A trans-spanning diphosphine ligand based on a m-terphenyl scaffold and its palladium and nickel complexes

Article abstract of DOI:10.1021/om049662d

The new diphosphine ligand 2,6-bis(2-((diphenylphosphino)methyl)phenyl)benzene (1) has been prepared for potential use as a terdentate pincer-type ligand, Upon examination of the coordination chemistry with representative palladium and nickel centers, however, it was discovered that it acts as a bidentate trans-spanning ligand upon coordination to form trans-[(1)PdCl₂] (2) and trans-[(1)NiCl₂] (3). Attempts to promote CH activation of the central benzene ring and produce a terdentate pincer binding mode for 1 were unsuccessful. Reaction of 2 and [Li(OEt₂)][B(C₆F ₆)₄] led to formation of the dicationic bis(chloro)-bridged dimer [(1)₂{Pd₂(μ ₂-Cl)₂}][B(C₆F₅)₄] ₂ (4), in which two ligands 1 have rearranged to span across the dichlorodipalladium core. The structures of air- and moisture-stable 2-4 were confirmed by X-ray crystallography.

Full text of DOI:10.1021/om049662d

Organometallics 2004, 23, 4215-4222
4215
A Tr a n s-Sp a n n in g Dip h osp h in e Liga n d Ba sed on a
m -Ter p h en yl Sca ffold a n d Its P a lla d iu m a n d Nick el
Com p lexes
Rhett C. Smith and J ohn D. Protasiewicz*
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106-7708
Received May 13, 2004
The new diphosphine ligand 2,6-bis(2-((diphenylphosphino)methyl)phenyl)benzene (1) has
been prepared for potential use as a terdentate pincer-type ligand. Upon examination of
the coordination chemistry with representative palladium and nickel centers, however, it
was discovered that it acts as a bidentate trans-spanning ligand upon coordination to form
trans-[(1)PdCl2] (2) and trans-[(1)NiCl2] (3). Attempts to promote CH activation of the central
benzene ring and produce a terdentate pincer binding mode for 1 were unsuccessful. Reaction
of 2 and [Li(OEt2)][B(C6F5)4] led to formation of the dicationic bis(chloro)-bridged dimer
[(1)2{Pd2(µ2-Cl)2}][B(C6F5)4]2 (4), in which two ligands 1 have rearranged to span across the
dichlorodipalladium core. The structures of air- and moisture-stable 2-4 were confirmed
by X-ray crystallography.
In tr od u ction
tions, hydrogen transfer, dehydrogenation, hydro-
amination, and polymerizations.7
The utility of phosphines as ligands in metal com-
plexes, and particularly the effect of phosphine selection
on catalysis, has made the synthesis and investigation
of novel phosphines an area of ongoing interest. Some
key phosphine properties to be considered are cone
angle,1 bite angle,2 and basicity.3 For example, struc-
tural and catalytic effects of unusually bulky phosphines
are a topic of current investigation.4 There have also
been a number of studies on wide bite angle2 and trans-
spanning bidentate phosphines5 which have demon-
strated advantageous effects on a number of catalytic
processes.2,5,6 Simple m-xylyl-anchored terdentate “pin-
cer” ligands (Scheme 1; A, D ) various donors such as
PR2, NR2, etc.) have attracted particular interest.7 One
advantage of such systems is that the preparation of
ligands featuring a variety of donor elements is readily
accomplished by starting from R,R′-dibromo-m-xylene
(Scheme 1, top left). Metal complexes featuring pincer
ligands have been employed in a number of studies such
as C-C, C-H, C-N, and C-Si bond activation, as gas
sensors, and as molecular switches. Catalytic applica-
tions studied include Heck and Suzuki coupling reac-
Although pincer ligands have been fruitfully employed
and exhibit rigid terdentate binding modes, such ligands
often lack an inherent three-dimensional ligand back-
bone that may be desirable for tuning catalytic pro-
cesses. The m-terphenyl scaffold has been used to access
a variety of interesting molecular geometries and coor-
dination environments.8 Recently Rabe has reported the
first pincer type ligands having a m-terphenyl backbone
(B with D ) OMe; Scheme 1) and their application in
the preparation of lanthanide complexes.9 As opposed
to following the process(es) outlined in Scheme 1 that
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10.1021/om049662d CCC: $27.50 © 2004 American Chemical Society
Publication on Web 07/29/2004

4216 Organometallics, Vol. 23, No. 18, 2004
Smith and Protasiewicz
Sch em e 1. P r ep a r a tion a n d Rea ctivity of (Left)
P in cer Liga n d s a n d (Righ t) P ossible P a r a llel
Tr a n sfor m a tion s w ith Ter p h en yl-Ba sed Liga n d sa
exhibit properties similar to those of the m-xylyl-based
pincer ligands but will have a more defined three-
dimensional backbone and a possibly C2-symmetric
(chiral) geometry. Herein is reported the synthesis of a
m-terphenyl-anchored diphosphine ligand and its use
in preparing new Ni and Pd chloride complexes. Struc-
tural characterizations of three of these complexes re-
veal a surprising flexibility in the ligand binding mode.
