J(RhH) 47 Hz, J(PH) 13 Hz]. On cooling to 190 K this pattern is
retained, and no high-field signals due to agostic CH3 groups were
observed. This data suggests that rapid exchange on the NMR
timescale is occurring between free and agostic CH3 groups (as
found for 12). On the IR timescale a broad stretch observed at
2671 cm-1 is assigned to the agostic CH3 groups.6 3 is closely related
to bis-phosphine and bis-N-heterocyclic carbene complexes of
Rh(III) and Ir(III), as reported by Caulton6 and Nolan7 that show
bis-agostic interactions.
Addition of tert-butylethene to 3 rapidly removes the hydrides to
afford a new complex of empirical formula [Rh(PtBuiBu2)2][BArF ]
4
4 (51% isolated yield, quantitative by in situ NMR spectroscopy).
Hydrogen loss is also promoted by vacuum, but this is much
slower (t1 ~15 h, 6 ¥ 10-2 mbar). As for 3, complex 4 crystallises
with two2independent cations in the unit cell, both of which are
disordered equally over crystallographically imposed inversion
centres. Fig. 2 shows the solid-state structure of one of these,
i
that demonstrates e-C–H activation of one of the Bu groups
to form a Rh(III)-metallacycle, Rh1–C3 2.151(15) A. Two
Fig. 2 Solid-state structure of one of the independent cations in
8,9
˚
-
the unit cell for 4‡. Disordered components and [BArF
]
anion are not
4
relatively close Rh1 ◊ ◊ ◊ C interactions from C7 and C15 (2.811(13)
shown. Thermal ellipsoids are given at the 30% probability level. The
hydride ligand on Rh1 was not located (see text). Rh1–C3, 2.152(15)
˚
and 2.981(13) A respectively) indicate supporting C–H agostic
interactions (IR: 2684 cm-1, vbr) similar to those observed in 3
and other Rh(III) and Ir(III) bis-agostic complexes.2,6,7 The Rh(III)
coordination environment is completed by a hydride (confirmed
by NMR spectrocopy) which was not located, but placed trans
to the agostic interaction from C7 on the basis of a gap in the
A; Rh1–C7, 2.811(13) A; Rh1–C15, 2.981(1◦3) A; Rh1–P1, 2.312(15) A,
˚
˚
˚
˚
◦
˚
Rh1–P2, 2.312(15) A; P1–C1–C2, 106.9(12) , P2–C13–C14, 116.3(11) ;
P2–C17–C18, 117.3(10)◦; P1–C5–C6, 116.1(11)◦.
steric constraints. Consistent with this 4 does not form a benzene
adduct, whereas 2 does form complexes with arenes.2
1
coordination sphere. In solution, the room temperature H and
1
31P{ H} NMR spectra show broad resonances. Cooling to 173 K
reveals four, closely related species by the observation of: four
hydride resonances grouped around d -22 that show coupling to
103Rh [J(RhH) ~ 54 Hz]; at least 4 broad peaks between d 0.25
and -0.34 (6 H total), assigned to agostic CH3 groups;8 and four
1
pairs of phosphine environments in the 31P{ H} NMR spectrum
that show mutual trans 31P–31P coupling [J(PP) ~300 Hz] on a
Rh(III) centre [J(RhP) ~115 Hz]. These low temperature data are
consistent with the observed solid-state structure. We suggest that
these isomers in solution differ in the position of C–H activation of
the diastereomeric iBu phosphine groups (e.g. C3, C4, see ESI for
diagram†). The structure of 4 is in contrast to that suggested for 2,
in which no cyclometallation was indicated by NMR spectroscopy
at low temperature. This facile cyclometallation9 on incorportation
of a bulky tert-butyl group is directly connected to Shaw’s “gem-
tert-butyl” effect,10 as well as the influence that steric bulk of
a phosphine has on the interaction of C–H bonds with metal
centres.6
Scheme 3 Anions not shown.
