Interconversion between zwitterionic and cationic rhodium(I) complexes of demonstrated value as catalysts in hydroformylation, silylformylation, and hydrogenation reactions. Dynamic 31P{1H} NMR studies of (η6-PhBPh3)-Rh+(DPPB) and [Rh(DPPB)2]+BPh4- in solution

Article abstract of DOI:10.1021/om960112s

Treatment of an orange solution of [Rh(COD)(DPPB)]⁺BF₄^- (2) in MeOH with 2 equiv of NaBPh₄ at room temperature (RT) afforded an orange precipitate, [Rh(COD)(DPPB)]⁺BPh₄^- (3), in 94% yield. Reaction of the cationic rhodium complex 3 with H₂ under ambient conditions in CH₂Cl₂ for 1 h gave the zwitterionic complex (η⁶-PhBPh₃)-Rh⁺(DPPB) (4) in quantitative yield. Although 3 is stable in the solid state, it has the propensity in solution to convert to the zwitterionic complexes (η⁶-PhBPh₃)-Rh⁺(COD) (1) and (η⁶-PhBPh₃)-Rh⁺-(DPPB) (4) along with a small amount of [Rh_x(DPPB)_2x]^x+[BPh₄ ^-]_x. Addition of 1, 2, and 4 equiv of DPPB to the CD₂Cl₂ solution of (η⁶-PhBPh₃)-Rh⁺(NBD) (6) under N₂ resulted in the formation of [Rh(NBD)(DPPB)]⁺BPh₄^- (7) and [Rh(DPPB)₂]⁺BPh₄^- (5) in ratios of 90/ 10, 57/43, and 0/100, respectively, while addition of 2 equiv of DPPB to the CD₂Cl₂ solution of 6, under an atmosphere of H₂ at RT, gave 4 and 5 in an ratio of 35/65. Slowed η⁶-PhBPh₃^- rotation about the (η⁶-PhBPh₃)^--Rh bond axis in (η⁶-PhBPh₃)^-Rh⁺(DPPB) (4) was established by a variable-temperature ³¹P{¹H} NMR study. Variable-temperature ³¹P{¹H} NMR spectra of [Rh(DPPB)₂]⁺BPh₄^- (5) along with the low-temperature ³¹P{1H} COSY and EXSY NMR spectra demonstrated the presence of an equilibrium between [Rh(DPPB)₂]⁺ (5α) and [Rh(DPPB)(μ-DPPB)]₂²⁺ (5β).

Full text of DOI:10.1021/om960112s

2496
Organometallics 1996, 15, 2496-2503
In ter con ver sion betw een Zw itter ion ic a n d Ca tion ic
Rh od iu m (I) Com p lexes of Dem on str a ted Va lu e a s
Ca ta lysts in Hyd r ofor m yla tion , Silylfor m yla tion , a n d
Hyd r ogen a tion Rea ction s. Dyn a m ic 31P {1H} NMR
Stu d ies of (η6-P h BP h 3)-Rh +(DP P B) a n d
-
[Rh (DP P B)2]+BP h 4 in Solu tion
Zhongxin Zhou,† Glenn Facey,† Brian R. J ames,*,‡ and Howard Alper*,†
Departments of Chemistry, University of Ottawa, 10 Marie Curie Street, Ottawa, Ontario,
Canada K1N 6N5, and University of British Columbia, Vancouver, British Columbia,
Canada V6T 1Z1
Received February 14, 1996X
Treatment of an orange solution of [Rh(COD)(DPPB)]+BF4 (2) in MeOH with 2 equiv of
-
NaBPh4 at room temperature (RT) afforded an orange precipitate, [Rh(COD)(DPPB)]+BPh4
-
(3), in 94% yield. Reaction of the cationic rhodium complex 3 with H2 under ambient
conditions in CH2Cl2 for 1 h gave the zwitterionic complex (η6-PhBPh3)-Rh+(DPPB) (4) in
quantitative yield. Although 3 is stable in the solid state, it has the propensity in solution
to convert to the zwitterionic complexes (η6-PhBPh3)-Rh+(COD) (1) and (η6-PhBPh3)-Rh+-
(DPPB) (4) along with a small amount of [Rhx(DPPB)2x]x+[BPh4-]x. Addition of 1, 2, and 4
equiv of DPPB to the CD2Cl2 solution of (η6-PhBPh3)-Rh+(NBD) (6) under N2 resulted in
the formation of [Rh(NBD)(DPPB)]+BPh4 (7) and [Rh(DPPB)2]+BPh4 (5) in ratios of 90/
10, 57/43, and 0/100, respectively, while addition of 2 equiv of DPPB to the CD2Cl2 solution
of 6, under an atmosphere of H2 at RT, gave 4 and 5 in an ratio of 35/65. “Slowed” η6-
PhBPh3- rotation about the (η6-PhBPh3)--Rh bond axis in (η6-PhBPh3)-Rh+(DPPB) (4) was
established by a variable-temperature 31P{1H} NMR study. Variable-temperature 31P{1H}
NMR spectra of [Rh(DPPB)2]+BPh4- (5) along with the low-temperature 31P{1H} COSY and
EXSY NMR spectra demonstrated the presence of an equilibrium between [Rh(DPPB)2]+
-
-
2+
(5R) and [Rh(DPPB)(µ-DPPB)]2 (5â).
