Dihydride complexes of the cobalt and iron group metals: An investigation of structure and dynamic behavior

Article abstract of DOI:10.1021/ja962702n

The previously reported cationic dihydride complexes (PP₃)MH₂⁺ (M = Co, Rh and Ir; PP₃ = P(CH₂-CH₂PPh₂)₃) have been prepared using improved synthetic methods. Variable-temperature ¹H and ³¹P NMR spectra of these complexes reveal complex dynamic behavior. The hydride region ¹H NMR spectra have been accurately simulated at all temperatures using a simple site permutation model after taking into consideration the opposite signs of the cis and trans H-P coupling constants. Partial deuteration of the hydride ligands in the rhodium and cobalt complexes is achieved by exposure to D₂. In the partially deuterated samples, no evidence is found for a bound dihydrogen ligand, but the involvement of a dihydrogen species in the dynamic process which interchanges the two hydride positions remains a mechanistic possibility, as indicated by a kinetic isotope effect k(H)/k(D) = 1.3(1). The partially deuterated samples exhibit large and temperature-dependent isotope effects on the ¹H NMR chemical shifts observed for the hydride resonances, which are attributed to isotopic perturbation of resonance. This arises from non-statistical occupation of the two different hydride sites and also leads to perturbation of the averaged H-P coupling constants. Similar observations have been made for the neutral iron complex (PP₃)FeH₂.

Full text of DOI:10.1021/ja962702n

12134
J. Am. Chem. Soc. 1996, 118, 12134-12140
Dihydride Complexes of the Cobalt and Iron Group Metals: An
Investigation of Structure and Dynamic Behavior
D. M. Heinekey* and Mirjam van Roon
Contribution from the Department of Chemistry, P.O. Box 351700, UniVersity of Washington,
Seattle, Washington 98195
ReceiVed August 2, 1996X
+
Abstract: The previously reported cationic dihydride complexes (PP3)MH2 (M ) Co, Rh and Ir; PP3 ) P(CH2-
CH2PPh2)3) have been prepared using improved synthetic methods. Variable-temperature 1H and 31P NMR spectra
1
of these complexes reveal complex dynamic behavior. The hydride region H NMR spectra have been accurately
simulated at all temperatures using a simple site permutation model after taking into consideration the opposite signs
of the cis and trans H-P coupling constants. Partial deuteration of the hydride ligands in the rhodium and cobalt
complexes is achieved by exposure to D2. In the partially deuterated samples, no evidence is found for a bound
dihydrogen ligand, but the involvement of a dihydrogen species in the dynamic process which interchanges the two
hydride positions remains a mechanistic possibility, as indicated by a kinetic isotope effect kH/kD ) 1.3(1). The
partially deuterated samples exhibit large and temperature-dependent isotope effects on the 1H NMR chemical shifts
observed for the hydride resonances, which are attributed to isotopic perturbation of resonance. This arises from
non-statistical occupation of the two different hydride sites and also leads to perturbation of the averaged H-P
coupling constants. Similar observations have been made for the neutral iron complex (PP3)FeH2.
The structure and reactivity of transition metal hydride
H-D coupling in partially deuterated samples (JH-D ) 18 Hz
for 2; JH-D ) 28 Hz for 1) was observed. In the case of
complex 2, an equilibrium between two isomeric forms was
postulated based on variable temperature NMR data. A trigonal
bipyramidal dihydrogen complex is believed to predominate at
higher temperatures, while an octahedral dihydride is favored
at low temperatures. X-ray diffraction data for 2 are consistent
with a dihydride structure in the solid state, but complex 1 has
been reported to have either a dihydrogen or a dihydride
structure in the solid state, depending on the nature of the
complexes is an area of great interest due to the central
importance of such species in catalytic processes. In this
context, the oxidative addition reaction of H2 with various
transition metal complexes to give dihydride complexes has been
extensively investigated. An additional area of research has
developed since the initial discovery of transition metal dihy-
drogen complexes by Kubas and co-workers.1 While a large
number of isolable dihydrogen complexes have been reported,2
definitive structural characterization data are available in only
a few cases. The difficulty of structural characterization in the
absence of neutron diffraction data has been described by
Kubas.3 Indirect magnetic resonance methods have been
developed based on the measurement of 1H relaxation rates (T1
data)4 or the measurement of JH-D in partially deuterated
samples. While the quantitative relationship between H-H
distance and the T1 of the bound H2 ligand is more complex
than first thought,5 qualitatively there is a clear correlation of
short H-H distances with rapid dipole-dipole relaxation (short
T1 values). The measurement of JH-D values of 10-35 Hz is
usually considered to be a definitive indication of the presence
of an intact dihydrogen ligand.6
-
counteranion employed. It is reported that the PF6 salt
-
crystallizes as the dihydrogen complex, while the BPh4 salt
adopts the dihydride structure.9
There is considerable precedent in the literature for rapid
equilibration of dihydrogen and dihydride complexes, such as
the tungsten complexes of Kubas and co-workers,10 cationic
[CpRu(diphos)]+ complexes,11 and complexes of [Cp*Os-
(CO)2]+.12 In all cases previously reported, considerable
structural reorganization accompanies the dihydrogen/dihydride
isomerization reaction. In contrast to these examples, the
dihydride/dihydrogen equilibration postulated for the rhodium
complex seems to involve relatively little rearrangement of the
phosphine coligands.
