Temperature-dependent elongation of the H–H bond in dihydrogen complexes of Ru(II) bearing an NHC ligand: Effect of the NHC and trans ligands

Article abstract of DOI:10.1016/j.ica.2018.08.015

Ruthenium hydride complexes bearing an N-heterocyclic carbene ligand [RuHCl(CO)(IMes)(PPh₃)(L/L′)] (L = py, 2; 4Mepy, 3; L′ = MeCN, 4; Me₃CCN, 5) have been synthesized in high-yields via reaction of [RuHCl(CO)(IMes)(PPh₃)] (1) with pyridyl ligands L (L = py and 4Mepy) or nitrile ligands L′ (L′ = MeCN and Me₃CCN). The ligands L/L′ are labile in all the ruthenium hydride complexes; they can be easily replaced by Lewis bases. The X-ray structures of complexes 2 and 3 show intramolecular π-π interactions between the aromatic ring of PPh₃, IMes, and pyridyl ligands. The protonation reaction of 2–5 gives the corresponding dihydrogen complexes of the type [RuCl(η²-H₂)(CO)(IMes)(PPh₃)(L/L′)][OTf] complexes (L = py, 6; 4Mepy, 7; L′ = MeCN, 8; Me₃CCN, 9). In all the dihydrogen complexes, H–H bond distances of η²-H₂ ligand is temperature-dependent: 0.98 ? to 0.93 ? in the temperature range of 183–233 K. Attempts to synthesize analogous ruthenium hydride complexes bearing phosphine ligands resulted in a mixture of cis and trans-[RuHCl(CO)(PPh₃)₂(L)] [L = py, 10/11 (trans_HCl/cis_HCl); 4Mepy, 12/13 (trans_HCl/cis_HCl)] complexes. A comparative study has been done to get an insight into the temperature-dependent H–H bond distances in complexes 6–8 by synthesizing analogous ruthenium dihydrogen complexes, [RuCl(η²-H₂)(CO)(PPh₃)₂(L)](OTf) (L = py, 15; 4Mepy, 17). All the complexes have been characterized using NMR spectroscopy. The X-ray crystal structures of complexes 2, 3, and 12 have also been determined.

Full text of DOI:10.1016/j.ica.2018.08.015

InorganicaChimicaActa483(2018)411–424
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Research paper
Temperature-dependent elongation of the HeH bond in dihydrogen
complexes of Ru(II) bearing an NHC ligand: Effect of the NHC and trans
ligands
Deep Mala, Balaji R. Jagirdar⁎, Yogesh P. Patil, Munirathinam Nethaji
Department of Inorganic & Physical Chemistry, Indian Institute of Science, Bangalore 560012, India
A R T I C L E I N F O
A B S T R A C T
Keywords:
Ruthenium hydride complexes bearing an N-heterocyclic carbene ligand [RuHCl(CO)(IMes)(PPh3)(L/L′)]
(L = py, 2; 4Mepy, 3; L′ = MeCN, 4; Me3CCN, 5) have been synthesized in high-yields via reaction of [RuHCl
(CO)(IMes)(PPh3)] (1) with pyridyl ligands L (L = py and 4Mepy) or nitrile ligands L′ (L′ = MeCN and Me3CCN).
The ligands L/L′ are labile in all the ruthenium hydride complexes; they can be easily replaced by Lewis bases.
The X-ray structures of complexes 2 and 3 show intramolecular π-π interactions between the aromatic ring of
PPh3, IMes, and pyridyl ligands. The protonation reaction of 2–5 gives the corresponding dihydrogen complexes
of the type [RuCl(η2-H2)(CO)(IMes)(PPh3)(L/L′)][OTf] complexes (L = py, 6; 4Mepy, 7; L′ = MeCN, 8; Me3CCN,
9). In all the dihydrogen complexes, HeH bond distances of η2-H2 ligand is temperature-dependent: 0.98 Å to
0.93 Å in the temperature range of 183–233 K. Attempts to synthesize analogous ruthenium hydride complexes
bearing phosphine ligands resulted in a mixture of cis and trans-[RuHCl(CO)(PPh3)2(L)] [L = py, 10/11
(transHCl/cisHCl); 4Mepy, 12/13 (transHCl/cisHCl)] complexes. A comparative study has been done to get an in-
sight into the temperature-dependent HeH bond distances in complexes 6–8 by synthesizing analogous ruthe-
nium dihydrogen complexes, [RuCl(η2-H2)(CO)(PPh3)2(L)](OTf) (L = py, 15; 4Mepy, 17). All the complexes
have been characterized using NMR spectroscopy. The X-ray crystal structures of complexes 2, 3, and 12 have
also been determined.
N-heterocyclic carbene
Ruthenium complexes
Dihydrogen complexes
Trans ligands
HeH bond distances
1. Introduction
complexes containing NHC as ancillary ligands have also been reported
to date [1e,12].
The study of ruthenium hydride and dihydrogen complexes is of
great interest in the field of homogeneous catalysis [1,2]. These com-
plexes play a crucial role in catalysis as reactive intermediates, cata-
lysts, and catalyst precursors [3]. A large number of catalytic reactions
have been reported using these complexes by several research groups
[2,4]. In particular, dihydrogen complexes are one of the most inter-
esting classes of compounds because of their resemblance to sigma
complexes [5–7]. Among different classes of dihydrogen complexes, the
chemistry of ruthenium dihydrogen complexes bearing phosphine an-
cillary ligands has been explored extensively [6,8].
The successful isolation of an N-heterocyclic carbene (NHC) by
Arduengo in 1991 opened up a new class of ligands to investigate their
transition metal chemistry [9]. The strong metal-carbon bond in metal
complexes bearing NHC ligands makes these complexes thermally
stable [10]. The first ruthenium NHC dihydrogen complex was reported
by Leitner and co-workers in 2003 [11]. Few other metal dihydrogen
In general, the nature of ancillary ligands in dihydrogen complexes
has a significant influence on their HeH bond distances [5f,7a,13].
D’Agostino and co-workers reported the synthesis of ruthenium com-
plexes [RuX(H2)(R2PCH2CH2PR2)2](PF6) (X]Cl, H; R]Ph, Et) and also
studied the influence of ligands, chloride as well as hydride, on the
HeH bond distances [13]. Spin-lattice relaxation time (T1) and H, D
coupling constant (1JHD) data indicate that the H-H distance in the
chloride complex was longer as compared to the hydride complex. Si-
milarly, Jagirdar and co-workers studied the influence of the electronic
properties of phosphine co-ligands in the ruthenium dihydrogen com-
plexes [RuCl(H2)(ArCH2)2PCH2CH2P(CH2Ar)2](BF4) (Ar]p-FC6H4,
C6H5, m-MeC6H4, p-MeC6H4, p-iPrC6H4). They reported that the elec-
tron donating nature of Ar group of the phosphine ligands results in a
small increment in the HeH bond distance (0.97– 1.03 Å) [7a]. We
recently reported that two [RuHCl(CO)(IMes)(PPh3)] fragments (1)
bridged by 4,4′-bipyridine, 1,2-bis(4-pyridyl)ethylene, and 1,2-bis(4-
⁎
Corresponding author.
E-mail address: jagirdar@iisc.ac.in (B.R. Jagirdar).
https://doi.org/10.1016/j.ica.2018.08.015
Received 4 July 2018; Received in revised form 15 August 2018; Accepted 15 August 2018
Availableonline19August2018
0020-1693/©2018ElsevierB.V.Allrightsreserved.

D. Mala et al.
InorganicaChimicaActa483(2018)411–424
pyridyl)ethane ligands form stable hydride complexes [{RuHCl(CO)
(IMes)(PPh3)}2(NN)] [NN = 4,4′-bipyridyl, 1,2-bis(4-pyridyl)ethylene,
1,2-bis(4-pyridyl)ethane] [14]. Subsequently, protonation of these bi-
metallic hydride complexes resulted in the corresponding dihydrogen
complexes [14]. However, the influence of IMes on the properties of
those bimetallic complexes could not be established. Herein, we present
a comparative study of the dihydrogen complexes bearing IMes, PPh3,
and different ligands trans to the bound H2 ligands.
2.2.3. Synthesis of [RuHCl(CO)(IMes)(PPh3)(Me3CCN)] (5)
Me3CCN (0.3 mL) was added to complex 1 (275 mg, 0.37 mmol) at
room temperature and stirred for 1 min. The product was washed using
hexanes (10 mL). The resulting off-white colored product of [RuHCl
(CO)(IMes)(PPh3)(Me3CCN)] complex (5) was washed twice with 5 mL
of hexanes and dried under vacuum (isolated yield = 250 mg, 87%).
