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Synthesis and Coordination Chemistry of the Novel Ligand N,N’-bis(2-pyridylmethyl)-bis(ethylacetate)-1,2-ethanediamine (debpn)

Qiao Zhang and Christian R. Goldsmith*Department of Chemistry and Biochemistry, Auburn University, AL 36849

Abstract

Heptacoordinate transition metal complexes are rare, due to the shortage of ligands that can enforce the unusual geometries associated with this coordination number. These compounds can potentially provide insights into associative ligand exchange mechanisms and enable modes of reactivity that are distinct from those seen for lower-coordinate analogs. We report the novel ligand N,N’-bis(2-pyridylmethyl)-bis(ethylacetate)-1,2-ethanediamine (debpn) and its complexes with first-row transition metal ions. With Mn(II), Fe(II), and Zn(II), debpn is hexadentate, binding through all four N-donors and the two esters. With the larger metal ions, debpn enables seven-coordinate geometries, with an exogenous water molecule completing the coordination.

Introduction

Ligand Synthesis

Syntheses of Metal Complexes

• MnII(ClO4)2(H2O) + debpn = [MnII(debpn)(H2O)](ClO4)2 52% yield

• FeII(OTf)2(MeCN)2 + debpn = [FeII(debpn)(H2O)](OTf)2 85% yield

• CoII(ClO4)2(H2O)6 + debpn = [CoII(debpn)(MeCN)](ClO4)2 42% yield

• NiII(ClO4)2(H2O) + debpn = [NiII(debpn)(MeCN)](ClO4)2 56% yield

• ZnII(ClO4)2(H2O) + debpn = [ZnII(debpn)](ClO4)2 69% yield

• Metal salt and debpn dissolved in acetonitrile (MeCN), stirred at 1 h under N2 at 295 K

• Crystallized from MeCN/EtherCrystal Structures

Crystal Structures (Continued) UV-Vis Spectra of Metal Complexes

Acknowledgements

Contact Information

Spectroscopy

Dr. John Gorden (crystallography)Prof. Eduardus Duin (EPR) Dr. Michael Meadows (NMR)

Funding

Auburn UniversityACS-Petroleum Research Fund

Qiao ZhangDepartment of Chemistry and Biochemistry

Auburn University, AL 36849Tel: (334) 844-7065

Email: qiao.zhang@auburn.edu

Prof. Christian R. GoldsmithDepartment of Chemistry and Biochemistry

Auburn University, AL 36849Tel: (334) 844-6463

Email: crgoldsmith@auburn.edu

In the overwhelming majority of first-row transition metal complexes, the metal ions are found in low coordinate geometries due to their relatively small ionic radii. Heptacoordinate complexes are observed, fleetingly, as intermediates in associative ligand exchange reactions. Stable seven-coordinate complexes can be prepared using polydentate ligands with a predisposition to chelate metal ions in unusual geometries (Figure 1). Higher-coordinate metal complexes have been investigated as potential catalysts for redox reactions and as contrast agents for magnetic resonance imaging. The novel ligand N,N’-bis(2-pyridylmethyl)-bis(ethylacetate)-1,2-ethanediamine (debpn) is neutrally charged and potentially hexadentate, binding metal ions through two amines, two pyridine rings, and two esters. The esters are anticipated to bind metals more weakly and have been investigated as substrate-docking sites in oxidants derived from the Mn(II) and Fe(II) compounds reported here.

(a) (b) (c)

Figure 1: Possible geometries of heptacoordinate metal centers: (a) pentagonal bipyramid, (b) mono-capped octahedron, (c) monocapped trigonal prism

H2N NH2

Br

O

O

KI, K2CO3,CH3CN RT 1d

2eq.

N

O

H2eq.

CH3OH RT 1h

NNNN

HNNHNN

NNNN

2eq. NaBH4

CH3OH RT overnight

NNNN

HNNHNN

O

O

O

O

debpn (31% yield)

• debpn prepared from bispicen in one step• Purified through column chromatography

Scheme 1: Ligand synthesis

• Mn(II) is heptacoordinate (pentagonal bipyramidal)

• Monoclinic• Space group: C2/c• a (Ǻ) = 13.5732(10)• b (Ǻ) = 9.5786(10) • c (Ǻ) = 22.716(2) • α (deg) = 90• β (deg) = 99.247(3)• γ (deg) = 90

• Co(II) is hexacoordinate (octahedral)

• Ligand is pentadentate• Triclinic• Space group: P-1• a (Ǻ) = 10.2905(7)• b (Ǻ) = 12.8039(9)• c (Ǻ) = 13.5556(9)• α (deg) = 72.2300(10)• β (deg) = 70.596(2)• γ (deg) = 73.3580(10)

• Ni(II) is hexacoordinate (octahedral)

• Ligand is pentadentate• Triclinic• Space group: P-1• a (Ǻ) = 10.3714(7)• b (Ǻ) = 12.6946(9)• c (Ǻ) = 13.5156(9)• α (deg) = 71.807(2)• β (deg) = 70.823(2)• γ (deg) = 73.280(2)

• Zn(II) is hexacoordinate (octahedral)

• Ligand is hexadentate• Monoclinic• Space group: C2/c• a (Ǻ) = 15.7195(14) • b (Ǻ) = 13.5426(12) • c (Ǻ) = 13.0717(12)• α (deg) = 90• β (deg) = 100.910(2)• γ (deg) = 90

• Mn(II) and Fe(II) complexes are seven-coordinate, with exogenous H2O bound to metal

• Co(II) and Ni(II) complexes are six-coordinate, with MeCN bound as exogenous ligand

• With larger metal ions, a predisposition towards pentagonal bipyramidal geometries

• Fe(II) is heptacoordinate (pentagonal bipyramidal)

• Orthorhombic• Space group: Fdd2• a (Ǻ) = 13.983(2)• b (Ǻ) = 47.723(8) • c (Ǻ) = 9.4858(15)• α (deg) = 90• β (deg) = 90• γ (deg) = 90

Figure 8:• Blue line: 0.4 mM Fe(II)

complex, ε = 1000 cm-1M-1 at 343 nm

• Red line: 1.5eq. Peracetic acid added, ε = 100 cm-1M-1 at 750 nm

Figure 9:• Beer’s Law work of Co(II)

complex, c = 0.5 mM (highest one)

• ε = 46 cm-1M-1 at 475 nm

Figure 10:• Beer’s Law work of Ni(II)

complex, c = 1.6 mM (highest one)

• ε = 16 cm-1M-1 at 550 nm and ε = 17 cm-1M-1 at 890 nm

Figure 1: [Mn(debpn)H2O)]2+

Figure 3: [Fe(debpn)(H2O)]2+

Figure 4: [Co(debpn)(MeCN)]2+

Figure 5: [Ni(debpn)(MeCN)]2+

Figure 6: [Zn(debpn)]2+