Bianchini and co-workers have reported novel cationic
dihydrogen complexes of the form (PP3)M(H2)+ (PP3 ) P(CH2-
CH2PPh2)3; M ) Co (1), Rh (2)).7,8 In both cases, substantial
We now report the outcome of further investigations of the
synthesis and properties of complexes 1 and 2, including a
1
1
detailed analysis of H and H{31P} NMR spectra of partially
deuterated derivatives, which indicates that these complexes are
correctly formulated as highly dynamic dihydride species. With
X Abstract published in AdVance ACS Abstracts, November 15, 1996.
(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) Recent reviews on dihydrogen complexes: (a) Heinekey, D. M.;
Oldham, W. J., Jr. Chem. ReV. 1993, 93, 913-926. (b) Morris, R. H.; Jessop,
P. G. Coord. Chem. ReV. 1992, 121, 155-284.
(3) Kubas, G. J. Acc. Chem. Res. 1988, 21, 120-128.
(4) Hamilton, D. G.; Crabtree, R. H. J. Am. Chem. Soc. 1988, 110, 4126-
4133.
(5) Desrosiers, P. J.; Cai, L.; Lin, Z.; Richards, R.; Halpern, J. J. Am.
Chem. Soc. 1991, 113, 4173-4184.
(6) Osmium complexes recently reported by Taube and co-workers have
very low values of JHD: (a) Hasegawa, T.; Li, Z.-W.; Parkin, S.; Hope, H.;
McMullan, R. K.; Koetzle, T. F.; Taube, H. J. Am. Chem. Soc. 1994, 116,
4352-4356. (b) Li, Z.-W.; Taube, H. J. Am. Chem. Soc. 1994, 116, 9506-
9513.
(7) Bianchini, C.; Mealli, C.; Peruzzini, M.; Zanobini, F. J. Am. Chem.
Soc. 1987, 109, 5548-5549.
(8) Bianchini, C.; Mealli, C.; Meli, A.; Peruzzini, M.; Zanobini, F. J.
Am. Chem. Soc. 1988, 110, 8725-8726.
(9) Bianchini, C.; Mealli, C.; Peruzzini, M.; Zanobini, F. J. Am. Chem.
Soc. 1992, 114, 5905-5906.
(10) Khalsa, G. R. K.; Kubas, G. J.; Unkefer, C. J.; Van Der Sluys, L.
S.; Kubat-Martin, K. A. J. Am. Chem. Soc. 1990, 112, 3855-3860.
(11) Chinn, M. S.; Heinekey, D. M. J. Am. Chem. Soc. 1990, 112, 5166-
5175
(12) Bullock, R. M.; Song, J.-S.; Szalda, D. J. Organometallics 1996,
15, 2504-2516.
S0002-7863(96)02702-3 CCC: $12.00 © 1996 American Chemical Society

Dihydride Complexes of the Cobalt and Iron Group Metals
J. Am. Chem. Soc., Vol. 118, No. 48, 1996 12135
1
suitable values of the H-P coupling constants, the H NMR
spectra can be simulated accurately at all temperatures using a
permuation model which interchanges the two hydride nuclei
HX and HY as well as permuting the phosphorus nuclei PM, PQ,
and PQ′.
A similar procedure has also been successful in the case of
the neutral iron analog (PP3)FeH2. A preliminary account of
some of our observations has appeared.13
Results
Synthesis. The compounds studied were prepared following
published procedures with modifications which are described
in the Experimental Section. An exception was the synthesis
of PP3RhH which was generated from (PPh3)4RhH14 Via ligand
exchange with PP3 in hot toluene. This method proved more
convenient and gave higher yields than the previously reported
synthesis starting from PP3RhCl.
NMR Observations: (a) [PP3CoH2]PF6 (1). The 1H NMR
spectrum of [PP3CoH2]PF6 in acetone-d6 exhibits a pseudo
doublet of quartets in the hydride region centered at δ -10.93
ppm, in good agreement with published data8 (Figure 1a).
Exposure of the sample to D2 leads to the observation of a new
signal at δ -10.73 ppm (Figure 1b) attributable to 1-d1. Signals
due to H2 and HD gas are observed at 4.54 (singlet) and 4.51
ppm (1:1:1 triplet, JHD ) 42 Hz) respectively. Over time, the
signal at -10.73 ppm continues to grow, while the signal at
-10.93 decreases and eventually disappears. The 1H{31P} NMR
spectrum shows two singlet resonances (Figure 1c); no H-D
coupling is observed in 1-d1. The separation of the two
resonances is temperature dependent. At 190 K the chemical
shifts are -10.56 (1-d1) and -10.86 ppm (1).
1
Figure 1. (a) Partial H NMR spectrum (hydride region) of complex
1 (300 MHz, 298 K) in acetone-d6 and (b) after exposure to D2 (1 atm)
for 11 h (500 MHz). (c) H{31P} spectrum (500 MHz).
1
When a sample of 1-d2 is placed under H2 gas, a resonance
corresponding to 1 appears in the hydride region after 5 min;
the signal continues to grow over time. A signal for 1-d1
appears only after 2.5 h and remains weak compared to the
signal for 1.
The T1 (min) for the hydride protons in 1 (500 MHz) was 95
ms at 226 K (see Figure 2). T1 values for the hydride in 1-d1
were typically about 10% greater than those observed for 1.
