Synthesis, structure determination, and hydroformylation activity of N-heterocyclic carbene complexes of rhodium

Article abstract of DOI:10.1139/v05-106

The effect of ancillary phosphine ligands on the structure and hydroformylation activity of Rh-N-heterocyclic carbene complexes of type [Rh(IMeS)(PR₃)(CO)Cl] and [Rh(SIMes)(PR₃)(CO)Cl] is described. Very high selectivities for the branched isomer (>95:5) were obtained in the hydroformylation of vinylarenes in all cases except for R = OPh. The new complexes were characterized spectroscopically and by X-ray crystallography.

Full text of DOI:10.1139/v05-106

1248
Rhodium(I) – (N-heterocyclic carbene) – diphosphine
complexes
Hongsui Sun, Xiao-Yan Yu, Paolo Marcazzan, Brian O. Patrick, and Brian R. James
Abstract: Reactions of [RhCl(COE)(IPr)]2 (1) and [RhCl(COE)(IMes)]2 (2) (COE = cyclooctene; IPr = N,N’-bis(2,6-
diisopropylphenyl)imidazolin-2-ylidene; IMes = N,N’-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene) with the diphosphines
Ph2P(CH2)nPPh2 and 1,2-bis(diphenylphosphino)benzene (dppbz) give the N-heterocyclic carbene (NHC) – diphosphine –
rhodium(I) complexes: RhCl(NHC)[Ph2P(CH2)nPPh2] [NHC = IPr, n = 1 (3); NHC = IMes, n = 1 (4); NHC = IPr, n = 2
(5); NHC = IMes, n = 2 (6); NHC = IPr, n = 4 (7); NHC = IMes, n = 4 (8)] and RhCl(NHC)(dppbz) [NHC = IPr (9);
1
NHC = IMes (10)]. All the complexes are characterized by H, 31P{1H}, and 13C{1H} NMR spectroscopy, elemental analy-
sis, and mass spectrometry. Complexes 3, 7, and 9 are also characterized crystallographically. In benzene solution, the
complexes decompose in the presence of O2 with formation of the diphosphine dioxide, whereas reaction with CO leads to
replacement of the NHC ligand to give known carbonyl–diphosphine complexes.
Key words: rhodium, N-heterocyclic carbenes, bis(diphenylphosphino) ligands, crystallography.
´
´
´
`
Resume : Les reactions du [RhCl(COE)(IPr)]2 (1) et du [RhCl(COE)(IMes)]2 (2) [COE = cyclooctene; IPr = N,N’-bis(2,6-
´
`
´
´
`
diisopropylphenyl)imidazoline-2-ylidene; IMes = N,N’-bis(2,4,6-trimethylphenyl)imidazoline-2-ylidene] avec les diphosphi-
´
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`
`
´
nes Ph2P(CH2)nPPh2 et 1,2-bis(diphenylphosphino)benzene (dppbz) conduisent a la formation de complexes carbene N-he-
´
terocyclique (NHC) – diphosphine – rhodium(I): RhCl(NHC)[Ph2P(CH2)nPPh2] [NHC = IPr, n = 1 (3); NHC = IMes, n = 1
(4); NHC = IPr, n = 2 (5); NHC = IMes, n = 2 (6); NHC = IPr, n = 4 (7); NHC = IMes, n = 4 (8)] et RhCl(NHC)(dppbz)
´ ´
´
´
´ ´
[NHC = IPr (9); NHC = IMes (10)]. Tous les complexes ont ete caracterises par des analyses elementaires, par spectrosco-
1
13
1
pie RMN du H, du 31P{1H} et du C{ H} et par spectrometrie de masse. Les complexes 3, 7 et 9 ont aussi ete caracteri-
´
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´
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ses par diffraction des rayons X. En solution dans le benzene, les complexes se decomposent en presence de O2 avec
´
formation d’oxyde de diphosphine, alors qu’une reaction avec le CO conduit au remplacement du ligand NHC pour
conduire aux complexes carbonyl–diphosphines connus.
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Mots-cles : rhodium, carbenes N-heterocycliques, ligands bis(diphenylphosphino), cristallographie.
´
[Traduit par la Redaction]
nated NHC ligands with or without ancillary phosphine
ligands. We have shown that RhCl(NHC)(P–N) and cis-
RhCl(NHC)(PPh3)2 complexes oxidatively add O2 to gener-
ate ‘‘standard’’ six-coordinate Rh(III)–peroxide species
(see Scheme 1), where P–N = P,N-chelated o-(diphenyl-
phosphino)-N,N-dimethylaniline and NHC = IPr or IMes;5
in an as yet unpublished work, our X-ray data for a five-
coordinate RhCl(IPr)2(O2) are also consistent with a perox-
ide.6 However, recent communications by Crudden and co-
workers3,4a report that RhCl(NHC)2(O2) (NHC = IPr or
IMes) and RhCl(IMes)(PPh3)(O2) are best described as
being Rh(I) singlet oxygen species. In efforts to understand
the nature of this important discrepancy, we decided to
synthesize Rh(I)–diphosphine–NHC complexes in the hope
that their reactivity toward O2 might prove helpful — a
hope that was not fulfilled!
