HRh(dppb)2, a powerful hydride donor

Article abstract of DOI:10.1021/om020421k

The Rh(I) and Rh(III) hydrides HRh(dppb)₂ and [HRh(dppb)₂(NCCH₃)](BF₄)₂ (where dppb is 1,2-(bis(diphenylphosphino)benzene) have been prepared, and a structural study of [HRh-(dppb)₂(NCCH₃)](BF₄)₂ has been completed. The latter complex is an octahedral complex with a trans arrangement of the hydride and acetonitrile ligands. A pK_a value of 9.4 was measured for this complex by equilibration of [Rh(dppb)₂](BF₄) with 4-bromoanilinium tetrafluoroborate in acetonitrile. [Rh(dppb)₂](BF₄) reacts with H₂ in the presence of Pt(dmpp)₂, which acts as a base, to form HRh(dppb)₂ and [HPt(dmpp)₂](BF₄) (where dmpp = 1,2-bis-(dimethylphosphino)propane). An equilibrium constant of 0.42 ± 0.2 was measured for this reaction. Using this equilibrium measurement and a thermodynamic cycle, the hydride donor ability (δG°_H-H-) of HRh(dppb)₂ was determined to be 34 kcal/mol. This value indicates that HRh(diphosphine)₂ complexes are powerful hydride donors. Similarly the pK_a value of HRh-(dppb)₂ was calculated to be 35 from a thermodynamic cycle that included the potential of the Rh(I/-I) couple (E_1/2 = -2.02 V vs ferrocene). These results combined with results from the literature suggest the following order of hydricity for five-coordinate, 18-electron hydrides: second row > third row > first row. Similarly an acidity order of second row ≥ first row > third row is deduced.

Full text of DOI:10.1021/om020421k

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Organometallics 2002, 21, 4833-4839
4833
HRh (d p p b)2, a P ow er fu l Hyd r id e Don or
Andrew J . Price,† Rebecca Ciancanelli,† Bruce C. Noll,† Calvin J . Curtis,‡
Daniel L. DuBois,*,‡ and M. Rakowski DuBois*,†
Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309,
and the National Renewable Energy Laboratory, 1617 Cole Boulevard,
Golden, Colorado 80401
Received May 28, 2002
The Rh(I) and Rh(III) hydrides HRh(dppb)2 and [HRh(dppb)2(NCCH3)](BF4)2 (where dppb
is 1,2-(bis(diphenylphosphino)benzene) have been prepared, and a structural study of [HRh-
(dppb)2(NCCH3)](BF4)2 has been completed. The latter complex is an octahedral complex
with a trans arrangement of the hydride and acetonitrile ligands. A pKa value of 9.4 was
measured for this complex by equilibration of [Rh(dppb)2](BF4) with 4-bromoanilinium
tetrafluoroborate in acetonitrile. [Rh(dppb)2](BF4) reacts with H2 in the presence of Pt(dmpp)2,
which acts as a base, to form HRh(dppb)2 and [HPt(dmpp)2](BF4) (where dmpp ) 1,2-bis-
(dimethylphosphino)propane). An equilibrium constant of 0.42 ( 0.2 was measured for this
reaction. Using this equilibrium measurement and a thermodynamic cycle, the hydride donor
ability (∆G°H-) of HRh(dppb)2 was determined to be 34 kcal/mol. This value indicates that
HRh(diphosphine)2 complexes are powerful hydride donors. Similarly the pKa value of HRh-
(dppb)2 was calculated to be 35 from a thermodynamic cycle that included the potential of
the Rh(I/-I) couple (E1/2 ) -2.02 V vs ferrocene). These results combined with results from
the literature suggest the following order of hydricity for five-coordinate, 18-electron
hydrides: second row > third row > first row. Similarly an acidity order of second row g
first row > third row is deduced.
In tr od u ction
logues, but no data could be obtained for the analogous
palladium hydrides, which are unstable with respect to
H2 elimination.4 For CpMo(CO)3H and CpW(CO)3H the
hydricity appears to be greater for W than for Mo,5 but
the hydricities of Cp2NbH3 and Cp2TaH3 have been
reported to be the same.6 This leaves the situation of
the relative hydricities of first, second, and third row
transition metals unclear, but suggests that the order-
ing may be different for different geometries and
ligands. In this paper we report the measurement of the
acidity and hydricity of HRh(dppb)2 and compare these
values with those determined previously for [HM-
(diphosphine)2]n complexes of cobalt (n ) 0) and plati-
num and nickel (n ) 1+). HRh(dppb)2 has the greatest
hydride donor ability of any of the complexes that we
have studied to date.
