Supramolecular assembly and transfer hydrogenation catalysis with ruthenium(II) complexes of 2,6-di(1H-pyrazol-3-yl)pyridine derivatives

Article abstract of DOI:10.1016/j.poly.2015.09.054

Two new tridentate ligands 2,6-bis(5-ethyl-1H-pyrazol-3-yl)pyridine and 2,6-bis(5-benzamido-1H-pyrazol-3-yl)pyridine, have been synthesized. These ligands have been used in a new series of six complexes of formula "RuCl₂(PPh₃)_2<

Full text of DOI:10.1016/j.poly.2015.09.054

Accepted Manuscript
Supramolecular Assembly and Transfer Hydrogenation Catalysis with Ruthe-
nium(II) Complexes of 2,6-Di(1H-pyrazol-3-yl)pyridine Derivatives
Thomas D. Roberts, Malcolm A. Halcrow
PII:
S0277-5387(15)00564-1
DOI:
http://dx.doi.org/10.1016/j.poly.2015.09.054
POLY 11568
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
21 July 2015
24 September 2015
Please cite this article as: T.D. Roberts, M.A. Halcrow, Supramolecular Assembly and Transfer Hydrogenation
Catalysis with Ruthenium(II) Complexes of 2,6-Di(1H-pyrazol-3-yl)pyridine Derivatives, Polyhedron (2015), doi:
http://dx.doi.org/10.1016/j.poly.2015.09.054
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School of Chemistry, University of Leeds,
Woodhouse Lane, Leeds LS2 9JT, U.K.
Supramolecular Assembly and Transfer Hydrogenation Catalysis with
Ruthenium(II) Complexes of 2,6-Di(1H-pyrazol-3-yl)pyridine Derivatives
Thomas D. Roberts and Malcolm A. Halcrow*
School of Chemistry, University of Leeds, Woodhouse Lane, Leeds LS2 9JT, UK
Tel +44 113 343 6506
Fax +44 113 343 6565
email m.a.halcrow@leeds.ac.uk
Dedicated to Malcolm Chisholm on the occasion of his 70th birthday.
Submission for the Malcolm Chisholm Special Issue
1

TOC Entry
Six new complexes of type “[RuCl(PPh3)2(LR)]Cl” have been prepared, where LR is a
disubstituted 2,6-di(1H-pyrazol-3-yl)pyridine derivative. Crystal structures have shown that
the chloride ligand in these compounds can be labile under ambient conditions, which is a
requirement for catalysis. One complex salt [Ru(OH2)(PPh3)2(LR)][PF6]2 (R = tBu) was also
obtained. All the complexes tested are moderately active towards transfer hydrogenation of
acetophenone, with the PF6‒ salt having a much higher activity than the chloride salts.
Keywords
Ruthenium; N-donor ligands; Crystal Structure; Hydrogen Bonding; Transfer Hydrogenation
2

ABSTRACT
Two new tridentate ligands 2,6-bis(5-ethyl-1H-pyrazol-3-yl)pyridine and 2,6-bis(5-
benzamido-1H-pyrazol-3-yl)pyridine, have been synthesized. These ligands have been used in
a new series of six complexes of formula “RuCl2(PPh3)2(LR)·nH2O” (n = 1 or 2) where LR is
2,6-bis(5-R-1H-pyrazol-3-yl)pyridine (R = Me, Et, tBu, NH2, NHC{O}tBu and NHC{O}Ph).
Crystal structures of [RuCl(PPh3)2(LMe)]Cl·MeOH and [Ru(OH2)(PPh3)2(LtBu)]Cl2·4CDCl3
contain six-coordinate complex centers with trans-phosphine ligands, and show that the
chloride ions can occupy the first or second coordination spheres of the complexes. The latter
structure demonstrates that the chloride ions in this type of compound can be labile under
ambient conditions, which is an essential pre-requisite for catalytic activity. Anion metathesis
yielded [Ru(OH2)(PPh3)2(LtBu)][PF6]2, which was also crystallographically characterized. All
the complexes (except air-sensitive [RuCl2(PPh3)2(LNH2)]) were screened for activity towards
transfer hydrogenation of acetophenone in refluxing 2-propanol. The chloride salt catalysts are
active but show a significant induction period, which may imply decomposition of the
complexes during the reaction. However the activity of the PF6‒ salt is much higher, which
shows that competition between chloride and substrate for the metal center is a significant
factor in catalysis by these compounds.
3

INTRODUCTION
Complexes of ligands derived from 2,6-di(pyrazol-3-yl)pyridine (3-bpp, Scheme 1) are finding
increasing use [1], in fields as diverse as spin-crossover compounds [2, 3], dye-sensitized solar
cells [4], emissive materials [5], metal ion separation [6] and catalysis [7-17]. In the latter
regard, a particular feature of 3-bpp derivatives are their relatively acidic pyrazolyl N‒H
groups, which are in close proximity to a coordinated metal ion. These can participate as
second-sphere proton donors during catalytic reactions [11, 18], leading to unusual reactivity
towards the reduction of small molecules for example [10]. Hydrogen bonding to these N‒H
groups could also serve to position a substrate molecule close to the metal reaction center
[15, 19].
<Insert Scheme 1 here>
A reaction where 3-bpp-containing catalysts have shown promise [18] is transfer
hydrogenation [20]. A number of groups have reported the reaction of [RuCl2(PPh3)3] with
L
R1,H and LR1,R2 derivatives (Scheme 1). Two types of compound are generally obtained from
these reactions, namely trans-[RuCl(PPh3)2(LR1,H)]Cl [11, 21], or cis-[RuCl2(PPh3)(LR1,R2)]
when R2 ≠ H [12-14]. A dinuclear product with deprotonated pyrazolyl groups,
[Ru2(PPAr3)2(µ-Cl)(µ-LR1,H‒H)2]Cl (Ar = aryl), was also reported in one case [16]. All these
types of complex can be useful pre-catalysts for the transfer hydrogenation of ketones.
Notably, it is unclear whether the ligand N‒H groups in LR1,H complexes of this type
participate Brønsted acid/base centers during the catalysis. On one hand, [RuCl2(PPh-
3)(LR1,R2)] complexes (R2 ≠ H) are generally more active towards transfer hydrogenation [12-
14] than [RuCl(PPh3)2(LR1,H)]Cl (R2 = H) [11], based on published reports. On the other hand,
other catalysts of the cis-[RuCl2(PPh3)L] type with 1H-pyrazolyl ‘L’ ligand donors have the
highest activities of all, and exhibit well-defined protonation/deprotonation cycles at their
pyrazolyl group that may contribute to their catalytic performance [14]. Moreover, a DF
4

