Full Paper doi.org/10.1002/ejic.202000550 EurJIC European Journal of Inorganic Chemistry Chromium(VI) NHC Complexes Chromium(VI) Bisimido Dichloro, Bisimido Alkylidene, and Chromium(V) Bisimido Iodo N-Heterocyclic Carbene Complexes Pradeep K. R. Panyam,[a] Laura Stöhr,[a] Dongren Wang,[a] Wolfgang Frey,[b] and Michael R. Buchmeiser*[a,c] Abstract: Reaction of CrCl2(N-tBu)2 with 1,3-dimethylimidazol- 2-ylidene (IMe), 1,3-dimethyl-4,5-dichloroimidazol-2-ylidene (IMeCl2), 1,3-di(2-propyl)imidazol-2-ylidene (IPr), 1,3-dimesity- limidazol-2-ylidene (IMes) and 1,3-bis(2,6-(2-Pr)2C6H3)imidazol- 2-ylidene (IDipp) yields the corresponding N-heterocyclic carb- ene (NHC) adducts CrCl2(IMe)(N-tBu)2 (1), CrCl2(IMeCl2)(N-tBu)2 (2), CrCl2(IPr)(N-tBu)2 (3), CrCl2(IMes)(N-tBu)2 (4) and CrCl2(ID- ipp)(N-tBu)2 (5). Likewise, reaction of CrCl2(N-2,6-(2-Pr)2C6H3)2 and CrCl2(N-adamantyl)2 with IMes yields CrCl2(N-2,6-(2- Pr)2C6H3)2(IMes) (6) and CrCl2(N-adamantyl)2(IMes) (7), respec- tively. Reaction of CrCl2(N-tBu)2 with the bidentate NHCs 1-R-3- (1-(2-LiO-C6H4))imidazol-2-ylidene yields the corresponding Introduction In view of the pivotal role of chromium in catalysis including the worldwide production of polyolefins[1] or in the tri- or te- tramerization of ethylene,[1d,2] we devoted our studies to the exploration of chromium(VI) N-heterocyclic carbene (NHC) com- plexes. Notably, incorporation of NHCs as ligands into high oxidation state 3d-metal species proved fruitful, as illustrated by the diverse complexes encompassing 3d metals in higher oxidation states such as VV, CrIII, CrV, MnV, or FeV featuring differ- ent ancillary ligands including oxo-, imido-, chloro- or nitride- moieties.[3] In continuation of our efforts towards the synthesis of group VI metal alkylidene NHC complexes,[4] we thrived to- wards the synthesis of chromium(VI) NHC alkylidene complexes. Indeed, chromium has been widely used both in coordination and organometallic chemistry for decades. Similarly, NHCs [a] P. K. R. Panyam, L. Stöhr, Dr. D. Wang, Prof. Dr. M. R. Buchmeiser Institut für Polymerchemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany E-mail: michael.buchmeiser@ipoc.uni-stuttgart.de [b] Dr. W. Frey Institut für Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany [c] Prof. Dr. M. R. Buchmeiser German Institutes of Textile and Fiber Research (DITF) Denkendorf, Körschtalstr. 26, 73770 Denkendorf, Germany Supporting information and ORCID(s) from the author(s) for this article are available on the WWW under https://doi.org/10.1002/ejic.202000550. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. · This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Eur. J. Inorg. Chem. 2020, 3673–3681 © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3673 pentacoordinated Cr(VI) complexes CrCl2(1-R-3-(1-(2-O- C6H4))imidazol-2-ylidene)2C6H3)2(IMes) (R = 2,4,6-(CH3)3C6H2, 8), (R = tBu, 9), (R = 2-phenyl-C6H4, 10). Reaction of the chro- mium(VI) complex Cr(N-2,6-(2-Pr)2-C6H3)2(CH2C(CH3)3)2 with 1,3- dimethylimidazol-2-ylidene·AgI yields the bimetallic silver ad- duct of the chromium alkylidene complex (11) along with the tetrahedral chromium(V) complex CrI(N-2,6-(2-Pr)2-C6H3)2(1,3- dimethylimidazol-2-ylidene) (12). Compounds 1–4, 7, 9–12 were characterized by single-crystal X-ray analysis. Finally, the chromium(VI) bisimido-amido complexes 13–14 bearing the N- 6-(2-(diethylboryl)phenyl)pyridyl-2-yl-motif are reported. alongside alkylidenes have also been used for about half a cen- tury and turned out to be particularly useful in the case of V, Mo, W, and Ru.[4d,5] However, to the best of our knowledge, till date chromium(VI) NHC complexes have not been reported, which is probably a result of the reducing power of the NHC and also weaker carbon metal bond in the high oxidation metal species. Though chromium is cut below molybdenum and tungsten in terms of the metal-carbene bond strength[6] and ease of synthesis, the synthesis of chromium bisimido dichloro NHC complexes was worth to be investigated. Here, we report the synthesis and structural characterization of the first examples of chromium(VI) complexes with both monodentate and bidentate NHCs alongside with chromium(VI) alkylidene complexes and chromium(VI) bisimido-amido com- plexes, bearing the 6-(2-(diethylboryl)phenyl)pyridyl-2-yl- motif. Results and Discussion Monodentate Chromium(VI) NHC Complexes Several novel Cr(VI) NHC complexes (1–7) bearing monodentate NHCs based on the chromium bisimido dichloro complexes[7] were synthesized as outlined in Scheme 1. Treatment of a pre- cooled (–34 °C) solution of the precursor in toluene with one equivalent of NHC, either in form of the free carbene or the NHC silver(I) iodide adduct, resulted in the formation of the chromium(VI) NHC complexes 1–7. All complexes were isolated as red-colored solids but decomposed upon standing in the solid state to brown solids at room temperature. However, they are stable at –34 °C both as solids and even in solution. The 1H https://doi.org/10.1002/ejic.202000550 http://creativecommons.org/licenses/by/4.0/ http://creativecommons.org/licenses/by/4.0/ Full Paper doi.org/10.1002/ejic.202000550 EurJIC European Journal of Inorganic Chemistry NMR spectra