Intramolecular 1,2 C−H Addition of o‐Methyl Groups to Form Unique Ruthenium Pincer Tuck‐in Complexes

Abstract

New RPNHP ligands containing 2,4‐xylyl (4mXPNHP) and mesityl (MesPNHP) groups on the phosphorus atoms were synthesized. 4mXPNHP reacts with [(cymene)RuCl2]2 followed by PMe3 to produce κ3‐4mXPNHPRu(PMe3)Cl2. MesPNHP reacts with [(cymene)RuCl2]2 to produce monomeric κ3‐MesPNHPRuCl2 that reacts with CO forming κ3‐MesPNHPRu(CO)Cl2. Surprisingly, dehydrohalogenation of these complexes results in the activation of ortho methyl groups of the pincer ligands, rather than formation of κ3‐RPNPRuLCl complexes. This results from transient κ3‐RPNPRuLCl formation followed by 1,2‐addition of an ortho C−H bond across the Ru‐amide bond. The transient amide complex of κ4‐4mXPNHPRu(PMe3)Cl was trapped with CO forming κ3‐4mXPNPRu(PMe3)(CO)Cl. In contrast, κ4‐MesPNHPRu(CO)Cl does not react with ligands to trap the expected amide complex of reverse C−H addition. Instead, CO and PMe3 displace the chloride ligand forming cationic complexes. In both cases, hemi‐lability of the pincer ligand was observed spectroscopically. The new complexes serve as precursors to moderately active catalysts for the acceptorless dehydrogenative coupling of n‐butanol.

Double Intramolecular 1,2 C–H Addition of o-Methyl Groups To Form Ruthenium Pincer Double Tuck-In Complexes

“…The structure of 6 is shown in Figure A, and relevant SCXRD metric parameters for the crystal structures in this study are provided in Table S2 in the Supporting Information. The structures of 6 and 7 have Ru–C bond lengths slightly longer than the precursor bond lengths (2.128(3)–2.175(3) Å) . Interestingly, the two Ru–C and Ru–P bonds in 6 differ by 0.05 Å, whereas in 7 , these bond lengths are identical (within error).…”

“…Ruthenium pincer complexes are excellent catalysts for a variety of organic transformations. − These catalysts are supported by various pincer ligands, but RPN H P (HN­(CH 2 CH 2 PR 2 ) 2 , R = alkyl or aryl) ligands are among the most commonly utilized pincer ligand class. RPN H P Ru complexes catalyze reactions such as acceptorless dehydrogenative coupling of alcohols, − ester hydrogenolysis, − dehydrogenative coupling of borane–amine adducts, − hydrogenation and dehydrogenation, − and several others. ,, Traditionally the active catalyst is generated in situ by the addition of a base. The iso -propyl and tert -butyl derivates of the RPNPRuH­(CO) complexes have been isolated in the absence of hydrogen (Figure A).…”

“…Recently, our group reported that RPN H PRuLX 2 complexes with ortho methylated aryl R groups react with a base to generate RPNPRuLCl intermediates that rapidly activate the ortho methyl groups, adding a C–H bond across the Ru–N bond and forming tuck-in complexes (Figure C) . This was the first report of intramolecular R group activation in this class of ruthenium complexes.…”

“…Mono tuck-in Ru pincer complexes 2a and 4 react with NaN­(SiMe 3 ) 2 to produce double tuck-in complexes 6 and 7 , respectively (Scheme ), in good yields. These reactions proceed by dehydrohalogenation of the complexes generating the amide and tuck-in intermediates that rapidly add a second C–H bond across the Ru–N bond via intramolecular 1,2 addition, forming double tuck-in pincer complexes.…”

“…Alternatively, 6 and 7 can be accessed via the addition of 2 equiv of NaN­(SiMe 3 ) 2 to the respective dichloride precursors in good yields, respectively. 5 also reacts with NaN­(SiMe 3 ) 2 , giving the double dehydrohalogenated complex 8 . We hypothesized that 8 would be a coordinatively unsaturated complex since 5 does not contain CO or PMe 3 .…”