Synthesis and characterization of the first examples of dicationic dihydrogen complexes of iron and ruthenium with the PF3 ligand

Article abstract of DOI:10.1002/ejic.200400130

New dicationic dihydrogen complexes of the type trans-[M(η²- H₂)(PF₃)(diphosphane)₂]²⁺ [M = Ru, diphosphane = dppm (Ph₂PCH₂PPh₂); M = Fe, Ru, diphosphane = dppe (Ph₂PCH₂CH₂PPh₂)] have been prepared from the precursor hydrides trans-[M(H)(PF ₃)(diphosphane)₂]⁺ upon reaction with HOTf. In the case of dppm, in addition to the trans-dihydrogen complex cis-[Ru(η²-H₂)(PF₃)(dppm)₂] ²⁺ was also obtained in the protonation reaction. The intact nature of the H-H bond in these derivatives has been established using the spin-lattice relaxation time measurements (short T₁ values) and the large J _H,D coupling constant of the H-D isotopomers. The H-H bond lengths and the stabilities of the dihydrogen complexes are discussed in terms of the π-acidity of the PF₃ ligand and compared with other systems possessing trans CO and CNH ligands. The trans-[Ru(η²-H ₂)(PF₃)(dppe)₂]²⁺ complex was found to be remarkably stable with respect to the loss of bound H₂ for a period of about 16 h. The H-D isotopomer of this complex exhibits small temperature variations in the J_H,D coupling constant. The X-ray crystal structure of trans-[Ru(H)(PF₃)(dppm)₂][BF ₄] has been determined. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400130

FULL PAPER
Synthesis and Characterization of the First Examples of Dicationic Dihydrogen
Complexes of Iron and Ruthenium with the PF3 Ligand
H. V. Nanishankar,[a] Munirathinam Nethaji,[a] and Balaji R. Jagirdar*[a]
Keywords: Dihydrogen complexes / HϪH activation / Hydride ligands / Iron / Phosphane ligands / Ruthenium
New dicationic dihydrogen complexes of the type trans-
[M(η2-H2)(PF3)(diphosphane)2]2+ [M = Ru, diphosphane =
dppm (Ph2PCH2PPh2); M = Fe, Ru, diphosphane = dppe
(Ph2PCH2CH2PPh2)] have been prepared from the precursor
hydrides trans-[M(H)(PF3)(diphosphane)2]+ upon reaction
with HOTf. In the case of dppm, in addition to the trans-
dihydrogen complex cis-[Ru(η2-H2)(PF3)(dppm)2]2+ was also
obtained in the protonation reaction. The intact nature of the
H−H bond in these derivatives has been established using
the spin−lattice relaxation time measurements (short T1
hydrogen complexes are discussed in terms of the π-acidity
of the PF3 ligand and compared with other systems pos-
sessing trans CO and CNH ligands. The trans-[Ru(η2-
H2)(PF3)(dppe)2]2+ complex was found to be remarkably
stable with respect to the loss of bound H2 for a period of
about 16 h. The H−D isotopomer of this complex exhibits
small temperature variations in the JH,D coupling constant.
The X-ray crystal structure of trans-[Ru(H)(PF3)(dppm)2][BF4]
has been determined.
values) and the large JH,D coupling constant of the H−D isoto- ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
pomers. The H−H bond lengths and the stabilities of the di-
Germany, 2004)
Introduction
CO ligand trans to the H2 imparts the high acidity to the
dihydrogen ligand. Morris and co-workers have reported an
iron dihydrogen complex trans-[Fe(η2-H2)(CO)(dppe)2]2ϩ
that was found to be quite acidic.[14] In addition to this
species, they also prepared a highly acidic dicationic dihy-
drogen complex, trans-[Ru(η2-H2)(CNH)(dppe)2]2ϩ that
was found to be stable with respect to the loss of protons
or H2 under strongly acidic conditions (excess triflic
acid).[13] A ligand that has similar π acidity to both CO and
CNH is PF3. Dihydrogen complexes with the PF3 ligand
are limited to only one example, the rhenium complex fac-
[Re(H2)(PPh3)(PF3)(dien)]ϩ.[15,16] As yet there are no re-
ports in the literature of dicationic dihydrogen complexes
of transition metals with the PF3 ligand. Therefore, in con-
tinuation of our investigations on the heterolytic activation
of molecular hydrogen we report here the first examples of
dicationic dihydrogen complexes of iron and ruthenium
The heterolytic activation of molecular hydrogen by tran-
sition metal centers to give a proton and a metal hydride is
one of the most intriguing reactions of dihydrogen com-
plexes. Highly electrophilic metal centers have been found
to greatly promote heterolysis of dihydrogen.[1,2] This acti-
vation pathway leads to interesting reactivity, including ca-
talysis and selective proton transfer to an ancillary li-
gand.[3,4] The ligand trans to the η2-H2 moiety has a signifi-
cant role in dictating the properties of the dihydrogen com-
plex, including the heterolytic cleavage of the bound
dihydrogen.
We have been interested in preparing systems that are
capable of activating H2 in a heterolytic fashion. Toward
this goal, we have reported certain dicationic dihydrogen
complexes of iron and ruthenium bearing trans-phosphite,
phosphane,[5Ϫ7] and nitrile[8] ligands that were found to be
quite acidic. Numerous examples of highly acidic η2-H2
complexes have been reported in the literature.[9Ϫ14]
Rocchini et al.[12] have reported two highly acidic dihydro-
gen complexes trans-[M(η2-H2)(CO)(dppp)2]2ϩ [M ϭ Ru,
Os; dppp ϭ 1,3-bis(diphenylphosphanyl)propane]. The ru-
thenium derivative was found to be stable only at low tem-
peratures whereas the osmium complex is stable even at
room temperature. The strong π-accepting tendency of the
with
a
trans-PF3 ligand trans-[M(η2-H2)(PF3)(diphos-
phane)2]2ϩ (M ϭ Fe, diphosphane ϭ dppe; M ϭ Ru, di-
phosphane ϭ dppm, dppe).
