Conjugated Triarylboryl Donor−Acceptor Systems Supported by 2,2′-Bipyridine: Metal Chelation Impact on Intraligand Charger Transfer Emission, Electron Accepting Ability, and “Turn-on” Fluoride Sensing

To investigate the impact of metal-chelation on intraligand charge transfer emission involving a triarylboron group, three 2,2′-bipyridine derivative molecules, 5,5′-bis(BMes2)-2,2′-bipy (B2bpy), 5-(BMes2)-5′-(NPh2)-2,2′-bipy (BNbpy), and 5,5′-bis(NPh2)-2,2′-bipy (N2bpy) have been synthesized, which can be described as donor-only, donor−acceptor, and acceptor-only systems. Each of these molecules displays distinctive electrochemical and photophysical properties with BNbpy and N2bpy being bright emitters and B2bpy being a strong electron acceptor. In addition, BNbpy displays a “turn-on” fluorescent response while B2bpy has a “turn-off” response upon binding with fluoride ions. These molecules can readily chelate to a PtPh2 or a PtCl2 group, producing square planar complexes Pt(B2bpy)Ph2 (Pt-1), Pt(B2bpy)Cl2 (Pt-1a), Pt(BNbpy)Ph2 (Pt-2), Pt(BNbpy)Cl2 (Pt-2a), and Pt(N2bpy)Ph2 (Pt-3) that have significantly altered electrochemical and photophysical properties from those of the free ligands. Metal chelation has been found to greatly enhance the electron accepting ability of the three ligands, especially B2bpy and BNbpy. The Ph and Cl auxiliary ligands have also been found to have a significant impact on the electrochemical and photophysical properties of the complexes. B2bpy complexes Pt-1 and Pt-1a are not luminescent at ambient temperature while BNbpy complexes Pt-2 and Pt-2a display room temperature phosphorescence in solution under air that has a similar “turn-on” response toward fluoride ions as the free BNbpy does but with a much more dramatic color switch (orange or red to blue-green). The persistent intraligand N→B charge transfer transition in the BNbpy complexes is believed to play a key role in their unique phosphorescent response toward fluorides. The complex Pt-3 displays a bright blue-green phosphorescence in solution at ambient temperature. Density functional theory computations established that the lowest electronic transition in the Pt(II) complex is from the Pt(II) d orbital and the auxiliary ligand to the π* orbital of the 2,2′-bipy derivative ligand

Benzene C−H Activation by Two Isomeric Platinum(II) Complexes of Bis(<i>N</i>-7-azaindolyl)methane

Two isomeric Pt(II) complexes, Pt(L1)(CH3)2 (1a) and Pt(L2)(CH3)2 (2a), based on two isomeric and fluorescent ligands of bis(N-7-azaindolyl)methane, L1 (symmetric) and L2 (asymmetric), have been synthesized and fully characterized by NMR and X-ray diffraction analyses. In the presence of [H(Et2O)2][BAr‘4] (Ar‘ = 3,5-bis(trifluoromethyl)phenyl), both 1a and 2a are capable of activating a benzene C−H bond readily at ambient temperature. The products from benzene activation by 1a and 2a have been isolated as [Pt(L1)Ph(SMe2)][BAr‘4] (1b) and [Pt(L2)Ph(SMe2)][BAr‘4] (2b). The structures of 1b and 2b have been determined by X-ray diffraction analyses. The phenyl ligand in 2b is bound exclusively trans to the pyrrole nitrogen atom of L2

Nonconjugated Dimesitylboryl-Functionalized Phenylpyridines and Their Cyclometalated Platinum(II) Complexes

To investigate possible through-space charge transfer transitions between a Pt(II) ion and a triarylborane unit, the two new nonconjugated molecules 1 and 2 have been synthesized and fully characterized. Compound 1 has a V-shaped geometry, while 2 has a U-shaped geometry. In 1 a BMes2Ar group and a ppy group (Mes = mesityl, ppy = phenylpyridine) are linked together by a SiPh2 unit, while in 2 these groups are joined together by a 1,8-naphthyl linker. The crystal structures of 1 and 2 were determined by single-crystal X-ray diffraction analyses. Their cyclometalated compounds Pt-1 and Pt-2 with a Pt(acac) unit chelated to the ppy site have been synthesized. Computational and experimental examinations on the photophysical properties of the free ligands and the Pt(II) compounds revealed that the molecular shape and geometry of the molecule have a distinct impact on the fluorescence and phosphorescence of these molecules. Pt-1 is a bright phosphorescent emitter with λem 490 nm and Φ = 66% while Pt-2 is very weakly emissive with λem 567 nm and Φ = ∼0.05%. Anions such as fluoride were found to have no impact on the phosphorescence of these two Pt(II) compounds, thus establishing that phosphorescence of these molecules does not involve a through-space charge transfer transition between the Pt(ppy)(acac) unit and the BMes2Ar unit

