Influence of the electronics of the phosphine ligands on the H-H bond elongation in dihydrogen complexes

Article abstract of DOI:10.1021/ic7016769

Five new monocationic dihydrogen complexes of ruthenium of the type trans-[RuCl(η²-H₂)(PP)₂][BF₄] (PP = bis-1,2(diarylphosphino)ethane, aryl = p-fluorobenzyl, 1a, benzyl, 2a, m-methylbenzyl, 3a, p-methylbenzyl, 4a, p-isopropylbenzyl, 5a) have been prepared by protonating the precursor hydride complexes trans-[RuCl(H)(PP) ₂] using HBF4·OEt₂. The dihydrogen complexes are quite stable and have been isolated in the solid state. The intact nature of the H-H bond in these derivatives has been established from the short spin-lattice relaxation times (T₁, ms) and observation of substantial H, D couplings in the HD isotopomers. The H-H bond distances (d_HH, A) increase systematically from 0.97 to 1.03 A as the electron-donor ability of the substituent on the diphosphine ligand increases from the p-fluorobenzyl to the p-isopropylbenzyl moiety. The d_HH in trans-[Ru(η²-H₂)(Cl)((C₆H ₅-CH₂)₂PCH₂CH₂P(CH ₂C₆H₅)₂)₂][BF ₄], 2a, was found to be 1.08(5) A by X-ray crystallography. In addition, two new 16-electron dicationic dihydrogen complexes of the type [Ru(η²-H₂)(PP)₂][OTf]₂ (PP = (ArCH₂)₂PCH₂CH₂P(CH ₂-Ar)₂, Ar = m-CH₃C₆H₄-, 6a, p-CH₃C₆H₄-, 7a) have also been prepared and characterized. These derivatives were found to possess elongated dihydrogen ligands.

Full text of DOI:10.1021/ic7016769

Inorg. Chem. 2008, 47, 548−557
Influence of the Electronics of the Phosphine Ligands on the H
Elongation in Dihydrogen Complexes
−H Bond
Saikat Dutta, Balaji R. Jagirdar,* and Munirathinam Nethaji
Department of Inorganic & Physical Chemistry, Indian Institute of Science,
Bangalore 560 012, India
Received August 26, 2007
Five new monocationic dihydrogen complexes of ruthenium of the type trans-[RuCl(
1,2(diarylphosphino)ethane, aryl p-fluorobenzyl, 1a, benzyl, 2a, m-methylbenzyl, 3a, p-methylbenzyl, 4a,
p-isopropylbenzyl, 5a) have been prepared by protonating the precursor hydride complexes trans-[RuCl(H)(PP)2]
using HBF4 OEt2. The dihydrogen complexes are quite stable and have been isolated in the solid state. The intact
nature of the H H bond in these derivatives has been established from the short spin lattice relaxation times (T1,
ms) and observation of substantial H, D couplings in the HD isotopomers. The H H bond distances (dHH, Å)
increase systematically from 0.97 to 1.03 Å as the electron-donor ability of the substituent on the diphosphine
ligand increases from the p-fluorobenzyl to the p-isopropylbenzyl moiety. The dHH in trans-[Ru(
2-H2)(Cl)((C6H5-
CH2)2PCH2CH2P(CH2C6H5)2)2][BF4], 2a, was found to be 1.08(5) Å by X-ray crystallography. In addition, two new
16-electron dicationic dihydrogen complexes of the type [Ru( (ArCH2)2PCH2CH2P(CH2-
2-H2)(PP)2][OTf]2 (PP
Ar)2, Ar m-CH3C6H4 , 6a, p-CH3C6H4 , 7a) have also been prepared and characterized. These derivatives were
found to possess elongated dihydrogen ligands.
η ) bis-
2-H2)(PP)2][BF4] (PP
)
‚
−
−
−
η
η
)
)
−
−
Introduction
for the oxidative addition of H2 to a metal center and find
out at what point the H-H bond can be considered to be
broken.
The binding of molecular hydrogen to a metal center
leads to elongation of the H-H bond and subsequently
to its cleavage along the reaction coordinate for the oxi-
dative addition of H2. There has been considerable interest
in the study of the activation of dihydrogen and map-
ping out the reaction coordinate for the homolytic cleavage
of H2 on a metal center. A large number of dihy-
drogen complexes reported to date possess H-H distances
ranging from 0.8 to 1.0 Å. Relatively fewer examples of
elongated dihydrogen complexes are known, wherein the
H-H distances fall in the range of 1.0-1.5 Å.1,2 In this
regard, the study of the molecules that possess elongated
dihydrogen moieties is of great interest because of the
relevance and significance in important catalytic processes
such as hydrogenation, hydrogenolysis, and hydroformyla-
tion. One of the current goals in this area is to study the
smooth gradation of the H-H distances along the continuum
In order to realize complexes with H-H distances in this
continuum, a very systematic study should be carried out
by subtly varying the electron-donor abilities of the coligands
in the complex such that the back-donation from the M(dπ)
to the H2(σ*) is increased in a systematic manner. For
example, employing coligands that can be fine tuned by way
of varying the substituents in a manner which will result in
increased back-donation can lead to dihydrogen complexes
with elongated H2 ligands along the reaction coordinate for
the oxidative addition of H2 to a metal center. This can be
achieved by employing phosphorus-based coligands since
their steric as well as electronic properties can be varied in
a very systematic manner. Kubas and co-workers studied the
influence of the electronics of the substituted and unsubsti-
tuted benzyl diphosphine ligands on the dihydrogen versus
dihydride coordination of a series of molybdenum complexes
of the type [Mo(H2)(CO){(RCH2)2PCH2CH2P(CH2R)}2].
They found that the binding mode (η2-H2 or dihydride) is
dictated by the nature of R: an η2-H2 complex is formed
when R is an electron-withdrawing group, whereas a
* To whom correspondence should be addressed. E-mail: jagirdar@
ipc.iisc.ernet.in.
(1) Kubas, G. J. Metal Dihydrogen and σ Bond Complexes; Kluwer
Academic/Plenum Publishers: New York, 2001.
(2) Heinekey, D. M.; Lledo´s, A.; Lluch, J. M. Chem. Soc. ReV. 2004, 33,
175-182.
548 Inorganic Chemistry, Vol. 47, No. 2, 2008
10.1021/ic7016769 CCC: $40.75
© 2008 American Chemical Society
Published on Web 12/15/2007

Influence of Phosphine Ligands on H-H Bond Elongation
dihydride results when R is an electron-donating group.3 We
earlier reported the influence of the sterics and electronics
of certain phosphorus-containing ligands (trans to H2 ligand)
on the properties of the bound dihydrogen.4 In this paper,
we report the synthesis and characterization of some new
dihydrogen complexes of ruthenium bearing the bis(1,2-
diarylphosphino)ethane ligands wherein the aryl group is a
benzyl moiety with a substituent (p-fluoro, H, m-methyl,
p-methyl, p-isopropyl). The electron-donor ability of the
chelating phosphine in this series of complexes can be varied
in a systematic manner by varying the substituent on the
benzyl group. In combination with a good π donor like a
chloride trans to the bound H2 and by use of a suitable
substituent, the donor ability of the chelating phosphine was
increased in small increments along the series resulting in
elongation of the H-H bonds in a systematic manner. In
addition, we also report the synthesis and characterization
of two new dicationic, 16-electron dihydrogen complexes
in which the H-H bonds are elongated. Preliminary aspects
of this work were communicated earlier.5
Experimental Section
General Procedures. All reactions were carried out under an
atmosphere of nitrogen or argon at room temperature using standard
Schlenk and inert atmosphere techniques unless otherwise noted.6
Solvents used for preparation of the dihydrogen complexes were
thoroughly saturated with H2 or Ar just before use. 1H and 31P NMR
spectral data were acquired using an Avance Bruker 400 MHz
spectrometer. The 31P NMR spectra were recorded relative to 85%
H3PO4 (aqueous solution) as an external standard. Variable-
temperature 1H T1 measurements were carried out at 400 MHz using
the inversion recovery method (180°-τ-90°).7 Mass spectral
analyses were carried out using a Micromass Q-TOF instrument at
the Department of Organic Chemistry, Indian Institute of Science.
