Disproportionation of bis(phosphaethenyl)pyridine iron(I) bromide induced by t BuNC

Article abstract of DOI:10.1021/om500901a

Redox behavior of Fe(I) complexes bearing a PNP-pincer-type phosphaalkene ligand, 2,6-bis[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-phosphaethenyl]pyridine (BPEP-Ph), is reported. The four-coordinate Fe(I) complex [FeBr(BPEP-Ph)] (1) readily reacts with 1 equiv of tBuNC in toluene at -35 °C to give [FeBr(tBuNC)(BPEP-Ph)] (2) with a 17e configuration. Complex 2 is fairly stable in neat benzene at room temperature, but smoothly undergoes disproportionation in the presence of added tBuNC to afford the Fe(0) complex [Fe(tBuNC)₂(BPEP-Ph)] (3) and the Fe(II) complex [FeBr₂(tBuNC)₄] (4) along with free BPEP-Ph. The single-crystal X-ray diffraction studies of 3 and 4 reveal distorted trigonal bipyramidal and square pyramidal arrangements around Fe, respectively. The yield of 3 increases up to 50%/2 when 2 equiv of tBuNC is added to the system. A disproportionation process involving a 19e intermediate is proposed.

Full text of DOI:10.1021/om500901a

Note
pubs.acs.org/Organometallics
Disproportionation of Bis(phosphaethenyl)pyridine Iron(I) Bromide
Induced by tBuNC
Ya-Fan Lin, Nobuko Ichihara, Yumiko Nakajima,*,† and Fumiyuki Ozawa*
International Research Center for Elements Science (IRCELS), Institute for Chemical Research, Kyoto University, Uji, Kyoto
611-0011, Japan
S
* Supporting Information
ABSTRACT: Redox behavior of Fe(I) complexes bearing a PNP-pincer-type phosphaalkene ligand, 2,6-bis[1-phenyl-2-(2,4,6-
tri-tert-butylphenyl)-2-phosphaethenyl]pyridine (BPEP-Ph), is reported. The four-coordinate Fe(I) complex [FeBr(BPEP-Ph)]
(1) readily reacts with 1 equiv of tBuNC in toluene at −35 °C to give [FeBr(tBuNC)(BPEP-Ph)] (2) with a 17e configuration.
Complex 2 is fairly stable in neat benzene at room temperature, but smoothly undergoes disproportionation in the presence of
added tBuNC to afford the Fe(0) complex [Fe(tBuNC)2(BPEP-Ph)] (3) and the Fe(II) complex [FeBr2(tBuNC)4] (4) along
with free BPEP-Ph. The single-crystal X-ray diffraction studies of 3 and 4 reveal distorted trigonal bipyramidal and square
pyramidal arrangements around Fe, respectively. The yield of 3 increases up to 50%/2 when 2 equiv of tBuNC is added to the
system. A disproportionation process involving a 19e intermediate is proposed.
This paper describes redox behavior of [FeBr(BPEP-Ph)]
(1) in solution. Recently, we carried out a related study on
[Fe(Mes)(BPEP-Ph)] and demonstrated a novel one-electron
reduction process induced by π-acid ligands (L = RNC and
CO). In this study, we found that tBuNC also induces a redox
reaction leading to disproportionation of the Fe(I) bromide
(1). A detailed analysis of the reaction stoichiometry indicates
that [FeBr(L)2(BPEP-Ph)], with a 19e configuration, serves as
a key intermediate for one-electron transfer.
INTRODUCTION
■
Transition metal complexes with an odd number of valence
electrons have attracted attention because of their crucial roles
in catalytic organic transformations and enzymatic reactions.1
However, structures and chemical properties of such complexes
have not been fully explored due to the instability arising from
their radical character. Indeed, in many cases, they are in situ
generated as transient species by either electrochemical or
photochemical methods.2 In this connection, rational design of
supporting ligands that enable the isolation of odd-electron
transition metal species has received continuous research
interest.3 Fe(I) complexes with 15e and 17e configurations
constitute the central subjects in such chemistry.4
Recently, we have reported that the four-coordinate Fe(I)
complexes [FeX(BPEP-Ph)] (X = Br, Mes (2,4,6-Me3C6H2))
are successfully stabilized by a PNP-pincer-type phosphaalkene
ligand, 2,6-bis[1-phenyl-2-(2,4,6-tri-tert-butylphenyl)-2-
phosphaethenyl]pyridine (BPEP-Ph).5 The BPEP-Ph ligand,
having PC double bonds at the 2,6-positions of pyridine,
possesses an extremely low-lying π* orbital around the
phosphorus atoms, thereby serving as a strong π-acceptor
toward transition metals.6−9 This particular ligand property
enables the isolation of Fe(I) complexes with a 15e
configuration.5
RESULTS AND DISCUSSION
■
Reaction of [FeBr(BPEP-Ph)] (1) with tBuNC. Complex 1
reacted with 1 equiv of tBuNC in toluene at −35 °C to give
[FeBr(tBuNC)(BPEP-Ph)] (2) with a 17e configuration,10
which was isolated as a dark green crystalline solid in 79% yield
(Scheme 1).