The zwitterionic rhodium complex 11 is an effective
in these reactions, and it was considered important to
investigate the solution behavior of the zwitterionic (η6-
PhBPh3)-Rh+(diene)n(DPPB)1-n and the cationic [Rh-
(diene)n(DPPB)2-n]+BPh4- (n ) 0, 1) complexes, in order
to gain insight into the reactions catalyzed by 1 (with
or without DPPB). Herein we report the preparation
and demonstrate the facile interconversion of these
rhodium complexes. The fluxionality of (η6-PhBPh3)--
Rh+(DPPB) and [Rh(DPPB)2]+BPh4- complexes is also
demonstrated by variable-temperature 31P{1H} NMR
spectra, which provide a detailed understanding of the
solution behavior of these complexes.
and versatile catalyst for a variety of carbonylation
reactions.2-9 This complex, either by itself or in the
presence of 1,4-bis(diphenylphosphino)butane (DPPB),
can effect the highly regioselective hydroformylation of
aryl and 1,1-disubstituted alkenes,2,3 allyl acetates,6
vinyl ethers,2 vinylsilanes,8 and vinyl sulfones and
sulfoxides,7 as well as R, â-unsaturated esters.4,9 Com-
plex 1 is also an excellent catalyst for the inter- and
intramolecular silylformylation of alkynes.10,11 Moder-
ate yields of acids can be realized by the carbonylation
of benzylic and allylic bromides with 1 under phase-
transfer conditions.12 Amines are isolated, usually in
high yield, by the hydrogenation of imines catalyzed by
1 and DPPB.13 Both cationic6 and zwitterionic12 inter-
mediates have been proposed as key catalytic species
† University of Ottawa.
‡ University of British Columbia.
X Abstract published in Advance ACS Abstracts, April 1, 1996.
(1) Schrock, R. R.; Osborn, J . A. Inorg. Chem. 1970, 9, 2339.
(2) Amer, I.; Alper, H. J . Am. Chem. Soc. 1990, 112, 3674.
(3) Zhou, J . Q.; Alper, H. J . Chem. Soc., Chem. Commun. 1991, 233.
(4) Alper, H.; Zhou, J . Q. J . Org. Chem. 1992, 57, 3729.
(5) Zhou, J . Q.; Alper, H. J . Org. Chem. 1992, 57, 3328.
(6) Alper, H.; Zhou, J . Q. J . Chem. Soc., Chem. Commun. 1993, 316.
(7) Totland, K.; Alper, H. J . Org. Chem. 1993, 58, 3326.
(8) Crudden, C. M.; Alper, H. J . Org. Chem. 1994, 59, 3091.
(9) Lee, C. W.; Alper, H. J . Org. Chem. 1995, 60, 499.
(10) Zhou, J . Q.; Alper, H. Organometallics 1994, 13, 1586.
(11) Monteil, F.; Matsuda, I.; Alper, H. J . Am. Chem. Soc. 1995, 117,
4419.
Resu lts a n d Discu ssion
P r ep a r a tion a n d In ter con ver sion of η6-P h BP h 3-
Coor d in a ted a n d Ca tion ic Rh od iu m Com p lexes.
Treatment of a clear orange solution of [Rh(COD)-
(DPPB)]+BF4- (2) with 2 equiv of NaBPh4 in MeOH at
room temperature (RT) afforded, in 94% yield, the
-
orange complex [Rh(COD)(DPPB)]+BPh4 (3), which
1
was characterized by H, 13C{1H}, and 31P{1H} NMR
(12) Amaratunga, S.; Alper, H. J . Organomet. Chem. 1995, 488, 25.
(13) Zhou, Z.; J ames, B. R.; Alper, H. Organometallics 1995, 14,
4209.
spectroscopy and elemental analysis. Its 1H and 13C-
{1H} NMR spectra were essentially those of the cation
S0276-7333(96)00112-4 CCC: $12.00 © 1996 American Chemical Society

Interconversion between Rh(I) Complexes
Organometallics, Vol. 15, No. 10, 1996 2497
Sch em e 1
Sch em e 2
to their noncoordinated analogs. The 1H NMR meta,
ortho, and para resonances of η6-PhBPh3 appear well
separated at δ 4.95, 5.63, and 6.81 ppm, respectively.
The 13C{1H} spectrum of 4 possesses four signals at δ
98.01, 103.30, 106.41, and 153.50 (m) ppm, respectively,
which are assigned to the coordinated η6-PhBPh3. The
31P{1H} doublet of coordinated DPPB in 4 moves down-
field to δ ) 38.7 ppm (1J Rh-P ) 200 Hz), compared to
1
that in the cationic complex 3 (δ ) 24.6 ppm, J Rh-P
)
143 Hz).
The low solubility of (η6-PhBPh3)-Rh+(COD) (1) in
any solvent at RT prevented an NMR investigation of
its reactivity. Thus, the closely related complex (η6-
PhBPh3)-Rh+(NBD) (6)1 was prepared, followed by
addition of 1 equiv of DPPB to a CD2Cl2 solution of 6
under N2, affording [Rh(NBD)(DPPB)]+BPh4- (7). After
the complete consumption of DPPB (in 60 min, as judged
-
of complex 2 and the BPh4 anion. The 31P{1H}
1
spectrum showed a doublet at δ ) 24.6 ppm with J Rh-P
) 143 Hz. Complex 3, soluble in CHCl3, CH2Cl2,
acetone, and THF but insoluble in MeOH and in hexane,
is stable in the solid state. In solution, however, 3 has
the propensity to convert to the zwitterionic complexes
(η6-PhBPh3)-Rh+(COD) (1) and (η6-PhBPh3)-Rh+(DPPB)
(4). When 3 was dissolved in CDCl3 and allowed to
1
by H and 31P NMR), 90% of 6 was converted to 7 (δ-
1
(31P{1H}) 27.5 ppm, J Rh-P ) 153 Hz) and [Rh(DP-
PB)2]+BPh4- (5) (a broad band at δ(31P{1H}) 21.3 ppm,
7/5 ) 9/1). Bubbling H2 through a CD2Cl2 solution of 7
for 1 h resulted in complete conversion to the zwitteri-
onic complex (η6-PhBPh3)-Rh+(DPPB) (4) and norbor-
nane. Reaction of 2 equiv of DPPB with 6 in CD2Cl2 at
RT under N2, for 150 min, resulted in the formation of
7 and 5 in the ratio of 57/43, while addition of 2 equiv
of DPPB to the CD2Cl2 solution of 6 under 1 atm of H2
at RT for 150 min gave 4 and 5 in the ratio of 35/65.