calculation of transfer hydrogenation by a [RuH(LR1,H)(PR3)2]+ center also concluded that
protonation of the ketone substrate by the ligand NH groups is a low-energy mechanistic
pathway [11]. Hence, from the available evidence, the activity of [Ru(LR1,R2)]2+ fragments
towards transfer hydrogenation appears to depend on the steric environment of the metal
center, as well as on participation of the tridentate ligand in the reaction.
Since we have access to a number of LR1,R2 ligands through our work on iron(II) complexes of
3-bpp and its derivatives [3], we decided to investigate their ruthenium chemistry. We report
here [RuCl(PPh3)2(LR1,H)]Cl complexes of six LR1,H derivatives, including two new ligands that
have not been synthesized before (Scheme 2). We were particularly interested in complexes of
L
R1,H bearing protic R1 substituents, LNH2,H and LNHCOR3,H (Scheme 1), which could enhance their
second sphere coordination properties by formation of chelating hydrogen bonds with a
substrate or anion [19]. Crystal structures obtained in this work have shown that the
coordination chemistry of “[RuCl(PPh3)2(LR1,H)]Cl” complexes is more varied than has been
reported up to now, while a preliminary survey of their activity towards transfer
hydrogenation is also described.
<Insert Scheme 2 here>
EXPERIMENTAL
The syntheses of 2,6-di(5-amino-1H-pyrazol-3-yl)pyridine (LNH2,H, Scheme 1) [19], 2,6-di(5-
methyl-1H-pyrazol-3-yl)pyridine (LMe,H) [22], 2,6-di(5-tert-butyl-1H-pyrazol-3-yl)pyridine
(LtBu,H) [19, 21], 2,6-bis(5-{tert-butylamido}-1H-pyrazol-3-yl)-pyridine (LNHCOtBu,H) [19] and
[RuCl2(PPh3)3] [23] followed the literature procedures. All other manipulations were carried
out in air, using reagent-grade solvents.
5

Synthesis of 2,6-di(3-oxo-pentanoyl)pyridine. Dimethyl pyridine-2,6-dicarboxylate (1.48 g,
7.6 mmol) and sodium methoxide (1.03 g, 19.0 mmol) were suspended in dry toluene (50
cm3) under a dry nitrogen atmosphere. Butan-2-one (1.37 g, 19.0 mmol) was then added, and
the mixture was stirred for 15 mins at room temperature before being heated at 60 ºC for 12
hrs. The yellow suspension was evaporated to dryness, and the residue was added to a
mixture of glacial acetic acid (15 cm3), water (25 cm3) and ice (25 g). When the ice had
melted the resultant yellow precipitate was recovered by filtration and dried in vacuo. Yield
1.37 g, 66 %. Found C, 65.8; H, 6.30; N, 5.15 %. Calcd. from C15H17NO4 C, 65.4; H, 6.22;
N, 5.09 %. Electrospray mass spectrum: m/z 298.1 ([Na(L)]+). 1H NMR (CDCl3 – spectrum
complicated by tautomeric isomerism of the dione groups): δ 0.93-1.37 (m, 6H, CH2CH3),
2.35-2.78 (m, 4H, CH2CH3), 7.92 (t, 8.1 Hz, 1H, Py H4), 8.01-8.26 (m, 2H, Py H3/5). 13C
NMR (CDCl3): δ 9.5 (CH2CH3), 32.7 (CH2CH3), 96.1 (COCH2CO), 124.1 (Py C3/5), 138.1
(Py C4), 151.8 (Py C2/6), 172.0 (EtC=O), 199.2 (PyC=O).
Synthesis of 2,6-di(5-ethyl-1H-pyrazol-3-yl)pyridine (LEt,H). 2,6-Di(3-oxo-
pentanoyl)pyridine (1.00 g, 3.6 mmol) was dissolved in 7:3 ethanol/acetic acid (50 cm3).
Hydrazine monohydrate (0.54 g, 10.7 mmol) was then added, and the solution was stirred at
room temperature for 20 hours. The ethanol was removed, and saturated sodium carbonate
solution was added to the residue. The resultant mixture was extracted with chloroform, and
the organic layer was washed with brine before being dried over Na2SO4 and evaporated to
leave a pale yellow solid. The product was recrystallized from methanol/diethyl ether. Found
65.7; H, 6.50; N, 24.5 %. Calcd. for C15H17N5·CH3OH C, 65.7; H, 6.76; N, 24.7 %.
Electrospray mass spectrum: m/z 290.10 ([Na(LEt,H)]+). 1H NMR (CDCl3): δ 1.06 (t, 7.3 Hz,
6H, CH2CH3), 2.50 (q, 7.3 Hz, 4H, CH2CH3), 6.19 (s, 2H, Pz H4), 7.11 (d, 7.8 Hz, 2H, Py
H3/5), 7.37 (t, 7.8 Hz, 2H, Py H4). 13C NMR (CDCl3): δ 13.5 (CH2CH3), 20.9 (CH2CH3),
100.9 (Pz C4), 117.7 (Py C3/5), 137.1 (Py C4), 143.8 (Pz C5), 148.1 (Pz C3), 154.5 (Py C2/6).
6

