Stable group 8 metal porphyrin mono- and bis(dialkylcarbene) complexes: synthesis, characterization, and catalytic activity

Abstract

Alkyl-substituted carbene (CHR or CR 2 , R ¼ alkyl) complexes have been extensively studied for alkylcarbene (CHR) ligands coordinated with high-valent early transition metal ions (a.k.a. Schrock carbenes or alkylidenes), yet dialkylcarbene (CR 2 ) complexes remain less developed with bis(dialkylcarbene) species being little (if at all) explored. Herein, several group 8 metal porphyrin dialkylcarbene complexes, including Fe-and Ru-mono(dialkylcarbene) complexes [M(Por)(Ad)] (1a,b, M ¼ Fe, Por ¼ porphyrinato dianion, Ad ¼ 2-adamantylidene; 2a,b, M ¼ Ru) and Os-bis(dialkylcarbene) complexes [Os(Por)(Ad) 2 ] (3ac), are synthesized and crystallographically characterized. Detailed investigations into their electronic structures reveal that these complexes are formally low-valent M(II)-carbene in nature. These complexes display remarkable thermal stability and chemical inertness, which are rationalized by a synergistic effect of strong metal-carbene covalency, hyperconjugation, and a rigid diamondoid carbene skeleton. Various spectroscopic techniques and DFT calculations suggest that the dialkylcarbene Ad ligand is unique compared to other common carbene ligands as it acts as both a potent s-donor and p-acceptor; its unique electronic and structural features, together with the steric effect of the porphyrin macrocycle, make its Fe porphyrin complex 1a an active and robust catalyst for intermolecular diarylcarbene transfer reactions including cyclopropanation (up to 90% yield) and X-H (X ¼ S, N, O, C) insertion (up to 99% yield) reactions. † Electronic supplementary information (ESI) available: Abbreviations, experimental procedures, characterization of compounds, computational details, Tables S1-S8, Fig. S1-S11, NMR spectra of compounds, Cartesian coordinates from DFT calculations, and CIF le for the crystal structure of 1b, 1a$Py, 2b$MeOH, 2az, and 3a. Scheme 1 Synthesis of Fe (a), Ru (b), and Os (c) dialkylcarbene complexes.This journal is a All values involving C1-3 are averaged between two Ad groups. b Average distances between M1 and pyrrolic nitrogens (N1-4). c L ax ¼ N5 (for 1a$Py) or O1 (for 2b$MeOH), or C1a (for 3a). View Article Online a E 1/2 values. 1a,b and 2a,b were measured in DMF with SCE as the reference electrode; E(Fc +/0 ) ¼ 0.49 V. 3a-c were measured in DCM with Ag/AgCl as the reference electrode; E(Fc +/0 ) ¼ 0.40 V. b E p,a . c E p,c . d Quasi-reversible. Scheme 4 Intermolecular diarylcarbene transfer reactions catalyzed by 1a. Product yields in red and recovery of 1a (blue) in parentheses. a Conditions: Ph 2 CN 2 (0.2 mmol), substrate (2 mmol), catalyst (0.004 mmol), DCM (1 mL), 40 C, 12 h, and under Ar. b Conditions: Ph 2 CN 2 (0.2 mmol, added over 3 h), substrate (2 mmol), catalyst (0.004 mmol), DCE (1 mL), 80 C, 12 h, and under Ar.Fig. 12 DFT-optimized structure of model complex 4b (left) and its LUMO (middle) and L + 3 orbitals (right).

Transition Metal Catalyzed Insertion Reactions with Donor/Donor Carbenes

“…This method used tosylhydrazones derived from acetophenones, alkyl aldehydes, and alkyl ketones without observing any Bamford–Stevens type elimination byproducts, suggesting that O−H insertion is faster than elimination without a metal catalyst. Similar work was recently accomplished by Che and co‐workers in O−H insertion reactions using phenyl/phenyl carbenes and iron porphyrin catalyst 38 to insert into phenyl, aryl, and alkyl alcohols (Table 5, entry 3) [64] . This methodology proved amenable to S−H insertion reactions as well, affording phenyl and aryl thioethers in excellent yields and one example of an alkoxy thioether in poor yield (Table 5, entry 4).…”

“…[51] Most recently,C he and co-workers utilized donor/ donor carbenes to study the crystallographic and electronic properties of group 8m etal porphyrin-based catalysts. [64] In their studies,t he Fe(TPFPP)Cl-2-diazoadamantane decomposition adduct 38 was shown to be particularly stable, suggesting to them that the catalyst may be proficient in typical C À Hi nsertion reaction catalysis.W ith 2mol %l oading of the iron porphyrin catalyst, they demonstrated that the donor/donor iron-carbenes were able to undergo insertion into allylic and benzylic CÀHb onds (Figure 12 B). Although these reactions afforded poor to moderate yields,t his work represents the first instance of catalytic intermolecular C sp 3 À Hinsertion with donor/donor carbenes.Although this recent publication does not report any diastereo-or enantioselective variants,itdoes expose fertile ground for future work.…”

“…Later efforts showed that ruthenium porphyrins were efficient in catalytic stereoselective intramolecular insertion reactions as well [51] . Most recently, Che and co‐workers utilized donor/donor carbenes to study the crystallographic and electronic properties of group 8 metal porphyrin‐based catalysts [64] . In their studies, the Fe(TPFPP)Cl‐2‐diazoadamantane decomposition adduct 38 was shown to be particularly stable, suggesting to them that the catalyst may be proficient in typical C−H insertion reaction catalysis.…”

“…Unfortunately, in most of these examples using copper catalysts, β‐hydride elimination is common. Most recently, Che and co‐workers were able to demonstrate N−H insertion reactions with iron porphyrin catalyst 38 , affording various tertiary and secondary amines in good yields from donor/donor carbenes (Table 4, entries 7–8) [64] . While significant progress has been made in the field of N−H insertion reactions, enantioselective variants of these reactions have yet to be successful.…”

Platinic metal porphyrins and hybrid nanomaterials embedding them as catalysts in chemical transformations

“…The reaction of N-H insertion of EDA to amines catalysed by μ-carbidodiruthenium complex yielded 100% product when starting from aniline, p-methylaniline and p-chloroaniline in toluene at 90 o C. Metalloporphyrin complexes with 2-adamantylidene axial ligands (Figure 12) display thermal and chemical stability due to the synergistic effect of the metal-carbene bond. 24 This type of compounds can act as catalysts for intermolecular diarylcarbene transfer reactions including cyclopropanation and X-H (X = S, N, O, C) insertion reactions. The best yields are provided by the compounds containing Fe as central metal ion, but the Ru-containing compound reached 92% yield in catalysing carbene transfer reactions, under mild conditions: dichloromethane, 40 o C and 12 hours.…”