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Jack Passmore, Ph.D., F.C.I.C.
Professor Emeritus
Department of Chemistry
University of New Brunswick
Fredericton, NB E3B 6E2
Canada

Tel: 1-506-453-4821
Fax: 1-506-453-4981
E-mail: passmore@unb.ca

Research Interests

Our aim is to prepare compounds that are counter intuitive, that is they appear to be impossible according to what was learned in the first year chemistry course, and fall into the following categories:

1. 1. as simple as possible
2. 2. have novel bonding
3. 3. possess novel properties
4. 4. simple and quantitative synthesis

Under this overarching philosophy, our current research is focused on the following areas:

1. The Chemistry of the Homopolyatomic Cations of groups 16-17.

It is a well known generalization that the second row elements tend to form homoatomic multiple π bonds, while a corresponding number of single bonds are favored among the heavier elements of these groups. Higher bond orders for the heavier elements of groups 13-15 can be achieved by employing bulky groups as substituents. A central focus of our group is the synthesis of new homopolyatomic cations of groups 16 and 17, many of which exhibit very high bond orders without requiring the stabilizing effects of bulky groups. We have previously published the discovery of the thermally stable (S₂I₄²⁺)[EF₆^-]₂ (E = As, Sb) which is one of the most highly npπ-npπ (n ≥ 3) bonded main group compound known, with a bond order approaching 3. Recent research in this area has focused on employing computational methods to further clarify the properties of the cation and the nature of its bonding. Currently we are extending the use of bonding models developed for S₂I₄²⁺ to other related species of groups 16 and 17, such as (S₇I⁺)[AsF₆^-], (Se₆I₂²⁺)[AsF₆^-]₂, and (Se₄I₄²⁺)[AsF₆^-]₂.

Fig. 1 Charge delocalization and bond energy gain leading to high S-S bond order in S2I42+.

Recent papers on S₂I₄²⁺ can be found here: Inorg. Chem. 2007, 46, 681-699 and here: Inorg. Chem. 2005, 44, 1660-1671

See also a related feature article in Chemical and Engineering News: Chem. Eng. News, 83, 12 (march 21), 49-50, 2005.

2. Reaction of naked cations with new interacting weak anions.

A) A new class of simple complexes of silicon analogues of the well known crown ethers.

The serendipitous isolation and x-ray crystal structure determination of Li(Me₂SiO)₆[Al_F] ([Al_F] = [Al(OC(CF₃)₂Ph)₄^-) resulted in the successfull synthesis of a number of salts of the type LiD_n[Al_F] (D = Me₂SiO, [Al_F]= polyfluoroalkoxyaluminate, n = 5 and 6). These provide the first examples of the preparation of alkali metal ion host-guest complexes of cyclic dimethylsiloxanes directly from their components, and imply that cyclic dimethylsiloxanes (D₅ and D₆) are acting as pseudo crown ethers. The counterpoise (CP) corrected binding energies (figure 4) for MD₆⁺ exhibit a remarkable similarity to that for 18-crown-6, i.e. both decreasing with increasing alkali metal cation size in gas phase. A difference between them can also clearly be seen in that the binding affinity of D₆ is about 100 kJmol^-1 less than that of 18-crown-6 at HF/3-21G level, which is associated with the low basicity of siloxanes.

Fig 2. Ortep Views of LiD6[Al], anions ommitted for clarity.	Fig 3. Counterpoise corrected binding energies of D6 and 18-crown-6 with alkali metals.

Papers on dimethylsiloxanes acting as pseudo crown ethers can be found here: Angew. Chem. Int. Ed. 2006, 45, 2773-2777 and here: Eur. J. Inorg. Chem. 2006, 4033-4036

See also a related highlight article in Angewandte Chemistry: Angew. Chem. Int. Ed., 2007, 46, 4610-4613.

B) The first example of Diiodine acting as donor.

Diiodine has extensive chemistry as a Lewis acid toward numerous Lewis bases, e.g. Me₃N to I₂ by donation of the lone pairs into the sigma antibonding LUMO of I₂. We have found the first clear example of I₂ acting as a donor towards Ag⁺ in (AgI₂)[MF₆] (s) (M = Sb, As) with the formation of planar (AgI₂⁺)_n polymeric chain as shown in Figure 4. A schematic representation of the covalent bonding is shown in Figure 5. This salt is formed almost quantatively by the reaction of Ag[MF₆] and I₂ in liquid SO₂ solution.
The zigzag planar geometry and the presence of positive charge on all atoms imply that molecular I₂ acts as a donor to Ag⁺ in (AgI₂⁺)_n, which is supported by bonding nature discussion in terms of VB and MO methods. The (AgI₂⁺)_n cation is also the first example in the solid phase in which a dihalogen molecule is coordinated to an uncomplexed metal cation.

Fig 4. (AgI2+)n cations in (AgI2)n·nMF6 (M = Sb, As).	Fig 5. Schematic representation of the covalent I2···Ag bonding in the (AgI2+)n cation.

The paper can be found here: Angew. Chem. Int. Ed., 2004, 43, 1995-1998.

3. Sulfur Nitrogen radicals and multiradicals

A) New cycloaddition reactions of SNS⁺ to give a new class of quinone-thiazyl hybrid radicals

Extension of the cycloaddition chemistry of SNS+ (See Chart 1) to the unsaturated quinones provides an unprecedented one step synthesis to both fused-ring systems, such as 1 and 2, and the only direct, essentially quantitative route to the –CNSNS ring system (c.f. 3++) which are not readily accessible by classical routes. Additionally, a major advancement exploits the inherent twofold cycloaddition and oxidation chemistry of SNS+ (c.f. ONO+), and provides a one step, high yield synthesis of 2+.

