Application of the dipyridylazaallyl ligand (2-py)CHNCH(2-py) (smif) to a series of first-row transition metals afforded (smif)2M n [n = 0, M = Fe (1), Co (2), Ni (3); n = +1, M = Co (2+)] and {(TMS)2NFe}2(smif)2 (4 2 ) via metathetical procedures. The Mössbauer spectrum of 1 (S = 0) and TDDFT calculations, including a UV−vis spectral simulation, reveal it to be a covalent, strong-field system with Δo estimated as ∼18 000 cm−1 and B ≈ 470 cm−1. (smif)2Co (2) has S = 1/2 according to SQUID data at 10 K. DFT calculations suggest that the odd electron is localized in a smif π* orbital, i.e., smif is redox-active. EPR-silent (smif)2Ni (3) has S = 1 (SQUID), and calculations show that the unpaired spins reside in the d z 2 and d x 2 −y 2 orbitals. X-ray structural parameters suggest that low-spin d6 1 and 2+ are relatively symmetric D 2d species, but 2 and 3 manifest a distortion in which one smif is canted in the plane perpendicular to the other. (smif)FeN(TMS)2 (4) is principally monomeric in solution, but reversibly dimerizes (K eq ≈ 10−4 M−1) via C−C bond formation in the azaallyl backbone to crystallize as {(TMS)2NFe}2(smif)2 (4 2 ). The azaallyl compounds possess extraordinary UV−vis absorptivities (ε ≈ 18000−52000) at 580 ± 15 nm and 406(25) nm that have been identified as intraligand bands with Cnb → smif π* character.

Aryl-bromide ligand precursors have been prepared with the potential to afford tetradentate chelates (2-pyridylmethyl)(3-x)N(CH(2)-2-Aryl)(x) (x = 1, 2) containing metal-aryl linkages that promise to impart stronger fields about first row transition metals. Oxidative addition to Ni(COD)(2) afforded two diamagnetic Ni(II) complexes, {kappa-C,N,N(py)-(2-pyridylmethyl)N(CH(2)(4-(t)Bu-phenyl-2-yl))(CH(2)(4-(t)Bu-phenyl-2-Br))}NiBr (1-Ni) and {(kappa-C,N,N(py)(2)-(2-pyridylmethyl)(2)N(CH(2)(4-(t)Bu-phenyl-2-yl))}NiBr (2-Ni) in 96% and 67% yield, respectively. Extending these synthetic efforts to iron provided {kappa-C,N,N(py)(2)-(2-pyridylmethyl)(2)N(CH(2)(4-(t)Bu-phenyl-2-yl))}FeBr (2-Fe, X-ray) in 91% yield via reduction of an adduct, {kappa-N,N(py)(2)-(2-pyridylmethyl)(2)N(CH(2)(4-(t)Bu-phenyl-2-Br))}FeBr(2) (3-Fe). 5-Coordinate 2-Fe possessed a pseudo-tbp structure, and SQUID magnetometry showed it to be S = 2 with significant zero field splitting (ZFS). 2-Fe was initially prepared via oxidative addition to Fe{N(TMS)(2)}(2)(THF) upon disproportionation to "Fe(0)" and 2 Fe{N(TMS)(2)}(3), but when this approach was attempted with Cr{N(TMS)(2)}(2)(THF)(2), the azaallyl complex {kappa-N,N(py)(2)-1,3-dipyridyl-2-azaallyl}CrN(TMS)(2) ((smif)CrN(TMS)(2), 4-Cr, X-ray), formed instead (>50%) via amine debenzylation. An alternative route consisting of addition of 1,3-di-2-pyridyl-2-azapropene to Cr{N(TMS)(2)}(2)(THF)(2) afforded 4-Cr in 74% yield. Pseudo-square planar 4-Cr was also S = 2 (SQUID) with marked ZFS. The dipyridylazaallyl ligand "smif" imparts a remarkable optical density to 4-Cr via intraligand bands at 675 nm (epsilon approximately 15,000 M(-1)cm(-1)) and 396 nm (epsilon approximately 27,000 M(-1)cm(-1)). The effective fields of the chelate complexes are discussed, and a comparison of smif to isoelectronic NHC ligands is given.

