Since about 1980, NMR spectroscopy of coordination compounds in solution has been increas- ingly used on a routine basis to address a multitude of new and older chemical problems. The introduction of two-dimensional correlation methods afforded quick access to parameters for relatively rare spin-1=2 nuclei. Further, three-dimensional solution structures can now routinely be solved, including not only their constitution but also all aspects of configuration, conform- ation, and intra- and intermolecular dynamics.
Although there are now hundreds of publications on the applications of solid-state NMR spectroscopy in coordination chemistry, this technique has not yet made a similar transition. It still remains mostly in the realm of ‘‘specialists,’’ often more interested in the physical properties itself than in their chemical significance. This is certainly partly due tothe additional equipment and knowledge required, but also due to the neglect of chemists who define structural chemistry as X-ray crystallography. At the moment, solid-state NMR exists only as a tool for bridging the gap to solution studies, thereby overlooking the inherent wealth of information available.
Naturally, but certainly not exclusively, solid-state NMR spectroscopy is the method of choice for all those materials that are neither crystalline nor soluble, e.g., coordination complexes
adsorbed or covalently linked to organic or inorganic polymer supports, and compounds in amorphous or glass phases.
2.1.4.2 Principles and Methodologies
Comprehensive treatments of solid-state NMR spectroscopy are available elsewhere.352–358 For this discussion, it is sufficient to express the principal spin interactions as the following sum:
HẳHZ ỵ HCS ỵHQ ỵHDIS ỵHJIS ð17ị
where the subscripts in the Hamiltonians denote the following relevant interactions: Z, Zeeman;
CS, chemical shift;359–361 Q, quadrupolar;362,363D, direct dipolar coupling; and J, indirect or scalar spin–spin coupling.364,365 As NMR measurements are usually carried out in strong magnetic fields, the Zeeman interaction is dominant and the other terms represent only modest perturbations. Only in cases where a quadrupolar nucleus is involved will the magnitude of the quadrupolar coupling constant, , be comparable to, or greater than, its Larmor frequency, L. All of the above interactions transform as tensors under rotation, and thus their magni- tudes depend on molecular orientation.355 The values of the familiar chemical shifts are determined not only by the position of the nucleus within the molecule, but also by the orientation of each molecule or crystallite with respect to the external magnetic field. In solution, where molecules are tumbling fast with respect to the Larmor frequency, thus sampling all possible orientations on a short timescale, chemical shifts and scalar coupling constants average to their isotropic values, and to zero for the traceless quadrupolar and dipolar interactions.
Given the angular dependence mentioned, it is obvious that the anisotropic spectra obtained from the condensed phase must be much richer in information, if more complicated. They contain all the essential geometrical information describing a molecule in terms of angles between the chemical shift, electric-field gradient, direct and indirect dipolar tensors. The angular dependence of the resonance frequencies can be separated from the molecular contribution by monitoring line positions as a function of a systematic rotation of the sample in the three orthogonal directions. Because of resolution restrictions, this can generally be realized only with single crystals, and the most accurate results are still obtained using this method.
In a powder sample, the orientations of the molecules are fixed within the rigid lattice but the crystallites are randomly distributed. Their resonance frequencies generally sum to form a broad
‘‘powder line,’’ representing the sum of their individual contributions. The frequency span encountered for such lines is often larger by orders of magnitude than the differences based on the position within the molecule itself, and the limited resolution is of general concern. Two different approaches to address this problem have been proposed: (i) line-narrowing techniques which simulate the tumbling of molecules either in the spin space using elaborate multipulse sequences,366–368 or in real space with macroscopic rotation of the sample around specific, so-called ‘‘magic,’’ angles369–372with the aim of observing an isotropic spectrum; (ii) application of additional frequency dimensions in two- and multidimensional experiments which correlate or separate the different anisotropic spin interactions.373 A combination is also possible, i.e., correlating an anisotropic spectrum with one of its isotropic counterparts, thus retaining the geometric information while benefitting from the high resolution of the latter.373
The first approach is well established and experiments employing magic-angle spinning (MAS) constitute the bulk of all reported studies. Fast rotation, relative to their frequency span, around an angle of 54 440 reduces the chemical shift and scalar coupling interactions to their isotropic averages and the dipolar interaction to zero. Averaging of the quadrupolar interaction, described by a rank 4 tensor, to zero requires an additional simultaneous rotation around the angle of 30 340which is achieved in the double-rotation (DOR) experiment.370,374,375
Alternatively, in the dynamic angle-spinning (DAS) 2D experiment, the rotor hops sequentially between the angles 37230 and 79 110.376–378Anisotropies at one angle cancel those at the other.
