Synthesis and structural characterization of a novel diosmium(III) compound: Os2(ap)4Cl2

Article abstract of DOI:10.1016/j.ica.2003.09.021

The reaction between Os₂(OAc)₄Cl₂ and Hap (Hap is 2-anilinopyridine) under prolonged refluxing conditions resulted in a new Os₂(III) compound, Os₂(ap)₄Cl₂ (1). The molecular structure of 1 was determined from a single crystal X-ray diffraction study, which revealed an Os-Os bond length of 2.396[1] ?, and a cis-(2,2) arrangement of the ap ligands. Also reported are magnetic, electrochemical and spectroscopic properties of 1.

Full text of DOI:10.1016/j.ica.2003.09.021

Inorganica Chimica Acta 357 (2004) 1313–1316
www.elsevier.com/locate/ica
Note
Synthesis and structural characterization of a novel
diosmium(III) compound: Os2(ap)4Cl2
a,
b
c
Tong Ren *, Damon A. Parrish b,1, Jeffrey R. Deschamps , Judith L. Eglin ,
a
a
b
b
Guo-Lin Xu , Wei-Zhong Chen , Martin H. Moore , Terence L. Schull ,
b
Steven K. Pollack , R. Shashidhar , A.P. Sattelberger
b
c
a
Department of Chemistry, University of Miami, 1301 Memorial Drive, Coral Gables, FL 33146, USA
b
Naval Research Laboratory, Washington, DC 20375, USA
c
Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Received 18 August 2003; accepted 4 September 2003
Abstract
The reaction between Os2(OAc)4Cl2 and Hap (Hap is 2-anilinopyridine) under prolonged refluxing conditions resulted in a new
Os2(III) compound, Os2(ap)4Cl2 (1). The molecular structure of 1 was determined from a single crystal X-ray diffraction study,
ꢀ
which revealed an Os–Os bond length of 2.396[1] A, and a cis-(2,2) arrangement of the ap ligands. Also reported are magnetic,
electrochemical and spectroscopic properties of 1.
Ó 2003 Elsevier B.V. All rights reserved.
Keywords: Diosmium(III); 2-Anilinopyridinate; Crystal structure
1. Introduction
ap ligands in Os2(ap)4Cl2. Indeed, crowding of the li-
gands in the metathesis reaction involving 6-fluoro-2-
hydroxypyrinate resulted in the elimination of one of the
axial chloro ligands and the concurrent reduction of the
Os62þ core to Os52þ [8]. Nevertheless, our recent work on
Ru2(ap)4(C2R)2 type compounds demonstrated the
feasibility of accommodating two axial ligands on a
M2(ap)4 core [9]. In this contribution, we described the
successful preparation of Os2(ap)4Cl2 and its structural
and electrochemical characterizations.
The paddlewheel structural motif, Os2(LL)4Cl2, is
prevalent in diosmium chemistry, where bidentate
bridging ligands LL are mostly l-O,O0-carboxylates
[1,2]. As a convenient starting material, Os2(OAc)4Cl2
undergoes either complete ligand metathesis to yield
Os2(LL)4Cl2 compounds with LL as l-O,N-amidates [3]
and l-N,N0-formamidinates [4,5], or partial metathesis
to yield Os2(LL)2(OAc)2Cl2 with LL as l-P,C-
Ph2PC6H4 [6]. One peculiar exception is Os2(ap)3Cl3
(ap ¼ 2-anilinopyridinate), which was obtained from
brief refluxing (4 h) of Os2(OAc)4Cl2 with four equiva-
lents of Hap in the presence of Me3SiCl in toluene [7].
Incomplete metathesis might be attributed to the po-
tential steric conflict between two axial chloro and four
2. Results and discussion
Synthesis of Os2(ap)4Cl2 is straightforward using a
technique similar to that previously described for
Ru2(ap)4Cl [10]: refluxing Os2(OAc)4Cl2 with eight
equivalents of Hap in toluene using a micro Soxhlet
extractor. The extractor is fit with a glass thimble con-
taining a 1:1 mixture of K2CO3 and sand, which serves
as the scavenger for acetic acid to drive the metathesis to
completion. Pioneered by the laboratory of Doyle et al.
* Corresponding author. Tel.: +1-305-2846617; fax: +1-305-2841880.
E-mail addresses: tren@miami.edu (T. Ren), dparrish@ccs.nrl.
navy.mil (D.A. Parrish).
1
To whom crystallographic correspondence should be addressed.
Tel.: +1-202-404-2141; fax: +1-202-767-6874.
0020-1693/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2003.09.021

