The Butterfly Dimer [(tBu3SiO)Cr]2 (μ-OSitBu3)2 and Its Oxidative Cleavage to (tBu3SiO)2 Cr(=N-N=CPh2)2 and (tBu3SiO)2 Cr=N(2,6-Ph2-C6H3) Page: 2,012
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Sydora et al.
Table 3. Comparison of Interatomic d(Cr...Cr) (A) and UEff (fB at 298 K) for Various Cr(II) Dimers
compound d(Cr.* Cr) M/eff ref{ [(Ph2PCH2SiMe2)2N]Cr}2(u-H)2
[CpCr(u-OtBu)]2
[(tBu3SiO)Cr]2(u-OSitBu3)2 (12)
[(Cy2N)Cr]2(u-NCy2)2
{ [(Ad)(3.5-Me2C6H3)N]2Cr}2( -N(Ad)(3,5-Me2C6H3))2
[('Pr2N)Cr]2(u-NPr2)2
[(thf)(Ph2N)Cr]2(u-NPh2)2
{ [(Ph2PCH2SiMe2)2N]Cr}2 u-Cl)2Table 4. Calculated Relative Energies (kcal/mol) of [(HO)Cr]2(u-OH)2
(1'2), a Model for (tBu3SiO)Cr]2(u-OSitBu3)2 (12), in Different Spin
States with the Calculated Magnetic Momentelectronic state
'A1
3B1
5A1
7B1
9A1MCSCF
(single shell)
basis basis
set I set II
0.00 0.00
0.29 0.29
0.87 0.85
1.73 1.68
2.84 2.76MCSCF
(double shell)
basis
set I
0.00
0.35
1.07
2.13
3.56MRMP2
(single shell)
basis basis
set I set II
0.00 0.00
0.44 0.44
1.32 1.32
2.68 2.63
4.58 4.482.64 2.1 19
2.65
2.64, 2.68
2.84
2.85
2.87
3.15
3.642.6
2.8
2.6
2.5
2.3
3.6
6.520
this work
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21
22
21
19[(tBu3SiO)Cr]2(u-OSitBu3)2 Cleavage by L. 1. (tBu3SiO)2-
CrL2 (1-L2). The Cr(II) dimer 12 proved to be susceptible
to cleavage. Treatment of 12 with various L (L = 4-picoline,4
p-tolunitrile, tBuCN, tBuNC, Ph2CO, and PMe3) afforded
(tBu3SiO)2CrL2 (1-L2) compounds, even with less than a
stoichiometric amount. To cleanly form 1-L2 from 12,
typically 4 equiv of the donor were used in pentane solvent,
as indicated in eq 2. Yellow crystalline trans-(tBu3SiO)2Cr-
(NCC6H4-p-Me) (1-(NCtol)2)[(tBu3SiO)Cr]2(l-OSitBu3)2 + 4L 3C
,ueff (in ,>B) 3.1 3.2 2.8 2.4 2.4 pentane
12Table 5. Active Space MOs Electronic Occupation for
[(HO)Cr]2(u-OH)2 (1'2), a Model for [(tBu3SiO)Cr]2(u-OSitBu3)2 (12),
Calculated with MCSCF Basis Set I
number of doubly occupied molecular orbitals
electronic state 4 3 2 1 0
'A1 27% 9% 53% 1% 4%52% 14%
- 85%23%
4%
100%9%
100%the major contribution to magnetic moment is from the triplet
and quintet states in a roughly 2:1 ratio: 28% for 'A1, 44%
for 3B1, 22% for 5A,, 5% for 7B1, and 1% for 9A,. It is
reassuring to note that the double-shell MCSCF (Basis Set
I) calculation reproduces the room-temperature moment (Ueff
= 2.8 MB) as observed experimentally and that all methods
provided reasonable calculated values for 1eff (2.4-3.2 MB),
as Table 4 indicates.
Since the Meff vs Tmeasurement of 12 was conducted, the
Boltzmann distribution of states was calculated as a function
of temperature, and the resulting average Meff vs T plot is
given in Figure 2b. The calculations manifest a roughly
similar profile of antiferromagnetism from 300 to 150 K,
but the 'A, state is basically the only one occupied at T <
20 K. The calculations, which were done at the single-shell
level for expediency, suggest that the description of the
antiferromagnetic coupling in 12 is inappropriately described
by a single J and that the observed moment of -1.1 MB at 0
K may indeed be due to impurities.
The natural orbital population shows different occupa-
tions for the active space MOs, as indicated in Table 5. This
variety of electron configurations is possible because the d
orbitals of Cr atoms are densely packed in 1'2. Interestingly,
the orbital occupancy for the 'A,, 3B,, and 5A, states suggests
a significant antiferromagnetic arrangement of the two
chromium d electrons, consistent with the observed magne-
tism.2(tBu3SiO)2CrL2
1-L2, L = 4-pic (>90%, NMR),
NCC6H4-p-Me (93%), (2)
tBuCN (~60%), tBuNC (83%),
Ph2CO (66%), PMe3
was assigned a D2h structure on the basis of a single IR
absorption at 2258 cm-1 corresponding to v(CN) and 1H
NMR resonances consistent with equivalent silox and p-
tolunitrile ligands. Its magnetic moment at room temperature
was 4.8 MB, consistent with a chromous center. Pink trans-
(tBu3SiO)2Cr(NCtBu)2 (1-(NCtBu)2), which was only pre-
pared on a small scale, showed related spectral characteris-
tics, including a 2272 cm-1 band in its IR spectrum and 1H
NMR resonances at 6 3.04 (v1/2 670 Hz) and 6 5.73 (v1/2
230 Hz) that occur with a 3:1 intensity ratio.
Orange (tBu3SiO)2Cr(CNtBu)2 (1-(CNtBu)2) exhibited one
broad resonance in the 1H NMR at 6 3.17 (v1/2 700 Hz),
but an upfield shoulder was apparent with an estimated
intensity consistent with the expected 3:1 ratio. Both Nujol
mull and solution IR spectra revealed a v(CN) absorption at
2192 cm-1 with a minor band at 2099 cm-'. Since tBuNC
is a strong a-donor and :t-acceptor and possesses a cor-
respondingly strong trans-influence, a cis-square planar
geometry is conceivable, but steric influences clearly favor
a trans arrangement; perhaps a mixture of these isomers is
evident. Alternately, the strong a-donating capability of
tBuNC could also lead to a distortion from square planar
geometry that would produce IR-active symmetric and
antisymmetric stretches. Recrystallizations did not appear to
change the IR; hence, minor impurities seem less likely but
cannot be ruled out. The room-temperature magnetic moment
of 1-(CNtBu)2 is 4.8 MB, consistent with a high-spin Cr(II)
center.
The intense blue color of (tBu3SiO)2Cr(PMe3)2 (1-(PMe3)2)
was distinct from the other derivatives, including the2012 Inorganic Chemistry, Vol. 45, No. 5, 2006
3B1
5A1
7B1
9A1
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Sydora, Orson L.; Kuiper, David S.; Wolczanski, Peter T.; Lobkovsky, Emil B.; Dinescu, Adriana & Cundari, Thomas R., 1964-. The Butterfly Dimer [(tBu3SiO)Cr]2 (μ-OSitBu3)2 and Its Oxidative Cleavage to (tBu3SiO)2 Cr(=N-N=CPh2)2 and (tBu3SiO)2 Cr=N(2,6-Ph2-C6H3), article, January 12, 2006; [Washington, DC]. (https://digital.library.unt.edu/ark:/67531/metadc75425/m1/5/: accessed March 28, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT College of Arts and Sciences.