Ruthenium hydride complexes of the hindered phosphine ligand tris(3-diisopropylphosphinopropyl)phosphine

Article abstract of DOI:10.1021/ic200492w

The synthesis and characterization of the novel hindered tripodal phosphine ligand P(CH₂CH₂CH₂PⁱPr ₂)₃ (P³P₃^iPr) (1) are reported, along with the synthesis and characterization of ruthenium chloro and hydrido complexes of 1. Complexes [RuCl(P³P₃ ⁱPr)][BPh₄] (2[BPh₄]), RuH₂(P ³P₃ⁱPr) (3), and [Ru(H₂)(H)(P ³P₃^iPr)][BPh₄] (4[BPh₄]) were characterized by crystallography. Complex 2 is fluxional in solution, and low-temperature NMR spectroscopy of the complex correlates well with two dynamic processes, an exchange between stereoisomers and a faster turnstile-type exchange within one of the stereoisomers.

Full text of DOI:10.1021/ic200492w

ARTICLE
pubs.acs.org/IC
Ruthenium Hydride Complexes of the Hindered Phosphine Ligand
Tris(3-diisopropylphosphinopropyl)phosphine
Mohan M. Bhadbhade,‡ Leslie D. Field,*,† Ryan Gilbert-Wilson,† Ruth W. Guest,§ and Paul Jensen§
†School of Chemistry and ‡Mark Wainwright Analytical Centre, The University of New South Wales, NSW 2052, Australia
§School of Chemistry, University of Sydney, NSW 2006, Australia
S Supporting Information
b
ABSTRACT: The synthesis and characterization of the novel
hindered tripodal phosphine ligand P(CH2CH2CH2PiPr2)3
(P3P3iPr) (1) are reported, along with the synthesis and
characterization of ruthenium chloro and hydrido complexes
of 1. Complexes [RuCl(P3P3 Pr)][BPh4] (2[BPh4]), RuH2(P3P3 Pr) (3), and [Ru(H2)(H)(P3P3iPr)][BPh4] (4[BPh4]) were
characterized by crystallography. Complex 2 is fluxional in solution, and low-temperature NMR spectroscopy of the complex
correlates well with two dynamic processes, an exchange between stereoisomers and a faster turnstile-type exchange within one of
the stereoisomers.
i
i
’ INTRODUCTION
This work reports a reasonable synthetic route to the hindered
ligand as well as the formation and characterization of the iron
and ruthenium chloro and hydrido complexes. Characterization
of these complexes allows analysis of the geometry around the
metal center as well as provides an initial assessment of the
chemistry of these metal complexes.
Polydentate phosphines have become important ligands for
controlling the stereochemistry of coordination complexes and
have also been used to solubilize metal catalysts.1 PP3 ligands, such
as P((CH2)nPR2)3 (n = 2, 3; R= Me, Ph), form metal complexes
with a range of applications, especially for complexes of iron and
ruthenium. Applications have ranged from the stabilization of
complexes containing η2-dihydrogen to the formation of stable
dinitrogen complexes with iron and ruthenium in both the 0
and þ2 oxidation states.2 The PP3-type ligand provides a strong
coordination environment and is able to coordinate to the metal at
up to four points through the four phosphine donors. Binding to
form complexes with octahedral, trigonal bipyramidal, and also
square-pyramidal geometry is possible with PP3-type ligands. In an
octahedral system, coordination of PP3-type ligands leaves two
free coordination sites for other ligands, and these are geometry
constrained by the ligand to be in a cis arrangement. It is known
that a cis arrangement of two ligands isa necessity for some partsof
common catalytic mechanisms, such as migratory insertion.3 Thus
PP3-type complexes often have higher catalytic activity, compared
to complexes with mono or bidentate ligands, which can form
inactive isomers where the two reactive ligands are in the unreac-
tive trans arrangement.4
The encapsulating nature of PP3 ligands and the presence of
sterically bulky groups on the terminal phosphines of PP3 ligands
also has the propensity to restrict access to the metal center and
enhance chemical reaction at any of the non-PP3 ligands.
There is now an expanding range of sterically encumbered,
polydentate ligands available,5 and we report here the synthesis
of the hindered tripodal tetradentate phosphine ligand
P(CH2CH2CH2PiPr2)3 (P3P3iPr). This ligand is a more hindered
version of the PP3-type ligand skeleton, P(CH2CH2CH2PR2)3
which is known with either ethyl or methyl substituents on the
terminal phosphine donors.6
’ RESULTS AND DISCUSSION
Syntheses of other PP3-type ligands with propylene bridges
between the apical and terminal phosphines have usually
involved the radical-initiated addition of a dialkylphoshine
(R2PꢀH) across the double bond of triallylphosphine. Synth-
eses of both P(CH2CH2CH2PMe2)3 and P(CH2CH2CH2-
PEt2)3 have been reported using this general approach.6e While
P(CH2CH2CH2PiPr2)3 (1) can be synthesized by this ap-
proach, the long reaction time and the tendency to form
PiPr2CH2CH2CH2PiPr2 as a reaction side product make it a
less than ideal synthesis.
i
P3P3 Pr (1) can be synthesized more efficiently using an
alternative approach via nucleophilic substitution of the halide
in tris(3-bromopropyl)phosphine with a dialkylphosphide
(Scheme 1). Tris(3-hydroxypropyl)phosphine was bromi-
nated using phosphorus tribromide to give the unstable tris-
(3-bromopropyl)phosphine. Tris(3-bromopropyl)phosphine
tends to form a mixture of oligomeric and polymeric products
on standing, probably by intramolecular or intermolecular
nucleophilic attack of the central phosphine on a brominated
γ-carbon, to form an insoluble solid mass within a few hours.
