Posts Tagged ‘carbon dioxide’

How does carbon dioxide coordinate to a metal?

Saturday, May 6th, 2017

Mention carbon dioxide (CO2) to most chemists and its properties as a metal ligand are not the first aspect that springs to mind. Here thought I might take a look at how it might act as such.

There are up to five binding modes with one metal that one might envisage:

  1. Bonded interaction with the metal via just one oxygen atom,
  2. Bonded interaction via just the central carbon atom,
  3. Bonded interaction via the π-face of one C=O double bond,
  4. A weaker non-bonded interaction via carbon, or
  5. via oxygen.

Search queries of the Cambridge structure database (CSD) for these five modes are illustrated below (dataDOI: 10.14469/hpc/2524), with the constraints being applied to how many bonds (of unspecified type) each atom carries, along with no disorder and no errors. Thus query 1 is constrained by 1-coordination on one oxygen, and two on the carbon and other oxygen. 

  1. This query yields four hits: 10.5517/ccvcdq9, 10.5517/cc12nq6n, 10.5517/cc12nq5m, 10.5517/cc12nq4l. The angle subtended at the central carbon of the CO2 ranges from 172-176°, a very modest bending of the linear CO2. There are no examples where the metal is bonded to both oxygens.
  2. The next category involves the metal binding just to the central carbon. Two examples are known, differentiated from O-coordination by a more acute angle at the central carbon of 121-132°.
  3. The π-coordinated type requires a slightly more complex search query, shown below. The π-complex is defined as adding one coordination to each of one oxygen and the carbon. 

    This reveals 16 examples:

    The sine of the angle subtended at the centroid of one C-O bond shows that for most of the examples, the metal is close to perpendicular to this bond. The angle subtended at the central carbon ranges from 128-138, rather larger than the examples where the metal is bound just to the carbon. I have picked these two for illustration. The first (dataDOI: 10.5517/cc86r17) contains both CO2 and CO coordinated to the metal.This one (dataDOI: 10.1021/ic101652e) contains a short metal-centroid distance of 1.78Å (as also does 10.5517/ccz34kr). 

    There are two examples where BOTH π-CO bonds are coordinated to a metal; 10.5517/ccqlv7c and 10.5517/ccqlv8d (Ni-centroid distance 1.9Å) but these are intriguing because the two π-complexes are co-planar and not orthogonal.

  4. The final two cases are defined in the CSD database by having not so much bonds between metal and either C or O, as close intermolecular contacts typical of e.g. hydrogen bonds. This one (dataDOI: 10.5517/cc12nq9r) is to Fe, with a metal-C distance of 2.87Å which is significantly shorter than the anticipated sum of the van der Waals radii of the two atoms. The next (dataDOI: 10.5517/cc12npn2) has a close approach of Co to O of 2.23Å. The angles subtended at the carbon range from 174-180°. There are no convincing examples of close non-bonded approaches of the metal to both oxygen atoms simultaneously.

It is striking that the searches (as defined above) reveal relatively few examples. This might simply be a result of how the compounds are indexed in the CSD, reflected in the coordination constraints applied in the searches. Nevertheless, we see three quite different types of ligand-metal coordination in which bonds can be said to form and a more diffuse spectrum of weaker interactions to carbon dioxide. As a metal ligand, it is certainly interesting! Several deserve their wavefunctions looked at and I might report back on this aspect.

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Posted in crystal_structure_mining | 2 Comments »

Solid carbon dioxide: hexacoordinate carbon?

Friday, September 17th, 2010

Carbon dioxide is much in the news, not least because its atmospheric concentration is on the increase. How to sequester it and save the planet is a hot topic. Here I ponder its solid state structure, as a hint to its possible reactivity, and hence perhaps for clues as to how it might be captured. The structure was determined (DOI 10.1103/PhysRevB.65.104103) as shown below.

The structure of solid carbon dioxide. Click for 3D

The two nominal double bond distances are 1.33Å, whilst a further four O…C contacts in the shape of a square complete the coordination (2.38Å each). All would probably agree that the central carbon is best described as hexa-coordinated. This is also a hot topic. For example, note the claim made recently to have created a hexa-coordinated carbon species by design (Synthesis and Structure of a Hexacoordinate Carbon Compound, DOI: 10.1021/ja710423d) based on a motif derived from an allene:

Designed hexacoordinate carbon. Click for 3D

This claim was supported by an unusual measured property, the electron density ρ(r) and its Laplacian in the putative O…C region. These two properties are one of those (relatively rare) meetings between experiment and quantum mechanics, and their usefulness has been noted in this blog on previous occasions. However, note that in this designed structure, the O…C distances are merely 2.65-2.7Å, significantly longer than in solid carbon dioxide! So carbon dioxide, in a form many of us are familiar with (solid), can certainly be justified as being described as having a hexacoordinate carbon (although we might draw the line at describing it as having hexavalent carbon).

If oxygen atoms can approach the carbon in CO2 to within ~2.4Å, an interesting question can be posed. How close can another carbon get to CO2 without actually reacting and forming a new molecule? C-C bonds, even weak ones, are so much more interesting than C-O bonds! It would have to be a particularly nucleophilic carbon, of course. A search of the August 2010 version of the Cambridge structural database (CSD) reveals no really close approaches of another carbon to CO2. Only about 8 weak examples are found, and here the C-C distances are ~3.0-3.2Å, with the O=C=O angle in the CO2 never less than 170°. In this context, there is an intriguing and very recent report (which has not yet made it into the searchable CSD) of the structure of CO2 trapped in a cavity next to what was claimed to be a molecule of 1,3-dimethyl cyclobutadiene, or CBD (see 10.1126/science.1188002 and the discussion of this article in my earlier blog post). The focus in that report was on the “Mona Lisa of organic chemistry”, namely the CBD unit. One feels that the structure of the adjacent CO2 was of lesser interest to the authors. According to a visual image of this system, the CBD and CO2 pair show quite an intimate approach via their carbon atoms (a ghostly C-C bond is clearly represented). This raises the interesting question of whether the description of this pair should be of two intimate but nevertheless separate and relatively unperturbed molecules not connected by a covalent bond (“more indicative of a strong van der Waals contact than of covalent bonding“) or of a pair fully bound by a covalent C-C bond between them?

The issue of what is an interaction, and what is a bond continues to raise its often controversial head. And quantum theory continues to provide a multitude of interpretations as well.

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Posted in Hypervalency, Interesting chemistry | 5 Comments »