Osmium Complexes With POP Pincer Ligands

Abstract

We are interested in complexes of platinum group metals with POP ligands such as 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene (xant(P i Pr 2 ) 2 ) and 4,6-bis(diisopropylphosphino)dibenzofuran (dbf(P i Pr 2 ) 2 ) and in the study of the similarities and differences between the 4d and 5d counterparts of each group, with the aim of finding new and more efficient catalysts for the conversion and storage of regenerative energy and in metal-promoted organic synthesis.POP-ether diphosphines are flexible, which allows them to change their coordination fashion for adapting to the requirement of the participating intermediates in multistep processes [1][2][3]. Thus for instance, Weller and coworkers have demonstrated that 9,9-dimethyl-4,5-bis(diphenylphosphino)xanthene (xant(PPh 2 ) 2 ) has the ability of changing its coordination mode in several relevant rhodium-and iridium-mediated processes, such as the hydroacylation [4] and carbothiolation [5] of alkenes and alkynes and the dehydrocoupling of amine-boranes [6].Platinum group metals certainly occupy a prominent place in pincer-metal chemistry [7][8][9]. Osmium is nevertheless a notable exception. 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. LEVELLING EFFECT OF THE METAL CENTERLigands dbf(P i Pr 2 ) 2 , xant(P i Pr 2 ) 2 , and xant(PPh 2 ) 2 react with OsCl 3 •3H 2 O in 2-propanol under reflux to provide the osmium(III) derivatives OsCl 3 {κ 3 -P,O,P-[dbf(P i Pr 2 ) 2 ]} (1), OsCl 3 {κ 3 -P,O,P-[xant(P i Pr 2 ) 2 ]} (2), and OsCl 3 {κ 3 -P,O,P-[xant(PPh 2 ) 2 ]} (3), which were isolated in high yields, according to Scheme 16.1, and characterized by X-ray diffraction analysis [60].The structures reveal that the rigid polycyclic backbones do not have any significant influence in the coordination of the diphosphines because OsCl 3 produces a levelling effect on the relevant coordinating parameters of the ligands, such as the separation between the phosphorous atoms or the P-M-P bite angle. Thus although t...

Rhodium-Mediated Dehydrogenative Borylation–Hydroborylation of Bis(alkyl)alkynes: Intermediates and Mechanism

“…The ether-diphosphine 4,5-bis­(diisopropylphosphino)­xanthene (xant­(P i Pr 2 ) 2 ) is a flexible ligand, which adapts its coordination mode to the electronic and steric requirements of each particular complex, in agreement with that observed for other POP ligands . This flexibility allows a rapid Z – E isomerization of the β-borylalkenyl group ( Z )-C­(Me)C­(Me)­Bpin coordinated with the fragment Rh­{xant­(P i Pr 2 ) 2 }, via rhodacyclopropene intermediates bearing a κ 2 -P,O,P-diphosphine .…”

Reactions of an Osmium(IV)-Hydroxo Complex with Amino-Boranes: Formation of Boroxide Derivatives

“…Compounds of this class are generally osmium­(II)-derivatives and only very recently was isolated a hydride-osmium­(IV)-hydroxo complex . In agreement with the marked ability of pincer ligands to stabilize uncommon species, it contains the POP group 4,5-bis­(diisopropylphosphino)­xanthene (xant­(P i Pr 2 ) 2 ) …”

Repercussion of a 1,3-Hydrogen Shift in a Hydride-Osmium-Allenylidene Complex

“… 13 According to such ability, xant(P i Pr 2 ) 2 is adapted to the requirements of the reactions in which its complexes participate, enabling the necessary geometrical transformations on the metal coordination sphere through changes in its coordination mode. 14 As proof-of-concept validation, besides species bearing the diphosphine κ 3 - P , O , P - mer coordinated, 15 complexes with the ligand in fashions κ 3 - P , O , P - fac , 16 κ 2 - P , P - cis , 17 and κ 2 - P , P - trans 14b , 18 have been also isolated. This is allowing to perform reactions 19 and to isolate compounds 20 initially forbidden, and as result, interesting catalysts for a wide range of processes are being discovered.…”

“…In addition, Weller’s group has demonstrated in the past years that POP-diphosphines are flexible hemilabile ligands . According to such ability, xant­(P i Pr 2 ) 2 is adapted to the requirements of the reactions in which its complexes participate, enabling the necessary geometrical transformations on the metal coordination sphere through changes in its coordination mode . As proof-of-concept validation, besides species bearing the diphosphine κ 3 - P , O , P - mer coordinated, complexes with the ligand in fashions κ 3 - P , O , P - fac , κ 2 - P , P - cis , and κ 2 - P , P - trans , have been also isolated.…”