Synthesis of 2,6-bis(5-{phenylamido}-1H-pyrazol-3-yl)-pyridine (LNHC{O}Ph,H). 2,6-Di(5-
amino-1H-pyrazol-3-yl)pyridine (2.00 g, 8.3 mmol) was suspended in dry acetonitrile (90
cm3) under a nitrogen atmosphere. Benzoyl chloride (4.00 g, 28.4 mmol) was added, and the
mixture was then heated at reflux for 48 hours. The off-white precipitate of LNHC{O}Ph,H·HCl
was collected by filtration, then suspended in a two-phase mixture of chloroform (140 cm3)
and saturated aqueous sodium carbonate (140 cm3). The mixture was heated at reflux for 2
days. The product formed a white precipitate in the aqueous layer which was filtered, washed
with acetone and dried. Yield 0.92 g, 33.0 %. Found 65.1; H, 4.10; N, 21.1 %. Calcd. for
C25H19N7O2·½H2O C, 65.5; H, 4.40; N, 21.4 %. Electrospray mass spectrum: m/z 472.20
([Na(LNHC{O}Ph,H)]+). 1H NMR ({CD3}2SO): δ 7.35 (s, 2H, Pz H4), 7.49-7.61 (m, 6H, Ph H3/5
and Ph H4), 7.81 (d, 7.7 Hz, 2H, Py H3/5), 7.97 (t, 7.7 Hz, 2H, Py H4), 8.05 (d, 6.0 Hz, 4H, Ph
H2/6), 11.1 (vbr s, 2H, NHC{O}), 13.0 (vbr s, 2H, Pz NH). 13C NMR ({CD3}2SO): δ 95.8 (Pz
C4), 118.4 (Py C3/5), 127.7 and 128.3 (Ph C2/6 and Ph C3/5), 131.6 (Ph C4), 134.0 (Pz C5),
138.8 (Py C4), 141.6 (Ph C1), 147.3 and 148.0 (Pz C3 and Py C2/6), 164.7 (C=O). IR (C=O)
•1
1672 cm .
Synthesis of [RuCl(PPh3)2(LR1,H)]Cl·nH2O (R1 = Me, 1; R1 = Et, 2; R1 = tBu, 3; R1 =
NHC{O}tBu, 4; R1 = NHC{O}Ph, 5; R1 = NH2, 6). The same basic method, as described
here for [RuCl(PPh3)2(LMe,H)]Cl, was followed for all the complexes in this study. A
suspension of [RuCl2(PPh3)3] (0.36 g, 0.4 mmol) and L1 (0.15 g, 0.4 mmol) in dry
dichloromethane (15 cm3) was stirred for 3 hours at room temperature. This yielded an
orange precipitate, which was collected by filtration. Alternatively, the products 3-6 were
soluble in the reaction mixture, and were precipated from it by careful addition of diethyl
ether. In either case, the resultant solids were washed repeatedly with diethyl ether to remove
excess PPh3, then dried in vacuo.
7

For [RuCl(PPh3)2(LMe,H)]Cl•H2O (1•H2O): Yield 0.23 g, 91 %. Found C, 61.9; H, 4.80;
N, 7.60; Cl, 7.38 %. Calcd. for C49H43Cl2N5P2Ru•H2O C, 61.7; H, 4.76; N, 7.34; Cl, 7.43 %.
Electrospray mass spectrum (MeCN): m/z 638.1 ([RuCl(PPh3)(LMe,H)]+), 900.2
([RuCl(PPh3)2(LMe,H)]+). 1H NMR (CD3OD): δ 2.21 (s, 6H, CH3), 6.37 (s, 2H, Pz H4), 7.01
(d, 7.7 Hz, 2H, Py H3/5), 7.09-7.35 (m, 31 H, Py H4 and P{C6H5}3). 31P NMR (CD3OD): δ
24.4.
For [RuCl(PPh3)2(LEt,H)]Cl•H2O (2•H2O): Yield 0.24 g, 67 %. Found C, 62.2; H, 5.10;
N, 7.40; Cl, 7.20 %. Calcd. for C51H47Cl2N5P2Ru•H2O C, 62.4; H, 5.03, N, 7.13; Cl, 7.22 %.
Electrospray mass spectrum (MeCN): m/z 928.2 ([RuCl(PPh3)2(LEt,H)]+). 1H NMR (CD3OD):
δ 1.06 (t, 7.6 Hz, 6H, CH2CH3), 2.56 (q, 7.6 Hz, 4H, CH2CH3), 6.40 (s, 2H, Pz H4), 7.00 (d,
7.8 Hz, 2H, Py H3/5), 7.08-7.32 (m, 31 H, Py H4 and P{C6H5}3). 31P NMR (CD3OD): δ 24.4.
For [RuCl(PPh3)2(LtBu,H)]Cl·2H2O (3•2H O): Yield 0.29 g, 77 %. Found C, 61.9; H,
2
5.40; N, 6.30; Cl, 6.80 %. Calcd. for C55H55Cl2N5P2Ru•2H2O C, 62.5; H, 5.63; N, 6.63; Cl,
6.71 %. Electrospray mass spectrum (MeCN): m/z 984.3 ([RuCl(PPh3)2(LtBu,H)]+). 1H NMR
(CDCl3): δ 1.14 (s, 18H, C{CCH3}3), 6.30 (s, 2H, Pz H4), 6.92 (d, 7.9 Hz, 2H, Py H3/5), 7.01-
7.18 (m, 31 H, Py H4 and P{C6H5}3), 12.05 (br s, 2H, NH). 31P NMR (CDCl3): δ 23.7.
For [RuCl(PPh3)2(LNHC{O}tBu,H)]Cl•H2O (4•H2O): Yield 0.11 g, 27 %. Found C, 60.5; H,
5.30; N, 9.20; Cl, 6.10 %. Calcd. for C57H57Cl2N7O2P2Ru•H2O C, 60.9; H, 5.29; N, 8.72; Cl,
6.31 %. Electrospray mass spectrum (dmso): m/z 886.2 ([RuCl(PPh3)(OSMe2)(LHC{O}Ph,H)]+).
1H NMR (CDCl3): δ 1.25 (s, 18H, C{CCH3}3), 7.20 (s, 2H, Pz H4), 7.22-7.46 (m, 24H) and
7.54-7.97 (m, 9H, Py H3/5, Py H4 and P{C6H5}3). 31P NMR (CDCl3): δ 34.8. IR (C=O) 1645
•1
cm .
For [RuCl(PPh3)2(LNHC{O}Ph,H)]Cl•H2O (5•H2O): Yield 0.25 g, 59 %. %. Found C, 63.3;
H, 4.30; N, 8.70 %. Calcd. for C61H49Cl2N7O2P2Ru•H2O C, 62.9; H, 4.42; N, 8.42 %.
Electrospray mass spectrum (dmso): m/z 663.2 ([Ru(OH2)2(OSMe2)(LHC{O}Ph,H‒H)]+), 680.5
([RuCl(OH2)(OSMe2)(LHC{O}Ph,H)]+), 697.3 ([RuCl2(OSMe2)(LHC{O}Ph,H‒H)]+). 1H NMR
8