Results and Discussion
Preparation and Properties of the Hydride Complexes
The new hydride complexes trans-[M(H)(PF3)(diphos-
phane)2][BF4] (M ϭ Fe, diphosphane ϭ dppe 1; M ϭ Ru,
diphosphane ϭ dppm 2, dppe 3) were prepared by substi-
tution of the bound H2 ligand in trans-[M(H)(η2-H2)(di-
phosphane)2][BF4] with excess PF3 gas [Equation (1)]. The
[a]
Department of Inorganic & Physical Chemistry,
Indian Institute of Science, Bangalore 560 012, India
Fax: (internat.) ϩ 80-2360-0683
E-mail: jagirdar@ipc.iisc.ernet.in
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DOI: 10.1002/ejic.200400130
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Dicationic Dihydrogen Complexes of Iron and Ruthenium
FULL PAPER
products were obtained as colorless solids that were puri- NMR spectroscopic data for these complexes are summar-
fied by crystallization from a CH2Cl2/Et2O mixture to give ized in Table 1. In order to get an insight into the structural
colorless crystals.
aspects, we carried out an X-ray crystallographic study of 2.
Structure of trans-[Ru(H)(PF3)(dppm)2][BF4] (2)
The ORTEP diagram of the trans-[Ru(H)(PF3)(dppm)2]ϩ
Ϫ
cation is shown in Figure 1. In addition to a discrete BF4
counterion, two molecules of H2O were also found (the
source of these water molecules could be the solvent used
for crystallization because strict measures were not taken to
exclude the moisture). The structure is a highly distorted
square pyramid defined by the four nearly coplanar dppm
phosphorus atoms, with the PF3 group almost perpendicu-
lar to this plane. The hydride ligand that occupies the sixth
coordination site on the metal was not located, although
(1)
1
its presence was confirmed by H NMR spectroscopy. The
RuϪP(dppm) bond lengths vary from 2.3302(18) to
1
The H NMR spectra of the hydride complexes show a
doublet of multiplets pattern for the hydride moiety due to
coupling with the phosphorus (trans and cis) and fluorine
nuclei. The JH,P
coupling constants were found to be
155.6 and 171.9 tHranzs whereas the JH,P coupling constants
were found to be 21.0 and 19.1 Hz forci2s and 3, respectively.
The 1H{31P} NMR spectra gave a quadruplet in the hydride
region due to H, F coupling, with JH,F equal to 33.1 and
34.4 Hz for 2 and 3, respectively. In the case of 1, a mul-
1
tiplet at δ ϭ Ϫ7.06 ppm in the H NMR spectrum with
an average JH,P coupling constant (H,Ptrans and H,Pcis) of
50.0 Hz was observed. The 31P{1H} NMR spectrum con-
sists of one doublet for the dppe P nuclei coupled to the
PF3 ligand and a quadruplet of quintets for the PF3 moiety
coupled to three fluorine and the four dppe P nuclei. The
JP,F coupling constant was found to be in the range
1267.0Ϫ1307.9 Hz, which is typical for compounds having
direct PϪF bonds.[17Ϫ20] The 19F NMR spectra of the hy-
dride complexes consist of a doublet of doublets for the PF3
ligand and a broad singlet for the BF4 counterion. The ion at the 50% probability level
Figure 1. ORTEP view of the trans-[(dppm)2Ru(H)(PF3)]ϩ (2) cat-
Ϫ
1
Table 1. H, 31P{1H}, and 19F NMR spectral data (δ) for trans-[M(H)(PF3)(diphosphane)2][BF4] complexes in CD2Cl2
Fragment
1
2
3
1H
δ(MϪH)
Ϫ7.06 (m, 1 H)
Ϫ2.95 (dm, 1 H)
155.6
21.0
Ϫ6.73 (dm, 1 H)
171.9
19.1
JH,P
(Hz)
50.0[a]
JH,Ptran(sHz)
cis
JH,F (Hz)
δ(CH2)
20.2
33.1
34.4
2.30 (m, 4 H)
2.60 (m, 4 H)
6.60Ϫ8.20 (m, 40 H)
77.9 (d, 4 P)
44.2
149.0 (q quint, 1 P)
1267.0
4.64 (m, 2 H)
4.97 (m, 2 H)
7.11Ϫ7.57 (m, 40 H)
Ϫ3.8 (d, 4 P)
32.1
132.9 (q quint, 1 P)
1307.9
2.29 (m, 4 H)
2.61 (m, 4 H)
6.66Ϫ7.30 (m, 40 H)
57.7 (d, 4 P)
29.2
126.8 (q quint, 1 P)
1304.6
δ(Ph)
31P
δ(diphosphane)
JP,P (Hz)
δ(PF3)
JP,F (Hz)
δ(PF3)
19F
Ϫ12.13 (dd)
Ϫ159.28 (br)
Ϫ19.00 (dd)
Ϫ161.18 (br)
Ϫ18.20 (dd)
Ϫ166.07 (br)
δ(BF4)
[a]
Average; no separate H,Ptrans and H,Pcis couplings seen.
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3049

H. V. Nanishankar, M. Nethaji, B. R. Jagirdar
FULL PAPER
˚
2.352(2) A whereas the RuϪP(PF3) distance was found to signals that were extensively broadened upon loss of H2 due
be 2.206(2) A. The bond lengths of certain analogous ru- to the generation of some FeIII species, amongst others. In
˚
˚
thenium derivatives fall in the following order: 2.3153(17) A an attempt to observe the dihydrogen complexes spectro-
{RuϪP(OMe)3 in trans-[Ru(H){P(OMe)3}(dppm)2][BF4]}[7] scopically, the addition of the protonating agent was done
˚
Ͼ
2.264(2)
A
{RuϪPF(OMe)2 in trans-[Ru(H){PF- at liquid N2 temperature and then the mixture was slowly
(OMe)2}(dppe)2][BF4]}[5] Ͼ 2.206(2) A {RuϪPF3 in trans- warmed up to 0 °C; the NMR spectral studies were also
[Ru(H)(PF3)(dppm)2][BF4]}. This order is a combined ef- carried out at this temperature. Under these conditions, we
fect of the progressive increase in the π-accepting ability as observed signals corresponding to the dihydrogen com-
well as the decrease in the steric crowding due to the re- plexes only in the case of 2.