Alkylaluminum Complexes Containing Pyridyl Amido Ligands. Syntheses, Structures, and NMR Spectroscopic Studies of [Al(CH₃)₂(NHCH₂-2-Py)]₂, [Al(CH₃)₂(NHCH₂-4-Py)]₂, [Al(CH₃)₂(HNCH₂-4-Py)Al(CH₃)₃]₂, and Al(CH₃)₃(NH₂CH₂-4-Py)Al(CH₃)₃

The reactions of Al(CH3)3 with 2-(aminomethyl)pyridine and 4-(aminomethyl)pyridine have been investigated. The reaction of Al(CH3)3 with 2-(aminomethyl)pyridine in a 1:1 ratio in toluene yields a cis dinuclear compound [Al(CH3)2(NHCH2-2-Py)]2 (1), where the aluminum center is five-coordinate. The reaction of Al(CH3)3 with 4-(aminomethyl)pyridine in a 1:1 ratio in toluene yields a four-coordinate dinuclear compound [Al(CH3)2(NHCH2-4-Py)]2 (2), while the reaction in a 2:1 ratio yields the adduct Al(CH3)3(NH2CH2-4-Py)Al(CH3)3 (4) initially which changes to the tetranuclear compound [Al(CH3)2(HNCH2-4-Py)Al(CH3)3]2 (3) in solution and in the solid state. Compound 3 can also be obtained by the reaction of Al(CH3)3 with 2 in a 2:1 ratio in toluene. Both cis and trans isomers are present in solution for compounds 2 and 3, but only the crystals of the trans products were obtained. The structures of compound 1 and the trans products of 2 and 3 were determined by X-ray diffraction analyses

Extending π-Conjugation of Triarylborons with a 2,2-Bpy Core: Impact of Donor−Acceptor Geometry on Luminescence, Anion Sensing, and Metal Ion Binding

Four new 2,2′-bipyridine-based molecules functionalized by either BMes2-phenyl (Mes = mesityl) or NPh2-phenyl5,5′-(p-BMes2-phenyl)2-2,2′-bpy (5,5′-BP2bpy, 1), 4,4′-(p-BMes2-phenyl)2-2,2′-bpy (4,4′-BP2bpy, 2), 4-(p-BMes2-phenyl)-4′-(p-NPh2-phenyl)-2,2′-bipy (4,4′-BPNPbpy, 4), and 4,4′-(p-NPh2-phenyl)2-2,2′-bpy (4,4′-NP2bpy, 5)have been synthesized. Their complexes with PtPh2 have been synthesized and fully characterized. The electronic and photophysical properties of the new molecules have been examined by electrochemical, absorption, and luminescence spectroscopic analysis and DFT calculations, which show significant differences from those of the related but smaller 2,2′-bpy derivatives functionalized directly by either BMes2 or NPh2 groups that we reported previously. Molecules 1, 2, and 4 and their Pt(II) complexes respond to fluoride ions in both absorption and emission modes. The donor−acceptor molecule 4 and its Pt(II) complex have a distinct fluorescence/phosphorescence turn-on response toward fluoride or cyanide ions. Molecules 1, 2, 4, and 5 also respond to Zn(II) ions in both absorption and emission modes. The diboryl molecules 1 and 2 have a distinct and contrasting fluorescence response toward Zn(II) ionturn-off for 1 and turn-on for 2demonstrating the significant impact of molecular geometry on metal ion binding and fluorescence

Racemic Atropisomeric N,N-Chelate Ligands for Recognizing Chiral Carboxylates via Zn(II) Coordination: Structure, Fluorescence, and Circular Dichroism