Introduction
A just-published special issue of European Journal of
Inorganic Chemistry demonstrates the growing interest in
transition-metal N-heterocyclic carbene (NHC) complexes.1
Seven of the papers in the issue are concerned with Rh–
NHC chemistry, including one from our group, which de-
scribes reactions of [RhCl(COE)(NHC)]2 complexes with H2
and with CO, where COE is cyclooctene and NHC is either
N,N’-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene (IPr) or
N,N’-bis(2,4,6-trimethylphenyl)imidazolin-2-ylidene (IMes).2
More generally, studies on Rh–NHC systems often pertain
to comparisons with the chemistry and catalytic activity of
Rh – tertiary phosphine systems, i.e., with replacement of PR3
ligand(s) by NHC ligand(s) or use of mixed NHC – tertiary
phosphine species.2–5 Of prime interest to our group is the
interaction of O2 with Rh(I) complexes containing coordi-
Received 5 May 2009. Accepted 7 July 2009. Published on the NRC Research Press Web site at canjchem.nrc.ca on 21 August 2009.
This tardy article is dedicated to Professor R.J. Puddephatt on the occasion of his 66th birthday (should have been submitted last year
for his 65th!), and in recognition of his contributions to the development of Canadian organometallic chemistry. This article is a belated
contribution to a Special Issue dedicated to Professor R.J. Puddephatt (Canadian Journal of Chemistry, January 2009).
H. Sun, X.-Y. Yu, P. Marcazzan, B.O. Patrick, and B.R. James.1 Department of Chemistry, The University of British Columbia,
Vancouver, BC V6T 1Z1, Canada.
1Corresponding author (e-mail: brj@chem.ubc.ca).
Can. J. Chem. 87: 1248–1254 (2009)
doi:10.1139/V09-118
Published by NRC Research Press

Sun et al.
1249
(s, 2H, m-IMes), 6.95–7.09 (m, 12H, o-Ph (4H) + m-Ph
(4H) + p-Ph (4H)?), 7.43–7.49 (m, 4H, m-Ph), 7.80–7.87
(m, 4H, o-Ph). 13C{H} NMR d: 19.45 (Me), 21.02 (Me),
21.61 (Me), 51.11 (dd, JCP = 18, JCP = 23, PCH2P), 122.93
(NCH), 122.97 (NCH), 128.99, 129.40, 130.31, 131.70,
131.92, 133.84 (d, JCP = 20, CPhP), 133.96 (d, JCP = 16,
CPhP), 135.35, 136.69, 136.98, 137.79, 138.07, 138.33,
138.54, 193.15 (ddd, JCP = 10, JCP = 47, JCRh = 129,
Experimental section
General procedures
All manipulations were performed under an atmosphere of
dry Ar using standard Schlenk techniques or in a glove box.
Solvents (Fisher Scientific) were dried using standard proce-
dures and, before use, were purged with a stream of Ar.
C6D6 (Cambridge Isotope Laboratory) was distilled from Na
and stored under Ar. The diphosphines were used as received
from Strem Chemicals. Complexes [RhCl(COE)(IPr)]2 (1)
and [RhCl(COE)(IMes)]2 (2) were prepared by our literature
procedure.5 NMR spectra were recorded in C6D6 at room
temperature (~20 8C) either on a Bruker AV 400 or AV 300
spectrometer, data being reported in ppm relative to TMS
with the solvent signals as internal references (1H, d 7.16;
13C, d 128.38); 31P{H} NMR shifts are relative to external
85% H3PO4. J values are given in Hz (s = singlet, d =
doublet, t = triplet, spt = septet, m = multiplet, br = broad,
ps = pseudo); assignments were sometimes aided by the use
A
B
Rh–C). 31P{H} NMR: AB portion of the PA, PB, Rh
(ABX) spin system simulated by the parameters dA = –
33.57, dB = –19.93, JAB = 107.0, JAX = 102.0, JBX
=
179.0 (see text). MS (EI) (m/z): 826 [M]+, 790 [M –
HCl]+, 303 [IMes – 1]+. Anal. calcd. for C46H46N2ClP2Rh:
C 66.81, H 5.61, N 3.39; found: C 66.8, H 5.6, N 3.3.
RhCl(IPr)(Ph2PCH2CH2PPh2) (5)
Complex 5 was prepared using 50.0 mg (0.04 mmol) of 1,
31.0 mg (0.08 mmol) of dppe, and 10 mL C6H6. Yield:
1
66 mg (91%). H NMR d: 0.40 (d, 6H, J = 6.8, Me), 0.90
1
of 13C{1H}-APT and H–13C{1H}–HSQC experiments. Mass
(d, 6H, J = 6.8, Me), 1.11 (d, 6H, J = 6.8, Me), 1.42–1.64
(m, 4H, CH2-dppe), 1.68 (d, 6H, J = 6.8, Me), 3.38 (m, 2H,
CHMe2), 3.81 (m, 2H, CHMe2), 6.81 (s, 2H, NCH), 6.87 (d,
2H, J = 7.4, m-IPr), 6.93 (t, 4H, J = 7.0, m-Ph), 7.02 (t, 2H,
J = 7.0, p-Ph), 7.07–7.14 (m, 6H, o-Ph (4H) + p-Ph (2H)?),
7.26 (t, 2H, J = 7.7, p-IPr), 7.35 (d, 2H, J = 7.6, m-IPr), 7.45
spectra were obtained on a Kratos MS-50 spectrometer oper-
ating in the EI mode or on a Bruker Biflex IV spectrometer
operating with a MALDI ion source. Elemental analyses
were done on a Carlo Erba EA 1108 elemental analyzer.
RhCl(IPr)(Ph2PCH2PPh2) (3)
(t, 4H,
J = 8.5, m-Ph), 7.73–7.78 (m, 4H, o-Ph).
