Synthesis and coordination chemistry of tridentate (PNN) amine enamido phosphine ligands with ruthenium

Article abstract of DOI:10.1039/c6dt00214e

Tridentate amine-imine-phosphine ligands, R₂PC₅H₇NC₂H₄NEt₂ [^RPNN(H)], where R = Prⁱ or Bu^t are synthesized using a straightforward protocol of condensation, deprotonation, and addition of a chlorodialkylphosphine. Multinuclear NMR spectroscopy shows the ligands exist exclusively in the enamine tautomeric form in solution. Treating these ligands with RuHCl(PPrⁱ₃)₂(CO) forms the desired coordination compounds, RuHCl[^RPNN(H)](CO), where the imine tautomeric form of the ligands coordinates to ruthenium. Deuterium labelling experiments show Ru-H/N-D scrambling occurs during ligand coordination. Treating the RuHCl[^RPNN(H)](CO) precursors with potassium tert-butoxide allows for the synthesis of two new ruthenium enamido-phosphine complexes, RuH[^RPNN](CO), which were fully characterized. The structure of one of the derivatives was confirmed by X-ray crystallography (R = Prⁱ). The reactivity of the enamido-phosphine complexes with H₂ and benzyl alcohol is also reported. For the enamido phosphine complex where R = Prⁱ, the reaction with H₂ is reversible and forms (RuH(CO)[^PriPNN(H)])₂(μ-H)₂, a hydride-bridged dimer that results from cooperative activation of H₂. The reactivity of both amine-enamido-phosphine ruthenium compounds with benzyl alcohol establishes that the complexes are catalyst precursors for acceptorless dehydrogenation (AD), although the turnover frequencies measured using both catalyst precursors are modest.

Full text of DOI:10.1039/c6dt00214e

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Synthesis and Coordination Chemistry of Tridentate (PNN) Amine
Enamido Phosphine Ligands with Ruthenium§
T. C. Wambach, C. Lenczyk, B. O. Patrick, and M. D. Fryzuk*††
Tridentate amineꢀimineꢀphosphine ligands, R2PC5H7NC2H4NEt2 [RPNN(H)], where R = Pri or But are synthesized
using a straightforward protocol of condensation, deprotonation, and addition of a chlorodialkylphosphine.
Multinuclear NMR spectroscopy shows the ligands exist exclusively in the enamine tautomeric form in solution.
Treating these ligands with RuHCl(PPri3)2(CO) forms the desired coordination compounds, RuHCl[RPNN(H)](CO),
where the imine tautomeric form of the ligands coordinates to ruthenium. Deuterium labelling experiments show
RuꢀH/ NꢀD scrambling occurs during ligand coordination. Treating the RuHCl[RPNN(H)](CO) precursors with
potassium tertꢀbutoxide allows for the synthesis of two new ruthenium enamidoꢀphosphine complexes,
RuH[RPNN](CO), which were fully characterized. The structure of one of the derivatives was confirmed by Xꢀray
crystallography (R = Pri). The reactivity of the enamidoꢀphosphine complexes with H2 and benzyl alcohol is also
reported. For the enamido phosphine complex where R = Pri, the reaction with H2 is reversible and forms
(RuH(CO)[PriPNN(H)])2(ꢀꢀH)2, a hydrideꢀbridged dimer that results from cooperative activation of H2. The
reactivity of both amineꢀenamidoꢀphosphine ruthenium compounds with benzyl alcohol establishes that the
complexes are catalyst precursors for acceptorless dehydrogenation (AD), although the turnover frequencies
measured using both catalyst precursors are modest.
Introduction
Coordination compounds featuring cooperative ligands are versatile platforms for developing
stoichiometric and catalytic reactivity.1ꢀ14 In particular, a pyridineꢀbased PNN pincer ligand
coordinated to Ru(II) (A in Scheme 1, (a)) has shown remarkable effectiveness in catalyzing a
series of coupling reactions facilitated by acceptorless dehydrogenation (AD);15ꢀ18 the production
of esters from primary alcohols via AD (Scheme 1 (b)) is one of many examples that illustrate
the usefulness of this catalyst system. The key equilibrium for loss of H2 occurs between the
§ Electronic supplementary information (ESI) available. CCDC reference number 1444765. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/xxxx
†† Department of Chemistry, The University of British Columbia, 2036 Main Mall, Vancouver, BC, Canada, V6T 1Z1. Email:
fryzuk@chem.ubc.ca; Fax: +1 604ꢀ822ꢀ2847; Tel: +1 604ꢀ822ꢀ2897
1

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Ru(II) dihydride A and the monohydride B. Interconversion of A and B involves combining a
proton from the methylene linker and a hydride on the Ru center of A to release H2 and form B.
Thus, the equilibrium between A, B, and H2 relies on a linkerꢀreactive cooperative ligand.
Subsequent to the discovery of the catalytic proprties of B, studies have shown that complexes
featuring a potentially reactive amido donor coordinated to Fe(II),19, 20 Ru(II),6, 8, 21 and Os(II)5, 8
are also active for AD catalysis.
Recent work from our laboratory has focused on exploring the coordination chemistry of
bidentate enamido phosphine ligands, which we discovered can react at both the donor and linker
positions when coordinated to either iridium or ruthenium.22, 23 An example is illustrated in the
lower portion of Scheme 1, wherein we examined a ruthenium enamidoꢀphosphine complex (C
in Scheme 2) as a simplified version of the dehydrogenated pyridine PNN ruthenium system B.
While H2 addition to C was possible to generate D, a different mechanism was operative that
involved direct protonation of the enamido donor by H2. Subsequent dissociation of the enamine
generated an intermediate (E in Scheme 1) that presumably was not thermally stable enough to
effect efficient AD catalysis.