Magnesium required for the synthesis of the Grignard reagents was
activated by the literature method.8 The ligands (ArCH2)2PCH2-
CH2P(CH2Ar)2 (Ar ) p-FC6H4, C6H5-, m-CH3C6H4-, p-CH3C6H4-,
p-iPrC6H4-, p-tBuC6H4-) and the metal complex [Ru(H)(Cl)-
(PPh3)3] were prepared adopting literature procedures.3,9 We
reported the synthesis and characterization of trans-[Ru(H)(Cl)-
((C6H5CH2)2PCH2CH2P(CH2C6H5)2)2], 2, recently.5 (p-iPrC6H4-
CH2)2PCH2CH2P(CH2-C6H4-p-iPr)2 and (p-tBuC6H4-CH2)2PCH2-
CH2P(CH2-C6H4-p-tBu)2 have not been reported previously.
Characterization Data for (p-iPrC6H4-CH2)2PCH2CH2P-
(CH2-C6H4-p-iPr)2. 1H NMR (CD2Cl2): δ 1.21 (d, 24H, ((CH3)2-
(3) Luo, X.-L.; Kubas, G. J.; Burns, C. J.; Eckert, J. Inorg. Chem. 1994,
33, 5219-5229.
(4) (a) Mathew, N.; Jagirdar, B. R.; Gopalan, R. S.; Kulkarni, G. U.
Organometallics 2000, 19, 4506-4517. (b) Mathew, N.; Jagirdar, B.
R.; Ranganathan, A. Inorg. Chem. 2003, 42, 187-197. (c) Nanishan-
kar, H. V.; Nethaji, M.; Jagirdar, B. R. Eur. J. Inorg. Chem. 2004,
3048-3056.
(5) Dutta, S.; Jagirdar, B. R. Inorg. Chem. 2006, 45, 7047-7049.
(6) (a) Shriver, D. F.; Drezdon, M. A. The Manipulation of Air SensitiVe
Compounds, 2nd ed.; Wiley: New York, 1986. (b) Herzog, S.;
Dehnert, J.; Luhder, K. In Technique of Inorganic Chemistry;
Johnassen, H. B., Ed.; Interscience: New York, 1969; Vol. VII.
(7) Hamilton, D. G.; Crabtree, R. H. J. Am. Chem. Soc. 1988, 110, 4126-
4133.
(8) Karen, V.; Barker, J. M.; Brown, N. H.; Skarnulis, A. J.; Sexton, A.
J. Org. Chem. 1991, 56, 698-703.
(9) (a) Schunn, R. A.; Wonchoba, E. R. Inorg. Synth. 1971, 13, 131-
134. (b) Hallman, P. S.; McGarvey, B. R.; Willkinson, G. J. Chem.
Soc. (A) 1968, 12, 3143-3150.
Inorganic Chemistry, Vol. 47, No. 2, 2008 549

Dutta et al.
CH)), 1.27 (br s, 4H, -CH2CH2-), 2.62 (d, 4H, CH2Ar, J(H,H) )
12.7 Hz), 2.70 (d, 4H, CH2Ar, J(H,H) ) 12.7 Hz), 2.85 (sep, 4H,
(CH3)2CH), J(H,H) ) 6.8 Hz), 7.10 (d, 8H, C6H4, J(H,H) ) 7.6
Hz), 7.23 (d, 8H, C6H4, J(H,H) ) 7.6 Hz). 31P{1H} NMR (CD2-
Cl2): δ -14.6 (s). QTOF-MS: m/z ) 621 [M+].
Characterization Data for (p-tBuC6H4-CH2)2PCH2CH2P-
(CH2-C6H4-p-tBu)2. 1H NMR (C6D6): δ 1.20 (br s, 4H,
-CH2CH2-), 1.29 (s, 36H, ((CH3)3C)), 2.65 (d, 4H, CH2Ar, J(H,H)
) 13.8 Hz,), 2.75 (d, 4H, CH2Ar, J(H,H) )13.7 Hz,), 6.99 (d, 8H,
C6H4, J(H,H) ) 7.8 Hz), 7.09 (d, 8H, C6H4, J(H,H) ) 7.8 Hz).
31P{1H} NMR (C6D6): δ -14.7 (s). QTOF-MS: m/z ) 678 [M+].
Preparation of trans-[Ru(H)(Cl)(PP)2] (PP ) (ArCH2)2-
PCH2CH2P(CH2Ar)2, Ar ) p-FC6H4, 1, C6H5-, 2, m-MeC6H4-,
3, p-MeC6H4-, 4, p-iPrC6H4-, 5). A mixture of [Ru(H)(Cl)-
(PPh3)3] (0.100 g, 0.108 mmol) and (p-FC6H4CH2)2PC2H4P-
(CH2C6H4p-F)2 (0.250 g, 0.475 mmol, ∼2.5 equiv) in toluene (10
mL) was refluxed for 10 min. The purple solution turned yellow
during this process. The reaction mixture was cooled to room
temperature, and the volatiles were removed under vacuum. The
yellowish-white solid product was washed using petroleum ether
(3 × 5 mL) and then dried under vacuum to obtain a yellowish-
white solid of trans-[Ru(H)(Cl)((p-FC6H4CH2)2PC2H4P(CH2C6H4-
p-F)2)2], 1. Yield: 0.065 g (50%). Compounds 2-5 were similarly
prepared. The synthetic details for complexes 2-5 are given in
the Supporting Information. The NMR spectral data of these
complexes have been collected in Table 1. The mass spectral data
of the complexes are as follows. 1: m/z ) 1154 [M+ - HCl]. 2:
m/z ) 1011 [M+ - Cl-]. 3: m/z ) 1121 [M+ - (H2 + Cl-)]. 4:
m/z ) 1157 [M+ - H+], 1122 [M+ - HCl]. 5: m/z ) 1346
[M+ - HCl].
Preparationoftrans-[Ru(η2-H2)(Cl)(PP)2][BF4](PP)(ArCH2)2-
PCH2CH2P(CH2Ar)2,Ar)p-FC6H4-,1a,C6H5-,2a,m-CH3C6H4-,
3a, p-CH3C6H4-, 4a, p-iPrC6H4-, 5a). A H2-saturated CH2Cl2
solution (10 mL) of trans-[Ru(H)(Cl)((p-FC6H4CH2)2PC2H4P-
(CH2C6H4p-F)2)2], 1 (0.050 g, 0.042 mmol), was cooled to 273 K.
