Electron-deficient linear tetranuclear ruthenium(I) carboxylate chains

Article abstract of DOI:10.1016/j.jorganchem.2015.11.005

The reaction of carboxylic acids RCOOH (R = ^tBu, Ph, Fc) with [Ru₃(CO)₁₂] in refluxing acetonitrile afforded dark purple/red solids. These reacted with phosphines in a 2:1 (Ru:P) ratio to form phosphine-substitued derivati

Full text of DOI:10.1016/j.jorganchem.2015.11.005

Journal of Organometallic Chemistry 802 (2016) 15e20
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Electron-deficient linear tetranuclear ruthenium(I) carboxylate chains
Bo Yang Chor, Rakesh Ganguly, Weng Kee Leong*
Division of Chemistry & Biological Chemistry, Nanyang Technological University, 21 Nanyang Link, 637371, Singapore
a r t i c l e i n f o
a b s t r a c t
The reaction of carboxylic acids RCOOH (R ¼ tBu, Ph, Fc) with [Ru3(CO)12] in refluxing acetonitrile
Article history:
Received 3 October 2015
Received in revised form
4 November 2015
Accepted 5 November 2015
Available online 17 November 2015
afforded dark purple/red solids. These reacted with phosphines in a 2:1 (Ru:P) ratio to form phosphine-
substitued derivatives of the electron-deficient tetraruthenium chains, viz., [Ru4(CO)8(
(where R0 ¼ Ph or OMe).
m
-OOCR)4(PR03)2]
© 2015 Elsevier B.V. All rights reserved.
Keywords:
Ruthenium
Carbonyl
Carboxylate
Chain
Electron-deficient
1. Introduction
acid in place of the phosphines have also been isolated from some
of the reactions of [Ru3(CO)12] with carboxylic acids [5e7,9]. Only
two examples of the “dimer of dimers” containing an unsupported
RueRu bond have been reported, however, that with R ¼ C6H3-3,5-
(CF3)2 and CO ligands in the axial positions reported by the group of
Petrukhina [6], and that with R ¼ 1-adamantyl (Ad) and PPh3 li-
gands in the axial positions reported by us [3]. These two tetrar-
uthenium complexes are interesting because they are electron-
deficient, with a total valence electron count of 64 compared to
the 66 required by the EAN rule. We now believe that these “dimer
of dimers” containing an unsupported RueRu bond may not be at
all uncommon; our findings on this point are reported here.
The reaction of [Ru3(CO)12] (1) with carboxylic acids (2) have
been reported to give polymeric, tetranuclear or dinuclear species
made up of diruthenium [Ru2(CO)4(
the identity of the acid and the reaction conditions used [1e16].
Polymeric complexes [{Ru2(CO)4( -OOCR)2}∞] were first reported
by Lewis et al. [12]. These comprised dinuclear [Ru2(CO)4(
OOCR)2] sawhorse units linked together by two RueO bonds, be-
tween an Ru atom of one unit and an O atom of the carboxylato
bridge of the other unit [5,7,8]. They dissolved reversibly in coor-
dinating solvents (S), such as, acetonitrile, tetrahydrofuran or pyr-
idine, to form dinuclear complexes. These were proposed to have
m-OOCR)2] units, depending on
m
m-
the formulation [Ru2(CO)4(
actions with phosphines (PR03) to form the dinuclear derivatives
[Ru2(CO)4(
-OOCR)2(PR03)2] [4,12]; a single crystal X-ray structural
determination on [Ru2(CO)4( -OOCAd)2(PPh3)2] confirmed it to
comprise a sawhorse diruthenium unit, with the two PPh3 ligands
occupying the axial positions [3].
m-OOCR)2(S)2], on the basis of their re-
2. Results and discussion
m
We have examined the products of the reaction of 1 with four
different carboxylic acids 2, and their reactivity; some of the
m
t
chemistry for R ¼ Ad (a) and Bu (b) have already been reported
[3,2]. Our results are summarised in Scheme 1.
