Ambi-valence taken literally: Ruthenium vs iron oxidation in (1,1′-diphosphinoferrocene)ruthenium(II) hydride and chloride complexes as deduced from spectroelectrochemistry of the heterodimetallic mixed-valent intermediates

Article abstract of DOI:10.1021/om1004258

Combining two different redox-active organometallic moieties, we prepared the compounds [(Cym)RuCl(dpf)](PF₆), with Cym = p-cymene = 1-isopropyl-4-methylbenzene, and the diphosphinoferrocenes (dpf) 1,1′-bis(diphenylphosphino)ferrocene (dppf; complex 3), 1,1′-bis(diisopropylphosphino)ferrocene (dippf; complex 4), and 1,1′-bis(diethylphosphino)ferrocene (depf; complex 5) as well as the structurally characterized hydride complex [(C₅Me₅) RuH(dippf)] (2). In contrast to the case for 2, with an approximately staggered ferrocene conformation, the chloride complexes 3-5 exhibit a syn-periplanar ferrocene arrangement due to a Cl...H(C₅H₄) interaction in the solid and in solution. The related new compounds [(Cym)RuH(dppf)](PF₆) (6) and trinuclear (μ-dpf)[(Cym)RuCl ₂)]₂ (7-9) were also obtained and identified by ¹H and ³¹P NMR spectroscopy. The redox behavior of 2-6 and of the known [(C₅Me₅)RuH(dppf)] (1) was investigated using cyclic voltammetry, spectroelectrochemistry in the UV/vis/near-IR and IR regions, and, in part, by EPR. The first oxidation of the areneruthenium compounds 3-6 occurs reversibly at the ferrocene site, while the reduction proceeds via an ECE two-electron pattern under chloride dissociation. These results are compared to those obtained for the pentamethylcyclopentadienide/ hydride complexes 1 and 2, which demonstrate unambiguously the ruthenium center as the site of the first electron loss. The different results for the two kinds of heterodimetallic d⁵/d⁶ mixed-valent intermediates, Fe^IIRu^III for 1⁺ and 2⁺ and Fe ^IIIRu^II for 3⁺-6⁺, are discussed with respect to the possible uses of such heterodinuclear systems in H ₂ conversion catalysis.

Full text of DOI:10.1021/om1004258

Organometallics 2010, 29, 5511–5516 5511
DOI: 10.1021/om1004258
Ambi-Valence Taken Literally: Ruthenium vs Iron Oxidation in
(1,10-Diphosphinoferrocene)ruthenium(II) Hydride and Chloride
Complexes as Deduced from Spectroelectrochemistry of the
Heterodimetallic “Mixed-Valent” Intermediates†
Torsten Sixt,† Monika Sieger,† Michael J. Krafft,† Denis Bubrin,† Jan Fiedler,‡ and
Wolfgang Kaim*,†
†Institut fu€r Anorganische Chemie, Universita€t Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany,
‡
and J. Heyrovsky Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic,
ꢁ
Dolejskova 3, CZ-18223 Prague, Czech Republic
ꢀ
Received May 5, 2010
Combining two different redox-active organometallic moieties, we prepared the compounds [(Cym)-
RuCl(dpf)](PF6), with Cym = p-cymene = 1-isopropyl-4-methylbenzene, and the diphosphinoferrocenes
(dpf) 1,10-bis(diphenylphosphino)ferrocene (dppf; complex 3), 1,10-bis(diisopropylphosphino)ferrocene
(dippf; complex 4), and 1,10-bis(diethylphosphino)ferrocene (depf; complex 5) as well as the structurally
characterized hydride complex [(C5Me5)RuH(dippf)] (2). In contrast to the case for 2, with an
approximately staggered ferrocene conformation, the chloride complexes 3-5 exhibit a syn-periplanar
ferrocene arrangement due to a Cl H(C5H4) interaction in the solid and in solution. The related new
3 3 3
compounds [(Cym)RuH(dppf)](PF6) (6) and trinuclear (μ-dpf)[(Cym)RuCl2)]2 (7-9) were also obtained
1
and identified by H and 31P NMR spectroscopy. The redox behavior of 2-6 and of the known
[(C5Me5)RuH(dppf)] (1) was investigated using cyclic voltammetry, spectroelectrochemistry in the UV/
vis/near-IR and IR regions, and, in part, by EPR. The first oxidation of the areneruthenium compounds
3-6 occurs reversibly at the ferrocene site, while the reduction proceeds via an ECE two-electron pattern
under chloride dissociation. These results are compared to those obtained for the pentamethylcyclopenta-
dienide/hydride complexes 1 and 2, which demonstrate unambiguously the ruthenium center as the site of
the first electron loss. The different results for the two kinds of heterodimetallic d5/d6 mixed-valent
intermediates, FeIIRuIII for 1þ and 2þ and FeIIIRuII for 3þ-6þ, are discussed with respect to the possible
uses of such heterodinuclear systems in H2 conversion catalysis.