63.8 mg (0.05 mmol) of 1 and 38.5 mg (0.10 mmol) of
dppm were dissolved in C6H6 (6 mL). The resulting yellow
solution became orange on being stirred at room temperature
for 5 h and was then concentrated to ~2 mL; hexane
(~2 mL) was then added to precipitate the product, which
was filtered off and dried in vacuo. Yield: 85 mg (93%). H
NMR d: 0.53 (6H, d, J = 6.7, Me), 1.00 (6H, d, J = 6.7,
13C{H} NMR d: 22.90, 24.34, 27.04, 27.17, 28.90, 29.71,
32.84 (d, JCP = 25, CH2), 33.17 (d, JCP = 23, CH2), 123.90,
124.86, 124.94, 127.83, 127.92, 127.98, 128.93, 129.16,
129.75, 133.64 (d, JCP = 12, CPhP), 135.16 (d, JCP = 12,
CPhP), 136.20, 136.60, 137.94, 138.25, 144.91, 149.61,
196.72 (dd, JCP ~ 48, JCRh = 121). 31P{H} NMR d: 62.10
1
2
1
1
(dd, JPP = 38, JRhP = 123), 65.84 (2JPP = 38, JRhP = 208).
MS (EI) (m/z): 924 [M]+, 536 [M – IPr]+, 387 [IPr – 1]+.
Anal. calcd. for C53H60N2ClP2Rh: C 68.81, H 6.54, N 3.03;
found: C 68.5, H 6.4, N 3.0.
Me), 1.16 (6H, d, J = 6.7, Me), 1.89 (d, 6H, J = 6.7, Me),
3.35 (br t, 2H, J
= 9.0, J
= 8.7, PCH2P), 3.62 (spt,
HPA
HPB
2H, J = 6.7, CHMe2), 3.74 (spt, 2H, J = 6.7, CHMe2), 6.79
(s, 2H, NCH), 6.88 (ps t, 6H, m-Ph (4H) + m-IPr (2H)?),
6.96–6.99 (m, 8H, o-Ph (4H) + p-Ph (4H)?), 7.23 (t, 2H,
3JHH = 7.6, p-IPr), 7.30–7.37 (ps t, 6H, m-Ph (4H) + m-IPr
(2H)?), 7.88–7.95 (m, 4H, o-Ph). 13C{H} NMR d: 22.87,
24.36, 26.87, 27.03, 29.09, 29.69, 48.29 (dd, JCP = 19,
JCP = 30, PCH2P), 123.78, 124.52, 124.91, 128.05, 128.21,
128.31, 128.88, 129.48, 129.77, 133.01 (d, JCP = 22, CPhP),
133.96 (d, JCP = 12, CPhP), 136.35, 136.65, 136.73, 137.02,
138.13, 144.89, 149.16, 195.45 (ddd, JCP = 10, JCP = 48,
RhCl(IMes)(Ph2PCH2CH2PPh2) (6)
Complex 6 was prepared using 75.0 mg (0.07 mmol) of 2,
54.0 mg (0.14 mmol) of dppe, and 10 mL C6H6. Yield:
1
98 mg (86%). H NMR d: 1.23–1.24 (m, 4H, CH2-dppe),
1.61 (s, 6H, o-Me), 2.23 (s, 6H, o-Me), 2.79 (s, 6H, p-Me),
6.23 (s, 2H, NCH), 6.60 (s, 2H, m-IMes), 6.85 (s, 2H, m-
IMes), 6.97–7.11 and 7.04–7.08 (m, 12H, o-Ph (4H) + m-
Ph(4H) + p-Ph (4H)?), 7.51 (t, 4H, J = 8, m-Ph), 7.80 (m,
4H, o-Ph). 13C{H} NMR d: 19.72 (Me), 21.56 (Me), 21.59
(Me), 32.05 (d, JCP = 25, CH2), 32.40 (d, JCP = 25, CH2),
123.16 (NCH), 127.77, 127.86, 127.93, 128.87, 128.93,
129.01, 129.14, 130.39, 134.52 (d, JCP = 11, CPhP), 135.04
(d, JCP = 10, CPhP), 135.58, 137.57, 138.49, 138.53, 139.04,
139.36. AB portion of the PA, PB, Rh (ABX) spin system
simulated by the parameters dA = –64.26, dB = –64.96,
JAB = 36.6, JAX = 125.7, JBX = 205.7 (see text). MS (EI)
(m/z): 840 [M]+, 804 [M – HCl]+, 536 [M – IMes]+, 303
[IMes – 1]+. Anal. calcd. for C47H48N2ClP2Rh: C 67.13,
H 5.76, N 3.33; found: C 67.0, H 5.8, N 3.23.
A
B
JCRh = 128, Rh–C). 31P{H} NMR: AB portion of the PA,
PB, Rh (ABX) spin system simulated by the parameters:
dA = –35.68, dB = –20.92, JAB = 101.0, JAX = 104.0, JBX
=
178.0 (see text). MS (EI) (m/z): 910 [M]+, 387 [IPr – 1]+.
Anal calcd. for C52H58N2ClP2Rh: C 68.55, H 6.42, N 3.08;
found: C 68.8, H 6.4, N 2.9. X-ray quality crystals of 3 con-
taining 0.5 mol C6H6 solvate were grown by layering a C6H6
solution of the compound with hexane.