To this solution, 54% HBF4‚OEt2 (5.7 µL, 0.043 mmol, 1 equiv)
was added, and the reaction mixture was stirred for 10 min. Solvent
was stripped under vacuum, and the solid residue of trans-[Ru(η2-
H2)(Cl)((p-FC6H4CH2)2PC2H4P(CH2C6H4p-F)2)2][BF4], 1a, was iso-
lated. Yield: 0.040 g (74%). The dihydrogen complexes 2a-5a
were prepared by a similar procedure. The synthetic details are given
in the Supporting Information. The NMR spectral data of these
complexes have been collected in Table 2. The mass spectral data
of the complexes are as follows. 1a: m/z ) 1189 [M+ - (H2 +
BF4-)], 1153 [M+ - (H2 + Cl- + BF4-)]. 2a: m/z ) 1046 [M+
- (H2 + BF4-)]. 3a: m/z ) 1122 [M+ - (H2 + Cl- + BF4-)]. 4a:
m/z ) 1122 [M+ - (H2 + Cl- + BF4-)].
Preparation of trans-[Ru(η2-HD)(Cl)(PP)2][BF4] (PP )
(ArCH2)2PCH2CH2P(CH2Ar)2, Ar ) p-FC6H4-, 1a-d1, C6H5-,
2a-d1, m-CH3C6H4-, 3a-d1, p-CH3C6H4-, 4a-d1, p-iPrC6H4-, 5a-
d1). Deuterium gas was purged through a solution of trans-[Ru-
(η2-H2)(Cl)((p-FC6H4CH2)2PC2H4P(CH2C6H4p-F)2)2][BF4] (0.020 g,
0.014 mmol) in 0.6 mL of CD2Cl2 for 30 min with the sample
(10) (a) Albeniz et al. proposed that isotope scrambling between a
dihydrogen complex and D2 gas takes place due to a combination of
the lability and acidity of the dihydrogen ligand: Albeniz, A. C.;
Heinekey, D. M.; Crabtree, R. H. Inorg. Chem. 1991, 30, 3632-3635.
(b) Bautista, M. T.; Earl, K. A.; Maltby, P. A.; Morris, R. H.;
Schweitzer, C. T.; Sella, A. J. Am. Chem. Soc. 1988, 110, 7031-
7036. (c) Chinn, M. S.; Heinekey, D. M.; Payne, N. G.; Sofield, C.
D. Organometallics 1989, 8, 1824-1826. (d) Earl, K. A.; Jia, G.;
Maltby, P. A.; Morris, R. H. J. Am. Chem. Soc. 1991, 113, 3027-
3039. (e) Chin, B.; Lough, A. J.; Morris, R. H.; Schweitzer, C. T.; D’
Agostino, C. Inorg. Chem. 1994, 33, 6278-6288.
550 Inorganic Chemistry, Vol. 47, No. 2, 2008

Influence of Phosphine Ligands on H-H Bond Elongation
Table 3. H, D Coupling Constant and the dHH (Å) Data for
trans-[Ru(η2-HD)(Cl)(PP)2][BF4] (PP ) (ArCH2)2PCH2CH2P(CH2Ar)2,
Ar ) p-FC6H4- (1a-d1), C6H5- (2a-d1), m-CH3C6H4- (3a-d1),
p-CH3C6H4- (4a-d1), p-iPrC6H4- (5a-d1)) and [Ru(η2-HD)(PP)2][OTf]2
(PP ) (ArCH2)2PCH2CH2P(CH2Ar)2, Ar ) m-CH3C6H4- (6a-d1),
p-CH3C6H4- (7a-d1))
compd no./Ar
δ(Ru-η2-HD) J(H,D), Hz J(HD,Pcis), Hz
dHH, Å
1a-d1/p-FC6H4-a -13.88 (1:1:1 t qnt) 26.4 ( 0.3 6.8 ( 0.2 0.975 ( 0.005
2a-d1/C6H5- -13.82 (1:1:1 t qnt) 25.4 ( 0.1 6.8 ( 0.1 0.992 ( 0.001
3a-d1/m-CH3C6H4- -13.81 (1:1:1 t)a
24.9 ( 0.1 1.001 ( 0.001
4a-d1/p-CH3C6H4- -13.95 (1:1:1 t qnt) 24.4 ( 0.2 6.8 ( 0.1 1.016 ( 0.003
5a-d1/p-iPrC6H4- -13.82 (1:1:1 t)a
23.4 ( 0.3 1.035 ( 0.005
6a-d1/m-CH3C6H4- -18.05 (1:1:1 t qnt) 22.4 ( 0.1 8.8 ( 0.3 1.041 ( 0.001
7a-d1/p-CH3C6H4- -18.07 (1:1:1 t qnt) 22.0 ( 0.2 7.8 ( 0.2 1.052 ( 0.003
1
a Obtained from H{31P} NMR spectra.
tube dipped in an ice-salt mixture. The sample tube was then set
on a rotary shaker for one-half an hour for deuterium incorporation.
The HD isotopomer10a trans-[Ru(η2-HD)(Cl)((p-FC6H4CH2)2PC2H4P-
(CH2C6H4p-F)2)2][BF4], 1a-d1, was observed by NMR spectroscopy
by nullifying the residual signal due to the η2-H2 species using an
inversion recovery pulse.10b-e Complexes 2a-d1, 3a-d1, 4a-d1, and
5a-d1 were prepared similarly. The HD coupling constant data are
summarized in Table 3.
Preparation of [Ru(H)(PP)2][OTf] (PP ) (ArCH2)2PCH2CH2P-
(CH2Ar)2, Ar ) m-CH3C6H4-, 6, p-CH3C6H4-, 7). To a CD2-
Cl2 solution (0.6 mL) of trans-[Ru(H)(Cl)((m-H3C-C6H4CH2)2-
PC2H4P(CH2C6H4-m-CH3)2)2], 3 (0.020 g, 0.017 mmol), in a 5 mm
NMR tube, cooled to 273 K, was added MeOTf (3.9 µL, 0.031
mmol), and it was shaken for 15 min. The colorless solution turned
yellow, resulting in formation of [Ru(H)((m-H3C-C6H4CH2)2-
PC2H4P(CH2C6H4-m-CH3)2)2][OTf], 6. Compound 7 was prepared
similarly. Both of these complexes were found to be quite unstable,
and attempts to isolate them resulted in the decomposed material;
therefore, they were characterized by NMR spectroscopy only in
solution. The NMR spectral data for these complexes are sum-
marized in Table 4.
Preparationof[Ru(η2-H2)(PP)2][OTf]2(PP)(ArCH2)2PCH2CH2P-
(CH2Ar)2, Ar ) m-CH3C6H4-, 6a, p-CH3C6H4-, 7a). The NMR
tubes containing solutions of the five-coordinate complexes, 6 and
7 prepared as described above, were cooled to 195 K, and then 2
equiv of HOTf (2.8 µL, 0.03 mmol) was added. The yellow-colored
solutions turned reddish-yellow. The dihydrogen complexes [Ru-
(η2-H2)(PP)2][OTf]2 (PP ) (ArCH2)2PCH2CH2P(CH2Ar)2, Ar )
m-CH3C6H4-, 6a, p-CH3C6H4-, 7a) were characterized in solution
by NMR spectroscopy. The NMR spectral features of these
complexes are summarized in Table 5.
