Inactive ruthenium(II)-xantphos complexes from attempted catalyzed lignin reactions

Article abstract of DOI:10.1016/j.inoche.2012.07.038

Tests on the use of Ru(H)₂(CO)(PPh₃)(xantphos) (1) as a hydrogenolysis catalyst for a lignin model compound in bromobenzene at 135 °C under Ar lead to formation of the corresponding, catalytically inactive, but fully characterized complexes, Ru(H)Br(CO)(xantphos) (2) and RuBr ₂(CO)(xantphos) (3). In 1, the xantphos [xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene] is bidentate with cis-coordinated P-atoms, whereas in 2 and 3 the xantphos is mer-P,O,P- coordinated with the P-atoms trans.

Full text of DOI:10.1016/j.inoche.2012.07.038

Inorganic Chemistry Communications 24 (2012) 11–15
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Inactive ruthenium(II)-xantphos complexes from attempted catalyzed
lignin reactions
⁎
Adam Wu, Brian O. Patrick, Brian R. James
Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada V6T 1Z1
a r t i c l e i n f o
a b s t r a c t
Article history:
Tests on the use of Ru(H)2(CO)(PPh3)(xantphos) (1) as a hydrogenolysis catalyst for a lignin model compound in
bromobenzene at 135 °C under Ar lead to formation of the corresponding, catalytically inactive, but fully charac-
terized complexes, Ru(H)Br(CO)(xantphos) (2) and RuBr2(CO)(xantphos) (3). In 1, the xantphos [xantphos=
4,5-bis(diphenylphosphino)-9,9-dimethylxanthene] is bidentate with cis-coordinated P-atoms, whereas in 2
and 3 the xantphos is mer-P,O,P-coordinated with the P-atoms trans.
Received 17 June 2012
Accepted 18 July 2012
Available online 26 July 2012
Keywords:
Ruthenium
© 2012 Elsevier B.V. All rights reserved.
Xantphos
Bromobenzene
Catalytic hydrogenolysis
Lignin models
Introduction
effective for hydrogenolysis of a polymer model for lignin, as shown in
Eq. (2), and clearly (as implied by the authors [3]), application of
We reported recently [1] on the use of Ru(H)2(CO)(PPh3)(xantphos)
(1), a known complex [2], as a catalyst for the hydrogenolysis of lig-
nin model compounds, as exemplified by the chemistry shown in
Eq. (1). The studies expanded on work of Bergman, Ellman et al. in
2010 [3], who reported that an in situ Ru(H)2(CO)(PPh3)3/xantphos
species catalyzed this remarkable conversion in toluene at 135 °C
under N2. The chemistry involves initial dehydrogenation of the
CH(OH) moiety to C(O), followed by hydrogenolysis of the CH2\OAr′
bond. Under similar conditions, but under a H2 atmosphere, the final ke-
tone product ArC(O)CH3 is hydrogenated to the corresponding alcohol
[1]; the initially formed ketone intermediate was also used as a sub-
strate, and was shown to undergo the hydrogenolysis reaction in the
presence of H2 [1]. The earlier report [3] also demonstrated that the
Ru-xantphos catalyst system was similarly
such hydrogenolysis cleavage catalysis within a lignin could provide
an attractive, valuable source of useful aromatic compounds. A major
problem in testing for this is finding compatibility of solubilities of
the catalyst and the lignin. The studies within the paper presented
here solve this compatibility problem, but unfortunately the selected
halogen-containing solvent destroys the active catalyst. The ob-
served chemistry (see Scheme 1) illustrates two (of three known)
coordination modes of xantphos.
þH2
ArCHðOHÞCH2OAr′ −→H2 ArCðOÞCH2OAr′ →
h
ArCðOÞCH3 þ HOAr′
ið1Þ
Ar ¼ a Ph moietywith or without OH and=or OMe substituents; Ar′ ¼ ðo−OMeÞC6H4
Experimental section
ð2Þ
All general solvents and reagents used in the syntheses were ‘re-
agent grade’ and were used as supplied by Aldrich or Fisher Scientific.
Praxair Ar (99.996%) was used as received.
