Mechanism of intramolecular transformations of nickel phosphanido hydride complexes

Abstract

In solution, nickel phosphanido hydride complexes ([NiH{P(Ar)(H)}(dtbpe)], Ar = Dmp, Mes*) undergo a degenerate intramolecular exchange, with the Ni-H and P-H hydrogens and both halves of the dtbpe moiety interchanging. This intramolecular rearrangement was shown to occur in three steps: first, the hydride proton migrates to phosphorus, then the P-Aryl moiety rotates around the P-Ni bond, and finally the back migration of one proton to Ni completes the process. Both migration and rotation were determined to be characterized by high barriers (on the NMR time scale) and to depend on the type of aryl group at the terminal phosphorus. Compared to that observed for the Ni complexes, the same isomeric preference, but with a slower rate of intramolecular rearrangement, is predicted for the corresponding Pt complexes. An opposite isomeric preference, however, is expected for the corresponding Pd complexes. Thus, it is likely that some of the catalytic reactions of Pd and Pt complexes are driven by the relative thermodynamic stabilities of their main forms.

[Ni(NHC)2] as a Scaffold for Structurally Characterized trans [H−Ni−PR2] and trans [R2P−Ni−PR2] Complexes

“…Identification as a trans ‐coupling by comparing it with the cis ‐splitting in [Ni(d t bpe){P(Dmp)H}H] [23a] was prevented by the absence of any resolvable couplings on the Ni−H signal of the latter (even at low temperature). Although the cis phosphido‐Ni−H splitting was likewise unresolved in the related complex [Ni(d t bpe){PMes*H}H] (Mes*=2,4,6‐ t Bu 3 C 6 H 2 ), [23b] this did show a coupling of comparable large magnitude (104 Hz) for J ( trans phosphine‐Ni−H) [26] . The best comparison involves the iridium bis‐PH 2 hydrido complex, [Ir(PEt 3 ) 2 (PH 2 ) 2 (CO)H], [25c] which shows large trans (42 Hz) and much smaller cis (7 Hz) values of 2 J (PH 2 ‐Ir−H).…”

Quantum Chemical Calculations of 31P NMR Chemical Shifts in Nickel Complexes: Scope and Limitations

“…The first example is that of the Ni complexes with terminal phosphanido (PR 2 ) ligands. , In general, for these complexes three types of isomers can be realized (Figure ) and an exact structure elucidation can be complicated. At first glance, the first form (NiH 2 –P) could be easily distinguished by the high-field shift of Ni–H protons, but it must be kept in mind that it may be in exchange or be extremely broadened, as there are also no clear “fingerprints” to prove or disprove this structure.…”

“…Moreover, for the Ni–PH 2 and NiH–PH isomers the situation is even worse: even the X-ray method does not allow localizing the two hydrogen atoms, and so the isomeric structure remains in question. Only a combination of low-temperature NMR experiments allowed one to establish the solution-state structure of the complex, which turned out to be the NiH-PH isomer. , …”

“…Only a combination of low-temperature NMR experiments allowed one to establish the solution-state structure of the complex, which turned out to be the NiH-PH isomer. 70,71 At the same time, the 31 P NMR shift calculations (PBE0/6-311G(2d,2p)//PBE0/6-31+G(d)) for possible structural hypotheses and comparison with experimental 31 P CSs unambiguously and relatively simply allows establishment of the isomeric structure of these types of complexes (Figure 7). As can be seen, the NiH 2 −P isomer can be easily excluded from consideration, as the deviation for the P 3 CS is more than 1440 ppm (Figure 7b).…”

Nickel Complexes in C‒P Bond Formation

“…Later, Ganushevich et al characterized, for the first time, the product of an oxidative addition of primary phosphane to a nickel(0) complex. The terminal phosphanido hydride nickel complex, [NiH{P(Dmp)(H)}(dtbpe)], where Dmp-2,6-dimesitylphenyl and dtbpe-1,2-bis(di-tert-butylphosphino)ethane, has been formed in this process [51][52][53]. Chiral metal complexes have been used to promote and control the asymmetric P-H addition reaction.…”