Requirements for Late-Stage Hydroboration of Pyridine N-Heterocyclic Carbene Iron(0) Complexes: The Role of Ancillary Ligands

Abstract

A series of zerovalent iron complexes were synthesized that contain allylic substituents attached to a 2,6-bis­(imidazol-2-ylidene)­pyridine pincer ligand. These species varied in the identity of their ancillary ligands and were used to study the requirements and limitations of late-stage hydroboration. While late-stage ligand functionalization can facilitate the incorporation of Lewis acidic boranes into a ligand scaffold, thereby alleviating Lewis acid/base incompatibilities of the free ligand, we identify and discuss complicating factors that arise from complexes containing labile M–L bonds.

An Atypically Bent Isocyanide at Iron in a Tetrapodal Penta‐Carbene Framework

“…Qualitative ligand field splitting for 1, symmetry labels are for the C 2V point group with Cartesian axis defined as shown with the y-axis perpendicular to the page. ligand, [29] shorter than those seen for FeÀ CN t Bu bonds at NHC ligated Fe II , [46][47][48] and similar to that seen at NHC ligated Fe 0 [49] (Figure 3). Moreover, the Fe1À C32 bond distance in 2 is comparable to those seen at a β-diketiminate supported Fe I bis(tert-butylisocyanide) complex (1.817(1), 1.821(1) Å), [50] as well as the Fe 0 complexes [Fe 0 (CN t Bu) 5 ] (1.803(10)-1.837(7) Å), [51] and ( iPr PDI)Fe(CN t Bu) 2 (1.8102(13) -1.8417(13) Å).…”

“…[56] The acute C32À N11À C33 bond angle of 142.67 (18)°in 2 is rare, as few examples with bond angles less than 150°are known and include examples of Fe II (139.65(16)°), [57] Fe I (148.3(7)°), [50] and Fe 0 (145.48(14)°, 142.35(17)°). [49,52] The XRD structure also reveals an Fe1À C32À N11 bond angle of 169.63 (16)°consistent with sp hybridization about the terminal C-atom of the isocyanide moiety.…”

“…This band is atypical as neutral isocyanide complexes generally exhibit a narrow absorption band of high intensity between 2000 and 2200 cm À 1 . [58] Broadening of isocyanide IR absorption bands has been observed for other complexes, for instance in the Fe 0 compounds ( allyl CNC)Fe(CN t Bu) 2 (ν CNR = 1812 cm À 1 ) [49] and Fe-(PMe 3 ) 2 (CN t Bu) 3 (ν CNR = 1980 cm À 1 ). [59] Broadening of the isocyanide stretching frequency can be understood considering the two resonance forms available to a metal-bound isocyanide ligand.…”

A guide to secondary coordination sphere editing

“…Notwithstanding, there are drawbacks that result from this approach. 82 Importantly, in cases where labile metalligand bonds are present, caution should be taken. In these instances, adduct formation of the type L -H n BR m (n + m = 3) has been noted, causing SCS borane incorporation routes to be impractical.…”

Distinct Reactivity Modes of a Copper Hydride Enabled by an Intramolecular Lewis Acid

“…Treating a C 6 H 6 solution of allyl TACN t Bu with 9-BBN (50 °C, 90 min) resulted in the successful in situ anti-Markovnikov hydroboration of the ligand, which when followed by metalation with CuI or Cu 2 (OTf) 2 (C 6 H 6 ) afforded ( BBN TACN t Bu )­Cu­(I) ( 2-I ) or ( BBN TACN t Bu )­Cu­(OTf) ( 2-OTf ) as white powders in 67% and 90% yields, respectively, over two steps. In contrast to the postmetalation hydroboration route, the premetalation hydroboration route provided the desired borane-appended Cu­(I) complexes in high yield …”