Isonitrile ruthenium and iron PNP complexes: synthesis, characterization and catalytic assessment for base-free dehydrogenative coupling of alcohols

Abstract

Neutral and ionic ruthenium and iron aliphatic PN^HP-type pincer complexes (PN^HP = NH(CH₂CH₂PiPr₂)₂) bearing benzyl, n-butyl or tert-butyl isocyanide ancillary ligands have been prepared and characterized. Reaction of [RuCl₂(PN^HP)]₂ with one equivalent CN-R per ruthenium center affords complexes [RuCl₂(PN^HP)(CNR)] (R = benzyl, 1a, R = n-butyl, 1b, R = t-butyl, 1c), with cationic [RuCl(PN^HP)(CNR)₂]Cl 2a–c as side-products. Dichloride species 1a–c react with excess NaBH₄ to afford [RuH(PN^HP)(BH₄)(CN-R)] 3a–c, analogues to benchmark Takasago catalyst [RuH(PN^HP)(BH₄)(CO)]. Reaction of 1a–c with a single equivalent of NaBH₄ results in formation of [RuHCl(PN^HP) (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^HP)(CN-R)] 5a and 5c. Deprotonation of 4c by KOtBu generates amido derivative [RuH(PNP)(CN-t-Bu)] (6, PNP = ⁻N(CH₂CH₂PiPr₂)₂), unstable in solution. Addition of excess benzylisonitrile to 4a provides cationic hydride [RuH(PN^HP) (CN-CH₂Ph)₂]Cl (7). Concerning iron chemistry, [Fe(PN^HP)Br₂] reacts with one equivalent of benzylisonitrile to afford [FeBr(PN^HP)(CNCH₂Ph)₂]Br (8). The outer-sphere bromide anion can be exchanged by salt metathesis with NaBPh₄ to generate [FeBr(PN^HP) (CNCH₂Ph)₂](BPh₄) (9). Cationic hydride species [FeH(PN^HP) (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^HP)(BH₄)(CO)] ≫ 3c; for robustness, [RuH(BH₄)(CO)(PN^HP)] > 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.