Novel rhodium complexes containing a bulky iminophosphine ligand and their use as catalysts for the hydroboration of vinylarenes

Article abstract of DOI:10.1016/j.ica.2005.10.043

Addition of o-Ph₂PC₆H₄CH{double bond, long}N-2,6-ⁱPr₂C₆H₃ (1) to [RhCl(coe)₂]₂ (coe = cis-cycloctene) gave several new iminophosphino rhodium(I) complexes i

Full text of DOI:10.1016/j.ica.2005.10.043

Inorganica Chimica Acta 359 (2006) 2771–2779
www.elsevier.com/locate/ica
Novel rhodium complexes containing a bulky iminophosphine
ligand and their use as catalysts for the hydroboration of vinylarenes
a
a
a
b
Daniel I. McIsaac , Stephen J. Geier , Christopher M. Vogels , Andreas Decken ,
a,
*
Stephen A. Westcott
a
Department of Chemistry, Mount Allison University, Sackville, Canada NB E4L 1G8
Department of Chemistry, University of New Brunswick, Fredericton, Canada NB E3B 5A3
b
Received 1 October 2005; accepted 17 October 2005
Available online 5 December 2005
Dedicated to Dr. Brian James in recognition of his outstanding contributions to chemistry and catalysis.
Abstract
Addition of o-Ph2PC6H4CH@N-2,6-iPr2C6H3 (1) to [RhCl(coe)2]2 (coe = cis-cycloctene) gave several new iminophosphino rho-
dium(I) complexes including [Rh(j2-o-Ph2PC6H4CH@N-2,6-iPr2C6H3)(l-Cl)]2 (2). Addition of 1 to Rh(acac)(coe)2 (acac = acetylaceto-
nato) gave [Rh(acac)(j2-o-Ph2PC6H4CH@N-2,6-iPr2C6H3)] (3) in yields of up to 75%. Complex 3 has been examined for its ability to
catalyze the hydroboration of a series of vinyl arenes. Reactions using catecholborane and pinacolborane seem to proceed largely
through a dehydrogenative borylation mechanism to give a number of boronated products.
ꢀ 2005 Elsevier B.V. All rights reserved.
Keywords: Borylation; Catalysis; Hydroboration; Iminophosphine; Rhodium compounds
1. Introduction
tion) where the boryl (BR2) fragment adds preferentially to
the least hindered carbon of the unsymmetrically substi-
tuted double bond. While borane adds rapidly to alkenes
at ꢀ80 ꢁC [2], some hydroborating agents, such as polyhe-
dral boranes, are slow to react even at room temperature.
Likewise, addition of H3B Æ THF to catechol affords cate-
cholborane (HBcat, cat = 1,2-O2C6H4), a relatively stable
hydroborating agent which adds to alkenes and alkynes
only at elevated temperatures (ca. 100 and 70 ꢁC, respec-
tively) [3]. HBcat decomposes thermally or by addition of
nucleophiles to give a number of boron-containing prod-
ucts [4].
That transition metals accelerate the addition of B–H
bonds to unsaturated organic moieties was initially
reported for catalyzed hydroborations of alkenes and alky-
nes using polyhedral boranes [5]. Ma¨nnig and No¨th [6] then
demonstrated that rhodium complexes could be used to
catalyze the hydroboration of alkenes with HBcat under
mild conditions and with chemoselectivity differing from
that of the uncatalyzed reaction. Indeed, in the catalyzed
One of the most important synthetic methodologies to
emerge from organic chemistry in the last century has been
the discovery that boron–hydrogen bonds add to unsatu-
rated organic molecules to form a class of compounds
known as organoboranes [1]. Organoboranes possess an
incredibly diverse chemistry and are remarkably useful
intermediates in organic synthesis. Indeed, organoboranes
can be transformed into any number of functional groups.
The simplest boron hydride agent is borane, BH3, which
reacts rapidly with unhindered alkenes to afford initially
monoalkylboranes, then dialkylboranes, and finally trial-
kylboranes. For sterically hindered alkenes, the second
and third hydroboration steps become increasingly slug-
gish. Addition occurs in a controlled cis-fashion (syn-addi-
*
Corresponding author. Tel.: +1 506 364 2372; fax: +1 506 364 2313.
E-mail address: swestcott@mta.ca (S.A. Westcott).
