Ullmann-Type and Related Redox Reactions of Nitrosyl Molybdenum Complexes Bearing a Large-Bite-Angle Diphosphine

Article abstract of DOI:10.1002/ejic.201501062

The reactions of ArX (X = Cl and Br) with [Mo(NO)(P?P)(NCMe)₃][BAr^F₄] P?P = 2,2′-bis(diphenylphosphanyl)diphenyl ether (DPEphos), BAr^F₄ = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate at 120 C resulted in the formation of biphenyl (through C_Ar-C_Ar reductive homocoupling) and the dinuclear halide salts [Mo₂(NO)₂(P?P)₂(NCMe)₂(μ-X)₂][BAr^F₄]₂ (X = Cl, 1; Br, 2). Complexes 1 and 2 potentially show cisoid (1c and 2c) and transoid (1t and 2t) regioisomerism with respect to the position of the NO ligand. The crystal-structure determinations of 1 and 2 revealed the presence of the transoid isomers 1t and 2t and M^I-M^I bonding in both cases. A proposed mechanism for the formation of 1t and 2t involves reductive C_Ar-C_Ar coupling to form biphenyl from two Ph-Mo^II centers. In addition, the complexes Mo(NO)(P?P)(CO)₂Cl (3) and [Mo(NO)(mer-κ³-P,P,O-DPEphos)Cl(PR₃)] (R = Me, 4; Ph, 5) were obtained through the reductions of [Mo₂(NO)₂(P?P)₂Cl₄(μ-Cl)₂]. The mild oxidation of [Mo⁰(NO)(P?P)(NCMe)₃][BAr^F₄] P?P = 2,2′-bis(diphenylphosphanyl)diphenyl ether, BAr^F₄ = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate with aryl halides affords dinuclear [Mo^I₂(NO)₂(P?P)₂(NCMe)₂(μ-X)₂][BAr^F₄]₂ species and biphenyl through an Ullmann-type dinuclear homocoupling process. The complexes are characterized spectroscopically and by X-ray diffraction studies.

Full text of DOI:10.1002/ejic.201501062

DOI: 10.1002/ejic.201501062
Full Paper
Nitrosyl Mo Complexes
Ullmann-Type and Related Redox Reactions of Nitrosyl
Molybdenum Complexes Bearing a Large-Bite-Angle
Diphosphine
[a]
[a]
[a]
[a]
Subrata Chakraborty, Rajesh Kunjanpillai, Olivier Blacque, and Heinz Berke*
Abstract: The reactions of ArX (X
=
Cl and Br) with to the position of the NO ligand. The crystal-structure determi-
F
[
Mo(NO)(P∩P)(NCMe) ][BAr ] P∩P = 2,2′-bis(diphenylphos- nations of 1 and 2 revealed the presence of the transoid isomers
3
4
F
I
I
phanyl)diphenyl ether (DPEphos), BAr 4 = tetrakis[3,5-bis(tri- 1t and 2t and M –M bonding in both cases. A proposed mecha-
fluoromethyl)phenyl]borate at 120 °C resulted in the formation nism for the formation of 1t and 2t involves reductive C –C
Ar Ar
II
of biphenyl (through C –C reductive homocoupling) and the coupling to form biphenyl from two Ph–Mo centers. In addi-
tion, the complexes Mo(NO)(P∩P)(CO) Cl (3) and [Mo(NO)(mer-
X = Cl, 1; Br, 2). Complexes 1 and 2 potentially show cisoid (1c κ -P,P,O-DPEphos)Cl(PR )] (R = Me, 4; Ph, 5) were obtained
Ar Ar
F
dinuclear halide salts [Mo (NO) (P∩P) (NCMe) (μ-X) ][BAr ]
2
2
2
2
2
4 2
2
3
(
3
and 2c) and transoid (1t and 2t) regioisomerism with respect through the reductions of [Mo (NO) (P∩P) Cl (μ-Cl) ].
2
2
2
4
2
Introduction
Organometallic complexes of group VI metals show an exten-
II
I
0
sive redox chemistry covering the M , M , and M oxidation
states; however, quite strong reducing agents are required to
II
0
go from M to M species. Mild reducing agents are expected
I
Scheme 1. Cu-mediated Ullmann coupling of aryl halide.
to stop the reduction process at the stage of the M species, as
0
are mild oxidizing agents when coming from M complexes. In
this study, it was found that aryl halides behave as mild oxid-
izing agents towards [Mo(NO)(P∩P)(NCMe)3] cations [P∩P =
ment of efficient Cu-mediated Ullmann-type reactions with vari-
+
[10,11]
ous ligands as additives.
onstrated Ullmann-type coupling reactions,
intermolecular chiral synthesis of biaryls.
Lately, several groups have dem-
2,2′-bis(diphenylphosphanyl)diphenyl ether (DPEphos)] and
[12–14]
including the
provoke C –C homocoupling to form the biaryl structural
[15]
Ar Ar
motif, which is the core of a wide range of functional molecules,
natural products, commercial dyes, and the backbones of vari-
In conjunction with Cu-mediated Ullmann-type coupling re-
actions, a modern prerequisite of metal-promoted organic
chemistry is the exploration of the cheap, low toxicity, and envi-
ronmentally benign middle transition elements Fe, Mo, and W
[
1]
ous ligands used for asymmetric catalysis. The first C –C
Ar Ar
bond formation was reported in 1901 by Ullmann and was
achieved through the reductive homocoupling of aryl halides
by employing stoichiometric amounts of finely divided copper
at high temperature (above 200 °C) to form biaryls and copper
halides (Scheme 1).[ Since then, the application of the Ullmann
reaction has become of paramount importance and has re-
vealed quite general applicability in the synthesis of many sym-
as surrogates for precious metal catalysts in homogeneous ca-
[16]
talysis.
