Heterobimetallic rhodium-gold halide and hydride complexes

Article abstract of DOI:10.1039/c0cc03261a

Heterobimetallic Rh^IAu^I halide complexes react with LiHBEt₃ to afford the corresponding hydride, and they are oxidised by halogen to afford thermally stable Rh^IIAu^II complexes. The hydride complex rea

Full text of DOI:10.1039/c0cc03261a

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Heterobimetallic rhodium–gold halide and hydride complexeswz
Thomas S. Teets, Markus P. Neumann and Daniel G. Nocera*
Received 14th August 2010, Accepted 18th November 2010
DOI: 10.1039/c0cc03261a
Heterobimetallic RhIAuI halide complexes react with LiHBEt3
to afford the corresponding hydride, and they are oxidised by
halogen to afford thermally stable RhIIAuII complexes. The
hydride complex reacts with acid and halogen sources to produce
H2 and HCl, respectively. The RhIIAuII complexes exhibit
optical and photochemical properties that are derived from the
bimetallic core.
complexes also show a single, strong absorption feature in the
visible region (Fig. S1–S4, ESIz), attributed to a ds* - ps
transition characteristic of d8ꢁ ꢁ ꢁd10 complexes.10 Full charac-
terization of all complexes is provided in ESI.z
Halogen oxidation of 1–4 affords RhIIAuII complexes 5–8
quantitatively, as shown in Scheme 1. A substantial upfield
shift of the 31P resonances of the RhIAuI precursor occurs
upon its treatment with PhICl2 or Br2. An apparent triplet
1
splitting pattern is observed, and JRh–P values are slightly
Photocatalytic hydrogen production from hydrohalic acids
can be mediated by phosphazane-bridged dirhodium
complexes, though ultimately the efficiency is limited by the
poor quantum yield of halogen (X2) photoelimination.1–3 We
have demonstrated the first systems that are able to photo-
reductively eliminate X2.4–7 Our interest in heterobimetallic
complexes, specifically those featuring rhodium, is rooted in
the potential to couple H2 generation1,2,8 with X2 elimination.
An initial foray into RhIIAuII heterobimetallic complexes,
bridged by the phosphazane [P(OCH2CF3)2]2NMe (tfepma),
yielded products whose thermal instability precluded reliable
examination of their thermal and photochemical reactivity.9
Motivated by our recent success with analogous iridium–gold
complexes,7 we report here a suite of thermally stable halide
and hydride-bound rhodium–gold complexes bridged by
bis(dicyclohexylphosphino)methane (dcpm).
smaller at 74–79 Hz. Whereas dppm-bridged 7 and 8
decompose above ꢀ20 1C to an intractable mixture of
products, dcpm-bridged 5 and 6 are stable in solution at room
temperature for at least 8 h with no observable change in the
31P spectrum.
Crystallographic characterisation of complexes 5-(PF6)y
and 6-(PF6)z reveals the presence of a metal–metal bond.
The thermal ellipsoid plots for complexes 1 and 5 are depicted
in Fig. 1. X-Ray structures of 2–4 and 6 are shown in
Fig. S6–S9, ESI.z The intermetallic distances of 2.9250(4) A
in 18 and 2.9187(2) A in 2** contract to 2.6812(4) A in 5 and
2.6960(3) A in 6, consistent with formation of a metal–metal
bond. The typical octahedral coordination environment of
metal–metal bonded d7 RhII and the square planar environ-
ment of d9 AuII centres are readily apparent in the crystal
structures.
