Reactivity of VOF3 with N-Heterocyclic Carbene and Imidazolium

Article pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Reactivity of VOF3 with N‑Heterocyclic Carbene and Imidazolium Fluoride: Analysis of Ligand−VOF3 Bonding with Evidence of a Minute π Back-Donation of Fluoride Ž iga Zupanek,†,§ Melita Tramšek,† Anton Kokalj,‡,§ and Gašper Tavčar*,†,§ †

Department of Inorganic Chemistry and Technology, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia Department of Physical and Organic Chemistry, Jožef Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia § Jožef Stefan International Postgraduate School, Jamova 39, 1000 Ljubljana, Slovenia Downloaded via UNIV OF SUNDERLAND on October 25, 2018 at 12:24:33 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Reaction of vanadium(V) oxide trifluoride (VOF3) and the new “naked” fluoride reagent [(LDipp)H][F] (LDipp = 1,3-bis(2,6-diisopropylphenyl)-1,3-dihydro-2H-imidazol-2-ylidene) leads to the isolation of [(LDipp)H][VOF4] (1) where the long sought discrete [VOF4]− anion was finally obtained. The neutral [(LDipp)VOF3] (2) complex was synthesized by a similar reaction between VOF3 and bulky N-heterocyclic carbene (NHC) ligand LDipp. In this context, we analyzed, by means of DFT calculations, intermolecular interactions between [(LDipp)VOF3] (2) complexes in the crystal structure and realized that these interactions have a significant effect on the V−Ftrans bond length. We further scrutinized ligand bonding within [(LDipp)VOF3] (2) and related complexes, because, in this kind of complexes, a rather short distance between CNHC and cis-halogen atoms has spurred some discussion about the type of interactions between them. We provide evidence of a minute π back-bonding into NHC ligands, which is larger for chloride [(NHC)VOCl3] than fluoride [(NHC)VOF3] complexes, although the fluoride ions are, counterintuitively and to a larger degree, involved in back-bonding than chloride ions. The influence of π back-bonding on V− Ftrans and V−Fcis bond lengths was also rationalized. Finally, the hydrolysis of [(LDipp)VOF3] (2) product was studied and [(LDipp)H][VO2F2] (3) salt was obtained and characterized as the most stable product in this system.

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INTRODUCTION Transition metal oxide fluorides and their anionic analogues are the subject of numerous research papers. Vanadium is one of the transition metals that is in the spotlight of research. It is involved in a variety of vanadium oxide fluoride compounds in different oxidation states (IV, V). Vanadium(IV) compounds are studied for the application of their magnetic or catalytic properties,1,2 whereas vanadium(V) oxide trifluoride (VOF3) is used as a reagent in different coupling reactions and asymmetric synthesis in organic chemistry.3 In a recent study, vanadium(V) dioxide fluoride (VO2F) is considered a suitable material for the use in Li ion batteries.4 Reactions of VOF3 with nitrogen and oxygen donor ligands usually lead to the formation of neutral VOF3 complexes, while use of phosphorus or arsenic ligands leads to the formation of varying amounts of products including [VOF3(OEPh3)2] (E = P or As) and compounds with [VOF4]− anion according to NMR spectroscopy.5 Reactions of VOF3 with fluoride donors lead to the formation of different anionic species including [VOF5]2−,6−9 [V2O2F8]2−,10 [V2O2F9]3−,11 and [V3O3F12]3−.11 The most common method for the preparation of such species is nowadays solvothermal synthesis; however, reduction of © XXXX American Chemical Society

vanadium(V) to vanadium(IV) or even vanadium(III) during such procedures is quite common.12−14 First of the prepared vanadium(V) oxide fluorides containing [VOF4]− anion was reported for the CsVOF4.15 Vibrational spectroscopy for the compound was interpreted as C4v symmetry for the discrete anion; however, the authors suggested a possible polymeric structure due to splitting of the bands in the infrared spectrum. Nevertheless, when the crystal structure of the salt was later determined, the anion was described as a polymeric ribbon of distorted [VOF4]− octahedra connected through fluorine bridges.16 Further studies of MVOF4 compounds with small cations (M = Na+, K+, Rb+, Cs+, Tl+, Ag+, NH3OH+) also led to polymeric crystal structures with chains of [VOF4]− anions connected through fluorine bridges.17−19 Studies with double protonated ethylene diamine (en) resulted in the structural characterization of [enH2][VOF4(H2O)]2 with a discrete anion in which vanadium is octahedrally coordinated due to a water molecule located trans to the oxygen atom in [VOF4]−.20 The same octahedral coordination was reported Received: August 22, 2018

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DOI: 10.1021/acs.inorgchem.8b02377 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

were recorded at 298 K. The chemical shifts of 1H and 13C were referenced to residual signals of deuterated solvent and are reported relative to tetramethylsilane (TMS) as primary reference. 19F signals were referenced to CFCl3 in deuterated benzene (C6D6) as an external standard, and 51V signals were referenced according to lock frequency and are given relative to the neat VOCl3. Raman Spectroscopy. Raman spectra were recorded at room temperature on a Horiba Jobin Yvon Labram-HR spectrometer coupled with an Olympus BXFM-ILHS microscope. Samples were excited by the 633 nm emission line of a 24.3 mW He−Ne laser with a power output of 14 mW on the sample. The solid samples were loaded inside the glovebox (compounds [(LDipp)H][VOF4] (1) and [(LDipp)VOF3] (2)) or in the air (compound [(LDipp)H][VO2F2] (3)) into 0.3 mm quartz capillaries, which were previously vacuum-dried. Crystal X-ray Structural Analysis. Details of the crystallographic data collection and refinement parameters are given in Table S3 in the Supporting Information. Crystal data for all compounds were collected on a Gemini A diffractometer equipped with an Atlas CCD detector, using graphite monochromated Cu Kα radiation. The data were treated using the CrysAlisPro software suite program package.26 Analytical absorption correction was applied to all data sets.27 Structures were solved with the SHELXT program.28 Structure refinement was performed with the SHELXL software29 implemented in the program package Olex2.30 Elemental Analysis. Compounds [(LDipp)H][VOF4] (1) and [(LDipp)VOF3] (2) were transferred into silver foils in the glovebox under a argon atmosphere. Samples with [(LDipp)H][VO2F2] (3) salt were prepared with a similar procedure in the laboratory. Carbon, hydrogen, and nitrogen contents were determined using a CHNS elemental analyzer vario EL cube (Elementar) operating in the CHN mode. Synthesis Procedure. Synthesis of [(LDipp)H][VOF4] (1). VOF3 (100 mg; 0.807 mmol) was loaded into a Schlenk flask and dissolved with 10 mL of CH3CN. A solution of [(LDipp)H][F] (330 mg; 0.808 mmol) in 15 mL of CH3CN was added to the orange VOF3 solution, which was left to stir for 16 h at ambient temperature. The solution was dried in vacuo, producing greenish brown powder used for Raman and NMR spectroscopic measurements and elemental analysis. Concentration and storage of the reaction mixture in CH3CN at −15 °C for 3 days yields crystals suitable for X-ray analysis. Yield: 407 mg (94.7%). 1H NMR (303.0 MHz, 25 °C, CD3CN): δ 8.94 (s, 1H, C2−H), 7.88 (s, 2H, CHCH), 7.66 (t, J = 7.6 Hz, 2H, p-ArH), 7.48 (d, J = 7.5 Hz, 4H, m-ArH), 2.42 (sept, J = 6.7 Hz, 4H, CHMe2), 1.27 (d, J = 6.7 Hz, 12H, CH3), 1.20 (d, J = 6.7 Hz, 12H, CH3); 19F NMR (285.1 MHz, 25 °C, CD3CN): δ 129.0 (oct, J = 87.0 Hz); 51V NMR (79.6 MHz, 25 °C, CD3CN): δ −796.9 (quint, J = 87.0 Hz). Elemental analysis calculated for C27H37N2OF4V (Mw = 532.54 g/ mol): C 60.90, H 7.00, N 5.26%; found: C 61.80, H 7.14, N 5.36%. Synthesis of [(LDipp)VOF3] (2). VOF3 (200 mg; 1.61 mmol) and Dipp (630 mg; 1.62 mmol) were loaded into a Schlenk flask. 15 mL of L THF was added to the reagent at ambient temperature. At first, the opaque deep orange solution became clear within a few hours. The reaction mixture was stirred for 16 h at ambient temperature. The solution was concentrated in vacuo, and crystals formed within 3 days by slow evaporation of the solvent under static vacuum. Crystals were washed with 5 mL of toluene. Yield: 653 mg (79%). 1H NMR (303.0 MHz, 25 °C, C6D6): δ 7.14−7.08 (m, 2H, p-ArH), 7.05−6.98 (m, 4H, m-ArH), 6.42 (s, 2H, CHCH), 2.82 (sept, J = 6.8 Hz, 4H, CHMe2), 1.42 (d, J = 6.8 Hz, 12H, CH3), 1.01 (d, J = 6.8 Hz, 12H, CH3); 13C NMR (76.2 MHz, 25 °C, C6D6): δ 146.6, 134.6, 131.4, 124.6, 123.0, 29.5, 26.1, 23.2; 19F NMR (285.1 MHz, 25 °C, C6D6): δ 177.7 (br m, 1F, F2), 173.8 (br m, 2F, F1 and F3); 51V NMR (79.6 MHz, 25 °C, C6D6): δ −733.5 (br m). Elemental analysis calculated for C27H36N2OF3V (Mw = 512.53 g/mol): C 63.27, H 7.08, N 5.47%; found: C 63.39, H 7.12, N 5.75%. Synthesis of [(LDipp)H][VO2F2] (3). VOF3 (100 mg, 0.807 mmol) was added to the solution of LDipp (314 mg, 0.808 mmol) in dry THF (10 mL) in a plastic container. The reaction mixture was stirred at ambient temperature for 12 h and later exposed to the outside atmosphere. After the whole solvent evaporated, residual solid was

