Manganese and rhenium formyl complexes of diphosphanylborane ligands: Stabilization of the formyl unit from intramolecular B-O bond formation

Article abstract of DOI:10.1002/ejic.201300482

Several manganese(I) and rhenium(I) carbonyl complexes of the chelating diphosphanylborane ligands Ph₂PCH(PPh₂)CH ₂B(C₈H₁₄) (1) and Ph₂PCH ₂CH[B(C₈H₁₄)]PPh₂ (2) were prepared to study Lewis acid assisted stoichiometric CO reductions. The phosphanylborane complexes fac,cis-Mn(CO)₃(1)(Br) (3a), fac,cis-Re(CO) ₃(1)(Br) (4a), fac,cis-Mn(CO)₃(2)(Br) (5a), and fac,cis-Re(CO)₃(2)(Br) (6a) were obtained from the reactions of Mn(CO)₅Br or Re(CO)₅Br with diphosphanylborane ligands 1 and 2. Further treatment of all four complexes with one equivalent of AgBF ₄ in CH₂Cl₂ in the dark followed by 1 atm of CO afforded the corresponding monocationic complexes [Mn(CO)₄(1)] [BF₄] (3b), [Re(CO)₄(1)][BF₄] (4b), [Mn(CO)₄(2)][BF₄] (5b), and [Re(CO)₄(2)] (BF₄) [6b]. An X-ray diffraction study confirmed the expected molecular structure of 3b. In addition to NMR and elemental analysis characterizations, the structures of these complexes could be unambiguously assigned based on two major ν(CO) IR bands, which established the cis-(CO)₄ local ligand environment. The cationic complexes 3b, 4b, 5b, and 6b were tested for stoichiometric CO reductions by reaction with NaHBEt₃ in chlorobenzene. However, the reductions of the complexes 3b and 4b produced mixtures of formyl and hydride species. The reaction of complexes 5b and 6b led to the formation of the stable formyl derivatives [Mn(CHO)(CO)₃(2)] (5c) and [Re(CHO)(CO)₃(2)] (6c), which were isolated in pure form and characterized by spectroscopic means. In addition, 5c could be characterized by a single-crystal X-ray study, which revealed the formyl structure and a stabilizing coordination of the pendant Lewis acidic B(C₈H₁₄) group to the formyl oxygen atom. Several manganese and rhenium formyl complexes containing chelating diphosphanylborane ligands were prepared and spectroscopically characterized. Exemplary single-crystal X-ray structures revealed the possibility for intramolecular Lewis acid support of the manganese- and rhenium-bound formyl groups by intramolecular B-O bond formation for cases with appropriate chelate ring sizes. Copyright

Full text of DOI:10.1002/ejic.201300482

FULL PAPER
DOI:10.1002/ejic.201300482
Manganese and Rhenium Formyl Complexes of
Diphosphanylborane Ligands: Stabilization of the Formyl
Unit from Intramolecular B–O Bond Formation
Rajkumar Jana,[a] Subrata Chakraborty,[a] Olivier Blacque,[a] and
Heinz Berke*[a]
Keywords: Rhenium / Manganese / Diphosphanylboranes / Hydrogenation / Formyl complexes
Several manganese(I) and rhenium(I) carbonyl complexes
of the chelating diphosphanylborane ligands Ph2PCH-
and elemental analysis characterizations, the structures of
these complexes could be unambiguously assigned based on
two major ν(CO) IR bands, which established the cis-(CO)4
local ligand environment. The cationic complexes 3b, 4b, 5b,
and 6b were tested for stoichiometric CO reductions by reac-
tion with NaHBEt3 in chlorobenzene. However, the re-
ductions of the complexes 3b and 4b produced mixtures of
formyl and hydride species. The reaction of complexes 5b
(PPh2)CH2B(C8H14
)
(1)
and
Ph2PCH2CH[B(C8H14)]-
PPh2 (2) were prepared to study Lewis acid assisted stoichio-
metric CO reductions. The phosphanylborane complexes
fac,cis-Mn(CO)3(1)(Br) (3a), fac,cis-Re(CO)3(1)(Br) (4a),
fac,cis-Mn(CO)3(2)(Br) (5a), and fac,cis-Re(CO)3(2)(Br) (6a)
were obtained from the reactions of Mn(CO)5Br or Re(CO)5-
Br with diphosphanylborane ligands 1 and 2. Further treat- and 6b led to the formation of the stable formyl derivatives
ment of all four complexes with one equivalent of AgBF4 in
CH2Cl2 in the dark followed by 1 atm of CO afforded the
corresponding monocationic complexes [Mn(CO)4(1)][BF4]
(3b), [Re(CO)4(1)][BF4] (4b), [Mn(CO)4(2)][BF4] (5b), and
[Re(CO)4(2)](BF4) [6b]. An X-ray diffraction study confirmed
the expected molecular structure of 3b. In addition to NMR
[Mn(CHO)(CO)3(2)] (5c) and [Re(CHO)(CO)3(2)] (6c), which
were isolated in pure form and characterized by spectro-
scopic means. In addition, 5c could be characterized by a sin-
gle-crystal X-ray study, which revealed the formyl structure
and a stabilizing coordination of the pendant Lewis acidic
B(C8H14) group to the formyl oxygen atom.
electrophilic metal carbonyl complex, could lead to catalytic
conversion. This approach is based on a two-catalyst sys-
tem, in which one metal complex acts as a scaffold for CO
reduction and the other delivers the hydride anion and
eventually also the proton as an H2 equivalent. Such new
approaches could thus be based on the chemistry of formal
heterolysis of H2 that has emerged in the last decade.
Introduction
The conversion of homogeneous syngas (CO + H2) to
C1–C3 oxygenates or higher organics still poses a great chal-
lenge for research in academia and industry.[1] Although
this field of research has been addressed for decades and
even peaked after the first oil crisis, real progress has not
been achieved yet. As the catalyst systems in a homogen-
eous process can readily be modified by changing the ligand
environment to tune activities and selectivities,[2] it seems
attractive to revive basic research on transition-metal-cata-
lyzed homogeneous CO reductions by utilizing newly estab-
lished approaches. The key challenges associated with this
chemistry, often denoted as homogeneous Fischer–Tropsch
chemistry, are C–H and C–C bond formations, for which
crucial reaction intermediates were obtained as mechanistic
models mostly by the reaction of main group and transi-
tion-metal complexes.[3] Recently, Bercaw et al.[4a] and
others[4b] proposed that a dual system, consisting of a late-
transition-metal hydrogen activator along with a relatively
Ambiphilic phosphanylborane ligands are unique for this
purpose because they contain nucleophilic phosphorus and
electrophilic boron center connected by an organic spacer
group.[5] The pendant Lewis acidic borane moiety can assist
in the delivery of a hydride ion to a metal-coordinated CO
ligand and the subsequent formation of a C–C bond, (tran-
siently) stabilize reaction intermediates by the affinity of the
Lewis acidic groups with B–O bond formations, and
eventually also function as a frustrated Lewis pair (FLP) in
combination with an externally added bulky base to hetero-
lytically cleave dihydrogen and deliver the hydride ion and
the proton to metal-bound CO groups.[5c–5e] However,
metal catalysts containing tethered Lewis acidic boron
groups are often not viable for catalytic applications as they
form strong B–O bonds, which are difficult to cleave and to
release the organic product. Nevertheless, the boron-linked
complexes are certainly important for obtaining models to
provide insights into the key challenges in this process.