Preparation of [Ru(η2-HD)(PP)2][OTf]2 (PP ) (ArCH2)2-
PCH2CH2P(CH2Ar)2, Ar ) m-CH3C6H4-, 6a-d1, p-CH3C6H4-,
7a-d1). To the NMR tubes containing the five-coordinate complexes,
6 and 7 at 195 K, prepared as described above, was added 2 equiv
of DOTf (2.8 µL, 0.03 mmol), and it was shaken for 10 min. The
HD isotopomers formed [Ru(η2-HD)(PP)2][OTf]2 (PP ) (ArCH2)2-
PCH2CH2P(CH2Ar)2, Ar ) m-CH3C6H4-, 6a-d1, p-CH3C6H4-, 7a-
d1) were characterized using NMR spectroscopy. The residual signal
due to the η2-H2 moiety in these derivatives were nullified using
an inversion recovery pulse.10b-e The HD coupling constant data
are summarized in Table 3.
X-ray Structure Determination of trans-[Ru(η2-H2)(Cl)-
((C6H5CH2)2PCH2CH2P(CH2C6H5)2)2][BF4], 2a. A suitable crystal
was mounted in a glass capillary. The unit cell parameters and
intensity data were collected at 293 K on a Bruker SMART APEX
CCD diffractometer equipped with a fine-focus Mo KR X-ray
source. The SMART software package was used for data acquisi-
Inorganic Chemistry, Vol. 47, No. 2, 2008 551

Dutta et al.
Table 5. 1H, 31P{1H}, and 19F NMR Spectral Data (δ) of [Ru(η2-H2)(PP)2][OTf]2 (PP ) (ArCH2)2PCH2CH2P(CH2Ar)2, Ar ) m-CH3C6H4- (6a),
p-CH3C6H4- (7a)) Complexes in CD2Cl2
1H
31P
19F
compd no./Ar
δ(Ru-η2-H2)
δ(CH2CH2)
δ(CH2Ar)
δ(Ar-R)
δ(Ar)
δ(PP)
δ(OTf)
6a/m-CH3C6H4-
7a/p-CH3C6H4-
-18.06 (br s, 2H)
-18.09 (br s, 2H)
0.86-1.22 (br s, 8H)
0.64-1.28 (br s, 8H)
2.65-4.09 (m, 16H)
2.70-4.04 (m, 16H)
2.31 (br m, 24H)
2.28 (br m, 24H)
6.51-7.94 (br m, 32H)
6.59-7.85 (br m, 32H)
52.9 (s)
54.7 (s)
-80.2 (s)
-80.1 (s)
Table 6. Crystallographic Data for
trans-[Ru(η2-H2)(Cl)((C6H5CH2)2PCH2CH2P(CH2C6H5)2)2][BF4], 2a
propriate Grignard reagent in Et2O at 273 K in fairly good
yields (eq 1). The magnesium metal required for preparation
of the Grignard reagents must be activated mechanically by
vigorously agitating under Ar for 48 h.8 The diphosphine
ligands are highly moisture sensitive in solution but stable
in the solid state in air for a few hours without getting
oxidized. The (p-tBu-C6H4CH2)2PCH2CH2P(CH2C6H4-p-t-
Bu)2 ligand was found to be extremely unstable in solution,
getting rapidly oxidized even in the presence of trace
quantities of oxygen. The oxidized ligand shows a singlet at
δ 20.1 in the 31P{1H} NMR spectrum.
formula
fw
cryst syst
space group
a, Å
b, Å
c, Å
R, deg
â, deg
γ, deg
V, Å3
C61H68BCl3F4P4Ru
1219.26
triclinic
Ph1
11.935(3)
14.451(4)
17.166(5)
77.478(4)
87.307(4)
79.995(4)
2846.2(13)
2
Z
D
calcd, g/cm3
T, K
1.423
293(2)
λ, Å
0.71073
0.581
0.0400
µ, mm-1
Ra
a
Rw
0.1033
a R ) Σ(|Fo| - |Fc|)/Σ|Fo|; Rw ) [Σw(|Fo| - |Fc|)2/Σw|Fo| ]1/2 (based
on reflections with I > 2σ(I)).
2
The Hammett constants (σ) of the substituents on the
benzyl moiety of the phosphine phosphorus systematically
vary along this series from the substituent: p-F (0.06), H
(0), m-CH3 (-0.07), p-CH3 (-0.17), p-iPr (not available),
p-tBu (-0.20), reflecting their electron-donor abilities in that
order.15 There is, however, no perfect correlation between
the Hammett constants and the 31P{1H} NMR chemical shifts
of these ligands.
Table 7. Selected Bond Lengths (Å) and Angles (deg) for
trans-[Ru(η2-H2)(Cl)((C6H5CH2)2PCH2CH2P(CH2C6H5)2)2][BF4] 2a
H(100)-H(101)
Ru(1)-P(1)
1.08(5)
2.3776(8)
2.3514(9)
83.37(3)
173.71(3)
95.39(3)
98.72(3)
178.21(3)
Ru(1)-P(3)
2.3755(9)
2.3571(8)
2.4453(8)
82.64(3)
88.08(3)
86.33(3)
86.14(3)
94.93(3)
Ru(1)-P(4)
Ru(1)-P(2)
Ru(1)-Cl(1)
P(1)-Ru(1)-P(2)
P(1)-Ru(1)-P(3)
P(1)-Ru(1)-P(4)
P(2)-Ru(1)-P(3)
P(2)-Ru(1)-P(4)
P(3)-Ru(1)-P(4)
P(1)-Ru(1)-Cl(1)
P(2)-Ru(1)-Cl(1)
P(3)-Ru(1)-Cl(1)
P(4)-Ru(1)-Cl(1)
Synthesis of trans-[Ru(H)Cl((ArCH2)2PCH2CH2P-
(CH2Ar)2)2] (Ar ) p-FC6H4-, 1, C6H5-, 2, m-CH3C6H4-,
3, p-CH3C6H4-, 4, p-iPrC6H4-, 5). The ruthenium hydride
chloride complexes (1-5) were prepared by reacting [Ru-
(H)Cl(PPh3)3] with the appropriate diphosphine ligand adopt-
ing the procedure reported by Manuel et al. for certain
analogous hydride derivatives (eq 2).16 These new hydride
complexes, isolated as yellow or reddish-yellow solids, are
stable in air for short periods of time. The 31P{1H} NMR
spectra consist of only one resonance for all the phosphorus
atoms, indicating the planar disposition of the chelating
phosphines and the hydride and the chloride ligands, in trans
conformation. Attempts to prepare the trans-[Ru(H)Cl((p-t-
Bu-C6H4CH2)2PCH2CH2P(CH2C6H4-p-tBu)2)2] complex were
unsuccessful; we always ended up getting the oxidized ligand
together with some unreacted starting ruthenium complex.
tion, and the SAINT software package was used for data re-
duction.11 Absorption corrections were made using the SADABS
program12 and empirical method.13 The structure was solved and
refined using the SHELX programs.14 The ruthenium atom position
was observed by the Patterson method, and the non-hydrogen atoms
were located by successive difference Fourier maps and refined
anisotropically. The hydrogen atoms attached to ruthenium were
located from the difference Fourier map and refined isotropically.
All other hydrogen atoms were generated in idealized positions and
refined in a riding model. The crystallographic data are summarized
in Table 6, and the pertinent bond lengths and angles are listed in
Table 7.
