1,4-Bis(phosphine)-2,5-difluoro-3,6-dihydroxybenzenes and their P-oxides: Syntheses, structures, ligating and electronic properties

Article abstract of DOI:10.1016/j.jorganchem.2008.07.023

Reactions of 1,4-difluoro-2,5-dimethoxybenzene with LDA (1:2) at low temperatures generated organodilithio intermediates; quenching the reaction mixtures with chlorophosphines ClPR₂ produced 1,4-bis(phosphino)-2,5-difluoro-3,6-dimethoxybenzenes 1a (R = Ph) and 1b (R = iPr). Demethylation of 1a-b was accomplished by BBr₃, yielding bis(phosphino)hydroquinones 2a-b. Treating 2a-b with excess hydrogen peroxide produced bis(phosphinyl)hydroquinones 3a-b. The binucleating properties of 2a were established by the formation of a bimetallic nickel complex upon reaction with Ph₂Ni(PMe₃)₂. Electrochemical activity of hydroquinones 2a-b and 3a-b was examined by cyclic voltammetry. In addition, compounds 2a, 3a and 3b were obtained in crystalline form and characterized by single-crystal X-ray diffraction. The influence of the fluorine substituents on the composition of the frontier orbitals of 2a and 3a was examined by computational methods.

Full text of DOI:10.1016/j.jorganchem.2008.07.023

Journal of Organometallic Chemistry 693 (2008) 3263–3272
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
1,4-Bis(phosphine)-2,5-difluoro-3,6-dihydroxybenzenes and their P-oxides:
Syntheses, structures, ligating and electronic properties
Louis R. Pignotti a, Natcharee Kongprakaiwoot a, William W. Brennessel b, Jonas Baltrusaitis c,
Rudy L. Luck a, Eugenijus Urnezius a,
*
a Department of Chemistry, Michigan Technological University, 1400 Townsend Dr, Houghton, MI 49931, United States
b Department of Chemistry, University of Rochester, B51 Hutchinson Hall, 120 Trustee Road, Rochester, NY 14627, United States
c Department of Chemistry and Central Microscopy Research Facility, University of Iowa, 76 Eckstein Medical Research Building, Iowa City, IA 5224, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 May 2008
Received in revised form 18 July 2008
Accepted 20 July 2008
Available online 29 July 2008
Reactions of 1,4-difluoro-2,5-dimethoxybenzene with LDA (1:2) at low temperatures generated organod-
ilithio intermediates; quenching the reaction mixtures with chlorophosphines ClPR2 produced 1,4-
bis(phosphino)-2,5-difluoro-3,6-dimethoxybenzenes 1a (R = Ph) and 1b (R = iPr). Demethylation of 1a–b
was accomplished by BBr3, yielding bis(phosphino)hydroquinones 2a–b. Treating 2a–b with excess
hydrogen peroxide produced bis(phosphinyl)hydroquinones 3a–b. The binucleating properties of 2a were
established by the formation of a bimetallic nickel complex upon reaction with Ph2Ni(PMe3)2. Electro-
chemical activity of hydroquinones 2a–b and 3a–b was examined by cyclic voltammetry. In addition,
compounds 2a, 3a and 3b were obtained in crystalline form and characterized by single-crystal X-ray
diffraction. The influence of the fluorine substituents on the composition of the frontier orbitals of 2a
and 3a was examined by computational methods.
Keywords:
Noninnocent ligands
Binucleating
Structures
Electrochemistry
Frontier orbitals
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
ligand has been reported [20]. The lack of broader interest in phos-
phine–quinoid ligands is most likely caused by their somewhat
Ligands containing p-quinone structural motifs are being
increasingly utilized in the design and construction of bimetallic
complexes and coordination polymers. Thus, simple p-quinone
linkages have been successfully used in producing multimetallic
assemblies containing early transition metals (Fig. 1A) [1–7]. Coor-
dination networks where the p-quinoid fragments function both as
complicated synthetic procedures [20,21] and by the fact that such
ligands can not be isolated in their oxidized (quinone) state. The
latter complication arises from the facile reaction of phosphines
with quinones [22], making the two moieties (phosphine and qui-
none) incompatible. However, phosphine–quinone ligands are
readily isolable in their reduced (hydroquinone) form, and their
degradation upon oxidation to quinone is prevented if the nucleo-
philicity of the phosphine moiety is neutralized by tying up the
lone pair of electrons on phosphorus in bonding [21,23–26].
Coordination polymers containing metal centers embedded
directly into the main chains of the macromolecules are actively
pursued as potentially highly tunable novel materials [27,28].
The presence of linked electroactive units (redox-active ligands
and metal centers) in such compounds presents an attractive
opportunity for developing molecular systems where numerous
stimuli/response features can be utilized using chemical, electrical
and optical methods. In particular, such compounds are being con-
sidered as potential molecular conductors and sensors [29]. While
several of quinoid-based ligands have been explored for this pur-
pose [30–33], the number of such ligands is still limited. Herein
we present syntheses, structural and electrochemical investiga-
tions of new 2,5-bis(phosphino)-p-hydroquinones (Fig. 2, 2a–b),
their P-oxides (Fig. 2, 3a–b), and initial studies of their ligation
properties (Figs. 2, 5). Theoretical analyses of frontier molecular
r
- (O-ligation) and p-ligands (via coordination of another transi-
tion metal containing fragment onto the quinone ring) have also
been extensively studied [8–11].
Polymetallic compounds possessing higher directionality and
increased stability are often obtained if binucleating p-quinoid li-
gands contain chelation sites (Fig. 1B) are used. Most of the cur-
rently used ligands allowing for the formation of compounds B
have either [O,O] (various dihydroxybenzoquinones [12]) or [O,N]
(2,5-bis(pyrazol-1-yl)-1,4-dihydroxybenzene or related derivatives
[13–19]) chelation pockets. Despite the extensive general usage of
phosphine-based ligands in coordination chemistry, binucleating
ligands based on the p-quinoid moiety appended by phosphine
substituents (i.e., having [P,O] chelating pockets) have received
only limited attention. To the best of our knowledge, only one
bimetallic complex supported by a 2,5-bis(phosphino)-p-quinoid
* Corresponding author. Tel.: +1 906 487 2055; fax: 1 906 487 2061.
E-mail address: urnezius@mtu.edu (E. Urnezius).
