Synthesis, characterization, and C-H activation reactions of novel organometallic O-donor ligated Rh(III) complexes

Article abstract of DOI:10.1016/j.jorganchem.2010.09.021

The synthesis and characterization of the O-donor ligated, air and water stable organometallic complexes trans- (2), and cis-(hfac-O,O) ₂Rh(CH₃)(py) (3), trans-(hfac-O,O)₂Rh(C ₆H₅)(py) (4), cis-(hfac-O,O)₂Rh(C ₆H₅)(py) (5), and cis-(hfac-O,O)₂Rh(Mes)(py) (6) (where hfac-O,O = κ²-O,O-1,1,1,5,5,5- hexafluoroacetylacetonato) are reported. These compounds are analogues to the O-donor iridium complexes that are active catalysts for the hydroarylation and C-H activation reactions as well as the bis-acetylacetonato rhodium complexes, which we recently reported. The trans-complex 2 undergoes a quantitative trans to cis isomerization in cyclohexane to form 3, which activates C-H bonds in both benzene and mesitylene to form compounds 5 and 6, respectively. All of these compounds are air and water stable and do not lead to decomposition products. Complex 5 promotes hydroarylation of styrene by benzene to generate dihydrostilbene.

Full text of DOI:10.1016/j.jorganchem.2010.09.021

Journal of Organometallic Chemistry 696 (2011) 551e558
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, characterization, and CeH activation reactions of novel organometallic
O-donor ligated Rh(III) complexes
,
William J. Tenn III a, Brian L. Conley a, Steven M. Bischof b, Roy A. Periana b
*
a Donald P. and Katherine B. Loker Hydrocarbon Research Institute, Department of Chemistry, University of Southern California, Los Angeles, CA 90089, United States
b The Scripps Energy Laboratories, Department of Chemistry, The Scripps Research Institute, Jupiter, FL 33458, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis and characterization of the O-donor ligated, air and water stable organometallic complexes
Received 15 July 2010
Received in revised form
3 September 2010
Accepted 7 September 2010
Available online 17 September 2010
trans-(2), and cis-(hfac-O,O)2Rh(CH3)(py)(3), trans-(hfac-O,O)2Rh(C6H5)(py)(4),cis-(hfac-O,O)2Rh(C6H5)(py)
(5), and cis-(hfac-O,O)2Rh(Mes)(py) (6) (where hfac-O,O ¼
k
2-O,O-1,1,1,5,5,5-hexafluoroacetylacetonato) are
reported. These compounds are analogues to the O-donor iridium complexes that are active catalysts for the
hydroarylationandCeH activationreactionsaswell asthebis-acetylacetonatorhodiumcomplexes, whichwe
recently reported. The trans-complex 2 undergoes a quantitative trans to cis isomerization in cyclohexane to
form 3, which activates CeH bonds in both benzene and mesitylene to form compounds 5 and 6, respectively.
All of these compounds are air and water stable and do not lead to decomposition products. Complex 5
promotes hydroarylation of styrene by benzene to generate dihydrostilbene.
Keywords:
CH activation
Hydroarylation
Rhodium
Ó 2010 Elsevier B.V. All rights reserved.
Hydrophenylation
Hydrocarbons
Catalysis
1. Introduction
functionalization [6], some other reasons for focus on O-donor
ligands are the weak
p-donor properties and electron-withdrawing
The design of highly efficient and selective catalysts for the
functionalization of arenes remains an important challenge [1]. One
approach to this challenge is based on the design of homogeneous
catalysts that operate by the CeH activation reaction [2]. In this
reaction, the CeH bond of unactivated hydrocarbons can selectively
react with homogeneous, ligated, metal complexes to break the
CeH bond under mild conditions to generate MeC intermediates
by a non-radical, concerted CeH bond cleavage. Currently, many
stoichiometric examples of this reaction are known; however, the
key challenge today when developing catalysts is coupling the CeH
activation reaction with functionalization of the MeC intermediate
to generate a useful product [2].
“hard” characteristics that could also facilitate both the CeH acti-
vation reaction and functionalization of the MeC intermediates [7].
Recently, weshowed[8] aclassof thermallyandacidstablek
2-acac-
O,O-donor ligated Ir(III) complexes (where acac ¼ acetylacetonato or
2,4-pentanedione) that are efficient catalysts for the regioselective
hydroarylation [9] of olefins via a reaction mechanism involving
reversible CeH activation and rate determining MeC functionalization
by olefin insertion by both experimental [10] and density functional
theory (DFT) studies [11]. The results from DFT calculations show that
unfavorable O-donor ligand-p
conflict”) with the d6 center could stabilize the transition state for
CeH bond cleavage through ligand -donation [12]; while the elec-
pto metal-dpinteractions (the so-called
“p
p
Developing the correct ligand is crucial in facilitating the
successful coupling of CeH activation and functionalization reac-
tions by mediating the properties of the metal center and stabi-
lizing the complex. In this regard, while N, P, and C-donor ligands
have been extensively examined [3], fewer examples exist for O-
donor ligated complexes for both early [4] and late [5] transition
metals. In addition, to the possibility that O-donor ligands could be
more stable to the acidic and oxidizing conditions required for
tron-withdrawing characteristics of the acac-O,O ligands could facili-
tate the rate determining olefin insertion step. Specifically, these
studies showed that substituting the eCH3 group on the acetylaceto-
nato ligands of the (acac-O,O)2Ir(III)(R)(L) complex (where R is an alkyl
or aryl group) with strongly withdrawing eCF3 groups as in cis-
(hfac-O,O)2Ir(R)(L) (where hfac-O,O ¼
k
2-O,O-1,1,1,5,5,5-hexafluo-
roacetylacetonato) could facilitate a facile insertion of the olefin into
the MePh bond [11b].