0020-1693/$ - see front matter ꢀ 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2005.10.043

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D.I. McIsaac et al. / Inorganica Chimica Acta 359 (2006) 2771–2779
hydroboration of 5-hexen-2-one, addition of HBcat
occurred preferentially at the C@C double bond, even in
the presence of the more reactive ketone group. Since this
seminal discovery, a considerable amount of research has
focussed on investigating the mechanism and scope of cat-
alyzed hydroboration reactions [7]. Although other transi-
tion metals have been found to catalyze hydroborations
with HBcat, rhodium catalysts are usually the most effec-
tive for reactions of vinylarenes [8]. The judicious choice
of the rhodium complex can be used to effectively generate
either the expected linear or the unusual branched hydrob-
oration product (Scheme 1). Unfortunately, these reactions
can suffer from poor selectivities or competing pathways
(i.e., hydrogenation) and, as a result, a considerable
amount of research has focussed on designing new catalyst
systems [9]. We report herein on the synthesis and reactiv-
ity of rhodium(I) complexes containing a bulky iminophos-
phine ligand.
Ar), 3.62 (sept, 4H, J = 7 Hz, C(H)Me2), 1.40 (d, 12H,
J = 7 Hz, Me), 0.88 (d, 12H, J = 7 Hz, Me); 31P{1H} d:
47.1 (d, JRhP = 201 Hz), 46.1 (d, JRhP = 200 Hz).
2.2. Preparation of [Rh(acac)(j2-o-Ph2PC6H4CH@N-
2,6-iPr2C6H3)] (3)
A mixture of 1 (213 mg, 0.47 mmol) and Rh(acac)(coe)2
(200 mg, 0.47 mmol) in toluene (3 mL) was heated at reflux
for 18 h. Upon removal of solvent the mixture was washed
with hexane (2 · 1 mL) and dried under vacuum to afford a
dark purple solid (Yield 230 mg, 75%). Spectroscopic
1
NMR data (in C6D6): H d: 8.32 (d, 1 H, JRhH = 4 Hz,
N@CH), 7.88 (m, 4H, Ar), 7.40 (d of d, 1H, J = 9 Hz,
Ar), 7.25 (t, 1H, J = 9 Hz, Ar), 7.19 (d, 2H, J = 9 Hz,
Ar), 7.08 (m, 6H, Ar), 6.98 (m, 3H, Ar), 5.18 (s, 1H,
CHC(O)Me), 3.73 (sept, 2H, J = 7 Hz, C(H)Me2), 1.60
(s, 3H, Me(O)C), 1.53 (s, 3H, C(O)Me), 1.42 (d, 6H,
J = 7 Hz, Me), 0.98 (d, 6H, J = 7 Hz, Me); 31P{1H} d:
50.7 (d, JRhP = 206 Hz); 13C{1H} d: 184.2, 183.9, 158.8
(d, JRhC = 8 Hz, C(H)N), 151.4, 141.4, 139.2 (d,
JPC = 19 Hz), 135.6 (d, JPC = 45 Hz), 134.0 (d,
JPC = 13 Hz), 132.7, 132.3 (d, JPC = 9 Hz), 130.0, 129.3
(d, JPC = 6 Hz), 128.9, 127.6 (d, JPC = 9 Hz), 125.9,
122.4, 99.2, 28.0, 26.6, 26.5, 24.4, 23.6. Anal. Calc. for
C36H39NOPRh: C, 66.35; H, 6.04; N, 2.15. Found: C,
66.98; H, 6.47; N, 1.87%.
2. Experimental
Reagents and solvents used were purchased from
Aldrich Chemicals. NMR spectra were recorded on a
JEOL JNM-GSX270 FT spectrometer. 1H NMR chemical
shifts are reported in ppm and referenced to residual sol-
vent protons in deuterated solvent at 270 MHz. 11B
NMR chemical shifts are reported in ppm and are refer-
enced to BF3 Æ OEt2 as an external standard at 87 MHz.
13C NMR chemical shifts are referenced to solvent carbon
resonances as internal standards at 68 MHz and are
reported in ppm. 13P NMR chemical shifts are reported
in ppm and are referenced to H3PO4 as an external stan-
dard at 109 MHz. Multiplicities are reported as singlet
(s), doublet (d), septet (sept), multiplet (m), and overlap-
ping (ov). Microanalyses for C, H, and N were carried
out at Guelph Chemical Laboratories (Guelph, ON). All
reactions were carried out under an atmosphere of
dinitrogen.