[a] Institute of Inorganic Chemistry, University of Zurich,
Winterthurerstrasse 190, 8057 Zürich, Switzerland
Fax: +41-44-6356802,
E-mail: hberke@aci.unizh.ch
Homepage: http://www.aci.uzh.ch/en/research-groups/emeriti/
berke-group/prof-dr-heinz-berke/?tx_wfqbe_pi1
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From the work of Bercaw and co-workers, it became evi- type [M(L)(CO)4]+ (M = Mn or Re, L = 1 or 2, two cis and
dent that both the number of phosphanylborane ligands in two trans CO groups). These kinds of metal complexes were
an electrophilic transition-metal carbonyl complex and the sought to act as new scaffolds for hydride transfer to CO
nature of the organic linker between the phosphorus and as primary steps for CO hydrogenation, whereas NaHBEt3
boron center have pronounced effects on the reduction was anticipated to deliver the hydride ion for the reduction
chemistry.[5c] To date, only monodentate monophosphanyl- reactions.
borane ligands containing a straight chain linker (C1–C4)
have been studied.[5c–5e] The analogous bidentate diphos-
phanylborane ligands have not yet been reported.
Results and Discussion
As our group has longstanding interest in hydrogen-
ation chemistry[6] and metal-bound CO reduction, we be-
came interested in new diphosphanylborane ligand systems.
Synthesis of MnI and ReI Diphosphanylborane Complexes
The ligand exchange reactions of the commercially avail-
We recently reported the synthesis and coordination able metal carbonyl precursors Mn(CO)5Br and Re(CO)5Br
properties of the diphosphanylborane ligands with the diphosphanylborane ligands 1 or 2 were very
Ph2PCH(PPh2)CH2B(C8H14) (1) and Ph2PCH2CH[B- much straightforward in tetrahydrofuran (THF), and the
(C8H14)]PPh2 (2, Figure 1) with relatively small bite reactions were completed within a few hours at 80 °C
angles.[7] Both the diphosphanylborane ligands were ob- (Scheme 1). When THF solutions containing an equimolar
tained from the reaction of the corresponding diphos- mixture of the metal precursor Mn(CO)5Br or Re(CO)5Br
phoalkenes (Ph2P)2CHCH2 or Ph2PCHCHPPh2 with 9- and the diphosphanylborane ligand 1 were heated to reflux
borabiyclo[3.3.1]nonane (9-BBN) under relatively drastic at 80 °C for ca. three hours, we obtained the desired diphos-
reaction conditions. These two ligands have monomeric phanylborane manganese and rhenium complexes fac,cis-
open structures both in the free ligand form and in the Mn(CO)3(1)(Br) (3a) and fac,cis-Re(CO)3(1)(Br) (4a)
metal complexes. Despite the large steric and ring strain through the expected chelate coordination of the ligand to
factors, both ligands coordinate to tungsten centers in a the metal centers and the replacement of the two cis CO
chelated fashion through the formation of either a five- ligands. The isolated yields of the complexes were 86 and
membered (for 2) or a four-membered (for 1) chelate ring. 93%, respectively. The 31P{1H} NMR signals were found at
The PPh2 groups are involved in primary coordination to δ = 36.33 and –14.75 ppm, and the 11B signals correspond-
the metal center, whereas the Lewis acidic B(C8H14) group ing to the tethered Lewis acidic B(C8H14) group were found
is present either as a free pendant in the secondary coordi- at δ = 58.3 and 53.0 ppm for 3a and 4a, respectively. The
nation sphere or involved in the secondary sphere coordina- elemental analyses were also satisfactory for these com-
tion of a hydride ligand leading to a three-center, two-elec- plexes. In such compounds, three νCO bands in facial geom-
tron (3c–2e) bond. These results prompted us to explore etry and two νCO bands in the meridional geometry are ob-
the CO hydrogenation chemistry of cationic manganese and served;[8] the related examples are the bis(phosphane) com-
rhenium phosphanylborane (PB) complexes.
plexes fac-[Re(CO)3(PPh3)2Br] (2029, 1949, and 1906 cm–1),
mer-[Re(CO)3(PPh3)2Br] (1949 and 1901 cm–1), and mer-
[Mn(CO)3(PPh3)2Br] (1948 and 1916 cm–1). In the FTIR
spectra, three νCO bands were found at 2013, 1942, and
1901 cm–1 for 3a and at 2019, 1936, and 1895 cm–1 for 4a
further confirming the formation of the fac,cis complexes
through the formation of a four-membered chelate ring
around the metal centers by the phosphanylborane ligand
1. In a similar procedure as above, the reaction of the di-
phosphanylborane ligand 2 with the metal precursors
Figure 1. The diphosphanylborane ligands Ph2PCH(PPh2)-
CH2B(C8H14) (1) and Ph2PCH2CH[B(C8H14)]PPh2 (2).
yielded the desired
complexes
fac,cis-[Mn(CO)3-
(2)(Br)] (5a) and fac,cis-[Re(CO)3(2)(Br)] (6a, Scheme 1).
To explore such transition-metal systems, we propose The 31P{1H} NMR signals for 5a were observed at δ =
3
here to study CO hydrogenation chemistry by employing 67.20 and 86.49 ppm as two doublets with JP,P = 22.7 Hz.
the PB ligands 1 and 2, which contain two P atoms, instead The presence of the tethered Lewis acidic B(C8H14) group
of one in all the earlier cases, and one boron center. We was confirmed from the broad 11B NMR signal at δ =
have prepared several cationic manganese and rhenium 51.3 ppm. The chelate coordination of the diphosphanyl-
phosphanylborane complexes containing highly electro- borane ligand 2 in 5a was confirmed by the three character-
philic CO groups and studied their reactivities towards the istic νCO bands in the FTIR spectra at 2010 (vs), 1940 (vs),
hydrogenation of metal-bound CO to formyl derivatives. and 1911 (vs) cm–1. The isolated yield of the complex was
The complexes were prepared from the reaction of commer- found to be 78%. For the rhenium complex 6a, two dia-
cially available metal carbonyl precursors Mn(CO)5Br and stereomers formed as indicated by the appearance of two
Re(CO)5Br with the diphosphanylborane ligands 1 and 2. sets of doublets in the 31P{1H} NMR spectrum [major
3
3
It is expected that the chelating behavior of these two li- (63%): δ = 26.14 (d, JP,P = 8 Hz, 1 P), 48.55 (d, JP,P
=
3
gands would lead to the formation of cis complexes of the 8 Hz, 1 P) ppm; minor (37%): δ = 30.37 (d, JP,P = 16 Hz,
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3
1 P), 44.82 (d, JP,P = 16 Hz, 1 P) ppm]. The formation However, the typical broad 11B NMR resonance corre-
of two diastereomers (as for ligand 2) was also observed sponding to the trialkyl boron group was also observed. All
previously in the tungsten complexes[7] and could be ex- these complexes were fully characterized by NMR spec-
plained on the basis of the two possible stereochemical po- troscopy and also provided satisfactory elemental analyses
sitions of the boryl group and Br ligands, which could (see Experimental Section). The identity of these complexes
either be cis or trans to each other in this case. However, was further supported by the FTIR spectra of 3b, 4b, 5b,
only one isomer was detected in the 31P{1H} NMR spec- and 6b, which exhibited two intense νCO bands at 2091 (vs)
trum of the analogous manganese complex 5a containing and 1987 (vs) cm–1 for 3b, 2108 (vs) and 1982 (vs) cm–1 for
the same PB ligand. Three νCO bands were observed in the 4b, 2092 (vs) and 2001 (vs) cm–1 for 5b, and at 2110 (vs)
FTIR spectrum of 6a at 2021 (vs), 1938 (vs), and 1897 (vs) and 2001 (vs) cm–1 for 6b. The frequencies are much higher
cm–1 in the expected region. The 11B NMR signal was ob- than those of the previously reported bis(phosphane) cat-
served at δ = 56.1 ppm. Details of the synthetic procedure ionic complexes[9] trans-[Re(CO)4(PPh3)2][BF4] (2000 cm–1),
and characterizations are provided in the Experimental Sec- trans-[Mn(CO)4(PPh3)2][BF4] (2090, 2040, and 1996 cm–1),
tion.
