Synthesis and characterization of hydrido carbonyl molybdenum and tungsten PNP pincer complexes

Article abstract of DOI:10.1021/om400254k

In the present study the Mo(0) and W(0) complexes [M(PNP)(CO)₃] as well as seven-coordinate cationic hydridocarbonyl Mo(II) and W(II) complexes of the type [M(PNP)(CO)₃H]⁺, featuring PNP pincer ligands based on 2,6-diaminopyridine, have been prepared and fully characterized. The synthesis of Mo(0) complexes [Mo(PNP)(CO)₃] was accomplished by treatment of [Mo(CO)₃(CH₃CN)₃] with the respective PNP ligands. The analogous W(0) complexes were prepared by reduction of the bromocarbonyl complexes [W(PNP)(CO)₃Br]⁺ with NaHg. These intermediates were obtained from the known dinuclear complex [W(CO) ₄(μ-Br)Br]₂, prepared in situ from W(CO)₆ and stoichiometric amounts of Br₂. Addition of HBF₄ to [M(PNP)(CO)₃] resulted in clean protonation at the molybdenum and tungsten centers to generate the Mo(II) and W(II) hydride complexes [M(PNP)(CO)₃H]⁺. The protonation is fully reversible, and upon addition of NEt₃ as base the Mo(0) and W(0) complexes [M(PNP)(CO)₃] are regenerated quantitatively. All heptacoordinate complexes exhibit fluxional behavior in solution. The mechanism of the dynamic process of the hydrido carbonyl complexes was investigated by means of DFT calculations, revealing that it occurs in a single step. The structures of representative complexes were determined by X-ray single-crystal analyses.

Full text of DOI:10.1021/om400254k

Article
pubs.acs.org/Organometallics
Synthesis and Characterization of Hydrido Carbonyl Molybdenum
and Tungsten PNP Pincer Complexes
†
†
‡
§
§
̈
̈
Ozgur Oztopcu, Christian Holzhacker, Michael Puchberger, Matthias Weil, Kurt Mereiter,
̈
Luis F. Veiros,∥ and Karl Kirchner*,†
†Institute of Applied Synthetic Chemistry, ‡Institute of Materials Chemistry, and §Institute of Chemical Technologies and Analytics,
Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, Austria
∥
́
́ ́
Centro de Quımica Estrutural, Instituto Superior Tecnico, Universidade Tecnica de Lisboa, Av. Rovisco Pais No. 1, 1049-001 Lisboa,
Portugal
S
* Supporting Information
ABSTRACT: In the present study the Mo(0) and W(0) complexes [M(PNP)(CO)3] as well as seven-coordinate cationic
hydridocarbonyl Mo(II) and W(II) complexes of the type [M(PNP)(CO)3H]+, featuring PNP pincer ligands based on 2,6-
diaminopyridine, have been prepared and fully characterized. The synthesis of Mo(0) complexes [Mo(PNP)(CO)3] was
accomplished by treatment of [Mo(CO)3(CH3CN)3] with the respective PNP ligands. The analogous W(0) complexes were
prepared by reduction of the bromocarbonyl complexes [W(PNP)(CO)3Br]+ with NaHg. These intermediates were obtained
from the known dinuclear complex [W(CO)4(μ-Br)Br]2, prepared in situ from W(CO)6 and stoichiometric amounts of Br2.
Addition of HBF4 to [M(PNP)(CO)3] resulted in clean protonation at the molybdenum and tungsten centers to generate the
Mo(II) and W(II) hydride complexes [M(PNP)(CO)3H]+. The protonation is fully reversible, and upon addition of NEt3 as
base the Mo(0) and W(0) complexes [M(PNP)(CO)3] are regenerated quantitatively. All heptacoordinate complexes exhibit
fluxional behavior in solution. The mechanism of the dynamic process of the hydrido carbonyl complexes was investigated by
means of DFT calculations, revealing that it occurs in a single step. The structures of representative complexes were determined
by X-ray single-crystal analyses.
Ni(II), Pd(II), and Pt(II) PNP complexes,13 various iron
INTRODUCTION
■
complexes acting as CO sensors14 and catalysts for the coupling
of aromatic aldehydes with ethyldiazoacetate,15 and several
pentacoordinated nickel complexes.16 Surprisingly, as far as
group 6 PNP complexes are concerned, only a few examples
have been described in the literature. A few years ago Haupt
and co-workers reported the synthesis of [M(PNP-Ph)(CO)3]
(M = Cr, Mo, W),12 while Walton and co-workers described
the synthesis of the dinuclear molybdenum complex
Tridentate PNP ligands in which the central pyridine-based
ring donor contains −CH2PR2 substituents in the two ortho
positions are widely utilized ligands in transition-metal
chemistry (e.g., Fe, Ru, Rh, Ir, Pd, Pt).1−10 As part of our
effort to create tridentate PNP pincer-type ligands in which the
steric, electronic, and stereochemical properties can be easily
varied, we have recently described the synthesis of a series of
modularly designed PNP ligands based on N-heterocyclic
diamines and R2PCl which contain both bulky and electron-rich
dialkylphosphines as well as various P−O bond containing
achiral and chiral phosphine units.11 In these PNP ligands the
central pyridine ring contains −NR′PR2 (R′ = H, alkyl, R =
alkyl, aryl) substituents in the two ortho positions. This
methodology was first developed for the synthesis of N,N′-
bis(diphenylphosphino)-2,6-diaminopyridine (PNP-Ph).12
With these types of PNP ligands, we have thus far studied
their reactivity toward different transition-metal fragments,
which resulted in the preparation of a range of new pincer
transition-metal complexes, including several new square-planar
[ M o ( P N P ) M o ( H P C y 2 ) C l 3 ] ( P N P
= 2 , 6 - b i s -
(dicyclohexylphosphinomethyl)pyridine).17 Most recently, di-
nuclear molybdenum and tungsten dinitrogen complexes
bearing bulky PNP pincer ligands were found to work as
effective catalysts for the formation of ammonia from
dinitrogen.18 Finally, Templeton and co-workers described
the synthesis of a series of hydrido carbonyl and halo carbonyl
tungsten pincer complexes featuring the silazane-based PNP
Received: March 25, 2013
Published: May 7, 2013
© 2013 American Chemical Society
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pincer-type ligand HN(SiMe2CH2PPh2)2.19 In a preliminary
study we have prepared cationic seven-coordinate halo carbonyl
molybdenum pincer complexes of the types [Mo(PNP-
iPr)(CO)3I]+ and [Mo(PNP-iPr)(CO)2(CH3CN)I]+.13 Here
we report on the synthesis, characterization, and reactivity of a
series of new hydrido carbonyl molybdenum(II) and tungsten-
(II) PNP pincer complexes.20
of [W(CO)4(μ-Br)Br]2 in CH2Cl2 at room temperature with
the PNP ligands 1a−c afforded on workup the seven-
coordinate tungsten(II) complexes [W(PNP)(CO)3Br]Br
(3a−c) in 60−80% yields (Scheme 3).
It has to be noted that the reaction of PNP ligands with
[Mo(CO)4(μ-Br)Br]2,21 prepared in situ from Mo(CO)6 and
Br2 in CH2Cl2 at −70 °C, affords the analogous seven-
coordinate molybdenum complexes [Mo(PNP)(CO)3Br]Br.
This has been demonstrated for the synthesis of [Mo(PNP-
Ph)(CO)3Br]Br (4a) and [Mo(PNP-iPr)(CO)3Br]Br (4b), as
illustrated in Scheme 3.
The solid-state structures of 3a and 4b were determined by
single-crystal X-ray diffraction. Molecular views of 3a and 4b
are depicted in Figures 1 and 2, respectively, with selected bond
distances and angles reported in the captions. While the Mo-
bonded bromide in 4b was clearly in an axial position, the
bromide in the tungsten complex 3a adopted an axial position
at about 86% occupancy (Br1 in Figure 1), while the remaining
14% exchanged places with the carbonyl group C32−O3.
Stirring complexes 3a−c with an excess of 10% sodium
amalgam in THF gave the desired W(0) complexes [W(PNP-
Ph)(CO)3] (5a), [W(PNP-iPr)(CO)3] (5b), and [W(PNP-
tBu)(CO)3] (5c) as yellow solids in 70−80% isolated yields
(Scheme 3). This methodology also yields the analogous
Mo(0) complexesl thus being an alternative method to that
described previously.13 The use of Zn as reducing reagent
turned out to be problematic, due to the formation of highly
RESULTS AND DISCUSSION
■
Molybdenum(0) and Tungsten(0) Complexes. We have
recently reported the synthesis of molybdenum tricarbonyl
complexes of the type [Mo(PNP)(CO)3] (2a−c) by reacting
[Mo(CO)3(CH3CN)3], prepared in situ by refluxing a solution
of [Mo(CO)6] in CH3CN for 4 h, with the PNP ligands PNP-
Ph (1a), PNP-iPr (1b), and PNP-tBu (1c) in 74−90% isolated
yields.13 The same procedure was followed with the N-
methylated PNP ligand PNPMe-iPr (1d), affording [Mo-
(PNPMe-iPr)(CO)3] (2d) in 80% yield (Scheme 1). The new
Scheme 1
soluble and, thus, difficult to remove bromozincate anions, e.g.,
[ZnBr3·solvent]− (solvent = acetone, THF) and ZnBr4
.
