ISSN:
1434-1948
Keywords:
Cooperative effects
;
Coordination modes
;
Nickel
;
Bimetallic complexes
;
π interactions
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Different cooperative binding modes of nitriles within the bimetallic pocket of a pyrazolate-based compartmental dinickel(II) site have been studied. The H3O2-bridged dinuclear complex 1 reacts with cyanamide to yield 4, in which a secondary hydrogencyanamido(1-) bridge spans the two metal centers at an unusually short metal-metal distance imposed by the primary ligand matrix. In 5, a single 2-cyanoguanidine (cnge) molecule is N-bound to one nickel(II) ion through its nitrile part and is coordinated to the adjacent metal site through an amido nitrogen. The characteristics of the coordination spheres of the metal centers suggest an additional side-on π-bonding interaction of the nitrile moiety with the second high-spin nickel(II) ion. This unusual interaction is corroborated by comparing the IR bands for the ν(C≡N) stretching vibration of 5 with those of complex 6, which has two end-on bound cnge molecules, and those of the related mononuclear complex 7, which lacks a second nickel(II) ion. The nature of the π-bonding interaction in 5 is further analyzed by DFT calculations on relevant model systems. Even though the π-bonding is found to be very weak, it does include some backbonding from occupied 3d MOs at the second high-spin nickel(II) ion to the π* MOs of the nitrile. Such an unconventional π-interaction is suggested to be enforced by the constrained fixation of the nitrile unit within the highly organized coordination pocket of the bimetallic framework. In contrast, the bifunctional 2-hydroxybenzonitrile is accommodated by the distinct binding of the nitrile and phenolate functions to the different metal centers in 8, which confirms that the simultaneous binding of both an OR-function and an end-on bound nitrile is indeed feasible within the active site pocket. Such a situation is reminiscent of the bimetallic effect that has been assumed to enable the cooperative hydration of nitriles at the dinickel(II) site of 1. Complexes 4·(ClO4)2, 5·(ClO4)2, 6·(ClO4)3, 7·(ClO4)(BPh4), and 8·(ClO4)2 have been characterized structurally by X-ray crystallography.
Additional Material:
10 Ill.
Type of Medium:
Electronic Resource
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