The reduction of these salts, using a variety of reducing agents has led to the isolation of the corresponding neutral radicals e.g. 3⁺⁺ to 3^·· or radical cations salts (e.g. 3⁺⁺ to 3^+· with AsF₆^- or SbF₆^-), which represents the first stable –CNSNS radical that does not undergo the typical rearrangement to the –CNSSN isomer. 1-3 are members of a new class of quinone-thiazyl hybrids, which exhibit highly spin delocalized radicals, where the quinone moiety provides enhanced intermolecular interactions in the solid state, inhibiting dimerization, while providing unique range of electronic and magnetic properties.

Related papers can be found here: Inorg. Chem. 2005, 44, 6524-6528. , here: Inorg. Chim. Acta 2008, 361, 521-539. and here: Chem. Commun. 2009, 6077-6079.

B) Tuning of the singlet - triplet gap

7π CNS radicals generally dimerize in the solid state to give diamagnetic π*-π* dimers. However, on melting sometimes paramagnetic liquids are obtained e.g. F₃CCSNSCCF₃, a rare example of a blue gas (See Fig. 6).

Fig 6. "Blue gas". Paramagnetic liquid F3CCSNSCCF3

The paper can be found here: Dalton Trans., 2000, 3365-3382.

The related (CF₃CNSSS)[AsF₆] is a paramagnetic solid containing isolated 7π CF₃CNSSS⁺ radical cations. However replacement of CF₃ by other groups (See Table 1) leads to weakly bonded dimers with ground states that contain a high percentage of diradical character i. e. open shell singlet character illustrated in Figures 7 and 8. In addition the low lying triplet excited state is thermally populated and is observed by EPR spectroscopy and the experimental singlet triplet gap is determined by variable temperature EPR studies.

Table 1. Species with thermally accessible triplet states

The related –CNSSN radicals show similar characteristics. Their low singlet-triplet gaps imply a very low in situ dimerization energy. The inter-radical S···S distances are longer than those found in (RCNSSN)₂ dimers [3.00 Å], which have higher dimerization energies [ca. 40 kJ mol^-1]. Thus the singlet triplet gaps have been “tuned” by the nature of the 7π CNS radical, the nature of R group, and the electrostatic environment.

Fig 7. Illustration of the formation of the (ClCNSSS)22+ dimer from two monomers.	Fig 8. Diagram of triplet and "open shell" singlet states

Recent papers on 7π CNS radicals can be found here: Dalton Trans., 2008, 4029-4037. here: Inorg. Chem., 2007, 46, 7756-7766. and here: Inorg. Chem., 2007, 46, 7436-7457.

C) O₂ -like diradical that retains its paramagnetism in the solid state

The MF₆^- (M = As, Sb) salts of (SSSNCCNSSS)²⁺ dication were readily prepared by a reaction of (CN)₂ with a 1:1 mixture of S₄[MF₆]₂ and S₈[MF₆]₂ salts in SO₂(l). The (SSSNCCNSSS)[Sb₂F₁₁]₂ salt was prepared by the addition of an excess of SbF₅ to (SSSNCCNSSS)[AsF₆]₂. The (SSSNCCNSSS)²⁺ dication does not adopt the classically bonded quinoidal structure, where the octet rule is obeyed, but forms a planar disjoint diradical where the unpaired electrons lie on separate rings. The unpaired electrons are only very weakly coupled affording a small singlet triplet gap (ΔE_ST = 2J) of < ± 2 cm^-1 with a triplet ground state.

Fig 9. (SSSNCCNSSS)[A]2 adopts a non-quinoidal structure

The (SSSNCCNSSS)²⁺ can be seen to be similar to O₂ in that they are both triplet-state molecules and do not adopt classical electron-paired structure. The unpaired electrons in O₂ are strongly coupled compared to weak coupling in (SSSNCCNSSS)²⁺. The unpaired electrons in (SSSNCCNSSS)²⁺ or O₂ do not pair in the solid state to form chemically bonded dimers or polymers and remain essentially paramagnetic albeit the magnetic susceptibilities of both decrease at very low temperatures because of intermolecular antiferromagnetic coupling. Different phases of solid O₂ have different magnetic properties as do the different salts of (SSSNCCNSSS)²⁺. As far as we are aware, O₂ and the salts of (SSSNCCNSSS)²⁺ are the only simple nonsterically hindered main-group isolated compounds to show paramagnetism in the solid state.

Fig 10. Similarities and differences between the electronic states of O2 and planar (SSSNCCNSSS)2+, and of [O2]2, and [(SSSNCCNSSS)2+]2 dimers.

The paper can be found here: Inorg. Chem., 2010, 49, 7861-7879.

4. New simple anions of oxides of the main group elements

With the combined use of high level quantum chemical calculations and solid state thermodynamic considerations we have been able to predict promising candidates for new classes of salts of hitherto unknown anions of oxides of the main group elements. These salts have a potential use in the ready reversible sequestering of environmentally important molecules e.g. CO₂, SO₂, with a reduced energy cost compared to the current processes employed by industry.

The current processes used for the capture of gaseous pollutants from industrial flue gas streams either create irreversible products or require high temperatures (T > 800 °C) to regenerate the trapping agent for reuse. The high energy requirements of the existing processes are counterproductive to their use in CO₂ capture for the remediation of greenhouse gases (GHGs), since the heating requirements are typically met through the combustion of fossil fuels. The development of effective solid sorbents for CO₂, SO₂ and NO_x with reduced energy requirements for absorption and regeneration is therefore highly desirable.

This is a relatively new research area in our group and there are currently post doctoral and graduate student positions available in this and other projects. For more information please contact passmore@unb.ca.

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Last update: 2010 Sep. 27