Rhodium hydride complexes are intermediates in a
variety of catalytic reactions.1 In general, rhodium
complexes appear to be more reactive than their cobalt
and iridium analogues, but the origin of this enhanced
reactivity is not fully understood. Although these dif-
ferences in reactivity may be related to thermodynamic
differences of the M-H bond, limited data are available.
Acidity data2 and homolytic bond dissociation energies3
for rhodium hydride complexes are scarce, and no data
are available on the relative ability of rhodium hydride
derivatives to act as hydride donors.
Previous studies of [HNi(diphosphine)2]+ and [HPt-
(diphosphine)2]+ complexes indicate that the platinum
complexes are more hydridic than their nickel ana-
Resu lts
† University of Colorado.
‡ National Renewable Energy Laboratory.
Syn th esis a n d Ch a r a cter iza tion of Com p lexes.
The complexes [Rh(dppb)2][BF4] and [Rh(dppe)2][BF4]
have been previously prepared.7,8 The dppb derivative
proved to be most suitable for our studies, for reasons
discussed below, and characterization data that were
not reported previously are presented here. The 31P
(1) (a) Osborn, J . A.; J ardine, F. H.; Young, J . F.; Wilkinson, G. J .
Chem. Soc. A 1966, 1711. (b) Vineyard, B. D.; Knowles, W. S.; Sabacky,
M. J .; Bachman, G. L.; Weinkauff, D. J . J . Am. Chem. Soc. 1977, 99,
5946. (c) Kagan, H. B.; Dang, T.-P. J . Am. Chem. Soc. 1972, 94, 6429.
(d) Evans, D.; Yagupsky, G.; Wilkinson, G. J . Chem. Soc. A 1968, 2660.
(e) Fowler, R.; Connor, H.; Baehl, R. A. Chem. Technol. 1976, 772.
(2) (a) Kristja´nsdo´ttir, S. S.; Norton, J . R. In Transition Metal
Hydrides; Dedieu, A., Ed.; VCH: New York, 1991; pp 309-359. (b)
Schrauzer, G. N.; Holland, R. J . J . Am. Chem. Soc. 1971, 93, 1505. (c)
Pearson, R. G.; Kresge, C. T. Inorg, Chem. 1981, 20, 1878. (d) Halpern,
J .; Riley, D. P.; Chan, A. S. C.; Pluth, J . J . J . Am. Chem. Soc. 1977,
99, 8055. (e) Ramasami, T.; Espenson, J . H. Inorg. Chem. 1980, 19,
1846. (f) Gillard, R. D.; Heaton, B. T.; Vaughan, D. H. J . Chem. Soc.,
A 1970, 3126.
(4) (a) Berning, D. E.; Noll, B. C.; DuBois, D. L. J . Am. Chem. Soc.
1999, 121, 11432-11447. (b) Berning, D. E.; Miedaner, A.; Curtis, C.
J .; Noll, B. C.; Rakowski DuBois, M.; DuBois, D. L. Organometallics
2001, 20, 1832-1839.
(5) Sarker, N.; Bruno, J . W. J . Am. Chem. Soc. 1999, 121, 2174-
2180.
(6) Sarker, N.; Bruno, J . W. Organometallics 2001, 20, 55-61
(7) Fordyce, W. A.;.Crosby, G. A. Inorg. Chem. 1982, 21 (4), 1455.
(8) van der Ploeg, A. F. M. J .; Van Koten, G. Inorg. Chim. Acta 1981,
51, 225-239.
(3) (a) Wayland. B. B.; Coffin, V. L.; Farnos, M. D. Inorg. Chem.
1988, 27, 2745. (b) Coffin, V. L.; Brennen, W.; Wayland, B. B. J . Am.
Chem. Soc. 1988, 110, 6063. (c) Wei, M.; Wayland, B. B. Organome-
tallics 1996, 15, 4681-4683. (d) Simo˜es, J . A. M.; Beauchamp, J . L.
Chem. Rev. 1990, 90, 629-688.
