Academic literature on the topic 'Cobalt-NHC complexes'

Deprotonation of 1 with Co{N(SiMe₃)₂}₂ affords aNHC-Co(ii) complex 2, whereas carbene transfer from 3 to Co{N(SiMe₃)₂}₂ enables access to complex 4.

The combination of simple cobalt salts and N-heterocyclic carbene (NHC) ligands has been highly effective in C–H functionalization, hydroarylation and cross-coupling catalysis, though displaying a strong dependence on the identity of the NHC ligand.

AbstractThe presented work describes the synthesis of new six- and seven-membered haloamidinium salts and their reaction with different metals. The isolated metal complexes were tested in a catalytic reaction. Two different synthetic routes were applied to prepare five different salts. Chloroamidinium salts were very water-sensitive in comparison to their corresponding bromoamidinium salts. Hence, the preparation of the less sensitive bromoamidinium salts was higher prioritized. The formed salts were converted with metal sources to N-heterocyclic carbene (NHC) metal complexes through an oxidative insertion into the C–X bond. This type of formation is less examined for the synthesis of extended NHC metal complexes. Pd(PPh3)4 and cobalt powder were applied as metal sources, whereby two palladium complexes were isolated, characterized, and their crystal and molecular structures determined. The palladium complexes were investigated in the Suzuki-Miyaura reaction and showed promising catalytic activity.

The hydrosilylation of alkynes is one of the most attractive and, at the same time, most challenging catalytic transformations, usually demanding the use of noble transition metals. We describe a catalytic system, based on cobalt(0) complex and bulky N-heterocyclic carbene (NHC) ligands, permitting the highly effective hydrosilylation of a broad scope of alkynes and silanes. The application of bulky NHC ligands allowed a decrease in the amount of cobalt necessary for an effective reaction run to 2.5 mol% and provided excellent selectivity towards challenging α-vinylsilanes. The developed method tolerates a number of substituted aryl, alkyl, and silyl acetylenes. Moreover, it is suitable for both tertiary and secondary silanes. Our findings confirm that steric hindrance around the metal center can effectively increase the activity of a catalyst and ensure better selectivity than those of analogous complexes bearing smaller ligands.

Earth-abundant first-row transition metals have seen a renaissance in chemistry in recent years due to their relatively low toxicity and cost in comparison to precious metals. Furthermore open-shell transition metal complexes exhibit useful one-electron redox processes which contrasts to their heavier d block anologues. This thesis aims to synthesize and analyse the structure and reactivity of low-coordiante first-row transition metal complexes of from groups 7-9 with an aim to utilize these species in catalysis. The divalent compound [Co{N(SiMe3)2}2] reacts with the primary phosphines PhPH2 in the presence of an NHC ligand (IMe4) to yield the phosphinidene bridged dimer [(IMe4)2Co(µ-PMes)]2. The complex has interesting magnetic properties due to strong antiferromagnetic coupling between the two cobalt(II) centres. Increasing the steric bulk of the NHC yielded carbene-phosphinidene adducts (NHC·PAr). This transformation was shown to be catalytic. The structure and reactivity of complexes of the type [(NHC)xMn{(N(SiMe3)2}2] were investigated. The complexes exhibit similar structural properties to their iron and cobalt analogues; however their reactivity has been shown to differ. The addition of primary phosphines to complexes of the type [(NHC)xMn{N(SiMe3)2}2] yielded a range of manganese phosphide complexes. [Mn{N(SiMe3)2}2] also reacts with imidazolium salts at elevated temperatures to yield heteroleptic manganese NHC complexes. The reaction of [Mn{N(SiMe3)2}2] with IPr·HCl afforded the abnormal carbene complex [(aIPr)Mn{N(SiMe3)2}µ-Cl]2. A new monoanionic bidentate ligand is reported which has shown to be a useful ligand system to stabilize three-coordiante iron(II) complex. The reaction of [(L)Fe(Br)] with mesitylmagnesium Grignard or n-butyllithium yield the iron hydrocarbyls [(L)Fe(Mes)] and [(L)Fe(nBu)] with the latter being stable to β-hydrogen elimination. Finally [(L)Fe(nBu)] has been utilized as a pre-catalyst in the hydrophosphination of internal alkynes, showing selectivity for the E-isomeric vinylphosphine.

