Dalton Transactions Blog

Professor Darrin Richeson’s group has carved out an interesting organometallic niche. Their papers feature low-valent metal complexes of pincer ligands combined with detailed computational analysis of the electronic structures. In their recent Dalton Transactions article they report unique coinage metal complexes of the “PN³P” ligand below.

This "PN3P" ligand

The judicious selection of this particular pincer framework resulted in complexes with geometric properties distinct from those of the same metals with other “PNP”-type ligands. Choosing the ligand with a NR instead of a NH spacer between the pyridine and phosphine moieties enhances the donor strength of the phosphines while rendering the N atoms inert towards deprotonation.

Subtle effects of this adjustment manifest themselves in dramatic changes in the solid-state coordination and geometric properties of their Cu and Ag complexes. Their reported CuBr complex is coordinated by one PN³P ligand and has a distorted trigonal pyramidal geometry. They contrast these to Cu(I) complexes of an analogous PN³P ligand with NH spacers, which coordinate two ligands and in one case displace the halide atom. Using CH₂ spacers, a previously reported Cu(I) structure exhibits T-shaped geometry and a non-coordinating triflate anion. The Cu(I) reported here shows a geometry much closer to trigonal pyramidal, with a coordinating triflate anion.

The second part of the paper discusses details of the electronic structure gleaned from DFT calculations. Particularly illuminating is a paragraph on page 19157, summarizing the particulars of the bonding as “a balance of bonding and antibonding interactions…diffuse polarized-π orbitals…(and) back donation.” Perhaps this seems summary seems unsurprising, but there are subtleties in the analysis.

Professor Richeson was my M.Sc. supervisor (I finished in his group six years ago). I write this in gratitude for his mentorship, and in appreciation of his group’s continuing work.

For more details read the full paper:

Coinage metal complexes supported by a “PN³P” scaffold
Gyandshwar Kumar Rao, Serge I. Gorelsky, Ilia Korobkov and Darrin Richeson
Dalton Trans., 2015,44, 19153-19162
DOI: 10.1039/C5DT03515E

Ian Mallov is currently a Ph.D. student in Professor Doug Stephan’s group at the University of Toronto. His research is focused on synthesizing new Lewis-acidic compounds active in Frustrated Lewis Pair chemistry. He grew up in Truro, Nova Scotia and graduated from Dalhousie University and the University of Ottawa, and worked in chemical analysis in industry for three years before returning to grad school.

To me the most interesting observation in the recent Dalton Transactions paper from the group of Professor Phil Power was their suggestion that secondary interactions between dicoordinate Fe(II) atoms and carbon atoms on their ligands probably have a significant effect on the magnetic moment of the complexes.

Specifically, they postulate that these interactions help to quench the orbital contribution to the magnetic moment, which is significant for other dicoordinate Fe(II) complexes studied.

But let’s take a step back.  Dicoordinate Iron(II) complexes were unknown until the 1980’s, thought to be too unstable to isolate and structurally characterize.  As the authors detail, examples were discovered gradually. All featuring large coordinating ligands bound through anionic C, N, or O donors.  Power reports a total of thirty currently known.

No one, it appears, has previously undertaken thorough magnetic studies.  Indeed, do you remember studying how crystal field theory applies to dicoordinate metal species in your introductory inorganic class?  I don’t.

The authors focus their attention on four species. Two of these feature large silylamido ligands and have solid-state N-Fe-N angles of 169^o and 172^o, the other have two large aryl ligands and exhibit slightly more bent geometries.  The authors support the evidence that a significant part of the measured temperature-dependent magnetic moment of these molecules arises from the orbital contribution – that is, from the motion of electrons around the iron nucleus, rather than arising only from the spin contribution, the electrons spinning about their own axes.

However, the less linear aryl iron(ll) complexes show the greater orbital contribution to the magnetic moment, which brings me back to the beginning.  This is a thorough paper; the authors also construct a spectrochemical series for the dicoordinate Fe(II) complexes and exactingly compare computed and experimental magnetic data.  But the original small structure-function observation fascinated me on my first reading.

