The reaction of the cobalt(I) complex [(TIMMN mes )Co I ](BPh 4 )(2)(TIMMN mes = tris-[2-(3-mesitylimidazolin-2-ylidene)methyl]amine) with 1-adamantylazide yields the cobalt(III) imido complex [(TIMMN mes )Co III -(NAd)](BPh 4 )( 3)w ith concomitant release of dinitrogen. The N-anchor in diamagnetic 3 features an unusual, planar tertiary amine,w hich results from repulsive electrostatic interaction with the filled d(z 2 )-orbital of the cobalt ion and negative hyperconjugation with the neighboring methylene groups.O ne-electron oxidation of 3 with [FeCp 2 ](OTf) provides access to the rare,h igh-valent cobalt(IV) imido complex [(TIMMN mes )Co IV (NAd)](OTf) 2 (4). Despite ah alflife of less than 1hat room temperature, 4 could be isolated at low temperatures in analytically pure form. Single-crystal Xray diffractometry and EPR spectroscopyc orroborate the molecular structure and the d 5 low-spin, S = 1 = 2 ,e lectron configuration. Ac omputational analysis of 4 suggests high covalency within the Co IV =NAdbond with non-negligible spin density located at the imido moiety,w hicht ranslates into substantial triplet nitrene character.

The first scandium phosphinoalkylidene complex was synthesized and structurally characterized. The complex has the shortest Sc-C bond lengths reported to date (2.089(3) Å). DFT calculations reveal the presence of a three center π interaction in the complex. This scandium phosphinoalkylidene complex undergoes intermolecular C-H bond activation of pyridine, 4-dimethylamino pyridine and 1,3-dimethylpyrazole at room temperature. Furthermore, the complex rapidly activates H under mild conditions. DFT calculations also demonstrate that the C-H activation of 1,3-dimethylpyrazole is selective for thermodynamic reasons and the relatively slow reaction is due to the need of fully breaking the chelating effect of the phosphino group to undergo the reaction whereas this is not the case for H.

The first phosphoniomethylidene complexes of scandium and lutetium, [LLn(CHPPh)X] (L = [MeC(NDIPP)CHC(NDIPP)Me]; Ln = Sc, X = Me, I, TfO; Ln = Lu, X = CHSiMe), have been synthesized and fully characterized. DFT calculations clearly demonstrate the presence of an allylic Ln, C, P π-type interaction in these complexes. X-ray diffraction indicates that the scandium iodide complex has the shortest Sc-C bond length to date (2.044(5) Å). These phosphoniomethylidene complexes readily convert into the ylide complexes, and the reactivity is affected by both X anion and Ln ion. The reaction of lutetium complex with imine shows a rapid insertion of imine into the Lu-C(alkylidene) bond. DFT calculations indicate that, although the bonding situation seems similar to that of the scandium analog, the strong negative charge at the alkylidene carbon is not sufficiently screened by one hydrogen in the lutetium complex because of a more ionic bonding, and therefore, the reactivity of the lutetium complex is much higher.

A new supporting ligand, tris-[2-(3-mesityl-imidazol-2-ylidene)methyl]amine (TIMMN Mes ), was developed and utilized to isolate an air-stable iron(V) complex bearing a terminal nitrido ligand, which was synthesized by one-electron oxidation from the iron(IV) precursor. Single-crystal X-ray diffraction analyses of both complexes reveal that the metal-centered oxidation is escorted by iron nitride (FeN) bond elongation, which in turn is accompanied by the accommodation of the high-valence iron center closer to the equatorial plane of a trigonal bipyramid. This contrasts with the previous observation of the only other literature-known Fe(IV)N/Fe(V)N redox pair, namely, [PhB-( tBu Im) 3 FeN] 0/+ . On the basis of 57 Fe Mossbauer, EPR, and UV/vis electronic absorption spectroscopy as well as quantum chemical calculations, we identified the lesser degree of pyramidalization around the iron atom, the Jahn−Teller distortion, and the resulting nature of the SOMO to be the decisive factors at play.

