Synthesis and coordination chemistry of boron-functionalized poly(pyrazolylpyridyl)borate with Ln(III) metals

A new family of lanthanide/boron-based complexes have been synthesized from the novel substituted poly(pyrazolylpyridyl)borate ligand containing phenyl substituents on the boron atom Ph2Bp py (L). The complexes have been fully characterized by analytical and spectroscopic methods. The single crystal X-ray structure and mass spectroscopy reveal the isostructural dinuclear complexes: [Ln2(L)2(NO3)2(OH)2] where Ln = Eu, Gd or Tb. Upon excitation of the Ln(III) complexes in DCM, their luminescence spectra display narrow f–f transitions. It is found that the ligand can efficiently sensitize the Tb(III) complex because of the better triplet state energy match to the Tb(III)– 5 D0 excited states.


Introduction
oron, the only non-metal of Group 13, displays a unique structural diversity in its combinations with transition metals [1].The enormous diversity of boron chemistry ranges from polyboranes [2] to carboranes [3] to metallaboranes [4] and metal complexes of organoboron ligands [5].In 1966, poly(pyrazolyl)borates were first reported by S. Trofimenko [6].The poly(pyrazolyl)borate ligands have been proven to be extremely popular amongst coordination chemists for a wide range of applications because of their ease of synthesis, ease of functionalisation, and the steric protection which they afford to metal centres [7].
Over the years poly(pyrazolyl)borates such as bidentate Bp ligands have been compared to -diketonates (Chart 1) [5].These 'first-generation' ligands are labile, rich in electron density and their coordination behavior is strongly dependent on the coordinating metal [8][9][10][11] In subsequent years, a new class of C 3 -substituted poly(pyrazolyl)borate ligands were developed in which additional coordinating groups have been attached to the pyrazolyl rings, completely changing the character of the simple first-generation ligands.Constituents such as pyridine [12].(Bp py in Chart 1), bipyridyl, [13] thioanisyl, [14] pyrazinyl, [15] and carboxamide [16] units have B been appended to the pyrazolyl C 3 positions of poly(pyrazolyl)borates, converting the ligands to polydentate chelates.
The Ward group prepared new ligands by the attachment of a 2-pyridyl substituent to each pyrazolyl ring at the C 3 position converting each 'arm' of the ligand to a tridentate chelate [17][18].Substitution of poly(pyrazolyl)borates is also confined largely to the boron atom [5].The potentially reactive B-H bonds in Bp py , may be replaced with either alkyl or aryl groups.Suitably sized ligand substituents can help to prevent agostic interaction through steric blocking of the space above and below the poly(pyrazolyl)borate ligands [19][20].Substituents on the boron atom of Bp ligands have shown great stability such as on [(Ph 2 Bp) 2 Ni].[21] Whilst the complex [(H 2 Bp)Ag] is air-sensitive, the analogue [(Ph 2 Bp)Ag] is very stable, [22] suggesting that the stability of these complexes result from a combination of steric effects and not just solely from the aromatic substituents on the pyrazolyl rings.
As part of our interest in luminescent lanthanide complexes, we describe the synthesis, crystal structure and luminescence behavior of a series of lanthanide complexes with the boron-substituted bis(pyrazolylpyridyl)borate ligand: Ph 2 Bp py (L) (Chart1) in which two bidentate arms are connected to a boron centre.The incorporation of two phenyl units at the boron centre has altered its sensitization properties to Eu(III) and Tb(III) ions.The X-ray crystal structure confirmed the general formulae of the complexes to be: [Ln 2 (L) 2 (NO 3 ) 2 (OH) 2 ] where Ln = Eu, Gd or Tb.

Experimental
The Boron ligand was prepared according to the method described by Trofimenko [19].