In an attempt to examine the prediction from the DFT studies, we
sought to synthesize the
where the eCH3 groups of the acac-O,O ligands were substituted
k
2-acac-O,O-donor ligated Ir(III) complexes
* Corresponding author. Tel.: þ1 561 228 2457.
E-mail address: rperiana@scripps.edu (R.A. Periana).
0022-328X/$ e see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.09.021

552
W.J. Tenn III et al. / Journal of Organometallic Chemistry 696 (2011) 551e558
with eCF3 groups. However, we could not obtain well characterized
materials using Ir and no analogs have been previously reported.
Given the usual parallels between Ir and Rh [13], we synthesized the
eCF3 analogs of the Rh(III) derivatives as a starting point for exam-
ining the chemistry of fluorinated
k
2-acac-O,O ligands and the
impact on CeH activation and olefin arylation.
Herein, we report the synthesis and characterization of cis-
(hfac-O,O)2Rh(CH3)(py) (3), and examine its reactivity with sp2 and
sp3 CeH bonds. The complex is competent for CeH activation and
the cis-(hfac-O,O)2Rh(Ph)(py) (5) and cis-(hfac-O,O)2Rh(Mes)(py)
(6) complexes generated by reaction with benzene and mesitylene,
respectively, were isolated and fully characterized. Since complex 5
is an analog of both the (acac-O,O)2Rh(Ph)(CH3OH) complex which
we recently reported [7d], and of the original (acac-O,O)2Ir(R)(L)
complexes [8,10], we contrast the reactivity of these compounds.
2. Results and discussion
Fig. 1. ORTEP structure of 1 (50% probability thermal ellipsoids). A molecule of co-
2.1. Synthesis of complexes
crystallized methanol and hydrogen atoms has been omitted for clarity. Selected bond
ꢀ
lengths (A): Rh(1)eCl(1), 2.2747(16); Rh(1)eO(1), 1.989(3); Rh(1)eO(2), 1.992(3); Rh
The syntheses of new Rh(III) complexes with the
b
-diketonate
are
(1)eO(3), 1.992(3); Rh(1)eO(4), 1.997(3); Rh(1)eO(5), 2.064(3). Selected bond angles
(ꢀ): O(1)eRh(1)eO(2), 94.55(11); O(2)eRh(1)eO(4), 86.36(13); C(11)eO(5)eRh(1),
119.3(3).
k
2-O,O-1,1,1,5,5,5-hexafluoroacetylacetonate
(hfac-O,O)
summarized in Scheme 1. The synthesis of 1 has been previously
reported [14]. Treatment of rhodium(III) trichloride hydrate with
hexafluoroacetylacetone in refluxing anhydrous ethanol for 4 h,
followed by recrystallization from methanol yields the complex
trans-(hfac-O,O)2Rh(CH3OH)(Cl) (1). Complex 1 is an air and water
stable mustard-yellow microcrystalline compound. The 1H NMR
spectrum of 1 shows a single resonance signal for the methine
proton of the hfac ligands at 6.60 ppm, which implies a symmetric
environment for the two hfac ligands as expected for the trans
geometry; whereas, a cis geometry would reveal two distinct
methine signals. Coordinated MeOH could not be detected due to
rapid scrambling with the solvent. Furthermore, analysis of the 19F
NMR revealed a singlet at 71.87 ppm for the eCF3, which is also
consistent with a symmetric trans environment of the hfac-O,O
ligands. This assignment was later confirmed by a low temperature,
single crystal, X-ray diffraction study of 1 as shown in Fig. 1.
Although the aquo analog of 1 was previously reported, no defini-
tive structure of the complex had been previously reported.
ꢀ
a bond length for the RheCH3 bond of 2.031(3) A. Interestingly,
comparison with two previously reported iridium species high-
lights the contraction of the metalemethyl bond in the rhodium
analog relative to the previously reported iridium derivatives: 1)
trans-(acac-O,O)2Ir(CH3)(py) [15] and 2) trans-(tropolone-O,O)2Ir
(CH3)(py) [16] where the IreCH3 bond lengths are 2.041 and
ꢀ
2.047 A, respectively (Fig. 2).
Having previously shown that our trans-bis-acac motif is active
for CeH activation after undergoing a trans to cis isomerization, we
were interested in generating the cis isomer of complex 2. Indeed,
heating complex 2 in cyclohexane at 130 ꢀC for 12 h induced the
desired trans to cis isomerization to yield the cis-(hfac-O,O)2Rh
(CH3)(py) isomer, complex 3 (Fig. 3). Carrying out the reaction in
a sealed NMR tube with a resealable PTFE valve containing
a coaxial, external standard of 1,3,5-trimethoxybenzene in CCl4,
revealed the trans to cis isomerization was quantitative by
comparison to the external standard and the methyl and methine
signals integration by 1H NMR after 12 h at 130 ꢀC. The isomeriza-
tion is also characterized by a downfield shift of the eCH3 signal to
2.48 ppm as a doublet due to coupling with the rhodium center
with a coupling constant of 2.5 Hz. Also, there is a minor contrac-
Complex 1 was heated with (CH3)2Hg in methanol at 75 ꢀC for
12 h, followed by treatment with pyridine. This afforded the methyl
complex, trans-(hfac-O,O)2Rh(CH3)(py) (2), which has also been
characterized by 1H, 13C NMR spectroscopy, elemental analysis, and
single crystal X-ray diffraction (Fig. 2). Analysis by 1H NMR revealed
a doublet at 2.43 ppm with a RheH coupling of 2.2 Hz corre-
sponding to the eCH3 group. X-ray diffraction studies revealed
ꢀ
tion of the RheN(C5H5) from 2.236(3) to 2.017(5) A in the crystal
Scheme 1. Synthesis of (hfac-O,O)2Rh(III) complexes.