2−
Complexes 5a−c were fully characterized by a combination of
1H, 13C{1H}, and 31P{1H} NMR spectroscopy, IR spectrosco-
py, and elemental analysis. Characteristic features of 5a−c
comprise, in the 13C{1H} NMR spectrum, two low-field triplet
resonances (1/2 ratio) in the ranges of 206−221 and 196−210
ppm assignable to the carbonyl carbon atoms trans and cis to
the pyridine nitrogen, respectively. The 31P{1H} NMR spectra
exhibit singlet resonances at 85.2, 106.6.1, and 131.6 ppm with
1JWP coupling constants of 315−329 Hz. The tungsten−
phosphorus coupling was observed as a doublet satellite due to
PNP ligand 1d was prepared in a three-step procedure
involving borane protection of the phosphine, a deprotona-
tion/alkylation step, and deprotection of the phosphine, as
shown in Scheme 2. The analogous tungsten complexes
Scheme 2
1
183W, 14% abundance with I = /2, superimposed over the
dominant singlet. The IR spectra show the typical three strong
to medium absorption bands of a mer CO arrangement in the
range of 2017−1760 cm−1 assignable to one weaker symmetric
and two strong asymmetric νCO stretching modes. The νCO
frequencies, in particular the symmetric CO stretch, is
indicative of the increasing electron donor strengths of the
PNP ligands and follow the order PNP-Ph < PNP-iPr ≈
PNPMe-iPr < PNP-tBu (Table 1). The CO stretching
frequencies are 1964 (2a, PNP-Ph), 1936 (2b, PNP-iPr),
1936 (2d, PNPMe-iPr), and 1922 cm−1 (2c, PNP-tBu). The
same order is found for the respective tungsten complexes. In
all complexes the PNP pincer ligand adopts the typical mer
coordination mode with no evidence for any fac isomers.22
In addition to the spectroscopic characterization, the solid-
state structures of 2d and 5b,c were determined by single-
crystal X-ray diffraction. Structural views are depicted in Figures
3−5, respectively, with selected bond distances and angles given
in the captions. The coordination geometry around the
tungsten center of 5b,c corresponds to a distorted octahedron
with P1−W−P2 and trans-CCO−W−CCO bond angles
154.43(4) and 165.7(2)° (5b), and 151.42(1) and
156.46(9)° (5c), respectively. For comparison, in the analogous
[W(PNP)(CO)3] can be prepared in a similar fashion but
require much longer reaction times (several days to prepare the
intermediate [W(CO)3(CH3CN)3]); moreover, the yields
turned out to be significantly lower (10−25%). It has to be
noted that already a few years ago Haupt and co-workers
reported the synthesis of [M(PNP-Ph)(CO3)] (M = Mo, W)
in 34 and 22% yields.12 We thus developed an alternative
method to obtain tungsten(0) complexes [W(PNP)(CO)3] via
the intermediacy of the known dinuclear complex [W(CO)4(μ-
Br)Br]2,21 prepared in situ from W(CO)6 and stoichiometric
amounts of Br2 in CH2Cl2 at −70 °C. Treatment of a solution
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Scheme 3
Figure 2. Structural view of [Mo(PNP-iPr)(CO)3Br]Br (4b) showing
50% thermal ellipsoids (Br− counterion omitted for clarity). Selected
bond lengths (Å) and bond angles (deg): Mo−C(18) = 2.037(2),
Mo−C(19) = 1.979(2), Mo−C(20) = 2.006(2), Mo−N(1) =
2.236(2), Mo−P(1) = 2.5242(5), Mo−P(2) = 2.5172(5), Mo−
Br(1) = 2.6713(3); P(1)−Mo−P(2) = 150.80(2), N(1)−Mo−P(1) =
75.67(4), N(1)−Mo−P(2) = 75.35(4), N(1)−Mo−C(18) = 85.95(7),
N(1)−Mo−C(19) = 77.73(6), N(1)−Mo−C(20) = 127.23(6),
N(1)−Mo−Br(1) = 84.22(4).
Figure 1. Structural view of [W(PNP-Ph)(CO)3Br]Br·CH3OH
(3a·CH3OH) showing 20% thermal ellipsoids (H atoms, Br−
counterion, solvent molecule and subordinate Br/CO positions
omitted for clarity). Selected bond lengths (Å) and bond angles
(deg): W−C(31) = 2.017(5), W−C(30) = 2.020(6), W−C(32) =
2.024(8), W−N(1) = 2.237(3), W−P(1) = 2.4955(10), W−P(2) =
2.4895(11), W−Br(1) = 2.6015(5); P(1)−W−P(2) = 152.34(4),
N(1)−W−P(1) = 77.44(9), N(1)−W−P(2) = 75.92(9), N(1)−W−
C(30) = 137.76(16), N(1)−W−C(31) = 82.35(16), N(1)−W−C(32)
= 150.87(8), N(1)−W−Br(1) = 82.28(9).
Table 1. Selected IR, 31P{1H} NMR, and 13C{1H} NMR Data
for [M(PNP)(CO)3] and [M(PNP)(CO)3H]+ (M = Mo, W)
complex
νCO, cm−1
δP, ppm
116.2
δCO, ppm
δH, ppm
2a
2b
2c
2d
5a
5b
5c
7a
7b
7c
7d
8a
8b
8c
1964, 1858, 1765
1936, 1809, 1790
1922, 1808, 1771
1936, 1810, 1795
1955, 1847, 1759
1929, 1805, 1784
1914, 1799, 1759
2042, 1940, 1937
2035, 1923, 1920
2019, 1937, 1916
2028, 1928, 1910
2038, 1963, 1918
2027, 1910, 1906
2021, 1934, 1897
228.4, 211.2
231.4, 216.9
233.1, 224.0
230.8, 217.9
206.0, 196.6
221.1, 210.6
224.7, 219.4
212.7, 203.2
212.3, 205.8
213.7, 209.5
210.8, 205.4
205.9, 196.6
205.5, 198.0
[Mo(PNP)(CO)3] complexes 2a−d the P1−Mo−P2 angles
are hardly affected by the size of the substituents of the
phosphorus atoms, being 155.0(2), 155.62(1), 155.3(1), and
151.73(1)°, respectively.12,13 The carbonyl−Mo−carbonyl
angles of the CO ligands trans to one another, on the other
hand, vary strongly with the bulkiness of the PR2 moiety (PNP-
Ph2 < PNP-iPr2 < PNPMe-iPr < PNP-tBu2) and decrease from
171.1(8)° in [Mo(PNP-Ph)(CO)3], to 166.03(5)° in [Mo-
(PNP-iPr)(CO)3], to 162.93(7)° in [Mo(PNPMe-iPr)(CO)3],
and finally to 156.53(4)° in [Mo(PNP-tBu)(CO)3].
Molybdenum(II) and Tungsten(II) Hydride Com-
plexes. Addition of HBF4 to a CH2Cl2 solution of [Mo-
(PNP)(CO)3] (2a−d) and [W(PNP)(CO)3] (5a−c) resulted
in an immediate color change from yellow to pale yellow
consistent with protonation at the tungsten and molybdenum
centers to generate tungsten(II) and molybdenum(II) hydride
complexes [Mo(PNP)(CO)3H]BF4 (7a−d) and [W(PNP)-
143.6
161.9
171.0
100.2
128.5
147.2
111.5, 97.8
142.3, 121.4
158.8, 142.8
166.1, 147.7
95.5, 84.8
125.9, 108.6
141.1, 126.8
−3.78
−4.98
−4.34
−5.49
−3.43
−4.83
−4.16
(CO)3H]BF4 (8a−c), respectively (Scheme 4). The proto-
nation is fully reversible, and upon addition of NEt3 as base the
Mo(0) and W(0) complexes [M(PNP)(CO)3] are re-formed
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Figure 5. Structural view of [W(PNP-tBu)(CO)3]·THF (5c·THF)
showing 50% thermal ellipsoids (H atoms and solvent molecule
omitted for clarity). Selected bond lengths (Å) and bond angles (deg):
W−C(22) = 1.941(3), W−C(23) = 1.997(2), W−C(24) = 2.001(2),
W−N(1) = 2.277(2), W−P(1) = 2.4583(5), W−P(2) = 2.4656(5);
P(1)−W−P(2) = 151.42(2), N(1)−W−P(1) = 76.52(4), N(1)−W−
P(2) = 76.00(4), N(1)−W−C(22) = 169.53(8), N(1)−W−C(23) =
113.16(8), N(1)−W−C(24) = 90.27(7), C(23)−W−C(24) =
156.46(9).
Figure 3. Structural view of [Mo(PNPMe-iPr)(CO)3] (2d) showing
50% thermal ellipsoids (H atoms are omitted for clarity; the complex is
1
mirror symmetric; symmetry code i for x, /2 − y, z). Selected bond
lengths (Å) and bond angles (deg): Mo−C(14) = 1.956(2), Mo−
C(15) = 2.0153(13), Mo−N(1) = 2.2589(15), Mo−P(1) = 2.3977(5),
Mo−P(2) = 2.4070(5); P(1)−Mo−P(2) = 155.25(2), N(1)−Mo−
P(1) = 77.74(4), N(1)−Mo−P(2) = 77.51(4), N(1)−Mo−C(15) =
98.52(4), C(15)−Mo−C(15i) = 162.93(7).
