This Review gives a comprehensive overview of the most topical weakly coordinating anions (WCAs) and contains information on WCA design, stability, and applications. As an update to the 2004 review, developments in common classes of WCA are included. Methods for the incorporation of WCAs into a given system are discussed and advice given on how to best choose a method for the introduction of a particular WCA. A series of starting materials for a large number of WCA precursors and references are tabulated as a useful resource when looking for procedures to prepare WCAs. Furthermore, a collection of scales that allow the performance of a WCA, or its underlying Lewis acid, to be judged is collated with some advice on how to use them. The examples chosen to illustrate WCA developments are taken from a broad selection of topics where WCAs play a role. In addition a section focusing on transition metal and catalysis applications as well as supporting electrolytes is also included.

Taming the Cationic Beast: Novel Developments in the Synthesis and Application of Weakly Coordinating Anions

The copper(I) and copper(II) complexes with the nitrogen donor ligands bis[(1-methylbenzimidazol-2-yl)methyl]amine (1-BB), bis[2-(1-methylbenzimidazol-2-yl)ethyl]amine (2-BB), N-acetyl-2-BB (AcBB), and tris[2-(1-methylbenzimidazol-2-yl)ethyl]nitromethane (TB) have been studied as models for copper nitrite reductase. The copper(II) complexes form adducts with nitrite and azide that have been isolated and characterized. The Cu(II)-(1-BB) and Cu(II)-AcBB complexes are basically four-coordinated with weak axial interaction by solvent or counterion molecules, whereas the Cu(II)-(2-BB) and Cu(II)-TB complexes prefer to assume five-coordinate structures. A series of solid state structures of Cu(II)-(1-BB) and -(2-BB) complexes have been determined. [Cu(1-BB)(DMSO-O)(2)](ClO(4))(2): triclinic, Po (No. 2), a = 9.400(1) A, b = 10.494(2) A, c = 16.760(2) A, alpha = 96.67(1) degrees, beta = 97.10(1) degrees, gamma = 108.45(1) degrees, V = 1534.8(5) A(3), Z = 2, number of unique data [I >/= 3sigma(I)] = 4438, number of refined parameters = 388, R = 0.058. [Cu(1-BB)(DMSO-O)(2)](BF(4))(2): triclinic, Po (No. 2), a = 9.304(5) A, b = 10.428(4) A, c = 16.834(8) A, alpha = 96.85(3) degrees, beta = 97.25(3) degrees, gamma = 108.21(2) degrees, V = 1517(1) A(3), Z = 2, number of unique data [I >/= 2sigma(I)] = 3388, number of refined parameters = 397, R = 0.075. [Cu(1-BB)(DMSO-O)(NO(2))](ClO(4)): triclinic, Po (No. 2), a = 7.533(2) A, b = 8.936(1) A, c = 19.168(2) A, alpha = 97.66(1) degrees, beta = 98.62(1) degrees, gamma = 101.06(1) degrees, V = 1234.4(7) A(3), Z = 2, number of unique data [I >/= 2sigma(I)] = 3426, number of refined parameters = 325, R = 0.081. [Cu(2-BB)(MeOH)(ClO(4))](ClO(4)): triclinic, Po (No. 2), a = 8.493(3) A, b = 10.846(7) A, c = 14.484(5) A, alpha = 93.71(4) degrees, beta = 103.13(3) degrees, gamma = 100.61(4) degrees, V = 1270(1) A(3), Z = 2, number of unique data [I>/= 2sigma(I)] = 2612, number of refined parameters = 352, R = 0.073. [Cu(2-BB)(N(3))](ClO(4)): monoclinic, P2(1)/n (No. 14), a = 12.024(3) A, b = 12.588(5) A, c = 15.408(2) A, beta = 101,90(2) degrees, V = 2282(1) A(3), Z = 4, number of unique data [I >/= 2sigma(I)] = 2620, number of refined parameters = 311, R = 0.075. [Cu(2-BB)(NO(2))](ClO(4))(MeCN): triclinic, Po (No. 2), a = 7.402(2) A, b = 12.500(1) A, c = 14.660(2) A, alpha = 68.14(1) degrees, beta = 88.02(2) degrees, gamma = 78.61(1) degrees, V = 1233.0(4) A(3), Z = 2, number of unique data [I>/= 2sigma(I)] = 2088, number of refined parameters = 319, R = 0.070. In all the complexes the 1-BB or 2-BB ligands coordinate the Cu(II) cations through their three donor atoms. The complexes with 2-BB appear to be more flexible than those with 1-BB. The nitrito ligand is bidentate in [Cu(2-BB)(NO(2))](ClO(4))(MeCN) and essentially monodentate in [Cu(1-BB)(DMSO-O)(NO(2))](ClO(4)). The copper(I) complexes exhibit nitrite reductase activity and react rapidly with NO(2)(-) in the presence of stoichiometric amounts of acid to give NO and the corresponding copper(II) complexes. Under the same conditions the reactions between the copper(I) complexes and NO(+) yield the same amount of NO, indicating that protonation and dehydration of bound nitrite are faster than its reduction. The NO evolved from the solution was detected and quantitated as the [Fe(EDTA)(NO)] complex. The order of reactivity of the Cu(I) complexes in the nitrite reduction process is [Cu(2-BB)](+) > [Cu(1-BB)](+) > [Cu(TB)](+) > [Cu(AcBB)](+).