Tris(3-bromopropyl)phosphine was used immediately in the
next step of the sequence without further purification. P(CH2-
CH2CH2PiPr2)3, P3P3 (1) was prepared in moderate yield
iPr
Received: March 9, 2011
Published: June 01, 2011
r
2011 American Chemical Society
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(51% from tris(3-hydroxypropyl)phosphine) by the nucleo-
philic substitution of bromide in the reaction of lithium diiso-
propylphosphide, (LiPiPr2) with tris(3-bromopropyl)phosphine
(Scheme 1). This method of synthesis is relatively direct and
clean and avoids the difficulty of the alternative radical route
which typically requires prolonged reaction times with volatile
and reactive secondary phosphines and the known formation of
unwanted byproduct.6e
grown from a THF/pentane solution of 2[BPh4] (Figure 1A),
and selected bond angles and lengths are given in Table 1.
The two structures within the asymmetric unit are sufficiently
similar that the crystallographic data of only one of the config-
urations, namely Ru1, is detailed here. Each asymmetric unit
within the crystal structure contains one THF molecule, giving
half a THF solvate for each metal complex.
The geometry of [RuCl(P3P3iPr)]þ (2) is a distorted square-
based pyramid with atoms Cl1, P1, P2, and P4 making up the
base and P3 at the apex (τ = 0.13).8 In this instance, the
PEꢀRuꢀPE angle, P1ꢀRu1ꢀP4, at 156.51(2)° is appreciably
closer to that of a square-based pyramid (180°) than a trigonal
bipyramid (120°). In addition, the RuꢀPT bond length,
Ru1ꢀP3, at 2.2536(6) Å is significantly shorter than the RuꢀPE
bond lengths Ru1ꢀP1 and Ru1ꢀP4 (2.4629(6) and 2.3892(7)
Å, respectively), and this is characteristic of square-based pyr-
amid geometry. One of the isopropyl methyl groups fills and
blocks the void under the base of the pyramid probably through
an anagostic (pseudoagostic) interaction (d(RuꢀH) = 2.637 Å
(Ru1) and 2.369 Å (Ru2); RuꢀCꢀH = 127.75°(Ru1) and
131.18°(Ru2)).9
iPr
In the 31P{1H} spectrum of P(CH2CH2CH2PiPr2)3, P3P3
(1), two resonances are observed at 1.9 and ꢀ34.7 ppm, and
these are assigned to the terminal phosphines and the central
phosphine respectively. Both resonances are singlets with no
discernible coupling between the two phosphine environments,
and this is consistent with data reported for other P3P3-type
ligands incorporating propylene bridges between the apical and
terminal phosphines.6e
Tripodal phosphorus ligands of the P((CH2)nPR2)3 (n = 2, 3)
type are well established as good ligands at ruthenium centers,7
and in this work, the P3P3iPr ligand 1 was successfully employed
in the synthesis of the five-coordinate chloro complex of ruthe-
nium [RuCl(P3P3iPr)]þ (2).
[RuCl(P3P3iPr)]þ. Addition of sodium tetraphenylborate to a
There are three structures of ruthenium with the analogous
tripodal tetradentate ligand (P(CH2CH2CH2PMe2)3),10 how-
ever, these are all six-coordinate complexes with approximate
octahedral geometry around the metal center.
iPr
tetrahydrofuran (THF) solution of P3P3 (1) and RuCl2-
(PPh3)3 afforded [RuCl(P3P3iPr)][BPh4] (2[BPh4]) as a pink
solid (Scheme 2). Crystals suitable for structural analysis were
The only comparative structure of a five-coordinate complex
of ruthenium with a tripodal tetradentate phosphine ligand is that
of [RuCl(P(CH2CH2PiPr2)3)][BPh4],2a which can be approxi-
mated to a distorted square-based pyramid in the same way as 2
(included in Figure 1B for comparison). In a similar fashion to 2,
[RuCl(P(CH2CH2PiPr2)3)][BPh4] also has one of the isopropyl
Scheme 1
Table 1. Selected Bond Lengths (Å) and Angles (°) for 2[Ru-
Cl(P3P3iPr)][BPh4] THF (2[BPh4])
3
Ru1ꢀCl1
2.4351(7)
2.4629(6)
2.3892(7)
165.53(2)
103.06(2)
89.20(2)
Ru1ꢀP2
Ru1ꢀP3
2.2618(7)
2.2536(6)
Ru1ꢀP1
Scheme 2
Ru1ꢀP4
Cl1ꢀRu1ꢀP2
Cl1ꢀRu1ꢀP3
P2ꢀRu1ꢀP1
P2ꢀRu1ꢀP4
P1ꢀRu1ꢀP4
Cl1ꢀRu1ꢀP1
Cl1ꢀRu1ꢀP4
P2ꢀRu1ꢀP3
P1ꢀRu1ꢀP3
P3ꢀRu1ꢀP4
89.28(2)
83.51(2)
91.38(2)
99.40(2)
104.00(2)
92.25(3)
156.51(2)
Figure 1. ORTEP plot (50% thermal ellipsoids) of: (A) 2[RuCl(P3P3iPr)][BPh4] THF (2[BPh4]) (see Supporting Information for more data) and for
3
comparison (B) [RuCl(P(CH2CH2PiPr2)3)][BPh4].2a Only one of the two complex cations in each asymmetric unit in shown. Selected hydrogen
atoms, tetraphenylborate anions, and THF solvate have been omitted for clarity.
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Scheme 4
Figure 2. Variable-temperature 31P{1H} NMR spectra for [RuCl-
(P3P3iPr)][BPh4] (2[BPh4]) (242.95 MHz, methylene chloride-d2).
Scheme 3
Figure 3. ORTEP plot (50% thermal ellipsoids) of one of the two
[RuH2(P3P3iPr)] (3) units within the asymmetric unit. Selected hydro-
gen atoms have been removed for clarity.
phosphines, displayed as an exchange-broadened singlet at
72.1 ppm. The reversal in relative chemical shifts from 2 to
those of [RuCl(P(CH2CH2PiPr2)3)][BPh4] is rationalized by
the five-membered ring effect in phosphorus metallocycles.
