Elsevier

Inorganica Chimica Acta

Volume 358, Issue 14, 15 November 2005, Pages 4163-4171
Inorganica Chimica Acta

Lewis base interaction with gallium hydrides: a computational study

Dedicated to Professor H. Schmidbaur on the occasion of his 70th birthday.
https://doi.org/10.1016/j.ica.2005.02.001Get rights and content

Abstract

The recent synthesis and structural characterization of the complex of 3,5-dimethyl-4-hydropyridyl-gallane 1 with the Lewis base 3,5-dimethylpyridine revealed an unusually large angle α = H–Ga–H, 127(2)°, at variance with expected steric effects of the bulky substituents at the tetrahedrally coordinated Ga center. This finding prompted us to study computationally gallium hydrides using density functional and post-Hartree–Fock methods. For 1, we estimated α at 131° from a calculation on 4-hydropyridyl-gallane, GaH2(Hpy). This value is reduced by 3° due to the interaction with Lewis base pyridine, to yield α = 128°, in excellent agreement with experiment. With an analysis of orbital interactions and a natural bond orbital analysis, we rationalized structural variations of donor–acceptor adducts LGaH2X where X is a substituent and L is a Lewis base. Angle α is mainly determined by the polarity of the Ga–X bond: the more electronegative substituent X, the larger α and the stronger the interaction of GaH2X with L. Interaction with a weak base L slightly distorts the initially planar geometry of the dihydride to a trigonal pyramidal form; for a strong base, the structure can become pseudo-tetrahedral.

Graphical abstract

Gallium hydrides and their donor–acceptor adducts LGaH2X, where X is a substituent and L is a Lewis base, have been studied theoretically using density functional and post-Hartree-Fock methods. With an analysis of orbital interactions and a natural bond orbital analysis, we rationalized structural variations (in particular, of the angle α = H–Ga–H) of these adducts due to electronic nature of X and L.

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Introduction

The chemistry of Group 13 elements was developed mostly by investigations on boron and aluminum compounds. Recently, gallium compounds are finding increased attention due to potential applications in semiconductor technologies [1], and the need for pertinent new precursors. This stimulated interest in the chemistry of gallium unraveled some remarkable features less common to boron and aluminum, such as the increased stability of unsaturated lower-valence states and hence the possibility to form metal-metal multiple bonds [2] or even pseudoaromatic 6-π systems [3], [4]. As simplest structural examples of low-valent states we mention the recent spectroscopic characterization of various gallium hydrides, GaH, GaH2, Ga2H2, GaHX (X = CO, CH3, NH2, PH2), in matrix-isolation experiments [5], [6], [7]. Attention to hydride derivatives of gallium is not limited to transient species, but extends to Lewis donor–acceptor complexes of gallane (GaH3)n, which itself remained evasive until 1989 [8].

Central to the chemistry of Group 13 elements is the Lewis acid character of trivalent hydrides, halides, or organometallic MR3 derivatives, where the Lewis acid strength decreases from boron to gallium [9]. Complexation is a key factor in the stabilization of the unsaturated parent hydrides, which is also reflected in the tendency of metal M to enter into MXM bridges (X = H, Cl, Me, NR2). The structures and properties of donor adducts of alane, (AlH3)n, and gallane, (GaH3)n, were recently reviewed [10], [11]. Many complexes of gallane or mono- and dichlorogallane have been well characterized and some are moderately stable. Such complexes gained prominence in preparative organogallium chemistry as much more convenient synthetic agents than their parent base-free hydrides [11].

Along with others [10], [11], several contributions from Schmidbaur’s group on Ga hydride, halide and chalcogenide complexes are worth noting in this context [12], [13], [14], [15]. Because Ga is more electronegative than Al, 1.82 versus 1.47 on the Allred–Rochow scale [16], the hydridic character is less pronounced in Ga hydrides, but can be activated by complexation with a Lewis base as demonstrated by Nogai and Schmidbaur [13], [15]. Recently, these authors showed dehydrogenative Ga–Ga coupling after activation of the Ga–H function by complexation of (HGaCl2)2 with a pyridine derivative, a weak Lewis base, which formed 1:1 or 2:1 donor–acceptor adducts [15]. The Lewis adduct (L′)GaH3 with L′ = 3,5-dimethylpyridine releases hydrogen and metallic Ga to rearrange to (L′)GaH2(L′H) where L′H is monohydrogenated in the 4-position, i.e., opposite to the N center of the heterocycle [15]. The authors discussed the structure of the complex (L′)GaH2(L′H) in terms of a pseudo-tetrahedral coordination of Ga and pointed out two noteworthy features [15]. The two bulky ligands L′H and L′ form a very small N–Ga–N angle of 103(1)°, while the H–Ga–H angle is “exceedingly large” at 127(2)° [15].