Introduction
formed by a heavier homologue, ruthenium, help to tolerate
formally uncharged ligands such as arenes.4,5 With regard
to the application potential, the ferrocenes have found
much interest not only as one-electron transfer and storage
moieties2,3,6 but also as molecular scaffolds for catalytically
relevant structures (involving, for example, the 1,10-disub-
stituted derivatives).7 On the other hand, the organoruthe-
nium complex fragments [Ru(CnRn)]x, n=5, 6, can have both
a direct catalytic function in coordination compounds8 and
The þIII/þII oxidation state change of the group 8
elements in their compounds is a most common and well-
investigated elementary reaction. This holds not only for the
atoms in solid-state materials or classical metal complexes
but also for organometallic systems.1 Whereas the lightest
homologue, iron, is best established through the remarkably
stable ferrocenium/ferrocene pair,2,3 the more inert bonds
† Part of the Dietmar Seyferth Festschrift.
(5) (a) Kaim, W.; Reinhardt, R.; Sieger, M. Inorg. Chem. 1994, 33,
4453. (b) Sarkar, B.; Kaim, W.; Fiedler, J.; Duboc, C. J. Am. Chem. Soc.
2004, 126, 14706.
(6) See, e.g.: Ornelas, C.; Aranzaes, J. R.; Cloutet, E.; Alves, S.;
Astruc, D. Angew. Chem. 2007, 119, 890; Angew. Chem., Int. Ed. 2007,
46, 872.
(1) (a) Elschenbroich, C. Organometallics; Wiley-VCH: Chichester,
U.K., 2005. (b) Crabtree, R. H. The Organometallic Chemistry of the
Transition Metals, 5th ed.; Wiley: Hoboken, NJ, 2009. (c) Geiger, W. E.
Organometallics 2007, 26, 5738.
(2) (a) Togni, A., Hayashi, T., Eds. Ferrocenes: Homogeneous Cata-
lysis, Organic Synthesis, Materials Science; VCH: Weinheim, Germany,
(7) (a) Hartwig, J. F. Acc. Chem. Res. 1998, 31, 852. (b) Siemeling, U.;
ꢁ
ꢁ
ꢁ
ꢀ
1995. (b) Stepnicka, P., Ed. Ferrocenes: Ligands, Materials and Biomole-
cules; Wiley: Chichester, U.K., 2008.
Auch, T.-C. Chem. Soc. Rev. 2005, 34, 584. (c) Arrayas, R. G.; Adrio, J.;
Carretero, J. C. Angew. Chem. 2006, 118, 7836; Angew. Chem., Int. Ed.
2006, 45, 7674.
(3) (a) Zanello, P. Inorganic Electrochemistry; RSC: Cambridge, U.K.,
2003. (b) Albinati, A.; Fabrizi de Biani, F.; Leoni, P.; Marchetti, L.; Pasquali,
M.; Rizzato, S.; Zanello, P. Angew. Chem. 2005, 117, 5847; Angew. Chem.,
Int. Ed. 2005, 44, 5701. (c) Kurosawa, M.; Nankawa, T.; Matsuda, T.;
Kubo, K.; Kurihara, M.; Nishihara, H. Inorg. Chem. 1999, 38, 5113.
(4) Melchart, M.; Sadler, P. J. In Bioorganometallics; Jaouen, G., Ed.;
Wiley-VCH: Weinheim, Germany, 2006; p 39.
(8) (a) Bruneau, C., Dixneuf, P., Eds. Ruthenium Catalysis and Fine
Chemistry; Springer: Berlin, 2004. (b) Ohkuma, T.; Utsumi, N.; Tsutsumi,
K.; Murata, K.; Sandoval, C.; Noyori, R. J. Am. Chem. Soc. 2006, 128, 8734.
(c) Zhou, H.; Li, Z.; Wang, Z.; Wang, T.; Xu, L.; He, Y.; Fan, Q.-H.; Pan, J.;
Gu, L.; Chan, A. S. C. Angew. Chem. 2008, 120, 8592; Angew. Chem., Int.
Ed. 2008, 47, 8464.
r
2010 American Chemical Society
Published on Web 08/02/2010
pubs.acs.org/Organometallics

5512 Organometallics, Vol. 29, No. 21, 2010
Sixt et al.
exhibit physiological effects in the field of anticancer
activity.4,9
Our earlier studies11 confirmed this claim, and similar assertions
were made recently for the halide species [(C5Me5)RuX-
Combining ferrocene (Fc) and organoruthenium moieties
within one molecular entity raises the question of the electron
transfer sequence for the two MIII/II transitions. This se-
quence, which would involve heterodimetallic mixed-valent
intermediates Fcþ/RuII or Fc/RuIII, is not a priori obvious; it
depends on the molecular configuration and environment,
and it is of significance for the expected overall reactivity.10
Following previous exploratory studies on Os/Fc and Ru/Fc
examples,11 we now present a more comprehensive report on
the heterometallic complexes 1-9, as obtained from reac-
tions between 1,10-bis(diorganophosphino)ferrocenes and
organoruthenium complex fragments.