A potential problem in obtaining solid-state NMR spectra involves the longitudinal relaxation times which tend to be long, thus compromising sensitivity. To overcome this, cross-polarization from abundant nuclei, such as1H, tothe dilute spin,S, under observation may be employed.379–383 Recycle delays are then determined by the T1of1H rather than ofS. In addition, the magnetiza- tion of Scan be increased up toa maximum ofH/S.
20 Nuclear Magnetic Resonance Spectroscopy
The quadrupolar-echo experiment represents the most widely used experiment for the observation of quadrupolar nuclei.384 For half-integer nuclei, it may be tuned to observe only the central transition (1/2–1/2), which is perturbed by the quadrupolar interaction only to second order, thus allowing the observation of less dominant anisotropic interactions. A significant improvement in sensitivity can be obtained by ncorporating a spin-echo method such as the Carr–Purcell–Meiboom–Gill sequence into the detection period.385–388The powder line-shape splits into a manifold of sidebands, from which information on the homogeneous and inhomogeneous interactions can be extracted from the line-shape of the sidebands and their envelope, respectively.
One- and two-dimensional multiple-quantum techniques have been introduced for the observa- tion of quadrupolar nuclei with half-integer spin. These methods have proven powerful in resolving overlapping resonances of multiple sites.389–395
2.1.4.3 Spin-1/2 Metal Nuclei
The importance of NMR for molecular structure determination rests primarily on the phenomenon of chemical shielding effects, which are particularly large for the heavy atoms. Isotropic metal chemical shifts obtained from MAS studies are often sufficient to solve most chemical problems, as there is now a large empirical body of data derived from solution NMR. However, taking the orientation dependence of the chemical shielding into account provides considerably more insight into the bonding at the metal center. Studies concerned with establishing the span of the metal chemical- shift anisotropy and its relation to coordination geometries and oxidation states have been reported.396–419 The topic was reviewed in the 1990s for thed- andp-block elements,396,397and in particular alsofor mercury compounds.398Procedures for intrumental set-up have been suggested.399 The span,420 ẳ11–33, of the 199Hg chemical shift tensors in [Hg(S-2,4,6-iPr3C6H2)2], [PPh4][Hg(S-2,4,6-iPr3C6H2)3], and [NMe4]2[Hg(SC6H4Cl)4] are found to be 4479, 1548, and 178 ppm, respectively.398 This decrease, which follows the sequence linear ML2>trigonal ML3>tetrahedral ML4, was alsoestablished for 195Pt shielding, based on the CSA contribution toT1relaxation in Pt0phosphine complexes.29 Interestingly, the sequence found forsp,sp2, and sp3 hybridized carbon is similar.421–423 Octahedral coordination environments generally show smaller spans, e.g., <120 ppm for M2PtCl6, MẳNa, K, NH4 complexes,409–412 whereas those associated with square-planar geometries can be very large,409,413–415 e.g., 10,414 ppm in K2PtCl4.409 Large shielding anisotropies were also observed for 2-coordinate tin(II) com- pounds,406–408as opposed to moderate ones in tin(IV) complexes.404–406
In bioinorganic applications,113Cd has attracted considerable interest,400–402due toits function as an effective spin-spy in monitoring the active site in metalloproteins where the native metals have poor spectroscopic properties. The elements of the shielding tensors as a function of the donor atoms, N, O, and S, and the coordination number have been determined, and empirical correlations to structure deduced.401–403
2.1.4.4 Quadrupolar Metal Nuclei
NMR spectra of quadrupolar nuclei are of particular interest, because they offer a unique opportunity to investigate both the electric-field gradient and the chemical-shift tensors at a common nuclear site simultaneously. Furthermore, many of the biologically relevant nuclei, e.g., 23Na, 25Mg, 39,41K, 55Mn, 59Co,424–427 61Ni, 63,65Cu,428 67Zn,22,429,430 95,97Mo,431,432 are quadrupolar. The measurement and interpretation of solid-state NMR spectra of these nuclei obtained on powders is still a challenge. New spectroscopic techniques385–395 and especially the impressive advances in computational chemistry, which make possible the calculation of both electric-field gradient and chemical-shift tensors, provide much-needed assistance. Solid-state NMR on quadrupolard- andp-metals was reviewed in the early 1990s.396
The Euler angles relating the electric-field gradient and chemical shielding tensors in the acetylacetonato complexes of beryllium,433aluminum,434and cobalt424,427have been determined.
