducts of Mo^VF₅ have been reported, i.e., MoF₅(NCCH₃) and MoF₅(NC₅H₅), no definitive evidence for the Lewis acid behaviour of Mo^VIF₆ towards organic bases has been obtained.[ ¹⁵ , ²⁴ , ²⁵ ] Herein, we report the synthesis and conclusive characterization of neutral adducts of MoF₆ with pyridine, which are the first reported adducts of MoF₆ with organic bases.

Results and Discussion

Synthesis and Properties of MoF₆(NC₅H₅) _n (n=1, 2)

The reaction of MoF₆ with pyridine in a 1 : 1 molar ratio in CH₂Cl₂ yielded an orange solid with clear orange supernatant and, when volatiles were removed, afforded bright orange solid MoF₆(NC₅H₅) (Equation 1). The solid was stable over several months at room temperature under exclusion of moisture, as determined by Raman spectroscopy. The 1 : 1 adduct is soluble in CH₃CN (orange solution) and pyridine (yellow solution), although when dissolved in pyridine MoF₆(NC₅H₅) was converted to MoF₆(NC₅H₅)₂. Isolated MoF₆(NC₅H₅) was sparingly soluble in CH₂Cl₂ or SO₂ forming yellow solutions, although MoF₆(NC₅H₅) exhibits higher solubility in CH₂Cl₂, when being prepared in this solvent, forming a bright orange solution.

(1)

The reaction of MoF₆ with 2 molar equivalents of pyridine in CH₂Cl₂ afforded the MoF₆(NC₅H₅)₂ adduct (Equation 1) as a bright yellow‐orange powder upon removal of volatiles. During the synthesis, the reaction mixture should not be allowed to warm to room temperature except for a brief moment, because decomposition is observed in CH₂Cl₂. The 1 : 2 adduct slowly degrades in the solid state at ambient temperature with additional Raman bands appearing after three days. The adduct is soluble in SO₂, pyridine, and sparingly soluble in CH₂Cl₂, all giving yellow solutions.

When heating a solution of MoF₆(NC₅H₅)₂ in pyridine to 60 °C the solution changes from bright yellow to a dull orange over the period of 1 h 20 min. The resulting solution did not show any ¹⁹F NMR resonances other than impurities, suggesting a paramagnetic decomposition product.

Molecular Geometries

The structures of MoF₆(NC₅H₅) and MoF₆(NC₅H₅)₂ were studied by X‐ray crystallography and gas‐phase optimizations at the B3LYP and MN15/cc‐pVTZ(C, H)/aug‐cc‐pVTZ(N, F)/aug‐cc‐pVTZ‐PP (Mo) levels of theory (aVTZ). Selected crystallographic parameters are listed in Table 1, and selected experimental and calculated metric parameters for both adducts are given in Table 2. The complete lists of metric parameters are given in Tables S1 and S2. Figure 1 shows the thermal ellipsoid plots of MoF₆(NC₅H₅) and MoF₆(NC₅H₅)₂ together with the respective optimized geometries. Figures S4 and S5 show the crystal packing of the two adducts with the π‐π stacking of the pyridine rings.

Table 1.

Crystallographic Data Collection and Refinement Parameters for MoF₆(NC₅H₅) _n (n=1, 2).

	MoF6(NC5H5)	MoF6(NC5H5)2
space group	triclinic, P $\bar{1}$	monoclinic, Cc
Z	2	4
a (Å)	6.8039(3)	12.2505(4)
b (Å)	7.9088(3)	7.9251(2)
c (Å)	7.9640(3)	12.5640(4)
α (°)	70.679(3)	90
β (°)	81.176(3)	104.358(4)
γ (°)	78.215(3)	90
R1 [I≥2σ(I)][a]	0.0499	0.0329
wR2 [I≥2σ(I)][b]	0.1450	0.0799
CCDC number	2370068	2370069

[a] $R_1=\sum \left|\left|F_0\right|-\left|F_c\right|\right|/\sum \left|F_o\right|$ .

[b] $w{R}_2={\left[\sum \left[w{\left({F_0}^2-{F_c}^2\right)}^2\right]/\sum w\left({F_0}^4\right)\right]}^{1/2}$ .

Table 2.

Selected metric parameters for the experimental and optimized structures (B3LYP and MN15) for MoF₆(NC₅H₅) and MoF₆(NC₅H₅)₂ with bond lengths in Å and angles in °.

	Exptl.	Calcd.
		B3LYP	MN15
MoF6(NC5H5)[a]
Mo−F(1)	1.846(5)	1.866	1.850
Mo−F(2)	1.854(3)	1.866	1.850
Mo−F(3)	1.861(4)	1.866	1.850
Mo−F(4)	1.836(5)	1.866	1.850
Mo−F(5)	1.861(4)	1.856	1.837
Mo−F(6)	1.859(4)	1.856	1.837
Mo−N	2.260(5)	2.346	2.278
MoF6(NC5H5)2 [a]
Mo−F(1)	1.875(2)	1.882	1.875
Mo−F(2)	1.879(3)	1.882	1.875
Mo−F(3)	1.895(3)	1.881	1.867
Mo−F(4)	1.897(2)	1.881	1.867
Mo−F(5)	1.898(2)	1.881	1.867
Mo−F(6)	1.897(3)	1.881	1.867
Mo−N(1)	2.331(4)	2.478	2.364
Mo−N(2)	2.326(3)	2.478	2.364
N(1)−Mo−N(2)	125.88(11)	126.4	125.7

[a] Atom numbering as shown in Figure 1.