The 1H, 31P{1H}, and 13C{1H} NMR spectra data of complex 5 have
been summarized below.
2. Experimental
2.2.4. Variable temperature (VT) NMR study of 2, 3, 4, and 5
Complexes 2–5 [20 mg, 2; 21 mg, 3; 19 mg, 4; 20 mg, 5 (each of
0.025 mmol)] were dissolved in 0.5 mL of CD2Cl2 in Schlenk NMR tubes
and the solutions were degassed by two freeze–pump-thaw cycles. The
NMR tubes were then flame-sealed under vacuum and then inserted
into an NMR probe at 293 K. The 1H and 31P{1H} NMR spectra were
recorded at each 5 or 10 K temperature interval (293–198 K). The
complete 1H, 31P{1H} NMR spectral data acquired at 198 K are given in
Table 1 and1 H, 31P{1H}, 13C{1H} NMR and IR spectral data obtained at
293 K are summarized below.
2.1. Materials and methods
All manipulations were carried out using standard Schlenk techni-
ques under N2 or Ar atmosphere. Solvents were dried using calcium
hydride (dichloromethane, acetonitrile, and pyridine), sodium benzo-
phenone ketyl (hexanes, pentane, toluene, THF, and Et2O) and distilled
under N2 or Ar atmosphere just before use. HOTf, DOTf, CDCl3, tol-d8,
pyvalonitrile (Me3CCN), and 4-methylpyridine were used as received
from Sigma-Aldrich. CD2Cl2 was dried and distilled over calcium hy-
dride and degassed by two consecutive cycles of freeze-pump-thaw.
Synthesis of 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene (IMes)
[15] and [RuHCl(CO)(PPh3)3] [16], [RuHCl(CO)(IMes)(PPh3)] [3a]
were carried out by following literature procedures. NMR spectra were
obtained using an Avance Bruker 400 MHz spectrometer. 1H and
13C{1H} NMR (100 MHz) spectra were referenced to the residual proton
signal of the deuterated solvents (5.32 ppm, CD2Cl2; 7.26 ppm, CDCl3;
2.08 ppm, tol-d8) and carbon signal of the deuterated solvents
(53.84 ppm, CD2Cl2; 77.16 ppm, CDCl3), respectively. The 31P{1H}
NMR spectra (161 MHz) were referenced to 85% H3PO4 (0.0 ppm, ex-
ternal standard). IR spectra were recorded using a Bruker Alpha FTIR
spectrometer and elemental analyses were obtained using Thermo
Finnigan Flash EA 1112 CHN analyzer.
2.2.5. Characterization data of [RuHCl(CO)(IMes)(PPh3)(py)] (2)
1H NMR (CD2Cl2, 293 K, ppm): δ = 8.50 (br s, 1H, py), 8.00 (br s,
1H, py), 7.00–7.21 (m, 15H, PPh3; 1H, py; s, 2H, IMes m-CH), 6.81 (s,
2H, IMes m-CH), 6.62 (s, 2H, NCH = CHN), 6.53 (br s, 2H, py), 2.28 (s,
6H, IMes p-CH3), 2.24 (s, 6H, IMes o-CH3), 2.10 (s, 6H, IMes o-CH3),
2
−13.28 (d, JHP = 15.5 Hz, 1H, Ru–H). 31P{1H} NMR (CD2Cl2, 293 K,
ppm): δ = 46.2 (s, 1P, PPh3). 13C{1H} NMR (CH2Cl2; CDCl3 ext. lock,
2
2
ppm): δ = 204.2 (d, JCP cis = 13.6 Hz, CO), 186.5 (d, JCP
trans = 102.0 Hz, IMes NCN), 149–153 (br. m, py),138.4 (s, IMes),
137.9 (s, IMes), 136.2 (s, IMes), 136.1 (s, IMes), 135.5 (d,
1JCP = 36.0 Hz, i-PPh3), 134.0 (d, JCP = 10.0 Hz, o-PPh3), 128.5 (s,
IMes), 128.4 (s, IMes), 128.4 (s, p-PPh3), 127.2 (d, JCP = 9.0 Hz, m-
2
3
PPh3), 123.1 (s, IMes NCH]CHN), 123.0 (s, IMes), 20.9 (s, IMes p-
CH3), 18.5 (s, IMes o-CH3), 18.4 (s, IMes o-CH3). IR (cm–1): ν(CO) 1877.
Anal. calcd for C45H45ClN3OPRu (811), C: 66.61, H: 5.59, N: 5.18.
found: C: 66.47, H: 5.62, N: 4.99.
2.2. Synthesis and characterization of [RuHCl(CO)(IMes)(PPh3)(L/L′)]
(L = py, 2; 4Mepy, 3; L′ = MeCN, 4; Me3CCN, 5)
2.2.1. Synthesis of [RuHCl(CO)(IMes)(PPh3)(L)] (L = py, 2; 4Mepy, 3)
Pyridine (py) or 4-methylpyridine (4Mepy) (1.5 mL) was added to
[RuHCl(CO)(IMes)(PPh3)] (1) (275 mg, 0.37 mmol) at room tempera-
ture and stirred for 1 min. The product was obtained via precipitation
by addition of 15 mL of hexanes. The resulting pale yellow colored
complexes were isolated in high yields (260 mg, 85%, 2; 285 mg, 90%,
3) through filtration and washed twice with 5 mL of hexanes and dried
under vacuum. The 1H, 31P{1H} NMR spectra were recorded at 293 K
and 198 K. The 13C{1H} NMR spectrum was recorded at room tem-
perature.
2.2.6. Characterization data of [RuHCl(CO)(IMes)(PPh3)(4Mepy) (3)
1H NMR (CD2Cl2, 293 K, ppm): δ = 8.21 (br. s, 1H, 4Mepy o-CH),
7.95 (br. s, 1H, 4Mepy o-CH), 7.00–7.22 (m, 15H, PPh3, 2H, IMes m-
CH), 6.83 (s, 2H, IMes m-CH), 6.65 (s, 2H, NCH]CHN), 6.37 (br. s, 2H,
4Mepy m-CH), 2.29 (s, 6H, IMes p-CH3), 2.25 (s, 6H, IMes o-CH3), 2.18
(s, 3H, 4Mepy p-CH3), 2.10 (s, 6H, IMes o-CH3), −13.19 (br. s, 1H, Ru-
H). 31P{1H} NMR (CD2Cl2, 293 K, ppm): δ = 46.4 (s, 1P, PPh3).
2
13C{1H} NMR (CH2Cl2; CDCl3 ext. lock, ppm): δ = 204.3 (d, JCP
2
cis = 13.6 Hz, CO), 186.4 (d, JCP trans = 102.0 Hz, IMes NCN),
150.1–153.0 (br. m, 4Mepy), 138.5 (s, IMes), 138.0 (s, IMes), 136.3 (s,
1
IMes), 136.1 (s, IMes), 135.6 (d, JCP = 36.0 Hz, i-PPh3), 134.1 (d,
2.2.2. Synthesis of [RuHCl(CO)(IMes)(PPh3)(MeCN)] (4)
2JCP = 10.0 Hz, o-PPh3), 128.5 (s, IMes), 128.4 (s, IMes), 128.4 (s, p-
3
A solution of IMes (200 mg, 0.66 mmol) in 8 mL of toluene was
added to a suspension of [RuHCl(CO)(PPh3)3] (420 mg, 0.44 mmol) in
8 mL of toluene. The reaction mixture was stirred at 303 K for 2 h. The
reaction mixture was concentrated up to 5 mL and filtered through a
filter frit. The filtrate was concentrated up to 0.5 mL under vacuum and
15 mL of hexanes was added. Upon cooling this solution in a low-
temperature bath (liq. N2 and acetone) and stirring for 5 min, a pre-
cipitate of orange-yellow colored complex 1 was obtained. The super-
natant was removed under a stream of N2 gas. Complex 1 was washed
with cold hexanes (2 × 4 mL) and dried under vacuum. MeCN (0.3 mL)
was added to complex 1 at room temperature and stirred for 1 min.
Addition of 10 mL of Et2O gave an off-white precipitate of [RuHCl(CO)
(IMes)(PPh3)(MeCN)] (4) which was washed with 5 mL of Et2O. The
product was washed again using Et2O (2 × 5 mL) and dried under va-
cuum (yield = 170 mg, 59%). The 1H, 31P{1H} (293 K) and 13C{1H}
NMR (room temperature) spectra of complex 4 were recorded and the
data have been summarized below.
