Transcript Group 14 - University of Ottawa

Multiple Bonding in Group 15
Like the group 14 compounds, catenation is not difficult with group 15.
Reduction of the monochloride can lead to a single bond:
2 Me2ECl + 2Na  Me2E-EMe2 + 2 NaCl
This works best with bismuth and antimony, but is generalized over the
entire group 15.
In the case of arsenic, elimination of HCl is possible:
Me2AsCl + Me2AsH  Me2As-AsMe2 + HCl
Pyridine Analogues
+ EX3
X2SnBu2 +
Sn
Bu 2
E
X
Analogues to pyridine for group 15 molecules have
been around long enough to have two trivial names:
Phosphabenzene, arsabenzene, stibabenzene,
bismuthabenzene
Phosphinine, arsinine, stibinine, bismuthinine
They are made by methathesis with a tin alkyl.
Substituted aryl rings are also known.
+ HX
E
Pyridine Analogues
Because they have a different basicity than pyridine, these ring systems
can be found as both s and h6 bonded ligands:
P
P
Mo(CO)3
Mo(CO)5
Phosphorous and arsenic see the most employment as heteroatoms, due
to their stronger bonds and greater participation in the aromatic system
(which is weaker than nitrogen in pyridine).
This trend in stability is rounded out by the Bi analogue being unstable.
Pyrrole Analogues
Pyrrole analogues with heavier members of group 15 are also known.
These analogues can be dealkylated and form Cp-like anions. The trend
in p system participation is the same as seen for the pyridine derivatives:
decreasing down the group.
ER + NaH
RH +
+
E Na
These ligands, including pyrrole, have been shown to h5 coordinate to
transition metal centres after the fashion of Cp.
Multiple Bonding in Group 15
There were only two published structures of diarsenes
Mes*AsAsCH(SiMe3)2 and [As{C(SiMe3)3}]2 by Cowley and coworkers.
J. Am.Chem. Soc. 1983, 105, 5506.
J. Chem. Soc., Dalton Trans. 1985, 383.
A Bi=Bi unit
Deep purple crystals
Bi–Bi = 2.8206(8) Å, Bi–Bi–C = 100.5(2)°
(Bi–Bi single bond = 2.990(2) Å in Ph2Bi–BiPh2)
propose three orthogonal 6p orbitals without significant hybridization leads to
a bond angle of approx. 90°
Stable as a crystal even in air for several hours.
Tokitoh, N.; Arai, Y.; Okazaki, R.; Nagase, S. Science 1997, 277, 78.
Extended to include Sb analogues Sb=Sb 2.642(1) Å in:J. Am. Chem.
Soc. 1998, 120, 433.
Double Bonds in Group 15
Using large, protective ligands, Powers has made a series of compounds
RECl2 and reduced them with magnesium to give single and double
bonds:
LiAr + ECl3  ArECl2 + LiCl
J. Am. Chem. Soc. 1999, 121(14), 3363.
Group 15 E=E
Notably, the ligand either has Me (this example) or iPr groups on the
ortho-position on the flanking rings:
ArECl2 + Mg  ArE=EAr + MgCl2
Group 15 E=E
Dipnictenes are intensely colored - range from yellow or orange yellow
to deep red-purple
Two bands more intense p-p* and less intense n-p*
The p-p* band shifts to longer wavelength as the group is descended consistent with a weakening of the p bond
Mechanism
The mechanism isn’t clear:
+ Mg
REX(MgX)
+ REX2
-MgX
REX2
+ Mg
R(X)E-E(X)R
- MgX
- MgX
R(X)E
+ REX2, -X
or dimerization
Mechanism (cont’d)
R(X)E-E(X)R
+ REX2
REX(MgX)
-MgX2
-MgX2
- MgX2
+ Mg
"ER"
possible reaarangement
to other products
RE=ER
dimerization
Bonding
Note that the angle at the pnictogen atom in dipnictenes narrows from
ca.. 113.6(2) in PhNNPh to 102.8(9) in Mes*PPMes* and 100.5(2) in
Bi compounds.
Ar
E
Ar
Aryl
E
Aryl
We expect the s-orbital character of the lone pairs also
increases with increasing atomic number such that at Sb or Bi
the lone pairs have mostly s-orbital character.
Accordingly, the s and p bonds comprising the Sb=Sb and
Bi=Bi double bonds are mostly composed from p-orbitals.
Multiple Bonding in Group 15
Bi-Bi bond = 2.8769(5) Å in
(CO)5W(BiR)2 (R = CH2SiMe3)
Compare to Bi=Bi = 2.82-2.83 Å
Organometallics 2003, 22, 2919.
Multiple Bonding in Group 15
unprecedented
opportunity to
examine a
dibismuthene
“before” and “after”
coordination
Bi-Bi-C bond angles in 1 are 109.3(2) and 109.54(18) - greater
Bi-Bi bond distance in 1 is 3.1442(7) Å - lengthened
Bi-Zr bond lengths (2.9903(10) and 3.0044(11) Å) may only
be compared with 3.190 Å, the sum of the bismuth and zirconium
covalent radii
J. AM. CHEM. SOC. 2005, 127, 7672
Multiple Bonding in Group 15
Bond options
(1) Cp2Zr is a 14-electron fragment while (CO)5W is a 16electron moiety
(2) electron-rich Cp ligands tend to donate electron density to
transition metals, whereas the strong p-acid (electron accepting)
CO ligands decrease the electron density at the metal.