Isonitrile ruthenium and iron PNP complexes: synthesis, characterization and catalytic assessment for base-free dehydrogenative coupling of alcohols. Issue 29 (1st July 2021)

• Record Type: Journal Article
• Authors: Nguyen, Duc Hanh; Merel, Delphine; Merle, Nicolas; Trivelli, Xavier; Capet, Frédéric; Gauvin, Régis M.
• Is Part Of: Dalton transactions. Volume 50:Issue 29(2021)
• Page Start: 10067
• Page End: 10081
• Publication Date: 2021-07-01
• Subjects: Chemistry, Inorganic -- Periodicals; Chemistry, Physical and theoretical -- Periodicals; Chemistry, Inorganic -- Periodicals; 546.05
• DOI: 10.1039/d1dt01722e
• Languages: English
• ISSNs: 1477-9226

Abstract

Abstract : Ruthenium and iron PNP pincer complexes bearing isocyanides ancillary ligands have been prepared, and assessed for alcohol acceptorless dehydrogenative coupling. Abstract : Neutral and ionic ruthenium and iron aliphatic PN H P-type pincer complexes (PN H P = NH(CH2 CH2 PiPr2 )2 ) bearing benzyl, n -butyl or tert -butyl isocyanide ancillary ligands have been prepared and characterized. Reaction of [RuCl2 (PN H P)]2 with one equivalent CN-R per ruthenium center affords complexes [RuCl2 (PN H P)(CNR)] (R = benzyl, 1a, R = n -butyl, 1b, R = t -butyl, 1c ), with cationic [RuCl(PN H P)(CNR)2 ]Cl 2a–c as side-products. Dichloride species 1a–c react with excess NaBH4 to afford [RuH(PN H P)(BH4 )(CN-R)] 3a–c, analogues to benchmark Takasago catalyst [RuH(PN H P)(BH4 )(CO)]. Reaction of 1a–c with a single equivalent of NaBH4 results in formation of [RuHCl(PN H P) (CN-R)] (4a–c ), from which 3a–c can be prepared upon reaction with excess NaBH4 . Use of one equivalent of NaHBEt3 with 4a and 4c affords bishydrides [Ru(H)2 (PN H P)(CN-R)] 5a and 5c . Deprotonation of 4c by KO t Bu generates amido derivative [RuH(PNP)(CN- t -Bu)] (6, PNP = − N(CH2 CH2 PiPr2 )2 ), unstable in solution. Addition of excess benzylisonitrile to 4a provides cationic hydride [RuH(PN H P) (CN-CH2 Ph)2 ]Cl (7 ). Concerning iron chemistry, [Fe(PN H P)Br2 ] reacts with one equivalent of benzylisonitrile to afford [FeBr(PN H P)(CNCH2 Ph)2 ]Br (8 ). The outer-sphere bromide anion can be exchanged by salt metathesis with NaBPh4 to generate [FeBr(PN H P) (CNCH2 Ph)2 ](BPh4 ) (9 ). Cationic hydride species [FeH(PN H P) (CN- t -Bu)2 ](BH4 ) (10 ) is prepared from consecutive addition of excess CN- t -Bu and NaBH4 on [Fe(PNP H )Br2 ]. Ruthenium complexes 3a–c are active in acceptorless alcohol dehydrogenative coupling into ester under base-free conditions. From kinetic follow-up, the trend in initial activity is 3a ≈ 3b > [RuH(PN H P)(BH4 )(CO)] ≫ 3c ; for robustness, [RuH(BH4 )(CO)(PN H P)] > 3a > 3b ≫ 3c . Hypotheses are given to account for the observed deactivation. Complexes 3b, 3c, 4a, 4c, 5c, 7, cis -8 and 9 were characterized by X-ray crystallography.