PPh3), 127.1 (d, JCP = 9.0 Hz, m-PPh3), 123.9 (s, IMes NCH]CHN),
123.1 (s, IMes NCH]CHN), 20.9 (s, IMes p-CH3), 18.5 (s, IMes o-CH3),
18.4 (s, IMes o-CH3), 20.5 (s, 4Mepy p-CH3). IR (cm–1): ν(CO) 1874.
Anal. calcd for C46H47ClN3OPRu (825), C: 66.94, H: 5.74, N: 5.09.
found: C: 66.91, H: 5.94, N: 5.21.
2.2.7. Characterization data of [RuHCl(CO)(IMes)(PPh3)(MeCN)] (4)
1H NMR (CD2Cl2, 293 K, ppm): δ = 7.21–7.36 (m, 15H, PPh3), 7.07
(s, 2H, IMes m-CH), 7.04 (s, 2H, NCH = CHN), 6.97 (s, 2H, IMes m-CH),
2.36 (s, 6H, IMes p-CH3), 2.30 (s, 6H, IMes o-CH3), 2.19 (s, 6H, IMes o-
CH3), 1.42 (s, 3H, MeCN), −15.97 (br. s, 1H, Ru-H). 31P{1H} NMR
(CD2Cl2, 293 K, ppm): δ = 44.6 (s, 1P, PPh3). 13C{1H} NMR (CH2Cl2;
2
CDCl3 ext. lock, ppm): δ = 203.0 (d, JCP cis = 13.5 Hz, CO), 186.9 (d,
2JCP trans = 102.5 Hz, IMes NCN), 138.5 (s, IMes), 138.1 (s, IMes),
137.0 (s, IMes), 136.7 (s, IMes), 135.6 (d, 1JCP = 37.0 Hz, i-PPh3), 134.4
(d, 2JCP = 10.7 Hz, o-PPh3), 128.8 (s, p-PPh3), 128.5 (s, IMes), 128.4 (s,
3
IMes), 127.4 (d, JCP = 9.0 Hz, m-PPh3), 122.9 (s, IMes NCH]CHN),
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InorganicaChimicaActa483(2018)411–424
116.9 (s, MeCN CN), 21.0 (s, IMes p-CH3), 18.6 (s, IMes o-CH3), 18.4 (s,
IMes o-CH3), 2.4 (s, MeCN CH3,). IR (cm–1): ν (CO) 1888 (KBr).
2.2.8. Characterization data of [RuHCl(CO)(IMes)(PPh3)(Me3CCN)] (5)
1H NMR (CD2Cl2, 293 K, ppm): δ = 7.19–7.35 (m,15H, PPh3), 7.07
(s, 2H, IMes m-CH), 7.05 (s, 2H, IMes m-CH), 7.00 (s, 2H, IMes
NCH = CHN), 2.40 (s, 6H, IMes p-CH3), 2.31 (s, 6H, IMes o-CH3), 2.21
(s, 6H, IMes o-CH3), 1.14 (s, 9H, Me3CN, CH3), −20.01 (br. s, 1H, Ru-
H). (Additionally, few peaks which could not be assigned were also
noted). 31P{1H} NMR (CD2Cl2, ppm): δ = 41.1 (s, 1P, PPh3). 13C{1H}
NMR (CH2Cl2; CDCl3 ext. lock, ppm): δ = 201.9 (d, 2JCP cis = 12.0 Hz,
CO), 187.6 (d, 2JCP trans = 103.0 Hz, IMes NCN), 138.4 (s, IMes), 137.4
1
(s, IMes), 136.6 (s, IMes), 136.3 (s, IMes),135.7 (d, JCP = 37.0 Hz, i-
2
PPh3), 134.5 (d, JCP = 11.0 Hz, o-PPh3), 129.4 (s, p-PPh3), 128.9 (s,
3
IMes), 127.7 (d, JCP = 9.0 Hz, m–PPh3), 126.4 (s, Me3CCN CN), 123.0
(s, IMes NCH]CHN), 28.4 (s, Me3CCN Me3C), 27.9 (s, Me3CCN CH3),
21.1 (s, IMes p-CH3), 18.6 (s, IMes o-CH3), 18.5 (s, IMes o-CH3). IR
(cm–1): ʋ (CO) 1900 (KBr).
2.2.9. Synthesis and characterization of [RuCl(η2-H2)(CO)(IMes)(PPh3)
(L/L′)](OTf) (L = py, 6; 4Mepy, 7; L′ = MeCN, 8; Me3CCN, 9)
Complexes 2–5 [18 mg, 2; 20 mg, 3; 19 mg, 4; 20 mg, 5;
(0.02–0.025 mmol)] were dissolved in 0.3 mL of CD2Cl2 in Schlenk
NMR tubes. These solutions were subjected to two freeze–pump-thaw
degassing cycles. HOTf solution (1.5–2.0 equiv. in 0.2 mL CD2Cl2) was
added to each of the solutions of 2–5 at liq. N2 temperature. The NMR
tubes were flame-sealed under vacuum at liq. N2 temperature and then
slowly warmed up to ∼193 K (liq. N2 and Et2O). In each case, the tube
was inserted into an NMR probe, which was pre-cooled and maintained
at 183 or 193 K. The dihydrogen complexes [RuCl(η2-H2)(CO)(IMes)
(PPh3)(L/L′)](OTf) (L = py, 6; 4Mepy, 7; L′ = MeCN, 8; Me3CCN, 9)
were characterized using NMR spectroscopy in the temperature ranges
of 183–233 K.
2.2.10. Characterization data of [RuCl(η2H2)(CO)(IMes)(PPh3)(py)]
[OTf] (6)
3
1H NMR (CD2Cl2, 203 K, ppm): δ = 8.40 (d, JH,H = 5.4 Hz, 1H, py
o-CH), 7.97 (d, 3JH,H = 5.4 Hz, 1H, py o-CH), 6.92–7.40 (m, 15H, PPh3;
2H, IMes NCH = CHN; 1H, py p-CH), 6.94 (t, 3JH,H = 5.4 Hz, 1H, py m-
CH), 6.33 (t, 3JH,H = 5.4 Hz, 1H, py m-CH), 2.25 (s, 6H, IMes CH3), 2.12
(s, 6H, IMes CH3), 1.93 (s, 6H, IMes CH3), −8.76 (br. s, 2H, Ru-H2). 31P
{1H} NMR (CD2Cl2, 213 K, ppm): δ = 29.3 (s, 1P, PPh3).
2.2.11. Characterization data of [RuCl(H2)(CO)(IMes)(PPh3)(4Mepy)]
[OTf] (7)
3
1H NMR (CD2Cl2; 193 K, ppm): δ = 8.18 (d, JH,H = 5.6 Hz, 1H,
4Mepy o-CH), 7.78 (d, 3JH,H = 5.6 Hz, 1H, 4Mepy o-CH), 6.94–7.40 (m,
21H, PPh3, IMes NCH = CHN, m-CH), 6.11 (d, JH,H = 5.6 Hz, 1H,
4Mepy m-CH), 6.57 (d, JH,H = 5.6 Hz, 1H, 4Mepy m-CH), 2.28 (6H,
3
3
IMes p-CH3), 2.14 (6H, 4Mepy o-CH3), 2.08 (3H, 4Mepy, p-CH3), 1.94
(6H, IMes o-CH3), −8.66 (br, 2H, Ru-H2). 31P{1H} NMR (CD2Cl2;
193 K, ppm): δ = 29.7 (s, 1P, PPh3).
2.2.12. Characterization data of [RuCl(η2H2)(CO)(IMes)(PPh3)(MeCN)]
[OTf] (8)
1H NMR (CD2Cl2, 198 K, ppm): δ = 6.98–7.19 (m, 15H, PPh3, 6H,
IMes NCH = CHN, m-CH), 2.38 (6H, IMes p-CH3), 2.23 (6H, IMes o-
CH3), 2.11 (6H, IMes o-CH3), 1.03 (s, 3H, MeCN) −9.12 (br. s, 2H, Ru-
H2). 31P{1H} NMR (CD2Cl2, 198 K, ppm): δ = 28.1 (s, 1P, PPh3).
