‘Pincer’ dicarbene complexes of some early transition metals and uranium

Abstract

The complexes [(C–N–C)MX_n(thf)_m] with the ‘pincer’ 2,6-bis(imidazolylidene)pyridine, (C–N–C) = 2,6-bis(arylimidazol-2-ylidene)pyridine, aryl = 2,6-Prⁱ₂C₆H₃, M = V, X = Cl, n = 2, m = 1 1a; M = Cr, X = Cl, n = 2, m = 0, 2a, X = Br, 2b; M = Mn, X = Br, n = 2, m = 0, 3; M = Nb, X = Cl, n = 3, m = 0, 4; and M = U, X = Cl, n = 4, m = 0, 5, were synthesised by (a) substitution of labile tmed (1a), thf (2a, 3, 5) or dme (4) by free (C–N–C) or by (b) reaction of the bisimidazolium salt (CH–N–CH)Br₂ with {Cr[N(SiMe₃)₂]₂(thf)₂} followed by amine elimination (2b). Attempted alkylation of 1a, 2, 3a and 4 with Grignard or alkyl lithiums gave intractable mixtures, and in one case [reaction of 1a with (mesityl)MgBr] resulted in exchange of Cl by Br (1b). Oxidation of 1a or [(C–N–C)VCl₃] with 4-methylmorpholine N-oxide afforded the trans-V(C–N–C)(=O)Cl₂, 6, which by reaction with AgBF₄ in MeCN gave trans-[V(C–N–C)(=O)(MeCN)₂][BF₄]₂, 7. Reaction of 1a with p-tolyl azide gave trans-V(C–N–C)(=N-p-tolyl)Cl₂8. The complex trans-Ti(C–N–C)(=NBu^t)Cl₂, 9, was prepared by substitution of the pyridine ligands in Ti(NBu^t)Cl₂(py)₃ by C–N–C.

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Introduction

The mer-coordinating ‘pincer’ ligands have been frequently used as versatile ancillary ligands in organometallic chemistry and catalysis.¹ In particular, the ‘pincer’ dicarbene ligand C–N–C (I, Scheme 1) provides robust Pd, Ru and Rh complexes by virtue of the strong σ-donating NHC functional groups flanking the central pyridine donor.²

Scheme 1 Metal complexes described in this paper: M = V, X = Cl, L = thf, n = 2, m = 1, 1a (method A); M = Cr, X = Cl, n = 2, m = 0, 2a (method A), X = Br, 2b, (method B); M = Mn, X = Br, n = 2, m = 0, 3; M = Nb, X = Cl, n = 3, m = 0, 4 (method A); M = U, X = Cl, n = 4, m = 0, 5, (method A); Ar = 2,6-Prⁱ₂C₆H₃. Reagents and conditions: (i) 4-methylmorpholine N-oxide–thf, X = Cl, Y = O, n = 2 or 3, m = 1; tolylN₃, X = Cl, Y = N-tolyl, n = 2, m = 1; (ii) AgBF₄, MeCN.

Some catalytic applications in the Heck reaction (Pd) and transfer hydrogenation of ketones (Ru) have been reported.³ Furthermore, unusual and reactive alkyl and dinitrogen complexes of Co(I) and Fe(0) with the C–N–C ligand have been recently described.⁴ NHC complexes of high oxidation state and early transition metals have been poorly investigated.⁵ Functionalised NHC ligand designs have been introduced in this area.⁶ Recently, M(C–N–C)Cl₃ complexes M = Ti(III), V(III), Cr(III) were synthesised and their ethylene polymerisation and oligomerisation activity studied.⁷

In this paper we report the synthesis and full characterisation of C–N–C complexes of Ti(IV), V(II), Cr(II), Mn(II), Nb(III) and U(IV) halides, and the oxidation of V(II) and V(III) complexes by oxo and imido transfer reagents. We also describe the structures of the previously prepared (C–N–C)MCl₃ M = Ti and V.⁷

Results and discussion

The substitution of labile ligands by C–N–C served as a general method for the synthesis of the early transition metal and uranium halide complexes that are described in this paper. The reactions were carried out in thf and the products isolated by crystallisation from thf/petroleum. In addition, the aminolysis from {Cr[N(SiMe₃)₂]₂(thf)₂} was successfully used for the preparation of the chromium(II) carbene complex 3b. All new complexes are moderately sensitive to air and dissolve without decomposition in polar non-protic solvents. Due to their paramagnetic nature they were characterised by elemental analysis, magnetic susceptibility and in some cases single-crystal X-ray diffraction. The structure of the V(II) complex 1a is shown in Fig. 1.