Synthesis, structure, and reactivity of model complexes of copper nitrite reductase

Die Schöne (WCA) und das (kationische) Biest: Neues aus der Chemie von und mit schwach koordinierenden Anionen

Silver(I) perchlorate or nitrate react readily with bis(di-phenylphosphanyl)-o-carborane to give the complexes [AgX{(PPh2)2C2B10H10}] [X = ClO4 (1), NO3, (2)]. The perchlorate ligand is weakly bound to the silver atom and thus can be displaced by other ligands affording the three-coordinated complexes [AgL{(PPh2)2C2B10H10}]ClO4 [L = PPh3 (3), PPh2Me (4), AsPh3 (5), C5H4NCOOH (6), C9H6NCOOH (7), SPPh3 (8)]. Compounds 3 and 4 can also be obtained by reaction of [Ag(OClO3)PR3] with the diphosphane. Treatment of complex 1 with bidentate ligands leads to the cationic four-coordinated [Ag(LL){(PPh2)2C2B10H10}]ClO4 [LL = (PPh2)2C2B10H10 (9), bipy (10), phen (11), (SPPh2)2CH2 (12)] or to the neutral [Ag(S2CNR2){(PPh2)2C2B10H10}] [NR2 = NEt2 (13), NC4H8 (14)]. The crystal structures of 11 and 12 have been established by X-ray crystallography. In both complexes the silver(I) atoms exhibit tetrahedral coordination by two phosphorus and two nitrogen or two sulfur atoms, respectively.

Synthesis of Silver(I) Complexes with the Bis(diphenylphosphanyl)‐o‐carborane Ligand. Crystal Structure of [Ag(phen){(PPh2)2C2B10H10}]ClO4 and [Ag{(SPPh2)2CH2}{(PPh2)2C2B10H10}]ClO4 · CH2Cl2

Trinuclear Au2Ag and Au2Cu Complexes with Mesityl Bridging Ligands. X-ray Structure of the Chain Polymer [{Au(μ-mes)AsPh3}2Ag](ClO4)

Publisher Summary This chapter discusses the perchlorate ion complexes. Coordination of perchlorate to transition and nontransition metal ions has been well established, though in most of the cases the metal-perchlorate bond is rather weak and has been termed as “semicoordination”. The highly symmetrical perchlorate ion is a hard base and falls into the category of relatively nonpolarizable and noncoordinating anions such as BF 4 ˉ and PF 6 ˉ. Its ability to coordinate strongly to soft metal ions is therefore limited. When coordination does occur, the perchlorate group may be linked to a metal ion in a monodentate and/or bidentate manner through oxygen atoms; the latter linkage can be either chelating. X-ray crystal structure analysis has unequivocally established coordination of perchlorate in a number of perchlorate complexes.

N.M.N. Gowda

Papers

Perchlorate Ion Complexes