4-Hydroxyaryl complexes of group 10 metals

Article abstract of DOI:10.1016/j.jorganchem.2017.10.012

Attempts have been made to synthesize group 10 metal complexes bearing σ-bonded 4-hydroxyaryl ligands that are potential precursors to metallaquinones. Treatment of 4-bromo-2,6-di-tert-butylphenol with [Pd(PPh₃)₄] led to formation of the phosphaquinone Ph₃P = C₆H₃^tBu₂-3,5-O-4 (1), whereas that with [M(PPh₃)₄] (M = Ni, Pt) yielded the biquinone [C₆H₃^tBu₂-3,5-O-4]₂ (2). The solid-state structure of 1 features a short C=O double bond and alternate phenyl C-C single and double bonds that are characteristic of quinoidal compounds. Alkylation of [NiCl₂(PPh₃)₂] with (C₆H₂Me₂-3,5-OSiMe₃-4)MgBr afforded cis-[Ni(PPh₃)₂Br(C₆H₂Me₂-3,5-OSiMe₃-4)] (3) that reacted with ⁿBu₄NF to yield [Ni(PPh₃)(C₆H₂Me₂-3,5-OH-4)]₂(μ-OH)₂ (4) containing a σ-bonded 4-hydroxyaryl ligand.

Full text of DOI:10.1016/j.jorganchem.2017.10.012

Journal of Organometallic Chemistry 853 (2017) 1e4
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Note
4-Hydroxyaryl complexes of group 10 metals
Shiu-Chun So, Wai-Man Cheung, Herman H.-Y. Sung, Ian D. Williams, Wa-Hung Leung*
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People's Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Attempts have been made to synthesize group 10 metal complexes bearing
s-bonded 4-hydroxyaryl
Received 18 July 2017
Received in revised form
14 September 2017
Accepted 13 October 2017
Available online 16 October 2017
ligands that are potential precursors to metallaquinones. Treatment of 4-bromo-2,6-di-tert-butylphe-
nol with [Pd(PPh3)4] led to formation of the phosphaquinone Ph3P ¼ C6Ht3Bu2-3,5-O-4 (1), whereas that
with [M(PPh3)4] (M ¼ Ni, Pt) yielded the biquinone [C6Ht3Bu2-3,5-O-4]2 (2). The solid-state structure of 1
features a short C¼O double bond and alternate phenyl C-C single and double bonds that are charac-
teristic of quinoidal compounds. Alkylation of [NiCl2(PPh3)2] with (C6H2Me2-3,5-OSiMe3-4)MgBr affor-
ded cis-[Ni(PPh3)2Br(C6H2Me2-3,5-OSiMe3-4)] (3) that reacted with nBu4NF to yield [Ni(PPh3)(C6H2Me2-
Keywords:
Group 10 metal
Quinone
3,5-OH-4)]2(
m
-OH)2 (4) containing a
s
-bonded 4-hydroxyaryl ligand.
© 2017 Elsevier B.V. All rights reserved.
4-Hydroxyaryl complex
Phosphaquinone
1. Introduction
reactions.
In an attempt to prepare 4-hydroxyaryl complexes, oxidative
addition of [M(PPh3)4] (M ¼ Ni, Pd, Pt) with substituted 4-
bromophenols was studied. In particular, the oxidative addition
with 4-bromo-2,6-di-tert-butylphenol was explored because all the
reported heteroatom-substituted quinoidal compounds A-C
contain the 2,6-di-tert-butylphenoxy moiety. In this work, we
found that whereas the reaction of 4-bromo-2,6-di-tert-butylphe-
nol with [Pd(PPh3)4] gave a phosphaquinone, 1, presumably via the
P-C elimination of a Pd(II) 4-hydroxyaryl intermediate, those with
[M(PPh3)4] (M ¼ Ni, Pt) afforded a biquinone, 2. A dinuclear Ni(II)
Quinoidal compounds have attracted considerable attention
because of their pivotal roles in biology, and chemistry, and ma-
terials science [1]. Although quinone derivatives are well docu-
mented, not many heteroatom-substituted quinoidal compounds
have been isolated. Examples of isolated quinone compounds
bearing C ¼ X (X ¼ B [2], P [3], Bi [4]) bonds are shown in Scheme 1.