Reaction of the Co I complex [(TIMMN mes )Co I ](PF 6 ) (1) (TIMMN mes = tris-[2-(3-mesityl-imidazolin-2-ylidene)-methyl]amine) with mesityl azide yields the Co III imide [(TIMMN mes )Co III (NMes)]-(PF 6 ) (2). Oxidation of 2 with [FeCp 2 ](PF 6 ) provides access to a rare Co III imidyl [(TIMMN mes )Co(NMes)]-(PF 6 ) 2 ( 3). Single-crystal X-ray diffractometry and EPR spectroscopy confirm the molecular structure of 3 and its S = 1 = 2 ground state. ENDOR, X-ray absorption spectroscopy and computational analyses indicate a ligand-based oxidation; thus, an imidyl-radical electronic structure for 3. Migratory insertion of one ancillary NHC to the imido ligand in 2 gives the Co I Nheterocyclic imine (4) within 12 h. Conversely, it takes merely 0.5 h for 3 to transform to the Co II congener (5). The migratory insertion in 2 occurs via a nucleophilic attack of the imido ligand at the NHC to give 4, whereas in 3, a nucleophilic attack of the NHC at the electrophilic imidyl ligand yields 5. The reactivity shunt upon oxidation of 2 to 3 confirms an umpolung of the imido ligand.

No abstract

As key intermediates in metal-catalyzed nitrogen-transfer chemistry, terminal imido complexes of iron have attracted significant attention for a long time. In search of versatile model compounds, the recently developed second-generation N-anchored tris-NHC chelating ligand tris-[2-(3-mesityl-imidazole-2-ylidene)-methyl]amine (TIMMNMes) was utilized to synthesize and compare two series of mid- to high-valent iron alkyl imido complexes, including a reactive Fe(V) adamantyl imido intermediate en route to an isolable Fe(V) nitrido complex. The chemistry toward the iron adamantyl imides was achieved by reacting the Fe(I) precursor [(TIMMNMes)FeI(N2)]+ (1) with 1-adamantyl azide to yield the corresponding trivalent iron imide. Stepwise chemical reduction and oxidation lead to the isostructural series of low-spin [(TIMMNMes)Fe(NAd)]0,1+,2+,3+ (2 Ad –5 Ad ) in oxidation states II to V. The Fe(V) imide [(TIMMNMes)Fe(NAd)]3+ (5 Ad ) is unstable under ambient conditions and converts to the air-stable nitride [(TIMMNMes)FeV(N)]2+ (6) via N–C bond cleavage. The stability of the pentavalent imide can be increased by derivatizing the nitride [(TIMMNMes)FeIV(N)]+ (7) with an ethyl group using the triethyloxonium salt Et3OPF6. This gives access to the analogous series of ethyl imides [(TIMMNMes)Fe(NEt)]0,1+,2+,3+ (2 Et –5 Et ), including the stable Fe(V) ethyl imide. Iron imido complexes exist in a manifold of different electronic structures, ultimately controlling their diverse reactivities. Accordingly, these complexes were characterized by single-crystal X-ray diffraction analyses, SQUID magnetization, and electrochemical methods, as well as 57Fe Mössbauer, IR vibrational, UV/vis electronic absorption, multinuclear NMR, X-band EPR, and X-ray absorption spectroscopy. Our studies are complemented with quantum chemical calculations, thus providing further insight into the electronic structures of all complexes.

A series of monomeric rare-earth metal silyl-thiophosphinoyl-alkylidene complexes [LLn{C(SiR )PPh S}] (5: Ln=Lu, R=Me; 6: Ln=Lu, R=Ph; 7: Ln=Y, R=Me; 8: Ln=Y, R=Ph; 9: Ln=Sm, R=Ph; 10: Ln=Sm, R=Me; 11: Ln=La, R=Ph; L=[MeC(NDIPP)CHC(Me)(NCH CH N(Me) )] , DIPP=2,6-(iPr) C H ) have been synthesized and structurally characterized. The influences of rare-earth metal ions, ancillary ligands, and alkylidene groups on the reactivity of complexes 5-11 and the related scandium complexes [LSc{C(SiR )PPh S}] (1: R=Me; 2: R=Ph) and [L'Sc{C(SiR )PPh S}] (3: R=Me; 4: R=Ph; L'=[MeC(NDIPP)CHC(Me)(NCH CH N(iPr) )] ) have been studied. Reactions of these rare-earth metal alkylidene complexes with PhCN give four kinds of products, the formation of which is dependent on the rare-earth metal ions, ancillary ligands, and alkylidene groups of the complexes. In the reactions with tBuNC, unusual C-P bond cleavage of the alkylidene group and C≡C triple bond formation occur. Complexes 10 and 11 also react with PhSiH to form hydrides, which subsequently undergo Ln-H addition to the C=N bond of the ancillary ligand L. DFT calculations have been used to analyze the bonding in complex 10, which exhibits a polarized three centers Sm-C-P π interaction, and to rationalize the reactivity by computing reaction mechanisms. The difference in reactivity of PhCN and tBuNC is due to the electron density delocalization that is enabled by the phenyl group rather than the tBu group.