The 5-membered ring effect is reasonably well documented
and describes how phosphorus nuclei involved in 5 member
metallocycles are significantly shifted to downfield compared
to their 4, 6, and 7 member metallocycle anaologues.11 This
effect is magnified for the central phosphorus PC in [RuCl-
(P(CH2CH2PiPr2)3)] since the central P is effectively part of
three five-membered metallocycles, and this rationalizes why
the two complexes, while chemically and structurally similar,
have 31P chemical shifts for the central P atom which differ by
almost 130 ppm.
methyl groups filling and blocking the sixth coordination site
under the base of the pyramid.
The metal-to-donor atom bond lengths of 2 are equivalent to,
or longer than, the analogous lengths in [RuCl(P(CH2CH2-
PiPr2)3)]þ. The PꢀRuꢀP bond angles are all greater in 2,
and the ClꢀRuꢀP bond angles are all more acute than in
[RuCl(P(CH2CH2PiPr2)3)]þ. Thus in [RuCl(P((CH2)n-
PiPr2)3)]þ complexes with n = 2, 3, the complex with 3-carbon
straps is a less strained complex and shows relaxation of the
ligand bite angles and a lengthening of the metal-to-donor
atom bond lengths.
As the temperature of the 31P{1H} NMR spectrum of
2[BPh4] is decreased, the spectra broaden and then sharpen
(Figure 2). At 243 MHz, as the temperature decreases to 220 K,
the single broad resonance for the terminal phosphines
separates into three distinct resonances representing the
terminal phosphines individually, giving four distinct reso-
nances representing the four different phosphine environ-
ments. At still lower temperatures, each of the resonances
eventually splits into two resonances of comparable intensity
(1:0.8) to give a total of 8 resonances, which we attribute to
two isomers of 2[BPh4]. The Cl resides either trans to the
apical phosphorus or trans to a terminal phosphorus (Isomer-
1 and Isomer-2) (Scheme 3).
In the 31P{1H} NMR spectrum of 2[BPh4] at room tempera-
ture, the signal for the three terminal phosphines appears as a
very broad resonance at 25.7 ppm, while the signal for the central
phosphine appears as a sharp quartet at 14.2 ppm with a splitting
of 36.4 Hz. A modest increase (25 K) in the temperature of the
NMR experiment resulted in an appreciable sharpening of the
resonances of the terminal phosphines. These spectra are
analogous to those of [RuCl(P(CH2CH2PiPr2)3)][BPh4],2a
and the broadness can be rationalized by the facile exchange of
the terminal phosphine environments.
It is interesting to note that the central phosphine resonance in
2[BPh4] appears to high field with respect to the terminal
phosphine signals. In the analogous compound with two-carbon
straps [RuCl(P(CH2CH2PiPr2)3)][BPh4], the 31P{1H} NMR
spectrum shows the central phosphine as a quartet at 142.9
ppm (splitting 15.2 Hz) to low field of the three terminal
In this system there are two exchange processes operating.
One which exchanges the terminal phosphorus environments,
and one which interchanges Isomer-1 and Isomer-2. At 199 K,
the three resonances of the terminal phosphines of Isomer-2
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2
(PA2, PB2, PC2) resolve with JPꢀP coupling observed, while in
Isomer-1 the terminal phosphine resonances remain broad with
no defined coupling. All of the resonances eventually sharpen at
178 K to display similar coupling patterns.
Simulation12 of the exchange-broadened 31P{1H} NMR spec-
trum of [RuCl(P3P3iPr)][BPh4] (2[BPh4]) was consistent with
an exchange process where k1 ≈ 3.5k2, and at 199 K, k1 ≈ 385
RuH2(P3P3iPr). Reaction of the ruthenium chloro complex
[RuCl(P3P3iPr)][BPh4] (2[BPh4]) with two equivalents of
KBEt3H afforded the dihydride complex RuH2(P3P3iPr) (3) as
a white crystalline solid (Scheme 4). Crystals suitable for
structural analysis were grown by slow evaporation of a toluene
solution of 3 under nitrogen (Figure 3) and selected bond angles
and lengths are given in Table 2.
and k2 ≈ 110 sꢀ1 and at 210 K with k1 ≈ 2135 and k2 ≈ 610 sꢀ1
.
The fact that the resonances of one isomer sharpen while the
resonances of the other remain broad suggests that the exchange
of the terminal phosphines is faster in one isomer (Isomer-1)
than in the other. The pattern and sequence of coalescences in
the variable temperature NMR spectra can be rationalized by a
model where exchange between the terminal phosphines in
Isomer-1 (k1) is fast compared to the interchange between the
isomers (k2, Scheme 3). We suggest that Isomer-1 is that in which
Cl is trans to the central phosphines since the exchange of the
terminal phosphines then involves simply a turnstile-type pro-
cess, with exchange of the terminal phosphines between adjacent
coordination sites.
The geometry of RuH2(P3P3iPr) (3) is a distorted octahedron
with the two hydrides in mutually cis coordination sites.
There are eight previously reported structures of ruthenium(II)
tetraphosphine dihydrides of which four have defined and
refined hydrides.10c,13 The RuꢀH bond lengths of 1.62(5) and
1.69(5) Å sit comfortably within the ranges provided by the other
4 structures, with RuꢀH bond lengths of 1.51 to 1.77 Å. Similarly
the HꢀRuꢀH bond angle of 91(2)° sits within the range of
previously reported structures with bond angles between 77 to
93°.
The structure of 3 is analogous to that of [RuH2-
(P(CH2CH2CH2PMe2)3)],10c which contains the related tetra-
dentate phosphine ligand with methyl substituents on the ter-
minal phosphines instead of isopropyl substituents. The average
of the RuꢀP bond lengths, 2.312 Å is slightly longer than for
[RuH2(P(CH2CH2CH2PMe2)3)] for which the average RuꢀP
bond length is 2.286 Å. This is probably due to the steric bulk of
the isopropyl substituents when compared to the methyl sub-
stituents, resulting in elongation of the core RuꢀP bonds.
