Imido complexes bridging ligands

Titanium imido complexes supported by amidinate ligands form an interesting and well-investigated class of early transition metal amidinato complexes. Metathetical reactions between the readily accessible titanium imide precursors Ti( = NR)Cl2(py)3 with lithium amidinates according to Scheme 84 afforded either terminal or bridging imido complexes depending on the steiic bulk of the amidinate anion. In solution, the mononuclear bis(pyridine) adducts exist in temperature-dependent, dynamic equilibrium with their mono(pyiidine) homologs and free pyridine. 

The Tc complex [TcI(N—Ar)s] (28) (see Section 5.2.2.1.2) can be reduced with Na° in THE to yield the green, nonbridged, dinuclear compound [Tc2(NAr)6] (46), in which three imido-ligands are bound to the Tc center and connected by a single bond to the second technetium. The molecule has a staggered, ethane-like structure and is diamagnetic. Reduction of (28) yields another homoleptic imido-complex of Tc, the imido-bridged, tetrahedral, dinuclear compound [Tc2(/u-NAr )2(NAr )4] (47) (Ar = 2,6-diisopropylphenyl). The conformation could be confirmed by X-ray structure analysis the assumption of a Tc Tc bond is confirmed by the... 

The other two classes of imido complex contain bridging imido ligands between two metal atoms as in [(/u,-NH)(/a-0) Mo-(0)[S2P(0Et)2l2 2], and between three metal atoms as in the iron cluster (37). 

Two binuclear complexes containing 2,2 -bipyridyl (bipy) have been reported in which the imido ligand appears to function in a bridging role. These are [0s2(NH)Cl2(bipy)4](C104)2, made from [Os(bipy)2(NH3)Cl]+ and cerium(TV), and Os2(NH2)(bipy)4, made from Os(bipy)2Cl2 and ammonia.173 The IR spectrum of the first complex has been recorded.294 The oxidation states seem too low for these to be true imido complexes and further investigation would certainly be of interest. 

To date, no terminal unsupported Ln-imido complexes, Ln=NR, (Ln=Ce-Lu) have been isolated and characterized in the solid state, despite the comparative fertility of this area for related early TMs [83]. Structurally authenticated Ln-imido and -phos-phinidene complexes in the literature, typically prepared by the deprotonation of Ln—NHR and Ln—PHR precursors, form dimers in the solid state with bridging NR and PR fragments [84]. It is noteworthy that the transient existence of unsupported Ln=NR and Ln=PR linkages had previously been observed, though could not be isolated [95]. In the case of the RE, the first terminal Sc-imido complex, [Sc(=NDipp)(T -NacNac-K -A, A, A ")(DMAP)] (T -NacNac = (N(Dipp)C(Me) CHC(Me)N(CH2CH2NMe2) DMAP = 4-dimethylaminopyridine, 45), was recently prepared, which is stabilized by a bespoke tri-coordinate variant of the ubiquitous NacNac ligand [96]. 

A deoxygenation reaction o-f the nitro compound by the metal is also probably involved when tetraphenylporphyrinate and phtalocyaninate derivatives o-f metals such as palladium or rhodium are used as catalysts [169,170]jpossibly -forming dimeric complexes having oxo and imido groups as bridging ligands. 

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