and the bis(monophosphanylborane) cationic complex-
es[10,5c]
trans-[Mn(CO)4{Ph2P(CH2)2B(C8H14)}2][BF4]
(1995 cm–1), trans-[Re(CO)4{Ph2P(CH2)3B(C8H14)}2][BF4]
(1999 cm–1),
trans-[Re(CO)4{Ph2PCH2B(C8H14)}2]-
[B(C6F5)4] (1998 cm–1), and trans-[Re(CO)4{Ph2PCH2B-
(C8H14)}2][B(C6F5)4] (1998 cm–1). This implies that the CO
ligands in 3b, 4b, 5b, and 6b are much more electrophilic
(labile) than the previously reported trans analogs and, con-
sequently, these cationic metal carbonyl complexes should
be suitable for metal-coordinated CO hydrogenations.
Scheme 1. Synthesis of the manganese and rhenium phosphanyl-
borane complexes.
Synthesis of the Cationic Manganese and Rhenium
Carbonyl Complexes
The phosphanylborane complexes 3a, 4a, 5a, and 6a,
containing a bromide ligand, were further reacted with one
equivalent of AgBF4 in dichloromethane in the dark in the
presence of 1 atm CO. The bromide abstraction by Ag+ fol-
lowed by the coordination of a new CO ligand gave the
desired monocationic carbonyl complexes [Mn(CO)4(1)]-
[BF4] (3b), [Re(CO)4(1)][BF4] (4b), [Mn(CO)4(2)][BF4] (5b),
Scheme 2. Synthesis of the cationic manganese and rhenium phos-
phanylborane complexes.
Single crystals of 3b suitable for X-ray diffraction (XRD)
and [Re(CO)4(2)][BF4] (6b, Scheme 2) in analytically pure study were obtained from slow diffusion of pentane into a
form. As the reactions of the rhenium complexes were light saturated dichloromethane solution at –25 °C. The struc-
sensitive, all the reactions were manipulated under complete ture analysis showed the expected chelate coordination of
darkness, and the reactions of the manganese complexes the diphosphanylborane 1 to the manganese center and
were also performed in darkness to avoid the possibility of substitution of the bromide ion by a CO ligand to form
the formation of mixtures of decomposition products. It the monocationic tetracarbonyl manganese complex. The
must be also noted that diastereomeric mixtures of the com- complex cation adopts a distorted octahedral geometry ow-
plex 6a yielded complex 6b containing only a single isomer, ing to the coordination of two PPh2 groups with a relatively
as supported by the 31P{1H} NMR signals at δ = 29.63 (br., small bite angle [P1–Mn1–P2 = 71.53(3)°] and the coordi-
1
1 P) and 46.12 (br., 1 P) ppm. In addition to the shifted H nation of four CO ligands. Two cis CO ligands and two
and 31P{1H} NMR signals, new singlet 19F NMR signals trans CO ligands are present in the structure. The trans C–
between δ = –153 and –156 ppm confirmed the presence of O bond lengths are 1.130(4) and 1.128(4) Å, and the two
–
a BF4 counteranion for all complexes. One sharp singlet cis–C–O bond lengths are 1.127(4) and 1.134(4) Å. The
11B NMR resonance between δ = –0.6 and 3.6 ppm was Mn–P bond lengths are 2.313(1) and 2.305(1) Å for Mn1–
also observed for the boron atom of the BF4– counteranion. P1 and Mn1–P2, respectively. As coordinating solvents were
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avoided throughout the synthesis of 3b, the pendant Lewis tion in toluene, 64.75 μL) at room temperature, the pale yel-
acidic boron center is tricoordinated and is not involved in low solution immediately turned deep red and cloudy owing
any intra- or intermolecular interactions. The presence of to the precipitation of NaBF4. An in situ measured 1H
the BF4– counteranion confirmed that the complex contains NMR spectrum (Figure 3) showed no evidence for the for-
a cationic MnI center. The thermal ellipsoid plot of the mation of a formyl species. Rather, two triplet hydride reso-
1
structure is represented in Figure 2. Selected bond lengths nances were found in the H NMR spectra at δ = –4.96
and angles are provided in Table 1. It is anticipated that the (2JP,H = 40.10 Hz) and –5.66 ppm (2JP,H = 44.02 Hz) with
structure of the remaining three complexes 4b, 5b, and 6b coupling of the hydride nuclei with both of the phosphorus
have similar coordination geometry, as indicated by the atoms of the diphosphanylborane ligand 1. This suggested
exemplary X-ray structure of 3b.
that the η2-binding of the diphosphanylborane ligand 1 in
the complex 3b remains intact in the in situ generated hy-
dride complexes. Owing to the occurrence of two hydride
resonances, we assumed that two isomers of the hydride
complex Mn(H)(CO)3(1) are present in solution with the
hydride ligand either cis or trans to the ring position of
the boryl group. The 31P{1H} NMR spectra revealed the
appearance of several other unidentified decomposition
products in addition to these two hydride species. The same
1
reaction was then repeated at –25 °C and monitored by H
and 31P{1H} NMR spectroscopy with a 300 MHz spec-
trometer. The reaction was very fast even at low tempera-
ture and the color change observed was the same as that of
1
the room temperature reactions. The H NMR spectrum
measured immediately after the addition of NaHBEt3 solu-
tion (1 m in toluene) at –25 °C is shown in Figure 4. In ad-
dition to the hydride species detected at room temperature,
a new formyl proton resonance was observed in the ex-
1
pected downfield region (δ = 13.28 ppm) in the H NMR
spectra. However, the formyl signal disappeared upon
warming the reaction mixture to room temperature, which
demonstrates the instability of the “room temperature” for-
myl species. As no significant changes in the chemical shifts
of the hydride species were observed, it is believed that the
hydride species formed at room temperature and at –25 °C
are same.
Figure 2. Thermal ellipsoid plot of the crystal structure obtained
from the single-crystal X-ray measurements of [Mn(CO)4(1)][BF4]·
CH2Cl2 (3b·CH2Cl2). Phenyl rings on phosphorus atoms and all
the hydrogen atoms are omitted for clarity.
Table 1. Selected bond lengths [Å] and angles [°] for 3b.
