SPANphos: Trans-spanning diphosphines as cis chelating ligands!

Article abstract of DOI:10.1039/b513870c

Several SPANphos ligands based on a spirobichroman backbone, introduced as a putative trans ligand, form compounds of the type [Rh(nbd)(SPANphos)]BF ₄ (1-6) in which both norbornadiene and SPANphos act as cis chelating ligands. The cyclooctadiene rhodium chloride derivatives form bimetallic complexes. Crystal structures for several of these compounds and free ligands are reported. Semiemperical AM1 and DFT calculations show that spirobichroman can assume several conformations, some of which are suitable for the formation of cis chelating SPANphos. All calculations on SPANphos complexes of PdCl ₂, PtCl₂ and Rh(CO)Cl show that the trans complex is more stable by 4-10 kcal mol^-1. The cis conformation in 1-6 is enforced by the cis chelating norbornadiene ligand. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b513870c

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PAPER
www.rsc.org/dalton | Dalton Transactions
SPANphos: trans-spanning diphosphines as cis chelating ligands!
Cristina Jime´nez-Rodr´ıguez, Francesc X. Roca, Carles Bo, Jordi Benet-Buchholz,
Eduardo C. Escudero-Ada´n, Zoraida Freixa and Piet W. N. M. van Leeuwen*
Received 29th September 2005, Accepted 31st October 2005
First published as an Advance Article on the web 18th November 2005
DOI: 10.1039/b513870c
Several SPANphos ligands based on a spirobichroman backbone, introduced as a putative trans
ligand, form compounds of the type [Rh(nbd)(SPANphos)]BF4 (1–6) in which both norbornadiene
and SPANphos act as cis chelating ligands. The cyclooctadiene rhodium chloride derivatives form
bimetallic complexes. Crystal structures for several of these compounds and free ligands are reported.
Semiemperical AM1 and DFT calculations show that spirobichroman can assume several
conformations, some of which are suitable for the formation of cis chelating SPANphos. All calculations
on SPANphos complexes of PdCl2, PtCl2 and Rh(CO)Cl show that the trans complex is more stable by
4–10 kcal mol−1. The cis conformation in 1–6 is enforced by the cis chelating norbornadiene ligand.
[Pd(C6H4-3-CN)(P(o-tolyl)3)Br]2, and [Rh(CO)2Cl]2 the corres-
Introduction
ponding trans complexes [PtCl2(SPANphos)], [PdCl2(SPAN-
phos)], [PdClMe(SPANphos)], [Pd(4-CNC6H4)Br(SPANphos)],
and [Rh(CO)Cl(SPANphos)] were isolated as the only product,
as confirmed by 1H and 31P NMR spectroscopy, and X-ray
diffraction.12 In many instances monodentate phosphines form
both cis and trans diphosphine complexes as the free energies
may be very similar.13,14 Stronger phosphorus donors and polar
solvent are more likely to yield cis complexes. If a carbon r-donor
is present in the complex, as in some of the examples above,
this will stabilize trans complexes relative to cis complexes, and
thus it is no surprise that trans complexes form. Isomerization
can be very slow as shown recently by Pringle and co-workers
for BISBI (substituted 2,2ꢀ-bis(phosphinomethyl)-1,1ꢀ-biphenyl)
complexes of PtCl2, which gave the cis and, surprisingly, also the
trans complex depending on the precursor platinum complex,15
while SPANphos gave trans complexes only, even when the cis
precursors [PtCl2(cod)] and [PdCl2(cod)] were used.
The reactivity of the trans complexes gave further information
about the trans preference of SPANphos. Oxidative addition
of MeI to trans-[Rh(CO)Cl(SPANphos)] does not take place,16
because one of the coordination sites is blocked by the backbone
and apparently isomerization to a cis diphosphine complex does
not occur to relieve this. Furthermore, the absence of any activity
in palladium catalysis of ethene and carbon monoxide seemed to
support our idea that cis complexes were not accessible.17
With excess of metal, bimetallic complexes were isolated. The
reaction of [Rh(CO)2Cl]2 with one equivalent of SPANphos
gave [Rh2(CO)2Cl2(SPANphos)] in which the ligand bridges
the two rhodium metals in a trans fashion over the folded
Rh2Cl2 moiety. This complex undergoes oxidative addition of
one molecule of MeI and provides a fast catalyst for methanol
carbonylation.16
The design and synthesis of diphosphine ligands exhibiting
coordination properties deviating from the standard cis chelating
ligands has been an intriguing and rewarding research goal for
several decades.1 Two groups of ligands were reported, viz. BISBI2
and Xantphos,3 the backbones of which favour bite angles of 110–
120◦ according to molecular mechanics calculations. However
rigid the backbone of Xantphos might seem, it still does form
square planar complexes with bite angles as low as 99◦ in cis
complexes4 and 164◦ in trans complexes.5 The phosphorus donor
atoms are very close to one another in Xantphos and one might
not expect bimetallic complexes to be formed, but nevertheless
bimetallic gold complexes have been identified.6 In particular, our
interest concerns the catalytic properties of such ligands; indeed,
Xantphos has led to a variety of catalysts exhibiting unusual
properties.
Likewise, exclusively trans-coordinating ligands have been a
research goal for several decades7 to which end Venanzi introduced
TRANSphos.8 The ligand turned out to be very flexible and
although it does form trans complexes, the full range of bite angles
was found experimentally and MM2 calculations supported this.
The TRAP ligands developed by Ito9 coordinate in a trans fashion,
but smaller bite angles were found and in view of their activity in
insertion reactions of square planar complexes one assumes that
cis complexes are accessible as well.
Recently we reported on a new trans-spanning diphosphine,
SPANphos, containing a spirobichroman backbone, for which
several trans organometallic complexes were identified confirming
a consistent preference for trans coordination.10 Simple modeling
had shown that SPANphos was ideally set up for forming
trans complexes. Several other examples of trans diphosphine
ligands (containing slightly flexible backbones) have appeared in
the literature.11 Thus, by reaction of one equivalent of SPAN-
phos per metal with [PtCl2(cod)], [PdCl2(cod)], [PdClMe(cod)],
While all results strengthened our belief that SPANphos was a
true trans ligand, we were awakened from this trance by several
results showing the contrary. Here we report on the first examples
in which SPANphos coordinates in a cis fashion and more careful
molecular modeling studies show that actually the energies of cis
and trans complexes differ only slightly.