for [C4N2H12]3[V2O2F8][VOF4(H2O)]2 in which a discrete [VOF4(H2O)]− unit is highly distorted as a result of the transeffect of the VO group on the lengthened V−OH2 bond.13 The focus of our work is twofold. First, we studied the reactivity of VOF3 with the goal to synthesize and structurally characterize discrete vanadium(V) oxide fluoride anionic species that are less common than their oligomeric or polymeric counterparts. Especially interesting is the [VOF4]− anion, which has not been previously structurally characterized as a discrete anion. Literature studies showed that the increase of the size of the used cation to 1-ethyl-3-methylimidazolium leads to the isolation of related dimeric [VOF4]− anion,10 suggesting the possible route to our goal. As this cation is related to the recently prepared “naked” fluoride reagent [(LDipp)H][F] (LDipp = 1,3-bis(2,6-diisopropylphenyl)-1,3dihydro-2H-imidazol-2-ylidene),21 we were fairly confident that its fluoride donor capabilities and steric properties could lead to the isolation of the product with the discrete [VOF4]− anion similarly as in the case of [GeF5]− and [SiF5]−.22 In addition to identification of discrete vanadium(V) anionic species in [(LDipp)H][VOF4] (1), we also synthesized and structurally characterized the neutral [(LDipp)VOF3] (2) complex. The second focus of this work is to analyze, by means of DFT calculations, ligand bonding within [(LDipp)VOF3] (2) and related complexes. Namely, in this kind of complexes, a rather short distance between CNHC and cis-halogen atoms spurred some interest and discussion about the type of interactions between them, with emphasis on back-bonding.23,24 We show that back-bonding is rather minute for NHC ligands and it mainly stems from the vanadium(V) ion, although F ions are also involved to a small extent. We further scrutinize the influence of crystal packing interactions on the V−Ftrans bond length and show that the hydrogen bonding interactions with LDipp ligands of neighboring complexes significantly elongate the V−Ftrans bond.

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EXPERIMENTAL SECTION

General Information. Most of the experiments were carried out under an inert atmosphere of dry argon, using standard Schlenk and glovebox (M. Braun) techniques, except for the preparation of [(LDipp)H][VO2F2] (3). Use of reaction vessels made of polymers is advised as HF starts to form during the slow decomposition of reactants. VOF3 (99%) was obtained from Alfa Aesar and used as received. LDipp and [(LDipp)H][F] were prepared according to a synthetic procedure in the literature.21,25 Diethyl ether, tetrahydrofuran (THF), and toluene were dried in a mixture of sodium and benzophenone until the solution turned deep purple, distilled under argon atmosphere, freeze−thawed, and stored over 3 Å molecular sieves. Acetonitrile (CH3CN) was degassed by freeze−thawing cycles and stored over 3 Å molecular sieves for at least 48 h prior to use. Deuterated NMR solvents were obtained from Deutero, stored in a glovebox, and dried over 3 Å molecular sieves. Glassware was ovendried overnight at 160 °C. Caution! Some care in handling these compounds is needed. VOF3 must be handled in a well ventilated hood and protective clothing must be worn at all times. During the reactions with VOF3, HF can form, which is corrosive and toxic. The experimentalist must become familiar with reagents and hazards associated with them. NMR Spectroscopy. Samples were loaded into NMR tubes in a glovebox under an argon atmosphere. NMR spectra (1H, 13C, 19F, 51 V) were recorded at the Slovenian NMR Centre at the National Institute of Chemistry, using an Agilent Technologies Unity Inova 300 MHz spectrometer with a broadband probe (1H at 303.0 MHz, 13 C at 76.2 MHz, 19F at 285.1 MHz, and 51V at 79.6 MHz). Samples B


Article

Inorganic Chemistry washed three times with 2 mL of stock THF. Crystals were obtained when the off-white solid (purity was proven by NMR spectroscopy) was dissolved in 3 mL of CH3CN and solvent slowly evaporated. Yield: 256 mg (62.1%). 1H NMR (303.0 MHz, 25 °C, CD3CN): δ 9.00 (t, J = 1.5 Hz, 1H, C2−H), 7.88 (d, J = 1.5 Hz, 2H, CHCH), 7.66 (t, J = 7.8 Hz, 2H, p-ArH), 7.48 (d, J = 7.7 Hz, 4H, m-ArH), 2.42 (sept, J = 6.8 Hz, 4H, CHMe2), 1.27 (d, J = 6.8 Hz, 12H, CH3), 1.20 (d, J = 6.8 Hz, 12H, CH3); 19F NMR (285.1 MHz, 25 °C, CD3CN): δ −34.1 (br m); 51V NMR (79.6 MHz, 25 °C, CD3CN): δ −594.8 (s). Elemental analysis calculated for C27H37N2O2F2V (Mw = 510.543 g/ mol): C 63.52, H 7.31, N 5.49%; found: C 62.99, H 7.32, N 5.48%.

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latter see section S6.1 in the Supporting Information). To this end, we utilized symmetrized geometries of the [(L)VOX3] complexes, which have CS symmetry. This point group has A′ and A″ irreducible representations, and the latter is appropriate for back-bonding analysis. The electron charge density associated with molecular orbitals of A″ symmetry is obtained as Nocc

ρA ″(r) =

COMPUTATIONAL METHODS

ΔρA ″(r) = ρAL″− M (r) − ρAL″(r) − ρAM″ (r)

where stands for A″ component of charge density of species S (S = L−M, L, or M) and the geometries of ligand L and counterpart M are kept the same as in the L−M complex. As to calculate the symmetry decomposed charge densities, we modified and extended the pp.x code of Quantum ESPRESSO. Intermolecular interactions between L−M complexes in the crystal structure were visualized by means of the charge density difference calculated as Ncmp

Δρ(r) = ρ tot (r) −

i=1

L→M

(5)

where ρ (r) is the charge density of the whole crystal, Ncmp is the number of L−M complexes in the unit cell, and ρi(r) is the charge density of the ith standalone complex, calculated with the same geometry as that in the crystal structure. Vibrational Calculations of Isolated Anions and Molecules. Molecular calculations (e.g., geometry optimization and vibrational frequency calculations for isolated [VOF4]− and [VO2F2]− anions and VOF3 molecule) were performed with the Gaussian09 program43 using in addition to the PBE exchange-correlation functional also the hybrid B3LYP functional44 and Møller−Plesset second-order perturbation theory (MP2).45 Electrons were described with all electron basis sets. We used triple-ζ basis sets augmented with polarization and diffuse functions, that is, aug-cc-pVTZ46−49 (for comparison between different methods and basis sets, see Tables S8− S10 in the Supporting Information).

