There
are few reports of activation of H2 across metal–phosphido
linkages, and all of the first-row metal examples use N-heterocyclic
phosphido donors. In this report, we highlight the discovery of H2 activation using first-row transition-metal phosphido complexes
with alkyl and aryl substituents. The complex [Mn(CO)4(μ-PPh2)]2 (1) was treated with H2 (125 °C, 33 h), affording [{Mn(CO)4}(μ-H)(μ-PPh2){Mn(CO)3(Ph2PH)}] (2).
Treating 2 with Mn2(CO)10 leads
to PH bond activation and formation of [{Mn(CO)4}(μ-H)(μ-PPh2){Mn(CO)4}] (3). The interconversion
of 1 to 3 is reversible, as indicated by
the treatment of 3 with free Ph2PH, giving 2 at 80 °C or 1 and H2 at 120
°C. The isopropyl analogue of 1, [Mn(CO)4(μ-P(iPr)2)]2 (5), was synthesized by the oxidative addition of [(iPr)2PP(iPr)2] (4) with Mn2(CO)10. The reactivity of 5 is analogous to that of 1, forming [{Mn(CO)4}(μ-H)(μ-P(iPr)2){Mn(CO)3((iPr)2PH}] (6) on
treatment with H2, which in turn reacts with Mn2(CO)10, quantitatively affording [{Mn(CO)4}(μ-H)(μ-P(iPr)2){Mn(CO)4}] (7).
The chemistry diverges upon use of the tBu substituent.
Treating Na[Mn(CO)5] with Cl(tBu)2P results in formation of the bis-(tBu2P) hexacarbonyl complex [Mn(CO)3(μ-PtBu2)]2 (8), a dark green
compound with a formal M–M double bond (2.5983(5) Å). 8 reacts sluggishly with H2 to form free tBu2PH and [MnH(CO)4(HPtBu2)] (10). The activation of H2 with 1 is incomplete even at high temperatures. In
contrast, facile activation of H2 occurs with [{Mn(CO)3(μ-PPh2)}2(μ-CO)] (1-CO) to yield 2 (84%, 70 °C, 10 h), implicating
thermally demanding CO dissociation from 1 as the first
step in the H2 activation. PCl bond activation under hydrogenative
conditions was also examined. The reactions between Mn2(CO)10 and ClPh2P or Cl(iPr)2P under 1 atm of H2 gave 3 (R = Ph)
or 7 (R = iPr) in 50–60% yield,
indicating the intermediacy of bisphosphido compounds. When Cl(tBu)2P was used instead, the compounds cis-[Mn(CO)4(H)((tBu2)P)2H)] (10), [Mn(CO)3(H)((tBu2)P)2H] (11), and diaxial-[Mn(CO)4((tBu2)PH)]2 (12) were isolated, indicating PCl
bond hydrogenation to phosphines using H2 and Mn2(CO)10.