Figure 1.

Thermal ellipsoid plots (50 % probability level) of a) MoF₆(NC₅H₅) and c) MoF₆(NC₅H₅)₂ and their B3LYP/aVTZP optimized structures b) and d), respectively. Optimized structures correspond to the local minima with the same conformations as the those of the experimental structures, i. e., conformations 1 (vide infra).

The 1 : 1 adduct, MoF₆(NC₅H₅) is isomorphous to WF₆(NC₅H₅), crystallizing in the triclinic P $\bar{1}$ space group, adopting a monocapped trigonal prismatic molecular structure. The MoF₆(NC₅H₅)₂ adduct adopts a bicapped trigonal prismatic geometry similar to that of WF₆(NC₅H₅)₂. However, the two 1 : 2 adducts are not isomorphous. The MoF₆(NC₅H₅)₂ adduct crystallizes in the non‐centrosymmetric, polar Cc space group, which manifests in the MoF₆(NC₅H₅)₂ molecules being oriented with the pyridine groups pointing in the same direction of the c‐axis (Figure S6). In contrast, WF₆(NC₅H₅)₂ crystallizes in the centrosymmetric Pnma space group with the pyridine groups alternating in their orientations (Figure S7). The valence geometry index for octacoordinated complexes, τ₈, ^[26] gives τ₈ values of 1.10 and 0.13 for opposite faces of for MoF₆(NC₅H₅)₂, which is consistent with the structure being bicapped trigonal prismatic (Figure S8).

Geometry optimizations at the DFT level of theory using the experimental structures as starting points gave minimum‐energy structures that reproduce the experimental structures well. The conformers obtained this way, however, are only local minima on the potential energy surface (vide infra). The τ₈ values from the optimized structure of MoF₆(NC₅H₅)₂ are 1.25 and 0 (B3LYP) for the opposing faces, which are in excellent agreement with those from the experimental structure. While the calculated M−F bond lengths agree closely with the experimental bond lengths for both adducts, the B3LYP Mo−N bond lengths are overestimated, as has been previously reported for other transition metal adducts. ^[21] The MN15 calculations better reproduce the dative Mo−N bonds (Table 2). DFT calculations for the MoF₆(NC₅H₅) adduct predict the Mo−F bond lengths of the capped square face to be 0.01 Å longer than the Mo−F bond lengths to the two distal fluorine atoms. The ranges of the experimental bond lengths overlap (distal fluorines: Mo−F(5,6) 1.859(4), 1.861(4) Å; square face: Mo−F(1,2,3,4) 1.836(5) to 1.861(4) Å), with the shortest Mo−F bond length being in the capped square face, suggesting that the solid‐state bond lengths are affected by packing in the crystal structure. In MoF₆(NC₅H₅)₂, the experimental Mo−F(1,2) bonds that are shared between the two capped square faces were found to be shorter than the other four Mo−F bonds, while DFT calculations predict the reverse trend (MN15) or essentially the same lengths (B3LYP). The Mo−N experimental bond lengths in the 1 : 2 adduct (2.331(4), 2.326(3) Å) are significantly larger than the one in the 1 : 1 adduct (2.260(5) Å), which is consistent with increased steric congestion for the octacoordinated adduct and is best reproduced by the MN15 computational results (MN15: 2.364 and 2.278 Å, respectively). The Mo−F bonds also lengthen with increasing coordination number; the bonds in free MoF₆ (exptl: 1.8147(6)–1.8201(8) Å) ^[27] increase upon addition of one and two pyridine moieties (1.836(4)–1.861(4) Å and 1.875(2)–1.898(2) Å, respectively). Interestingly, the M−N (M=W or Mo) bond length (2.251(7) Å) in WF₆(NC₅H₅) is indistinguishable of that in MoF₆(NC₅H₅).