Lately, our group has developed several Mo- and W-
based highly efficient hydrogenation catalysts, which showed
that complexes of middle transition elements can also take over
2]
functions that are normally attributed solely to platinum-group
[17]
metal centers.
In this regard, the exploitation of a suitable
[
3]
metric and unsymmetric biaryls and polyaryls.
group VI complex in Ullmann-type C –C coupling reactions
Ar Ar
Nevertheless, the harsh reaction conditions, stoichiometric
amount of the Cu reagent, and longer reaction times typically
required for Ullmann coupling have motivated the search for
would also be highly desirable. Furthermore, from a mechanis-
tic point of view, the observation of C –C bond formations
Ar Ar
[18]
through dinuclear reductive elimination are quite rare.
In
[
4,5]
milder variations,
and Pd catalyzed C–C cross-coupling reac-
2001, Anderson and co-workers demonstrated the reductive
tions have become a viable synthetic route to such biaryl
elimination of 1,2-diphenylethane from the “A-frame” dinuclear
[
6–9]
cores.
However, Pd systems suffer from toxicity and cost is-
–
–
[
Pd (CH Ph) (μ-Cl)(μ-dppm) ]X [X = Cl , PF ; dppm = 1,1-bis(di-
2
2
2
2
6
sues, which prompted researchers to refocus on the develop-
[19]
phenylphosphanyl)methane] system.
The elimination of di-
phenylethane was envisaged to occur through mononuclear re-
ductive elimination, in which the eliminated organic groups
must first become bonded to the same palladium center. Bera
and co-workers demonstrated Suzuki cross-coupling reactions
[
a] Department of Chemistry, University of Zürich,
Winterthurerstrasse 190, 8057 Zürich, Switzerland
E-mail: hberke@chem.uzh.ch
http://www.chem.uzh.ch/research/emeriti/berke/curriculumVitae.html
Eur. J. Inorg. Chem. 2016, 103–110
103
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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of aryl halides catalyzed by a dipalladium(I) system [Pd L ][BF ]
4 2
signals in the 31P1H NMR spectrum could be grouped into pairs
2
2
[
L
=
[(5,7-dimethyl-1,8-naphthyridin-2-yl)-amino]carbonyl- on the basis of their coupling constants [1t: δ = 37.8 and
[
20]
2
2
ferrocene]. One of the proposed catalytic steps was the dinu- 27.1 ppm ( J = 108.7 Hz); 1c: δ = 34.2 and 29.9 ppm ( J
clear reductive elimination of C–C bonds from two Pd centers 109.3 Hz)], which indicated the inequivalence of the two phos-
to give coupled Pd centers. However, solid mechanistic evi- phorus atoms of one bidentate ligand in both isomers. This is
=
P, P
P, P
II
I
dence for the proposed catalytic step could not be provided. presumably because of the “twisted” conformation of the rigid
Johnson and co-worker synthesized tetraphenylene from bi- large-bite-angle DPEphos ligand, which also induces asymme-
I
I
phenylene mediated by a dinuclear Ni –Ni species, and the C– try in the chlorido bridges. Crystallization from a chloroben-
C bond formation occurred in a dinuclear fashion, as supported zene/pentane mixture at room temperature afforded single
[
21]
by a deuterium labeling study and crossover experiments,
crystals of 1t suitable for an X-ray diffraction study, which re-
and Jones and co-workers demonstrated mono- vealed that the cationic dinuclear unit of 1t consists of two
a
nuclear mechanism for the catalytic conversion of biphenylene [Mo(NO)(P∩P)(NCMe)] units with the NO ligands of the two
to tetraphenylene through reductive C–C elimination by apply- complex units in a transoid arrangement and held together by
[
22]
ing Pt and Pd metal centers.
the two μ-Cl bridges (Figure 1). The twisted conformation of
we reported the preparation of the DPEphos ligand forces one phenyl ring of the backbone to
[
23]
In a previous publication,
F
[
Mo(NO)(P∩P)(NCMe) ][BAr ] BArF
=
tetrakis[3,5-bis(tri- be located more axially cisoid to the acetonitrile ligand, and the
3
4
4
fluoromethyl)phenyl]borate, which could be employed as an ef- other phenyl ligand is arranged more equatorially between the
fective catalyst for hydrosilylation reactions of various alde- MeCN and NO ligands to generate helical twists of the rigid
hydes, ketones, and imines. Herein, we demonstrate the unique DPEphos backbones with opposite helicities at both Mo cen-
–
use of this complex for an Ullmann-type two-centered reduc- ters. The bridging chlorido ligands support the 18e configura-
[
24]
tive homocoupling process of aryl halides (X = Cl and Br) to tion of the M–M fragment
form biphenyl and dinuclear Mo –Mo bonded species.
and the overall diamagnetism.