1H (300 MHz) and 31P{1H} (122 MHz) NMR spectra were recorded
at room temperature (r.t., ~20 °C) on a Bruker spectrometer. Residual
deuterated solvent proton, relative to external SiMe4 [4], or external
85% H3PO4 was used as reference (s=singlet, m=multiplet, t=triplet;
⁎ Corresponding author. Tel.: +1 604 822 6645; fax: +1 604 822 2847.
E-mail address: brj@chem.ubc.ca (B.R. James).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.07.038

12
A. Wu et al. / Inorganic Chemistry Communications 24 (2012) 11–15
Table 1
J values are given in Hz). Deuterated solvents were purchased from
Cambridge Isotope Laboratories, Inc. IR bands were recorded on a Perkin
Elmer Frontier FT-IR spectrometer using an ATR sampling surface.
Electrospray ionization mass spectra in the positive ion mode (ESI/
MS+) were recorded on a Bruker Esquire-LC ion trap instrument, with
MeOH solution of samples being infused into the ion-source by a syringe
pump at a flow rate of 200 μL/min. Elemental analyses were performed
using a Carlo Erba EA1108 elemental analyzer.
Ru(H)2(CO)(PPh3)(xantphos) (1) was synthesized from Ru(H)2-
(CO)(PPh3)3 [5] by our recently reported method [1], and 2-
(2-methoxyphenoxy)-1-phenylethanol [the lignin model compound —
see Eq. (1) where Ar=Ph, and Ar′=(o-OMe)C6H4] was made via a
literature method [3]. Lignin samples available were: an alkali lignin
(from Aldrich), an Indulin AT kraft lignin (from MeadWestvaco Corp.),
a lignin from Lignol Energy Corp., a pyrolytic lignin, and a CH2Cl2-soluble
fraction lignin (the last two being provided by Prof. J. Kadla, Faculty of
Forestry, Univ. of BC).
X-ray crystallographic data for complexes 2 and 3.
Complex
2⋅0.5 C6H6
3⋅0.5 CH2Cl2
Empirical formula
Mw
Crystal size (mm)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
C43H36O2P2RuBr
827.64
C41H34O2P2RuBr2Cl2
952.41
0.02×0.12×0.22
Monoclinic
C2/c(#15)
33.9363(7)
12.3216(3)
19.4164(4)
90
120.267(2)
90
7012.2(3)
8
1.568
1.72
55,481
10,314
0.046
0.040, 0.065
1.02
0.10×0.16×0.31
Monoclinic
P21/c(#14)
10.2234(2)
13.3706(3)
27.2002(5)
90
91.910(1)
90
3716.0(1)
4
1.702
2.84
50,639
10,920
0.050
0.038, 0.074
1.07
γ (°)
V (Å3)
Z value
Dcalc (g cm−3
)
μ (mm−1
)
Reflections collected
Unique reflections
Rint
R1, wR2 (I>2σ(I))
Goodness of fit
Synthesis of Ru(H)Br(CO)(xantphos) (2)
Complex 1 (100 mg, 0.10 mmol) and C6H5Br (5 mL) were charged
in a Schlenk flask, which after three freeze-pump-thaw cycles was
filled with Ar to 1 atm. The mixture was heated with stirring at
120 °C for 1 h, and then cooled to r.t. Addition of hexanes (50 mL)
precipitated a white powder that was collected, dissolved in CH2Cl2
(5 mL), and then reprecipitated with hexanes (50 mL); the product
was filtered off, washed with hexanes (2×10 mL), and dried in vacuo
at 70 °C. Yield=28 mg (35%). 1H NMR (CD2Cl2): δ−11.98 (Ru–H, t,
1H, JHP=19.7), 1.60 and 1.76 (xantphos-CH3, s, 3H each), 7.18–7.90
(Ar–H, m, 26 H). 31P{1H} NMR (CD2Cl2): δ 48.3. IR: 1929 cm−1 (νCO).
ESI/MS+: 709 [M–Br]+. Anal. Calcd. for C40H33BrO2P2Ru⋅H2O: C, 59.56;
H, 4.37. Found: C, 59.6; H, 4.3.
of the lignin model compound or a lignin; the 31P{1H} spectra re-
vealed formation of complex 2 (δ 48.1) and free PPh3 (δ 5.2), followed
by conversion of 2 to complex 3 (δ 42.5).
