2,2′-Bis((di-tert-butylphosphino)methyl)-1,1′-biphenyl (ditbi): A bulky analogue of bisbi. The crystal structure of [Rh2Cl2(1,5-cod)2(μ-ditbi)]

Article abstract of DOI:10.1016/j.jorganchem.2004.06.029

Diphosphine 2,2′-bis(di-tert-butylphosphino)methyl)-1,1′ -biphenyl (ditbi) is synthesised by the addition of Bu₂^tP(BH₃)Li to 2,2′-bis(bromomethyl)-1,1′-biphenyl, followed by deprotection with diethylamine. Treatment of [Rh₂ Cl₂(1,5-cod)z], with ditbi gives [Rh₂ Cl₂(1,5-cod)₂(μ-ditbi)] (2) as confirmed by its X-ray crystal structure determination. Hydroformylation of 1-hexene using [Rh(acac)(CO)₂]/ditbi as catalyst gave n- and iso-heptanal in a ratio of 1:1.

Full text of DOI:10.1016/j.jorganchem.2004.06.029

Journal of Organometallic Chemistry 689 (2004) 2975–2978
www.elsevier.com/locate/jorganchem
2,20-Bis((di-tert-butylphosphino)methyl)-1,10-biphenyl (ditbi):
a bulky analogue of bisbi. The crystal structure of
[Rh2Cl2(1,5-cod)2(l-ditbi)]
Lee J. Higham ,1, Katie Heslop, Paul G. Pringle , A. Guy Orpen
*
*
School of Chemistry, University of Bristol, Bristol BS8 1TS, UK
Received 18 June 2004; accepted 21 June 2004
Available online 30 July 2004
Abstract
Diphosphine 2,20-bis(di-tert-butylphosphino)methyl)-1,10-biphenyl (ditbi) is synthesised by the addition of Bu2t PðBH3ÞLi to 2,20-
bis(bromomethyl)-1,10-biphenyl, followed by deprotection with diethylamine. Treatment of [Rh2Cl2(1,5-cod)2], with ditbi gives
[Rh2Cl2(1,5-cod)2(l-ditbi)] (2) as confirmed by its X-ray crystal structure determination. Hydroformylation of 1-hexene using
[Rh(acac)(CO)2]/ditbi as catalyst gave n- and iso-heptanal in a ratio of 1:1.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Diphosphine; Hydroformylation; Rhodium; Synthesis and characterisation
1. Introduction
2. Results and discussion
The diphosphine bisbi ((2,20-bis(diphenylphosphi-
no)methyl)-1,10-biphenyl) [1] has attracted much atten-
tion because it is an outstanding ligand for
hydroformylation catalysis: its rhodium complexes are
highly active under mild conditions and n:i product ra-
tios of 66:1 have been achieved [2]. The flexibility of
the bisbi backbone enables this ligand to span angles
at the metal ranging from 90–120ꢁ and it is this property
that may be at the root of its success [3]. We are interested
in other ligands that contain the bisbi backbone and
here we report the bulky diphosphine ditbi (1).
The synthesis of ditbi was carried out according to
Scheme 1. Difficulties were encountered in the final bo-
rane deprotection step including the regeneration of
Ph2PH and the formation of many by-products, as evi-
denced by the complicated 31P NMR spectra. Several
amines (morpholine, diazabicyclo[2.2.2]octane (DAB-
CO), triethylamine and diethylamine) were investigated
as deprotecting reagents and the reactions were carried
out at different temperatures, concentrations of amine
and reaction times; the most convenient procedure used
diethylamine under the conditions given in the Section
4.3.
Reaction of ditbi with one equivalent of [Rh2Cl2(1,5-
cod)2] in CH2Cl2 yielded an orange solution which 31P
NMR spectroscopy showed contained a single P-con-
taining species with a chemical shift of 40.6 ppm
(1JRh–P =137.7 Hz). Orange crystals that deposited from
this solution were suitable for X-ray crystallography.
The crystal structure (see Fig. 1 and Tables 1 and 2)
*
Corresponding authors.
E-mail address: lee.higham@ucd.ie (L.J. Higham).
1
Present address: Chemistry, University College Dublin, Belfield,
Dublin, Ireland. Tel.: +35-317-162-316; fax: +35-317-162-308.
0022-328X/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2004.06.029