10.1021/om020421k CCC: $22.00 © 2002 American Chemical Society
Publication on Web 10/01/2002

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4834 Organometallics, Vol. 21, No. 22, 2002
Price et al.
Ta ble 1. Selected Bon d Dista n ces (Å) a n d An gles
(d eg) for [HRh (d p p b)2(CH3CN)](BF 4)2‚2CH3CN
Ta ble 2. Cr ysta l Da ta for
[HRh (d p p b)2(CH3CN)](BF 4)2‚2 CH3CN
Bond Distances
formula
C132H116B4F16N6P8Rh2
2587.13
triclinic
Rh(1)-H(1)
Rh(1)-P(1)
Rh(1)-P (3)
N(1)-C(61)
P(1)-C(3)
1.51(5)
Rh(1)-N(1)
Rh(1)-P (2)
Rh(1)-P(4)
C(1)-C(2)
P(2)-C(4)
2.119 (4)
fw (amu)
2.3493(10)
2.3621(11)
1.130(6)
2.393(7)
2.3493 (11)
2.3613 (11)
2.215 (6)
cryst syst
unit cell dimens
a (Å)
13.8411(18)
20.305(3)
22.832(3)
103.609(3)
98.905(3)
96.007(3)
6094.5(14)
P1h
2.462 (7)
b (Å)
c (Å)
Bond Angles
77.64 (6) N(1)-Rh(1)-P(2) 86.17 (6)
76.70 (16) N(1)-Rh(1)-P(4) 80.88 (6)
R (deg)
N(1)-Rh(1)-P(1)
N(1)-Rh(1)-P(3)
â (deg)
γ (deg)
volume, Å3
space group
Z
H(1)-Rh(1)-P(1) 118.9 (2)
H(1)-Rh(1)-P(2) 79.8 (18)
H(1)-Rh(1)-P(4) 92.4 (17)
P(1)-Rh(1)-P(4)
P(3)-Rh(1)-P(4)
H(1)-Rh(1)-P(3)
88.9 (17)
H(1)-Rh(1)-N(1) 177.0 (17)
97.33 (4)
83.71 (4)
2
density, calc (mg/m-3
λ(Mo KR) (Å)
temp (K)
)
1.410
P(1)-Rh(1)-P(2)
P(2)-Rh(1)-P(3)
81.68 (4)
96.80 (4)
0.71073
130 (2)
scan type
θ range
φ and ω scans
0.93 < θ < 26.37
24 831 (R(int) ) 0.0535)
18 215
semiempirical from
equivalents
0.0814
0.1557
1.031
1.562 and -1.018
NMR spectrum of [Rh(dppb)2][BF4] exhibits a doublet
centered at 62.5 ppm (1J P-Rh ) 134 Hz). This chemical
shift and coupling constant are consistent with a chelat-
ing phosphine bound to Rh(I).9 The electrospray mass
spectrum (m/z ) 995) is also consistent with a [Rh-
(dppb)2]+ cation.
no. of ind reflns
no. of reflns obsd
abs coeff, mm-1
Ra
b
Rw
GOFc
Reaction of [Rh(dppb)2][BF4] with LiAlH4 in toluene
gives the neutral rhodium hydride complex HRh(dppb)2
in fair yield. The 31P NMR spectrum consists of a
doublet of doublets centered at 59.3 ppm (1J P-Rh ) 142
largest peak in final diff
map (e-/Å3)
) R1 ) ∑||Fo| - |Fc||/∑|Fo|. Rw ) [∑[w(Fo - Fc2)2]/
a
2
2
R
∑[w(Fo2)2]]1/2
.
GOF ) S ) [∑[w(Fo - Fc2)2]/(M - N)]1/2 where M
3
2
is the number of reflections and N is the number of parameters
refined.
2
1
Hz, J P-H ) 18 Hz). The hydride region of the H NMR
spectrum shows a doublet of pentets at -9.4 ppm as
expected for a fluxional five-coordinate species (1J Rh-H
2
) 12 Hz, J P-H ) 18 Hz). The infrared absorption
spectrum has a band at 1868 cm-1 assigned to the
Rh-H stretch. The deuterated analogue, DRh(dppb)2,
was prepared by reacting LiEt3BD with [Rh(dppb)2]-
[BF4]. A new infrared absorption band is observed at
1343 cm-1 for the deuterium-labeled complex, and that
at 1868 cm-1 is absent, consistent with the expected
shift in the infrared spectrum.