Scheme 4
arrangements are typically characterized by a markedly lower
total energy.25 Hence, interconversions between these various
structures are quite facile. The fluxional behavior of complexes
1
7a−d and 8a−c was evident in variable-temperature H and
1
31P{1H} NMR spectra. At room temperature, the H NMR
spectrum of complexes 7 and 8 confirmed the presence of one
hydride ligand, which appeared in the range of −3.89 to −5.36
ppm either as a well-resolved doublet of doublets (8b,c) or as
triplets (7a−d, 8a).
Figure 4. Structural view of [W(PNP-iPr)(CO)3]·THF·1/2C6H14
(5b·THF·1/2C6H14) showing 30% thermal ellipsoids (H atoms,
solvent molecules, and alternative orientation of iPr group C(16a)−
C(15a)−C(17a) omitted for clarity). Selected bond lengths (Å) and
bond angles (deg): W−C(18) = 2.014(5), W−C(19) = 2.015(5), W−
C(20) = 1.934(4), W−N(1) = 2.257(3), W−P(1) = 2.4080(12), W−
P(2) = 2.4013(12); P(1)−W−P(2) = 154.43(4), N(1)−W−P(1) =
77.30(9), N(1)−W−P(2) = 77.18(9), N(1)−W−C(18) = 100.8(2),
N(1)−W−C(19) = 93.3(2), N(1)−W−C(20) = 175.8(2), C(18)−
W−C(19) = 165.7(2).
At −60 °C, all hydride resonances appear as a well-resolved
doublet of doublets with one large and one small coupling
constant of about 21−36 and 47−53 Hz, respectively. As an
1
example, the variable-temperature 300 MHz H NMR spectra
of the hydride region of [Mo(PNP-Ph)(CO)3H]BF4 (7a) are
shown in Figure 6. At low temperatures the hydride signal
constitutes the X part of an AMX spin system, giving rise to a
doublet of doublets which, at elevated temperatures in the fast
exchange regime, becomes a simple A2X spin system where the
X part exhibits a triplet resonance.
quantitatively (Scheme 4). All hydride complexes are thermally
robust pale yellow solids that are air stable in the solid state but
slowly decompose in solution. Characterization was accom-
plished by elemental analysis and by 1H, 13C{1H}, and 31P{1H}
NMR and IR spectroscopy (Table 1). The recording of a
13C{1H} NMR spectrum of 8c was precluded due to the poor
solubility of this complex in most common solvents.
Seven-coordinate complexes are well-known for their
fluxional behavior in solution,23,24 since typically none of the
idealized geometries such as capped prism, capped octahedron,
and pentagonal bipyramid or any of the less symmetrical
In the 13C{1H} NMR spectrum of complexes 7a−d and 8a−
d the most noticeable resonances are two low-field resonances
of the carbonyl carbon atoms trans and cis to the pyridine
nitrogen observed as two triplets in a 1:2 ratio. At room
temperature, no 31P{1H} NMR signals could be detected for
molybdenum complexes 7a−d and the tungsten complex 8a
due to their fluxional behavior. At −60 °C, however, the
31P{1H} NMR spectra of all complexes 8 give rise to two
doublets with a large geminal coupling constant of about 80 Hz,
which is indicative of the phosphorus atoms being in mutually
trans positions. The IR spectra of 7 and 8 show three strong to
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Figure 8. Structural view of [Mo(PNPMe-iPr)(CO)3H]BF4·CH2Cl2
−
(7d·CH2Cl2) showing 50% thermal ellipsoids (BF4 counterion and
CH2Cl2 omitted for clarity). Selected bond lengths (Å) and bond
angles (deg): Mo−H(1) = 1.63(2), Mo−C(20) = 2.0440(13), Mo−
C(21) = 2.0239(13), Mo−C(22) = 1.9856(13), Mo−N(1) =
2.2462(10), Mo−P(1) = 2.4425(3), Mo−P(2) = 2.4776(3); P(1)−
Mo−P(2) = 154.61(1), N(1)−Mo−P(1) = 77.29(3), N(1)−Mo−
P(2) = 77.31(3), N(1)−Mo−C(20) = 94.27(4), N(1)−Mo−C(21) =
89.76(4), N(1)−Mo−C(22) = 162.26(4), N(1)−Mo−H(1) =
145.7(8), C(22)−Mo−H(1) = 52.0(8).
1
Figure 6. Variable-temperature 300 MHz H NMR spectra of the
hydride region of [Mo(PNP-Ph)(CO)3H]BF4 (7a) in CD2Cl2.
medium absorption νCO bands of the one symmetric and the
two asymmetric vibration modes (Table 1), which again are
typical for a mer CO arrangement.
In addition to the spectroscopic characterization, the solid-
state structures of 7a,d and 8a were determined by single-
crystal X-ray diffraction. Structural diagrams are depicted in
Figures 7−9, respectively, with selected bond distances given in
Figure 9. Structural view of [W(PNP-Ph)(CO)3H]BF4 (8a) showing
50% thermal ellipsoids (BF4− counterion and alternative orientation of
C(32)−O(3) and H(1) omitted for clarity). Selected bond lengths
(Å) and bond angles (deg): W−H(1) = 1.65(5), W−C(30) =
2.033(2), W−C(31) = 2.039(2), W−C(32) = 2.022(4), W−N(1) =
2.2316(15), W−P(1) = 2.4444(5), W−P(2) = 2.4459(5); P(1)−W−
P(2) = 154.32(2), N(1)−W−P(1) = 77.17(4), N(1)−W−P(2) =
77.29(4), N(1)−W−C(30) = 87.78(7), N(1)−W−C(31) = 86.88(7),
N(1)−W−C(32) = 162.35(11), N(1)−W−H(1) = 143.5(17),
C(32)−W−H(1) = 53.9(17).
Figure 7. Structural view of [Mo(PNP-Ph)(CO)3H]BF4 (7a) showing
50% thermal ellipsoids (BF4− counterion and alternative orientation of
C(32)−O(3) and H(1) omitted for clarity). Selected bond lengths
(Å) and bond angles (deg): Mo−H(1) = 1.67(4), Mo−C(30) =
2.0368(15), Mo−C(31) = 2.0465(15), Mo−C(32) = 2.017(3), Mo−
N(1) = 2.2357(11), Mo−P(1) = 2.4409(4), Mo−P(2) = 2.4489(4);
P(1)−Mo−P(2) = 154.64(1), N(1)−Mo−P(1) = 77.39(3), N(1)−
Mo−P(2) = 77.39(3), N(1)−Mo−C(30) = 87.62(5), N(1)−Mo−
C(31) = 86.97(5), N(1)−Mo−C(32) = 163.73(9), N(1)−Mo−H(1)
= 146.0(15), C(32)−Mo−H(1) = 50.2(15).
hydride bond length in the three complexes averages 1.65 Å
(1.64−1.70 Å), the mean bond angle H−M−P to the nearest P
atom is 67° (65−69°), and the mean bond angle H−M−C
between hydride and the equatorial carbonyl group is 53° (50−
55°). The hydride to carbonyl C atom distance (1.62 Å in 7d)
and the almost linear attachment of the equatorial carbonyl
group (Mo1−C22−O3 = 179° in 7d) do not indicate a
significant bonding interaction between hydride and the
adjacent carbonyl C atom in the three structurally characterized
hydrido carbonyl complexes.
the captions. The coordination geometry around the
molybdenum center corresponds to a distorted capped
octahedron, in which a hydride ligand occupies the capping
position of an octahedral face. The crystal structure showed the
tridentate PNP ligand to be bound meridionally with three
carbonyl ligands filling the remaining three sites. The carbonyl
trans to nitrogen was pushed toward one of the phosphine
ligands to accommodate the hydride ligand. The metal−
The mechanism of the dynamic process of the hydrido
carbonyl complexes was investigated by means of DFT
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calculations26 for the molybdenum and tungsten complexes
[Mo(PNP)(CO)3H]+ (7a−c) and [W(PNP)(CO)3H]+ (8a−
c). The free energy profile for the “pseudorotation” is depicted
in Figure 10. The optimized structures of 8a,c and the
Figure 12. Front (left) and side views (right) of the optimized
structures (DFT/B3LYP) of the tungsten complex [W(PNP-tBu)-
(CO)3H]+ (8c; top) and the transition state TS (bottom) with most
tBu carbon atoms and hydrogen atoms omitted for clarity.
hydride ligand. Accordingly, In the case of the molybdenum
and tungsten PNP complexes bearing the less bulky Ph and iPr
substituents (7a,b and 8a,b), the C30−W1−C31 angle changes
from about 177° in the ground-state structure to 166° in TS,
where both trans CO ligands are bent toward the PNP ligand
(Figure 11). On the other hand, in the case of the molybdenum
and tungsten PNP complexes bearing the bulky tBu
substituents (7c and 8c) the situation is somewhat different.
The C30−W1−C31 angle increases from 169° in the minima
to 178° in TS, while one of the two trans CO ligands is severely
bent away from the PNP ligand and the other one is bent
toward the PNP ligand (Figure 12). In the course of all these
transformations also the H1−W1−C32 angles change from
about 55° in 7a−c and 8a−c to roughly 43° in the TS,
respectively. Bond distances are hardly affected by the
interconversion. It is interesting to note that, in agreement
with the X-ray structures of 7a, 7d, and 8a (Figures 6−8), the
distance between H1 and C32 is rather short, being in the range
of 1.80−1.66 Å. A Wiberg index27 of 0.20, in 8c, seems to
indicate an attractive interaction between the hydride and the
neighboring CCO atom (C32). Most importantly, the free
energy barriers ΔG⧧ are 18.6 and 21.3 kcal/mol for 7c and 8c,
respectively, containing the bulky tBu substituents. In the case
of all other complexes the free energy barrier is lower, being in
the range of 12.5−16.1 kcal/mol, as shown in Figure 9. These
results corroborate a process that can be stopped in the
temperature range employed in the NMR studies and a more
facile process in the case of the Mo species, as observed.
