Heterobimetallic complexes of rhodium dibenzotetramethylaza[14]annulene [(tmtaa)RH-M]: Formation, structures, and bond dissociation energetics

Article abstract of DOI:10.1021/ic502414r

A rhodium(II) dibenzotetramethylaza[14]annulene dimer ([(tmtaa)Rh]₂) undergoes metathesis reactions with [CpCr(CO)₃]₂, [CpMo(CO)₃]₂, [CpFe(CO)₂]₂, [Co(CO)₄]₂, and [Mn(CO)₅]₂ to form (tmtaa)Rh-M complexes (M = CrCp(CO)₃, MoCp(CO)₃, FeCp(CO)₂, Co(CO)₄, or Mn(CO)₅). Molecular structures were determined for (tmtaa)Rh-FeCp(CO)₂, (tmtaa)Rh-Co(μ-CO)(CO)₃, and (tmtaa)Rh-Mn(CO)₅ by X-ray diffraction. Equilibrium constants measured for the metathesis reactions permit the estimation of several (tmtaa)Rh-M bond dissociation enthalpies (Rh-Cr = 19 kcal mol^-1, Rh-Mo = 25 kcal mol^-1, and Rh-Fe = 27 kcal mol^-1). Reactivities of the bimetallic complexes with synthesis gas to form (tmtaa)Rh-C(O)H and M-H are surveyed.

Full text of DOI:10.1021/ic502414r

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Article
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Heterobimetallic Complexes of Rhodium
Dibenzotetramethylaza[14]annulene [(tmtaa)Rh-M]: Formation,
Structures, and Bond Dissociation Energetics
Gregory H. Imler, Garvin M. Peters, Michael J. Zdilla,* and Bradford B. Wayland*
Department of Chemistry, Temple University, 1901 North 13th Street, Philadelphia, Pennsylvania 19122, United States
S
* Supporting Information
ABSTRACT: A rhodium(II) dibenzotetramethylaza[14]annulene
dimer ([(tmtaa)Rh]2) undergoes metathesis reactions with [CpCr-
(CO)3]2, [CpMo(CO)3]2, [CpFe(CO)2]2, [Co(CO)4]2, and [Mn-
(CO)5]2 to form (tmtaa)Rh-M complexes (M = CrCp(CO)3,
MoCp(CO)3, FeCp(CO)2, Co(CO)4, or Mn(CO)5). Molecular
structures were determined for (tmtaa)Rh-FeCp(CO)2, (tmtaa)Rh-
Co(μ-CO)(CO)3, and (tmtaa)Rh-Mn(CO)5 by X-ray diffraction.
Equilibrium constants measured for the metathesis reactions permit
the estimation of several (tmtaa)Rh-M bond dissociation enthalpies
(RhCr = 19 kcal mol−1, RhMo = 25 kcal mol−1, and RhFe = 27
kcal mol−1). Reactivities of the bimetallic complexes with synthesis gas to form (tmtaa)Rh-C(O)H and M-H are surveyed.
((tmtaa)Rh-M) produced by metathesis reactions of [(tmtaa)-
Rh]2 (1) with [CpCr(CO)3]2 (2), [CpMo(CO)3]2 (3),
[CpFe(CO)2]2 (4), [Co(CO)4]2 (5), and [Mn(CO)5]2 (6).
Minimum energy structure optimized density functional theory
(DFT) computations reproduce the primary structural features
of the bimetal complexes, including an unexpected semi-
bridging carbonyl in the molecular structure of (tmtaa)Rh-
Co(μ-CO)(CO)3.
INTRODUCTION
■
2−
The easily prepared, low cost N4 macrocyclic ligand
dibenzotetramethylaza[14]annulene dianion (tmtaa) has been
suggested as a substitute for porphyrins.1−3 A rhodium(II)
tmtaa dimer ([(tmtaa)Rh]2, 1) has recently been reported to
react with synthesis gas (H2/CO) in toluene and pyridine to
form large equilibrium concentrations of a metallo-formyl
complex ((tmtaa)Rh-C(O)H) (Figure 1).3 Rhodium(II)
RESULTS AND DISCUSSION
■
The closed shell dimers [CpCr(CO)3]2, [CpMo(CO)3]2, and
[Mn(CO)5]2, which have only terminal CO ligands, are
unambiguously described as having M-M bonds, but the
presence of bridging CO units complicates the description of
the bonding in [CpFe(CO)2]2 and [Co(CO)4]2.23,24 Each of
these dimers can homolyze to a metal-centered radical,25 and
for the purpose of unifying and simplifying the presentation in
this Article, all of the dimers (2−6) will be described as if the
M-M bond were present.