I
I
Complex 1t crystallizes in the triclinic P 1¯ space group. The
asymmetric unit of the crystals contains the cationic part of
t, the [BAr ] counteranions, and pentane and chlorobenzene
F
–
1
4
Results and Discussion
solvate molecules. The average Mo–P bond length is 2.6188 Å,
and the bridging Mo–Cl distances are 2.409(17) and
2.4106(16) Å. The Mo1–Mo1 bond length of 3.0162(10) Å lies in
Preparation of Dinuclear [Mo (NO) (P∩P) (NCMe)
2
2
2
2
F
[25]
(
μ-X) ][BAr ] Complexes
the range expected for bridge-supported Mo–Mo contacts.
2
4 2
The formation of the cisoid isomer 1c with the nitrosyl ligands
F
The treatment of [Mo(NO)(P∩P)(NCMe) ][BAr ] with chloro-
3
4
of the dinuclear species on the same side was also evident from
benzene at 120 °C for 30 min resulted in the formation of an
isomeric mixture of the dinuclear [Mo (NO) (P∩P) (NCMe) (μ-
Cl) ][BAr ] (1c and 1t) complexes in 92 % yield (Scheme 2).
3
1
1
1
analysis of the P H COSY and 1D NOE spectra. In the H NMR
2
2
2
2
spectrum, the methyl protons of the attached CH CN ligand
F
3
2
4 2
appeared at δ = 1.9 and 1.8 ppm in a 1:1 intensity ratio (as-
signed to 1t and 1c, respectively). The composition of the iso-
meric mixture of 1c and 1t was further confirmed by elemental
Scheme 2. Reaction scheme for the formation of biphenyl and dinuclear
molybdenum complexes of type 1 and 2.
Figure 1. Molecular structure of the cationic part of 1t. Thermal ellipsoids are
drawn at the 50 % probability level. All hydrogen atoms, phenyl rings, pent-
The 31P1H NMR spectrum of 1 showed four doublet signals
at δ = 37.8 (1t), 34.2 (1c), 29.9 (1c) and 27.1 ppm (1t); this
supports the formation of the dinuclear isomers 1c and 1t (1:1
ratio, as revealed by the 31P1H NMR spectra), which differ in the
relative position of the nitrosyl ligands in the dinuclear arrange-
ments, namely, cisoid (1c) and transoid (1t). The four doublet
F
–
ane and C H Cl solvate molecules, and [BAr 4] counteranions are omitted
6
5
for clarity. Selected bond lengths [Å] and angles [°]: Mo1–N1 1.779(6), Mo1–
N2 2.188(6), Mo1–Cl1 2.4189(17), Mo1–Cl1i 2.4106(16), Mo1–P1 2.6270(17),
Mo1–P2 2.6106(17), Mo1–Mo1 3.0162(10), N1–O1 1.178(7), N1–Mo1–N2
1
1
77.43(2), P1–Mo1–P2 97.92(5), P1–Mo1–Cl1 167.85(6), Cl1–Mo1–Cli
02.70(5), Cl1–Mo1–Mo1i 51.23(4), Cl1–Mo1i–Mo1 51.47(4). Symmetry opera-
tion, i: –x, 1 – y, 1 – z.
Eur. J. Inorg. Chem. 2016, 103–110
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104 © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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analysis. The formation of 1t or 1c was mechanistically ex- Mo1 bond length of 3.0614(7) Å is in the range expected for a
plained on the basis of the oxidative addition of chlorobenzene bridge-supported Mo–Mo contact and is very close to that of
F
[25]
to the cationic complex [Mo(NO)(P∩P)(NCMe) ][BAr ] through 1t.
The crystallographic data of 1t and 2t are given in
3
4
the replacement of one MeCN ligand. Subsequently, the di- Table 1. The GC–MS analysis of the pentane-extracted part of
nuclear adduct with chlorido bridges is envisaged to undergo the 2t and 2c reaction mixture revealed the presence of bi-
dinuclear elimination of biphenyl through C –C coupling to phenyl as a product from the clear mononuclear fragment at
Ar Ar
I
I
generate the Mo –Mo bonded isomers 1c and 1t. Thus, the m/z = 154. The formation of the dinuclear species of types 1
biphenyl formation is anticipated to result from dinuclear re- and 2 could be envisaged according to Scheme 3. First, Ar–X
ductive coupling and is supported by the conspicuous mono- (X = Cl, Br) exchanges with a labile MeCN ligand of
+
nuclear fragment at m/z = 154 in the GC–MS spectra. However, [Mo(NO)(P∩P)(NCMe) ] and subsequent oxidative addition
3
II
another more complex reaction path with mononuclear C –C
leads to Mo halophenyl species of type I-1, which could then
Ar Ar
bond formation can also be envisaged, as shown in Scheme 3.
dimerize to the dinuclear intermediates I-2. Supported by the
To put the production of biphenyl and the formation of the presence of the large-bite-angle diphosphine, the reductive
dinuclear species of type 1 on more general grounds, we also elimination of diphenyl could occur from I-2 or I-3 to generate
F
probed the reaction of [Mo(NO)(P∩P)(NCMe) ][BAr ] with the metal–metal bonds of 1 and 2. A mononuclear pathway
3
4
[
26]
bromobenzene at 120 °C in tetrahydrofuran (THF). This reaction invoked by Osakada et al.
cannot be excluded but seems
3
1 1
was monitored by P H NMR spectroscopy, which revealed the less probable. In this pathway, two molecules of the oxidatively
appearance of four doublet signals at δ = 37.4 and 26.1 ppm added I-1 may undergo “symmetrization” through halide and
2
2
II
(
JP, P = 107.1 Hz) and δ = 33.8 and 29.6 ppm ( J = 108.4 Hz). aryl exchange to form the two Mo species of type I-3 and
P, P
This is consistent with the formation of an isomeric mixture of I-3′; after reductive elimination from I-3 and comproportiona-
F
the dinuclear species [Mo (NO) (P∩P) (NCMe) (μ-Br) ][BAr ]
2
2
2
2
2
4 2
(
2t and 2c) if it is assumed that the phosphorus atoms at each
molybdenum center of 2t and 2c are inequivalent (Scheme 2).
