Electrochemistry of Hexaphospha Chromocene: A stable Chromocene Anion

1. Electrochemistry of Hexaphospha Chromocene: A stable Chromocene Anion Matthias Zeller,a,b Ulrich Zenneck a aInstitut für Anorganische Chemie, Universität Erlangen-Nürnberg, Egerlandstrasse 1, 91058 Erlangen, Germany. fax: Int. code +(9131)852-7367, e-mail: zenneck@chemie.uni-erlangen.de b current address: Youngstown State University, Department of Chemistry, 1 University Plaza, Youngstown, Ohio 44555-3663, USA, e-mail: mzeller@cc.ysu.edu Introduction: The incorporation of heteroatoms like phosporous in coordinated -ligands is able to influence the properties of organometallic complexes to a remarkable extend. Those heteroatoms are able to interact directly with the metal center and a pronounced change of not only reactivity, but also redox properties is to be expected. In recent years a growing number of mono and oligophospha ferrocene complexes have become accessible and first electrochemical investigations on their redox behavior have been reported.1 For these compounds the inclusion of the phosphorus atoms causes an anodic shift for both the oxidation and reduction potentials. Simultaneously the monocations are destabilized when compared with their parent cyclopentadienyl compounds. A recent publication by Ellis and Schleyer reported about a remarkably stable all inorganic titanocene dianion (P5)2Ti2–, 2 which is a 16 VE complex. This report prompted us to present here our observations on the electrochemistry of paramagnetic hexaphospha metallocenes. 3 Synthesis: Properties: • 2 is very sensitive and can be stored only as a solid under inert gas. • 1 is a stable compound and can be stored for months in air. Chromocene itself is pyrophoric! • The redox potentials for 1 are shifted by ca 2V (compared to Cp2Cr). This is the highest value ever measured for any oligophospha metallocene. The anion 1– is stable in the absence of air and humidity • 2 shows no pronounced anodic shift. It is easily oxidized Conclusions: • Chromocene 1 follows general trend of oligophospha metallocenes; shows extreme anodic shift of redox potentials • Manganocene 2 behaves opposite; oxidation to 16 VE cation easy (but irreversible) • phosphorous rich metallocenes seem to prefer a 16 VE configuration Left: EPR spectrum of [(t-Bu2C2P3)2Cr]–K+ (1–K+); top: measured spectrum in toluene at 273 K (9.44 GHz, 20.1 mW, 0.21 G modulation amplitude); bottom: simulated spectrum (<g> = 2.0047, <a(53Cr, 9.55%)> = 16.3 G, <a(31P)> = 2  3.3 G, 2  1.9 G, 2  0.0 G). Right: CV spectra of 1 (CH2Cl2), 0.1mol/l [n-BuN][PF6], 20 mV/s GC electrode, 0C). • a) Lemoine, P.; Gross, M.; Braunstein, P.; Mathey, F.; Deschamps, B.; Nelson, J. H.; Organometallics1984, 3, 1303-1307. b) Hu, D Dissertation, University of Heidelberg 1990. c) Elvers, A. Dissertation, University of Erlangen Nuremberg 1998. d) Bartsch, R.; Datsenko, S.; Ignatiev, N. V.; Muller, C.; Nixon, J. F.; Pickett, C. J. J. Organometal. Chem. 1997, 529(1-2), 375-378. • Urnius E.; Brennessel W. W.; Cramer C. J.; Ellis J. E.; von Rague Schleyer, P. Science2002, 295, 832-834. • Synthesis and properties of 2, see at: Clark, T.; Heinemann, F. W.; Hennemann, M.; Zeller, M.; Zenneck, U. Angew. Chem. Int. Ed. Engl.2000, 39, 2087-2091.