IR spectroscopy of hydrides and its application to hydrogen bonding and proton transfer studies

Natalia V. Belkova, Lina M. Epstein, Oleg A. Filippov and Elena S. Shubina

DOI: 10.1039/9781849734899-00001

This review is devoted to the IR spectra of transition metal and main group element hydrides focusing on the nMH bands and their response to hydrogen bonding and proton transfer. The aim is to show what advantages can be provide by variable temperature IR spectroscopy in structure and reactivity studies of hydrides.

Introduction

Transition metal and main group element hydrides keep attracting research interest because of their importance in different chemical and biochemical processes. Their chemical behaviour, and transformations, are usually studied by NMR spectroscopy. IR is employed mostly for traditional compound characterisation upon synthesis, however it provides information that would not otherwise be readily obtainable. Indeed 1H NMR is very convenient for studying hydrides since hydride resonances appear in a strong field without overlap with other hydrogens and there are distinct signals of both classical and non-classical (η2-H2) complexes. Development and availability of variable temperature NMR set-ups gave another advantage to NMR spectroscopy and made it a widespread (almost routine) technique. In contrast, variable temperature IR measurements are still not widely used. However IR spectroscopy operates at a different time-scale (10-11–10-14 s) and thus can be used to study processes, which are too fast on the NMR timescale (101–10-6 s). This includes regular IR or FTIR spectroscopy not to mention recently developed time resolved IR techniques. Among the processes conveniently studied by IR spectroscopy are complexation (formation of intermolecular adducts) and proton transfer. The latter is an important step in many catalytic or stoichiometric transformations of hydrides and is one of our research interests.

The aim of this report is to show what advantages can be provided by variable temperature IR spectroscopy and our approach to study the reactivity of transition metal and main group element hydrides. We focus on the vMH bands and their response to hydrogen bonding and proton transfer, but will also mention behaviour of other bands such as vOH, vCOetc. These studies gain a lot from combination with NMR and quantum chemical and frequency calculations, to which we will refer as well.

M-H stretching vibrations

Transition metal complexes containing terminal hydrido ligands exhibit stretching (vMH) and deformational (δMH) vibrations at 2200–1600 and 800–600 cm-1, respectively. The δMH vibrations are not characteristic but mixed with other modes. Their region is more difficult to access due to the overlap with vibrations of other ligands and solvents commonly used in inorganic/organometallic chemistry. Therefore δMH will not be discussed here.

In the case of di- and polyhydrides the first question is the assignment of the vMH bands. Calculations of simple homoleptic hydrides (MH2 and MH2 · H2 complexes (M=Ti, V, Cr) and comparison with the matrix isolation IR measurements have shown that symmetric (vs) and antisymmetric (vas) M-H stretching vibrational bands in IR spectra can appear near to each other and differ widely (from 2 to 10 times) in intensity, the intensity of the vsMH band being lower than that of the vasMH band. This work has been expanded by L. Andrews and co-workers who a trapped variety of homo leptic metal hydrides obtained by the reaction of transition metal atoms (from group 3 to 12) with dihydrogen for matrix isolation IR spectroscopic studies. However, these "naked" metal hydrides and dihydrogen complexes are fundamentally different from organometallic hydride complexes. Being unsaturated, they exhibit high reactivity, e.g. interact differently with molecular H2. The spectroscopic and theoretical data on these compounds are discussed in a review. They confirm the early observations (above) that very often only more intense vas bands are observed in the IR spectra of hydrides.

The analysis of IR and Raman spectra of bis-cyclopentadienyl metal hydrides Cp2MHn (Cp=η5-C5H5; M=Re (n=1); Mo, W (n=2); Nb, Ta (n=3))3 having C2v symmetry led to the conclusion that for Cp2MH2 only symmetric stretching vibrations appear in both spectra. The measurements for (η5-C5D5)2MH2 derivatives have shown very little [MATHEMATICAL EXPRESSION OMITTED] band shift as a result of the Cp-ring deuteration evidencing the absence of vibrational coupling with other modes. However, the authors noticed unusually high bandwidth (in solution) and splitting (in matrices) of vMH bands for almost all the spectra measured. The proposed explanation for this observation was the steric interference with the Cp-rings during M-H vibrations that broadens MH stretching bands inhomogeneously in solution or splits them in matrices when Cp rotation is frozen. Recently we reported for the first time on the observation of such conformational equilibrium for [Cp*MoH(PMe3)3]n+ (Cp*=η5-C5Me5; n=0, 1). The two temperature and solvent dependant nMH bands in IR spectra (Fig. 1 gives an example of the neutral hydride) were assigned to the two rotamers originating from hindered rotation of the Cp*-ring. Despite the seemingly small difference in the geometry of the two rotamers (change of C-ring center-Mo-H dihedral angle by ca. 20°) the Mo-H bond lengths and the calculated vMoH frequencies are different (Fig. 2). A detailed look into the structure reveals different lengths of Mo-H ... HCH2(ring) contacts, which are ca. 0.1 Å shorter for the lower energy rotamers.

IR and polarized Raman measurements performed for Os(L)H2(Mes) (Mes=2,4,6Me3C6H3; L=CO, CH3CN) have shown that for...