Reported herein are new Fe bis-alkynyl complexes [Fe III (L)-(C 2 R) 2 ]BPh 4 based on tetraimine macrocycle (

A pair of novel Fe-alkynyl complexes, [Fe III (HMTI)(C 2 SiEt 3 ) 2 ]ClO 4 (2) and [Fe II (HMTI)(C 2 SiEt 3 )(NCCH 3 )]ClO 4 (3), is described herein. Reaction of Fe(meso-HMC)Cl(ClO 4 )]ClO 4 (1) with lithiated triethylsilylacetylene and subsequent exposure to oxygen yielded the bis-alkynyl 2 containing the dehydrogenated tetraimine macrocycle (HMTI). Reduction of 2 by mossy zinc in acetonitrile yielded the mono-alkynyl 3. The structures of 2 and 3 were determined using single-crystal X-ray diffraction. Analysis of visible absorption and electrochemical data establishes the redox-active nature of the HMTI macrocycle, and indicates significant interactions between the Fe dπ and tetraimine π* orbitals. These deductions are further supported by density functional theory calculations.

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.

tetraazacyclotetradecane)iron(III) chloride, [FeCl 2 (C 16 H 36 N 4 )]Cl or cis-[FeCl 2 (rac-HMC)]Cl (1), and dichlorido (5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane)iron(III) tetrachloridoferrate, [FeCl 2 (C 16 H 36 N 4 )][FeCl 4 ] or trans-[FeCl 2 (meso-HMC)][FeCl 4 ] (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) .

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.