Bond
Length
Bonds
Angle
Mn1–P1
Mn1–P2
C1–O1
C2–O2
C3–O3
C4–O4
2.313(1)
2.305(1)
1.130(4)
1.127(4)
1.128(4)
1.134(4)
P1–Mn1–P2
P1–Mn1–C2
P2–Mn1–C4
C2–Mn1–C4
C3–Mn1–C1
71.53(3)
The reaction mixture was then transferred to a glove box,
filtered through a celite bed, and the solvents were removed
completely. Recrystallization of the solid mixture of com-
pounds from a chlorobenzene/pentane solution produced
few yellow single crystals suitable for X-ray diffraction stud-
ies. The hydride complex fac,cis-[Mn(H)(CO)3(1)] (3c) crys-
98.54(10)
96.38(10)
93.61(14)
178.18(13)
¯
tallizes in the triclinic P1 space group. The hydride atom
was located in a difference Fourier map and freely refined.
All other hydrogen positions were calculated after each cy-
cle of refinement by using a riding model. The thermal el-
Reactions of [Mn(CO)4(1)][BF4] (3b) and [Re(CO)4(1)][BF4]
(4b) with NaHBEt3
When a [D5]chlorobenzene solution of 3b (50 mg, lipsoid plot of the crystal structure is given in Figure 5. As
0.065 mmol) was treated dropwise with NaHBEt3 (1 m solu- expected from the in situ measured 31P{1H}spectrum, the
Figure 3. 1H NMR spectrum (C6D5Cl, 400 MHz, room temp., in situ) measured immediately after mixing a C6D5Cl solution of 3b
(50 mg, 0.065 mmol) with NaHBEt3 (1 m solution in toluene, 64.75 μL) at room temperature.
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1
Figure 4. H NMR spectrum (C6D5Cl, 300 MHz, –25 °C, in situ) measured immediately after mixing a C6D5Cl solution of 3b (50 mg,
0.065 mmol) with NaHBEt3 (1 m solution in toluene, 64.75 μL) at –25 °C.
diphosphanylborane ligand 1 is present as a chelator to the HMn atom is cis to the phosphane groups. The Mn1–H
manganese center, and one of the labile CO groups (trans bond length is 1.54 Å, and the Mn1–P1 and Mn1–P2 bond
to each other) was replaced by a hydride ligand. Two CO lengths are 2.2639(4) and 2.2557(4) Å, respectively. The P1–
groups remain cis to each other and trans to the phosphane Mn1–P2 bite angle is 73.05(2)°, which is very close to that
groups, and another CO ligand and the HMn are trans. The in the crystal structure of the cationic complex 3b. The teth-
ered Lewis acidic B(C8H14) group was found to be intact in
the complex, and remains free in the coordination sphere
without involvement in intra- or intermolecular interac-
tions. Selected bond lengths and angles are provided in
Table 2.
Table 2. Selected bond lengths [Å] and angles [°] for 3c.
Bond
Length
Bonds
Angle
Mn1–P1
Mn1–P2
C1–O1
C2–O2
C3–O3
Mn1–H
2.2639(4)
2.2557(4)
1.148(2)
1.142(2)
1.146(2)
1.54(2)
P1–Mn1–P2 73.05(2)
C2–Mn1–C3 92.52(8)
P1–Mn1–C2 94.84(6)
P2–Mn1–C3 96.19(6)
C1–Mn1–H 175.2(9)
Figure 5. Thermal ellipsoid plot of the crystal structure obtained
from the single-crystal X-ray measurements of fac,cis-[Mn(H)-
(CO)3(1)] (3c). Phenyl rings on phosphorus atoms and all the hy-
drogen atoms except the hydride ligand are omitted for clarity.
In a similar way, when a C6D5Cl solution of 4b (50 mg,
0.055 mmol) was treated with NaHBEt3 (1 m solution in
Figure 6. 1H NMR spectrum (C6D5Cl, 400 MHz, room temp., in situ) measured immediately after mixing a C6D5Cl solution of 4b
(50 mg, 0.055 mmol) with NaHBEt3 (1 m solution in toluene, 55.34 μL) at room temperature.
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toluene, 55.34 μL) at room temperature, the colorless solu-
tion immediately turned yellow, which indicated the forma-
tion of a new species in solution. An in situ 1H NMR spec-
tra showed a singlet resonance at δ = 13.59 ppm corre-
sponding to a metal formyl species [Re(CHO)(CO)3(1)] and
a triplet hydride resonance at δ = –3.68 ppm (2JP,H
=
26.4 Hz) corresponding to a rhenium hydride complex
Scheme 3. Reduction of [Mn(CO)4(2)](BF4) (5b) and [Re(CO)4-
(2)](BF4) (6b) with NaHBEt3 (1 m solution in toluene) at room tem-
perature.
[Re(H)(CO)3(1)] analogous to the previously discussed
1
manganese complex fac,cis-[Mn(H)(CO)3(1)] (3c). The H
NMR spectrum obtained from the reaction mixture is
shown in Figure 6. The hydride species could be formed
either by decarbonylation of the formyl species or by substi-
tution of one of the labile CO ligands from 4b by a hydride
ion from the NaHBEt3 source. In addition to the mentioned
formyl and hydride species, several other unidentified de-
composition products were detected in the 31P{1H} NMR
spectrum. After one day at room temperature, the 1H NMR
spectrum of the reaction mixture showed the presence of
both the formyl and the hydride species in solution and
additional new decomposition products. The reaction at
–25 °C gave similar products including the metal formyl
and metal hydride species. Unfortunately, neither the formyl
species nor the hydride species could be isolated from the
reaction mixture and analyzed further.
counteranion to form NaBF4, which precipitated in chloro-
benzene solution. The in situ reaction monitored by 1H and
31P{1H} NMR over a few days at room temperature
showed no evidence for decomposition products. As the for-
myl species was stable in solution over a few days, the reac-
tion mixture was filtered through a celite bed, all the sol-
vents were removed completely under reduced pressure, and
the obtained solid was washed several times with pentane.
The analytically pure formyl complex could be isolated and
stored at –25 °C for at least a few weeks. The 31P{1H}
3
NMR signals of 5b were shifted to δ = 87.89 (d, JP,P
=
3
22.7 Hz) and 88.69 ppm (d, JP,P = 22.7 Hz) upon hydride
transfer to the CO ligand. A typical 11B NMR signal was
found at δ = 43.6 ppm [br., B(C8H14)]. The shift of ca.
40 ppm with respect to the freely pendant boron group in
5b [δ(11B) = 87.10 ppm] clearly indicates a tetrahedral boron
atom and coordination of the boron center to the formyl
group.
Stoichiometric Reduction of [Mn(CO)4(2)][BF4] (5b) and
[Re(CO)4(2)][BF4] (6b) with NaHBEt3
In a Young NMR tube, a NaHBEt3 solution (1 m in tolu-
The crystal structure of 5c was also determined (thermal
ene, 135.66 μL, 0.135 mmol) was added dropwise to a ellipsoid plot shown in Figure 8). The single crystals for X-
C6D5Cl solution (0.5 mL) of [Mn(CO)4(2)][BF4] (5b, ray diffraction were grown from the diffusion of pentane
100 mg, 0.135 mmol), and the mixture was shaken vigor- into a saturated chlorobenzene solution of 5c at room tem-
1
ously. From the in situ monitoring by H NMR, the stoi- perature. The structural analysis revealed the identity of the
chiometric conversion to
a
single formyl species neutral manganese formyl complex, which contains the che-
[Mn(CHO)(CO)3(2)] (5c, Scheme 3) was observed in ad- lating diphosphanylborane ligand 2, two CO ligands (cis to
dition to the precipitation of the NaBF4 salt. The down- each other and trans to the PPh2 groups), and another CO
field-shifted broad singlet 1H NMR resonance at δ = ligand and the formyl group (trans to each other). As ex-
14.59 ppm was assigned to the formyl proton of the ex- pected, one of the trans CO ligands was found to be con-
pected manganese formyl species 5c (Figure 7). In addition, verted into the formyl group and remained attached to the
the absence of a 19F NMR signal indicated the formation metal center as a ligand. The distance between the formyl
–
of a neutral formyl species and the removal of the BF4
carbon atom and the manganese center (Mn1–C4) is
Figure 7. 1H NMR spectrum (C6D5Cl, 400 MHz, room temp., formyl region) of the manganese formyl complex [Mn(CHO)(CO)3(2)]
(5c).