W.J. Tenn III et al. / Journal of Organometallic Chemistry 696 (2011) 551e558
553
Initial investigations with this molecule revealed that a decompo-
sition pathway was readily accessible leading to intractable
rhodium materials. Upon heating in arene solutions or methanol, 4
was found to decompose readily and generate biphenyl (identified
by GCeMS and NMR analysis) and a new (hfac-O,O)2Rh complex
which we were unable to characterize. Attempts to carry out the
trans to cis isomerization analogous to cases of 2 and 3 were also
unsuccessful and readily afforded the decomposition products of
biphenyl and uncharacterized (hfac-O,O)Rh complexes.
2.2. Hydrocarbon activation
The reactivity of (hfac-O,O)2Rh(III) complexes towards CeH
bonds was subsequently studied. Heating a solution of 3 in benzene
at 190 ꢀC for 14 h readily generates the cis-(hfac-O,O)2Rh(Ph)(py)
(5) species, which can be isolated in 78% yield; as shown in
Scheme 2. The 1H NMR spectrum revealed the disappearance of the
distinctive doublet of the Rh-CH3 resonance at 2.48 ppm, which is
accompanied by the appearance of two new multiplets at 7.10 and
6.36 ppm corresponding to a new set of phenyl protons with an
integration of 3:2, respectively. Monitoring the reaction in C6D6
using a sealed NMR tube with resealable PTFE valve reveals an
upfield 1:1:1 triplet at 0.18 ppm due to the formation of CH3D.
Some decomposition of the starting material is observed, especially
when the reactions were carried out at temperatures greater than
190 ꢀC. Complex 5 has been fully characterized by 1H, 13C NMR
spectroscopy, and elemental analysis.
Inacloselyrelatedreaction tothe onereported above(Scheme 2),
after complex 3 is heated in mesitylene at 190 ꢀC for 14 h it is con-
verted to the cis-(hfac-O,O)2Rh(Mes)(Py) (6) species. Compound 6
was obtained in 51% yield from the reaction of 3. Complex 6 can be
characterized by two methine signals at 5.96 and 5.68 ppm corre-
sponding to the cis geometry and the two double of doublets at 5.05
and 4.46 ppm representing the methylene linker bound to the
rhodium in the 1H NMR. Further analysis of the 1H NMR spectrum
after reaction in a sealed J-Young NMR tube revealed the generation
of CH3D when run in deuterated mesitylene again characterized by
the upfield 1:1:1 triplet at 0.18 ppm. We were able to further char-
acterize 6 by 13C NMR spectroscopy, high-resolution mass spec-
trometry, and X-ray crystallography (Fig. 4).
Fig. 2. ORTEP plot of 2 (50% probability thermal ellipsoids) with hydrogen atoms
removed for clarity. Selected bond lengths (A): Rh(1)eN(1), 2.236(3); Rh(1)eC(16),
2.031(3); Rh(1)eO(1), 1.998(2); Rh(1)eO(2), 2.001(2); Rh(1)eO(3), 2.007(2); Rh(1)eO
(4), 2.003(2). Selected bond angles (ꢀ): O(1)eRh(1)eO(2), 94.58(9); O(1)eRh(1)eO(3),
85.12(9); N(1)eRh(1)eC(16), 178.88(11).
ꢀ
structures upon isomerization of and the RheCH3 bond from 2.031
ꢀ
(3) to 2.026(5) A, although this is minor and within experimental
error. This is most likely a trans influence phenomenon and
exchange of the trans methyl/pyridine moiety for both the methyl
and pyridine groups in an orientation trans to the oxygen from the
“hard” hfac-O,O ligands leads to the contraction in the bond
distances due to electron withdrawal effects of the eCF3 groups
mediated by the rhodium center.
We were also interested in making the phenyl analog of 2, by
treating 1 with Ph2Hg, instead of Me2Hg, in CHCl3/CH3OH at 100 ꢀC
for 12 h, followed by treatment with pyridine, gave the air and
water stable phenyl complex trans-(hfac-O,O)2Rh(Ph)(py) (4).
2.3. Catalytic CeH activation
Afterestablishingthat3 canactivatethe sp2 andsp3 CeHbondsof
benzene and mesitylene to generate the respective aryl complexes,
we examined the catalytic CeH activation of benzene with 3. The
rate of H/D exchange was monitored in a reaction mixture of C6H6
and toluene-d8 (1:1 v/v), at 190 ꢀC catalyzed by the methanol analog
of 2, trans-(hfac-O,O)2Rh(CH3)(CH3OH) (2-CH3OH), which is used as
the crude residue mixture (see Experimental section). The reaction
Fig. 3. ORTEP structure of 3 (50% probability thermal ellipsoids) with hydrogen atoms
ꢀ
removed for clarity. Selected bond lengths (A): Rh(1)eN(1), 2.017(5); Rh(1)eC(16),
2.026(5); Rh(1)eO(1), 1.996(4); Rh(1)eO(2), 2.017(4); Rh(1)eO(3), 2.171(4); Rh(1)eO
(4), 2.000(3). Selected bond angles (ꢀ): O(1)eRh(1)eO(2), 94.34(15); O(3)eRh(1)eO(4),
91.98(14); O(2)eRh(1)eN(1), 176.34(16); O(3)eRh(1)eO(3), 177.18(19).
Scheme 2. Hydrocarbon activation by 3.

554
W.J. Tenn III et al. / Journal of Organometallic Chemistry 696 (2011) 551e558
expected reactivity and experimental simplicity we limited the
feasibility studies to liquid styrene. A 2 mL Schlenk tube fitted
with resealable PTFE valve was charged with 5.7 mg
a
(8.47 ꢁ 10ꢂ3 mmol) of 5, styrene (0.2 mL), and benzene (1 mL) and
placed in a temperature regulated oil bath set at 190 ꢀC for 30 min
(Scheme 3). The contents of the reactor were then analyzed by
GCeMS followed by evaporation of the volatiles and dissolution in
benzene-d6 for NMR analysis. The complex was found to promote
stoichiometric hydroarylation to generate dihydrostilbene as
compared to an internal standard of cyclohexane. Short reaction
times (<1 h) led primarily to dihydrostilbene. Monitoring the
reaction for extended periods (>1 h) revealed the production of
significant quantities of polystyrene, as evidenced by 1H NMR and
MS analysis. Given the relatively low rates with this system, we did
not extend the studies to other olefins.