Exp er im en ta l Section
All manipulations were carried out in a drybox under an
atmosphere of N2. Anhydrous dichloromethane was purchased
from Acros and used as received. Acetonitrile was distilled
from CaH2 under nitrogen; all other solvents were distilled
from sodium benzophenone ketyl prior to use. Literature
routes were used to prepare 2,2′′-dimethyl-m-terphenyl,11a
2,2′′-bis(bromomethyl)-m-terphenyl,11c and PdCl2(NCPh)2.11f
[Li(OEt2)2.5][B(C6F5)4] was purchased from Boulder Scientific.
NMR spectra were recorded on a Varian Gemini instrument
operating at 300 MHz for proton measurements and 121.5
MHz for phosphorus. Proton and phosphorus spectra are
referenced to residual solvent signals or 85% phosphoric acid,
respectively. Variable-temperature 31P NMR spectra were
recorded in CD2Cl2 between -80 and +55 °C at a field strength
of 121.5 MHz. Variable-temperature 1H NMR spectra were
recorded in CDCl3 between -45 and +55 °C on a Varian Inova
600 MHz NMR instrument.
2,6-{2-(P h 2P CH2)C6H4}2C6H3 (1). To a solution of diphenyl-
chlorophosphine (3.05 g, 13.8 mmol) in THF (30 mL) was added
freshly cut Li metal (0.200 g, 31.2 mmol). The resultant
solution was stirred for 4 h at room temperature, over which
time the mixture became warm, lithium was consumed, and
a red color developed. The resultant red solution was decanted
from the excess Li pieces and cooled to -78 °C. To the chilled
solution was added a solution of 2,2′′-bis(bromomethyl)-m-
terphenyl (2.50 g, 6.01 mmol) in THF (30 mL) via cannula.
The resultant solution was warmed to room temperature and
stirred for an additional 6 h. All volatiles were removed in
vacuo, and the residue was taken up in 40 mL of diethyl ether.
This solution was washed with two 10 mL aliquots of water
and then dried over Na2SO4. Removal of volatiles under
reduced pressure produced a white powder. Final purification
was affected by recrystallization from a saturated CH2Cl2
solution at -35 °C to give 1 as white needlelike crystals (2.82
g, 60.3%). 1H NMR (CDCl3): δ 3.44 (s, 4H), 7.05-7.24 (m, 31H),
7.31 (t, 1H, J ) 7 Hz). 31P NMR (CDCl3): δ -8.8. Mp: 148-
151 °C.
[(1)P d Cl2] (2). A solution of 1 (0.279 g, 0.449 mmol) and
PdCl2(NCPh)2 (0.144 g, 0.449 mmol) in CH2Cl2 (30 mL) was
stirred at room temperature for 10 min. All volatiles were
removed under reduced pressure. The solid was rinsed with
n-pentane, and then diethyl ether, to afford 2 as a yellow
powder (0.349 g, 97.2%). Analytically pure crystalline 2 was
obtained by diffusion of tetramethylsilane into a saturated
dichloromethane solution. 1H NMR (C6D6): δ 3.06 (d, 2H, J )
12 Hz), 4.80 (d, 2H, J ) 12 Hz), 5.96 (d, 2H, J ) 8 Hz), 6.68 (t,
1H, J ) 8 Hz), 7.10-7.31 (m, 28H), 8.02 (s, 1H). 31P NMR
(CDCl3): δ 16.1. Anal. Calcd for C44H36Cl2P2Pd: C, 65.73; H,
4.51. Found: C, 65.58; H, 4.36.
a
D ) various donor groups such as NR2, PR2, OR, etc.
involve activation of the CH bond of a benzene ring,
successful entry into these complexes was affected by
use of the lithium salt of the 2,6-(o-anisyl)2C6H3 ligand.
The simple synthetic methodology10 for the prepara-
tion of m-terphenyls affords an opportunity for their
development as attractive scaffolds upon which to build
ligand frameworks. Specifically, by tethering metal-
coordinating substituents to each of the flanking aryl
rings (Scheme 1, right), ligands may be elaborated that
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1975, 584-585. (f) March, F. C.; Mason, R.; Thomas, K. M.; Shaw, B.
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Zambonelli, L.; Venanzi, L. M. Helv. Chim. Acta 1977, 60, 2815-2823.
(h) Bachechi, F.; Zambonelli, L.; Venanzi, L. M. Helv. Chim. Acta 1977,
60, 2815-2823. (i) Kamer, P. C. J .; van Leeuwen, P. W. N. M.; Reek,
J . N. H. Acc. Chem. Res. 2001, 34, 895-904.
(6) (a) Shaw, B.; Perera, S. D. Chem. Commun. 1998, 1863-1864.
