Copper square planar complexes

Square planar complexes, in which the four bonds are directed toward the comers of a square, are more common. Certain complexes of copper(II) and nickel(II) show this geometry it is characteristic of die complexes of Pd2+ and Pt2+, including Pt(NH3)42+. 

Splitting of d-orbitals in square planar complexes of copper(II), nickel(II) and cobalt(II). Y. Nishida andS. Kida, Coord. Chem. Rev., 1979, 27, 275-298 (94). 

X-ray crystallographic data has become available on the commercially important 1 1 copper complex azo dyes. The symmetrical dihydroxyazo ligand (16) forms the 1 1 square planar complex (17).13 A pyridine molecule occupies the fourth coordination site since this complex facilitated the formation of crystals suitable for X-ray diffraction. The complex (17) actually exists as an unusual trimer which is held together by long bridging interactions between the copper atom in one molecule and one of the hydroxy oxygen atoms from the adjacent molecules. 

The square-planar complex (34) NiCI2-(P-/i-Bu3)2 was a better catalyst than the tetrahedral complex NiBr2 (PPh3)2 for hydrosilation of styrene with trichlorosilane at temperatures of 150°-170°C. A nickel(0) complex, Ni[P(OPh)3]4, was as good as NiCl2(NC5H5)4, which was best among known nickel catalysts for this reaction. Addition of copper(I) chloride... 

The results obtained by cyclic voltammetry clearly show that upon oxidation or reduction of the central metal copper, the macrocycle is set in motion. Upon oxidation of 6(4)+, the resulting tetrahedrally coordinated Cu(II) is unstable as Cu(II) forms stable square planar complexes or higher coordination (five or six). Therefore, the macrocycle pirouettes around the axle permitting the restoration of a stable coordination, that is pentacoordination by the 2,2, 6 2"-terpyridine and 2, 2 -bipyridine... 

The selectivity of these reagents for copper(II) over iron(III), which is an important aspect of their successful commercial acceptance, is attributable to the fact that the former metal gives a specifically structured, essentially square-planar complex of the type (15).150... 

Among several chiral cyclic and acyclic diamines, (R,R)-cyclohexane-l,2-diamine-derived salen ligand (which can adopt the gauche conformation) was most effective in providing high enantioselectivity [38]. Further, the introduction of substituents at the 3,4, 5 and 6 positions on the aromatic ring of catalyst 39c was not advantageous, and resulted in low enantioselectivity [32,37,39]. The metal ions from first-row transition metals - particularly copper(II) and cobalt(II) - that could form square-planar complexes, produced catalytically active complexes for the asymmetric alkylation of amino ester enolates [38]. 

Phthalocyanine is a divalent ligand with a geometry appropriate for forming a four-coordinated square-planar complex and was thought ideal for generating an Au(II) species. The action of AuBr on molten 1,3-diiminoisoindoline in the absence of solvent indeed yields a neutral gold phthalocyanine (63). The EPR spectra of the complex in 1-chloronaphthalene at 77 K clearly showed the presence of Au(II), a d9 ion. The g value is 2.065, comparable to 2.042 for copper and 2.093 for silver phthalocyanine (64, 80). 

Under certain conditions, ethylenediamine itself may form square planar complexes with nickel(II). Sone and Kato (89) reported that yellow Ni(en)2" is formed when [Ni(en)2(H20)2]" is heated in alcohol. The bis-ethylenediaminenickel(II) complex, Ni(en)2(AgIBr)2, and the corresponding platinum(II) and palladium(II) derivatives are isomorphous, indicating square planar nickel in this complex (58), The anions may be changed to Cul2 , Is", Pbls , and B.glr without destroying the planarity of nickel. Cobalt (II) and copper(II) also form square planar complexes of this type. These square planar species appear to be stable only in the solid state for they decompose when dissolved in water (58). 

Nuclear quadrupole coupling constants have been measured for copper in six square-planar complexes, five with sulphur donors and the other with S,0 donors. ° The preparation of Cu(thiaz)X2 (X = Cl or Br), Cu(thiaz)2X2 (X = Cl or Br) and Co(thiaz)2X2 [X = Cl, Br, or I thiaz = 5-(2-hydroxyethyl)-4-methylthiazole] have been isolated and generally appear to contain MX " ions. An e.p.r. study of bis-(l,l-diethyl-3-benzoyl-selenoureato)copper(ii) (145) has been reported. ... 

Some new combinations of chiral ligands with different Lewis acids have been lately evaluated in catalytic asymmetric 1,3-DC reactions of nitrones. When the complex derived from copper(II) triflate and bis(oxazoline) 72 was used as chiral catalyst in the cycloaddition of nitrone 66 and crotonate 68, both endo and exo isomers were obtained with very high enantioselectivities (7 3 dr > 99% ee). In this reaction, the presence of molecular sieves 4 A (MS) was crucial as in their absence the nitrone decomposed and almost no cycloadduct was obtained <04TL9581>. Sibi et al. found that square planar complexes derived from copper triflate and some chiral bisoxazolines favour the COZ-exo approach in the 1,3-DC of nitrone... 

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