Magnesium Complexes with Isomeric Pyrazol-4-ylidene and Imidazol-2-ylidene Ligands

Dimagnesium(I) complexes [{( Ar nacnac)Mg} 2 ], where Ar nacnac = HC(MeCNAr) 2 , Ar = Dip = 2,6-i Pr 2 (cid:0) C 6 H 3 , Ar = Dep = 2,6-Et 2 (cid:0) C 6 H 3 , Ar = Mes = 2,4,6-Me 3 (cid:0) C 6 H 2 , react with iodoarenes in oxidative addition reactions. With iodobenzene, magnesium phenyl and magnesium iodide complex fragments were obtained, and from a reaction with 4-iodo-1,2,3,5-tetramethylpyrazolium iodide, [ Me PZI]I, the pyrazol-4-ylidene complex [( Dip nacnac)MgI( Me PZ)] was structurally characterised, alongside other products. The isomeric imidazol-2-ylidene complex [( Dip nacnac)MgI( Me NHC)], where Me NHC is 1,3,4,5-tetramethylimidazol-2-ylidene, was prepared and characterised. X-ray crystal structure determinations and DFT computational studies have been carried out to compare the related complexes. The results show that the Me PZ ligand is higher in energy and more nucleophilic than its more common isomeric Me NHC carbene.


Introduction
[3] The class of imidazol-2-ylidenes, A (Figure 1), is the most commonly used type, but a range of remote or mesoionic carbenes have shown advantageous properties, such as stronger σ-donor abilities, albeit often at the expense of a significantly lower stability of the free carbene species in comparison to A. [1,2,4] Dyker, Bertrand and co-workers introduced the cyclic bent allenes B and C as carbene species, [5,6] see Figure 1.These compounds have been called (cyclic) bent allenes, (remote) carbenes, mesoionic carbenes, carbodicarbenes, carbenoids, zwitterions, and possibly other interpretations, which highlights the different viewpoints that the unusual bonding aspects, structure, and properties in these species can be described as. [7,8]Related to this, Huynh and co-workers reported pyrazol-4-ylidene (pyrazolin-4-ylidene) complexes of palladium, e. g., D, which were prepared by oxidative addition of iodopyrazolium salts to palladium(0) complexes. [9,10]These species contain aliphatic or aromatic substituents on the carbon centres adjacent to the central nucleophilic carbon donor.