Results and Discussion
Ligand Synthesis. A set of bis-1,2(dibenzylphosphino)-
ethane ligands of the type (ArCH2)2PCH2CH2P(CH2Ar)2 (Ar
) p-FC6H4-, C6H5-, m-CH3C6H4-, p-CH3C6H4-, p-i-
PrC6H4-, p-tBuC6H4-) were prepared following the proce-
dure reported by Kubas and co-workers.3 These ligands were
obtained by treatment of Cl2PCH2CH2PCl2 with the ap-
1
The H NMR chemical shifts for the hydride hydrogen
fall in the range from -19.63 to -19.98; the chemical shifts
do not correlate with the Hammett constants, although the
electron-donor ability increases from the p-F to the p-iPr
substituent in this series of complexes. The 1H NMR spectra
consist of a quintet resonance for the hydride hydrogen due
to coupling with the four cis-phosphorus atoms with J(H,
(11) SMART and SAINT, version 6.22a; Bruker AXS: Madison, WI, 1999.
(12) Sheldrick, G. M. SADABS, version 2; Multiscan Absorption Correction
program; University of Go¨ttingen: Go¨tingen, Germany, 2001.
(13) Blessing, R. H. Acta Crystallogr., Sect. A 1995, 51, 33-38.
(14) Sheldrick, G. M. SHELXL-97 Program for the solution of Crystal
Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997.
(15) Blackburn, E. V.; Tanner, D. D. J. Am. Chem. Soc. 1980, 102, 692-
697.
(16) Manuel, J.-T.; Carmen, M. P.; Valerga, P. Inorg. Chem. 1994, 33,
3515-3520.
552 Inorganic Chemistry, Vol. 47, No. 2, 2008

Influence of Phosphine Ligands on H-H Bond Elongation
Pcis) ) 19.5-20.0 Hz and the two methylene hydrogens of
the benzyl groups are diastereotopic showing up as four
doublets with geminal couplings ranging from 12.6 to 17.6
Hz. However, in the case of the p-iPr complex, three doublets
are well resolved whereas the fourth one overlaps with other
signals. Similar spectral characteristics were noted earlier
for a series of molybdenum complexes bearing benzylphos-
phine-based coligands.3 The 31P{1H} NMR chemical shifts
experience upfield shifts, albeit small, with increasing
basicities of the chelating phosphine ligand.
Preparation and Properties of trans-[Ru(η2-H2)(Cl)-
((ArCH2)2PCH2CH2P(CH2Ar)2)2][BF4] (Ar ) p-FC6H4-,
1a, C6H5-, 2a, m-CH3C6H4-, 3a, p-CH3C6H4-, 4a,
p-iPrC6H4-, 5a). Protonation of the hydride complexes 1-5
using HBF4‚OEt2 resulted in the dihydrogen complexes 1a-
5a (eq 3). These dihydrogen complexes were found to be
quite stable, and therefore, isolation in the solid state was
possible in the cases of 1a-4a, whereas 5a was characterized
in solution only by NMR spectroscopy. The products were
obtained in fairly good yields. Crystals suitable for X-ray
crystallographic study were obtained in the case of 2a.
Figure 1. Plot of T1 (400 MHz) versus temperature for complexes 1a-
5a.
parabolic behavior as shown in Figure 1.17 The T1 minima
together with the H-H distances (dHH (slow and fast), Å)
calculated from T1 (min) have been tabulated in the Sup-
porting Information.18 T1 (min) ranges from 14 to 22 ms.
More definitive evidence for the intact nature of the H-H
bond in these derivatives was obtained from the J(H,D)
couplings for the HD isotopomers. The HD isotopomers 1a-
d1, 2a-d1, 3a-d1, 4a-d1, and 5a-d1 were prepared by treating
the η2-H2 complexes with D2(g) for long periods of time at
273 K. Due to the overlapping of signals of the η2-H2 and
1
η2-HD species, the H NMR spectral signal appeared as a
broad singlet with a shoulder upfield to the broad signal.
1
Nullification of the η2-H2 signal in the H NMR spectrum
by the inversion recovery pulse sequence using the relation-
ship T1 ) τnull/ln 2 and the known T1 value of the dihydrogen
complex at room temperature10b-e gave the 1:1:1 triplet which
is expected for the HD isotopomer. Each of the triplet signals
was further split into quintets due to HD-Pcis couplings. As
a representative example, the HD and HD-Pcis couplings in
the case of 4a-d1 are shown in Figure 2.
The H-H distances (dHH, Å) in complexes 1a-5a were
calculated from the inverse relationship between the J(H,D)
and the dHH19 and are listed in Table 3. dHH varies from 0.97
Å to 1.03 Å with a systematic increment, albeit small from
1a to 5a. The increase in the H-H distances along this series
of dihydrogen complexes is a reflection of the electron-donor
ability of the diphosphine ligand (Hammett constants: p-F
(0.06), H (0), m-CH3 (-0.07), p-CH3 (-0.17), p-iPr (not
available)), increasing along this order: Ar ) p-FC6H4- <
C6H5- < m-CH3C6H4 < p-CH3C6H4- < p-iPrC6H4-. A plot
of the J(H,D) versus the appropriate σ values of these ligands
is shown in Figure 3. It is interesting to note that Axenrod
et al. observed a linear correlation between the 15N-H
coupling constants and the appropriate Hammett σ constants
The 1H NMR spectra of the dihydrogen complexes consist
of a broad singlet for the bound H2 ligand in the range from
δ -13.78 to -13.88. As in the precursor hydrides, the
benzylic hydrogens in the dihydrogen complexes are dias-
tereotopic, showing up either as four doublets or as broad
multiplets. The 31P{1H} NMR spectra consist of only one
resonance for all four phosphorus atoms, indicating that the
four P atoms of the chelating phosphines are equivalent and
that the trans disposition of the Cl and the dihydrogen ligand
is retained upon protonation. The 31P{1H} NMR chemical
shifts are upfield shifted with respect to the starting hydride
derivatives.
(17) (a) Luo, X.-L; Crabtree, R. H. Inorg. Chem. 1990, 29, 2788-2791.
(b) Luo, X.-L.; Howard, J. A. K.; Crabtree, R. H. Magn. Reson. Chem.
1991, 29, S89-S93. (c) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards,
R.; Halpern, J. J. Am. Chem. Soc. 1991, 113, 4173-4184.
(18) Morris, R. H.; Wittebort, R. J. Magn. Reson. Chem. 1997, 35, 243-
250.
The intact nature of the H-H bond in these complexes
1
was established from the variable-temperature H spin-
(19) (a) Heinekey, D. M.; Luther, T. A. Inorg. Chem. 1996, 35, 4396-
4399. (b) Maltby, P. A.; Schlaf, M.; Steinbeck, M.; Lough, A. J.;
Morris, R. H.; Klooster, W. T.; Koetzle, T. F.; Srivastava, R. C. J.
Am. Chem. Soc. 1996, 118, 5396-5407.
lattice relaxation time (T1, ms; 400 MHz) measurements. The
T1 data have been deposited in the Supporting Information.
A plot of T1 (ms) versus temperature (K) shows the typical
Inorganic Chemistry, Vol. 47, No. 2, 2008 553

Dutta et al.
) 25.2 Hz, dHH ) 1.00 Å), and (c) trans-[Ru(η2-H2)Cl(Cy2-
PCH2CH2PCy2)]+ (J(H,D) ) 16.0 Hz, dHH ) 1.15 Å).19b,23
This data suggests that there is a correlation between the
donor abilities of the substituents on the phosphine phos-
phorus and the J(H,D) and in turn the dHH. In addition to an
attempt by Kubas and co-workers to study the reaction
coordinate for H2 cleavage by the electronic control of the
dihydrogen versus the dihydride coordination in certain
molybdenum complexes,3 there has been only one other
systematic study,24 although to a lesser extent, of an
investigation of the type presented in the current work. The
dHH shows a great deal of variation in the dihydrogen
complexes reported to date, from 0.82 Å all the way up to
1.5 Å along the reaction coordinate for the activation of
H2.1,2,17c,18,19b,25,26 The study of the binding and elongation
of H2 and the arrested intermediate states on its way to
oxidative addition to a metal center is extremely important
because it serves as a prototype for the other σ-bond
activation processes, e.g., C-H bond and Si-H bond.
