Electronic properties of 4,4′,5,5′-tetramethyl-2,2′-biphosphinine (tmbp) in the redox series fac-[Mn(Br)(Co)3(tmbp)], [Mn(Co)3(tmbp)]2, and [Mn(Co)3(tmbp)]-: Crystallographic, spectroelectrochemical, and DFT computational study

Article abstract of DOI:10.1021/ic0206894

Stepwise electrochemical reduction of the complex fac-[Mn(Br)(CO)₃(tmbp)] (tmbp = 4,4′,5,5′-tetramethyl-2,2′-biphosphinine) produces the dimer [Mn(CO)₃(tmbp)]₂ and the five-coordinate anion [Mn(CO)₃(tmbp)]^-. All three members of the redox series have been characterized by single-crystal X-ray diffraction. The crystallographic data provide valuable insight into the localization of the added electrons on the (carbonyl)manganese and tmbp centers. In particular, the formulation of the two-electron-reduced anion as [Mn⁰(CO)₃(tmbp^-)]^- also agrees with the analysis of its IR v(CO) wavenumbers and with the results of density functional theoretical (DFT) MO calculations on this compound. The strongly delocalized π-bonding in the anion stabilizes its five-coordinate geometry and results in the appearance of several mixed Mn-to-tmbp charge-transfer/IL(tmbp) transitions in the near-UV-vis spectral region. A thorough voltammetric and UV-vis/IR spectroelectrochemical study of the reduction path provided evidence for a direct formation of [Mn(CO)₃(tmbp)]^- via a two-electron ECE mechanism involving the [Mn(CO)₃(tmbp)]^., radical transient. At ambient temperature [Mn(CO)₃(tmbp)]^- reacts rapidly with nonreduced fac-[Mn(Br)(CO)₃(tmbp)] to produce [Mn(CO)₃(tmbp)]₂. Comparison with the analogous 2,2′-bipyridine complexes has revealed striking similarity in the bonding properties and reactivity, despite the stronger π-acceptor character of the tmbp ligand.

Full text of DOI:10.1021/ic0206894

Inorg. Chem. 2003, 42, 4442−4455
Electronic Properties of 4,4′,5,5′-Tetramethyl-2,2′-biphosphinine (tmbp)
in the Redox Series fac-[Mn(Br)(CO)3(tmbp)], [Mn(CO)3(tmbp)]2, and
[Mn(CO)3(tmbp)]-: Crystallographic, Spectroelectrochemical, and DFT
Computational Study
Frantisˇek Hartl* and Taasje Mahabiersing
Institute of Molecular Chemistry, UniVersiteit Van Amsterdam, Nieuwe Achtergracht 166,
1018 WV Amsterdam, The Netherlands
Pascal Le Floch,* Francois Mathey, Louis Ricard, and Patrick Rosa
Laboratoire “He´te´roe´le´ments et Coordination”, URA CNRS 1499, DCPH, Ecole Polytechnique,
91128 Palaiseau Cedex, France
Stanislav Za´lisˇ
J. HeyroVsky´ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic,
DolejsˇkoVa 3, 182 23 Prague 8, Czech Republic
Received November 25, 2002
Stepwise electrochemical reduction of the complex fac-[Mn(Br)(CO)3(tmbp)] (tmbp ) 4,4′,5,5′-tetramethyl-2,2′-
biphosphinine) produces the dimer [Mn(CO)3(tmbp)]2 and the five-coordinate anion [Mn(CO)3(tmbp)]-. All three
members of the redox series have been characterized by single-crystal X-ray diffraction. The crystallographic data
provide valuable insight into the localization of the added electrons on the (carbonyl)manganese and tmbp centers.
0
In particular, the formulation of the two-electron-reduced anion as [Mn (CO)3(tmbp-)]- also agrees with the analysis
of its IR ν(CO) wavenumbers and with the results of density functional theoretical (DFT) MO calculations on this
compound. The strongly delocalized π-bonding in the anion stabilizes its five-coordinate geometry and results in
the appearance of several mixed Mn-to-tmbp charge-transfer/IL(tmbp) transitions in the near-UV−vis spectral region.
A thorough voltammetric and UV−vis/IR spectroelectrochemical study of the reduction path provided evidence for
a direct formation of [Mn(CO)3(tmbp)]- via a two-electron ECE mechanism involving the [Mn(CO)3(tmbp)]• radical
transient. At ambient temperature [Mn(CO)3(tmbp)]- reacts rapidly with nonreduced fac-[Mn(Br)(CO)3(tmbp)] to
produce [Mn(CO)3(tmbp)]2. Comparison with the analogous 2,2′-bipyridine complexes has revealed striking similarity
in the bonding properties and reactivity, despite the stronger π-acceptor character of the tmbp ligand.
Introduction
efficient inter- and intramolecular energy- and electron-trans-
fer processes.2 By contrast, only a few analogous Mn(I)
compounds [Mn(L)m(CO)4-m(R-diimine)]n (m ) 1-3; n )
0, +1) have been prepared and studied so far, even though
Since the mid-1970s the coordination chemistry of Re(I)
carbonyls with chelated R-diimine ligands such as 2,2′-
bipyridine (bpy), [Re(L)m(CO)4-m(R-diimine)]n (m ) 0-2;
n ) 0, +1; L ) e.g. donor solvent, halide, phosphine), has
received wide interest. Many of these complexes have been
extensively investigated owing to their electrocatalytic
properties in CO2 reduction1 and ability to participate in
(1) See for example: (a) Johnson, F. P. A.; George, M. W.; Hartl, F.;
Turner, J. J. Organometallics 1996, 15, 3374-3387. (b) Christensen,
P. A.; Hamnett, A.; Muir, A. V. G.; Timney, J. A. J. Chem. Soc.,
Dalton Trans. 1992, 1455-1463. (c) Calzaferri, G.; Hadener, K.; Li,
J. W. J. Photochem. Photobiol., A 1992, 64, 259-262. (d) Koike, K.;
Hori, H.; Ishizuka, M.; Westwell, J. R.; Takeuchi, K.; Ibusuki, T.;
Enjouji, K.; Konno, H.; Sakamoto, K.; Ishitani, O. Organometallics
1997, 16, 5724-5729.
* Authors to whom correspondence should be addressed. E-mail:
hartl@science.uva.nl (F.H.); lefloch@poly.polytechnique.fr (P.L.F.).
4442 Inorganic Chemistry, Vol. 42, No. 14, 2003
10.1021/ic0206894 CCC: $25.00 © 2003 American Chemical Society
Published on Web 06/12/2003

4,4′,5,5′-Tetramethyl-2,2′-biphosphinine
their photochemistry and redox reactivity are equally chal-
lenging.3,4 Whereas light excitation into their metal-to-diimine
charge-transfer transitions typically initiates CO-loss and/or
isomerization reactions,3c,d,4a,h,i their largely diimine-based
electrochemical reduction often results in the cleavage of a
Mn-L bond.3g,5 For strongly bound L such as P(OMe)3 or
CNBut the reversibility of the reduction process increases
with the larger substitution number m.4c,5b In this case,
instead, reductive isomerization of the various cis/cis and
fac isomers (for given m) can occur.4c,g
In the Amsterdam group, fundamental studies of the pho-
tochemistry, photophysics, and redox properties of transition
metal R-diimine carbonyls, including the Re(I) and Mn(I)
species,2d,3,5,6 have recently been extended to the class of
coordination compounds with 2,2′-biphosphinine and 1,4-
diphospha-1,3-butadiene ligands, the phosphorus equivalents
of widely employed 2,2′-bipyridine or 1,4-diaza-1,3-butadi-
ene, respectively. This area is still largely unexplored, even
though the coordination chemistry of phosphinines is a
rapidly developing field with a promising future.7 The first
joint study with the Palaiseau group8 focused on the novel
intriguing transition metal clusters [Os3(CO)10-m(tmbp)(PPh3)m]
(m ) 0, 1; tmbp ) 4,4′,5,5′-tetramethyl-2,2′-biphosphinine,
available9 since 1991). These compounds possess structural,
bonding, and physicochemical properties markedly different
from those of the analogous clusters with the significantly
more basic 2,2′-bipyridine and other R-diimine ligands. Most
remarkably, the tmbp ligand is bound in a doubly bridging
fashion as dianion to the electron-deficient cluster core with
one Os-Os bond split. Differently from the 2,2′-bipyridine
derivative, [Os3(CO)10(tmbp)] is photostable and its electro-
chemical reduction ultimately results in a CO-loss reaction
rather than fragmentation of the cluster core. Obviously, sys-
tematic comparative electro- and photochemical investiga-
tions of corresponding biphosphinine and bipyridine transi-
tion metal complexes are needed to understand the mutual
differences and to illustrate the potential of the former com-
pounds in the areas of physical coordination chemistry that
have been so far the domain of their R-diimine analogues.
In conformity with this approach, in this article we present
results of the combined crystallographic, spectroelectro-
chemical, and theoretical study of a three-member redox
series based on the complex fac-[Mn(Br)(CO)3(tmbp)]. The
data are compared with those for the corresponding 2,2′-
bipyridine complex and its reduction products reported
elsewhere.4c,5,10,11 The synthesis of fac-[Mn(Br)(CO)3(tmbp)]
was first described by Mathey and Le Floch in 1996.12 Its
two-electron reduction is shown in this work to produce the
anion [Mn(CO)3(tmbp)]- that belongs to the remarkable
family of formally 16e complexes with five-coordinate
geometry stabilized by strong π-bonding in the metalla-
cycle.13 In the literature [Mn(CO)3(tmbp)]- has been identi-
fied in the [Mn2(CO)6(tmbp)] adduct with the {Mn(CO)3}+
fragment, whose structure has been established by X-ray
crystallography.14 The thorough description of the bonding
properties of [Mn(CO)3(tmbp)]-, supported by the density
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F. Coord. Chem. ReV. 2001, 216, 127-154. (d) Stufkens, D. J.; Vlcˇek,
Jr., A. Coord. Chem. ReV. 1998, 177, 127-179. (e) Ziessel, R.; Juris,
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Inorganic Chemistry, Vol. 42, No. 14, 2003 4443

