16-Electron elongated dihydrogen complex stabilized by agostic interaction

Article abstract of DOI:10.1021/ic061143a

A 16-electron dicationic dihydrogen complex [Ru(η²- H...H)(PP)₂][OTf]₂ [4; PP = (C₆H ₅CH₂)₂PCH₂CH₂P(CH ₂C₆H₅)₂] has been prepared and characterized by protonating the precursor hydride complex [Ru(H)(PP) ₂)][OTf] (2) using HOTf. The hydride and dihydrogen complexes are stabilized via agostic interaction of the ortho C-H fragment of the phenyl ring on the benzyl group. The intact nature of the H-H bond in this derivative was established from the short spin-lattice relaxation time and the observation of a substantial J(H,D) of 22.0 Hz for the HD isotopomer. The H-H bond distance calculated from J(H,D) is 1.05 A, which falls under the category of elongated dihydrogen ligands.

Full text of DOI:10.1021/ic061143a

Inorg. Chem. 2006, 45, 7047−7049
16-Electron Elongated Dihydrogen Complex Stabilized by Agostic
Interaction
Saikat Dutta and Balaji R. Jagirdar*
Department of Inorganic & Physical Chemistry, Indian Institute of Science,
Bangalore 560 012, India
Received June 23, 2006
A 16-electron dicationic dihydrogen complex [Ru(
[OTf]2 [4; PP (C6H5CH2)2PCH2CH2P(CH2C6H5)2] has been
η
2-H‚‚‚H)(PP)2]-
)
prepared and characterized by protonating the precursor hydride
complex [Ru(H)(PP)2)][OTf] (2) using HOTf. The hydride and
dihydrogen complexes are stabilized via agostic interaction of the
ortho C
intact nature of the H
from the short spin lattice relaxation time and the observation of
a substantial J(H,D) of 22.0 Hz for the HD isotopomer. The H
−H fragment of the phenyl ring on the benzyl group. The
−H bond in this derivative was established
−
−H
bond distance calculated from J(H,D) is 1.05 Å, which falls under
the category of elongated dihydrogen ligands.
at δ -25.44 for the hydride moiety. The benzyl CH2
hydrogens appear as four independent doublets at δ 3.68,
3.29, 3.22, and 2.90, with three of the geminal J(H,H) of
2
14.6 Hz and the fourth 12.6 Hz. In addition, the spectrum
shows a well-resolved doublet at δ 6.67 (integrates to 8H)
that is upfield-shifted from the phenyl resonances (δ 6.92-
The binding of molecular hydrogen to a transition-metal
center leads to the elongation of the H-H bond along the
reaction coordinate for the oxidative addition of dihydrogen.
While a large number of dihydrogen complexes reported to
date have H-H distances (dHH, Å) between 0.8 and 1.0 Å,
there are fewer examples of complexes that possess elongated
dihydrogen moieties (dHH between 1.0 and 1.5 Å).1,2 The
elongated dihydrogen complexes represent arrested interme-
diates in the important process of oxidative addition of
dihydrogen to a metal center. We recently started a program
to study the smooth gradation of the H-H distances in certain
ruthenium dihydrogen complexes along the reaction coor-
dinate for the oxidative addition of dihydrogen. In this
Communication, we report an unprecedented example of a
16-electron, dicationic elongated dihydrogen complex of
ruthenium that is stabilized by an agostic Ru‚‚‚H-C interac-
tion.