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RESULTS AND DISCUSSION Synthesis and Structural Characterization of [(LDipp)H][VOF4] (1). A reaction of VOF3 with [(LDipp)H][F] in acetonitrile (CH3CN) gives the [(LDipp)H][VOF4] (1) salt in almost quantitative yields. The product was recrystallized from the acetonitrile solution as [(LDipp)H][VOF4]·2CH3CN (1a). The structure of one asymmetric unit is presented in Figure 1. X-ray structural analysis reveals an imidazolium [(LDipp)H]+ cation and discrete [VOF4]− anion. To the best of our knowledge, this is the first structural identification of this discrete anion. The product crystallizes as a solvate with nitrogen atoms of the CH3CN molecules turned away from the vanadium(V) center negating any possibility of the coordination to the vanadium(V) ion like in the aforementioned compound with [VOF4(H2O)]− anion.13,20 In contrast, CH3CN molecules are not present in noncrystalline powder as revealed by NMR and elemental analysis. In the crystal structure, the orientation of the [VOF4]− anion is disordered. Two preferred orientations of the anion are observed, with occupancy of the first domain (A) being 73% and of the second domain (B) being 27%. The O1A−V−O1B angle between domains is 131.51(3)°. Both domains of the [VOF4]− anion show a square pyramidal arrangement of apical oxygen and basal fluorine atoms, with the vanadium(V) center having the same position for both orientations. Herein we describe

where the sum runs over all the occupied molecular orbitals of the L− M complex and the geometry of the ligand is kept the same as that in the L−M complex. The integral of eq 1 was calculated using the molecularpdos.x code42 of Quantum ESPRESSO. The L→M donation can be obtained analogously to eq 1 by projecting to occupied σ orbitals of the ligand. However, the σ donating power of a ligand (QσL→M) was instead estimated from the Bader charge of the M counterpart (qM) and the π back-bonding (Qπ*L←M), i.e.

Q σL → M = Q πL*← M − qM

∑ ρi (r)

tot

(1)

i=1

(4)

ρSA″(r)

Nocc

∑ |⟨ϕπ *|ψi⟩|2

(3)

i∈A″

and the A″ component of charge density difference is then calculated as

DFT Calculations of Crystal Structure. Calculations of crystal structures were performed in the framework of density functional theory (DFT), using the generalized gradient approximation of Perdew−Burke−Ernzerhof (PBE),31 plane-wave basis set with ultrasoft pseudopotential,32,33 and the PWscf code from the Quantum ESPRESSO distribution.34 Kohn−Sham orbitals were expanded in a plane-wave basis set up to a kinetic energy cutoff of 50 Ry (400 Ry for the charge density cutoff); these cutoffs yield well converged results. All degrees of freedom including the unit cell size and shape were relaxed using a variable-cell Broyden−Fletcher−Goldfarb−Shanno optimization. Brillouin-zone integrations were performed using only the gamma k-point. To better describe intermolecular interactions in the crystal structure, we also utilized consistently applied empirical dispersion correction of Grimme, known as D2.35,36 The results obtained by this scheme will be referred to by the PBE-D denotation. Electronic Structure and Bonding Analysis. In addition to the full crystal structure, we also calculated standalone [(L)VOX3] complexes, VOX3 molecule, and [VOX4]− and [VO2X2]− anions (X = F, Cl, or Br) at the PBE/plane-wave level of theory. To this end, we used a “molecule in a box” approach with a large cubic box of 25 Å size and Makov−Payne correction.37 This approach was utilized to perform electronic structure and Bader charge analyses. The latter was performed using the Bader code38,39 by generating charge densities with the PAW (projector-augmented-wave) potentials40 and 1000 Ry kinetic energy cutoff for charge density. Charge density difference plots and molecular graphics were produced by the XCRYSDEN graphical package.41 The bonding analysis of standalone [(L)VOX3] complexes was performed as described below (L stands for ligand, whereas, for the sake of brevity, the VOX3 fragment will be designated by M). The L← M back-donation (Qπ*L←M) into the unoccupied π* orbital of the ligand (ϕπ*) was estimated by projecting occupied molecular orbitals of the whole L−M complex (ψi) onto the virtual π* orbital of the ligand, i.e. Q πL*← M =

∑ |ψi(r)|2

(2) L←M

donation and Qπ* back-donation are where both Qσ considered as positive numbers, whereas the Bader charge qM is typically negative (this implies that usually QσL→M > Qπ*L←M). The electron charge transfer associated with L←M back-donation can be visualized using the symmetry decomposed charge density difference and the associated charge-displacement analysis (for the C


Article

Inorganic Chemistry

plane of basal fluorine atoms toward the center of the pyramid by 0.446(3) Å. Further description of bond lengths, angles, anion geometry, and bond valence sum of both domains of the anion are provided in Table S4 in the Supporting Information. To evaluate the stability of the [VOF4]− anion, the [(LDipp)H][VOF4] (1) salt was dissolved in acetonitrile solutions with 10 or 30 equiv of water. Isolation of the products after 16 h experiment and subsequent analysis of their 19 F and 51V NMR spectra revealed that, in both instances, partial hydrolysis of the [VOF4 ]− anion does occur. Decomposition of the anion leads to the formation of the [VO2F2]− anion (Figures S5 and S6, and Table S1 in the Supporting Information). Interestingly, prolonged reaction times do not completely convert starting [VOF4]− and the initial ratio between anions is kept even after 5 days. The addition of another equivalent of [(LDipp)H][F] does not lead to the formation of known isolated [VOF5]2− anion6 but rather a mixture of [(LDipp)H][VOF4] (1) and unreacted starting imidazolium fluoride according to 19F NMR spectroscopy (Figure S7 in the Supporting Information). Prolonged reaction times do not yield any additional components. We attribute the lack of observation of the octahedral [VOF5]2− anion to the bulkiness of the imidazolium [(LDipp)H]+ cation. Namely, we showed in our previous publication22 that the role of the bulky cation in stabilizing the pentacoordinated monovalent anions against the hexacoordinated divalent ones is due to steric hindrance as well as electrostatic effects. Synthesis and Structural Characterization of [(LDipp)VOF3] (2). Using a bulky neutral LDipp in a reaction with VOF3 in ethers (THF, diethyl ether) leads to the formation of the expected [(LDipp)VOF3] (2). To obtain pure crystals of the neutral complex and remove any anionic vanadium oxide fluoride species, crystals were washed with small amounts of CH3CN. LDipp is an N-heterocyclic carbene (NHC) that has become since its isolation a widely utilized ligand for late transition metals in low and medium oxidation states, while oxophilic metal species in their highest oxidation states received considerably less attention.50−53 Our [(LDipp)VOF3] (2) complex is the first reported coordination compound of VOF3 with NHC and the second structurally determined NHC

Figure 1. Asymmetric unit of the [(LDipp)H][VOF4]·2CH3CN (1a). Ellipsoids are drawn at 50% probability. For clarity reasons, only domain A of the [VOF4]− anion is shown and “wingtips” of LDipp are shaded. Selected bond lengths [Å] and angles [deg]: V1−O1A 1.560(2), V1−F1A 1.776(2), V1−F2A 1.815(2), V1−F3A 1.807(2), V1−F4A 1.794(2), O1A−V1−F1A 107.0(1), O1A−V1−F2A 104.5(1), O1A−V1−F3A 103.1(1), O1A−V1−F4A 103.4(1). Detailed descriptions of bond lengths, angles, and anion geometry can be found in Table S4 in the Supporting Information.

only more representable results of the dominant domain A; for structural details of domain B, see Table S4 in the Supporting Information. The V−O bond is close to perpendicular to the plane of the imidazolium ring, and basal fluorine atoms are almost parallel to this plane with the dihedral angle between both planes being 10.42(6)°. The length of the V−O bond is 1.560(2) Å, and four basal V−F bonds are between 1.776(2) and 1.815(2) Å. O−V−F angles are between 103.1(1)° and 107.0(1)°, and trans F−V−F angles are 149.8(1)° and 152.1(1)°. The corresponding geometry index τ5 is 0.038, indicating that the geometry of the anion is close to square pyramid, although the vanadium(V) ion is dislocated from the

