Doublet Ground State in a Vanadium(II) Complex: Redox and Coordinative Noninnocence of Tripodal Ligand Architecture.

We report on the geometric and electronic structures of a series of V2+/3+ tren-bridged iminopyridine complexes [tren = tris(2-aminoethyl)amine] that enable the observation of an unexpected doublet ground state for a nominally 3d3 species. Tren undergoes condensation reactions with picolinaldehyde or methyl-6-formylnictonate to form the respective tripodal ligand sets of (py)3tren and (5-CO2Mepy)3tren. The (py)3tren ligand is coordinated to V2+ and V3+ metal centers to form complex salts [1-H](OTf)2 and [1-H](OTf)3, respectively (OTf- = CF3SO3-). For [1-H]2+, strong metal-ligand π-covalency with respect to the V2+ (3d3) and iminopyridine ligands weakens its interelectronic repulsion. For [1-H]3+, the bridgehead nitrogen of the tren scaffold forms a seventh coordinate covalent bond with a V3+ (3d2) metal center. The coordination of (5-CO2Mepy)3tren to a V2+ metal center results in the redox noninnocent and heptacoordinate compound [1-CO2Me](OTf)2 with a doublet (S = 1/2) ground state that we support with magnetic susceptibility and spectroscopy measurements. The complexes are uniformly characterized experimentally with single-crystal X-ray diffraction, electronic absorbance, and electrochemistry, and electronic structures are corroborated by computational techniques. We present a new computational procedure that we term the spin-optimized approximate pair (SOAP) method that enables the visualization and quantification of electron-electron interactions.

Electronic Structures of Cr(III) and V(II) Polypyridyl Systems: Undertones in an Isoelectronic Analogy.

A recently reported description of the photophysical properties of V2+ polypyridyl systems has highlighted several distinctions between isoelectronic, d3, Cr3+, and V2+ tris-homoleptic polypyridyl complexes of 2,2'-bipyridine (bpy) and 1,10-phenanthroline (phen). Here, we combine theory and experimental data to elucidate the differences in electronic structures. We provide the first crystallographic structures of the V2+ complexes [V(bpy)3](BPh4)2 (V-1B) and [V(phen)3](OTf)2 (V2) and observe pronounced trigonal distortion relative to analogous Cr3+ complexes. We use electronic absorption spectroscopy in tandem with TD-DFT computations to assign metal-ligand charge transfer (MLCT) properties of V-1B and V2 that are unique from the intraligand transitions, 4(3IL), solely observed in Cr3+ analogues. Our newly developed natural transition spin density (NTρα,ß) plots characterize both the Cr3+ and V2+ absorbance properties. A multideterminant approach to DFT assigns the energy of the 2E state of V-1B as stabilized through electron delocalization. We find that the profound differences in excited state lifetimes for Cr3+ and V2+ polypyridyls arise from differences in the characters of their lowest doublet states and pathways for intersystem crossing, both of which stem from trigonal structural distortion and metal-ligand π-covalency.

Long-Lived Mixed 2MLCT/MC States in Antiferromagnetically Coupled d3 Vanadium(II) Bipyridine and Phenanthroline Complexes.

Exploration of [V(bpy)3]2+ and [V(phen)3]2+ (bpy = 2,2'-bipyridine; phen = 1,10-phenanthroline) using electronic spectroscopy reveals an ultrafast excited-state decay process and implicates a pair of low-lying doublets with mixed metal-to-ligand charge-transfer (MLCT) and metal-centered (MC) character. Transient absorption (TA) studies of the vanadium(II) species probing in the visible and near-IR, in combination with spectroelectrochemical techniques and computational chemistry, lead to the conclusion that after excitation into the intense and broad visible 4MLCT ← 4GS (ground-state) absorption band (ε400-700 nm = 900-8000 M-1 cm-1), the 4MLCT state rapidly (τisc < 200 fs) relaxes to the upper of two doublet states with mixed MLCT/MC character. Electronic interconversion (τ ∼ 2.5-3 ps) to the long-lived excited state follows, which we attribute to formation of the lower mixed state. Following these initial dynamics, GS recovery ensues with τ = 430 ps and 1.6 ns for [V(bpy)3]2+ and [V(phen)3]2+, respectively. This stands in stark contrast with isoelectronic [Cr(bpy)3]3+, which rapidly forms a long-lived doublet metal-centered (2MC) state following photoexcitation and lacks strong visible GS absorption character. 2MLCT character in the long-lived states of the vanadium(II) species produces geometric distortion and energetic stabilization, both of which accelerate nonradiative decay to the GS compared to [Cr(bpy)3]3+, where the GS and 2MC are well nested. These conclusions are significant because (i) long-lived states with MLCT character are rare in first-row transition-metal complexes and (ii) the presence of a 2MLCT state at lower energy than the 4MLCT state has not been previously considered. The spin assignment of charge-transfer states in open-shell transition-metal complexes is not trivial; when metal-ligand interaction is strong, low-spin states must be carefully considered when assessing reactivity and decay from electronic excited states.