Addition of 4 equiv of DPPB to the CD2Cl2 solution of 6
under N2 at RT afforded complex 5, quantitatively, in
60 min (cf. Scheme 1). The composition of [Rh(DP-
PB)2]+BPh4-, (5) was confirmed by the isolation of
analytically pure compound via an independent route.
stand in air for 7 days, H, 13C{1H}, and 31P{1H} NMR
1
spectra of the solution indicated complete conversion of
3 to 1, 4, and 1,4-bis(diphenylphosphino)butane dioxide
(δ(31P{1H}) 32.0 (s) ppm), along with some unidentified
[Rhx(DPPB)2x]x+ species (4/DPPB dioxide/[Rhx(DPPB)2x]x+
) 32/42/26). Similar observations were made previously
- 14
when an acetone solution of [Rh(COD)(PPh3)2)]+BPh4
- 15
or [Rh(P(OR)3)5]+BPh4
was exposed to air. When a
CDCl3 solution of 3 was kept under N2 at room tem-
perature for 6 days, 4 was also formed as the major
product together with 1 and some phosphine-coordinat-
ed rhodium complexes (4/[Rhx(DPPB)2x]x+ ) 51/49), one
of which was [Rh(DPPB)2]+BPh4-, (5; cf. Scheme 1 and
Hydrogenation of [Rh(COD)(DPPB)]+BF4- (2) in MeOH
at RT (20 min) followed by the addition of 1 equiv of
DPPB in CH2Cl2 afforded the known cationic complex
[Rh(DPPB)2]+BF4- (8)20-23 as an orange-red solid, after
removal of volatiles under vacuum. Addition of 4 equiv
of NaBPh4 (in MeOH) to the clear MeOH solution of 8
afforded the orange-red [Rh(DPPB)2]+BPh4- (5) in 99%
yield (cf. Scheme 2). Interestingly, about 15% of 5 was
converted to the zwitterionic (η6-PhBPh3)-Rh+(DPPB)
(4) and DPPB, after a THF or CH2Cl2 solution of 5 was
heated to 40 °C for 2 h.
Reaction of (η6-PhBPh3)-Rh+(NBD) (6) with 200 psi/
200 psi of H2/CO at room temperature in CD2Cl2 or CH2-
Cl2 afforded a very air-sensitive red solution containing
rhodium carbonyl complexes, [Rhx(CO)y]z+[BPh4-]z (9),
and aldehydes (δ(1H) 10.01, 9.67, 9.64 ppm, δ(13C{1H})
202.7, 202.4, 192.7 ppm , ν(CdO) ) 1717 cm-1). This
solution possesses three strong infrared ν(CO) bands at
1
discussion below). The complexity of H NMR spectra
of the solution in both the aerobic and anaerobic systems
prevented us from determining the ratio of 1 and 4. In
both cases free 1,5-cyclooctadiene was detected by GC.
Reaction of complex 3 with H2 at ambient pressure
and temperature, in CH2Cl2 for 1 h, quantitatively
afforded the zwitterionic complex (η6-PhBPh3)-Rh+-
(DPPB) (4) as deep red prismatic crystals and cyclooc-
tane (cf. Scheme 1). This complex is soluble in CH2Cl2
and THF and partially soluble in CHCl3. The η6-
PhBPh3 coordination in 4 is clearly demonstrated by the
1H, 13C{1H}, and 31P{1H} NMR spectra. As observed
in analogous complexes,15-19 all protons and carbons on
η6-PhBPh3 show significantly high-field shifts compared
(14) Schrock, R. R.; Osborn, J . A. J . Am. Chem. Soc. 1971, 93, 2397.
(15) Haines, L. M. Inorg. Chem. 1971, 10, 1685.
(16) Nolte, M. J .; Gafner, G.; Haines, L. M. J . Chem. Soc., Chem.
Commun. 1969, 1406.
(17) Albano, P.; Aresta, M.; Manassero, M. Inorg. Chem. 1980, 19,
1069.
(18) Longato, B.; Pilloni, G.; Graziani, R.; Casellato, U. J . Orga-
nomet. Chem. 1991, 407, 369.
(19) Aresta, M.; Quaranta, E.; Albinati, A. Organometallics 1993,
12, 2032.
(20) Pignolet, L. H.; Doughty, D. H.; Nowicki, S. C.; Anderson, M.
P.; Casalnuovo, A. L. J . Organomet. Chem. 1980, 202, 211.
(21) Anderson, M. P.; Pignolet, L. H. Inorg. Chem. 1981, 20, 4101.
(22) Brown, J . M.; Chaloner, P. A.; Kent, A. G.; Murrer, B. A.;
Nicholson, P. N.; Parker, D.; Sidebottom, P. J . J . Organomet. Chem.
1981, 216, 263.
(23) J ames, B. R.; Mahajan, D. Can. J . Chem. 1979, 57, 180.

2498 Organometallics, Vol. 15, No. 10, 1996
Zhou et al.
Sch em e 3
Sch em e 4
2072, 2042, and 1875 cm-1, respectively, suggesting the
existence of both terminal and bridging CO in the
solution.24 A similar solution resulted from the reaction
of (η6-PhBPh3)-Rh+(COD) (1) with 200 psi/200 psi of H2/
CO at 40 oC for 24 h. Attempts to isolate these carbonyl
complexes, however, failed. Bubbling CO through the
solution of complex 6 for 2 h at RT resulted in the color
changing from pale yellow to orange-red. An infrared
spectrum of this solution showed three strong carbonyl
stretching bands at 2076, 2051, and 1975 cm-1, respec-
tively, while the 1H NMR spectrum indicated the
presence of a large amount of 6. Passing N2 through
the above solution at RT for 1 h resulted in bleaching
of the above solution and the disapearance of ν(CO)
bands from the IR spectrum, indicating the presence of
a reversible process between complex 6 and [Rh-
(NBD)x(CO)y]+BPh4- (10; cf. Scheme 3). Similar obser-
vations were obtained after complex 6 was exposed to
200 psi of CO for 20 min and then to 1 atm of N2.
