Silane(silyl) and Bis(silyl)hydrido Manganese Complexes with Different Mn···H···Si Interaction: Observation of Gradual Si–H Bond Activation on the Metal Center

Abstract

Three manganesecarbonyl complexes having a xanthenebased silane(silyl) or bis(silyl) chelate ligand were synthesized and characterized. The single-crystal X-ray analysis of these complexes demonstrated that an SiH bond is sequentially activated on the metal according to ligand substitution of CO with xanthene-oxygen or η 6 -toluene.The SiH bond activation of hydrosilanes induced by transition-metal complexes is an attractive process for its broad application to the synthesis of organosilicon compounds (e.g., hydrosilylation).1 Generally, it is proposed that this bond activation can be achieved by interaction of the SiH · bond with a coordinatively unsaturated metal center.1 A possible reaction mechanism of the SiH activation suggested by Schubert and co-workers based on their comprehensive study (Scheme 1) involves the generation of silane complexes A and B, both having a 3c2e MHSi bond, as key intermediates.

[(dmpe)2MnH(C2H4)] as a Source of a Low-Coordinate Ethyl Manganese(I) Species: Reactions with Primary Silanes, H2, and Isonitriles

“…Complex 4a is the first structurally characterized disilyl hydride complex of manganese with nonchelating silyl ligands, although Tobita et al previously reported a series of disilyl hydride and silyl hydrosilane complexes in which both silicon-based donors are tethered by a xanthene backbone (Figure ). The Mn–Si distance of 2.3851(2) Å in 4a is within the range previously observed for manganese silyl hydride complexes (2.254(1)–2.4702(9) Å) and is shorter than those in Tobita’s most closely related disilyl hydride complex ( T3 in Figure : d (Mn–Si) = 2.4026(7)–2.4069(7) Å) …”

“…35 The Mn−Si distance of 2.3851(2) Å in 4a is within the range previously observed for manganese silyl hydride complexes (2.254(1)−2.4702(9) Å) 36 and is shorter than those in Tobita's most closely related disilyl hydride complex (T3 in Figure 5: d(Mn−Si) = 2.4026(7)−2.4069(7) Å). 35 The central position of the hydride ligand between the two silyl ligands in 4a,b appears to be maintained on the NMR time scale in solution, given that the 1 H NMR spectra indicate C 2 symmetry between 25 and −87 °C. In the solid state (for 4a), the Si(1)−H(1A) distance is 1.9163(8) Å (calculated 1.92 Å), which is significantly longer than a typical Si−H single bond (1.48 Å) 37 but much shorter than the sum of the van der Waals radii (3.1 Å).…”

J(Si,H) Coupling Constants of Activated Si–H Bonds

“…From its positive/negative slope one can conclude that the individual 1 J (Si,H t ) and J (Si,H br ) coupling constants are of same/opposite sign in 2 and 5 , respectively . Since 1 J (Si,H t ) < 0, the sign of J (Si,H br ) must be negative (−63 Hz) in 2 and positive (+15 Hz) in 5 .…”

“…From its positive/negative slope one can conclude that the individual 1 J(Si,H t ) and J(Si,H br ) coupling constants are of same/opposite sign in 2 and 5, respectively. 90 Since 1 J(Si,H t ) < 0, 17 the sign of J(Si,H br ) must be negative (−63 Hz) in 2 and positive (+15 Hz) in 5. 3 and reflect again the promotion of the oxidative addition of the Si−H bond to the metal atom by electron-withdrawing substituents at the silicon atom.…”

“…These nonclassical hydrosilane complexes can be considered as arrested intermediates along the oxidative addition pathway. − This is illustrated in Figure by selected benchmark systems, where the σ-complex Cr­(CO) 5 SiH 2 Ph 2 ( 1 ) and the classical silyl hydride Fe­(CO) 4 H­(SiCl 3 ) ( 7 ) represent the starting point and terminus of this process. The propagation along the oxidative addition pathway can then be considered as a continuum of electronic structures. − Accordingly, the d 6 manganese complex (CH 3 )­CpMn­(CO) 2 (HSiHPh 2 ) ( 2 ) can be characterized as a σ-complex but shows already an enhanced M···Si interaction relative to 1 . ,, Hence, 1 and 2 can be distinguished by their η 1 /η 2 (Si–H)­M coordination modes, respectively . The M···Si interaction increases further in (CH 3 )­CpMn­(CO) 2 (HSiFPh 2 ) ( 3 ), , which displays an electron-withdrawing substituent at the silicon atom of the hydrosilane ligand yielding an asymmetric oxidative addition product (ASOAP). , A similar scenario can be also observed in case of the early transition metal hydrosilane complexes such as the d 2 -titanium complexes Cp 2 Ti­(PMe 3 )­(HSiHPh 2 ) ( 4 ) and Cp 2 Ti­(PMe 3 )­(HSiHPhCl) ( 5 ) .…”