˚
duction in the cone angles[21,22] of the trans-phosphorus li-
gands. The dppm bite angles P(1)ϪRu(1)ϪP(2) and of H2 resulted in
The protonation of 2 with HOTf under an atmosphere
a
mixture of trans-[Ru(η2-H2)-
P(3)ϪRu(1)ϪP(4) are 71.87(6)° and 70.48(5)°, respectively. (PF3)(dppm)2]2ϩ (5) and cis-[Ru(η2-H2)(PF3)(dppm)2]2ϩ (6)
The P(1)ϪC(25)ϪP(2) and P(3)ϪC(50)ϪP(4) bond angles in a ratio of 1:3. We have previously observed a similar be-
are 96.6(3)° and 94.4(3)°, respectively, falling at the lower havior in certain analogous ruthenium complexes with trans
end of the range of PϪCϪP angles (92Ϫ133°) that the phosphite ligands.[7] The complexes of the type trans-
dppm ligand exhibits in its complexes. The crystallographic [Ru(H)[P(OR)3](dppm)2][BF4] isomerize under thermal
data are given in the Exp. Sect. and selected bond lengths conditions (in the presence of small amounts of acid im-
and angles are summarized in Table 2.
purities in the solvent) to give a mixture of the trans and
the cis hydride phosphite derivatives. We traced the origin
of this isomerization to the lability of the trans phosphite
ligand. The addition of acid accelerated the isomerization
reaction in those derivatives resulting in the corresponding
trans and cis dihydrogen complexes trans/cis-[Ru(η2-
˚
Table 2. Selected bond lengths (A) and angles (°) for trans-
[Ru(H)(PF3)(dppm)2][BF4] (2)
Ru(1)ϪP(1)
Ru(1)ϪP(2)
Ru(1)ϪP(3)
Ru(1)ϪP(4)
Ru(1)ϪP(5)
P(5)ϪF(1)
2.3302(18)
2.3481(17) H2)[P(OR)3](dppm)2][BF4]2.[7] The mechanism of the iso-
2.352(2)
2.3437(18)
2.206(2)
1.554(4)
merization in the current work, however, seems to be differ-
ent. The PF3 ligand was not found to be labile; refluxing
solutions containing the starting trans hydride complexes
did not give any isomerized species, thus suggesting that the
isomerization occurs only upon addition of the acid. We
monitored the loss of the bound H2 ligand and found that
it was faster in the case of 5 than 6. A plausible mechanism
could involve the loss of H2 ligand from the trans dihydro-
gen complex, rearrangement of the five-coordinate species,
followed by the re-binding of the H2 cis to the PF3 moiety.
When the protonation was carried out under an Ar atmos-
phere, we found no evidence of isomerization except for the
loss of H2 ligand from 5; a singlet at δ ϭ 4.6 ppm due to
free H2 was observed in the 1H NMR spectrum in this case.
This suggests that the isomerization is effected upon loss of
H2 from the trans dihydrogen complex followed by the re-
binding of H2 cis to PF3 only when there is excess H2. The
P(5)ϪF(2)
P(5)ϪF(3)
1.552(5)
1.549(5)
71.87(6)
70.48(5)
165.15(6)
168.11(6)
100.70(7)
96.78(7)
93.78(7)
95.05(7)
P(1)ϪRu(1)ϪP(2)
P(3)ϪRu(1)ϪP(4)
P(1)ϪRu(1)ϪP(3)
P(2)ϪRu(1)ϪP(4)
P(1)ϪRu(1)ϪP(5)
P(2)ϪRu(1)ϪP(5)
P(3)ϪRu(1)ϪP(5)
P(4)ϪRu(1)ϪP(5)
Protonation Reactions of trans-[Fe(H)(PF3)(dppe)2][BF4]
and trans-[Ru(H)(PF3)(dppm)2][BF4].
The protonation of the hydrides with HOTf in CD3CN mechanism is illustrated in Scheme 1.
or with HBF4·Et2O in CD2Cl2 did not afford the dihydro-
gen complexes; even with large excesses of the acid, no reac-
tion took place. These experiments indicate that the pKa of
the protonating agent must be less than Ϫ2.4 in order to
obtain the dihydrogen complexes since the pKa of HOTf in
CH3CN is 2.6[23] and that of HBF4·Et2O is Ϫ2.4.[24] When
the hydrides were protonated with excess triflic acid
under H2 atmosphere in CD2Cl2 at room temperature,
the corresponding dihydrogen complexes trans-[Fe-
(η2-H2)(PF3)(dppe)2]2ϩ (4) and trans-[Ru(η2-H2)(PF3)-
(dppm)2]2ϩ (5) were presumably formed, although we found
that they lose the H2 ligand quite rapidly thus precluding
the NMR spectroscopic characterization of these com-
plexes. The singlet at δ ϭ 4.6 ppm in the 1H NMR spectrum
due to free H2 grew in intensity in these experiments. In the
case of protonation of 1, the resulting mixture gave NMR Scheme 1
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Dicationic Dihydrogen Complexes of Iron and Ruthenium
FULL PAPER
Another possible mechanism for the isomerization could (JH ,P
ϭ 56.4 Hz). We have observed such strong coup-
trans
2
involve the protonation of one of the dppm P atoms, open- lings in the past.[5Ϫ7] The 31P{1H} NMR spectrum displays
ing up of the chelate arms to give RuϪ(η1-dppmHϩ), fol- a single signal for all the four dppm phosphorus atoms indi-
lowed by the isomerization. The dppmHϩ could then pro- cating that they are coplanar. On the other hand, cis-
tonate the hydride ligand and close its arm. The pKa1 of [Ru(η2-H2)(PF3)(dppm)2]2ϩ (6) shows a broad singlet at δ
1
dppmHϩ has been reported to be 3.81.[25] The protonated Ϫ4.86 ppm for the bound H2 ligand in the H NMR spec-
dppm P ([Ph2PCH2PHPh2]ϩ) undergoes a dramatic down- trum and two multiplets for the dppm P atoms in the
field shift with respect to the bound dppm P chemical shifts 31P{1H} NMR spectrum. The NMR spectroscopic data for
in the 31P NMR spectroscopy, as has been observed by Rot- these derivatives are summarized in Table 3.
tink and Angelici earlier.[26] We did not find any evidence
for a dangling dppmHϩ in our 31P NMR spectral studies.
