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 (Pym^R-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-Pym^Me-iPr)­(CO)₄] and [M­(CO)₄-μ₂-(κ²P,N-Pym^Ph-iPr)­M­(CO)₄], respectively. Heterobimetallic complexes of the type [M¹(CO)₄-μ₂-(κ²P,N-Pym^Me-iPr)­M²(CO)₄] were obtained by reacting mononuclear complexes [M¹(κ²P,N-Pym^Me-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-Pym^tBu-iPr)­(CO)₃] containing an η²-C_aryl–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-Pym^tBu-iPr)­(CO)₃] and the hydrido carbonyl complex [Mo­(κ³P,C,P-Pym^tBu-iPr)­(CO)₃(H)], and with tungsten the hydrido carbonyl complex [W­(κ³P,C,P-Pym^tBu-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-Pym^tBu-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.