Ligand assisted carbon dioxide activation and hydrogenation using molybdenum and tungsten amides

Article abstract of DOI:10.1039/c5dt00278h

The hepta-coordinated isomeric M(NO)Cl₃(PN^HP) complexes {M = Mo, 1a(syn,anti); W, 1b(syn,anti), PN^HP = (iPr₂PCH₂CH₂)₂NH, (H_N atom of PN^HP syn and anti to the NO ligand)} and the paramagnetic species M(NO)Cl₂(PN^HP) (M = Mo, 2a(syn,anti); W, 2b(syn,anti)) could be prepared via a new synthetic pathway. The pseudo trigonal bipyramidal amides M(NO)(CO)(PNP) {M = Mo, 3a; W, 3b; [PNP]^- = [(iPr₂PCH₂CH₂)₂N]^-} were reacted with CO₂ at room temperature with CO₂ approaching the MN double bond in the equatorial (CO,NO,N) plane trans to the NO ligand and forming the pseudo-octahedral cyclic carbamates M(NO)(CO)(PNP)(OCO) (M = Mo, 4a(trans); W = 4b(trans)). DFT calculations revealed that the approach to form the 4b(trans) isomer is kinetically determined. The amine hydrides M(NO)H(CO)(PN^HP) {M = Mo, 5a(cis,trans); W, 5b(cis,trans)}, obtained by H₂ addition to 3a,b, insert CO₂ (2 bar) at room temperature into the M-H bond generating isomeric mixtures of the η¹-formato complexes M(NO)(CO)(PN^HP)(η¹-OCHO), (M = Mo, 6a(cis,trans); M = W, 6b(cis,trans)). Closing the stoichiometric cycles for sodium formate formation the 6a,b(cis,trans) isomeric mixtures were reacted with 1 equiv. of Na[N(SiMe₃)₂] regenerating 3a,b. Attempts to turn the stoichiometric formate production into catalytic CO₂ hydrogenation using 3a,b in the presence of various types of sterically congested bases furnished yields of formate salts of up to 4%. This journal is

Full text of DOI:10.1039/c5dt00278h

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Ligand assisted carbon dioxide activation and
hydrogenation using molybdenum and tungsten
amides†
Cite this: Dalton Trans., 2015, 44,
6560
Subrata Chakraborty, Olivier Blacque and Heinz Berke*
H
The hepta-coordinated isomeric M(NO)Cl
3
(PN P) complexes {M = Mo, 1a(syn,anti); W, 1b(syn,anti),
H
H
PN P = (iPr
2
PCH
2
CH
2
)
2
NH, (H
N
atom of PN P syn and anti to the NO ligand)} and the paramagnetic
H
2
species M(NO)Cl (PN P) (M = Mo, 2a(syn,anti); W, 2b(syn,anti)) could be prepared via a new synthetic
pathway. The pseudo trigonal bipyramidal amides M(NO)(CO)(PNP) {M = Mo, 3a; W, 3b; [PNP]−
=
−
[
(iPr
2
PCH
2
CH
2
)
2
N] } were reacted with CO
2
at room temperature with CO
2
approaching the MvN double
bond in the equatorial (CO,NO,N) plane trans to the NO ligand and forming the pseudo-octahedral cyclic
carbamates M(NO)(CO)(PNP)(OCO) (M = Mo, 4a(trans); W = 4b(trans)). DFT calculations revealed that
the approach to form the 4b(trans) isomer is kinetically determined. The amine hydrides M(NO)H(CO)-
H
(
PN P) {M = Mo, 5a(cis,trans); W, 5b(cis,trans)}, obtained by H
2
addition to 3a,b, insert CO
2
(2 bar) at
1
room temperature into the M–H bond generating isomeric mixtures of the η -formato complexes
M(NO)(CO)(PN P)(η -OCHO), (M = Mo, 6a(cis,trans); M = W, 6b(cis,trans)). Closing the stoichiometric
cycles for sodium formate formation the 6a,b(cis,trans) isomeric mixtures were reacted with 1 equiv. of
H
1
Received 20th January 2015,
Accepted 27th February 2015
Na[N(SiMe
CO hydrogenation using 3a,b in the presence of various types of sterically congested bases furnished
yields of formate salts of up to 4%.
3 2
) ] regenerating 3a,b. Attempts to turn the stoichiometric formate production into catalytic
DOI: 10.1039/c5dt00278h
2
www.rsc.org/dalton
Introduction
Carbon dioxide is one of the main carbon sources and
although it is abundant and inexpensive, its utilization as feed-
1
stock for commodity chemicals is insufficiently explored. CO
2
2
Fig. 1 Possible binding modes of CO to a metal centre.
being a thermodynamically and kinetically stable molecule a
promising approach to overcome its often too low reactivity
2
,3a
would be its metal mediated “activation”.
years have seen tremendous efforts to activate and reduce CO2
and to employ it as a C building block for chemical
In fact recent
1
3
synthesis. Activation by binding to a metal center and
subsequent reaction modes of the CO depend on the type of
2
the CO
nation as depicted in Fig. 1.
Lately metal ligand cooperation (MLC) has been explored as
a way to activate H or small molecules with C–H, N–H, O–H
moieties. MLC would be a promising approach for CO acti-
2 2
binding. There are three possible ways of CO coordi-
1
b
2
2
Fig. 2 Metal–ligand cooperation in CO activation.
4
,5
2
vation, but reported examples of such CO2 activation are
scarce. Only recently Sanford and coworkers have demon-
strated that (PNN)Ru(H)(CO) pincer systems take up CO2 at
room temperature involving C–C bond formation between the
Departent of Chemistry, University of Zürich, Winterthurerstrasse 190, 8057 Zürich,
Switzerland. E-mail: hberke@chem.uzh.ch
†
Electronic supplementary information (ESI) available. CCDC 888517, 888516,
60496 and 960495. For ESI and crystallographic data in CIF or other electronic
unsaturated arm of the pincer ligand and the electrophilic
9
6
format see DOI: 10.1039/c5dt00278h
carbon atom of CO (Fig. 2). Milstein and coworkers investi-
2
6560 | Dalton Trans., 2015, 44, 6560–6570
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2
gated related types of reversible CO binding employing un-
7
saturated PNP pincer complexes of Ru and Re (Fig. 2).
Metal-free” CO activation was accomplished using Fru-
“
2
8
strated Lewis Pair (FLP) type chemistry. In this context it
seems also worth mentioning that a transition metal based
n 6 5 3 2
L Re-H/B(C F ) systems can activate CO in a FLP type
manner, where the metal hydride species took the role of the
9
FLP Lewis base component. Activation of CO is anticipated
2
to be preceding its hydrogenation, which may then lead to
formic acid, but also can continue in the reduction sequence
forming products with higher hydrogen contents, which like
formic acid could in principle function as reversible H
2
storage
to obtain
formic acid is exothermic (ΔH = −8 kcal mol ), it is endergo-
1
0
2 2
materials. Though the reaction of H with CO
−
1
−
1
nic with ΔG = 8 kcal mol due to gaseous starting materials.
H
Scheme 1 Preparation of the isomeric mixture of M(II)(NO)Cl
3
(PN P),
H
Therefore an additional thermodynamic driving force has to 1a,b(syn,anti)) and reduction to M(I)(NO)Cl (PN P), 2a,b(syn,anti) com-
2
be provided, like for instance pressure and the addition of a plexes (M = Mo,W).
base inducing salt formation or stabilizing acid–base hydrogen
3
bonding interactions causing a shift of the equilibrium
towards the formic acid side.