(CDCl3): δ 7.20 (s, 2H, Pz H4), 7.22-7.46 (m, 24H) and 7.54-7.97 (m, 9H) (Py H3/5, Py H4
and P{C6H5}3), 7.70 (t, 7.7 Hz, 1H, Py H4), 7.79 (d, 7.7 Hz, 2H, Py H3/5). 31P NMR (CDCl3):
•1
δ 36.5. IR (C=O) 1681 cm .
For [RuCl(PPh3)2(LNH2,H)]Cl•2H2O (6•2H2O): Yield 0.13 g, 35 %. Found C, 57.6; H,
4.30; N, 10.2; Cl, 7.50 %. Calcd. for C47H41Cl2N7P2Ru•2H2O C, 58.0; H, 4.66; N, 10.1; Cl,
7.28 %. Electrospray mass spectrum (MeCN): m/z 657.5 ([RuCl(PPh3)(OH2)(LNH2,H)]+), 680.5
([RuCl(PPh3)(NCMe)(LNH2,H)]+). 1H NMR (CDCl3): δ 5.05 (br s, 4H, NH2), 5.35 (s, 2H, Pz
H4), 6.84-7.09 (m, 18H) and 7.33-7.70 (m, 17H) (Py H3/5, Py H4 and P{C6H5}3). 31P NMR
(CDCl3): δ 29.1.
Synthesis of [Ru(OH2)(PPh3)2(LtBu,H)][PF6]2·H2O (7·H2O)
A mixture of 3•2H2O (0.10 g, 0.1 mmol) and AgPF6 (0.05 g, 0.2 mmol) in dichloromethane
(15 cm3) was stirred for 1 hour at room temperature. The white AgCl precipitate was then
removed by filtration, and the filtrate evaporated to dryness. Yield 0.11 g, 91 %. Found C,
51.6; H, 4.40; N, 5.30 %. Calcd. for C55H57F12N5OP4Ru•H2O C, 51.8; H, 4.66; N, 5.49 %. 1H
NMR (CD3OD): δ 1.22 (s, 18H, C{CCH3}3), 6.53 (s, 2H, Pz H4), 7.03-7.35 (m, 33H, Py H3/5,
•
Py H4 and P{C6H5}3). 31P NMR (CD3OD): δ 23.9 (s, PPh ), •144.6 (hept, 751 Hz, PF ).
3
6
Single crystal X-ray structure determinations
Single crystals of [RuCl(PPh3)2(LMe,H)]Cl·MeOH (1·MeOH) and
[Ru(H2O)x(NCMe)1‒x(PPh3)2(LtBu,H)][PF6]2 (7) were grown by slow diffusion of di-isopropyl
ether into solutions of the complexes in methanol and acetonitrile, respectively. Crystals of
[Ru(OH2)(PPh3)2(LtBu,H)]Cl2·4CDCl3 (3·4CDCl3) slowly crystallized from a NMR sample of
that compound in CDCl3. Diffraction data were collected with an Agilent Supernova dual-
source diffractometer using either monochromated Cu-Kα (λ = 1.54184 Å) or monochromated
9

Mo-Kα radiation (λ = 0.71073 Å). Experimental details of each structure determination are
given in Table 1. The structures were solved by direct methods (SHELXS97 [24]), and
developed by full least-squares refinement on F2 (SHELXL97 [24]). Crystallographic figures
were prepared using X-SEED [25].
<Insert Table 1 here>
X-ray structure determination of [RuCl(PPh3)2(LMe,H)]Cl·MeOH (1·MeOH). No disorder
is present in this structure, and no restraints were applied to the refinement. All non-H atoms
were refined anisotropically, while H atoms were placed in calculated positions and refined
using a riding model.
X-ray structure determination of [Ru(OH2)(PPh3)2(LtBu,H)]Cl2·4CDCl3 (3·H2O·4CDCl3).
The asymmetric unit contains half a formula unit, with half a complex molecule with Ru(1),
N(2), C(5) and O(15) lying on the C2 axis ¼, y, ¼; and, one chloride ion and two chlorofom
molecules lying on general positions. No disorder is present in the model, and no restraints
were applied during refinement. All non-H atoms were refined anisotropically, while C- and
N-bound H atoms were placed in calculated positions and refined using a riding model. The
unique H atom on the water ligand, H(15), was located in the Fourier map and allowed to
refine, with Uiso = 1.5xUeq of O(15).
X-ray structure determination of [Ru(H2O)x(NCMe)1‒x(PPh3)2(LtBu,H)][PF6]2 (7; x =
0.61). One metal coordination site is occupied by a disordered mixture of water and
‒
acetonitrile ligands. This is coupled to disorder in one of the two PF6 ions, one of whose
orientations is positioned to hydrogen bond to the partial water site. The occupancies of the
combined solvent ligand/anion disorder orientations refined to 0.61 (water+anion A):0.39
(acetonitrile+anion B). No restraints were applied to the solvent ligands, but the refined
10