In the 31P{1H} NMR spectrum of RuH2(P3P3iPr) (3), the
signal for the two terminal phosphines PE appears as a doublet of
Table 2. Selected Bond Lengths (Å) and Angles (°) for
[RuH2(P3P3iPr)] (3)
Ru1 -H1
1.62(5)
Ru1 ꢀH2
1.69(5)
Ru1 ꢀP1
Ru1 ꢀP4
2.2785(13)
2.3502(12)
71.7(17)
75.4(17)
88.4(15)
85.0(15)
89.45(4)
105.33(4)
93.42(4)
91(2)
Ru1 ꢀP2
2.3162(12)
2.3032(12)
85.1(19)
176.7(18)
175.7(16)
90.7(16)
146.44(4)
94.75(4)
107.62(4)
Ru1 ꢀP3
H1ꢀRu1ꢀP2
H1ꢀRu1ꢀP3
H2ꢀRu1ꢀP2
H2ꢀRu1ꢀP3
P2ꢀRu1ꢀP1
P2ꢀRu1ꢀP4
P1ꢀRu1ꢀP4
H1ꢀRu1ꢀH2
H1ꢀRu1ꢀP1
H1ꢀRu1ꢀP4
H2ꢀRu1ꢀP1
H2ꢀRu1ꢀP4
P2ꢀRu1ꢀP3
P1ꢀRu1ꢀP3
P3ꢀRu1ꢀP4
Figure 4. Selected high-field region of 1H NMR (600 MHz, benzene-d6) RuH2(P3P3iPr) (3) with coupling tree.
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Scheme 5
Table 3. Selected Bond Lengths (Å) and Angles (°) for
[Ru(H2)(H)(P3P3iPr)][BPh4].EtOH 4[BPh4]
H2 ꢀH3
0.66(6)
1.62(6)
2.2890(9)
2.4270(8)
75(2)
Ru1 ꢀH1
1.59(3)
Ru1 ꢀH2
Ru1 ꢀH3
1.75(4)
Ru1 ꢀP1
Ru1 ꢀP3
Ru1 ꢀP2
Ru1 ꢀP4
2.3756(9)
2.3591(9)
97.4(18)
86.3(12)
178.7(12)
161(2)
H1ꢀRu1ꢀH2
H2ꢀRu1ꢀH3
H1ꢀRu1ꢀP2
H1ꢀRu1ꢀP4
H2ꢀRu1ꢀP2
H2ꢀRu1ꢀP4
H3ꢀRu1ꢀP2
H3ꢀRu1ꢀP4
P1ꢀRu1ꢀP3
P2ꢀRu1ꢀP3
P3ꢀRu1ꢀP4
H1ꢀRu1ꢀH3
H1ꢀRu1ꢀP1
H1ꢀRu1ꢀP3
H2ꢀRu1ꢀP1
H2ꢀRu1ꢀP3
H3ꢀRu1ꢀP1
H3ꢀRu1ꢀP3
P1ꢀRu1ꢀP2
P1ꢀRu1ꢀP4
P2ꢀRu1ꢀP4
23(2)
75.1(12)
74.1(12)
89(2)
106(2)
79(2)
175.4(14)
89.2(13)
89.15(3)
93.56(3)
148.79(3)
89.2(13)
90.1(13)
92.36(3)
104.61(3)
106.33(3)
Figure 5. ORTEP plot (50% thermal ellipsoids) of the complex cation
of [Ru(H2)(H)(P3P3iPr)][BPh4] EtOH (4[BPh4]) (see Supporting
3
Information for more data) within the asymmetric unit. Selected
hydrogen atoms have been removed for clarity.
doublets at 49.2 ppm, the signal for the terminal phosphine PT is
a doublet of triplets at 28.5 ppm, and the central phosphine PC
signal appears as a doublet of triplets at 0.4 ppm.
Figure 6. 31P{1H} NMR spectrum (242.9 MHz, THF-d8) of
[Ru(H2)(H)(P3P3iPr)][BPh4] (4[BPh4]) at 298, 244, and 215 K.
The 1H NMR resonances for the two hydrido ligands of 3 both
appear as doublets of triplets of doublets of doublets at ꢀ9.43
and ꢀ12.50 ppm due to coupling to the 4 phosphorus nuclei in 3
different environments and to each other (Figure 4). The
coupling constants 2JHꢀP are 59.6 Hz, 24.2 and 18.8 Hz for the
resonance at ꢀ9.43 ppm and 63.2 Hz, 34.0 and 15.0 Hz for the
dihydrogen ligand was not refined. There have, however, been
examples of ruthenium cis hydride dihydrogen complexes with
all ligands refined, including the dihydrogen ligand, and these
include [Ru(H)(H2)(X)(PiPr3)2] (X = benzoquinoline, 5)16
and [RuH(H2)(o-C6H5py)(PiPr3)2][BArf] 6[BArF].17 The
Ru1ꢀH1 bond length for 4 (1.59(3) Å) is comparable to that
of 5 (1.54(4) Å) and 6[BArF] (1.528(20) Å). Likewise the
dihydrogen bond distances Ru1ꢀH2 and RuꢀH3 for 4 of
1.62(6) and 1.75(4) Å, respectively, are similar but slightly
elongated compared to those for 5 (1.57(5) and 1.68(4) Å)
and 6[BArF] (1.564(20) and 1.547(21) Å). This elongation
is probably caused by the differences in the donor atom
in the coordination site trans to dihydrogen, with 5 being
carbon and 6[BArF] being nitrogen as opposed to phos-
phorus in 4.
2
resonance at ꢀ12.50 ppm, with the JHꢀH coupling constant
between the two hydrides of 6.2 Hz.