2.3. Preparation of [Rh(catBcat)(j2-o-Ph2PC6H4CH@N-
2,6-iPr2C6H3)] (4)
A mixture of 3 (140 mg, 0.22 mmol) and B2cat3 (74 mg,
0.22 mmol) in Et2O (4 mL) was allowed to react for 18 h. A
purple solid was collected by suction filtration and dried
under vacuum (Yield 62 mg, 35%). Spectroscopic NMR
1
data (in CDCl3): H d: 7.81 (d, 1H, JRhH = 5 Hz, N@CH),
7.57–7.49 (ov m, 5H, Ar), 7.42 (m, 8H, Ar), 7.24–7.13 (ov
m, 4H, Ar), 5.24 (m, 4H, Ar), 6.62 (m, 4H, Ar), 3.14 (sept,
2H, J = 7 Hz, C(H)Me2), 1.34 (d, 6H, J = 7 Hz, Me), 0.73
(d, 6H, J = 7 Hz, Me); 11B{1H} d: 13.7 (s); 31P{1H} d: 50.6
(d, JRhP = 210 Hz). Anal. Calc. for C43H40BNO4PRh: C,
66.25; H, 5.18; N, 1.80. Found: C, 66.62; H, 5.35; N, 1.98%.
2.1. Preparation of [Rh(j2-o-Ph2PC6H4CH@N-
2,6-iPr2C6H3)(l-Cl)]2 (2)
A mixture of 1 (122 mg, 0.29 mmol) and [RhCl(coe)2]2
(100 mg, 0.14 mmol) in toluene (3 mL) was allowed to react
for 18 h. Upon removal of solvent the mixture was washed
with hexane (2 · 3 mL) and dried under vacuum to afford a
dark purple solid (Yield 180 mg, 55%). Selected NMR data
2.4. General procedure for the catalyzed hydroboration of
vinylarenes
The appropriate hydroboration reagent (1.2 equiva-
lents), in 0.5 mL of C6D6, was added to a 0.5 mL C6D6
solution of 3 and the vinylarene. Reactions were allowed
1
(in C6D6): H d: 8.10 (d, 2H, JRhH = 4 Hz, N@CH), 7.70
(m, 8H, Ar), 7.24–7.02 (ov m, 20H, Ar), 6.87 (m, 6H,
O
B
H
O
O
O
+
O
B
HB
O
5 mol %
RhCl(PPh )
H
5 mol %
[Rh(coe) Cl] /2PPh
3
3 3
2
2
HBcat
Scheme 1. The rhodium catalyzed hydroboration of styrene with HBcat.

D.I. McIsaac et al. / Inorganica Chimica Acta 359 (2006) 2771–2779
2773
to proceed for 18 h and then monitored by multinuclear
N
R
NMR spectroscopy.
N
N
N
tBu
PPh2
PPh2
2.5. X-ray crystallography
PPh2
Crystals of 1 were grown from saturated MeOH and
crystals of 2 and 3 from toluene solutions at ꢀ5 ꢁC. Single
crystals were coated with Paratone-N oil, mounted using a
glass fibre and frozen in the cold stream of the goniometer.
A hemisphere of data were collected on a Bruker AXS P4/
SMART 1000 diffractometer using x and h scans with a
scan width of 0.3ꢁ and 30 s exposure times. The detector
distances were 5 (1 and 2) and 6 cm (3). The data were
reduced [10a] and corrected for absorption [10b]. The
structures were solved by direct methods and refined by
full-matrix least squares on F2 [10c]. All non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were
located in Fourier difference maps and refined
isotropically.
tBu
c
b
a
AcO
AcO
CO2R
iPr
iPr
MeO
O
N
N
PPh2
PPh2
d
e
Fig. 1. P,N ligands used in the catalysed hydroboration of vinylarenes.
with respect to the diphenylphosphine group. These struc-
tures are in contrast to o-Ph2PC6H4CH@NC6H4OH-o
[15d] where the nitrogen is closer to the phosphorus atom
3. Results and discussion
3.1. Synthesis and structures
as the molecule assumes
N(20)@C(19) double bond of 1 is 1.267(2) A is well within
the range observed for these types of compounds.