Scheme 1
2

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Et2
N
Et2
N
H
(- H2)
(a)
N
Ru CO
N
H
Ru
CO
(+ H2)
H
H
P
P
But2
But2
H
H
R
B
A
O
catalyst
+ 2H2 (b)
2
OH
R
O
R
N
PPri3
PPri3
Pri
Pri
O
H
C
+ H2
Pri
Pri
Ru
P
H
CO
N
Ru
H
P
Pri2
Pri2
PPri3
Ru
- H2
+ 2 H2
D
C
H
H
- 2 H2
H2
C
O
PPri2
E
Pri
NH
Pri
In this paper, we outline the modular synthesis of two new PNN ligands, their coordination
chemistry with ruthenium(II), as well as the catalytic AD reactivity of two new ruthenium
complexes. Building off our previous studies, the new PNN ligand system generates a tridentate
donor set that more closely mimics the dehydrogenated pyridineꢀbased PNN ruthenium complex
B by incorporation of a CH2CH2NEt2 moiety as shown in Scheme 2. Like the bidentate imineꢀ
phosphine donors explored previously, this new ligand type coordinated to ruthenium (F) can
engage in AD type reactions and also adds and releases H2. However, as will be shown, the
catalytic activity is much lower than what was reported for the pyridineꢀbased system.
Scheme 2
Et2
N
Et2
N
N
H
Ru CO
N
Ru CO
H
H
P
P
R2
But2
F
B
3

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Results and Discussion
Synthesis of the tridentate PNN ligands 2a (R = But) and 2b (R = Pri) is straightforward. For
the first step, utilization of N1,N1ꢀdiethylethaneꢀ1,2ꢀdiamine in a condensation reaction with
cyclopentanone generates 1 in modest yield and acceptable purity for the next step (Scheme 3).
Deprotonation of 1 with LDA and subsequent treatment with a chlorodialkylphosphine generates
the desired tridentate ligands. A notable feature of these compounds, as established by
multinuclear NMR spectroscopy, is that both 2a and 2b exist in the enamine tautomeric form in
31
solution. P{1H} NMR spectra of both 2a and 2b show singlet resonances at δ ꢀ19.6 and ꢀ18.6,
respectively. Resonances in the 1H NMR spectrum at δ 5.5 for 2a and δ 5.3 for 2b correspond to
the NꢀH group proton of the enamine tautomer.
Reaction of either 2a or 2b with RuHCl(PPri3)2(CO) results in coordination of the ligands to
ruthenium and release of two equiv of PPri3 to form new Ru(II) complexes, as evidenced by
singlet resonances in the 31P{1H} NMR spectra at δ 109.6 (3a) and 94.2 (3b), respectively
(Scheme 3). After workup, both compounds are isolated in good yield (55ꢀ84%) as analytically
pure colorless solids. Interestingly, the solubilities of 3a and 3b are significantly different: the diꢀ
tertꢀbutyl derivative 3a is partially soluble in THF and insoluble in other nonꢀpolar solvents such
as benzene, hexanes and pentane, whereas the diisopropyl complex 3b is partially soluble in
diethyl ether, and readily dissolves in benzene.
Multinuclear NMR spectroscopy is consistent with formation of the desired ruthenium
1
hydride complexes 3a and 3b. The H NMR spectrum of 3a features a doublet resonance at δ ꢀ
16.3 (2JPH = 23.5 Hz) assigned to the hydride ligand attached to ruthenium. Also diagnostic is a
4

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1
resonance at δ 4.0 that a Hꢀ13C HSQC NMR experiment shows correlates to a doublet 13C
resonance at δ 57.3 (1JPC = 10.3 Hz). These data support assignment of this resonance at δ 4.0 as
the αꢀCꢀH of the ligand, and establishes that the imine tautomeric form of the ligand is bound to
Ru for 3a. Similarly, NMR spectroscopy establishes the imine tautomer of the ligand
coordinates to ruthenium for 3b. Interestingly, as only one αꢀCꢀH resonance is observed, this
implies that only one diastereomer is present for both complexes; unfortunately, we were unable
to determine if this αꢀCꢀH is syn to the RuꢀH or to the RuꢀCl in solution.
The formation of both 3a and 3b requires tautomerization of the ligand given that the free
ligand exists in the enamine form and upon coordination the imine form is present. We
envisioned two ways tautomerization of the ligand could occur: the first is via a metalꢀmediated
pathway where the ruthenium hydride complex activates the NꢀH bond of the ligand, and the
second is, tautomerization of the free ligand prior to coordination (without NꢀH activation). To
probe these possibilities, d1-2b was synthesized by addition of d1-methanol to the potassium salt
of 2b (see Supporting Information) and reacted with RuHCl(CO)(PiPr3)2 to synthesize d1-3b; this
is shown in the lower portion of Scheme 3. Monitoring the reaction by 1H and 2H NMR under a
variety of conditions show deuterium scrambled between the αꢀCꢀH and the RuꢀH positions. No
evidence for deuterium scrambling into other positions was observed, and furthermore, no
scrambling of deuterium is observed prior to coordination. While the data collected to date do
not definitively establish the mechanism for formation of the imine form of the ligand to Ru,
they suggest a complex mechanism that involves NꢀH activation by the RuꢀH moiety.
Having 3a and 3b in hand, synthesis of enamidoꢀphosphine complexes via deprotonation was
undertaken. Reacting either 3a or 3b with potassium tertꢀbutoxide results in a slow reaction over
several hours to form 4a and 4b, respectively (Scheme 3). The transformation from 3 to 4 is
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accompanied by a color change to deep red; in addition, new upfield singlet resonances in the
31P{1H} NMR spectra are observed. For example, the resonance at δ 109.6, assigned to 3a,
converts to a new resonance at δ 92.3 (4a). A similar shift in the 31P{1H} NMR spectrum occurs
upon deprotonation of 3b.
