Tuning coordination modes of pyridine/thioether Schiff base (NNS) ligands to mononuclear manganese carbonyls

Article abstract of DOI:10.1039/c4dt00600c

We have investigated the coordination modes of NNS Schiff base, thioether ligands to manganese(i) carbonyls. The ligands contain ortho substituted pyridines (H, CH₃, OCH₃, fluorophenyl) and varying substituents (H, CH₃) at the Schiff base linkage. In general, reaction of [Mn(CO)₅Br] with a tridentate NNS ligand in CH ₂Cl₂ affords species in which the thioether-S may be bound or unbound to the manganese center, depending on the steric and electronic substitution in the ligand framework; as a result, the complexes exhibit two or three carbonyl ligands, respectively. Aldehyde-derived ligand frames ( _R1N_HNS) generally afford complexes of type [( _RNNS)Mn(CO)₃Br] (1_CO, 2_CO, 3 _CO; R = H, OCH₃, CH₃) that exhibit incomplete ligation of the chelate (S not bound) in X-ray structures. In contrast, use of the iminomethyl ligand (N_MeNS) affords a complex of formula [(N _MeNS)Mn(CO)₂Br] (4_CO), in which the mixed N/thioether-S stabilizes the {Mn(CO)₂}⁺ fragment. In solid state IR spectra, complexes of type [(_RNNS)Mn(CO)₃Br] (1_CO through 3_CO) afford three ν(CO) in the range ～2060-1865 cm^-1; the dicarbonyl complex [(N_MeNS)Mn(CO) ₂Br] (4_CO) exhibits two carbonyl stretches in the range ～1920-1845 cm^-1. Prolonged storage of the tricarbonyl [( _MeNNS)Mn(CO)₃Br] (3_CO) in presence of trace dioxygen affords the dibromide species [(_MeNNS)Mn(Br)₂] (3_Br), in which the thioether S reliably binds to the Mn(ii) center. Complexes 1_CO-3_CO exhibit simple, diamagnetic ¹H NMR spectra in CD₂Cl₂. The S-ligated complex 4_CO exhibits spectra consistent with a mixture of an S-bound (6-coordinate) and S-unbound (5-coordinate) species as represented by [(N _MeNS)Mn(CO)₂Br] ? [(N_MeNS)Mn(CO) ₂Br]. Lastly, we obtained crystal structures of the S-bound and S-unbound conformers derived from the same ligand-the fluorophenyl derived _FPhNNS, namely [(_FPhNNS)Mn(CO)₃Br] (5 _CO-a) and [(_FPhNNS)Mn(CO)₂Br] (5 _CO-b). This report represents several examples of a thioether-stabilized {Mn(CO)₂}⁺ fragment, a deviation from the usual 'piano stool' Mn(i) tricarbonyl motif. We highlight that coordination of these NNS ligands to Mn(i) carbonyls occurs on a soft conformational landscape, and that ligand substituents can be rationally employed to favor the desired coordination mode.

Full text of DOI:10.1039/c4dt00600c

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Tuning coordination modes of pyridine/thioether
Schiff base (NNS) ligands to mononuclear
manganese carbonyls†
Cite this: Dalton Trans., 2014, 43,
10725
Simone E. A. Lumsden, Gummadi Durgaprasad, Keren A. Thomas Muthiah and
Michael J. Rose*
We have investigated the coordination modes of NNS Schiff base, thioether ligands to manganese(I)
carbonyls. The ligands contain ortho substituted pyridines (H, CH3, OCH3, fluorophenyl) and varying
substituents (H, CH3) at the Schiff base linkage. In general, reaction of [Mn(CO)5Br] with a tridentate NNS
ligand in CH2Cl2 affords species in which the thioether-S may be bound or unbound to the manganese
center, depending on the steric and electronic substitution in the ligand framework; as a result, the
complexes exhibit two or three carbonyl ligands, respectively. Aldehyde-derived ligand frames (R1NHNS)
generally afford complexes of type [(RNNS)Mn(CO)3Br] (1CO, 2CO, 3CO; R = H, OCH3, CH3) that exhibit
incomplete ligation of the chelate (S not bound) in X-ray structures. In contrast, use of the iminomethyl
ligand (NMeNS) affords a complex of formula [(NMeNS)Mn(CO)2Br] (4CO), in which the mixed N/thioether-S
stabilizes the {Mn(CO)2}+ fragment. In solid state IR spectra, complexes of type [(RNNS)Mn(CO)3Br]
(1CO through 3CO) afford three ν(CO) in the range ∼2060–1865 cm−1; the dicarbonyl complex [(NMeNS)-
Mn(CO)2Br] (4CO) exhibits two carbonyl stretches in the range ∼1920–1845 cm−1. Prolonged storage of
the tricarbonyl [(MeNNS)Mn(CO)3Br] (3CO) in presence of trace dioxygen affords the dibromide species
[(MeNNS)Mn(Br)2] (3Br), in which the thioether S reliably binds to the Mn(II) center. Complexes 1CO–3CO
exhibit simple, diamagnetic 1H NMR spectra in CD2Cl2. The S-ligated complex 4CO exhibits spectra
consistent with a mixture of an S-bound (6-coordinate) and S-unbound (5-coordinate) species as rep-
resented by [(NMeNS)Mn(CO)2Br] ↔ [(NMeNS)Mn(CO)2Br]. Lastly, we obtained crystal structures of the
S-bound and S-unbound conformers derived from the same ligand – the fluorophenyl derived FPhNNS,
namely [(FPhNNS)Mn(CO)3Br] (5CO-a) and [(FPhNNS)Mn(CO)2Br] (5CO-b). This report represents several
examples of a thioether-stabilized {Mn(CO)2}+ fragment, a deviation from the usual ‘piano stool’ Mn(I)
tricarbonyl motif. We highlight that coordination of these NNS ligands to Mn(I) carbonyls occurs on a soft
conformational landscape, and that ligand substituents can be rationally employed to favor the desired
coordination mode.
Received 26th February 2014,
Accepted 28th April 2014
DOI: 10.1039/c4dt00600c
www.rsc.org/dalton
nyls have found use in carbon–carbon bond forming reactions,4
Introduction
as well as the study of redox-active ligands.5 Organometallic
The coordination chemistry of manganese(I) carbonyls has species of manganese carbonyls have also been explored in
been explored because of their utility as catalysts (both homo- catalytic hydrosilylations6 and alcoholysis of silanes.7 More
geneous1 and heterogeneous2), in the detection and delivery of recently, manganese carbonyls derived from bpy (2,2′-bipyri-
carbon monoxide,3 as well as studies in fundamental coordi- dine) have been used in place of rhenium carbonyls in
nation chemistry. Coordination complexes of manganese carbo- the electrocatalytic reduction of CO2 to CO,8 a promising
direction for energy-related catalysis by earth abundant
metals.9
Department of Chemistry, The University of Texas at Austin, USA.
E-mail: mrose@cm.utexas.edu
†Electronic supplementary information (ESI) available: CIF files for all X-ray
structures; VT 1H NMR spectrum of 1CO (Fig. S1); disorder model for 5CO-b
(Fig. S2). CCDC 988783–988788. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c4dt00600c
In our research, we are interested in the fundamental
coordination chemistry of multi-dentate ligands bearing nitro-
gen and sulfur donors to mononuclear metal carbonyls of the
late first row. The low-spin, d6 configuration of Mn(I) carbonyls
makes them amenable to isolation and solution studies. The
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carbonyls of Mn(I) are isoelectronic with their Fe(II) counter- releasing CO reagent molecules (photoCORMs).23 Related
parts, and therefore exhibit some similar structural and donor photoactive manganese(I) tricarbonyls were also derived from
preferences. However mononuclear Fe(II) carbonyls derived diazopyridine, diimine N2, and N3 ligands.23–25 Most relevant
from coordinating ligands with nitrogen and sulfur donors – to this work are complexes derived from similar Schiff base
and that are free from stabilizing phosphines – are less rep- ligands with a pendant thioether-S. In the two cases of [(LNNS)-
resented in literature. Such iron complexes (especially mono- Mn(CO)3]+ (L = pmtpm and qmtpm; NNS and QNNS in our
nuclear species) can be challenging to isolate, requiring low nomenclature), the thioether remained unbound.23 Indeed, to
temperatures and dark conditions.10–15 In contrast, the Mn(I) date there is just a single example of a mixed N/S(thioether)
carbonyls are thermally stable under ambient conditions in ligand fully bound in a mononuclear complex, namely the
non-coordinating solvents, and exhibit a reduced sensitivity to complex [(py(CH2)SPh)Mn(CO)3Br].26 Also, irrespective of
light. Therefore, as an entry point to iron chemistry, we wished ligand choice (N2S, N2, or N3) in all of these cases, only the tri-
to explore the coordination of a tridentate NNS ligand frame carbonyl ‘piano stool’ congeners were isolated.
with manganese carbonyls.