Interestingly, no olefinic products, such as stilbenes, which
could result from b-hydride elimination of the products from the
metal center, were detected in the analysis of the reaction mixture.
Our previously reported hydroarylation system based on the (acac-
O,O)2Ir(R)(L) motif also showed no formation of
nation products [8,10]. In both systems, this is likely due to the
occurrence of reversible and unproductive -hydride elimination
b-hydride elimi-
b
reactions. The previously reported TpRu(CO)(CH3CN)(R) complex
(where Tp ¼ trispyrazolylborate) [19], also showed the lack of
b
-hydride elimination reactions when R ¼ ethylbenzene to
generate styrene; however, on the substitution of ethyl to propyl-
or hexylbenzene facile -hydride elimination reactions were seen.
b
This effect was attributed to the rapid dissociation of higher order
olefins from the Ru center. We are currently examining these
b-hydride elimination reactions in more detail to understand the
relative differences in selectivities to products and isomers.
Fig. 4. ORTEP plot of 6 (50% probability thermal ellipsoids) with selected hydrogens
ꢀ
removed for clarity. Selected bond lengths (A): Rh1eN1, 2.021(4); Rh1eC16, 2.062(4);
2.5. RheCH3 bond distance
Rh(1)eO(1), 2.202(3); Rh(1)eO(2), 2.006(3); Rh(1)eO(3), 2.002(3); Rh(1)eO(4), 2.016
(3). Selected bond angles (ꢀ): O(1)eRh(1)eO(2), 92.03(11); O(3)eRh(1)eO(4), 94.18
(11); O(4)eRh(1)eN(1), 174.76(12); O(1)eRh(1)eC(16), 177.98(15); Rh(1)eC(16)eC(17),
114.7(3).
We reasoned that our lack of catalytic activity, compared to the
previously reported Ir systems, might be due to the significant
increase in electron deficiency at the Rh center. Our previously
reported prediction [11] suggested that making the metal center
more electron deficient should facilitate the olefin insertion step
and improve overall catalytic rates. However, those calculations
were performed on the (hfac-O,O)2Ir(III) complexes, and even with
the similarities between Rh and Ir, it was not obvious how this
would affect catalysis. In the case of Rh, only the stoichiometric
reaction between benzene and styrene to generate dihydrostilbene
is observed. Thus, the (hfac-O,O)2RheCH3 metric parameters were
compared to known RheCH3 complexes to understand the lack of
reactivity.
mixture is stable over 11.5 h and obtains 114 turn-over-numbers
(TON) with a turn-over-frequency (TOF) of 2.8 ꢁ 10ꢂ3 sꢂ1 based on
added catalyst by monitoring deuterium incorporation into benzene
via GCeMS using a program developed with Microsoft Excel (see
Experimentalsectionfor furtherexplanation)[17]. Inaddition, when
the reaction was carried out using the pyridyl complex, 2, a TOF of
2.0 ꢁ 10ꢂ3 sꢂ1 was observed. These catalytic rates are roughly
equivalent to our previously reported cis-(acac-O,O)2Ir(Ph)(py)
system, which yielded a TOF of 2 ꢁ10ꢂ4 sꢂ1 at 160 ꢀC for exchange
between toluene and benzene, given the temperature difference
between the two runs (190 versus 160 ꢀC, respectively). Unfortu-
nately, this is slower than the trans (acac-O,O)2Ir isomer which, as
previous studies have shown reacts via the same transition state for
CeH activation as the cis isomer but since it has a less stable ground
state, is more active for H/D exchange [11,18]. The inhibition by
pyridine, a more tightly binding ligand, also supports a metal-
mediated mechanism involving pre-equilibrium dissociation of
a labile ligand from the metal center to form a reactive, 5-coordinate
intermediate followed by hydrocarbon coordination followed by
CeH bond cleavage.
Interestingly, when we compare the RheCH3 bond distances
(Table 2) from X-Ray studies of numerous Rh complexes, we find
that both the trans- and cis-(hfac-O,O)2Rh complexes have two of
ꢀ
the shortest RheCH3 bond distances at 2.031(3) and 2.026(5) A,
respectively. The short RheCH3 bond distance highlights the
2.4. Hydroarylation of styrene with benzene
Having demonstrated the stoichiometric and catalytic capability
of 3 for the CeH activation reaction of benzene, we examined the
competency of 5 for hydroarylation reactions. Based on the higher
Scheme 3. Anti-Markovnikov hydroarylation of styrene promoted by 5.

W.J. Tenn III et al. / Journal of Organometallic Chemistry 696 (2011) 551e558
555
Table 1
Crystal data and summary of collection and refinement details for 1e3, and 6.