Figure 1. Reaction of [(tmtaa)Rh]2 with CO and H2.
porphyrins and related chelates and macrocycles provide
precedents for reactions of H2/CO that produce η1-carbon-
bonded formyl complexes.4−17 Several approaches attempting
to advance the utilization of this type of metal-formyl fragment
in the production of organic oxygenates invoke reactions that
occur at the formyl group, such as hydrogenation using a
second catalyst.18,19 Bimetal catalyst systems for alkene
hydroformylation that involve intermolecular processes illus-
trate the efficacy of this type of strategy.20−22 Heterobimetallic
complexes can be anticipated to occur in bimetal catalyst
systems as nonproductive traps for active metal catalyst species.
Reactivity studies of bimetallic compounds are important to
determine whether or not these intermetallic intermediates are
capable of returning the active catalyst units back into the
productive cycle.
Toluene solutions of the RhII-RhII bonded dimer (1)
undergo metathesis reactions with 2, 3, 4, 5, and 6 to produce
equilibrium distributions with heterobimetallic complexes of
the general form (tmtaa)Rh-M, where M = CpCr(CO)3 (7),
CpMo(CO)3 (8), CpFe(CO)2 (9), Co(CO)4 (10), and
Mn(CO)5 (11), respectively (eq 1).
[(tmtaa)Rh]2 + M‐M ⇌ 2(tmtaa)Rh‐M
(1)
Reactions of dimers 2−6 usually involve thermal or photo
homolysis to form monomeric d5, d7, and d9 metal-centered
This Article reports on the formation, structural features, and
dissociation energetics for a series of bimetallic complexes
Received: October 1, 2014
© XXXX American Chemical Society
A
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radicals. Thermal homolysis of compounds 1−6 into
monomers occurs with reported dissociation enthalpies ranging
from 15.5 to 33 kcal mol−1.2,26−28 Thermal reactions of the
more strongly bonded dimer species that pass through metallo-
radicals are slow because the dissociation enthalpies directly
contribute to the activation parameters. Kinetics of the dimer
reactions are often greatly increased by light-induced dimer
homolysis. Both the π to σ* and the σ to σ* electronic
transitions associated with M-M single-bonded complexes place
an electron in the antibonding M-M σ molecular orbital and
give photoinduced dimer homolysis.29
The observed 1H NMR line width (Δν1/2(obs)) is given as the
sum of the natural line width and the line width resulting from
dissociation into paramagnetic components (Δν1/2(obs)
=
Δν1/2(nat) + Δν1/2(ex)). Equation 5 gives the general relationship
of the contribution of this exchange to the NMR line width
(Δν1/2(ex)) with the electron−proton coupling constant (AH)
and the diamagnetic (τd) and paramagnetic lifetimes (τp).
Equation 5 simplifies to eq 6 (T2−1 = kex) for the case in which
each exchange event results in nuclear spin relaxation. This
limiting case occurs when the electron−nuclear coupling
constant (A) is large enough such that (Aτp/2)2 ≫ 1.
The lowest energy pathway for metathesis reactions of the
form in reaction 1 is most likely a radical chain reaction that
proceeds through an associative radical interchange (eqs 2,
3).30,31 The Rh(II) center in [(tmtaa)Rh]2 is a 5-coordinate,
16-electron site that can bind to M• and facilitate radical
interchange (eqs 2, 3).