Inspection of crystallographically characterized thioether
Mn(I) carbonyls reveals several interesting trends. Cationic
complexes of Mn(I) carbonyls with fully bound thioether-S
ligation have been previously reported,16,17 as in the case of
[(LS3)Mn(CO)3]+, where the identities of LS3 are given below.
Select examples of complete LS2 ligation to neutral complexes
also exist, as in the case of [(LS2)Mn(CO)3X] (R = CH3, Ph;
X = Cl, Br, respectively).18 However, incomplete ligation of
thioether chelates is very common, especially in neutral com-
plexes. For example, the supporting tripodal ligand in [(PS2)-
Mn(CO)3Br] and the tetrapodal [(LS4)Mn(CO)3Cl] and leave
one or two (respectively) thioether-S donor(s) unbound.19,20
This occurs even in some cationic complexes: the crown-type
complexes [([12]aneS4)Mn(CO)3]+ and [([15]aneS3/5)Mn(CO)3]+
also exhibit one and two unbound thioether(s) (respectively).21
In this work, we attempted to circumvent difficulties associ-
ated with iron carbonyl chemistry, and utilize the more reliably
Treatment of [([12]aneS3/4)Mn(CO)3]+ with Me3NO did provide
the dicarbonyl species [([12]aneS4/4)Mn(CO)2]+, wherein all 4
mononuclear manganese congeners to explore the fundamen-
thioether-S were bound.21 Indeed, the authors of this report
tal coordination chemistry of NNS type Schiff base ligands
with metal carbonyls. As dimers readily form across thiolato-S
bridges, we have also chosen a thioether moiety to allow for a
sulfur donor, but to retain mononuclear motifs. Our work is
focused on ligand optimization to ensure complete ligation of
the chelate to the metal center, as well as structural and spec-
troscopic characterization of the resulting complexes. We show
that careful selection of the ligand substituents (see substi-
tuted NNS framework above) leads to not only the fully bound
NNS coordination motif, but also to a new example of a
thioether-stabilized {Mn(CO)2}+ motif in the obligatory cis
arrangement.
noted this was the first report of a thioether-stabilized {Mn-
(CO)2}+ fragment, which is typically only stabilized by phos-
phines and other strong σ-donating, π-accepting ligands. The
{Mn(CO)2}+ fragment formed spontaneously under metalation
conditions, although photolysis or exchange of coordinating
solvent (MeCN, THF) can readily afford the {Mn(CO)2}+ unit in
some cases.22
Experimental
General procedures
The manganese starting material [Mn(CO)5Br] was obtained
from Strem Chemicals. All other reagents were obtained from
Sigma Aldrich or Acros and used without further purification.
For ligand preparation, solvents (MeOH, CH2Cl2, AcOH) were
purchased from Fisher and used without further purification.
During the course of this work, Mascharak and co-workers For metalation reactions, the CH2Cl2, Et2O and pentane were
reported the preparation of several manganese(I) carbonyls dried over an alumina column under argon atmosphere. All
derived from Schiff base ligands, and their utility as photo- metalations and crystallizations were performed under N2
10726 | Dalton Trans., 2014, 43, 10725–10738
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atmosphere. Deuterated solvents were obtained from Cam- TMS): 8.66 d (1H), 8.39 d (1H), 7.79 t (1H), 7.36 dt (1H), 7.25 d
bridge Isotopes or Acros Organics. Several aldehyde-derived (1H), 7.14 m (2H), 6.72 d (1H), 2.37 s (3H), 2.34 (3H). HRMS
ligands (NNS and MeNNS) were prepared according to literature calcd: 241.0799, 242.0878, 243.0956; found: 241.0802,
reports.27,28 We independently devised an analogous synthesis 242.0878, 243.0959.
of [(NNS)Mn(CO)3Br] compared with a recently published
Complexes
report.23
[(NNS)Mn(CO)3Br] (1CO). Under N2 atmosphere, 0.130 g
(0.471 mmol) of [Mn(CO)5Br] was stirred in 10 mL of DCM in a
Ligands
OMeNNS. 1-(6-Methoxy-2-pyridyl)-N-(2-methylsulfanylphenyl)- round bottom flask until a clear yellow-orange solution is
methanimine. A batch of 6-methoxy-2-pyridinecarboxaldehyde obtained. In
a separate round bottom flask, 0.110 g
(2.00 g, 14.6 mmol) was dissolved in 20 mL of MeOH. To this (0.482 mmol) of ligand was dissolved in another 10 mL of
stirred solution was added a solution of 2-(methylthio)aniline CH2Cl2 to generate a yellow solution. The ligand/CH2Cl2 solu-
(2.03 g, 14.6 mmol) in 15 mL of MeOH. The solution was heated tion was added dropwise to the [Mn(CO)5Br] solution. The
to reflux temperature for 2 h, at which point it was cooled to mixture was stirred overnight and there was a slow color
ambient temperature and solvent evaporated under a stream change from yellow to a dark red solution. The dark solution
of N2 gas. The resulting tacky solid was triturated 2× with Et2O was subjected to vapor diffusion of pentane at room tempera-
to yield an orange solid. Selected IR bands (ν in cm−1): 1623 w, ture, which resulted in small black/red crystals suitable for X-
1587 m, 1567 s, 1462 s, 1439 s, 1321 m, 1138 w, 800 vs, 750 vs, ray diffraction. Yield: 71 mg (34%). Selected IR bands (ν in
1
725 vs, 635 w, 538 m. H NMR in CDCl3 (δ from TMS): 8.44 s cm−1, νCO in italics): 2023 m, 1918 s, 1867 s, 1590 w (νCN), 1415
(1H), 7.90 d (1H), 7.68 t (1H), 7.23 br (1H), 7.16 br (1H), 7.08 w, 962 w, 767 s, 581 m. UV/vis in CH2Cl2, λ in nm (ε in cm−1
br (1H), 6.82 d (1H), 6.75 br (1H), 4.00 s (3H), 2.47 s br (3H).
M
−1): 323 (19 900); also see Table 3. 1H NMR in CD2Cl2 (δ from
HRMS calcd: 257.0749, 258.0827, 259.0905; found: 257.0756, TMS): 9.27 s (1H), 8.99 s (1H), 8.48 s (1H), 8.08 m (2H),
258.0833, 259.0903. NMR analysis indicated unreacted (∼30%) 7.78–7.32 m (3H), 2.54 s (3H). Elemental Analysis, calcd:
aldehyde that was not separated by fractional crystallization C 42.97, H 2.70, N 6.26; found: C 42.82, H 2.71, N 6.50.
or chromatography. The mixture was used in subsequent
metalations.
[(MeONNS)Mn(CO)3Br] (2CO). Under N2 atmosphere, 0.078 g
(0.284 mmol) of [Mn(CO)5Br] was stirred in 10 mL of DCM in a
FPhNNS.
