Phenyl{bis(triphenylphosphanemethylenido)}borane: A zwitterionic chelating ligand exhibiting allyl-type coordination

Article abstract of DOI:10.1002/anie.200351056

When complexed to transition-metal atoms, the zwitterionic ligand phenyl-{bis(triphenylphosphanemethylenido)}borane behaves like a neutral analogue of the η³allyl ligand. Syntheses and X-ray crystal structures of the ligand and its complexes with ZrCl₄ and PdCl₂ (depicted) are described (Pd = cyan, B = yellow, P = pink, Cl = green, C=white).

Full text of DOI:10.1002/anie.200351056

Angewandte
Chemie
A Zwitterionic Chelating Ligand
product from the reaction intended to bond Ph3PCH2 to a
borylidene bridged bis(indenyl)zirconium species similar to
the ones reported by Reetz et al.[5] The unusual allyl-type
coordination between the boron-bridged diylide ligand and
zirconium prompted us to pursue the synthesis of the ligand
by a more direct route to determine if additional coordination
chemistry with this novel, zwitterionic, chelating ligand might
be accessed.
Phenyl{bis(triphenylphosphanemethylenido)}-
borane: A Zwitterionic Chelating Ligand
Exhibiting Allyl-Type Coordination**
Feilong Jiang, Pamela J. Shapiro,* Faysal Fahs, and
Brendan Twamley
Boron-bridged diylide compounds similar to the ligand in
1 were reported in 1986 by Bestmann and Arenz,[6] who
prepared the compounds by treating alkyldichloroboranes
with four equivalents of phosphonium ylide. Related tri-
phenyl- and tri-tert-butyl-phosphiniminatoborane derivatives
were reported by Dehnicke and co-workers[7] and by Stephan
and co-workers.[8] Until now, however, the coordination of
these molecules to metal atoms has not been reported.
Initial efforts to prepare the ligand in complex 1 by
treating PhBCl2 with either four equivalents of Ph3PCH2 or
with two equivalents of the ylide in the presence of triethyl-
amine were unsuccessful and afforded only the ylide adduct
PhCl2B(CH2PPh3). Therefore, we used the lithium salt of the
ylide[9] to prepare the desired ligand 2 as a benzene-soluble
yellow powder in 68% yield [Eq. (1)].
The ability of phosphorus ylides to form stable organometallic
complexes with both main-group metal atoms and transition-
metal atoms has been known for some time.[1] The dipolar
resonance form available to these molecules, by placing a
formal negative charge on the carbon, makes them very
effective Lewis bases that in some cases have been substituted
for phosphane ligands to form even more active transition-
metal organometallic catalysts.[2]
Recent work in our group employing Ph3PCH2 as a Lewis
base adduct for the boron in borylidene bridged ansa-
zirconocene complexes[3] led to the serendipitous discovery
of an unusual bonding motif between a boron-bridged
bis(ylide) ligand and zirconium tetrachloride. Complex 1
(Figure 1)[4] was isolated reproducibly with a yield of 14% as a
Two independent but structurally similar molecules were
found in the asymmetric unit of the crystal (Figure 2).[4] In
both molecules of 2 one triphenylphosphane group adopts a
syn orientation and the other an anti orientation relative to
the boron-diylide backbone instead of the double syn
arrangement that they exhibit in the zirconium complex.
À
The B C(ylide) bond lengths (av. 1.521(7) Š) are at the low
À
end of the 1.50–1.64 Š range for B C single bonds in the
Cambridge Crystallographic Database[10] and noticeably
À
shorter than the B C(phenyl) bond in each molecule (av.
À
1.598(2) Š). The P C(ylide) bond lengths (av. 1.689(2) Š) are
Figure 1. ORTEP drawing of complex 1 with thermal ellipsoids at the
À
30% probability level. Selected bond lengths [Š] and angles [8]: Zr1 B1
À
À
À
2.754(4), Zr1 C19 2.434(3), Zr1 C26 2.469(3), Zr Cl1 2.4068(11),
À
À
À
À
Zr1 Cl2 2.4360(11), Zr1 Cl3 2.4980(11), Zr1 Cl4 2.4076(12), B1 C19
À
À
À
À
1.544(6), B1 C26 1.532(5), B1 C20 1.587(5), P1 C19 1.748(3), P2
C26 1.734(4), C26-B1-C19 110.0(3), C26-Zr-C19 61.83(12), Cl1-Zr-Cl3
179.23(4), Cl2-Zr-Cl4 106.64(4).