Treatment of (η6-PhBPh3)-Rh+(NBD) (6) with H2 (no
DPPB or CO) gave rhodium black.
Sch em e 5
F lu xion a lity of (η6-P h BP h 3)-Rh +(DP P B) (4). The
conformational preferences and dynamic behavior of η6-
arene complexes have been subjects of longstanding
interest to theoretical and experimental chemists.25-31
The rotation of an MLn fragment around the η6-
arene-M bond axis is a well-known dynamic process
and the rotational barrier is usually very low,25,26,28,29
unless exceptional steric32 and/or electronic33 factors
exist. In the latter case, this dynamic process has been
unequivocally shown to be “slowed” on the NMR time
scale at accessible temperatures.31
η2 a η6 fashion in (η-PhBPh3)-Zr+(CH2Ph)3 (cf. Scheme
4),36 and of the ethylene proton exchange in {[(C2H4)2-
Rh(η6-Ph)]2BPh2}+[O3SCF3]-.19 Another interesting and
unexplained “complicated exchange process”37 is worth
noting. The crystal structure of Co(PMe3)2BPh4 clearly
shows one phenyl ring η6-coordinated to cobalt with a
mirror plane bisecting the two PMe3 ligands.37 The
fluxionality of this molecule, however, in CD2Cl2 re-
sulted in the disappearance of the PMe3 signal in the
31P{1H} spectrum and of all phenyl resonances in the
13C{1H} spectrum of (η6-PhBPh3)-Co+(PMe3)2, as well
The fluxionality of η-tetraphenylborate-metal com-
plexes has been previously demonstrated by NMR
experiments.19,34-37 These include the observations of
1
as an ill-resolved H NMR spectrum at 295 K. At 183
K, a singlet appeared at 8.3 ppm in the 31P{1H}
spectrum and a well-resolved 13C{1H} spectrum along
-
hapticity exchange, with a phenyl ring of BPh4 being
1
with a partially resolved H spectrum were observed.37
coordinated to zirconium in an η2 a η3 fashion in
Cp′2Zr+Me(BPh4-) (Cp′2 ) 1,1′-(CH2)2(indenyl)2)35 or an
These observations are in agreement with the Co(PMe3)2
moiety “creeping” among the four phenyl rings as shown
in Scheme 5. So far, no “slowed” η6-PhBPh3 rotation
about the η6-PhBPh3-MLn bond axis has been described
in the literature, although many η6-PhBPh3-MLn com-
plexes have been isolated and characterized by crystal-
lographic and/or spectroscopic methods.15-19,34,37-47
(24) Hughes, R. P. In Comprehensive Organometallic Chemistry;
Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press:
Oxford, U.K., 1982; Vol. 5, p 277.
(25) Albright, T. A. Acc. Chem. Res. 1982, 15, 149.
(26) Albright, T. A.; Hoffmann, R.; Tse, Y. C.; D’Ottavio, T. J . Am.
Chem. Soc. 1979, 101, 3812.
(27) Albright, T. A.; Hofmann, P.; Hoffmann, R. J . Am. Chem. Soc.
1977, 99, 7546.
(28) Albright, T. A.; Burdett, J . K.; Whangbo, M. H. In Orbital
Interactions in Chemistry; Wiley: New York, 1985.
(29) Radonovich, L. J .; Koch, F. J .; Albright, T. A. Inorg. Chem. 1980,
19, 3373.
(30) Hunter, G.; Weakley, T. J . R.; Mislow, K.; Wong, M. G. J . Chem.
Soc., Dalton Trans. 1986, 577.
(31) McGlinchey, M. J . Adv. Organomet. Chem. 1992, 34, 285.
(32) Pomeroy, R. K.; Harrison, D. J . J . Chem. Soc., Chem. Commun.
1980, 661.
(33) Acampora, M.; Ceccon, A.; Farra, M. D.; Giacometti, G.; Rigatti,
G. J . Chem. Soc., Perkin Trans. 1977, 2, 483.
(35) Horton, A. D.; Frijns, J . H. G. Angew. Chem., Int. Ed. Engl.
1991, 30, 1152.
(36) Bochmann, M.; Karger, G.; J aggar, A. J . J . Chem. Soc., Chem.
Commun. 1990, 1038.
(37) de Carvalho, L. C. A.; Dartiguenave, M.; Dartiguenave, Y.;
Beauchamp, A. L. J . Am. Chem. Soc. 1984, 106, 6848.
(38) Herrmann, W. A.; Anwander, R.; Riepl, H.; Scherer, W.;
Whitaker, C. R. Organometallics 1993, 12, 4342.
(39) Oro, L. A.; Pinilla, E.; Tenajas, M. L. J . Organomet. Chem. 1978,
148, 81.
(40) Bennett, M. A.; Hann, E. J . J . Organomet. Chem. 1977, 124,
213.
(34) Bochmann, M. Angew. Chem., Int. Ed. Engl. 1992, 31, 1181.

Interconversion between Rh(I) Complexes
Organometallics, Vol. 15, No. 10, 1996 2499
Ch a r t 1
dissymmetric conformer adopted by complex 4 at low
temperature can be found from structural data for the
analogous (η6-PhBPh3)-Rh+L2 complexes (L ) P(O-
Me)3;15,16 L2 ) 1,2-bis(diphenylphosphino)ethane (DP-
PE),17 1,1′-bis(diphenylphosphino)ferrocene (DPPF)18).
