Thin Film Electrodeposition of Ir ( III ) Cyclometallated Complexes

Novel electropolymerizable Ir(III) cyclometallated complexes have been synthesized and characterized. In these complexes the cyclometallated ligands are either 2-phenylpyridine H(PhPy) or benzothiazole-triphenylamine H(BzTh-tpa), while the Ir(III) coordination sphere is completed by a Schiff base substituted with a triphenylamine fragment. A complete electrochemical study has been conducted on all complexes, in order to verify the feasibility of electropolymerization and to elucidate the role of the specific position of the triphenylamine moiety in the molecular structure. Homogeneous thin films of Ir(III) metallopolymers have been successfully obtained through electropolymerization process.


Introduction
Metallopolymers represent a fast rising class of functional materials due to their combined specific properties induced by metal centers and the processability and low cost of synthetic polymers [1].Since the first reported poly(vinylferrocene) [2], metallopolymers (especially those containing d-and f-block metals) have been used in optoelectronic (super/semiconductors, electrochromic, and luminescent devices) and catalysis processes.Many polymerization techniques have been employed in the synthesis of metallopolymers, including "living" polymerization routes and self-assembling methods [3].Nevertheless, the controlled deposition of high quality polymeric films on different substrates, a key step towards functional applications, still represents a challenge.
In the case of deposition of metallopolymers, electropolymerization presents several advantages over the different film generating methods based on transition metal complexes, like layer-by-layer assembly [4], spin-casting [5], drop-casting [6], or electrostatic binding in the case of polyelectrolyte films [7].In the electropolymerization process, the polymer formation and its deposition occur simultaneously, avoiding limitation such as scarce solubility of polymeric networks and leading to controllable film composition, thickness, and surface coverage [8].
The first reports on the electrodeposition of organometallic monomers are referred to the reductive electropolymerization of vinyl-substituted polypyridine complexes [9].Up to date, different electropolymerizable groups such as thiophene [10,11], pyrrole [12], aromatic amines [13][14][15], and diphenylamino [16,17] have been introduced in organometallic compounds.The obtained neutral and ionic complexes of different metal centers (Ru, Os, Fe, and Ir) [8] have afforded the in situ preparation and the deposition of metallopolymeric films on different conducting substrates.
In this context, a series of electropolymerizable cyclometallated square planar photoconductive Pd(II) and Pt(II) complexes, containing as ancillary ligand a Schiff base substituted with a triphenylamine group, were recently reported by our group (Scheme 1).The use of this specifically functionalized Schiff base ligands has been the key factor in the induction of an electropolymerization process during repetitive potential scans, allowing the deposition of highly stable and homogeneous electroactive and photoconductive Pd(II) and Pt(II) metallopolymers [18].The choice of the triphenylamine substituent as a potential electropolymerizable fragment has been motivated by its ability to form cross-linked conjugated polymeric networks, favouring isotropic charge transport and showing high electrochemical stability [16,17,19].
In order to extend the developed electrodeposition method towards octahedral complexes, we have incorporated the triphenylamine electropolymerizable Schiff base H(O ∧ N) tpa (Figure 1) into Ir(III) cyclometallated complexes.Therefore, herein a series of new cyclometallated Ir(III) complexes containing the electropolymerizable Schiff base H(O ∧ N) tpa is reported, together with corresponding model complexes as direct comparison, where the H(O ∧ N) C6 Schiff base ligand is lacking the electropolymerizable group (Figure 1).
A complete electrochemical study has been performed on the novel complexes I-IV, in order to establish their redox behaviour and to probe the electrodeposition feasibility as high quality thin films.

Materials and Methods
All commercially available chemicals were purchased from Aldrich Chemical Co. and were used without further purification.IR spectra (KBr pellets) were recorded on a Perkin-Elmer Spectrum One FT-IR spectrometer. 1 H-NMR spectra were recorded on a Bruker WH-300 spectrometer in deuterated solvents with TMS as an internal standard.Elemental analyses were performed with a Perkin-Elmer 2400 analyzer CHNS/O.Melting points were examined with a Leica DMLP polarising microscope equipped with a Leica DFC280 camera and a CalCTec (Italy) heating stage.
All potentials were measured using an Epsilon electrochemical analyser.Voltammetry experiments were performed in a 3 mL cell of dry and degassed (N 2 ) dichloromethane solution using tetrabutylammonium hexafluorophosphate (0.1 M) as a supporting electrolyte, a Pt disk working as electrode, a Pt wire as counterelectrode, and an Ag wire as a pseudoreference electrode.Voltammograms were recorded at a 100 mV s −1 scan rate from ca. 10 −3 M complex solutions.Redox potentials are given relative to a ferrocene/ferrocenium (Fc/Fc + ) redox couple used as an internal reference.Estimation of HOMO/LUMO energy values was performed taking into account −4.8 eV for the couple Fc/Fc + [20].