N-((6-(4-Fluorophenyl)pyridin-2-yl)methylene)-2- round bottom flask until a clear yellow-orange solution is
separate round bottom flask, 0.071
(methylthio)aniline. A small batch of 6-(4-fluorophenyl)-2-pyri- obtained. In
a
g
dinecarboxaldehyde (0.289 g, 1.434 mmol) was dissolved in (0.275 mmol) of MeONNS ligand was dissolved in another 10 mL
25 mL of MeOH. To this stirred solution was added a solution DCM to generate a pale yellow solution. The ligand/DCM solu-
of 2-(methylthio)aniline dissolved in 5 mL of MeOH. The reac- tion was added dropwise to the [Mn(CO)5Br] solution. The
tion mixture was gently refluxed for 1 h, and then the solvent mixture was stirred overnight and there was a slow color change
removed in vacuo. The resulting residue was washed with a from yellow to a dark blood-red solution. The dark solution was
small portion of cold pentane, and the product was collected subjected to vapor diffusion of pentane at room temperature,
as a yellow solid. Yield: 351 mg (76%). Selected IR bands (ν in which resulted in small red cubic crystals suitable for X-ray
cm−1): 1622 m, 1600 m, 1586 m, 1451 s, 1351 w, 1213 s, 810 vs, diffraction. Yield: 70 mg (54%). Selected IR bands (ν in cm−1
,
1
642 w, 566 s. H NMR in CDCl3 (δ in ppm from TMS): 8.64 s νCO in italics): 2056 s, 1934 s, 1905 vs, 1619 w (νCN), 1478 s,
(1H), 8.25 d (1H), 8.03 dd (2H), 7.90–7.84 m (2H), 7.73 d (1H), 1306 m, 957 w, 627 s, 455 w. UV/vis in CH2Cl2, λ in nm (ε in
7.32 s br (1H), 7.16 t (1H), 7.10 m br (2H) 6.70 s br (1H), 2.46 s cm−1
M
−1): 354 (4070); also see Table 3. Elemental Analysis,
(3H). HRMS calcd: 321.0862, 322.0940, 323.1018; found: calcd: C 42.79, H 2.96, N 5.87; found: C 40.47, H 3.09, N 5.95.
321.0860, 322.0939, 323.1010. [(MeNNS)Mn(CO)3Br] (3CO). Under inert atmosphere, 0.130 g
MeNS. 2-(Methylthio)-N-(1-(pyridin-2-yl)ethylidene)aniline. (0.474 mmol) of [Mn(CO)5Br] was stirred in 15 mL of CH2Cl2
N
The 2-(methylthio)aniline (0.500 g, 3.59 mmol) was diluted in in a round bottom flask until completely dissolved. Separately,
5 mL of acetic acid, and to this solution was added 2-acetylpyri- 0.106 g (0.437 mmol) of MeNNS was diluted in 15 mL of CH2Cl2.
dine (0.434 g, 3.59 mmol) in 5 mL of AcOH. Next, a slurry of The ligand/CH2Cl2 solution was added dropwise to the [Mn-
ZnCl2 (0.489 g, 3.59 mmol) in 5 mL of AcOH was added, and (CO)5Br] solution. The mixture was stirred overnight and there
the reaction refluxed for 1 h to generate an orange solution was a slow color change from yellow to dark red-brown. The solu-
(when hot). Upon cooling a yellow solid forms, which was col- tion precipitated some dark material and was filtered with a
lected by filtration and washed with several portions of Et2O. celite pipette to extract the fine precipitate from solution. The
The solid was then dissolved in CH2Cl2 (25 mL) and washed 2× solution was placed into a Schlenk flask and placed in 4 °C
in a separatory funnel with 0.25 M K2C2O4, and 2× with water. which resulted in large red-orange blocks suitable for X-ray
The organic layer was dried over Na2SO4 and evaporated to diffraction. Yield: 144 mg (72%). Selected IR bands (ν in cm−1
,
dryness by rotary evaporation to afford a yellow oil. Yield: νCO in italics): 2019 m, 1925 vs, 1618 w (νCN), 1579 w, 1263 w,
0.49 g (56%). Selected IR bands (ν in cm−1): 3052 w, 2919 w, 799 m, 626 s. UV/vis in CH2Cl2, λ in nm (ε in cm−1 M−1): 314
1635 s, 1578 m, 1565 m, 1466 s, 1434 s, 1301 m, 993 w, 965 w, (7490), 488 (1530); also see Table 3. 1H NMR in CD2Cl2 (δ from
780 vs, 737 vs, 649 w, 621 m. 1H NMR in CDCl3 (δ in ppm from TMS): 8.46 s (1H), 7.91 t (1H), 7.76 t (2H), 7.52 d (1H), 7.39 m
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(2H), 7.31 t (1H), 3.13 s (3H), 2.53 s (3H). Elemental Analysis, solution: slow vapor diffusion of Et2O (1 to 3 days) afforded
calcd: C 44.27, H 3.06, N 6.07; found: C 43.94, H 3.12, N 6.00. black needles of 5CO-b (S bound) suitable for X-ray diffraction.
[(MeNNS)Mn(Br)2] (3Br). Method A. A reaction was prepared Yield: 73 mg (Yield: 46%). Selected IR bands (ν in cm−1, νCO in
according to the procedure for 3CO stated above (3CO collected italics): 1934 s, 1872 vs, 1607 m (νCN), 1474 s, 1171 s, 839 w,
as precipitate), and the filtrate was subjected to vapor 749 vs, 519 w. UV/vis in CH2Cl2, λ in nm (ε in cm−1 M−1): 332
diffusion of Et2O. This generated red crystals suitable for X-ray (3250), 612 (495); also see Table 3. Elemental Analysis, calcd: C
analysis. As the yield was <10%, we developed an alternative 49.14, H 2.95, N 5.46; found: C 48.84, H 2.82, N 5.47.
preparation. Method B. Under ambient atmosphere, 0.060 g
Physical measurements
(0.218 mmol) of [Mn(CO)5Br] is stirred in 20 mL of CH2Cl2 in a
round bottom flask until completely dissolved. Separately, 1H NMR spectra of ligands were collected on Varian DirecDrive
0.050 g (0.206 mmol) of MeNNS is diluted in 20 mL of Et2O. 400 MHz spectrometer and chemical shifts were referenced to
The CH2Cl2 solution of the Mn carbonyl was distributed to a TMS. 1H NMR spectra of the manganese complexes were
series of small test tubes. Next, the Et2O solution of ligand was recorded on a 500 MHz Varian Inova instrument fitted with a
carefully layered on top of the Mn solution, and the mixture variable temperature apparatus. UV/vis absorption spectra
was capped and allowed to diffuse slowly for 1 week. This pro- were obtained using an Ocean Optics USB2000+XRS-ES detec-
cedure afforded large red diamond shaped crystals suitable for tor probed with a PX2 pulsed xenon lamp. Solutions of the
X-ray diffraction. Yield: 27 mg (29%). Selected IR bands (ν in complexes were prepared at ∼0.1 mM in 1 cm quartz cuvettes
cm−1): 1586 m (νCN), 1378 m, 962 m, 794 s, 766 s, 738 s, in an argon drybox, and absorbances were measured with the
579 m. Magnetic moment (solid state, 298 K): μeff = 4.9 μB. aid of fiber optic feedthroughs. Infrared spectra of complexes
Elemental Analysis, calcd: C 36.79, H 3.09, N 6.13; found: C in the solid state were recorded under ambient atmosphere
36.78, H 3.05, N 6.02.
on a Bruker Alpha spectrometer equipped with a diamond
[(NMeNS)Mn(CO)2Br] (4CO). Under inert atmosphere, 0.146 g ATR crystal. The solid state magnetic moment of 3Br was
(0.531 mmol) of [Mn(CO)5Br] was stirred in 10 mL of CH2Cl2 determined using a Johnson-Mathey magnetic susceptibility
in a round bottom flask until completely dissolved. Separately, balance.