0022-328X/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2008.07.023

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2.2. Syntheses
2.2.1. 1,4-Bis(diphenylphosphino)-2,5-difluoro-3,6-dimethoxybenzene
(1a)
To a cold (ꢁ95 °C) stirred solution of 1,4-difluoro-2,5-dimeth-
oxybenzene (1.00 g, 5.74 mmol) in THF (120 mL) 2.0 equiv. of
freshly prepared LDA (in THF, 20 mL) was added. After stirring
the reaction mixture for 5 min at -95 °C, a solution of ClPPh2
(2.5 equiv.) in THF (30 mL) was slowly (over 30 min) added. The
mixture was stirred at ꢁ95 °C for 1 h and then allowed to slowly
warm up while stirring overnight. The resulting solution was fil-
tered, and all volatiles were removed under reduced pressure to
yield a pale yellow solid. Washing it with hot deoxygenated meth-
anol (100 mL) yielded 1a as white powder. Yield: 2.329 g (74.8%).
M.p.: 156–157 °C. 1H NMR (acetone-d6): d 7.40 (m, 20H), 3.48 (s,
Fig. 1. General structural motifs of coordination polymers built on p-quinoid
ligands. M = transition metal; E = donor atom (O, N, or P) positioned 1 or 2 bonds
away from the central ring.
3
4
6H). 31P NMR (CDCl3): d ꢁ20.4 (dd, JPF = 20 Hz, JPF = 9 Hz). 19F
3
4
NMR (CDCl3): d ꢁ120.1 (dd, JFP = 20 Hz, JFP = 9 Hz). HRMS (FAB):
MH+ Calc. for C32H27F2O2P2: 543.1454. Found: 543.1456.
Fig. 2. New 2,5-bis(phosphino)- and 2,5-bis(phosphinyl)-hydroquinones, and
bimetallic nickel complex.
2.2.2. 1,4-Bis(diisopropylphosphino)-2,5-fluoro-3,6-
dimethoxybenzene (1b)
The synthesis of 1b was accomplished by employing ClPiPr2 in-
stead of ClPPh2 in the procedure used for 1a; 2.00 g (11.74 mmol)
of 1,4-difluoro-2,5-dimethoxybenzene was used. The resulting
light yellow solid was redissolved in hot deoxygenated methanol
(70 mL), and light yellow crystals of 1b precipitated on standing
at ꢁ10 °C overnight. Yield: 2.934 g (62.9%). M.p.: 91–92 °C. 1H
NMR (CDCl3): d 3.84 (s, 6H, (OCH3)), 2.48 (m, 4H, (PCH)), 1.14
orbitals of compounds 2a–3a and their fluorine-free analogs, 2aH–
3aH are also presented and discussed.
2. Experimental
3
3
2.1. General procedures
(dd, 12H, JPH = 17 Hz, JHH = 7 Hz, (CH3)), 0.92 (dd, 12H,
3
3JPH = 12 Hz, JHH = 7 Hz, (CH3)). 13C NMR (CDCl3): d 154.0 (dm,
Synthetic preparations and procedures were carried out by
using suitably adapted Schlenk techniques or in a dry box under
a N2 atmosphere. Low temperature reactions were performed in
Schlenk flasks immersed in ethanol/liquid N2 slush, contained in
a Dewar flask. Solvents were purified before use by distillation
from Na-benzophenone ketyl (ether, THF, hexanes, benzene) or
CaH2 (toluene, 2-propanol, dichloromethane) or Na (pyridine) un-
der a N2 atmosphere. Dry methanol was obtained by distillation
from Mg(OCH3)2 under a N2 atmosphere. Deoxygenated methanol
and ethanol were obtained by bubbling nitrogen through the sol-
vent for 1 h. Commercially available chlorophosphines were puri-
fied prior to usage: ClPPh2 was distilled under reduced pressure;
ClPiPr2 was frozen and degassed while thawing under vacuum (re-
peated 3 times). Lithium diisopropylamide (LDA) was freshly pre-
1JCF = 244 Hz (CF)), 147.2 (m, Carom-OCH3), 121.9 (m (Carom-P)),
1
62.0 (s (OCH3)), 24.0 (d, JPC = 12 Hz (PCH(CH3)2)), 21.4 (d,
2
2JPC = 24 Hz (CH3)), 20.9 (d, JPC = 11 Hz (CH3)). 31P NMR (CDCl3):
d 4.8 (s, br). 19F NMR (CDCl3): d ꢁ121.3 (s, br). HRMS (FAB): MH+
Calc. for C20H35F2O2P2: 407.2080. Found: 407.2081.
2.2.3. 1,4-Bis(diphenylphosphino)-2,5-difluoro-3,6-dihydroxybenzene
(2a)
To
a stirred solution of 1,4-bis(diphenylphosphino)-2,5-di-
fluoro-3,6-dimethoxybenzene (1a; 1.00 g, 1.85 mmol) in 80 mL of
dichloromethane at -80 °C BBr3 (0.80 mL, 8.49 mmol) was added.
The mixture was stirred at -80 °C for 10 min and then allowed to
warm up to room temperature overnight. Volatiles were removed
under reduced pressure to yield a white solid which was subse-
quently treated with dry methanol (30 mL) at 0 °C. The resulting
suspension was removed from the cooling bath and then was stir-
red at room temperature overnight. All volatiles were removed in
vacuo, and 2-propanol (50 mL) and NEt3 (1.20 mL, 8.49 mmol)
were added to the remaining solid. After 2 h of stirring, the mixture
was filtered leaving a light yellow solid which was redissolved in
40 mL of a 1:1 mixture of pyridine and 2-propanol. Yellow crystals
of 2aꢂ2C5H5N (suitable for X-ray analysis) were formed after the
solution was kept at ꢁ28 °C for 2 days; a second fraction of crystals
was collected from the mother liquor after ten days at ꢁ28 °C. To-
tal yield: 0.646 g (68.1%). M.p.: 233–234 °C. 1H NMR (CDCl3): d 7.45
(m, 8H), 7.35 (m, 12H). 31P NMR (CDCl3): d ꢁ30.6 (m). 19F NMR
pared immediately prior to use according to
a published
procedure [34]. Triethylamine was distilled from CaH2 under nitro-
gen. Nickel complexes NiCl2(PMe3)2 [35] and Ph2Ni(PMe3)2 [36]
were prepared utilizing literature procedures. 1H, 13C, 31P, and
19F NMR spectra were recorded on Varian INOVA spectrometer
operating at 400, 100, 161, and 376 MHz, respectively. Spectra
are referenced to tetramethylsilane (1H and 13C), 85% H3PO4 as
an external reference (31P), and CFCl3 as an internal reference in
a sealed capillary tube (19F). Low resolution mass spectrometric
measurements were performed on Shimadzu QP5050A instru-
ment; high resolution mass spectrometric measurements were
performed at Mass Spectrometry Facility at Michigan State Univer-
sity (East Lansing, MI 48824-1319). Elemental analyses were per-
formed at Galbraith Laboratories Inc.