Two racemic atropisomeric N,N′-chelate ligands, bis{3,3′-[N-Ph-2-(2′-py)indolyl]} (1) and bis{3,3′-N-4-[N-2-(2′-py)indolyl]phenyl-2-(2′-py)indolyl} (2), have been found to be able to distinguish the enantiomers of Zn((R)-BrMeBu)2 and Zn((S)-BrMeBu)2 where BrMeBu = O2CCH(Br)CHMe2, with a distinct and intense CD spectral response at approximately the 10 μM concentration range. Computational studies established that the (R)-1-Zn((R)-BrMeBu)2 or (S)-1-Zn((S)-BrMeBu)2 diastereomer is more stable than (R)-1-Zn((S)-BrMeBu)2 or (S)-1-Zn((R)-BrMeBu)2. In addition, computational studies showed that the CD spectra of (S)-1-Zn((S)-BrMeBu)2 and (S)-1-Zn((R)-BrMeBu)2 are similar. 1H NMR spectra confirmed that these two diastereomers exist in solution in about a 2:1 ratio for both complexes of 1 and 2. The distinct CD response of the racemic ligands 1 and 2 toward the chiral zinc(II) carboxylate is therefore attributed to the preferential formation of one diastereomer. The binding modes of the zinc(II) salt with ligands 1 and 2 were established by the crystal structures of the model compounds 1-Zn(tfa)2 and 2-Zn(tfa)2 (tfa = CF3CO2−), where the ZnII ion is chelated by the two central pyridyl groups in the ligand. Fluorescent titration experiments with various zinc(II) salts showed that the fluorescent spectrum of the atropisomeric ligand displays an anion-dependent change. The zinc(II) binding strength to the N,N′-chelate site of the atropisomeric ligand has been found to play a key role in the selective recognition of different chiral zinc(II) carboxylate derivatives by the racemic atropisomeric ligands

Diboron and Triboron Compounds Based on Linear and Star-Shaped Conjugated Ligands with 8-Hydroxyquinolate Functionality: Impact of Intermolecular Interaction and Boron Coordination on Luminescence

New 8-R-quinoline functionalized linear and star-shaped conjugated molecules have been synthesized using Suzuki−Miyaura coupling methods (R = MeO, L1−L5; R = CH3OCH2O, L1‘−L5‘). When treated with HCl, L1‘−L5‘ are converted readily to the corresponding 8-hydroxyquinoline compounds L(OH)1−L(OH)5 which react readily with BPh3 in refluxing THF to produce the corresponding polyboron chelate compounds B1−B5 in good yields. L1−L5 and B1−B5 display similar thermal stability with Td at ∼300 °C. Experimental and molecular orbital calculation results showed that the chelation by boron stabilizes the LUMO level of the ligand and narrows the HOMO−LUMO gap, resulting in the blue emission of the ligands and the green or orange emission of the boron compounds. Crystal structures of L1, L3, and L5 showed that these molecules have layered arrangements in the solid state with significant intermolecular π−π interactions. The linear diboron B5 displays concentration and temperature-dependent emission in solution, attributable to intermolecular interactions. The properties of a monoboron compound BPh2(5-Ph-8-MeO-q) (B0) and its corresponding free ligand L0 were investigated and compared to the closely related diboron compound B1 and the ligand L1, which revealed that the increase of the number of chromophores linked by an aromatic group has a significant impact on thermal stability and the HOMO and LUMO energy levels

Conjugated Triarylboryl Donor−Acceptor Systems Supported by 2,2′-Bipyridine: Metal Chelation Impact on Intraligand Charger Transfer Emission, Electron Accepting Ability, and “Turn-on” Fluoride Sensing

