Metal Stannylene Complexes

A "free" stannylene is a tin compound in which the tin center has a formal oxidation state of two, Sn (II), and two groups bound to it, SnR2.  We want to synthesize metal complexes in which these "free" stannylenes act as a ligand to the metal center.  To describe this chemistry, we first need to discuss the basic structure of free stannylenes (in comparison with the well known, albeit very reactive, free carbenes).  We can then move on to how a stannylene group could interact with a metal as a ligand.  At that point, we'll talk about some of the synthetic routes that we hope will allow us to make metal stannylene complexes.

Here are a couple of references concerning general organometallic chemistry.
 

Structure and  Bonding

            Free Carbenes

            Free Stannylenes

            Stannylene Complexes

            Deciding Which Compounds to Make

Synthesis

            Stannylene Route

            Dianion Route

References

Structure and Bonding

Free Carbenes.

Since stannylenes are related to the well-known carbene family of compounds, by studying what is already known about carbenes we can learn a lot about stannylenes.  Look at the structures of the two carbene groups pictured below to explore the bonding in the carbene group.
 
 

Above left is pictured a singlet carbene.  Instead of a full octet, the carbene carbon only has six electrons and so is electronically unsaturated.  The hybridization of  the central carbon atom is sp2.  In a singlet carbene both of the non-bonding electrons are located in the lower-energy sp2 hybridized orbital and the higher-energy p-orbital is left empty.

Above right is pictured a triplet carbene.  The carbene carbon still only has six electrons, but now the distribution of the electrons in the compound is different.  In a triplet carbene the two non-bonding electrons are unpaired; each electron inhabits its own orbital.

Free carbenes and metal complexes in which they act as ligands have been extensively studied.  Below are listed a few references to look at if you are interested.
 

Free Stannylenes.

Free stannylene compounds have also been made (references about free stannylenes).  Due to relativistic effects the energy of the nominally sp2 hybridized orbital is considerably lower in energy than the p-orbital and so the ground state structure of free stannylenes is always a singlet.  An example of a free stannylene that has been synthesized and isolated by Michael Lappert as a (relatively) stable compound is shown below.5
 
 

Stannylene Complexes

Researchers have previously made some stannylene complexes.  Here are some references to some of the previous work.

As a free stannylene approaches a metal center, different orbital interactions can occur.  One possible interaction is that of s-donation from the filled sp2-hybridized orbital of the tin into an empty s-symmetry orbital of the metal center.

 

A second interaction is that of p-acceptance of a lone-pair from a filled d-orbital of the metal into the empty p-orbital of the tin.

Of course, from the metal's point-of-view the top interaction is one of s-acceptance and the bottom interaction is one of p-donation.

These two types of bonding are inherently different in nature.  Examine, for instance, the effect of rotation about the M-Sn bond axis on the overlap between the two s-symmetry orbitals.

There is no difference; the s-symmetry interaction is immune to rotation about the bond axis.  However, upon the same rotation of the M-Sn bond, the p-interaction is lost completely because the filled d-orbital is orthogonal to the empty p-orbital.

 

Deciding Which Compounds to Make

To determine what structure we expect to have for a given organometallic complex where stannylene is acting as a ligand, we must know what frontier orbitals are available on the transition metal fragment.  Drawn below is an electronically unsaturated tungsten fragment -- Cp2W (click here for an explanation of the Cp abbreviation).  In terms of the 18-electron rule, the tungsten only has 16 electrons surrounding it (5 from each of the Cp ligands and 6 from the W itself).  Since the free stannylene has a lone pair (in the sp2 hybridized orbital) which it could donate to the tungsten, formation of a bond between the tin and the tungsten centers will result in tungsten having 18 electrons surrounding it (5 from each of the Cp ligands, 6 from the W itself, and 2 from the stannylene ligand).
 

