Tris-chelates

Dichloro neutral bis-chelates of hexacoordinate silicon were obtained by two methods directly (35) by transsilylation of Af-dimethylamino-O-(trimethylsilyl)acylimidates (1) with tetrachlorosilane, or by rearrangement as described in the previous section (70). The presence of two chloro ligands was utilized for further substitution, to form hexacoordinate silicon tris-chelates, by reaction with bis(trimethylsilyl)-precursors 71 and 72, as shown in Eqs. (31) and (32). The products (73, 74, and 75), are hexacoordinate neutral silicon tris-chelates, and 75 is the first reported zwitterionic tris-chelate with three different chelate rings.73 

The crystal structures of 73a, 74a, and 75, each representing one of the three types of tris-chelates, were reported. The structures are depicted in Figs. 39-41, and selected bond lengths and angles are listed in Table XXIV. It is evident that all three molecules have distorted octahedral geometries in the solid-state. 

The dynamic behavior of tris-chelates was of considerable interest due to the additional constraint of a third bidentate ligand, relative to the extensively studied bis-chelates, with two monodentate ligands (Section III.A.3). Also the question of whether N-Si dissociation takes place during multi-step ligand-site exchange processes was considered. 

Bond Distance Bond Distance Bond Distance 

Compound 74a has Ci symmetry, and therefore displays four A-methyl signals in the H and 13C NMR spectra. There are three reasonable, 

---

Figure B The absolute configuration of the optical isomers of a metal tris-chelates complex such as [Co(en)3]. (a) A configuration and (b) A configuration.

The simplest systems are the tris(chelates) of phen and bipy... 

Tris-chelates with bidentate S-donors are made conventionally (Rh—S 2.368 A for Rh(S2CNEt2)3)... 

Dynamics of intramolecular metal-centred rearrangement reactions of tris-chelate complexes. L. H. Pignolet, Top. Curr. Chem., 1975,56,93-137 (85). 

Encapsulation of [Co(bpy)3]2+ within zeolite frameworks has also been shown to have a remarkable influence on the electronic spin state of the complex.240 Distortions imparted on the tris-chelate complex by the confines of the zeolite supercage are found to be responsible for stabilizing the unusual low-spin electronic ground state.241,242 The [Co(bpy)3]3+/2+ couple has been measured for the encapsulated complex and it has been found that the complexes remain within the zeolite and do not exchange with the bulk solution.243 Electrochemistry of [Co(bpy)3]3+/2+ immobilized within a sol-gel has also been studied.244... 

The [Co(phen)3]3+ complex is photoactive and a powerful oxidant in its excited state. The ion has no H-bonding groups and hence is considerably more hydrophobic1279 than hexaamine relatives. These properties have proven particularly useful. Aryl and alkyl substituted [Co(phen)3]3+ complexes have received a great deal of attention due to their ability to intercalate within the helical structure of DNA through a combination of electrostatic and hydrophobic forces. The chirality of the tris-chelate complex is crucial in determining the degree of association between the complex and... 

The synthesis and characterization of tris-chelate Ir111 complexes of 2-(acetylamino)benzoic acid, 2-(benzoylamino)benzoic acid, 2-[(2-aminophenylamino)carbonyl]benzoic acid, maleanilic acid, malea-l-naphthalanilic acid, and 2-[(phenylamino)carbonyl]benzoic acid have been described.245... 

An spontaneous assembly of 15 components is reported in the synthesis of [Ag9L6](OTf)9 where L is the hexadentate, tris-chelating 6,6-bis[2-(6-methylpyridine)]-3j3 -bipyrazine. The structure consists of a 3x3 grid of Ag-atoms (average Ag" Ag = 3.72(3)A), sandwiched between two almost perpendicular sets of three parallel L ligands (Figure 10).641... 

