Enhancing electron accepting ability of triarylboron via π-conjugation with 2,2′-bipy and metal chelation: 5,5′-bis(BMes 2)-2,2′-bipy and its metal complexes

Article abstract of DOI:10.1021/ja0725652

The synthesis of a new diboron compound, 5,5′-bis(BMes₂)-2,2′-bipy (B2bipy, 1), has been accomplished. The conjugation of the 2,2′-bipy unit with the two BMes₂ groups in 1 has been found to greatly enhance the electron-accepting ability of the boron centers, compared to the biphenyl analogue. 1 is an effective chelate ligand for metal ions and three metal complexes; namely, [Cu^I(B2bipy)(PPh₃)₂][BF₄], 2, Pt^II(B2bipy)Ph2, 3a, and Pt^II(B2bipy)Me₂, 3b, have been synthesized. The binding of metal ions to B2bipy was found to further significantly increase the electron-accepting ability/Lewis acidity of the boron centers. Cyclic voltammetry diagrams revealed that the free ligand and all three complexes display two reversible reduction peaks that are separated by 0.31-0.40 V. The first reduction potential (E_1/2) for 1, 2, 3a, and 3b was found to be -1.69, -1.36, -1.34, and -1.38 V, respectively, versus FeCp₂^+/0 in DMF, which supports that the electron-accepting ability of the metal complexes is much greater than that of BMes(C₆F₅)₂. The strong electron-accepting ability/Lewis acidity of the metal complexes and the free ligand was manifested by their ability to sequentially bind to 2 equiv of F- ions in non-alcoholic solvents with K₁ ≥ 10⁸ and 109 M^-1 for 1 and 3a, respectively, and to 1 equiv of F- ions in the presence of methanol/ethanol or water. The crystal structures of 1, its 2:1 fluoride adduct [NBu₄]2[1F₂], and 3a were established by X-ray diffraction analyses. The 1:1 fluoride adduct of 2, [Cu^I(B2bipyF)(PPh₃)₂], 2F, isolated directly from the reaction of 2 with NBu4F in a 4:1 mixture of CH₃OH and CH₂Cl₂ was characterized. All metal complexes are intensely colored and display a characteristic metal-to-ligand (B2bipy) charge transfer absorption band in the visible region, which was found to be highly sensitive toward binding by anions such as fluorides. Copyright

Full text of DOI:10.1021/ja0725652

Published on Web 05/26/2007
Enhancing Electron Accepting Ability of Triarylboron via π-Conjugation with
2,2′-Bipy and Metal Chelation: 5,5′-Bis(BMes2)-2,2′-bipy and Its Metal
Complexes
Yi Sun, Nicole Ross, Shu-Bin Zhao, Krista Huszarik, Wen-Li Jia, Rui-Yao Wang,
Donal Macartney, and Suning Wang*
Department of Chemistry, Queen’s UniVersity, Kingston, Ontario K7L 3N6, Canada
Received April 12, 2007; E-mail: wangs@chem.queensu.ca
Triarylboron compounds with a high Lewis acidity/electron-
accepting ability are highly sought-after materials because of their
potential applications in olefin polymerization,1 nonlinear optics,2
organic light emitting diodes (OLEDs),3 and anion sensors.4 For
applications as effective and competitive electron transport materials
in OLEDs, in addition to being chemically and thermally stable,
triarylborons are required to have a deep LUMO level with a strong
electron-accepting ability. Stable triarylborons with a high Lewis
acidity are also desired for use as fluoride sensors in aqueous or
alcohol solvents to overcome the competing binding of protons for
fluoride ions.4 B(C6F5)3 (E1/2red ) ∼-1.17 V vs FeCp2
)
+/0 5a and a
number of diboron molecules based on fluorinated phenyl or
biphenyl possess the strongest Lewis acidity among triarylborons.1,5
However, their general instability toward ambient air renders them
unsuitable for applications in OLEDs or sensors. Stable triarylboron
compounds usually contain two or three ortho-substituted aryl
groups such as mesityl to protect the boron center from nucleophiles
such as H2O. This approach, however, sacrifices the Lewis acidity
of the boron center due to the electron-donating nature of mesityls,
Figure 1. Left: cyclic voltammetry diagrams of 1, 2, 3a, and 3b versus
Ag/AgCl reference electrode (E1/2ox of FeCp2 ) 0.55 V) in DMF (0.10 M
NBu4PF6, scan rate ) 2 V/s for 1 and 3a and 0.5 V/s for 2 and 3b). Right:
crystal structures of 1 and 3a.
red
red1
red2
as evidenced by the typical E1/2 values (-2.0 to -2.8 V vs
reduction peaks (E1/2
) -1.69 V, E1/2
) -2.07 V vs
FeCp2+/0) of BMes2(Ar).