3
7.36), with a J(H,H) ) 6.8 Hz. The 19F NMR spectrum
shows a single resonance at δ -79.83 for the OTf-
counterion. These spectral features and the observation of
the elimination of MeCl points to the formation of a five-
coordinate, 16-electron hydride complex, which is stabilized
by a Ru‚‚‚H-C agostic interaction in the sixth coordinate
site on the metal center.5 A similar agostic benzyl interaction
(3) To a 2-propanol solution (10 mL) of [RuH(Cl)(PPh3)3] (0.100 g, 0.108
mmol) was added (C6H5CH2)2PCH2CH2P(CH2C6H5)2 (0.098 g, 0.216
mmol), and the reaction mixture was refluxed for 2 h, during which
time a yellow product precipitated out. The solution was filtered after
it had cooled to room temperature, and the precipitate containing the
product trans-[RuH(Cl)(((C6H5CH2)2PCH2CH2P(CH2C6H5)2)2)] (1)
i
was washed with PrOH (5 mL) and dried in vacuo. Yield: 0.065 g
1
(57.0%). QTOF-MS: m/z ) 1011 [M+ - Cl-]. H NMR (CD2Cl2):
δ -19.63 (qnt, 1H, Ru-H, J(H,P) ) 19.5 Hz), 0.60 (br s, 4H,
PCH2CH2P), 1.10 (br s, 4 H, PCH2CH2P), 2.89 (d, 4H, CH2C6H5,
J(H,H) ) 13.6 Hz), 3.13 (d, 4H, CH2C6H5, J(H,H) ) 13.6 Hz), 3.22
(d, 4H, CH2C6H5, J(H,H) ) 13.7 Hz), 4.33 (d, 4H, CH2C6H5, J(H,H)
) 14.6 Hz), 7.14-7.56 (m, 40H, C6H5). 31P{1H} NMR (CD2Cl2): δ
63.1 (s, 4P).
Treatment of trans-[Ru(Cl)H(PP)2]3 [1; PP ) (C6H5CH2)2-
PCH2CH2P(CH2C6H5)2] with 1 equiv of MeOTf in CH2Cl2
at room temperature resulted in the unexpected elimination
of MeCl instead of CH4, accompanied by the formation of
a new 16-electron hydride complex, [Ru(H)(PP)2][OTf] (2;
eq 1).4 The 1H NMR spectrum of this species shows a quintet
(4) Characterization of [Ru(H)(((C6H5CH2)2PCH2CH2P(CH2C6H5)2)2)]-
[OTf] (2). QTOF-MS: m/z ) 1009 [M+ - (H2 + OTf-)], 555 [M+
- (((C6H5CH2)2PCH2CH2P(CH2C6H5)2) + OTf-)]. 1H NMR (CD2-
Cl2): δ -25.44 (qnt, 1H, Ru-H, J(H,P) ) 20.2 Hz), 0.75 (br s, 4H,
PCH2CH2P), 1.19 (br s, 4 H, PCH2CH2P), 2.90 (d, 4H, CH2C6H5,
J(H,H) ) 14.6 Hz), 3.22 (d, 4H, CH2C6H5, J(H,H) ) 12.7 Hz), 3.29
(d, 4H, CH2C6H5, J(H,H) ) 14.6 Hz), 3.68 (d, 4H, CH2C6H5, J(H,H)
) 14.6 Hz), 6.67 (d, 8H, Ru‚‚‚H-C, J(H,H) ) 6.8 Hz), 6.92-7.36
(m, 32H, C6H5, and hydrogens of nonagostic phenyl rings). 31P{1H}
NMR (CD2Cl2): δ 59.0 (s, 4P). 19F (CD2Cl2): δ -79.83 (s, OTf).
* 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.
10.1021/ic061143a CCC: $33.50
Published on Web 08/03/2006
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 18, 2006 7047

COMMUNICATION
was reported earlier by Luo et al.5b In contrast to [RuH-
(dppp)2] [dppp ) Ph2P(CH2)3PPh2] reported by Saburi et al.,5a
which has the hydride in the cis position with respect to the
agostic site, in complex 2, the hydride and the agostic site
are mutually trans to one another. This is based on the
observation of a single resonance for all of the four phosphine
phosphorus atoms in the 31P{1H} NMR spectrum (δ 59.0)
The H-C moiety on the ortho position of the phenyl ring is
the donor for this agostic interaction.