(b) van der Veen, L. A.; Kamer, P. C. J .; van Leeuwen, P. W. N. M.
Angew. Chem., Int. Ed. 1999, 38, 336-338. (c) van der Veen, L. A.;
Keeven, P. K.; Kamer, P. C. J .; van Leeuwen, P. W. N. M. Dalton 2000,
2105-2112.
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40, 3750-3781. (b) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003,
103, 1759-1792.
(8) (a) Twamley, B.; Haubrich, S. T.; Power, P. Adv. Organomet.
Chem. 1999, 1-65. (b) Clyburne, J . A. C.; McMullen, N. Coord. Chem.
Rev. 2000, 210, 73-99. (c) Robinson, G. H. Acc. Chem. Res. 1999, 32,
773. (d) Hart, H. Pure Appl. Chem. 1993, 65, 27-34.
[(1)NiCl2] (3). A solution of 1 (0.071 g, 0.11 mmol) and
NiCl2(DME)12 (0.025 g, 0.11 mmol) in CH2Cl2 (3 mL) was
allowed to stand at room temperature for 8 h, over which time
(9) (a) Rabe, G.; Zhang-Presse, M.; Riederer, F. A.; Yap, G. P. A.
Inorg. Chem. 2003, 42, 3527-3533. (b) Rabe, G. W.; Zhang-Presse, M.;
Golen, J . A.; Rheingold, A. L. Acta Crystallogr., Sect. E 2003, E59,
m1102-m1103. (c) Rabe, G. W.; Zhang-Presse, M.; Riederer, F. A.;
Golen, J . A.; Incarvito, C. D.; Rheingold, A. L. Inorg. Chem. 2003, 42,
7587-7592. (d) Rabe, G. W.; Rhatigan, B.; Golen, J . A.; Rheingold, A.
L. Acta Crystallogr., Sect. E 2003, E59, m99-m101. (e) Rabe, G. W.;
Zhang-Presse, M.; Yap, G. P. A. Acta Crystallogr., Sect. E 2002, E58,
m434-m435. (f) Rabe, G. W.; Berube, C. D.; Yap, G. P. A. Inorg. Chem.
2001, 40, 4780-4784.
(11) (a) Vinod, T. K.; Hart, H. J . Org. Chem. 1990, 55, 5461-5466.
(b) Voegtle, F.; Schunder, L. J ustus Liebigs Ann. Chem. 1969, 721,
129-132. (c) Hammerschmidt, E.; Bieber, W.; Voegtle, F. Chem. Ber.
1978, 111, 2445-2447. (d) J osel, H. P.; Loehr, H. G.; Engel, A.; Rapp,
J .; Voegtle, F.; Franken, S.; Puff, H., J . Incl. Phenom. 1985, 3, 43-50.
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Chem. Soc. 1938, 60, 882-884.
(10) Saednya, A.; Hart, H. Synthesis 1996, 1455-1458.

A Trans-Spanning Diphosphine Ligand
Organometallics, Vol. 23, No. 18, 2004 4217
Ta ble 1. Cr ysta l Da ta a n d Str u ctu r e Refin em en t Deta ils for 2-4
2
3
4
empirical formula
fw
C
44H36Cl2P2Pd
C44H36Cl2NiP2
756.28
298
0.710 73
monoclinic
P21/n
C
146H96B2Cl2F40P4Pd2
803.97
298
0.710 73
monoclinic
P21/n
3039.43
298
0.710 73
triclinic
P1h
temp (K)
wavelength (Å)
cryst syst
space group
unit cell dimens
a (Å)
13.123(2)
10.3739(18)
27.945(8)
90.00
101.182(12)
90.00
3732.2(14)
4
1.431
12.9651(18)
10.3559(15)
28.028(5)
90.00
101.978(14)
90.00
3681.2(10)
4
1.365
14.379(3)
15.2099(19)
16.078(2)
88.431(12)
82.665(15)
82.108(17)
3454.3(10)
1
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (A3)
Z
calcd density (Mg/m3)
1.461
0.450
1528
abs coeff (mm-1
F(000)
)
0.757
1640
0.790
1568
cryst size (mm)
0.30 × 0.24 × 0.12
yellow plate
2.74-48.00
-1 < h < 15
-11 < k < 1
-31 < l < 31
5848
0.34 × 0.24 × 0.16
violet block
2.74-48.00
-1 < h < 14
-1 < k < 11
-32 < l < 31
5738
0.28 × 0.14 × 0.10
yellow block
2.74-48.00
-1 < h <16
-17 < k < 17
-18 < l < 18
10 809
cryst color and shape
θ range, data collecn (deg)
limiting indices
no. of rflns collected
no. of indep rflns
refinement method
no. of data/restraints/params
goodness of fit on F2
final R indices (I > 2σ(I))a,b
R1
3768
3735
5547
full-matrix least squares on F2
5738/0/442
1.051
5848/0/437
1.083
10809/0/858
1.042
0.0533
0.0917
0.0469
0.0864
0.0787
0.1873
wR2
R indices (all data)
R1
wR2
0.1077
0.1115
0.0979
0.1075
0.1756
0.2327
a
b
2
2
R(F) ) ∑||Fo| - |Fc||/∑|Fo|. Rw(F2) ) [∑{w(Fo - Fc2)2}/∑{w(Fo2)2}]0.5; w-1 ) σ2(Fo2) + (aP)2 + bP, where P ) [Fo + 2Fc2]/3 and a and
b are constants adjusted by the program.