FeCp2+/0), attributable to the consecutive reduction of the two boron
red1
One way to enhance the Lewis acidity or the electron-accepting
ability of protected triarylboron is to link two or more boron centers
to a π-conjugate unit. In fact, Kaim and co-workers have shown
nearly two decades ago that, by connecting two BMes2 groups with
a phenyl or a biphenyl, the Lewis acidity of the molecule can be
substantially enhanced.6a Yamaguchi and co-workers have dem-
onstrated that the attachment of three BMes2 groups to a B(9,10-
centers. The E1/2
of 1 is ∼0.27 V more positive than that of
BMes2-(C6H4)2-BMes2 (ca. -1.96 V vs FeCp2+/0),6a thus confirming
that bipy is indeed more effective than biphenyl in enhancing the
electron-accepting ability of the boron center. The difference
between the first and the second reduction potential, ∆E1/2red (0.38
V), of 1 is 0.13 V greater than that of BMes2-(C6H4)2-BMes2 (0.25
V), indicative of greater electronic communication between the two
red
anthryl)3 core can shift E1/2 to -1.66 V vs FeCp2+/0, thus
red1
boron centers in 1. It is noteworthy that E1/2
of 1 is similar to
significantly increasing the electron-accepting ability.6b Shirota and
that of BMes(C6F5)2 (-1.72 V vs FeCp2+/0).5a The electronegative
py rings and the coplanarity of the bipy unit that allows effective
conjugation of the two boron centers in 1 are clearly responsible
for its high Lewis acidity, relative to the biphenyl analogue. MO
calculations have in fact confirmed that there is an extensive
π-conjugation between the B atoms and the bipy unit (see
Supporting Information).
Noda have established that 5,5′-bis(BMes2)-2,2′-bithiophene has a
red1
strong electron-accepting capability (E1/2
) -1.76 V vs Ag/
AgNO3), enabling its successful use as an electron transport material
in OLEDs.3a Compared to phenyl, biphenyl, or thiophene, pyridyl
or bipyridyl linkers should be more effective in enhancing Lewis
acidity of triarylboron due to the electronegative nitrogen atoms.
2,2′-Bipy is especially attractive as a linker because of the possibility
of further tuning the electron-accepting ability of the boron center
by metal chelation. On the basis of these considerations, we have
explored the synthesis of 5,5′-bis(BMes2)-2,2′-bipy (B2bipy, 1) and
examined the impact of metal chelation on Lewis acidity and
photophysical properties of 1. The preliminary results are presented
herein.
The synthesis of 1 was accomplished by the reaction of BMes2F
with 5,5′-(Li)2-2,2′-bipy in a 2.2:1 ratio at -100 °C in ∼58% yield.
The two pyridyl rings are coplanar, as revealed by the crystal
structure shown in Figure 1. 1 is stable toward ambient air and
H2O in solution and the solid state. Its cyclic voltammetry (CV)
diagram in DMF has two reversible and highly reproducible
Like 2,2′-bipy, 1 is an effective chelate ligand for metal ions as
demonstrated by a Cu(I) complex, [Cu(B2bipy)(PPh3)2][BF4] (2),
obtained from the reaction of [Cu(CH3CN)2(PPh3)2][BF4] with 1,
and two Pt(II) complexes, Pt(B2bipy)R2, R ) Ph, 3a, R ) Me, 3b,
obtained from the reactions of the corresponding [Pt(SMe2)R2]n with
1. The choice of these two metal ions is based on the fact that they
are well-known to display metal-to-ligand (diimine) charge transfer
(MLCT) transitions.7,8 2 and 3a are air stable in solution and the
solid state, while 3b appears to be sensitive to air. The crystal
structure of 3a determined by X-ray diffraction shows that the Pt-
(II) center has a typical square planar geometry (Figure 1). Like
the free ligand 1, all complexes also display two reversible and
highly reproducible reduction peaks in their CV diagrams (Figure
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10.1021/ja0725652 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
∼2.5 equiv of F- to form the 2:1 adduct, and the conversion of the
2:1 adduct to the 1:1 adduct after extraction by D2O. Using the
UV-vis titration data, the F- binding constants K1 and K2 in CH2-
Cl2 were determined to be g109 and ∼106 M-1 for 3a, respectively
(see Supporting Information).