Institute of Chemical Research of Catalonia (ICIQ), Av. Pa¨ısos Catalans 16,
43007, Tarragona, Spain. E-mail: pvanleeuwen@iciq.es; Fax: 34 977920221;
Tel: 34 977920200
2 6 8 | Dalton Trans., 2006, 268–278
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2
nbd), 1.6 (6H, s, CH3), 2.2 (12H, s, CH3), 2.5 (2H, d, JH,H
=
Experimental
14 Hz, CH2), 2.9 (2H, d, 2JH,H = 14 Hz, CH2), 3.3 (2H, br, nbd),
3.6 (2H, br t, nbd), 5.1 (2H, br t, nbd), 6.4 (2H, t, 3JH,H = 5.1 Hz,
Ar), 6.7 (2H, m, Ar), 7.0 (2H, m, Ar), 7.1 (2H, m, Ar), 7.3 (4H, m,
Ar), 7.5 (2H, t, 3JH,H = 7.5 Hz, Ar), 7.6 (2H, t, 3JH,H = 7.5 Hz, Ar),
General
All reactions were carried out under an argon atmosphere
using standard Schlenk techniques. Solvents were obtained
from Sigma-Aldrich and dried with an SPS system of IT-inc.
[Rh(nbd)2]BF4 was obtained from Alfa Aesar. The diphosphines,
SPANphos, SPANPOP, SPANDBP, SPANEt, SPANiPr, SPANCy
and SPANtBu were prepared by procedures analogous to those
reported in the literature.10 NMR spectra unless otherwise stated
were recorded at the following frequencies: 400.13 MHz (1H),
100.63 MHz (13C), 161.98 MHz (31P). 13C and 31P NMR spectra
were recorded using broad band proton decoupling. Chemical
shifts of 1H and 13C NMR spectra are reported in ppm downfield
from TMS, used as internal standard. Chemical shifts of 31P NMR
spectra are referred to H3PO4 as external standard. Signals are
quoted as s (singlet), d (doublet), t (triplet) vt (virtual triplet)
3
3
7.9 (2H, d, JH,H = 7.8 Hz, Ar), 8.0 (2H, d, JH,H = 7.8 Hz, Ar);
31P{ H} d 11.6 (d, J = 149.1 Hz); 13C{ H} 16.67 (s), 20.7 (s), 29.11
(s), 30.01 (s), 31.7 (s), 32.67 (s), 33.47 (s), 49.80 (s), 52.56 (br, s),
74.25 (m), 79.57 (m), 105.5 (s), 106.7 (s), 122 (vt, J = 3.0 Hz), 122.8
(vt, J = 2.6 Hz), 128.3 (vt, J = 4.9 Hz), 128.5 (s), 128.6 (s), 128.8
(vt, J = 4.4 Hz), 130.6 (vt, J = 6.2 Hz), 131.2 (m), 131.8 (s), 132.6
(vt, J = 3.9 Hz), 132.7 (vt, J = 4.1 Hz), 133.6 (vt, J = 3.5 Hz),
141.3 (vt, J = 4.9 Hz), 142.5 (vt, J = 5.1 Hz), 152.1 (vt, J =
4.3 Hz). MS m/z 895 (M+).
1
1
[Rh(nbd)(SPANEt)]BF4 (4). [Rh(nbd)2]BF4 (0.03 g, 0.08 mmol)
and SPANEt (0.041 g, 0.08 mmol) were dissolved in THF (2 ml).
The dark red solution was stirred at room temperature for 2 h.
The solvent was removed under vacuum and the yield of the solid
was 87%. Attempts to obtain crystals were unsuccessful. NMR
(average coupling constant presented J = [nJP,C
+
n+2JP,C ]/2), m
(multiplet), br (broad). Mass spectra were run by electrospray
impact on a Waters LCT Premier spectrometer.
1
2
3
(400 MHz, CDCl3): H d 0.72 (6H, dt, JH,P = 7.5 Hz, JH,H
=
18.1 Hz, CH3CH2), 0.86 (6H, dt, 2JH,P = 7.3 Hz, 3JH,H = 18.1 Hz,
CH3CH2), 0.99 (4H, m, CH2CH3), 1.27 (6H, s, CH3), 1.39 (6H, s,
CH3), 1,59 (6H, m, CH2CH3), 2.03 (2H, d, 2JH,H = 14 Hz, CH2),
2.36 (6H, s, CH3), 2.43 (2H, d, 2JH,H = 14 Hz, CH2), 4.06 (2H, br,
nbd), 4.92 (2H, br, nbd), 4.96 (2H, br, nbd), 6.96 (2H, br, Ar), 7.10
Synthesis
[Rh(nbd)(SPANphos)]BF4 (1). [Rh(nbd)2]BF4 (0.03 g,
0.08 mmol) and SPANphos (0.0563 g, 0.08 mmol) were dissolved
in THF (2 ml). The solution was stirred at room temperature for
1 h. The solvent was removed under vacuum. Recrystallisation
from CH2Cl2–THF–hexane afforded dark red crystals in 95%
(2H, br, Ar); 31P{ H} d 17.9 (d, J = 151.4 Hz); 13C{ H} d 7.34 (s),
9.21 (s), 13.0 (vt, J = 13.5 Hz) 15.1 (vt, J = 12.7 Hz), 20.9 (s), 27.1
(s), 28.0 (s), 32.6 (s), 52.71 (s), 54.0 (s) 68.80 (br), 80.3 (br), 81.1
(br), 106.20 (s), 119.5 (vt, J = 18.3 Hz), 126.8 (s), 129.98 (vt, J =
5.2 Hz), 136.86 (br), 151.56 (s). MS m/z 707 (M+)
1
1
1
yield. NMR (400 MHz, CDCl3): H, d 1.2 (2H, br t, nbd), 1.4
(6H, s, CH3), 1.5 (6H, s, CH3), 2.08 (6H, s, CH3), 2.2 (2H, d,
2JH,H = 14 Hz, CH2), 2.6 (2H, d, 2JH,H = 14 Hz, CH2), 3.4 (2H, br,
nbd), 3.6 (2H, br, nbd), 3.9 (2H, br t, nbd), 6.4 (2H, m, Ar), 6.8
(4H, m, Ar), 7.0 (6H, m, Ar), 7.1 (2H, m, Ar), 7.6 (6H, m, Ar),
8.1 (4H, m, Ar); 31P{ H}, d 26.05 (d, 1JRh,P = 154.7 Hz); 13C{ H}
d 22.61 (s), 25.58 (s), 29.56 (s), 32.22 (s), 49.80 (s), 51.55 (s), 73.97
(m), 79.77 (m), 104.84 (s), 127.94 (vt, J = 5 Hz), 128.11 (s), 129.26
(s), 129.48 (t, J = 5.2 Hz), 130.64 (s), 131.77 (vt, J = 5.3 Hz),
131.97 (vt, J = 3.6 Hz), 132.35 (br s), 134.15 (vt, J = 2.4 Hz),
136.97 (vt, J = 7.4 Hz), 151.21 (vt, J = 4 Hz). MS m/z 899 (M+).