Chelating aldehydes such as 2-pyridine carbaldehyde and 2-dimethylamino benzaldehyde improve the stability of the aldehyde complexes via N,0 chelation. NMR studies show that the complexes are present in solution without an excess of aldehyde and can be formed in the presence of donor ligands. The X-ray structures showed longer and weaker Zn—O bonds when more than one chelating ligand was present. IR demonstrates the variation in C=0 bond strengths and how the environment of the zinc ion will influence potential catalytic activity via reaction rates or pathways. Tetrahedral chelate complexes, and octahedral bis- and tris-chelate complexes, were isolated.843... 

Pignolet, L. H. Dynamics of Intramolecular Metal-Centered Rearrangement Reactions of Tris-Chelate Complexes. 56, 91-137 (1975). 

All such complexes can be thought of as potential. .irreversible labels of bio-molecules. A tris-chelate such as [Os(dipyridyl) 3]2+ will not behave as a reagent which has substitutional properties. Thus one limitation on the tjqpe of complex may well be that at least one or possibly two ligands should be monodentate as in [Pt(ethylenediamine)Cl2]. 

A large number of complexes have been prepared in which metalla-/3-diketonate anions coordinate to either metal ions or B(X)(Y) moieties. The chemistry of such metal complexes has been reviewed previously (7). To summarize briefly, tris-chelate complexes (3) of Al(III) or Ga(III)... 

Besides tris-chelates such as (58)-(63), Le Bozec and co-workers have also studied several tetrahedral complexes of 4,4 -disubstituted 2,2 -bipy ligands.129-131 1,340 nm EFISHG was applied... 

In a later publication Tor et al. reported for the first time that coordination compounds, such as tris-chelate Ru(II) complexes, are good substrates for C-C... 

Rhenium(III) tris-chelates are formed by the reaction of 2-(diphenylphosphinomethyl)-4-methylphenol, PpOH (258a), or 2-diphenylphosphinophenol, P2-OH (258b) with [ReCl3(PPh3)2(CH3CN)] in a 1 3 ratio. [Re(Pi-0)3] is stable as solid and in solutions while [Re(P2-0)3] is easily oxidized in air giving the rhenium(V) 0x0 complex [Re0(P2-0)2(0P2-0]. This behavior is also reflected in the electrochemical behavior of the compounds, where each one well-defined redox couple for one-electron oxidation and reduction steps are observed for both compounds. A second oxidative wave (Re /Re ) is only reversible for [Re(Pi-0)3]. 

Reduction of [ReOCl3(PPh3)2] with sodium dithionite in the presence of a bidentate isocyanide ligand and the isolation of a cationic tris-chelate of the composition [Re CNC3H60C2H40C3H6NC 3]" has been reported. The product was isolated as its [BPh4] salt and characterized based on elemental analysis and spectroscopic data. " ... 

Chelating diazene coordination has been observed for hydralazine, thiobenzoylhydrazine, 2-hydrazinopyridine, and 2-hydrazinopyrimidines. A cationic tris-chelate of the composition [Re(HNNN2CgH5)3] has been reported as the product of the reduction of perrhenate with excess of hydralazine in methanol. It can be isolated as a green solid by precipitation with Na[BPh4]. ... 

A series of compounds has been isolated with thiobenzoylhydrazine. The reaction with perrhenate in acidic solution yields after recrystallization from DMF the neutral tris-chelate [Re(NHNC(S)Ph)3] DMF, which is also formed from [ReOCl3(PPh3)2] in neutral ethanol/benzene... 

The PFg" salts of [Ru(bpy)2(110)] and [Ru(110)3] and analogous complexes containing 4,4 -bis(substituted) ferrocenyl ligands (110 ), have been synthesized and characterized the tris(chelate) complexes are either poorly soluble or insoluble. Electropolymerization of [Ru(110 )3][PF6]2 produces an electrochromic film. The complex [Ru(bpy)2(lll)] undergoes electropolymerization on Pt and glassy carbon electrodes, although the related complex [Ru(bpy)2(112)] does not. Electrochemical and spectroscopic properties of the films indicate that they form by both head-to-tail and tail-to-tail monomer coupling. ... 

---