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L.J. Higham et al. / Journal of Organometallic Chemistry 689 (2004) 2975–2978
BH3
PBut2
Br
PBut2
PBut2
BH3
t
excess Et2NH
+
2
Bu 2P
THF
Li
Br
PBut2
BH3
1
0.5 equiv.
[Rh2Cl2(cod)2]
1 equiv. [Rh2Cl2(cod)2]
Cl
Cl
But2
Rh
0.5 equiv.
[Rh2Cl2(cod)2]
Rh
But2
P
P
PBut2
But2
P
Rh
Cl
2
3
Scheme 1. Synthesis of ditbi (1) and its reactions with [Rh2Cl2(cod)2].
Table 2
Crystal and refinement data for [Rh2Cl2(1,5-cod)2(l-ditbi)] (2)ÆCHCl3
Crystal
[Rh2Cl2(1,5-cod)
(l-ditbi)] (2)ÆCHCl3
Colour, habit
Size (mm)
Formula
Orange block
0.4·0.35·0.2
C47H73Cl5P2Rh2
1083.06
Formula weight
Crystal system
Space group (No.)
Monoclinic
P21/c(14)
12.0851(14)
20.188(2)
23.201(3)
95.553(2)
5633.8(11)
4
˚
a (A)
˚
b (A)
˚
c (A)
b (ꢁ)
Fig. 1. Crystal structure of [Rh2Cl2(1,5-cod)2(l-ditbi)] (2).
3
˚
V (A )
Z
l (mmꢀ1
)
0.907
27474
Reflections collected
Unique data
Rint
Table 1
˚
8832
0.0662
Bond distances (A) and angles (ꢁ) for [Rh2Cl2(1,5-cod)2(l-ditbi)] (2)
˚
Bond lengths (A)
Angles (ꢁ)
Final R1[I>2r(I)]
Max, min difference
˚
map features (eA
0.0814
0.78, ꢀ0.98
Rh(1)–P(1)
Rh(2)–P(2)
Rh(1)–Cl(1)
Rh(2)–Cl(2)
P(1)–C(13)
P(2)–C(14)
C(1)–C(2)
2.390(3)
2.407(3)
2.381(3)
2.370(3)
1.860(10)
1.853(11)
1.399(16)
1.407(15)
1.398(15)
1.475(19)
Cl(1)–Rh(1)–P(1)
Cl(2)–Rh(2)–P(2)
C(35)–Rh(1)–C(40)
C(13)–P(1)–C(31)
C(14)–P(2)–C(26)
C(7)–C(13)–P(1)
C(28)–C(31)–C(30)
89.80(10)
88.40(11)
81.3 (5)
ꢀ3
)
102.3(5)
102.5(5)
121.6(7)
110.7(10)
confirmed the binuclear structure of 2. Binuclear
complexes have previously been reported when 1,n-
bis(diphenylphosphino)alkanes (n=3–6) reacted with
[Rh2Cl2-(diolefin)2] [4].
A space-filling model of 2 (see Fig. 2) shows the twisted
bisbi backbone and the crowded nature of the structure.
C(2)–C(8)
C(34)–C(35)
C(38)–C(44)