[Rh(nbd)(SPANiPr)]BF4 (5). [Rh(nbd)2]BF4 (0.03 g, 0.08 mmol)
and SPANiPr (0.045 g, 0.08 mmol) were dissolved in THF (2 ml).
The dark red solution was stirred at room temperature for 2 h.
The solvent was removed under vacuum. Recrystallisation from
CHCl3–diethyl ether afforded red crystals in 98% yield. NMR
1
1
1
(400 MHz, CD2Cl2): H d 0.91 (12H, m, CH3CH), 1.35 (6H, s,
CH3), 1.39 (6H, br, CH3CH), 1.39 (2H, br, CH3CH), 1.42 (6H, s,
2
3
CH3), 1.50 (2H, br, nbd), 1,51 (6H, dd, JH,P = 7.4 Hz, JH,H
=
16.4 Hz, CH3CH), 2.10 (2H, d, 2JH,H = 14 Hz, CH2), 2.34 (6H, s,
[Rh(nbd)(SPANPOP)]BF4 (2). [Rh(nbd)2 ]BF4 (0.03 g,
0.08 mmol) and SPANPOP (0.0560 g, 0.08 mmol) were dissolved in
THF (2 ml). The orange solution was stirred at room temperature
for 1 h. The solvent was removed under vacuum and the compound
was obtained in 92% yield. Attempts to obtain crystals were
unsuccessful. NMR (400 MHz, CD2Cl2): 1H d 1.1 (2H, br t, nbd),
1.4 (6H, s, CH3), 2.03 (12H, s, CH3), 2.09 (2H, d, 2JH,H = 14 Hz,
CH2), 2.7 (2H, d, 2JH,H = 14 Hz, CH2), 3.2 (2H, br, nbd), 3.5 (2H,
2
CH3), 2.47 (2H, d, JH,H = 14 Hz, CH2), 2.50 (2H, sp, CH3CH),
3.66 (2H, br, nbd), 3.93 (2H, br, nbd), 4.64 (2H, br t, nbd), 6.9
(2H, br, Ar), 7.13 (2H, br, Ar); 31P{ H} d 26.2 (d, J = 147.1 Hz);
1
13C{ H} d 19.4 (s), 20.4 (s), 21.2 (vt, J = 2.3 Hz), 21.8 (vt, J =
1
3.7 Hz), 25.0 (vt, J = 9 Hz), 28.9 (s), 32.1 (s), 50.4 (s), 52 (s), 71.5
(br), 72 (br), 104.6 (s), 127.1 (s), 129.4 (br s), 132.2 (t, J = 3.5 Hz),
134.8 (br), 150.8 (vt, J = 2.4 Hz). MS m/z 763 (M+)
3
br t, nbd), 4.1 (2H, br t, nbd), 6.3 (2H, d, JH,H = 9.3 Hz, Ar),
[Rh(nbd)(SPANCy)]BF4 (6). [Rh(nbd)2]BF4 (0.03 g, 0.08 mmol)
and SPANCy (0.0582 g, 0.08 mmol) were dissolved in THF
(2 ml). The solution was stirred at room temperature for 1 h.
The solvent was removed under vacuum. Recrystallisation from
CHCl3–diethyl ether afforded dark red crystals in 92% yield. NMR
(400 MHz, CDCl3): 1H d 1.37 (6H, s, CH3), 1.47 (6H, s, CH3), 2.37
(6H, s, CH3), 0.95–2.52 (50H, br, Cy, nbd, CH2), 3.56 (2H, br,
nbd), 3,68 (2H, br, nbd), 4.5 (2H, br, nbd), 6.82 (2H, br, Ar), 7.21
6.7 (2H, t, 3JH,H = 7.3 Hz, Ar), 6.8 (4H, m, Ar), 7.1 (4H, m, Ar),
7.2 (6H, m, Ar), 7.4 (2H, dt, 4JH,P = 1.2 Hz, 3JH,H = 7.5 Hz, Ar);
1
1
1P{ H} d −3.2 (br), −10 (br);13C{ H} d 14.5 (s), 15.6 (s), 24.6 (s),
30.8 (s), 32.5 (s), 38.9 (s), 67.0 (s), 115–135 (br). MS m/z 927 (M+).
[Rh(nbd)(SPANDBP)]BF4 ] (3). [Rh(nbd)2 ]BF4 (0.03 g,
0.08 mmol) and SPANDBP (0.060 g, 0.08 mmol) were dissolved in
THF (2 ml). The dark red solution was stirred at room temperature
for 2 h. The solvent was removed under vacuum and the compound
was obtained in 89% yield. Attempts to obtain crystals were
(2H, br, Ar) 31P{ H} d 15.50 (br), 16.87 (d, J = 168.7 Hz), 23.09
1
(br); 13C{ H} (230 K, 125.05 MHz) d 25.5–34 (br), 37.4 (br), 47.8
1
1
unsuccessful. NMR (400 MHz, CD2Cl2): H d 1.05 (2H, br t,
(s), 51.3 (s), 51.7 (s), 52.1 (s), 54.0 (s), 65.5 (s), 67.4 (s), 68.0 (s),
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75.4 (s), 103.8 (s), 117.0 (s), 117.3 (s), 117.5 (s), 127.3 (s), 127.7 (s),
128.3 (s), 129.3 (s), 130.6 (d, J = 2.8 Hz), 130.9 (d, J = 5.5 Hz),
133.0 (d, J = 5.5 Hz), 137.7 (d, J = 4.0 Hz), 150.4 (d, J = 6.5 Hz),
152.3 (d, J = 7.4 Hz) MS m/z 923 (M+).
Computational details
The geometry of spirobichroman conformers was optimized
using the semiempircal AM1 hamiltonian as implemented in
ArgusLab,20 and at the B3LYP/6-31G level using Gaussian03.21
Palladium, platinum and rhodium complexes were fully opti-
mized using the Amsterdam Density Functional (ADF2004.01)
program developed by Baerends et al.22,23 We used the local
VWN24 exchange-correlation potential to optimize geometries,
while energies were evaluated by single point calculations in-
cluding nonlocal Becke’s exchange correction25 and Perdew’s
correlation correction26 (BP86). Relativistic corrections were in-
troduced by scalar-relativistic Zero Order Regular Approximation
(ZORA).27–29 A triple-f plus polarization basis set was used
on all atoms. For non-hydrogen atoms a relativistic frozen-core
potential was used, including 3d for palladium and rhodium, 4d
for platinum, 2p for phosphorus and 1s for carbon and oxygen.