Although elongation of the H-H bond length in the
complexes reported herein is not very appreciable, it is
instructive to note that it is possible to design systems such
that the H-H bond could be substantially elongated and at
the same time in an incremental order as if mapping out the
reaction coordinate for the oxidative addition of H2 to a metal
center. Efforts toward this goal are in progress in our
laboratories.
Figure 2. 1H NMR spectrum (hydride region) of trans-[Ru(η2-HD)(Cl)-
((C6H5-CH2)2PCH2CH2P(CH2C6H5)2)2][BF4] 2a-d1 (400 MHz, 293 K) in
CD2Cl2.
X-ray Crystal Structure of trans-[Ru(η2-H2)(Cl)-
((C6H5CH2)2PCH2CH2P(CH2C6H5)2)2][BF4], 2a. The ORTEP
diagram of the cation is shown in Figure 4. Pertinent bond
lengths and angles are summarized in Table 7. The structure
consists of an octahedron with the four phosphine phosphorus
atoms lying in a square plane and the chloride approximately
perpendicular to this plane. The sixth coordination site is
occupied by the dihydrogen ligand which is bound in an η2-
fashion. The hydrogen atoms of the H2 ligand were located
Figure 3. Plot of J(H,D) of 1a-d1-4a-d1 versus Hammett constants (σ)
of the substituent on the benzyl ligand.
in a series of substituted aniline-15N derivatives.20 It was
found that the magnitude of the coupling constants decreased
with an increase in the donor property of the substituent.
Similarly, Hess and co-workers21 found a linear relationship
between the methyl 13C-1H coupling constants and Hammett
σ constants of the substituents for a series of substituted
toluenes, anisoles, and other similar aromatic compounds.
In addition, Cobley and Pringle established a correlation
between the Pt-P bond strength and the Hammett substituent
(23) The Hammett constants (σp and σm) have been obtained from studies
of compound XC6H4COOH: (a) Hansch, C.; Leo, A.; Taft, W. R.
Chem. ReV. 1991, 91, 165-195. (b) Jaffe, H. H. Chem. ReV. 1953,
53, 191-261. (c) Hammett constant data are not available for the
cyclohexyl substituent.
(24) Cappellani, E. P.; Drouin, S. D.; Jia, G.; Maltby, P. A.; Morris, R.
H.; Schweitzer, C. T. J. Am. Chem. Soc. 1994, 116, 3375-3388.
(25) (a) Crabtree, R. H. Acc. Chem. Res. 1990, 23, 95-101. (b) Jessop, P.
G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155-284. (c)
Heinekey, D. M.; Oldham, W. J., Jr. Chem. ReV. 1993, 93, 155-284.
(d) Morris, R. H. Can. J. Chem. 1996, 74, 1907-1915. (e) Kuhlman,
R. Coord. Chem. ReV. 1997, 167, 205-232. (f) Sabo-Etienne, S.;
Chaudret, B. Coord. Chem. ReV. 1998, 178-180, 381-407. (g)
Esteruelas, M. A.; Oro, L. A. Chem. ReV. 1998, 98, 577-588. (h) Jia,
G.; Lau, C.-P. Coord. Chem. ReV. 1999, 192, 83-108. (i) Esteruelas,
M. A.; Oro, L. A. AdV. Organomet. Chem 2001, 47, 1-59.
(26) (a) Matthews, S. L.; Heinekey, D. M. J. Am. Chem. Soc. 2006, 128,
2615-2620. (b) Yousufuddin, M.; Wen, T. B.; Mason, S. A.;
McIntyre, G. J.; Jia, G.; Bau, R. Angew. Chem., Int. Ed. 2005, 44,
7227-7230. (c) Grellier, M.; Vendier, L.; Chaudret, B.; Albinati, A.;
Rizzato, S.; Mason, S.; Sabo-Etienne, S. J. Am. Chem. Soc. 2005,
127, 17592-17593. (d) Nanishankar, H. V.; Dutta, S.; Nethaji, M.;
Jagirdar, B. R. Inorg. Chem. 2005, 44, 6203-6210. (e) Ingleson, M.
J.; Brayshaw, S. K.; Mahon, M. F.; Ruggiero, G. D.; Weller, A. S.
Inorg. Chem. 2005, 44, 3162-3171. (f) Pons, V.; Heinekey, D. M. J.
Am. Chem. Soc. 2003, 125, 8428-8429. (g) Heinekey, D. M.; Law,
J. K.; Schultz, S. M. J. Am. Chem. Soc. 2001, 123, 12728-12729. (h)
Fang, X.; Huhmann-Vincent, J.; Scott, B. L.; Kubas, G. J. J.
Organomet. Chem. 2000, 609, 95-103.
1
constants via examination of the J(Pt,P) and σ values.22
The complexes 3a-5a can be categorized as elongated
dihydrogen complexes since the dHH in these derivatives is
g1.0 Å. The elongated dihydrogen complexes represent the
arrested intermediate states of the oxidative addition of H2
to a metal center. Some of the complexes that are pertinent
to the current work are (a) trans-[Ru(η2-H2)Cl(Ph2PCH2CH2-
PPh2)]+ (J(H,D) ) 25.9 Hz, dHH ) 0.99 Å; σp ) 0.009, σm
) 0.218), (b) trans-[Ru(η2-H2)Cl(Et2PCH2CH2PEt2)]+ (J(H,D)
(20) Axenrod, T.; Pregosin, P. S.; Wieder, M. J.; Milne, G. W. A. J. Am.
Chem. Soc. 1969, 91, 3681-3682.
(21) Yoder, C. H.; Tuck, R. H.; Hess, R. E. J. Am. Chem. Soc. 1969, 91,
539-543.
(22) Cobley, C. J.; Pringle, P. G. Inorg. Chim. Acta 1997, 265, 107-115.
554 Inorganic Chemistry, Vol. 47, No. 2, 2008

Influence of Phosphine Ligands on H-H Bond Elongation
Ar)2, Ar ) m-CH3C6H4-, 6, p-CH3C6H4-, 7) via MeCl
elimination (eq 4).
1
The H NMR spectra of these species show a quintet for
the hydride hydrogen that is upfield shifted in comparison
to the precursor hydrides at δ -25.03 and -24.94, respec-
tively, for 6 and 7. The 31P{1H} NMR spectra of 6 and 7
exhibit a single resonance at δ 59.1 and 59.0, respectively.
The appearance of a distinct doublet at δ 6.23 (J(H,H) )
6.8 Hz) which is upfield shifted from the remaining phenyl
hydrogens (δ 6.72-7.73) could be assigned to the ortho
hydrogens of the p-methylbenzyl moiety in the case of
complex 7 interacting with the metal in an agostic fashion
in the sixth coordination site. Attempts to isolate 7 resulted
in intractable species as ascertained by NMR spectroscopy,
which can be attributed to the weak agostic interaction. This
is in contrast to the case of [Ru(H)((C6H5CH2)2PCH2CH2P-
(CH2C6H5)2)2][OTf] which is isolable in the solid state.5 On
the other hand, we found no evidence of agostic interaction
in 6. It is interesting to note that the sixth coordination site
on the metal in 6 is vacant, and attempts to isolate this species
in the solid state resulted in extensive decomposition
products. The sterically crowded substituted benzyl ligands
in complexes 6 and 7 lead to the lack of and weak agostic
interaction, respectively; this is reflected in their instabilities.