Scheme 3
NEt2
N
NEt2
NH
2a R = But
2b R = Pri
1) LDA
2) ClPR2
PR2
1
RuHCl(CO)(PiPr3)2
Et2
N
Et2
N
KOBut
Cl
N
Ru
CO
N
Ru
CO
H
H
P
R2
P
R2
4a R = But
4b R = Pri
3a R = But
3b R = Pri
Et2
N
NEt2
ND
RuHCl(CO)(PiPr3)2
Cl
N
Ru
CO
H/D
PR2
P
Pri2
H/D
d1-2b
d1-3b
The solution NMR data for 4a and 4b are consistent with formation of Ru(II) enamidoꢀ
13
hydride complexes; evidence for this is a resonance at δ 90.0 in the C APT NMR spectrum of
31
4a, which couples to P (1JPC = 41.3 Hz) and is assigned to the αꢀC of the enamide phosphine
1
unit. Supporting this assignment, no cross correlation to a proton in the Hꢀ13C HSQC NMR
spectrum is observed for this 13C NMR resonance. A diagnostic feature in the 1H NMR spectrum
2
of 4a is a resonance at δ ꢀ25.2 (d, JPH = 25.5 Hz), which is indicative of a ruthenium hydride
(RuꢀH) ligand trans to an empty coordination site, and is consistent with the square pyramidal
6

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geometry proposed; similar features are present for 4b. Elemental analysis of both compounds
confirms their composition and purity.
Storing a concentrated pentane solution of 4a at ꢀ35 °C overnight results in formation of
crystals that were analyzed by Xꢀray diffraction. Figure 1 depicts an ORTEP representation of
the solidꢀstate molecular structure of 4a. Overall, the bond lengths and angles are similar to other
enamide phosphine complexes previously reported from our laboratory.22, 23 One notable
difference is the Ru1ꢀN1 bond length is contracted for the tridentate system (2.042(3) Å). For
comparison, ruthenium complexes featuring bidentate enamido phosphine ligands have RuꢀN
bond lengths between 2.1287(2) Å and 2.143(2) Å.22 Unfortunately, a solidꢀstate structure of the
ruthenium deprotonated pyridine pincer B remains unreported to date, therefore, a direct
comparison to this complex is not possible.
In previous studies from our lab, pairing bidentate enamide phosphine ligands with iridium
and ruthenium revealed the cyclopentylꢀlinked enamide phosphine unit is able to cooperate with
the metal by accepting a proton during H2 or ROꢀH bond activation. In every case where a proton
is shuttled to the linker of these ligands, evidence suggests that the N donor of the ligand is
protonated and dissociates from the metal. Tautomerization of the dissociated arm transfers a
proton between the N donor and the αꢀC of the linker (e.g., E to D in Scheme 1).
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Figure 1. An ORTEP drawing of the solidꢀstate molecular structure of 4a is shown with 50% thermal
ellipsoids. The hydrogen atoms have been omitted for clarity except for H50, which was located in the
difference map. The structure of 4a is disordered about a mirror plane; a simplified structure that does not
include the modelled disorder is shown here. Selected bond lengths (Å) and bond angles (°): Ru1ꢀN1
2.042(3), Ru1ꢀN2 2.235(2), Ru1ꢀP1 2.2912(6), Ru1ꢀC16 1.844(3), N1ꢀC1 1.370(4), C1ꢀC2 1.358(3), C2ꢀP1
1.795(2), C16ꢀO1 1.164(3), N1ꢀC1ꢀC2 122.1(2), C1ꢀC2ꢀP1 113.91(2), N1ꢀRu1ꢀP1 161.73(6), C16ꢀRu1ꢀP1
95.59(8), N1ꢀRu1ꢀN2 79.28(9), P1ꢀRu1ꢀN2 161.73(6), N1ꢀRu1ꢀC16 174.0(2).
For 4a and 4b, the potentially reactive enamido nitrogen is tethered to Ru with flanking amine
and phosphine donors, which should disfavor dissociation and therefore change the mechanism
for linker reactivity. With this in mind, we explored the reactivity of 4a and 4b with H2.
Unexpectedly, the diꢀtertꢀbutyl complex 4a does not react with dihydrogen at room
temperature (Scheme 4). Instead, after 24 hours at 100 °C, some partial conversion to an upfield,
unidentified singlet resonance in the 31P{1H} NMR spectrum (δ 32.2) is observed. No new
1
hydride resonances appear in the H NMR spectrum, which suggests the unidentified product
does not contain an added equivalent of H2. Given the similar donor set and sterics of 4a
compared to the pyridineꢀbased complex A, this apparent lack reaction with H2 was
unanticipated.
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Scheme 4
NEt2
Pri2
P
Et2
N
Et2
N
H
Cl
H2
+ H2
- H2
KHBEt3
O
N
H
C
N
N
Ru CO
Ru
Ru
no reaction
N
Ru
CO
H
C
R = But
H
O
H
H
P
R2
P
P
Pri2
Pri2
Et2N
5
4a R = But
4b R = Pri
3b
On the other hand, at room temperature, the diisopropyl derivative 4b reacts slowly with H2.
Monitoring the reaction by 31P{1H} NMR spectroscopy (Supporting Information) shows
formation of three new resonances in addition to a resonance at δ 76.4 corresponding to starting
31
material (4b). A singlet in the P{1H} NMR spectrum at δ 104.3 assigned as 5 is observed
initially. As the reaction proceeds to completion, the resonance for starting material and the
signal assigned to 5 at δ 104.3 convert into two new doublet resonances at δ 78.7 and 90.1 (2JPP =
1
6.9 Hz), which correspond to the final product, as yet unidentified. An H NMR spectrum,
recorded while monitoring the conversion of 4b to the new products shows resonances in the
hydride region from δ ꢀ7.6 to ꢀ11.6 (Supporting Information). As will be established, two
resonances at δ ꢀ7.6 and ꢀ11.6, which integrate 1:1 relative to one another, correspond to the
hydride ligands of 5. The triplet splitting pattern of the resonance at δ ꢀ11.6 is due to two
bridging hydrides coupled to two equivalent 31P nuclei (2JPH = 16.4 Hz). The resonance at δ ꢀ7.6
31
is assigned to the terminal hydrides, which also show a large P coupling constant consistent
with the trans disposition of these hydride ligands to the phosphine donors (2JPH = 61.0 Hz).
While these observations are consistent with the dimeric structure proposed in Scheme 4, the
1
complexity of the H NMR spectra recorded during H2 addition to 4b prohibited definitive
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identification of 5. Importantly, a resonance corresponding to the αꢀCꢀH proton of the ligand
could not be identified.
