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

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posted on 2018-06-04, 19:51 authored by Gerald Tomsu, Matthias Mastalir, Ernst Pittenauer, Berthold Stöger, Günter Allmaier, Karl Kirchner
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)42-(κ2P,N-PymPh-iPr)­M­(CO)4], respectively. Heterobimetallic complexes of the type [M1(CO)42-(κ2P,N-PymMe-iPr)­M2(CO)4] were obtained by reacting mononuclear complexes [M12P,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.