Figure 10. Free energy profile (in kcal/mol) for the “pseudorotation”
of CO and hydride ligands in the complexes [M(PNP)(CO)3H]+ (M
= W, Mo).
Figure 11. Front (left) and side views (right) of the optimized
structures (DFT/B3LYP) of the tungsten complex [W(PNP-Ph)-
(CO)3H]+ (8a; top) and the transition state TS (bottom) with most
phenyl carbon atoms and hydrogen atoms omitted for clarity.
CONCLUSION
■
In the present study the Mo(0) and W(0) complexes
[M(PNP)CO)3] as well as seven-coordinate cationic hydrido
carbonyl and halo carbonyl Mo(II) and W(II) complexes of the
type [M(PNP)(CO)3Br]+ and [M(PNP)(CO)3H]+ featuring
PNP pincer ligands based on 2,6-diaminopyridine were
prepared and fully characterized. The synthesis of the Mo(0)
complexes [Mo(PNP)CO)3] was accomplished by treatment of
[Mo(CO)3(CH3CN)3] with the respective PNP ligands. The
analogous W(0) complexes were prepared by reduction of the
bromo carbonyl complexes [W(PNP)(CO)3Br]+ with NaHg.
These intermediates were obtained from the known dinuclear
complex [W(CO)4(μ-Br)Br]2, prepared in situ from W(CO)6
and stoichiometric amounts of Br2. Addition of HBF4 to
[M(PNP)(CO)3] resulted in protonation at the tungsten and
corresponding transition states TS are shown in Figures 11 and
12. In the fluxional process, the CO and the hydride ligands in
the PNP plane exchange positions in a single-step path. During
that process the rest of the molecule has to change in order to
accommodate the overall transformations associated with the
pseudorotation. The main geometry change that happens along
the path is the hydride ligand moving from the PNP plane to
the perpendicular plane, i.e., the plane of the three CO ligands,
in the transition state (TS) and then back to the PNP plane
again but on the other side of the CO ligand. Thus, in TS, the
two trans CO ligands have to create enough space to allow the
presence of an extra ligand in the NCCC plane, opening the
corresponding OC−M−CO angle and bending away from the
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N,N′-Bis(diisopropylphosphino)-N,N′-methyl-2,6-diamino-
pyridine (PNPMe-iPr; 1d). 1d·BH3 (5.00 g, 12.59 mmol) was refluxed
for 72 h in 100 mL of Et2NH. After removal of the solvent under
reduced pressure the remaining oil was dissolved in THF, filtered
through Celite, and obtained as a yellow oil after evaportaion of the
solvent under reduced pressure. The crude product was purified by
recrystallization from acetonitrile to afford 2d as a white solid. Anal.
Calcd for C19H37N3P2: C, 61.77; H, 10.09; N, 11.37. Found: C, 61.69;
H, 10.16; N, 11.44. Yield: 3.25 g (70%). 1H NMR (δ, CD2Cl2, 20 °C):
7.22 (t, J = 8.0 Hz, 1H, py4), 6.64 (bs, py3,5), 3.04 (d, J = 2.3 Hz, 6H,
NCH3), 2.22 (bs, 4H, CH(CH3)2), 1.10 (dd, J = 6.9 Hz, J = 16.9 Hz,
12H, CH(CH3)2), 0.98 (dd, J = 7.0 Hz, J = 12.1 Hz, 12H, CH(CH3)2).
13C{1H} NMR (δ, CD2Cl2, 20 °C): 160.4 (s, py2,6), 136.9 (s, py4),
molybdenum centers to formally generate the Mo(II) and
W(II) hydride complexes [M(PNP)(CO)3H]+. The proto-
nation is fully reversible, and upon addition of NEt3 as base the
Mo(0) and W(0) complexes [M(PNP)(CO)3] are re-formed
quantitatively. All seven-coordinate complexes exhibit fluxional
behavior in solution, since none of the idealized geometries
(capped prism, capped octahedron, and pentagonal bipyramid)
or any of the less symmetrical arrangements are typically
characterized by a markedly lower total energy. The mechanism
of the dynamic process of the hydrido carbonyl complexes was
investigated by means of DFT calculations, revealing that it
occurs in a single step. Thereby the CO and the hydride ligands
which are situated in the PNP plane are interconverted. The
structures of representative complexes were determined by X-
ray single-crystal analyses.
99.1 (d, J = 22.3 Hz, py3,5), 33.7 (bs, NCH3), 26.2 (d, J = 15.3 Hz,
CH(CH3)2), 19.4 (s, CH(CH3)2), 19.2 (s, CH(CH3)2), 19.0 (s,
CH(CH3)2). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 81.3 (bs).
[Mo(PNPMe-iPr)(CO)3] (2d). A suspension of Mo(CO)6 (714 mg
2.7 mmol) in acetonitrile (10 mL) was refluxed for 4 h. After that
PNPMe-iPr (1d; 1.00 g, 2.7 mmol) was added and the mixture was
refluxed for an additional 12 h. The solvent was then removed under
reduced pressure, and the product was washed twice with diethyl ether
and dried under vacuum. Yield: 1.19 g (80%). Anal. Calcd for
C22H37MoN3O3P2: C, 48.09; H, 6.79; N, 7.65. Found: C, 48.15; H,
EXPERIMENTAL SECTION
■
General Considerations. All manipulations were performed
under an inert atmosphere of argon by using Schlenk techniques.
The solvents were purified according to standard procedures.28 The
ligands and complexes N,N′-bis(diphenylphosphino)-2,6-diaminopyr-
idine (PNP-Ph; 1a), N,N′-bis(diisopropylphosphino)-2,6-diaminopyr-
idine (PNP-iPr; 1b), N,N′-bis(di-tert-butylphosphino)-2,6-diamino-
pyridine (PNP-tBu; 1c), [Mo(PNP-Ph)(CO)3] (2a), [Mo(PNP-
iPr)(CO)3] (2b), and [Mo(PNP-tBu)(CO)3] (2c) were prepared
according to the literature.13 The deuterated solvents were purchased
1
6.82; N, 7.61. H NMR (δ, CD2Cl2, 20 °C): 7.40 (t, J = 7.6 Hz, 1H,
py), 6.30 (d, J = 7.6 Hz, 2H, py), 3.01 (s, 6H, NCH3), 2.47 (m, 4H,
CH(CH3)2), 1.30 (m, 12H, CH(CH3)2), 1.09 (m, 12H, CH(CH3)2).
13C{1H} NMR (δ, CD2Cl2, 20 °C): 230.8 (t, J = 6.0 Hz, CO), 217.9 (t,
J = 10.8 Hz, CO), 174.9 (t, J = 2.6 Hz, py), 137.8 (s, py), 96.7 (t, J =
2.2 Hz, py), 33.9 (t, J = 1.8 Hz, N(CH3)2), 32.9 (t, J = 9.0 Hz,
CH(CH3)2), 29.6 (s, N(CH3)2), 19.2 (t, J = 7.5 Hz, CH(CH3)2), 18.1
(s, CH(CH3)2). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 171.0 (s). IR
(ATR, cm−1): 1936 (νCO), 1810 (νCO), 1795 (νCO).
from Aldrich and dried over 4 Å molecular sieves. H, 13C{1H}, and
1
31P{1H} NMR spectra were recorded on Bruker AVANCE-250 and
AVANCE-300 DPX spectrometers and were referenced to SiMe4 and
H3PO4 (85%), respectively.
[W(PNP-Ph)(CO)3Br]Br (3a). To a suspension of W(CO)6 (2.0 g,
5.68 mmol) in CH2Cl2 (30 mL) was added Br2 (292 μL, 5.68 mmol)
at −70 °C, and the mixture was stirred for 1 h at that temperature and
for an additional 1 h at room temperature. After that, PNP-Ph (1a;
2.72 g, 5.68 mmol) was added and the mixture was stirred for 5 h at
room temperature. After removal of the solvent under reduced
pressure, a yellow solid was obtained, which was washed with a 1/9
methanol/diethyl ether mixture and dried under vacuum. Yield: 4.11 g
(80%). Anal. Calcd for C32H25Br2N3O3P2W: C, 42.46; H, 2.78; N,
N,N′-Bis(diisopropylphosphino-borane)-2,6-diaminopyri-
dine (PNP-iPr; 1b·BH3). BH3·THF (43.1 mL, 1.0 M, 43.06 mmol)
was added slowly to a solution of 1b (7.00 g, 20.50 mmol) in 100 mL
of dry THF. After the mixture was stirred for 30 min at room
temperature, the solvent was evaporated under reduced pressure. The
white solid was further dried under vacuum for 2 h to give the product
in quantitative yield. Anal. Calcd for C17H39B2N3P2: C, 55.32; H,
1
10.65; N, 11.39. Found: C, 55.28; H, 10.70; N, 11.43. H NMR (δ,
CDCl3, 20 °C): 7.32 (t, J = 7.9 Hz, 1H, py4), 6.26 (d, J = 7.9, 2H,
py3,5), 4.58 (d, J = 8.4 Hz, 2H, NH), 2.62 (septd, J = 6.9 Hz, J = 13.7
Hz, 4H, CH(CH3)2), 1.16 (m, 24H, CH(CH3)2), 0.60 to −0.15 (bs,
6H, BH3). 13C{1H} NMR (δ, CDCl3, 20 °C): 154.4 (s, py2,6), 140.1 (s,
py4), 103.1 (s, py3,5), 24.5 (d, J = 36.3 Hz, CH(CH3)2), 17.2 (d, J = 4.3
Hz, CH(CH3)2), 17.0 (s, CH(CH3)2). 31P{1H} NMR (δ, CDCl3, 20
°C): 88.5 (br, m).