Results and Discussion
3.1.Synthesis.The two first synthesized Ir(III) model complexes I and II contain the PhPy fragment as cyclometallated ligands and the Schiff bases H(O ∧ N) C6 [21] and H(O ∧ N) tpa [18], respectively, as ancillary ligands.In order to complete the comparing series of Ir(III) complexes and study the influence of the position of the TPA fragment on the coordinated ligands with respect to the electropolymerization process, the two cyclometallated benzothiazoletriphenylamine Ir(III) complexes III and IV, containing the same Schiff bases H(O ∧ N) C6 and H(O ∧ N) tpa , were synthesized.
The synthesis of all the complexes I-IV (Scheme 2) was achieved starting from their corresponding dichlorobridged dimer.In particular, complexes I and II were obtained starting from the already described chlorobridged dimer 1 [22].In the case of complexes III and IV, after synthesizing the cyclometallating ligand H(BzTh-tpa) [23], the corresponding dichlorobridged cyclometallated Ir(III) intermediate 2 was obtained by modification of the reported procedure [23].
Finally, through bridge splitting reaction between the respective bridged intermediate and the appropriate  substituted Schiff base, in the presence of an excess of trimethylamine for the complete deprotonation of the ancillary ligand, a single isomer of the final complexes I-IV was obtained in overall high yields.

Electrochemical Studies.
Complexes I-IV redox behaviour was studied by cyclic voltammetry.The measured oxidation potentials and the estimated HOMO levels are reported in Table 1, together with the H(BzTh-tpa) ligand oxidation potential added for direct comparison.The corresponding obtained cyclic voltammograms are shown in Figure 2.
Considering the oxidation behaviour of the model complexes I-II (Figure 2(a)), the single reversible one-electron wave seen in the case of complex I can be attributed to the Schiff base H(O ∧ N) C6 oxidation, while the two consecutive oxidation waves observed in the cyclic voltammogram of II are typical of the triphenylamine moiety of the H(O ∧ N) tpa Schiff base [18].For the two reversible oxidation waves observed in the case of complex III, the first one is probably associated with the Schiff base (O ∧ N) C6 , while the second reversible wave can reasonably be attributed to the oxidation of the cyclometallated ligand [24], H(BzTh-tpa) showing, in the solvent window explored (−1.2-1.45V), a single reversible oxidation at ca. +545 mV versus Fc/Fc + .
Regarding the oxidation properties of complex IV, a specific and unambiguous attribution of the oxidation processes on a molecular scale resulted to be unfeasible due to the overlap of the oxidation waves of the various fragments.However, according to the potential value of the first quasireversible wave, this oxidation process can be reasonably attributed to the first oxidation process of the H(O ∧ N) tpa Schiff base ligand.For all complexes I-IV, no reduction wave was recorded in the allowed solvent window (−1.2-1.45V).
As expected for complex I, no electropolymerization process was observed upon repetitive potential scan cycles, but neither for complex II containing the (O ∧ N) tpa triphenylamine substituted Schiff base nor for complex III, where the triphenylamine fragment is on the cyclometallated ligand, such electropolymerization processes were recorded (Figure 3).The slight increase in the current intensity visible on the respective cyclic voltammograms is only due to minor accumulation of oxidized specie at the surface electrode.
From these observations, we can conclude that for the triphenylamine moieties incorporated either in the Schiff base (complex II) or in the cyclometallated ligand (BzThtpa) (complex III) the electropolymerization is prevented, probably due to electronic factors induced by the nature of the other ligands surrounding the Ir(III) center.Indeed, as previously reported, the presence of electrowithdrawing substituents in paraposition with respect to triphenylamine fragments highly controls the electropolymerization process in which these units are involved [25].
On the contrary, upon repetitive oxidation scans, a distinctive regular increase in current is observed in the cyclic voltammogram of IV, which is typical of the electropolymerization of the substituted triphenylamine fragment (Figure 4(a)) [23].In this case, the synergistic electronic contribution of all substituents on both the cyclometallated and the ancillary ligands produces an overall withdrawing effect able to allow the electropolymerization of the triphenylamine fragment of the (O ∧ N) tpa Schiff base.
Once the polymer was formed, the modified Pt disk working electrode covered with complex IV electropolymerized film was immersed in a freshly distilled dichloromethane solution (0.1 M electrolytic solution).The recorded cyclic voltammogram proved the electroactivity and stability of the deposited film (Figure 4(b)).
Electrogenerated thin films of IV were deposited on ITO etched glass substrates.A cyclic voltammogram recorded upon electropolymerization of IV onto an ITO glass substrate is shown in Figure 5 together with the photography of the obtained deposited thin film.