a Z configuration. The
˚
There has been recent considerable interest in the syn-
thesis of late metal complexes containing P,N-bidentate
ligands as catalysts for the hydroboration of vinylarenes
[8,11–13]. For example, rhodium quinap (quinap =
1-(2-diphenylphosphino-1-napthyl)isoquinoline, Fig. 1(a))
complexes have been shown to catalyse the addition of
HBcat to 2,4-dimethylstyrene to selectively give the corre-
sponding branched isomer [13l]. Similar work with rho-
dium quinazolinap (Fig. 1(b)) [9a] and pyphos
(Fig. 1(c)) [9d] complexes has also been reported and
these catalysts show excellent selectivity in favour of the
branched products. However, a cationic rhodium system
using carbohydrate iminophosphine ligands (Fig. 1(d))
was reported in the catalysed hydroboration of 4-methyl-
styrene with HBcat to give equimolar amounts of the
branched and linear products [13i]. Addition of HBcat
to 2,3,4,5-pentafluorostyrene using this iminophosphine
catalyst system gave predominant formation of the corre-
sponding terminal product. In order to gain an under-
standing of the factors that influence selectivities in
these reactions we have prepared rhodium complexes con-
taining a bulky hemilabile ligand (Fig. 1(e)) and examined
their ability to catalyse the hydroboration of a wide range
of vinylarenes.
Addition of 1 to [RhCl(coe)2]2 (coe = cis-cyclooctene)
lead to the formation of several new iminophosphinorho-
dium complexes, as indicated by 31P{1H} NMR spectros-
copy and the observation of doublets with JP–Rh
=
200 Hz. While we were not able to isolate these products,
dimeric 2 has been characterized by a single crystal X-ray
diffraction study. The molecular structure of 2 is shown
in Fig. 3, crystallographic data in Table 1, and select bond
distances and angles in Table 3. The Rh–P bond distances
˚
˚
of Rh(1)–P(3) 2.1543(7) A and Rh(2)–P(4) 2.1507(7) A are
somewhat shorter than typical donor phosphine Rh–P
interactions in related 4-coordinate Rh(I) structures [16].
For instance, the average Rh–P interaction in [Rh(dippe)
(l-Cl)]2 (dippe = 1,2-bis(diisopropylphosphino)ethane) is
Ligand 1 [14] (Fig. 1(e)) was prepared as described pre-
viously and the molecular structure is shown in Fig. 2.
Crystallographic data is provided in Table 1. Bond dis-
tances and angles are within the range reported for other
iminophosphine ligands [15] and are provided in Table 2.
In comparison to the related compound o-
Ph2PC6H4CH@NC6H4OMe-o [15c], both iminophosphines
have a very similar arrangement with planar C(14)–
C(19)@N(20)–C(21) imino units with an E configuration
Fig. 2. A view of compound 1, with displacement ellipsoids drawn at the
30% probability level. H atoms have been omitted for clarity.

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D.I. McIsaac et al. / Inorganica Chimica Acta 359 (2006) 2771–2779
Table 1
Crystallographic data collection parameters for 1–3
Complex
Formula
Fw
1
2 Æ 2MePh
C76H80Cl2N2P2Rh2
1360.08
monoclinic
P21/c
23.4312(17)
12.9412(10)
22.3225(16)
90
93.039(1)
90
6759.3(9)
4
1.337
0.40 · 0.25 · 0.15
173(1)
Mo Ka (k = 0.71073)
0.658
3
C31H32NP
449.55
triclinic
C36H39NO2PRh
651.56
triclinic
Crystal system
Space group
ꢀ
ꢀ
P1
P1
˚
a (A)
9.8753(11)
11.8954(14)
12.6795(15)
66.576(2)
74.721(2)
74.208(2)
1294.6(3)
2
1.153
0.35 · 0.30 · 0.20
213(1)
Mo Ka (k = 0.71073)
0.125
11.8139(16)
12.2718(17)
14.306(2)
75.580(2)
67.240(2)
66.349(2)
1740.9(4)
2
1.243
0.30 · 0.15 · 0.15
173(1)
Mo Ka (k = 0.71073)
0.566
˚
b (A)
˚
c (A)
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
qcalc (mg mꢀ3
)
Crystal size (mm3)
Temperature (K)
Radiation
l (mmꢀ1
)
Total reflections
Total unique reflections
Number of variables
Rint
8400
5540
426
0.0175
46142
14985
811
0.0341
12725
12725
385
H range (ꢁ)
1.78–27.49
0.505/ꢀ0.168
1.102
0.0463
0.1356
0.87–27.50
1.057/ꢀ0.564
1.038
0.0346
0.0883
1.55–25.00
2.174/ꢀ0.633
1.073
0.0577
0.1945
ꢀ3
˚
Largest difference peak/hole (e A
)
S(GoF) on F2
a
R1 (I > 2r(I))
b
wR2 (all data)
P
P
a
R1 = ||Fo| ꢀ |Fc||/ |Fo|.