[Ru(H2)(H)(P3P3iPr)][BPh4]. A solution of LiAlH4 (∼1.5 M) in
THF was added dropwise to a THF solution of 2[BPh4] to the
point where the color change from pink to colorless was
complete. Ethanol was added, and the resulting orange suspen-
sion worked up to afford [Ru(H2)(H)(P3P3iPr)][BPh4] 4[BPh4]
as an orange crystalline solid (Scheme 5). Crystals suitable for
structural analysis were grown from a THF/pentane solution of
4[BPh4] (Figure 5), and selected bond angles and bond lengths
are given in Table 3.
[Ru(H2)(H)(P3P3iPr)][BPh4] (4[BPh4]) is clearly fluxional
in solution. At low temperature (215 K) there are 4 resonances
for the coordinated phosphines (at 30.3, 21.3, 13.7, and 4.3 ppm,
Figure 6). The appearance of four resonances is consistent with
the solid-state structure where the two mutually trans phos-
phines are not equivalent due to puckering of the six-mem-
bered metallocyclic rings. At low temperature, the two
mutually trans phosphines (PB and PC) exhibit a large resolved
The geometry of [Ru(H2)(H)(P3P3iPr)]þ (4) is a distorted
octahedral with the hydride and dihydrogen ligands in mutually
cis coordination sites. There are two other structures of dihydro-
gen hydrido ruthenium complexes where the hydrido and
dihydrogen ligands are in the cis arrangement with four other
phosphine donors, [Ru(H2)(H)(PPh2Me)4]14 and [Ru(H2)-
(H)(P(CH2CH2PPh2)3)]þ.15 In both of these complexes, the
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Figure 7. Selected high-field 1H{31P} NMR spectrum (700 MHz, THF-d8) of partially deuterated [Ru(H2)(H)(P3P3iPr)][BPh4] (4[BPh4]) at 200 K
with resolution enhancement.
Table 4. Crystal Data Refinement Details for 2[BPh4], 3, and 4[BPh4]
2[BPh4]
3
4[BPh4]
chemical formula
C
104H168OB2Cl2P8Ru2
C27H62P4Ru
611.72
C52H86BOP4Ru
956.94
formula mass
2000.82
triclinic
12.280(2)
19.093(4)
23.928(4)
111.889(3)
98.687(3)
90.272(3)
5134.6(16)
150(2)
P1
crystal system
monoclinic
17.099(2)
17.3858(17)
21.692(3)
90.00
monoclinic
16.4283(7)
17.7729(6)
18.1170(6)
90.00
a/Å
b/Å
c/Å
R/°
β/°
γ/°
V (Å3)
100.723(4)
90.00
105.8920(10)
90.00
6335.9(13)
150(2)
P2(1)/c
8
5087.6(3)
150(2)
temperature/K
space group
P2(1)/n
4
Z
2
μ(Mo KR) (mmꢀ1
)
0.517
0.711
0.468
N
49295
43788
60050
Nind
Rint
23041
11053
11063
0.0245
0.0999
0.0561
Final R1 values (I > 2σ(I))
Final wR(F2) values (I > 2σ(I))
Final R1 values (all data)
Final wR(F2) values (all data)
Goodness of fit on F2
0.0322
0.0426
0.0468
0.0754
0.1098
0.0975
0.0496
0.0770
0.0670
0.0851
0.1417
0.1054
1.034
0.790
1.095
1
coupling (about 180 Hz). As the temperature is raised to
244 K, the resonances for PB and PC broaden and coalesce, and
this is probably due to ring-flipping of the ligand backbone. At
higher temperatures (above 298 K), there is mutual exchange
of all three of the terminal phosphines (a turnstile-type
exchange), and the signals for the terminal phosphines are
averaged to a single broad resonance. Modeling of the
exchanges at 244 K indicates that the exchange of magnetism
between the mutually trans phosphines (ring flip) occurs at
a rate of about 3000 sꢀ1 and that the turnstile exchange of
the terminal phosphines occurs at a significantly slower rate
(about 300 sꢀ1).
At 298 K, the H NMR resonances of the hydrido and
dihydrogen ligands of 4[BPh4] appear as a single broad reso-
nance at ꢀ8.57 ppm, indicating fast exchange between the
hydrido and dihydrogen ligands. At 195 K the signal resolves to
a broad 2-proton resonance at ꢀ7.44 ppm, assigned to the
dihydrogen ligand and a phosphorus-coupled doublet of
triplets at ꢀ10.3 ppm (2JHꢀP of 57 and 32 Hz) for the hydrido
resonance. Further evidence for this assignment comes from
the T1 values for these two high field resonances at 180 K with
the dihydrogen resonance having a T1 of 68 ( 3 ms and the
hydrido resonance a T1 of 594 ( 12 ms. It is characteristic
of dihydrogen hydrido metal complexes that the dihydrogen
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Inorganic Chemistry
ARTICLE
ligand has a significantly faster relaxation time than the
hydrido ligand.6a
Under an atmosphere of deuterium gas, the dihydrogen ligand
substituents may permit the chemistry of PP3 metal complexes
to be tuned to access catalytic reactivity not possible with
stronger ligand sets.