RhIAuI halide complexes are prepared in the stepwise
The optical spectra of 5-(PF6) and 6-(PF6), depicted in
Fig. 2, are similar to those of the isostructural iridium–gold
complexes but red-shifted.7 Substitution of the chlorides in 5
with bromides in 6 causes a noticeable bathochromic shift in
the absorption features, indicative of LMCT character in the
excited states.
procedure outlined in Scheme 1, generating heterobimetallic
ꢀ
complexes 1–4, which are isolated as either PF6 (1–3) or
OTfꢀ (4) salts in isolated yields of 74–83%. The RhIAuI
complexes are readily identified by their 31P{1H} NMR spec-
tra, which show two AA0XX0 multiplets largely reminiscent of
the splitting observed for isostructural IrIAuI complexes,7 with
1
observed JRh–P values of 113–120 Hz. The RhIAuI halide
The LMCT character of 5 and 6 suggests that a halogen
elimination photochemistry may be accessible. Indeed,
irradiation of acetonitrile solutions of complexes 5-(PF6) or
6-(PF6) with the UV light in the presence of 2,3-dimethyl-1,3-
butadiene (DMBD) as a halogen trap results in smooth
Scheme 1
Department of Chemistry, Massachusetts Institute of Technology,
6-335, 77 Massachusetts Ave., Cambridge, MA 02139-4307, USA.
E-mail: nocera@mit.edu; Fax: +1 617 253 7670;
Tel: +1 617 253 5537
w This article is part of a ChemComm ‘Hydrogen’ web-based themed
issue.
z Electronic supplementary information (ESI) available: Experimental
details, additional spectra and crystallographic tables. CCDC
789594–789600. For ESI and crystallographic data in CIF or other
electronic format see DOI: 10.1039/c0cc03261a
Fig. 1 X-Ray crystal structures for complexes 1 (left) and 5 (right),
depicted at the 50% probability level with hydrogen atoms,
counterions, and solvents of crystallization omitted.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 1485–1487 1485

Fig. 4 Spectral evolution for the photolysis of 6 with monochromatic
320 nm light in acetonitrile with 2.2 M DMBD at 283 K. Spectra
shown from 0 to 8 min; 2 min interval for the first spectrum and 1 min
intervals afterward.
Fig. 2 Room temperature UV-vis absorption spectra of 5-(PF6)
) and 6-(PF6) ( ) recorded in CH2Cl2 solution.
(
Scheme 2
used to prepare a gold(I) hydride,11 though in our case the
hydride is bound to rhodium owing to the strong tendency of
gold(I) to remain in a linear, two-coordinate geometry. The most
definitive evidence for the presence of a hydride ligand comes
1
from the H NMR spectrum (Fig. S10, ESIz), which shows a
Fig. 3 Spectral evolution for the photolysis of 5 with monochromatic
320 nm light in acetonitrile with 2.2 M DMBD at 283 K. Traces shown
from 0 to 39 min in 3 min intervals.
symmetric nine-line multiplet centered at ꢀ4.41 ppm for the
1
hydride bound to rhodium. H{31P} NMR spectra provided
additional insight into the splitting pattern. A fully decoupled
1H{31P} spectrum shows a doublet splitting pattern, with
1JH–Rh = 12 Hz (Fig. S11, ESIz). With a lower decoupling
power, only short-range 31P coupling is observed, resulting in
conversion to the corresponding RhIAuI complexes 1-(PF6)
and 2-(PF6). Fig. 3 shows the evolution of the UV-vis spectra
during the photoconversion of a 40 mM solution of chloride
complex 5 to 1 when irradiated at 320 nm in the presence of
2.2 M DMBD. Well-anchored isosbestic points are maintained
at 400 and 440 nm, and the final spectrum matches an
authentic spectrum of 1. The product-forming quantum yield
(Fp) for this transformation was found to be 9.4(8)% in 2.2 M
DMBD. The quantum efficiency is minimally responsive to the
DMBD concentration in the range of 0.55 to 4.4 M (Table S3,
ESIz).
2
an apparent quartet splitting pattern indicative of a JH–P
coupling constant that is also 12 Hz (Fig. S12, ESIz). The
remainder of the nine-line splitting pattern is generated by
3
considering long-range JH–P coupling with a value of 6 Hz.
This collection of coupling constants predicts
a
1 : 2 : 4 : 6 : 6 : 6 : 4 : 2 : 1 nonet, which is precisely what is
observed. The optical spectrum of 9 (Fig. S5, ESIz) is
consistent with the formulation of a d8ꢁꢁꢁd10 electronic structure.