2.2.13. Characterization
data
of
[RuCl(η2H2)(CO)(IMes)(PPh3)
(Me3CCN)][OTf] (9)
1H NMR (CD2Cl2, 193 K, ppm): δ −9.54 (br. s, 2H, Ru-H2). The re-
gion of the spectrum downfield to TMS was comprised of peaks that were
not well resolved. Therefore, peak assignments could not be made with
certainty. 31P{1H} NMR (CD2Cl2, 193 K, ppm): δ = 22.0 (s, 1P, PPh3).
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InorganicaChimicaActa483(2018)411–424
2.2.14. Synthesis of [RuHCl(CO)(PPh3)2(py)] (10/11)
temperature. The NMR tube was flame sealed at liq. N2 temperature
and then it was slowly warmed to ∼193 K (liq. N2 and diethylether)
and inserted into an NMR probe which was pre-cooled and maintained
at 193 K. The dihydrogen complexes [RuCl(η2-H2)(CO)(PPh3)2(L)](OTf)
(L = py, 14/15; 4Mepy, 16/17) were characterized by NMR spectro-
scopy at low temperature (193–263 K).
Addition of 1 mL of pyridine (py) to [RuHCl(CO)(PPh3)3] (220 mg,
0.23 mmol) at room temperature resulted in the formation of a greyish
white precipitate in 30–40 min. Addition of excess hexane gave more
quantity of precipitate. The supernatant liquid was removed completely
and the precipitate was washed twice with 5 mL of hexanes. The pre-
cipitate containing the isomer products 10 and 11 was dried under
vacuum and isolated in a high yield (150 mg, yield = 85%). The 1H, 31
P
2.2.19. NMR data for [RuCl(H2)(CO)(PPh3)2(py)](OTf) (14/15)
14: 1H NMR (CD2Cl2, 223 K, 400 MHz): δ = 7.88 (br s, 1H, py, o-
CH), 7.71 (br s, 1H, py, o-CH), 7.33–7.47 (m, 30H, PPh3; 1H, py, p-CH),
6.82 (s, 1H, py, m-CH), 6.48 (s, 1H, py, m-CH), −7.49 (br. s, 2H, Ru-
H2). 31P{1H} NMR (CD2Cl2, 293 K, 161 MHz): δ = 30.8 (s, 2P, PPh3).
15: 1H NMR (CD2Cl2, 223 K, 400 MHz): δ = 8.18 (m, 2H, py, o-CH),
7.33–7.47 (m, 30H, PPh3; 1H, py, p-CH), 7.04 (br s, 2H, py, m-CH),
−7.88 (br. s, 2H, Ru-H2). 31P{1H} NMR (CD2Cl2, 293 K, 161 MHz):
δ = 30.0 (s, 2P, PPh3).
{1H}, and 13C{1H} NMR and IR spectra were recorded for character-
ization (Table 4). NMR spectroscopy revealed that the resultant product
is a mixture of two isomers [RuHCl(CO)(PPh3)2(Py)] [10/11(transHCl
cisHCl = 85/15)].
/
2.2.15. Characterization data of [RuHCl(CO)(PPh3)2(py)] (10)
13C{1H} NMR (CDCl3, room temperature, 100 MHz): δ = 205.4 (t,
2JCP 16 Hz, CO), 153.1–157.2 (m, py), 135.2 (s, PPh3), 134.0 (t,
2
1JCP = 5.9 Hz, i-PPh3), 129.3 (t, PPh3), 127.9 (t, JCP = 4 Hz, o-PPh3),
122.9–123.6 (m, py). IR (cm–1): ν(CO). Anal. calcd for C42H36ClNOP2Ru
2.2.20. NMR data for [RuCl(H2)(CO)(PPh3)2(4Mepy)](OTf) (16/17)
16: 1H NMR (CD2Cl2, 223 K, 400 MHz): δ = 7.73 (d, 5.8 Hz, 1H,
4Mepy, o-CH), 7.20–7.50 (m, 30H, PPh3; 2H, 4Mepy, o-CH p-CH), 6.62
(d, 4.4 Hz, 1H, 4Mepy, m-CH), 6.28 (d, 5.8 Hz, 1H, 4Mepy, m-CH)
−7.50 (br s, 2H, Ru-H2). 31P{1H} NMR (CD2Cl2, 293 K, 161 MHz):
δ = 30.9 (s, 2P, PPh3).
(769), C: 65.58, H: 4.72, N: 1.82. Found: C: 65.41, H: 4.59, N: 1.79.
2.2.16. Synthesis of [RuHCl(CO)(PPh3)2(4Mepy)] (12/13)
Complexes 12/13 were also prepared and isolated in a similar
manner to that of complexes 10/11 using 2 mL of 4Mepy and [RuHCl
(CO)(PPh3)3] (575 mg, 0.6 mmol). The resulting mixture of isomers was
isolated in a high yield (∼400 mg, 85%). The 1H, 31P{1H}, and 13C{1H}
NMR and IR spectra were recorded for characterization of 12/13. The
1H and 31P{1H} NMR and IR data for complexes 10–13 are given in
Supplementary Material.
17: 1H NMR (CD2Cl2, 223 K, 400 MHz): δ = 7.20–7.50 (m, 30H,
PPh3), −7.82 (br s, 2H, Ru-H2) (4MePy peaks of complex 17 were
overlapped with signals of unreacted hydride precursor 12 31P{1H}
NMR (CD2Cl2, 223 K, 161 MHz): δ = 29.6 (s, 2P, PPh3).
2.3. X-ray crystal structure determination of complexes 2, 3 and 12
2.2.17. Characterization data of [RuHCl(CO)(PPh3)2(4Mepy)] (12)
13C{1H} NMR (CDCl3, room temperature, 100 MHz): δ = 205.4 (t,
2JCP 16.0 Hz, CO), 152.5–156.7 (m, 4Mepy), 147.0 (s, PPh3), 134.5(t,
Pale yellow colored crystals of complexes 2, 3, and 12 suitable for a
diffraction study were chosen and picked carefully under a microscope.
The diffraction data were collected at 100 K for complexes 2 and 3 and
at 293 K for complex 12. The unit cell parameters and intensity data
were collected using a BRUKER SMART APEX CCD diffractometer
equipped with a fine focus Mo-Kα X-ray source. The SMART software
was used for data acquisition and the SAINT program was used for data
reduction. The structures were solved by direct methods using SHELX-
97. The complexes 2 and 3 crystallized in the monoclinic P21/c space
group with four molecules in a unit cell. However, complex 12 crys-
tallized in the monoclinic P21 space group. All the non-hydrogen atoms
were refined anisotropically and the refinement was carried out against
F2 with the help of SHELX-97 [17]. The hydride hydrogen atoms were
located on a difference Fourier map while all the other hydrogen atoms
2
1JCP = 5.9 Hz, i-PPh3), 129 (s, PPh3), 127.8 (t, JCP = 4.5 Hz, o-PPh3),
124.6 (m, 4Mepy), 20.6 (4Mepy). IR (cm–1): ν(CO). Anal. calcd for
C
43H38ClNOP2Ru (783), C: 65.94, H: 4.89, N: 1.79; Found: C: 65.56, H:
4.71, N: 1.81.
2.2.18. Synthesis of [RuCl(η2-H2)(CO)(PPh3)2(L)](OTf) [L = py, 14/15;
4Mepy, 16/17]
[RuHCl(CO)(PPh3)2(L)]
(L = py,
10/11;
4Mepy,
12/13)
(0.02–0.025 mmol) complexes were dissolved in 0.3 mL of CD2Cl2 in a
Schlenk NMR tube. Each solution was then subjected to two cycles of
freeze–pumpthaw degassing. 3–5 equiv. of HOTf solution in 0.2 mL of
CD2Cl2 was added to each of the solution of 10/11 or 12/13 at liq. N2
Table 2
Crystallographic data of complexes 2, 3, and 12.