3
4
(pyridine-d5):
d
ꢁ125.5 (dd, JFP = 18 Hz, JFP = 10 Hz). HRMS
(FAB): MH+ Calc. for C30H23F2O2P2: 515.1141. Found: 515.1139.
Electrochemical measurements (CV) were performed using a
CHI420 electrochemical analyzer. A standard 3-electrode cell fitted
to operate under air-tight conditions was used. A glassy carbon
disk (3 mm), Ag/AgNO3 and Pt wire were used as working, refer-
ence, and counter electrodes, respectively. Measurements for 2a–
b (scan rate of 0.5 V/s) and 3a–b (scan rate 0.1 V/s) were performed
in ꢀ1 mM solutions in CH2Cl2 containing 0.1 M NBu4PF6 as sup-
porting electrolyte.
2.2.4. 1,4-Bis(diisopropylphosphino)-2,5-difluoro-3,6-
dihydroxybenzene (2b)
A solution of 1,4-bis(diisopropylphosphino)-2,5-difluoro-3,6-
dimethoxybenzene (1b; 3.30 g, 8.2 mmol) in CH2Cl2 (200 mL)
was cooled to ꢁ80 °C and BBr3 (3.9 mL, 41 mmol) was added. The
reaction mixture was stirred overnight while slowly attaining
room temperature. All volatiles were removed under reduced pres-

L.R. Pignotti et al. / Journal of Organometallic Chemistry 693 (2008) 3263–3272
3265
Fig. 3. Thermal ellipsoid plots (30%) of 2a, 3a and 3b.
Fig. 4. Cyclic voltammetry plots of 2a and 3a.
Fig. 5. Frontier orbitals (contour plots) of 2a.
sure, the flask was cooled in an ice bath, and 50 mL of methanol
was added to the residual solid. The resulting slurry was stirred
overnight while attaining room temperature. All volatiles were re-
moved under reduced pressure, followed by addition (at 0 °C) of 2-
propanol (100 mL) and triethylamine (5.4 mL, 38.4 mmol). After
stirring for 2 days at room temperature all the volatiles were re-
moved under reduced pressure and THF (100 mL) was added. The
resulting slurry was then filtered, all volatiles were removed under
vacuum, and the remaining pale orange solid was washed with two
60 mL portions of diethyl ether. Yield: 1.862 g (59.8%). M.p.: 216–
218 °C. 1H NMR (CDCl3): d 6.68 (s (br), 2H (OH)), 2.53 (m, 4H
3
3
(PCH)), 1.16 (dd, 12H, JPH = 18 Hz, JHH = 7 Hz (CH3)), 0.94 (dd,
12H, JPH = 14 Hz, JHH = 7 Hz). 13C NMR (CDCl3): d 148.1 (dm,
3
3
1JCF = 240 Hz, CF), 140.8 (m, COH), 113.3 (m, CaromP), 23.4 (s,
(CH)), 21.2 (d, JCP = 21 Hz (CH3), 20.5 (d, JCP = 20 Hz, (CH3)). 31P
2
2
NMR (CDCl3): d ꢁ11.7 (m). 19F NMR (pyridine-d5): d ꢁ127.9 (dd,
3JFP = 47 Hz, JFP = 9 Hz). HRMS (FAB): MH+ Calc. for C18H31F2O2P2:
4
379.1767. Found: 379.1766.

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L.R. Pignotti et al. / Journal of Organometallic Chemistry 693 (2008) 3263–3272
2.2.5. 1,4-Bis(diphenylphosphinyl)-2,5-difluoro-3,6-dihydroxybenzene
(3a)
of a 0.1 mm diameter glass fiber and mounted on a Bruker SMART
APEX II CCD diffractometer for data collection at 243.0(2) K. At-
tempts at lower temperatures resulted in multiply twinned data,
suggesting that the crystals suffered from thermal stress. A preli-
minary set of cell constants and an orientation matrix were calcu-
lated from 385 reflections harvested from three sets of 20 frames.
These initial sets of frames were oriented such that orthogonal
wedges of reciprocal space were surveyed. The data collection
(Caution! Acetone/H2O2 mixtures can form explosive products.
Protecting shields and other necessary safety measures should be
used.) Hydrogen peroxide (5 mL, 30% in H2O) was added to a solu-
tion of 1,4-bis(diphenylphosphino)-2,5-difluoro-1,4-dihydroxy-
benzene (2a; 0.836 g, 1.6 mmol) in 70 mL of acetone. The
solution was allowed to stir overnight. Subsequent filtration/dry-
ing in vacuo yielded 0.558 g of a fine white powder 3a. An addi-
tional 0.150 g of 3a was recovered after the volume of the filtrate
was reduced to 50 mL followed by cooling to ꢁ28 °C for several
days. Combined yield: 0.708 g (81.0%). M.p.: 275–277 °C. 1H NMR
(CDCl3): d 7.79 (m, 8H), 7.62 (m, 4H), 7.51 (m, 8H). 31P NMR
(CDCl3): d 42.2 (s). 19F NMR (acetone): d ꢁ125.2 (s (br)). HRMS
(FAB): MH+ Calc. for C30H23F2O4P2: 547.1040. Found: 547.1035.
was carried out using Mo K
a radiation with a frame time of 30 s
and a detector distance of 5.02 cm. A randomly oriented region
of reciprocal space was surveyed: four major sections of frames
were collected with 0.50° steps in
x at four different u settings
and a detector position of ꢁ33° in 2h. The intensity data were cor-
rected for absorption [37]. Final cell constants were calculated
from the xyz centroids of 3107 strong reflections from the actual
data collection after integration [38]. The space group P21/n was
determined based on systematic absences and intensity statistics.
A direct-methods solution was calculated using SIR97 [39] which
provided most non-hydrogen atoms from the E-map. Full-matrix
least squares/differences Fourier cycles were performed using
SHELXL-97 [40] which located the remaining non-hydrogen atoms.
All non-hydrogen atoms were refined with anisotropic displace-
ment parameters. The hydrogen atom of the OH group was found
from the difference map, and then was refined positionally and
thermally with respect to the oxygen atom. All other hydrogen
atoms were placed in ideal positions and refined as riding atoms
with relative isotropic displacement parameters.