The frontier orbitals (generally considered the HOMO and the first couple of LUMOs) of Cp2W are shown below in the orientation necessary for both a s-symmetry interaction and a p-symmetry interaction with the stannylene ligand.  Notice that in this orientation there is a steric interaction between the other groups on the stannylene and the two Cp ligands.

 

By rotation of the stannylene ligand by 90°, one can relieve the steric strain.  On the other hand, the p-backdonation from the W to the Sn is cut off (as described above).

The question is, which effect will win?  Is the steric strain large enough to force the stannylene ligand to rotate?  Without the p-backdonation, the Sn center has only 6 valence electrons.  However, if the Sn can receive electron density from somewhere else, perhaps this wouldn't matter so much.  In fact, the Sn can receive electron density by p-donation from the lone pairs of the N centers bound to it.

Since we want to synthesize metal-stannylene complexes which have the intact p-interaction, we should try to minimize the factors which will favor the rotated structure.  For instance, choosing less bulky substituents on the tin center would reduce the steric strain between the Cp's on the tungsten and the groups on the stannylene.  Also, by picking substituents on the tin which do not have lone pairs to donate into the tin center maybe we can force the tin to get its electron density from the tungsten d-orbitals.

Synthesis

Now that we have a pretty good idea of the type of compounds that we want to make, we can discuss a little bit about the different routes that could be used to synthesize the compounds.  Two major routes spring to mind as being possible:  the first is what we could call the stannylene route and the second we could call the dianion route.
 

The Stannylene Route

In the stannylene route we generate a free stannylene and then allow it to react with an unsaturated transition metalloorganic compound (or its precursor).  In our case, this route may have problems because we want non-bulky stannylenes which do not have any stabilization from neighboring p-donor atoms (such as N in the Lappert stannylene described above).  Such stannylenes are likely to be quite reactive and so chemistry with them will be challenging.

In the general example given here, the precursor to the tungsten fragment discussed above (the carbon monoxide adduct of Cp2W) is irradiated with ultraviolet light (hn) which results in loss of CO followed by reaction with the free stannylene.  Free stannylenes can be generated by treatment of the diorganotin dihalides (R2SnX2) with sodium naphthalide (NaNp) which is a common reducing agent.

 

The Dianion Route

In the dianion route, the starting materials are also diorganotin dihalides.  However, instead of reducing with sodium naphthalide, the reduction is done with  the pure metal (e.g. sodium) itself.  This results in formation of the metal salt of a diorganotin dianion.

In our proposed synthesis these dianions would react with a metal dihalide to form the stannylene species.

In the summer of 1999, Kerstin Wolf and Angel Vargas started down the synthetic pathway for the synthesis of Bz2SnLi2, tBu2SnLi2, Ph2SnLi2, and Bu2SnLi2. They were quite successful in the preparation of the diorganotin dihalides; however, the generation of the dianion species did not proceed as easily as indicated in the literature.  Through careful work it was found that varying the reaction conditions resulted in different physical and chemical observations.  Unfortunately, we do not believe that we have yet observed the tin dianion species as of yet.

It is also possible to allow a transition metal anion to react with the diorganotin dihalide.  This route has recently been explored by Angel Vargas in my laboratory, and initial results look promising.  Specifically, Angel has attempted to generate the lithiotungstenocene hydride shown below and treat it with Ph2SnCl2.  This reaction generated a new compound which we are now trying to identify. 

The starting point for my group's research for the summer of 2000 will be these anionic synthetic routes.  Our initial work will be based upon the work of Wolf and Vargas from last summer and this year.  The success of their reactions should provide a good launching point to move further along the pathway to synthesizing transition metal stannylenes.  Using their notebook descriptions as well as literature descriptions13-20  for the preparation of the organotin dihalides, disodium salts and transition metal anions, we will prepare these compounds.  Assuming that all goes well, we then move on to our own chemistry of making novel metal stannylenes.

 

References

General Organometallic Chemistry:

1.  Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, 1987.