L.J. Higham et al. / Journal of Organometallic Chemistry 689 (2004) 2975–2978
2977
Aldrich and purged with nitrogen prior to use. C6D6 and
CDCl3 were degassed with nitrogen and stored over mo-
lecular sieves. [Rh2Cl2(1,5-cod)2] was synthesised ac-
cording to a literature procedure [6]. The NMR
spectra were recorded at 23 ꢁC on a Jeol ecp300 spec-
trometer with chemical shifts relative to tetramethylsi-
1
lane for H and 13C spectra and 85% H3PO4 for 31P
spectra.
4.2. Synthesis of But2PH ꢁ BH3
To But2PH (5.00 g, 34.0 mmol) in THF (50 cm3) at
room temperature, was slowly added BH3 ÆTHF (40
cm3 of a 1 M solution in THF, 40 mmol). The reaction
mixture was stirred for 1 h and then the solvent and oth-
er volatiles were removed under reduced pressure. The
residue was dissolved in n-pentane (40 cm3) and the so-
lution was set-aside at 0 ꢁC for 7 days to give the crys-
talline product (5.03 g, 92%). 31P NMR (CDCl3): 48.3
Fig. 2. Space-filling model of [Rh2Cl2(1,5-cod)2(l-ditbi)] (2).
The bulk of the PBut2 groups may explain why ditbi
seems reluctant to cis-chelate; addition of ditbi to 0.5
equivalents of [Rh2Cl2(1,5-cod)2] gave a species charac-
terised in solution as the mononuclear ditbi complex 3
from the 31P NMR spectrum. The coordinated phos-
phine appears as a doublet at d 38.6 ppm (1JRh–P
145.7) and the non-coordinated phosphine at d 28.2
ppm. Treatment of solutions of 3 with 0.5 equivalents
of [Rh2Cl2(1,5-cod)2] gave 2.
1
(q, JB–P =53.0 Hz) ppm.
=
4.3. Synthesis of 2,20-bis((di-tert-butylphosphino)me-
thyl)-1,10-biphenyl (ditbi) (1)
We compared ditbi and bisbi in rhodium-catalysed
hydroformylation of hex-1-ene. Under the reaction con-
ditions employed (9.0 bar H2/CO, 36 ꢁC, 11 h) the alk-
ene had completely reacted but the n:iso ratio of
product aldehydes was found to be ca. 1:1 with ditbi.
Others have shown [2] that using bisbi under similar
conditions led to an n:i ratio of greater than 66:1
1.25 g (7.80 mmol) of But2PH ꢁ BH3 was dissolved in
THF (20 cm3) and cooled to 0 ꢁC. To this was added
n-BuLi (4.9 cm3, 1.6 M solution in hexanes, 7.8 mmol).
The resulting pale yellow solution was allowed to warm
to room temperature and stirred for 1 h. The solution
was then cooled to 0 ꢁC, and bis(bromomethyl)-1,10-bi-
phenyl (1.33 g, 3.90 mmol) was added as a solid, generat-
ing a yellow solution which became colourless after 5
min. The ice-bath was removed and the reaction mixture
allowed to warm to room temperature. After 2 h, the
mixture was heated to 35 ꢁC and then a large excess of
diethylamine (20 cm3, 14.14 g, 0.20 mol) was added by
syringe to give a suspension which was then stirred for
a further 60 h. The volatiles were then removed and
CH2Cl2 (50 cm3) added. The solution was filtered from
the ammonium salt and then the solvent removed to yield
an oil. Methanol (30 cm3) was added to this oil to give a
colourless solution which was filtered from a small quan-
tity of an oily solid and then cooled to 0 ꢁC overnight.
The resultant white solid product was filtered off and
dried (0.66 grams, 51.3%). EI mass spectrum: m/z 470
(M+). Elemental composition to within 1.7 ppm. 1H
NMR (CDCl3): 7.69 (m, 2H, ArH), 7.0–7.3 (m, 6H,
3. Conclusion
The bulky diphosphine ditbi has been made and
shown to bridge in a dirhodium(I) complex. The great
steric bulk of ditbi perhaps inhibits the formation of
cis-chelates which may explain the low n:i ratio in the
hydroformylation of 1-hexene.
4. Experimental
4.1. General methods
All operations were carried out under a N2 atmos-
phere using standard Schlenk-line techniques. Toluene,
CH2Cl2 and THF were dried using the Grubbs drying
system [5] and purged with nitrogen; CH3OH was ref-
luxed and distilled under nitrogen from Mg/I2. Et3N
and Et2NH were dried and distilled from P2O5 and
KOH respectively. Anhydrous n-pentane, 2,20-
bis(bromomethyl)-1,10-biphenyl, morpholine, diazabicy-
clo[2.2.2]octane (DABCO) and [Rh(acac)(CO)2] were all
purchased from Aldrich. 1-Hexene was purchased from
3
ArH), 2.59 (m, 4H, CH H b), 1.03 (d, 18H, JP–H =11.0
3
Hz, CH3), 0.77 (d, 18H, JP–H =10.7 Hz, CH3) ppm.
1
13C NMR (CDCl3): 25.6 (d, JP–C =22.5 Hz, C H2),
1
29.7 (m, 13.3 Hz, CH3), 31.4 (d, JP–C =20.7 Hz, C
1
Me3), 32.0 (d, JP–C =23.0 Hz, C Me3), 125.3–130.4 (m,
1
CH, aromatic) 139.7 (d, JP–C =12.1 Hz, o C 2), 141.3
(br, ipso C 1) ppm. 31P NMR: 31.6 (s) ppm. IR (solid,
cmꢀ1): m(aryl-H) 3059 (w), 756 (s), 737 (s), m(C‚C)
1597 (m), m(C–H) 1469 (m), m(aryl-P) 1432 (m).