restraints P‒F = 1.57(2) and trans-F...F = 3.14(2) Å were applied to the disordered anion.
Two phenyl rings on the same PPh3 ligand are also disordered, over two equally occupied
sites which refined successfully without restraints. The phenyl group disorder is driven by a
close contact between C(61B) and C(61Bi) (symmetry code (i) 1‒x, y, 1.5‒z), which are
2.07(3) Å apart. To avoid this clash, one molecule in this pair will have the 'A' orientation and
the other the 'B' orientation, probably with a random distribution through the crystal. All non-
H atoms except the disordered F atoms were refined anisotropically. C- and N-bound H atoms
were placed in calculated positions and refined using a riding model. The water H atoms were
not located and are not included in the final model, but are accounted for in the density and
F(000) calculations. The highest residual Fourier peak (+1.6 e.Å‒3) and trough (‒1.0 e.Å‒3)
‒
are both associated with the PF6 ions.
Other measurements.
Elemental microanalyses were performed by the University of Leeds School of Chemistry
microanalytical service. 1H, 13C and 31P NMR spectra employed a Bruker DPX300
spectrometer, operating at 300.2, 75.5 and 121.5 MHz respectively. Electrospray mass
spectra were obtained on a Bruker MicroTOF spectrometer, from dmso or MeCN feed
solutions. The presence of sodium in some molecular ions reflects the use of a Na[O2CH]
calibrant. Diffuse reflectance IR spectra were run using a Perkin Elmer SpectrumOne
spectrophotometer. Dry solvents were obtained from the University of Leeds solvent
purification service.
Gas chromatograpy analyses were performed using a Bruker 430-GC equipped with a CP-
8400 autosampler. Separation was achieved using a BR-5 column (30 m x 0.25 mm (ID) x
•1
0.25 µm film thickness) with carrier gas flow rate of 2.0 cm3min . The temperature ramp
11

•1
•1
from 50 to 310 oC was 10 ºCmin for reactions involving acetophenone, and 20 oCmin for
reactions involving 3,4-dimethoxyacetophenone and 4-nitroacetophenone where better
resolution between starting materials and products was observed. The injection volume was 1
µL with a split ratio of 10. The response factors for the standard, substrate and product were
calculated using an appropriate calibration for this analyser and column.
RESULTS AND DISCUSSION
The new ligand LEt,H (Scheme 1) was prepared by the usual route for 5’,5’’-disubstituted 3-
bpp derivatives [1, 2, 26]; that is, by a Claisen condensation of dimethyl 2,6-dipicolinate with
2 equiv butan-2-one, followed by treatment of the resultant bis-1,3-dione with excess
hydrazine. The synthesis of LNHC{O}Ph was adapted from our previously published method for
L
NHC{O}tBu [18], by acylation of LNH2,H (Scheme 1) with benzoyl chloride. These, and the other
previously reported derivatives LMe,H [22], LtBu,H [19, 21] and LNHCOtBu,H [19], were each
reacted with 1 equiv [RuCl2(PPh3)3] to afford complexes of empirical formula
“RuCl2(PPh3)2(LR1,H)·nH2O” (R1 = Me, 1; R1 = Et, 2; R1 = tBu, 3; R1 = NHC{O}tBu, 4; R1 =
NHC{O}Ph, 5; R1 = NH2, 6; Scheme 2). While complex 3 has been described previously [21],
the other complexes in this series are new. Bulk samples of 1-6 all contain 1 or 2 equiv H2O
by microanalysis (n = 1 or 2). This water, which is also evident by IR spectroscopy, may be
lattice solvent and/or part of the ruthenium inner coordination sphere, according to the
crystallographic data described below. Hydrated 3·2H2O-6·2H2O are soluble in many organic
solvents, including chlorinated solvents which is unusual for a chloride salt of a metal
complex. In contrast 1·H2O and 2·H2O, bearing smaller peripheral hydrophobic substituents,
only dissolve in polar alcohol solvents. While 1·H2O -5·H2O are air-stable, 6·2H2O
decomposes in air in the solid state and in solution to one or more ruthenium(III) complex
12

products, as evidenced by a change in color to dark green and a broadening of its NMR
spectrum. That may reflect the influence of the ligand NH2 substituents, which have a strongly
donating resonance contribution and are also readily deprotonated. Both these properties
would promote oxidation of the ruthenium center, as observed ([Fe(LNH2,H)2]2+ is also air-
sensitive, for the same reasons [19]).
Single crystals of two of the above compounds were obtained, which had the following
compositions: [RuCl(PPh3)2(LMe,H)]Cl·MeOH (1·MeOH); and
[Ru(OH2)(PPh3)2(LtBu,H)]Cl2·4CDCl3 (3·H2O·4CDCl3). Both complexes exhibit the expected
six-coordinate geometry, which is distorted by the narrow LR1,H bite angle of 77-78° (Tables 2
and 3). Notably, the ruthenium coordination sphere in the two structures is different; while
1·MeOH contains coordinated chloride (Fig. 1), this has been displaced by a water molecule
in 3·H2O·4CDCl3 (Fig. 2). The complex molecules in 1·MeOH associate into centrosymmetric
hydrogen-bonded dimers through N‒H…Cl hydrogen bonding to the chloride ligand; the Cl‒
counterions hydrogen bond to the other pyrazolyl N‒H groups, and to the methanol solvent
(Fig. 1 and Table 4). This mode of association is also found in crystal structures of two other
[RuCl(PPh3)2(LR1,H)]Cl complexes (R1 = nBu [11] or tBu [21]).
In contrast, crystals of 3·H2O·4CDCl3 are composed of discrete
{[Ru(OH2)(PPh3)2(LtBu,H)]Cl2·4CDCl3} assemblies, with both Cl‒ ions accepting one N‒H…Cl
and one O‒H…Cl hydrogen bond, and two weaker C‒D…Cl interactions from the
chloroform solvent (Fig. 2 and Table 4). This strong association of the chloride ions with the
complex through second-sphere interactions may explain the unexpected solubility of 3·2H2O
(and 4·H2O-6·2H2O) in weakly polar solvents. Such a dicationic structure has not been
observed before in complexes of type [RuCl(PPh3)2L]+ (L = 2,2’:6’,2’’-terpyridine, LR1,H or
another meridional tridentate N-donor ligand) [11, 21, 27]. In particular, the published
13

