Dynamic processes in cis dihydrogen/hydride complexes of ruthenium [6]

Full text of DOI:10.1021/ja993952h

6498
J. Am. Chem. Soc. 2000, 122, 6498-6499
Dynamic Processes in cis Dihydrogen/Hydride
Complexes of Ruthenium
D. Michael Heinekey,* Heather Mellows, and Tom Pratum
Department of Chemistry, UniVersity of Washington
Box 351700, Seattle, Washington 98195-1700
ReceiVed NoVember 9, 1999
ReVised Manuscript ReceiVed March 13, 2000
Since the isolation and characterization of the first transition
metal dihydrogen complexes,1 the possibility that a fluxional
polyhydride complex might also contain a dihydrogen ligand has
been actively investigated.2 Of particular interest in this context
is the dynamic behavior of complexes containing one dihydrogen
and one hydride ligand, which are expected to be prototypical of
the larger class of polyhydrides. Excluding the large group of
known complexes with hydride and dihydrogen ligands trans to
each other, which exhibit relatively high barriers to site exchange
between hydride and dihydrogen ligands,3 we focus on complexes
with hydride and dihydrogen ligands in cis positions. Several
complexes of this type have been reported. All show interesting
dynamic properties, exhibiting exchange of hydrogen environ-
ments between the dihydrogen and the hydride ligands, as revealed
by variable temperature NMR studies. Examples of this situation
include the iridium complex [IrH(H2)(bq)(PPh3)2]+ (bq ) ben-
zoquinolate), which was reported by Crabtree and co-workers to
undergo hydrogen site exchange between the dihydrogen and
1
Figure 1. Partial 750 MHz H{31P} NMR spectrum of a mixture of 1,
1-d1, and 1-d2 (298 K).
hydrido-chloride complexes (PCy3)2Ru(bipy)HCl and (PCy3)2Ru-
(phen)HCl in good yield.9 Treatment of (PCy3)2RuH(H2)Cl with
carbon monoxide gives immediate and quantitative conversion
to (PCy3)2Ru(CO)2HCl. The hydrido-chloride complexes react
with NaBH4 or LiAlH4 in THF to give the corresponding
dihydride complexes.10 Protonation with (CF3SO2)CH2 affords
cationic complexes [(PCy3)2Ru(bipy)(H2)H]+ (1) and [(PCy3)2-
Ru(phen)(H2)H]+ (2). Protonation of the dicarbonyl dihydride
complex with [H(Et2O)2][B(Ar)4] (Ar ) 3,5-(CF3)2C6H3) affords
[(PCy3)2Ru(CO)2(H2)H]+ (3).11
hydride ligands with ∆Gq ) 10 kcal/mol.4 An iron complex
240
[FeP(CH2CH2CH2PMe2)3(H2)H]+, which has ∆Gq200 ) 9.1 kcal/
mol for permutation of the hydrogen environments was reported
by Field and Bampos.5 In these and other reported examples6 of
cis dihydrogen/hydride complexes, interconversion of the hydride
and dihydrogen environments also requires significant rearrange-
ment on the part of the ancillary ligands, which might be expected
to contribute substantially to the activation energy for the process.
To isolate the hydride/dihydrogen exchange process from other
ligand rearrangements, a dihydride complex with cis hydride
ligands related by a mirror plane is required. Such a species can
then be protonated to give a cationic dihydrogen/hydride complex
where a dynamic process could readily interconvert the hydrogen
environments with very limited movement by the ancillary
ligands.7
We now report our investigations of the synthesis, structure,
and dynamic behavior of ruthenium dihydrogen/hydride com-
plexes of the form [(PCy3)2RuH(H2)(L2)]+. Using very low-
temperature NMR spectroscopy, we have succeeded for the first
time in measuring the rate of these very rapid dynamic processes.
Reaction of (PCy3)2RuH(H2)Cl8 with bidentate nitrogen ligands
such as 2,2′-bipyridine and 1,10-phenanthroline affords the
Formulation of these species as dihydrogen hydride complexes
is based primarily upon the observation of H-D coupling upon
partial deuteration of the hydride ligands. Exposure of a THF-d8
solution of complex 1 to D2 leads to rapid incorporation of
deuterium, as evidenced by the appearance of new resonances in
1
the hydride region of the H NMR spectrum (Figure 1).