of the synthesized complexes showed the protons corresponding to imidazol-2-ylidene confirming the formation of the NHC adduct. The 13C NMR spectra display a characteristic carbene resonance in the range of 183 < δ < 192 ppm, confirm- ing the formation of the Cr–carbene bond. We were not able to carry out a full assignment of the 1H-NMR spectra of the brown solids owing to the mixture of different compounds but we could predict the reduction of one imido to an amido group by the presence of two signals corresponding to an N-tBu and NH-tBu moiety, respectivley. However, our efforts to recrystallize the brown solids were not fruitful and hence any solid conclu- sion about the molecular structure of the decomposed product (brown solids) could not be established at this point of time. Scheme 1. Synthesis of monodentate chromium(VI) NHC complexes. The molecular structures of the chromium complexes 1–7 were established by single-crystal X-ray analysis, which also al- lowed identifying structural similarities. Unlike complexes 1 and 2, the chromium complexes 3, 4 and 7 have both chloride li- gands trans to each another, attributable to the steric influence of the bulky IMes ligand compared to that of its methyl coun- terpart. The Cr–C bond length slightly increases with increasing size of the NHC from Cr-1 to Cr-7. Crystal suitable for single- crystal X-ray analysis were grown from a mixture of CH2Cl2 and pentane at –34 °C. Complex 1 (Figure 1) crystallizes in the monoclinic space group C2/c with a = 2379.7(8) pm, b = 970.1(3) pm, c = 2144.1(7) pm, α = γ = 90°, � = 95.586(2)° (Z = 8). Complex 1 adopts a distorted square pyramidal geometry (τ5 = 0.44) around the metal center with the chloride ligands cis to each other, attributable to the small IMe-ligand occupying the apex posi- tion (Figure 1). Complex 2 (Figure 2) crystallizes in the monoclinic space group P21/n with a = 898.7(5) pm, b = 1151.6(6) pm, c = 2481.9(12) pm, α = γ = 90°, � = 100.013(2)° (Z = 4). As in com- plex 1, the two chloride ligands are also arranged cis to each Eur. J. Inorg. Chem. 2020, 3673–3681 www.eurjic.org © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3674 Figure 1. Single crystal X-ray structure of 1. Relevant bond lengths [pm] and bond angles [°]: Cr1–C1 208.9, Cr1–Cl1 236.7, Cr1–Cl2 237.6, Cr1–N3 163.2, Cr1–N4 164.4; Cl1–Cr1–Cl2 83.81(2), Cl1–Cr1–C1 160.44(4), Cl1–Cr1–N3 92.66(5), Cl1–Cr1–N4 96.09(5), Cl2–Cr1–C1 79.73(4), Cl2–Cr1–N3 133.67(5), Cl2–Cr1–N4 115.84(5), C1–Cr1–N3 91.13(6), C1–Cr1–N4 100.53(6), N3–Cr1–N4 110.47(6). Thermal ellipsoids are set at a 50 % probability level. other due to the low steric demand of the IMeCl2-ligand. The geometry at the metal center is distorted square pyramidal (τ5 = 0.405) with the NHC occupying the apex position. Com- plex 3 (Figure 3) crystallizes in the monoclinic space group P21/c with a = 1481.14(7) pm, b = 1117.8(7) pm, c = 1392.6(6) pm, α = γ = 90°, � = 96.180(4)° (Z = 4). The geometry at the metal center is also distorted trigonal bipyramidal (τ5 = 0.52) with the NHC occupying the apex position. Figure 2. Single crystal X-ray structure of 2. Relevant bond lengths [pm] and bond angles [°]: Cr1–Cl1 236.6, Cr1–Cl2 234.6, Cr1–C1 208.3, Cr1–N3 163.3, Cr1–N4 164.6; Cl1–Cr1–Cl2 85.04(2), Cl1–Cr1–C1 76.91(3), Cl1–Cr1–N3 134.47(4), Cl1–Cr1–N4 115.04(4), Cl2–Cr1–C1 158.80(4), Cl2–Cr1–N3 89.05(4), Cl2–Cr1–N4 97.92(4), C1–Cr1–N3 95.66(5), C1–Cr1–N4 99.73(5), N3–Cr1–N4 110.49(5). Thermal ellipsoids are set at a 50 % probability level. Full Paper doi.org/10.1002/ejic.202000550 EurJIC European Journal of Inorganic Chemistry Figure 3. Single crystal X-ray structure of 3. Relevant bond lengths [pm] and bond angles [°]: Cr1–C1 211.3, Cr1–Cl1 235.2, Cr1–Cl2 234.5, Cr1–N3 163.3, Cr1–N4 163.4; Cl1–Cr1–Cl2 161.29(4), Cl1–Cr1–C1 81.5(1), Cl1–Cr1–N3 92.1(1), Cl1–Cr1–N4 97.8(1), Cl2–Cr1–C1 80.7(1), Cl2–Cr1–N3 95.0(1), Cl2–Cr1–N4 95.4(1), C1–Cr1–N3 129.9(1), C1–Cr1–N4 117.3(1), N3–Cr1–N4 112.8(2). Ther- mal ellipsoids are set at a 50 % probability level. Complex 4 (Figure 4) crystallizes in the monoclinic space group P21/c with a = 807.8(7) pm, b = 2231.1(14) pm, c = 1733.2(11) pm, α = γ = 90°, � = 100.752(2)° (Z = 4). The geome- try at the metal center is also distorted trigonal bipyramidal (τ5 = 0.68) with the NHC occupying the apex position. Complex 7 (Figure 5) crystallized from a saturated solution in CH2Cl2 and pentane. It decomposes quickly and the measured data con- tains two independent Cr complexes in the asymmetric unit with one strongly disordered adamantane moiety. 