To confirm the structure of 5, 3b was reacted with KBEt3H, which allowed for isolation of
analytically pure samples of 5 in moderate yield (Scheme 4). The synthesis of 5 confirms the
3
3
identification of a resonance at δ 3.3 (dd, JPH = 11.8 Hz, JHH = 7.3 Hz), which is diagnostic of
an αꢀCꢀH of the ligand (see Supporting Information for NMR spectra). Interestingly, we do not
observe diastereomeric mixtures via this route, which is unusual since the dimer could form in
meso and rac forms. The modest yield is due to loss of H2 from 5 to form 4b, which is washed
away during purification. To confirm the ability of 5 to lose H2, 5 was dissolved in C6D6 and
1
sealed in a J. Young tube. Under these conditions, evolution of H2 is observed by H NMR
spectroscopy, as is formation of 4b. Thus, similar to B, the enamide phosphine unit of 4b can
participate in reversible activation of H2, a desirable feature of a potential catalyst for AD of
primary alcohols.
To explore the potential of 4a and 4b to act as catalyst precursors, they were each reacted with
excess benzyl alcohol, respectively. Treating 4a with 100 equivalents of benzyl alcohol,
dissolved in tolueneꢀd8, in a J. Young NMR tube vented to an Ar manifold results in 80 %
conversion to benzyl benzoate over several days at 110 °C, corresponding to a TON of 40 and a
TOF of 0.2 hꢀ1. The conversion profile shows normal catalytic behavior without inhibition
(Supporting Information). To compare the catalytic activity of 4a directly with the Milstein preꢀ
catalyst system (A and B in Scheme 1), the previously reported conditions for AD of benzyl
alcohol to benzyl benzoate were used (1000 equiv benzyl alcohol at reflux in toluene).24 With 4a
after 4 hours approximately 0.7 % conversion to benzyl benzoate occurs. Heating for 22 hours
resulted the conversion to 1.3 %. Identical results are obtained for 4b under closely matched
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conditions. In comparison to A (TOF = 117 hꢀ1) the rate of catalysis recorded for 4a and 4b is
approximately 600 times slower.
Conclusion
The synthesis, reactivity, and coordination chemistry of two new tridentate amineꢀenamidoꢀ
phosphine ligands with ruthenium are reported. Similar to our previously developed cooperative
scaffolds, these ligands feature a reactive enamidoꢀphosphine linker. While the diꢀtertꢀbutyl
derivative 4a does not react at room temperature with H2, it does act as a catalyst precursor for
AD of benzyl alcohol at elevated temperatures, albeit very slowly. Interestingly, the diisopropyl
derivative 4b does react with H2 at room temperature and also displays virtually the identical
catalytic efficiency for AD of benzyl alcohol as 4a. The formation of the dimeric hydride
complex 5 upon addition of H2 to 4b must be a steric effect and points to the delicate balance in
electronics and sterics in AD reactivity. Based on these results, simply matching the donor
environments to existing, efficient ruthenium systems that are effective for AD, for example
comparing 4a and 4b to the pyridineꢀbased PNNꢀRu system A and B, is not an effective ligand
design strategy for discovery of new catalysts for this particular process. We continue to
examine these modular ligand systems and their coordination chemistry with the goal of
uncovering new chemistry.
Experimental Section
Synthesis of 2-(cyclopentylideneamino)-N,N-diethylethan-1-amine (1). To activated 4 Å
molecular sieves (12.0 g) sieves, N,Nꢀdiethylethylenediamine (11.62 g, 100 mmol) was added
under a positive flow of N2 using a syringe. Cyclopentanone (8.41 g, 100 mmol) was added in a
similar fashion immediately afterword. The reaction vessel was placed in an 80 °C oil bath,
sealed using the Kontes valve, and left to react without stirring for 18 hours. After this time, the
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1
reaction was cooled to room temperature and a H NMR spectrum of a small aliquot was
recorded to ensure complete consumption of cyclopentanone and N,Nꢀdiethylethylenediamine. A
complex mixture of products was observed at this point. For work up, in the open air, diethyl
ether (20 mL) was added to the reaction vessel and the resulting mixture was transferred into a
sinter glass frit (to separate the reaction mixture from molecular sieves). The molecular sieves
were washed five times with diethyl ether (20 mL). The volatiles were then removed under
vacuum and the product was distilled using an oil bath temperature of 95 °C and a short path
distillation column (~ 5 cm) under reduced pressure (1.6ꢀ4.0 x 10ꢀ1 torr). The product was
1
obtained as colorless oil (yield 16.7 g, 46 %). H NMR (CDCl3, 400.0 MHz, 298 K): δ 1.0 (t,
3
2
3
3JHH = 7.1 Hz, 6H, NꢀEtCH3), 1.7 (dt, JHH = 6.6 Hz, JHH = 13.5 Hz, 2H, CH2), 1.8 (dd, JHH
=
3
2
3
6.9 Hz, JHH = 13.0 Hz, 2H, CH2), 2.1 (t, JHH = 7.2 Hz, 2H, CH2), 2.3 (t, JHH = 6.9 Hz, 2H,
CH2), 2.4 (q, 3JHH = 7.1 Hz, 4H, NꢀEtCH2), 2.7 (m, 2H, CH2), 3.2 (m, 2H, CH2). 13C APT NMR
(CDCl3 100.6 MHz, 298 K): δ 11.6 (s, NꢀEtCH3), 24.0 (s, CH2), 24.7 (s, CH2), 28.8 (s, CH2),
36.2 (s, CH2), 47.3 (s, NꢀEtCH2), 52.0 (s, CH2), 53.4 (s, CH2), 180.5 (s, NꢀCimine). HRMSꢀEI
(m/z) calculated for C11H22N2: 182.17830, Found: 182.17845.