Protonation Reaction of trans-[Ru(H)(PF3)(dppe)2][BF4]
In addition, since the pKa of the protonating agent needs
The protonation of 3 was attempted with various pro-
to be lower than Ϫ2.4 in order to obtain the dihydrogen
tonating agents such as HCl, HBr, HI, CF3COOH, and
complexes, we can rule out this mechanism.
HBF4·Et2O; once again, the behavior of this hydride was
The protonation of 2 in CD2Cl2 followed by the addition
similar to that of 1 and 2. The dihydrogen complex trans-
of CD3CN gave trans-[Ru(CD3CN)(PF3)(dppm)2]2ϩ (major
[Ru(η2-H2)(PF3)(dppe)2]2ϩ (7) was obtained upon pro-
product)[27] and the precursor hydride 2 (minor product).
tonation with excess triflic acid. Titration of the hydride
The observation of a small amount of the precursor hydride
complex with triflic acid revealed that it requires 50 equiva-
could be due to a part of 2 remaining unchanged under
lents of the acid for the complete conversion of the hydride
these conditions. These results are summarized in Scheme 2.
to the dihydrogen complex. This suggests that the pKa of 7
must be much less than Ϫ5 since the aqueous pKa of HOTf
has been estimated to be about Ϫ5.[12]
Basallote and co-workers[28] obtained similar results in
the protonation of CpRuHL complexes (L ϭ dppm, dppe,
2PPh3) with various acids. They suggested that the lack of
formation of the dihydrogen complexes in the presence of a
large excess of HCl, HBr, or CF3COOH was unlikely to be
due to kinetic reasons because they observed protonation
of certain other metal hydrides with these acids to be usu-
ally faster than with HBF4·Et2O. The failure of the pro-
tonation was related to the thermodynamics of the process.
There have been other reports in the literature of the forma-
tion of dihydrogen complexes upon protonation of the hy-
drides with both strong and weak acids.[29,30] The fact that
protonation of our hydride complexes failed with all acids
except HOTf, and that the complete conversion of the hy-
Scheme 2
drides to the corresponding η2-H2 derivatives requires large
excesses of the acid (HOTf), indicates that our dihydrogen
trans-[Ru(η2-H2)(PF3)(dppm)2]2ϩ (5) gives a broad doub- complexes are extremely strong acids, being roughly similar
let at δ ϭ Ϫ1.75 ppm in the H NMR spectrum for the to HOTf.[31] It is intriguing to note that 7 was found to be
1
bound H2 ligand coupled to the trans-PF3 ligand stable at room temperature under H2 atmosphere for about
1
Table 3. H, 31P{1H}, and 19F NMR spectral data (δ) for [M(η2-H2)(PF3)(diphosphane)2]2ϩ complexes in CD2Cl2
Fragment
5
6
7
1H
δ(RuϪH2)
Ϫ1.75 (br. d, 2 H)
56.4
Ϫ4.86 (br. s, 2 H)
Ϫ4.33 (br. d, 2 H)
63.0
JH,P
(Hz)
trans
δ(CH2)
4.86 (m, 2 H)
5.08 (m, 2 H)
6.35Ϫ7.76 (m, 40 H)
Ϫ15.6 (d, 4 P)
5.93 (m, 4 H)
2.83 (m, 4 H)
3.00 (m, 4 H)
6.24Ϫ7.82 (m, 40 H)
49.3 (d, 4 P)
δ(Ph)
δ(diphosphane)
6.35Ϫ7.76 (m, 40 H)
Ϫ13.9 (m, 2 P)
Ϫ16.2 (m, 2 P)
45.0
130.8 (m, 1 P)
1286.0
Ϫ20.80 (d)
Ϫ91.10 (br)
Ϫ144.25 (br)
31P
JP,P (Hz)
δ(PF3)
J(P,F) (Hz)
δ(PF3)
δ(OTf)
δ(BF4)
40.0
29.2
134.4 (q quint, 1 P)
1261.5
Ϫ16.20 (d)
Ϫ90.54 (br)
Ϫ160.14 (br)
127.4 (q quint, 1 P)
1262.5
Ϫ5.50 (d)
Ϫ91.71 (br)
Ϫ144.77 (br)
19F
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H. V. Nanishankar, M. Nethaji, B. R. Jagirdar
FULL PAPER
16 h after which time it begins to decompose. We found
evidence for some dppeHϩ in the 31P NMR spectrum
(singlet at δ ϭ 10.0 ppm).[14] We attempted to isolate 7 by
precipitation with Et2O; our attempts, however, failed, and
only the starting hydride complex 3 was recovered. In this
case, the dihydrogen complex 7 protonates the Et2O to
give Et2OHϩ.
Table 4. Variable-temperature spinϪlattice relaxation times (T1, ms;
400 MHz) of the dihydrogen ligand in [M(η2-H2)(PF3)-
[a]
(diphosphane)2]2ϩ complexes in CD2Cl2
T (K)
T1(5)
T1(6)
T1(7)
298
284
283
273
263
258
243
18.0
17.3
12.3
10.1
10.8
8.7
7.2
7.5
The NMR spectral features of 7 (Table 3) are quite simi-
13.0
lar to those of 5; the JH ,P
coupling constant of 63.0 Hz
trans
2
is even larger than the values (49Ϫ51 Hz) we found earlier
15.9
20.2
in the cases of trans-[Ru(η2-H2)(P)(diphosphane)2]2ϩ com-
11.5
13.0
8.7
8.7
plexes [diphosphane ϭ dppe, P ϭ PF(OR)2; diphosphane ϭ 233
dppm, P ϭ P(OR)3, PF(OR)2].[5Ϫ7] This stronger coupling
228
223
28.9
14.4
18.7
10.8
14.4
could be due to the presence of the more electronegative F
200
substituents in the PF3 complex, which results in an enor-
[a]
mous enhancement of the ‘‘s’’ character of the bonding or-
Italicized data indicate T1 minima.
bitals in comparison to the PF(OR)2 complexes if it is as-
sumed that the coupling is dominated by the Fermi contact
term.[32] No observable H2ϪF couplings were observed. In
an attempt to look for H2ϪF couplings in the H NMR
Table 5. dHH values from T1 (min) (400 MHz) for [M(η2-
1
H2)(PF3)(diphosphane)2]2ϩ complexes in CD2Cl2
spectrum, we decoupled all the P nuclei and obtained a
˚
˚
T1 min
dHH (slow) A
dHH (fast) A
broad singlet, indicating that the JH ,F coupling constant is
2
smaller than the line-width of the signal.
