Electrochemical oxidation of cis-[Mo(CO)4(2,2′-bipyridine)] coupled with ligand substitution reactions in non-aqueous solvents

The Best of Both Worlds: Combining the Power of MICs and WCAs To Generate Stable and Crystalline CrI -Tetracarbonyl Complexes with π-Accepting Ligands

Here we present stable and crystalline chromium(I) tetracarbonyl complexes with pyridyl-MIC (MIC=mesoionic carbene) ligands and weakly coordinating anions (WCA=[Al(ORF )4 ]- , RF =C(CF3 )3 and BArF =[B(ArF )4 ]- , ArF =3,5-(CF3 )2 C6 H3 ). The complexes were fully characterized via crystallographic, spectroscopic and theoretical methods. The influence of counter anions on the IR and EPR spectroscopic properties of the CrI complexes was investigated, and the electronic innocence versus non-innocence of WCAs was probed. These are the first examples of stable and crystalline [Cr(CO)4 ]+ complexes with a chelatingπ - ${\pi -}$ accepting ligand, and the data presented here are of relevance for both the photochemical and the electrochemical properties of these classes of compounds.

Electrochemical Oxidation of W(CO)4(LL): Generation, Characterization, and Reactivity of [W(CO)4(LL)]+ (LL=α-diimine ligands)

The electrochemistry of a series of W(CO)4(LL) complexes, where LL is an aromatic α-diimine ligand, was examined in coordinating and weakly coordinating media using several techniques. These compounds undergo metal-centred one-electron oxidations and the electrogenerated radical cations undergo a range of subsequent chemical steps, the nature of which depends on the substituents of the α-diimine ligand and the presence of coordinating species. In CH2Cl2/TBAPF6, where TBAPF6 is n-tetrabutylammonium hexaflurophosphate, the bulk oxidations are partially reversible at scan rates of 0.25 V s−1; the resulting tungsten(i) radicals react via disproportionation and loss of carbonyl, the rate constants for which were measured by double-potential step chronocoulometry. Large-amplitude a.c. voltammetry experiments suggest that the one-electron oxidized species are in equilibrium with the corresponding disproportionation products. Steric crowding of the metal centre prolongs the lifetime of the radical cations, allowing the infrared spectroelectrochemical characterization of two [W(CO)4(LL)]+ species. Electrogenerated [W(CO)4(LL)]+ cations are highly susceptible to attack by potential ligands; oxidations performed in CH3CN/TBAPF6, for example, were chemically irreversible. Kinetic studies in weakly coordinating media show that near-stoichiometric amounts of added pyridine and acetonitrile are enough to greatly diminish the reversibility of the bulk oxidations; the dominant path of the coupled chemistry depends on the ligand strength, with substitution being the major reaction with added pyridine, whereas disproportionation is favoured by the presence of acetonitrile. A reaction scheme that provides an overall framework of the reactions followed by the radical cations is presented and discussed in the context of the previously observed chemistry of the molybdenum analogues.

New Polypyridine Ligands Functionalized with Redox-Active Fe(II) Organometallic Fragments

We report in this Communication the isolation and characterization, including structure determinations, of 2,2',6',2"-terpyridine (2) and 2,2'-bipyridine (3) ligands bearing two redox-active "(eta2-dppe)(eta5-C5Me5)FeC[triple bond]C-" moieties grafted to the 5 and 5" positions of terpy or to the 5 and 5' positions of bipy. These "metalloligands" have been complexed with Ru(II) and Mo(0), providing new heterotrinuclear complexes displaying intense absorptions around 700 and 600 nm, respectively, for the Fe2Ru/terpy and Fe2Mo/bipy species. In both cases, the Fe(II)/Fe(III) oxidation potentials of the free ligands became more positive by more than 50 mV upon complexation.

Spectroscopy and electrochemistry of new 6,6′-disubstituted-4,4′-bipyrimidine molybdenum(0) and tungsten(0) tetracarbonyl complexes

A new family of tetracarbonyl molybdenum(0) and tungsten(0) complexes based on new 6,6′-disubstituted-4,4′-bipyrimidine ligands was synthesized and characterized. The visible region of the absorption spectrum of each complex is dominated by a metal-to-ligand charge transfer band significantly lower in energy than the corresponding transition in 2,2′-bipyridine tetracarbonyl metal complexes. The 6,6′-substituents create a larger π-electronic system in the substituted bipyrimidines and are consequently better π acceptors than even the parent 4,4′-bipyrimidine. The absorption bands are shifted bathochromically with a decrease in solvent polarity.Key words: molybdenum and tungsten, nitrogen ligands, tetracarbonyl metal complexes, absorption spectroscopy, electrochemistry.