RhCl(IMes)(Ph2PCH2PPh2) (4)
The synthetic procedure for complexes 4–10 follows that
given for 3; here, 55.0 mg (0.05 mmol) of 2, 38.6 mg
(0.10 mmol) of dppm, and C6H6 (5 mL) were used. Yield:
RhCl(IPr)[Ph2P(CH2)4PPh2] (7)
1
74 mg (89%). H NMR d: 1.73 (s, 6H, o-Me), 2.22 (s, 6H,
Complex 7 was prepared by using 63.8 mg (0.05 mmol)
of 1, 42.7 mg (0.10 mmol) of dppb, and 10 mL C6H6. Yield:
o-Me), 2.77 (s, 6H, p-Me), 3.40 (br t, 2H, JHP = 9.1, JHP
=
8.8, PCH2P), 6.25 (s, 2H, NCH), 6.60 (s, 2H,Am-IMes), 6B.88
76 mg (80%). H NMR d: 0.59 (br s, 6H, Me), 1.05 (m,
1
Published by NRC Research Press

1250
Can. J. Chem. Vol. 87, 2009
12H, Me), 1.29 (br s, 6H, Me), 1.72 (br s, 4H, CH2), 1.85
(br s, 4H, CH2), 3.56 (br s, 4H, CHMe2), 6.89 (s, 2H,
NCH), 6.98–7.08 (m, 16H, Ar), 7.30–7.41 (m, 10H, Ar).
13C{H} NMR d: 21.86 (Me), 23.89 (CH2), 23.97, 25.14,
25.58 (d, JCP = 20, PCH2), 27.34 (Me), 29.64 (Me), 29.73
(Me), 30.24 (CH2), 36.17 (d, JCP = 25, PCH2), 123.61,
124.05, 125.20, 127.15 (d, JCP = 9, CPhP), 128.30, 128.69,
128.93, 129.03, 129.61, 131.42, 131.51, 131.58, 133.36 (d,
JCP = 11, CPhP), 145.10, 150.55. 31P{H} NMR d: 13.85
6H, o-Me), 2.16 (s, 6H, o-Me), 2.74 (s, 6H, p-Me), 6.21 (s,
2H, NCH), 6.53 (s, 2H, m-IMes), 6.66–6.71 (m, 2H, o-
C6H4), 6.74 (s, 2H, m-IMes), 6.95–7.04 (m, 12H, o-Ph
(4H) + m-Ph (4H) + p-Ph (4H)), 7.25 (ps t, 2H, m-C6H4),
7.67 (t, 4H, J = 8.6, m-Ph), 7.79 (m, 4H, o-Ph).
13C{H} NMR d: 18.58, 20.79, 122.48, 127.09, 127.18,
127.25, 127.34, 127.81, 127.96, 128.16, 128.37, 128.88 (d,
JCP = 3, CPhP?), 129.21 (d, JCP = 4, CPhP?), 129.65, 130.57,
130.70, 131.68, 131.83, 133.99, 134.11, 134.22, 134.32,
134.68, 136.38, 136.80, 137.53, 137.64, 138.02, 138.36,
192.32 (dd, JCP ~ 9, JCRh = 118, Rh–C). 31P{H} NMR: AB
portion of the PA, PB, Rh (ABX) spin system simulated by
the parameters dA = –66.77, dB = –67.33, JAB = 39.60,
JAX = 206.8, JBX = 125.6 (see text). MS (EI) (m/z): 888
[M]+, 303 [IMes – 1]+. Anal. calcd. for C51H48ClN2P2Rh:
C 68.90, H 5.45, N 3.15; found: C 69.0, H 5.3, N 3.3.
1
2
1
(2JPP = 48, JRhP = 114), 48.94 (dd, JPP = 48, JRhP = 208).
MS (MALDI) (m/z): 952 [M]+. Anal. calcd. for
C55H64ClN2P2Rh: C 69.30, H 6.77, N 2.94; found: C 69.5,
H 6.7, N 3.1. X-ray quality crystals of 7 were grown by
layering a C6H6 solution of the compound with hexane.
RhCl(IMes)[Ph2P(CH2)4PPh2] (8)
The yellow complex 8 was obtained using 75.0 mg
(0.07 mmol) of 2, 58.0 mg (0.14 mmol) of dppb, and
10 mL of benzene. Yield: 91 mg (77%). H NMR d: 1.91
X-ray crystallographic analysis of complexes 3, 7, and 9
Measurements were made at 173.0(1) K on a Bruker X8
APEX diffractometer with graphite-monochromated Mo-Ka
1
(s, 6H, o-Me), 2.09 (m, 4H, CH2), 2.26 (s, 6H, o-Me), 2.46
(br s, 4H, CH2), 2.83 (s, 6H, p-Me), 6.29 (s, 2H, NCH), 6.75
(s, 2H, m-IMes), 6.90–7.11 (m, 14H, o-Ph (4H) + m-Ph
(4H) + p-Ph (4H) + m-IMes (2H)?), 7.32 (t, 4H, J = 8, m-
Ph), 7.62 (m, 4H, o-Ph). 13C{H} NMR d: 19.90 (Me), 21.89
(Me), 22.48 (Me) 24.08 (CH2), 32.74 (d, J = 18, P–CH2),
32.99 (CH2), 122.03, 122.05, 123.11, 126.84 (d, JCP = 10,
CPhP), 127.64, 127.82, 129.13, 129.54 (d, JCP = 8, CPhP),
133.44, 134.39, 134.65, 134.92, 135.43, 135.68, 137.38,
137.41, 138.20, 138.60, 138.90, 144.34. 31P{H} NMR d:
˚
radiation (0.71 069 A). Data were collected and integrated
using the Bruker SAINT software package7 and were cor-
rected for absorption effects using the multi-scan SADABS
technique.8 All structures were solved by direct methods,9
and refinements were performed using the SHELXTL pro-
gram.10 All non-hydrogen atoms were refined anisotropically
and all H atoms were included in calculated positions but
not refined.
