Mixed-Valent Ru2(II,III) and Oxidized Ru2(III,III) Phosphine and Carbene Adducts and their Magnetic Properties
Our lab was the first to isolate and structurally characterize a phosphine diadduct of a mixed-valent diruthenium-tetracarboxylate. In most cases, reaction of the diruthenium core with phosphines or diphosphines leads to a disassembly of the core and Ru-Ru bond cleavage as mentioned in Section 1. By using a sterically bulky phosphine, such as tri-cyclohexylphosphine (PCy3), which has a large Tolman cone angle, we were able to isolate the kinetic, [Ru2(μ
-O2CCH3)4(PCy3)2]+, product before the complex converts to the more thermodynamically stable form in which the phosphine migrates equatorially, displacing some of the acetates and inducing bond-cleavage. Structurally this complex displays the longest Ru-Ru bond length of any Ru2(II,III) tetracarboxylate to date. An electrochemical (cyclic voltammetry) study of the diadduct revealed an accessible Ru2(III,III) state at E1/2 = 0.52 V vs. Fc+/0 (reversible process) which was unprecedented. Numerous attempts to isolate and characterize this oxidized product chemically have failed. To date, no Ru2(III,III) tetracarboxylate as been prepared.
Most of the mixed-valent diruthenium-tetraacetate complex adducts, [Ru2(μ-O2CCH3)4L2]+ (where L = a weak to modestly strong σ-donor such as H2O, DMF, DMSO and pyridine), display variable temperature magnetic behaviour consistent with 3 unpaired electrons and a large zero-field splitting (ZFS), due to the accidental near-degeneracy of the HOMO ((π*δ*)3). When very strong σ-donor/π-acceptor ligands such as phosphines are introduced in the axial position, we have shown that the near-degeneracy of the π*δ* HOMO can be removed somewhat over the temperature range 3-300K with a Boltzmann distribution of states resulting. Cotton has shown that a Boltzman distribution could be induced in a diruthenium(II,III) tetra-N,N'-diarylformamidinate (DArF) type paddle-wheel complexes by changing the equatorial bridges from the meta to the para isomer, i.e., where Ar = m or p-C6H4OMe. Ours is the first example of inducing such a property by varying only the axial ligand. We also presented variable-temperature structural evidence that showed the changeover from a π*2δ* to a π*3 HOMO as the temperature was dropped (consistent with the magnetic measurements).
We are currently attempting to isolate and structurally characterize the first fully oxidized Ru2(III,III) tetracarboxylate adduct by using bulk electrolysis at a potential greater than 0.55 V vs. Fc+/0. We are having an H-cell with Pt-mesh (or Pt-plate) electrodes constructed, which will allow us, by using a potentiostat or power module, to electro-synthesize [Ru2(μ-O2CCH3)4(PCy3)2]2+ in one half of the cell (with reduced products in the other half). The [Ru2(μ-O2CCH3)4(PCy3)2]2+ cation will be precipitated from solution with an appropriate anion, such as PF6- , BF4- , Cl- , ClO4- , etc., and recrystallized. Once this compound is prepared, we will investigate its variable temperature magnetic susceptibility and electronic properties to assess the nature of the HOMO to determine whether the complex is paramagnetic (π*2) or diamagnetic (δ*2). We will then extend this method to the preparation of other "fully" oxidized phosphine adducts and possibly even carbene adducts. Additional mixed-valent Ru2(II,III) adducts will be synthesized by reactions with other phosphines that have large cone-angles, such as P(tBu)3, P(Bz)3, P(o-Tol)3, etc. Within the restriction of the large cone angle, we hope to vary the σ-donor/π-acceptor properties sufficiently to be useful in observing trends in the magnetic susceptibility measurements. Carbene adducts of paddle-wheel complexes have only been studied once before in the case of a dirhodium(II,II) complex, Rh(μ-O2CR)4(carbene)2, where the carbene was an imidazol-2-ylidene. This complex has important implications in the catalytic trans-formation of α-diazo ketones into polycyclic organic frameworks. As carbenes of this type are relatively stable and easy to synthesize, we will synthesize their Ru2(II,III) adducts. As some carbenes have comparable σ-donor/π-acceptor properties to phosphines, their properties in terms of structure, electrochemistry, electronic properties and disassembly abilities should prove interesting as will their magnetic properties and catalytic potential.