exchanges for D2, partially incorporating deuterium into the
1
metal-bound hydrogens. In the H NMR spectrum at 200 K,
’ EXPERIMENTAL SECTION
the dihydrogen (or hydrogen deuteride) resonance at about
ꢀ10 ppm appears as a superimposition of signals (Figure 7) due
to the different isotopomers. The two 3-line resonances corre-
sponding to the species with coordinated HꢀD can be resolved
and the JHD coupling of 31 Hz extracted. JHD greater than 15 Hz
is considered to be characteristic of dihydrogen complexes,18
clearly confirming the presence of η2-coordinated HꢀD in this
species. Using the most recent interrelationship between HꢀH
General Information. All manipulations were carried out using
standard Schlenk and glovebox techniques under a dry atmosphere of
nitrogen. Solvents were dried and distilled under nitrogen or argon using
standard procedures21 and stored in glass ampules fitted with Youngs
Teflon taps. Benzene was dried over sodium wire before distillation from
sodium/benzophenone, while ethanol was distilled from diethoxymag-
nesium. THF (inhibitor free) and pentane were dried and deoxygenated
using a Pure Solv 400-4-MD (Innovative Technology) solvent purifica-
tion system. Deuterated solvents THF-d8, toluene-d8, and benzene-d6
were dried over and distilled from sodium/benzophenone and were
vacuum distilled immediately prior to use. Dichlorotris(triphenyl-
phosphine)ruthenium(II),22 and diisopropylphosphine23 were prepared
by literature methods. Tris(3-hydroxypropyl)phosphine was purchased
from Strem and used without further purification. LiAlH4 was purchased
from Aldrich, and a concentrated solution in THF prepared by Soxhlet
extraction. Air-sensitive NMR samples were prepared in an argon- or
nitrogen-filled glovebox or on a high-vacuum line by vacuum transfer of
bond distance (dHH) and HꢀD coupling (JHD) of dHH
=
1.47ꢀ0.0175 JHD Å determined by Gusev,19 dHH is determined
to be 0.927 Å. This is longer than the value determined from the
crystal structure of [Ru(H2)(H)(P3P3iPr)][BPh4] (4[BPh4]) of
0.66(6) Å but expected, as crystallographic techniques are
notorious for giving foreshortened dHH because of rapid H2
rotation/vibration.18
[Ru(H2)(H)(P3P3iPr)][BPh4] (4[BPh4]) is remarkably
stable and survives unchanged under a nitrogen atmosphere
indefinitely without substitution of the H2, even after several
freezeꢀpumpꢀthaw cycles. The lack of ready N2 substitution
probably reflects the fact that steric crowding from the bulky
P3P3iPr ligand makes binding the smaller H2 more preferable than
the bulkier N2.
1
solvent into an NMR tube fitted with a concentric Teflon valve. H,
13C{1H}, and 31P{1H} spectra were recorded on Bruker DPX300,
Avance III 400, Avance III 500, Avance III 600, or Avance III 700
NMR spectrometers operating at 300, 400, 500, 600, and 700 MHz for
1H, 100.61 or 150.92 MHz for 13C{1H}, and 121.49, 161.98, 202.49, and
242.95 MHz for 31P{1H} respectively. All NMR spectra were recorded
at 298 K, unless stated otherwise. 1H and 13C{1H} NMR spectra were
referenced to residual solvent resonances. 31P{1H} NMR spectra were
referenced to external neat trimethyl phosphite at 140.85 ppm. Dynamic
NMR simulations were performed using WinDNMR: Dynamic NMR
Spectra for Windows.12 T1 calculations were performed using the curve
fitting applications of Origin 8.1, by OriginLab.24 Microanalyses were
carried out at the Campbell Microanalytical Laboratory, University of
Otago, New Zealand. Details of the X-ray analyses are given in (Table 4).
Treatment of [Ru(H2)(H)(P3P3iPr)][BPh4] (4[BPh4]) with
potassium tert-butoxide in d8-THF results in clean deprotonation
and formation of RuH2(P3P3iPr) (3). Conversely, treatment of
3 with triflic acid in d8-THF in the presence of BPh4ꢀ results in
formation of 4[BPh4], and this acid/base behavior is consistent
with that observed for other hydrido and dihydrido complexes of
ruthenium and iron.20
iPr
Synthesis of P(CH2CH2CH2PiPr2)3, P3P3 (1); P(CH2CH2-
’ CONCLUSIONS
CH2Br)3. Phosphorus tribromide (4.5 mL, 0.048 mol) was added
dropwise to a stirring suspension of tris(3-hydroxypropyl)phosphine
(7.2 g, 0.035 mol) in DCM (30 mL) under nitrogen. The reaction
mixture was stirred at room temperature for 18 h. Saturated aqueous
sodium carbonate solution (approximately 30 mL) was added to the
reaction mixture until all effervescence ceased. The organic layer was
separated and dried over anhydrous sodium sulfate. The solution was
filtered, and the solvent was removed under reduced pressure to give
tris(3-bromopropyl)phosphine as a clear liquid (10.0 g, 73%). The crude
P(CH2CH2CH2Br)3 was used immediately in the next step without
further purification. 31P NMR (162 MHz, benzene-d6): δ ꢀ34.1 (1P, sept,
2JPꢀH = 7 Hz). 1H{31P} NMR (400 MHz, benzene-d6): δ 2.99 (6H, m,
CH2Br); 1.56 (6H, m, PCH2); 1.06 (6H, m, CH2CH2CH2). 13C{1H}
NMR (101 MHz, benzene-d6): δ 34.7 (d, 3JCꢀP = 14 Hz, CH2Br); 29.4 (d,
1JCꢀP = 16 Hz, PCH2); 25.5 (d, 2JCꢀP = 15 Hz, CH2CH2CH2).
The new sterically hindered, tripodal tetradentate ligand
P3P3iPr (1) was synthesized and used in the synthesis of a series
of stable ruthenium compounds. The 5-coordinate chloro com-
plex [RuCl(P3P3iPr)]þ (2) was characterized crystallographically
and by multinuclear NMR spectroscopy, with low-temperature
31P{1H} NMR spectroscopy being used to explore the exchange
mechanisms between its various isomers. Complex 2 was reacted
with potassium triethylborohydride to produce RuH2(P3P3
)
iPr
(3). Complex 2 was also reacted with lithium aluminum hydride,
followed by reaction with ethanol to produce the stable hydrido
dihydrogen species [Ru(H2)(H)(P3P3iPr)]þ (4). Complexes 3
and 4 were characterized both crystallographically and by multi-
nuclear NMR spectroscopy.