The reaction was complete in 1 h, and the isomeric product
mixture was obtained in 82 % yield.
1
In the H NMR spectrum, the methyl protons of the aceto-
nitrile ligand appeared at δ = 2.3 and 2.4 ppm in an approxi-
mate 1:1 intensity ratio for 2t and 2c, respectively. The slow
diffusion of pentane into a concentrated C H Cl solution at
6
5
room temperature produced deep red crystals of 2t, which
were suitable for an X-ray diffraction study. Complex 2t is iso-
structural to 1t and crystallizes in the triclinic space group P 1¯ .
I
2+
The molecular structure of [Mo (NO)(P∩P)(NCMe)(μ-Br)]
was
2
similar to that of 1t; two [Mo(NO)(P∩P)(NCMe)] cationic units
are held together by two μ-Br bridges with bridging Mo–Br
bond lengths of 2.5043(8) and 2.5211(8) Å, and the mono-
nuclear units have a transoid arrangement (Figure 2). The unit
cell additionally contains two BAr 4 anions and three chloro-
benzene solvate molecules. The Mo–P distances are in the ex-
pected range, and the P–Mo–P angles are 96.99(4)°. The Mo1–
Figure 2. Molecular structure of the cationic part of 2t. Thermal ellipsoids are
drawn at the 50 % probability level. All hydrogen atoms, phenyl rings, C H Cl
6 5
solvate molecules, and [BArF
] counteranions are omitted for clarity. Selected
–
4
bond lengths [Å] and angles [°]: Mo1–N1 1.773(4), Mo1–N2 2.201(4), Mo1–
Br1 2.5043(8), Mo1–Br1i 2.5211(8), Mo1–P1 2.6212(12), Mo1–P2 2.6119(12),
Mo1–Mo1 3.0614(7), N1–O1 1.185(6), N1–Mo1–N2 176.53(18), P1–Mo1–P2
F –
9
6.99(4), P1–Mo1–Br1 172.79(3), Br1–Mo1–Bri 104.94(2), Br1–Mo1–Mo1i
52.720(19), Br1–Mo1i–Mo1 52.220(19). Symmetry operation, i: –x, 1 – y, 1 – z.
I
2+
Scheme 3. Alternative reaction courses for the formation of [Mo (NO)(P∩P)(NCMe)(μ-Br)]
2
and biphenyl involving the oxidative addition to I-1 followed by
dinuclear reductive elimination of biphenyl or scrambling to form I-3 and I-3′ after the oxidative addition to I-1.
Eur. J. Inorg. Chem. 2016, 103–110 www.eurjic.org © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
105

Full Paper
tion with I-3′, the isomeric dinuclear complexes are formed fol- supported the equivalence of the P atoms of the DPEphos li-
lowed by concomitant reductive elimination of biphenyl from gands and the apparent absence of stereoisomers in solution.
0
+
I-3. The resulting [Mo (NO)(P∩P)(NCMe)] species compropor- The IR spectrum showed strong absorptions at ν˜ = 2028 and
tionates with I-3′ to give the dinuclear complexes of type 1 or 1957 cm– assigned to the ν stretching vibration in addition
1
CO
–1
2
.
to the νNO stretching frequency at ν˜ = 1630 cm .
Table 1. Crystallographic data for 1t and 2t.[a]
1t
2t
Empirical formula
C
2
C
6
3393.10
183(2)
76
(C32
Cl
H
62Cl
2
Mo
2
N
4
O
4
P
4
·
C
76
H
62Br
H
2 2 4 4 4
Mo N O P ·
H12BF24)·C
5
H
12
·
2(C32 12BF24)·3(C H Cl)
6 5
H
5
Formula weight /g mol–1
Temperature /K
3634.98
183(1)
Wavelength /Å
0.71073
0.71073
Crystal system, space group triclinic, P1¯
triclinic, P1¯
13.6800(2)
17.6953(3)
19.7075(4)
113.853(2)
98.357(2)
98.951(2)
4193.93(13)
1, 1.439
a /Å
16.0776(9)
17.0716(13)
18.0135(14)
105.257(7)
115.011(7)
98.938(5)
4116.7(6)
1, 1.369
b /Å
c /Å
α /°
β /°
γ /°
Volume /Å
3
Z, density (calcd.) /Mg m–
Absorption coefficient /
mm–
3
0
Scheme 4. Synthetic access to various low-valent Mo complexes bearing the
0.344
0.817
large-bite-angle DPEphos ligand.
