Synthesis and Characterization of Osmium(II) Compounds of Stoichiometry (C₅Me₅)OsL₂Br, (C₅Me₅)OsL₂H, and (C₅Me₅)Os(NO)Br₂

Abstract

The preparations of several new (pentamethylcyclopentadienyl)osmium(II) complexes from the osmium(III) compound (C5Me5)2Os2Br4 are described; among these are phosphine and alkene complexes of stoichiometry (C5Me5)OsL2Br and (C5Me5)OsL2H as well as the nitrosyl complex (C5Me5)Os(NO)Br2. Treatment of (C5Me5)2Os2Br4 with PPh3 in ethanol or PMe3 in dichloromethane affords the osmium(II) complexes (C5Me5)OsL2Br, where L = PPh3 or PMe3; the 1,5-cyclooctadiene complex (C5Me5)Os(cod)Br can be made similarly in ethanol. Treatment of either the PPh3 or cod complex with other tertiary phosphines in refluxing heptane affords several other compounds of this class: (C5Me5)OsL2Br, where L = PEt3, 1/2 Me2PCH2PMe2, 1/2 Me2PCH2CH2PMe2, or 1/2 Ph2PCH2PPh2. These bromoosmium(II) species serve as excellent starting materials for the preparation of other osmium(II) complexes. For example, treatment with NaBH4 in ethanol or with NaOMe in methanol affords the hydrides (C5Me5)OsL2H, where L = PMe3, PEt3, PPh3, 1/2 cod, 1/2 Me2PCH2PMe2, 1/2 Me2PCH2CH2PMe2, or 1/2 Ph2PCH2PPh2. Interestingly, treatment of (C5Me5)Os(PMe3)2Br with NaBH4 in refluxing ethanol affords the dihydride cation [(C5Me5)Os(PMe3)2H2 +], which can be deprotonated with methyllithium in tetrahydrofuran to afford the electrically neutral hydride (C5Me5)Os(PMe3)2H. This hydride complex is expected to be one of the most basic transition metal complexes known. Finally, treatment of (C5Me5)2Os2Br4 with nitric oxide in dichloromethane yields the osmium(II) complex (C5Me5)Os(NO)Br2. IR, NMR, and mass spectra of the new complexes are described. A secondary 13C/12C isotope effect on the 31P NMR chemical shifts of ca. 0.025 ppm is noted in several compounds. Comparisons of these osmium(II) compounds with analogous ruthenium species suggests that the former have stronger metal−ligand bonds, are slower to undergo nucleophilic substitution reactions, and are stronger reducing agents.

Synthesis, characterization, and crystal structures of the osmium triflate complexes Cp∗Os(P–P)(OTf) and [Cp∗Os(P–P)(OH2)][OTf]

“…These patterns can be analyzed in terms of ABX spin systems (A, B = 31 P, X = 13 C), where the phosphorus chemical shifts are different because only one of the two phosphorus atoms bears a 13 C-labeled carbon. Simulations show that the one-bond 13 C/ 12 C secondary isotope effect on the 31 P NMR chemical shift is about 0.025 ppm; this value is similar to those seen in other systems [16,19]. Single resonances were seen in the 31 P{ 1 H} NMR spectrum for all three compounds.…”

“…We have previously described the conversion of the bromo complexes Cp * Os(P-P) 2 Br to their corresponding hydrides Cp * Os(P-P)H, where P-P = bis(dimethylphosphino)methane (dmpm), dppm = bis(diphenylphosphino)-methane (dppm), or 1,2-bis(dimethylphosphino)ethane (dmpe) [16]. Treatment of these hydride complexes with one to two equivalents of methyl trifluoromethanesulfonate in pentane results in loss of methane and generation of the triflate compounds Cp * Os(dmpm)(OTf) (1), Cp * Os(dppm)-(OTf) (2), and Cp * Os(dmpe)(OTf) (3), in 59%, 69%, and 86% yield, respectively.…”

Ruthenium vs. Osmium Complexes as Catalysts for Atom Transfer Radical Addition Reactions

“…The major diamagnetic species in solution (36 % yield, as determined with the internal standard mesitylene) was found to be the known complex [Cp*OsBr(PPh 3 ) 2 ] (3) (Scheme 1). [12] Scheme 1. Reduction of complex 2 by Mg.…”

“…We have not tried to optimize this synthetic procedure since complex 3 is more conveniently obtained by reaction of [Cp*OsBr 2 ] 2 with PPh 3 . [12] However, we have performed a single-crystal X-ray analysis of 3 (Figure 1). For comparison, we have also examined the solid-state structure of the precursor 2 (Figure 2).…”

The contrasting chemical reactivity of potent isoelectronic iminopyridine and azopyridine osmium(ii) arene anticancer complexes

“…The current work appears to be the first detection of an osmium(II) hydride NMR peak for an osmium hydride in an aqueous solution. [56][57][58][59][60][61][62][63][64] These findings suggest that osmium arene complexes can modulate the redox balance in cells by a novel mechanism. 95 1 H NMR studies show that complexes 6 and 14 are more effective in oxidising NADH than 8 and 16 after 24 h incubation at 310K ( Figure S5).…”