Reaction of [Rh(dppb)2][BF4] in acetonitrile with a
stoichiometric amount of ethereal tetrafluoroboric acid
gave a new compound formulated as [HRh(dppb)2(CH3-
CN)][BF4]2. This compound is a colorless solid, with a
doublet in the 31P NMR spectrum at 56.8 ppm (1J P-Rh
) 93 Hz). The phosphorus spectrum is consistent with
an octahedral structure in which the hydride and
acetonitrile ligands are trans and all four phosphorus
atoms are equivalent. Supporting this structural as-
signment is the observation that the hydride resonance
F igu r e 1. Perspective drawing and numbering scheme for
the cation of [HRh(dppb)2(CH3CN)][BF4]2‚2CH3CN. Ther-
mal ellipsoids are shown at 50% probability.
1
in the H NMR spectrum is a doublet of pentets (-14.6
1
2
of 2.119(4) Å is normal for a second row metal complex
ppm, J Rh-H ) 10 Hz, J P-H ) 7.5 Hz). A broad and
weak infrared band at 1993 cm-1 is tentatively assigned
to the Rh-H stretch.
of acetonitrile.12 The Rh-P bond lengths (Rh-Pav
)
2.3555(11) Å) are slightly shorter than other reported
trans Rh(III)-P bonds13 (e.g., Rh-Pav ) 2.398(4) Å in
Rh(PPh3)3Cl3).13a The hydride ligand was located by a
difference Fourier map, and a Rh-H bond distance of
1.51(5) Å was determined.
Crystallization of [HRh(dppb)2(CH3CN)][BF4]2 from a
mixture of acetonitrile and diethyl ether gave crystals
that were suitable for X-ray diffraction studies. Selected
bond distances and angles and crystallographic data are
shown Tables 1 and 2, respectively. The molecular
structure of [HRh(dppb)2(CH3CN)][BF4]2 confirms the
assigned octahedral structure as shown in Figure 1. The
four phosphorus atoms bend toward the hydride ligand
as indicated by an average N-Rh-P bond angle of
92.83(10)°. This distortion is expected for both electronic
and steric reasons10 and is similar to that observed for
[HCo(dppe)2(CH3CN)][BF4]2.11 The Rh-N bond length
Electr och em ica l Stu d ies of [Rh (d ip h osp h in e)2]-
[BF 4]. A reversible two-electron reduction is observed
(10) (a) Elian, M.; Hoffmann, R. Inorg. Chem. 1975, 14, 1058-1076.
(b) Wander, S. A.; Miedaner, A.; Noll, B. C.; Barkley, R. M.; DuBois,
D. L. Organometallics 1996, 15, 3360-3373.
(11) Ciancanelli, R.; Noll, B. C.; DuBois, D. L.; Rakowski DuBois,
M. J . Am. Chem. Soc. 2002, 124, 2984-2992.
(12) Storhoff, B. N.; Lewis, H. C., J r. Coord. Chem. Rev. 1977, 23,
1-29.
(13) (a) Skapski, A. C.; Stephens, F. A. J . Chem. Soc., Dalton Trans.
1973, 1789. (b) Allen, F. H.; Chang, G.; Cheung, K. K.; Lai, T. F.; Lei,
L. M.; Pidcock, A. Chem. Commun. 1970, 1297.
(9) Garrou, P. E. Chem. Rev. 1981, 81, 229-266.

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HRh(dppb)2, a Powerful Hydride Donor
Organometallics, Vol. 21, No. 22, 2002 4835
In the calculation of this equilibrium constant, the
participation of the solvent, acetonitrile, was neglected,
i.e., the Keq1 ) [BH+][Rh]/[HRh] (where Rh ) [Rh-
(dppb)2]+ and HRh ) [HRh(dppb)2(CH3CN)]2+). The pKa
value of [HRh(dppb)2(CH3CN)](BF4)2 in acetonitrile (9.4
( 0.3) was calculated by subtracting the log of this
equilibrium constant from the pKa value of 4-bromo-
anilinium (9.6).16 This approach results in a pKa value
that accurately reflects that [HRh(dppb)2(CH3CN)]2+
will dissociate a proton in acetonitrile solution only
slightly more than protonated 4-bromoaniline. It also
allows the accurate prediction of the free energy for the
reaction of this complex with other bases whose pKa-
(BH+) values are known in acetonitrile.