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1.988(1) Å, which is comparable to that of the previously Table 3. Bond lengths [Å] and angles [°] for 5c.
reported neutral manganese formyl complex trans-[Mn-
Bond
Length
Bonds
Angle
(PPh3)2(CO)3(CHOB{C6F5}3)][10] [Mn–C4 = 1.994(2) Å]
with an intermolecular B–O interaction in the secondary
sphere. Also in the present case, the tethered Lewis acidic
Mn1–C4
Mn1–C1
Mn1–C2
1.988(1)
1.839(2)
1.813(2)
1.813(2)
2.2860(4)
2.2876(4)
1.027(16)
1.265(2)
1.140(2)
1.134(2)
1.143(2)
1.599(2)
P1–Mn1–P2
C2–Mn1–C3
P1–Mn1–C2
P2–Mn1–C3
Mn1–C4–O4
C4–O4–B1
C17–B1–O4
C4–Mn1–P1
C4–Mn1–P2
C4–Mn1–C1
87.19(4)
89.61(7)
93.94(5)
91.86(5)
135.61(10)
133.02(11)
109.41(10)
83.33(4)
87.19(4)
175.70(7)
B(C8H14) group is involved in intramolecular coordination Mn1–C3
Mn1–P1
from the formyl oxygen atom to the Lewis acidic boron
Mn1–P2
center of the tethered B(C8H14) group. The B1–O4 distance
C4–H4
is 1.599(2) Å. The related distance for the above reported
C4–O4
complex was 1.593 Å. The formyl C4–O4 bond length is
1.265(2) Å, which is much longer than the CϵO triple bond
of a CO group. The C1–O1 bond length trans to the formyl
group is 1.140(2) Å, and the C2–O2 and C3–O3 bond
lengths are 1.143(2) and 1.134(2) Å, respectively. The di-
phosphanylborane ligand 2 showed a tridentate type chela-
tion through the phosphorus atoms and the coordinated
boron atom, which led to a five-membered, a six-mem-
bered, and a seven-membered chelate ring. The P1–Mn1–
P2 bite angle is 84.57(1)°, the Mn1–C4–O4 angle is
135.61(10)°, the C4–O4–B1 angle is 133.02(11)°, and the
C4–Mn1–C1 angle is 175.70(7)°. All selected bond lengths
and angles are given in Table 3.
C1–O1
C3–O3
C2–O2
O4–B1
broad 11B NMR signal was found at δ = 51.2 ppm for the
9-boranorbornyl group B(C8H14), which indicates a freely
pendant boron Lewis acid not coordinated to the formyl
group. This observation emphasizes the importance of an
exact match of the ring size to establish the coordination of
the boron center to the formyl group. Several attempts to
crystallize the formyl species 6c and characterize it by X-
ray diffraction remained unsuccessful because of the oily
consistence of the precipitate from chlorobenzene/pentane
solutions.
Figure 8. Thermal ellipsoid plot of the crystal structure obtained
from the single-crystal X-ray measurements of [Mn(CHO)(CO)3(2)]·
0.5(C6H5Cl) [5c·0.5(C6H5Cl)]. The solvent molecule C6H5Cl, the
phenyl rings on the phosphorus atoms, and all hydrogen atoms
except the formyl H atom are omitted for clarity.
Figure 9. Formyl region of the 1H NMR spectrum (C6D5Cl,
400 MHz, room temperature) of the rhenium formyl complex [Re-
(CHO)(CO)3(2)] (6c).
The addition of one equivalent of NaHBEt3 (1 m in tolu-
ene) to a C6D5Cl solution of [Re(CO)4(2)][BF4] (6b,
1
Scheme 3) led to the appearance of a new H NMR reso-
nance at δ = 15.06 (Figure 9) and 31P NMR resonances at
δ = 40.67 and 45.93 ppm, in addition to the precipitation
Conclusions
1
of NaBF4. The downfield-shifted H NMR resonance at δ
This work is the first report on the synthesis, characteri-
= 15.06 ppm was attributed to the proton of the expected zation, and reactivity of manganese(I) and rhenium(I) carb-
rhenium formyl species [Re(CHO)(CO)3(2)] (6c). The for- onyl complexes with chelating diphosphanylborane ligands.
myl complex is stable in solution over a few weeks. Follow- The cis-enforcing nature of the diphosphanylborane ligands
ing a workup procedure similar to that for 5c, the analyti- makes the metal-bound CO ligands even more labile in
cally pure rhenium formyl complex 6c was obtained. A comparison to those in the complexes of monodentate
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tor) by using a single wavelength Enhance X-ray source with Mo-
Kα radiation (λ = 0.71073 Å).[11] The selected suitable single crys-
tals were mounted by using polybutene oil on the top of a glass
fiber fixed on a goniometer head and immediately transferred to
the diffractometer. Pre-experiment, data collection, data reduction,
and analytical absorption corrections[12] were performed with the
CrysAlisPro program suite.[11] The crystal structures were solved
with SHELXS97[13] by using direct methods. The structure refine-
ments were performed by full-matrix least-squares on F2 with
SHELXL97.[13] All programs used during the crystal structure de-
phosphane ligands. The cationic complexes 3b, 4b, 5b, and
6b containing highly electrophilic CO groups (νCO
≈ 2100 cm–1) were prepared and fully characterized. All
four cationic complexes were tested for stoichiometric hy-
dride transfer to metal-bound CO by reacting them with
NaHBEt3. The reduction of the manganese complex 3b and
rhenium complex 4b gave mixtures of products that in-
cluded formyl species and hydride species. The correspond-
ing formyl complexes were unstable, apparently because of
a lack of stabilization either from the metal center or by termination process are included in the WINGX software.[14]
PLATON[15] was used to check the result of the X-ray analyses.
intramolecular attachment of the boron Lewis acid to the
formyl group. In accord with general observations on for-
myl complexes, the heavy metal rhenium formyl complexes
appear to be more stable than the manganese formyl com-
plexes, and parallel to this, the four-membered MPCP ring
seemed to be too rigid to allow the boron center to reach
the formyl group in the corresponding formyl complexes
and to achieve additional stabilization. However, reduction
of 5b and 6b produced the stable formyl species 5c and 6c
upon treatment with NaHBEt3. The structural analysis of
5c revealed that one of the trans CO groups in 5b and 6b
was attacked by the hydride reagent to build up a formyl
group, which is further stabilized by a strong intramolecular
B–O bond formation. The backbone of the coordinated PB
ligand 1 is too rigid and it does not support formyl forma-
tion, whereas ligand 2 with a more flexible backbone in the
metal-coordinated form was suited to the stabilization of
the manganese and rhenium formyl unit through a correct
fit of its structure, which enabled the coordination of the
formyl oxygen atom to the Lewis acidic boron center.