[Rh2(cod)2(SPANiPr)Cl2] (7). Synthesis of: [Rh(cod)Cl]2
(0.020 g, 0.04 mmol) and SPANiPr (0.023 g, 0.04 mmol) were
dissolved in THF (2 ml) under a stream of Ar. The orange
solution was stirred at room temperature for 3 h. The solvent was
removed under vacuum. Recrystallisation from CH2Cl2–diethyl
ether afforded yellow crystals. NMR (400 MHz, CD2Cl2): 31P d 34
(br), 37 (br). MS m/z 1025 (M+ − Cl)
[Rh2(cod)2(SPANDBP)Cl2] (8). [Rh(cod)Cl]2] (0.020 g,
0.04 mmol) and SPANDBP (0.028 g, 0.04 mmol) were dissolved
in THF (2 ml) under a stream of Ar. The orange solution was
stirred at room temperature for 3 h. The solvent was removed
under vacuum. Recrystallisation from CH2Cl2–diethyl ether
1
afforded yellow crystals. NMR (400 MHz, CD2Cl2): H d 1.37
Results and discussion
(6H, s), 1.53 (6H, s), 1.55–1.9 (4H, m), 2.00 (6H, s), 2.03–2.23
(6H, m), 2.52 (2H, d, J = 14.8), 4.26 (2H, br t), 4.96 (2H, dt,
J = 6.9 Hz, J = 12.6 Hz), 5.4 (2H, dd, J = 7.6 Hz), 6.59 (2H, dd,
J = 1.6 Hz, J = 11.4 Hz), 6.85 (2H, ddd, J = 2.9 Hz), 7.0 (2H,
d, J = 1.1 Hz), 7.24 (2H, dd, J = 1.6 Hz, J = 11.4 Hz), 7.44 (2H,
ddd, = 2.9 Hz), 7.55 (2H, dd (t), J = 7.5 Hz), 7.76 (2H, d, J =
7.6 Hz), 7.85 (2H, d J = 7.6 Hz), 8.4 (4H, dt, J = 7.2 Hz, J =
13.2 Hz); 31P d 18.8 (d, J = 144.5 Hz). MS m/z 1157 (M+ − Cl)
Synthesis and characterization
Since the first report on SPANphos as a trans spanning diphos-
phine, we continued our work on the synthesis of organometallic
complexes to determine the scope of the ligand in both coordina-
tion chemistry and catalytic reactions. In all reactions in which no
preference for a cis or trans coordination is imposed by the metal
precursor, SPANphos formed exclusively trans complexes.
Initially putative “cis” precursors failed to give bidentate
complexes. For instance, when the SPANphos was reacted
with one equivalent of [Rh(acac)(CO)2], in which the acety-
lacetonate ligand imposes a cis coordination, we obtained
[Rh(acac)(CO)(SPANphos)] in which SPANphos acts as a mon-
odentate ligand (Scheme 1).10
X-Ray structure determinations
Crystals of 1, 5, 6, 7 and 8 were obtained as described in the
experimental (synthesis). Crystals of SPANiPr and SPANtBu were
obtained by slow evaporation at room temperature of CH2Cl2–
CH3OH. SPANPOP was obtained by precipitation in THF as
mother liquor. The measured crystals were prepared under inert
conditions immersed in perfluoropolyether as protecting oil for
manipulation.
However, when one equivalent of SPANphos (phosphine sub-
stituent = PPh2) was reacted with [Rh(nbd)2]BF4 in THF, the
mononuclear compound [Rh(nbd)(SPANphos)]BF4 (1) in which
SPANphos occupies two cis coordination sites was obtained in
95% yield. Likewise were obtained compounds 2–6 carrying,
respectively, the substituents POP (2), DBP (3), Et (4), i-Pr (5)
and Cy (6). The 1H-NMR spectra all witness the liberation
of one norbornadiene molecule. All 31P NMR spectra, except
the broad ones of 6 and 2 exhibit a single resonance with
2JP,Rh = 144–163 Hz as is to be expected for complexes having
C2 symmetry, at least on the NMR time scale. These spectral
data are in accordance with both cis and (most likely, oligomeric)
trans complexes. The mass spectra all gave the mass of the
molecular cation, Rh(nbd)(SPAN)+. The 13C NMR spectra of
compounds 1, 3–5 show many triplets in the aromatic and aliphatic
region of the backbone due to coupling of 13C with 31P nuclei,
which is due to a relatively strong mutual coupling between the
phosphorus nuclei and the fact that the chemical shifts are equal
(virtual triplets). Often this is taken as a proof of a trans P–
Data collection. Measurements were made on a Bruker-
Nonius diffractometer equipped with a APPEX 2 4 K CCD area
detector, a FR591 rotating anode with Mo-Ka radiation, Montel
mirrors as monochromator and a Kryoflex low temperature device
(T = −173 ◦C). Full-sphere data collection was used with x and
u scans.
Programs used: Data collection Apex2 V. 1.0–22 (Bruker-
Nonius 2004), data reduction Saint + Version 6.22 (Bruker-Nonius
2001) and absorption correction SADABS V. 2.10 (2003).
Structure solution and refinement. SHELXTL Version 6.10
(Sheldrick, 2000) was used.18 Compound 1 which includes in the
crystal structure highly disordered tetrahydrofuran molecules was
treated with Squeeze (Platon) in order to avoid modelling the
disordered molecules.19
CCDC reference numbers 286677 (1), 286678 (5), 286679 (6),
286681 (7), 286682 (8), 286683 (SPANiPr), 286684 (SPANtBu),
286685 (SPANPOP).