Crystallographic characterisation verifies the identity of
9,ww as shown in the thermal ellipsoid plot in Fig. 5. The
Photoconversion of bromide-substituted 6 to its RhIAuI
precursor 2 is likewise facile with 320 nm excitation, as gleaned
from the UV-vis traces in Fig. 4. Isosbestic points are
maintained at 416 and 429 nm, and again quantitative photo-
reduction is apparent. For this conversion, the quantum yield,
Fp, increases significantly as the [DMBD] is increased, as
shown in the inset of Fig. 4 and the data listed in Table S3,
ESI.z The Fp at 0.055 M DMBD is 3.1(2)%, and a value of
18.2(5)% is attained when the DMBD concentration is 4.4 M.
Having established the halogen elimination photochemistry
of rhodium–gold complexes, we next sought to prepare
hydride-bound analogues that would be likely intermediates
in a HX-splitting scheme. As depicted in Scheme 2, treatment
of chloride-bound RhIAuI complex 1-(PF6) with a stoichio-
metric amount of LiHBEt3 (Super-Hydrides) yields the
hydride complex [RhIAuI(dcpm)2(CO)H](PF6) (9), which
was isolated in 86% yield. An analogous strategy has been
Fig. 5 Thermal ellipsoid plot for complex 9, depicted at the 50%
probability level with carbon-bound hydrogen atoms, counterions,
and solvents of crystallization omitted.
c
1486 Chem. Commun., 2011, 47, 1485–1487
This journal is The Royal Society of Chemistry 2011

In addition, a hydride-bound RhIAuI complex is accessed
1
by treating chloride complex 1 with LiHBEt3. H NMR and
X-ray crystallography clearly reveal the presence of
a
rhodium-bound hydride, unprecedented in rhodium–gold
heterobimetallic chemistry. The rhodium-hydride moiety is
quite reactive, producing HCl upon treatment with PhICl2
and H2 upon treatment with acid. Our interest in new
platforms for HX-splitting is ongoing, and will likely lead us
to pursue other mixed-metal hydrido-halide complexes.
Research was supported by NSF Grant CHE-0750239.
Grants from the NSF (CHE-9808061 and DBI-9729592)
support the Department of Chemistry Instrumentation
Facility. T.S.T. acknowledges the Fannie and John Hertz
Foundation for a graduate research fellowship.
Scheme 3
intermetallic distance of 2.8556(5) A in 9 is only slightly
shorter than the one found in 1 and is consistent with a weak
metal–metal interaction, as observed in other d8ꢁ ꢁ ꢁd10
complexes.4,7,9,12 The coordination geometries around the metal
centres are also reminiscent of these previous examples.
Additionally, the trans influence of the hydride ligand, which
was located in the difference map, is evident when comparing
the structure of hydride complex 9 to those of halide complexes
1 and 2. The Rh–C internuclear distance, which is 1.828(4) A in
1 and 1.807(3) A in 2, lengthens to 1.885(4) A in 9.
Notes and references
y Crystallographic data for 5-(PF6)ꢁCH2Cl2: C52H94AuCl5F6OP5Rh,
M = 1481.25, monoclinic, P21/c, a = 14.6050(13), b = 20.5013(17),
c = 20.6567(18), b = 95.374(2)1, V = 6157.9(9), Z = 4, m =
3.048 mmꢀ1, T = 100(2) K, R1 = 0.0552, wR2 = 0.0899 (based on
all reflections), GooF = 1.060, reflections measured = 122 466,
unique reflections = 16 655, Rint = 0.0685.
z Crystallographic data for 6-(PF6)ꢁCH2Cl2: C52H94AuBr3Cl2F6OP5Rh,
M = 1614.63, monoclinic, P21/c, a = 14.6332(13), b = 20.6673(18),
c = 20.6771(18), b = 95.422(2)1, V = 6225.4(9), Z = 4, m =
4.812 mmꢀ1, T = 100(2) K, R1 = 0.0461, wR2 = 0.0829 (based on
all reflections), GooF = 1.041, reflections measured = 143 038,
unique reflections = 18 943, Rint = 0.0525.