Stable metallaquinones (quinone in which one of the oxygen atoms
is replaced by a transition metal) remain elusive. By contrast,
transition metal complexes with
p-bonded quinoidal ligands that
have been used as building blocks for polymeric organometallic
networks are well known [5].
hydroxyaryl complex [Ni(PPh3)(C6H2Me2-3,5-OH-4]2(m-OH)2 (4)
has been synthesized by desilylation of [Ni(PPh3)2Br(C6H2Me2-3,5-
Heavy element-containing quinoidal compounds are generally
synthesized from the hydroxyaryl precursors. For example, reaction
of the RPCl2 (R ¼ 2,4,6-tri-tert-butylphenyl) with Li(C6Ht4Bu2-3,5-O-
4), followed by dechlorination afforded the phosphaquinone B [3],
whereas the bismuthaquninone C (Scheme 1) was obtained by
reaction of [(N^C^N)BiCl2] (N^C^N ¼ 2,6-(Me2NCH2)2C6H3) with
Li(C6Ht4Bu2-3,5-O-4) [4]. Milstein and co-workers reported that
deprotonation of a Ru(II) complex bearing a hydroxy-PCP pincer
ligand yielded a Ru(0) quinoid species (D) that is readily converted
to the zwitterionic form (E) by changing the solvent polarity
(Scheme 2) [6]. This finding prompted us to synthesize transition
metal 4-hydroxyaryl complexes and to explore their deprotonation
OSiMe3-4] (3) and characterized by X-ray crystallography.
2. Experimental section
2.1. General considerations
All manipulations were carried out under nitrogen by standard
Schlenk techniques. Solvents were dried, distilled and degassed
before use. NMR spectra were recorded on a Bruker AV 400 MHz
NMR spectrometer operating at 400, 100 and 162.0 MHz for 1H, 13
C
and 31P, respectively. Chemical shifts (
reference to SiMe4 (1H and 13C), and H3PO4
d
, ppm) were reported with
(
31P). Infrared spectra
(KBr) were recorded on a Perkin-Elmer 16 PC FT-IR spectropho-
tometer. Elemental analyses were performed by Medac Ltd., Surrey,
United Kingdom. The compounds [M(PPh3)4] (M ¼ Ni [7], Pd [8], Pt
* Corresponding author.
E-mail address: chleung@ust.hk (W.-H. Leung).
https://doi.org/10.1016/j.jorganchem.2017.10.012
0022-328X/© 2017 Elsevier B.V. All rights reserved.

2
S.-C. So et al. / Journal of Organometallic Chemistry 853 (2017) 1e4
solution in THF, 1.6 mmol) at 0 ꢀC. The resulting orange mixture was
stirred for 15 min at 0 ꢀC and the volatiles were removed in vacuo.
Methanol was added and the resulting suspension was cooled to
0
ꢀC. The yellow precipitate was collected by vacuum filtration,
washed with two portions of cold methanol (10 mL), and dried
under high vacuum. Recrystallization from CH2Cl2/hexanes affor-
ded orange crystals which were suitable for X-ray diffraction
analysis. Yield: 1.1 g, 80%. 1H NMR (400 MHz, C6D6):
d
¼ 0.17 (s, 9H,
CH3), 2.72 (s, 6H, CH3), 5.93 (s, 2H, Ph), 7.10 (m, 18H, Ph), 7.82 (m,
12H, Ph) ppm. 31P {1H} NMR (CDCl3):
¼ 19.50 (s) ppm. Anal. Calcd
for C47H47BrNiOP2Si: C, 55.88; H, 4.69. Found: C, 55.73; H, 4.73.