structure of the same complex, 3·2CHCl3, has a coordinated chloride ligand [21]. Importantly,
the displacement of the chloride ligand by solvent in crystals of 3·H2O·4CDCl3 demonstrates
that the chloride ligand in 1-5 is labile, as required for catalysis. However, in the light of these
results, it is unclear whether 2·H2O and 4·H2O-6·2H2O (which were not crystallized) have
[RuCl(PPh3)2(LR1,H)]Cl·nH2O or [Ru(OH2)(PPh3)2(LR1,H)]Cl2·(n‒1)H2O formulations in the
solid state.
<Insert Figures 1 and 2, and Tables 1-3, here>
Notably, 1H and 31P NMR spectra of 1·H2O -6·2H2O all contain peaks from only a single
compound, showing that solvolysis of the chloride ligand is rapid on the NMR timescale. The
31P chemical shifts of the complexes follow the trend:
3 (δ 23.7) < 1 (24.4) = 2 (24.4) < 6 (29.1) < 4 (34.8) < 5 (36.5)
This is broadly consistent with the inductive properties of the ligand R1 substitutents, with 3
(R1 = tBu) having the most electron-donating substituents and 5 (R1 = NHC{O}Ph) the most
electron-withdrawing. More electron-donating substituents will increase the basicity of the
pyrazolyl N-donor atoms in the LR1,H ligand [28], thus making the Ru centre more electron-
rich and shielding the coordinated 31P nuclei. The degree of dissociation of the coordinated
chloride in solution, which could vary between the compounds, may also have a bearing on
these data however. While strong molecular ions corresponding to the intact complex
[RuCl(PPh3)2(LR1,H)]+ were observed for 1-3 by ES-MS, the mass spectra of 4-6 were more
complicated and showed species of type [RuCl(solv)(PPh3)(LR1,H)]+ or [RuCl(solv)2(LR1,H)]+
(solv = solvent). This suggests [RuCl(PPh3)2(LR1,H)]+ complexes with heteroatom R1
substituents may be more labile towards ligand substitution.
The only other [Ru(solv)(PPh3)2L]2+ (solv = H2O or MeCN; L = a meridional tris-N-donor
ligand) complexes to have been crystallographically characterized are salts of more weakly
14

‒
interacting ClO4 or PF6‒ anions [29]. Hence, in the light of the above results, 1·H2O -5·H2O
were treated with 2 equiv AgPF6 to exchange the chloride ions for more weakly associating
‒
PF6 . These reactions proved sluggish; significant chloride remained in the materials by
microanalysis after reactions at room temperature, while reactions under reflux led to mixtures
of products. Only one of the desired cationic products was obtained cleanly by this procedure,
namely [Ru(OH2)(PPh3)2(LtBu,H)][PF6]2•H O (7•H O). Recrystallization of this material from
2
2
acetonitrile/di-isopropyl ether yielded single crystals of formula
[Ru(OH2)x(NCMe)1‒x(PPh3)2(LtBu,H)][PF6]2 (x ≈ 0.6), in which some of the coordinated water
in the crude material had been exchanged by acetonitrile solvent (Fig. 3). Apart from the
solvent disorder, the complex cation in the structure of 7 has a similar coordination geometry
to the [Ru(OH2)(PPh3)2(LtBu,H)]2+ center in 3·H2O·4CDCl3 (Table 3). The pyrazolyl N‒H
‒
groups in 7 each donate weaker N‒H…F hydrogen bonds to a different PF6 ion (Table 4).
One of these anions is also disordered, with the major disorder site being positioned to
hydrogen bond to the partial water ligand (Fig. 3).
<Insert Figure 3 here>
All the complexes except 6·2H2O (because of its air-sensitivity) were screened for activity
towards transfer hydrogenation of acetophenone to 1-phenylethanol in refluxing 2-propanol,
following a protocol taken from the literature [11]. All the compounds were active although
1·H2O-5·H2O exhibited an extended induction period, only giving significant yields (52-81 %)
after 24 hrs reaction with a 1 mol% catalyst loading (Table 5). That may indicate that the
complexes themselves are inactive, but slowly decompose in the refluxing ethanol to
catalytically active ruthenium nanoparticles [30]. Only 5·H2O gave a detectable amount of
product at shorter reaction times, although reaction for 24 hrs was still required to afford a
comparable yield to the other complexes. These data contrast with the literature compound
[RuCl(PPh3)2(LnBu,H)]Cl where, under similar conditions, yields above 90 % were obtained
15

after only 6 hrs reaction with 0.25 mol% catalyst [11]. In contrast, the activity of 7·H2O was
much higher, with 60 % conversion being observed after 2 hrs and the reaction being almost
complete after 4 hrs (Table 5). Compound 7·H2O is a more active catalyst for this reaction
than other published [RuCl(PPh3)2(LR1,H)]Cl complexes [11], but it is less active than catalysts
of the [RuCl2(PPh3)(LR1,R2)] type [12-14].
<Insert Table 5 here>
Three of the complexes were also screened against two other substrates with inductively
donating (3,4-dimethoxyacetophenone) or withdrawing (4-nitroacetophenone) substituents
(Table 6). Reactions with 3,4-dimethoxyacetophenone were lower yielding than
acetophenone, which is consistent with some previous reports [12, 13], but were also more
rapid for two of the complexes. In contrast, 4-nitroacetophenone gave zero or trace product
yields with all three of the catalysts examined.
<Insert Table 6 here>
CONCLUSION
Six new complexes of the “[RuCl(PPh3)2(LR1,H)]Cl” type (1-6) have been prepared. Single
crystal structures of two of the compounds had the compositions [RuCl(PPh3)2(LR1,H)]Cl·solv
and [Ru(OH2)(PPh3)2(LR1,H)]Cl2·solv (solv = solvent), demonstrating that the chloride ligand
in complexes of this type can be labile under ambient conditions. This is a new observation to
our knowledge, that is an essential first step for the activity of these compounds towards
catalysis. However, it also means that the precise chemical structures of these complexes are
uncertain, since bulk samples of 1-6 all contain 1 or 2 equiv of water by microanalysis when
isolated under ambient conditions. This water content could be metal-coordinated, or simply
included in the crystal lattice, in different compounds. Metathesis of the chloride ions in 1-5
16