[OTf]2 (4; eq 3).7 The dihydrogen ligand appears as a broad
The variable-temperature (VT) 1H NMR spectroscopy of
this sample resulted in the broadening of the doublet signal
(agostic hydrogen) at low temperature, eventually becoming
a broad singlet and at the same time shifted upfield in the
temperature range of 293-223 K. The four sets of doublets
for the benzyl CH2’s also underwent line broadening at low
temperatures. Further cooling to 183 K resulted in de-
coalescence of the signal due to the agostic hydrogen into
two broad singlets at δ 6.37 and 6.64 signaling the freezing
of the o-hydrogen exchange process. This complex is stable
in the solid state; however, it is only moderately stable in
solution at 293 K. In the CH2Cl2 solvent, it picks up a
chloride from the solvent to give chloride hydride complex
1 over a period of 12 h. It is only partially soluble in most
solvents, thus causing difficulty in growing crystals. To
establish the agostic interaction with certainty, we reacted
the hydride complex 2 with 1 equiv of CH3CN. The reaction
mixture immediately turned colorless from yellow, and yet
another new hydride derivative was obtained, trans-[Ru(H)-
(CH3CN)(PP)2][OTf] (3; eq 2).6 The hydride signal of
1
singlet in the H NMR spectrum at δ -17.97. Remarkably
enough, the agostic interaction was left undisturbed upon
protonation. The Ru‚‚‚H-C agostic hydrogen appears as a
doublet [2J(H,H) ) 6.8 Hz] at δ 6.77, upfield from the
remaining phenyl resonances (δ 6.83-7.86). The dihydrogen
ligand and the agostic site are trans-disposed as evidenced
by 31P{1H} NMR spectroscopy (δ 54.0, singlet). The 19F
NMR spectrum of 4 shows only one signal at δ -79.71.
The observation of only one signal in the 19F NMR spectrum
1
and the signals for the benzyl agostic interaction in the H
NMR spectrum, along with the formation of the CH3CN
complex (eq 2), strongly suggests that a potential coordinat-
ing anion like the triflate is not bound to the metal in the
sixth coordination site. The complex 4 has limited stability,
thus precluding further reactivity studies. With time, it picks
up a chloride from the solvent, resulting in a dihydrogen
chloride complex that we are currently attempting to isolate
and characterize.
To establish the intact nature of the H-H bond in this
derivative, we carried out the VT 1H spin-lattice relaxation
time (T1, ms) measurements for the bound dihydrogen.8 A
T1 (min) (400 MHz) of 23.1 ms was obtained at 253 K from
which the H-H bond distances for the fast- and slow-
spinning regimes were calculated to be 0.90 and 1.14 Å,
respectively.9 Unambiguous establishment of the intact nature
of the H-H bond was achieved by the partial incorporation
of deuterium to form the HD isotopomer of complex 4 and
measurement of J(H,D). This was done by reacting 2 with 1
equiv of DOTf at 233 K to generate a mixture of [Ru(η2-
H‚‚‚D)(PP)2][OTf]2 (major) and 4 (minor) derivatives. We
complex 2 at δ -25.44 disappeared, whereas a new quintet
1
recorded the H NMR spectrum of the HD isotopomer in
for the hydride ligand of the acetonitrile complex formed at
1
the temperature range 293-223 K by nullifying the residual
signal of the η2-H2 species by an inversion-recovery pulse
(Figure 1).10 At 293 K, the 1H NMR spectrum in the hydride
region shows a very well-resolved 1:1:1 triplet due to H-D
coupling at δ -18.00; each one of the 1:1:1 triplet signals
was further split into a quintet as a result of HD-P coupling.
δ -17.52 in the H NMR spectrum. CH3CN appears as a
singlet at δ 2.32.
Protonation of 2 using 1 equiv of HOTf in CD2Cl2 resulted
in a new dicationic dihydrogen complex [Ru(η2-H‚‚‚H)(PP)2]-
(5) Previous instances of the observation of agostic interactions in
somewhat similar systems: (a) Saburi, M.; Fujii, T.; Shibusawa, T.;
Masui, D.; Ishii, Y. Chem. Lett. 1999, 1343-1344 and references cited
therein. (b) Luo, X.-L. L.; Kubas, G. J.; Burns, C. J.; Eckert, J. Inorg.
Chem. 1994, 33, 5219-5229. (c) King, W. A.; Scott, B. L.; Eckert,
J.; Kubas, G. J. Inorg. Chem. 1999, 38, 1069-1084 and references
cited therein.