Despite the apparent structural similarity of the two ligands L′H and L′, one has to keep in mind that they form two rather different Ga–N bonds, differing by 0.17 Å [15]. Therefore, one may view the structure of the complex (L′)GaH2(L′H) as trigonal pyramidal with the base L′ in apical position and a largely flattened GaH2(L′H) foundation. Indeed, the sum γ of the three basal angles is 347° [15]. This is to be compared with γ = 328.5° in the ideal tetrahedron whereas γ = 360° for a completely flat base; also note γ = 352° in (L′)GaH3 [13]. In this hydride complexes, the ligand L′ in axial position exhibits N–Ga–H angles close to 100°. Still, many four-coordinated Ga complexes feature a geometry at Ga that is close to tetrahedral [11]; for instance, in the related chloride compounds (L)GaCl2H and (L)GaCl3, γ is reduced to 341–343° and 336°, respectively [13]. The degree of flatness of the GaClnH3−n foundation decreases from n = 0 to n = 3, so that (L)GaCl3 exhibits almost ideal tetrahedral angles at the Ga center, in the range 106–112°.

To rationalize quantitatively the structural differences of donor–acceptor complexes of Ga hydrides and to link these differences to chemical properties, we would like to address theoretically the following questions: (i) How do substituents at the Ga atom affect the angle H–Ga–H? (ii) Is there a correlation between the strength of the donor–acceptor bond and the coordination geometry of the Ga center (tetrahedral versus flattened)? (iii) How do Lewis base ligands affect the hydridic character of Ga–H bonds?

We will show that the first question can be answered rather well by combining fragment orbital considerations with a natural bond orbital (NBO) analysis of the many-electron wave function available from electronic structure calculations. In response to the second question, we offer results of accurate quantum chemistry calculations. For the third question, we can touch in the present context only on one aspect, namely whether the charge redistribution in the Lewis donor adduct shows a propensity for releasing a hydride. Of course, for each reaction system, a number of other factors affecting the reactivity at each stage of a complex mechanism should be taken into account.

Section snippets

Computational methods

The electronic structure calculations were carried out with the program package Gaussian03 [17]. The geometries were optimized at the hybrid-DFT B3LYP level (Becke’s three-parameter hybrid density functional [18] combined with the Lee–Yang–Parr correlation functional [19]) using the standard 6-311G(d,p) basis set. For the resulting structures, we calculated binding energies of base ligands in single-point fashion at the CCSD(T) level (coupled cluster theory with single and double excitations

Effect of substituents X on the polarity of a Ga–H bond

Unlike ionic AlH3, gallane GaH3 has considerable covalent character – a consequence of the larger electronegativity of Ga as compared to Al (Section 1). In halogen-substituted hydrides of less electropositive metallic elements, the polarity of the M–H bond may be inverted due to electronegative substituents; an example is HGeCl3 which sometimes acts as a weak acid [13]. In the same way, one expects that electron-donor or electron-acceptor groups attached to the metal center affect the polarity

Conclusions

We have shown that the interaction of gallium hydrides with Lewis bases distorts the initially planar geometry of the hydride to induce a trigonal pyramidal form. The degree of distortion depends on the strength of the Lewis base and ultimately leads to a pseudo-tetrahedral structure. The interaction with weak bases, such as substituted pyridines, thus yields a small perturbation only, where the geometry of the adduct is largely determined by that of the free substrate. Therefore, the unusually

Acknowledgements

Prof. H. Schmidbaur brought the peculiar structure of substituted gallanes to our attention and we dedicate this study to him on the occasion of his 70th birthday. This work was supported by Fonds der Chemischen Industrie.

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