Results and discussion
Established previously [1,3] is that the lignin model compound
2-(2-methoxyphenoxy)-1-phenylethanol (0.20 M in toluene-d8), on
treatment with 5 mol% Ru(H)2(CO)(PPh3)(xantphos) (1) at 135 °C for
20 h under Ar, undergoes quantitative hydrogenolysis to acetophenone
and 2-methoxyphenol, according to the chemistry outlined in Eq. (1).
Corresponding tests, made on several lignin samples [1], revealed the
absence (by 1H NMR data) of hydrogenolysis products, but the lignins
were insoluble in toluene and this was considered a major factor
leading to non-reactivity. Increased solubility of lignins in more
polar solvents, and the availability of a CH2Cl2-soluble fraction lignin,
subse-quently led us to test bromobenzene as solvent under otherwise
identical reaction conditions; the lignin and the model compound were
soluble in C6D5Br, which has a higher boiling point (156 °C) than
toluene (110.6 °C). However, again no hydrogenolysis products were
detected, and it was soon realized by examining 31P{1H} NMR spectra
that new, catalytically inactive Ru complexes had been formed, and
these were readily isolated from reaction of 1 with C6H5Br at 120 °C
under Ar, as shown in Scheme 1. Bromine extraction from the solvent
first generates trans-RuH(Br)(CO)(xantphos) (2) and then, after longer
heating, trans-RuBr2(CO)(xantphos) (3); 2 and 3 are isolated in 35% and
56% yields, respectively. Neither 2 nor 3 is active for hydrogenolysis of
2-(2-methoxyphenoxy)-1-phenylethanol under the conditions noted
above where 1 was an active catalyst [1].
X-ray quality crystals of 2 and 3 were obtained and analyzed. Both
structures have an associated 0.5 mole of solvate (C6H6 and CH2Cl2,
respectively) per mole of complex. The distorted octahedral struc-
tures (Figs. 1 and 2) reveal mer-P,O,P-coordinated xantphos with
trans-phosphines, with trans hydrido and bromo ligands in 2 and
trans bromo ligands in 3. The coordination geometry of 2 is similar to
that of Ru(H)Cl(PPh3)(xantphos) [11], but with the PPh3 replaced by
CO. The P\Ru\P xantphos bite angles are similar (160.2° in 2, and
156.4° in the latter complex) and are significantly less than linear, as
the xantphos backbone constrains the trans binding geometry. The
Ru\P bond lengths of the xantphos are essentially the same in both
complexes, whereas the Ru\O length in 2 (trans to CO) is ~0.04 Å
shorter than in the hydrido(chloro) complex (where the O-atom is
trans to PPh3); as expected, the Ru\Br bond (2.624 Å) is significantly
longer than the Ru\Cl bond (2.521 Å). Other neutral and cationic,
Synthesis of RuBr2(CO)(xantphos) (3)
The method was identical to that given above for 2, except that
the mixture was heated for 3 h; the product was a yellow powder.
Yield=50 mg (56%). 1H NMR (CD2Cl2):
δ
1.78 (xantphos-CH3,
s, 6H), 7.20–7.90 (Ar–H, m, 26H). 31P{1H} NMR (CD2Cl2): δ 42.3. IR:
1951 cm−1 (νCO). ESI/MS+: 709 [M−2Br+H]+, 787 [M−Br]+
Anal. Calcd. for C40H32Br2O2P2Ru: C, 55.38; H, 3.72. Found: C, 55.2;
H, 3.8.
.
X-Ray structural determinations
Crystals of 2 and 3 were grown, respectively, by layering hexanes
onto C6H6 and CH2Cl2 solutions of the complexes. The data were col-
lected at −183°C on a Bruker X8 APEX II diffractometer with graphite
monochromated Mo-Kα radiation of 0.71073 Å, and processed using
the Bruker SAINT software package [6]; the data were corrected for
absorption effects using the multi-scan technique (SADABS) [7], as
well as for Lorentz and polarization effects. The structures were
solved by direct methods [8], with all non H-atoms and the hydride
ligand of 2 being refined anisotropically; the other H-atoms, were
placed in calculated positions. All refinements were performed
using the SHELXL-97 [9] via the WinGX interface [10]. Some crystal
data are given in Table 1, with full details being available as Supple-
mentary Material.