7 crystallizes in the monoclinic space group P21/c with a = 1001.42(9) pm, b = 2482.6(3) pm, c = 3264.6(3) pm, α = γ = 90°, � = 91.725(7)° (Z = 8). The geometry at the metal center is distorted trigonal bipyramidal (τ5 = 0.73) with the NHC occupying the apex posi- tion. Figure 4. Single crystal X-ray structure of 4. Relevant bond lengths [pm] and bond angles [°]: Cr1–C1 216.1, Cr1–Cl1 233.4, Cr1–Cl2 233.1, Cr1–N3 163.1, Cr1–N4 163.2; Cl1–Cr1–Cl2 166.46(3), Cl1–Cr1–C1 83.69(6), Cl1–Cr1–N3 92.60(8), Cl1–Cr1–N4 93.82(8), Cl2–Cr1–C1 82.90(6), Cl2–Cr1–N3 95.79(8), Cl2– Cr1–N4 92.46(8), C1–Cr1–N3 120.3(1), C1–Cr1–N4 125.7(1), N3–Cr1–N4 114.0(1). Thermal ellipsoids are set at a 50 % probability level. Eur. J. Inorg. Chem. 2020, 3673–3681 www.eurjic.org © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3675 Figure 5. Single crystal X-ray structure of 7. Relevant bond lengths [pm] and bond angles [°]: Cr1–C1 213.9, Cr1–Cl1 234.5, Cr1–Cl2 233.8, Cr1–N3 163.0, Cr1–N4 164.0; Cl1–Cr1–Cl2 167.1(1), Cl1–Cr1–C1 83.8(2), Cl1–Cr1–N3 95.7(3), Cl1–Cr1–N4 92.9(3), Cl2–Cr1–C1 83.4(2), Cl2–Cr1–N3 91.8(3), Cl2–Cr1–N4 93.0(3), C1–Cr1–N3 120.0(4), C1–Cr1–N4 123.3(4), N3–Cr1–N4 116.7(5). Ther- mal ellipsoids are set at a 50 % probability level. Bidentate O-Chelating Chromium NHC Complexes In addition to the chromium(VI) NHC complexes with monod- entate ligands, we were also interested in the synthesis of com- plexes with bidentate carbene ligands. We envisioned the che- lating effect of the bidentate O-chelating NHC to enhance the stability of the complexes as reported for this class of ligands.[8] Bidentate NHCs with non-chelating N-substituents, i.e. N-tert- butyl, N-2,4,6-trimethylphenyl, and N-2-phenylphenyl) were syn- thesized according to reported methods.[4b] As depicted in Scheme 2, the imidazolinium salts were deprotonated by em- ploying LiHMDS in toluene for one hour and then added to the chromium precursor CrCl2(N-tBu)2 at –34 °C followed by stirring at room temperature for one hour. The corresponding com- plexes were isolated as red colored solids in 54–71 % yield. Scheme 2. Synthesis of bidentate O-chelating chromium NHC complexes. Full Paper doi.org/10.1002/ejic.202000550 EurJIC European Journal of Inorganic Chemistry Complexes 8–10 showed resonances for the carbene in the 13C NMR spectra between 190 < δ < 200 ppm, confirming the for- mation of the corresponding Cr-carbene bonds. Generally, the resonances of the Cr-carbene with bidentate NHCs were slightly downfield shifted compared to those of the monodentate NHCs. Single crystals of 9 and 10 suitable for X-ray diffraction stud- ies were obtained by layering pentane over saturated CH2Cl2 solution of the complex. Complex 9 (Figure 6) crystallizes in the orthorhombic space group P21 with a = 1049.54(5) pm, b = 1667.47(6) pm, c = 1695.78(8) pm, α = � = γ = 90° (Z = 4). The geometry at the metal center is distorted square pyramidal (τ5 = 0.11) with the NHC occupying the apex position. Figure 6. Single crystal X-ray structure of 9. Relevant bond lengths [pm] and bond angles [°]: Cr1–N3 164.9, Cr1–N4 163.7, Cr1–Cl1 238.0, Cr1–O1 193.7, Cr1–C1 210.6; N3–Cr1–N4 112.3(1), N3–Cr1–Cl1 96.61(8), N3–Cr1–O1 104.6(1), N3–Cr1–C1 111.8(1), N4–Cr1–Cl1 89.73(8), N4–Cr1–O1 143.0(1), N4–Cr1–C1 89.2(1), Cl1–Cr1–O1 83.70(6), Cl1–Cr1–C1 149.64(8), O1–Cr1–C1 79.1(1). Ther- mal ellipsoids are set at a 50 % probability level. Likewise, complex 10 (Figure 7) crystallizes in the monoclinic space group P21/c with a = 1293.69(5) pm, b = 1291.46(5) pm, c = 1853.01(7) pm, α = γ = 90°, � = 95.19(2)° (Z = 4). The geometry at the metal center is distorted square pyramidal (τ5 = 0.26) with the NHC occupying the apex position. Chromium Alkylidene Complexes Recently, Theopold and co-workers[6,9] reported on the success- ful preparation of chromium(VI) alkylidene complexes stabilized by PPh3; results were in line with those reported by Gibson et al.[10] In view of the most favorable properties of Mo and W imido/ oxo alkylidene NHC complexes,[4d,11] we attempted the synthe- sis of chromium(VI) alkylidene complexes stabilized by NHC li- gands (Scheme 3). Interestingly, reaction of the precursor Cr(N- 2,6-(2-Pr)2-C6H3)2(CH2C(CH3)3)2 [7a,10] with the silver(I) iodide ad- duct of IMe unexpectedly yielded the bimetallic adduct 11, which upon prolonged stirring at room temperatures decom- posed to yield the tetrahedral chromium(V) complex CrI(N-2,6- (2-Pr)2-C6H3)2(1,3-dimethylimidazol-2-ylidene) (12). By contrast, Eur. J. Inorg. Chem. 2020, 3673–3681 www.eurjic.org © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3676 Figure 7. Single crystal X-ray structure of 10. Relevant bond lengths [pm] and bond angles [°]: Cr1–N3 163.6, Cr1–N4 165.4, Cr1–Cl1 236.0, Cr1–O1 195.2, Cr1–C1 207.0; N3–Cr1–N4 111.77(6), N3–Cr1–Cl1 90.92(4), N3–Cr1–O1 136.70(5), N3–Cr1–C1 87.29(6), N4–Cr1–Cl1 99.47(4), N4–Cr1–O1 111.51(5), N4–Cr1–C1 106.68(6), Cl1–Cr1–O1 83.48(3), Cl1–Cr1–C1 152.49(4), O1–Cr1–C1 79.18(5). Thermal ellipsoids are set at a 50 % probability level. under the same reaction conditions, we could not obtain the expected products, even in trace quantities, employing other NHCs including IMeCl2, IMes or IMesH2. Complexes 11 and 12 are extremely sensitive, both in solution and in the solid state, even under high vacuum. However, a freshly prepared sample of complex 11 was characterized by 1H NMR spectroscopy. The 1H NMR spectrum of 11 shows a downfield shifted proton at δ = 13.5 ppm (J = 127.9 Hz) corresponding to the Cr-alkylidene proton. Notably, the alkylidene proton is significantly upfield shifted compared to the corresponding Cr-THF (15 ppm)[10] and Scheme 3. Synthesis of chromium(V) bisimido iodo NHC and chromium(VI) bisimido alkylidene NHC complexes. Full Paper doi.org/10.1002/ejic.202000550 EurJIC European Journal of Inorganic Chemistry Cr-PPh3 (14.5 ppm)[6] complexes. We also performed single- crystal X-ray analysis on