CCDC-929474 (for 3b), -929475 (for 3c), and -929476 (for 5c) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
General Procedure for the Synthesis of 3a, 4a, 5a, and 6a: A weighed
quantity of Mn(CO)5Br or Re(CO)5Br, an equimolar amount of
diphosphanylborane ligand 1 or 2, and THF were placed in sealed
tubes under a nitrogen atmosphere. The reaction mixtures were
heated to reflux for ca. 3 h at 80 °C. The sealed tubes were cooled
to room temperature and transferred to the glove box. All solvents
were removed under vacuum to obtain yellow (for manganese com-
plexes) and white solids (for rhenium complexes). The obtained
solids were extracted several times with diethyl ether. The combined
ether extracts were concentrated to minimum volume and kept at
–25 °C to obtain crystals. The crystals obtained from four different
reactions were characterized by 1H, 31P, 11B NMR, and IR spec-
troscopy and elemental analyses.
fac,cis-Mn(CO)3[Ph2PCH(PPh2)CH2B(C8H14)](Br) (3a): Yield
86%. FTIR (solid, ATR): ν = 2013 (vs, νCO), 1942 (vs, νCO), 1901
˜
(vs, νCO) cm–1. 31P{1H} NMR ([D8]THF, 162 MHz): δ = 36.33 (s,
PPh2) ppm. 11B NMR ([D8]THF, 96.28 MHz): δ = 58.3 [br.,
B(C8H14)] ppm. 1H NMR ([D8]THF, 400 MHz): δ = 7.36–7.66 (m,
20 H, PhH), 2.25 (m, 1 H, CH), 2.14 (m, 2 H, CH2), 1.78–1.83 [m,
14 H, B(C8H14)] ppm. C37H37BBrMnO3P2 (737.29): calcd. C 60.27,
H 5.06; found C 60.38, H 5.17.
Experimental Section
General Considerations: All experiments were performed under an
atmosphere of nitrogen by using either dry glove box or Schlenk
techniques. Reagent grade solvents were dried with sodium benzo-
phenone (tetrahydrofuran, diethyl ether, and pentane) and calcium
hydride (dichloromethane and chlorobenzene) and distilled prior
to use under a N2 atmosphere. Deuterated solvents were dried with
sodium benzophenone ketyl ([D8]THF) or calcium hydride
(C6D5Cl, CD2Cl2) and distilled by a freeze–pump–thaw cycle prior
to use. NaHBEt3 (1 m toluene solution), 9-borabicyclo[3.3.1]-
nonane (0.5 m solution), Mn(CO)5Br, and Re(CO)5Br were pur-
chased from commercially available sources and used without fur-
ther purifications. NMR spectra were measured with Varian Gem-
ini-300 (300 MHz for 1H and 96.28 MHz for 11B) and Bruker DRX
fac,cis-Re(CO)3[Ph2PCH(PPh2)CH2B(C8H14)](Br) (4a): Yield
93%. FTIR (solid, ATR): ν = 2019 (vs, νCO), 1936 (vs, νCO), 1895
˜
(vs, νCO) cm–1. 31P{1H} NMR ([D8]THF, 162 MHz): δ = –14.75 (s)
ppm. 11B NMR ([D8]THF, 96.28 MHz): δ = 53.0 [br., B(C8H14)]
ppm. 1H NMR ([D8]THF, 400 MHz): δ = 7.37–7.67 (m, 20 H,
PhH), 2.19 (m, 1 H, CH), 2.01 (m, 2 H, CH2), 1.54–1.82 [m, 14 H,
B(C8H14)] ppm. C37H37BBrO3P2Re (868.56): calcd. C 51.16, H
4.29; found C 51.38, H 4.21.
fac,cis-Mn(CO)3{Ph2PCH2CH[B(C8H14)]PPh2}(Br) (5a): Yield
78%. FTIR (solid, ATR): ν = 2010 (vs, νCO), 1940 (vs, νCO), 1911
˜
(vs, νCO) cm–1. 31P{1H} NMR ([D8]THF, 162 MHz): δ = 67.20 (d,
3JP,P = 22.7 Hz, 1 P), 86.49 (d, 3JP,P = 22.7 Hz, 1 P) ppm. 11B NMR
([D8]THF, 96.28 MHz): δ = 51.3 [br., B(C8H14)] ppm. 1H NMR
([D8]THF, 400 MHz): δ = 6.95–8.05 (m, 20 H, PhH), 3.62 (m, 1 H,
CH), 3.22 (m, 2 H, CH2), 1.32–1.84 [m, 14 H, B(C8H14)] ppm.
C37H37BBrMnO3P2 (737.29): calcd. C 60.27, H 5.06; found C
59.98, H 4.93.
1
1
400 spectrometers (400.1 MHz for H, 162.0 MHz for 31P). All H
chemical shifts are expressed in ppm relative to tetramethylsilane
(TMS), 31P{1H} chemical shifts are relative to 85% H3PO4, and
11B chemical shifts are relative to BF3·OEt2 as an external stan-
dard. Signal patterns are as follows: s singlet, d doublet, t triplet,
q quartet, m multiplet. IR spectra were obtained either by attenu-
ated total reflectance (ATR) or KBr methods with a Bio-rad FTS-
45 instrument. Elemental analyses were carried out at the Anorgan-
isch-Chemisches Institut of the University of Zurich.
fac,cis-Re(CO)3{Ph2PCH2CH[B(C8H14)]PPh2}(Br) (6a): Yield
96%. FTIR (solid, ATR): ν = 2021 (vs, νCO), 1938 (vs, νCO), 1897
˜
(vs, νCO) cm–1. 31P{1H} NMR ([D8]THF, 162 MHz): Major (63%):
3
3
Crystal Structure Determination: The data collection and structure-
refinement data for 3b, 3c, and 5c are presented in Table 4. Single-
crystal X-ray diffraction data were collected at 183(2) K with an
Xcalibur diffractometer (Agilent Technologies, Ruby CCD detec-
δ = 26.14 (d, JP,P = 8 Hz, 1 P), 48.55 (1P, d, JP,P = 8 Hz); minor
3
3
(37%): δ = 30.37 (d, JP,P = 16 Hz, 1 P), 44.82 (d, JP,P = 16 Hz, 1
P) ppm. 11B NMR ([D8]THF, 96.28 MHz): δ = 56.1 [br., B(C8H14)]
ppm. 1H NMR ([D8]THF, 400 MHz): δ = 7.13–7.85 (m, 20 H,
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Table 4. Crystallographic data for 3b, 3c, and 5c.
