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Coordination versus Coupling of Dicyanamide in Molybdenum and Manganese Pyrazole Complexes

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posted on 04.06.2012, 00:00 by Marta Arroyo, Patricia Gómez-Iglesias, Jose Miguel Martín-Alvarez, Celedonio M. Alvarez, Daniel Miguel, Fernando Villafañe
The reactions of cis-[MoCl­(η3-methallyl)­(CO)2(NCMe)2] (methallyl = CH2C­(CH3)­CH2) with Na­(NCNCN) and pz*H (pzH, pyrazole, or dmpzH, 3,5-dimethylpyrazole) lead to cis-[Mo­(η3-methallyl)­(CO)2(pz*H)­(μ-NCNCN2N,N)]2 (pzH, 1a; dmpzH, 1b), where dicyanamide is coordinated as bridging ligand. Similar reactions with fac-[MnBr­(CO)3(NCMe)2] lead to the pyrazolylamidino complexes fac-[Mn­(pz*H)­(CO)3(NHC­(pz*)­NCN-κ2N,N)] (pzH, 2a; dmpzH, 2b), resulting from the coupling of pyrazol with one of the CN bonds of dicyanamide. The second CN bond of dicyanamide in 2a undergoes a second coupling with pyrazole after addition of 1 equiv of fac-[MnBr­(CO)3(pzH)2], yielding the dinuclear doubly coupled complex [{fac-Mn­(pzH)­(CO)3}2(μ-NHC­(pz)­NC­(pz)=NH-κ4N,N,N,N)]Br (3). The crystal structure of 3 reveals the presence of two isomers, cis or trans, depending on whether the terminal pyrazoles are coordinated at the same or at different sides of the approximate plane defined by the bridging bis-amidine ligand. Only the cis isomer is detected in the crystal structure of the perchlorate salt of the same bimetallic cation (4), obtained by metathesis with AgClO4. All the N-bound hydrogen atoms of the cations in 3 or 4 are involved in hydrogen bonds. Some of the C–N bonds of the pyrazolylamidino ligand have a character intermediate between single and double, and theoretical studies were carried out on 2a and 3 to confirm its electronic origin and discard packing effects. Calculations also show the essential role of bromide in the planarity of the tetradentate ligand in the bimetallic complex 3.