using AgPF6 was sluggish, possibly reflecting the supramolecular complexation of Cl‒ by the
complexes’ N‒H donors (Figs. 1 and 2), and only one compound of type
[Ru(solv)(PPh3)2(LR1,H)][PF6]2 (R1 = tBu; 7·H2O) was obtained in pure form.
All the compounds in this work, except 6·2H2O which was not studied, show activity towards
transfer hydrogenation of acetophenone in refluxing 2-propanol. The activities of the chloride
salts 1·H2O-5·H2O were lower than the related literature compound [RuCl(PPh3)2(LR1,H)]Cl
[11], however, and required a significant induction period that may indicate decomposition of
the complexes in the refluxing alcohol medium [30]. The reason for this difference is unclear,
but it might relate to the presence of water in the (pre)catalyst complexes. Transfer
hydrogenation reactions often function well in water [20], and the higher activity of 7·H2O
shows that water does not poison catalysis in this system (see below). However, the crystal
structure of 3·4CDCl3 suggests that the presence of water might promote second-sphere
binding of chloride to the complex, which would then inhibit coordination of the substrate.
That suggestion is supported by two further pieces of evidence. First, is the low activity of
1·H2O-6·2H2O towards anion metathesis, showing that they retain chloride unusually
strongly. Second, is the catalytic activity of 7·H2O, which is substantially higher than for
1·H2O-5·H2O and for [RuCl(PPh3)2(LR1,H)]Cl. That shows that water is not inimical to
catalysis in this system, per se, and proves that catalysis is substantially enhanced by removal
of chloride ions from the system.
Finally, although our data are preliminary, it is intriguing that 5·H2O is the only chloride salt
that does not exhibit an extended initiation, although its catalysis is still slow. That may
indicate that the introduction of additional hydrogen bonding groups to the periphery of
“[RuCl(PPh3)2(LR1,H)]Cl” complexes may enhance their catalyst properties, which was our
17

goal at the outset of this study. Our current work aims to build on these results to design new
chloride-free hydrogenation catalysts based on LR1,H supporting ligands.
SUPPLEMENTARY DATA
CCDC 1411237, 1411238 and 1411239 contain the supplementary crystallographic data for
1·MeOH, 3·4CDCl3 and 7, respectively. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic
Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk.
ACKNOWLEDGEMENTS
This work was funded by the University of Leeds (Brotherton scholarship to TDR). We also
thank Dr C. E. Willans (University of Leeds) for the use of the gas chromatography analyser.
18

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21

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23

Table 1. Experimental details for the single crystal structure determinations in this work.
1·MeOH
3·H2O·4CDCl3
7
Formula
Mr
C50H47Cl2N5OP2Ru
967.84
triclinic
C59H61Cl14N5OP2Ru
1515.44
monoclinic
P21/n
C55.78H57.39F12N5.39O0.61P4Ru
1265.99
monoclinic
C2/c
Crystal system
Space group
P
1
a (Å)
b (Å)
c (Å)
α (°)
11.9494(7)
12.4660(7)
16.6969(10)
79.907(5)
78.706(5)
64.883(6)
2196.5(2)
2
12.1641(5)
10.8190(4)
25.7429(10)
–
91.863(3)
–
41.2547(14)
12.9487(6)
21.7812(4)
–
96.097(3)
–
β (°)
γ (°)
V (Å3)
3386.1(2)
2
11569.6(7)
8
Z
T (K)
120(2)
100(2)
110(2)
ρcalc (g.cm–3)
1.463
1.486
1.454
µ (mm–1)
5.048a
0.874b
3.953a
Measured reflections
Independent reflections
Rint
17211
8261
0.034
15626
7979
0.033
25671
11030
0.046
Observed reflections [I > 2σ(I)]
Data, restraints, parameters
R1(I > 2σ(I))c, wR2(all data)d
GOF
7254
6455
9133
8261, 0, 554
0.031, 0.080
1.025
7979, 0, 378
0.055, 0.107
1.135
11030, 18, 820
0.069, 0.200
1.013
∆ρmin, ∆ρmax (e.Å–3)
–0.73, 0.91
–1.35, 1.65
–1.07, 1.56
aData collected using Cu-Kα radiation. bData collected using Mo-Kα radiation. R = Σ [Fo – Fc] / ΣFo. wR = [Σw(Fo2 – Fc2)2 / ΣwFo ]
c
d
4 1/2
24

Table 2. Selected bond lengths and angles in the crystal structure of 1·MeOH (Å, º).
Ru(1)‒N(2)
Ru(1)‒N(9)
Ru(1)‒N(15)
1.9888(19)
2.1100(17)
2.0556(18)
Ru(1)‒Cl(20)
Ru(1)‒P(21)
Ru(1)‒P(40)
2.4721(5)
2.3650(6)
2.3906(6)
N(2)‒Ru(1)‒N(9)
N(2)‒Ru(1)‒N(15)
N(2)‒Ru(1)‒Cl(20)
N(2)‒Ru(1)‒P(21)
N(2)‒Ru(1)‒P(40)
N(9)‒Ru(1)‒N(15)
N(9)‒Ru(1)‒Cl(20)
N(9)‒Ru(1)‒P(21)
77.75(7)
78.42(7)
170.60(5)
91.02(6)
89.06(6)
156.16(7)
111.63(5)
88.50(5)
N(9)‒Ru(1)‒P(40)
N(15)‒Ru(1)‒Cl(20)
N(15)‒Ru(1)‒P(21)
N(15)‒Ru(1)‒P(40)
Cl(20)‒Ru(1)‒P(21)
Cl(20)‒Ru(1)‒P(40)
P(21)‒Ru(1)‒P(40)
88.88(5)
92.20(5)
91.25(5)
91.42(5)
89.799(19)
90.559(19)
177.293(18)
25