2.2.6. 1,4-Bis(diisopropylphosphinyl)-2,5-difluoro-3,6-
dihydroxybenzene (3b)
To a stirring solution of 1,4-bis(diisopropylphosphino)-2,5-di-
fluoro-3,6-dihydroxybenzene (2b; 0.563 g, 1.49 mmol) in 20 mL
of acetone was added 0.75 mL of 30% H2O2 (5 equiv.). The solution
turned clear dark orange as it was allowed to stir overnight.
Removing all volatiles in vacuo yielded 0.569 g of a pale orange so-
lid 3b (>90% pure by NMR; crude yield ꢀ90%). Further purification
can be performed by crystallizations from diethyl ether/acetone
(1:1, ꢁ28 °C, 4–5 days) or methanol (ꢁ28 °C, 4–5 days), producing
3b in the form of white crystalline substance in ꢀ70% yield. M.p.:
193–196 °C. 1H NMR (CDCl3): d 11.82 (s, 2H (OH)), 2.41 (m,
Compounds 3a–b were characterized using crystal preparation,
data collection and refinement procedures described in our previ-
ous reports [41]. Crystals of 3a–b suitable for X-ray analysis were
obtained by crystallization from methanol solutions. In 3a–b, the
3
3
4H(CH)), 1.29 (dd, 12H, JCP = 17 Hz, JHH = 7 Hz (CH3)), 1.17 (dd,
12H, JCP = 18 Hz, JHH = 7 Hz (CH3)). 13C NMR (CDCl3): d 145.3
(dm, JCF = 241 Hz, (CF)), 143.4 (m (COH)), 105.7 (dm, JCP = 74 Hz
3
3
1
1
1
(CaromP)), 27.8 (d, JCP = 66 Hz (PCH)), 16.2 (s (br), (CH3)), 14.9 (s
hydrogen atom of the OH group was freely refined resulting in
0
(br), (CH3)). 31P NMR (CDCl3): d 72.8 (s). 19F NMR (CDCl3): d
ꢁ129.5 (s (br)). HRMS (FAB): MH+ Calc. for C18H31F2O4P2:
411.1666. Found: 411.1669.
O-H distances of 0.91(3), 0.83(3) and 0.82(3) ÅA and UISO values
for the H atom of 0.083(9), 0.064(9) and 0.062(9) for 3a and the
two independent molecules of 3b, respectively. In both cases the
remaining H atoms were refined as described above for 2a.
2.2.7. [Ph(PMe3)Ni-(P,O)-l{(1,4-(PPh2)2-2,5-F2-3,6-O2-C6)}-(P,O)-
Ni(PMe3)Ph] (5)
2.4. Calculations
A solution of 2a (0.246 g, 0.479 mmol) in 20 mL of THF was
slowly added to a solution of Ph2Ni(PMe3)2 (0.35 g, 0.959 mmol)
in 20 mL of THF. The reaction mixture was heated at reflux for
4 h, followed by the removal of all volatiles under reduced pres-
sure. The remaining yellow solid 5 was washed with 10 mL of tol-
uene. Yield: 0.262 g (58.4%). 1H NMR (C6D6): d 7.10 (m, 30H), 2.14
Density functional theory (DFT) calculations were performed
using GAUSSIAN 03 software package revision B.01 [42]. Geometries
were optimized using spin restricted calculations with B3LYP ex-
change correlation functional and 6-31G(d) basis set as imple-
mented in GAUSSIAN 03 [43,44]. All stationary points were verified
as minima using vibrational frequency calculations at the same le-
vel of theory. Tight convergence cutoffs were used in geometry
optimization calculations. Additional single point calculations
were performed at B3LYP/6-31G(d) geometry using R3BLYP ex-
change correlation functional with TZVP basis set [45]. Molecular
orbital (MO) compositions were calculated using AOMix [46,47]
software package and the Mulliken scheme [48–51]. Molecular
orbitals were visualized using Chemcraft software package [52].
2
(s (br), 9H). 31P NMR: d 24.9 (dm, JPP = 309 Hz, PPh2), ꢁ12.8 (dm,
2JPP = 309 Hz, PMe3). 19F NMR: d ꢁ130.3 (d, 3JFP = 36 Hz). Anal. Calc.
for C48H48F2Ni2O2P4: C, 61.58; H, 5.17. Found: C, 60.91; H, 4.96%
(repeated elemental analyses were consistently slightly low on
carbon).
2.2.8. NiCl2(PMe3)2
A solution of PMe3 (16.8 mL, 1.0 M in THF) was added to a stir-
red solution of NiCl2 ꢂ 6H2O (2.00 g, 8.41 mmol in 30 mL of deoxy-
genated ethanol), yielding a dark red solution immediately. After
stirring the reaction mixture overnight, volatiles were removed un-
der vacuo to give a red solid which was redissolved in 30 mL of
warm ethanol. The solution was cooled in an ice bath for 1 h,
resulting in the formation of dark red crystals which were isolated
by filtration. A second fraction of crystals was obtained after the fil-
trate was kept at ꢁ28 °C for 2 weeks. Yield (ꢀ55%) and m.p. (201–
202 °C) matched the reported values [35] well.
3. Results and discussion
3.1. Syntheses
Recently we reported on the preparation of several new polyd-
entate phosphine [41,53] and mixed donor [P,S] [54] ligands. The
synthetic method utilized in such syntheses relies on the sequen-
tial generation of organodilithio intermediates from selected tetra-
substituted benzenes. In particular, low temperature (ꢁ95 °C)
2.3. X-ray crystallography
double
deprotonation
of
1,4-dibromo-2,5-difluorobenzene
[53,54], 1,2-dibromo-4,5-difluorobenzene [41], or 1-bromo-2-
chloro-4,5-difluorobenzene [41] with lithium diisopropylamide
Compound 2a was analyzed at the X-ray crystallographic facil-
ity, at the University of Rochester. A crystal was placed onto the tip

L.R. Pignotti et al. / Journal of Organometallic Chemistry 693 (2008) 3263–3272
3267
(LDA) was possible due to the relative acidity of the aromatic pro-
tons flanked by F/Br or F/Cl substituents on the same phenyl ring.
