Iron and Chromium Complexes Containing Tridentate Chelates Based on Nacnac and Imino- and Methyl-Pyridine Components: Triggering CX Bond Formation

Nacnac-based tridentate ligands containing
a pyridyl-methyl and
a 2,6-dialkyl-phenylamine (i.e., (2,6-R2-C6H3NC­(Me)­CHC­(Me)­NH­(CH2py); R = Et,
{Et­(nn)­PM}­H; R = iPr, {iPr­(nn)­PM}­H) were synthesized by condensation routes. Treatment
of M­{N­(TMS)2}­THFn (M = Cr, n = 2;
M = Fe, Co, n = 1; TMS = trimethylsilane; THF = tetrahydrofuran)
with {iPr­(nn)­PM}­H) afforded {iPr­(nn)­PM}­MN­(TMS)2 (1-MiPr; M = Cr, Fe); {Et­(nn)­PM}­MN­(TMS)2 (1-MEt; M = Fe, Co) was similarly obtained.
{R­(nn)­PM}­FeBr (R = iPr, Et; 2-FeR) were prepared from FeBr2 and {R­(nn)­PM}­Li,
and alkylated to generate {R­(nn)­PM}­FeneoPe (R = iPr, Et; 3-FeR). Carbonylation
of 3-FeR provided {iPr­(nn)­PM}­Fe­(COneoPe)­CO (4-FeiPr), and carbonylations of 1-FeR (R = Et, iPr) and 1-CriPr induced deamination to afford
{R­(nn)­PI}­Fe­(CO)2 (R = iPr, 5-FeiPr; Et, 5-FeEt), where PI is pyridine-imine, and {κ2-N,N-pyrim-pyr}­Cr­(CO)4 (6-CriPr), in which the aryl-amide
side of the nacnac attacked the incipient PI group. Carbon–carbon
bonds were formed at the imine carbon of the {R­(nn)­PI} ligand. Addition
of [{iPr­(nn)­PI}2–]­(K+(THF)x)2 to FeCl3 generated {iPr­(nn)­CHpy}2Fe2Cl2 (7-FeiPr), and TMSN3 induced the deamination of 1-FeEt, but with disproportionation to provide
{[Et­(nn)­CHpy]2}Fe (8-FeEt). Ph2CN2 induced C–C bond formation with 1-FeiPr via its thermal degradation
to ultimately afford {iPr­(nn)­CHpy}2(FeNCPh2)2 (9-FeiPr). The compounds were examined by X-ray
crystallography (1-MiPr,
M = Cr, Fe; 1-CoEt; 2-FeiPr; 4-FeiPr; 5-FeiPr; 6-CriPr; 7-FeiPr; 8-FeEt; 9-FeiPr), Mössbauer spectroscopy,
and NMR spectroscopy. Structural parameters assessing redox noninnocence
are discussed, as are structural and mechanistic consequences of the
various electronic environments.