Oxidation solvento complexes

In the earliest authentic halocarbon complex (1982), o-diiodobenzene was found to chelate to cationic Ir(III) as shown in diagram (5)." An earlier proposed example proved to be misidentified when the crystal structure was carried out. To be stable, any such complex must resist oxidative addition, hence the use of an oxidation state, Ir(III), that is only oxidized with difficulty. The normally rather weakly basic halocarbon lone pairs are often reluctant to bind, but chelation and involvement of the least electronegative hahde, iodine, favor binding as does the cationic character of the complex. A series of such complexes was soon found, including complexes of fr(I)" and a series of weakly bound dichloromethane complexes for certain systems." These solvento complexes can be very labile and so find use as precursors for binding of other weakly basic hgands. Even fluorocarbon complexes proved viable." A review of the area is available. It now seems... 

Lanthanide Complexes with Multidentate Ligands Lanthanide Oxide/Hydroxide Complexes Lanthanides Coordination Chemistry Solvento Complexes of the Lanthanide Ions Trivalent Chemistry Cyclopentadienyl. 

McNeese and Mueller have shown35 that [VCKSchiff Base)] complexes lose halide in reactions with thallium tetrafluoroborate in acetone. The corresponding solvento-complexes are generated along with thallium chloride. The use of silver salts in such reactions could lead to oxidation of the starting material. 

The formation of solvento-complexes as a result of redox transformations of coordinated ligands is not r tricted to N-donor ligands reactions with other ligands are well known. For example, oxidation of the dimethyl sulfoxide ligand in [Co(NH3)5(DMSO)]3+ can be effected by treatment with KMn04 or CI2 in aqueous solution. The sulfoxide oxidizes to the sulfone which exhibits a much lower donor ability than Me2SO and, as a result of hydrolysis, is expelled from the complex generating the aqua-complex. 

The synthesis of zirconium solvento-complexes through oxidation of one of the alkyl groups in [ZrCp2R2l with silver tetraphenylborate has been demonstrated by Jordan and coworkers 3-75 (r - Me, CH2Ph and solv = MeCN, THF), Eq. 3.14 ... 

NMR spectroscopy was used to monitor ethane liberated in the synthesis of [Zr(MeCN)Cp2Me][BPh4] by this method. Oxidation of the methyl ligand apparently proceeds via intermediate formation of AgCH3. The X-ray crystal structure of the acetonitrile solvento-complex, [Zr(NCMe)Cp2(CH2Ph)]+, has been determined. Figure 3.3. 

In some cases, a metal ion exhibits different coordination numbers in different oxidation states. It may be possible, then, to take advantage of the change in coordination number to form solvento-complexes. For example, platinum(II) complexes typically exhibit a coordination number of four, whereas, the coordination number is usually six for platinum(IV) complexes. Electrochemical oxidation of [PtCl2(PR3)2] (PR3 = various phosphines) in acetonitrile was once thought to lead to the formation of... 

In conclusion, the generation of solvento-complexes as reactive intermediates is extensively used in synthesis. Three distinct approaches have been developed to provide solvent-binding sites (i) conversion of ligands into products with low solubility or their transformation into volatile compounds, (ii) chemical transformations of coordinated ligands where the products of the reaction are poorer donors and are expelled from the coordination sphere of the complex, and (Hi) manipulation of the metal oxidation state to affect ligand-binding properties. 

Synthesis of solvento-complexes by metal oxidation in non-aqueous solvents... 

In order to prepare a homoleptic solvento-complex by metal oxidation in a non-aqueous medium, the reduction products must not compete with the solvent molecules as ligands. Nitrosonium perchlorate, N(DC104, is a suitable oxidant. 4,15 Reactions have been performed in acetonitrile, nitromethane and ethylacetate. In acetonitrile this oxidant converts copper powder into the copper(I) and copper(II) homoleptic solvento-complexes, [Cu(MeCN)4][C104] and [Cu(MeCN)4][C104]2. On boiling an acetonitrile solution of these complexes with metallic copper, reduction of the copper(II)... 

Silver salts containing anions of strong adds are suitable reagents for synthesis of solvento-complexes by metal oxidation in non-aqueous solvents. Metallic gold in acetonitrile reacts with silver perchlorate to produce [Au(MeCN)4][C104], Eq. 4.3 ... 