In the case of 59Cothe results arise from single-crystal NMR.427 Euler angles relating the two interactions have also been retrieved from the manifolds of the spinning sidebands of the central and satellite transitions, observed in 51V MAS spectra of a series of ortho- and meta- vanadates.434,436 Reports on the chemical shielding anisotropies for quadrupolar nuclei are becoming more frequent in the early 2000s.424–428,432–439
2.1.4.5 Ligand Nuclei 2.1.4.5.1 1H NMR
The concentrated presence, high abundance, high gyromagnetic ratio, and low chemical-shift dispersion of the proton make this nucleus a difficult one: homogeneous broadening due to strong homonuclear dipole–dipole couplings lead to featureless absorptions much broader than its chemical-shift range. As a consequence of the homogeneous nature of the broadening, slow- spinning MAS only scales the interaction and does not resolve the spectrum into an isotropic part and a spinning side-band manifold. Ultrafast spinning is mandatory; otherwise homonuclear decoupling sequences—either alone368 or in combination366,368 with magic-angle spinning—
would have to be employed. Fortunate cases exist where one particular interaction dominates the dipolar Hamiltonian.
Molecular hydrogen complexes are again special, since the strong dipolar coupling due to the short HH distance dominates all other interactions. Zilmet al.270,440reported the solid-state1H NMR spectrum of W(H2)(CO)3(PCy3)2 showing a distorted Pake-pattern, from which inform- ation on the HH separation and the chemical-shift anisotropy of the hydrogen atoms could be obtained. The temperature dependence indicated an in-plane motion of 16.440 Similar results were also obtained for Mo,270Mn441, and Ru270hydrogen complexes.
2.1.4.5.2 2H NMR
The deuterium quadrupolar coupling constant is a sensitive measure of the magnitude of the electric-field gradient at the nucleus, and is therefore affected by the bonding situation. The deviation from axial symmetry sheds light on the bonding mode, e.g., in distinguishing molecular hydrogen complexes from classical hydrides. The facile isotope substitution for hydride and hydrogen ligands renders the observation of deuterium an alternative to1H. In general, motional properties of ligands are easily assessed with the 2H NMR method.
A highly asymmetric quadrupolar tensor has been found for OsCl2(D2)(CO)(PiPr3)2, together with characteristic line-shapes in the MAS side-bands, originating from interference of dipolar and quadrupolar interactions.442 From the strength of the dipolar interaction, DD distances could be obtained as well as the relative orientations of the tensors.442 Solid-state 2H NMR data have also been reported for W(D2)(CO)3(PiPr3)2and interpreted as a motionally averaged quadrupolar tensor of axial symmetry due to significant molecular motion.443 Coherent D2
rotational tunneling and incoherent D2dynamics were shown to affect the2H NMR lineshapes of nonclassical Ru(D2) complexes.444,445
Studies on Zeise’s salt and the complex Pt(2-C2D4)(PPh3)2ruled out rotation of the ethylene ligand based on the observed 2H quadrupole coupling constants, which are comparable in magnitude to those found in rigid olefins.446
2.1.4.5.3 13C NMR
Structural data on coordination compounds can be gathered from 13C CP–MAS spectra of the carbon nuclei in the backbone of ligands.447Here, however, we restrict the discussion to carbon nuclei directly bound to the metal center, i.e., -bonded CO,448–459 CN,428,429,460–465
acetylide,414,415 alkyl466,467 and aryl ligands,447,466 and the -bonded alkenes, alkynes,446,466–469
cyclopentadienyls, and arenes.470–474
Metal carbonyl complexes are at the center of many areas of chemistry. Interest has focussed on their dynamic processes,447,449but also on the shielding tensor itself.454–458With respect tothe latter, the three modes (1to3) of CO bonding have been investigated;455CO coordination is characterized by 13C tensors spanning ca. 400 ppm and 200 ppm for terminal and bridging COs, respectively.
