Dinickel complex

The side-on structure (3) has been established in two dinickel complexes which have very complicated structures involving lithium atoms also in association with the bridging It also... 

Pyrazolato-bridged bis(tacn) ligands and their dinickel complexes are mentioned in Sections 6.3.4.12.6 and 6.3.4.12.7. 

In most cases the Ni11 ions are low spin and in square planar coordination environment. Dinickel(II) complexes [Ni2L(pyrazolate)] of dinucleating ligands like (725) and dinickel complexes such as (726) feature two nearly reversible electrode processes at very negative potential,... 

Thiolate-bridged dinickel complexes and, in particular, heterobimetallic Ni/Fe complexes have attracted much interest as model systems for the hydrogenase enzymes. A review covering this... 

Type (820) dinickel complexes offer the opportunity of substrate binding within the bimetallic pocket, and highly preorganized complexes of this type have also been employed as model systems for the urease metalloenzyme (see Section 6.3.4.12.7). The Ni—Ni separation in type (820) complexes can be... 

In the case of macrocyclic tacn side arms, stability constants of type (820) dinickel complexes are considerably higher and six-coordination can be achieved due to the binding of additional coligands, either at both Ni11 (in (828))2031 or at only one of them (in (829)).2082... 

Crystal structures of dinickel complexes with 0,0-bridging and 0,N-bridging (O-methylated) carbamate have been reported.2074,20 (874) produces one equivalent of ammonia upon heating in methanol/water solution.2082... 

The observation that a,/ -unsaturated carboxylate ligands can be readily accommodated in the binding pocket led us to study an orienting reaction between the dinickel complex 101 and 2,3-dimethylbutadiene (Scheme 10). However, the reaction did not proceed, even when the reaction mixture was heated at 210°C for 24 h. The inhibition of the Diels-Alder reaction can be traced to the limited space in the binding pocket of 101. 

Similarly, the dinuclear Ni(II) complexes of bis-monocyclic ligands, 20, were prepared by using [Ni(2,3,2-tet)](C104)2, formaldehyde, and NH2—(CH2) —NH2(n = 2,3,4). No interactions between the metal centers were observed in these dinickel complexes, either (26, 27). 

These dicobalt and dinickel complexes are of interest, since there is an unusual bridge binding of one water molecule at the expense of the two electronic pairs in the oxygen atom. Moreover, a simultaneous terminal and bridge coordination of the carboxyl groups is characteristic in this kind of compound. 

Specific active site stmctural features of urease are the bimetalhc arrangement with a Ni Ni distance of 3.5-3.7 A and nonsymmetric N/O-rich coordination environment, the bridging carbamate (often modeled by a bridging carboxylate), and the presence of a hydrolytically active Ni-bound hydroxide or water. Relevant dinickel complexes that emulate at least part of these features are introduced in this chapter. Of course, the ability to bind urea is a prerequisite for urease-like activity, and different urea-binding modes were observed in synthetic model compounds. Those model complexes and artificial systems that mediate the decomposition of urea are discussed in Section III. 

An even shorter Ni Ni distance of 3.17 A was observed in a related dinickel complex (5) that has longer chelate arms of the ligand scaffold and a ()0,-phenolato)-( a,-hydroxo) core (56). Addition of acid was suggested to protonate the bridging hydroxide with subsequent coordination of water molecules and six-coordinate metal ions, with pAia 8.5 for the Ni-bound water derived from a pH-metric titration. Unfortunately, the protonated product was not further characterized. 

An unusual situation where urea forms a single atom bridge between the proximate metal ions through its carbonyl-O atom was found in 56 (Fig. 11 cf. also 38) (111). Although one might assume enhanced polarization of the substrate when the two metal ions act as a tweezer, the relevance of this rare urea coordination mode, which hitherto has been observed in only one other dinickel complex (see Section III.B.4), for the mechanism of the enzyme remains questionable. 

N-bridging cyanate in low yields (17-23% after heating for 24 h Scheme 11). Conversion was found to proceed at comparable rates in ethanol or acetonitrile, and it was thus concluded that hydrolytic processes by traces of water do not play a role. Cyanate formation also was observed with Af-methylurea or 7/,7/-dimethylurea, but not with Af,A -dimethylurea or tetramethylurea, which shows that at least one NH2 group is essential for the elimination reaction to occur. A possible interpretation is that bridging of urea over the binuclear core through its O atom and one amino N atom is a key step for the conversion (58). This finding is in line with the discovery of urea-to-cyanate transformation for several pyrazolate-based dinickel complexes with urea bound in the N,0-bridging mode (see below). 

The H3O2 bridged dinickel complex (21) accommodates urea in an N,0 bridging mode after deprotonation, giving 63 (113). Binding constants were determined as = 4.3 0.4M in acetone and 2.7 0.5 M in MeCN (Scheme 12) (74). 

In methanol solution, the terminal water ligands in the phtalazine-based dinickel complex 32 (see Section 11.B.2) can be replaced by O-bound urea to give 75 (Fig. 15). When acetonitrile is used as the solvent instead of methanol, one of the substrate molecules shifts to an unusual single atom O-bridging position in 76 (the only other known example in dinickel chemistry being 56). Solid-state (IR) spectra reveal a shift of the C=0 stretching frequency of urea from 1690 to 1663 cm in 75 or 1661 cm in 76upon coordination to the dinickel sites. Due to... 

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