2
1
1
12.38 (dd, JPP = 48, JRhP = 118), 54.04 (2JPP = 48, JRhP
=
Crystal data for 3
207). MS (MALDI) (m/z): 868 [M]+. Anal. calcd. for
C49H52ClN2P2Rh: C 67.70, H 6.04, N 3.23; found: C 67.9,
H 6.0, N 3.3.
C52H58N2ClP2RhÁ0.5C6H6, fw = 949.31, orange tablet,
size 0.15 mm  0.20 mm  0.50 mm, monoclinic, space
˚
˚
group P 21/n (No. 14), a = 11.0616(2) A, b = 21.9905(4) A,
3
˚
˚
c = 20.1095(4) A, b = 90.774(1)8, V = 4891.2(2) A , Z = 4,
Dcalcd = 1.291 g cm–3, m = 0.507 mm–1. F(000) = 1988.00,
62 508 total reflections (11 674 independent of symmetry,
8726 for I > 2s(I)), 558 variables, residuals (refined on F 2,
all data): R1 = 0.038, wR2 = 0.088.
RhCl(IPr)(Ph2PC6H4PPh2) (9)
The orange complex 9 was obtained using 55.0 mg
(0.04 mmol) of 1 and 44.0 mg (0.10 mmol) of dppbz in
5 mL of C6H6. Crystalline material contained 0.5 mol sol-
vate C6H6. Yield: 82 mg, 96%. Dried material was of the
1
Crystal data for 7
formulation C57H60ClN2P2Rh. H NMR d: 0.70 (d, 6H, J =
6.7, Me), 0.90 (d, 6H, J = 6.7, Me), 1.10 (d, 6H, J = 6.7,
Me), 1.65 (d, 6H, J = 6.5, Me), 3.37 (m, 2H, CHMe2), 3.91
(m, 2H, CHMe2), 6.72 (br s, 2H, NCH), 6.80–6.82 (m, 4H,
m-Ph), 6.89–6.95 (br t, 6H, J ~ 6.5), 7.09–7.15 (m, 8H),
7.25 (d, 2H, J = 7.6), 7.43 (ps t, 6H), 7.81 (m, 4H).
13C{H} NMR d: 22.73, 24.27, 26.99, 27.14, 29.21, 29.62,
123.63, 124.99, 127.87, 127.97, 128.32, 128.51, 128.69,
C55H64N2ClP2Rh, fw
= 952.33, orange plate, size
0.20 mm  0.25 mm  0.50 mm, monoclinic, space group
˚
˚
P 21/n (No. 14), a = 11.1595(6) A, b = 20.971(1) A, c =
3
˚
˚
20.938(1) A, b = 95.693(2)8, V = 4875.8(5) A , Z = 4,
Dcalcd = 1.299 g cm–3, m = 0.509 mm–1. F(000) = 2000.00,
69 860 total reflections (11 679 independent of symmetry,
9737 for I > 2s(I)), 558 variables; residuals (refined on F 2,
all data): R1 = 0.037, wR2 = 0.067.
128.92, 129.10, 129.81, 133.10, 133.22, 135.13 (d, JCP
=
11, CC H P?), 137.10 (d, JCP = 45, CPhP?), 138.55, 139.57
6
4
Crystal data for 9
(d, JCP = 36, CPhP?), 144.47, 149.05, 194.96 (ddd, JCP
=
A
11, JCP = 48, JCRh = 122, Rh–C). 31P{H} NMR d: 65.96
C57H60N2ClP2RhÁ0.5C6H6, fw = 1011.32, orange plate,
B
2
1
1
(dd, JPP = 39, JRhP = 205), 71.36 (2JPP = 39, JRhP = 124).
MS (EI) (m/z): 972 [M]+, 387 [IPr – 1]+. Anal. calcd:
C 70.35, H 6.22, N 2.88; found: C 70.6, H 6.3, N 2.7. The
layering method given for complex 3 yielded X-ray quality
crystals of 9 again containing 0.5 mol C6H6 solvate.
size 0.04 mm  0.15 mm  0.25 mm, triclinic, space group
˚
˚
P-1 (No. 2), a = 12.6856(8) A, b = 17.438(1) A, c =
˚
24.693(2) A, a = 80.911(3)8, b = 81.812(2)8, g
=
86.156(3)8, V = 5333.1(6) A , Z = 4, Dcalcd = 1.256 g cm–3,
m = 0.476 cm–1. F(000) = 2116.00, 69 026 total reflections
(18 891 independent of symmetry, 14 480 for I > 2s(I)),
1200 variables, residuals (refined on F2, all data): R1 =
0.061, wR2 = 0.103. The material crystallized with two mol-
ecules of the Rh-complex, one C6H6 molecule and one addi-
tional solvent molecule, in the asymmetric unit; the solvent
3
˚
RhCl(IMes)(Ph2PC6H4PPh2) (10)
The orange complex 10 was obtained using 55.0 mg
(0.05 mmol) of 2 and 44.0 mg (0.10 mmol) of dppbz in
1
5 mL of C6H6. Yield: 71 mg (80%). H NMR d: 1.62 (s,
Published by NRC Research Press

Sun et al.
1251
resembled hexane, but could not be modeled satisfactorily
and was thus removed from the model, with the SQUEEZE
program11 used to correct data for any residual electron den-
sity found in the space formerly occupied by the solvent.
Scheme 2. Synthesis of complexes and their numbering.
Scheme 1. Reactions of O2 with some Rh – N-heterocyclic carbene
(NHC) – phosphine complexes.