1
2
3
6
Empirical formula
Formula weight
Crystal color
C11H6ClF12O5Rh
584.50
Brown
C16H10F12NO4Rh
611.16
Orange
C16H10F12NO4Rh
611.16
Yellow
C24H18F12NO4Rh
715.34
Orange
Temperature (K)
Crystal system
Space group
123(2)
Orthorhombic
Pca2(1)
143(2)
Triclinic
Pꢂ1
153(2)
Monoclinic
P2(1)/c
143(2)
Monoclinic
P2(1)/c
0.5 ꢁ 0.4 ꢁ 0.2
7.9459(8)
38.117(4)
8.8400(9)
90.00
96.506(2)
90.00
2660.2(5)
4
1.786
0.759
Crystal size (mm)
0.50 ꢁ 0.29 ꢁ 0.10
0.19 ꢁ 0.09 ꢁ 0.02
10.8849(12)
10.9005(12)
10.9681(12)
117.822(2)
108.698(2)
99.217(2)
1012.91(19)
2
0.20 ꢁ 0.18 ꢁ 0.01
ꢀ
a (A)
8.6144(10)
8.0941(15)
ꢀ
b (A)
19.986(2)
11.7027(13)
90.00
90.00
90.00
2014.8(4)
4
29.962(6)
8.7770(17)
90.00
97.310(3)
90.00
2111.2(7)
4
ꢀ
c (A)
a
b
g
(ꢀ)
(ꢀ)
(ꢀ)
3
ꢀ
V (A )
Z
Dcalcd (g/cm3)
2.029
1.117
2.004
0.977
1.923
0.937
m
(mmꢂ1
)
Absorp. correction
Reflect. collected
Independent reflections
R1
None
11495
None
6284
None
12955
None
16330
4341 [R(int) ¼ 0.0298]
4373 [R(int) ¼ 0.0225]
4737 [R(int) ¼ 0.0511]
5987 [R(int) ¼ 0.0380]
0.0494
0.0448
0.0849
0.0543
wR2
0.0842
0.0921
0.1387
0.1309
GOF
1.042
1.056
1.043
1.043
Refinement method
Full-matrix least-squares on F2
Full-matrix least-squares on F2
Full-matrix least-squares on F2
Full-matrix least-squares on F2
significant electron-withdrawing capabilities that hfac-O,O ligands
have on mediating the electronic character of the rhodium.
Although it was reported that an increase in the electron deficiency
of the metal center should facilitate the olefin insertion step with
minimal effect on the CeH activation barrier [11], the reactivity of
the (hfac-O,O)2Rh complex reveals that the interplay between the
two barriers can have a profound impact on catalysis. We are
therefore exploring further modifications of the ligand system to
improve the reactivity of this class of catalysts.
cause irreversible CNS damage, sensitization, weight loss, immu-
nological disease and other serious effects. Dimethylmercury is
volatile and dangerous concentrations can readily build up in
poorly ventilated areas. Work with these toxic compounds must
not begin until a full assessment of the risks has been made and
suitable protocols established.
4.2. General methods
All air and water sensitive procedures were carried out either in
an MBraun inert atmosphere glovebox or using standard Schlenk
techniques under argon. The glovebox atmosphere was maintained
by periodic nitrogen purges and monitored by an oxygen analyzer
{O2 < 15 ppm}. Methanol was dried from Mg/I2 and benzene was
distilled from Na/benzophenone under dinitrogen. All deuterated
solvents (Cambridge Isotopes) were used as received. 1,1,1,5,5,5-
hexafluoroacetylacetone (Synquest) was stored under argon.
GCeMS analysis was performed on a Shimadzu GCeMS QP5000
(ver. 2) equipped with cross-linked methyl silicone gum capillary
column (DB5) for liquid samples and on a GasPro column for gas
analyses. The retention times of the products were confirmed by
comparison to authentic samples. NMR spectra were obtained on
a Bruker AM-360 spectrometer, measured at 360.138 MHz for 1H
and 90.566 MHz for 13C or on a Bruker AC-250 spectrometer,
measured at 250.134 MHz for 1H and 62.902 MHz for 13C, or Varian
Mercury-400 spectrometer, measured at 399.96 MHz for 1H,
3. Conclusion
Here we have prepared several new Rh(III) compounds based on
hard, O-donor, bis-hexafluoroacetylacetonato ligands. This is an
extension of our previous work on Rh and Ir systems based on
the bis-acetylacetonato motif. The cis-(hfac-O,O)2Rh(CH3)(py) (3)
species is produced in near quantitative yield via trans to cis
isomerization. Reaction of 3 with benzene or mesitylene generates
the corresponding phenyl- or mesityl-complexes in good yields.
The phenyl analog, 5, was found to promote stoichiometric, anti-
Markovnikov hydroarylation of styrene with benzene to generate
dihydrostilbene; unfortunately, no catalytic hydroarylation prod-
ucts were observed due to lack of catalyst activity. We are currently
working on modifications to the Rh complexes in an attempt to
improve their catalyst activity. Also, we are working towards
expanding our Ir system containing hfac-O,O ligands for the
development of non-free radical reactions for the functionalization
of MeR bonds.
19
100.57 MHz for 13C, and 376.34 MHz for F at 25 ꢀC. All chemical
shifts are reported in units of ppm and referenced to the residual
proteated solvent unless otherwise stated. High-resolution mass
spectra were obtained by UCLA Pasarow Mass Spectrometry
Laboratory on an ESI mass spectrometer. Elemental analysis was
performed by Desert Analytics (Columbia Analytical Services) of
Tucson, Arizona.
4. Experimental section
4.1. Warning!
Organomercury compounds are highly toxic! There is a danger
of cumulative effects. These compounds may cause serious and
irreversible effects on skin contact. These compounds may be fatal
if absorbed through the skin e even in small amounts. A single drop
may cause serious injury or potentially be fatal. They may cause
metal fume fever if inhaled or swallowed. Chronic exposure may
4.3. X-ray crystallography
Table 1 summarizes the crystallographic data for all structurally
characterized compounds. X-ray data were collected at 85 K for
complexes 1, 2, and 6 at 149 K for complex 3, on a SMART APEX CCD

556
W.J. Tenn III et al. / Journal of Organometallic Chemistry 696 (2011) 551e558
Table 2
These reactions are easier to dry and yields averaged w48%. A
common impurity is free hfac ligand as characterized by a 1H NMR
resonance upfield of the methine proton in the bound ligand.
Recrystallization from methanol gives trans-(hfac-O,O)2Rh(Cl)
ꢀ
RheCH3 bond distances (A) for various Rh(III) methyl complexes.
