Pyrazolylamidino Ligands from Coupling of Acetonitrile and Pyrazoles: A Systematic Study

Mixed pyrazole–acetonitrile
complexes, both neutral fac-[ReBr­(CO)3(NCMe)­(pz*H)] (pz*H = pzH, pyrazole; dmpzH, 3,5-dimethylpyrazole;
or indzH, indazole) and cationic fac-[Re­(CO)3(NCMe)­(pz*H)2]­A (A = BF4, ClO4, or OTf), are described. Their role as the only starting products
to obtain final pyrazolylamidino complexes fac-[ReBr­(CO)3(NHC­(Me)­pz*-κ2N,N)] and fac-[Re­(CO)3(pz*H)­(NHC­(Me)­pz*-κ2N,N)]­A, respectively, is examined. Other
products involved in the processes, such as fac-[ReBr­(CO)3(pz*H)2], fac-[Re­(CO)3(NCMe)­(NHC­(Me)­pz*-κ2N,N)]­A, and fac-[Re­(CO)3(pz*H)2(OTf)] are also described. Warming CD3CN solutions of fac-[Re­(CO)3(NCMe)­(pz*H)2]­A at 40
°C gives cleanly the pyrazolylamidino complexes [Re­(CO)3(pz*H)­(NHC­(Me)­pz*-κ2N,N)]­A as the only products, pointing to an intramolecular process.
This is confirmed by carrying out reactions in the presence of one
equivalent of a pyrazole different from that coordinated, which affords
complexes where the pyrazolylamidino ligand contains only the pyrazole
previously coordinated. When the reactions lead to an equilibrium
mixture of the final and starting products, the reverse reaction gives
the same equilibrium mixture, which indicates that the coupling reaction
of pyrazoles and nitriles to obtain pyrazolylamidino ligands is a
reversible intramolecular process. A systematic study of the possible
factors which may affect the reaction gives the following results:
(a) the yields of the direct reactions are higher for lower temperatures;
(b) the tendency of the pyrazoles to give pyrazolylamidino complexes
follows the sequence indzH > pzH > dmpzH; and (c) the reaction
rates do not depend on the nature of the anion even when a large excess
is added. The presence of a small amount of aqueous solution of NaOH
catalyzes the reaction. Thus, addition of 0.5–1% of NaOH (aq)
to solutions of fac-[ReBr­(CO)3(NCMe)­(pz*H)]
(in CD3CN) or fac-[Re­(CO)3(NCMe)­(pz*H)2]­A (in CD3CN, CD3NO2 or (CD3)2CO) allowed the syntheses of the corresponding
pyrazolylamidino complexes [ReBr­(CO)3(NHC­(Me)­pz*-κ2N,N)] or [Re­(CO)3(pz*H)­(NHC­(Me)­pz*-κ2N,N)]­A with better yields, more rapidly,
and in milder conditions.