Synthesis and molecular structures
We set out to explore if small pyrazol-4-ylidenes can be stabilised by electropositive main group metals.Dimagnesium(I) complexes [11] seemed to be good candidates for their generation.These highly reducing complexes of general formula LMgMgL, where L is a sterically demanding, typically chelating anionic N-ligand such as a β-diketiminate, have demonstrated the ability to reduce a range of challenging carbon-based substrates including, for example, the reversible reduction of selected alkenes, [12] and conversion of C 60 to its hexaanion. [13]11b] Their ability to activate a range of CÀ F bonds [14] to afford (nucleophilic) organomagnesium complexes show their suitability for challenging organic halides, RX, where X = halogen.Similarly, rare magnesium(0) complexes react with RX species such as nBuI, PhI and PhF to furnish organomagnesium complexes in (homogeneous) molecular reactions to Grignardlike species. [15]Thus, it should be straightforward activating RX species of heavier halides with magnesium(I) complexes and this has been our past experience with this compound class, e. g., the unsuitability of using chlorinated solvents with these compounds due to rapid CÀ Cl reduction.In comparison, related dizinc(I) compounds also react with RX species and, for example, show slow reaction at room temperature with EtI and EtBr in aromatic solvents, but not with PhI. [16]Outside of molecular low oxidation state complex chemistry, the mecha-nisms of heterogeneous Grignard reagent formation from magnesium metal have been widely studied and are complicated by the heterogenous nature of the system relying on a surface reaction on magnesium metal. [17]n preliminary reactions with iodoarenes, we have treated [{( Ar nacnac)Mg} 2 ] 1, where Ar nacnac = HC(MeCNAr) 2 , Ar = Dip = 2,6-iPr 2 À C 6 H 3 1 a, [18] Ar = Dep = 2,6-Et 2 À C 6 H 3 1 b, [19] Ar = Mes = 2,4,6-Me 3 À C 6 H 2 1 c, [20] with iodobenzene in deuterated benzene at room temperature.These reactions proceeded rapidly and cleanly in aromatic solvents for Ar = Dip and Mes, and slightly slower for Ar = Dep, to yield either [( Dip nacnac)MgPh] 2 a [21] and poorly soluble [{( Dip nacnac)MgI} 2 ] 3 a [22] for Ar = Dip, or the asymmetrically bridged complexes [{( Ar nacnac)Mg} 2 (μ-Ph)(μ-I)] for Ar = Dep (4 b) or Mes (4 c), respectively, as judged by NMR spectroscopy, see Scheme 1.The slightly different outcomes in these reactions are due to steric effects with [( Dip nacnac)MgPh] 2 a showing a monomeric structure, [21] whereas 4 b and 4 c form stable asymmetric bridging species according to NMR spectroscopic studies.The asymmetrically bridged species 4 b, c are furthermore thermally stable and the ligands do not rapidly redistribute easily, i. e., the Schlenk-equilibrium appears to be supressed.For example, they show no significant decomposition or redistribution after 16 hours at 100 °C in deuterated benzene.However, small quantities of the respective iodide complexes [{( Ar nacnac)MgI} 2 ] 3 b, c can be formed alongside 4 b, c.For example, storing 4 c for a prolonged time in solution or as part of mixtures will eventually form some [{( Mes nacnac)MgI} 2 ] 3 c [20] that is driven by crystallisation.So far, we have not been able to structurally characterise 4 b or 4 c, or crystallise analytically pure crops.NMR spectroscopy revealed the asymmetry of the resonances for the aryl groups in 4 b, c above and below the β-diketiminate magnesium plane, and the resonances for the bridging phenyl group are similar to those found in the related complex [{( Mes nacnac)Mg} 2 (μ-Ph)(μ-H)]. [23]A range of different coordination modes of bridging phenyl groups was very recently revealed for related β-diketiminate calcium complexes. [24]These reactions again show the higher reactivity of dimagnesium(I) compounds, here towards PhI, compared with those of dizinc(I) complexes. [16]s suitable starting materials towards pyrazol-4-ylidene complexes, we prepared two small 1,2,3,5-tetraalkyl-4-iodopyrazolium iodide salts from the respective 4-iodopyrazole and iodomethane, as described previously. [9,10,25]The reactions of these salts [ Me PZI]I 5 a (4-iodo-1,2,3,5-tetramethylpyrazolium iodide) and [ Et PZI]I 5 b (3,5-diethyl-4-iodo-1,2-dimethylpyrazolium iodide), see Scheme 2, with one equivalent of dimagnesium(I) complexes [{( Ar nacnac)Mg} 2 ] 1 a, b in toluene, benzene or hexane proceeded slowly at room temperature.The reaction mixtures are suspensions, due to the poor solubility of the salts, that slowly lose the yellow colour of the magnesium complexes and form product mixtures dominated by [{( Ar nacnac)MgI} 2 ] 3 a, b and other magnesium iodide species, see Scheme 2. Similar product mixtures were obtained when the reactions were carried out at elevated temperatures.The cationic nature of the iodopyrazolium salts 5 a, b should allow for a fast reduction with dimagnesium(I) complexes, as with iodobenzene, but the poor solubility of the salts render these reactions relatively slow in practice.NMR spectroscopic studies from in-situ reactions typically showed 1 H NMR resonances of four to five main species that contain β-diketiminate ligands in the reaction mixtures.Similarly, the precipitates from these reactions also showed to be product mixtures of several complexes including large quantities of poorly soluble [{( Ar nacnac)MgI} 2 ] 3 a, b. 13 C{ 1 H} NMR spectra of these mixtures could not be assigned and provided no observed unusual 13 C NMR resonance.Separation attempts by fractionalised crystallisation failed to yield pure crops of compounds so far and increasingly afforded further formation and precipitation of [{( Ar nacnac)MgI} 2 ] 3 a, b over time.However, from these reactions, several complexes could be structurally characterised, including the target complex [( Dip nacnac)MgI( Me PZ)] 6 a, see  1 for the cation.For comparison we have also prepared and structurally characterised the 2-iodo-imidazolium iodide salt [ Me NHCI]I 8 (2-iodo-1,3,4,5-tetramethylimidazolium iodide) from Me NHC and iodine in THF, see Figure 5, and selected bond distances are collected in Table 1.As part of this study [ Me PZH]I was also structurally characterised and is presented in the supporting information.The bond distances within the rings of 7 ab, 7 ba and 8 are not largely different from those in the respective carbene species, vide infra.
For more detailed comparison to 6 a we have prepared the imidazol-2-ylidene magnesium complexes [( Dip nacnac)MgI( Me NHC)] 9 a and [( Dip nacnac)MgI( Et NHC)] 9 b, with Et NHC = 1,3-diethyl-4,5-dimethylimidazol-2-ylidene, from [( Dip nacnac)MgI(OEt 2 )] with the respective free N-heterocyclic carbene in toluene, see Scheme 3, and Figures 6 and 7. NHC ligands are commonly employed in organometallic main group chemistry and a range of examples are known for magnesium. [3]or example, monomeric complexes with N,N'-chelating ligands and a halide [26] or reactive anionic ligands [27] have been prepared.NHCs can even coordinate to, and be activated by, magnesium(I) complexes, [28] and a related reduced magnesium complex with a cyclic (alkyl)(amino)carbene radical anion has been characterised. [29]Both complexes 9 a and 9 b show the expected structures with distorted tetrahedrally coordinated Mg centres.Metrical data for complex 9 a, which is a constitutional (structural) isomer of 6 a, is presented in more detail in the next section.Complex 9 a shows expected NMR spectroscopic properties and a carbene resonance at δ 181.0 ppm in the 13 C{ 1 H} NMR spectrum.Complexes 9 a and 9 b show two septets and four doublets for the hydrogen environments of the isopropyl groups in 1 H NMR spectra.At room temperature, several resonances are broad and significantly sharpen when   (10).Selected distances for the cation are collected in Table 1.
recorded at 75 °C, more so for the Me NHC derivative 9 a compared with Et NHC analogue 9 b.