The complexes, [Fe(py>4]X2 (X = Br, 1), have been prepared by oxidation of metallic iron with bromine or iodine in methanol. Pyridine is added to the resultant solution and the pyridine complexes precipitated.22 Interaction between metallic zinc or cadmium and an excess of arsenic pentafluoride in liquid sulfur dioxide leads to the formation of solvento-complexes containing two or four SO2 ligands depending on the experimental conditions.23/24 The reactions studied are summarized as follows, Eq.4.4 ... 

Homoleptic solvento-complexes can be prepared by electrochemical oxidation of metals in non-aqueous solvents. For example, [V(DMSO)6][BF4l2 and [In(MeCN)6][BF4]3 have been synthesized25,26 by vanadium or indium oxidation in dimethyl sulfoxide or acetonitrile in the presence of HBF4. Gram quantities of these solvento-complexes may be obtained with yields of 80-90% based on the dissolved metal. 

The preparation of homoleptic solvento-complexes by metal oxidation with ArN2 or Ph3C+ salts in acetonitrile has been proposed.27 However, such procedures are not recommended as routine preparative methods. 

Certain low-valent metal carbonyls are oxidized easily with protic adds in non-aqueous media. Thus, [MofCO) ] in diglyme is oxidized by 100% formic acid with the formation of [Mo2(02CH)4). In a related reaction, [Mo(S03CF3)3] is prepared by boiling a [Mo(CO>6) suspension in an excess of HSO3CF3.28 It is reasonable to assume that if a similar reaction was performed in the presence of a strong donor solvent, ttie reaction would lead to the formation of a homoleptic solvento-complex. 

An additional technique is based on the interaction of metal oxides with acids or acid anhydrides in non-aqueous solvents.78,79 This method is convenient but has not been widely used for the synthesis of homoleptic solvento-complexes in preparative coordination chemistry. 

Nitrosonium salts containing weak donor anions are employed for the oxidation of metals in the preparation of both metal salts and homoleptic solvento-complexes. Thus, one of the first syntheses in which a nitrosonium salt was employed as the oxidant was reported by Seel in 1950, Eq. 8.5 ... 

More recently the oxidation of metals by nitrosonium salts in acetonitrile has been used to synthesize homoleptic solvento-complexes. Thus, the complexes, [Cu(MeCN)4]X (X = BF4, C104), l have been obtained by oxidation of metallic copper in acetonitrile with NOX the perchlorate, NOCIO4, in MeCN oxidizes metallic gold to form [Au(MeCN)4][C104], 3 Eq. 8.6 ... 

Examples of the oxidation of metal complexes ly thionitrosonium salts, NSX, are uncommon but some useful reactions have been reported. Thus, the reaction between [ReBrfCOls] and NSSbFg leads to formation of [Re(CO)5(NS)][SbF6] in low yield.4l In addition, interaction between [Ct(CO)3(C H6)] and NSPFs results in formation Z of the solvento-complex, [Cr(MeCN)5(NS)P+. 

It has been shown 4 that Ph3CBp4 can function as a one-electron oxidant and is able to oxidize metallic copper with formation of the homoleptic solvento-complex, [Cu(MeCN)4][BF4]. Yamazaki S has reported that the phosphine and phosphite complexes of palladium(n), [Pd(PPh3)4]X2 (X = BF4, PF, and Pd P(OPh)3 4][PF6]2, have been synthesized in 60% and... 

The tetrafluoroborate complex, [Cr(BF4)(NO)2(T -Cp)], is converted into the more stable salt, [Cr(NO)2(MeCN)(Ti5-Cp)][BF4L on treatment with MeCN. 5 Similarly, the interaction of the isoelectroniccomplex, [Fe(CO)2(Ti5-Cp)l2, with p-toluenesulphonic add in acetonitrile leads to oxidative deavage of the Fe-Fe bond and formation of the acetonitrile solvento-complex, [Fe(CO)2(MeCN)(Ti5-Cp)][C)3SC6H4Me].8 For this reaction it was reported that the protic add was responsible for oxidation of the complex. However, the evolution of molecular hydrogen was not observed. 

If oxidative decarbonylation is carried out in the presence of donor solvents, the reaction can lead to the formation of solvento-complexes. For... 

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