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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
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10.1021/om020421k CCC: $22.00 © 2002 American Chemical Society
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