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T. Ren et al. / Inorganica Chimica Acta 357 (2004) 1313–1316
Table 1
ꢀ
Selected bond lengths (A) and angles (°) for both independent mole-
cules of 1
Os(1)–Os(1A)
2.3976(5)
2.5215(13)
2.069(5)
2.050(5)
2.074(5)
2.023(5)
2.3943(6)
2.5503(16)
2.067(5)
2.030(5)
2.068(4)
2.024(4)
Os(1)–Cl(1)
Os(1)–N(1)
Os(1)–N(2)
Os(1)–N(3)
Os(1)–N(4)
Os(2)–Os(2A)
Os(2)–Cl(2)
Os(2)–N(5)
Os(2)–N(6)
Os(2)–N(7)
Os(2)–N(8)
N(1)–Os(1)–Os(1A)
N(2)–Os(1)–Os(1A)
N(3)–Os(1)–Os(1A)
N(4)–Os(1)–Os(1A)
Os(1A)–Os(1)–Cl(1)
N(5)–Os(2)–Os(2A)
N(6)–Os(2)–Os(2A)
N(7)–Os(2)–Os(2A)
N(8)–Os(2)–Os(2A)
Os(2A)–Os(2)–Cl(2)
90.24(11)
87.79(11)
88.96(11)
88.79(12)
176.57(4)
90.16(14)
88.05(13)
89.50(13)
88.62(14)
177.17(4)
Fig. 1. ORTEP plot of molecule A at 30% probability level; hydrogen
atoms were omitted for clarity.
[11,12], this technique eliminates potential side reactions
caused by the direct contact between the Os-containing
species and the base. The previously reported compound
Os2(ap)3Cl3 was described as dark blue with intense
absorptions at 870 and 620 nm [7]. Similarly,
Os2(ap)4Cl2 has a royal blue color resulted from strong
absorptions at 880 and 630 nm. In addition, it has a
strong absorption at 450 nm that is absent in
Os2(ap)3Cl3.
rationale is that both axial chloro ligands remain Os2-
bound during the metathesis process of the bridging
ligands, and their presence enforces a symmetric ar-
rangement.
The Os–Os bond lengths determined for molecules A
ꢀ
ꢀ
(2.3976(5) A) and B (2.3943(6) A) are nearly identical
within experimental errors, and consistent with the
presence of an Os–Os triple bond [1]. In comparison, the
The molecular structure of Os2(ap)4Cl2 was eluci-
dated from the single crystal X-ray diffraction studies.
ꢁ
ꢀ
Crystallized in the P1 space group, the asymmetric unit
Os–Os bond lengths are 2.4672(6) A in Os2(DPhF)4Cl2
ꢀ
(DPhF is diphenylformamidinate) [4] and 2.379(2) A in
contains the halves of two independent molecules, and
each half is related to the other via a crystallographic
inversion center that bisects the Os–Os bond. The
ORTEP representation of the molecule A is shown in
Fig. 1, and selected bond distances and angles for both
molecules A and B are listed in Table 1. Since each ap
ligand has two distinct N-donor centers, i.e., pyridine N
(Np) and aniline N (Na), there are four possible ligand
arrangements for a M2(ap)4 core, as depicted in Scheme
1. The (4,0) arrangement has been dominant in both
the Ru2(ap)4Lax and Ru2(ap)4(Lax)2 type compounds,
where Lax is an axial ligand [13]. In contrast,
Os2(ap)4Cl2 adopts the cis-(2,2) geometry. A possible
Os2(hpp)4Cl2 (Hhpp is 1,3,4,6,7,8-hexahydro-2H-pyrim-
ido[1,2-a]pyrimidine) [5]. Notably, the order of Os–Os
bond lengths, DPhF > ap > hpp, is the exact opposite of
the donor strength of N,N0-bridging ligands,
hpp > ap > DPhF, which was previously determined on
the basis of electrode potential data for Ru2(LL)4
complexes [14]. The Os–Cl bond length, on the other
hand, increases with the increasing donor strength: hpp
(2.667(4) A) > ap (2.522(1) and 2.550(2) A) > DPhF
ꢀ
(2.478(3) A). Two factors contribute to the observed
trend: (1) the Os2 center with a stronger donor is elec-
tron rich, and hence less receptive to the chloro ligand as
ꢀ
ꢀ
Np
Np
Np
N
N
a
N
Np
N
a
a
a
N
Np
N
Np
N
a
Np
N
Np
a
a
a
M
M
M
M
M
M
M
M
N
Np
a
Np
Np
N
a
N
Np
N
Np
N
a
a
a
Np
Np
N
a
N
N
Np
Np
N
a
a
a
ap ligand
Scheme 1. Possible ligand arrangements in a M2(ap)4 compound; Na and Np denote anilino and pyridino nitrogen centers, respectively.