For Ar ) p-FC6H4, the negative inductive effect of the
fluorine makes the ortho-H of p-FC6H4CH2- less electron
donating compared to the others; therefore, in this case, we
noted no reaction between MeOTf and trans-[Ru(H)Cl((p-
FC6H4CH2)2PCH2CH2P(CH2-p-FC6H4)2)2]. Kubas et al. ear-
lier found in a series of molybdenum complexes of the type
[Mo(CO)((ArCH2)2PCH2CH2P(CH2Ar)2)2] that agostic in-
teraction is favored only for Ar ) C6H5- and m-CH3C6H4-,
which has been attributed to the favorable sterics, whereas
other aryl groups gave intractable products.3 Complexes 6
and 7 are stable for up to 10-12 h at room temperature in
solution after which time they pick up a chloride from the
solvent resulting in recovery of the hydride chloride deriva-
tives 3 and 4, respectively. In order to establish the existence
of a vacant site or the agostic interaction with certainty in 6
and 7, respectively, we treated these derivatives with CH3-
CN. In both cases, the nitrile products trans-[Ru(H)(CH3-
CN)(PP)2][OTf] (PP ) (ArCH2)2PCH2CH2P(CH2Ar)2, Ar )
m-CH3C6H4-, 8, p-CH3C6H4-, 9) were obtained almost
instantaneously (eq 5). In addition, the 19F NMR spectra of
complexes 6 and 7 show only one signal (δ -79.0)
suggesting that there is only one type of triflate, namely,
counterion, rather than a counterion and a bound triflate.
Figure 4. ORTEP view of trans-[Ru(η2-H2)(Cl)((C6H5CH2)2PCH2CH2P-
(CH2C6H5)2)2][BF4], 2a, at the 50% probability level.
from the successive difference Fourier maps. H(100)-H(101)
was found to be 1.08(5) Å, which can be categorized as an
elongated dihydrogen ligand. This distance is slightly longer
(taking into account the standard deviations) than that
obtained from J(H,D) couplings (see above). The two Ru-H
bond distances (Ru(1)-H(100) and Ru(1)-H(101)) are,
respectively, 1.63(3) and 1.59(3) Å. The four phosphorus
atoms and ruthenium atom are in a plane. The Ru-P
distances are in the range from 2.3514(9) to 2.3776(8) Å,
and the average of these distances is 2.3654 Å. The bite
angles P(1)-Ru(1)-P(2) and P(3)-Ru(1)-P(4) are 83.37-
(3)° and 82.64(3)°, respectively. The Ru(1)-Cl(1) bond
length is 2.4453(8) Å which is longer by 0.038 Å in
comparison to the dppe analog trans-[Ru(η2-H2)Cl(dppe)2]-
[PF6] reported by Morris et al.10e This is a reflection of the
greater donor ability of the chelating phosphine in 2a
compared to that of dppe (Ph2PCH2CH2PPh2) in Morris’
complex. In the family of dihydrogen complexes with the
[Ru(PP)2] backbone, 2a is only the fifth complex to be
crystallographically characterized.27 The dihydrogen hydro-
gens in the trans-[Ru(η2-H2)Cl(dppe)2][PF6] derivative were
not located. dHH obtained from X-ray crystallography in the
case of trans-[Ru(η2-H2)H(dppe)2][BPh4] is 0.83(8) Å.28 Our
attempts to obtain single crystals suitable for neutron
diffraction studies were unsuccessful.
Preparation and Properties of [Ru(η2-H2)(PP)2][OTf]2
(PP ) (ArCH2)2PCH2CH2P(CH2Ar)2, Ar ) m-CH3C6H4-,
6a, p-CH3C6H4-, 7a). We recently reported the synthesis
and protonation studies of a five-coordinate, 16-electron
complex stabilized by agostic interaction, [Ru(H)((C6H5-
CH2)2PCH2CH2P(CH2C6H5)2)2][OTf].5 This compound was
obtained from the corresponding six-coordinate hydride
chloride complex by reaction with MeOTf via elimination
of MeCl. Similarly, reaction of trans-[Ru(H)Cl((ArCH2)2-
PCH2CH2P(CH2Ar)2)2] (Ar ) m-CH3C6H4-, 3, p-CH3C6H4-,
4) with MeOTf resulted in the five-coordinate, 16-electron
complexes[Ru(H)(PP)2][OTf](PP)(ArCH2)2PCH2CH2P(CH2-
(27) (a) Jimenez-Tenorio, M.; Puerta, M. C.; Valerga, P. J. Chem. Soc.,
Chem. Commun. 1993, 1750-1751. (b) Schlaf, M.; Lough, A. J.;
Morris, R. H. Organometallics 1997, 16, 1253-1259.
(28) Albinati, A.; Klooster, W. J.; Koetzle, T. F.; Fortin, J. B.; Ricci, J. S.;
Eckert, J.; Fong, T. P.; Lough, A. J.; Morris, R. H.; Golombek, A. P.
Inorg. Chim. Acta 1997, 259, 351-357.
1
Variable-temperature H NMR spectroscopy of 7 resulted
in the broadening of the doublet signal at δ 6.23 (agostic
hydrogen) at low temperature, eventually becoming a broad
singlet at 243 K. The signal was also slightly upfield shifted.
Inorganic Chemistry, Vol. 47, No. 2, 2008 555

Dutta et al.
Scheme 1
Further cooling to 223 K resulted only in the broad signal
merging with the baseline; we could not discern any fluxional
process.
Protonation of the five-coordinate complexes 6 and 7 using
HOTf in CD2Cl2 gave the dicationic dihydrogen complexes
[Ru(η2-H2)((ArCH2)2PCH2CH2P(CH2Ar)2)2][OTf]2 (Ar )
m-MeC6H4-, 6a, p-MeC6H4-, 7a). The bound H2 ligand in
these complexes appears as a broad singlet that is downfield
shifted with respect to the precursor hydrides. 31P{1H} NMR
spectroscopy showed only a single resonance for all four
phosphine P atoms, evidencing that the structural motif is
not perturbed upon protonation. The agostic interaction noted
in complex 7 disappeared upon protonation. No evidence
for the binding of either the triflate counterion or the solvent
in the sixth coordination site of the metal in freshly prepared
solutions of 6a and 7a was found. The 19F NMR spectra of
7 and 7a showed only the resonance of the triflate counterion
that remained invariant even at low temperatures (see
Supporting Information). In addition, protonation of 7 in 1,2-
dichloroethane (external lock, acetone-d6) did not show
significant differences in the spectral features of 7a compared
to that performed in CD2Cl2 (see Supporting Information).
This further rules out the binding of the solvent in the vacant
site around the metal center. However, with time, 6a and 7a
pick up a chloride from the solvent CD2Cl2 to generate the
six-coordinate complexes 3a and 4a (Scheme 1).