Figure 2. Asymmetric unit of the [(LDipp)VOF3] (2). Ellipsoids are drawn at 50% probability. For clarity reasons, hydrogen atoms are omitted and “wingtips” of LDipp are shaded. Only domain A of the disordered isopropyl groups are shown. Selected bond lengths [Å] and angles [deg]: V1−O1 1.558(2), V1−F1 1.771(1), V1−F2 1.804(1), V1−F3 1.781(1), V1−C2 2.141(2), V2−O2 1.561(1), V2−F4 1.776(1), V2−F5 1.802(1), V2−F6 1.784(1), V2−C31 2.139(2), O1−V1−F1 104.61(8), O1−V1−F2 110.08(7), O1−V1−F3 103.99(8), O1−V1−C2 100.22(7), O2−V1−F4 104.40(7), O2−V1−F5 108.77(6), O2−V1−F6 105.01(7), O2−V1−C31 98.95(7). Further descriptions of bond lengths and angles are listed in Table S5 in the Supporting Information. D


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Inorganic Chemistry complex with metal oxide fluoride (MOF). Interestingly, the first reported NHC−MOF compound contained a metal from the same d-block group: niobium.54 The molecular structure of neutral [(LDipp)VOF3] (2) complex is shown in Figure 2. In the crystal structure, the asymmetric unit consists of two separate [(LDipp)VOF3] (2) subunits with the 86.58(7)° dihedral angle between both planes of NHC rings. Both subunits are connected by hydrogen interactions, which consequently prevent disorder. However, positional disorder of two isopropyl groups (iPr) is present within the first subunit. Additional information on this disorder is provided in Table S6 in the Supporting Information. Due to substantial structural similarities between subunits, only the results of subunit 1 are reported here, but the information about subunit 2 is available in Table S5 in the Supporting Information. The coordination sphere of the vanadium(V) center is distorted square pyramidal, where three fluorine atoms form the corners of the distorted square with the fourth corner being occupied by the CNHC atom from the LDipp ligand. The oxygen atom is in the apical position of the pyramid with the V1−O1 bond length being 1.558(2) Å. Although the τ5 index with a value of 6.7·10−5 indicates square pyramidal geometry of the complex, the distances between the opposite basal atoms differ significantly (F1−F3 3.428(2) Å and F2−C2 3.808(2) Å), which is obviously due to different chemical characteristics of F and C atoms. The V−F bond trans to the V−CNHC bond is slightly elongated (V1−F2 1.804(1) Å) in comparison to the cis-fluorides (V1−F1 1.771(1) Å and V1−F3 1.781(1) Å). Recently, it was shown in the trifluoridoorganogold(III) complex [AuF3(SIMes)] (SIMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolidin-2-ylidene) that, in contrast to Fcis, the Ftrans ligand involved in such elongated metal−Ftrans bond is capable of substitution and could be utilized for selective fluorination reactions.55 However, the elongated V−Ftrans bond could be due to crystal packing effects rather than electron donating nature of the CNHC atom of the LDipp ligand, which we will show in the Computational Crystal Structure Results section (below). As for the orientation of the VOF3 fragment, the axis linking two Fcis atoms is almost perpendicular to the plane of the NHC ring and these fluorine atoms are bent toward the LDipp ligand with the C2−V1−F angle being 82.79(6)° and 82.48(6)° for F1 and F3, respectively. Bending of the cis-atoms toward the NHC ligand was also observed in the previously published neutral square pyramidal VOCl3 complex with LIMes = 1,3-bis(2,4,6-trimethylphenyl)-1,3dihydro-2H-imidazol-2-ylidene (C1−V1−Cl1 81.04(6)° and C1−V1−Cl3 82.20(6)°).23 Further comparison of the two analogous structures surprisingly reveals that the V−CNHC distance with uncertainty considered is practically the same in [(LDipp)VOF3] (2) (C2−V1 2.141(2) Å) and in [(LIMes)VOCl3] (C1−V1 2.137(2) Å) despite the fact that the fluorine atom is more electronegative than chlorine and has a smaller van der Waals radius (F 1.46 Å; Cl 1.82 Å).56 Moreover, literature results with VOCl3 also report surprisingly short Clcis−C2 distances that could be attributed to unusual interactions between them.23 However, recent results of theoretical calculations for [(NHC)NbCl5] complexes showed that there is no direct back-donation from Cl to CNHC, but rather the niobium metal center exploits the electronic charge of the Cltrans atom for back-donation, yet even this type of back-donation is rather minute.24 Due to the short Fcis−CNHC distance (F1−C2 2.601(2) Å and F3−C2 2.599(2) Å), we were curious if a similar back-donation effect can be seen for

the [(LDipp)VOF3] (2) complex, and the corresponding results are presented in the Electronic Structure and Bonding Analysis of Standalone Complexes section (below), where we show that our results concerning back-donation are in agreement with recent finding of Ciancaleoni et al.24 for [(LMe)NbCl5] (LMe = 1,3-dimethyl-1,3-dihydro-2H-imidazol-2-ylidene) and in variance with that of Abernethy et al.,23 who claimed significant back-donation for [(LMe)VOCl3]. Increasing the molar ratio between VOF3 and LDipp ligand to 2:1 did not afford any kind of other possible oligomeric products. According to the 19F NMR spectrum, only [(LDipp)VOF3] (2), [(LDipp)H][VOF4] (1), and unreacted starting VOF3 are present in the reaction mixture (Figure S8 in the Supporting Information), showing that the steric effect of the LDipp is prevalent and vanadium oxide fluoride oligo- or polymers could not be formed despite the indication that oligomeric d-block metal fluorides could be stronger acids than their parent monomers.57 Likewise, the reaction with a molar ratio of 1:2, with an additional equivalent of LDipp ligand, did not yield any new species. In the 1H and 19F NMR spectra of the reaction mixture, only signals of neutral complex [(LDipp)VOF3] (2) and unreacted starting LDipp were observed (Figure S9 in the Supporting Information). We have also tried to prepare rare molecular VO2F complex with LDipp in the reaction of [(LDipp)VOF3] (2) with (Me3Si)2O following the procedure for the preparation of the known neutral [(L)VO2F] (L = bipy, py, phen, tmen) complexes,58 but the reaction did not proceed in proposed acetonitrile, tetrahydrofuran, or (Me3Si)2O as a solvent. NMR spectroscopy showed only the starting material with a small amount of [(LDipp)H][VOF4] (1). When assessing stability of [(LDipp)VOF3] (2), we found that hydrolysis of the complex led to the formation of a mixture of [(LDipp)H][VOF4] (1) and [(LDipp)H][VO2F2] (3) according to the following equation: 2[(LDipp)VOF3] + H 2O → [(LDipp)H][VOF4 ] + [(LDipp)H][VO2 F2]

(6)

In the 19F NMR spectrum of the hydrolysis experiment, also a signal at δ = −139 ppm can be observed (Figure S10 in the Supporting Information), which can be explained as the impurity obtained during decomposition of the [(LDipp)VOF3] (2). The breakdown of the complex proceeds through the formation of HF which in turn slowly reacts with the glass reaction vessel, forming SiF4. The decomposition cascade ends with the reaction of SiF4 with [(LDipp)H][F] in the solution, yielding a [SiF5 ] − anion. Release of HF during the decomposition of VOF3 in acetonitrile solution, followed by the formation of SiF4, was already observed.59 When the reaction was repeated in the reaction vessel made from FEP (fluorinated ethylene propylene), we did not detect the peak at −139 ppm, proving not only the in situ formation of HF during the hydrolysis according to the mechanism proposed in Scheme S1 in the Supporting Information but also that SiO2 from the glassware is indeed the source of the proposed [SiF5]− impurity. Synthesis and Structural Characterization of [(LDipp)H][VO2F2] (3). Compounds with a [VO2F2]− anion are usually obtained from the reaction between vanadium oxides or other vanadates with hydrofluoric acid,60−62 but such an approach usually yields an anion in oligomeric or polymeric form in practically all known crystal structures, except [Ph4M][VO2F2] E