5
6
7
10.1
7.2
13.0
0.99
0.94
1.03
0.79
0.79
0.81
Rate Constant for the Decomposition of trans-[Ru(η2-H2)-
(PF3)(dppe)2]2؉ (7)
We studied the kinetics of the decomposition of 7 by
monitoring the decrease in the integrals of the dihydrogen
complex for the η2-H2 ligand in the 1H NMR spectrum and
the dppe phosphorus in the 31P{gated 1H} NMR spectrum.
From the kinetics data, the pseudo-first-order rate constant
was found to be 4 ϫ 10Ϫ5 sϪ1 with a half-life of about
4.45 h. The residual material in the NMR tube after 16 h
consisted of dppeHϩ (ca. 46%), trans-[RuCl(PF3)(dppe)2]ϩ
(8; ca. 36%), and trans-[Ru(OTf)(PF3)(dppe)2]ϩ (9; Ͻ5%)
along with certain other unidentifiable RuIII species. The
source of the chloride in 8 is the solvent, CD2Cl2.
(dppm)2]2ϩ 6-D1, and trans-[Ru(η2-HD)(PF3)(dppe)2]2ϩ 7-
D1 derivatives, respectively. The H NMR spectrum of the
1
hydride region of 5-D1 and 7-D1 is expected to show a
doublet of 1:1:1 triplets pattern: the doublet due to the
coupling of H with the trans phosphorus, and 1:1:1 triplet
from the coupling of the proton with deuterium. Figure 2a
displays this pattern in the case of 7-D1. An overlap of the
signals is the cause of the poor resolution of the six lines.
Upon decoupling the phosphorus atoms, the 1H NMR
spectrum shows an approximate 1:1:1 triplet pattern due to
Variable Temperature Spin؊Lattice Relaxation Time
Measurements.
The variable temperature spinϪlattice relaxation times
(T1) for the η2-H2 moiety were determined; the data are
summarized in Table 4. T1 minima of 10.1, 7.2, and 13.0
ms (at 400 MHz and 273 K) were obtained for 5, 6, and
7, respectively. The HϪH distances for the slow- and fast-
rotation regimes thus calculated[33] from the T1 minima are
tabulated in Table 5.
HD Isotopomers
In an attempt to prepare the HD isotopomers of 5, 6, and
7, we purged the respective CD2Cl2 solutions containing the
H2 complexes with HD gas (generated from NaH and
D2O). Despite purging with HD for a considerable period
of time, no observable HD isotopomer was obtained.
Therefore, we reacted the starting hydride complexes 2 and
3 with excess F3CSO3D in CD2Cl2 to obtain the trans-
Figure 2. (a) 1H NMR spectrum of trans-[Ru(η2-
1
HD)(PF3)(dppe)2]2ϩ (7-D1); (b) H{31P} NMR spectrum of trans-
[Ru(η2-HD)(PF3)(dppm)2]2ϩ 5-D1, cis-[Ru(η2-HD)(PF3)- [Ru(η2-HD)(PF3)(dppe)2]2ϩ (7-D1)
3052
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Eur. J. Inorg. Chem. 2004, 3048Ϫ3056

Dicationic Dihydrogen Complexes of Iron and Ruthenium
FULL PAPER
HϪD coupling (Figure 2b). The complex 6-D1 exhibits only
The effect of the ligand trans to H2 on the properties of
a 1:1:1 triplet pattern due to the presence of a trans phos- the dihydrogen complex, for example dHH and stability, can
phane that is not as strong an acceptor as PF3. The JH,D be interpreted in terms of the σ donation (η2-H2 Ǟ empty
coupling constants were found to be 27.0, 28.0, and 30.0 Hz metal d orbital) and the back-donation (filled metal d or-
at 273 K for 5-D1, 6-D1, and 7-D1, respectively. The HϪH bital Ǟ σ* orbital of η2-H2) components. Woska et al.[41]
distances calculated from the HϪD coupling constant[34,35] have determined the stereoelectronic parameters of a series
˚
are 0.97, 0.96, and 0.92 A for 5-D1, 6-D1, and 7-D1, respec- of phosphorus ligands and concluded that PF3 is the weak-
tively. In addition, we carried out variable temperature JH,D est σ donor and the strongest π acid. Thus, the trans-PF3
measurements for 7-D1; the data are summarized in Table 6 ligand exerts a high trans influence due to its strong π-acid-
and show an interesting temperature dependence.
ity; in addition, it is also expected to stabilize the metal d
orbitals resulting in reduced overlap with the antibonding
orbitals of the H2. Consequently, a strengthening of the
HϪH bond and a weakening of the MϪH2 interaction oc-
curs giving rise to dihydrogen complexes with short dHH
values. The dihydrogen complexes 5, 6, and 7 show HϪH
Table 6. Temperature dependence of JH,D for trans-[Ru(η2-
H2)(PF3)(dppe)2]2ϩ in CD2Cl2
˚
˚
distances of 0.97, 0.96, and 0.92 A, respectively, at 273 K.
T (K)
JH,D
dHH (A)
These distances, rather surprisingly, are not appreciably
short. We have also measured the JH,D coupling constant
of 7-d1 at various temperatures and obtained a dHH of 0.88
298
273
258
243
228
213
29.8
30.0
30.4
31.0
31.4
32.0
0.922
0.919
0.912
0.902
0.895
0.885
˚
A at 213 K. This distance is similar to the HϪH distance
found in the case of trans-[Ru(η2-H2)(CO)(dppp)2]2ϩ (0.85
[12]
˚
A) reported by Rocchini et al.
and trans-[Ru(η2-
H2)(CNH)(dppm)2]2ϩ (0.88 A) reported by Morris and co-
workers.[13] Both of these compounds are unstable with re-
spect to the loss of H2. Since 7 loses H2 as noted earlier,
we attempted to determine the equilibrium constant for the
substitution of the bound H2 with CH3CN as this could
provide insight into the relative MϪH2 bond strength for
the complexes prepared in this study. However, we found
that CH3CN substitutes H2 completely even under stoichio-
metric conditions (1:1 molar ratio, complex:CH3CN) thus
precluding the measurement of the equilibrium constant.