2
P arrangement,30 but simulation shows that for JP,P > 20 Hz
n
n+2
and the common JP,C and
J
coupling constants already
P,C
apparent triplets will be observed. Indeed, the apparent coupling
n
constants range from 1 through 5 Hz, the average of JP,C (0–
n+2
For crystallographic data in CIF or other electronic format see
DOI: 10.1039/b513870c
20) and
J
(0–10), which often have opposite signs.31,32 The
P,C
appearance of virtual triplets for cis complexes of nickel has been
2 7 0 | Dalton Trans., 2006, 268–278
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(1JP,Rh = 168.7 Hz).34 This suggests that different phosphorus nuclei
of 6 or isomers of 6 are present, which rapidly equilibrate. On
cooling to 250 K two broad doublets start to grow at 14.26 and
23.34 ppm. When the temperature reaches 230 K, in addition
to the appearance of a minor doublet at 26.7 ppm (1JP,Rh
=
146.4 Hz), it becomes evident that the two doublets are in fact
doublets of doublets (1JP,Rh = 156 and 138 Hz respectively, 2JP,P
=
30 Hz). The presence of two doublets of doublets proves that
the two phosphorus atoms are inequivalent and the value of
2
the coupling constant JP,P indicates the relative cis position to
one another. Thus, at 230 K the equilibrium is slow enough
to distinguish a structure of 6 in which the C2 symmetry has
disappeared. At low temperature the 13C NMR spectra should
show only doublets due to the coupling with one phosphorus
nucleus only, as the difference in chemical shifts of 9 ppm removes
the virtual coupling effect. Unfortunately, the presence of a
mixture of symmetrical and unsymmetrical compounds renders
a complex 13C NMR spectra in which the signals cannot be easily
attributed. According to simulations already 0.1 ppm suffices to
remove the triplet character, at the spectrometer frequency and
coupling constants under consideration.35 At high temperature
we observe not only intramolecular exchange, but also exchange
with the minor absorption at 26.7 ppm. We assign a symmetrical
structure to the complex giving rise to this doublet in which the
C2 symmetry is retained. In compounds 1 and 3–5 the doublet
observed in the 31P NMR spectra can be attributed to a static
symmetric structure or to an asymmetric structure in which the
exchange is faster than in 6 and 2; the latter would not be unlikely
for the sterically less hindered phenyl, isopropyl or ethyl derivatives
(1, 4 and 5) but less so for the bulky ligand in 3. The X-ray
structures obtained elucidated how a cis C2 symmetric complex
may lose its symmetry and convert into a C1 symmetric complex.
Scheme 1
+
reported before and hence care must be taken when using this
as a criterion.33
The 31P NMR spectra of 6 in CDCl3 (Fig. 1) at different
temperatures provide most information. At 300 K only two broad
signals are observed and a trace amount of doublet at ∼17 ppm
Thus the cations Rh(nbd)2 react with SPANphos derivatives
under replacement of one norbornadiene ligand rendering cis
complexes. Reaction of the chloro-bridged dimer [(cod)RhCl]2
usually leads to cleavage of the bridge for monodentate ligands and
replacement of cod when cis diphosphines are used.36 Reaction
of [(cod)RhCl]2 with SPANDBP and SPANiPr gave dinuclear
compounds 7 and 8 in which the chloro bridges have been cleaved
but cyclooctadiene remains coordinated to the rhodium metal. In
both compounds the two phosphine donors coordinate each to
one (cod)rhodium chloride moiety.
X-Ray structures
X-Ray single-crystal structures were obtained for the mononuclear
cis compounds 1, 5, and 6, for the bis rhodium compounds 7 and 8,
and for the free ligands SPANiPr, SPANtBu and SPANPOP, while
that of SPANphos10 has been reported previously. For compound 6
two pseudopolymorphic structures with a similar arrangement of
the compound were determined (6 and 6b). Due to the similarity
only 6 will be reported. The obtained molecular structures are
represented in Fig. 2 (the numbering of the spirobichroman, which
is relevant for the discussion, is represented in 5 and is identical for
all the molecules in the series). Crystal data are listed in Tables 1
and 2. Selected bond distances and angles are listed in Table 3.
Compounds 5 and 6 exhibit a molecular symmetry close to
C2 with the spirobichroman backbone arranged symmetrically
close to the plane of coordination of the rhodium atom. The
Fig. 1 Variable-temperature 31P spectra of [Rh(nbd)(SPANCy)]BF4 (6)
in CDCl3.
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Table 1 Crystal data for compounds 1, 5 and 6
Compound
1
5
6
Formula
C54H54BF4O2P2Rh
BF4/4THF (Squeeze)
986.63
C44H64BCl6F4O2P2Rh
BF4/2CHCl3
1089.31
C56H80BCl6F4O2P2Rh
BF4/2CHCl3
1249.56
Anion/solvents in crystal
Formula weight
Crystal size/mm
Crystal color
T/K
0.50 × 0.50 × 0.50
0.40 × 0.20 × 0.20
0.40 × 0.30 × 0.20
Red
Red
Red
100
100
100
Crystal system
Space group
Monoclinic
P21/c
Monoclinic
P21/c
Monoclinic
P21/n
˚
a/A
12.3904(5)
19.1948(8)
24.8906(9)
93.6620(10)
5907.7(4)
13.0532(8)
19.6411(12)
19.8764(13)
94.746(2)
5078.4(6)
4
1.425
0.764
39.54
12.4171(6)
27.9184(14)
17.4627(9)
104.4700(10)
5861.7(5)
4
1.416
0.672
39.64
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z
4
Dc/g cm−3
1.109 (with 4THF: 1.353)
0.389
39.54
l/mm−1
◦
hmax
/
Refl. measured
Unique reflections (Rint
Absorp. correct.
Transmission min./max.
