A new PC(sp3)P ligand and its coordination chemistry with low-valent iron, cobalt and nickel complexes

Article abstract of DOI:10.1039/c4dt00639a

A new PC(sp³)P ligand N,N′-bis(diphenylphosphino) dipyrromethane [PCH₂P] (1) was prepared and its iron, cobalt and nickel chemistry was explored. Two pincer-type complexes [PCHP]Fe(H)(PMe ₃)₂ (2) and [PCHP]Co(PMe₃)₂ (4) were synthesized in the reaction of 1 with Fe(PMe₃)₄ and Co(Me)(PMe₃)₄. 1 reacted with Co(PMe₃) ₄ and Ni(PMe₃)₄ to afford Co(0) and Ni(0) complexes [PCH₂P]Co(PMe₃)₂ (3) and [PCH ₂P]Ni(PMe₃)₂ (5). The structures of complexes 2-5 were determined by X-ray diffraction.

Full text of DOI:10.1039/c4dt00639a

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A new PC(sp3)P ligand and its coordination
chemistry with low-valent iron, cobalt and nickel
complexes†
Cite this: Dalton Trans., 2014, 43,
8595
Received 3rd March 2014,
Accepted 11th April 2014
Gengyu Zhu, Xiaoyan Li, Guoqiang Xu, Lin Wang and Hongjian Sun*
DOI: 10.1039/c4dt00639a
www.rsc.org/dalton
A
new PC(sp3)P ligand N,N’-bis(diphenylphosphino)dipyrro-
methane [PCH2P] (1) was prepared and its iron, cobalt and nickel
chemistry was explored. Two pincer-type complexes [PCHP]Fe(H)-
(PMe3)2 (2) and [PCHP]Co(PMe3)2 (4) were synthesized in the reac-
tion of 1 with Fe(PMe3)4 and Co(Me)(PMe3)4. 1 reacted with Co-
(PMe3)4 and Ni(PMe3)4 to afford Co(0) and Ni(0) complexes
[PCH2P]Co(PMe3)2 (3) and [PCH2P]Ni(PMe3)2 (5). The structures of
complexes 2–5 were determined by X-ray diffraction.
Fig. 1 DPM-based PC(sp3)P ligand and its coordination patterns.
The “pincer era” began from the initial investigations of PCP
ligands by Shaw1 in the late 1970s. However, exploration of the
PC(sp3)P ligands has been overshadowed for quite a long time
by their aromatic counterparts.2 This imbalance originated
from not only the greater synthetic convenience for access to
diverse ligands, but also the greater thermal and confor-
mational stability of PC(sp2)P ligands compared to PC(sp3)P
ligands.3 Nevertheless, more recent work4 has clearly demon-
strated that the sp3-hybridized carbon coordinated to the
metal centre highly influences the reactivity of the pincer
complex and many interesting transformations have been
spotted.5 This makes the PC(sp3)P pincer system more and
more attractive for further developments.
on a dipyrromethane (DPM) scaffold pioneered by Ozerov and
coworkers6b has caught attention for two reasons (Fig. 1). First,
its construction is very straightforward, by taking advantage of
the facile N–P bond formation. More importantly, the pyrrole
ring as a rigid linker makes the communication between the
metal centre and the C(sp3)–H bond more efficient, which
increases the chance for cyclometalation.6b,8b However, since
the initial studies no further exploration has been made on
this type of PC(sp3)P ligand because C(sp3)–H cleavage is a
challenge. Herein, we report a new DPM-based PC(sp3)P ligand
and its first pincer complexes with iron and cobalt. We also
describe the formation of its cobalt(0) and nickel(0) complexes
where it was utilized as a bidentate ligand.
So far, the known PC(sp3)P ligands are relatively limited,5
and research in this field has mainly been focused on plati-
num group metals6–10 and nickel11 chemistry. Due to our long-
standing efforts in the chemistry of iron, cobalt and nickel,12
we have set out to study the activation of the C(sp3)–H bond in
pincer ligands by these metals. Several PC(sp3)P pincer com-
plexes of iron and cobalt were reported in our previous
work,12a but the examples of this type are still very limited.
