The Effect of Remote Donor Substituents on the Properties of Alkoxy and Amino Fischer Carbene Complexes of Tungsten

Abstract

Dedicated to Prof. Wolfgang Kaim on the Occasion of his 70th BirthdayTungsten Fischer ethoxy-and dimethylaminocarbene complexes [W{C(X)(C 6 H 4 -4-R}(CO) 5 ] (X = OEt: series a; or X = NMe 2 : series b) are synthesized from phenyl substrates containing remote tertiary amino substituents R' 2 N with R' = Me (2), Ph (3) or C 6 H 4 Br-4 (4). The π-delocalization and carbene-stabilizing effects of the distant tertiary amine donor substituent are investigated by NMR and IR spectroscopy, electrochemistry and quantum chemical calculations. A significant transfer of electron density from the remote 4-R' 2 N substituent to the carbene C atom in the ethoxycarbene complexes is supported by NMR data and the solid-state structure of 2 a. This is strongly attenuated in the amino-substituted carbene complex 2 c and irrelevant in the dimethylaminocarbene complexes of series b, where the electron demand of the electrophilic carbene center is satisfied by the directly attached dimethylamino substituent. Quantum chemical calculations and IR spectroelectrochemistry on complexes 1 a--4 a verify carbene-centered reductions and a ligand based oxidation of complex 2 b.

Roles played by carbene substituents during ligand transfer reactions between tungsten fischer carbene complexes and [Pt(COD)Cl2]

“…Unlike 1a, complex 2a is far more stable, highlighting a supportive electronic donor role played by the remote NMe2 substituent in 2a, as was the case for [W(CO)5{C(OEt)(4-C6H4-NMe2)}], whereby electron density is donated on demand via the phenylene linker to the electrophilic carbene carbon. [19] The cationic tris(carbene) complexes have Cl − or [W(CO)5Cl] − as the counter ions, which can readily be exchanged for PF6 − (see Experimental section). The [W(CO)5Cl] − anion results from the release of a chloro ligand from Pt(II) on substitution with the third carbene ligand and its subsequent reaction with the resulting, coordinatively unsaturated {W(CO)5} fragment.…”

“…According to the NMR and IR data as well as the redox properties of the Pt complexes, the differences between coordinated oxy-and aminocarbene ligands of the W-precursors also prevail in the Pt complexes. [19] Remarkably, stronger electron donation from the dimethylaminocarbene as compared to the oxycarbene ligands even outweighs the positive, outer charge. In addition, the presence of the remote 4-NR'2 donor directs the first oxidation of the aminocarbene complexes away from the metal carbene entity to the triarylamine constituent.…”

“…[37] The precursors, [W(CO)5{C(X)p-C6H4NR}] X=OEt (a), NMe2 (b), R=H(1ʹ), NMe2 (2ʹ), NPh2 (3ʹ) and N(C6H4Br)2 (4ʹ), were synthesised according to the literature. [19] Silica gel 60 (particle size 0.0063−0.200 mm) was used as resin for all separations in column chromatography. NMR spectra were recorded on Bruker Ultrashield Plus 400 AVANCE 3 and Bruker Ultrashield 300 AVANCE 3 spectrometers using CD2Cl2 as solvent at 25 °C.…”

“…[12][13][14][15][16] Recently FC complexes [W{C(X)(C6H4-4-R)}(CO)5] (X = OEt: a series; X = NMe2: b series) were synthesized in our laboratories from phenyl substrates containing remote tertiary amino substituents. [17][18][19] In addition, [W{C(X)(C6H5)}(CO)5] (a,b), were also synthesized to aid as reference complexes to probe the effects and magnitude of electron donation by remote tertiary nitrogen substituents. The importance of π-resonance effects within carbene substituents is an emerging area of research and aims at fine-tuning of the extent of electron donation and the distribution of electron density in the FC carbene ligand.…”

“…The importance of π-resonance effects within carbene substituents is an emerging area of research and aims at fine-tuning of the extent of electron donation and the distribution of electron density in the FC carbene ligand. [16,[19][20][21] With the above tungsten complexes of particularly electron-rich ethoxy-and dimethylaminocarbene ligands available, we now set out to probe and study their transferability to Pt(II) precursors.…”