Sch em e 2. P r ep a r a tion of 1
deep violet crystals formed. The crystals were collected by
filtration and dried to afford 0.088 g of 3 (83%). 1H NMR
(CD2Cl2): δ 3.48 (virtual quartet, 4H, 2J + 4J ) 7 Hz), 6.02 (d,
2H, J ) 8 Hz), 6.77 (t, 1H, J ) 8 Hz), 7.00-7.60 (m, 28H),
8.93 (s, 1H). 31P NMR (CDCl3): δ 4.9. Anal. Calcd for C44H36
-
Cl2NiP2: C, 69.88; H, 4.80. Found: C, 69.71; H, 4.66.
[(1)2P d 2Cl2][B(C6F 5)4]2 (4). A solution of 2 (0.112 g, 0.140
mmol) and [Li(OEt2)2.5][B(C6F5)4] (0.122 g, 0.140 mmol) in
CH2Cl2 was allowed to stand at room temperature for 24 h,
over which time yellow crystals formed in the reaction vessel.
Cooling the mixture to -35 °C caused further crystallization.
The crystals were collected by filtration and dried in vacuo to
1
afford 4 (0.157 g, 74.0%). H NMR (CD2Cl2): δ 4.10 (m, 4H),
4.79 (m, 4H), 5.91 (d, 4H, J ) 8 Hz), 6.47 (t, 2H, J ) 8 Hz),
6.65, (s, 2H), 6.74-6.98 (m, 24H), 7.12-7.39 (m, 20H), 7.49-
7.59 (m, 8H), 7.81-7.89 (m, 4H). 31P NMR (CD2Cl2): δ 46.3.
Anal. Calcd for C136H72B2Cl2F40P4Pd2: C, 56.42; H, 2.51.
Found: C, 55.74; H, 2.24.
Resu lts a n d Discu ssion
The synthesis of 1 was carried out in 40% yield over
three steps, utilizing readily accessible starting materi-
als (Scheme 2). The key precursor, 2,2′′-bis(bromo-
methyl)-m-terphenyl,11c was chosen to allow for the
future development of other new ligands featuring
various donor elements upon reaction at the benzyl
bromide functionalities, by analogy to the methods used
to prepare previously reported m-xylyl-based pincer
ligands. In the current example, transformation to
diphosphine 1 was effected by reaction of the dibromide
with lithium diphenylphosphide (Scheme 2). Ligand 1
displays a single 31P NMR resonance at δ -8.8 ppm in
CDCl3.
X-r a y Cr ysta llogr a p h y. Diffraction data were collected
with a Siemens P4 instrument (Mo KR radiation (λ ) 0.710 73
Å)). Crystals were mounted onto glass fibers using epoxy.
Crystals were judged to be acceptable on the basis of ω scans
and rotation photography. A random search located reflections
to generate the reduced primitive cell, and cell lengths were
corroborated by axial photography. Additional reflections with
2θ values near 25° were appended to the reflection array and
yielded the refined cell constants. Data were collected as
presented in Table 1 and were corrected for absorption
(empirical ψ scans). Computations were performed using
SHELXTL version 6.1 (Bruker AXS). Further details may be
found within the CIF files provided as Supporting Information.
Complexes of 1 with palladium and nickel were
obtained upon mixing 1 with PdCl2(NCPh)2 and NiCl2-
(12) Nylander, L. R.; Pavkovic, S. F. Inorg. Chem. 1970, 9, 1959-
1960.

4218 Organometallics, Vol. 23, No. 18, 2004
Smith and Protasiewicz
F igu r e 1. ORTEP drawing (30% probability ellipsoids) of
the molecular structure of 2. Hydrogen atoms are omitted
for clarity.
Sch em e 3. P r ep a r a tion of 2 a n d 3
(DME) at room temperature in dichloromethane (Scheme
3). Analyses of these reaction mixtures by 31P NMR
spectroscopy revealed a single major product in each
case with resonances at δ 16.1 and 4.9 ppm in CDCl3
for the palladium and nickel complexes, respectively.
Complexes 2 and 3 were readily isolated in 97 and 83%
yields as pure materials that gave the correct analysis
F igu r e 2. Further views of 2 emphasizing the conforma-
tion of 1.