Among all the known PC(sp3)P ligands, a particular one based
The new ligand N,N′-bis(diphenylphosphino)dipyrro-
methane [PCH2P] (1) was prepared according to a slightly
modified procedure6b developed by Ozerov for the original
ligand P2CH2 (Scheme 1). Reaction of 2 equiv. of Ph2PCl with
deprotonated dipyrromethane in THF afforded compound 1
which was isolated as a colourless viscous oil in 62% yield by
column chromatography on silica gel under N2. 1 was charac-
1
terized by H, 31P, and 13C NMR in solution. It displayed one
School of Chemistry and Chemical Engineering, Key Laboratory of Special Functional
Aggregated Materials, Ministry of Education, Shandong University, Shanda Nanlu
27, 250199 Jinan, People’s Republic of China. E-mail: hjsun@sdu.edu.cn;
Fax: +86 531 88361350
†Electronic supplementary information (ESI) available: Experimental details and
characterization of all compounds; crystal structure data for 2, 3, 4 and 5. CCDC
986904–986907. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4dt00639a
Scheme 1 Synthesis of the ligand [PCH2P].
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Fig. 3 Molecular structure of complex 2. The thermal ellipsoids are dis-
played at the 50% probability level, and hydrogen atoms except for Fe–
H are omitted for clarity. Selected bond lengths (Å) and angles [°]: Fe1–
H1 1.64(6), Fe1–C29 2.166(5), Fe1–P1 2.156(2), Fe1–P2 2.164(2), Fe1–P3
2.240(2), Fe1–P4 2.264(2); C29–Fe1–H1 92(2), C29–Fe1–P3 175.7(2),
C29–Fe1–P4 91.3(2), P3–Fe1–H1 84(2), P1–Fe1–P2 148.40(6), P1–Fe1–
P3 95.10(6), P1–Fe1–P4 105.65(6), P2–Fe1–P4 102.51(6), P3–Fe1–P4
92.89(6).
Scheme 2 Preparation of complexes 2–5.
Fe–C(sp3) bonds.13 Both Fe–P4 distance (2.264(2) Å) and Fe–P3
distance (2.240(2) Å) are longer than Fe1–P1 distance (2.156(2)
Å) and Fe1–P2 distance (2.164(2) Å), presumably due to the
strong trans-influence of the hydrido H and C(sp3) atoms
being greater than that of the phosphorus atoms.
The reaction of 1 with Co(PMe3)4 was different (Scheme 2).
After stirring for 24 h in THF at 25 °C, complex [PCH2P]Co-
(PMe3)2 (3) was isolated as dark red blocks in the yield of 69%.
Complex 3 is a paramagnetic species, and showed no ν(Co–H)
stretching bands in the IR spectrum, indicating that the clea-
vage of the C(sp3)–H had not happened. The structure of
complex 3 was determined by X-ray diffraction analysis (Fig. 4).
singlet resonance at δ 35.1 ppm in the 31P{1H} NMR spectrum.
The protons of the CH2 linkage resonate at δ 4.81 ppm as a
1
singlet in the H NMR spectrum, and the carbon of the CH2
linkage resonates at δ 25.4 ppm as a triplet with JCP = 18 Hz in
the 13C NMR spectrum.
We investigated its coordination chemistry with iron, cobalt
and nickel (Scheme 2). When 1 was treated with 1 equiv. of
Fe(PMe3)4 in THF, the solution turned dark brown after 24 h.
Complex [PCHP]Fe(H)(PMe3)2 (2) was isolated as golden yellow
needles from diethyl ether. A typical ν(Fe–H) stretching band
was found at 1922 cm−1 in the IR spectrum. The characteristic
1
hydride signal of 2 in the H NMR spectrum (Fig. 2) as a tdd
peak was found at −13.36 ppm with JPH coupling constants of
71.1, 32.7 and 20.7 Hz. A single crystal X-ray structure analysis
of 2 established the structural details associated with the
coordination of the new PC(sp3)P ligand. In the molecular
structure of 2 (Fig. 3), iron is centred in a distorted octahedral
geometry. The axial angle P1–Fe1–P2 is 148.40(6)°, greatly
deviating from 180°. [C29Fe1P3P4H1] are in the equatorial
plane. The Fe1–C29 distance (2.166(5) Å) is within the range of
Fig. 4 Molecular structure of complex 3. The thermal ellipsoids are dis-
played at the 50% probability level, and hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles [°]: Co1–P1 2.208(1), Co1–
P2 2.235(1), Co1–P3 2.136(1), Co1–P4 2.141(1); P1–Co1–P2 104.87(5),
P1–Co1–P3 112.58(5), P1–Co1–P4 105.82(5), P2–Co1–P3 110.35(5), P2–
Co1–P4 115.35(5), P3–Co1–P4 107.88(4).
Fig. 2 Hydride resonance of complex 2.
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The cobalt(0) centre is coordinated to two phosphorus atoms
of the [PCH2P] ligand and two PMe3 molecules. The environ-
ment of the cobalt centre is close to an idealized tetrahedral
geometry. The Co1–C35 distance (ca. 3.34 Å) is much longer
than those of the known Co–C(sp3) bonds (2.03–2.15 Å).14 This
suggests that no bond interaction exists between these two
atoms, thus 1 [PCH2P] only served as a neutral bidentate
ligand in this case.