Synthesis of [PNN]But(H) (2a). 2ꢀ(Cyclopentylideneamino)ꢀN,Nꢀdiethylethanꢀ1ꢀamine (1)
(4.94 g, 27.1 mmol) was dissolved in THF (30 mL). In a separate reaction vessel, LDA was
prepared by combining a small excess of diisopropyl amine (4.56 mL, 32.5 mmol), dissolved in
THF (30 mL), with 1.6 M nꢀbutyl lithium (16.9 mL, 27.1 mmol) at ꢀ 78 ºC. After addition of nꢀ
butyl lithium, the mixture was warmed to room temperature and allowed to stir for 30 min. The
flask containing LDA was then reꢀcooled to ꢀ78 °C. The previously prepared THF solution of 1
was added dropwise to the cold LDA solution. After the addition was complete, the reaction was
brought to room temperature and allowed to stir for 30 min. The resulting mixture was cooled to
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ꢀ78 ºC for a final time and a solution of diꢀtertꢀbutylchlorophosphine (5.15 mL, 27.1 mmol) in
THF (100 mL) was added to the cooled flask dropwise. The mixture was warmed to room
temperature and subsequently heated to reflux for 26 hours. Monitoring the reaction by 31P{1H}
NMR spectroscopy shows the disappearance of the resonance for diꢀtertꢀbutylchlorophosphine
and formation of the desired product. After the reaction is complete, the solvent was removed
under vacuum. The resulting oil was treated with toluene (20 mL). A slurry forms, which was
filtered through Celite. From the filtrate, the volatiles were removed under vacuum to give a
crude yellowꢀbrown oil. This oil was distilled using a short path (~ 5 cm) distillation apparatus
at 8.3 x 10ꢀ2 torr, at an oil bath temperature of 160 °C. The distillation apparatus was actively
heated using a heat gun to transfer the product to the receiving flask (6.31 g, 78 %). 31P{1H}
1
NMR (C6D6, 161.9 MHz, 298 K):
δ
5.8 (s, 1P). H NMR (C6D6, 298 K):
δ
0.9 (t, 3JHH = 7.1 Hz,
6H, NꢀEtCH3), 1.3 (d, 3JPH = 11.6 Hz, 18H, PꢀButCH3), 1.8 (m, 2H, CH2), 2.3 (m, 8H, 2x CH2,
2x NꢀEtCH2), 2.7 (t, 3JHH = 6.8 Hz, 2H, CH2), 2.9 (q, 3JHH = 6.2 Hz, 2H, CH2), 5.5 (dd, J= 5.7
Hz, J = 11.5 Hz, 1H, NꢀH). 13C APT NMR (C6D6, 100.6 MHz, 298 K):
δ 12.6 (s, NꢀEtCH3),
23.4 (s, CH2), 24.9 (d, 1JPC = 54.6 Hz, PꢀButC), 31.3 (d, 2JPC = 14.3 Hz, PꢀButCH3), 32.5 (d, JPC
= 10.9 Hz, CH2), 35.7 (d, 3JPC = 5.8 Hz, CH2), 43.7 (s, CH2), 47.5 (s, NꢀEtCH2), 54.1 (s, CH2),
96.4 (d, 1JPC = 15.1 Hz, ꢀC), 161.3 (d, 2JPC = 27.6 Hz, NꢀCenamine). Unidentified impurities were
observed at δ 33.1 (d) and 36.5 (s). HRMSꢀEI (m/z) calculated for C23H36NP (M+): 326.28509,
Found: 326.28489.
Synthesis of [PNN]Pri(H) (2b). 2ꢀ(Cyclopentylideneamino)ꢀN,Nꢀdiethylethanꢀ1ꢀamine (1)
(6.11g, 33.5 mmol) was dissolved in diethyl ether (50 mL). In a separate reaction vessel, LDA
was prepared by combining diisopropyl amine (4.70 mL, 33.5 mmol) dissolved in diethyl ether
(20 mL) with 1.6 M nꢀbutyl lithium (21.0 mL, 33.5 mmol) at ꢀ78 °C. After addition of nꢀbutyl
13

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lithium, the mixture was warmed to room temperature and allowed to stir for 30 min. The flask
containing LDA was then reꢀcooled to ꢀ78 °C. The previously prepared diethyl ether solution of
imine 1 was added dropwise to the chilled LDA solution. After the addition was complete, the
reaction was brought to room temperature and allowed to stir for 30 min. The mixture was
cooled to ꢀ 78 °C for a final time and a solution of chlorodiisopropylphosphine (5.34 mL, 33.5
mmol) in diethyl ether (100 mL) was added to the reaction dropwise. The mixture was warmed
to room temperature and allowed stir overnight. After this time the reaction was complete, the
solvent was removed under vacuum and the resulting paste was suspended in hexanes. The
slurry was filtered through celite and taken to dryness. The resulting yellowꢀbrown oil was
distilled using a short path distillation apparatus at 8.6 x 10ꢀ2 torr. The oil bath temperature was
140ꢀ147 °C, and the final product is a cloudy, colorless oil (5.59 g, 56 %). 31P{1H} NMR (C6D6,
1
3
161.9 MHz, 298 K): δ ꢀ18.6 (s, 1P). H NMR (C6D6, 298 K): δ 0.9 (t, JHH = 7.1 Hz, 6H, Nꢀ
3
EtCH3), 1.1 (dd, 3JHH = 7.0 Hz, 3JPH = 11.7 Hz, 6H, PꢀPriCH3), 1.2 (dd, JHH = 7.1 Hz, 15.2 Hz,
6H, PꢀPriCH3), 1.8 (dt, 3JHH = 7.3 Hz, 1JHH = 14.4 Hz, 2H, CH2), 1.9 (d. sept. 3JHH = 7.0 Hz, 2JPH
= 3.6 Hz, 2H, PꢀPriCH), 2.3 (m, 6H, NꢀEtCH2, CH2), 2.4 (m, 2H, CH2), 2.5 (m, 2H, CH2), 2.9
(dd, 3JHH = 6.2 Hz, 3JHH = 12.3 Hz, 2H, CH2), 5.3 (dd, JPH = 4.6 Hz, J = 10.3 Hz, NꢀH). 13C APT
NMR (C6D6, 100.6 MHz, 298 K): δ 12.6 (s, NꢀEtCH3), 20.5 (s, PꢀPriCH3), 21.0 (s, PꢀPriCH3)
1
23.1 (s, CH2), 23.5 (d, JPC = 7.9 Hz, PꢀPriCH), 33.3 (d, JPC = 5.6 Hz, CH2), 33.4 (d, JPC = 5.4
Hz, CH2), 43.5 (d, JPC = 1.6 Hz, CH2), 47.4 (s, NꢀEtCH2), 53.8 (s, CH2), 95.8 (d, 1JPC = 10.9 Hz,
2
ꢀC), 160.9 (d, JPC = 25.6 Hz, NꢀCenamine). Anal. Calcd for C17H35N2P: C 68.41; H, 11.82 N,
9.39. Found: C 68.45; H 12.00; N 9.08. HRMSꢀEI (m/z) calculated for C17H35NP (M+):
298.25379, Found: 298.25370.