The bulky P3P3 ligand is among the most sterically en-
iPr
iPr
cumbered PP3-type ligands so far synthesized. While the P3P3
LiPiPr2. Lithium phosphide was prepared following a modified
method by Fryzuk et al.25 n-Butyllithium (1.5 M in hexane, 50 mL,
0.097 mol) was added to diisopropylphosphine (8.3 g, 0.070 mol) in
THF (40 mL) with stirring. This procedure resulted in a bright-yellow
solution which was used directly in the next step.
ligand, outlined in this paper, forms stable tetradentate 5- and
6-coordinate complexes with ruthenium, the complexes are
hindered, and they are fluxional and hemilabile in solution. This
behavior is typical of many complexes where the metalꢀP bonds
are weakened because the bulky ligand substituents restrict the
phosphorus donors from gaining optimal access to the metal
center.
iPr
P3P3 (1). The lithium diisopropylphosphide solution from the
previous step was added to a stirring solution of tris(3-bromopropyl)-
phosphine (10 g, 0.023 mol) in THF (approximately 100 mL). The
reaction mixture was left to stir at room temperature for 18 h. The
solvent was removed under reduced pressure, and benzene (20 mL) was
added, followed by deaerated water (30 mL) which was added with care
The synthetic approach to the P3P3iPr ligand is generic, and it
is possible to introduce a range of substituents on the terminal
phosphorus using this method. Other bulkier and tailored
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Inorganic Chemistry
ARTICLE
at 0 °C until all excess lithium phosphide had been destroyed. Benzene
(10 mL) was added, and the mixture was stirred for 2 h. The organic
layer was decanted, dried over anhydrous sodium sulfate, and filtered to
give a yellow solution. The solvent was removed under reduced pressure,
and the resulting yellow oil was heated under vacuum (0.4 mbar) at
100 °C to remove volatile byproducts, leaving tris(3-diisopropylpho-
sphinopropyl)phosphine as a yellow oil (9.0 g, 18 mmol, 51% from
tris(3-hydroxypropyl)phosphine). 31P{1H} NMR (162 MHz, benzene-
d6): δ 1.9 (3P, s, PiPr2); ꢀ34.7 (1P, s, P(CH2)3). 1H{31P} NMR (400
MHz, benzene-d6): δ 1.75 (6H, m, CH2CH2CH2); 1.58 (6H, m,
CH(CH3)2); 1.53 (6H, m, PCH2); 1.42 (6H, t, 3JHꢀH = 8 Hz, PCH2);
1.05 (18H, d, 3JHꢀH = 7 Hz, CH(CH3)2); 1.03 (18H, d, 3JHꢀH = 7 Hz,
CH(CH3)2). 13C{1H} NMR (101 MHz, benzene-d6): δ 29.8 (dd,
JCꢀP = 12 Hz, 15 Hz, PCH2); 25.4 (dd, JCꢀP = 20 Hz, 14 Hz,
THF-d8): δ 33.4 (s, CH(CH3)2) ; 33.3 (s, CH(CH3)2); 31.4 (dd, JCꢀP =
3.5 Hz, JCꢀP = 18 Hz, PCH2); 30.2 (t, JCꢀP = 16 Hz, PCH2); 30.0
(t, JCꢀP = 3.7 Hz, CH2CH2CH2); 29.8 (q, JCꢀP = 9 Hz, PCH2); 23.8
(t, JCꢀP = 4.7 Hz, CH2CH2CH2); 23.2 (d, JCꢀP = 5 Hz, CH(CH3)2);
22.9 (dd, JCꢀP = 11 Hz, JCꢀP = 7 Hz, PCH2); 21.2 (s, CH(CH3)2); 20.5
(s, CH(CH3)2); 20.1 (s, CH(CH3)2); 20.0 (s, CH(CH3)2); 18.8
(s, CH(CH3)2).
Synthesis of [Ru(H2)(H)(P3P3iPr)][BPh4] (4[BPh4]). A concen-
trated solution of LiAlH4 in THF was added dropwise to a solution of
[RuCl(P3P3iPr)][BPh4] (2[BPh4]) (0.16 g, 0.16 mmol) in THF (3 mL)
until there was a color change from pink to colorless with a white
suspension. Ethanol was added carefully, dropwise, until effervescence
had ceased (about four drops) and the color of the reaction mixture had
turned to orange, then an additional four drops of ethanol was added.
The reaction mixture was filtered through Celite, and the solvent
removed under reduced pressure. The orange powder was washed with
pentane (10 mL) to give [Ru(H2)(H)(P3P3iPr)][BPh4] (0.050 g,
0.051 mmol, 33% yield). Anal. found: C, 65.80; H, 8.98; C51H83P4-
RuB.C4H8O (MW 1004.10) requires: C, 65.79; H, 9.13. 31P{1H} NMR
(203 MHz, THF-d8): δ 24.7 (3P, s br, PA/B/C); 5.4 (1P, q, 2JPꢀP = 37.5
Hz, PD). 31P{1H} NMR (203 MHz, THF-d8, 180K): δ 31.3 (1P, d br,
2JPꢀP = 185 Hz, PA); 21.3 (1P, d br, 2JPꢀP = 185 Hz, PA); 13.7 (1P, s br,
PC); 4.3 (1P, s br, PD). 1H NMR (400 MHz, THF-d8): δ 7.20 (8H, m,
BPhortho); 6.82 (8H, m, BPhmeta); 6.68 (4H, m, BPhpara); 2.1ꢀ
1.9 (12H, m, CH(CH3)2/CH2); 1.9ꢀ1.8 (6H, m, CH2); 1.65ꢀ1.55
(6H, m, CH2); 1.25ꢀ1.05 (36H, m, CH(CH3)2); ꢀ8.57 (3H, s br,
CH2CH2CH2); 24.1 (dd, JCꢀP = 20 Hz, 10 Hz, PCH2); 23.7 (d, 1JCꢀP
14 Hz, CH(CH3)2); 20.4 (d, 2JCꢀP = 16 Hz, CH(CH3)2); 19.0 (d, 2JCꢀP
=
=
10 Hz, CH(CH3)2). HRMS (EI) m/z: [M þ H]þ 509.3716 (calcd
509.3724)
Synthesis of [RuCl(P3P3iPr)][BPh4] (2[BPh4]). Tris(3-diisopro-
pylphosphinopropyl)phosphine P3P3iPr (1) (456 mg, 0.896 mmol) was
added to a brown solution of dichlorotris(triphenylphosphine)ruthenium-
(II) (860 mg, 0.896 mmol) in THF (approximately 30 mL) resulting in an
immediate color change to green. A stoichiometric amount of sodium
tetraphenylborate (306 mg, 0.894 mmol) was added, and the solution
slowly turned red with stirring. After 3 h, the solvent was removed under
reduced pressure to give a pink solid which was recrystallized twice from
THF layered with pentane (300 mg, 53%). Crystals suitable for X-ray
diffraction were collected. Anal. found: C, 63.71; H, 8.27; C51H80BClP4Ru
(MW 964.41) requires: C, 63.52; H, 8.36%. 31P{1H} NMR (162 MHz,
THF-d8): δ 25.7 (3P, br, PE/T); 14.2 (1P, q, 2JP(C)ꢀP(B/P) = 36.4 Hz, PC).