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L.J. Higham et al. / Journal of Organometallic Chemistry 689 (2004) 2975–2978
4.4. Synthesis of [Rh2Cl2(1,5-cod)2(l-ditbi)] (2)
difference map. Attempts to model this disorder pro-
duced unsatisfactory results and an improved model
was obtained by applying a diffuse solvent correction
to the data using the Squeeze routine in the Platon suite
of software [7].
To a solution of ditbi (0.19 g, 0.4 mmol) in CH2Cl2
(10 cm3) was added [Rh2Cl2(1,5-cod)2] (0.16 g, 0.4
mmol). The resulting deep-red solution was stirred for
1 h and then the volatiles were removed. The residue
was dissolved in CDCl3 (2 cm3) and after 2 weeks red
1
crystals of 2 had formed. 31P NMR: 40.6 (d, JP–Rh
=
5. Supplementary material
137.7 Hz) ppm. FAB mass spectrum: m/e 987 ([MNa]+).
Crystallographic data for the structural analysis has
been deposited with the Cambridge Crystallographic
Data Centre, CCDC No. 241328 for 2. Copies of this
information may be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2
1EZ, UK; fax: +44(1223)336033 or e-mail: deposit@
ccdc.cam.ac.uk or www.ccdc.cam.ac.uk/conts/retriev-
ing.html.
4.5. Catalytic hydroformylation of 1-hexene
The diphosphine ditbi (9.0 mg, 0.019 mmol) was add-
ed to a solution of [Rh(acac)(CO)2] (5.0 mg, 0.019
mmol) in anhydrous C6D6 (3 cm3) and the emerald
green solution became orange. The solution was trans-
ferred under nitrogen to the autoclave which was then
evacuated and refilled 3 times with a 1:1 mixture of H2
and CO. The reaction mixture was stirred for 1 h at
36 ꢁC and 9.0 bar of syngas. 1-Hexene (1.2 cm3, 9.6
mmol) was injected into the autoclave and the reaction
monitored by syngas uptake. After 11 h the uptake
was complete but the conditions were maintained for a
further 5 h. The pressure was released slowly and the
autoclave purged with nitrogen. The ratio of line-
ar:branched products was calculated by integrating the
1H NMR signals for the aldehyde protons at 9.22 (n)
and 9.15 (i) in C6D6.
Acknowledgement
We would like to thank BP Chemicals for financial
support, the Leverhulme Trust for a Research Fellow-
ship (to PGP) and Johnson Matthey for the loan of
rhodium compounds.
References
4.6. X-ray crystal structure of [Rh2Cl2(1,5-cod)2(l-
ditbi)] (2)
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X-ray diffraction experiments on [Rh2Cl2(1,5-
cod)2(l-ditbi)] (2) as a chloroform solvate were carried
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(b) P.C.J. Kamer, P.W.N.M. Van Leeuwen, J.N.H. Reek, Acc.
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