0.108 g (0.446 mmol) of NMeNS was diluted in 10 mL of
CH2Cl2. The ligand/CH2Cl2 solution was added dropwise to the
[Mn(CO)5Br] solution. The mixture stirred overnight and there
X-ray diffraction data collection and crystal structure
refinement
was a slow color change from yellow to dark red-orange. This The data were collected on either a Rigaku AFC12 diffracto-
red-orange solution was subjected to vapor diffusion of Et2O at meter with a Saturn 724+ CCD or on a Nonius Kappa CCD
room temperature, which resulted in black needles suitable for diffractometer using a Bruker AXS Apex II detector, both using
X-ray diffraction. Yield: 152 mg (66%). Selected IR bands (ν in a graphite monochromator with MoKα radiation. Reduced
cm−1, νCO in italics): 1918 s, 1847 s, 1594 w (νCN), 1430 m, 1256 temperatures were maintained using an Oxford Cryostream
1
w, 965 m. H NMR in CD2Cl2: see main text. UV/vis in CH2Cl2, low temperature device. Data reduction were performed using
λ in nm (ε in cm−1 M−1): 319 (7450), 560 (510); also see the Rigaku Crystal Clear version 1.40.2. Structures were solved
Table 3. 1H NMR in CD2Cl2 (δ from TMS): two species by direct methods using SIR973 and refined by full-matrix
observed; see main text. Elemental Analysis, calcd: C 44.36, least-squares on F2 with anisotropic displacement para-
H 3.26, N 6.47; found: C 43.53, H 3.30, N 6.33.
meters for the non-H atoms using SHELXL-97.4. Structure
Subspecies of FPhNNS: [(FPhNNS)Mn(CO)3Br] (5CO-a) and analysis was aided by use of the programs PLATON985 and
[(FPhNNS)Mn(CO)2Br] (5CO-b). Under N2 atmosphere, 0.113 g WinGX.6. The hydrogen atoms on carbon were calculated
(0.411 mmol) of [Mn(CO)5Br] was stirred in 10 mL of DCM in a in ideal positions with isotropic displacement parameters set
round bottom flask until a clear yellow-orange solution is to 1.2Ueq of the attached atom (1.5Ueq for methyl hydrogen
obtained. In
a
separate round bottom flask, 0.108
g
atoms). For 5CO-b, a large peak persisted in the difference
(0.335 mmol) of FPhNNS ligand was dissolved in another 10 mL electron density map 2.4 Å from Mn. Additional but much
DCM to generate a yellow solution. The ligand/DCM solution smaller peaks were observed near Br1. These peaks were
was added dropwise to the [Mn(CO)5Br] solution. The mixture interpreted to be from a five coordinate Mn complex with one
stirred overnight and there was a slow color change from Br atom and one carbonyl group present in small quantities
yellow to a dark maroon-red solution. The reaction was split within the crystal. As such, the structure was modeled as a
into two equal portions. Portion A. The first dark solution was disordered species where the lone carbonyl group resides
subjected to vapor diffusion of pentane at room temperature near the Br atom of the six coordinate species. The Br atom of
(12–24 h), which resulted in small red cubic crystals of 5CO-a (S the five coordinate species resides between the two carbonyl
unbound) suitable for X-ray diffraction. Yield: 68 mg (Yield: groups of the six coordinate complex. The relative occupancies
44%). Selected IR bands (ν in cm−1, νCO in italics): 2021 s, 1943 of the two species were estimated by assigning the variable x
s, 1914 vs, 1603 w (νCN), 1423 m, 1224 s, 846 vs, 733 s, 545 m. to the site occupancy for Br1 and (1 − x) to the site occupancy
UV/vis in CH2Cl2, λ in nm (ε in cm−1 M−1): 497 (1120). Elemen- for Br1a.
A common isotropic displacement parameter
tal Analysis, calcd: C 48.82, H 2.79, N 5.18; found: C 48.67, H was refined for the two Br atoms while refining the variable x.
2.70, N 5.21. Portion B. To the second portion of the reaction In this way, the site occupancy for Br1 refined to 96(1)%.
10728 | Dalton Trans., 2014, 43, 10725–10738
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Table 1 Crystal data and refinement parameters
2CO [(OMeNNS)-
Mn(CO)3Br]
3CO·CH2Cl2 [(MeNNS)- 3Br [(MeNNS)-
Mn(CO)3Br]·CH2Cl2 Mn(Br)2]
4CO [(NMeNS)-
Mn(CO)2Br]
5CO-a [(FPhNNS)-
Mn(CO)3Br]
5CO-b [(FPhNNS)-
Mn(CO)2Br]
Formula
FW
Color
Habit
Size (mm)
T (K)
C
17H14N2O4BrSMn C18H16N2O3BrSMn
C14H14N2Br2SMn C16H14BrMnN2O2S C22H15N2O3SBrFMn C21H15N2O2SBrFMn
477.21
Red
Plate
546.14
Red
Block
457.09
Orange
Block
433.20
Black
Needle
541.27
Orange
Plate
512.14
Black
Needles
0.17 × 0.13 × 0.08
100(2)
0.36 × 0.21 × 0.09 0.65 × 0.41 × 0.11
163(2)
0.30 × 0.14 × 0.09 0.27 × 0.21 × 0.12 0.34 × 0.23 × 0.11
163(2)
173(2)
153(2)
153(2)
λ (Å)
0.71073
Triclinic
P1
0.71073
Triclinic
P1
0.71073
Monoclinic
P21/n
11.838(3)
10.124(3)
13.654(4)
90
102.610(7)
90
1596.9(8)
4
0.71073
Monoclinic
P21/n
8.7752(19)
12.659(3)
14.644(3)
90
92.798(5)
90
1624.8(6)
4
0.71073
Monoclinic
P21/n
7.9074(5)
21.6449(14)
12.5018(6) Å
90
96.094(2)
90
2127.7(2)
4
0.71073
Monoclinic
P21/c
13.942(5)
11.412(5)
14.573(4)
90
117.66(3)
90
2053.7(13)
4
Lattice
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
ˉ
ˉ
7.5300(3)
7.6723(3)
17.7327(6)
87.250(2)
79.371(2)
66.350(2)
921.93(6)
2
8.7613(3)
10.8921(4)
13.0888(5)
66.319(2)
79.790(2)
69.483(2)
1070.35(7)
2
γ (°)
V (Å3)
Z
dcalc (g cm−3
)
1.719
3.021
1.122
1.695
2.852
1.031
1.901
5.952
0.999
1.771
3.409
1.112
1.690
2.632
1.121
1.656
2.718
1.110
μ (mm−1
)
GOF on F2
R indices [I > 2σ(I)] R1 = 0.0232
wR2 = 0.0676
R indices all data R1 = 0.0273
wR2 = 0.0700
R1 = 0.0350
wR2 = 0.0792
R1 = 0.0412
wR2 = 0.0842
R1 = 0.0872
wR2 = 0.2012
R1 = 0.1217
wR2 = 0.2242
R1 = 0.0310
wR2 = 0.0735
R1 = 0.0365
wR2 = 0.0759
R1 = 0.0284
wR2 = 0.0661
R1 = 0.0411
wR2 = 0.0710
R1 = 0.0553
wR2 = 0.1205
R1 = 0.0725
wR2 = 0.1280
The ratio of the six coordinate species in the crystal to the intermediate was de-metalated by treatment with aqueous
five coordinate species is 24 : 1. After an initial refinement, oxalate, allowing extraction of the free ligand into the organic
the coordinates for O1a and C2a were fixed along with their dis- layer.
placement parameters in the final refinement model. The data
collection and refinement parameters are listed in Table 1.
We pursued the metalation of NNS-type ligands with [Mn-
(CO)5Br] under reported conditions (CH2Cl2, room tempera-
ture, N2 atmosphere).17–23 In general, the reaction of a yellow
solutions of NNS and [Mn(CO)5Br] generated a dark red color
over the course of 15 min to 1 hour. Vapor diffusion of
pentane or Et2O afforded crystalline material that ranged from
black to translucent orange. We found that ligation of the
thioether-S donor was dependent on the nature of the Schiff
base R-group. As shown in Scheme 1, metalations of aldehyde-
derived ligands afford the NNS bound complex of type [(RNNS)-
Mn(CO)3Br]. Coincidentally, the same reaction was reported by
Mascharak and co-workers during preparation of this manu-
script using the same NNS ligand, as well as its quinoline con-
Results and discussion
Syntheses
The NNS ligands derived from aldehyde starting materials
(denoted R1NNS) were prepared under reported conditions
(condensation in MeOH).27,28 The novel iminomethyl-substi-
tuted ligands (ketone starting materials; denoted R1NMeNS)
were prepared by a zinc-templated reaction (AcOH, ZnCl2,
reflux) with 2-(methylthio)aniline. The resulting zinc-bound
Scheme 1 Metalation of substituted NNS ligands with [Mn(CO)5Br]; the coordination mode is dependent on R2.