Molecular orbital analysis depicts the CNC(nb) backbone of the smif (1,3-di-(2-pyridyl)-2-azaallyl) ligand as having singlet diradical and/or ionic character where electrophilic or nucleophilic attack is plausible. Reversible dimerization of (smif)Fe{N(SiMe3)2} (1) to [{(Me3Si)2N}Fe]2(μ-κ(3),κ(3)-N,py2-smif,smif) (2) may be construed as diradical coupling. A proton transfer within the backbone-methylated, and o-pyridine-methylated smif of putative ((b)Me2(o)Me2smif)FeN(SiMe3)2 (8) provides a route to [{(Me3Si)2N}Fe]2(μ-κ(4),κ(4)-N,py2,C-((b)Me,(b)CH2,(o)Me2(smif)H))2 (9). A 3 + 2 cyclization of ditolyl-acetylene occurs with 1, leading to the dimer [{2,5-di(pyridin-2-yl)-3,4-di-(p-tolyl-2,5-dihydropyrrol-1-ide)}FeN(SiMe3)2]2 (11), and the collateral discovery of alkyne cyclotrimerization led to a brief study that identified Fe(N(SiMe3)2(THF) as an effective catalyst. Nucleophilic attack by (smif)2Fe (13) on (t)BuNCO and (2,6-(i)Pr2C6H3)NCO afforded (RNHCO-smif)2Fe (14a, R = (t)Bu; 14b, 2,6-(i)PrC6H3). Calculations suggested that (dpma)2Fe (15) would favorably lose dihydrogen to afford (smif)2Fe (13). H2-transfer to alkynes, olefins, imines, PhN═NPh, and ketones was explored, but only stoichiometric reactions were affected. Some physical properties of the compounds were examined, and X-ray structural studies on several dinuclear species were conducted.

A series of Werner complexes featuring the tridentate ligand smif, that is, 1,3-di-(2-pyridyl)-2-azaallyl, have been prepared. Syntheses of (smif)(2)M (1-M; M = Cr, Fe) were accomplished via treatment of M(NSiMe(3))(2)(THF)(n) (M = Cr, n = 2; Fe, n = 1) with 2 equiv of (smif)H (1,3-di-(2-pyridyl)-2-azapropene); ortho-methylated ((o)Mesmif)(2)Fe (2-Fe) and ((o)Me(2)smif)(2)Fe (3-Fe) were similarly prepared. Metatheses of MX(2) variants with 2 equiv of Li(smif) or Na(smif) generated 1-M (M = Cr, Mn, Fe, Co, Ni, Zn, Ru). Metathesis of VCl(3)(THF)(3) with 2 Li(smif) with a reducing equiv of Na/Hg present afforded 1-V, while 2 Na(smif) and IrCl(3)(THF)(3) in the presence of NaBPh(4) gave [(smif)(2)Ir]BPh(4) (1(+)-Ir). Electrochemical experiments led to the oxidation of 1-M (M = Cr, Mn, Co) by AgOTf to produce [(smif)(2)M]OTf (1(+)-M), and treatment of Rh(2)(O(2)CCF(3))(4) with 4 equiv Na(smif) and 2 AgOTf gave 1(+)-Rh. Characterizations by NMR, EPR, and UV-vis spectroscopies, SQUID magnetometry, X-ray crystallography, and DFT calculations are presented. Intraligand (IL) transitions derived from promotion of electrons from the unique CNC(nb) (nonbonding) orbitals of the smif backbone to ligand π*-type orbitals are intense (ε ≈ 10,000-60,000 M(-1)cm(-1)), dominate the UV-visible spectra, and give crystals a metallic-looking appearance. High energy K-edge spectroscopy was used to show that the smif in 1-Cr is redox noninnocent, and its electron configuration is best described as (smif(-))(smif(2-))Cr(III); an unusual S = 1 EPR spectrum (X-band) was obtained for 1-Cr.