Osmium‐Mediated CH and CC Bond Cleavage of a Phenolic Substrate: p‐Quinone Methide and Methylene Arenium Pincer Complexes

Abstract

The diphosphine 2,4,6-(CH(3))(3)-3,5-(iPr(2)PCH(2))(2)C(6)OH (1) reacts with [OsCl(2)(PPh(3))(3)] in presence of an excess of triethylamine to yield the isomeric para-quinone methide derivatives [Os{4-(CH(2))-1-(O)-2,6-(CH(3))(2)-3,5-(iPr(2)PCH(2))(2)C(6)}(Cl)(H)(PPh(3))] (2 and 3), which differ in the positions of the mutually trans hydride and chloride ligands. Complex 2 reacts with CO to afford the dicarbonyl species [Os{1-(O)-2,4,6-(CH(3))(3)-3,5-(iPr(2)PCH(2))(2)C(6)}(Cl)(CO)(2)] (4), which results from hydride insertion into the quinonic double bond. Protonation of 2 and 3 leads to the formation of the methylene arenium derivative [Os{4-(CH(2))-1-(OH)-2,6-(CH(3))(2)-3,5-(iPr(2)PCH(2))(2)C(6)}(Cl)(H)(PPh(3))][OSO(2)CF(3)] (5 a). The diphosphine 1 reacts with [OsCl(2)(PPh(3))(3)] at 100 degrees C under H(2) to afford [Os{1-(OH)-2,6-(CH(3))(2)-3,5-(iPr(2)PCH(2))(2)C(6)}(Cl)(H(2))(PPh(3))] (6), a PCP pincer complex resulting formally from C(sp(2))--C(sp(3)) cleavage of the C--CH(3) group in 1. C--C hydrogenolysis resulting in the same complex is achieved by heating 2 under H(2) pressure. Reaction of the diphosphine substrate with [OsCl(2)(PPh(3))(3)] under H(2) at lower temperature allows the observation of a methylene arenium derivative resulting from C--H activation, [Os{4-(CH(2))-1-(OH)-2,6-(CH(3))(2)-3,5-(iPr(2)PCH(2))(2)C(6)}(Cl)(2)(H)] (7). This compound reacts with PPh(3) in toluene to afford the ionic derivative [Os{4-(CH(2))-1-(OH)-2,6-(CH(3))(2)-3,5-(iPr(2)PCH(2))(2)C(6)}(Cl)(H)(PPh(3))]Cl (5 b). X-ray diffraction studies have been carried out on compounds 2, 3, 4, 5 b, 6, and 7, which allows the study of the structural variations when going from methylene arenium to quinone methide derivatives.

Cyclometalation Using d-Block Transition Metals: Fundamental Aspects and Recent Trends

“…332 Activation of a C alkyl -H bond in the diphosphine ligand precursor 195 proceeds in analogy to its ruthenium homolog (cf. Scheme 70) and is only successful if the reaction is performed under H 2 atmosphere (Scheme 82), 334 emphasizing again the relevance of (transiently formed) osmium hydride species for the C-H bond activation process. Metal chelation as imposed in cyclometalation reactions offers interesting opportunities, either to enhance the oxidative addition propensity, thus increasing the reactivity at the metal center for activating even highly unreactive bonds, or to decelerate reactions in order to identify intermediates and to establish pertinent reaction pathways.…”

Osmium Complexes With POP Pincer Ligands

“…In fact, the most rigid dibenzofuran stabilizes the OsH 4 -species and prevents the formation of a counterpart dimer of 32. Thus under argon, the addition of 1 equivalent of HBF 4 •OEt 2 to 8 in dichloromethane leads to the compressed dihydride-dihydrogen complex [OsCl(H•••H)(η 2 -H 2 ){κ 3 -P,O,P-[dbf(P i Pr 2 ) 2 ]}]BF 4 (33). In contrast to 31, this dbf(P i Pr 2 ) 2 is moderately stable under argon and does not release H 2 to dimerize.…”

“…The scarce development of the pincer-osmium chemistry is probably due to the rooted belief that osmium is not useful in catalysis because it is a reductant and prefers to be coordinatively saturated and forms redox isomers with greater metal-carbon multiplicity [10-13]. However, recent findings have proved that osmium can be a promising alternative to the metals classically used in catalysis [14][15][16][17][18][19][20][21][22][23].The chemistry of the pincer-osmium complexes has been mainly focused on a few PCP derivatives reported by the groups of Gusev [24-28], Jia [29][30][31] and Milstein [32,33] and PNP complexes described by the groups of Caulton [34-37], Gusev [38-41], and Jia [42], along with some CCC [43-46], CCN [47], CNC [48,49], CNN [50-52], CNO [53], and NNN [54-59] species.We synthesized the diphosphines xant(P i Pr 2 ) 2 and dbf(P i Pr 2 ) 2 in 2010 [60], and subsequently started our research program on POP complexes of platinum group metals [3,[60][61][62][63][64][65], including osmium [60,[66][67][68][69][70][71]. In this chapter, we summarize the results obtained for this element until now.…”

“…The chemistry of the pincer-osmium complexes has been mainly focused on a few PCP derivatives reported by the groups of Gusev [24-28], Jia [29][30][31] and Milstein [32,33] and PNP complexes described by the groups of Caulton [34-37], Gusev [38-41], and Jia [42], along with some CCC [43-46], CCN [47], CNC [48,49], CNN [50-52], CNO [53], and NNN [54-59] species.…”

Prototropic rearrangements in cycloheptatrienyl PCP pincer iridium complexes

“…25 Exocyclic carbene complexes have been reported, 17,18, 26 and internal metal-carbon double bonded compounds have been discussed as metal quinone methide complexes and metallaquinones. [27][28][29] The first known internal metal carbene, or better termed alkylidene, was described by Shaw, 30 who thermolysed the hydrochloride 1 to form the internal iridium pincer carbene complex 2 in low yield a Institut für Anorganische Chemie, Universität Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany. E-mail: hermann.mayer@ uni-tuebingen.de; Fax: +49 (0)7071 292436; Tel: +49 (0)7071 2976229 b Institut für Physikalische und Theoretische Chemie, Universität Tübingen, Auf der Morgenstelle 8, 72076 Tübingen, Germany (Scheme 1).…”