Hartl et al.
Synthesis of [Mn(CO)3(tmbp)]- (4) as Sodium Salt. A solution
of sodium naphthalenide (10 mL, 0.8 mmol) in DME was added
in a glovebox to 2 (186 mg, 0.4 mmol). The resulting red-brown
solution was stirred for 5 min. The solvent was then removed in
vacuo. The red solid obtained was washed with Et2O (5 mL) and
dried overnight at room temperature to sublime off any remaining
naphthalene. Single crystals suitable for X-ray diffraction analysis
were grown inside a sealed tube filled with a ca. 10 mg of anion
4 in 2 mL of Et2O, heated to 35 °C on a water bath, and allowed
to cool overnight. Yield: ca. 60%. Complex 4 is too air-sensitive
to give satisfactory results of elemental analysis.
functional theoretical (DFT) MO treatment (notably, with
ADF2000.2 and Gaussian 98 software packages), has been
one of the major goals of this study.
Experimental Section
All reactions and experiments were performed under an atmo-
sphere of dry nitrogen or argon, using Schlenk techniques.
Solvents of analytical grade (all Acros) were freshly distilled
from sodium benzophenone (hexane, diethyl ether (Et2O), tetrahy-
drofuran (THF), and dimethoxyethane (DME)), sodium wire
(toluene), CaH2 (butyronitrile (PrCN)), and P2O5 (CH2Cl2). Dioxane-
d6, CD2Cl2, and CDCl3 were purchased in sealed ampules, opened
in a glovebox, and dried over 4 Å Linde molecular sieves.
The ligand 4,4′,5,5′-tetramethyl-2,2′-biphosphinine (tmbp; 1) was
synthesized according to the reported procedure.9 The precursor
complex [Mn(CO)5Br] was prepared by bromination of [Mn2(CO)10],
as described elsewhere. Ph3SnCl (Acros) was purified by sublima-
tion. The supporting electrolyte tetrabutylammonium hexafluoro-
phosphate (Bu4NPF6; Aldrich) was recrystallized twice from
absolute ethanol and dried overnight under vacuum at 353 K for
12 h before use.
Synthesis of fac-[Mn(Br)(CO)3(tmbp)] (2). Toluene (20 mL)
was added to [Mn(CO)5Br] (0.28 g, 1 mmol) and tmbp (1) (0.25 g,
1 mmol). The resulting pale yellow solution was heated at 60 °C
for 2 h. A precipitate formed during the reaction. It was collected
by filtration, washed with hexane (10 mL) and Et2O (10 mL), and
then dried in vacuo. Complex 2 was obtained as a pale-orange solid.
Yield: 0.42 g (90%).
1
31P NMR (C4D8O): δ 231.30. H NMR (C4D8O): δ 2.47 and
2.50 (s, 12H, 4 Me), 8.19 (vt, AA′XX′, 2H, ΣJ(P-H) ) 25.4 Hz,
H
3,3′), 8.44 (vt, AA′XX′, 2H, ΣJ(P-H) ) 17.45 Hz, H6,6′). 13C NMR
(C4D8O): δ 24.3 and 26.1 (s, Me), 122.7 (vt, AXX′, ΣJ(P-C) )
15.3 Hz, C5,5′ or C4,4′), 126.6 (vt, AXX′, ΣJ(P-C) ) 19.7, C3,3′ or
C6,6′), 135.5 (vt, AXX′, ΣJ(P-C) ) 20.4 Hz, C6,6′ or C3,3′), 140.0
(vt, AXX′, ΣJ(P-C) ) 11.9 Hz, C5,5′ or C4,4′), 142.3 (vt, AXX′,
ΣJ(P-C) ) 73.6 Hz, C2,2′), 234.7 (bs, COs).
Synthesis of fac-[Mn(SnPh3)(CO)3(tmbp)] (5). Ph3SnCl (155
mg, 0.4 mmol) was added at room temperature to a solution of
anion 4 (0.4 mmol) in DME, prepared as described above. After
10 min, the color of the solution turned red. The solvent was
evaporated and the red solid obtained was washed with hexane (3
× 10 mL) and dissolved in CH2Cl2 (20 mL). The solution was
filtrated over dry Celite and evaporated to dryness to yield
analytically pure complex 5. Yield: 147 mg (50%).
1
31P NMR (CD2Cl2): δ 230.95. H NMR (CD2Cl2): δ 2.44 and
CAUTION! The solutions of light-sensitive 2 must be kept in
the dark to avoid photochemical decomposition via formation of
[Mn(CO)3(tmbp)]2 (3), as was testified by IR spectroscopy. Irradia-
tion of complex 2 (ca. 10-2 M) in THF at 293 K with filtered 450
nm light of a medium-pressure Hg lamp results in complete
photodecomposition within less than 1 min.
31P NMR (CDCl3): δ 225.1. 1H NMR (CDCl3): δ 2.52 (s, 12H,
4 Me), 8.28 (vd, AA′XX′, 2H, ΣJ(P-H) ) 23.3 Hz, H3,3′), 8.50
(vd, AA′XX′, 2H, ΣJ(P-H) ) 24.8 Hz, H6,6′). 13C NMR (CDCl3):
δ 22.9 and 24.4 (s, Me), 131.4 (vt, AXX′, ΣJ(P-C) ) 49.3 Hz,
2.46 (s, 12H, 4 Me of C14H16P2), 6.97-7.17 (m, 15H, H of SnPh3),
8.14 (vd, AA′XX′, 2H, ΣJ(P-H) ) 17.0 Hz, H3,3′ or H6,6′ of
C14H16P2), 8.20 (vd, AA′XX′, 2H, ΣJ(P-H) ) 23.4 Hz, H6,6′ or
H
3,3′ of C14H16P2). 13C NMR (CD2Cl2): δ 22.6 (s, Me of C14H16P2),
24.3 (vt, AXX′, ΣJ(P-C) ) 9.6 Hz, Me of C14H16P2), 127.5 (s,
4J(Sn-C) ) 10.1 Hz, HCpara of SnPh3), 127.7 (s, 3J(Sn-C) ) 41.8
Hz, HCmeta of SnPh3), 130.1 (AXX′, ΣJ(P-C) ) 12.7 Hz, C3,3′ of
C14H16P2), 134.4 (AXX′, ΣJ(P-C) ) 22.6 Hz, C5,5′ or C4,4′ of
C14H16P2), 136.8 (s, 2J(Sn-C) )34.5 Hz, HCortho of SnPh3), 142.1
(AXX′, ΣJ(P-C) ) 29.4 Hz, C6,6′ of C14H16P2), 143.8 (t, J(P-C)
) 1.5 Hz, Cipso of SnPh3), 147.2 (vt, AXX′, ΣJ(P-C) ) 12.6 Hz,
C4,4′ or C5,5′ of C14H16P2), 152.0 (vt, AXX′, ΣJ(P-C) ) 71.9 Hz,
C3,3′), 138.3 (vt, AXX′, ΣJ(P-C) ) 29.1 Hz, C5,5′ or C4,4′), 144.6
(vt, AXX′, ΣJ(P-C) ) 21.0 Hz, C4,4′ or C5,5′), 147.9 (vt, AXX′,
ΣJ(P-C) ) 16.0 Hz, C6,6′), 155.2 (vt, AXX′, ΣJ(P-C) ) 64.8 Hz,
C2,2′ of C14H16P2). IR (KBr): ν(CO) 1997 (s), 1932 (s, br) cm-1
.
C
2,2′), 219.3 (bs, COs). IR (CH2Cl2): ν(CO) 2041 (s), 1987 (m),
MS-IE m/z (assignment; relative intensity): 734/736 (M+ for
118Sn and 120Sn; 2), 650/652 (M+ - 3CO for 118Sn and 120Sn; 4),
377 (100). Anal. Calcd for C35H31MnO3P2Sn: C, 57.18; H, 4.25.
Found: C, 57.05; H, 4.28.
1950 (m) cm-1. Anal. Calcd for C17H16BrMnO3P2: C, 43.90; H,
3.47. Found: C, 43.91; H, 3.36.
Synthesis of [Mn(CO)3(tmbp)]2 (3). A solution of sodium
naphthalenide (5 mL, 0.4 mmol) in DME was added in a glovebox
to complex 2 (186 mg, 0.4 mmol). The resulting red solution was
stirred for 5 min. The volume was then reduced, and the solution
was left standing overnight. The violet precipitate was then collected
by filtration and washed with dry hexane (2 × 5 mL). In order to
remove NaBr, complex 3 was extracted with CH2Cl2 (5 mL).
Heating complex 3 in THF in a sealed tube at 60 °C overnight
gave dark-red crystals suitable for X-ray diffraction analysis.
Yield: ca. 80%.
31P NMR (dioxane-d8): δ 248.2. 1H NMR (dioxane-d8): δ 2.40
and 2.47 (s, 12H, 4 Me), 7.48 (m, AA′XX′, 2H, ΣJ(P-H) ) 23.5
Hz, H3,3′), 7.90 (m, AA′XX′, 2H, ΣJ(P-H) ) 18.0 Hz, H6,6′). IR
(KBr): ν(CO) 2001 vs, 1958 (s), 1936 (s-m), 1904.5 (s) cm-1. Anal.
Calcd for C34H32Mn2O6P4: C, 53.01; H, 4.19. Found: C, 52.60;
H, 4.35.
Spectroscopic Measurements. NMR spectra were recorded on
a Bruker AC-200 SY spectrometer operating at 200.12 MHz for
1H, 50.32 MHz for 13C, and 81.91 MHz for 31P nuclei. Solvent
peaks are used as an internal reference relative to Me4Si at 0 ppm
1
for the H and 13C chemical shifts (ppm); 31P chemical shifts are
relative to H3PO4 (85%) external reference. All listed coupling
constants are in hertz. Abbreviations used: s ) singlet, d ) doublet,
t ) triplet, m ) multiplet, v ) virtual, b ) broad. IR spectra were
obtained with a Bio-Rad FTS-7 FTIR spectrometer (16 scans,
resolution of 2 cm-1), UV-vis spectra with a software-updated
Perkin-Elmer Lambda 5 spectrophotometer, and mass spectra with
a Shimadzu GC-MS QP 1000 spectrometer (at 70 eV, by the direct
inlet method). Elemental analyses were carried out at the “Service
d’analyse du CNRS”, Gif-sur-Yvette, France.
X-ray Structure Determinations. Crystals of complex 2 were
grown by slow evaporation of a dichloromethane solution at room
temperature. Dimer 3 was crystallized from THF at 60 °C in a tube
sealed under nitrogen. Crystals of extremely air-sensitive anion 4
(14) Le Floch, P.; Maigrot, N.; Ricard, L.; Charrier, C.; Mathey, F. Inorg.
Chem. 1995, 34, 5070-5072.
(15) Quick, M. H.; Angelici, R. J. Inorg. Synth. 1979, 19, 160-161, 163.
4444 Inorganic Chemistry, Vol. 42, No. 14, 2003

