Homodinuclear complexes

In the homodinuclear complex [ Pd(dppp) 2(//-biim)](OTf)2 (dppp = bis(diphenylphosphino)-propane, biim = 2,2 -biimidazole, 0Tf=0S02CF3) a biim2 ligand spans the two palladium(II) centers.535... 

Alternative stabilizing forces (Z) include (i) complexation with metal alkyls (or hydrides), forming hetero-and homodinuclear complexes such as [Cp 2M-(/i-Me)2AlMe2] Y and [(Gp 2M-Me)2(/i-Me)] Y (Figure... 

Spectroscopic data for mixed-metal complexes are consistent with data for structures in which the alkyne bridges the heteronuclear metal-metal bond in a p.-p2,r fashion, as is found in the homodinuclear complexes [Co2 ( U-alkyne)(CO)6]. This has been confirmed by X-ray crystallography in several cases.1619,21,46-50... 

The ESR spectrum of the Cu —Zn heterodinuclear complex gave the well-defined ESR parameters (gy = 2.10, gJ. = 2.24,14 l = 117 G, and IA LI = 124 G), which indicate that the Cu(ll) ion in the complex has a trigonal-bipyramidal environment and a ground state (154). The observation of well-defined ESR spectmm also confirms that the complex retains its imidazolate-bridged Cu —Zn heterodinuclear structure and that it is not a 1 1 mixture of Cu —Cu and Zn Zn homodinuclear complexes. 

The Cp2Zrl- C2H2-B interaction is probably electrostatic. It corresponds to an internal ion pair situation (Zrl-C2 2.600(3) A, C1-C2-B 119.2(2)° and Zrl-C2-B 155.0(2)°) analogous of the complexes described previously (see Section 3.1). It seems that this distorted C2v - methane-like coordination geometry is ca. 5 kcal moH less stable than ideal tetrahedral geometry. Certainly the gain of energy afforded by the ion pair formation is responsible for the stability of homodinuclear complex (59). 

The considerable nucleophilicity of the OH bridges in the anionic complexes [ R2M(/t-OH) 2](NBu4)2 (M = Ni, Pd, or Pt) is indicated by their reactivity toward weak acids such as pyrazoles, with formation of homodinuclear complexes of the formula [ R2M(/x-pz ) 2](NBu4)2 (Scheme 1) (60-66). 

In each case products are the parent homodinuclear complexes, M-> 1,10-phen — 1,10-phenanthroiine. 

To enhance the kinetics of the C—H activation process, Wayland and co-workers prepared homodinuclear complexes consisting of two (porphyrinato)-Rh(II) complexes tethered by different spacers (170-172). This significantly accelerates the C—H bonds activation process by diminishing the unfavorable entropy contribution to the activation barrier [Fig. 59(a)]. Within one dinuclear complex, one Rh-site receives a hydride ligand and the other Rh-site receives an alkyl ligand (Fig. 60). 

A series of j8-diketonate Schiff base complexes (87 M = Ni, Cu, Pd, VO n=10, 12, 16) having a half disc-shape, was synthesised [164]. All the complexes, except the oxovanadium complexes, were mesomorphic (Colh), at relatively low temperatures. This approach was also applied to half-discshaped homodinuclear and heterodinuclear 1,3,5-triketonate Schiff base complexes, 88, and 1,3,5,7-tetraketonate Schiff base dicopper complexes, 89 [165]. The homodinuclear complexes 88, with M = M = Cu, X = OR, were mesomorphic showing Colh phases, whereas those with four chains (88 X = H) were not liquid-crystalline. Amongst the five heterodinuclear prepared (88 X = OR,n=14, Y = —CH2—CH2—, M = Cu, Pd, Ni, Mn, Co and M =Ni), only those with M = Cu, M = Ni and M = Pd, M =Ni, showed a mesophase, namely a Colh phase. A Colh phase was also observed for a structurally related compound, 89, (n=12, X = OR). 

Despite the publication of these papers, as was indicated in the introduction, most of the later publications have focused on the calculation of dinuclear complexes employing the broken-symmetry approach proposed by Noodleman et al. In this approach the J value involves the calculation of the energy difference between the high-spin state and a low-spin solution that corresponds to a broken symmetry wavefunction in the case of symmetric homodinuclear complexes. From now on, it will be employed the expressions for the Hamiltonians indicated in Eqs. (1) and (2) Eq. (3) was kept for historical reasons. A general expression, (see Eq. 4) can be proposed for any dinuclear complex using the original broken-symmetry approach proposed by Noodleman [26] ... 