πΔν1/2(ex) = T −1 = τ −1[(AHτp/2)2][1 + (AHτp/2)2]−1
2
d
(5)
(6)
πΔν1/2(ex) = T −1 = τ −1 = kex
2
d
The mean lifetime for the diamagnetic species (τd) that
results from the observed T2−1 yields the apparent rate constant
(2)
M‐M ⇌ 2M•
−1
(τd
= kex) for bond homolysis events that produce
paramagnetic species with efficient nuclear relaxation ((Aτp/
2)2 ≫ 1). Activation parameters for homolytic dissociation of
(tmtaa)Rh-CrCp(CO)3 were obtained by application of
M• + [(tmtaa)Rh]2 ⇌ M‐Rh(tmtaa) + (tmtaa)Rh•
(3)
transition state theory (K⧧ = kex(h/kT); −RT ln K⧧ = ΔG⧧
Formation and Reactions of (tmtaa)Rh-CrCp(CO)3.
ex
= ΔH⧧ − TΔS⧧).6,32−36 Using a Δν1/2(nat) of 1.0 Hz for the Cp
hydrogens of 7 yields the apparent activation parameters of
Reaction of [(tmtaa)Rh]2 (1) in toluene with a three-fold
1
excess of [CpCr(CO)3]2 (2) produces a H NMR-measurable
ΔH⧧ = 21 (1) kcal mol−1 and ΔS⧧ = 19.6 (1.2) cal K−1
equilibrium with the metathesis product CpCr(CO)3 (7) (K4 =
0.19 (0.05), ΔG4° = 0.98 kcal mol−1) (eq 4) in a period of
hours at 298 K in the absence of light.
ex,4
mol−41 for homolytic dissociation of (tmtaa)Rh-CrCp(CO)3
(Figure 3). Activation enthalpies for homolytic bond
[(tmtaa)Rh]2 + [CpCr(CO)3]2
⇌ 2(tmtaa)Rh‐CrCp(CO)3
(4)
Reaction 4 is relatively fast for this type of process when light
is absent. The dissociation free enthalpy for 2 is small (15.5 kcal
mol−1),27 which makes 2 a facile source of Cp(CO)3Cr•, but
reaction 4 is observed to be further accelerated by visible and
UV irradiation.
The temperature dependence of Δν1/2 (Hz) for the η5-
cyclopentadienyl ligand hydrogens in (tmtaa)Rh-CrCp(CO)3 is
1
shown in Figure 2. The observed increase in the η5-Cp H
NMR full line width at half height for (tmtaa)Rh-CrCp(CO)3
as the temperature is elevated is ascribed to lifetime broadening
that results from dissociation of the diamagnetic (tmtaa)Rh-
CrCp(CO)3 complex into paramagnetic (S = 1/2) metal-
centered radicals (tmtaa)Rh• and •CrCp(CO)3.
Figure 3. Determination of activation parameters for homolytic
dissociation of (tmtaa)Rh-CrCp(CO)3 into (tmtaa)Rh• and
1
(CO)3CpCr• in d8-toluene by H NMR line width measurements of
the cyclopentadienide hydrogens (ΔH‡4 = 21 (1) kcal mol−1, ΔS‡
=
ex,4
19.6 (1.2) cal K−1 mol−1).
dissociation (ΔH⧧app) in a low viscosity medium like toluene
are about 2 kcal mol−1 larger than the bond dissociation
enthalpies (BDE; ΔH°),34,35,37,38 which places the Rh-Cr BDE
in (tmtaa)Rh-CrCp(CO)3 at ∼19 kcal mol−1.
An estimate for the Rh-Cr BDE in (tmtaa)Rh-CrCp(CO)3 is
alternately obtained from the equilibrium studies in toluene for
reaction 4. Using ΔG4° (298 K) of 0.98 kcal mol−1, Rh(II)-
Rh(II) BDE for [(tmtaa)Rh]2 of 22 kcal mol−1, and
[CpCr(CO)3]2 Cr-Cr BDE of 15.5 kcal mol−1,27 and assuming
that ΔS4° is approximately zero, results in an estimate of 18 kcal
mol−1 for the Rh-Cr BDE compared to 19 kcal mol−1 estimated
by bond homolysis kinetics (Figure 3).
Exhaustive efforts to obtain single crystals of (tmtaa)Rh-
CrCp(CO)3 for X-ray diffraction were unsuccessful. The small
equilibrium constant for reaction 4 precluded obtaining
(tmtaa)Rh-CrCp(CO)3 in the absence of [(tmtaa)Rh]2 and
Figure 2. Temperature dependence of the 1H NMR line width for the
cyclopentadienyl ligand in (tmtaa)Rh-CrCp(CO)3.