both complexes, which unambiguously confirmed their molecular structure. Single crystals of 11 and 12 suitable for single-crystal X-ray analysis were obtained by layering an almost saturated solution of CH2Cl2 with n-pentane followed by storage at –40 °C for a week. Complex 11 crystalli- zes in the monoclinic space group C2/c with a = 3037.53(17) pm, b = 1199.04(6) pm, c = 2331.63(11) pm, α = γ = 90°, � = 99.003(3)° (Z = 8). The geometry at the chromium metal center is distorted tetrahedral (τ4 = 0.906) with a slightly larger N3–Cr1– N4 angle of 117.5(2)° compared to the metal precursor Cr(N- 2,6-(2-Pr)2-C6H3)2(CH2C(CH3)3)2 (111.71(8)°). From Figure 8 it is evident that only one neopentyl ligand is attached to chro- mium and the Cr–C6 bond length is 190.5 pm, which is consid- erably shorter than the single bond lengths in the precursor Cr(N-2,6-(2-Pr)2-C6H3)2(CH2C(CH3)3)2 [10] (203.0(2) and 204.2(2) pm) but similar to that of the earlier reported Cr-alkylidene- PPh3 complexes (184.8(5) pm).[6] Figure 8. Single crystal X-ray structure of 11. Relevant bond lengths [pm] and bond angles [°]: Cr1–I1 266.0, Cr1–C6 190.5, Cr1–N3 165.2, Cr1–N4 164.5, Ag1–I1 286.4, Ag1–C6 225.3, Ag1–C1 212.0; N3–Cr1–N4 117.5(2), N3–Cr1–C6 103.5(3), N3–Cr1–I1 109.9(2), N4–Cr1–C6 105.4(3), N4–Cr1–I1 106.4(2), I1–Cr1– C6 114.4(2), Cr1–C6–Ag1 43.2(2), Cr1–I1–Ag1 56.38(2), C6–Ag1–C1 135.3(2), I1–Ag1–C1 127.6(2). Thermal ellipsoids are set at a 50 % probability level. Likewise, complex 12 (Figure 9) crystallizes in the monoclinic space group P21/n with a = 1133.50(5) pm, b = 1623.64(7) pm, c = 1707.94(7) pm, α = γ = 90°, � = 103.616(2)° (Z = 4). The geometry at the metal center is also distorted tetrahedral (τ4 = 0.913). Chromium(VI) Bisimido-Amido-Complexes Bearing the 6-(2-(Diethylboryl)phenyl)pyridyl-2-yl-Motif We also synthesized CrVI bisimido-amido complexes containing the 6-(2-(diethylboryl)phenyl)pyridyl-2-yl-group. Indeed, group 4 metal complexes containing the (6(–2-(diethylboryl)phen- yl)pyrid-2-yl-amido motif have been demonstrated to be useful pre-catalysts, which upon activation with methylalumoxane (MAO) form highly active catalysts for the homopolymerization of ethylene and styrene,[12] and the copolymerization of ethyl- ene with norborn-2-ene and which, in selected cases, allow for Eur. J. Inorg. Chem. 2020, 3673–3681 www.eurjic.org © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3677 Figure 9. Single crystal X-ray structure of 12. Relevant bond lengths [pm] and bond angles [°]: Cr1–I1 260.62, Cr1–C1 205.5, Cr1–N3 166.2, Cr1–N4 166.6; I1–Cr1–C1 102.69(7), I1–Cr1–N3 108.87(7), I1–Cr1–N4 111.59(7), C1–Cr1–N3 107.1(1), C1–Cr1–N4 105.9(1), N3–Cr1–N4 119.2(1). Thermal ellipsoids are set at a 50 % probability level. the formation of poly(ethylene)-co-poly(norborn-2-ene) with both vinyl insertion and ring opening metathesis polymeriza- tion (ROMP) derived structures within the same polymer chain.[12,13] Such polymers can, at least in principle, be further used for the synthesis of functional polyolefins.[14] This (reversi- ble) switch between VIP and ROMP is induced by an α-H+ elimi- nation triggered by the 2-pyridylamido group and is controlled by the dialkylboryl group and concurring aminoborane forma- tion.[13a,13d,15] In view of these most favorable properties, we were interested in the question, whether the corresponding chromium imido and amido complexes would be accessible, too. Chromium N-6(–2-(diethylboryl)phenyl)pyrid-2-yl-imido-N′- tert-butylimido-N′′-tert-butylamido chloride (3) was prepared in 83 % isolated yield via reaction of bis(N-tert-butylimido) chro- mium(VI) dichloride (1) with 6(–2-(diethylboryl)phenyl)pyridine- 2-amine (2, Scheme 4). Subsequent reaction with neophylmag- nesium chloride yielded N-6(–2-(diethylboryl)phenyl)pyrid-2-yl- imido-N′-tert-butylimido-N′-tert-butylamido neophyl chromium (4) in 63 % isolated yield. However, all attempts to prepare the corresponding chromium(VI) imido bisamido alkylidene com- plexes failed. Scheme 4. Synthesis of chromium(VI) bisimido-amido complexes. Full Paper doi.org/10.1002/ejic.202000550 EurJIC European Journal of Inorganic Chemistry Conclusion We have demonstrated the synthesis of the first examples of high-oxidation state chromium(VI) NHC complexes bearing both mono- and bidentate NHCs in substantial variability in both the imido and NHC ligands. Our efforts towards the syn- thesis of chromium(VI)-alkylidene NHC complexes were not suc- cessful so far as the reaction yields the chromium-silver alkyl- idene adduct, which further transform to a chromium(V) bisim- ido iodo NHC complex. In addition, we succeeded in synthesiz- ing chromium(VI) complexes bearing two different imido li- gands ((6(–2-(diethylboryl)phenyl)pyrid-2-yl-amido and tert- butyl imido) that were so far inaccessible. However, the subse- quent α-hydrogen elimination from those complexes to yield the alkylidene complexes did not occur. Further studies on the synthesis of chromium(VI) alkylidene NHC complexes are still underway in our laboratories. Experimental Section General Unless otherwise noted, all reactions were performed under the exclusion of air and moisture by standard Schlenk techniques. Reac- tions involving metal complexes were performed in a nitrogen-filled glove box (MBraun Labmaster 130). Glassware was stored at 120 °C overnight and cooled in an evacuated antechamber. 