Synthesis and Characterization of Hydrotris(pyrazolyl)borate Dihydrogen/Hydride Complexes of Rhodium and Iridium

Abstract

Protonation of TpM(PR 3 )H 2 (M ) Rh, Ir) complexes with HBF 4 ‚Et 2 O or [H(Et 2 O) 2 ][B(Ar) 4 ] (Ar ) 3,5-(CF 3 ) 2 C 6 H 3 ) affords cationic complexes which exhibit a single hydride resonance at all accessible temperatures in the 1 H NMR spectrum. Formulation as fluxional dihydrogen/hydride complexes is indicated by short T 1 (min) values of ca. 22 ms (Ir) and 7 ms (Rh). The relaxation times are consistent with H-H bond lengths of 0.88-1.11 Å in the iridium complexes and 0.73-0.92 Å in the rhodium complexes depending on the relative rate of the dihydrogen rotational motion. In the case of the iridium complexes, partial substitution of the hydride positions with deuterium or tritium results in large temperature-dependent isotope shifts and resolvable J H-D or J H-T coupling constants. Analysis of the chemical shift and coupling constant data as a function of temperature is consistent with a preference for the heavy hydrogen isotope to occupy the hydride rather than the dihydrogen site. This analysis also provides the limiting chemical shifts of the dihydrogen and hydride ligands as well as the 1 J H-D coupling constant (ca. 25 Hz) in the bound dihydrogen ligand.Since the first report of a stable molecular hydrogen complex by Kubas, 1 the possibility that a fluxional polyhydride complex might also contain a dihydrogen ligand has been actively investigated. 2 In general, transition metal polyhydride complexes are characterized by high coordination numbers (CN 7-9) and high formal oxidation states. 3 Because several structures of nearly equivalent energy are available to seven-, eight-, and nine-coordinate complexes, rapid permutation of the hydride positions is often observed by 1 H NMR spectroscopy. As a result, structural characterization in solution depends upon indirect methods, in which the observed NMR parameters are a population-weighted average of all the hydride environments. For example, Crabtree and co-workers have employed T 1 measurements to detect short H-H contacts in a range of polyhydride complexes, including [Ir(PCy 3 ) 2 H 6 ] + and Fe-(PEtPh 2 ) 3 H 4 . 4 A quantitative treatment of relaxation in polyhydride complexes has been developed which allows useful structural information to be obtained from T 1 (min) data. [4][5][6] We have previously reported the structure and properties of cationic iridium complexes of the form [CpIr(L)H 3 ]BF 4 (L ) various PR 3 ), which have been shown to adopt iridium(V) trihydride structures in the solid state. 7,8 These complexes undergo a rapid hydride rearrangement which leads to a single hydride resonance in the 1 H NMR spectrum above ca. 220 K. However, at very low temperatures, spectra consistent with the solid state structure are obtained.In this paper we investigate the effect of substituting the Cp ligand of [CpIr(L)H 3 ]BF 4 complexes with the hydrotris(1-pyrazolyl)borate (Tp) 9 ligand. The new Tp complexes, [TpIr-(L)(H 2 )H]BF 4 (L ) PMe 3 , PPh 3 ), are formulated as dihydrogen/ hydride complexes, although only a single hydride resonance is observed...

A Computational Study of Rhodium Pincer Complexes with Classical and Nonclassical Hydride Centres as Catalysts for the Hydroamination of Ethylene with Ammonia

“…[11] The nonclassically bound H 2 moiety is a good leaving group, that is, for the introduction of ethylene or ammonia. We chose iridium [12] and rhodium [13] compounds of type A with synthetically available NCN and PCP pincer ligands 2,5-bis(dimethylaminomethyl)benzene and 2,5-bis(dimethylphosphanylmethyl)benzene, respectively, for the investigation of key hydroamination reaction steps.…”

“…Valence bond structures for the generation of active catalyst species 5 and 17 using trans-1 and cis-1 as catalyst precursors, respectively. Path I: trans-1, 2, 3, 5; Path II: trans-1, 2, 6, 7, 9, 5; Path III: cis-1, 10, 11, 12, 5; Path IV: cis-1, 10,11,13,14,15,5;10,13,14,15,16,17;10,13,14,18,19,22 or 23,17. Structures in grey could not be localised.…”

“…While attempting to synthesise trans-or cis-1, one might instead arrive at 10 as the product, though the energy differences are low enough to be reversed by solvent effects. As with 2, competitive pathways resulting from either the addition of NH 3 or C 2 H 4 to 10 exist and, accordingly, the analogous routes lead to 5 by Path III: Addition of NH 3 (11), s-bond metathesis (12), and H 2 -elimination (5) and Path IV: addition of C 2 H 4 (13), insertion into the RhÀH bond (14), addition of NH 3 (15), and s-bond metathesis with concomitant elimination of ethane (5). The loss of hydrogen is the first step in Paths I to IV.…”

“…The reaction sequence cis-1, 10,13,14,18,19,22,17 of Path VI has the lowest activation barriers (TS13-14 3.4 kcal mol ). Additionally, the intermediates of Path VI are more stable than those of the other paths, so for the case of hydrogen loss of cis-1 the active species 17 will be formed.…”

Development of 2nd Row Molybdenum Dearomatization Agents

“…[51][52][53][54][55][56] Moreover, Tp complexes are less prone to oxidative additions, an important consideration given the highly reducing nature of the metal centers in question. 53,[55][56][57][58] With a Tp foundation in place, the rest of the ligand set was considered. Initially the achiral {TpRe(CO)2} fragment was explored due to the relative synthetic ease to access the complex, and its similarity to previous CpRe -η 2 -aromatic complexes.…”