Nanosecond Metal-to-Ligand Charge-Transfer State in an Fe(II) Chromophore: Lifetime Enhancement via Nested Potentials.

Examples of Fe complexes with long-lived (≥1 ns) charge-transfer states are limited to pseudo-octahedral geometries with strong σ-donor chelates. Alternative strategies based on varying both coordination motifs and ligand donicity are highly desirable. Reported herein is an air-stable, tetragonal FeII complex, Fe(HMTI)(CN)2 (HMTI = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene), with a 1.25 ns metal-to-ligand charge-transfer (MLCT) lifetime. The structure has been determined, and the photophysical properties have been examined in a variety of solvents. The HMTI ligand is highly π-acidic due to low-lying π*(C═N), which enhances ΔFe via stabilizing t2g orbitals. The inflexible geometry of the macrocycle results in short Fe-N bonds, and density functional theory calculations show that this rigidity results in an unusual set of nested potential energy surfaces. Moreover, the lifetime and energy of the MLCT state depends strongly on the solvent environment. This dependence is caused by modulation of the axial ligand-field strength by Lewis acid-base interactions between the solvent and the cyano ligands. This work represents the first example of a long-lived charge transfer state in an FeII macrocyclic species.

Enabling Valence Delocalization in Iron(III) Macrocyclic Complexes through Ring Unsaturation.

The complexes [FeIII(HMC)(C2DMA)2]CF3SO3 ([2]OTf) and [FeIII(HMTI)(C2Y)2]CF3SO3 ([3a-c]OTf) have been prepared and thoroughly characterized (HMC = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; HMTI = 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene; Y = Fc (ferrocenyl, [3a]OTf), 4-(N,N-dimethyl)anilino (DMA, [3b]OTf), or 4-(N,N-bis(4-methoxyphenyl)anilino (TPA, [3c]OTf); OTf- = CF3SO3-)). Vibrational and electronic absorption spectroelectrochemical analyses following one-electron oxidation of the ethynyl substituent Y revealed evidence of strong coupling in the resultant mixed valent species for all HMTI-based complexes. However, the analogous mixed valent ion based on [2]OTf appeared to be more localized. Thus, the tetra-imino macrocycle HMTI has enabled significant valence delocalization along the -C2-FeIII-C2- bridge. Electron paramagnetic resonance and Mössbauer spectroscopic studies of [3b]OTf reveal that the π-acidity of HMTI lowers the energy of the FeIII dπ orbitals compared to the purely σ-donating HMC. This observation provides a basis for the interpretation of the macrocycle-dependent valence (de)localization.

Bis-Alkynyl Complexes of Fe(III) Tetraaza Macrocycles─A Tale of Two Rings.

Reported herein are new Fe bis-alkynyl complexes [FeIII(L)(C2R)2]BPh4 based on tetraimine macrocycle (L = HMTI = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradeca-1,3,8,10-tetraene; 1a-1c; R = C6H5 (a), C10H9 (b), SiMe3 (c)) and tetraamine macrocycle (L = HMC = meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; 2a-2c). These complexes have been characterized using single-crystal X-ray diffraction, electronic absorption spectroscopy, and cyclic and differential pulse voltammetry. Spectroelectrochemical studies of 1a and 2a allowed for investigation of the FeII oxidation state, which revealed a strong dependence on the nature of the macrocycle for both the energies of the FeII to C2Ph metal-to-ligand charge transfer (MLCT) and the ν(C≡C). The ν(C≡C) was further influenced by the oxidation state, though sensitivity to the formal metal oxidation state was much higher in the case of 2a than in 1a. These findings are rationalized on the basis of the relative energies of the formally metal-centered orbitals via density functional theory calculations.

Phenylene as an efficient mediator for intermetallic electronic coupling.

The new compound [(NC)Ru2(ap)4]2(µ-1,4-C6H4) (ap = 2-anilinopyridinate) was prepared to address the open question of whether a 1,4-phenylene bridge can mediate intermetallic electronic coupling. As a manifestation of strong coupling, hole delocalization between the Ru2 centers on the IR time scale (10-14 s) was established using spectroelectrochemistry. An orbital mechanism for coupling was elaborated with DFT analysis.

Geometric isomers of dichloridoiron(III) complexes of CTMC (5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradecane).