By means of methodology previously employed in our studies
of iridium dihydrogen/hydride complexes,7 an analysis of the
isotope effects on the chemical shifts and the H-D couplings
for 1-d1 (JH-D ) 5.5 Hz) and 1-d2 (JH-D ) 6.7 Hz) leads to the
conclusion that there is a nonstatistical distribution of deuterium
between the dihydrogen and the hydride environments, with a
slight preference for deuterium to occupy the hydride site. The
H-D coupling in the dihydrogen ligand (1JH-D) is calculated to
be ∼19 Hz. A similar analysis for complex 2 is consistent with
1JH-D ) ∼17 Hz in the dihydrogen ligand. On the basis of the
known inverse correlation of 1JH-D values with H-H distance,12
(1) Kubas, G. J.; Ryan, R. R.; Swanson, B. I.; Vergamini, P. J.; Wasserman,
H. J. J. Am. Chem. Soc. 1984, 106, 451-452.
(2) Compare Gusev, D. G.; Kuhlman, R. L.; Renkema, K. B.; Eisenstein,
O.; Caulton, K. G. Inorg. Chem. 1996, 35, 6775-6783 and references therein.
(3) Jessop, P. G.; Morris, R. H. Coord. Chem. ReV. 1992, 121, 155-284
(4) Crabtree, R. H.; Lavin, M.; Bonneviot, L. J. Am. Chem. Soc. 1986,
108, 4023-4037.
(9) The hydridochloride (PCy3)2Ru(CO)2HCl has been previously re-
ported: Christ, M. L.; Sabo-Etienne, S.; Chaudret, B. Organometallics 1994,
13, 3800-3804. Data for new hydrido-chloride complexes: (PCy3)2Ru(bipy)-
1
HCl: H NMR (δ, THF-d8, PCy3 resonances omitted): 10.2, d, 1H.; 8.9, d,
1H.; 8.0, d, 1H.; 7.9, d, 1H.; 7.5, t, 1H.; 7.32, d, 1H.; 7.28, d, 1H.; 6.8, t, 1H.;
-11.5, t, 1H, Ru-H, JHP ) 28 Hz. (PCy3)2Ru(phen)HCl: 10.6, d, 1H.; 9.3,
d, 1H.; 8.2, d, 1H.; 7.9, d, 1H.; 7.8, dd, 1H.; 7.3, t, 1H.; 7.28, d, 1H.; 6.8, t,
1H.;-11.4, t, 1H, Ru-H, JHP ) 27 Hz.
(5) Bampos, N.; Field, L. D. Inorg. Chem. 1990, 29, 587-588.
(6) Other reported cis dihydrogen/hydride complexes: Bianchini, C.;
Peruzzini, M.; Zanobini, F. J. Organomet. Chem. 1988, 354, C19-C22. Jia,
G.; Drouin, S. D.; Jessop, P. G.; Lough, A. J.; Morris, R. H. Organometallics
1993, 12, 906-916. Bianchini, C.; Perez, P. J.; Peruzzini, M.; Zanobini, F.;
Vacca, A. Inorg. Chem. 1991, 30, 279-287. See also ref 3, page 200.
(7) Two such compounds have been reported in the literature, [TpIr(PR3)-
(H2)H]+ and [(PR3)4Ru(H2)H]+. In neither case was a barrier to exchange
measured, but it was estimated to be 4.5 kcal/mol for the iridium case. Oldham,
W. J., Jr.; Hinkle, A. S.; Heinekey, D. M. J. Am. Chem. Soc. 1997, 119,
11028-11036. Gusev, D. G.; Hu¨bener, R.; Burger, P.; Orama, O.; Berke, H.
