Hydrogen-bonded adducts between neutral molecules and [Mo(η³-methallyl)(CO)₂(HOC(py)₃)]⁺: snapshots of a deprotonation

Abstract

Hydrogen-bonded adducts between the title metal alcohol complex and several neutral bases, showing different degrees of H⁺ transfer, display distinctive νCO bands.

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AH⋯B and A⋯HB adducts of the AH hydrogen-bond donor (the acid) and the B hydrogen-bond acceptor (the base) are intermediate situations along the reaction coordinate in H⁺ transfer, the simplest chemical reaction.¹ For a set of adducts showing different degrees of H⁺ transfer between a given hydrogen-bond donor and different acceptors, could conventional characterization methods reveal their intermediate properties? The development of supramolecular systems able to modulate a response upon guest binding is a challenging area.²Hydrogen-bond donor groups in the periphery of metal complexes have been used for second-sphere binding of molecular or ionic guests.³ Metal complexes can be interrogated by a variety of techniques, so the choice of a metal fragment as the hydrogen bond donor could make the sought characterization a more complete one. In particular, the position of the νCO IR bands of metal carbonyl complexes is highly sensitive to the electron density at the metal center. Our previous attempts to employ fac-[Re(CO)₃(Hpz)₃]BAr′₄ (Hpz = generic pyrazole, Ar′ = 3,5-bis(trifluoromethyl)phenyl) for IR sensing of anions resulted in very small variations in the position of the IRνCO bands.⁴ We hypothesized that, for an XH hydrogen bond donor group with X directly bound to the metal, those νCO IR bands could also report on the strength of the hydrogen bond formed between XH and an external acceptor.

When high binding strength and selectivity are the goal in the design of an artificial host using hydrogen bonding for guest binding, multipoint interactions involving several geometrically convergent donor groups are employed.³ Here, to reduce complexity, a single-point interaction will be used. To avoid that this would result in too weak an interaction, an OH group, a stronger acid, was chosen instead of N–H groups, the one most often employed in supramolecular hosts. Since alcohols coordinated to low-valent transition metal fragments are very labile ligands, we have employed a tridentate ligand featuring, besides the hydroxyl group, three 2-pyridyl (py) groups,⁵ which are good donors for a variety of metal centers.^5,6 Compound [Mo(η³-methallyl)(CO)₂(HOC(py)₃)]BAr′₄ (1H) was prepared by reaction of tris(2-pyridyl)carbinol⁷ with [MoCl(η³-methallyl)(CO)₂(NCMe)₂]⁸ and NaBAr′₄⁹ (Scheme 1). The BAr′₄ tetraarylborate was chosen due to its low coordinating character.^3d The reaction of 1H with KN(SiMe₃)₂ in THF afforded the deprotonated alkoxo form, complex 1, and both compounds were characterized by IR (νCO bands at: 1H: 1956 and 1869 cm⁻¹, 1: 1918 and 1820 cm⁻¹), NMR and X-ray diffraction (Fig. 1). In both solid state structures, the tridentate ligand is coordinated through the nitrogens of two of the py groups—one trans to one of the CO groups and the other trans to the methallyl group, and the oxygen, and the third pyridine remains un-coordinated. At low temperature, the ¹H NMR spectra indicate that the same geometry exists in solution, with three chemically non-equivalent py groups and an asymmetric η³-methallyl ligand, whereas at room temperature a dynamic process renders apparently equivalent the two coordinated py groups. In the structure of 1H there is an intramolecular hydrogen bond between the coordinated OH and the nitrogen of the un-coordinated pyridine, characterized by O⋯N = 2.415(3) Å and OHN = 132.87(7)°.

Scheme 1 Synthesis of compounds 1H and 1.

Fig. 1 Thermal ellipsoid (30%) plot of the cation in 1H (a) and compound 1 (b).

The change in the ¹H chemical shift of the OH signal upon dilution reveals the presence of additional, intermolecular hydrogen bonds in solution. To shed some light on the hydrogen bonding in 1H, compound 1′H, differing from 1H only in that the dangling group is a phenyl ring rather than a pyridine, was synthesized and characterized in solution and solid state. Dilution of 1′H did not shift the OH ¹H NMR signal, demonstrating that intermolecular hydrogen bonds in 1H are of the OH⋯N type. Additionally, 1′H was found to be isostructural with 1H, showing that the adoption of the N,N′,O coordination mode in 1H is not dictated by the intramolecular hydrogen bond.

IR (νCO) monitoring of a THF (a solvent of very low basicity) solution of 1H showed a gradual (and reversible) transformation into 1, highlighting the acidity of the OH ligand (Fig. 2).

Fig. 2 IR νCO (cm⁻¹) in THF solution of: (a) 1H; (b) after 15 min, mixture of 1H and 1; (c) 1.

