The Butterfly Dimer [(tBu3SiO)Cr]2(μ-OSitBu3)2 and Its Oxidative Cleavage to (tBu3SiO)2Cr(N−NCPh2)2 and (tBu3SiO)2CrN(2,6-Ph2-C6H3)

Sydora, Orson L.; Kuiper, David S.; Wolczanski, Peter T.; Lobkovsky, Emil; Dinescu, and Adriana; Cundari, Thomas R.

doi:10.1021/ic051481w

Cited by 41 publications

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“…The structure of Cr(OSi t Bu 3 ) 2 (N-2,6-diphenylphenyl) similarly demonstrated trigonal geometry at the Cr center and a short Cr−N bond distance of 1.649(2) Å. 32 Solution magnetic moments of compounds 2 and 3, obtained using the Evans method, 34 were found to be μ obs = 2.5 ± 0.3 μ B and μ obs = 3.3 ± 0.4 μ B , respectively, in comparison with 2.7−2.8 μ B values observed for Cr(OSi t Bu 3 ) 2 (NR). 32 These values are indicative of triplet ground states, which are consistent with DFT calculations (see below).…”

Section: ■ Results and Discussionmentioning

confidence: 93%

“…The solution magnetic moment of compound 1 was found to be μ obs = 2.5 ± 0.3 μ B (2.8 μ B in Cr 2 (OSi t Bu 3 ) 4 ), indicative of some antiferromagnetic interaction between the Cr(II) centers. As the electronic structure of the related compound Cr 2 (OSi t Bu 3 ) 4 has been studied in great detail, 32 it was not pursued in this work.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…This finding is consistent with the experimental and calculated spin states in the related CrNR′ species supported by siloxide ligands. 32 On the basis of the large thermodynamic preference for the triplet state that matches the experimental magnetic data, we focused on the triplet for the structural and electronic analysis. A trigonalplanar arrangement about Cr was observed in the optimized structures of 2 and 3 ( Figure 4).…”

Section: ■ Results and Discussionmentioning

confidence: 99%

“…For [Cr(OSi t Bu 3 ) 2 ], it was also reported that bulky azides form mono(imido) species while nonbulky azides form bis(imido) complexes at room temperature. 32 Cr(VI) bis(imido) complexes 5 and 6 are diamagnetic species that were characterized by 1 H and 13 C NMR (and 19 F for 6) spectroscopy, IR spectroscopy, X-ray crystallography, and elemental analysis. NMR spectroscopy is consistent with an effective C 2v symmetry in solution, displaying a single type of t Bu group and a phenyl group for both [OR] ligands.…”

Section: Organometallicsmentioning

confidence: 99%

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Catalytic Formation of Asymmetric Carbodiimides at Mononuclear Chromium(II/IV) Bis(alkoxide) Complexes