To investigate the impact of metal-chelation on intraligand charge transfer emission involving a triarylboron group, three 2,2′-bipyridine derivative molecules, 5,5′-bis(BMes2)-2,2′-bipy (B2bpy), 5-(BMes2)-5′-(NPh2)-2,2′-bipy (BNbpy), and 5,5′-bis(NPh2)-2,2′-bipy (N2bpy) have been synthesized, which can be described as donor-only, donor−acceptor, and acceptor-only systems. Each of these molecules displays distinctive electrochemical and photophysical properties with BNbpy and N2bpy being bright emitters and B2bpy being a strong electron acceptor. In addition, BNbpy displays a “turn-on” fluorescent response while B2bpy has a “turn-off” response upon binding with fluoride ions. These molecules can readily chelate to a PtPh2 or a PtCl2 group, producing square planar complexes Pt(B2bpy)Ph2 (Pt-1), Pt(B2bpy)Cl2 (Pt-1a), Pt(BNbpy)Ph2 (Pt-2), Pt(BNbpy)Cl2 (Pt-2a), and Pt(N2bpy)Ph2 (Pt-3) that have significantly altered electrochemical and photophysical properties from those of the free ligands. Metal chelation has been found to greatly enhance the electron accepting ability of the three ligands, especially B2bpy and BNbpy. The Ph and Cl auxiliary ligands have also been found to have a significant impact on the electrochemical and photophysical properties of the complexes. B2bpy complexes Pt-1 and Pt-1a are not luminescent at ambient temperature while BNbpy complexes Pt-2 and Pt-2a display room temperature phosphorescence in solution under air that has a similar “turn-on” response toward fluoride ions as the free BNbpy does but with a much more dramatic color switch (orange or red to blue-green). The persistent intraligand N→B charge transfer transition in the BNbpy complexes is believed to play a key role in their unique phosphorescent response toward fluorides. The complex Pt-3 displays a bright blue-green phosphorescence in solution at ambient temperature. Density functional theory computations established that the lowest electronic transition in the Pt(II) complex is from the Pt(II) d orbital and the auxiliary ligand to the π* orbital of the 2,2′-bipy derivative ligand

Comparative Study on Tetrahedral and Tripodal Luminescent Silane and Methane Compounds with a 2,2‘-Dipyridylamino Group

A comparative study on methane and silane derivatives that contain either one or four 2,2‘-dipyridylamino group (dpa) functionalized groups has been carried out. Six new compounds, (p-dpa-phenyl)triphenylsilane (1), (p-dpa-phenyl)triphenylmethane (2), tetra(p-dpa-phenyl)silane (3), tetra(p-dpa-phenyl)methane (4), tetra(p-dpa-biphenyl)silane (5), and tetra(p-dpa-biphenyl)methane (6), have been synthesized using Suzuki coupling, Ullmann condensation methods, or simple substitution reactions. The structures of 3 and 4 have been determined by X-ray diffraction analyses. Thermal and luminescent properties have been investigated, which revealed that these new compounds are luminescent in the violet−blue region and the methane derivatives in general have a higher thermal stability than the corresponding silane analogues. The electronic properties of the new compounds were investigated experimentally and theoretically by molecular orbital calculations, which revealed that there is a subtle difference between the methane derivatives and their silane analogues

Impact of Cyclometalation and π-Conjugation on Photoisomerization of an N,C-Chelate Organoboron Compound

N,C-Chelate four-coordinate boron compounds that contain a B(ppy)Mes2 unit (ppy = 2-phenylpyridyl, Mes = mesityl) are a new class of photochromic molecules discovered recently by our group that undergo photoisomerization upon exposure to light. To examine the influence of a covalently bound transition metal ion on the photochromic properties of this class of boron compounds, a new molecule (L1) that contains two linearly conjugated ppy units has been synthesized. A BMes2 group was attached to L1 via chelation with one of the ppy units, producing a new four-coordinate boron compound, B1. The reactions of B1 with PtPh2(DMSO)2 produced a Pt(II) cyclometalated compound, Pt1, where a PtPh(DMSO) unit is bound to the second ppy unit of B1. Replacement of DMSO in Pt1 by p-t-Bu-pyridine provided a new compound, Pt2. A third Pt(II) compound, Pt3, where a Pt(dpm) group (dpm = dipivaloylmethane) is chelated to the second ppy site of B1, was also synthesized successfully. The crystal structures of B1 and Pt3 have been determined by single-crystal X-ray diffraction analyses. The photophysical and photochromic properties of B1 and Pt1–Pt3 have been examined. Experimental and computational studies established that Pt(II) cyclometalation to B1 stabilizes a 3LC state that involves π→π* transitions localized on the ppy–ppy conjugated backbone. This 3LC state of the Pt(II) compounds is highly phosphorescent, with quantum efficiencies being 0.16, 0.13, and 0.45 for Pt1, Pt2, and Pt3, respectively, in toluene and at ambient temperature. The B(ppy)Mes2 chromophore in all three Pt(II) compounds has been found to undergo photoisomerization in a similar manner to that of B1, but with a much lower quantum efficiency than B1. Deactivation of the photoisomerization process by the 3LC state has been found to be most likely responsible for the low photoisomerization quantum efficiency of the Pt(II) compounds