1H and 13C NMR spectra were recorded on a Bruker Avance III 400 spectrometer at 400 and 100 MHz, respectively. Chemical shifts are reported in ppm from tetramethylsilane with the solvent resonance resulting from residual solvent protons (CDCl3: 7.26 ppm, CD2Cl2 5.32 ppm) as reference. Data are reported as follows: chemical shift, multiplic- ity (s = singlet, d = doublet, t = triplet, q = quartet, quint = quintet, sept = septet, br = broad, m = multiplet), coupling constants (Hz) and integration. High-resolution mass spectra were recorded at the Institute of Organic Chemistry, University of Stuttgart, Germany. CH2Cl2, diethyl ether, toluene and pentane were dried by using an MBraun SPS-800 solvent purification system with alumina drying columns and stored over 4 Å Linde type molecular sieves. THF, benzene and DME were distilled from Na prior to use and stored over 4 Å Linde type molecular sieves and Selexsorb®. Deuterated solvents were filtered through activated alumina and stored over 4 Å Linde type molecular sieves inside the glove box. 1,3-Dimesityl- imidazolium tetrafluoroborate,[16] 1,3-diisopropylimidazolium chlor- ide,[17] 1-mesityl-3-(2-hydroxyphenyl)-4,5-dihydroimidazolium tetra- fluoroborate,[18] 1-(tert-butyl)-3-(2-hydroxyphenyl)-4,5-dihydroimid- azolium tetrafluoroborate,[4b] 1-([1,1′-biphenyl-2-yl])-3-(2-hydroxy- phenyl)-4,5-dihydroimidazolium chloride,[4b] the silver iodide ad- duct of 1,3-dimethylimidazol-2-ylidene,[19] 1,3-dimesitylimidazol-2- ylidene,[20] 1,3-diisopropylimidazol-2-ylidene[17] were synthesized according to published procedures. 1,3-Diisopropylimidazolium chloride was additionally washed with boiling acetone, dried by azeotropic distillation with toluene and then dried in vacuo over- night before being transferred into the glove box. The chromium precursors (N-tBu)2CrCl2),[21] Cr(N-2,6-diisopropylphenyl)2Cl2,[7a,7d] Cr(NAd)2Cl2,[7c] Cr(N-2,6-diisopropylphenyl)2(CH2CMe3)2 [10] were synthesized using a slightly modified literature procedure. Deposition Numbers 1996759 (for 1), 1991460 (for 2), 1996758 (for 3), 1996756 (for 4), 1996757 (for 7), 2007984 (for 9), 1996760 (for 10), 2007983 (for 11), and 1996761 (for 12) contain the supplemen- tary crystallographic data for this paper. These data are provided Eur. J. Inorg. Chem. 2020, 3673–3681 www.eurjic.org © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3678 free of charge by the joint Cambridge Crystallographic Data Centre and Fachinformationszentrum Karlsruhe Access Structures service www.ccdc.cam.ac.uk/structures. CrCl2(IMe)(N-tBu)2 (1): (N-tBu)2CrCl2 (50 mg, 0.189 mmol, 1.0 equiv.) was dissolved in 3 mL of toluene. The dark red solution was cooled to –34 °C, then AgI·1,3-dimethylimidazol-2-ylidene (63 mg, 0.189 mmol, 1.0 equiv.) was added. The resulting reaction mixture was warmed to room temperature, stirred for 2 h and then filtered through Celite. The filtrate was evaporated to dryness under reduced pressure and the residue was washed with pentane (2 × 5 mL) and diethyl ether (1 × 5 mL). A red solid was obtained, which was dried in vacuo. Yield: 37 mg (54 %); 1H NMR (400 MHz, CDCl3) δ = 7.00 (m, 1H, NHC backbone), 6.87 (s, 1H, NHC backbone), 3.95 (s, 3H, NCH3), 3.89 (s, 3H, NCH3), 1.60 (s, 18H, 2N-tBu). 13C NMR (101 MHz, CD2Cl2) δ = 183.60 (NCN-NHC), 123.13,122.93 (CH=CH- NHC), 79.94 (NCMe3), 30.60 (NMe), 29.67 (CMe3). Elemental analysis calcd. for C13H26Cl2CrN4: C 43.22, H 7.25, N 15.51. Found: C 42.82, H 7.39, N 15.12. Orange crystals suitable for X-ray analysis were ob- tained by layering a saturated solution of complex 1 in CH2Cl2 with diethyl ether and storage at –34 °C for few days. CrCl2(IMeCl2)(N-tBu)2 (2): (N-tBu)2CrCl2 (50 mg, 0.189 mmol, 1.0 equiv.) was dissolved in 3 mL of toluene. The dark red solution was cooled to –34 °C; then, AgI·4,5-dichloro-1,3-dimethylimidazol- 2-ylidene (75.5 mg, 0.189 mmol, 1.0 equiv.) was added. The reaction mixture was warmed to room temperature, stirred for 2 h; then, filtered through Celite. The filtrate was concentrated under reduced pressure and the residue was washed with pentane (2 × 5 mL) and diethyl ether (1 × 5 mL) yielding a red solid, which was dried in vacuo. Yield: 48 mg (59 %); 1H NMR (400 MHz, C6D6) δ = 3.56 (s, 6H, 2NMe2), 1.52 (s, 18H, 2NtBu); 13C NMR (101 MHz, C6D6) δ = 191.98 (NCN-NHC), 116.16 (CH=CH-NHC), 79.10 (NCMe3), 36.22 (NMe), 29.57 (CMe3). Elemental analysis calcd. for C13H24Cl4CrN4: C 36.30, H 5.62, N 13.02. Found: C 36.28, H 5.65, N 12.93. Dark red colored crystals were obtained from a mixture of CH2Cl2 and pent- ane at –34 °C for few days. CrCl2(IPr)(N-tBu)2 (3): (NtBu)2CrCl2 (50 mg, 0.189 mmol, 1.0 equiv.) was dissolved in 3 mL of toluene giving a dark red solution and the solution was cooled to –34 °C. To the chilled solution, 1,3-diisopro- pylimidazol-2-ylidene (28.8 mg, 0.189 mmol, 1.0 equiv.) was added, the reaction mixture was warmed to room temperature, stirred for 2 h and then filtered through Celite. The filtrate was evaporated to dryness under reduced pressure and the residue was washed with pentane (2 × 5 mL) and diethyl ether (1 × 5 mL). A pale red solid was obtained, which was dried in vacuo. Yield: 53 mg (67 %); 1H NMR (400 MHz, CDCl3) δ = 6.46 (s, 2H, NHC backbone), 5.61–5.51 (sept, 2H, NCHMe2), 1.65 (s, 18H, 2N-tBu), 1.40 (d, J = 8 Hz, 12H, NCHMe2). 