Attempted catalyzed hydrogenolysis reactions
2-(2-Methoxyphenoxy)-1-phenylethanol (0.10 mmol) or 25 mg
of a lignin sample, 5 mol% of 1, and 0.5 mL C6D5Br were added to a
J-Young NMR tube. After three freeze-pump-thaw cycles, the tube
was filled with 1 atm of Ar. The 1H NMR spectra, recorded at r.t.,
prior to and after heating for 20 h at 135 °C, revealed no conversion

A. Wu et al. / Inorganic Chemistry Communications 24 (2012) 11–15
13
Scheme 1. Reaction of 1 to form 2 and 3.
Ru-hydrido-(P,O,P-xantphos) structures have been appropriately drawn
with trans P-atoms, although in some of these structures the P\Ru\P
angle of the xantphos moiety is as small as 120°; the decrease requires in-
creased hinging of the xantphos as exemplified by a marked decrease in
the angle between the aromatic planes and a reduction in the P\O\P
angle [11,12]. The acyl complexes Ru(CO)(xantphos)[C(O)CX=C(Ph)O],
where X=H or OC6H4(OMe), isolated from reaction of 1 with lignin
models akin to those shown in Eq. (1) but with the CH2 replaced by
CH(CH2OH), also contain mer-P,O,P-xantphos with the P\Ru\P being
~160° [1].
The above structural data illustrate the flexibility of the xantphos
ligand even when mer-coordinated, the most common bonding
mode of xantphos. The data refute the description of xantphos as hav-
ing a rigid backbone [14,15] and, worth noting in this regard, is that
in complexes 1 [2], Ru(H)2(CO)2(xantphos) [2], Ru(H)F(CO)(PPh3)-
(xantphos) [16], and Ru(H)2(CO)(NHC)(xantphos) and derivatives
(NHC=N-heterocyclic carbene) [17], the xantphos is cis-P,P-bonded.
Further, there are two complexes, cis-RuCl2(S-dmso)(xantphos) [18]
and the catecholate species Ru(OC6H4O)(CO)(xantphos) [1] that
contain fac-P,O,P-xantphos with P\Ru\P angles of 105.5° and 102,7°,
respectively; the Ru\O bond length in these complexes is 2.16 Å,
whereas the corresponding values in the mer-P,O,P-xantphos species
is 2.25–2.33 Å [1,11,12,14].
The coordination geometry of 3 is very similar to that of complex 2,
but with the equal length, trans Ru\Br bonds being ~0.1 Å shorter than
the one in 2, which is trans to the hydride, this being consistent with the
greater trans influence of H− versus Br− [13]. The RuCl2(=CHPh)(xantphos)
The NMR data for 2 and 3 in CD2Cl2 are consistent with the solid state
structures; for 2, the Ru\H triplet signal at δ −11.98 (2JHP=19.7) com-
pares with the doublet of triplets seen for Ru(H)Cl(PPh3)(xantphos)
complex is also similar to
3 in having the mer-xantphos and
trans-dihalide ligands [14]: the P\Ru\P angles are similar (163.9° in
3 and 161.0° in the carbene complex), as are the X\Ru\X angles
(X_Br, 165.7°; Cl, 165.1°).
2
(δ −16.22, JHP =27.2 and 23.9 in C6D6) [11]; doublets of triplets
2
in the range δ −1.5 to −23 with JHP values between 31 and
Fig. 1. ORTEP diagram of Ru(H)Br(CO)(xantphos) (2) with selected bond lengths (Å) and angles (°): H(1)\Ru 1.54(3), O(1)\Ru 2.2085(12), P(1)\Ru 2.2984(5), P(2)\Ru
2.2947(5), Br(1)\Ru 2.6239(2), C(40)\Ru 1.8075(18); C(40)\Ru\O(1) 173.69(7), C(40)\Ru\P(2) 95.68(6), O(1)\Ru\P(2) 82.32(3), C(40)\Ru\P(1) 98.41(6), O(1)\
Ru\P(1) 82.18(3), P(2)\Ru\P(1) 160.203(17), C(40)\Ru\Br(1) 101.29(6), O(1)\Ru\Br(1) 84.93(4), P(2)\Ru\Br(1) 98.078(14), P(1)\Ru\Br(1) 92.739(13).