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Scheme 2 Metalation of MeNNS with [Mn(CO)5Br] to form [(MeNNS)Mn(CO)3Br] (3CO) and subsequent conversion to the Mn(II) dibromide species
[(MeNNS)Mn(Br)2] (3Br).
gener.23 This trend of the unbound thioether-S is largely inde- base moiety. Because this distorts the otherwise planar, conju-
pendent of the R-group at the ortho position of the pyridine gated π-system of the ligand frame, this indicates a strong pre-
moiety (H, CH3, OCH3). Conversely, substitution of a methyl ference of the Mn(I) center for the back-bonding CO ligand
group at the Schiff base affords the fully ligated complex versus the σ-donor only character of the thioether-S. There is,
[(NMeNS)Mn(CO)2Br]. Binding of the thioether is likely encour- however, a somewhat close contact between the unbound S
aged by steric repulsion between the Schiff base –CH3 group donor and the adjacent carbonyl – the S⋯C(O) distance is
and the aryl(methyl)thioether unit in the S-unbound state ∼3.9 Å. The three carbonyl ligands are bound in a facial
(vide infra, X-ray section).
arrangement (as nearly always found in tricarbonyls), and
The manganese carbonyls are generally stable in CH2Cl2 exhibit Mn–C(O) distances of 1.806(3), 1.781(4), 1.802(3) Å –
solution at 298 K under N2, even in presence of trace amounts the shortest of which is bound trans to the Br− ion. The Mn–Br
of O2. One exception is the 2-methylpyridine complex [(MeNNS)- distance of 2.5338(5) Å is predictably long due to repulsion
Mn(CO)3Br], which crystallized in sufficient purity only at between the low-valent Mn(I) and bromide ion, and in a
−20 °C. At ambient temperature (and with trace O2), this similar range to other Mn(I) carbonyl bromides.
complex transforms to the Mn(II) dibromide complex [(MeNNS)-
Mn(Br)2], in which the chelate is fully ligated (Scheme 2).
[(OMeNNS)Mn(CO)3(Br)] (2CO). The X-ray structure of the
methoxy-appended complex 2CO is also shown in Fig. 1.
Overall, the complex exhibits similarity to 1CO in that the
coordination geometry is pseudo-octahedral and the thioether
X-ray structures
[(HNNS)Mn(CO)3(Br)] (1CO). During the course of this work, moiety remains unbound; the three carbonyl ligands are also
the X-ray structure of 1CO was reported by another research arranged in the same facially coordinating fashion. The Mn–
group.23 For the purpose of comparison to the other com- Npy bond distance of 2.0844(15) Å is slightly longer than that
plexes, we briefly describe the metrics of the reported complex found in 1CO (Mn–Npy = 2.054(2) Å), likely due to the steric
here. Similar to 2CO (Fig. 1), the manganese center in 1CO is repulsion due to the ortho methoxy group. The repulsion effect
overall pseudo-octahedral, with the two N donors of the ligand is not apparent in the Mn–NSB bond distances found in 2CO
bound to the metal center. The Mn–Npy and Mn–NSB bond dis- [2.0446(15) Å) and 1CO (2.049(2) Å]. The methoxy-O is located
tances [2.054(2) and 2.049(2) Å, respectively] are typical for low- 3.207 Å away from the Mn center, distinctly outside the range
spin, Mn(I) nitrogen bonds in complexes bearing the 2-imino- of normal Mn–O bonding. Notably, the distance between the
methene-pyridyl moiety. The thioether-S is not bound to the methoxy-O and the C-donor of the adjacent carbonyl is quite
manganese center, with the aryl(thioether) moiety rotated at a short – only 2.66 Å. As found in 1CO, there is a subtle inter-
dihedral angle of 65.9° away from the planar pyridyl/Schiff action between the unbound thioether and a carbonyl ligand.
This interaction appears to be more pronounced in the case of
2CO [S⋯C(O) ≈ 3.34 Å] versus that observed in 1CO [S⋯C(O) ≈
3.9 Å]. This appears to occur in 2CO due to a slightly greater
twist of the aryl(methyl)thioether moiety (70.0°) versus that in
1CO (65.9°). Interestingly, the largest differences in carbonyl
ligand metrics between 2CO and 1CO occur at the carbonyl partici-
pating in the interaction with the thioether S moiety. While the
Mn–C–O bond angles are nearly invariant in each complex
[174.03(18)°, 174.86(16)°], in 2CO the Mn–C(O) and (Mn)C–O
bonds [1.798(2), 1.126(3) Å] are significantly longer than the
same distances in 1CO [1.781(4), 1.148(4) Å]. This suggests the
S⋯C(O) interaction is significant, and generates weaker bonding
along the Mn–C–O axis due to partial donation of sulfur lone
pair to the Lewis acidic carbonyl C. The remaining bond dis-
Fig. 1 ORTEP diagram (50% ellipsoids) of [(OMeNNS)Mn(CO)3Br] (2CO).
tances to the carbonyl and bromide ligands are unremarkable.
H atoms are omitted for clarity.
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Fig. 2 ORTEP diagrams of the molecular structures of [(MeNNS)Mn(CO)3Br] (3CO, 50% ellipsoids) and [(MeNNS)Mn(Br)2] (3Br, 30% ellipsoids). H atoms
are omitted for clarity.
[(MeNNS)Mn(CO)3(Br)] (3CO). The X-ray structure of the 2.4881(19) Å] bond distances found in 3Br are longer than in the
2-methylpyridine derived manganese carbonyl 3CO is shown corresponding manganese carbonyl, due to the high-spin con-
below in Fig. 2. This manganese species exhibits a similar figuration of the complex (vide infra, Spectroscopy section). Of
binding mode and overall coordination geometry to 1CO and note is the fact that this complex is monomeric; many dihalide
2CO. The steric effect of the 2-methyl group at the pyridine complexes of Mn(II) with bidentate or tridentate ligands
moiety is quite pronounced, resulting in a long Mn–Npy bond exhibit a dimeric motif via a μ-(X)2 (where X = Cl, Br) bridge. It
of 2.097(2) Å, versus the corresponding Mn–Npy distance of is likely that dimerization is prevented by the combined steric
2.0844(15) Å and 2.054(2) Å in 2CO and 1CO, respectively. effects of the 2-methylpyridine and thioether S–CH3 units.
However the Schiff base N-donor remains bound at approxi- Close inspection of the structural database reveals numerous
mately the same distance (2.045(2) Å) as found in 1CO and 2CO
.
examples of dimerized Mn2Br2 complexes.29–33 However, the
And although the –CH3 protons were not experimentally arrangement of the capping ligands is always parallel and
located in the diffraction map, the model suggests that the close-packed – a configuration not possible with the two
CH3⋯C(O) distance is short – in the range of 2.6 to 2.7 Å – and pendant methyl units (–SCH3 and orthopy-CH3).
less than the sum of the van der Waals radii (∼2.8 Å).