Alkoxy groups are known to direct ortho-lithiations on the phenyl
rings, including the dilithiations of some ethers [55]. Not surpris-
ingly, double deprotonation at the CH3O/F flanked aromatic posi-
tions and the formation of organodilithium intermediate (Eq. (1),
step 1) are facile
P-oxidation is quite surprising, given the fact that o-phosphinophe-
nols typically are quite air sensitive [61] and some of them form
P-oxides almost instantly upon exposure to air [57,62]. Detailed
investigations addressing the oxidation of phosphorus centers in
o-phosphinophenols and in phosphine-appended hydroquinones
are currently unavailable. Our theoretical investigations (discussed
in Section 3.4.) suggest that such behavior of 2a–b towards O2 may
ð1Þ
when 1,4-difluoro-2,5-dimethoxybenzene is reacted with LDA (2
equivalents), as judged by the relatively high isolated yields
(>60%) of 1,4-bis(phosphino)-2,5-difluoro-3,6-dimethoxybenzenes
1a–b (Eq. (1)). Similar p-dialkoxybenzenes appended with phos-
phine groups at the 2- and 5-positions have been known for some
time [56]. Compounds 1a–b represent methyl-protected forms of
highly substituted 1,4-hydroquinones. They are white/pale yellow
crystalline solids that are stable to oxidation when handled in air.
Conversion of 1a–b into the corresponding hydroquinones 2a–b is
relatively facile using BBr3 as the O-dealkylating reagent, as such
procedures are typical for analogous dealkylations producing
be attributed to the presence of fluorine substituents on the central
phenyl ring as they have a direct influence on the composition of
the HOMO in 2a.
Oxidation of the P centers in 2a–b is facile when excess of 30%
H2O2 is used as an oxidizing reagent (Eq. (1), last step); the phos-
phine oxides 3a–b conveniently precipitate out of the reaction
mixture. They are white, crystalline solids, soluble in polar organic
solvents (particularly in alcohols). Our attempts to further oxidize
hydroquinones 3a–b into corresponding quinones have not been
successful so far (oxidizing reagents used: [Cp2Fe]PF6, NaIO4, Ce-
(NH4)2(NO3)6, potassium nitrosodisulfonate, and others).
ð2Þ
o-phosphinophenols [21,57]. Compound 2b is isolated in lower
yield, most likely due to the higher stability of phosphonium salts
(2bꢂnHBr) formed by protonation of more basic dii-
sopropylphosphino groups during the dealkylation procedure
[21,57]. Solution 31P NMR data for compounds 2a (d ꢁ30.6) and
2b (d ꢁ11.7) resemble chemical shift values reported for o-diphe-
nylphosphinophenols (d ꢀ ꢁ26) [58,59] and o-diisopropylphos-
phinophenols (d ꢀ ꢁ22) [60].
We have started investigating the ligating properties of 2a–b
and 3a–b. Our preliminary results suggest that compound 2a func-
tions as an effective binucleating ligand as judged by the formation
of organometallic nickel complexes 4 and 5 (Eq. (2)). When heated,
a THF solution of 2a and Ph2Ni(PMe3)2 (1:2 ratio) slowly (hours)
changes color from deep red to orange to yellow. The progress of
the reaction monitored by 31P NMR can be assessed by the disap-
pearance of signals for the starting nickel complex (Ph2Ni(PMe3)2,
d ꢁ10.8) and for ligand 2a. A new set of signals seen in the spec-
trum (d 24.7 (br), ꢁ40.7 (br)) is attributed to the formation of
bimetallic complex 4 (Eq. (2)) (not isolated). This assignment is
based on a comparison of the spectral signature of 4 to the signals
observed for similar monometallic o-phosphinophenolate methyl-
nickel-(bis)trimethylphosphine complex (d 27.0 (br), ꢁ28.5(br))
[60], where the broadness of the signals is attributed to the flux-
ional behavior of the PMe3 ligands at the formally pentacoordinate
nickel center [36]. The lability of the PMe3 ligands in 4 is also illus-
trated by the fact that removing the volatiles in vacuo during the
reaction workup produces bimetallic complex 5 (yellow powder)
(Eq. (2)) where only one PMe3 ligand is coordinated to four-coordi-
nate nickel center. Despite numerous attempts we have not been
Phosphorus centers in hydroquinones 2a–b are only moderately
sensitive to oxidation by atmospheric oxygen. Thus, the presence
of P-oxide 3a (31P NMR d 42.2) was evident after about 48 h of
exposure of a solution of 2a in CDCl3 to air. Continued monitoring
of the relative 2a/3a signal intensities in the 31P NMR spectrum re-
vealed that approximate 50% conversion of 2a into 3a was
achieved after 4 weeks. The final spectrum also contained small
(<10%) quantities of species with d values of 39.0 and ꢁ31.7, which
we assigned to
a product of partial oxidation of 2a (Ph2-
P-C6(OH)2F2-P(O)Ph2) (also verified by the presence of strong peak
at m/z = 530 (M+) in mass-spectrometric measurements of the
reaction mixture). Similar reactivity pattern was also observed
for phosphorus centers in 2b. Such relative stability of 2a–b to

3268
L.R. Pignotti et al. / Journal of Organometallic Chemistry 693 (2008) 3263–3272
Table 2
able to obtain single crystals of 5 suitable for crystallographic
investigations. The strongest evidence of the assignment of 5 as a
bimetallic quinonate complex comes from a comparison of its 31P
Selected bond distances and angles of 2a, 3a and 3b (data for one of the two unique
molecules of 3b is listed, labels for isostructural atoms for 3b are given in
parentheses)
2
NMR spectral signature (d 24.9 (dm, JPP = 309 Hz, PPh2), ꢁ12.8
0
2
Bond distances (AÅ)
2a
3a
3b
(dm, JPP = 309 Hz, PMe3)) to those of monometallic organonickel
P1–C1 (P11–C11)
C1–C2 (C11–C12)
C3–O3 (C13–O13)
P1–O1 (P11–O11)
1.8435(14)
1.3930(19)
1.3548(16)
1.813(2)
1.385(3)
1.353(2)
1.4936(16)
1.833(2)
1.388(3)
1.353(2)
1.5002(17)
trimethylphosphine o-phosphinophenolates, where a large value
for 2JPP is indicative for the trans arrangement of the two phospho-
rus centers. The complex phenylnickel-2-(diphenylphosphino)phe-
nolate(O,P)(trimethylphosphine) displays very similar chemical
shift (31P NMR, d 23.6 (d), ꢁ13.4 (d)) and coupling constant
(2JPP = 303 Hz) values in solution at ꢁ60 °C; broad signals are re-
ported for the room temperature spectrum [36]. By contrast, we
observed sharp signals in the spectra of 5 recorded at room tem-
perature (with some multiplet character due to unresolved
19F–31P couplings). Differences in fluxional behavior (responsible
for the broadening of NMR signals) among similar nickel [P,O]
complexes have been noted before [36], but the reasons behind
this phenomenon are not yet known.