In addition to the conformations observed experimentally, several minima were found by DFT (B3LYP and MN15) calculations for the two MoF₆ adducts (Figures 2 and 3). The lowest calculated energy structures differ from those observed in the crystal structures by rotation of the pyridine ligands along the Mo−N bonds. In the crystal structure of MoF₆(NC₅H₅), pyridine adopts a conformation where the pyridine is eclipsed with the two opposing Mo−F bonds, as was observed for WF₆(NC₅H₅) (conformer 1 in Figure 2). However, in the DFT‐predicted lowest‐energy structure, the pyridine is staggered with respect to the distal Mo−F bonds (conformer 2 in Figure 2), which is more stable by 4.94 (B3LYP) or 5.85 kJ/mol (MN15) in free energy. This parallels conformer 2 of WF₆(NC₅H₅) being 4.23 kJ/mol lower (B3LYP) in energy than its conformation 1. ^[21] The crystal structure of MoF₆(NC₅H₅)₂ shows the pyridines both staggered towards the opposing MoF₂ groups, resulting in a close to coplanar arrangement of the two pyridine rings. As expected from the steric interference of the two pyridine rings in the experimentally observed conformation, the two pyridine groups are perpendicular to each other in the calculated global energy minimum structure (conformer 3 in Figure 3), which is only 2.80 (B3LYP) or 1.19 kJ/mol (MN15) lower in free energy than conformer 1. The discrepancy between the experimental and predicted lowest‐energy conformer is likely due to crystal packing effects and the stacking of the pyridine rings, which can easily overwrite the small calculated energy difference. When using MN15, conformation 2 is predicted to be essentially isoenergetic with conformation 1 (0.28 kJ/mol higher in free energy), while conformation 2 is predicted to be slightly more stable than 1 when using B3LYP (1.46 kJ/mol lower in free energy). Conformer 4 is the only local‐minimum structure that is not capped trigonal prismatic while still having exclusively real frequencies, i. e., a trigonal dodecahedral structure, being 50.59 (B3LYP) or 54.27 kJ/mol (MN15) higher in free energy than conformer 1. These calculations indicate that the MoF₆ in neutral adducts exhibit preferences for adopting capped trigonal prismatic structures over other 7‐ or 8‐coordinated geometries.

Figure 2.

DFT predicted gas‐phase geometries (local minima) of MoF₆(NC₅H₅) at the B3LYP and MN15/aVTZ level of theory.

Figure 3.

DFT predicted gas‐phase geometries (local minima) of MoF₆(NC₅H₅)₂ at the B3LYP and MN15/aVTZ level of theory.

Vibrational Spectroscopy

Raman and infrared spectra were obtained for MoF₆(NC₅H₅) and MoF₆(NC₅H₅)₂ in the solid state at room temperature (Figure 4). The experimental vibrational frequencies are correlated to the calculated vibrational frequencies for the optimized geometry corresponding to the experimentally observed structure, i. e., conformation 1, at the B3LYP or MN15/aVTZ level of theory. Selected experimental and calculated vibrational frequencies at the B3LYP level of theory are listed in Table 3 whereas complete lists are given in Tables S3 and S4. Both adducts have molecular C _2v point symmetry with only the A₂ modes being infrared inactive and all modes being Raman active. Calculated frequencies tend to overestimate the experimental values as was also the case with the WF₆(NC₅H₅) adduct and its derivatives. ^[21] The MN15 functional results in a more substantial overestimation of vibrational frequencies, particularly those of the MoF₆ moiety. Therefore, the B3LYP‐calculated frequencies are used in the discussion.

Figure 4.

Raman spectra (lower trace) and IR spectra (upper trace) of MoF₆(NC₅H₅) (top) and MoF₆(NC₅H₅)₂ (bottom).

Table 3.

Selected Raman active vibrational frequencies (in cm⁻¹) of the MoF₆ pyridine adducts.

Mode Description[a]	MoF6(NC5H5)
	Raman	infrared	B3LYP
vs(NC5), v(A1)[b]	1023(40)	1021[m]	1040(32)[6]
vas(MoF6), v(B2)		682sh	696(<1)[249]
vs(MoF2), v(A1)	674(100)	669[s]	691(46)[176]
vas(MoF4), v(B1)		641[vs]	670(<0.1)[192]
vs(MoF4)−vs(MoF2), v(A1)	651(11)	648[vs]	665(15)[101]
vas(MoF4), v(A2)	566(3)	570sh	567(0)[0]
vas(MoF2)+vas(MoF4), v(B2)	530(4)	517[m]	537(3)[2]
	MoF6(NC5H5)2
	Raman	infrared	B3LYP
vs(NC5), v(A1)	1021(20)		1034(21)[<1]
vs(NC5), v(B2)		1018[m]	1034(1)[8]
vas(MoF4), v(B1)		635sh	658(<1)[190]
vs(MoF6), v(A1)	640(100)		653(107)[42]
vas(MoF6), v(A1)	606(35)	601[vs]	617(37)[269]
vas(MoF4), v(B2)		567[vs]	617(<1)[51]
vas(MoF4)−vas(MoF2), v(B1)	536(3)	535sh	542(2)[<1]
vas(MoF4), v(A2)	487(1)	499sh	506(2)[0]

[a] Mulliken symbols with respect to C _2v point symmetry of the adducts. Choice of coordinate systems is given in Tables S3 and S4. [b] The v_s(NC₅), mode for free NC₅H₅ has vibrational frequencies of 990(100) {Raman}, ^[28] 995[vs] (infrared), ^[29] and 1013(26)[5] cm⁻¹ {calculated, B3LYP}.

