Dedicated to Professor Michael Veith on the occasion of his 60th birthdayThe violet, formally 16 valence electron (VE) sandwich complexes 1 [1] are folded along the B···B vector of the heterocycle, as are the corresponding green iron complexes (folding angle a = 41.38).[2] This effect causes a unique reactivity leading to classic 18 VE complexes. Coordination of the donor ligands DCO and DCN-R at the ruthenium center yields yellow 2 with decreased folding angles (< 208).[1a] The incorporation of boranediyl ([DBH] from BH 3 ·THF) results in the formation of ruthenacarboranes 3 and likewise the incorporation of sulfur (from H 2 S) gives ruthenathiacarboranes. With phosphines, 1 forms donor-acceptor complexes 1-PH 2 R (R = H, Ph), however, the triorganylphosphine adducts 1-PR 3 (R = Me, Ph) are unstable. With tert-butylphosphaalkyne DP C-CMe 3 , no adduct but its incorporation into 1 is observed, however, it is not yet known which of the possible isomers of the resulting ruthenaphosphacarborane is formed.[1b]Herein we report the insertion of terminal alkynes into the heterocycle of 1, which leads to the 18 VE complexes 4 with the novel h 7 -4-borataborepine as a six-electron (6e) ligand, whereas with diorganylacetylenes the formation of boratabenzene complexes 5 occurs.Treatment of 1 a with 3-phenyl-1-propyne (Scheme 1) in hexane yields the yellow, relatively air-stable solid 4 a, which exhibits one broad signal at d = 29 ppm in the 11 B NMR spectrum (downfield compared to that of 1 a, d = 21.7 ppm). The absence of a second 11 B NMR signal in the upfield region indicates that the uptake of the alkyne did not give a ruthenacarborane with an apical boron atom, such as 3. In the EI-MS spectra, a cutoff peak at m/z 630 [M + ] shows that the 1:1 product does not lose the alkyne (which would occur an adduct of 2). A weak signal at m/z 532 [4 aÀBCH 2 SiMe 3 ]+ was tentatively assigned to the complex [Cp*Ru(boratabenzene)] (5 a; Cp* = C 5 Me 5 ). In addition, tribenzylbenzene was identified by MS as a side product. The structure of 4 a (Figure 1), established by a single-crystal X-ray diffraction, [3] demonstrates the formation of a novel sandwich complex with the 4-borataborepine ligand, as a result of the insertion of the C C unit of the alkyne into a BÀC bond of 1 a.[5] As the B1ÀC2 and B3ÀC2 bonds in 1 a are equivalent, both bonds react with the alkyne yielding enantiomers, which are found in the crystal structure.[3]The seven-membered ring in 4 a is less folded along the B···B vector (a = 268), and the RuÀB separations (2.527 and 2.542 ) are significantly elongated compared to those in 1 a.[5] The Ru À C bond lengths of the C 5 B 2 ring vary between 2.203 (RuÀC4) and 2.345 (RuÀC2). Because of the bonding between the ruthenium center and the larger ring, the bonds to the exocyclic a-atoms are tilted towards the ruthenium, with the exception of those to C16 and C20, which are bound to the boron atoms, which tilt in the opposite direction.The influence of the bulky silyl groups in 4 a is evident when compared with 4 b, o...