Bond angles (°)
P1–C1–C2 (P11–C11–C12)
C1–C2–F2 (C11–C12–F12)
O3–C3–C2 (O13–C13–C12)
C1–C2–C3 (C11–C12–C13)
O1–P1–C1 (O11–P11–C11)
126.99(10)
119.42(12)
120.36(12)
122.82(12)
122.66(14)
118.08(17)
118.09(17)
124.86(17)
107.05(9)
124.07(16)
118.50(18)
118.17(19)
124.81(19)
106.92(10)
shown in Fig. 3). Most of the bond distances and angles are unex-
ceptional. The oxygen and fluorine substituents are in the plane of
3.2. Structural investigations
the central phenyl ring, while phosphorus atom P1 is displaced
0
from it by 0.129 ÅA. The –PPh2 group in 2a is rotated around the
To the best of our knowledge, hydroquinones 2a–b constitute
the second and third examples of compounds having 1,4-bis(phos-
phino)-2,5-dihydroxybenzene motifs reported in the literature;
data about 1,4-bis(phosphinyl)-2,5-dihydroxybenzenes analogous
to 3a–b are also quite limited [63]. Also, no structural characteriza-
tions of such compounds have been reported previously. We have
obtained single crystals of 2a and 3a–b suitable for X-ray diffrac-
tion experiments, and determined their crystal and molecular
structures (general crystallographic data are summarized in
Table 1).
C1–P1 bond so that one of the Ph substituents has the plane of
the aromatic ring directly facing the fluorine on the central phenyl
0
ring. This is illustrated by the F2–C4 distance of 2.86 ÅA, which is
substantially shorter then the 0 sum of van der Waals radii for C
and F, estimated around 3.05 ÅA [64]. The proximity of F2 and C4
is also most likely responsible for the relatively large C2–C1–P1 an-
gle (126.99(10)°). We have previously observed similar arrange-
ments of the –PPh2 groups attached to the phenyl ring
containing fluorine substituents at ortho positions [41]; the under-
lying reasons for these apparent C–F interactions in 2a are unclear
at the moment.
Crystals of 3a and of 3b do not contain co-crystallized solvent.
Two independent molecules comprising the unit cell of 3b have
very small differences, thus only one of them is shown in Fig. 3
and discussed. The oxygen atoms from the phosphinyl groups in
3a–b are engaged in hydrogen bonding with the protons from
the OH groups of the hydroquinone, which is a common feature
for hydroxyl groups attached to aromatic fragments with o-phos-
Thermal ellipsoid plots of hydroquinones 2a, 3a and 3b are
presented in Fig. 3, with selected bond distances and angles sum-
marized in Table 2. Crystals of 2a contain pyridine as a co-crystal-
lized solvent. The molecule of hydroquinone 2a lies on
a
crystallographic inversion center; thus, one half of the molecule
is unique. The two disordered co-crystallized pyridine molecules
(one unique) per hydroquinone molecule interact with the hydro-
xyl groups of 2a through [-Nꢂ ꢂ ꢂH–O–] hydrogen bonding (not
Table 1
Summary of crystallographic data for 2a and 3a–b
Compound
2a ꢂ 2C5H5N
40H32F2N2O2P2
672.62
P21/n
13.0046(19)
9.4524(14)
14.216(2)
90
99.791(2)
90
1722.0(4)
2
1.297
0.175
3a
3b
Formula
Formula weight
Space group
a (Å)
b (Å)
c (Å)
C
C30H22F2O4P2
546.42
C18H30F2O4P2
410.36
ꢀ
ꢀ
P1
P1
8.747(5)
8.824(5)
9.375(5)
74.732(5)
75.109(5)
71.469(5)
649.9(6)
7.5409(9)
11.243(2)
14.178(2)
69.56(2)
74.96(1)
89.64(1)
1083.0(3)
2
1.258
a
(°)
b (°)
c
(°)
V (Å3)
Z
2
qcalc (g/cm3)
1.396
0.217
l
(Mo K
a
, mmꢁ1
)
0.236
T (K)
243.0(2)
293(2)
293(2)
k (Å)
0.71073
0.71069
0.71073
h Range (°)
Measured/independent reflections
1.96–32.03
5991
2.29–22.47
1553
1.59–22.47
2469
Final R indices [I > 2
R indices (all data)
r
(I)]a
R1 = 0.0556, wR2b = 0.1590
R1 = 0.0786, wR2 = 0.1788
0.430, ꢁ0.318
1.060
R1 = 0.0295, wR2c = 0.0787
R1 = 0.0329, wR2 = 0.0817
0.143, ꢁ0.214
1.061
R1 = 0.034, wR2d = 0.093
R1 = 0.041; wR2 = 0.098
0.253, ꢁ0.285
1.064
Largest difference in peak and hole
Goodness-of-fit on F2
a
R1
=
R
||Fo| ꢁ |Fc||/
R|Fo|.
P
P
2
2
2
wR2 ¼ ½ ½wðFo2 ꢁ FcÞ ꢃ= ½wðFo2Þ ꢃꢃ1=2, where w ¼ 1=½r2ðFo2Þ þ ð0:0900PÞ þ 0:3145Pꢃ and P ¼ 1=3 maxð0; Fo2Þ þ 2=3F2c .
b
c
2
w ¼ 1=½r2ðF2oÞ þ ð0:0411PÞ þ 0:2412Pꢃ.
