3,5-Bis(diphenylphosphinoethyl)pyrazolate ligand (PNNPC2) and its dirhodium complexes: Comparison with related quadridentate dinucleating diphenylphosphinomethyl (PNNPPy) and phthalazine derivatives (PNNPPh)

Dirhodium carbonyl complex with the 3,5-bis(diphenylphosphinoethyl)pyrazolato ligand (PNNPC2), [(μ-κ2:κ2-PNNPC2)Rh2(CO)3]BF4, is prepared and its reactivity is studied as compared with the previously reported 3,5-bis(diphenylphosphinomethyl)pyrazolate (PNNP), [(μ-κ2:κ2-PNNP){Rh(CO)2}2]BF4, and 1,4-bis(diphenylphosphinomethyl)phthalazine (PNNPPh) derivatives, [(μ-κ2:κ2-PNNPPh){Rh(CO)2}2](BF4)2. The three quadridentate ligands are different in the size of the central ring and the charge; six-membered ring/neutral (PNNPC2) vs. five-membered ring/mono-negative (PNNP) vs. six-membered ring/neutral (PNNPPh). The number of the carbonyl ligands (n) in the dirhodium carbonyl complexes, [(μ-PNNP)Rh2(CO)n](BF4)x, is dependent on the dinucleating ligand: n = 2 (PNNPPh), 3 (PNNPC2) and 4 (PNNPPy). The three dirhodium carbonyl complexes serve as 4e-acceptors, and their reactivities turn out to be very similar as can be seen from formation of the analogous, unique tetranuclear μ4-acetylide ([(μ-PNNP)2{Rh(CO)}4(μ4–CC–R)](BF4)x) and μ4-dicarbide complexes ([(μ-PNNP)2{Rh(CO)}4(μ4–C2)](BF4)x).