Boronic acid(esters) have been well recognized as an indispensable coupling partner in the Suzuki-Miyaura cross coupling reactions producing a vast spectrum of molecules, applicable in the diverse field ranging from medicinal to materials sciences.[1] Transition metal catalyzed synthesis of boronic esters from diborons with the assistance of bases is a well-established methodology[2]. In this thesis, the cobalt-N-Heterocyclic carbene complexes catalyzed borylation of organic compounds and interaction of diazabutadienes with diboron compounds will be discussed. (i) In the first section, Co(IMes)2Cl2 catalyzed borylation of aryl halides will be discussed. [3a] The robust protocol, operating under mild condition facilitate the borylation of a diverse range of aryl halides with great efficacy, which includes the challenging aryl chlorides. The preliminary mechanistic studies suggest that base-bis(pinacolato)diboron adduct reduces the Co(IMes)2Cl2 complex to generate Co(IMes)2Cl complex, which acts as an active catalytic species. (ii) The second section deals with catalytic synthesis of primary and secondary alkyl boronic esters using alkyl halides. [3b] The in situ generated Co-NHC complex, in assistance with base and diboron compound, produces the corresponding borylated product from alky halides. The reaction proceeds under very mild conditions and covers a wide range of alkyl halides, including chlorides having different functional groups. (iii) In the third section, development in selective hydroboration of vinyl arenes and aliphatic alkenes will be discussed. [3c] Catalyzed by Co(I)NHC complex, the alkene hydroboration by pinacol borane gives Markovnikov selective product with good selectivity, where the regio-selectivity is controlled by phenyl substituent. In absence of that, complete inversion in the selectivity has been observed. The preliminary mechanistic cycle suggests that the catalytic cycle proceeds via oxidative addition of pinacol borane to [Co] followed by alkene insertion and reduction elimination steps. (iv) The last section discusses the interaction of diazabutadiene molecules with diboron compounds. [3d] The diazabutadiene derivatives have been observed to completely cleave the B-B bond of Bis(catacolato)diboron and Bis(dithiocatacolato)diboron. The preliminary findings hint towards homolytic cleavage of the B-B bond by concerted interaction of the two nitrogen atoms of diazabutadiene with the two boron atoms of the diboron from the same face. References: [1] Boronic Acids-Preparation and Applications in Organic Synthesis, Medicine and Materials, 2nd ed.; Hall, D. G., Ed.; Wiley-VCH: Weinheim, 2011. [2] Neeve, E. C.; Geier, S. J.; Mkhalid, I. A. I.; Westcott, S. A.; Marder, T. B. Diboron(4) Compounds: From Structural Curiosity to Synthetic Workhorse. Chem. Rev. 2016, 116, 9091-9161. [3] (a) Verma, P. K.; Mandal, S.; Geetharani, K. ACS Catal. 2018, 8, 4049-4054. (b) Verma, P. K.; Prasad, K. S.; Varghese, D.; Geetharani, K. Org. Lett. 2020, 22, 4, 1431-1436. (c) Verma, P. K.; Setulekshmi, A. S.; Geetharani, K. Org. Lett. 2018, 20, 7840-7845. (d) Verma, P. K.; Meher, N. K.; Geetharani, K. Accepted for publication in Chem. Commun., Manuscript ID: CC-COM-06-2021-002881.R2.

The reduction of azobenzene 1 with catalyst 2 was reported (J. Am. Chem. Soc. 2012, 134, 11330) by Alexander T. Radosevich at Pennsylvania State University, representing a unique example of a nontransition metal-based two-electron redox catalysis platform. Wolfgang Kroutil at the University of Graz found (Angew. Chem. Int. Ed. 2012, 51, 6713) that diketone 4 was converted to piperidinium 5 with very high stereoselectivity using a transaminase followed by reduction over Pd/C. Dennis P. Curran at the University of Pittsburgh reported (Org. Lett. 2012, 14, 4540) that NHC-borane 7 is a convenient reducing agent for aldehydes and ketones, showing selectivity for the former as in the monoreduction of 6 to 8. A catalytic reduction of esters to ethers with Fe3(CO)12 and TMDS, as in the conversion of 9 to 10, was developed (Chem. Commun. 2012, 48, 10742) by Matthias Beller at the Leibniz-Institute for Catalysis. Meanwhile, iridium catalysis was used (Angew. Chem. Int. Ed. 2012, 51, 9422) by Maurice Brookhart at the University of North Carolina at Chapel Hill for the reduction of esters to aldehydes with diethylsilane (e.g., 11 to 12). As an impressive example of selective reduction, Ohyun Kwon at UCLA reported (Org. Lett. 2012, 14, 4634) the conversion of ester 13 to aldehyde 14, leaving the malonate moiety intact. The cobalt complex 16 was found (Angew. Chem. Int. Ed. 2012, 51, 12102) by Susan K. Hanson at Los Alamos National Laboratory to be an effective catalyst for C=O, C=N, and C=C bond hydrogenation, including the conversion of alkene 15 to 17. The use of frustrated Lewis pair catalysis for the low-temperature hydrogenation of alkenes such as 18 was developed (Angew. Chem. Int. Ed. 2012, 51, 10164) by Stefan Grimme at the University of Bonn and Jan Paradies the Karlsruhe Institute of Technology. Guanidinium nitrate was found (Chem. Commun. 2012, 48, 6583) by Kandikere Ramaiah Prabhu at the Indian Institute of Science to catalyze the hydrazine-based reduction of alkenes such as 20. The hydrogenation of thiophenes is difficult for a number of reasons, but now Frank Glorius at the University of Münster has developed (J. Am. Chem. Soc. 2012, 134, 15241) an effective system for the highly enantioselective catalytic hydrogenation of thiophenes and benzothiophenes, including 22.