Complex 2 was fairly stable in neat C6D6 at room
temperature, but in the presence of tBuNC (2.3 equiv) the
same complex was smoothly converted to the Fe(0) complex
[Fe(tBuNC)2(BPEP-Ph)] (3) and the Fe(II) complex
[FeBr2(tBuNC)4] (4)11 along with free BPEP-Ph. Complexes
Received: September 1, 2014
Published: October 27, 2014
© 2014 American Chemical Society
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Note
are located at the apical and equatorial positions, respectively.
The P1−Fe−P2 bond (134.84(4)°) is significantly bent,
whereas the N2−Fe−C5 bond (175.19(12)°) retains the
linearity. Thus, the geometry around the Fe atom is described
as a distorted trigonal bipyramid (τ = 0.67).12 The P−C lengths
(1.703(3) and 1.715(3) Å) are comparable to those of 1
(1.713(6) and 1.719(6) Å).5a On the other hand, Fe−P1, Fe−
P2, and Fe−N2 bonds of 2 (2.1987(9), 2.1952(10), and
1.974(3) Å) are slightly shorter than those of 1 (2.2716(17),
2.2883(17), and 2.035(5) Å), respectively. The IR spectrum
exhibited a strong νCN band at 2107 cm−1 (ATR).
Scheme 1
Identification of [Fe(tBuNC)2(BPEP-Ph)] (3). Complex 3
is a diamagnetic species, showing a singlet signal at δ 257.8 in
the 31P{1H} NMR spectrum; the chemical shift is in a typical
1
region of phosphaalkene compounds. The H NMR signals of
3 and 4 were isolated as a dark green and reddish-brown solid
in 52% and 46% yields, respectively. Therefore, we concluded
that half of 2 was converted to 3, whereas the other half was
converted to 4 and BPEP-Ph. It is likely that 4 and BPEP-Ph
are afforded from an Fe(II) dibromide species, generated by
disproportionation of 2 in the presence of tBuNC. As an
indirect evidence, we confirmed that [FeBr2(BPEP-Ph)] (5)5a
instantly reacted with tBuNC (4.1 equiv) in benzene at room
temperature to afford 4 and BPEP-Ph in 77% and 63% isolated
yields, respectively.
two tBuNC ligands appeared at δ 0.82 and 0.95, independently.
The tBu substituents at the ortho-positions of Mes* (2,4,6-
(tBu)3C6H2) groups on phosphorus displayed two singlets at δ
1.59 and 1.98, whereas those at the para-position showed one
singlet at δ 1.39. These signal patterns are consistent with Cs
symmetry of the complex. The IR spectrum exhibited two
strong νCN bands at 2094 and 2041 cm−1 (ATR).
Figure 2 shows the X-ray structure of 3, having a mirror
plane including Fe, C3, N1, C5, C4, N2, C8, and N3 atoms.
Unlike the reaction with tBuNC, treatment of 1 with PhCN
as a much weaker π-acid ligand resulted in simple coordination
to form [FeBr(PhCN)(BPEP-Ph)]5a even when an excess
amount of PhCN was employed. On the other hand, treatment
of 1 with CO (1 atm) in C6D6 at room temperature caused
disproportionation. In this case, [FeBr2(BPEP-Ph)] (5),
[Fe(CO)2(BPEP-Ph)] (6), and BPEP-Ph were isolated from
the reaction solution in 14%, 39%, and 15% yields, respectively.
Free BPEP-Ph is the product generated from Fe(II) dibromide
and CO along with [FeBr2(CO)4],11 as confirmed by an
independent experiment using isolated [FeBr2(BPEP-Ph)] (5).
Identification of [FeBr(tBuNC)(BPEP-Ph)] (2). Com-
plexes 2 and 3 in Scheme 1 are new compounds and identified
by elemental analysis, IR and/or NMR spectroscopy, and X-ray
diffraction analysis.
Figure 2. X-ray crystal structure of [Fe(tBuNC)2(BPEP-Ph)] (3) with
50% probability ellipsoids. Hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Fe−P1 2.1381(8), Fe−
N3 1.999(3), Fe−C3 1.813(4), Fe−C4 1.838(5), P1−C1 1.713(3),
C1−C2 1.427(4), C3−Fe−N3 170.99(16), P1−Fe−P1′ 146.08(5),
Fe−C3−N1 179.6(4), Fe−C4−N2 179.1(4), C3−N1−C5 169.8(5),
C4−N2−C8 176.1(4).