A
co-crystallized solvent molecule (0.5 C6H6) is not shown.

14
A. Wu et al. / Inorganic Chemistry Communications 24 (2012) 11–15
Fig. 2. ORTEP diagram of RuBr2(CO)(xantphos) (3) with selected bond lengths (Å) and angles (°): O(1)\Ru 2.1930(19), P(1)\Ru 2.3360(7), P(2)\Ru 2.3312(7), Br(1)\Ru
2.5201(3), Br(2)\Ru 2.5225(3), C(41)\Ru 1.819(3); C(41)\Ru\O(1) 177.43(10), C(41)\Ru\P(2) 99.69(9), O(1)\Ru\P(2) 82.49(5), C(41)\Ru\P(1) 95.30(9), O(1)\
Ru\P(1) 82.68(5), P(2)\Ru\P(1) 163.91(3), C(41)\Ru\Br(1) 95.45(9), O(1)\Ru\Br(1) 83.23(5), P(2)\Ru\Br(1) 88.050(19), P(1)\Ru\Br(1) 96.378(19), C(41)\
Ru\Br(2) 97.98(9), O(1)\Ru\Br(2) 83.54(5), P(2)\Ru\Br(2) 84.873(19), P(1)\Ru\Br(2) 87.278(19), Br(1)\Ru\Br(2) 165.699(14). A co-crystallized solvent molecule (0.5
CH2Cl2) is not shown.
18 Hz are reported for RuH(OTf)(PPh3)(xantphos) and the cationic
[Ru(H)(PPh3)(xantphos)L]+ species in CD2Cl2, where L=η2-O2,
MeCN, H2O [11].
Appendix A. Supplementary material
The CCDC numbers 886568 and 886567 contain crystallographic
data for 2· 0.5 C6H6 and 3· 0.5 CH2Cl2, respectively. These data can
be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data related to this article can be found online at http://dx.doi.org/10.
1016/j.inoche.2012.07.038.
The xantphos 31P{1H} NMR singlets for 2 and 3 in CD2Cl2 are seen
at δ 48.3 and 42.3, respectively, comparable to the δ 46.7 doublet of
Ru(H)Cl(PPh3)(xantphos) measured in C6D6 [11]. The 31P{1H} sin-
glet for RuCl2(=CHPh)(xantphos) in CD2Cl2 is seen at δ 36.26 [14],
and the corresponding doublets for the neutral and cationic species
containing PPh3 mentioned in the previous paragraph also appear
2
in the δ 51.4–44.4 region with JPP values of 33–19 Hz. Of note, the
lowest δ and 2JPP values are for the η2–O2 complex, which was classi-
fied by structural data as a peroxide making this complex formally a
RuIV species, although this was not mentioned in the report [11].
The ESI-mass spectral data and the elemental analyses support the
formulations of 2 and 3, although addition of a solvated H2O was
needed for a satisfactory experimental elemental analysis for 2; the
presence of H2O was seen qualitatively in the 1H NMR spectrum at δ
1.54. The νCO IR band for 3 was 22 cm−1 higher than that for 2, con-
sistent with the greater electronegativity of Br vs. H [19], which is
expected to reduce the extent of Ru\CO π-backbonding; the hydride
stretch in 2 was not detected.
The conversion of a transition metal hydride ligand to the corre-
sponding chloride in solvents such as CHCl3 and CH2Cl2 (or the deu-
terated analogues) is well documented and is generally thought to
involve radical processes [20], but there are examples involving
bromobenzene in which both radical [21] and oxidative-addition
[22] mechanisms have been suggested.
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Acknowledgement
We thank the NSERC Lignoworks Network for funding.

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