We earlier found that the H2 ligand in complexes of the
type trans-[Ru(η2-H2)[P(OR)3](dppm)2]2ϩ and cis-[Ru(η2-
H2)[PF(OR)2](dppm)2]2ϩ is labile; however, upon loss of the
H2 ligand (typically loss of H2 starts to occur after
24Ϫ36 h), the dihydrogen complex could be recovered by
purging the solutions containing the five-coordinate species
with H2 gas.[7] Considering the greater π-acidity of the PF3
ligand in comparison to P(OR)3 or PF(OR)2, it is not sur-
prising to find that 7 is stable only for about 16 h before it
starts to decompose. Thus, the π-acidities of the CO, CNH,
˚
There have been a few reports on the unusual tempera-
ture dependence of JH,D coupling in the case of elongated
dihydrogen complexes, for example [RuCp*(H···D)-
(dppm)]ϩ,[36]
trans-[Os(H)(H···D)(dppe)2]ϩ,[37]
trans-
[Os(Cl)(H···D)(dppe)2]ϩ,[36] and [RuCp/Cp*(H···D/T)-
(PP)]ϩ.[38] Based on the theoretical studies by Gelabert et
al.,[39] it was interpreted that the varying thermal popu-
lation of the vibrational excited states of the M-H2 unit re-
sults in the unusual temperature dependence of JH,D of the
elongated dihydrogen complexes. The data presented in
Table 6 show that the variations in JH,D with temperature
are small and close to the error in the determination of the
HϪD coupling constant.
H؊H Distances and the Stabilities of the Dihydrogen
Complexes
The HϪH distances obtained from the T1(min) data fall and PF3 ligands account for the trend in the stability of the
˚
in the range 1.03Ϫ0.75 A (see Table 5) for the slow- and MϪH2 bond for the above three complexes, which increases
the fast-rotation regimes. A correlation between the HϪH in the order CO Յ CNH Ͻ PF3. We found that the pro-
distances calculated from JH,D and T1(min) and ∆G‡ for the tonation of 1 with HOTf resulted in extensive broadening
1
loss of the H2 ligand from the metal center has been re- of the H NMR signals due to the presence of FeIII species
ported by Gusev et al.[40] The dHH values calculated from that are generated quite rapidly from a highly unstable iron
JH,D in the current work are of the order of 0.92Ϫ0.97 A dihydrogen complex. We believe that the origin of the FeIII
˚
(273 K) which are intermediate between the values of 1.03 species is a dihydrogen complex since, upon its (FeIII spe-
˚
and 0.75 A obtained using the slow- and the fast-spinning cies) generation, a singlet at δ ϭ 4.6 ppm was observed,
approximations of H2. Morris and Wittebort[33] have sug- indicating the evolution of H2. This observation further
gested that HϪH distances obtained from JH,D that are in- supports the earlier suggestion that the MϪH2 interaction
termediate between those derived from the T1(min) data becomes stronger on going down the group, as has been
indicate that librational motion significantly influences the found earlier.[12,42] We speculate that a compound of for-
H2 relaxation process. In addition to these complexes, sev- mula trans-[Os(η2-H2)(PF3)(diphosphane)2]2ϩ, if realized,
eral examples have been reported earlier where such a situ- could be isolable. Efforts toward this goal are in progress in
ation has been observed.[33]
our laboratories.
Eur. J. Inorg. Chem. 2004, 3048Ϫ3056
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3053

H. V. Nanishankar, M. Nethaji, B. R. Jagirdar
FULL PAPER
BF4]. ES-MS of 3: m/z ϭ 897 [Mϩ Ϫ PF3 ϩ HBF4]. The NMR
spectroscopic data of 2 and 3 are given in Table 1.
Conclusion
The complexes trans-[Ru(η2-H2)(PF3)(dppm)2]2ϩ (5),
cis-[Ru(η2-H2)(PF3)(dppm)2]2ϩ (6), and trans-[Ru(η2-
H2)(PF3)(dppe)2]2ϩ (7) are formulated as dihydrogen com-
plexes based on their short T1 values and the observation
of substantial HϪD coupling in the corresponding HD iso-
topomers. trans-[Ru(η2-HD)(PF3)(dppe)2]2ϩ shows a tem-
perature-dependent JH,D coupling, although this effect is
only small. All of these derivatives, including trans-[Fe(η2-
H2)(PF3)(dppe)2]2ϩ (4), are unstable at room temperature.
The dihydrogen complexes are formed only upon addition
of excess HOTf to the precursor hydrides and they pro-
tonate Et2O in solution, indicating the high acidities of
these derivatives.
Attempt to Prepare trans-[Fe(η2-H2)(PF3)(dppe)2]2؉ (4): A sample
of trans-[Fe(H)(PF3)(dppe)2][BF4] (15 mg) was placed in a 5 mm
NMR tube capped with a septum. The tube was then evacuated
and filled with H2 in three cycles. The hydride complex was then
dissolved in 0.6 mL of CD2Cl2 and the solution was frozen in a
liquid-nitrogen bath. About 20 µL (16 equiv.) of triflic acid (HOTf)
was added and the tube was warmed up to 273 K slowly. It was
1
then transferred to the NMR probe precooled to 273 K. H NMR
spectroscopy revealed a decrease in the intensity of the hydride res-
onances, whereas the 31P{1H} NMR spectrum showed the appear-
ance of an intense signal at δ ϭ 10.0 ppm due to dppeHϩ. At the
same time, the lock signal deteriorated, signaling the formation of
some FeIII species. No evidence of the formation of a dihydrogen
complex was apparent, although the singlet at δ ϭ 4.6 ppm due to
free H2 increased in intensity. Upon addition of a further 10 µL (8
equiv.) of HOTf, the lock signal deteriorated further and all the 1H
NMR signals broadened. The intensity of the dppeHϩ in the 31P
NMR spectrum increased significantly.
Experimental Section
Preparation of trans- and cis-[Ru(η2-H2)(PF3)(dppm)2]2؉ (5 and 6):
The preparation of these two complexes was carried out as de-
scribed above. Upon addition of 30 µL (60 equiv.) of HOTf the
tube was warmed up to 273 K and then transferred to the NMR
probe precooled to 273 K. The dihydrogen complexes formed were
observed by NMR spectroscopy. The NMR spectroscopic data of
5 and 6 are summarized in Table 3.