Interestingly, reaction of the polymer with a phosphine in a 2:1
(Ru:P) ratio afforded the tetraruthenium species [Ru4(CO)8(m-
OOCR)4(PR3)2]. These can be regarded as “dimer of dimers” with
two dinuclear units linked via either RueO interactions, or an un-
supported RueRu bond (Fig. 1) [10,11]. Analogues with a carboxylic
The reaction of 1 and 2 in refluxing acetonitrile gave yellow
solutions which, upon the removal of volatiles, gave either dark
purple (3a and 3b) or dark red (3c and 3d) solids. These solids
dissolved in various solvents to give orange-yellow solutions but
reverted to their original colours upon removal of the solvent.
Further characterisation of 3c and 3d other than by IR spectroscopy
was hampered by their tendency to form an orange solid. For
example, 3c gradually turned orange when left in air and more
readily so when heated (80 ꢀC) under vacuum. The IR
* Corresponding author.
E-mail address: chmlwk@ntu.edu.sg (W.K. Leong).
http://dx.doi.org/10.1016/j.jorganchem.2015.11.005
0022-328X/© 2015 Elsevier B.V. All rights reserved.

16
B.Y. Chor et al. / Journal of Organometallic Chemistry 802 (2016) 15e20
Fig. 1. RueRu vs RueO interactions found in tetraruthenium chain complexes [1,3,5e7,9e11,13e16].
Scheme 1.
characteristics of the orange solid 4c are similar to that for the
previously reported benzoate polymer [{Ru2(CO)4( -OOCPh)2}∞]
depending on the synthetic methodology and the work-up
procedure.
m
and hence, we believe that it is also polymeric [9]. A dichloro-
methane solution of 3d similarly afforded an orange precipitate 4d
on standing. Although the IR spectrum is identical to that for dark
red 3d, we believe that 4d is also a polymer. In contrast, both 3a and
3b remained purple upon heating in vacuum and showed no pre-
cipitation when left standing in a dichloromethane solution. Their
mass spectra shows ions corresponding to both diruthenium and
tetraruthenium fragments but not higher nuclearities such as a
hexaruthenium.
That 3a and 3b are more stable in the tetranuclear form may
possibly be due to steric crowding; it has been reported that piv-
alate complexes resist polymerization because of steric crowding at
the
a position of the acid [9]. A longer RueRu chain is also pre-
sumed to be less favoured as that will require even more electron-
deficiency. Interestingly, for R ¼ C6H3-3,5-(CF3)2, it has been re-
ported that both a tetranuclear species containing an unsupported
RueRu bond, and a polymeric species containing RueO in-
teractions, could be formed [6]. Taken together, we believe that
these suggest that polymeric 4 containing RueO linkages is the
more favoured form but a sterically bulky R group can disfavor this.
The reaction of polymeric 4 with phosphines has been described
as a depolymerization reaction, in which the diruthenium species
was first formed, whatever the Ru:P ratio [10,11]. Irrespective of the
true nature of 3 and 4, we have observed that both undergo a
similar depolymerization reaction with PPh3, except that when the
It had been assumed earlier that the complexes 3a and 3b were
polymeric [3,2]. The 1H NMR spectrum of 3b, however, exhibits a
resonance at 1.25 ppm, which can be assigned to a bound pivalic
acid similar to that reported for the diruthenium species
[Ru2(CO)4(m
-OOCtBu)2(tBuCOOH)2] [9]; weak IR bands at
~1600e1700 cmꢁ1 can also be attributed to the carbonyl stretches
of the bound pivalic acid group. Complex 3b thus probably has the
formulation [Ru4(CO)8(
m
-OOCtBu)4(tBuCOOH)2]. The mass spec-
Ru:P ratio was 2:1, the linear tetranuclear chains [Ru4(CO)8(
OOCR)4(PPh3)2] (5) were obtained as dark blue/violet solids beside
the diruthenium species [Ru2(CO)4(
-OOCtBu)2(PPh3)2] (6). With
an excess of the phosphine, only 6 was formed. The P(OMe)3
analogue [Ru4(CO)8(
-OOCtBu)4{P(OMe)3}2] (5b′) has also been
m-
trum of an acetonitrile solution, on the other hand, shows frag-
ments corresponding to substitution of the acid ligands by
acetonitrile. We therefore believe that all the solids 3 are tetrar-
m
uthenium species with the formulation [Ru4(CO)8(m-OOCR)4(S)2];
m
the intense purple/red colours are due to the presence of an un-
supported, electron-deficient RueRu bond, and they are weakly
coordinated in the axial positions by a solvent molecule (like
acetonitrile or dichloromethane [17]), or a carboxylic acid. They are
probably mixtures of solvent- and acid-coordinated species,
similarly synthesised from 3b. We have observed (by spot TLC) that
6 was first formed, and over a longer reaction time, the tetrar-
uthenium species 5 resulted, with a concomitant decrease in the
amount of 6.
