Activation of H₂ with Dinuclear Manganese(I)-Phosphido Complexes

Abstract

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.

Bench‐Stable Dinuclear Mn(I) Catalysts in E‐Selective Alkyne Semihydrogenation: A Mechanistic Investigation**

“…The reaction of 2 with equimolar dpa in toluene‐ d 8 was monitored over various temperatures. The reaction was slow at 80 °C and mostly contained unreacted 2 (>90 %) even after 44 h. The products were identified as [{Mn(CO) 4 }( μ ‐H)( μ ‐P( i Pr) 2 ){Mn(CO) 3 ( κ‐P‐ hpa )}] ( 5 ), [Mn 2 (CO) 9 { κ‐P ‐ hpa }] ( 7 ), [{Mn(CO) 4 }( μ ‐H)( μ ‐P( i Pr) 2 ){Mn(CO) 3 (HP( i Pr) 2 )}] ( 8 ), [28] and Mn 2 (CO) 10 owing to the presence of 8 [28] . Further heating to 105 °C (12 h) caused the disappearance of 8 and the growth of 5 and 7 in addition to the formation of a new mononuclear Mn hydride complex fac ‐Mn(CO) 3 (H)( κ 2 ‐P ‐η 2 ‐ hpa ) ( 4 ).…”

“…( i ) First, the alkyne reacts with 2 via a ligand substitution reaction to form 11 . The stereochemistry of 11 can take many forms based on the known substitution chemistry with 2 , 5 and 8 and other related analogs [5,28,50] . The isomer given is the lowest energy isomer obtained from DFT calculations (Figures 5 and S51).…”

“…We also showed that phosphido substituents affect H 2 activation chemistry at dinuclear Mn 2 -bis-phosphido complexes. [28] Therefore, we hypothesized an activity relationship between phosphine substituent in the E-SASH reaction catalyzed by [{Mn(CO) 4 } 2 (μ-H)(μ-PR 2 )] complexes and prepared [{Mn(CO) 4 } 2 (μ-H)(μ-P(iPr) 2 )] (2), among others.…”

“…As shown by us previously, bench‐stable complexes such as 1 can be prepared in a single step from commercially available starting materials, namely HPR 2 and Mn 2 (CO) 10 (Scheme 1 and TOC graphic). This allows simple gram‐scale access to potentially exciting catalyst diversity [28] . Together, these attributes motivated us to explore dinuclear Mn 2 H systems for catalytic applications and uncover novel reactivity and coordination chemistry.…”

“…This allows simple gram-scale access to potentially exciting catalyst diversity. [28] Together, these attributes motivated us to explore dinuclear Mn 2 H systems for catalytic applications and uncover novel reactivity and coordination chemistry.…”