31P{1H} NMR (243 MHz, 177.6 K, methylene chloride-d2): δ 75.1
(Isomer-2, 1P, m, PB); 72.0 (Isomer-1, 1P, m, PB); 15.4 (Isomer-2, 1P,
ddd, 2JP(B)ꢀP(D) = 45 Hz, 2JP(A)ꢀP(D) = 32 Hz, 2JP(C)ꢀP(D) = 32 Hz, PD);
14.0 (Isomer-1, 1P, dt, m, PD); 4.1 (Isomer-1, 1P, dm, 2JP(A)ꢀP(C) = 227
Hz, PA); 2.3 (Isomer-2, 1P, ddd, 2JP(A)ꢀP(C) = 234 Hz, 2JP(A)ꢀP(D) =31Hz,
2JP(A)ꢀP(B) = 18 Hz, PA); ꢀ4.3 (Isomer-2, 1P, ddd, 2JP(A)ꢀP(C) = 234 Hz,
2JP(A)ꢀP(D) = 31 Hz, 2JP(A)ꢀP(B) = 29 Hz, PC); ꢀ8.1 (Isomer-1, 1 P, dm,
2JP(A)ꢀP(C) = 227 Hz, PC); 1H NMR (400 MHz, THF-d8): δ 7.29 (8H, m,
BPhortho); 6.86 (8H, m, BPhmeta); 6.72 (4H, m, BPhpara); 2.64 (6H, sep,
3JHꢀH = 6.9 Hz, CH(CH3)2); 1.96 (6H, m, CH2CH2CH2); 1.83 (6H, m,
PE/TCH2); 1.39 (9H, d, CH(CH3)); 1.12 (9H, d, CH(CH3)); 1.06 (6H,
m, PCCH2). 13C{1H} NMR (101 MHz, THF-d8): δ 165.6 (m, BPhipso);
137.6 (s, BPhortho); 126.1 (m, BPhmeta); 122.2 (s, BPhpara); 30.8
(m, CH(CH3)2); 29.1 (m, PE/TCH2); 26.7 (m, CH2CH2CH2); 21.3
(s, CH(CH3)); 20.9 (s, CH(CH3)); 20.6 (m, PCCH2).
1
Ru(H2)(H)). H NMR (600 MHz, THF-d8, 195 K, high field only):
2
δ ꢀ7.44 (2H, s br, Ru(H2)); δ ꢀ10.29 (1H, dt br, JHꢀP = 57 Hz,
2JHꢀP = 32 Hz, Ru(H)). 13C{1H} NMR (151 MHz, THF-d8): δ 165.1
(m, BPhipso) 137.0 (s, BPhmeta); 125.4 (m, BPhortho); 121.6 (s,
BPhpara); 30.2 (s br, CH(CH3)2); 29.0 (d, 1JCꢀP = 32 Hz, PE/TCH2);
25.9 (m, PCCH2); 21.1 (s, CH2CH2CH2); 19.8 (s, CH(CH3)2); 19.2
(s, CH(CH3)2).
’ ASSOCIATED CONTENT
S
Supporting Information. A CIF file with crystallographic
b
data for compounds [RuCl(P3P3iPr)][BPh4] THF (2[BPh4]),
3
[RuH2(P3P3iPr)] (3), and Ru(H2)(H)(P3P3iPr)][BPh4] EtOH
3
(4[BPh4]). This material is available free of charge via the
Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: L.Field@unsw.edu.au. Telephone: þ61 2 9385 2700.
Synthesis of Ru(P3P3iPr)H2 (3). A suspension of potassium
triethylborohydride (0.068 g, 0.49 mmol) and [RuCl(P3P3iPr)][BPh4]
(2[BPh4]) (0.22 g, 0.23 mmol) was stirred in toluene (10 mL) over-
night. The color of the pink suspension changed to a faint yellow. The
suspension was filtered through Celite, and the solvent was removed
under reduced pressure to give Ru(P3P3iPr)H2 (3) as a white crystalline
powder (0.101 g, 0.165 mmol, 72% yield). Crystals suitable for X-ray
diffraction were grown by slow evaporation of a toluene solution
under an atmosphere of nitrogen. Anal. found: C, 52.92; H, 10.51;
C27H62P4Ru (MW 611.75) requires: C, 53.01; H, 10.22. 31P{1H} NMR
’ ACKNOWLEDGMENT
The authors wish to thank Dr. Hsiu Lin Li and Dr. Alison
Magill for technical assistance and discussions. The authors also
thank the Australian Research Council for financial support, and
R.G.-W. thanks the Australian Government and the University of
New South Wales for postgraduate scholarships.
2
(121.49 MHz, benzene-d6): δ 49.2 (2P, dd, JP(E)ꢀP(C) = 28.5 Hz,
’ REFERENCES
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(1P, dt, PC). H NMR (400 MHz, toluene-d8): δ 2.2ꢀ2.0 (2H, m,
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