B
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[CpCr(CO)3]2, which reduced the probability of obtaining a
crystal of 7. Fourier transform infrared spectroscopy (FTIR) for
a KBr pellet of the reaction mixture containing (tmtaa)Rh-
CrCp(CO)3 shows a new CO stretching band at 1766 cm−1
that is indicative of an intermetal-bridging CO unit. DFT
calculations39 were carried out using the B3LYP functional and
a mixed basis set (6-31G* for C, H, N, O, and Cr; 3-21G for
Rh) for (tmtaa)Rh-CrCp(CO)3. DFT computations for 7
starting from an all-terminal CO structure reached a computed
minimum energy structure that contains a bridging CO unit
consistent with the CO stretching frequency of 1766 cm−1
(Figure 4). Calculated Mulliken atomic charges from DFT
to the formyl complex (tmtaa)Rh-C(O)H is effectively
complete, and the (tmtaa)Rh-H concentration is too small
3
1
for observation by H NMR. Favorable thermodynamics for
reaction 9 also give high conversion of [CpCr(CO)3]2 to
40
Cp(CO)3Cr-H at a pressure of 0.5 atm of H2 at 298 K.
Formation and Reactions of (tmtaa)Rh-MoCp(CO)3.
Reaction of [(tmtaa)Rh]2 (1) with [CpMo(CO)3]2 (3) in the
absence of light slowly produces a 1H NMR observable
equilibrium with (tmtaa)Rh-MoCp(CO)3 (8) over a period of
days (eq 10). Reaction 10 is greatly accelerated by light and
reaches completion in a period of minutes when carried out in a
Rayonet RMR-400 photochemical reactor with a 300 nm light
source.
[(tmtaa)Rh]2 + [CpMo(CO)3]2
⇌ 2(tmtaa)Rh‐MoCp(CO)3
(10)
A BDE for Rh-Mo of 25 kcal mol−1 is estimated for 8 from
the equilibrium constant for reaction 10 at 298 K in toluene
(K10(298 K) = 7.8 × 10−2, ΔG10° (298 K) = 1.5 kcal mol−1) along
with the Rh-Rh and Mo-Mo BDEs for [(tmtaa)Rh]2 (22 kcal
mol−1)2 and [CpMo(CO)3]2 (30
3 kcal mol−1),26
respectively. The Rh-Mo BDE in (tmtaa)Rh-MoCp(CO)3 is
estimated at 25 3 kcal mol−1 assuming that ΔS10° is close to
zero. The Rh-Mo BDE in 8 is about 7 kcal mol−1 larger than
the Rh-Cr BDE in 7, which is in the range expected for second
transition series metal complexes compared to analogous first
transition series metal species. Minimum energy structural
optimization by DFT (B3LYP, 3-21G)39 anticipates a semi-
bridging carbonyl in complex 8 (Figure 5), and calculated
Mulliken atomic charges for this unit show a smaller positive
charge on the semibridging CO (C = 0.162, O = −0.312)
compared to that of the terminal CO units (Cavg = 0.201, Oavg
=
−0.305).
Solutions of 3 in equilibrium with 1 and 8 in toluene were
charged with 706 Torr of a 50:50 mixture of CO/H2, and the
Figure 4. Minimum energy structural optimization of (tmtaa)Rh-
CrCp(CO) by DFT (B3LYP, 3-21G/6-31G*) and a table of selected
internuclear distances.
show a smaller positive charge on the carbon of the
semibridging CO (C = 0.197, O = −0.317) compared to that
of the terminal CO units (Cavg = 0.257, Oavg = −0.320).
Toluene solutions of 7 with [(tmtaa)Rh]2 and [CpCr-
(CO)3]2 in a vacuum-adapted NMR tube were charged with 1.0
atm of a 1:1 molar mixture of H2 and CO, and changes in the
1
solution composition were followed by H NMR. At these
conditions, 7 is fully consumed to form (tmtaa)Rh-C(O)H and
[CpCr(CO)3]2 (eq 7). There are mutiple pathways to
accomplish this transformation, but the distribution of species
is defined by equilibria described by eqs 7−9.