A large deviation from axial symmetry is observed for the2-mode, and it is interesting to note that in Fe(CO)5the shielding of the equatorial ligands deviates slightly from axial symmetry.455
Copper(I) cyanides have been extensively studied by 13C solid-state NMR, and reveal axial chemical-shift tensors plus the novel coupling constants 1J(63,65Cu, 13C) and Cu–C separations from the dipolar coupling.428,460 CuCN has a linear polymeric structure [–Cu–CN–]n, with the cyanides subject to‘‘head–tail’’ disorder.460
22 Nuclear Magnetic Resonance Spectroscopy
The low-frequency isotropic chemical shift normally observed for -bonded alkyl organometallic carbon is due to one particularly shielded component, whose orientation with respect to the carbon–metal bond could not be determined.466For methyl groups, the span,of the shielding tensor is larger than for organic compounds and depends strongly on the other ligands present in the complex.466
Powder as well as MAS studies are available for several2-bonded olefins.446,466–469The spans of the chemical-shift tensors are reduced with respect to the free olefins, which is discussed in terms of the Dewar–Chatt–Duncanson model of-donation and-back-bonding.446,468The CC bond lengths and the orientations of the shielding tensor elements are available from dipolar- chemical shift methods and 2-D spin-echo experiments on the doubly13C labelled olefins.445,469
5-cyclopentadienyl and 6-benzene ligands of transition-metal complexes,466,470–473
but also some derivatives of alkali or main-group elements,474exhibit single13C resonances and shielding tensors of axial symmetry at room temperature. Both observations point to relatively fast rotations around the respective 5- and 6-fold local rotor axis.470,471
2.1.4.5.4 15N NMR
The question of nitrogen vs. carbon in cyanides as a donor atom has been investigated by 15N solid-state NMR.428,460,461This problem could also be addressed from the metal side, given that a suitable spin-1/2 metal is involved.475 Interest in the antitumor reagent cis-[PtCl2(NH3)2] prompted studies on the15N characteristics of this and related types of platinum complexes.476–478 Cobaloximes with aniline and pyridine ligands have been investigated, yielding the orientations of the nitrogen shift and the cobalt electric-field gradient tensors with respect to the molecular frame, plus the signs and magnitudes of1J(59Co, 15N) scalar and the59Co quadrupole coupling constants.479Nitroso and aryl-nitroso metalloporphyrins have been studied with interest towards O2binding.480,481
2.1.4.5.5 17O NMR
Oxygen has been studied in conjunction with 13C in transition-metal carbonyls and is often found to be more sensitive to structural changes than the latter.450,455–458
2.1.4.5.6 19F NMR
After some initial MAS studies of tin fluorides,405this nucleus seems tohave attained ‘‘sleeping beauty’’ status. New interest in fluorination catalysis may awaken some interest. Potential exists for crystal structure refinements, where often Fand OHare indistinguishable.482
2.1.4.5.7 31P NMR
31P in phosphines, phosphates, and their metal complexes remains the most studied ligand nucleus. Developments up to 1992 have been summarized by Davies and Dutremez.483 MAS studies often show isotropic chemical shifts and isotropic scalar coupling constants very similar to those derived from solution studies.484–488
The problem of1vs.2-coordination in phosphaalkene platinum chemistry has been solved by
31P MAS NMR.489,490 Whereas the solution (31P) value and the smallish J(195Pt,31P) coupling constant were consistent with the2bonding mode, (29),1-coordination, (30), was unambigously assigned in the solid from the high-frequency chemical shift and the large one-bond coupling to platinum, in agreement with an X-ray crystal structure determination.491
