Singlet–Triplet Gaps of Cobalt Nitrosyls: Insights from Tropocoronand Complexes

Abstract

A DFT study of cobalt-nitrosyl [n,n]tropocoronand (TC-n,n) complexes has revealed a sharp reduction of singlet-triplet gaps as the structures change from near-square-pyramidal 10 (for n = 3) to trigonal-bipyramidal with an equatorial NO (for n = 5, 6). For n = 6, low-energy triplet states may result in enhanced reactivity, which would account for the failure to isolate [Co(TC-6,6)(NO)] as a stable species. reinvestigation of the molecule. The calculations also revealed a decreasing singlet-triplet gap and low-energy triplet states as the coordination geometry changed from the nearly SQP TC-3,3 complex to the TBP TC-5,5 and TC-6,6 complexes. Thermally accessible triplet states are a distinct possibility for 6,6)(NO)], which would explain its apparent instability. The lowest-energy singlet and triplet states of the [Co(TCn,n)(NO)] complexes (n = 3-6) were optimized with the B3LYP, OLYP, and PW91 functionals and the 6-311G(d,p) basis set. In general, the hybrid functional B3LYP led to a broken-symmetry 45 M S = 0 solution as the ground state, whereas the pure functionals Scheme 1. Dianion of an [n,n]tropocoronand (n,n-TC) ligand.PW91 and OLYP led to closed-shell ground states. Interestingly, 50 depending on the starting point of the optimizations, two distinct triplet states, denoted T 1 and T 2 in the discussion below, could be obtained. The T 1 state may be described as low-spin S = ½ Co(II) ferromagnetically coupled to a NO radical, and the T 2 state may as a high-spin S = 2 Co(III) antiferromagnetically coupled to an S 55 = 1 NO -anion. Table 1

Multireference Perturbation Theory with Cholesky Decomposition for the Density Matrix Renormalization Group

“…[Co(TC-3,3)(NO)], the smallest member of a homologous series of cobalt complexes with n = 3 to 5, was previously reported to be paramagnetic, unlike its higher homologues and other cobalt nitrosyl complexes, 76 , 78 which contradicts a recent DFT and experimental study. 79 Metal-nitrosyl bonds are known to show a large amount of static correlation (see, e.g., ref ( 86 )), which cannot be reliably described by single-reference methods such as DFT. Due to this fact and the controversy on the spin state, we study with DMRG-SCF and CD-DMRG-NEVPT2 the electronic structure and spin-state energetics of [Co(TC-3,3)(NO)].…”

Multireference Approaches to Spin‐State Energetics of Transition Metal Complexes Utilizing the Density Matrix Renormalization Group

“…published a spin‐state study of two mononuclear transition metal compounds, [Fe(C 3 N 2 H 5 ) 2 (H 2 O)] (complex 4 in Figure ) and a cobalt tropocoronand nitrosyl complex ([Co(TC‐3,3)(NO)], complex 5 in Figure ), to demonstrate the capabilities of a DMRG‐NEVPT2 implementation that utilizes the Cholesky decomposition to reduce the cost of integral evaluation . In both cases, they used separately optimized geometries available from previous works . On the basis of one‐and two‐orbital entropies (see above), active spaces of 14 electrons in 18 orbitals and 22 electrons in 22 orbitals were chosen for 4 and 5 , respectively.…”

Spin Changes Accompany Ultrafast Structural Interconversion in the Ground State of a Cobalt Nitrosyl Complex

“…The different electronic characters of the two structures require the exchange dynamics to be described by an adiabatic ground-state potential energy surface (PES) constructed from diabatic PESs for the TBP and SP isomers (Figure 1). [ Most cobalt nitrosyl complexes adopt either the bent Co-N-O (triplet) [6][7] or linear Co-N-O (singlet) structures at thermal equilibrium, [8] but in selected cases the two geometries are close enough in energy that both are accessible at ambient temperature. [6,9] The chosen complexes are such examples.…”

“…[4] TheT BP and SP geometries are associated with singlet and triplet states,respectively.Inthe SP complex, the {d yz ; p * y NO ðÞ }o rbital is stabilized through interactions with the other ligands,b ringing it close enough in energy to the {d xz ; sp 2 N ðÞ }MOthat atriplet spin-state becomes the ground electronic configuration. [6] Thee xistence of ap aramagnetic compound is supported by magnetic susceptibility and 31 P-{ 1 H} NMR signal broadening measurements described in Supporting Information (SI). Thed ifferent electronic characters of the two structures require the exchange dynamics to be described by an adiabatic ground-state potential energy surface (PES) constructed from diabatic PESs for the TBP and SP isomers ( Figure 1).…”

“…Thed ifferent electronic characters of the two structures require the exchange dynamics to be described by an adiabatic ground-state potential energy surface (PES) constructed from diabatic PESs for the TBP and SP isomers ( Figure 1). Most cobalt nitrosyl complexes adopt either the bent Co-N-O (triplet) [6,7] or linear Co-N-O (singlet) structures at thermal equilibrium, [8] but in selected cases the two geometries are close enough in energy that both are accessible at ambient temperature. [6,9] Thec hosen complexes are such examples.Steady state IR absorption spectroscopy in the N À Os tretching region differentiates between the two forms of the complex, with the NO band of the triplet complex shifted to lower wavenumber because greater localization of the p*e lectron density on the nitrosyl ligand gives doublebonded NO À character.…”