Complexes
2
3
12
Empirical formula
Formula weight
Temperature (K)
λ(Mo Kα) [Å]
Crystal system
Space group
C45H45ClN3OPRu
811.33
100(2)
0.71073
Monoclinic
P 21/c
C46H47ClN3OPRu
825.36
100(2)
0.71073
Monoclinic
P 21/c
C43H38ClNOP2Ru
783.20
293(2)
0.71073
Monoclinic
P21
a [Å]
18.2661(10)
18.8910(8)
12.249(6)
b [Å]
14.4692(7)
14.4394(6)
25.133(13)
c [Å]
15.2464(8)
15.3921(7)
12.593(7)
α [°]
90
90
90
β [°]
105.656(1)
90
3880.1(3)
108.495(1)
90
3981.7(3)
105.282(11)
90
3740(3)
γ [°]
V [Å3]
Z, Calculated density [Mg/m3]
Absorption coefficient, μ (mm−1
Reflections collected/unique
Goodness-of-fit on F^2
Final R indices [I > 2σ (I)]
R indices (all data)
4, 1.389
0.553
31,163/7615 [Rint = 0.0431]
1.058
R1 = 0.0322, wR2 = 0.0783
R1 = 0.0381, wR2 = 0.0816
4, 1.377
0.540
80368/7813 [Rint = 0.0293]
1.153
R1 = 0.0279, wR2 = 0.0625
R1 = 0.0330, wR2 = 0.0673
4, 1.391
0.611
26,831/14,229 [Rint = 0.1443]
0.982
R1 = 0.0952, wR2 = 0.1805
R1 = 0.2329, wR2 = 0.2429
)
414

D. Mala et al.
InorganicaChimicaActa483(2018)411–424
coordination geometry around the metal center in complexes 2 and 3
could be described as a distorted octahedron. Crystal structures of both
the complexes reveal that the chloride and carbonyl, hydride and py or
4Mepy, and IMes and PPh3 are mutually trans to each other. A large
deviation from the expected bond angle of 180° for C(1)(IMes)eRu(1)
eP(1)(PPh3) (∼165°) was noted. In both the complexes, intramolecular
π-π stacking interactions involving mesityl group of IMes, pyridyl, and
one of the phenyl groups of PPh3 were observed (centroid to centroid
distance: 3.5–3.6 Å) (Supplementary Material). The Ru-N bond lengths
in both the complexes are comparatively longer [2.299(2) Å for com-
plex 2 and 2.287(2) Å for complex 3] than the previously reported
pyridyl or substituted pyridyl ruthenium complexes (< 2.250 Å) [18].
The dihedral angles between Py and IMes, N1eC1eRu1eN3 and
N2eC1eRu1eN3 in complex 2 are respectively, −13.69° and 165.04°.
Similarly, the dihedral angles between 4Mepy and IMes are
N1eC1eRu1eN3 = −13.79° and N2eC1eRu1eN3 = 165.07° in case
of complex 3, which shows that IMes and pyridyl ligands are nearly
parallel to each other. Selected bond lengths and angles are summar-
ized in Table 3.
Scheme 1. Synthesis of [RuHCl(CO)(IMes)(PPh3)(L)] (L = py, 2; 4Mepy, 3).
were fixed using a riding model. The crystallographic parameters are
summarized in Table 2.
3. Results and discussion
3.1. Synthesis and characterization of [RuHCl(CO)(IMes)(PPh3)(L)]
(L = py, 2; 4Mepy, 3) complexes
Reaction of complex 1 with excess ligand L [L = pyridine (py) or 4-
methylpyridine (4Mepy)] at room temperature afforded [RuHCl(CO)
(IMes)(PPh3)(L)] (L = py, 2; 4Mepy, 3) complexes in high yields
(85–90%) (Scheme 1). Complexes 2 and 3 were characterized by 1H,
31P{1H}, 13C{1H} NMR, and IR spectroscopy, elemental analysis, and X-
ray crystallography. These complexes are stable in the solid state for
about 5–6 h in air but stable for longer periods of time (> 6 months)
under an inert atmosphere. Exposure of solutions of these complexes to
air led to their rapid decomposition. In addition to the intractable ru-
thenium species, IMes.HCl and OPPh3 could be identified among the
decomposed products using NMR spectroscopy.
3.2. Variable temperature (VT) NMR spectral study of [RuHCl(CO)(IMes)
(PPh3)(L)] (L = py, 2; 4Mepy, 3)
The 1H NMR spectral signals of the ligands L (L = py or 4Mepy)
trans to the hydride in complexes 2 and 3 are broad in nature at 293 K.
These signals sharpen upon cooling the samples from 293 to 198 K. The
1H–1H COSY spectrum of complex 2 at 198 K indicates that signals of all
the ortho and the meta-protons of the py ligand are distinguishable as
compared to the broad features observed at 293 K. In complex 2, five
sets of aromatic proton signals were observed for the py ligand at 198 K.
Similarly, four sets of aromatic proton signals were observed for 4Mepy
ligand in complex 3 at 198 K. The 1H NMR spectral signals of py and
4Mepy clearly suggests that the rotation around the Ru-N bond is re-
stricted at low temperature (198 K) in both the complexes. Partial
1H–1H COSY spectrum and VT 1H NMR spectral stack plot of complex 2
are shown in Fig. 2. VT 1H and 31P{1H} NMR spectral stack plots of
complex 3 have been deposited in the Supplementary Material. It is also
clear from the crystal structures that the steric environment of both the
ortho and the meta-carbon atoms of py or 4Mepy are different due to π-π
stacking (Supplementary Material).
Light yellow colored crystals were obtained by slow diffusion of
hexane into saturated solutions of complexes 2 and 3 in toluene at
273 K over a period of 10–12 days. The ORTEP view of complex 2 is
shown in Fig. 1 and of complex 3 in Supplementary Material. The
Attempts to synthesize [RuHCl(CO)(IMes)(PPh3)(Coll)] complex
(Coll = 2,4,6-collidine) were unsuccessful. The reaction did not occur
even when neat ligand was used instead of a stoichiometric quantity as
a solution in a solvent. This could be ascribed to the sterically bulky
nature of the 2,4,6-collidine ligand.
3.3. Synthesis and characterization of [RuHCl(CO)(IMes)(PPh3)(L′)]
(L′ = MeCN, 4; Me3CCN, 5) complexes
Initial attempts to isolate the nitrile complexes [RuHCl(CO)(IMes)
(PPh3)(L′)] (L′ = MeCN, 4; Me3CCN, 5) from the reaction of complex 1
with the nitrile ligand L′ (L′ = MeCN or Me3CCN), were unsuccessful
due to the labile nature of the nitrile ligand in solution. The ligands L′
Fig. 1. ORTEP view of [RuHCl(CO)(IMes)(PPh3)(py)] (2) complex.
Table 3
Selected bond distances (Å) and bond angles (°) of complexes 2 and 3.
Bond distances (Å)
(2)
(3)
Bond angles (°)
(2)
(3)
Ru(1)eC(1)
Ru(1)eC(1A)
Ru(1)eCl(1)
Ru(1)eP(1)
Ru(1)eH(1)
Ru(1)eN(3)
C(1A)eO(1A)
2.109(2)
1.801(6)
2.480(1)
2.336(1)
1.470(3)
2.299(2)
1.170(8)
2.104(2)
1.798(5)
2.483(2)
2.338(1)
1.540(2)
2.287(2)
1.175(4)
C(1)eRu(1)eP(1)
C(1)eRu(1)eN(3)
P(1)eRu(1)eN(3)
N(3)eRu(1)eH(1)
C(1A)eRu(1)eCl(1)
C(1A)eRu(1)eC(1)
P(1)eRu(1)eCl(1)
P(1)eRu(1)eH(1)
165.2(1)
100.5(1)
93.1(1)
178.1(11)
178.0(2)
89.9(15)
86.5(1)
164.7(1)
101.3(1)
93.1(1)
175.8(9)
178.1(1)
89.7(1)
86.6(1)
82.9(9)
85.1(10)
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D. Mala et al.
InorganicaChimicaActa483(2018)411–424
Table 4
Numbering scheme for the complexes.
could be easily removed under vacuum with concomitant recovery of
complex 1. Initially, the reaction of complex 1 with L′ (L′ = MeCN or
Me3CCN) was monitored by 1H and 31P{1H} NMR spectroscopy in the
solution state without attempting isolation of products (Supplementary
Material).
5a) were obtained by the displacement of the Cl ligand in 4 and 5 by
MeCN or Me3CCN ligands, respectively. A partial 1H NMR spectral stack
plots of Ru-H and methyl protons of ligand L′ (L′ = MeCN, 4; Me3CCN,
5) upon addition of different quantities of L′ have been deposited in the
Supplementary Material.