J. Am. Chem. Soc. 1997, 119, 3716-3731.
(10) The dihydride (PCy3)2Ru(CO)2H2 has been previously reported.9 Data
for new dihydride complexes: (PCy3)2Ru(bipy)H2: 1H NMR: (δ, THF-d8,
PCy3 signals omitted): 9.5, d, 2H.; 8.1, d, 2H.; 7.3, t, 2H.; 6.9 ppm, t, 2H.;
-13.9, t, JHP ) 29 Hz, 2H, Ru-H. (PCy3)2Ru(phen)H2: 9.8, d, 2H.; 7.9, d,
2H.; 7.7, s, 2H.; 7.4, dd, 2H: -13.4, t, 2H, Ru-H, JHP ) 28.4 Hz.
(11) Data for complex 1: 1H NMR (δ, THF-d8, PCy3 resonances omit-
ted): 9.5, d, 2H.; 9.0, d 2H.; 8.3, t, 2H.; 7.7, t, 2H.; Ru-H3 -11.1, t, 3H, JHP
) 11.4 Hz. 31P NMR: 38.5 br s. Complex 2: 1H NMR (δ, THF-d8, PCy3
resonances omitted): 9.58 d, 2H.; 8.7 d 2H.; 8.2, s, 2H.; 8.0, dd, 2H.; Ru-H3
1
(8) Wilhelm, T. E.; Belderrain, T. R.; Brown, S. N.; Grubbs, R. H.
Organometallics 1997, 18, 3867-3869.
-10.9, t, 3H, JHP ) 14 Hz. Complex 3: H NMR (240 K, δ, CD2Cl2, PCy3
resonances omitted): Ru-H3 -16.8 br t, 3H, JHP not resolved.
10.1021/ja993952h CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/22/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 27, 2000 6499
the H-H distance in the dihydrogen ligand in complexes 1 and
2 is found to be ∼1.1-1.2 Å.
The observation of a single hydride resonance for the two
distinct proton envrinonments in 1 and 2 at ambient temperature
is consistent with a rearrangement process which is rapid on the
NMR time scale. In an attempt to obtain the limiting chemical
shifts for the dihydrogen and hydride resonances and the rate of
1
exchange, the low temperature H NMR spectra of 1 and 2 and
partially deuterated samples were examined down to 170 K in
THF-d8. While substantial broadening of the hydride resonance
was observed, no low-temperature limiting spectra could be
obtained. The broadening of the resonances at low temperature
is attributed to efficient dipole-dipole relaxation, leading to short
T1 and T2 values.14 This rapid relaxation problem persists to a
13
lesser extent even with heavily deuterated samples, in which the
only proton resonances in the hydride region are due to 1-d2 or
2-d2. These results exemplify a fundamental limitation to using
1H NMR spectroscopy to probe very rapid dynamic processes in
cases where short H-H distances lead to efficient dipole-dipole
relaxation.
We next examined complex 3, motivated by the recent report
of Kubas and co-workers that binding dihydrogen trans to CO
ligands limits the degree of H-H bond lengthening.15 We
reasoned that a shorter, stronger H-H bond in the bound
dihydrogen might lead to a higher barrier for the site exchange
process. Samples of 3 were prepared as above by protonation of
the corresponding neutral dihydride. While 1 and 2 are stable in
solution at room temperature, complex 3 is only stable under
hydrogen below 240 K. At 240 K, a single broad hydride
resonance was observed for 3. Mixtures of 3/3-d1/3-d2 were
prepared by exposure of solutions of 3 to D2. The observed JH-D
value is 10.5 Hz for both 3-d1 and 3-d2, consistent with a statistical
distribution of deuterium and JH-D ) 31.5 Hz in the dihydrogen
ligand. This corresponds to an H-H distance of ∼0.9 Å.12
Examination of the hydride resonances of 3/3-d1/3-d2 by very low
temperature 1H NMR spectroscopy showed line broadening
attributable to relaxation processes as noted above for 1 and 2.
The dynamic process in complex 3 also is amenable to
examination by 13C NMR spectroscopy. Samples of 3 enriched
in 13C were prepared by treatment of (PCy3)2RuH(H2)Cl with
13CO, followed by reduction and protonation as above. The 13C
NMR spectrum of enriched 3 at 168 K exhibits a single sharp
triplet resonance in the carbonyl region at 196.3 ppm, consistent
with coupling to two 31P nuclei (JC-P ) 6.5 Hz). Upon lowering
the temperature to 130 K in CDFCl2/CDF2Cl (2:1), broadening
of the signal and ultimately decoalescence was observed to give
two broad resonances centered at 195 and 197 ppm (Figure 2).