Fig. 1 ORTEP representation of the molecular structure of 1a (50% probability ellipsoids). Hydrogen atoms and two thf molecules are omitted for clarity. Important bond lengths (Å) and angles (°): C(6)–V(1) 2.209(9), C(25)–V(1) 2.162(9), Cl(1)–V(1) 2.443(3), Cl(2)–V(1) 2.416(3), N(1)–V(1) 2.084(7), O(1)–V(1) 2.145(5); N(1)–V(1)–C(6) 75.2(3), N(1)–V(1)–C(25) 74.7(3), C(25)–V(1)–C(6) 149.6(3), Cl(2)–V(1)–Cl(1) 173.73(10).

Complex 1a features a distorted octahedral metal centre with the ‘pincer’ ligand occupying meridional sites. The two chlorides are mutually trans and the sixth coordination site is filled by one thf molecule. The vanadium lies on the plane defined by the pyridine and the NHC rings; the bulky aromatic substituents of the NHC are virtually perpendicular to the coordination plane (80.5–84.5°). The N–C bond lengths and the N–C–N angles of the NHC are virtually identical to those observed for the free ligand. The two V–C carbene bond lengths are slightly different [2.209(9) and 2.162(9) Å] and within the range for V–C σ(sp³) single bonds [2.124–2.254 Å].⁸ The Cl–V–Cl angle is 173.73(10)° with the chlorine atoms pointing away from the pyridine ring; there is no obvious interaction of any of the chlorine atoms with the carbene carbons.

For comparison purposes the structures of the previously reported (C–N–C)VCl₃ and (C–N–C)TiCl₃ and of the niobium complex 4 were also determined; an ORTEP diagram of the former is given in Fig. 2; structural data for the latter two are included in the ESI.†

Fig. 2 ORTEP representation of the molecular structure of (C–N–C)VCl₃ (50% probability ellipsoids). Hydrogen atoms are omitted for clarity. Important bond lengths (Å) and angles (°): C(2)–N(3) 1.348(3), C(2)–N(2) 1.368(3), C(2)–V(1) 2.144(3), C(21)–V(1) 2.147(3), N(1)–V(1) 2.160(2), Cl(3)–V(1) 2.2578(9); C(2)–V(1)–N(1) 72.77(10), C(2)–V(1)–Cl(1) 91.82(8), Cl(1)–V(1)–Cl(2) 167.17(3).

In all three complexes the distorted octahedral geometry with occupation of the meridional sites by the ‘pincer’ is a common feature. The Ti–C bonds [2.211(5) and 2.191(5) Å] are slightly longer than the V(III)–C bonds [2.114(3) and 2.191(5) Å] and, surprisingly, shorter than other Ti(IV)–NHC bonds reported in the literature;^5f they are longer than typical Ti (IV)–CH₃ σ(sp³) bonds (ca. 2.175 Å)^9a–c and some Ti–C σ(sp²) bonds [2.060–2.070 Å].^9d,e The Nb–C bonds in 4 are within the range 2.203(6)–2.206(6) Å and are shorter than other Nb–C σ(sp³)^10a–c and Nb–C σ(sp²)^10d,e bonds, possibly due to some backbonding. In common with the other known metal (III) trihalide ‘pincer’ complexes,^4a,7 the metal–chloride bond trans to the pyridine is significantly shorter than those to the mutually trans chlorides. The ligand bite angle increases in the order V(II) > V(III) > Ti(III) > Nb(III) [74.9(3), 72.89(10), 71.32(16), 69.6(2)°, respectively] showing an inverse correlation with the metal covalent radius; the trend can be rationalised by assuming that as the size of the metal increases it moves further out of pocket of the ‘pincer’ ligand.

The structures of the chromium and manganese bromide complexes 2b and 3 have also been determined by single-crystal X-ray diffraction; an ORTEP diagram of the manganese complex is shown in Fig. 3, while details of the structure of the chromium complex have been included in the ESI.†

Fig. 3 ORTEP representation of the molecular structure of 3 (50% probability ellipsoids) showing one refined orientation of the molecule. Hydrogen atoms and one thf molecule are omitted for clarity. Important bond lengths (Å) and angles (°): Mn(1A)–C(13A) 2.206(2), Mn(1A)–C(21A) 2.210(2), Mn(1A)–Br(1A) 2.5458(7), Mn(1A)–Br(2A) 2.5708(7), Mn(1A)–N(3A) 2.2574(16); C(13A)–Mn(1A)–C(21A) 143.58(8), C(13A)–Mn(1A)–N(3A) 74.99(7), C(21A)–Mn(1A)–N(3A) 71.60(6), Br(1A)–Mn(1A)–Br(2A) 124.57(3), N(3A)–Mn(1A)–Br(2A) 84.52(4), C(21A)–Mn(1A)–Br(2A) 93.48(6), C(13A)–Mn(1A)–Br(2A) 97.41(6).

