Modulation of Molybdenum-Centered Redox Potentials and Electron-Transfer Rates by Sulfur versus Oxygen Ligation

Abstract

Temperature-dependent measurements of potential, E degrees', and electron-transfer rate constant, k(s,h), are reported for electrochemical reduction (in 0.3 M TBAPF(6)/CH(3)CN) of a series of oxomolybdenum(V) complexes, [(Tp)MoO(X,Y)], where Tp = hydrotris(3,5-dimethyl-1-pyrazolyl)borate and X,Y is a series of bidentate 1,2-disubstituted aliphatic or aromatic ligands in which oxygen donors are replaced sequentially by sulfur. E degrees' values shift in the positive direction, and k(s,h) values increase as O is replaced by S and as the framework of the ligand is changed from aliphatic to aromatic. The electrochemical enthalpy of activation, measured under conditions of zero driving force as DeltaH= -R partial differential[ln(k(s,h))]/ partial differential(1/T) and corrected for an outer-shell component by the mean spherical approximation, is approximately 10 kJ mol(-1) larger for complexes with O versus S donors and with an aliphatic versus aromatic ligand framework. Thus, the rate of Mo(V/IV) electron transfer is modulated primarily by differences in inner-shell reorganization. Following a recent description of electronic structure contributions to electron-transfer reactivity (Kennepohl, P.; Solomon, E. I. Inorg. Chem. 2003, 42, 679 ff), it is concluded that more effective charge distribution over the entire molecular structure, as mediated by electronic relaxation in S versus O and aromatic versus aliphatic systems, is responsible for the influence of ligand structure on the kinetics and thermodynamics of Mo-centered electron transfer. There is no evidence, based on experimentally measured pre-exponential factors, that sulfur donors or an aromatic ligand framework are more effective than their structural counterparts in facilitating electronic coupling between the electrode and the Mo d(xy) redox orbital.

Rhenium(V) Complexes with Thiolato and Dithiolato Ligands: Synthesis, Structures, and Monomerization Reactions

“…Thus, the reactivity decreases in those complexes with the Me group or aliphatic backbone. Similar effects have been observed in a number of examples. − …”

“…Similar effects have been observed in a number of examples. [52][53][54][55][56] One can examine the influence of changes in the size of the chelate ring on reactivity toward monomer formation. This change can affect interactions between the donor atoms of the chelate ligand and the rhenium atom by varying the bite angle and the conformation of the chelate ring.…”

Platinum(II) Diimine Complexes with Halide/Pseudohalide Ligands and Dangling Trialkylamine or Ammonium Groups

“…E pa increases from 0.46 to 0.60 V as the scan rate is varied from 0.025 to 1.0 V/s; over the same scan rate range, E pc decreases by only 0.05 mV. This pattern of behavior is very similar to that found for cooperative two-electron reagents ,,− and is consistent with the formation of an organized structure, such as a five- or six-coordinate complex with intramolecular Pt–piperidyl interactions, prior to or during anodic electron transfer. Such interactions are likely to be weaker in the case of Pt(phen)(pip 2 NCN)Cl because of the lower electrophilicity of the metal center compared to Pt(tpy)(pip 2 NCN) + .…”

Intramolecular NH···Pt Interactions of Platinum(II) Diimine Complexes with Phenyl Ligands

“…The pip 2 NCligands of Pt(pipNC) 2 adopt a cis configuration with 80. 53 For each diimine complex, the pipNCand pipNHC groups adopt the more sterically favored head-to-tail conformation in which the piperidyl groups are directed toward opposite faces of the coordination plane. Interestingly, in crystals of Pt(pipNC) 2 (phen), the piperidyl N atoms are directed away from the Pt atom [Pt 3 3 3 N4, 4.668(7) A ˚; Pt 3 3 3 N3, 4.726 (7) A ˚], forming short intramolecular contacts with phenyl groups [N4 3 3 3 H29, 2.54 A ˚; N3 3 3 3 H17, 2.47 A ˚].…”

“…Although the chemical irreversibility of the oxidation process near 0.45 V prevents assessment of the electrochemical reversibility or extraction of precise thermodynamic information, it is noteworthy that the peak potential of the anodic process shows a characteristic dependence on the sweep rate, with E pa undergoing a substantial anodic shift with an increase in the scan rate. ,,, For example, E pa increases from 0.42 to 0.77 V as the scan rate is varied from 0.05 to 4.0 V/s. Over the same scan rate range, E pc of the cathodic process near −0.3 V decreases by only 0.06 mV (Figure S6 in the Supporting Information).…”