1.2- Dithiolenes function

A main focus of preparing metal dithiolenes functionalized with thiophene units has been for their incorporation into conjugated organic materials. On first glance, it would seem that the combination of metal dithiolenes and conjugated polymers could produce attractive new materials for use in such applications as field effect transistors and NIR optical materials. However, several challenges remain before these materials can be applied to useful... 

Title Dithiolene Functionalized Polymer Membrane for Olefin/Paraffin Separation... 

In biological systems Mo is present as the Fe/Mo cofactor of the nitrogenase enzymes (2) and of the multitude of oxidoreductases (3). In the latter the common molybdopterin cofactor (4), in addition to a dithiolene functionalized pyranopterin (5) ligand (Fig. 1), contains terminal oxo ligands and in the case of xanthine oxidase both oxo and thio ligands. Some aspects of molybdenum sulfur chemistry discussed in this work may be relevant to the biosynthesis of the molybdopterin cofactor and the function of xanthine oxidase (6). 

The cyclic voltammograms of the metal-dithiolene capped porphyrazines 147-149, 152, and 153 reveal a reversible oxidation couple arising from the peripheral [N2-M-S2] functionality, formally written as M(nyni) (M = Pt, Pd) (29). These oxidations occur at approximately +0.2 V (vs Fc+/Fc) for the Pd(bdt) (148), Pd(dmid) (149), Pt(tdt) (152), and Pt(dmit) (153) capped porphyrazines, respectively (Table XXVII). However, all... 

Synthesis AND Structural Characterization of Thiophene-Functionalized Metal Dithiolenes... 

Herein, we will review the preparation and study of this class of functionalized metal dithiolenes, with particular emphasis on the relationships between functionality and solid-state structure, and the overall effect on material properties. Discussion will go beyond the application of this class of thiolene complexes as molecular materials, and equal attention will be given to the effectiveness of various structural motifs when designing these species as polymeric precursors. 

Other Thiophene-Functionalized Metal Dithiolene Complexes... 

Three main options, described in Scheme 2, exist for the conversion of dithiodiketones to dithiolenes (i) reaction with a transition metal carbonyl or other reactive zerovalent material (ii) reduction to an ethylenedithiolate and reaction as described above for these species and (iii) reaction with a metal salt to form a cationic dithiolene and reduction of the reaction product to the neutral compounds, whereby a suitable solvent (such as methanol) may serve the function of the reducing agent for in situ generation of the neutral species. 

Whereas complexes of ethylenedithiolate H2C2S212 are typically prepared by the reductive S-dealkylation of c -H2C2(SCH2Ph)2 (Section II.B), a viable alternative route involves base hydrolysis of l,3-dithiol-2-one, H2C2S2(CO).The parent H2C2S2CO can in turn be prepared on a multigram scale from chloroacetal-dehyde (82). This l,3-dithiol-2-one can be functionalized via deprotonation followed by C-alkylation (72), thus opening the way to a variety of functional dithiolenes (Eq. 7). 

A versatile route to RS-substituted dithiolenes entails S-alkylation of the trithiocarbonate dmit2 (see Section II.C.l), which provides an efficient means to introduction of diverse functionality to the dithiolene backbone. Subsequent to S-alkylation, the resulting S=CS2C2(SR)2 is converted to the dithiocarbonate 0=CS2C2(SR)2 with Hg(OAc)2 in acetic acid (63, 83, 84). Such dithiocarbo-nates are more easily hydrolyzed than the trithiocarbonates (72, 85). This approach has been used for the synthesis of Ni[S2C2(S(CH2) Me)2]2 (n = 2-11) (86) and related complexes with pendant alkene substituents (Eq. 8) (87). 

There are roughly 421 reports of homoleptic bis(dithiolene) units based on transition metal elements. The approximate tally of the structures as a function of central metal atom is outlined in Fig. 2. The examples predominantly contain late transition metals. The majority of complexes are Ni based, partially because of interest in these complexes for materials applications. Other common central elements are Cu, Pd, Pt, Au, and Zn. There are also a few Fe and Co complexes and a small number of structures based on Cr, Mn, Ag, Cd, and Hg. 

Finally, a third dithiolene ligand model has been utilized with success in order to understand the electronic structure and spectroscopy of a number of oxo-molybdenum mono- and bis(dithiolenes) (23). This dithiolene ligand bonding description utilizes the symmetric and antisymmetric out-of-plane Sop p 7t orbitals, in addition to the corresponding in-plane symmetric and antisymmetric S p p orbitals. Ab initio and density functional theory (DFT) calculations have been performed on the simple dithiolene dianion, [S2C2H2]2 (23), in order to illustrate the details of this four orbital model and electron density contours of the four MOs are presented in Fig. 4. These calculations result in an isolated set of four filled dithiolene orbitals, and these are the ligand... 

