Elongated Dihydrogen Versus Compressed Dihydride Complexes: The Temperature Dependence of the H–D Spin–Spin Coupling Constant as a Criterion To Distinguish between Them

Abstract

To be able to propose experimental tests to distinguish elongated dihydrogen transition-metal complexes from compressed dihydride transition-metal complexes, a thorough density functional study of the electronic structure in combination with quantum nuclear dynamics calculations have been performed for complexes [Cp*Ru(H2PCH2PCH2(H2)]+ and [CpRe(CO)2H2]. The results of this study suggest that elongated dihydrogen complexes and compressed dihydride complexes have different properties and that it should be possible to distinguish between them experimentally. In particular, different behavior is predicted with respect to 1) the sign of the isotope geometric effect on the H-H distance at 0 K, 2) the temperature dependence of the H-H distance, and 3) the temperature dependence of the H-D spin-spin coupling constant in 1H NMR spectroscopy.

Activation of Molecular Hydrogen

“…3,[10][11][12][13][14][15]17,[21][22][23][24]30,[34][35][36][37][38][39][40][43][44][45][46][47][48][49][50] Although the d HH ranges shown are arbitrary, each category of complexes has distinct properties. The d HH is relatively short (0.8-1.0 A ), and H 2 is reversibly bound, in "true" H 2 complexes best exemplified by W(CO) 3 14,34,48,[49][50][51] were first clearly identified in 1991 in ReH 5 (H 2 )(PR 3 ) 2 where neutron diffraction showed a d HH of 1.357(7) A . 49 Complexes with d HH > 1.…”

“…3 A are now viewed as "compressed hydrides," with NMR features differing from elongated H 2 complexes; for example, J HD increases with temperature for the former and decreases for the latter. 51 These are relative terms since a near continuum of d HH has been observed.…”

Unprecedented flexibility of the >TiSi< group for the addition of H2

“…(i) There is a striking similarity of the molecular geometries of I (strongly bound dihydrogen complex, with the partially elongated H-H bond at 0.80 A ˚, which is due to significant acidity of the poorly coordinated Ti site), and of the transition state that leads to isomer II (the transition state shows the HÁ Á ÁH separation of 1.05 A ˚, typical for so called 'elongated dihydrogen complexes'); 19 we anticipate that the elongation of the H-H bond from 0.80 to 1.05 A ˚is largely compensated by the formation of the bonding multicenter HÁ Á ÁSi and HÁ Á ÁTi interactions. A similar situation occurs for the VI -VII transformation, where the H-H bond length changes from 0.83 A ˚(for VI) to the 1.075 A ˚(for the transition state).…”

Controlling the binding of dihydrogen using ruthenium complexes containing N-mono-functionalised 1,4,7-triazacyclononane ligand systems

“…The first investigations of an intramolecular deprotonation process were reported by Crabtree et al 16,17 and Morris et al 18,19 in the study of coordination complexes of iridium, and thereafter so-called 'dihydrogen bonding' has become an active area of research. [20][21][22][23][24][25][26][27] One method of intramolecular deprotonation is to employ a classical 'capping' ligand such as cyclopentadienyl, functionalised with a pendant arm, 28,29 of which several instances have been reported in ruthenium chemistry. [30][31][32] As an alternative, 1,4,7-triazacyclononane (tacn) has received increased attention in recent years 33 and ruthenium complexes of 1,4,7triazacyclononane are numerous.…”