General Remarks: All reactions were carried out under N2 at room
temperature using standard Schlenk[43,44] and inert atmosphere
techniques unless otherwise noted. Solvents used for the prep-
aration of dihydrogen complexes were thoroughly saturated with
either H2 or Ar just before use. The 1H, 31P, and 19F NMR spectro-
scopic data were obtained using AMX Bruker 400 MHz or Avance
Bruker 400 MHz instruments. The shift of the residual protons of
the deuterated solvent was used as an internal reference. Variable-
temperature proton T1 measurements were carried out at 400 MHz
using the inversion recovery method (180°-τ-90° pulse sequence at
each temperature)[45] and the data are summarized in Table 4; the
dHH values obtained from the T1 minima are given in Table 5. All
31P NMR spectra were proton decoupled, unless otherwise speci-
fied. 31P NMR chemical shifts were measured relative to 85%
Preparation of trans-[Ru(η2-H2)(PF3)(dppe)2]2؉ (7): The dihydrogen
complex 7 was prepared as described above starting from trans-
[Ru(H)(PF3)(dppe)2][BF4], except that the protonation was carried
out at room temperature and a large excess of HOTf (100 µL, 80
equiv.) was used. The resulting solution was analyzed by NMR
spectroscopy. The NMR spectroscopic data of 7 are summarized
in Table 3.
H3PO4 (as an aqueous solution, external reference) in CD2Cl2. 19
F
NMR spectra were recorded with respect to CFCl3 (external refer-
ence) in CD2Cl2. Satisfactory elemental analysis data for the
samples could not be obtained because of the formation of non-
combustible metal fluorides during combustion. The mass spectral
studies of the compounds were carried out using a Micromass Q-
TOF instrument. 1,2-Bis(diphenylphosphanyl)ethane[46] (dppe),
bis(diphenylphosphanyl)methane[47] (dppm), trans-[Fe(H)(η2-
Attempted Protonation of trans-[Ru(H)(PF3)(dppe)2][BF4] with HCl/
HBr/HI/CF3CO2H: The protonation of trans-[Ru(H)(PF3)-
(dppe)2][BF4] was attempted with various other acids such as HCl,
HBr, HI, and CF3CO2H in addition to HOTf as described above.
The
corresponding
dihydrogen
complex
trans-[Ru(η2-
H2)(PF3)(dppe)2]2ϩ (7) was obtained only in the case of HOTf,
whereas only the starting hydride was recovered with the other ac-
ids. The addition of a large excess of acid (HCl, HBr, HI, and
CF3CO2H) also did not afford the corresponding dihydrogen com-
plex.
H2)(dppe)2][BF4], trans-[Ru(H)(η2-H2)(dppe)2][BF4],[48]
trans-
were prepared accord-
[49]
[Ru(H)(η2-H2)(dppm)2][BF4],[7] and PF3
ing to literature procedures.
Preparation of trans-[Fe(H)(PF3)(dppe)2][BF4] (1): trans-[Fe(H)(η2-
H2)(dppe)2][BF4] (0.100 g, 0.1 mmol) was dissolved in CH2Cl2
(8 mL) under an atmosphere of H2. PF3 gas was then purged
through this solution at a steady rate for about 10 min after which
time it was stirred for 2 h. Concentration of the reaction mixture
to about 2 mL and addition of Et2O (20 mL) caused the complete
precipitation of a cream-colored product of 1. The product was
dried in vacuo and recrystallized from a CH2Cl2 solution by slow
diffusion of Et2O over a period of several days. Yield 0.082 g (75%).
ES-MS: m/z ϭ 941 [Mϩ Ϫ BF4], 853 [Mϩ Ϫ PF3 ϩ BF4]. The
NMR spectroscopic data of 1 are summarized in Table 1.
Attempt to Isolate trans-[Ru(η2-H2)(PF3)(dppe)2]2؉ (7): The dihy-
drogen complex 7 was prepared as described above. NMR spec-
troscopy revealed the complete conversion of the starting hydride
into the dihydrogen complex. In an attempt to isolate it, excess
Et2O was added to the reaction mixture to precipitate the colorless
dihydrogen complex. The supernatant solution was carefully re-
moved with a syringe and the precipitate was washed a few times
with Et2O and then dried under a stream of H2 gas. The NMR
spectrum revealed the presence of only the starting hydride com-
plex trans-[Ru(H)(PF3)(dppe)2][BF4] and no other species, suggest-
ing that complete deprotonation had taken place upon addition of
Et2O. The dihydrogen complex was found to be stable with respect
to deprotonation only in the presence of a large excess of acid.
Preparation of trans-[Ru(H)(PF3)(diphosphane)2][BF4] (diphos-
phane ؍ dppm 2, dppe 3): These compounds were prepared in a
similar manner to that of 1 starting from trans-[Ru(H)(η2-H2)(di-
Measurement of the Rate Constant for the Decomposition of
phosphane)2][BF4]. Yields of 2 and 3 were 0.075 g (69%) and trans-[Ru(η2-H2)(PF3)(dppe)2]2؉ (7):
A
sample of trans-
0.080 g (73%), respectively. ES-MS of 2: m/z ϭ 871 [Mϩ Ϫ PF3 ϩ [Ru(H)(PF3)(dppe)2][BF4] (6 mg) was placed in a 5 mm NMR tube
3054
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Eur. J. Inorg. Chem. 2004, 3048Ϫ3056

Dicationic Dihydrogen Complexes of Iron and Ruthenium
FULL PAPER
capped with a septum. The tube was evacuated and filled with dihy-
The structure was solved and refined using the SHELX program
drogen gas in three cycles. The hydride complex was then dissolved suite.[52] The Ru atom position was located from the Patterson map
in CD2Cl2 (0.6 mL). Excess HOTf (15 µL, 30 equiv.) was added
and the tube was transferred immediately to the NMR probe. The
1H and the 31P{gated 1H} NMR spectra were recorded at intervals
of 30 min. The rate constant for the decomposition of trans-[Ru(η2-
and the non-hydrogen atoms were located from successive differ-
ence-Fourier maps and were refined anisotropically. All other hy-
drogen atoms were fixed in idealized positions and refined in a
riding model. Additional details of data collection and structure
H2)(PF3)(dppe)2]2ϩ was determined from the integrals of the η2-H2 refinements are listed in Table 7.
signal with respect to the residual CDHCl2 resonance (δ ϭ 5.32
1
ppm) in the H NMR spectra and those of the dppe P signal with
Table 7. Crystallographic data for trans-[Ru(H)(PF3)(dppm)2]-
[BF4] (2)
respect to externally added [Ph3PH][OTf] (δ ϭ 7.11 ppm) in
the 31P{gated 1H} NMR spectra. After about 16 h, the NMR
spectra showed the formation of dppeHϩ (ca. 40%), trans-
Formula
Mol. wt.