Both contain five-coordinate metal centres in distorted square-pyramidal geometries with apical bromides. There is an approximate C_s symmetry arising from the plane containing the metal, the two bromides and the pyridine nitrogen. Even though the position of the manganese atom in the structure of 3 is disordered, modelling of the disorder allowed the calculation of bond lengths and angles in the complex. The metal–C bond lengths are 2.206(2) and 2.210(2) Å for the Mn(II) and 2.122(10) and 2.125(10) Å for Cr(II). The apical M–Br is slightly longer than the basal. The bite angle of the ligand is in the range 71.6–74.9° (Mn) and 74.9–76.3° (Cr). The metals are positioned out of the basal plane (0.3873 Å for the Mn complex and 0.3114 Å for the Cr complex). Complex 3 constitutes a rare example of a structurally characterised Mn–NHC complex.^11a,b The Mn–NHC bond lengths in 3 are in agreement with those previously reported; furthermore they are longer than Mn–C σ(sp³) [2.145–2.175 Å]^11c,d and Mn–C σ(sp²) [2.039 Å].^11e Tetrahedral chromium(II)–NHC complexes have been previously reported,¹² with Cr–NHC bond length [2.175(3) Å] slightly longer than those in 2b within the observed esds. The Cr(II)–CH₃ σ(sp³) bond lengths are within the range 2.150–2.168 Å.^12b,c

The structure of the uranium complex is shown in Fig. 4.

Fig. 4 ORTEP representation of the molecular structure of 5 (50% probability ellipsoids). Hydrogen atoms, one thf molecule and Prⁱ groups are omitted for clarity. Important bond lengths (Å) and angles (°): C(2)–U(1) 2.573(5), C(21)–U(1) 2.587(5), N(1)–U(1) 2.653(4), Cl(1)–U(1) 2.6486(12), Cl(2)–U(1) 2.6725(12), Cl(3)–U(1) 2.6314(13), Cl(4)–U(1) 2.5936(13); C(2)–U(1)–N(1) 62.47(14), C(21)–U(1)–N(1) 62.30(14), Cl(1)–U(1)–Cl(2) 145.55(4), Cl(4)–U(1)–Cl(3) 81.86(4), Cl(4)–U(1)–Cl(2) 83.44(4), Cl(3)–U(1)–Cl(2) 126.22(4).

Complex 5 is only the second example of a U(IV) N-heterocyclic carbene complex^13a even though U(III) and U(VI) complexes have been described recently.^13b–d The metal adopts a distorted seven-coordinate geometry with an approximate C₂ axis passing through the pyridine N atom and the uranium.¹⁴ The U–Cl bonds are within 2.587–2.673 Å. The U–C carbene bond lengths in 5 [2.573(5)–2.587(5) Å] are shorter than those observed previously for U(IV) [2.636(9) Å], U(III) [2.672(5)–2.789(14) Å] and U(VI) complexes (2.64 Å). However, they are longer than other known U–C σ(sp³)-alkyl [2.405–2.539 Å]^13e–g and U–C σ(sp²)-vinyl (2.435 Å)^13h bonds.

Oxidation reactions

Despite the fact that good σ-donors are expected to stabilise high oxidation state metal centres, only a few examples of NHC complexes in high oxidation states have been reported.^5a–d The interaction of free NHCs with high oxidation state metal precursors was the method of choice for the synthesis of the majority of known high oxidation state NHC complexes,⁵ but it suffers from unwanted side reactions (metal reduction, ligand oxidation). Alternatively, the oxidation of lower oxidation state NHC complexes has not been explored. In this section we describe the application of the latter strategy to the synthesis of high oxidation state NHC complexes.

The reaction of (C–N–C)VCl₃ with 4-methylmorpholine N-oxide in thf at room temperature gave rise to the light green, paramagnetic V(IV)–oxo complex 6. The same product was also obtained, albeit in lower yields, by the reaction of 1a with the same oxidant. Complex 6 was characterised by analytical and spectroscopic methods. The V=O absorption in the IR spectrum is found at 974 cm⁻¹. Compared to other V(IV)–oxo complexes¹⁵ the position of this absorption is at the lower end of the observed range and is in agreement with decreased V=O π-bonding due to the strong σ-donating NHCs. The structure of 6 was determined crystallographically and a diagram of the molecule is given in Fig. 5.