Synthesis of the complexes
The complexes were prepared by combining equimolar quantities of Eu(NO 3 ) 3 , Gd(NO 3 ) 3 or Tb(NO 3 ) 3 with the ligand in MeOH and left to stand for 1 hr during which time a precipitate was formed.The precipitate was filtered off and washed with MeOH to give a powder. 7

Spectro-fluorimetric measurements
UV-visible spectra were recorded on a Varian Cary 50 conc UV-visible spectrophotometer in range 250-800 nm.Quartz cuvettes of 1 cm path length were used and solvent background corrections were applied.Steady state fluorescence spectra in solid and solution states were recorded on an Edinburogh FS5 fluorimeter.The solid state

Crystallography
Single crystals of the complex GdL was obtained as detailed above.Data was collected at 100 K, using Bruker APEX-II CCD diffractometer equipped with Mo-Kα radiation.Absorption corrections were applied in each case using SADABS.[28] The structures were solved by direct methods or heavy atom Patterson methods and refined by full matrix least squares methods on F 2 using SHELXL-97.[29] The hydrogen atoms were generated geometrically with isotropic thermal parameters.Crystallography data (excluding the structure factors) for the reported structure have been deposited with the Cambridge Crystallographic Data Centre.

Results and discussion
The synthesis of the new bi(pyrazolylpyridyl)borate ligand: Ph 2 Bp py (L) is outlined in Scheme 1.The key intermediates for formation of the ligand is 3-(2-pyridyl)pyrazole, which is prepared from 2-acetylpyridine, in the usual two-step procedure for conversion of acetyl groups to pyrazole groups.[23] Then, L was prepared by reaction of 3-(2-pyridyl)pyrazole with KBPh 4 in a melt at 190 °C until benzene vapour is no more evolved.[24] Satisfactory elemental analysis, FAB mass spectra, [11] B NMR and 1 H NMR spectra confirmed the formulation of the ligand.Reaction of L with nitrate salts of Eu(III), Gd(III) or Tb(III) in MeOH afforded white precipitates respectively.X-ray quality crystals were grown by slow evaporation of their solutions in dichloromethane, DCM.The complexes were satisfactorily characterised by FAB mass spectrometry.For example, the fragment: {[M 2 (L) 2 (OH) 2 (NO 3 )]} 2+ was observed at m/z 1310, 1316 and 1318 respectively for Eu(III), Gd(III) or Tb(III) complexes.The crystal structure of [Gd 2 (L) 2 (NO 3 ) 2 (OH) 2 ]•CH 2 Cl 2 (GdL) in Figure 1 has proved to be a dinuclear complex in which each ligand donates two bidentate sites to one metal ion, with the two metal centres bridged through two hydroxyl groups.The coordination environment about each metal centre is 8-coordinate and best described as a square antiprismatic, having an equatorial belt of four N atoms from [L] -with two bridging hydroxyl groups and one bidentate chelating (NO 3 ) -in the axial positions on either side of the MN 4 plane.The four N-donor atoms from each [L] -ligand form a plane, with the Gd(III) centre lying 0.33 Å out of the N 4 plane.The angle between the two Gd(NN) planes is 14.3°.The metal-ligand bond distances exhibit a Gd-N(py) bonds are on average significantly longer than the Gd-N(pz) bonds and are comparable to previously reported complexes.[12][13] Selected bond distances and angles are listed in Table 1.
It is also worth noting that replacement of the hydrogen atoms in Bp by two phenyl rings in Ph 2 Bp py (L) changes the coordination behaviour of the ligand compared to functionalized Bp which yielded mononuclear complexes with Ln(III) ions.[12] The steric effects caused by the phenyl rings play a big role in the new structure.Intra-molecular O-H••• hydrogen-bonding interactions (~2.9 Å) are observed, involving one phenyl ring and the coordinating hydroxyl group, which clearly plays an important role in stabilising this complex.The steric interactions within the Gd(III) complex are shown in Figure 1   Table 1.Selected bond lengths (Å) and angles (˚) for GdL.

Photo-physical properties
The absorption spectrum of L in DCM is shown in Figure 2. It shows two strong transitions at 250 and 285 nm which can be attributed to ligand-centred   * transitions from (pyrazolyl/pyridine) units [25].