With respect to the oxo-molybdenum mono(dithiolenes), cpM are the Mo d orbitals (e.g.,

atomic sulfur p orbital functions. Experimental studies have shown that the dominant contributor to the oscillator strengths of all transitions are integrals of the form (xnMXn) (310-312), which, in the limit of no overlap between the two sulfur p orbitals on different atoms reduces to (311). 

Here, C and C n are the sulfur atomic orbital coefficients in v ia and ib, rL is the position vector, and (%n % n) is an overlap integral. The upshot of this is that the same type of dithiolene MO (e.g., (ground-state and excited-state wave functions for enhanced CT transition intensity, and this intensity is a direct consequence of the dmetai — dithiolene covalency. 

The unique electronic structure of these (L-A3)MoO(dithiolene) complexes arises from two basic factors. The first is the strong axial a- and Ji-donor properties of the terminal oxo ligand, which dominates the ligand field and predetermines the energy of the Mo-based dxz, dyz, and dzi acceptor orbitals. The second is the equatorial dithiolene sulfur donors, from which the low-energy LMCT transitions arise. Dithiolene covalency contributions to the electroactive C, or redox, orbital can be directly probed via the relative oscillator strengths of the / —> ixy and /fp —> (/", transitions (see above). These three wave functions may be expanded in terms of Mo- and dithiolene sulfur-based functions ... 

Figure 20. Plot of the reduction potential of (L-A 3)MoO(dithiolene) complexes as a function of calculated Mulliken charge per S atom of the dithiolene. [Adapted from (20).]...

Square-planar metallo(diimine)(dithiolene) complexes generally display intense, solvatochromatic absorptions in the visible region of the spectrum that are not found in the corresponding metallo-bis(dithiolene) or metallo-bis (diimine) complexes. Futhermore, the LLCT transition energy does not vary appreciably as a function of the metal ion. Extended Hiickel calculations on Ni, Pt, and Zn metallo(diimine)(dithiolene) complexes indicate that the HOMO is comprised almost entirely of dithiolene orbital character (Figure 2), while the LUMO was found to possess essentially all diimine n orbital character (112, 252, 268). In stark contrast to the spectra of square-planar Ni and Pt metallo (diimine)(dithiolene) complexes, the psuedo-tetrahedral complexes of Zn possess extremely weak LLCT transitions. Now, it is of interest to discuss the differences in LLCT intensity as a function of geometry from a MO point of view. This discussion should help to explain important orientation-dependent differences in photoinduced electron delocalization and charge separation. 

Figure 24 displays the high energy (E > 25,000 cm-1) region of the room temperature electronic absorption spectrum for Zn(bpy)(tdt), where bpy = 2,2 -bipyridine. The LLCT transition occurs at 22,470 cm-1 (445 nm) with very weak absorption intensity (e = 72 M 1cm 1). The origin of the weak LLCT is a function of the symmetry of this psuedo-tetrahedral complex. A MO diagram for Zn(bpy)(tdt), derived from extended Hiickel calculations, is presented in Fig. 25. Irrespective of whether the metallo(diimine)(dithiolene) complex is square-planar or psuedo-tetrahedral, the point symmetry is C2V, and all intermediate geometries possess C2 symmetry. When the dithiolene and diimine planes are orthogonal (psuedo-tetrahedral geometry) the HOMO — LUMO transition represents a b2 —> b one-electron promotion and is electric dipole forbidden. However, the HOMO —> LUMO transition in a square-planar...

Furthermore, this overlap should vary as a function of the projection of the dithiolene n orbitals onto the diimine n orbitals, and this possesses a simple cos 0 dependence. 

Therefore, the intensity of the transition as a function of the dithiolene-diimine torsion angle should approximate a cos2 0 function, and... 

Extended Hiickel calculations have been used to probe the nature of the HOMO and LUMO wave functions for Pt(diimine)(dithiolene) complexes (252). These calculations reveal a HOMO orbital composition for Pt(bpy)(mnt), which is 27% Pt, and 72% dithiolene. The LUMO for this complex contains dominant contributions from the bpy ligand with 2% Pt and 98% bpy character. The appreciable degree to which Pt orbitals contribute to the HOMO is the origin of the MMLL CT description for these complexes. These results compare... 

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