(dpf)]nþ 10b,c
.
Herein we present further spectroelectrochemical and EPR
evidence and describe another derivative, [(C5Me5)RuH-
(dippf)] (2; dippf =1,10-bis(diisopropylphosphino)ferrocene).
Compound 2 has been structurally characterized, as have the
complexes [(Cym)RuCl(dpf)](PF6) (Cym=p-cymene=1-iso-
propyl-4-methylbenzene) with dpf = dppf (complex 3),14 dippf
(complex 4), and 1,10-bis(diethylphosphino)ferrocene (depf;
complex 5) with a neutral arene instead of a cyclopenta-
dienide coligand at ruthenium. Complexes of the (Cym)ClRuþ
fragment with R-diimine chelate ligands were reported to
exhibit a characteristic ECE reduction behavior with loss of
chloride,5 while the dpf ligand family may be described as
“noninnocent”,15 exhibiting the potential for redox activity.
The compounds [(Cym)RuH(dppf)](PF6) (6) and (μ-dpf)-
[(Cym)RuCl2)]2 (7-9; dpf = dppf, depf, dippf) were also
obtained and identified by NMR spectroscopy. Complex 7
was reported earlier,16 but claims of a reduction to Ru0
species could not be substantiated spectroscopically. The
osmium analogue [(Cym)OsCl(dppf)](PF6) (10) of the
known11a,14 3 has been described previously.11b
Experimental Section
Instrumentation. EPR spectra in the X band were recorded
with a Bruker System ESP 300. 1H and 31P NMR spectra were
taken on a Bruker AC 250 spectrometer. IR spectra were
obtained using a Philips PU 9800 FT-IR instrument. UV/vis/
near-IR absorption spectra were recorded on a Bruins Instru-
ments Omega 10 spectrophotometer. Cyclic voltammetry was
carried out in 0.1 M Bu4NPF6 solution using a three-electrode
configuration (glassy-carbon working electrode, Pt counter
electrode, Ag reference) and a PAR 273 potentiostat and func-
tion generator. The ferrocene/ferrocenium (Fc/Fcþ) couple
served as internal reference. Electron consumption was assessed
by comparing integrated peak areas. Spectroelectrochemistry
was performed using an optically transparent thin-layer elec-
trode (OTTLE) cell.17a A two-electrode capillary served to
generate intermediates for X-band EPR studies.17b
A catalysis of the energy-producing dihydrogen oxidation to
Hþ or of the reversed process is of interest, both for technical
processes, especially fuel cell reactions, and for microbiological
activity.12 Biochemically, this reaction is catalyzed by hydro-
genase enzymes, several of which were shown to possess oligo-
metal active sites such as Fe,Ni-heterodimetallic structures.13
Synthetic efforts for molecular systems effecting the reaction
H2 f 2Hþ þ 2e- via “redox-switch” models have been des-
cribed,10a,d,e,h involving inter alia the Fe,Ru-heterodimetallic
hydride complex [(C5Me5)RuH(dppf)] (1; dppf=1,10-bis(diphe-
nylphosphino)ferrocene), which was shown to reduce methyl-
viologen with H2.10a In that work by Hembre et al. it was
suggested that the oxidation of 1, which is crucial for the catalytic
process, involves the ruthenium and not the ferrocene iron.10a
Syntheses. The dpf ligands were obtained by an established
procedure19b unless they were commercially available. Complex
1 was prepared according to the literature.10a
(9) (a) Yan, Y. K.; Melchart, M.; Habtemariam, A.; Sadler, P. J.
Chem. Commun. 2005, 4764. (b) Ronconi, L.; Sadler, P. J. Coord. Chem.
Rev. 2007, 251, 1633. (c) Dyson, P. J.; Hartinger, C. Chem. Soc. Rev. 2009,
38, 391. (d) Jakupec, M. A.; Galanski, M.; Arion, V. B.; Hartinger, C. G.;
Keppler, B. K. Dalton Trans. 2008, 183.
(10) (a) Hembre, R. T.; McQueen, J. S.; Day, V. W. J. Am. Chem. Soc.
1996, 118, 798. (b) Shaw, A. P.; Norton, J. R.; Buccella, D.; Sites, L. A.;
Kleinbach, S. S.; Jarem, D. A.; Bocage, K. M.; Nataro, C. Organometallics
2009, 28, 3804. (c) Paim, L. A.; Batista, A. A.; Dias, F. M.; Golfeto, C. C.;
Ellena, J.; Siebald, H. G. L.; Ardisson, J. D. Transition Met. Chem. 2009, 34,
949. (d) Gloaguen, F.; Rauchfuss, T. B. Chem. Soc. Rev. 2009, 38, 100.