et al. 2015

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Herein we report the synthesis of Cr imido complexes in bis(alkoxide) ligand environments and their nitrene transfer reactivity with isocyanides. The reaction of Cr 2 (OR) 4 (OR = OC t Bu 2 Ph) with bulky aryl or alkyl azide results in the formation of the trigonal-planar Cr(IV) mono(imido) complexes Cr(OR) 2 (NR 1 ), whereas less bulky aryl azides form the Cr(VI) bis(imido) complexes Cr(OR) 2 (NR 1 ) 2 . Cr(IV) mono-(imido) complexes undergo facile reaction with 1 equiv of 2,6-dimethylphenyl isocyanide (CNR 2 ) to form the corresponding carbodiimides R 1 NCNR 2 . In contrast, no reaction of Cr(OR) 2 (NR 1 ) 2 complexes with CNR 2 is observed. The reaction of Cr(OR) 2 (NR 1 ) with excess isocyanide leads to the isolation of the Cr(II) complex Cr(OR) 2 (CNR 2 ) 4 , along with the observation of the anticipated carbodimide product. Cr(OR) 2 (CNR 2 ) 4 , which can also be obtained by treating Cr 2 (OR) 4 with 4 equiv of isocyanide, reacts with azides N 3 R 1 (R 1 = adamantyl, mesityl) to produce the respective carbodiimides. Catalytic formation of carbodiimides R 1 NCNR 2 is observed from the mixtures of azides R 1 N 3 (R 1 = mesityl, 2,6-diethylphenyl, 2-isopropylphenyl, adamantyl) and several different aryl isocyanides CNR 2 using 2.5 mol % of Cr 2 (OR) 4 . ■ INTRODUCTIONCarbodiimides are widely used organic compounds that hydrolyze readily and thus serve as excellent dehydration agents. Large-scale applications of carbodiimides include as stabilizers and antihydrolysis agents in polymers production, as coupling agents in peptide synthesis, and in various organic transformations. 1−6 Carbodiimides are synthesized by a plethora of transition-metal-catalyzed routes, 7 including oxidative coupling of amines with isocyanides 8 and coupling of two isocyanates R′NCO with loss of 1 equiv of CO 2 , 9 as well as by transition-metal-free stoichiometric routes such as the reaction of isocyanates with phosphinimines (aza-Wittig) 10 and by tinmediated addition of silylamines to isocyanates. 11 Coupling of an [NR] (nitrene) group (originating in organoazides) with isocyanide is among the most attractive methods to synthesize carbodiimides, as (1) a large variety of azides are easily accessible using simple synthetic procedures, (2) both symmetrical and asymmetrical carbodiimides can be synthesized, and (3) the only side product in this reaction is the environmentally friendly dinitrogen. Therefore, numerous efforts have been made to make carbodiimides via coupling of isocyanides and organoazides. Most of these coupling reactions take place at low-coordinate late-transition-metal catalysts stabilized by nitrogen, carbon, or phosphorus ligands, where the MNR functionality is suggested to serve as the reactive intermediate. 12−21 The combination of the soft, electron-rich late transition metals and the hard π-donating imido functionality make the imido functionality reactive and thus susceptible to transfer. In contrast, imido compounds of earlier transition metals are significantly more stable, because the higher oxidation stat...

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Section: ■ Results and Discussionmentioning

confidence: 93%

Section: ■ Results and Discussionmentioning

confidence: 99%

Section: ■ Results and Discussionmentioning

confidence: 99%

Section: Organometallicsmentioning

confidence: 99%

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Catalytic Formation of Asymmetric Carbodiimides at Mononuclear Chromium(II/IV) Bis(alkoxide) Complexes

et al. 2015

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“…The terminal Cr(2A) À O(1A) and Cr(1A) À O(4A) bonds have a length of 1.934(4) and 1.984(4) , respectively, similar to other chromium siloxide complexes. [21] The bridging Cr-O(2A) and Cr-O(3A) distances are slightly longer than the terminal CrÀO bonds. Besides the terminal and bridging chromium-oxygen bonds in 1, two À OSi(OtBu) 3 ligands coordinate through a OtBu fragment to satisfy the distorted square-planar geometry; these distances are longer (2.109(4) and 2.167(4) , respectively).…”

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confidence: 99%

Polymerization of Ethylene by Silica‐Supported Dinuclear Cr^III Sites through an Initiation Step Involving CH Bond Activation

et al. 2014

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The insertion of an olefin into a preformed metal-carbon bond is a common mechanism for transition-metal-catalyzed olefin polymerization. However, in one important industrial catalyst, the Phillips catalyst, a metal-carbon bond is not present in the precatalyst. The Phillips catalyst, CrO3 dispersed on silica, polymerizes ethylene without an activator. Despite 60 years of intensive research, the active sites and the way the first CrC bond is formed remain unknown. We synthesized well-defined dinuclear Cr(II) and Cr(III) sites on silica. Whereas the Cr(II) material was a poor polymerization catalyst, the Cr(III) material was active. Poisoning studies showed that about 65 % of the Cr(III) sites were active, a far higher proportion than typically observed for the Phillips catalyst. Examination of the spent catalyst and isotope labeling experiments showed the formation of a Si-(μ-OH)-Cr(III) species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at Cr(III) O bonds.