Fig. 5 ORTEP representation of the molecular structure of 6 (50% probability ellipsoids). Only one out of two identical molecules in the asymmetric unit is shown. Hydrogen atoms are omitted for clarity. Important bond lengths (Å) and angles (°): C(2)–V(1) 2.169(10), C(21)–V(1) 2.182(11), O(1)–V(1) 1.600(5), N(1)–V(1) 2.275(7), Cl(1)–V(1) 2.398(3), Cl(2)–V(1) 2.421(3); Cl(1)–V(1)–Cl(2) 161.97(10), C(2)–V(1)–N(1) 71.6(3), C(21)–V(1)–N(1) 70.4(3).

The geometry around the metal is octahedral with the ‘pincer’ ligand occupying mer sites and the chlorides being mutually trans. The V(IV)–C bond lengths are in the range 2.169(10)–2.182(11) Å, comparable to the corresponding V–C bond lengths of 1a and (C–N–C)VCl₃; the V=O bond [1.600(5) Å] is in the range reported for similar V(IV)=O complexes.¹⁶ The trans chlorides deviate from linearity [161.97(10)°] in this case pointing away from the oxo-ligand, presumably in order to minimise interelectronic repulsions.

The chloride ligands in 6 can be easily abstracted with AgBF₄ in acetonitrile, giving good yields of the salt 7 comprising a V(IV)–oxo pincer complex dication. Characterisation of 7 was accomplished by analytical and crystallographic methods. A diagram of the molecule is shown in Fig. 6.

Fig. 6 ORTEP representation of the cation in 7 (50% probability ellipsoids). Hydrogen atoms, BF₄⁻ anions and one MeCN molecule are omitted for clarity. Important bond lengths (Å) and angles (°): V(1)–C(1) 2.129(3), V(1)–C(21) 2.136(3), V(1)–N(3) 2.236(2), V(1)–O(1) 1.5913(19), V(1)–N(7) 2.091(2); C(1)–V(1)–N(3) 72.04(9), C(21)–V(1)–N(3) 71.59(9).

The octahedral geometry is maintained in 7; the acetonitrile ligands occupy trans positions and the V-carbene bond lengths [2.129(3)–2.136(3) Å] are shorter than those observed in 6. The origin of this shortening is not clear but it may be due to increased ionic interaction of the carbene ligand with the metal. The V=O bond length is comparable to the one observed in 6. As expected, the V–N bond length which is trans to the oxo ligand is longer than the V–acetonitrile bond lengths due to the increased trans influence of the oxo ligand.

The reaction of 1a with p-tolylazide gave the brown paramagnetic V(IV)–tolylimido complex 8 which was characterised by analytical and spectroscopic methods. The V=N(o-tolyl) bond stretching was observed at 970 cm⁻¹ in the infrared spectrum, while the magnetic susceptibility is almost equal to the values measured for 6 and 7, in agreement with one unpaired electron. All data obtained for 8 point to a monomeric six coordinate complex, most likely analogous to 6. We were unable to obtain the crystal structure of 8 due to facile solvent loss from the crystals.

The related Ti(IV) analogue of 8 was prepared by substitution of the labile pyridine ligands in Ti(NBu^t)Cl₂(pyridine)₃ with the free C–N–C. Complex 9 was characterised by spectroscopic, analytical and diffraction methods. The ¹H NMR data support the presence of a plane of symmetry perpendicular to the plane of the ‘pincer’ and therefore the chloride ligands are mutually trans. The structure of the molecule was confirmed crystallographically and is shown in Fig. 7.

Fig. 7 ORTEP representation of the molecular structure of 9 (50% probability ellipsoids). Hydrogen atoms and disorder of the Prⁱ and Bu^t groups are omitted for clarity. Important bond lengths (Å) and angles (°): Ti(1)–N(6) 1.677(5), Ti(1)–C(13) 2.281(6), Ti(1)–C(21) 2.286(6), Ti(1)–N(3) 2.332(5), Ti(1)–Cl(2) 2.415(2), Ti(1)–Cl(1) 2.433(2); C(21)–Ti(1)–N(3) 69.77(18), C(13)–Ti(1)–N(3) 70.82(19), Cl(2)–Ti(1)–Cl(1) 161.42(7).

The octahedral titanium centre is coordinated by the ‘pincer’ ligand in the usual way, two trans-chlorides and one tert-butylimido group. The Ti–C bond lengths are in the range 2.281(6)–2.286(6) Å, significantly longer than in (C–N–C)TiCl₃ [2.191(5)–2.211(5) Å]. The origin of this elongation is unclear. The Cl1–Ti1–Cl2 angle is 161.42(7)° with the chlorides pointing towards the metal, possibly in order to minimise interelectronic repulsions.

The Ti–NHC bond in 9 is relatively inert to a variety of reagents. For example the ¹H NMR spectrum of 9 in C₆D₆ remains unchanged in the presence of excess tmed or PMe₃, but quickly decomposes by the addition of water. In addition, 6 and 7 do not oxidise simple organic substrates under mild conditions.