Computational studies
To gain further insights into the isomeric carbene species and their magnesium complexes, we carried out a DFT study at the M06/def2-TZVP level of theory of a cut-back model of complexes   16); I1À Mg1À I2 106.69(2),N1À Mg1À N5 95.87 (6).Selected distances for the cation are collected in Table 1.   1.
substituents, [( Me nacnac)MgI( Me PZ)] 6 x and [( Me nacnac)MgI( Me NHC)] 9 x, and the free isomeric pyrazol-4ylidene Me PZ and N-heterocyclic carbene Me NHC.The optimised geometries of the molecules agree well with the available data from X-ray diffraction.Bond distance data of 6 a, x, 9 a, x, Me PZ, and Me NHC is summarised in Figure 8 together with NPA and QTAIM charges and Wiberg bond indices.The range of bond lengths in Me PZ and for the coordinated fragment in 6 a, x are close to those found in related Pd complexes such as D. [10] The nitrogen centres in Me PZ (sum of angles 359.1°) and for the coordinated species in 6 x (359.5°mean) are almost planar, whereas those obtained from the X-ray structure of 6 a are planar within error margins (359.8(8),360.0(8)).It is worth noting that the related examples B and C show significantly pyramidalised nitrogen centres, for example with a respective sum of angles of 349°(mean) for B. [5] These effects from the nature of the substituents in 3,5-position, and other properties including Wiberg bond indices, are in line with those found in a computational study, including on models of B and C. [5] π-donor substituents in 3 and 5-position of the ring have also been shown to significantly decrease the aromaticity of the heterocycles. [7]heme 3. Synthesis of NHC-complexes 9. Selected orbitals with energy levels of Me PZ and Me NHC are presented in Figure 9 and selected orbitals of 6 x and 9 x are shown in Figure 10.Me PZ shows a higher HOMO and lower LUMO energy level compared with Me NHC and, accordingly, a smaller HOMO-LUMO gap (4.91 eV versus 6.59 eV).Inspecting the range of orbitals and Wiberg bond indices (Figure 8) allows us to see why descriptions including cyclic bent allenes and carbenes have been used for Me PZ and related species, [5][6][7][8][9]30] and highlights differences and similarities to the more common imidazol-2-ylidenes. Bothsmall molecules show a carbene-type lone pair in their HOMOs and this is reflected in MgÀ C interactions in 6 x (HOMO-5 and minor in HOMO-9) and 9 x (HOMO-7 and minor in HOMO-9), see Figure 10.Because the pairs Me PZ, and Me NHC, and 6 x and 9 x, are isomers we compared their energies.Me PZ is significantly less stable (ΔG = + 45.4 kcal/mol, ΔH = + 46.1 kcal/mol) than Me NHC, although this difference is reduced by around 10 kcal/mol when these nucleophilic molecules are coordinated to the ( Me nacnac)MgI fragment (ΔG = + 36.2 kcal/mol, ΔH = + 36.5 kcal/mol).The Me -PZ molecule and ligand show a significant negative charge on its carbene carbon atom from both NPA and QTAIM methods, whereas the carbene centre in Me NHC is approximately charge neutral according to NPA or significantly positively charged based on QTAIM analysis (Figure 8).Independent of the method, the charge of the carbon donor centre in Me PZ is significantly more negative than that for Me NHC.Calculated charges for magnesium and iodine atoms agree well between the two methods.The MgÀ C bond is slightly shorter in 6 x compared with 9 x for both the DFT and X-ray data.Laplacian maps from QTAIM studies (Figure 11) show a similar overall picture between these molecules, and the bond critical point on the MgÀ C bond path shows a slightly higher electron density value for 6 x compared with 9 x.Taken together, all these observations are in line with pyrazol-4-ylidenes acting as significantly better σ-donors in comparison with Arduengo-type N-heterocyclic carbenes.[1,2,4]