Ta ble 2. Com p a r ison of Select Bon d Len gth s (Å)
a n d An gles (d eg) for 2 (M ) P d ) a n d 3 (M ) Ni)
2
3
M(1)-Cl(1)
M(1)-Cl(2)
M(1)-P(1)
M(1)-P(2)
2.2813(18)
2.3028(16)
2.3201(18)
2.3233(17)
2.1602(13)
2.1761(12)
2.2434(14)
2.2381(14)
1
for the compositions of 1‚PdCl2 and 1‚NiCl2. H NMR
spectra of these materials were not definitive for
complete structural assignments, owing to the presence
of complex and broadened signals (vide infra).
Cl(1)-M(1)-Cl(2)
P(1)-M(1)-P(2)
Cl(1)-M(1)-P(1)
Cl(1)-M(1)-P(2)
Cl(2)-M(1)-P(1)
Cl(2)-M(1)-P(2)
169.44(8)
172.97(7)
90.79(7)
92.68(7)
88.43(6)
87.05(6)
166.42(6)
174.78(5)
92.11(5)
90.44(5)
87.72(5)
88.73(5)
Single crystals of each material were grown from
diffusion of either tetramethylsilane (2) or n-pentane
(3) into dichloromethane solutions of material at room
temperature. The X-ray structural analysis of these
compounds revealed coordination of 1 in a bidentate
trans-spanning mode with P-M-P bite angles of
172.97(7)° in 2 and 174.77(5)° in 3 (Figure 1 shows a
view of 2). Crystals of 2 and 3 are isomorphous and thus
have very similar structural parameters (Tables 1 and
2). The trans-spanning mode of 1 in these structures
has some unusual structural consequences, as eluci-
dated by the additional views of 2 shown in Figure 2.
These representations highlight the facts that the
central benzene ring lies roughly parallel to the PdCl2P2
plane and that H(1) (in its calculated position) and the
palladium atom lie directly above one another at a
distance of 3.48 Å (3.51 Å for the Ni complex). Addition-
ally, one of the chlorine atoms (Cl(2)) lies directly below
the center of the middle benzene ring of 1 at a relatively
short distance of 3.33 Å. Short intramolecular M-Cl‚‚‚
arene distances are not uncommon. A particularly
striking example of the impact of a Pd-Cl‚‚‚arene
interaction is seen in the structure of [LPdCl][PdCl4]
(L ) 6,6′-dimesityl-2,2′:6′′,2′′-terpyridine), where large
distortions of the square-planar palladium center and
a very short Cl‚‚‚centroid distance of 3.29 Å are ob-
served.13 Despite the proximity of H(1) to the metal
centers in 2 and 3, clearly the metals have not inserted
into this C-H bond. Two of the benzyl protons (H(13A)
and H(20A)) are about 2.84 Å from Cl(2) (the hydrogen
atoms being in their generated locations) in the X-ray
structure of 2 (Figure 2; dashed lines indicate putative
CH‚‚‚Cl contacts). While this distance is somewhat
outside the range for significant hydrogen-bonding
interactions (see comparisons to 4), the marked 1.74
ppm difference in chemical shifts for the protons that
are directed inward toward Cl(2) (δ 4.80) and those that
are directed outward (δ 3.06) suggest that in solution
such interactions may be more significant. Alterna-
tively, or in addition to these interactions, the location
of these same protons under the central benzene ring
may impart shifts due to ring anisotropy effects. Either
(13) Onoda, A.; Kawakita, K.; Okamura, T.-A.; Yamamoto, H.;
Ueyama, N. Acta Crystallogr., Sect. E 2003, 59, m291-m293.

A Trans-Spanning Diphosphine Ligand
Organometallics, Vol. 23, No. 18, 2004 4219
F igu r e 3. Comparison of partial 1H NMR (300 MHz)
spectra for the free ligand 1 (top) versus those of complex
2 (bottom) (recorded in C6D6; singlet resonances at 7.15
ppm correspond to residual C6D5H).
way, there are notable shift differences between the
“inside” (A) and “outside” (B) protons on C(13) and C(20)
of cyclic 2.
F igu r e 4. Temperature dependence of 1H NMR (600 MHz,
CDCl3) signals for aromatic protons of 2.
Although 2 and 3 appear to display well-defined
ligand environments in the solid state, analysis of their
1H NMR spectra is not simple. These spectra display
mixtures of broad and sharp signals, which are highly
temperature and solvent dependent. While we do not
have a comprehensive explanation of these spectra at
this time, some details are notable and one possibility
can be forwarded. These spectral features and their
respective interpretations are thus presented in part to
allow construction of a probable picture for the fluxion-
ality of 2.
First, Figure 3 illustrates that the 1H NMR spectrum
for 1 in C6D6 shows a large dispersion in chemical shifts
upon its binding to the palladium center. These signals
are attributed to the protons on the central benzene ring
of 1, due to its enforced proximity to the metal center.
The proton that resides almost directly above the Pd
atom (H(1)) resonates as a singlet at 8.02 ppm. The
other protons for this ring resonate as a doublet at 5.96
ppm for H(3) and H(5) and a triplet at 6.68 ppm (J ) 8
Hz) for H(4) and are easily assigned on the basis of the
nature of the coupling patterns.