C50H48BF7O2P5Ru
1080.61
orthorhombic
P212121
11.845(5)
16.252(6)
27.617(10)
90.00
90.00
90.00
5316(3)
4
1.350
[Ru(Cl)(PF3)(dppe)2]ϩ
[Ru(OTf)(PF3)(dppe)2]ϩ (9; Ͻ5%).
(8;
ca.
36%),
and
trans-
Crystal system
Space group
8: 1H NMR (CD2Cl2): δ ϭ 3.01 (m, 4 H, Ph2PCH2CH2PPh2),
6.81Ϫ7.91 (m, 40 H, Ph2PCH2CH2PPh2) ppm. 19F NMR
(CD2Cl2): δ ϭ Ϫ18.57 (d, PF3) ppm. 31P{1H} NMR (CD2Cl2): δ ϭ
64.3 (d, JP,P ϭ 41.0 Hz, Ph2PCH2CH2PPh2), 127.9 (q quint, JP,F ϭ
1308.0 Hz, PF3) ppm.
˚
a (A)
˚
b (A)
˚
c (A)
α (°)
β (°)
γ (°)
9: 1H NMR (CD2Cl2): δ ϭ 2.94 (m, 2 H, Ph2PCH2CH2PPh2), 3.13
3
˚
(m,
2
H, Ph2PCH2CH2PPh2), 6.81Ϫ7.91 (m, 40 H,
V (A )
Z
Ph2PCH2CH2PPh2) ppm. 19F NMR (CD2Cl2): δ ϭ Ϫ21.81 (d,
PF3), Ϫ75.70 (br. s, OTf Ϫ coordinated) ppm. 31P{1H} NMR
(CD2Cl2): δ ϭ 46.6 (d, JP,P ϭ 41.3 Hz, Ph2PCH2CH2PPh2), 132.1
(q quint, JP,F ϭ 1312.0 Hz, PF3) ppm.
Dcalcd (g/cm3)
T (K)
293(2)
0.71073
0.506
0.0607
0.1726
˚
λ (A)
µ (mmϪ1
)
R[a]
Attempt to Determine the Equilibrium Constant for the Substitution
of η2-H2 with CH3CN in trans-[Ru(η2-H2)(PF3)(dppe)2]2؉: The di-
hydrogen complex trans-[Ru(η2-H2)(PF3)(dppe)2]2ϩ was prepared
starting from trans-[Ru(H)(PF3)(dppe)2][BF4] (20 mg) and HOTf
(50 µL, 30 equiv.) as described above. After ensuring that the start-
ing hydride complex had been completely converted into the dihy-
drogen complex, the solution was frozen in a liquid-nitrogen bath.
Then, CH3CN (1 µL, 1 equiv.) was added and the NMR tube was
slowly warmed up to 253 K. It was then transferred to the NMR
probe precooled to 253 K. NMR spectroscopy revealed the com-
plete disappearance of the resonances due to the dihydrogen com-
plex and the appearance of signals due to the nitrile complex trans-
[Ru(CH3CN)(PF3)(dppe)2]2ϩ (10) thus precluding the measurement
of the equilibrium constant for the substitution of the coordinated
H2 ligand with CH3CN.
[b]
Rw
[a]
[b]
2 1/2
R ϭ (Fo Ϫ Fc)/Fo.
Rw ϭ [w(Fo Ϫ Fc)2/wFo ] [based on
reflections with I Ͼ 2σ(I)].
CCDC-231476 contains the supplementary crystallographic
data for this paper. These data can be obtained free of
charge at www.ccdc.cam.ac.uk/conts/retrieving.html [or from
the Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; Fax: ϩ44-1223-336033; E-mail:
deposit@ccdc.cam.ac.uk].
Acknowledgments
1
10: H NMR (CD2Cl2): δ ϭ 1.45 (s, 3 H, CH3CN), 3.05 (m, 4 H,
Financial support from the Council of Scientific and Industrial
Research, India, is gratefully acknowledged. Acquisition of a
400 MHz NMR spectrometer was supported by the Department of
Science and Technology, India, under the ‘‘FIST’’ program.
Ph2PCH2CH2PPh2), 6.91Ϫ7.91 (m, 40 H, Ph2PCH2CH2PPh2)
ppm. 19F NMR (CD2Cl2): δ ϭ Ϫ13.51 (d, PF3), Ϫ91.03 (br. s, OTf),
Ϫ144.62 (br. s, BF4) ppm. 31P{1H} NMR (CD2Cl2): δ ϭ 38.2 (d,
JP,P ϭ 42.5 Hz, Ph2PCH2CH2PPh2), 132.8 (q quint, JP,F
1258.0 Hz, PF3) ppm.
ϭ
Observation of the HD Isotopomers, trans-[Ru(η2-HD)-
(PF3)(dppm)2]2؉ (5-D1), cis-[Ru(η2-HD)(PF3)(dppm)2]2؉ (6-D1), and
trans-[Ru(η2-HD)(PF3)(dppe)2]2؉ (7-D1): The HD isotopomers
were prepared using similar procedures as the corresponding H2
complexes except that appropriate quantities of DOTf were used.
The HD isotopomers formed were observed by NMR spectroscopy.
The temperature dependent JH,D data for trans-[Ru(η2-
H2)(PF3)(dppe)2]2ϩ are summarized in Table 6.
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Eur. J. Inorg. Chem. 2004, 3048Ϫ3056
www.eurjic.org
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3055

H. V. Nanishankar, M. Nethaji, B. R. Jagirdar
FULL PAPER
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Received February 17, 2004
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´
´
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Early View Article
Published Online May 25, 2004
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3056
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
Eur. J. Inorg. Chem. 2004, 3048Ϫ3056