The Raman and infrared spectra of MoF₆(NC₅H₅) and MoF₆(NC₅H₅)₂ contain bands associated with the pyridine ligands, which are shifted with respect to those of free pyridine. The most diagnostic pyridine band is that of the ring breathing mode, which shifts to higher frequencies upon adduct formation (Raman: free pyridine 991, MoF₆(NC₅H₅) 1023, MoF₆(NC₅H₅)₂ 1021 cm⁻¹). Any expected vibrational coupling for the pyridine modes in MoF₆(NC₅H₅)₂ is predicted to be very small, below the resolution of experimental Raman (2 cm⁻¹) and infrared (4 cm⁻¹) spectra.

The most intense Raman band for MoF₆(NC₅H₅) appears at 674 cm⁻¹ (corresponding infrared band: 669 cm⁻¹), which mainly corresponds to the v_s (MoF₂) of the distal difluoro moiety (calcd. 691 cm⁻¹). This symmetric Mo−F stretching frequency is red‐shifted with respect to v_s (MoF₆) of free MoF₆ (742 cm⁻¹), ^[30] reflecting the longer, more ionic Mo−F bonding upon adduct formation. Coordination of a second pyridine ligand results in a further low‐frequency shift to 640 cm⁻¹ for v_s (MoF₆) of MoF₆(NC₅H₅)₂ (calcd. 653 cm⁻¹). Intense infrared bands at 648 and 641 cm⁻¹ for MoF₆(NC₅H₅) (calcd. 670, 665 cm⁻¹) and at 601 and 567 cm⁻¹ for MoF₆(NC₅H₅)₂ (calcd. 617, 617 cm⁻¹) are assigned to asymmetric stretching modes.

NMR Spectroscopy

The ¹⁹F NMR spectrum of MoF₆(NC₅H₅) in CH₂Cl₂ shows a broad singlet resonance at 294.7 ppm (Δν_1/2=160 Hz) at 22 °C that broadens to Δν_1/2=240 Hz and shifts to 259.9 ppm at −80 °C, indicating rapid exchange of the two fluorine environments that are expected based on the capped trigonal prismatic structure observed by X‐ray crystallography. The 1 : 2 adduct, MoF₆(NC₅H₅)₂, also shows a singlet resonance in the ¹⁹F NMR spectrum at room temperature in CH₂Cl₂ at 251.7 ppm (Δν_1/2=400 Hz), reflecting the exchange between the two expected fluorine environments. At −80 °C, the exchange can be slowed down to allow for the observation of a triplet at 228.9 ppm and a quintet at 158.0 ppm (² J _F–F=121 Hz) in a 2 : 1 ratio (Figure 5), representing the two different fluorine environments observed in the crystal structure. Variable‐temperature NMR spectroscopy shows that the triplet and quintet start to lose their multiplicity at −70 °C and coalesce between −40 °C and −30 °C (Figure S12a). Upon raising the temperature the relative integration of the two resonances deviates from the expected 2 : 1 ratio observed at −80 and −70 °C to ratios of 4 : 1.54 (−60 °C) and 4 : 1.3 (−50 °C), suggesting a secondary exchange process. While the determination of accurate activation enthalpy and entropy values are thwarted by the secondary exchange process, analysis of the low‐temperature signals yielded an approximate activation energy of 46 kJ mol⁻¹ for the exchange of the two fluorine environments based on an Arrhenius plot (see Supporting Information). The significant high‐frequency shift of the room‐temperature resonance for MoF₆(NC₅H₅)₂ suggests a dissociative equilibrium between MoF₆(NC₅H₅)₂ and MoF₆(NC₅H₅).

Figure 5.

¹⁹F NMR spectrum of MoF₆(NC₅H₅)₂ at −80 °C in CH₂Cl₂ showing a A₂X₄ spin system.

In the ¹H and ¹³C NMR spectra of both adducts, single sets of resonances are observed for ortho, meta and para nuclei of the adducted pyridine. The fact that only a single set of three resonances is observed for the 1 : 2 adduct at room temperature, as well as low temperature, indicates rapid rotation of the pyridine rings along the Mo−N bonds, consistent with the small calculated energy difference between the conformers.

NBO Analysis

The natural population analysis (NPA) charges and Wiberg Bond Indices (WBIs) were calculated for MoF₆(NC₅H₅) and MoF₆(NC₅H₅)₂, as well as MoF₆ in the O_h ground state minimum and D _3h transition state (Table 4). The NPA charge on Mo is larger for MoF₆ in the trigonal prismatic structure (+2.48) than in the octahedral ground‐state geometry (+2.40). As pyridine ligands coordinate to trigonal prismatic MoF₆, the charge becomes less positive with values of +2.36 (1 : 1 adduct) and +2.28 (1 : 2 adduct), as a consequence of negative charge donation from the ligand. The charges on the F atoms become more negative upon coordination of pyridine, consistent with increasing ionic contributions to Mo−F bonding. The total charge transferred from pyridine to the molybdenum fluoride in MoF₆(NC₅H₅) is 0.29 and in MoF₆(NC₅H₅)₂ it is 0.25 per pyridine. The WBI for Mo−N is almost half of that for Mo−F, indicating a strong dative bond with the weakest dative bond being found for MoF₆(NC₅H₅)₂ (0.33). For WF₆(NC₅H₅), the WBI for the W−F bond is 0.72–0.76 and for the W−N bond is 0.36, which are both slightly lower than those in MoF₆(NC₅H₅). ^[21]

Table 4.

