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Ligand field influence on the electronic and magnetic properties of quasi-linear two-coordinate iron(II) complexes†‡ Nicholas F. Chilton,*a Hao Lei,b Aimee M. Bryan,b Fernande Grandjean,c Gary J. Long*c and Philip P. Power*b The 2 to 300 K magnetic susceptibilities of Fe{N(SiMe2Ph)2}2, 1, Fe{N(SiMePh2)2}2, 2, and the diaryl i
i
complex Fe(ArPr 4)2, 3, where ArPr 4 is C6H3-2,6(C6H3-2,6-Pri2)2 have been measured. Initial fits of these properties in the absence of an independent knowledge of their ligand field splitting have proven problematic. Ab initio calculations of the CASSCF/RASSI/SINGLE-ANISO type have indicated that the orbital energies of the complexes, as well as those of Fe(ArMe6)2, 4, where ArMe6 is C6H3-2,6(C6H2-2,4,6-Me3)2), are in the order dxy ≈ dx2−y2 < dxz ≈ dyz < dz2, and the iron(II) complexes in this ligand field have the (dxy, dx2−y2)3(dxz, dyz)2(dz2)1 ground electronic configuration with a substantial orbital contribution to their effective magnetic moments. An ab initio-derived ligand field and spin–orbit model is found to yield an excellent simulation of the observed magnetic properties of 1–3. The calculated ligand field strengths of these ligands are placed in the broader context of common coordination ligands in hypothetical twocoordinate linear iron(II) complexes. This yields the ordering I− < H− < Br− ≈ PMe3 < CH3− < Cl− ≈ i i C(SiMe3)3− < CN− ≈ SArPr 6− < ArPr 4− < ArMe6− ≈ N3− < NCS− ≈ NCSe− ≈ NCBH3− ≈ MeCN ≈ H2O ≈ NH3 < Received 28th April 2015, Accepted 15th May 2015
NO3− ≈ THF ≈ CO ≈ N(SiMe2Ph)2− ≈ N(SiMePh2)2− < F− ≈ N(H)ArPr 6− ≈ N(SiMe3)Dipp− < OArPr 4−. The i
i
DOI: 10.1039/c5dt01589h
magnetic susceptibility of the bridged dimer, [Fe{N(SiMe3)2}2]2, 5, has also been measured between 2 and 300 K and a fit of χMT with the isotropic Heisenberg Hamiltonian, Ĥ = −2JŜ1·Ŝ2 yields an antiferromagnetic
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exchange coupling constant, J, of −131(2) cm−1.
Introduction Stable, crystalline, two-coordinate, open shell (d1–d 9) transition metal complexes have been known for almost three decades.1 Currently, about one hundred such complexes have been characterised but, despite their rarity, they have been shown to have a rich chemistry, which is continuing to develop rapidly.2,3 They are also of interest because their magnetic properties4–8 indicate an extensive orbital contribution to their magnetic moments.4 This extensive contribution occurs because, when two monodentate ligands are bonded in a a School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK. E-mail: [email protected] b Department of Chemistry, University of California at Davis, One Shields Avenue, Davis, California 95616, USA. E-mail: [email protected] c Department of Chemistry, Missouri University of Science and Technology, University of Missouri, Rolla, Missouri 65409-0010, USA. E-mail: [email protected] † Dedicated to the memory of Prof. Ken Wade FRS, esteemed colleague and scientist. ‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/ c5dt01589h
This journal is © The Royal Society of Chemistry 2015
linear fashion to the cation, the orbital contribution to the magnetic moment may be unquenched and yield an effective magnetic moment, µeff, that is larger than the spin-only moment. This unquenched contribution is especially apparent in iron(II)4–8 and cobalt(II)9,10 complexes in which either unquenched ground state or excited state orbital angular momenta are often observed. Although the first crystalline, two-coordinate iron(II) complexes were synthesised in the 1980s, their geometries were generally found to be non-linear, and their magnetic properties were not thoroughly investigated.11–13 It was only after a study4 of the magnetism of the linear, two-coordinate iron(II) alkyl complex, Fe{C(SiMe3)3}2,14,15 that the essentially unquenched nature of the orbital contribution to the moment was realised. This contribution was confirmed by magnetic studies5 on Fe(NBut2)2 and studies on the related linear coi i ordinated Fe{N(H)ArPr 6}2 complex,6 where ArPr 6 is C6H3-2,6(C6H2-2,4,6-Pri3)2), and the bent-geometry Fe{N(H)ArMe6}2 complex, where ArMe6 is C6H3-2,6-(C6H2-2,4,6-Me3)2),6 both of which indicated that bending the N–Fe–N coordination from 180° to ca. 141° resulted in a large reduction of µeff from 7.5 to
Dalton Trans.
View Article Online
Published on 18 May 2015. Downloaded by Nanyang Technological University on 27/05/2015 07:50:48.
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5.8µB at 300 K in a 1 T applied field, a reduction that is consistent with considerable quenching of the orbital magnetic moment.6 Currently, ca. 30 two-coordinate iron(II) complexes have been structurally characterised in the crystalline state,1 but only four have a strictly linear coordination geometry. These linear complexes have been found to have large orbital moments.4,6,7 A similar number of complexes display modest, i.e.,
Published on 18 May 2015. Downloaded by Nanyang Technological University on 27/05/2015 07:50:48.
