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ꢁꢀꢀꢀ
P. Mayer and H.-C. Böttcher: Crystal and molecular structure of trans-[RhCl(CO)(P Bu2Ph)2]ꢂ
ꢂ1031
labile complexes was confirmed by the formation of the
Till now, the Vaska-type complex 2 has not been
compounds [Fe(CO)4(py)] and [Fe(CO)4(pz)] (pyꢀ=ꢀpyridine, characterized by single-crystal structure analysis. During
pzꢀ=ꢀpyrazine), which were obtained by adding the corre- our experiments we obtained single crystals of 2 suitable
sponding N-donor ligands to the THF solutions. Finally, for an X-ray diffraction study and thus we confirmed its
the composition of both compounds was confirmed by molecular structure. Crystals of 2 were obtained from
single-crystal X-ray diffraction [6]. During the dissolution dichloromethane, using methanol as the antisolvent, by
of the metal carbonyl, a splitting of the dinuclear complex the diffusion method at room temperature overnight. The
was assumed, according to the reaction shown in Eq. (2):
crystals belong to the orthorhombic space group Pbca
with four molecules in the unit cell. A view of the mole-
cule is shown in Fig. 1, together with some selected bond
lengths and bond angles in the caption.
[Fe2 (CO)9 ]+ THF → “[Fe(CO)4(THF)]”+[Fe(CO)5 ]
(2)
In the hope of coordinating the free phos-
phane, compare Eq. (1), with equimolar amounts of
“[Fe(CO)4(THF)]” we combined a THF solution of 1 with a
solution of [Fe2(CO)9] in the same solvent in a molar ratio
of 1:2 at room temperature. Immediately, a color change
of the resulting mixture to orange was observed. The
immediate investigation of the THF solution by 31P{1H}
NMR showed no doublet at δꢀ=ꢀ62.6 ppm (d, JRhPꢀ=ꢀ99.4 Hz)
corresponding to the hypothetical dinuclear complex 2,
and no singlet belonging to the free PtBu2Ph ligand was
found. Instead, a new species exhibiting a doublet at
δꢀ=ꢀ60.5 ppm (d, JRhPꢀ=ꢀ125.4 Hz) was present. This doublet
signal was somewhat broadened, hinting at conforma-
tional exchange processes. In 1990, a paper on the ste-
reodynamics of complexes trans-[MCl(CO)(PtBu2Ph)2]
(Mꢀ=ꢀRh, Ir) described temperature-dependent decoales-
cence phenomena in their NMR spectra [7]. The complex
The coordination sphere around the rhodium atom
can be best described as nearly square planar with the
bulky phosphane ligands in the trans position to each
other. The bonding characteristics are in good agreement
with those reported for trans-[RhCl(CO)(PPh3)2] [10]. As
often described for similar complexes, the chlorido and
the carbonyl ligand in crystals of 2 are disordered. Since
the rhodium atom is located on an inversion center, these
ligands occupy each of the mutually trans positions with
50% probability. Obviously, this phenomenon seems to
play an important role in the crystal structures of other
closely related compounds with the composition trans-
[MCl(L)(PR3)2] (Mꢀ=ꢀRh, Ir; Lꢀ=ꢀO2, N2, CO, SO) [11–13].
In summary, we have described an unexpected
synthesis of the Vaska-type complex trans-[RhCl(CO)
(PtBu2Ph)2] (2) in high yield. The product was obtained by
reacting trans-[RhCl2(PtBu2Ph)2] (1) with [Fe2(CO)9] in THF,
where the metal carbonyl acted as a carbonylating agent
toward the rhodium(II) starting complex. The molecular
31
trans-[RhCl(CO)(PtBu2Ph)2] showed the same P{1H} NMR
data (δꢀ=ꢀ61.3 ppm, d, JRhPꢀ=ꢀ127.0 Hz) as we found during
our investigations. Therefore, we had to accept that our
efforts to separate the phosphane by formation of a
complex failed. Instead, a reductive carbonylation of 1
occurred, with COCl2 as a possible oxidation product (not
1
proved). The product 2 was further characterized by H
NMR and IR spectroscopy as well as by elemental analysis
31
13
(see Experimental section). The P{1H} and C{1H} DNMR
spectra of 2 have already been reported elsewhere [7, 8].
An analysis of the P{1H} DNMR spectra of 2 revealed a
31
decoalescence into four subspectra corresponding to four
diastereomeric conformations. The dominant subspec-
tra were assigned to the two conformations in which the
phenyl rings are mutually anti and oriented on the same
1
(or opposite) sides of the coordination plane [7]. The H
NMR spectrum of 2 showed a peculiarity: the signal cor-
responding to the tert-butyl protons was observed as a
virtual triplet pattern caused by the trans arrangement
of the phosphane ligands and giving rise to an A18XX′A′18
spin system. In the absence of a P–P coupling (for an A18X
system), a doublet is found in standard cases with a cou-
Fig. 1:ꢀMolecular structure of 2 in the crystal. Displacement
ellipsoids are drawn at the 50% probability level. Selected bond
lengths (Å) and angles (deg): Rh1–Cl1 2.455(2), Rh1–P1 2.3771(4),
Rh1–C15 1.697(6), C15–O1 1.163(6), P1–C1 1.8367(17), P1–C7
1.8999(18), P1–C11 1.8953(18); Cl1–Rh1–P1 91.86(6), Cl1i–Rh1–C15
177.21(15), P1–Rh1–C15 93.34(13), Rh1–P1–C1, 112.39(6), Rh1–
P1–C7, 117.43(6), Rh1–P1–C11, 108.72(6). Atoms are generated by
symmetry operation i: 1ꢁ−ꢁx, 1ꢁ−ꢁy, 1ꢁ−ꢁz.
3
pling JPH in the range of 12–15 Hz. A similar pattern was
observed for trans-[IrCl(CO)(PtBu2Ph)2] [9].
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Mayer, Peter