13C NMR (101 MHz, C6D6) δ = 190.05 (NCN-NHC), 115.63 (CH=CH-NHC), 79.58 (NCMe3), 30.48 (NMe), 29.55 (NCMe3), 22.94 (NCMe2). Elemental analysis calcd. for C17H34Cl2CrN4: C 48.92, H 8.21, N 13.42. Found: C 48.86, H 8.17, N 13.31. Orange crystals suitable for X-ray analysis were obtained by the layering the saturated solu- tion of complex 3 in CH2Cl2 with pentane and storage at –34 °C for few days. CrCl2(IMes)(N-tBu)2 (4): Cr(N-tBu)2Cl2 (50 mg, 0.189 mmol, 1.0 equiv.) was dissolved in 3 mL of toluene and the solution was cooled to –34 °C. Next, IMes (57.5 mg, 0.189 mmol, 1.0 equiv.) was dissolved in toluene and the solution was cooled to –34 °C. Both solutions were combined, the resulting mixture was stirred at room temperature for 2 h and then filtered through Celite. The filtrate was concentrated under reduced pressure and repeatedly washed with pentane (2 × 5 mL) and diethyl ether (2 × 5 mL) to obtain an orange colored solid. Yield: 77 mg (72 %); 1H NMR (400 MHz, CDCl3) https://www.ccdc.cam.ac.uk/services/structures?id=doi:10.1002/ejic.202000550 https://www.ccdc.cam.ac.uk/services/structures?id=doi:10.1002/ejic.202000550 https://www.ccdc.cam.ac.uk/services/structures?id=doi:10.1002/ejic.202000550 www.ccdc.cam.ac.uk/structures Full Paper doi.org/10.1002/ejic.202000550 EurJIC European Journal of Inorganic Chemistry δ = 7.26 (s, 2H, CH=CH-NHC), 7.17 (m, 4H, m-Ar-Mes), 2.58 (s, 12H, Me-Mes), 2.52 (s, 6H, Me-Mes), 1.36 (s, 18H, 2N-tBu); 13C NMR (101 MHz, CDCl3) δ = 191.49 (NCN-NHC), 138.50 (Ar), 136.94 (Ar), 136.66 (Ar), 128.84 (Ar), 122.22 (CH=CH-NHC), 79.22 (NCMe3), 29.07 (CMe3), 20.95 (p-Me-Mes), 19.33 (o-Me-Mes). Elemental analysis calcd. for C29H42Cl2CrN4: C 61.15, H 7.43, N 9.84. Found: C 61.12, H 7.44, N 9.80. Red colored crystals were obtained from a mixture of CH2Cl2, pentane and few drops of toluene at –34 °C for few days. CrCl2(IDipp)(N-tBu)2 (5): Cr(NtBu)2Cl2 (50 mg, 0.189 mmol, 1.0 equiv.) was dissolved in 3 mL of toluene and the solution was cooled to –34 °C. Next, Dipp (73.5 mg, 0.189 mmol, 1.0 equiv.) was dissolved in toluene and cooled to –34 °C. Both solutions were com- bined; the resulting mixture was stirred at room temperature for 2 h and filtered through Celite. The filtrate was then concentrated under reduced pressure and repeatedly washed with pentane (2 × 5 mL) and diethyl ether (2 × 5 mL) to obtain a pink colored solid. Yield: 58 mg (47 %); 1H NMR (400 MHz, CDCl3) δ = 7.40–7.36 (m, 2H, Ar), 7.27 (s, 2H, CH=CH-NHC), 7.25 (m, 2H, Ar), 7.07 (s, 2H, Ar), 3.35–3.25 (sept, 4H, 4CHMe2), 1.37–1.36 (d, J = 6.56 Hz, 12H, CHMe2), 1.09–1.07 (d, J = 6.92Hz, 12H, CHMe2), 1.02 (s, 18H, 2NtBu); 13C NMR(101 MHz, CDCl3) δ = 192.29 (NCN-NHC), 147.56 (Ar), 136.57 (Ar), 129.78 (Ar), 123.69 (Ar), 123.24 (Ar), 78.59 (NCMe3), 29.31 (CMe3), 28.62(CHMe2), 26.55 (CHMe2), 22.82 (CHMe2). Elemental analysis calcd. for C35H54Cl2CrN4: C 64.30, H 8.33, N 8.57. Found: C 64.13, H 8.41, N 8.47. CrCl2(N-2,6-(2-Pr)2C6H3)2(IMes) (6): Cr(N-2,6-diisopropylphen- yl)2Cl2 (50 mg, 0.106 mmol, 1.0 equiv.) was dissolved in 3 mL of toluene and the solution was cooled to –34 °C. IMes (32.2 mg, 0.106 mmol, 1.0 equiv.) was also dissolved in toluene and cooled to –34 °C. Both solutions were combined, the resulting reaction mixture was stirred at room temperature for 2 h and then filtered through Celite. The filtrate was then evaporated to dryness under reduced pressure and washed repeatedly with pentane (2 × 5 mL) and diethyl ether (2 × 5 mL) to obtain a black colored solid. Yield: 36 mg (44 %); 1H NMR (400 MHz, C6D6) δ = 6.87–6.85 (m, 4H, Ar), 6.79–6.75 (m, 2H, Ar), 6.70–6.69 (m, 4H, Ar), 6.16 (s, 2H, CH=CH- NHC), 3.73–3.67 (sept, 4H, 4CHMe2), 2.54 (s, 12H, Me-Mes), 1.96 (s, 6H, Me-Mes), 1.12(d, J = 4Hz, 24H, 4CHMe2). 13C NMR (101 MHz, CD2Cl2) δ = 192.61 (NCN-NHC), 160.70 (Ar), 147.41, 138.16, 137.44, 136.42, 134.35, 130.23, 129.78, 129.01, 123.18, 122.83, 28.05, 24.94, 20.66, 18.92, 18.89. Elemental analysis calcd. for C45H58Cl2CrN4: C 69.48; H 7.52, N 7.20. Found: C 69.50; H 7.67; N 7.16. CrCl2(N-adamantyl)2(IMes) (7): Cr(NAd)2Cl2 (50 mg, 0.119 mmol, 1.0 equiv.) was dissolved in 3 mL of toluene and the solution was cooled to –34 °C. IMes (36.1 mg, 0.119 mmol, 1.0 equiv.) was also dissolved in toluene and the solution was cooled to –34 °C. The IMes solution was then added dropwise to the one of Cr(NAd)2Cl2, the resulting reaction mixture was stirred at room temperature for 3 h and then filtered through Celite. The filtrate was then evapo- rated to dryness under reduced pressure and repeatedly washed with pentane (2 × 5 mL) and diethyl ether (2 × 5 mL) to obtain a black colored solid. Yield: 51 mg (61 %); 1H NMR (400 MHz, CD2Cl2) δ = 7.07 (s, 2H, CH=CH-NHC), 6.95 (m, 4H, Ar), 2.32 (s, 12H, Me-Mes), 2.30 (s, 6H, Me-Mes), 2.20 (m, 4H, Ad), 1.98–1.96 (m, 6H, Ad), 1.72– 1.66 (m, 12H, Ad), 1.55–1.52 (m, 8H, Ad). The recording of 13C NMR spectra was impossible as the complex decomposes immediately in the solution. Despite numerous efforts, inconsistent