[*] Prof. P. J. Shapiro, Dr. F. Jiang, F. Fahs, Dr. B. Twamley
Department of Chemistry
University of Idaho
Moscow, ID 83844–2343 (USA)
Fax: (+1)208-885-5785
E-mail: shapiro@uidaho.edu
Figure 2. ORTEP drawings of two independent molecules of 2. Ther-
mal ellipsoids are at the 30% probability level. Selected bond lengths
À
À
À
[**] This work was supported by The Dow Chemical Company and
partially by the donors of the Petroleum Research Fund, adminis-
tered by the American Chemical Society. We are grateful to Dr. Alex
Blumenfeld for his assistance with NMR data acquisition.
[Š] and angles [8]: B1 C19 1.513(5), B1 C26 1.530(5), B1 C20
À
À
À
À
1.595(5), B2 C63 1.522(5), B2 C70 1.518(5), B2 C64 1.600(5), P1
À
À
À
C19 1.691(3), P2 C26 1.689(3), P3 C63 1.693(3), P4 C70 1.682(3),
C19-B1-C26 124.1(3), C63-B2-C70 122.9(3).
Angew. Chem. Int. Ed. 2003, 42, 2651 – 2653
DOI: 10.1002/anie.200351056
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Communications
À
longer than the P C bond of free Ph3PCH2 (1.661(8) Š),
À
consistent with a reduction in P C(ylide) p-bonding. These
À
bonds are still substantially shorter than the P C(ylide) bond
length of 1.7919(14) Š in the donor–acceptor complex
Ph3PCH2B(C6F5)3.[11] A PM3 calculation on the molecule
gave an energy optimized conformation with both phospho-
nium moieties in syn positions. The HOMO of the molecule
consists almost entirely of nonbonding p-orbital density on
the ylide carbons, which should be conducive to their
coordination to metals. The HOMO-1 is p-bonding between
the boron and these two carbons, with no contribution from
either phosphorus atom. Significantly, the HOMO and
HOMO-1 orbitals for the ligand are essentially identical in
appearance to the corresponding molecular orbitals of an allyl
ligand.[12]
Complex 1 was prepared directly by reacting 2 with
ZrCl4(SMe2)2 in toluene [Eq. (2)]. The complex is exception-
Figure 3. ORTEP drawing of complex 3 with thermal ellipsoids at the
À
30% probability level. Selected bond lengths [Š] and angles [8]: Pd1
À
À
À
C26 2.140(4), Pd1 C19 2.094(4), Pd1 B1 2.200(5), Pd1 Cl1
À
2.3705(11), Pd1 Cl2 2.3604, Cl1-Pd1-Cl2 93.45(4), P1-C19-Pd 112.8(2),
P2-C26-Pd 128.8(2).
does in the zirconium complex (0.08 Š av. difference in 3
versus 0.3 Š av. difference in 1). The PdII complex is also
considerably more stable than the ZrIV complex both in
solution and upon storage as a solid under N2 at ambient
temperature.
ally sensitive to protonolysis, forming [Ph3PCH3]Cl, and must
be stored at low temperature (À308C) under N2. Another X-
ray crystallographic analysis of the complex was undertaken
to confirm that the structure of this complex was identical to
that of the species obtained earlier by an indirect route.
The structure of 1 (Figure 1) can be regarded as pseudo-
octahedral, with the ylide carbons occupying two sites of an
equatorial plane, Cl2and Cl4 at the other two equatorial
positions, and Cl1 and Cl3 at trans axial positions. Although
In summary, ligand 2 behaves like a boron-containing
analogue of a h3-allyl ligand when attached to both early and
late transition-metal systems. As a neutral, zwitterionic
ligand, 2 offers an interesting alternative to other neutral
bidentate ligands containing phosphanes,[14] imines,[15] and
nucleophilic carbenes[16] that are currently popular in tran-
sition-metal-mediated catalysis. Furthermore, it should be
possible to fine-tune the electronic and steric properties of the
ligand by varying the substituent on boron and the phospho-
nium ylide components.