NPA charges and WBIs of MoF₆ and its pyridine adducts calculated at the B3LYP/aVTZ level of theory.

	MoF6 (Oh)	MoF6 (D3h)	MoF6(NC5H5)	MoF6(NC5H5)2
	charges
Mo	+2.40	+2.48	+2.36	+2.28
F	−0.40	−0.41	−0.42(F5,6) −0.45(F1,2,3,4)	−0.46(F3,4,5,6) −0.48(F1,2)
N	–	–	−0.43	−0.40
NC5H5	–	–	+0.29	+0.25
	WBI
Mo−F	0.84	0.83	0.76–0.80	0.71–0.74
Mo−N	–	–	0.38	0.33

Molecular Electrostatic Potentials (MEPs)

The MEP isosurfaces of MoF₆ in the O_h and D _3h structures, MoF₆(NC₅H₅), and MoF₆(NC₅H₅)₂ are shown in Figure 6. For octahedral MoF₆, the eight regions of high electrostatic potential (EP), i. e., σ‐holes, have values of 94 kJ mol⁻¹. In comparison, trigonal prismatic MoF₆ (D _3h ) has three regions of substantially higher EP (σ‐holes) with values of 256 kJ mol⁻¹, localized in each of the square faces, which is consistent with the MoF₆(NC₅H₅) _n adducts preferring the capped trigonal prismatic geometry as the pyridine can coordinate more readily to the higher EP region. Once a pyridine coordinates to MoF₆, the two remaining σ‐holes in the square faces of the monocapped trigonal prismatic structure are lowered to 95 kJ mol⁻¹ in MoF₆(NC₅H₅), which is similar to that of the σ‐holes on free octahedral MoF₆ and affords the coordination of a second pyridine. In MoF₆(NC₅H₅)₂ the open coordination site has an electrostatic potential value of −13 kJ mol⁻¹, which prevents a third pyridine from coordinating and rationalizes why there is no evidence for a 1 : 3 adduct even with excess pyridine. The size of the σ‐holes is paralleled by the NPA charges on Mo, with the Mo in MoF₆ (D _3h ) having the most positive charge and σ‐hole and MoF₆(NC₅H₅)₂ having the least positive charge on Mo and the lowest EP on the σ‐hole.

Figure 6.

Molecular electrostatic potential surfaces (MEPs) calculated at the 0.001 e bohr⁻³ isosurfaces of MoF₆ (O_h and D _3h ), MoF₆(NC₅H₅), and MoF₆(NC₅H₅)₂. The arrows indicated areas of relatively high electrostatic potential values. Calculations were done at the B3LYP/aTVZ level of theory.

Molecular Orbitals and Electronic Transition

The molecular orbital representing the Mo−N bond in MoF₆(NC₅H₅) is represented by the HOMO‐2, in which the lone pair on N interacts with the d_z ² orbital on Mo (Figure S18). For the MoF₆(NC₅H₅)₂ adduct, the Mo−N bonding orbitals are represented by the HOMO‐2 and HOMO‐5 orbitals (Figure S19) which transform as B₂ and A₁ in the C _2v point group.

Frontier orbital energies and corresponding absorption energies, calculated from the HOMO‐LUMO gap using regular DFT and the time‐dependent (TD) DFT methods, are listed in Table 5. Figure 7 shows the frontier molecular orbitals for MoF₆ O_h , MoF₆(NC₅H₅), MoF₆(NC₅H₅)₂. For MoF₆(NC₅H₅), MoF₆(NC₅H₅)₂, the HOMOs are localized on the pyridines, whereas the LUMOs are mainly localized on the MoF₆ moiety.

Table 5.

Calculated HOMO and LUMO energies, HOMO‐LUMO energy gaps (ΔE_HOMO/LUMO), the HOMO‐LUMO transition using time‐dependent (TD) DFT, and corresponding wavelengths (λ_HOMO/LUMO and λ_TDDFT).

		MoF6 (Oh)	MoF6(NC5H5)	MoF6(NC5H5)2
MO Energy[a]	HOMO	−12.46	−8.54	−7.90
(eV)	LUMO	−5.98	−4.63	−3.85
ΔEHOMO/LUMO	DFT[a]	6.48	3.90	4.05
(eV)	TDDFT[b] (f)[c]	–	4.08 (0)	4.22 (0)

[a] Calculated at the B3LYP/aVTZ level of theory. [b] Calculated at the camB3LYP/def2‐TZVP. [c] Calculated oscillator strength, f, is given in parentheses.

Figure 7.