(e) Ringenberg, M. R.; Kokatam, S. L.; Heiden, Z. M.; Rauchfuss, T. B.
J. Am. Chem. Soc. 2008, 130, 788. (f) Heiden, Z. M.; Rauchfuss, T. B.
J. Am. Chem. Soc. 2009, 131, 3593. (g) Smith, K-T.; Rømming, C.; Tilset,
M. J. Am. Chem. Soc. 1993, 115, 8681. For a general overview on bimetallic
cooperativity related to H2 see: (h) Gray, T. G.; Veige, A. S.; Nocera, D. G.
J. Am. Chem. Soc. 2004, 126, 9760.
(11) (a) Sixt, T.; Fiedler, J.; Kaim, W. Inorg. Chem. Commun. 2000, 3,
80. (b) Kaim, W.; Sixt, T.; Weber, M.; Fiedler, J. J. Organomet. Chem. 2001,
637-639, 167.
(12) (a) Fukuzumi, S. Eur. J. Inorg. Chem. 2008, 1351. (b) Ogo, S.;
Kabe, R.; Hayashi, H.; Harada, R.; Fukuzumi, S. Dalton Trans. 2006, 4657.
(c) Hayashi, H.; Ogo, S.; Abura, T.; Fukuzumi, S. J. Am. Chem. Soc. 2003,
125, 14266. (d) Ogo, S.; Kabe, R.; Uehara, K.; Kure, B.; Nishimura, T.;
Menon, S. C.; Harada, R.; Fukuzumi, S.; Higuchi, Y.; Ohhara, T.; Tamada, T.;
Kuroki, R. Science 2007, 316, 585.
[(C5Me5)RuH(dippf)]10b (2). A 53 mg amount (0.086 mmol) of
18
[(C5Me5)RuCl2]2 and 60 mg (0.199 mmol) of aluminum
bronze were suspended in 5 mL of degassed toluene and stirred
with 86 mg (0.206 mmol) of dippf19 for 3 days at room
temperature. The filtrate obtained after separation was treated
with 27 mg (1.21 mmol) of sodium in 5 mL of methanol, and the
mixture was heated to reflux for 4 h. Removal of solvent,
redissolution in a very small amount of toluene, layering with
CH3OH, and cooling to -17 °C produced an orange precipitate
(14) (a) Jensen, S. B.; Rodger, S. J.; Spicer, M. D. J. Organomet.
Chem. 1998, 556, 151. (b) Mai, J.-F.; Yamamoto, Y. J. Organomet. Chem.
1998, 560, 223. See also: (c) Yee Ng, S.; Fang, G.; Kee Leong, W.; Yoong
Goh, L.; Garland, M. V. Eur. J. Inorg. Chem. 2007, 452. (d) Yee Ng, S.; Kee
Loeng, W.; Yoong Goh, L.; Webster, R. D. Eur. J. Inorg. Chem. 2007, 463.
(15) Ward, M. D.; McCleverty, J. A. Dalton Trans. 2002, 275.
(16) Estevan, F.; Lahuerta, P.; Latorre, J.; Sanchez, A.; Sieiro, C.
Polyhedron 1987, 6, 473.
ꢁ
(17) (a) Krejcik, M.; Danek, M.; Hartl, F. J. Electroanal. Chem.
Interfacial Electrochem. 1991, 317, 179. (b) Kaim, W.; Ernst, S.; Kasack, V.
J. Am. Chem. Soc. 1990, 112, 173.
(18) Oshima, N.; Suzuki, H.; Moro-Oka, Y. Chem. Lett. 1984, 1161.
(19) (a) Bishop, J. J.; Davison, A.; Katcher, M. L.; Lichtenberg,
D. W.; Merrill, R. E.; Smart, J. C. J. Organomet. Chem. 1971, 27, 241.
(b) Butler, I. R.; Cullen, W. R.; Kim, T.-J. Synth. React. Inorg. Met.-Org.
Chem. 1985, 15, 109.
(13) McGlynn, S. E.; Mulder, D. W.; Shepard, E. M.; Broderick,
J. B.; Peters, J. W. Dalton Trans. 2009, 4274.

Article
Organometallics, Vol. 29, No. 21, 2010 5513
(82 mg, 73%). Anal. Calcd for C32H52FeP2Ru (655.63 g/mol):
C, 58.62; H, 7.99. Found: 58.63; H, 7.99. For NMR see Tables
S1 and S2 (Supporting Information).