À
the Zr B distance of 2.754(4) Š is significantly longer than
distances between the zirconium and the ylide carbon atoms
(av. 2.45(2) Š), it is well within the sum of the van der Waals
radii of the two atoms.[13] Ab initio calculations at the 3-21G*
level on the model complex [(H3PCH)2BH]ZrCl4 affords an
energy optimized geometry that is remarkably close to that of
2. When an initial geometry that places the Zr, B, and ylide
carbon atoms in the same plane is used, the final optimized
Experimental Section
All manipulations were carried out under an argon or nitrogen
atmosphere. NMR spectra were recorded on Bruker AMX 300 and
Bruker AVANCE 500 spectrometers. H3PO4(aq, 80%) and BF3·Et2O
were used as external references for 31P and 11B NMR assignments,
respectively. 1H chemical shifts were referenced to residual proton
signals from the deuterated solvent. 13C NMR chemical shifts were
À
geometry bends the boron toward zirconium with a Zr B
À
distance of 2.754 Š and Zr C(ylide) distances of 2.453 Š.
also referenced to the solvent peaks. Ph3PCH2[17] and Zr(SMe2)2Cl4
[18]
To determine if ligand 2 and its allyl-type binding mode
could be extended to other metal atoms, 2 was reacted with
trans-[PdCl2(SMe2)2] to give 3 (Figure 3). An allyl-type
bonding motif between the ligand and the palladium is
again observed. In this case, however, one phosphonium
group is anti and the other is syn, making the complex
asymmetric. The presence of two different, weakly coupled
phosphorus signals in the 31P NMR spectrum, and the
presence of two different methine proton signals with differ-
ent couplings to phosphorus in the 1H NMR spectrum
indicate that the asymmetric geometry of the complex is
retained in solution. The boron atom approaches the metal
center more closely relative to the ylide carbons in 3 than it
were prepared as described in the literature. Elemental analyses were
determined by Desert Analytics (Tuscon, AZ). Theoretical calcula-
tions were performed with the MacSpartan Plus program (1996,
Wavefunction, Inc., Irvine CA).
1: A solution of Zr(SMe2)2Cl4 (373 mg, 1.04 mmol) in 30 mL toluene
was added to a stirring solution of 2 (667 mg, 1.04 mmol) in toluene
(150 mL) at À788C. The reaction was allowed to warm to room
temperature and a light yellow precipitate began to form. After 18 h,
the precipitate was isolated by filtration and dried in vacuo (yield:
1
736 mg, 74%). H NMR (300 MHz, CDCl3, 2 58C): d = 7.60–7.54 (m,
12H; PC6H5), 7.51–7.45 (m, 6H; PC6H5), 7.38–7.33 (m, 12H; PC6H5),
6.63 (tt, 3J(H,H) = 7.3 Hz, 4J(H,H) = 1.5 Hz, 1H; BC6H5) 6.42(dd„
4
3
3J(H,H) = 6.8 Hz, J(H,H) = 1.5 Hz, 2H; BC6H5), 6.30 (t, J(H,H) =
7.4 Hz, 2H; BC6H5) 4.23 ppm (d, 2J(P,H) = 7.9 Hz, 2H; BCHP);
2652
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Angew. Chem. Int. Ed. 2003, 42, 2651 – 2653

Angewandte
Chemie
13C{1H} NMR (125.7 MHz, CDCl3, 2 85C): d = 134.5 (pseudo t,
J(C,P) = 4.9 Hz; P(C6H5)3), 134.1 (d, J(C,P) = 10.4 Hz; P(C6H5)3),
133.0 (s, B(C6H5)), 131.2(d, J(P,C) = 12.5 Hz; P(C6H5)3), 129.3
(pseudo t, J(C,P) = 6.0 Hz; P(C6H5)3), 126.5 (s; B(C6H5)), 125.8 ppm
(s; B(C6H5)). No 13C resonance was observed for the ylide methine
carbons although a cross peak at d = 51 ppm with the ylide methine
P1, a = 11.199(2), b = 12.298(3), c = 17.622(4) Š, a = 85.84(3),
ꢀ
b = 84.40(3), g = 68.07(3)8, V= 2238.8(10), Z = 2, 1calc
=
1.419 mgmÀ3, 2qmax = 56.88, m = 0.706 mmÀ1, 496 parameters,
11106 independent reflections (Rint = 0.0669), 7779 with I >
¯
2(s)I, R1 = 0.0597, wR2 = 0.1276. 2: 203 K, triclinic, P1, a =
9.925(2), b = 19.215(4), c = 19.233(4) Š, a = 102.03(3), b =
hydrogens was observed in
a
heteronuclear multiple-quantum
92.66(3), g = 103.61(3)8, V= 3469.0(14), Z = 4, 1calc
=
coherence (HMQC) experiment. 31P{1H} (121.4 MHz, CD2Cl2,
258C): d = 27.0 ppm; 11B NMR (160.4 MHz, CD2Cl2, 2 85C): d =
33 ppm. X-ray quality crystals of 1 were grown from a concentrated
CH2Cl2 solution cooled at À308C for several days. Elemental analysis
calcd (%) for C44H37BCl4P2Zr·CH2Cl2: C 55.97 , H 4.07; found
C 56.03, H 4.12.