Read the full article now:

Ligand field influence on the electronic and magnetic properties of quasi-linear two-coordinate iron(II) complexes
Nicholas F. Chilton, Hao Lei, Aimee M. Bryan, Fernande Grandjean, Gary J. Long and Philip P. Power
Dalton Trans., 2015, 44, 11202-11211
DOI: 10.1039/C5DT01589H

Ian Mallov is currently a Ph.D. student in Professor Doug Stephan’s group at the University of Toronto. His research is focused on synthesizing new Lewis-acidic compounds active in Frustrated Lewis Pair chemistry. He grew up in Truro, Nova Scotia and graduated from Dalhousie University and the University of Ottawa, and worked in chemical analysis in industry for three years before returning to grad school.

From early on, we are taught to look at linear mathematical relationships as dependent variables that are determined by the value of an independent variable.

The reason that I find the phenomenon of hysteresis fascinating is that it is the violation of the above concept. By definition, the value of a dependent variable showing hysteresis depends not only on the value of the independent variable but also on whether the independent variable has been increasing or decreasing before it arrives at the value at which a measurement is taken. The dependent variable has a ‘memory’ of its path.

Examples of hysteresis are rare on a molecular level. One of several fascinating aspects of the recent Dalton Transactions paper by Guy Jameson and colleagues is that the (temperature-dependent) magnetism exhibited by the authors’ Fe(III) spin-crossover (SCO) complex shows two hysteresis loops; an example from Jameson and colleagues is shown below.

In transition metal complexes where both high spin and low spin states are possible, SCO occurs when a complex switches from one spin state to the other. Unsurprisingly, this is temperature-dependent, hence the temperature dependence of the magnetism. Hysteresis in magnetic measurements of SCO compounds is known, but Jameson and colleagues report a number of rare properties for the Fe(III) compound that they investigate, [Fe(qsal-Br)₂]NO₃•2MeOH (where qsal-Br denotes the (N-8-quinolyl)-5-bromo-salicylaldimate ligand).

The vast majority of complexes in which SCO occurs exhibit the phenomenon in a single step — at a certain temperature, all of the molecules in a bulk sample will convert from one spin state to the other. This is only the fifth example of a mononuclear Fe(III) complex that shows full SCO that occurs in two or more discrete increments where symmetry is lost within the molecular structure. In the case of this Fe(III) complex, the temperature range over which some metal nuclei are high spin, and some are low spin (in this case 50% each), is the largest ever reported for a mixed spin-state Fe(III): that is, 96 K.

Here, both steps exhibit hysteresis. The spin-state transitions occur at different temperatures when the authors start with the complex at 300 K and cool it, versus when they warm the complex from 4 K.

Read the full article at:

Abrupt two-step and symmetry breaking spin crossover in an iron(III) complex: an exceptionally wide [LS–HS] plateau
David J. Harding, Wasinee Phonsri, Phimphaka Harding, Keith S. Murray, Boujemaa Moubaraki and Guy N. L. Jameson
Dalton Trans., 2015, DOI: 10.1039/C4DT03184A

Ian Mallov is currently a Ph.D. student in Professor Doug Stephan’s group at the University of Toronto. His research is focused on synthesizing new Lewis-acidic compounds active in Frustrated Lewis Pair chemistry. He grew up in Truro, Nova Scotia and graduated from Dalhousie University and the University of Ottawa, and worked in chemical analysis in industry for three years before returning to grad school.

In fall 2008 I visited Boston, where a good friend gave me a tour of Harvard’s chemical laboratories.  Passing by a small office, I saw through the window a thin elderly man in glasses hunched over a desk.  He was turned away from me but the sign on his office said William N. Lipscomb.

Since Lipscomb’s astonishing groundwork beginning in the 1940’s, boranes have provided fascinating examples of the diversity and possibilities of chemical bonding.  Today, boron’s Lewis acidity is widely exploited in catalysis and Frustrated Lewis Pair (FLP) chemistry.