Advances in the efficiency of homogeneous hydrogenation both 1a,b(syn,anti) are given in Scheme 1. The isomers of 1a-
catalysis of CO
2
were recently reported with late transition (syn,anti) and 1b(syn,anti) were found to be reasonably soluble
1
1,12
31
1
metal complexes,
opments it is one of the aims of this research to replace mixture of 1a(syn,anti) revealed two singlets at δ 69.3 ppm (1a-
2 2
but somewhat different from these devel- only in CH Cl . The P{ H} NMR spectra of the isomeric
1
3–15
16,17
the platinum group metals ruthenium,
rhodium
and (syn)) and 71 ppm (1a(anti)) in a ratio of 7 : 3 indicating equiv-
1
8,19
20
iridium
denum and tungsten based reductions of CO
objectives. Despite some recent achievements in molybdenum mixture of 1b(syn,anti) exhibited also two singlets, but in this
with non-precious metals. In this regard molyb- alence of both phosphorus atoms of each isomer of 1a(syn,
3
1
1
are challenging anti) (Fig. 3, left). The P{ H} NMR spectra of the isomeric
2
2
1
1
and tungsten catalysed homogeneous hydrogenations and case along with tungsten satellites at δ 56.8 ppm (s, JP–W (d,
related hydrosilylations, and our group’s recent accomplish- satellite) = 184.8 Hz, 1b(syn)) and 58.3 (s, JP–W (d, satellite) =
2
2
1
2
3a
ments in bifunctional imine hydrogenations,
ionic hydrogenation of imines,
genations and nitrile hydrogenations, the homogeneous
the stepwise 189.3 Hz, 1b(anti)) assigned to the equivalent phosphorus
Osborn type olefin hydro- atoms of each syn and anti isomer (Fig. 3, right).
2
3b,c
2
4
25
In the IR spectra of 1a,b(syn,anti) the isomers could not be
hydrogenation of carbon dioxide could not be achieved with distinguished, which showed only one sharp band for the
−
1
compounds of these metals till date.
molybdenum and tungsten complexes at 1653 and 1596 cm ,
1
In this paper a carbon dioxide activation mode supported respectively, attributed to the νNO vibrations. The H NMR
by MLC could be established as shown in Fig. 2. Furthermore, spectra of the isomeric mixtures of 1a,b(syn,anti) exhibited
stoichiometric hydrogenation of CO turned out to be accessi- several doubled signals for the methyl, methyne and methy-
2
ble using the previously reported molybdenum and tungsten lene protons appearing in the expected region with strong
2
3a
amides 3a,b.
overlap of the signals of the syn and anti isomers impeding
their distinct assignment. In the H NMR spectra a conspicu-
1
ous signal appeared at δ 4.4 ppm, which was assigned to the
HN atom of 1a(syn). However, the H atom of 1a(anti) could
not be assigned in the H NMR spectrum presumably owing to
N
Results and discussion
1
Preparation of M(II) and M(I) (M = Mo, W) complexes bearing
the quite low concentration of this isomer and overlap of the
signal with the signal of the CH Cl solvent. The H atoms of
i
(
Pr PCH CH ) NH ligand
2 2 2 2
2
2
N
H
Reaction of (iPr PCH CH ) NH (PN P) ligand with the M(NO)- the isomeric mixture of 1b(syn,anti) appeared as broad sing-
2
2
2 2
1
Cl
in THF to precipitation of the isomeric mixtures of Mo(NO)- based on the different intensity these could be assigned to the
3 2
(NCMe)
complexes (M = Mo, W) led at room temperature lets at δ 4.47 ppm and 5.50 ppm in the H NMR spectra and
H
H
Cl (PN P), 1a(syn,anti) and W(NO)Cl (PN P), 1b(syn,anti)) 1b(syn) and 1b(anti) isomers, respectively. The virtual triplet
3
3
1
3
1
v
in moderate yields (1a(syn,anti), 55%; 1b(syn,anti), 62%) signal in the C{ H} NMR at δ 55.9 (t, JC–P = 3.6 Hz) ppm for
Scheme 1).
isomeric mixture of 1a(syn,anti) was attributed to the carbon
The syn and anti notation for the isomers of 1a,b(syn,anti) atoms of 1a(syn) isomer adjacent to the nitrogen atom of the
(
H
correspond to the relative position of the H
N
atom at the PN P ligand and the related signals of 1b(anti) could not be
H
13
1
internal N atom of the PN P ligand with respect to the NO detected. On the other hand the virtual C{ H} NMR triplets
v
ligand. The spectroscopic yields of syn and anti isomers of at δ 55.9 (t, JC–P = 5.9 Hz) ppm for 1b(syn) and at 55 ppm
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3
1
1
H
Fig. 3
P{ H} NMR spectra of the isomeric mixture of Mo(NO)Cl
3
(PN P), 1a(syn,anti) {left, 1a(syn), 70%; 1a(anti), 30%} and the isomeric mixture of
Cl
H
W(NO)Cl
3
(PN P), 1b(syn,anti) {right, 1b(syn), 70%; 1b(anti), 30%} at room temperature in CD
2
2
.
v
(
t, JC–P 4.8 Hz) for 1b(anti) could be assigned to the N-adjacent
carbon atoms. Several attempts failed to grow suitable single
crystals for X-ray diffraction studies of any isomer of the 1a,b-
(
syn,anti) mixture, but 1a,b(syn,anti) could be fully character-
1
31
1
13
1
ized by H NMR, P{ H} NMR, C{ H} NMR, IR and mass
spectrometry, and the compositions were established by
elemental analyses.
1
a,b(syn,anti) are heptacoordinated diamagnetic species
H
2
Fig. 4 Molecular structures of M(NO)Cl (PN P), {M = Mo; 2a(anti) (left);
with the metal centers in +II oxidation states. Our initial
attempts to deprotonate the N–H moiety of 1a(syn,anti) apply-
ing NaN(SiMe ) to obtain 16 e unsaturated amido complex
3 2
W, 2b(syn) (right)} Thermal ellipsoids are drawn at the 50% probability
level. Selected bond distances (Å) and bond angles (°) for 2a(anti): Mo1–
N2 2.255(2), Mo1–N1 1.855(3), Mo1–Cl1 2.5021(7), Mo1–P1 2.5372(7),
−
23a
failed, which led to inseparable mixtures of products and N2–H2 0.91, P2–Mo1–P 1156.25(2), P2–Mo1–Cl2 100.64(2), P2–Mo1–
could not be identified. Attempts to reduce 1a,b(syn,anti) by Cl1 87.04(2), Mo1–N1–O1 178.7(3). Selected bond distances(Å) and bond
angles (°) for 2b(syn): W1–N2 2.232(4), W1–N1B 1.811(19), W1–P1
1
-electron reducing agents led to paramagnetic M(+I) species
2
.5123(13), W1–Cl2B 2.381(6), N2–H 0.79 (4), P2–W1–P1 157.23(4),
(M = Mo, W). Further reduction to M(0) species seemed
P2–W1–N2 78.82(13), W1–N1B–O1B 178(3), N1B–W1–Cl2B 172.2(8).
difficult with the given strongly electron donating coordinative
environment.
The reaction of 1a,b(syn,anti) with approximately one equi-
valent of 1% Na/Hg in THF at room temperature resulted in isomers. Both of them crystallized in the orthorhombic space
the formation of green isomeric mixtures of Mo(I)(NO)- group P2 2 2 . The molecular structures revealed neutral
1
1 1
H
H
Cl (PN P) complexes (2a(syn,anti)) or of W(I)(NO)Cl (PN P) pseudo-octahedral complexes. Perspective views with selected
2
2
(
2b(syn,anti)) in 74% and 78% yields, respectively (Scheme 1). bond distances and bond angles of 2a(anti) and 2b(syn) are
2
a,b(syn,anti) were found quite soluble in THF and CH
2
Cl
2
,
P
shown in Fig. 4. The two phosphorus atoms and the nitrogen
atom of the PN P ligand and the two chloride ligands attached
3
1
H
but were sparingly soluble in toluene and benzene. In the
1
1
{
H} and H NMR spectra of the isomeric mixtures of 2a,b(syn, to the metal center occupy “equatorial” positions in both
anti) no signals could be observed due to the paramagnetic cases. The HN proton and the nitrosyl ligand were found to be
1
31
nature of these complexes. The inaccessibility of H and
{
P
disposed anti indicating the presence of the anti 2a(anti)
1
H} NMR spectra impeded estimates of spectroscopic yields isomer. No NO/Cl disorder was found in the structure of
of the syn and anti isomers in solution. Thus, the isomeric 2a(anti), but despite this, the presence of isomeric mixtures in
mixtures of 2a,b(syn,anti) were characterized by IR and EPR solution with one prevailing component could not fully be
spectroscopy, elemental analyses and X-ray diffraction studies. ruled out.
