Tetrazadiene complexes

Pair-of-dimer effects, chromium, 43 287-289 Palladium alkoxides, 26 316 7t-allylic complexes of, 4 114-118 [9JaneS, complexes, 35 27-30 112-16]aneS4 complexes, 35 53-54 [l5]aneS, complexes, 35 59 (l8)aneS4 complexes, 35 66-68 associative ligand substitutions, 34 248 bimetallic tetrazadiene complexes, 30 57 binary carbide not reported, 11 209 bridging triazenide complex, structure, 30 10 carbonyl clusters, 30 133 carboxylates... 

In contrast, tetrazadienes, RN=N—N=NR, which are unknown in the free state, are found in a growing number of transition metal complexes. The ligands can be generated in situ from organic azides or diazonium cations and, following the discovery of Fe(MeNNNNMeXCO)3 by Dekker and Knox in 1967 (59), an extensive range of transition metal tetrazadiene complexes has been synthesized. Finally, several papers report theoretical calculations on the stability and structure of N4 ligands bound to transition metals (196-198). 

Tetrazadiene complexes are noted for their brilliant color and for their stability, which is much greater than that of analogous 1,4-diazabutadiene derivatives. 

Salient examples of the applications of spectroscopic techniques to the study of tetrazadiene complexes are given below. In most instances, further details are given when the complexes concerned are discussed in Section III,E. 

In the absence of spectra for the free ligands relatively little use has been made of infrared and Raman spectroscopy in the study of tetrazadiene complexes. However prima facie evidence that the n-acceptor ability of 1,4-dimethyltetrazadiene is comparable to that of a pair of carbonyl ligands is provided by the observation that the degeneracy weighted average v(CO) frequency for Fe(MeNNNNMe)(CO)3 is the same (2028.6 cm ) as that of Fe(CO)5 213). Infrared data [v(NC) 2168 and 2146 cm ] for Ni(ArNNNNAr)(BuNC)2 complexes point to a similar conclusion 167). 

Selected Bond Lengths and Angles for Tetrazadiene Complexes... 

In marked contrast to the parent ligands, which are unknown in the free state, tetrazadiene complexes are extremely stable. In particular they are much less susceptible to oxidation, thermal decomposition, or hydrolysis... 

Under the influence of UV radiation the cobalt complex CofPhNNNNPhXCjHj) undergoes a remarkable transformation to the diimine derivative Co(HNC6H4NPh)(C5H5) (93, 96). More details of this and some related reactions are given in Section III,E,3. Exchange reactions between tetrazadiene complexes and free aryl azides lead to partial or complete substitution (169). 

Finally, the ability of tetrazadiene ligands to transfer between metal centers via bridged intermediates has been utilized as a route to new tetrazadiene complexes 170, 171). Further discussion of these exchange and transfer reactions is given in Section III,E,4. 

No tetrazene or tetrazadiene complexes have been reported to date for members of the manganese, technetium, rhenium triad. 

The first tetrazadiene complex, Fe(MeNNNNMe)(CO)3, which was isolated from the mixture of products obtained on treatment of Fe2(CO)g with methyl azide, is a volatile orange solid of remarkable stability (59). On... 

The first nickel tetrazadiene complex, Ni(C6FjNNNNC6F5XC8H,2), obtained from Ni(CgHj2)2 and pentafluorophenyl azide, reacts with... 

Scheme II. Tentative mechanism for the formation of iridium tetrazadiene complexes [IrlArNNNNArXCOXPPhjlj]. Adapted with permission from Can. J. Chem. 52, 3387 (1974).

Scheme 12. Mechanisms proposed for the formation of nickel tetrazadiene complexes Ni(RNNNNR)L2- Adapted from J. Chetn. Soc., Dalton Trans., p. 1805 (1972).

No simple palladium tetrazadiene complexes have been isolated to date reactions between Pd(PPh3)4 and organic azides afford polymeric palladium phosphine complexes (232). The apparent instability of palladium tetrazadiene complexes has been tentatively attributed to the relatively low basicity of palladium, which does not permit sufficient n back donation to the tetrazadiene ligand (32). However, the bimetallic species Ni(p-tol-NNNN-tol-p)2PdL2 (L = Bu NC or PEtj) have been obtained from reactions between Pd(norbornene)3 and Ni(p-tol-NNNN-tol-p)2 in the presence of Bu NC or PEt3 at 0°C (170, 171). 

To date no tetrazene or tetrazadiene complexes of the coinage metals— copper, silver, and gold—appear to have been reported. 

No examples of tetrazadiene complexes are known for members of this triad. However, all three elements form complexes with tetrazene ligands R N—N=N—NR2. 

---