Conclusion
We have studied the reactions of dimagnesium(I) complexes [{( Ar nacnac)Mg} 2 ] 1 with the iodoarenes iodobenzene and two 4iodopyrazolium iodide salts.In preliminary reactions with iodobenzene, Grignard-type species [{( Ar nacnac)Mg} 2 (μ-Ph)(μ-I)], Ar = Dep (4 b) or Mes (4 c), were obtained according to NMR spectroscopy, whereas for Ar = Dip, the complexes  and structural study between pyrazol-4-ylidene, Me PZ, and imidazol-2-ylidene, Me NHC, including their magnesium complexes confirmed the higher thermodynamic energy and more nucleophilic character of Me PZ over Me NHC.The pyrazol-4ylidene ligand shows cyclic bent allene character that provides a nucleophilic carbene donor functionality, and the study highlights the differences and similarities between the two species.

Experimental Section
General considerations.All manipulations, except the syntheses of the iodopyrazoles, were carried out using standard Schlenk and glove box techniques under an atmosphere of high purity argon or dinitrogen.Benzene, toluene, n-hexane and THF were either dried and distilled under inert gas over LiAlH 4 , sodium or potassium, or taken from an MBraun solvent purification system and degassed prior to use.CDCl 3 and DMSO-d 6 were used as received for stable organic molecules. 1H and 13 C{ 1 H} NMR spectra were recorded on a Bruker AVII 400 or Bruker AV III 500 spectrometer in deuterated benzene and were referenced to the residual 1 H or 13 C{ 1 H} resonances of the solvent used.Selected NMR spectra are collected in the supporting information.Details on the molecular structures from single crystal X-ray diffraction (Deposition Numbers 2177891-2177897) and computational studies are provided in the supporting information.

Figure 8 .
Figure 8. Selected data for complexes 6 a, x (top) and 9 a, x (bottom); R = Dip (a), Me (x).Bond distances from DFT studies (black values) and single crystal X-ray diffraction (blue values) are given in Å. Red values show natural population analysis (NBO) charges, purple values report QTAIM charges, and green values provide Wiberg bond indices.Values in parentheses are for the calculated uncoordinated carbene species.

Figure 9 .
Figure 9. Selected orbitals and energy levels of Me PZ and Me NHC (isovalue 0.09).