Molecular and Electronic Structures of Homoleptic Six-Coordinate Cobalt(I) Complexes of 2,2′:6′,2″-Terpyridine, 2,2′-Bipyridine, and 1,10-Phenanthroline. An Experimental and Computational Study

Abstract

The crystal structures of nine homoleptic, pseudooctahedral cobalt complexes, 1-9, containing either 2,2':6',2″-terpyridine (tpy), 4,4'-di-tert-butyl-2,2'-bipyridine ((t)bpy), or 1,10-phenanthroline (phen) ligands have been determined in three oxidation levels, namely, cobalt(III), cobalt(II), and, for the first time, the corresponding presumed cobalt(I) species. The intraligand bond distances in the complexes [Co(I)(tpy(0))2](+), [Co(I)((t)bpy(0))3](+), and [Co(I)(phen(0))3](+) are identical, within experimental error, not only with those in the corresponding trications and dications but also with the uncoordinated neutral ligands tpy(0), bpy(0), and phen(0). On this basis, a cobalt(I) oxidation state assignment can be inferred for the monocationic complexes. The trications are clearly low-spin Co(III) (S = 0) species, and the dicationic species [Co(II)(tpy(0))2](2+), [Co(II)((t)bpy(0))3](2+), and [Co(II)(phen(0))3](2+) contain high-spin (S = (3)/2) Co(II). Notably, the cobalt(I) complexes do not display any structural indication of significant metal-to-ligand (t2g → π*) π-back-donation effects. Consistent with this proposal, the temperature-dependent molar magnetic susceptibilities of the three cobalt(I) species have been recorded (3-300 K) and a common S = 1 ground state confirmed. In contrast to the corresponding electronic spectra of isoelectronic (and isostructural) [Ni(II)(tpy(0))2](2+), [Ni(II)(bpy(0))3](2+), and [Ni(II)(phen(0))3](2+), which display d → d bands with very small molar extinction coefficients (ε < 60 M(-1) cm(-1)), the spectra of the cobalt(I) species exhibit intense bands (ε > 10(3) M(-1) cm(-1)) in the visible and near-IR regions. Density functional theory (DFT) calculations using the B3LYP functional have validated the experimentally derived electronic structure assignments of the monocations as cobalt(I) complexes with minimal cobalt-to-ligand π-back-bonding. Similar calculations for the six-coordinate neutral complexes [Co(II)(tpy(•))2](0) and [Co(II)(bpy(•))2(bpy(0))](0) point to a common S = (3)/2 ground state, each possessing a central high-spin Co(II) ion and two π-radical anion ligands. In addition, the excited-states and ground state magnetic properties of [Co(I)(tpy(0))2][Co(I-)(CO)4] have been explored by variable-temperature variable-magnetic-field magnetic circular dichroism (MCD) spectroscopy. A series of strong signals associated with the paramagnetic monocation exhibit pronounced C-term behavior indicative of the presence of metal-to-ligand charge-transfer bands [in contrast to d-d transitions of the nickel(II) analogue]. Time-dependent DFT calculations have allowed assignment of these transitions as Co(3d) → π*(tpy) excitations. Metal-to-ligand charge-transfer states intermixing with the Co(d(8)) multiplets explain the remarkably large (and negative) zero-field-splitting parameter D obtained from SQUID and MCD measurements. Ground-state electron- and spin-density distributions of [Co(I)(tpy(0))2](+) have been investigated by multireference electronic structure methods: co...

Structure and Electronic Properties of an Expanded Terpyridine Complex of Nickel(II) [Ni(ddpd)2](BF4)2

“…Closely related pseudooctahedral nickel(II) complexes are [Ni(tpy) 2 ] 2+ ( 2 2+ ), [Ni(bipy) 3 ] 2+ ( 3 2+ ), and [Ni(phen) 3 ] 2+ ( 4 2+ ), (bipy = 2,2′‐bipyridine; phen = 1,10‐phenanthroline) (Scheme ).…”

A Redox‐Active Bridging Ligand to Promote Spin Delocalization, High‐Spin Complexes, and Magnetic Multi‐Switchability

“…This compound was characterized by single-crystal X-ray diffraction at 120 K ( Figure 1; Supporting Information, Table S1), confirming the envisioned structure with two crystallographically equivalent Co(tpy) 2+ units connected by tphz. [9] TheC o À N pyz bond distance (Co À N2) between Co and tphz is 1.94 while the other two CoÀ N py distances (CoÀN1 and CoÀN3) are significantly longer due to the rigidity of the bridging ligand (Table 1). Each Co ions reside in an octahedral environment involving three nitrogen atoms from the bridging ligand and another three nitrogen atoms from capping terpyridine.T he Co À Nb ond distances between the metal and tpy (Co À N4,N5,N6) range from 1.87 to 1.99 ,indicating aCo II center in its low spin (LS) state.…”

A Redox‐Active Bridging Ligand to Promote Spin Delocalization, High‐Spin Complexes, and Magnetic Multi‐Switchability

“…Each Co ions reside in an octahedral environment involving three nitrogen atoms from the bridging ligand and another three nitrogen atoms from capping terpyridine. The Co−N bond distances between the metal and tpy (Co−N4,N5,N6) range from 1.87 to 1.99 Å, indicating a Co II center in its low spin (LS) state . The Co−N pyz bond distance (Co−N2) between Co and tphz is 1.94 Å while the other two Co−N py distances (Co−N1 and Co−N3) are significantly longer due to the rigidity of the bridging ligand (Table ).…”