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Polymerization of Ethylene by Silica‐Supported Dinuclear Cr^III Sites through an Initiation Step Involving CH Bond Activation

et al. 2014

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The insertion of an olefin into a preformed metalcarbon bond is a common mechanism for transition-metalcatalyzed olefin polymerization. However, in one important industrial catalyst, the Phillips catalyst, a metal-carbon bond is not present in the precatalyst. The Phillips catalyst, CrO 3 dispersed on silica, polymerizes ethylene without an activator. Despite 60 years of intensive research, the active sites and the way the first CrÀC bond is formed remain unknown. We synthesized well-defined dinuclear Cr II and Cr III sites on silica. Whereas the Cr II material was a poor polymerization catalyst, the Cr III material was active. Poisoning studies showed that about 65 % of the Cr III sites were active, a far higher proportion than typically observed for the Phillips catalyst. Examination of the spent catalyst and isotope labeling experiments showed the formation of a Si-(m-OH)-Cr III species, consistent with an initiation mechanism involving the heterolytic activation of ethylene at Cr III ÀO bonds.

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The Butterfly Dimer [(^tBu₃SiO)Cr]₂(μ-OSi^tBu₃)₂ and Its Oxidative Cleavage to (^tBu₃SiO)₂Cr(N−NCPh₂)₂ and (^tBu₃SiO)₂CrN(2,6-Ph₂-C₆H₃)

Abstract: Treatment of CrCl 2 (THF) 2 with NaOSi t Bu 3 afforded the butterfly dimer [( t Bu 3 SiO)Cr] 2 (µ-OSi t Bu 3 ) 2 (1 2 ), whose d(CrCr) of 2.658(31) Å and magnetism were indicative of strong antiferromagnetic coupling. A Boltzmann distribution of low-energy 1 A

Cited by 41 publications

References 75 publications

Catalytic Formation of Asymmetric Carbodiimides at Mononuclear Chromium(II/IV) Bis(alkoxide) Complexes

Catalytic Formation of Asymmetric Carbodiimides at Mononuclear Chromium(II/IV) Bis(alkoxide) Complexes

Polymerization of Ethylene by Silica‐Supported Dinuclear Cr^III Sites through an Initiation Step Involving CH Bond Activation

Polymerization of Ethylene by Silica‐Supported Dinuclear Cr^III Sites through an Initiation Step Involving CH Bond Activation

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The Butterfly Dimer [(tBu3SiO)Cr]2(μ-OSitBu3)2 and Its Oxidative Cleavage to (tBu3SiO)2Cr(N−NCPh2)2 and (tBu3SiO)2CrN(2,6-Ph2-C6H3)

Abstract: Treatment of CrCl 2 (THF) 2 with NaOSi t Bu 3 afforded the butterfly dimer [( t Bu 3 SiO)Cr] 2 (µ-OSi t Bu 3 ) 2 (1 2 ), whose d(CrCr) of 2.658(31) Å and magnetism were indicative of strong antiferromagnetic coupling. A Boltzmann distribution of low-energy 1 A

Cited by 41 publications

References 75 publications

Catalytic Formation of Asymmetric Carbodiimides at Mononuclear Chromium(II/IV) Bis(alkoxide) Complexes

Catalytic Formation of Asymmetric Carbodiimides at Mononuclear Chromium(II/IV) Bis(alkoxide) Complexes

Polymerization of Ethylene by Silica‐Supported Dinuclear CrIII Sites through an Initiation Step Involving CH Bond Activation

Polymerization of Ethylene by Silica‐Supported Dinuclear CrIII Sites through an Initiation Step Involving CH Bond Activation

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The Butterfly Dimer [(^tBu₃SiO)Cr]₂(μ-OSi^tBu₃)₂ and Its Oxidative Cleavage to (^tBu₃SiO)₂Cr(N−NCPh₂)₂ and (^tBu₃SiO)₂CrN(2,6-Ph₂-C₆H₃)

Polymerization of Ethylene by Silica‐Supported Dinuclear Cr^III Sites through an Initiation Step Involving CH Bond Activation

Polymerization of Ethylene by Silica‐Supported Dinuclear Cr^III Sites through an Initiation Step Involving CH Bond Activation