Winter, Rainer F.
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DFT modeling of Spectral and Redox Properties of Di-and Tetranuclear Ruthenium Transition Metal Complexes with Bridging Ligands
The electronic structures of di-and tetranuclear transition metal complexes with bridging ligands (tetracyanoethene, tetracyano-p-quinodimethane, divinylphenylene and tetrakis(4-styryl)ethene) were calculated by density functional (DFT) method. DFT method was used for calculations of IR frequencies in different oxidation states and EPR parameters of radical ions. The observed electronic transitions of closed shell systems were assigned by TD DFT. The different aspects of bridge mediated metal-metal interaction are discussed. ©2009 American Institute of Physics
Ruthenium Complexes with Vinyl, Styryl, and Vinylpyrenyl Ligands: A Case of Non-innocence in Organometallic Chemistry
We herein describe a systematic account of mononuclear ruthenium vinyl complexes L−{Ru}−CHCH−R where the phosphine ligands at the (PR‘3)2Ru(CO)Cl{Ru} moiety, the coordination number at the metal (L = 4-ethylisonicotinate or a vacant coordination site) and the substituent R (R = nbutyl, phenyl, 1-pyrenyl) have been varied. Structures of the enynyl complex Ru(CO)Cl(PPh3)2(η1:η2-nBuHCCHCCnBu), which results from the coupling of the hexenyl ligand of complex 1a with another molecule of 1-hexyne, of the hexenyl complexes (nBuCHCH)Ru(CO)Cl(PiPr3)2 (1c) and (nBuCHCH)Ru(CO)Cl(PPh3)2(NC5H4COOEt-4) (1b), and of the pyrenyl complexes (1-Pyr-CHCH)Ru(CO)Cl(PiPr3)2 (3c) and (1-Pyr-CHCH)Ru(CO)Cl(PPh3)3 (3a-P) have been established by X-ray crystallography. All vinyl complexes undergo a one-electron oxidation at fairly low potentials and a second oxidation at more positive potentials. Anodic half-wave or peak potentials show a progressive shift to lower values as π-conjugation within the vinyl ligand increases. Carbonyl band shifts of the metal-bonded CO ligand upon monooxidation are significantly smaller than is expected of a metal-centered oxidation process and are further diminished as the vinyl CHCH entity is incorporated into a more extended π-system. ESR spectra of the electrogenerated radical cations display negligible g-value anisotropies and small deviations of the average g-value from that of the free electron. The vinyl ligands thus strongly contribute to or even dominate the anodic oxidation processes. This renders them a class of truly “non-innocent” ligands in organometallic ruthenium chemistry. Experimental findings are fully supported by quantum chemical calculations: The contribution of the vinyl ligand to the HOMO increases from 46% (Ru-vinyl delocalized) to 84% (vinyl dominated) as R changes from nbutyl to 1-pyrenyl.
Divinylphenylene-Bridged Diruthenium Complexes Bearing Ru(CO)Cl(PiPr3)2 Entities
The divinylphenylene-bridged diruthenium complexes (E,E)-[{(PiPr3)2(CO)ClRu}2(μ-HCCHC6H4CHCH-1,3)] (m-2) and (E,E)-[{(PiPr3)2(CO)ClRu}2(μ-HCCHC6H4CHCH-1,4)] (p-2) have been prepared and compared to their PPh3-containing analogues m-1 and p-1. The higher electron density at the metal atoms increases the contribution of the metal end groups to the bridge-dominated occupied frontier orbitals and stabilizes the various oxidized forms with respect to those of m-1 and p-1. This has been confirmed and quantified electrochemically, because the two reversible oxidation waves were observed at considerably lower potentials than for the PPh3 complexes. Owing to their greater stability, the one- and two-electron-oxidized forms m-2n+ and p-2n+ of both complexes could be generated and spectroscopically characterized inside an optically transparent thin layer electrolysis cell. UV/vis/near-IR and ESR spectroelectrochemistry indicates that the oxidation processes are centered at the organic bridging ligand. σ-Bonded divinylphenylenes thus constitute an unusual class of “noninnocent” ligands for organometallic compounds. Electronic transitions observed for the mono- and dioxidized forms closely resemble those of donor-substituted phenylenevinylene compounds, including oligo(phenylenevinylenes) (OPVs) and poly(phenylenevinylene) (PPV) in the respective oxidation states. Strong ESR signals and nearly isotropic g tensors are observed for the monocations in fluid and frozen solutions. The metal contribution to the redox orbitals is illustrated by a shift of the CO stretching bands to notably higher energies upon stepwise oxidation. The shifts strongly exceed those observed for the PPh3 containing, six-coordinated species (E,E)-[{(PPh3)2(CO)Cl(L)Ru}2(μ-HCCHC6H4CHCH)]n+ (L = substituted pyridine). IR spectroelectrochemistry reveals the presence of two electronically different transition-metal moieties in m-2+, while they resemble each other more closely in p-2+. Differences in electronic coupling are illustrated by the charge distribution parameters calculated from the spectra. Bulk electrolysis experiments confirm the results from the in situ spectroelectrochemistry and the overall stoichiometry of the redox processes. Quantum-chemical calculations were performed in order to provide insight into the nature and composition of the frontier orbitals. The electronic transitions observed for the neutral forms were assigned by TD DFT. IR frequencies calculated for m-2 and p-2 in their various oxidation states retrace the experimental observations. They fail, however, in the case of m-2+, where a symmetrical structure is calculated, as opposed to the distinctly asymmetric electron distribution observed by IR spectroscopy. Geometry-optimized structures were calculated for all accessible oxidation states. The structural changes following stepwise oxidation agree well with the experimental findings: e.g., a successive low-energy shift of the CC stretching vibration of the bridge. The radical cation m-2+ displays a broad composite electronic absorption band at low energy that extends into the mid-IR region.