The electronic spectra of solid 5b-d all showed a strong

B.Y. Chor et al. / Journal of Organometallic Chemistry 802 (2016) 15e20
17
absorption maximum at 540e590 nm. As in the complexes 3,
dissolution in a number of organic solvents gave orange/yellow
solutions which reverted to their original colour upon solvent
removal. This was reflected in the electronic spectrum of 5d, for
example, in which peaks at 338 nm and 590 nm were replaced by a
new peak at 353 nm in dichloromethane. We attribute this to
deficient, linear tetranuclear chains [Ru4(CO)8(
This thus represents a general synthetic strategy to this unusual
class of chain complexes.
m
-OOCR)4(PPh3)2].
4. Experimental
reversible formation of a dinuclear solvent adduct [Ru2(CO)4(m-
4.1. General procedure
OOCR)2(PPh3) (S)]. This is in agreement with our previous
computational studies in which we have shown that the LUMO in
complex 5a was located primarily at the unsupported RueRu bond
and hence this bond was susceptible to nucleophilic attack [3]. As
expected, the reaction of 5b with P(OMe)3 afforded the
All reactions were performed under an argon atmosphere using
standard Schlenk techniques. All reagents were from commercial
sources and used as supplied without further purification. Reaction
mixtures were separated by preparative thin-layer chromatog-
raphy (TLC) on 20 cm ꢂ 20 cm plates precoated with silica gel
K60F254, purchased from Merck. Infrared spectra were recorded on
a Bruker Alpha FT-IR spectrometer. ESI spectra were recorded on a
Waters UPLC-Q-TOF mass spectrometer. 1H and 31P{1H} NMR
spectra were recorded on a Bruker BBFO400 spectrometer. 1H
chemical shifts were referenced to the residual proton resonances
of the respective deuterated solvents, and 31P chemical shifts were
referenced to 85% H3PO4 as an external standard. Elemental ana-
lyses were performed by the microanalytical laboratory at the
Nanyang Technological University. Uvevis spectra were recorded
on a Varian Cary 100 spectrometer. The syntheses and character-
ization data for the pivalate analogue is presented here, while those
for the benzoate and ferrocenecarboxylate analogues are available
in the supporting information.
asymmetrically-substituted dinuclear species [Ru2(CO)4(m
-OOCt-
Bu)2(PPh3) (P(OMe)3)] (7), which was found to exist in equilibrium
with the symmetrically-substituted homologues 6b and 6b′
(Scheme 2); the equilibrium was established within a day, and the
equilibrium constant has been estimated (on the basis of NMR
integration ratios) to be ~4.6. This observation is also consistent
with ligands in the axial positions being significantly less tightly
bound than those in radial positions.
Crystallographic discussion
All the chains 5 have been characterised by single crystal X-ray
crystallographic studies; that for 5a has been reported earlier [3].
The crystal structure of 6b has also been determined. The ORTEP
plot of 5b illustrating its molecular structure is given in Fig. 2, while
that for 6b, together with selected bond parameters, is given in
Fig. 3. Two different isomorphs were obtained for 5b′; red mono-
clinic crystals and red/green dichroic orthorhombic crystals.
Essential bond parameters for the tetraruthenium chains 5,
together with a common atomic numbering scheme, are tabulated
in Table 1. The molecules of 5b, 5b′(monoclinic), 5c and 5d possess
crystallographic inversion symmetry about the unsupported
Ru2eRu20 bond. As has already been observed for 5a, the central
RueRu bond is significantly longer than the outer RueRu bonds,
consistent with bond orders of 1.0 and 1.5, respectively. There is
also apparent asymmetry in the RueCO bond lengths; > 1.85 Å for
Ru1eCO and <1.85 Å for Ru2eCO. Interestingly, the nCO pattern of
the IR spectrum of crystalline 5c is different from that of the
powdered form obtained from, say, initial synthesis. Both gave the
same solution-phase IR spectrum, however (Figs. S9 and S10).