Article

Inorganic Chemistry (M = P, As),61 [{VO(salen)}2(μ-F)][VO2F2],63 and [Et4N]6[HV22O54(VO2F2)].64 We were able to prepare crystals of [(LDipp)H][VO2F2] (3) salt through the reaction of LDipp and VOF3 in THF. After initial stirring, the solution was left standing with a slow diffusion of air into the reaction mixture. Apparently, moisture from the atmosphere is enough to decompose neutral complex [(LDipp)VOF3] (2) according to eq 6. Isolation of pure [(LDipp)H][VO2F2] (3) from a dried combination of salts is possible due to its poor solubility in THF in comparison to the [(LDipp)H][VOF4] (1). The crystal structure of [(LDipp)H][VO2F2] (3) contains an imidazolium [(LDipp)H]+ cation and a discrete [VO2F2]− anion; its asymmetric unit is presented in Figure 3. Bond distances in

mask many of the bands contributed by vanadium(V) oxide fluorides species. Raman spectra of [(LDipp)H][VOF4] (1), [(LDipp)VOF3] (2), and [(LDipp)H][VO2F2] (3) are presented in Figure 4. We compared spectra of [(LDipp)H][VOF4] (1)

Figure 4. Raman spectra of [(LDipp)H][VOF4] (1), [(LDipp)VOF3] (2), and [(LDipp)H][VO2F2] (3) with assigned bands for V−O and V−F vibrations.

and [(LDipp)H][VO2F2] (3) with recently published [(LDipp)H][GeF5]22 in order to first identify bands of the cation and in turn determine bands in the spectra contributed by vanadium oxide fluoride anions. The V−O stretching band can be assigned to the bands at 1030 cm−1 for the [VOF4]− anion and at 971 cm−1 for the [VO2F2]− anion. This is a characteristic region for the identifications of V−O vibrations in vanadium oxide fluoride species. Bands at 636 and 631 cm−1 can be assigned to V−F vibrations. The most intense bands observed in Raman spectra for V−O and V−F vibrations together with calculated vibrations are listed in Table 1. The complete lists of calculated vibrations of isolated [VOF4]− and [VO2F2]− anions and VOF3 are presented in Tables S8−S10 in the Supporting Information.

Figure 3. Asymmetric unit of the [(LDipp)H][VO2F2] (3). Ellipsoids are drawn at 50% probability. For clarity reasons, “wingtips” of LDipp are shaded. Selected bond lengths [Å]: V1−O1 1.615(1), V1−F1 1.801(1), O1−V1−O1i 106.49(9), F1−V1−F1i 108.88(7), O1−V1− F1 109.80(5), O1−V1−F1i 110.94(5). Additional descriptions of bond lengths and angles are listed in Table S7 in the Supporting Information. Symmetry code: (i) 1/2 − x, 3/2 − y, z.

Table 1. PBE/aug-cc-pVTZ Calculated Frequencies for Isolated Species Compared to Experimental Frequencies of the Strongest Raman Active Modes of [VOF4]− and [VO2F2]− Anions and VOF3

the tetrahedral anion are 1.615(1) Å for V−O and 1.801(1) Å for V−F. Angles deviate from the ideal tetrahedral angle with the smallest O1−V−O1i being 106.49(9)° and the largest O1−V−F1i being 110.94(5)°. In our case, we did not observe any orientational disorder reported in the literature with the same anionic species,63,64 probably because hydrogen interactions locked the anion in position and consequently prevented the disorder. The synthesis of [(LDipp)H][VO2F2] (3) was from its inception onward performed in air, thus proving that both product salts of eq 6 are air stable. Still further hydrolysis of [(LDipp)H][VO2F2] (3) was tested, but the [VO2F2]− anion did not produce any side product or show any other signs of reaction with water after 16 h according to NMR spectroscopy (Table S2 in the Supporting Information). Raman Spectroscopy. Raman spectroscopy was used in order to detect vanadium(V) oxide fluoride species in the samples. Assignation of these species in the spectra was challenging due to the large number of bands of the imidazolium [(LDipp)H]+ cation or neutral LDipp ligand that

frequencies [cm−1] [VOF4]− νs (VF4) ν (VO) VOF3 νs (VF3) ν (VO) [VO2F2]− νs (VF2) νa (VF2) νa (VO2) νs (VO2)

PBE/aug-cc-pVTZ

experimental

609 1023

636,a 630b 1030,a 1012b

698 1075, 1044d

721.5c 973,e 1057.8c

615 647 965 966

631,a 631c 664c 960c 971,a 969c

a

This work. bReference 10. cReference 65. dCorresponds to standalone [(LMe)VOF3] complex. eThis work, corresponds to [(LDipp)VOF3] (2). F


Article

Inorganic Chemistry

Table 2. Chemical Shifts Observed in 19F and 51V NMR Spectra of the Compounds [(LDipp)H][VOF4] (1), [(LDipp)VOF3] (2), and [(LDipp)H][VO2F2] (3) Given for Deuterated Acetonitrile (CD3CN), Dichloromethane (DCM-d2), Tetrahydrofuran (THF-d8), and Benzene (C6D6) δ [ppm] 19

[(LDipp)H][VOF4] (1)

F F 19 F 19 F

(CD3CN) (DCM-d2) (THF-d8) (C6D6)

129.0 125.6 127.4

51

(CD3CN) (DCM-d2) (THF-d8) (C6D6)

−796.9 −793.5 −790.8

19

V V 51 V 51 V 51

[(LDipp)VOF3] (2) 177.0 179.5 173.3 178.0

(2F), (2F), (2F), (1F),

−714.9 −722.3 −728.3 −733.5

142.3 144.6 169.3 173.8

(1F) (1F) (1F) (2F)

[(LDipp)H][VO2F2] (3) −34.1 −29.1 −29.8

−594.8 −597.4 −594.1

complexes with nitrogen based ligands.5 The distinct 19F NMR resonances correspond to two fluorine nuclei that are cis to the CNHC in the heterocyclic ring and one fluorine nucleus, which is in a trans position. The latter fluorine atom interacts with solvent molecules as the chemical shift of its peak changes noticeably between solvents (Table 2). Similar 1H and 19F NMR results can also be observed in much more polar solvents (CD3CN, THF-d8), which shows that, in solution, those molecules do not displace LDipp as a ligand. The 51V NMR spectrum in C6D6 shows a broad peak corresponding to the [(LDipp)VOF3] (2) at δV = −733.5 ppm, which is in line with the shifts reported for other neutral complexes of VOF3.5 The slightly crystalline powder of [(LDipp)H][VO2F2] (3) obtained after reaction in moist air was transferred from a polyethylene container into an NMR tube and dissolved with addition of either CD3CN, DCM-d2, or THF-d8, producing an almost colorless solution in all cases. The 1H NMR spectrum of the salt in any of the solvent is consistent with the presence of the imidazolium cation. In the 1H NMR spectrum in CD3CN, a triplet is observed for the proton at the C2 position of the cation at δH = 9.0 ppm (JH−H = 1.5 Hz) (Figure S3 in the Supporting Information). In the 19F NMR spectrum, a well-defined octet can be observed at δF = −29.8 ppm (JF−V = 272.1 Hz) in THF-d8, whereas, in CD3CN or DCM-d2, the chemical shift is similar (Table 2), while the multiplet structure is not resolved (Figures S3 and S4 in the Supporting Information). The chemical shift value corresponds to ionic compounds with the [VO2F2]− anion.58,63 Signals in the 51V NMR spectra support similar conclusions. A well-defined triplet resulting from coupling to two equivalent 19F nuclei is observed in THF-d8 at δV = −594.1 ppm (JV−F = 272.9 Hz), which correlates to previously reported values for [VO2F2]− anions.58,63 Meanwhile, only broad multiplets are observed in CD3CN or DCM-d2 at a similar chemical shift (Table 2). Computational Crystal Structure Results. Periodic plane-wave DFT calculations reproduce the crystal structure of [(LDipp)VOF3] (2) in fair agreement with experiment. Comparison between calculated and experimental unit cell parameters is provided in Table S11 in the Supporting Information, whereas a few relevant bond lengths and angles are compared in Table S12 in the Supporting Information. Notably, crystal structure calculations reproduce the experimentally observed elongated V−Ftrans bond with respect to V−Fcis bonds. Analysis reveals that V−Ftrans bond elongation is due to crystal packing effects rather than the electron donating nature of the CNHC atom from the LDipp ligand. In particular, in the calculated crystal structure, three CH fragments of LDipp ligands from neighboring complexes are within the distance of

