Isonitrile Coordination to Pincer Iridium Hydrido Chlorides

Abstract

Isonitriles are useful reagents that participate in many cycloaddition or multicomponent reactions of interest in organic synthesis, which are often catalyzed by transition metal complexes. Being good electron donors, RN=C bonds become activated and prone to the nuclephilic attack upon coordination to electrophilic (Lewis acidic) metal centers. Herein we explore the complexation of ^tBuNC to a series of iridium hydrido chlorides supported by benzene-based pincer ligands, (\({\text{R}}{{{\text{-}}}^{{^{t}{\text{Bu}}}}}{\text{PZCZP}}\))IrH(Cl) (\({\text{R}}{{{\text{-}}}^{{^{t}{\text{Bu}}}}}{\text{PZCZP}}\) = κ³‑C₆H₃–2,6-(ZP^tBu₂)₂, where R = H (I–III), EtCO₂–(IV), and Z = CH₂ (I), CH₂, O (II), O (III, IV)), that occurs instantaneously and quantitatively in solution yielding one of possible isomers Ia–IVa. Single crystal X-ray diffraction studies for Ia–IIIa confirmed the apical coordination of isonitrile ligand trans to hydride ligand suggested by NMR studies. Perusal of the structural data suggests stronger binding of ^tBuNC to PCP-supported iridium center in complex Ia in comparison to PCOP/POCOP species IIa–IIIa. Structural data also reveal a distortion in the P–Ir–P arrangement caused by ^tBuNC docking in apical position that is the most noticeable for asymmetric PCOP version IIa. The computational analysis of vibrational frequencies unveils an essential coupling ν_Ir-H and ν_N=С modes that produce two strong bands of similar intensity in the ν_N=C region for Ia. Weaker isonitrile binding in IIa–IVa with shorter N=С and longer N–\({{{\text{C}}}^{{^{t}{\text{Bu}}}}}\) /Ir–C_CN bonds and a plethora of intramolecular interactions result in a different degree of vibrational mixing for Ir‒H and N=C stretches as well as lead to an intense bands due to a Fermi resonance of low frequency modes involving vibrations of N–C–^tBu fragment. Thus, the raw experimental IR data should be taken with care as an indicator of RN=С activation, paying attention to the vibrational coupling. An estimation of band position for the “true” (uncoupled) ν_NC vibration gives a small high frequency shift for Ia (+16 cm^–1) but low frequency shifts for other complexes becoming less negative in the order IIa–IIa–IVa (PCOP–POCOP– EtCO₂-POCOP). Overall the results obtained show the influence of the pincer ligand on the complex picture of isonitrile complexes structure and spectra, and suggest that strong binding does not always mean a strong activation.