P
P
2
1=2
2
2
b
wR2 ¼ ð ½wðF 2o ꢀ F c2Þ ꢁ= ½wF 4oꢁÞ
,
where
w ¼ 1=½r2ðF 2oÞ þ ð0:0824PÞ þ ð0:0732PÞꢁ ð1Þ,
1=½r2ðF 2oÞ þ ð0:0426PÞ þ ð3:1194PÞꢁ ð2Þ
and
2
w ¼ 1=½r2ðF o2Þ þ ð0:134PÞ þ ð0:2957PÞꢁ ð3Þ, where P ¼ ðmaxðF o2; 0Þ þ 2F 2c Þ=3.
Table 2
˚
Selected bond lengths (A) and angles (ꢁ) for 1
P–C(7)
P–C(1)
P–C(13)
C(19)–N(20)
N(20)–C(21)
1.8260(17)
1.8297(16)
1.8363(16)
1.2668(19)
1.4280(19)
C(7)–P–C(1)
103.39(7)
C(7)–P–C(13)
C(1)–P–C(13)
103.07(7)
101.91(7)
120.78(13)
117.26(13)
121.46(14)
121.0(9)
C(26)–C(21)–N(20)
C(22)–C(21)–N(20)
N(20)–C(19)–C(14)
N(20)–C(19)–H(19)
C(19)–N(20)–C(21)
118.68(13)
˚
2.1821(13) A [16b]. Furthermore, the Rh–P bond distances
Fig. 3. A view of complex 2, with displacement ellipsoids drawn at the
30% probability level. H atoms and solvent molecules have been omitted
for clarity.
in related rhodium(I) cationic complexes containing 1
range from 2.1934(11) to 2.2361(6) A [17]. Rhodium phos-
phorus bond distances similar to 2 have been found, how-
˚
ever, in related diphosphite complexes [18a,18b] and in
[Rh(dfepe)(l-Cl)]2
(dfepe = (C2F5)2PCH2CH2P(C2F5)2)
planar environment. Interestingly, cationic diolefin rho-
dium(I) complexes containing 1 have been shown to
undergo conformational change of the iminophosphine
ligands in solution, whereby inverting the position of the
six-membered chelate ring relative to the N–Rh–P plane
[20]. It is possible that this process is responsible for gener-
ating isomers of 2 and making isolation problematic.
[18c], which contains electron-withdrawing fluoroalkyl-
phosphines. The Rh–N distances of Rh(1)–N(23)
˚
˚
1.996(2) A and Rh(2)–N(55) 2.005(2) A are similar to those
reported in other rhodium imine complexes [19]. The
Rh2(l-Cl)2 core is strictly planar with a Rh–Rh distance
˚
of 3.4806(3) A and the metals occupying a distorted square

D.I. McIsaac et al. / Inorganica Chimica Acta 359 (2006) 2771–2779
2775
Table 3
Selected bond lengths (A) and angles (ꢁ) for 2
˚
Rh(1)–N(23)
Rh(1)–P(3)
Rh(1)–Cl(2)
Rh(1)–Cl(1)
1.996(2)
2.1543(7)
2.3469(7)
2.4325(7)
2.005(2)
N(23)–Rh(1)–P(3)
N(23)–Rh(1)–Cl(2)
P(3)–Rh(1)–Cl(2)
N(23)–Rh(1)–Cl(1)
P(3)–Rh(1)–Cl(1)
Cl(2)–Rh(1)–Cl(1)
N(55)–Rh(2)–P(4)
N(55)–Rh(2)–Cl(1)
P(4)–Rh(2)–Cl(1)
N(55)–Rh(2)–Cl(2)
P(4)–Rh(2)–Cl(2)
90.02(6)
175.79(6)
93.35(2)
94.23(6)
175.37(2)
82.48(2)
89.72(6)
175.76(6)
92.78(3)
95.62(6)
174.66(3)
Rh(2)–N(55)
Rh(2)–P(4)
Rh(2)–Cl(1)
Rh(2)–Cl(2)
C(22)–N(23)
N(23)–C(24)
C(54)–N(55)
C(54)–H(54)
2.1507(7)
2.3490(7)
2.4582(7)
1.301(3)
1.467(3)
1.300(3)
0.9500
N(55)–C(56)
1.454(3)
Cl(1)–Rh(2)–Cl(2)
Rh(2)–Cl(1)–Rh(1)
Rh(1)–Cl(2)–Rh(2)
C(16)–P(3)–C(4)
C(16)–P(3)–C(10)
C(4)–P(3)–C(10)
C(16)–P(3)–Rh(1)
C(4)–P(3)–Rh(1)
C(10)–P(3)–Rh(1)
81.89(2)
93.41(2)
92.80(2)
102.85(12)
102.51(12)
102.79(12)
113.22(8)
118.78(9)
114.70(8)
C(48)–P(4)–C(42)
C(48)–P(4)–C(36)
C(42)–P(4)–C(36)
C(48)–P(4)–Rh(2)
C(42)–P(4)–Rh(2)
C(36)–P(4)–Rh(2)
C(22)–N(23)–Rh(1)
C(24)–N(23)–Rh(1)
102.50(13)
103.19(13)
102.90(13)
113.85(9)
120.75(9)
111.67(9)
131.69(17)
116.08(15)
Fig. 4. A view of complex 3, with displacement ellipsoids drawn at the
30% probability level. H atoms have been omitted for clarity.