−
1
Sharp bands at 1595 and 1547 cm for the isomeric mixtures
On the other hand the crystal structure analysis of 2b(syn)
of 2a,b(syn,anti) were assigned to νNO vibrations, respectively, revealed that the structure had trans NO/Cl disorder over two
which appeared at lower wave numbers than those of 1a,b(syn, positions with site occupancy factors of 0.698(8) and 0.302(8).
anti) indicating a higher degree of back-donation from the This NO/Cl disorder again supports the presence of the iso-
metal center to the nitrosyl ligand. Single crystals suitable for meric mixtures with respect to the H position and the nitrosyl
N
X-ray diffraction studies were grown from the isomeric mix- ligand and the presence of the syn and anti isomer in an
tures of 2a,b(syn,anti) from concentrated toluene–pentane approximately 7 : 3 ratio in the solid state. An inversion at the
mixtures at −30 °C. The crystal structure determination N atom with prototopic rearrangement at this atom seemed at
revealed preferential crystallization of the 2a(anti) and 2b(syn) least not to occur in the solid state. This would be also in
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accord with the assignment of the same syn/anti ratio for the isolated in pure form in quantitative yields. Monitoring the
precursors 1a,b(syn,anti). The isomeric mixtures of 2a,b- CO addition reactions visually, a purple color appeared indi-
2
(
syn,anti) are rare paramagnetic complexes with the metal cating the presence of short-lived intermediates, which were
5
centers in +I oxidation state and of low-spin d configurations. not isolable and are proposed to be charge transfer complexes.
To probe the paramagnetic nature, solution EPR measure- This type of reaction of CO with 3a,b to form carbamates 4a,b-
2
ments were carried out on 2a,b(syn,anti) in toluene at room (trans) is closely related to the 2 + 2 cycloaddition reactions of
temperature. The spectra are displayed in the ESI section early transition metal imido complexes with alkynes, olefins
2
6
(
Fig. S1†).
and ketones.
proton of the example of a thermodynamically favored insertion of alkynes
PN P ligand backbone of 2a,b(syn,anti) employing into the V–amide bond of cationic amido imido vanadium
Hessen and coworkers demonstrated an
We then attempted deprotonation of the H
N
H
2
7
Na[N(SiMe ) ], which led to decomposition and the solution complexes rather than a 2 + 2 cycloaddition reaction. There-
3
2
turned black immediately. Moreover, attempts were also fore, an insertion reaction of CO
2
across the M–amide bond
acti-
PNP) {M = Mo, 3a; W, 3b} amides via a new synthetic route vation product is unique in its structure and is therefore in its
starting from the 2a,b(syn,anti). way of formation presumed to be distinct from the insertion
Reduction of 2a,b(syn,anti) in the presence of 1 bar CO and reactions of early transition M–N bonds.
undertaken to prepare the previously reported M(NO)(CO)- could alternatively be envisaged. Nevertheless, this CO
2
2
3a
(
3
1
1
1
% Na/Hg at room temperature showed the formation of
The P{ H} NMR spectra of 4a,b(trans) at room tempera-
several unidentified species instead of the expected unique ture exhibited sharp singlets at δ 61.9 and 56.4 ppm, respect-
H
complex of an isomeric carbonyl chloride M(NO)(CO)Cl(PN P) ively, indicating equivalence of the phosphorus atoms of the
(
M = Mo, W), which prevented concomitant deprotonation to metal attached PNP ligand and under the given conditions
obtain the requested M(NO)(CO)(PNP) (M = Mo, 3a; W, 3b). also the absence of the theoretically possible isomeric pro-
Nevertheless M(NO)(CO)(PNP) 3a,b could be prepared ducts 4a,b(cis) with the CO approach from the NO side of
starting 3a,b. When the reactions of 3a,b with CO (2 bar) were carried
2
2
3a
according to our previously published procedure
from M(NO)(CO)
2
H
31
1
4 3
(ClAlCl ) and PN P ligand followed by depro- out at −60 °C and pursued by P{ H} NMR spectroscopy,
tonation with NaN(SiMe
and their hydrogenation capability.
3
)
2
and tested their reactivity with CO
2
additional weak signals were seen at δ 62.7 and at 57.9 ppm
besides those of 4a,b(trans) indicating formation of kinetic
mixtures of the 4a,b(trans,cis) isomers (7 : 3 ratios in both
cases of compounds). These mixtures equilibrated at room
temperature into the thermodynamically more stable 4a,b-
Reactivity of the M(NO)(CO)(PNP) {M = Mo, 3a W, 3b} amide
complexes with CO
Reaction of M(NO)(CO)(PNP) {M = Mo, 3a W, 3b; PNP =
N(CH CH PiPr } with CO (2 bar) at room temperature in
2
(trans) as the sole products based on the reversibility of the
CO additions to 3a,b from the NO side. 4a,b(cis) could
2
2
2
2
)
2
2
31
1
thus not be observed by P{ H} NMR spectroscopy at room
temperature in solution and could also not be isolated
THF led to formation of the 2 + 2 addition products across the
MvN bonds, the carbamate complexes Mo,W(NO)(CO)-
(
Scheme 2).
(PNP)(OCO) 4a,b(trans) (Scheme 2). The specification “trans”
1
The H NMR spectra of 4a,b(trans) showed several distinct
refers to the orientation of the CO approach transoid to the
NO ligand of 3a,b. The orange 4a,b(trans) complexes could be
2
signals for the methyl, methylene and methyne protons in the
expected region supporting also the absence of isomers. The
IR spectra of 4a,b(trans) displayed ν(NO) and ν(CO) bands
at 1593, 1554 cm− and at 1902, 1879 cm , respectively.
1
−1
Additional strong absorptions appeared for 4a,b(trans) at 1732
and 1741 cm− , respectively, and were assigned to the ν
1
CO2
1
3
1
2
vibration of the attached CO molecule. In the C{ H} NMR
spectra of 4a,b(trans) sharp singlets at δ 156.4 and 159.3 ppm
were attributed to the CCO2 atoms along with multiplets at
δ 247 and 250.6 ppm for the CCO atoms of the carbonyl ligands.
The CCH2 atoms adjacent to the nitrogen atoms of the PNP
ligand of 4a,b(trans) were observed as virtual triplets at δ 53.2
v
v
(
t, JCP = 3.6 Hz) and at 57.7 (t, JC–P = 3.6 Hz) ppm, respectively.
Single crystals of 4b(trans) suitable for an X-ray diffraction
study were obtained by layering pentane over a concentrated
THF solution. The structural model obtained from the diffrac-
tion study is depicted in Fig. 5, which revealed a pseudo
octahedral geometry. The W1–N2 bond distance was found to
be 2.2603(19) Å. Upon CO
2
addition it became significantly
elongated by about 0.2 Å with respect to the approximate
WvN bond of 3b.
2
Scheme 2 CO activation by molybdenum and tungsten amides 3a,b
and hydrides 5a,b(cis,trans).