Figure 1 shows the crystal structure of 2. The BPEP-Ph
ligand is coordinated with the Fe(I) center via P1, N2, and P2
atoms in a meridional arrangement. The Br and tBuNC ligands
The bond angles of C3−Fe−N3 and P1−Fe−P1′ are
170.99(16)° and 146.08(5)°, respectively. Thus, the complex
adopts a distorted square pyramidal configuration around Fe (τ
= 0.42).12 The bond lengths of Fe−P1 (2.1381(8) Å), Fe−N3
(1.999(3) Å), Fe−C3 (1.813(4) Å), and Fe−C4 (1.838(5) Å)
are comparable to those of 2. The P1−C1 bond length
(1.713(3) Å) is also very close to that of 2.
Examination of the Disproportionation Process. We
have confirmed that the Fe(I) bromide [FeBr(BPEP-Ph)] (1),
having a 15e configuration, readily combines with 1 equiv of
tBuNC to form the five-coordinate complex [FeBr(tBuNC)-
(BPEP-Ph)] (2), having a 17e configuration. This complex is
fairly stable in neat solvent at room temperature. On the other
hand, in the presence of excess tBuNC, the same complex
undergoes disproportionation to afford [Fe(tBuNC)2(BPEP-
Ph)] (3), [FeBr2(tBuNC)4] (4), and free BPEP-Ph. To ensure
the reaction stoichiometry, we next examined the disproportio-
nation reaction of 2 with changing the amounts of added
tBuNC.
Figure 1. X-ray crystal structure of [FeBr(tBuNC)(BPEP-Ph)]·C6H6
(2) with 50% probability ellipsoids. Hydrogen atoms and a crystal
solvent (benzene) are omitted for clarity. Selected bond lengths (Å)
and angles (deg): Fe−P1 2.1987(9), Fe−P2 2.1952(10), Fe−N2
1.974(3), Fe−C5 1.854(3), P1−C1 1.703(3), P2−C2 1.715(3), C5−
N1 1.154(4), N1−C6 1.466(4), C1−C3 1.449(4), C2−C4 1.431(4),
P1−Fe−P2 134.84(4), N2−Fe−C5 175.19(12), Fe−C5−N1
179.5(3), C5−N1−C6 173.8(3).
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Note
Figure 3 shows the relationship between the yield of 3 and
the amount of tBuNC added to the system. The formation of 3
(tBuNC)2(BPEP-Ph)] (A), which undergoes one-electron
transfer with 2. Although such a disproportionation process
involving a 19e Fe(I) species has been documented for several
instances including [Fe(C5H5)(benzene)] and [Fe-
(CO)3(PPh3)2]+,13 most of them are caused by σ-donor
ligands such as nitriles, pyridines, and phosphines, where the
ligand coordination enhances the reducing ability of 19e
species, enabling the subsequent electron transfer. In contrast,
disproportionation promoted by π-acid ligands has been
scarcely documented.14 In our present system, it seems
reasonable that BPEP-Ph as a strong π-acceptor ligand reduces
the reduction potential of the electron-acceptor unit (2),
thereby facilitating the one-electron transfer. We already
confirmed a remarkably positive shift of reduction potential
caused by BPEP-Ph for [FeBr2(BPEP-Ph)] (5).5b
Figure 3. Plot of the yield of 3 against the amount of tBuNC added to
the solution of 2 (8.5 mM) in C6D6 at room temperature.
EXPERIMENTAL SECTION
■
Reaction of 1 with tBuNC (1 equiv). A solution of 1 (14.0 mg,
0.015 mmol) in toluene (0.4 mL) was cooled to −35 °C in a glovebox
freezer, and tBuNC (2.0 μL, 0.018 mmol) was slowly added by means
of a syringe. The solution was filtrated through a Celite pad and
concentrated to dryness under vacuum. The residue was washed with
hexane (1.5 mL × 4) and dried under vacuum to give analytically pure
2 (12.1 mg, 0.012 mmol, 79%). 2: IR (ATR): 2107 cm−1 (νCN). Anal.
Calcd for C60H80N2P2BrFe: C, 70.17; H, 7.85; N, 2.73. Found: C,
70.36; H, 7.81; N, 2.49.
1
as a diamagnetic species was followed by H NMR spectros-
copy using an internal standard. It is seen that the amount of 3
increases with increasing amount of tBuNC and becomes
constant at 50% yield/2 when over 2 equiv/2 of tBuNC is
added. Hence the reaction stoichiometry given in Scheme 1 was
established.