4,4′,5,5′-Tetramethyl-2,2′-biphosphinine
Table 1. Crystallographic Data and Experimental Parameters for
Complexes 2-4
IR Spectroelectrochemistry. IR spectroelectrochemical experi-
ments at variable temperatures were carried out with previously
described optically transparent thin-layer electrochemical (OTTLE)
cells,22,23 equipped with a Pt minigrid working electrode (32 wires/
cm) and CaF2 windows. Potential control during electrolyses was
achieved with a PA4 potentiostat (EKOM, Czech Republic). The
solutions were ca. 5 × 10-3 M in complex 2 and contained 3 ×
10-1 M Bu4NPF6. A thin-layer cyclic voltammogram was recorded
in the course of each spectroelectrochemical experiment.
2
3
4
formula
Mw
C
17H16BrMnO3P2 C34H32Mn2O6P4
C25H36MnNaO5P2
556.41
465.09
770.36
cryst
red plate
red plate
red plate
cryst size (mm) 0.20 × 0.14 × 0.10 0.20 × 0.16 × 0.07 0.18 × 0.16 × 0.10
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
P21/c
15.1080(5)
12.5770(6)
10.0060(8)
90.00
99.591(3)
90.00
1874.70(19)
4
monoclinic
C2/c
triclinic
P1h
22.3415(9)
10.6463(3)
16.0877(7)
90.0000(10)
118.7330(10)
90.0000(10)
3355.4(2)
4
9.024(5)
10.130(5)
15.833(5)
96.830(5)
90.140(5)
103.770(5)
1395.1(11)
2
Density Functional Calculations. The ground-state elec-
tronic structure of anion 4 was calculated by density functional
theory (DFT) methods, using Amsterdam Density Functional
(ADF2000.2)24,25 and Gaussian 9826 software packages. Within the
ADF program Slater-type orbital (STO) basis sets of double-ú
quality with polarization functions for H atoms and triple-ú quality
with additional polarization functions for C, N, O, P, and Mn were
employed. Inner shells were represented by the frozen core
approximation (1s for C, N, O; 1s, 2s, 2p for P, Mn). Within ADF,
the functional used included Becke’s gradient correction27 to the
local exchange expression in conjunction with Perdew’s gradient
correction28 to local density approximation (LDA) with Vosko-
Wilk-Nusair (VWN) parametrization29 of electron-gas data (ADF/
BP). Within Gaussian 98, B3LYP hybrid functionals30 and the
6-311G** basis set31 for H, C, N, O, P, and Mn atoms were used
(G98/B3LYP). The low-lying excited states of closed-shell com-
plex 4 were calculated using the time-dependent DFT (TD-DFT)
method (both ADF/BP and G98/B3LYP). Calculations on 4 were
done in constrained Cs symmetry, with the Mn center and Cap
positioned in the plane of symmetry.
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
D
calc (g cm-3
)
1.648
1.525
1.325
F000
928
3.018
150.0(1)
1576
0.988
150.0(1)
60.0
584
0.635
150.0(1)
55.0
-11.11; -11.13;
-20.20
9407
6345
4643
0.0340
same
same
µ (cm-1
T (K)
)
max 2Θ (deg) 60.1
hkl ranges
0.21; 0.17; -14.13 -31.16; -14.13;
-22.22
10854
4815
4221
0.0364
same
same
reflns collected 5458
indep reflns 5458
reflns included 4559
a
Rint
0.0000
refinement type Fsqd
hydrogen atoms mixed
params refined 239
212
343
R1b
0.0540
0.1541
>2σ(I)
1.216
0.0265
0.0924
same
0.0459
0.1293
same
wR2c
criterion
GOF
1.145
1.057
diff peak/hole 0.505(0.094)/
0.510(0.169)/
-0.533(0.169)
0.768(0.071)/
-0.491(0.071)
(e Å3)
-0.628(0.094)
2
2
2
a Rint ) ∑|Fo - Fo (mean)|/∑Fo . b The unweighted agreement factor
Results and Discussion
2
R1 ) ∑||Fo| - |Fc||/∑|Fo|. c The weighted agreement factor wR2 ) [∑w(Fo
- Fc )2/∑Fo ]1/2, where w ) 1/[σ2(Fo ) + (xP)2 + yP], P ) (Fo
+
2
4
2
2
Crystal Structure of fac-[Mn(Br)(CO)3(tmbp)] (2). The
syntheses of the title biphosphinine complexes 2-4 were
conducted with the readily available ligand 4,4′,5,5′-tetra-
methyl-2,2′-biphosphinine (tmbp) 1. Complex 2 was prepared
from equal amounts of 1 and [Mn(CO)5Br] according to
2
2Fc )/3.
were obtained from a solution in diethyl ether, heated at 35 °C
overnight in a tube sealed under nitrogen. The tube was broken in
a glovebox, and crystals were protected with Paratone oil for
handling and submitted to X-ray diffraction analysis. Data were
collected on a Nonius Kappa CCD diffractometer using a Mo KR
(λ ) 0.71070 Å) X-ray source and a graphite monochromator.
Experimental details are presented in Table 1. The crystal structures
were solved using SIR 9716 and SHELXL-9717 software. ORTEP
drawings were made using ORTEP-3 for Windows.18
Cyclic Voltammetry. Conventional cyclic voltammograms were
recorded with a PAR model 283 potentiostat, using an airtight, light-
protected, single-compartment cell placed in a Faraday cage. A Pt
disk (0.4 mm diameter) working electrode was polished with a 0.25
µm diamond paste between scans. Coiled Pt and Ag wires served
as auxiliary and pseudoreference electrodes, respectively. The
concentration of the studied complexes was typically 10-3 mol
dm-3. Ferrocene (Fc; BDH) or, for complex 3, cobaltocenium
hexafluorophosphate (Cc+PF6-; Aldrich) were added as internal
standards.19,20 The position of the ferocene/ferrocenium (Fc/Fc+)
couple toward various reference electrodes can be found else-
where.21 For comparison, E1/2(Cc/Cc+) ) -1.34 V vs Fc/Fc+.20
(21) Pavlishchuk, V. V.; Addison, A. W. Inorg. Chem. 2000, 298, 97-
102.
(22) Krejcˇ´ık, M.; Daneˇk, M.; Hartl, F. J. Electroanal. Chem. Interfacial
Electrochem. 1991, 317, 179-187.
(23) Hartl, F.; Luyten, H.; Nieuwenhuis, H. A.; Schoemaker, G. Appl.
Spectrosc. 1994, 48, 1522-1528.
(24) Fonseca Guerra, C.; Snijders, J. G.; te Velde, G.; Baerends, E. J. Theor.
Chem. Acc. 1998, 99, 391-403.
(25) van Gisbergen, S. J. A.; Snijders, J. G.; Baerends, E. J. Comput. Phys.
Commun. 1999, 118, 119-138.
(26) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.;
Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels,
A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone,
V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.;
Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.;
Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.;
Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Stefanov, B. B.; Liu, G.;
Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R.
L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara,
A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.;
Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.6; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(16) Altomare, A.; Burla, M. C.; Camalli, M.; Gascarno, G.; Giacovazzo,
C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. SIR97,
an integrated package of computer programs for the solution and
refinement of crystal structures using single-crystal data, 1999.
(17) Sheldrick, G. M. SHELXL-97; Universita¨t Go¨ttingen: Go¨ttingen, 1998.
(18) Farrugia, L. J. ORTEP-3 for Windows. J. Appl. Crystallogr. 1997,
30, 565.
(27) Becke, A. D. Phys. ReV. A 1988, 38, 3098-3100.
(28) (a) Perdew, J. P. Phys. ReV. B 1986, 33, 8822-8824. (b) Perdew, J.
P. Phys. ReV. B 1986, 34, 7406 (erratum).
(29) Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200.
(30) Stephens, P. J.; Devlin, F. J.; Cabalowski, C. F.; Frisch, M. J. J. Phys.
Chem. 1994, 98, 11623-11627.
(19) Gritzner, G.; Ku˚ta, J. Pure Appl. Chem. 1984, 56, 461-466.
(20) Stojanovic, R. S.; Bond, A. M. Anal. Chem. 1993, 65, 56-64.
(31) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56,
2257-2261.
Inorganic Chemistry, Vol. 42, No. 14, 2003 4445

Hartl et al.
Figure 1. ORTEP drawing of complex 2. Hydrogen atoms are omitted
for clarity. Ellipsoids are scaled to enclose 50% of the electron density.
The crystallographic labeling is arbitrary and different from the numbering
used for assignment of the 13C NMR signals. Selected bond distances (Å):
P1-Mn1, 2.259(1); P2-Mn1, 2.254(1); Mn1-C15, 1.848(4); Mn1-C16,
1.875(4); C15-O1, 1.116(5); C16-O2, 1.083(5); P1-C1, 1.701(4);
C1-C2, 1.389(6); C2-C3, 1.409(5); C3-C4, 1.398(5); C4-C5, 1.398(5);
C5-C6, 1.470(5); C6-P2, 1.719(4); P2-C10, 1.704(4); C10-C9,
1.392(6); C9-C8, 1.413(6); C8-C7, 1.397(5); C7-C6, 1.396(5).
Bond angles (deg): C1-P1-C5, 105.2(2); C6-P2-C10, 105.4(2);
P1-Mn1-P2, 79.12(4); P2-Mn1-C15, 92.9(1); C15-Mn1-C16, 94.1-
(2); C15-Mn1-C17, 87.1(6); C16-Mn1-C17, 87.6(6); C15-Mn1-Br1,
89.3(1); C16-Mn1-Br1, 89.5(1); C16-Mn1-P1, 93.9(1); P1-Mn-Br1,
88.95(4); P2-Mn1-C17, 90.6(6).
Figure 2. ORTEP drawing of dimer 3. Hydrogen atoms are omitted for
clarity. Ellipsoids are scaled to enclose 50% of the electron density. Selected
bond distances (Å): P1-C1, 1.713(2); C1-C2, 1.396(2); C2-C3, 1.409-
(2); C3-C4, 1.394(2); C4-C5, 1.397(2); C5-C6, 1.459(2); C6-C7, 1.398-
(2); C7-C8, 1.390(2); C8-C9, 1.417(2); C9-C10, 1.388(2); C10-P2,
1.709(1); P1-Mn1, 2.2362(4); P2-Mn1, 2.2226(4); Mn1-C16, 1.793(2);
C16-O2, 1.150(2); Mn1-C15, 1.824(2); C15-O1, 1.147(2); Mn1-C17,
1.814(2); C17-O3, 1.149(2); Mn1-Mn2, 3.0599(5). Bond angles (deg):
C6-P2-C10, 104.87(7); C1-P1-C5, 104.55(7); P1-Mn1-P2, 78.69(1);
P1-Mn1-C16, 96.96(5); P1-Mn1-C15, 92.91(5); P1-Mn1-C17, 166.45-
(5); P2-Mn1-C15, 170.25(5); C15-Mn1-C16, 91.18(7); C16-Mn1-
C17, 91.75(7).
eq 1. The fac geometry of 2 can be easily deduced from the
31P NMR spectrum that shows a single resonance at 225.1
ppm. No traces of the mer isomer were detected in the 31P
NMR spectrum, which should have produced a classical AB
system.
equatorial CO ligands in 2 is also evidenced by a shortening
of the CO bonds: 1.083(5) and 1.116(5) Å in 2 compared
to 1.14(2) and 1.19(2) Å in fac-[Mn(I)(CO)3(bpy)], and
1.122(18) and 1.148(19) Å in the above diphosphite complex.
Crystal Structures of the Reduction Products [Mn-
(CO)3(tmbp)2]2 (3) and [Mn(CO)3(tmbp)][Na(OEt2)2] (4).
Chemical reduction of complex 2 with 1 molar equiv of
sodium naphthalenide in DME at room temperature produces
rapidly dimer 3 (eq 2), which was obtained as a poorly
soluble dark red solid. The limited solubility precluded
recording of a 13C NMR spectrum.
The 1H and 13C NMR data of complex 2 also support the
proposed structure. Definitive evidence has been given by a
single-crystal X-ray diffraction study (Table 1). An ORTEP
view of 2 is presented in Figure 1, together with the most
significant bond distances and angles. Not unusual for this
type of complex, the manganese environment in 2 is nearly
perfectly octahedral, with C-Mn-C and P-Mn-C bond
angles ranging between 87.1(6)° and 92.9(1)°, respectively.
The tmbp ligand in 2 is not significantly distorted compared
to its noncoordinate cis form,9 revealing that its aromaticity
has remained preserved. In both cases the internal P-C bonds
are slightly longer (e.g., P1-C5 ) 1.728(4) Å in 2 and
1.736(4) Å in cis-1) than the two external ones (P1-C1 )
1.701(4) Å in 2 and 1.716(5) Å in cis-1). The inter-ring C-C
bond lengths are almost identical: 1.470(5) Å in 2 vs
1.490(8) Å in cis-1. The Mn-COeq bond lengths are
particularly indicative of the strong π-acceptor ability of
biphosphinine compared to classical phosphites and 2,2′-
bipyridine. Thus, Mn-Ceq bond distances in 2 (1.848(4) and
1.875(4) Å) are apparently longer than those found in related
complexes fac-[Mn(I)(CO)3(bpy)] (1.77(2) and 1.80(2) Å)10
and fac-[Mn(Br)(CO)3{((MeO)2PO)2SiMe2}] (1.810(4) and
1.832(14) Å).32 The weaker π-back-bonding toward the two
The dimeric structure of complex 3 could not be estab-
lished on the simple basis of the available 1H and 31P NMR
data. Again, definitive evidence has been provided by an
X-ray crystallographic study (Table 1). Figure 2 presents an
ORTEP view of dimer 3, together with a list of selected of
bond lengths and angles. The structure of the dimer resembles
that of [Mn2(CO)10].33 Both tmbp ligands bind as chelates
and do not bridge, unlike bridging diphosphine34 or 2-phos-
phinyl-phosphinine35 ligands in dinuclear manganese carbo-
nyls. The coordination geometry of both Mn centers is nearly
octahedral. The two equatorial planes are twisted with respect
(33) (a) Churchill, M. R.; Amoh, K. N.; Wasserman, H. J. Inorg. Chem.
1981, 20, 1609-1611. (b) Martin, M.; Rees, B.; Mitschler, A. Acta
Crystallogr., Sect. B 1982, 38, 6-15.
(32) Sum, V.; Patel, M. T.; Kee, T. P.; Thornton-Pett, M. Polyhedron 1992,
11, 1743-1754.
4446 Inorganic Chemistry, Vol. 42, No. 14, 2003

