Accessing δ‑B–H Coordinated Complexes of Rh(I) and Ir(I) Using Mono- and Dihydroboranes: Cooperative Stabilization by a Phosphoramidate Coligand

Reactions
of the four-membered irida­(I)- and rhoda­(I)­heterocycles
[M­{κ2-N,O-Xyl­(N)­P­(O)­(OEt)2}­(η4-COD)] (M = Rh (1), Ir (2)) with electrophilic borane reagents are investigated. Treatment
of 1 or 2 with 9-borabicyclo[3.3.1]­nonane
(HBC8H14) gives [M­{κ2-N,H-Xyl­(N)­P­(OH(BC8H14)(OEt)2}­(η4-COD)] (M = Rh (5), Ir (6)). Complexes 1 and 2 display differing reactivity toward HBMes2 and H2BMes: reaction with HBMes2 provides
an equilibrium mixture of [M­{κ2-N,H-Xyl­(N)­P­(OHB­(Mes)2)­(OEt)2}­(η4-COD)] (M = Rh (7), Ir (8)) and the precursor
complex 1 or 2. For H2BMes, borane
coordination was found to be irreversible and the five-coordinate
bis­(B–H) complexes [M­{κ3-N,H,H-Xyl­(N)­P­(OH2BMes)­(OEt)2}­(η4-COD)] (M = Rh (9), Ir (10)) were accessed. Complexes 9 and 10 are highly fluxional in solution; this is ascribed
to rapid rotation of the 1,5-cyclooctadiene (COD) coligand. Treatment
of 1 or 2 with Piers’ borane, HB­(C6F5)2, also provides the corresponding
ring-expanded metalla­(I)­cycles [M­{κ2-N,H-Xyl­(N)­P­(OHB­(C6F5)2)­(OEt)2}­(η4-COD)] (M = Rh (11), Ir (12)). In the absence of a stabilizing [M]··H–B
interaction, treatment of 1 or 2 with B­(C6F5)3 provides the corresponding η5-iminocyclohexadienyl complexes [M­{η5-Xyl­(N)­P­(O··B­(C6F5)3)­(OEt)2}­(η4-COD)] (M = Rh (21), Ir (22)), which
result from phosphine oxide donation to the electron-deficient boron
center.