Scheme 2 illustrates a plausible process for disproportiona-
tion. The first step should be the coordination of 2 with tBuNC
Reaction of 2 with with tBuNC. A solution of 2 (4.2 mg, 0.0041
mmol) in C6D6 (0.4 mL) was charged into an NMR sample tube, and
a C6D6 solution of tBuNC (0.48 M, 19.5 μL, 0.0094 mmol) was added.
The mixture was allowed to stand at room temperature and examined
Scheme 2
1
at intervals by H NMR spectroscopy using mesitylene (0.20 M in
C6D6, 7.0 μL, 0.0014 mmol) as an internal standard. The reaction
attained a steady state after ca. 6 h. The mixture was concentrated to
dryness and extracted with hexane. The hexane extract was subjected
to column chromatography (Al2O3) to give BPEP-Ph (1.3 mg, 0.0016
mmol, 39%; eluent: hexane/toluene, 4:1) and [Fe(tBuNC)2(BPEP-
Ph)] (3) (2.2 mg, 0.0021 mmol, 52%; eluent: Et2O), respectively. On
the other hand, the residue remaining after hexane extraction was
dissolved in Et2O. Slow evaporation of the Et2O solution at room
temperature afforded [FeBr2(tBuNC)4]10 (4) (1.0 mg, 0.0019 mmol,
46%). 3: 1H NMR (C6D6, 298 K): δ 0.82 (s, 9H, tBuNC), 0.95 (s, 9H,
tBuNC), 1.39 (s, 18H, p-tBu), 1.59 (s, 18H, o-tBu), 1.98 (s, 18H, o-
tBu), 7.02−7.12 (m, 10H, Ph), 7.21 (br, 1H, 4-Py), 7.44 (s, 2H,
Mes*), 7.65 (s, 2H, Mes*), (d, JHH = 8.0 Hz, 2H, 3-Py). 13C{1H}
3
NMR (C6D6, 298 K): δ 31.4, 31.7, 32.0, 34.7, 35.1, 35.6, 39.5, 39.8,
56.3, 114.6, 121.8, 122.0, 122.2, 124.4, 125.1, 127.4, 128.5, 131.8,
134.8, 136.4, 145.7, 151.6, 155.4, 159.0, 161.2. 31P{1H} NMR (C6D6,
298 K): δ 257.8 (s). IR (ATR): 2094, 2041 cm−1 (νCN). Anal. Calcd
for C65H89N3P2Fe: C, 75.78; H, 8.71; N, 4.08. Found: C, 74.98; H,
(L) to form [FeBr(tBuNC)2(BPEP-Ph)] (A), with a 19e
configuration, because the addition of tBuNC is the essential
requirement to start the reaction. Then, A undergoes one-
electron transfer with 2 to afford [FeBr2(tBuNC)(BPEP-Ph)
(B) and [Fe(tBuNC)2(BPEP-Ph)] (3). Finally, complex B
reacts with tBuNC to generate [FeBr2(tBuNC)4] (4) and free
BPEP-Ph.
1
8.82; N, 3.94. 4: H NMR (C6D6, 298 K): δ 1.07 (br, 18H, tBuNC),
1.26 (br, 18H, tBuNC). 13C{1H} NMR (C6D6, 300 K): δ 31.1, 31.2,
57.7, 58.1, 162.9, 169.1. IR (ATR): 2165 cm−1 (νCN).
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental section (supplementary); X-ray data (CIF) for 2
and 3. This material is available free of charge via the Internet at
http://pubs.acs.org.
CONCLUSIONS
■
We have shown that [FeBr(BPEP-Ph)] (1) undergoes
disproportionation via one-electron transfer in the presence
of tBuNC. Complex 1 readily combines with 1 equiv of tBuNC,
but the resulting complex [FeBr(tBuNC)(BPEP-Ph)] (2), with
a 17e configuration, is sufficiently stable to be isolated as a
crystalline solid. On the other hand, when an additional tBuNC
is present, disproportionation giving [Fe(tBuNC)2(BPEP-Ph)]
(3), [FeBr2(tBuNC)4] (4), and free BPEP-Ph takes place. This
reaction very probably involves the 19e intermediate [FeBr-
AUTHOR INFORMATION
Corresponding Authors
*E-mail: yumiko-nakajima@aist.go.jp.
*E-mail: ozawa@scl.kyotou.ac.jp.
■
Present Address
†National Institute of Advanced Industrial Science and
Technology (AIST), Tsukuba 305-8565, Japan.
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Note
(10) Complexes 3 and 4 were not detected in the reaction solution
by NMR spectroscopy.
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a MEXT Grant-in-Aid for
Scientific Research on Innovative Areas “Stimuli-Responsive
Chemical Species” (No. 24109010).
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