F igu r e 5. Temperature dependence of 1H NMR (600 MHz,
CDCl3) signals for benzyl protons of 2.
Second, all of the remaining aromatic signals are ill-
defined, and some are broadened and lost into the
baseline between 6.5 and 8 ppm. The nature of the
spectra are solvent and temperature dependent. In
solvents of increasing polarity (C6D6 , CDCl3 < CD2Cl2
≈ DMSO-d6), additional, albeit broad, resonances are
resolved. These observations may imply an intercon-
version that is rapid (on the 1H NMR time scale) to
another species that is more polar. Variable-tempera-
ture 31P NMR spectra recorded between -80 and +55
°C in CD2Cl2 did not reveal any additional resonances
or notable peak broadening/sharpening, making it un-
likely that a major change in the phosphine coordination
mode (from trans to cis, for example) is occurring. Proton
NMR spectra were also obtained between -45 and +55
°C, in CDCl3. These spectra revealed a marked sharp-
ening of the broad resonances as the sample was cooled
to -25 °C, with little change noted upon further cooling
(Figures 4 and 5). No concentration dependence was
noted for this behavior, ruling out bimolecular processes.
The spectra shown in Figure 4 show that in CDCl3 the
“simple” picture for the central arene protons H(1) and
H(4) is complicated by the resolution of additional minor
signals. These signals, although fully resolved, suggest
the presence of a minor isomer in solution that is in
rapid equilibrium with a major form. In particular, a
new signal near 8 ppm grows in as a doublet, suggesting
that perhaps this might be attributable to the proton
H(3) or H(5). If true, this result would indicate that the

4220 Organometallics, Vol. 23, No. 18, 2004
Smith and Protasiewicz
Sch em e 5. P r ep a r a tion of 4
Sch em e 4. Str u ctu r es of Isom er s of 2 a n d 2′a
a
Only key hydrogen atoms are shown for clarity.
position of the central benzene ring has moved in
relationship to the PdCl2P2 “plane” and placed these
2
protons in a position to feel the impact of the filled dz
orbital on palladium (or the presence of a chloride
ligand) and, hence, have a downfield shift. Unfortu-
nately, similar statements for the other protons of this
ring are harder to claim, due to the lack of resolution
and the fact that much of the spectrum in this area is
dominated by the broad aromatic signals of the two
other aromatic rings of the terphenyl group and the four
other aromatic rings on the phosphorus atoms. Regard-
less, it appears that the fluxional process involves
movement of nearly all protons of the ligand.
of the fluxional behavior. The energy difference between
2 and 2′ is estimated to be only 1.4 kcal/mol by these
calculations (calculations for 2 were performed by
constraining ligand geometry and for 2′ by using no
constraints). Such a small energy difference could easily
be modulated by either solvent effects or crystal-packing
forces. As the two isomers do not significantly change
the phosphorus environments, only a single averaged
signal would be expected (and is seen). Due to the
incomplete resolution of all of the signals involved, the
limited solubility of 2 in CDCl3 and C6D6, and the
corresponding lack of good 13C NMR data, a more
complete picture of the solution dynamics of 2 is
unavailable at this time.
In the course of studies to explain the dynamic
behavior of 2, the possibility that dissociation of one of
the chloride ligands and formal η2 binding of the central
benzene was examined. These efforts led to the isolation
of 4 (below) and helped rule out halide dissociation as
being responsible for the variable-temperature NMR
behavior of 2.
On the basis of the proximity of H(1) to the metal
centers in 2 and 3 (M-H(1) ) ca. 3.5 Å), and the
reactivity of other pincer metal-phosphine complexes,
it was anticipated that insertion of the metal center into
the C(1)-H(1) bond might be induced. Many pincer-type
ligands often undergo insertion under relatively mild
conditions, such as refluxing solutions of the ligand with
metal salts in solution. Compound 2 was thus subjected
to similar conditions. However, no new species were
observed in the 31P NMR spectrum after heating to 100
°C in either dichloromethane or benzene in a sealed
vessel for 48 h, or even after heating to 140 °C in DMF
under N2 for 24 h. Since the desired insertion process
would necessitate elimination of HCl, the aforemen-
tioned reactions were repeated in the presence of excess
triethylamine as a base to help promote the reaction.
However, the same results were obtained as in the base-
free trials. Thermolysis of solid-state samples has also
been reported to lead to intramolecular C-H bond
insertion by metal phosphine complexes.14 However,
progressive heating of solid 2 under nitrogen or in vacuo
did not appear to produce the desired species either.
Monitoring such thermolysis reactions by 31P NMR
spectroscopy revealed only 2 up to the decomposition
point of the compound to an intractable black tar at
around 300 °C.
Third, and perhaps most revealing, is the variable-
temperature behavior of the benzyl protons (Figure 5).