F igu r e 2. Cyclic voltammogram of [Rh(dppb)2]+ in aceto-
nitrile/0.3 M n-Bu4NPF6 at a scan rate of 50 mV/s.
A similar equilibrium measurement was carried out
for the reaction of [HRh(dppe)2(CH3CN)][BF4]2 with
4-bromoaniline in acetonitrile, and the equilibrium
constant was determined to be 4.1 ( 0.6. This corre-
sponds to a pKa value of 9.0 ( 0.3.
at -2.02 V vs ferrocene for [Rh(dppb)2]+ in acetonitrile
as shown by the cyclic voltammogram in Figure 2. The
observed reduction wave is diffusion-controlled, as
indicated by a linear plot of the peak current vs the
square root of the scan rate, and the observed peak-to-
peak separation of 33 mV at 50 mV/s is near the 30 mV
value expected for a two-electron reduction.14 The ratio
of the cathodic to anodic peak currents of 0.95 at 50
mV/s is consistent with a fully reversible process. The
geometry of the square-planar Rh(I) complex is expected
to change to a distorted tetrahedron upon reduction
based on the structures of other four-coordinate d8 and
d10 complexes.4,15 These rearrangements are rapid on
the CV time scale at scan rates of 50-4000 mV/s. Sim-
ilar electrochemical behavior is observed for [Rh(dppe)2]-
[BF4], which was studied for comparison. A reversible
two-electron reduction is observed at -2.12 V vs Fc.
Reversible waves were not observed in the cyclic volta-
Hyd r icity of HRh (d p p b)2. The hydride donor ability
of HRh(dppb)2 was obtained from equilibrium studies
in benzonitrile of the reaction of [Rh(dppb)2](BF4) with
H2 in the presence of Pt(dmpp)2 to form HRh(dppb)2 and
[HPt(dmpp)2](BF4) (pKa ) 30.4), the reverse of reaction
2.17
HRh(dppb)2 + [HPt(dmpp)2]+ h [Rh(dppb)2]+ +
Pt(dmpp)2 + H2 (2)
The relative ratios of the four metal complexes were
determined by integration of the appropriate 31P NMR
resonances. The activity of hydrogen at 1.0 atm was
taken as 1. An equilibrium constant of 2.4 ( 1.0 was
measured for reaction 2 from a series of four experi-
ments carried out in the reverse direction as described
in the Experimental Section. The reversibility of reac-
tion 2 was established by purging the solution with N2
after equilibrium had been reached under hydrogen. In
this case HRh(dppb)2 disappeared with reformation of
[Rh(dppb)2]+.
mmograms of HRh(dppb)2 or [HRh(dppb)2(CH3CN)]2+
.
Th er m od yn a m ic St u d ies of t h e R h -H Bon d .
Acid ity of [HRh (d ip h osp h in e)2(CH3CN)][BF 4]2 De-
r iva tives. As discussed above, [Rh(dppb)2][BF4] reacts
quantitatively with a stoichiometric amount of tetra-
fluoroboric acid to produce [HRh(dppb)2(CH3CN)][BF4]2.
However, an equilibrium is observed when 4-bromo-
anilinium tetrafluoroborate is used as the acid. To con-
firm that the system is at equilibrium, the reaction was
also run in the reverse direction, i.e., starting from [H-
Rh(dppb)2(CH3CN)][BF4]2 and 4-bromoaniline, as shown
in reaction 1. This reaction can be conveniently followed
by 31P NMR spectroscopy. Integration of the resonances
of the two rhodium complexes provides a convenient
method for determining the ratio of [HRh(dppb)2(CH3-
CN)][BF4]2 to [Rh(dppb)2][BF4]. Integrations reached
constant values over a period of 8 h. The conjugate pair
of 4-bromoanilinium tetrafluoroborate and 4-bromoa-
niline were used in a 20-fold excess in a 1:1 ratio. Under
these conditions, the protonation of [Rh(dppb)2][BF4]
makes a negligible contribution to the ratio of proto-
nated and unprotonated 4-bromoaniline. The results of
four experiments, two in each direction, gave an equi-
librium constant of 1.5 ( 0.3 for reaction 1.