Acknowledgements
The EPSRC and University of Oxford are thanked for support.
Dr Nick Rees for assistance with NMR experiments.
Notes and references
¯
‡ Crystallographic data: 3: C56H68BF24P2Rh, M = 1372.76, Triclinic, P1
The room temperature NMR data for 4 suggest a fluxional pro-
cess is occurring, and although a Rh(III) complex in the solid-state
and at low temperature, it reacts with H2, NCMe and H3B·NMe3 as
˚
˚
˚
(Z = 2), a = 12.88050(10) A,◦b = 13.05280(10) ◦A, c = 22.3154(2) A, a =
103.1981(3)◦ , b = 94.4213(4) , g = 118.4125(4) , V = 3136.24(4) A , T =
150(2) K, 26387 unique reflections [Rint = 0.0193]. Final R1 = 0.0560 [I >
3
˚
¯
if a Rh(I) species: giving 3, trans-[Rh(NCMe)2(PtBuiBu2)2][BArF ]
2s(I)]. 4: C56H66BF24P2Rh, M = 1370.75, Triclinic, P1(Z◦= 2), a = 12.9373(2)
4
◦
˚
˚
˚
A, b = 13.0919(2) A, c = 22.2056(3) A, a = 72.9625(7) , b = 85.6901(6) ,
g = 60.4717(6)◦, V = 3118.05(8), T = 150(2) K, 22967 unique reflections
[Rint = 0.0252]. Final R1 = 0.0722 [I > 2s(I)]. Selected NMR data (CD2Cl2;
2
5
and [Rh(h -H3B·NMe3)(PtBuiBu2)2][BArF ] 63 respectively
4
(Scheme 3). This suggests the fluxional process is one that rapidly
equilibrates Rh(I) with Rh(III)-cyclometallated species by re-
versible H–C(sp3) bond cleavage. Others have previously observed
such reactivity,4,9,11 notably Caulton and co-workers who reported
that the 12-electron Rh(I) complex, [(tBu2PCH2SiMe2)2N]Rh, is in
rapid equilibrium with a cyclometallated Rh(III) hydride. Unlike
2, complex 4 does not promote C–X activation with aryl halides,
which we suggest is due to the inability to form an intermediate
Rh(I) h-complex prior to oxidative cleavage,2 presumably due to
298 K; 1H, 500 MHz; 31P{ H}, 202 MHz): Compound 3: 1H: d 7.72 (br, 8H,
1
BArF4), 7.56 (br, 4H, BArF4), 1.95–1.79 (m, 8H, iBu{CH/CH2}), 1.73–1.67
(m, 4H, iBu{CH2}), 1.16 (apparent t, 18H, J = 7, tBu{Me}), 0.89 (d, 12H,
3JHH = 6.5, iBu{Me}), 0.81 (d, 12H, 3JHH = 6.5, iBu{Me}), -22.03 (dt, 2H,
1JRhH = 47.1, JPH = 13.3, RhH). Selected 1H{ P@56.9 ppm}: d -22.03
2
31
(d, 2H, JRhH = 47.3, RhH). 31P{ H}: d 56.9 (d, JRhP = 109).1H (190 K):
1
1
d 7.71 (br, 8H, BArF4), 7.53 (br, 4H, BArF4), 1.76 (br, 8H, Bu{CH2}),
i
1.54 (br, 4H, iBu{CH}), 1.04 (br, 18H, tBu{Me}), 0.75 (d, 3JHH = 4.6, 12H,
iBu{Me}), 0.67 (d, 3JHH = 4.2, 12H, iBu{Me}), -21.48 (dt (br), 1JRhH = 44.9,
2JPH = 11.7, 2H, RhH). 31P{ H} (190 K): d 57.2 (br d, 1JRhP = 105). ESI-
1
7438 | Dalton Trans., 2010, 39, 7437–7439
This journal is
The Royal Society of Chemistry 2010
©
Sewell, Laura J.