[(Cym)RuCl(dpf)](PF6) (3-5). A suspension containing 200
mg (0.328 mmol) of [(Cym)RuCl2]2 in 20 mL of a CH3OH/
CH3CN mixture was treated with 174 mg (0.653 mmol) of
TlNO3. After it was stirred for 30 min, this preparation was
added to 0.685 mmol of the corresponding dpf ligand, dissolved
in 30 mL of methanol (depf, dippf) or THF (dppf). After 1 h and
the removal of most solvent the oily residue was redissolved in
methanol and precipitated with diethyl ether and the residue this
time was dissolved in ethanol/water (4/1). Excess ammonium
hexafluorophosphate precipitated the product, which was
washed with wet ethanol and water and dried in vacuo.
dpf = dppf (3; 298 mg, 56%).14 Anal. Calcd for C44H42ClF6-
FeP3Ru (934.34 g/mol): C, 54.48; H, 4.36. Found: 54.00; H,
4.39. For NMR see Tables S1 and S2.
dpf = dippf (4; 248 mg, 52%). Anal. Calcd for C32H50ClF6-
FeP3Ru (834.03 g/mol): C, 46.08; H, 6.04. Found: 45.37; H,
6.26. For NMR see Tables S1 and S2.
dpf = depf (5; 255 mg, 49%). Anal. Calcd for C28H42ClF6Fe-
P3Ru (777.92 g/mol): C, 43.23; H, 5.44. Found: 42.78; H, 5.32.
For NMR see Tables S1 and S2.
Figure 1. Molecular structure of 2 in the crystal state.
in analogy to the dppf complex 1 reported by Hembre et al.10a
The areneruthenium hydride complex 6 was prepared from the
chloride complex 3 using LiBEt3H; other approaches were un-
successful. The heterotrinuclear compounds 7,16 8, and 9 were
obtained from [Ru(Cym)Cl2]2 and the corresponding 1,10-di-
phosphinoferrocenes in a 1:1 ratio under mild conditions. Using
higher temperatures or activation with silver or thallium(I) salts,
[(Cym)RuH(dppf)](PF6) (6). An orange solution of 301 mg
(310 mmol) of 314 in 50 mL of THF was cooled to 205 K and
treated with 0.39 mL of a commercially available 1 M LiBEt3H
solution in THF for 1 h. Warming to room temperature, stirring
for 4 h, removing the solvent in vacuo, washing three times with
15 mL of water and 3 mL of diethyl ether each, dissolving in
acetonitrile, filtering over Celite, and eventual drying under
vacuum yielded 248 mg (0.265 mmol, 86%) of 6 as a yellow
solid. Anal. Calcd for C44H43F6FeP3Ru (935.64 g/mol): C,
56.48; H, 4.63. Found: C, 56.16; H, 4.90. MS: m/z 791.12
[M - PF6]þ. UV-vis (CH2Cl2): 320 (sh), 440 (sh) nm.
(μ-dpf)[(Cym)RuCl2)]2 (7-9). A mixture containing 78 mg
(0.126 mmol) of [(Cym)RuCl2]2 and 0.120 mmol of the dpf
ligand were stirred for 6 h in 20 mL of CH3OH (dippf, depf) or
acetone (dppf). The resulting precipitate was collected by filtra-
tion, washed with methanol, and dried in vacuo.
-
followed by Cl-/PF6 anion exchange, produced the hetero-
dinuclear systems 3-5. The NMR spectra of 3-5, 7-9, and the
hydride complexes 2 and 6 confirm the composition of the
compounds (Tables S1 and S2) and shed light on the P-sub-
stituent (Ph, iPr, Et) and Ru-ligand effects (H vs Cl). The 1Hand
31P chemical shifts not only reflect the lowered symmetry of the
dinuclear systems 1-6 and the anticipated substituent effects,
e.g. Cl- vs more electron-rich hydride, cymene vs more electron-
rich pentamethylcyclopentadienide, P-phenyl vs more electron-
rich P-isopropyl, but also point to specific inter- and intra-
dpf = dppf (7) was reported earlier.16 For NMR see Tables
S1 and S2.
dpf = depf (8; 24 mg, 24%). Anal. Calcd for C38H56Cl4Fe-
P2Ru2 (974.60 g/mol): C, 46.83; H, 5.79. Found: 46.05; H, 5.95.
For NMR see Tables S1 and S2.
dpf = dippf (9; 46 mg, 22%). Anal. Calcd for C42H64Cl4Fe-
P2Ru2 (1030.70 g/mol): C, 48.94; H, 6.26. Found: 48.71; H, 6.31.
For NMR see Tables S1 and S2.
molecular interactions. One such case is the Cl H attraction
3 3 3
between chlororuthenium and two H(cyclopentadienyliron)
atoms, which causes 1H NMR deshielding of one set of protons
and is attributed to a crystallographically confirmed (cf. below)
intramolecular bridging, as illustrated in Figure 4.