Parameters
120149
100953
119031
)
31008 (0.0296)
SADABS (Bruker)
0.7228/1.0000
847
28230 (0.0335)
SADABS (Bruker)
0.8009/1.0000
581
30269 (0.0377)
SADABS (Bruker)
0.6924/1.0000
668
R1/wR2 [I > 2r(I)]
R1/wR2 (all data)
Goodness-of-fit (F2)
0.0350/0.1148
0.0437/0.1191
1.082
0.835/−0.638
0.0335/0.0814
0.0481/0.0899
1.036
1.106/−0.809
0.0466/0.1232
0.0610/0.1342
1.040
2.123/−2.059
−3
˚
Peak, hole/e A
Table 2 Crystal data for compounds 7, 8, SPANiPr, SPANtBu and SPANPOP
Compound
7
8
SPANiPr
SPANtBu
SPANPOP
Formula
C51H78Cl2O2P2Rh2
—
1061.79
C66.6H69.7Cl13O2P2Rh2
4.5CHCl3
1631.48
C35H54O2P2
—
568.72
0.20 × 0.20 × 0.20
Colorless
100
C39H62O2P2
—
624.83
0.20 × 0.10 × 0.05
Colorless
100
C47H42O2P2
—
732.75
0.20 × 0.20 × 0.10
Colorless
100
Anion/solvents in crystal
Formula weight
Crystal size/mm
Crystal color
T/K
0.10 × 0.10 × 0.05
0.20 × 0.10 × 0.10
Yellow
Yellow
100
Monoclinic
P21/c
100
Triclinic
¯
P1
Crystal system
Space group
Triclinic
¯
P1
Monoclinic
C2/c
Monoclinic
P21/c
˚
a/A
12.3860(15)
18.114(2)
21.617(3)
90
97.392(4)
90
4809.9(10)
4
1.466
14.0174(19)
14.210(2)
19.617(3)
100.226(3)
109.755(3)
101.837(3)
3467.6(8)
2
10.8544(10)
11.1005(11)
15.1731(15)
85.280(2)
89.538(2)
63.872(2)
1635.0(3)
2
39.0066(13)
11.6480(5)
16.9525(6)
90
101.8800(10)
90
7537.4(5)
8
1.101
11.7640(5)
24.9675(11
12.5726(6)
90
96.9550(10)
90
3665.6(3)
4
1.328
˚
b/A
˚
c/A
a/◦
b/◦
c /◦
3
˚
V/A
Z
Dc/g cm−3
1.563
1.066
38.25
1.155
0.162
40.00
l/mm−1
0.903
36.03
0.146
39.49
0.166
39.50
◦
hmax
/
Refl. measured
Unique reflections (Rint
Absorp. correct.
Transmission min./max.
Parameters
70795
60660
32900
60102
17439
)
20080 (0.0645)
SADABS (Bruker)
0.6919/1.0000
546
33678 (0.0418)
SADABS (Bruker)
0.5817/1.0000
874
18323 (0.0317)
SADABS (Bruker)
0.5978/1.0000
366
20428 (0.0466)
SADABS (Bruker)
0.7870/1.0000
406
10811 (0.0251)
SADABS (Bruker)
0.5381/1.0000
484
R1/wR2 [I > 2r(I)]
R1/wR2 (all data)
Goodness-of-fit (F2)
0.0683/0.1741
0.1104/0.1957
1.055
2.282/−2.311
0.0806/0.1884
0.1396/0.2369
1.018
3.049/−3.367
0.0536/0.1668
0.0631/0.1721
1.096
1.486/−1.267
0.0417/0.1150
0.0528/0.1223
1.039
0.691/−0.300
0.0402/0.1116
0.0507/0.1187
1.034
0.424/−0.282
−3
˚
Peak, hole/e A
rhodium atom is coordinated in a slightly distorted square planar
geometry. In 5 the planes P1–Rh1–P2 and double bond–Rh1–
double bond are rotated approximately 11◦ and in 6 approximately
13◦. In solution only a small fraction of compound 6 has the
symmetric structure giving rise to a singlet in the 31P NMR
spectrum; vide infra for the nonsymmetric structure. In solution
symmetric structures have been found for compounds 1, 3, 4 and 5.
Compound 1 shows interesting differences in the solid state
compared to compounds 5 and 6. It is arranged differently, crystal-
lizing in a molecular C1 symmetry with the spirobichroman ligand
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◦
˚
Table 3 Selected bond distances (A) and angles ( ) for 1, 5, 6, 7 and 8
Rh1–P1
Rh1–P2
Rh1–C(A)a
2.1708(12)
2.1828(11)
2.1865(14)
Rh1–C(B)a
2.1861(12)
2.1870(11)
2.1925(13)
Rh1–C(C)a
2.2246(12)
2.1937(11)
2.1925(15)
Rh1–C(D)a
2.2444(13)
2.2024(10)
2.1961(13)
1
5
6
2.3013(3)
2.3854(3)
2.3832(4)
2.3463(3)
2.3717(3)
2.3776(4)
Rh1–P1
2.3637(10)
2.2945(10)
Rh2–P2
2.3673(11)
2.2974(9)
7
8
P1–Rh1–P2
97.892(11)
95.594(9)
C8–P1–Rh1
118.17(4)
111.95(3)
110.43(5)
CX–P1–Rh1b
111.72(4)
111.33(4)
CY–P1–Rh1b
114.24(4)
114.70(4)
C19–P2–Rh1b
100.74(3)
107.95(3)
CXꢀ–P2–Rh1b
125.80(4)
110.84(4)
CYꢀ–P2–Rh1b
114.56(4)
116.99(3)
1
5
6
96.312(13)
113.86(4)
113.94(4)
110.06(5)
112.81(4)
115.36(4)
C19–P2–Rh2
114.69(13)
120.59(10)
CXꢀ–P2–Rh2b
107.24(17)
108.66(11)
CYꢀ–P2–Rh2b
124.20(13)
120.32(11)
7
8
113.70(11)
121.55(12)
123.12(14)
110.83(14)
109.65(15)
120.25(14)
a The olefinic carbon atoms of norbornadiene have been named A, B, C and D. b The a-carbons to the phosphorous non belonging to spirobichroman
have been named X and Y.
Fig. 2 ORTEP Plots (ellipsoids drawn in 50% probability levels) of the analyzed structures. The dimeric compounds are drawn using partly spheres and
hydrogen atoms are omitted to fully appreciate the geometry of the complexes. Compound 5 shows the labeling scheme of the relevant atoms used for all
molecules.
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out of plane of the planar coordination sphere of the rhodium
atom, actually located at one face of the coordination plane. The
Rh–P bonds in 1 have shorter distances than those in 5 and 6.
Short values have also been observed in the dimeric compound
8. The shorter distances are detected in ligands containing
phosphorus atoms linked to three sp2-hybridized carbon atoms,
but the shorter distances may also be due to less steric hindrance
in phenyl substituted phosphines compared to isopropyl and
cyclohexyl substituted phosphines. These effects are probably due
to the changes in the electronic properties of the phosphorous
atoms. In the case of 1 the rhodium atom is also coordinated
in a slightly distorted square planar geometry (The planes
P1–Rh1–P2 and double bond–Rh1–double bond are rotated
approximately 8◦).
An interesting difference detected in compound 1 is a pyra-
midization of the sp2-hybridized atom connecting C19 with
P2. The angle between the plane defined by the aromatic ring
containing C19 and the bond P2–C19 is 17.5◦. The corresponding
angle with respect to P1 is 1.2◦, which is within the standard values.