In conclusion, we have shown that the rigid C–N–C dicarbene ligand is ideal for stabilising certain low and high oxidation state early transition metal complexes bearing metal–NHC bonds. In particular, the V–NHC bond is compatible with oxidising reagents but susceptible to hydrolysis. A comparison of the metal–carbene bond lengths observed in the series of the reported complexes does not help in clarifying the nature of the early metal–NHC bond (covalent with or without backbonding and varying degree of electrostatic interaction). However, first indications show that the coordinated NHC is neither susceptible to external nucleophilic attack nor showing any lability in the presence of other good donors (possibly due to the rigid nature of the tridentate ligand). Further work on the use of the C–N–C as a spectator ligand with electropositive metals and the reactivity of the complexes reported here is in progress.

Experimental

Elemental analyses were carried out by the London Metropolitan University microanalytical laboratory. NMR data were recorded on Bruker AMX-300, AM-300 and DPX-400 spectrometers, operating at 300 MHz and 400 MHz, respectively (¹H). The spectra were referenced internally using the signal from the residual protio-solvent (¹H) or the signals of the solvent (¹³C). Magnetic susceptibility measurements at room temperature were carried out using a Johnson-Matthey balance. All manipulations involving air sensitive materials were carried out under vacuum or N₂ using standard Schlenkware techniques. All solvents used were dried by continuous reflux under N₂ and distillation from suitable drying agents immediately prior to use; the light petroleum had bp 40–60 °C. Commercial chemicals were from Aldrich, Lancaster and Avocado. The following starting materials were prepared following literature methods: [Cr(N(SiMe₃)₂](thf)₂],¹⁷ (CH–N–CH)Br₂,¹⁸ (CHmes–N–CHmes)Br₂,¹⁸ VCl₂(tmed)₂,¹⁹ VCl₃(C–N–C),⁷ Ti(NBu^t)Cl₂(py)₃,²⁰ FeBr₂(C–N–C).²¹

[2,6-Bis{3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine, C–N–C 3a

The imidazolium salt (CH–N–CH)Br₂ (6.86 g, 9.93 mmol) was suspended in thf (100 cm³) and cooled to −78 °C. A solution of KN(SiMe₃)₂ (4.32 g, 21.6 mmol) in thf (100 cm³) was cooled to −78 °C, and added to the vigorously stirred reaction solution. The mixture was allowed to warm slowly to room temperature overnight. After evaporation of the solvent at reduced pressure, the residue was dissolved in toluene (∼100 cm³) and filtered through Celite. The solvent volume was reduced to ∼20 cm³, and light petroleum (40 cm³) was added. Cooling to −30 °C overnight gave a cream solid and a red solution. The solution was filtered and the solid washed with light petroleum (∼20 cm³). The product was obtained as a cream solid (4.06 g, 77%). δ_H (C₆D₆) 1.09 (d, 12H, J = 6.9 Hz, Me), 1.18 (d, 12H, J = 6.8 Hz, Me), 2.90 [sept., 4H, J = 6.9 Hz, CH(CH₃)₂], 6.62 (d, 2H, J = 1.8 Hz, imidazole backbone CH), 7.05 (t, 1 H, J = 8.0 Hz, central pyridine CH), 7.14 (d, 4H, J = 7.4 Hz, aromatic CH), 7.25 (dd, 2H, J = 6.9, 8.4 Hz; central aromatic CH), 8.06 (d, 2H, J = 1.7 Hz, imidazole backbone CH), 8.43 (d, 2H, J = 8.0 Hz, pyridine CH). δ_C (C₆D₆) 24.03 (Me), 24.42 (Me), 28.60 [CH(CH₃)₂], 111.70 (aromatic CH), 116.25 (aromatic CH), 122.82 (aromatic CH), 128.53 (aromatic CH), 129.29 (aromatic CH), 138.72 (aromatic C), 140.69 (aromatic C), 146.20 (aromatic CH), 152.62 (aromatic C), 220.34 (carbene C).

[2,6-Bis{3-(2,4,6-trimethylphenyl)imidazol-2-ylidene]pyridine, C^mes–N–C^mes

The reaction was carried out as described above, using the imidazolium salt (CHmes–N–CHmes)Br₂ (2.78 g, 4.00 mmol) and KN(SiMe₃)₂ (1.68 g, 8.42 mmol). For solubility reasons, benzene was used in place of toluene in the work up, giving the carbene as a cream solid (1.07 g, 60%). δ_H (C₆D₆) 2.12 (s, 12H, Me), 2.14 (s, 6H, Me), 6.44 (d, 2H, J = 1.4 Hz, imidazole backbone CH), 6.79 (s, 4H, aromatic CH), 7.09 (t, 1H, J = 7.6 Hz, central pyridine CH), 8.11 (d, 2H, J = 1.4 Hz), 8.54 (d, 2H, J = 7.6 Hz. δ_C (C₆D₆) 18.07 (Me), 21.00 (Me), 111.48 (aromatic CH), 116.49 (aromatic CH), 119.66 (aromatic C), 121.47 (aromatic CH), 129.18 (aromatic CH), 135.29 (aromatic C), 137.61 (aromatic C), 140.68 (aromatic CH), 152.70 (aromatic C), 219.75 (carbene).