ꢀ
Molecule
Bond dist. (A)
Ref.
cis-(hfac-O,O)2Rh(CH3)(py)
2.026(5)
2.031(3)
2.031(6)
2.031(8)
2.044(4)
2.050(3)
2.05(3)
2.059(3)
2.062(3)
2.075(3)
2.076(8)
2.085(7)
2.091(4)f
2.102(3)
2.115(13)
2.137(20)
2.156(5)
This one
This one
[20]
[21]
[22]
[23]
[24]
[25]
[25]
[25]
[26]
[25]
[27]
[28]
[29]
[30]
[31]
(CH3OH).1HNMR (CD3OD, 400 MHz):
(CD3OD, ref. to CFCl3, 360 MHz):
d
6.60(s, 2H, hfacC3H).19FNMR
trans-(hfac-O,O)2Rh(CH3)(py)
(Oetaethylporphinato)Rh(CH3)
(octa-n-pentylphthalocyaninato)Rh(CH3)
(PCP)Rh(CH3)Cla
d
71.87 (s, 12F, hfac-CF3). 13C {1H}
NMR (C6D6,100 MHz):
d
179.1 (q, 2JCF ¼ 146.8 Hz, hfac C]O),116.8 (q,
1JCF ¼ 1132.8 Hz, hfac-CF3), 93.3 (s, hfac methine). Single crystals
suitable for X-ray diffraction were obtained from a concentrated
solution in methanol stored at ꢂ30 ꢀC overnight.
(POCOP)Rh(CH3)(FBF3)b
TpRh(PPh3)(CH3)Ic
cis-[(acac-O,O)Rh(PPh3)2(CH3)(CH3CN)][BPh4]
cis-[Rh(BA)(PPh3)2(CH3)(CH3CN)][BPh4]d
cis-[(acac-O,O)Rh(PPh3)2(CH3)(NH3)][BPh4]
(fctfa)Rh(CO)(PPh3)(CH3)Ie
4.5. Synthesis of trans-(hfac-O,O)2Rh(CH3)(py) (2)
[(acac-O,O)Rh(PPh3)2(CH3)(CH3CN)][BPh4]
Cp*Rh(Me2SO)(CH3)2
Approximately 1.0 g of 1 was dissolved in 15 mL of methanol in
a 30 mL Schlenk tube fitted with a resealable PTFE valve. This
compound is highly soluble and does not require much methanol to
dissolve large quantities of the compound. Dimethylmercury was
added ina 2:1 molar ratiowithrespect tothe compound. The reaction
mixture was then heated for 14 h at 75 ꢀC under argon after 5
successive freezeepumpethaw cycles to degas the solution. Excess
dimethylmercury and methanol were removed in vacuo after the
reaction was complete. The condensed material in the trap was
treated with HNO3 to convert excess dimethylmercury to (CH3)Hg
(NO3) for safer disposal. The remaining residue in the reaction flask
was redissolved in a minimal amount of methanol and kept at ꢂ30 ꢀC
overnight to precipitate methylmercuric chloride (grayish black)
which wasremoved by filtration. 2-MeOHwasisolatedby filtration of
the mercuric salt precipitates and removal of the solvent from the
filtrate by high vacuum in the glovebox. 1H NMR of 2-MeOH (CDCl3,
cis,cis-(dmb)Rh(CH3)2(I)(CO)g
h
(ArN]C(CH3)eC(CH3)]NAr)Rh(CO)(CH3)I2
trans-(BPHA)Rh(P(OPh)3)2(CH3)Ii
(
PyPCP)Rh(CH3)(PEt3)Ij
a
PCP ¼ 1-((diethylamino)methyl)-3-((di-tertbutylphosphino)methyl)-2,4,6-
trimethylbenzene.
b
POCOP ¼ C6H3(CH3)[OP(iPr)2]2.
c
Tp ¼ trispyrazoleborate.
d
BA ¼ benzoylacetonato.
e
fctfa ¼ ferrocenoylacetonato.
f
Average of the methyl bond lengths.
g
Only one of two methyls is reported due to methyl/carbonyl disorder in the
crystal structure.
h
Ar ¼ 2-iPrC6H4.
i
BPHA ¼ N-benzoyl-N-phenylhydroxyamino.
j
PytPCP ¼ pyrrolylphoshinoxylene.
400 MHz):
d 6.4 (s, 2H, hfac methine), 3.5 (s, 3H, bound methanol
diffractometer with graphite-monochromated Mo-K
a radiation
CH3), 3.0 (d, 3H, Rh-CH3). Neat pyridine was added to the methanol
adduct and the solution immediately turned red. Heating 65 ꢀC for
30 min converts the methanol adduct to the pyridyl complex.
Analytical TLC revealed an impurity that stuck to neutral alumina
when run in CH2Cl2. Preparatory TLC or an alumina plug was
employed at this point to remove to isolate the product. Excess
pyridine was removed in vacuo. When nearly all the pyridine was
removed, a small amount of chloroform was added to dissolve the
solid formed in the flask and the remaining traces of pyridine. This
solution was then fully evaporated. Crystals suitable for X-ray
diffraction were grown from methanol at low temperature (ꢂ30 ꢀC).
ꢀ
(
l
¼ 0.71073 A). The cell parameters for each compound were
obtained from the least-squares refinement of the spots (from 60
collected frames) using the SMART program [32]. In each case,
a hemisphere of data was collected up to a resolution of 0.75 A, and
ꢀ
intensity data were processed using the SAINT PLUS program [33].
Empirical absorption corrections were applied using SADABS [34],
and all calculations for the structural analyses were carried out
using the SHELXTL package [35]. Initial positions of the Rh atoms
were located by direct methods, and the rest of the atoms were
found using conventional heavy atom techniques. The structures
were refined by least-squares methods using data in the range of
2è ¼ 3.5e55.0ꢀ. All non-hydrogen atoms in the three complexes
were anisotropically refined, and calculated hydrogen positions
were varied in a riding manner along with their attached carbons.
There was some disorder present in the fluorines of the eCF3
groups in compounds 3 and 6.