2
3a
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square with W–N, N–C, C–O and O–W contact lengths of 2.144,
2.399, 1.184 and 2.780 Å, respectively. These bond lengths
mark an early transition state of the 2e + 2e reaction. The C–N
interaction is the main new contact in the transition state,
which is also represented in Fig. 6 (right), where large orbital
coefficients appear between the C(CO
2
) and the N atoms. The
other new contact of the O(CO ) atom to the W center is still
2
very distant. Despite this feature that asymmetry prevails in
the bond forming process, the reaction coordinate was found
to be continuous and not two-stage without intermediate
generated by consecutive bond formations.
H
Reactivity of the hydride complexes M(NO)H(CO)(PN P)
(
M = Mo, 5a(cis,trans); W, 5b(cis,trans)) with CO
2
Then we also tested the CO
2
reactivity of the previously repor-
Fig. 5 Molecular structure of 4b(trans). Thermal ellipsoids are drawn at
the 50% probability level. All hydrogen atoms are omitted for clarity. ted
Selected bond distances [Å] and bond angles [°]: W1–N1 1.789(2), W1–
C1 1.969(3), W1–N2 2.2603(19), W1–O3 2.2155(18), C1–O1 1.159(3), C2–
O3 1.283(3), C2–O4 1.208(3), N1–W1–C1 88.61(10), N2–W1–O3
2
3a
isomeric mixtures of the amine hydride compounds
H
M(NO)H(CO)(PN P) (5a,b(cis,trans)), which were obtained by
addition to 3a,b from both sides of the MvN bond. The cis
and trans notation indicate trans and cis attack of the H mole-
H
2
2
1
72.47(8), N2–C2–O3 108.0 (2) P1–W1–P2 157.60(2).
cule with respect to the NO ligand leading to the trans H/NO
complexes (5a,b(trans)) or to the trans H/CO complexes (5a,b-
(
cis)). However, the reactions to 5a(cis,trans) with H
2
were
The transition state of the reaction between W(NO)(CO)- found to be equilibria showing at room temperature an
(
PNP) 3b and CO
2
could be modelled by DFT calculations (see approximate 1 : 1 : 1 mixture of 5a(cis)/5a(trans)/3a. Therefore,
ESI†).
the reaction with CO (2 bar) had to be carried out using the
2
‡
−1
H
Reasonably low activation energies ΔG = 14.4 kcal mol
in situ generated mixture of Mo(NO)H(CO)(PN P) (5a(cis,
and 14.1 kcal mol− were calculated for the CO and NO side trans)), which led to the formate products Mo(NO)(CO)(PN P)
1
H
1
approaches of CO2 onto 3b to give 4b(trans) and 4b(cis), (η -OCHO) 6a(cis,trans) and the cyclic carbamate species
respectively. Remarkably, both approaches are kinetically 4a(trans) in an approx. 1 : 1 : 1 ratio as indicated by C{ H}
almost not distinct. The free enthalpies of both reactions were and P{ H} NMR spectroscopy (Scheme 2). Since the three
1
3
1
3
1
1
−
1
determined to be −7.7 kcal mol (4b(trans)) and −5.6 kcal different kinds of reactions (2 kinds of CO insertions into the
2
−
1
mol (4b(cis)) indicating that 4b(trans) is the thermodynami- M–H bonds and CO addition) did not reveal a change in the
2
cally more favored and stable product. The calculated lower ratios from the starting materials to the products, we have to
free energy value for 4b(cis) looks plausible in view of the fact assume that all involved reaction rates are about the same. The
1
that the CO
2
approaches from the NO side are experimentally formate protons of 6a(trans) and 6a(cis) appeared in the
H
equilibria. The calculated activation barrier for the reverse NMR spectrum at δ 8.1 and 8.5 ppm, respectively (1 : 1 ratio).
1
3
1
reaction from 4b(cis) to form 3b and CO seems thus to be still Signals at δ 170 and 168 ppm in the C{ H} NMR spectra were
2
in energetic reach for a thermal process at room temperature assigned to the Cformate nuclei of the 6a(cis,trans) isomers,
ΔG = 19.8 kcal mol− ). Drawings of the TS structure of the respectively. 6a(cis,trans) and 4a(trans) were not in equili-
1
(
CO2 approach to 3b (Fig. 6 (left)) and the HOMO of the TS brium, but despite this attempts to separate these complexes
Fig. 6 (right)) are shown in Fig. 6 (see also ESI†). According to turned out to be unsuccessful.
(
the calculations the TS consists of a strongly distorted TS
Similarly, the reaction of the 5b(cis,trans) mixture, which is
not an equilibrium reaction allowing this mixture to be iso-
lated, with CO resulted at room temperature in the rapid for-
2
1
mation of the isomeric η -formate complexes W(NO)(CO)-
PN P)(η -OCHO) (6a,b(trans)) (Scheme 2) appearing in a
: 1 ratio. A strong signal at δ 51.3 ppm (with tungsten satel-
lites) in the P{ H} NMR spectrum was assigned to 6b(trans)
along with a weak signal at δ 52.6 ppm for 6b(cis). In the H
H
1
28
(
9
3
1
1
1
NMR spectrum the formate protons of 6b(trans,cis) were
observed at δ 8.5 and 8.0 ppm, respectively, and signals at δ
1
3
1
2
44.7 and 164.3 ppm in the C( H) NMR spectra were attribu-
ted to the carbonyl ligand and the Cformate atom of the major
isomer 6b(trans). Supposedly due to a too low concentration of
Fig. 6 Geometry of the transition state (TS) of the approach of CO
b (left) and the TS HOMO−3 orbital for the CO 2e + 2e addition reac-
tion (right).
2
to
1
3
6b(cis), the C NMR resonances of the carbonyl and formate
3
2
carbon atoms of the minor 6b(cis) isomer could not be
6564 | Dalton Trans., 2015, 44, 6560–6570
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observed. In the IR spectrum of the 6b(cis,trans) mixture a isomer 6b(cis) by ΔE = −4.4 kcal mol− (see Scheme 2 and
1
band at 1612 cm− was attributed to the ν (CO ) vibration in ESI†), which seems to match well reality. The fact that in the
1
as
2
1
accord with earlier assignments of η -O-formato tungsten com- crystallization experiment of the 6b(cis,trans) mixture 6b(cis)
plexes. Additional IR bands at 1863 and 1578 cm of the was found to be less soluble than 6b(trans) can now be attribu-
2
8
−1
6
b(cis,trans) mixture were assigned to the ν(CO) and ν(NO) ted to a secondary crystallization effect based on the men-
ligand vibrations of both compounds showing that both com- tioned hydrogen bonding system in the solid state causing
pounds cannot be distinguished in the 1500 to 1900 cm−
region of the IR spectra.
1
lower solubility of the minor constituent in solution. In
addition it should be noted that the proton at N1 and the
Despite the fact that 6b(cis) was the minor constituent of formate ligand of the structure of 6b(cis) are in anti position,
the isomeric mixture in solution, tiny crystals of this minor which somewhat contradicts the fact that in 5a,b(trans) and
component could precipitated from solution by slow diffusion 5a,b(cis) they are expected to be syn due to the Z stereo-
of pentane into a concentrated THF solution. The X-ray diffrac- chemistry of the H
2
addition and the expected retention of the
tion study revealed a pseudo-octahedral framework with cis configuration at the metal center associated with the CO
2
nitrosyl and formate ligands (Fig. 7). The asymmetric unit of insertion process. Therefore the proton at N1 is assumed to
this minor isomer 6b(cis) contained two crystallographically have undergone an inversion process when forming the crys-
independent tungsten species displaying intermolecular tals of 6b(cis), which seems likely on the basis of our DFT cal-
3
1
hydrogen bonding between the protic HN atom of one mole- culations, which revealed only minor energetic differences
cule and the Oformato of an adjacent molecule forming linear (approx. 2 kcal mol− ) between the syn and anti structures in
chains in the three dimensional lattice. It should be noted favour of the anti arrangements.