Scheme 1. Examples of heteroatom-substituted quinoidal compounds [2e4].
d
2.5. Synthesis of [Ni(PPh3)(C6H2Me2-2,6-OH-4)]2(
mꢁOH)2 (4)
To a solution of 3 (100 mg, 0.12 mmol) in THF (10 mL) was added
1.5 equivalents of nBu4NF,H2O (0.18 mL of a 1 M solution in THF,
0.18 mmol) at ꢁ78 ꢀC. The resulting mixture was then warmed to
room temperature and stirred overnight. The volatiles were
removed in vacuo. The residue was washed with hexanes and then
extracted with MeCN. Recrystallization from MeCN/Et2O afforded
yellow crystals which were suitable for X-ray diffraction. Yield:
Scheme 2. A PCP pincer-based ruthenaquionone [5].
33 mg, 52%. 1H NMR (400 MHz, CD3CN):
(s, 6H, CH3), 5.85 (br, 2H, OH), 5.92 (s, 4H, Ph), 7.24e7.37 (m, 30H,
Ph) ppm. 31P {1H} NMR (CDCl3):
¼ 21.20 (s) ppm. Anal. Calcd for
52H50Ni2O4P2: C, 68.01; H, 5.49. Found: C, 68.16; H, 5.28 .
d
¼ ꢁ4.0 (br, 2H, OH), 2.98
[9]) and [Ni(PPh3)2Cl2] [10] were prepared according to literature
methods. 2,6-Dimethyl-4-trimethylsiloxyphenylmagnesium bro-
mide was prepared by treatment of 1-bromo-2,6-dimethyl-4-
trimethylsiloxybenzene, which was obtained from 4-bromo-3,5-
dimethylphenol and chorotrimethylsilane, with Mg in tetrahydro-
furan (THF).
d
C
2.6. X-ray crystallography
Crystal data and experimental details for 1, 3, and 4 are sum-
marized in Table 1. Preliminary examinations and intensity data
collection were carried out on a Bruker SMART APEX 1000 CCD
diffractometer. The collected frames were processed with the
software SAINT. The data was corrected for absorption using the
program SADABS [11]. Structures were solved by direct methods
and refined by full-matrix least-squares on F2 using the SHELXTL
software package [12]. Unless stated otherwise, non-hydrogen
atoms were refined with anisotropic displacement parameters.
Carbon-bonded hydrogen atoms were included in calculated po-
sitions and refined in the riding mode using SHELXL97 default
parameters. CCDC 1539784, 1540324 and 1540325 contains the
supplementary crystallography data for 1, 3, and 4, respectively.
2.2. Reaction of [Pd(PPh3)4] with 4-bromo-2,6-di-tert-butylphenol
To a suspension of [Pd(PPh3)4] (230 mg, 0.2 mmol) in toluene
(10 mL) was added 1 equivalent of 4-bromo-2,6-di-tert-butylphe-
nol (57 mg, 0.2 mmol). The resulting solution was refluxed for 2 d,
and the volatiles were removed in vacuo. The residue was washed
with hexanes and then extracted with CH2Cl2. Recrystallization
from CH2Cl2/hexanes afforded yellow crystals that were charac-
terized as the phosphaquinone Ph3P ¼ C6Ht3Bu2-3,5-O-4 (1) by X-
ray crystallography. Yield: 50 mg, 54%. 1H NMR (400 MHz, CDCl3):
d
¼ 1.31 (s, 18H, CH3), 6.84 (s, 2H, Ph), 7.60e7.72 (m, 15H, Ph) ppm.
13C {1H} NMR (100 MHz, CDCl3):
¼ 28.82 (s), 34.63 (s), 122.11 (s),
122.99 (s), 140.15 (s), 176.21 (s) ppm. 31P {1H} NMR (CDCl3):
d
d
¼ 20.74 (s) ppm. Anal. Calcd for C32H35OP: C, 82.37; H, 7.56.
Table 1
Found: C, 81.50; H, 7.24.
Crystallographic data and experimental details for 1, 3 and 4.