1
4.64. Found: C, 42.29; H, 2.79; N, 4.55. H NMR (δ, acetone-d6, 20
°C): 10.85 (bs, 2H, NH), 7.78 (bs, 11H, py, Ph), 7.60 (bs, 10H, Ph),
7.43 (d, J = 7.6 Hz, 2H, py). 13C{1H} NMR (δ, CDCl3, 20 °C): 225.4
(t, J = 10.3 Hz, CO), 210.3 (t, J = 8.6 Hz, CO), 159.7 (d, J = 7.5 Hz,
py), 159.6 (d, J = 7.5 Hz, py), 133.1 (s, py), 131.9 (s, Ph), 131.9 (s,
Ph), 129.0 (d, J = 5.0 Hz, Ph), 128.2 (d, J = 5.4 Hz, Ph), 103.1 (s, py).
31P{1H} NMR (δ, acetone-d6, 20 °C): 85.2. IR (ATR, cm−1): 2030
(νCO), 1958 (νCO), 1933 (νCO).
N,N′-Bis(diisopropylphosphino-borane)-N,N′-methyl-2,6-di-
aminopyridine (PNPMe-iPr; 1d·BH3). To a solution of 1b·BH3 (7.45
g, 20.19 mmol) in THF (50 mL) at −20 °C was slowly added n-BuLi
(17.0 mL, 2.5 M, 41.39 mmol). The reaction mixture was allowed to
reach room temperature and was stirred for 2 h. Methyl iodide (3.15
mL, 50.46 mmol) was then added slowly via syringe. After the mixture
was stirred for 12 h at room temperature, the reaction was quenched
with a saturated NH4Cl solution (100 mL) and 5 mL of concentrated
NH3. The aqueous phase was extracted twice with CH2Cl2, and the
combined organic phases were washed with 25 mL of brine and dried
over Na2SO4. The solvent was removed under reduced pressure to
afford 1d·BH3 as a yellow oil. The crude product was purified via flash
chromatography using silica gel and THF to give the product as a
white solid. Anal. Calcd for C19H43B2N3P2: C, 57.56; H, 10.91; N,
[W(PNP-iPr)(CO)3Br]Br (3b). This complex was prepared
analogously to 3a with W(CO)6 (2.0 g, 5.68 mmol) and PNP-iPr
(1b; 1.95 g, 5.69 mmol) as starting materials. Yield: 3.06 g (70%).
Anal. Calcd for C20H33Br2N3O3P2W: C, 31.23; H, 4.32; N, 5.46.
Found: C, 31.13; H, 4.42; N, 5.50. 1H NMR (δ, CD2Cl2, 20 °C): 9.32
(bs, 2H, NH), 7.66 (t, J = 7.2 Hz, 1H, py), 6.67 (d, J = 8.1 Hz, 2H,
py), 3.54 (m, 2H, CH(CH3)2), 2.88 (m, 2H, CH(CH3)2), 1.43 (m,
24H, CH(CH3)2). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 212.2 (t, J =
10.1 Hz, CO), 191.3 (t, J = 7.6 Hz, CO), 161.0 (s, py), 142.2 (s, py),
101.9 (s, py), 30.6 (t, J = 15.0 Hz, CH(CH3)2), 29.3 (t, J = 15.3 Hz,
CH(CH3)2), 19.2 (s, CH(CH3)2), 19.1 (s, CH(CH3)2), 18.9 (s,
CH(CH3)2), 17.7 (s, CH(CH3)2). 31P{1H} NMR (δ, CD2Cl2, 20 °C):
106.6. IR (ATR, cm−1): 2023 (νCO), 1953 (νCO), 1922 (νCO).
[W(PNP-tBu)(CO)3Br]Br (3c). This complex was prepared
analogously to 3a with W(CO)6 (2.0 g, 5.68 mmol) and PNP-tBu
(1c) (2.26 g, 5.69 mmol) as starting materials. Yield: 2.51 g (60%).
Anal. Calcd for C24H41Br2N3O3P2W: C, 34.93; H, 5.01; N, 5.09.
Found: C, 34.81; H, 5.02; N, 5.20. 1H NMR (δ, CD2Cl2, 20 °C): 9.11
(s, 2H, NH), 7.74 (d, J = 7.8 Hz, 2H, py), 7.23 (t, J = 8.7 Hz, 1H, py),
1.70 (d, J = 8.2 Hz, 9H, C(CH3)3), 1.68 (d, J = 7.3 Hz, 9H, C(CH3)3),
1.54 (d, J = 7.5 Hz, 9H, C(CH3)3), 1.48 (d, J = 7.1 Hz, 9H, C(CH3)3).
1
10.58. Found: C, 57.62; H, 10.89; N, 10.61. Yield: 5.05 g (63%). H
NMR (δ, CDCl3, 20 °C): 7.48 (t, J = 8.0 Hz, 1H, py4), 6.49 (d, J = 8.0,
2H, py3,5), 3.17 (d, J = 7.9 Hz, 6H, NCH3), 2.80 (septd, J = 7.0 Hz, J =
21.2 Hz, 4H, CH(CH3)2), 1.22 (dd, J1 = 6.9 Hz, J2 = 16.5 Hz, 12H,
CH(CH3)2), 1.03 (dd, J = 7.0 Hz, J = 15.11 Hz, 12H, CH(CH3)2),
0.70 to −0.30 (bs, 6H, BH3). 13C{1H} NMR (δ, CDCl3, 20 °C): 156.9
(s, py2,6), 139.1 (s, py4), 105.8 (s, py3,5), 37.6 (s, NCH3), 25.6 (d, J =
36.2 Hz, CH(CH3)2), 17.8 (s, CH(CH3)2), 17.2 (s, CH(CH3)2).
31P{1H} NMR (δ, CDCl3, 20 °C): 105.9 (br, m).
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13C{1H} NMR (δ, CDCl3, 20 °C): 229.9 (t, J = 14.8 Hz, CO), 212.9
(t, J = 7.6 Hz, CO), 162.6 (t, J = 4.2 Hz, py), 153.3 (d, J = 5.8 Hz, py),
149.9 (s, py), 103.8 (s, py), 46.0 (t, J = 7.2 Hz, C(CH3)3), 44.6 (t, J =
7.2 Hz, C(CH3)3), 31.9 (s, C(CH3)3), 30.4 (s, C(CH3)3), 26.9 (s,
C(CH3)3). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 131.6. IR (ATR, cm−1):
2019 (νCO), 1941 (νCO), 1909 (νCO).
C(CH3)3). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 147.2 (s, JW−P = 329
Hz). IR (ATR, cm−1): 1914 (νCO), 1799 (νCO), 1759 (νCO).
Alternative Synthesis of [Mo(PNP-iPr)(CO)3] (2b). This
complex was prepared analogously to 5a with 4b (88 mg, 0.13
mmol) and NaHg (9 mg, 0.39 mmol) as starting materials. Yield: 61
mg (90%). All spectral data for 2a are identical with those of the
authentic sample reported previously.13
[Mo(PNP-Ph)(CO)3Br]Br (4a). This complex was prepared
analogously to 3a with Mo(CO)6 (300 mg, 1.14 mmol) and PNP-
Ph (1a; 570 mg, 1.20 mmol) as starting materials. Yield: 775 mg
(83%). Anal. Calcd for C32H25Br2MoN3O3P2: C, 47.03; H, 3.08; N,
[Mo(PNP-Ph)(CO)3H]BF4 (7a). To a solution of 2a (200 mg, 0.30
mmol) in CH2Cl2 (10 mL) was added HBF4 ((46 μL, 0.45 mmol, 54%
solution in Et2O) at room temperature. After the solution was stirred
overnight, a pale yellow precipitate was formed, which was collected
on a glass frit, washed with diethyl ether, and dried under vacuum.
Yield: 193 mg (85%). Anal. Calcd for C32H26BF4MoN3O3P2: C, 51.57;
1
5.14. Found: C, 46.95; H, 3.12; N, 5.01. H NMR (δ, acetone-d6, 20
°C): 8.70 (s, 2H, NH), 7.59 (m, 5H, Ph), 7.47 (m, 5H, Ph), 7.16 (t, J
= 7.2 Hz, 1H, py), 7.01 (m, 5H, Ph), 6.85 (m, 5H, Ph), 6.56 (d, J = 7.8
Hz, 2H, py). 31P{1H} NMR (δ, acetone-d6, 20 °C): 125.3. IR (ATR,
cm−1): 2040 (νCO), 1975 (νCO), 1875 (νCO).