Presumably, several conformers are present in the non-crystalline
form, which may account for the broadening and extra peaks
observed in the spectrum. A similar broadening was also observed
in the spectrum of 5d but not that of 5b.
4.2. Reaction of 1 and 2b
Cluster 1 (210.9 mg, 0.330 mmol) and 2b (202.7 mg,1.984 mmol)
were refluxed in CH3CN (50 mL) for 18 h under an argon atmo-
sphere. After removal of the solvent, the residue was taken up in
DCM and the resulting orange solution filtered through silica gel.
The filtrate was evaporated to dryness to afford 3b as a dark purple
solid.
Yield ¼ 222.5 mg (87%).
IR (DCM): nCO 2042 (vs),1990 (m), 1960 (vs),1924 (w); nCOO 1742
(vw), 1670 (w), 1606 (vw), 1558 (m), 1485 (w), 1422 (w) cmꢁ1. IR
(KBr): nCO 2034 (vs), 1999 (m), 1979 (vs), 1947 (s); nCOO 1547 (m),
1484 (m), 1421 (m) cmꢁ1. 1H NMR (CDCl3):
d
1.14 (s, tBuCOO, 36H),
d
1.25 (s, tBuCOOH, 18H) ppm.
ESI-MSþ
m/z
(MeCN):
1015.80
[Ru4(CO)8(m-
OOCC4H9)3þ2MeCN]þ, 972.78 [Ru4(CO)8(
m
-OOCC4H9)3þMeCN]þ,
933.76 [Ru4(CO)8(
m
-OOCC4H9)3]þ, 497.90 [Ru2(CO)4(
m
-OOCC4H9)þ
2MeCN]þ, 456.87 [Ru2(CO)4(OOCC4H9)þMeCN]þ. Elemental Anal-
ysis (%) calcd for C38H56O20Ru4.CH2Cl2: C 35.43, H 4.42; found: C
35.36, H 4.71.
3. Concluding remarks
In this work, we have shown that the reaction of [Ru3(CO)12
with various carboxylic acids appear to afford initially either tet-
raruthenium [Ru4(CO)8( -OOCR)4(S)2] or polymeric [{Ru2(CO)4(
]
4.3. Synthesis of [Ru2(CO)4(m
-OOCtBu)2(PPh3)2], 6b
m
m-
OOCR)2}∞] complexes, depending on the nature of the acid. Irre-
spective of the exact nature of the product, however, they reacted
with phosphines in a 2:1 (Ru:P) ratio to afford electronically
Complex 3b (15.9 mg, 0.0307 mmol) and PPh3 (18.6 mg,
0.0709 mmol) were stirred in DCM for 15 min. Separation of the
crude product by TLC, with DCM: Hex ¼ 1:2 as eluent, gave a yellow
Scheme 2.

18
B.Y. Chor et al. / Journal of Organometallic Chemistry 802 (2016) 15e20
Fig. 2. ORTEP plot of Ru4(CO)8(
m
-OOCtBu)4(PPh3)2 (5b). Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms have been omitted for clarity.
Fig. 3. ORTEP plot of Ru2(CO)4(m
-OOCtBu)2(PPh3)2 (6b). Thermal ellipsoids are drawn at the 50% probability level, and hydrogen atoms have been omitted for clarity. Selected bond
lengths (Å) and angles (ꢀ): Ru(1)-Ru(2) ¼ 2.6978(2), Ru(2)-P(2) ¼ 2.4373(5), Ru(1)-P(1) ¼ 2.4284(5), Ru(1)-O(5) ¼ 2.1157(13), Ru(1)-O(7) ¼ 2.1295(13), Ru(2)-O(6) ¼ 2.1532(13),
Ru(2)-O(8) ¼ 2.1286(13); Ru(1)-Ru(2)-P(2) ¼ 169.475(13), Ru(2)-Ru(1)-P(1) ¼ 167.572(13), Ru(2)-Ru(1)-O(5) ¼ 83.09(4), Ru(2)-Ru(1)-O(7) ¼ 81.51(4), Ru(1)-Ru(2)-O(6) ¼ 82.10(4),
Ru(1)-Ru(2)-O(8) ¼ 83.72(4), O(7)-Ru(1)-O(5) ¼ 84.38(5), O(6)-Ru(2)-O(8) ¼ 83.77(5).
solid upon removal of solvent. NMR and IR spectral values were
comparable with the literature [18].