2.3. Reaction of [M(PPh3)4] (M ¼ Ni, Pt) with 4-bromo-2,6-di-tert-
1$0.125(C4H8O)
3$CH2Cl2
4$2CH3CN
butylphenol
Formula
Fw
C32H35OP
475.58
10.8259(2)
18.4075(3)
14.8443(2)
90
94.853(2)
90
2947.53(8)
4
monoclinic
1.072
P21/n
173.15
0.973
C47H47BrNiOP2Si
941.42
C52H50Ni2O4P2
1000.39
14.01551(11)
12.82811(10)
27.58082(19)
90
97.8616(7)
90
4912.22(6)
4
monoclinic
1.353
P21/n
100.0(2)
1.947
2096
A mixture of [Ni(PPh3)4] or [Pt(PPh3)4] (0.2 mmol) and 4-bromo-
2,6-di-tert-butylphenol (0.2 mol) in toluene (10 mL) was heated at
reflux for 2 d, and the volatiles were removed in vacuo. The residue
was washed with hexanes and then extracted with CH2Cl2.
Recrystallization from CH2Cl2/Et2O/hexanes afforded yellow crys-
tals that were identified as the biquione [C6Ht3Bu2-3,5-O-4]2 (2) by
X-ray crystallography. Yield: 40%. 1H NMR (400 MHz, CDCl3):
a [Å]
b [Å]
c [Å]
11.7312(4)
12.2073(5)
17.6080(5)
95.260(3)
93.322(3)
116.250(4)
2238.28(14)
2
triclinic
1.397
P-1
99.98(6)
3.969
a
b
g
[o]
[o]
[o]
V [Å3]
Z
Cryst system
rcalcd [g$cmꢁ1
Space group
T [K]
d
¼ 1.35 (s, 36H, CH3), 7.70 (s, 4H, Ph) ppm. 13C {1H} NMR (100 MHz,
]
CDCl3):
d
¼ 28.95 (s), 35.38 (s), 125.38 (s), 135.49 (s), 149.78 (s),
185.92 (s) ppm. Anal. Calcd for C28H40O2: C,82.30; H, 9.87. Found: C,
81.79; H, 10.04.
m
[mmꢁ1
F(000)
No. of reflns
]
1020
8830
4910
972
12411
7872
19228
8757
2.4. Synthesis of cis-[Ni(PPh3)2Br(C6H2Me2-2,6-OSiMe3-4)] (3)
No. of indep reflns
R(int)
0.0626
0.0290
0.0201
R1, wR2 (I > 2
R1, wR2 (all data)
GoF
s
(I))
0.0598, 0.1355
0.0971, 0.1560
1.002
0.0446, 0.1205
0.0555, 0.1264
1.002
0.0268, 0.0657
0.0337, 0.06823
1.002
To a solution of [NiCl2(PPh3)2] (1 g, 1.60 mmol) in THF (10 mL)
was
added
1
equivalent
of
2,6-dimethyl-4-
trimethylsiloxyphenylmagnesium bromide (3.2 mL of 0.5
M

S.-C. So et al. / Journal of Organometallic Chemistry 853 (2017) 1e4
3
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
3. Results and discussion
3.1. Reactions of [M(PPh3)4] with 4-bromo-2,6-di-tert-butylphenol
No reaction was found between [Pd(PPh3)4] and 4-bromo-2,6-
di-tert-butylphenol in toluene at room temperature. Refluxing
[Pd(PPh3)4] with 4-bromo-2,6-di-tert-butylphenol in toluene for
2 d led to isolation of yellow crystals that were identified as the
phosphaquinone Ph3P ¼ C6H3t Bu2-3,5-O-4 (1) (Scheme 3) by X-ray
crystallography. The formation of 1 presumably involves the
oxidative addition of 4-bromo-2,6-di-tert-butylphenol to Pd(0) and
subsequent P-C elimination of a Pd(II) hydroxyaryl intermediate.