How to elucidate and control the redox sequence in vinylbenzoate and vinylpyridine bridged diruthenium complexes
Vinylbenzoate-bridged diruthenium complexes (RHC[double bond, length as m-dash]CH)(CO)(PiPr3)2Ru(μ-4-OOCC6H4–CH[double bond, length as m-dash]CH)RuCl(CO)(PiPr3)2 (R = Ph, 3a or CF3, 3b) and vinylpyridine-bridged (η6-p-cymene)Cl2Ru(μ-NC5H4-4-CH[double bond, length as m-dash]CH)RuCl(CO)(PiPr3)2 (3c) have been prepared from their monoruthenium precursors and investigated with respect to the sequence of the individual redox steps and electron delocalization in their partially and fully oxidized states. Identification of the primary redox sites rests on the trends in redox potentials and the EPR, IR and Vis/NIR signatures of the oxidized radical cations and is correctly reproduced by quantum chemical investigations. Our results indicate that the trifluoropropenyl complex 3b has an inverse FMO level ordering (Ru1-bridge-Ru2 > terminal vinyl-Ru1 site) when compared to its styryl substituted counterpart 3a such that the primary oxidation site in these systems can be tuned by the choice of the terminal alkenyl ligand. It is further shown that the vinylbenzoate bridge is inferior to the vinylpyridine one with regard to charge and spin delocalization at the radical cation level. According to quantum chemical calculations, the doubly oxidized forms of these complexes have triplet diradical ground states and feature two interconnected oxidized vinyl ruthenium subunits.
Six-Membered N-Heterocyclic Carbenes with a 1,1′-Ferrocenediyl Backbone : Bulky Ligands with Strong Electron-Donor Capacity and Unusual Non-Innocent Character
Abstract
The stable, crystalline N-heterocyclic diaminocarbene fc[N(CH2tBu)-C-N(CH2tBu)] (2d, fc = 1,1′-ferrocenediyl) was prepared by deprotonation of its formamidinium precursor fc[N(CH2tBu)-CH-N(CH2tBu)][BF4] (1d) and used for the preparation of the 16 valence electron complexes [Mo(2d)(CO)4], [RhCl(2d)(cod)] (cod = 1,5-cyclooctadiene) and [RhCl(2d)(CO)2]. 1d, 2d and [RhCl(2d)(cod)] were structurally characterised by single-crystal X-ray diffraction studies. The electrochemical properties of 2d, its 2-adamantyl analogue 2c, its complex [RhCl(2d)(CO)2] and of the precursors 1d and 1,1′-bis(neopentylamino)ferrocene were investigated by electrochemistry. The carbenes are easily oxidised to the corresponding radical cation, whose persistent nature is unprecedented in the chemistry of N-heterocyclic carbenes. The spin density is located at the Fe atom and the carbene C atom according to the results of EPR spectroscopic studies and DFT calculations.
Ligand-Centered Oxidations and Electron Delocalization in a Tetranuclear Complex of a Tetradonor-Substituted Olefin
The tetrakis(4-styryl)ethene (TSTE 4−)-bridged tetraruthenium complex [{(P i Pr 3) 2(CO)ClRu} 4{μ 4-(CHCHC 6H 4) 4(CC)}] undergoes four consecutive oxidations at low potential. The ligand-dominated nature of these processes is confirmed by spectroscopic and quantum-chemical investigations.
Oxidative Perhydroxylation of [closo-B12H12]2- ; to the Stable Inorganic Cluster Redox System [B12(OH)12]2-/.- : Experiment and Theory
It′s radical: A surprisingly simple reaction converts the normally inert prototypical hydridoborate cluster [closo-B12H12]2− not only to [closo-B12(OH)12]2− but also, at less-elevated temperatures, to the new [hypocloso-B12(OH)12].− radical anion (see figure). Both approximately icosahedral closo and hypocloso cluster ions were structurally characterized as the cesium salts and their properties were assessed experimentally and theoretically.
