Electronic Structure of the 16 Valence Electron Fragments M(CO)3(PR3)2 (M = Mo, W; R = iPr, Cy) in Their Complexes with H2, THF, and Three .pi.-Conjugated Dinucleating Ligands: Electrochemistry and Spectroscopy of Different Oxidation States

Abstract

complexes trans,mer-[(PR3)2(CO)3M]2(«-L) with the symmetrically bridging ligands w-L = pyrazine (pz), 4,4'bipyridine (bp) and 3,6-bis(4-pyridyl)-1,2,4,5-tetrazine (4,4'-bptz) were studied by cyclic voltammetry and by IR, UV/vis/near-IR, and EPR spectroscopy. Oxidation of the electron-rich and dissociatively labile (Mo > W) systems yields M(I) species, including stable (Kc > 106 *) mixed-valent d5/d6 cations {[(PPrsMCO^Mhlpz)}"1". These mixed-valent complex ions exhibit complete delocalization on the vibrational time scale and show similar spectroscopic features as the structurally related Creutz-Taube ion {[(HgNisRuht/i-pz)}5"1". Reversible twoelectron oxidation processes and thus no mixed-valent states were observed for the bp-and 4,4'-bptz-bridged ditungsten systems. Whereas the oxidation of (PCy3)2(CO)3W(?y2-H2) is irreversible with the assumed loss of a proton, the solvates (PR3)2(CO)3M(THF) are reversibly oxidized and reduced, the latter process requiring rather negative potentials. In nonpolar solvents the neutral dinuclear complexes display very intense and strikingly narrow charge transfer bands in the near-infrared region, suggesting very little geometrical change between ground and MLCT excited states. One-electron reduction of the dinuclear complexes produces EPR-detectable radical anion complexes which show the loss of one PR3 ligand per metal center, i.e. the preference for a 16 + <5 rather than an 18 + valence electron configuration. The tungsten-centered oxidation of complexes (PR3)2(CO)3W(L) is facilitated in the order L = mpz+, pz, H2, 4,4'-bptz, bp, and THF, illustrating quantitatively the well-balanced donor and a* acceptor characteristics of the H2 ligand. From the results of this study it appears that the (PR3)2-(CO)3M fragments are suited to bind H2 because of a very finely tuned combination of -acceptor and fairly strong but not excessive -donor characteristics, in addition to the proper amount of steric shielding.

Dihydrogen complexes as prototypes for the coordination chemistry of saturated molecules

“…The pK a can become as low as Ϫ6, i.e., 2 -H 2 can become more acidic than sulfuric acid as shown by Morris (25,26,82) and later Jia (36). Electron-deficient cationic H 2 complexes with electron withdrawing ligands such as CO and short H-H bonds (Ͻ0.9 Å), i.e., [Re(H 2 )(CO) 4 ϩ becomes acidic enough to protonate weakly basic ethers (89). Such ability is relevant to processes such as ionic hy- drogenation and the function of metalloenzymes such as hydrogenases (H 2 ases).…”

Activation of Molecular Hydrogen

“…Crabtree 65 but on oxidation to [W(CO) 3 (PCy 3 ) 2 (H 2 )] þ it becomes acidic enough to protonate weakly basic ethers. 66 As will be shown below, such ability is relevant to processes such as ionic hydrogenation and the function of metalloenzymes such as hydrogenases.…”

Multielectron transfer process for molecular conversions: Oxidative polymerization with vanadium catalysts

“…[1][2][3][4][5]. Such studies have shown that multi-ET process is applicable to the multielectron reduction of substrates such as 9, N, and C q , which would allow for artificial photosynthetic model system (Refs.…”