NMR Spectroscopy. The bulk of the [(LDipp)H][VOF4] (1) product was transferred to NMR tubes and dissolved in deuterated solvents (acetonitrile (CD3CN), dichloromethane (DCM-d2), or tetahydrofuran (THF-d8)), forming a deep orange solution. In the 1H NMR spectra in all solvents, we observed characteristic signals for the imidazolium [(LDipp)H]+ cation. A signal is present in the 19F NMR spectrum of CD3CN at δF = 129.0 ppm (JF−V = 86.9 Hz), which is split into an octet by scalar coupling to a quadrupolar 51V nucleus (I = 7/2) and a corresponding quintet is observed in the 51V spectrum at δV = −796.9 ppm (JV−F = 86.9 Hz) (Figure S1 in the Supporting Information). Corresponding signals have similar chemical shifts also in other used solvents (Table 2). Chemical shifts and shapes of these signals correspond well to those previously reported for [Ph4As][VOF4], despite the use of a different deuterated solvent.5 Interestingly, it has been reported that the 19 F chemical shift of [(C2H5)4N][VOF4] in deuterated chloroform (CDCl3) is δF = −28.21 ppm.66 However, we assigned this value to the [VO2F2]− anion instead (later in the NMR Spectroscopy section). The presence of well-resolved multiplets in 19F and 51V NMR spectra implies that the [VOF4]− anion in solution retains its symmetrical shape observed in the crystal structure. Note that a similar result was obtained when weakly coordinating DCM-d2 solvent was used (Table 2), meaning that most likely there are no CD3CN molecules in the coordination sphere of vanadium(V) in the solution. NMR spectra of the dried bulk product obtained from a straightforward VOF3 and LDipp reaction mixture in polar solvents (CD3CN, THF-d8) show the expected signals of [(LDipp)VOF3] (2) and additional signals indicating the presence of impurities in the form of [(LDipp)H][VOF4] (1) salt, which forms with spontaneous decomposition of VOF3 and release of HF.59 To eliminate the presence of traces of this salt in solution, crystals of [(LDipp)VOF3] (2) were loaded into an NMR tube and deuterated nonpolar solvent C6D6 was added, in which [(LDipp)H][VOF4] (1) is only scarcely soluble. The solution turned pale yellow despite only moderate solubility of the [(LDipp)VOF3] (2) product in this nonpolar solvent. Doublets are observed in the 1H NMR spectrum for the two methyl groups in the isopropyl of the LDipp ligand in the complex, which are noticeably split (δH = 1.42 and 1.01 ppm) (Figure S2 in the Supporting Information). This is in agreement with previously reported values for coordinated LDipp ligand.67−69 Two broad signals are present in the 19F NMR spectrum at δF = 178.0 and 173.8 ppm with an approximate integral ratio of 1:2, respectively. Our observation is comparable with values reported for neutral VOF 3 G


Article

Inorganic Chemistry

CNHC and cis-halogen atoms, which are well below the sum of respective van der Waals radii,56 induced some interest and discussion about the type of interactions between them.23,24 These include an unusual CNHC←Clcis back-donation23 as well as a more standard CNHC←metal back-bonding.24 For the latter type of back-bonding, it is noteworthy that the V ion should not be able to back-donate electron charge, because, in current complexes, it is in a +5 oxidation state and therefore formally without valence electrons. Nevertheless, the charge density plot instead reveals a high valence electron density around the V nucleus (Figure 6). Bader analysis further reveals

2.35 Å from Ftrans (Figure 5a), and the charge density difference plot clearly shows hydrogen bond like interactions

Figure 5. PBE-D/plane-wave calculated structure of [(LDipp)VOF3] (2); for clarity, only a subset of atoms in the unit cell is drawn (note that atoms are drawn on the basis of atomic and not ionic radii). (a) In the crystal structure, three H atoms of neighboring LDipp ligands are within the distance of 2.35 Å from Ftrans and these H···F interactions cause the V−Ftrans bond to elongate by 0.04 Å with respect to the standalone [(LDipp)VOF3] (2) complex. (b) Electron charge density difference plot, calculated as the difference between the electronic density of the whole crystal minus the density of the individual standalone [(LDipp)VOF3] (2) complexes (eq 5). Electron excess regions are colored red, while electron deficit regions are blue (isosurfaces are drawn at ±0.002 e/Bohr3). H···F interactions are evident from the kidney-like electron accumulation region near the Ftrans atom and the charge deficit regions at the neighboring H atoms (marked by yellow arrows).

Figure 6. Valence electron density of the [(CO)VOCl3] complex plotted on the C−V−Cltrans plane; contours are drawn in a linear scale from 0 to 0.2 e/Bohr3 with the increment of 0.05 e/Bohr3. Notice the high electron density around the V ion, although it is formally in a +5 oxidation state. Bader population analysis reveals that the charge of the V ion in [(CO)VOCl3] is +1.93; hence, it contains about 3 valence electrons.

between the three neighboring H atoms and Ftrans (Figure 5b). If, instead of the whole crystal structure, only a single isolated [(LDipp)VOF3] (2) complex is considered in calculations, then the V−Ftrans distance is reduced by 0.04 Å, thus becoming slightly shorter than the V−Fcis distances. Electronic Structure and Bonding Analysis of Standalone Complexes. We analyzed ligand bonding within [(LDipp)VOF3] (2) and some related complexes, shown in Scheme 1 (most are model complexes used for the sake of argument), by means of PBE/plane-wave calculations. In this context, it is notable that the rather short distances between

that the V5+ ion contains about 3 valence electrons in various currently considered [(L)VOX3] complexes (Table 3), standalone VOX3 molecule and isolated oxide fluoride [VOX4]− and [VO2X2]− anions (Table S13 in the Supporting Information). On this basis, it is not surprising that back-donation in organometallic complexes with a d0 metal center was observed and is commonly explained for the carbonyl molecule as a model ligand.70−72 In order to determine whether for [(LDipp)VOF3] (2) complex the back-donation of charge into the ligand’s unoccupied π* states takes place and to what extent and how this back-donation affects the V−F bond lengths, there are two prerequisites: (i) a compound that can be used as a reference for evaluating the back-donation into π* and (ii) a large enough pool of model compounds as to avoid spurious chance correlations. Furthermore, to make calculations more trackable, we utilized a smaller LMe ligand (LMe = 1,3-dimethyl1,3-dihydro-2H-imidazol-2-ylidene) in favor of the larger LDipp ligand and consider the symmetrized geometry of the complex within the CS point group. It should be noted, though, that this symmetry constrained structure is only by 2.4 kJ/mol less stable than the relaxed low-symmetry structure of [(LMe)VOF3]. As for the first item, we started from the work of Ciancaleoni et al.,24 who showed that back-donation is significant for a model [(CO)NbCl5] complex; using charge-displacement analysis, they obtained the value of 0.13 e− and attributed it to a combination of standard CO←Nb as well as the unusual CO←Clcis back-donation. Our estimate of the cumulative CO←NbCl5 back-donation, obtained from the projection of occupied molecular orbitals onto unoccupied 2π orbitals of CO (note that the 2π of CO is doubly degenerate), eq 1, is

Scheme 1. Skeletal Structures of Standalone [(L)VOX3] Model Complexes (L = LMe, CO, CN− and X = F, Cl, Br), Used in the Computational Analysis to Disentangle Ligand−VOX3 Bonding

H


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Inorganic Chemistry

Table 3. Bader Charges (q) of VOX3 Fragments and Their Constituent Ions as Well as π Back-Donation (Qπ*L←M) and σ Donation (QσL→M) for Standalone High-Symmetry [(L)VOX3] Complexes (L = LMe, CO, CN− and X = F, Cl, Br), Calculated at the PBE/Plane-Wave Level of Theory q(VOX3)

q(V)

q(O)

q(Xcis)

q(Xtrans)

Qπ*L←M

QσL→M

(L )VOF3 (CO)VOF3 [(CN)VOF3]−

−0.302 −0.049 −0.344

+2.209 +2.265 +2.243

−0.687 −0.647 −0.715

−0.605 −0.567 −0.619

−0.615 −0.533 −0.634

0.025 0.027 0.000

0.327 0.076 0.344

(LMe)VOCl3 (CO)VOCl3 [(CN)VOCl3]−

−0.320 −0.017 −0.377

+1.901 +1.929 +1.944

−0.671 −0.612 −0.673

−0.515 −0.448 −0.529

−0.519 −0.438 −0.588

0.051 0.174 0.000

0.371 0.191 0.377

(LMe)VOBr3 (CO)VOBr3 [(CN)VOBr3]−

−0.285 +0.030 −0.354

+1.794 +1.792 +1.830

−0.665 −0.621 −0.674

−0.470 −0.377 −0.480

−0.473 −0.388 −0.549

0.065 0.247 0.000

0.349 0.217 0.354

Me

Figure 7. Analysis of L←M back-donation for [(CO)VOCl3] (left), [(LMe)VOCl3] (middle), and [(LMe)VOF3] (right). A″ symmetry resolved charge density differences, ΔρA″(r) of eq 4, shown in 3D with isosurfaces at ±0.001 e/Bohr3 (top row) and in 2D on the C−V−Xcis plane (X = Cl or F) with nine contours in a linear scale from −0.002 to +0.002 e/Bohr3 (bottom row). The blue (red) color represents electron deficit (excess) regions; i.e., electron charge moved from blue to red regions (indicated by green arrows).