Charge Delocalization in a Heterobimetallic Ferrocene−(Vinyl)Ru(CO)Cl(PiPr3)2 System
Ru(CH═CHFc)Cl(CO)(PiPr3)2 (Fc = ferrocenyl, (η5-C5H4)Fe(η5-C5H5)), 1, has been prepared by hydroruthenation of ethynylferrocene and characterized by NMR, IR, ESI-MS, and Moessbauer spectroscopy and by X-ray crystallography. Complex 1 features conjoined ferrocene and (vinyl)ruthenium redox sites and undergoes two consecutive reversible oxidations. Pure samples of crystalline, monooxidized 1•+ have been prepared by chemical oxidation of 1 with the ferrocenium ion. Structural comparison with 1 reveals an increase of Fe−C and Fe−Cpcentr. bond lengths and ring tilting of the Cp decks, as is typical of ferrocenium ions, but also a discernible lengthening of the Ru−C(CO) and Ru−P bonds and a shortening of the Ru−C(vinyl) bond upon oxidation. This supports the general idea of charge delocalization over both redox sites in 1•+. Band shifts of the charge-sensitive IR labels (ν(CO) for Ru, ν(C−H, Cp) for Fc), the rather small g-anisotropy in the ESR spectrum of 1•+, and the results of quantum chemical calculations indicate that in solution the positive charge partly resides on the vinyl ruthenium moiety. Comparison of IR shifts in the solid state and in solution and the quadrupole splitting in the Moessbauer spectrum of powdered 1•+ point to a larger extent of charge localization on the ferrocenyl site in solid samples. This is probably due to CH···F hydrogen bonding interactions between the cyclopentadienyl hydrogen atoms of the radical cations and the PF6− counterions. Monooxidized 1•+ displays low-energy electronic absorption bands at 1370 and 2150 nm. According to quantum chemical calculations, the underlying transitions are largely localized on the ferrocene part of the molecule with only little charge transfer into the vinyl ruthenium subunit. The second oxidation is more biased toward the (vinyl)ruthenium site.
Towards New Organometallic Wires: Tetraruthenium Complexes Bridged by Phenylenevinylene and Vinylpyridine Ligands
The tetranuclear complexes [{(PiPr3)2(CO)ClRu(μ-CH[DOUBLE BOND]CHpy)Ru Cl(CO)(PPh3)2}2(μ-CH[DOUBLE BOND]CH-C6H4- CH[DOUBLE BOND]CH-1,4)] (3 a) and [{(PiPr3)2(CO)ClRu(μ-CH[DOUBLE BOND]CHpy)RuCl(CO)(PPh3)2}2(μ-CH[DOUBLE BOND]CH-C6H4-CH[DOUBLE BOND]CH-1,3)] (3 b), which contain vinylpyridine ligands that connect peripheral Ru(PiPr3)2(CO)Cl units to a central divinylphenylene-bridged diruthenium core, have been prepared and investigated. These complexes, in various oxidation states up to the tetracation level, have been characterized by standard electrochemical and spectroelectrochemical techniques, including IR, UV/Vis/NIR and ESR spectroscopy. A comparison with the results for the vinylpyridine-bridged dinuclear complex [PiPr3)2(CO)ClRu(μ-CH[DOUBLE BOND]CHpy)RuCl(CO)(PPh3)2(CH[DOUBLE BOND]CHPh)] (6) and the divinylphenylene-bridged complexes [{(EtOOCpy)(CO)Cl(PPh3)2Ru}2(μ-CH[DOUBLE BOND]CH-C6H4-CH[DOUBLE BOND]CH-1,4)] (8 a) and [{(EtOOCpy)(CO)Cl(PPh3)2Ru}2(μ-CH[DOUBLE BOND]CH-C6H4-CH[DOUBLE BOND]CH-1,3)] (8 b), which represent the outer sections (6) or the inner core (8 a,b) of complexes 3 a,b, and with the mononuclear complex [(EtOOCpy)(CO)(PPh3)2RuCl(CH[DOUBLE BOND]CHPh)] (7) indicate that every accessible oxidation process is primarily centred on one of the vinyl ligands, with smaller contributions from the metal centres. The experimental results and quantum chemical calculations indicate charge- and spin-delocalization across the central divinylphenylenediruthenium part of 3 a,b or the styrylruthenium unit of 6, but not beyond. The energy gap between the higher lying styryl- or divinylphenylenediruthenium-based and the lower occupied vinylpyridineruthenium-based orbitals increases in the order 6<3 b<3 a and thus follows the conjugation within the non-heteroatom-substituted aromatic vinyl ligand.