Crystallography. Single crystals were obtained by layering a
toluene solution of 2 with methanol at 256 K, by cooling of a
saturated methanol solution of 4 to 276 K, or by slowly evaporating
a methanolic solution of 5 at room temperature. The crystals were
analyzed using a Syntex P21 diffractometer at 173 K with graphite-
Structures. The compounds 2-5 could be crystallized for
structure analysis. Complex 3 and its structure were reported
previously.14a Crystallographic data of compounds 2, 4, and
5 are summarized in Table S3, and selected bond parameters
are given in Tables S4 and S5 (Supporting Information).
Figures 1-4 and Figure S2 (Supporting Information) illus-
trate the structural situation in the hydride 2 and in two of
the cymene/chloride complex salts: viz., complexes 5 and 4.
Compounds 2-5 all contain ruthenium chelated by the
1,10-diphosphinoferrocene ligands with the hydride/chloride
and (C5Me5-)/(Cym) 6e ligands in an approximately per-
pendicular position. The chelate bite angles P-Ru-P are ca.
95°, and the P-Ru-Cl angles for complexes 3-5 are be-
tween 81 and 88°, in agreement with the preferential “octa-
hedral” coordination of the 4d6 center ruthenium(II). The
˚
monochromated KR radiation (0.710 73 A). Direct method refine-
ment using available programs20a was used; the C-H hydrogen
atoms were included in calculated positions. In 2 the metal hydride
H atom was located but refined at a realistic21 distance of 159 pm at
last refinement. For complex 4 with two independent molecules in
the unit cell an absorption correction procedure20b led to significant
-
improvement. Complex 5 exhibited a disorder of the PF6 ions
involving two edge-sharing octahedra.
Results and Discussion
Synthesis and NMR Spectroscopy. Different from a recent
report,10b the neutral hydride compound 2 was prepared here
Ru-Cl bond lengths do not vary significantly; the Ru Fe
3 3 3
and Ru-P distances are longer for the dippf complexes with
the space-demanding isopropyl groups.
(20) (a) SHELXTLN, Version 5.1; Bruker AXS. (b) Walker, N.; Stuart,
D. Acta Crystallogr. 1983, A39, 158.
The structure characterization of compound 2 allowed us
to find the hydride ligand at a reasonable Ru-H distance
of about 159 pm.21 Figure S1 (Supporting Information)
~
(21) Ohkuma, T.; Koizumi, M. M.; Muniz, K.; Hilt, G.; Kabuto, C.;
Noyori, R. J. Am. Chem. Soc. 2002, 124, 6508.

5514 Organometallics, Vol. 29, No. 21, 2010
Sixt et al.
Figure 4. Cl H interactions in the molecular structure of 5
(M = Ru; P substituents omitted for clarity).
3 3 3
Figure 2. View along the Cp-Fe-Cp axis in the molecular
structure of 2.
Figure 5. Cyclic voltammogram of 4 at 298 K in 0.1 M
Bu4NPF6/THF.
Figure 3. Molecular structure of 5 in the crystal state.
illustrates the particular position of the hydride in compound 2
and its shielding by the isopropyl groups. Figures 1 and 2 show
that the ferrocene-cyclopentadienide rings are a little less stag-
gered (27°) in 2 than in the related 1 (33°; 36° is the value for
“ideal” staggering). Whereas the hydride compounds 1 and 2
thus show near staggering of the ferrocene-cyclopentadienide
rings, the chloride complex ions exhibit an almost synperiplanar
arrangement with twist angles below 6°. This conformation is
Figure 6. Cyclic voltammogram of 6 at 298 K in 0.1 M
Bu4NPF6/CH3CN.
and 6 show two such typical responses, and Table S6
(Supporting Information) summarizes the potentials.
According to our experiments the cyclopentadienide/hy-
dride complexes 1 and 2 first exhibit a reversible one-electron
oxidation, followed by irreversible processes. The reversibil-
ity of the second oxidation (to “RuIV”) and the quasi-
reversibility of the third oxidation of 1 as reported by
Hembre et al.10a could not be reproduced. Changing the π-
accepting P-phenyl to the more σ donating P-isopropyl
groups in going from 1 to 2 causes a decrease of the oxidation
potential by about 270 mV.
favored because of an intramolecular H Cl H bridging
3 3 3 3 3 3
which has already been mentioned in the discussion of solution
1H NMR results. Cl-H distances of about 270 pm and the
particular conformation (Figure 4) point to a weak but struc-
ture-determining interaction.
The cymene ligands in 4 and 5 are nearly planar and show
the isopropyl group oriented toward the chloride ligand.