0.14 e−, thus being in good agreement with the aforementioned value. However, in the current work, we consider vanadium complexes instead. An analogous calculation for the vanadium [(CO)VCl5] complex reveals that back-donation is even larger, 0.19 e−, whereas, for the pentacoordinated [(CO)VOCl3], back-donation is 0.17 e−. This latter complex was used as our reference for back-donation. We would use a more closely related [(CO)VOF3] instead; however, CO interacts too weakly with VOF3. The resulting C−V bond length is 2.59 Å, and due to such weak interaction, backdonation is only 0.03 e−. As for the second item, we analyzed the back-donation and bond lengths for nine [(L)VOX3] complexes (L = LMe, CO, CN− and X = F, Cl, Br) (Scheme 1). The choice of these ligands is motivated by the observation that CO is a ligand archetypical for back-donation, whereas, for the CN− ligand, little (if any) back-donation is expected (see ref 24). As for halide ions, Cl and Br are less electronegative than F, and correspondingly, we can expect larger back-bonding for chloride and bromide complexes. Table 3 indeed reveals that these expectations are correct.

As mentioned above, CO←VOCl3 back-bonding of 0.17 e− is significant, but to disentangle its local characteristics, the symmetry resolved charge density differences (i.e., A″ component) are depicted in Figure 7. The ΔρA″(r) plots clearly show that, in this case, back-bonding mainly stems from Clcis ions. Note also that the red electron charge accumulation lobes around the C atom point toward Clcis ions and the blue charge deficit lobes around Clcis ions point toward the C atom; this is a clear indication of direct CO←Clcis back-bonding interaction. A similar charge difference pattern was previously reported for the [(CO)NbCl5] complex.24 Further analysis reveals that back-donation originates also from the O ion of VOCl3; however, due to symmetry reasons ΔρA″(r) cannot show it, because the O ion lies on the A″ nodal plane. However, CO←O back-donation can be anticipated from the plot of total charge density difference (Figure S24 in the Supporting Information) and also by analyzing the backdonation onto two degenerate 2π orbitals of CO; the backdonation onto the 2π orbital compatible with CO←Clcis backbonding is 0.11 e−, whereas the back-donation onto the other 2π orbital, compatible with CO←O back-bonding, is 0.06 e−. I


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Inorganic Chemistry

Table 4. V−Xcis, V−Xtrans, V−O, and V−C Bond Lengths and C−V−Xcis Angles for Standalone High-Symmetry [(L)VOX3] Complexes, Calculated at the PBE/Plane-Wave Level of Theory d(V−Xcis) [Å]

d(V−Xtrans) [Å]

d(V−O) [Å]

d(V−C) [Å]

∠(C−V−Xcis) [deg]

(L )VOF3 (CO)VOF3 [(CN)VOF3]−

1.797 1.751 1.798

1.781 1.732 1.813

1.581 1.575 1.583

2.184 2.592 2.119

80.1 76.1 82.6

(LMe)VOCl3 (CO)VOCl3 [(CN)VOCl3]−

2.268 2.225 2.261

2.215 2.175 2.268

1.572 1.572 1.573

2.138 2.182 2.076

78.3 76.8 80.9

(LMe)VOBr3 (CO)VOBr3 [(CN)VOBr3]−

2.437 2.401 2.425

2.371 2.334 2.438

1.570 1.570 1.571

2.130 2.122 2.076

78.4 75.3 80.8

Me

affinity is just the opposite: CN− > LMe ≫ CO. Notice that, for CN− and LMe ligands, σ donation is considerably larger than the π back-donation, whereas, for CO, the two display about the same magnitude. The type of C−V bonding can be further inferred from total charge density plots, shown in Figure S24 in the Supporting Information. Table 3 further reveals that the ability of VOX3 fragments to back-donate the charge to CO and LMe ligands follows the VOBr3 > VOCl3 > VOF3 order, which is what one would expect on the basis of chemical softness and electronegativity of Br, Cl, and F. The results of the population analysis (Table 3) and the structural parameters presented in Table 4 allow us to rationalize the dependence of V−Xtrans and V−Xcis bond distances on donating/back-donating characteristics of the ligand. Figure 8 plots the dependence of the V−Xtrans bond

In contrast to [(CO)VOCl3], where back-donation predominantly comes from Clcis ions, for the [(LMe)VOCl3] complex, back-bonding is not only much smaller (0.05 e−) but by and large stems instead from the metal V ion (see the corresponding plots in Figure 7). Larger back-donation for the [(CO)VOCl3] can be attributed in part to a triple bond of CO and, consequently, doubly degenerated virtual 2π orbital due to which CO←O back-bonding is possible. Finally, for [(LMe)VOF3], back-bonding is even smaller (0.03 e−). Surprisingly, however, the pattern of back-bonding appears to be a combination of the two aforedescribed cases. The ΔρA″(r) plots clearly reveal (Figure 7, right column) that, while the majority of back-bonding stems from the metal V ion, there is also a noticeable contribution coming from all three F ions (note the blue charge deficit 3D isosurfaces located at F ions and the intense blue region around the Fcis ion in the 2D contour plot). That back-donation in the case of [(LMe)VOF3] is smaller than that of [(LMe)VOCl3] is reasonable, because Cl is chemically softer (more polarizable) and less electronegative than F. However, for precisely the same two reasons, it is counterintuitive why F ions are to a larger extent involved in back-bonding than Cl ions. Perhaps, the larger involvement of F ions in back-donation can be to some extent attributed to a smaller Fcis−CNHC distance (2.58 Å) compared to the Clcis−CNHC distance (2.79 Å). Nevertheless, despite the fact that F ions appear to be more involved in back-donation than Cl ions in [(LMe)VOX3], the situation is qualitatively different from the case of the CO ligand; in particular, in [(LMe)VOF3], the blue charge deficit lobes of the Fcis ions do not point toward the C atom as in the case of the CO ligand (see the respective 2D contour plots in Figure 7). In addition to the just described analysis of back-bonding in terms of molecular-orbital projections, we also performed charge-displacement analysis of the three compounds considered in Figure 7 (see section S6.1 and Figure S25 in the Supporting Information), which corroborates the findings described above. Having described the local characteristics of back-donation for the three cases considered in Figure 7, let us now discuss the CN− ligand and the effect of the type of halide ion X− (X = F, Cl, or Br). The results of the Bader charge analysis of [(L)VOX3] complexes and the respective σ donations and π back-donations are given in Table 3. For all three [(CN)VOX3]− complexes, orbital projections predict no π backdonation (the resulting numbers are on the order of 10−5 e−). The order of back-bonding affinity for the three ligands is therefore CO ≫ LMe > CN−, whereas the order of donating

Figure 8. Correlation between the V−Xtrans bond lengths in [(L)VOX3] complexes and the Bader charges of Xtrans ions; L = CO, LMe, or CN− and X = F, Cl, or Br. Note that the more the Xtrans anion is negatively charged, the longer is the V−Xtrans bond.

length on the Bader charge of the Xtrans ion; this figure clearly reveals that the more the Xtrans ion is negatively charged the longer is the V−Xtrans bond (a similar dependence exits if the Bader charge of VOX3 is considered instead of the charge of Xtrans, not shown). Due to different sizes of F, Cl, and Br ions, each of them displays its own dependence, but the three regression lines are roughly parallel to each other (the slopes are −0.74, −0.62, and −0.64 Å/e for F, Cl, and Br, respectively). This plot further reveals that, for a given ligand, the magnitude of the negative charge of Xtrans increases, as expected, in the order of F > Cl > Br. Another notable J