4,4′,5,5′-Tetramethyl-2,2′-biphosphinine
to each other, the dihedral angle Θ reaching 40°, similar to
[Mn2(CO)10] with Θ ) 50.2°.33b Another important com-
parison concerns the Mn-Mn bond distance, which is rather
long in 3 (3.0599(5) Å) compared to [Mn2(CO)10] (2.895(1)
Å). As expected, π-back-bonding to the tmbp ligand in the
dimer reduction product is strong. This phenomenon is
revealed by elongated internal P-C bond distances (1.735-
(1) and 1.740(1) Å in 3 compared to 1.728(4) and 1.719(4)
Å in 2), concomitantly with a slightly shortened inter-ring
C-C bond distance (1.459(2) Å in 3 vs 1.470(5) Å in 2)
and Mn-P distances (2.2226(4) and 2.2362(4) in 3 compared
to 2.254(1) and 2.259(1) Å in 2). Such differences are in
good agreement with the character of the π*(tmbp) lowest
unoccupied molecular orbital (LUMO) that is antibonding
with regard to the P-CR bonds and bonding for the inter-
ring C-C bond between the two phosphinine rings.36
Importantly, the apparently longer internal P-C bonds
(1.784(1) and 1.788(2) Å) and shorter inter-ring C-C bond
(1.440(2) Å) for the noncoordinate radical anion of tmbp
(1•-)37 testify that the tmbp ligand in dimer 3 cannot be
viewed as (partly) reduced. The added electrons still largely
reside on the Mn centers, being localized in a new σ(Mn-Mn)
orbital. Nevertheless, there is crystallographic evidence that
delocalization of the Mn-tmbp π-bonding in 3 is not
negligible, causing the above-mentioned elongation of the
Mn-Mn bond compared to [Mn2(CO)10], although steric
hindrance of the tmbp ligands may also play a role. Similarly
to [Mn2(CO)10], Mn-to-CO π-back-bonding in 3 dominates,
being stronger toward the axial than equatorial carbonyls
(Mn1-C16, 1.793(2) Å, vs Mn1-C15, 1.824(2) Å).
Figure 3. ORTEP drawing of anion 4. Hydrogen atoms are omitted for
clarity. Ellipsoids are scaled to enclose 50% of the electron density. The
crystallographic labeling is arbitrary and different from the numbering used
for assignment of the 13C NMR signals. Selected bond distances (Å):
P1-C1, 1.728(3); C1-C2, 1.376(4); C2-C3, 1.420(4); C3-C4, 1.380(4);
C4-C5, 1.398(4); C5-C6, 1.434(4); C6-C7, 1.399(4); C7-C8, 1.383(4);
C8-C9, 1.419(4); C9-C10, 1.386(4); C10-P2, 1.728(3); P1-Mn1,
2.180(1); P2-Mn1, 2.201(1); Mn1-C16, 1.773(3); C16-O2, 1.172(3);
Mn1-C15, 1.779(3); C15-O1, 1.166(3); Mn1-C17, 1.781(3); C17-O3,
1.167(3); O3-Na4, 2.429(2); O4-Na4, 2.321(2); O5-Na4, 2.286(3).
Bond angles (deg): C1-P1-C5, 103.3(1); C6-P2-C10, 103.2(1);
P1-Mn1-P2, 78.19(4); P2-Mn1-C16, 88.1(1); P1-Mn1-C15,
116.1(1); C16-Mn1-C17, 94.0(1); C17-Mn1-C15, 95.9(1).
The reduction of complex 2 with 2 equiv of sodium
naphthalenide was again carried out in DME at room
temperature (eq 3). In contrast to dimer 3, the resulting
anionic complex [Mn(CO)3(tmbp)]- (4) is well soluble in
DME and THF and could be fully characterized by NMR
1
techniques (31P, H, 13C). It should be noted that anion 4 is
also readily obtained by reduction of dimer 3 with 2 equiv
of sodium naphthalenide.
Figure 4. Polymeric arrangement of the sodium salt of anion 4 in the
crystal lattice. The two Et2O ligands at Na+ have been omitted for clarity.
sodium cation being linked to three carbonyl ligands of three
different [Mn(CO)3(tmbp)]- molecules. The nearly trigonal
bipyramidal coordination sphere of sodium is completed by
two diethyl ether ligands. The polymeric structure of anion
4 is schematically shown in Figure 4. A close inspection of
bond distances in the tmbp ligand reveals a significant
localization of electron density in the π*(tmbp) LUMO. As
argued above, occupation of this orbital results in elongated
internal P-C bonds (1.751(3) and 1.757(3) Å) and a
shortened inter-ring C-C distance (1.434(2) Å), close to the
The five-coordinate structure of anion 4 has been un-
equivocally confirmed by X-ray crystallographic study (Table
1). Suitable crystals of 4 were slowly grown from a diethyl
ether solution under strictly inert conditions. An ORTEP view
of the sodium salt of 4 is presented in Figure 3, together
with a list of the most relevant bond distances and angles.
Complex 4 adopts a distorted square pyramidal geometry
around Mn. It crystallizes as a coordination polymer, each
(35) Waschbu¨sch, K.; Le Floch, P.; Ricard, L.; Mathey, F. Chem. Ber./
Recl. 1997, 130, 843-849.
(34) (a) Reimann, R. H.; Singleton, E. J. Organomet. Chem 1972, 38, 113-
119. (b) Colton, R.; Commons, C. J. Aust. J. Chem. 1975, 28, 1673-
1680. (c) Lemke, F. R.; Kubiak, C. P. J. Chem. Soc., Chem. Commun.
1985, 1729-1730. (d) Morgan, C. A.; Fanwick, P. E.; Rothwell, I. P.
Inorg. Chim. Acta 1994, 224, 105-111.
(36) Rosa, P.; Ricard, L.; Le Floch, P.; Mathey, F.; Sini, G.; Eisenstein,
O. Inorg. Chem. 1998, 37, 3154-3158.
(37) Choua, S.; Sidorenkova, H.; Berclaz, T.; Geoffroy, M.; Rosa, P.;
Me´zailles, N.; Ricard, L.; Mathey, F.; Le Floch, P. J. Am. Chem. Soc.
2000, 122, 12227-12234.
Inorganic Chemistry, Vol. 42, No. 14, 2003 4447

