Cooperative B─H Bond Activation: A Direct Route to Monometallic Diborane(5) and Dihydroborate Complexes

To study the role of hemilabile ligands in B─H bond activation, κ²-N,E-chelated iridium and rhodium complexes, [Cp*M(κ¹-E-C₅H₄NE)(κ²-N,E-C₅H₄NE)] (2a: M = Ir, E = S; 2b: M = Ir, E = Se; 2c: M = Rh, E = S; 2b: M = Rh, E = Se) have been synthesized by salt metathesis reactions of [Cp*MCl₂]₂ (1a: M = Ir, 1a: M = Rh) and potassium salts of thio-/seleno-pyridine ligands. The strained four-membered rings in 2a-d indicate their potential for activating small molecules. Indeed, treatment of 2a-b with [BH₃⋅THF] afforded monometallic diborane(5) complexes, [(Cp*Ir^IIIH){κ³-B,B,Se-(B₂H₄-C₅H₄NE)}] (3a: E = S, 3b: E = Se) and transition metal (TM)-dihydroborate complexes, [(η⁴-C₅Me₅H)Ir^I{κ²-H,H,E-(H₃B-C₅H₄NE))] (4a: E = S, 4b: E = Se). Complexes 3a-b, containing a [Cp*IrH] unit, stabilize the non-classical [B₂H₄(C₅H₄NE)]^- fragment, presumably generated through B─H activation at the Ir─N bond and subsequent dehydrogenation. In contrast, the Rh analogues 2c-d yielded dihydroborate rhodium(I) complexes, [(η⁴-C₅Me₅H)Rh^I{κ³-H,H,E-(H₃B-C₅H₄NE)}] (4c: E = S, 4d: E = Se) through B─H bond insertion into the Rh─N bonds followed by cyclopentadienyl ring slippage. In an attempt to synthesize a rhodium analogue of 3b, complex 4d is treated with [BH₃⋅THF] which generated an unsymmetrical bis-zwitterionic rhodium complex, [Cp*Rh(μ₃-Se){BH₂(C₅H₄NSe)}₂], 5 comprising a bicyclo [4.4.0] cage.