(6) Characterization of trans-[Ru(H)(CH3CN)(((C6H5CH2)2PCH2CH2P-
(CH2C6H5)2)2)][OTf] (3). 1H NMR (CD2Cl2): δ -17.52 (qnt, 1H, Ru-
H, J(H,P) ) 20.4 Hz), 0.91 (br s, 4H, PCH2CH2P), 1.23 (br s, 4 H,
PCH2CH2P), 2.32 (s, 3H, CH3CN), 2.82 (d, 4H, CH2C6H5, J(H,H) )
13.6 Hz), 3.00 (d, 4H, CH2C6H5, J(H,H) ) 13.6 Hz), 3.29 (d, 4H,
CH2C6H5, J(H,H) ) 13.6 Hz), 3.39 (d, 4H, CH2C6H5, J(H,H) ) 13.6
Hz), 6.81-7.75 (m, 40H, C6H5). 31P{1H} NMR (CD2Cl2): δ 55.0 (s,
4P). 19F (CD2Cl2): δ -79.84 (s, OTf).
(7) Characterizationof[Ru(η2-H‚‚‚H)(((C6H5CH2)2PCH2CH2P(CH2C6H5)2)2)]-
[OTf]2 (4). 1H NMR (CD2Cl2): δ -17.97 (br s, 2H, Ru-η2H2), 0.80
(br s, 4H, PCH2CH2P), 0.94 (br s, 4 H, PCH2CH2P), 2.63 (d, 4H,
CH2C6H5, J(H,H) ) 14.7 Hz), 2.95 (d, 4H, CH2C6H5, J(H,H) ) 14.7
Hz), 3.94 (d, 4H, CH2C6H5, J(H,H) ) 14.7 Hz), 4.04 (d, 4H, CH2C6H5,
J(H,H) ) 13.7 Hz), 6.77 (d, 8H, Ru‚‚‚H-C, J(H,H) ) 6.8 Hz), 6.83-
7.86 (m, 32H, C6H5, and hydrogens of nonagostic phenyl rings). 31P-
{1H} NMR (CD2Cl2): δ 54.0 (s, 4P). 19F (CD2Cl2): δ -79.71 (s,
OTf-).
(8) Hamilton, D. G.; Crabtree, R. H. J. Am. Chem. Soc. 1988, 110, 4126-
4133.
(9) Morris, R. H.; Wittebort, R. J. Magn. Reson. Chem. 1997, 35, 243-
250.
7048 Inorganic Chemistry, Vol. 45, No. 18, 2006

COMMUNICATION
The dihydrogen complex 4 belongs to the class of
elongated dihydrogen complexes. The ruthenium fragment
is dicationic and is formally a 16-electron center stabilized
via an agostic interaction (trans to the bound dihydrogen)
with the H-C fragment on the ortho position of the phenyl
ring. We also studied the temperature dependence of J(H,D)
in [Ru(η2-H‚‚‚D)(PP)2][OTf]2 and found a modest variation,
increasing from 22.0 Hz at 293 K to 24.0 Hz at 233 K. This
behavior has been observed for other elongated dihydrogen
complexes as well.2
Chaudret et al.16 reported certain 16-electron ruthenium
complexes of the type [Ru(H)X(η2-H2)(PR3)2]. dHH in this
series of compounds was found to be in the range 1.0-1.03
Å. In one of the complexes, [RuH(H2)(o-C6H5py)(PR3)2]-
[BArf], Ru‚‚‚H-C (H-C of the phenyl) agostic interaction
was noted.16a This compound was isolated and characterized
by NMR spectroscopic, X-ray crystallographic, and theoreti-
cal studies.
1
Figure 1. VT H NMR spectral stack plot of the hydride region of [Ru-
(η2-H‚‚‚D)(PP)2][OTf]2 in CD2Cl2.
