Linear Tetranickel String Complexes with Mixed Supported Ligands and Mixed‐Valence Units [Ni₂]³⁺: Synthesis, Crystal Structure, and Magnetic Studies

Abstract

The synthesis, crystal structures, and magnetic properties of linear tetranickel string complexes supported by mixed 2‐(α‐pyridylamino)‐1,8‐naphthyridine (Hpyany) and N‐(p‐tolylsulfonyl)dipyridyldiamine (H2tsdpda) ligands are reported. In comparing the crystal structure of [Ni4(pyany)2(tsdpda)2Cl] (1) with that of [Ni4(pyany)2(tsdpda)2Cl(H2O)](PF6) (2), the one‐electron‐reduced compound 1 displays shorter Ni(3)–Ni(4) (ca. 2.28 Å) and longer Ni(3)–N (ca. 2.02 Å) bond lengths. Similar trends have also been observed for axial NCS–‐substituted derivatives [Ni4(pyany)2(tsdpda)2(NCS)] (3) and [Ni4(pyany)2(tsdpda)2(NCS)2] (4). These structural variations indicate the formation of a mixed‐valence [Ni2]3+ unit and a three‐electron, two‐center Ni(4)–Ni(3) σ bond. Magnetic measurements of 2 and 4 show that both terminal Ni(1) and Ni(4) ions are in the high‐spin states (S = 1) and are antiferromagnetically coupled. The one‐electron‐reduced complexes 1 and 3, however, exhibit a delocalized mixed‐valence [Ni2]3+ unit (S = 3/2), which is antiferromagnetically coupled with the terminal high‐spin NiII ion.

Determination of the Ni–Ni Bonding Strength in Metal-String Complexes Using Head-to-Head Nanorods and Electrochemical Surface-Enhanced Raman Spectroscopy

“…17 The calculated outcomes using these methods failed in obtaining correct energies of MOs for 18 This results in three electrons, two metal center bonds (Figure 6), and a short Ni−Ni bond. 27 The vibrational wavenumber for ν Ni−Ni is 327 cm −1 , a stronger bond than in [Ni 2 ] 5+ ; this may be explained that the extra two electrons filling the M−L σ* orbitals (Figure 6 Limited by the computational capacity, method B3LYP/ LanL2DZ(Ni), 3-21G was used to calculate the energies of MOs in [Ni 3 (dpa) 4 Cl 2 ] 1−,0,1+ and TD-DFT to obtain the vertical transitions.…”

“…Hence, [Ni 2 ] 3+ is a delocalized mixed valence Ni + and Ni 2+ that three unpaired electrons occupy the M–L σ* and σ* orbitals . This results in three electrons, two metal center bonds (Figure ), and a short Ni–Ni bond . The vibrational wavenumber for ν Ni–Ni is 327 cm –1 , a stronger bond than in [Ni 2 ] 5+ ; this may be explained that the extra two electrons filling the M–L σ* orbitals (Figure ), intrinsically antiorbitals in complex, result in weakening Ni–ligand bonding which results in strengthening the Ni–Ni bond.…”