2
w ¼ 1=½r2ðF2oÞ þ ð0:0553PÞ þ 0:4283Pꢃ.
d

L.R. Pignotti et al. / Journal of Organometallic Chemistry 693 (2008) 3263–3272
3269
Table 3
Table 4
Selected bond distances and angles of 2a, 2aH, 3a and 3aH structures calculated at
Frontier orbitals of 2a and their compositions calculated at B3LYP/TZVP//B3LYP/
B3LYP/6-31G(d) level of theory
6-31G(d) level of theory
0
Bond distances (ÅA)
2a
3a
HOMO(ꢁ1)
HOMO
LUMO
LUMO(+1)
Calculated
X-ray
Calculated
X-ray
Energy (eV)
C, H atoms on Ph rings
s
p
d
ꢁ6.20
48.2
1.4
46.0
0.8
ꢁ5.98
31.1
1.5
29.2
0.5
ꢁ1.56
26.7
2.6
23.5
0.6
ꢁ1.07
81.6
0.0
79.9
2.0
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
P1–C1
C1–C2
C3–O3
P1–O1
1.86
1.39
1.36
1.8435(14)
1.3930(19)
1.3548(16)
1.83
1.39
1.34
1.52
1.813(2)
1.385(3)
1.353(2)
1.4936(16)
P atoms
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
45.5
8.7
36.6
0.2
32.1
6.0
25.9
0.2
11.8
0.0
7.9
3.9
12.4
0.0
10.4
2.1
Bond angles (°)
P1–C1–C2
C1–C2–F2
O3–C3–C2
C1–C2–C3
O1–P1–C1
126.9
120.5
120.1
120.9
126.99(10)
119.42(12)
120.36(12)
122.82(12)
122.6
118.7
117.7
124.4
108.0
122.66(14)
118.08(17)
118.09(17)
124.86(17)
107.05(9)
F atoms
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
0.1
0.0
0.1
0.0
0.5
0.0
0.5
0.0
1.2
0.0
1.2
0.0
0.1
0.0
0.4
0.0
2aH
1.85
1.40
1.37
3aH
1.82
1.40
1.35
1.52
P1–C1
C1–C2
C3–O3
P1–O1
Bond angles (°)
P1–C1–C2
C1–C2–F(H)2
O3–C3–C2
C1–C2–C3
O1–P1–C1
OH group atoms
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
0.4
0.1
0.3
0.0
9.7
0.2
9.6
0.0
0.9
0.0
0.8
0.0
0.5
0.0
0.5
0.0
124.4
119.3
121.9
121.6
121.7
120.8
117.5
122.0
109.8
C atoms on inner ring
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
5.8
1.3
4.3
0.2
26.5
0.2
25.7
0.6
59.4
0.0
57.7
1.7
5.4
4.9
0.1
0.4
phinyl/phosphoryl substituents [62,65]. The other bond distances
and angles (Table 2) are unexceptional.
tion process. Simple p-quinones are well known to react with ter-
tiary phosphines, forming (irreversibly) zwitterionic adducts (Eq.
(3)) [22]. Therefore, an oxidation of the central hydroquinone cores
in 2a–b into quinones is most likely accompanied by the immedi-
ate addition of a –PR2 group from another 2a–b molecule, produc-
ing complex adducts.
3.3. Electrochemistry
The redox activities of compounds 2a–b and 3a–b were investi-
gated by cyclic voltammetry. CV profiles of compounds 2a/2b and
3a/3b are very similar, thus only the data for 2a and 3a is pre-
sented and discussed.
The cyclic voltammogram of 2a contains broad irreversible
waves in the oxidative scan (Fig. 4, left). Such a profile is most
likely dictated by chemical reactions which accompany the oxida-
ð3Þ
Fig. 6. Frontier orbitals (contour plots) of 3a.

3270
L.R. Pignotti et al. / Journal of Organometallic Chemistry 693 (2008) 3263–3272
A cyclic voltammogram for hydroquinone 3a displays an irre-
tion of the hydroquinone moiety will be directly influenced by this
interaction. Similar CV profiles were reported for hydroquinones
where proton-accepting (amine, carboxylate) substituents are
present on the quinone moiety [15,67].
versible peak at 0.96 V, which is associated with a peak at 0.55 V
on the return scan (Fig. 4, right). The non-reversibility of the oxida-
tion process here is most likely caused by the coupling of electron/
proton transfers during the oxidation process. The nature of the
hydroquinone/quinone oxidation (2eꢁ/2H+ transfer in one step or
two successive 1eꢁ/1H+ transfers) is well known to be proton-
dependant [66]. The hydroxylic protons in 3a–b are engaged in
strong hydrogen bonding with the adjacent P@O oxygens (as seen
in X-ray structures), thus the H+ transfer associated with the oxida-
3.4. Calculations
3.4.1. Structural parameters and calculated frontier molecular orbitals
(FMOs)
The selected structural parameters of 2a and 3a structures opti-
mized at B3LYP/6-31G(d) level of theory are shown in Table 3. Cal-
culated structural parameters of 2a and 3a are compared with the
X-ray structural data. The calculated geometries of 2a and 3a agree
with the experimentally obtained ones well within 2.7% error
(Table 3). Fluorine-free analogs of 2a and 3a (2aH and 3aH) were
also examined in order to estimate the effects of fluorine substitu-
tion on the structural and electronic properties. The calculated
structural parameters of 2aH and 3aH are also presented in Table 3.
The frontier orbitals of 2a and 3a calculated at B3LYP/TZVP//
B3LYP/6-31G(d) level of theory are shown in Figs. 5 and 6 with cor-
responding orbital composition shown in Tables 4 and 5, respec-
tively. In 2a the electron density of the HOMO is localized on the
electron rich inner ring, Ph rings, P atoms and OH centers (Table
4). The latter two localize HOMO electron density mainly in p-orbi-
tals (25.9% and 9.6% of all HOMO density) and these atoms can be
involved in metal ion coordination by donating electrons into read-
ily available d-orbitals. This agrees well with the ligating proper-
ties of 2a reported in this paper.
Table 5
Frontier orbitals of 3a and their compositions calculated at B3LYP/TZVP//B3LYP/
6-31G(d) level of theory
HOMO(ꢁ1)
ꢁ7.33
55.6
0.3
54.7
0.6
HOMO
LUMO
LUMO(+1)
Energy (eV)
C, H atoms on Ph rings
s
p
d
ꢁ5.89
0.6
0.1
0.5
0.0
ꢁ1.84
28.4
0.0
28.2
0.7
ꢁ1.36
84.3
0.2
82.0
2.0
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
P@O atoms
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
1.4
0.0
0.7
0.6
0.2
0.0
-0.1
0.2
17.4
0.2
12.5
4.7
12.7
0.0
9.7
3.8
F atoms
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
3.9
0.0
3.9
0.0
4.6
0.0
4.6
0.0
1.3
0.0
1.3
0.0
0.0
0.0
0.0
0.0
OH group atoms
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
0.9
0.0
0.9
0.0
32.0
0.0
32.0
0.0
1.4
0.1
1.3
0.0
0.9
0.8
0.1
0.0
Remarkably, the HOMO on 3a is located preferably on the cen-
tral ring carbon atoms, OH groups and F atoms (Table 5). Thus,
based on the electron distribution only in the HOMO it appears
that the coordination of metal ions to 3a would occur preferen-
tially at rather unusual [O,F] pockets and not at [O,(O@P)]. How-
ever, the ligation at the later site can not be excluded, as the
oxygen atom in the P@O functionality contributes significantly to
C atoms on inner ring
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
38.2
0.1
37.6
0.5
62.6
0.1
61.1
1.5
51.6
0.0
50.1
1.5
2.2
0.0
2.7
0.1
Fig. 7. Frontier orbitals (contour plots) of 2aH.