Preparation of the complexes

Method A. A solution of the carbene in thf was added to a solution or suspension of the metal halide precursor in the same solvent at −78 °C. After warming the mixture to room temperature, stirring for 30–60 min and filtering, the volatiles were removed under reduced pressure and the solid residue was washed with light petroleum and dried under vacuum. Crystallisation can be performed by layering a thf solution with light petroleum.

Method B. To a suspension of the (CH–N–CH)Br₂ in thf was added dropwise at −78 °C a solution of the metal amide in the same solvent. The reaction mixture was allowed to reach room temperature and stirred for 3–4 h. Evaporation of the volatiles under reduced pressure left the product as a solid residue which was washed with light petroleum and dried under vacuum. Recrystallisation can be carried out by layering thf solutions with light petroleum.

[2,6-Bis{3-(2,6-diisopropylphenyl)imidazol-2-ylidene}]pyridine vanadium dichloride tetrahydrofuran 1a

This was prepared following method A from the carbene C–N–C (266 mg, 0.50 mmol) in thf (5 cm³) and [VCl₂(tmeda)₂] (0.177 g, 0.50 mmol) in thf (5 cm³) at −78 °C. The mixture was stirred for 10 min at −78 °C, then allowed to warm to room temperature, giving a dark green solution. Layering with light petroleum overnight led to the formation of crystals suitable for X-ray diffraction. Yield: 0.355 g, 98%. Mp 130–132 °C (decomp.). Found: C, 64.10; H, 6.63; N, 9.48%. Calc. for C₃₉H₄₉N₅Cl₂OV: C, 64.55; H, 6.81; N, 9.65%. μ_eff = 3.79 μ_B.

[2,6-Bis{3-(2,6-diisopropylphenyl)imidazol-2-ylidene}]pyridine chromium dichloride 2a

To a suspension of CrCl₂ (0.231 g, 1.88 mmol) in thf (25 cm³) was added a solution of C–N–C (1.0 g, 1.88 mmol) in thf (75 cm³) at −78 °C. The reaction mixture was stirred for 15 min at −78 °C and then allowed to reach room temperature and stirred for 16 h. Removal of the volatiles under reduced pressure gave 2a as a purple solid. Under prolonged exposure to vacuum the solid turns green. Yield: 1.256 g, 92%. Mp 101–102 °C. Found: C, 64.57, H, 6.75, N, 9.60. Calc. for C₃₉H₄₉N₅OCrCl₂: C, 64.45, H, 6.80, N, 9.64%. μ_eff = 4.99 μ_B.

[2,6-Bis{3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine chromium dibromide 2b

This was prepared following method B from (CH–N–CH)Br₂ (0.385 g, 1 mmol) and {Cr[N(SiMe₃)₂]₂(thf)₂} (0.540 g, 1 mmol). The product was crystallised from a thf solution by slow diffusion of light petroleum. Found: C, 57.51; H, 6.05; N, 8.12%. Calc. for C₃₅H₄₁N₅Br₂Cr·1.5C₄H₈O: C, 57.82; H, 6.27; N, 8.22%. μ_eff = 4.97 μ_B.

[2,6-Bis{3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine manganese dibromide 3

This was prepared following method A from MnBr₂ (0.607 g, 2.8 mmol) and C–N–C (1.50 g, 2.8 mmol). Yield: 2.15 g (yellow powder) 93%. Mp 106–108 °C (decomp.). Crystals were grown by slow vapour diffusion of light petroleum into a thf solution. Found: C, 57.11; H, 6.00; N, 8.47. Calc. for C₃₉H₄₉N₅OMnBr₂: C, 57.22; H, 6.03, N; 8.56%. μ_eff = 5.87 μ_B.

[2,6-Bis{3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine niobium trichloride 4

This was prepared following method A from NbCl₃(dme) (0.65 g, 2.25 mmol) and C–N–C (1.98 g, 2.25 mmol). Filtration of the dark blue/purple solution followed by evaporation of the volatiles under reduced pressure gave 4 as a purple solid residue. X-Ray quality crystals were obtained by layering a thf solution with light petroleum. Yield: 1.56 g, 95%. Mp 260–263 °C (decomp.). Found: C, 57.58; H, 5.79; N, 9.42. Calc. for C₃₉H₄₉N₅ONbCl₃: C, 57.51; H, 5.65; N, 9.58%. μ_eff = 2.04 μ_B.