(tmtaa)Rh‐CrCp(CO)3 + H2 + 1/2CO
⇌ (tmtaa)Rh‐C(O)H + H‐CrCp(CO)3
(7)
(tmtaa)Rh‐H + CO ⇌ (tmtaa)Rh‐C(O)H
(8)
[CpCr(CO)3]2 + H2 ⇌ 2Cp(CO)3Cr‐H
(9)
The equilibrium constant for the reaction of (tmtaa)Rh-H
with CO (eq 8) to produce a rhodium formyl complex
(tmtaa)Rh-C(O)H in toluene (K8(298 K) = (10.8 (0.3)) × 103,
ΔG8° (298 K) = −5.5 kcal mol−1) is sufficiently large such that
at a pressure of 0.5 atm of CO the conversion of (tmtaa)Rh-H
Figure 5. Minimum energy structural optimization of (tmtaa)Rh-
MoCp(CO)3 by DFT (B3LYP, 3-21G/6-31G*) and a table of selected
internuclear distances.
C
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1
subsequent slow reactions were followed by H NMR for a
period of weeks. When the sample reached equilibrium through
reactions 8, 11, and 12, compounds 3, (tmtaa)Rh-C(O)H, and
1
Cp(CO)3Mo-H were observed in the H NMR spectrum.
(tmtaa)Rh‐MoCp(CO)3 + H2 + 1/2CO
⇌ (tmtaa)RhC(O)H + Cp(CO)3Mo‐H
(11)
[CpMo(CO)3]2 + H2 ⇌ 2Cp(CO)3Mo‐H
(12)
The equilibrium constant for the reaction of [CpMo(CO)3]2
with H2 to form Cp(CO)2Mo-H (eq 12) was evaluated by
integration of the 1H NMR (K12(298 K) ≅ 1.7; ΔG12° (298 K) ≅
−0.3 kcal mol−1). An estimate of 67 (3) kcal mol−1 for the Mo-
H BDE in Cp(CO)2Mo-H is obtained by using a Mo-Mo BDE
of 30 (3) kcal mol−1 for [CpMo(CO)3]2 along with an H-H
BDE of 104 kcal mol−1 and using ΔH12° ≅ ΔG12° (298 K).26
This estimate of 67 (3) kcal mol−1 for the Mo-H BDE in
Cp(CO)2Mo-H compares favorably with prior published
estimates in the range of 66−69 kcal mol−1.41,42 The Mo-H
BDE is ∼7 kcal mol−1 larger than the Cr-H BDE in
Cp(CO)2M-H, which is in the range expected for second
versus first transition series metal hydrides.
The rhodium formyl complex ((tmtaa)Rh-C(O)H) is the
only observed product that contains the (tmtaa)Rh unit
because K8(298 K) is very large. Removing H2/CO gases from
solution by three freeze−pump−thaw cycles results in the
formyl complex completely reverting to a mixture of 1 and 8
during a period of one week, but Cp(CO)3Mo-H did not
decrease at an observable rate over this time. Reaction of
[MoCp(CO)3]2 with 700 Torr of a 50:50 mole fraction ratio of
CO/H2 in the absence of 1 did not produce 1H NMR-
observable concentrations of Cp(CO)3Mo-H over a period of
one week. Thermal reaction of [MoCp(CO)3]2 with H2/CO
must be catalyzed by the presence of a (tmtaa)Rh species such
as (tmtaa)Rh-H.
Figure 6. ORTEP representation of (tmtaa)Rh-FeCp(CO)2 (9)
structure from single crystal X-ray diffraction with thermal ellipsoids
shown on non-hydrogen atoms at 50% probability level, hydrogen
atoms shown as open circles, and energy minimization by DFT
computations (B3LYP, 3-21G/6-31G*). A table of selected inter-
nuclear distances observed in the molecular structure and calculated by
DFT is also included.
Cp ligand on iron and smaller dihedral angles (23.4°) on the
alternate half. These dihedral angles define the tilting of the
nonplanar saddle shape of the tmtaa macrocycle, and the
inequivalency here shows that there is more pronounced tilting
of the diiminato fragment on one-half of tmtaa to accommodate
the steric demands of the η5-Cp ligand on iron.
The equilibrium constant for reaction 13 was directly
1
evaluated by integration of the H NMR for each species in
reaction 13 (K13(298 K) = 0.19; ΔG13° (298 K) = 0.98 kcal
mol−1). An estimate of 27 (5) kcal mol−1 for the Rh-Fe BDE in
(tmtaa)Rh-FeCp(CO)2 is obtained by using the Rh-Rh BDE of
22 kcal mol−1 for [(tmtaa)Rh]2 and the electrochemical
estimate for the Fe-Fe BDE for [CpFe(CO)2]2 (33 5 kcal
mol−1)26 and assuming that ΔH13° ≅ ΔG13° = 0.98 kcal mol−1.