Cyclic Voltammetry. The complexes 1-6 exhibit charac-
teristic cyclic voltammograms in solution involving two
oxidation processes and, in the case of chloro derivatives, a
reduction coupled to a chloride release (cf. below). Figures 5
The cymene/chloride complexes also exhibit a first rever-
sible oxidation, albeit at higher potentials. The difference

Article
Organometallics, Vol. 29, No. 21, 2010 5515
Scheme 1
Scheme 2
between the dppf and dippf complexes (3 vs 4) amounts to
only 110 mV. Whereas no reduction of the hydride com-
plexes 1 and 2 was observed within the available potential
range, the cationic chloride complexes 3-5 are reduced in a
two-electron fashion with a back-oxidation in cyclic voltam-
metry at less negative potentials.11 The underlying ECE
reductive elimination/EEC oxidative addition mechanism
between [M(CnRn)Cl(L)]þ and [M(CnRn)(L)] forms is well-
known for such complexes (Scheme 1).5,22
The hydrido complex cation 6 exhibits a well-behaved
reversible one-electron oxidation near that of the chloride
precursor 3, which is attributed to a RuII-ferrocenium
configuration on the basis of EPR silence and absorption
spectrum (cf. below). A second wave for irreversible oxida-
tion with double intensity (Figure 6) is assigned to a proton-
releasing oxidation of ruthenium. The trinuclear complexes
7-9 show a similar behavior on the anodic side; they are also
reduced in an irreversible multielectron step (Scheme 2). as
was noted before for compound 7.16
Figure 7. EPR spectrum of electrogenerated 2þ at 4 K in glassy
frozen 0.1 M Bu4NPF6/THF.
EPR response under those conditions. Table S7 (Supporting
Information) gives available g values.
It is thus obvious from the values in Table S7 that the
hydride cations 1þ and 2þ are ferrocene/Ru(III) species,
whereas the oxidized complexes [(Cym)RuCl(dpf)]2þ are
ferrocenium/RuII systems. The effects of substitution are
only small, however, and it is most revealing that the osmium
analogue 10þ of 3þ exhibits an almost identical ferrocenium-
type EPR response11b despite the much higher spin-orbit
coupling constant of that metal.
IR Spectroelectrochemistry. Infrared spectroelectrochemical
studies of metal complexes have often focused on stretching
bands of ligands such as CO, CN, and NO with multiple bonds
because of the band intensity and the favorable, nonfingerprint
spectral region around 2000 cm .
-1 25 However, metal-hydride
vibrations also occur frequently in that region,21 albeit with
lower intensity. We therefore took advantage of the reversibility
of the oxidation processes for the ruthenium hydrides 1, 2, and 6
and studied the response of the M-H stretching band. Unfortu-
nately, complex 6with(Cym)Ru2þ exhibited a very low intensity
Ru-H stretching absorption at about 1950 cm-1 which shifted
only slightly to about 1975 cm-1 on spectroelectrochemical oxi-
dation. Compounds 1 and 2 involving electron-rich (C5Me5)-
Ruþ showed a better response (Figure 8, Table S8 (Supporting
Information)), but even in this case the intensity was severely
diminished in the oxidized (=dipositive) state, indicating a
rather small dipole moment change of the Ru-H vibration.
The high-energy shift of about 50 cm-1 for the Ru-H stretch on
oxidation of 1 or 2 reflects the oxidation of ruthenium and the
enhanced Coulombic attraction, while the smaller shift for 6 is in
agreement with a remote, ferrocene-based oxidation.
EPR Spectroscopy. To identify the site of electron removal
on oxidation, we have employed EPR spectroscopy. Although
both iron(II) in ferrocene and ruthenium(II) centers are oxidized
fairly easily, the resulting EPR signals in glassy frozen solution
will differ significantly. Whereas organometallic ruthenium(III)
centers in a coordination environment as in the complexes
described here should lead to relatively small g anisotropy,
Δg=g1 - g3, with individual g components between 1.5 and
3.0,23 the ferrocenium system with its special symmetry and d
orbital splitting is distinguished by broad EPR signals, often
observable only at 4 K, with large g anisotropy Δg, especially
one component at about g = 4; ferrocenium itself has g1,2
4.36 and g3=1.28.24
=
Figure 7 shows the low-temperature (4 K) spectrum for the
cyclopentadienide/hydride cation 2þ, taken from in situ
electrolytic oxidation of the precursor complex in THF/0.1
M Bu4NPF6. A representative EPR spectrum of the oxidized
cymene/chloride complex 3þ at 4 K has been shown
earlier;11a the cymene/hydride system 6þ did not exhibit an
UV/Vis/Near-IR Spectroelectrochemistry. Similarly to
EPR spectra, absorption spectra of oxidized species should
indicate whether the ruthenium(II) center or the ferrocene
iron is oxidized. Regular ruthenium(III) centers are expected
to exhibit weak long-wavelength (vis/near-IR) ligand-field
(22) Kaim, W. In New Trends in Molecular Electrochemistry; Pombeiro,
A. J. L., Ed.; Fontis Media: Lausanne, Switzerland, 2004, p 127.
(23) Winter, R.; Hornung, F. M. Organometallics 1999, 18, 4005.