In the 1H NMR spectra of 4 and 5, a variation in the chemical shift of
the hydride and methyl moieties of the ligand (MeCN or Me3CCN) was
noted as the concentration of the free ligand (MeCN or Me3CCN) was
varied. When excess nitrile was used, a new doublet for the hydride
ligand at δ −13.35 ppm (d, 2JPH = 19.4 Hz in case of MeCN ligand) and
In solution, the nitrile ligands in these complexes were found to be
labile. The ligands L′ could be easily removed under vacuum with
concomitant recovery of complex 1, which made their isolation diffi-
cult. However, in the solid state, these ligands are tightly bound to the
metal center. Considering these observations, attempts were made to
isolate complexes 4 and 5 by carrying out the reactions in neat MeCN or
Me3CCN which led to the precipitation of these complexes. Thus,
complexes 4 and 5 were isolated in the solid state in this manner
(Scheme 2). The isolated products were characterized by 1H, 31P{1H},
and 13C{1H} NMR and IR spectroscopy. The 1H NMR spectrum is
comprised of a broad singlet for the hydride hydrogen at δ −15.97 and
−20.01 ppm for complexes 4 and 5, respectively. The 13C{1H} NMR
spectrum showed a doublet for CO at δ 203.0 ppm (2JP,C cis: 13.5 Hz;
Ru-CO, 4), 201.9 ppm (2JP,C cis: 13.5 Hz; Ru-CO, 5), and a doublet for
2
at δ −13.45 ppm (d, JPH = 18.3 Hz in case of Me3CCN ligand) were
also observed along with signals of 4 and 5 (Supplementary Material).
Esteruelas and co-workers reported that the reaction of [Os
(H)2(Cl)2(PiPr3)2] with MeCN lead to the formation of [Os(Cl)2(η2-H2)
(MeCN)(PiPr3)2] and [OsCl(η2-H2)(MeCN)2(PiPr3)2]Cl complexes. The
formation of [OsCl(η2-H2)(MeCN)2(PiPr3)2](Cl) was due to the dis-
placement of one of the chloride ligands of [Os(Cl)2(η2-H2)(MeCN)
(PiPr3)2] by MeCN [18d]. Similarly, in the present work, new hydride
complexes [RuH(CO)(IMes)(PPh3)(L′)2]Cl (L′ = MeCN, 4a; Me3CCN,
416

D. Mala et al.
InorganicaChimicaActa483(2018)411–424
Fig. 2. Partial 1H–1H COSY NMR spectrum at 198 K (left) and VT 1H NMR spectral stack plot (right) of complex 2.
Scheme 2. Synthesis of complexes 4 and 5.
carbene carbon at δ 186.9 ppm (2JP,C trans:102.5 Hz, Ru-NCN, 4),
187.5 ppm (2JP,C trans:104.0 Hz, Ru-NCN, 5). These values are con-
sistent with retention of ligand environment around the ruthenium
metal center. Selected spectral data of all the complexes have been
given in Supplementary Material. Attempts to crystallize these com-
plexes by slow evaporation in THF afforded only colorless crystals of
IMes.HCl salt (Supplementary Material). X-ray structure determination
of IMes.HCl revealed that it is a different polymorph form than that
reported in the literature earlier (Supplementary Material) [19].
from δ 1.42 (s, 293 K) to 1.11 ppm (s, 198 K) was also noted. A partial
VT NMR spectral stack plot of 4 is shown in Fig. 3.
A systematic study was carried out to unravel the concentration and
temperature-dependent behavior of chemical shifts of Ru-H and L′
(L′ = MeCN or Me3CCN) for both the complexes
4 and 5 (see
Supplementary Material). This study also gives an indication of the
existence of an equilibrium between complexes 1 and 4 or 5: complex 1
at room temperature due to labile nature of L′ (L′ = MeCN, Me3CCN)
and complexes 4 or 5 at low temperature due to binding of L′. Selected
NMR spectral data (at 293 K and 198 K) and IR carbonyl frequency data
(at room temperature) of all the complexes have been summarized in
Table S10 (Supplementary Material).
3.4. Variable temperature (VT) NMR spectral study of [RuHCl(CO)(IMes)
(PPh3)(L′)] (L′ = MeCN, 4; Me3CCN, 5)
An unusually large temperature-dependent chemical shifts were
observed for the Ru-H and methyl protons of L′ (L′ = MeCN and
Me3CCN) in complexes 4 and 5. In the 1H NMR spectrum of complex 4,
the Ru-H signal appeared as a broad singlet at δ −15.97 ppm in CD2Cl2
at 293 K. A substantial downfield shift (Δδ = 3.39 ppm) from δ −15.95
3.5. Equilibrium between complexes 1 and 4 or 5 in solution
Variable temperature NMR spectral study of complexes 4 and 5 was
carried out in tol-d8 in the temperature range of 363–193 K for 4 and
343–203 K for 5. In case of complex 4, at 363 K, the RueH signal ap-
peared at −23.37 ppm (d, 2JH,P = 24.2 Hz) which is comparable to the
chemical shift of RueH of complex 1 (−23.89 ppm, d, 2JH,P = 24.3 Hz).
2
(br. s, 293 K) to −12.58 ppm (d, JPH = 20.0 Hz,) was noted for the
hydride moiety at 198 K. An upfield shift for methyl signal of MeCN
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D. Mala et al.
InorganicaChimicaActa483(2018)411–424
Fig. 3. VT partial 1H NMR spectral stack plot for Ru-eH of [RuHCl(CO)(IMes)
(PPh3)(MeCN)] (4) in CD2Cl2.
Fig. 4. VT Partial 1H NMR spectral stack plot of [RuHCl(CO)(IMes)(PPh3)
(Me3CCN)] (5) in tol-d8.
Upon cooling a sample of complex 4, the 1H NMR spectral signal of Ru-
H merged with the baseline at 243 K and then reappeared at a
temperature < 243 K; at 193 K, this signal re-appeared as a doublet at
of thermodynamic parameters of these reactions have been summarized
in Supplementary Material.
VT NMR spectral study was carried out for the mixture of complexes
1 and 5 (∼1:1) in CD2Cl2. In the 1H NMR spectrum, an average broad
singlet at δ −22.0 ppm was noted for Ru-H at 293 K instead of the two
separate signals for both the complexes (Fig. 5). However, at 208 K, two
separate doublets corresponding to Ru-H of complexes 1 and 5 ap-
peared (Supplementary Material). The 2D-EXSY spectrum also con-
firmed that both the hydride signals are related via exchange of the
Me3CCN ligand. A correlation between both the Ru-H signals was ob-
served as cross peaks (Fig. 5).
2
−11.88 ppm (d, JH,P = 20.5 Hz). Similarly, the signal due to the me-
thyl group of MeCN ligand underwent an upfield shift of 0.49 ppm
(from 0.85 ppm at 363 K to 0.36 ppm at 193 K) (Supplementary
Material).
These observations indicate the existence of an equilibrium between
complexes 1 and 4; at high temperature, the equilibrium is shifted to
the left and at low temperature, it is shifted to the right (Scheme 3).
Thermodynamic parameters were obtained for complex 4 using the VT
NMR spectral data. The ΔH and ΔS values were calculated from a plot of
RlnKeq versus 1/T and found to be −5.75 kcalmol−1 and
−20.79 cal mol−1 K−1, respectively.
3.6. Lability of trans ligands in [RuHCl(CO)(IMes)(PPh3)(L)] (L = py, 2;
4Mepy, 3; MeCN, 4; Me3CCN, 5)
Similarly, in case of complex 5, at 343 K, the Ru-H signal appeared
2
at −23.43 ppm (d, JH,P = 24.1 Hz) which is comparable to the che-
As discussed earlier, the trans nitrile ligands in complexes 4 and 5
are labile; under vacuum complex 1 could be recovered as established
using NMR spectroscopy. However, ligands L (L = py and 4Mepy) were
bound tightly in complexes 2 and 3 even under vacuum. On the other
hand, addition of any excess Lewis base to complexes 2–5 led to the
displacement of the ligand L/L′ (L = py, 4Mepy; L′ = MeCN, Me3CCN)
trans to the hydride. For example, rapid displacement of MeCN or
Me3CCN (4 or 5) was noted upon addition of excess py or 4Mepy to
these complexes. In comparison, displacement of py or 4Mepy (2 or 3)
by MeCN or Me3CCN was found to be slower. These observations in-
dicate that ligands MeCN and Me3CCN (complexes 4 and 5) are highly
labile compared to py or 4Mepy (complexes 2 and 3). Based on these
experiments, it was found that the order of lability of the ligands in
complexes 2–5 is Me3CCN > MeCN > py∼4Mepy. This could be at-
tributed to steric and electronic effects. The greater lability of Me3CCN
could be ascribed to the high steric encumbrance that it experiences in
the sixth coordination site of the metal center in complex 5. In com-
plexes 2 and 3, intramolecular π-πstacking interactions which are evi-
dent from the crystal structure stabilize the py and 4Mepy ligands and
2
mical shift of RueH of complex 1 (−23.89 ppm, d, JH,P = 24.3 Hz).