Although this equilibrium study was performed in
benzonitrile for solubility reasons, we believe that it
provides a good estimate of the equilibrium value in
acetonitrile as well. A similar heterolytic cleavage
reaction of [Pt(dmpp)2](BF4)2 with hydrogen and tri-
ethylamine was performed in both benzonitrile and
acetonitrile and gave equilibrium constants of 3.2 ( 0.5
and 1.7 ( 0.3, respectively, with corresponding values
of ∆GoH- of 51.4 and 51.0 kcal/mol, respectively. This
indicates that the use of benzonitrile in place of aceto-
nitrile should not introduce a large error in ∆GoH-. We
estimate the difference in this value in the two solvents
to be less than 0.8 kcal/mol on the basis of this result
and previous results that compared equilibria involving
proton transfer reactions in both solvents.4
[HRh(dppb)2(CH3CN)]2+
+ B h [Rh(dppb)2]+ +
BH+ + CH3CN (1)
B ) BrC6H4NH2
(15) Longato, B.; Riello, L.; Bandoli, G.; Pilloni, G. Inorg. Chem.
1999, 37, 2818.
(16) Kristja´nsdo´ttir, S. S.; Moody, A. E.; Weberg, R. T.; Norton, J .
R. Organometallics 1988, 7, 1983-1987.
(17) Curtis, C. J .; Miedaner, A.; Ellis, W. W.; DuBois, D. L. J . Am.
Chem. Soc. 2002, 124, 1918-1925.
(14) Bard, A. J .; Faulkner, L. R. Electrochemical Methods; J ohn
Wiley & Sons: New York, 1980; pp 218 and 229.

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4836 Organometallics, Vol. 21, No. 22, 2002
Price et al.
Reaction 2 can be combined with reactions 3 and 4 to
produce a thermodynamic cycle whose sum is the
heterolytic cleavage of the Rh-H bond of HRh(dppb)2,
reaction 5.
isoelectronic complexes of the formula [HM(diphos-
phine)2]n+, where M ) Co, Rh, n ) 0, and M ) Ni, Pt,
n ) 1.4,11,17 It has been necessary to introduce some
slight variations in the diphosphine ligands used in the
comparisons of this series. The dppe and dppb ligands
appeared to be the most suitable for thermodynamic
studies of the acidity and hydricity of the rhodium
hydrides, because they are among the least electron
donating commercially available diphosphine ligands.
It was therefore anticipated that HRh(dppe)2 and HRh-
(dppb)2 would be less hydridic than most other HRh-
(diphosphine)2 complexes. This feature is important
because the determination of hydricity by the heterolytic
cleavage of hydrogen required a base whose conjugate
acid has a pKa value of approximately 31. The base
therefore had to be (1) a strong base, (2) noncoordinating
to rhodium, and (3) not so bulky that the reaction was
too slow for steric reasons. We were fortunate to find
that Pt(dmpp)2 fit these requirements, and HPt(dmpp)2
reacted cleanly with HRh(dppb)2 to give a readily
measured equilibrium mixture as shown in reaction 2.
Unfortunately, the reaction of HRh(dppe)2 with the
same platinum reagent resulted in ligand exchange and
decomposition reactions, and an accurate equilibrium
measurement was not possible for the dppe complex.
Acid ities of M(III) Hyd r id es. The pKa values of the
rhodium(III) derivatives [HRh(dppe)2(CH3CN))]2+ (9.0
( 0.3) and [HRh(dppb)2(CH3CN)]2+ (9.4 ( 0.3) are
nearly the same, and both values are smaller than that
of [HCo(dppe)2(CH3CN)]2+ (11.3).11 The isoelectronic
complexes HRh(dmg)2(PPh3) and HCo(dmg)2(PBu3) show
a similar trend, with pKa values of 9.5 and 10.5,
respectively, in 50:50 water/methanol mixtures.2e Com-
parisons of octahedral rhodium(III) and iridium(III)
hydrides have been made in studies of oxidative addition
reactions of protic acids to MX(CO)(PR3)2 complexes in
methanol (where M ) Rh or Ir and X ) halide). The
studies indicate that Rh complexes are also slightly
more acidic (by 0 to 1 pKa unit) than the analogous Ir
complexes.2c These data taken together would indicate
that the differences in acidity between octahedral Co,
Rh, and Ir complexes in the +3 oxidation state are
small, with Rh hydrides being slightly more acidic than
both the analogous Co and Ir derivatives. For the
diphosphine complexes studied here, deprotonation is
also accompanied by cleavage of the M-solvent bond. It
may be that the small difference in pKa values is a
consequence of the loss of coordinated solvent molecules
that accompanies the deprotonation of the octahedral
complexes. In contrast, the isoelectronic octahedral
complexes, H2Fe(CO)4, H2Ru(CO)4, and H2Os(CO)4,
show a systematic decrease in acidity as one descends
the periodic table with pKa values of 11.4, 18.7, and 20.8,
respectively, in acetonitrile.19 In this case no ligand loss
accompanies deprotonation. It is clear that more data
are needed to fully understand the factors governing the
relative acidity within triads of metal octahedral com-
plexes.