1
F(000)
1706
1816
Crystal size /mm
θ range /°
0.38 × 0.28 × 0.06
2.82 to 25.03
43069
14519 (Rint = 0.0745)
99.8
0.50 × 0.24 × 0.11
2.08 to 27.48
68102
19225 (Rint = 0.0372)
100.0
1
The H NMR spectrum of 3 in CD Cl at room temperature
2
2
Reflections collected
Reflections unique
Completeness to θ /%
Absorption correction
Max./min. transmission
revealed several signals in the expected aromatic region for the
attached DPEphos ligand. Single crystals suitable for X-ray dif-
fraction studies were obtained from a toluene/pentane mixture
at room temperature. The X-ray structure analysis showed that
the two carbonyl ligands and the two phosphorus atoms of the
DPEphos ligand occupy the equatorial plane (Figure 3). The
trans NO/Cl axis was disordered with a site-occupancy ratio of
analytical
analytical
0.978 and 0.914
10291/238/1064
1.179
0.916 and 0.720
15522/325/1242
1.053
Data/restraints/parameters
Goodness-of-fit on F2
Final R
I > 2σ(I)]
and wR
data)
1
and wR
2
indices
0.1156, 0.2968
0.0884, 0.2602
[
R
0
.776:0.224(1). The composition of the compound was further
confirmed by elemental analysis.
1
2
indices (all
0.1467, 0.3276
0.1061, 0.2807
Largest diff. peak and
6.031 and –1.128
2.392 and –3.954
The monodentate phosphine complexes Mo(NO)(P–O–P)-
PR )Cl (R = Me, 4; Ph, 5) were prepared by the reactions of
3
hole /e Å–3
(
[
a] The unweighted R factor is R
1
= Σ(F
.
o
– F
c
)/ΣF
o
and the weighted R factor
[Mo (NO) (P∩P) Cl (μ-Cl) ] with 1 equiv. of the PR derivative
2
2
2
4
2
3
2
2 2
2 21/2
is wR
2
= Σw(F
o
– F
c
) /Σw(F
o
)
(R = Me and Ph) in the presence of excess 1 % Na/Hg (5 equiv.)
in THF at room temperature and isolated as red (4, 50 %) and
orange (5, 55 %) solids in moderate yields. It should be men-
tioned at this point that the employment of 2 equiv. of these
monophosphines did not allow the further incorporation of
0
Preparation of Low-Valent DPEphos Mo Complexes
Although the soft oxidation of the [Mo(NO)(P∩P)(NCMe)3]+ phosphine ligands, apparently owing to the bulkiness of the
3
1 1
complexes by aryl halides led to the dinuclear species of type DPEphos ligand. The P H NMR spectrum of 4 displayed a dou-
1
or 2, stronger reducing agents such as 1 % Na/Hg reduce the blet of doublet signal for the coordinated P–O–P ligand at δ =
[
23]
2
dinuclear [Mo (NO) (P∩P) Cl (μ-Cl) ] complex to various low- 45 ppm ( J = 12 Hz) along with a triplet resonance at lower
valent Mo complexes in the presence of strongly coordinating field (δ = 26 ppm, J = 12 Hz) for the PMe ligand. On the
π-accepting or σ-donating ligands such as CO and PR (R = Me other hand, 5 exhibited a triplet signal at δ = 74 ppm ( J
2
2
2
4
2
P, P
0
2
P, P
3
2
=
3
P, P
and Ph). The reaction of [Mo (NO) (P∩P) Cl (μ-Cl) ] with excess 12 Hz) for the attached triphenylphosphine ligand, and the
2
2
2
4
2
1
% Na/Hg (5 equiv.) in the presence of 1 bar of carbon monox- phosphorus resonance of the DPEphos ligand was transformed
ide at room temperature produced Mo(NO)(P∩P)(CO) Cl (3, into two doublet of doublet signals (ABX spin system) with a
2
2
Scheme 4). However, the yield was quite low (20 %) owing to strong trans J coupling of 165 Hz at δ = 43 and 37 ppm
the formation of other unidentified products. Nevertheless, 3 ( J = 12 Hz). The inequivalence of the two phosphorus atoms
could be prepared in excellent yield (76 %) by a modified syn- within the η chelate arises from a rigid conformation of the
P, P
2
P, P
3
[
27]
thetic procedure from the Mo(NO)(CO) (ClAlCl ) precursor
complexed ligand, as is typical for large-bite-angle diphos-
4
3
and the DPEphos ligand at 70 °C in THF. The 31P1H NMR spec- phines, for which the four P-bound phenyl rings adopt pseudo-
trum of the resulting mixture displayed only a sharp singlet axial and pseudoequatorial positions that result in different
resonance at δ = 16.6 ppm owing to the presence of 3 and conformations at each P atom with the consequence that inver-
Eur. J. Inorg. Chem. 2016, 103–110
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106
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 3. Molecular structures of 3 (left), 4 (middle), and 5 (right). Thermal ellipsoids are drawn at the 30 % (for 3 and 4) and 50 % (for 5) probability levels.
All hydrogen atoms and solvent molecules are omitted for clarity. The trans NO/Cl ligands are disordered with a site-occupancy ratio of 0.776:0.224. Selected
bond lengths [Å] and angles [°]: 3: Mo1–N1A 1.815(2), Mo1–C1 2.0172(16), Mo1–C2 2.0282(18), Mo1–Cl1A 2.4554(8), Mo1–P1 2.6011(4), Mo1–P2 2.6209(4),
N1A–O3A 1.283(2), C1–O1 C2–O2, N1A–Mo1–Cl1A 169.91(7), N1A–Mo1–C2 85.34(8), P1–Mo1–P2 96.039(13), P1–Mo1–C2 175.17(5), P1–Mo1–C1 85.96(5); 4:
Mo1–N1B 1.771(14), Mo1–O2 2.266(2), Mo1–P1 2.3740(11), Mo1–P2 2.4432(10), Mo1–P3 2.4599(9), N1B–O1B 1.27(3), N1B–Mo1–Cl1B 176.4(4), P1–Mo1–N1B
8
4.4(4), P2–Mo1–P3 152.23(3), P1–Mo1–P3 104.62(4); 5: Mo1–N1A 1.745(4), Mo1–O1 2.2914(10), Mo1–P1 2.4681(3), Mo1–P2 2.4840(3), Mo1–P3 2.4225(4), N1A–
O1A 1.198(4), N1A–Mo1–Cl1A 176.89(16), P1–Mo1–N1A 95.41(15), P1–Mo1–P3 101.565(11), P1–Mo1–P2 151.326(13).
