2-(Methylamido)pyridine–Borane: A Tripod κ³-N,H,H Ligand in Trigonal Bipyramidal Rhodium(I) and Iridium(I) Complexes with an Asymmetric Coordination of Its BH₃Group

Abstract

The complexes [M(κ(3)-N,H,H-mapyBH3)(cod)] (M = Rh, Ir; HmapyBH3 = 2-(methylamino)pyridine-borane; cod = 1,5-cyclooctadiene), which contain a novel anionic tripod ligand coordinated to the metal atom through the amido N atom and through two H atoms of the BH3 group, were prepared by treating the corresponding [M2(μ-Cl)2(cod)2] (M = Rh, Ir) precursor with K[mapyBH3]. X-ray diffraction studies and a theoretical Quantum Theory of Atoms in Molecules analysis of their electron density confirmed that the metal atoms of both complexes are in a very distorted trigonal bipyramidal coordination environment, in which two equatorial sites are asymmetrically spanned by the H-B-H fragment. While both 3c-2e BH-M interactions are more κ(1)-H (terminal σ coordination of the B-H bond) than κ(2)-H,B (agostic-type coordination of the B-H bond), one BH-M interaction is more agostic than the other, and this difference is more marked in the iridium complex than in the rhodium one. This asymmetry is not evident in solution, where the cod ligand and the BH3 group of these molecules participate in two concurrent dynamic processes of low activation energies (variable-temperature NMR and density functional theory studies), namely, a rotation of the cod ligand that interchanges its two alkene fragments (through a square pyramidal transition state) and a rotation of the BH3 group about the B-N bond that equilibrates the three B-H bonds (through a square planar transition state). While the cod rotation has similar activation energy in 2 and 3, the barrier to the BH3 group rotation is higher in the iridium complex than in the rhodium one.

Umpolung at Boron: Ancillary‐Ligand‐Induced Formation of Boron‐Based Donor Ligands from Phosphine‐Boranes

“…The observation of one band at 2083 cm À1 for the stretching mode of the cyanidol igand and two bands at 1922 and 1871 cm À1 for the CO stretching mode of the carbonyl ligands in the IR spectrum of 3b-CN are in agreement with ac yanido dicarbonyl complex. The connectivity in 3b-CN wasfinally con- (2);P4-B1-P5 112.28 (8),P3-Fe1-P6 93.03(1),P 1-Fe1-P2 71.31(1), H1Fe1-Fe1-C1177.2 (9).…”

“…Neutral phosphine‐boranes are best described as Lewis acid/base adducts (R 3 P→BH 3 ) that consequently have been used as ligands in transition metal complexes since the 1980s . Recently, they have attracted attention as potential hydrogen‐storage materials, metal‐free catalysts and ambiphilic ligands …”

“…[1][2][3][4][5][6] Recently,t hey have attracted attention as potential hydrogenstoragem aterials, metal-free catalysts and ambiphilic ligands. [1,3,[7][8][9][10][11] The reactivity of coordinated phosphine-boranes in transition metal complexes has been studied in differentc ontexts (Scheme 1, top). For example, Weller and co-workers showed that coordination of ancillary ligandss uch as CO or PMe 3 to complexes with ac helatingp hosphine-borane such as [Ru(Cp*)(h 2 -H 3 B·dppm)]PF 6 leads to ac hange of the borane binding mode from h 2 (type A)t oh 1 (type B).…”

Octahedral manganese(i) and ruthenium(ii) complexes containing 2-(methylamido)pyridine–borane as a tripod κ3N,H,H-ligand

“…Therefore, while the M-H200 and M-H300 interactions are equivalent in complex 1, they are different in complex 2. As it has been previously shown that the Shimoi and the agostic attachments of a B-H bond to a metal atom have similar electron density at the M-H bcp (ρ b ) and similar Laplacian of the electron density at the M-H bcp (∇ 2 ρ b ), but different ellipticity at the bcp (ε b ), that of agostic-type being significantly greater than that of 14,22 the data given in Table 3 indicate that the type of interaction existing between the M atom of 1 and 2 and the borane B-H bonds is intermediate between those of Shimoi and agostic types and that, in complex 2, the interaction of the B-H200 bond with the Ru atom is more agostic than that of the B-H300 bond.…”

“…1), 14 which are the first metal complexes derived from 2-(methylamino)pyridine-borane (HmapyBH 3 ). In solution at room temperature, the BH 3 group of these complexes rotates about the B-N bond averaging the three B-H 14 The analytical and spectroscopic data of the ruthenium complex 2 confirmed its composition and the presence of one hydride, one phosphane, one CO, and one mapyBH 3 ligand, but they did not help to unambiguously assign the ligand positions, which were subsequently determined by XRD (see below).…”