Metal-centred luminescence
The triplet state energy ( 3 *) of L can be determined from the phosphorescence spectrum of its Gd(III) complex recorded in a mixture of ethanol/methanol solution at 77K.Gd(III) ion lacks a low lying electronic acceptor level (>30,000 cm -1 ), so its luminescence features are associated with those of the coordinated ligand.From this emission spectrum, the ligand-based triplet state energy of GdL is 28,800 cm -1 (347 nm).The luminescence from the Eu(III) and Tb(III) complexes in DCM solution are presented in Figure 3. Excitation of the EuL at 345 nm in DCM results in intense red luminescence of Eu(III) showing characteristic peaks from the 5 D 0  7 F j transitions of Eu(III) at 558, 603, 626, 662 and 711 nm.This implies that energy-transfer from the ligand triplet state to the emissive 5 D 0 level of Eu(III) (ca.17500 cm -1 ) is favourable.The presence of the 5 D 0  7 F 0 transition proves the presence of the Eu(III) centres in a non-centrosymmetric site in EuL which agrees with the X-ray crystal structures.[26] Upon excitation of the Tb(III) complex, TbL, at 345 nm in CH 2 Cl 2 , its luminescence spectrum displays strong green emission from the 5 D 4  7 F 6-2 transitions at 632, 592, 550 and 496 nm.The energy difference between the triplet state energy of L and the 5 D 4 level (20,500 cm -1 ) of Tb(III) is equal to ca. 8,300 cm - 1 .According to Latva's empirical rule, optimal sensitization for the Tb(III) luminescence requires an energy gap E( 3 *-5 D 4 ) more than 1,850 cm -1 .[27] Therefore, L can sensitise the Tb(III) centre efficiently.By comparison to the Eu(III) complex, the situation is different.According to Latva's empirical rule, for optimal ligand to Eu(III) energy transfer, the energy gap should be no more than 2,500 cm -1 .In fact the calculated energy differenceE( 3 *-5 D 0 ) is higher than this at around 11,300 cm -1 .Therefore, the ligands can poorly transfer energy to the 5 D 0 excited states and hence the luminescence from Eu(III) is weak.This result is in agreement with their observed lifetime ( obs ) measurements.The phosphorescence decays of the Eu(III) and Tb(III) complexes with L in DCM are mono-exponential.The luminescence lifetime of Tb(III) is longer than that from Eu(III).[(10 3 M -1 cm -1 ) in MeCN Lifetime at f-f (ms) [a]   L

Conclusion
The new boron-based ligand, L, which is derived from the well-known bis(pyrazolyl)borate core, has been synthesised and fully characterised.The X-ray crystal structure and MS analysis revealed the isostructural dinuclear complexes: [Ln 2 (L) 2 (NO 3 ) 2 (OH) 2 ], Ln = Eu, Gd and Tb.The metal centres are bridged through two hydroxyl groups.The steric effects caused by the aromatic substituents on the boron centre has played a big role in stabilizing the structures, through intra-molecular O-H••• hydrogen-bonding interactions.The photophysical investigations in DCM gave evidence that L is able to achieve ligand to metal energy transfer and intense red emission from Eu(III) and green emission from Tb(III) centres are observed in the visible region.

2
Bp py Bp py Tp Bp phosphorescence lifetime measurements for the Ln(III) complexes were measured on Perkin-Elmer LS50B Fluorimeter at RT and 77 K.

1 .
Scheme 1. Schematic route for the synthesis of Ph 2 Bp.
(Bottom).Other ••• and •••CH stacking interactions between the aromatic rings and the solvent molecules, were observed in the molecular packing.

Figure 1
Figure 1 Top: The crystal structure of GdL showing the two hydroxyl bridges.Bottom: The -interactions within the molecule.

Figure 2 .
Figure 2. The absorption spectra of L and its Ln(III) complexes (1x10 -5 M) in DCM solutions at room temperature.

Table 2 .
Summary of the photophysical data for L and its Ln(III) complexes.