Remarkable behavior of a bifunctional alkynylborane zirconocene complex toward donor ligands and acetylenes

Treatment of a [(trimethylsilylethynyl)alkenyl]ZrCp2 complex with Piers' borane [HB(C6F5)2] resulted in the clean formation of the Zr/B complex [η(2)-(C6F5)2B-C≡C-SiMe3]ZrCp2 (3). This compound shows some unique reaction patterns. With a variety of typical donor ligands (carbon monoxide, an isonitrile, a nitrile, THF-d8,) it forms the respective adducts 5 (four examples characterized by X-ray diffraction). Toward terminal alkynes (RC≡CH) compound 3 behaves as a Zr(II) metal Lewis base/boron Lewis acid frustrated Lewis pair (FLP) and undergoes typical regioselective 1,2-Zr/B addition reactions to yield the metallaheterocyclic products 7 (four examples characterized by X-ray diffraction). Compound 3 also undergoes characteristic 1,4-Zr/B FLP addition to a silyl-substituted conjugated enyne (to give 9) and also to 1,4-bis(trimethylsilyl)butadiyne to yield a metallacyclic butatriene derivative 10 (both 9 and 10 were characterized by X-ray diffraction). Eventually, compound 3 reacted with mes2P-C≡CH to give the metallacycloallenoid product 15, which was probably formed by means of 1,2-Zr/B FLP addition followed by alkynyl shift from boron to zirconium and reductive coupling. Treatment of 3 with tolane gave the closely related zirconacycloallenoid complex 16 by carbon carbon coupling. Both the products 15 and 16 were characterized by X-ray diffraction.