Formation of (tmtaa)Rh-Co(μ-CO)(CO)3. Reaction of
[(tmtaa)Rh]2 (1) with an excess of [Co(CO)4]2 (5) in toluene
occurs immediately and completely to form (tmtaa)Rh-Co(μ-
CO)(CO)3 (10). Excess [Co(CO)4]2 was removed by
evacuation of the toluene solution followed by vacuum
sublimation; however, subsequent IR (KBr pellet) showed
traces of dicobalt octacarbonyl remaining. The reverse reaction
of 10 to form 1 and 5 was not observed after removing most of
the excess [Co(CO)4]2, indicating a large equilibrium constant
for the formation of 10. Crystals of 10 were obtained from slow
diffusion of pentane into a toluene solution, and the molecular
structure was determined by X-ray diffraction. An important
structural feature of 10 is the presence of a semibridging
carbonyl ligand supported by a Co-Rh single bond. The
semibridging carbonyl results from the filled rhodium dπ
donating into the π* of CO (Figure 7). The dπ to π* donation
is reflected in the structure by an increased carbon-oxygen bond
length for the semibridging CO (1.169 Å) compared to the
average bond length of the terminal carbonyls (1.140 Å). DFT
calculations (B3LYP, 3-21G/6-31G*)39 for 10 converge to an
energy-minimized structure that mimics the observed bridging
CO unit (Figure 8). Analysis of the Mulliken atomic charges
from the DFT shows a smaller positive charge on the
Formation and Reactions of (tmtaa)Rh-FeCp(CO)2.
Toluene solutions of [(tmtaa)Rh]2 (1) in the absence of light
react very slowly with excess [η5-CpFe(CO)2]2 (4) over 18
days to produce an equilibrium distribution with the
heterobimetallic complex (tmtaa)Rh-FeCp(CO)2 (9) (eq 13).
Reaction 13 is accelerated in ambient laboratory light, and the
reaction is completed after irradiation for 10 min in a Rayonet
RMR-400 photochemical reactor with a 300 nm source.
[(tmtaa)Rh]2 + [CpFe(CO)2]2
⇌ (tmtaa)Rh‐FeCp(CO)2
(13)
Crystals of heterobimetallic complex in solutions of 9 were
grown from slow diffusion of pentane in toluene, and the
molecular structure was determined by single crystal X-ray
diffraction. An ORTEP diagram for 9 at the 50% probability
level is shown in Figure 6 along with an energy-minimized
structure from DFT computations (B3LYP, 3-21G/6-31G*).
The presence of a d7-d7 RhII-FeI bond (263 nm), an η5-Cp, and
all terminal CO ligands are the primary structural features
observed in the molecular structure. The energy-minimized
structure from DFT (B3LYP, 3-21G/6-31G*)39 computations
also shows all of the most important structural features for 9
(Figure 6).
Dihedral angles looking down the N-Cbenzene single bond of
tmtaa in 9 show an inequivalency present in the ligand with
larger dihedral angles (31.8°) in the half of tmtaa opposite the
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CO stretching mode centered at 1880 cm−1 diagnostic of
43,44
−
Co(CO)4 .
(tmtaa)Rh‐Co(μ‐CO)(CO)3 + 2pyr
⇌ [(tmtaa)Rh(pyr)2]+ + [Co(CO)4]−
(14)
Solutions of 10 were charged with 200 Torr of ethene,
resulting in the formation of an insoluble product that formed
crystals in an NMR tube that were then used to obtain the
molecular structure by X-ray diffraction (Figure 9). The
Figure 7. Drawing of the semibridging carbonyl in (tmtaa)Rh-Co(μ-
CO)(CO)3 showing the donation of rhodium dπ into the CO π*
orbital.
Figure 9. Crystal structure of ionic bridged ethyl complex and drawing
of cationic (tmtaa)Rh fragment with bond distances labeled in
angstroms.
product of the reaction of 10 with ethene is an ionic species
with a cationic (tmtaa)Rh fragment, in which a CH2CH2 group
bridges the rhodium and methine carbon centers, and a
[Co(CO)4]− counteranion. The diiminato fragment bridged to
rhodium lost the delocalized π system, which is demonstrated
in the structure by shorter N-C double bonds and longer C-C
single bonds as shown in Figure 9. The nucleophilicity of the
methine position was previously reported by Cotton.45
Formation and Reactions of (tmtaa)Rh-Mn(CO)5.