elemental analysis data were obtained. Nonetheless, the molecular structure was established by single crystal analysis. Red colored crystals were obtained from a mixture of CH2Cl2 and pentane at –34 °C for few days. The crystals quickly decomposed at room temperature. Eur. J. Inorg. Chem. 2020, 3673–3681 www.eurjic.org © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3679 CrCl2(1-Mes-3-(1-(2-O-C6H4))imidazol-2-ylidene)2C6H3)2(IMes) (8): To a chilled (–34 °C) suspension of 1-mesityl-3-(2-hydroxy- phenyl)-4,5-dihydro-1H-imidazolium chloride (L1 , 59.7 mg, 0.188 mmol, 1.0 equiv.) in 1 mL of toluene was added a chilled solution of LiHMDS (63 mg, 0.376 mmol, 2.0 equiv.) in 1 mL of toluene. The resulting mixture was stirred at room temperature for 1 h, then filtered through Celite and cooled to –34 °C. This solution was then added dropwise to a chilled solution of Cr(N-tBu)2Cl2 (50 mg, 0.188 mmol, 1.0 equiv.) in 1 mL of toluene. The resulting mixture was stirred at room temperature for 2 h and then filtered through celite. The filtrate was evaporated to dryness under re- duced pressure and then washed repeatedly washed with pentane (2 × 5 mL) and diethyl ether (2 × 5 mL) to obtain A pink colored solid. Yield: 60 mg (61 %). 1H NMR (400 MHz, CDCl3) δ = 7.14–7.12 (dd, J = 1.4, 8.08Hz, 1H, Ar), 7.09–7.05 (m, 1H, Ar), 6.95–6.94 (m, 2H,Ar), 6.89–6.86 (dd, J = 1.42, 7.76Hz, 1H, Ar), 6.73–6.69 (td, 1H, Ar), 5.30 (0.5 CH2Cl2) 4.28–4.23 (t, J = 9.96Hz, 2H, CH2-NHC), 3.93– 3.88 (t, J = 9.94Hz, 2H, CH2-NHC), 2.39 (s, 6H, 2CH3-Ar), 2.28 (s, 3H,CH3-Ar), 1.12 (s, 18H, 2N-tBu). 13C NMR (101 MHz, CD2Cl2) δ = 202.5 (NCN-NHC), 157.4, 139.0, 136.4, 136.2, 132.3, 130.1, 125.7, 119.4, 116.5, 116.5, 78.8, 67.8, 49.1, 29.8, 25.6, 20.61, 20.59, 18.24, 18.22. All bidentate complexes crystallized with trapped sol- vents, which were difficult to remove even under high vacuum (0.001 mbar). 1H NMR indicated the trapping of substoichiometric quantities (0.5 mol) of CH2Cl2 along with 8. Elemental analysis calcd. for C26H37ClCrN4O·0.5CH2Cl2: C 57.71, H 6.95, N 10.16. Found: C 57.81, H 6.931, N 10.25. CrCl2(1-tBu-3-(1-(2-O-C6H4))imidazol-2-ylidene)2C6H3)2(IMes) (9): To a chilled (–34 °C) suspension of 1-(tert-butyl)-3-(2-hydroxy- phenyl)-4,5-dihydro-1H-imidazolium chloride (L2 , 47.9 mg, 0.188 mmol, 1.0 equiv.) in 1 mL of toluene was added a chilled solution of LiHMDS (63 mg, 0.376 mmol, 2.0 equiv.) in 1 mL of toluene. The resulting mixture was stirred at room temperature for 1 h, then filtered through celite and stored in the freezer at –34 °C. The free carbene was then added dropwise to the chilled solution of Cr(N-tBu)2Cl2 (50 mg, 0.188 mmol, 1.0 equiv.) in 1 mL of toluene, the resulting mixture was stirred at room temperature for 2 h and then filtered through Celite. The filtrate was evaporated to dryness under reduced pressure and washed repeatedly with pentane (2 × 5 mL) and diethyl ether (2 × 5 mL) to obtain a pale red colored solid. Yield: 62 mg (71 %). 1H NMR (400 MHz, CDCl2) δ = 6.98–6.96 (t, J = 4, 8Hz, 1H, Ar), 6.84 (t, J = 8Hz, 1H, Ar), 6.79 (t, J = 8Hz, 1H, Ar), 6.70–6.66 (t, J = 8Hz, 1H, Ar), 4.00–3.91 (m, 4H, CH2-CH2-NHC), 1.59 (s, 9H, NCMe3), 1.41 (s, 18H, N-tBu). The molecule crystallizes from diethyl ether with one molecule of diethyl ether trapped that cannot be removed even under high vacuum. Elemental analysis calcd. for C21H35ClCrN4O. C4H10O: C 57.62, H 8.70, N 10.75. Found: C 57.22, H 8.635, N 10.40. The molecular structure was confirmed by single-crystal X-ray analysis. Dark red colored crystals suitable for single-crystal X-ray analysis were obtained from a solution of CH2Cl2 and pentane with a few drops of diethyl ether at –34 °C. CrCl2(1–2-phenyl-C6H4-3-(1-(2-O-C6H4))imidazol-2-ylid- ene)2C6H3)2(IMes) (10): To a chilled (–34 °C) suspension of 1-(2- phenyl-C6H4)-3-(2-hydroxyphenyl)-4,5-dihydro-1H-imidazolium chloride (L3, 66 mg, 0.188 mmol, 1.0 equiv.) in 1 mL of toluene was added to a chilled solution of LiHMDS (63 mg, 0.376 mmol, 2.0 equiv.) in 1 mL of toluene at –34 °C. The resulting mixture was stirred for 1 h at room temperature, then filtered through Celite and stored at –34 °C. The free carbene was then added dropwise to a chilled solution of Cr(N-tBu)2Cl2 (50 mg, 0.188 mmol, 1.0 equiv.) in 2 mL of toluene and the resulting mixture was stirred at room tem- perature for 2 h, then filtered through celite. The filtrate was evapo- rated to dryness under reduced pressure and washed repeatedly Full Paper doi.org/10.1002/ejic.202000550 EurJIC European Journal of Inorganic Chemistry washed with pentane (2 × 5 mL) and diethyl ether (2 × 5 mL) to obtain a dark red colored solid. Yield: 57 mg (54 %). One molecule of toluene co-crystallized along with the complex. 