A 1H–THF adduct could be isolated by slow crystallization of a CH₂Cl₂ solution of 1H containing a large excess of THF. Its ¹H NMR featured the signals of a THF molecule slightly upfield from those of free THF, and the results of an X-ray diffraction study revealed the hydrogen bond showed in Fig. 3 as a dashed line. The formation of this OH⋯O hydrogen bond takes place at the expense of the OH⋯N hydrogen bond with the dangling pyridine ring; actually, the pyridine ring is rotated so that its N atom is away from the Mo-coordinated OH group. The same feature has been found in the majority of adducts discussed below. The ¹H NMR titration of a CD₂Cl₂ solution of 1H–THF with the cyclic urea 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPU), an excellent hydrogen bond-acceptor, showed the gradual shift of the THF signals towards the position of free THF (Fig. 4). On addition of DMPU, the broad OH signals shift downfield, reflecting the formation of stronger hydrogen bonds, and soon broadens and disappear, becoming visible at low temperature. The crystalline adduct 1H–DMPU was isolated by slow diffusion of hexane from an equimolar mixture of 1H and DMPU in CH₂Cl₂ and characterized by IR, NMR and X-ray diffraction (ESI†). Whereas the IRνCO bands of 1H–THF were indistinguishable from those of 1H, indicating the very weak interaction between THF and the OH group in this 1 :  1 adduct, those of 1H–DMPU (1952 and 1864 cm⁻¹) occur at wavenumber values significantly lower. This stronger hydrogen bond interaction is largely due to the significant contribution of an urea resonance form with a single C–O bond and a negative charge on oxygen, as shown by the shift to lower frequency of the ureaνCO band (1558 cm⁻¹ in 1H–DMPU, 1630 cm⁻¹ in free DMPU). The O⋯O hydrogen bond distance in 1H–DMPU, 2.491(5) Å, shorter than in 1H–THF (2.588(4) Å), is attributed to the stronger hydrogen bond interaction in the former.

Fig. 3 Thermal ellipsoid (30%) plot of the cation in 1H–THF (a) and 1H–DBU (b).

Fig. 4 ¹H NMR (CD₂Cl₂) in the 4.0–1.2 ppm region, showing the displacement of the THF molecule by DMPU in 1H–THF.

From equimolar solutions of 1H and pyridine (py), triethylamine (TEA) or 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), the 1 : 1 adducts 1H–py, 1H–TEA and 1H–DBU respectively were isolated by crystallization and characterized by IR (see Table 1), NMR and X-ray diffraction. In the solid state, 1 :  1 adducts were encountered, now with the nitrogen atom of the neutral base involved in the hydrogen bonding. The hydrogen atom was found to be closer to O in 1H–THF and 1H–DMPU, and closer to N in 1H–py, 1H–TEA and 1H–DBU, consistent with the higher basicity of the N-bases. Nonetheless, it must be noted that the addition of equimolar amounts of the bases to 1H does not result in deprotonation to yield 1; rather, distinct, isolable 1 : 1 adducts are formed.

Table 1 IR νCO (cm⁻¹) data in CH₂Cl₂ solution

1H	1H–THF	1H–DMPU	1H–py	1H–TEA	1H–DBU	1
1956	1956	1852	1945	1930	1927	1918
1869	1869	1864	1856	1837	1832	1820

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In summary, 1 : 1 adducts were formed between the alcohol complex 1H and THF, DMPU, py, TEA and DBU. Despite the fact that these are neutral molecules (therefore, there is no ion–ion attraction) and there is a single hydrogen bond, the adducts could be isolated and characterized in solid state and in CH₂Cl₂ solution, where their IRνCO bands are intermediate between those of 1H and 1, with νCO decreasing in the order 1H ≈ 1H–THF > 1H–DMPU > 1H–py > 1H–TEA > 1H–DBU > 1, indicating a variation in the degree of H⁺ transfer from 1H to the neutral molecule in the same order. Since the timescale of IR spectroscopy makes it a fast technique, these pairs of bands correspond each to a single species, not to averages of 1H and 1, which, when present, give rise to distinct, well separate pairs of bands, like those seen when 1H was dissolved in THF. Thus, simple IR monitoring in solution using the νCO region reports the relative extent of H⁺ transfer from an acidic metal carbonyl complex to different bases.

We thank Principado de Asturias (Grants IB05-069 and IB08-104 administered by FICYT), Ministerio de Ciencia y Tecnología (CTQ2006-07036/BQU) and Ministerio de Ciencia e Innovación (CTQ2009-12366/BQU).

Notes and references

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Footnotes

† Electronic supplementary information (ESI) available: Experimental section, crystal structure determination and molecular structure representation of 1′H; 1H–DMPU, 1H–py and 1H–TEA; ¹H NMR of 1H at different concentrations, VT ¹H NMR of 1H–DMPU and ¹H NMR titration showing the displacement of THF by DMPU in 1H–THF. CCDC reference numbers 786107 (1H), 786108 (1), 786109 (1H–THF), 786110 (1H–DBU), 786111 (1H–DMPU), 786112 (1H–py), 786113 (1H–TEA) and 786114 (1′H). For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c0ce00688b

‡ Present address: Fundación ITMA, Parque Tecnológico de Asturias, 33248 Llanera, Spain. E-mail: m.morales@itma.es

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