Both trans and cis iron-CTMC complexes, namely, trans-dichlorido[(5SR,7RS,12RS,14SR)-5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradecane]iron(III) tetrachloridoferrate, [Fe(C14H32N4)Cl2][FeCl4] (1a), the analogous chloride methanol monosolvate, [Fe(C14H32N4)Cl2]Cl·CH3OH (1b), and cis-dichlorido[(5SR,7RS,12SR,14RS)-5,7,12,14-tetramethyl-1,4,8,11-tetraazacyclotetradecane]iron(III) chloride, [Fe(C14H32N4)Cl2]Cl (2), were successfully synthesized and structurally characterized using X-ray diffraction. The coordination geometry of the macrocycle is dependent on the stereoisomerism of CTMC. The packing of these complexes appears to be strongly influenced by extensive hydrogen-bonding interactions, which are in turn determined by the nature of the counter-anions (1a versus 1b) and/or the coordination geometry of the macrocycle (1a/1b versus 2). These observations are extended to related ferric cis- and trans-dichloro macrocyclic complexes.

Spectroelectrochemical and Computational Analysis of a Series of Cycloaddition-Retroelectrocyclization-Derived Donor-Acceptor Chromophores.

The [2+2] cyclcoaddition (CA) and subsequent retroelectrocyclization (RE) reactions are useful in constructing nonplanar donor-acceptor chromophores that exhibit nonlinear optical properties and intramolecular charge-transfer transitions. However, both the infrared (IR) and visible-near IR (vis-NIR) spectroelectrochemical responses of CA-RE-derived chromophores are rarely explored in depth. Reported in this contribution is a comprehensive IR and vis-NIR spectroelectrochemical study of the CA-RE adducts of DMAP-C2n-NAPiPr of both tetracyanoethene (TCNE) and tetracyanoquinodimethane (TCNQ) and companion time-dependent density functional theory (TD-DFT) analysis of the bands observed. Specifically, DMAP-C2n-NAPiPr (1a, n = 1; 1b n = 2; DMAP = N,N-dimethylaniline; NAPiPr = N-isopropyl-1,8-naphthalimide) react with TCNE to yield the tetracyanobutadiene (TCBD) derivatives (2a and 2b, respectively) and with TCNQ to yield the dicyanoquinodimethane (DCNQ) derivatives (3a and 3b, respectively). IR spectroelectrochemical studies showed the emergence/intensification of new CN stretches upon reductions. Ultraviolet-vis-NIR (UV-vis-NIR) spectroelectrochemical study of 3 revealed a partial bleach of the charge-transfer (CT) bands, originally appearing in the neutral species, and the emergence of new CT bands originating from NAPiPr to the reduced DCNQ moiety. UV-vis-NIR spectroelectrochemical study of 2, surprisingly, indicated a very minimal change upon reductions. Dynamic changes were observed in the mid-IR absorption for C≡C and C≡N for both 2 and 3, indicative of enhanced asymmetry and the formation of ion pairs on the dicyano bridge. DFT and TD-DFT analyses were used to obtain the semi-quantitative pictures of the frontier orbitals of 1-3 and elucidate the origin of the transient features observed spectroelectrochemically for the 1e- and 2e- reduced species.

Crystal structures of two (5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane)iron(III) complexes.

Reported in this contribution are the synthesis and crystal structures of two new FeIII complexes of 5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane (HMC), namely, dichlorido(5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane)iron(III) chloride, [FeCl2(C16H36N4)]Cl or cis-[FeCl2(rac-HMC)]Cl (1), and dichlorido(5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane)iron(III) tetrachloridoferrate, [FeCl2(C16H36N4)][FeCl4] or trans-[FeCl2(meso-HMC)][FeCl4] (2). Single-crystal X-ray diffraction studies revealed that both 1 and 2 adopt a pseudo-octahedral geometry, where the macrocycles adopt folded and planar geometries, respectively. The chloride ligands in 1 are cis to each other, while those in 2 have a trans configuration. The relevant bond angles in 1 deviate substantially from an ideal octahedral coordination geometry, with the angles between the cis substituents varying from 81.55 (5) to 107.56 (4)°, and those between the trans-ligating atoms varying from 157.76 (8) to 170.88 (3)°. In contrast, 2 adopts a less strained configuration, in which the N-Fe-N angles vary from 84.61 (8) to 95.39 (8)° and the N-Fe-Cl angles vary from 86.02 (5) to 93.98 (5)°.