A pincer complex of cobalt bearing this ligand was obtained
in the following reaction (Scheme 2). Treatment of 1 with an
equimolar amount of Co(Me)(PMe3)4 in diethyl ether at 25 °C
furnished complex [PCHP]Co(PMe3)2 (4) in a yield of 84% as a
red powder which can be crystallized in diethyl ether at 0 °C.
The existence of the Co–CH moiety in 4 was evidenced by a
doublet at δ 3.88 ppm in the 1H NMR spectrum. A pair of
doublets at δ 0.76 and 1.06 ppm in the 1H NMR spectrum indi-
cated that the PMe3 ligands are not chemically identical. X-ray
crystallography confirmed a slightly distorted trigonal bipyra-
midal structure of complex 4 (Fig. 5). C35 and P2 are trans-
oriented in the axial positions and the C35–Co1–P2 angle is
176.65(7)°. Two five-membered metallacycles with remarkable
ring bending (sum of the internal bond angles is 530.4° and
531.9° respectively) are formed through coordination of the
two phosphine arms and the metalated C(sp3) atom. The Co1–
C35 distance of 2.140(3) Å is within the range of Co–C(sp3)
bonds (2.03–2.15 Å).14
Fig. 6 Molecular structure of complex 5. The thermal ellipsoids are dis-
played at the 50% probability level, and hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles [°]: Ni1–P1 2.199(1), Ni1–P2
2.217(1), Ni1–P3 2.149(1), Ni1–P4 2.141(1); P1–Ni1–P2 106.83(4), P1–
Ni1–P3 107.35(4), P1–Ni1–P4 111.99(4), P2–Ni1–P3 114.80(4), P2–Ni1–
P4 109.68(4), P3–Ni1–P4 111.99(4).
Scheme 3 Attempts on the hydrosilylation of benzaldehyde catalyzed
by 2.
The reaction of
1 with Ni(PMe3)4 or Ni(Me)2(PMe3)3
afforded the same product [PCH2P]Ni(PMe3)2 (5) (Scheme 2).
Complex 5 was isolated as orange crystals in high yield in both
reactions. The molecular configuration is depicted in Fig. 6.
The nickel atom is situated in a distorted tetrahedral coordi-
nation sphere. The P1–Ni1–P2 (106.83(4)°), P1–Ni1–P3
(107.35(4)°), P1–Ni1–P4 (111.99(4)°), P2–Ni1–P3 (114.80(4)°),
P2–Ni1–P4 (109.68(4)°) and P3–Ni1–P4 (111.99(4)°) angles are
all close to 109.5°. The four Ni–P bond lengths of 2.1989(12) Å,
2.2167(11) Å, 2.1490(11) Å, and 2.1408(12) Å are comparable
with literature values.12c Complex 5 can be viewed as a nickel
version of Complex 2. It is noticeable that only a Fe(PMe3)4
formed PC(sp3)P pincer-type complex with ligand 1 in these
three zero valent species (Fe(PMe3)4, Co(PMe3)4, and Ni(PMe3)4).
Iron hydride complexes have been playing important roles
in many catalytic systems.15 Recently, several groups, including
ours, have been interested in developing iron-catalysed hydro-
silylation of aldehydes and ketones.12c,16 Therefore, we also
examined if complex 2 would catalyse similar reactions. Unfor-
tunately, this proved to be unsuccessful although many
attempts had been made (Scheme 3).
In conclusion, a new PC(sp3)P pincer ligand based on a
dipyrromethane backbone was synthesized and its coordi-
nation chemistry with iron, cobalt and nickel was investigated.
The activation of the C(sp3)–H bond is strongly metal-depen-
dent. Further studies into the coordination chemistry of this
ligand and the properties of its resulting complexes are
ongoing in our laboratory.
We gratefully acknowledge the financial support by NSF
China no. 21072113 and 21372143 and Shandong Province
Natural Science Foundation ZR2010BZ002.
Fig. 5 Molecular structure of complex 4. The thermal ellipsoids are dis-
played at the 50% probability level, and hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles [°]: Co1–C35 2.140(3),
Co1–P1 2.2053(9), Co1–P2 2.2119(9), Co1–P3 2.1452(9), Co1–P4
2.1529(8); C35–Co1–P2 176.65(7), P1–Co1–P3 121.59(3), P1–Co1–P4
116.93(3), P3–Co1–P4 117.14(3), C34–C35–C36 109.1(2).
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