1H NMR (400 MHz, CD2Cl2) δ = 7.71–7.69 (m, 2H, Ar), 7.53–7.45 (6H, Ar), 7.40–7.36 (m, 1H, Ar), 7.24–7.22 (m, 2H, Ar), 7.18–7.16 (m, 3H, Ar), 7.04–7.02 (m, 1H, Ar), 6.95–6.93 (dd, J = 8Hz, 1H, Ar), 6.87–6.84 (dd, J = 4, 8Hz, 1H, Ar), 6.71–6.66 (m, 1H, Ar), 4.24–4.16 (m, 1H, CH2-CH2- NHC), 3.74–3.67 (m, 1H, CH2-CH2-NHC), 3.63–3.56 (m, 1H, CH2-CH2- NHC), 3.46–3.38 (m, 1H, CH2-CH2-NHC), 2.34 (s, 3H, Ar-CH3 from tolu- ene), 1.31 (s, 9H, N-tBu), 1.04 (s, 9H, N-tBu); 13C NMR (101 MHz, CD2Cl2) δ = 201.9 (NCN-NHC), 157.3, 139.7, 139.1, 138.0, 137.7, 131.7, 131.3, 130.3, 129.5, 129.0, 128.92, 128.90, 128.2, 125.8, 125.2, 119.3, 116.5, 116.4, 79.7(NCMe3), 78.4(NCMe3), 49.3(CH2-CH2-NHC), 30.1(NCMe3), 29.4 (NCMe3). One molecule of CH2Cl2 is trapped in the crystal as confirmed by single-crystal X-ray analysis, which can- not be removed even under high vacuum (0.001 mbar). Red colored crystals were obtained by the layering of pentane to a saturated solution of complex 10 in CH2Cl2 and storage at –34 °C. Elemental analysis calcd. for C29H35ClCrN4O·[] CH2Cl2: C 57.38, H 5.94, N 8.92. Found: C 57.36, 5.88, 8.81. Complex 11: Cr(N-2,6-diisopropylphenyl)2(CH2CMe3)2 (50 mg, 0.0918 mmol,1 equiv.) was dissolved in 1 mL of THF and cooled to –34 °C. Next, solid AgI·1,3-dimethylimidazol-2-ylidene (30.3 mg, 0.0918 mmol, 1 equiv.) was added. The reaction mixture was stirred at room temperature for 33 h and then evaporated to dryness un- der reduced pressure. The residue was then dissolved in 5 mL of pentane and filtered through a short pad of Celite. All volatiles were removed in vacuo. Analytically pure 11 was obtained via crystalliza- tion from pentane at –34 °C. Yield: 17 mg (33 %). 1H NMR (400 MHz, [D8]THF) δ = 13.01 (s,1H, Cr=CH), 7.11 (s, 2H, NHC backbone), 6.87 (m, 3H, Ar), 6.81 (m, 3H, Ar), 3.99–3.93 (m, 2H, CH(CH3)2), 3.72 (s, 6H, 2N(CH3)), 3.70–3.66 (m, 2H, CH(CH3)2), 1.16 (br, 9H, tBu), 1.15–1.11 (m, 12H, 2CHMe2), 0.95–0.93 (d, J = 8Hz, 6H, CHMe2), 0.70- 0.69 (d, J = 4Hz, 6H, CHMe2). 13C NMR analysis and elemental analysis were impossible due to the extreme sensitivity of the complex in solution and also in the solid state. Crystals suitable for single-crystal X-ray analysis were grown from a mixture of CH2Cl2 and pentane at –34 °C over a course of few days. Chromium N-6(–2-(diethylboryl)phenyl)pyrid-2-yl-imido-N′-tert- butylimido-N′′-tert-butylamido chloride (13): To a solution of 6(– 2-(diethylboryl)phenyl)pyridine-2-amine (0.50 g, 2.10 mmol) in di- ethyl ether (12 mL) was added n-butyllithium (1.31 mL, 1.6 m in hexane, 2.10 mmol, 1.0 equiv.) at –37 °C. The solution was stirred at room temperature for three hours, then the precipitated solid was filtered off, washed with cold pentane and dried in vacuo. The lithium salt (0.52 g, 1.80 mmol, 1 equiv.) was dissolved in diethyl ether (10 mL) and added at –34 °C to a solution of bis(N-tert-butyl- imido) chromium(VI) dichloride (1, 0.42 g, 1.80 mmol, 1 equiv.) in diethyl ether (10 mL). After stirring for one hour, triethylamine (0.25 mL, 1.80 mmol, 1 equiv.) was added. The purple solution was stirred overnight at room temperature. The solvent was removed, and the crude product was extracted with pentane, LiCl was filtered off. Pure product was obtained by recrystallization from cold pent- ane. Yield: 0.77 g (1.49 mmol, 83 %). 1H-NMR (C6D6): δ = 11.18 (s, 1H, NH) 7.77 (m, 1H, Ar-H), 7.59 (m, 2H, Ar-H), 7.40 (m, 1H, Ar-H), 7.22 (m, 1H, Ar-H), 7.05 (t, 1H, J = 8 Hz, Ar-H), 6.84 (m, 1H, Ar-H), 1.42–1.38 (m, 2H, CH2), 1.21 (s, 18H, CCH3), 1.18–1.12 (m, 2H, CH2), 0.71 (t, 6H, CH3). 13C-NMR (C6D6): δ = 163.6, 155.9, 140.2, 137.3, 130.4; 129.4, 125.7; 121.7, 116.0, 111.2, 80.4, 33.7, 31.1, 30.0, 15.6, 10.4, 1.4. Elemental analysis: calculated for C23H36BClCrN4: C 59.18, H 7.77, N 12.00, found: C 59.01, H 8.03, N 11.88. N-6(–2-(Diethylboryl)phenyl)pyrid-2-yl-imido-N′-tert-butylim- ido-N′′-tert-butylamido neophyl chromium (14): 2-Methyl-2- Eur. J. Inorg. Chem. 2020, 3673–3681 www.eurjic.org © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim3680 phenylpropylmagnesium chloride (0.57 mL, 0.5 m, 0.3 mmol, 1 equiv.) was added dropwise to a solution of chromium N-6(–2- (diethylboryl)phenyl)pyrid-2-yl-imido-N′-tert-butylimido-N′′-tert-bu- tylamido chloride (0.15 g, 0.3 mmol, 1 equiv.) in diethyl ether (10 mL) at –34 °C. The orange solution was stirred at room tempera- ture overnight. Then the solvent was removed under reduced pres- sure and the crude product was extracted with pentane. At –34 °C, 3,5-lutidine (0.05 mg, 0.4 mmol, 1.5 equiv.) was added and the solu- tion was stirred at room temperature overnight. The precipitated solid was filtered off and the product recrystallized from pentane. Yield: 0.11 g (0.02 mmol, 63.1 %). 1H-NMR (C6D6): δ = 10.05 (s, 1H, NH), 7.84–7.80 (m, 1H, Ar-H), 7.64–7.62 (d, 2H, Ar-H), 7.45–7.39 (m, 3H, Ar-H), 7.26–7.20 (m, 3H, Ar-H), 7.08–7.04 (m, 3H, J = 7.9 Hz, Ar- H), 6.86 (dd, 1H, J = 7.5 Hz, Ar-H), 3.14 (s, 2H, CrCH2), 1.54 (s, 18H, CCH3), 1.40–1.25 (m, 4H, BCH2), 1.21 (t, 6H, CH3). 13C-NMR (C6D6): δ = 162.5, 156.1, 152.1, 139.8, 139.5, 138.0, 132.0, 130.2, 130.1, 126.1, 125.7, 125.5, 121.3, 112.8, 108.1, 77.9, 74.7, 41.2, 32.9, 31.6, 15.6, 14.2, 10.5, 1.4. Elemental analysis calculated for C33H49BCrN4: C 70.20, H 8.75, N 9.92, found: C 69.85, H 9.04, N 9.64. Acknowledgments Financial support provided by the Deutsche Forschungsge- meinschaft (DFG, German Research Foundation, project number 358283783 - CRC 1333) is gratefully acknowledged. Open access funding enabled and organized by Projekt DEAL. Keywords: Chromium · Alkylidenes · Carbene ligands · Imido complexes [1] a) T. Agapie, Coord. Chem. Rev. 2011, 255, 861–880; b) K. A. Alferov, G. 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