Upon cooling the sample of complex 5, the 1H NMR spectral signal of
RueH merged with the baseline at 263 K and then reappeared as a
2
doublet at −12.22 ppm (d, JH,P = 20.0 Hz) at 193 K. Similarly, the
signal due to the methyl group of Me3CCN ligand underwent an upfield
shift of 0.42 ppm from 0.87 ppm at 343 K to 0.45 ppm at 193 K. All
temperature-dependent chemical shifts of Ru-H moiety and MeCN and
Me3CCN ligands of complexes 4 and 5 have been plotted and deposited
in the Supplementary Material. A variable temperature NMR spectral
stack plot of complex 5 is shown in Fig. 4. These observations are also
indicative of an equilibrium between complexes 1 and 5 (Scheme 3).
The thermodynamic parameters, in this case, were also calculated and
deposited in the Supplementary Material.
The negative sign of entropy and enthalpy suggest that the reaction
is feasible at low temperature. The ΔG values for both the reactions
were calculated to be 0.34 kcal/mol and 0.35 kcal/mol for complexes 4
and 5, respectively at 293 K and −1.53 kcal/mol and −1.96 kcal/mol
for complexes 4 and 5, respectively at 203 K. Details of the calculations
Scheme 3. Equilibrium between complex 1 and 4 or 5.
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D. Mala et al.
InorganicaChimicaActa483(2018)411–424
Fig. 5. 2 VT partial 1H NMR spectra for RueH signal of 1 and 5 (left) and 2D-EXSY NMR spectrum of the Ru-H region of complexes 1 and 5 in CD2Cl2 at 208 K (right).
Scheme 4. Lability of trans ligands in complexes 2–5.
hence provide better stability to these complexes. The possibility of π-π
stacking interactions in the solution state could not be ascertained since
the 1H NMR spectral signals of py and 4Mepy ligands at 293 K in
complexes 2 and 3 are broad. The broad nature of the signals could be
attributed to the rotation of these ligands around the Ru-N bond. The
bulky IMes ligand in complexes 2–5 imposes great steric crowding
around the ruthenium center. In addition to the steric factor, the elec-
tronic effect of trans ligands is also crucial for their binding. Pyridyl
ligands are better donors as compared to nitrile ligands, which could
make the nitrile ligands more labile as compared to pyridyl ligands in
this case. Solid and cone angles of Py (solid angle:1.90° and cone angle:
91.6°) and MeCN (solid angle: 1.60° and cone angle: 83.69°) suggest
greater steric hindrance in case of Py as compared to MeCN as a ligand
[20]. However, stronger binding of pyridyl ligand to the metal center in
comparison to nitrile ligands could be ascribed to its better donor
ability and intramolecular π-π stacking. The lability studies of com-
plexes 2–5 is shown schematically in Scheme 4.
comparison, complex 1 is less sterically crowded than [RuHClCO
(IMes)2] [21]. Thus, complex 1 reacts with MeCN and PMe3 (Schemes 4
and 5). However, the ruthenium center in complex 4 experiences en-
ough steric crowding that makes the MeCN ligand labile.
Addition of excess PMe3 to solutions of 2–5 in CDCl3 resulted in the
formation of [RuHCl(CO)(IMes)(PPh3)(PMe3)] via elimination of trans
ligands (py, 4Mepy, MeCN, Me3CCN), which further confirm the labile
nature of trans ligands. Upon elimination of labile ligands, [RuHCl(CO)
(IMes)(PPh3)(PMe3)] complex transforms to [RuHCl(CO)(IMes)
(PMe3)2] and [RuH(CO)(IMes)(PMe3)3](Cl) via substitution of PPh3 and
chloride ligands with another two PMe3 ligands. The reaction pro-
ceeded similarly to the one as reported earlier by us and shown in
Scheme 5 [14].
3.7. Synthesis and characterization of dihydrogen complexes bearing an
NHC ligand
Peterson and co-workers reported a similar five-coordinate complex
[RuHClCO(IMes)2] which does not react with MeCN or PMe3 ligand due
to the presence of two sterically demanding IMes groups [21]. In
Protonation of the complexes 2–5 using 1.5–2.0 equiv. of HOTf at
room temperature did not show any evidence for the formation of the
corresponding dihydrogen complexes using NMR spectroscopy. Signals
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D. Mala et al.
InorganicaChimicaActa483(2018)411–424
Scheme 5. Reaction of complexes 1–5 with PMe3.
Scheme 6. Synthesis of [RuCl(η2-H2)(CO)(IMes)(PPh3)(L)][OTf] complexes (L = py, 6; 4Mepy, 7; MeCN, 8; Me3CCN, 9).
for the imidazolium salt (IMes.HOTf) and free H2 (s, δ 4.59 ppm) were
noted in the 1H NMR spectrum. This observation indicates that the
dihydrogen complexes, if formed, are unstable at room temperature.
Therefore, protonation reactions of hydride complexes (2–5) were
carried out using HOTf (1.5–2.0 equiv.) at low temperature
(183–193 K) (Scheme 6). In this case, we obtained the dihydrogen
complexes [RuCl(η2-H2)(CO)(IMes)(PPh3)(L)]OTf (L = Py, 6; 4Mepy, 7;
MeCN, 8; Me3CCN, 9) 6–9 which were found to be stable in the tem-
perature range of 183–233 K. Upon warming beyond 233 K, they de-
composed via elimination of H2. In addition, protonation of complex 4
was also carried out in presence of excess ligand (MeCN). We noted that
the dihydrogen complex 8 was stable up to 253 K in the presence of
excess ligand. The 1H NMR spectra of complexes 6–9 are comprised of a
broad singlet in the range of −8.66 to −9.55 ppm for the η2-H2 ligand.
The intact nature of the H–H bond in these derivatives was established
by measurement of VT 1H spin-lattice relaxation time (T1, ms,
dihydrogen complexes have previously been reported in the literature
[5e,23]. This type of behavior of 1JHD was primarily noted for elongated
dihydrogen (dHH = 1.0–1.3 Å) and compressed dihydride complexes
(dHH = 1.3–1.6 Å). However, this behavior is rather unusual in case of
true dihydrogen (dHH = < 1.0 Å) or dihydride complexes
1
(dHH = > 1.6 Å). In the literature, temperature-dependence of JHD in
elongated dihydrogen complexes has been rationalized by taking into
consideration, the temperature-dependence characteristic of the H-H
distance [23]. For example, Heinekey and co-workers reported tem-
1
perature-dependent JHD values for [IrCp*(dmpm)H2][B(C6F5)4]2
(dmpm = bis(dimethylphosphino)methane) (9.0 Hz at 303 K; 7.3 Hz at
223 K) [23a]. This type of behavior was ascribed to the temperature-
dependent equilibrium between the two isomeric species: dihydrogen
and dihydride. Mort and Autschbach further supported this observation
using theoretical calculations [23c]. Similarly, an osmium dihydrogen
complex reported by Maltby et al., [Os(dppe)2Cl(H2)](PF6)
1
1
400 MHz) and JHD in the corresponding η2-HD isotopomers. Although
(dppe = Ph2PCH2CH2PPh2) in which JHD value increases with an in-
crease in sample temperature (13.6 Hz at 253 K; 14.2 Hz at 308 K) [5e].
we did not obtain T1 minima, it is apparent that the short T1 values
(Supplementary Material) indicate the intact nature of the HeH bond in
these complexes..
