PNP-Pincer Complexes of Osmium

Comparison with Isoelectronic (PCP)Ir and (PNP)Ir+ Units

Nicholas Lease, Elizabeth M. Pelczar, Tian Zhou, Santanu Malakar, Thomas J. Emge, Faraj Hasanayn, Karsten Krogh-Jespersen, Alan S Goldman

Research output: Contribution to journalArticle

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Abstract

Several complexes of (tBu 4PNP)Os (1) (tBu 4PNP = C5NH3-2,6-(CH2PtBu2)2) are reported. 1 is isoelectronic with (tBu 4PCP)Ir (2), which has played a leading role in homogeneous catalytic alkane dehydrogenation; the (tBu 4PNP)Os complexes were investigated in this context. (tBu 4PNP)OsH4 (1-H4) is analogous to (tBu 4PCP)IrH4 (2-H4), but while 2-H4 has some character of a dihydrogen dihydride, 1-H4 is unambiguously a tetrahydride. Ethylene reacts with 1-H4 to afford trans-(tBu 4PNP)OsH2(C2H4) (1-H2(C2H4)). At 25 °C, 1-H2(C2H4) readily undergoes reversible ethylene insertion into an Os-H bond to yield (tBu 4PNP)OsH(C2H5) (1-EtH). DFT calculations indicate that alkane C-H addition to 1 is thermodynamically much more favorable than addition to 2. The favorable thermodynamics of 1-(alkyl)H, however, disfavor reductive elimination and formation of the free Os(0) fragment that is required for a catalytic cycle analogous to that reported for 2. The much greater favorability of C-H or H-H addition to 1 as compared with Ir(I) fragment 2 would typically be attributed to the lower oxidation state of 1. However, H2 addition to the perfectly isoelectronic [(tBu 4PNP)Ir(I)]+ cation is even more favorable than addition to 1; thus the thermodynamic differences result from the difference of the pincer ligand (PNP vs PCP) rather than the different metal centers (Os(0) vs Ir(I)). Although H2 addition to Ir(I) is as favorable as addition to Os(0), addition of a second molecule of H2 (to give tetrahydrides) is much more favorable for (tBu 4PNP)Os. NBO analysis indicates that the MH2/MH4 additions are oxidative, whereas the M/MH2 transformations are reductive.

Original languageEnglish
Pages (from-to)314-326
Number of pages13
JournalOrganometallics
Volume37
Issue number3
DOIs
Publication statusPublished - Feb 12 2018

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Osmium
Alkanes
osmium
Thermodynamics
Dehydrogenation
Discrete Fourier transforms
Cations
Metals
Ligands
Oxidation
Molecules
alkanes
ethylene
fragments
dihydrides
thermodynamics
dehydrogenation
insertion
elimination
cations

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

Cite this

PNP-Pincer Complexes of Osmium : Comparison with Isoelectronic (PCP)Ir and (PNP)Ir+ Units. / Lease, Nicholas; Pelczar, Elizabeth M.; Zhou, Tian; Malakar, Santanu; Emge, Thomas J.; Hasanayn, Faraj; Krogh-Jespersen, Karsten; Goldman, Alan S.

In: Organometallics, Vol. 37, No. 3, 12.02.2018, p. 314-326.

Research output: Contribution to journalArticle

Lease, N, Pelczar, EM, Zhou, T, Malakar, S, Emge, TJ, Hasanayn, F, Krogh-Jespersen, K & Goldman, AS 2018, 'PNP-Pincer Complexes of Osmium: Comparison with Isoelectronic (PCP)Ir and (PNP)Ir+ Units', Organometallics, vol. 37, no. 3, pp. 314-326. https://doi.org/10.1021/acs.organomet.7b00738
Lease, Nicholas ; Pelczar, Elizabeth M. ; Zhou, Tian ; Malakar, Santanu ; Emge, Thomas J. ; Hasanayn, Faraj ; Krogh-Jespersen, Karsten ; Goldman, Alan S. / PNP-Pincer Complexes of Osmium : Comparison with Isoelectronic (PCP)Ir and (PNP)Ir+ Units. In: Organometallics. 2018 ; Vol. 37, No. 3. pp. 314-326.
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title = "PNP-Pincer Complexes of Osmium: Comparison with Isoelectronic (PCP)Ir and (PNP)Ir+ Units",
abstract = "Several complexes of (tBu 4PNP)Os (1) (tBu 4PNP = C5NH3-2,6-(CH2PtBu2)2) are reported. 1 is isoelectronic with (tBu 4PCP)Ir (2), which has played a leading role in homogeneous catalytic alkane dehydrogenation; the (tBu 4PNP)Os complexes were investigated in this context. (tBu 4PNP)OsH4 (1-H4) is analogous to (tBu 4PCP)IrH4 (2-H4), but while 2-H4 has some character of a dihydrogen dihydride, 1-H4 is unambiguously a tetrahydride. Ethylene reacts with 1-H4 to afford trans-(tBu 4PNP)OsH2(C2H4) (1-H2(C2H4)). At 25 °C, 1-H2(C2H4) readily undergoes reversible ethylene insertion into an Os-H bond to yield (tBu 4PNP)OsH(C2H5) (1-EtH). DFT calculations indicate that alkane C-H addition to 1 is thermodynamically much more favorable than addition to 2. The favorable thermodynamics of 1-(alkyl)H, however, disfavor reductive elimination and formation of the free Os(0) fragment that is required for a catalytic cycle analogous to that reported for 2. The much greater favorability of C-H or H-H addition to 1 as compared with Ir(I) fragment 2 would typically be attributed to the lower oxidation state of 1. However, H2 addition to the perfectly isoelectronic [(tBu 4PNP)Ir(I)]+ cation is even more favorable than addition to 1; thus the thermodynamic differences result from the difference of the pincer ligand (PNP vs PCP) rather than the different metal centers (Os(0) vs Ir(I)). Although H2 addition to Ir(I) is as favorable as addition to Os(0), addition of a second molecule of H2 (to give tetrahydrides) is much more favorable for (tBu 4PNP)Os. NBO analysis indicates that the MH2/MH4 additions are oxidative, whereas the M/MH2 transformations are reductive.",
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AU - Lease, Nicholas