PAPER
Cite this: DOI: 10.1039/c5dt01589h
View Journal
Ligand field influence on the electronic and magnetic properties of quasi-linear two-coordinate iron(II) complexes†‡ Nicholas F. Chilton,*a Hao Lei,b Aimee M. Bryan,b Fernande Grandjean,c Gary J. Long*c and Philip P. Power*b The 2 to 300 K magnetic susceptibilities of Fe{N(SiMe2Ph)2}2, 1, Fe{N(SiMePh2)2}2, 2, and the diaryl i
i
complex Fe(ArPr 4)2, 3, where ArPr 4 is C6H3-2,6(C6H3-2,6-Pri2)2 have been measured. Initial fits of these properties in the absence of an independent knowledge of their ligand field splitting have proven problematic. Ab initio calculations of the CASSCF/RASSI/SINGLE-ANISO type have indicated that the orbital energies of the complexes, as well as those of Fe(ArMe6)2, 4, where ArMe6 is C6H3-2,6(C6H2-2,4,6-Me3)2), are in the order dxy ≈ dx2−y2 < dxz ≈ dyz < dz2, and the iron(II) complexes in this ligand field have the (dxy, dx2−y2)3(dxz, dyz)2(dz2)1 ground electronic configuration with a substantial orbital contribution to their effective magnetic moments. An ab initio-derived ligand field and spin–orbit model is found to yield an excellent simulation of the observed magnetic properties of 1–3. The calculated ligand field strengths of these ligands are placed in the broader context of common coordination ligands in hypothetical twocoordinate linear iron(II) complexes. This yields the ordering I− < H− < Br− ≈ PMe3 < CH3− < Cl− ≈ i i C(SiMe3)3− < CN− ≈ SArPr 6− < ArPr 4− < ArMe6− ≈ N3− < NCS− ≈ NCSe− ≈ NCBH3− ≈ MeCN ≈ H2O ≈ NH3 < Received 28th April 2015, Accepted 15th May 2015
NO3− ≈ THF ≈ CO ≈ N(SiMe2Ph)2− ≈ N(SiMePh2)2− < F− ≈ N(H)ArPr 6− ≈ N(SiMe3)Dipp− < OArPr 4−. The i
i
DOI: 10.1039/c5dt01589h
magnetic susceptibility of the bridged dimer, [Fe{N(SiMe3)2}2]2, 5, has also been measured between 2 and 300 K and a fit of χMT with the isotropic Heisenberg Hamiltonian, Ĥ = −2JŜ1·Ŝ2 yields an antiferromagnetic
www.rsc.org/dalton
exchange coupling constant, J, of −131(2) cm−1.
Introduction Stable, crystalline, two-coordinate, open shell (d1–d 9) transition metal complexes have been known for almost three decades.1 Currently, about one hundred such complexes have been characterised but, despite their rarity, they have been shown to have a rich chemistry, which is continuing to develop rapidly.2,3 They are also of interest because their magnetic properties4–8 indicate an extensive orbital contribution to their magnetic moments.4 This extensive contribution occurs because, when two monodentate ligands are bonded in a a School of Chemistry, The University of Manchester, Oxford Road, Manchester M13 9PL, UK. E-mail: [email protected] b Department of Chemistry, University of California at Davis, One Shields Avenue, Davis, California 95616, USA. E-mail: [email protected] c Department of Chemistry, Missouri University of Science and Technology, University of Missouri, Rolla, Missouri 65409-0010, USA. E-mail: [email protected] † Dedicated to the memory of Prof. Ken Wade FRS, esteemed colleague and scientist. ‡ Electronic supplementary information (ESI) available. See DOI: 10.1039/ c5dt01589h
This journal is © The Royal Society of Chemistry 2015
linear fashion to the cation, the orbital contribution to the magnetic moment may be unquenched and yield an effective magnetic moment, µeff, that is larger than the spin-only moment. This unquenched contribution is especially apparent in iron(II)4–8 and cobalt(II)9,10 complexes in which either unquenched ground state or excited state orbital angular momenta are often observed. Although the first crystalline, two-coordinate iron(II) complexes were synthesised in the 1980s, their geometries were generally found to be non-linear, and their magnetic properties were not thoroughly investigated.11–13 It was only after a study4 of the magnetism of the linear, two-coordinate iron(II) alkyl complex, Fe{C(SiMe3)3}2,14,15 that the essentially unquenched nature of the orbital contribution to the moment was realised. This contribution was confirmed by magnetic studies5 on Fe(NBut2)2 and studies on the related linear coi i ordinated Fe{N(H)ArPr 6}2 complex,6 where ArPr 6 is C6H3-2,6(C6H2-2,4,6-Pri3)2), and the bent-geometry Fe{N(H)ArMe6}2 complex, where ArMe6 is C6H3-2,6-(C6H2-2,4,6-Me3)2),6 both of which indicated that bending the N–Fe–N coordination from 180° to ca. 141° resulted in a large reduction of µeff from 7.5 to
Dalton Trans.
View Article Online
Published on 18 May 2015. Downloaded by Nanyang Technological University on 27/05/2015 07:50:48.
Paper
5.8µB at 300 K in a 1 T applied field, a reduction that is consistent with considerable quenching of the orbital magnetic moment.6 Currently, ca. 30 two-coordinate iron(II) complexes have been structurally characterised in the crystalline state,1 but only four have a strictly linear coordination geometry. These linear complexes have been found to have large orbital moments.4,6,7 A similar number of complexes display modest, i.e.,