IR (DCM): nCO 2036 (vs), 1985 (m), 1963 (s), 1917 (w) cmꢁ1. IR
(KBr): nCO 2028 (vs), 1988 (m), 1967 (s), 1930 (w) cmꢁ1 1H NMR
(C6D6):
1.13 (s, tBu, 36H), 7.02e7.05 (m, Ph,18H), 7.63e7.68 (m, Ph,
12H) ppm. 31P{1H} NMR (C6D6): 8.6 ppm. ESI-MSþ m/z: 1558.00
Mþ, [M-tBuCOO]þ,
1455.95 594.96 [M-Ru2(CO)4(
OOCtBu)2(PPh3)-tBuCOO]þ. Elemental Analysis (%) calcd for
64H66O16P2Ru4: C 49.36, H 4.27; found: C 49.60, H 4.26. UVevis
(300e900 nm, solid, lmax): 586, 350 nm.
.
Yield ¼ 21.8 mg (68%).
d
IR (DCM): nCO 2022 (vs), 1978 (m), 1949 (s), 1918 (w) cmꢁ1. 1H
d
NMR (CDCl3):
d
0.79 (s, tBu, 18H), 7.37e7.63 (m, Ph, 30H) ppm. 31
P
m-
{1H} NMR (CDCl3):
d
13.6 ppm. 31P{1H} NMR (C6D6):
d 13.4 ppm.
ESI-MSþ m/z: 1041.09 Mþ, 1064.47 [MþNa]þ, 1013.10 [MꢁCO]þ.
Elemental Analysis (%) calcd for C50H48O8P2Ru2: C 57.69, H 4.65;
found: C 57.93, H 4.95.
C
4.5. Synthesis of [Ru2(CO)4(
m
-OOCtBu)2{P(OMe)3}2], 6b0
4.4. Synthesis of [Ru4(CO)8(m
-OOCtBu)4(PPh3)2], 5b
Complex 3b (12.3 mg, 0.0238 mmol) and P(OMe)3 (5.6
mL,
0.0476 mmol) were stirred in DCM for 4 h. Separation of the crude
product by TLC, with DCM: Hex ¼ 3:1 as eluent, gave a yellow solid
upon removal of solvent.
Complex 3b (17.4 mg, 0.0474 mmol) and PPh3 (4.4 mg,
0.0237 mmol) [Ratio of Ru: PPh3 ¼ 2:1] was stirred in DCM for 18 h.
After removal of the solvent, the residue was separated by TLC with
DCM as eluent. Two products were isolated; a yellow solid identi-
Yield ¼ 6.9 mg (38%).
fied as [Ru2(CO)4(
m
-OOCtBu)2(PPh3)2] (6b, 6.2 mg, 13%) and a dark
violet solid identified as[Ru4(CO)8(
m
-OOCtBu)4(PPh3)2] (5b,
4.6. Synthesis of [Ru4(CO)8(m
-OOCtBu)4{P(OMe)3}2], 5b0
23.6 mg, 64%). The dark violet solid was crystallized by slow
evaporation of a diethyl ether solution at 4 ꢀC, which gave dichroic
red/green crystals of diffraction quality.
Complex 3b (33.8 mg, 0.0654 mmol) and P(OMe)3 (3.75
0.0318 mmol) [Ratio of Ru:P ¼ 2:1] were stirred in DCM for 16 h.
mL,

B.Y. Chor et al. / Journal of Organometallic Chemistry 802 (2016) 15e20
19
Table 1
Bond lengths (Å) and angles (ꢀ) in 5.