T. Ren et al. / Inorganica Chimica Acta 357 (2004) 1313–1316
1315
3. Conclusion
-2/-3
On the basis of the one-electron oxidation potential,
Os2(ap)4Cl2 is more electron deficient than Os2(hpp)4-
Cl2, and consequently a better candidate for forming
stable complexes with more electron-donating alkynyl
ligands. With both chloro ligands semi-exposed as a
result of the cis-(2,2) geometry, Os2(ap)4Cl2 should un-
dergo faster axial ligand metathesis reactions than
Os2(DPhF)4Cl2 and the alkynylation chemistry will be
investigated in the near future.
-1/-2
0/-1
+1/0
0.5
0
-0.5
-1
-1.5
-2
-2.5
-3
Fig. 2. Cyclic voltammogram of 1 recorded in 0.20 M THF solution of
Bu4NPF6 at the scan rate of 0.10 V/s.
4. Experimental
a donor and (2) the Os2 center of higher electron density
also has a stronger Os–Os bond, which results in a
weaker bond in the trans-position, the Os–Cl bond.
Among four independent Os–N bond lengths for mol-
ecule A, the Os–Na bonds (Os(1)–N(2) and Os(1)–N(4))
are significantly shorter than the Os–Np bonds (Os(1)–
N(1) and Os(1)–N(3)), and the same is true for molecule
B.
In comparison with Os2 compounds supported by
the hpp and DPhF ligands, Os2(ap)4Cl2 has a fairly
large room temperature effective moment of 2.76 lB
that is consistent with a S ¼ 1 ground state. A rea-
sonable ground state configuration for Os2(ap)4Cl2 is
r2p4d2pꢀ2, the same configuration ascribed to
Os2(DPhF)4Cl2 [4]. It is noteworthy that, despite the
structural similarity to the aforementioned compounds,
Os2(hpp)4Cl2 has a ground state configuration of
r2p4d2dꢀ2 [5].
4.1. General conditions, reagents and instruments
Os2(OAc)4Cl2 [17] and 2-anilinopyridine [18] were
prepared as previously described. Anhydrous organic
solvents were obtained from Aldrich and used as
received. Absorption spectra were obtained with a
Perkin–Elmer Lambda-900 UV–Vis–NIR spectropho-
tometer. The magnetic susceptibility was measured at
294 K with a Johnson Matthey Mark-I Magnetic
Susceptibility Balance. Cyclic voltammograms were
recorded in 0.2 M (n-Bu)4NPF6 solution (THF, N2-
degassed) on a CHI620A voltammetric analyzer with a
glassy carbon working electrode (diameter ¼ 2 mm), a
Pt-wire auxiliary electrode and a Ag wire pseudo ref-
erence electrode, and the electrode potentials were re-
ported vs. E(Fcþ/Fc). The concentration of Os2(ap)4Cl2
was 1.0 mM.
Os2(ap)4Cl2 exhibits rich features in the cyclic vol-
tammogram (CV) shown in Fig. 2, which consists of
four one-electron waves. In analogy to the redox
chemistry of other Os2(III) species [4,5], the most anodic
wave is assigned as the oxidation of the Os2-core, and
the adjacent wave as the first reduction of the Os2 core
(Eq. (1)). The second reduction, ca. 0.90 V more nega-
tive than the first, is also attributed to a further reduc-
tion of the Os2 core. Although not noted for
Os2(hpp)4Cl2 and Os2(DPhF)4Cl2, a second reduction of
the Ru2(ap)4 core was observed recently, and the po-
tential difference between two consecutive reductions,
i.e., E(Ru2II;I/Ru2II;II) ) E(Ru2II;II/RuI2I;III), is about )1 V
[15]. The nature of the most cathodic couple at )2.30 V
remains uncertain. The E1=2(+1/0) of Os2(ap)4Cl2, 0.031
V, is in the middle of those observed for Os2(hpp)4Cl2
()0.492 V vs. Fcþ/Fc) [16] and Os2(DPhF)4Cl2 (Epa, 0.60
V, vs. Fcþ/Fc) [4], reaffirming the aforementioned order
of donor strength
4.2. Synthesis of Os2(ap)4Cl2 (1)
A round bottom flask was charged with Os2(OAc)4-
Cl2 (0.24 g, 0.35 mmol) and 2-anilinopyridine (0.51 g, 3
mmol), and 40 ml of toluene, and a micro Soxhlet ex-
traction condenser with a glass thimble containing 1:1
K2CO3/sand was mounted to the flask. The reaction
mixture was heated to reflux in open air for 3 days,
during which the initial brown suspension turned to
dark purple within the first 2 h, and then to deep blue.
Reaction mixture was filtered to remove a small amount
brownish residues that are impurities from the
Os2(OAc)4Cl2 synthesis, and toluene was distilled from
the filtrate to yield a deep blue residue. After the re-
moval of excess Hap via vacuum sublimation, the resi-
due was recrystallized from 20 ml of hot CH3OH to
yield 0.22 g (56% based on Os) of blue crystalline
material. Data for 1. FAB-MS: 1129, [MHþ]; molar
susceptibility (vmol), 3.23 ꢃ 10ꢂ3 emu; leff ¼ 2:76 lB.
Vis–NIR, kmax(nm, e (Mꢂ1 cmꢂ1), THF): 452 (4540), 632
(7820), 770(sh), 880 (1930). Electrochemical, E1=2/V (vs.
Fcþ/Fc), DEp=V , ibackward=iforward: +1/0, 0.032, 0.103,
þeꢂ
þeꢂ
1ꢂ
½Os2ðIII; IVÞꢁ1þ ꢀ ½Os2ðIII; IIIÞꢁ ꢀ ½Os2ðIII; IIÞꢁ
þeꢂ
þeꢂ
3ꢂ00
ꢀ ½Os2ðII; IIÞꢁ2ꢂ ꢀ ½Os2ðI; IIÞꢁ
ð1Þ

