Coordination versus Coupling of Dicyanamide in Molybdenum and Manganese Pyrazole Complexes

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-NCNCN-κ2N,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.