AU - Pelczar, Elizabeth M.

AU - Zhou, Tian

AU - Malakar, Santanu

AU - Emge, Thomas J.

AU - Hasanayn, Faraj

AU - Krogh-Jespersen, Karsten

AU - Goldman, Alan S

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N2 - Several complexes of (tBu 4PNP)Os (1) (tBu 4PNP = C5NH3-2,6-(CH2PtBu2)2) are reported. 1 is isoelectronic with (tBu 4PCP)Ir (2), which has played a leading role in homogeneous catalytic alkane dehydrogenation; the (tBu 4PNP)Os complexes were investigated in this context. (tBu 4PNP)OsH4 (1-H4) is analogous to (tBu 4PCP)IrH4 (2-H4), but while 2-H4 has some character of a dihydrogen dihydride, 1-H4 is unambiguously a tetrahydride. Ethylene reacts with 1-H4 to afford trans-(tBu 4PNP)OsH2(C2H4) (1-H2(C2H4)). At 25 °C, 1-H2(C2H4) readily undergoes reversible ethylene insertion into an Os-H bond to yield (tBu 4PNP)OsH(C2H5) (1-EtH). DFT calculations indicate that alkane C-H addition to 1 is thermodynamically much more favorable than addition to 2. The favorable thermodynamics of 1-(alkyl)H, however, disfavor reductive elimination and formation of the free Os(0) fragment that is required for a catalytic cycle analogous to that reported for 2. The much greater favorability of C-H or H-H addition to 1 as compared with Ir(I) fragment 2 would typically be attributed to the lower oxidation state of 1. However, H2 addition to the perfectly isoelectronic [(tBu 4PNP)Ir(I)]+ cation is even more favorable than addition to 1; thus the thermodynamic differences result from the difference of the pincer ligand (PNP vs PCP) rather than the different metal centers (Os(0) vs Ir(I)). Although H2 addition to Ir(I) is as favorable as addition to Os(0), addition of a second molecule of H2 (to give tetrahydrides) is much more favorable for (tBu 4PNP)Os. NBO analysis indicates that the MH2/MH4 additions are oxidative, whereas the M/MH2 transformations are reductive.

AB - Several complexes of (tBu 4PNP)Os (1) (tBu 4PNP = C5NH3-2,6-(CH2PtBu2)2) are reported. 1 is isoelectronic with (tBu 4PCP)Ir (2), which has played a leading role in homogeneous catalytic alkane dehydrogenation; the (tBu 4PNP)Os complexes were investigated in this context. (tBu 4PNP)OsH4 (1-H4) is analogous to (tBu 4PCP)IrH4 (2-H4), but while 2-H4 has some character of a dihydrogen dihydride, 1-H4 is unambiguously a tetrahydride. Ethylene reacts with 1-H4 to afford trans-(tBu 4PNP)OsH2(C2H4) (1-H2(C2H4)). At 25 °C, 1-H2(C2H4) readily undergoes reversible ethylene insertion into an Os-H bond to yield (tBu 4PNP)OsH(C2H5) (1-EtH). DFT calculations indicate that alkane C-H addition to 1 is thermodynamically much more favorable than addition to 2. The favorable thermodynamics of 1-(alkyl)H, however, disfavor reductive elimination and formation of the free Os(0) fragment that is required for a catalytic cycle analogous to that reported for 2. The much greater favorability of C-H or H-H addition to 1 as compared with Ir(I) fragment 2 would typically be attributed to the lower oxidation state of 1. However, H2 addition to the perfectly isoelectronic [(tBu 4PNP)Ir(I)]+ cation is even more favorable than addition to 1; thus the thermodynamic differences result from the difference of the pincer ligand (PNP vs PCP) rather than the different metal centers (Os(0) vs Ir(I)). Although H2 addition to Ir(I) is as favorable as addition to Os(0), addition of a second molecule of H2 (to give tetrahydrides) is much more favorable for (tBu 4PNP)Os. NBO analysis indicates that the MH2/MH4 additions are oxidative, whereas the M/MH2 transformations are reductive.

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