Frontier molecular orbitals (DFT/B3LYP) of MoF₆ O_h (left), MoF₆(NC₅H₅) (centre), MoF₆(NC₅H₅)₂ (right). Isosurface values are drawn at 0.02 e Å⁻³.

Going from MoF₆(NC₅H₅) to MoF₆(NC₅H₅)₂, the HOMO and LUMO energies increase, resulting in similar HOMO‐LUMO gaps for the two adducts (3.90 and 4.05 eV), which are significantly smaller than that in MoF₆ (6.48 eV). TDDFT calculations (using the B3LYP/aVTZ conformation 1 geometry) at the camB3LYP/def2‐TZVP level of theory (Tables S7 and S8) show that the HOMO‐LUMO (MO #55 to 56 for 1 : 1, 76 to 77 for 1 : 2) transitions are 4.08 (1 : 1 adduct) and 4.22 eV (1 : 2 adduct). Although these represent ligand‐to‐metal charge transfer (LMCT) transitions for both adducts, they are symmetry‐forbidden A₂‐symmetric, with zero oscillator strength and, hence, cannot be causing the intense colour of the adducts. For MoF₆(NC₅H₅), the two transitions with a significant oscillator strength (f=0.05) are the HOMO to LUMO+3 (B₂, 5.56 eV, 223 nm), which is a pyridine‐based π‐π transition, and HOMO‐2 to LUMO+2 (A₁, f=0.1, 6.33 eV, 196 nm), which is an excitation from a fully delocalized MO to a MoF₆‐based MO (Figure S20). For MoF₆(NC₅H₅)₂, the two transitions with the largest oscillator strengths are HOMO‐2 to LUMO+1 (B₂, 4.65 eV, 266 nm, f=0.23) and HOMO‐5 to LUMO+2 (B₂, 5.72 eV, 217 nm, f=0.2), which are both excitations from fully delocalized MOs to MoF₆‐based MOs (Figure S21). Calculations of spin‐orbit‐coupling corrected excited state energies are given in Tables S9 and S10 and show that such corrections need to be considered; an excited state of 297 nm with oscillator strength of 0.015 is most likely expected to be observable (vacuum‐UV spectroscopy) for MoF₆(NC₅H₅), but overall the oscillator strengths remain low for these compounds. The largest oscillator strength (0.14) is predicted for a 197 nm transition. For MoF₆(NC₅H₅)₂, a notable excitation is at 334 nm with an oscillator strength of 0.015. The transition with the highest oscillator strength (0.22) occurs at 269 nm. The calculated electronic transitions close to the visible region of light for the adducts are in agreements with the yellow and orange color of the compounds, whereas MoF₆ is colourless.

Conclusions

Despite the significant oxidizing strength of MoF₆, the first neutral Lewis acid base adducts of MoF₆ have been synthesized and structurally characterized with pyridine as the organic ligand, i. e., MoF₆(NC₅H₅) and MoF₆(NC₅H₅)₂. These adducts are only the second type of neutral adducts of a transition metal hexafluoride other than those of WF₆. Single‐crystal X‐ray diffraction revealed that both adducts adopt capped trigonal prismatic structures, mirroring the geometries of the tungsten analogues. The molecular electrostatic potential surfaces (DFT/B3LYP level of theory) provide a rationale for the preference of the capped trigonal prismatic structure, as the σ‐holes on a trigonal prismatic MoF₆ are significantly larger than those of free octahedral MoF₆. Addition of pyridine reduces the size of the remaining σ‐holes, with a negative potential on the uncapped square face of MoF₆(NC₅H₅)₂, preventing the coordination of a third pyridine. Raman spectroscopy showed red shifts of the Mo−F stretching frequencies upon addition of pyridine. At room temperature, MoF₆(NC₅H₅) and MoF₆(NC₅H₅)₂ are fluxional at the NMR time scale with singlet resonances in the ¹⁹F NMR spectra. For the 1 : 2 adduct, triplet and quintet resonances could be observed at −80 °C for the two ¹⁹F environments of MoF₆(NC₅H₅)₂ allowing for the estimate of the energy barrier of exchange (46 kJ mol⁻¹), whereas exchange was still rapid for MoF₆(NC₅H₅) at that temperature. The isolation of these pyridine adducts suggests that further Lewis acid base chemistry of MoF₆ is possible.

Experimental Section

Caution! Elemental F₂ and MoF₆ are highly toxic and corrosive, rapidly evolving HF upon exposure to moisture. Appropriate safety measures should be taken during their handling.

Materials and Apparatus

Reactors were made of heat‐sealed and flared 1/4 in. o.d. tetrafluoroethene‐hexafluoropropene copolymer (FEP) reactors attached to a stainless‐steel valve. The reactors were evacuated for at least 7 h and passivated with 100 % F_2(g) for at least 8 h. The solvents were volatile enough to be distilled through a Pyrex vacuum line equipped with PTFE Chemglass stopcocks. The solvents dichloromethane, SO₂, pyridine and acetonitrile were vacuum distilled on the glass vacuum line. In contrast, MoF₆ was handled on a prepassivated nickel/316 stainless‐steel vacuum line equipped with 316 stainless‐steel valves (Autoclave Engineers). A drybox (Omni Lab, Vacuum Atmospheres) with dry nitrogen atmosphere was used for handling solids.