Table 3. Selected bond lengths and angles in the crystal structures of 3·H2O·4CDCl3 and 7 (Å,
º). The Table is formatted to facilitate comparison of the two structures. Symmetry code: (ii)
−
−
½ x, y, ½ z.
3·H2O·4CDCl3
Ru(1)‒N(2)
Ru(1)‒N(7)
7
1.979(4)
2.065(3)
Ru(1)‒N(2)
Ru(1)‒N(9)
Ru(1)‒N(18)
Ru(1)‒O(26A)/N(26B)
1.997(4)
2.097(4)
2.095(4)
2.171(7)/2.125(12
)
Ru(1)‒O(15)
Ru(1)‒P(16)
2.148(4)
2.4113(8)
Ru(1)‒P(29)
Ru(1)‒P(48)
2.4096(11)
2.4100(12)
N(2)‒Ru(1)‒N(7) 77.85(8)
N(2)‒Ru(1)‒N(9)
77.10(16)
N(2)‒Ru(1)‒N(18)
77.69(16)
N(2)‒Ru(1)‒O(15) 180
N(2)‒Ru(1)‒P(16) 90.10(2)
N(2)‒Ru(1)‒O(26A)/N(26B)
N(2)‒Ru(1)‒P(29)
174.5(2)/173.2(4)
88.92(11)
N(2)‒Ru(1)‒P(48)
N(9)‒Ru(1)‒N(18)
N(9)‒Ru(1)‒O(26A)/N(26B)
N(9)‒Ru(1)‒P(29)
N(9)‒Ru(1)‒P(48)
91.24(12)
N(7)‒Ru(1)‒N(7ii) 155.69(16)
N(7)‒Ru(1)‒O(15) 102.15(8)
N(7)‒Ru(1)‒P(16) 88.50(8)
N(7)‒Ru(1)‒P(16ii) 91.54(8)
154.74(15)
98.3(2)/109.4(4)
86.97(11)
90.71(11)
N(18)‒Ru(1)‒O(26A)/N(26B)
N(18)‒Ru(1)‒P(29)
107.0(2)/95.7(4)
91.02(11)
N(18)‒Ru(1)‒P(48)
91.38(11)
O(15)‒Ru(1)‒P(16) 89.90(2)
O(26A)/N(26B)‒Ru(1)‒P(29)
O(26A)/N(26B)‒Ru(1)‒P(48)
P(29)‒Ru(1)‒P(48)
93.7(2)/89.6(3)
85.9(2)/90.5(3)
177.58(4)
P(16)‒Ru(1)‒P(16ii 179.79(5)
)
26

Table 4. Hydrogen bond parameters for the crystal structures in this work (Å, º). Symmetry
−
−
−
code: (i) 1 x, 1 y, 1 z.
D‒H
H…A
D…A
D‒H…A
1·MeOH
N(10)‒H(10)…Cl(59)
N(16)‒H(16)…Cl(20i)
O(61)‒H(61)‒Cl(59)
0.88
0.88
0.84
2.26
2.48
2.32
3.119(2)
3.316(2)
3.126(2)
166.8
158.8
162.1
3·4CDCl3
N(8)‒H(8)…Cl(35)
O(15)‒H(15A)…Cl(35)
C(36)‒H(36)‒Cl(35)
C(40)‒H(40)‒Cl(35)
0.88
0.88(4)
1.00
2.31
2.26(4)
2.63
3.146(3)
3.1281(16)
3.461(4)
3.343(4)
159.8
167(4)
140.1
157.7
1.00
2.40
7
N(10)‒H(10)…F(79A)
0.88
0.88
0.88
‒
2.10
2.26
2.14
‒
2.952(9)
3.088(12)
3.019(6)
3.140(12)
161.5
155.8
174.3
‒
N(10)‒H(10)…F(79B)
N(19)‒H(19)…F(69)
O(26A)…F(79A)
27

Table 5. Catalysis of the transfer hydrogenation of acetophenone to 1-phenylethanol by the
complexes in this work (iso-propanol, K[OiPr], 80 °C).
catalyst
loading
(mol %)
0.2
yield (%)a
1 hr
20 hrs
0
3·2H2O
3·2H2O
0
0
0.5
48
3·2H2O
1.0
0
84
catalyst
loading
(mol %)
1.0
yield (%)a
2 hrs
0
8 hrs
0
24 hrs
64
1·H2O
1.0
2·H2O
0
0
80
1.0
3·2H2O
0
0
81
1.0
1.0
1.0
4·H2O
5·H2O
0
19
59
0
32
80b
52
64
86
7·H2O
aYields by GC analysis. b80 % yield observed after 4 hrs reaction.
28

Table 6. Catalysis of the transfer hydrogenation of acetophenone to 1-phenylethanol by the
complexes in this work (1 mol% catalyst, iso-propanol, K[OiPr], 80 °C).
catalyst
Substratea
yield (%)b
8 hrs
0
24 hrs
80
18
0
2·H2O
2·H2O
2·H2O
3·2H2O
3·2H2O
3·2H2O
5·H2O
5·H2O
5·H2O
I
II
III
I
20
0
0
11
4
32
0
81
30
4
64
27
0
II
III
I
II
III
0
aI = acetophenone; II = 3,4-dimethoxyacetophenone; III = 4-nitroacetophenone. bYields by
GC analysis.
29