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Inorganic Chemistry

Figure 9. (a) Correlation between the V−Xtrans and V−Xcis bond lengths in standalone [(L)VOX3] complexes; the solid slanted line represents the slope of 1 and the points of the [(CN)VOX3]− complexes lie on it due to absence of back-bonding to the CN− ligand, whereas, for CO and LMe ligands, the back-bonding shortens the V−Xtrans bond and correspondingly these points lie on a green-dashed line which displays the smaller slope of 0.93 (i.e., the V−Xtrans bond is slightly shorter than the V−Xcis bonds). (b) Correlation between the back-donation into virtual π* orbitals and the difference between the V−Xcis and V−Xtrans bond lengths, plotted only for CO (red dashed line) and LMe (gray dashed line) ligands, because the back-bonding is absent for CN− ligand. For a given ligand, the larger is the back-donation, the shorter is the V−Xtrans bond with respect to the V−Xcis bond.

power solely from bond lengths obtained from the crystal structure.

observation from Tables 3 and 4 is that, for a given VOX3 counterpart, the larger is the back-bonding to the ligand, the smaller is the C−V−Xcis angle; in other words, the tilting of Xcis ions toward the ligand increases with back-donation, thus displaying the CO > LMe > CN− trend (i.e., the larger is the tilt, the smaller is the C−V−Xcis angle). As to scrutinize the effect of back-bonding on the V−Xtrans versus V−Xcis bond distances, we compare the two distances to each other in Figure 9a. For the [(CN)VOX3]− complexes, which lack any back-donation, the two distances are roughly the same and the respective points lie on the y = x line. In contrast, in [(CO)VOX3] and [(LMe)VOX3] complexes, backbonding apparently shortens the V−Xtrans bond with respect to the V−Xcis bond, and correspondingly, these points lie on a dashed line which displays a slightly smaller slope. To further corroborate this observation, Figure 9b shows how the difference between V−Xcis and V−Xtrans bond lengths depends on π back-donation (the dependence is plotted only for CO and LMe ligands, because back-donation is absent for the CN− ligand). It is clearly evident from the figure that, for a given ligand, the larger is the back-donation, the shorter is the V− Xtrans bond with respect to the V−Xcis bond. Our computational results agree with previous results of [(L)NbCl5] complexes, where the Nb−Cltrans bond is longer in the presence of σ donating ligands and shorter in the presence of π acceptor ligands.24 But currently, we also scrutinized the effect of back-bonding on the difference between the V−Xcis and V−Xtrans bond lengths. There is, however, one provision. Namely, the above rationalization of a ligand’s donating and back-donating characteristics on V−Xtrans and V−Xcis bond distances of VOX3 holds for standalone complexes. We showed above that, in the crystal structure, the crystal packing effects may have a significant influence on V−X bond lengths (Figure 5); for [(LDipp)VOF3], they elongate the V−Ftrans bond by 0.04 Å. An estimate based on the correlation between the V−Xtrans bond lengths and Bader charges of the VOF3 counterpart (not shown) reveals that such an elongation is equivalent to a ligand’s donation of about 0.15 e−. This clearly indicates that care should be exercised when estimating the ligand’s donating

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CONCLUSIONS We studied the reactivity of vanadium(V) oxide trifluoride (VOF3) with the goal to synthesize and structurally characterize unknown or rare discrete vanadium(V) oxide fluoride species. To this end, the use of a sterically hindering cation or ligand was found to be crucial for the isolation of such species. A suitable choice for the preparation of a discrete [VOF4]− anion proved to be the recently prepared imidazolium fluoride salt: [(LDipp)H][F].21 In the straightforward reaction, the salt reacted with VOF3 and resulted in a product from which the elusive discrete [VOF4]− anion was finally structurally characterized. Synthesis of the neutral [(LDipp)VOF3] (2) complex was achieved by a similar reaction between VOF3 and a bulky neutral LDipp ligand, making it the second structurally characterized neutral compound of NHC with metal oxide fluorides (MOF). This example shows a viable route for the preparation of various [(NHC)MOF] complexes. Moreover, we studied by DFT calculations intermolecular interactions between [(LDipp)VOF3] complexes in the crystal structure as well as intramolecular ligand bonding within the complex. To shed some light onto ligand bonding, we analyzed intramolecular interactions within several standalone [(L)VOX3] model complexes for L = CO, LMe, or CN− and X = F, Cl, or Br. The acceptor affinity of the three ligands follows, as expected, the CO ≫ LMe > CN− order, with the latter lacking any L←VOX3 back-bonding. We showed that, for standalone complexes, (i) the V−Xtrans bond length is proportional to the charge of the Xtrans anion (and also to the charge of the VOX3 fragment), i.e., the larger is the net donation of charge from the ligand, the longer is the V−Xtrans bond; (ii) for a given ligand, the magnitude of negative charge of the Xtrans anion increases, as expected, in the order of F > Cl > Br; and (iii) the backdonation of charge into the ligand shortens the V−Xtrans bond with respect to the V−Xcis bond, that is, for a given acceptor ligand (i.e., CO and LMe in the current case), back-bonding increases in the VOF3 < VOCl3 < VOBr3 order and the larger K


Article

Inorganic Chemistry is the π back-donation, the shorter is the V−Xtrans bond with respect to the V−Xcis bond. The local characteristics of π backbonding were further scrutinized for [(CO)VOCl3], [(LMe)VOCl3], and [(LMe)VOF3] model complexes. We showed that CO←VOCl3 back-bonding is significant and mainly stems from cis-chloride and oxide ions, indicating a direct CO←Clcis interaction. In contrast, for [(LMe)VOCl3] and [(LMe)VOF3] complexes, the back-donation is minute and predominantly involves the vanadium cation. While back-donation in the case of [(LMe)VOF3] is smaller than that of [(LMe)VOCl3], the fluoride ions are, counterintuitively, to a larger extent involved in back-bonding than chloride ions. Furthermore, we analyzed the influence of intermolecular interactions between [(LDipp)VOF3] (2) complexes in the crystal structure and showed that they elongate the V−Ftrans bond by 0.04 Å. On the basis of our analysis of ligand bonding, such an elongation is equivalent to a ligand’s donation of about 0.15 e− in the case of [(L)VOX3] complexes. This clearly indicates that care should be exercised when estimating the ligand’s donating power solely from bond lengths obtained from the crystal structure. Finally, we showed that the hydrolysis reaction of [(LDipp)VOF3] (2) leads to the formation of [(LDipp)H][VO2F2] (3) species. Noteworthy, the crystal structure of the discrete tetrahedral [VO2F2]− anion, as observed in [(LDipp)H][VO2F2] (3), is rarely seen in the literature. With this study, we extended the applicability of fluoride donating [(LDipp)H][F] salt in the preparation of a novel anionic species beyond the Lewis acid fluorides of Group 14.22 In the current paper, we successfully utilized its properties in reactions with d-block metal oxide fluorides and proved that it is a suitable and versatile reagent for preparation and structural characterization of the [VOF4]− anion. We are encouraging the use of the aforementioned reagent for the preparation of similarly scarce discrete anionic species or possibly even more obscure anionic species with mixed halides.

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the Slovenian Research Agency (ARRS) for the financial support of the Research Programs P1-0045 (Inorganic Chemistry and Technology), P2-0393 (Advanced Materials for Low Carbon and Sustainable Society), and PR-06163 (Young Researcher Program). The authors would also like to thank Assist. Prof. Dr. Maja Ponikvar-Svet and Mira Zupančič for performing CHN elemental analysis and the Slovenian NMR Centre of National Institute of Chemistry for their resources and support. A.K. acknowledges helpful discussions with Dr. Matic Lozinšek.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02377. Experimental NMR data, crystallographic data and structural parameters, additional vibrational data, and computational results (PDF) Accession Codes

CCDC 1861391−1861393 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ž iga Zupanek: 0000-0002-7587-463X Melita Tramšek: 0000-0003-3712-5168 Anton Kokalj: 0000-0001-7237-0041 Gašper Tavčar: 0000-0001-9891-6153 L


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Reactivity of VOF3 with N-Heterocyclic Carbene and Imidazolium

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