[2,6-Bis{3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine uranium tetrachloride 5

This was prepared following method A from UCl₄ (0.300 g, 0.79 mmol) and C–N–C (0.42 g, 0.79 mmol). Yield: 0.36 g, 50% of yellow–green solid. X-Ray quality crystals were obtained by layering of a dilute thf solution with light petroleum. Found: C, 46.02; H, 4.86; N, 7.51. Calc. for C₃₅H₄₁N₅UCl₄: C, 46.12; H, 4.53; N, 7.68%.

trans-[2,6-Bis{3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine oxovanadium dichloride 6

To a stirred suspension of (C–N–C)VCl₃ (1.10 g, 1.60 mmol) in THF (100 cm³) under a counterflow of dry N₂ was added N-methylmorpholine N-oxide (0.206 g, 1.76 mmol) and the reaction was stirred for 16 h at room temperature. A green solution with a dark precipitate was obtained which was diluted with THF (50 cm³), filtered through Celite and concentrated in vacuo, to give 6 as a green solid. Yield: 0.965 g, 90%. X-Ray quality crystals were grown by layering a THF solution with light petroleum. Mp 286–288 °C (decomp.). Found: C, 62.54; H, 6.24; N, 10.54. Calc. for C₃₅H₄₁N₅OCl₂V: C, 62.78; H, 6.17; N, 10.46%. IR (Nujol mull): 1623 (w), 1595 (w), 1448 (s), 1291 (m), 1151 (w), 974 (m), 801 (m), 761 (m), 722 (s), 336 (m). MS (ES⁺, MeCN): 633.1 ([M − Cl]⁺, 100%).

trans-[2,6-Bis{3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine bis(acetonitrile) oxovanadium bis(tetrafluoroborate) 7

To a mixture of 6 (0.150 g, 0.22 mmol) and AgBF₄ (0.092 g, 0.47 mmol) was added acetonitrile (15 cm³), and the reaction was stirred at room temperature for 4 h. The white precipitate that was formed was removed by filtration through Celite and the blue solution was evaporated to dryness. Yield: 0.191 g, 97%. X-Ray quality crystals were grown by slow diffusion of diethyl ether into an acetonitrile solution. Mp 145–146 °C. Found: C, 54.74, H, 5.41, N, 11.65. Calc. for C₃₉H₄₇N₇B₂F₈V: C, 54.82; H, 5.54; 11.48%; IR (Nujol mull): 2361 (m), 2343 (w), 2292 (w), 1630 (s), 1594 (w), 1300 (s), 1261 (m), 1075 (m), 1020 (m), 976 (w), 803 (s), 778 (w) cm⁻¹. μ_eff = 1.79 μ_B.

trans-[2,6-Bis{3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine vanadium (p-tolylimido) dichloride 8

To a stirred suspension of 1a (0.200 g, 0.28 mmol) in THF (20 cm³) under a counterflow of dry N₂ was added p-tolyl azide (0.037 g, 0.28 mmol) and the reaction mixture was stirred for 16 h at room temperature. The brown solution was filtered through Celite and evaporated to dryness, affording 8 as a brown solid. Mp 169–170 °C. Yield: 0.150 g, 72%. Found: C, 66.32; H, 6.50; N, 10.97. Calc. for C₄₂H₄₈N₆Cl₂V: C, 66.49; H, 6.38; N, 11.08%. IR (Nujol mull): 1623 (w), 1592 (w), 1304 (m), 1169 (w), 1153 (w), 970 (w), 722 (s). MS (ES⁺, MeCN): 722.3 ([M − Cl]⁺, 100%). μ_eff = 1.66 μ_B.

trans-[2,6-Bs{3-(2,6-diisopropylphenyl)imidazol-2-ylidene]pyridine titanium (tert-butylimido) dichloride 9