(b) [PP3RhH2]BF4 (2). Consistent with the observations of
Bianchini and co-workers,7 we find that the 1H NMR spectrum
of 2 (hydride region) at ambient temperature exhibits only a
single resonance (d of quintets at -7.52 ppm). Upon lowering
the temperature the signal broadens and disappears into the
baseline below 190 K. At 173 K two separate signals appear
in the hydride region due to non-equivalence of the two hydride
ligands. We were able to supercool a sample of 2 in THF-d8
down to 160 K, and observed two well-resolved doublets with
a line width of 65 Hz at -4.93 (JHP ) -139 Hz) and -10.01
ppm (JHP ) -135 Hz). Using selective 31P decoupling it was
determined that the downfield resonance is due to the proton
trans to the bridgehead phosphorus atom (PA) and the upfield
resonance is due to the proton trans to the terminal phosphorus
atom (PM).
Figure 2. T1 (ms) vs T(K) for PP3Co(H)2+ in acetone-d6 at 500 MHz.
partially overlapping the signal due to 2 at -7.52 ppm (Figure
3a). The 1H {31P} spectrum (Figure 3b) of this mixture exhibits
two doublet resonances, with the doublet pattern due to a 14.7
Hz coupling to 103Rh. There is no evidence for H-D coupling
in the resonance due to the HD species. The separation between
the signals for 2 and 2-d1 is temperature dependent. At 340 K,
∆δ ) 161 ppb, increasing to 209 ppb at 277 K.
(c) [PP3IrH2]Cl. The room temperature 1H NMR spectrum
of [PP3IrH2]Cl (3) exhibits two broad doublets in the hydride
region at -7.62 and -12.55 ppm, consistent with the observa-
tions of Bianchini et al.15 Upon lowering the temperature the
When a sample of 2 is placed under a D2 atmosphere a signal
due to [PP3RhHD]+ (2-d1) is observed to grow in at -7.33 ppm,
(13) Heinekey, D. M.; Liegeois, A.; van Roon, M. J. Am. Chem. Soc.
1994, 116, 8388-8389.
(14) Ahmad, N.; Levison, J. J.; Robinson, S. D.; Uttley, M. F. Inorg
Synth. 1972, 15, 45-64.
(15) Bianchini, C.; Peruzzini, M.; Zanobini, F. J. Organomet. Chem.
1987, 326, C79-C82.

12136 J. Am. Chem. Soc., Vol. 118, No. 48, 1996
Heinekey and Van Roon
1
Figure 5. Partial H NMR spectrum (hydride region) of complex 4
and 4-d1 (500 MHz, 298 K) in THF-d8.
at -10.85 ppm (JHP ) 41.5 Hz; JHP ) 3.7 Hz); the resonance
M
A
for 4-d1 appears at -10.58 ppm (JHP ) 42.1 Hz; JHP ) 5.5
M
A
Hz).
Below 170 K (in THF-d8) the signal decoalesces into two
broad (ν1/2 ) 160 Hz) resonances at -7.2 (HY) and -14.2 ppm
(HX). Individual couplings could not be resolved even at 140
K in CDFCl2.
1
Figure 3. (a) Partial H NMR spectrum (hydride region) of complex
2 (500 MHz, 298 K) in THF-d8 after exposure to D2 (1 atm) for 7 h.
(b) H{31P} spectrum (500 MHz).
1
Discussion
Structure of the Cobalt and Rhodium Complexes. The
1
high-temperature H {31P}NMR spectra in the hydride region
of partially deuterated samples of complexes 1 and 2 show no
evidence for H-D coupling (Figures 1 and 3). In the case of
the cobalt complex, the line width of the hydride resonance due
to 1-d1 is ca. 14 Hz. The rhodium complex 2-d1 gives a line
width of ca. 6 Hz. While it is possible that a small H-D
coupling remains unresolved in these spectra, such a coupling
would be outside the range usually associated with dihydrogen
complexes.
1
Figure 4. Partial H NMR spectrum (hydride region) of complex 3
(200 MHz, 260 K) in THF-d8.
Additional support for the previous formulation of 1 as a
dihydrogen complex was provided by relaxation time data. Since
no H-D coupling could be detected, it is necessary to reconsider
the reported T1 data for complex 1. Halpern and co-workers
have recently examined in detail the use of T1 measurements
to determine the structure of transition metal hydride and
dihydrogen complexes.5 It is clear from this work that all
sources of relaxation, including relaxation due to metal nuclei
which have high magnetogyric ratios (γ), must be taken into
account. Since cobalt has a very high γ, protons directly bonded
to cobalt are quite rapidly relaxed. In the case of complex 1,
assuming a dihydride structure with a Co-H distance of 1.55
Å and a H-Co-H angle of 80°, the calculated T1 (min, 500
MHz) using the published solid state structure of 1 is ca. 97
ms.9 Using this structural model, the proton relaxation is
primarily due to the cobalt nuclear spin, with smaller contribu-
tions from the adjacent hydride, the ligand 31P nuclei, and the
ortho hydrogen atoms of the phenyl rings. Alternatively, a
dihydrogen ligand bound to cobalt with an H-H distance of
0.90 Å is predicted to have a T1(min) of ca. 7 ms, with the
relaxation now dominated by H-H contributions.
peaks sharpen and at 260 K resolve into two well-resolved
multiplets (Figure 4). The downfield resonance is due to the
hydride trans to the bridgehead phosphorus PA (J
) -100
HYPA
Hz, J
) J
) +13.0 Hz), the upfield resonance is due
HYPM
HYPQ
to the hydride trans to the terminal phosphorus PM (J
)
HXPA
+8.3 Hz, JH P ) -110 Hz, JH P ) +19.7 Hz). The JH H
X
M
X
Q
X Y
coupling constant accounts for the residual coupling of 3.6 Hz.
