Comprehensive Studies of Magnetic Transitions and Spin–Phonon Couplings in the Tetrahedral Cobalt Complex Co(AsPh₃)₂I₂

Abstract

A combination of inelastic neutron scattering (INS), far-IR magnetospectroscopy (FIRMS), and Raman magneto-spectroscopy (RaMS) has been used to comprehensively probe magnetic excitations in Co(AsPh 3 ) 2 I 2 (1), a reported singlemolecule magnet (SMM). With applied field, the magnetic zero-field splitting (ZFS) peak (2D′) shifts to higher energies in each spectroscopy. INS placed the ZFS peak at 54 cm −1 , as revealed by both variable-temperature (VT) and variable-magnetic-field data, giving results that agree well with those from both far-IR and Raman studies. Both FIRMS and RaMS also reveal the presence of multiple spin−phonon couplings as avoided crossings with neighboring phonons. Here, phonons refer to both intramolecular and lattice vibrations. The results constitute a rare case in which the spin− phonon couplings are observed with both Raman-active (g modes) and far-IR-active phonons (u modes; space group P2 1 /c, no. 14, Z = 4 for 1). These couplings are fit using a simple avoided crossing model with coupling constants of ca. 1−2 cm −1 . The combined spectroscopies accurately determine the magnetic excited level and the interaction of the magnetic excitation with phonon modes. Density functional theory (DFT) phonon calculations compare well with INS, allowing for the assignment of the modes and their symmetries. Electronic calculations elucidate the nature of ZFS in the complex. Features of different techniques to determine ZFS and other spin-Hamiltonian parameters in transition-metal complexes are summarized.

Incorporating Highly Anisotropic Four-Coordinate Co(II) Ions within One-Dimensional Coordination Chains

“…Complete active space self-consistent field (CASSCF) followed by second-order N-electron valence perturbation theory (NEVPT2) was carried out on model complexes with an active space of CAS­(7,5) to compute the spin-Hamiltonian parameters (Table S7, S8, SI). This methodology has been widely used to compute the spin-Hamiltonian (SH) parameters in various open-shell transition metal complexes. , NEVPT2 (CASSCF) computed D values are −29.6(−32.8) cm –1 and −31.6(−34.8) cm –1 for complexes 1 and 2 respectively. The computed E / D values are 0.05(0.06) and 0.04(0.04) for complexes 1 and 2 , respectively, at the NEVPT2­(CASSCF) level of theory (Table , Figure S9, SI).…”

Spectroscopic techniques to probe magnetic anisotropy and spin–phonon coupling in metal complexes

“…The ground state ϕ 1,2 are mostly M S = ±1/2 which are mixed with M S = ±3/2, as a result of x ≠ y in 1 (i.e., rhombic ZFS parameter E). In other transition metal complexes such as Co(AsPh 3 ) 2 I 2 35 not discussed in this article, the ϕ 1,2 and ϕ 3,4 levels are reversed of that in (a), as its D < 0. That is, ϕ 3,4 is the ground level, while ϕ 1,2 is the excited level.…”

“…[107][108][109][110][111][112]115 We have sought to probe spin-phonon coupling by spectroscopies. 31,33,35,53,54,124 This article discusses our recent efforts to use a combination of far-IR and Raman magneto-spectroscopies (FIRMS and RaMS, respectively), and inelastic neutron scattering (INS) to study magnetic anisotropies 31,33,35,40,[53][54][55][124][125][126][127] and spinphonon couplings in metal complexes, determining magnetic excited states and coupling constants. Sometimes, the combined use of FIRMS with high-field EPR (HFEPR) leads to the determination of spin-Hamiltonian parameters D, E and g for transition metal complexes.…”

“…30,[38][39][40][41] Sometimes, simulating HFEPR spectra or combining HFEPR and DC data fitting may measure larger ZFS, [42][43][44][45][46] although using the DC data fitting here may not give reliable ZFS parameters. Using FIRMS, 31,33,35,37,40,[47][48][49][50][51][52][53][54][55][56][57][58][59][60][61][62][63][64][65][66] frequency-domain-Fourier-transform-terahertz-EPR spectroscopy (FD-FT-THz-EPR), 54,67,68 and INS [69][70][71][72][73][74][75][76][77][78][79] to determine magnetic excited levels and anisotropy has been reported. It should be pointed out ...…”

2,2′-Diphosphino- and 2,2′-Diarsenotolanes and Their Fe-, Co-, and Ni-Complexes: Pnictogen-Dependent Cyclization Tendencies and Metal-Dependent Stability and Reactivity Patterns

“…Upon reaction between 1-As and CoBr 2 , a turquoise κ 2 - As , As -chelate complex ( 3-Co , λ max = 777, 680, and 649 nm) with an uncoordinated alkyne moiety was isolated (see Scheme ) and its structure was ascertained by single-crystal X-ray diffraction (see Figure ). As expected for the nearly tetrahedral cobalt ion in 3-Co (τ 4 = 0.87), an effective magnetic moment of μ eff = 4.58 μ B was determined in CD 2 Cl 2 (Evans method), indicative of the S = 3/2 ground state . This interpretation was confirmed by DFT studies, that is, the hypothetical doublet state ( S = 1/2) was found to be higher in energy by 19.8 kcal/mol in comparison to the quartet state (see the Supporting Information for details).…”