Synthesis, characterization, and reactivity of Fe complexes containing cyclic diazadiphosphine ligands: The role of the pendant base in heterolytic cleavage of H 2

Tianbiao Liu, Shentan Chen, Molly J. O'Hagan, Mary Rakowski Dubois, R Morris Bullock, Daniel L DuBois

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The iron complexes CpFe(P Ph 2N Bn 2)Cl (1-Cl), CpFe(P Ph 2N Ph 2)Cl (2-Cl), and CpFe(P Ph 2C 5)Cl (3-Cl)(where P Ph 2N Bn 2 is 1,5-dibenzyl-1,5-diaza-3,7-diphenyl-3,7-diphosphacyclooctane, P Ph 2N Ph 2 is 1,3,5,7-tetraphenyl-1,5- diaza-3,7-diphosphacyclooctane, and P Ph 2C 5 is 1,4-diphenyl-1,4-diphosphacycloheptane) have been synthesized and characterized by NMR spectroscopy, electrochemical studies, and X-ray diffraction. These chloride derivatives are readily converted to the corresponding hydride complexes [CpFe(P Ph 2N Bn 2)H (1-H), CpFe(P Ph 2N Ph 2)H (2-H), CpFe(P Ph 2C 5)H (3-H)] and H 2 complexes [CpFe(P Ph 2N Bn 2)(H 2)]BAr F 4, [1-H 2]BAr F 4, (where BAr F 4 is B[(3,5-(CF 3) 2C 6H 3) 4] -), [CpFe(P Ph 2N Ph 2)(H 2)]BAr F 4, [2-H 2]BAr F 4, and [CpFe(P Ph 2C 5)(H 2)]BAr F 4, [3-H 2]BAr F 4, as well as [CpFe(P Ph 2N Bn 2)(CO)]BAr F 4, [1-CO]Cl. Structural studies are reported for [1-H 2]BAr F 4, 1-H, 2-H, and [1-CO]Cl. The conformations adopted by the chelate rings of the P Ph 2N Bn 2 ligand in the different complexes are determined by attractive or repulsive interactions between the sixth ligand of these pseudo-octahedral complexes and the pendant N atom of the ring adjacent to the sixth ligand. An example of an attractive interaction is the observation that the distance between the N atom of the pendant amine and the C atom of the coordinated CO ligand for [1-CO]BAr F 4 is 2.848 Å, considerably shorter than the sum of the van der Waals radii of N and C atoms. Studies of H/D exchange by the complexes [1-H 2] +, [2-H 2] +, and [3-H 2] + carried out using H 2 and D 2 indicate that the relatively rapid H/D exchange observed for [1-H 2] + and [2-H 2] + compared to [3-H 2] + is consistent with intramolecular heterolytic cleavage of H 2 mediated by the pendant amine. Computational studies indicate a low barrier for heterolytic cleavage of H 2. These mononuclear Fe II dihydrogen complexes containing pendant amines in the ligands mimic crucial features of the distal Fe site of the active site of the [FeFe]-hydrogenase required for H-H bond formation and cleavage.

Original languageEnglish
Pages (from-to)6257-6272
Number of pages16
JournalJournal of the American Chemical Society
Issue number14
Publication statusPublished - Apr 11 2012


ASJC Scopus subject areas

  • Chemistry(all)
  • Catalysis
  • Biochemistry
  • Colloid and Surface Chemistry

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