Hartl et al.
Table 2. Redox Potentials (V vs Fc/Fc+) of fac-[Mn(Br)(CO)3(tmbp)],
Its Reduction Products, and Corresponding 2,2′-Bipyridine Complexes5
red complex [Mn(SnPh3)(tmbp)(CO)3] (5) (eq 4). All NMR
data and elemental analyses have confirmed the proposed
formulation. Furthermore, the singlet phosphorus signal in
the 31P NMR spectrum of 5 confirms the fac geometry. More
detailed physicochemical study of the interesting “inorga-
nometallic” complex 5 was, however, out of the scope of
this work.
b
c
complex a
T (K) (solvent)
Ep,c
Ep,a
[Mn(Br)(CO)3(tmbp)] (2)
293 (THF)
293 (PrCN)
218 (PrCN)
293 (THF)
223 (THF)
293 (THF)
293 (THF)
223 (THF)
293 (THF)e
293 (PrCN)
218 (PrCN)
293 (THF)
223 (THF)
-1.67
-1.60
-1.76
-1.89
-2.03
-1.80
-2.05
-2.21
-2.54g
f
+0.86
+0.92
+0.92
f
[Mn(Br)(CO)3(bpy)]
f
[Mn(CO)3(tmbp)]2 (3)d
[Mn(CO)3(bpy)]2
-0.13
-0.61
-0.53
-0.97
-0.90
-0.76
-1.53
-1.34
[Mn(CO)3(tmbp)]- (4)
f
f
f
[Mn(CO)3(bpy)]-
a Experimental conditions: cyclic voltammetry (V ) 100 mV/s); 10-3
M tmbp complexes; 3 × 10-1 M Bu4NPF6 as supporting electrolyte; Pt
disk working electrode. E1/2 (Fc/Fc+) ) + 0.57 V (THF, 293 K) and +0.42
V (PrCN, 293 K) vs SCE. b Cathodic peak potential; irreversible reduction.
c Anodic peak potential; irreversible oxidation. d Produced in situ from
[Mn(Br)(CO)3(tmbp)] and Na[Mn(CO)3(tmbp)]. e Solution of Na[Mn(CO)3-
(tmbp)]. f Not recorded. g Irreversible cathodic wave R2, not shown in
Figure 2.
values for noncoordinated radical anion 1•- (P-C, 1.784(1)
and 1.788(2) Å; C-C, 1.440(2) Å)37 but still significantly
different from those for dianion 12- (P-C, 1.815(1) and
1.821(3) Å; C-C, 1.401(4) Å).38 The Mn-P bonds in anion
4 are notably shorter (2.180(1) and 2.201(2) Å) compared
to 2 (2.254(1) and 2.259(1) Å) and 3 (2.2226(4) and 2.2362-
(4) Å). The Mn-C and C-O bond lengths in 4 are quite
similar to those observed39 for the 18e anions [Mn-I(CO)5]-
and [Mn-I(PR3)(CO)4]-. However, a direct comparison is
risky in this case, as the effects of the sodium coordination
are difficult to assess with accuracy. (Note that the ν(CO)
wavenumbers of Na[Mn(CO)3(tmbp)] are smaller compared
to those of Bu4N[Mn(CO)3(tmbp)] (Table 3), indicating a
larger charge on the CO ligands in the sodium salt.) The
combined crystallographic data point to the formulation of
the two-electron-reduced anion 4 as [Mn0(CO)3(tmbp-)]-
with a strongly delocalized π-bonding in the Mn(tmbp)
metallacycle rather than [Mn-I(CO)3(tmbp)]- or [MnI(CO)3-
(tmbp2-)]-. A similar situation is encountered in the square
planar complex [Ru0(tmbp-)2]2- (C-C, 1.436(3) Å; P-C,
1.740(2) Å).40 Interestingly, the electronic properties of the
anionic tmbp ligand in anion 4 are largely preserved in the
adduct [Mn2(CO)6(tmbp)], where the {Mn(CO)3}+ moiety
binds to the delocalized five-membered Mn-tmbp metalla-
cycle of [Mn(CO)3(tmbp)]-, with the bond lengths P-C
(internal) of 1.760(2) and 1.762(2) Å and C-C (inter-ring)
of 1.443(3) Å.14
Spectroelectrochemical Study of Complexes 2-4. Chemi-
cal reduction of the complex [Mn(Br)(CO)3(tmbp)] (2) was
shown to produce dimer 3 and anion 4 on overall consump-
tion of 1 and 2 molar equiv, respectively, of the one-electron-
reducing agent. However, description of the intimate mech-
anism of these electron-transfer reactions required more
thorough examination. The redox properties and details of
the reduction path of complex 2 were investigated by
conventional cyclic voltammetry and IR/UV-vis spectro-
electrochemistry at variable temperatures. The redox poten-
tials and recorded spectroscopic data (IR ν(CO) wavenum-
bers and UV-vis absorption maxima) for the studied redox
series are presented in Tables 2 and 3, respectively. The
corresponding literature data for 2,2′-bipyridine derivatives
of complexes 2-4 have been added for comparison.
Cyclic Voltammetry. The cyclic voltammogram of com-
plex 2 in THF at 293 K is depicted in Figure 5a. Oxidation
of 2 at the potential of the anodic peak O1 is chemically
irreversible at moderate scan rates (0.1-20 V/s) in the
temperature range 293-218 K. The same result was obtained
in butyronitrile. The oxidation was not investigated in detail.
In the cathodic region, the peak R1 belongs to chemically
irreversible reduction of complex 2. Determination5b,41 of the
apparent number of electrons, napp, transferred on the time
scale defined by V ) 0.1-20 V/s, was not accurate owing
to adsorption of [Mn(Br)(CO)3(tmbp)] at disk ultramicro-
electrodes. However, napp ≈ 2 has been deduced from the
analysis of the voltammetric response (see below). No anodic
counterpeak (RO1) due to reoxidation of the primary 1e-
reduction product, the radical anion [Mn(Br)(CO)3(tmbp)]•-
(eq 5), was observed under the above experimental conditions
for oxidation. Instead, two other anodic peaks arose on the
reverse scan, viz., O2 due to reoxidation of the 2e-reduced
anion 4 and, more positively, small O3 due to reoxidation
of the 1e-reduced dimer 3 (see Figure 5a). This assignment
Trapping Reaction of Anion 4 with Ph3SnCl. Even
though a significant part of the charge resides on the tmbp
ligand, anion 4 is still sufficiently reactive to undergo
reactions with main-group electrophiles. Its trapping reaction
with Ph3SnCl in DME at room temperature yielded the stable
(38) Rosa, P.; Mezailles, N.; Ricard, L.; Mathey, F.; Le Floch, P. Angew.
Chem., Int. Ed. 2001, 40, 4476-4479.
(39) (a) Corraine, S. M.; Lai, C. K.; Zhen, Y. Q.; Churchill, M. R.; Buttrey,
L. A.; Ziller, J. W.; Atwood, J. D. Organometallics 1992, 11, 35-40.
(b) Riley, P. E.; Davis, R. E. Inorg. Chem. 1980, 19, 159-165.
(40) Rosa, P.; Mezailles, N.; Ricard, L.; Mathey, F.; Le Floch, P.; Jean, Y.
Angew. Chem., Int. Ed. 2001, 40, 1251-1253.
(41) Amatore, C.; Azzabi, M.; Calas, P.; Jutand, A.; Lefrou, C.; Rollin, Y.
J. Electroanal. Chem. Interfacial Electrochem. 1990, 288, 45-63.
4448 Inorganic Chemistry, Vol. 42, No. 14, 2003

4,4′,5,5′-Tetramethyl-2,2′-biphosphinine
Table 3. IR ν(CO) and UV-Vis Data for fac-[Mn(Br)(CO)3(tmbp)] (2), Its Reduction Products 3 and 4, and Corresponding 2,2′-Bipyridine (bpy)
Complexes3b,c,5
complex
T (K)
ν(CO) (cm-1
)
λ
max (nm) [ꢀmax (M-1 cm-1)]
[Mn(Br)(CO)3(tmbp)] (2)
293a
2038 (s), 1982 (m), 1944 (m)
2041 (s), 1985 (m), 1949 (m)
2041 (s), 1985 (m), 1949 (m)
2023 (s), 1935 (m), 1914 (m)
2021 (s), 1932 (s-m), 1913 (s-m)
2027 (s), 1934 (s), 1919 (s)
310, 368 (sh), 377 [12000], 444 [6000]
293a,c
193b,c
293a
[Mn(Br)(CO)3(bpy)]
375, 430
273, 357 (sh), 392
293a,c
193b,c
293a
[Mn(I)(CO)3(bpy)]
[Mn(CO)3(tmbp)]2 (3)
2019 (s), 1933 (m), 1916 (m)
385, 445
293a
2009 (vs), 1968 (s), 1941 (s-m), 1910 (m)
2007 (vs), 1966 (s), 1941 (s-m), 1904 (m)
2007 (vs), 1964 (s),d 1905 (m)
1975 (m), 1963 (w), 1936 (s), 1886 (m), 1866 (m)
1974 (m), 1963 (w), 1930 (s), 1877 (m), 1855 (m)
1927 (s), 1854 (m), 1814 (m-w)
1933 (s), 1857 + 1848 (s-m, br)
1928 (s), 1844 (s-m, br)
1926 (s), 1843 (s-m, br)
1916 (s), 1815 (s, br)
1909 (s), 1811 (s, br)
1917 (s), 1823 (m), 1815 (sh)
295sh, 332 (sh), 378 [20000], 431 (sh), 533 [8500]
293a,c
193b,c
293a,c
193b,c
293a,e
293a,f
293a,c
193b,c
293a,c
193b,c
135g
[Mn(CO)3(bpy)]2
395, 455, 650, 740 (sh), 840
321, 411 [15000], 540 [2500]
[Mn(CO)3(tmbp)]- (4)
[Mn(CO)3(bpy)]-
213, 273, 352, 466 (sh), 548, 607 (sh), 676 (sh), 761 (sh)
560
a In THF. b In PrCN. c With added supporting electrolyte Bu4NPF6 (3 × 10-1 M). d Hidden under the band of 4 at 1926 cm-1
.
e As the Na+ salt with
CO‚‚‚Na+ interactions (see Figure 3). f As the Bu4N+ salt. g In 2-MeTHF.
formed by the zero-electron coupling reaction between anion
4 and yet nonreduced complex 2 (eq 8), which was proven
in a separate experiment. The latter reaction, however, does
not occur on the subsecond time scale of cyclic voltammetry
(V g 100 mV/s), as indicated by the absence of the cathodic
peak due to the reduction of dimer 3 (peak R3 in Figure
5c). The cathodic process R1 in Figure 5a therefore corre-
sponds to the total transfer of two electrons (napp ≈ 2),
resulting in the formation of anion 4 via the ECE sequence
as described by eqs 5-7. According to the data in Table 2,
the cathodic peak R3 lies 130 mV more negatively than R1
(THF solution, 293 K, V ) 100 mV/s). This result implies
that the second-order rate constant for the zero-electron
coupling reaction following eq 8 must be less than a few
hundred M-1 s-1.42 Evidence for the proposed ECE(C)
E
C
[Mn(Br)(CO)3(tmbp)] + e- w [Mn(Br)(CO)3(tmbp)]•-
(5)
Figure 5. Cyclic voltammograms of parent complex 2 (a) and its reduction
products, anion 4 (b) and dimer 3 (c), all in THF. Conditions used: 3 ×
[Mn(Br)(CO)3(tmbp)]•- f [Mn(CO)3(tmbp)]• + Br-
10-1 M Bu4NPF6 as supporting electrolyte, T ) 293 K, V ) 100 mV s-1
,
Pt disk microelectrode (0.4 mm diameter),
(6)
(7)
E
C
[Mn(CO)3(tmbp)]• + e- w [Mn(CO)3(tmbp)]-
[Mn(CO)3(tmbp)]- + [Mn(Br)(CO)3(tmbp)] f
has been facilitated by recording separately cyclic voltam-
mograms of (i) anion 4 (see Figure 5b), obtained by reduction
of complex 2 with Na/Hg, and (ii) dimer 3 (see Figure 5c),
formed in situ from equimolar amounts of [Mn(Br)(CO)3-
(tmbp)] and Na[Mn(CO)3(tmbp)] (precipitated NaBr was
filtered off). The results allow us to conclude that the radical
anion [Mn(Br)(CO)3(tmbp)]•- is short-lived and rapidly
expels the Br- ligand (eq 6). According to the data in Table
2 (peak potentials R3 vs O2), the electrode potential for the
five-coordinate redox couple [Mn(CO)3(tmbp)]•/[Mn(CO)3-
(tmbp)]- probably lies more positively than that for the six-
coordinate redox couple [Mn(Br)(CO)3(tmbp)]•-/[Mn(Br)-
(CO)3(tmbp)]. The radicals [Mn(CO)3(tmbp)]•, produced
from one-electron-reduced [Mn(Br)(CO)3(tmbp)]•- by fast
dissociation of Br-, are therefore instantaneously further
reduced at the cathodic potential R1 to give anion 4 (eq 7)
and do not concomitantly dimerize to 3. Instead, dimer 3 is
[Mn(CO)3(tmbp)]2 + Br- (8)
reduction path of complex 2 also comes from IR spectro-
electrochemical experiments at variable temperatures, as
described hereinafter.
Reduction of dimer 3 at R3 (Figure 5c) produces anion 4,
as revealed by the appearance of the anodic peak O2 on the
reverse scan. The intimate mechanism is undoubtedly less
simple than described by eq 9, but no detailed study of this
process was conducted. The five-coordinate anion 4 is further
reduced about 750 mV more negatively than parent complex
2 (Table 2). The cathodic step R2 (not shown in Figure 5)
(42) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706-723.
Inorganic Chemistry, Vol. 42, No. 14, 2003 4449