1H NMR (CDCl3, 400 MHz):
d 8.49 (d, 2H, o-py), 7.93 (t,1H, p-py), 7.54
(t, 2H, m-py), 6.14 (s, 2H, hfac methine), 2.43 (d, 3H, RheCH3,
2JRhH ¼ 2.2 Hz).19F NMR (CD3OD, ref. to CFCl3, 360 MHz):
d 72.2 (s,12F,
175.2 (q, JFC ¼ 35.6 Hz,
hfac-CF3). 13C {1H} NMR (C6D6, 100 MHz):
d
2
hfac C]O), 148.9 (m, py), 138.8 (m, py), 125.8 (s, py), 115.3 (q,
1JFC ¼ 285.4 Hz, hfac-CF3),92.2(m, hfacmethine), 5.9(d,1JRhC ¼ 16 Hz,
CH3). Anal. Calcd. for RhC16H10F12NO4: C, 31.44; H, 1.65; N, 2.29.
Found: C, 31.58; H, 1.58; N, 2.24.
4.4. Synthesis of trans-(hfac-O,O)2Rh(Cl)(CH3OH) (1)
ThissynthesiscanbefollowedasreportedbyChattorajandSievers
[14] by heating 4.0 g RhCl3(H2O)3 and 11.0 mL 1,1,1,5,5,5-hexa-
fluoropentanedione (Hhfac) in 50 mL absolute ethanol under argon
or dinitrogen. However, this makes the workup rather difficult. After
removing the bulk solvent in vacuo, the residue must be thoroughly
dried,eitherbyexposuretovacuum(30 mTorr)forw24 horbygently
heating at 40e50 ꢀC on a high vacuum line (2 mTorr) for several
hours. Upon removal of all excess ethanol, deionized water was
added. A large excess of water was added as the desired product is
very insoluble. The complex precipitated from solution as mustard-
yellow microcrystals, which were thenwashed on a medium porosity
frit with copious amounts of water. A second crop of crystals was also
recovered from the mother-liquor as they precipitated. The crops of
crystals were then combined. Smaller scale reactions were run using
1.0 g RhCl3$(H2O)x and 3.0 mL Hhfac in 15 mL of absolute ethanol.
4.6. Synthesis of cis-(hfac-O,O)2Rh(CH3)(py) (3)
Quantitative isomerization of the trans-complex, 2, to the cis-
analog, 3, occurs upon heating 2 in cyclohexane under argon.
Complex 2 (45.0 mg) was dissolved upon heating in cyclohexane
(6.0 mL) at 95 ꢀC and after 14 h was completely converted to the cis
isomer, 3, as evidenced by 1H NMR. The yield of the reaction was
measured by comparison with an internal standard (1,3,5-trime-
thoxybenzene). Decomposition may occur if the solution is not fully
degassed prior to carrying out the isomerization procedure. 1H
NMR (CDCl3, 400 MHz):
d 8.26 (d, 2H, o-py), 7.88 (t, 1H, p-py), 7.43
(t, 2H, m-py), 6.22 (s, 1H, hfac methine), 5.99 (s, 1H, hfac methine),
2
2.48 (d, 3H, RheCH3, JRhH ¼ 2.5 Hz). 19F NMR (CDCl3, ref. to CFCl3,
360 MHz):
d 76.32 (s, 3F, hfac-CF3), 74.79 (s, 3F, hfac-CF3), 74.70 (s,
3F, hfac-CF3), 74.37 (s, 3F, hfac-CF3). 13C {1H} NMR (CDCl3,

W.J. Tenn III et al. / Journal of Organometallic Chemistry 696 (2011) 551e558
557
2
400 MHz):
d
177.3 (q, hfac C]O, JCF ¼ 35.0), 175.3 (m, hfac C]O,
from the MS analysis, using a program developed with Microsoft
EXCEL [36]. An important assumption made with this method is that
there are no isotope effects on the fragmentation pattern for the
various benzene isotopologs. Fortunately, because the parent ion of
benzene is relatively stable towards fragmentation, it can be used
reliably to quantify the exchange reactions. The mass range from 78 to
84 (for benzene) was examined for each reaction and compared to
a control reaction where no metal catalyst was added. The program
was calibrated with known mixtures of benzene isotopologs. The
results obtained by this method are reliable to within ꢃ2.5%. Thus,
analysis of a mixture of C6H6, C6D6 and C6H5D1 prepared in a molar
ratio of 40:50:10 resulted in a calculated ratio of 41.2(C6H6):47.5
(C6D6):9.9(C6H5D1). Catalytic H/D exchangereactions were thus run for
sufficient reaction times to be able to detect changes >5% exchange.
The methanol analog of complex 2, trans-(hfac-O,O)2Rh(CH3)(CH3OH),
was used for most H/D exchange experiments unless otherwise stated.
In a typical experiment, a 2 mL Schlenk tube fitted with
a resealable PTFE valve was charged with 5 mg of methanol analog
2, benzene (1.0 mL), and toluene-d8 (1.0 mL) under an atmosphere
of argon. The tube was then placed in a temperature controlled oil
bath maintained at 190 ꢀC and deuteration was then measured via
GCeMS analysis as described previously.
overlapping resonances), 151.7 (s, o-py), 138.7 (s, p-py), 125.6 (s, m-
py), 117.4 (m, CF3, 1JCF ¼ 284 Hz), 116.4 (m, CF3, 1JCF ¼ 284 Hz), 115.9
1
1
(q, CF3, JCF ¼ 284 Hz), 115.7 (q, CF3, JCF ¼ 284 Hz), 92.1 (s, hfac
methine), 89.0 (s, hfac methine), 7.2 (d, RheCH3, 1JRhC ¼ 26 Hz).