1
In the present work, we noted that the JHD value increases from
27.3 to 30.3 Hz upon raising the temperature of the sample from 183 to
233 K (Supplementary Material). This behavior could be rationalized by
considering the lability of the trans ligands. It was observed that the
trans ligand binds to the metal center strongly at low temperature
(183–193 K) and is either weakly bound or labile at high temperature
(233 K). As a result, the HeH bond distance in dihydrogen complexes
6–9 is dependent on the temperature. The HeH distances (dHH, Å) were
The HD isotopomers [RuCl(η2-HD)(CO)(IMes)(PPh3)(L)][OTf]
(L = py, 6-d; 4Mepy, 7-d; MeCN, 8-d; Me3CCN, 9-d) were synthesized
similar to 6–9 using DOTf instead of HOTf. The 1H{31P} NMR spectra
show 1:1:1 triplets upon nullification of the residual signal of η2–H2
moiety in the corresponding dihydrogen complexes using the inversion
recovery pulse [22]. Complex 7-d showed
a 1:1:1 triplet at δ
1
−8.72 ppm due to an η2-HD ligand (Fig. 6).
calculated from the measurement of JHD of the HD isotopomer of all
In the present study, a small variation in 1JHD coupling constant for
the η2-HD isotopomers was noted as a function of temperature. The
1JHD values increased with increase in the temperature (Supplementary
Material). Temperature-dependent 1JHD coupling constants of
the complexes [6a]. Similar trends were noted in the variation of the
HeH distance with temperature for the complexes 6–8. The variation in
the pattern in case of complex 9 could be ascribed to the poor binding
of the H2 ligand to the metal center as compared to the other trans
420

D. Mala et al.
InorganicaChimicaActa483(2018)411–424
Fig. 6. Partial 1H NMR spectrum of [RuCl(η2-HD)(CO)(IMes)(PPh3)(4Mepy)][OTf] (7-d) in CD2Cl2 at 233 K.
complexes 6–9 are examples of true dihydrogen complexes. Tempera-
ture dependence of the H-H distance in these dihydrogen complexes is a
manifestation of the lability of the trans ligands.
With a view to establish that the variation of dHH is a manifestation of
the lability of trans ligands, we attempted to synthesize the phosphine
analogues of complexes 6–7 and measure the H-H distances in them.
3.8. Synthesis and characterization of phosphine complexes
Attempts were made to prepare phosphine analogues of [RuHCl
(CO)(IMes)(PPh3)(L)] (L = py, 2; 4Mepy, 3) of the type [RuHCl(CO)
(PPh3)2(L)] (py and 4Mepy). Malecki and co-workers reported that
refluxing a mixture of [RuHCl(CO)(PPh3)3] and py in methanol affords
[RuHCl(CO)(PPh3)2(py)] (transHCl) (10) [18a]. Herein, attempts were
made to prepare and isolate its cis-isomer [RuHCl(CO)(PPh3)2(py)]
(cisHCl) (11); the cis-isomer has structural similarity to complex 2.
Reaction of [RuHCl(CO)(PPh3)3] in neat py or 4Mepy gave a mix-
ture of cis and trans isomers [RuHCl(CO)(PPh3)2(L)] [(L) = py, 10/11
(transHCl/cisHCl); 4Mepy, 12/13 (transHCl/cisHCl)] complexes at room
temperature (Scheme 7). Structures of the complexes 10–13 were es-
tablished by NMR and IR spectroscopy (Supplementary Material).
Complex 12 was also structurally characterized using X-ray
Fig. 7. Temperature versus dHH (Å) plot for 6-d, 7-d, 8-d and 9-d.
ligands, which results in a decrease in the HeH distance.
The H-H distance (dHH) in complexes 6–9 were found to be
0.98–0.93 Å in the temperature range of 183–233 K (Fig. 7) [6a]. Thus,
Scheme 7. Synthesis of ruthenium hydride complexes bearing phosphine ligands.
421

D. Mala et al.
InorganicaChimicaActa483(2018)411–424
Table 5
1JHD (Hz) and dHH (Å) data of 14-d, 15-d, 16-d, and 17-d.
Temp (K)
14-d
15-d
16-d
17-d
1JHD
dHH
1JHD
dHH
1JHD
dHH
1JHD
dHH
213
223
233
243
253
263
28.5
28.8
29.3
29.3
29.3
29.3
0.96
0.96
0.95
0.95
0.95
0.95
30.0
30.3
30.5
30.6
30.6
–
0.93
0.93
0.93
0.93
0.93
–
28.5
28.8
29.2
29.4
29.4
29.4
0.96
0.96
0.95
0.95
0.95
0.95
30.4
30.5
30.6
30.5
30.6
–
0.93
0.93
0.93
0.93
0.93
–
1
Experimental error for JHD is 0.2 to 0.4 Hz.
4Mepy and CO, hydride and chloride, and both PPh3 ligands are mu-
tually trans to each other. RueN bond length in complex 12 is com-
paratively shorter [2.187(17) Å] than the RueN bond length in [RuHCl
(CO)(IMes)(PPh3)(L)] complexes (L = py, 2; 4Mepy, 3) [RueN = 2.299
(2) Å, 2; 2.287(2)]. However, this bond length is comparable to the
previously reported ruthenium pyridyl complexes reported in the lit-
erature [18a,18b]. Crystallographic details are given in Table 2.
Fig. 8. ORTEP view of [RuHCl(CO)(PPh3)2(4Mepy)] (12).
3.9. Synthesis of ruthenium dihydrogen complexes bearing phosphine
ligands
crystallography. In the 1H NMR spectrum of the isomeric complex
mixture 10/11, the hydride hydrogen appears at δ −13.27 ppm (t,
2
2JH,P = 19.0 Hz; 10, major) and −12.49 ppm (t, JH,P = 18.0 Hz; 11,
Synthesis of [RuCl(η2-H2)(CO)(PPh3)2(L)]OTf [L = py, 14/15,
(transHCl/cisHCl); 4Mepy, 16/17, (transHCl/cisHCl) was carried out similar
to that of 6–9 at low temperature (193 K), since these dihydrogen
complexes are also unstable at room temperature (Scheme 8). The VT
spin-lattice relaxation time (T1, 400 MHz, Supplementary Material) and
1JHD values (Table 5) of the HD isotopomers [RuCl(η2-HD)(CO)
(PPh3)2(4Mepy)]OTf (16-d/17-d, major/minor) indicate the intact
nature of the HeH bond in these complexes. 1H{31P} NMR spectra of
HD isotopomers of 16-d/17-d are comprised of two triplets with in-
tensity ratios of 1:1:1 (Fig. 9).
From Table 5, it is apparent that the H-H distances (0.95–0.96 Å) are
invariant with a change in temperature as in complexes 14 and 16 and
0.93 Å in complexes 15 and 17. Since chloride is a stronger σ donor
compared to py or 4Mepy, the H-H distance in complexes 14 and 16 are
slightly longer (0.95–0.96 Å) than in 15 and 17 (0.93 Å).Whereas,
complexes [RuCl(η2-H2)(CO)(IMes)(PPh3)(L)][OTf] (L = py, 6; 4Mepy,
7) which are analogous to complexes 15 and 17, show temperature-
dependent HeH bond distances (0.93–0.98 Å in the temperature range
of 183–243 K). Complexes 15 and 17 have the PPh3 ligand in place of
the IMes ligand. IMes is a strong sigma donor as compared to PPh3. In
addition, IMes is sterically bulkier than PPh3. As it was discussed ear-
lier, presence of the bulky IMes ligand in complexes 6–7 imposes steric
minor). The 31P{1H} NMR spectrum shows two singlets at δ 46.1 (10,
major) and 44.1 ppm (11, minor). In the 1H NMR spectrum of 12/13,
2
two triplets at −13.28 ppm (t, JH,P = 19.0 Hz; 12, major) and
2
−12.48 ppm (t, JH,P = 19.0 Hz; 13, minor) were noted for the Ru-H
ligand (Supplementary Material). The 31P{1H} NMR spectrum showed
two singlets at δ 46.0 (12, major) and 43.9 ppm (13, minor). The
13C{1H} NMR spectrum showed a triplet at δ 205.4 ppm for the CO
ligand due to coupling (2JC,P = 16.0 Hz) with two cis phosphorus atoms
for complex 12. However, due to the small amount of complex 13
present in solution, the signal for its CO ligand could not be detected in
the 13C{1H} NMR spectrum. The IR stretching frequencies for CO li-
gands in 10–13 were noted at 1938 cm−1 10; 1917 cm−1
, , 11;
1907 cm−1, 12; 1890 cm−1, 13; these are greater than those of IMes
analogues (1877 cm−1, 2; 1874 cm−1, 3) which is consistent with the
stronger σ donor ability of IMes over the PPh3 ligand.
Yellow colored crystals of complex 12 were obtained via slow
evaporation of solvent from a saturated solution of 12 in methanol at
room temperature [18a]. The structure was established by X-ray crys-
tallography. ORTEP view of complex 12 is shown in Fig. 8. The co-
ordination geometry around the metal center could be described as a
distorted octahedron. Crystal structure of complex 12 reveals that
Scheme 8. Synthesis of ruthenium dihydrogen complexes bearing phosphine ligands.
422

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InorganicaChimicaActa483(2018)411–424
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