Fig. 1. The experimental (upper) and simulated (lower)
31P{1H} NMR spectra of RhCl(IPr)(dppm) (3). dP = –35.68,
dP = –20.92, JAB = 101.0 Hz, J
= 104.0 Hz, AJRhP = 178.0 Hz.
RhPA
B
B
Reactivity of the complexes
The complexes (~0.05 mmol) were dissolved in C6D6
(1 mL), and a sample (~0.7 mL) of the solution was placed
in a J-Young NMR tube and subjected to an atmosphere of
O2, CO, or H2 at room temperature; 31P{1H} NMR spectral
changes were then monitored.
Results and discussion
Synthesis and spectroscopic characterization of
complexes
The diphosphines Ph2P(CH2)nPPh2 (n = 1, dppm; n =2,
dppe; n = 4, dppb) and 1,2-bis(diphenylphosphino)benzene
(dppbz) react at room temperature with [RhCl(COE)(NHC)]2
complexes in benzene to give yellow RhCl(NHC)(P–P)
species in 77%–96% isolated yields (NHC = IPr or IMes, and
P–P is the chelating diphosphine). All eight complexes (3–10)
have been synthesized (see Scheme 2) and well characterized
The two singlets seen for the m-protons of the IMes ligand
similarly imply inequivalence of these protons within one
ring; the same description has been noted for the analogous
cis-RhCl(IMes)(PPh3)2 complex.4j Where possible, we have
assigned the m- and p-protons of the IPr ligand, the m-protons
of the IMes ligand, and the phenyl protons of the PPh2
groups, based on (i) expected splitting patterns (e.g., the
m-H of IPr and of IMes are doublets and singlets, respec-
tively, and the p-H of IPr is a triplet),2,5,14 (ii) data for free
IPr and IMes,15, and (iii) data for the free diphosphines.16
The more tentative assignments of these aromatic protons
are indicated by a query (?) in the Experimental section.
The CH2 protons of the carbon backbone of the diphos-
phine ligand in 5–8 appear as a multiplet or broad singlets.
Those of dppm in 3 and 4 are seen as a broadened triplet,
which is in fact a set of overlapping doublets that result
from two equivalent protons coupling to the two inequiva-
lent P atoms; the 31P-decoupled spectra showed a slightly
broadened singlet.
The 31P{1H} NMR spectra reveal the expected inequiva-
lent P atoms. For 5 and 7–9, first-order spectra show dou-
blets of doublets with JPP values of 48–38 Hz and JRhP
values of 123–114 and 208–205 Hz (for the P atom cis and
trans to Cl, respectively4d,4e,5). For the other complexes, the
spectra are more complex ABX systems but are readily si-
mulated. Figure 1 shows the data for 3 and those of 4 (the
other dppm derivative) are very similar: the JPP values are
now ~100 Hz, with JRhP values of ~100 and 180 Hz, respec-
tively, for the P atoms cis and trans to Cl. The ABX spectra
of 6 (Fig. 2) and 10 are similar to each other, with JPP and
1
by elemental analysis, H, 31P{1H}, and 13C{1H} NMR spec-
troscopy, mass spectrometry and, for 3, 7, and 9, by X-ray
crystallography.
The solution NMR data for the complexes are generally
straightforward, and most of the shifts are readily assigned
based on previously reported data, for example, for cis-
RhCl(NHC)(PPh3)2 and RhCl(NHC)(P–N) species (see
1
Scheme 1).5 All the complexes show a H singlet for NCH
between dH 6.79–6.89 and 6.21–6.29 for the IPr- and IMes-
containing species, respectively. The IPr species 3, 5, and 9
show four sets of doublets for inequivalent IPr–Me groups,
with coupling of ~6.7 Hz to a IPr–methine proton; the dou-
blets indicate restricted rotation about the Rh–C bond or,
less likely, the Nimid–Carene bond.12 Complex 7 shows broad
signals for these methyls, presumably because of the relative
greater flexibility of the dppb chelate ring, which might al-
low for more rotational motion.13 The Me signals are some-
times shifted downfield (e.g., to dH 1.89 for 3), which could
result from a deshielding effect from a phenyl ring of the di-
phosphine (see the following section for the structure of 3).
The IPr–methine protons appear as two septets (for 3), two
multiplets (for 5), or one broad singlet (for 7). For each of
the IMes species 4, 6, 8, and 10, the Me protons are seen as
three singlets at dH ~2.8, ~2.2, and between 1.9 and 1.6, im-
plying that the two o-Me groups on one aromatic ring are
inequivalent, consistent again with some restricted rotation.
Published by NRC Research Press

1252
Can. J. Chem. Vol. 87, 2009
Fig. 2. The experimental (upper) and simulated (lower)
Fig. 3. ORTEP diagram of RhCl(IPr)(dppm) (3) with 50% prob-
31P{1H} NMR spectra of RhCl(IMes)(dppe) (6) dP = –64.26,
ability thermal ellipsoids; H atoms are omitted for clarity. Selected
A
˚
dP = –64.96, JAB = 36.6 Hz, J
= 125.7 Hz, JRhP = 205.7 Hz.
bond lengths (A) and angles (8): Rh1—C1, 2.067(2); Rh1—Cl1,
RhPA
B
B
2.3793(6); Rh1—P1, 2.2005(6); Rh1—P2, 2.2516(6); P1—C28,
1.854(2); P2—C28, 1.845(2); C1–Rh1–Cl1, 90.30(6); Cl1–Rh1–P2,
93.94(2); P2–Rh1–P1, 72.81(2); P1–Rh1–C1, 103.07(6); C1–Rh1–
P2, 175.28(6); Cl1–Rh1–P1, 166.32(2).