While in C6D6 two signals are observed for H(13A)/
H(20A) and H(13B)/H(20B), these signals appear as very
broad signals in CDCl3 at 300 MHz at room tempera-
ture. Examination of 2 by variable-temperature 1H
NMR spectroscopy at higher field (600 MHz) revealed
the behavior shown in Figure 5. Cooling solutions of 2
to -35 °C allowed resolution of the broad signals into
two sets of peaks. The relative ratio of the larger pair
of signals to the smaller pair of signals is 3:1. As seen
in C6D6, there is a ca. 1.5 ppm difference between two
types of benzyl protons (the “inside” and “outside”
protons). The more downfield resonances are again
attributed to the “inside” benzyl protons. One of these
signals, at 4.95 ppm, resolves into a triplet at low
temperature (apparent J values for all four signals are
1
12-13 Hz), possibly indicating coupling to the I ) /2
31P nucleus or the presence of another isomer. These
data suggest that two isomers are present and that each
isomer has distinct environments for its “inside” and
“outside” benzyl protons. At higher temperature (up to
55 °C) the signals get even broader and seem to
disappear into the baseline. Such observations probably
indicate that the exchange process involves H(“in”,
major)/H(“out” minor) and H(“out”, minor)/H(“in” minor)
pairs of signals, as increasing exchange between nearest
neighboring signals would have led to some sharpening
of the signals at higher temperatures.
Insights to this perplexing set of data were gained
by semiempirical (SPARTAN ’04, PM3) molecular mod-
eling of 2. Minimization of 2 starting with the X-ray
structure coordinates resulted in the conformational
isomer 2′ depicted in the right-hand side of Scheme 4.
Transformation of 2 to 2′ involves rotation about one of
the Pd-P bonds and concomitant displacement of the
central benzene ring of 1 in relationship to the PdCl2P2
plane. Such a movement of the central benzene ring
might account for the complexity of the aromatic signals
shown in Figure 4. In addition, this isomer would be
expected to have a dipole moment different from that
of 2 and thus could rationalize the solvent dependency
(14) Baltensperger, U.; Gunter, J . R.; Kagi, S.; Kahr, G.; Marty, W.
Organometallics 1983, 2, 571-578.

A Trans-Spanning Diphosphine Ligand
Organometallics, Vol. 23, No. 18, 2004 4221
Ch a r t 1
1
Ta ble 3. Select Bon d Len gth s (Å) a n d An gles (d eg)
for 4
3, 4 displays sharp H NMR resonances and is consist-
ent with its formulation as [(1)PdCl][B(C6F5)4]. Com-
pound 4 is also properly analyzed for this formula.
However, X-ray analysis of a crystal grown by diffusion
of n-pentane into a dichloromethane solution of 4
revealed an unexpected rearrangement of phosphine
binding sites. The actual structure consists of a chlorine-
bridged dipalladium core in which each Pd center is
coordinated to one of the phosphine binding sites from
each diphosphine 1 (Table 3 and Figure 6). A Pd‚‚‚Pd
distance of 3.60Å is realized in this structure.
Pd(1)-P(1)
Pd(1)-P(2)
2.2576(26)
2.2785(26)
Pd(1)-Cl
Pd(1)-Cl(A)
2.4057(24)
2.4142(25)
P(1)-Pd(1)-P(2)
P(2)-Pd(1)-Cl
97.04(10) Pd(1)-Cl-Pd(1A)
91.56(9) P(1)-Pd(1)-Cl
96.26(9)
165.67(10)
P(2)-Pd(1)-Cl(A) 166.51(10)
P(1)-Pd(1)-Cl(A) 90.24(9)
Cl-Pd(1)-Cl(A)
83.74(8)
Although this type of cross-core bridging is fairly
common for some other M2Cl2 cores (i.e., common “A-
frame” ligands such as a recently reported and structur-
ally characterized Rh2Cl2-core bridging diphosphine),15
structurally characterized compounds are rarer for
palladium. Other structurally characterized examples
of bridging across a single Pd2Cl2 core have been
reported that do not feature a diphosphine as the
bridging moiety.16 In contrast to 4, the previous ex-
amples featured long and highly flexible linking groups.
Comparisons to the commonly employed “PCHP” pincer
type ligands (Chart 1, top) are natural. This particular
ligand, in its nonmetalated form, and its disilver
complexes offer M‚‚‚M distances comparable to those
seen in 4.17a Some other examples shown in Chart 1
illustrate longer M‚‚‚M distances.17b-d
Interestingly, the orientation of the Pd2Cl2 core in 4
directs the chlorine atoms toward the face of the central
benzene ring of 1, but at a distance (3.66 Å) greater than
that observed in 2 (3.33 Å). Finally, the benzyl protons
in 4 are found to resonate as two sets of multiplets at δ
4.10 and 4.79. The latter signals are again ascribed to
those being vicinal to the chloride ligands (ca. 2.60 Å in
the X-ray structure of 4). These short intramolecular
F igu r e 6. ORTEP drawing of the molecular structure of
4. Hydrogen atoms and [B(C6F5)4]- anions are omitted for
clarity.
