Tetranuclear Complexes of [Fe(CO)₂(C₅H₅)]⁺ with TCNX Ligands (TCNX = TCNE, TCNQ, TCNB):  Intramolecular Electron Transfer Alternatives in Compounds (μ₄-TCNX)[ML_n]₄

Abstract

The complexes {(mu4-TCNX)[Fe(CO)2(C5H5)]4}(BF4)4 were prepared as light-sensitive materials from [Fe(CO)2(C5H5) (THF)](BF4) and the corresponding TCNX ligands (TCNE = tetracyanoethene, TCNQ=7,7,8,8-tetracyano-p-quinodimethane, TCNB=1,2,4,5-tetracyanobenzene). Whereas the TCNE and TCNQ complexes are extremely easily reduced species with reduction potentials>+0.3 V vs ferrocenium/ferrocene, the tetranuclear complex of TCNB exhibits a significantly more negative reduction potential at about -1.0 V. Even for the complexes with strongly pi-accepting TCNE and TCNQ, the very positive reduction potentials, the unusually high nitrile stretching frequencies>2235 cm(-1), and the high-energy charge-transfer transitions indicate negligible metal-to-ligand electron transfer in the ground state, corresponding to a largely unperturbed (TCNX degrees)(FeII)4 formulation of oxidation states as caused by orthogonality between the metal-centered HOMO and the pi* LUMO of TCNX. Mössbauer spectroscopy confirms the low-spin iron(II) state, and DFT calculations suggest coplanar TCNE and TCNQ bridging ligands in the complex tetracations. One-electron reduction to the 3+ forms of the TCNE and TCNQ complexes produces EPR spectra which confirm the predominant ligand character of the then singly occupied MO through isotropic g values slightly below 2, in addition to a negligible g anisotropy of frozen solutions at frequencies up to 285 GHz and also through an unusually well-resolved solution X band EPR spectrum of {(mu4-TCNE)[Fe(CO)2(C5H5)]4}3+ which shows the presence of four equivalent [Fe(CO)2(C5H5)]+ moieties through 57Fe and 13C(CO) hyperfine coupling in nonenriched material. DFT calculations reproduce the experimental EPR data. A survey of discrete TCNE and TCNQ complexes [(mu4-TCNX)(MLn)4] exhibits a dichotomy between the systems {(mu4-TCNX)[Fe(CO)2(C5H5)]4}4+ and {(mu4-TCNQ)[Re(CO)3(bpy)]4}4+ with their negligible metal-to-ligand electron transfer and several other compounds of TCNE or TCNQ with Mn, Ru, Os, or Cu complex fragments which display evidence for a strong such interaction, i.e., an appreciable value delta in the formulation {(mu4-TCNXdelta-)[Mx+delta/4Ln]4}. Irreversibility of the first reduction of {(mu4-TCNB)[Fe(CO)2(C5H5)]4}(BF4)4 precluded spectroelectrochemical studies; however, the high-energy CN stretching frequencies and charge transfer absorptions of that TCNB analogue also confirm the exceptional position of the complexes {(mu4-TCNX)[Fe(CO)2(C5H5)]4}(BF4)4.

Synthesis and Characterization of Redox‐Active Charge‐Transfer Complexes with 2,3,5,6‐Tetracyanopyridine (TCNPy) for the Photogeneration of Pyridinium Radicals

“…They can coordinate in a π fashion or may also act as bridging ligands, thereby connecting up to four metal centers through σ bonds 2, 5, 6. In this context, TCNX ligands are either found to function as neutral π acceptors, as stable monoanionic radicals, or as dianions and, thus, can play an essential role as electron-transfer mediators in inorganic and organometallic chemistry 5, 6, 7, 8. In addition, the charge-transfer interactions of TCNX acceptor systems with various kinds of electron donors can lead to a wide variety of supramolecular binding motifs, including π—π stacking and the assembly of ionic layer structures 2, 9.…”

Variable coordination of redox-active TCNB in discrete and polymeric ferrocenylcopper(i) complexes: structures and spectroelectrochemical behaviour

“…Although infrared absorption bands from CN stretching vibrations can be informative for elucidating the electronic structures of electrogenerated intermediates, [2][3][4][5] especially for studies of metal-TCNX interaction in the ground state, 1 the technique was of only limited use in the present case. For complex 1, the IR band in the CN stretching region around 2000 cm −1 was not detectable when measured in the solid state or in dichloromethane solution.…”

“…The low intensity of certain ν CN absorption bands has been discussed before. [2][3][4][5] The limited stability of 1 after oxidation or reduction prevented us from obtaining reliable spectroelectrochemical data in the IR region. For complex 2 a very weak broad band at ν CN = 2244 cm −1 was observed when measured in KBr.…”

“…TCNE = tetracyanoethene, TCNQ = 7,7,8,8-tetracyanoquinodimethane, or TCNB = 1,2,4,5-tetracyanobenzene (Chart 1), are very useful and versatile ligands in coordination chemistry for σ/π-type coordination, due to their tendency to form polymeric networks utilizing various coordination modes, and due to exceptional electronic and magnetic properties. [1][2][3][4][5][6] The special properties of metal compounds with TCNX as ligands arise from the facile reduction of the TCNX molecules to radical (TCNX •− ) or dianionic (TCNX 2− ) forms. While TCNB is well known to get involved in π stacking arrangements, [6][7][8][9][10] it can also serve as a ligand in various ways, i.e.…”

“…The transition metal chemistry of TCNE and TCNQ is characterized by the σ-polarization and potentially strong π-back donation from d-electron rich transition metal centres to the very low lying π* molecular orbitals of TCNE or TCNQ. A dichotomy has been recognised 11 in that electron-rich and potentially redox-active metal fragments may 2 or may not 5 engage in ground state electron transfer when bound to TCNE or TCNQ, i.e., these ligands may exhibit innocent or non-innocent behaviour. 12 The aromatic and less strongly π accepting TCNB is not only a component of reactions leading to phthalocyanines; 13 it is also a candidate for the stabilisation of metal complexes having non-reduced ("innocent") TCNX ligands, thus allowing for studies of metal-TCNB interactions.…”