In summary, we synthesized and characterized a dicationic
dihydrogen complex 4 in which the H-H bond is elongated
(1.05 Å). This species is stabilized by an agostic interaction
trans to the dihydrogen ligand. It is of interest to note that,
although the metal fragment is dicationic and formally a 16-
electron center, the bound dihydrogen is significantly
elongated, which is contrary to what one would expect based
on the bonding principles of these types of novel molecules.1
In addition, a potential coordinating anion like OTf- remains
as a counterion rather than binding to the metal. Recently,
Pons and Heinekey reported an elongated dihydrogen
complex of iridium [Ir(Cp*)(η2-H2)(dppm)]2+ (dmpm ) Me2-
PCH2PMe2) wherein the metal fragment is dicationic.17
Efforts are underway to systematically study the effect of
increasing the electron donor ability of the diphosphine ligand
on dHH with a view to realize complexes with even more
elongated dihydrogen ligands.
J(H,D) was found to be 22.0 Hz,11 which gives a H-H
distance (dHH) calculated from the inverse relationship
between dHH and J(H,D) of 1.05 Å.12 This distance seems to
be more consistent with the slow than the fast rotation regime
of the bound dihydrogen.9 J(HD,Pcis) was found to be 8.0
Hz, which is within the range (7.2-8.0 Hz) reported for
complexes of the type trans-[RuCl(η2-HD)(diphosphine)2]+
by Morris et al.10b
In the family of complexes of the type trans-[Ru(η2-H2)-
(H)(PP)2]+ (PP ) dppe, depe) reported by Morris et al.,13
J(H,D) was found to be in the range 32-33 Hz, which gives
a dHH of 0.88-0.86 Å. On the other hand, in complexes with
a good π donor like chloride, trans-[RuCl(η2-HD)(PP)2]+,
dHH was found to be 1.15 Å (PP ) dcype), 1.04 Å (PP )
depe), and 0.98 Å (PP ) dppe). The decrease in dHH reflects
a decrease in the donor abilities of the diphosphine ligands
from dcype to depe to dppe. On the basis of the T1 data,
Mezzetti et al. reported that the bound dihydrogen ligand in
[RuX(η2-H2)(PP)2]+ (PP ) dppp) complexes was elongated.14
Our group reported several examples of dicationic dihydro-
gen complexes with the [Ru(PP)2] backbone of the type [Ru-
(η2-H2)(L)(PP)2]2+ (L ) RCN, P(OR)3, PF(OR)2, PF3; PP
) dppm, dppe), and in all of those cases, dHH was found to
be less than 1.0 Å.15
Acknowledgment. We are grateful to the Department of
Science and Technology, India, for financial support and also
for funding of the procurement of a 400-MHz NMR
spectrometer under the “FIST” program. We thank the
Department of Organic Chemistry, Indian Institute of Sci-
ence, for the mass spectral data.
Supporting Information Available: Synthesis and character-
ization details for the compounds, VT T1 data, J(H,D) data, NMR
spectra of the reactions, and mass spectra (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
(10) (a) 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. (b) Chin, B.; Lough, A. J.; Morris, R. H.; Schweitzer, C. T.;
D’Agostino, C. Inorg. Chem. 1994, 33, 6278-6288. (c) Chinn, M.
S.; Heinekey, D. M.; Payne, N. G.; Sofield, C. D. Organometallics
1989, 8, 1824-1826.
IC061143A
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Organometallics 2000, 19, 4506-4517. (b) Majumdar, K. K.; Nan-
ishankar, H. V.; Jagirdar, B. R. Eur. J. Inorg. Chem. 2001, 1847-
1853. (c) Mathew, N.; Jagirdar, B. R.; Ranganathan, A. Inorg. Chem.
2003, 42, 187-197. (d) Nanishankar, H. V.; Nethaji, M.; Jagirdar, B.
R. Eur. J. Inorg. Chem. 2004, 3048-3056.
(11) The 1H{31P} NMR spectrum of the HD isotopomer gave a 1:1:1 triplet
with a J(H,D) of 22.0 Hz, consistent with that obtained in the 1H NMR
spectrum. See the Supporting Information.
(12) (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.
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Schweitzer, C. T.; Sella, A.; Zubkowski, J. J. Am. Chem. Soc. 1991,
113, 4876-4887.
(14) Barthazy, P.; Broggini, D.; Mezzetti, A. Can. J. Chem. 2001, 79, 904-
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Inorganic Chemistry, Vol. 45, No. 18, 2006 7049