The slowed η6-phenyl ring rotation in 4, in contrast to
the reported analogs,15-18 is presumably due to the size
of the chelating ring, where the seven-membered chelate
ring may force the phenyl groups on each phosphorus
to be relatively closer to the phenyl groups of η6-PhBPh3
(cf. Chart 1A). This may impose a higher rotational
barrier to η6-PhBPh3 about the (η6-PhBPh3)-Rh bond
axis.
F igu r e 1. Variable-temperature 31P{1H} NMR spectra of
(η6-PhBPh3)-Rh+(DPPB) (4) at 121.45 MHz in CD2Cl2/
toluene-d8 (2/1): (a) 295 K; (b) 233 K; (c) 213 K; (d) 194 K;
(e) 183 K; (f) 168 K.
Figure 1 illustrates the VT 31P{1H} NMR spectra of
(η6-PhBPh3)-Rh+(DPPB) (4) in CD2Cl2/toluene-d8 at
121.45 MHz. At 295 K, 4 exhibits a sharp doublet at δ
1
) 38.7 ppm with J Rh-P ) 200 Hz. As the temperature
Dyn a m ic Beh a vior of [Rh (DP P B)2]+BP h 4- (5). It
is well-known that the catalytic activity of the cationic
[Rh(PPh2(CH2)nPPh2)2]+ (n ) 1-6) complexes is de-
pendent on the size of the chelating ring.23,48-50 These
complexes possess different solid-state structures,21,22,51
solution behavior,21,22,52 and chemical reactivity,20,22,52-54
as n is varied from 1 to 6. The reaction of these
complexes with CO and with H2 illustrates major
reactivity differences.20,22,50,52-54 Reversible addition of
H2 gave cis-[(H)2Rh(PPh2(CH2)nPPh2)2]+ with n ) 3,
while the n ) 1 and 2 cationic rhodium precursors were
unreactive toward H2 and a mixture of hydrides was
formed when n ) 4.53 The simple monocarbonyl adducts
[Rh(PPh2(CH2)nPPh2)2CO]+ were isolated for n ) 1 and
353-55 and no CO adduct was formed for n ) 2, while a
mixture containing some binuclear DPPB-bridged com-
plex [Rh2(PPh2(CH2)4PPh2)3(CO)4]+ was obtained when
n ) 4.20,53 In addition, variable-temperature 31P NMR
data indicated that the geometry of [Rh(PPh2(CH2)n-
PPh2)2]+ cations in solution varied from square planar
(n ) 2) to solvated trigonal bipyramidal with a solvent
molecule occupying an equatorial position (n ) 3) and
to unidentifiable, complicated species (n ) 4),21 although
decreases, the doublet gradually broadens and coalesces
at 194 K. A further decrease of the temperature to 168
K results in the disappearance of the resonance at δ )
38.7 ppm and the appearance of two sets of doublets at
1
δ ) 58.4 and 19.2 ppm, respectively, with J Rh-P ≈ 193
Hz. The same spectra were obtained in neat CD2Cl2 at
T > 183 K. This dynamic process was reversible as the
solution was warmed. We were unable to lower the
temperature further in order to resolve the two doublets
to the expected two sets of doublets of doublets, due to
the limitation of the NMR probe and the solvents used.
These observations may be attributed to the “slowed”
rotation of the coordinated phenyl ring about the (η6-
PhBPh3)-Rh bond axis (cf. Chart 1A). Rapid rotation
of η6-PhBPh3 about the (η6-PhBPh3)-Rh bond axis at
T > 194 K establishes the time-averaged symmetry
plane (cf. Chart 1B) and gives a single phosphorus
resonance (cf. Figure 1a-c). With T < 194 K, this
rotation is slowed down and a conformation without the
symmetry plane is adopted by complex 4 (cf. Chart 1C);
and two sets of doublets are observed in the 31P{1H}
spectrum (cf. Figure 1e,f). The anticipated P-P cou-
pling is obscured due to the incomplete freezing of this
rotation at the accessible temperature. Evidence for the
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1980, 19, 1191.
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Theopold, K. H. J . Am. Chem. Soc. 1991, 113, 893.
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Chem., Int. Ed. Engl. 1992, 31, 1235.
(49) Doughty, D. H.; Pignolet, L. H. J . Am. Chem. Soc. 1978, 100,
7083.
(50) Doughty, D. H.; Pignolet, L. H. In Homogeneous Catalysis with
Metal Phosphine Complexes; Pignolet, L. H., Ed.; Plenum Press: New
York, 1983; p 343.
(51) Hall, M. C.; Kilbourn, B. T.; Taylor, K. A. J . Chem. Soc. A 1970,
2539.
(52) Slack, D. A.; Greveling, I.; Baird, M. C. Inorg. Chem. 1979, 18,
3125.
(53) J ames, B. R.; Mahajan, D. Can. J . Chem. 1980, 58, 996.
(54) Pignolet, L. H.; Doughty, D. H.; Nowicki, S. C.; Casalnuovo, A.
L. Inorg. Chem. 1980, 19, 2172.
(47) Calderazzo, F.; Pampaloni, G.; Rocchi, L.; Englert, U. Organo-
metallics 1994, 13, 2592.
(55) Sanger, A. R. J . Chem. Soc., Dalton Trans. 1977, 120.

2500 Organometallics, Vol. 15, No. 10, 1996
Zhou et al.
F igu r e 2. Variable-temperature 31P{1H} NMR spectra of [Rh(DPPB)2]+BPh4- (5) at 202.46 MHz in CD2Cl2: (a) 323 K; (b)
300 K; (c) 270 K; (d) 247 K; (e) 224 K; (f) 198 K; (g) 176 K.
Ch a r t 2
in the chemical shift range of -5 to +45 ppm (Figure
2d-f). These resonances eventually became well-
resolved multiplets centered at 40.4, 31.3, 20.7, and -2.2
ppm, with an integration ratio of 2.5/1/1/2.5, respec-
tively, at 176 K (Figure 2g). This process was also
reversible with increasing solution temperature.