Mixed hydrido-halide complexes have been shown to be
intermediates in HX-splitting mediated by dirhodium
complexes.2 With the goal of accessing a mixed hydrido-halide
RhIIAuII complex, 9 was treated with one equivalent of
PhICl2. 31P NMR of the reaction mixture showed a ca. 1 : 1
mixture of RhIAuI chloride complex 1 and the RhIIAuII
complex 5, suggesting that hydride 9 is completely consumed
with a sub-stoichiometric amount of PhICl2. Indeed, treat-
ment of hydride-bound 9 with 0.5 equivalents of PhICl2
furnishes 1 cleanly. The same reaction, carried out in benzene
in the presence of 2,6-lutidine, leads to precipitation of
2,6-lutidinium hydrochloride (LutH+Clꢀ) and incomplete
consumption of 9, demonstrating that HCl is produced
during the reaction. Hydride complex 9 reacts rapidly with
LutH+Clꢀ in solvents such as acetonitrile, where both are
substantially soluble. The overall reactivity of complex 9 is
summarised in Scheme 3. Such reactivity is well-documented
for late-transition metal hydride complexes,13 and precludes
the isolation of mixed hydrido-halide RhIIAuII complexes.
Further support of the reaction pathways depicted in
Scheme 3 is provided by GC analysis of the headspace gases
evolved from each of the reactions. For both reactions, GC
measurements demonstrated that H2 was the exclusive gaseous
product. Reaction of 9 with one equivalent of LutH+Clꢀ
yielded 0.84 ꢂ 0.08 equivalents of H2, in good agreement with
the expected stoichiometry. The reaction of 9 with 0.5 equiva-
lents of PhICl2 produced 0.38 ꢂ 0.03 equivalents of H2
(relative to 9), again reasonably close to the expected 0.5
equivalents.
8 Crystallographic data for 1-(PF6)ꢁCH2Cl2: C52H94AuCl3F6OP5Rh,
M = 1410.35, monoclinic, P21/n, a = 15.031(2), b = 21.469(3), c =
,
20.325(3), b = 102.251(2)1, V = 6409.6(16), Z = 4, m = 2.844 mmꢀ1
T = 100(2) K, R1 = 0.0401, wR2 = 0.0978 (based on all reflections),
GooF = 1.071, reflections measured = 145 895, unique reflections =
18 727, Rint = 0.0514.
** Crystallographic data for 2-(PF6)ꢁEt2O: C55H102AuBrF6O2P5Rh,
M = 1444.00, monoclinic, P21/n, a = 15.0137(12), b = 21.2486(17),
c = 20.3833(16), b = 102.5770(10)1, V = 6346.6(9), Z = 4, m =
3.379 mmꢀ1, T = 100(2) K, R1 = 0.0390, wR2 = 0.0640 (based on all
reflections), GooF = 1.034, reflections measured = 147 939, unique
reflections = 19 764, Rint = 0.0492.
ww Crystallographic data for 9ꢁ2.5(C6H6): C66H108AuF6OP5Rh,
ꢀ
M = 1486.25, triclinic, P1, a = 13.5672(15), b = 14.0811(15), c =
19.974(2), a = 76.123(2)1, b = 82.666(2)1, g = 66.756(2), V =
3401.4(6), Z = 2, m = 2.570 mmꢀ1, T = 100(2) K, R1 = 0.0394,
wR2 = 0.0723 (based on all reflections), GooF = 1.117, reflections
measured = 78 297, unique reflections = 19 683, Rint = 0.0395.
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sized and they exhibit active thermal oxidative-addition and
photochemical reductive chemistries. In contrast to
a
previous RhIIAuII complex,9 [RhIIAuII(dcpm)2(CO)X3](PF6)
(X = Cl, Br) are thermally robust, allowing for the photo-
chemical characterisation of RhAu heterobimetallic com-
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complexes with UV light in the presence of a halogen trap
results in clean photoreduction to the corresponding RhIAuI
complex. These complexes are consistent with the emerging
trend that d7–d9 complexes can efficiently eliminate halogen
when irradiated.
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c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 1485–1487 1487