97732-08-6Relevant academic research and scientific papers
Redox-induced hapticity changes: Effect of substituents on arene bending in a series of rhodium complexes
Bowyer, Walter J.,Merkert, Jon W.,Geiger, William E.,Rheingold, Arnold L.
, p. 191 - 198 (2008/10/08)
Electrochemical measurements have been performed on a series of rhodium arene complexes of the formula [(MenC6H6-n)RhCp*]2+ (n = 0-6; Cp* = η5-C5Me5). In general, two reduction waves are observed, corresponding to the Rh(III)/Rh(II) and Rh(II)/Rh(I) oxidation state changes. The Rh(I) complexes have been proposed to have a η4-bonded arene, and this has been confirmed by X-ray crystallographic measurements on the hexamethylbenzene complex 6. (η4-C6Me6)RhCp* crystallizes in the space group Pnma with a = 16.724 (4) A?, b = 15.149 (4) A?, c = 7.963 (2) A?, V = 2017.5 (9) A?-3, and Z = 4. The arene hinge angle is 41.8°. For nine of the complexes studied, both the Rh(II) monocation and the neutral Rh(I) complex have been detected. In all cases, the lifetimes of the Rh(II) complexes were less than the corresponding Rh(III) or Rh(I) complexes. Half-lives of the Rh(II) monocations steadily increased as the number of methyl groups on the arene increased, from about 5 ms for [(C6H6)RhCp*]+ to >1800 s for [(C6Me6)RhCp*]+. The standard reduction potential for the Rh(III)/Rh(II) couple decreased by about 28 mV for each Me group on the arene ring, and an analysis of previous literature data shows that this is the expected substituent effect when substituting Me for H in an arene complex. E° values for the Rh(II)/Rh(I) couples are, however, virtually independent of the degree of arene methylation. The implication of this observation is that the arene hapticity change is thermodynamically favored with an increasing degree of arene methylation. The "natural" lowering of the potential due to the inductive effects of added Me groups is thus counterbalanced by a raising of the potential due to increased stabilization of the η4 structure. The amount of stabilization of the η4 structure is about 33 mV/Me group or about 0.7 (kcal/mol)/Me. The favoring of the η4 structure with a higher degree of arene substitution may be due to decreased loss of resonance energies in the bending of the more highly substituted arene complexes.
Electrochemically Induced Changes in Hapticity in Mixed-Sandwich Compounds of Iridium and Rhodium
Bowyer, Walter J.,Geiger, William E.
, p. 5657 - 5663 (2007/10/02)
The electrochemical reductions of two sandwich complexes, 5-C5Me5)M(η6-C6Me6)>2+ (M=Rh, Ir), are reported.The reduction of the rhodium complex occurs by two reversible one-electron transfer having E0 values separated by several hundred millivolts.The iridium complex reduces in a single two-electron step with cyclic voltammetric peak separations very dependent on electrode material.The neutral species of both metals have been isolated and the 1H NMR spectra indicate the structure 5-C5Me5)M(η4-C6Me6)>.Thus these complexes undergo reversible η6->4 isomerization of the arene coordination with the transfer of two electrons.The relationship between the E0 values and the structure change is discussed, and a model is favored in which the slippage from η6- to η4-arene coordination occurs during the second electron transfer.