The analysis of the spectrum recorded at 176 K
(Figure 2g) was at first complicated by the overlap of
the resonances centered at 40.4 and -2.2 ppm. At-
tempts to assign this spectrum using any single species
with a general formula of [Rhx(DPPB)y]x+ failed. There-
fore, exchange(s) between different Rh-DPPB com-
plexes is (are) almost certainly occurring in this solution.
The 31P{1H} COSY data (Figure 3) clearly show the
coupling connectivities between the individual 31P reso-
nances in the same species. On this basis there appears
to be two species present. The 31P{1H} EXSY data
(Figure 4) show that the two species are in slow
exchange with each other. Although the assignment is
not completely unambiguous due to some resonance
overlap, the cross-peaks in the COSY and EXSY data
are mutually exclusive. As shown in Figure 3, reso-
nance D, centered at 31.3 ppm, is coupled with reso-
nances M (strong), G (moderate), and A (weak), respec-
tively. Resonance G centered at 20.7 ppm is coupled
with A (strong), D (moderate), and M (weak), respec-
tively, clearly demonstrating that resonances A, D, G,
and M belong to the phosphines from the same species.
This is confirmed by the 31P{1H} EXSY data, which do
not show any exchange between sites D and G (Figure
4). Therefore, the other resonances (a, d, g, m) are
assigned to another species. Computer simulation of
the ADGMX and the adgmx multiplets gives the pa-
rameters reported in Scheme 6, which are consistent
with the presence of a distorted-square-planar mono-
meric rhodium complex (5R) and a dimeric rhodium
these complexes all possess the same general formula
in the solid state (Chart 2).21,23,51-55 “A combination of
solvated species and dimeric/ polynuclear species” was
suggested by Anderson and Pignolet in the case of n )
4.21
In order to clarify the solution behavior of [Rh-
(DPPB)2]+X- (X- ) BF4- and BPh4-), VT 31P{1H} NMR
experiments, along with low-temperature 31P{1H} COSY
and EXSY experiments, were performed. The VT
31P{1H} NMR spectra of [Rh(DPPB)2]+BPh4- (5) in CD2-
Cl2 are shown in Figure 2. The 31P{1H} resonance of
-
[Rh(DPPB)2]+X- (X- ) BF4 and BPh4-) at 300 K
appeared as a broad band (202.46 MHz, Figure 2b) or a
broadened doublet (121.45 MHz) at about 21.5 ppm.
This resonance, however, was resolved to a doublet at
202.46 MHz at 323 K (Figure 2a), while the broadened
doublet at 121.45 MHz became a sharp doublet (1J RhP
) 136 Hz) at 313 K. As the temperature decreases, the
single 31P resonance disappeared at 270 K (Figure 2c),
and then gradually well-separated resonances appeared

Interconversion between Rh(I) Complexes
Organometallics, Vol. 15, No. 10, 1996 2501
-
F igu r e 3. 31P{1H} COSY spectrum of [Rh(DPPB)2]+BPh4 (5) at 202.46 MHz in CD2Cl2 at 176 K.
-
complex (5â) in a ratio of 100/75. The solvated trigonal-
bipyramidal species as observed for a solution of [Rh-
(PPh2(CH2)3PPh2)2]+ and [Rh(DPPF)2]+ complexes (cf.
Chart 2)21,56 is ruled out, because a different coupling
pattern would have resulted. The same spectra were
obtained when the more strongly coordinating solvents
THF-d8 and acetone-d6 were used, respectively, which
confirms the above assumption. It is important to point
out that no resonances for an uncoordinated phosphine
(δ(31P{1H} -16 ppm)23 are present in the VT 31P NMR
spectra. This rules out the presence of the monoden-
tate-coordinated DPPB with one end dangling. The VT
resonances of BPh4 always remained sharp in the
temperature range of 176-300 K, as the 1H and 13C-
{1H} resonance pattern of [Rh(DPPB)2]+ changed with
a change in temperature.
In conclusion, this study has demonstrated that the
zwitterionic (η6-PhBPh3)-Rh+(diene)n(DPPB)1-n and the
-
cationic [Rh(diene)n(DPPB)2-n]+BPh4 (n ) 0, 1) com-
plexes are interconvertible and coexist in solution. The
size of the chelating ring probably rendered a “slowed”
-
η6-PhBPh3 rotation about the (η6-PhBPh3)--Rh bond
axis in (η6-PhBPh3)-Rh+(DPPB) (4). This is the first
example showing rotational fluxionality in η-tetraphe-
nylborate-coordinated complexes. The VT 31P{1H} and
low-temperature 31P{1H} COSY and EXSY NMR spec-
tra of [Rh(DPPB)2]+BPh4-, (5) establish the coexistence
of [Rh(DPPB)2]+ (5R) and [Rh(DPPB)(µ-DPPB)]22+ (5â)
in solutions of complex 5. It is clear that the solution
geometries of [Rh(PPh2(CH2)nPPh2)]+ are the “expected”
square planar for n ) 2, solvated trigonal bipyramidal
for n ) 3 (cf. Chart 2),21 and a mixture of mono- and
binuclear rhodium species for n ) 4 (cf. Scheme 6).
-
31P NMR spectra are independent of the anion (BF4
and BPh4-), which excludes possible exchange between
-
coordinated phosphine and an η-PhBPh3 coordinated
1
species. This was further confirmed by the VT H and
13C{1H} NMR spectra of [Rh(DPPB)2]+BPh4- (5) in CD2-
Cl2, which demonstrated that the 1H and 13C{1H}
(56) Casellato, U.; Corain, B.; Graziani, R.; Longato, B.; Pilloni, G.