Hartl et al.
is chemically completely irreversible, and the corresponding
radical product [Mn(CO)3(tmbp)]2- is probably highly
unstable.
1/2[Mn(CO)3(tmbp)]2 + e- w [Mn(CO)3(tmbp)]- (9)
Reoxidation of anion 4 at O2 results in the recovery of
dimer 3. This is evidenced by the cathodic peak R3 due to
the reduction of the dimer that arose during scan reversal
beyond O2 (Figure 5b). Obviously, oxidation of 4 produces
initially the five-coordinate radicals [Mn(CO)3(tmbp)]• (eq
10). Dimer 3 is then formed by two possible coupling
reactions. In the first case the radicals [Mn(CO)3(tmbp)]•
directly dimerize (eq 11). This step, in contrast to the
reduction path of 2 (in particular eq 7), is essentially possible
at the electrode potential O2.
Figure 6. IR spectral changes in the ν(CO) region, recorded in the course
of the reduction of complex 2 in PrCN at 193 K within a spectroelectro-
chemical cell.23 Symbols used: (V) complex 2; (b) dimer 3; (9) anion 4.
[Mn(CO)3(tmbp)]- w [Mn(CO)3(tmbp)]• + e- (10)
2[Mn(CO)3(tmbp)]• f [Mn(CO)3(tmbp)]2
(11)
any detectable trace of less negatively positioned cathodic
peaks due to the reduction of [Mn(THF)(CO)3(tmbp)]+,
[Mn(PrCN)(CO)3(tmbp)]+, or complex 2. This result implies
that the oxidation of anion 4 yields dimer 3 exclusively via
the direct radical coupling reaction (eq 7). In contrast to iso-
electronic radicals [Mn(CO)3(bpy)]• that form adducts with,
e.g., phosphines or coordinating solvent,3c,5b the five-coor-
dinate radicals [Mn(CO)3(tmbp)]• are less capable of coor-
dinating a Lewis base (eq 12), resembling more [Mn(CO)5]•
in this regard. This difference may have its origin in the
weakly distorted square pyramidal geometry of [Mn(CO)3-
(bpy)]• with axial vacancy having a considerable 4pz(Mn)
character.43
Alternatively, the five-coordinate radicals [Mn(CO)3-
(tmbp)]• may first bind a donor solvent molecule (Sv) and
convert to the six-coordinate radicals [Mn(Sv)(CO)3(tmbp)]•
(eq 12). When formed, the latter radicals will directly oxidize
at the anodic potential O2 to [Mn(Sv)(CO)3(tmbp)]+ (eq 13).
Similarly to the reduction path of 2, the cation [Mn(Sv)-
(CO)3(tmbp)]+ will then react rapidly with yet nonoxidized
anion 4 to give dimer 3 (eq 14). It should be noted that the
latter ECEC path (eqs 10 and 12-14) operates in the case
of electrochemical oxidation of [Mn(CO)3(bpy)]-.5b In order
[Mn(CO)3(tmbp)]• + Sv T [Mn(Sv)(CO)3(tmbp)]• (12)
Finally, oxidation of dimer 3 at the anodic peak O3 in
Figures 5a and 5c in the presence of Br- ions released during
the reduction of [Mn(Br)(CO)3(tmbp)] (eqs 6 and 8) results
in the recovery of parent complex 2 (eq 16). This oxidation
process, monitored by IR spectroscopy, was separately
confirmed by chemical oxidation of [Mn(CO)3(tmbp)]2 with
[Mn(Sv)(CO)3(tmbp)]• w [Mn(Sv)(CO)3(tmbp)]+ + e-
(13)
[Mn(Sv)(CO)3(tmbp)]+ + [Mn(CO)3(tmbp)]- f
[Mn(CO)3(tmbp)]2 + Sv (14)
-
-
a ferrocenium salt (PF6 or BF4 ) in THF containing 3 ×
10-1 M Bu4NBr.
to discriminate between the two possible paths producing
dimer 3, the oxidation of anion 4 was studied by cyclic
voltammetry in THF at 213 K, and in 2:3 THF/PrCN (v/v)
at 293 K. If anion 4 were oxidized via steps 10, 12, and 13,
the coupling reaction 14 would be, most likely, sufficiently
slow under these conditions5b to allow detection of the
oxidation products [Mn(THF)(CO)3(tmbp)]+ and [Mn(PrCN)-
(CO)3(tmbp)]+, respectively, on the reverse scan beyond the
anodic peak O2 of anion 4 (see Figure 5b). The latter process
was also studied in the presence of Bu4NBr (3 × 10-1 M) at
293 K to form rapidly complex 2 according to eq 15. As
argued above for the reduction path, complex 2 will not
produce a detectable amount of dimer 3 in reaction with
anion 4 (eq 8) at V g 0.1 V s-1. However, in all three cases
1/2[Mn(CO)3(tmbp)]2 - e- + Br- w [Mn(Br)(CO)3(tmbp)]
(16)
IR Spectroelectrochemistry. The ECEC(E) reduction
path of complex 2 (eqs 5-8 and 9) can be conveniently
followed also in situ with the aid of IR spectroscopy, by
monitoring changes in the ν(CO) region during the elec-
trolysis. In butyronitrile at 193 K, the reduction of the parent
compound produces directly observable anion 4, together
with some dimer 3 (Figure 6). This result confirms the ECE
part of the reduction path described by eqs 5-7. According
to the electrode potentials in Table 2, dimer 3 is reduced
sufficiently more negatively than complex 2 (∆Ep,c ) 130
mV) to exclude observation of anion 4 in the early stage of
the electrolysis, in case the latter species is formed via the
alternative ECCE reduction path described by eqs 5, 6, 11,
[Mn(Sv)(CO)3(tmbp)]+ + Br- f
[Mn(Br)(CO)3(tmbp)] + Sv (15)
only the cathodic peak R3 of dimer 3 was observed on the
reverse scan beyond the anodic peak O2 (Figure 5b), without
(43) Rosa, A.; Ricciardi, G.; Baerends, E. J.; Stufkens, D. J. Inorg. Chem.
1998, 37, 6244-6254.
4450 Inorganic Chemistry, Vol. 42, No. 14, 2003

4,4′,5,5′-Tetramethyl-2,2′-biphosphinine
Figure 7. IR spectral changes in the ν(CO) region, recorded in the course
of the reduction of complex 2 in PrCN at 293 K within a spectroelectro-
chemical cell.22 Symbols used: (V) complex 2; (b) dimer 3; (9) anion 4.
Figure 8. UV-vis spectra of parent complex 2 (a) and its reduction
products, anion 4 (b) and dimer 3 (c), all in THF at 293 K.
Table 4. Comparison of Selected Experimental and DFT Calculated
Bond Lengths (Å) and Angles (deg) in [Mn(CO)3(tmbp)]- (4)
and 9. The cathodic step (eq 9) indeed occurs at the end of
the electrolysis of 2, when the potential of the working
electrode reached the value of the cathodic peak R3 (Figure
5). Apparently, the thermal coupling reaction 8 is signifi-
cantly inhibited at 193 K, although not completely, resulting
in coexistence of all three members of the redox series,
precursor 2, dimer 3, and anion 4, in the electrolyzed solution.
In butyronitrile at 243 K, anion 4 is also formed prior to
dimer 3, but the coupling reaction (eq 8) occurs with a lower
activation energy and dimer 3 therefore dominates over anion
4 until the reduction of 3 itself begins (eq 9).
The IR spectroelectrochemical response to the reduction
of complex 2 at 293 K is less simple (Figure 7). In agreement
with the rapid coupling reaction 8, anion 4 is initially not
observed. It only starts to appear due to the subsequent
reduction of dimer 3 at the cathodic wave R3. It is
noteworthy that during its reduction the ν(CO) bands of
complex 2 become broadened and shift to smaller wave-
numbers. This feature is even more pronounced in THF. It
may reflect an interaction of complex 2 with a reduced
species, probably dimer 3, for the ν(CO) bands of the latter
product are also broadened and shifted to larger wavenum-
bers until complex 2 disappears. Ultimately, dimer 3 is again
reduced to anion 4. However, at 293 K this step also
generates some unassigned minor side products absorbing
at 2000-1970 cm-1 and 1910-1880 cm-1.
To record UV-vis spectra of pure dimer 3 and anion 4,
precursor 2 is preferably rapidly reduced in THF with 1%
Na/Hg, with the formation of pure anion 4 (Figure 8b).
Mixing of equimolar amounts of anion 4 and nonreduced
complex 2 (coupling reaction 8) then gives pure dimer 3
(Figure 8c), as was proved by IR spectroscopy.
Ground-State Structure of the Five-Coordinate Anion
[Mn(CO)3(tmbp)]- (4). Selected bond distances and angles
of anion 4, calculated and optimized with the ADF2000.2
and Gaussian 98 program packages, are listed together with
the corresponding experimental data in Table 4. It is
important to note that also the methyl substituents on the
tmbp ligand with their steric and electronic influence were
included to provide the best model for the description of the
bond
Mn-P
ADF/BP
G98/B3LYP
exptla
2.202
1.782
1.791
1.748
1.785
1.390
1.426
1.396
1.398
1.438
1.174
2.225
1.758
1.785
1.749
1.780
1.382
1.434
1.390
1.408
1.433
1.169
2.201
1.776
1.781
1.728
1.754
1.381
1.420
1.382
1.398
1.434
1.168
Mn-C (CO)eq
Mn-C (CO)ap
P1-C1
P1-C5
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C-O (CO)
angle
ADF/BP
G98/B3LYP
exptla
P1-Mn-P2
Mn-P1-C1
Mn-P1-C5
P1-C5-C6
P1-C5-C4
C1-P1-C5
P1-C1-C2
C1-C2-C3
C2-C3-C4
C15-Mn-C17
78.2
138.5
118.8
112.0
120.6
102.7
124.8
122.9
122.2
98.8
78.3
138.8
118.0
112.7
120.3
102.9
124.9
122.7
122.2
98.0
78.19
137.6
118.3
112.1
120.5
103.3
124.5
123.0
121.8
95.9
a Symmetry-averaged values.
ground-state electronic structure and bonding properties of
the complex. Apart from the goal of the density functional
theoretical (DFT) study, it is interesting to compare results
obtained with both DFT methods to demonstrate their
benefits and mutual differences. Table 4 shows that both
ADF/BP and G98/B3LYP data describe qualitatively well
the experimental geometry determined by X-ray diffraction.
The largest discrepancy regards the OC-Mn-CO angles,
which may be ascribed to the CO‚‚‚Na+ interactions in the
sodium salt of anion 4 not considered in the theoretical
model. ADF/BP overestimates the P1-C5 distance by 0.031
Å, while the other experimental distances are reproduced
within 0.02 Å. The G98/B3LYP data are slightly more
accurate in description of the crystallographic results. The
optimized geometry of 4 was employed in the following
single point electronic structure calculations.
Table 5 summarizes the calculated energies and characters
of the frontier molecular orbitals of anion 4. Both theoretical
Inorganic Chemistry, Vol. 42, No. 14, 2003 4451