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Synthesis of RuHCl[(PNN)But(H)](CO) (3a). RuHCl(PPri3)2(CO) (0.500 g, 1.1 mmol) was
combined with 2a (0.460 g, 1.41 mmol) in a Kontes sealed reaction vessel fitted with a stir bar.
THF was added (5 mL) and the reaction was heated to 65 ºC for 19 hours. During this time, the
reaction lightened in color and an off white precipitate forms, which corresponds to the product.
The reaction was taken to dryness and the remaining solids were suspended in pentane and
collected on a glass frit. The precipitate was washed with pentane until it no longer lightened in
1
color (yield: 0.44 g, 84%). 31P{1H} NMR (CD2Cl2, 161.9 MHz, 298 K): δ 109.6 (s, 1P). H
2
3
NMR (CD2Cl2, 400 MHz, 298 K): δ ꢀ16.3 (d, JPC = 23.5 Hz, 1H, RuꢀH), 1.0 (t, JHH = 7.2 Hz,
3H, NꢀEtCH3), 1.3 (t, 3JHH = 7.1 Hz, 3H, NꢀEtCH3), 1.3(1) (br.d, 3JPH = 10.5 Hz, 9H, PꢀButCH3),
3
1.4 (d, JPH = 13.3 Hz, 9H, PꢀButCH3), 2.1 (m, 2H, CH2), 2.2 (m, 2H, CH2), 2.3 (m, 1H, CH2),
2.5(5) (m, 1H, CH2), 2.6(3) (m, 1H, CH2), 2.9 (dqd, J = 2.5 Hz, J = 6.7 Hz, J = 13.6 Hz, 1H, Nꢀ
EtCH2), 3.3 (br. dq, J = 7.2 Hz, J = 14.3 Hz, 1H, NꢀEtCH2), 3.4 (m, 2H, CH2
ꢀEtCH2), 3.5
(m, 2H, CH2/ NꢀEtCH2), 3.6 (m, 1H, CH2), 4.0 (m, 1H, ꢀCH). 13C APT NMR (CD2Cl2 100.6
MHz, 298 K): δ 8.7 (s, NꢀEtCH3), 10.8 (s, NꢀEtCH3), 27.0 (d, JPC =6.8 Hz, CH2), 27.3 (d, JPC
=
7.2 Hz, CH2), 28.0 (d, JPC = 5.9 Hz, CH2), 29.9 (identified by HSQC, PꢀButCH3), 31.7 (d, 2JPC =
1
1
3.7 Hz, PꢀButCH3), 37.7 (d, JPC = 19.5 Hz, PꢀButC), 38.0 (d, JPC = 14.7 Hz, PꢀButC), 49.7 (s,
NꢀEtCH2), 51.6 (s, CH2), 53.3 (s, NꢀEtCH2), 56.9 (d, 1JPC = 9.8 Hz, ꢀCH), 57.8 (s, CH2), 187.0
2
2
(d, JPC = 7.8 Hz, NꢀCimine), 209.4 (d, JPC = 15.7 Hz, RhꢀCO). ATRꢀFTIR ν(CO) (cmꢀ1): 1890.
Anal. Calcd for: C20H41ClN2ORu, C, 48.32, H, 8.19, N, 5.68 Found: C, 48.32, H, 7.98, N, 5.59.
Synthesis of RuHCl[(PNN)Pri(H)](CO) (3b). RuHCl(PPri3)2(CO) (0.324 g, 0.666 mmol) was
combined with an excess of 2b (0.239 g, 0.800 mmol) in a vial with a small stir bar.
Diethylether (10 mL) was added to the mixture, and the resulting orange slurry was stirred for 2
days at room temperature. During this time, the slurry lightened from bright orange to light
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yellow. The solvent volume was reduced by ~ 50% and the reaction was cooled to ꢀ35 °C for 30
minutes prior to filtration through a sintered glass frit. The white solid collected by filtration was
washed with a minimal amount of diethyl ether (yield: 0.18 g, 59 %). 31P{1H} NMR (C6D6,
1
2
161.9 MHz, 298 K): δ 95.5 (s, 1P). H NMR (C6D6, 400 MHz, 298 K): δ ꢀ15.1 (d, JPH = 22.1
Hz, 1H, RuꢀH), 0.8 (dd, 3JHH = 6.7 Hz, 3JPH = 14.9 Hz, 3H, PꢀPriCH3), 0.9 (t, 3JHH = 7.2 Hz, 3H,
3
3
3
NꢀEtCH3), 1.0 (dd, JHH = 6.8 Hz, JPH =16.0 Hz, 3H, PꢀPriCH3), 1.01 (t, JHH = 7.2 Hz, 3H, Nꢀ
3
3
EtCH3), 1.2 (dd, JHH = 7.3 Hz, JPH = 13.3 Hz, 3H, PꢀPriCH3), 1.3 (m, 2H, CH2), 1.4 (m 2H,
CH2), 1.5 (dd, 3JHH = 7.4 Hz, 3JPH = 15.6 Hz, 3H, PꢀPriCH3), 1.6 (m, 3H, CH2, PꢀPriCH), 2.2 (m,
1H, NꢀEtCH2), 2.6 (m, 2H, CH2/ PꢀPriCH), 2.7 (m, 1H, CH2), 3.0 (m, 1H, CH2), 3.1 (m, 1H, Nꢀ
3
3
3
EtCH2), 3.2 (dt, JHH = 4.3 Hz, JHH = 11.9 Hz, 1H, CH2), 3.7 (sept, JHH = 7.1 Hz, 2H, Nꢀ
3
EtCH2), 3.9 (td, 3JHH = 4.1 Hz, JPH = 11.1 Hz, 1H, ꢀCH). 13C APT NMR (C6D6, 100.6 MHz,
298 K): δ 8.9 (s, NꢀEtCH3), 10.7 (s, NꢀEtCH3), 18.1 (d, 2JPC = 4.8 Hz, PꢀPriCH3), 19.3 (d, 2JPC =
2.4 Hz, 2x PꢀPriCH3), 21.3 (d, 2JPC = 2.0 Hz, PꢀPriCH3), 22.7 (d, 1JPC = 26.2 Hz, PꢀPriCH), 24.6
1
(d, JPC = 6.8 Hz, CH2), 26.5 (d, JPC = 7.5 Hz, CH2), 27.8 (d, JPC = 6.1 Hz, CH2), 28.7 (d, JPC
=
23.6 Hz, PꢀPriCH), 49.9 (d, JPC = 0.7 Hz, NꢀEtCH2), 51.3 (s, CH2), 53.1 (d, JPC = 1.6 Hz, Nꢀ
EtCH2), 57.9 (d, 1JPC = 8.0 Hz, ꢀCH), 58.7 (d, JPC = 1.3 Hz, CH2), 187.0 (d, 2JPC = 7.9 HZ, Nꢀ
C
imine), RuꢀCO resonance not resolved from baseline. ATRꢀFTIR ν(CO) (cmꢀ1): 1888. Anal.