1.223 mgmÀ3 2qmax = 508, m = 0.156 mmÀ1
,
,
847 parameters,
12207 independent reflections ( Rint = 0.0819), 6826 with I >
2(s)I, R1 = 0.0629, wR2 = 0.1174. 3·CH2Cl2: 82K, monoclinic,
P21/n, a = 11.479(2), b = 17.301(4), c = 20.108(4) Š, b =
91.64(3)8, V= 3992.1(14), Z = 4, 1calc = 1.499 mgmÀ3, 2qmax
55.28, m = 0.846 mmÀ1, 478 parameters, 9215 independent reflec-
tions (Rint = 0.0939), 6758 with I > 2(s)I, R1 = 0.0560, wR2 =
0.1110. Hydrogen atoms on C19, C26 (1, 2) and C63 and C70
(2) were located on the difference map and then added
=
2: PhBCl2 (635 mg, 4.00 mmol) was injected by syringe into a stirring
solution of Ph3PCHLi (2.26 g, 8.02 mmol) in toluene (120 mL) at
À788C. The reaction mixture was stirred for 8 h at room temperature
affording an orange suspension that was isolated by filtration, washed
with pentane (2” 30 mL) and dried in vacuo (yield: 1.74 g, 68%).
1H NMR (300 MHz, C6D6, 2 58C): d = 7.7–7.6 (br m,14H; C6H5), 7.0–
6.9 (m, 21H; C6H5), 1.50 ppm (d, 2H, 2J(P,H) = 13.8 Hz; PCH);
13C{1H} NMR (125.7 MHz, C6D6, 2 58C): 134.9 (d, J(C,P) = 9.5 Hz),
133.3 (s), 131.3 (d, J(C,P) = 2.8 Hz), 130.7 (d, J(C,P) = 2.5 Hz) 129.0
(d, J(C,P) = 12 Hz), 127.0 (s), 124.9 ppm (s). Avery broad 13C signal at
d = 34 ppm is attributed to the ylide methine carbons. 31P{1H} NMR
(121.4 MHz, C6D6, 2 58C): d = 18.2ppm; 11B NMR (160.4 MHz, C6D6,
258C): d = 50 ppm. X-ray quality crystals of 2 were obtained by
layering pentane over a concentrated benzene solution of the ligand
and storing the mixture at 48C for several days. Elemental analysis
calcd (%) for C44H37BP2: C 82.76, H 5.84; found: C 2.70, H 5.88.
3: Toluene (120 mL) was added to a mixture of Pd(SMe2)2Cl2
(483 mg, 1.60 mmol) and 2 (1.02g, 1.60 mmol) and the reaction
mixture was stirred for 36 h at room temperature. A dark-orange
slurry formed, and the precipitate was isolated by filtration, washed
with toluene (30 mL), and dried under vacuum to afford a light brown
geometrically using a riding model. There are residuals of 1.68
À3
and 1.03 eŠ
ca. 1 Š from the Zr center in 1·CH2Cl2.
CCDC 202606–202608 (1–3, respectively) contain the supple-
mentary crystallographic data for this paper. These data can be
obtained free of charge via www.ccdc.cam.ac.uk/conts/retrie-
ving.html (or from the Cambridge Crystallographic Data Centre,
12, Union Road, Cambridge CB21EZ, UK; fax: (+ 44)1223-
336-033; or deposit@ccdc.cam.ac.uk).
[5] M. T. Reetz, M. Willuhn, C. Psiorz, R. Goddard, J. Chem. Soc.
Chem. Commun. 1999, 1105.
[6] H.-J. Bestmann, T. Arenz, Angew. Chem. 1986, 98, 571; Angew.
Chem. Int. Ed. Engl. 1986, 25, 559.
[7] M. Möhlen, B. Neumüller, K. Dehnicke, Z. Anorg. Allg. Chem.
1998, 624, 177.
[8] S. Courtenay, J. Y. Mutus, R. W. Schurko, D. W. Stephan, Angew.