These values of the coupling constants were obtained by
comparing simulated spectra with the observed resonance
positions and intensities.
Coalescence of the two hydride signals was not observed at
any accessible temperature (up to 360 K in DMSO-d6) and no
incorporation of deuterium was observed after prolonged
exposure of 3 to D2 gas at 360 K.
(d) PP3FeH2 (4). A variable-temperature 1H NMR study of
this dihydride as well as a room temperature 1H NMR spectrum
of a mixture of 4 and PP3FeHD (4-d1) in toluene-d8 have been
published.16 In order to separate the signals for 4 and 4-d1 and
to resolve all high-temperature coupling constants, a spectrum
was taken at 500 MHz (Figure 5). The resonance for 4 appears
The reported value for T1(min) of 19 ms for complex 1 was
(16) Bianchini, C.; Laschi, F.; Peruzzini, M.; Ottaviani, F. M.; Vacca,
A.; Zanello, P. Inorg. Chem. 1990, 29, 3394-3402
measured at 203 K on a 300-MHz instrument. This value would

Dihydride Complexes of the Cobalt and Iron Group Metals
J. Am. Chem. Soc., Vol. 118, No. 48, 1996 12137
correspond to a T1(min) of ca. 32 ms at 500 MHz.5 At 226 K
(500 MHz), we find a T1(min) value of 95 ms for the hydride
protons in 1 (see Figure 2). At all temperatures, the single
hydride in 1-d1 gives a T1 value about 10% greater than that
observed for 1. These results confirm that hydride-hydride
interactions are a small contribution to relaxation.
Our measurements of the hydride T1 in complex 1 are
consistent with the dihydride formulation and are inconsistent
with the formulation of complex 1 as a dihydrogen complex.
These data combined with the lack of resolvable H-D coupling
in 1-d1 and 2-d1 indicates conclusively that these species are
correctly formulated as dihydride complexes. A similar con-
clusion has been recently communicated by Bianchini and
co-workers.17 The previously reported JH-D values for 1 (18
Hz) and 2 (28 Hz) are now believed to result from mis-
interpretation of overlapping spectra arising from unusually
large isotope effects on the chemical shift of the hydride
resonance in 1-d1 and 2-d1. We now attribute these large
chemical shift differences to isotopic perturbation of resonance
(Vide infra).
It is important to note that our observations of monodeuterated
species require a mechanism for atom exchange. For example,
we find that complex 1-d2 reacts quickly with H2 gas to initially
give a mixture of 1 and 1-d2, and then more slowly forms 1-d1.
While we have not studied the mechanism of the latter reaction,
we postulate that an intermolecular proton (deuteron) exchange
process is facilitated by adventitious base (such as water).
Variable-Temperature NMR Observations. The octahe-
dral dihydride structure of these complexes provides for two
inequivalent hydride sites: one trans to the unique P atom of
the tetradentate ligand, the other cis to the unique P atom. We
therefore expect two resonances in the hydride region of the
1H NMR spectrum. With the exception of the iridium complex,
all complexes studied exhibit a single resonance for the hydride
protons at ambient temperature, indicating a rapid dynamic
process which interchanges the two hydride positions (eq 1).
Low-temperature spectra showing the separate hydride reso-
nances have been previously reported for M ) Rh,7 Ru,18 and
Os.19
complexes.20 Our observations are consistent with those
reported7 by Bianchini et al., but we have collected data at
slightly lower temperatures and observe correspondingly reduced
line widths.
1
At room temperature, we find that the H NMR spectrum
(hydride region) of the iridium dihydride complex (3) consists
of two broad doublets centered at -7.62 and -12.55 ppm as
reported by Bianchini.15 At 260 K all couplings can be resolved
to give two complex multiplets (Figure 4). The splitting pattern
of the downfield resonance (due to the hydride trans to PA) arises
from a large trans coupling to the bridgehead phosphorus (JH P
Y
A
) -100 Hz) and equivalent couplings to the three terminal
phosphorus atoms (JH P ) JH P ) +13.0 Hz). The upfield
Y
M
Y Q
resonance (due to the hydride trans to PM) shows a large doublet
splitting due to coupling to the trans terminal phosphorus (JH P
X
M
) -110 Hz), a smaller triplet splitting due to coupling to the
two cis terminal phosphorus atoms PQ (JH P ) +19.7 Hz), and
X
Q
a small doublet coupling to the cis bridgehead phosphorus (JH P
X
A
) +8.3 Hz). The JH H coupling constant accounts for the
X
Y
residual coupling of 3.6 Hz. A similar splitting pattern was
reported in the low temperature spectrum of the osmium
analog.19
Previous investigations of the 1H NMR spectra of iron
complex 4 were reported in deuterated toluene.16 We find that
by recording spectra in THF-d8 we were able to freeze out the
separate hydride signals at 170 K. They appear as two broad
(ν1/2 ) 160 Hz) resonances at -7.2 (HY) and -14.2 ppm (HX).
No H-P couplings could be resolved.