Calcd for: C20H41ClN2ORu, C, 46.60, H, 7.82, N, 6.04; Found: C, 46.62, H, 7.78, N, 6.18.
Synthesis of RuH[(PNN)But](CO) (4a). RuHCl[(PNN)tBu(H)](CO) (3a) (0.200 g, 0.41 mmol)
was combined with potassium tertꢀbutoxide (0.046 g, 0.41 mmol) in a vial in the glove box.
31
THF (5 mL) was added. Upon stirring, the reaction turned dark red. After three hours, P{1H}
NMR spectroscopy indicated the reaction was complete. Volatiles were removed under vacuum
and the resulting red solid was extracted with pentane (3 x 2 mL) until no red material remained
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in the reaction vial. The red pentane solution was filtered through Celite and taken to dryness
(yield: 0.16 g, 93 %). The red powder that resulted was recrystallized by dissolving it in a
minimal amount of pentane and storing the solution at ꢀ 35 ºC overnight. Suitable crystals for
elemental analysis and Xꢀray diffraction studies were grown in this manner. 31P{1H} NMR
1
(C6D6, 161.9 MHz, 298 K): δ 92.3 (s, 1P). H NMR (C6D6, 400 MHz, 298 K): δ ꢀ25.2 (d, 2JPH
=
3
25.5 Hz, 1H, RuꢀH), 0.94 (t, JHH = 7.2 Hz, 3H, NꢀEtCH3), 1.0 (t, 3JHH = 7.1 Hz, 3H, NꢀEtCH3),
1.4 (d, 3JPH = 13.2 Hz, 9H, PꢀButCH3), 1.5 (d, 3JPH = 12.6 Hz, 9H, PꢀButCH3), 2.1 (m, 1H, CH2),
2.3 (m, 3H, CH2, NꢀEtCH2), 2.4 (m, 4H, CH2, NꢀEtCH2), 2.6 (m, 3H, CH2, NꢀEtCH2), 2.7 (td,
1H, NꢀEtCH2), 3.1 (m, 1H, CH2), 3.2 (m, 1H, CH2). 13C APT NMR (C6D6, 100.6 MHz, 298 K):
δ 11.0 (s, NꢀEtCH3), 11.3 (s, NꢀEtCH3), 28.7 (d, JPC = 6.8 Hz, CH2), 28.9 (d, JPC = 2.4 Hz, CH2),
29.9 (d, JPC = 4.2 Hz, PꢀButCH3), 30.4 (d, JPC = 4.3 Hz, PꢀButCH3), 31.4 (s, NꢀEtCH2), 34.9 (d,
1JPC =24.1 Hz, PꢀButC), 40.7 (d, 1JPC = 22.1 Hz, PꢀButC), 48.7 (s, NꢀEtCH2), 49.1 (s, CH2), 54.6
(s, CH2), 60.0 (s, CH2), 90.0 (s, 1JPC = 41.3 Hz, ꢀC), 180.2 (d, 2JPC = 26.0 Hz, NꢀCenamide), 207.6
2
(d, JPC = 12.1 Hz, RuꢀCO). ATRꢀFTIR ν(CO) (cmꢀ1): 1875. Anal. Calcd for: C20H39N2OPRu,
C, 52.73, H, 8.63, N, 6.15 Found: C, 52.93, H, 8.46, N, 6.02.
Synthesis of RuH[(PNN)Pri](CO) (4b). RuHCl[(PNN)iPr(H)](CO) (3b) (0.050 g, 0.11 mmol)
was combined with potassium tertꢀbutoxide (0.012 g, 0.11 mmol) in a vial in the glove box.
THF (2 mL) was added. Upon stirring, the reaction turned dark red. The mixture was stirred for
31
three hours, at which point P{1H} NMR spectroscopy indicated the reaction was complete.