Table 2. Crystallographic data for 3, 4, and 5.
3
4
5
Empirical formula
C
38
H
28ClMoNO
4
P
2
·C
7
H
8
2(C39
1626.11
183(2)
H
37ClMoNO
2
P
3
)·C
4
H
10
O
C
54 2 3
H43ClMoNO P
Formula weight /g mol–1
848.08
183(2)
962.19
183(2)
Temperature /K
Wavelength /Å
Crystal system, space group
a /Å
b /Å
c /Å
α /°
0.71073
monoclinic, P2
11.5883(1)
13.8586(1)
24.9475(2)
90
0.71073
monoclinic, P2
10.2639(1)
21.2569(3)
18.3320(3)
90
0.71073
monoclinic, P2
19.3956(3)
12.6738(2)
20.4746(3)
90
1
/c
1
/n
1
/n
β /°
101.155(1)
90
3930.82(5)
4, 1.433
0.528
105.192(2)
90
3859.88(10)
2, 1.399
0.571
114.639(2)
90
4574.74(14)
4, 1.397
0.493
γ /°
Volume /Å3
Z, density (calcd.) /Mg m–3
–
1
Absorption coefficient /mm
F(000)
1736
1676
1976
Crystal size /mm
θ range /°
0.28 × 0.12 × 0.12
2.58 to 30.51
67377
12004 (Rint = 0.0321)
99.9
analytical
0.944 and 0.891
9437/3/498
0.26 × 0.16 × 0.07
2.49 to 26.37
47560
7882 (Rint = 0.0490)
99.9
0.44 × 0.34 × 0.20
2.57 to 30.51
96389
13957 (Rint = 0.0307)
99.9
Reflections collected
Reflections unique
Completeness to θ /%
Absorption correction
Max./min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
analytical
analytical
0.964 and 0.899
5943/166/547
1.052
0.935 and 0.870
11579/4/569
1.080
1.081
Final R
and wR
Largest diff. peak and hole /e Å
1
and wR
2
indices [I > 2σ(I)]
0.0323, 0.0809
0.0450, 0.0834
0.602 and –0.688
0.0472, 0.1100
0.0696, 0.1160
1.528 and –0.725
0.0270, 0.0714
0.0361, 0.0733
0.393 and –0.610
R
1
2
(all data)
–
3
sions are not permitted. The molecular structures of 4 and 5 and the Mo–O bond lengths lie within the range 2.27–2.29 Å.
1
were determined by X-ray diffraction studies (Table 2) and are Furthermore, 4 and 5 could be characterized fully by H NMR
displayed in Figure 3 with selected bond lengths and angles. In spectroscopy, and their compositions were confirmed by ele-
both structures, the chelating phosphine ligand is tridentate mental analysis.
through the P,O,P atoms in a meridional arrangement with trans
P–Mo–P angles of 152.23(3)° for 4 and 151.326(13)° for 5. The
coordination of the oxygen atom is presumably preferred to
allow the complexes to achieve 18-electron configurations
Conclusions
[
28]
0
and is in accord with observations for related (DPEphos)Ru We have discovered the mild oxidation of [Mo (NO)-
F
complexes. The Mo–Pchelate bond lengths (2.44–2.48 Å) are (P∩P)(NCMe) ][BAr ] with aryl halides (ArX; X = Cl, Br) to form
3
4
slightly longer than the Mo–PR bond lengths (ca. 2.37–2.42 Å), biphenyl and the dinuclear species [MoI (NO) (P∩P) -
3
2
2
2
Eur. J. Inorg. Chem. 2016, 103–110
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107
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F
(
NCMe) (μ-X) ][BAr ] (X = Cl, 1c and 1t; Br, 2c and 2t) through of C
6
H
5
Br (relative to the complex) was added into the THF solution.