To a solution of Ti(NBu^t)Cl₂(py)₃ (0.161 g, 0.38 mmol) in thf (5 cm³) was added a solution of C–N–C (0.200 g, 0.38 mmol) in the same solvent (15 cm³). The mixture was stirred at room temperature for three days. Evaporation of the volatiles under reduced pressure, afforded 9 as an orange solid. Yield: 0.285 g, 95%. X-Ray quality crystals were grown by slow diffusion of light petroleum into a thf solution of 9. Found: C, 65.01, H, 7.05, N, 11.60%. Calc. for C₃₉H₅₀N₆Cl₂Ti: C, 64.91; H, 6.98; N, 11.65%. δ_H (C₆D₆) 7.03–7.23 (6H, m, aromatic), 6.80 (2H, br s, aromatic), 6.62 (1H, br t, J = 6.0 Hz, aromatic), 6.43 (2H, d, J = 7.5 Hz, imidazol-2-ylidene CH), 6.36 (2H, d, J = 1.5 Hz, imidazol-2-ylidene CH), 3.27 (4H, septet, J = 6.5 Hz, Prⁱ CH), 1.68 and 1.02 (each 12H, d, J = 6.5 Hz, Prⁱ CH₃), 0.54 [9H, s, C(CH₃)₃]. δ_C (C₆D₆) 200.72 (C, carbene), 148.18, 146.76, 137.42 (C, Ar); 141.96, 130.29, 128.34, 124.88, 124.57, 123.53 (CH, Ar); 114.55, 107.79 (CH, imidazol-2-ylidene); 31.91 (CH₃, Bu^t); 28.62, 25.87 (CH₃, Prⁱ); 25.47 (C, Bu^t); 24.16 (CH, Prⁱ). IR (Nujol mull): 2360 (m), 2342 (w), 1621 (m), 1598 (m), 1481 (s), 1410 (s), 1377 (s), 1286 (m), 1250 (w), 1149 (w), 1107 (m), 1037 (m), 1016 (m), 799 (s), 778 (w), 769 (w), 740 cm⁻¹

Crystallography

A summary of the crystal data, data collection and refinement parameters for compounds 1a,(C–N–C)VCl₃, 3, 5, 6, and 7 are given in Table 1; in addition, detailed data for the crystallographic characterisation of 1b, 2b, 4 and [(C–N–C)TiCl₃ have been included in the ESI.†

Table 1 Crystallographic data for the complexes 1a, (C–N–C)VCl₃, 3, 5, 6, 7 and 9

	1a	(C–N–C)VCl3	3	5	6	7	9
Chemical formula	C47H65Cl2N5O3V	C39H49Cl3N5OV	C39H49Br2MnN5O	C39H49Cl4N5OU	C35H41Cl2N5OV	C39H47B2F8N7OV	C39H49Cl2N6Ti
M r	869.88	761.12	818.59	983.66	669.57	895.45	720.64
Crystal system	Orthorhombic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Orthorhombic
Space group	Pna21	P21/c	P21/c	P21/c	P21/n	P21/c	P212121
a/Å	10.306(3)	10.8934(12)	10.6948(8)	10.7146(8)	16.699(5)	16.851(3)	10.802(9)
b/Å	32.019(11)	18.7259(17)	18.7188(14)	18.384(2)	19.125(5)	12.707(4)	13.630(13)
c/Å	13.807(5)	18.381(2)	19.2039(19)	20.229(4)	22.226(6)	21.564(4)	32.45(2)
β/°	90	94.051(8)	93.796(9)	93.628(10)	97.012(4)	100.152(13)	90
V/Å^3	4556(3)	3740.2(7)	3836.1(6)	3976.7(9)	7045(3)	4545.1(18)	4778(7)
Z	4	4	4	4	8	4	4
T/K	120(2)	120(2)	95(2)	120(2)	120(2)	120(2)	120(2)
μ/mm^−1	0.380	0.518	2.464	4.388	0.467	0.292	0.319
No. of data collected	21996	19040	26916	37286	34114	63406	33304
No. of unique data	6436	8522	6754	7713	7369	10899	10871
R int	0.1892	0.0570	0.0661	0.0447	0.1383	0.073	0.1118
Final R(|F|) for Fo > 2σ(Fo)	0.0656	0.0571	0.0874	0.0370	0.0763	0.0604	0.0944
Final R(F^2) for all data	0.0915	0.1282	0.2042	0.0715	0.2073	0.1382	0.1944

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All data sets were collected on an Enraf Nonius KappaCCD area detector diffractometer with rotating anode FR591 and an Oxford Cryosystems low-temperature device operating in omega scanning mode with phi and omega scans to fill the Ewald sphere. The crystals were mounted on a glass fibre with silicon grease, from Fomblin vacuum oil. The programs used for control and integration were Collect, Scalepack and Denzo.²² All solutions and refinements were performed using the WinGX package and all software packages within.²³ The SQUEEZE algorithm²⁴ was used to remove highly disordered water molecule in 6 and thf molecules in 9 after attempts to satisfactorily model the disorder failed.

CCDC reference numbers 282396–282406.

For crystallographic data in CIF or other electronic format see DOI: 10.1039/b512133g

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Footnote

† Electronic supplementary information (ESI) available: Full crystallographic data and ORTEP diagrams for the structurally characterised species. See DOI: 10.1039/b512133g

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