[(NMeNS)Mn(CO)2(Br)] (4CO). The tridentate NNS ligand
However, there is no resulting perturbation of the Mn–C–O bearing a methyl substituent at the Schiff base linkage in 4CO
bond metrics in 3CO versus 2CO or 1CO that can be attributed to is fully ligated in NNS fashion to the manganese center
the infringing methyl unit. Interestingly, in 3CO the S⋯C(O) (Fig. 3). As a result, the Mn–Npy and Mn–NSB bonds in 4CO are
interaction (∼3.26 Å) is closer than in 2CO or 1CO, but unlike much shorter (1.982(2) and 2.029(2) Å, respectively) than those
2CO there is no significant change in Mn–C–O bond metrics in 1CO, presumably due to the chelation effect of all three
versus 1CO. The remaining Mn–C(O), C–O and Mn–Br distances bound donor atoms. The Mn–S bond distance of 2.301(8) Å is
are listed in Table 2.
similar to that found in the only other mixed N/S(thioether)
[(MeNNS)Mn(Br)2] (3Br). The X-ray structure of the dibromide manganese carbonyl [2.3449(6), 2.3467(7) Å].26 The Mn–S bond
complex [(MeNNS)Mn(Br)2] is shown in Fig. 2 (right side). length in 4CO is also much shorter than in the Mn(II) dibro-
Both the Mn–N [2.246(8), 2.245(8) Å] and Mn–Br [2.4877(18), mide species described herein [2Br, 2.6839(13); 3Br, 2.645(3) Å].
Table 2 Selected bond distances (Å) and bond angles (°)
R1, R2
1COa H, H
2CO OCH3, H
3CO CH3, H
3Br CH3, H
4CO H, CH3
5CO-a FPh, H
5CO-b FPh, H
Mn–Npy
Mn–NSB
Mn–SMe
Mn–Br
2.054(2)
2.049(2)
—
2.5338(5)
—
2.0844(15)
2.0446(15)
—
2.5216(3)
—
2.097(2)
2.045(2)
—
2.5314(5)
—
2.246(8)
2.245(8)
2.645(3)
2.4877(18)
1.987(2)
2.029(2)
2.3001(8)
2.5564(7)
—
2.0954(16)
2.0488(16)
—
2.5250(4)
—
2.100(4)
2.016(4)
2.3069(14)
2.5690(10)
—
Mn–Br
2.4781(19)
Mn–C(O) trans SB
(Mn)C–O trans SB
Mn–C(O) trans Br
(Mn)C–O trans Br
Mn–C(O) trans py
(Mn)C–O trans py
Aryl-(SMe) Dihedral
S⋯C(O)Br
1.806(3)
1.148(3)
1.781(4)
1.148(4)
1.802(3)
1.148(3)
65.92
1.815(2)
1.140(2)
1.798(2)
1.126(3)
1.810(2)
1.140(3)
70.00
1.809(3)
1.139(4)
1.803(3)
1.143(3)
1.811(3)
1.142(4)
68.46
—
—
—
—
—
—
9.33
—
—
1.789(3)
1.155(3)
1.789(3)
1.131(3)
—
—
31.92
1.824(2)
1.144(2)
1.802(2)
1.132(3)
1.799(2)
1.147(2)
64.64
1.805(5)
1.145(5)
1.799(4)
1.135(5)
—
—
5.18
3.900
2.646
3.338
2.666
3.258
∼2.65
—
2.694
4.128
2.947
—
3.097
R1⋯C(O)SB
a Values taken from ref. 23.
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Fig. 3 ORTEP diagram (50% ellipsoids) of the molecular structure of
[(NMeNS)Mn(CO)2Br]. H atoms are omitted for clarity.
Interestingly, binding of the thioether-S induces a twist in the
conjugated ligand frame, exhibiting a dihedral angle of 32.06°
from the Schiff base to the aryl(thioether) moiety. In 4CO, the
origin of thioether ligation is likely steric: in 1CO, the distance
from the unbound and ‘twisted’ aryl–S–CH3 to the imine-H is
∼3.0 Å. In the case of 4CO, replacement of the imine-H with
imine-CH3 would decrease this contact to within the van der Fig. 4 Infrared spectra of [(HNNS)Mn(CO)3Br] (1CO
, top red line),
[(NMeNS)Mn(CO)2Br] (4CO, middle blue line) and [(MeNNS)Mn(CO)3Br]
(3CO, bottom green line) at 298 K in the solid state.
Waals radii, disallowing protrusion of the thioether in the
direction of the Schiff base linkage. Thus, binding of the
S-donor and concomitant release of one carbonyl ligand is pre-
ferred in 4CO. The two remaining carbonyl ligands are bound
in cis fashion (Mn–C(O) = 1.789(3), 1.789(3) Å), as is often obli-
apparent. Overall, due to the varying coordination geometries,
gatory in manganese dicarbonyls. The remaining coordination we unambiguously assigned each CO stretch. Future com-
site is occupied by bromide at a distance of 2.5564(7) Å.
plexes could be structurally assigned on this basis (IR) in the
absence of X-ray structural characterization.
1H NMR spectra of 1CO, 3CO and 4CO. This set of complexes
represents all of the structural motifs found in the X-ray struc-
tures that are reported in this work. The properties of several
complexes in CD2Cl2 solution were investigated with regard to
the bound/unbound thioether unit. We found that two com-
Spectroscopic characterization
Infrared spectra. The IR spectrum of [(HNNS)Mn(CO)3Br]
(1CO) is shown in Fig. 4 (bottom red line), wherein the carbonyl
stretches at 2023, 1917, and 1867 cm−1 are due to the three
chemically distinct carbonyls in the complex. Comparison of
this IR with that of the fully ligated dicarbonyl complex
[(NMeNS)Mn(CO)2Br] (4CO) allows identification of the highest
energy (and lowest intensity) peak at 2023 cm−1 in 1CO as the
CO ligand trans from the Npy donor (absent in 4CO). The high
energy of this stretch is presumably due to the strong π-accep-
tor capacity of the pyridine moiety. In the same vein, we assign
1
plexes (1CO, 3CO) exhibited H NMR spectra consistent with a
single species in solution (according to their X-ray structure),
whereas one complex (4CO) exhibited two species in solution.
Below we discuss the straightforward cases of 1CO and 3CO, fol-
lowed by a comparative interpretation of the more complex
phenomena observed for 4CO
.
1
the lowest energy ν(CO) stretches (1867 cm−1 in 1CO
;
The H NMR spectrum of 1CO in CD2Cl2 at room tempera-
ture is shown in Fig. 5. The broadness of the peaks and lack of
observed multiplicities were not resolved by lower temperature
spectra (see ESI, Fig. S1†). Nonetheless, the spectrum indicates
the presence of a single species in solution, and the unbound
state of the thioether-S is indicated by the nearly unchanged
value of the S–CH3 peak in the complex (2.54 ppm) versus that
of the free NNS ligand (2.48 ppm). The furthest downfield
1847 cm−1 in 4CO) as the carbonyl ligand trans from the Br−
donor, due to its greater π-donor strength (greater π*(CO) elec-
tron density). The remaining ν(CO) stretches are of nearly iden-
tical value (1917 cm−1, 1CO; 1918 cm−1, 4CO; 1918 cm−1, 5CO),
owing to their analogous chemical environment trans from the
NSB donor. The other tricarbonyls (2CO, 3CO) follow the same
trend. The IR spectrum of the 2-methylpyridine species 3CO is
shown at the bottom of Fig. 4. Although it is a tricarbonyl like
1CO, the spectrum of 3CO does not exhibit three distinct ν(CO)
bands. Rather, the two lower energy stretches (assigned as
trans to Br, and trans to NSB) appear largely merged near
1925 cm−1. The steric (or agostic) perturbation of the CO
ligands’ environment(s) by the ortho methyl group may be
peaks are indicative of the imino NvC(H) (9.27 ppm) in 1CO
,
which is shifted significantly downfield from the free ligand
(δfree = 8.57 ppm). This claim is supported by the spectrum of
the iminomethyl complex 4CO (vide infra), which exhibits no
Schiff base H (singlet) in the 8.5–9.5 ppm region. The C–H at
the ortho position of the bound pyridine experiences a smaller
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Fig. 5 1H NMR spectra (500 MHz) of [(NNS)Mn(CO)3Br] (1CO, top panel) and [(MeNNS)Mn(CO)3Br] (3CO, bottom panel) in CD2Cl2 at 25 °C.