2
2
4 2
The resulting mixture was heated at 120 °C for 1 h. The reaction was
monitored by 31P1H NMR spectroscopy to ensure the completion
of the reaction. After the completion of the reaction, the mixture
was filtered, and the solvent was evaporated in vacuo. The obtained
oily residue was washed with the minimum amount of pentane to
remove the biphenyl-containing excess bromobenzene. Finally, a
an Ullmann-type dinuclear homocoupling process. The di-
nuclear complexes 1c/1t and 2c/2t were characterized spectro-
scopically and by X-ray diffraction studies. We have also de-
scribed the synthetic access to several DPEphos-containing low-
0
valent complexes, namely, Mo (NO)(P∩P)(CO) Cl (3) and
2
0
Mo (NO)(P–O–P)(PR )Cl (R = Me, 4; Ph, 5), through the reduc-
tions of the dinuclear Mo compounds. This work, particularly
3
red dinuclear isomeric mixture of 2t and 2c was obtained in 84 %
II
1
yield after drying in vacuo. H NMR (400 MHz, [D ]THF, 293 K): δ =
8
F
the Mo-complex-mediated homocoupling process to form bi- 8.2 (m, ph), 8.0 (m, Ph), 7.8 (s, BAr ), 7.7 (m, Ph), 7.6 (m, Ph), 7.5 (s,
phenyl, render the idea that Mo and W catalysts can be devel- BAr ) 7.2 (m), 7.0 (m), 2.4 (s, CH , 2t), 2.3 (s, CH , 2c) ppm. P H
4
F
31 1
4
3
3
2
oped for catalytic C –C bond formations through further li- NMR (162 MHz, [D
]THF, 293 K): δ = 37.4 (d, JP, P = 107.1 Hz, 2t),
6.1 (d, J = 107.1 Hz, 2t), 33.8 (d, J = 108.4 Hz, 2c), 29.6 (d,
Ar Ar
8
2
2
2
gand tuning efforts and the involvement of a suitable redox
couple.
P, P
P, P
2
JP, P = 108.4 Hz, 2c) ppm.
Mo(NO)(P∩P)(CO) Cl (3): To a solution of Mo(NO)(CO) (ClAlCl )
2
4
3
(
0.20 g, 0.49 mmol) in THF (15 mL) in a Schlenk tube equipped with
Experimental Section
a Young tap, a solution of DPEphos (0.265 g, 0.49 mmol) in THF
(5 mL) was added. The resulting mixture was heated at 70 °C for
General Considerations: All manipulations were performed under
an atmosphere of nitrogen by standard Schlenk techniques or in a
glovebox. All reagent-grade solvents were dried according to stand-
ard laboratory procedures with CaH (C D Cl and CD Cl ) and dis-
3
1 1
3
h. After the completion of the reaction, as indicated by P H
NMR spectroscopy, the red solution was filtered, and the solvent
was evaporated to dryness. The solid residue was washed with
pentane and then extracted with toluene. The concentrated toluene
solution was layered with pentane to afford tiny yellow crystals of
2
6
5
2
2
tilled through freeze–pump–thaw cycles before use. [Mo(NO)-
F
(
P∩P)(NCMe) ][BAr ], [Mo (NO) (P∩P) Cl (μ-Cl) ], and Mo(NO)-
3 4 2 2 2 4 2
3
after several days, yield (76 %, 280 mg). IR: ν˜ = 1630 (NO),
(
CO) (ClAlCl ) were prepared according to literature proce-
4
3
–1 1
[23,27]
2028(CO), 1957 (CO) cm . H NMR (500 MHz, CD
7
H) ppm. P H NMR (125 MHz, CD
C
Cl , 300 K): δ =
2 2
dures.
All other chemicals were purchased from commercial
.58 (Ph), 7.40 (m, Ph), 7.18 (m, Ph), 7.08 (m, Ph), 6.64 (m, DPEphos
sources and used without further purifications. The NMR spectra
31 1
1
Cl , 300 K): δ = 16.63 (s) ppm.
2 2
were recorded with Varian Mercury 200 (200.1 MHz for H, 81.0 MHz
3
1
1
13
38
H
28ClMoNO (755.97): calcd. C 60.37, H 3.73, N 1.85; found C
P
4 2
for P), Varian Gemini-300 ( H at 300.1 MHz, C at 75.4 MHz),
1
31
60.42, H 3.92, N 1.68.
Bruker-DRX 500 (500.2 MHz for H, 202.5 MHz for P, 125.8 MHz for
1
3
C), and Bruker-DRX 400 spectrometers (400.1 MHz for 1 H,
3
[
[
1
Mo(NO)(κ -P,P,O-DPEphos)Cl(PMe )] (4): [Mo (NO) (P∩P) Cl ]-
3
2
2
2
4
62.0 MHz for 31P, 100.6 MHz for C). The H and C H chemical
13
1
13 1
1
μ-Cl] (0.13 g, 0.084 mmol) was added to a stirred suspension of
2
shifts are expressed in ppm relative to tetramethylsilane (TMS), and
% Na/Hg (10 mg, 0.42 mmol) in THF (5 mL) in a Schlenk tube with
the 31P H chemical shifts are relative to 85 % H PO as an external
1
3
4
a Young tap, and PMe (0.03 mL, 9 μL) was introduced with a syr-
3
standard. The signal patterns are as follows: s, singlet; d, doublet; t,
triplet; q, quartet; m, multiplet. The IR spectra were obtained by
attenuated total reflectance (ATR) or KBr methods with a Bio-rad
FTS-45 instrument. The elemental analyses were performed at the
Anorganisch-Chemisches Institut of the University of Zürich. The
GC–MS spectra were recorded with a Varian Saturn 2000 spectrom-
eter equipped with Varian 450-GC chromatograph Phenomenex ZB-
inge. The resulting solution was stirred for 6 h at room temperature.
The resulting red solution was filtered, the solvent was removed in
vacuo, and the residue was washed with pentane. The obtained
red solid was extracted with toluene and then with benzene. The
concentrated benzene solution of 4 was layered with pentane and
kept in a fridge to afford pure red crystals of 4, yield 50 %. IR: ν˜ =
–1
1
1
546 (NO) cm . H NMR (400 MHz, C D , 300 K): δ = 7.5–7.44 (m,
6 6
5
ms (30 m), Brechbühler company; gradient 70–270 °C.