Data quality at these low temperatures is poor, in part due to
the low solubility of complex 3. Comparison of the observed
spectra to computer simulated spectra reveals that most of the
line broadening is due to slowing of the exchange process, but
there is additional line broadening at the lowest temperatures
which may arise from viscosity and relaxation effects. An
Figure 2. Partial 125 MHz 13C NMR spectra at various temperatures
for complex 3 enriched with 13C at the carbonyl positions. Calculated
line shapes are shown below the experimental data.
approximate free energy of activation, ∆Gq120 ) 5.5 kcal/mol can
be derived from these data. This is the lowest rearrangement
barrier measured to date for a dynamic polyhydride and is actually
comparable to reported barriers for rotation around slightly
hindered C-C and C-N single bonds.16
If we assume that the slightly stretched H-H bond in the bound
dihydrogen has retained most of its substantial bond energy,17 it
is quite remarkable that this fairly strong interaction is being
disrupted at ∼103 sec-1 at 130 K. We speculate that the reaction
must be highly concerted, with a substantial degree of bond
formation and bond breaking in a transition state which has
approximate 2-fold symmetry. Complete scrambling of all three
H atoms also requires rapid rotation of the bound dihydrogen
ligand.18 In the representation below, the fate of a labeled atom
indicated with an asterisk is depicted.
The factors which determine the structure and dynamic behav-
ior of complexes of this type are subtle and not well understood.
It has been reported that a neutral complex very closely related
to the species reported here, that is, (PCy3)2Ru(Ph-py)(H2)H (Ph-
py ) o-C6H4C5H4N), is also a dihydrogen/hydride complex. In
this case, H-D coupling was not detected, and the hydride and
dihydrogen ligands exhibit a large exchange coupling.19 We are
continuing to investigate complexes related to 1-3 to gain a better
understanding of their structures and dynamic behavior.
1
(12) An inverse correlation between H-H distance and JH-D has been
reported by Morris and co-workers: 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. A very similar analysis has
been reported by Heinekey and Luther: Luther, T. A.; Heinekey, D. M. Inorg.
Chem. 1996, 35, 4396-4399. Luther, T. A.; Heinekey, D. M. Inorg. Chem.
1998, 37, 127-132. An extension of this correlation to include dihydride
complexes has been recently reported: Gru¨ndemann, S.; Limbach, H. H.;
Buntkowsky, G.; Sabo-Etienne, S.; Chaudret, B. J. Phys. Chem. A 1999, 103,
4752-4754.
Acknowledgment. This research was supported by the NSF. Acquisi-
tion of the 750 MHz NMR spectrometer was supported by the NSF and
the Murdoch Charitable Trust.
JA993952H
(13) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am.
Chem. Soc. 1991, 113, 4173-4184.
(14) Low-temperature line broadening in dynamic dihydrogen/hydride
complexes has been investigated: Yao, W.; Faller, J. W.; Crabtree, R. H.
Inorg. Chim. Acta 1997, 259, 71-76.
(17) 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.
(18) With the exception of d2 metal complexes, barriers to the rotation of
bound dihydrogen are generally very low. Compare Jalon, F. A.; Otero, A.;
Manzano, B. R.; Villasenor, E.; Chaudret, B. J. Am. Chem. Soc. 1995, 117,
10123-10124 and references therein.
(15) King, W. A.; Scott, B. L.; Eckert, J.; Kubas, G. J. Inorg. Chem. 1999,
38, 1069-1084.
(16) Sternell, S. In Dynamic Nuclear Magnetic Resonance Spectroscopy;
Jackman, L. M., Cotton, F. A., Eds.; Academic Press: New York, 1975; p.
163-202.
(19) Guari, Y.; Sabo-Etienne, S.; Chaudret, B. J. Am. Chem. Soc. 1998,
120, 4228-4229.