Chun Y. Liu

Steps on the way to the first dirhodium tetracarboxylate with no axial ligation: synthetic lessons and a plethora of Rh2(O2CR)4L2−n compounds, n=0, 1, 2

We report the preparation and structural features of thirteen new dirhodium tetracarboxylato compounds. Twelve of these were synthesized in early attempts to create Rh 2 (TiPB) 4 , where TiPB = 2,4,6-triisopropylbenzoate, the first dirhodium tetracarboxylate without axial ligation, the structure of which is also reported here. Several challenges in the synthesis of such a molecule are identified and discussed. Although axial interactions are of paramount importance for Rh 2 4+ units, Rh-Rh bond lengths are quite insensitive to the nature, presence or absence of axial ligation. However, the effect of axial ligation is clearly observed using electronic spectroscopy; Rh 2 (TiPB) 4 shows an extremely low energy π*(Rh 2 ) → σ*(Rh 2 ) transition. The energy of this transition is put into the context of previously unreported spectroscopy of Rh 2 (O 2 CCPh 3 ) 4 bis-adducts.

Perturbation of the Charge Density between Two Bridged Mo2 Centers: The Remote Substituent Effects

A series of terephthalate-bridged dimolybdenum dimers with various formamidinate ancillary ligands, denoted as [Mo2(ArNCHNAr)3]2(μ-O2CC6H4CO2) (Ar = p-XC6H4, with X = OCH3 (1), CH3 (2), F (3), Cl (4), OCF3 (5), and CF3 (6)), has been synthesized and studied in terms of substituent effects on electron delocalization between the two dimetal sites. X-ray structural analyses show that these complexes share the same molecular scaffold with the para-substituents (X) being about 8 Å away from the Mo2 center. It is found that the remote substituents have the capability to tune the electronic properties of the complexes. For the series 1 to 6, the metal-metal bond distances (d(Mo-Mo)) decrease slightly and continuously; the potential separations (ΔE(1/2)) for the two successive one-electron oxidations decrease constantly, and the metal to ligand transition energies (λ(max)) increase in order. More interestingly, the two types of methine protons, H(∥) on the horizontal and H(⊥) on the vertical ligands with respect to the plane defined by the Mo-Mo bond vectors and bridging ligand, display separate resonant signals δ(∥) and δ(⊥) in the NMR spectra. The displacements of the chemical shifts, Δδ(∥-⊥) = δ(∥) - δ(⊥), are getting smaller as the substituents vary from electron-donating to -withdrawing. These results show that the peripheral groups on the [Mo2] units function to fine-tune the metal-metal interactions crossing the bridging ligand. The experimental parameters, ΔE(1/2), λ(max), and Δδ(∥-⊥), which are linearly related with the Hammett constants (σ(X)) of the X groups, can be used to probe the charge density on the two [Mo2] units and the electronic delocalization between them.

A mixed-valence compound with one unpaired electron delocalized over four molybdenum atoms in a cyclic tetranuclear ion

The first oxidation of a species derived from a compound having two linked, quadruply-bonded Mo2 units has been performed and [[cis-Mo2(DAniF)2]2(mu-Cl)4]PF6, 2, has been isolated and characterized in many ways; it has one unpaired electron and a fully delocalized structure where the Mo-Mo distances increase from 2.1191(4) A in the reduced species to 2.1453(3) A in 2 and the Kc of 1.3 x 10(9) is three orders of magnitude larger than that of the Creutz-Taube ion.