Coordination versus Coupling of Dicyanamide in Molybdenum and Manganese Pyrazole Complexes

The reactions of <i>cis</i>-[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 <i>cis</i>-[Mo­(η³-methallyl)­(CO)₂(pz*H)­(μ-<i>N</i>CNC<i>N</i>-κ²<i>N,N</i>)]₂ (pzH, <b>1a</b>; dmpzH, <b>1b</b>), where dicyanamide is coordinated as bridging ligand. Similar reactions with <i>fac</i>-[MnBr­(CO)₃(NCMe)₂] lead to the pyrazolylamidino complexes <i>fac</i>-[Mn­(pz*H)­(CO)₃(<i>N</i>HC­(pz*)­NCN-κ²<i>N,N</i>)] (pzH, <b>2a</b>; dmpzH, <b>2b</b>), resulting from the coupling of pyrazol with one of the CN bonds of dicyanamide. The second CN bond of dicyanamide in <b>2a</b> undergoes a second coupling with pyrazole after addition of 1 equiv of <i>fac</i>-[MnBr­(CO)₃(pzH)₂], yielding the dinuclear doubly coupled complex [{<i>fac</i>-Mn­(pzH)­(CO)₃}₂(μ-<i>N</i>HC­(pz)­NC­(pz)=<i>N</i>H-κ⁴<i>N,N,N,N</i>)]Br (<b>3</b>). The crystal structure of <b>3</b> 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 (<b>4</b>), obtained by metathesis with AgClO₄. All the N-bound hydrogen atoms of the cations in <b>3</b> or <b>4</b> 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 <b>2a</b> and <b>3</b> 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 <b>3