Table 4
˚
Selected bond lengths (A) and angles (ꢁ) for 3
As mentioned previously, rhodium phosphine com-
plexes are versatile and efficient catalysts for the hydrobo-
ration of alkenes using catecholborane (HBcat). Among
this group, complexes of the type Rh(acac)(P2)
(acac = acetylacetonato; P2 = diphosphine) are unique in
that they are the only catalyst precursors to have been used
in the hydroboration of a tetrasubstituted alkene [7d].
These complexes are also active and selective catalysts for
the hydroboration of a wide range of vinylarenes using
HBcat, giving predominantly the corresponding branched
isomers. In this study, we have prepared the analogous rho-
dium acetylacetonato complex bearing the bulky imino-
phosphine ligand 1.
Rh–N(1)
Rh–O(37)
Rh–O(33)
Rh–P
P–C(4)
P–C(15)
P–C(9)
2.015(3)
2.018(3)
2.077(3)
2.1692(11)
1.812(4)
1.823(4)
1.836(5)
1.287(5)
1.486(5)
N(1)–C(2)
N(1)–C(21)
N(1)–Rh–O(37)
N(1)–Rh–O(33)
O(37)–Rh–O(33)
N(1)–Rh–P
O(37)–Rh–P
O(33)–Rh–P
C(4)–P–C(15)
C(4)–P–C(9)
C(15)–P–C(9)
C(4)–P–Rh
C(15)–P–Rh
C(9)–P–Rh
178.37(14)
89.96(13)
89.57(12)
88.07(10)
92.43(9)
177.66(9)
105.03(18)
101.90(19)
103.06(19)
111.08(14)
118.81(14)
115.14(13)
115.0(3)
Addition of 1 to Rh(acac)(coe)2 [21] gave the corre-
sponding
complex
[Rh(acac)(j2-o-Ph2PC6H4CH@N-
2,6-iPr2C6H3)] (3) in yields of up to 75%. The 31P{1H}
NMR spectra showed a doublet with coupling to rhodium
with JP–Rh = 206 Hz. Complex 3 has also been character-
ized by a single crystal X-ray diffraction study (Fig. 4).
The rhodium atom lies in a distorted square planar geom-
etry. While the rhodium oxygen distance of Rh–O(33) of
C(2)–N(1)–C(21)
C(2)–N(1)–Rh
C(21)–N(1)–Rh
N(1)–C(2)–C(3)
132.1(3)
112.6(3)
127.0(4)
˚
2.077(3) A is slightly longer than that found in other rho-
dium acetylacetonato complexes [22], the Rh–O(37) bond
˚
length of 2.018(3) A is somewhat shorter. This result is pre-
those observed for the acetylacetonato precursors. In an
elegant study, Marder and co-workers [23c] found that
addition of B2cat3 to Rh(acac)(P2) led to the zwitterionic
complexes Rh(g6-catBcat)(P2) in high yields, along with
concomitant formation of acacBcat. In this study we have
found that addition of B2cat3 to 3 gave the zwitterionic
compound [Rh(catBcat)(j2-o-Ph2PC6H4CH@N-2,6-iPr2-
C6H3)] (4). The 31P{1H} NMR spectrum shows a doublet
with coupling to rhodium at JP–Rh = 210 Hz and the
11B{1H} NMR has a sharp peak at 14 ppm, consistent with
the boron atom being four coordinate [23]. All attempts to
grow single crystals of 4 suitable for X-ray diffraction stud-
sumably due to the steric crowding caused by the bulky
imine group, leading to an asymmetry of the steric demand
of the iminophosphine ligand. Selected bond distances and
angles are provided in Table 4.