The 1H NMR spectrum of 1 displayed a sharp singlet at
d
¼ 1.31 ppm
due to the tert-butyl group; no O-H signal was observed. The 31P
{1H} NMR spectrum showed a singlet at
d
¼ 20.74 ppm. The
observation of the CO signal at
trum together with the IR
d
¼ 176 ppm in the 13C NMR spec-
n
(C-O) band at 1578 cmꢁ1 is indicative of
Fig. 1. Molecular structure of 1. Hydrogen atoms are omitted for clarity. Thermal el-
lipsoids are drawn at 30% probability.
the C¼O double bond character in 1.
The molecular structure of 1 is shown in Fig. 1. Selected bond
lengths are listed in Scheme 4. The C-O bond length of 1.273(2) Å is
close to those in related quinone compounds, such as [C6Ht3Bu2-3,5-
O-4]2 [1.228 Å] [13]. The C-C bonds in the 6-member carbocyclic
ring displays a long-short-long pattern [1.415(4), 1.374(4), and
1.460(5) Å] that is characteristic of quinoidal compounds. The P-C
distance in 1 of 1.755(3) Å is similar to that of the P¼C bond in B
[1.750(2) Å] [3]. Taken together, the structural data in 1 are
consistent with the formulation of a phosphaquinone.
Scheme 4. Selected bond lengths [Å] in 1.
Unlike the Pd analogue, the reactions of [M(PPh3)4] (M ¼ Ni, Pt)
4-bromo-2,6-di-tert-butylphenol in refluxing toluene led to for-
mation of 3,30,5,50-tetra-tert-butyl-4,40-diphenoquinone (2)
(Scheme 3) that was characterized by NMR spectroscopy and
elemental analyses. The biquinone 2 was apparently formed by
oxidation of the biphenol precursor (3,5-tBu2-4-OHC6H2)2 that is
derived from metal-mediated homo-coupling of the 3,5-di-tert-
butyl-4-hydroxyphenyl groups. At this stage, it is not clear why the
reactions of 4-bromo-2,6-di-tert-butylphenol with the Ni and Pt
complexes led to C-C coupling, whereas P-C elimination occurred at
the Pd centre.
Et2O/hexanes afforded air-stable orange crystals characterized as
trans-[Ni(C6H2Me2-2,6-OSiMe3-4)Br(PPh3)2] (3) (Scheme 5), the
bromide ligand of which apparently came from the Grignard re-
agent. Attempts to prepare a Ni(II) diaryl complex by alkylation of
[Ni(PPh3)2Cl2] with excess (C6H2Me2-2,6-OSiMe3-4)MgBr failed.
The 1H NMR spectrum of 3 displayed a singlet at
d
¼ 0.17 ppm due
to the trimethylsilyl protons. The 31P {1H} NMR spectrum displayed
a singlet at
d
¼ 19.50 ppm, consistent with the trans arrangement of
the two phosphine ligands. Fig. 2 shows the molecular structure of
3. The geometry around Ni(II) is pseudo square planar with the two
PPh3 ligand trans to each other. The Ni-C distance of 1.907(3) Å is
typical for Ni(II) phenyl complexes.
3.2. Ni(II) hydroxyaryl complex
Treatment of 3 with nBu4NF,H2O in THF resulted in formation of
a yellow precipitate that was characterized as a dinuclear hydroxo-
bridged Ni(II) 4-hydroxyaryl complex [Ni(PPh3)(C6H2Me2-2,6-OH-
We next attempted to synthesize hydroxyaryl complexes by
transmetallation with Grignard reagents. Treatment of
[Ni(PPh3)2Cl2] with the trimethylsilyl-protected Grignard reagent
(C6H2Me2-2,6-OSiMe3-4)MgBr in THF resulted in an immediate
color change from maroon to orange. Recrystallization from CH2Cl2/
4)]2(m-OH)2 (4) (Scheme 5). Thus, the desilylation of 3 also resulted
in substitution of the bromide ligand by hydroxide (presumably
derived from nBu4NF,H2O) and elimination of the PPh3 ligand. The
structure of 4 that features a Ni2(OH)2 core is shown in Fig. 3. The
geometry around each Ni is pseudo square planar with the aryl
ligand trans to the
m-hydroxo group. The Ni-O bond lengths in the
OH
H
O
PPh3
Ph3P
ii
i
[NiCl2(PPh3)2]
Ni
Br
Ni
OSiMe3
Ni
PPh3
O
PPh3
H
3
HO
4
Scheme 5. Synthesis of the Ni(II) 4-hydroxyaryl complex 4. Reagents: (i) (C6H2Me2-
2,6-OSiMe3-4)MgBr, THF; (ii) nBu4NF,H2O, THF.