Inorg. Chem. 1990, 29, 1193.

2502 Organometallics, Vol. 15, No. 10, 1996
Zhou et al.
-
F igu r e 4. 31P{1H} EXSY spectrum of [Rh(DPPB)2]+BPh4 (5) at 202.46 MHz in CD2Cl2 at 176 K.
pulse sequence modified to incorporate continuous 1H decou-
pling. The data consist of 64 slices, each with an acquisition
time of 0.011 s using a spectral width of 11.6 kHz. Each slice
was collected using 128 transients with a 0.5 s relaxation
delay. The data were zero-filled to 512 points in each domain
and Fourier-transformed using sine bell weighting. The EXSY
spectrum was recorded using a standard phase-sensitive
(TPPI) NOESY pulse program modified to incorporate con-
tinuous 1H decoupling. The data consist of 200 slices, each
with an acquisition time of 0.044 s using a spectral width of
11.6 kHz. Each of the slices was collected using 16 transients
with a 0.5 s relaxation delay. The data were zero-filled to 512
points in each domain and Fourier-transformed using sine
squared weighting. RhCl3‚3H2O, [Rh(COD)(DPPB)]+BF4- (2),
and other chemicals were purchased from Aldrich and were
used as received. [Rh(COD)Cl]2,57 (η6-PhBPh3)-Rh+(COD) (1),1
and (η6-PhBPh3)Rh(NBD) (6)1 were prepared by following
literature procedures.
These findings are likely of significance in homogeneous
catalysis involving zwitterionic and/or cationic rhodium
complexes.
Exp er im en ta l Section
Gen er a l Con sid er a tion s. The syntheses and manipula-
tions of solutions were performed under a nitrogen or carbon
monoxide atmosphere with standard Schlenk-line techinques.
Solvents were dried and purified by standard methods.
Infrared spectra were run on a Bomem MB-100 FT-IR spec-
trometer. All 1H 13C{1H}, and 31P{1H} NMR spectra were
recorded on a Bruker AMX-500, Varian XL-300, or Gemini 200
MHz spectrometer using CDCl3 or CD2Cl2 as the solvent.
Variable-temperature 31P{1H} NMR spectra (reported with
respect to 85% aqueous H3PO4, downfield shifts being positive)
were acquired on the Bruker operating at 202.46 MHz using
the decoupling coil of an inverse detection probe or on the
Varian spectrometer operating at 121.45 MHz using a stan-
dard broad-band probe. The temperature was regulated
within (0.5 °C. The 31P{1H} COSY NMR data were collected
(Bruker AMX-500) using a standard magnitude COSY-45
Gen er a l P r oced u r e for t h e R ea ct ion of (η6-P h B-
P h 3)-Rh +(NBD) (6) w ith CO or H2/CO u n d er High P r es-
su r e. In a 45-mL Parr autoclave fitted with a glass liner and
(57) Cramer, R. Inorg. Synth. 1974, 15, 14.

Interconversion between Rh(I) Complexes
Organometallics, Vol. 15, No. 10, 1996 2503
Sch em e 6. 31P {1H} NMR Da ta for 5r a n d 5â a t
of CH2Cl2 in a Schlenk tube. The system was first flushed
with nitrogen and then with hydrogen. Continuous stirring
of this solution for 60 min under H2 at RT resulted in a color
change from orange-red to deep red and the disappearance of
COD, which was converted to cyclooctane (1H NMR). Hexanes
(1 mL) was then added slowly after the reaction solution was
concentrated to about 3 mL. This solution was allowed to
stand under N2 at RT overnight. The resulting deep red
prisms were filtered, washed with cold CH2Cl2 (2 × 3 mL),
176 K
1
and air-dried (0.13 g, 99%): mp >202 °C dec; H NMR (CD2-
Cl2, 22 °C) δ 7.34-6.98 (m, 35H, Ph), 6.81 (t, J ) 6.2 Hz, 1H,
Hpara of η-PhsB), 5.63 (d, J ) 6.2 Hz, 2H, Hortho of η-PhsB),
4.95 (t, J ) 6.2 Hz, 2H, Hmeta of η-PhsB), 2.17 (s, broad, 4H,
-CH2-), 1.72 (s, broad, 4H, -CH2-); 13C{1H} NMR (CD2Cl2,
22 °C) δ 161.57 (q, J B-C ) 50.2 Hz, Cipso of BsPh), 153.50 (m,
C
ipso of η-PhsB), 138.59 (m), 136.52, 132.87 (t, J P-C ) 5.0 Hz),
130.15, 128.55 (t, J P-C ) 5.0 Hz), 126.46, 123.20 (phenyl
carbons), 106.41, 103.30, 98.01 (η-PhsB carbons), 29.47 (t, J P-C
) 16.2 Hz, PsCH2s), 24.39 (sCH2s); 31P{1H} NMR (CD2Cl2,
22 °C) δ 38.7 (d, J RhP ) 200.3 Hz). Anal. Calcd for C52H48
BP2Rh: C, 73.60; H, 5.70. Found: C, 73.29; H, 5.47.
-
Syn th esis of [Rh (DP P B)2]+BF 4- (8).20-23 Because of the
low solubility of [Rh(COD)Cl]2 in acetone, alternative methods
-
were used here to synthesize [Rh(DPPB)2]+BF4
.
Meth od A. To a solution of [Rh(COD)Cl]2 (0.0826 g, 0.17
mmol) in 10 mL of CH2Cl2 and 2 mL of MeOH was added
0.0853 g (0.44 mmol) of AgBF4 in 2 mL of acetone. The
resulting precipitate was filtered, and DPPB (0.29 g, 0.68
mmol) was added to the filtrate. Hydrogenation of the reaction
mixture under 200 psi of H2 at RT afforded a deep red solution.