As an alternative route to such a product, attempts
were made to obtain a Pd0 complex by reaction of 1 with
Pd2(dba)3. These reactions produced multiple products,
and Pd metal was observed in the reaction vessels.
Reduction of 2 by heating of a DMF solution in the
presence of Zn metal did produce a single soluble
species, as ascertained by 31P NMR spectroscopy (δ
27.8), but only in low yields and in conjunction with Pd
black.
(15) Ini, S.; Oliver, A. G.; Tilley, T. D.; Bergman, R. G. Organo-
metallics 2001, 20, 3839-3841.
(16) (a) Hambley, T. W.; Raguse, B.; Ridley, D. D. Aust. J . Chem.
1985, 38, 1455-1460. (b) Kleij, A. W.; Gebbink, R. J . M. K.; van den
Nieuwenhuijzen, P. A. J .; Kooijman, H.; Lutz, M.; Spek, A. L.; van
Koten, G. Organometallics 2001, 20, 634-647.
(17) (a) Caruso, F.; Camalli, M.; Rimml, H.; Venanzi, L. M. Inorg.
Chem. 1995, 34, 673-679. (b) van der Boom, M. E.; Gozin, M.; Ben-
David, Y.; Shimon, L. J . W.; Frolow, F.; Kraatz, H.-B.; Milstein, D.
Inorg. Chem. 1996, 35, 7068-7073. (c) Dilworth, J . R.; Zheng, Y.;
Griffiths, D. V. J . Chem. Soc., Dalton Trans. 1999, 1877-1881. (d)
Albrecht, M.; Dani, P.; Lutz, M.; Spek, A. L.; van Koten, G. J . Am.
Chem. Soc. 2000, 122, 11822-11833.
As another strategy undertaken to enhance the
reactivity of 2, and as a means to address the possibility
of halide dissociation as an explanation for the compli-
1
cated H NMR spectra of 2, the reaction of [Li(OEt2)2.5]-
[B(C6F5)4] with 2 was examined. Reaction of 2 with 1
equiv of [Li(OEt2)2.5][B(C6F5)4] in dichloromethane af-
fords a new species (4) that is characterized by a single
31P NMR resonance at δ 46.3 (Scheme 5). Unlike 2 and

4222 Organometallics, Vol. 23, No. 18, 2004
Smith and Protasiewicz
on the arene).17d If true, then 2 with its long C(1)-
H(1)‚‚‚Cl(1) distance of 4.51 Å could be limiting the
activation of the C(1)-H(1) bond. Either way, it appears
that the successful synthesis of complexes bearing this
type of m-terphenyl-based ligand in a true “pincer”
binding mode may require the premetalation approach.9
In conclusion, a representative diphosphine based on
a m-terphenyl scaffold has been prepared. Diphosphine
complexes of NiCl2 and PdCl2 display a good degree of
thermal stability and have thus far not been observed
to undergo C-H bond insertion analogous to that
observed in simple pincer complexes. X-ray structural
analyses of neutral Ni and Pd complexes indicate that
the diphosphine binds in a trans geometry with a bite
angle near 180°. X-ray analysis of a dicationic Pd
complex revealed a dimeric structure in which the
diphosphine is engaged in an unusual cross-core bridg-
ing mode. Investigations featuring other donor func-
tionalities built upon this framework are underway to
explore the reactivity and possible pincer binding modes
of this motif.
Sch em e 6. Ca lcu la ted Str u ctu r es for th e
Con ver sion of 2 to 5
CH‚‚‚Cl contacts are very reminiscent of the weak
hydrogen bonding noted for D (Chart 1, dCH‚‚‚Cl ) 2.66
Å).17d
It is interesting to now examine the factors that might
prevent the metalation of the vicinal C-H bond of 1.
The putative pincer binding mode of 1 at a palladium
center was modeled using SPARTAN (PM3). Analysis
of the resulting structure showed no evidence for
unusual bond lengths or angles about the metal center
(5; Scheme 6). This calculation, in conjunction with that
for 2 (above), allows an estimate of -1.3 kcal/mol for
∆Hrxn for the transformation of 2 to 5. In addition to
the marginal enthalpy of reaction for the insertion
process, the nature of the chelate structure may place
some constraints that result in a prohibitively high
reaction barrier. For example, 2 may not allow a
sufficiently close Pd‚‚‚H distance required for insertion.
Alternatively, it may be that the key step requires a
close CH‚‚‚Cl interaction for the resulting incipient
release of HCl (and concomitant attack on metal center
Ack n ow led gm en t. We thank the National Science
Foundation (Grant No. CHE-0202040) for support and
Dr. Philip Mountford (Oxford University) for enlighten-
ing discussions.
Su p p or tin g In for m a tion Ava ila ble: X-ray crystallo-
graphic files in CIF format for the structures of 2-4. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OM049662D