Although phosphinorhodium acetylacetonato com-
plexes are active and selective catalysts for the hydrobora-
tion of a wide range of alkenes, the catalyst resting state in
these systems is believed to be the zwitterionic complexes
Rh(g6-catBcat)(P2), arising from the redistribution of sub-
stituents on HBcat [7d,23]. Indeed, hydroborations using
these zwitterionic complexes give similar selectivities to

2776
D.I. McIsaac et al. / Inorganica Chimica Acta 359 (2006) 2771–2779
ies proved unsuccessful. Compound 4 rapidly decomposed
in chloroform or methylene chloride and as a result, we
were unable to obtain useful 13C{1H} NMR data. This
result is not surprising as rhodium(I) complexes are well
know to activate the C–Cl bond in these solvents [24].
Complexes 2 and 3 also decomposed in chlorinated
solvents.
In order to compare these new complexes against other
rhodium catalysts, we have examined 3 for its ability to cat-
alyze the hydroboration of a series of vinylarenes. Catalytic
studies were not done with the chloride dimer due to diffi-
culties in isolating one discrete complex. Initial studies with
3 were done on 4-vinylanisole using HBcat in C6D6 and
this reaction (Scheme 3), surprisingly, gave a mixture of
products including the linear hydroboration product 4-
1
3.2. Catalysed hydroborations
MeOC6H4CH2CH2Bcat (25% by H NMR spectroscopy),
the alkenyl boronate ester trans-4-MeOC6H4CH=CHBcat
(25%) and the diborated product 4-MeOC6H4CH2CH(B-
cat)2 (50%). Interestingly, unlike reactions using other
phosphinorhodium acetylacetonato complexes, the
branched product 4-MeOC6H4CH(Bcat)CH3 was not
observed to any significant degree. The diborated product
presumably arises from hydroboration of the alkenyl bor-
onate ester, which is generated from a dehydrogenative
borylation reaction [25]. Dehydrogenative borylations are
generally believed to occur via initial coordination of the
alkene to the metal centre followed by insertion into the
Rh–B, and not the Rh–H bond, with subsequent b-H elim-
ination to give the corresponding alkenyl boronate ester
and an active rhodium dihydride species. Although the lin-
ear product could be generated from a traditional catalyzed
The mechanism of the catalyzed hydroboration reaction
has been the subject of considerable debate [11]. For reac-
tions using commonly employed RhCl(PPh3)3, one plausi-
ble mechanism has been postulated to occur via initial loss
of phosphine to give ꢀRhCl(PPh3)2ꢁ, which in turn oxida-
tively adds the borane to the metal centre to give an unsat-
urated hydrido boryl intermediate. This is followed by
coordination of the alkene and insertion into either the
Rh–H or Rh–B bond [6] with subsequent reductive elimi-
nation to afford the corresponding organoboronate ester
product (Scheme 2). The unusual regioselectivity observed
in hydroborations of vinylarenes is believed to arise when
the rhodium centre can best stabilize a benzylic intermedi-
ate [10].
RhCl(PPh3)3
RCH2CH2(Bcat)
CH(Bcat)RCH3
-PPh3
HBcat
'RhCl(PPh3)2'
PPh3
PPh3
PPh3
catB
catB
catB
Rh
Cl
Rh
Cl
Rh
Cl
H3CRHC
RCH2CH2
H
PPh3
PPh3
PPh3
PPh3
catB
H
R
Rh
R
Cl
PPh3
Scheme 2. A plausible mechanism for the catalysed hydroboration of alkenes using RhCl(PPh3)3.
O
O
Bcat
Bcat
Bcat
5 mol% 3
+
BH
MeO
-H2
MeO
MeO
HBcat
H2
Bcat
MeO
Scheme 3. Catalyzed addition of 4-vinylanisole using HBcat and 3.

D.I. McIsaac et al. / Inorganica Chimica Acta 359 (2006) 2771–2779
2777
O
Bpin
5 mol% 3
+
BH
MeO
O
MeO
HBpin
Scheme 4. Catalyzed addition of 4-vinylanisole using HBpin and 3.
F
F
F
Bpin
F
F
Bpin
F
F
F
F
F
F
F
F
F
F
50%
5%
5 mol% 3
1. 5 HBpin
F
F
F
F
F
Bpin
Bpin
F
F
F
F
F
20%
25%
Scheme 5. Catalyzed addition of 2,3,4,5,6-C6H5CH=CH2 using HBpin and 3.
hydroboration reaction, it is also plausible that this prod-
uct arises from the catalyzed hydrogenation of alkenyl bor-
onate ester via the aforementioned rhodium dihydride
intermediate.
dent and further work is therefore needed to understand
the factors controlling product selectivities.