Chem. 2002, 114, 516; Angew. Chem. Int. Ed. 2002, 41, 498.
[9] E. J. Corey, J. Kang, J. Am. Chem. Soc. 1982, 104, 4724.
[10] F. H. Allen, O. Kennard, Chem. Des. Autom. News 1993, 8(1), 1;
F. H. Allen, O. Kennard, Chem. Des. Autom. News 1993, 8(1), 31.
[11] S. Döring, G. Erker, R. Fröhlich, O. Meyer, K. Bergander,
Organometallics 1998, 17, 2183.
[12] R. H. Crabtree, The Organometallic Chemistry of the Transition
Metals, 3rd. ed., John Wiley & Sons, New York, 2001, p. 122.
[13] A. Bondi, J. Phys. Chem. 1964, 68, 441.
[14] J. Ledford, C. S. Schultz, D. P. Gates, P. S. White, J. M. DeSi-
mone, M. Brookhart, Organometallics 2001, 20, 5266.
[15] S. D. Ittel, L. K. Johnson, M. Brookhart, Chem. Rev. 2000, 100,
1169.
[16] a) W. A. Herrmann, C.-P. Reisinger, M. Spiegler, J. Organomet.
Chem. 1998, 557, 93; b) A. J. Arduengo, T. Bannenberg, Strem
Chem. 2002, XVIV, 1, and references therein.
1
solid (yield: 758 mg, 58%): H NMR (300 MHz, CD2Cl2, 2 58C): d =
8.0–7.8 (m, 5H; C6H5), 7.7–7.3 (m, 28H; C6H5), 6.99–6.91 (m, 2H;
2
2
C6H5), 3.10 (d, J(P,H) = 13.3 Hz, 1H; PCH), 3.05 ppm (d, J(P,H) =
4.2Hz, 1H; PC H). 13C{1H} (125.7 MHz, CD2Cl2, 2 58C): d = 135.94 (d,
J(C,P) = 3.0 Hz), 135.15 (d, J(C,P) = 9.8 Hz), 134.63 (d, J(C,P) =
11 Hz), 133.98 (d, J(C,P) = 2.9 Hz), 133.21 (d, J(C,P) = 2.6 Hz),
133.0, 131.2(d, J(C,P) = 13 Hz), 129.85 (d, J(C,P) = 12Hz), 129.2
(br d, J(C,P) = 11 Hz) 128.0 (s), 126.9 (s), 126.5 (s), 125.8 ppm (s). No
13C resonance was observed for the ylide methine carbons although a
cross peak at d = 31 ppm with the ylide methine hydrogens was
observed in an HMQC experiment. 31P{1H} NMR: (121.4 MHz,
4
4
CD2Cl2, 2 58C): d = 29.0 (d, J(P,P) = 3.1 Hz), 26.0 ppm (d, J(P,P) =
3.3 Hz). 11B NMR (160.4 MHz, CD2Cl2, 2 58C): d = 35 ppm. X-ray
quality crystals of 3 were grown from a CH2Cl2 solution cooled at
À308C for several days. Elemental analysis calcd (%) for
C44H37BCl2P2Pd·CH2Cl2: C 60.00, H 4.33; found: C 58.01, H 4.46.
[17] H. Schmidbaur, H. Stuhler, W. Vornberger, Chem. Ber. 1972,
105, 1084.
[18] E. C. Lund, T. Livinghouse, Organometallics 1990, 9, 2426.
Received: January 30, 2003 [Z51056]
Keywords: allyl ligands · boranes · palladium · ylides · zirconium
.
[1] H. Schmidbaur, Angew. Chem. 1983, 95, 980; Angew. Chem. Int.
Ed. Engl. 1983, 22, 907, and references therein.
[2] a) K. Alexander, O. Starzewski, J. Witte, Angew. Chem. 1988,
100, 861; Angew. Chem. Int. Ed. Engl. 1988, 27, 839; b) K.
Alexander, O. Starzewski, L. Born, Organometallics 1992, 11,
2701.
[3] P. J. Shapiro, Eur. J. Inorg. Chem. 2001, 321, and references
therein.
[4] Crystal data for 1·CH2Cl2 and 2 and 3·CH2Cl2 was collected by
using a Bruker SMART APEX diffractometer with monochro-
mated MoKa radiation (l = 0.71073 Š): 1·CH2Cl2: 81 K, triclinic,
Angew. Chem. Int. Ed. 2003, 42, 2651 – 2653
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