Pyridine Functionalized N-Heterocyclic Silane Complexes of Iridium and Rhodium–An Unexpected Change in Coordination

Abstract

A new pyridine functionalized N-heterocyclic silane with ambident reactivity as a ligand has been synthesized and characterized by NMR spectroscopy ( 1 H, 13 C{ 1 H}, 29 Si), mass spectrometry, elemental analysis, and X-ray crystallography. This ligand reacts with iridium and rhodium cod precursors (cod = 1,5-cyclooctadiene) to yield two new complexes that possess divergent, and unexpected, binding properties. In particular, no oxidative addition occurs at the intraligand Si-H unit. With the iridium(I) center, the ligand acts as a tertiary amino-pyridine chelator, whereas coordination of rhodium(I) to the ligand occurs with arene πcomplexation on an opposite side. The latter interaction yields an unprecedented, electronically induced, coordination change at the silicon center over a long spatial distance. The new iridium and rhodium compounds are of high interest as they provide two potentially different reactions sites in one complex and as these sorts of complexes are known to activate small molecules like dihydrogen or silanes. Hence, the compounds are promising candidates for applications in tandem catalysis. Furthermore, the rhodium complex might be utilized for molecular switches, sensors, and comparable applications.

Synthesis and Characterization of New N‐Heterocyclic Silylazides

“…In contrast to the 1 H NMR assessment however, the rather similar Si–N1 bond lengths in all compounds do not support the assumptions based on NMR spectroscopic data concerning significant changes in the electronic structure at the central Si atom. The apparent elongation of the Si1–N3 bonds in all three compounds compared to the respective Si1–N2 bonds (Table ) reflects the reported trans ‐influence of the coordinated pyridine N1 atom . The observed Si1–Cl1/Cl2 (compound 4 ) and Si1–N4/N7 bond lengths (compounds 3 and 5 ) are in accordance with literature known organosilyl chlorides and azides , .…”

“…Additionally to the Si1–N2/N3 bonds forming the N‐heterocyclic silane and the respective chlorides, hydride, and azides, a dative interaction of the pyridine moiety with Si1–N1 bond lengths of 2.009(3) ( 3 ), 1.994(3) ( 4 ), and 1.9904(14) Å ( 5 ) is observed. All Si1–N bond lengths compare well to related compounds, reported in the literature …”

“…Comparing 1 H NMR spectra of 3 , 4 and 5 , signal H 19 (see Supporting Information or Figure ) of the aromatic proton in ortho position to the pyridine‐N appears to shift considerably through variation of the coordinated chlorides and azides. Regarding compound 4 , the signal for H 19 exhibits a distinct downfield shift ( δ = 8.67 ppm) in comparison to the respective signals of the mono‐ ( 3 , δ = 7.85 ppm) and di‐azide ( 5 , δ = 8.13 ppm) and the mono‐chloride ( 2 , δ = 8.08 ppm) . These differences in shift suggest significant changes in the electronic structure with regard to the central Si atom.…”

“…Synthetically, all three silanes are derived from the diamine precursor 1 . The precursor is synthesized in nearly quantitative yield according to a four‐step literature procedure . The consecutive conversion with SiCl 4 and 1,4‐diazabicyclo[2.2.2]octane (DABCO) as a base affords compound 4 in 84 % yield (Scheme ).…”

“…Synthesis of compounds 3 – 5 . Precursors 1 and 2 were synthesized according to literature procedures . Reactions and conditions: (i) TMSN 3 (xs.…”