Isonitrile Ruthenium and Iron PNP Complexes: Synthesis, Characterization and Catalytic Assessment for Base-Free Dehydrogenative Coupling of Alcohols

Neutral and ionic ruthenium and iron aliphatic PNPH-type pincer complexes (PNPH= NH(CH2CH2PiPr2)2) bearing benzyl, n-butyl or tert-butyl isocyanide ancillary ligands have been prepared and characterized. Reaction of [RuCl2(PNPH)]2 with one equivalent CN-R per ruthe-nium center affords complexes [Ru(PNPH)Cl2(CNR)] (R= benzyl, 1a, R= n-butyl, 1b, R= t-butyl, 1c), with cationic [Ru(PNPH)(Cl)(CNR)2]Cl 2a-c as side-products. Complexes 2a-c are selectively prepared upon reaction of [RuCl2(PNPH)]2 with 2 equiva-lents of isonitrile per ruthenium center. Dichloride species 1a-c react with excess NaBH4 to afford [Ru(PNPH)(H)(BH4)(CN-R)] 3a-c, analogues to benchmark Takasago catalyst [Ru(PNP)(H)(BH4)(CO)]. Reaction of 1a-c with a single equivalent of NaBH4 under protic conditions results in formation of hydrido chloride derivatives [Ru(PNPH)(H)(Cl)(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(PNPH)(H)2(CN-R)] 5a and 5c. In the case of bulkier t-butylisonitrile, two isomers were observed by NMR, with the PNP framework in either meridional or facial confor-mation. Deprotonation of 4c by KOtBu generates amido derivative [Ru(PNP’)(H)(CN-t-Bu)] (6, PNP’= -N(CH2CH2PiPr2)2), unstable in solution. Addition of excess benzylisonitrile to 4a provides cationic hydride [Ru(PNPH)(H)(CN-CH2Ph)2]Cl (7). Concerning iron chemis-try, [Fe(PNPH)Br2] reacts one equivalent benzylisonitrile to afford [Fe(PNPH)(Br)(CNCH2Ph)2]Br (8). The outer-sphere bromide anion can be exchanged by salt metathesis with NaBPh4 to generate [Fe(PNPH)(Br)(CNCH2Ph)2](BPh4) (9). Cationic hydride species [Fe(PNPH)(H)(CN-t-Bu)2](BH4) (10) is prepared from consecutive addition of excess CN-t-Bu and NaBH4 on [Fe(PNPH)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 > [Ru(PNPH)(H)(BH4)(CO)] >> 3c; for robustness, [Ru(H)(BH4)(CO)(PNPH)] > 3a > 3b >> 3c. Hy-potheses 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.