The intact nature of the H-H bond in complexes 6a and
7a was established from the variable-temperature 1H spin-
lattice relaxation time measurements (Table S3, Supporting
Information) and the J(H,D) couplings of the corresponding
HD isotopomers 6a-d1 and 7a-d1. The HD and HD-Pcis
couplings (J(H,D) ) 22.0 ( 0.2) in the case of complex
7a-d1 are shown in Figure 5.
dHH for these complexes were found to be 1.04 and 1.05
Å, respectively. It is interesting to note that 6a and 7a are
unprecedented examples of five-coordinate, 16-electron,
dicationic elongated dihydrogen complexes of ruthenium.
Related examples of 16-electron, elongated dihydrogen
complexes of ruthenium with agostic interactions of the type
[Ru(H)(X)(η2-H2)(PCy3)2] (X ) Cl, Br, SR) have been
reported by Chaudret and co-workers previously.29
Figure 5. 1H NMR spectrum of the hydride region of [Ru(η2-HD)((p-
MeC6H4-CH2)2PCH2CH2P(CH2 C6H4-p-Me)2)2][OTf]2, 7a-d1.
donation. The M(dπ) to H2(σ*) back-donation can weaken
the H-H bond, but the degree of back-donation controls
H-H bond cleavage due to the population of the σ* orbital
of the bound H2.30 Kubas et al. established the binding of
the dihydrogen ligand (intact H-H bond) in [Mo(η2-H2)-
(CO)(dppe)2] (dppe ) Ph2PCH2CH2PPh2), whereas H2
undergoes oxidative addition in molybdenum complexes of
the type [Mo(η2-H2)(CO)(R2PCH2CH2PR2)2] (R ) Et, Bui).31
Although the cone angles of PPh3 and PiBu3 are very similar,
145° and 143°, respectively,32 the dihydrogen versus dihy-
dride binding is dictated by the electronics rather than sterics
(cone angles in this case).33 The extent of back-donation is
controlled by the diphosphine basicity, which in turn directs
the H-H bond elongation; no steric influence in the form
of cone angles of the phosphines was found to play a role.
(29) (a) Toner, A.; Matthes, J.; Gru¨ndemann, S.; Limbach, H.-H.; Chaudret,
B.; Clot, E.; Sabo-Etienne, S. Proc. Natl. Acad. Sci. 2007, 104, 6945-
6950. (b) Toner, A.; Gru¨ndemann, S.; Clot, E.; Limbach, H.-H.;
Donnadieu, B.; Sabo-Etienne, S.; Chaudret, B. J. Am. Chem. Soc. 2000,
122, 6777-6778.
(30) (a) Saillard, J.-Y.; Hoffmann, R. J. Am. Chem. Soc. 1984, 106, 2006-
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Electronic Influence of Coligands on the H-H Bond
Elongation. The well-established two-component binding
involves H2 to M(d) σ donation and M(dπ) to H2(σ*) back-
556 Inorganic Chemistry, Vol. 47, No. 2, 2008

Influence of Phosphine Ligands on H-H Bond Elongation
As a consequence of the flexibility of the benzyl groups,
the effective cone angle of P(CH2C6H5)3 was found to be
142°, which is very similar to that of PiBu3.34 Therefore, in
Conclusions
The protonation reactions of the hydride complexes of the
type trans-[Ru(H)Cl((ArCH2)2PCH2CH2P(CH2Ar)2)2] (Ar )
p-FC6H4-, 1, C6H5-, 2, m-CH3C6H4-, 3, p-CH3C6H4-, 4,
p-iPrC6H4-, 5) with HBF4‚OEt2 result in the corresponding
dihydrogen complexes trans-[Ru(η2-H2)(Cl)((ArCH2)2PCH2-
CH2P(CH2Ar)2)2][BF4]. The H-H bond distances, dHH, vary
in small increments from 0.97 to 1.03 Å, correlating with
the donor properties of the substituents (Hammett constants)
on the benzyl moiety of the chelating phosphine ligands.
Thus, it should be possible to construct dihydrogen com-
plexes along the reaction coordinate for the oxidative addition
of H2 to a metal center by suitably altering the coligands in
a systematic manner such that the donor ability of the
coligands increases in small increments.
Reaction of trans-[Ru(H)Cl((ArCH2)2PCH2CH2P(CH2-
Ar)2)2] (Ar ) m-CH3C6H4-, 3, p-CH3C6H4-, 4) with MeOTf
results in five-coordinate species of the type [Ru(H)-
((ArCH2)2PCH2CH2P(CH2Ar)2)2][OTf] via elimination of
MeCl. Protonation of these complexes results in the corre-
sponding five-coordinate elongated dihydrogen complexes
[Ru(η2-H2)((ArCH2)2PCH2CH2P(CH2Ar)2)2][OTf]2 which are
unprecedented.
i
the Bu2PCH2CH2PiBu2 ligand and its benzyl counterpart,
despite having similar steric properties, weaker basicity of
the benzyl diphosphine ligand is expected to stabilize the
η2-H2 binding mode. The reaction coordinate for the oxida-
tive addition of H2 to the molybdenum center in the
molybdenum-diphosphine complexes reported by Kubas et
al. is rather flat until it is quite close to cleavage of the H-H
bond. This indicates that there will be very little change in
the H-H bond distance until bond cleavage is imminent.
On the other hand, the series of cationic complexes reported
by Morris et al. [MH(η2-H2)(R2PCH2CH2PR2)]+ (M ) Fe,
Ru) do not show H-H bond cleavage as the substituent is
varied in a similar fashion.33 This means that H-H is farther
away from the point of cleavage on the reaction coordinate.
The positive charge on the molecule could stabilize η2-H2
binding over dihydride formation via reduced Ru(dπ)-H2-
(σ*) back-donation. Recent work of Heinekey and co-
workers on a family of complexes of the type [Ru(Cp)/
(Cp*)(η2-H2)(PP)]+ showed that the phosphine ligand variation
had a profound effect on the temperature dependence of the
J(H,D).35 They found that the variations in the electronic
properties of the chelating phosphines resulted in subtle
changes in the Ru-H2 binding, suggesting that such ligand
variation will have a significant effect on the vibrational
potential of the bound H2. In the series of complexes
presented in this report, 1a-5a, the H-H distance increases
from 1a to 5a reflecting the increasing basicity of the
diphosphine or a decrease in the Hammett constants of the
benzyl substituent from p-fluorobenzyl to p-iso-propylbenzyl.
The incremental increase, although small, follows an ex-
pected trend.
Acknowledgment. We thank the Department of Science
& Technology, India, with gratitude for financial support.
We also thank the DST, India, for funding the procurement
of the X-ray diffractometer and a 400 MHz NMR spectrom-
eter under the “FIST” grant. We are grateful to the Depart-
ment of Organic Chemistry, I.I.Sc., for providing access to
the mass spectral facility.
Supporting Information Available: X-ray crystallographic data
for trans-[Ru(η2-H2)(Cl)((C6H5CH2)2PCH2CH2P(CH2C6H5)2)2][BF4],
2a, in CIF format, details of the synthesis of the complexes, VT
1H spin-lattice relaxation time (T1, ms; 400 MHz) data for the
dihydrogen complexes, VT NMR spectral stack plots, and the mass
spectral data for the complexes. This material is available free of
charge via the Internet at http://pubs.acs.org.
(34) Barron, A. R. J. Chem. Soc., Dalton Trans. 1988, 3047-3050.
(35) Law, J. K.; Mellows, H.; Heinekey, D. M. J. Am. Chem. Soc. 2002,
124, 1024-1030.
IC7016769
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