Hartl et al.
Table 5. ADF/BP Calculated One-Electron Energies and Percentage Composition of Selected Occupied and Unoccupied Frontier Molecular Orbitals of
[Mn(CO)3(tmbp)]- (4), Expressed in Terms of the Constituent Fragments (%)
MO
E (eV)
Mn
tmbp
COap
COeq
character
unoccupied
30a′′
38a′
29a′′
37a′
2.18
1.37
1.33
0.79
2 (pz); 24 (dxz)
35
60
93
96
6
4
27
10
2
π* tmbp + d + COeq
π* tmbp + d
π* tmbp
2
2
2
3 (py); 2 (dz ); 4 (dx -y ); 15 (dxy)
3 (dxz); 2 (dyz)
2
2
1 (dx -y ); 1 (dxy)
1
π* tmbp
occupied
36a′
28a′′
35a′
34a′
2
2
2
-0.94
-1.50
-1.78
-2.29
-2.37
9 (py); 7 (dz ); 13 (dx -y ); 3 (dxy)
2 (pz); 2 (dxz); 40 (dyz)
49
36
12
11
71
2
11
15
0
16
8
14
25
5
π* tmbp + d
d + π tmbp + CO
d
d + COeq
π tmbp
2
4 (px); 15 (dz ); 40 (dxy)
1 (px); 46 (dz ); 13 (dx -y ); 2 (dxy)
2 (pz); 16 (dyz)
2
2
2
27a′′
6
that the two electrons added in the course of the reduction
of complex 2 are nearly equally divided between the tmbp
ligand and the {Mn(CO)3}+ moiety, producing anion 4,
which can be viewed as [Mn0(CO)3(tmbp-)]-. This localized-
valence formulation receives support from X-ray crystal-
lography (see above) and IR spectroscopy. In the latter case,
the average CO force constant kav calculated44 for the Bu4N+
salt of anion 4 (about 1420 N m-1, based on data in Table
3) is close to the average kav value for the isoelectronic five-
coordinate anions [MnI(CO)3(DBCat)]- (1498 N m-1; DBCat
) 3,5-di-tert-butylcatecholate),13k and [Mn-I(CO)3(dppe)]-
(1320 N m-1; dppe ) 1,2-diphenylphosphinoethane).45
By contrast, the 37a′ LUMO of anion 4 (Figure 10A) is
almost exclusively the second lowest unoccupied orbital of
free tmbp, π2*(tmbp). The calculated HOMO-LUMO
energy gap (1.73 eV, Table 5) does not deviate much from
the experimental potential difference between the electro-
chemical oxidation and reduction of anion 4 (1.57 V, Table
2); however, solvation effects and the irreversible nature of
both redox processes present an obstacle for direct compari-
son.
Figure 9. Qualitative molecular orbital diagrams for the isoelectronic
anions [Mn(CO)3(ChL)]- (ChL ) tmbp (4) and 2,2′-bipyridine11) calculated
by the ADF/BP method.
The lower-lying occupied orbitals 28a′′ (HOMO-1), 35a′
(HOMO-2) and 34a′ (HOMO-3) are predominantly man-
ganese d orbitals, with a significant admixture of a π*(CO)
character and contribution from the bonding π(tmbp) orbitals.
The higher-lying unoccupied orbitals 29a′′ (LUMO + 1) and
38a′ (LUMO + 2) again mainly reside on the π* system of
the tmbp ligand, while the 30a′′ (LUMO + 3) is highly
delocalized over the whole Mn(tmbp) metallacycle.
The strongly delocalized character of the π-bonding
between the tmbp ligand and the {Mn(CO)3} moiety in anion
4 is the main factor responsible for the five-coordinate
geometry and absence of interaction with Lewis bases (e.g.,
donor solvents). On the other hand, oxidative addition of
electrophiles is facile. As examples may serve reactions with
Ph3Sn+ or Mn+(CO)3(tmbp) (as halide complexes), giving
rise to the formation of Sn-Mn and Mn-Mn bonds,
respectively. An extraordinary example from the literature
is the η5-bonding between the five-membered Mn(PCCP)
metallacycle in anion 4 and the {Mn(CO)3}+ fragment in
the dinuclear complex [Mn2(CO)6(tmbp)], prepared by a
thermal reaction between [Mn2(CO)10] and tmbp.14
Figure 10. ADF/BP-calculated 37a′ LUMO (A) and 36a′ HOMO (B) of
the anion [Mn(CO)3(tmbp)]- (4).
methods give qualitatively the same composition of the
frontier orbitals. Therefore, only the ADF/BP results are
discussed below. The ADF/BP qualitative molecular orbital
diagram is shown in Figure 9 (right), and the highest
occupied and lowest unoccupied molecular orbitals (HOMO
and LUMO) are shown in Figure 10. Similar pictures have
been obtained for G98/BLY3P.
The 36a′ HOMO of anion 4 (Figure 10B) is strongly
delocalized over the Mn(tmbp) metallacycle, having a
π-bonding character with 44% contribution of the π1*(tmbp)
LUMO and little contribution (5%) of the π(tmbp) HOMO,
the frontier orbitals of free nonreduced tmbp. This means
(44) Braterman, P. S. Metal Carbonyl Spectra; Academic Press: London,
1975.
(45) Kuchynka, D. J.; Amatore, C.; Kochi, J. K. J. Organomet. Chem. 1987,
328, 133-154.
4452 Inorganic Chemistry, Vol. 42, No. 14, 2003

4,4′,5,5′-Tetramethyl-2,2′-biphosphinine
Table 6. Selected Calculated Singlet Excitation Energies for
[Mn(CO)3(tmbp)]- (4) with Oscillator Strength Larger than 0.005
general, these three transitions have a prevailing MLCT
character, though also the IL (tmbp) component cannot be
neglected. This particularly applies for the largely ππ*(tmbp)
27a′′ (HOMO - 4) to 38a′ (LUMO + 2) orbital excitation
as the 24% contribution to the highest lying of the three
electronic transitions.
ADF/BP
transition
G98/B3LYP
transition
exptl
energy oscillator energy oscillator transitions
state
compositiona
(eV)
strength
(eV)
strength
(eV)b
2.29
1A′ 94% (36a′ f 37a′)
1.87
0.009
2.20
0.025
Comparison with Analogous Mn-bpy Complexes.
Halide Complexes. Parent complex 2 closely resembles fac-
[Mn(X)(CO)3(bpy)] (X ) halide, bpy ) 2,2′-bipyridne). In
the latter halide complexes the π-back-bonding toward the
CO ligands is stronger, as revealed by shorter Mn_C and
longer C_O bonds for10 X ) I compared to the values for
complex 2 (see above), and by their smaller IR CO-stretching
wavenumbers (see Table 3). This difference is attributed to
the larger π-acceptor capacity of the tmbp ligand compared
to 2,2′-bipyridine, encountered also in other pairs of com-
plexes such as9 [Cr(CO)4(ChL)] (ChL ) tmbp, bpy)]. Indeed,
complex 2 is electrochemically reduced about 200 mV less
negatively than fac-[Mn(Br)(CO)3(bpy)] (see Table 2). In
both cases the LUMO resides predominantly on the π*
system of the redox-active ligand, which allows a straight-
forward comparison.
1A′′ 71% (28a′′ f 37a′)
28% (36a′ f 29a′′)
2.43
2.72
2.78
0.003
0.084
0.064
2.68
2.87
2.93
0.011
0.094
0.088
1A′′ 61% (36a′ f 29a′′)
21% (28a′′ f 37a′)
3.02
3.86
1A′ 71% (36a′ f 38a′)
22% (28a′′ f 29a′′)
1A′ 51% (28a′′ f 29a′′) 3.29
32% (35a′ f 38a′)
0.091
0.113
0.199
3.45
3.73
3.92
0.117
0.117
0.094
1A′ 55% (35a′ f 38a′)
15% (28a′′ f 29a′′)
3.60
1A′′ 34% (34a′ f 29a′′)
24% (27a′′ f 38a′)
3.74
a Compositions of electronic transitions are expressed in terms of
contributing excitation between ground-state Kohn-Sham molecular orbit-
als, see Table 5 and Figure 9. b Corresponding λmax (nm) values given in
Table 3.
Electronic Absorption Spectrum of Anion [Mn(CO)3-
(tmbp)]- (4). The strongly delocalized nature of the π-bond-
ing between the metal carbonyl moiety and the tmbp ligand
in anion 4 also determines the character of the low-lying
singlet electronic transitions in this complex. The experi-
mental and TD-DFT computed data are summarized in Table
6. Obviously, the theoretical methods reproduce reasonably
well the energetic positions and intensities of the absorption
bands in the electronic spectrum of anion 4 depicted in Figure
8b. The ADF/BP transition energies are lower by about
0.20-0.35 eV compared to the experimental values, while
a fairly good agreement (within 0.10-0.15 eV) has been
obtained with G98/BLY3P. The transition energies can be
influenced by the surrounding medium, in particular the
solvent dipoles that are neglected in the TD-DFT calculations
performed on gas-phase molecules at 0 K.
The broad absorption band of relatively low intensity with
a maximum at 540 nm is shown to correspond to the lowest-
energy singlet electronic transition that is nearly exclusively
36a′ (HOMO) to 37a′ (LUMO) and has a mixed MLCT/IL
(IL ) tmbp-centered, predominantly π1*-to-π2*) character.
The intense asymmetric band at 411 nm belongs mainly
to two close-lying allowed electronic transitions, again of
mixed MLCT/IL characters: predominantly (61%) 36a′
(HOMO) to 29a′′ (LUMO + 1), followed by predominantly
(71%) 36a′ (HOMO) to 38a′ (LUMO + 2). Both transitions
also have a component that involves excitation from 28a′′
(HOMO - 1). An additional, eight to nine times weaker
transition, whose main component (71%) involves 28a′′
(HOMO - 1) to 37a′ (LUMO) MLCT excitation, is probably
responsible for the broadening of the 411 nm band at the
low-energy side.
Both complex 2 and fac-[Mn(X)(CO)3(bpy)] (X ) halide)
are photoreactive. At room temperature, they produce the
corresponding dimers [Mn(CO)3(ChL)]2 (ChL ) tmbp (3),
bpy) upon irradiation into the lowest-energy absorption bands
around 450 nm (Table 3). Whereas the photochemistry of
fac-[Mn(X)(CO)3(bpy)] was studied in detail and the forma-
tion of the intermediate species [Mn(Sv)(X)(CO)2(bpy)] (Sv
) donor solvent) and mer-[Mn(X)(CO)3(bpy)] along the
reaction pathways toward [Mn(CO)3(bpy)]2 has been firmly
established,3c-e for complex 2 only preliminary data are
available. Time-resolved and temperature-dependent studies
need to be conducted to prove whether photoinduced CO-
loss and isomerization reactions occur prior to the formation
of dimer 3 also in this case. The similar bonding properties
of complex 2 and fac-[Mn(Br)(CO)3(bpy)] are also reflected
in the identical reduction pathways. In both cases, no
corresponding transient radical anionic complexes were
detected; the axial Br- ligand is expelled readily, probably
due to a “charge leakage” from the reduced chelating ligand
(ChL) to the metal-halide bond.46 Apparently, in the six-
coordinate complexes the strong π-acceptor character is only
confined to the neutral tmbp ligand. The initial cathodic
process consumes two electrons, as the Br- cleavage reaction
is coupled to the concomitant one-electron reduction of the
five-coordinate radical transients.
Dimers. Even though the crystal structure of [Mn(CO)3-
(bpy)]2 is not available to allow direct comparison with dimer
3 in the solid state, some remarks can be made on the grounds
of the IR and UV-vis spectra (see Table 3). The structure
of the two dimers in solution is not completely identical, as
testified by the different IR ν(CO) patterns. For [Mn(CO)3-
(bpy)]2 the geometry is close to eclipsed (C2V; five ν(CO)
bands identified). The related dimer trans,cis-[Ru(Me)(CO)2-
Finally, the intense absorption band at 321 nm (only partly
shown in Figure 8b) contains contributions from three
allowed combined electronic transitions. They originate in
predominantly 28a′′ (HOMO - 1), 35a′ (HOMO - 2), and
34a′ (HOMO - 3), respectively, and are directed toward
29a′′ (LUMO + 1) and/or 38a′ (LUMO + 2) levels. In
(46) Klein, A.; Vogler, C.; Kaim, W. Organometallics 1996, 15, 236-
244.
Inorganic Chemistry, Vol. 42, No. 14, 2003 4453