HRh(dppb)2 + [HPt(dmpp)2]+ h [Rh(dppb)2]+ +
Pt(dmpp)2 + H2 (2)
Pt(dmpp)2 + H+ h [HPt(dmpp)2]+
H2 f H+ + H-
(3)
(4)
(5)
HRh(dppb)2 f [Rh(dppb)2]+ + H-
∆G°H- ) -1.37 log Keq2 -1.37pKa(3) + 76.0 kcal/mol
(6)
The free energies associated with reactions 2 (-0.5 kcal/
mol, -1.37 pKeq 2), 3 (-41.6 kcal/mol, calculated using
a pKa value of 30.4 for [HPt(dmpp)2]+),16 and 4 (76.0
kcal/mol, the free energy associated with the heterolytic
cleavage of hydrogen in acetonitrile)17,18 can therefore
be used in eq 6 to calculate the hydride donor ability,
∆G°H-, of HRh(dppb)2 (34 kcal/mol). The uncertainty
of this value is estimated to be approximately (1.4 kcal/
mol on the basis of the uncertainty of the equilibrium
constant ((0.8 kcal/mol) and the pKa value of [HPt-
(dmpp)2]+ ((0.6 kcal/mol).
Acid ity of HRh (d p p b)2. The acidity of HRh(dppb)2
can be calculated using eq 10. This equation is based
on the sequence of reactions 5, 7, and 8. Reaction 5 is
the hydride donor ability of HRh(dppb)2, which was
determined as described in the preceding paragraph.
Reaction 7 is the reduction of [Rh(dppb)2]+ to [Rh(dppb)2]-
(E1/2 ) -2.02 V vs ferrocene in aetonitrile), and reaction
8 is the oxidation of the hydride ion to H+ in acetonitrile
(-79.6 kcal/mol when referenced to the ferrocene/
ferrocenium couple).4a,18 The sum of these reactions is
the deprotonation of HRh(dppb)2, eq 9, and the free
energy for this process is the sum of the free energies
associated with these three reactions as shown in eq
10. The resulting free energy value of 47 kcal/mol
corresponds to a pKa value of 35 ( 1.3. The estimated
uncertainty arises from the uncertainty in the free
energy associated with reaction 5 ((1.4 kcal/mol) and
the uncertainty in the reduction potential of reaction 7
((0.4 kcal/mol).
HRh(dppb)2 f [Rh(dppb)2]+ + H-
[Rh(dppb)2]+ + 2 e- f [Rh(dppb)2]-
(5)
(7)
H- f H+ + 2 e-
(8)
(9)
HRh(dppb)2 f [Rh(dppb)2]+ + H+
∆G°H+ ) ∆G°H- - 46.1E°(I/-I) - 79.6 (10)
Acid ities of M(I) Hyd r id es. The acidity of HRh-
(dppb)2 (pKa ) 35) is greater than that observed for
HCo(dppe)2 (pKa ) 38).11 The difference is due in part
to the greater electron-withdawing ability of the dppb
Discu ssion
Thermodynamic parameters have now been deter-
mined for a series of first, second, and third row
(19) Moore, E. J .; Sullivan, J . M.; Norton, J . R. J . Am. Chem. Soc.
1986, 108, 2257. (b) J ordan, R. F.; Norton, J . R. J . Am. Chem. Soc.
1982, 104, 1255.
(18) Wayner, D. D. M.; Parker, V. D. Acc. Chem. Res. 1993, 26, 287-
294.