1
H, 3.52; N, 5.64. Found: C, 51.66; H, 3.59; N, 5.70. H NMR (δ,
acetone-d6, −60 °C): 9.25 (s, 2H, NH), 8.23 (m, 13H, Ph, py), 7.93
(m, 8H, Ph), 6.99 (d, J = 8.0 Hz, 2H, py), −3.78 (dd, 2JHP = 21.6 Hz,
2JHP = 48.5 Hz, 1H, MoH). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 212.7
(t, J = 11.2 Hz, CO), 203.2 (t, J = 8.4 Hz, CO), 158.4 (d, J = 6.1 Hz,
py), 158.1 (d, J = 8.3 Hz, py), 141.6 (s, py), 135.3 (s, py), 134.5 (d, J =
55.6 Hz, Ph), 131.8 (s, Ph), 130.7 (d, J = 13.7 Hz, Ph), 129.0 (d, J =
11.1 Hz, Ph), 102.2 (d, J = 8.3 Hz, py). 31P{1H} NMR (δ, acetone-d6,
−60 °C): 111.5 (d, 2JPP = 88.4 Hz), 97.8 (d, 2JPP = 88.4 Hz). IR (ATR,
cm−1): 2042 (νCO), 1940 (νCO), 1937 (νCO).
[Mo(PNP-iPr)(CO)3Br]Br (4b). This complex was prepared
analogously to 3a with Mo(CO)6 (500 mg, 1.89 mmol) and PNP-
iPr (1b; 678 mg, 1.98 mmol) as starting materials. Yield: 965 mg
(75%). Anal. Calcd for C20H33Br2MoN3O3P2: C, 35.26; H, 4.88; N,
6.17. Found: C, 35.13; H, 4.92; N, 6.20. 1H NMR (δ, CDCl3, 20 °C):
9.08 (s, 2H, NH), 7.24 (d, J = 9.9 Hz, 2H, py), 7.10 (t, J = 7.4 Hz, 1H,
py), 3.55 (m, 2H, CH(CH3)2), 2.96 (m, 2H, CH(CH3)2). 1.44 (m,
24H, CH(CH3)2). 13C{1H} NMR (δ, CDCl3, 20 °C): 234.6 (t, J = 7.2
Hz, CO), 218.3 (t, J = 11.5 Hz, CO), 175.1 (s, py), 175.0 (s, py), 160.0
(t, J = 4.9 Hz, py), 142.1 (s, py), 102.7 (s, py), 30.8 (d, J = 12.8 Hz,
CH(CH3)2), 30.6 (d, J = 14.9 Hz, CH(CH3)2), 30.3 (d, J = 12.4 Hz,
CH(CH3)2), 30.1 (d, J = 11.6 Hz, CH(CH3)2), 19.4 (s, CH(CH3)2),
19.3 (s, CH(CH3)2), 19.1 (s, CH(CH3)2), 18.1 (s, CH(CH3)2).
31P{1H} NMR (δ, CDCl3, 20 °C): 126.7. IR (ATR, cm−1): 2030
(νCO), 1967 (νCO), 1936 (νCO).
[W(PNP-Ph)(CO)3] (5a). A solution of 3a (200 mg, 0.22 mmol) in
THF (15 mL) was stirred in the presence of NaHg (10%) (16 mg,
0.66 mmol) for 8 h at room temperature. The solvent was then
removed under reduced pressure. The residue was redissolved in
acetone (10 mL), and the solution was filtered through Celite. After
removal of the solvent under reduced pressure, a yellow solid was
obtained, which was washed twice with diethyl ether (10 mL) and
dried under vacuum. Yield: 140 mg (85%). Anal. Calcd for
C32H25N3O3P2W: C, 51.57; H, 3.38; N, 5.64. Found: C, 51.63; H,
3.41; N, 5.50. 1H NMR (δ, acetone-d6, 20 °C): 7.64 (t, J = 8.6 Hz, 1H,
py), 7.44 (bs, 8H, Ph), 7.30 (m, 12H, Ph), 6.43 (d, J = 7.6 Hz, 2H, py),
5.93 (d, J = 8.9 Hz, 2H, NH). 13C{1H} NMR (δ, acetone-d6, 20 °C):
206.0 (t, J = 4.6 Hz, CO), 196.6 (t, J = 9.1 Hz, CO), 159.6 (d, J = 4.7
Hz, py), 159.3 (d, J = 5.5 Hz, py), 142.1 (s, py), 132.0 (s, Ph), 131.0
(d, J = 12.4 Hz, Ph), 129.1 (d, J = 11.5 Hz, Ph), 127.9 (d, J = 6.6 Hz,
Ph), 102.0 (d, J = 10.7 Hz, py). 31P{1H} NMR (δ, acetone-d6, 20 °C):
100.2 (s, JW−P = 327 Hz). IR (ATR, cm−1): 1955 (νCO), 1847 (νCO),
1759 (νCO).
[Mo(PNP-iPr)(CO)3H]BF4 (7b). This complex was prepared
analogously to 7a with 2b (200 mg, 0.38 mmol) and HBF4 (78 μL,
0.58 mmol) as starting materials. Yield: 200 mg (87%). Anal. Calcd for
C20H34BF4MoN3O3P2: C, 39.43; H, 5.63; N, 6.90. Found: C, 39.53; H,
5.71; N, 6.99. 1H NMR (δ, acetone-d6, −60 °C): 8.86 (d, J = 41.2 Hz,
2H, NH), 8.00 (t, J = 8.2 Hz, 1H, py), 6.95 (d, J = 2.91 Hz, 1H, py),
6.92 (d, J = 2.91 Hz, 1H, py), 3.27 (m, 4H, CH(CH3)2), 1.83 (m, 24H,
2
2
CH(CH3)2), −4.98 (dd, JHP = 36.7 Hz, JHP = 51.0 Hz, 1H, MoH).
13C{1H} NMR (δ, CD2Cl2, 20 °C): 212.3 (t, J = 11.3 Hz, CO), 205.8
(t, J = 9.0 Hz, CO), 159.8 (d, J = 3.9 Hz, py), 159.7 (d, J = 3.9 Hz, py),
141.5 (s, py), 100.5 (d, J = 7.5 Hz, py), 31.5 (d, J = 29.5 Hz,
CH(CH3)2), 17.8 (d, J = 2.3 Hz, CH(CH3)2). 31P{1H} NMR (δ,
2
2
acetone-d6, −60 °C): 142.3 (d, JPP = 90.4 Hz), 121.4 (d, JPP = 90.4
Hz). IR (ATR, cm−1): 2035 (νCO), 1923 (νCO), 1920 (νCO).
[Mo(PNP-tBu)(CO)3H]BF4 (7c). This complex was prepared
analogously to 7a with 2c (240 mg, 0.42 mmol) and HBF4 (85 μL,
0.62 mmol) as starting materials. Yield: 243 mg (87%). Anal. Calcd for
C24H42BF4MoN3O3P2: C, 43.33; H, 6.36; N, 6.32. Found: C, 43.43; H,
1
6.42; N, 6.28. H NMR (δ, acetone-d6, −60 °C): 8.06 (bs, 2H, NH),
7.62 (t, J = 8.3 Hz, 1H, py), 6.62 (d, J = 7.9 Hz, 2H, py), 1.82 (d, J =
5.8 Hz, 18H, C(CH3)3), 1.80 (d, J = 6.5 Hz, 18H, C(CH3)3), −4.34
2
2
(dd, JHP = 20.7 Hz, JHP = 47.8 Hz, 1H, MoH). 13C{1H} NMR (δ,
acetone-d6, 20 °C): 213.7 (t, J = 12.1 Hz, CO), 209.5 (t, J = 9.4 Hz,
CO), 160 (d, J = 7.0 Hz, py), 141.8 (s, py), 137.9 (s, py), 101.5 (d, J =
6.5 Hz, py), 97.3 (s, py), 41.2 (d, J = 15.8 Hz, C(CH3)3), 24.8 (d, J =
7.1 Hz, C(CH3)3). 31P{1H} NMR (δ, acetone-d6, −60 °C): 158.8 (d,
2
2JPP = 81.5 Hz), 142.8 (d, JPP = 81.5 Hz). IR (ATR, cm−1): 2019
[W(PNP-iPr)(CO)3] (5b). This complex was prepared analogously
to 5a with 3b (500 mg, 0.65 mmol) and NaHg (45 mg, 1.95 mmol) as
starting materials. Yield: 315 mg (80%). Anal. Calcd for
C20H33N3O3P21W: C, 39.43; H, 5.46; N, 6.90. Found: C, 39.51; H,
5.42; N, 6.84. H NMR (δ, CD2Cl2, 20 °C): 7.15 (t, J = 8.2 Hz, 1H,
py), 6.15 (d, J = 7.9 Hz, 2H, py), 5.50 (bs, 2H, NH), 2.40 (m, 4H,
CH(CH3)2), 1.24 (m, 24H, CH(CH3)2). 13C{1H} NMR (δ, CD2Cl2,
20 °C): 221.1 (t, J = 2.0 Hz, CO), 210.6 (t, J = 7.1 Hz, CO), 162.5 (t, J
= 8.5 Hz, py), 137.1 (s, py), 96.6 (t, J = 3.1 Hz, py), 32.60 (t, J = 12.8
Hz, CH(CH3)2), 18.5 (t, J = 1.7 Hz, CH(CH3)2), 18.1 (t, J = 2.9 Hz,
CH(CH3)2). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 128.5 (s, JW−P = 315
Hz). IR (ATR, cm−1): 1929 (νCO), 1805 (νCO), 1784 (νCO).
[W(PNP-tBu)(CO)3] (5c). This complex was prepared analogously
to 5a with 3c (350 mg, 0.42 mmol) and NaHg (30 mg, 1.27 mmol) as
starting materials. Yield: 215 mg (77%). Anal. Calcd for
C24H41N3O3P2W: C, 43.32; H, 6.21; N, 6.32. Found: C, 43.23; H,
(νCO), 1937 (νCO), 1916 (νCO).
[Mo(PNPMe-iPr)(CO)3H]BF4 (7d). This complex was prepared
analogously to 7a with 2d (67 mg, 0.11 mmol) and HBF4 (23 μL, 0.17
mmol) as starting materials. Yield: 63 mg (90%). Anal. Calcd for
C22H38BF4MoN3O3P2: C, 41.46; H, 6.01; N, 6.59. Found: C, 41.55; H,
1
6.12; N, 6.47. H NMR (δ, acetone-d6, −60 °C): 7.93 (t, J = 8.2 Hz,
1H, py), 6.65 (d, J = 8.2 Hz, 2H, py), 3.41 (s, 6H, N(CH3)), 3.25 (m,
4H, CH(CH3)2), 1.24 (d, J = 7.00 Hz, 6H, CH(CH3)2), 1.22 (d, J =
7.80 Hz, 12H, CH(CH3)2), 1.16 (d, J = 7.00 Hz, 6H, CH(CH3)2),
−5.49 (dd, 2JHP = 20.9 Hz, 2JHP = 49.3 Hz, 1H, MoH). 13C{1H} NMR
(δ, CD2Cl2, 20 °C): 210.8 (t, J = 10.8 Hz, CO), 205.4 (t, J = 9.5 Hz,
CO), 161 (t, J = 5.2 Hz, py), 141.9 (s, py), 100.7 (s, py), 35.3 (s,
NCH3), 32.3 (d, J = 24.9 Hz, CH(CH3)2), 19.3 (d, J = 7.6 Hz,
CH(CH3)2), 17.9 (s, CH(CH3)2). 31P{1H} NMR (δ, acetone-d6, −60
2
2
°C): 166.1 (d, JPP = 85.4 Hz), 147.7 (d, JPP = 85.4 Hz). IR (ATR,
1
cm−1): 2028 (νCO), 1928 (νCO), 1910 (νCO).
6.42; N, 6.48. H NMR (δ, CD2Cl2, 20 °C): 7.37 (bs, 2H, NH), 7.18
(t, J = 8.0 Hz, 1H, py), 6.29 (d, J = 7.5 Hz, 2H, py), 1.41 (t, J = 5.6 Hz,
36H, C(CH3)3). 13C{1H} NMR (δ, acetone-d6, 20 °C): 224.7 (t, J =
6.4 Hz,CO), 219.4 (t, J = 7.0 Hz, CO), 162.6 (d, J = 6.7 Hz, py), 137.8
(s, py), 97.1 (s, py), 40.7 (t, J = 7.5 Hz, C(CH3)3), 25.8 (d, J = 7.3 Hz,
[W(PNP-Ph)(CO)3H]BF4 (8a). This complex was prepared
analogously to 7a with 5a (210 mg, 0.28 mmol) and HBF4 (58 μL,
0.42 mmol) as starting materials. Yield: 186 mg (80%). Anal. Calcd for
C32H26BF4N3O3P2W: C, 46.13; H, 3.15; N, 5.04. Found: C, 46.23; H,
3049
dx.doi.org/10.1021/om400254k | Organometallics 2013, 32, 3042−3052

Organometallics
Article
Table 2. Details for the Crystal Structure Determinations of Compounds 3a·CH3OH, 4b, 2d, 5b·THF·1/2C6H14, 5c·THF, 7a,
7d·CH2Cl2, and 8a
3a·CH3OH
4b
2d
5b·THF·1/2C6H14
formula
C33H29Br2N3O4P2W
937.20
C20H33Br2MoN3O3P2
681.19
C22H37MoN3O3P2
549.43
C27H48N3O4P2W
724.47
fw
cryst size, mm
0.16 × 0.06 × 0.05
orange prism
triclinic
0.59 × 0.20 × 0.18
yellow plate
monoclinic
C2/c (No. 15)
29.6913(15)
10.7919(5)
16.7764(8)
90
0.24 × 0.14 × 0.10
yellow prism
orthorhombic
Pnma (No. 62)
18.6552(5)
12.1772(3)
10.9035(2)
90
0.50 × 0.40 × 0.20
yellow plate
monoclinic
P21/c (No. 14)
10.1399(10)
15.5053(18)
20.036(2)
90
color, shape
cryst syst
space group
P1 (No. 2)
̅
a, Å
10.1741(6)
13.1580(7)
14.1040(8)
99.456(3)
95.412(3)
104.306(3)
1786.63(17)
296(2)
b, Å
c, Å
α, deg
β, deg
97.954(1)
90
90
94.485(5)
90
γ, deg
V, Å3
90
5323.9(4)
173(2)
2476.93(10)
100(2)
3140.5(6)
100(2)
T, K
Z
2
8
4
4
ρcalcd, g cm−3
μ(Mo Kα), mm−1
F(000)
1.742
1.700
1.473
1.532
5.598
3.640
0.687
3.815
908
2720
1144
1468
abs corr
multiscan, 0.77−0.56
1.96−30.00
46633
multiscan, 0.50−0.32
2.30−30.00
36362
multiscan, 0.93−0.83
2.18−30.07
20758
multiscan, 0.75−0.46
1.66−30.00
77629
θ range, deg
no. of rflns measd
Rint
0.060
0.022
0.033
0.042
no. of unique rflns
no. of rflns with I > 2σ(I)
no. of params/restraints
10399
7732
3770
9124
7880
6562
3328
6686
393/6
294/96
167/0
335/75
a
R1 (I > 2σ(I))
0.0385
0.0273
0.0217
0.0398
R1 (all data)
0.0625
0.0351
0.0276
0.0689
wR2 (I > 2σ(I))
0.0822
0.0693
0.0507
0.0805
wR2 (all data)
min/max diff Fourier peaks, e Å−3
0.0884
0.0734
0.0534
0.0907
−1.39/1.56
5c·THF
−1.01/0.97
7a
−0.32/0.43
7d·CH2Cl2
−1.80/1.85
8a
formula
C28H49N3O4P2W
737.48
C32H26BF4MoN3O3P2
745.25
C23H40BCl2F4MoN3O3P2
722.17
C32H26BF4N3O3P2W
833.16
fw
cryst size, mm
0.30 × 0.17 × 0.15
yellow block
monoclinic
P21/n (No. 14)
8.9480(2)
16.2069(4)
21.6874(5)
90
0.42 × 0.35 × 0.28
yellow block
orthorhombic
Pbca (No. 61)
17.4696(5)
15.5290(4)
23.3940(6)
90
0.32 × 0.30 × 0.14
yellow plate
monoclinic
P21/n (No. 14)
8.4324(3)
23.4895(7)
16.2384(5)
90
0.24 × 0.08 × 0.06
yellow column
orthorhombic
Pbca (No. 61)
17.5150(4)
15.5022(3)
23.3387(5)
90
color, shape
cryst syst
space group
a, Å
b, Å
c, Å
α, deg
β, deg
95.539(1)
90
90
104.815(2)
90
90
γ, deg
V, Å3
90
90
3130.41(13)
100(2)
6346.5(3)
100(2)
3109.46(17)
100(2)
6336.9(2)
100(2)
T, K
Z
4
8
4
8
ρcalcd, g cm−3
μ(Mo Kα), mm−1
F(000)
1.565
1.560
1.543
1.747
3.828
0.576
0.751
3.809
1496
3008
1480
3264
abs corr
multiscan, 0.60−0.51
1.89−30.00
59364
multiscan, 0.85−0.78
1.96−30.00
147535
multiscan, 0.75−0.63
2.17−30.00
52608
multiscan, 0.75−0.56
1.96−30.00
103531
θ range, deg
no. of rflns measd
Rint
0.023
0.030
0.030
0.032
no. of unique rflns
no. of rflns with I > 2σ(I)
no. of params/restraints
9120
9239
9052
9225
8540
8380
8280
8120
355/0
440/0
366/0
440/0
a
R1 (I > 2σ(I))
0.0202
0.0252
0.0216
0.0200
R1 (all data)
0.0226
0.0302
0.0253
0.0260
wR2 (I > 2σ(I))
0.0470
0.0595
0.0504
0.0385
3050
dx.doi.org/10.1021/om400254k | Organometallics 2013, 32, 3042−3052

Organometallics
Article
Table 2. continued
5c·THF
0.0479
7a
7d·CH2Cl2
8a
wR2 (all data)
0.0636
−0.71/0.50
0.0525
−0.43/0.45
0.0406
−0.87/0.74
min/max diff Fourier peaks, e Å−3
−1.03/1.40
a
2
2
R1 = ∑||Fo| − |Fc||/∑|Fo|; wR2 = {∑[w(Fo − Fc2)2]/∑[w(Fo )2]}1/2
.
1
3.12; N, 5.20. H NMR (δ, acetone-d6, −60 °C): 9.74 (bs, 2H, NH),
with the procedure SQUEEZE of the program PLATON. 5b has also
an orientation-disordered isopropyl group. The hydride complexes 7a
and 8a form an isostructural pair, [Mo/W(PNP-Ph)(CO)3H]BF4,
space group Pbca, with very similar unit cell dimensions, bond lengths,
and bond angles. Both structures show a disorder of the equatorial CO
group (C32−O3) and the hydride H atom (H1), which appear in two
approximately equivalent positions left and right of the plane bisecting
the complexes perpendicular to the pyridine ring. Due to the very
good quality of the diffraction data of both complexes and due to good
separations of the respective atom positions, it was possible to refine
the approximately half-occupied positions of the hydride H atoms
without restraints. The left/right ratio of the population parameter of
CO and H in the molybdenum complex 7a is 0.540(3)/0.460(3) and
in the tungsten complex 8a is 0.586(4)/0.414(4) (for C32, O3, H1/
C32′, O3′, H1′; cf. Figures 7 and 9, which depict only the dominant
nonprimed part). In contrast, the analogous molybdenum hydrido
carbonyl complex 7d·CH2Cl2 was perfectly ordered and gave on
refinement with high-quality diffraction data a hydride position in very
good agreement with complexes 7a and 8a, fully supporting the split
atom refinements of these two crystal structures.
Computational Details. All calculations were performed using the
GAUSSIAN 09 software package32 on the Phoenix Linux Cluster of
the Vienna University of Technology and the B3LYP functional33
without symmetry constraints. The optimized geometries were
obtained with the Stuttgart/Dresden ECP (SDD) basis set34 to
describe the electrons of the tungsten and molybdenum atoms. For all
other atoms a standard 6-31g** basis set was employed.35 All
geometries were optimized without symmetry constraints. Frequency
calculations were performed to confirm the nature of the stationary
points, yielding one imaginary frequency for the transition states and
none for the minima. Each transition state was further confirmed by
following its vibrational mode downhill on both sides and obtaining
the minima presented on the energy profiles. All energies reported are
Gibbs free energies and thus contain zero-point, thermal, and entropy
effects at 298 K and 1 atm pressure. A natural population analysis
(NPA)36 and the resulting Wiberg indices27 were used to study the
electronic structure and bonding of the optimized species. The NPA
analysis was performed with the NBO 5.0 program.37
8.15 (s, 8H, Ph), 8.25 (t, J = 8.4 Hz, 1H, py), 8.15 (m, 8H, Ph), 7.94
(m, 12H, Ph), 7.04 (d, J = 8.0 Hz, 2H, py), −3.43 (dd, 2JHP = 22.4 Hz,
2JHP = 53.8 Hz 1H, WH). 13C{1H} NMR (δ, acetone-d6, 20 °C): 205.9
(t, J = 9.7 Hz, CO), 196.6 (t, J = 8.7 Hz, CO), 159.5 (d, J = 5.9 Hz,
py), 159.3 (d, J = 5.9 Hz, py), 142.1 (s, py), 132.0 (s, py), 130.9 (d, J =
13.0 Hz, Ph), 129.5 (d, J = 13.7 Hz, Ph), 129.1 (d, J = 12.1 Hz, Ph),
127.9 (d, J = 4.6 Hz, Ph), 102.0 (d, J = 9.4 Hz, py). 31P{1H} NMR
2
2
(δ,acetone-d6, −60 °C): 95.5 (d, JPP = 85.9 Hz), 84.8 (d, JPP = 85.9
Hz). IR (ATR, cm−1): 2038 (νCO), 1963 (νCO), 1918 (νCO).
[W(PNP-iPr)(CO)3H]BF4 (8b). This complex was prepared
analogously to 8a with 5b (200 mg, 0.33 mmol) and HBF4 (20 μL,
0.49 mmol) as starting materials. Yield: 207 mg (90%). Anal. Calcd for
C20H34BF4N3O3P2W: C, 34.46; H, 4.92; N, 6.03. Found: C, 34.55; H,
1
5.02; N, 6.10. H NMR (δ, CD2Cl2, 20 °C): 8.47 (bs, 2H, NH), 7.50
(t, J = 8.5 Hz, 1H, py), 6.67 (d, J = 7.7 Hz, 2H, py), 2.41 (m, 4H,
2
CH(CH3)2), 1.30 (m, 28H, CH(CH3)2), −4.83 (dd, JHP = 23.3 Hz,
2JHP = 54.7 Hz, 1H, WH). 13C{1H} NMR (δ, CD2Cl2, 20 °C): 205.5
(t, J = 8.5 Hz, CO), 198.0 (t, J = 8.1 Hz, CO), 160.1 (d, J = 3.9 Hz,
py), 141.6 (s, py), 100.4 (d, J = 6.4 Hz, py), 31.9 (d, J = 25.5 Hz,
CH(CH3)2), 18.0 (d, J = 4.2 Hz, CH(CH3)2). 31P{1H} NMR (δ,
2
2
CD2Cl2, 20 °C): 125.9 (d, JPP = 80.0 Hz), 108.6 (d, JPP = 80.0 Hz).
IR (ATR, cm−1): 2027 (νCO), 1910 (νCO), 1906 (νCO).
[W(PNP-tBu)(CO)3H]BF4 (8c). This complex was prepared
analogously to 8a with 5c (200 mg, 0.30 mmol) and HBF4 (18 μL,
0.45 mmol) as starting materials. Yield: 207 mg (90%). Anal. Calcd for
C24H42BF4N3O3P2W: C, 38.27; H, 5.62; N, 5.58. Found: C, 38.29; H,
1
5.52; N, 5.52. H NMR (δ, acetone-d6, 20 °C): 8.00 (bs, 2H, NH),
7.60 (t, J = 6.7 Hz, 1H, py), 6.75 (d, J = 8.3 Hz, 2H, py), 1.59 (s, 18H,
C(CH3)3), 1.53 (s, 18H, C(CH3)3). −4.16 (dd, 2JHP = 37.8 Hz, 2JHP
=
50.8 Hz, 1H, WH). 31P{1H} NMR (δ, CD2Cl2, 20 °C): 141.1 (d, JPP
2
= 83.6 Hz), 126.8 (d, JPP = 83.6 Hz). IR (ATR, cm−1): 2021 (νCO),
2
1934 (νCO), 1897 (νCO).
X-ray Structure Determination. Single crystals of the complexes
3a, 4b, 2d, 5b,c, 7a,d, and 8a suitable for X-ray diffraction were mainly
obtained by the solvent/antisolvent liquid−liquid diffusion method at
room temperature using CH2Cl2/diethyl ether (4b, 2d, 7a,d, 8a) or
THF/n-hexane (5b,c), while 3a was crystallized from methanol at −20
°C. The crystals of 3a, 5b,c, and 7d were solvates (3a·CH3OH,
5b·THF·1/2hexane, 5c·THF, 7d·CH2Cl2). X-ray diffraction data were
collected at T = 100 K on a Bruker Kappa APEX-2 CCD area detector
diffractometer using graphite-monochromated Mo Kα radiation (λ =
0.71073 Å) and φ- and ω-scan frames covering complete spheres of
the reciprocal space with θmax = 30°. Corrections for absorption and λ/
2 effects were applied using the program SADABS.29 After structure
solution with the program SHELXS97 refinement on F2 was carried
out with the program SHELXL97.30 Non-hydrogen atoms were
refined anisotropically. Most hydrogen atoms were placed in calculated
positions and thereafter treated as riding. Crystal data are reported in
Table 2, and detailed structural data are given in CIF format in the
Supporting Information. Variata are as follows. The solid-state
structure of 3a (3a·CH3OH) contained methanol and eventually
some water disordered in a large oval infinite channel along the a axis
embodying also the free bromide anions. The contribution of this
solvent to the structure factors was removed with the procedure
SQUEEZE of the program PLATON.31 This structure shows also a Br
by CO and a complementary CO by Br substitution disorder
(equatorial C32−O3 by Br1′ and axial Br1 by C32′ and O3′ in 86:14
proportion). The crystal structure of 5b contains an ordered THF and
a disordered n-hexane solvent molecule, the latter across a center of
inversion; the solid is therefore 5b·THF·1/2C6H14. The contribution of
the n-hexane solvent molecule to the structure factors was removed
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files giving complete crystallographic data and technical
details for complexes 3a, 4b, 2d, 5b,c, 7a,d, and 8a. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail for K.K.: kkirch@mail.tuwien.ac.at.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by the Austrian Science Fund (FWF)
(Project No. P24202-N17) and by Fundaca̧ o para a Ciencia e
Tecnologia, FCT (PEst-OE/QUI/UI0100/2011), is gratefully
acknowledged
̂
̃
3051
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Organometallics
Article
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Martins, A. M. Eur. J. Inorg. Chem. 2007, 1103. (c) Shafiq, F.; Szaldza,
D. J.; Creutz, C.; Bullock, R. M. Organometallics 2000, 19, 824.
(d) Calvo, M.; Gomez-Sal, P.; Manzanero, A.; Royo, P. Polyhedron
1998, 17, 1081. (e) Brammer, L.; Zhao, D.; Bullock, R. M.; McMullan,
R. K. Inorg. Chem. 1993, 32, 4819. (f) Protasiewicz, J. D.; Theopold, K.
H. J. Am. Chem. Soc. 1993, 115, 5559. (g) Caffyn, A. J. M.; Feng, S. G.;
Dierdorf, A.; Gamble, A. S.; Eldredge, P. A.; Vossen, M. R.; White, P.
S.; Templeton, J. L. Organometallics 1991, 10, 2842.
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