This torsion also affects the planarity of the aromatic ring which
is slightly distorted in direction of the bending out of plane of
C19–P2. An explanation for the distortions of the molecule in
this area is probably the CH/p interaction between the aromatic
ring C15–C16–C17–C18–C19–C20 of the spirobichroman and
an olefinic hydrogen atom of the norbornadiene ligand.37 The
distance between the carbon atom at norbornadiene and the center
to define the conformation of each ring it is assumed that the
two sets of atoms C3–C4–C9–O1 and C14–C15–C20–O2 are each
in one same plane due to the sp2-hybridization of the central
atoms, and carbon atoms C1, C2 and C13 may be out of these
˚
planes. The out of plane deviations (in A) for the single atoms C1,
C2 and C1, C13 with respect to these defined planes have been
calculated to determine boat, twist and envelope conformations.
A boat conformation will be found if both atoms are out of plane
˚
at the same side with a deviation >0.2 A. A twist conformation
will be found if both atoms are showing small deviations out of
plane at opposite sides of the plane. An envelope conformation will
be found if only one atom deviates significantly from this plane.
The envelope conformation can be found at the spiro atom (C1)
[= envelope (spiro)] and at the CH2-atom (C2, C13) [= envelope
(CH2)]. The detected conformations for the described atoms are
resumed at Table 4.
A comparison of the conformations in 1, 5 and 6 of the spiro
rings shows in 1 a boat/envelope (CH2) conformation and in 5
and 6 a boat/boat conformation. In a spiro bichroman molecule,
4,4,4ꢀ,4ꢀ,7,7ꢀ-hexamethyl-2,2ꢀ-spirobichroman, a twist/twist con-
formation has been found.38 The ligands SPANiPr and SPANtBu
display a twist/envelope (spiro) conformation and the ligand
SPANPOP containing the large planar substituents at phosphorus
has, like spirobichroman, a twist/twist conformation. The dimeric
compounds 7 and 8 reveal an envelope (spiro)/envelope (spiro)
conformation. All the analyzed compounds adopt a concave
arrangement of the spiro rings except the dimeric compound 8
which shows a distal orientation of the rings.
˚
of the aromatic ring is 3.26 A (the uncorrected distance between
˚
center of the aromatic ring and the hydrogen atom is 2.47 A).
Similar distances for CH/p interaction between benzene and
The conformations found in the analyzed structures show that
SPANphos ligands are highly flexible and can adopt different
conformations depending on the needs of the molecules or
complexes to be formed. If, due to the fixed geometry of the
norbornadiene, the resulting monomeric complex allows only cis
geometry, the bichroman backbone will adopt the appropriate
conformation to reach the required orientation and form the cis
structure.
methane are calculated in the region of 3.6–3.8 A.37c Additionally,
˚
a p/p interaction between the aromatic ring C4–C5–C6–C7–C8–
C9 of the spirobichroman and one of the phenyl groups bonded
at P2 can be detected. The approximate distance of this contact is
3.8 A.37d Both, CH/p and p/p interaction, need to be considered in
˚
order to describe the unexpected C1 symmetry of the compound 1.
Such a structure would give rise to two different absorptions in 31
P
NMR and most likely 2 and 6 in solution assume a conformation
related to this one, rapidly equilibrating at room temperature with
a C2 symmetric conformer.
The “bite angles” (P–Rh–P) of the cis chelating complexes are
all around 97◦, i.e. only slightly more opened than the expected
angles for an ideal square planar system (see Table 3). The almost
equal distances of the metal to the four olefinic carbon atoms
of norbornadiene represents a symmetric arrangement without
distortions.
A comparison of the Rh–P–C angles for all complexes gives
similar values except for compound 1 and compounds 7 and 8.
The differences in 1 may be caused by the tilting of the backbone
described above. The differences in the angles of compounds 7
and 8 are probably due to steric effects of the substituents at
phosphorus.
Computational chemistry
In view of the various conformations that the two six-membered
rings of SPANphos ligands can assume, a too simple approach for
bite angle map calculations using MM2 can lead one easily astray.
Therefore we had a look at spirobichroman compounds at higher
levels of theory. Spirobichroman 9 (Scheme 2, 2,2ꢀ-spirobi[2H-1-
benzopyran], 3,3ꢀ,4,4ꢀ-tetrahydro-4,4,4ꢀ,4ꢀ-tetramethyl-) was stud-
ied with the use of semiempirical (AM1) and DFT (B3LYP/6-
31G) methods.
In order to understand the ability of SPANphos to form cis
complexes an exact analysis of the conformations of the spiro
rings in spirobichroman in the cis complexes, the free ligands
and the dimeric complexes is necessary. The hexacyclic rings in
the spiro compounds can adopt a boat conformation, a twist
conformation and an envelope conformation. Additionally to
the different conformations, the six membered rings can adopt
concave or distal orientations relative to one another. In order
Scheme 2
Seven local minima were identified within 10 kcal mol−1 from the
global minima exhibiting the ring conformations also encountered
in the X-ray studies. On both levels of theory three distinct
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conformers show identical relative stability; the conformers can
be described as envelope (spiro)/envelope (spiro), twist/twist and
envelope (CH2)/envelope (CH2). The global minimum coincides
with the crystal structure found for bischromane framework
which has a perfect twist/twist structure, also found for the
free SPANPOP ligand (Table 5, conformation D). This structure
can evolve, without a significant energy change, to an envelope
(spiro)/envelope (spiro) conformer which was found for free
SPANphos10 as well as for compound 7 (Table 5, conformation C).
The third conformer, almost degenerate to D and C, corresponds
to an envelope (CH2)/envelope (CH2) conformation (Table 5,
conformation E), only characterized in trans compounds. The
next lowest in energy is a structure that contains one 2H,3,4-
dihydrobenzopyran in a boat conformation and the other ring
in a twist conformation (Table 5, conformation B). A very
similar arrangement is found in the solid state structure of
compound 1, which should be described as boat/envelope (CH2)
(vide supra).
The next lowest in energy shows both rings in a boat confor-
mation (Table 5, conformation A), which is exactly the structure
displayed by cis compounds 5 and 6 in the solid state. The asym-
metric conformation F, has been observed in crystal structures of
the free ligands SPANiPr and SPANtBu. Conformation G, the
highest energy backbone conformation, in which the phosphorus
substituents are at a large distance, was observed for bimetallic
compound 8. In this fashion the interaction between the two
(cod)Rh(PR2)Cl fragments is minimized, although the backbone
strain is higher than in the other conformations. In summary,
all the possible conformations of the spirobichroman moiety
displayed in the calculated minima have been observed in crystal
structures of either free ligands or SPAN-containing complexes.