Static disorder in crystals represents a problem which can lead to a crystallographic symmetry higher than its molecular symmetry, thus hiding important structural features. In this respect the presence of a PHPd agostic interaction in the dinuclear Pd(I) complex (31) was established492in solution and shown to remain intact in the solid phase, as evidenced from the four quite different31P parameters of the four P ligands, which were in good agreement with their solution values.493The centrosymmetrical crystal structure suggested that only half of the molecule would be independent.493
CHR1 P
R2
(29)
CHR1
P R2 Pt
(30) P Ru OTf
PPh2OH Ph2
OTf
(28)
Pt
PtBu2
Pd
PtBu2
Pd PHtBu2
Bu2HtP
H +
(31)
Due topacking effects, molecules in the solid state generally reveal lower symmetry relative tothe solution phase, and often there are as many resonances resolved as there are sites in the coordination complex.484–487Spin–spin coupling, unobservable in solution, can be employed for structural assign- ments. Unless they are related by an inversion center, spins are magnetically inequivalent even if they are related by symmetry planes or axes. Although they have the same isotropic chemical shifts, they do have different orientations of their chemical-shift tensor. In cases where such spins show dipolar interactions (direct and/or indirect), a spinning-frequency-dependent splitting of resonances is observed in their MAS spectra, thus offering insight into their molecular structure.494–496
The magnitudes of the one-bond metal–phosphorus coupling constants represent a rich source of structural information.483–490,495–498
The anisotropy of the indirect 199Hg–31P spin–spin coupling constant has been determined.497–499 Additional information on metal-ligand one-bond coupling constants can be gained from MAS spectra of phosphorus ligands coordinated to quadrupolar metal nuclei. In solution, T1values for the latter nuclei are extremely short, unless they are in a cubic environment, leading to ‘‘self-decoupling’’ effects. In the solid phase, theseT1
Co N
N N
N PPh3
X
O H O
O H O
CH3
CH3 H3C
H3C
OC Mn
OC CO
L CH2Ph
OC
X = CH3 1J(Co,P) = 225 (2) Hz X = Cl 1J(Co,P) = 371 (2) Hz
L = PPh3 1J(Mn,P) = 216 (4) Hz L = PCy3 1J(Mn,P) = 220 (2) Hz L = P(p-Tol)3 1J(Mn,P) = 196 (3) Hz
(32) (33)
24 Nuclear Magnetic Resonance Spectroscopy
values are long, even for quadrupolar nuclei, and coupling to phosphorus ligands is observed.
One-bond scalar couplings have been reported for the following spin >1/2 nuclei: 55Mn,500–502
59Co,500,50263,65Cu,504–51193Nb,500,51295,97Mo,501,51399Ru,514105Pd,515115In.517Within the limits of the high-field approximation, i.e., L> Q, the observed 1J values are readily interpreted.
Occasionally, first-order perturbation theory or even a full treatment of the Hamiltonians is necessary.500,504,517–520It is worth emphasizing that, given the utility of1J(M,L), such information is otherwise not available.
Twonice examples are shown in (32) and (33). The cobaloxime complexes of the type (32) are especially informative,503as they reveal the expected change in 1J(59Co,31P) as a function of the transinfluence of the ligand X.81,82
70
80
90
δ(31P) 70
80 δ(31P) 90
Figure 11 Contour plot showing the isotropic part of the 162.0 MHz 2D31P CP/MAS exchange spectrum (mixẳ1s) recorded for cis-[PdCl2{P(OC6H4-o-Me)}2]. Two crystalline modifications of this complex are indicated by full and dotted lines, respectively. The corresponding part of the conventional31P CP/MAS spectrum is plotted above. A column taken at the position indicated by the arrows and shown to the left, exhibits the complete set of six105Pd(Iẳ5/2, 22.2%) satellites associated with the lowest frequency resonance
with1J(105Pd,31P)ẳ420 (3) Hz.