1
here that the H atom and the formate group were found to be
disposed anti in the solid state structure, which was somewhat
N
Stoichiometric hydrogenation of carbon dioxide
2 N
surprising when cis addition of H to 3a,b is taken into con- The 6a,b(cis,trans) mixtures can be deprotonated at the H
sideration, followed by CO2 insertion into the M–H bond atom employing 1 equiv. of Na[N(SiMe ) ] at room temperature
3 2
occurring with retention of the configuration at the metal with regeneration of 3a,b. Sodium formate was formed in
centers (Scheme 2). One possible reason for the thermodyna- addition in accord with eqn (1) closing thus a stoichiometric
mically favored re-orientation of the HN atom by a prototopic cycle of consecutive reaction steps of H and CO additions in
2
2
rearrangement could be the adoption of an energetically pre- a “pseudo-catalytic” reaction course. The permanent presence
ferred intermolecular hydrogen bonding network requiring of a base seemed not to interfere with 6b(cis,trans) and to
anti disposition of the HN atom and the formate ligand in the initiate side reactions, but the preferential addition of CO2
crystal lattice.
over the H
2 2
addition to 3a,b seemed in the presence of a H –
In addition to these findings DFT calculations support the CO
2
mixture a great obstacle to establish a genuine catalytic
observation that the major trans NO/formate isomer 6b(trans) reaction course. Therefore, if catalysis should be accomplish-
is thermodynamically more stable than the trans CO/formate able, it could be envisaged at higher temperatures making the
CO
2
addition to some extent reversible enabling thus under
catalytic conditions H2 before CO2 addition to 3a,b
(
Scheme 3).
Being aware of this situation, we attempted catalysis and
carried out at 140 °C 3 types of hydrogenation experiments of
CO
CO
2
applying the simultaneous presence of H
(10 bar) and of catalytic amounts of 3a,b or of the 5b-
2
(70 bar) and
2
(cis,trans) mixture (5 mol% loading) in the presence of Na-
[
N(SiMe ] as stoichiometric agent. After 15 h all the experi-
3 2
)
ments revealed formation of HCOONa according to eqn (1),
but the yields were low, only 4%, 2% and 3%, respectively, as
1
examined by H NMR (DMF as internal standard) (Table 1). To
improve the yield of the formate salt, we varied the type of the
base using DBU, KOtBu or NEt in the presence of 3a as a cata-
3
lyst with otherwise the same conditions as above. However, the
yield of the formate salts [HCOO][DBUH] (3.5%), HCOOK
(
2.5%) or HCOOH–NEt (0.5%), did not increase (Table 1). Fur-
3
6 5 3 3 6 5 3
thermore, the addition of B(C F ) or the Et SiH–B(C F )
mixture as co-catalysts led in the presence of 3a and
Fig. 7 Molecular structure of 6b(cis). Thermal ellipsoids are drawn at
the 50% probability level. All hydrogen atoms are omitted for clarity
except H1. Selected bond distances [Å] and bond angles [°]: W1–N1
Na[N(SiMe ) ] even to a decrease in yields.
3
2
2
At this stage it can be stated that CO hydrogenation is in
2.287(5), W1–C1 1.930(8), W1–N2 1.792(6), W1–O3 2.185(5), C1–O1
principle possible, but for effective such catalyses further
tuning efforts have to be taken into consideration, presumably
1
.172(8), C2–O3 1.271(7), C2–O4 1.213(8), N1–W1–C1 91.6(3), N2–W1–
O3 101.9(2), N1–W1–N2 178.3 (3) P1–W1–P2 158.32(7).
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NMR spectroscopy (DMF internal standard), the solution was
still found to contain white particles even after addition of
excess D
2
O. Therefore, formation of another species is antici-
(note that
pated to be produced in the hydrogenation of CO
2
the product sodium formate is expected to be soluble in D O).
2
The nature of this species could however not be unravelled,
but in principle it should be added to the overall yield of these
catalytic reactions.
Conclusions
In conclusion we have reported a new synthetic route for the
H
preparation of isomeric diamagnetic M(NO)Cl (PN P) (M =
3
Mo, 1a(syn,anti); W, 1b(syn,anti)) complexes and their
H
reduction to rare paramagnetic species M(NO)Cl
2
(PN P) {M =
Mo, 2a(syn,anti); W, 2b(syn,anti)}. Furthermore, we have
shown that the excessively reduced pentacoordinate amides
2 2 2 2
Mo,W(0)(PNP)(NO)(CO) (3a,b; PNP = (iPr PCH CH ) N)) can
2
Scheme 3 Schematic cycle for the CO hydrogenation using 3a,b.
activate CO2 in a metal–ligand assisted way via 2e + 2e
additions across the MvN bonds forming the pseudo-octa-
hedral 4a,b(trans) compounds. The CO approach occurs ther-
2
Table 1 Hydrogenation of CO
2
modynamically controlled from the CO ligand side. Kinetically
there is practically no distinction between the sides of attack.
The amine hydride species M(NO)(CO)H(PN P) (M = Mo, 5a-
cat ð5 mol%Þ
H
Co2 þ H2 þ Na½NðSiMe3Þ2ꢀ ꢁꢁꢁꢁꢁꢁ! HCOONa þ HNðSiMe3Þ2 ð1Þ
140 °C;THF
ð10 barÞ
ð70 barÞ
(
cis,trans) W, 5b(cis,trans)) formed by H2 addition to 3a,b
1
a
b
2
reacted with CO at room temperature to produce η -formato
Entry
Cat
Base
Time (h)
Yield (%)
H
1
complexes M(NO)(CO)(PN P)(η -OCHO) {M = Mo, 6a(cis,trans);
W, 6b(cis,trans)}. Stoichiometric hydrogenations of CO2 to
formate salts could thus be accomplished via elimination of
the formate ligands of type 6 complexes induced through the
presence of a strong base regenerating the amides 3a,b. Cataly-
1
2
3
4
5
6
7
8
9
1
3a
3b
Na[N(SiMe
Na[N(SiMe
Na[N(SiMe
DBU
KOtBu
3
Et N
Na[N(SiMe
TMP
Na[N(SiMe
Na[N(SiMe
3
)
3
)
3
)
2
2
2
]
]
]
15
15
15
15
15
15
17
15
15
22
4
2
3
3.5
2.5
0.50
0.14
—
0.70
0.30
5b(cis,trans)
3a
3a
3a
—
3a
c
2
tic hydrogenation of CO was then also approached at some-
what elevated temperatures and pressures. However, the
3
)
2
]
3a/BCF
3a/Et SiH/BCF
3
3
)
)
2
]
]
apparently too high stability of the CO addition products of
2
0
3
2
type 4 are anticipated to largely block the catalytic reaction
a
Unless and otherwise stated 10 bar CO
40 °C temperature and 5 mol% catalyst with respect to base were
2 2
, 70 bar H
, THF as solvent, course and cause low yields. This conclusion rendered the
1
idea that in these Mo and W systems catalysis could eventually
be achieved by tuning the 4 type complexes for reversibility in
b
1
used as reaction conditions. Yield on c the basis of
H NMR
integration using DMF as internal standard. No catalyst was used.