Consistent with the strong binding by the first F- is the
observation that 2 and 3a are able to form the 1:1 adduct with excess
F- in the presence of methanol as established by UV-vis and NMR
experiments. In fact, the addition of excess TBAF to 2 in a CH2-
Cl2/CH3OH solution (1:4) resulted in the quantitative precipitation
of the yellow 1:1 adduct Cu(B2bipyF)(PPh3)2, 2F, which has been
fully characterized, further confirming the formation of the 1:1
adduct in methanol. The inability of the complex to bind to the
second F- in the presence of methanol is again due to the decreased
Lewis acidity of the 1:1 adduct. The unusual ability of 2 and 3a to
bind to F- in the presence of methanol confirms unequivocally the
exceptionally high Lewis acidity of the boron centers in the
complexes. 1 is also capable of binding 1 equiv of F- in the
presence of alcohol (K ≈ 104 M-1) or 2 equiv of F- in non-alcoholic
solvents (K1 g 108 M-1, K2 ≈ 106 M-1), which can be monitored
by either absorption or fluorescence spectra. The structure of the
2:1 adduct [NBu4]2[1F2] was established by X-ray diffraction.
Nonetheless, the metal complexes are clearly more attractive for
anion binding/sensing applications because of their high Lewis
acidity and the distinct visual MLCT-based color change.
Figure 2. The MLCT region of the UV-vis titration spectra of 3a (3.2 ×
10-5 M) by TBAF in CH2Cl2. The red region corresponds to the spectral
change with the addition of 1 equiv of F-. Inset: visual color change of 3a
(1.0 × 10-3 M) with the addition of 1 equiv of F- (middle) and 3 equiv of
F- in CH2Cl2 and the reverse color switching after addition of H2O (middle).
1). Most remarkable is that the reduction peaks of the complexes
red1
are much more positive than those of 1 (e.g., E1/2
is -1.36 V
for 2, -1.34 V for 3a, and -1.38 V for 3b vs FeCp2+/0) and close
to that of B(C6F5)3 (∼-1.17 V),5a demonstrating that the electron-
accepting ability of B2bipy is much further enhanced by metal
chelation. Similar change in reduction potential of substituted
bipyridines upon coordination has been observed previously.9 Also
noteworthy is the difference of E1/2red1 and E1/2red2 for the free ligand
and the complexes: 0.31 V for 2, less than that of 1; 0.40 V for 3a
and 0.38 V for 3b, similar to that of 1 (0.38 V).
Acknowledgment. We thank the Natural Sciences and Engi-
neering Research Council of Canada for financial support.
Supporting Information Available: Synthetic and experimental
details, full characterization data, CV diagrams, UV-vis spectra, NMR
and UV-vis titration data by TBAF, and all crystal data. This material
is available free of charge via the Internet at http://pubs.acs.org.
In contrast to the colorless free ligand 1, the metal complexes
are all intensely colored orange for 2, red for 3a, and burgundy for
3b in the solid state and in CH2Cl2 due to a broad metal-to-ligand
(B2bipy) charge transfer band in the visible region. The MLCT
band shifts to higher energy with increasing solvent polarity.
Compared to the bipy chelate analogues, the MLCT band of the
B2bipy complexes is red-shifted by ∼100 nm (e.g., in CH2Cl2, Pt-
(bipy)Ph2, λMLCT ) 438 nm; 3a, λMLCT ) 542 nm), consistent with
a deeper LUMO (π*) of the B2bipy ligand. Because the MLCT
transition involves the π* orbital of B2bipy with significant
contributions from the B atoms, it was anticipated to be sensitive
to anion binding to the B centers, thus its possible use as an effective
probe for anions. To test this idea, we chose to examine F- binding
to 2 and 3a, due to the fact that protected triarylborons are known
to bind to F- with a high selectivity.4 To our delight, 2 and 3a
were found to change color instantly upon the addition of NBu4F
(TBAF) in organic solvents, and the resulting color is dependent
on the ratio of [F-]/[complex].
In non-alcoholic solvents, two stages of color change were
observed for both compounds; the addition of ∼1 equiv of F-
changes the color of 2 from orange to yellow, 3a from red to orange,
and further addition of ∼1.5 equiv of F- changes 2 from yellow to
colorless, 3a from orange to light yellow. This two-stage color
change can be attributed to the formation of a 1:1 (F-:complex)
adduct and a 2:1 adduct, respectively, and the sequential quenching
of the MLCT band of the free complex and the 1:1 intermediate,
as illustrated by the UV-vis spectral change and the corresponding
color change of 3a with F- addition in Figure 2. Interestingly, the
addition of H2O to the solution of the 2:1 adduct reversed the color
to that of the 1:1 adduct but not the original complex, an indication
that the first F- is tightly bound to the boron center. This observation
was also confirmed by 1H and 19F NMR titration experiments which
showed the nearly quantitative formation of the 1:1 product after
the addition of 1 equiv of F- to the complex, the requirement of
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