The BH₅ molecule, described as BH₃ with sigma-bonded dihydrogen bound in an H₂ manner, lies at the confluence of many currents of boron chemistry: the activation of hydrogen by FLP’s using borane Lewis acids; three-centre-two-electron bonding; H₂ complexes, and the comparison of main group and transition metal chemistry.

In a recent paper in Dalton Transactions, authors Szieberth, Szpisjak, Turczel and Konczol describe BH₅ as “rare.”  This is an understatement.  As they report, its existence was confirmed in 1994 by infra-red spectroscopy in an argon matrix at 10-25K temperatures.

In this paper, they present the modelling of the BH₅ complex using Natural Bond Order analysis.  Although this has been reported before, the authors use this paper to discern a unique and significant contribution to the stability of η² H₂ borane complexes: the back-donation of electron density from the B-H (or B-R) s-bonds into the σ* orbital of the bound H₂, just as electron density from d-orbitals is donated to the H-H σ* orbital in transition metal H₂ complexes.  Their description of the Lewis structure of BH₅ as a BH₃/H₂ adduct featuring a three-centre-two electron bond accounts for 99.1% of the electron density.

William Lipscomb passed away on April 14, 2011 at the age of 91.  But I am sure he would be pleased that work within his research area remains vigorously active.

Read the original paper:

The stability of η2-H2 borane complexes – a theoretical investigation
László Könczöl, Gábor Turczel, Tamás Szpisjaka and Dénes Szieberth
Dalton Trans., 2014,43, 13571-13577

Ian Mallov is currently a Ph.D. student in Professor Doug Stephan’s group at the University of Toronto. His research is focused on synthesizing new Lewis-acidic compounds active in Frustrated Lewis Pair chemistry. He grew up in Truro, Nova Scotia and graduated from Dalhousie University and the University of Ottawa, and worked in chemical analysis in industry for three years before returning to grad school.

How do you combine rare-earth metals, extremely specific energy transfers, and luminescent properties to investigate changes in enzymes? New methods often arise from unique confluences of existing knowledge. In their recent paper, the Natrajan group from the University of Manchester exploit known properties of easily-obtained chemical products to present a clever new biosensory technique .

The unique aspect is the use of upconverting phosphors (UCPs) in combination with enzymes. UCPs are luminescent particles, often based on rare-earth metals, which can be excited by multiple photons absorbed in the near-infra-red region (750-1400 nm wavelengths). Post excitation, they emit a photon of light in the higher-energy visible spectrum, thus the energetic process is known as up-conversion. While enzymes have high specificities and sensitivities to substrates, UCP’s have the advantage of excitation in the near-infra-red region without autofluorescence. In combination, enzymes and UCPs provide several direct advantages over simple biosensory fluorescence measurements.

In the current paper, NaYF4:Yb:Tm was the UCP used to probe the redox properties of the enzyme pentaerythritol tetranitrate reductase (PETNR) and Forster Resonance Energy Transfer (FRET), involving energy transfer between two chromophores, was used to excite the UCP. In this case, transfer of energy from the absorbance band of the flavin mononucleotide core of the PETNR enzyme and the emission band of the UCP, which are very close in wavelength, allow FRET to occur. Since a second emission band in the near-IR region originates from this UCP, this was normalized so that the other band, varying with the enzyme concentration, could be measured against it. When the PETNR underwent a two-electron reduction, it negated its ability to undergo FRET, resulting in the loss of the emission band at 460 nm, rendering the solution colourless. The researchers demonstrated that this new technique can be used with either the full PETNR enzyme or the mononucleotide flavin core alone, indicating that this can be applied to a wider range of systems.

Find out more and download the article now:
Ratiometric detection of enzyme turnover and flavin reduction using rare-earth upconverting phosphors
Dalton Trans., 2014, DOI: 10.1039/C4DT00356J

Ian Mallov is currently a Ph.D. student in Professor Doug Stephan’s group at the University of Toronto. His research is focused on synthesizing new Lewis-acidic compounds active in Frustrated Lewis Pair chemistry. He grew up in Truro, Nova Scotia and graduated from Dalhousie University and the University of Ottawa, and worked in chemical analysis in industry for three years before returning to grad school.