CO
2
additions. It is finally worth mentioning that this study
demonstrated for the first time that reductions of CO2
by attempts to make the CO adducts of type 4 more labile. are within reach utilizing middle, non-noble transition metal
2
It should be mentioned at this point that since strong bases compounds as catalysts.
are known to react with CO particularly at high temperatures
2
and pressures, a control experiment was also carried out in the
absence of the metal catalyst applying only Na[N(SiMe ) ] as
the catalyst and keeping all the other conditions the same. The
yield of sodium formate was in this case only 0.14%, which is
3
2
Experimental section
General procedures
significantly lower than the yields obtained in the presence All experiments were carried out under an atmosphere of nitro-
of 3a,b.
gen using either dry glove box or Schlenk techniques. Reagent
It deserves mentioning that after the CO2 hydrogenation grade solvents benzene, tetrahydrofuran, pentane, toluene,
experiments (applying especially the NaN(SiMe3)2 base), the diethyl ether were dried with sodium benzophenone and dis-
reaction mixtures were found to contain a considerable tilled prior to use under N atmosphere. CH Cl and Et N were
2
2
2
3
amount of white precipitate in the THF reaction mixture. dried over calcium hydride and distilled. Deuterated solvents
When THF was evaporated in vacuo and the residue was dis- were dried with sodium benzophenone ketyl (THF-d , toluene-
8
1
solved in D
2
O to quantify the yield of the formate salt by H d8 and C D ) and calcium hydride (CD Cl ) and distilled
6
6
2
2
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via freeze–pump–thaw cycle prior to use. The M(NO)- NCH
2
, syn,anti), 3.19–3.05 (m, CH), 2.30–2.23 (m, –PCH
2
),
2
9
23a
13
1
Cl (NCMe) ,
M(NO)(CO)(PNP),
{M = Mo, W; PNP = 2.09–2.06 (m, PCH ), 1.47–1.19 (m, CH ).
C{ H} NMR
): δ = 55.9 (t, C–P = 5.9 Hz, NCH
3
2
2
3
i
30
v
N(CH
2
CH
2
iPr
2
)
2
} complexes and HN(CH
2
CH
2
Pr
2
)
2
were pre- (100.6 MHz, CD
2
Cl
2
J
2
,
v
pared according to literature procedures. KOtBu, Na[N(SiMe
3
)
2
]
1b(anti)), 55 (t, JC–P = 4.8 Hz, NCH , 1b(anti)), 27.8 (m, CH),
2
3
1
1
and DBU were purchased from commercially available sources 27.3 (m, –PCH ), 20 (m, CH ). P{ H} NMR (400 MHz,
2
3
1
and used without further purification. The NMR spectra were CD
2
Cl
2
): 56.8 [s, JP–W (d, satellite) = 184.8 Hz, 1b(syn)], 58.3 [s,
1
−1
measured with
a
Varian Mercury 200 spectrometer (at
JP–W (d, satellite) = 189.3 Hz, 1b(anti)]. IR (cm , ATR): 1596
1
31
2
00.1 MHz for H, at 81.0 MHz for P), with Varian Gemini- (νNO), Anal. Calcd for C H Cl N OP W: C, 30.72; H, 5.96;
16 37 3 2 2
1
13
3
00 instrument ( H at 300.1 MHz, C at 75.4 MHz), with N, 4.48. Found: C, 30.72; H, 5.88; N, 4.55.
1
Bruker-DRX 500 spectrometer (500.2 MHz for H, 202.5 MHz
H
3
1
13
Preparation of Mo(NO)Cl
2
(PN P), 2a(syn,anti)
for P, 125.8 MHz for C) and Bruker-DRX 400 spectrometer
1
31
13
(
400.1 MHz for H, 162.0 MHz for P, 100.6 MHz for C). All The isomeric mixture of 1a(syn,anti) (0.030 g, 0.056 mmol)
1
13
1
chemical shifts for H and C{ H} are expressed in ppm rela- was added to a stirred suspension of approximately 1%
tive to tetramethylsilane (TMS) and for P{ H} relative to 85% sodium amalgam (0.0013 g, 0.057 mmol) in 15 mL THF. Stir-
PO as an external standard reference. Signal patterns are as ring was continued overnight at room temperature to ensure
3
1
1
H
3
4
followed: s, singlet; d, doublet; t, triplet; q, quartet; m, multi- the completion of the reaction. The final supernatant green
plet; v, virtual triplet. IR spectra were obtained either ATR or solution was filtered off from the mercury containing residue
KBr methods using Bio-rad FTS-45 instrument. Elemental and evaporated to dryness. The resulting residue was washed
analyses were carried out at Anorganisch-Chemisches Institut with pentane and extracted with THF. Finally the isomeric
of the University of Zurich.
mixture of 2a(syn,anti) was obtained as a green powder after
removal of THF in vacuo. Yield 74%. IR (cm− , ATR): 1595
(νNO), Anal. Calcd for C16H37Cl MoN OP : C, 38.26; H, 7.42; N,
2 2 2
1
H
Preparation of isomeric mixture of Mo(NO)Cl (PN P),
3
1
a(syn,anti)
To a solution of Mo(NO)Cl
0 mL of THF, 0.16 g (196 mg, 0.64 mmol) (iPr
5.58. Found: C, 37.88; H, 7.12; N, 5.29.
3
(NCMe)
2
(0.200 g, 0.636 mmol) in
PCH CH NH
H
Preparation of W(NO)Cl (PN P), 2b(syn,anti)
2
1
2
2
2 2
)
ligand in 5 mL THF was added. The resulting mixture was kept 0.075 g (0.12 mmol) of the isomeric mixture of 1b(syn,anti)
stirring overnight to obtain a light green precipitate and the was added to a stirred suspension of 1% sodium amalgam
solution becomes dark green. Then the solvent was separated (0.003 g, 0.13 mmol) in 15 mL THF. Stirring was continued
from the precipitate and the precipitate was washed with overnight at room temperature to ensure the completion of the
minimum amount of THF and extracted with dichloromethane reaction. The final supernatant solution was filtered off from
and dried in vacuo to obtain the powdery isomeric mixture of the mercury containing residue and evaporated to dryness.
1
a(syn,anti) in 55% yield.
The residue was washed with pentane and extracted with THF
): δ = 4.4 (broad singlet, NH, and later with toluene. Recrystallisation from cold toluene–
1
H NMR (400 MHz, CD
2
Cl
2
1
(
CH
2 2
a(syn)), 3.63–3.5 (m, NCH ), 3.23–3.21 (m, NCH ), 3.1–3.02 pentane solution afforded green crystals of 2b(syn,anti). Yield
m, CH), 2.3–2.2 (m, –PCH ), 2.05 (m, PCH ), 1.52–1.39 (m, 78%. IR (cm− , ATR): 1547 (νNO). Anal. Calcd for W(NO)
1
2
2
1
3
1
3
, 1a(syn)), 1.34–1.30 (m, CH
3
, 1a(anti)). C{ H} NMR Cl
2
(PNP): C, 32.56; H, 6.32; N, 4.75. Found: C, 32.71; H, 6.22;
v
(100.6 MHz, CD
2
Cl
2
): δ = 55.9 (t, JC–P = 3.6 Hz, NCH
2
, 1a(syn)), N, 4.78.
3
1
1
2
9 (m, CH), 28 (m, PCH ), 21 (m, CH ), 20.4 (m, CH ). P{ H}
2
3
3
i
General procedure for the preparation of
M(NO)(CO)(PNP)(OCO), (M = Mo, 4a(trans); W, 4b(trans))
NMR (400 MHz, CD
2
Cl
2
): 69.3 (s, Pr
2
P, 1a(syn)), 71 (s,
a(anti)). IR (cm− , ATR): 1653 (νNO), expected m/z: 538,
1
1
observed m/z: 538.2. Anal. Calcd for C H Cl MoN OP : M(NO)(CO)(PNP) (0.065 mmol, M = Mo, 3a; W, 3b) was dis-
1
6
37
3
2
2
C, 35.74; H, 6.94; N, 5.21. Found: C, 36.03; H, 6.39; N, 5.67.
8
solved in 0.5 mL THF-d in a J. Young NMR tube. The tube was
taken out from the glove box and frozen with liquid nitrogen.
The nitrogen atmosphere was removed via a freeze–pump–
H
Preparation of isomeric mixture of W(NO)Cl
b(syn,anti)
3
(PN P),
1
thaw cycle and the tube was filled with 2 bar of CO
(200 mg, 0.5 mmol) was dissolved in 10 mL sealed. Formation of 4a(trans) or 4b(trans) took place immedi-
(0.52 mmol) of aminodiphosphine ately within five minutes in quantitative yield.