31 1
Ph), 7.35 (m, Ph), 7.22 (m, Ph), 7.01 (m, Ph), 6.8 (m, Ph) ppm. P H
2
2
F
NMR (162 MHz, C D , 300 K): δ = 45 (dd, J = 12.0 Hz), 26 (t, J
=
[
[
Mo(NO)(P∩P)(NCMe) μ-Cl ][BAr ] (1t and 1c): A solution of
6
6
P, P
P, P
2
2
F
4 2
12.0 Hz) ppm. 13C H NMR (100.6 MHz, C D , 300 K): δ = 159.7 (d,
1
Mo(NO)(P∩P)(NCMe) ][BAr ] (50 mg, 0.03 mmol) in chlorobenzene
6 6
3
4
J = 17.9 Hz, C H OP), 134 (m, Ph), 130 (s, Ph), 128 (m, Ph), 124 (s,
was heated at 120 °C for 30 min. The resulting solution was moni-
6 4
tored by 31P H NMR spectroscopy to ensure the completion of the
reaction. The resulting mixture was filtered, and the solvents were
evaporated to dryness. The obtained red oily residue was washed
twice with pentane to remove the biphenyl side product, and finally
a pure dinuclear isomeric mixture of 1t and 1c was obtained as a
1
C H OP), 118 (s, C H OP), 21 (m, CH ) ppm. C H ClMoNO P
6 4 6 4 3 39 37 2 3
(776.05): calcd. C 60.36, H 4.81, N 1.80; found C 60.45, H 5.04, N,
1.62.
3
[
Mo(NO)(κ -P,P,O-DPEphos)Cl(PPh )] (5): [Mo (NO) (P∩P) Cl ]-
3 2 2 2 4
[
μ-Cl] (0.30 g, 0.195 mmol) and PPh (0.102 g, 0.39 mmol) were
2
3
1
red solid in 92 % yield after drying in vacuo. H NMR (500 MHz,
added to a suspension of 1 % Na/Hg (22 mg, 0.96 mmol) in THF
10 mL). The resulting mixture was stirred overnight at room tem-
F
C D Cl, 293 K): δ = 8.2 (s, BAr ), 8.0 (m, Ph), 7.9 (m, Ph), 7.7 (m, Ph),
6
5
4
(
F
7
.6 (s, BAr ), 7.5 (m, Ph), 7.4–7.3 (m), 7.2–7.1 (m), 6.9 (m), 1.9 (s,
CH , 1t), 1.8 (s, CH , 1c) ppm. P H NMR (202 MHz, C D Cl, 293 K):
δ = 37.8 (d, J = 108.7 Hz, 1t), 27.1 (d, J = 108.5 Hz, 1t), 34.2
4
perature. After the completion of the reaction, the red solution was
filtered, the solvent was removed in vacuo, and the residue was
washed with pentane. The crude product was extracted with tolu-
ene and then with benzene. The concentrated benzene solution
was layered with pentane and kept at room temperature for several
days to afford orange crystals of pure 5 in 55 % yield. IR: ν˜ = 1561
3
1 1
3
3
6 5
2
2
P, P
P, P
2
2
(d, JP, P
= 109.3 Hz, 1c), 29.9 (d, JP, P = 109.1 Hz, 1c)
ppm. C140H B Cl F Mo N O P (3208): calcd. C 52.41, H 2.70, N
86
2
2
48
2 4 4 4
1
.75; found C 52.85, H 2.80, N 1.75. The presence of an isomeric
31
mixture of 1c and 1t was confirmed by 1D NOE and P COSY
–1 1
(
NO) cm . H NMR (400 MHz, C D , 300 K): δ = 8.2 (m, Ph), 7.86–
6 6
experiments.
7.76 (m, Ph), 7.73–7.68 (m, Ph), 7.40 (br s, Ph), 7.06–7.03 (m, Ph),
F
[
(
Mo(NO)(P∩P)(NCMe) μ-Br ][BAr ] (2t and 2c): [Mo(NO)-
7.0–6.96 (m, Ph), 6.83–6.79 (m, DPEphos), 6.76–6.72 (m, DPEphos
2
2
4 2
F
31 1
P∩P)(NCMe) ][BAr ] (200 mg, 0.12 mmol) was dissolved in THF
H), 6.68–6.64 (m, DPEphos H) ppm. P H NMR (162 MHz, C D ,
3
4
6 6
(10 mL) in Schlenk tube equipped with a Young tap, and 10 equiv.
300 K): δ = 75 (t, 2JP, P = 11.2 Hz, PPh ), 45 (dd, 2JP, P = 165.3 Hz,
3
Eur. J. Inorg. Chem. 2016, 103–110
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108
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
2JP, P = 11.2 Hz, DPEphos), 39 (dd, J = 165.3 Hz, 2JP, P = 11.22 Hz,
2
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1
1
[
(
5
4
43
2 3
2
4
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X-ray Diffraction Analyses: The single-crystal X-ray diffraction data
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The
[
32]
structures were solved by direct methods by using SHELXS97.
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[32]
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peaks (greater than 5 e Å ) were observed at chemically meaning-
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3
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Acknowledgments
The authors thank the Swiss National Science Foundation and
the University of Zürich for financial support.
4
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Keywords: Redox chemistry · Reductive coupling · C–C
coupling · Molybdenum · Phosphane ligands · Biaryls
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Received: September 16, 2015
Published Online: December 2, 2015
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