L.R. Pignotti et al. / Journal of Organometallic Chemistry 693 (2008) 3263–3272
3271
Fig. 8. Frontier orbitals (contour plots) of 3aH.
Table 6
Table 7
Frontier orbitals of 2aH and their compositions calculated at B3LYP/TZVP//B3LYP/6-
31G(d) level of theory
Frontier orbitals of 3aH and their compositions calculated at B3LYP/TZVP//B3LYP/6-
31G(d) level of theory
HOMO(ꢁ1)
HOMO
LUMO
LUMO(+1)
HOMO(ꢁ1)
HOMO
LUMO
LUMO(+1)
Energy (eV)
C, H atoms on Ph rings
s
p
d
ꢁ6.16
17.4
0.9
16.2
0.3
ꢁ5.56
14.2
2.2
11.8
0.3
ꢁ1.06
30.8
2.9
27.1
0.8
ꢁ0.94
89.6
0.3
87.2
2.1
Energy (eV)
C, H atoms on Ph rings
s
p
d
ꢁ7.26
57.5
0.2
56.7
0.6
ꢁ5.71
1.2
0.0
1.3
0.0
ꢁ1.72
32.7
0.3
31.6
0.8
ꢁ1.38
85.4
0.6
82.7
2.0
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
P atoms
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
22.8
4.6
18.1
0.1
26.1
4.7
21.0
0.4
12.4
0.3
9.0
7.4
0.1
5.0
2.3
P@O atoms
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
1.7
0.0
1.2
0.5
0.5
0.0
0.0
0.4
15.0
0.1
10.5
4.4
11.1
0.0
8.4
3.4
3.1
H atoms
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
0.4
0.2
0.2
0.4
0.1
0.1
0.0
0.1
0.2
0.0
0.2
0.2
0.2
0.2
0.0
0.2
H atoms
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
0.1
0.0
0.1
0.0
0.1
0.0
0.1
0.0
0.3
0.1
0.2
0.0
0.1
0.1
0.0
0.0
OH group atoms
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
15.1
0.0
15.0
0.0
13.8
0.0
13.8
0.0
0.5
0.0
0.5
0.0
0.0
-0.1
0.1
OH group atoms
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
0.0
0.0
0.0
0.0
33.2
0.0
33.2
0.0
0.5
0.1
0.4
0.0
0.4
0.3
0.1
0.0
0.0
C atoms on inner ring
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
44.5
0.3
43.3
0.8
45.8
0.3
44.7
0.9
56.1
-0.1
54.8
1.4
2.9
0.1
2.6
0.1
C atoms on inner ring
s
p
d
Pop. (%)
Orbitals:
Orbitals:
Orbitals:
40.7
0.1
40.2
0.4
64.9
0.0
63.5
1.4
51.5
0.2
50.0
1.3
3.1
0.0
3.8
0.1
the composition of the LUMO and can function as an acceptor
ligand.
with fluorination by 0.08 eV, the HOMO and the LUMO orbital
energies are lower in the fluorinated compounds by 0.42 and
0.5 eV, respectively. Thus, the presence of highly electronegative
fluorine atom clearly stabilizes the HOMO and the LUMO levels
in 2a rendering it more stable than the unfluorinated counterpart
2aH. Similarly, the HOMO and LUMO energies of 3a and 3aH are
ꢁ5.98 and ꢁ1.84 eV and ꢁ5.71 and ꢁ1.72 eV, respectively (Tables
5 and 7). Here the HOMO and LUMO orbitals in fluorinated com-
pounds are shifted by 0.27 and 0.12 eV towards lower energy val-
ues with the HOMO–LUMO gap increasing with fluorination by
3.4.2. Effect of fluorine atoms on the stability of the compounds
To investigate any increased stability of 2a and 3a we per-
formed DFT calculations using hydrogens instead of fluorine atoms
on the central ring (compounds 2aH and 3aH). Frontier orbitals of
2aH and 3aH calculated at B3LYP/TZVP//B3LYP/6-31G(d) level of
theory are shown in Figs. 7 and 8 with corresponding orbital com-
position shown in Tables 6 and 7, respectively.
The calculated energies of the HOMO and the LUMO orbitals in
2a and 2aH are ꢁ5.98 and ꢁ1.56 eV and ꢁ5.56 and ꢁ1.06 eV,
respectively (Tables 4 and 6, respectively) with the HOMO–LUMO
gaps at 4.42 and 4.5 eV. While the HOMO–LUMO gap decreases
0.15 eV. Such batho- and hypsochromic shifts in fluorinated p-con-
jugated organic molecules have previously been attributed to an
interplay between inductive and mesomeric effects of fluorine
atoms on the specific molecular backbone [68].

3272
L.R. Pignotti et al. / Journal of Organometallic Chemistry 693 (2008) 3263–3272
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A comparison of the occupied orbitals also provides insights
(1989) 1733.
into the stability of 2a and 3a as compared with unfluorinated
compounds 2aH and 3aH. HOMO in 2a has 31.1% of total electron
density located on Ph rings (Fig. 4 and Table 4), whereas only 14.2%
is located in 2aH (Fig. 5 and Table 5). The orbitals involved are pre-
dominantly p-type, and there is strong electron delocalization be-
tween the fluorine atoms, the inner ring, the phosphorus atoms
and the phenyl carbons in 2a, but not in 2aH. In 3a and 3aH case
this effect is less pronounced and primarily HOMO(ꢁ1) orbitals
are involved into the interactions of inner ring with phosphorus
atoms as well as Ph groups.
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4. Conclusion
Compounds containing 2,5-bis(phosphine)- and 2,5-(bis)phos-
phinyl-p-hydroquinone motifs can be conveniently synthesized.
They represent a relatively unexplored class of noninnocent binu-
cleating ligands, which should be suitable for the construction of
highly linear polymetallic assemblies. The electronic properties of
the ligands are shown to be sensitive to the nature of the substit-
uents at the remaining two positions on the hydroquinone ring,
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Supplementary material
CCDC 687861, 681176 and 682455 contains the supplementary
crystallographic data for 2a, 3a and 3b. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Acknowledgment
Support by Michigan Technological University is acknowledged.
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