downfield shift upon metal binding (δbound = 8.99 ppm; δfree
=
−40 °C (not shown), the peaks revert to broader linewidths due
8.70). This claim is supported by the spectrum of the ortho- to the limited solubility of 4CO in CD2Cl2. As envisaged from
methylpyridine complex 3CO (vide infra), which exhibits no the X-ray structure of 4CO (thioether-S bound), the complex
ortho Hpy (doublet) peak near between ∼8.5 and 9.0 ppm.
should exhibit two resonances in the alkyl region, emanating
The 1H NMR spectra of [(MeNNS)Mn(CO)3Br] (3CO) in from (i) the NvC(CH3) unit (2.34 ppm in the free ligand), and
CD2Cl2 is shown at the bottom of Fig. 5. This complex exhibits (ii) the thioether moiety (δfree = 2.43 ppm); however, four peaks
a
much sharper spectrum, and is broadened by VT are observed in this region. Similarly, a set of eight resonances
experiments at lower temperature (ESI, Fig. S2†). This complex are expected in the aromatic region, but a more complicated
exhibits a NvC(H) resonance at 8.46 ppm, which is quite set of >10 peaks is observed. One readily identifiable aromatic
similar compared to the free MeNNS ligand (δfree = 8.55 ppm). feature(s) should be the ortho-H of the bound pyridine,
In the alkyl region, the complex exhibits two resonances observed near ∼9 ppm. Two broad peaks are observed in this
(each 3H, singlet) corresponding to the S–CH3 and o-CH3(py) region (9.29, 8.98 ppm) in a 1.3 : 1 ratio. This suggested that
units. These peaks are not readily assigned in the spectrum two species were present in solutions of 4CO in CD2Cl2.
of the free ligand (2.63, 2.47 ppm; 3H, singlets) due to
Overall, integration of the spectrum of 4CO at −20 °C
their close proximity. As the pyridine unit is bound to the Mn (optimum resolution) reveals that the two distinct sets of
center in all structurally characterized complexes, we ascribe peaks are present in a roughly 1.3 : 1 ratio; the ratio is tempera-
the downfield-shifted resonance at 3.13 ppm to the Mn-bound ture independent in the accessible range (RT → −40 °C). In
ortho CH3(py) unit. We do not ascribe this resonance to the the alkyl region, the furthest downfield methyl resonance at
S–CH3 unit (instead ascribed to the feature at 2.53 ppm), as 3.01 ppm is significantly different than the furthest downfield
only a Mn-bound thioether unit would exhibit a feature resonance in the free ligand (2.43 ppm). Based on the X-ray
near ∼3 ppm (vide infra). The X-ray structure of 3CO (Fig. 2) structure of 4CO that exhibits a Mn-bound thioether-S, we
also strongly argues against thioether-S binding in solution assign the peak at 3.01 ppm as the Mn-bound S–CH3 unit. By
(crystallized from CH2Cl2, NMR studies performed in CD2Cl2). integration, the corresponding Schiff base (NvC)CH3 unit is
In contrast, we do present evidence of thioether-S binding in observed at 2.47 ppm. The furthest upfield peak in the spec-
the case of 4CO (vide infra), which does exhibit a Mn-bound trum of 4CO (2.39 ppm) is quite similar to the Schiff base
thioether unit in the X-ray structure.
methyl peak in the free ligand (2.34 ppm), and we assign it as
1
The H NMR spectra of [(NMeNS)Mn(CO)2Br] (4CO) is shown the (NvC)CH3 unit in a second species. As the one remaining
in Fig. 6. The room temperature spectrum of 4CO (top) exhibits alkyl resonance at 2.54 ppm is quite similar to that of the
broad peaks devoid of observable multiplicities. The resonance unbound thioether in the free ligand (2.43), we assign it as
peaks become somewhat sharper upon decreasing the temp- belonging to this second species in solution in which the
erature from RT → −40 °C. However, no changes in the thioether-S is unbound. Based on integration values, we assign
number of peaks nor chemical shifts are observed. Below the peak at 8.98 to the S-bound (S trans to py) species and the
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Fig. 6 Variable temperature 1H NMR spectra (500 MHz) of solution species derived from dissolving crystals of [(NMeNS)Mn(CO)2Br] (4CO) in CD2Cl2.
9.29 feature to the S-unbound species (CO trans to py). Due to routine, analytical UV/vis measurements, we noted a drastic
lack of observable multiplicities, the assignment of the change in color upon dilution of red solutions of 4CO to a
remaining aromatic peaks remains ambiguous.
blue/violet color. This indicated a possible change in relative
We hypothesized that the second species in solution (i.e. proportions of species (5- and 6-coordinate) as a function
“S-unbound”, not crystallographically characterized) was the of concentration.
five-coordinate species as shown in Scheme 3. To our
The fact that the peaks do not coalesce at any tested temp-
knowledge, there are three authentic precedents (i.e. no redox erature indicates that the kinetics of the equilibrium do not
active ligand) regarding the isolation and X-ray structural occur on the NMR timescale. This is not surprising in light of
characterization of five-coordinate Mn(I) carbonyls; all are the low-spin, d6 kinetically inert configuration of the pseudo-
deep blue, diamagnetic species supported by P4 phosphine octahedral manganese center, which would prevent rapid
donor sets (2 × dppe, depe, etc.) in rigid square pyramidal interconversion between conformers. The octahedral ligand
orientation.34–36 The 1H NMR experiments described above field splitting would be amplified by the t2g type interactions
(∼10–100 mM concentrations) indicated that the relative pro- among the metal–carbonyl {dp(Mn)–π*(CO)} bonding units.
portions of the two populations were not affected by tempera- Thus, there are two distinct, kinetically stable populations of
ture (RT → −40 °C). However, in the course of performing species during the NMR experiment in CD2Cl2. The reason(s)
for the preferential crystallization of the S-bound conformer
from CH2Cl2 in the synthetic procedure is not clear at this
time.
The observed broadness of the peaks in 1CO and 4CO is
most likely due to the conformational flexibility of the
unbound aryl-thioether unit. While the broad linewidths in
the case of 4CO may be related to the slow interconversion of
the two species (S-bound and S-unbound), the reason for the
peak broadness in 1CO remains unclear. As a corollary, the
reason for the structural rigidity in the case of 3CO (viz. a vis
sharp H NMR spectra) also is not clear. It is possible that the
Scheme 3 Speciation observed between diamagnetic 6- and 5-coordi-
nate species as observed in the case of 4CO in 1H NMR experiments
1
in CD2Cl2.
ortho CH3(py) unit serves as a structural “anchor” that fixes
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Scheme 4 Schematic representation regarding the observed broad linewidths of 1CO and 4CO (but not 3CO) in 1H NMR experiments in CD2Cl2.
the ligand framework in position via an agostic interaction
The structural parameters of 5CO-a versus 5CO-b warrant
with the CO ligand trans to the NSB donor. The interaction of several comments. In the S-unbound state 5CO-a, the fluorophe-
the 2-methylpyridine moiety with the carbonyl ligand (i.e. nyl unit is quite close to the proximal carbonyl: the ipso
pyCH3⋯CuO) may account for the anomalously large down- carbon (C14) is located at a distance of ∼2.95 Å from the carbo-
field shift (∼0.5 ppm) observed for the ortho CH3(py) unit, nyl (C20). This same carbonyl unit exhibits the longest Mn–
which is 2 atoms away from the metal binding site. Note that C(O) distance (1.824(2) Å) of all of the complexes reported here,
by comparison, the Schiff base methyl unit (also 2 atoms away likely due to the close proximity of the fluorophenyl unit. The
from the metal binding N) exhibits an almost immeasurable FPh unit is also twisted out of the conjugated plane by 62.1°,
shift (∼0.1 ppm) upon ligation. As a third case in point, we and even more strikingly it diverges 8° from the expected
observe that the ortho-CH3 bearing 3CO exhibits unusually low linear alignment across the Cpy–Cipso–CF axis. In contrast, the
solubility in CH2Cl2 solution as compared to all other com- S-bound conformer 5CO-b exhibits a more ‘relaxed’ orientation
plexes – even the unsubstituted 1CO, which would be expected of the fluorophenyl unit. The Cipso⋯C(O) distance is elongated
to exhibit the lowest solubility. And although this might be an to 3.097 Å, and the phenyl unit (although still twisted 62.8°
anecdotal coincidence, it may instead be related to its relative out of the conjugated plane), adopts a more conventional,
rigidity (or lack of conformational motion) in solution. Pre- linear orientation along the Cpy–Cipso–CF axis (178.85°). The IR
liminary DFT calculations, however, did not reveal any orbital spectra of the complexes is also consistent with these findings.
overlap or shared MO density between the ortho CH3(py) unit Complex 5CO-a (the tricarbonyl) exhibits three CO stretches at
and the carbonyl ligand (Scheme 4).
