Ligand-Enforced Switch of the Coordination Mode in Low-Valent Group 6 Carbonyl Complexes Containing Pyrimidine-Based Bisphosphines

The reaction of M­(CO)6 (M = Cr, Mo, W) with N,N′-bis­(diisopropylphosphine)-N,N′-dimethylpyrimidine-4,6-diamines
(PymR-iPr) bearing R = Me, Ph, tBu substituents in the 2-position was investigated. The
pyrimidine-based bisphosphine ligands with R = Me, Ph reacted with
M­(CO)6 to yield mononuclear and homobimetallic complexes
of the types [M­(κ2P,N-PymMe-iPr)­(CO)4] and [M­(CO)4-μ2-(κ2P,N-PymPh-iPr)­M­(CO)4], respectively. Heterobimetallic
complexes of the type [M1(CO)4-μ2-(κ2P,N-PymMe-iPr)­M2(CO)4] were obtained by reacting
mononuclear complexes [M1(κ2P,N-PymMe-iPr)­(CO)4] with 1 equiv
of the respective M2(CO)6. Replacing these substituents
by a bulky tBu group led to a switch in the coordination
mode of the pyrimidine ligand. In the case of chromium, the complex
[Cr­(κ3P,CH,P-PymtBu-iPr)­(CO)3] containing an η2-Caryl–H agostic bond was selectively formed and no C–H
bond cleavage took place. However, in the case of molybdenum, the
reaction led to the formation of an inseparable mixture of the agostic
complex [Mo­(κ3P,CH,P-PymtBu-iPr)­(CO)3] and the hydrido carbonyl
complex [Mo­(κ3P,C,P-PymtBu-iPr)­(CO)3(H)], and with tungsten the
hydrido carbonyl complex [W­(κ3P,C,P-PymtBu-iPr)­(CO)3(H)] was
exclusively formed. In the presence of strong bases both complexes
could not be deprotonated. In addition, hydride abstraction with Ph3C+PF6– failed. However,
treatment of [W­(κ3P,C,P-PymtBu-iPr)­(CO)3(H)] with Ph3C+PF6– in a benzene/THF mixture
resulted in protonation of one of the pyrimidine nitrogen atoms to
form two isomeric cationic hydride species. X-ray structures of representative
complexes are presented.