While the temperatures of coalescence vary depending upon
the central metal, in all cases except M ) Ir, the hydride
resonances broaden and coalesce and at high temperature give
a single resonance. The chemical shift of this resonance is found
at the average of the chemical shifts for each proton (eq 2):
δHH ) 1/2(δH + δH )
(2)
X
Y
The doublet of quartets splitting pattern at high temperature
arises from coupling of the hydrides to one bridgehead and three
terminal phosphorus atoms (in complex 2 a doublet of quintets
is observed due to JH-Rh ≈ JH-P ). The value of the averaged
M
coupling constant is described by eqs 3 and 4:
Jdoublet ) 1/2(JH P + JH P
)
Y A
(3)
X
A
Jquartet
)
In the case of the cobalt complex 1, we observed only a single
resonance in the hydride region of the H NMR spectrum at
1
1/6(JH P + JH P + JH P ′ + JH P + JH P + JH P ′) (4)
X
M
X
Q
X
Q
Y
M
Y
Q
Y Q
ambient temperature, as noted above. Other than slight line
broadening, the spectrum is unaffected by cooling the sample
to 170 K. For the rhodium analog 2, separate signals can be
frozen out below 175 K in THF-d8; at 160 K we observed two
well-resolved doublets with a line width of 65 Hz at -4.93 (JHP
) -139 Hz) and -10.01 ppm (JHP ) -135 Hz). Using
selective 31P decoupling it was determined that the upfield
resonance (HX) is due to the proton trans to a terminal
phosphorus atom (PM), and the downfield resonance (HY) is
due to the proton trans to the bridgehead phosphorus atom, PA.
The smaller coupling constants of each hydride to the phos-
phorus atoms cis to it could not be resolved. The sign of these
two bond H-P couplings is presumed to be negative, based on
the observations of Field and co-workers on closely related Rh
These equations were used to calculate those HP coupling
constants that could not be resolved at low temperature (Vide
infra).
Isotopic Perturbation of Resonance. In all of the dihydride
complexes where high-temperature averaged spectra have been
reported, the signal due to the d1 species is observed at
substantially lower field than the corresponding signal for the
d0 species. This isotope effect is remarkable in both magnitude
and direction, in that most known dihydrogen and polyhydride
complexes exhibit upfield isotope shifts of modest magnitude
upon partial deuteration.21 Exceptions to this general trend are
found in complexes where a rapidly established equilibrium
between two different proton sites is perturbed by deuteration.
This phenomenon is denoted isotopic perturbation of resonance
(17) Bianchini, C.; Elsevier, C. J.; Ernsting, J. M.; Perruzini, M.;
Zanobini, F. Inorg. Chem. 1995, 34, 84-92.
(18) Bianchini, C.; Perez, P. J.; Perruzini, M.; Zanobini, F.; Vacca, A.
Inorg. Chem. 1991, 30, 279-287.
(20) Partridge, M. G.; Messerle, B. A.; Field, L. D. Organometallics
1995, 14, 3527-3530.
(21) Luo, X. L.; Crabtree, R. H. J. Am. Chem. Soc. 1990, 112, 4813-
(19) Bianchini, C.; Linn, K.; Masi, D.; Polo, A.; Perruzini, M.; Vacca,
A.; Zanobini, F. Inorg. Chem. 1993, 32, 2366-2376.
4821

12138 J. Am. Chem. Soc., Vol. 118, No. 48, 1996
Heinekey and Van Roon
Table 2. Coupling Constants for the Dihydride Complexesa
Table 1. Isotope Shifts and Calculated Equilibrium Constants
(300 K)
M
JH
JH
JH
JH
JH
JH
JH
P
X Q
H
P
P
P
P
P
X
Y
Y
A
Y
M
Y
Q
X
A
X
M
M
δHX - δHY, ppm
∆δ(300 K), ppb
K (300 K)
Fe
-5
-26.6
-71
+19.3
+11
Co
Rh
Ir
∼5(est)
200
193
1.17
1.16
Ru -3
+13
+20
-70
+26
Osb -5.2
Rh -5
-48.7 +14.6 +14.6
+5.6
-55.9 +28.2
5.08
4.93
7.0
-139
+16.7 +16.7
+13 +13
+6.0 -136
+8.3 -110
+2.2
+19.7
Fe
270
1.17
Ir
-3.6 -100
a Bold type indicates values obtained from experimental data; other
values obtained from computer simulations. b Data from ref 19.
(IPR) and arises from non-statistical occupation of inequivalent
proton sites.22
to eqs 3 and 4, respectively. For complex 2, partial deuteration
leads to K ) 1.16, as determined from the IPR effects noted
above. The corresponding isotopic perturbation of coupling
causes a small change in the H-P coupling observed at high
temperatures.
(5)
While the low-temperature spectra of complex 2 do not show
any fine structure even at 160 K, the actual H-P couplings
can be calculated from the above equations and the values of
the H-Ptrans couplings obtained from the low-temperature data.
The only assumption made was that coupling of HY to all
terminal phosphorus atoms was the same, as was found
experimentally for the analogous Ir and Os species. The
calculated couplings are shown in Table 2. The small values
of the coupling constants are consistent with the line widths of
the doublet resonances observed for complex 2 at 160 K (ν1/2
) 65 Hz).