2021, 1943 and 1914 cm−1, while 5CO-b (dicarbonyl) exhibits
two CO stretches at 1934 and 1872 cm−1. These are also con-
sistent with the ν(CO) values for 1CO and 4CO depicted in Fig. 4
(Scheme 5).
S-bound and -unbound conformers derived from FPhNNS
Lastly, we wished to determine the effect of a much larger sub-
stituent at the pyridine ortho position. Under identical reaction
conditions, metalation of the fluorophenyl derived ligand
FPhNNS has led to structural characterization of both confor-
Electronic absorption spectra
mers (S bound/unbound). Fig. 7 below shows conclusively that In making routine UV/vis measurements, we noticed blue or
the fluorophenyl ligand can support both binding modes. Red violet colors observed in dilute solutions of several of the
crystals (blocks) of the tricarbonyl complex 5CO-a (S unbound) complexes – namely 2CO, 4CO and 5CO-b (λabs = 611, 560,
were obtained by fast vapor diffusion of pentane (∼12 h to 1 d) 612 nm respectively); the dicarbonyls 4CO and 5CO-b crystallized
into the reaction mixture in CH2Cl2. Alternatively, crystalliza- as black crystals, which upon crushing in KBr afforded grayish-
1
tion of the same reaction mixture via slow vapor diffusion of violet powders. [Note that 4CO showed multiple species in H
Et2O (2 to 3 days) afforded crystals of 5CO-b (black needles, S NMR experiments.] In contrast, species exhibiting orange to
bound). The 1H NMR spectrum of 5CO-a reveals a single species red solutions were correlated with complexes that exhibited a
in solution over the course of one experiment (minutes to single set of peaks in the 1H NMR spectrum (1CO, 3CO). Table 3
1–2 hours).
summarizes the collected UV/vis data.
Fig. 7 ORTEP diagrams (50% ellipsoids) of the structural conformers of the FPhNNS ligand: [(FPhNNS)Mn(CO)3Br] (5CO-a, left) and [(FPhNNS)Mn-
(CO)2Br] (5CO-b, right). For a description of the full disorder model found for 5CO-b, see X-ray section in Experimental and ESI (Fig. S2†).
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Scheme 5 Preparation and sequential isolation of [(FPhNNS)Mn(CO)3Br] (5CO-a) and [(FPhNNS)Mn(CO)2Br] (5CO-b).
Table 3 UV/vis data (wavelengths in nm, ε-values in M−1 cm−1) of
manganese carbonyls determined in dry CH2Cl2 under argon atmos-
phere (298 K)
CH3 or OCH3) induces a ‘puckering’ of the bound NNS frag-
ment; this tension is released upon binding of the sulfur in
the dicarbonyl. For R2 (Schiff base substituent), conversion of
the tricarbonyl to the dicarbonyl is accelerated by even a small
steric change (R2 = H → CH3), due to steric repulsion of the
S–CH3 unit with NvC(CH3) in the unbound state. This results
in several examples of a {Mn(CO)2}+ fragment supported by
a neutral, thioether-containing ligand.
λ1 in nm
λ2 in nm
Complex
Color
(ε in M−1 cm−1
)
(ε in M−1 cm−1
)
1CO [(NNS)Mn(CO)3Br]
2CO [(OMeNNS)Mn(CO)3Br] Blue
3CO [(MeNNS)Mn(CO)3Br]
4CO [(NMeNS)Mn(CO)2Br]
5COa [(FPhNNS)Mn(CO)3Br] Red
5COb [(FPhNNS)Mn(CO)2Br] Teal
Orange 323 (20 000)
354 (4000)
Yellow 314 (7490)
—
611 (4000)
488 (1570)
560 (800)
497 (1120)
612 (500)
Violet
319 (7450)
325 (6300)
332 (3250)
Summary
— No peak observed in 450–650 nm region.
We summarize our findings in the following points:
(a) Consistent with others’ findings, thioether-S donors are
generally not favorable donors in manganese(I) carbonyl systems.
(b) Substitution of an iminomethyl moiety can enforce
binding of the thioether-S, likely due to unfavorable steric
interactions in the unbound conformation of the ligand.
(c) In the iminomethyl ligand (NMeNS), 1H NMR studies
reveal an equilibrium of the 6-coordinate (S bound) and a
second [possibly 5-coordinate (S unbound)] species. The 1H
NMR spectra of the aldehyde-derived NNS complexes reveal a
single conformer – the S-unbound species.
We hypothesize that the blue species observed with 4CO in
CH2Cl2 (where two species were observed in 1H NMR, vide
supra) may be the 5-coordinate species in which the thioether-
S is unbound. There are several previous reports of 5-coordi-
nate Mn(I) carbonyls that have been structurally characterized:
[(PNMeP)(dppm)Mn(CO)]+, [(dppe)2Mn(CO)]+ and [(depe)2Mn-
(CO)]+.34–36 Each complex was described as “dark blue” or
“deep blue” by the reporting authors (no UV/vis data reported),
both in solution and in crystalline form. It is possible that the
blue species observed in this work under dilute conditions in
non-coordinating solvent (dry CH2Cl2, argon atmosphere) is
also such a 5-coordinate species. It also is possible that at
higher concentrations, Mn⋯Mn interactions or intermolecular
Mn⋯S(unbound) associations prevent accumulation of sufficient
quantities of the 5-coordinate species. We are further investi-
gating the nature of the blue species by more detailed spectro-
scopies and structural characterization attempts.
(d) A significant electronic and steric substitution at the
pyridine ortho position (fluorophenyl) leads to isolation of
both S-bound and S-unbound species with resolution provided
by kinetics (12 h versus 3 days crystallization time) and/or
choice of crystallization solvent (pentane vs. Et2O).
(e) We provide several new examples of the {Mn(CO)2}+ frag-
ment stabilized by FPhNNS and NMeNS; such dicarbonyls are
usually stabilized only by phosphine ligands.
(f) A rational design of the ligand framework (R1, R2) may
now lead to the desired NNS coordination motif in manganese
carbonyls.
Conclusion
Overall, the sequential fashion of the isolation of 5CO-a and
5CO-b (and structural characterization of both conformers), as
1
well as H NMR and UV/vis measurements allow us to postu-
late a step-wise scheme to explain all of the results observed
Acknowledgements
herein. Metalation of [Mn(CO)5Br] with NNS-type ligands This research was supported by startup funds to MJR provided
affords an initial product wherein the ligand is bound in NNS by the UT Austin College of Natural Sciences, the Welch Foun-
fashion (S unbound). Depending on the nature of both R1 and dation (F-1822), the ACS Petroleum Research Fund (53542-
R2, conversion to the S-bound species can be fast, slow or not DN13). SEL was the recipient of an Undergraduate Research
observed. In the case of R1, conversion of the tricarbonyl to Fellowship sponsored by the UT Austin Office of Research. The
the dicarbonyl is mildly accelerated by a large change in authors gratefully acknowledge Angela Spangenberg and Steve
sterics. A very bulky R1 substituent (i.e. for R1 = FPh, but not Sorey for technical assistance in acquiring VT NMR spectra.
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We also thank Dr Vince Lynch for assistance in the collection
of X-ray data and refinement of structures.
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