Scheme 3. Reactions of [M(PPh3)4] with 4-bromo-2,6-di-tert-butylphenol.

4
S.-C. So et al. / Journal of Organometallic Chemistry 853 (2017) 1e4
[av. 2.1407 Å] are slightly shorter than those in the latter. An
attempt has been made to deprotonate 4 with base. Unfortunately,
the treatment of 4 in THF with 1 equivalent of Et4NOH led to im-
mediate formation of a dark insoluble precipitate, indicating that
the complex has decomposed.
In summary, we found that reaction of [Pd(PPh3)4] with 4-
bromo-2,6-di-tert-butylphenol afforded
a phosphaqunione, A,
presumably via reductive elimination of a Pd(II) 4-hydroxyaryl in-
termediate, whereas a bi-quinone, B, was obtained for the Ni and Pt
analogues. A Ni(II) complex bearing a 4-hydroxyaryl ligand, 4, has
been prepared by alkylation of [NiCl2(PPh3)2] with (C6H2Me2-2,6-
OSiMe3-4)MgBr, followed by desilylation. The synthesis of 4-
hydroxyaryl complexes with more substitutionally inert transition
metals (e.g. Os) by this method and the study of their deprotonation
are underway.
Acknowledgment
The financial support from the Hong Kong Research Grants
Council (project no. 602913) and the Hong Kong University of Sci-
ence and Technology is gratefully acknowledged.
Fig. 2. Molecular structure of 3. Hydrogen atoms are omitted for clarity. Thermal el-
lipsoids are drawn at 30% probability. Selected bond lengths [Å]: Ni(1)-C(1) 1.907(3),
Ni(1)-P(1) 2.2255(8), Ni(1)-P(2) 2.2146(9), Ni(1)-Br(1) 2.3645(6).
Supporting information available
Crystallographic data for compounds 1, 3 and 4 have been
deposited with the Cambridge Crystallographic Data Centre, CCDC
No. 1539784, 1540324 and 1540325, respectively, in CIF format.
Copies of this information may be obtained free of charge from the
Director, CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK (fax: þ44-
1223-336033; e-mail: deposit@ccdc.cam.ac.uk).
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Fig. 3. Molecular structure of 4. Hydrogen atoms are omitted for clarity. Thermal el-
lipsoids are drawn at 30% probability. Selected bond lengths [Å] and angles [deg]:
Ni(1)-P(1) 2.1436(4), Ni(1)-O(1) 1.9491(10), Ni(1)-O(2), 1.8979(10), Ni(1)-C(1)
1.9012(15), Ni(2)-P(2) 2.1377(4), Ni(2)-O(1) 1.9164(10), Ni(2)-O(2) 1.9304(10), Ni(2)-
C(11) 1.8998(15); Ni(1)-O(1)-Ni(2) 92.91(4), Ni(1)-O(2)-Ni(2) 94.09(5).
[11] Bruker SMART and SAINTþ, Version 6.02a; Siemens Analytical X-ray In-
Ni2(OH)2 core are in the range of 1.8979(10)-1.9304(10) Å and the
O-Ni-O angles are 92.91(4) and 94.09(5)o. The Ni-C distance in 4 [av.
1.901 Å] is almost the same as that in 3, whereas the Ni-P distances
struments Inc., Madison, Wisconsin, USA, 1998.
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