The trans H-P coupling constants are presumed to have
negative signs, based on results reported by Field and co-workers
on closely related rhodium complexes.20 Our results require
that the relative signs of the cis and trans H-P couplings are
opposite. The sign of the H-H coupling is believed to be
negative, based on the results of a polarization transfer experi-
ment reported for a rhodium dihydride complex.23
In the monodeuterated compound the deuterium atom has a
site preference, which causes a perturbation of the equilibrium
in eq 5. It can be shown that the magnitude of the isotope shift
(δHD - δHH or ∆δ) is dependent on both K and the difference
in chemical shifts of the two hydrite sites according to eq 6:
1 - K
2(K + 1)
∆δ )
(δX - δY)
(6)
[
]
The chemical shift difference between HX and HY has been
determined for the Rh, Ir, Fe, Ru, and Os dihydride complexes
and ranges from ca. 5 to 7 ppm. The separate hydride
resonances could not be frozen out for PP3CoH2+, but a chemical
shift difference of ca. 5 ppm was assumed in order to calculate
Keq. Using selective 31P decoupling it was determined that the
downfield resonance in these complexes is due to the proton
trans to the bridgehead phosphorus. Since the observed isotope
shift upon deuteration is downfield, we conclude that deuterium
concentrates in the site trans to the terminal phosphorus (labeled
as HX). No equilibrium constant was calculated for the Ir
analog, since the fast exchange spectrum was not observed. The
calculated values for K are tabulated in Table 1.
Mechanism of Hydride Permutation. All of the dihydride
complexes of the form [PP3MH2]n+ exhibit dynamic behavior,
which leads to averaging on the NMR time scale of the two
hydride environments and also renders equivalent the three
terminal phosphorus atoms. We have studied in detail the
rhodium complex 2. For compound 2, the coupling constants
derived above were used to simulate spectra for various rates
of exchange at 15 temperatures ranging from 170 to 300 K.
The permutation matrix employed in the calculations permutes
PM with the two PQ nuclei while also exchanging the positions
of the hydride ligands. An Eyring plot of these data gives ∆Hq
) 6.8 ( 0.3 kcal/mol and ∆Sq ) -8 ( 1 eu, leading to
Isotopic Perturbation of Coupling. The non-statistical site
preference which leads to deuterium concentration in the HX
site (trans to the terminal phosphorus) also leads to measurable
perturbation of the averaged high-temperature H-P coupling
constants. These effects are subtle, and careful measurement
in well-resolved spectra is required. An example of this effect
is the observation that JHP increases from 3.7 Hz in 4 to 5.5 Hz
in 4-d1 (see Figure 5).
A perturbation of the JHP was also observed upon deuteration
in rhodium complex 2. At 300 K, the small H-P coupling in
2 is 13.5 Hz, which combined with the 14.7 Hz coupling to Rh
gives rise to a pseudoquintet. In 2-d1 the small H-P coupling
is reduced to 11.2 Hz; a doublet of quartets is observed (see
Figure 3a). At 320 K the small H-P coupling in 2-d1 is 11.4
Hz. This can be quantified by use of eqs 7 and 8:
∆Gq
) 9.2 ( 0.5 kcal/mol. The small negative entropy
(300K)
of activation and retention of H-P coupling at all temperatures
is consistent with a degenerate intramolecular process.
We have made a similar study of PP3RuH2 including the low-
temperature limiting spectrum, which we have obtained in THF-
d8 at 200 K. The rate constants obtained from simulated spectra
in the range 200-300 K were used in an Eyring plot which
gives ∆Hq ) 10 ( 0.5 kcal/mol, ∆Sq ) -2 ( 2 eu, and
∆Gq(300K) ) 9.4 ( 0.7 kcal/mol. In the case of the iron complex
PP3FeH2 (4), we have obtained very limited low-temperature
data and are not able to extract all of the static couplings. From
the available data, an approximate value of ∆Gq(300K) ) 7.2 kcal/
mol for the hydride rearrangement process was obtained.
JH P + K(JH P )
Y A
X
A
Jdoublet
)
(7)
(8)
1 + K
Jquartet
)
JH P + JH P + JH P ′ + K(JH P + JH P + JH P ′
)
Y Q
X
M
X
Q
X
Q
Y
M
Y
Q
These activation energy values can be compared to ∆Gq
(300K)
3K + 3
) 12 kcal/mol reported for the osmium analog PP3OsH2.19 It
can be concluded based on these data that the trend in activation
energy is Os > Ru > Fe. Within the cobalt group a similar
trend is observed (based on coalescence temperatures) with Ir
If K ) 1, all couplings contribute equally to the averaged
coupling observed at high temperature and eqs 6 and 7 reduce
1
(22) IPR has been invoked to explain the H NMR spectra of partially
deuterated iridium hydride complexes: Heinekey, D. M.; Oldham, W. J.,
Jr. J. Am. Chem. Soc. 1994, 116, 3137-3138.
(23) Bowers, C. R.; Weitekamp, D. P. J. Am. Chem. Soc. 1987, 109,
5541-5542.

Dihydride Complexes of the Cobalt and Iron Group Metals
J. Am. Chem. Soc., Vol. 118, No. 48, 1996 12139
mechanism outlined below:
> Rh > Co. While the nature of the central metal atom seems
to be the primary factor determining the facility of these
rearrangements, changes in the tetradentate ligand also have an
effect. Field and co-workers report an activation energy of 15.3
kcal/mol for hydride site exchange in PP3FeH2 (PP3 ) P(CH2-
CH2CH2PMe2)3).24
This mechanism would require relatively little movement of the
heavy atoms. Rapid rotation of the dihydrogen ligand would
facilitate the exchange process. Some M-H bond breaking is
required, so the observed trends in the activation energy for
rearrangement are consistent with the reported observation that
M-H bonds are stronger for the second- and third-row metals
than for first-row metals.30
Hydride site exchange in complexes of the general type H2M-
(PR3)4 has been the subject of extensive mechanistic study.25
While the topology of the dynamic process is well understood,
detailed understanding of the mechanism is limited. Based on
permutational considerations, Muetterties and co-workers pro-
posed mechanisms described as a tetrahedral jump or trigonal
twist for various metal and ligand combinations.25 Only recently
has the possibility been considered that a dihydrogen species
could be on the reaction coordinate. Studies by Berke and co-
workers26 of hydride site exchange in complexes of the form
Re(CO)(NO)H2(PR3)2 using a combination of experimental and
computational methods suggest the intermediacy of a dihydrogen
complex in this rearrangement.