2.  Crabtree, R. H. The Organometallic Chemistry of the Transition Metals, 2nd ed. Wiley, New York, 1994.

Free Carbenes:

3.  Bruice, P. Y. Organic Chemistry, 2nd ed. Prentice Hall, New Jersey, 1998, pp. 154-155.

4.  Carey, F. A.; Sundberg, R. J. Advanced Organic Chemistry, Part B pp. 511-541.

Metal Carbene Complexes:

5.  Collman, J. P.; Hegedus, L. S.; Norton, J. R.; Finke, R. G. Principles and Applications of Organotransition Metal Chemistry; University Science Books: Mill Valley, 1987.

6.  Marsella, J. A.; Folting, K.; Huffman, J. C.; Caulton, K. G. J. Am. Chem. Soc.  1981, 103, 5596.

Free Stannylenes:

7.  Lappert, M. F.; Rowe, R. S. Coord. Chem. Rev. 1990, 100, 267.

Stannylene Complexes:

8.  Petz, W. Chem. Rev. 1986, 86, 1019.

9.  Lappert, M. F.; Rowe, R. S. Coord. Chem. Rev. 1990, 100, 267.

10.  Holt, M. S.; Wilson, W. L.; Nelson, J. H. Chem. Rev. 1989, 89, 11.

11.  Nakazawa, H.; Yamaguchi, Y.; Kawamura, K.; Miyoshi, K. Organometallics 1997, 16, 4626.

12.  Weidenbruch, M.; Stilter, A.; Saak, W.; Peters, K.; von Schnering, H. G. J. Organomet. Chem. 1998, 560, 125.

Synthesis:

13.  Shriver, D. F.; Drezdzon, M. The Manipulation of Air-Sensitive Compounds, Wiley, New York, 1986.

14.  Herrmann, W. A.; Salzer, A. eds. Synthetic Methods of Organometallic and Inorganic Chemistry, Thieme, New York, 1996, vols. 1-8.

15.  Comprehensive Organometallic Chemistry; 1st ed.; Wilkinson, G.; Stone, F. G. A.; Abel, E. W., Ed.; Pergamon Press: New York, 1982; Vol. 2.

16.  Kraus, C. A.; Greer, W. N. J. Am. Chem. Soc. 1925, 47, 2568.

17.  Schumann, H.; Schumann, I. Gmelin Handbook of Inorganic Chemistry; Springer Verlag: New York, 1981; Vols. 6-8.

18.  Comprehensive Organometallic Chemistry; 1st ed.; Wilkinson, G.; Stone, F. G. A.; Abel, E. W., Ed.; Pergamon Press: New York, 1982; Vol. 3.

19.  Inorganic Syntheses, Grimes, R. N., ed.; Wiley-Interscience:  Chichester, 1992; Vol. 29.

20.  Synthetic Methods of Organometallic and Inorganic Chemistry (Herrmann/Brauer) Herrmann, W. A.; Salzer, A. eds.; Georg Thieme Verlag:  Stuttgart, 1996; Vol. 3.

Definition of the Cp abbreviation:

The abbreviation Cp stands for cyclopentadienyl.  This is a very common ligand in organometallic chemistry.  The Cp anion is a very stable anion because the negative charge is delocalized in a 6-electron cyclic p-system.  As you know from benzene, 6-electron cyclic p-systems are aromatic and show extremely high stability.  In fact, the Cp anion is so stable that the conjugate acid (cyclopentadiene or CpH) has a pKa of around 16.  In other words, CpH has about the same acidity as water!  The Cp anion has an accessible p-cloud of electrons with which a transition metal can interact.  In addition, since it is an anion it will have a strong coulombic attraction to a metal cation.  These two factors combined result in very strong bonding between the Cp anion and the metal cation.  In terms of electron counting, the Cp group counts as 6 electrons when using the charged (or ionic) counting system and 5 electrons when using the neutral (or radical) counting system.