Volatiles were removed under vacuum and the resulting red solid was extracted with hexanes (3
x 2 mL) until no red material remained in the reaction vial. The red hexanes solution was filtered
through Celite and taken to dryness (yield 0.030 g, 65 %). 31P{1H} NMR (C6D6, 161.9 MHz,
1
2
298 K): δ 76.4 (s, 1P). H NMR (C6D6, 400 MHz, 298 K): δ ꢀ24.3 (d, JPH = 29.0 Hz, 1H, Ruꢀ
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3
3
H), 0.9 (t, JHH = 7.2 Hz, 3H, NꢀEtCH3), 1.0 (t, JHH = 7.1 Hz, 3H, NꢀEtCH3), 1.1(6) (m, 6H, Pꢀ
PriCH3), 1.2 (dd, 3JHH = 6.0 Hz, 3JPH = 15.1 Hz, 3H, PꢀPriCH3), 1.4 (dd, 3JHH = 7.0 Hz, 3JPH = 1.8
Hz, 3H, PꢀPriCH3), 1.9 (m, 1H, PꢀPriCH), 2.0 (m, 1H, CH2), 2.4 (m, 10H, PꢀPriCH, CH2, 2x Nꢀ
EtCH2), 2.5 (m, 1H, NꢀEtCH2), 2.7 (m, 1H, NꢀEtCH2), 3.1 (m, 2H, CH2). 13C APT NMR (C6D6,
100.6 MHz, 298 K): δ 10.7 (s, NꢀEtCH3), 11.6 (s, NꢀEtCH3), 18.6 (s, PꢀPriCH3), 18.9 (s, 3JPC =
1.5 Hz, 2x PꢀPriCH3), 19.7 (d, 3JPC = 4.2 Hz, PꢀPriCH3), 24.1 (d, 1JPC = 33.0 Hz, PꢀPriCH), 26.2
(d, 1JPC = 29.4 Hz, PꢀPriCH), 28.6 (d, JPC = 15.7 Hz, CH2), 28.7 (d, JPC = 6.8 Hz, CH2), 28.9 (s,
CH2), 48.9 (s, NꢀEtCH2), 49.1 (s, CH2), 54.4 (s, NꢀEtCH2), 59.9 (s, CH2), 88.7 (d, 1JPC = 44.1 Hz,
2
2
ꢀC), 180.6 (d, JPC = 18.4 Hz, NꢀCenamide), 206.9 (d, JPC = 8.0 Hz, RuꢀCO). Anal. Calcd for:
C18H35N2OPRu, C, 50.57, H, 8.25, N, 6.55 Found: C, 50.41, H, 8.36, N, 6.62. ATRꢀFTIR ν(CO)
(cmꢀ1): 1874.
Synthesis of (RuH(CO)[(PNN)iPr(H)])(ꢀ-H)2 (5). In a vial equipped with a small stir bar, 3b
(0.190 g, 0.409 mmol) was combined with KBEt3H (0.057 g, 0.409 mmol) dissolved in toluene
(5 mL). The reaction mixture immediately turned deep red. After 10 minutes, the reaction
mixture was filtered through a glass fiber filter packed into a pipette. The volatiles were removed
under vacuum to give a red solid. On a sintered glass filter (medium), the red solids were washed
with cold pentane until all of the red compound, presumably 4b, washed away and a yellow
powder remained, which corresponds to the desired product (0.080 g, 0.19 mmol, 45%).
1
31P{1H} NMR (C6D6, 161.9 MHz, 298 K): δ 104.7 (s). H NMR (C6D6, 400 MHz, 298 K): δ ꢀ
11.6 (ddd, J = 20.7, 15.1, 4.8 Hz, 2H, RuꢀH), ꢀ7.6 (dt, J = 61.8, 6.1 Hz, 2H, RuꢀH), 1.0(4) –
0.9(5) (m, 6H, PꢀPriCH3), 1.0(6) – 1.0(2) (m, 6H, PꢀPriCH3), 1.1 (dd, J = 12.0, 7.1 Hz, 6H, Pꢀ
PriCH3), 1.2 (t, J = 7.1 Hz, 12H, NꢀEtCH3), 1.3(9) – 1.2(1) (m, 7H, CH2/ PꢀPriCH), 1.5(5) (dd, J
= 14.8, 7.2 Hz, 6H, PꢀPriCH3), 1.6(9) – 1.5(9) (m, 6H, CH2/ PꢀPriCH), 1.8 (dt, J = 18.3, 9.2 Hz,
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2H, CH2), 2.0 (pd, J = 7.1, 4.7 Hz, 2H, CH2), 2.2 (dd, J = 17.7, 7.4 Hz, 2H, CH2), 2.7 (q, J = 7.1
Hz, 8H, NꢀEtCH2), 2.8 (ddd, J = 12.6, 10.4, 5.8 Hz, 2H, CH2), 3.3 (dd, J = 11.8, 7.3 Hz, 2H, αꢀ
CH), 3.7 (ddd, J = 12.6, 9.9, 4.7 Hz, 2H, CH2), 3.9 (td, J = 10.4, 5.8 Hz, 2H, CH2), 4.8 (td, J =
10.5, 4.5 Hz, 2H, CH2). 13C APT NMR (C6D6, 100.6 MHz, 298 K): δ 12.9 (s, NꢀEtCH3), 17.3 (d,
J = 6.7 Hz, PꢀPriCH3), 19.4 (d, J = 3.7 Hz, PꢀPriCH3 ), 19.6 (d, J = 3.5 Hz, PꢀPriCH3), 21.0 (s, Pꢀ
PriCH3), 23.2 (d, J = 16.7 Hz, PꢀPriCH), 25.6 (d, J = 6.6 Hz, CH2), 26.6 (d, J = 23.5 Hz, Pꢀ
PriCH), 27.3 (d, J = 5.9 Hz, CH2), 27.7 (d, J = 5.2 Hz, CH2), 47.9 (s, NꢀEtCH2), 49.1 (d, J = 22.1
Hz, PꢀPriCH), 52.2 (s, CH2), 52.9 (d, J = 13.5 Hz, αꢀCH), 59.4 (s, CH2), 187.7 (d, J = 10.2 Hz,
NꢀCimine), 212.1 (d, J = 13.1 Hz, RuꢀCO). Anal. Calcd. for C36H74N4O2P2Ru2: C, 50.33; H, 8.68;
N, 6.52. Found: C, 50.52; H, 8.62; N, 6.73.
Acknowledgements
We thank the Natural Sciences and Engineering Research Council of Canada (NSERC) for
funding this work.
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Simple condensation and deprotonation sequences
allow the synthesis of modular tridentate ligands that
mimic the coordination environment of the famous
Milstein dehydrogenation system B when coordinated
to Ru(II) as shown with F.
Et2
N
Et2
N
N
H
Ru CO
N
Ru CO
H
H
P
P
R2
But2
F
B