Photon Factory Activity Report 2014 #32 (2015) B

AR-NW2A/2014P004

Mixed Valency in Hydrogen-Bonded Dimers of Half-Lantern Dirhodium Complexes
Jin-Long and Masahiro EBIHARA*
Department of Chemistry and Biomolecular Science, Faculty of Engineering, Yanagido, Gifu 5011193, Japan
1 Introduction
Mixed-valence complexes have received muchattention in coordination chemistry over the last fourdecades. Recently, a few examples of mixed valency inhydrogen-bonded dimer complexes have been reported.
In 2007 Tadokoro and co-workers reported a hydrogenbonded dimerized complex [ReIIICl2(PBu3)2(Hbim)]2(H2bim = biimidazole) that showed two successive oneelectron oxidation and two successive one-electron
reduction processes. They explained that the firstoxidation step generated the asymmetrically protonatedmixed-valence
species[ReIVCl2(PBu3)2(bim)]III+[Re Cl2(PBu3)2(H2bim)] . Synchronized motion of protonand electron stabilized this type of mixed-valencecompound. We have synthesized half-lantern-type
dirhodium complexes with two biimidazole ligands[Rh2(O2CR)2(H2bim)2(L)2]2+ (R = Bu ([1(L)2]2+), Pr([2(L)2]2+)) and their deprotonated dimerized species[Rh2(O2CR)2(Hbim)2(L)2]2 (R = Bu ([1'(PPh3)2]2), Pr([2'(PPh3)2]2)). The latter complexes formed hydrogenbonded dimers with four complementary NH···N
hydrogen bonds.
2 Experiment[1Cl2] and [2Cl2] was obtained by the reaction ofH2bim with Rh2(O2CR)4] (R = Pr and Bu), respectively, in
1,2-dichloroethane and consecutive treatment with
HCl/MeOH. [1](PF6)2 was afforded in a similar method tothat for [1Cl2] using HPF6 instead of HCl. [1](PF6)2reacted with PPh3 to afford [1(PPh3)2](PF6)2. Protonabstruction from [1(PPh3)2](PF6)2 and [2(PPh3)2](PF6)2 by
KOH/MeOH solution gave hydrogen bonded dimers of[1ʹ′(PPh3)2] and [2ʹ′(PPh3)2], respectively. The crystals of[1Cl2], [2Cl2], [1(PPh3)2]Cl2, [1ʹ′(PPh3)2] and [2ʹ′(PPh3)2]
were obtained and their X-ray structures were determined.
The X-ray data collection of [2ʹ′(PPh3)2] was carried outin KEK.
3 Results and Discussion 	  The structure of [1ʹ′(PPh3)2]2 shown in Figure 1 is ahydrogen-bonded dimer of the half-lantern complex. TheNH of each biimidazolate (Hbim–) ligand donates ahydrogen to the N of Hbim– on the neighboring complex
and then four hydrogen bonds connect two dirhodiumcomplexes. The structure of [2ʹ′(PPh3)2]2 is very similar to[1ʹ′(PPh3)2]2. Dihedral angles between two Hbim– ligands
on each dirhodium unit are ca 16°. These angles hindered
coplanar self-complementary hydrogen bonds between
the biimidazolate ligands on the different dirhodium units.
To form hydrogen bonds between the noncoplanarbiimidazolate ligands, the Rh2–Rh2ʹ′ axis in [1ʹ′(PPh3)2]2was rotated 25.7° from the eclipsed position with theRh1–Rh1ʹ′ around the two-fold axis, and the dihedral
angle between the biimidazolate ligands is 34.90(13)°.
The CV of [1](PF6)2 in CH3CN did not show any clear
redox process between –2.1 V and 1.9 V. However,[1(PPh3)2](PF6)2 and [1ʹ′(PPh3)2]2 in CH2Cl2 showed someoxidation processes The biimidazole complex monomer[1(PPh3)2](PF6)2 exhibits one reversible (E1/2 = 0.208 V)
and two irreversible oxidation waves (Epa = 0.484 V and0.697 V). The first reversible oxidation wave can be
assigned to oxidation of the dirhodium core Rh24+/Rh25+
and the second one to Rh25+/Rh26+. On the other hand, thebiimidazolate complex dimer [1ʹ′(PPh3)2]2 shows two setsof two successive reversible oxidation waves and one
irreversible oxidation wave. This indicated that relatively
stable mixed-valence state existed for [1ʹ′(PPh3)2]2.
The ESR spectra of the oxidized [1ʹ′(PPh3)2]2+ andoptimized structure of the model complex cations
suggested that stability of the mixed valence state is due
to proton coupled electron motion. We have proposed thecomplex cation that consist of the oxidized andcompletely deprotonated [Rh2(O2CR)2(bim)2(PPh3)2]–hydrogenbondedwith
thedicationof[Rh2(O2CR)2(H2bim)2(PPh3)2]2+.

Fig. 1: The structure of [1ʹ′(PPh3)2]2
Acknowledgement
This work was supported by JSPS KAKENHI Grant
Numbers 22550058 and 26410068.
References[1] Jin-Long et al., Inorg. Chem. 54, 2331 (2015).
* ebihara@gifu-u.ac.jp