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T. Ren et al. / Inorganica Chimica Acta 357 (2004) 1313–1316
0.93; 0/)1, )1.068, 0.180, 0.63; )1/)2, )1.967, 0.128,
0.67; )2/)3, )2.301, 0.108, 0.93.
Acknowledgements
This work is supported by the Office of Naval Re-
search (Contract No. N00014-03-1-0531 to T.R.). T.R.
also thanks the American Society for Engineering Ed-
ucation for a Summer Faculty Fellowship during his
stay at the Naval Research Laboratory.
4.3. X-ray data collection, processing, and structure
analysis and refinement
Single crystals of compound 1 were grown via slow
evaporation of a saturated CH3OH/CH2Cl2 solution of
1. Data were collected on a Bruker SMART 1K CCD
diffractometer using graphite monochromated Mo Ka
radiation. Crystal was cooled using a Rigaku X-Stream
Cryogenic Crystal Cooler. Corrections were applied for
Lorentz, polarization, and absorption effects. The
structure was solved and refined with the aid of the
programs in the SHELXTL-PLUS system of programs [19].
The full-matrix least-squares refinement on F 2 included
atomic coordinates and anisotropic thermal parameters
for all non-H atoms except those of the solvent molecule.
The solvent molecule is disordered over a special position
and was refined with isotropic thermal parameters only.
All of the H atoms were included using a riding model.
Crystal data of 1: MW ¼ 1142.11; triclinicic, space
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ꢁ
ꢀ
ꢀ
group P1, a ¼ 10.397(2) A, b ¼ 10.801(2) A,
3
ꢀ
ꢀ
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,
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,
ꢀ
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5. Supplementary material
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Crystallographic data (excluding structure factors)
have been deposited with the Cambridge Crystallo-
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Os2(ap)4Cl2. Copies of this information may be obtained
free of charge from the director, CCDC, 12 Union
Road, Cambridge, CB2 1EZ, UK.
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