(24) Elschenbroich, C.; Bilger, E.; Ernst, R. D.; Wilson, D. R.;
Kralik, M. S. Organometallics 1985, 4, 2068.
(25) Best, S. T. In Spectroelectrochemistry; Kaim, W., Klein, A., Eds.;
RSC: Cambridge, U.K., 2008; p 1.

5516 Organometallics, Vol. 29, No. 21, 2010
Sixt et al.
Figure 8. IR spectroelectrochemical oxidation of 2 at 298 K in
0.1 M Bu4NPF6/THF. Spectra were collected during the poten-
tial scan at the first oxidation peak.
(LF) transitions between occupied and unoccupied d orbi-
tals.26a Ferrocenium systems, on the other hand, are well-
known to exhibit one weak absorption band at about 600
nm, involving a ligand-to-metal charge transfer (LMCT, 2E1u)
transition.26 The precursors to these oxidations do not interfere
with the long-wavelength features because the compounds 1-6
have their lowest energy bands in the UV region.
UV/vis spectroelectrochemistry of compounds 1-5 was per-
formed in 0.1 M Bu4NPF6 solutions in an optically transparent
thin-layer electrolytic (OTTLE) cell. The results are illustrated in
Figure 9 and given in Table S9 (Supporting Information).
The data from OTTLE spectroelectrochemistry confirm
the results from EPR spectroscopy, assigning on initial
ferrocene oxidation for the cymene/chloride and cymene/
hydride complexes (λmax ∼630 nm, ε < 1000 M-1 cm-1) and
a first ruthenium oxidation for the hydride compounds 1þ
and 2þ (λmax ∼900 nm, Figure S3 (Supporting Information)).
Hembre et al.10a have attributed this latter transition to an
intramolecular metal-to-metal charge-transfer (MM0CT)
transition from FeII to RuIII which would involve a distance
of about 440 pm without conjugation. In our view a Ru-
based ligand field transition26a is also plausible.
Figure 9. UV/vis spectroelectrochemical oxidation of 6 at 298
K in 0.1 M Bu4NPF6/CH2Cl2. Spectra were collected during the
potential scan at the first oxidation peak.
entities.27b In this report we have shown how cationic
hydridoruthenium groups can also be stabilized; otherwise,
such groups are typically prone to facile decay via proton
loss or via disproportionation.10b,g The control of the ruthe-
nium and, by implication, of the ferrocene iron oxida-
tion state through the organic 6e ligand ηn-CnR0n in [(ηn-
CnR0n)RuH(dpf)]k is responsible for the unambiguously
established ambi-valence, RuII-FeIII or RuIII-FeII, on oxi-
dation of ruthenium(II)-ferrocene precursors. Electron-rich
-
C5Me5 as coligand was found to favor the latter alterna-
tive, while complexes with neutral p-cymene yielded
ruthenium(II)-ferrocenium configurations, both with RuH
and RuCl complexes. Reactivity studies of these potential
heterodimetallic “redox-switch” catalysts10a in either con-
figuration with H2, CO2, or other relevant targets10 will be
the next research step.
Acknowledgment. This work was supported by the
Deutsche Forschungsgemeinschaft and the Fonds der
Chemischen Industrie. Support from the EU (COST
D35) and the Grant Agency of the Czech Republic
(Grants 203/09/0705, 203/08/1157) is also gratefully ac-
knowledged. We thank Drs. W. Schwarz and F. Lissner
for X-ray diffraction measurements.
Summary
Due to the rigid chelate framework of the dpf ligands
and due to their electronic capacity to act as noninnocent
ligands (i.e., as dpf or dpfþ), these systems can stabilize
unusual metal configurations,27a including copper(I)-quinone
Supporting Information Available: CIF files giving X-ray
crystallographic data for compounds 2, 4 and 5, Scheme S1
(syntheses), Tables S1-S9, and Figures S1 (space-filling model
of 2), S2 (molecular structure of 4), and S3 (UV-vis spectrum
of 2þ). This material is available free of charge via the Internet at
http://pubs.acs.org.
(26) (a) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.;
Elsevier: Amsterdam, 1984. (b) Prins, R. J. Chem. Soc., Chem. Commun.
1970, 280.
(27) (a) Pilloni, G.; Toffoletti, A.; Bandoli, G.; Longato, B. Inorg.
Chem. 2006, 45, 10321. (b) Roy, S.; Sarkar, B.; Bubrin, D.; Niemeyer, M.;
ꢁ
Zalis, S.; Lahiri, G. K.; Kaim, W. J. Am. Chem. Soc. 2008, 130, 15230.