2
and
WCl (NO)(NCMe)
of THF. 0.160
iPr PCH CH NH was added via a syringe. After a few
minutes stirring a light red precipitate was observed and the 2H, –NCH
reaction was allowed to stir overnight. Then the solution was 2.25–2.17(m, 2H, CH), 2.08–2.03 (m, 2H, CH ), 1.81–1.72 (m,
3
2
g
1
(
2
2
2
)
2
4a(trans). H NMR (400 MHz, THF-d
8
): δ = 3.47–3.35 (m,
2
), 2.85–2.78 (m, 2H, –NCH ), 2.42–2.36 (m, 2H, CH),
2
2
removed partially under vacuum and the reddish residue was 2H, –PCH
2
), 1.35–1.28 (m, 12H, CH
3 3
), 1.23–1.15 (m, 12H, CH ).
1
3
1
filtered off and washed with THF. The obtained residue was
C{ H} NMR (100.62 MHz THF-d ): δ = 247 (m, CO), 156.4 (s,
8
v
v
extracted with dichloromethane and the solvent was removed CO , 53.2 (t, J = 3.6 Hz, NCH ), 23.79 (s, CH), 19.83 (t, J
2
)
CP
2
C–P
3
1
in vacuo to obtained the isomeric mixture of 1b(syn,anti) in = 6 Hz, CH
2
), 16.59 (s, CH
3
), 16.21 (s, CH
3
), 15.74 (s, CH
3
).
P
1
2 2 8
Cl ): δ = 4.47 (broad singlet, { H} NMR (161.97 MHz, THF-d
): δ = 61.9 (s). IR (cm−1, KBr):
1
6
2% yield. H NMR (400 MHz, CD
NH, 1b(syn)), 5.5 (broad singlet, NH, 1b(anti)) 3.71–3.57 (m, 1593 (νNO), 1732 (νCO), 1902 (νCO). Anal. Calcd for
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C
18
H
36MoN
2
O
4
P
2
: C, 43.03; H, 7.22; N, 5.58. Found: C, 42.67; immediately upon shaking the Young NMR tube in quantitat-
ive yield in a 1 : 9 ratio. Suitable orange crystals were obtained
H, 7.28; N, 5.34.
1
4
b(trans). H NMR (400 MHz, THF-d
H, –NCH ), 2.92–2.85 (m, 2H, –NCH ), 2.52–2.46 (m, 2H, CH), solution of the product. Yield (by P{ H} NMR): 10% (6b(cis)),
.28–2.20(m, 2H, CH), 2.11–2.06 (m, 2H, –PCH ), 1.81–1.76 (m, 90% (6b(trans)). H NMR (400.1 MHz, THF-d ): δ = 8.52 (s, 1H,
8
): δ = 3.58–3.49 (m, for diffraction upon layering pentane to a concentrated THF
3
1
1
2
2
2
2
2
1
2
8
H, –PCH
2
), 1.37–1.29 (m, 12H, –CH
3
), 1.25–1.11 (m, 12H, OCH, 6b(trans)), 8.0 (s, 1H, OCH, 6b(cis)), 2.93 (m, 4H,
). C{ H} NMR (75.45 MHz, THF-d ): δ = 250.6 (m, CO), –NCH ), 2.41–2.26 (m, 4H, –CH), 1.66–1.62 (m, 4H, CH P),
1.35–1.25 (m, 24H, CH3). C{ H} NMR (100.6 MHz, THF-d ):
1
3
1
–
1
1
7
CH
3
8
2
2
v
v
13
1
59.3 (s, CO , 57.7 (t, J
0.9 Hz, CH), 26.87 (t, JCP = 12.2 Hz, CH), 23.62 (t, JCP
.7 Hz, PCH
= 3.6 Hz, NCH ), 27.53 (t, J
=
=
2
)
C–P
2
CP
8
v
v
v
δ 244.7 (s, CO), 164.3 (s, CO
), 13.1 Hz, CH), 24.01 (t,
2
H
)
, 57.7 (s, NCH
2
), 24.5 (t, JCP
=
=
v
v
V
2
), 19.66 (t,
J
CP = 5.9 Hz, CH
3
), 19.05(s, CH
3
JCP = 9.5 Hz, CH), 19.08 (t, J
CP
3
1
1
31
1
1
J
8.61 (s, CH3). P{ H} NMR (161.97 MHz, THF-d ): 56.4 [s, 8.3 Hz, CH ), 17.2 (s, CH ), 14.80 (s, CH ). P{ H} NMR
8 2 3 3
1
PW (d, satellite) = 308.2 Hz]. IR (cm−1, KBr): 1554 (νNO), 1741 (161.97 MHz, THF-d
8
): 51.3 (s, 6b(trans)), 52.6 (s, 6b(cis)). IR
W: C, 36.63; (cm , KBr): 1578 (νNO), 1612 (νOCHO), 1863 (νCO). Anal. Calcd
−
1
(
ν
CO2), 1879 (νCO). Anal. Calcd for C18
H, 6.15; N, 4.75. Found: C, 36.29; H, 6.27; N, 4.72.
Reaction of 3a and 3b with CO
0.03 mmol) (M = Mo, 3a; W; 3b) was dissolved in 0.5 mL
H
36
N
2
O
4
P
2
for C H N O P W: C, 36.50; H, 6.47; N, 4.73. Found:
1
8
38 2 4 2
2
at 213 K. M(NO)(CO)(PNP) C, 36.25; H, 6.41; N, 4.53.
(
General procedure for the reaction of the formate complexes
mixture of 6a(cis) and 6a(trans)) and (mixture of 6b(cis) and
b(trans)) with Na[N(SiMe ) ]
THF-d in a J. Young NMR tube. The tube was taken out of the
glove box and frozen with liquid nitrogen. The nitrogen atmos-
phere was removed via freeze–pump–thaw cycle. The tube was
8
(
6
3
2
filled with 2 bar of CO2 under frozen condition and sealed. The mixture of the formate complexes (0.03 mmol) of molyb-
Then the tube was immediately placed in a 300 MHz NMR denum (mixture of 6a(cis) and 6a(trans) along with 4b(trans))
3
1
1
spectrometer preset at −60 °C and P{ H} NMR spectra was and tungsten (mixture of 6b(cis) and 6b(trans)) was dissolved
recorded. in THF in a J. Young NMR tube. Then 1 equiv. of Na-
at [N(SiMe ] was added. The mixture was shaken vigorously.
3
1
1
P{ H} NMR data for the reaction of 3a with CO
2
3 2
)
3
1
1
31
1
2
8
13 K: P{ H} NMR (121.48 MHz, 213 K, THF-d ): δ = 63.1 (s, The P{ H} NMR spectra revealed immediate regeneration of
formation of 4a(trans)), 62.7 (s, formation of 4a(cis)).
3a or 3b. Then the reaction mixture was dried in vacuo and
3
1
1
P{ H} NMR data for the reaction of 3b with CO
2
at 213 K: extracted with pentane to remove the 3a or 3b. The remaining
3
1
1
1
P{ H} NMR (121.48 MHz, 213 K, THF-d
8
): δ = 58.1 (s, for- solid insoluble in pentane was dissolved in D
2
O. The H NMR
mation of 4b(trans)), 57.9 (s, formation of 4b(cis)).
Preparation of Mo(NO)(CO)(PN P)(η -OCHO), {mixture of
spectra in D O revealed the formation of HCOONa.