JRhP values close to those seen in the first-order spectra of 5
and 7–9. Spectra akin to those of Fig. 2 have been noted
previously for analogous cis-RhCl(IMes)[P(OPh)3]2 spe-
cies.4e
The expected ddd pattern (coupling to Rh and inequiva-
lent P atoms) in the 13C{1H} spectra for the Rh–C signals is
seen at d ~195 for 3, 4, and 9, with JRhC being ~125 Hz and
the JPC values being ~10 and ~50 Hz. For 5 and 10, only
somewhat broader dd patterns are seen, whereas for 6–8
these signals are not seen and are presumably broadened
into the baseline by some exchange processes, reflecting a
relatively labile Rh–carbene bond (see the last section on
the reactivity toward CO); the flexibility of the dppb ring
might account in part for the lack of the Rh–C signal in 7
and 8. The non-appearance of carbene-carbon resonances in
Rh–NHC species is not unusual.2,4e,5 There is no question
about the identity of the complexes, which are all essentially
square-planar Rh(I) species, with some distortion induced by
the chelating phosphines, as seen particularly in the structure
of 3, the dppm derivative (see the following section). For 3
and 4, the 13C{1H} signal for the PCH2P carbon is seen as a
doublet of doublets at d ~50 with JCP values for the inequi-
valent P atoms in the 18–30 Hz range. For complexes 5–10,
13C{1H} signals for the C atoms attached to P atoms are
seen as doublets in the expected regions dC 25–36 and 125–
139, respectively, for alkyl and aryl carbons; uncertain as-
signments are again indicated by ?.
Fig. 4. ORTEP diagram of RhCl(IPr)(dppb) (7) with 50% probabil-
ity thermal ellipsoids; H atoms are omitted for clarity. Selected
˚
bond lengths (A) and angles (8): Rh1—C1, 2.0484(15); Rh1—Cl1,
2.3846(4); Rh1—P1, 2.2888(4); Rh1—P2, 2.2043(4); P1—C28,
1.8427(18); P2—C31, 1.8681(16); C1–Rh1–Cl1, 86.01(4); Cl1–
Rh1–P1, 82.569(15); P1–Rh1–P2, 91.046(15); P2–Rh1–C1,
100.39(4); C1–Rh1–P1, 168.34(4); Cl1–Rh1–P2, 173.587(15).
Mass spectral data reveal the parent peak for all the com-
plexes using either the EI mode (for 3–6, 9, and 10) or the
MALDI-TOF ion source (for 7 and 8).
Cl1–Rh1–P2 and P1–Rh1–C1 bond angles are 93.94 and
103.07(6)8, respectively. In 7, the more flexible dppb allows
for a P1–Rh–P2 bond angle of 91.058. Steric interactions be-
tween the IPr and a PPh2–phenyl ring are evident in all three
structures, this accounting for cis C–Rh–P angles of ~1008.
The Rh–C and Rh–Cl bond lengths are in the established
range for such chloro(carbene)–Rh(I or III) complex-
es.2,5,4d,17–21 In all three complexes, the Rh–P bond trans to
Structural characterization of 3, 7, and 9
The crystal structures of 3, 7, and 9 (all containing IPr)
were established by X-ray analysis; the molecular structures
and selected bond lengths and angles are shown in Figs. 3,
4, and 5, respectively. Complex 3 crystallized with one-half
of a benzene molecule, whereas 9 crystallized as two inde-
pendent molecules with one benzene molecule and one addi-
tional solvent molecule (likely hexane) in the asymmetric
unit; 7 was nonsolvated. The essentially square-planar struc-
tures show some distortion, particularly in the dppm of 3
where the P1–Rh–P2 bond angle is 72.818 and the adjacent
˚
the carbene is 0.04–0.08 A longer than the one cis to the
carbene, a finding compatible with data for cis-RhCl(I-
4d
˚
Mes)(PPh3)2, where the difference is about 0.09 A. Our
data also agree with a finding from Crudden’s group that
Published by NRC Research Press

Sun et al.
1253
Fig. 5. ORTEP diagram of RhCl(IPr)(dppbz) (9) with 50% prob-
ability thermal ellipsoids; the other independent molecule in the
asymmetric unit, and H atoms, are omitted for clarity. Selected
strated previously,22 although the chelating diphosphine li-
+
+
gands are retained when Rh(dppm)2 and Rh(dppe)2 react
in solution with O2 at ambient conditions to form the respec-
tive cis-Rh(P–P)2(O2)+ peroxide species, where (P–P) is the
chelating diphosphine.13b Our findings at this stage on the
reactivity of O2 toward the RhCl(NHC)(P–P) complexes un-
fortunately cannot contribute to the question of the nature of
the coordinated O2 in Rh–NHC complexes with and without
an ancillary phosphine ligand, in which singlet oxygen has
been suggested (see Introduction).3,4a Studies on such sys-
tems are ongoing.
˚
bond lengths (A) and angles (8): Rh1—C1, 2.101(3); Rh1—Cl1,
2.3929(8); Rh1—P2, 2.2239(8); Rh1—P1, 2.1796(9); P1—C28,
1.852(3); P2—C33, 1.832(3); C1–Rh1–Cl1, 91.58(8); Cl1–Rh1–P2,
83.92(3); P2–Rh1–P1, 85.30(3); P1–Rh1–C1, 99.66(8); C1–Rh1–
P2, 173.14(9); Cl1–Rh1–P1, 173.73(3).
