N‐Heterocyclic Silylenes as Ligands in Transition Metal Carbonyl Chemistry: Nature of Their Bonding and Supposed Innocence

Abstract

A study on the reactivity of the N‐heterocyclic silylene Dipp2NHSi (1,3‐bis(diisopropylphenyl)‐1,3‐diaza‐2‐silacyclopent‐4‐en‐2‐yliden) with the transition metal complexes [Ni(CO)4], [M(CO)6] (M=Cr, Mo, W), [Mn(CO)5(Br)] and [(η5‐C5H5)Fe(CO)2(I)] is reported. We demonstrate that N‐heterocyclic silylenes, the higher homologues of the now ubiquitous NHC ligands, show a remarkably different behavior in coordination chemistry compared to NHC ligands. Calculations on the electronic features of these ligands revealed significant differences in the frontier orbital region which lead to some peculiarities of the coordination chemistry of silylenes, as demonstrated by the synthesis of the dinuclear, NHSi‐bridged complex [{Ni(CO)2(μ‐Dipp2NHSi)}2] (2), complexes [M(CO)5(Dipp2NHSi)] (M=Cr 3, Mo 4, W 5), [Mn(CO)3(Dipp2NHSi)2(Br)] (9) and [(η5‐C5H5)Fe(CO)2(Dipp2NHSi‐I)] (10). DFT calculations on several model systems [Ni(L)], [Ni(CO)3(L)], and [W(CO)5(L)] (L=NHC, NHSi) reveal that carbenes are typically the much better donor ligands with a larger intrinsic strength of the metal–ligand bond. The decrease going from the carbene to the silylene ligand is mainly caused by favorable electrostatic contributions for the NHC ligand to the total bond strength, whereas the orbital interactions were often found to be higher for the silylene complexes. Furthermore, we have demonstrated that the contribution of σ‐ and π‐interaction depends significantly on the system under investigation. The σ‐interaction is often much weaker for the NHSi ligand compared to NHC but, interestingly, the π‐interaction prevails for many NHSi complexes. For the carbonyl complexes, the NHSi ligand is the better σ‐donor ligand, and contributions of π‐symmetry play only a minor role for the NHC and NHSi co‐ligands.

Activation of CO Using a 1,2‐Disilylene: Facile Synthesis of an Abnormal N‐Heterocyclic Silylene

“…The IR spectrum of 3 exhibits four CO stretching bands ( ν =2005, 1900, 1870, 1835 cm −1 ), all of which are at higher wavenumbers than typically found for related cis ‐bis(NHC) complexes of the Mo(CO) 4 fragment [22] . Similar differences have been reported for the IR spectra of normal NHSi vs. NHC complexes of group 6 metal carbonyl fragments, with the conclusion that NHCs are more nucleophilic than N‐hetereocyclic silylenes [23] . It seems very likely that both silylene donor centers of ligand 2 are similarly poorer σ‐donors than either NHCs or a NHCs.…”

Cooperative B–H and Si–H Bond Activations by κ2-N,S-Chelated Ruthenium Borate Complexes

“…2b Thus, on the basis of the solid-state structure of 5a and all the spectroscopic and mass data, we believe that 5b is a structural isomer of 5a with phosphine trans to S of the benzothiazolyl group 27a,29 and the silyl group having a high trans influence (Scheme 2). 32 The computed HOMO−LUMO energy gap of 5b was slightly higher than those of 5a and 1 (3.584 eV for 5b, 3.363 eV for 5a and 3.229 eV for 1). The HOMO-5 of 5a showed that the d orbital of the Ru atom is engaged in bonding with the p orbital of the Si atom and is in an antibonding interaction with B−H bond donation (Figure 6a,b).…”

A square planar silylene nickel four-membered ring

“…In general, four-membered [M-Si(II)] 2 -rings (M = transition metal) are rare [50][51][52][53] and only a few examples are known for Ni. [54][55][56][57][58] However, the bonding situation and the nature of the silylene is different in those complexes (Scheme 2).…”

“…In analogy to the procedure in small Scheme 2 Literature known Si 2 Ni 2 compounds and the title complex. [54][55][56][57] four-membered ring molecules which was intensely investigated, 63 the force constant of the out-of-plane motion is determined according to the method of Wilson, Decius and Cross. 64 Using the G-matrix element G oop = 1/2(μ Si + μ Ni ), 63 the unusually large force constant value of 1.92 mdyn•Å −1 is thus obtained, which is indicative of a rather rigid Ni 2 Si 2 framework.…”