General procedures for the CO hydrogenations. A steel
2
2
H
1
6
a(cis) and 6a(trans)}. 3a (0.015 g, 0.033 mmol) in 0.5 mL THF autoclave was charged under nitrogen with the catalysts
was frozen in a J. Young NMR tube. The nitrogen atmosphere (0.1 mmol, 5 mol% with respect to base) dissolved in 0.5 mL
was removed via a freeze–pump–thaw cycle. Then the tube was of THF and 20 equiv. of Na[N(SiMe ] (1 M in THF). The tube
filled with 2 bar of H and sealed and kept for three days to was pressurized with 40 bar H and allowed to stir at room
allow formation of 5a(cis) and 5a(trans). After formation of the temperature for half an hour. After this period of time 10 bar
equilibrium mixture of hydride complexes 5a(cis) and of CO was charged. Then the total pressure was kept at 80 bar
a(trans) along with 3a (1 : 1 : 1), the hydrogen pressure was filling with H pressure. The reaction mixture was heated to
3 2
)
2
2
2
5
2
slowly released and CO (2 bar) was purged into that mixture 140 °C (Caution!!: pressure rises to 100 bar). After certain reac-
2
instantaneously. Immediate formation of 6a(cis) and 6a(trans) tion times, the autoclave was cooled to room temperature and
was observed along with 4a(trans) in a ratio of 1 : 1 : 1 as the pressure was released. The THF solvent was evaporated to
3
1
1
revealed by the P{ H} NMR.
Selected spectroscopic data: H NMR (400.1 MHz, THF-d
dryness and D O was added to dissolve the solid residue.
Dimethylformamide (2 µL) was added to the reaction mixtures
2
1
8
)
data: δ = 8.5 (s, 1H, OCH, 6a(cis)), 8.1 (s, 1H, OCHO, 6a(trans)). as an internal standard for the determination of the yield by
1
3
1
1
1
C{ H} NMR (100.6 MHz, THF-d ): δ 248 (s, CO, 6a(trans)),
H NMR spectroscopy. H NMR (300 MHz, D O): δ = 8.3 (s, 1H,
2
8
2
6
46.5 (s, CO, 6a(cis)), 170 (s, OCHO, 6a(cis)), 168.8 (s, OCHO, HCOONa).
3
1
1
2
a(trans)), 156 (s, CO , 4a(trans)). P{ H} NMR (161.97 MHz,
X-ray diffraction analyses
THF-d ): 60.9 (s, 6a(trans)), 60.0 (s, 6b(cis)). Since the resulting
8
product contains mixtures of 6a(cis), 6a(trans) and 4a(trans) The data collection and structure refinement data for 2a(anti),
which could not be separated, elemental analyses could not be 2b(syn), 4b(trans) and 6b(cis) are presented in (Table 2).
provided.
Single-crystal X-ray diffraction data were collected at 183(2) K
H
1
W(NO)(CO)(PN P)(η -OCHO), {mixture of 6b(cis) and 6b- on a Xcalibur diffractometer (Agilent Technologies, Ruby CCD
trans)}. A solution of the mixture of 5b(cis) and 5b(trans) detector) for all compounds using a single wavelength
(
3
2a
(0.020 g, 0.036 mmol) in 0.5 mL THF was frozen in a J. Young Enhance X-ray source with MoKα radiation (λ = 0.71073 Å).
NMR tube. The nitrogen atmosphere was removed via a freeze– The selected suitable single crystals were mounted using poly-
pump–thaw cycle. Then the tube was filled with 2 bar of CO butene oil on the top of a glass fiber fixed on a goniometer
and sealed. Formation of 6b(cis) and 6b(trans) took place head and immediately transferred to the diffractometer. Pre-
2
6568 | Dalton Trans., 2015, 44, 6560–6570
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Dalton Transactions
Paper
a
Table 2 Crystallographic data for compounds 2a(anti), 2b(syn), 4b(trans) and 6b(cis)
2a(anti)
2b(syn)
4b(trans)
6b(cis)
CCDC
960495
960496
888516
888517
8(C18H N O P W), C H O
38 2 4 2 4 8
4810.46
183(2)
0.71073
Empirical formula
Formula weight (g mol
Temperature (K)
Wavelength (Å)
C
16
H37Cl
2
MoN
2
OP
2
C
16
H37Cl
2 2 2
N OP W
18 36
C H N
2
O
4 2
P W
−
1
)
502.26
183(2)
0.71073
590.16
183(2)
0.71073
590.28
183(2)
0.71073
Crystal system, space group
Orthorhombic, P2
7.3971(1)
13.2575(2)
23.8535(4)
90
90
90
2339.24(6)
4, 1.426
0.933
1
2 2
1 1
Orthorhombic, P2
7.4129(1)
13.2003(2)
23.8800(5)
90
90
90
2336.72(7)
4, 1.678
5.316
1
2
1
2
1
Monoclinic, P2
7.8847(1)
26.3644(4)
11.5872(2)
90
106.979(2)
90
2303.70(7)
4, 1.702
5.178
1
/c
Tetragonal, I4
36.3651(6)
36.3651(6)
14.8550(3)
90
90
90
19644.6(8)
4, 1.626
4.859
1
/a
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
3
Volume (Å )
−3
Z, density (calcd) (Mg m
)
−
1
Abs coefficient (mm
F(000)
)
1044
1172
1176
9632
3
Crystal size (mm )
0.30 × 0.11 × 0.06
2.88 to 32.58
55 639
8495/[Rint = 0.0485]
99.9
0.12 × 0.10 × 0.06
2.88 to 28.28
16 728
5804/[Rint = 0.0372]
99.9
0.33 × 0.27 × 0.20
2.70 to 28.28
37 089
5720/[Rint = 0.0263]
99.9
0.10 × 0.08 × 0.03
2.44 to 25.68
27 807
9319/[Rint = 0.0800]
99.9
θ range (°)
Reflections collected
Reflections unique
Completeness to θ (%)
Absorption correction
Max/min transmission
Analytical
Analytical
Analytical
Analytical
0.951 and 0.799
7529/0/225
1.030
0.765 and 0.644
5804/61/257
0.895
0.432 and 0.275
5574/0/252
1.294
0.995 and 0.987
5971/0/510
0.987
Data/restraints/parameters
2
Goodness-of-fit on F
Final R
1
and wR
2
indices [I > 2σ(I)]
0.0351, 0.0852
0.0420, 0.0872
−0.02(3)
0.0329, 0.0488
0.0473, 0.0505
−0.007(7)
0.0203, 0.0400
0.0212, 0.0403
0.0488, 0.0619
0.0972, 0.0744
R
1
and wR
Absolute structure parameter
2
indices (all data)
Largest diff. peak and hole (e Å−
3
)
1.107 and −0.613
2.044 and −1.158
0.657 and −1.231
1.241 and −0.845
a
2
o
2 2
2 2 1/2
The unweighted R-factor is R
1
= ∑(F
o
− F
c
o
)/∑F ; I > 2σ(I) and the weighted R-factor is wR
2
= {∑w(F
− F
c
) /∑w(Fo ) }
.
experiment, data collection, data reduction and analytical
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3
2b
absorption corrections
were performed with the program
Pro 32a
suite CrysAlis
.
The crystal structures were solved with
3
2c
SHELXS97 using direct methods. The structure refinements
2
were performed by full-matrix least-squares on F with
3
2c
SHELXL97.
All programs used during the crystal structure
3
2d
determination process are included in the WINGX software.
3
2e
PLATON was used to check the result of the X-ray analyses
3
2f
and DIAMOND was used for the molecular graphics. CCDC
8
2
88516 (for 4b(trans)), 888517 (for 6b(cis)), 960495 (for
a(anti)) and 960496 (for 2b(syn)) contain the supplementary
crystallographic data (excluding structure factors) for this
paper.
Acknowledgements
Financial support from the Swiss National Science Foundation
and the University of Zürich are gratefully acknowledged.
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6570 | Dalton Trans., 2015, 44, 6560–6570
This journal is © The Royal Society of Chemistry 2015