Osmabenzenes from Osmacycles Containing an η²‐Coordinated Olefin

Abstract

Treatment of HC[triple bond]CC(CH3)(OH)CH=CH2 with [OsCl2(PPh3)3] in dichloromethane yielded the eta2-olefin-coordinated osmacycle [Os{CH=C(PPh3)C(=CH2)-eta2-CH=CH2}Cl2(PPh3)2] (9). Transformations of osmacycle 9 by treatment with benzonitrile under various conditions have been investigated. Reaction of 9 with excess benzonitrile at room temperature afforded the dicationic osmacycle [Os{CH=C(PPh3)C(=CH2)-eta2-CH=CH2}(PhCN)2(PPh3)2]Cl2 (11) by ligand substitution, which reacted further to the intramolecularly coordinated eta2-allene complex [Os{CH=C(PPh3)C(CH3)=(eta2-C=CH2)}(PhCN)2(PPh3)2]Cl2 (12). In contrast, heating a chloroform solution of 9 to the reflux temperature in the presence of excess benzonitrile generated osmabenzene [Os{CHC(PPh3)C(CH3)CHCH}(PhCN)2(PPh3)2]Cl2 (14). Complexes 11, 12 and 14 are in fact isomers. In the absence of excess benzonitrile, the isolated dicationic 12 and 14 readily dissociate the benzonitrile ligands in solution to produce the neutral complex [Os{CH=C(PPh3)C(CH3)=(eta2-C=CH2)}Cl2(PPh3)2] (13) and the monocationic osmabenzene [Os{CHC(PPh3)C(CH3)CHCH}Cl(PhCN)(PPh3)2]BPh4 (15), respectively. Mechanisms for the formation of osmabenzene 14 from 11 and 12 are proposed based on DFT calculations.

Interconversion of Metallabenzenes and Cyclic η2‐Allene‐Coordinated Complexes

“…Ligand substitution of E with additional 8‐hydroxyquinoline gives F and subsequent intermolecular S N Ar reaction and oxidation reaction could generate osmabenzene 5 . Although we have failed to observe the intermediate E , the formation of metallabenzene from a cyclic η 2 ‐allene‐coordinated complex has been previously reported and DFT calculations have also been performed before to elucidate the mechanism 10c. In addition, increasing the stability of metallabenzene by introducing 8‐hydroxyquinoline substitution to the metallabenzene ring has also been confirmed by our recent study 9a…”

“…We have demonstrated that a similar cyclic η 2 ‐allene complex, [Os{CHC(PPh 3 )C(CH 3 )=(η 2 ‐CCH 2 )}(PhCN) 2 (PPh 3 ) 2 ]Cl 2 , could isomerize to metallabenzene when CHCl 3 was employed as solvent in the reaction 10c. By contrast, complex 4 is very stable in CHCl 3 under refluxing condition.…”

Synthesis of Aromatic Aza-metallapentalenes from Metallabenzene via Sequential Ring Contraction/Annulation

“…Inspired by this observation, we initially attempted to react osmium complexes with a number of nitriles such as benzonitrile or acetonitrile in the hope of obtaining the corresponding aza-osmapentalynes. However, the expected reactions did not occur, only osmabenzenes or other osmacycles were detected, which have been previously reported 24 25 . Therefore, the preparation of an aza-metallapentalene via the synthesis of an aza-osmapentalyne followed by protonation is unfeasible.…”

Recent Advances in Metallaaromatic Chemistry

“…Since these early reports the study of metallabenzenes has flourished. [39][40][41][42][43][44] The third row transition metalsa re well-represented with examples incorporating osmium, [45][46][47][48][49][50][51][52][53][54][55][56][57][58][59][60][61] iridium, [62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78][79][80] platinum [81][82][83][84] and, most recently,r henium. [85][86][87] Metallabenzenes of one second row transition metal, ruthenium, are also known.…”