An Efficient Method for the Synthesis of Boratrane Complexes of Late Transition Metals

In a quest for efficient precursors for the synthesis of boratrane complexes of late transition metals, we have developed a useful synthetic method using [L'M(mu-Cl)Cl-x](2) as precursors (L' = eta(6)-p-cymene, M= Ru, x= 1; L'= COD, M= Rh, x= 0 and L'= Cp*, M= Ir or Rh, x= 1; COD= 1,5-cyclooctadiene, Cp*= eta(5)-C5Me5). For example, treatment of Na[(H3B) bbza] or Na[(H2B) mp(2)] (bbza= bis(benzothiazol-2-yl)amine; mp = 2-mercaptopyridyl) with [L'M(mu-Cl)Cl-x](2) yielded [(eta(6)-p-cymene) RuBH{(NCSC6H4)(NR)}(2)] (2; R= NCSC6H4), [{N(NCSC6H4)(2)} RhBH{(NCSC6H4)(NR)}(2)] (3; R= NCS-C6H4), [(eta(6)-pcymene) RuBH(L)(2)] (5; L= C5H4NS), and [Cp* MBH(L)(2)] (6 and 7; L= C5H4NS, M= Ir or Rh). In order to delineate the significance of the ligands, we studied the reactivity of [(COD) Rh(mu-Cl)](2) with Na[(H3B) bbza], which led to the formation of the isomeric agostic complexes [(eta(4)-COD) Rh(mH) BHRh(C14H8N3S2)(3)], 4a and 4b, in parallel to the formation of 16-electron square-pyramidal rhodaboratrane complex 3. Compounds 4a and 4b show two different geometries, in which the Rh - B bonds are shorter than in the reported Rh agostic complexes. The new compounds have been characterized in solution by various spectroscopic analyses, and their structural arrangements have been unequivocally established by crystallographic analyses. DFT calculations provide useful insights regarding the stability of these metallaboratrane complexes as well as their M -> B bonding interactions.