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, κ 2 ‐ N,E‐ chelated iridium and rhodium complexes, [Cp*M(κ 1 ‐ E ‐C 5 H 4 NE)(κ 2 ‐ N,E ‐C 5 H 4 NE)] ( 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*MCl 2 ] 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 [BH 3 ⋅THF] afforded monometallic diborane(5) complexes, [(Cp*Ir III H){κ 3 ‐ B,B,Se ‐(B 2 H 4 ‐C 5 H 4 NE)}] ( 3a : E = S, 3b : E = Se) and transition metal (TM)‐dihydroborate complexes, [( η 4 ‐C 5 Me 5 H)Ir I {κ 2 ‐ H,H,E ‐(H 3 B‐C 5 H 4 NE))] ( 4a : E = S, 4b : E = Se). Complexes 3a ‐ b , containing a [Cp*IrH] unit, stabilize the non‐classical [B 2 H 4 (C 5 H 4 NE)] − 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, [( η 4 ‐C 5 Me 5 H)Rh I {κ 3 ‐ H,H,E ‐(H 3 B‐C 5 H 4 NE)}] ( 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 [BH 3 ⋅THF] which generated an unsymmetrical bis‐zwitterionic rhodium complex, [Cp*Rh( μ 3 ‐Se){BH 2 (C 5 H 4 NSe)} 2 ], 5 comprising a bicyclo [4.4.0] cage.