Conclusion
In contrast to previous reports, we find that the cationic
complexes [(PP3)CoH2]+ and [(PP3)RhH2]+ are correctly for-
mulated as dihydrides. The large isotope shifts in the 1H NMR
spectra observed upon deuteration have been quantitatively
analyzed in terms of isotopic perturbation effects. A mechanism
for the hydride rearrangement involving a transition state with
substantial H-H interactions is proposed based on the observed
kinetic isotope effect.
In principle, mechanistic insight can be gained by measure-
ment of the kinetic isotope effect upon replacement of hydride
ligands with deuterium. To this end, we have studied carefully
the 1H{31P} NMR spectra of mixtures of 2 and 2-d1. By
inspection, the spectra in the temperature range 260-300 K
show that the rearrangement is slightly faster for 2. Simulation
of the observed line shapes leads to kH/kD ) 1.3(1). Within
experimental error, this isotope effect is temperature indepen-
dent. This is the first report of a kinetic isotope effect in an
intramolecular hydride rearrangement of this type. It should
be noted that our measurement differs from other data on
processes such as reductive elimination reactions, where a
dideuteride is usually compared to a dihydride complex. In our
Experimental Section
All syntheses and chemical manipulations were conducted under Ar
or N2 using standard Schlenk and vacuum line techniques. Toluene,
tetrahydrofuran, diethyl ether, pentane, and heptane were distilled from
NaK/benzophenone; ethanol was distilled from magnesium (all under
dry nitrogen). Distilled water, dimethylformamide, and acetone were
purged with argon before use. Deuterated solvents were purchased
from Cambridge Isotope Laboratories, degassed, and stored in Pyrex
bulbs fitted with Kontes vacuum valves. Tetrahydrofuran-d8 and
benzene-d6 were dried over NaK/benzophenone, and acetone-d6 was
dried over activated molecular sieves.
1
case, we chose to examine a monodeuteride so that H NMR
spectra of both the dihydride and hydridodeuteride could be
recorded in the same sample, eliminating systematic errors such
as temperature variations. Had we been able to accurately
measure the rate of rearrangement for the dideuteride in
comparison to the dihydride complex, the KIE would have been
larger.
The observation of a KIE for the rearrangement process
suggests that the M-H interactions change significantly between
the ground state and the transition state. Our observations can
be put in context by noting that isotope effects for reductive
elimination of hydrogen are generally modest, and in some cases
inverse effects have been reported (kH/kD < 1).27 A highly
relevant comparison can be made to the observations reported
by Hoff and co-workers, who found kH/kD ) 1.08 ( 0.04 for
the conversion of a tungsten dihydride complex to the corre-
sponding dihydrogen species.28 Based on our observed KIE,
we postulate that the rearrangement reaction in 2 may proceed
Via a transition state with some degree of H-H bonding. The
tetradentate ligand system used here is known to stabilize five
coordinate trigonal bipyramidal complexes,29 so we suggest the
1H NMR spectra were obtained on Bruker AC-200, AF-300, and
WM-500 spectrometers. Chemical shifts were referenced against
residual protio solvent and are reported in ppm relative to TMS. 31P-
{1H} NMR spectra were recorded on the Bruker AC-200 instrument
at 81 MHz with chemical shifts relative to an external 85% H3PO4
standard. 1H{31P} spectra were recorded on the Bruker WM-500
1
instrument referenced to TMS. Variable-temperature H{31P} NMR
experiments used a Bruker B-VT 1000 temperature controller with
copper/constantan thermocouple. All temperatures were calibrated
using the 1H chemical shifts of methanol.31 Simulations of NMR spectra
were obtained on a Macintosh Quadra 900 using the Dynamac program.
CoCl2 was obtained from Aldrich Chemicals, dried under vacuum
at 180 °C for 24 h and stored under Ar. Et3OPF6, NaBH4, PPh3
(Aldrich), and P(CH2CH2PPh2)3 (Strem Chemicals) were used without
further purification. RhCl3‚3H2O was prepared from rhodium residues
following the procedure of Anderson and co-workers.32 HB-
16
(Ar′)4‚(Et2O)2,33 (PPh3)4RhH,14 PP3CoH,34 PP3IrCl,15 and PP3FeH2
were prepared following literature procedures.
In the six-coordinate complexes PA indicates the bridgehead phos-
phorus, PQ and PQ′ the two axial PPh2 groups, and PM the equatorial
PPh2 group. The 1H NMR spectra of all compounds studied are
consistent with previously published data.
(24) Field, L. D.; Bampos, N.; Messerle, B. A. Magn. Reson. Chem.
1991, 29, 36-39.