Conclusions
Novel biscyclometallated Ir(III) complexes (I-IV), bearing (PhPy) or (BzTh-tpa) as cyclometallated ligand as well as substituted Schiff bases as (O ∧ N) chelated ancillary ligand, have been synthesized.A complete electrochemical study has been performed on all complexes.It has been observed that electropolymerization of the triphenylamine fragment is highly dependent onto the nature of the ligands incorporated within the coordination sphere of the metal ion.Indeed, the introduction of the triphenylamine fragment on the Schiff base (O ∧ N) tpa ligand in the presence of the cyclometallated (BzTh-tpa) ligands favoured the electropolymerization process.Upon repetitive potential scans, high quality polymeric thin films of complex IV have been electrodeposited on ITO covered glass electrodes.The presence of the triphenylamine fragment in the Schiff base represents thus a necessary condition in order to induce electropolymerization.However, the described results of the electrochemical study proved that this condition is not sufficient, as demonstrated by the lack of electropolymerization in the case of complex II bearing (PhPy) fragments as cyclometallated ligands.Furthermore, investigations of the photophysical and electrochromic properties of these metallopolymers are in progress, in order to test the potential application of Ir(III) thin films in electroluminescent and/or electrochromic devices.

Experimental Details
5.1.Synthesis of Complex I. Intermediate complex 1 [22] (150 mg, 0.14 mmol) was solubilized in ca.50 mL of a 1 : 1 mixture of acetone and chloroform. 2 eqs. of H(O ∧ N) C6 [21] (87 mg, 0.28 mmol) and 30 eqs. (0.3 mL, 4.2 mmol) of trimethylamine were added to the solution.The reaction mixture was stirred at reflux for 72 h.The solvent was evaporated and the organic phase extracted with dichloromethane/water and dried over sodium sulphate.The crude product was purified by precipitation from dichloromethane/-hexane. The solid was filtered off, washed with hexane, and dried under vacuum.

Synthesis of Complex
II. 1 (100 mg, 0.09 mmol) was solubilized in ca.50 mL of a 1 : 1 mixture of acetone and chloroform. 2 eqs. of H(O ∧ N) tpa [18] (88 mg, 0.18 mmol) and 30 eqs. (0.2 mL, 2.8 mmol) of trimethylamine were added to the solution.The reaction mixture was stirred at reflux for 72 h.The solvent was evaporated and the organic phase extracted with dichloromethane/water and dried over sodium sulphate.The crude product was purified by precipitation from dichloromethane/hexane.The solid was filtered off, washed with n-hexane, and dried under vacuum.
Red solid; yield 80% (145 mg); m.p. > 250 ∘ C;    [21] and 30 eqs. (0.06 mL, 0.9 mmol) of trimethylamine were added to the solution.The reaction mixture was stirred at reflux for 48 h. and at r.t. for 48 h.The solvent was evaporated and the organic phase extracted with dichloromethane/water and dried over sodium sulphate.The crude product was purified by precipitation from dichloromethane/cold ethanol.The solid was filtrated, washed with cold methanol, and dried under vacuum.

Synthesis of Complex IV.
Complex IV was prepared following the same procedure reported for complex III with the following modifications: intermediate 2 (96 mg, 0.048 mmol) and 2 eqs. of H(O ∧ N) tpa [18] (46 mg, 0.096 mmol) were treated with 30 eqs. of trimethylamine (86 mg, 1.44 mmol).

Figure 1 :
Figure 1: Molecular structures of the synthesized Ir(III) complexes I-IV and the O-, N-coordinated ligands.

Figure 3 :Figure 4 :
Figure 3: Cyclic voltammograms of (a) II and (b) III obtained from six consecutive oxidation scans.