Side-on binding

Figure 10 End-on and side-on binding observed for metal complexes of nitriles, and some probable reaction consequences.

A. Karlsson, J. V. Parales, R. E. Parales, D. T. Gibson, H. Eklund, S. Ramaswamy, Crystal structure of naphthalene dioxygenase Side-on binding of dioxygen to iron. Science 299 (2003) 1039. 

Side-on/side-on binding is found with group 1-4 metals, lanthanides, and actinides. One complex of this type is [ (thf)3Li 2(/i-ry 77 -N2)] (1), which may be a dication or a monocat-ion. Complexes like this generally result from the reaction of a transition metal complex with N2, and conceptually may be envisioned as the result of electron transfer to N2. Such complexes are known for y 32,33 p>y 34 35 pj. 36 Nd. Several group 4 complexes of this type... 

In other complexes, N2 is bound both end-on and side-on. In some end-on/side-on complexes, each metal plays its customary role, with an alkali metal bound side-on and a late transition metal bound end-on." " In one very unusual compound, though, lithium atoms are at the ends of N2 and nickel coordinates to the side. In some homometallic clusters, N2 is bound end-on to some metals and side-on to others. One of the best-characterized examples of end-on/side-on binding is in [(NPN)Ta]2(N2) (2), " in which the unsymmetric binding of N2 activates it toward... 

Karlsson A, Parales JV, Parales RE et al (2003) Crystal stmcture of naphthalene dioxygenase side-on binding of dioxygen to iron. Science 299 1039-1042... 

Protonation of metal hydrides may often go via the pathway of Eq. 2. which also shows the side-on binding of the H2 ligand resulting from proton transfer. 

The Nl-Bl separation is consistent with an elongated single bond, while the N3-H2 and N3-N2 separations are indicative of hydrogen bonding. This complex also mediates Nj-centered redox reactivity, and in the presence of CoCp 2 and collidinium hydrochloride as a reductant and proton donor, respectively, ammonia was confirmed as a reduction product. The side-on binding mode and N-N bond scission reaction is of potential relevance to the Nj-fixation cycles since hydrazine is a known byproduct of N2 reduction to ammonia by V-nitrogenase [37]. Future work on this scaffold will target redox transformations of other small molecule substrates amenable to reductive activation, for instance, CO2, N2, and CO, and seek to elucidate the intimate role of each Lewis partner to effect substrate activation. 

Catalytic mechanisms for tyrosinase activity have been proposed (7) based on both biochemical and model system studies (see later). The former include key recent kinetic studies from Itoh (50) and Casella (51). Mechanistic details have also been suggested based on computational studies (52,53). (Scheme 3) shows a well-accepted proposed mechanism, which includes phenolase and catecholase cycles. Here, tyrosinase is present in three redox forms during the cycle. The reduced deoxy Cu(I)-Cu(I) entity binds O2 to generate the oxyform Cu(II)-02 -Cu(II) in which the peroxide group occurs in a side-on -binding mode the... 

From biochemical studies of Klinman and co-workers, both enzymes proceed via similar activated intermediates, most recently postulated to be a superoxide-copper(ll) species (4,66). Chen and Solomon s computational studies also invoke a Cu(ll)-(02 ) (possibly with side-on -binding) substrate hydrogen-abstracting active species (67). Their overall mechanisms proposed differ in some details. That proposed by Klinman is given in (Scheme 4) (4,66). Other possible alternative oxidizing intermediates (ie, a Cu(II)-OOH complex or a Cu(H)-(0 ) Cu(III)-(0 )) are still under debate. 

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