[PP3CoH2]PF6 (1). PP3CoH (30 mg, 0.041 mmol) was dissolved
in 5 mL of diethyl ether to give a bright yellow solution. Dropwise
(25) For summaries of the extensive early work on this problem, cf.:
(a) Muetterties, E. L. Acc. Chem. Res. 1970, 3, 266-273. (b) Jesson, J. P.;
Muetterties, E. L. In Dynamic Nuclear Magnetic Resonance Spectroscopy;
Jackman, L. M., Ed.; Academic Press: New York, 1975; p 277. (c) Jesson,
J. P.; Meakin, P. Acc. Chem. Res. 1973, 6, 269-275.
(26) Bakhmutov, V.; Bu¨rgi, T.; Burger, P.; Ruppli, U.; Berke, H.
Organometallics 1994, 13, 4203-4213.
.
addition of HPF6 Et2O (freshly prepared by stoichiometric reaction of
ethanol with Et3OPF6) immediately affords a reddish brown precipitate.
(29) Sacconi, L.; Mani, F. Transition Met. Chem. 1984, 8, 179-252.
(30) Cf.: Wang, D.; Angelici, R. J. J. Am. Chem. Soc. 1996, 118, 935-
942 and references therein.
(27) Bullock, R. M. In Transition Metal Hydrides; Dedieu, A., Ed.;
VCH: New York, 1992; p 296. See also: Abu-Hasanayn, F.; Krogh-
Jesperson, K.; Goldman, A. S. J. Am. Chem. Soc. 1993, 115, 8019-8023
and references therein.
(28) Zhang, K.; Gonzalez, A. A.; Hoff, C. D. J. Am. Chem. Soc. 1989,
111, 3627-3632.
(31) Van Geet, A. L. Anal. Chem. 1970, 42, 679-680.
(32) Anderson, S. N.; Basolo, F. Inorg. Synth. 1963, 7, 214-220.
(33) Brookhart, M.; Grant, B.; Volpe, A. F., Jr. Organometallics 1992,
11, 3920-3922.
(34) Ghilardi, C. A.; Midollini, S.; Sacconi, L. Inorg. Chem. 1975, 14,
1790-1795.

12140 J. Am. Chem. Soc., Vol. 118, No. 48, 1996
Heinekey and Van Roon
Addition of HPF6 was continued until the supernatant was completely
colorless. The supernatant was removed via cannula and the precipitate
washed with 3 × 5 mL of diethyl ether, then dried in vacuo. Yield
95%.
[PP3CoHD]PF6 (1-d1). An NMR tube was charged with 8 mg of
[PP3CoH2]PF6 and 0.5 mL of acetone-d6. The frozen sample was put
under D2 (0.5 atm) and the NMR tube was then flame sealed. NMR
spectra were acquired at t ) 0, 7, 11, and 16 h. Prolonged exposure
to D2 combined with periodic replacement of the gas with fresh D2
leads to complete deuteration of the hydride positions.
PP3RhH. A solution of 250 mg (0.37 mmol) of PP3 in 10 mL of
warm toluene was added to a solution of 420 mg (0.36 mmol) of
(PPh3)4RhH in 40 mL of warm toluene. The resulting yellow solution
was stirred at 90 °C for 5 h, then for 14 h at room temperature. Upon
reduction in volume and cooling to -18 °C a yellow microcrystalline
solid precipitated. The supernatant was removed by cannula, and the
solid product washed with diethyl ether and pentane, then dried in
vacuo. Yield 200 mg (72%).
[PP3RhH2]BF4 (2). To a solution of 36 mg (0.046 mmol) of PP3-
RhH in 5 mL of THF at 0 °C under H2 atmosphere was added an excess
(0.1 mL) of HBF4‚Et2O. Addition of 20 mL of ethanol, followed by
cooling to -18 °C for 48 h affords complex 2 as colorless microcrystals.
The supernatant was cannulated off and the product washed three times
with cold diethyl ether and dried in vacuo. Yield: 30 mg (76%).
Complex 2-d1 was prepared by exposure of 2 to D2, as outlined above
for the cobalt complex.
Synthesis of [PP3IrH2]Cl. To a bright yellow solution of 50 mg
(0.056 mmol) of PP3IrCl in 40 mL of ethanol and 10 mL of
dimethylformamide was added 80 mg of solid NaBH4. The solution
was stirred overnight, turning pale yellow. The excess of NaBH4 and
-
any BH4 adduct formed were hydrolyzed by addition of 3 drops of
concentrated HCl. NMR samples were obtained by evaporating 5 mL
1
of the solution to dryness and redissolving the residue in THF-d8. H
NMR (THF-d8, 265K): δ 8.0 (br s, 12H, Ph); δ 7.50-6.70 (m, 18H,
Ph); δ 3.55-2.75 (m, 12H, aliphatic); δ -7.62 (ddq, 1H, JHPA ) -100
Hz, JHP ) JHP ) +13.0 Hz, JH
) + 3.6 Hz, Ir-HY); δ -12.55
H
X Y
M
Q
(dddt, 1H, JHP ) 8.3 Hz, JHP ) -110 Hz, JHP ) +19.7 Hz, JH
H
X Y
A
M
Q
) +3.6, Ir-HX).
Acknowledgment. We thank the National Science Founda-
tion for support of this research. Partial support was also
provided by the Petroleum Research Foundation, administered
by the American Chemical Society. We thank Dr. Tom Pratum,
Dr. Amber S. Hinkle, and Dr. T. G. P. Harper for their technical
assistance.
JA962702N