All solvents used (acetonitrile, dichloromethane, pyridine) were dried over 3 Å sieves except for SO₂ which was dried over CaH₂. All solvents were kept in glass bulbs with either J‐Young or Chemglass stopcocks.

Synthesis of MoF₆(NC₅H₅) _n (n=1, 2)

Excess MoF₆ (0.130 g, 0.619 mmol) was distilled onto pyridine (0.038 g, 0.48 mmol) in CH₂Cl₂ (0.567 g) at −196 °C. The reaction was started at −85 °C and warmed up to room temperature to afford MoF₆(NC₅H₅) as a bright orange powder with a 97 % yield (0.135 g, 0.467 mmol). ¹⁹F NMR (in CH₂Cl₂ at 22 °C) δ 294.7 ppm and (at −80 °C) 259.9 ppm {br, s, MoF₆(NC₅H₅)}. Impurities of [MoF₇]⁻ (267.3 ppm, 1.9 mol %) and MoOF₄(NC₅H₅) (142.3 ppm, 0.3 mol %) at 22 °C; and of MoF₆ (280.5 ppm, ¹ J _MoF=49 Hz, 2.1 mol %), [MoF₇]⁻ (266.5 ppm, 0.2 mol %), unknown impurity (262.6 ppm, 4 % integration with respect to MoF₆(NC₅H₅)) and MoOF₄(NC₅H₅) (141.1 ppm, 1.7 mol %) at −80 °C, small amount of white precipitate at −80 °C. ¹H NMR (in CH₂Cl₂ at 22 °C) 8.56 ppm {H1, d, ³ J _HH=5.0 Hz}, 7.59 ppm {H3, t, ³ J _HH=7.5 Hz}, 7.15 ppm {H2, t, ³ J _HH=6.8 Hz}. ¹³C NMR (in CH₂Cl₂ at −30 °C) 144.3 ppm {C1, d, ¹ J=188.1 Hz}, 142.4 ppm {C3, dt, ¹ J _CH=166.8 Hz, ² J _CH=6.2 Hz}, 126.2 ppm {C2, dt, ¹ J _CH=170.2 Hz, ² J _CH=5.4 Hz}.

The synthesis for MoF₆(NC₅H₅)₂ used pyridine (0.076 g, 0.96 mmol) to MoF₆ (0.090 g, 0.43 mmol) in CH₂Cl₂ (0.289 g) afforded MoF₆(NC₅H₅)₂ (0.164 g, 0.44 mmol, 100 % yield) as a yellow‐orange powder which slowly decomposed over a few days at room temperature within the drybox. ¹⁹F NMR (in CH₂Cl₂ at 22 °C) δ 251.7 ppm {br, s, MoF₆(NC₅H₅)₂} and (at −80 °C) 228.9 ppm {t, ² J _FF=121 Hz, MoF₆(NC₅H₅)₂} and 158.0 ppm {q, ² J _FF=121 Hz, MoF₆(NC₅H₅)₂}. Impurities of MoOF₄(NC₅H₅) (142.5 ppm, 0.5 mol %) and [MoOF₅]⁻ (F_eq, 130.4 ppm, d, ² J _FF=48 Hz, 1.6 mol %) at 22 °C; and of [MoF₇]⁻ (269.0 ppm, 1.5 mol %) and [MoOF₅]⁻ (F_eq, 129.2 ppm, d, ² J _FF=51 Hz, 0.8 mol %) at −80 °C, small amount of white precipitate at −80 °C. ¹H NMR (in CH₂Cl₂ at −20 °C) 7.98 ppm {H1, d, ³ J _HH=4.9 Hz} 6.97 ppm {H2, t, ³ J _HH=7.05 Hz} 6.55 ppm {H3, t, ³ J _HH=5.7 Hz}. ¹³C NMR (in CH₂Cl₂ at −30 °C) 143.76 ppm {C1, ddd, ¹ J _CH=184.6 Hz, ² J _CH=12.3 Hz} 139.7 ppm {C3, dt, ¹ J _CH=166.5 Hz, ² J _CH=5.7 Hz} 125.0 ppm {C2, dt, ¹ J _CH=168.3 Hz, ² J _CH=5.9 Hz}.

X‐ray Crystallography. Crystal Growth and Mounting

0.011 g of MoF₆(NC₅H₅) was dissolved in pyridine to afford a yellow solution consisting of dissolved MoF₆(NC₅H₅)₂ that, when kept at −35 °C over two days, afforded a cluster of orange crystals. The solvents were removed under dynamic vacuum.

Bright orange crystals of MoF₆(NC₅H₅) were grown by reacting 0.038 g (0.48 mmol) of pyridine with 0.128 g (0.610 mmol) of MoF₆ in 0.372 g of dichloromethane to afford a bright orange solution which was then cooled down from −3 to −35 °C. Note that MoF₆(NC₅H₅) dissolves in dichloromethane when synthesized but redissolving the solid in dichloromethane only shows partial solubility.