A further step in complexity was aecomplished by designing ligands able to eomplex more than one cation eryptand 8 (Fig. 8), for example, forms a homodinuclear complex with silver or copper.The erystal stracture of the dinuclear Cu(I) eomplex formed with the hexaimine macrobicycle 8 shows the inelusion of the two copper ions, which are tetracoordinated at each end of the cavity, the Cu...Cu distance being very large (11.07 A). The small hexaimine macrobicycle 9 (Fig. 9) forms a Cu(l) dinuclear complex in which the Cu(I)... Cu(l) distance is short (2.45 A). In the reduced hgand 10 (Fig. 10), the Cu(I) is not stable, and only the [Cu2 (lO)] " cryptate was obtained. The Cu... Cu distance in this complex is 2.42 A, and on the basis of ESW measurements, an average redox state (di Cu ) was postulated rather than a Cu(I)... Cu(II) dinuclear complex. ... 

Fig. 8 Homodinuclear complex of the hexaimine crx ptand 8 containing a diphenjl methane spacei...

Some elegant work related to this area is that reported by Geiger and coworkers [20, 176] who employed a range of in situ spectroscopic techniques including FTIR to study the oxidation of (fulv) Mn(CO)2 2(g-dppm), 1, where fulv = CioHg " and dppm = bis(diphenylphosphino)methane and related homodinuclear complexes. 

In addition, the authors were able to analyze the carbonyl infrared spectral shifts for the range of closely related dinuclear systems in terms of a charge distribution parameter, Ap, which was found to correlate Knearly with the separation potentials of the successive one-electron oxidations of the dinuclear systems. Overall, the authors clearly showed the power of electrochemistry and in situ spectro-electrochemical techniques with respect to probing site-to-site interactions in homodinuclear complexes. Further studies of inorganic systems are described in Volume 7 of the Encyclopedia. 

When one metal ion, let us say copper(II) without loss of generality, senses another copper(II) ion, under the condition J < kT and J > A and hTf, no obvious physical mechanism can be conceived that can increase the electron relaxation rate of the former copper ion. To first approximation, the electronic relaxation is not affected by magnetic coupling in homodinuclear complexes [20-23]. 

Alkyl halides also add to dinudear compounds. Additions can occur across the metal-metal bond of homodinuclear complexes, as shown in Equation 7.22, or across the metal-metal bond of heterodinudear compounds, as shown in Equation 7.23. Oxidative additions of dihaloalkanes across two metal centers to generate bridging methylene complexes - have been reported for complexes of iron (Equation 7.24) and osmium. Addition of a higher alkyl dihalide to an odd-electron, dinudear cobalt system (Equation 7.25) " has also been reported. In this case, the stereochemical outcome suggests that the first substitution process occurs by a radical mechanism. 

The Schiff-base macrocycle H2L1733 (Eq. 8.39) with increased coordination capacity has been assembled in the presence of lanthanum(III) acetate from 6,6 -bis(aminomethyl)-2,2 -bipyridyl and 2,6-diformyl-4-methylphenol and isolated as the homodinuclear complex [La2(L1733)(02CCH3)4]4CH3CN-3H20 [103]. X-Ray crystallographic study of the latter showed that all ten donor atoms of the macrocycle are involved in coordination to two identical La centres. A coordination number of 10 for each La is completed by two bidentate acetate anions. Reduc-... 

A particularly notable homodinuclear complex is the dititanium compound 3 displayed in Figure 4.1. For this complex, a remarkably short Ti-Ti distance of 2.362 A was found, which is undoubtedly in part enforced by the bridging cydooctatetraene ligand. 

A method frequently used in the synthesis of homodinuclear complexes is the dimination of halide ligands by reduction. This strategy is applicable in the synthesis of heterodinudear complexes if bridging ligands effect the prearrangement of the two metal centers, as is exemplified by the synthesis of the Ti-Rh complex 19, which was obtained by Slaughter and Wblczanski upon Na/Hg reduction of a halide precursor (Scheme 4.4) [21]. Its most characteristic structural element is the extremely short... 

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