Synthesis, spectroscopic properties and electrochemistry of Rh 2 (ap) 4 (R) where R = CH 3 or C 6 H 5 and ap = 2-anilinopyridinate anion John L. Bear a, * , Baocheng Han a , Yulan Li a , Siyabonga Ngubane a , Eric Van Caemelbecke a,b , Karl M. Kadish a, * a Department of Chemistry, University of Houston, Houston, TX 77204-5641, United States b Houston Baptist University, 7502 Fondren Road, Houston, TX 77074-3298, United States article info Article history: Received 31 October 2008 Accepted 12 March 2009 Available online 20 March 2009 Keywords: Dirhodium r-bonded Electrochemistry Spectroelectrochemistry X-ray structure ESR abstract Two sigma-bonded Rh 2 5+ complexes, Rh 2 (ap) 4 (C 6 H 5 ) and Rh 2 (ap) 4 (CH 3 ), where ap = 2-anilinopyridinate anion were synthesized and examined as to their electrochemical and spectroscopic properties in non- aqueous media. Rh 2 (ap) 4 (C 6 H 5 ) was also structurally characterized. The presence of the phenyl axial ligand on Rh 2 (ap) 4 (C 6 H 5 ) does not bring about a significant structural effect on the ap framework as com- pared to Rh 2 (ap) 4 Cl or Rh 2 (ap) 4 (C„CH) but the Rh–Rh and Rh–C axial bond lengths are both elongated due to the strong electron-donating effect of the phenyl group. The two sigma-bonded dirhodium compounds each undergo a single reversible one-electron reduction to in situ generate a stable Rh 2 4þ complex and two reversible one-electron oxidations to give stable compounds in Rh 2 6þ and Rh 2 7þ oxidation states. These redox reactions occur at E 1/2 = 0.79, 0.28 and 0.93 V for R = CH 3 and 0.71, 0.35 and 0.95 V for R=C 6 H 5 . The ESR spectrum of Rh 2 (ap) 4 (CH 3 ) has g \ and g || values of 2.17 and 1.89 while Rh 2 (ap) 4 (C 6 H 5 ) has an ESR spectrum with signals at 2.17 (g \ ) and 2.00 (g || ), consistent with the odd electron in each dimetal unit being localized on only one of the two metal centers in Rh 2 5þ . Ó 2009 Published by Elsevier Ltd. 1. Introduction Since the first isolation of Rh 2 (O 2 CCH 3 ) 4 Cl in 1976 [1], the syn- thesis and characterization of about three dozen Rh 2 5þ complexes with different anionic bridging ligands (L) have been reported in the literature [2,3]. Dirhodium(II,III) complexes of this type have been incorporated into molecular structures [4–6] while Rh 2 (L) 4 and [Rh 2 (L) 4 ] + have been used as catalysts in a variety of reactions [7–9]. A Rh 2 5þ species has also been proposed as an intermediate in the formation of a bis(phenyl)dirhodium(III) caprolactamate [10]. Our own interest has been in the electrochemical and structural characterization of [Rh 2 (L) 4 ] + where L = dpf [11], ap [12–14], and F x ap [15] (x = 1, 2, 3, 5). These and other dirhodium compounds with Rh 2 5þ cores can bind a variety of neutral and anionic ligands [2], with the Rh–Rh bond lengths [2,16,17] and electrochemical properties [12,13] being affected by the type and number of axial ligands. For example, the metal–metal distance varies from 2.316 Å in [Rh 2 (O 2 CCH 3 ) 4 (H 2 O) 2 ]ClO 4  H 2 O [18] to 2.510 Å in [Rh 2 (O 2 CC 2 H 5 ) 4 (PPr i 3 ) 2 ]SbF 6 [19]. The redox potentials of the dirhodium center also vary as a function of the bridging and axial ligands and differ for the Rh 2 5þ=4þ and Rh 2 5þ=6þ processes by as much as 850 mV. Only a few dirhodium compounds with r-bonded alkyl axial ligands are known, examples being Rh 2 (ap) 4 (C„CR) [13] where R = H, C 6 H 5 ,C 5 H 11 ,C 4 H 8 C„CH and, to our knowledge, there are no reports in the literature for these type compounds with r-bonded aryl ligands. When bound to [Rh 2 (L) 4 ] + , alkyl or aryl r-donor groups such as CH 3 or C 6 H 5 would be expected to interact primarily with the Rh–Rh molecular orbitals of axial symmetry, such as was observed for the C„CR derivatives [13], but the extent of interaction and changes in spectroscopic and/or electrochemical properties with changes in axial coordination should also depend upon the type of ligand r-bonded to Rh 2 5þ . This point is examined in the present study which describes the synthesis and characterization of two new previously unreported Rh 2 5þ com- plexes containing r-bonded axial ligands, Rh 2 (ap) 4 (CH 3 ) and Rh 2 (ap) 4 (C 6 H 5 ). The electrochemical and spectroscopic properties of each are measured in CH 2 Cl 2 and the structure of Rh 2 (ap) 4 (C 6 H 5 ) is presented. 2. Experimental 2.1. Chemicals HPLC grade CH 2 Cl 2 obtained from Aldrich Co. was distilled over phosphorous pentoxide (P 2 O 5 ). Spectroscopic grade THF from Al- drich Co. was purified by distillation from sodium/benzophenone under Ar prior to use. Analytical grade n-hexane from Aldrich Co. was used as received. Tetra-n-butylammonium perchlorate (TBAP) purchased from Fluka Chemical Co. was twice recrystallized from absolute ethanol. LiC 6 H 5 and LiCH 3 were obtained from Aldrich Co. and used as received. 0277-5387/$ - see front matter Ó 2009 Published by Elsevier Ltd. doi:10.1016/j.poly.2009.03.012 * Corresponding authors. Tel.: +1 713 743 2740; fax: +1 713 743 2745. E-mail addresses: jbear@uh.edu (J.L. Bear), kkadish@uh.edu (K.M. Kadish). Polyhedron 28 (2009) 1551–1555 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly