Tuning "kappticity" of tripodal ligands

Article abstract of DOI:10.1039/b516127d

The synthesis and structural characterization of a series of tripodal tris(phosphine) ligands, containing SiMe₂ elbow groups, is described. The significant steric congestion in these ligands, due to the silylmethyl substituents, is manifest both in the solid-state structures and in the solution NMR spectra of the free ligands. Variable temperature ¹H{ ³¹P} NMR studies of one of the ligands, CH₃C(SiMe ₂PEt₂)₃ (4b) gave an estimated barrier to rotation around the Si-C_apical bonds of approximately 10.4 kcal mol^-1. Octahedral κ²- and κ³- molybdenum complexes of these ligands also demonstrate the impact of the additional bulk imparted by the SiMe₂ substituents, and the high Lewis basicity of these phosphines, with subtle changes at the apical and phosphine substituents changing the overall coordination chemistry observed. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b516127d

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www.rsc.org/dalton | Dalton Transactions
Tuning “kappticity” of tripodal ligands†
Dawn M. Friesen,a Owen J. Bowles,a Robert McDonalda,b and Lisa Rosenberg*a
Received 15th November 2005, Accepted 23rd March 2006
First published as an Advance Article on the web 6th April 2006
DOI: 10.1039/b516127d
The synthesis and structural characterization of a series of tripodal tris(phosphine) ligands, containing
SiMe2 elbow groups, is described. The significant steric congestion in these ligands, due to the
silylmethyl substituents, is manifest both in the solid-state structures and in the solution NMR spectra
31
of the free ligands. Variable temperature 1H{ P} NMR studies of one of the ligands,
CH3C(SiMe2PEt2)3 (4b) gave an estimated barrier to rotation around the Si–Capical bonds of
approximately 10.4 kcal mol−1. Octahedral j2- and j3-molybdenum complexes of these ligands also
demonstrate the impact of the additional bulk imparted by the SiMe2 substituents, and the high Lewis
basicity of these phosphines, with subtle changes at the apical and phosphine substituents changing the
overall coordination chemistry observed.
Introduction
Multidentate phosphine ligands play an important role in
transition metal coordination chemistry and catalysis.1 Among
these multifunctional, chelating ligands, tripodal tris(phosphine)
compounds such as triphos (1,1,1-tris(diphenylphosphino-
methyl)ethane, 1)2 have been exhaustively exploited and studied,1,3
due to the significant control they can offer over coordination
environments at a metal centre, by anchoring three ancillary
phosphine groups in mutually cis positions on one face of the
metal. Considerable effort has been devoted to tailoring this useful
ligand structure for a wide range of applications: tris(phosphine)
tripods have been prepared with modified apical groups,4 apical
pendant groups,5 and pendant substituents at phosphorus.6 How-
ever, the only examples of tripodal polyphosphines with other
than –CH2– groups a to phosphorus in the chelate backbone
are compounds 27 and 3a,8,9 and the potential impact of such
modified elbow groups on the coordination chemistry of tripodal
tris(phosphine) ligands remains relatively unexplored. Also, de-
spite the prevalence of triphos-derived ligands in coordination
chemistry, relatively few complexes of this popular tripodal
framework with dialkyl substituents at phosphorus have been
reported. We present here the synthesis and characterization of a
series of tripodal tris(phosphine) ligands containing SiMe2 elbow
groups, including both Ph and Et substituents at phosphorus,
and an evaluation of the electronic and steric properties of these
bulky triphos analogues based on their molybdenum coordination
chemistry.
Results
Ligand synthesis and characterization
Bulky triphos analogs 3a–b and 4a–b were prepared as outlined in
Scheme 1. 1,1,1-Tris(dimethylsilyl)ethane (5b) was not previously
described in the literature, despite its structural similarity to the
core of carbon-based triphos, CH3C(CH2PPh2)3 (1). The Si–Br
bonds in 6a–b were substituted by three equivalents of lithium
diphenyl- or diethylphosphide to give compounds 3a–b and 4a–b
in good yield.10
Fig. 1 and 2 show molecular structures determined for crystals
of the methyl-capped tripods 6b and 3b (see Table 1 for selected
bond lengths and angles). A key feature of the solid-state structures
of H-capped tripods 6a and 3a is a widening and flattening of the
tripodal trisilylmethane core, in particular relative to the solid-
state structure of HC(CH2PPh2)3,11 which complements tripod
conformation in minimizing interactions between silylmethyl
elbow groups on adjacent arms of the tripods.9 However, solid-
state structures of the Me-capped tripods 6b (Br-substituted) and
3b (PPh2-substituted) do not exhibit this “splaying”, despite the
presence of the SiMe2 elbows. The average Si–Capical–Si angles in
6b and 3b are both 110.6◦, and, accordingly, the central carbon
aDepartment of Chemistry, University of Victoria, P.O. Box 3065, Victoria,
British Columbia, Canada V8W 3V6. E-mail: lisarose@uvic.ca; Fax: 250-
721-7147; Tel: 1-250-721-7173
bX-Ray Crystallography Laboratory, Department of Chemistry, University
of Alberta, Edmonton, Alberta, Canada T6G 2G2
† Electronic supplementary information (ESI) available: Experimental
details including the solvolytic and oxidative sensitivities of 3 and 4 and
the mechanism of phosphine exchange in 3b. See DOI: 10.1039/b516127d
˚
in these Me-capped tripods is approximately 0.2 A further from
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distortion of the tetrahedral core. The resulting crowding renders
the relative conformations of tripod arms in the methyl-capped
compounds more sensitive than those in the corresponding H-
capped tripods to the size of the non-methyl substituents at silicon,
i.e. H, Br, PEt2, PPh2. For example, the solid-state structure of
methyl-capped, Br-substituted tripod 6b has a threefold rotation
axis coincident with the apical CH3–C bond (Fig.1, symmetry
operations are given in footnote a of Table 1). One methyl group
on each silicon elbow in 6b points down toward the base of
the tripod, approximately anti to the apical CH3–C bond, while
the second methyl group points back toward the bromine atom
on an adjacent arm. This “gearing” of the silyl methyl groups
around the threefold central axis minimizes the interactions of
these groups both with each other and with the apical methyl
substituent. The H-capped, Br-substituted tripod 6a adopts a
more relaxed, unsymmetric, syn-anti-anti conformation, but an
analogous, geared, C3-symmetric structure is observed for the H-
capped tripod 3a, with its larger PPh2-substituents.9a Meanwhile,
for the Me-capped, PPh2-substituted tripod 3b, gearing of SiMe
groups and bulky PPh2 substituents is not sufficient to offset
interactions between these groups and the methyl group at the apex
of this tetrahedral Si3 tripod. Dihedral angles of −42◦, −46◦, and
85◦ between the apical methyl carbon and the three phosphorus
atoms in C1-symmetric 3b give rise to the slightly “staggered”
structure illustrated in Fig. 3. Two of the diphenylphosphine
Scheme 1
Table 1 Selected bond lengths and angles for Me-capped tripods 6b and
3b
6ba, CH3C(SiMe2Br)3
3b, CH3C(SiMe2PPh2)3
˚
Bond lengths/A
C1–C2
C1–Si
1.577(11)
1.905(2)
C1–C2
C1–Si1
C1–Si2
C1–Si3
1.567(3)
1.923(2)
1.911(2)
1.918(2)
2.3151(9)
2.2948(9)
2.3063(9)
1.874(3)
1.871(3)
1.868(3)
1.882(2)
1.870(2)
1.872(2)
Fig. 1 Molecular structure of CH3C(Me2SiBr)3, 6b. Shown is one of two
enantiomeric forms, present in equal abundance in the crystal lattice.
Si–Br
2.2670(10) Si1–P1
Si2–P2
Si3–P3
Si–C3
Si–C4
1.890(6)
1.814(7)
Si1–C3
Si1–C4
Si2–C5
Si2–C6
Si3–C7
Si3–C8
Bond angles/◦
Si–C1–Siꢀ
110.6(2)
Si1–C1–Si2
Si1–C1–Si3
Si2–C1–Si3
P1–Si1–C1
P2–Si2–C1
P3–Si3–C1
110.03(11)
109.69(11)
112.09(11)
107.60(7)
112.26(7)
107.77(7)
104.64(11)
101.34(11)
101.32(11)
Br–Si–C1
Br–Si–C3
109.38(5)
103.60(18) C11–P1–C21
C31–P2–C41
C51–P3–C61
Torsional angles/◦
Br–Si–C1–C2
83.29(8)
P1–Si1–C1–C2
P2–Si2–C1–C2
P3–Si3–C1–C2
−42.07(15)
85.41(15)
−46.50(15)
a Primed atoms are related to unprimed ones via the rotational symmetry
operation (1 − y, x − y, z); double-primed atoms are related to unprimed
ones via the operation (1 − x + y, 1 − x, z) (both operations represent
opposite-handed one-third rotations about the crystallographic threefold
rotational axis (2/3, 1/3, z)).
Fig. 2 Molecular structure of CH3C(SiMe2PPh2)3, 3b.
the plane containing the three silicon atoms than in 6a and 3a:
˚
˚
0.600(7) A in 6b and 0.602(2) A in 3b. Thus the repulsions between
HMe(apical) and HSiMe in methyl-capped 6b and 3b do not allow
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arms. The increase is not large enough to allow resolution of
non-equivalent chemical shifts for distinct SiMe groups at low
temperature (183 K). Low temperature 1H NMR spectra of the H-
capped, PEt2-substituted tripod 4a show slight broadening of the
SiMe2 signal similar to that observed for 6b, while the SiMe2 signal
for Me-capped, PEt2-substituted 4b decoalesces into two broad
singlets of equal intensity, similar to the decoalescence previously
observed for H-capped, PPh2-substituted 3a,9a consistent with
slow exchange of SiMe groups between two inequivalent sites, in a
ground state solution structure of pseudo-C3 symmetry. Rotation
around the Capical-Si bonds is required for this exchange of SiMe
groups, and the barrier to this rotation was determined by line-
Fig. 3 Views illustrating C1 symmetry and the distinct environments for
SiMe groups in the molecular structure of 3b.
substituents in 3b (P1 and P3) are canted in the same direction
(counter-clockwise), while the third phosphine fragment (P2) has
the opposite cant (clockwise). This arrangement distinguishes P1
by placing it in a relatively unhindered environment, with the
phenyl groups pointing out and away from the bulk of the tripod
core. The remaining phosphine substituents (P2, P3) are canted
towards each other, which alleviates the crowding of SiMe2 groups
on these two arms of the tripod, with a partial “gearing” of the
four phenyl groups, similar to that seen for 3a. For the arms
bearing P2 and P3, one methyl group on each silicon atom is
anti relative to the CH3–C bond (MeA), while the second methyl
group is approximately gauche (MeB). The SiMe groups on the
third tripod arm (MeC) are staggered between those on the other
two arms, which optimizes the distance between the methyl and
phenyl substituents. Silicon-phosphorus bond lengths in 3b are
1–2% longer than those in 3a, which is also consistent with the
1
31
shape analysis of the SiMe peaks in the H{ P} NMR spectra
recorded for 4b at low temperature (Fig. 4). An energy of activation
(Ea) of 10.7 kcal mol−1 for this two-site exchange is similar to that
for 3a (Ea = 10.9 kcal mol−1),9a which contains the smaller apical
group (H) but larger PPh2-substituents. Finally, the SiMe2 signal
31
in the 1H{ P} NMR for a cooled sample of methyl-capped, PPh2-
substituted 3b undergoes an apparent decoalescence at 195 K
giving three broad singlets, consistent with a ground state
1
increased congestion around the core of 3b. 31P{ H} NMR spectra
of 3b in d8-toluene at low temperature are consistent with a ground
state solution structure analogous to this solid-state structure (see
Electronic Supporting Information (ESI) for details).†
Also sensitive to the combined steric influence of substituents
at the tripod apex and elbows are the barriers to tripod arm
rotation around the Capical–Si bonds for these compounds in
solution, as illustrated in Scheme 2. While 1H NMR spectroscopy
of H-capped tripod 6a (Br-derivative) shows a sharp singlet for
the SiMe2 protons even when the sample is cooled to 183 K,
1
consistent with rapid arm rotation, the SiMe2 signal in the H
Fig. 4 The SiMe2 region of (a) room temperature and (b) low temperature
1H{ P} NMR spectra of CH3C(SiMe2PEt2)3, 4b, in d8-toluene. (c)
NMR of compound 6b (Me-capped, Br-derivative) in toluene-
d8 shows some broadening when the sample is cooled, which
suggests that simply replacing the apical hydrogen in 6a with a
methyl group increases the barrier to rotation around the tripod
31
Calculated low temperature spectrum. The vertical scale for (b) and (c) is
40× that for (a). Signals due to an impurity of the disubstituted compound
CH3C(Si(CH3)2Br){Si(CH3)2PEt2}2 are marked with “*”.
Scheme 2
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solution structure of low symmetry, not inconsistent with its solid-
state structure.12
Molybdenum carbonyl complexes of the tris(phosphine) tripods
The j2-complexes 8a–b and 9a–b formed readily from the addition
of ligands 3a–b and 4a–b, respectively, to Mo(pip)2(CO)4 (pip =
piperidine) (eqn (1)), and were isolated as pale yellow solids, after
removal of the solvent and free piperidine under vacuum. The
Fig. 5 Molecular structure of {j2-HC(SiMe2PPh2)3}Mo(CO)4 (8a) show-
ing the chair conformation of the six member chelate ring. Non-hydrogen
atoms are represented by Gaussian ellipsoids at the 20% probability level.
The hydrogen atom attached to the methyne carbon (C10) is shown with
an arbitrarily small thermal parameter; all other hydrogens are not shown.
For clarity, only the ipso carbons from the phenyl groups are shown.
1
31P{ H} NMR spectra for all four j2-complexes show two singlets
in an approximately 2 : 1 ratio, with the shift of the lower intensity
peaks corresponding closely to that for free tripod. The solid-state
structure of 8a (Fig. 5, bond distances and angles in Table 2)
illustrates an approximate chair conformation adopted by the
six member chelate ring, as anticipated for these j2-molybdenum
complexes.13
As shown in Scheme 3, the j2-complexes 9a–b, containing PEt2-
substituted ligands, convert to the corresponding j3-complexes
10a–b, with loss of one equivalent of CO, with heating in benzene
or toluene. Complexes 10a–b were isolated from the reactions
6
of ligands 4a and 4b with Mo(CO)6 or Mo(g -mesitylene)(CO)3
1
in refluxing toluene. The 31P{ H} NMR spectra of these j3-
(1)
complexes each show a singlet, surrounded by a low intensity sextet
arising from coupling of the three equivalent phosphorus nuclei
to 95Mo (I = 5/2, 15.72% natural abundance, 1JMo–P ≈ 111 Hz in
both complexes). The molecular structures of 10a–b are shown in
Table 2 Selected bond lengths and angles for 8a, 10a, and 10b
8a: (j2-3a)Mo(CO)4
10aa: (j3-4a)Mo(CO)3
10b: (j3-4b)Mo(CO)3
˚
Bond lengths/A
Mo–P1
Mo–P2
P1–Si1
P2–Si2
P3–Si3
Mo–C1
Mo–C2
Mo–C3
Mo–C4
Si1–C10
Si2–C10
Si3–C10
2.5473(5)
2.5497(5)
2.3022(7)
2.3103(7)
2.2772(8)
1.986(2)
1.973(2)
2.032(2)
2.031(2)
Mo–P
2.5582(5)
Mo–P1
Mo–P2
Mo–P3
Si1–P1
Si2–P2
Si3–P3
Mo–C1
Mo–C2
Mo–C3
Si1–C4
Si2–C4
Si3–C4
2.5577(4)
2.5554(4)
2.5547(4)
2.2947(5)
2.2923(5)
2.2979(6)
1.9604(16)
1.9550(16)
1.9657(16)
1.9028(16)
1.9103(16)
1.9028(16)
SiA–P
SiB–P
2.3245(8)
2.277(3)
Mo–C1
1.955(2)
1.8981(19)
1.8890(19)
1.9162(19)
SiA–C10
SiB–C10
1.8886(9)
1.909(3)
Bond angles/◦
P1–Mo–P2
91.109(16)
P–Mo–Pꢀ
90.388(15)
P1–Mo–P2
P1–Mo–P3
P2–Mo–P3
Si1–C4–Si2
Si1–C4–Si3
Si2–C4–Si3
Mo–P1–Si1
Mo–P2–Si2
Mo–P3–Si3
C11–P1–C13
C21–P2–C23
C31–P3–C33
91.964(12)
90.251(12)
89.236(12)
112.42(7)
113.51(8)
112.74(8)
117.360(18)
116.676(18)
116.390(18)
101.23(8)
100.94(7)
100.96(8)
Si1–C10–Si2
Si1–C10–Si3
Si2–C10–Si3
Mo–P1–Si1
Mo–P2–Si2
118.02(10)
110.58(9)
112.38(10)
119.11(2)
124.24(2)
SiA–C10–SiAꢀ
SiB–C10–SiBꢀ
114.98(7)
112.45(11)
Mo–P–SiA
Mo–P–SiB
116.33(2)
113.77(7)
C11–P1–C21
C31–P2–C41
C51–P3–C61
99.47(9)
103.56(9)
106.37(10)
C11–P1–C13
100.92(10)
Torsional angles/◦
P1–Si1–C10–H
P2–Si2–C10–H
P2–Si3–C10–H
55.8
P–SiA–C10–H
P–SiB–C10–H
−169.5
P1–Si1–C4–C5
P2–Si2–C4–C5
P3–Si3–C4–C5
−170.08(9)
−166.58(10)
−168.36(9)
−65.5
159.8
22.9
a While the solid-state structure of 10a is highly symmetric, there is some disorder in the crystals. The SiA and SiB labels represent alternate atom positions
arising from the presence of a tripod with an opposite “twist”. Atoms bearing an “A” label are refined at 5/6 occupancy, and “B” atoms are refined at
1/6 occupancy giving a local 5 : 1 disorder. However, due to the presence of an inversion center in the unit cell (centrosymmetric space group R3), the
5 : 1 disorder also occurs as a 1 : 5 disorder in the invertomer, resulting in an overall equal distribution of the tripod “hands”.
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Fig. 7 Molecular structure of {j3-CH3C(SiMe2PEt2)3}Mo(CO)3 (10b).
coordinated triphos ligands on molybdenum carbonyl fragments,
but we note that mCO values for our j2-complexes 8–9 are also
consistently lower than those recorded for (R3P)2Mo(CO)4 with
R = Et or Ph, respectively (entries 13–18). Among the few
examples of tris(phosphine) molybdenum complexes exhibiting
mCO close to those observed for 10a–b are a series of j3-
triphosphacyclododecane complexes (e.g. entry 10), with all-alkyl
substituents at each phosphorus,19 and a complex containing the
very bulky phosphine P{(H)NPri}3 (entry 3). This latter ligand,
which is probably a good p-acid, should limit back-bonding to
the carbonyl ligands, yet the carbonyl stretching frequencies are
consistent with strong back-bonding from this P3 molybdenum
fragment. We note the much larger P–Mo–P angles of 94◦ in this
complex: although the strongly donating SiMe2 elbow groups,
in concert with the ethyl substituents at phosphorus in tripods
4a–b, probably do render our tris(phosphine) ligands very strong
r donors, we cannot rule out the influence of differences in
coordination geometries on our observed mCO values. For example,
there is very little variation in mCO for j3-triphos derivatives with
a range of different substituents at phosphorus (entries 4–9),
though we note each example contains at least one aryl group
at phosphorus. However, available crystallographic data suggests
these triphos ligands also routinely impose P–Mo–P angles ≤85◦
in their j3-complexes, while ligands 4a–b give P–Mo–P angles of
about 90◦ in complexes 10a–b, and {cyclo-(PriPC3H6)3} (entry 10)
imposes an average P–Mo–P angle of 88◦. Finally, consistent with
a high Lewis basicity of the PEt2-substituted tripod arms of 4a–
b, which should render both the tripod and the ancillary CO
ligands more stable to ligand substitution, the j2-complexes 9a–
b do not react with approximately equimolar amounts of PMe3,
Scheme 3
Fig. 6 and 7, respectively, and relevant bond lengths and angles are
listed in Table 2. Despite the conformational constraints imposed
by coordination of all three bulky tripod arms in these complexes
(vide infra), the geometry at Mo is undistorted octahedral, with
chelate bite angles of approximately 90◦. Reactions of 8a–b or 3a–
b analogous to those shown in Scheme 3 did not give j3-complexes
of the PPh2-substituted ligands 3a–b, even after prolonged reflux
of reaction mixtures in toluene.14
Fig. 6 View of the {j3-HC(SiMe2PEt2)3}Mo(CO)3 molecule (10a) slightly
offset along the C10–H bond axis, illustrating crystallographically-
imposed threefold symmetry. Atoms marked with a prime (ꢀ) character
are at (−y, x − y, z) and those marked with a double prime (ꢀꢀ) are at (−x +
y, −x, z).
even after 12 h of heating at 85 ◦C, as determined by 31P{ H}
1
NMR spectroscopic monitoring of mixtures of these complexes
with 1–2 equiv of PMe3 in d8-toluene in sealed NMR tubes. The
only reaction observed in these experiments is partial conversion
of the j2-complexes to j3, with loss of CO (eqn (2)),20 which also
occurs with heating in the absence of PMe3 (vide supra). Mixtures
of the PPh2-substituted j2-complexes 8a–b and approximately 2–3
equiv of PMe3 in d8-toluene in sealed NMR tubes showed, after
4 h of heating at 85 ◦C, signals corresponding to small amounts of
the products 11a–b, resulting from substitution of one CO ligand
by PMe3 (eqn (3)), perhaps indicative of a slightly lower Lewis
basicity of 3a–b relative to 4a–b.21
Discussion
Carbonyl stretching frequencies in the infrared spectra of
the molybdenum carbonyl phosphine complexes 8–10 (see Table 3)
apparently point to strong Lewis basicity of the ligands 3–4.
In particular, the j3-tripodal complexes 10a–b exhibit signifi-
cantly lower stretching frequencies than is observed for a wide
range of tris(phosphine) analogs (entries 1–2, 4–9 in Table 3),
indicating a weaker CO bond due to increased back-bonding
from molybdenum to CO. We have not found infrared data for j2-
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temperature NMR data point to at least the same degree of
congestion in 3b and 6b. The crystal structures of j3-complexes
10a–b (Fig. 6 and 7) show puckering in the 5-membered chelate
rings, which helps offset the Si–Me interactions on adjacent arms,
and P–Et conformations that direct half of the ethyl groups well
away from the SiMe2 groups, while the remaining ethyl groups
intercalate between SiMe2 groups on each arm. Replacement of
ethyl groups on phosphorus with phenyl groups would clearly
place more strain on these j3-structures, particularly for the flatter,
H-capped ligand 3a, leading to less stable complexes.
(2)
Greater stability of j2-complexes of the PPh2-substituted lig-
ands 3a–b relative to their putative j3-complexes may explain why
we have not observed coordination of that final tripod arm to yield
(j3-3)Mo(CO)3 complexes. However, particularly high barriers to
rotation around tripod arms in ligands 3a–b, as discussed above,
may also influence the rates of formation of these j3-complexes,
which presumably occurs via the j2-complexes. As shown in
Scheme 4, rotation around a Si–Capical bond is required to pull
(3)
The SiMe2 elbow groups in tripods 3 and 4 also present
steric constraints that affect the relative stabilities of j2- and j3-
complexes incorporating these ligands, more than for analogous
complexes of tripods with CH2 elbow groups. This is illustrated
by the structure of the j2-complex 8a (Fig. 5), containing PPh2-
substituted, H-capped ligand 3a. An unusually wide angle of
118◦ at the tripodal ligand apex (Si(1)–C(10)–Si(2)) in the chelate
ring in 8a indicates the distortion from tetrahedral required to
accommodate approximately gauche conformations of SiMe2 and
PPh2 groups on adjacent arms of the j2-ligand, and the third,
un-coordinated tripod arm is twisted well out of the way of the
Mo coordination sphere. The j3-coordination of 3 or 4 requires
anti conformations of all three phosphorus donors with respect
to the apical substituent (H or Me), which forces Si–Me elbow
substituents on adjacent tripod arms to point directly at each
other. This all-anti tripod conformation has been shown to be
sterically disfavored for the H-capped free ligand 3a and its
brominated precursor 6a,9 and our crystallographic and variable
Scheme 4
Table 3 Infrared data for selected molybdenum carbonyl 3◦ phosphine complexes. Crystallographic data included where available
Entry
Complex
mCO/cm−1
P–Mo–P avg/◦
C–Mo–C avg/◦
Ref.
fac-L3Mo(CO)3 where L3 is:
(PPh3)3
1
2
3
4
5
6
7
8
9
1934, 1835
1937, 1841
1918, 1815
1937, 1844
1931, 1844
1934, 1838
1934, 1840
1937, 1845
1927, 1844, 1826
15
15
16
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
(PEt3)3
{P(N(H)Pri)3}3
94
84
83
83
87
85
89
87
6b
6b
6b
17
5c
5c
j3-CH3C(CH2PPh2)3
j3-CH3C(CH2P(Et)Ph)3
j3-CH3C(CH2P(CH2Ph)Ph)3
j3-CH3C(CH2P(4-tBuPh)2)3
j3-HOCH2C(CH2PPh2)3
j3-
MeSO2OCH2C(CH2PPh2)3
j3-{cyclo-(PriPC3H6)3}
j3-HC(SiMe2PEt2)3 (10a)
j3-CH3C(SiMe2PEt2)3 (10b)
10
11
12
1915, 1813
1904, 1802, 1773 (sh)
1911, 1816, 1802
88
90
90
87
86
86
18
This work
This work
cis-L2Mo(CO)4 where L2 is:
13
14
15
16
17
18
(PPh3)2
(PEt3)2
2023, 1929, 1911, 1899
2016, 1915, 1900, 1890
2015, 1917, 1876 (br)
2010, 1909, 1883, 1871
2002, 1892, 1881, 1859
2005, 1881 (br), 1857
15
15
j2-HC(SiMe2PPh2)3 (8a)
j2-CH3C(SiMe2PPh2)3 (8b)
j2-HC(SiMe2PEt2)3 (9a)
j2-CH3C(SiMe2PEt2)3 (9b)
91
89
This worka
This work
This work
This work
a These deviate somewhat from previously reported values of 2018, 1950(sh), 1925, 1880 cm−1.8 In particular, we see no stretch at 1950 cm−1. We note that
the elemental analysis reported in reference 8 for this complex is 6.4% low in carbon. It may be that this sample contained some Mo–CO impurities.
2676 | Dalton Trans., 2006, 2671–2682
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the unbound tripod arm into the metal’s coordination sphere.
Perhaps even more importantly, the 6-membered metallacycle of
the j2-precursor must undergo a chair to boat conformational
change before the phosphine on the third arm can coordinate to the
metal. This involves Si–P bond rotations that may be significantly
hindered by the SiMe2 and PPh2 interactions. Of course, these
kinetic barriers to chair–boat inversion and single arm rotation
should also render j3-complexes of elbow-substituted tripods such
as 3 and 4 considerably inert to arm-dissociation reactions, as is
perhaps demonstrated by the lack of reaction of 10a–b with PMe3
(eqn (3)).
are reported in ppm at ambient temperature unless otherwise
stated. H chemical shifts are referenced to residual protonated
1
solvent peaks at 7.15 ppm (C6D5H), 2.09 ppm (PhCD2H), and
7.24 (CHCl3). 13C chemical shifts are referenced to C6D6 at
1
128.4 ppm and CDCl3 at 77.5 ppm. H, 13C, and 29Si chemical
shifts are reported relative to tetramethylsilane, 31P chemical
shifts are reported relative to 85% H3PO4(aq). Microanalysis was
performed by Canadian Microanalytical Service Ltd., Delta,
BC, Canada. IR spectra were recorded on a Perkin Elmer FTIR
Spectrum 1000 spectrophotometer using KBr pellets (unless
otherwise noted). Mass spectrometry was carried out by Mr
David McGillivray in the Department of Chemistry, University
of Victoria, and by Mr Wayne Buchannon in the Department of
Chemistry, University of Manitoba.
Conclusion
The silylmethyl elbow substituents in tripodal tridentate phos-
phine ligands 3 and 4 provide sufficient bulk to render tripod
conformations extremely sensitive to variations in the size of
the apical and phosphorus substituents. The presence of strongly
donating silicon groups a- to phosphorus significantly enhances
the Lewis basicity of these phosphines. Both of these features
influence the molybdenum coordination chemistry of ligands 3 and
4: our studies suggest that there is delicate energetic and kinetic
balance between j2 and j3 coordination of these bulky ligands.
Preparation of ligands
Synthesis of CH3C(SiMe2H)3 (5b). Under a flow of nitrogen,
magnesium powder (10.9 g, 0.450 mol) and THF (anhydrous,
150 mL) were stirred together in a 500 mL two-necked RB
flask equipped with a magnetic stir bar, reflux condenser,
and pressure equalizing dropping funnel. Chlorodimethylsilane
(50 mL, 0.450 mol) was added to the RB flask by syringe. 1,1,1-
Trichloroethane (15 mL, 0.150 mol) in THF (50 mL) was placed in
the dropping funnel and added to the RB flask over a period of 2 h,
with the dropping rate adjusted to maintain gentle reflux during
the addition. The reaction mixture was stirred under reflux for
an additional 4 h, then cooled to room temperature, poured over
500 mL crushed ice and allowed to stand for 1 h, then stirred for an
additional hour. The mixture was filtered through glass wool into
a 500 mL separatory funnel and the aqueous and organic layers
separated. The aqueous layer was extracted with 2 × 30 mL of
hexanes, and the organic layer washed with 2 × 30 mL of distilled
water. The combined organic fractions were dried over MgSO4,
and then gravity filtered to remove the drying agent. Solvents were
removed on a rotary evaporator, and the remaining liquid was
distilled under reduced pressure. The fraction distilling between
60–80 ◦C (10 mmHg) was collected, giving 5b as a clear oil in
Experimental
General conditions, reagents, and instruments
Unless otherwise noted, all reactions and manipulations were
performed under nitrogen in an MBraun Unilab 1200/780
glovebox or using conventional Schlenk techniques. Toluene was
dried by distillation from sodium under argon; benzene, pentane,
hexanes, tetrahydrofuran, and ether were distilled from
sodium/benzophenone under argon. Deuterated solvents were
purchased from Canadian Isotope Labs (CIL), freeze–pump–thaw
degassed and vacuum transferred from over sodium/benzophe-
none (benzene-d6, toluene-d8) or calcium hydride (chloroform-
d1) before use. Chlorodimethylsilane, 1,1,1-trichloroethane,
bromine, triphenylphosphine oxide, and n-butyllithium (1.6 M in
hexanes) were purchased from Aldrich Chemical Co. and used as
received without further purification. [AgI·PMe3]4 was purchased
from Acros Chemical Co. and used as received. Diphenyl-
and diethylphosphine were purchased from Strem Chemicals
as 10 wt% solutions in hexanes, and the concentration checked
against a known quantity of triphenylphosphine oxide by
≥97% purity.25 Yield: 13.9 g (45%). H NMR (300 MHz, C6D6,
1
3
d): 4.137 (sept, JHH = 3.7 Hz, SiH), 1.071 (s, 3H, CCH3), 0.129
(d, 3JHH = 3.7 Hz, 18H, SiCH3). 13C{ H} NMR (75 MHz, C6D6,
1
d): 13.56 (s, CCH3), −3.95 (s, SiCH3), −5.54 (s, CCH3). 29Si{ H}
1
NMR (99 MHz, C6D6, d): −9.20 (s, SiCH3). IR (thin film on NaCl
plate, cm−1): 2103 (mSi–H). MS (EI, 70 eV) m/z: 203 ([M − H], 7%),
189 ([M − CH3], 32%), 144 ([M − Si(CH3)2H], 100%), 129 ([M −
Si(CH3)3H], 81%), 85 ([M − 2Si(CH3)2H], 21%), 73 ([Si(CH3)3],
65%), 59 ([Si(CH3)2H], 74%), 40 ([SiC], 78%).
1
31P{ H} NMR before use. Tris(dimethylsilyl)methane (5a),22
tris(bromodimethylsilyl)methane (6a),9a,22 tris(diphenylphos-
6
phinodimethylsilyl)methane (3a),9a (g -mesitylene)tricarbonyl-
molybdenum(0)23
and
bis(piperidyl)tetracarbonylmolybde-
Synthesis of CH3C(SiMe2Br)3 (6b). To a 500 mL three-neck
round bottom flask equipped with a magnetic stir bar, pressure
equalizing dropping funnel and two gas inlet adapters was added
5b (10.50 g, 51.34 mmol) in benzene (20 mL). A 3.0 M solution
of bromine (0.15 mol, 7.9 mL Br2 total) in benzene was added
dropwise over 2.5 h to the contents of the round bottom flask.
A stream of nitrogen was passed through the flask and allowed
to exit via a dispersion tube submerged in a 1 M solution of cis-
cyclooctene in toluene (400 mL), then through a 1 cm layer of
NaOH pellets, to trap product HBr(g) and any unreacted Br2(g).
num(0)24 were prepared according to literature methods. Lithium
diphenylphosphide and lithium diethylphosphide were prepared
by the reaction of equimolar amounts of n-butyllithium and the
desired dialkylphosphine in hexanes. NMR spectra were acquired
on a Bruker AC 300 operating at 300.133 MHz for 1H, and
75.469 MHz for 13C; a Bruker AMX 360 operating at 360.13 MHz
for 1H, 90.565 MHz for 13C, 145.784 MHz for 31P, and 71.550 MHz
1
for 29Si; a Bruker Avance 500 operating at 500.133 MHz for H,
202.430 MHz for 31P, and 99.361 MHz for 29Si. Chemical shifts
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Once addition of bromine was complete, the mixture was allowed
to stir for 30 min. The solvent was removed in vacuo to give
a pale orange solid, which was purified by sublimation under
vacuum with the aid of a heat gun to give a white crystalline
solid. Yield: 18.19 g (80.3%). The product was resublimed (77 ◦C,
5 × 10−3 mmHg) to obtain a sample for elemental analysis. Mp
(uncorrected) 225 ◦C (sublimes). 1H NMR (300 MHz, C6D6,
Found: C 67.82, H 6.72.27 MS (EI, 70 eV) m/z: 571 ([M − PPh2],
11%), 386 ([M − 2PPh2], 11%), 201 ([M − 3PPh2], 19%).
Synthesis of CH3C(SiMe2PEt2)3 (4b). In a Schlenk tube
equipped with a magnetic stir bar, 6b (1.62 g, 3.67 mmol) was
dissolved in 3 mL THF and cooled in an ice bath. In a second
Schlenk tube, LiPEt2 (1.01 g, 10.6 mmol) was dissolved in THF
(25 mL), and slowly added to the cooled solution of 6b by cannula.
After the LiPEt2 addition was complete, the ice bath was removed,
and the mixture was allowed to stir under nitrogen. After 1 h, the
solvent was removed under vacuum to give a yellow paste. The
paste was extracted with dry hexanes (25 mL) and filtered by
cannula; the solvent was removed from the filtrate under vacuum.
The cloudy, pale yellow oil was dissolved in pentane (6 mL) and
filtered through Celite to remove remaining solid LiBr. Pentane
was removed under vacuum to give 4b as a yellow oil in >95%
1
d): 1.091 (s, 3H, CCH3), 0.689 (s, 18H, SiCH3). 13C{ H} NMR
(75 MHz, C6D6, d): 14.6 (s, CCH3), 10.3 (s, CCH3), 5.8 (s, SiCH3).
1
29Si{ H} NMR (71 MHz, C6D6, d): 28.4 (s, SiCH3). Anal. Calcd
for C8H21Br3Si3: C 21.78, H 4.80. Found: C 21.85, H 4.75. MS (EI,
70 eV) m/z: 427 ([M − CH3], 32%), 361 ([M − Br], 100%), 224
([M − Br, Si(CH3)2Br], 61%), 139 ([Si(CH3)2Br], 21%), 85 ([M −
Br, 2 Si(CH3)2Br], 38%), 73 ([Si(CH3)3], 37%).
purity (by H NMR).26 Yield: 1.44 g (87%). Bp > 130 ◦C (5 ×
1
Synthesis of HC(SiMe2PEt2)3 (4a). In a Schlenk tube equipped
with a magnetic stir bar, 6a (1.49 g, 3.48 mmol) was dissolved in
3 mL THF and cooled in an ice bath. In a second Schlenk tube,
LiPEt2 (0.98 g, 10 mmol) was dissolved in THF (25 mL), and slowly
added to the cooled solution of 6a by cannula. After the LiPEt2
addition was complete, the ice bath was removed, and the mixture
allowed to stir under nitrogen. After 1.5 h, the solvent was removed
under vacuum to give a pale yellow paste. The paste was extracted
with dry hexanes (30 mL) and filtered by cannula; the solvent was
removed from the filtrate under vacuum. The cloudy, pale yellow
oil was dissolved in pentane (6 mL) and filtered through Celite to
remove remaining solid LiBr. Pentane was removed under vacuum
to give 4a as a pale yellow oil in >95% purity.26 Yield: 1.17 g (76%).
1H NMR (360 MHz, C6D6, d): 1.56 (m, 6 H, PCH2CH3), 1.43 (m,
10−3 mmHg). 1H NMR (360 MHz, C6D6, d): 1.63 (m, 6 H,
PCH2CH3), 1.41 (s, 3H, CCH3), 1.40 (m, 6 H, PCH2CH3), 1.19
(dt, 3JPH = 15.1 Hz, 3JHH = 7.6 Hz, 18 H, PCH2CH3), 0.41 (s, 18 H,
1
SiCH3). 13C{ H} NMR (90 MHz, C6D6, d): 16.8 (q, 3JCP = 8.6 Hz,
2
CCH3), 15.6 (m, PCH2CH3), 14.3 (m, PCH2CH3), 2.9 (q, JCP
=
11.9 Hz, C(CH3)), −0.5 (td, 4JCP = 4.2 Hz, 3JCP = 3.7 Hz, SiCH3).
31P{ H} NMR (145 MHz, C6D6, d): −81.4 (s, PEt2). 29Si{ H}
NMR (99 MHz, C6D6, d): 6.65 (complex m, SiCH3). MS (EI,
70 eV) m/z: 439 ([M − Et], 70%), 379 ([M − PEt2], 68%), 321 ([M −
SiMe2PEt2], 14%), 291 ([M − 2PEt2], 14%). For the disubstituted
tripod CH3C(SiMe2PEt2)2(SiMe2Br): 1H NMR (360 MHz, C6D6,
d): 1.65–1.50 (m, 6 H, PCH2CH3), 1.45–1.30 (m, 6 H, PCH2CH3),
1.33 (s, 3H, CCH3), 1.20–1.10 (m, 18 H, PCH2CH3), 0.71 (s, 6 H,
SiCH3Br), 0.44 (br s, 6 H, SiCH3PEt2), 0.35 (br s, 6 H, SiCH3PEt2).
1
1
3
3
6 H, PCH2CH3), 1.18 (dt, JPH = 15.1 Hz, JHH = 7.6 Hz, 18 H,
1
31P{ H} NMR (145 MHz, C6D6, d): −82.6 (s, PEt2).
3
PCH2CH3), 0.47 (s, 18 H, SiCH3), −0.12 (q, JPH = 3.9 Hz, 1H,
1
HCSi3). 13C{ H} NMR (90 MHz, C6D6, d): 15.2 (m, PCH2CH3),
14.0 (m, PCH2CH3), 1.1 (q, 2JCP = 12.4 Hz, HCSi3), 1.0 (m, SiCH3).
Preparation of molybdenum complexes
1
1
31P{ H} NMR (145 MHz, C6D6, d): −80.2 (s, PEt2). 29Si{ H}
NMR (99 MHz, C6D6, d): 2.56 (complex m, SiCH3). MS (EI,
70 eV) m/z: 425 ([M − Et], 100%), 365 ([M − PEt2], 72%), 275
([M − 2PEt2], 20%), 187 ([M − 3PEt2], 12%).
Synthesis of j2-(HC(SiMe2PPh2)3)Mo(CO)4 (8a). Compound
3a (0.51 g, 0.68 mmol) and Mo(pip)2(CO)4 (0.26 g, 0.68 mmol)
were suspended in 30 mL dry toluene in a Schlenk tube. The
headspace was evacuated and the yellow suspension stirred under
static vacuum in an oil bath for 2 h at 45 ◦C, after which the
solvent was removed in vacuo. The resulting sticky golden brown
foam was washed with hexanes (10 mL), and the washings were
decanted by cannula filtration to give 8a as a pale yellow powder
that was dried under vacuum. Yield: 0.32 g (49%). Spectroscopic
and physical properties are in agreement with those reported in
the literature.8
Synthesis of CH3C(SiMe2PPh2)3 (3b). In a Schlenk tube
equipped with a magnetic stir bar, LiPPh2 (2.613 g, 13.60 mmol)
was dissolved in 25 mL THF to give a clear red solution. A solution
of 6b (1.996 g, 4.524 mmol) in 5 mL THF was added to the
LiPPh2 solution by syringe, and stirred overnight under nitrogen.
The solvent was removed under vacuum to give an orange paste,
which was extracted with toluene (25 mL) and filtered by cannula
to remove lithium bromide. Toluene was removed under vacuum
to leave a yellow foam, which was washed with 3 × 10 mL dry
pentane to give the product as a white powder. This was collected
on a glass frit and dried under vacuum. Yield 2.86 g (83%). Mp
(uncorrected) 166–168 ◦C. 1H NMR (360 MHz, C6D6, d): 7.68 (m,
12 H, o-C6H5), 7.04 (br m, 18 H, p-, m-C6H5), 2.05 (s, 3H, CCH3),
Synthesis of j2-(CH3C(SiMe2PPh2)3)Mo(CO)4 (8b). This
compound was prepared as described above for compound 8a,
using the following reagents and amounts: compound 3b (0.40 g,
0.53 mmol), Mo(pip)2(CO)4 (0.20 g, 0.51 mmol), 25 mL benzene.
Yield: 0.33 g (69%).28 Mp 125 ◦C (decomp). 1H NMR (360 MHz,
CDCl3, d): 7.59 (m, 4H, o-C6H5PMo), 7.52 (m, 4H, o-C6H5PMo),
7.41 (m, 4H, o-C6H5P), 7.4–7.2 (m, 18H, p-, m-C6H5P/PMo),
1
0.43 (br s, 18 H, SiCH3). 13C{ H} NMR (90 MHz, C6D6, d): 136.9
3
(m, Cipso), 135.4 (m, Cortho), 128.1 (m, Cmeta), 127.7 (s, Cpara), 19.6
1.36 (br s, 3H, CCH3), 0.48 (d, JPH = 2.9 Hz, 6H, SiCH3),
(q, 3JCP = 7.8 Hz, CCH3), 5.0 (q, 2JCP = 12.7 Hz, CCH3), 1.4 (td,
0.45 (d, JPH = 6.1 Hz, 6H, SiCH3PMo), 0.14 (d, JPH
=
3
3
1
1
4JCP = 4.3 Hz, 2JCP = 4.6 Hz, SiCH3). 31P{ H} NMR (145 MHz,
2.5 Hz, 6H, MoPSiCH3). 13C{ H} NMR (90 MHz, CDCl3, d):
1
2
2
C6D6, d): −50.9 (s, PPh2). 29Si{ H} NMR (99 MHz, C7D8, d): 8.28
215.2 (dd, JCP(trans) = 23.1 Hz, JCP(cis) = 9.1 Hz, CO), 214.7 (t,
2JCP(cis) = 7.1 Hz, CO), 208.4 (t, 2JCP(cis) = 7.2 Hz, CO), 137.5 (dd,
(complex m, SiCH3). Anal. Calcd for C44H51P3Si3: C 69.81, H 6.79;
2678 | Dalton Trans., 2006, 2671–2682
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3
1JCP = 21.1 Hz, JCP = 6.8 Hz, ipso-C6H5PMo), 137.3 (d, 1JCP
=
7.2 Hz, CO), 20.5 (br d, 1JCP = 15.5 Hz, MoPCH2CH3), 18.5 (d,
1JCP = 14.2 Hz, MoPCH2CH3), 16.1 (br d, 3JCP = 7.3 Hz, CCH3),
21.7 Hz, ipso-C6H5PMo), 135.1 (d, 2JCP = 20.7 Hz, o-C6H5PMo),
1
2
1
2
135.1 (d, JCP = 13.1 Hz, ipso-C6H5P), 135.0 (d, JCP = 12.1 Hz,
15.2 (d, JCP = 17.8 Hz, PCH2CH3), 13.9 (d, JCP = 18.5 Hz,
2
o-C6H5PMo), 134.0 (d, JCP = 11.4 Hz, o-C6H5P), 128.8 (s, p-
PCH2CH3), 11.1 (s, MoPCH2CH3), 10.0 (s, MoPCH2CH3), 2.5
3
2
C6H5P), 128.4 (s, p-C6H5P), 128.4 (d, JCP = 7.1 Hz, m-C6H5P),
(m, CCH3), 1.5 (br s, SiCH3), −0.5 (d, JCP = 5.3 Hz, SiCH3),
3
1
128.1 (s, p-C6H5P), 128.1 (d, JCP = 6.7 Hz, m-C6H5P), 128.0 (d,
−1.0 (s, SiCH3). 31P{ H} NMR (145 MHz, C6D6, d): −55.5 (s,
3JCP = 7.5 Hz, m-C6H5P), 18.6 (d, JCP = 10.0 Hz, CCH3), 8.0
2P, MoPEt2), −78.5 (s, 1P, PEt2). 29Si{ H} NMR (99 MHz, C6D6,
3
1
(q, 2JCP = 9.1 Hz, CCH3), 3.6 (d, 2JCP = 11.7 Hz, SiCH3PMo), 2.1
d): 5.64 (dt, 1JSiP = 29.8 Hz, 3JSiP = 6.6 Hz, 1Si, SiPfree), 2.95 (m,
2Si, SiPMo). MS (FAB, mNBA matrix) m/z: 648 ([M − EtH],
25%), 606 ([M − SiMe3], 22%), 590 ([M − PEt2], 22%), 578 ([M −
SiMe3, − CO], 26%), 550 ([M − SiMe3, − 2CO], 61%), 522 ([M −
SiMe3, − 3CO], 100%), 506 ([M − (CH3CSiMe2PEt2)], 28%). IR
(KBr disk, mCO, cm−1): 2005 (s), 1881 (br, s), 1857 (s).
2
2
(d, JCP = 5.8 Hz, SiCH3PMo), 0.7 (d, JCP = 10.0 Hz, SiCH3).
31P{ H} NMR (145 MHz, CDCl3, d): −18.9 (s, 2P, PMo), −50.4
1
(s, 1P, P). 29Si{ H} NMR (99 MHz, C6D6, d): 7.66 (dt, JSiP
=
1
1
3
40.4 Hz, JSiP = 8.1 Hz, 1Si, SiPfree), 7.14 (m, 2Si, SiPMo). MS
(FAB, mNBA matrix) m/z: 966 ([M], 2%), 782 ([M − PPh2], 21%),
753 ([M − PPh2, − CO], 40%), 725 ([M − PPh2, − 2CO], 38%),
697 ([M − PPh2, − 3CO], 68%), 669 ([M − PPh2, − 4CO], 39%).
IR (KBr disk, mCO, cm−1): 2010 (s), 1909 (s), 1883 (s), 1871 (s).
Synthesis of j3-(HC(SiMe2PEt2)3)Mo(CO)3 (10a). This com-
pound can be prepared from Mo(CO)6 (method (a)) but we have
obtained higher yields using Mo(mesitylene)(CO)3 as a precursor
(method (b)). Method (a). Under a flow of nitrogen, a thick walled
glass reaction vessel with a J. Young valve (100 mL capacity) was
charged with a magnetic stir bar, Mo(CO)6 (0.499 g, 1.90 mmol),
HC(SiMe2PEt2)3 (0.843 g, 1.85 mmol), and 30 mL dry toluene.
The headspace was evacuated and the vessel sealed under static
vacuum, then heated in an oil bath at 120 ◦C for 18 h. After cooling
to room temperature, solvent was removed in vacuo to give a yellow
paste, which was washed with hexanes (2 × 10 mL) to give j3-
[HC(SiMe2PEt2)3]Mo(CO)3 as a white powder that was isolated
by filtration and dried under vacuum. Yield 0.267 g (22.7%).
Method (b). Under nitrogen, a 250 mL Schlenk flask was charged
with 4a (610 mg, 1.34 mmol), and Mo(mesitylene)(CO)3 (402 mg,
1.34 mmol). Toluene (90 mL) was added by syringe, the flask was
equipped with a reflux condenser, and the clear yellow solution
was heated at reflux for 2 h. When the solution cooled, the solvent
was removed under vacuum to leave a pale yellow solid. Repeated
washing with hexanes (4 × 10 mL) and toluene (2 × 10 mL)
ultimately allowed isolation of 10a as a white microcrystalline
powder, which was dried under vacuum. Yield 0.455 g (54%). Mp
Synthesis of j2-(HC(SiMe2PEt2)3)Mo(CO)4 (9a). In a Schlenk
tube, 4a (0.43 g, 0.94 mmol) was dissolved in 30 mL dry toluene.
Mo(pip)2(CO)4 (0.37 g, 0.97 mmol) was added under a flow of
nitrogen, and the cloudy yellow suspension was allowed to stir
at RT under static vacuum for four days, until all of the solid
had dissolved to give a clear yellow solution. The solvent was
removed in vacuo to give a pale yellow paste that was extracted
into 10 mL hexanes and filtered by cannula. The hexanes was
removed under vacuum, and the residue washed with 5 mL of ice
cold pentane to give 9a as a pale yellow powder which was dried
◦
under vacuum.29 Yield 0.32 g (51%). Mp 123–130 C. H NMR
(360 MHz, C6D6, d): 1.70–1.51 (overlapping m, 6H, PCH2CH3),
1.50–1.21 (overlapping m, 6H, PCH2CH3), 1.14–0.95 (overlapping
m, 18H, PCH2CH3), 0.42 (d, 3JPH = 3.2 Hz, 6H, SiCH3(free)), 0.18
1
(d, 3JPH = 2.2 Hz, 12H, SiCH3(Mo)), 0.12 (dt, 3JPH = 3.6 Hz, 3JPH
=
1.8 Hz, 1H, CH). 13C{ H} NMR (90 MHz, C6D6, d): 216.1 (dd,
1
2JCP(trans) = 22.2 Hz, JCP(cis) = 12.3 Hz CO), 213.4 (t, JCP(cis)
=
=
2
2
2
1
6.3 Hz, CO), 211.8 (t, JCP(cis) = 9.5 Hz, CO), 19.7 (dd, JCP
11.3 Hz, 3JCP = 8.0 Hz, MoPCH2CH3), 19.1 (dd, 1JCP = 13.5 Hz,
3JCP = 8.9 Hz, MoPCH2CH3), 15.0 (d, 1JCP = 16.4 Hz, PCH2CH3),
13.9 (d, 2JCP = 16.3 Hz, PCH2CH3), 10.6 (s, MoPCH2CH3), 10.2
◦
1
220 C (decomp). H NMR (300 MHz, C6D6, d): 1.90–1.70 (m,
12 H, PCH2CH3), 1.08 (dt, 3JPH = 13.5 Hz, 3JHH = 7.3 Hz, 18 H,
3
PCH2CH3), 0.06 (br d, JPH = 1.9 Hz, 18 H, SiCH3), −1.01 (q,
2
2
(s, MoPCH2CH3), 3.2 (dt, JCP = 11.0 Hz, JCP = 9.6 Hz, CH),
4JPH = 16.2 Hz, 1H, CH). 31P{ H} NMR (121 MHz, C6D6, d):
1
2
2
1.8 (d, JCP = 8.2 Hz, SiCH3), 1.4 (br s, SiCH3), 0.3 (d, JCP
=
−56.9 (s, PEt2), also see sext, 1J95Mo31P = 109 Hz. Anal. Calcd for
C22H49MoO3P3Si3: C 41.63, H 7.78; Found: C 41.32, H 8.08. IR
(KBr disk, mCO, cm−1): 1904 (s), 1802 (s), 1773 (sh).
4.4 Hz, SiCH3). 31P{ H} NMR (145 MHz, C6D6, d): −49.3 (s, 2P,
1
MoPEt2), −81.2 (s, 1P, PEt2). 29Si{ H} NMR (99 MHz, C6D6, d):
1
1.91 (dt, 1JSiP = 29.8 Hz, 3JSiP = 8.7 Hz, 1Si, SiPfree), 0.03 (m, 2Si,
SiPMo). MS (FAB, mNBA matrix) m/z: 592 ([M − SiMe3], 18%),
564 ([M − SiMe3, − CO], 11%), 536 ([M − SiMe3, − 2CO], 36%),
508 ([M − SiMe3, − 3CO], 92%), 506 ([M − (HCSiMe2PEt2)],
100%). IR (KBr disk, mCO, cm−1): 2002 (s), 1883 (br s), 1859 (s).
Synthesis of j3-(CH3C(SiMe2PEt2)3)Mo(CO)3 (10b). Under
a flow of nitrogen, a thick walled glass reaction vessel with a
Teflon J. Young valve (100 mL capacity) was charged with a mag-
netic stir bar, Mo(CO)6 (0.999 g, 3.78 mmol), CH3C(SiMe2PEt2)3
(2.5 g, 3.8 mmol), and 30 mL dry toluene. The headspace was
evacuated and the vessel sealed under static vacuum, then heated in
an oil bath at 120 ◦C for 18 h. After the mixture was cooled to room
temperature, solvent was removed in vacuo to give a yellow paste,
which was washed with pentane (20 mL). A pale yellow powder
was isolated by filtration, washed repeatedly with hexanes and
toluene and dried under vacuum. Yield 0.80 g (33%). Mp 227 ◦C
(decomp). 1H NMR (360 MHz, C6D6, d): 1.85 (m, 6 H, PCH2CH3),
1.76 (m, 6 H, PCH2CH3), 1.08 (dt, 3JPH = 13.3 Hz, 3JHH = 7.4 Hz,
Synthesis of j2-(CH3C(SiMe2PEt2)3)Mo(CO)4 (9b). This com-
pound was prepared as described above for compound 9a, using
the following reagents and amounts: compound 4b (0.55 g,
1.2 mmol), Mo(pip)2(CO)4 (0.45 g, 1.2 mmol), 30 mL toluene.
Yield 0.32 g (41%) yellow powder, in ≥95% purity (by 1H NMR).29
Mp 87–105 ◦C. 1H NMR (360 MHz, C6D6, d): 1.9–1.2 (overlapping
m, 12H, PCH2CH3), 1.34 (s, CCH3), 1.2–0.9 (overlapping m, 18H,
3
3
PCH2CH3), 0.30 (d, JHP = 2.9 Hz, 6H, SiCH3), 0.27 (d, JHP
=
4.3 Hz, 6H, SiCH3), 0.09 (d, 3JHP = 1.8 Hz, 6H, SiCH3). 13C{ H}
18 H, PCH2CH3), 0.77 (q, JPH = 1.8 Hz, 3H, CCH3), 0.01 (d,
1
4
NMR (90 MHz, C6D6, d): 216.3 (dd, 2JCP(trans) = 20.1 Hz, 2JCP(cis)
10.9 Hz, CO), 212.7 (t, 2JCP(cis) = 6.9 Hz, C≡O), 211.9 (t, 2JCP(cis)
=
=
3JPH = 1.8 Hz, 18 H, SiCH3). 13C{ H} NMR (90 MHz, C6D6, d):
1
220.7 (m, CO), 19.1 (m, PCH2CH3), 13.1 (q, 3JCP = 5.3 Hz, CCH3),
Dalton Trans., 2006, 2671–2682 | 2679
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2
9.9 (s, x1/2 ∼ 5 Hz, PCH2CH3), 2.6 (q, JCP = 16.6 Hz, CCH3),
free CH3C{SiMe2PPh2}3 (d −50.9, ∼25%), cis-Mo(PMe3)2(CO)4
(d −16.4), trans-Mo(PMe3)2(CO)4 (d −6.3), and PMe3 (d −62.3),
and signals due to an unidentified species (d −17.5).
0.2 (s, x1/2 ∼ 5 Hz, SiCH3). 31P{ H} NMR (145 MHz, C6D6, d):
1
−57.3 (s, PEt2), also see sext, 1J95Mo31P = 110.8 Hz. 29Si{ H} NMR
1
(99 MHz, C6D6, d): 2.16 (complex m, SiCH3). Anal. Calcd for
C23H51MoO3P3Si3: C 42.58, H 7.92; Found: C 42.72, H 8.04. MS
(FAB, mNBA matrix) m/z: 650 ([M], 24%), 622 ([M − CO], 46%),
594 ([M − 2CO], 100%). IR (KBr disk, mCO, cm−1): 1911, 1816,
1802.
Reaction of j2-(HC(SiMe2PEt2)3)Mo(CO)4 (9a) with PMe3.
Compound 9a (29 mg, 0.044 mmol), PMe3 (0.038 mmol). After
◦
1
12 h at 85 C, 31P{ H} NMR showed only 9a (d −49.1, −81.2),
PMe3 (d −62.3), and trace amounts of HPEt2 (d −55.6). Small
white crystals on the walls of the NMR tube were identified as
10a, indicating that ∼10% of the original complex had converted
from j2 to j3 coordination, with concomitant liberation of CO.
Other reactions
Conversion of j2-(HC(SiMe2PEt2)3)Mo(CO)4 (9a) to j3-
(HC(SiMe2PEt2)3)Mo(CO)3 (10a). Compound 9a (9 mg,
0.01 mmol) was dissolved in 0.5 mL C7D8 in a 5 mm NMR tube
equipped with a Teflon J. Young valve, and degassed by one freeze–
pump–thaw cycle. The sample was heated in an oil bath at 85 ◦C
and removed periodically to monitor the progress of reaction by
Reaction of j2-(CH3C(SiMe2PEt2)3)Mo(CO)4 (9b) with PMe3.
Compound 9b (28 mg, 0.041 mmol), PMe3 (0.099 mmol). After
16 h at 85 ◦C, 31P{ H} NMR showed 9b (d −55.4 and −78.6,
1
∼60%), 10b (d −57.0, ∼30%), PMe3 (d −62.3), and trace amounts
of HPEt2 (d −55.6). Peaks potentially representing ∼15% of the
total tripod complex were also observed, but were unassigned (d
−54.8 (s), −55.7 (s)).
1
31P{ H} NMR spectroscopy. After 22.5 h, 38% of 9a had converted
to the j3 complex 10a.
Reaction of j3-(CH3C(SiMe2PEt2)3)Mo(CO)3 (10b) with PMe3.
Compound 10b (32 mg, 0.049 mmol), PMe3 (0.007 mmol). After
Conversion of j2-(CH3C(SiMe2PEt2)3)Mo(CO)4 (9b) to j3-
(CH3C(SiMe2PEt2)3)Mo(CO)3 (10b). Compound 9b (21 mg,
0.031 mmol) was dissolved in 0.5 mL C7D8 in a 5 mm NMR
tube equipped with a Teflon J. Young valve, and degassed by one
freeze–pump–thaw cycle. The sample was heated in an oil bath
at 85 ◦C and removed periodically to monitor the progress of
12 h at 85 ◦C, 31P{ H} NMR showed no change; only 10b (d
1
−57.0), and PMe3 (d −62.3) were detected.
Line-shape analysis of 4b and related calculations
1
reaction by 31P{ H} NMR spectroscopy. After 12 h, 32% of 9b
31
Line-shape analysis was carried out on the SiMe region of 1H{ P}
had converted to the j3 complex 10b.
NMR (500 MHz) spectra recorded for compound 4b in toluene-
d8 at 200, 195, 190, 185, and 180 K, using a modified version
of DNMR330a contained in SpinWorks 2.3.30b Rate constants (k)
for the two-site exchange were determined iteratively at each
temperature, giving well-matched simulated and experimental
spectra. Estimated errors in k varied from 2–6%, temperatures
are 5 K. Thermodynamic parameters were obtained from the
slope and intercept of an Eyring plot (ln (k/T) vs. 1/T, R2 =
0.9972): DH‡ = 10.4 kcal mol−1, DS‡ = 10.9 cal K mol−1. These
yielded DG‡(300 K) = 7.1 kcal mol−1 0.2 kcal mol−1 (2.8% error).
Energy of activation, Ea = 10.7 kcal mol−1, was determined from
the slope of an Arrhenius plot (ln k vs. 1/T, R2 = 0.9973). DG‡
was also calculated for decoalescence of the SiMe signals in the
Reactions of molybdenum tripod complexes with PMe3. Ap-
proximately 30 mg of the desired tripod–molybdenum complex
was placed in a sealable NMR tube, and attached via a glass
“T” connector to a 10 mL RB flask containing an appropriate
amount of [AgI·PMe3]4. The entire apparatus was connected
to a vacuum line and placed under vacuum, and toluene-d8
(0.5 mL) was vacuum transferred into the NMR tube. Under
static vacuum, the NMR tube was cooled in liquid nitrogen,
and the RB flask containing [AgI·PMe3]4 was warmed with a
heat gun until evolution of PMe3 could no longer be observed.
After allowing the PMe3 to collect in the NMR tube, the tube
was flame sealed under static vacuum, and warmed to room
temperature. The actual amount of PMe3 present in each tube
31
variable temperature 1H{ P} NMR spectra (500 MHz) recorded
1
was determined by 31P{ H} NMR, relative to the known amount
for compound 4b using the value for the rate constant kC (where
kC = pDmC/(2)1/2) in the Eyring equation
1
of tripod–molybdenum complex (delay time D1 = 20 s). 31P{ H}
NMR spectra were rerecorded after heating the tubes in an oil
bath at 85 ◦C for 12–16 h. Percent conversions are reported with
respect to the overall amount of tripodal ligand present in the
sample.
DG‡ = −RTC ln[(kCh)/(kBTC)],
where R = the gas constant, TC = coalescence temperature,
DmC = peak separation at the low-temperature limit, h = Planck’s
constant, and kB = Boltzmann constant. For the two-site exchange
of SiMe groups in 4b: TC = 184 K 5 K, DmC = 218 Hz, DG‡ =
8.2 0.2 kcal mol−1.
Reaction of j2-(HC(SiMe2PPh2)3)Mo(CO)4 (8a) with PMe3.
Compound 8a (20 mg, 0.021 mmol), PMe3 (0.039 mmol). After
12 h at 85 ◦C, 31P{ H} NMR showed 8a (d −21.4 and −52.4,
1
∼67%), (j2-HC{SiMe2PPh2}3)(PMe3)Mo(CO)3 (d −19.2 d, −24.6
t, and −51.3 s, ∼31%), PMe3 (d −62.3), and a small amount of
HPPh2 (d −40.4).
X-Ray crystallographic studies
Single crystals suitable for X-ray diffraction were grown from
hexanes (6b) or toluene (3b, 8a, 10a–b) solutions at −20 ◦C under
nitrogen, and mounted on glass fibres in hydrocarbon oil. Selected
crystal data and structural refinement details are listed in Table 4;
further details are given in the ESI.†
Reaction of j2-(CH3C(SiMe2PPh2)3)Mo(CO)4 (8b) with PMe3.
Compound 8b (38 mg, 0.039 mmol), PMe3 (0.10 mmol). After
12 h at 85 ◦C, 31P{ H} NMR showed 8b (d −17.9 and −50.2,
1
∼58%), (j2-CH3C{SiMe2PPh2}3)(PMe3)Mo(CO)3 (d − 13.7 d,
−23.7 t, and −49.3 s, ∼7%), (j3-CH3C{SiMe2PPh2}3)Mo(CO)3,
CCDC reference numbers 289572–289576.
2680 | Dalton Trans., 2006, 2671–2682
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Table 4 Crystallographic experimental details
Compound
3b
6b
8a
10a
10b
Formula
Mr
C44H51P3Si3
757.03
C8H21Br3Si3
441.25
C54H57MoO4P3Si3
1043.12
C22H49MoO3P3Si3
634.73
C23H51MoO3P3Si3
648.76
Crystal size/mm
Crystal system
Space group
0.43 × 0.27 × 0.24
0.88 × 0.19 × 0.16
0.66 × 0.22 × 0.21
0.40 × 0.29 × 0.23
0.44 × 0.40 × 0.25
Monoclinic
P21/na
11.5080(5)
17.2076(7)
Triclinic
Hexagonal
Monoclinic
Trigonal
¯
P1 (No. 2)
P63/m (No. 176)
9.3209(9)
P21/c (No. 14)
14.8482(8)
14.8803(8)
R3 (No. 146)
17.1523(3)
˚
a/A
12.2968(11)
13.0128(12)
13.7299(12)
104.2431(15)
100.4996(14)
95.9048(14)
2068.5(3)
2
˚
b/A
˚
c/A
11.143(2)
23.9239(13)
9.2856(4)
16.6511(7)
a/◦
b/◦
c /◦
94.4133(10)
90.3365(7)
3
˚
V/A
838.4(2)
2
5270.2(5)
4
1.315
2365.84(12)
3
1.337
3297.3(2)
4
1.307
Z
qcalcd/g cm−3
1.215
0.261
1.748
7.404
−80
l/mm−1
0.451
0.703
0.674
Temperature/◦C
Max. 2h/◦
−80
−80
−80
−80
52.78
52.72
3156
52.78
52.70
52.78
Total data collected
Unique data, Rint
Obsd data [I ≥
2r(I)]
10365
8299, 0.0256
5997
40529
10791, 0.0342
9105
5046
2083, 0.0167
2080
21617
6733, 0.0191
6370
608, 0.0341
519
Restraints/params
S(F2) [all data]b
R1(F) [I ≥ 2r(I)]c
wR2(F2) [all data]d
0/451
1.007
0.0461
0.1260
0/38
1.145
0.0254
0.0575
0/587
1.035
0.0310
0.0831
0/105
1.108
0.0178
0.0445
0/299
1.064
0.0210
0.0587
−3
˚
Drmax, Drmin/e A
0.443, −0.399
0.371, −0.359
0.520, −0.426
0.316, −0.228
0.358, −0.350
ꢀ
a An alternate setting of P21/c (No. 14). b S = [ w(Fo − Fc2)2/(n − p)]1/2 (n = number of data; p = number of parameters varied; w = [r2(Fo2) + (a0P)2 +
2
a1P]−1 where P = [Max(Fo2, 0) + 2Fc2]/3; for 3b, a0 = 0.0731, a1 = 0; for 6b, a0 = 0.0174, a1 = 0.5075; for 8a, a0 = 0.0427, a1 = 2.3909; for 10a, a0 =
ꢀ
ꢀ
ꢀ
ꢀ
0.0243, a1 = 0.8197; for 10b, a0 = 0.0306, a1 = 1.4152). c R1 = ||Fo| − |Fc||/ |Fo|. d wR2 = [ w(Fo − Fc2)2/ w(Fo4)]1/2
.
2
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b516127d
23, 2488–2502; (d) D. M. Jenkins, A. J. Di Bilio, M. J. Allen, T. A. Betley
and J. C. Peters, J. Am. Chem. Soc., 2002, 124, 15336–15350.
5 Examples of tripods with varied apical substituents include: (a) C.
Bianchini, P. Frediani and V. Sernau, Organometallics, 1995, 14, 5458;
(b) R. A. Findeis and L. H. Gade, Eur. J. Inorg. Chem., 2003, 99–
110; (c) P. Schober, R. Soltek, G. Huttner, L. Zsolnai and K. Heinze,
Eur. J. Inorg. Chem., 1998, 1407–1415.
6 Examples of tripods with a range of substituents at phosphorus include:
(a) A. M. Arif, J. G. Hefner, R. A. Jones and B. R. Whittlesey, Inorg.
Chem., 1986, 25, 1080–1084; (b) O. Walter, T. Klein, G. Huttner and
L. Zsolnai, J. Organomet. Chem., 1993, 458, 63–81; (c) H. Heidel,
J. Scherer, A. Asam, G. Huttner, O. Walter and L. Zsolnai, Chem.
Ber., 1995, 128, 293–301; (d) C. Becker, L. Dahlenburg and S. Kerstan,
Z. Naturforsch., B: Chem. Sci., 1993, 48, 577–582; (e) M. Sulu, L. M.
Venanzi, T. Gerfin and V. Gramlich, Inorg. Chim. Acta, 1998, 270,
499–510.
Acknowledgements
Financial support for this work was provided by the NSERC of
Canada (operating grants to L.R. and a postgraduate scholarship
to D.M.F.) and start-up grants from the University of Manitoba
and the University of Victoria. We thank Chris Greenwood
(University of Victoria) for obtaining spectra on the Bruker
Avance 500 NMR spectrometer.
7 Y. M. Yao, C. J. A. Daley, R. McDonald and S. H. Bergens,
Organometallics, 1997, 16, 1890–1896.
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2 The tripod 1,1,1-tris(diphenylphosphinomethyl)ethane is also some-
times referred to as “tpdme”. The linear tridentate phosphine
bis(2,2ꢀ-diphenylphosphinoethyl)phenyl phosphine has also been called
“triphos”, but in this article we use this name only in reference to the
tripodal ligand 1.
8 A. G. Avent, D. Bonafoux, C. Eaborn, S. K. Gupta, P. B. Hitchcock
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10 Relative sensitivities of 3–4 to solvolysis of the P–Si bond and to
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3 C. Bianchini, A. Meli, M. Peruzzini, F. Vizza and F. Zanobini, Coord.
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4 Examples of tripods with non-carbon apical groups include: (a) S.
Herold, A. Mezzetti, L. M. Venanzi, A. Albinati, F. Lianza, T. Gerfin
and V. Gramlich, Inorg. Chim. Acta, 1995, 235, 215–231; (b) T. G.
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12 Details of this variable temperature NMR study and proposed mecha-
nisms for phosphine exchange in 3b are included in the ESI†.
13 Among structurally characterized j2-triphos complexes, the six-
member chelate ring has been found in both chair and boat confor-
mations, but the former is more common. (a) R. M. Kirchner, R. G.
Little, K. D. Tau and D. W. Meek, J. Organomet. Chem., 1978, 149, C15–
C18; (b) D. H. Gibson, H. Y. He and M. S. Mashuta, Organometallics,
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2001, 20, 1456–1461; (c) S. C. Lin, C. P. Cheng, T. Y. Lee, T. J. Lee and
S. M. Peng, Acta Crystallogr., Sect. C: Cryst. Struct. Commun., 1986,
42, 1733–1736; (d) W. O. Siegl, S. J. Lapporte and J. P. Collman, Inorg.
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23 B. Nicholls and M. C. Whiting, J. Chem. Soc., 1959, 551.
24 D. J. Darensbourg and R. L. Kump, Inorg. Chem., 1978, 17, 2680–2682.
25 1H NMR spectra of compound 5b consistently show 2–3% of
SiMe-containing impurities, even after multiple fractional distil-
lations. Analysis of the EI-MS trace for
a sample of 5b
14 The j2-(3a) Mo(CO)4 complex was previously reported to have been
prepared from the addition of 3a to Mo(CO)6 (ref. 8), but our attempts
to repeat this synthesis using 3a or 3b gave only intractable tan powders,
which we presume to be polymers consisting of multiple Mo centres
linked by l-j1 or l-j2 tripodal ligands.
suggests that these contaminants are primarily composed of
H(SiMe2)5H (292 m/z) and a compound with the general structure
CH3C{(SiMe2)xH}{(SiMe2)yH}{(SiMe2)zH} (x + y + z = 7436 m/z),
both of which show fragmentation patterns similar to 5b. These
impurities were easily removed from derivatives of 5b in subsequent
synthetic steps.
15 F. A. Cotton, Inorg. Chem., 1964, 3, 702.
16 A. Tarassoli, H. J. Chen, V. S. Allured, T. G. Hill, R. C. Haltiwanger,
M. L. Thompson and A. D. Norman, Inorg. Chem., 1986, 25, 3541–
3544.
17 A. Muth, O. Walter, G. Huttner, A. Asam, L. Zsolnai and C. Emmerich,
J. Organomet. Chem., 1994, 468, 149–163.
18 S. J. Coles, P. G. Edwards, J. S. Fleming and M. B. Hursthouse, J. Chem.
Soc., Dalton Trans., 1995, 1139–1145.
26 Compounds 4a–b are high-boiling oils that have been so
far consistently contaminated with ∼3–4% disubstituted tripod
RC(SiMe2PEt2)2(SiMe2Br) (for R = H, dP −81.3; for R = CH3, dP
−82.6), and traces of other, unidentified SiMe-containing impurities.
We continue to modify the reaction and work-up to avoid these
impurities.
27 Microanalysis samples of 3b were clean by 1H NMR but this analysis
was consistently low in carbon over repeated attempts, and with the
use of V2O5 as a combustion agent. This may be due to SiC formation
during the combustion process.
19 P. G. Edwards, J. S. Fleming and S. S. Liyanage, J. Chem. Soc., Dalton
Trans., 1997, 193–197.
20 In the reaction of 9a in the presence of PMe3, crystals of the j3-
complex 10a formed on the wall of the sealed NMR tube during heating.
28 Solution samples of this compound consistently contain a fine sus-
pension of intractable tan powder, which cannot be entirely removed,
even when the solutions are filtered through Celite. We believe this
insoluble by-product is an oligomeric or polymeric complex formed
from intermolecular reaction of the unbound phosphine from the j2
product with itself, or with residual Mo starting material, based on the
presence of two distinct carbonyl bands in the IR spectrum (KBr disk,
mCO, cm−1): 1923 (s) 1822 (s).
1
The 31P{ H} NMR spectrum showed only j2-complex throughout the
reaction, but the intensity of these signals gradually diminished relative
to the signal for free PMe3. The crystals were isolated by decanting
the original d8-toluene solution, and their identity was confirmed by
1
31P{ H} NMR.
21 After 12 h of heating at 85 ◦C, the 8a sample still showed only 11a as
product (31% conversion) but the 8b sample showed free Me-capped
tripod 3b, cis and trans-(PMe3)2Mo(CO)3, and another unidentified
species, along with 11b (7%).
29 The extreme solubility of 9a–b in pentane has precluded their purifica-
tion for microanalysis. Impurities can be seen in the SiMe2 region of
the 1H NMR (≤5%).
22 (a) C. Eaborn, P. B. Hitchcock and P. D. Lickiss, J. Organomet. Chem.,
1983, 252, 281–288; (b) L. H. Gade, C. Becker and J. W. Lauher, Inorg.
Chem., 1993, 32, 2308–2314.
30 (a) D. A. Kleier and G. Binsch, J. Magn. Reson., 1970, 3, 146; (b) K.
Marat, SpinWorks, University of Manitoba, Winnipeg, MB, 2004.
2682 | Dalton Trans., 2006, 2671–2682
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The Royal Society of Chemistry 2006
©