3b
3c
5c
Empirical formula
Formula weight /gmol–1
Temperature /K
Wavelength /Å
Crystal system, space group
a /Å
b /Å
c /Å
α /°
C38H37B2F4MnO4P2·CH2Cl2
C37H38BMnO3P2
658.36
183(2)
(C38H38BMnO4P2)2·C6H5Cl
1485.30
183(2)
857.10
183(2)
0.71073
monoclinic, P21/c
10.6759(4)
35.1572(8)
11.8633(5)
90
0.71073
triclinic, P1
0.71073
triclinic, P1
¯
¯
9.8427(5)
9.9301(4)
18.8448(6)
98.206(3)
8.6414(2)
10.5320(2)
21.0251(5)
81.103(2)
β /°
116.535(5)
90
3983.7(3)
4, 1.429
0.604
95.053(3)
87.009(2)
74.356(2)
1820.35(7)
1, 1.355
0.529
γ /°
110.305(4)
1690.84(12)
2, 1.293
0.520
688
Volume /Å3
Z, density (calcd.) /Mgm–3
Abs. coefficient /mm–1
F(000)
1760
774
Crystal size /mm
θ range /°
0.50ϫ0.45ϫ0.28
3.01–30.51
52370
12120/Rint = 0.0418
99.7
0.38ϫ0.35ϫ0.29
2.83–30.51
31391
10292/Rint = 0.0406
99.9
analytical
0.900 and 0.866
8616/0/401
1.034
0.0429, 0.1129
0.0529, 0.1208
0.666, –0.305
0.50ϫ0.26ϫ0.15
2.94–30.51
33454
11072/Rint = 0.0407
99.7
analytical
0.917 and 0.773
9318/0/455
1.046
0.0387, 0.0992
0.0481, 0.1060
0.546, –0.363
Reflections collected
Unique reflections
Completeness to θ /%
Absorption correction
Max. and min. transmission
Data/restraints/parameters
Goodness-of-fit on F2
analytical
0.896 and 0.808
10000/36/515
1.088
Final R1 and wR2 indices [IϾ2σ(I)][a] 0.0770, 0.1861
R1 and wR2 indices (all data)
0.0919, 0.1966
1.116, –0.729
Largest diff. peak and hole /eÅ–3
[a] The unweighted R factor is R1 = Σ(Fo – Fc)/ΣFo; IϾ2σ(I) and the weighted R factor is wR2 = [Σw(Fo – Fc ) /Σw(Fo2)2]1/2
.
2
2 2
PhH), 3.09 (m, 2 H, CH2), 2.63 (m, 1 H, CH), 1.63–1.75 [m, 14 H, (CD2Cl2, 300 MHz): δ = 86.7 [br., B(C8H14)], 0.99 (s, BF4) ppm.
B(C8H14)] ppm. C37H37BBrO3P2Re (868.56): calcd. C 51.16, H
4.29; found C 51.53, H 4.48.
1H NMR (CD2Cl2, 400 MHz): δ = 7.29–8.44 (m, 20 H, PhH), 2.74
(m, 3 H, CH and CH2), 1.27–1.73 [m, 14 H, B(C8H14)] ppm. 19F
NMR (CD2Cl2, 376.5 MHz):
C38H37B2F4O4P2Re (903.47): calcd. C 50.52, H 4.13; found C
50.38, H 4.13.
δ = –153.03 (s, BF4) ppm.
General Procedure for the Synthesis of Cationic Manganese and
Rhenium Complexes 3b, 4b, 5b, and 6b: A Teflon-stoppered J. Young
Schlenk tube was charged with the bromide precursor, AgBF4
(1 equiv.), a stirbar, and CH2Cl2 (50 mL). The vessels were sealed
and removed from the glove box. The reaction mixtures were imme-
diately frozen in liquid nitrogen, evacuated to degas, and 1.5 atm
CO was introduced to the vessels. After the injection of CO gas,
the vessels were covered with aluminum foil to protect the reaction
mixtures from light. After 30 min, the yellow solution became col-
orless (for manganese complexes) and precipitation of AgBr oc-
curred, and then the mixtures were stirred overnight at room tem-
perature. The reaction mixtures were boil-degassed to remove ex-
cess CO and filtered through a small celite bed to afford a pale
yellow (manganese complexes) or colorless (rhenium complexes)
solution. The filtrates were dried in vacuo to afford the desired
white or pale yellow cationic manganese tetracarbonyl or white cat-
ionic rhenium tetracarbonyl complexes.
[Mn(CO)4{Ph2PCH2CH{B(C8H14)}PPh2}][BF4] (5b): Yield 84%.
FTIR (solid, KBr): ν = 2092(vs, νCO), 2001(vs, νCO) cm–1. 31P{1H}
˜
NMR (CD2Cl2, 162 MHz): δ = 74.14 (br s, 1 P, PPh2), 87.58 (br s,
1 P, PPh2) ppm. 11B NMR (CD2Cl2, 96.28 MHz): δ = 87.10 [br.,
B(C8H14)], –0.62 (s, BF4) ppm. 19F NMR (CD2Cl2, 376.5 MHz): δ
= –153.01 (s) ppm. 1H NMR (CD2Cl2, 400 MHz): δ = 7.43–7.62
(m, 20 H, PhH), 3.91 (m, 1 H, CH), 2.86 (m, 2 H, CH2), 1.45–1.82
[m, 14 H, B(C8H14)] ppm. C38H37B2F4MnO4P2 (772.20): calcd. C
59.10, H 4.83; found C 59.18, H 4.71.
[Re(CO)4{Ph2PCH2CH{B(C8H14)}PPh2}][BF4] (6b): Yield 68%.
FTIR (solid, ATR): ν = 2110 (vs, νCO), 2001 (vs, νCO) cm–1.
˜
31P{1H} NMR (CD2Cl2, 162 MHz): δ = 29.63 (br., 1 P), 46.12 (br.,
1 P) ppm. 11B NMR (CD2Cl2, 96.28 MHz): δ = 64.7 [br.,
B(C8H14)], 0.41 (BF4) ppm. 1H NMR (CD2Cl2, 400 MHz): δ =
7.47–7.71 (m, 20 H, PhH), 3.28 (m, 1 H, CH), 2.75 (m, 2 H, CH2),
1.43–1.65 [m, 14 H, B(C8H14)] ppm. 19F NMR (CD2Cl2,
[Mn(CO)4{Ph2PCH(PPh2)CH2B(C8H14)}][BF4] (3b): Yield 73%.
FTIR (solid, ATR): ν = 2091 (vs, νCO), 1987 (vs, νCO) cm–1.
˜
31P{1H} NMR (CD2Cl2, 162 MHz): δ = 31.68 (s, PPh2) ppm. 11B
188.14 MHz):
δ
=
–152.69
(s,
BF4)
ppm.
NMR (CD2Cl2, 96.28 MHz): δ = 87.0 [br., B(C8H14)], 3.6 (s, BF4) C38H37B2F4O4P2Re·CH2Cl2 (903.47 + 84.9): calcd. C 47.35, H
ppm. 1H NMR (CD2Cl2, 400 MHz): δ = 7.41–7.73 (m, 20 H, PhH), 3.98; found C 47.18, H 3.73.
1.90 (m, 1 H, CH), 2.12 (m, 2 H, br., CH2), 1.36–1.86 [m, 14 H,
Reduction of [Mn(CO)4{Ph2PCH(PPh2)CH2B(C8H14)}][BF4] (3b)
B(C8H14)] ppm. 19F NMR (CD2Cl2, 376.5 MHz): δ = –156.03 (s,
with NaHBEt3: NaHBEt3 (1 m solution in toluene, 64.75 μL,
BF4) ppm. C38H37B2F4MnO4P2 (772.20): calcd. C 59.10, H 4.83;
0.065 mmol) was added to a Young NMR tube containing a solu-
found C 58.94, H 4.93.
tion of 3b (50 mg, 0.065 mmol) in C6D5Cl (ca. 0.5 mL), and the
[Re(CO)4{Ph2PCH(PPh2)CH2B(C8H14)}][BF4] (4b): Yield 71%.
mixture was stirred vigorously before the NMR measurements. Im-
mediately after mixing, the pale yellow solution turned deep red
and conversion to two hydride species was observed in addition to
FTIR (solid, KBr): ν = 2108 (vs, νCO), 1982 (vs, νCO) cm–1. 31P{1H}
˜
NMR (CD2Cl2, 162 MHz): δ = –20.95 (s, PPh2) ppm. 11B NMR
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several other unidentified products. Two triplet hydride resonances
were found at δ = –4.96 and –5.66 ppm in the in situ measured H
(m, 20 H, PhH), 3.16–3.39 (m, 3 H, CH and CH2), 1.50–1.88 [m,
14 H, B(C8H14)] ppm. C38H37BF4MnO4P2 (761.15): calcd. C 59.91,
H 4.90; found C 61.05, H 5.41.