Reaction of [(tmtaa)Rh]2 (1) with an excess of [Mn(CO)5]2
(6) in toluene proceeds in the dark over several weeks to
achieve equilibrium with (tmtaa)Rh-Mn(CO)5 (11) (eq 15), or
11 may be formed much more rapidly by photolysis (Rayonet
RMR-400 photochemical reactor, 300 nm source).
Figure 8. ORTEP representation of the (tmtaa)Rh-Co(CO)4 structure
from single crystal X-ray diffraction with thermal ellipsoids shown on
non-hydrogen atoms at 50% probability level, hydrogen atoms shown
as open circles, and energy minimization by DFT computations
(B3LYP, 3-21G/6-31G*). A table of selected internuclear distances
observed in the molecular structure and calculated by DFT is also
included.
[(tmtaa)Rh]2 + [Mn(CO)5]2 ⇌ 2(tmtaa)Rh‐Mn(CO)5
(15)
The molecular structure of 11 determined by X-ray
diffraction shows only terminal CO ligands (Figure 10). The
carbon monoxide trans to rhodium has shorter Mn-C (0.06 Å)
and longer C-O (0.01 Å) bond lengths compared to that of the
cis-oriented carbonyl, which is ascribed to larger π backbonding
for the Mn-CO unit trans to rhodium. Infrared spectroscopy
also shows only terminal CO stretches for 11 (1950−2150
cm−1).
semibridging carbonyl (C = 0.206, O = −0.305) compared to
that of the terminal carbonyls (Cavg = 0.308, Oavg = −0.281),
which is consistent with the bonding model. The infrared
spectrum of 10 in a KBr pellet has a peak at 1820 cm−1 that is
attributed to the CO stretching vibration of the semibridging
carbonyl. The analogous porphyrin complex (OEP)Rh-Co-
(CO)4 shows only terminal CO stretches (νCO = 2054, 1996,
and 1965 cm−1) in the infrared spectrum.
SUMMARY AND CONCLUSIONS
■
Transition metal complexes that are sources of d5, d7, and d9
17-electron metal-centered radicals, including [CpCr(CO)3]2,
[CpMo(CO)3]2, [CpFe(CO)2]2, [Co(CO)4]2, and [Mn-
(CO)5]2, react with rhodium(II) dibenzotetramethylaza[14]-
annulene dimers to form (tmtaa)Rh-M complexes (M =
CrCp(CO)3, MoCp(CO)3, FeCp(CO)2, Co(CO)4, and Mn-
(CO)5). The molecular structures for (tmtaa)Rh-FeCp(CO)2,
(tmtaa)Rh-Co(μ-CO)(CO)3, and (tmtaa)Rh-Mn(CO)5 were
determined by single crystal X-ray diffraction. The structure of
(tmtaa)Rh-Co(μ-CO)(CO)3 has a semibridging carbon mon-
oxide supported by a Rh-Co bond. DFT calculations correctly
mimic the observed bridging CO in (tmtaa)Rh-Co(μ-CO)-
Solutions of 10 in pyridine with a small excess of
[Co(CO)4]2 immediately disproportionate into [(tmtaa)Rh-
(pyr)2]+ and the cobalt tetracarbonyl anion (eq 14).
1
Disproportionation is not observed by H NMR for [(tmtaa)-
Rh]2 in pyridine, but 1 reacts via heterolytic pathways when
treated with pyridinium chloride to form [(tmtaa)Rh-
(pyr)2]+[Cl]− and (tmtaa)Rh-H. The cationic fragment
[(tmtaa)Rh(pyr)2]+ shows an identical set of peaks with either
cobalt tetracarbonyl or chloride anion, and the cobalt
tetracarbonyl anion is observed by the FTIR (KBr pellet) T2
E
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constants evaluated for the metathesis reactions show that the
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form (tmtaa)Rh-CHO and M-H derivatives, which makes these
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
Corresponding Authors
*E-mail: mzdilla@temple.edu.
*E-mail: bwayland@temple.edu.
■
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENTS
■
This research was supported by the National Science
Foundation through Grant CHE-1362016 and assisted by
major research instrumentation Grant CNS-09-58854.
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