Thus, although four out of six atoms are retained in one plane and
a rigid spiro center holds the two rings together the flexibility is
high.
We have calculated the distances between the two hydrogen
atoms in the spirobichroman backbone that become phosphorus
atoms in SPANphos. Indeed, the distances in boat/boat confor-
mations are much shorter than in the envelope conformations (see
Table 5). Still, considerable distortions are needed to fit either
a cis or a trans complex. In addition to the P–P distance one
should also consider the resulting angle that the lone pairs of the
donor atoms adopt in these conformations. If metal coordination
requires a deviation from these values it will also imply further
distortions of the backbone. Accuracy of the molecular mechanics
calculations of organometallics are hampered by the fact that not
many parameters have been introduced in such programs and often
estimated parameters are used. In natural bite angle calculations3,39
it is assumed that it suffices to set the P–M–P bending frequency
to zero and that other parameters are available to do an accurate
MM2 calculation of the backbone energies. This would be true
if only organic backbone atoms were involved, which are indeed
accurately parameterized. Still several other force constants play a
role and large errors may result. M–P–C bending force constants
and dihedral force constants involving metal or phosphine should
have relatively trustworthy values when the “natural” angles of the
backbone don’t fit the metal ligand bond.40 Furthermore, due to
changes in electronics, for instance when considering cis and trans
positions, the force constants will not be as general as they are in
organic compounds.41
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Table 6 Relative energy (kcal.mol−1) of cis/trans complexes of (SPANphos)MCl2 (M = Pd, Pt) and (SPANphos)RhCl(CO)
(SPANphos)MCl2
(SPAN phos)RhCl(CO)
Relative energy/kcal mol−1
trans
cis
trans
cis
M = Pd
M = Pd
M = Pt
b angle/◦
M = Pd
M = Pd
M = Pd
SPAN-PH2
Full system
Full system
2.4
0.0
0.0
trans
159.2
171.6
171.5
0.0
14.8
9.5
0.0
0.0
2.2
17.9
cis
trans
156.6
170.0
cis
92.6
98.4
SPAN-PH2
Full system
Full system
94.9
102.9
100.7
In order to estimate how much energy it might cost to enforce
SPANphos to act as a cis ligand we have calculated the energies
of the cis and trans isomers of two previously reported trans
complexes, (SPANphos)PtCl2 and (SPANphos)Rh(CO)Cl, and
a hypothetical (SPANphos)PdCl2, using a DFT method. DFT
calculations were also done for the complexes containing PH2
as a model ligand instead of PPh2 in SPANphos. On the DFT
level of calculation and the use of PH2 model substituents the
energies of the cis and trans isomers are very similar, trans being
more stable for rhodium and cis for palladium. In these PH2
models the lack of the steric bulk introduced by the phosphine
substituent allows us to analyze the backbone strain and the
cis/trans preference separately. Using an equivalent methodology,
it was found previously that for PtCl2(PH3)2 in the gas phase
the trans complex was consistently more stable, and only when
solvents were included13 a preference for cis was found. In our
case for PH2 model ligands, the cis/trans energy difference is quite
small, and this suggests that the backbone strain in both isomers
is similar. Due to the small energy difference, it may be concluded
that in a polar environment also our PH2 models will prefer a
cis configuration. Calculations on the full complexes revealed the
additional effect of the phenyl phosphine substituents. In the gas-
phase a preference for the trans configuration by 9.5 kcal.mol−1
for platinum dichloride, 14.8 kcal.mol−1 for palladium dichloride
and 18 kcal mol−1 for the rhodium chloride monocarbonyl was
found. The phenyl substituents affect stability and geometries as
well. Note how the bite angle changed when the full systems
were considered (Table 6). For the trans platinum dichoride
complex the agreement between X-ray parameter (171.9◦) and
the computed geometry (171.5◦) is excellent. Harvey calculated
for PtCl2(PPh3)2 that the trans isomer should be 4.5 kcal.mol−1
more stable in the gas phase. For SPANphos we calculate a
difference of 9.5 kcal.mol−1, part of which may result from the
interaction of the neighbouring phenyl groups in the cis complex.
From these data we conclude that at most 10 kcal.mol−1 backbone
strain may be needed to enforce cis coordination of SPANphos.
Normally such a difference would preclude the formation of such
a complex, but the presence of another strongly bound ligand such
as norbornadiene to cationic Rh(I) suffices to obtain cis complexes.
A second provision to be made is that both calculations and
experiments supply ample evidence that in polar solvents cis com-
plexes are by far more stable, making cis complexes more readily
accessible.
plexes when a bidentate ligand is used that can assume the trans
conformation. The non-reactivity of trans-(SPANphos)Pd(CH3)+
in the common insertion chemistry of palladium cis diphosphine
complexes suggested that cis configurations were not in reach
energetically.17 Catalytic activity drops rapidly even when only
a few kcal mol−1 are added as extra barrier. In view of the
above results and the observations by Eberhard et al.15 it may
be worthwhile to investigate whether the absence of activity is due
to a high barrier of cis–trans isomerization, or a high energy of the
cis complex.
Conclusions
The conformations found in the analyzed structures show that
SPANphos ligands are highly flexible and can adopt different
orientations depending on the needs of the formed molecules or
complexes. The flexibility of the spirobichroman backbone is well-
known, but initially we had thought that by the introduction of
the bulky PR2 only trans or nearly trans complexes or bimetallic
with transitions metals would be obtained. If the complex to be
formed due to either a fixed geometry, (nbd)Rh+ providing an
extreme case, or an intrinsic preference for cis complexes of some
transition metal complexes, the spirobichroman ligand will adopt
a conformation enabling the required orientation to form the cis
structure. Everything being equal, SPANphos retains a preference
for trans complexes, but the energetic preference is not as high as
we initially thought.
From a viewpoint of reactivity in catalysis the flexibility of the
ligands may be even more interesting, as too rigid complexes
may not be willing to undergo any reaction, as for instance the
absence of oxidative addition of MeI to (SPANphos)Rh(CO)Cl
or insertion reaction of palladium(II) SPANphos complexes. At
present we are investigating ligands that may be more strongly
trans directing than SPANphos and the reaction with (nbd)2Rh+
reported above provides a quick test to see whether we are on the
right track!
References
1 P. C. J. Kamer, P. W. N. M. van Leeuwen and J. N. H. Reek, Acc. Chem.
Res., 2001, 34, 895.
2 T. J. Devon, G. W. Phillips, T. A. Puckette, J. L. Stavinoha and J. J.
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2 7 8 | Dalton Trans., 2006, 268–278
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