Hartl et al.
(iPr-DAB)]2 (iPr-DAB ) N,N′-diisopropyl-1,4-diaza-1,3-
butadiene) is also eclipsed in solution (three (plus one very
weak) ν(CO) bands identified), but turns nearly anti-eclipsed
in the solid state, and the Ru-to-CO π-back-bonding becomes
considerably weaker.47a Dimer 3 preserves the ν(CO) pattern
in solution, but the corresponding wavenumbers increase by
5-10 cm-1. These data point to a less staggered geometry
compared to the solid state. The stronger metal-to-diimine
and weaker metal-to-CO π-back-bonding in the eclipsed
geometry probably originates from a more pronounced
overlap between the π+ and π- combinations of the occupied
dπ(metal) and empty π*(R-diimine) orbitals.47a The UV-
vis spectra of [Mn(CO)3(bpy)]2 and trans,cis-[Ru(Me)(CO)2-
(iPr-DAB)]2 exhibit a characteristic intense metal-to-diimine
charge-transfer band at a low energy (λmax ) 840 and 745
nm, respectively)3c,5a,47a that is indicative of the strong
π-interaction between the coplanar R-diimine ligands in the
(nearly) eclipsed geometry.47a In the UV-vis spectrum of
dimer 3 (Figure 8c), such a band is present at a higher energy
(λmax ) 533 nm), which is in qualitative agreement with a
larger difference between its oxidation and reduction poten-
tials compared to the other two dimers.5a,47 This difference
may point to a more staggered geometry of dimer 3, pre-
sumably due to a steric hindrance between the tmbp ligands
that causes their mutual displacement (Figure 2) and is partly
responsible for the fairly long Mn-Mn distance (see above).
On the other hand, care must be taken when comparing
electronic spectra of analogous bpy and tmbp complexes
without a theoretical support, as demonstrated below for
[Mn(CO)3(bpy)]- and anion 4.
Similar to the parent halide complexes, the formally neutral
tmbp ligand in dimer 3 is clearly a stronger π-acceptor than
bpy in [Mn(CO)3(bpy)]2, which is again reflected in the larger
ν(CO) wavenumbers and less negative reduction potential
of dimer 3 (Tables 2 and 3). Along the reduction paths, both
dimers are formed by a zero-electron coupling reaction
between the parent halide complexes and the corresponding
two-electron-reduced five-coordinate anions. This reaction
could be slowed down, but not completely inhibited, at 193
K in butyronitrile. In the case of the bpy complexes, no
dimerization occurs at 135 K in 2-Me-THF.3c,5b
Five-Coordinate Anions. The formally 16e anion 4 and
its equivalent [Mn(CO)3(bpy)]- are the most interesting
members of the redox series. Their direct structural com-
parison is possible, as the crystal structure of [Mn(CO)3(bpy)]-
has also been solved recently for its unusual Na+(bpy)(Et2O)
salt.48 Even though details on the electronic structure of
[Mn(CO)3(bpy)]- and related five-coordinate complexes will
be published in a forthcoming paper,11 a short synopsis is
presented already here.
and monoclinic, respectively, for the bpy compound),48 which
is most likely dictated by the different coordination environ-
ments of the sodium cations. Like anion 4 (see above), also
[Mn(CO)3(bpy)]- reacts readily with Ph3SnCl to afford
photoreactive fac-[Mn(SnPh3)(CO)3(bpy)].49 Binding of do-
nor solvent molecules (THF, PrCN, DMF) to both anions
is, however, prevented. The unfavorable coordination of a
sixth donor ligand is attributed to the strongly delocalized
π-bonding in the Mn-bpy/tmbp chelate rings. Figure 9
documents that the occupied and empty frontier π and π*
orbitals of anion 4 have their counterparts in [Mn(CO)3(bpy)]-,
even though the corresponding energies may differ. The
highest occupied and lowest unoccupied molecular orbitals,
the 36a′ HOMO and 37a′ LUMO of anion 4 (Table 5) lie at
lower energies (by 0.8 and 0.27 eV, respectively) than the
corresponding levels of [Mn(CO)3(bpy)]-, in agreement with
the similar differences in the oxidation and reduction
potentials of the anions (Table 2). The participation of tmbp
in the 36a′ HOMO is slightly smaller than that of bpy in the
30a′ HOMO of [Mn(CO)3(bpy)]-, viz., 49% vs 57%. On
the other hand, the tmbp ligand contributes notably more to
the three lower-lying metal-based occupied molecular orbit-
als. Consequently, the participation of the basal and apical
CO ligands in these orbitals is reduced by more than 10%
in anion 4, which nicely corresponds with its larger ν(CO)
wavenumbers compared to those of [Mn(CO)3(bpy)]- (Table
3). The 31a′ LUMO of [Mn(CO)3(bpy)]- is fairly π-delo-
calized, with 68% bpy contribution,11 while that of anion 4
resides nearly exclusively on the tmbp ligand (96%). The
36a′ f 37a′ HOMO f LUMO electronic transition in anion
4 therefore obtains a mixed MLCT/IL character, while the
corresponding 30a′ f 31a′ excitation for [Mn(CO)3(bpy)]-
can be assigned as a delocalized πML f π*ML transition with
a limited charge-transfer character. The assignment of the
electronic transitions of the anions 4 and [Mn(CO)3(bpy)]-
in the near-UV-vis region (Table 2) is qualitatively well
supported by the time-dependent DFT calculations. With the
exception of the HOMO f LUMO transition and the
corresponding lowermost absorption band, the higher-lying
electronic transitions of anion 4 (Table 6) show large mixing
and the absorption bands cannot be ascribed to any individual
excitation. A very similar situation is encountered11 in the
case of [Mn(CO)3(bpy)]-. The differences in its UV-vis
spectrum compared to that of anion 4 (Table 3) arise from
a plethora of variables: energies and characters of the
occupied and empty frontier orbitals, mixing of electronic
transitions and their oscillator strength, and energetic dif-
ference between individual excitations.
Concluding, the density functional theory in combination
with crystallographic, redox, and spectroscopic data allows
thorough analysis of bonding properties and electronic
structure of highly delocalized complexes with noninnocent
ligands, such as anion 4 with the 4,4′,5,5′-tetramethyl-2,2′-
biphosphinine ligand. Interestingly, comparison of the studied
series of complexes 2-4 with analogous 2,2′-bipyridine
The anions 4 and [Mn(CO)3(bpy)]- show a similar crystal
structure, described most accurately as a distorted square
pyramid with the chelating ligand at the basis. Their space
groups and crystal systems are, however, different (P21/c
(47) (a) tom Dieck, H.; Rohde, W.; Behrens, U. Z. Naturforsch. 1989, 44b,
158-168. (b) Hartl, F.; Aarnts, M. P.; Nieuwenhuis, H. A.; van
Slageren, J. Coord. Chem. ReV. 2002, 230, 107-125.
(49) (a) Andre´a, R. R.; de Lange, W. G. J.; Stufkens, D. J.; Oskam, A.
Inorg. Chim. Acta 1988, 149, 77. (b) Andre´a, R. R.; de Lange, W. G.
J.; Stufkens, D. J.; Oskam, A. Inorg. Chem. 1989, 28, 318.
(48) Rosa, P. Ph.D. Thesis, EÄ cole Polytechnique, Palaiseau, France, 2000.
4454 Inorganic Chemistry, Vol. 42, No. 14, 2003

4,4′,5,5′-Tetramethyl-2,2′-biphosphinine
Supporting Information Available: (a) Figure SI-1 showing
the polymeric structure of the Na+(OEt2)2 salt of anion 4 (PLATON
view). (b) X-ray structure determination for complexes 2-4,
including tables of crystal data, atomic coordinates and equiv-
alent isotropic displacement parameters, bond distances and ang-
les for all non-hydrogen atoms, anisotropic displacement param-
eters, and hydrogen coordinates. Crystallographic data in CIF
format. This material is available free of charge via the Internet at
http://pubs.acs.org.
compounds has revealed similar bonding properties and redox
reactivity, independent of the stronger π-acceptor character
of the phosphorus ligand.
Acknowledgment. We are thankful for the financial
support provided by the Council for Chemical Sciences of
the Netherlands Organization for Scientific Research (CW-
NWO), the CNRS, and EÄ cole Polytechnique. This work is
also part of the European collaboration COST Action D14,
Project D14/0001/99, supported by Grant No. OC.D14.20
(The Ministry of Education of the Czech Republic).
IC0206894
Inorganic Chemistry, Vol. 42, No. 14, 2003 4455