Reaction of a benzene solution of RhCl(NHC)(dppe)
[NHC = IPr (5), IMes (6)] with CO at ambient conditions
rapidly precipitated a yellow solid, which was recrystallized
by layering of a CH2Cl2 solution of the compound with hex-
ane. The material, however, was shown by elemental analy-
sis and 31P{1H} NMR and IR spectroscopies to be the
known compound RhCl(CO)(dppe).24,25 Reactions of CO
with the dppm and dppb analogues (complexes 3, 4, 7, and
8) also resulted in displacement of the NHC ligand by CO
1
as shown by in situ H NMR, when resonances of the free
carbenes were generated.26
The Rh products were not investigated, but are pre-
sumed to be the well-known dimeric, bridged-diphosphine
complexes trans-[RhCl(CO)]2(m-P–P)2 that are formed
with dppm and dppb.25 The substitution of an NHC ligand
by CO, to the best of our knowledge, has not been re-
ported, and implies a relatively labile NHC ligand; in-
deed, substitution of IMes by PPh3 has been reported (see
Rh–P bonds in cis-RhCl(NHC)(PPh3)2 complexes are mar-
˚
ginally shorter (by up to 0.03 A) than corresponding bonds
in RhCl(PPh3)3 and trans-RhCl(CO)(PPh3)2;4d in the case of
˚
RhCl(IPr)(dppbz) (9), the difference is up to 0.08 A. Data
from Herrmann’s group on other chloro(carbene)phosphine–
phosphite–Rh(I) complexes18 also support this finding,
which is likely general within analogous Rh–carbene and
Rh–phosphine species.
the following).3,4f Within Rh–NHC–carbonyl systems,
2,14,27
RhCl(CO)(NHC)2
and RhCl(CO)2(NHC)2,19,28 com-
Reactivity of the complexes
plexes have been synthesized by reacting CO with Rh–
NHC precursors, whereas RhCl(CO)(PPh3)(NHC) has
been made from RhCl(PPh3)2(NHC) and CO.3,4d,4f,4j
Complexes 3–10 are all highly soluble in benzene, tol-
uene, CH2Cl2, CHCl3, and polar aprotic solvents, but are in-
soluble in solvents such as hexane. In the solid state, these
complexes are air-stable, but in solution (e.g., in C6H6/
C6D6), they are sensitive to air or oxygen and rapidly de-
compose at room temperature with formation of free diphos-
phine dioxide, as demonstrated by in situ 31P{1H} NMR
data; singlets were seen at dP = 24.4, 12.2, 31.7, and 32.0,
respectively, for the dioxides of dppm, dppe, dppb, and
dppbz, in agreement with literature data,22,23 and were iden-
tical to values measured by in situ oxidation of the free di-
phosphine in C6D6 with H2O2. The reactivity toward O2 of
these diphosphine complexes is markedly different to that
of the analogous cis-RhCl(NHC)(PPh3)2 complex, which
under the same conditions undergoes irreversible oxidative
addition to form a Rh(III)–peroxide that contains, however,
trans-PPh3 ligands (see Scheme 1)5; this peroxide is stable
in both the solid state, and even in solution shows no sign
of decomposition to OPPh3 over 24 h in air.5 The necessa-
rily cis-chelating diphosphines clearly do not allow for for-
mation at ambient conditions of a stable peroxide, although
the chelated P–N ligand mentioned in the Introduction
(Scheme 1) does so.5 Whether oxidation of the diphosphines
occurs via an intermediate with coordinated O2 requires de-
tailed low-temperature studies. Loss of a chelating diphos-
phine as the dioxide during oxygenation of a Rh(I)–
tris(pyrazolyl)borate–dppe complex in solution in attempts
to study formation of peroxide species has been demon-
The RhCl(NHC)(P–P) complexes in benzene solution do
not react with H2 at ambient conditions. Known hydrido com-
plexes of the type Rh(H)2Cl(NHC)2 and [Rh(H)2Cl(NHC)]2
have been made by reaction of H2 with Rh(I)–NHC precur-
sors,2,3,14,27 whereas Rh–hydrido–NHC species have also
been generated via intramolecular C–H activation of the
Me group of IMes.14,29 Hydrido derivatives of Rh–NHC–
phosphine species have not yet been reported.
Supplementary data
Supplementary data for this article are available on the
journal Web site (http://canjchem.nrc.ca) or may be pur-
chased from the Depository of Unpublished Data, Document
Delivery, CISTI, National Research Council Canada, Build-
ing M-55, 1200 Montreal Road, Ottawa, ON K1A 0R6,
Canada. DUD 3999. For more information on obtaining
material refer to http://cisti-icist.nrc-cnrc.gc.ca/eng/ibp/cisti/
collection/unpublished-data.html. CCDC numbers 739246–
739248 contain the crystallographic data for complexes 3,
7, and 9, respectively. These data can be obtained, free of
charge, via www.ccdc.cam.ac.uk/conts/retrieving.html (or
from the Cambridge Crystallographic Data Centre, 12 Union
Road, Cambridge CB2 1EZ, UK; fax +44 1223 336033; or
deposit@ccdc.cam.ac/uk).
Published by NRC Research Press

1254
Can. J. Chem. Vol. 87, 2009
(13) (a) Zhou, Z.; Facey, G.; James, B. R.; Alper, H. Organome-
tallics 1996, 15 (10), 2496–2503. doi:10.1021/om960112s.;
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Acknowledgment
We thank the Natural Sciences and Engineering Research
Council of Canada (NSERC) for financial support.
(14) Huang, J.; Stevens, E. D.; Nolan, S. P. Organometallics
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