4. Conclusions
Reactions of 4-vinylanisole using pinacolborane
(HBpin, pin = 1,2-O2C2Me4) and 3 gave exclusive forma-
tion of the corresponding linear ꢀhydroboration productꢁ
4-MeOC6H4CH2CH2Bpin after 24 hours (Scheme 4). How-
We have found that addition of o-Ph2PC6H4CH@
N-2,6-iPr2C6H3 (1) to [RhCl(coe)2]2 (coe = cis-cycloctene)
gave several new iminophosphino rhodium(I) complexes
including [Rh(j2-o-Ph2PC6H4CH@N-2,6-iPr2C6H3)(l-Cl)]2
(2). Addition of 1 to Rh(acac)(coe)2 (acac = acetylacetonato)
gave [Rh(acac)(j2-o-Ph2PC6H4CH@N-2,6-iPr2C6H3)] (3),
which could be converted to the zwitterionic [Rh(catB-
cat)(j2-o-Ph2PC6H4CH@N-2,6-iPr2C6H3)] (4) by addition
of B2cat3. Complex 3 has been examined for its ability to
catalyze the hydroboration of a series of vinyl arenes.
Reactions using catecholborane and pinacolborane seem
to proceed largely through a dehydrogenative borylation
mechanism to give a number of boronated products.
Future work will investigate other iminophosphine rho-
dium complexes for their ability to catalyze the hydrobora-
tion of vinylarenes and to gain insight into the steric and
electronic requirements needed for the metal to facilitate
these unusual dehydrogenative borylation reactions.
1
ever, monitoring this addition by H NMR spectroscopy
showed the formation of a transient alkenyl boronate ester
4-MeOC6H4CH@CHBpin at the beginning of the reaction.
This observation suggests that reactions using bulky 3 are
occurring predominantly through a competing dehydroge-
native borylation reaction. Similar chemistry was observed
using both HBpin and HBcat for reactions with 2,4,6-trim-
ethylstyrene, a-methylstyrene and b-methylstyrene. Reac-
tions using preformed zwitterionic 4 also gave similar
regioselectivities with both HBcat and HBpin.
Reactions with 4-fluorostyrene, 2,3,4,5,6-perfluorosty-
rene, and 2-naphthylstyrene and HBcat all gave compli-
cated product distributions, including varying amounts of
the corresponding branched hydroboration products. For
instance, reactions of highly activated 2,3,4,5,6-perfluoro-
styrene with HBcat and 3 gave up to 50% of 2,3,4,5,6-
C6F5CH(Bcat)CH3. Interestingly, reactions of 2,3,4,5,6-
perfluorostyrene with HBpin and 3 also gave several prod-
ucts, including the linear hydroboration product, diborated
and hydrogenation products and even the branched prod-
uct 2,3,4,5,6-C6F5CH(Bpin)CH3 (50%) (Scheme 5). A
recent report [26] describes the formation of similar
branched products from hydroborations of vinylarenes
using cyclooctadiene rhodium(I) catalysts and HBpin.
Reactions with 3, however, appear to be substrate depen-
5. Supplementary data
Crystallographic information has also been deposited
with the Cambridge Crystallographic Data Centre (CCDC
Nos. 285186–285188). Copies of the data can be obtained
free of charge via www.ccdc.cam.ac.uk/conts/retriev-
ing.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).

2778
D.I. McIsaac et al. / Inorganica Chimica Acta 359 (2006) 2771–2779
(d) F.Y. Kwong, Q. Yang, T.C.W. Mak, A.S.C. Chan, K.S. Chan, J.
Acknowledgements
Org. Chem. 67 (2002) 2769;
(e) T.A. Betley, J.C. Peters, Angew. Chem., Int. Ed. Engl. 42 (2003)
2385;
(f) C. Borriello, M.E. Cucciolito, A. Panunzi, F. Ruffo, Inorg. Chim.
Acta 353 (2003) 238;
Thanks are gratefully extended to the Canada Research
Chairs Programme, Canadian Foundation for Innovation-
Atlantic Innovation Fund, Natural Science and Engineer-
ing Research Council of Canada, and Mount Allison Uni-
versity for financial support. We also thank ARMAC
(Halifax) and Roger Smith and Dan Durant for expert
technical assistance.
`
(g) G. Chelucci, G. Orru, G.A. Pinna, Tetrahedron 59 (2003) 9471;
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