An aluminium trough was cooled between −50 and −80 °C with N_2(g) that was generated from a Dewar filled with N_2(l) by passing a flow of nitrogen gas through the liquid. The crystals were picked with a nylon cryo‐loop previously dipped in perfluorinated polyether oil (Fomblin Z‐15).

Data Collection and Reduction

The Rigaku SuperNova diffractometer equipped with a Dectris Pilatus 3R 200K−A hybrid‐pixel‐array detector, a four‐circle κ goniometer, an Oxford Cryostream 800, and sealed MoKα and CuKα X‐ray sources where the MoKα source (λ=0.71073 Å) at −173 °C was used. CrysAlisPro was used to process the data. ^[31]

The crystals were centered on a Rigaku SuperNova diffractometer equipped with a Dectris Pilatus 3R 200 K−A hybrid‐pixel‐array detector, a four‐circle κ goniometer, an Oxford Cryostream 800, and sealed Mo Kα and Cu Kα X‐ray sources. Data were collected using the Mo Kα source (λ=0.71073 Å) at −173 °C. Crystals were screened for quality before a preexperiment was run to determine the unit cell, and a data collection strategy was calculated based on the determined unit cell and intensity of the preliminary data. The data were processed using CrysAlisPro, which applied the necessary Lorentz and polarization corrections to the integrated data and scaled the data. A numerical (Gaussian‐grid) absorption correction was generated based on the indexed faces of the crystal.

Structure Solution and Refinement

The intrinsic phasing method (ShelXT) and least‐squares refinement (ShelXL) were used for the refinement. Structure and solution were done with Olex2 (version 1.5).[ ³² , ³³ , ³⁴ ] Non‐H atoms were refined anisotropically, and recommended weights for the atoms were determined before H atoms were introduced using a riding model (HFIX). The maximum and minimum electron densities in the Fourier difference maps were located close to the Mo atoms.

Deposition Numbers 2370068 and 2370069 contain the supplementary crystallographic data for this paper. These data are provided free of charge by the Cambridge Crystallographic Data Centre.

Raman Spectroscopy

Raman spectra were recorded in the solid state at 21 °C in flame‐sealed melting point capillaries with a Bruker RFS‐100 Raman spectrometer outfitted with a quartz beam‐splitter and liquid‐N₂‐cooled germanium detector. The 1064 nm line of a Nd:YAG laser was used for excitation of the sample, and backscattered (180°) radiation was sampled. The usable Stokes range of the collected data was 85–3500 cm⁻¹. The spectral resolution is 2 cm⁻¹ and a laser power of 100 mW was used.

Infrared Spectroscopy

Infrared spectra were recorded on solid samples at 21 °C inside a dry box using a Bruker Alpha‐P FT‐IR spectrometer outfitted with a KBr beam splitter, a RT‐DLATGS detector, and a single‐bounce ATR module. A spectral resolution of 4 cm⁻¹ was used.

NMR Spectroscopy

The NMR samples were in a heat sealed (under dynamic vacuum) 4 mm o.d. FEP tube that is then inserted into a 5 mm o.d. glass NMR tube and run on a Bruker Avance II 300 MHz spectrometer with a 5 mm broadband probe. All spectra were run unlocked in dichloromethane with an external reference (CFCl₃ for ¹⁹F and Si(CH₃)₄ for ¹H and ¹³C at 21 °C).

Computational Methods

Geometry optimizations and the corresponding vibrational frequencies were performed in the gas phase at the DFT/B3LYP (or MN15) level of theory using Gaussian 16. The basis set used is the aug‐cc‐pVTZ on N, O, F, and Mo with a Stuttgart pseudopotential also included on Mo, and the cc‐pVTZ basis set on H and C (aVTZ). ^[35] Subsequent calculations were done with the optimized geometry that matched the experimental conformations. Natural bond order (NBO) analysis was done using NBO (version 6.0). ^[36] Gaussview and Avogadro were used to visualize vibrational modes and molecular orbitals.[ ³⁷ , ³⁸ ] Time‐dependent DFT (TDDFT) was done at the camB3LYP/def2‐TZVP level of theory using the Gaussian 16 program package. ^[39] The TDDFT calculations with spin‐orbit coupling were done at the camB3LYP/ZORA‐def2‐TZVP level of theory using ORCA 5.0.[ ⁴⁰ , ⁴¹ ]

Supporting Information Summary

The data that support the findings of this study are available in the supplementary material of this article.

Conflict of Interests

The authors declare no conflict of interest.

Supporting information

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Supporting Information

van Hoeve M. D., Bell R., O'Donnell F., Hazendonk P., Wetmore S. D., Gerken M., Chem. Eur. J. 2024, 30, e202402749. 10.1002/chem.202402749

Data Availability Statement

The data that support the findings of this study are available in the supplementary material of this article.