Synthesis and characterization of a family of M²⁺ complexes supported by a trianionic ONO³⁻ pincer-type ligand: towards the stabilization of high-spin square-planar complexes

Abstract

High-spin square-planar molecular compounds are rare. In an effort to access this unique combination of geometry and spin state, we report the synthesis of a series of M(II) compounds stabilized by a trianionic pincer-type ligand, highlighting the formation of a high-spin square-planar Co(II) complex. Low-temperature, variable-frequency EPR measurements reveal that the ground electronic state of the Co(II) analogue is a highly anisotropic Kramers doublet (effective g values 7.35, 2.51, 1.48). This doublet can be identified with the lowest doublet of a quartet, S = 3/2 spin state that exhibits a very large ZFS, D ≥ 50 cm(-1). The observation of an effective g value considerably greater than the largest spin-only value 6, demonstrates that the orbital angular moment is essentially unquenched along one spatial direction. Density Functional Theory (DFT) and time-dependent DFT calculations reveal the electronic configurations of the ground and excited orbital states. A qualitative crystal field description of the geff tensor shows that it originates from the spin-orbit coupling acting on states obtained through the transfer of a β electron from the doubly occupied xy to the singly-occupied {xz/yz} orbitals.

Solid State Collapse of a High-Spin Square-Planar Fe(II) Complex, Solution Phase Dynamics, and Electronic Structure Characterization of an Fe(II)2 Dimer

“…Contributing to this exclusive list, we reported the synthesis and characterization of the high-spin square-planar Fe­(II) trianionic pincer complex {[CF 3 -ONO]­FeCl}­{Li­(Sv) 2 } 2 ( 2 ; Sv = THF, Et 2 O). , Complex 2 breaks the mold of FeO 4 square-planar S = 2 cores. Instead for 2 the core comprises an FeO 2 NCl with a square plane (τ 4 = 0.06) consisting of a trianionic [CF 3 -ONO] 3– pincer-type ligand ,− and a chloride.…”

“…A 1 H NMR spectrum of 3 in C 6 D 6 exhibits eight paramagnetic resonances in the −25 to 40 ppm range and bears a striking resemblance to the spectrum of 2. 19,20 Confirmation of the identity of complex 3 in the solid state comes from elemental analysis and its single crystal X-ray structure.…”

Reactions of Late First‐Row Transition Metal (Fe‐Zn) Dichlorides with a PGeP Pincer Germylene

“…As spin crossovers are driven by molecular changes, a very small stretch of some metal–ligand bond lengths provokes in this case a (subtle) weakening of the ligand field that favors the S 3/2 spin state at increasing temperatures. For cobalt(II), both TD and LS SQ complexes are well represented in the literature, [27] but HS SQ complexes are rare [23b, 28] …”

Trianionic Pincer Complexes of Niobium and Tantalum as Precatalysts for ROMP of Norbornene

“…Among the numerous trianionic pincer and pincer-type ligands known, the [CF 3 –ONO] 3– ligand, in particular, engenders unique reactivity to metal complexes. The combination of weakly basic fluoro alkoxides and a π-donating amido donor within the [CF 3 –ONO] 3– motif generates metal complexes with interesting structure/bonding relationships − and reactivity. ,, Specific to metal–carbon multiple bonds, the lone pair of electrons on the central amido donor within the [CF 3 –ONO] 3– ligand can align with adjacent M = C or MC π-bonds, thereby enhancing their nucleophilicity via the inorganic enamine effect. ,, …”