4.7. Synthesis of trans-(hfac-O,O)2Rh(Ph)(py) (4)
In a Schlenk tube fitted with a resealable PTFE valve, 1 (0.10 g)
was heated with diphenyl mercury (0.10 g, w1.2 mol eq) in 1:1
CHCl3/CH3OH (15 mL) at 100 ꢀC for 12 h after 5 freezeepumpethaw
cycles and filled with argon. The solution became dark orange. The
solvent was then removed in vacuo. The residue was treated with
7 mL of dry pyridine and the solution was heated for 15 min at 50 ꢀC.
The solution was concentrated by removal of solvent, then 1.0 mL of
water was added and a yellow/orange compound precipitated. The
solid was filtered and washed with two 5 mL portions of cold water.
The compound was then dried on the vacuum line and stored under
argon. The compound was found to be air and water stable. 1H NMR
(CDCl3, 250 MHz):
m-py), 7.14 (m, 3H, Ph-H), 7.05 (d, 2H, Ph-H), 5.91 (s, 2H, hfac
methine). 19F NMR (CD3OD, ref. to CFCl3, 360 MHz):
74.98 (s, 12F,
d 8.62 (d, 2H, o-py), 7.96 (t, 1H, p-py), 7.57 (t, 2H,
d
hfac-CF3). Anal. Calcd for C21H18F12NO4Rh: C, 37.47; H, 1.80; N, 2.08.
Found: C, 37.73; H, 1.80; N, 2.02.
4.11. Hydroarylation of styrene with benzene
4.8. Synthesis of cis-(hfac-O,O)2Rh(Ph)(py) (5)
A 2 mL Schlenk tube fitted with a resealable PTFE valve was
charged with 5.7 mg (8.47 ꢁ 10ꢂ3 mmol) of 5, styrene (0.2 mL), and
benzene (1 mL) and placed in a temperature regulated oil bath set
at 190 ꢀC for 30 min. The contents of the reactor were then
analyzed by GCeMS followed by evaporation of the volatiles and
dissolution in benzene-d6 for NMR analysis. The retention times,
mass spectra, and NMR spectra of the species detected were
compared to that of authentic samples.
In a Schlenk flask with a resealable PTFE valve, 3 (0.25 mg) was
heated in neat benzene (15 mL) under argon at 190 ꢀC for 24 h.
Removal of the volatiles followed by sublimation of this compound at
w60 ꢀC left behind some Rh metal and afforded cis-(hfac)2Rh(Ph)(py)
in 75% yield.1H NMR (CDCl3, 400 MHz):
d 8.17 (d, 2H, o-py), 7.87 (t,1H,
p-py), 7.36 (t, 2H, m-py), 7.10 (m, m/p-phenyl overlaping), 6.86 (d, o-
phenyl), 6.09(s, 2H, hfacmethine), 6.06(s, 2H, hfacmethine).13CNMR
{1H} (CDCl3, 100 MHz):
d 177.1 (m, hfac C]O, overlapping reso-
Acknowledgment
nances),152.4 (Ph or Py),147.7 (d, i-phenyl 1JRhC ¼ 28.6 Hz),139.0 (Ph
or Py), 134.4 (Ph or Py),127.7 (Ph or Py), 125.5 (Ph or Py),125.2 (Ph or
Py), 118.0 (m, hfac-CF3 overlapping resonances), 92.3 (s, hfac
methine), 89.5 (s, hfac methine). Anal. Calcd for C21H18F12NO4Rh: C,
37.47; H, 1.80; N, 2.08. Found: C, 37.80; H, 2.00; N, 2.08.
The authors acknowledge Muhammed Yousufuddin, Timothy J.
Stewart, and Prof. Robert Bau for solving the crystal structures. Dr.
Brian G. Hashiguchi and Dr. Kapil S. Lokare are thanked for their
helpful discussions during the preparation of this manuscript. We
thank the National Science Foundation (CHE-0328121), Chevron
Corporation, the Loker Hydrocarbon Research Institute, the
University of Southern California, The Scripps Research Institute,
and Center for Catalytic Hydrocarbon Functionalization, a DOE
Energy Frontier Research Center (DOE DE-SC000-1298) for finan-
cial support.
4.9. Synthesis of cis-(hfac-O,O)2Rh(Mes)(py) (6)
In a Schlenk flask with a resealable PTFE valve, 3 (0.25 mg) was
heated in neat mesitylene (10 mL) under argon at 195 ꢀC for 14 h.
After removal of the solvent cis-(hfac-O,O)2Rh(Mes)(py) was iso-
latedin 51% yield bysublimation at 60 ꢀC.1HNMR (CDCl3, 400 MHz):
d
8.26 (d, 2H, o-py), 7.90 (t, 1H, p-py), 7.45 (t, 2H, m-py), 6.79 (s, 1H,
Appendix A. Supporting information
p-mesityl), 6.69 (s,1H, o-mesityl), 5.96 (s, 1H, hfac methine), 5.68 (s,
1H, hfac methine), 5.05 (dd, 1H, mesityl methylene), 4.46 (dd, 1H,
mesityl methylene), 2.15 (s, 3H, mesityl methyl). 13C NMR (CDCl3,
CCDC 790213, 790214, 790215, and 790216 contain the
supplementary crystallographic data for this paper. This data can be
obtained free of charge from The Cambridge Crystallographic Data
Center via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2010.09.021.
100 MHz):
d 176.0 (m, C]O hfac overlapping resonances), 151.7,
145.1, 138.7, 137.6, 128.0, 127.3, 125.7 (phenyl and mesityl aromatic
carbons), 117.0 (m, hfac-CF3 overlapping resonances), 91.4 (s, hfac
methine), 89.1 (s, hfac methine), 30.9 (s, mesityl methyl), 30.7 (s,
mesityl methyl), 20.9 (d, mesityl methylene,1JRhC ¼ 25.1 Hz). HR-MS
(ESI): Calculated for C24H19NO4F12Rhþ1 (M þ H) 716.0172, found
716.0170. Single crystals suitable for X-ray diffraction were grown
from evaporation of a concentrated solution in methanol.
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