O
C
O
C
O
O
O
O
C
C
O
O
O
C
Ru1'
O
P’
Ru2'
O
Ru2
P
Ru1
O
C
C
O
C
O
O
O
Bond parameter
5a [3]
5b
5b0 (monoclinic)
2.6761(3)
5b0 (orthorhombic)a
5c
5d
Ru1eRu2
2.6851(5)
2.6796(2)
2.6756(3)
2.6724(5)
2.6985(4)
2.6732(3)
2.8954(3)
2.2967(8)
Ru2eRu20
RueP
2.8754(7)
2.3945(9)
2.8663(4)
2.3967(6)
2.8721(5)
2.3050(9)
2.8874(7)
2.3781(13)
2.8549(5)
2.3915(9)
2.3012(8)
Ru1eO
2.110(3);
2.138(3)
2.076(3);
2.089(3)
1.851(4);
1.855(4)
1.821(5);
1.838(4)
171.44(3)
2.1158(16);
2.1288(15)
2.0910(16);
2.0939(17)
1.855(2);
1.857(2)
1.837(3);
1.839(3)
173.893(15)
2.123(3);
2.134(3)
2.101(3);
2.107(2)
1.864(4);
1.856(4)
1.818(4);
1.845(4)
168.63(3)
2.114(2); 2.125(2)
2.123(2); 2.137(2)
2.105(2); 2.116(2)
2.105(2); 2.110(2)
1.857(3); 1.868(3)
1.862(3); 1.858(3)
1.833(3); 1.845(3)
1.841(3); 1.845(3)
167.74(2)
2.120(3);
2.129(3)
2.103(3);
2.134(3)
1.855(5);
1.863(5)
1.830(5);
1.838(5)
166.77(3)
2.115(2);
2.136(2)
2.066(2);
2.101(2)
1.854(4);
1.859(3)
1.835(4);
1.838(4)
169.20(2)
Ru2eO
Ru1eCO
Ru2eCO
PeRu1eRu2
Ru1eRu2eRu20
169.60(2)
165.570(12)
162.510(18)
171.917(12)
164.538(18)
165.10(2)
162.620(16)
162.888(12)
a
Molecule has no crystallographic centre of symmetry; second set of values is for the right half of the molecule.
Separation of the crude product by TLC with DCM as eluent gave a
pink solid. Crystals of X-ray diffraction quality were grown by slow
evaporation of a diethyl ether solution at 4 ꢀC.
was processed and corrected for Lorentz and polarisation effects
with SAINT [20], and for absorption effects with SADABS [21].
Structural solution and refinement were carried out with the
SHELXTL suite of programs [22]. There was a disorder of one of the
pivalate group for the orthorhombic crystal of 5b′. This was
modelled with two alternative positions, and appropriate distance
and thermal parameter restraints were applied. Crystal and
refinement data are summarized in Table S2.
Yield ¼ 21.8 mg (54%).
IR (KBr): nCO 2038 (vs), 1982 (s), 1927 (m) cmꢁ1. IR (DCM): nCO
2048 (vs), 1986 (s), 1977 (s), 1917 (w) cmꢁ1. 1H NMR (C6D6):
d 3.41
t
(d, OMe, 18H, J ¼ 11.6 Hz), 1.18 (s, Bu, 36H) ppm. 31P{1H} NMR
(C6D6):
d
108.5 ppm. ESI-MSþ m/z: 1281.88 Mþ, 1179.81
[M-tBuCOO]þ. Elemental Analysis (%) calcd for C34H54O22P2Ru4: C
31.88, H 4.25; found: C 31.75, H 4.10. UVevis (300e900 nm, solid,
lmax): 548, 324 nm.
Acknowledgements
This work was supported by Nanyang Technological University
and the Ministry of Education (Research Grant No. M4011158).
4.7. Synthesis of [Ru2(CO)4(m
-OOCtBu)2(PPh3){P(OMe)3}], 7
Complex 5d (5.7 mg, 0.00421 mmol) was added into 4 mL of
C6D6 in an NMR tube. P(OMe)3 (1.04 mg, 0.00841 mmol) was added
and the orange-red solution immediately turned yellow. The NMR
tube was left to stir for 10 min and analysis showed the formation of
7. However, 7 was not isolated as it was found to equilibrate in
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2015.11.005.
solution with its symmetric homologues [Ru2(CO)4(
m
-OOCt-
References
Bu)2(PPh3)2] (6a) and [Ru2(CO)4(
m
-OOCtBu)2{P(OMe)3}2] (6a′). To
determine the equilibrium constant, 6a (2.9 mg, 0.00274 mmol)
and 6a′ (2.1 mg, 0.00274 mmol) were dissolved in C6D6 and added
into an NMR tube and left to stir at rt in a water bath. The reaction
was monitored by NMR spectroscopy. NMR analysis showed that 7
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t
1H NMR (C6D6):
d
1.07 (s, Bu, 18H), 3.56 (d, OMe, J ¼ 11.6 Hz),
7.03e7.76 (aromatic) ppm. 31P{1H} NMR (C6D6):
d 128.3 (d,
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