1
NMR spectrum. The hydride species were identified as two isomers
of the manganese complex [Mn(H)(CO)3(1)], in which the hydride
ligand can either be cis or trans to the ring position of the boryl
group. A formyl resonance was also observed along with the two
triplet hydride resonances when the same reaction was repeated
at –25 °C. However, only the formyl resonance disappeared upon
warming of the reaction mixture to room temperature. After fil-
tration through a small celite bed followed by removal of the sol-
vents, the reaction mixture gave a red solid mixture of compounds.
The solid mixture was then recrystallized from a chlorobenzene/
pentane solution to obtain a few red single crystals of the manga-
nese hydride complex fac,cis-[Mn(H)(CO)3(1)] (3c) suitable for sin-
gle-crystal X-ray diffraction studies. However, all the other prod-
ucts formed could not be isolated and analyzed separately. Some
selected in situ measured NMR values are reported here. 1H NMR
(C6D5Cl, 400 MHz, reaction at room temperature): δ = –4.96 (t,
Reduction of [Re(CO)4{Ph2PCH2CH(B{C8H14})PPh2}][BF4] (6b)
with NaHBEt3: In a Young NMR tube, complex 6b (100 mg,
0.111 mmol) was dissolved in C6D5Cl (0.5 mL). NaHBEt3 (1 m
solution in toluene, 110.68 μL, 0.135 mmol) was added dropwise,
and the mixture was stirred vigorously. The in situ measured 1H
and 31P{1H} NMR spectra indicated the stoichiometric conversion
to the desired formyl complex [Re(CHO)(CO)3(2)] (6c). The formyl
complex is stable in solution over a few weeks. The workup was
performed in a similar procedure as for 5c to obtain analytically
pure rhenium formyl complex 6c, yield 68%. FTIR (solid, ATR):
ν = 2001 (vs, νCO), 1905 (vs, νCO), 1860 (vs, νCO) cm–1. 31P{1H}
˜
NMR (C6D5Cl, 162 MHz): δ = 40.67 (s, 1 P, PPh2), 45.93 (s, 1 P,
PPh2) ppm. 11B NMR (C6D5Cl, 96.28 MHz): δ = 51.2 [br.,
1
B(C8H14)] ppm. H NMR (C6D5Cl, 400 MHz): δ = 15.06 (s, 1 H,
formyl), 7.25–7.47 (m, 20 H, PhH), 2.65 (m, 2 H, CH2), 3.12 (m, 1
H, CH), 1.60–2.08 [m, 14 H, B(C8H14)] ppm. C38H38BO4P2Re
(817.43): calcd. C 55.78, H 4.68; found C 55.62, H 4.53.
2
2JP,H = 40.10 Hz, hydride), –5.66 (t, JP,H = 44.02 Hz, hydride)
ppm. 31P{1H} NMR (C6D5Cl, 162 MHz, reaction at room tem-
perature): δ = 74.00, 58.04, 56.42 (major), 52.28, 40.77, 39.14, 4.28,
–0.61 (major), –8.44 and –8.69 ppm. 1H NMR (C6D5Cl, 300 MHz,
reaction at –25 °C): δ = 13.28 (d, J = 20.7 Hz, formyl species),
–4.50 (t, 2JP,H = 40.8 Hz, hydride species), –5.70 (t, 2JP,H = 45.6 Hz,
hydride species) ppm.
Acknowledgments
Financial support from the Swiss National Science Foundation
(SNF), Lanxess AG, Leverkusen, Germany, the Funds of the Uni-
versity of Zurich, and the Deutsche Forschungsgemeinschaft
(DFG) and SNF within the project “Forschergruppe 1175 – Un-
conventional Approaches to the Activation of Dihydrogen” are
gratefully acknowledged.
Reduction of [Re(CO)4{Ph2PCH(PPh2)CH2B(C8H14)}](BF4) (4b)
with NaHBEt3: To a Young NMR tube containing a solution of 4b
(50 mg, 0.055 mmol) in C6D5Cl (ca. 0.5 mL), NaHBEt3 (1 m solu-
tion in toluene, 55.34 μL, 0.055 mmol) was added, and the mixture
was stirred vigorously before the NMR measurements. The color-
less solution immediately turned yellow, and conversion to a formyl
{likely to be [Re(CHO)(CO)3(1)]} and a hydride species was ob-
served in addition to other unidentified decomposition products.
The triplet hydride resonance confirmed the presence of the diphos-
phanylborane ligand 1 in the hydride complex. Therefore, the hy-
dride species was identified as the rhenium hydride complex
[Re(H)(CO)3(1)], analogous to the manganese hydride complex
fac,cis-[Mn(H)(CO)3(1)] (3c). However, the products formed in the
reaction mixture could not be isolated and analyzed separately. Se-
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lected in situ measured data: H NMR (C6D5Cl, 400 MHz, reac-
tion at room temperature): δ = 13.59 (s, formyl species), –3.68 (t,
2JP,H = 26.4 Hz, hydride species) ppm. 31P{1H} NMR (C6D5Cl,
162 MHz, reaction at room temperature): δ = 37.33, 32.52, 29.60,
–2.79, –6.33, –7.45, –11.04, –13.79, –17.01, –18.99, –43.25 (major)
ppm.
Reduction of [Mn(CO)4{Ph2PCH2CH(B{C8H14})PPh2}][BF4] (5b)
with NaHBEt3: In a Young NMR tube, 5b (100 mg, 0.135 mmol)
was dissolved in C6D5Cl (0.5 mL). NaHBEt3 (1 m solution in tolu-
ene, 135.66 μL, 0.135 mmol) was added dropwise, and the mixture
was stirred vigorously. The in situ measured 1H and 31P{1H} NMR
spectra indicated the stoichiometric conversion to the desired for-
myl complex [Mn(CHO)(CO)3(2)] (5c). The formyl complex was
stable in solution over a few weeks. The reaction mixture was trans-
ferred to the glove box, filtered through a small celite bed, and all
the solvents were removed to obtain a yellow color solid. The solid
was washed several times with pentane and dried under reduced
pressure. The complex was recrystallized several times from a chloro-
benzene/pentane solution to obtain the pure formyl complex 5c,
yield 51%. FTIR (solid, ATR): ν = 2013 (vs, νCO), 1947 (vs, νCO),
˜
1920 (vs, νCO) cm–1. 31P{1H} NMR (C6D5Cl, 162 MHz): δ = 87.89
3
3
(d, JP,P = 22.7 Hz, 1 P), 88.69 (d, JP,P = 22.7 Hz, 1 P) ppm. 11B
NMR (C6D5Cl, 96.28 MHz): δ = 43.6 [br., B(C8H14)] ppm. 1H
NMR (C6D5Cl, 400 MHz): δ = 14.59 (s, 1 H, formyl), 7.03–7.54
Eur. J. Inorg. Chem. 2013, 4574–4584
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