Removal of volatiles under vacuum resulted in an orange-red
solid, which was recrystallized from CH2Cl2/n-pentane (0.31
g, 91%).
Meth od B. [Rh(COD)(DPPB)]+BF4- (0.052 g, 0.072 mmol)
was dissolved in 15 mL of MeOH in a Schlenk tube. The
system was first flushed with nitrogen and then with hydro-
gen. Continuous stirring of this solution for 20 min under H2
at RT, followed by the addition of 0.031 g (0.072 mmol) of
DPPB in 10 mL of CH2Cl2, resulted in a solution color change
from orange-red to deep red. This reaction mixture was stirred
for another 2 h. Removal of volatiles afforded 8 (0.072 g, 96%).
stirring bar was added (η6-PhBPh3)-Rh+(NBD) (6; 50 mg) and
CH2Cl2 (5 mL) or CD2Cl2 (1 mL). The CO line was flushed
three times with CO, and the autoclave was fill-vented three
times with CO to displace the air; subsequently, the pressure
was increased to 200 psi with CO. When H2 was required,
the pressure was increased to 400 psi by filling with H2, after
the H2 line was flushed three times. The solution was stirred
in the autoclave at RT for the desired period of time. The
excess CO (or H2/CO) was released and the system disas-
sembled, and the reaction solution was transferred to a
Schlenk tube or an NMR tube under an atmosphere of N2 or
CO and subjected to IR and NMR tests.
Gen er a l P r oced u r e for t h e R ea ct ion of (η6-P h -
BP h 3)-Rh +(NBD) (6) w ith CO, H2, or H2/CO u n d er Am bi-
en t P r essu r e. In a 25-mL Schlenk tube with a stirring bar,
or an NMR tube, was added (η6-PhBPh3)-Rh+(NBD) (6; 50 mg)
and CH2Cl2 (5 mL) or CD2Cl2 (1 mL). The system was frozen
and thawed three times under N2 first and then CO, H2, or
H2/CO was slowly bubbled through for the desired period of
time. The reaction solution was analyzed by IR and NMR.
Syn th esis of [Rh (DP P B)2]+BP h 4- (5). Addition of excess
NaBPh4 (0.13 g, 0.38 mmol) in 5 mL of MeOH to a deep red
solution of [Rh(DPPB)2]+BF4-, (8; 0.083 g, 0.079 mmol) in 15
mL of MeOH resulted in the formation of an orange-red
precipitate. The precipitate was collected and washed three
times with 10 mL of H2O followed by 10 mL of MeOH and
then air-dried (0.10 g, 99%): mp 234-236 °C dec; 1H NMR
(CD2Cl2, 27 °C) δ 7.40-7.10 (m, broad, PsPh), 7.33 (m, Hortho
of BsPh), 7.04 (t, J ) 7.0 Hz, Hmeta of BsPh), 6.87 (t, J ) 7.0
Hz, Hpara of BsPh) (total 60H), 2.13, 1.66 (broad signals, 16H,
-
Syn th esis of [Rh (COD)(DP P B)]+BP h 4 (3). An excess
of NaBPh4 (0.23 g, 0.66 mmol) in 5 mL of methanol was added,
drop-by-drop, into a clear orange-yellow solution of [Rh-
(COD)(DPPB)]+BF4- (2; 0.21 g, 0.29 mmol) in 20 mL of MeOH.
An orange-red precipitate soon formed, and after the solution
was stirred for about 10 min, the precipitate was collected and
washed three times with 20 mL of H2O, followed by 20 mL of
MeOH, and then air-dried (0.26 g, 94%): mp 167-168 °C dec;
1H NMR (CDCl3, 22 °C) δ 7.48, 7.40 (m, 28H, PsPh and Hortho
of BsPh), 6.98 (t, J ) 7.0 Hz, 8H, Hmeta of B-Ph), 6.83 (t, J )
7.0 Hz, 4H, Hpara of BsPh), 4.37 (s, broad, 4H, CHdCH), 2.30,
2.17, 1.50 (broad signals, 16H, -CH2-); 13C{1H} NMR (CDCl3,
22 °C) δ 164.10 (q, J B-C ) 49.6 Hz, Cipso of BsPh), 136.32,
133.05, 132.95, 132.84, 132.28 (m), 131.67, 129.49, 129.39,
129.29, 125.40, 125.35, 121.48 (phenyl carbons), 100.50 (m,
CHdCH), 31.50 (m), 30.38, 24.67 (-CH2-); 31P{1H} NMR
(CDCl3, 22 °C) δ 24.6 (d, J RhP ) 143.1 Hz). Anal. Calcd for
-CH2-); 13C{1H} NMR (CD2Cl2, 27 °C) δ 164.70 (q, J B-C
)
49.9 Hz, Cipso of BsPh), 136.23, 134.9 (m), 133.19, 130.61,
128.68, 125.97, 125.91, 125.86, 122.03 (phenyl carbons), 31.16
(broad), 25.02 (-CH2-); 31P{1H} NMR (CD2Cl2, 27 °C) δ 21.5
(s, broad, at 202.46 MHz), 21.5 (broadened doublet, at 121.45
MHz). Anal. Calcd for C80H76BP4Rh: C, 75.36; H, 6.01.
Found:, C, 75.04; H, 5.85.
Ack n ow led gm en t. We are indebted to the Natural
Sciences and Engineering Research Council of Canada
for the award of a collaborative project grant in support
of this research.
C
60H60BP2Rh: C, 75.32; H, 6.32. Found: C, 75.13; H, 6.15.
Syn th esis of (η6-P h BP h 3)-Rh +(DP P B) (4). [Rh(COD)-
(DPPB)]+BPh4- (3; 0.15 g, 0.16 mmol) was dissolved in 10 mL
OM960112S